ReviewArticle Slack, Slick, and Sodium-Activated Potassium...
15
Hindawi Publishing Corporation ISRN Neuroscience Volume 2013, Article ID 354262, 14 pages http://dx.doi.org/10.1155/2013/354262 Review Article Slack, Slick, and Sodium-Activated Potassium Channels Leonard K. Kaczmarek Departments of Pharmacology and Cellular and Molecular Physiology, Yale University School of Medicine, 333 Cedar Street, New Haven, CT 06520-8066, USA Correspondence should be addressed to Leonard K. Kaczmarek; [email protected] Received 24 February 2013; Accepted 18 April 2013 Academic Editors: Y. Bozzi, A. Kulik, and W. Van Drongelen Copyright © 2013 Leonard K. Kaczmarek. is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. e Slack and Slick genes encode potassium channels that are very widely expressed in the central nervous system. ese channels are activated by elevations in intracellular sodium, such as those that occur during trains of one or more action potentials, or following activation of nonselective cationic neurotransmitter receptors such as AMPA receptors. is review covers the cellular and molecular properties of Slack and Slick channels and compares them with findings on the properties of sodium-activated potassium currents (termed K Na currents) in native neurons. Human mutations in Slack channels produce extremely severe defects in learning and development, suggesting that K Na channels play a central role in neuronal plasticity and intellectual function. 1. Introduction A key determinant of the many varied types of firing patterns that can be observed in excitable cells is the number and types of potassium channels that are expressed in the plasma mem- brane. In contrast to the relatively low number of genes for other channels that regulate excitability (10 known genes each for voltage-activated sodium and calcium channels), there are at least 77 known genes that encode alpha subunits of potassium channels [1]. Moreover, in contrast to sodium and calcium channels, in which the entire pore-forming channel is formed from one polypeptide, most potassium channels are tetramers that result from the coassembly of either the same or different subunits from the same family. is finding, coupled with the fact that most potassium channel genes have multiple splice isoforms and that the subunits themselves are subject to a variety of posttranslational modifications, means the potassium channel superfamily is capable of producing an extraordinarily large variety of channels with different kinetic behaviors and voltage dependences. e pore-forming -subunits of potassium channel can be divided into voltage-dependent (K v ) subunits, inward rectifier (K ir ) subunits, two pore (K 2P ) subunits (which assemble as dimers rather than tetramers), those activated by intracellular calcium (K Ca subunits), and those activated by intracellular sodium (K Na subunits). While the existence of K Na channels has been recognized since the early 1980s, it is only in the last ten years or so that their molecular identity has been known. Research over this time has revealed that rather small changes in the function of these channels can have devastating effects on neuronal development and intellectual function [2, 3]. is review will summarize our current understanding of this class of channels and how they contribute to the electrical and cellular properties of nerve cells. 2. History of K Na Channels Potassium channels that are activated by an increase in cytoplasmic levels of sodium ions, commonly termed K Na channels, were first reported in cardiomyocytes [7]. Since that time, K Na channels recorded at the single-channel level, or macroscopic currents that can be attributed to K Na channels, have been described in a wide variety of mammalian neu- rons [8–19], and some of these studies have been reviewed previously [20, 21]. High levels of K Na currents have been found in neurons of the dorsal root ganglion [5, 22, 23]. e existence and general properties of K Na currents have been relatively well conserved throughout evolution and have been extensively studied in larval lamprey spinal cord neurons [24– 28]. ey have been described in a wide variety of neurons
Transcript of ReviewArticle Slack, Slick, and Sodium-Activated Potassium...
Hindawi Publishing CorporationISRN NeuroscienceVolume 2013 Article ID 354262 14 pageshttpdxdoiorg1011552013354262
Review ArticleSlack Slick and Sodium-Activated Potassium Channels
Leonard K Kaczmarek
Departments of Pharmacology and Cellular and Molecular Physiology Yale University School of Medicine 333 Cedar StreetNew Haven CT 06520-8066 USA
Correspondence should be addressed to Leonard K Kaczmarek leonardkaczmarekyaleedu
Received 24 February 2013 Accepted 18 April 2013
Academic Editors Y Bozzi A Kulik and W Van Drongelen
Copyright copy 2013 Leonard K Kaczmarek This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited
The Slack and Slick genes encode potassium channels that are very widely expressed in the central nervous systemThese channelsare activated by elevations in intracellular sodium such as those that occur during trains of one or more action potentials orfollowing activation of nonselective cationic neurotransmitter receptors such as AMPA receptors This review covers the cellularand molecular properties of Slack and Slick channels and compares them with findings on the properties of sodium-activatedpotassium currents (termed KNa currents) in native neurons Humanmutations in Slack channels produce extremely severe defectsin learning and development suggesting that KNa channels play a central role in neuronal plasticity and intellectual function
1 Introduction
A key determinant of the many varied types of firing patternsthat can be observed in excitable cells is the number and typesof potassium channels that are expressed in the plasmamem-brane In contrast to the relatively low number of genes forother channels that regulate excitability (10 known genes eachfor voltage-activated sodium and calcium channels) thereare at least 77 known genes that encode alpha subunits ofpotassium channels [1] Moreover in contrast to sodium andcalcium channels in which the entire pore-forming channelis formed from one polypeptide most potassium channelsare tetramers that result from the coassembly of either thesame or different subunits from the same familyThis findingcoupledwith the fact thatmost potassium channel genes havemultiple splice isoforms and that the subunits themselves aresubject to a variety of posttranslational modifications meansthe potassium channel superfamily is capable of producing anextraordinarily large variety of channels with different kineticbehaviors and voltage dependences
The pore-forming 120572-subunits of potassium channel canbe divided into voltage-dependent (Kv) subunits inwardrectifier (Kir) subunits two pore (K2P) subunits (whichassemble as dimers rather than tetramers) those activatedby intracellular calcium (KCa subunits) and those activatedby intracellular sodium (KNa subunits) While the existence
of KNa channels has been recognized since the early 1980sit is only in the last ten years or so that their molecularidentity has been known Research over this time has revealedthat rather small changes in the function of these channelscan have devastating effects on neuronal development andintellectual function [2 3] This review will summarize ourcurrent understanding of this class of channels and how theycontribute to the electrical and cellular properties of nervecells
2 History of KNa Channels
Potassium channels that are activated by an increase incytoplasmic levels of sodium ions commonly termed KNachannels were first reported in cardiomyocytes [7] Since thattime KNa channels recorded at the single-channel level ormacroscopic currents that can be attributed to KNa channelshave been described in a wide variety of mammalian neu-rons [8ndash19] and some of these studies have been reviewedpreviously [20 21] High levels of KNa currents have beenfound in neurons of the dorsal root ganglion [5 22 23] Theexistence and general properties of KNa currents have beenrelatively well conserved throughout evolution and have beenextensively studied in larval lamprey spinal cordneurons [24ndash28] They have been described in a wide variety of neurons
2 ISRN Neuroscience
pArg409Gln
93
77
407Ser pAla913Thr
RCK1
438
517
684
RCK2
806
351
360 1124
1099
1096 1221
P
Figure 1 Schematic diagram of a Slack KNa channel subunitdepicting the relative positions of the two RCK domains consensusPKC phosphorylation sites (depicted simply as red numbers cor-responding to the positions of the amino acids) the position ofthe regulatory PKC site Serine 407 and the positions of the twocharacterizedmutations that give rise tomalignantmigrating partialseizures of infancy (blue) (modified from [2])
in the nervous system of invertebrates including antennal-lobe neurons of the sphinx moth Manduca sexta [29]Kenyon cells of mushroom bodies in crickets [30] pressure119875 neurons in the leech [31] and bag cell neurons that regulatereproductive behavior in the sea hare Aplysia californica [32]
Although the major focus of studies of KNa currentshas been in the nervous system KNa channels have beendescribed in a variety of cardiac cells including guinea pigmyocytes and rat myoblasts [7 33 34] They have also beenfound in guinea pig gastric myocytes [35] mouse diaphragmmuscle [36] circular smooth muscle of the opossum loweresophageal sphincter [37] and the thick ascending limb ofmouse kidney [38] as well as in Xenopus oocytes [39]
3 The Slack Channel
Two genes that encode KNa channels are now known andthese have been termed Slack and Slick [4 40 41] ThetermSlack is derived from ldquosequence likeA calcium-activatedK channelrdquo because part of the pore domain and the fol-lowing S6 domain of Slack is similar to that of the Slo1(BK) large-conductance calcium-activated potassium chan-nel [40] Overall however there is only 7 identity betweenSlack and Slo1 The Slack channel has also been termed theSlo22 channel and also erroneously listed as KCa41 Thenomenclature for the human Slack gene is KCNT1
The overall structure of the potassium channel is similarto that of the voltage-dependent Kv family of potassiumchannels in that there are six hydrophobic membrane-spanning domains S1ndashS6 with a pore P-domain between S5-S6 (Figure 1) With a predicted length of 1237 amino acids itis however very much larger than any of the Kv subunitsand the membrane-spanning domains together representonly one seventh of the entire sequence The majority ofthe Slack sequence represents a very large cytoplasmic C-terminal domainWithin this C-terminal region there are twopredicted RCK (regulators of conductance of K+) domains
that have also been found in Slo1 channels [42] X-raycrystallographic studies of the C-terminal domain of chickenSlack channels have confirmed that the structure of the RCKdomains is likely to resemble those of Slo1 channels [43]
Although the Slack channel was first cloned and ex-pressed in 1998 [40] it was not found to be regulated byinternal sodium ions until 2003 [41] When expressed eitherin mammalian cell lines or Xenopus oocytes and excisedinto solutions that have equal concentrations of potassiumon both sides of the membrane Slack channels have a rel-atively large unitary conductance (sim180 pS) [4 41] but theconductance is reduced by over 50 in physiological solu-tions [40] As has been found for KNa channels in neuronsand other cells Slack channels in expression systems havemultiple subconductance states
As is the case for Kv and Slo1 subunits Slack cur-rents increase with depolarization and in single-channelrecordings the open probability of Slack channels increaseswith depolarization Slack differs from these other channelshowever in that there are no charged residues in the S4transmembrane domain which is the principal voltage sensorof these other channels The residues that confer voltagesensitivity on Slack channels are not yet known
In excised patches the half-maximal concentration foractivation of Slack channels by sodium ions at the cytoplas-mic surface is sim40mM (Figures 2(a) and 2(b)) [4 41] Aswill be described later this EC
50value may however not
represent the situation within intact cells [5] A region withinthe second RCK domain has been identified as a site thatconfers sensitivity to sodium ions [44]This region resemblesone found in inwardly rectifying Kir3 family channels whichcan also be activated by sodium ions [45] Mutation ofeither an aspartate residue (D818N) or a histidine (H823A)within this region in Slack lowers sodium sensitivity by anorder of magnitude [44] This finding suggests that as inSlo1 channels conformational changes in the RCK domainsbrought about by ion binding lead to channel opening
Overall the biophysical characteristics of Slack channelsin heterologous expression systems resemble those reportedfor KNa channels in native tissues which have been found tohave EC
50values for sodium activation of 7ndash80mM and to
have unitary conductances from 100 to 200 pS with multiplesubconductance states
4 Slack Isoforms
The rates of activation KNa currents vary substantially indifferent types of neurons At least in part such diversity mayarise from different Slack channel subunits that are generatedby alternative splicing of SlackRNA Five different transcriptsfrom the rat Slack gene have been described and theseare predicted to produce Slack channels that differ in theircytoplasmic amino termini [46] Two of these transcriptsSlack-A and Slack-B have been expressed functionally andbeen found to have very different kinetics of activation Thefirst isoform to be identified [40] which is now termed Slack-B has a long N-terminal domain making it the largest potas-sium channel subunit currently known When expressed
ISRN Neuroscience 3
0mV minus80mV+80mV 0mV
0mM Na
90mM Na
0mM Na200pA
25ms40K+
119894130K+
119900 30Clminus
(a)
12
1
08
06
04
02
0
[Na] (mM)300100101
119868119868
max
(b)
15pA25ms
0mM Na
50mM Na
2
50mM Na
(c)
[Na] (mM)300100101
12
1
08
06
04
02
0
Nor
mal
ized
NP o
(d)
Figure 2 Activation of Slack and Slick channels by cytoplasmic sodium (a) Representative patch of macroscopic Slack currents recordedat 0 and minus80mV in an excised inside-out patch from Slack-transfected CHO cells in the presence and absence of 90mM sodium (b)Concentration-response relationship of sodium activation of Slack current in excised patches Currents were normalized those recordedin 90mM sodium (c) Excised patch recording at minus80mV from a CHO cell transfected with Slick The cytoplasmic face of the patch wasperfused with a solution containing either 0 or 50mM intracellular sodium (d) Concentration-response relationship of sodium activation ofSlick channels in excised patches Figures modified from [4]
inXenopus oocytes ormammalian cell lines Slack-B channelsactivate slowly over hundreds of milliseconds [4 40 41]
The N-terminal domain of the other characterized splicevariant Slack-A is smaller than that of Slack-B and thesequence of this N-terminal domain very closely resemblesthe N-terminus of the other KNa channel subunit Slick (seenext section) Neuronal expression of Slack-A and Slack-B channels is driven by independent promoters [46] Incontrast to the very slowly-activating Slack-B channels Slack-A channels activate very rapidly upon depolarizationThedif-ferent kinetic behaviors of Slack-A and Slack-B channels arealso evident in single-channel recordings in patches excisedfrom Xenopus oocytes expressing these subunitsThe unitaryconductance of the two isoforms is identical The gating ofSlack-A channels is however very rapid During sustaineddepolarization brief transient openings to fully open state areinterspersedwith repeated opening to subconductance statesIn contrast the mean open time of Slack-B channels is about6 times longer and subconductance states although they
are very evident are less frequent than in Slack-A channels[46]
The potential effects of the expression of either Slack-Aor Slack-B on neuronal firing patterns have been addressed innumerical simulations Neurons in which potassium currentsare dominated by a Slack-A-like current adapt very rapidly torepeated or maintained stimulation In contrast Slack-B cur-rents allow neurons to fire rhythmically during maintainedstimulation and allow the rate of adaptation rate to vary withthe intensity of stimulation [46]
5 Slick a Second KNa Channel
The second known gene that encodes a KNa channel wastermed Slick for sequence like an intermediate conductanceK channel because it is closely related to the Slack geneand Slack channels have conductance that is intermedi-ate between that of Slo1 (BK) calcium-activated potassium
4 ISRN Neuroscience
channels and most other potassium channels [4] The Slickchannel has also been termed the Slo21 channel and thehuman Slick gene is KCNT2 Overall the sequence of Slickis sim74 identical to that of Slack and the transmembranedomains and the RCK domains of Slick and Slack are almostidentical The greatest divergence between Slack and Slickoccurs at their distal C-terminal regions
The predicted cytoplasmic N-terminus of Slick is similarto that of the Slack-A isoform Consistent with this fact thecharacteristics of Slick currents recorded inmammalian cellsor Xenopus oocytes resemble those of Slack-A Althoughthe unitary conductance of Slick channels (sim140 pS) isslightly smaller than that of Slack channels Slick channelsactivate very rapidly with depolarization and have multi-ple subconductance states [4] The estimated half-maximalconcentration for activation of Slick by sodium 89mMis greater than that for Slack channels (Figures 2(c) and2(d)) Moreover in contrast to Slack channels which havean absolute requirement for Na+ for channel opening Slickchannels have a basal level of activity in the absence of sodium[4]
In addition to their sodium sensitivity both Slick andSlack channels can be activated in increases in intracellularchloride levels [4 47] Under physiological conditions how-ever Slick channels are markedly more sensitive to this anion[4] The domains that confer sensitivity of these channels tochloride are not known and mutations within a region thatresembles the ldquocalcium bowlrdquo of Slo1 channels and whichhas been established to contribute to the calcium sensitivityof Slo1 channels [48] have not been found to influence thesensitivity of Slack or Slick to either sodium or chloride ions[4]
The Slick channel also differs from Slack in having aconsensus ATP binding site just C-terminal to the secondRCK domain This site renders Slack channel activity to besensitive to cytoplasmic ATP levels The presence of 5mMATP reduces Slick whole cell currents or channel activityin excised patches [4] Further description of the effects ofATP on Slick channels will be provided in a later section onphysiological modulation of KNa channels
6 Heteromer Formation between Slack-Band Slick Subunits
Avariety of lines of evidence indicate that the Slack-B subunitand Slick can coassemble to form heteromeric channelsthat differ in their properties from those of either subunitexpressed alone [49] Moreover assembly of heteromericchannels appears to be specific for the Slack-B isoform anddoes not occur with Slack-A which as described abovediffers from Slack-B only in its N-terminal cytoplasmicdomain When Slick and Slack-B are coexpressed at a 1 1ratio either in Xenopus oocytes or in mammalian HEK293cells there is an 18ndash25-fold increase in current amplitudecompared to expression of either subunit alone [49] Thisincrease in current is associated with a comparable increasein levels of the channel subunits in plasma membrane asassayed by a surface biotinylation assay No differences in
total cellular levels of either protein are however foundwhenthe subunits are expressed singly or in combination with theother subunit
Themacroscopic currents of the Slack-BSlick heteromersdiffer from those of the homomers [49] For example thetime for the SlickSlack-B currents to reach 90 activationis significantly longer than that for either Slick or Slack-Bhomomeric currents In addition the unitary conductanceof the Slack-BSlick channels is intermediate between that ofSlack (sim180 pS) and Slick (sim140 pS) A direct demonstrationof this fact was obtained by making a pore mutant of theSlick channel Slick-EE (Q276E Y279E) that increases itsunitary conductance to sim500 pS Coexpression of Slick-EEwithwild-type Slack-B resulted in channels that had a unitaryconductance of sim325 pS a value that is clearly distinct fromthat for either channel alone providing definitive electro-physiological evidence for the formation of SlickSlack-Bheteromers
Coimmunoprecipitation experiments have also demon-strated that Slack-B forms a protein complex with Slick inboth rat brain tissue and heterologous expression systems[49] No coimmunoprecipitation could however be foundfor epitope-tagged Slick-A and Slick subunits This is con-sistent with electrophysiological findings demonstrating thatcoexpression of Slack-A with Slick does not increase whole-oocyte currents compared to expression of either subunitalone and that no intermediate conductance single-channelactivity can be observed when Slack-A is expressed withthe Slick-EE mutant Moreover coexpression of Slack-A withSlick does not increase levels of surface expression of eithersubunit
The specificity of SlackSlick interactions for the Slack-Bisoform is dependent on the extended N-terminal domainof Slack-B This has been demonstrated by constructing achimeric channel that replaced the cytoplasmic N-terminaldomain of Slick with that of Slack-B Coexpression of thismodified Slick channel with wild-type Slick channels inoocytes produced a 30-fold increase in whole-oocyte cur-rents which is similar to the increase obtained with Slack-Bsubunits [49] Thus the Slack-B N-terminal domain plays akey role in trafficking andor the level of plasma membraneexpression of heteromeric KNa channels
7 Localization of Slack and Slick Channels inthe Central Nervous System
Slack transcripts of about 45 kb and 75 kb are abundantlyexpressed in the brain and in the kidney [40] By the tech-nique of in situ hybridization in sections of adult rat brainSlack is very highly expressed in neurons but no staining isevident in glial cells Strong hybridization is found through-out the brain including the cerebral cortex hippocampusdeep cerebellar nuclei cerebellar Purkinje cells reticulartegmental nucleus of the pons preoptic nucleus substantianigra and auditory brainstem nuclei Neurons within thethalamus and hypothalamus were found to have a moremoderate level of staining [40] Subsequent quantificationof expression of mRNA levels by probes that are specific
ISRN Neuroscience 5
for either Slack-B or Slack-A isoforms found that bothisoforms are present throughout the brain but that thehighest levels of both isoforms are detected in brainstem andolfactory bulb [46]
Immunocytochemical staining using an antibody to theSlack-B specific N-terminus confirmed the localization ofSlack-B in neurons of the brainstem and olfactory bulb of rat[50] Prominent immunoreactivity was found in the olfactorybulb red nucleus and deep cerebellar nuclei as well asin vestibular and oculomotor nuclei and in the trigeminalsystem and reticular formation where intense staining occursin both cell bodies and axonal fibers As with the in situhybridization studies labeling was prominent in auditorybrainstem nuclei Although staining was also evident inneurons of the thalamus substantia nigra and amygdala theonly region of the cerebral cortex where the Slack-B isoformwas found was in the frontal cortex [50]
Immunolocalization studies have also been carried outusing an antibody targeted against a region in the cytoplasmicC-terminus of Slack [46] This would be expected to recog-nize themultiple Slack isoforms that differ at the N-terminusbut not to recognize Slick channels Immunostaining withthis ldquopan-Slackrdquo antibody found strong labeling of neuronsthroughout the nervous system of mice This included theregions previously identified with the Slack-B antibody aswell as other locations such as the dendrites of hippocampalneurons and olfactory bulb glomeruli [46]
Messenger RNA for the Slick subunit has awider distribu-tion throughout the body than does Slack mRNA and a Slicktranscript of sim69 kB is expressed in rat heart where no SlackmRNA has been found [4] Like Slack however the highestlevels of Slick mRNA are found in the brain Slick mRNA andimmunoreactivity overlap considerably with those for Slackand are found at high levels in olfactory bulb midbrain brainstem hippocampus throughout the cerebral cortex withhigh expression in primary somatosensory and visual regions[51] Like Slack high Slick immunoreactivity is found inneurons of sensory pathways including those in the auditorybrainstem The wide colocalization of Slick with the Slack-Bisoform suggests that the native KNa channels in these regionsare likely to be heteromers of these two subunits
Studies have also found Slack channels to be expressedin neurons of the peripheral nervous system Slack immuno-labeling is found in over 90 of neurons of the dorsal rootganglion and has been localized to both the somata andthe axonal tracts of small- medium- and large-diameterneurons [5 23] Localization to axonal tracts has also beendetected in electrophysiological experiments on peripheralaxons of Xenopus neurons for which a correlation has beenfound between the localization of KNa channels and voltage-dependent sodium channels [52]
8 Pharmacology of KNa Channels
A very common test for the presence of KNa channels inneurons and other cells has been to measure the effects ofreplacement of external sodium by lithium ions on the netoutward current Lithium is a much weaker activator of KNa
channels than sodium Because lithium readily enters cellsthough voltage-dependent sodium channels usually withminimal effects on the inward currents lithium replace-ment in voltage-clamp experiments reduces the net out-ward currents if KNa currents are present In current clampexperiments lithium substitution may alter the shape ofaction potentials or firing patterns in a manner consistentwith partial block of KNa currents The majority of theexperiments described later in the section on contributionsof KNa channels to neuronal firing patterns have tested theeffects of lithium substitution
Studies on native KNa channels in cardiac cells found thatseveral antiarrhythmic drugs inhibit these channels in cardiaccells [53ndash55] Some of these compounds such as quinidinebepridil and clofilium have been also found to be veryeffective and reversible blockers of Slack and Slick channelsexpressed in oocytes and mammalian cells [4 56 57] but arenon-specific in that they also act on a variety of other channeltypes
The pharmacological properties of Slack and Slick chan-nels are similar to each other but differ from those of manyother potassium channels including their closest molec-ular relative the Slo1 channel Slack and Slick are onlyweakly sensitive to the general potassium channel blockertetraethylammonium ions (TEA) [4] External barium ionsproduce a time- and voltage-dependent block of Slack andSlick channels In contrast both subunits are insensitiveto blockers of calcium-activated potassium channels suchas apamin iberiotoxin paxilline and charybdotoxin andto a wide range of blockers of other classes of potassiumchannels [4 57] Although Slick channels are sensitive tocytoplasmic ATP levels they are insensitive to glibenclamideand diazoxide an inhibitor and an activator respectively ofclassical KATP channels [4]
A variety of compounds that activate Slack and Slickchannels are also knownThefirst of these to be describedwasbithionol which in Slack-expressing HEK cells produces arobust increase in the amplitude of Slack currents with anEC50
of 077120583M [56] Similar bithionol-induced increasesin current are observed for Slack-expressing oocytes and fornative KNa currents in neurons of the auditory brainstem [5658] Bithionol reversibly activates Slack channels even whenapplied to the extracellular face of excised patches indicatingthat it acts on Slack channels relatively directly The increasein current occurs though a very marked bithionol-inducedchange in the voltage dependence of Slack channels such thatfull activation is already observed at potentials as negativeas minus40mV [56] Bithionol is however not selective forKNa channels in that it also activates Slo1 calcium-activatedpotassium channels [56 59]
A screen of pharmacologically active compounds usinga rubidium flux assay against Slack channels expressed inCHO cells has revealed other activators [59] These includeriluzole loxapine an antipsychotic agent and niclosamidean anthelmintic agent Loxapine was found to be moreselective than bithionol in that it has no effect of Slo1 calcium-activated potassium channels Electrophysiological experi-ments confirmed that loxapine is effective on recombinanthuman and rat Slack channels and that it activates native KNa
6 ISRN Neuroscience
channels in isolated neurons of the rat dorsal root ganglion[59]
9 Cellular Regulation of KNa Channels
The activity of Slack and Slick channels as well as ofheteromeric SlackSlick channels is potently regulated by avariety of cellular signaling pathways including G-protein-coupled receptors linked to activation of PKA or PKC [2328 30 49 60 61] direct phosphorylation of the channelssubunits [2 49 60] changes in cytoplasmic ATP levels[4] cytoplasmic NAD+ levels [5] PIP2 [62] estradiol [63]hypoxia [41 64] and the fragile X mental retardation proteinFMRP [6 32] This section will summarize these findings
91 Modulation of Slack and Slick by Protein Kinase CTreatment ofmammalian cells orXenopus oocytes expressingSlack channels with diacylglycerol or phorbol ester activatorsof protein kinase C (PKC) leads to a 2-3-fold increase incurrent amplitude and a slowing of the rate of activation[2 60] This effect of PKC has been found to result from thephosphorylation of a single serine residue S407 located in aPKC consensus phosphorylation site in the ldquohingerdquo region ofthe cytoplasmic C-terminus between the S6 transmembranedomain and the first RCK domain [2] Mutation of this serineto an alanine (S407A) which renders the site incapable ofundergoing phosphorylation results in apparently normalSlack currents that are entirely insensitive to PKC activationIn contrast mutation of the twelve other putative PKCphosphorylation sites in the Slack channel (see Figure 1)does not prevent the PKC-induced increase in Slack current[2]
In contrast to Slack channels the action of PKC onSlick channels is to produce a decrease in current amplitude[60] As in Slack this effect is likely to be mediated by amodification within the C-terminus In particular a chimericchannel that contains the N-terminus and membrane span-ning regions of Slack but the cytoplasmic C-terminus ofSlick responds to PKC activation with a decrease in currentMoreover direct application of a constitutively active formof PKC directly to the cytoplasmic face of excised patchescontaining these chimeric channels reduces their activity[60] Nevertheless the specific sites at which Slick subunitsare modified in response to PKC activation are not yetknown
Heteromeric Slack-BSlick channels respond to activationof PKC in a manner that is quite distinct from that of eithersubunit expressed alone [49] In Xenopus oocytes expressingboth subunits application of PKC activators potently sup-presses current bysim90This ismuch greater than the degreeof suppression measured for Slick subunits alone (sim50)and cannot therefore be explained by any linear combinationof Slick or Slack-B currents The finding that heteromericSlack-BSlick channels are regulated even more potently byPKC activation than are the homomeric channels constitutesone of the many pieces of evidence supporting selectiveheteromer formation between these subunits [49]
92 Modulation of Slack and Slick by G Protein CoupledReceptors Slack and Slick have been coexpressed in Xenopusoocytes with the M1 muscarinic receptor and the mGluR1metabotropic glutamate receptor [60]These are G120572q proteincoupled receptors that lead to the activation of PKC andformation of IP3 Accordingly activation of these receptorsleads to an increase in Slack currents and a suppression ofSlick currents consistent with modulation of these chan-nels by PKC There is widespread colocalization of thesereceptors with the KNa subunits throughout the nervoussystem suggesting that modulation of KNa currents by thesereceptors is likely to be widespread [60] Direct modulationof KNa channels by mGluR1 has been observed in nativeneurons within the lamprey spinal cord [28] In these cellsactivation of mGluR1 produces a strong suppression ofcurrent as expected for Slick or Slack-BSlick heteromersand this suppression is prevented by pharmacological inhibi-tion of PKC Interestingly chelation of intracellular calciumwith the chelator BAPTA (12-bis(2-aminophenoxy)ethane-NNN1015840N1015840-tetraacetic acid) prevented modulation of theKNa current by PKC but unmasked an mGluR1-dependentactivation of KNa current by an independent pathway whosemechanism is not yet known [28]
Modulation of Slack channels by G protein-coupledreceptors has been studied using a novel label-free technologyto detect changes in the distribution of mass close to theplasma membrane [61] The characteristic change in massthat normally follows activation of native receptors in HEK-293 cells is significantly modified in amplitude and timingby the coexpression of Slack channels further supporting thefinding of modulation of KNa current by these receptors [61]
93 Modulation of K119873119886
Currents by Cyclic AMP The activityof KNa channels in Kenyon cells isolated from the mushroombody of the cricket (Gryllus bimaculatus) is enhanced uponapplication of either the neurotransmitter octopamine or themembrane-permeable analog cyclic AMP analog 8-Br-cyclic-AMP [30] The increase in channel activity was found to beattenuated by the protein kinase A (PKA) inhibitor H-89Conversely KNa channel activity in these cells is decreasedby the transmitter dopamine or by the cyclic GMP analog8-Br-cyclic GMP and this decrease in open probability isantagonized by the protein kinaseG (PKG) inhibitor KT5823These findings have led to the proposal that modulation ofKNa currents play a role in olfactory learning circuits thatmediate reward and punishment signals in insects which areregulated by octopamine and dopamine respectively [30]
The Slack subunit is expressed at high levels in pri-mary afferent nociceptor neurons located in the dorsal rootganglion [5 23] and KNa currents can readily be recordedin these cells [5 22 23] Inflammatory substances suchas prostaglandin E2 that activate the PKA pathway greatlyincrease the excitability of these neurons sensitizing them tothermal and mechanical stimulation In their resting statethe neurons adapt rapidly to maintained depolarizationstypically generating only one or two action potentials at theonset of depolarization After an elevation of cyclic AMPlevels that activates PKA adaptation is greatly reduced such
ISRN Neuroscience 7
that sustained firing can be recorded throughout a prolongeddepolarization In major part the effects of PKA can beattributed to a decrease in Slack KNa current and suppressionof Slack expression using anti-Slack siRNA produces a reduc-tion in adaptation similar to that produced by PKA activation[23]
In contrast to the effects of PKC on Slack channels thatwere described above the effects of PKA on Slack are indirectand do not involve phosphorylation of the channel subunititself [23 65] Slack channels in HEK-293 cells are insensitiveto cyclic AMP analogs or to the PKA inhibitor KT5720 andapplication of the catalytic subunit of PKA to the cytoplasmicface of Slack channels excised from transfected cells or dorsalroot ganglion neurons has no effect on channel openingInstead activation of PKA decreases Slack current amplitudeby rapidly internalizing Slack channels from the plasmamembrane into internal organelles [23] Such internalizationwas detected by confocal microscopy of neurons expressingSlack channels tagged at their N-terminus with the greenfluorescent protein GFP Elevations of cyclic AMP reducedcolocalization of the labeled channels with a cell surfacemarker A reduction in native Slack channels at the plasmamembrane following PKA activation was also confirmedusing a surface biotinylation assay [23]
94 Modulation of Slack and Slick by PIP2 A variety ofion channels have been shown to be regulated by phos-phatidylinositol 45-biphosphate (PIP2) a phospholipid thatis localized to the inner lipid leaflet of the plasma membrane[66] Exogenously applied PI(45)P2 as well as its isoformPI(34)P2 increases the amplitude of Slack and Slick currentsexpressed in Xenopus oocytes and pharmacological agentsthat are expected to reduce endogenous PIP2 levels reducecurrent amplitude [62] Regulation by PIP2 was found torequire specific lysine residues in the C-termini of thechannelsMutation of these lysines to alanine (K339A in Slackand K306A in Slick) produced functional KNa currents thatwere insensitive to the activating effects of PIP2 [62]
95 Suppression of Slick Channels by Cytoplasmic ATP TheSlick subunit differs from Slack in that it contains a consensusATP binding site just C-terminal to the second RCK domain[4] The current density of Slick currents but not Slackcurrents is reduced by sim80 by the presence of 5mM ATPat the cytoplasmic face of the channels The same full effectis observed with ATP120574S a slowly hydrolyzable ATP analogand the nonhydrolyzable analog AMP-PNP but not withADP Suppression by ATP of Slick currents at all membranevoltages can also be readily detected in excised patches [4]
Confirmation that the effect of ATP on Slick channelsis mediated by the consensus ATP binding site in the C-terminal domain was obtained by site-directed mutagenesisto replace the glycine at residue 1032 in this site with a serine[4]Themutant G1032S Slick channel was fully functional butfailed to decrease channel activity in response to ATP Thesensitivity of Slick channels to ATP levels suggests that theiractivity could be enhanced during periods of high metabolicdemand when cellular ATP levels fall
96 Modulation of Slack by NAD+ KNa channels can read-ily be detected in cell-attached patch-clamp recordings onneurons [5 16 58] This may be surprising given that thecytoplasmic sodium concentration in resting cells is relativelylow (ltsim10mM see Section 10 below) and as described earlierthe EC
50for activation of these channels in excised patches
is relatively high (typically sim40ndash80mM) This discrepancy isresolved by the finding that the open probability and sodiumsensitivity of KNa channels are greatly reduced following exci-sion ofmembrane patches [5 16] suggesting that rundown ofKNa activity in excised patches may result from loss of somecytoplasmic factor
A major cytoplasmic factor that regulates both Slack andSlick channels has been found to be nicotinamide adeninedinucleotideNAD+ [5] Both subunits have a putativeNAD+-binding site within their second RCK domain Application ofNAD+ to the cytoplasmic face of KNa channels excised fromdorsal root ganglion neurons increases their open probabilityand reduces themeasured EC
50for activation of sodium from
sim50mM to sim17mM (Figure 3) NAD+ as well as NADP+but not NADH also increased the open probability of Slackchannels excised from a transfected mammalian cell lineMutation of glycine in the putative NAD+-binding site ofSlack (G792A) resulted in a loss of the ability of NAD+to potentiate activity [5] These findings suggest that KNachannels play a much more prominent role in the normalphysiology of excitable cells than would be suggested by theirproperties in isolated patches
97 Modulation of Slack by Estradiol The open probability ofchannels reconstituted from lipid bilayers from extracts of ratcortex has been found to be increased by 17120573-estradiol [63]Although these channels were not explicitly shown to be KNachannels a similar effect was reported for whole-cell currentsrecorded in Slack-expressing HEK-293 cells Activation wasnot prevented by tamoxifen a blocker of classical estrogenreceptors but may be a consequence of direct binding of 17120573-estradiol to Slack channels themselves [63]
98 Modulation of Slack Channels by Hypoxia Duringhypoxic conditions cytoplasmic levels of internal sodiumand chloride ions as well as those of H+ and CO
2 rise It
has been suggested that activation of KNa channels couldact to hyperpolarize cells during hypoxia providing a pro-tective mechanism [7] This conjecture is supported by thefinding that nematodes lacking the Slack ortholog in thisspecies are significantly more sensitive to hypoxic deaththan are wild-type animals [41] Further support for a rolefor Slack channels in the response to hypoxia comes fromrecordings of Slack channel activity in Xenopus oocytesexposed to increasing concentrations ofH+ andCO
2 demon-
strating that elevations of sodium activate Slack channelsunder cellular conditions comparable to those expected inischemiahypoxia [64]
99 Interactions of Slack with the Fragile X Mental Retarda-tion Protein FMRP Fragile X syndrome the most commoninherited form of intellectual disability in humans is caused
8 ISRN Neuroscience
1
08
06
04
02
0
[Na] (mM)10010
Control (small neurons)NAD
Nor
mal
ized
NP o
Figure 3 NAD+ increases the sodium sensitivity of KNa channelsexcised from adult rat dorsal root ganglion neurons Concentration-response relationship provides estimates of EC
50for sodium activa-
tion of 50mM and 17mM in the absence and presence of NAD+respectively Figure modified from [5]
by loss of expression of the RNA-binding protein FMRP(fragile X mental retardation protein) FMPR is known to berequired for normal activity-dependent protein translationin neurons Yeast two-hybrid and coimmunoprecipitationexperiments have shown that the cytoplasmic C-terminus ofthe Slack subunit interacts with this protein [6] MoreoverSlack-FMRP complexes also contain messenger RNAs thatare targets of FMRP such as the mRNAs encoding Map1band Arc No association of mRNAs with Slack can howeverbe found in mice lacking FMRP indicating that these arebound to Slack despite its interaction with FMRP A peptidearray assay revealed confirmed that FMRP binds selectivelyto sequences at the distal C-terminus of Slack [6]
FMRP is a potent activator of Slack channel activity [6]Application of recombinant FMRP(1ndash298) which containsthemajority of protein-protein interaction domains of FMRPto the cytoplasmic face of Slack channels in excised patchesreversibly increases channel opening by two- to threefold(Figure 4(a)) This is associated with the almost completeelimination of openings to subconductance states which areprominent in untreated patches (Figure 4(b)) FMRP wasfound to have no effect on the channel activity of a Slacktruncation mutant (Slack-BΔ804) which lacks sites found tointeract with FMRP in biochemical assays [6]
Support for a role for FMRP in regulating the amplitude ofnative KNa currents has also come from recordings of neuronsfrom animals lacking the fmr1 gene which encodes FMPR[6] As will be described later KNa channels account for amajor component of potassium current in principal neuronsof the medial nucleus of the trapezoid body (MNTB) Whileloss of FMRP does not affect total levels of Slack proteinthe level of the KNa component of current in these cells issubstantially reduced in the FMRP knockout mice compared
to that in wild-type animals consistent with an activating rolefor this RNA-binding protein [6]
The interaction between Slack and FMRP is likely tobe evolutionarily conserved Both proteins have been foundto be coexpressed in bag cell (BC) neurons which regulatereproductive behaviors in mollusk Aplysia [32] FMRP andSlack immunoreactivity are colocalized at the periphery ofisolated BC neurons and the two proteins can be reciprocallycoimmunoprecipitatedThe native Slack current in these cellswas identified using an siRNA approach and intracellularinjection of FMRP(1ndash298) induced a KNa current that in itsproperties matches the Slack current As for Slack channelsin heterologous expression systems addition of FMRP(1ndash298) to the inside-out patches containing native AplysiaKNa channels increased channel opening In current clamprecordings FMRP(1ndash298) produced a narrowing of actionpotentials
Downregulation of Slack expression in BC neurons usingan siRNA approach had an interesting effect on long-termchanges in the excitability of these cells [32] In untreatedcells brief stimulation of these neurons produces a charac-teristic prolonged (sim30min) discharge that is followed bya prolonged (sim18 hr) inhibitory state Recovery from thisinhibitory state requires new protein synthesis and is blockedby the protein synthesis inhibitor anisomycin Anti-SlacksiRNA did not alter the ability of BC neurons to undergothe normal discharge but like inhibition of protein synthesisprevented recovery from the inhibitory state [32] Althoughother interpretations are possible these findings raise thepossibility that the activity of Slack channels is linked to theregulation of protein translation through the FMRP-RNApathway
As will be described later point mutations in humanSlack channels can produce devastating effects on intellectualdevelopment [2 3] A variety of experiments have demon-strated that activation of ion channels can influence cytoplas-mic processes independently of ion flux through the channels[67] The interaction between the C-terminus of Slack andFMRP raises the hypothesis that conformational changesin the C-terminus could represent such a ldquononconductingrdquofunction of Slack that links neuronal firing to changes inprotein translation
10 Sodium Entry Pathways ThatActivate KNa Channels
Estimates of cytoplasmic sodium concentrations in restingunstimulated neurons obtained using sodium-sensitivemicroelectrodes and ratiometric dyes provide valuesbetween sim4mM and 15mM [68] As noted earlier in thesection on NAD+ regulation of Slack channels this maybe sufficient to provide significant activation of native KNachannels in intact neurons even under resting conditions[5] Internal sodium levels rise in response to stimulationprincipally as a result of entry through voltage-dependentsodium channels and ionotropic ligand-gated receptors suchas AMPA and NMDA glutamate receptors [69] Stimulationparticularly repetitive tetanic stimulation produces local
ISRN Neuroscience 9
Control RecoveryC
1
2
3
C
1
2
3
10pA
2 s
10pA
200ms
S S S S S S S
FMRP(1ndash298)
(a)
2520151050
10000
1000
100
pA
C1
2
3
Recovery
2520151050
10000
1000
100
pA
C1
2
3
S
S
S(N
)
(N)
Control
2520151050
10000
1000
100
pA
C
1 2 3
S
S
S
(N)
FMRP(1ndash298)
(b)
Figure 4 FMRP alters gating of Slack-B channels in inside-out patches (a) Recording of a patch containing 3-4 Slack channels before duringand after transient application of FMRP(1ndash298) to the cytoplasmic face of the patch Top traces show representative 10 s examples of recordingatminus40mVand bottom traces show expanded time views of the same patch Arrowsmarked S indicate the occurrence of subconductance statesthat are suppressed by FMRP(1ndash298) (b) All-points amplitude histograms of Slack channel activity plotted on a logarithmic scale before andafter application of FMRP(1ndash298) and after washout C marks the closed state S indicates subconductance states and numbers on peaks inthe histograms refer to the number of fully open Slack channels Modified from [6]
increases in sodium concentrations that can rise to levelsbetween 45 and 100mM [70 71] In particular high levelscan be attained in restricted compartments such as activedendritic spines
Although the upstroke of action potentials in mostneurons is driven by rapidly-inactivating voltage-dependentsodium currents these currents are transient Evidencesuggest that the much smaller persistent component ofsodium current that does not inactivate with maintaineddepolarization is responsible both for sustained elevationsof internal sodium during stimulation [69] and for selectiveactivation of KNa channels [72 73] It has been found thatin patches excised from somata of rat neurons sodium entrythrough persistent tetrodotoxin-sensitive sodium channels isthe dominant factor that leads to activation of KNa channelsin the same patch [73] Activation of KNa channels by sodiumentry in the isolated patches was enhanced by veratridinewhich prolongs sodium channel openings and was observedin the absence of sodium at the cytoplasmic face of thepatches Moreover activation appeared to be independent ofthe amplitude of the much larger transient sodium current
[73] Although the molecular basis for persistent sodiumchannels is not fully understood further support for a selec-tive activation of KNa currents by entry through persistentchannels has also been found in electrocytes of electric fish inwhich the time course of KNa currents during depolarizationsreflects that of the persistent sodium current and not that ofthe very much larger transient sodium current [74]
A number of studies have documented that KNa currentsare also activated by sodium entry through ionotropic gluta-mate receptors particularly AMPA receptors Slack subunitscan be coimmunoprecipitated with GluR23 AMPA receptorsusing rat brain synaptosomal fractions or lysates of the lam-prey central nervous system [27] Moreover Slack subunitshave been found to bind directly to the PDZ domain of PSD-95 (postsynaptic density-95 protein) a major componentof the postsynaptic density of glutamatergic synapses andto colocalize with PSD-95 in cultures of mouse corticalneurons [75] In electrophysiological studies of lampreyneurons application of AMPA was been found to activate aKNa current with pharmacological properties of Slack [25]Activation of this KNa current regulates both the amplitude
10 ISRN Neuroscience
and the decay rate of AMPA-induced synaptic currents pro-viding a feedback loop that decreases the size of glutamatergicpostsynaptic potentials [27]
Activation of KNa currents following activation of AMPAreceptors has also been proposed in studies of neurons of thelocus coeruleus to which application of glutamate producesa burst of action potentials followed by period of inhibitionlasting 30ndash45 seconds [76] The potassium current responsi-ble for the period of inhibition was selectively activated byAMPA or the AMPA-receptor agonist quisqualate but not byother stimuli that excite these neurons Consistent with thiscurrent being a KNa current it was attenuated by lowering ofexternal sodium concentrations but not affected by externalcalcium levels and it was potentiated by the KNa channelopener bithionol [76]
While persistent voltage-dependent sodium channels andAMPA receptors may represent the primary sodium entrypathways for activation of KNa channels there exist otherpathways that allow sodium ions to enter excitable cellsand whose role has been less explored Slack subunits areabundantly expressed in mitral cells of the olfactory bulb[46 50] In these neurons deletion of the gene for a majorvoltage-dependent potassiumchannelKv13 produces a com-pensatory increase in levels of Slack-B protein and in SlackKNa currents identified using an anti-Slack siRNA technique[77] Activation of the KNa current during depolarizationof these cells could only be significantly suppressed by acombination of tetrodotoxin to block voltage-dependentsodium channels and ZD7288 a blocker of H-channelswhich are nonselective cation channels that flux both sodiumand potassium ions Either drug alone failed to significantlysuppress KNa currents suggesting that sodium entry throughH-channels plays a role in KNa channel activation in theseneurons [77]
11 Contribution of KNa Channels toNeuronal Firing Patterns
The component of whole-cell potassium current in neuronsthat can be attributed to Slack channel subunits has beenidentified in mitral cells of the olfactory bulb and mediumspiny neuron of the striatum using siRNAs to suppress trans-lation of SlackmRNA [72 77] Such experiments indicate thatSlack KNa channels are responsible for a very major compo-nent of the total delayed outward current in neurons underphysiological conditions [72] The specific contributions thatKNa channels make to the electrical personality of individualtypes of neurons may differ in different cell types
One of the earliest effects to be described for activationof KNa currents is to produce a slow afterhyperpolarization(sAHP) following a burst of action potentials This was firstdescribed in pyramidal neurons of the cat sensorimotorcortex [11 78] and similar slow sodium-dependent sAHPsthat endure for many seconds have been found in a widevariety of neuronal types [79ndash84]These have been the subjectof earlier reviews [20 21] Such slow potentials have beenfound to influence the timing of regular bursts of actionpotentials such as the 3ndash12 per minute spindle waves that
occur at the onset of sleep in certain neurons within thethalamus [80] Slow activation of KNa during repetitive firingalso produces an adaptation of firing rate during bursts ofaction potentials [23] and such adaptation is predicted bynumerical simulations that incorporate KNa currents [46 85]
In some neurons such as neocortical pyramidal neuronsand hippocampal CA1 neurons a significant amount of KNacurrent can also be activated by single spikes and contributeto action potentials repolarization or to the amplitude ofdepolarizing afterpotentials that follow single spikes andare generated by persistent sodium currents [83 86] Inthese cases KNa current activation may be detected withinsim5msec following a single-action potential A recent studyhas examined themechanismbywhich the cells of the electricorgan (electrocytes) of certain electric fish are able to generatesustained firing at frequencies greater than 500Hz [74] Slackchannels are expressed at high levels in these cells and theirdelayed potassium current is almost exclusively a KNa currentthat is activated by a persistent sodium current Numericalsimulations have indicated that the use of KNa current torepolarize the very large action potentials of these cells allowsthe cells to fire at such high rates with minimal energyexpenditure [74]
A set of neurons that have been particularly instructivein studies of the role of KNa channels in shaping neuronalfiring are those that control locomotion in the spinal cord oflamprey [24ndash28 87] In addition to the transient KNa currentthat is activated by AMPA receptors and that was describedearlier [25 27] these neurons appear to have two distinctcomponents ofKNa current that are triggered by action poten-tial firingmdasha transient component that follows a single spikeand a slower more sustained component [24 26 87] Therapid transient KNa current is activated by the sodium influxthrough tetrodotoxin-sensitive channel during the actionpotentials and may play a role in determining the amplitudeand duration of individual action potentials The sustainedKNa current is not abolished by tetrodotoxin suggesting thatit is selectively activated by an independent sodium entrypathway and may underlie the calcium-independent sAHPthat occurs during repetitive bursting in lamprey spinal cordneurons [24]
Experiments with neurons that fire at high rates withvery high temporal accuracy have provided another potentialbiological role for KNa channels [58] Slack and Slick areexpressed at high levels in neurons of the medial nucleusof the trapezoid (MNTB) within the auditory brainstem[6 50 51] These neurons fire at rates up to sim800Hzand accurate timing of their action potentials is requiredfor transmission of information about the localization ofsound stimuli KNa channels are readily be detected in bothcell-attached and excised patches on the somata of MNTBneurons and they are localized appropriately for activationby sodium influx through AMPA receptors or voltage-dependent sodium channels [58] Current-clamp recordingsfrom MNTB neurons and numerical simulations indicatethat as KNa channels become activated temporal accuracyof firing is improved This occurs because the voltage-dependence of Slack and Slick channels allows them to opennear the resting potential of MNTB neurons increasing
ISRN Neuroscience 11
the resting conductance and decreasing the membrane timeconstant An increase in temporal fidelity of MNTB neuronscan also be produced by treatment with the pharmacologicalKNa activator bithionol [58]
12 Mutations in Slack Channels Result inEpilepsy and Intellectual Disabilities
De novo human mutations that produce single-amino-acidalterations in Slack channels produce devastating effectson development and intellectual function [2 3] One setof Slack mutations results in malignant migrating partialseizures of infancy (MMPSI) [2]This disease is characterizedby pharmacoresistant seizures in the first 6 months of lifethat arise randomly and ldquomigraterdquo from one brain regionto another While the occurrence of seizures abates withage MMPSI results in near-total arrest of psychomotordevelopment Exome sequencing revealed that at least 50of MMPSI patients have mutations in the large cytoplasmicC-terminal domain of Slack [2]
Two mutations in the human Slack gene that produceMMPSI are R428Q and A934T which correspond to themutations R409Q and A913T in the highly conserved ratSlack sequence [2] Expression of these mutants in Xenopusoocytes produced Slack currents with voltage dependenceand kinetic behavior that matched that of wild-type Slackchannels but with a current amplitude that was increased 2-3-fold over that of wild-type Slack This increase in currentlikely results because the mutations produce channels thatare either more prone to undergo phosphorylation by PKC(or less likely to be dephosphorylated) or that mimic theconformation induced by PKC activation One of theMMPSImutations R409Q lies adjacent to the phosphorylation siteS407 that as described earlier enhances current by 2-3-fold when phosphorylated by PKC Consistent with thisinterpretation current amplitude of the two MMPSI muta-tions could not be further increased upon PKC activa-tion [2] In other respects the MMPSI currents resembledthat of wild-type channels with unitary conductance andsodiumdependencesmatching those of wild-type Slack BothMMPSI mutant channels however had reduced openings tosubconductance states compared to the wild-type channels[2]
A second disease that has been attributed to a differentset of mutations in the C-terminal domain of Slack chan-nels is autosomal dominant nocturnal frontal lobe epilepsy(ADNFLE) [3] This is a childhood-onset focal epilepsysyndrome in which motor seizures arise during sleep Onsetof ADNFLE in the patients with Slack mutations occurred ata mean age of 6 years and was associated with mild to severeintellectual disability andor severe psychiatric problemsTheeffects of these ADNFLE mutations on the function of Slackchannels have however not been investigated Interestinglyanother known genetic cause of ADNFLE is mutations insubunits of the neuronal nicotinic acetycholine receptor Theacetycholine receptor ADNFLE mutations do not howeverproduce intellectual disability which contrasts strikingly tothe effects of the Slack mutations [3]
It is possible that the very severe outcomes of mutationsin Slack occur solely because of changes in the amplitudeof Slack currents An increase in total potassium currentin rapidly firing interneurons could reduce the firing rateof these cells predisposing the nervous system to hyper-excitability The relatively modest increase in current thatoccurs with theMMPSImutations [2] however suggests thatother processes may also be affected As described earlierthe large C-terminal cytoplasmic domain of Slack in whichMMPSI and ADNFLE mutations are located interacts withFMRP and its cargo mRNAs [6] Thus it is possible thatin addition to directly altering the excitability of centralneuronsmutations in Slack disrupt the regulation of activity-dependent protein translation in neurons
13 Conclusions
This review has covered themolecular and cellular propertiesof Slack and Slick two potassium channel subunits thatprovide neurons and other cells with KNa channels It shouldbe pointed out that additional potassium channels that areactivated by cytoplasmic sodiummay also exist For examplesome of the native KNa currents that have been described inneurons do not match Slack or Slick in their single-channelconductances or pharmacological properties [88]These mayperhaps represent inward rectifier potassium channels thatare activated by sodium [45 89] and the physiological rolesof the sodium sensitivity of these channels require furtherinvestigation Nevertheless Slack and Slick which are verywidely expressed in the brain appear to play dominant rolesin shaping the firing properties of verymany types of neuronsHumanmutations in these channels produce profound effectson neuronal function and development suggesting that per-haps their biological role may extend beyond simply settingthe level of neuronal excitability and that these channels mayinfluence cytoplasmic biochemical pathways that regulatedevelopment plasticity and intellectual function
References
[1] B Hille Ionic Channels of Excitable Membranes Sinauer Asso-ciates Sunderland Mass USA 3rd edition 2001
[2] G Barcia M R Fleming A Deligniere et al ldquoDe novo gain-of-function KCNT1 channel mutations cause malignant migratingpartial seizures of infancyrdquo Nature Genetics vol 44 pp 1255ndash1259 2012
[3] S E Heron K R SmithM Bahlo et al ldquoMissensemutations inthe sodium-gated potassium channel gene KCNT1 cause severeautosomal dominant nocturnal frontal lobe epilepsyrdquo NatureGenetics vol 44 pp 1188ndash1190 2012
[4] A Bhattacharjee W J Joiner M Wu Y Yang F J Sigworthand L K Kaczmarek ldquoSlick (Slo2 1) a rapidly-gating sodium-activated potassium channel inhibited by ATPrdquo The Journal ofNeuroscience vol 23 no 37 pp 11681ndash11691 2003
[5] T J Tamsett K E Picchione and A Bhattacharjee ldquoNAD+activates KNa channels in dorsal root ganglion neuronsrdquo TheJournal of Neuroscience vol 29 no 16 pp 5127ndash5134 2009
12 ISRN Neuroscience
[6] M R Brown J Kronengold V R Gazula et al ldquoFragileX mental retardation protein controls gating of the sodium-activated potassium channel SlackrdquoNatureNeuroscience vol 13no 7 pp 819ndash821 2010
[7] M Kameyama M Kakei and R Sato ldquoIntracellular Na+activates a K+ channel in mammalian cardiac cellsrdquoNature vol309 no 5966 pp 354ndash356 1984
[8] C R Bader L Bernheim and D Bertrand ldquoSodium-activatedpotassium current in cultured avian neuronesrdquoNature vol 317no 6037 pp 540ndash542 1985
[9] K Hartung ldquoPotentiation of a transient outward current byNa+influx in crayfish neuronesrdquo Pflugers Archiv vol 404 no 1 pp41ndash44 1985
[10] S E Dryer J T Fujii and A R Martin ldquoA Na+-activatedK+ current in cultured brain stem neurones from chicksrdquo TheJournal of Physiology vol 410 pp 283ndash296 1989
[11] P C Schwindt W J Spain and W E Crill ldquoLong-lastingreduction of excitability by a sodium-dependent potassiumcurrent in cat neocortical neuronsrdquo Journal of Neurophysiologyvol 61 no 2 pp 233ndash244 1989
[12] C Haimann L Bernheim D Bertrand and C R BaderldquoPotassium current activated by intracellular sodium in quailtrigeminal ganglion neuronsrdquo Journal of General Physiology vol95 no 5 pp 961ndash979 1990
[13] S E Dryer ldquoNa+-activated K+ channels and voltage-evokedionic currents in brain stem and parasympathetic neurones ofthe chickrdquoThe Journal of Physiology vol 435 pp 513ndash532 1991
[14] M Saito and C F Wu ldquoExpression of ion channels andmutational effects in giant Drosophila neurons differentiatedfrom cell division-arrested embryonic neuroblastsrdquoThe Journalof Neuroscience vol 11 no 7 pp 2135ndash2150 1991
[15] T M Egan D Dagan J Kupper and I B Levitan ldquoPropertiesand rundown of sodium-activated potassium channels in ratolfactory bulb neuronsrdquoThe Journal of Neuroscience vol 12 no5 pp 1964ndash1976 1992
[16] C Haimann J Magistretti and B Pozzi ldquoSodium-activatedpotassium current in sensory neurons a comparison of cell-attached and cell-free single-channel activitiesrdquo Pflugers Archivvol 422 no 3 pp 287ndash294 1992
[17] N Dale ldquoA large sustained Na+- and voltage-dependent K+current in spinal neurons of the frog embryordquo The Journal ofPhysiology vol 462 pp 349ndash372 1993
[18] B V Safronov and W Vogel ldquoProperties and functions of Na+-activated K+ channels in the soma of rat motoneuronesrdquo TheJournal of Physiology vol 497 no 3 pp 727ndash734 1996
[19] U Bischoff W Vogel and B V Safronov ldquoNa+-activated K+channels in small dorsal root ganglion neurones of ratrdquo TheJournal of Physiology vol 510 part 3 pp 743ndash754 1998
[20] S E Dryer ldquoNa+-activated K+ channels a new family of large-conductance ion channelsrdquo Trends in Neurosciences vol 17 no4 pp 155ndash160 1994
[21] A Bhattacharjee and L K Kaczmarek ldquoFor K+ channels Na+ isthe new Ca2+ rdquo Trends in Neurosciences vol 28 no 8 pp 422ndash428 2005
[22] S B Gao Y Wu C X Lu Z H Guo C H Li and J P DingldquoSlack and Slick KNa channels are required for the depolarizingafterpotential of acutely isolated medium diameter rat dorsalroot ganglion neuronsrdquoActa Pharmacologica Sinica vol 29 no8 pp 899ndash905 2008
[23] M O Nuwer K E Picchione and A Bhattacharjee ldquoPKA-induced internalization of Slack KNa channels produces dorsal
root ganglion neuron hyperexcitabilityrdquo The Journal of Neuro-science vol 30 no 42 pp 14165ndash14172 2010
[24] D Hess E Nanou and A El Manira ldquoCharacterization of Na+-activated K+ currents in larval lamprey spinal cord neuronsrdquoJournal of Neurophysiology vol 97 no 5 pp 3484ndash3493 2007
[25] E Nanou and A El Manira ldquoA postsynaptic negative feedbackmediated by coupling between AMPA receptors and Na+-activated K+ channels in spinal cord neuronesrdquo EuropeanJournal of Neuroscience vol 25 no 2 pp 445ndash450 2007
[26] PWallen B Robertson L Cangiano et al ldquoSodium-dependentpotassium channels of a Slack-like subtype contribute to theslow afterhyperpolarization in lamprey spinal neuronsrdquo TheJournal of Physiology vol 585 no 1 pp 75ndash90 2007
[27] E Nanou A Kyriakatos A Bhattacharjee L K Kaczmarek GParatcha and A El Manira ldquoNa+-mediated coupling betweenAMPA receptors and KNa channels shapes synaptic transmis-sionrdquo Proceedings of the National Academy of Sciences of theUnited States of America vol 105 no 52 pp 20941ndash20946 2008
[28] E Nanou and A El Manira ldquoMechanisms of modulation ofAMPA-induced Na+-activated K+ current by mGluR1rdquo Journalof Neurophysiology vol 103 no 1 pp 441ndash445 2010
[29] A R Mercer and J G Hildebrand ldquoDevelopmental changes inthe density of ionic currents in antennal-lobe neurons of thesphinx moth Manduca sextardquo Journal of Neurophysiology vol87 no 6 pp 2664ndash2675 2002
[30] K Aoki K Kosakai and M Yoshino ldquoMonoaminergic modu-lation of the Na+-activated K+ channel in kenyon cells isolatedfrom the mushroom body of the cricket (Gryllus bimaculatus)brainrdquo Journal of Neurophysiology vol 100 no 3 pp 1211ndash12222008
[31] G Klees P Hochstrate and P W Dierkes ldquoSodium-dependentpotassium channels in leech P neuronsrdquo Journal of MembraneBiology vol 208 no 1 pp 27ndash38 2005
[32] Y Zhang M R Brown C Hyland et al ldquoRegulation ofneuronal excitability by interaction of fragile X mental retar-dation protein with Slack potassium channelsrdquo The Journal ofNeuroscience vol 32 pp 15318ndash15327 2012
[33] C Lawrence and G C Rodrigo ldquoA Na+-activated K+ cur-rent (IKNa) is present in guinea-pig but not rat ventricularmyocytesrdquo Pflugers Archiv vol 437 no 6 pp 831ndash838 1999
[34] M Zhou Z Liu C Hu Z Zhang and Y Mei ldquoDevelopmentalregulation of a Na+-activated fast outward K+ current in ratmyoblastsrdquo Cellular Physiology and Biochemistry vol 14 no 4ndash6 pp 225ndash230 2004
[35] C K Young H S Jae M K Tong et al ldquoSodium-activatedpotassium current in guinea pig gastric myocytesrdquo Journal ofKorean Medical Science vol 22 no 1 pp 57ndash62 2007
[36] L Re V Moretti L Rossini and P Giusti ldquoSodium-activatedpotassium current in mouse diaphragmrdquo FEBS Letters vol 270no 1-2 pp 195ndash197 1990
[37] Y Zhang and W G Paterson ldquoFunctional evidence for Na+-activatedK+ channels in circular smoothmuscle of the opossumlower esophageal sphincterrdquo American The Journal of Physiol-ogy vol 292 no 6 pp G1600ndashG1606 2007
[38] M Paulais S Lachheb and J Teulon ldquoANa+- and Clminus-activatedK+ channel in the thick ascending limb of mouse kidneyrdquoJournal of General Physiology vol 127 no 2 pp 205ndash215 2006
[39] T M Egan D Dagan J Kupper and I B Levitan ldquoNa+-activated K+ channels are widely distributed in rat CNS and inXenopus oocytesrdquo Brain Research vol 584 no 1-2 pp 319ndash3211992
ISRN Neuroscience 13
[40] W J Joiner M D Tang L Y Wang et al ldquoFormation ofintermediate-conductance calcium-activated potassium chan-nels by interaction of Slack and Slo subunitsrdquo Nature Neuro-science vol 1 no 6 pp 462ndash469 1998
[41] A Yuan C M Santi A Wei et al ldquoThe sodium-activatedpotassium channel is encoded by a member of the Slo genefamilyrdquo Neuron vol 37 no 5 pp 765ndash773 2003
[42] Y Jiang A Pico M Cadene B T Chait and R MacKinnonldquoStructure of the RCK domain from the E coli K+ channeland demonstration of its presence in the human BK channelrdquoNeuron vol 29 no 3 pp 593ndash601 2001
[43] P Yuan M D Leonetti A R Pico Y Hsiung and R MacK-innon ldquoStructure of the human BK channel Ca2+-activationapparatus at 30 A resolutionrdquo Science vol 329 no 5988 pp182ndash186 2010
[44] Z Zhang A Rosenhouse-Dantsker Q Y Tang S Noskov andD E Logothetis ldquoThe RCK2 domain uses a coordination sitepresent in Kir channels to confer sodium sensitivity to Slo22channelsrdquoThe Journal of Neuroscience vol 30 no 22 pp 7554ndash7562 2010
[45] J L Sui K W Chan and D E Logothetis ldquoNa+ activationof the muscarinic K+ channel by a G-protein-independentmechanismrdquo Journal of General Physiology vol 108 no 5 pp381ndash391 1996
[46] M R Brown J Kronengold V R Gazula et al ldquoAmino-terminiisoforms of the Slack K+ channel regulated by alternative pro-moters differentiallymodulate rhythmic firing and adaptationrdquoThe Journal of Physiology vol 586 no 21 pp 5161ndash5179 2008
[47] A Yuan M Dourado A Butler N Walton A Wei and LSalkoff ldquoSLO-2 a K+ channel with an unusual Clminus dependencerdquoNature Neuroscience vol 3 no 8 pp 771ndash779 2000
[48] M Schreiber and L Salkoff ldquoA novel calcium-sensing domainin the BK channelrdquo Biophysical Journal vol 73 no 3 pp 1355ndash1363 1997
[49] H Chen J Kronengold Y Yan et al ldquoThe N-terminal domainof slack determines the formation and trafficking of slickslackheteromeric sodium-activated potassium channelsrdquo The Jour-nal of Neuroscience vol 29 no 17 pp 5654ndash5665 2009
[50] A Bhattacharjee L Gan and L K Kaczmarek ldquoLocalization ofthe Slack potassium channel in the rat central nervous systemrdquoJournal of Comparative Neurology vol 454 no 3 pp 241ndash2542002
[51] A Bhattacharjee C A A von Hehn X Mei and L KKaczmarek ldquoLocalization of the Na+-activated K+ channel slickin the rat central nervous systemrdquo Journal of ComparativeNeurology vol 484 no 1 pp 80ndash92 2005
[52] D S Koh P Jonas and W Vogel ldquoNa+-activated K+ channelslocalized in the nodal region of myelinated axons of XenopusrdquoThe Journal of Physiology vol 479 no 2 pp 183ndash197 1994
[53] K Mori T Saito Y Masuda and H Nakaya ldquoEffects of classIII antiarrhythmic drugs on the Na+-activated K+ channels inguinea-pig ventricular cellsrdquo British Journal of Pharmacologyvol 119 no 1 pp 133ndash141 1996
[54] K Mori S Kobayashi T Saito Y Masuda and H NakayaldquoInhibitory effects of class I and IV antiarrhythmic drugs onthe Na+- activated K+ channel current in guinea pig ventricularcellsrdquo Naunyn-Schmiedebergrsquos Archives of Pharmacology vol358 no 6 pp 641ndash648 1998
[55] Y Li T Sato and M Arita ldquoBepridil blunts the shortening ofaction potential duration caused by metabolic inhibition viablockade of ATP-sensitive K+ channels and Na+- activated K+
channelsrdquo Journal of Pharmacology and ExperimentalTherapeu-tics vol 291 no 2 pp 562ndash568 1999
[56] B Yang V K Gribkoff J Pan et al ldquoPharmacological activationand inhibition of Slack (Slo2 2) channelsrdquoNeuropharmacologyvol 51 no 4 pp 896ndash906 2006
[57] M D Tejada K Stolpe A K Meinild and D A KlaerkeldquoClofilium inhibits Slick and Slack potassium channelsrdquo Biolog-ics vol 6 pp 465ndash470 2012
[58] B Yang R Desai and L K Kaczmarek ldquoSlack and slick KNachannels regulate the accuracy of timing of auditory neuronsrdquoThe Journal of Neuroscience vol 27 no 10 pp 2617ndash2627 2007
[59] B Biton S Sethuramanujam K E Picchione et al ldquoTheantipsychotic drug loxapine is an opener of the sodium-activated potassium channel slack (Slo2 2)rdquo The Journal ofPharmacology and ExperimentalTherapeutics vol 340 pp 706ndash715 2012
[60] C M Santi G Ferreira B Yang et al ldquoOpposite regulation ofSlick and Slack K+ channels by neuromodulatorsrdquo The Journalof Neuroscience vol 26 no 19 pp 5059ndash5068 2006
[61] M R Fleming and L K Kaczmarek ldquoUse of optical biosensorsto detect modulation of Slack potassium channels by G protein-coupled receptorsrdquo Journal of Receptors and Signal Transduc-tion vol 29 no 3-4 pp 173ndash181 2009
[62] M D Tejada L J Jensen and D A Klaerke ldquoPIP(2) mod-ulation of Slick and Slack K(+) channelsrdquo Biochemical andBiophysical Research Communications vol 424 pp 208ndash2132012
[63] L Zhang M Sukhareva J L Barker et al ldquoDirect bindingof estradiol enhances Slack (sequence like a calcium-activatedpotassium channel) channelsrsquo activityrdquo Neuroscience vol 131no 2 pp 275ndash282 2005
[64] V A Ruffin X Q Gu D Zhou et al ldquoThe sodium-activatedpotassium channel Slack is modulated by hypercapnia andacidosisrdquo Neuroscience vol 151 no 2 pp 410ndash418 2008
[65] M O Nuwer K E Picchione and A Bhattacharjee ldquocAMP-dependent kinase does not modulate the Slack sodium-activated potassium channelrdquo Neuropharmacology vol 57 no3 pp 219ndash226 2009
[66] B C Suh and B Hille ldquoRegulation of ion channels by phos-phatidylinositol 45-bisphosphaterdquo Current Opinion in Neuro-biology vol 15 no 3 pp 370ndash378 2005
