PHARMACOLOGY OF THE DENDRITIC ACTION OF …€¦ · Dendritic and somatic actions of acetylcholine...

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PHARMACOLOGY OF THE DENDRITIC ACTION OF ACETYLCHOLINE AND FURTHER OBSERVATXONS ON THE

SOMATIC DISINHIBITION IN THE RAT HIPPOCAMPUS IN SITU

C. ROVIRA, Y. BEN-ARI,* E. CHERUBINI,~ K. KRNJEVIC$ and N. ROPERT$

Laboratoire de Physiologie Nerveuse, DCpartment de Neurophysiologie Appliquee, C.N.R.S. 91 i90, GiEsur-Yvette, France

Abstract-In rats under urethane anaesthesia various cholinergic agonists and antagonists were micro- iontophorctically applied in the pyramidal layer of area CA1 and in the apical dendrites. using a twin set of multibarreled micropipettes. Thus, the somatic population spike and the dendritic negative field (field excitarorypostsynaptic potential) evoked by commissural stimulation and the effect of iontophore- tic agents at either site were recorded simultaneously.

Using this double recording system, several observations were made which confirmed the previous conclusions that acetylcholine has an important disinhibitory action at the level of the pyramidal cell bodies. Both muscarinic and nicotinic agents enhanced the somatic population spikes without altering the dendritic field excitatory postsynaptic potential. Local ejection of atropine, or scopolamine readily depressed or abolished the action of muscarinic agents (such as muscarine, metacholine or acetylcholine) without reducing the effects of nicotinic agents. Conversely, on several occasions, brief applications of curare or dihydro-/$-erythroidine selectively reduced the effects of dimethylphenylpiperazinium or tetra- methyl~~mm~~niurn.

Local appii~tions of acetyicholine in the apical dendrites reduced the field excitatory postsynaptic potential and the somatic population spike recorded simultaneously. This effect was reproduced by muscarinic agents (notably muscarine or metacho~ine)~ in contrast nicotinic agents (notably dimethyl- phenylpiper~tzinium or tetramethyiammonium) increased the fiefd excitatory postsynaptic potential and the simultaneously recorded somatic spike. This effect had a slow onset and a maximal increase was observed toward the end of a 20 s application. These actions were pharmacologically specific since local ejection of scopolamine or atropine completely blocked the action of muscarinic agents whereas curare or dihydro-fi-erythroidine selectively reduced the enhancement of the field excitatory postsynaptic potential produced by dimethylphenylpiperazinium or tetramethylammonium.

In summary. both muscarinic and nicotinic receptors are involved in the facilitatory action of cholinomimetics at the cell soma, whereas muscarinic and nicotinic agents have an opposite actions on the dendritic tield excitatory postsynaptic potentials.

Extensive ~ln~itornic~~l and biochemical studies have demonstrated the existence of a cholinergic afferent pathway which originates in the medial septum and diagonal band complex and projects both to the immediate vicinity of the pyramidal and granular layers and to various dendritic segments.29,3’,3’,40 Recent

intracellular observations made irk citro and in aira have provided strong evidence in favour of a post-

synaptic site of action of ACh.7.A.1”.” Hounsgaard’” first reported that local applications of ACh in the apical dendrites of CAI in the hippocampai slice

* To whom reprint requests should be addressed. f PWWN c~&ir~.s~ Dep;irtment of nutrition and Food

Sciences. M.I.T. Cambridge. MA 01139. U.S.A. + PW.WI~ &~WSS: McIntyre Bldg. McGill University,

365 Drummond Street. Montreal. Quebec H3G IYb Canada.

/thh~~~intio~~~: ACh. acctylcholinc: GABA, y-aminobu-

tpte: IPSP. inhibitory postsynaptic potential; EPSP. excitatory postsynaptic potential: DMPP. diphenylmethyl- pipera7inium: MeCh. metacholine: DHE. dihydro-/l-erythroidine: TMA. tetramethylammonium.

