Purinergic Neurotransmlssion

Adenosine triphosphate (ATP) is a primitive exaacellu- lar signaling molecule and is now established as a cotrans- mitter in most nerve types in the peripheral and central nervous systems (CNS). There is pwinergic cotransmission from sympathetic nerves (together with noradrenaline [NA] and neumpeptide Y), parasympathetic nerves (together with acetylcholine [ACh]). sensory-motor nerves (together with calcitonin gen~c la ted peptide and substance P), enteric nerves (together with nitric oxide (NO] and vasoactive intes- tinal polypeptide [VIP]) and in subpopulations of central neurons with glutamate. dopamine, NA, y-aminobutyric acid, and 5-hydroxytryptamine. Postjunctional purinergic P2X ionotropic receptors are involved in neurotransmission both at neuroeffector junctions in the periphery and synapses in ganglia and in the CNS, whereas prejunctional modula- tion of neurotransmission is mediated largely by P 1 (A 1 sub- types) and PZY G protein-coupled receptors.

An increase in the role of ATP as a cotransmittcr has been demonstrated in pathologic conditions including interstitial cystitis and hypertension. In addition to rapid signaling during neurotransmission. ATP appears to act as a long-term (uophic) signaling molecule during development and rcgener- ation, sometimes synerpistically with growth factors. Puriner- gic mechawsensosy transduction has been pmposed where ATP released from epithelial cells closely associated with sensory nerve terminals labeled with P2X, receptors leads to activation of sensory nerve activity related to physiologic events such as bladder voiding, or to nociccption. or both. The thempcutic potential of purinoceptor agonists and antag- onists. ATP transport inhibitors and promoters. and adenosine uiphosphtase (ATPase) inhibitors is being explored.

It is now more than 30 years since purinergic neuro- transmission was proposed by Burnstock in 1972, but it is only in the past few years that ATP has become widely accepted as a neurotransmitter [I]. The early evidence that nonadrenergic, noncholinergic (NANC) nerves used ATP as a transmitter when supplying the smooth muscle of the intestine and bladder was based on the criteria generally regarded as necessary for establishing a substance as a

Geoffrey Bumstock Autonomic Ncumienec Institute

M o n , United Kingdom

neurotransmitter. The criteria was summarized by Eccles in 1964 as the following: synthesis and storage of transmitter in nerve terminals; release of transmitter during nerve stim- ulation; postjunctional responses to exogenous transmitters that mimic responses to nerve stimulation; enzymes that inactivate the transmitter andlor an uptake system for the transmitter or its breakdown products; and drugs that produce parallel blocking of potentiating effects on the responses of both exogenous transmitter and nerve stimula- tion. There is evidence for neuronal synthesis, storage, and release of ATP and for inactivation of released ATP by cctoATPases. The model proposed for purinergic neuro- transmission in 1972 is shown in Figure 14.1.

Receptors for ATP an implicit in the process of puriner- gic neurotransmission, and the early proposal by Burnstock in 1978 to distinguish receptors for adenosine (Pl- purinoceptors) from those for ATP (PZ-purinoceptors) was the start of many subsequent studies of both PI- and P2- purinoceptor subtypes. In 1994, on the basis of molecular cloning, transduction mechanisms, and selective agonist potencies, Abbracchio and Burnstock made the proposal, which has been widely adopted, that P2-purinoceptors should be considered in terms of two major families: a PZX receptor family. which are ligand-gated ion channel receptors. and a P2Y receptor family, which a n G protein-coupled receptors. This subdivision into ionotropic and metabotropic families brings receptors for ATP in line with most of the other major neurotransmitters (Table 14.1). Four subtypes of the P1 receptor, seven subtypes of the P2X receptor family. and seven subtypes of the P2Y receptor family are currently recognized and have been characterid (Table 14.2) [2. 31.

Following the proposal by Burnstock in 1976 that nerves might store surd release more than one transmitter, then is now considerable experimental support for ATP as a cotransmitter with classical transmitters and neuropeptides in most major nerve types (Fig. 14.2) (4.5.61. ATP r e l d from nerves may also act as a pnjunctional modulator of cotnnsmitter release (often through adenosine) or cis a postjunctional modulator of cotransmitter actions.