[67] L K Kaczmarek ldquoNon-conducting functions of voltage-gatedion channelsrdquo Nature Reviews Neuroscience vol 7 no 10 pp761ndash771 2006
[68] C R Rose ldquoNa+ signals at central synapsesrdquoNeuroscientist vol8 no 6 pp 532ndash539 2002
[69] C R Rose and B R Ransom ldquoRegulation of intracellularsodium in cultured rat hippocampal neuronesrdquo The Journal ofPhysiology vol 499 part 3 pp 573ndash587 1997
[70] C R Rose and A Konnerth ldquoNmda receptor-mediated Na+signals in spines and dendritesrdquoThe Journal of Neuroscience vol21 no 12 pp 4207ndash4214 2001
[71] N Zhong V Beaumont and R S Zucker ldquoRoles for mito-chondrial and reverse mode Na+Ca2+ exchange and the plas-malemma Ca2+ ATPase in post-tetanic potentiation at crayfishneuromuscular junctionsrdquo The Journal of Neuroscience vol 21no 24 pp 9598ndash9607 2001
[72] G Budelli T A Hage AWei et al ldquoNa+-activated K+ channelsexpress a large delayed outward current in neurons duringnormal physiologyrdquoNature Neuroscience vol 12 no 6 pp 745ndash750 2009
14 ISRN Neuroscience
[73] T A Hage and L Salkoff ldquoSodium-activated potassium chan-nels are functionally coupled to persistent sodiumcurrentsrdquoTheJournal of Neuroscience vol 32 pp 2714ndash2721 2012
[74] M RMarkham L K Kaczmarek andHH Zakon ldquoA sodium-activated potassium channe supports high frequency firing andreduces energetic costs during rapid modulations of actionpotential amplituderdquo Journal of Neurophysiology vol 109 no 7pp 1713ndash1723 2013
[75] S Uchino H Wada S Honda et al ldquoSlo2 sodium-activated K+channels bind to the PDZ domain of PSD-95rdquo Biochemical andBiophysical Research Communications vol 310 no 4 pp 1140ndash1147 2003
[76] T Zamalloa C P Bailey and J Pineda ldquoGlutamate-inducedpost-activation inhibition of locus coeruleus neurons is medi-ated by AMPAkainate receptors and sodium-dependent potas-sium currentsrdquo British Journal of Pharmacology vol 156 no 4pp 649ndash661 2009
[77] S Lu P Das D A Fadool and L K Kaczmarek ldquoThe slacksodium-activated potassium channel provides a major outwardcurrent in olfactory neurons of Kv13-- super-smeller micerdquoJournal of Neurophysiology vol 103 no 6 pp 3311ndash3319 2010
[78] R C Foehring P C Schwindt andW E Crill ldquoNorepinephrineselectively reduces slowCa2+- andNa+-mediated K+ currents incat neocortical neuronsrdquo Journal of Neurophysiology vol 61 no2 pp 245ndash256 1989
[79] M Kubota and N Saito ldquoSodium- and calcium-dependentconductances of neurones in the zebra finch hyperstriatumventrale pars caudale in vitrordquo The Journal of Physiology vol440 pp 131ndash142 1991
[80] U Kim and D A Mccormick ldquoFunctional and ionic propertiesof a slow afterhyperpolarization in ferret perigeniculate neuronsin vitrordquo Journal of Neurophysiology vol 80 no 3 pp 1222ndash1235 1998
[81] V M Sandler E Puil and DW F Schwarz ldquoIntrinsic responseproperties of bursting neurons in the nucleus principalistrigemini of the gerbilrdquoNeuroscience vol 83 no 3 pp 891ndash9041998
[82] M V Sanchez-Vives L G Nowak and D A McCormick ldquoCel-lular mechanisms of long-lasting adaptation in visual corticalneurons in vitrordquoThe Journal of Neuroscience vol 20 no 11 pp4286ndash4299 2000
[83] S Franceschetti T Lavazza G Curia et al ldquoNa+-activatedK+ current contributes to postexcitatory hyperpolarization inneocortical intrinsically bursting neuronsrdquo Journal of Neuro-physiology vol 89 no 4 pp 2101ndash2111 2003
[84] V F Descalzo L G Nowak J C Brumberg D A McCormickand M V Sanchez-Vives ldquoSlow adaptation in fast-spikingneurons of visual cortexrdquo Journal of Neurophysiology vol 93no 2 pp 1111ndash1118 2005
[85] J Benda L Maler and A Longtin ldquoLinear versus nonlinearsignal transmission in neuron models with adaptation currentsor dynamic thresholdsrdquo Journal of Neurophysiology vol 104 no5 pp 2806ndash2820 2010
[86] X Liu and L S Leung ldquoSodium-activated potassium conduc-tance participates in the depolarizing afterpotential following asingle action potential in rat hippocampal CA1 pyramidal cellsrdquoBrain Research vol 1023 no 2 pp 185ndash192 2004
[87] L Cangiano P Wallen and S Grillner ldquoRole of apamin-sensitive KCa channels for reticulospinal synaptic transmissionto motoneuron and for the afterhyperpolarizationrdquo Journal ofNeurophysiology vol 88 no 1 pp 289ndash299 2002
[88] Q Y Liu A E Schaffner and J L Barker ldquoKainate induces anintracellular Na+-activated K+ current in cultured embryonicrat hippocampal neuronesrdquo The Journal of Physiology vol 510part 3 pp 721ndash734 1998
[89] I Rishal T Keren-Raifman D Yakubovich et al ldquoNa+ pro-motes the dissociation between G120572GDP and G120573120574 activating Gprotein-gated K+ channelsrdquoThe Journal of Biological Chemistryvol 278 no 6 pp 3840ndash3845 2003
Submit your manuscripts athttpwwwhindawicom
Neurology Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Alzheimerrsquos DiseaseHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentSchizophrenia
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neural Plasticity
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Parkinsonrsquos Disease
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentAutism
Sleep DisordersHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neuroscience Journal
Epilepsy Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Psychiatry Journal
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Computational and Mathematical Methods in Medicine
Depression Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Cardiovascular Psychiatry and NeurologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
2 ISRN Neuroscience
pArg409Gln
93
77
407Ser pAla913Thr
RCK1
438
517
684
RCK2
806
351
360 1124
1099
1096 1221
P
Figure 1 Schematic diagram of a Slack KNa channel subunitdepicting the relative positions of the two RCK domains consensusPKC phosphorylation sites (depicted simply as red numbers cor-responding to the positions of the amino acids) the position ofthe regulatory PKC site Serine 407 and the positions of the twocharacterizedmutations that give rise tomalignantmigrating partialseizures of infancy (blue) (modified from [2])
in the nervous system of invertebrates including antennal-lobe neurons of the sphinx moth Manduca sexta [29]Kenyon cells of mushroom bodies in crickets [30] pressure119875 neurons in the leech [31] and bag cell neurons that regulatereproductive behavior in the sea hare Aplysia californica [32]
Although the major focus of studies of KNa currentshas been in the nervous system KNa channels have beendescribed in a variety of cardiac cells including guinea pigmyocytes and rat myoblasts [7 33 34] They have also beenfound in guinea pig gastric myocytes [35] mouse diaphragmmuscle [36] circular smooth muscle of the opossum loweresophageal sphincter [37] and the thick ascending limb ofmouse kidney [38] as well as in Xenopus oocytes [39]
3 The Slack Channel
Two genes that encode KNa channels are now known andthese have been termed Slack and Slick [4 40 41] ThetermSlack is derived from ldquosequence likeA calcium-activatedK channelrdquo because part of the pore domain and the fol-lowing S6 domain of Slack is similar to that of the Slo1(BK) large-conductance calcium-activated potassium chan-nel [40] Overall however there is only 7 identity betweenSlack and Slo1 The Slack channel has also been termed theSlo22 channel and also erroneously listed as KCa41 Thenomenclature for the human Slack gene is KCNT1
The overall structure of the potassium channel is similarto that of the voltage-dependent Kv family of potassiumchannels in that there are six hydrophobic membrane-spanning domains S1ndashS6 with a pore P-domain between S5-S6 (Figure 1) With a predicted length of 1237 amino acids itis however very much larger than any of the Kv subunitsand the membrane-spanning domains together representonly one seventh of the entire sequence The majority ofthe Slack sequence represents a very large cytoplasmic C-terminal domainWithin this C-terminal region there are twopredicted RCK (regulators of conductance of K+) domains
that have also been found in Slo1 channels [42] X-raycrystallographic studies of the C-terminal domain of chickenSlack channels have confirmed that the structure of the RCKdomains is likely to resemble those of Slo1 channels [43]
Although the Slack channel was first cloned and ex-pressed in 1998 [40] it was not found to be regulated byinternal sodium ions until 2003 [41] When expressed eitherin mammalian cell lines or Xenopus oocytes and excisedinto solutions that have equal concentrations of potassiumon both sides of the membrane Slack channels have a rel-atively large unitary conductance (sim180 pS) [4 41] but theconductance is reduced by over 50 in physiological solu-tions [40] As has been found for KNa channels in neuronsand other cells Slack channels in expression systems havemultiple subconductance states
As is the case for Kv and Slo1 subunits Slack cur-rents increase with depolarization and in single-channelrecordings the open probability of Slack channels increaseswith depolarization Slack differs from these other channelshowever in that there are no charged residues in the S4transmembrane domain which is the principal voltage sensorof these other channels The residues that confer voltagesensitivity on Slack channels are not yet known
In excised patches the half-maximal concentration foractivation of Slack channels by sodium ions at the cytoplas-mic surface is sim40mM (Figures 2(a) and 2(b)) [4 41] Aswill be described later this EC
50value may however not
represent the situation within intact cells [5] A region withinthe second RCK domain has been identified as a site thatconfers sensitivity to sodium ions [44]This region resemblesone found in inwardly rectifying Kir3 family channels whichcan also be activated by sodium ions [45] Mutation ofeither an aspartate residue (D818N) or a histidine (H823A)within this region in Slack lowers sodium sensitivity by anorder of magnitude [44] This finding suggests that as inSlo1 channels conformational changes in the RCK domainsbrought about by ion binding lead to channel opening
Overall the biophysical characteristics of Slack channelsin heterologous expression systems resemble those reportedfor KNa channels in native tissues which have been found tohave EC
50values for sodium activation of 7ndash80mM and to
have unitary conductances from 100 to 200 pS with multiplesubconductance states
4 Slack Isoforms
The rates of activation KNa currents vary substantially indifferent types of neurons At least in part such diversity mayarise from different Slack channel subunits that are generatedby alternative splicing of SlackRNA Five different transcriptsfrom the rat Slack gene have been described and theseare predicted to produce Slack channels that differ in theircytoplasmic amino termini [46] Two of these transcriptsSlack-A and Slack-B have been expressed functionally andbeen found to have very different kinetics of activation Thefirst isoform to be identified [40] which is now termed Slack-B has a long N-terminal domain making it the largest potas-sium channel subunit currently known When expressed
ISRN Neuroscience 3
0mV minus80mV+80mV 0mV
0mM Na
90mM Na
0mM Na200pA
25ms40K+
119894130K+
119900 30Clminus
(a)
12
1
08
06
04
02
0
[Na] (mM)300100101
119868119868
max
(b)
15pA25ms
0mM Na
50mM Na
2
50mM Na
(c)
[Na] (mM)300100101
12
1
08
06
04
02
0
Nor
mal
ized
NP o
(d)
Figure 2 Activation of Slack and Slick channels by cytoplasmic sodium (a) Representative patch of macroscopic Slack currents recordedat 0 and minus80mV in an excised inside-out patch from Slack-transfected CHO cells in the presence and absence of 90mM sodium (b)Concentration-response relationship of sodium activation of Slack current in excised patches Currents were normalized those recordedin 90mM sodium (c) Excised patch recording at minus80mV from a CHO cell transfected with Slick The cytoplasmic face of the patch wasperfused with a solution containing either 0 or 50mM intracellular sodium (d) Concentration-response relationship of sodium activation ofSlick channels in excised patches Figures modified from [4]
inXenopus oocytes ormammalian cell lines Slack-B channelsactivate slowly over hundreds of milliseconds [4 40 41]
The N-terminal domain of the other characterized splicevariant Slack-A is smaller than that of Slack-B and thesequence of this N-terminal domain very closely resemblesthe N-terminus of the other KNa channel subunit Slick (seenext section) Neuronal expression of Slack-A and Slack-B channels is driven by independent promoters [46] Incontrast to the very slowly-activating Slack-B channels Slack-A channels activate very rapidly upon depolarizationThedif-ferent kinetic behaviors of Slack-A and Slack-B channels arealso evident in single-channel recordings in patches excisedfrom Xenopus oocytes expressing these subunitsThe unitaryconductance of the two isoforms is identical The gating ofSlack-A channels is however very rapid During sustaineddepolarization brief transient openings to fully open state areinterspersedwith repeated opening to subconductance statesIn contrast the mean open time of Slack-B channels is about6 times longer and subconductance states although they
are very evident are less frequent than in Slack-A channels[46]
The potential effects of the expression of either Slack-Aor Slack-B on neuronal firing patterns have been addressed innumerical simulations Neurons in which potassium currentsare dominated by a Slack-A-like current adapt very rapidly torepeated or maintained stimulation In contrast Slack-B cur-rents allow neurons to fire rhythmically during maintainedstimulation and allow the rate of adaptation rate to vary withthe intensity of stimulation [46]
5 Slick a Second KNa Channel
The second known gene that encodes a KNa channel wastermed Slick for sequence like an intermediate conductanceK channel because it is closely related to the Slack geneand Slack channels have conductance that is intermedi-ate between that of Slo1 (BK) calcium-activated potassium
4 ISRN Neuroscience
channels and most other potassium channels [4] The Slickchannel has also been termed the Slo21 channel and thehuman Slick gene is KCNT2 Overall the sequence of Slickis sim74 identical to that of Slack and the transmembranedomains and the RCK domains of Slick and Slack are almostidentical The greatest divergence between Slack and Slickoccurs at their distal C-terminal regions
The predicted cytoplasmic N-terminus of Slick is similarto that of the Slack-A isoform Consistent with this fact thecharacteristics of Slick currents recorded inmammalian cellsor Xenopus oocytes resemble those of Slack-A Althoughthe unitary conductance of Slick channels (sim140 pS) isslightly smaller than that of Slack channels Slick channelsactivate very rapidly with depolarization and have multi-ple subconductance states [4] The estimated half-maximalconcentration for activation of Slick by sodium 89mMis greater than that for Slack channels (Figures 2(c) and2(d)) Moreover in contrast to Slack channels which havean absolute requirement for Na+ for channel opening Slickchannels have a basal level of activity in the absence of sodium[4]
In addition to their sodium sensitivity both Slick andSlack channels can be activated in increases in intracellularchloride levels [4 47] Under physiological conditions how-ever Slick channels are markedly more sensitive to this anion[4] The domains that confer sensitivity of these channels tochloride are not known and mutations within a region thatresembles the ldquocalcium bowlrdquo of Slo1 channels and whichhas been established to contribute to the calcium sensitivityof Slo1 channels [48] have not been found to influence thesensitivity of Slack or Slick to either sodium or chloride ions[4]
The Slick channel also differs from Slack in having aconsensus ATP binding site just C-terminal to the secondRCK domain This site renders Slack channel activity to besensitive to cytoplasmic ATP levels The presence of 5mMATP reduces Slick whole cell currents or channel activityin excised patches [4] Further description of the effects ofATP on Slick channels will be provided in a later section onphysiological modulation of KNa channels
6 Heteromer Formation between Slack-Band Slick Subunits
Avariety of lines of evidence indicate that the Slack-B subunitand Slick can coassemble to form heteromeric channelsthat differ in their properties from those of either subunitexpressed alone [49] Moreover assembly of heteromericchannels appears to be specific for the Slack-B isoform anddoes not occur with Slack-A which as described abovediffers from Slack-B only in its N-terminal cytoplasmicdomain When Slick and Slack-B are coexpressed at a 1 1ratio either in Xenopus oocytes or in mammalian HEK293cells there is an 18ndash25-fold increase in current amplitudecompared to expression of either subunit alone [49] Thisincrease in current is associated with a comparable increasein levels of the channel subunits in plasma membrane asassayed by a surface biotinylation assay No differences in
total cellular levels of either protein are however foundwhenthe subunits are expressed singly or in combination with theother subunit
Themacroscopic currents of the Slack-BSlick heteromersdiffer from those of the homomers [49] For example thetime for the SlickSlack-B currents to reach 90 activationis significantly longer than that for either Slick or Slack-Bhomomeric currents In addition the unitary conductanceof the Slack-BSlick channels is intermediate between that ofSlack (sim180 pS) and Slick (sim140 pS) A direct demonstrationof this fact was obtained by making a pore mutant of theSlick channel Slick-EE (Q276E Y279E) that increases itsunitary conductance to sim500 pS Coexpression of Slick-EEwithwild-type Slack-B resulted in channels that had a unitaryconductance of sim325 pS a value that is clearly distinct fromthat for either channel alone providing definitive electro-physiological evidence for the formation of SlickSlack-Bheteromers
Coimmunoprecipitation experiments have also demon-strated that Slack-B forms a protein complex with Slick inboth rat brain tissue and heterologous expression systems[49] No coimmunoprecipitation could however be foundfor epitope-tagged Slick-A and Slick subunits This is con-sistent with electrophysiological findings demonstrating thatcoexpression of Slack-A with Slick does not increase whole-oocyte currents compared to expression of either subunitalone and that no intermediate conductance single-channelactivity can be observed when Slack-A is expressed withthe Slick-EE mutant Moreover coexpression of Slack-A withSlick does not increase levels of surface expression of eithersubunit
The specificity of SlackSlick interactions for the Slack-Bisoform is dependent on the extended N-terminal domainof Slack-B This has been demonstrated by constructing achimeric channel that replaced the cytoplasmic N-terminaldomain of Slick with that of Slack-B Coexpression of thismodified Slick channel with wild-type Slick channels inoocytes produced a 30-fold increase in whole-oocyte cur-rents which is similar to the increase obtained with Slack-Bsubunits [49] Thus the Slack-B N-terminal domain plays akey role in trafficking andor the level of plasma membraneexpression of heteromeric KNa channels
7 Localization of Slack and Slick Channels inthe Central Nervous System
Slack transcripts of about 45 kb and 75 kb are abundantlyexpressed in the brain and in the kidney [40] By the tech-nique of in situ hybridization in sections of adult rat brainSlack is very highly expressed in neurons but no staining isevident in glial cells Strong hybridization is found through-out the brain including the cerebral cortex hippocampusdeep cerebellar nuclei cerebellar Purkinje cells reticulartegmental nucleus of the pons preoptic nucleus substantianigra and auditory brainstem nuclei Neurons within thethalamus and hypothalamus were found to have a moremoderate level of staining [40] Subsequent quantificationof expression of mRNA levels by probes that are specific
ISRN Neuroscience 5
for either Slack-B or Slack-A isoforms found that bothisoforms are present throughout the brain but that thehighest levels of both isoforms are detected in brainstem andolfactory bulb [46]
Immunocytochemical staining using an antibody to theSlack-B specific N-terminus confirmed the localization ofSlack-B in neurons of the brainstem and olfactory bulb of rat[50] Prominent immunoreactivity was found in the olfactorybulb red nucleus and deep cerebellar nuclei as well asin vestibular and oculomotor nuclei and in the trigeminalsystem and reticular formation where intense staining occursin both cell bodies and axonal fibers As with the in situhybridization studies labeling was prominent in auditorybrainstem nuclei Although staining was also evident inneurons of the thalamus substantia nigra and amygdala theonly region of the cerebral cortex where the Slack-B isoformwas found was in the frontal cortex [50]
Immunolocalization studies have also been carried outusing an antibody targeted against a region in the cytoplasmicC-terminus of Slack [46] This would be expected to recog-nize themultiple Slack isoforms that differ at the N-terminusbut not to recognize Slick channels Immunostaining withthis ldquopan-Slackrdquo antibody found strong labeling of neuronsthroughout the nervous system of mice This included theregions previously identified with the Slack-B antibody aswell as other locations such as the dendrites of hippocampalneurons and olfactory bulb glomeruli [46]
Messenger RNA for the Slick subunit has awider distribu-tion throughout the body than does Slack mRNA and a Slicktranscript of sim69 kB is expressed in rat heart where no SlackmRNA has been found [4] Like Slack however the highestlevels of Slick mRNA are found in the brain Slick mRNA andimmunoreactivity overlap considerably with those for Slackand are found at high levels in olfactory bulb midbrain brainstem hippocampus throughout the cerebral cortex withhigh expression in primary somatosensory and visual regions[51] Like Slack high Slick immunoreactivity is found inneurons of sensory pathways including those in the auditorybrainstem The wide colocalization of Slick with the Slack-Bisoform suggests that the native KNa channels in these regionsare likely to be heteromers of these two subunits
Studies have also found Slack channels to be expressedin neurons of the peripheral nervous system Slack immuno-labeling is found in over 90 of neurons of the dorsal rootganglion and has been localized to both the somata andthe axonal tracts of small- medium- and large-diameterneurons [5 23] Localization to axonal tracts has also beendetected in electrophysiological experiments on peripheralaxons of Xenopus neurons for which a correlation has beenfound between the localization of KNa channels and voltage-dependent sodium channels [52]
8 Pharmacology of KNa Channels
A very common test for the presence of KNa channels inneurons and other cells has been to measure the effects ofreplacement of external sodium by lithium ions on the netoutward current Lithium is a much weaker activator of KNa
channels than sodium Because lithium readily enters cellsthough voltage-dependent sodium channels usually withminimal effects on the inward currents lithium replace-ment in voltage-clamp experiments reduces the net out-ward currents if KNa currents are present In current clampexperiments lithium substitution may alter the shape ofaction potentials or firing patterns in a manner consistentwith partial block of KNa currents The majority of theexperiments described later in the section on contributionsof KNa channels to neuronal firing patterns have tested theeffects of lithium substitution
Studies on native KNa channels in cardiac cells found thatseveral antiarrhythmic drugs inhibit these channels in cardiaccells [53ndash55] Some of these compounds such as quinidinebepridil and clofilium have been also found to be veryeffective and reversible blockers of Slack and Slick channelsexpressed in oocytes and mammalian cells [4 56 57] but arenon-specific in that they also act on a variety of other channeltypes
The pharmacological properties of Slack and Slick chan-nels are similar to each other but differ from those of manyother potassium channels including their closest molec-ular relative the Slo1 channel Slack and Slick are onlyweakly sensitive to the general potassium channel blockertetraethylammonium ions (TEA) [4] External barium ionsproduce a time- and voltage-dependent block of Slack andSlick channels In contrast both subunits are insensitiveto blockers of calcium-activated potassium channels suchas apamin iberiotoxin paxilline and charybdotoxin andto a wide range of blockers of other classes of potassiumchannels [4 57] Although Slick channels are sensitive tocytoplasmic ATP levels they are insensitive to glibenclamideand diazoxide an inhibitor and an activator respectively ofclassical KATP channels [4]
A variety of compounds that activate Slack and Slickchannels are also knownThefirst of these to be describedwasbithionol which in Slack-expressing HEK cells produces arobust increase in the amplitude of Slack currents with anEC50
of 077120583M [56] Similar bithionol-induced increasesin current are observed for Slack-expressing oocytes and fornative KNa currents in neurons of the auditory brainstem [5658] Bithionol reversibly activates Slack channels even whenapplied to the extracellular face of excised patches indicatingthat it acts on Slack channels relatively directly The increasein current occurs though a very marked bithionol-inducedchange in the voltage dependence of Slack channels such thatfull activation is already observed at potentials as negativeas minus40mV [56] Bithionol is however not selective forKNa channels in that it also activates Slo1 calcium-activatedpotassium channels [56 59]
A screen of pharmacologically active compounds usinga rubidium flux assay against Slack channels expressed inCHO cells has revealed other activators [59] These includeriluzole loxapine an antipsychotic agent and niclosamidean anthelmintic agent Loxapine was found to be moreselective than bithionol in that it has no effect of Slo1 calcium-activated potassium channels Electrophysiological experi-ments confirmed that loxapine is effective on recombinanthuman and rat Slack channels and that it activates native KNa
6 ISRN Neuroscience
channels in isolated neurons of the rat dorsal root ganglion[59]
9 Cellular Regulation of KNa Channels
The activity of Slack and Slick channels as well as ofheteromeric SlackSlick channels is potently regulated by avariety of cellular signaling pathways including G-protein-coupled receptors linked to activation of PKA or PKC [2328 30 49 60 61] direct phosphorylation of the channelssubunits [2 49 60] changes in cytoplasmic ATP levels[4] cytoplasmic NAD+ levels [5] PIP2 [62] estradiol [63]hypoxia [41 64] and the fragile X mental retardation proteinFMRP [6 32] This section will summarize these findings
91 Modulation of Slack and Slick by Protein Kinase CTreatment ofmammalian cells orXenopus oocytes expressingSlack channels with diacylglycerol or phorbol ester activatorsof protein kinase C (PKC) leads to a 2-3-fold increase incurrent amplitude and a slowing of the rate of activation[2 60] This effect of PKC has been found to result from thephosphorylation of a single serine residue S407 located in aPKC consensus phosphorylation site in the ldquohingerdquo region ofthe cytoplasmic C-terminus between the S6 transmembranedomain and the first RCK domain [2] Mutation of this serineto an alanine (S407A) which renders the site incapable ofundergoing phosphorylation results in apparently normalSlack currents that are entirely insensitive to PKC activationIn contrast mutation of the twelve other putative PKCphosphorylation sites in the Slack channel (see Figure 1)does not prevent the PKC-induced increase in Slack current[2]
In contrast to Slack channels the action of PKC onSlick channels is to produce a decrease in current amplitude[60] As in Slack this effect is likely to be mediated by amodification within the C-terminus In particular a chimericchannel that contains the N-terminus and membrane span-ning regions of Slack but the cytoplasmic C-terminus ofSlick responds to PKC activation with a decrease in currentMoreover direct application of a constitutively active formof PKC directly to the cytoplasmic face of excised patchescontaining these chimeric channels reduces their activity[60] Nevertheless the specific sites at which Slick subunitsare modified in response to PKC activation are not yetknown
Heteromeric Slack-BSlick channels respond to activationof PKC in a manner that is quite distinct from that of eithersubunit expressed alone [49] In Xenopus oocytes expressingboth subunits application of PKC activators potently sup-presses current bysim90This ismuch greater than the degreeof suppression measured for Slick subunits alone (sim50)and cannot therefore be explained by any linear combinationof Slick or Slack-B currents The finding that heteromericSlack-BSlick channels are regulated even more potently byPKC activation than are the homomeric channels constitutesone of the many pieces of evidence supporting selectiveheteromer formation between these subunits [49]
92 Modulation of Slack and Slick by G Protein CoupledReceptors Slack and Slick have been coexpressed in Xenopusoocytes with the M1 muscarinic receptor and the mGluR1metabotropic glutamate receptor [60]These are G120572q proteincoupled receptors that lead to the activation of PKC andformation of IP3 Accordingly activation of these receptorsleads to an increase in Slack currents and a suppression ofSlick currents consistent with modulation of these chan-nels by PKC There is widespread colocalization of thesereceptors with the KNa subunits throughout the nervoussystem suggesting that modulation of KNa currents by thesereceptors is likely to be widespread [60] Direct modulationof KNa channels by mGluR1 has been observed in nativeneurons within the lamprey spinal cord [28] In these cellsactivation of mGluR1 produces a strong suppression ofcurrent as expected for Slick or Slack-BSlick heteromersand this suppression is prevented by pharmacological inhibi-tion of PKC Interestingly chelation of intracellular calciumwith the chelator BAPTA (12-bis(2-aminophenoxy)ethane-NNN1015840N1015840-tetraacetic acid) prevented modulation of theKNa current by PKC but unmasked an mGluR1-dependentactivation of KNa current by an independent pathway whosemechanism is not yet known [28]
Modulation of Slack channels by G protein-coupledreceptors has been studied using a novel label-free technologyto detect changes in the distribution of mass close to theplasma membrane [61] The characteristic change in massthat normally follows activation of native receptors in HEK-293 cells is significantly modified in amplitude and timingby the coexpression of Slack channels further supporting thefinding of modulation of KNa current by these receptors [61]
93 Modulation of K119873119886
Currents by Cyclic AMP The activityof KNa channels in Kenyon cells isolated from the mushroombody of the cricket (Gryllus bimaculatus) is enhanced uponapplication of either the neurotransmitter octopamine or themembrane-permeable analog cyclic AMP analog 8-Br-cyclic-AMP [30] The increase in channel activity was found to beattenuated by the protein kinase A (PKA) inhibitor H-89Conversely KNa channel activity in these cells is decreasedby the transmitter dopamine or by the cyclic GMP analog8-Br-cyclic GMP and this decrease in open probability isantagonized by the protein kinaseG (PKG) inhibitor KT5823These findings have led to the proposal that modulation ofKNa currents play a role in olfactory learning circuits thatmediate reward and punishment signals in insects which areregulated by octopamine and dopamine respectively [30]
The Slack subunit is expressed at high levels in pri-mary afferent nociceptor neurons located in the dorsal rootganglion [5 23] and KNa currents can readily be recordedin these cells [5 22 23] Inflammatory substances suchas prostaglandin E2 that activate the PKA pathway greatlyincrease the excitability of these neurons sensitizing them tothermal and mechanical stimulation In their resting statethe neurons adapt rapidly to maintained depolarizationstypically generating only one or two action potentials at theonset of depolarization After an elevation of cyclic AMPlevels that activates PKA adaptation is greatly reduced such
ISRN Neuroscience 7
that sustained firing can be recorded throughout a prolongeddepolarization In major part the effects of PKA can beattributed to a decrease in Slack KNa current and suppressionof Slack expression using anti-Slack siRNA produces a reduc-tion in adaptation similar to that produced by PKA activation[23]
In contrast to the effects of PKC on Slack channels thatwere described above the effects of PKA on Slack are indirectand do not involve phosphorylation of the channel subunititself [23 65] Slack channels in HEK-293 cells are insensitiveto cyclic AMP analogs or to the PKA inhibitor KT5720 andapplication of the catalytic subunit of PKA to the cytoplasmicface of Slack channels excised from transfected cells or dorsalroot ganglion neurons has no effect on channel openingInstead activation of PKA decreases Slack current amplitudeby rapidly internalizing Slack channels from the plasmamembrane into internal organelles [23] Such internalizationwas detected by confocal microscopy of neurons expressingSlack channels tagged at their N-terminus with the greenfluorescent protein GFP Elevations of cyclic AMP reducedcolocalization of the labeled channels with a cell surfacemarker A reduction in native Slack channels at the plasmamembrane following PKA activation was also confirmedusing a surface biotinylation assay [23]
94 Modulation of Slack and Slick by PIP2 A variety ofion channels have been shown to be regulated by phos-phatidylinositol 45-biphosphate (PIP2) a phospholipid thatis localized to the inner lipid leaflet of the plasma membrane[66] Exogenously applied PI(45)P2 as well as its isoformPI(34)P2 increases the amplitude of Slack and Slick currentsexpressed in Xenopus oocytes and pharmacological agentsthat are expected to reduce endogenous PIP2 levels reducecurrent amplitude [62] Regulation by PIP2 was found torequire specific lysine residues in the C-termini of thechannelsMutation of these lysines to alanine (K339A in Slackand K306A in Slick) produced functional KNa currents thatwere insensitive to the activating effects of PIP2 [62]
95 Suppression of Slick Channels by Cytoplasmic ATP TheSlick subunit differs from Slack in that it contains a consensusATP binding site just C-terminal to the second RCK domain[4] The current density of Slick currents but not Slackcurrents is reduced by sim80 by the presence of 5mM ATPat the cytoplasmic face of the channels The same full effectis observed with ATP120574S a slowly hydrolyzable ATP analogand the nonhydrolyzable analog AMP-PNP but not withADP Suppression by ATP of Slick currents at all membranevoltages can also be readily detected in excised patches [4]
Confirmation that the effect of ATP on Slick channelsis mediated by the consensus ATP binding site in the C-terminal domain was obtained by site-directed mutagenesisto replace the glycine at residue 1032 in this site with a serine[4]Themutant G1032S Slick channel was fully functional butfailed to decrease channel activity in response to ATP Thesensitivity of Slick channels to ATP levels suggests that theiractivity could be enhanced during periods of high metabolicdemand when cellular ATP levels fall
96 Modulation of Slack by NAD+ KNa channels can read-ily be detected in cell-attached patch-clamp recordings onneurons [5 16 58] This may be surprising given that thecytoplasmic sodium concentration in resting cells is relativelylow (ltsim10mM see Section 10 below) and as described earlierthe EC
50for activation of these channels in excised patches
is relatively high (typically sim40ndash80mM) This discrepancy isresolved by the finding that the open probability and sodiumsensitivity of KNa channels are greatly reduced following exci-sion ofmembrane patches [5 16] suggesting that rundown ofKNa activity in excised patches may result from loss of somecytoplasmic factor
A major cytoplasmic factor that regulates both Slack andSlick channels has been found to be nicotinamide adeninedinucleotideNAD+ [5] Both subunits have a putativeNAD+-binding site within their second RCK domain Application ofNAD+ to the cytoplasmic face of KNa channels excised fromdorsal root ganglion neurons increases their open probabilityand reduces themeasured EC
50for activation of sodium from
sim50mM to sim17mM (Figure 3) NAD+ as well as NADP+but not NADH also increased the open probability of Slackchannels excised from a transfected mammalian cell lineMutation of glycine in the putative NAD+-binding site ofSlack (G792A) resulted in a loss of the ability of NAD+to potentiate activity [5] These findings suggest that KNachannels play a much more prominent role in the normalphysiology of excitable cells than would be suggested by theirproperties in isolated patches
97 Modulation of Slack by Estradiol The open probability ofchannels reconstituted from lipid bilayers from extracts of ratcortex has been found to be increased by 17120573-estradiol [63]Although these channels were not explicitly shown to be KNachannels a similar effect was reported for whole-cell currentsrecorded in Slack-expressing HEK-293 cells Activation wasnot prevented by tamoxifen a blocker of classical estrogenreceptors but may be a consequence of direct binding of 17120573-estradiol to Slack channels themselves [63]
98 Modulation of Slack Channels by Hypoxia Duringhypoxic conditions cytoplasmic levels of internal sodiumand chloride ions as well as those of H+ and CO
2 rise It
has been suggested that activation of KNa channels couldact to hyperpolarize cells during hypoxia providing a pro-tective mechanism [7] This conjecture is supported by thefinding that nematodes lacking the Slack ortholog in thisspecies are significantly more sensitive to hypoxic deaththan are wild-type animals [41] Further support for a rolefor Slack channels in the response to hypoxia comes fromrecordings of Slack channel activity in Xenopus oocytesexposed to increasing concentrations ofH+ andCO
2 demon-
strating that elevations of sodium activate Slack channelsunder cellular conditions comparable to those expected inischemiahypoxia [64]
99 Interactions of Slack with the Fragile X Mental Retarda-tion Protein FMRP Fragile X syndrome the most commoninherited form of intellectual disability in humans is caused
8 ISRN Neuroscience
1
08
06
04
02
0
[Na] (mM)10010
Control (small neurons)NAD
Nor
mal
ized
NP o
Figure 3 NAD+ increases the sodium sensitivity of KNa channelsexcised from adult rat dorsal root ganglion neurons Concentration-response relationship provides estimates of EC
50for sodium activa-
tion of 50mM and 17mM in the absence and presence of NAD+respectively Figure modified from [5]
by loss of expression of the RNA-binding protein FMRP(fragile X mental retardation protein) FMPR is known to berequired for normal activity-dependent protein translationin neurons Yeast two-hybrid and coimmunoprecipitationexperiments have shown that the cytoplasmic C-terminus ofthe Slack subunit interacts with this protein [6] MoreoverSlack-FMRP complexes also contain messenger RNAs thatare targets of FMRP such as the mRNAs encoding Map1band Arc No association of mRNAs with Slack can howeverbe found in mice lacking FMRP indicating that these arebound to Slack despite its interaction with FMRP A peptidearray assay revealed confirmed that FMRP binds selectivelyto sequences at the distal C-terminus of Slack [6]
FMRP is a potent activator of Slack channel activity [6]Application of recombinant FMRP(1ndash298) which containsthemajority of protein-protein interaction domains of FMRPto the cytoplasmic face of Slack channels in excised patchesreversibly increases channel opening by two- to threefold(Figure 4(a)) This is associated with the almost completeelimination of openings to subconductance states which areprominent in untreated patches (Figure 4(b)) FMRP wasfound to have no effect on the channel activity of a Slacktruncation mutant (Slack-BΔ804) which lacks sites found tointeract with FMRP in biochemical assays [6]
Support for a role for FMRP in regulating the amplitude ofnative KNa currents has also come from recordings of neuronsfrom animals lacking the fmr1 gene which encodes FMPR[6] As will be described later KNa channels account for amajor component of potassium current in principal neuronsof the medial nucleus of the trapezoid body (MNTB) Whileloss of FMRP does not affect total levels of Slack proteinthe level of the KNa component of current in these cells issubstantially reduced in the FMRP knockout mice compared
to that in wild-type animals consistent with an activating rolefor this RNA-binding protein [6]
The interaction between Slack and FMRP is likely tobe evolutionarily conserved Both proteins have been foundto be coexpressed in bag cell (BC) neurons which regulatereproductive behaviors in mollusk Aplysia [32] FMRP andSlack immunoreactivity are colocalized at the periphery ofisolated BC neurons and the two proteins can be reciprocallycoimmunoprecipitatedThe native Slack current in these cellswas identified using an siRNA approach and intracellularinjection of FMRP(1ndash298) induced a KNa current that in itsproperties matches the Slack current As for Slack channelsin heterologous expression systems addition of FMRP(1ndash298) to the inside-out patches containing native AplysiaKNa channels increased channel opening In current clamprecordings FMRP(1ndash298) produced a narrowing of actionpotentials
Downregulation of Slack expression in BC neurons usingan siRNA approach had an interesting effect on long-termchanges in the excitability of these cells [32] In untreatedcells brief stimulation of these neurons produces a charac-teristic prolonged (sim30min) discharge that is followed bya prolonged (sim18 hr) inhibitory state Recovery from thisinhibitory state requires new protein synthesis and is blockedby the protein synthesis inhibitor anisomycin Anti-SlacksiRNA did not alter the ability of BC neurons to undergothe normal discharge but like inhibition of protein synthesisprevented recovery from the inhibitory state [32] Althoughother interpretations are possible these findings raise thepossibility that the activity of Slack channels is linked to theregulation of protein translation through the FMRP-RNApathway
As will be described later point mutations in humanSlack channels can produce devastating effects on intellectualdevelopment [2 3] A variety of experiments have demon-strated that activation of ion channels can influence cytoplas-mic processes independently of ion flux through the channels[67] The interaction between the C-terminus of Slack andFMRP raises the hypothesis that conformational changesin the C-terminus could represent such a ldquononconductingrdquofunction of Slack that links neuronal firing to changes inprotein translation
10 Sodium Entry Pathways ThatActivate KNa Channels
Estimates of cytoplasmic sodium concentrations in restingunstimulated neurons obtained using sodium-sensitivemicroelectrodes and ratiometric dyes provide valuesbetween sim4mM and 15mM [68] As noted earlier in thesection on NAD+ regulation of Slack channels this maybe sufficient to provide significant activation of native KNachannels in intact neurons even under resting conditions[5] Internal sodium levels rise in response to stimulationprincipally as a result of entry through voltage-dependentsodium channels and ionotropic ligand-gated receptors suchas AMPA and NMDA glutamate receptors [69] Stimulationparticularly repetitive tetanic stimulation produces local
ISRN Neuroscience 9
Control RecoveryC
1
2
3
C
1
2
3
10pA
2 s
10pA
200ms
S S S S S S S
FMRP(1ndash298)
(a)
2520151050
10000
1000
100
pA
C1
2
3
Recovery
2520151050
10000
1000
100
pA
C1
2
3
S
S
S(N
)
(N)
Control
2520151050
10000
1000
100
pA
C
1 2 3
S
S
S
(N)
FMRP(1ndash298)
(b)
Figure 4 FMRP alters gating of Slack-B channels in inside-out patches (a) Recording of a patch containing 3-4 Slack channels before duringand after transient application of FMRP(1ndash298) to the cytoplasmic face of the patch Top traces show representative 10 s examples of recordingatminus40mVand bottom traces show expanded time views of the same patch Arrowsmarked S indicate the occurrence of subconductance statesthat are suppressed by FMRP(1ndash298) (b) All-points amplitude histograms of Slack channel activity plotted on a logarithmic scale before andafter application of FMRP(1ndash298) and after washout C marks the closed state S indicates subconductance states and numbers on peaks inthe histograms refer to the number of fully open Slack channels Modified from [6]
increases in sodium concentrations that can rise to levelsbetween 45 and 100mM [70 71] In particular high levelscan be attained in restricted compartments such as activedendritic spines
Although the upstroke of action potentials in mostneurons is driven by rapidly-inactivating voltage-dependentsodium currents these currents are transient Evidencesuggest that the much smaller persistent component ofsodium current that does not inactivate with maintaineddepolarization is responsible both for sustained elevationsof internal sodium during stimulation [69] and for selectiveactivation of KNa channels [72 73] It has been found thatin patches excised from somata of rat neurons sodium entrythrough persistent tetrodotoxin-sensitive sodium channels isthe dominant factor that leads to activation of KNa channelsin the same patch [73] Activation of KNa channels by sodiumentry in the isolated patches was enhanced by veratridinewhich prolongs sodium channel openings and was observedin the absence of sodium at the cytoplasmic face of thepatches Moreover activation appeared to be independent ofthe amplitude of the much larger transient sodium current
[73] Although the molecular basis for persistent sodiumchannels is not fully understood further support for a selec-tive activation of KNa currents by entry through persistentchannels has also been found in electrocytes of electric fish inwhich the time course of KNa currents during depolarizationsreflects that of the persistent sodium current and not that ofthe very much larger transient sodium current [74]
A number of studies have documented that KNa currentsare also activated by sodium entry through ionotropic gluta-mate receptors particularly AMPA receptors Slack subunitscan be coimmunoprecipitated with GluR23 AMPA receptorsusing rat brain synaptosomal fractions or lysates of the lam-prey central nervous system [27] Moreover Slack subunitshave been found to bind directly to the PDZ domain of PSD-95 (postsynaptic density-95 protein) a major componentof the postsynaptic density of glutamatergic synapses andto colocalize with PSD-95 in cultures of mouse corticalneurons [75] In electrophysiological studies of lampreyneurons application of AMPA was been found to activate aKNa current with pharmacological properties of Slack [25]Activation of this KNa current regulates both the amplitude
10 ISRN Neuroscience
and the decay rate of AMPA-induced synaptic currents pro-viding a feedback loop that decreases the size of glutamatergicpostsynaptic potentials [27]
Activation of KNa currents following activation of AMPAreceptors has also been proposed in studies of neurons of thelocus coeruleus to which application of glutamate producesa burst of action potentials followed by period of inhibitionlasting 30ndash45 seconds [76] The potassium current responsi-ble for the period of inhibition was selectively activated byAMPA or the AMPA-receptor agonist quisqualate but not byother stimuli that excite these neurons Consistent with thiscurrent being a KNa current it was attenuated by lowering ofexternal sodium concentrations but not affected by externalcalcium levels and it was potentiated by the KNa channelopener bithionol [76]
While persistent voltage-dependent sodium channels andAMPA receptors may represent the primary sodium entrypathways for activation of KNa channels there exist otherpathways that allow sodium ions to enter excitable cellsand whose role has been less explored Slack subunits areabundantly expressed in mitral cells of the olfactory bulb[46 50] In these neurons deletion of the gene for a majorvoltage-dependent potassiumchannelKv13 produces a com-pensatory increase in levels of Slack-B protein and in SlackKNa currents identified using an anti-Slack siRNA technique[77] Activation of the KNa current during depolarizationof these cells could only be significantly suppressed by acombination of tetrodotoxin to block voltage-dependentsodium channels and ZD7288 a blocker of H-channelswhich are nonselective cation channels that flux both sodiumand potassium ions Either drug alone failed to significantlysuppress KNa currents suggesting that sodium entry throughH-channels plays a role in KNa channel activation in theseneurons [77]
11 Contribution of KNa Channels toNeuronal Firing Patterns
The component of whole-cell potassium current in neuronsthat can be attributed to Slack channel subunits has beenidentified in mitral cells of the olfactory bulb and mediumspiny neuron of the striatum using siRNAs to suppress trans-lation of SlackmRNA [72 77] Such experiments indicate thatSlack KNa channels are responsible for a very major compo-nent of the total delayed outward current in neurons underphysiological conditions [72] The specific contributions thatKNa channels make to the electrical personality of individualtypes of neurons may differ in different cell types
One of the earliest effects to be described for activationof KNa currents is to produce a slow afterhyperpolarization(sAHP) following a burst of action potentials This was firstdescribed in pyramidal neurons of the cat sensorimotorcortex [11 78] and similar slow sodium-dependent sAHPsthat endure for many seconds have been found in a widevariety of neuronal types [79ndash84]These have been the subjectof earlier reviews [20 21] Such slow potentials have beenfound to influence the timing of regular bursts of actionpotentials such as the 3ndash12 per minute spindle waves that
occur at the onset of sleep in certain neurons within thethalamus [80] Slow activation of KNa during repetitive firingalso produces an adaptation of firing rate during bursts ofaction potentials [23] and such adaptation is predicted bynumerical simulations that incorporate KNa currents [46 85]
In some neurons such as neocortical pyramidal neuronsand hippocampal CA1 neurons a significant amount of KNacurrent can also be activated by single spikes and contributeto action potentials repolarization or to the amplitude ofdepolarizing afterpotentials that follow single spikes andare generated by persistent sodium currents [83 86] Inthese cases KNa current activation may be detected withinsim5msec following a single-action potential A recent studyhas examined themechanismbywhich the cells of the electricorgan (electrocytes) of certain electric fish are able to generatesustained firing at frequencies greater than 500Hz [74] Slackchannels are expressed at high levels in these cells and theirdelayed potassium current is almost exclusively a KNa currentthat is activated by a persistent sodium current Numericalsimulations have indicated that the use of KNa current torepolarize the very large action potentials of these cells allowsthe cells to fire at such high rates with minimal energyexpenditure [74]
A set of neurons that have been particularly instructivein studies of the role of KNa channels in shaping neuronalfiring are those that control locomotion in the spinal cord oflamprey [24ndash28 87] In addition to the transient KNa currentthat is activated by AMPA receptors and that was describedearlier [25 27] these neurons appear to have two distinctcomponents ofKNa current that are triggered by action poten-tial firingmdasha transient component that follows a single spikeand a slower more sustained component [24 26 87] Therapid transient KNa current is activated by the sodium influxthrough tetrodotoxin-sensitive channel during the actionpotentials and may play a role in determining the amplitudeand duration of individual action potentials The sustainedKNa current is not abolished by tetrodotoxin suggesting thatit is selectively activated by an independent sodium entrypathway and may underlie the calcium-independent sAHPthat occurs during repetitive bursting in lamprey spinal cordneurons [24]
Experiments with neurons that fire at high rates withvery high temporal accuracy have provided another potentialbiological role for KNa channels [58] Slack and Slick areexpressed at high levels in neurons of the medial nucleusof the trapezoid (MNTB) within the auditory brainstem[6 50 51] These neurons fire at rates up to sim800Hzand accurate timing of their action potentials is requiredfor transmission of information about the localization ofsound stimuli KNa channels are readily be detected in bothcell-attached and excised patches on the somata of MNTBneurons and they are localized appropriately for activationby sodium influx through AMPA receptors or voltage-dependent sodium channels [58] Current-clamp recordingsfrom MNTB neurons and numerical simulations indicatethat as KNa channels become activated temporal accuracyof firing is improved This occurs because the voltage-dependence of Slack and Slick channels allows them to opennear the resting potential of MNTB neurons increasing
ISRN Neuroscience 11
the resting conductance and decreasing the membrane timeconstant An increase in temporal fidelity of MNTB neuronscan also be produced by treatment with the pharmacologicalKNa activator bithionol [58]
12 Mutations in Slack Channels Result inEpilepsy and Intellectual Disabilities
De novo human mutations that produce single-amino-acidalterations in Slack channels produce devastating effectson development and intellectual function [2 3] One setof Slack mutations results in malignant migrating partialseizures of infancy (MMPSI) [2]This disease is characterizedby pharmacoresistant seizures in the first 6 months of lifethat arise randomly and ldquomigraterdquo from one brain regionto another While the occurrence of seizures abates withage MMPSI results in near-total arrest of psychomotordevelopment Exome sequencing revealed that at least 50of MMPSI patients have mutations in the large cytoplasmicC-terminal domain of Slack [2]
Two mutations in the human Slack gene that produceMMPSI are R428Q and A934T which correspond to themutations R409Q and A913T in the highly conserved ratSlack sequence [2] Expression of these mutants in Xenopusoocytes produced Slack currents with voltage dependenceand kinetic behavior that matched that of wild-type Slackchannels but with a current amplitude that was increased 2-3-fold over that of wild-type Slack This increase in currentlikely results because the mutations produce channels thatare either more prone to undergo phosphorylation by PKC(or less likely to be dephosphorylated) or that mimic theconformation induced by PKC activation One of theMMPSImutations R409Q lies adjacent to the phosphorylation siteS407 that as described earlier enhances current by 2-3-fold when phosphorylated by PKC Consistent with thisinterpretation current amplitude of the two MMPSI muta-tions could not be further increased upon PKC activa-tion [2] In other respects the MMPSI currents resembledthat of wild-type channels with unitary conductance andsodiumdependencesmatching those of wild-type Slack BothMMPSI mutant channels however had reduced openings tosubconductance states compared to the wild-type channels[2]
A second disease that has been attributed to a differentset of mutations in the C-terminal domain of Slack chan-nels is autosomal dominant nocturnal frontal lobe epilepsy(ADNFLE) [3] This is a childhood-onset focal epilepsysyndrome in which motor seizures arise during sleep Onsetof ADNFLE in the patients with Slack mutations occurred ata mean age of 6 years and was associated with mild to severeintellectual disability andor severe psychiatric problemsTheeffects of these ADNFLE mutations on the function of Slackchannels have however not been investigated Interestinglyanother known genetic cause of ADNFLE is mutations insubunits of the neuronal nicotinic acetycholine receptor Theacetycholine receptor ADNFLE mutations do not howeverproduce intellectual disability which contrasts strikingly tothe effects of the Slack mutations [3]
It is possible that the very severe outcomes of mutationsin Slack occur solely because of changes in the amplitudeof Slack currents An increase in total potassium currentin rapidly firing interneurons could reduce the firing rateof these cells predisposing the nervous system to hyper-excitability The relatively modest increase in current thatoccurs with theMMPSImutations [2] however suggests thatother processes may also be affected As described earlierthe large C-terminal cytoplasmic domain of Slack in whichMMPSI and ADNFLE mutations are located interacts withFMRP and its cargo mRNAs [6] Thus it is possible thatin addition to directly altering the excitability of centralneuronsmutations in Slack disrupt the regulation of activity-dependent protein translation in neurons
13 Conclusions
This review has covered themolecular and cellular propertiesof Slack and Slick two potassium channel subunits thatprovide neurons and other cells with KNa channels It shouldbe pointed out that additional potassium channels that areactivated by cytoplasmic sodiummay also exist For examplesome of the native KNa currents that have been described inneurons do not match Slack or Slick in their single-channelconductances or pharmacological properties [88]These mayperhaps represent inward rectifier potassium channels thatare activated by sodium [45 89] and the physiological rolesof the sodium sensitivity of these channels require furtherinvestigation Nevertheless Slack and Slick which are verywidely expressed in the brain appear to play dominant rolesin shaping the firing properties of verymany types of neuronsHumanmutations in these channels produce profound effectson neuronal function and development suggesting that per-haps their biological role may extend beyond simply settingthe level of neuronal excitability and that these channels mayinfluence cytoplasmic biochemical pathways that regulatedevelopment plasticity and intellectual function
References
[1] B Hille Ionic Channels of Excitable Membranes Sinauer Asso-ciates Sunderland Mass USA 3rd edition 2001
[2] G Barcia M R Fleming A Deligniere et al ldquoDe novo gain-of-function KCNT1 channel mutations cause malignant migratingpartial seizures of infancyrdquo Nature Genetics vol 44 pp 1255ndash1259 2012
[3] S E Heron K R SmithM Bahlo et al ldquoMissensemutations inthe sodium-gated potassium channel gene KCNT1 cause severeautosomal dominant nocturnal frontal lobe epilepsyrdquo NatureGenetics vol 44 pp 1188ndash1190 2012
[4] A Bhattacharjee W J Joiner M Wu Y Yang F J Sigworthand L K Kaczmarek ldquoSlick (Slo2 1) a rapidly-gating sodium-activated potassium channel inhibited by ATPrdquo The Journal ofNeuroscience vol 23 no 37 pp 11681ndash11691 2003
[5] T J Tamsett K E Picchione and A Bhattacharjee ldquoNAD+activates KNa channels in dorsal root ganglion neuronsrdquo TheJournal of Neuroscience vol 29 no 16 pp 5127ndash5134 2009
12 ISRN Neuroscience
[6] M R Brown J Kronengold V R Gazula et al ldquoFragileX mental retardation protein controls gating of the sodium-activated potassium channel SlackrdquoNatureNeuroscience vol 13no 7 pp 819ndash821 2010
[7] M Kameyama M Kakei and R Sato ldquoIntracellular Na+activates a K+ channel in mammalian cardiac cellsrdquoNature vol309 no 5966 pp 354ndash356 1984
[8] C R Bader L Bernheim and D Bertrand ldquoSodium-activatedpotassium current in cultured avian neuronesrdquoNature vol 317no 6037 pp 540ndash542 1985
[9] K Hartung ldquoPotentiation of a transient outward current byNa+influx in crayfish neuronesrdquo Pflugers Archiv vol 404 no 1 pp41ndash44 1985
[10] S E Dryer J T Fujii and A R Martin ldquoA Na+-activatedK+ current in cultured brain stem neurones from chicksrdquo TheJournal of Physiology vol 410 pp 283ndash296 1989
[11] P C Schwindt W J Spain and W E Crill ldquoLong-lastingreduction of excitability by a sodium-dependent potassiumcurrent in cat neocortical neuronsrdquo Journal of Neurophysiologyvol 61 no 2 pp 233ndash244 1989
[12] C Haimann L Bernheim D Bertrand and C R BaderldquoPotassium current activated by intracellular sodium in quailtrigeminal ganglion neuronsrdquo Journal of General Physiology vol95 no 5 pp 961ndash979 1990
[13] S E Dryer ldquoNa+-activated K+ channels and voltage-evokedionic currents in brain stem and parasympathetic neurones ofthe chickrdquoThe Journal of Physiology vol 435 pp 513ndash532 1991
[14] M Saito and C F Wu ldquoExpression of ion channels andmutational effects in giant Drosophila neurons differentiatedfrom cell division-arrested embryonic neuroblastsrdquoThe Journalof Neuroscience vol 11 no 7 pp 2135ndash2150 1991
[15] T M Egan D Dagan J Kupper and I B Levitan ldquoPropertiesand rundown of sodium-activated potassium channels in ratolfactory bulb neuronsrdquoThe Journal of Neuroscience vol 12 no5 pp 1964ndash1976 1992
[16] C Haimann J Magistretti and B Pozzi ldquoSodium-activatedpotassium current in sensory neurons a comparison of cell-attached and cell-free single-channel activitiesrdquo Pflugers Archivvol 422 no 3 pp 287ndash294 1992
[17] N Dale ldquoA large sustained Na+- and voltage-dependent K+current in spinal neurons of the frog embryordquo The Journal ofPhysiology vol 462 pp 349ndash372 1993
[18] B V Safronov and W Vogel ldquoProperties and functions of Na+-activated K+ channels in the soma of rat motoneuronesrdquo TheJournal of Physiology vol 497 no 3 pp 727ndash734 1996
[19] U Bischoff W Vogel and B V Safronov ldquoNa+-activated K+channels in small dorsal root ganglion neurones of ratrdquo TheJournal of Physiology vol 510 part 3 pp 743ndash754 1998
[20] S E Dryer ldquoNa+-activated K+ channels a new family of large-conductance ion channelsrdquo Trends in Neurosciences vol 17 no4 pp 155ndash160 1994
[21] A Bhattacharjee and L K Kaczmarek ldquoFor K+ channels Na+ isthe new Ca2+ rdquo Trends in Neurosciences vol 28 no 8 pp 422ndash428 2005
[22] S B Gao Y Wu C X Lu Z H Guo C H Li and J P DingldquoSlack and Slick KNa channels are required for the depolarizingafterpotential of acutely isolated medium diameter rat dorsalroot ganglion neuronsrdquoActa Pharmacologica Sinica vol 29 no8 pp 899ndash905 2008
[23] M O Nuwer K E Picchione and A Bhattacharjee ldquoPKA-induced internalization of Slack KNa channels produces dorsal
root ganglion neuron hyperexcitabilityrdquo The Journal of Neuro-science vol 30 no 42 pp 14165ndash14172 2010
[24] D Hess E Nanou and A El Manira ldquoCharacterization of Na+-activated K+ currents in larval lamprey spinal cord neuronsrdquoJournal of Neurophysiology vol 97 no 5 pp 3484ndash3493 2007
[25] E Nanou and A El Manira ldquoA postsynaptic negative feedbackmediated by coupling between AMPA receptors and Na+-activated K+ channels in spinal cord neuronesrdquo EuropeanJournal of Neuroscience vol 25 no 2 pp 445ndash450 2007
[26] PWallen B Robertson L Cangiano et al ldquoSodium-dependentpotassium channels of a Slack-like subtype contribute to theslow afterhyperpolarization in lamprey spinal neuronsrdquo TheJournal of Physiology vol 585 no 1 pp 75ndash90 2007
[27] E Nanou A Kyriakatos A Bhattacharjee L K Kaczmarek GParatcha and A El Manira ldquoNa+-mediated coupling betweenAMPA receptors and KNa channels shapes synaptic transmis-sionrdquo Proceedings of the National Academy of Sciences of theUnited States of America vol 105 no 52 pp 20941ndash20946 2008
[28] E Nanou and A El Manira ldquoMechanisms of modulation ofAMPA-induced Na+-activated K+ current by mGluR1rdquo Journalof Neurophysiology vol 103 no 1 pp 441ndash445 2010
[29] A R Mercer and J G Hildebrand ldquoDevelopmental changes inthe density of ionic currents in antennal-lobe neurons of thesphinx moth Manduca sextardquo Journal of Neurophysiology vol87 no 6 pp 2664ndash2675 2002
[30] K Aoki K Kosakai and M Yoshino ldquoMonoaminergic modu-lation of the Na+-activated K+ channel in kenyon cells isolatedfrom the mushroom body of the cricket (Gryllus bimaculatus)brainrdquo Journal of Neurophysiology vol 100 no 3 pp 1211ndash12222008
[31] G Klees P Hochstrate and P W Dierkes ldquoSodium-dependentpotassium channels in leech P neuronsrdquo Journal of MembraneBiology vol 208 no 1 pp 27ndash38 2005
[32] Y Zhang M R Brown C Hyland et al ldquoRegulation ofneuronal excitability by interaction of fragile X mental retar-dation protein with Slack potassium channelsrdquo The Journal ofNeuroscience vol 32 pp 15318ndash15327 2012
[33] C Lawrence and G C Rodrigo ldquoA Na+-activated K+ cur-rent (IKNa) is present in guinea-pig but not rat ventricularmyocytesrdquo Pflugers Archiv vol 437 no 6 pp 831ndash838 1999
[34] M Zhou Z Liu C Hu Z Zhang and Y Mei ldquoDevelopmentalregulation of a Na+-activated fast outward K+ current in ratmyoblastsrdquo Cellular Physiology and Biochemistry vol 14 no 4ndash6 pp 225ndash230 2004
[35] C K Young H S Jae M K Tong et al ldquoSodium-activatedpotassium current in guinea pig gastric myocytesrdquo Journal ofKorean Medical Science vol 22 no 1 pp 57ndash62 2007
[36] L Re V Moretti L Rossini and P Giusti ldquoSodium-activatedpotassium current in mouse diaphragmrdquo FEBS Letters vol 270no 1-2 pp 195ndash197 1990
[37] Y Zhang and W G Paterson ldquoFunctional evidence for Na+-activatedK+ channels in circular smoothmuscle of the opossumlower esophageal sphincterrdquo American The Journal of Physiol-ogy vol 292 no 6 pp G1600ndashG1606 2007
[38] M Paulais S Lachheb and J Teulon ldquoANa+- and Clminus-activatedK+ channel in the thick ascending limb of mouse kidneyrdquoJournal of General Physiology vol 127 no 2 pp 205ndash215 2006
[39] T M Egan D Dagan J Kupper and I B Levitan ldquoNa+-activated K+ channels are widely distributed in rat CNS and inXenopus oocytesrdquo Brain Research vol 584 no 1-2 pp 319ndash3211992
ISRN Neuroscience 13
[40] W J Joiner M D Tang L Y Wang et al ldquoFormation ofintermediate-conductance calcium-activated potassium chan-nels by interaction of Slack and Slo subunitsrdquo Nature Neuro-science vol 1 no 6 pp 462ndash469 1998
[41] A Yuan C M Santi A Wei et al ldquoThe sodium-activatedpotassium channel is encoded by a member of the Slo genefamilyrdquo Neuron vol 37 no 5 pp 765ndash773 2003
[42] Y Jiang A Pico M Cadene B T Chait and R MacKinnonldquoStructure of the RCK domain from the E coli K+ channeland demonstration of its presence in the human BK channelrdquoNeuron vol 29 no 3 pp 593ndash601 2001
[43] P Yuan M D Leonetti A R Pico Y Hsiung and R MacK-innon ldquoStructure of the human BK channel Ca2+-activationapparatus at 30 A resolutionrdquo Science vol 329 no 5988 pp182ndash186 2010
[44] Z Zhang A Rosenhouse-Dantsker Q Y Tang S Noskov andD E Logothetis ldquoThe RCK2 domain uses a coordination sitepresent in Kir channels to confer sodium sensitivity to Slo22channelsrdquoThe Journal of Neuroscience vol 30 no 22 pp 7554ndash7562 2010
[45] J L Sui K W Chan and D E Logothetis ldquoNa+ activationof the muscarinic K+ channel by a G-protein-independentmechanismrdquo Journal of General Physiology vol 108 no 5 pp381ndash391 1996
[46] M R Brown J Kronengold V R Gazula et al ldquoAmino-terminiisoforms of the Slack K+ channel regulated by alternative pro-moters differentiallymodulate rhythmic firing and adaptationrdquoThe Journal of Physiology vol 586 no 21 pp 5161ndash5179 2008
[47] A Yuan M Dourado A Butler N Walton A Wei and LSalkoff ldquoSLO-2 a K+ channel with an unusual Clminus dependencerdquoNature Neuroscience vol 3 no 8 pp 771ndash779 2000
[48] M Schreiber and L Salkoff ldquoA novel calcium-sensing domainin the BK channelrdquo Biophysical Journal vol 73 no 3 pp 1355ndash1363 1997
[49] H Chen J Kronengold Y Yan et al ldquoThe N-terminal domainof slack determines the formation and trafficking of slickslackheteromeric sodium-activated potassium channelsrdquo The Jour-nal of Neuroscience vol 29 no 17 pp 5654ndash5665 2009
[50] A Bhattacharjee L Gan and L K Kaczmarek ldquoLocalization ofthe Slack potassium channel in the rat central nervous systemrdquoJournal of Comparative Neurology vol 454 no 3 pp 241ndash2542002
[51] A Bhattacharjee C A A von Hehn X Mei and L KKaczmarek ldquoLocalization of the Na+-activated K+ channel slickin the rat central nervous systemrdquo Journal of ComparativeNeurology vol 484 no 1 pp 80ndash92 2005
[52] D S Koh P Jonas and W Vogel ldquoNa+-activated K+ channelslocalized in the nodal region of myelinated axons of XenopusrdquoThe Journal of Physiology vol 479 no 2 pp 183ndash197 1994
[53] K Mori T Saito Y Masuda and H Nakaya ldquoEffects of classIII antiarrhythmic drugs on the Na+-activated K+ channels inguinea-pig ventricular cellsrdquo British Journal of Pharmacologyvol 119 no 1 pp 133ndash141 1996
[54] K Mori S Kobayashi T Saito Y Masuda and H NakayaldquoInhibitory effects of class I and IV antiarrhythmic drugs onthe Na+- activated K+ channel current in guinea pig ventricularcellsrdquo Naunyn-Schmiedebergrsquos Archives of Pharmacology vol358 no 6 pp 641ndash648 1998
[55] Y Li T Sato and M Arita ldquoBepridil blunts the shortening ofaction potential duration caused by metabolic inhibition viablockade of ATP-sensitive K+ channels and Na+- activated K+
channelsrdquo Journal of Pharmacology and ExperimentalTherapeu-tics vol 291 no 2 pp 562ndash568 1999
[56] B Yang V K Gribkoff J Pan et al ldquoPharmacological activationand inhibition of Slack (Slo2 2) channelsrdquoNeuropharmacologyvol 51 no 4 pp 896ndash906 2006
[57] M D Tejada K Stolpe A K Meinild and D A KlaerkeldquoClofilium inhibits Slick and Slack potassium channelsrdquo Biolog-ics vol 6 pp 465ndash470 2012
[58] B Yang R Desai and L K Kaczmarek ldquoSlack and slick KNachannels regulate the accuracy of timing of auditory neuronsrdquoThe Journal of Neuroscience vol 27 no 10 pp 2617ndash2627 2007
[59] B Biton S Sethuramanujam K E Picchione et al ldquoTheantipsychotic drug loxapine is an opener of the sodium-activated potassium channel slack (Slo2 2)rdquo The Journal ofPharmacology and ExperimentalTherapeutics vol 340 pp 706ndash715 2012
[60] C M Santi G Ferreira B Yang et al ldquoOpposite regulation ofSlick and Slack K+ channels by neuromodulatorsrdquo The Journalof Neuroscience vol 26 no 19 pp 5059ndash5068 2006
[61] M R Fleming and L K Kaczmarek ldquoUse of optical biosensorsto detect modulation of Slack potassium channels by G protein-coupled receptorsrdquo Journal of Receptors and Signal Transduc-tion vol 29 no 3-4 pp 173ndash181 2009
[62] M D Tejada L J Jensen and D A Klaerke ldquoPIP(2) mod-ulation of Slick and Slack K(+) channelsrdquo Biochemical andBiophysical Research Communications vol 424 pp 208ndash2132012
[63] L Zhang M Sukhareva J L Barker et al ldquoDirect bindingof estradiol enhances Slack (sequence like a calcium-activatedpotassium channel) channelsrsquo activityrdquo Neuroscience vol 131no 2 pp 275ndash282 2005
[64] V A Ruffin X Q Gu D Zhou et al ldquoThe sodium-activatedpotassium channel Slack is modulated by hypercapnia andacidosisrdquo Neuroscience vol 151 no 2 pp 410ndash418 2008
[65] M O Nuwer K E Picchione and A Bhattacharjee ldquocAMP-dependent kinase does not modulate the Slack sodium-activated potassium channelrdquo Neuropharmacology vol 57 no3 pp 219ndash226 2009
[66] B C Suh and B Hille ldquoRegulation of ion channels by phos-phatidylinositol 45-bisphosphaterdquo Current Opinion in Neuro-biology vol 15 no 3 pp 370ndash378 2005
[67] L K Kaczmarek ldquoNon-conducting functions of voltage-gatedion channelsrdquo Nature Reviews Neuroscience vol 7 no 10 pp761ndash771 2006
[68] C R Rose ldquoNa+ signals at central synapsesrdquoNeuroscientist vol8 no 6 pp 532ndash539 2002
[69] C R Rose and B R Ransom ldquoRegulation of intracellularsodium in cultured rat hippocampal neuronesrdquo The Journal ofPhysiology vol 499 part 3 pp 573ndash587 1997
[70] C R Rose and A Konnerth ldquoNmda receptor-mediated Na+signals in spines and dendritesrdquoThe Journal of Neuroscience vol21 no 12 pp 4207ndash4214 2001
[71] N Zhong V Beaumont and R S Zucker ldquoRoles for mito-chondrial and reverse mode Na+Ca2+ exchange and the plas-malemma Ca2+ ATPase in post-tetanic potentiation at crayfishneuromuscular junctionsrdquo The Journal of Neuroscience vol 21no 24 pp 9598ndash9607 2001
[72] G Budelli T A Hage AWei et al ldquoNa+-activated K+ channelsexpress a large delayed outward current in neurons duringnormal physiologyrdquoNature Neuroscience vol 12 no 6 pp 745ndash750 2009
14 ISRN Neuroscience
[73] T A Hage and L Salkoff ldquoSodium-activated potassium chan-nels are functionally coupled to persistent sodiumcurrentsrdquoTheJournal of Neuroscience vol 32 pp 2714ndash2721 2012
[74] M RMarkham L K Kaczmarek andHH Zakon ldquoA sodium-activated potassium channe supports high frequency firing andreduces energetic costs during rapid modulations of actionpotential amplituderdquo Journal of Neurophysiology vol 109 no 7pp 1713ndash1723 2013
[75] S Uchino H Wada S Honda et al ldquoSlo2 sodium-activated K+channels bind to the PDZ domain of PSD-95rdquo Biochemical andBiophysical Research Communications vol 310 no 4 pp 1140ndash1147 2003
[76] T Zamalloa C P Bailey and J Pineda ldquoGlutamate-inducedpost-activation inhibition of locus coeruleus neurons is medi-ated by AMPAkainate receptors and sodium-dependent potas-sium currentsrdquo British Journal of Pharmacology vol 156 no 4pp 649ndash661 2009
[77] S Lu P Das D A Fadool and L K Kaczmarek ldquoThe slacksodium-activated potassium channel provides a major outwardcurrent in olfactory neurons of Kv13-- super-smeller micerdquoJournal of Neurophysiology vol 103 no 6 pp 3311ndash3319 2010
[78] R C Foehring P C Schwindt andW E Crill ldquoNorepinephrineselectively reduces slowCa2+- andNa+-mediated K+ currents incat neocortical neuronsrdquo Journal of Neurophysiology vol 61 no2 pp 245ndash256 1989
[79] M Kubota and N Saito ldquoSodium- and calcium-dependentconductances of neurones in the zebra finch hyperstriatumventrale pars caudale in vitrordquo The Journal of Physiology vol440 pp 131ndash142 1991