Yi

preparation reduce the excitatory postsynaptic potentials (EPSP) and provided evidence that this action was best explained by a presynaptic reduction of the

release of excitatory transmitter. This was confirmed and extended in the same preparation more

recently. AZ In the intact animal, Ben-Ari ef (11.~ have

shown that microiontophoretic application of ACh in the pyramidal layer reduces the probably GABA-ergic inhibitory postsynaptic potential (IPSP) and its associated conductance change without reducing the response elicited by local application of GABA. Therefore ACh appears to modulate ptesyn~pticaily the excitatory and inhibitory drives which impinge upon the dendritic and somatic elements of the hippocampus.

In the present report. we have studied the effects of dendritic and somatic applications of muscarinic and nicotinic agents in situ. To gain a better understand- ing of the dual modulatory action of ACh. we have used a method which was recently developed in this laboratory and which makes it possible to record simultaneous in situ somatic and dendritic potentials and

to apply m~croionto~horetically various agents at

either site.” This study has been reported else~hcre in brief.f*3”

EXPERIMENTAL PROCEDURES

Forty adult male Wistar rats (20@3OOg) were studied. They had access to food and water ad lib and were housed in cages under diurnal lighting conditions with lights on from 08.00 h. Experimental procedures were carried out under general anaesthesia with urethane (2~.3g/kg i.p.); The core temperature was maintained at 37”.-38°C and the heart rate was continuously monitored. The animal was placed in a conventional stereotaxic frame and bipolar electrodes were introduced to stimulate the ventral hippocampal commissure and fimbria (details in ref. 6). Seven barrelled micropipettes (4-7 pm tip diameter) were introduced through the overlying cortex to the regio superior of Ammon’s horn (CAI, ref. 27). The surface was then covered with agar (4% w/v in saline). In 20 experiments, the dendritic and somatic potentials were recorded simultaneously using a twin set of multibarrelled pipettes. This method has been described in detail elsewhere.’ ’ In brief. conventional multibarrelled electrodes were first prepared, then another multibarrelled pipette was bent with a microforge (approsi- mately 20-30” from vertical) and glued with dental cement to a (straight) muttibarrelled assembly. The vertical distance between the tips varied from 10&600 blrn depending on the dendritic segment under investi~tio~l: however, routinely the distance was 220-25O~im. so that one elcc- trode was placed in the pyramidal layer, the other (deeper) one was situated optimally to record the negative field (EPSP) generated in the stratum radiatum by activation of Schaffer collaterals.

The central barrel of each set contained NaCl (3 M) or pontamine sky blue (2:, w!v in 0.5 M Na acetate) for the determination of the electrode position at the end of the experiment. I5 The remaining barrels contained, in various permutations, the following drugs: acetylcholine f I M. ACh), muscarine Cl (0.05 M, MUSC); acetyl-/?-methylcho- line C1 (0.1 M, MeCh); atropine sulphate (1 mM in 165 mM NaCl); scopolamine hydrobromide (I mM in 165 NaCli: hexamethonium Br (10 mM); gailamine trieth~od~dc (0.5 mM); mecamytamine (1 M); dihydro-~-ery~hro~d~nc (5 mM in 165 mM NaCL DHE, kindly donated by A. Ttautman); d-tubocurarine Cl (10 mM in 165 mM NaCII; tetramethylammonium hydroxide (1 M, TMA); diphenyl- methylpiperazinium iodide (0.1 M in 165 mM NaCL DMPP); eserine sulphate, 0.1 M); Benzylpenicillin (G) Na (0.1 M in 165 mM NaCl); bicucullinc methiodide (IOmM in 165 mM NaCl, Pierce); picrotoxin (5 mM in 165 mM NaCl); GABA (1 M). Unless otherwise indicated all drugs were purchased from Sigma.

At the end of the experiment, the rat was intracardiailj perfused with saline, foltowed by a 10% formalin sotution and the brains processed for acetylcholinesterase staining. using the method adapted by Lewis,” and Nissi or Klijver-Barrera stains to verify the location of stimulating and recording electrodes.