FIGURE, 14,1 Sdremelic q r c n ~ n ~ t j a n & pwioergir: neunanmculqr ~ s i m dqimiog Ihe s g m i % w. micas and: inactivlm d itUenbbin= triphcaphms, and ant-larim +rough, p@DcrioW adelrtleine (PI) -. I M d W with permimion from Bnrnflt&, G 19W. P d o q i c fotr;msmt95ian. Bruin Res B d . -355-3g.l

-r F a b d m @ c ~ m e a e ~ -

ACh Nid i i i c Mrrse~rinic M w k type MI-W Neuronal type

GABA GAB& GAS& ia~tmnw W A mOl~rnrCn%

Kaiaate W A

5-IdT 5-1-m~ I~ IA~ 5-mac 5-m4 5-mm 5-HT;o 5-m7 P Z Y ~ ~ ~ ~ ~ ~

Mndikd Fmm Abbsadia. M, F,, and a. B m e . 1 M . Purinugic aignWg: Patimphysjolngir%l rok~ Jjm J. Phmmd. 78:LILI45.

5-W, Shydrwyrryprarmine: A C ~ L wkbdint: M A . 2 (mimelhy1)pknyl~tk mid; RTP, & m i n e triphasphte; GMA. yunlnobutyric =id; N W A . K m a h y l - t + ~ .

these junction ~ n ~ s were ;blocked by guanethidine, whkh p m s the releast b q r n l p a k t i c neumstnimrs, at the rime it was smpk i ig that adnemceptur amtagnniats w m irrdkctive. It was m m than 24) y r w s b d m ~ l d v c &mmsiiti;mrim of the ATP receptor by ~ ~ ~ y l c r a e PcTP was shnw to Mock the EJPq when it became clear that we wle looking at the neswes to ATP aa a cotrmsmiWLs in sympathetic nlerves [TI. & b e of ATP w w a h n to be abolishad by emdotaxin. guaddiiae, and, after destrw; tim Of sympathetic nmes, by 6 - h y d r o x y d o ~ but rn by ~aserpim, which blmkd chs smnd slow n-@c phase d tbe resjmnx but not the iniltial fast phase. Spritz- ing BZ"P mm single smooth muscle cells of 6.le v& ddm- a s mimicked the W, whereas sprimd NA dimd not.

Sympuuhetic purimgic ~obramissiorr dm has h e n h m m M ~JA a variety d blmd vesmls, The pmpr~rtiaul of NA 4, A W is exmmdy variable in the sympathetic m e s supplying the difhent Moad vessePa. The pusinqic mmt is reldvdy tnitrar in rabbit ear a d rat tail aeter- k, ia t..na& ~~ in the rabbit q h m w artmy, and bas k e n claimed to be h~ sole hmsrniwr in s y m w i c nervts supplying artepinles in the mmmy md the mb mumal plexus ,of the intestine, w b m NA rekascd h m these mme& acts as a mQdulam of ATP dwm.

SYMPAt3HETK NEKVES Pamymipathetic rreaves supplying the urinary b W

Thn first hint about sympathetic pmimqic m n s m l s urn ACh and MT as aobms~m. in variable pqmtiorus aioa was irr an &le publishled by Burn-k d H o l m TI d i f i i t species; and by andsgy with ~ymptP.realc in 1962, in which they recwded mcimtq jui~crim~ p&~- rrmes, ATP again b g h FZX iomtmpic -tors, tiah @Jh) in snrooth muscle cctls o€ the vas ddhem w b m s the slow cornpent dthe mpmt i s mdiakd by through stimulation d tb hgppscric m. Although a u - t e t a ~ -tor, in this case m w d c . There i s

h m i n q r i d d c n r d b - i u a w u l d d s

Bmin 1- spleen IWim bbMcr

L.uw liver; bmir TtSLfg. hemt

= m e wmm. NTS+ m m

s y ~ i c n e u m CNS. as. d m

sMne tvidtm ra sclggtsi p,masp-, pllk-i- ~ ~ ~ s e n s m y a t n r t ~ a l l a c e r s l s ~ ~ coaansmissicm to Tesjmnce d s in the heart d " a x a r ~ " ~ v i t v ~ ~ ~ a t i a r o f ~ ~ s .