[80] U Kim and D A Mccormick ldquoFunctional and ionic propertiesof a slow afterhyperpolarization in ferret perigeniculate neuronsin vitrordquo Journal of Neurophysiology vol 80 no 3 pp 1222ndash1235 1998
[81] V M Sandler E Puil and DW F Schwarz ldquoIntrinsic responseproperties of bursting neurons in the nucleus principalistrigemini of the gerbilrdquoNeuroscience vol 83 no 3 pp 891ndash9041998
[82] M V Sanchez-Vives L G Nowak and D A McCormick ldquoCel-lular mechanisms of long-lasting adaptation in visual corticalneurons in vitrordquoThe Journal of Neuroscience vol 20 no 11 pp4286ndash4299 2000
[83] S Franceschetti T Lavazza G Curia et al ldquoNa+-activatedK+ current contributes to postexcitatory hyperpolarization inneocortical intrinsically bursting neuronsrdquo Journal of Neuro-physiology vol 89 no 4 pp 2101ndash2111 2003
[84] V F Descalzo L G Nowak J C Brumberg D A McCormickand M V Sanchez-Vives ldquoSlow adaptation in fast-spikingneurons of visual cortexrdquo Journal of Neurophysiology vol 93no 2 pp 1111ndash1118 2005
[85] J Benda L Maler and A Longtin ldquoLinear versus nonlinearsignal transmission in neuron models with adaptation currentsor dynamic thresholdsrdquo Journal of Neurophysiology vol 104 no5 pp 2806ndash2820 2010
[86] X Liu and L S Leung ldquoSodium-activated potassium conduc-tance participates in the depolarizing afterpotential following asingle action potential in rat hippocampal CA1 pyramidal cellsrdquoBrain Research vol 1023 no 2 pp 185ndash192 2004
[87] L Cangiano P Wallen and S Grillner ldquoRole of apamin-sensitive KCa channels for reticulospinal synaptic transmissionto motoneuron and for the afterhyperpolarizationrdquo Journal ofNeurophysiology vol 88 no 1 pp 289ndash299 2002
[88] Q Y Liu A E Schaffner and J L Barker ldquoKainate induces anintracellular Na+-activated K+ current in cultured embryonicrat hippocampal neuronesrdquo The Journal of Physiology vol 510part 3 pp 721ndash734 1998
[89] I Rishal T Keren-Raifman D Yakubovich et al ldquoNa+ pro-motes the dissociation between G120572GDP and G120573120574 activating Gprotein-gated K+ channelsrdquoThe Journal of Biological Chemistryvol 278 no 6 pp 3840ndash3845 2003
Submit your manuscripts athttpwwwhindawicom
Neurology Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Alzheimerrsquos DiseaseHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentSchizophrenia
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neural Plasticity
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Parkinsonrsquos Disease
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentAutism
Sleep DisordersHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neuroscience Journal
Epilepsy Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Psychiatry Journal
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Computational and Mathematical Methods in Medicine
Depression Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Neurodegenerative Diseases
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Cardiovascular Psychiatry and NeurologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
ISRN Neuroscience 3
0mV minus80mV+80mV 0mV
0mM Na
90mM Na
0mM Na200pA
25ms40K+
119894130K+
119900 30Clminus
(a)
12
1
08
06
04
02
0
[Na] (mM)300100101
119868119868
max
(b)
15pA25ms
0mM Na
50mM Na
2
50mM Na
(c)
[Na] (mM)300100101
12
1
08
06
04
02
0
Nor
mal
ized
NP o
(d)
Figure 2 Activation of Slack and Slick channels by cytoplasmic sodium (a) Representative patch of macroscopic Slack currents recordedat 0 and minus80mV in an excised inside-out patch from Slack-transfected CHO cells in the presence and absence of 90mM sodium (b)Concentration-response relationship of sodium activation of Slack current in excised patches Currents were normalized those recordedin 90mM sodium (c) Excised patch recording at minus80mV from a CHO cell transfected with Slick The cytoplasmic face of the patch wasperfused with a solution containing either 0 or 50mM intracellular sodium (d) Concentration-response relationship of sodium activation ofSlick channels in excised patches Figures modified from [4]
inXenopus oocytes ormammalian cell lines Slack-B channelsactivate slowly over hundreds of milliseconds [4 40 41]
The N-terminal domain of the other characterized splicevariant Slack-A is smaller than that of Slack-B and thesequence of this N-terminal domain very closely resemblesthe N-terminus of the other KNa channel subunit Slick (seenext section) Neuronal expression of Slack-A and Slack-B channels is driven by independent promoters [46] Incontrast to the very slowly-activating Slack-B channels Slack-A channels activate very rapidly upon depolarizationThedif-ferent kinetic behaviors of Slack-A and Slack-B channels arealso evident in single-channel recordings in patches excisedfrom Xenopus oocytes expressing these subunitsThe unitaryconductance of the two isoforms is identical The gating ofSlack-A channels is however very rapid During sustaineddepolarization brief transient openings to fully open state areinterspersedwith repeated opening to subconductance statesIn contrast the mean open time of Slack-B channels is about6 times longer and subconductance states although they
are very evident are less frequent than in Slack-A channels[46]
The potential effects of the expression of either Slack-Aor Slack-B on neuronal firing patterns have been addressed innumerical simulations Neurons in which potassium currentsare dominated by a Slack-A-like current adapt very rapidly torepeated or maintained stimulation In contrast Slack-B cur-rents allow neurons to fire rhythmically during maintainedstimulation and allow the rate of adaptation rate to vary withthe intensity of stimulation [46]
5 Slick a Second KNa Channel
The second known gene that encodes a KNa channel wastermed Slick for sequence like an intermediate conductanceK channel because it is closely related to the Slack geneand Slack channels have conductance that is intermedi-ate between that of Slo1 (BK) calcium-activated potassium
4 ISRN Neuroscience
channels and most other potassium channels [4] The Slickchannel has also been termed the Slo21 channel and thehuman Slick gene is KCNT2 Overall the sequence of Slickis sim74 identical to that of Slack and the transmembranedomains and the RCK domains of Slick and Slack are almostidentical The greatest divergence between Slack and Slickoccurs at their distal C-terminal regions
The predicted cytoplasmic N-terminus of Slick is similarto that of the Slack-A isoform Consistent with this fact thecharacteristics of Slick currents recorded inmammalian cellsor Xenopus oocytes resemble those of Slack-A Althoughthe unitary conductance of Slick channels (sim140 pS) isslightly smaller than that of Slack channels Slick channelsactivate very rapidly with depolarization and have multi-ple subconductance states [4] The estimated half-maximalconcentration for activation of Slick by sodium 89mMis greater than that for Slack channels (Figures 2(c) and2(d)) Moreover in contrast to Slack channels which havean absolute requirement for Na+ for channel opening Slickchannels have a basal level of activity in the absence of sodium[4]
In addition to their sodium sensitivity both Slick andSlack channels can be activated in increases in intracellularchloride levels [4 47] Under physiological conditions how-ever Slick channels are markedly more sensitive to this anion[4] The domains that confer sensitivity of these channels tochloride are not known and mutations within a region thatresembles the ldquocalcium bowlrdquo of Slo1 channels and whichhas been established to contribute to the calcium sensitivityof Slo1 channels [48] have not been found to influence thesensitivity of Slack or Slick to either sodium or chloride ions[4]
The Slick channel also differs from Slack in having aconsensus ATP binding site just C-terminal to the secondRCK domain This site renders Slack channel activity to besensitive to cytoplasmic ATP levels The presence of 5mMATP reduces Slick whole cell currents or channel activityin excised patches [4] Further description of the effects ofATP on Slick channels will be provided in a later section onphysiological modulation of KNa channels
6 Heteromer Formation between Slack-Band Slick Subunits
Avariety of lines of evidence indicate that the Slack-B subunitand Slick can coassemble to form heteromeric channelsthat differ in their properties from those of either subunitexpressed alone [49] Moreover assembly of heteromericchannels appears to be specific for the Slack-B isoform anddoes not occur with Slack-A which as described abovediffers from Slack-B only in its N-terminal cytoplasmicdomain When Slick and Slack-B are coexpressed at a 1 1ratio either in Xenopus oocytes or in mammalian HEK293cells there is an 18ndash25-fold increase in current amplitudecompared to expression of either subunit alone [49] Thisincrease in current is associated with a comparable increasein levels of the channel subunits in plasma membrane asassayed by a surface biotinylation assay No differences in
total cellular levels of either protein are however foundwhenthe subunits are expressed singly or in combination with theother subunit
Themacroscopic currents of the Slack-BSlick heteromersdiffer from those of the homomers [49] For example thetime for the SlickSlack-B currents to reach 90 activationis significantly longer than that for either Slick or Slack-Bhomomeric currents In addition the unitary conductanceof the Slack-BSlick channels is intermediate between that ofSlack (sim180 pS) and Slick (sim140 pS) A direct demonstrationof this fact was obtained by making a pore mutant of theSlick channel Slick-EE (Q276E Y279E) that increases itsunitary conductance to sim500 pS Coexpression of Slick-EEwithwild-type Slack-B resulted in channels that had a unitaryconductance of sim325 pS a value that is clearly distinct fromthat for either channel alone providing definitive electro-physiological evidence for the formation of SlickSlack-Bheteromers
Coimmunoprecipitation experiments have also demon-strated that Slack-B forms a protein complex with Slick inboth rat brain tissue and heterologous expression systems[49] No coimmunoprecipitation could however be foundfor epitope-tagged Slick-A and Slick subunits This is con-sistent with electrophysiological findings demonstrating thatcoexpression of Slack-A with Slick does not increase whole-oocyte currents compared to expression of either subunitalone and that no intermediate conductance single-channelactivity can be observed when Slack-A is expressed withthe Slick-EE mutant Moreover coexpression of Slack-A withSlick does not increase levels of surface expression of eithersubunit
The specificity of SlackSlick interactions for the Slack-Bisoform is dependent on the extended N-terminal domainof Slack-B This has been demonstrated by constructing achimeric channel that replaced the cytoplasmic N-terminaldomain of Slick with that of Slack-B Coexpression of thismodified Slick channel with wild-type Slick channels inoocytes produced a 30-fold increase in whole-oocyte cur-rents which is similar to the increase obtained with Slack-Bsubunits [49] Thus the Slack-B N-terminal domain plays akey role in trafficking andor the level of plasma membraneexpression of heteromeric KNa channels
7 Localization of Slack and Slick Channels inthe Central Nervous System
Slack transcripts of about 45 kb and 75 kb are abundantlyexpressed in the brain and in the kidney [40] By the tech-nique of in situ hybridization in sections of adult rat brainSlack is very highly expressed in neurons but no staining isevident in glial cells Strong hybridization is found through-out the brain including the cerebral cortex hippocampusdeep cerebellar nuclei cerebellar Purkinje cells reticulartegmental nucleus of the pons preoptic nucleus substantianigra and auditory brainstem nuclei Neurons within thethalamus and hypothalamus were found to have a moremoderate level of staining [40] Subsequent quantificationof expression of mRNA levels by probes that are specific
ISRN Neuroscience 5
for either Slack-B or Slack-A isoforms found that bothisoforms are present throughout the brain but that thehighest levels of both isoforms are detected in brainstem andolfactory bulb [46]
Immunocytochemical staining using an antibody to theSlack-B specific N-terminus confirmed the localization ofSlack-B in neurons of the brainstem and olfactory bulb of rat[50] Prominent immunoreactivity was found in the olfactorybulb red nucleus and deep cerebellar nuclei as well asin vestibular and oculomotor nuclei and in the trigeminalsystem and reticular formation where intense staining occursin both cell bodies and axonal fibers As with the in situhybridization studies labeling was prominent in auditorybrainstem nuclei Although staining was also evident inneurons of the thalamus substantia nigra and amygdala theonly region of the cerebral cortex where the Slack-B isoformwas found was in the frontal cortex [50]
Immunolocalization studies have also been carried outusing an antibody targeted against a region in the cytoplasmicC-terminus of Slack [46] This would be expected to recog-nize themultiple Slack isoforms that differ at the N-terminusbut not to recognize Slick channels Immunostaining withthis ldquopan-Slackrdquo antibody found strong labeling of neuronsthroughout the nervous system of mice This included theregions previously identified with the Slack-B antibody aswell as other locations such as the dendrites of hippocampalneurons and olfactory bulb glomeruli [46]
Messenger RNA for the Slick subunit has awider distribu-tion throughout the body than does Slack mRNA and a Slicktranscript of sim69 kB is expressed in rat heart where no SlackmRNA has been found [4] Like Slack however the highestlevels of Slick mRNA are found in the brain Slick mRNA andimmunoreactivity overlap considerably with those for Slackand are found at high levels in olfactory bulb midbrain brainstem hippocampus throughout the cerebral cortex withhigh expression in primary somatosensory and visual regions[51] Like Slack high Slick immunoreactivity is found inneurons of sensory pathways including those in the auditorybrainstem The wide colocalization of Slick with the Slack-Bisoform suggests that the native KNa channels in these regionsare likely to be heteromers of these two subunits
Studies have also found Slack channels to be expressedin neurons of the peripheral nervous system Slack immuno-labeling is found in over 90 of neurons of the dorsal rootganglion and has been localized to both the somata andthe axonal tracts of small- medium- and large-diameterneurons [5 23] Localization to axonal tracts has also beendetected in electrophysiological experiments on peripheralaxons of Xenopus neurons for which a correlation has beenfound between the localization of KNa channels and voltage-dependent sodium channels [52]
8 Pharmacology of KNa Channels
A very common test for the presence of KNa channels inneurons and other cells has been to measure the effects ofreplacement of external sodium by lithium ions on the netoutward current Lithium is a much weaker activator of KNa
channels than sodium Because lithium readily enters cellsthough voltage-dependent sodium channels usually withminimal effects on the inward currents lithium replace-ment in voltage-clamp experiments reduces the net out-ward currents if KNa currents are present In current clampexperiments lithium substitution may alter the shape ofaction potentials or firing patterns in a manner consistentwith partial block of KNa currents The majority of theexperiments described later in the section on contributionsof KNa channels to neuronal firing patterns have tested theeffects of lithium substitution
Studies on native KNa channels in cardiac cells found thatseveral antiarrhythmic drugs inhibit these channels in cardiaccells [53ndash55] Some of these compounds such as quinidinebepridil and clofilium have been also found to be veryeffective and reversible blockers of Slack and Slick channelsexpressed in oocytes and mammalian cells [4 56 57] but arenon-specific in that they also act on a variety of other channeltypes
The pharmacological properties of Slack and Slick chan-nels are similar to each other but differ from those of manyother potassium channels including their closest molec-ular relative the Slo1 channel Slack and Slick are onlyweakly sensitive to the general potassium channel blockertetraethylammonium ions (TEA) [4] External barium ionsproduce a time- and voltage-dependent block of Slack andSlick channels In contrast both subunits are insensitiveto blockers of calcium-activated potassium channels suchas apamin iberiotoxin paxilline and charybdotoxin andto a wide range of blockers of other classes of potassiumchannels [4 57] Although Slick channels are sensitive tocytoplasmic ATP levels they are insensitive to glibenclamideand diazoxide an inhibitor and an activator respectively ofclassical KATP channels [4]
A variety of compounds that activate Slack and Slickchannels are also knownThefirst of these to be describedwasbithionol which in Slack-expressing HEK cells produces arobust increase in the amplitude of Slack currents with anEC50
of 077120583M [56] Similar bithionol-induced increasesin current are observed for Slack-expressing oocytes and fornative KNa currents in neurons of the auditory brainstem [5658] Bithionol reversibly activates Slack channels even whenapplied to the extracellular face of excised patches indicatingthat it acts on Slack channels relatively directly The increasein current occurs though a very marked bithionol-inducedchange in the voltage dependence of Slack channels such thatfull activation is already observed at potentials as negativeas minus40mV [56] Bithionol is however not selective forKNa channels in that it also activates Slo1 calcium-activatedpotassium channels [56 59]
A screen of pharmacologically active compounds usinga rubidium flux assay against Slack channels expressed inCHO cells has revealed other activators [59] These includeriluzole loxapine an antipsychotic agent and niclosamidean anthelmintic agent Loxapine was found to be moreselective than bithionol in that it has no effect of Slo1 calcium-activated potassium channels Electrophysiological experi-ments confirmed that loxapine is effective on recombinanthuman and rat Slack channels and that it activates native KNa
6 ISRN Neuroscience
channels in isolated neurons of the rat dorsal root ganglion[59]
9 Cellular Regulation of KNa Channels
The activity of Slack and Slick channels as well as ofheteromeric SlackSlick channels is potently regulated by avariety of cellular signaling pathways including G-protein-coupled receptors linked to activation of PKA or PKC [2328 30 49 60 61] direct phosphorylation of the channelssubunits [2 49 60] changes in cytoplasmic ATP levels[4] cytoplasmic NAD+ levels [5] PIP2 [62] estradiol [63]hypoxia [41 64] and the fragile X mental retardation proteinFMRP [6 32] This section will summarize these findings
91 Modulation of Slack and Slick by Protein Kinase CTreatment ofmammalian cells orXenopus oocytes expressingSlack channels with diacylglycerol or phorbol ester activatorsof protein kinase C (PKC) leads to a 2-3-fold increase incurrent amplitude and a slowing of the rate of activation[2 60] This effect of PKC has been found to result from thephosphorylation of a single serine residue S407 located in aPKC consensus phosphorylation site in the ldquohingerdquo region ofthe cytoplasmic C-terminus between the S6 transmembranedomain and the first RCK domain [2] Mutation of this serineto an alanine (S407A) which renders the site incapable ofundergoing phosphorylation results in apparently normalSlack currents that are entirely insensitive to PKC activationIn contrast mutation of the twelve other putative PKCphosphorylation sites in the Slack channel (see Figure 1)does not prevent the PKC-induced increase in Slack current[2]
In contrast to Slack channels the action of PKC onSlick channels is to produce a decrease in current amplitude[60] As in Slack this effect is likely to be mediated by amodification within the C-terminus In particular a chimericchannel that contains the N-terminus and membrane span-ning regions of Slack but the cytoplasmic C-terminus ofSlick responds to PKC activation with a decrease in currentMoreover direct application of a constitutively active formof PKC directly to the cytoplasmic face of excised patchescontaining these chimeric channels reduces their activity[60] Nevertheless the specific sites at which Slick subunitsare modified in response to PKC activation are not yetknown
Heteromeric Slack-BSlick channels respond to activationof PKC in a manner that is quite distinct from that of eithersubunit expressed alone [49] In Xenopus oocytes expressingboth subunits application of PKC activators potently sup-presses current bysim90This ismuch greater than the degreeof suppression measured for Slick subunits alone (sim50)and cannot therefore be explained by any linear combinationof Slick or Slack-B currents The finding that heteromericSlack-BSlick channels are regulated even more potently byPKC activation than are the homomeric channels constitutesone of the many pieces of evidence supporting selectiveheteromer formation between these subunits [49]
92 Modulation of Slack and Slick by G Protein CoupledReceptors Slack and Slick have been coexpressed in Xenopusoocytes with the M1 muscarinic receptor and the mGluR1metabotropic glutamate receptor [60]These are G120572q proteincoupled receptors that lead to the activation of PKC andformation of IP3 Accordingly activation of these receptorsleads to an increase in Slack currents and a suppression ofSlick currents consistent with modulation of these chan-nels by PKC There is widespread colocalization of thesereceptors with the KNa subunits throughout the nervoussystem suggesting that modulation of KNa currents by thesereceptors is likely to be widespread [60] Direct modulationof KNa channels by mGluR1 has been observed in nativeneurons within the lamprey spinal cord [28] In these cellsactivation of mGluR1 produces a strong suppression ofcurrent as expected for Slick or Slack-BSlick heteromersand this suppression is prevented by pharmacological inhibi-tion of PKC Interestingly chelation of intracellular calciumwith the chelator BAPTA (12-bis(2-aminophenoxy)ethane-NNN1015840N1015840-tetraacetic acid) prevented modulation of theKNa current by PKC but unmasked an mGluR1-dependentactivation of KNa current by an independent pathway whosemechanism is not yet known [28]
Modulation of Slack channels by G protein-coupledreceptors has been studied using a novel label-free technologyto detect changes in the distribution of mass close to theplasma membrane [61] The characteristic change in massthat normally follows activation of native receptors in HEK-293 cells is significantly modified in amplitude and timingby the coexpression of Slack channels further supporting thefinding of modulation of KNa current by these receptors [61]
93 Modulation of K119873119886
Currents by Cyclic AMP The activityof KNa channels in Kenyon cells isolated from the mushroombody of the cricket (Gryllus bimaculatus) is enhanced uponapplication of either the neurotransmitter octopamine or themembrane-permeable analog cyclic AMP analog 8-Br-cyclic-AMP [30] The increase in channel activity was found to beattenuated by the protein kinase A (PKA) inhibitor H-89Conversely KNa channel activity in these cells is decreasedby the transmitter dopamine or by the cyclic GMP analog8-Br-cyclic GMP and this decrease in open probability isantagonized by the protein kinaseG (PKG) inhibitor KT5823These findings have led to the proposal that modulation ofKNa currents play a role in olfactory learning circuits thatmediate reward and punishment signals in insects which areregulated by octopamine and dopamine respectively [30]
The Slack subunit is expressed at high levels in pri-mary afferent nociceptor neurons located in the dorsal rootganglion [5 23] and KNa currents can readily be recordedin these cells [5 22 23] Inflammatory substances suchas prostaglandin E2 that activate the PKA pathway greatlyincrease the excitability of these neurons sensitizing them tothermal and mechanical stimulation In their resting statethe neurons adapt rapidly to maintained depolarizationstypically generating only one or two action potentials at theonset of depolarization After an elevation of cyclic AMPlevels that activates PKA adaptation is greatly reduced such
ISRN Neuroscience 7
that sustained firing can be recorded throughout a prolongeddepolarization In major part the effects of PKA can beattributed to a decrease in Slack KNa current and suppressionof Slack expression using anti-Slack siRNA produces a reduc-tion in adaptation similar to that produced by PKA activation[23]
In contrast to the effects of PKC on Slack channels thatwere described above the effects of PKA on Slack are indirectand do not involve phosphorylation of the channel subunititself [23 65] Slack channels in HEK-293 cells are insensitiveto cyclic AMP analogs or to the PKA inhibitor KT5720 andapplication of the catalytic subunit of PKA to the cytoplasmicface of Slack channels excised from transfected cells or dorsalroot ganglion neurons has no effect on channel openingInstead activation of PKA decreases Slack current amplitudeby rapidly internalizing Slack channels from the plasmamembrane into internal organelles [23] Such internalizationwas detected by confocal microscopy of neurons expressingSlack channels tagged at their N-terminus with the greenfluorescent protein GFP Elevations of cyclic AMP reducedcolocalization of the labeled channels with a cell surfacemarker A reduction in native Slack channels at the plasmamembrane following PKA activation was also confirmedusing a surface biotinylation assay [23]
94 Modulation of Slack and Slick by PIP2 A variety ofion channels have been shown to be regulated by phos-phatidylinositol 45-biphosphate (PIP2) a phospholipid thatis localized to the inner lipid leaflet of the plasma membrane[66] Exogenously applied PI(45)P2 as well as its isoformPI(34)P2 increases the amplitude of Slack and Slick currentsexpressed in Xenopus oocytes and pharmacological agentsthat are expected to reduce endogenous PIP2 levels reducecurrent amplitude [62] Regulation by PIP2 was found torequire specific lysine residues in the C-termini of thechannelsMutation of these lysines to alanine (K339A in Slackand K306A in Slick) produced functional KNa currents thatwere insensitive to the activating effects of PIP2 [62]
95 Suppression of Slick Channels by Cytoplasmic ATP TheSlick subunit differs from Slack in that it contains a consensusATP binding site just C-terminal to the second RCK domain[4] The current density of Slick currents but not Slackcurrents is reduced by sim80 by the presence of 5mM ATPat the cytoplasmic face of the channels The same full effectis observed with ATP120574S a slowly hydrolyzable ATP analogand the nonhydrolyzable analog AMP-PNP but not withADP Suppression by ATP of Slick currents at all membranevoltages can also be readily detected in excised patches [4]
Confirmation that the effect of ATP on Slick channelsis mediated by the consensus ATP binding site in the C-terminal domain was obtained by site-directed mutagenesisto replace the glycine at residue 1032 in this site with a serine[4]Themutant G1032S Slick channel was fully functional butfailed to decrease channel activity in response to ATP Thesensitivity of Slick channels to ATP levels suggests that theiractivity could be enhanced during periods of high metabolicdemand when cellular ATP levels fall
96 Modulation of Slack by NAD+ KNa channels can read-ily be detected in cell-attached patch-clamp recordings onneurons [5 16 58] This may be surprising given that thecytoplasmic sodium concentration in resting cells is relativelylow (ltsim10mM see Section 10 below) and as described earlierthe EC
50for activation of these channels in excised patches
is relatively high (typically sim40ndash80mM) This discrepancy isresolved by the finding that the open probability and sodiumsensitivity of KNa channels are greatly reduced following exci-sion ofmembrane patches [5 16] suggesting that rundown ofKNa activity in excised patches may result from loss of somecytoplasmic factor
A major cytoplasmic factor that regulates both Slack andSlick channels has been found to be nicotinamide adeninedinucleotideNAD+ [5] Both subunits have a putativeNAD+-binding site within their second RCK domain Application ofNAD+ to the cytoplasmic face of KNa channels excised fromdorsal root ganglion neurons increases their open probabilityand reduces themeasured EC
50for activation of sodium from
sim50mM to sim17mM (Figure 3) NAD+ as well as NADP+but not NADH also increased the open probability of Slackchannels excised from a transfected mammalian cell lineMutation of glycine in the putative NAD+-binding site ofSlack (G792A) resulted in a loss of the ability of NAD+to potentiate activity [5] These findings suggest that KNachannels play a much more prominent role in the normalphysiology of excitable cells than would be suggested by theirproperties in isolated patches
97 Modulation of Slack by Estradiol The open probability ofchannels reconstituted from lipid bilayers from extracts of ratcortex has been found to be increased by 17120573-estradiol [63]Although these channels were not explicitly shown to be KNachannels a similar effect was reported for whole-cell currentsrecorded in Slack-expressing HEK-293 cells Activation wasnot prevented by tamoxifen a blocker of classical estrogenreceptors but may be a consequence of direct binding of 17120573-estradiol to Slack channels themselves [63]
98 Modulation of Slack Channels by Hypoxia Duringhypoxic conditions cytoplasmic levels of internal sodiumand chloride ions as well as those of H+ and CO
2 rise It
has been suggested that activation of KNa channels couldact to hyperpolarize cells during hypoxia providing a pro-tective mechanism [7] This conjecture is supported by thefinding that nematodes lacking the Slack ortholog in thisspecies are significantly more sensitive to hypoxic deaththan are wild-type animals [41] Further support for a rolefor Slack channels in the response to hypoxia comes fromrecordings of Slack channel activity in Xenopus oocytesexposed to increasing concentrations ofH+ andCO
2 demon-
strating that elevations of sodium activate Slack channelsunder cellular conditions comparable to those expected inischemiahypoxia [64]
99 Interactions of Slack with the Fragile X Mental Retarda-tion Protein FMRP Fragile X syndrome the most commoninherited form of intellectual disability in humans is caused
8 ISRN Neuroscience
1
08
06
04
02
0
[Na] (mM)10010
Control (small neurons)NAD
Nor
mal
ized
NP o
Figure 3 NAD+ increases the sodium sensitivity of KNa channelsexcised from adult rat dorsal root ganglion neurons Concentration-response relationship provides estimates of EC
50for sodium activa-
tion of 50mM and 17mM in the absence and presence of NAD+respectively Figure modified from [5]
by loss of expression of the RNA-binding protein FMRP(fragile X mental retardation protein) FMPR is known to berequired for normal activity-dependent protein translationin neurons Yeast two-hybrid and coimmunoprecipitationexperiments have shown that the cytoplasmic C-terminus ofthe Slack subunit interacts with this protein [6] MoreoverSlack-FMRP complexes also contain messenger RNAs thatare targets of FMRP such as the mRNAs encoding Map1band Arc No association of mRNAs with Slack can howeverbe found in mice lacking FMRP indicating that these arebound to Slack despite its interaction with FMRP A peptidearray assay revealed confirmed that FMRP binds selectivelyto sequences at the distal C-terminus of Slack [6]
FMRP is a potent activator of Slack channel activity [6]Application of recombinant FMRP(1ndash298) which containsthemajority of protein-protein interaction domains of FMRPto the cytoplasmic face of Slack channels in excised patchesreversibly increases channel opening by two- to threefold(Figure 4(a)) This is associated with the almost completeelimination of openings to subconductance states which areprominent in untreated patches (Figure 4(b)) FMRP wasfound to have no effect on the channel activity of a Slacktruncation mutant (Slack-BΔ804) which lacks sites found tointeract with FMRP in biochemical assays [6]
Support for a role for FMRP in regulating the amplitude ofnative KNa currents has also come from recordings of neuronsfrom animals lacking the fmr1 gene which encodes FMPR[6] As will be described later KNa channels account for amajor component of potassium current in principal neuronsof the medial nucleus of the trapezoid body (MNTB) Whileloss of FMRP does not affect total levels of Slack proteinthe level of the KNa component of current in these cells issubstantially reduced in the FMRP knockout mice compared
to that in wild-type animals consistent with an activating rolefor this RNA-binding protein [6]
The interaction between Slack and FMRP is likely tobe evolutionarily conserved Both proteins have been foundto be coexpressed in bag cell (BC) neurons which regulatereproductive behaviors in mollusk Aplysia [32] FMRP andSlack immunoreactivity are colocalized at the periphery ofisolated BC neurons and the two proteins can be reciprocallycoimmunoprecipitatedThe native Slack current in these cellswas identified using an siRNA approach and intracellularinjection of FMRP(1ndash298) induced a KNa current that in itsproperties matches the Slack current As for Slack channelsin heterologous expression systems addition of FMRP(1ndash298) to the inside-out patches containing native AplysiaKNa channels increased channel opening In current clamprecordings FMRP(1ndash298) produced a narrowing of actionpotentials
Downregulation of Slack expression in BC neurons usingan siRNA approach had an interesting effect on long-termchanges in the excitability of these cells [32] In untreatedcells brief stimulation of these neurons produces a charac-teristic prolonged (sim30min) discharge that is followed bya prolonged (sim18 hr) inhibitory state Recovery from thisinhibitory state requires new protein synthesis and is blockedby the protein synthesis inhibitor anisomycin Anti-SlacksiRNA did not alter the ability of BC neurons to undergothe normal discharge but like inhibition of protein synthesisprevented recovery from the inhibitory state [32] Althoughother interpretations are possible these findings raise thepossibility that the activity of Slack channels is linked to theregulation of protein translation through the FMRP-RNApathway
As will be described later point mutations in humanSlack channels can produce devastating effects on intellectualdevelopment [2 3] A variety of experiments have demon-strated that activation of ion channels can influence cytoplas-mic processes independently of ion flux through the channels[67] The interaction between the C-terminus of Slack andFMRP raises the hypothesis that conformational changesin the C-terminus could represent such a ldquononconductingrdquofunction of Slack that links neuronal firing to changes inprotein translation
10 Sodium Entry Pathways ThatActivate KNa Channels
Estimates of cytoplasmic sodium concentrations in restingunstimulated neurons obtained using sodium-sensitivemicroelectrodes and ratiometric dyes provide valuesbetween sim4mM and 15mM [68] As noted earlier in thesection on NAD+ regulation of Slack channels this maybe sufficient to provide significant activation of native KNachannels in intact neurons even under resting conditions[5] Internal sodium levels rise in response to stimulationprincipally as a result of entry through voltage-dependentsodium channels and ionotropic ligand-gated receptors suchas AMPA and NMDA glutamate receptors [69] Stimulationparticularly repetitive tetanic stimulation produces local
ISRN Neuroscience 9
Control RecoveryC
1
2
3
C
1
2
3
10pA
2 s
10pA
200ms
S S S S S S S
FMRP(1ndash298)
(a)
2520151050
10000
1000
100
pA
C1
2
3
Recovery
2520151050
10000
1000
100
pA
C1
2
3
S
S
S(N
)
(N)
Control
2520151050
10000
1000
100
pA
C
1 2 3
S
S
S
(N)
FMRP(1ndash298)
(b)
Figure 4 FMRP alters gating of Slack-B channels in inside-out patches (a) Recording of a patch containing 3-4 Slack channels before duringand after transient application of FMRP(1ndash298) to the cytoplasmic face of the patch Top traces show representative 10 s examples of recordingatminus40mVand bottom traces show expanded time views of the same patch Arrowsmarked S indicate the occurrence of subconductance statesthat are suppressed by FMRP(1ndash298) (b) All-points amplitude histograms of Slack channel activity plotted on a logarithmic scale before andafter application of FMRP(1ndash298) and after washout C marks the closed state S indicates subconductance states and numbers on peaks inthe histograms refer to the number of fully open Slack channels Modified from [6]
increases in sodium concentrations that can rise to levelsbetween 45 and 100mM [70 71] In particular high levelscan be attained in restricted compartments such as activedendritic spines
Although the upstroke of action potentials in mostneurons is driven by rapidly-inactivating voltage-dependentsodium currents these currents are transient Evidencesuggest that the much smaller persistent component ofsodium current that does not inactivate with maintaineddepolarization is responsible both for sustained elevationsof internal sodium during stimulation [69] and for selectiveactivation of KNa channels [72 73] It has been found thatin patches excised from somata of rat neurons sodium entrythrough persistent tetrodotoxin-sensitive sodium channels isthe dominant factor that leads to activation of KNa channelsin the same patch [73] Activation of KNa channels by sodiumentry in the isolated patches was enhanced by veratridinewhich prolongs sodium channel openings and was observedin the absence of sodium at the cytoplasmic face of thepatches Moreover activation appeared to be independent ofthe amplitude of the much larger transient sodium current
[73] Although the molecular basis for persistent sodiumchannels is not fully understood further support for a selec-tive activation of KNa currents by entry through persistentchannels has also been found in electrocytes of electric fish inwhich the time course of KNa currents during depolarizationsreflects that of the persistent sodium current and not that ofthe very much larger transient sodium current [74]
A number of studies have documented that KNa currentsare also activated by sodium entry through ionotropic gluta-mate receptors particularly AMPA receptors Slack subunitscan be coimmunoprecipitated with GluR23 AMPA receptorsusing rat brain synaptosomal fractions or lysates of the lam-prey central nervous system [27] Moreover Slack subunitshave been found to bind directly to the PDZ domain of PSD-95 (postsynaptic density-95 protein) a major componentof the postsynaptic density of glutamatergic synapses andto colocalize with PSD-95 in cultures of mouse corticalneurons [75] In electrophysiological studies of lampreyneurons application of AMPA was been found to activate aKNa current with pharmacological properties of Slack [25]Activation of this KNa current regulates both the amplitude
10 ISRN Neuroscience
and the decay rate of AMPA-induced synaptic currents pro-viding a feedback loop that decreases the size of glutamatergicpostsynaptic potentials [27]
Activation of KNa currents following activation of AMPAreceptors has also been proposed in studies of neurons of thelocus coeruleus to which application of glutamate producesa burst of action potentials followed by period of inhibitionlasting 30ndash45 seconds [76] The potassium current responsi-ble for the period of inhibition was selectively activated byAMPA or the AMPA-receptor agonist quisqualate but not byother stimuli that excite these neurons Consistent with thiscurrent being a KNa current it was attenuated by lowering ofexternal sodium concentrations but not affected by externalcalcium levels and it was potentiated by the KNa channelopener bithionol [76]
While persistent voltage-dependent sodium channels andAMPA receptors may represent the primary sodium entrypathways for activation of KNa channels there exist otherpathways that allow sodium ions to enter excitable cellsand whose role has been less explored Slack subunits areabundantly expressed in mitral cells of the olfactory bulb[46 50] In these neurons deletion of the gene for a majorvoltage-dependent potassiumchannelKv13 produces a com-pensatory increase in levels of Slack-B protein and in SlackKNa currents identified using an anti-Slack siRNA technique[77] Activation of the KNa current during depolarizationof these cells could only be significantly suppressed by acombination of tetrodotoxin to block voltage-dependentsodium channels and ZD7288 a blocker of H-channelswhich are nonselective cation channels that flux both sodiumand potassium ions Either drug alone failed to significantlysuppress KNa currents suggesting that sodium entry throughH-channels plays a role in KNa channel activation in theseneurons [77]
11 Contribution of KNa Channels toNeuronal Firing Patterns
The component of whole-cell potassium current in neuronsthat can be attributed to Slack channel subunits has beenidentified in mitral cells of the olfactory bulb and mediumspiny neuron of the striatum using siRNAs to suppress trans-lation of SlackmRNA [72 77] Such experiments indicate thatSlack KNa channels are responsible for a very major compo-nent of the total delayed outward current in neurons underphysiological conditions [72] The specific contributions thatKNa channels make to the electrical personality of individualtypes of neurons may differ in different cell types
One of the earliest effects to be described for activationof KNa currents is to produce a slow afterhyperpolarization(sAHP) following a burst of action potentials This was firstdescribed in pyramidal neurons of the cat sensorimotorcortex [11 78] and similar slow sodium-dependent sAHPsthat endure for many seconds have been found in a widevariety of neuronal types [79ndash84]These have been the subjectof earlier reviews [20 21] Such slow potentials have beenfound to influence the timing of regular bursts of actionpotentials such as the 3ndash12 per minute spindle waves that
occur at the onset of sleep in certain neurons within thethalamus [80] Slow activation of KNa during repetitive firingalso produces an adaptation of firing rate during bursts ofaction potentials [23] and such adaptation is predicted bynumerical simulations that incorporate KNa currents [46 85]
In some neurons such as neocortical pyramidal neuronsand hippocampal CA1 neurons a significant amount of KNacurrent can also be activated by single spikes and contributeto action potentials repolarization or to the amplitude ofdepolarizing afterpotentials that follow single spikes andare generated by persistent sodium currents [83 86] Inthese cases KNa current activation may be detected withinsim5msec following a single-action potential A recent studyhas examined themechanismbywhich the cells of the electricorgan (electrocytes) of certain electric fish are able to generatesustained firing at frequencies greater than 500Hz [74] Slackchannels are expressed at high levels in these cells and theirdelayed potassium current is almost exclusively a KNa currentthat is activated by a persistent sodium current Numericalsimulations have indicated that the use of KNa current torepolarize the very large action potentials of these cells allowsthe cells to fire at such high rates with minimal energyexpenditure [74]
A set of neurons that have been particularly instructivein studies of the role of KNa channels in shaping neuronalfiring are those that control locomotion in the spinal cord oflamprey [24ndash28 87] In addition to the transient KNa currentthat is activated by AMPA receptors and that was describedearlier [25 27] these neurons appear to have two distinctcomponents ofKNa current that are triggered by action poten-tial firingmdasha transient component that follows a single spikeand a slower more sustained component [24 26 87] Therapid transient KNa current is activated by the sodium influxthrough tetrodotoxin-sensitive channel during the actionpotentials and may play a role in determining the amplitudeand duration of individual action potentials The sustainedKNa current is not abolished by tetrodotoxin suggesting thatit is selectively activated by an independent sodium entrypathway and may underlie the calcium-independent sAHPthat occurs during repetitive bursting in lamprey spinal cordneurons [24]
Experiments with neurons that fire at high rates withvery high temporal accuracy have provided another potentialbiological role for KNa channels [58] Slack and Slick areexpressed at high levels in neurons of the medial nucleusof the trapezoid (MNTB) within the auditory brainstem[6 50 51] These neurons fire at rates up to sim800Hzand accurate timing of their action potentials is requiredfor transmission of information about the localization ofsound stimuli KNa channels are readily be detected in bothcell-attached and excised patches on the somata of MNTBneurons and they are localized appropriately for activationby sodium influx through AMPA receptors or voltage-dependent sodium channels [58] Current-clamp recordingsfrom MNTB neurons and numerical simulations indicatethat as KNa channels become activated temporal accuracyof firing is improved This occurs because the voltage-dependence of Slack and Slick channels allows them to opennear the resting potential of MNTB neurons increasing
ISRN Neuroscience 11
the resting conductance and decreasing the membrane timeconstant An increase in temporal fidelity of MNTB neuronscan also be produced by treatment with the pharmacologicalKNa activator bithionol [58]
12 Mutations in Slack Channels Result inEpilepsy and Intellectual Disabilities
De novo human mutations that produce single-amino-acidalterations in Slack channels produce devastating effectson development and intellectual function [2 3] One setof Slack mutations results in malignant migrating partialseizures of infancy (MMPSI) [2]This disease is characterizedby pharmacoresistant seizures in the first 6 months of lifethat arise randomly and ldquomigraterdquo from one brain regionto another While the occurrence of seizures abates withage MMPSI results in near-total arrest of psychomotordevelopment Exome sequencing revealed that at least 50of MMPSI patients have mutations in the large cytoplasmicC-terminal domain of Slack [2]
Two mutations in the human Slack gene that produceMMPSI are R428Q and A934T which correspond to themutations R409Q and A913T in the highly conserved ratSlack sequence [2] Expression of these mutants in Xenopusoocytes produced Slack currents with voltage dependenceand kinetic behavior that matched that of wild-type Slackchannels but with a current amplitude that was increased 2-3-fold over that of wild-type Slack This increase in currentlikely results because the mutations produce channels thatare either more prone to undergo phosphorylation by PKC(or less likely to be dephosphorylated) or that mimic theconformation induced by PKC activation One of theMMPSImutations R409Q lies adjacent to the phosphorylation siteS407 that as described earlier enhances current by 2-3-fold when phosphorylated by PKC Consistent with thisinterpretation current amplitude of the two MMPSI muta-tions could not be further increased upon PKC activa-tion [2] In other respects the MMPSI currents resembledthat of wild-type channels with unitary conductance andsodiumdependencesmatching those of wild-type Slack BothMMPSI mutant channels however had reduced openings tosubconductance states compared to the wild-type channels[2]
A second disease that has been attributed to a differentset of mutations in the C-terminal domain of Slack chan-nels is autosomal dominant nocturnal frontal lobe epilepsy(ADNFLE) [3] This is a childhood-onset focal epilepsysyndrome in which motor seizures arise during sleep Onsetof ADNFLE in the patients with Slack mutations occurred ata mean age of 6 years and was associated with mild to severeintellectual disability andor severe psychiatric problemsTheeffects of these ADNFLE mutations on the function of Slackchannels have however not been investigated Interestinglyanother known genetic cause of ADNFLE is mutations insubunits of the neuronal nicotinic acetycholine receptor Theacetycholine receptor ADNFLE mutations do not howeverproduce intellectual disability which contrasts strikingly tothe effects of the Slack mutations [3]
It is possible that the very severe outcomes of mutationsin Slack occur solely because of changes in the amplitudeof Slack currents An increase in total potassium currentin rapidly firing interneurons could reduce the firing rateof these cells predisposing the nervous system to hyper-excitability The relatively modest increase in current thatoccurs with theMMPSImutations [2] however suggests thatother processes may also be affected As described earlierthe large C-terminal cytoplasmic domain of Slack in whichMMPSI and ADNFLE mutations are located interacts withFMRP and its cargo mRNAs [6] Thus it is possible thatin addition to directly altering the excitability of centralneuronsmutations in Slack disrupt the regulation of activity-dependent protein translation in neurons
13 Conclusions
This review has covered themolecular and cellular propertiesof Slack and Slick two potassium channel subunits thatprovide neurons and other cells with KNa channels It shouldbe pointed out that additional potassium channels that areactivated by cytoplasmic sodiummay also exist For examplesome of the native KNa currents that have been described inneurons do not match Slack or Slick in their single-channelconductances or pharmacological properties [88]These mayperhaps represent inward rectifier potassium channels thatare activated by sodium [45 89] and the physiological rolesof the sodium sensitivity of these channels require furtherinvestigation Nevertheless Slack and Slick which are verywidely expressed in the brain appear to play dominant rolesin shaping the firing properties of verymany types of neuronsHumanmutations in these channels produce profound effectson neuronal function and development suggesting that per-haps their biological role may extend beyond simply settingthe level of neuronal excitability and that these channels mayinfluence cytoplasmic biochemical pathways that regulatedevelopment plasticity and intellectual function
References
[1] B Hille Ionic Channels of Excitable Membranes Sinauer Asso-ciates Sunderland Mass USA 3rd edition 2001
[2] G Barcia M R Fleming A Deligniere et al ldquoDe novo gain-of-function KCNT1 channel mutations cause malignant migratingpartial seizures of infancyrdquo Nature Genetics vol 44 pp 1255ndash1259 2012
[3] S E Heron K R SmithM Bahlo et al ldquoMissensemutations inthe sodium-gated potassium channel gene KCNT1 cause severeautosomal dominant nocturnal frontal lobe epilepsyrdquo NatureGenetics vol 44 pp 1188ndash1190 2012
[4] A Bhattacharjee W J Joiner M Wu Y Yang F J Sigworthand L K Kaczmarek ldquoSlick (Slo2 1) a rapidly-gating sodium-activated potassium channel inhibited by ATPrdquo The Journal ofNeuroscience vol 23 no 37 pp 11681ndash11691 2003
[5] T J Tamsett K E Picchione and A Bhattacharjee ldquoNAD+activates KNa channels in dorsal root ganglion neuronsrdquo TheJournal of Neuroscience vol 29 no 16 pp 5127ndash5134 2009
12 ISRN Neuroscience
[6] M R Brown J Kronengold V R Gazula et al ldquoFragileX mental retardation protein controls gating of the sodium-activated potassium channel SlackrdquoNatureNeuroscience vol 13no 7 pp 819ndash821 2010
[7] M Kameyama M Kakei and R Sato ldquoIntracellular Na+activates a K+ channel in mammalian cardiac cellsrdquoNature vol309 no 5966 pp 354ndash356 1984
[8] C R Bader L Bernheim and D Bertrand ldquoSodium-activatedpotassium current in cultured avian neuronesrdquoNature vol 317no 6037 pp 540ndash542 1985
[9] K Hartung ldquoPotentiation of a transient outward current byNa+influx in crayfish neuronesrdquo Pflugers Archiv vol 404 no 1 pp41ndash44 1985
[10] S E Dryer J T Fujii and A R Martin ldquoA Na+-activatedK+ current in cultured brain stem neurones from chicksrdquo TheJournal of Physiology vol 410 pp 283ndash296 1989
[11] P C Schwindt W J Spain and W E Crill ldquoLong-lastingreduction of excitability by a sodium-dependent potassiumcurrent in cat neocortical neuronsrdquo Journal of Neurophysiologyvol 61 no 2 pp 233ndash244 1989
[12] C Haimann L Bernheim D Bertrand and C R BaderldquoPotassium current activated by intracellular sodium in quailtrigeminal ganglion neuronsrdquo Journal of General Physiology vol95 no 5 pp 961ndash979 1990
[13] S E Dryer ldquoNa+-activated K+ channels and voltage-evokedionic currents in brain stem and parasympathetic neurones ofthe chickrdquoThe Journal of Physiology vol 435 pp 513ndash532 1991
[14] M Saito and C F Wu ldquoExpression of ion channels andmutational effects in giant Drosophila neurons differentiatedfrom cell division-arrested embryonic neuroblastsrdquoThe Journalof Neuroscience vol 11 no 7 pp 2135ndash2150 1991
[15] T M Egan D Dagan J Kupper and I B Levitan ldquoPropertiesand rundown of sodium-activated potassium channels in ratolfactory bulb neuronsrdquoThe Journal of Neuroscience vol 12 no5 pp 1964ndash1976 1992
[16] C Haimann J Magistretti and B Pozzi ldquoSodium-activatedpotassium current in sensory neurons a comparison of cell-attached and cell-free single-channel activitiesrdquo Pflugers Archivvol 422 no 3 pp 287ndash294 1992
[17] N Dale ldquoA large sustained Na+- and voltage-dependent K+current in spinal neurons of the frog embryordquo The Journal ofPhysiology vol 462 pp 349ndash372 1993
[18] B V Safronov and W Vogel ldquoProperties and functions of Na+-activated K+ channels in the soma of rat motoneuronesrdquo TheJournal of Physiology vol 497 no 3 pp 727ndash734 1996
[19] U Bischoff W Vogel and B V Safronov ldquoNa+-activated K+channels in small dorsal root ganglion neurones of ratrdquo TheJournal of Physiology vol 510 part 3 pp 743ndash754 1998
[20] S E Dryer ldquoNa+-activated K+ channels a new family of large-conductance ion channelsrdquo Trends in Neurosciences vol 17 no4 pp 155ndash160 1994
[21] A Bhattacharjee and L K Kaczmarek ldquoFor K+ channels Na+ isthe new Ca2+ rdquo Trends in Neurosciences vol 28 no 8 pp 422ndash428 2005
[22] S B Gao Y Wu C X Lu Z H Guo C H Li and J P DingldquoSlack and Slick KNa channels are required for the depolarizingafterpotential of acutely isolated medium diameter rat dorsalroot ganglion neuronsrdquoActa Pharmacologica Sinica vol 29 no8 pp 899ndash905 2008
[23] M O Nuwer K E Picchione and A Bhattacharjee ldquoPKA-induced internalization of Slack KNa channels produces dorsal
root ganglion neuron hyperexcitabilityrdquo The Journal of Neuro-science vol 30 no 42 pp 14165ndash14172 2010
[24] D Hess E Nanou and A El Manira ldquoCharacterization of Na+-activated K+ currents in larval lamprey spinal cord neuronsrdquoJournal of Neurophysiology vol 97 no 5 pp 3484ndash3493 2007
[25] E Nanou and A El Manira ldquoA postsynaptic negative feedbackmediated by coupling between AMPA receptors and Na+-activated K+ channels in spinal cord neuronesrdquo EuropeanJournal of Neuroscience vol 25 no 2 pp 445ndash450 2007
[26] PWallen B Robertson L Cangiano et al ldquoSodium-dependentpotassium channels of a Slack-like subtype contribute to theslow afterhyperpolarization in lamprey spinal neuronsrdquo TheJournal of Physiology vol 585 no 1 pp 75ndash90 2007
[27] E Nanou A Kyriakatos A Bhattacharjee L K Kaczmarek GParatcha and A El Manira ldquoNa+-mediated coupling betweenAMPA receptors and KNa channels shapes synaptic transmis-sionrdquo Proceedings of the National Academy of Sciences of theUnited States of America vol 105 no 52 pp 20941ndash20946 2008
[28] E Nanou and A El Manira ldquoMechanisms of modulation ofAMPA-induced Na+-activated K+ current by mGluR1rdquo Journalof Neurophysiology vol 103 no 1 pp 441ndash445 2010
[29] A R Mercer and J G Hildebrand ldquoDevelopmental changes inthe density of ionic currents in antennal-lobe neurons of thesphinx moth Manduca sextardquo Journal of Neurophysiology vol87 no 6 pp 2664ndash2675 2002
[30] K Aoki K Kosakai and M Yoshino ldquoMonoaminergic modu-lation of the Na+-activated K+ channel in kenyon cells isolatedfrom the mushroom body of the cricket (Gryllus bimaculatus)brainrdquo Journal of Neurophysiology vol 100 no 3 pp 1211ndash12222008
[31] G Klees P Hochstrate and P W Dierkes ldquoSodium-dependentpotassium channels in leech P neuronsrdquo Journal of MembraneBiology vol 208 no 1 pp 27ndash38 2005
[32] Y Zhang M R Brown C Hyland et al ldquoRegulation ofneuronal excitability by interaction of fragile X mental retar-dation protein with Slack potassium channelsrdquo The Journal ofNeuroscience vol 32 pp 15318ndash15327 2012
[33] C Lawrence and G C Rodrigo ldquoA Na+-activated K+ cur-rent (IKNa) is present in guinea-pig but not rat ventricularmyocytesrdquo Pflugers Archiv vol 437 no 6 pp 831ndash838 1999
[34] M Zhou Z Liu C Hu Z Zhang and Y Mei ldquoDevelopmentalregulation of a Na+-activated fast outward K+ current in ratmyoblastsrdquo Cellular Physiology and Biochemistry vol 14 no 4ndash6 pp 225ndash230 2004
[35] C K Young H S Jae M K Tong et al ldquoSodium-activatedpotassium current in guinea pig gastric myocytesrdquo Journal ofKorean Medical Science vol 22 no 1 pp 57ndash62 2007
[36] L Re V Moretti L Rossini and P Giusti ldquoSodium-activatedpotassium current in mouse diaphragmrdquo FEBS Letters vol 270no 1-2 pp 195ndash197 1990
[37] Y Zhang and W G Paterson ldquoFunctional evidence for Na+-activatedK+ channels in circular smoothmuscle of the opossumlower esophageal sphincterrdquo American The Journal of Physiol-ogy vol 292 no 6 pp G1600ndashG1606 2007
[38] M Paulais S Lachheb and J Teulon ldquoANa+- and Clminus-activatedK+ channel in the thick ascending limb of mouse kidneyrdquoJournal of General Physiology vol 127 no 2 pp 205ndash215 2006
[39] T M Egan D Dagan J Kupper and I B Levitan ldquoNa+-activated K+ channels are widely distributed in rat CNS and inXenopus oocytesrdquo Brain Research vol 584 no 1-2 pp 319ndash3211992
ISRN Neuroscience 13
[40] W J Joiner M D Tang L Y Wang et al ldquoFormation ofintermediate-conductance calcium-activated potassium chan-nels by interaction of Slack and Slo subunitsrdquo Nature Neuro-science vol 1 no 6 pp 462ndash469 1998
[41] A Yuan C M Santi A Wei et al ldquoThe sodium-activatedpotassium channel is encoded by a member of the Slo genefamilyrdquo Neuron vol 37 no 5 pp 765ndash773 2003
[42] Y Jiang A Pico M Cadene B T Chait and R MacKinnonldquoStructure of the RCK domain from the E coli K+ channeland demonstration of its presence in the human BK channelrdquoNeuron vol 29 no 3 pp 593ndash601 2001
[43] P Yuan M D Leonetti A R Pico Y Hsiung and R MacK-innon ldquoStructure of the human BK channel Ca2+-activationapparatus at 30 A resolutionrdquo Science vol 329 no 5988 pp182ndash186 2010
[44] Z Zhang A Rosenhouse-Dantsker Q Y Tang S Noskov andD E Logothetis ldquoThe RCK2 domain uses a coordination sitepresent in Kir channels to confer sodium sensitivity to Slo22channelsrdquoThe Journal of Neuroscience vol 30 no 22 pp 7554ndash7562 2010
[45] J L Sui K W Chan and D E Logothetis ldquoNa+ activationof the muscarinic K+ channel by a G-protein-independentmechanismrdquo Journal of General Physiology vol 108 no 5 pp381ndash391 1996
[46] M R Brown J Kronengold V R Gazula et al ldquoAmino-terminiisoforms of the Slack K+ channel regulated by alternative pro-moters differentiallymodulate rhythmic firing and adaptationrdquoThe Journal of Physiology vol 586 no 21 pp 5161ndash5179 2008
[47] A Yuan M Dourado A Butler N Walton A Wei and LSalkoff ldquoSLO-2 a K+ channel with an unusual Clminus dependencerdquoNature Neuroscience vol 3 no 8 pp 771ndash779 2000
[48] M Schreiber and L Salkoff ldquoA novel calcium-sensing domainin the BK channelrdquo Biophysical Journal vol 73 no 3 pp 1355ndash1363 1997
[49] H Chen J Kronengold Y Yan et al ldquoThe N-terminal domainof slack determines the formation and trafficking of slickslackheteromeric sodium-activated potassium channelsrdquo The Jour-nal of Neuroscience vol 29 no 17 pp 5654ndash5665 2009
[50] A Bhattacharjee L Gan and L K Kaczmarek ldquoLocalization ofthe Slack potassium channel in the rat central nervous systemrdquoJournal of Comparative Neurology vol 454 no 3 pp 241ndash2542002
[51] A Bhattacharjee C A A von Hehn X Mei and L KKaczmarek ldquoLocalization of the Na+-activated K+ channel slickin the rat central nervous systemrdquo Journal of ComparativeNeurology vol 484 no 1 pp 80ndash92 2005
[52] D S Koh P Jonas and W Vogel ldquoNa+-activated K+ channelslocalized in the nodal region of myelinated axons of XenopusrdquoThe Journal of Physiology vol 479 no 2 pp 183ndash197 1994
[53] K Mori T Saito Y Masuda and H Nakaya ldquoEffects of classIII antiarrhythmic drugs on the Na+-activated K+ channels inguinea-pig ventricular cellsrdquo British Journal of Pharmacologyvol 119 no 1 pp 133ndash141 1996
[54] K Mori S Kobayashi T Saito Y Masuda and H NakayaldquoInhibitory effects of class I and IV antiarrhythmic drugs onthe Na+- activated K+ channel current in guinea pig ventricularcellsrdquo Naunyn-Schmiedebergrsquos Archives of Pharmacology vol358 no 6 pp 641ndash648 1998
[55] Y Li T Sato and M Arita ldquoBepridil blunts the shortening ofaction potential duration caused by metabolic inhibition viablockade of ATP-sensitive K+ channels and Na+- activated K+
channelsrdquo Journal of Pharmacology and ExperimentalTherapeu-tics vol 291 no 2 pp 562ndash568 1999
[56] B Yang V K Gribkoff J Pan et al ldquoPharmacological activationand inhibition of Slack (Slo2 2) channelsrdquoNeuropharmacologyvol 51 no 4 pp 896ndash906 2006
[57] M D Tejada K Stolpe A K Meinild and D A KlaerkeldquoClofilium inhibits Slick and Slack potassium channelsrdquo Biolog-ics vol 6 pp 465ndash470 2012
[58] B Yang R Desai and L K Kaczmarek ldquoSlack and slick KNachannels regulate the accuracy of timing of auditory neuronsrdquoThe Journal of Neuroscience vol 27 no 10 pp 2617ndash2627 2007
[59] B Biton S Sethuramanujam K E Picchione et al ldquoTheantipsychotic drug loxapine is an opener of the sodium-activated potassium channel slack (Slo2 2)rdquo The Journal ofPharmacology and ExperimentalTherapeutics vol 340 pp 706ndash715 2012
[60] C M Santi G Ferreira B Yang et al ldquoOpposite regulation ofSlick and Slack K+ channels by neuromodulatorsrdquo The Journalof Neuroscience vol 26 no 19 pp 5059ndash5068 2006
[61] M R Fleming and L K Kaczmarek ldquoUse of optical biosensorsto detect modulation of Slack potassium channels by G protein-coupled receptorsrdquo Journal of Receptors and Signal Transduc-tion vol 29 no 3-4 pp 173ndash181 2009
[62] M D Tejada L J Jensen and D A Klaerke ldquoPIP(2) mod-ulation of Slick and Slack K(+) channelsrdquo Biochemical andBiophysical Research Communications vol 424 pp 208ndash2132012
[63] L Zhang M Sukhareva J L Barker et al ldquoDirect bindingof estradiol enhances Slack (sequence like a calcium-activatedpotassium channel) channelsrsquo activityrdquo Neuroscience vol 131no 2 pp 275ndash282 2005
[64] V A Ruffin X Q Gu D Zhou et al ldquoThe sodium-activatedpotassium channel Slack is modulated by hypercapnia andacidosisrdquo Neuroscience vol 151 no 2 pp 410ndash418 2008
[65] M O Nuwer K E Picchione and A Bhattacharjee ldquocAMP-dependent kinase does not modulate the Slack sodium-activated potassium channelrdquo Neuropharmacology vol 57 no3 pp 219ndash226 2009
[66] B C Suh and B Hille ldquoRegulation of ion channels by phos-phatidylinositol 45-bisphosphaterdquo Current Opinion in Neuro-biology vol 15 no 3 pp 370ndash378 2005
[67] L K Kaczmarek ldquoNon-conducting functions of voltage-gatedion channelsrdquo Nature Reviews Neuroscience vol 7 no 10 pp761ndash771 2006
[68] C R Rose ldquoNa+ signals at central synapsesrdquoNeuroscientist vol8 no 6 pp 532ndash539 2002
[69] C R Rose and B R Ransom ldquoRegulation of intracellularsodium in cultured rat hippocampal neuronesrdquo The Journal ofPhysiology vol 499 part 3 pp 573ndash587 1997
[70] C R Rose and A Konnerth ldquoNmda receptor-mediated Na+signals in spines and dendritesrdquoThe Journal of Neuroscience vol21 no 12 pp 4207ndash4214 2001
[71] N Zhong V Beaumont and R S Zucker ldquoRoles for mito-chondrial and reverse mode Na+Ca2+ exchange and the plas-malemma Ca2+ ATPase in post-tetanic potentiation at crayfishneuromuscular junctionsrdquo The Journal of Neuroscience vol 21no 24 pp 9598ndash9607 2001
[72] G Budelli T A Hage AWei et al ldquoNa+-activated K+ channelsexpress a large delayed outward current in neurons duringnormal physiologyrdquoNature Neuroscience vol 12 no 6 pp 745ndash750 2009
14 ISRN Neuroscience
[73] T A Hage and L Salkoff ldquoSodium-activated potassium chan-nels are functionally coupled to persistent sodiumcurrentsrdquoTheJournal of Neuroscience vol 32 pp 2714ndash2721 2012
[74] M RMarkham L K Kaczmarek andHH Zakon ldquoA sodium-activated potassium channe supports high frequency firing andreduces energetic costs during rapid modulations of actionpotential amplituderdquo Journal of Neurophysiology vol 109 no 7pp 1713ndash1723 2013
[75] S Uchino H Wada S Honda et al ldquoSlo2 sodium-activated K+channels bind to the PDZ domain of PSD-95rdquo Biochemical andBiophysical Research Communications vol 310 no 4 pp 1140ndash1147 2003
[76] T Zamalloa C P Bailey and J Pineda ldquoGlutamate-inducedpost-activation inhibition of locus coeruleus neurons is medi-ated by AMPAkainate receptors and sodium-dependent potas-sium currentsrdquo British Journal of Pharmacology vol 156 no 4pp 649ndash661 2009
[77] S Lu P Das D A Fadool and L K Kaczmarek ldquoThe slacksodium-activated potassium channel provides a major outwardcurrent in olfactory neurons of Kv13-- super-smeller micerdquoJournal of Neurophysiology vol 103 no 6 pp 3311ndash3319 2010
[78] R C Foehring P C Schwindt andW E Crill ldquoNorepinephrineselectively reduces slowCa2+- andNa+-mediated K+ currents incat neocortical neuronsrdquo Journal of Neurophysiology vol 61 no2 pp 245ndash256 1989
[79] M Kubota and N Saito ldquoSodium- and calcium-dependentconductances of neurones in the zebra finch hyperstriatumventrale pars caudale in vitrordquo The Journal of Physiology vol440 pp 131ndash142 1991
[80] U Kim and D A Mccormick ldquoFunctional and ionic propertiesof a slow afterhyperpolarization in ferret perigeniculate neuronsin vitrordquo Journal of Neurophysiology vol 80 no 3 pp 1222ndash1235 1998
[81] V M Sandler E Puil and DW F Schwarz ldquoIntrinsic responseproperties of bursting neurons in the nucleus principalistrigemini of the gerbilrdquoNeuroscience vol 83 no 3 pp 891ndash9041998
[82] M V Sanchez-Vives L G Nowak and D A McCormick ldquoCel-lular mechanisms of long-lasting adaptation in visual corticalneurons in vitrordquoThe Journal of Neuroscience vol 20 no 11 pp4286ndash4299 2000
[83] S Franceschetti T Lavazza G Curia et al ldquoNa+-activatedK+ current contributes to postexcitatory hyperpolarization inneocortical intrinsically bursting neuronsrdquo Journal of Neuro-physiology vol 89 no 4 pp 2101ndash2111 2003
[84] V F Descalzo L G Nowak J C Brumberg D A McCormickand M V Sanchez-Vives ldquoSlow adaptation in fast-spikingneurons of visual cortexrdquo Journal of Neurophysiology vol 93no 2 pp 1111ndash1118 2005
[85] J Benda L Maler and A Longtin ldquoLinear versus nonlinearsignal transmission in neuron models with adaptation currentsor dynamic thresholdsrdquo Journal of Neurophysiology vol 104 no5 pp 2806ndash2820 2010
[86] X Liu and L S Leung ldquoSodium-activated potassium conduc-tance participates in the depolarizing afterpotential following asingle action potential in rat hippocampal CA1 pyramidal cellsrdquoBrain Research vol 1023 no 2 pp 185ndash192 2004
[87] L Cangiano P Wallen and S Grillner ldquoRole of apamin-sensitive KCa channels for reticulospinal synaptic transmissionto motoneuron and for the afterhyperpolarizationrdquo Journal ofNeurophysiology vol 88 no 1 pp 289ndash299 2002
[88] Q Y Liu A E Schaffner and J L Barker ldquoKainate induces anintracellular Na+-activated K+ current in cultured embryonicrat hippocampal neuronesrdquo The Journal of Physiology vol 510part 3 pp 721ndash734 1998
[89] I Rishal T Keren-Raifman D Yakubovich et al ldquoNa+ pro-motes the dissociation between G120572GDP and G120573120574 activating Gprotein-gated K+ channelsrdquoThe Journal of Biological Chemistryvol 278 no 6 pp 3840ndash3845 2003
Submit your manuscripts athttpwwwhindawicom
Neurology Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentSchizophrenia
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neural Plasticity
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Parkinsonrsquos Disease
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentAutism
Sleep DisordersHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Epilepsy Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Cardiovascular Psychiatry and NeurologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
4 ISRN Neuroscience
channels and most other potassium channels [4] The Slickchannel has also been termed the Slo21 channel and thehuman Slick gene is KCNT2 Overall the sequence of Slickis sim74 identical to that of Slack and the transmembranedomains and the RCK domains of Slick and Slack are almostidentical The greatest divergence between Slack and Slickoccurs at their distal C-terminal regions
The predicted cytoplasmic N-terminus of Slick is similarto that of the Slack-A isoform Consistent with this fact thecharacteristics of Slick currents recorded inmammalian cellsor Xenopus oocytes resemble those of Slack-A Althoughthe unitary conductance of Slick channels (sim140 pS) isslightly smaller than that of Slack channels Slick channelsactivate very rapidly with depolarization and have multi-ple subconductance states [4] The estimated half-maximalconcentration for activation of Slick by sodium 89mMis greater than that for Slack channels (Figures 2(c) and2(d)) Moreover in contrast to Slack channels which havean absolute requirement for Na+ for channel opening Slickchannels have a basal level of activity in the absence of sodium[4]
In addition to their sodium sensitivity both Slick andSlack channels can be activated in increases in intracellularchloride levels [4 47] Under physiological conditions how-ever Slick channels are markedly more sensitive to this anion[4] The domains that confer sensitivity of these channels tochloride are not known and mutations within a region thatresembles the ldquocalcium bowlrdquo of Slo1 channels and whichhas been established to contribute to the calcium sensitivityof Slo1 channels [48] have not been found to influence thesensitivity of Slack or Slick to either sodium or chloride ions[4]
The Slick channel also differs from Slack in having aconsensus ATP binding site just C-terminal to the secondRCK domain This site renders Slack channel activity to besensitive to cytoplasmic ATP levels The presence of 5mMATP reduces Slick whole cell currents or channel activityin excised patches [4] Further description of the effects ofATP on Slick channels will be provided in a later section onphysiological modulation of KNa channels
6 Heteromer Formation between Slack-Band Slick Subunits
Avariety of lines of evidence indicate that the Slack-B subunitand Slick can coassemble to form heteromeric channelsthat differ in their properties from those of either subunitexpressed alone [49] Moreover assembly of heteromericchannels appears to be specific for the Slack-B isoform anddoes not occur with Slack-A which as described abovediffers from Slack-B only in its N-terminal cytoplasmicdomain When Slick and Slack-B are coexpressed at a 1 1ratio either in Xenopus oocytes or in mammalian HEK293cells there is an 18ndash25-fold increase in current amplitudecompared to expression of either subunit alone [49] Thisincrease in current is associated with a comparable increasein levels of the channel subunits in plasma membrane asassayed by a surface biotinylation assay No differences in
total cellular levels of either protein are however foundwhenthe subunits are expressed singly or in combination with theother subunit
Themacroscopic currents of the Slack-BSlick heteromersdiffer from those of the homomers [49] For example thetime for the SlickSlack-B currents to reach 90 activationis significantly longer than that for either Slick or Slack-Bhomomeric currents In addition the unitary conductanceof the Slack-BSlick channels is intermediate between that ofSlack (sim180 pS) and Slick (sim140 pS) A direct demonstrationof this fact was obtained by making a pore mutant of theSlick channel Slick-EE (Q276E Y279E) that increases itsunitary conductance to sim500 pS Coexpression of Slick-EEwithwild-type Slack-B resulted in channels that had a unitaryconductance of sim325 pS a value that is clearly distinct fromthat for either channel alone providing definitive electro-physiological evidence for the formation of SlickSlack-Bheteromers
Coimmunoprecipitation experiments have also demon-strated that Slack-B forms a protein complex with Slick inboth rat brain tissue and heterologous expression systems[49] No coimmunoprecipitation could however be foundfor epitope-tagged Slick-A and Slick subunits This is con-sistent with electrophysiological findings demonstrating thatcoexpression of Slack-A with Slick does not increase whole-oocyte currents compared to expression of either subunitalone and that no intermediate conductance single-channelactivity can be observed when Slack-A is expressed withthe Slick-EE mutant Moreover coexpression of Slack-A withSlick does not increase levels of surface expression of eithersubunit
The specificity of SlackSlick interactions for the Slack-Bisoform is dependent on the extended N-terminal domainof Slack-B This has been demonstrated by constructing achimeric channel that replaced the cytoplasmic N-terminaldomain of Slick with that of Slack-B Coexpression of thismodified Slick channel with wild-type Slick channels inoocytes produced a 30-fold increase in whole-oocyte cur-rents which is similar to the increase obtained with Slack-Bsubunits [49] Thus the Slack-B N-terminal domain plays akey role in trafficking andor the level of plasma membraneexpression of heteromeric KNa channels