RESULTS

Depth profile of somatic and den&tic jeld potrntiak

With a twin set of recording electrodes in area CAI

(inter tip distance 220-250 pm), the sequence of events

during 3 10 V commissural s~im~lI~t~~1~ (c),1 i.j.5 ma

duration at I Hz) was as fdl~~~: it ncgatk field WA

first recorded appr~~xirn~~tet~ 1 X%i ;im benen& t&a cortical surface. corresponding to the field EPSP in the basal dendrites.’ At the level of the pyramidal cell

layer (X00 /urn from the surface). ;s p~~sit~vc wave and a negative population spike w~‘rc rc~rded: thcsc ark

believed to reflect mainly the ;IO~IW >)f the inhibitor!

synapses impinging upon the pyr‘mt~dal somat:\ ’ and the synchronous discharge of a targt: rlumher of pyre- midal cells.’ During the experimenti. electrodes wcrc

positioned ultb reference to the 131 r;imidal layer. b! relying on the response evoked by the ejection of AC”h and GABA.’ ’ ,‘A The leading &clrode, Z% Jim deeper. recorded the field induced in the str;ltum

radiatum. whereas the other was loc;trud in VI’ ver!

close to the pyramidal layer. The rlcndritic response consisted of a small triphasic w;~e r?he :tAi‘crent fiber

volley? followed by a large negative lieid \vhich probably reflects the synaptic activation 01‘ pyr;imidal cells.‘,’ Thu spatial distribution of :hcse fields is in keeping u\ith previous irf riro ~xperun~nts in other specics.‘~“,” The correct positiomnp ,I!‘ rhc r.licroelec-

trodes has been confirmed repcated!~ b\; histotogic:tf

meirns.

~~.s~~~~~i~it#r~, &~ct cjf c~ho(irr7rt7irtr~~ri~~~. lising the t&-in set of electrodes. wc have made the foIlowing observations. which support the conclusion that ACh has an important disinhibitory action at the pyramidal layer,’ and suggest that both muscarinic and nico-

tinic agents depress recurrent inhibition (possibly by :I different mechanism, see Discussion). Firstly. there

was no significant enhancement of the dendritic field EPSP at the time when chohnomimctics applied at

the somatic level produced a iargc enhancement of the populati(~n spike (see Fig. i tiw .AfX TMA. similar observations were made with mL~scarinc, metacholine ;md DMPP). The latter. thercforc, cannot he ascribed to an enhanced excitatoq action at the dendritic level due, for instance. to diffusion of the agents.

Secondly. when two stimuli arc appiied to the fim- brju in quick succession there is a pronounced de-

pression of the second population spike followed by a

facilitation. This depression is not .tssociated with significant changes in the dendritic tield EPSP (Fig. IBL Post-spike depression of hippocampal pyramids in- cludes ;I component of after-hyperpolarization,“” but an important role of GABAergic synaptic inhibition is indicated by the ready block by agents such a~ bicu- culline, picrotoxine or penicillin without changes simultaneously recorded in the field EPSP (not iflus- trated). The consistent reduction of the ‘paired spikes’ inhibition by muscarinic (Fig. 1D) and nicotinic agents (Fig. I E) supports the hypothesis that cholinomimetics depress recurrent inhibition (also see ref. 16).

Dendritic and somatic actions of acetylcholine

Fig. I. Simultaneous recordings from stratum pyramidale (sP) and stratum radiatum (sR) in the CAI layer (intertip distance 250 pm): local iontophoresis of cholinomimetic agents was made in sP (asterisk). A. A single shock was applied to the fimbria (2V. I Hz): acetylcholine (ACh), tetramethylammonium

(TMA) and butyrylcholine (BuCh) enhance the (somatic) population spike without altering the dendritic

field EPSP recorded simultaneously. B. Paired pulse inhibition: tw’o pulses were applied to the fimbria

at various intervals The first pulse evoked in the pyramidal layer a positive wave and a superimposed

negative population spike. In stratum radiatum. a trace of the population spike is seen on the negative

wave. When the second pulse occurred 20-150 ms after the first, the second population spike was

reduced; for longer delays. the latter was increased. C to E: disinhibitory action of cholimimeti~s applied

in stratum pyramidal. The amplitude of the population spike evoked by the first pulse was taken as

1004,, and the second spike was measured at various intervals. ACh. metacholine (MeCh) and dimethyi-

phenylpiperazinium (DMPP) produce a reduction of the inhibition; the simultaneously recorded dendri-

tic EPSP was not altered (not shown). In this and following Figures the traces represent single sweeps,

tic agents, As shown in Figs 2 C,D, and Table 1, atropine (or scopolamine) applied iontophoretically antagonized selectively the action of ACh, metacholine or muscarine but not the effects of DMPP. or TMA (not illustrated). Conversely, on several occasions, brief applications of curare (Fig. 2B and