rialogic compomnt; of autonomic d. Cnkitonin SLNSORY-MOTOR NERVES p t - ~ p~ptide and s u b part w611 d l l s h t d to

mbst in smmymotm rum- and in same mbpopula-

Tnairrpic-srLaio-dhraa*-dh ~ . l v l n n y d ~ a r c p a r t d u l c ~ d ~ s y s m ; b u r w m i r d y b ~ g u t d ~ ~ h i n d m ~ spmedtlwe inhiwkmeumasm M u a d h n e u d l t i a ~ that differs from that whkh formar the s y m p k t b a r d p m s p p & m a ~ ~ a n d a p p r a r c o ~ r a n ~ ~ l ~ . b t h C ~ # t l ~ ~ D f i n u b a i e ~ i n t b e ~ e n d m r ~ - h b shDwnas W d ~ m ~ A T P , No, uempqmk Y , A C h , a n d ~ y d m x ~ k . ~ d t h e s e ~ ~ e c t a t h e ~ v ~ t u r e a n d ~ ~ v a w - mpwr %tiDng

T l w m h i c - q x k m - M M dM-- iprhemyrmtr iem- mu& - d die gukrmcplrs flex= The Gkmid dn$-dhaz ncrvas has bmm examined In Wl. A s u b p c p u ~ 9f tltese irttraanrral e M c ntr~tg prwkks NANC inhibitmy inneMdon oftbe of el mmMh musrckltpeansllbdy th#~mqjammmimrsm r d e s s e d R w a W - : A r n ~ ~ t n m i

jwlctim pntials, NO a h inhibitmy pptloaide W w i t h r ~ ~ t i m e ~ . ~ V W ~ s ~ m k m ~ . m ~ ~ d h ~ ~ h m y c m & k d A y i n B i & m n t ~ d t h e @ a n d i o d i f - fmm s p b ; for armple, in same sphhcbm, h NANC Mbitory nerve3 targcIy we VIP and o t h h g e l y ust NO. w* in nontsphinc& regions of the imsclnr, ATP t9 m a d a p r o m d

ATP mexiax and i s dead witb A t 3 in llllotw nerves s q p i y i ~ ~ ~~ mrrscle ad, i d m i in kvdoping myamka A n , ming dmqh Pm rcqmm, is qwlly effedive with A&. aahg thm@ nicotinic ~~. Hnwew, in adnla, tbt umnmitter ~ I E 6f ATP b lacking. but tb ATP dm.wd acts boOh as a pastjunctimal p~tent ia toroftknicotinic ~ctaPnsaPAa d p r a p r r j u a e t b d m x b h l m o f A C b m h ! e a f k r i E s b t m k c h m t n ~ #MI &on MI p,qjuWlimI P1 rmxpmL

WUWTK WFUNEKZC TRA-ION IN GANWA AND BRAIN

TWO articles phlisbed in ?Vmm in 1992 made a fdrmg impregsiort on the EW%UW&W wmmunity, boch showing hat excitatory pixmynaptic p m i d s wcxe rmrmckbd

. . b

ATP and blocked by k F2 mcqmx antap& 9vramirr+

h~zacrmwdthcguineapigce~Iac~l icm,pndt lwothtr ixmumd #he rar d i a l &nula. S b that time. purlna- ~ ~ a a n s n r i a s i c m l m b a t t ~ ~ b ~ gangliaatdmhrrcgioasdthbrain.Alhwghthe:di~ butiw a n d ~ ~ r p I e s d P 1 mzptmia them have b e e n ~ z a d f a r r n a n y ~ t k & o f P 2 ~ ~ k o ~ e g h t d . Hmwvm, there is growing mi& for multiple roks for ATPmxpms in ganglia and b the CNS. Wide bistribticm of im-~i ty to tbe ami- for PW2, P 2 L and P2Grecepms including F2Xm d hramultimers and, to a leiisex aterat. PZX, arPd PWJ ~ a r s , a s w e l l a s P 2 Y , ~ l c a ~ b e r m ~ i i n m m y d i f f e r e n t ~ X a p n c d t b e b m b a r r d s p ~ d d ~ h e n ccmlabed with e ~ y s b t o ~ Syrmpk phergic ~ r n i s s r h also h a bea dmmWaW in tbe myenteric phm Some eltgaat s m h have sttown a ~ d i g a i b l r t i s m d ~ t F ' 2 ~ ~ h differtntdbintheinnerearpndinthemtina

GLIAL CULS

P Z ~ s u b t y p c s h a v e b e e n i b t d h d o ~ ~ ~ clostly awxiated with rreumna hdudirog a m h x y k ~ . dip ~ ~ ~ E Y W L M w m n cells. and tnttric glid d l s . GZiat =Us have &a ghDm &I rzlease ATIP.