7 Localization of Slack and Slick Channels inthe Central Nervous System
Slack transcripts of about 45 kb and 75 kb are abundantlyexpressed in the brain and in the kidney [40] By the tech-nique of in situ hybridization in sections of adult rat brainSlack is very highly expressed in neurons but no staining isevident in glial cells Strong hybridization is found through-out the brain including the cerebral cortex hippocampusdeep cerebellar nuclei cerebellar Purkinje cells reticulartegmental nucleus of the pons preoptic nucleus substantianigra and auditory brainstem nuclei Neurons within thethalamus and hypothalamus were found to have a moremoderate level of staining [40] Subsequent quantificationof expression of mRNA levels by probes that are specific
ISRN Neuroscience 5
for either Slack-B or Slack-A isoforms found that bothisoforms are present throughout the brain but that thehighest levels of both isoforms are detected in brainstem andolfactory bulb [46]
Immunocytochemical staining using an antibody to theSlack-B specific N-terminus confirmed the localization ofSlack-B in neurons of the brainstem and olfactory bulb of rat[50] Prominent immunoreactivity was found in the olfactorybulb red nucleus and deep cerebellar nuclei as well asin vestibular and oculomotor nuclei and in the trigeminalsystem and reticular formation where intense staining occursin both cell bodies and axonal fibers As with the in situhybridization studies labeling was prominent in auditorybrainstem nuclei Although staining was also evident inneurons of the thalamus substantia nigra and amygdala theonly region of the cerebral cortex where the Slack-B isoformwas found was in the frontal cortex [50]
Immunolocalization studies have also been carried outusing an antibody targeted against a region in the cytoplasmicC-terminus of Slack [46] This would be expected to recog-nize themultiple Slack isoforms that differ at the N-terminusbut not to recognize Slick channels Immunostaining withthis ldquopan-Slackrdquo antibody found strong labeling of neuronsthroughout the nervous system of mice This included theregions previously identified with the Slack-B antibody aswell as other locations such as the dendrites of hippocampalneurons and olfactory bulb glomeruli [46]
Messenger RNA for the Slick subunit has awider distribu-tion throughout the body than does Slack mRNA and a Slicktranscript of sim69 kB is expressed in rat heart where no SlackmRNA has been found [4] Like Slack however the highestlevels of Slick mRNA are found in the brain Slick mRNA andimmunoreactivity overlap considerably with those for Slackand are found at high levels in olfactory bulb midbrain brainstem hippocampus throughout the cerebral cortex withhigh expression in primary somatosensory and visual regions[51] Like Slack high Slick immunoreactivity is found inneurons of sensory pathways including those in the auditorybrainstem The wide colocalization of Slick with the Slack-Bisoform suggests that the native KNa channels in these regionsare likely to be heteromers of these two subunits
Studies have also found Slack channels to be expressedin neurons of the peripheral nervous system Slack immuno-labeling is found in over 90 of neurons of the dorsal rootganglion and has been localized to both the somata andthe axonal tracts of small- medium- and large-diameterneurons [5 23] Localization to axonal tracts has also beendetected in electrophysiological experiments on peripheralaxons of Xenopus neurons for which a correlation has beenfound between the localization of KNa channels and voltage-dependent sodium channels [52]
8 Pharmacology of KNa Channels
A very common test for the presence of KNa channels inneurons and other cells has been to measure the effects ofreplacement of external sodium by lithium ions on the netoutward current Lithium is a much weaker activator of KNa
channels than sodium Because lithium readily enters cellsthough voltage-dependent sodium channels usually withminimal effects on the inward currents lithium replace-ment in voltage-clamp experiments reduces the net out-ward currents if KNa currents are present In current clampexperiments lithium substitution may alter the shape ofaction potentials or firing patterns in a manner consistentwith partial block of KNa currents The majority of theexperiments described later in the section on contributionsof KNa channels to neuronal firing patterns have tested theeffects of lithium substitution
Studies on native KNa channels in cardiac cells found thatseveral antiarrhythmic drugs inhibit these channels in cardiaccells [53ndash55] Some of these compounds such as quinidinebepridil and clofilium have been also found to be veryeffective and reversible blockers of Slack and Slick channelsexpressed in oocytes and mammalian cells [4 56 57] but arenon-specific in that they also act on a variety of other channeltypes
The pharmacological properties of Slack and Slick chan-nels are similar to each other but differ from those of manyother potassium channels including their closest molec-ular relative the Slo1 channel Slack and Slick are onlyweakly sensitive to the general potassium channel blockertetraethylammonium ions (TEA) [4] External barium ionsproduce a time- and voltage-dependent block of Slack andSlick channels In contrast both subunits are insensitiveto blockers of calcium-activated potassium channels suchas apamin iberiotoxin paxilline and charybdotoxin andto a wide range of blockers of other classes of potassiumchannels [4 57] Although Slick channels are sensitive tocytoplasmic ATP levels they are insensitive to glibenclamideand diazoxide an inhibitor and an activator respectively ofclassical KATP channels [4]
A variety of compounds that activate Slack and Slickchannels are also knownThefirst of these to be describedwasbithionol which in Slack-expressing HEK cells produces arobust increase in the amplitude of Slack currents with anEC50
of 077120583M [56] Similar bithionol-induced increasesin current are observed for Slack-expressing oocytes and fornative KNa currents in neurons of the auditory brainstem [5658] Bithionol reversibly activates Slack channels even whenapplied to the extracellular face of excised patches indicatingthat it acts on Slack channels relatively directly The increasein current occurs though a very marked bithionol-inducedchange in the voltage dependence of Slack channels such thatfull activation is already observed at potentials as negativeas minus40mV [56] Bithionol is however not selective forKNa channels in that it also activates Slo1 calcium-activatedpotassium channels [56 59]
A screen of pharmacologically active compounds usinga rubidium flux assay against Slack channels expressed inCHO cells has revealed other activators [59] These includeriluzole loxapine an antipsychotic agent and niclosamidean anthelmintic agent Loxapine was found to be moreselective than bithionol in that it has no effect of Slo1 calcium-activated potassium channels Electrophysiological experi-ments confirmed that loxapine is effective on recombinanthuman and rat Slack channels and that it activates native KNa
6 ISRN Neuroscience
channels in isolated neurons of the rat dorsal root ganglion[59]
9 Cellular Regulation of KNa Channels
The activity of Slack and Slick channels as well as ofheteromeric SlackSlick channels is potently regulated by avariety of cellular signaling pathways including G-protein-coupled receptors linked to activation of PKA or PKC [2328 30 49 60 61] direct phosphorylation of the channelssubunits [2 49 60] changes in cytoplasmic ATP levels[4] cytoplasmic NAD+ levels [5] PIP2 [62] estradiol [63]hypoxia [41 64] and the fragile X mental retardation proteinFMRP [6 32] This section will summarize these findings
91 Modulation of Slack and Slick by Protein Kinase CTreatment ofmammalian cells orXenopus oocytes expressingSlack channels with diacylglycerol or phorbol ester activatorsof protein kinase C (PKC) leads to a 2-3-fold increase incurrent amplitude and a slowing of the rate of activation[2 60] This effect of PKC has been found to result from thephosphorylation of a single serine residue S407 located in aPKC consensus phosphorylation site in the ldquohingerdquo region ofthe cytoplasmic C-terminus between the S6 transmembranedomain and the first RCK domain [2] Mutation of this serineto an alanine (S407A) which renders the site incapable ofundergoing phosphorylation results in apparently normalSlack currents that are entirely insensitive to PKC activationIn contrast mutation of the twelve other putative PKCphosphorylation sites in the Slack channel (see Figure 1)does not prevent the PKC-induced increase in Slack current[2]
In contrast to Slack channels the action of PKC onSlick channels is to produce a decrease in current amplitude[60] As in Slack this effect is likely to be mediated by amodification within the C-terminus In particular a chimericchannel that contains the N-terminus and membrane span-ning regions of Slack but the cytoplasmic C-terminus ofSlick responds to PKC activation with a decrease in currentMoreover direct application of a constitutively active formof PKC directly to the cytoplasmic face of excised patchescontaining these chimeric channels reduces their activity[60] Nevertheless the specific sites at which Slick subunitsare modified in response to PKC activation are not yetknown
Heteromeric Slack-BSlick channels respond to activationof PKC in a manner that is quite distinct from that of eithersubunit expressed alone [49] In Xenopus oocytes expressingboth subunits application of PKC activators potently sup-presses current bysim90This ismuch greater than the degreeof suppression measured for Slick subunits alone (sim50)and cannot therefore be explained by any linear combinationof Slick or Slack-B currents The finding that heteromericSlack-BSlick channels are regulated even more potently byPKC activation than are the homomeric channels constitutesone of the many pieces of evidence supporting selectiveheteromer formation between these subunits [49]
92 Modulation of Slack and Slick by G Protein CoupledReceptors Slack and Slick have been coexpressed in Xenopusoocytes with the M1 muscarinic receptor and the mGluR1metabotropic glutamate receptor [60]These are G120572q proteincoupled receptors that lead to the activation of PKC andformation of IP3 Accordingly activation of these receptorsleads to an increase in Slack currents and a suppression ofSlick currents consistent with modulation of these chan-nels by PKC There is widespread colocalization of thesereceptors with the KNa subunits throughout the nervoussystem suggesting that modulation of KNa currents by thesereceptors is likely to be widespread [60] Direct modulationof KNa channels by mGluR1 has been observed in nativeneurons within the lamprey spinal cord [28] In these cellsactivation of mGluR1 produces a strong suppression ofcurrent as expected for Slick or Slack-BSlick heteromersand this suppression is prevented by pharmacological inhibi-tion of PKC Interestingly chelation of intracellular calciumwith the chelator BAPTA (12-bis(2-aminophenoxy)ethane-NNN1015840N1015840-tetraacetic acid) prevented modulation of theKNa current by PKC but unmasked an mGluR1-dependentactivation of KNa current by an independent pathway whosemechanism is not yet known [28]
Modulation of Slack channels by G protein-coupledreceptors has been studied using a novel label-free technologyto detect changes in the distribution of mass close to theplasma membrane [61] The characteristic change in massthat normally follows activation of native receptors in HEK-293 cells is significantly modified in amplitude and timingby the coexpression of Slack channels further supporting thefinding of modulation of KNa current by these receptors [61]
93 Modulation of K119873119886
Currents by Cyclic AMP The activityof KNa channels in Kenyon cells isolated from the mushroombody of the cricket (Gryllus bimaculatus) is enhanced uponapplication of either the neurotransmitter octopamine or themembrane-permeable analog cyclic AMP analog 8-Br-cyclic-AMP [30] The increase in channel activity was found to beattenuated by the protein kinase A (PKA) inhibitor H-89Conversely KNa channel activity in these cells is decreasedby the transmitter dopamine or by the cyclic GMP analog8-Br-cyclic GMP and this decrease in open probability isantagonized by the protein kinaseG (PKG) inhibitor KT5823These findings have led to the proposal that modulation ofKNa currents play a role in olfactory learning circuits thatmediate reward and punishment signals in insects which areregulated by octopamine and dopamine respectively [30]
The Slack subunit is expressed at high levels in pri-mary afferent nociceptor neurons located in the dorsal rootganglion [5 23] and KNa currents can readily be recordedin these cells [5 22 23] Inflammatory substances suchas prostaglandin E2 that activate the PKA pathway greatlyincrease the excitability of these neurons sensitizing them tothermal and mechanical stimulation In their resting statethe neurons adapt rapidly to maintained depolarizationstypically generating only one or two action potentials at theonset of depolarization After an elevation of cyclic AMPlevels that activates PKA adaptation is greatly reduced such
ISRN Neuroscience 7
that sustained firing can be recorded throughout a prolongeddepolarization In major part the effects of PKA can beattributed to a decrease in Slack KNa current and suppressionof Slack expression using anti-Slack siRNA produces a reduc-tion in adaptation similar to that produced by PKA activation[23]
In contrast to the effects of PKC on Slack channels thatwere described above the effects of PKA on Slack are indirectand do not involve phosphorylation of the channel subunititself [23 65] Slack channels in HEK-293 cells are insensitiveto cyclic AMP analogs or to the PKA inhibitor KT5720 andapplication of the catalytic subunit of PKA to the cytoplasmicface of Slack channels excised from transfected cells or dorsalroot ganglion neurons has no effect on channel openingInstead activation of PKA decreases Slack current amplitudeby rapidly internalizing Slack channels from the plasmamembrane into internal organelles [23] Such internalizationwas detected by confocal microscopy of neurons expressingSlack channels tagged at their N-terminus with the greenfluorescent protein GFP Elevations of cyclic AMP reducedcolocalization of the labeled channels with a cell surfacemarker A reduction in native Slack channels at the plasmamembrane following PKA activation was also confirmedusing a surface biotinylation assay [23]
94 Modulation of Slack and Slick by PIP2 A variety ofion channels have been shown to be regulated by phos-phatidylinositol 45-biphosphate (PIP2) a phospholipid thatis localized to the inner lipid leaflet of the plasma membrane[66] Exogenously applied PI(45)P2 as well as its isoformPI(34)P2 increases the amplitude of Slack and Slick currentsexpressed in Xenopus oocytes and pharmacological agentsthat are expected to reduce endogenous PIP2 levels reducecurrent amplitude [62] Regulation by PIP2 was found torequire specific lysine residues in the C-termini of thechannelsMutation of these lysines to alanine (K339A in Slackand K306A in Slick) produced functional KNa currents thatwere insensitive to the activating effects of PIP2 [62]
95 Suppression of Slick Channels by Cytoplasmic ATP TheSlick subunit differs from Slack in that it contains a consensusATP binding site just C-terminal to the second RCK domain[4] The current density of Slick currents but not Slackcurrents is reduced by sim80 by the presence of 5mM ATPat the cytoplasmic face of the channels The same full effectis observed with ATP120574S a slowly hydrolyzable ATP analogand the nonhydrolyzable analog AMP-PNP but not withADP Suppression by ATP of Slick currents at all membranevoltages can also be readily detected in excised patches [4]
Confirmation that the effect of ATP on Slick channelsis mediated by the consensus ATP binding site in the C-terminal domain was obtained by site-directed mutagenesisto replace the glycine at residue 1032 in this site with a serine[4]Themutant G1032S Slick channel was fully functional butfailed to decrease channel activity in response to ATP Thesensitivity of Slick channels to ATP levels suggests that theiractivity could be enhanced during periods of high metabolicdemand when cellular ATP levels fall
96 Modulation of Slack by NAD+ KNa channels can read-ily be detected in cell-attached patch-clamp recordings onneurons [5 16 58] This may be surprising given that thecytoplasmic sodium concentration in resting cells is relativelylow (ltsim10mM see Section 10 below) and as described earlierthe EC
50for activation of these channels in excised patches
is relatively high (typically sim40ndash80mM) This discrepancy isresolved by the finding that the open probability and sodiumsensitivity of KNa channels are greatly reduced following exci-sion ofmembrane patches [5 16] suggesting that rundown ofKNa activity in excised patches may result from loss of somecytoplasmic factor
A major cytoplasmic factor that regulates both Slack andSlick channels has been found to be nicotinamide adeninedinucleotideNAD+ [5] Both subunits have a putativeNAD+-binding site within their second RCK domain Application ofNAD+ to the cytoplasmic face of KNa channels excised fromdorsal root ganglion neurons increases their open probabilityand reduces themeasured EC
50for activation of sodium from
sim50mM to sim17mM (Figure 3) NAD+ as well as NADP+but not NADH also increased the open probability of Slackchannels excised from a transfected mammalian cell lineMutation of glycine in the putative NAD+-binding site ofSlack (G792A) resulted in a loss of the ability of NAD+to potentiate activity [5] These findings suggest that KNachannels play a much more prominent role in the normalphysiology of excitable cells than would be suggested by theirproperties in isolated patches
97 Modulation of Slack by Estradiol The open probability ofchannels reconstituted from lipid bilayers from extracts of ratcortex has been found to be increased by 17120573-estradiol [63]Although these channels were not explicitly shown to be KNachannels a similar effect was reported for whole-cell currentsrecorded in Slack-expressing HEK-293 cells Activation wasnot prevented by tamoxifen a blocker of classical estrogenreceptors but may be a consequence of direct binding of 17120573-estradiol to Slack channels themselves [63]
98 Modulation of Slack Channels by Hypoxia Duringhypoxic conditions cytoplasmic levels of internal sodiumand chloride ions as well as those of H+ and CO
2 rise It
has been suggested that activation of KNa channels couldact to hyperpolarize cells during hypoxia providing a pro-tective mechanism [7] This conjecture is supported by thefinding that nematodes lacking the Slack ortholog in thisspecies are significantly more sensitive to hypoxic deaththan are wild-type animals [41] Further support for a rolefor Slack channels in the response to hypoxia comes fromrecordings of Slack channel activity in Xenopus oocytesexposed to increasing concentrations ofH+ andCO
2 demon-
strating that elevations of sodium activate Slack channelsunder cellular conditions comparable to those expected inischemiahypoxia [64]
99 Interactions of Slack with the Fragile X Mental Retarda-tion Protein FMRP Fragile X syndrome the most commoninherited form of intellectual disability in humans is caused
8 ISRN Neuroscience
1
08
06
04
02
0
[Na] (mM)10010
Control (small neurons)NAD
Nor
mal
ized
NP o
Figure 3 NAD+ increases the sodium sensitivity of KNa channelsexcised from adult rat dorsal root ganglion neurons Concentration-response relationship provides estimates of EC
50for sodium activa-
tion of 50mM and 17mM in the absence and presence of NAD+respectively Figure modified from [5]
by loss of expression of the RNA-binding protein FMRP(fragile X mental retardation protein) FMPR is known to berequired for normal activity-dependent protein translationin neurons Yeast two-hybrid and coimmunoprecipitationexperiments have shown that the cytoplasmic C-terminus ofthe Slack subunit interacts with this protein [6] MoreoverSlack-FMRP complexes also contain messenger RNAs thatare targets of FMRP such as the mRNAs encoding Map1band Arc No association of mRNAs with Slack can howeverbe found in mice lacking FMRP indicating that these arebound to Slack despite its interaction with FMRP A peptidearray assay revealed confirmed that FMRP binds selectivelyto sequences at the distal C-terminus of Slack [6]
FMRP is a potent activator of Slack channel activity [6]Application of recombinant FMRP(1ndash298) which containsthemajority of protein-protein interaction domains of FMRPto the cytoplasmic face of Slack channels in excised patchesreversibly increases channel opening by two- to threefold(Figure 4(a)) This is associated with the almost completeelimination of openings to subconductance states which areprominent in untreated patches (Figure 4(b)) FMRP wasfound to have no effect on the channel activity of a Slacktruncation mutant (Slack-BΔ804) which lacks sites found tointeract with FMRP in biochemical assays [6]
Support for a role for FMRP in regulating the amplitude ofnative KNa currents has also come from recordings of neuronsfrom animals lacking the fmr1 gene which encodes FMPR[6] As will be described later KNa channels account for amajor component of potassium current in principal neuronsof the medial nucleus of the trapezoid body (MNTB) Whileloss of FMRP does not affect total levels of Slack proteinthe level of the KNa component of current in these cells issubstantially reduced in the FMRP knockout mice compared
to that in wild-type animals consistent with an activating rolefor this RNA-binding protein [6]
The interaction between Slack and FMRP is likely tobe evolutionarily conserved Both proteins have been foundto be coexpressed in bag cell (BC) neurons which regulatereproductive behaviors in mollusk Aplysia [32] FMRP andSlack immunoreactivity are colocalized at the periphery ofisolated BC neurons and the two proteins can be reciprocallycoimmunoprecipitatedThe native Slack current in these cellswas identified using an siRNA approach and intracellularinjection of FMRP(1ndash298) induced a KNa current that in itsproperties matches the Slack current As for Slack channelsin heterologous expression systems addition of FMRP(1ndash298) to the inside-out patches containing native AplysiaKNa channels increased channel opening In current clamprecordings FMRP(1ndash298) produced a narrowing of actionpotentials
Downregulation of Slack expression in BC neurons usingan siRNA approach had an interesting effect on long-termchanges in the excitability of these cells [32] In untreatedcells brief stimulation of these neurons produces a charac-teristic prolonged (sim30min) discharge that is followed bya prolonged (sim18 hr) inhibitory state Recovery from thisinhibitory state requires new protein synthesis and is blockedby the protein synthesis inhibitor anisomycin Anti-SlacksiRNA did not alter the ability of BC neurons to undergothe normal discharge but like inhibition of protein synthesisprevented recovery from the inhibitory state [32] Althoughother interpretations are possible these findings raise thepossibility that the activity of Slack channels is linked to theregulation of protein translation through the FMRP-RNApathway
As will be described later point mutations in humanSlack channels can produce devastating effects on intellectualdevelopment [2 3] A variety of experiments have demon-strated that activation of ion channels can influence cytoplas-mic processes independently of ion flux through the channels[67] The interaction between the C-terminus of Slack andFMRP raises the hypothesis that conformational changesin the C-terminus could represent such a ldquononconductingrdquofunction of Slack that links neuronal firing to changes inprotein translation
10 Sodium Entry Pathways ThatActivate KNa Channels
Estimates of cytoplasmic sodium concentrations in restingunstimulated neurons obtained using sodium-sensitivemicroelectrodes and ratiometric dyes provide valuesbetween sim4mM and 15mM [68] As noted earlier in thesection on NAD+ regulation of Slack channels this maybe sufficient to provide significant activation of native KNachannels in intact neurons even under resting conditions[5] Internal sodium levels rise in response to stimulationprincipally as a result of entry through voltage-dependentsodium channels and ionotropic ligand-gated receptors suchas AMPA and NMDA glutamate receptors [69] Stimulationparticularly repetitive tetanic stimulation produces local
ISRN Neuroscience 9
Control RecoveryC
1
2
3
C
1
2
3
10pA
2 s
10pA
200ms
S S S S S S S
FMRP(1ndash298)
(a)
2520151050
10000
1000
100
pA
C1
2
3
Recovery
2520151050
10000
1000
100
pA
C1
2
3
S
S
S(N
)
(N)
Control
2520151050
10000
1000
100
pA
C
1 2 3
S
S
S
(N)
FMRP(1ndash298)
(b)
Figure 4 FMRP alters gating of Slack-B channels in inside-out patches (a) Recording of a patch containing 3-4 Slack channels before duringand after transient application of FMRP(1ndash298) to the cytoplasmic face of the patch Top traces show representative 10 s examples of recordingatminus40mVand bottom traces show expanded time views of the same patch Arrowsmarked S indicate the occurrence of subconductance statesthat are suppressed by FMRP(1ndash298) (b) All-points amplitude histograms of Slack channel activity plotted on a logarithmic scale before andafter application of FMRP(1ndash298) and after washout C marks the closed state S indicates subconductance states and numbers on peaks inthe histograms refer to the number of fully open Slack channels Modified from [6]
increases in sodium concentrations that can rise to levelsbetween 45 and 100mM [70 71] In particular high levelscan be attained in restricted compartments such as activedendritic spines
Although the upstroke of action potentials in mostneurons is driven by rapidly-inactivating voltage-dependentsodium currents these currents are transient Evidencesuggest that the much smaller persistent component ofsodium current that does not inactivate with maintaineddepolarization is responsible both for sustained elevationsof internal sodium during stimulation [69] and for selectiveactivation of KNa channels [72 73] It has been found thatin patches excised from somata of rat neurons sodium entrythrough persistent tetrodotoxin-sensitive sodium channels isthe dominant factor that leads to activation of KNa channelsin the same patch [73] Activation of KNa channels by sodiumentry in the isolated patches was enhanced by veratridinewhich prolongs sodium channel openings and was observedin the absence of sodium at the cytoplasmic face of thepatches Moreover activation appeared to be independent ofthe amplitude of the much larger transient sodium current
[73] Although the molecular basis for persistent sodiumchannels is not fully understood further support for a selec-tive activation of KNa currents by entry through persistentchannels has also been found in electrocytes of electric fish inwhich the time course of KNa currents during depolarizationsreflects that of the persistent sodium current and not that ofthe very much larger transient sodium current [74]
A number of studies have documented that KNa currentsare also activated by sodium entry through ionotropic gluta-mate receptors particularly AMPA receptors Slack subunitscan be coimmunoprecipitated with GluR23 AMPA receptorsusing rat brain synaptosomal fractions or lysates of the lam-prey central nervous system [27] Moreover Slack subunitshave been found to bind directly to the PDZ domain of PSD-95 (postsynaptic density-95 protein) a major componentof the postsynaptic density of glutamatergic synapses andto colocalize with PSD-95 in cultures of mouse corticalneurons [75] In electrophysiological studies of lampreyneurons application of AMPA was been found to activate aKNa current with pharmacological properties of Slack [25]Activation of this KNa current regulates both the amplitude
10 ISRN Neuroscience
and the decay rate of AMPA-induced synaptic currents pro-viding a feedback loop that decreases the size of glutamatergicpostsynaptic potentials [27]
Activation of KNa currents following activation of AMPAreceptors has also been proposed in studies of neurons of thelocus coeruleus to which application of glutamate producesa burst of action potentials followed by period of inhibitionlasting 30ndash45 seconds [76] The potassium current responsi-ble for the period of inhibition was selectively activated byAMPA or the AMPA-receptor agonist quisqualate but not byother stimuli that excite these neurons Consistent with thiscurrent being a KNa current it was attenuated by lowering ofexternal sodium concentrations but not affected by externalcalcium levels and it was potentiated by the KNa channelopener bithionol [76]
While persistent voltage-dependent sodium channels andAMPA receptors may represent the primary sodium entrypathways for activation of KNa channels there exist otherpathways that allow sodium ions to enter excitable cellsand whose role has been less explored Slack subunits areabundantly expressed in mitral cells of the olfactory bulb[46 50] In these neurons deletion of the gene for a majorvoltage-dependent potassiumchannelKv13 produces a com-pensatory increase in levels of Slack-B protein and in SlackKNa currents identified using an anti-Slack siRNA technique[77] Activation of the KNa current during depolarizationof these cells could only be significantly suppressed by acombination of tetrodotoxin to block voltage-dependentsodium channels and ZD7288 a blocker of H-channelswhich are nonselective cation channels that flux both sodiumand potassium ions Either drug alone failed to significantlysuppress KNa currents suggesting that sodium entry throughH-channels plays a role in KNa channel activation in theseneurons [77]
11 Contribution of KNa Channels toNeuronal Firing Patterns
The component of whole-cell potassium current in neuronsthat can be attributed to Slack channel subunits has beenidentified in mitral cells of the olfactory bulb and mediumspiny neuron of the striatum using siRNAs to suppress trans-lation of SlackmRNA [72 77] Such experiments indicate thatSlack KNa channels are responsible for a very major compo-nent of the total delayed outward current in neurons underphysiological conditions [72] The specific contributions thatKNa channels make to the electrical personality of individualtypes of neurons may differ in different cell types
One of the earliest effects to be described for activationof KNa currents is to produce a slow afterhyperpolarization(sAHP) following a burst of action potentials This was firstdescribed in pyramidal neurons of the cat sensorimotorcortex [11 78] and similar slow sodium-dependent sAHPsthat endure for many seconds have been found in a widevariety of neuronal types [79ndash84]These have been the subjectof earlier reviews [20 21] Such slow potentials have beenfound to influence the timing of regular bursts of actionpotentials such as the 3ndash12 per minute spindle waves that
occur at the onset of sleep in certain neurons within thethalamus [80] Slow activation of KNa during repetitive firingalso produces an adaptation of firing rate during bursts ofaction potentials [23] and such adaptation is predicted bynumerical simulations that incorporate KNa currents [46 85]
In some neurons such as neocortical pyramidal neuronsand hippocampal CA1 neurons a significant amount of KNacurrent can also be activated by single spikes and contributeto action potentials repolarization or to the amplitude ofdepolarizing afterpotentials that follow single spikes andare generated by persistent sodium currents [83 86] Inthese cases KNa current activation may be detected withinsim5msec following a single-action potential A recent studyhas examined themechanismbywhich the cells of the electricorgan (electrocytes) of certain electric fish are able to generatesustained firing at frequencies greater than 500Hz [74] Slackchannels are expressed at high levels in these cells and theirdelayed potassium current is almost exclusively a KNa currentthat is activated by a persistent sodium current Numericalsimulations have indicated that the use of KNa current torepolarize the very large action potentials of these cells allowsthe cells to fire at such high rates with minimal energyexpenditure [74]
A set of neurons that have been particularly instructivein studies of the role of KNa channels in shaping neuronalfiring are those that control locomotion in the spinal cord oflamprey [24ndash28 87] In addition to the transient KNa currentthat is activated by AMPA receptors and that was describedearlier [25 27] these neurons appear to have two distinctcomponents ofKNa current that are triggered by action poten-tial firingmdasha transient component that follows a single spikeand a slower more sustained component [24 26 87] Therapid transient KNa current is activated by the sodium influxthrough tetrodotoxin-sensitive channel during the actionpotentials and may play a role in determining the amplitudeand duration of individual action potentials The sustainedKNa current is not abolished by tetrodotoxin suggesting thatit is selectively activated by an independent sodium entrypathway and may underlie the calcium-independent sAHPthat occurs during repetitive bursting in lamprey spinal cordneurons [24]
Experiments with neurons that fire at high rates withvery high temporal accuracy have provided another potentialbiological role for KNa channels [58] Slack and Slick areexpressed at high levels in neurons of the medial nucleusof the trapezoid (MNTB) within the auditory brainstem[6 50 51] These neurons fire at rates up to sim800Hzand accurate timing of their action potentials is requiredfor transmission of information about the localization ofsound stimuli KNa channels are readily be detected in bothcell-attached and excised patches on the somata of MNTBneurons and they are localized appropriately for activationby sodium influx through AMPA receptors or voltage-dependent sodium channels [58] Current-clamp recordingsfrom MNTB neurons and numerical simulations indicatethat as KNa channels become activated temporal accuracyof firing is improved This occurs because the voltage-dependence of Slack and Slick channels allows them to opennear the resting potential of MNTB neurons increasing
ISRN Neuroscience 11
the resting conductance and decreasing the membrane timeconstant An increase in temporal fidelity of MNTB neuronscan also be produced by treatment with the pharmacologicalKNa activator bithionol [58]
12 Mutations in Slack Channels Result inEpilepsy and Intellectual Disabilities
De novo human mutations that produce single-amino-acidalterations in Slack channels produce devastating effectson development and intellectual function [2 3] One setof Slack mutations results in malignant migrating partialseizures of infancy (MMPSI) [2]This disease is characterizedby pharmacoresistant seizures in the first 6 months of lifethat arise randomly and ldquomigraterdquo from one brain regionto another While the occurrence of seizures abates withage MMPSI results in near-total arrest of psychomotordevelopment Exome sequencing revealed that at least 50of MMPSI patients have mutations in the large cytoplasmicC-terminal domain of Slack [2]
Two mutations in the human Slack gene that produceMMPSI are R428Q and A934T which correspond to themutations R409Q and A913T in the highly conserved ratSlack sequence [2] Expression of these mutants in Xenopusoocytes produced Slack currents with voltage dependenceand kinetic behavior that matched that of wild-type Slackchannels but with a current amplitude that was increased 2-3-fold over that of wild-type Slack This increase in currentlikely results because the mutations produce channels thatare either more prone to undergo phosphorylation by PKC(or less likely to be dephosphorylated) or that mimic theconformation induced by PKC activation One of theMMPSImutations R409Q lies adjacent to the phosphorylation siteS407 that as described earlier enhances current by 2-3-fold when phosphorylated by PKC Consistent with thisinterpretation current amplitude of the two MMPSI muta-tions could not be further increased upon PKC activa-tion [2] In other respects the MMPSI currents resembledthat of wild-type channels with unitary conductance andsodiumdependencesmatching those of wild-type Slack BothMMPSI mutant channels however had reduced openings tosubconductance states compared to the wild-type channels[2]
A second disease that has been attributed to a differentset of mutations in the C-terminal domain of Slack chan-nels is autosomal dominant nocturnal frontal lobe epilepsy(ADNFLE) [3] This is a childhood-onset focal epilepsysyndrome in which motor seizures arise during sleep Onsetof ADNFLE in the patients with Slack mutations occurred ata mean age of 6 years and was associated with mild to severeintellectual disability andor severe psychiatric problemsTheeffects of these ADNFLE mutations on the function of Slackchannels have however not been investigated Interestinglyanother known genetic cause of ADNFLE is mutations insubunits of the neuronal nicotinic acetycholine receptor Theacetycholine receptor ADNFLE mutations do not howeverproduce intellectual disability which contrasts strikingly tothe effects of the Slack mutations [3]
It is possible that the very severe outcomes of mutationsin Slack occur solely because of changes in the amplitudeof Slack currents An increase in total potassium currentin rapidly firing interneurons could reduce the firing rateof these cells predisposing the nervous system to hyper-excitability The relatively modest increase in current thatoccurs with theMMPSImutations [2] however suggests thatother processes may also be affected As described earlierthe large C-terminal cytoplasmic domain of Slack in whichMMPSI and ADNFLE mutations are located interacts withFMRP and its cargo mRNAs [6] Thus it is possible thatin addition to directly altering the excitability of centralneuronsmutations in Slack disrupt the regulation of activity-dependent protein translation in neurons
13 Conclusions
This review has covered themolecular and cellular propertiesof Slack and Slick two potassium channel subunits thatprovide neurons and other cells with KNa channels It shouldbe pointed out that additional potassium channels that areactivated by cytoplasmic sodiummay also exist For examplesome of the native KNa currents that have been described inneurons do not match Slack or Slick in their single-channelconductances or pharmacological properties [88]These mayperhaps represent inward rectifier potassium channels thatare activated by sodium [45 89] and the physiological rolesof the sodium sensitivity of these channels require furtherinvestigation Nevertheless Slack and Slick which are verywidely expressed in the brain appear to play dominant rolesin shaping the firing properties of verymany types of neuronsHumanmutations in these channels produce profound effectson neuronal function and development suggesting that per-haps their biological role may extend beyond simply settingthe level of neuronal excitability and that these channels mayinfluence cytoplasmic biochemical pathways that regulatedevelopment plasticity and intellectual function
References
[1] B Hille Ionic Channels of Excitable Membranes Sinauer Asso-ciates Sunderland Mass USA 3rd edition 2001
[2] G Barcia M R Fleming A Deligniere et al ldquoDe novo gain-of-function KCNT1 channel mutations cause malignant migratingpartial seizures of infancyrdquo Nature Genetics vol 44 pp 1255ndash1259 2012
[3] S E Heron K R SmithM Bahlo et al ldquoMissensemutations inthe sodium-gated potassium channel gene KCNT1 cause severeautosomal dominant nocturnal frontal lobe epilepsyrdquo NatureGenetics vol 44 pp 1188ndash1190 2012
[4] A Bhattacharjee W J Joiner M Wu Y Yang F J Sigworthand L K Kaczmarek ldquoSlick (Slo2 1) a rapidly-gating sodium-activated potassium channel inhibited by ATPrdquo The Journal ofNeuroscience vol 23 no 37 pp 11681ndash11691 2003
[5] T J Tamsett K E Picchione and A Bhattacharjee ldquoNAD+activates KNa channels in dorsal root ganglion neuronsrdquo TheJournal of Neuroscience vol 29 no 16 pp 5127ndash5134 2009
12 ISRN Neuroscience
[6] M R Brown J Kronengold V R Gazula et al ldquoFragileX mental retardation protein controls gating of the sodium-activated potassium channel SlackrdquoNatureNeuroscience vol 13no 7 pp 819ndash821 2010
[7] M Kameyama M Kakei and R Sato ldquoIntracellular Na+activates a K+ channel in mammalian cardiac cellsrdquoNature vol309 no 5966 pp 354ndash356 1984
[8] C R Bader L Bernheim and D Bertrand ldquoSodium-activatedpotassium current in cultured avian neuronesrdquoNature vol 317no 6037 pp 540ndash542 1985
[9] K Hartung ldquoPotentiation of a transient outward current byNa+influx in crayfish neuronesrdquo Pflugers Archiv vol 404 no 1 pp41ndash44 1985
[10] S E Dryer J T Fujii and A R Martin ldquoA Na+-activatedK+ current in cultured brain stem neurones from chicksrdquo TheJournal of Physiology vol 410 pp 283ndash296 1989
[11] P C Schwindt W J Spain and W E Crill ldquoLong-lastingreduction of excitability by a sodium-dependent potassiumcurrent in cat neocortical neuronsrdquo Journal of Neurophysiologyvol 61 no 2 pp 233ndash244 1989
[12] C Haimann L Bernheim D Bertrand and C R BaderldquoPotassium current activated by intracellular sodium in quailtrigeminal ganglion neuronsrdquo Journal of General Physiology vol95 no 5 pp 961ndash979 1990
[13] S E Dryer ldquoNa+-activated K+ channels and voltage-evokedionic currents in brain stem and parasympathetic neurones ofthe chickrdquoThe Journal of Physiology vol 435 pp 513ndash532 1991
[14] M Saito and C F Wu ldquoExpression of ion channels andmutational effects in giant Drosophila neurons differentiatedfrom cell division-arrested embryonic neuroblastsrdquoThe Journalof Neuroscience vol 11 no 7 pp 2135ndash2150 1991
[15] T M Egan D Dagan J Kupper and I B Levitan ldquoPropertiesand rundown of sodium-activated potassium channels in ratolfactory bulb neuronsrdquoThe Journal of Neuroscience vol 12 no5 pp 1964ndash1976 1992
[16] C Haimann J Magistretti and B Pozzi ldquoSodium-activatedpotassium current in sensory neurons a comparison of cell-attached and cell-free single-channel activitiesrdquo Pflugers Archivvol 422 no 3 pp 287ndash294 1992
[17] N Dale ldquoA large sustained Na+- and voltage-dependent K+current in spinal neurons of the frog embryordquo The Journal ofPhysiology vol 462 pp 349ndash372 1993
[18] B V Safronov and W Vogel ldquoProperties and functions of Na+-activated K+ channels in the soma of rat motoneuronesrdquo TheJournal of Physiology vol 497 no 3 pp 727ndash734 1996
[19] U Bischoff W Vogel and B V Safronov ldquoNa+-activated K+channels in small dorsal root ganglion neurones of ratrdquo TheJournal of Physiology vol 510 part 3 pp 743ndash754 1998
[20] S E Dryer ldquoNa+-activated K+ channels a new family of large-conductance ion channelsrdquo Trends in Neurosciences vol 17 no4 pp 155ndash160 1994
[21] A Bhattacharjee and L K Kaczmarek ldquoFor K+ channels Na+ isthe new Ca2+ rdquo Trends in Neurosciences vol 28 no 8 pp 422ndash428 2005
[22] S B Gao Y Wu C X Lu Z H Guo C H Li and J P DingldquoSlack and Slick KNa channels are required for the depolarizingafterpotential of acutely isolated medium diameter rat dorsalroot ganglion neuronsrdquoActa Pharmacologica Sinica vol 29 no8 pp 899ndash905 2008
[23] M O Nuwer K E Picchione and A Bhattacharjee ldquoPKA-induced internalization of Slack KNa channels produces dorsal
root ganglion neuron hyperexcitabilityrdquo The Journal of Neuro-science vol 30 no 42 pp 14165ndash14172 2010
[24] D Hess E Nanou and A El Manira ldquoCharacterization of Na+-activated K+ currents in larval lamprey spinal cord neuronsrdquoJournal of Neurophysiology vol 97 no 5 pp 3484ndash3493 2007
[25] E Nanou and A El Manira ldquoA postsynaptic negative feedbackmediated by coupling between AMPA receptors and Na+-activated K+ channels in spinal cord neuronesrdquo EuropeanJournal of Neuroscience vol 25 no 2 pp 445ndash450 2007
[26] PWallen B Robertson L Cangiano et al ldquoSodium-dependentpotassium channels of a Slack-like subtype contribute to theslow afterhyperpolarization in lamprey spinal neuronsrdquo TheJournal of Physiology vol 585 no 1 pp 75ndash90 2007
[27] E Nanou A Kyriakatos A Bhattacharjee L K Kaczmarek GParatcha and A El Manira ldquoNa+-mediated coupling betweenAMPA receptors and KNa channels shapes synaptic transmis-sionrdquo Proceedings of the National Academy of Sciences of theUnited States of America vol 105 no 52 pp 20941ndash20946 2008
[28] E Nanou and A El Manira ldquoMechanisms of modulation ofAMPA-induced Na+-activated K+ current by mGluR1rdquo Journalof Neurophysiology vol 103 no 1 pp 441ndash445 2010
[29] A R Mercer and J G Hildebrand ldquoDevelopmental changes inthe density of ionic currents in antennal-lobe neurons of thesphinx moth Manduca sextardquo Journal of Neurophysiology vol87 no 6 pp 2664ndash2675 2002
[30] K Aoki K Kosakai and M Yoshino ldquoMonoaminergic modu-lation of the Na+-activated K+ channel in kenyon cells isolatedfrom the mushroom body of the cricket (Gryllus bimaculatus)brainrdquo Journal of Neurophysiology vol 100 no 3 pp 1211ndash12222008
[31] G Klees P Hochstrate and P W Dierkes ldquoSodium-dependentpotassium channels in leech P neuronsrdquo Journal of MembraneBiology vol 208 no 1 pp 27ndash38 2005
[32] Y Zhang M R Brown C Hyland et al ldquoRegulation ofneuronal excitability by interaction of fragile X mental retar-dation protein with Slack potassium channelsrdquo The Journal ofNeuroscience vol 32 pp 15318ndash15327 2012
[33] C Lawrence and G C Rodrigo ldquoA Na+-activated K+ cur-rent (IKNa) is present in guinea-pig but not rat ventricularmyocytesrdquo Pflugers Archiv vol 437 no 6 pp 831ndash838 1999
[34] M Zhou Z Liu C Hu Z Zhang and Y Mei ldquoDevelopmentalregulation of a Na+-activated fast outward K+ current in ratmyoblastsrdquo Cellular Physiology and Biochemistry vol 14 no 4ndash6 pp 225ndash230 2004
[35] C K Young H S Jae M K Tong et al ldquoSodium-activatedpotassium current in guinea pig gastric myocytesrdquo Journal ofKorean Medical Science vol 22 no 1 pp 57ndash62 2007
[36] L Re V Moretti L Rossini and P Giusti ldquoSodium-activatedpotassium current in mouse diaphragmrdquo FEBS Letters vol 270no 1-2 pp 195ndash197 1990
[37] Y Zhang and W G Paterson ldquoFunctional evidence for Na+-activatedK+ channels in circular smoothmuscle of the opossumlower esophageal sphincterrdquo American The Journal of Physiol-ogy vol 292 no 6 pp G1600ndashG1606 2007
[38] M Paulais S Lachheb and J Teulon ldquoANa+- and Clminus-activatedK+ channel in the thick ascending limb of mouse kidneyrdquoJournal of General Physiology vol 127 no 2 pp 205ndash215 2006
[39] T M Egan D Dagan J Kupper and I B Levitan ldquoNa+-activated K+ channels are widely distributed in rat CNS and inXenopus oocytesrdquo Brain Research vol 584 no 1-2 pp 319ndash3211992
ISRN Neuroscience 13
[40] W J Joiner M D Tang L Y Wang et al ldquoFormation ofintermediate-conductance calcium-activated potassium chan-nels by interaction of Slack and Slo subunitsrdquo Nature Neuro-science vol 1 no 6 pp 462ndash469 1998
[41] A Yuan C M Santi A Wei et al ldquoThe sodium-activatedpotassium channel is encoded by a member of the Slo genefamilyrdquo Neuron vol 37 no 5 pp 765ndash773 2003
[42] Y Jiang A Pico M Cadene B T Chait and R MacKinnonldquoStructure of the RCK domain from the E coli K+ channeland demonstration of its presence in the human BK channelrdquoNeuron vol 29 no 3 pp 593ndash601 2001
[43] P Yuan M D Leonetti A R Pico Y Hsiung and R MacK-innon ldquoStructure of the human BK channel Ca2+-activationapparatus at 30 A resolutionrdquo Science vol 329 no 5988 pp182ndash186 2010
[44] Z Zhang A Rosenhouse-Dantsker Q Y Tang S Noskov andD E Logothetis ldquoThe RCK2 domain uses a coordination sitepresent in Kir channels to confer sodium sensitivity to Slo22channelsrdquoThe Journal of Neuroscience vol 30 no 22 pp 7554ndash7562 2010
[45] J L Sui K W Chan and D E Logothetis ldquoNa+ activationof the muscarinic K+ channel by a G-protein-independentmechanismrdquo Journal of General Physiology vol 108 no 5 pp381ndash391 1996
[46] M R Brown J Kronengold V R Gazula et al ldquoAmino-terminiisoforms of the Slack K+ channel regulated by alternative pro-moters differentiallymodulate rhythmic firing and adaptationrdquoThe Journal of Physiology vol 586 no 21 pp 5161ndash5179 2008
[47] A Yuan M Dourado A Butler N Walton A Wei and LSalkoff ldquoSLO-2 a K+ channel with an unusual Clminus dependencerdquoNature Neuroscience vol 3 no 8 pp 771ndash779 2000
[48] M Schreiber and L Salkoff ldquoA novel calcium-sensing domainin the BK channelrdquo Biophysical Journal vol 73 no 3 pp 1355ndash1363 1997
[49] H Chen J Kronengold Y Yan et al ldquoThe N-terminal domainof slack determines the formation and trafficking of slickslackheteromeric sodium-activated potassium channelsrdquo The Jour-nal of Neuroscience vol 29 no 17 pp 5654ndash5665 2009
[50] A Bhattacharjee L Gan and L K Kaczmarek ldquoLocalization ofthe Slack potassium channel in the rat central nervous systemrdquoJournal of Comparative Neurology vol 454 no 3 pp 241ndash2542002
[51] A Bhattacharjee C A A von Hehn X Mei and L KKaczmarek ldquoLocalization of the Na+-activated K+ channel slickin the rat central nervous systemrdquo Journal of ComparativeNeurology vol 484 no 1 pp 80ndash92 2005
[52] D S Koh P Jonas and W Vogel ldquoNa+-activated K+ channelslocalized in the nodal region of myelinated axons of XenopusrdquoThe Journal of Physiology vol 479 no 2 pp 183ndash197 1994
[53] K Mori T Saito Y Masuda and H Nakaya ldquoEffects of classIII antiarrhythmic drugs on the Na+-activated K+ channels inguinea-pig ventricular cellsrdquo British Journal of Pharmacologyvol 119 no 1 pp 133ndash141 1996
[54] K Mori S Kobayashi T Saito Y Masuda and H NakayaldquoInhibitory effects of class I and IV antiarrhythmic drugs onthe Na+- activated K+ channel current in guinea pig ventricularcellsrdquo Naunyn-Schmiedebergrsquos Archives of Pharmacology vol358 no 6 pp 641ndash648 1998
[55] Y Li T Sato and M Arita ldquoBepridil blunts the shortening ofaction potential duration caused by metabolic inhibition viablockade of ATP-sensitive K+ channels and Na+- activated K+
channelsrdquo Journal of Pharmacology and ExperimentalTherapeu-tics vol 291 no 2 pp 562ndash568 1999
[56] B Yang V K Gribkoff J Pan et al ldquoPharmacological activationand inhibition of Slack (Slo2 2) channelsrdquoNeuropharmacologyvol 51 no 4 pp 896ndash906 2006
[57] M D Tejada K Stolpe A K Meinild and D A KlaerkeldquoClofilium inhibits Slick and Slack potassium channelsrdquo Biolog-ics vol 6 pp 465ndash470 2012
[58] B Yang R Desai and L K Kaczmarek ldquoSlack and slick KNachannels regulate the accuracy of timing of auditory neuronsrdquoThe Journal of Neuroscience vol 27 no 10 pp 2617ndash2627 2007
[59] B Biton S Sethuramanujam K E Picchione et al ldquoTheantipsychotic drug loxapine is an opener of the sodium-activated potassium channel slack (Slo2 2)rdquo The Journal ofPharmacology and ExperimentalTherapeutics vol 340 pp 706ndash715 2012
[60] C M Santi G Ferreira B Yang et al ldquoOpposite regulation ofSlick and Slack K+ channels by neuromodulatorsrdquo The Journalof Neuroscience vol 26 no 19 pp 5059ndash5068 2006
[61] M R Fleming and L K Kaczmarek ldquoUse of optical biosensorsto detect modulation of Slack potassium channels by G protein-coupled receptorsrdquo Journal of Receptors and Signal Transduc-tion vol 29 no 3-4 pp 173ndash181 2009
[62] M D Tejada L J Jensen and D A Klaerke ldquoPIP(2) mod-ulation of Slick and Slack K(+) channelsrdquo Biochemical andBiophysical Research Communications vol 424 pp 208ndash2132012
[63] L Zhang M Sukhareva J L Barker et al ldquoDirect bindingof estradiol enhances Slack (sequence like a calcium-activatedpotassium channel) channelsrsquo activityrdquo Neuroscience vol 131no 2 pp 275ndash282 2005
[64] V A Ruffin X Q Gu D Zhou et al ldquoThe sodium-activatedpotassium channel Slack is modulated by hypercapnia andacidosisrdquo Neuroscience vol 151 no 2 pp 410ndash418 2008
[65] M O Nuwer K E Picchione and A Bhattacharjee ldquocAMP-dependent kinase does not modulate the Slack sodium-activated potassium channelrdquo Neuropharmacology vol 57 no3 pp 219ndash226 2009
[66] B C Suh and B Hille ldquoRegulation of ion channels by phos-phatidylinositol 45-bisphosphaterdquo Current Opinion in Neuro-biology vol 15 no 3 pp 370ndash378 2005
[67] L K Kaczmarek ldquoNon-conducting functions of voltage-gatedion channelsrdquo Nature Reviews Neuroscience vol 7 no 10 pp761ndash771 2006
[68] C R Rose ldquoNa+ signals at central synapsesrdquoNeuroscientist vol8 no 6 pp 532ndash539 2002
[69] C R Rose and B R Ransom ldquoRegulation of intracellularsodium in cultured rat hippocampal neuronesrdquo The Journal ofPhysiology vol 499 part 3 pp 573ndash587 1997
[70] C R Rose and A Konnerth ldquoNmda receptor-mediated Na+signals in spines and dendritesrdquoThe Journal of Neuroscience vol21 no 12 pp 4207ndash4214 2001
[71] N Zhong V Beaumont and R S Zucker ldquoRoles for mito-chondrial and reverse mode Na+Ca2+ exchange and the plas-malemma Ca2+ ATPase in post-tetanic potentiation at crayfishneuromuscular junctionsrdquo The Journal of Neuroscience vol 21no 24 pp 9598ndash9607 2001
[72] G Budelli T A Hage AWei et al ldquoNa+-activated K+ channelsexpress a large delayed outward current in neurons duringnormal physiologyrdquoNature Neuroscience vol 12 no 6 pp 745ndash750 2009
14 ISRN Neuroscience
[73] T A Hage and L Salkoff ldquoSodium-activated potassium chan-nels are functionally coupled to persistent sodiumcurrentsrdquoTheJournal of Neuroscience vol 32 pp 2714ndash2721 2012
[74] M RMarkham L K Kaczmarek andHH Zakon ldquoA sodium-activated potassium channe supports high frequency firing andreduces energetic costs during rapid modulations of actionpotential amplituderdquo Journal of Neurophysiology vol 109 no 7pp 1713ndash1723 2013
[75] S Uchino H Wada S Honda et al ldquoSlo2 sodium-activated K+channels bind to the PDZ domain of PSD-95rdquo Biochemical andBiophysical Research Communications vol 310 no 4 pp 1140ndash1147 2003
[76] T Zamalloa C P Bailey and J Pineda ldquoGlutamate-inducedpost-activation inhibition of locus coeruleus neurons is medi-ated by AMPAkainate receptors and sodium-dependent potas-sium currentsrdquo British Journal of Pharmacology vol 156 no 4pp 649ndash661 2009
[77] S Lu P Das D A Fadool and L K Kaczmarek ldquoThe slacksodium-activated potassium channel provides a major outwardcurrent in olfactory neurons of Kv13-- super-smeller micerdquoJournal of Neurophysiology vol 103 no 6 pp 3311ndash3319 2010
[78] R C Foehring P C Schwindt andW E Crill ldquoNorepinephrineselectively reduces slowCa2+- andNa+-mediated K+ currents incat neocortical neuronsrdquo Journal of Neurophysiology vol 61 no2 pp 245ndash256 1989
[79] M Kubota and N Saito ldquoSodium- and calcium-dependentconductances of neurones in the zebra finch hyperstriatumventrale pars caudale in vitrordquo The Journal of Physiology vol440 pp 131ndash142 1991
[80] U Kim and D A Mccormick ldquoFunctional and ionic propertiesof a slow afterhyperpolarization in ferret perigeniculate neuronsin vitrordquo Journal of Neurophysiology vol 80 no 3 pp 1222ndash1235 1998
[81] V M Sandler E Puil and DW F Schwarz ldquoIntrinsic responseproperties of bursting neurons in the nucleus principalistrigemini of the gerbilrdquoNeuroscience vol 83 no 3 pp 891ndash9041998
[82] M V Sanchez-Vives L G Nowak and D A McCormick ldquoCel-lular mechanisms of long-lasting adaptation in visual corticalneurons in vitrordquoThe Journal of Neuroscience vol 20 no 11 pp4286ndash4299 2000
[83] S Franceschetti T Lavazza G Curia et al ldquoNa+-activatedK+ current contributes to postexcitatory hyperpolarization inneocortical intrinsically bursting neuronsrdquo Journal of Neuro-physiology vol 89 no 4 pp 2101ndash2111 2003
[84] V F Descalzo L G Nowak J C Brumberg D A McCormickand M V Sanchez-Vives ldquoSlow adaptation in fast-spikingneurons of visual cortexrdquo Journal of Neurophysiology vol 93no 2 pp 1111ndash1118 2005
[85] J Benda L Maler and A Longtin ldquoLinear versus nonlinearsignal transmission in neuron models with adaptation currentsor dynamic thresholdsrdquo Journal of Neurophysiology vol 104 no5 pp 2806ndash2820 2010
[86] X Liu and L S Leung ldquoSodium-activated potassium conduc-tance participates in the depolarizing afterpotential following asingle action potential in rat hippocampal CA1 pyramidal cellsrdquoBrain Research vol 1023 no 2 pp 185ndash192 2004
[87] L Cangiano P Wallen and S Grillner ldquoRole of apamin-sensitive KCa channels for reticulospinal synaptic transmissionto motoneuron and for the afterhyperpolarizationrdquo Journal ofNeurophysiology vol 88 no 1 pp 289ndash299 2002
[88] Q Y Liu A E Schaffner and J L Barker ldquoKainate induces anintracellular Na+-activated K+ current in cultured embryonicrat hippocampal neuronesrdquo The Journal of Physiology vol 510part 3 pp 721ndash734 1998
[89] I Rishal T Keren-Raifman D Yakubovich et al ldquoNa+ pro-motes the dissociation between G120572GDP and G120573120574 activating Gprotein-gated K+ channelsrdquoThe Journal of Biological Chemistryvol 278 no 6 pp 3840ndash3845 2003
Submit your manuscripts athttpwwwhindawicom
Neurology Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentSchizophrenia
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
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Neural Plasticity
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Parkinsonrsquos Disease
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentAutism
Sleep DisordersHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Epilepsy Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Cardiovascular Psychiatry and NeurologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
ISRN Neuroscience 5
for either Slack-B or Slack-A isoforms found that bothisoforms are present throughout the brain but that thehighest levels of both isoforms are detected in brainstem andolfactory bulb [46]
Immunocytochemical staining using an antibody to theSlack-B specific N-terminus confirmed the localization ofSlack-B in neurons of the brainstem and olfactory bulb of rat[50] Prominent immunoreactivity was found in the olfactorybulb red nucleus and deep cerebellar nuclei as well asin vestibular and oculomotor nuclei and in the trigeminalsystem and reticular formation where intense staining occursin both cell bodies and axonal fibers As with the in situhybridization studies labeling was prominent in auditorybrainstem nuclei Although staining was also evident inneurons of the thalamus substantia nigra and amygdala theonly region of the cerebral cortex where the Slack-B isoformwas found was in the frontal cortex [50]
Immunolocalization studies have also been carried outusing an antibody targeted against a region in the cytoplasmicC-terminus of Slack [46] This would be expected to recog-nize themultiple Slack isoforms that differ at the N-terminusbut not to recognize Slick channels Immunostaining withthis ldquopan-Slackrdquo antibody found strong labeling of neuronsthroughout the nervous system of mice This included theregions previously identified with the Slack-B antibody aswell as other locations such as the dendrites of hippocampalneurons and olfactory bulb glomeruli [46]
Messenger RNA for the Slick subunit has awider distribu-tion throughout the body than does Slack mRNA and a Slicktranscript of sim69 kB is expressed in rat heart where no SlackmRNA has been found [4] Like Slack however the highestlevels of Slick mRNA are found in the brain Slick mRNA andimmunoreactivity overlap considerably with those for Slackand are found at high levels in olfactory bulb midbrain brainstem hippocampus throughout the cerebral cortex withhigh expression in primary somatosensory and visual regions[51] Like Slack high Slick immunoreactivity is found inneurons of sensory pathways including those in the auditorybrainstem The wide colocalization of Slick with the Slack-Bisoform suggests that the native KNa channels in these regionsare likely to be heteromers of these two subunits
Studies have also found Slack channels to be expressedin neurons of the peripheral nervous system Slack immuno-labeling is found in over 90 of neurons of the dorsal rootganglion and has been localized to both the somata andthe axonal tracts of small- medium- and large-diameterneurons [5 23] Localization to axonal tracts has also beendetected in electrophysiological experiments on peripheralaxons of Xenopus neurons for which a correlation has beenfound between the localization of KNa channels and voltage-dependent sodium channels [52]
8 Pharmacology of KNa Channels
A very common test for the presence of KNa channels inneurons and other cells has been to measure the effects ofreplacement of external sodium by lithium ions on the netoutward current Lithium is a much weaker activator of KNa
channels than sodium Because lithium readily enters cellsthough voltage-dependent sodium channels usually withminimal effects on the inward currents lithium replace-ment in voltage-clamp experiments reduces the net out-ward currents if KNa currents are present In current clampexperiments lithium substitution may alter the shape ofaction potentials or firing patterns in a manner consistentwith partial block of KNa currents The majority of theexperiments described later in the section on contributionsof KNa channels to neuronal firing patterns have tested theeffects of lithium substitution
Studies on native KNa channels in cardiac cells found thatseveral antiarrhythmic drugs inhibit these channels in cardiaccells [53ndash55] Some of these compounds such as quinidinebepridil and clofilium have been also found to be veryeffective and reversible blockers of Slack and Slick channelsexpressed in oocytes and mammalian cells [4 56 57] but arenon-specific in that they also act on a variety of other channeltypes
The pharmacological properties of Slack and Slick chan-nels are similar to each other but differ from those of manyother potassium channels including their closest molec-ular relative the Slo1 channel Slack and Slick are onlyweakly sensitive to the general potassium channel blockertetraethylammonium ions (TEA) [4] External barium ionsproduce a time- and voltage-dependent block of Slack andSlick channels In contrast both subunits are insensitiveto blockers of calcium-activated potassium channels suchas apamin iberiotoxin paxilline and charybdotoxin andto a wide range of blockers of other classes of potassiumchannels [4 57] Although Slick channels are sensitive tocytoplasmic ATP levels they are insensitive to glibenclamideand diazoxide an inhibitor and an activator respectively ofclassical KATP channels [4]
A variety of compounds that activate Slack and Slickchannels are also knownThefirst of these to be describedwasbithionol which in Slack-expressing HEK cells produces arobust increase in the amplitude of Slack currents with anEC50
of 077120583M [56] Similar bithionol-induced increasesin current are observed for Slack-expressing oocytes and fornative KNa currents in neurons of the auditory brainstem [5658] Bithionol reversibly activates Slack channels even whenapplied to the extracellular face of excised patches indicatingthat it acts on Slack channels relatively directly The increasein current occurs though a very marked bithionol-inducedchange in the voltage dependence of Slack channels such thatfull activation is already observed at potentials as negativeas minus40mV [56] Bithionol is however not selective forKNa channels in that it also activates Slo1 calcium-activatedpotassium channels [56 59]
A screen of pharmacologically active compounds usinga rubidium flux assay against Slack channels expressed inCHO cells has revealed other activators [59] These includeriluzole loxapine an antipsychotic agent and niclosamidean anthelmintic agent Loxapine was found to be moreselective than bithionol in that it has no effect of Slo1 calcium-activated potassium channels Electrophysiological experi-ments confirmed that loxapine is effective on recombinanthuman and rat Slack channels and that it activates native KNa
6 ISRN Neuroscience
channels in isolated neurons of the rat dorsal root ganglion[59]
9 Cellular Regulation of KNa Channels
The activity of Slack and Slick channels as well as ofheteromeric SlackSlick channels is potently regulated by avariety of cellular signaling pathways including G-protein-coupled receptors linked to activation of PKA or PKC [2328 30 49 60 61] direct phosphorylation of the channelssubunits [2 49 60] changes in cytoplasmic ATP levels[4] cytoplasmic NAD+ levels [5] PIP2 [62] estradiol [63]hypoxia [41 64] and the fragile X mental retardation proteinFMRP [6 32] This section will summarize these findings
91 Modulation of Slack and Slick by Protein Kinase CTreatment ofmammalian cells orXenopus oocytes expressingSlack channels with diacylglycerol or phorbol ester activatorsof protein kinase C (PKC) leads to a 2-3-fold increase incurrent amplitude and a slowing of the rate of activation[2 60] This effect of PKC has been found to result from thephosphorylation of a single serine residue S407 located in aPKC consensus phosphorylation site in the ldquohingerdquo region ofthe cytoplasmic C-terminus between the S6 transmembranedomain and the first RCK domain [2] Mutation of this serineto an alanine (S407A) which renders the site incapable ofundergoing phosphorylation results in apparently normalSlack currents that are entirely insensitive to PKC activationIn contrast mutation of the twelve other putative PKCphosphorylation sites in the Slack channel (see Figure 1)does not prevent the PKC-induced increase in Slack current[2]
In contrast to Slack channels the action of PKC onSlick channels is to produce a decrease in current amplitude[60] As in Slack this effect is likely to be mediated by amodification within the C-terminus In particular a chimericchannel that contains the N-terminus and membrane span-ning regions of Slack but the cytoplasmic C-terminus ofSlick responds to PKC activation with a decrease in currentMoreover direct application of a constitutively active formof PKC directly to the cytoplasmic face of excised patchescontaining these chimeric channels reduces their activity[60] Nevertheless the specific sites at which Slick subunitsare modified in response to PKC activation are not yetknown
Heteromeric Slack-BSlick channels respond to activationof PKC in a manner that is quite distinct from that of eithersubunit expressed alone [49] In Xenopus oocytes expressingboth subunits application of PKC activators potently sup-presses current bysim90This ismuch greater than the degreeof suppression measured for Slick subunits alone (sim50)and cannot therefore be explained by any linear combinationof Slick or Slack-B currents The finding that heteromericSlack-BSlick channels are regulated even more potently byPKC activation than are the homomeric channels constitutesone of the many pieces of evidence supporting selectiveheteromer formation between these subunits [49]
92 Modulation of Slack and Slick by G Protein CoupledReceptors Slack and Slick have been coexpressed in Xenopusoocytes with the M1 muscarinic receptor and the mGluR1metabotropic glutamate receptor [60]These are G120572q proteincoupled receptors that lead to the activation of PKC andformation of IP3 Accordingly activation of these receptorsleads to an increase in Slack currents and a suppression ofSlick currents consistent with modulation of these chan-nels by PKC There is widespread colocalization of thesereceptors with the KNa subunits throughout the nervoussystem suggesting that modulation of KNa currents by thesereceptors is likely to be widespread [60] Direct modulationof KNa channels by mGluR1 has been observed in nativeneurons within the lamprey spinal cord [28] In these cellsactivation of mGluR1 produces a strong suppression ofcurrent as expected for Slick or Slack-BSlick heteromersand this suppression is prevented by pharmacological inhibi-tion of PKC Interestingly chelation of intracellular calciumwith the chelator BAPTA (12-bis(2-aminophenoxy)ethane-NNN1015840N1015840-tetraacetic acid) prevented modulation of theKNa current by PKC but unmasked an mGluR1-dependentactivation of KNa current by an independent pathway whosemechanism is not yet known [28]
Modulation of Slack channels by G protein-coupledreceptors has been studied using a novel label-free technologyto detect changes in the distribution of mass close to theplasma membrane [61] The characteristic change in massthat normally follows activation of native receptors in HEK-293 cells is significantly modified in amplitude and timingby the coexpression of Slack channels further supporting thefinding of modulation of KNa current by these receptors [61]
93 Modulation of K119873119886
Currents by Cyclic AMP The activityof KNa channels in Kenyon cells isolated from the mushroombody of the cricket (Gryllus bimaculatus) is enhanced uponapplication of either the neurotransmitter octopamine or themembrane-permeable analog cyclic AMP analog 8-Br-cyclic-AMP [30] The increase in channel activity was found to beattenuated by the protein kinase A (PKA) inhibitor H-89Conversely KNa channel activity in these cells is decreasedby the transmitter dopamine or by the cyclic GMP analog8-Br-cyclic GMP and this decrease in open probability isantagonized by the protein kinaseG (PKG) inhibitor KT5823These findings have led to the proposal that modulation ofKNa currents play a role in olfactory learning circuits thatmediate reward and punishment signals in insects which areregulated by octopamine and dopamine respectively [30]
The Slack subunit is expressed at high levels in pri-mary afferent nociceptor neurons located in the dorsal rootganglion [5 23] and KNa currents can readily be recordedin these cells [5 22 23] Inflammatory substances suchas prostaglandin E2 that activate the PKA pathway greatlyincrease the excitability of these neurons sensitizing them tothermal and mechanical stimulation In their resting statethe neurons adapt rapidly to maintained depolarizationstypically generating only one or two action potentials at theonset of depolarization After an elevation of cyclic AMPlevels that activates PKA adaptation is greatly reduced such
ISRN Neuroscience 7
that sustained firing can be recorded throughout a prolongeddepolarization In major part the effects of PKA can beattributed to a decrease in Slack KNa current and suppressionof Slack expression using anti-Slack siRNA produces a reduc-tion in adaptation similar to that produced by PKA activation[23]
In contrast to the effects of PKC on Slack channels thatwere described above the effects of PKA on Slack are indirectand do not involve phosphorylation of the channel subunititself [23 65] Slack channels in HEK-293 cells are insensitiveto cyclic AMP analogs or to the PKA inhibitor KT5720 andapplication of the catalytic subunit of PKA to the cytoplasmicface of Slack channels excised from transfected cells or dorsalroot ganglion neurons has no effect on channel openingInstead activation of PKA decreases Slack current amplitudeby rapidly internalizing Slack channels from the plasmamembrane into internal organelles [23] Such internalizationwas detected by confocal microscopy of neurons expressingSlack channels tagged at their N-terminus with the greenfluorescent protein GFP Elevations of cyclic AMP reducedcolocalization of the labeled channels with a cell surfacemarker A reduction in native Slack channels at the plasmamembrane following PKA activation was also confirmedusing a surface biotinylation assay [23]
94 Modulation of Slack and Slick by PIP2 A variety ofion channels have been shown to be regulated by phos-phatidylinositol 45-biphosphate (PIP2) a phospholipid thatis localized to the inner lipid leaflet of the plasma membrane[66] Exogenously applied PI(45)P2 as well as its isoformPI(34)P2 increases the amplitude of Slack and Slick currentsexpressed in Xenopus oocytes and pharmacological agentsthat are expected to reduce endogenous PIP2 levels reducecurrent amplitude [62] Regulation by PIP2 was found torequire specific lysine residues in the C-termini of thechannelsMutation of these lysines to alanine (K339A in Slackand K306A in Slick) produced functional KNa currents thatwere insensitive to the activating effects of PIP2 [62]
95 Suppression of Slick Channels by Cytoplasmic ATP TheSlick subunit differs from Slack in that it contains a consensusATP binding site just C-terminal to the second RCK domain[4] The current density of Slick currents but not Slackcurrents is reduced by sim80 by the presence of 5mM ATPat the cytoplasmic face of the channels The same full effectis observed with ATP120574S a slowly hydrolyzable ATP analogand the nonhydrolyzable analog AMP-PNP but not withADP Suppression by ATP of Slick currents at all membranevoltages can also be readily detected in excised patches [4]
Confirmation that the effect of ATP on Slick channelsis mediated by the consensus ATP binding site in the C-terminal domain was obtained by site-directed mutagenesisto replace the glycine at residue 1032 in this site with a serine[4]Themutant G1032S Slick channel was fully functional butfailed to decrease channel activity in response to ATP Thesensitivity of Slick channels to ATP levels suggests that theiractivity could be enhanced during periods of high metabolicdemand when cellular ATP levels fall
96 Modulation of Slack by NAD+ KNa channels can read-ily be detected in cell-attached patch-clamp recordings onneurons [5 16 58] This may be surprising given that thecytoplasmic sodium concentration in resting cells is relativelylow (ltsim10mM see Section 10 below) and as described earlierthe EC
50for activation of these channels in excised patches
is relatively high (typically sim40ndash80mM) This discrepancy isresolved by the finding that the open probability and sodiumsensitivity of KNa channels are greatly reduced following exci-sion ofmembrane patches [5 16] suggesting that rundown ofKNa activity in excised patches may result from loss of somecytoplasmic factor
A major cytoplasmic factor that regulates both Slack andSlick channels has been found to be nicotinamide adeninedinucleotideNAD+ [5] Both subunits have a putativeNAD+-binding site within their second RCK domain Application ofNAD+ to the cytoplasmic face of KNa channels excised fromdorsal root ganglion neurons increases their open probabilityand reduces themeasured EC