Table 1) or DHE (Table 1) blocked the action of DMPP or TMA without significantly reducing the action of muscarinic agents. Prolonged applications of the antagonists-in particular, curare-tended directly to promote population spikesI This selective

blockade was also obtained without significant changes in the dendritic field EPSP (not illustrated). As shown in Table I. other blockers, notably hexamethonium or mecamylamine, did not produce clear-

cut effects whereas. unexpectedly. gallamine selectively blocked muscarinic agents. The present observations which suggest a clear-cut distinction between

mus~lrinic and nicotinic actions are at variance with earlier observations. This is a topic for discussion.

Dtwdritic uppiicutions c$ acetylcholine

Inhibition cf the j’idd escitutory postsynaptic potrn-

tial. When applied at the level of the apical dendrites in the region of activated synapses (see below), ACh consistently reduced the negative field (presumably generated by EPSPs). The magnitude of ACh effects

was variable; however, even with high doses of current, the reduction of the negative field was usually

less than 50% (Figs 3,4.6). These effects of Afh were not due to a current artefact since application of

stronger control current did not produce a similar

effect (Fig. 3). As shown in Fig. 3. Iocal appli~~~tion of eserine also produced a similar effect. in addition it

potentiated the effects of ACh (not illustrated). Using the double-electrode system, the contrast

between somatic and dendritic applications of ACh was particularly evident, Thus, whereas somatic appli-

cations of ACh enhanced the negative population spike (with little or no effects on the dendritic field, Fig. l), dendritic applications of ACh produced a sharp reduction of the population spike even when

there was only a minimal (l&20‘?<,) reduction of the EPSP (Fig. 4). Furthermore. the efficacy of ACh was

also evident in paired-pulse experiments. since the de-

crease of the somatic population spike induced by dendritic application of ACh was associated with a reduction of the emcacy of the somatic recurrent inhibition.

Phurmmlogy of the trcetylcholinr dendritk qfict. In order to determine whether this inhibitory effect of ACh was mediated by muscarinic or nicotinic agents, we tested various agonists and antagonists. In contrast to somatic applications, where muscarinic and

Tab

le

I.

Eff

ects

of

som

atic

an

d de

ndri

tic

iont

opho

retic

ap

plic

atio

ns

of a

ntag

onis

ts

on t

he a

ctio

n of

cho

linom

imet

ks

Ant

agon

ist

If

DO

%

WA

)

Som

atic

ap

plic

atio

n N

O

Ant

agon

ism

ef

fect

s n

Mus

catin

ic

-Atr

opin

e

-Sco

pola

min

e

13

IS

25-2

00

Mus

cari

ne

DM

PP,

TM

A

9 M

&h,

A

Ch

Z-2

00

Mus

cari

ne

DM

PP,

TM

A

6 M

eCh,

A

Ch

Dos

e (n

.4)

Den

driti

c ap

plic

atio

n D

irec

t ef

fect

on

fie

ld

EPS

P A

ntag

onis

m

No

effe

cts

__.

20-1

50

20-1

50

Enh

ance

men

t

Mild

en

hanc

emen

t

Mus

cari

ne

MeC

h,

AC

h M

usca

rine

M

eCh,

A

Ch

DM

PP-T

MA

DM

PP-T

MA

Nic

otin

ic

-&tu

bocu

rari

ne

-Dih

ydro

-&er

y-

thro

idin

e G

alla

min

e

6 15

--w

D

MPP

-TM

A

Mus

cari

ne

5 20

-X

Mild

D

MPP

-TM

A

Mus

cari

ne

MeC

h,

AC

h en

hanc

emen

t M

eCh,

A

Ch

4 5%

250

DM

PP-T

MA

M

usca

rine

3

25.. 2

00

NIL

D

MPP

-TM

A

MeC

h.

AC

h M

eCh.