50for activation of sodium from
sim50mM to sim17mM (Figure 3) NAD+ as well as NADP+but not NADH also increased the open probability of Slackchannels excised from a transfected mammalian cell lineMutation of glycine in the putative NAD+-binding site ofSlack (G792A) resulted in a loss of the ability of NAD+to potentiate activity [5] These findings suggest that KNachannels play a much more prominent role in the normalphysiology of excitable cells than would be suggested by theirproperties in isolated patches
97 Modulation of Slack by Estradiol The open probability ofchannels reconstituted from lipid bilayers from extracts of ratcortex has been found to be increased by 17120573-estradiol [63]Although these channels were not explicitly shown to be KNachannels a similar effect was reported for whole-cell currentsrecorded in Slack-expressing HEK-293 cells Activation wasnot prevented by tamoxifen a blocker of classical estrogenreceptors but may be a consequence of direct binding of 17120573-estradiol to Slack channels themselves [63]
98 Modulation of Slack Channels by Hypoxia Duringhypoxic conditions cytoplasmic levels of internal sodiumand chloride ions as well as those of H+ and CO
2 rise It
has been suggested that activation of KNa channels couldact to hyperpolarize cells during hypoxia providing a pro-tective mechanism [7] This conjecture is supported by thefinding that nematodes lacking the Slack ortholog in thisspecies are significantly more sensitive to hypoxic deaththan are wild-type animals [41] Further support for a rolefor Slack channels in the response to hypoxia comes fromrecordings of Slack channel activity in Xenopus oocytesexposed to increasing concentrations ofH+ andCO
2 demon-
strating that elevations of sodium activate Slack channelsunder cellular conditions comparable to those expected inischemiahypoxia [64]
99 Interactions of Slack with the Fragile X Mental Retarda-tion Protein FMRP Fragile X syndrome the most commoninherited form of intellectual disability in humans is caused
8 ISRN Neuroscience
1
08
06
04
02
0
[Na] (mM)10010
Control (small neurons)NAD
Nor
mal
ized
NP o
Figure 3 NAD+ increases the sodium sensitivity of KNa channelsexcised from adult rat dorsal root ganglion neurons Concentration-response relationship provides estimates of EC
50for sodium activa-
tion of 50mM and 17mM in the absence and presence of NAD+respectively Figure modified from [5]
by loss of expression of the RNA-binding protein FMRP(fragile X mental retardation protein) FMPR is known to berequired for normal activity-dependent protein translationin neurons Yeast two-hybrid and coimmunoprecipitationexperiments have shown that the cytoplasmic C-terminus ofthe Slack subunit interacts with this protein [6] MoreoverSlack-FMRP complexes also contain messenger RNAs thatare targets of FMRP such as the mRNAs encoding Map1band Arc No association of mRNAs with Slack can howeverbe found in mice lacking FMRP indicating that these arebound to Slack despite its interaction with FMRP A peptidearray assay revealed confirmed that FMRP binds selectivelyto sequences at the distal C-terminus of Slack [6]
FMRP is a potent activator of Slack channel activity [6]Application of recombinant FMRP(1ndash298) which containsthemajority of protein-protein interaction domains of FMRPto the cytoplasmic face of Slack channels in excised patchesreversibly increases channel opening by two- to threefold(Figure 4(a)) This is associated with the almost completeelimination of openings to subconductance states which areprominent in untreated patches (Figure 4(b)) FMRP wasfound to have no effect on the channel activity of a Slacktruncation mutant (Slack-BΔ804) which lacks sites found tointeract with FMRP in biochemical assays [6]
Support for a role for FMRP in regulating the amplitude ofnative KNa currents has also come from recordings of neuronsfrom animals lacking the fmr1 gene which encodes FMPR[6] As will be described later KNa channels account for amajor component of potassium current in principal neuronsof the medial nucleus of the trapezoid body (MNTB) Whileloss of FMRP does not affect total levels of Slack proteinthe level of the KNa component of current in these cells issubstantially reduced in the FMRP knockout mice compared
to that in wild-type animals consistent with an activating rolefor this RNA-binding protein [6]
The interaction between Slack and FMRP is likely tobe evolutionarily conserved Both proteins have been foundto be coexpressed in bag cell (BC) neurons which regulatereproductive behaviors in mollusk Aplysia [32] FMRP andSlack immunoreactivity are colocalized at the periphery ofisolated BC neurons and the two proteins can be reciprocallycoimmunoprecipitatedThe native Slack current in these cellswas identified using an siRNA approach and intracellularinjection of FMRP(1ndash298) induced a KNa current that in itsproperties matches the Slack current As for Slack channelsin heterologous expression systems addition of FMRP(1ndash298) to the inside-out patches containing native AplysiaKNa channels increased channel opening In current clamprecordings FMRP(1ndash298) produced a narrowing of actionpotentials
Downregulation of Slack expression in BC neurons usingan siRNA approach had an interesting effect on long-termchanges in the excitability of these cells [32] In untreatedcells brief stimulation of these neurons produces a charac-teristic prolonged (sim30min) discharge that is followed bya prolonged (sim18 hr) inhibitory state Recovery from thisinhibitory state requires new protein synthesis and is blockedby the protein synthesis inhibitor anisomycin Anti-SlacksiRNA did not alter the ability of BC neurons to undergothe normal discharge but like inhibition of protein synthesisprevented recovery from the inhibitory state [32] Althoughother interpretations are possible these findings raise thepossibility that the activity of Slack channels is linked to theregulation of protein translation through the FMRP-RNApathway
As will be described later point mutations in humanSlack channels can produce devastating effects on intellectualdevelopment [2 3] A variety of experiments have demon-strated that activation of ion channels can influence cytoplas-mic processes independently of ion flux through the channels[67] The interaction between the C-terminus of Slack andFMRP raises the hypothesis that conformational changesin the C-terminus could represent such a ldquononconductingrdquofunction of Slack that links neuronal firing to changes inprotein translation
10 Sodium Entry Pathways ThatActivate KNa Channels
Estimates of cytoplasmic sodium concentrations in restingunstimulated neurons obtained using sodium-sensitivemicroelectrodes and ratiometric dyes provide valuesbetween sim4mM and 15mM [68] As noted earlier in thesection on NAD+ regulation of Slack channels this maybe sufficient to provide significant activation of native KNachannels in intact neurons even under resting conditions[5] Internal sodium levels rise in response to stimulationprincipally as a result of entry through voltage-dependentsodium channels and ionotropic ligand-gated receptors suchas AMPA and NMDA glutamate receptors [69] Stimulationparticularly repetitive tetanic stimulation produces local
ISRN Neuroscience 9
Control RecoveryC
1
2
3
C
1
2
3
10pA
2 s
10pA
200ms
S S S S S S S
FMRP(1ndash298)
(a)
2520151050
10000
1000
100
pA
C1
2
3
Recovery
2520151050
10000
1000
100
pA
C1
2
3
S
S
S(N
)
(N)
Control
2520151050
10000
1000
100
pA
C
1 2 3
S
S
S
(N)
FMRP(1ndash298)
(b)
Figure 4 FMRP alters gating of Slack-B channels in inside-out patches (a) Recording of a patch containing 3-4 Slack channels before duringand after transient application of FMRP(1ndash298) to the cytoplasmic face of the patch Top traces show representative 10 s examples of recordingatminus40mVand bottom traces show expanded time views of the same patch Arrowsmarked S indicate the occurrence of subconductance statesthat are suppressed by FMRP(1ndash298) (b) All-points amplitude histograms of Slack channel activity plotted on a logarithmic scale before andafter application of FMRP(1ndash298) and after washout C marks the closed state S indicates subconductance states and numbers on peaks inthe histograms refer to the number of fully open Slack channels Modified from [6]
increases in sodium concentrations that can rise to levelsbetween 45 and 100mM [70 71] In particular high levelscan be attained in restricted compartments such as activedendritic spines
Although the upstroke of action potentials in mostneurons is driven by rapidly-inactivating voltage-dependentsodium currents these currents are transient Evidencesuggest that the much smaller persistent component ofsodium current that does not inactivate with maintaineddepolarization is responsible both for sustained elevationsof internal sodium during stimulation [69] and for selectiveactivation of KNa channels [72 73] It has been found thatin patches excised from somata of rat neurons sodium entrythrough persistent tetrodotoxin-sensitive sodium channels isthe dominant factor that leads to activation of KNa channelsin the same patch [73] Activation of KNa channels by sodiumentry in the isolated patches was enhanced by veratridinewhich prolongs sodium channel openings and was observedin the absence of sodium at the cytoplasmic face of thepatches Moreover activation appeared to be independent ofthe amplitude of the much larger transient sodium current
[73] Although the molecular basis for persistent sodiumchannels is not fully understood further support for a selec-tive activation of KNa currents by entry through persistentchannels has also been found in electrocytes of electric fish inwhich the time course of KNa currents during depolarizationsreflects that of the persistent sodium current and not that ofthe very much larger transient sodium current [74]
A number of studies have documented that KNa currentsare also activated by sodium entry through ionotropic gluta-mate receptors particularly AMPA receptors Slack subunitscan be coimmunoprecipitated with GluR23 AMPA receptorsusing rat brain synaptosomal fractions or lysates of the lam-prey central nervous system [27] Moreover Slack subunitshave been found to bind directly to the PDZ domain of PSD-95 (postsynaptic density-95 protein) a major componentof the postsynaptic density of glutamatergic synapses andto colocalize with PSD-95 in cultures of mouse corticalneurons [75] In electrophysiological studies of lampreyneurons application of AMPA was been found to activate aKNa current with pharmacological properties of Slack [25]Activation of this KNa current regulates both the amplitude
10 ISRN Neuroscience
and the decay rate of AMPA-induced synaptic currents pro-viding a feedback loop that decreases the size of glutamatergicpostsynaptic potentials [27]
Activation of KNa currents following activation of AMPAreceptors has also been proposed in studies of neurons of thelocus coeruleus to which application of glutamate producesa burst of action potentials followed by period of inhibitionlasting 30ndash45 seconds [76] The potassium current responsi-ble for the period of inhibition was selectively activated byAMPA or the AMPA-receptor agonist quisqualate but not byother stimuli that excite these neurons Consistent with thiscurrent being a KNa current it was attenuated by lowering ofexternal sodium concentrations but not affected by externalcalcium levels and it was potentiated by the KNa channelopener bithionol [76]
While persistent voltage-dependent sodium channels andAMPA receptors may represent the primary sodium entrypathways for activation of KNa channels there exist otherpathways that allow sodium ions to enter excitable cellsand whose role has been less explored Slack subunits areabundantly expressed in mitral cells of the olfactory bulb[46 50] In these neurons deletion of the gene for a majorvoltage-dependent potassiumchannelKv13 produces a com-pensatory increase in levels of Slack-B protein and in SlackKNa currents identified using an anti-Slack siRNA technique[77] Activation of the KNa current during depolarizationof these cells could only be significantly suppressed by acombination of tetrodotoxin to block voltage-dependentsodium channels and ZD7288 a blocker of H-channelswhich are nonselective cation channels that flux both sodiumand potassium ions Either drug alone failed to significantlysuppress KNa currents suggesting that sodium entry throughH-channels plays a role in KNa channel activation in theseneurons [77]
11 Contribution of KNa Channels toNeuronal Firing Patterns
The component of whole-cell potassium current in neuronsthat can be attributed to Slack channel subunits has beenidentified in mitral cells of the olfactory bulb and mediumspiny neuron of the striatum using siRNAs to suppress trans-lation of SlackmRNA [72 77] Such experiments indicate thatSlack KNa channels are responsible for a very major compo-nent of the total delayed outward current in neurons underphysiological conditions [72] The specific contributions thatKNa channels make to the electrical personality of individualtypes of neurons may differ in different cell types
One of the earliest effects to be described for activationof KNa currents is to produce a slow afterhyperpolarization(sAHP) following a burst of action potentials This was firstdescribed in pyramidal neurons of the cat sensorimotorcortex [11 78] and similar slow sodium-dependent sAHPsthat endure for many seconds have been found in a widevariety of neuronal types [79ndash84]These have been the subjectof earlier reviews [20 21] Such slow potentials have beenfound to influence the timing of regular bursts of actionpotentials such as the 3ndash12 per minute spindle waves that
occur at the onset of sleep in certain neurons within thethalamus [80] Slow activation of KNa during repetitive firingalso produces an adaptation of firing rate during bursts ofaction potentials [23] and such adaptation is predicted bynumerical simulations that incorporate KNa currents [46 85]
In some neurons such as neocortical pyramidal neuronsand hippocampal CA1 neurons a significant amount of KNacurrent can also be activated by single spikes and contributeto action potentials repolarization or to the amplitude ofdepolarizing afterpotentials that follow single spikes andare generated by persistent sodium currents [83 86] Inthese cases KNa current activation may be detected withinsim5msec following a single-action potential A recent studyhas examined themechanismbywhich the cells of the electricorgan (electrocytes) of certain electric fish are able to generatesustained firing at frequencies greater than 500Hz [74] Slackchannels are expressed at high levels in these cells and theirdelayed potassium current is almost exclusively a KNa currentthat is activated by a persistent sodium current Numericalsimulations have indicated that the use of KNa current torepolarize the very large action potentials of these cells allowsthe cells to fire at such high rates with minimal energyexpenditure [74]
A set of neurons that have been particularly instructivein studies of the role of KNa channels in shaping neuronalfiring are those that control locomotion in the spinal cord oflamprey [24ndash28 87] In addition to the transient KNa currentthat is activated by AMPA receptors and that was describedearlier [25 27] these neurons appear to have two distinctcomponents ofKNa current that are triggered by action poten-tial firingmdasha transient component that follows a single spikeand a slower more sustained component [24 26 87] Therapid transient KNa current is activated by the sodium influxthrough tetrodotoxin-sensitive channel during the actionpotentials and may play a role in determining the amplitudeand duration of individual action potentials The sustainedKNa current is not abolished by tetrodotoxin suggesting thatit is selectively activated by an independent sodium entrypathway and may underlie the calcium-independent sAHPthat occurs during repetitive bursting in lamprey spinal cordneurons [24]
Experiments with neurons that fire at high rates withvery high temporal accuracy have provided another potentialbiological role for KNa channels [58] Slack and Slick areexpressed at high levels in neurons of the medial nucleusof the trapezoid (MNTB) within the auditory brainstem[6 50 51] These neurons fire at rates up to sim800Hzand accurate timing of their action potentials is requiredfor transmission of information about the localization ofsound stimuli KNa channels are readily be detected in bothcell-attached and excised patches on the somata of MNTBneurons and they are localized appropriately for activationby sodium influx through AMPA receptors or voltage-dependent sodium channels [58] Current-clamp recordingsfrom MNTB neurons and numerical simulations indicatethat as KNa channels become activated temporal accuracyof firing is improved This occurs because the voltage-dependence of Slack and Slick channels allows them to opennear the resting potential of MNTB neurons increasing
ISRN Neuroscience 11
the resting conductance and decreasing the membrane timeconstant An increase in temporal fidelity of MNTB neuronscan also be produced by treatment with the pharmacologicalKNa activator bithionol [58]
12 Mutations in Slack Channels Result inEpilepsy and Intellectual Disabilities
De novo human mutations that produce single-amino-acidalterations in Slack channels produce devastating effectson development and intellectual function [2 3] One setof Slack mutations results in malignant migrating partialseizures of infancy (MMPSI) [2]This disease is characterizedby pharmacoresistant seizures in the first 6 months of lifethat arise randomly and ldquomigraterdquo from one brain regionto another While the occurrence of seizures abates withage MMPSI results in near-total arrest of psychomotordevelopment Exome sequencing revealed that at least 50of MMPSI patients have mutations in the large cytoplasmicC-terminal domain of Slack [2]
Two mutations in the human Slack gene that produceMMPSI are R428Q and A934T which correspond to themutations R409Q and A913T in the highly conserved ratSlack sequence [2] Expression of these mutants in Xenopusoocytes produced Slack currents with voltage dependenceand kinetic behavior that matched that of wild-type Slackchannels but with a current amplitude that was increased 2-3-fold over that of wild-type Slack This increase in currentlikely results because the mutations produce channels thatare either more prone to undergo phosphorylation by PKC(or less likely to be dephosphorylated) or that mimic theconformation induced by PKC activation One of theMMPSImutations R409Q lies adjacent to the phosphorylation siteS407 that as described earlier enhances current by 2-3-fold when phosphorylated by PKC Consistent with thisinterpretation current amplitude of the two MMPSI muta-tions could not be further increased upon PKC activa-tion [2] In other respects the MMPSI currents resembledthat of wild-type channels with unitary conductance andsodiumdependencesmatching those of wild-type Slack BothMMPSI mutant channels however had reduced openings tosubconductance states compared to the wild-type channels[2]
A second disease that has been attributed to a differentset of mutations in the C-terminal domain of Slack chan-nels is autosomal dominant nocturnal frontal lobe epilepsy(ADNFLE) [3] This is a childhood-onset focal epilepsysyndrome in which motor seizures arise during sleep Onsetof ADNFLE in the patients with Slack mutations occurred ata mean age of 6 years and was associated with mild to severeintellectual disability andor severe psychiatric problemsTheeffects of these ADNFLE mutations on the function of Slackchannels have however not been investigated Interestinglyanother known genetic cause of ADNFLE is mutations insubunits of the neuronal nicotinic acetycholine receptor Theacetycholine receptor ADNFLE mutations do not howeverproduce intellectual disability which contrasts strikingly tothe effects of the Slack mutations [3]
It is possible that the very severe outcomes of mutationsin Slack occur solely because of changes in the amplitudeof Slack currents An increase in total potassium currentin rapidly firing interneurons could reduce the firing rateof these cells predisposing the nervous system to hyper-excitability The relatively modest increase in current thatoccurs with theMMPSImutations [2] however suggests thatother processes may also be affected As described earlierthe large C-terminal cytoplasmic domain of Slack in whichMMPSI and ADNFLE mutations are located interacts withFMRP and its cargo mRNAs [6] Thus it is possible thatin addition to directly altering the excitability of centralneuronsmutations in Slack disrupt the regulation of activity-dependent protein translation in neurons
13 Conclusions
This review has covered themolecular and cellular propertiesof Slack and Slick two potassium channel subunits thatprovide neurons and other cells with KNa channels It shouldbe pointed out that additional potassium channels that areactivated by cytoplasmic sodiummay also exist For examplesome of the native KNa currents that have been described inneurons do not match Slack or Slick in their single-channelconductances or pharmacological properties [88]These mayperhaps represent inward rectifier potassium channels thatare activated by sodium [45 89] and the physiological rolesof the sodium sensitivity of these channels require furtherinvestigation Nevertheless Slack and Slick which are verywidely expressed in the brain appear to play dominant rolesin shaping the firing properties of verymany types of neuronsHumanmutations in these channels produce profound effectson neuronal function and development suggesting that per-haps their biological role may extend beyond simply settingthe level of neuronal excitability and that these channels mayinfluence cytoplasmic biochemical pathways that regulatedevelopment plasticity and intellectual function
References
[1] B Hille Ionic Channels of Excitable Membranes Sinauer Asso-ciates Sunderland Mass USA 3rd edition 2001
[2] G Barcia M R Fleming A Deligniere et al ldquoDe novo gain-of-function KCNT1 channel mutations cause malignant migratingpartial seizures of infancyrdquo Nature Genetics vol 44 pp 1255ndash1259 2012
[3] S E Heron K R SmithM Bahlo et al ldquoMissensemutations inthe sodium-gated potassium channel gene KCNT1 cause severeautosomal dominant nocturnal frontal lobe epilepsyrdquo NatureGenetics vol 44 pp 1188ndash1190 2012
[4] A Bhattacharjee W J Joiner M Wu Y Yang F J Sigworthand L K Kaczmarek ldquoSlick (Slo2 1) a rapidly-gating sodium-activated potassium channel inhibited by ATPrdquo The Journal ofNeuroscience vol 23 no 37 pp 11681ndash11691 2003
[5] T J Tamsett K E Picchione and A Bhattacharjee ldquoNAD+activates KNa channels in dorsal root ganglion neuronsrdquo TheJournal of Neuroscience vol 29 no 16 pp 5127ndash5134 2009
12 ISRN Neuroscience
[6] M R Brown J Kronengold V R Gazula et al ldquoFragileX mental retardation protein controls gating of the sodium-activated potassium channel SlackrdquoNatureNeuroscience vol 13no 7 pp 819ndash821 2010
[7] M Kameyama M Kakei and R Sato ldquoIntracellular Na+activates a K+ channel in mammalian cardiac cellsrdquoNature vol309 no 5966 pp 354ndash356 1984
[8] C R Bader L Bernheim and D Bertrand ldquoSodium-activatedpotassium current in cultured avian neuronesrdquoNature vol 317no 6037 pp 540ndash542 1985
[9] K Hartung ldquoPotentiation of a transient outward current byNa+influx in crayfish neuronesrdquo Pflugers Archiv vol 404 no 1 pp41ndash44 1985
[10] S E Dryer J T Fujii and A R Martin ldquoA Na+-activatedK+ current in cultured brain stem neurones from chicksrdquo TheJournal of Physiology vol 410 pp 283ndash296 1989
[11] P C Schwindt W J Spain and W E Crill ldquoLong-lastingreduction of excitability by a sodium-dependent potassiumcurrent in cat neocortical neuronsrdquo Journal of Neurophysiologyvol 61 no 2 pp 233ndash244 1989
[12] C Haimann L Bernheim D Bertrand and C R BaderldquoPotassium current activated by intracellular sodium in quailtrigeminal ganglion neuronsrdquo Journal of General Physiology vol95 no 5 pp 961ndash979 1990
[13] S E Dryer ldquoNa+-activated K+ channels and voltage-evokedionic currents in brain stem and parasympathetic neurones ofthe chickrdquoThe Journal of Physiology vol 435 pp 513ndash532 1991
[14] M Saito and C F Wu ldquoExpression of ion channels andmutational effects in giant Drosophila neurons differentiatedfrom cell division-arrested embryonic neuroblastsrdquoThe Journalof Neuroscience vol 11 no 7 pp 2135ndash2150 1991
[15] T M Egan D Dagan J Kupper and I B Levitan ldquoPropertiesand rundown of sodium-activated potassium channels in ratolfactory bulb neuronsrdquoThe Journal of Neuroscience vol 12 no5 pp 1964ndash1976 1992
[16] C Haimann J Magistretti and B Pozzi ldquoSodium-activatedpotassium current in sensory neurons a comparison of cell-attached and cell-free single-channel activitiesrdquo Pflugers Archivvol 422 no 3 pp 287ndash294 1992
[17] N Dale ldquoA large sustained Na+- and voltage-dependent K+current in spinal neurons of the frog embryordquo The Journal ofPhysiology vol 462 pp 349ndash372 1993
[18] B V Safronov and W Vogel ldquoProperties and functions of Na+-activated K+ channels in the soma of rat motoneuronesrdquo TheJournal of Physiology vol 497 no 3 pp 727ndash734 1996
[19] U Bischoff W Vogel and B V Safronov ldquoNa+-activated K+channels in small dorsal root ganglion neurones of ratrdquo TheJournal of Physiology vol 510 part 3 pp 743ndash754 1998
[20] S E Dryer ldquoNa+-activated K+ channels a new family of large-conductance ion channelsrdquo Trends in Neurosciences vol 17 no4 pp 155ndash160 1994
[21] A Bhattacharjee and L K Kaczmarek ldquoFor K+ channels Na+ isthe new Ca2+ rdquo Trends in Neurosciences vol 28 no 8 pp 422ndash428 2005
[22] S B Gao Y Wu C X Lu Z H Guo C H Li and J P DingldquoSlack and Slick KNa channels are required for the depolarizingafterpotential of acutely isolated medium diameter rat dorsalroot ganglion neuronsrdquoActa Pharmacologica Sinica vol 29 no8 pp 899ndash905 2008
[23] M O Nuwer K E Picchione and A Bhattacharjee ldquoPKA-induced internalization of Slack KNa channels produces dorsal
root ganglion neuron hyperexcitabilityrdquo The Journal of Neuro-science vol 30 no 42 pp 14165ndash14172 2010
[24] D Hess E Nanou and A El Manira ldquoCharacterization of Na+-activated K+ currents in larval lamprey spinal cord neuronsrdquoJournal of Neurophysiology vol 97 no 5 pp 3484ndash3493 2007
[25] E Nanou and A El Manira ldquoA postsynaptic negative feedbackmediated by coupling between AMPA receptors and Na+-activated K+ channels in spinal cord neuronesrdquo EuropeanJournal of Neuroscience vol 25 no 2 pp 445ndash450 2007
[26] PWallen B Robertson L Cangiano et al ldquoSodium-dependentpotassium channels of a Slack-like subtype contribute to theslow afterhyperpolarization in lamprey spinal neuronsrdquo TheJournal of Physiology vol 585 no 1 pp 75ndash90 2007
[27] E Nanou A Kyriakatos A Bhattacharjee L K Kaczmarek GParatcha and A El Manira ldquoNa+-mediated coupling betweenAMPA receptors and KNa channels shapes synaptic transmis-sionrdquo Proceedings of the National Academy of Sciences of theUnited States of America vol 105 no 52 pp 20941ndash20946 2008
[28] E Nanou and A El Manira ldquoMechanisms of modulation ofAMPA-induced Na+-activated K+ current by mGluR1rdquo Journalof Neurophysiology vol 103 no 1 pp 441ndash445 2010
[29] A R Mercer and J G Hildebrand ldquoDevelopmental changes inthe density of ionic currents in antennal-lobe neurons of thesphinx moth Manduca sextardquo Journal of Neurophysiology vol87 no 6 pp 2664ndash2675 2002
[30] K Aoki K Kosakai and M Yoshino ldquoMonoaminergic modu-lation of the Na+-activated K+ channel in kenyon cells isolatedfrom the mushroom body of the cricket (Gryllus bimaculatus)brainrdquo Journal of Neurophysiology vol 100 no 3 pp 1211ndash12222008
[31] G Klees P Hochstrate and P W Dierkes ldquoSodium-dependentpotassium channels in leech P neuronsrdquo Journal of MembraneBiology vol 208 no 1 pp 27ndash38 2005
[32] Y Zhang M R Brown C Hyland et al ldquoRegulation ofneuronal excitability by interaction of fragile X mental retar-dation protein with Slack potassium channelsrdquo The Journal ofNeuroscience vol 32 pp 15318ndash15327 2012
[33] C Lawrence and G C Rodrigo ldquoA Na+-activated K+ cur-rent (IKNa) is present in guinea-pig but not rat ventricularmyocytesrdquo Pflugers Archiv vol 437 no 6 pp 831ndash838 1999
[34] M Zhou Z Liu C Hu Z Zhang and Y Mei ldquoDevelopmentalregulation of a Na+-activated fast outward K+ current in ratmyoblastsrdquo Cellular Physiology and Biochemistry vol 14 no 4ndash6 pp 225ndash230 2004
[35] C K Young H S Jae M K Tong et al ldquoSodium-activatedpotassium current in guinea pig gastric myocytesrdquo Journal ofKorean Medical Science vol 22 no 1 pp 57ndash62 2007
[36] L Re V Moretti L Rossini and P Giusti ldquoSodium-activatedpotassium current in mouse diaphragmrdquo FEBS Letters vol 270no 1-2 pp 195ndash197 1990
[37] Y Zhang and W G Paterson ldquoFunctional evidence for Na+-activatedK+ channels in circular smoothmuscle of the opossumlower esophageal sphincterrdquo American The Journal of Physiol-ogy vol 292 no 6 pp G1600ndashG1606 2007
[38] M Paulais S Lachheb and J Teulon ldquoANa+- and Clminus-activatedK+ channel in the thick ascending limb of mouse kidneyrdquoJournal of General Physiology vol 127 no 2 pp 205ndash215 2006
[39] T M Egan D Dagan J Kupper and I B Levitan ldquoNa+-activated K+ channels are widely distributed in rat CNS and inXenopus oocytesrdquo Brain Research vol 584 no 1-2 pp 319ndash3211992
ISRN Neuroscience 13
[40] W J Joiner M D Tang L Y Wang et al ldquoFormation ofintermediate-conductance calcium-activated potassium chan-nels by interaction of Slack and Slo subunitsrdquo Nature Neuro-science vol 1 no 6 pp 462ndash469 1998
[41] A Yuan C M Santi A Wei et al ldquoThe sodium-activatedpotassium channel is encoded by a member of the Slo genefamilyrdquo Neuron vol 37 no 5 pp 765ndash773 2003
[42] Y Jiang A Pico M Cadene B T Chait and R MacKinnonldquoStructure of the RCK domain from the E coli K+ channeland demonstration of its presence in the human BK channelrdquoNeuron vol 29 no 3 pp 593ndash601 2001
[43] P Yuan M D Leonetti A R Pico Y Hsiung and R MacK-innon ldquoStructure of the human BK channel Ca2+-activationapparatus at 30 A resolutionrdquo Science vol 329 no 5988 pp182ndash186 2010
[44] Z Zhang A Rosenhouse-Dantsker Q Y Tang S Noskov andD E Logothetis ldquoThe RCK2 domain uses a coordination sitepresent in Kir channels to confer sodium sensitivity to Slo22channelsrdquoThe Journal of Neuroscience vol 30 no 22 pp 7554ndash7562 2010
[45] J L Sui K W Chan and D E Logothetis ldquoNa+ activationof the muscarinic K+ channel by a G-protein-independentmechanismrdquo Journal of General Physiology vol 108 no 5 pp381ndash391 1996
[46] M R Brown J Kronengold V R Gazula et al ldquoAmino-terminiisoforms of the Slack K+ channel regulated by alternative pro-moters differentiallymodulate rhythmic firing and adaptationrdquoThe Journal of Physiology vol 586 no 21 pp 5161ndash5179 2008
[47] A Yuan M Dourado A Butler N Walton A Wei and LSalkoff ldquoSLO-2 a K+ channel with an unusual Clminus dependencerdquoNature Neuroscience vol 3 no 8 pp 771ndash779 2000
[48] M Schreiber and L Salkoff ldquoA novel calcium-sensing domainin the BK channelrdquo Biophysical Journal vol 73 no 3 pp 1355ndash1363 1997
[49] H Chen J Kronengold Y Yan et al ldquoThe N-terminal domainof slack determines the formation and trafficking of slickslackheteromeric sodium-activated potassium channelsrdquo The Jour-nal of Neuroscience vol 29 no 17 pp 5654ndash5665 2009
[50] A Bhattacharjee L Gan and L K Kaczmarek ldquoLocalization ofthe Slack potassium channel in the rat central nervous systemrdquoJournal of Comparative Neurology vol 454 no 3 pp 241ndash2542002
[51] A Bhattacharjee C A A von Hehn X Mei and L KKaczmarek ldquoLocalization of the Na+-activated K+ channel slickin the rat central nervous systemrdquo Journal of ComparativeNeurology vol 484 no 1 pp 80ndash92 2005
[52] D S Koh P Jonas and W Vogel ldquoNa+-activated K+ channelslocalized in the nodal region of myelinated axons of XenopusrdquoThe Journal of Physiology vol 479 no 2 pp 183ndash197 1994
[53] K Mori T Saito Y Masuda and H Nakaya ldquoEffects of classIII antiarrhythmic drugs on the Na+-activated K+ channels inguinea-pig ventricular cellsrdquo British Journal of Pharmacologyvol 119 no 1 pp 133ndash141 1996
[54] K Mori S Kobayashi T Saito Y Masuda and H NakayaldquoInhibitory effects of class I and IV antiarrhythmic drugs onthe Na+- activated K+ channel current in guinea pig ventricularcellsrdquo Naunyn-Schmiedebergrsquos Archives of Pharmacology vol358 no 6 pp 641ndash648 1998
[55] Y Li T Sato and M Arita ldquoBepridil blunts the shortening ofaction potential duration caused by metabolic inhibition viablockade of ATP-sensitive K+ channels and Na+- activated K+
channelsrdquo Journal of Pharmacology and ExperimentalTherapeu-tics vol 291 no 2 pp 562ndash568 1999
[56] B Yang V K Gribkoff J Pan et al ldquoPharmacological activationand inhibition of Slack (Slo2 2) channelsrdquoNeuropharmacologyvol 51 no 4 pp 896ndash906 2006
[57] M D Tejada K Stolpe A K Meinild and D A KlaerkeldquoClofilium inhibits Slick and Slack potassium channelsrdquo Biolog-ics vol 6 pp 465ndash470 2012
[58] B Yang R Desai and L K Kaczmarek ldquoSlack and slick KNachannels regulate the accuracy of timing of auditory neuronsrdquoThe Journal of Neuroscience vol 27 no 10 pp 2617ndash2627 2007
[59] B Biton S Sethuramanujam K E Picchione et al ldquoTheantipsychotic drug loxapine is an opener of the sodium-activated potassium channel slack (Slo2 2)rdquo The Journal ofPharmacology and ExperimentalTherapeutics vol 340 pp 706ndash715 2012
[60] C M Santi G Ferreira B Yang et al ldquoOpposite regulation ofSlick and Slack K+ channels by neuromodulatorsrdquo The Journalof Neuroscience vol 26 no 19 pp 5059ndash5068 2006
[61] M R Fleming and L K Kaczmarek ldquoUse of optical biosensorsto detect modulation of Slack potassium channels by G protein-coupled receptorsrdquo Journal of Receptors and Signal Transduc-tion vol 29 no 3-4 pp 173ndash181 2009
[62] M D Tejada L J Jensen and D A Klaerke ldquoPIP(2) mod-ulation of Slick and Slack K(+) channelsrdquo Biochemical andBiophysical Research Communications vol 424 pp 208ndash2132012
[63] L Zhang M Sukhareva J L Barker et al ldquoDirect bindingof estradiol enhances Slack (sequence like a calcium-activatedpotassium channel) channelsrsquo activityrdquo Neuroscience vol 131no 2 pp 275ndash282 2005
[64] V A Ruffin X Q Gu D Zhou et al ldquoThe sodium-activatedpotassium channel Slack is modulated by hypercapnia andacidosisrdquo Neuroscience vol 151 no 2 pp 410ndash418 2008
[65] M O Nuwer K E Picchione and A Bhattacharjee ldquocAMP-dependent kinase does not modulate the Slack sodium-activated potassium channelrdquo Neuropharmacology vol 57 no3 pp 219ndash226 2009
[66] B C Suh and B Hille ldquoRegulation of ion channels by phos-phatidylinositol 45-bisphosphaterdquo Current Opinion in Neuro-biology vol 15 no 3 pp 370ndash378 2005
[67] L K Kaczmarek ldquoNon-conducting functions of voltage-gatedion channelsrdquo Nature Reviews Neuroscience vol 7 no 10 pp761ndash771 2006
[68] C R Rose ldquoNa+ signals at central synapsesrdquoNeuroscientist vol8 no 6 pp 532ndash539 2002
[69] C R Rose and B R Ransom ldquoRegulation of intracellularsodium in cultured rat hippocampal neuronesrdquo The Journal ofPhysiology vol 499 part 3 pp 573ndash587 1997
[70] C R Rose and A Konnerth ldquoNmda receptor-mediated Na+signals in spines and dendritesrdquoThe Journal of Neuroscience vol21 no 12 pp 4207ndash4214 2001
[71] N Zhong V Beaumont and R S Zucker ldquoRoles for mito-chondrial and reverse mode Na+Ca2+ exchange and the plas-malemma Ca2+ ATPase in post-tetanic potentiation at crayfishneuromuscular junctionsrdquo The Journal of Neuroscience vol 21no 24 pp 9598ndash9607 2001
[72] G Budelli T A Hage AWei et al ldquoNa+-activated K+ channelsexpress a large delayed outward current in neurons duringnormal physiologyrdquoNature Neuroscience vol 12 no 6 pp 745ndash750 2009
14 ISRN Neuroscience
[73] T A Hage and L Salkoff ldquoSodium-activated potassium chan-nels are functionally coupled to persistent sodiumcurrentsrdquoTheJournal of Neuroscience vol 32 pp 2714ndash2721 2012
[74] M RMarkham L K Kaczmarek andHH Zakon ldquoA sodium-activated potassium channe supports high frequency firing andreduces energetic costs during rapid modulations of actionpotential amplituderdquo Journal of Neurophysiology vol 109 no 7pp 1713ndash1723 2013
[75] S Uchino H Wada S Honda et al ldquoSlo2 sodium-activated K+channels bind to the PDZ domain of PSD-95rdquo Biochemical andBiophysical Research Communications vol 310 no 4 pp 1140ndash1147 2003
[76] T Zamalloa C P Bailey and J Pineda ldquoGlutamate-inducedpost-activation inhibition of locus coeruleus neurons is medi-ated by AMPAkainate receptors and sodium-dependent potas-sium currentsrdquo British Journal of Pharmacology vol 156 no 4pp 649ndash661 2009
[77] S Lu P Das D A Fadool and L K Kaczmarek ldquoThe slacksodium-activated potassium channel provides a major outwardcurrent in olfactory neurons of Kv13-- super-smeller micerdquoJournal of Neurophysiology vol 103 no 6 pp 3311ndash3319 2010
[78] R C Foehring P C Schwindt andW E Crill ldquoNorepinephrineselectively reduces slowCa2+- andNa+-mediated K+ currents incat neocortical neuronsrdquo Journal of Neurophysiology vol 61 no2 pp 245ndash256 1989
[79] M Kubota and N Saito ldquoSodium- and calcium-dependentconductances of neurones in the zebra finch hyperstriatumventrale pars caudale in vitrordquo The Journal of Physiology vol440 pp 131ndash142 1991
[80] U Kim and D A Mccormick ldquoFunctional and ionic propertiesof a slow afterhyperpolarization in ferret perigeniculate neuronsin vitrordquo Journal of Neurophysiology vol 80 no 3 pp 1222ndash1235 1998
[81] V M Sandler E Puil and DW F Schwarz ldquoIntrinsic responseproperties of bursting neurons in the nucleus principalistrigemini of the gerbilrdquoNeuroscience vol 83 no 3 pp 891ndash9041998
[82] M V Sanchez-Vives L G Nowak and D A McCormick ldquoCel-lular mechanisms of long-lasting adaptation in visual corticalneurons in vitrordquoThe Journal of Neuroscience vol 20 no 11 pp4286ndash4299 2000
[83] S Franceschetti T Lavazza G Curia et al ldquoNa+-activatedK+ current contributes to postexcitatory hyperpolarization inneocortical intrinsically bursting neuronsrdquo Journal of Neuro-physiology vol 89 no 4 pp 2101ndash2111 2003
[84] V F Descalzo L G Nowak J C Brumberg D A McCormickand M V Sanchez-Vives ldquoSlow adaptation in fast-spikingneurons of visual cortexrdquo Journal of Neurophysiology vol 93no 2 pp 1111ndash1118 2005
[85] J Benda L Maler and A Longtin ldquoLinear versus nonlinearsignal transmission in neuron models with adaptation currentsor dynamic thresholdsrdquo Journal of Neurophysiology vol 104 no5 pp 2806ndash2820 2010
[86] X Liu and L S Leung ldquoSodium-activated potassium conduc-tance participates in the depolarizing afterpotential following asingle action potential in rat hippocampal CA1 pyramidal cellsrdquoBrain Research vol 1023 no 2 pp 185ndash192 2004
[87] L Cangiano P Wallen and S Grillner ldquoRole of apamin-sensitive KCa channels for reticulospinal synaptic transmissionto motoneuron and for the afterhyperpolarizationrdquo Journal ofNeurophysiology vol 88 no 1 pp 289ndash299 2002
[88] Q Y Liu A E Schaffner and J L Barker ldquoKainate induces anintracellular Na+-activated K+ current in cultured embryonicrat hippocampal neuronesrdquo The Journal of Physiology vol 510part 3 pp 721ndash734 1998
[89] I Rishal T Keren-Raifman D Yakubovich et al ldquoNa+ pro-motes the dissociation between G120572GDP and G120573120574 activating Gprotein-gated K+ channelsrdquoThe Journal of Biological Chemistryvol 278 no 6 pp 3840ndash3845 2003
Submit your manuscripts athttpwwwhindawicom
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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Research and TreatmentSchizophrenia
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
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Research and TreatmentAutism
Sleep DisordersHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Cardiovascular Psychiatry and NeurologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
6 ISRN Neuroscience
channels in isolated neurons of the rat dorsal root ganglion[59]
9 Cellular Regulation of KNa Channels
The activity of Slack and Slick channels as well as ofheteromeric SlackSlick channels is potently regulated by avariety of cellular signaling pathways including G-protein-coupled receptors linked to activation of PKA or PKC [2328 30 49 60 61] direct phosphorylation of the channelssubunits [2 49 60] changes in cytoplasmic ATP levels[4] cytoplasmic NAD+ levels [5] PIP2 [62] estradiol [63]hypoxia [41 64] and the fragile X mental retardation proteinFMRP [6 32] This section will summarize these findings
91 Modulation of Slack and Slick by Protein Kinase CTreatment ofmammalian cells orXenopus oocytes expressingSlack channels with diacylglycerol or phorbol ester activatorsof protein kinase C (PKC) leads to a 2-3-fold increase incurrent amplitude and a slowing of the rate of activation[2 60] This effect of PKC has been found to result from thephosphorylation of a single serine residue S407 located in aPKC consensus phosphorylation site in the ldquohingerdquo region ofthe cytoplasmic C-terminus between the S6 transmembranedomain and the first RCK domain [2] Mutation of this serineto an alanine (S407A) which renders the site incapable ofundergoing phosphorylation results in apparently normalSlack currents that are entirely insensitive to PKC activationIn contrast mutation of the twelve other putative PKCphosphorylation sites in the Slack channel (see Figure 1)does not prevent the PKC-induced increase in Slack current[2]
In contrast to Slack channels the action of PKC onSlick channels is to produce a decrease in current amplitude[60] As in Slack this effect is likely to be mediated by amodification within the C-terminus In particular a chimericchannel that contains the N-terminus and membrane span-ning regions of Slack but the cytoplasmic C-terminus ofSlick responds to PKC activation with a decrease in currentMoreover direct application of a constitutively active formof PKC directly to the cytoplasmic face of excised patchescontaining these chimeric channels reduces their activity[60] Nevertheless the specific sites at which Slick subunitsare modified in response to PKC activation are not yetknown
Heteromeric Slack-BSlick channels respond to activationof PKC in a manner that is quite distinct from that of eithersubunit expressed alone [49] In Xenopus oocytes expressingboth subunits application of PKC activators potently sup-presses current bysim90This ismuch greater than the degreeof suppression measured for Slick subunits alone (sim50)and cannot therefore be explained by any linear combinationof Slick or Slack-B currents The finding that heteromericSlack-BSlick channels are regulated even more potently byPKC activation than are the homomeric channels constitutesone of the many pieces of evidence supporting selectiveheteromer formation between these subunits [49]
92 Modulation of Slack and Slick by G Protein CoupledReceptors Slack and Slick have been coexpressed in Xenopusoocytes with the M1 muscarinic receptor and the mGluR1metabotropic glutamate receptor [60]These are G120572q proteincoupled receptors that lead to the activation of PKC andformation of IP3 Accordingly activation of these receptorsleads to an increase in Slack currents and a suppression ofSlick currents consistent with modulation of these chan-nels by PKC There is widespread colocalization of thesereceptors with the KNa subunits throughout the nervoussystem suggesting that modulation of KNa currents by thesereceptors is likely to be widespread [60] Direct modulationof KNa channels by mGluR1 has been observed in nativeneurons within the lamprey spinal cord [28] In these cellsactivation of mGluR1 produces a strong suppression ofcurrent as expected for Slick or Slack-BSlick heteromersand this suppression is prevented by pharmacological inhibi-tion of PKC Interestingly chelation of intracellular calciumwith the chelator BAPTA (12-bis(2-aminophenoxy)ethane-NNN1015840N1015840-tetraacetic acid) prevented modulation of theKNa current by PKC but unmasked an mGluR1-dependentactivation of KNa current by an independent pathway whosemechanism is not yet known [28]
Modulation of Slack channels by G protein-coupledreceptors has been studied using a novel label-free technologyto detect changes in the distribution of mass close to theplasma membrane [61] The characteristic change in massthat normally follows activation of native receptors in HEK-293 cells is significantly modified in amplitude and timingby the coexpression of Slack channels further supporting thefinding of modulation of KNa current by these receptors [61]
93 Modulation of K119873119886
Currents by Cyclic AMP The activityof KNa channels in Kenyon cells isolated from the mushroombody of the cricket (Gryllus bimaculatus) is enhanced uponapplication of either the neurotransmitter octopamine or themembrane-permeable analog cyclic AMP analog 8-Br-cyclic-AMP [30] The increase in channel activity was found to beattenuated by the protein kinase A (PKA) inhibitor H-89Conversely KNa channel activity in these cells is decreasedby the transmitter dopamine or by the cyclic GMP analog8-Br-cyclic GMP and this decrease in open probability isantagonized by the protein kinaseG (PKG) inhibitor KT5823These findings have led to the proposal that modulation ofKNa currents play a role in olfactory learning circuits thatmediate reward and punishment signals in insects which areregulated by octopamine and dopamine respectively [30]
The Slack subunit is expressed at high levels in pri-mary afferent nociceptor neurons located in the dorsal rootganglion [5 23] and KNa currents can readily be recordedin these cells [5 22 23] Inflammatory substances suchas prostaglandin E2 that activate the PKA pathway greatlyincrease the excitability of these neurons sensitizing them tothermal and mechanical stimulation In their resting statethe neurons adapt rapidly to maintained depolarizationstypically generating only one or two action potentials at theonset of depolarization After an elevation of cyclic AMPlevels that activates PKA adaptation is greatly reduced such
ISRN Neuroscience 7
that sustained firing can be recorded throughout a prolongeddepolarization In major part the effects of PKA can beattributed to a decrease in Slack KNa current and suppressionof Slack expression using anti-Slack siRNA produces a reduc-tion in adaptation similar to that produced by PKA activation[23]
In contrast to the effects of PKC on Slack channels thatwere described above the effects of PKA on Slack are indirectand do not involve phosphorylation of the channel subunititself [23 65] Slack channels in HEK-293 cells are insensitiveto cyclic AMP analogs or to the PKA inhibitor KT5720 andapplication of the catalytic subunit of PKA to the cytoplasmicface of Slack channels excised from transfected cells or dorsalroot ganglion neurons has no effect on channel openingInstead activation of PKA decreases Slack current amplitudeby rapidly internalizing Slack channels from the plasmamembrane into internal organelles [23] Such internalizationwas detected by confocal microscopy of neurons expressingSlack channels tagged at their N-terminus with the greenfluorescent protein GFP Elevations of cyclic AMP reducedcolocalization of the labeled channels with a cell surfacemarker A reduction in native Slack channels at the plasmamembrane following PKA activation was also confirmedusing a surface biotinylation assay [23]
94 Modulation of Slack and Slick by PIP2 A variety ofion channels have been shown to be regulated by phos-phatidylinositol 45-biphosphate (PIP2) a phospholipid thatis localized to the inner lipid leaflet of the plasma membrane[66] Exogenously applied PI(45)P2 as well as its isoformPI(34)P2 increases the amplitude of Slack and Slick currentsexpressed in Xenopus oocytes and pharmacological agentsthat are expected to reduce endogenous PIP2 levels reducecurrent amplitude [62] Regulation by PIP2 was found torequire specific lysine residues in the C-termini of thechannelsMutation of these lysines to alanine (K339A in Slackand K306A in Slick) produced functional KNa currents thatwere insensitive to the activating effects of PIP2 [62]
95 Suppression of Slick Channels by Cytoplasmic ATP TheSlick subunit differs from Slack in that it contains a consensusATP binding site just C-terminal to the second RCK domain[4] The current density of Slick currents but not Slackcurrents is reduced by sim80 by the presence of 5mM ATPat the cytoplasmic face of the channels The same full effectis observed with ATP120574S a slowly hydrolyzable ATP analogand the nonhydrolyzable analog AMP-PNP but not withADP Suppression by ATP of Slick currents at all membranevoltages can also be readily detected in excised patches [4]
Confirmation that the effect of ATP on Slick channelsis mediated by the consensus ATP binding site in the C-terminal domain was obtained by site-directed mutagenesisto replace the glycine at residue 1032 in this site with a serine[4]Themutant G1032S Slick channel was fully functional butfailed to decrease channel activity in response to ATP Thesensitivity of Slick channels to ATP levels suggests that theiractivity could be enhanced during periods of high metabolicdemand when cellular ATP levels fall
96 Modulation of Slack by NAD+ KNa channels can read-ily be detected in cell-attached patch-clamp recordings onneurons [5 16 58] This may be surprising given that thecytoplasmic sodium concentration in resting cells is relativelylow (ltsim10mM see Section 10 below) and as described earlierthe EC
50for activation of these channels in excised patches
is relatively high (typically sim40ndash80mM) This discrepancy isresolved by the finding that the open probability and sodiumsensitivity of KNa channels are greatly reduced following exci-sion ofmembrane patches [5 16] suggesting that rundown ofKNa activity in excised patches may result from loss of somecytoplasmic factor
A major cytoplasmic factor that regulates both Slack andSlick channels has been found to be nicotinamide adeninedinucleotideNAD+ [5] Both subunits have a putativeNAD+-binding site within their second RCK domain Application ofNAD+ to the cytoplasmic face of KNa channels excised fromdorsal root ganglion neurons increases their open probabilityand reduces themeasured EC
50for activation of sodium from
sim50mM to sim17mM (Figure 3) NAD+ as well as NADP+but not NADH also increased the open probability of Slackchannels excised from a transfected mammalian cell lineMutation of glycine in the putative NAD+-binding site ofSlack (G792A) resulted in a loss of the ability of NAD+to potentiate activity [5] These findings suggest that KNachannels play a much more prominent role in the normalphysiology of excitable cells than would be suggested by theirproperties in isolated patches
97 Modulation of Slack by Estradiol The open probability ofchannels reconstituted from lipid bilayers from extracts of ratcortex has been found to be increased by 17120573-estradiol [63]Although these channels were not explicitly shown to be KNachannels a similar effect was reported for whole-cell currentsrecorded in Slack-expressing HEK-293 cells Activation wasnot prevented by tamoxifen a blocker of classical estrogenreceptors but may be a consequence of direct binding of 17120573-estradiol to Slack channels themselves [63]
98 Modulation of Slack Channels by Hypoxia Duringhypoxic conditions cytoplasmic levels of internal sodiumand chloride ions as well as those of H+ and CO
2 rise It
has been suggested that activation of KNa channels couldact to hyperpolarize cells during hypoxia providing a pro-tective mechanism [7] This conjecture is supported by thefinding that nematodes lacking the Slack ortholog in thisspecies are significantly more sensitive to hypoxic deaththan are wild-type animals [41] Further support for a rolefor Slack channels in the response to hypoxia comes fromrecordings of Slack channel activity in Xenopus oocytesexposed to increasing concentrations ofH+ andCO
2 demon-
strating that elevations of sodium activate Slack channelsunder cellular conditions comparable to those expected inischemiahypoxia [64]
99 Interactions of Slack with the Fragile X Mental Retarda-tion Protein FMRP Fragile X syndrome the most commoninherited form of intellectual disability in humans is caused
8 ISRN Neuroscience
1
08
06
04
02
0
[Na] (mM)10010
Control (small neurons)NAD
Nor
mal
ized
NP o
Figure 3 NAD+ increases the sodium sensitivity of KNa channelsexcised from adult rat dorsal root ganglion neurons Concentration-response relationship provides estimates of EC
50for sodium activa-
tion of 50mM and 17mM in the absence and presence of NAD+respectively Figure modified from [5]
by loss of expression of the RNA-binding protein FMRP(fragile X mental retardation protein) FMPR is known to berequired for normal activity-dependent protein translationin neurons Yeast two-hybrid and coimmunoprecipitationexperiments have shown that the cytoplasmic C-terminus ofthe Slack subunit interacts with this protein [6] MoreoverSlack-FMRP complexes also contain messenger RNAs thatare targets of FMRP such as the mRNAs encoding Map1band Arc No association of mRNAs with Slack can howeverbe found in mice lacking FMRP indicating that these arebound to Slack despite its interaction with FMRP A peptidearray assay revealed confirmed that FMRP binds selectivelyto sequences at the distal C-terminus of Slack [6]
FMRP is a potent activator of Slack channel activity [6]Application of recombinant FMRP(1ndash298) which containsthemajority of protein-protein interaction domains of FMRPto the cytoplasmic face of Slack channels in excised patchesreversibly increases channel opening by two- to threefold(Figure 4(a)) This is associated with the almost completeelimination of openings to subconductance states which areprominent in untreated patches (Figure 4(b)) FMRP wasfound to have no effect on the channel activity of a Slacktruncation mutant (Slack-BΔ804) which lacks sites found tointeract with FMRP in biochemical assays [6]
Support for a role for FMRP in regulating the amplitude ofnative KNa currents has also come from recordings of neuronsfrom animals lacking the fmr1 gene which encodes FMPR[6] As will be described later KNa channels account for amajor component of potassium current in principal neuronsof the medial nucleus of the trapezoid body (MNTB) Whileloss of FMRP does not affect total levels of Slack proteinthe level of the KNa component of current in these cells issubstantially reduced in the FMRP knockout mice compared
to that in wild-type animals consistent with an activating rolefor this RNA-binding protein [6]
The interaction between Slack and FMRP is likely tobe evolutionarily conserved Both proteins have been foundto be coexpressed in bag cell (BC) neurons which regulatereproductive behaviors in mollusk Aplysia [32] FMRP andSlack immunoreactivity are colocalized at the periphery ofisolated BC neurons and the two proteins can be reciprocallycoimmunoprecipitatedThe native Slack current in these cellswas identified using an siRNA approach and intracellularinjection of FMRP(1ndash298) induced a KNa current that in itsproperties matches the Slack current As for Slack channelsin heterologous expression systems addition of FMRP(1ndash298) to the inside-out patches containing native AplysiaKNa channels increased channel opening In current clamprecordings FMRP(1ndash298) produced a narrowing of actionpotentials
Downregulation of Slack expression in BC neurons usingan siRNA approach had an interesting effect on long-termchanges in the excitability of these cells [32] In untreatedcells brief stimulation of these neurons produces a charac-teristic prolonged (sim30min) discharge that is followed bya prolonged (sim18 hr) inhibitory state Recovery from thisinhibitory state requires new protein synthesis and is blockedby the protein synthesis inhibitor anisomycin Anti-SlacksiRNA did not alter the ability of BC neurons to undergothe normal discharge but like inhibition of protein synthesisprevented recovery from the inhibitory state [32] Althoughother interpretations are possible these findings raise thepossibility that the activity of Slack channels is linked to theregulation of protein translation through the FMRP-RNApathway
As will be described later point mutations in humanSlack channels can produce devastating effects on intellectualdevelopment [2 3] A variety of experiments have demon-strated that activation of ion channels can influence cytoplas-mic processes independently of ion flux through the channels[67] The interaction between the C-terminus of Slack andFMRP raises the hypothesis that conformational changesin the C-terminus could represent such a ldquononconductingrdquofunction of Slack that links neuronal firing to changes inprotein translation
10 Sodium Entry Pathways ThatActivate KNa Channels
Estimates of cytoplasmic sodium concentrations in restingunstimulated neurons obtained using sodium-sensitivemicroelectrodes and ratiometric dyes provide valuesbetween sim4mM and 15mM [68] As noted earlier in thesection on NAD+ regulation of Slack channels this maybe sufficient to provide significant activation of native KNachannels in intact neurons even under resting conditions[5] Internal sodium levels rise in response to stimulationprincipally as a result of entry through voltage-dependentsodium channels and ionotropic ligand-gated receptors suchas AMPA and NMDA glutamate receptors [69] Stimulationparticularly repetitive tetanic stimulation produces local
ISRN Neuroscience 9
Control RecoveryC
1
2
3
C
1
2
3
10pA
2 s
10pA
200ms
S S S S S S S
FMRP(1ndash298)
(a)
2520151050
10000
1000
100
pA
C1
2
3
Recovery
2520151050
10000
1000
100
pA
C1
2
3
S
S
S(N
)
(N)
Control
2520151050
10000
1000
100
pA
C
1 2 3
S
S
S
(N)
FMRP(1ndash298)
(b)
Figure 4 FMRP alters gating of Slack-B channels in inside-out patches (a) Recording of a patch containing 3-4 Slack channels before duringand after transient application of FMRP(1ndash298) to the cytoplasmic face of the patch Top traces show representative 10 s examples of recordingatminus40mVand bottom traces show expanded time views of the same patch Arrowsmarked S indicate the occurrence of subconductance statesthat are suppressed by FMRP(1ndash298) (b) All-points amplitude histograms of Slack channel activity plotted on a logarithmic scale before andafter application of FMRP(1ndash298) and after washout C marks the closed state S indicates subconductance states and numbers on peaks inthe histograms refer to the number of fully open Slack channels Modified from [6]
increases in sodium concentrations that can rise to levelsbetween 45 and 100mM [70 71] In particular high levelscan be attained in restricted compartments such as activedendritic spines
Although the upstroke of action potentials in mostneurons is driven by rapidly-inactivating voltage-dependentsodium currents these currents are transient Evidencesuggest that the much smaller persistent component ofsodium current that does not inactivate with maintaineddepolarization is responsible both for sustained elevationsof internal sodium during stimulation [69] and for selectiveactivation of KNa channels [72 73] It has been found thatin patches excised from somata of rat neurons sodium entrythrough persistent tetrodotoxin-sensitive sodium channels isthe dominant factor that leads to activation of KNa channelsin the same patch [73] Activation of KNa channels by sodiumentry in the isolated patches was enhanced by veratridinewhich prolongs sodium channel openings and was observedin the absence of sodium at the cytoplasmic face of thepatches Moreover activation appeared to be independent ofthe amplitude of the much larger transient sodium current
[73] Although the molecular basis for persistent sodiumchannels is not fully understood further support for a selec-tive activation of KNa currents by entry through persistentchannels has also been found in electrocytes of electric fish inwhich the time course of KNa currents during depolarizationsreflects that of the persistent sodium current and not that ofthe very much larger transient sodium current [74]
A number of studies have documented that KNa currentsare also activated by sodium entry through ionotropic gluta-mate receptors particularly AMPA receptors Slack subunitscan be coimmunoprecipitated with GluR23 AMPA receptorsusing rat brain synaptosomal fractions or lysates of the lam-prey central nervous system [27] Moreover Slack subunitshave been found to bind directly to the PDZ domain of PSD-95 (postsynaptic density-95 protein) a major componentof the postsynaptic density of glutamatergic synapses andto colocalize with PSD-95 in cultures of mouse corticalneurons [75] In electrophysiological studies of lampreyneurons application of AMPA was been found to activate aKNa current with pharmacological properties of Slack [25]Activation of this KNa current regulates both the amplitude
10 ISRN Neuroscience
and the decay rate of AMPA-induced synaptic currents pro-viding a feedback loop that decreases the size of glutamatergicpostsynaptic potentials [27]
Activation of KNa currents following activation of AMPAreceptors has also been proposed in studies of neurons of thelocus coeruleus to which application of glutamate producesa burst of action potentials followed by period of inhibitionlasting 30ndash45 seconds [76] The potassium current responsi-ble for the period of inhibition was selectively activated byAMPA or the AMPA-receptor agonist quisqualate but not byother stimuli that excite these neurons Consistent with thiscurrent being a KNa current it was attenuated by lowering ofexternal sodium concentrations but not affected by externalcalcium levels and it was potentiated by the KNa channelopener bithionol [76]
While persistent voltage-dependent sodium channels andAMPA receptors may represent the primary sodium entrypathways for activation of KNa channels there exist otherpathways that allow sodium ions to enter excitable cellsand whose role has been less explored Slack subunits areabundantly expressed in mitral cells of the olfactory bulb[46 50] In these neurons deletion of the gene for a majorvoltage-dependent potassiumchannelKv13 produces a com-pensatory increase in levels of Slack-B protein and in SlackKNa currents identified using an anti-Slack siRNA technique[77] Activation of the KNa current during depolarizationof these cells could only be significantly suppressed by acombination of tetrodotoxin to block voltage-dependentsodium channels and ZD7288 a blocker of H-channelswhich are nonselective cation channels that flux both sodiumand potassium ions Either drug alone failed to significantlysuppress KNa currents suggesting that sodium entry throughH-channels plays a role in KNa channel activation in theseneurons [77]
11 Contribution of KNa Channels toNeuronal Firing Patterns
The component of whole-cell potassium current in neuronsthat can be attributed to Slack channel subunits has beenidentified in mitral cells of the olfactory bulb and mediumspiny neuron of the striatum using siRNAs to suppress trans-lation of SlackmRNA [72 77] Such experiments indicate thatSlack KNa channels are responsible for a very major compo-nent of the total delayed outward current in neurons underphysiological conditions [72] The specific contributions thatKNa channels make to the electrical personality of individualtypes of neurons may differ in different cell types
One of the earliest effects to be described for activationof KNa currents is to produce a slow afterhyperpolarization(sAHP) following a burst of action potentials This was firstdescribed in pyramidal neurons of the cat sensorimotorcortex [11 78] and similar slow sodium-dependent sAHPsthat endure for many seconds have been found in a widevariety of neuronal types [79ndash84]These have been the subjectof earlier reviews [20 21] Such slow potentials have beenfound to influence the timing of regular bursts of actionpotentials such as the 3ndash12 per minute spindle waves that
occur at the onset of sleep in certain neurons within thethalamus [80] Slow activation of KNa during repetitive firingalso produces an adaptation of firing rate during bursts ofaction potentials [23] and such adaptation is predicted bynumerical simulations that incorporate KNa currents [46 85]
In some neurons such as neocortical pyramidal neuronsand hippocampal CA1 neurons a significant amount of KNacurrent can also be activated by single spikes and contributeto action potentials repolarization or to the amplitude ofdepolarizing afterpotentials that follow single spikes andare generated by persistent sodium currents [83 86] Inthese cases KNa current activation may be detected withinsim5msec following a single-action potential A recent studyhas examined themechanismbywhich the cells of the electricorgan (electrocytes) of certain electric fish are able to generatesustained firing at frequencies greater than 500Hz [74] Slackchannels are expressed at high levels in these cells and theirdelayed potassium current is almost exclusively a KNa currentthat is activated by a persistent sodium current Numericalsimulations have indicated that the use of KNa current torepolarize the very large action potentials of these cells allowsthe cells to fire at such high rates with minimal energyexpenditure [74]
A set of neurons that have been particularly instructivein studies of the role of KNa channels in shaping neuronalfiring are those that control locomotion in the spinal cord oflamprey [24ndash28 87] In addition to the transient KNa currentthat is activated by AMPA receptors and that was describedearlier [25 27] these neurons appear to have two distinctcomponents ofKNa current that are triggered by action poten-tial firingmdasha transient component that follows a single spikeand a slower more sustained component [24 26 87] Therapid transient KNa current is activated by the sodium influxthrough tetrodotoxin-sensitive channel during the actionpotentials and may play a role in determining the amplitudeand duration of individual action potentials The sustainedKNa current is not abolished by tetrodotoxin suggesting thatit is selectively activated by an independent sodium entrypathway and may underlie the calcium-independent sAHPthat occurs during repetitive bursting in lamprey spinal cordneurons [24]
Experiments with neurons that fire at high rates withvery high temporal accuracy have provided another potentialbiological role for KNa channels [58] Slack and Slick areexpressed at high levels in neurons of the medial nucleusof the trapezoid (MNTB) within the auditory brainstem[6 50 51] These neurons fire at rates up to sim800Hzand accurate timing of their action potentials is requiredfor transmission of information about the localization ofsound stimuli KNa channels are readily be detected in bothcell-attached and excised patches on the somata of MNTBneurons and they are localized appropriately for activationby sodium influx through AMPA receptors or voltage-dependent sodium channels [58] Current-clamp recordingsfrom MNTB neurons and numerical simulations indicatethat as KNa channels become activated temporal accuracyof firing is improved This occurs because the voltage-dependence of Slack and Slick channels allows them to opennear the resting potential of MNTB neurons increasing
ISRN Neuroscience 11
the resting conductance and decreasing the membrane timeconstant An increase in temporal fidelity of MNTB neuronscan also be produced by treatment with the pharmacologicalKNa activator bithionol [58]
12 Mutations in Slack Channels Result inEpilepsy and Intellectual Disabilities
De novo human mutations that produce single-amino-acidalterations in Slack channels produce devastating effectson development and intellectual function [2 3] One setof Slack mutations results in malignant migrating partialseizures of infancy (MMPSI) [2]This disease is characterizedby pharmacoresistant seizures in the first 6 months of lifethat arise randomly and ldquomigraterdquo from one brain regionto another While the occurrence of seizures abates withage MMPSI results in near-total arrest of psychomotordevelopment Exome sequencing revealed that at least 50of MMPSI patients have mutations in the large cytoplasmicC-terminal domain of Slack [2]
Two mutations in the human Slack gene that produceMMPSI are R428Q and A934T which correspond to themutations R409Q and A913T in the highly conserved ratSlack sequence [2] Expression of these mutants in Xenopusoocytes produced Slack currents with voltage dependenceand kinetic behavior that matched that of wild-type Slackchannels but with a current amplitude that was increased 2-3-fold over that of wild-type Slack This increase in currentlikely results because the mutations produce channels thatare either more prone to undergo phosphorylation by PKC(or less likely to be dephosphorylated) or that mimic theconformation induced by PKC activation One of theMMPSImutations R409Q lies adjacent to the phosphorylation siteS407 that as described earlier enhances current by 2-3-fold when phosphorylated by PKC Consistent with thisinterpretation current amplitude of the two MMPSI muta-tions could not be further increased upon PKC activa-tion [2] In other respects the MMPSI currents resembledthat of wild-type channels with unitary conductance andsodiumdependencesmatching those of wild-type Slack BothMMPSI mutant channels however had reduced openings tosubconductance states compared to the wild-type channels[2]
A second disease that has been attributed to a differentset of mutations in the C-terminal domain of Slack chan-nels is autosomal dominant nocturnal frontal lobe epilepsy(ADNFLE) [3] This is a childhood-onset focal epilepsysyndrome in which motor seizures arise during sleep Onsetof ADNFLE in the patients with Slack mutations occurred ata mean age of 6 years and was associated with mild to severeintellectual disability andor severe psychiatric problemsTheeffects of these ADNFLE mutations on the function of Slackchannels have however not been investigated Interestinglyanother known genetic cause of ADNFLE is mutations insubunits of the neuronal nicotinic acetycholine receptor Theacetycholine receptor ADNFLE mutations do not howeverproduce intellectual disability which contrasts strikingly tothe effects of the Slack mutations [3]
It is possible that the very severe outcomes of mutationsin Slack occur solely because of changes in the amplitudeof Slack currents An increase in total potassium currentin rapidly firing interneurons could reduce the firing rateof these cells predisposing the nervous system to hyper-excitability The relatively modest increase in current thatoccurs with theMMPSImutations [2] however suggests thatother processes may also be affected As described earlierthe large C-terminal cytoplasmic domain of Slack in whichMMPSI and ADNFLE mutations are located interacts withFMRP and its cargo mRNAs [6] Thus it is possible thatin addition to directly altering the excitability of centralneuronsmutations in Slack disrupt the regulation of activity-dependent protein translation in neurons
13 Conclusions
This review has covered themolecular and cellular propertiesof Slack and Slick two potassium channel subunits thatprovide neurons and other cells with KNa channels It shouldbe pointed out that additional potassium channels that areactivated by cytoplasmic sodiummay also exist For examplesome of the native KNa currents that have been described inneurons do not match Slack or Slick in their single-channelconductances or pharmacological properties [88]These mayperhaps represent inward rectifier potassium channels thatare activated by sodium [45 89] and the physiological rolesof the sodium sensitivity of these channels require furtherinvestigation Nevertheless Slack and Slick which are verywidely expressed in the brain appear to play dominant rolesin shaping the firing properties of verymany types of neuronsHumanmutations in these channels produce profound effectson neuronal function and development suggesting that per-haps their biological role may extend beyond simply settingthe level of neuronal excitability and that these channels mayinfluence cytoplasmic biochemical pathways that regulatedevelopment plasticity and intellectual function
References
[1] B Hille Ionic Channels of Excitable Membranes Sinauer Asso-ciates Sunderland Mass USA 3rd edition 2001
[2] G Barcia M R Fleming A Deligniere et al ldquoDe novo gain-of-function KCNT1 channel mutations cause malignant migratingpartial seizures of infancyrdquo Nature Genetics vol 44 pp 1255ndash1259 2012
[3] S E Heron K R SmithM Bahlo et al ldquoMissensemutations inthe sodium-gated potassium channel gene KCNT1 cause severeautosomal dominant nocturnal frontal lobe epilepsyrdquo NatureGenetics vol 44 pp 1188ndash1190 2012
[4] A Bhattacharjee W J Joiner M Wu Y Yang F J Sigworthand L K Kaczmarek ldquoSlick (Slo2 1) a rapidly-gating sodium-activated potassium channel inhibited by ATPrdquo The Journal ofNeuroscience vol 23 no 37 pp 11681ndash11691 2003
[5] T J Tamsett K E Picchione and A Bhattacharjee ldquoNAD+activates KNa channels in dorsal root ganglion neuronsrdquo TheJournal of Neuroscience vol 29 no 16 pp 5127ndash5134 2009
12 ISRN Neuroscience
[6] M R Brown J Kronengold V R Gazula et al ldquoFragileX mental retardation protein controls gating of the sodium-activated potassium channel SlackrdquoNatureNeuroscience vol 13no 7 pp 819ndash821 2010
[7] M Kameyama M Kakei and R Sato ldquoIntracellular Na+activates a K+ channel in mammalian cardiac cellsrdquoNature vol309 no 5966 pp 354ndash356 1984
[8] C R Bader L Bernheim and D Bertrand ldquoSodium-activatedpotassium current in cultured avian neuronesrdquoNature vol 317no 6037 pp 540ndash542 1985
[9] K Hartung ldquoPotentiation of a transient outward current byNa+influx in crayfish neuronesrdquo Pflugers Archiv vol 404 no 1 pp41ndash44 1985