A

Ch

3 25

-150

M

usca

rine

l3

MPP

3

25-1

00

NIL

no

t cl

ear

+

Mec

amyl

amin

e H

exam

ethi

oniu

m

MeC

h,

AC

h 4

‘5-l

50

N

ot

clea

r -+

3

25-1

00

Not

cl

ear

--+

4 20

-100

N

IL*

DM

PP-T

MA

M

e<%

. A

Ch

2 25

-200

N

IL

Not

cl

ear

-+

Sum

mar

y of

the

eff

ects

pro

duce

d by

som

atic

or

den

driti

e ap

pkat

ion

of t

holin

ergi

c an

tago

nist

s.

The

num

ber

of t

rial

s is

fir

st i

ndic

ated

, fo

llow

ed

hq t

he d

osag

e fi

n n

A).

T

he

dire

ct

effe

cts

upon

th

e fi

eld

EPS

P of

th

e an

tago

nist

s ar

e in

dica

ted

as

wel

i as

th

e ch

olin

omim

etic

s ag

ents

w

hich

w

ere

anta

goni

zed

(ant

agon

ism

) or

no

t SK

I:, &

&t+

i *A

pplic

atio

ns

of m

ecam

ylam

ine

in r

hc d

endr

ites

prom

oted

ro

petlt

ive

paro

xysm

al

nega

tive

pote

ntia

ls

afte

r pr

olon

ged

(510

m

in)

appl

icat

ion.


N SplkeS @ ATROPINE 80nA(+408)

AChaOnA 0MPPlOO”A Mech lOO”A

AC-h fJO”A DMPPlOOr!A MeChlOOnA

Fig, 2. Specific antagonism by the muscarinic and nicotinic blockers at the somatic level. A c‘. The

population spike (0) and positive wave (LI) amplitude. and the number of spikes were measured durmg

local application of ACh. DMPP and MeCh in a control period (A) and during local applications of

d-tubocurare (B) and atropine (C). Note that curare had very little effect by itself. but markedl) sup-

pressed the effect of DMPP. whereas atropine strongly antagonized the action of ACh and MeCh. In D.

the responses obtained with several doses of muscarine were clearly reduced by a local application of

scopolamine.

nicotinic agents had a comparable effect (see above. and ref. 35). opposite effects were clearly evident in the dendrites: muscarinic agents reduced and nicoti- nit agents augmented the field. Thus. local application of metacholine or muscarine (Figs 3.5 and 6)

readily produced a reduction of the dendritic field. The action of muscarine had a somewhat longer onset and recovery was slower (Fig. 5B). Furthermore, as shown in Fig. SB, the reduction of the dendritic field was accotnpanied by a marked reduction of the soma-

tic population spikes. In contrast, local applications of tetr~~methyl~~mmonium. or DMPP, caused a pro- gressive increase of the duration of the EPSP as well

as its amplitude (see Figs 3.5 and 6). The double recording system permitted direct demonstration of concomitant enhancement of the somatic population spikes (Fig. 5A). DMPP was slow to take effect- usually the maximal response was observed at the end of a 70 s application--and recovery was also slow.

These opposite actions of muscarinic and nicotinic agents in the apical dendrites were highly reproduc- ible being seen consistently in 22 rats.

The action of ACh was much diminished by local application of scopolamine or a&opine (Fig, 6B). thereby conlirming the primarily muscarinic nature of its effect. These drugs also completely antagonized the effects of MeCh (Fig. 6. Table I) or muscarine (not

illustrated) without affecting the enhanccmei~t of the field EPSP produced by nicotinic agents such as DMPP or TMA. Atropine and scopolamine :rlso tended to enhance directly the field EPSP (Table I). Conversely, local applications of curare (Fig. 6) or DHE (not illustrated) blocked the effects of DMPP (or TMA) without significantly reducing the action of muscarinic agonists (Fig. 6). Three other Lmtagonists were tested on the field EPSP: gallamine and hrxa- methonium had no clear-cut effects, whereas mecamylamine on four occasions blocked the :Iction of

DMPP without reducing the efftct of rnus~lrinjc

agents (Table i ). Interestingly. applic~iti(~lls of mecamylamine promoted recurrent paroxysmal negative

potentials which were not observed with other Ltntag- onists.

Deptl~ prqji/r of’ nwscurirw md rricmfink ~~c~riorl.s. To

determine the spatial distribution of ACh-sensitive sites, the effects of cholinergic agents were tested at

various levels in stratum radiatum. As sho~cn in Fig. 7. ACh produced a reduction of the ncgativc licld

(and the somatic population spike) when applied at the site from which the maximal negative E:PSP field

was recorded (220-250btm deep to the pyramidal layer. confirmed by hist~~logic~l~ means). This site. therefore, corresponds to the region of activated excitatory synapses of Schaffer collaterals. A similar depth

Fig. 4, Effects of dcndritic applications of acetylcholine QXI the negative dendritic field and the .WwntUic population spike double recording system. in A, we show traces obtain& simultaneously from straW% p~~rn~da~~ <SPf and ~~~~t~~ ~a~jatnrn (SR), ACb was applied in stratum sadiatum {asterisk] for 20% The dendritic negative wave (SR) is only sIi@tly reduced but the somatic popufation spike is practical@ abolished. The results from thirn experiment tire represented graphically in the lower part of the EjiWe.


@ DMPP (dendrites)

i! I . . I , , I.

0 ,“, 40 60

DMPP 150nA

43 MUSCARfNE ldendriterl

t t MUSSARINE fOOnA

Fig, 5. Opposite effects of diphenylmethylpi~ra~in~um (A) and muscarine (B) applications in stratum radiatum. Soma- tic (SP) and dendritic (SR) recording were simuitaneous; results from a single experiment in which the position of the double electrode was held constant. Drugs were applied for 30 s. Note that the field EPSP and somatic population spike are increased by DMPP, whereas mus-

carine has opposite effects.

Fig. 6. Specific antagonism by the muscarine and nicotine blockers of the dendritic action of cholinomimetics The negative wave evoked by stimulating the fimhria (3 Y. i Hz) was clearly enhanced by DMPP and depressed by ACh or MeCh (A). Atropine (B) blocked the action of ACh and MeCh without affecting so much the response to DMPP; d-tubocurare (C) abolished the action of DMPP.

profile was obtained with muscarine {to = 3 rats, cf. Fig. 7). In contrast, the actions of DMPP were more wide-spread. enhancement of the negative fields also being obtained above or below the site of activated synapses.

DISCUSSION

Chotinomimetic actions al thr somutic lewd

Additional tests of ACh and related agents applied at the level of the CA1 pyramidal layer and the simultaneous recording of somatic and dendritic potentials confirmed and extended the conclusion that ACh has

Fig. 7. Depth profifes of dendritic actions of chofinomimetics. The agents were applied successively ifor 20 s, once per minute) at intervals of 50 pm.

a disinhibitory action which is mediated hq both muscarinic and nicotinic receptors.9*‘“.z2 The pharmacological specificity of the action of choiinomimc-

tics is in contrast more controversial in the literature.

Thus, in the recent study of Roper? & Krnjevic.” local application of ACh antagonists (in particular.

curare) had a powerful direct excitatory efiect and

therefore the pharmacological specificity was largelq

assessed by means of systemic administration of antagonists. Both scopolamine and atropine given in

high doses (up to 80mg;kg) depressed the action of muscarine and ACh without blocking the effects of

DMPP. In those cases in which curare or DHE could be iontophoretically applied (i.e. without inducing an

enhancement of the population spike [s]), the action

of DMPP was not reduced; DHE actually appeared selectively to block muscarinic agents.” In contrast. in

the present series of experiments, ACh antagonists had a less powerful immediate excitatory action. and it was therefore possible to demonstrate ;I ?;electivc antagonism by iontophoresis. Muscarininc actions

were consistently blocked in a selective manner fi.2.

without significantly alteration in the action of nicoti- nit agents); conversely, the action of DMPP or TMA

was selectively blocked on several occasions by curare or DHE. The reason for this discrepancy is not clear: it may result from the use of lower ~oncentratior3s of antagonists. although a different drug-sensitivity oi

Wistar. as opposed to Sprague-Dawley rats (used

earlier), cannot be excluded. The present study. therefore. suggests a more cfear-

cut pharmacological distinction between muscarinic

and nicotinic actions than previously. although, in agreement with the conclusions reached by Ropert & Krnjevic,“5 hippo~~~mpal somatic ACh receptors can-

not be categorized to a single conventional scheme

(sir. the effects of gallamine), Further studies :Irt’ required particularly in order to clarify whether II

similar mechanism underlies the disinhib~tion produced by muscarinic and nicotinic agents. lntcrest-

ingly. nicotinic agonists are more potent than musar- inic ones in depressing the positive field” as well ;fs

the paired pulse inhibition (Fig. 2). This may indicate . .