[10] S E Dryer J T Fujii and A R Martin ldquoA Na+-activatedK+ current in cultured brain stem neurones from chicksrdquo TheJournal of Physiology vol 410 pp 283ndash296 1989
[11] P C Schwindt W J Spain and W E Crill ldquoLong-lastingreduction of excitability by a sodium-dependent potassiumcurrent in cat neocortical neuronsrdquo Journal of Neurophysiologyvol 61 no 2 pp 233ndash244 1989
[12] C Haimann L Bernheim D Bertrand and C R BaderldquoPotassium current activated by intracellular sodium in quailtrigeminal ganglion neuronsrdquo Journal of General Physiology vol95 no 5 pp 961ndash979 1990
[13] S E Dryer ldquoNa+-activated K+ channels and voltage-evokedionic currents in brain stem and parasympathetic neurones ofthe chickrdquoThe Journal of Physiology vol 435 pp 513ndash532 1991
[14] M Saito and C F Wu ldquoExpression of ion channels andmutational effects in giant Drosophila neurons differentiatedfrom cell division-arrested embryonic neuroblastsrdquoThe Journalof Neuroscience vol 11 no 7 pp 2135ndash2150 1991
[15] T M Egan D Dagan J Kupper and I B Levitan ldquoPropertiesand rundown of sodium-activated potassium channels in ratolfactory bulb neuronsrdquoThe Journal of Neuroscience vol 12 no5 pp 1964ndash1976 1992
[16] C Haimann J Magistretti and B Pozzi ldquoSodium-activatedpotassium current in sensory neurons a comparison of cell-attached and cell-free single-channel activitiesrdquo Pflugers Archivvol 422 no 3 pp 287ndash294 1992
[17] N Dale ldquoA large sustained Na+- and voltage-dependent K+current in spinal neurons of the frog embryordquo The Journal ofPhysiology vol 462 pp 349ndash372 1993
[18] B V Safronov and W Vogel ldquoProperties and functions of Na+-activated K+ channels in the soma of rat motoneuronesrdquo TheJournal of Physiology vol 497 no 3 pp 727ndash734 1996
[19] U Bischoff W Vogel and B V Safronov ldquoNa+-activated K+channels in small dorsal root ganglion neurones of ratrdquo TheJournal of Physiology vol 510 part 3 pp 743ndash754 1998
[20] S E Dryer ldquoNa+-activated K+ channels a new family of large-conductance ion channelsrdquo Trends in Neurosciences vol 17 no4 pp 155ndash160 1994
[21] A Bhattacharjee and L K Kaczmarek ldquoFor K+ channels Na+ isthe new Ca2+ rdquo Trends in Neurosciences vol 28 no 8 pp 422ndash428 2005
[22] S B Gao Y Wu C X Lu Z H Guo C H Li and J P DingldquoSlack and Slick KNa channels are required for the depolarizingafterpotential of acutely isolated medium diameter rat dorsalroot ganglion neuronsrdquoActa Pharmacologica Sinica vol 29 no8 pp 899ndash905 2008
[23] M O Nuwer K E Picchione and A Bhattacharjee ldquoPKA-induced internalization of Slack KNa channels produces dorsal
root ganglion neuron hyperexcitabilityrdquo The Journal of Neuro-science vol 30 no 42 pp 14165ndash14172 2010
[24] D Hess E Nanou and A El Manira ldquoCharacterization of Na+-activated K+ currents in larval lamprey spinal cord neuronsrdquoJournal of Neurophysiology vol 97 no 5 pp 3484ndash3493 2007
[25] E Nanou and A El Manira ldquoA postsynaptic negative feedbackmediated by coupling between AMPA receptors and Na+-activated K+ channels in spinal cord neuronesrdquo EuropeanJournal of Neuroscience vol 25 no 2 pp 445ndash450 2007
[26] PWallen B Robertson L Cangiano et al ldquoSodium-dependentpotassium channels of a Slack-like subtype contribute to theslow afterhyperpolarization in lamprey spinal neuronsrdquo TheJournal of Physiology vol 585 no 1 pp 75ndash90 2007
[27] E Nanou A Kyriakatos A Bhattacharjee L K Kaczmarek GParatcha and A El Manira ldquoNa+-mediated coupling betweenAMPA receptors and KNa channels shapes synaptic transmis-sionrdquo Proceedings of the National Academy of Sciences of theUnited States of America vol 105 no 52 pp 20941ndash20946 2008
[28] E Nanou and A El Manira ldquoMechanisms of modulation ofAMPA-induced Na+-activated K+ current by mGluR1rdquo Journalof Neurophysiology vol 103 no 1 pp 441ndash445 2010
[29] A R Mercer and J G Hildebrand ldquoDevelopmental changes inthe density of ionic currents in antennal-lobe neurons of thesphinx moth Manduca sextardquo Journal of Neurophysiology vol87 no 6 pp 2664ndash2675 2002
[30] K Aoki K Kosakai and M Yoshino ldquoMonoaminergic modu-lation of the Na+-activated K+ channel in kenyon cells isolatedfrom the mushroom body of the cricket (Gryllus bimaculatus)brainrdquo Journal of Neurophysiology vol 100 no 3 pp 1211ndash12222008
[31] G Klees P Hochstrate and P W Dierkes ldquoSodium-dependentpotassium channels in leech P neuronsrdquo Journal of MembraneBiology vol 208 no 1 pp 27ndash38 2005
[32] Y Zhang M R Brown C Hyland et al ldquoRegulation ofneuronal excitability by interaction of fragile X mental retar-dation protein with Slack potassium channelsrdquo The Journal ofNeuroscience vol 32 pp 15318ndash15327 2012
[33] C Lawrence and G C Rodrigo ldquoA Na+-activated K+ cur-rent (IKNa) is present in guinea-pig but not rat ventricularmyocytesrdquo Pflugers Archiv vol 437 no 6 pp 831ndash838 1999
[34] M Zhou Z Liu C Hu Z Zhang and Y Mei ldquoDevelopmentalregulation of a Na+-activated fast outward K+ current in ratmyoblastsrdquo Cellular Physiology and Biochemistry vol 14 no 4ndash6 pp 225ndash230 2004
[35] C K Young H S Jae M K Tong et al ldquoSodium-activatedpotassium current in guinea pig gastric myocytesrdquo Journal ofKorean Medical Science vol 22 no 1 pp 57ndash62 2007
[36] L Re V Moretti L Rossini and P Giusti ldquoSodium-activatedpotassium current in mouse diaphragmrdquo FEBS Letters vol 270no 1-2 pp 195ndash197 1990
[37] Y Zhang and W G Paterson ldquoFunctional evidence for Na+-activatedK+ channels in circular smoothmuscle of the opossumlower esophageal sphincterrdquo American The Journal of Physiol-ogy vol 292 no 6 pp G1600ndashG1606 2007
[38] M Paulais S Lachheb and J Teulon ldquoANa+- and Clminus-activatedK+ channel in the thick ascending limb of mouse kidneyrdquoJournal of General Physiology vol 127 no 2 pp 205ndash215 2006
[39] T M Egan D Dagan J Kupper and I B Levitan ldquoNa+-activated K+ channels are widely distributed in rat CNS and inXenopus oocytesrdquo Brain Research vol 584 no 1-2 pp 319ndash3211992
ISRN Neuroscience 13
[40] W J Joiner M D Tang L Y Wang et al ldquoFormation ofintermediate-conductance calcium-activated potassium chan-nels by interaction of Slack and Slo subunitsrdquo Nature Neuro-science vol 1 no 6 pp 462ndash469 1998
[41] A Yuan C M Santi A Wei et al ldquoThe sodium-activatedpotassium channel is encoded by a member of the Slo genefamilyrdquo Neuron vol 37 no 5 pp 765ndash773 2003
[42] Y Jiang A Pico M Cadene B T Chait and R MacKinnonldquoStructure of the RCK domain from the E coli K+ channeland demonstration of its presence in the human BK channelrdquoNeuron vol 29 no 3 pp 593ndash601 2001
[43] P Yuan M D Leonetti A R Pico Y Hsiung and R MacK-innon ldquoStructure of the human BK channel Ca2+-activationapparatus at 30 A resolutionrdquo Science vol 329 no 5988 pp182ndash186 2010
[44] Z Zhang A Rosenhouse-Dantsker Q Y Tang S Noskov andD E Logothetis ldquoThe RCK2 domain uses a coordination sitepresent in Kir channels to confer sodium sensitivity to Slo22channelsrdquoThe Journal of Neuroscience vol 30 no 22 pp 7554ndash7562 2010
[45] J L Sui K W Chan and D E Logothetis ldquoNa+ activationof the muscarinic K+ channel by a G-protein-independentmechanismrdquo Journal of General Physiology vol 108 no 5 pp381ndash391 1996
[46] M R Brown J Kronengold V R Gazula et al ldquoAmino-terminiisoforms of the Slack K+ channel regulated by alternative pro-moters differentiallymodulate rhythmic firing and adaptationrdquoThe Journal of Physiology vol 586 no 21 pp 5161ndash5179 2008
[47] A Yuan M Dourado A Butler N Walton A Wei and LSalkoff ldquoSLO-2 a K+ channel with an unusual Clminus dependencerdquoNature Neuroscience vol 3 no 8 pp 771ndash779 2000
[48] M Schreiber and L Salkoff ldquoA novel calcium-sensing domainin the BK channelrdquo Biophysical Journal vol 73 no 3 pp 1355ndash1363 1997
[49] H Chen J Kronengold Y Yan et al ldquoThe N-terminal domainof slack determines the formation and trafficking of slickslackheteromeric sodium-activated potassium channelsrdquo The Jour-nal of Neuroscience vol 29 no 17 pp 5654ndash5665 2009
[50] A Bhattacharjee L Gan and L K Kaczmarek ldquoLocalization ofthe Slack potassium channel in the rat central nervous systemrdquoJournal of Comparative Neurology vol 454 no 3 pp 241ndash2542002
[51] A Bhattacharjee C A A von Hehn X Mei and L KKaczmarek ldquoLocalization of the Na+-activated K+ channel slickin the rat central nervous systemrdquo Journal of ComparativeNeurology vol 484 no 1 pp 80ndash92 2005
[52] D S Koh P Jonas and W Vogel ldquoNa+-activated K+ channelslocalized in the nodal region of myelinated axons of XenopusrdquoThe Journal of Physiology vol 479 no 2 pp 183ndash197 1994
[53] K Mori T Saito Y Masuda and H Nakaya ldquoEffects of classIII antiarrhythmic drugs on the Na+-activated K+ channels inguinea-pig ventricular cellsrdquo British Journal of Pharmacologyvol 119 no 1 pp 133ndash141 1996
[54] K Mori S Kobayashi T Saito Y Masuda and H NakayaldquoInhibitory effects of class I and IV antiarrhythmic drugs onthe Na+- activated K+ channel current in guinea pig ventricularcellsrdquo Naunyn-Schmiedebergrsquos Archives of Pharmacology vol358 no 6 pp 641ndash648 1998
[55] Y Li T Sato and M Arita ldquoBepridil blunts the shortening ofaction potential duration caused by metabolic inhibition viablockade of ATP-sensitive K+ channels and Na+- activated K+
channelsrdquo Journal of Pharmacology and ExperimentalTherapeu-tics vol 291 no 2 pp 562ndash568 1999
[56] B Yang V K Gribkoff J Pan et al ldquoPharmacological activationand inhibition of Slack (Slo2 2) channelsrdquoNeuropharmacologyvol 51 no 4 pp 896ndash906 2006
[57] M D Tejada K Stolpe A K Meinild and D A KlaerkeldquoClofilium inhibits Slick and Slack potassium channelsrdquo Biolog-ics vol 6 pp 465ndash470 2012
[58] B Yang R Desai and L K Kaczmarek ldquoSlack and slick KNachannels regulate the accuracy of timing of auditory neuronsrdquoThe Journal of Neuroscience vol 27 no 10 pp 2617ndash2627 2007
[59] B Biton S Sethuramanujam K E Picchione et al ldquoTheantipsychotic drug loxapine is an opener of the sodium-activated potassium channel slack (Slo2 2)rdquo The Journal ofPharmacology and ExperimentalTherapeutics vol 340 pp 706ndash715 2012
[60] C M Santi G Ferreira B Yang et al ldquoOpposite regulation ofSlick and Slack K+ channels by neuromodulatorsrdquo The Journalof Neuroscience vol 26 no 19 pp 5059ndash5068 2006
[61] M R Fleming and L K Kaczmarek ldquoUse of optical biosensorsto detect modulation of Slack potassium channels by G protein-coupled receptorsrdquo Journal of Receptors and Signal Transduc-tion vol 29 no 3-4 pp 173ndash181 2009
[62] M D Tejada L J Jensen and D A Klaerke ldquoPIP(2) mod-ulation of Slick and Slack K(+) channelsrdquo Biochemical andBiophysical Research Communications vol 424 pp 208ndash2132012
[63] L Zhang M Sukhareva J L Barker et al ldquoDirect bindingof estradiol enhances Slack (sequence like a calcium-activatedpotassium channel) channelsrsquo activityrdquo Neuroscience vol 131no 2 pp 275ndash282 2005
[64] V A Ruffin X Q Gu D Zhou et al ldquoThe sodium-activatedpotassium channel Slack is modulated by hypercapnia andacidosisrdquo Neuroscience vol 151 no 2 pp 410ndash418 2008
[65] M O Nuwer K E Picchione and A Bhattacharjee ldquocAMP-dependent kinase does not modulate the Slack sodium-activated potassium channelrdquo Neuropharmacology vol 57 no3 pp 219ndash226 2009
[66] B C Suh and B Hille ldquoRegulation of ion channels by phos-phatidylinositol 45-bisphosphaterdquo Current Opinion in Neuro-biology vol 15 no 3 pp 370ndash378 2005
[67] L K Kaczmarek ldquoNon-conducting functions of voltage-gatedion channelsrdquo Nature Reviews Neuroscience vol 7 no 10 pp761ndash771 2006
[68] C R Rose ldquoNa+ signals at central synapsesrdquoNeuroscientist vol8 no 6 pp 532ndash539 2002
[69] C R Rose and B R Ransom ldquoRegulation of intracellularsodium in cultured rat hippocampal neuronesrdquo The Journal ofPhysiology vol 499 part 3 pp 573ndash587 1997
[70] C R Rose and A Konnerth ldquoNmda receptor-mediated Na+signals in spines and dendritesrdquoThe Journal of Neuroscience vol21 no 12 pp 4207ndash4214 2001
[71] N Zhong V Beaumont and R S Zucker ldquoRoles for mito-chondrial and reverse mode Na+Ca2+ exchange and the plas-malemma Ca2+ ATPase in post-tetanic potentiation at crayfishneuromuscular junctionsrdquo The Journal of Neuroscience vol 21no 24 pp 9598ndash9607 2001
[72] G Budelli T A Hage AWei et al ldquoNa+-activated K+ channelsexpress a large delayed outward current in neurons duringnormal physiologyrdquoNature Neuroscience vol 12 no 6 pp 745ndash750 2009
14 ISRN Neuroscience
[73] T A Hage and L Salkoff ldquoSodium-activated potassium chan-nels are functionally coupled to persistent sodiumcurrentsrdquoTheJournal of Neuroscience vol 32 pp 2714ndash2721 2012
[74] M RMarkham L K Kaczmarek andHH Zakon ldquoA sodium-activated potassium channe supports high frequency firing andreduces energetic costs during rapid modulations of actionpotential amplituderdquo Journal of Neurophysiology vol 109 no 7pp 1713ndash1723 2013
[75] S Uchino H Wada S Honda et al ldquoSlo2 sodium-activated K+channels bind to the PDZ domain of PSD-95rdquo Biochemical andBiophysical Research Communications vol 310 no 4 pp 1140ndash1147 2003
[76] T Zamalloa C P Bailey and J Pineda ldquoGlutamate-inducedpost-activation inhibition of locus coeruleus neurons is medi-ated by AMPAkainate receptors and sodium-dependent potas-sium currentsrdquo British Journal of Pharmacology vol 156 no 4pp 649ndash661 2009
[77] S Lu P Das D A Fadool and L K Kaczmarek ldquoThe slacksodium-activated potassium channel provides a major outwardcurrent in olfactory neurons of Kv13-- super-smeller micerdquoJournal of Neurophysiology vol 103 no 6 pp 3311ndash3319 2010
[78] R C Foehring P C Schwindt andW E Crill ldquoNorepinephrineselectively reduces slowCa2+- andNa+-mediated K+ currents incat neocortical neuronsrdquo Journal of Neurophysiology vol 61 no2 pp 245ndash256 1989
[79] M Kubota and N Saito ldquoSodium- and calcium-dependentconductances of neurones in the zebra finch hyperstriatumventrale pars caudale in vitrordquo The Journal of Physiology vol440 pp 131ndash142 1991
[80] U Kim and D A Mccormick ldquoFunctional and ionic propertiesof a slow afterhyperpolarization in ferret perigeniculate neuronsin vitrordquo Journal of Neurophysiology vol 80 no 3 pp 1222ndash1235 1998
[81] V M Sandler E Puil and DW F Schwarz ldquoIntrinsic responseproperties of bursting neurons in the nucleus principalistrigemini of the gerbilrdquoNeuroscience vol 83 no 3 pp 891ndash9041998
[82] M V Sanchez-Vives L G Nowak and D A McCormick ldquoCel-lular mechanisms of long-lasting adaptation in visual corticalneurons in vitrordquoThe Journal of Neuroscience vol 20 no 11 pp4286ndash4299 2000
[83] S Franceschetti T Lavazza G Curia et al ldquoNa+-activatedK+ current contributes to postexcitatory hyperpolarization inneocortical intrinsically bursting neuronsrdquo Journal of Neuro-physiology vol 89 no 4 pp 2101ndash2111 2003
[84] V F Descalzo L G Nowak J C Brumberg D A McCormickand M V Sanchez-Vives ldquoSlow adaptation in fast-spikingneurons of visual cortexrdquo Journal of Neurophysiology vol 93no 2 pp 1111ndash1118 2005
[85] J Benda L Maler and A Longtin ldquoLinear versus nonlinearsignal transmission in neuron models with adaptation currentsor dynamic thresholdsrdquo Journal of Neurophysiology vol 104 no5 pp 2806ndash2820 2010
[86] X Liu and L S Leung ldquoSodium-activated potassium conduc-tance participates in the depolarizing afterpotential following asingle action potential in rat hippocampal CA1 pyramidal cellsrdquoBrain Research vol 1023 no 2 pp 185ndash192 2004
[87] L Cangiano P Wallen and S Grillner ldquoRole of apamin-sensitive KCa channels for reticulospinal synaptic transmissionto motoneuron and for the afterhyperpolarizationrdquo Journal ofNeurophysiology vol 88 no 1 pp 289ndash299 2002
[88] Q Y Liu A E Schaffner and J L Barker ldquoKainate induces anintracellular Na+-activated K+ current in cultured embryonicrat hippocampal neuronesrdquo The Journal of Physiology vol 510part 3 pp 721ndash734 1998
[89] I Rishal T Keren-Raifman D Yakubovich et al ldquoNa+ pro-motes the dissociation between G120572GDP and G120573120574 activating Gprotein-gated K+ channelsrdquoThe Journal of Biological Chemistryvol 278 no 6 pp 3840ndash3845 2003
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Research and TreatmentSchizophrenia
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Parkinsonrsquos Disease
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Research and TreatmentAutism
Sleep DisordersHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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ISRN Neuroscience 7
that sustained firing can be recorded throughout a prolongeddepolarization In major part the effects of PKA can beattributed to a decrease in Slack KNa current and suppressionof Slack expression using anti-Slack siRNA produces a reduc-tion in adaptation similar to that produced by PKA activation[23]
In contrast to the effects of PKC on Slack channels thatwere described above the effects of PKA on Slack are indirectand do not involve phosphorylation of the channel subunititself [23 65] Slack channels in HEK-293 cells are insensitiveto cyclic AMP analogs or to the PKA inhibitor KT5720 andapplication of the catalytic subunit of PKA to the cytoplasmicface of Slack channels excised from transfected cells or dorsalroot ganglion neurons has no effect on channel openingInstead activation of PKA decreases Slack current amplitudeby rapidly internalizing Slack channels from the plasmamembrane into internal organelles [23] Such internalizationwas detected by confocal microscopy of neurons expressingSlack channels tagged at their N-terminus with the greenfluorescent protein GFP Elevations of cyclic AMP reducedcolocalization of the labeled channels with a cell surfacemarker A reduction in native Slack channels at the plasmamembrane following PKA activation was also confirmedusing a surface biotinylation assay [23]
94 Modulation of Slack and Slick by PIP2 A variety ofion channels have been shown to be regulated by phos-phatidylinositol 45-biphosphate (PIP2) a phospholipid thatis localized to the inner lipid leaflet of the plasma membrane[66] Exogenously applied PI(45)P2 as well as its isoformPI(34)P2 increases the amplitude of Slack and Slick currentsexpressed in Xenopus oocytes and pharmacological agentsthat are expected to reduce endogenous PIP2 levels reducecurrent amplitude [62] Regulation by PIP2 was found torequire specific lysine residues in the C-termini of thechannelsMutation of these lysines to alanine (K339A in Slackand K306A in Slick) produced functional KNa currents thatwere insensitive to the activating effects of PIP2 [62]
95 Suppression of Slick Channels by Cytoplasmic ATP TheSlick subunit differs from Slack in that it contains a consensusATP binding site just C-terminal to the second RCK domain[4] The current density of Slick currents but not Slackcurrents is reduced by sim80 by the presence of 5mM ATPat the cytoplasmic face of the channels The same full effectis observed with ATP120574S a slowly hydrolyzable ATP analogand the nonhydrolyzable analog AMP-PNP but not withADP Suppression by ATP of Slick currents at all membranevoltages can also be readily detected in excised patches [4]
Confirmation that the effect of ATP on Slick channelsis mediated by the consensus ATP binding site in the C-terminal domain was obtained by site-directed mutagenesisto replace the glycine at residue 1032 in this site with a serine[4]Themutant G1032S Slick channel was fully functional butfailed to decrease channel activity in response to ATP Thesensitivity of Slick channels to ATP levels suggests that theiractivity could be enhanced during periods of high metabolicdemand when cellular ATP levels fall
96 Modulation of Slack by NAD+ KNa channels can read-ily be detected in cell-attached patch-clamp recordings onneurons [5 16 58] This may be surprising given that thecytoplasmic sodium concentration in resting cells is relativelylow (ltsim10mM see Section 10 below) and as described earlierthe EC
50for activation of these channels in excised patches
is relatively high (typically sim40ndash80mM) This discrepancy isresolved by the finding that the open probability and sodiumsensitivity of KNa channels are greatly reduced following exci-sion ofmembrane patches [5 16] suggesting that rundown ofKNa activity in excised patches may result from loss of somecytoplasmic factor
A major cytoplasmic factor that regulates both Slack andSlick channels has been found to be nicotinamide adeninedinucleotideNAD+ [5] Both subunits have a putativeNAD+-binding site within their second RCK domain Application ofNAD+ to the cytoplasmic face of KNa channels excised fromdorsal root ganglion neurons increases their open probabilityand reduces themeasured EC
50for activation of sodium from
sim50mM to sim17mM (Figure 3) NAD+ as well as NADP+but not NADH also increased the open probability of Slackchannels excised from a transfected mammalian cell lineMutation of glycine in the putative NAD+-binding site ofSlack (G792A) resulted in a loss of the ability of NAD+to potentiate activity [5] These findings suggest that KNachannels play a much more prominent role in the normalphysiology of excitable cells than would be suggested by theirproperties in isolated patches
97 Modulation of Slack by Estradiol The open probability ofchannels reconstituted from lipid bilayers from extracts of ratcortex has been found to be increased by 17120573-estradiol [63]Although these channels were not explicitly shown to be KNachannels a similar effect was reported for whole-cell currentsrecorded in Slack-expressing HEK-293 cells Activation wasnot prevented by tamoxifen a blocker of classical estrogenreceptors but may be a consequence of direct binding of 17120573-estradiol to Slack channels themselves [63]
98 Modulation of Slack Channels by Hypoxia Duringhypoxic conditions cytoplasmic levels of internal sodiumand chloride ions as well as those of H+ and CO
2 rise It
has been suggested that activation of KNa channels couldact to hyperpolarize cells during hypoxia providing a pro-tective mechanism [7] This conjecture is supported by thefinding that nematodes lacking the Slack ortholog in thisspecies are significantly more sensitive to hypoxic deaththan are wild-type animals [41] Further support for a rolefor Slack channels in the response to hypoxia comes fromrecordings of Slack channel activity in Xenopus oocytesexposed to increasing concentrations ofH+ andCO
2 demon-
strating that elevations of sodium activate Slack channelsunder cellular conditions comparable to those expected inischemiahypoxia [64]
99 Interactions of Slack with the Fragile X Mental Retarda-tion Protein FMRP Fragile X syndrome the most commoninherited form of intellectual disability in humans is caused
8 ISRN Neuroscience
1
08
06
04
02
0
[Na] (mM)10010
Control (small neurons)NAD
Nor
mal
ized
NP o
Figure 3 NAD+ increases the sodium sensitivity of KNa channelsexcised from adult rat dorsal root ganglion neurons Concentration-response relationship provides estimates of EC
50for sodium activa-
tion of 50mM and 17mM in the absence and presence of NAD+respectively Figure modified from [5]
by loss of expression of the RNA-binding protein FMRP(fragile X mental retardation protein) FMPR is known to berequired for normal activity-dependent protein translationin neurons Yeast two-hybrid and coimmunoprecipitationexperiments have shown that the cytoplasmic C-terminus ofthe Slack subunit interacts with this protein [6] MoreoverSlack-FMRP complexes also contain messenger RNAs thatare targets of FMRP such as the mRNAs encoding Map1band Arc No association of mRNAs with Slack can howeverbe found in mice lacking FMRP indicating that these arebound to Slack despite its interaction with FMRP A peptidearray assay revealed confirmed that FMRP binds selectivelyto sequences at the distal C-terminus of Slack [6]
FMRP is a potent activator of Slack channel activity [6]Application of recombinant FMRP(1ndash298) which containsthemajority of protein-protein interaction domains of FMRPto the cytoplasmic face of Slack channels in excised patchesreversibly increases channel opening by two- to threefold(Figure 4(a)) This is associated with the almost completeelimination of openings to subconductance states which areprominent in untreated patches (Figure 4(b)) FMRP wasfound to have no effect on the channel activity of a Slacktruncation mutant (Slack-BΔ804) which lacks sites found tointeract with FMRP in biochemical assays [6]
Support for a role for FMRP in regulating the amplitude ofnative KNa currents has also come from recordings of neuronsfrom animals lacking the fmr1 gene which encodes FMPR[6] As will be described later KNa channels account for amajor component of potassium current in principal neuronsof the medial nucleus of the trapezoid body (MNTB) Whileloss of FMRP does not affect total levels of Slack proteinthe level of the KNa component of current in these cells issubstantially reduced in the FMRP knockout mice compared
to that in wild-type animals consistent with an activating rolefor this RNA-binding protein [6]
The interaction between Slack and FMRP is likely tobe evolutionarily conserved Both proteins have been foundto be coexpressed in bag cell (BC) neurons which regulatereproductive behaviors in mollusk Aplysia [32] FMRP andSlack immunoreactivity are colocalized at the periphery ofisolated BC neurons and the two proteins can be reciprocallycoimmunoprecipitatedThe native Slack current in these cellswas identified using an siRNA approach and intracellularinjection of FMRP(1ndash298) induced a KNa current that in itsproperties matches the Slack current As for Slack channelsin heterologous expression systems addition of FMRP(1ndash298) to the inside-out patches containing native AplysiaKNa channels increased channel opening In current clamprecordings FMRP(1ndash298) produced a narrowing of actionpotentials
Downregulation of Slack expression in BC neurons usingan siRNA approach had an interesting effect on long-termchanges in the excitability of these cells [32] In untreatedcells brief stimulation of these neurons produces a charac-teristic prolonged (sim30min) discharge that is followed bya prolonged (sim18 hr) inhibitory state Recovery from thisinhibitory state requires new protein synthesis and is blockedby the protein synthesis inhibitor anisomycin Anti-SlacksiRNA did not alter the ability of BC neurons to undergothe normal discharge but like inhibition of protein synthesisprevented recovery from the inhibitory state [32] Althoughother interpretations are possible these findings raise thepossibility that the activity of Slack channels is linked to theregulation of protein translation through the FMRP-RNApathway
As will be described later point mutations in humanSlack channels can produce devastating effects on intellectualdevelopment [2 3] A variety of experiments have demon-strated that activation of ion channels can influence cytoplas-mic processes independently of ion flux through the channels[67] The interaction between the C-terminus of Slack andFMRP raises the hypothesis that conformational changesin the C-terminus could represent such a ldquononconductingrdquofunction of Slack that links neuronal firing to changes inprotein translation
10 Sodium Entry Pathways ThatActivate KNa Channels
Estimates of cytoplasmic sodium concentrations in restingunstimulated neurons obtained using sodium-sensitivemicroelectrodes and ratiometric dyes provide valuesbetween sim4mM and 15mM [68] As noted earlier in thesection on NAD+ regulation of Slack channels this maybe sufficient to provide significant activation of native KNachannels in intact neurons even under resting conditions[5] Internal sodium levels rise in response to stimulationprincipally as a result of entry through voltage-dependentsodium channels and ionotropic ligand-gated receptors suchas AMPA and NMDA glutamate receptors [69] Stimulationparticularly repetitive tetanic stimulation produces local
ISRN Neuroscience 9
Control RecoveryC
1
2
3
C
1
2
3
10pA
2 s
10pA
200ms
S S S S S S S
FMRP(1ndash298)
(a)
2520151050
10000
1000
100
pA
C1
2
3
Recovery
2520151050
10000
1000
100
pA
C1
2
3
S
S
S(N
)
(N)
Control
2520151050
10000
1000
100
pA
C
1 2 3
S
S
S
(N)
FMRP(1ndash298)
(b)
Figure 4 FMRP alters gating of Slack-B channels in inside-out patches (a) Recording of a patch containing 3-4 Slack channels before duringand after transient application of FMRP(1ndash298) to the cytoplasmic face of the patch Top traces show representative 10 s examples of recordingatminus40mVand bottom traces show expanded time views of the same patch Arrowsmarked S indicate the occurrence of subconductance statesthat are suppressed by FMRP(1ndash298) (b) All-points amplitude histograms of Slack channel activity plotted on a logarithmic scale before andafter application of FMRP(1ndash298) and after washout C marks the closed state S indicates subconductance states and numbers on peaks inthe histograms refer to the number of fully open Slack channels Modified from [6]
increases in sodium concentrations that can rise to levelsbetween 45 and 100mM [70 71] In particular high levelscan be attained in restricted compartments such as activedendritic spines
Although the upstroke of action potentials in mostneurons is driven by rapidly-inactivating voltage-dependentsodium currents these currents are transient Evidencesuggest that the much smaller persistent component ofsodium current that does not inactivate with maintaineddepolarization is responsible both for sustained elevationsof internal sodium during stimulation [69] and for selectiveactivation of KNa channels [72 73] It has been found thatin patches excised from somata of rat neurons sodium entrythrough persistent tetrodotoxin-sensitive sodium channels isthe dominant factor that leads to activation of KNa channelsin the same patch [73] Activation of KNa channels by sodiumentry in the isolated patches was enhanced by veratridinewhich prolongs sodium channel openings and was observedin the absence of sodium at the cytoplasmic face of thepatches Moreover activation appeared to be independent ofthe amplitude of the much larger transient sodium current
[73] Although the molecular basis for persistent sodiumchannels is not fully understood further support for a selec-tive activation of KNa currents by entry through persistentchannels has also been found in electrocytes of electric fish inwhich the time course of KNa currents during depolarizationsreflects that of the persistent sodium current and not that ofthe very much larger transient sodium current [74]
A number of studies have documented that KNa currentsare also activated by sodium entry through ionotropic gluta-mate receptors particularly AMPA receptors Slack subunitscan be coimmunoprecipitated with GluR23 AMPA receptorsusing rat brain synaptosomal fractions or lysates of the lam-prey central nervous system [27] Moreover Slack subunitshave been found to bind directly to the PDZ domain of PSD-95 (postsynaptic density-95 protein) a major componentof the postsynaptic density of glutamatergic synapses andto colocalize with PSD-95 in cultures of mouse corticalneurons [75] In electrophysiological studies of lampreyneurons application of AMPA was been found to activate aKNa current with pharmacological properties of Slack [25]Activation of this KNa current regulates both the amplitude
10 ISRN Neuroscience
and the decay rate of AMPA-induced synaptic currents pro-viding a feedback loop that decreases the size of glutamatergicpostsynaptic potentials [27]
Activation of KNa currents following activation of AMPAreceptors has also been proposed in studies of neurons of thelocus coeruleus to which application of glutamate producesa burst of action potentials followed by period of inhibitionlasting 30ndash45 seconds [76] The potassium current responsi-ble for the period of inhibition was selectively activated byAMPA or the AMPA-receptor agonist quisqualate but not byother stimuli that excite these neurons Consistent with thiscurrent being a KNa current it was attenuated by lowering ofexternal sodium concentrations but not affected by externalcalcium levels and it was potentiated by the KNa channelopener bithionol [76]
While persistent voltage-dependent sodium channels andAMPA receptors may represent the primary sodium entrypathways for activation of KNa channels there exist otherpathways that allow sodium ions to enter excitable cellsand whose role has been less explored Slack subunits areabundantly expressed in mitral cells of the olfactory bulb[46 50] In these neurons deletion of the gene for a majorvoltage-dependent potassiumchannelKv13 produces a com-pensatory increase in levels of Slack-B protein and in SlackKNa currents identified using an anti-Slack siRNA technique[77] Activation of the KNa current during depolarizationof these cells could only be significantly suppressed by acombination of tetrodotoxin to block voltage-dependentsodium channels and ZD7288 a blocker of H-channelswhich are nonselective cation channels that flux both sodiumand potassium ions Either drug alone failed to significantlysuppress KNa currents suggesting that sodium entry throughH-channels plays a role in KNa channel activation in theseneurons [77]
11 Contribution of KNa Channels toNeuronal Firing Patterns
The component of whole-cell potassium current in neuronsthat can be attributed to Slack channel subunits has beenidentified in mitral cells of the olfactory bulb and mediumspiny neuron of the striatum using siRNAs to suppress trans-lation of SlackmRNA [72 77] Such experiments indicate thatSlack KNa channels are responsible for a very major compo-nent of the total delayed outward current in neurons underphysiological conditions [72] The specific contributions thatKNa channels make to the electrical personality of individualtypes of neurons may differ in different cell types
One of the earliest effects to be described for activationof KNa currents is to produce a slow afterhyperpolarization(sAHP) following a burst of action potentials This was firstdescribed in pyramidal neurons of the cat sensorimotorcortex [11 78] and similar slow sodium-dependent sAHPsthat endure for many seconds have been found in a widevariety of neuronal types [79ndash84]These have been the subjectof earlier reviews [20 21] Such slow potentials have beenfound to influence the timing of regular bursts of actionpotentials such as the 3ndash12 per minute spindle waves that
occur at the onset of sleep in certain neurons within thethalamus [80] Slow activation of KNa during repetitive firingalso produces an adaptation of firing rate during bursts ofaction potentials [23] and such adaptation is predicted bynumerical simulations that incorporate KNa currents [46 85]
In some neurons such as neocortical pyramidal neuronsand hippocampal CA1 neurons a significant amount of KNacurrent can also be activated by single spikes and contributeto action potentials repolarization or to the amplitude ofdepolarizing afterpotentials that follow single spikes andare generated by persistent sodium currents [83 86] Inthese cases KNa current activation may be detected withinsim5msec following a single-action potential A recent studyhas examined themechanismbywhich the cells of the electricorgan (electrocytes) of certain electric fish are able to generatesustained firing at frequencies greater than 500Hz [74] Slackchannels are expressed at high levels in these cells and theirdelayed potassium current is almost exclusively a KNa currentthat is activated by a persistent sodium current Numericalsimulations have indicated that the use of KNa current torepolarize the very large action potentials of these cells allowsthe cells to fire at such high rates with minimal energyexpenditure [74]
A set of neurons that have been particularly instructivein studies of the role of KNa channels in shaping neuronalfiring are those that control locomotion in the spinal cord oflamprey [24ndash28 87] In addition to the transient KNa currentthat is activated by AMPA receptors and that was describedearlier [25 27] these neurons appear to have two distinctcomponents ofKNa current that are triggered by action poten-tial firingmdasha transient component that follows a single spikeand a slower more sustained component [24 26 87] Therapid transient KNa current is activated by the sodium influxthrough tetrodotoxin-sensitive channel during the actionpotentials and may play a role in determining the amplitudeand duration of individual action potentials The sustainedKNa current is not abolished by tetrodotoxin suggesting thatit is selectively activated by an independent sodium entrypathway and may underlie the calcium-independent sAHPthat occurs during repetitive bursting in lamprey spinal cordneurons [24]
Experiments with neurons that fire at high rates withvery high temporal accuracy have provided another potentialbiological role for KNa channels [58] Slack and Slick areexpressed at high levels in neurons of the medial nucleusof the trapezoid (MNTB) within the auditory brainstem[6 50 51] These neurons fire at rates up to sim800Hzand accurate timing of their action potentials is requiredfor transmission of information about the localization ofsound stimuli KNa channels are readily be detected in bothcell-attached and excised patches on the somata of MNTBneurons and they are localized appropriately for activationby sodium influx through AMPA receptors or voltage-dependent sodium channels [58] Current-clamp recordingsfrom MNTB neurons and numerical simulations indicatethat as KNa channels become activated temporal accuracyof firing is improved This occurs because the voltage-dependence of Slack and Slick channels allows them to opennear the resting potential of MNTB neurons increasing
ISRN Neuroscience 11
the resting conductance and decreasing the membrane timeconstant An increase in temporal fidelity of MNTB neuronscan also be produced by treatment with the pharmacologicalKNa activator bithionol [58]
12 Mutations in Slack Channels Result inEpilepsy and Intellectual Disabilities
De novo human mutations that produce single-amino-acidalterations in Slack channels produce devastating effectson development and intellectual function [2 3] One setof Slack mutations results in malignant migrating partialseizures of infancy (MMPSI) [2]This disease is characterizedby pharmacoresistant seizures in the first 6 months of lifethat arise randomly and ldquomigraterdquo from one brain regionto another While the occurrence of seizures abates withage MMPSI results in near-total arrest of psychomotordevelopment Exome sequencing revealed that at least 50of MMPSI patients have mutations in the large cytoplasmicC-terminal domain of Slack [2]
Two mutations in the human Slack gene that produceMMPSI are R428Q and A934T which correspond to themutations R409Q and A913T in the highly conserved ratSlack sequence [2] Expression of these mutants in Xenopusoocytes produced Slack currents with voltage dependenceand kinetic behavior that matched that of wild-type Slackchannels but with a current amplitude that was increased 2-3-fold over that of wild-type Slack This increase in currentlikely results because the mutations produce channels thatare either more prone to undergo phosphorylation by PKC(or less likely to be dephosphorylated) or that mimic theconformation induced by PKC activation One of theMMPSImutations R409Q lies adjacent to the phosphorylation siteS407 that as described earlier enhances current by 2-3-fold when phosphorylated by PKC Consistent with thisinterpretation current amplitude of the two MMPSI muta-tions could not be further increased upon PKC activa-tion [2] In other respects the MMPSI currents resembledthat of wild-type channels with unitary conductance andsodiumdependencesmatching those of wild-type Slack BothMMPSI mutant channels however had reduced openings tosubconductance states compared to the wild-type channels[2]
A second disease that has been attributed to a differentset of mutations in the C-terminal domain of Slack chan-nels is autosomal dominant nocturnal frontal lobe epilepsy(ADNFLE) [3] This is a childhood-onset focal epilepsysyndrome in which motor seizures arise during sleep Onsetof ADNFLE in the patients with Slack mutations occurred ata mean age of 6 years and was associated with mild to severeintellectual disability andor severe psychiatric problemsTheeffects of these ADNFLE mutations on the function of Slackchannels have however not been investigated Interestinglyanother known genetic cause of ADNFLE is mutations insubunits of the neuronal nicotinic acetycholine receptor Theacetycholine receptor ADNFLE mutations do not howeverproduce intellectual disability which contrasts strikingly tothe effects of the Slack mutations [3]
It is possible that the very severe outcomes of mutationsin Slack occur solely because of changes in the amplitudeof Slack currents An increase in total potassium currentin rapidly firing interneurons could reduce the firing rateof these cells predisposing the nervous system to hyper-excitability The relatively modest increase in current thatoccurs with theMMPSImutations [2] however suggests thatother processes may also be affected As described earlierthe large C-terminal cytoplasmic domain of Slack in whichMMPSI and ADNFLE mutations are located interacts withFMRP and its cargo mRNAs [6] Thus it is possible thatin addition to directly altering the excitability of centralneuronsmutations in Slack disrupt the regulation of activity-dependent protein translation in neurons
13 Conclusions
This review has covered themolecular and cellular propertiesof Slack and Slick two potassium channel subunits thatprovide neurons and other cells with KNa channels It shouldbe pointed out that additional potassium channels that areactivated by cytoplasmic sodiummay also exist For examplesome of the native KNa currents that have been described inneurons do not match Slack or Slick in their single-channelconductances or pharmacological properties [88]These mayperhaps represent inward rectifier potassium channels thatare activated by sodium [45 89] and the physiological rolesof the sodium sensitivity of these channels require furtherinvestigation Nevertheless Slack and Slick which are verywidely expressed in the brain appear to play dominant rolesin shaping the firing properties of verymany types of neuronsHumanmutations in these channels produce profound effectson neuronal function and development suggesting that per-haps their biological role may extend beyond simply settingthe level of neuronal excitability and that these channels mayinfluence cytoplasmic biochemical pathways that regulatedevelopment plasticity and intellectual function
References
[1] B Hille Ionic Channels of Excitable Membranes Sinauer Asso-ciates Sunderland Mass USA 3rd edition 2001
[2] G Barcia M R Fleming A Deligniere et al ldquoDe novo gain-of-function KCNT1 channel mutations cause malignant migratingpartial seizures of infancyrdquo Nature Genetics vol 44 pp 1255ndash1259 2012
[3] S E Heron K R SmithM Bahlo et al ldquoMissensemutations inthe sodium-gated potassium channel gene KCNT1 cause severeautosomal dominant nocturnal frontal lobe epilepsyrdquo NatureGenetics vol 44 pp 1188ndash1190 2012
[4] A Bhattacharjee W J Joiner M Wu Y Yang F J Sigworthand L K Kaczmarek ldquoSlick (Slo2 1) a rapidly-gating sodium-activated potassium channel inhibited by ATPrdquo The Journal ofNeuroscience vol 23 no 37 pp 11681ndash11691 2003
[5] T J Tamsett K E Picchione and A Bhattacharjee ldquoNAD+activates KNa channels in dorsal root ganglion neuronsrdquo TheJournal of Neuroscience vol 29 no 16 pp 5127ndash5134 2009
12 ISRN Neuroscience
[6] M R Brown J Kronengold V R Gazula et al ldquoFragileX mental retardation protein controls gating of the sodium-activated potassium channel SlackrdquoNatureNeuroscience vol 13no 7 pp 819ndash821 2010
[7] M Kameyama M Kakei and R Sato ldquoIntracellular Na+activates a K+ channel in mammalian cardiac cellsrdquoNature vol309 no 5966 pp 354ndash356 1984
[8] C R Bader L Bernheim and D Bertrand ldquoSodium-activatedpotassium current in cultured avian neuronesrdquoNature vol 317no 6037 pp 540ndash542 1985
[9] K Hartung ldquoPotentiation of a transient outward current byNa+influx in crayfish neuronesrdquo Pflugers Archiv vol 404 no 1 pp41ndash44 1985
[10] S E Dryer J T Fujii and A R Martin ldquoA Na+-activatedK+ current in cultured brain stem neurones from chicksrdquo TheJournal of Physiology vol 410 pp 283ndash296 1989
[11] P C Schwindt W J Spain and W E Crill ldquoLong-lastingreduction of excitability by a sodium-dependent potassiumcurrent in cat neocortical neuronsrdquo Journal of Neurophysiologyvol 61 no 2 pp 233ndash244 1989
[12] C Haimann L Bernheim D Bertrand and C R BaderldquoPotassium current activated by intracellular sodium in quailtrigeminal ganglion neuronsrdquo Journal of General Physiology vol95 no 5 pp 961ndash979 1990
[13] S E Dryer ldquoNa+-activated K+ channels and voltage-evokedionic currents in brain stem and parasympathetic neurones ofthe chickrdquoThe Journal of Physiology vol 435 pp 513ndash532 1991
[14] M Saito and C F Wu ldquoExpression of ion channels andmutational effects in giant Drosophila neurons differentiatedfrom cell division-arrested embryonic neuroblastsrdquoThe Journalof Neuroscience vol 11 no 7 pp 2135ndash2150 1991
[15] T M Egan D Dagan J Kupper and I B Levitan ldquoPropertiesand rundown of sodium-activated potassium channels in ratolfactory bulb neuronsrdquoThe Journal of Neuroscience vol 12 no5 pp 1964ndash1976 1992
[16] C Haimann J Magistretti and B Pozzi ldquoSodium-activatedpotassium current in sensory neurons a comparison of cell-attached and cell-free single-channel activitiesrdquo Pflugers Archivvol 422 no 3 pp 287ndash294 1992
[17] N Dale ldquoA large sustained Na+- and voltage-dependent K+current in spinal neurons of the frog embryordquo The Journal ofPhysiology vol 462 pp 349ndash372 1993
[18] B V Safronov and W Vogel ldquoProperties and functions of Na+-activated K+ channels in the soma of rat motoneuronesrdquo TheJournal of Physiology vol 497 no 3 pp 727ndash734 1996
[19] U Bischoff W Vogel and B V Safronov ldquoNa+-activated K+channels in small dorsal root ganglion neurones of ratrdquo TheJournal of Physiology vol 510 part 3 pp 743ndash754 1998
[20] S E Dryer ldquoNa+-activated K+ channels a new family of large-conductance ion channelsrdquo Trends in Neurosciences vol 17 no4 pp 155ndash160 1994
[21] A Bhattacharjee and L K Kaczmarek ldquoFor K+ channels Na+ isthe new Ca2+ rdquo Trends in Neurosciences vol 28 no 8 pp 422ndash428 2005
[22] S B Gao Y Wu C X Lu Z H Guo C H Li and J P DingldquoSlack and Slick KNa channels are required for the depolarizingafterpotential of acutely isolated medium diameter rat dorsalroot ganglion neuronsrdquoActa Pharmacologica Sinica vol 29 no8 pp 899ndash905 2008
[23] M O Nuwer K E Picchione and A Bhattacharjee ldquoPKA-induced internalization of Slack KNa channels produces dorsal
root ganglion neuron hyperexcitabilityrdquo The Journal of Neuro-science vol 30 no 42 pp 14165ndash14172 2010
[24] D Hess E Nanou and A El Manira ldquoCharacterization of Na+-activated K+ currents in larval lamprey spinal cord neuronsrdquoJournal of Neurophysiology vol 97 no 5 pp 3484ndash3493 2007
[25] E Nanou and A El Manira ldquoA postsynaptic negative feedbackmediated by coupling between AMPA receptors and Na+-activated K+ channels in spinal cord neuronesrdquo EuropeanJournal of Neuroscience vol 25 no 2 pp 445ndash450 2007
[26] PWallen B Robertson L Cangiano et al ldquoSodium-dependentpotassium channels of a Slack-like subtype contribute to theslow afterhyperpolarization in lamprey spinal neuronsrdquo TheJournal of Physiology vol 585 no 1 pp 75ndash90 2007
[27] E Nanou A Kyriakatos A Bhattacharjee L K Kaczmarek GParatcha and A El Manira ldquoNa+-mediated coupling betweenAMPA receptors and KNa channels shapes synaptic transmis-sionrdquo Proceedings of the National Academy of Sciences of theUnited States of America vol 105 no 52 pp 20941ndash20946 2008
[28] E Nanou and A El Manira ldquoMechanisms of modulation ofAMPA-induced Na+-activated K+ current by mGluR1rdquo Journalof Neurophysiology vol 103 no 1 pp 441ndash445 2010
[29] A R Mercer and J G Hildebrand ldquoDevelopmental changes inthe density of ionic currents in antennal-lobe neurons of thesphinx moth Manduca sextardquo Journal of Neurophysiology vol87 no 6 pp 2664ndash2675 2002
[30] K Aoki K Kosakai and M Yoshino ldquoMonoaminergic modu-lation of the Na+-activated K+ channel in kenyon cells isolatedfrom the mushroom body of the cricket (Gryllus bimaculatus)brainrdquo Journal of Neurophysiology vol 100 no 3 pp 1211ndash12222008
[31] G Klees P Hochstrate and P W Dierkes ldquoSodium-dependentpotassium channels in leech P neuronsrdquo Journal of MembraneBiology vol 208 no 1 pp 27ndash38 2005
[32] Y Zhang M R Brown C Hyland et al ldquoRegulation ofneuronal excitability by interaction of fragile X mental retar-dation protein with Slack potassium channelsrdquo The Journal ofNeuroscience vol 32 pp 15318ndash15327 2012
[33] C Lawrence and G C Rodrigo ldquoA Na+-activated K+ cur-rent (IKNa) is present in guinea-pig but not rat ventricularmyocytesrdquo Pflugers Archiv vol 437 no 6 pp 831ndash838 1999
[34] M Zhou Z Liu C Hu Z Zhang and Y Mei ldquoDevelopmentalregulation of a Na+-activated fast outward K+ current in ratmyoblastsrdquo Cellular Physiology and Biochemistry vol 14 no 4ndash6 pp 225ndash230 2004
[35] C K Young H S Jae M K Tong et al ldquoSodium-activatedpotassium current in guinea pig gastric myocytesrdquo Journal ofKorean Medical Science vol 22 no 1 pp 57ndash62 2007
[36] L Re V Moretti L Rossini and P Giusti ldquoSodium-activatedpotassium current in mouse diaphragmrdquo FEBS Letters vol 270no 1-2 pp 195ndash197 1990
[37] Y Zhang and W G Paterson ldquoFunctional evidence for Na+-activatedK+ channels in circular smoothmuscle of the opossumlower esophageal sphincterrdquo American The Journal of Physiol-ogy vol 292 no 6 pp G1600ndashG1606 2007
[38] M Paulais S Lachheb and J Teulon ldquoANa+- and Clminus-activatedK+ channel in the thick ascending limb of mouse kidneyrdquoJournal of General Physiology vol 127 no 2 pp 205ndash215 2006
[39] T M Egan D Dagan J Kupper and I B Levitan ldquoNa+-activated K+ channels are widely distributed in rat CNS and inXenopus oocytesrdquo Brain Research vol 584 no 1-2 pp 319ndash3211992
ISRN Neuroscience 13
[40] W J Joiner M D Tang L Y Wang et al ldquoFormation ofintermediate-conductance calcium-activated potassium chan-nels by interaction of Slack and Slo subunitsrdquo Nature Neuro-science vol 1 no 6 pp 462ndash469 1998
[41] A Yuan C M Santi A Wei et al ldquoThe sodium-activatedpotassium channel is encoded by a member of the Slo genefamilyrdquo Neuron vol 37 no 5 pp 765ndash773 2003
[42] Y Jiang A Pico M Cadene B T Chait and R MacKinnonldquoStructure of the RCK domain from the E coli K+ channeland demonstration of its presence in the human BK channelrdquoNeuron vol 29 no 3 pp 593ndash601 2001
[43] P Yuan M D Leonetti A R Pico Y Hsiung and R MacK-innon ldquoStructure of the human BK channel Ca2+-activationapparatus at 30 A resolutionrdquo Science vol 329 no 5988 pp182ndash186 2010
[44] Z Zhang A Rosenhouse-Dantsker Q Y Tang S Noskov andD E Logothetis ldquoThe RCK2 domain uses a coordination sitepresent in Kir channels to confer sodium sensitivity to Slo22channelsrdquoThe Journal of Neuroscience vol 30 no 22 pp 7554ndash7562 2010
[45] J L Sui K W Chan and D E Logothetis ldquoNa+ activationof the muscarinic K+ channel by a G-protein-independentmechanismrdquo Journal of General Physiology vol 108 no 5 pp381ndash391 1996
[46] M R Brown J Kronengold V R Gazula et al ldquoAmino-terminiisoforms of the Slack K+ channel regulated by alternative pro-moters differentiallymodulate rhythmic firing and adaptationrdquoThe Journal of Physiology vol 586 no 21 pp 5161ndash5179 2008
[47] A Yuan M Dourado A Butler N Walton A Wei and LSalkoff ldquoSLO-2 a K+ channel with an unusual Clminus dependencerdquoNature Neuroscience vol 3 no 8 pp 771ndash779 2000
[48] M Schreiber and L Salkoff ldquoA novel calcium-sensing domainin the BK channelrdquo Biophysical Journal vol 73 no 3 pp 1355ndash1363 1997
[49] H Chen J Kronengold Y Yan et al ldquoThe N-terminal domainof slack determines the formation and trafficking of slickslackheteromeric sodium-activated potassium channelsrdquo The Jour-nal of Neuroscience vol 29 no 17 pp 5654ndash5665 2009
[50] A Bhattacharjee L Gan and L K Kaczmarek ldquoLocalization ofthe Slack potassium channel in the rat central nervous systemrdquoJournal of Comparative Neurology vol 454 no 3 pp 241ndash2542002
[51] A Bhattacharjee C A A von Hehn X Mei and L KKaczmarek ldquoLocalization of the Na+-activated K+ channel slickin the rat central nervous systemrdquo Journal of ComparativeNeurology vol 484 no 1 pp 80ndash92 2005
[52] D S Koh P Jonas and W Vogel ldquoNa+-activated K+ channelslocalized in the nodal region of myelinated axons of XenopusrdquoThe Journal of Physiology vol 479 no 2 pp 183ndash197 1994
[53] K Mori T Saito Y Masuda and H Nakaya ldquoEffects of classIII antiarrhythmic drugs on the Na+-activated K+ channels inguinea-pig ventricular cellsrdquo British Journal of Pharmacologyvol 119 no 1 pp 133ndash141 1996
[54] K Mori S Kobayashi T Saito Y Masuda and H NakayaldquoInhibitory effects of class I and IV antiarrhythmic drugs onthe Na+- activated K+ channel current in guinea pig ventricularcellsrdquo Naunyn-Schmiedebergrsquos Archives of Pharmacology vol358 no 6 pp 641ndash648 1998
[55] Y Li T Sato and M Arita ldquoBepridil blunts the shortening ofaction potential duration caused by metabolic inhibition viablockade of ATP-sensitive K+ channels and Na+- activated K+
channelsrdquo Journal of Pharmacology and ExperimentalTherapeu-tics vol 291 no 2 pp 562ndash568 1999
[56] B Yang V K Gribkoff J Pan et al ldquoPharmacological activationand inhibition of Slack (Slo2 2) channelsrdquoNeuropharmacologyvol 51 no 4 pp 896ndash906 2006
[57] M D Tejada K Stolpe A K Meinild and D A KlaerkeldquoClofilium inhibits Slick and Slack potassium channelsrdquo Biolog-ics vol 6 pp 465ndash470 2012
[58] B Yang R Desai and L K Kaczmarek ldquoSlack and slick KNachannels regulate the accuracy of timing of auditory neuronsrdquoThe Journal of Neuroscience vol 27 no 10 pp 2617ndash2627 2007
[59] B Biton S Sethuramanujam K E Picchione et al ldquoTheantipsychotic drug loxapine is an opener of the sodium-activated potassium channel slack (Slo2 2)rdquo The Journal ofPharmacology and ExperimentalTherapeutics vol 340 pp 706ndash715 2012
[60] C M Santi G Ferreira B Yang et al ldquoOpposite regulation ofSlick and Slack K+ channels by neuromodulatorsrdquo The Journalof Neuroscience vol 26 no 19 pp 5059ndash5068 2006
[61] M R Fleming and L K Kaczmarek ldquoUse of optical biosensorsto detect modulation of Slack potassium channels by G protein-coupled receptorsrdquo Journal of Receptors and Signal Transduc-tion vol 29 no 3-4 pp 173ndash181 2009
[62] M D Tejada L J Jensen and D A Klaerke ldquoPIP(2) mod-ulation of Slick and Slack K(+) channelsrdquo Biochemical andBiophysical Research Communications vol 424 pp 208ndash2132012
[63] L Zhang M Sukhareva J L Barker et al ldquoDirect bindingof estradiol enhances Slack (sequence like a calcium-activatedpotassium channel) channelsrsquo activityrdquo Neuroscience vol 131no 2 pp 275ndash282 2005
[64] V A Ruffin X Q Gu D Zhou et al ldquoThe sodium-activatedpotassium channel Slack is modulated by hypercapnia andacidosisrdquo Neuroscience vol 151 no 2 pp 410ndash418 2008
[65] M O Nuwer K E Picchione and A Bhattacharjee ldquocAMP-dependent kinase does not modulate the Slack sodium-activated potassium channelrdquo Neuropharmacology vol 57 no3 pp 219ndash226 2009
[66] B C Suh and B Hille ldquoRegulation of ion channels by phos-phatidylinositol 45-bisphosphaterdquo Current Opinion in Neuro-biology vol 15 no 3 pp 370ndash378 2005
[67] L K Kaczmarek ldquoNon-conducting functions of voltage-gatedion channelsrdquo Nature Reviews Neuroscience vol 7 no 10 pp761ndash771 2006
[68] C R Rose ldquoNa+ signals at central synapsesrdquoNeuroscientist vol8 no 6 pp 532ndash539 2002
[69] C R Rose and B R Ransom ldquoRegulation of intracellularsodium in cultured rat hippocampal neuronesrdquo The Journal ofPhysiology vol 499 part 3 pp 573ndash587 1997
[70] C R Rose and A Konnerth ldquoNmda receptor-mediated Na+signals in spines and dendritesrdquoThe Journal of Neuroscience vol21 no 12 pp 4207ndash4214 2001
[71] N Zhong V Beaumont and R S Zucker ldquoRoles for mito-chondrial and reverse mode Na+Ca2+ exchange and the plas-malemma Ca2+ ATPase in post-tetanic potentiation at crayfishneuromuscular junctionsrdquo The Journal of Neuroscience vol 21no 24 pp 9598ndash9607 2001
[72] G Budelli T A Hage AWei et al ldquoNa+-activated K+ channelsexpress a large delayed outward current in neurons duringnormal physiologyrdquoNature Neuroscience vol 12 no 6 pp 745ndash750 2009
14 ISRN Neuroscience
[73] T A Hage and L Salkoff ldquoSodium-activated potassium chan-nels are functionally coupled to persistent sodiumcurrentsrdquoTheJournal of Neuroscience vol 32 pp 2714ndash2721 2012
[74] M RMarkham L K Kaczmarek andHH Zakon ldquoA sodium-activated potassium channe supports high frequency firing andreduces energetic costs during rapid modulations of actionpotential amplituderdquo Journal of Neurophysiology vol 109 no 7pp 1713ndash1723 2013
[75] S Uchino H Wada S Honda et al ldquoSlo2 sodium-activated K+channels bind to the PDZ domain of PSD-95rdquo Biochemical andBiophysical Research Communications vol 310 no 4 pp 1140ndash1147 2003
[76] T Zamalloa C P Bailey and J Pineda ldquoGlutamate-inducedpost-activation inhibition of locus coeruleus neurons is medi-ated by AMPAkainate receptors and sodium-dependent potas-sium currentsrdquo British Journal of Pharmacology vol 156 no 4pp 649ndash661 2009
[77] S Lu P Das D A Fadool and L K Kaczmarek ldquoThe slacksodium-activated potassium channel provides a major outwardcurrent in olfactory neurons of Kv13-- super-smeller micerdquoJournal of Neurophysiology vol 103 no 6 pp 3311ndash3319 2010
[78] R C Foehring P C Schwindt andW E Crill ldquoNorepinephrineselectively reduces slowCa2+- andNa+-mediated K+ currents incat neocortical neuronsrdquo Journal of Neurophysiology vol 61 no2 pp 245ndash256 1989
[79] M Kubota and N Saito ldquoSodium- and calcium-dependentconductances of neurones in the zebra finch hyperstriatumventrale pars caudale in vitrordquo The Journal of Physiology vol440 pp 131ndash142 1991
[80] U Kim and D A Mccormick ldquoFunctional and ionic propertiesof a slow afterhyperpolarization in ferret perigeniculate neuronsin vitrordquo Journal of Neurophysiology vol 80 no 3 pp 1222ndash1235 1998
[81] V M Sandler E Puil and DW F Schwarz ldquoIntrinsic responseproperties of bursting neurons in the nucleus principalistrigemini of the gerbilrdquoNeuroscience vol 83 no 3 pp 891ndash9041998
[82] M V Sanchez-Vives L G Nowak and D A McCormick ldquoCel-lular mechanisms of long-lasting adaptation in visual corticalneurons in vitrordquoThe Journal of Neuroscience vol 20 no 11 pp4286ndash4299 2000
[83] S Franceschetti T Lavazza G Curia et al ldquoNa+-activatedK+ current contributes to postexcitatory hyperpolarization inneocortical intrinsically bursting neuronsrdquo Journal of Neuro-physiology vol 89 no 4 pp 2101ndash2111 2003
[84] V F Descalzo L G Nowak J C Brumberg D A McCormickand M V Sanchez-Vives ldquoSlow adaptation in fast-spikingneurons of visual cortexrdquo Journal of Neurophysiology vol 93no 2 pp 1111ndash1118 2005
[85] J Benda L Maler and A Longtin ldquoLinear versus nonlinearsignal transmission in neuron models with adaptation currentsor dynamic thresholdsrdquo Journal of Neurophysiology vol 104 no5 pp 2806ndash2820 2010
[86] X Liu and L S Leung ldquoSodium-activated potassium conduc-tance participates in the depolarizing afterpotential following asingle action potential in rat hippocampal CA1 pyramidal cellsrdquoBrain Research vol 1023 no 2 pp 185ndash192 2004
[87] L Cangiano P Wallen and S Grillner ldquoRole of apamin-sensitive KCa channels for reticulospinal synaptic transmissionto motoneuron and for the afterhyperpolarizationrdquo Journal ofNeurophysiology vol 88 no 1 pp 289ndash299 2002
[88] Q Y Liu A E Schaffner and J L Barker ldquoKainate induces anintracellular Na+-activated K+ current in cultured embryonicrat hippocampal neuronesrdquo The Journal of Physiology vol 510part 3 pp 721ndash734 1998
[89] I Rishal T Keren-Raifman D Yakubovich et al ldquoNa+ pro-motes the dissociation between G120572GDP and G120573120574 activating Gprotein-gated K+ channelsrdquoThe Journal of Biological Chemistryvol 278 no 6 pp 3840ndash3845 2003
Submit your manuscripts athttpwwwhindawicom
Neurology Research International
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BioMed Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentSchizophrenia
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neural Plasticity
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Parkinsonrsquos Disease
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentAutism
Sleep DisordersHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neuroscience Journal
Epilepsy Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Psychiatry Journal
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Depression Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Neurodegenerative Diseases
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Cardiovascular Psychiatry and NeurologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
8 ISRN Neuroscience
1
08
06
04
02
0
[Na] (mM)10010
Control (small neurons)NAD
Nor
mal
ized
NP o
Figure 3 NAD+ increases the sodium sensitivity of KNa channelsexcised from adult rat dorsal root ganglion neurons Concentration-response relationship provides estimates of EC
50for sodium activa-
tion of 50mM and 17mM in the absence and presence of NAD+respectively Figure modified from [5]
by loss of expression of the RNA-binding protein FMRP(fragile X mental retardation protein) FMPR is known to berequired for normal activity-dependent protein translationin neurons Yeast two-hybrid and coimmunoprecipitationexperiments have shown that the cytoplasmic C-terminus ofthe Slack subunit interacts with this protein [6] MoreoverSlack-FMRP complexes also contain messenger RNAs thatare targets of FMRP such as the mRNAs encoding Map1band Arc No association of mRNAs with Slack can howeverbe found in mice lacking FMRP indicating that these arebound to Slack despite its interaction with FMRP A peptidearray assay revealed confirmed that FMRP binds selectivelyto sequences at the distal C-terminus of Slack [6]
FMRP is a potent activator of Slack channel activity [6]Application of recombinant FMRP(1ndash298) which containsthemajority of protein-protein interaction domains of FMRPto the cytoplasmic face of Slack channels in excised patchesreversibly increases channel opening by two- to threefold(Figure 4(a)) This is associated with the almost completeelimination of openings to subconductance states which areprominent in untreated patches (Figure 4(b)) FMRP wasfound to have no effect on the channel activity of a Slacktruncation mutant (Slack-BΔ804) which lacks sites found tointeract with FMRP in biochemical assays [6]
Support for a role for FMRP in regulating the amplitude ofnative KNa currents has also come from recordings of neuronsfrom animals lacking the fmr1 gene which encodes FMPR[6] As will be described later KNa channels account for amajor component of potassium current in principal neuronsof the medial nucleus of the trapezoid body (MNTB) Whileloss of FMRP does not affect total levels of Slack proteinthe level of the KNa component of current in these cells issubstantially reduced in the FMRP knockout mice compared
to that in wild-type animals consistent with an activating rolefor this RNA-binding protein [6]
The interaction between Slack and FMRP is likely tobe evolutionarily conserved Both proteins have been foundto be coexpressed in bag cell (BC) neurons which regulatereproductive behaviors in mollusk Aplysia [32] FMRP andSlack immunoreactivity are colocalized at the periphery ofisolated BC neurons and the two proteins can be reciprocallycoimmunoprecipitatedThe native Slack current in these cellswas identified using an siRNA approach and intracellularinjection of FMRP(1ndash298) induced a KNa current that in itsproperties matches the Slack current As for Slack channelsin heterologous expression systems addition of FMRP(1ndash298) to the inside-out patches containing native AplysiaKNa channels increased channel opening In current clamprecordings FMRP(1ndash298) produced a narrowing of actionpotentials
Downregulation of Slack expression in BC neurons usingan siRNA approach had an interesting effect on long-termchanges in the excitability of these cells [32] In untreatedcells brief stimulation of these neurons produces a charac-teristic prolonged (sim30min) discharge that is followed bya prolonged (sim18 hr) inhibitory state Recovery from thisinhibitory state requires new protein synthesis and is blockedby the protein synthesis inhibitor anisomycin Anti-SlacksiRNA did not alter the ability of BC neurons to undergothe normal discharge but like inhibition of protein synthesisprevented recovery from the inhibitory state [32] Althoughother interpretations are possible these findings raise thepossibility that the activity of Slack channels is linked to theregulation of protein translation through the FMRP-RNApathway
As will be described later point mutations in humanSlack channels can produce devastating effects on intellectualdevelopment [2 3] A variety of experiments have demon-strated that activation of ion channels can influence cytoplas-mic processes independently of ion flux through the channels[67] The interaction between the C-terminus of Slack andFMRP raises the hypothesis that conformational changesin the C-terminus could represent such a ldquononconductingrdquofunction of Slack that links neuronal firing to changes inprotein translation
10 Sodium Entry Pathways ThatActivate KNa Channels
Estimates of cytoplasmic sodium concentrations in restingunstimulated neurons obtained using sodium-sensitivemicroelectrodes and ratiometric dyes provide valuesbetween sim4mM and 15mM [68] As noted earlier in thesection on NAD+ regulation of Slack channels this maybe sufficient to provide significant activation of native KNachannels in intact neurons even under resting conditions[5] Internal sodium levels rise in response to stimulationprincipally as a result of entry through voltage-dependentsodium channels and ionotropic ligand-gated receptors suchas AMPA and NMDA glutamate receptors [69] Stimulationparticularly repetitive tetanic stimulation produces local
ISRN Neuroscience 9
Control RecoveryC
1
2
3
C
1
2
3
10pA
2 s
10pA
200ms
S S S S S S S
FMRP(1ndash298)
(a)
2520151050
10000
1000
100
pA
C1
2
3
Recovery
2520151050
10000
1000
100
pA
C1
2
3
S
S
S(N
)
(N)
Control
2520151050
10000
1000
100
pA
C
1 2 3
S
S
S
(N)
FMRP(1ndash298)
(b)
Figure 4 FMRP alters gating of Slack-B channels in inside-out patches (a) Recording of a patch containing 3-4 Slack channels before duringand after transient application of FMRP(1ndash298) to the cytoplasmic face of the patch Top traces show representative 10 s examples of recordingatminus40mVand bottom traces show expanded time views of the same patch Arrowsmarked S indicate the occurrence of subconductance statesthat are suppressed by FMRP(1ndash298) (b) All-points amplitude histograms of Slack channel activity plotted on a logarithmic scale before andafter application of FMRP(1ndash298) and after washout C marks the closed state S indicates subconductance states and numbers on peaks inthe histograms refer to the number of fully open Slack channels Modified from [6]
increases in sodium concentrations that can rise to levelsbetween 45 and 100mM [70 71] In particular high levelscan be attained in restricted compartments such as activedendritic spines
Although the upstroke of action potentials in mostneurons is driven by rapidly-inactivating voltage-dependentsodium currents these currents are transient Evidencesuggest that the much smaller persistent component ofsodium current that does not inactivate with maintaineddepolarization is responsible both for sustained elevationsof internal sodium during stimulation [69] and for selectiveactivation of KNa channels [72 73] It has been found thatin patches excised from somata of rat neurons sodium entrythrough persistent tetrodotoxin-sensitive sodium channels isthe dominant factor that leads to activation of KNa channelsin the same patch [73] Activation of KNa channels by sodiumentry in the isolated patches was enhanced by veratridinewhich prolongs sodium channel openings and was observedin the absence of sodium at the cytoplasmic face of thepatches Moreover activation appeared to be independent ofthe amplitude of the much larger transient sodium current
[73] Although the molecular basis for persistent sodiumchannels is not fully understood further support for a selec-tive activation of KNa currents by entry through persistentchannels has also been found in electrocytes of electric fish inwhich the time course of KNa currents during depolarizationsreflects that of the persistent sodium current and not that ofthe very much larger transient sodium current [74]
A number of studies have documented that KNa currentsare also activated by sodium entry through ionotropic gluta-mate receptors particularly AMPA receptors Slack subunitscan be coimmunoprecipitated with GluR23 AMPA receptorsusing rat brain synaptosomal fractions or lysates of the lam-prey central nervous system [27] Moreover Slack subunitshave been found to bind directly to the PDZ domain of PSD-95 (postsynaptic density-95 protein) a major componentof the postsynaptic density of glutamatergic synapses andto colocalize with PSD-95 in cultures of mouse corticalneurons [75] In electrophysiological studies of lampreyneurons application of AMPA was been found to activate aKNa current with pharmacological properties of Slack [25]Activation of this KNa current regulates both the amplitude
10 ISRN Neuroscience
and the decay rate of AMPA-induced synaptic currents pro-viding a feedback loop that decreases the size of glutamatergicpostsynaptic potentials [27]
Activation of KNa currents following activation of AMPAreceptors has also been proposed in studies of neurons of thelocus coeruleus to which application of glutamate producesa burst of action potentials followed by period of inhibitionlasting 30ndash45 seconds [76] The potassium current responsi-ble for the period of inhibition was selectively activated byAMPA or the AMPA-receptor agonist quisqualate but not byother stimuli that excite these neurons Consistent with thiscurrent being a KNa current it was attenuated by lowering ofexternal sodium concentrations but not affected by externalcalcium levels and it was potentiated by the KNa channelopener bithionol [76]
While persistent voltage-dependent sodium channels andAMPA receptors may represent the primary sodium entrypathways for activation of KNa channels there exist otherpathways that allow sodium ions to enter excitable cellsand whose role has been less explored Slack subunits areabundantly expressed in mitral cells of the olfactory bulb[46 50] In these neurons deletion of the gene for a majorvoltage-dependent potassiumchannelKv13 produces a com-pensatory increase in levels of Slack-B protein and in SlackKNa currents identified using an anti-Slack siRNA technique[77] Activation of the KNa current during depolarizationof these cells could only be significantly suppressed by acombination of tetrodotoxin to block voltage-dependentsodium channels and ZD7288 a blocker of H-channelswhich are nonselective cation channels that flux both sodiumand potassium ions Either drug alone failed to significantlysuppress KNa currents suggesting that sodium entry throughH-channels plays a role in KNa channel activation in theseneurons [77]
11 Contribution of KNa Channels toNeuronal Firing Patterns
The component of whole-cell potassium current in neuronsthat can be attributed to Slack channel subunits has beenidentified in mitral cells of the olfactory bulb and mediumspiny neuron of the striatum using siRNAs to suppress trans-lation of SlackmRNA [72 77] Such experiments indicate thatSlack KNa channels are responsible for a very major compo-nent of the total delayed outward current in neurons underphysiological conditions [72] The specific contributions thatKNa channels make to the electrical personality of individualtypes of neurons may differ in different cell types
One of the earliest effects to be described for activationof KNa currents is to produce a slow afterhyperpolarization(sAHP) following a burst of action potentials This was firstdescribed in pyramidal neurons of the cat sensorimotorcortex [11 78] and similar slow sodium-dependent sAHPsthat endure for many seconds have been found in a widevariety of neuronal types [79ndash84]These have been the subjectof earlier reviews [20 21] Such slow potentials have beenfound to influence the timing of regular bursts of actionpotentials such as the 3ndash12 per minute spindle waves that
occur at the onset of sleep in certain neurons within thethalamus [80] Slow activation of KNa during repetitive firingalso produces an adaptation of firing rate during bursts ofaction potentials [23] and such adaptation is predicted bynumerical simulations that incorporate KNa currents [46 85]
In some neurons such as neocortical pyramidal neuronsand hippocampal CA1 neurons a significant amount of KNacurrent can also be activated by single spikes and contributeto action potentials repolarization or to the amplitude ofdepolarizing afterpotentials that follow single spikes andare generated by persistent sodium currents [83 86] Inthese cases KNa current activation may be detected withinsim5msec following a single-action potential A recent studyhas examined themechanismbywhich the cells of the electricorgan (electrocytes) of certain electric fish are able to generatesustained firing at frequencies greater than 500Hz [74] Slackchannels are expressed at high levels in these cells and theirdelayed potassium current is almost exclusively a KNa currentthat is activated by a persistent sodium current Numericalsimulations have indicated that the use of KNa current torepolarize the very large action potentials of these cells allowsthe cells to fire at such high rates with minimal energyexpenditure [74]
A set of neurons that have been particularly instructivein studies of the role of KNa channels in shaping neuronalfiring are those that control locomotion in the spinal cord oflamprey [24ndash28 87] In addition to the transient KNa currentthat is activated by AMPA receptors and that was describedearlier [25 27] these neurons appear to have two distinctcomponents ofKNa current that are triggered by action poten-tial firingmdasha transient component that follows a single spikeand a slower more sustained component [24 26 87] Therapid transient KNa current is activated by the sodium influxthrough tetrodotoxin-sensitive channel during the actionpotentials and may play a role in determining the amplitudeand duration of individual action potentials The sustainedKNa current is not abolished by tetrodotoxin suggesting thatit is selectively activated by an independent sodium entrypathway and may underlie the calcium-independent sAHPthat occurs during repetitive bursting in lamprey spinal cordneurons [24]
Experiments with neurons that fire at high rates withvery high temporal accuracy have provided another potentialbiological role for KNa channels [58] Slack and Slick areexpressed at high levels in neurons of the medial nucleusof the trapezoid (MNTB) within the auditory brainstem[6 50 51] These neurons fire at rates up to sim800Hzand accurate timing of their action potentials is requiredfor transmission of information about the localization ofsound stimuli KNa channels are readily be detected in bothcell-attached and excised patches on the somata of MNTBneurons and they are localized appropriately for activationby sodium influx through AMPA receptors or voltage-dependent sodium channels [58] Current-clamp recordingsfrom MNTB neurons and numerical simulations indicatethat as KNa channels become activated temporal accuracyof firing is improved This occurs because the voltage-dependence of Slack and Slick channels allows them to opennear the resting potential of MNTB neurons increasing
ISRN Neuroscience 11
the resting conductance and decreasing the membrane timeconstant An increase in temporal fidelity of MNTB neuronscan also be produced by treatment with the pharmacologicalKNa activator bithionol [58]
12 Mutations in Slack Channels Result inEpilepsy and Intellectual Disabilities
De novo human mutations that produce single-amino-acidalterations in Slack channels produce devastating effectson development and intellectual function [2 3] One setof Slack mutations results in malignant migrating partialseizures of infancy (MMPSI) [2]This disease is characterizedby pharmacoresistant seizures in the first 6 months of lifethat arise randomly and ldquomigraterdquo from one brain regionto another While the occurrence of seizures abates withage MMPSI results in near-total arrest of psychomotordevelopment Exome sequencing revealed that at least 50of MMPSI patients have mutations in the large cytoplasmicC-terminal domain of Slack [2]
Two mutations in the human Slack gene that produceMMPSI are R428Q and A934T which correspond to themutations R409Q and A913T in the highly conserved ratSlack sequence [2] Expression of these mutants in Xenopusoocytes produced Slack currents with voltage dependenceand kinetic behavior that matched that of wild-type Slackchannels but with a current amplitude that was increased 2-3-fold over that of wild-type Slack This increase in currentlikely results because the mutations produce channels thatare either more prone to undergo phosphorylation by PKC(or less likely to be dephosphorylated) or that mimic theconformation induced by PKC activation One of theMMPSImutations R409Q lies adjacent to the phosphorylation siteS407 that as described earlier enhances current by 2-3-fold when phosphorylated by PKC Consistent with thisinterpretation current amplitude of the two MMPSI muta-tions could not be further increased upon PKC activa-tion [2] In other respects the MMPSI currents resembledthat of wild-type channels with unitary conductance andsodiumdependencesmatching those of wild-type Slack BothMMPSI mutant channels however had reduced openings tosubconductance states compared to the wild-type channels[2]
A second disease that has been attributed to a differentset of mutations in the C-terminal domain of Slack chan-nels is autosomal dominant nocturnal frontal lobe epilepsy(ADNFLE) [3] This is a childhood-onset focal epilepsysyndrome in which motor seizures arise during sleep Onsetof ADNFLE in the patients with Slack mutations occurred ata mean age of 6 years and was associated with mild to severeintellectual disability andor severe psychiatric problemsTheeffects of these ADNFLE mutations on the function of Slackchannels have however not been investigated Interestinglyanother known genetic cause of ADNFLE is mutations insubunits of the neuronal nicotinic acetycholine receptor Theacetycholine receptor ADNFLE mutations do not howeverproduce intellectual disability which contrasts strikingly tothe effects of the Slack mutations [3]
It is possible that the very severe outcomes of mutationsin Slack occur solely because of changes in the amplitudeof Slack currents An increase in total potassium currentin rapidly firing interneurons could reduce the firing rateof these cells predisposing the nervous system to hyper-excitability The relatively modest increase in current thatoccurs with theMMPSImutations [2] however suggests thatother processes may also be affected As described earlierthe large C-terminal cytoplasmic domain of Slack in whichMMPSI and ADNFLE mutations are located interacts withFMRP and its cargo mRNAs [6] Thus it is possible thatin addition to directly altering the excitability of centralneuronsmutations in Slack disrupt the regulation of activity-dependent protein translation in neurons
13 Conclusions
This review has covered themolecular and cellular propertiesof Slack and Slick two potassium channel subunits thatprovide neurons and other cells with KNa channels It shouldbe pointed out that additional potassium channels that areactivated by cytoplasmic sodiummay also exist For examplesome of the native KNa currents that have been described inneurons do not match Slack or Slick in their single-channelconductances or pharmacological properties [88]These mayperhaps represent inward rectifier potassium channels thatare activated by sodium [45 89] and the physiological rolesof the sodium sensitivity of these channels require furtherinvestigation Nevertheless Slack and Slick which are verywidely expressed in the brain appear to play dominant rolesin shaping the firing properties of verymany types of neuronsHumanmutations in these channels produce profound effectson neuronal function and development suggesting that per-haps their biological role may extend beyond simply settingthe level of neuronal excitability and that these channels mayinfluence cytoplasmic biochemical pathways that regulatedevelopment plasticity and intellectual function
References
[1] B Hille Ionic Channels of Excitable Membranes Sinauer Asso-ciates Sunderland Mass USA 3rd edition 2001
[2] G Barcia M R Fleming A Deligniere et al ldquoDe novo gain-of-function KCNT1 channel mutations cause malignant migratingpartial seizures of infancyrdquo Nature Genetics vol 44 pp 1255ndash1259 2012
[3] S E Heron K R SmithM Bahlo et al ldquoMissensemutations inthe sodium-gated potassium channel gene KCNT1 cause severeautosomal dominant nocturnal frontal lobe epilepsyrdquo NatureGenetics vol 44 pp 1188ndash1190 2012
[4] A Bhattacharjee W J Joiner M Wu Y Yang F J Sigworthand L K Kaczmarek ldquoSlick (Slo2 1) a rapidly-gating sodium-activated potassium channel inhibited by ATPrdquo The Journal ofNeuroscience vol 23 no 37 pp 11681ndash11691 2003
[5] T J Tamsett K E Picchione and A Bhattacharjee ldquoNAD+activates KNa channels in dorsal root ganglion neuronsrdquo TheJournal of Neuroscience vol 29 no 16 pp 5127ndash5134 2009
12 ISRN Neuroscience
[6] M R Brown J Kronengold V R Gazula et al ldquoFragileX mental retardation protein controls gating of the sodium-activated potassium channel SlackrdquoNatureNeuroscience vol 13no 7 pp 819ndash821 2010
[7] M Kameyama M Kakei and R Sato ldquoIntracellular Na+activates a K+ channel in mammalian cardiac cellsrdquoNature vol309 no 5966 pp 354ndash356 1984
[8] C R Bader L Bernheim and D Bertrand ldquoSodium-activatedpotassium current in cultured avian neuronesrdquoNature vol 317no 6037 pp 540ndash542 1985
[9] K Hartung ldquoPotentiation of a transient outward current byNa+influx in crayfish neuronesrdquo Pflugers Archiv vol 404 no 1 pp41ndash44 1985
[10] S E Dryer J T Fujii and A R Martin ldquoA Na+-activatedK+ current in cultured brain stem neurones from chicksrdquo TheJournal of Physiology vol 410 pp 283ndash296 1989
[11] P C Schwindt W J Spain and W E Crill ldquoLong-lastingreduction of excitability by a sodium-dependent potassiumcurrent in cat neocortical neuronsrdquo Journal of Neurophysiologyvol 61 no 2 pp 233ndash244 1989
[12] C Haimann L Bernheim D Bertrand and C R BaderldquoPotassium current activated by intracellular sodium in quailtrigeminal ganglion neuronsrdquo Journal of General Physiology vol95 no 5 pp 961ndash979 1990
[13] S E Dryer ldquoNa+-activated K+ channels and voltage-evokedionic currents in brain stem and parasympathetic neurones ofthe chickrdquoThe Journal of Physiology vol 435 pp 513ndash532 1991
[14] M Saito and C F Wu ldquoExpression of ion channels andmutational effects in giant Drosophila neurons differentiatedfrom cell division-arrested embryonic neuroblastsrdquoThe Journalof Neuroscience vol 11 no 7 pp 2135ndash2150 1991
[15] T M Egan D Dagan J Kupper and I B Levitan ldquoPropertiesand rundown of sodium-activated potassium channels in ratolfactory bulb neuronsrdquoThe Journal of Neuroscience vol 12 no5 pp 1964ndash1976 1992
[16] C Haimann J Magistretti and B Pozzi ldquoSodium-activatedpotassium current in sensory neurons a comparison of cell-attached and cell-free single-channel activitiesrdquo Pflugers Archivvol 422 no 3 pp 287ndash294 1992
[17] N Dale ldquoA large sustained Na+- and voltage-dependent K+current in spinal neurons of the frog embryordquo The Journal ofPhysiology vol 462 pp 349ndash372 1993
[18] B V Safronov and W Vogel ldquoProperties and functions of Na+-activated K+ channels in the soma of rat motoneuronesrdquo TheJournal of Physiology vol 497 no 3 pp 727ndash734 1996
[19] U Bischoff W Vogel and B V Safronov ldquoNa+-activated K+channels in small dorsal root ganglion neurones of ratrdquo TheJournal of Physiology vol 510 part 3 pp 743ndash754 1998
[20] S E Dryer ldquoNa+-activated K+ channels a new family of large-conductance ion channelsrdquo Trends in Neurosciences vol 17 no4 pp 155ndash160 1994
[21] A Bhattacharjee and L K Kaczmarek ldquoFor K+ channels Na+ isthe new Ca2+ rdquo Trends in Neurosciences vol 28 no 8 pp 422ndash428 2005
[22] S B Gao Y Wu C X Lu Z H Guo C H Li and J P DingldquoSlack and Slick KNa channels are required for the depolarizingafterpotential of acutely isolated medium diameter rat dorsalroot ganglion neuronsrdquoActa Pharmacologica Sinica vol 29 no8 pp 899ndash905 2008
[23] M O Nuwer K E Picchione and A Bhattacharjee ldquoPKA-induced internalization of Slack KNa channels produces dorsal
root ganglion neuron hyperexcitabilityrdquo The Journal of Neuro-science vol 30 no 42 pp 14165ndash14172 2010
[24] D Hess E Nanou and A El Manira ldquoCharacterization of Na+-activated K+ currents in larval lamprey spinal cord neuronsrdquoJournal of Neurophysiology vol 97 no 5 pp 3484ndash3493 2007
[25] E Nanou and A El Manira ldquoA postsynaptic negative feedbackmediated by coupling between AMPA receptors and Na+-activated K+ channels in spinal cord neuronesrdquo EuropeanJournal of Neuroscience vol 25 no 2 pp 445ndash450 2007
[26] PWallen B Robertson L Cangiano et al ldquoSodium-dependentpotassium channels of a Slack-like subtype contribute to theslow afterhyperpolarization in lamprey spinal neuronsrdquo TheJournal of Physiology vol 585 no 1 pp 75ndash90 2007
[27] E Nanou A Kyriakatos A Bhattacharjee L K Kaczmarek GParatcha and A El Manira ldquoNa+-mediated coupling betweenAMPA receptors and KNa channels shapes synaptic transmis-sionrdquo Proceedings of the National Academy of Sciences of theUnited States of America vol 105 no 52 pp 20941ndash20946 2008
[28] E Nanou and A El Manira ldquoMechanisms of modulation ofAMPA-induced Na+-activated K+ current by mGluR1rdquo Journalof Neurophysiology vol 103 no 1 pp 441ndash445 2010
[29] A R Mercer and J G Hildebrand ldquoDevelopmental changes inthe density of ionic currents in antennal-lobe neurons of thesphinx moth Manduca sextardquo Journal of Neurophysiology vol87 no 6 pp 2664ndash2675 2002
[30] K Aoki K Kosakai and M Yoshino ldquoMonoaminergic modu-lation of the Na+-activated K+ channel in kenyon cells isolatedfrom the mushroom body of the cricket (Gryllus bimaculatus)brainrdquo Journal of Neurophysiology vol 100 no 3 pp 1211ndash12222008
[31] G Klees P Hochstrate and P W Dierkes ldquoSodium-dependentpotassium channels in leech P neuronsrdquo Journal of MembraneBiology vol 208 no 1 pp 27ndash38 2005
[32] Y Zhang M R Brown C Hyland et al ldquoRegulation ofneuronal excitability by interaction of fragile X mental retar-dation protein with Slack potassium channelsrdquo The Journal ofNeuroscience vol 32 pp 15318ndash15327 2012
[33] C Lawrence and G C Rodrigo ldquoA Na+-activated K+ cur-rent (IKNa) is present in guinea-pig but not rat ventricularmyocytesrdquo Pflugers Archiv vol 437 no 6 pp 831ndash838 1999
[34] M Zhou Z Liu C Hu Z Zhang and Y Mei ldquoDevelopmentalregulation of a Na+-activated fast outward K+ current in ratmyoblastsrdquo Cellular Physiology and Biochemistry vol 14 no 4ndash6 pp 225ndash230 2004
[35] C K Young H S Jae M K Tong et al ldquoSodium-activatedpotassium current in guinea pig gastric myocytesrdquo Journal ofKorean Medical Science vol 22 no 1 pp 57ndash62 2007
[36] L Re V Moretti L Rossini and P Giusti ldquoSodium-activatedpotassium current in mouse diaphragmrdquo FEBS Letters vol 270no 1-2 pp 195ndash197 1990
[37] Y Zhang and W G Paterson ldquoFunctional evidence for Na+-activatedK+ channels in circular smoothmuscle of the opossumlower esophageal sphincterrdquo American The Journal of Physiol-ogy vol 292 no 6 pp G1600ndashG1606 2007
[38] M Paulais S Lachheb and J Teulon ldquoANa+- and Clminus-activatedK+ channel in the thick ascending limb of mouse kidneyrdquoJournal of General Physiology vol 127 no 2 pp 205ndash215 2006
[39] T M Egan D Dagan J Kupper and I B Levitan ldquoNa+-activated K+ channels are widely distributed in rat CNS and inXenopus oocytesrdquo Brain Research vol 584 no 1-2 pp 319ndash3211992
ISRN Neuroscience 13
[40] W J Joiner M D Tang L Y Wang et al ldquoFormation ofintermediate-conductance calcium-activated potassium chan-nels by interaction of Slack and Slo subunitsrdquo Nature Neuro-science vol 1 no 6 pp 462ndash469 1998
[41] A Yuan C M Santi A Wei et al ldquoThe sodium-activatedpotassium channel is encoded by a member of the Slo genefamilyrdquo Neuron vol 37 no 5 pp 765ndash773 2003
[42] Y Jiang A Pico M Cadene B T Chait and R MacKinnonldquoStructure of the RCK domain from the E coli K+ channeland demonstration of its presence in the human BK channelrdquoNeuron vol 29 no 3 pp 593ndash601 2001
[43] P Yuan M D Leonetti A R Pico Y Hsiung and R MacK-innon ldquoStructure of the human BK channel Ca2+-activationapparatus at 30 A resolutionrdquo Science vol 329 no 5988 pp182ndash186 2010
[44] Z Zhang A Rosenhouse-Dantsker Q Y Tang S Noskov andD E Logothetis ldquoThe RCK2 domain uses a coordination sitepresent in Kir channels to confer sodium sensitivity to Slo22channelsrdquoThe Journal of Neuroscience vol 30 no 22 pp 7554ndash7562 2010
[45] J L Sui K W Chan and D E Logothetis ldquoNa+ activationof the muscarinic K+ channel by a G-protein-independentmechanismrdquo Journal of General Physiology vol 108 no 5 pp381ndash391 1996
[46] M R Brown J Kronengold V R Gazula et al ldquoAmino-terminiisoforms of the Slack K+ channel regulated by alternative pro-moters differentiallymodulate rhythmic firing and adaptationrdquoThe Journal of Physiology vol 586 no 21 pp 5161ndash5179 2008
[47] A Yuan M Dourado A Butler N Walton A Wei and LSalkoff ldquoSLO-2 a K+ channel with an unusual Clminus dependencerdquoNature Neuroscience vol 3 no 8 pp 771ndash779 2000
[48] M Schreiber and L Salkoff ldquoA novel calcium-sensing domainin the BK channelrdquo Biophysical Journal vol 73 no 3 pp 1355ndash1363 1997
[49] H Chen J Kronengold Y Yan et al ldquoThe N-terminal domainof slack determines the formation and trafficking of slickslackheteromeric sodium-activated potassium channelsrdquo The Jour-nal of Neuroscience vol 29 no 17 pp 5654ndash5665 2009
[50] A Bhattacharjee L Gan and L K Kaczmarek ldquoLocalization ofthe Slack potassium channel in the rat central nervous systemrdquoJournal of Comparative Neurology vol 454 no 3 pp 241ndash2542002
[51] A Bhattacharjee C A A von Hehn X Mei and L KKaczmarek ldquoLocalization of the Na+-activated K+ channel slickin the rat central nervous systemrdquo Journal of ComparativeNeurology vol 484 no 1 pp 80ndash92 2005
[52] D S Koh P Jonas and W Vogel ldquoNa+-activated K+ channelslocalized in the nodal region of myelinated axons of XenopusrdquoThe Journal of Physiology vol 479 no 2 pp 183ndash197 1994
[53] K Mori T Saito Y Masuda and H Nakaya ldquoEffects of classIII antiarrhythmic drugs on the Na+-activated K+ channels inguinea-pig ventricular cellsrdquo British Journal of Pharmacologyvol 119 no 1 pp 133ndash141 1996
[54] K Mori S Kobayashi T Saito Y Masuda and H NakayaldquoInhibitory effects of class I and IV antiarrhythmic drugs onthe Na+- activated K+ channel current in guinea pig ventricularcellsrdquo Naunyn-Schmiedebergrsquos Archives of Pharmacology vol358 no 6 pp 641ndash648 1998
[55] Y Li T Sato and M Arita ldquoBepridil blunts the shortening ofaction potential duration caused by metabolic inhibition viablockade of ATP-sensitive K+ channels and Na+- activated K+
channelsrdquo Journal of Pharmacology and ExperimentalTherapeu-tics vol 291 no 2 pp 562ndash568 1999
[56] B Yang V K Gribkoff J Pan et al ldquoPharmacological activationand inhibition of Slack (Slo2 2) channelsrdquoNeuropharmacologyvol 51 no 4 pp 896ndash906 2006
[57] M D Tejada K Stolpe A K Meinild and D A KlaerkeldquoClofilium inhibits Slick and Slack potassium channelsrdquo Biolog-ics vol 6 pp 465ndash470 2012
[58] B Yang R Desai and L K Kaczmarek ldquoSlack and slick KNachannels regulate the accuracy of timing of auditory neuronsrdquoThe Journal of Neuroscience vol 27 no 10 pp 2617ndash2627 2007
[59] B Biton S Sethuramanujam K E Picchione et al ldquoTheantipsychotic drug loxapine is an opener of the sodium-activated potassium channel slack (Slo2 2)rdquo The Journal ofPharmacology and ExperimentalTherapeutics vol 340 pp 706ndash715 2012
[60] C M Santi G Ferreira B Yang et al ldquoOpposite regulation ofSlick and Slack K+ channels by neuromodulatorsrdquo The Journalof Neuroscience vol 26 no 19 pp 5059ndash5068 2006
[61] M R Fleming and L K Kaczmarek ldquoUse of optical biosensorsto detect modulation of Slack potassium channels by G protein-coupled receptorsrdquo Journal of Receptors and Signal Transduc-tion vol 29 no 3-4 pp 173ndash181 2009
[62] M D Tejada L J Jensen and D A Klaerke ldquoPIP(2) mod-ulation of Slick and Slack K(+) channelsrdquo Biochemical andBiophysical Research Communications vol 424 pp 208ndash2132012
[63] L Zhang M Sukhareva J L Barker et al ldquoDirect bindingof estradiol enhances Slack (sequence like a calcium-activatedpotassium channel) channelsrsquo activityrdquo Neuroscience vol 131no 2 pp 275ndash282 2005
[64] V A Ruffin X Q Gu D Zhou et al ldquoThe sodium-activatedpotassium channel Slack is modulated by hypercapnia andacidosisrdquo Neuroscience vol 151 no 2 pp 410ndash418 2008
[65] M O Nuwer K E Picchione and A Bhattacharjee ldquocAMP-dependent kinase does not modulate the Slack sodium-activated potassium channelrdquo Neuropharmacology vol 57 no3 pp 219ndash226 2009
[66] B C Suh and B Hille ldquoRegulation of ion channels by phos-phatidylinositol 45-bisphosphaterdquo Current Opinion in Neuro-biology vol 15 no 3 pp 370ndash378 2005
[67] L K Kaczmarek ldquoNon-conducting functions of voltage-gatedion channelsrdquo Nature Reviews Neuroscience vol 7 no 10 pp761ndash771 2006
[68] C R Rose ldquoNa+ signals at central synapsesrdquoNeuroscientist vol8 no 6 pp 532ndash539 2002
[69] C R Rose and B R Ransom ldquoRegulation of intracellularsodium in cultured rat hippocampal neuronesrdquo The Journal ofPhysiology vol 499 part 3 pp 573ndash587 1997
[70] C R Rose and A Konnerth ldquoNmda receptor-mediated Na+signals in spines and dendritesrdquoThe Journal of Neuroscience vol21 no 12 pp 4207ndash4214 2001
[71] N Zhong V Beaumont and R S Zucker ldquoRoles for mito-chondrial and reverse mode Na+Ca2+ exchange and the plas-malemma Ca2+ ATPase in post-tetanic potentiation at crayfishneuromuscular junctionsrdquo The Journal of Neuroscience vol 21no 24 pp 9598ndash9607 2001
[72] G Budelli T A Hage AWei et al ldquoNa+-activated K+ channelsexpress a large delayed outward current in neurons duringnormal physiologyrdquoNature Neuroscience vol 12 no 6 pp 745ndash750 2009
14 ISRN Neuroscience
[73] T A Hage and L Salkoff ldquoSodium-activated potassium chan-nels are functionally coupled to persistent sodiumcurrentsrdquoTheJournal of Neuroscience vol 32 pp 2714ndash2721 2012
[74] M RMarkham L K Kaczmarek andHH Zakon ldquoA sodium-activated potassium channe supports high frequency firing andreduces energetic costs during rapid modulations of actionpotential amplituderdquo Journal of Neurophysiology vol 109 no 7pp 1713ndash1723 2013
[75] S Uchino H Wada S Honda et al ldquoSlo2 sodium-activated K+channels bind to the PDZ domain of PSD-95rdquo Biochemical andBiophysical Research Communications vol 310 no 4 pp 1140ndash1147 2003
[76] T Zamalloa C P Bailey and J Pineda ldquoGlutamate-inducedpost-activation inhibition of locus coeruleus neurons is medi-ated by AMPAkainate receptors and sodium-dependent potas-sium currentsrdquo British Journal of Pharmacology vol 156 no 4pp 649ndash661 2009
[77] S Lu P Das D A Fadool and L K Kaczmarek ldquoThe slacksodium-activated potassium channel provides a major outwardcurrent in olfactory neurons of Kv13-- super-smeller micerdquoJournal of Neurophysiology vol 103 no 6 pp 3311ndash3319 2010
[78] R C Foehring P C Schwindt andW E Crill ldquoNorepinephrineselectively reduces slowCa2+- andNa+-mediated K+ currents incat neocortical neuronsrdquo Journal of Neurophysiology vol 61 no2 pp 245ndash256 1989
[79] M Kubota and N Saito ldquoSodium- and calcium-dependentconductances of neurones in the zebra finch hyperstriatumventrale pars caudale in vitrordquo The Journal of Physiology vol440 pp 131ndash142 1991
[80] U Kim and D A Mccormick ldquoFunctional and ionic propertiesof a slow afterhyperpolarization in ferret perigeniculate neuronsin vitrordquo Journal of Neurophysiology vol 80 no 3 pp 1222ndash1235 1998
[81] V M Sandler E Puil and DW F Schwarz ldquoIntrinsic responseproperties of bursting neurons in the nucleus principalistrigemini of the gerbilrdquoNeuroscience vol 83 no 3 pp 891ndash9041998
[82] M V Sanchez-Vives L G Nowak and D A McCormick ldquoCel-lular mechanisms of long-lasting adaptation in visual corticalneurons in vitrordquoThe Journal of Neuroscience vol 20 no 11 pp4286ndash4299 2000
[83] S Franceschetti T Lavazza G Curia et al ldquoNa+-activatedK+ current contributes to postexcitatory hyperpolarization inneocortical intrinsically bursting neuronsrdquo Journal of Neuro-physiology vol 89 no 4 pp 2101ndash2111 2003
[84] V F Descalzo L G Nowak J C Brumberg D A McCormickand M V Sanchez-Vives ldquoSlow adaptation in fast-spikingneurons of visual cortexrdquo Journal of Neurophysiology vol 93no 2 pp 1111ndash1118 2005
[85] J Benda L Maler and A Longtin ldquoLinear versus nonlinearsignal transmission in neuron models with adaptation currentsor dynamic thresholdsrdquo Journal of Neurophysiology vol 104 no5 pp 2806ndash2820 2010
[86] X Liu and L S Leung ldquoSodium-activated potassium conduc-tance participates in the depolarizing afterpotential following asingle action potential in rat hippocampal CA1 pyramidal cellsrdquoBrain Research vol 1023 no 2 pp 185ndash192 2004
[87] L Cangiano P Wallen and S Grillner ldquoRole of apamin-sensitive KCa channels for reticulospinal synaptic transmissionto motoneuron and for the afterhyperpolarizationrdquo Journal ofNeurophysiology vol 88 no 1 pp 289ndash299 2002
[88] Q Y Liu A E Schaffner and J L Barker ldquoKainate induces anintracellular Na+-activated K+ current in cultured embryonicrat hippocampal neuronesrdquo The Journal of Physiology vol 510part 3 pp 721ndash734 1998
[89] I Rishal T Keren-Raifman D Yakubovich et al ldquoNa+ pro-motes the dissociation between G120572GDP and G120573120574 activating Gprotein-gated K+ channelsrdquoThe Journal of Biological Chemistryvol 278 no 6 pp 3840ndash3845 2003
Submit your manuscripts athttpwwwhindawicom
Neurology Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Alzheimerrsquos DiseaseHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentSchizophrenia
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neural Plasticity
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Parkinsonrsquos Disease
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentAutism
Sleep DisordersHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Epilepsy Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Psychiatry Journal
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Depression Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Neurodegenerative Diseases
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Cardiovascular Psychiatry and NeurologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
ISRN Neuroscience 9
Control RecoveryC
1
2
3
C
1
2
3
10pA
2 s
10pA
200ms
S S S S S S S
FMRP(1ndash298)
(a)
2520151050
10000
1000
100
pA
C1
2
3
Recovery
2520151050
10000
1000
100
pA
C1
2
3
S
S
S(N
)
(N)
Control
2520151050
10000
1000
100
pA
C
1 2 3
S
S
S
(N)
FMRP(1ndash298)
(b)
Figure 4 FMRP alters gating of Slack-B channels in inside-out patches (a) Recording of a patch containing 3-4 Slack channels before duringand after transient application of FMRP(1ndash298) to the cytoplasmic face of the patch Top traces show representative 10 s examples of recordingatminus40mVand bottom traces show expanded time views of the same patch Arrowsmarked S indicate the occurrence of subconductance statesthat are suppressed by FMRP(1ndash298) (b) All-points amplitude histograms of Slack channel activity plotted on a logarithmic scale before andafter application of FMRP(1ndash298) and after washout C marks the closed state S indicates subconductance states and numbers on peaks inthe histograms refer to the number of fully open Slack channels Modified from [6]
increases in sodium concentrations that can rise to levelsbetween 45 and 100mM [70 71] In particular high levelscan be attained in restricted compartments such as activedendritic spines
Although the upstroke of action potentials in mostneurons is driven by rapidly-inactivating voltage-dependentsodium currents these currents are transient Evidencesuggest that the much smaller persistent component ofsodium current that does not inactivate with maintaineddepolarization is responsible both for sustained elevationsof internal sodium during stimulation [69] and for selectiveactivation of KNa channels [72 73] It has been found thatin patches excised from somata of rat neurons sodium entrythrough persistent tetrodotoxin-sensitive sodium channels isthe dominant factor that leads to activation of KNa channelsin the same patch [73] Activation of KNa channels by sodiumentry in the isolated patches was enhanced by veratridinewhich prolongs sodium channel openings and was observedin the absence of sodium at the cytoplasmic face of thepatches Moreover activation appeared to be independent ofthe amplitude of the much larger transient sodium current
[73] Although the molecular basis for persistent sodiumchannels is not fully understood further support for a selec-tive activation of KNa currents by entry through persistentchannels has also been found in electrocytes of electric fish inwhich the time course of KNa currents during depolarizationsreflects that of the persistent sodium current and not that ofthe very much larger transient sodium current [74]
A number of studies have documented that KNa currentsare also activated by sodium entry through ionotropic gluta-mate receptors particularly AMPA receptors Slack subunitscan be coimmunoprecipitated with GluR23 AMPA receptorsusing rat brain synaptosomal fractions or lysates of the lam-prey central nervous system [27] Moreover Slack subunitshave been found to bind directly to the PDZ domain of PSD-95 (postsynaptic density-95 protein) a major componentof the postsynaptic density of glutamatergic synapses andto colocalize with PSD-95 in cultures of mouse corticalneurons [75] In electrophysiological studies of lampreyneurons application of AMPA was been found to activate aKNa current with pharmacological properties of Slack [25]Activation of this KNa current regulates both the amplitude
10 ISRN Neuroscience
and the decay rate of AMPA-induced synaptic currents pro-viding a feedback loop that decreases the size of glutamatergicpostsynaptic potentials [27]
Activation of KNa currents following activation of AMPAreceptors has also been proposed in studies of neurons of thelocus coeruleus to which application of glutamate producesa burst of action potentials followed by period of inhibitionlasting 30ndash45 seconds [76] The potassium current responsi-ble for the period of inhibition was selectively activated byAMPA or the AMPA-receptor agonist quisqualate but not byother stimuli that excite these neurons Consistent with thiscurrent being a KNa current it was attenuated by lowering ofexternal sodium concentrations but not affected by externalcalcium levels and it was potentiated by the KNa channelopener bithionol [76]
While persistent voltage-dependent sodium channels andAMPA receptors may represent the primary sodium entrypathways for activation of KNa channels there exist otherpathways that allow sodium ions to enter excitable cellsand whose role has been less explored Slack subunits areabundantly expressed in mitral cells of the olfactory bulb[46 50] In these neurons deletion of the gene for a majorvoltage-dependent potassiumchannelKv13 produces a com-pensatory increase in levels of Slack-B protein and in SlackKNa currents identified using an anti-Slack siRNA technique[77] Activation of the KNa current during depolarizationof these cells could only be significantly suppressed by acombination of tetrodotoxin to block voltage-dependentsodium channels and ZD7288 a blocker of H-channelswhich are nonselective cation channels that flux both sodiumand potassium ions Either drug alone failed to significantlysuppress KNa currents suggesting that sodium entry throughH-channels plays a role in KNa channel activation in theseneurons [77]
11 Contribution of KNa Channels toNeuronal Firing Patterns
The component of whole-cell potassium current in neuronsthat can be attributed to Slack channel subunits has beenidentified in mitral cells of the olfactory bulb and mediumspiny neuron of the striatum using siRNAs to suppress trans-lation of SlackmRNA [72 77] Such experiments indicate thatSlack KNa channels are responsible for a very major compo-nent of the total delayed outward current in neurons underphysiological conditions [72] The specific contributions thatKNa channels make to the electrical personality of individualtypes of neurons may differ in different cell types
One of the earliest effects to be described for activationof KNa currents is to produce a slow afterhyperpolarization(sAHP) following a burst of action potentials This was firstdescribed in pyramidal neurons of the cat sensorimotorcortex [11 78] and similar slow sodium-dependent sAHPsthat endure for many seconds have been found in a widevariety of neuronal types [79ndash84]These have been the subjectof earlier reviews [20 21] Such slow potentials have beenfound to influence the timing of regular bursts of actionpotentials such as the 3ndash12 per minute spindle waves that
occur at the onset of sleep in certain neurons within thethalamus [80] Slow activation of KNa during repetitive firingalso produces an adaptation of firing rate during bursts ofaction potentials [23] and such adaptation is predicted bynumerical simulations that incorporate KNa currents [46 85]
In some neurons such as neocortical pyramidal neuronsand hippocampal CA1 neurons a significant amount of KNacurrent can also be activated by single spikes and contributeto action potentials repolarization or to the amplitude ofdepolarizing afterpotentials that follow single spikes andare generated by persistent sodium currents [83 86] Inthese cases KNa current activation may be detected withinsim5msec following a single-action potential A recent studyhas examined themechanismbywhich the cells of the electricorgan (electrocytes) of certain electric fish are able to generatesustained firing at frequencies greater than 500Hz [74] Slackchannels are expressed at high levels in these cells and theirdelayed potassium current is almost exclusively a KNa currentthat is activated by a persistent sodium current Numericalsimulations have indicated that the use of KNa current torepolarize the very large action potentials of these cells allowsthe cells to fire at such high rates with minimal energyexpenditure [74]
A set of neurons that have been particularly instructivein studies of the role of KNa channels in shaping neuronalfiring are those that control locomotion in the spinal cord oflamprey [24ndash28 87] In addition to the transient KNa currentthat is activated by AMPA receptors and that was describedearlier [25 27] these neurons appear to have two distinctcomponents ofKNa current that are triggered by action poten-tial firingmdasha transient component that follows a single spikeand a slower more sustained component [24 26 87] Therapid transient KNa current is activated by the sodium influxthrough tetrodotoxin-sensitive channel during the actionpotentials and may play a role in determining the amplitudeand duration of individual action potentials The sustainedKNa current is not abolished by tetrodotoxin suggesting thatit is selectively activated by an independent sodium entrypathway and may underlie the calcium-independent sAHPthat occurs during repetitive bursting in lamprey spinal cordneurons [24]
Experiments with neurons that fire at high rates withvery high temporal accuracy have provided another potentialbiological role for KNa channels [58] Slack and Slick areexpressed at high levels in neurons of the medial nucleusof the trapezoid (MNTB) within the auditory brainstem[6 50 51] These neurons fire at rates up to sim800Hzand accurate timing of their action potentials is requiredfor transmission of information about the localization ofsound stimuli KNa channels are readily be detected in bothcell-attached and excised patches on the somata of MNTBneurons and they are localized appropriately for activationby sodium influx through AMPA receptors or voltage-dependent sodium channels [58] Current-clamp recordingsfrom MNTB neurons and numerical simulations indicatethat as KNa channels become activated temporal accuracyof firing is improved This occurs because the voltage-dependence of Slack and Slick channels allows them to opennear the resting potential of MNTB neurons increasing
ISRN Neuroscience 11
the resting conductance and decreasing the membrane timeconstant An increase in temporal fidelity of MNTB neuronscan also be produced by treatment with the pharmacologicalKNa activator bithionol [58]
12 Mutations in Slack Channels Result inEpilepsy and Intellectual Disabilities
De novo human mutations that produce single-amino-acidalterations in Slack channels produce devastating effectson development and intellectual function [2 3] One setof Slack mutations results in malignant migrating partialseizures of infancy (MMPSI) [2]This disease is characterizedby pharmacoresistant seizures in the first 6 months of lifethat arise randomly and ldquomigraterdquo from one brain regionto another While the occurrence of seizures abates withage MMPSI results in near-total arrest of psychomotordevelopment Exome sequencing revealed that at least 50of MMPSI patients have mutations in the large cytoplasmicC-terminal domain of Slack [2]
Two mutations in the human Slack gene that produceMMPSI are R428Q and A934T which correspond to themutations R409Q and A913T in the highly conserved ratSlack sequence [2] Expression of these mutants in Xenopusoocytes produced Slack currents with voltage dependenceand kinetic behavior that matched that of wild-type Slackchannels but with a current amplitude that was increased 2-3-fold over that of wild-type Slack This increase in currentlikely results because the mutations produce channels thatare either more prone to undergo phosphorylation by PKC(or less likely to be dephosphorylated) or that mimic theconformation induced by PKC activation One of theMMPSImutations R409Q lies adjacent to the phosphorylation siteS407 that as described earlier enhances current by 2-3-fold when phosphorylated by PKC Consistent with thisinterpretation current amplitude of the two MMPSI muta-tions could not be further increased upon PKC activa-tion [2] In other respects the MMPSI currents resembledthat of wild-type channels with unitary conductance andsodiumdependencesmatching those of wild-type Slack BothMMPSI mutant channels however had reduced openings tosubconductance states compared to the wild-type channels[2]
A second disease that has been attributed to a differentset of mutations in the C-terminal domain of Slack chan-nels is autosomal dominant nocturnal frontal lobe epilepsy(ADNFLE) [3] This is a childhood-onset focal epilepsysyndrome in which motor seizures arise during sleep Onsetof ADNFLE in the patients with Slack mutations occurred ata mean age of 6 years and was associated with mild to severeintellectual disability andor severe psychiatric problemsTheeffects of these ADNFLE mutations on the function of Slackchannels have however not been investigated Interestinglyanother known genetic cause of ADNFLE is mutations insubunits of the neuronal nicotinic acetycholine receptor Theacetycholine receptor ADNFLE mutations do not howeverproduce intellectual disability which contrasts strikingly tothe effects of the Slack mutations [3]
It is possible that the very severe outcomes of mutationsin Slack occur solely because of changes in the amplitudeof Slack currents An increase in total potassium currentin rapidly firing interneurons could reduce the firing rateof these cells predisposing the nervous system to hyper-excitability The relatively modest increase in current thatoccurs with theMMPSImutations [2] however suggests thatother processes may also be affected As described earlierthe large C-terminal cytoplasmic domain of Slack in whichMMPSI and ADNFLE mutations are located interacts withFMRP and its cargo mRNAs [6] Thus it is possible thatin addition to directly altering the excitability of centralneuronsmutations in Slack disrupt the regulation of activity-dependent protein translation in neurons
13 Conclusions
This review has covered themolecular and cellular propertiesof Slack and Slick two potassium channel subunits thatprovide neurons and other cells with KNa channels It shouldbe pointed out that additional potassium channels that areactivated by cytoplasmic sodiummay also exist For examplesome of the native KNa currents that have been described inneurons do not match Slack or Slick in their single-channelconductances or pharmacological properties [88]These mayperhaps represent inward rectifier potassium channels thatare activated by sodium [45 89] and the physiological rolesof the sodium sensitivity of these channels require furtherinvestigation Nevertheless Slack and Slick which are verywidely expressed in the brain appear to play dominant rolesin shaping the firing properties of verymany types of neuronsHumanmutations in these channels produce profound effectson neuronal function and development suggesting that per-haps their biological role may extend beyond simply settingthe level of neuronal excitability and that these channels mayinfluence cytoplasmic biochemical pathways that regulatedevelopment plasticity and intellectual function
References
[1] B Hille Ionic Channels of Excitable Membranes Sinauer Asso-ciates Sunderland Mass USA 3rd edition 2001
[2] G Barcia M R Fleming A Deligniere et al ldquoDe novo gain-of-function KCNT1 channel mutations cause malignant migratingpartial seizures of infancyrdquo Nature Genetics vol 44 pp 1255ndash1259 2012
[3] S E Heron K R SmithM Bahlo et al ldquoMissensemutations inthe sodium-gated potassium channel gene KCNT1 cause severeautosomal dominant nocturnal frontal lobe epilepsyrdquo NatureGenetics vol 44 pp 1188ndash1190 2012
[4] A Bhattacharjee W J Joiner M Wu Y Yang F J Sigworthand L K Kaczmarek ldquoSlick (Slo2 1) a rapidly-gating sodium-activated potassium channel inhibited by ATPrdquo The Journal ofNeuroscience vol 23 no 37 pp 11681ndash11691 2003
[5] T J Tamsett K E Picchione and A Bhattacharjee ldquoNAD+activates KNa channels in dorsal root ganglion neuronsrdquo TheJournal of Neuroscience vol 29 no 16 pp 5127ndash5134 2009
12 ISRN Neuroscience
[6] M R Brown J Kronengold V R Gazula et al ldquoFragileX mental retardation protein controls gating of the sodium-activated potassium channel SlackrdquoNatureNeuroscience vol 13no 7 pp 819ndash821 2010
[7] M Kameyama M Kakei and R Sato ldquoIntracellular Na+activates a K+ channel in mammalian cardiac cellsrdquoNature vol309 no 5966 pp 354ndash356 1984
[8] C R Bader L Bernheim and D Bertrand ldquoSodium-activatedpotassium current in cultured avian neuronesrdquoNature vol 317no 6037 pp 540ndash542 1985
[9] K Hartung ldquoPotentiation of a transient outward current byNa+influx in crayfish neuronesrdquo Pflugers Archiv vol 404 no 1 pp41ndash44 1985
[10] S E Dryer J T Fujii and A R Martin ldquoA Na+-activatedK+ current in cultured brain stem neurones from chicksrdquo TheJournal of Physiology vol 410 pp 283ndash296 1989
[11] P C Schwindt W J Spain and W E Crill ldquoLong-lastingreduction of excitability by a sodium-dependent potassiumcurrent in cat neocortical neuronsrdquo Journal of Neurophysiologyvol 61 no 2 pp 233ndash244 1989
[12] C Haimann L Bernheim D Bertrand and C R BaderldquoPotassium current activated by intracellular sodium in quailtrigeminal ganglion neuronsrdquo Journal of General Physiology vol95 no 5 pp 961ndash979 1990
[13] S E Dryer ldquoNa+-activated K+ channels and voltage-evokedionic currents in brain stem and parasympathetic neurones ofthe chickrdquoThe Journal of Physiology vol 435 pp 513ndash532 1991
[14] M Saito and C F Wu ldquoExpression of ion channels andmutational effects in giant Drosophila neurons differentiatedfrom cell division-arrested embryonic neuroblastsrdquoThe Journalof Neuroscience vol 11 no 7 pp 2135ndash2150 1991
[15] T M Egan D Dagan J Kupper and I B Levitan ldquoPropertiesand rundown of sodium-activated potassium channels in ratolfactory bulb neuronsrdquoThe Journal of Neuroscience vol 12 no5 pp 1964ndash1976 1992
[16] C Haimann J Magistretti and B Pozzi ldquoSodium-activatedpotassium current in sensory neurons a comparison of cell-attached and cell-free single-channel activitiesrdquo Pflugers Archivvol 422 no 3 pp 287ndash294 1992
[17] N Dale ldquoA large sustained Na+- and voltage-dependent K+current in spinal neurons of the frog embryordquo The Journal ofPhysiology vol 462 pp 349ndash372 1993
[18] B V Safronov and W Vogel ldquoProperties and functions of Na+-activated K+ channels in the soma of rat motoneuronesrdquo TheJournal of Physiology vol 497 no 3 pp 727ndash734 1996
[19] U Bischoff W Vogel and B V Safronov ldquoNa+-activated K+channels in small dorsal root ganglion neurones of ratrdquo TheJournal of Physiology vol 510 part 3 pp 743ndash754 1998
[20] S E Dryer ldquoNa+-activated K+ channels a new family of large-conductance ion channelsrdquo Trends in Neurosciences vol 17 no4 pp 155ndash160 1994
[21] A Bhattacharjee and L K Kaczmarek ldquoFor K+ channels Na+ isthe new Ca2+ rdquo Trends in Neurosciences vol 28 no 8 pp 422ndash428 2005
[22] S B Gao Y Wu C X Lu Z H Guo C H Li and J P DingldquoSlack and Slick KNa channels are required for the depolarizingafterpotential of acutely isolated medium diameter rat dorsalroot ganglion neuronsrdquoActa Pharmacologica Sinica vol 29 no8 pp 899ndash905 2008
[23] M O Nuwer K E Picchione and A Bhattacharjee ldquoPKA-induced internalization of Slack KNa channels produces dorsal
root ganglion neuron hyperexcitabilityrdquo The Journal of Neuro-science vol 30 no 42 pp 14165ndash14172 2010
[24] D Hess E Nanou and A El Manira ldquoCharacterization of Na+-activated K+ currents in larval lamprey spinal cord neuronsrdquoJournal of Neurophysiology vol 97 no 5 pp 3484ndash3493 2007
[25] E Nanou and A El Manira ldquoA postsynaptic negative feedbackmediated by coupling between AMPA receptors and Na+-activated K+ channels in spinal cord neuronesrdquo EuropeanJournal of Neuroscience vol 25 no 2 pp 445ndash450 2007
[26] PWallen B Robertson L Cangiano et al ldquoSodium-dependentpotassium channels of a Slack-like subtype contribute to theslow afterhyperpolarization in lamprey spinal neuronsrdquo TheJournal of Physiology vol 585 no 1 pp 75ndash90 2007
[27] E Nanou A Kyriakatos A Bhattacharjee L K Kaczmarek GParatcha and A El Manira ldquoNa+-mediated coupling betweenAMPA receptors and KNa channels shapes synaptic transmis-sionrdquo Proceedings of the National Academy of Sciences of theUnited States of America vol 105 no 52 pp 20941ndash20946 2008
[28] E Nanou and A El Manira ldquoMechanisms of modulation ofAMPA-induced Na+-activated K+ current by mGluR1rdquo Journalof Neurophysiology vol 103 no 1 pp 441ndash445 2010
[29] A R Mercer and J G Hildebrand ldquoDevelopmental changes inthe density of ionic currents in antennal-lobe neurons of thesphinx moth Manduca sextardquo Journal of Neurophysiology vol87 no 6 pp 2664ndash2675 2002
[30] K Aoki K Kosakai and M Yoshino ldquoMonoaminergic modu-lation of the Na+-activated K+ channel in kenyon cells isolatedfrom the mushroom body of the cricket (Gryllus bimaculatus)brainrdquo Journal of Neurophysiology vol 100 no 3 pp 1211ndash12222008
[31] G Klees P Hochstrate and P W Dierkes ldquoSodium-dependentpotassium channels in leech P neuronsrdquo Journal of MembraneBiology vol 208 no 1 pp 27ndash38 2005
[32] Y Zhang M R Brown C Hyland et al ldquoRegulation ofneuronal excitability by interaction of fragile X mental retar-dation protein with Slack potassium channelsrdquo The Journal ofNeuroscience vol 32 pp 15318ndash15327 2012
[33] C Lawrence and G C Rodrigo ldquoA Na+-activated K+ cur-rent (IKNa) is present in guinea-pig but not rat ventricularmyocytesrdquo Pflugers Archiv vol 437 no 6 pp 831ndash838 1999
[34] M Zhou Z Liu C Hu Z Zhang and Y Mei ldquoDevelopmentalregulation of a Na+-activated fast outward K+ current in ratmyoblastsrdquo Cellular Physiology and Biochemistry vol 14 no 4ndash6 pp 225ndash230 2004
[35] C K Young H S Jae M K Tong et al ldquoSodium-activatedpotassium current in guinea pig gastric myocytesrdquo Journal ofKorean Medical Science vol 22 no 1 pp 57ndash62 2007
[36] L Re V Moretti L Rossini and P Giusti ldquoSodium-activatedpotassium current in mouse diaphragmrdquo FEBS Letters vol 270no 1-2 pp 195ndash197 1990
[37] Y Zhang and W G Paterson ldquoFunctional evidence for Na+-activatedK+ channels in circular smoothmuscle of the opossumlower esophageal sphincterrdquo American The Journal of Physiol-ogy vol 292 no 6 pp G1600ndashG1606 2007
[38] M Paulais S Lachheb and J Teulon ldquoANa+- and Clminus-activatedK+ channel in the thick ascending limb of mouse kidneyrdquoJournal of General Physiology vol 127 no 2 pp 205ndash215 2006
[39] T M Egan D Dagan J Kupper and I B Levitan ldquoNa+-activated K+ channels are widely distributed in rat CNS and inXenopus oocytesrdquo Brain Research vol 584 no 1-2 pp 319ndash3211992
ISRN Neuroscience 13
[40] W J Joiner M D Tang L Y Wang et al ldquoFormation ofintermediate-conductance calcium-activated potassium chan-nels by interaction of Slack and Slo subunitsrdquo Nature Neuro-science vol 1 no 6 pp 462ndash469 1998
[41] A Yuan C M Santi A Wei et al ldquoThe sodium-activatedpotassium channel is encoded by a member of the Slo genefamilyrdquo Neuron vol 37 no 5 pp 765ndash773 2003
[42] Y Jiang A Pico M Cadene B T Chait and R MacKinnonldquoStructure of the RCK domain from the E coli K+ channeland demonstration of its presence in the human BK channelrdquoNeuron vol 29 no 3 pp 593ndash601 2001
[43] P Yuan M D Leonetti A R Pico Y Hsiung and R MacK-innon ldquoStructure of the human BK channel Ca2+-activationapparatus at 30 A resolutionrdquo Science vol 329 no 5988 pp182ndash186 2010
[44] Z Zhang A Rosenhouse-Dantsker Q Y Tang S Noskov andD E Logothetis ldquoThe RCK2 domain uses a coordination sitepresent in Kir channels to confer sodium sensitivity to Slo22channelsrdquoThe Journal of Neuroscience vol 30 no 22 pp 7554ndash7562 2010
[45] J L Sui K W Chan and D E Logothetis ldquoNa+ activationof the muscarinic K+ channel by a G-protein-independentmechanismrdquo Journal of General Physiology vol 108 no 5 pp381ndash391 1996
[46] M R Brown J Kronengold V R Gazula et al ldquoAmino-terminiisoforms of the Slack K+ channel regulated by alternative pro-moters differentiallymodulate rhythmic firing and adaptationrdquoThe Journal of Physiology vol 586 no 21 pp 5161ndash5179 2008
[47] A Yuan M Dourado A Butler N Walton A Wei and LSalkoff ldquoSLO-2 a K+ channel with an unusual Clminus dependencerdquoNature Neuroscience vol 3 no 8 pp 771ndash779 2000
[48] M Schreiber and L Salkoff ldquoA novel calcium-sensing domainin the BK channelrdquo Biophysical Journal vol 73 no 3 pp 1355ndash1363 1997
[49] H Chen J Kronengold Y Yan et al ldquoThe N-terminal domainof slack determines the formation and trafficking of slickslackheteromeric sodium-activated potassium channelsrdquo The Jour-nal of Neuroscience vol 29 no 17 pp 5654ndash5665 2009
[50] A Bhattacharjee L Gan and L K Kaczmarek ldquoLocalization ofthe Slack potassium channel in the rat central nervous systemrdquoJournal of Comparative Neurology vol 454 no 3 pp 241ndash2542002
[51] A Bhattacharjee C A A von Hehn X Mei and L KKaczmarek ldquoLocalization of the Na+-activated K+ channel slickin the rat central nervous systemrdquo Journal of ComparativeNeurology vol 484 no 1 pp 80ndash92 2005
[52] D S Koh P Jonas and W Vogel ldquoNa+-activated K+ channelslocalized in the nodal region of myelinated axons of XenopusrdquoThe Journal of Physiology vol 479 no 2 pp 183ndash197 1994
[53] K Mori T Saito Y Masuda and H Nakaya ldquoEffects of classIII antiarrhythmic drugs on the Na+-activated K+ channels inguinea-pig ventricular cellsrdquo British Journal of Pharmacologyvol 119 no 1 pp 133ndash141 1996
[54] K Mori S Kobayashi T Saito Y Masuda and H NakayaldquoInhibitory effects of class I and IV antiarrhythmic drugs onthe Na+- activated K+ channel current in guinea pig ventricularcellsrdquo Naunyn-Schmiedebergrsquos Archives of Pharmacology vol358 no 6 pp 641ndash648 1998
[55] Y Li T Sato and M Arita ldquoBepridil blunts the shortening ofaction potential duration caused by metabolic inhibition viablockade of ATP-sensitive K+ channels and Na+- activated K+
channelsrdquo Journal of Pharmacology and ExperimentalTherapeu-tics vol 291 no 2 pp 562ndash568 1999
[56] B Yang V K Gribkoff J Pan et al ldquoPharmacological activationand inhibition of Slack (Slo2 2) channelsrdquoNeuropharmacologyvol 51 no 4 pp 896ndash906 2006
[57] M D Tejada K Stolpe A K Meinild and D A KlaerkeldquoClofilium inhibits Slick and Slack potassium channelsrdquo Biolog-ics vol 6 pp 465ndash470 2012
[58] B Yang R Desai and L K Kaczmarek ldquoSlack and slick KNachannels regulate the accuracy of timing of auditory neuronsrdquoThe Journal of Neuroscience vol 27 no 10 pp 2617ndash2627 2007
[59] B Biton S Sethuramanujam K E Picchione et al ldquoTheantipsychotic drug loxapine is an opener of the sodium-activated potassium channel slack (Slo2 2)rdquo The Journal ofPharmacology and ExperimentalTherapeutics vol 340 pp 706ndash715 2012
[60] C M Santi G Ferreira B Yang et al ldquoOpposite regulation ofSlick and Slack K+ channels by neuromodulatorsrdquo The Journalof Neuroscience vol 26 no 19 pp 5059ndash5068 2006
[61] M R Fleming and L K Kaczmarek ldquoUse of optical biosensorsto detect modulation of Slack potassium channels by G protein-coupled receptorsrdquo Journal of Receptors and Signal Transduc-tion vol 29 no 3-4 pp 173ndash181 2009
[62] M D Tejada L J Jensen and D A Klaerke ldquoPIP(2) mod-ulation of Slick and Slack K(+) channelsrdquo Biochemical andBiophysical Research Communications vol 424 pp 208ndash2132012
[63] L Zhang M Sukhareva J L Barker et al ldquoDirect bindingof estradiol enhances Slack (sequence like a calcium-activatedpotassium channel) channelsrsquo activityrdquo Neuroscience vol 131no 2 pp 275ndash282 2005
[64] V A Ruffin X Q Gu D Zhou et al ldquoThe sodium-activatedpotassium channel Slack is modulated by hypercapnia andacidosisrdquo Neuroscience vol 151 no 2 pp 410ndash418 2008
[65] M O Nuwer K E Picchione and A Bhattacharjee ldquocAMP-dependent kinase does not modulate the Slack sodium-activated potassium channelrdquo Neuropharmacology vol 57 no3 pp 219ndash226 2009
[66] B C Suh and B Hille ldquoRegulation of ion channels by phos-phatidylinositol 45-bisphosphaterdquo Current Opinion in Neuro-biology vol 15 no 3 pp 370ndash378 2005
[67] L K Kaczmarek ldquoNon-conducting functions of voltage-gatedion channelsrdquo Nature Reviews Neuroscience vol 7 no 10 pp761ndash771 2006
[68] C R Rose ldquoNa+ signals at central synapsesrdquoNeuroscientist vol8 no 6 pp 532ndash539 2002
[69] C R Rose and B R Ransom ldquoRegulation of intracellularsodium in cultured rat hippocampal neuronesrdquo The Journal ofPhysiology vol 499 part 3 pp 573ndash587 1997
[70] C R Rose and A Konnerth ldquoNmda receptor-mediated Na+signals in spines and dendritesrdquoThe Journal of Neuroscience vol21 no 12 pp 4207ndash4214 2001
[71] N Zhong V Beaumont and R S Zucker ldquoRoles for mito-chondrial and reverse mode Na+Ca2+ exchange and the plas-malemma Ca2+ ATPase in post-tetanic potentiation at crayfishneuromuscular junctionsrdquo The Journal of Neuroscience vol 21no 24 pp 9598ndash9607 2001
[72] G Budelli T A Hage AWei et al ldquoNa+-activated K+ channelsexpress a large delayed outward current in neurons duringnormal physiologyrdquoNature Neuroscience vol 12 no 6 pp 745ndash750 2009
14 ISRN Neuroscience
[73] T A Hage and L Salkoff ldquoSodium-activated potassium chan-nels are functionally coupled to persistent sodiumcurrentsrdquoTheJournal of Neuroscience vol 32 pp 2714ndash2721 2012
[74] M RMarkham L K Kaczmarek andHH Zakon ldquoA sodium-activated potassium channe supports high frequency firing andreduces energetic costs during rapid modulations of actionpotential amplituderdquo Journal of Neurophysiology vol 109 no 7pp 1713ndash1723 2013
[75] S Uchino H Wada S Honda et al ldquoSlo2 sodium-activated K+channels bind to the PDZ domain of PSD-95rdquo Biochemical andBiophysical Research Communications vol 310 no 4 pp 1140ndash1147 2003
[76] T Zamalloa C P Bailey and J Pineda ldquoGlutamate-inducedpost-activation inhibition of locus coeruleus neurons is medi-ated by AMPAkainate receptors and sodium-dependent potas-sium currentsrdquo British Journal of Pharmacology vol 156 no 4pp 649ndash661 2009
[77] S Lu P Das D A Fadool and L K Kaczmarek ldquoThe slacksodium-activated potassium channel provides a major outwardcurrent in olfactory neurons of Kv13-- super-smeller micerdquoJournal of Neurophysiology vol 103 no 6 pp 3311ndash3319 2010
[78] R C Foehring P C Schwindt andW E Crill ldquoNorepinephrineselectively reduces slowCa2+- andNa+-mediated K+ currents incat neocortical neuronsrdquo Journal of Neurophysiology vol 61 no2 pp 245ndash256 1989
[79] M Kubota and N Saito ldquoSodium- and calcium-dependentconductances of neurones in the zebra finch hyperstriatumventrale pars caudale in vitrordquo The Journal of Physiology vol440 pp 131ndash142 1991
[80] U Kim and D A Mccormick ldquoFunctional and ionic propertiesof a slow afterhyperpolarization in ferret perigeniculate neuronsin vitrordquo Journal of Neurophysiology vol 80 no 3 pp 1222ndash1235 1998
[81] V M Sandler E Puil and DW F Schwarz ldquoIntrinsic responseproperties of bursting neurons in the nucleus principalistrigemini of the gerbilrdquoNeuroscience vol 83 no 3 pp 891ndash9041998
[82] M V Sanchez-Vives L G Nowak and D A McCormick ldquoCel-lular mechanisms of long-lasting adaptation in visual corticalneurons in vitrordquoThe Journal of Neuroscience vol 20 no 11 pp4286ndash4299 2000
[83] S Franceschetti T Lavazza G Curia et al ldquoNa+-activatedK+ current contributes to postexcitatory hyperpolarization inneocortical intrinsically bursting neuronsrdquo Journal of Neuro-physiology vol 89 no 4 pp 2101ndash2111 2003
[84] V F Descalzo L G Nowak J C Brumberg D A McCormickand M V Sanchez-Vives ldquoSlow adaptation in fast-spikingneurons of visual cortexrdquo Journal of Neurophysiology vol 93no 2 pp 1111ndash1118 2005
[85] J Benda L Maler and A Longtin ldquoLinear versus nonlinearsignal transmission in neuron models with adaptation currentsor dynamic thresholdsrdquo Journal of Neurophysiology vol 104 no5 pp 2806ndash2820 2010
[86] X Liu and L S Leung ldquoSodium-activated potassium conduc-tance participates in the depolarizing afterpotential following asingle action potential in rat hippocampal CA1 pyramidal cellsrdquoBrain Research vol 1023 no 2 pp 185ndash192 2004
[87] L Cangiano P Wallen and S Grillner ldquoRole of apamin-sensitive KCa channels for reticulospinal synaptic transmissionto motoneuron and for the afterhyperpolarizationrdquo Journal ofNeurophysiology vol 88 no 1 pp 289ndash299 2002
[88] Q Y Liu A E Schaffner and J L Barker ldquoKainate induces anintracellular Na+-activated K+ current in cultured embryonicrat hippocampal neuronesrdquo The Journal of Physiology vol 510part 3 pp 721ndash734 1998
[89] I Rishal T Keren-Raifman D Yakubovich et al ldquoNa+ pro-motes the dissociation between G120572GDP and G120573120574 activating Gprotein-gated K+ channelsrdquoThe Journal of Biological Chemistryvol 278 no 6 pp 3840ndash3845 2003
Submit your manuscripts athttpwwwhindawicom
Neurology Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Alzheimerrsquos DiseaseHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentSchizophrenia
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neural Plasticity
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Parkinsonrsquos Disease
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentAutism
Sleep DisordersHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Epilepsy Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Depression Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Cardiovascular Psychiatry and NeurologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
10 ISRN Neuroscience
and the decay rate of AMPA-induced synaptic currents pro-viding a feedback loop that decreases the size of glutamatergicpostsynaptic potentials [27]
Activation of KNa currents following activation of AMPAreceptors has also been proposed in studies of neurons of thelocus coeruleus to which application of glutamate producesa burst of action potentials followed by period of inhibitionlasting 30ndash45 seconds [76] The potassium current responsi-ble for the period of inhibition was selectively activated byAMPA or the AMPA-receptor agonist quisqualate but not byother stimuli that excite these neurons Consistent with thiscurrent being a KNa current it was attenuated by lowering ofexternal sodium concentrations but not affected by externalcalcium levels and it was potentiated by the KNa channelopener bithionol [76]
While persistent voltage-dependent sodium channels andAMPA receptors may represent the primary sodium entrypathways for activation of KNa channels there exist otherpathways that allow sodium ions to enter excitable cellsand whose role has been less explored Slack subunits areabundantly expressed in mitral cells of the olfactory bulb[46 50] In these neurons deletion of the gene for a majorvoltage-dependent potassiumchannelKv13 produces a com-pensatory increase in levels of Slack-B protein and in SlackKNa currents identified using an anti-Slack siRNA technique[77] Activation of the KNa current during depolarizationof these cells could only be significantly suppressed by acombination of tetrodotoxin to block voltage-dependentsodium channels and ZD7288 a blocker of H-channelswhich are nonselective cation channels that flux both sodiumand potassium ions Either drug alone failed to significantlysuppress KNa currents suggesting that sodium entry throughH-channels plays a role in KNa channel activation in theseneurons [77]
11 Contribution of KNa Channels toNeuronal Firing Patterns
The component of whole-cell potassium current in neuronsthat can be attributed to Slack channel subunits has beenidentified in mitral cells of the olfactory bulb and mediumspiny neuron of the striatum using siRNAs to suppress trans-lation of SlackmRNA [72 77] Such experiments indicate thatSlack KNa channels are responsible for a very major compo-nent of the total delayed outward current in neurons underphysiological conditions [72] The specific contributions thatKNa channels make to the electrical personality of individualtypes of neurons may differ in different cell types
One of the earliest effects to be described for activationof KNa currents is to produce a slow afterhyperpolarization(sAHP) following a burst of action potentials This was firstdescribed in pyramidal neurons of the cat sensorimotorcortex [11 78] and similar slow sodium-dependent sAHPsthat endure for many seconds have been found in a widevariety of neuronal types [79ndash84]These have been the subjectof earlier reviews [20 21] Such slow potentials have beenfound to influence the timing of regular bursts of actionpotentials such as the 3ndash12 per minute spindle waves that
occur at the onset of sleep in certain neurons within thethalamus [80] Slow activation of KNa during repetitive firingalso produces an adaptation of firing rate during bursts ofaction potentials [23] and such adaptation is predicted bynumerical simulations that incorporate KNa currents [46 85]
In some neurons such as neocortical pyramidal neuronsand hippocampal CA1 neurons a significant amount of KNacurrent can also be activated by single spikes and contributeto action potentials repolarization or to the amplitude ofdepolarizing afterpotentials that follow single spikes andare generated by persistent sodium currents [83 86] Inthese cases KNa current activation may be detected withinsim5msec following a single-action potential A recent studyhas examined themechanismbywhich the cells of the electricorgan (electrocytes) of certain electric fish are able to generatesustained firing at frequencies greater than 500Hz [74] Slackchannels are expressed at high levels in these cells and theirdelayed potassium current is almost exclusively a KNa currentthat is activated by a persistent sodium current Numericalsimulations have indicated that the use of KNa current torepolarize the very large action potentials of these cells allowsthe cells to fire at such high rates with minimal energyexpenditure [74]
A set of neurons that have been particularly instructivein studies of the role of KNa channels in shaping neuronalfiring are those that control locomotion in the spinal cord oflamprey [24ndash28 87] In addition to the transient KNa currentthat is activated by AMPA receptors and that was describedearlier [25 27] these neurons appear to have two distinctcomponents ofKNa current that are triggered by action poten-tial firingmdasha transient component that follows a single spikeand a slower more sustained component [24 26 87] Therapid transient KNa current is activated by the sodium influxthrough tetrodotoxin-sensitive channel during the actionpotentials and may play a role in determining the amplitudeand duration of individual action potentials The sustainedKNa current is not abolished by tetrodotoxin suggesting thatit is selectively activated by an independent sodium entrypathway and may underlie the calcium-independent sAHPthat occurs during repetitive bursting in lamprey spinal cordneurons [24]
Experiments with neurons that fire at high rates withvery high temporal accuracy have provided another potentialbiological role for KNa channels [58] Slack and Slick areexpressed at high levels in neurons of the medial nucleusof the trapezoid (MNTB) within the auditory brainstem[6 50 51] These neurons fire at rates up to sim800Hzand accurate timing of their action potentials is requiredfor transmission of information about the localization ofsound stimuli KNa channels are readily be detected in bothcell-attached and excised patches on the somata of MNTBneurons and they are localized appropriately for activationby sodium influx through AMPA receptors or voltage-dependent sodium channels [58] Current-clamp recordingsfrom MNTB neurons and numerical simulations indicatethat as KNa channels become activated temporal accuracyof firing is improved This occurs because the voltage-dependence of Slack and Slick channels allows them to opennear the resting potential of MNTB neurons increasing
ISRN Neuroscience 11
the resting conductance and decreasing the membrane timeconstant An increase in temporal fidelity of MNTB neuronscan also be produced by treatment with the pharmacologicalKNa activator bithionol [58]
12 Mutations in Slack Channels Result inEpilepsy and Intellectual Disabilities
De novo human mutations that produce single-amino-acidalterations in Slack channels produce devastating effectson development and intellectual function [2 3] One setof Slack mutations results in malignant migrating partialseizures of infancy (MMPSI) [2]This disease is characterizedby pharmacoresistant seizures in the first 6 months of lifethat arise randomly and ldquomigraterdquo from one brain regionto another While the occurrence of seizures abates withage MMPSI results in near-total arrest of psychomotordevelopment Exome sequencing revealed that at least 50of MMPSI patients have mutations in the large cytoplasmicC-terminal domain of Slack [2]
Two mutations in the human Slack gene that produceMMPSI are R428Q and A934T which correspond to themutations R409Q and A913T in the highly conserved ratSlack sequence [2] Expression of these mutants in Xenopusoocytes produced Slack currents with voltage dependenceand kinetic behavior that matched that of wild-type Slackchannels but with a current amplitude that was increased 2-3-fold over that of wild-type Slack This increase in currentlikely results because the mutations produce channels thatare either more prone to undergo phosphorylation by PKC(or less likely to be dephosphorylated) or that mimic theconformation induced by PKC activation One of theMMPSImutations R409Q lies adjacent to the phosphorylation siteS407 that as described earlier enhances current by 2-3-fold when phosphorylated by PKC Consistent with thisinterpretation current amplitude of the two MMPSI muta-tions could not be further increased upon PKC activa-tion [2] In other respects the MMPSI currents resembledthat of wild-type channels with unitary conductance andsodiumdependencesmatching those of wild-type Slack BothMMPSI mutant channels however had reduced openings tosubconductance states compared to the wild-type channels[2]
A second disease that has been attributed to a differentset of mutations in the C-terminal domain of Slack chan-nels is autosomal dominant nocturnal frontal lobe epilepsy(ADNFLE) [3] This is a childhood-onset focal epilepsysyndrome in which motor seizures arise during sleep Onsetof ADNFLE in the patients with Slack mutations occurred ata mean age of 6 years and was associated with mild to severeintellectual disability andor severe psychiatric problemsTheeffects of these ADNFLE mutations on the function of Slackchannels have however not been investigated Interestinglyanother known genetic cause of ADNFLE is mutations insubunits of the neuronal nicotinic acetycholine receptor Theacetycholine receptor ADNFLE mutations do not howeverproduce intellectual disability which contrasts strikingly tothe effects of the Slack mutations [3]
It is possible that the very severe outcomes of mutationsin Slack occur solely because of changes in the amplitudeof Slack currents An increase in total potassium currentin rapidly firing interneurons could reduce the firing rateof these cells predisposing the nervous system to hyper-excitability The relatively modest increase in current thatoccurs with theMMPSImutations [2] however suggests thatother processes may also be affected As described earlierthe large C-terminal cytoplasmic domain of Slack in whichMMPSI and ADNFLE mutations are located interacts withFMRP and its cargo mRNAs [6] Thus it is possible thatin addition to directly altering the excitability of centralneuronsmutations in Slack disrupt the regulation of activity-dependent protein translation in neurons
13 Conclusions
This review has covered themolecular and cellular propertiesof Slack and Slick two potassium channel subunits thatprovide neurons and other cells with KNa channels It shouldbe pointed out that additional potassium channels that areactivated by cytoplasmic sodiummay also exist For examplesome of the native KNa currents that have been described inneurons do not match Slack or Slick in their single-channelconductances or pharmacological properties [88]These mayperhaps represent inward rectifier potassium channels thatare activated by sodium [45 89] and the physiological rolesof the sodium sensitivity of these channels require furtherinvestigation Nevertheless Slack and Slick which are verywidely expressed in the brain appear to play dominant rolesin shaping the firing properties of verymany types of neuronsHumanmutations in these channels produce profound effectson neuronal function and development suggesting that per-haps their biological role may extend beyond simply settingthe level of neuronal excitability and that these channels mayinfluence cytoplasmic biochemical pathways that regulatedevelopment plasticity and intellectual function
References
[1] B Hille Ionic Channels of Excitable Membranes Sinauer Asso-ciates Sunderland Mass USA 3rd edition 2001
[2] G Barcia M R Fleming A Deligniere et al ldquoDe novo gain-of-function KCNT1 channel mutations cause malignant migratingpartial seizures of infancyrdquo Nature Genetics vol 44 pp 1255ndash1259 2012
[3] S E Heron K R SmithM Bahlo et al ldquoMissensemutations inthe sodium-gated potassium channel gene KCNT1 cause severeautosomal dominant nocturnal frontal lobe epilepsyrdquo NatureGenetics vol 44 pp 1188ndash1190 2012
[4] A Bhattacharjee W J Joiner M Wu Y Yang F J Sigworthand L K Kaczmarek ldquoSlick (Slo2 1) a rapidly-gating sodium-activated potassium channel inhibited by ATPrdquo The Journal ofNeuroscience vol 23 no 37 pp 11681ndash11691 2003
[5] T J Tamsett K E Picchione and A Bhattacharjee ldquoNAD+activates KNa channels in dorsal root ganglion neuronsrdquo TheJournal of Neuroscience vol 29 no 16 pp 5127ndash5134 2009
12 ISRN Neuroscience
[6] M R Brown J Kronengold V R Gazula et al ldquoFragileX mental retardation protein controls gating of the sodium-activated potassium channel SlackrdquoNatureNeuroscience vol 13no 7 pp 819ndash821 2010
[7] M Kameyama M Kakei and R Sato ldquoIntracellular Na+activates a K+ channel in mammalian cardiac cellsrdquoNature vol309 no 5966 pp 354ndash356 1984
[8] C R Bader L Bernheim and D Bertrand ldquoSodium-activatedpotassium current in cultured avian neuronesrdquoNature vol 317no 6037 pp 540ndash542 1985
[9] K Hartung ldquoPotentiation of a transient outward current byNa+influx in crayfish neuronesrdquo Pflugers Archiv vol 404 no 1 pp41ndash44 1985
[10] S E Dryer J T Fujii and A R Martin ldquoA Na+-activatedK+ current in cultured brain stem neurones from chicksrdquo TheJournal of Physiology vol 410 pp 283ndash296 1989
[11] P C Schwindt W J Spain and W E Crill ldquoLong-lastingreduction of excitability by a sodium-dependent potassiumcurrent in cat neocortical neuronsrdquo Journal of Neurophysiologyvol 61 no 2 pp 233ndash244 1989
[12] C Haimann L Bernheim D Bertrand and C R BaderldquoPotassium current activated by intracellular sodium in quailtrigeminal ganglion neuronsrdquo Journal of General Physiology vol95 no 5 pp 961ndash979 1990
[13] S E Dryer ldquoNa+-activated K+ channels and voltage-evokedionic currents in brain stem and parasympathetic neurones ofthe chickrdquoThe Journal of Physiology vol 435 pp 513ndash532 1991
[14] M Saito and C F Wu ldquoExpression of ion channels andmutational effects in giant Drosophila neurons differentiatedfrom cell division-arrested embryonic neuroblastsrdquoThe Journalof Neuroscience vol 11 no 7 pp 2135ndash2150 1991
[15] T M Egan D Dagan J Kupper and I B Levitan ldquoPropertiesand rundown of sodium-activated potassium channels in ratolfactory bulb neuronsrdquoThe Journal of Neuroscience vol 12 no5 pp 1964ndash1976 1992
[16] C Haimann J Magistretti and B Pozzi ldquoSodium-activatedpotassium current in sensory neurons a comparison of cell-attached and cell-free single-channel activitiesrdquo Pflugers Archivvol 422 no 3 pp 287ndash294 1992
[17] N Dale ldquoA large sustained Na+- and voltage-dependent K+current in spinal neurons of the frog embryordquo The Journal ofPhysiology vol 462 pp 349ndash372 1993
[18] B V Safronov and W Vogel ldquoProperties and functions of Na+-activated K+ channels in the soma of rat motoneuronesrdquo TheJournal of Physiology vol 497 no 3 pp 727ndash734 1996
[19] U Bischoff W Vogel and B V Safronov ldquoNa+-activated K+channels in small dorsal root ganglion neurones of ratrdquo TheJournal of Physiology vol 510 part 3 pp 743ndash754 1998
[20] S E Dryer ldquoNa+-activated K+ channels a new family of large-conductance ion channelsrdquo Trends in Neurosciences vol 17 no4 pp 155ndash160 1994