that mcotlmc agents have a purely disinhib~tor~ action,“.“’ whereas a double mechanism \+ ould underline the muscarinic action, i.c. presynaptic reduction in GABA release- as well as post-synaptic facilitation.‘,“. ’ ~ It is. however. also possible that

muscarinic and nicotinic receptors have a different anatomical distribution (see, for instance. ref. 37). muscarinic agents would reduce the release of GABA. whereas nicotinic ones would disinhibjt the pyramidal cells by means of a more conventional postsynaptic inhibition of the GABAergic interneurones.

IMuscurilzic. Bearing in mind the limitations of extracellular recording, our observations that ACh and other muscarinics depress the amplitude (and duration) of the negative field evoked in stratiam

(In the other hand, there arc m;~;~~~ c.tzrmpirs UI

cholinergic presynaptic receptors th.ir reduce trans.

mitter release; indeed, muscarine r’~‘ce;~tors :tre kno\vn to diminish the release of ACh ~tst:Ji‘ from ccntraf cholinergic terminals..‘O*“‘, ” ” Furthermore. mus-

carinc receptors appear to be presenr 1~~~ ;I variety oJ other terminals (noradrenergic, dopamrnergic. ete.1. and generally they tend also to reduce the release of

whatever tr~insmitter they usuatl~ sto;; (ior ;I rt~ic\t,. see Starke”)

.3’ri orirk. Our observations that nlcotmic agents

increase the EPSP tield (especially its duration) are in agreement with a general tendency ii&, presynaptic

nicotine receptors to facilitate transmitter release.3” Indeed. there are several examples oi‘ clpposite mus-

carinic and nicotinic actions on the s‘lrne populatton of nerve terminals. on peripheral ~lor~~dre~lerg~~

fibres.““, ” serotoninergic fibres in the hypothala- mus.” dopaminergic terminals from the striatum” and choline@ fibres of the myenteric plexus. “’

There is practically no evidence :tvailahie about the

postulated presynaptic mechanism of nicotinic fad%- tation. One possibility might be that ‘PvlA (and per-

haps DMPP) have a TEA -or -I-aminopyridine-like action; by dire&y blocking the K-channels respon- sible for delayed rectification. these .tgcntb enhance

Ca influx at terminals and thus enhance transmitter re,ease,? 1. !?. 28 However. the acticrn nf DMPP and TMA can be blocked by a specific nir:otine rmtapon-

ist, which would indicate a receptor-media&cd action. Another possible mechanism \vould hc J nicotine receptor-tnodul;ttion of terminal Ca current. similar

but opposite m direction to that rrcentty descrihcd by Dunlap 81 Fishbach.”

.ludging by the predominantly facllitatory eHtct of medial scptai stimulation on C ‘.9. i population spikes”,“’ 2s IVeIl as its greater sensit~vit! to mu+

carine rather Ihan nicotine antagonisrs’i the lrlost important functional site of cholinerglc modulation would stem to he at the level !>1 the p).r:unidal somat;t. However. recent observations made in this

lclborxtor> tlsi~zg the double regurding sl;>tem’” sug-

gest that septal stimulation also reduce< the dentlritic field EPSP recorded in CAI, The medial septum may thus modulate excitatory synaptic transmission at the level of the dendrites in nddition In i!~ disi~~ilibjt~)r~

action on the ccl1 hod\.

.-~~~rlci~,lrd~ntlerll.s The visit of E. C3sruhrnr to Git-sur-

Yvette was made possible by ;t Grant from the ~~~lli~d~~ti~~i~

pour la Recherche Mkdicale. and that of K. Krnjcvic nnd N. Ropert by financial support of the C’ommission pour la Recherche France-Qlkbccoise. We are grateful to C;. Chnr-

ton. G. Levesyue and G. Chilini for ttwhnicai assistance.

Da&tic and somatic actions of a~etyicholine 105

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(Accqrfed 22 July 1982)

PHARMACOLOGY OF THE DENDRITIC ACTION OF …€¦ · Dendritic and somatic actions of acetylcholine...

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