[21] A Bhattacharjee and L K Kaczmarek ldquoFor K+ channels Na+ isthe new Ca2+ rdquo Trends in Neurosciences vol 28 no 8 pp 422ndash428 2005
[22] S B Gao Y Wu C X Lu Z H Guo C H Li and J P DingldquoSlack and Slick KNa channels are required for the depolarizingafterpotential of acutely isolated medium diameter rat dorsalroot ganglion neuronsrdquoActa Pharmacologica Sinica vol 29 no8 pp 899ndash905 2008
[23] M O Nuwer K E Picchione and A Bhattacharjee ldquoPKA-induced internalization of Slack KNa channels produces dorsal
root ganglion neuron hyperexcitabilityrdquo The Journal of Neuro-science vol 30 no 42 pp 14165ndash14172 2010
[24] D Hess E Nanou and A El Manira ldquoCharacterization of Na+-activated K+ currents in larval lamprey spinal cord neuronsrdquoJournal of Neurophysiology vol 97 no 5 pp 3484ndash3493 2007
[25] E Nanou and A El Manira ldquoA postsynaptic negative feedbackmediated by coupling between AMPA receptors and Na+-activated K+ channels in spinal cord neuronesrdquo EuropeanJournal of Neuroscience vol 25 no 2 pp 445ndash450 2007
[26] PWallen B Robertson L Cangiano et al ldquoSodium-dependentpotassium channels of a Slack-like subtype contribute to theslow afterhyperpolarization in lamprey spinal neuronsrdquo TheJournal of Physiology vol 585 no 1 pp 75ndash90 2007
[27] E Nanou A Kyriakatos A Bhattacharjee L K Kaczmarek GParatcha and A El Manira ldquoNa+-mediated coupling betweenAMPA receptors and KNa channels shapes synaptic transmis-sionrdquo Proceedings of the National Academy of Sciences of theUnited States of America vol 105 no 52 pp 20941ndash20946 2008
[28] E Nanou and A El Manira ldquoMechanisms of modulation ofAMPA-induced Na+-activated K+ current by mGluR1rdquo Journalof Neurophysiology vol 103 no 1 pp 441ndash445 2010
[29] A R Mercer and J G Hildebrand ldquoDevelopmental changes inthe density of ionic currents in antennal-lobe neurons of thesphinx moth Manduca sextardquo Journal of Neurophysiology vol87 no 6 pp 2664ndash2675 2002
[30] K Aoki K Kosakai and M Yoshino ldquoMonoaminergic modu-lation of the Na+-activated K+ channel in kenyon cells isolatedfrom the mushroom body of the cricket (Gryllus bimaculatus)brainrdquo Journal of Neurophysiology vol 100 no 3 pp 1211ndash12222008
[31] G Klees P Hochstrate and P W Dierkes ldquoSodium-dependentpotassium channels in leech P neuronsrdquo Journal of MembraneBiology vol 208 no 1 pp 27ndash38 2005
[32] Y Zhang M R Brown C Hyland et al ldquoRegulation ofneuronal excitability by interaction of fragile X mental retar-dation protein with Slack potassium channelsrdquo The Journal ofNeuroscience vol 32 pp 15318ndash15327 2012
[33] C Lawrence and G C Rodrigo ldquoA Na+-activated K+ cur-rent (IKNa) is present in guinea-pig but not rat ventricularmyocytesrdquo Pflugers Archiv vol 437 no 6 pp 831ndash838 1999
[34] M Zhou Z Liu C Hu Z Zhang and Y Mei ldquoDevelopmentalregulation of a Na+-activated fast outward K+ current in ratmyoblastsrdquo Cellular Physiology and Biochemistry vol 14 no 4ndash6 pp 225ndash230 2004
[35] C K Young H S Jae M K Tong et al ldquoSodium-activatedpotassium current in guinea pig gastric myocytesrdquo Journal ofKorean Medical Science vol 22 no 1 pp 57ndash62 2007
[36] L Re V Moretti L Rossini and P Giusti ldquoSodium-activatedpotassium current in mouse diaphragmrdquo FEBS Letters vol 270no 1-2 pp 195ndash197 1990
[37] Y Zhang and W G Paterson ldquoFunctional evidence for Na+-activatedK+ channels in circular smoothmuscle of the opossumlower esophageal sphincterrdquo American The Journal of Physiol-ogy vol 292 no 6 pp G1600ndashG1606 2007
[38] M Paulais S Lachheb and J Teulon ldquoANa+- and Clminus-activatedK+ channel in the thick ascending limb of mouse kidneyrdquoJournal of General Physiology vol 127 no 2 pp 205ndash215 2006
[39] T M Egan D Dagan J Kupper and I B Levitan ldquoNa+-activated K+ channels are widely distributed in rat CNS and inXenopus oocytesrdquo Brain Research vol 584 no 1-2 pp 319ndash3211992
ISRN Neuroscience 13
[40] W J Joiner M D Tang L Y Wang et al ldquoFormation ofintermediate-conductance calcium-activated potassium chan-nels by interaction of Slack and Slo subunitsrdquo Nature Neuro-science vol 1 no 6 pp 462ndash469 1998
[41] A Yuan C M Santi A Wei et al ldquoThe sodium-activatedpotassium channel is encoded by a member of the Slo genefamilyrdquo Neuron vol 37 no 5 pp 765ndash773 2003
[42] Y Jiang A Pico M Cadene B T Chait and R MacKinnonldquoStructure of the RCK domain from the E coli K+ channeland demonstration of its presence in the human BK channelrdquoNeuron vol 29 no 3 pp 593ndash601 2001
[43] P Yuan M D Leonetti A R Pico Y Hsiung and R MacK-innon ldquoStructure of the human BK channel Ca2+-activationapparatus at 30 A resolutionrdquo Science vol 329 no 5988 pp182ndash186 2010
[44] Z Zhang A Rosenhouse-Dantsker Q Y Tang S Noskov andD E Logothetis ldquoThe RCK2 domain uses a coordination sitepresent in Kir channels to confer sodium sensitivity to Slo22channelsrdquoThe Journal of Neuroscience vol 30 no 22 pp 7554ndash7562 2010
[45] J L Sui K W Chan and D E Logothetis ldquoNa+ activationof the muscarinic K+ channel by a G-protein-independentmechanismrdquo Journal of General Physiology vol 108 no 5 pp381ndash391 1996
[46] M R Brown J Kronengold V R Gazula et al ldquoAmino-terminiisoforms of the Slack K+ channel regulated by alternative pro-moters differentiallymodulate rhythmic firing and adaptationrdquoThe Journal of Physiology vol 586 no 21 pp 5161ndash5179 2008
[47] A Yuan M Dourado A Butler N Walton A Wei and LSalkoff ldquoSLO-2 a K+ channel with an unusual Clminus dependencerdquoNature Neuroscience vol 3 no 8 pp 771ndash779 2000
[48] M Schreiber and L Salkoff ldquoA novel calcium-sensing domainin the BK channelrdquo Biophysical Journal vol 73 no 3 pp 1355ndash1363 1997
[49] H Chen J Kronengold Y Yan et al ldquoThe N-terminal domainof slack determines the formation and trafficking of slickslackheteromeric sodium-activated potassium channelsrdquo The Jour-nal of Neuroscience vol 29 no 17 pp 5654ndash5665 2009
[50] A Bhattacharjee L Gan and L K Kaczmarek ldquoLocalization ofthe Slack potassium channel in the rat central nervous systemrdquoJournal of Comparative Neurology vol 454 no 3 pp 241ndash2542002
[51] A Bhattacharjee C A A von Hehn X Mei and L KKaczmarek ldquoLocalization of the Na+-activated K+ channel slickin the rat central nervous systemrdquo Journal of ComparativeNeurology vol 484 no 1 pp 80ndash92 2005
[52] D S Koh P Jonas and W Vogel ldquoNa+-activated K+ channelslocalized in the nodal region of myelinated axons of XenopusrdquoThe Journal of Physiology vol 479 no 2 pp 183ndash197 1994
[53] K Mori T Saito Y Masuda and H Nakaya ldquoEffects of classIII antiarrhythmic drugs on the Na+-activated K+ channels inguinea-pig ventricular cellsrdquo British Journal of Pharmacologyvol 119 no 1 pp 133ndash141 1996
[54] K Mori S Kobayashi T Saito Y Masuda and H NakayaldquoInhibitory effects of class I and IV antiarrhythmic drugs onthe Na+- activated K+ channel current in guinea pig ventricularcellsrdquo Naunyn-Schmiedebergrsquos Archives of Pharmacology vol358 no 6 pp 641ndash648 1998
[55] Y Li T Sato and M Arita ldquoBepridil blunts the shortening ofaction potential duration caused by metabolic inhibition viablockade of ATP-sensitive K+ channels and Na+- activated K+
channelsrdquo Journal of Pharmacology and ExperimentalTherapeu-tics vol 291 no 2 pp 562ndash568 1999
[56] B Yang V K Gribkoff J Pan et al ldquoPharmacological activationand inhibition of Slack (Slo2 2) channelsrdquoNeuropharmacologyvol 51 no 4 pp 896ndash906 2006
[57] M D Tejada K Stolpe A K Meinild and D A KlaerkeldquoClofilium inhibits Slick and Slack potassium channelsrdquo Biolog-ics vol 6 pp 465ndash470 2012
[58] B Yang R Desai and L K Kaczmarek ldquoSlack and slick KNachannels regulate the accuracy of timing of auditory neuronsrdquoThe Journal of Neuroscience vol 27 no 10 pp 2617ndash2627 2007
[59] B Biton S Sethuramanujam K E Picchione et al ldquoTheantipsychotic drug loxapine is an opener of the sodium-activated potassium channel slack (Slo2 2)rdquo The Journal ofPharmacology and ExperimentalTherapeutics vol 340 pp 706ndash715 2012
[60] C M Santi G Ferreira B Yang et al ldquoOpposite regulation ofSlick and Slack K+ channels by neuromodulatorsrdquo The Journalof Neuroscience vol 26 no 19 pp 5059ndash5068 2006
[61] M R Fleming and L K Kaczmarek ldquoUse of optical biosensorsto detect modulation of Slack potassium channels by G protein-coupled receptorsrdquo Journal of Receptors and Signal Transduc-tion vol 29 no 3-4 pp 173ndash181 2009
[62] M D Tejada L J Jensen and D A Klaerke ldquoPIP(2) mod-ulation of Slick and Slack K(+) channelsrdquo Biochemical andBiophysical Research Communications vol 424 pp 208ndash2132012
[63] L Zhang M Sukhareva J L Barker et al ldquoDirect bindingof estradiol enhances Slack (sequence like a calcium-activatedpotassium channel) channelsrsquo activityrdquo Neuroscience vol 131no 2 pp 275ndash282 2005
[64] V A Ruffin X Q Gu D Zhou et al ldquoThe sodium-activatedpotassium channel Slack is modulated by hypercapnia andacidosisrdquo Neuroscience vol 151 no 2 pp 410ndash418 2008
[65] M O Nuwer K E Picchione and A Bhattacharjee ldquocAMP-dependent kinase does not modulate the Slack sodium-activated potassium channelrdquo Neuropharmacology vol 57 no3 pp 219ndash226 2009
[66] B C Suh and B Hille ldquoRegulation of ion channels by phos-phatidylinositol 45-bisphosphaterdquo Current Opinion in Neuro-biology vol 15 no 3 pp 370ndash378 2005
[67] L K Kaczmarek ldquoNon-conducting functions of voltage-gatedion channelsrdquo Nature Reviews Neuroscience vol 7 no 10 pp761ndash771 2006
[68] C R Rose ldquoNa+ signals at central synapsesrdquoNeuroscientist vol8 no 6 pp 532ndash539 2002
[69] C R Rose and B R Ransom ldquoRegulation of intracellularsodium in cultured rat hippocampal neuronesrdquo The Journal ofPhysiology vol 499 part 3 pp 573ndash587 1997
[70] C R Rose and A Konnerth ldquoNmda receptor-mediated Na+signals in spines and dendritesrdquoThe Journal of Neuroscience vol21 no 12 pp 4207ndash4214 2001
[71] N Zhong V Beaumont and R S Zucker ldquoRoles for mito-chondrial and reverse mode Na+Ca2+ exchange and the plas-malemma Ca2+ ATPase in post-tetanic potentiation at crayfishneuromuscular junctionsrdquo The Journal of Neuroscience vol 21no 24 pp 9598ndash9607 2001
[72] G Budelli T A Hage AWei et al ldquoNa+-activated K+ channelsexpress a large delayed outward current in neurons duringnormal physiologyrdquoNature Neuroscience vol 12 no 6 pp 745ndash750 2009
14 ISRN Neuroscience
[73] T A Hage and L Salkoff ldquoSodium-activated potassium chan-nels are functionally coupled to persistent sodiumcurrentsrdquoTheJournal of Neuroscience vol 32 pp 2714ndash2721 2012
[74] M RMarkham L K Kaczmarek andHH Zakon ldquoA sodium-activated potassium channe supports high frequency firing andreduces energetic costs during rapid modulations of actionpotential amplituderdquo Journal of Neurophysiology vol 109 no 7pp 1713ndash1723 2013
[75] S Uchino H Wada S Honda et al ldquoSlo2 sodium-activated K+channels bind to the PDZ domain of PSD-95rdquo Biochemical andBiophysical Research Communications vol 310 no 4 pp 1140ndash1147 2003
[76] T Zamalloa C P Bailey and J Pineda ldquoGlutamate-inducedpost-activation inhibition of locus coeruleus neurons is medi-ated by AMPAkainate receptors and sodium-dependent potas-sium currentsrdquo British Journal of Pharmacology vol 156 no 4pp 649ndash661 2009
[77] S Lu P Das D A Fadool and L K Kaczmarek ldquoThe slacksodium-activated potassium channel provides a major outwardcurrent in olfactory neurons of Kv13-- super-smeller micerdquoJournal of Neurophysiology vol 103 no 6 pp 3311ndash3319 2010
[78] R C Foehring P C Schwindt andW E Crill ldquoNorepinephrineselectively reduces slowCa2+- andNa+-mediated K+ currents incat neocortical neuronsrdquo Journal of Neurophysiology vol 61 no2 pp 245ndash256 1989
[79] M Kubota and N Saito ldquoSodium- and calcium-dependentconductances of neurones in the zebra finch hyperstriatumventrale pars caudale in vitrordquo The Journal of Physiology vol440 pp 131ndash142 1991
[80] U Kim and D A Mccormick ldquoFunctional and ionic propertiesof a slow afterhyperpolarization in ferret perigeniculate neuronsin vitrordquo Journal of Neurophysiology vol 80 no 3 pp 1222ndash1235 1998
[81] V M Sandler E Puil and DW F Schwarz ldquoIntrinsic responseproperties of bursting neurons in the nucleus principalistrigemini of the gerbilrdquoNeuroscience vol 83 no 3 pp 891ndash9041998
[82] M V Sanchez-Vives L G Nowak and D A McCormick ldquoCel-lular mechanisms of long-lasting adaptation in visual corticalneurons in vitrordquoThe Journal of Neuroscience vol 20 no 11 pp4286ndash4299 2000
[83] S Franceschetti T Lavazza G Curia et al ldquoNa+-activatedK+ current contributes to postexcitatory hyperpolarization inneocortical intrinsically bursting neuronsrdquo Journal of Neuro-physiology vol 89 no 4 pp 2101ndash2111 2003
[84] V F Descalzo L G Nowak J C Brumberg D A McCormickand M V Sanchez-Vives ldquoSlow adaptation in fast-spikingneurons of visual cortexrdquo Journal of Neurophysiology vol 93no 2 pp 1111ndash1118 2005
[85] J Benda L Maler and A Longtin ldquoLinear versus nonlinearsignal transmission in neuron models with adaptation currentsor dynamic thresholdsrdquo Journal of Neurophysiology vol 104 no5 pp 2806ndash2820 2010
[86] X Liu and L S Leung ldquoSodium-activated potassium conduc-tance participates in the depolarizing afterpotential following asingle action potential in rat hippocampal CA1 pyramidal cellsrdquoBrain Research vol 1023 no 2 pp 185ndash192 2004
[87] L Cangiano P Wallen and S Grillner ldquoRole of apamin-sensitive KCa channels for reticulospinal synaptic transmissionto motoneuron and for the afterhyperpolarizationrdquo Journal ofNeurophysiology vol 88 no 1 pp 289ndash299 2002
[88] Q Y Liu A E Schaffner and J L Barker ldquoKainate induces anintracellular Na+-activated K+ current in cultured embryonicrat hippocampal neuronesrdquo The Journal of Physiology vol 510part 3 pp 721ndash734 1998
[89] I Rishal T Keren-Raifman D Yakubovich et al ldquoNa+ pro-motes the dissociation between G120572GDP and G120573120574 activating Gprotein-gated K+ channelsrdquoThe Journal of Biological Chemistryvol 278 no 6 pp 3840ndash3845 2003
Submit your manuscripts athttpwwwhindawicom
Neurology Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Alzheimerrsquos DiseaseHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentSchizophrenia
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neural Plasticity
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Parkinsonrsquos Disease
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentAutism
Sleep DisordersHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neuroscience Journal
Epilepsy Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Psychiatry Journal
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Computational and Mathematical Methods in Medicine
Depression Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Brain ScienceInternational Journal of
StrokeResearch and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neurodegenerative Diseases
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Cardiovascular Psychiatry and NeurologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
ISRN Neuroscience 11
the resting conductance and decreasing the membrane timeconstant An increase in temporal fidelity of MNTB neuronscan also be produced by treatment with the pharmacologicalKNa activator bithionol [58]
12 Mutations in Slack Channels Result inEpilepsy and Intellectual Disabilities
De novo human mutations that produce single-amino-acidalterations in Slack channels produce devastating effectson development and intellectual function [2 3] One setof Slack mutations results in malignant migrating partialseizures of infancy (MMPSI) [2]This disease is characterizedby pharmacoresistant seizures in the first 6 months of lifethat arise randomly and ldquomigraterdquo from one brain regionto another While the occurrence of seizures abates withage MMPSI results in near-total arrest of psychomotordevelopment Exome sequencing revealed that at least 50of MMPSI patients have mutations in the large cytoplasmicC-terminal domain of Slack [2]
Two mutations in the human Slack gene that produceMMPSI are R428Q and A934T which correspond to themutations R409Q and A913T in the highly conserved ratSlack sequence [2] Expression of these mutants in Xenopusoocytes produced Slack currents with voltage dependenceand kinetic behavior that matched that of wild-type Slackchannels but with a current amplitude that was increased 2-3-fold over that of wild-type Slack This increase in currentlikely results because the mutations produce channels thatare either more prone to undergo phosphorylation by PKC(or less likely to be dephosphorylated) or that mimic theconformation induced by PKC activation One of theMMPSImutations R409Q lies adjacent to the phosphorylation siteS407 that as described earlier enhances current by 2-3-fold when phosphorylated by PKC Consistent with thisinterpretation current amplitude of the two MMPSI muta-tions could not be further increased upon PKC activa-tion [2] In other respects the MMPSI currents resembledthat of wild-type channels with unitary conductance andsodiumdependencesmatching those of wild-type Slack BothMMPSI mutant channels however had reduced openings tosubconductance states compared to the wild-type channels[2]
A second disease that has been attributed to a differentset of mutations in the C-terminal domain of Slack chan-nels is autosomal dominant nocturnal frontal lobe epilepsy(ADNFLE) [3] This is a childhood-onset focal epilepsysyndrome in which motor seizures arise during sleep Onsetof ADNFLE in the patients with Slack mutations occurred ata mean age of 6 years and was associated with mild to severeintellectual disability andor severe psychiatric problemsTheeffects of these ADNFLE mutations on the function of Slackchannels have however not been investigated Interestinglyanother known genetic cause of ADNFLE is mutations insubunits of the neuronal nicotinic acetycholine receptor Theacetycholine receptor ADNFLE mutations do not howeverproduce intellectual disability which contrasts strikingly tothe effects of the Slack mutations [3]
It is possible that the very severe outcomes of mutationsin Slack occur solely because of changes in the amplitudeof Slack currents An increase in total potassium currentin rapidly firing interneurons could reduce the firing rateof these cells predisposing the nervous system to hyper-excitability The relatively modest increase in current thatoccurs with theMMPSImutations [2] however suggests thatother processes may also be affected As described earlierthe large C-terminal cytoplasmic domain of Slack in whichMMPSI and ADNFLE mutations are located interacts withFMRP and its cargo mRNAs [6] Thus it is possible thatin addition to directly altering the excitability of centralneuronsmutations in Slack disrupt the regulation of activity-dependent protein translation in neurons
13 Conclusions
This review has covered themolecular and cellular propertiesof Slack and Slick two potassium channel subunits thatprovide neurons and other cells with KNa channels It shouldbe pointed out that additional potassium channels that areactivated by cytoplasmic sodiummay also exist For examplesome of the native KNa currents that have been described inneurons do not match Slack or Slick in their single-channelconductances or pharmacological properties [88]These mayperhaps represent inward rectifier potassium channels thatare activated by sodium [45 89] and the physiological rolesof the sodium sensitivity of these channels require furtherinvestigation Nevertheless Slack and Slick which are verywidely expressed in the brain appear to play dominant rolesin shaping the firing properties of verymany types of neuronsHumanmutations in these channels produce profound effectson neuronal function and development suggesting that per-haps their biological role may extend beyond simply settingthe level of neuronal excitability and that these channels mayinfluence cytoplasmic biochemical pathways that regulatedevelopment plasticity and intellectual function
References
[1] B Hille Ionic Channels of Excitable Membranes Sinauer Asso-ciates Sunderland Mass USA 3rd edition 2001
[2] G Barcia M R Fleming A Deligniere et al ldquoDe novo gain-of-function KCNT1 channel mutations cause malignant migratingpartial seizures of infancyrdquo Nature Genetics vol 44 pp 1255ndash1259 2012
[3] S E Heron K R SmithM Bahlo et al ldquoMissensemutations inthe sodium-gated potassium channel gene KCNT1 cause severeautosomal dominant nocturnal frontal lobe epilepsyrdquo NatureGenetics vol 44 pp 1188ndash1190 2012
[4] A Bhattacharjee W J Joiner M Wu Y Yang F J Sigworthand L K Kaczmarek ldquoSlick (Slo2 1) a rapidly-gating sodium-activated potassium channel inhibited by ATPrdquo The Journal ofNeuroscience vol 23 no 37 pp 11681ndash11691 2003
[5] T J Tamsett K E Picchione and A Bhattacharjee ldquoNAD+activates KNa channels in dorsal root ganglion neuronsrdquo TheJournal of Neuroscience vol 29 no 16 pp 5127ndash5134 2009
12 ISRN Neuroscience
[6] M R Brown J Kronengold V R Gazula et al ldquoFragileX mental retardation protein controls gating of the sodium-activated potassium channel SlackrdquoNatureNeuroscience vol 13no 7 pp 819ndash821 2010
[7] M Kameyama M Kakei and R Sato ldquoIntracellular Na+activates a K+ channel in mammalian cardiac cellsrdquoNature vol309 no 5966 pp 354ndash356 1984
[8] C R Bader L Bernheim and D Bertrand ldquoSodium-activatedpotassium current in cultured avian neuronesrdquoNature vol 317no 6037 pp 540ndash542 1985
[9] K Hartung ldquoPotentiation of a transient outward current byNa+influx in crayfish neuronesrdquo Pflugers Archiv vol 404 no 1 pp41ndash44 1985
[10] S E Dryer J T Fujii and A R Martin ldquoA Na+-activatedK+ current in cultured brain stem neurones from chicksrdquo TheJournal of Physiology vol 410 pp 283ndash296 1989
[11] P C Schwindt W J Spain and W E Crill ldquoLong-lastingreduction of excitability by a sodium-dependent potassiumcurrent in cat neocortical neuronsrdquo Journal of Neurophysiologyvol 61 no 2 pp 233ndash244 1989
[12] C Haimann L Bernheim D Bertrand and C R BaderldquoPotassium current activated by intracellular sodium in quailtrigeminal ganglion neuronsrdquo Journal of General Physiology vol95 no 5 pp 961ndash979 1990
[13] S E Dryer ldquoNa+-activated K+ channels and voltage-evokedionic currents in brain stem and parasympathetic neurones ofthe chickrdquoThe Journal of Physiology vol 435 pp 513ndash532 1991
[14] M Saito and C F Wu ldquoExpression of ion channels andmutational effects in giant Drosophila neurons differentiatedfrom cell division-arrested embryonic neuroblastsrdquoThe Journalof Neuroscience vol 11 no 7 pp 2135ndash2150 1991
[15] T M Egan D Dagan J Kupper and I B Levitan ldquoPropertiesand rundown of sodium-activated potassium channels in ratolfactory bulb neuronsrdquoThe Journal of Neuroscience vol 12 no5 pp 1964ndash1976 1992
[16] C Haimann J Magistretti and B Pozzi ldquoSodium-activatedpotassium current in sensory neurons a comparison of cell-attached and cell-free single-channel activitiesrdquo Pflugers Archivvol 422 no 3 pp 287ndash294 1992
[17] N Dale ldquoA large sustained Na+- and voltage-dependent K+current in spinal neurons of the frog embryordquo The Journal ofPhysiology vol 462 pp 349ndash372 1993
[18] B V Safronov and W Vogel ldquoProperties and functions of Na+-activated K+ channels in the soma of rat motoneuronesrdquo TheJournal of Physiology vol 497 no 3 pp 727ndash734 1996
[19] U Bischoff W Vogel and B V Safronov ldquoNa+-activated K+channels in small dorsal root ganglion neurones of ratrdquo TheJournal of Physiology vol 510 part 3 pp 743ndash754 1998
[20] S E Dryer ldquoNa+-activated K+ channels a new family of large-conductance ion channelsrdquo Trends in Neurosciences vol 17 no4 pp 155ndash160 1994
[21] A Bhattacharjee and L K Kaczmarek ldquoFor K+ channels Na+ isthe new Ca2+ rdquo Trends in Neurosciences vol 28 no 8 pp 422ndash428 2005
[22] S B Gao Y Wu C X Lu Z H Guo C H Li and J P DingldquoSlack and Slick KNa channels are required for the depolarizingafterpotential of acutely isolated medium diameter rat dorsalroot ganglion neuronsrdquoActa Pharmacologica Sinica vol 29 no8 pp 899ndash905 2008
[23] M O Nuwer K E Picchione and A Bhattacharjee ldquoPKA-induced internalization of Slack KNa channels produces dorsal
root ganglion neuron hyperexcitabilityrdquo The Journal of Neuro-science vol 30 no 42 pp 14165ndash14172 2010
[24] D Hess E Nanou and A El Manira ldquoCharacterization of Na+-activated K+ currents in larval lamprey spinal cord neuronsrdquoJournal of Neurophysiology vol 97 no 5 pp 3484ndash3493 2007
[25] E Nanou and A El Manira ldquoA postsynaptic negative feedbackmediated by coupling between AMPA receptors and Na+-activated K+ channels in spinal cord neuronesrdquo EuropeanJournal of Neuroscience vol 25 no 2 pp 445ndash450 2007
[26] PWallen B Robertson L Cangiano et al ldquoSodium-dependentpotassium channels of a Slack-like subtype contribute to theslow afterhyperpolarization in lamprey spinal neuronsrdquo TheJournal of Physiology vol 585 no 1 pp 75ndash90 2007
[27] E Nanou A Kyriakatos A Bhattacharjee L K Kaczmarek GParatcha and A El Manira ldquoNa+-mediated coupling betweenAMPA receptors and KNa channels shapes synaptic transmis-sionrdquo Proceedings of the National Academy of Sciences of theUnited States of America vol 105 no 52 pp 20941ndash20946 2008
[28] E Nanou and A El Manira ldquoMechanisms of modulation ofAMPA-induced Na+-activated K+ current by mGluR1rdquo Journalof Neurophysiology vol 103 no 1 pp 441ndash445 2010
[29] A R Mercer and J G Hildebrand ldquoDevelopmental changes inthe density of ionic currents in antennal-lobe neurons of thesphinx moth Manduca sextardquo Journal of Neurophysiology vol87 no 6 pp 2664ndash2675 2002
[30] K Aoki K Kosakai and M Yoshino ldquoMonoaminergic modu-lation of the Na+-activated K+ channel in kenyon cells isolatedfrom the mushroom body of the cricket (Gryllus bimaculatus)brainrdquo Journal of Neurophysiology vol 100 no 3 pp 1211ndash12222008
[31] G Klees P Hochstrate and P W Dierkes ldquoSodium-dependentpotassium channels in leech P neuronsrdquo Journal of MembraneBiology vol 208 no 1 pp 27ndash38 2005
[32] Y Zhang M R Brown C Hyland et al ldquoRegulation ofneuronal excitability by interaction of fragile X mental retar-dation protein with Slack potassium channelsrdquo The Journal ofNeuroscience vol 32 pp 15318ndash15327 2012
[33] C Lawrence and G C Rodrigo ldquoA Na+-activated K+ cur-rent (IKNa) is present in guinea-pig but not rat ventricularmyocytesrdquo Pflugers Archiv vol 437 no 6 pp 831ndash838 1999
[34] M Zhou Z Liu C Hu Z Zhang and Y Mei ldquoDevelopmentalregulation of a Na+-activated fast outward K+ current in ratmyoblastsrdquo Cellular Physiology and Biochemistry vol 14 no 4ndash6 pp 225ndash230 2004
[35] C K Young H S Jae M K Tong et al ldquoSodium-activatedpotassium current in guinea pig gastric myocytesrdquo Journal ofKorean Medical Science vol 22 no 1 pp 57ndash62 2007
[36] L Re V Moretti L Rossini and P Giusti ldquoSodium-activatedpotassium current in mouse diaphragmrdquo FEBS Letters vol 270no 1-2 pp 195ndash197 1990
[37] Y Zhang and W G Paterson ldquoFunctional evidence for Na+-activatedK+ channels in circular smoothmuscle of the opossumlower esophageal sphincterrdquo American The Journal of Physiol-ogy vol 292 no 6 pp G1600ndashG1606 2007
[38] M Paulais S Lachheb and J Teulon ldquoANa+- and Clminus-activatedK+ channel in the thick ascending limb of mouse kidneyrdquoJournal of General Physiology vol 127 no 2 pp 205ndash215 2006
[39] T M Egan D Dagan J Kupper and I B Levitan ldquoNa+-activated K+ channels are widely distributed in rat CNS and inXenopus oocytesrdquo Brain Research vol 584 no 1-2 pp 319ndash3211992
ISRN Neuroscience 13
[40] W J Joiner M D Tang L Y Wang et al ldquoFormation ofintermediate-conductance calcium-activated potassium chan-nels by interaction of Slack and Slo subunitsrdquo Nature Neuro-science vol 1 no 6 pp 462ndash469 1998
[41] A Yuan C M Santi A Wei et al ldquoThe sodium-activatedpotassium channel is encoded by a member of the Slo genefamilyrdquo Neuron vol 37 no 5 pp 765ndash773 2003
[42] Y Jiang A Pico M Cadene B T Chait and R MacKinnonldquoStructure of the RCK domain from the E coli K+ channeland demonstration of its presence in the human BK channelrdquoNeuron vol 29 no 3 pp 593ndash601 2001
[43] P Yuan M D Leonetti A R Pico Y Hsiung and R MacK-innon ldquoStructure of the human BK channel Ca2+-activationapparatus at 30 A resolutionrdquo Science vol 329 no 5988 pp182ndash186 2010
[44] Z Zhang A Rosenhouse-Dantsker Q Y Tang S Noskov andD E Logothetis ldquoThe RCK2 domain uses a coordination sitepresent in Kir channels to confer sodium sensitivity to Slo22channelsrdquoThe Journal of Neuroscience vol 30 no 22 pp 7554ndash7562 2010
[45] J L Sui K W Chan and D E Logothetis ldquoNa+ activationof the muscarinic K+ channel by a G-protein-independentmechanismrdquo Journal of General Physiology vol 108 no 5 pp381ndash391 1996
[46] M R Brown J Kronengold V R Gazula et al ldquoAmino-terminiisoforms of the Slack K+ channel regulated by alternative pro-moters differentiallymodulate rhythmic firing and adaptationrdquoThe Journal of Physiology vol 586 no 21 pp 5161ndash5179 2008
[47] A Yuan M Dourado A Butler N Walton A Wei and LSalkoff ldquoSLO-2 a K+ channel with an unusual Clminus dependencerdquoNature Neuroscience vol 3 no 8 pp 771ndash779 2000
[48] M Schreiber and L Salkoff ldquoA novel calcium-sensing domainin the BK channelrdquo Biophysical Journal vol 73 no 3 pp 1355ndash1363 1997
[49] H Chen J Kronengold Y Yan et al ldquoThe N-terminal domainof slack determines the formation and trafficking of slickslackheteromeric sodium-activated potassium channelsrdquo The Jour-nal of Neuroscience vol 29 no 17 pp 5654ndash5665 2009
[50] A Bhattacharjee L Gan and L K Kaczmarek ldquoLocalization ofthe Slack potassium channel in the rat central nervous systemrdquoJournal of Comparative Neurology vol 454 no 3 pp 241ndash2542002
[51] A Bhattacharjee C A A von Hehn X Mei and L KKaczmarek ldquoLocalization of the Na+-activated K+ channel slickin the rat central nervous systemrdquo Journal of ComparativeNeurology vol 484 no 1 pp 80ndash92 2005
[52] D S Koh P Jonas and W Vogel ldquoNa+-activated K+ channelslocalized in the nodal region of myelinated axons of XenopusrdquoThe Journal of Physiology vol 479 no 2 pp 183ndash197 1994
[53] K Mori T Saito Y Masuda and H Nakaya ldquoEffects of classIII antiarrhythmic drugs on the Na+-activated K+ channels inguinea-pig ventricular cellsrdquo British Journal of Pharmacologyvol 119 no 1 pp 133ndash141 1996
[54] K Mori S Kobayashi T Saito Y Masuda and H NakayaldquoInhibitory effects of class I and IV antiarrhythmic drugs onthe Na+- activated K+ channel current in guinea pig ventricularcellsrdquo Naunyn-Schmiedebergrsquos Archives of Pharmacology vol358 no 6 pp 641ndash648 1998
[55] Y Li T Sato and M Arita ldquoBepridil blunts the shortening ofaction potential duration caused by metabolic inhibition viablockade of ATP-sensitive K+ channels and Na+- activated K+
channelsrdquo Journal of Pharmacology and ExperimentalTherapeu-tics vol 291 no 2 pp 562ndash568 1999
[56] B Yang V K Gribkoff J Pan et al ldquoPharmacological activationand inhibition of Slack (Slo2 2) channelsrdquoNeuropharmacologyvol 51 no 4 pp 896ndash906 2006
[57] M D Tejada K Stolpe A K Meinild and D A KlaerkeldquoClofilium inhibits Slick and Slack potassium channelsrdquo Biolog-ics vol 6 pp 465ndash470 2012
[58] B Yang R Desai and L K Kaczmarek ldquoSlack and slick KNachannels regulate the accuracy of timing of auditory neuronsrdquoThe Journal of Neuroscience vol 27 no 10 pp 2617ndash2627 2007
[59] B Biton S Sethuramanujam K E Picchione et al ldquoTheantipsychotic drug loxapine is an opener of the sodium-activated potassium channel slack (Slo2 2)rdquo The Journal ofPharmacology and ExperimentalTherapeutics vol 340 pp 706ndash715 2012
[60] C M Santi G Ferreira B Yang et al ldquoOpposite regulation ofSlick and Slack K+ channels by neuromodulatorsrdquo The Journalof Neuroscience vol 26 no 19 pp 5059ndash5068 2006
[61] M R Fleming and L K Kaczmarek ldquoUse of optical biosensorsto detect modulation of Slack potassium channels by G protein-coupled receptorsrdquo Journal of Receptors and Signal Transduc-tion vol 29 no 3-4 pp 173ndash181 2009
[62] M D Tejada L J Jensen and D A Klaerke ldquoPIP(2) mod-ulation of Slick and Slack K(+) channelsrdquo Biochemical andBiophysical Research Communications vol 424 pp 208ndash2132012
[63] L Zhang M Sukhareva J L Barker et al ldquoDirect bindingof estradiol enhances Slack (sequence like a calcium-activatedpotassium channel) channelsrsquo activityrdquo Neuroscience vol 131no 2 pp 275ndash282 2005
[64] V A Ruffin X Q Gu D Zhou et al ldquoThe sodium-activatedpotassium channel Slack is modulated by hypercapnia andacidosisrdquo Neuroscience vol 151 no 2 pp 410ndash418 2008
[65] M O Nuwer K E Picchione and A Bhattacharjee ldquocAMP-dependent kinase does not modulate the Slack sodium-activated potassium channelrdquo Neuropharmacology vol 57 no3 pp 219ndash226 2009
[66] B C Suh and B Hille ldquoRegulation of ion channels by phos-phatidylinositol 45-bisphosphaterdquo Current Opinion in Neuro-biology vol 15 no 3 pp 370ndash378 2005
[67] L K Kaczmarek ldquoNon-conducting functions of voltage-gatedion channelsrdquo Nature Reviews Neuroscience vol 7 no 10 pp761ndash771 2006
[68] C R Rose ldquoNa+ signals at central synapsesrdquoNeuroscientist vol8 no 6 pp 532ndash539 2002
[69] C R Rose and B R Ransom ldquoRegulation of intracellularsodium in cultured rat hippocampal neuronesrdquo The Journal ofPhysiology vol 499 part 3 pp 573ndash587 1997
[70] C R Rose and A Konnerth ldquoNmda receptor-mediated Na+signals in spines and dendritesrdquoThe Journal of Neuroscience vol21 no 12 pp 4207ndash4214 2001
[71] N Zhong V Beaumont and R S Zucker ldquoRoles for mito-chondrial and reverse mode Na+Ca2+ exchange and the plas-malemma Ca2+ ATPase in post-tetanic potentiation at crayfishneuromuscular junctionsrdquo The Journal of Neuroscience vol 21no 24 pp 9598ndash9607 2001
[72] G Budelli T A Hage AWei et al ldquoNa+-activated K+ channelsexpress a large delayed outward current in neurons duringnormal physiologyrdquoNature Neuroscience vol 12 no 6 pp 745ndash750 2009
14 ISRN Neuroscience
[73] T A Hage and L Salkoff ldquoSodium-activated potassium chan-nels are functionally coupled to persistent sodiumcurrentsrdquoTheJournal of Neuroscience vol 32 pp 2714ndash2721 2012
[74] M RMarkham L K Kaczmarek andHH Zakon ldquoA sodium-activated potassium channe supports high frequency firing andreduces energetic costs during rapid modulations of actionpotential amplituderdquo Journal of Neurophysiology vol 109 no 7pp 1713ndash1723 2013
[75] S Uchino H Wada S Honda et al ldquoSlo2 sodium-activated K+channels bind to the PDZ domain of PSD-95rdquo Biochemical andBiophysical Research Communications vol 310 no 4 pp 1140ndash1147 2003
[76] T Zamalloa C P Bailey and J Pineda ldquoGlutamate-inducedpost-activation inhibition of locus coeruleus neurons is medi-ated by AMPAkainate receptors and sodium-dependent potas-sium currentsrdquo British Journal of Pharmacology vol 156 no 4pp 649ndash661 2009
[77] S Lu P Das D A Fadool and L K Kaczmarek ldquoThe slacksodium-activated potassium channel provides a major outwardcurrent in olfactory neurons of Kv13-- super-smeller micerdquoJournal of Neurophysiology vol 103 no 6 pp 3311ndash3319 2010
[78] R C Foehring P C Schwindt andW E Crill ldquoNorepinephrineselectively reduces slowCa2+- andNa+-mediated K+ currents incat neocortical neuronsrdquo Journal of Neurophysiology vol 61 no2 pp 245ndash256 1989
[79] M Kubota and N Saito ldquoSodium- and calcium-dependentconductances of neurones in the zebra finch hyperstriatumventrale pars caudale in vitrordquo The Journal of Physiology vol440 pp 131ndash142 1991
[80] U Kim and D A Mccormick ldquoFunctional and ionic propertiesof a slow afterhyperpolarization in ferret perigeniculate neuronsin vitrordquo Journal of Neurophysiology vol 80 no 3 pp 1222ndash1235 1998
[81] V M Sandler E Puil and DW F Schwarz ldquoIntrinsic responseproperties of bursting neurons in the nucleus principalistrigemini of the gerbilrdquoNeuroscience vol 83 no 3 pp 891ndash9041998
[82] M V Sanchez-Vives L G Nowak and D A McCormick ldquoCel-lular mechanisms of long-lasting adaptation in visual corticalneurons in vitrordquoThe Journal of Neuroscience vol 20 no 11 pp4286ndash4299 2000
[83] S Franceschetti T Lavazza G Curia et al ldquoNa+-activatedK+ current contributes to postexcitatory hyperpolarization inneocortical intrinsically bursting neuronsrdquo Journal of Neuro-physiology vol 89 no 4 pp 2101ndash2111 2003
[84] V F Descalzo L G Nowak J C Brumberg D A McCormickand M V Sanchez-Vives ldquoSlow adaptation in fast-spikingneurons of visual cortexrdquo Journal of Neurophysiology vol 93no 2 pp 1111ndash1118 2005
[85] J Benda L Maler and A Longtin ldquoLinear versus nonlinearsignal transmission in neuron models with adaptation currentsor dynamic thresholdsrdquo Journal of Neurophysiology vol 104 no5 pp 2806ndash2820 2010
[86] X Liu and L S Leung ldquoSodium-activated potassium conduc-tance participates in the depolarizing afterpotential following asingle action potential in rat hippocampal CA1 pyramidal cellsrdquoBrain Research vol 1023 no 2 pp 185ndash192 2004
[87] L Cangiano P Wallen and S Grillner ldquoRole of apamin-sensitive KCa channels for reticulospinal synaptic transmissionto motoneuron and for the afterhyperpolarizationrdquo Journal ofNeurophysiology vol 88 no 1 pp 289ndash299 2002
[88] Q Y Liu A E Schaffner and J L Barker ldquoKainate induces anintracellular Na+-activated K+ current in cultured embryonicrat hippocampal neuronesrdquo The Journal of Physiology vol 510part 3 pp 721ndash734 1998
[89] I Rishal T Keren-Raifman D Yakubovich et al ldquoNa+ pro-motes the dissociation between G120572GDP and G120573120574 activating Gprotein-gated K+ channelsrdquoThe Journal of Biological Chemistryvol 278 no 6 pp 3840ndash3845 2003
Submit your manuscripts athttpwwwhindawicom
Neurology Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Alzheimerrsquos DiseaseHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentSchizophrenia
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neural Plasticity
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Parkinsonrsquos Disease
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentAutism
Sleep DisordersHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neuroscience Journal
Epilepsy Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Psychiatry Journal
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Computational and Mathematical Methods in Medicine
Depression Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Brain ScienceInternational Journal of
StrokeResearch and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neurodegenerative Diseases
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Cardiovascular Psychiatry and NeurologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
12 ISRN Neuroscience
[6] M R Brown J Kronengold V R Gazula et al ldquoFragileX mental retardation protein controls gating of the sodium-activated potassium channel SlackrdquoNatureNeuroscience vol 13no 7 pp 819ndash821 2010
[7] M Kameyama M Kakei and R Sato ldquoIntracellular Na+activates a K+ channel in mammalian cardiac cellsrdquoNature vol309 no 5966 pp 354ndash356 1984
[8] C R Bader L Bernheim and D Bertrand ldquoSodium-activatedpotassium current in cultured avian neuronesrdquoNature vol 317no 6037 pp 540ndash542 1985
[9] K Hartung ldquoPotentiation of a transient outward current byNa+influx in crayfish neuronesrdquo Pflugers Archiv vol 404 no 1 pp41ndash44 1985
[10] S E Dryer J T Fujii and A R Martin ldquoA Na+-activatedK+ current in cultured brain stem neurones from chicksrdquo TheJournal of Physiology vol 410 pp 283ndash296 1989
[11] P C Schwindt W J Spain and W E Crill ldquoLong-lastingreduction of excitability by a sodium-dependent potassiumcurrent in cat neocortical neuronsrdquo Journal of Neurophysiologyvol 61 no 2 pp 233ndash244 1989
[12] C Haimann L Bernheim D Bertrand and C R BaderldquoPotassium current activated by intracellular sodium in quailtrigeminal ganglion neuronsrdquo Journal of General Physiology vol95 no 5 pp 961ndash979 1990
[13] S E Dryer ldquoNa+-activated K+ channels and voltage-evokedionic currents in brain stem and parasympathetic neurones ofthe chickrdquoThe Journal of Physiology vol 435 pp 513ndash532 1991
[14] M Saito and C F Wu ldquoExpression of ion channels andmutational effects in giant Drosophila neurons differentiatedfrom cell division-arrested embryonic neuroblastsrdquoThe Journalof Neuroscience vol 11 no 7 pp 2135ndash2150 1991
[15] T M Egan D Dagan J Kupper and I B Levitan ldquoPropertiesand rundown of sodium-activated potassium channels in ratolfactory bulb neuronsrdquoThe Journal of Neuroscience vol 12 no5 pp 1964ndash1976 1992
[16] C Haimann J Magistretti and B Pozzi ldquoSodium-activatedpotassium current in sensory neurons a comparison of cell-attached and cell-free single-channel activitiesrdquo Pflugers Archivvol 422 no 3 pp 287ndash294 1992
[17] N Dale ldquoA large sustained Na+- and voltage-dependent K+current in spinal neurons of the frog embryordquo The Journal ofPhysiology vol 462 pp 349ndash372 1993
[18] B V Safronov and W Vogel ldquoProperties and functions of Na+-activated K+ channels in the soma of rat motoneuronesrdquo TheJournal of Physiology vol 497 no 3 pp 727ndash734 1996
[19] U Bischoff W Vogel and B V Safronov ldquoNa+-activated K+channels in small dorsal root ganglion neurones of ratrdquo TheJournal of Physiology vol 510 part 3 pp 743ndash754 1998
[20] S E Dryer ldquoNa+-activated K+ channels a new family of large-conductance ion channelsrdquo Trends in Neurosciences vol 17 no4 pp 155ndash160 1994
[21] A Bhattacharjee and L K Kaczmarek ldquoFor K+ channels Na+ isthe new Ca2+ rdquo Trends in Neurosciences vol 28 no 8 pp 422ndash428 2005
[22] S B Gao Y Wu C X Lu Z H Guo C H Li and J P DingldquoSlack and Slick KNa channels are required for the depolarizingafterpotential of acutely isolated medium diameter rat dorsalroot ganglion neuronsrdquoActa Pharmacologica Sinica vol 29 no8 pp 899ndash905 2008
[23] M O Nuwer K E Picchione and A Bhattacharjee ldquoPKA-induced internalization of Slack KNa channels produces dorsal
root ganglion neuron hyperexcitabilityrdquo The Journal of Neuro-science vol 30 no 42 pp 14165ndash14172 2010
[24] D Hess E Nanou and A El Manira ldquoCharacterization of Na+-activated K+ currents in larval lamprey spinal cord neuronsrdquoJournal of Neurophysiology vol 97 no 5 pp 3484ndash3493 2007
[25] E Nanou and A El Manira ldquoA postsynaptic negative feedbackmediated by coupling between AMPA receptors and Na+-activated K+ channels in spinal cord neuronesrdquo EuropeanJournal of Neuroscience vol 25 no 2 pp 445ndash450 2007
[26] PWallen B Robertson L Cangiano et al ldquoSodium-dependentpotassium channels of a Slack-like subtype contribute to theslow afterhyperpolarization in lamprey spinal neuronsrdquo TheJournal of Physiology vol 585 no 1 pp 75ndash90 2007
[27] E Nanou A Kyriakatos A Bhattacharjee L K Kaczmarek GParatcha and A El Manira ldquoNa+-mediated coupling betweenAMPA receptors and KNa channels shapes synaptic transmis-sionrdquo Proceedings of the National Academy of Sciences of theUnited States of America vol 105 no 52 pp 20941ndash20946 2008
[28] E Nanou and A El Manira ldquoMechanisms of modulation ofAMPA-induced Na+-activated K+ current by mGluR1rdquo Journalof Neurophysiology vol 103 no 1 pp 441ndash445 2010
[29] A R Mercer and J G Hildebrand ldquoDevelopmental changes inthe density of ionic currents in antennal-lobe neurons of thesphinx moth Manduca sextardquo Journal of Neurophysiology vol87 no 6 pp 2664ndash2675 2002
[30] K Aoki K Kosakai and M Yoshino ldquoMonoaminergic modu-lation of the Na+-activated K+ channel in kenyon cells isolatedfrom the mushroom body of the cricket (Gryllus bimaculatus)brainrdquo Journal of Neurophysiology vol 100 no 3 pp 1211ndash12222008
[31] G Klees P Hochstrate and P W Dierkes ldquoSodium-dependentpotassium channels in leech P neuronsrdquo Journal of MembraneBiology vol 208 no 1 pp 27ndash38 2005
[32] Y Zhang M R Brown C Hyland et al ldquoRegulation ofneuronal excitability by interaction of fragile X mental retar-dation protein with Slack potassium channelsrdquo The Journal ofNeuroscience vol 32 pp 15318ndash15327 2012
[33] C Lawrence and G C Rodrigo ldquoA Na+-activated K+ cur-rent (IKNa) is present in guinea-pig but not rat ventricularmyocytesrdquo Pflugers Archiv vol 437 no 6 pp 831ndash838 1999
[34] M Zhou Z Liu C Hu Z Zhang and Y Mei ldquoDevelopmentalregulation of a Na+-activated fast outward K+ current in ratmyoblastsrdquo Cellular Physiology and Biochemistry vol 14 no 4ndash6 pp 225ndash230 2004
[35] C K Young H S Jae M K Tong et al ldquoSodium-activatedpotassium current in guinea pig gastric myocytesrdquo Journal ofKorean Medical Science vol 22 no 1 pp 57ndash62 2007
[36] L Re V Moretti L Rossini and P Giusti ldquoSodium-activatedpotassium current in mouse diaphragmrdquo FEBS Letters vol 270no 1-2 pp 195ndash197 1990
[37] Y Zhang and W G Paterson ldquoFunctional evidence for Na+-activatedK+ channels in circular smoothmuscle of the opossumlower esophageal sphincterrdquo American The Journal of Physiol-ogy vol 292 no 6 pp G1600ndashG1606 2007
[38] M Paulais S Lachheb and J Teulon ldquoANa+- and Clminus-activatedK+ channel in the thick ascending limb of mouse kidneyrdquoJournal of General Physiology vol 127 no 2 pp 205ndash215 2006
[39] T M Egan D Dagan J Kupper and I B Levitan ldquoNa+-activated K+ channels are widely distributed in rat CNS and inXenopus oocytesrdquo Brain Research vol 584 no 1-2 pp 319ndash3211992
ISRN Neuroscience 13
[40] W J Joiner M D Tang L Y Wang et al ldquoFormation ofintermediate-conductance calcium-activated potassium chan-nels by interaction of Slack and Slo subunitsrdquo Nature Neuro-science vol 1 no 6 pp 462ndash469 1998
[41] A Yuan C M Santi A Wei et al ldquoThe sodium-activatedpotassium channel is encoded by a member of the Slo genefamilyrdquo Neuron vol 37 no 5 pp 765ndash773 2003
[42] Y Jiang A Pico M Cadene B T Chait and R MacKinnonldquoStructure of the RCK domain from the E coli K+ channeland demonstration of its presence in the human BK channelrdquoNeuron vol 29 no 3 pp 593ndash601 2001
[43] P Yuan M D Leonetti A R Pico Y Hsiung and R MacK-innon ldquoStructure of the human BK channel Ca2+-activationapparatus at 30 A resolutionrdquo Science vol 329 no 5988 pp182ndash186 2010
[44] Z Zhang A Rosenhouse-Dantsker Q Y Tang S Noskov andD E Logothetis ldquoThe RCK2 domain uses a coordination sitepresent in Kir channels to confer sodium sensitivity to Slo22channelsrdquoThe Journal of Neuroscience vol 30 no 22 pp 7554ndash7562 2010
[45] J L Sui K W Chan and D E Logothetis ldquoNa+ activationof the muscarinic K+ channel by a G-protein-independentmechanismrdquo Journal of General Physiology vol 108 no 5 pp381ndash391 1996
[46] M R Brown J Kronengold V R Gazula et al ldquoAmino-terminiisoforms of the Slack K+ channel regulated by alternative pro-moters differentiallymodulate rhythmic firing and adaptationrdquoThe Journal of Physiology vol 586 no 21 pp 5161ndash5179 2008
[47] A Yuan M Dourado A Butler N Walton A Wei and LSalkoff ldquoSLO-2 a K+ channel with an unusual Clminus dependencerdquoNature Neuroscience vol 3 no 8 pp 771ndash779 2000
[48] M Schreiber and L Salkoff ldquoA novel calcium-sensing domainin the BK channelrdquo Biophysical Journal vol 73 no 3 pp 1355ndash1363 1997
[49] H Chen J Kronengold Y Yan et al ldquoThe N-terminal domainof slack determines the formation and trafficking of slickslackheteromeric sodium-activated potassium channelsrdquo The Jour-nal of Neuroscience vol 29 no 17 pp 5654ndash5665 2009
[50] A Bhattacharjee L Gan and L K Kaczmarek ldquoLocalization ofthe Slack potassium channel in the rat central nervous systemrdquoJournal of Comparative Neurology vol 454 no 3 pp 241ndash2542002
[51] A Bhattacharjee C A A von Hehn X Mei and L KKaczmarek ldquoLocalization of the Na+-activated K+ channel slickin the rat central nervous systemrdquo Journal of ComparativeNeurology vol 484 no 1 pp 80ndash92 2005
[52] D S Koh P Jonas and W Vogel ldquoNa+-activated K+ channelslocalized in the nodal region of myelinated axons of XenopusrdquoThe Journal of Physiology vol 479 no 2 pp 183ndash197 1994
[53] K Mori T Saito Y Masuda and H Nakaya ldquoEffects of classIII antiarrhythmic drugs on the Na+-activated K+ channels inguinea-pig ventricular cellsrdquo British Journal of Pharmacologyvol 119 no 1 pp 133ndash141 1996
[54] K Mori S Kobayashi T Saito Y Masuda and H NakayaldquoInhibitory effects of class I and IV antiarrhythmic drugs onthe Na+- activated K+ channel current in guinea pig ventricularcellsrdquo Naunyn-Schmiedebergrsquos Archives of Pharmacology vol358 no 6 pp 641ndash648 1998
[55] Y Li T Sato and M Arita ldquoBepridil blunts the shortening ofaction potential duration caused by metabolic inhibition viablockade of ATP-sensitive K+ channels and Na+- activated K+
channelsrdquo Journal of Pharmacology and ExperimentalTherapeu-tics vol 291 no 2 pp 562ndash568 1999
[56] B Yang V K Gribkoff J Pan et al ldquoPharmacological activationand inhibition of Slack (Slo2 2) channelsrdquoNeuropharmacologyvol 51 no 4 pp 896ndash906 2006
[57] M D Tejada K Stolpe A K Meinild and D A KlaerkeldquoClofilium inhibits Slick and Slack potassium channelsrdquo Biolog-ics vol 6 pp 465ndash470 2012
[58] B Yang R Desai and L K Kaczmarek ldquoSlack and slick KNachannels regulate the accuracy of timing of auditory neuronsrdquoThe Journal of Neuroscience vol 27 no 10 pp 2617ndash2627 2007
[59] B Biton S Sethuramanujam K E Picchione et al ldquoTheantipsychotic drug loxapine is an opener of the sodium-activated potassium channel slack (Slo2 2)rdquo The Journal ofPharmacology and ExperimentalTherapeutics vol 340 pp 706ndash715 2012
[60] C M Santi G Ferreira B Yang et al ldquoOpposite regulation ofSlick and Slack K+ channels by neuromodulatorsrdquo The Journalof Neuroscience vol 26 no 19 pp 5059ndash5068 2006
[61] M R Fleming and L K Kaczmarek ldquoUse of optical biosensorsto detect modulation of Slack potassium channels by G protein-coupled receptorsrdquo Journal of Receptors and Signal Transduc-tion vol 29 no 3-4 pp 173ndash181 2009
[62] M D Tejada L J Jensen and D A Klaerke ldquoPIP(2) mod-ulation of Slick and Slack K(+) channelsrdquo Biochemical andBiophysical Research Communications vol 424 pp 208ndash2132012
[63] L Zhang M Sukhareva J L Barker et al ldquoDirect bindingof estradiol enhances Slack (sequence like a calcium-activatedpotassium channel) channelsrsquo activityrdquo Neuroscience vol 131no 2 pp 275ndash282 2005
[64] V A Ruffin X Q Gu D Zhou et al ldquoThe sodium-activatedpotassium channel Slack is modulated by hypercapnia andacidosisrdquo Neuroscience vol 151 no 2 pp 410ndash418 2008
[65] M O Nuwer K E Picchione and A Bhattacharjee ldquocAMP-dependent kinase does not modulate the Slack sodium-activated potassium channelrdquo Neuropharmacology vol 57 no3 pp 219ndash226 2009
[66] B C Suh and B Hille ldquoRegulation of ion channels by phos-phatidylinositol 45-bisphosphaterdquo Current Opinion in Neuro-biology vol 15 no 3 pp 370ndash378 2005
[67] L K Kaczmarek ldquoNon-conducting functions of voltage-gatedion channelsrdquo Nature Reviews Neuroscience vol 7 no 10 pp761ndash771 2006
[68] C R Rose ldquoNa+ signals at central synapsesrdquoNeuroscientist vol8 no 6 pp 532ndash539 2002
[69] C R Rose and B R Ransom ldquoRegulation of intracellularsodium in cultured rat hippocampal neuronesrdquo The Journal ofPhysiology vol 499 part 3 pp 573ndash587 1997
[70] C R Rose and A Konnerth ldquoNmda receptor-mediated Na+signals in spines and dendritesrdquoThe Journal of Neuroscience vol21 no 12 pp 4207ndash4214 2001
[71] N Zhong V Beaumont and R S Zucker ldquoRoles for mito-chondrial and reverse mode Na+Ca2+ exchange and the plas-malemma Ca2+ ATPase in post-tetanic potentiation at crayfishneuromuscular junctionsrdquo The Journal of Neuroscience vol 21no 24 pp 9598ndash9607 2001
[72] G Budelli T A Hage AWei et al ldquoNa+-activated K+ channelsexpress a large delayed outward current in neurons duringnormal physiologyrdquoNature Neuroscience vol 12 no 6 pp 745ndash750 2009
14 ISRN Neuroscience
[73] T A Hage and L Salkoff ldquoSodium-activated potassium chan-nels are functionally coupled to persistent sodiumcurrentsrdquoTheJournal of Neuroscience vol 32 pp 2714ndash2721 2012
[74] M RMarkham L K Kaczmarek andHH Zakon ldquoA sodium-activated potassium channe supports high frequency firing andreduces energetic costs during rapid modulations of actionpotential amplituderdquo Journal of Neurophysiology vol 109 no 7pp 1713ndash1723 2013
[75] S Uchino H Wada S Honda et al ldquoSlo2 sodium-activated K+channels bind to the PDZ domain of PSD-95rdquo Biochemical andBiophysical Research Communications vol 310 no 4 pp 1140ndash1147 2003
[76] T Zamalloa C P Bailey and J Pineda ldquoGlutamate-inducedpost-activation inhibition of locus coeruleus neurons is medi-ated by AMPAkainate receptors and sodium-dependent potas-sium currentsrdquo British Journal of Pharmacology vol 156 no 4pp 649ndash661 2009
[77] S Lu P Das D A Fadool and L K Kaczmarek ldquoThe slacksodium-activated potassium channel provides a major outwardcurrent in olfactory neurons of Kv13-- super-smeller micerdquoJournal of Neurophysiology vol 103 no 6 pp 3311ndash3319 2010
[78] R C Foehring P C Schwindt andW E Crill ldquoNorepinephrineselectively reduces slowCa2+- andNa+-mediated K+ currents incat neocortical neuronsrdquo Journal of Neurophysiology vol 61 no2 pp 245ndash256 1989
[79] M Kubota and N Saito ldquoSodium- and calcium-dependentconductances of neurones in the zebra finch hyperstriatumventrale pars caudale in vitrordquo The Journal of Physiology vol440 pp 131ndash142 1991
[80] U Kim and D A Mccormick ldquoFunctional and ionic propertiesof a slow afterhyperpolarization in ferret perigeniculate neuronsin vitrordquo Journal of Neurophysiology vol 80 no 3 pp 1222ndash1235 1998
[81] V M Sandler E Puil and DW F Schwarz ldquoIntrinsic responseproperties of bursting neurons in the nucleus principalistrigemini of the gerbilrdquoNeuroscience vol 83 no 3 pp 891ndash9041998
[82] M V Sanchez-Vives L G Nowak and D A McCormick ldquoCel-lular mechanisms of long-lasting adaptation in visual corticalneurons in vitrordquoThe Journal of Neuroscience vol 20 no 11 pp4286ndash4299 2000
[83] S Franceschetti T Lavazza G Curia et al ldquoNa+-activatedK+ current contributes to postexcitatory hyperpolarization inneocortical intrinsically bursting neuronsrdquo Journal of Neuro-physiology vol 89 no 4 pp 2101ndash2111 2003
[84] V F Descalzo L G Nowak J C Brumberg D A McCormickand M V Sanchez-Vives ldquoSlow adaptation in fast-spikingneurons of visual cortexrdquo Journal of Neurophysiology vol 93no 2 pp 1111ndash1118 2005
[85] J Benda L Maler and A Longtin ldquoLinear versus nonlinearsignal transmission in neuron models with adaptation currentsor dynamic thresholdsrdquo Journal of Neurophysiology vol 104 no5 pp 2806ndash2820 2010
[86] X Liu and L S Leung ldquoSodium-activated potassium conduc-tance participates in the depolarizing afterpotential following asingle action potential in rat hippocampal CA1 pyramidal cellsrdquoBrain Research vol 1023 no 2 pp 185ndash192 2004
[87] L Cangiano P Wallen and S Grillner ldquoRole of apamin-sensitive KCa channels for reticulospinal synaptic transmissionto motoneuron and for the afterhyperpolarizationrdquo Journal ofNeurophysiology vol 88 no 1 pp 289ndash299 2002
[88] Q Y Liu A E Schaffner and J L Barker ldquoKainate induces anintracellular Na+-activated K+ current in cultured embryonicrat hippocampal neuronesrdquo The Journal of Physiology vol 510part 3 pp 721ndash734 1998
[89] I Rishal T Keren-Raifman D Yakubovich et al ldquoNa+ pro-motes the dissociation between G120572GDP and G120573120574 activating Gprotein-gated K+ channelsrdquoThe Journal of Biological Chemistryvol 278 no 6 pp 3840ndash3845 2003
Submit your manuscripts athttpwwwhindawicom
Neurology Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Alzheimerrsquos DiseaseHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentSchizophrenia
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neural Plasticity
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Parkinsonrsquos Disease
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentAutism
Sleep DisordersHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neuroscience Journal
Epilepsy Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Psychiatry Journal
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Computational and Mathematical Methods in Medicine
Depression Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Brain ScienceInternational Journal of
StrokeResearch and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neurodegenerative Diseases
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Cardiovascular Psychiatry and NeurologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
ISRN Neuroscience 13
[40] W J Joiner M D Tang L Y Wang et al ldquoFormation ofintermediate-conductance calcium-activated potassium chan-nels by interaction of Slack and Slo subunitsrdquo Nature Neuro-science vol 1 no 6 pp 462ndash469 1998
[41] A Yuan C M Santi A Wei et al ldquoThe sodium-activatedpotassium channel is encoded by a member of the Slo genefamilyrdquo Neuron vol 37 no 5 pp 765ndash773 2003
[42] Y Jiang A Pico M Cadene B T Chait and R MacKinnonldquoStructure of the RCK domain from the E coli K+ channeland demonstration of its presence in the human BK channelrdquoNeuron vol 29 no 3 pp 593ndash601 2001
[43] P Yuan M D Leonetti A R Pico Y Hsiung and R MacK-innon ldquoStructure of the human BK channel Ca2+-activationapparatus at 30 A resolutionrdquo Science vol 329 no 5988 pp182ndash186 2010
[44] Z Zhang A Rosenhouse-Dantsker Q Y Tang S Noskov andD E Logothetis ldquoThe RCK2 domain uses a coordination sitepresent in Kir channels to confer sodium sensitivity to Slo22channelsrdquoThe Journal of Neuroscience vol 30 no 22 pp 7554ndash7562 2010
[45] J L Sui K W Chan and D E Logothetis ldquoNa+ activationof the muscarinic K+ channel by a G-protein-independentmechanismrdquo Journal of General Physiology vol 108 no 5 pp381ndash391 1996
[46] M R Brown J Kronengold V R Gazula et al ldquoAmino-terminiisoforms of the Slack K+ channel regulated by alternative pro-moters differentiallymodulate rhythmic firing and adaptationrdquoThe Journal of Physiology vol 586 no 21 pp 5161ndash5179 2008
[47] A Yuan M Dourado A Butler N Walton A Wei and LSalkoff ldquoSLO-2 a K+ channel with an unusual Clminus dependencerdquoNature Neuroscience vol 3 no 8 pp 771ndash779 2000
[48] M Schreiber and L Salkoff ldquoA novel calcium-sensing domainin the BK channelrdquo Biophysical Journal vol 73 no 3 pp 1355ndash1363 1997
[49] H Chen J Kronengold Y Yan et al ldquoThe N-terminal domainof slack determines the formation and trafficking of slickslackheteromeric sodium-activated potassium channelsrdquo The Jour-nal of Neuroscience vol 29 no 17 pp 5654ndash5665 2009
[50] A Bhattacharjee L Gan and L K Kaczmarek ldquoLocalization ofthe Slack potassium channel in the rat central nervous systemrdquoJournal of Comparative Neurology vol 454 no 3 pp 241ndash2542002
[51] A Bhattacharjee C A A von Hehn X Mei and L KKaczmarek ldquoLocalization of the Na+-activated K+ channel slickin the rat central nervous systemrdquo Journal of ComparativeNeurology vol 484 no 1 pp 80ndash92 2005
[52] D S Koh P Jonas and W Vogel ldquoNa+-activated K+ channelslocalized in the nodal region of myelinated axons of XenopusrdquoThe Journal of Physiology vol 479 no 2 pp 183ndash197 1994
[53] K Mori T Saito Y Masuda and H Nakaya ldquoEffects of classIII antiarrhythmic drugs on the Na+-activated K+ channels inguinea-pig ventricular cellsrdquo British Journal of Pharmacologyvol 119 no 1 pp 133ndash141 1996
[54] K Mori S Kobayashi T Saito Y Masuda and H NakayaldquoInhibitory effects of class I and IV antiarrhythmic drugs onthe Na+- activated K+ channel current in guinea pig ventricularcellsrdquo Naunyn-Schmiedebergrsquos Archives of Pharmacology vol358 no 6 pp 641ndash648 1998
[55] Y Li T Sato and M Arita ldquoBepridil blunts the shortening ofaction potential duration caused by metabolic inhibition viablockade of ATP-sensitive K+ channels and Na+- activated K+
channelsrdquo Journal of Pharmacology and ExperimentalTherapeu-tics vol 291 no 2 pp 562ndash568 1999
[56] B Yang V K Gribkoff J Pan et al ldquoPharmacological activationand inhibition of Slack (Slo2 2) channelsrdquoNeuropharmacologyvol 51 no 4 pp 896ndash906 2006
[57] M D Tejada K Stolpe A K Meinild and D A KlaerkeldquoClofilium inhibits Slick and Slack potassium channelsrdquo Biolog-ics vol 6 pp 465ndash470 2012
[58] B Yang R Desai and L K Kaczmarek ldquoSlack and slick KNachannels regulate the accuracy of timing of auditory neuronsrdquoThe Journal of Neuroscience vol 27 no 10 pp 2617ndash2627 2007
[59] B Biton S Sethuramanujam K E Picchione et al ldquoTheantipsychotic drug loxapine is an opener of the sodium-activated potassium channel slack (Slo2 2)rdquo The Journal ofPharmacology and ExperimentalTherapeutics vol 340 pp 706ndash715 2012
[60] C M Santi G Ferreira B Yang et al ldquoOpposite regulation ofSlick and Slack K+ channels by neuromodulatorsrdquo The Journalof Neuroscience vol 26 no 19 pp 5059ndash5068 2006
[61] M R Fleming and L K Kaczmarek ldquoUse of optical biosensorsto detect modulation of Slack potassium channels by G protein-coupled receptorsrdquo Journal of Receptors and Signal Transduc-tion vol 29 no 3-4 pp 173ndash181 2009
[62] M D Tejada L J Jensen and D A Klaerke ldquoPIP(2) mod-ulation of Slick and Slack K(+) channelsrdquo Biochemical andBiophysical Research Communications vol 424 pp 208ndash2132012
[63] L Zhang M Sukhareva J L Barker et al ldquoDirect bindingof estradiol enhances Slack (sequence like a calcium-activatedpotassium channel) channelsrsquo activityrdquo Neuroscience vol 131no 2 pp 275ndash282 2005
[64] V A Ruffin X Q Gu D Zhou et al ldquoThe sodium-activatedpotassium channel Slack is modulated by hypercapnia andacidosisrdquo Neuroscience vol 151 no 2 pp 410ndash418 2008
[65] M O Nuwer K E Picchione and A Bhattacharjee ldquocAMP-dependent kinase does not modulate the Slack sodium-activated potassium channelrdquo Neuropharmacology vol 57 no3 pp 219ndash226 2009
[66] B C Suh and B Hille ldquoRegulation of ion channels by phos-phatidylinositol 45-bisphosphaterdquo Current Opinion in Neuro-biology vol 15 no 3 pp 370ndash378 2005
[67] L K Kaczmarek ldquoNon-conducting functions of voltage-gatedion channelsrdquo Nature Reviews Neuroscience vol 7 no 10 pp761ndash771 2006
[68] C R Rose ldquoNa+ signals at central synapsesrdquoNeuroscientist vol8 no 6 pp 532ndash539 2002
[69] C R Rose and B R Ransom ldquoRegulation of intracellularsodium in cultured rat hippocampal neuronesrdquo The Journal ofPhysiology vol 499 part 3 pp 573ndash587 1997
[70] C R Rose and A Konnerth ldquoNmda receptor-mediated Na+signals in spines and dendritesrdquoThe Journal of Neuroscience vol21 no 12 pp 4207ndash4214 2001
[71] N Zhong V Beaumont and R S Zucker ldquoRoles for mito-chondrial and reverse mode Na+Ca2+ exchange and the plas-malemma Ca2+ ATPase in post-tetanic potentiation at crayfishneuromuscular junctionsrdquo The Journal of Neuroscience vol 21no 24 pp 9598ndash9607 2001
[72] G Budelli T A Hage AWei et al ldquoNa+-activated K+ channelsexpress a large delayed outward current in neurons duringnormal physiologyrdquoNature Neuroscience vol 12 no 6 pp 745ndash750 2009
14 ISRN Neuroscience
[73] T A Hage and L Salkoff ldquoSodium-activated potassium chan-nels are functionally coupled to persistent sodiumcurrentsrdquoTheJournal of Neuroscience vol 32 pp 2714ndash2721 2012
[74] M RMarkham L K Kaczmarek andHH Zakon ldquoA sodium-activated potassium channe supports high frequency firing andreduces energetic costs during rapid modulations of actionpotential amplituderdquo Journal of Neurophysiology vol 109 no 7pp 1713ndash1723 2013
[75] S Uchino H Wada S Honda et al ldquoSlo2 sodium-activated K+channels bind to the PDZ domain of PSD-95rdquo Biochemical andBiophysical Research Communications vol 310 no 4 pp 1140ndash1147 2003
[76] T Zamalloa C P Bailey and J Pineda ldquoGlutamate-inducedpost-activation inhibition of locus coeruleus neurons is medi-ated by AMPAkainate receptors and sodium-dependent potas-sium currentsrdquo British Journal of Pharmacology vol 156 no 4pp 649ndash661 2009
[77] S Lu P Das D A Fadool and L K Kaczmarek ldquoThe slacksodium-activated potassium channel provides a major outwardcurrent in olfactory neurons of Kv13-- super-smeller micerdquoJournal of Neurophysiology vol 103 no 6 pp 3311ndash3319 2010
[78] R C Foehring P C Schwindt andW E Crill ldquoNorepinephrineselectively reduces slowCa2+- andNa+-mediated K+ currents incat neocortical neuronsrdquo Journal of Neurophysiology vol 61 no2 pp 245ndash256 1989
[79] M Kubota and N Saito ldquoSodium- and calcium-dependentconductances of neurones in the zebra finch hyperstriatumventrale pars caudale in vitrordquo The Journal of Physiology vol440 pp 131ndash142 1991
[80] U Kim and D A Mccormick ldquoFunctional and ionic propertiesof a slow afterhyperpolarization in ferret perigeniculate neuronsin vitrordquo Journal of Neurophysiology vol 80 no 3 pp 1222ndash1235 1998
[81] V M Sandler E Puil and DW F Schwarz ldquoIntrinsic responseproperties of bursting neurons in the nucleus principalistrigemini of the gerbilrdquoNeuroscience vol 83 no 3 pp 891ndash9041998
[82] M V Sanchez-Vives L G Nowak and D A McCormick ldquoCel-lular mechanisms of long-lasting adaptation in visual corticalneurons in vitrordquoThe Journal of Neuroscience vol 20 no 11 pp4286ndash4299 2000
[83] S Franceschetti T Lavazza G Curia et al ldquoNa+-activatedK+ current contributes to postexcitatory hyperpolarization inneocortical intrinsically bursting neuronsrdquo Journal of Neuro-physiology vol 89 no 4 pp 2101ndash2111 2003
[84] V F Descalzo L G Nowak J C Brumberg D A McCormickand M V Sanchez-Vives ldquoSlow adaptation in fast-spikingneurons of visual cortexrdquo Journal of Neurophysiology vol 93no 2 pp 1111ndash1118 2005
[85] J Benda L Maler and A Longtin ldquoLinear versus nonlinearsignal transmission in neuron models with adaptation currentsor dynamic thresholdsrdquo Journal of Neurophysiology vol 104 no5 pp 2806ndash2820 2010
[86] X Liu and L S Leung ldquoSodium-activated potassium conduc-tance participates in the depolarizing afterpotential following asingle action potential in rat hippocampal CA1 pyramidal cellsrdquoBrain Research vol 1023 no 2 pp 185ndash192 2004
[87] L Cangiano P Wallen and S Grillner ldquoRole of apamin-sensitive KCa channels for reticulospinal synaptic transmissionto motoneuron and for the afterhyperpolarizationrdquo Journal ofNeurophysiology vol 88 no 1 pp 289ndash299 2002
[88] Q Y Liu A E Schaffner and J L Barker ldquoKainate induces anintracellular Na+-activated K+ current in cultured embryonicrat hippocampal neuronesrdquo The Journal of Physiology vol 510part 3 pp 721ndash734 1998
[89] I Rishal T Keren-Raifman D Yakubovich et al ldquoNa+ pro-motes the dissociation between G120572GDP and G120573120574 activating Gprotein-gated K+ channelsrdquoThe Journal of Biological Chemistryvol 278 no 6 pp 3840ndash3845 2003
Submit your manuscripts athttpwwwhindawicom
Neurology Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Alzheimerrsquos DiseaseHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentSchizophrenia
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neural Plasticity
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Parkinsonrsquos Disease
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentAutism
Sleep DisordersHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neuroscience Journal
Epilepsy Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Psychiatry Journal
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Computational and Mathematical Methods in Medicine
Depression Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Brain ScienceInternational Journal of
StrokeResearch and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neurodegenerative Diseases
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Cardiovascular Psychiatry and NeurologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
14 ISRN Neuroscience
[73] T A Hage and L Salkoff ldquoSodium-activated potassium chan-nels are functionally coupled to persistent sodiumcurrentsrdquoTheJournal of Neuroscience vol 32 pp 2714ndash2721 2012
[74] M RMarkham L K Kaczmarek andHH Zakon ldquoA sodium-activated potassium channe supports high frequency firing andreduces energetic costs during rapid modulations of actionpotential amplituderdquo Journal of Neurophysiology vol 109 no 7pp 1713ndash1723 2013
[75] S Uchino H Wada S Honda et al ldquoSlo2 sodium-activated K+channels bind to the PDZ domain of PSD-95rdquo Biochemical andBiophysical Research Communications vol 310 no 4 pp 1140ndash1147 2003
[76] T Zamalloa C P Bailey and J Pineda ldquoGlutamate-inducedpost-activation inhibition of locus coeruleus neurons is medi-ated by AMPAkainate receptors and sodium-dependent potas-sium currentsrdquo British Journal of Pharmacology vol 156 no 4pp 649ndash661 2009
[77] S Lu P Das D A Fadool and L K Kaczmarek ldquoThe slacksodium-activated potassium channel provides a major outwardcurrent in olfactory neurons of Kv13-- super-smeller micerdquoJournal of Neurophysiology vol 103 no 6 pp 3311ndash3319 2010
[78] R C Foehring P C Schwindt andW E Crill ldquoNorepinephrineselectively reduces slowCa2+- andNa+-mediated K+ currents incat neocortical neuronsrdquo Journal of Neurophysiology vol 61 no2 pp 245ndash256 1989
[79] M Kubota and N Saito ldquoSodium- and calcium-dependentconductances of neurones in the zebra finch hyperstriatumventrale pars caudale in vitrordquo The Journal of Physiology vol440 pp 131ndash142 1991
[80] U Kim and D A Mccormick ldquoFunctional and ionic propertiesof a slow afterhyperpolarization in ferret perigeniculate neuronsin vitrordquo Journal of Neurophysiology vol 80 no 3 pp 1222ndash1235 1998
[81] V M Sandler E Puil and DW F Schwarz ldquoIntrinsic responseproperties of bursting neurons in the nucleus principalistrigemini of the gerbilrdquoNeuroscience vol 83 no 3 pp 891ndash9041998
[82] M V Sanchez-Vives L G Nowak and D A McCormick ldquoCel-lular mechanisms of long-lasting adaptation in visual corticalneurons in vitrordquoThe Journal of Neuroscience vol 20 no 11 pp4286ndash4299 2000
[83] S Franceschetti T Lavazza G Curia et al ldquoNa+-activatedK+ current contributes to postexcitatory hyperpolarization inneocortical intrinsically bursting neuronsrdquo Journal of Neuro-physiology vol 89 no 4 pp 2101ndash2111 2003
[84] V F Descalzo L G Nowak J C Brumberg D A McCormickand M V Sanchez-Vives ldquoSlow adaptation in fast-spikingneurons of visual cortexrdquo Journal of Neurophysiology vol 93no 2 pp 1111ndash1118 2005
[85] J Benda L Maler and A Longtin ldquoLinear versus nonlinearsignal transmission in neuron models with adaptation currentsor dynamic thresholdsrdquo Journal of Neurophysiology vol 104 no5 pp 2806ndash2820 2010
[86] X Liu and L S Leung ldquoSodium-activated potassium conduc-tance participates in the depolarizing afterpotential following asingle action potential in rat hippocampal CA1 pyramidal cellsrdquoBrain Research vol 1023 no 2 pp 185ndash192 2004
[87] L Cangiano P Wallen and S Grillner ldquoRole of apamin-sensitive KCa channels for reticulospinal synaptic transmissionto motoneuron and for the afterhyperpolarizationrdquo Journal ofNeurophysiology vol 88 no 1 pp 289ndash299 2002
[88] Q Y Liu A E Schaffner and J L Barker ldquoKainate induces anintracellular Na+-activated K+ current in cultured embryonicrat hippocampal neuronesrdquo The Journal of Physiology vol 510part 3 pp 721ndash734 1998
[89] I Rishal T Keren-Raifman D Yakubovich et al ldquoNa+ pro-motes the dissociation between G120572GDP and G120573120574 activating Gprotein-gated K+ channelsrdquoThe Journal of Biological Chemistryvol 278 no 6 pp 3840ndash3845 2003
Submit your manuscripts athttpwwwhindawicom
Neurology Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Alzheimerrsquos DiseaseHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentSchizophrenia
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neural Plasticity
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Parkinsonrsquos Disease
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentAutism
Sleep DisordersHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neuroscience Journal
Epilepsy Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Psychiatry Journal
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Computational and Mathematical Methods in Medicine
Depression Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Brain ScienceInternational Journal of
StrokeResearch and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neurodegenerative Diseases
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Cardiovascular Psychiatry and NeurologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Submit your manuscripts athttpwwwhindawicom
Neurology Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Alzheimerrsquos DiseaseHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentSchizophrenia
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neural Plasticity
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Parkinsonrsquos Disease
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Research and TreatmentAutism
Sleep DisordersHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neuroscience Journal
Epilepsy Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Psychiatry Journal
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Computational and Mathematical Methods in Medicine
Depression Research and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Brain ScienceInternational Journal of
StrokeResearch and TreatmentHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Neurodegenerative Diseases
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Cardiovascular Psychiatry and NeurologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