TIE exprssion of m d t r c r s atld mccptm iu the m m m i e nemw sptem d m w s mmkd plssricity duing d t u e m n t and @ng in che nerves that remain afw t r a u m a o r ~ , ~ & d ~ i n f l u m m d h o r n w m s , d i n V a r i w d i ~ ~ ~ o s [ X ] . ~ ~ ~ r n h e r e h role d ATP is signilicanrly enbmxl in diwase-h exmqde. in interstitial cystitis d dmmud bhldm, ihc puriaecgie component of ~y~ oaaangmigaioa i s imzasd by up lo 40%. Thae i s a significantly p n b m role WATP cmmpmd with N A h vwk~us U d WAS ofspw- ranwusly h-ivt rats-

LONG-TERM W P H [ C l SIGNALING

in ddi t im to the examples of shorr-arra s i w m g b mibed d l r , thtn are ww many examples o€ plrrimgk

~ h u v e b t t a ~ ~ a v g ~ y c a r s ~ i a g A T P o n ~ ~ ~ 9 . Lq+k 1995,thePLXsmmp ~ w ~ c l o n e d a n d s b o w n r n b ~ ~ m ~ n e u m a s i n s w x o r y ~ ~ S ~ ~ y m t e r - ~ i n t k m g w a m ~ y ~ & w f m t h c P P I ~ ~ d a ~ ~ n C r v C p ~ r r r t i a n h a s b t e n d e v e ~ t o i m m k l k ~ d p l r r i a e r $ i e ~ ~ A T P a n d ~ ~ a p p l i e d t o c h e tmgae were &OWP b preftreawly &Y& *- e n t ~ i a ~ ~ x l a u l ; b d f l b O t t b t w t e B k r s i n I h e w t y e

A ~ ~ T o t ~ ~ ~ chclhm i visceral ixpm iny0Ivd in Initintion d pin wms p p s d b y Bmwackia 1 9 9 9 , ~ ~ Y w a s ~ ~ ~ ~ u f ~ , s a c h a s t b e ~ , s a l i m - y ~ . a l l d ~ a n d g 1 l ~ ~ , ~ 3 S ~ a a d ~ , ~ t o t h e ~ O f r n f m m r h e l ~ ~ ~ I k u r f i ~ b i f - r u s a S t o r h e ~ t b d i a l s m ~ a y m w e p l e x u s t o ~ P W G g . ~ ~ , ~ b a t 6 , - - ~ t a [ W i d ~ ~ ~ m $ . A T P ~ b e e n ~ b b e ~ ~ ~ ~ U ~ i n t b e ~ M a d d a , gut, and wetm, aad PaX, recepcods have ken ideotified msubegi the lTa l~ in~cqum.RbeandingbaP2.X3 l m t d m m ~ ~ i r b a s b a m s ~ ~ ~ m i i c t r t r i t i o c l ~ i s i m p d d a n d t t r a t ~ o f ~ 6 b t r s r a =*reapar&=gaat,-tiogthtF'%=-q- t r n o n ~ ~ ~ e b e ~ ~ ~ a ~ ~ a n d a mdeptiw rde. SimilaFty, P2XS rmqKtm on pra)seeiods ~ f ~ h m t k ~ ~ i a ~ y b g ~ ~ ~ ~ l a r h e l ~ o f l h h r a g m a y a l ~ m a d i a t e ~ pbysid@c wants, p r b ~ Involvtd in prcmxivt reqonsex t o ~ ~ Q r t o ~ i o a s g s s e 9 .

FlAURE DEVELOPMENE

F u m r e ~ ~ ~ ~ y m ~ ~ ~ c ~ - d i n g in tk ~~ on ~ l o @ c and hhaviaral ~ m d l c m g - ~ n a p h i c m l t s d p u ~ i n a s d w t ; m i & ~ m i n ~ ~ ~ ~ y i t l ~ i c ~ o p - Rlmt and in r q m m h n .

Tbtre is Likely to be sub&mtial inktest in T& thecap%- tic &rdapmt of purinqic agents fix a variety obdi~asles of the n e n m system. The therapeutic s t m k g b are likely to extend beyond the dere1opment of selective agonist~ and antagonists for d E h t F?? receptor aubtypts, €o the developmeat of agars hat mb-01 thre expmaicm af P2 receptors, to the d e v e l w t of inhibitors d wmdJula~ ATP b r e ~ w n , and ta the Bevd~lent of 14TP mm enhancm and inhibitors. The intmactim of p b r g i c signaling witb other estabEi&ed signaling systems in the m o w system dm will be m i m m t way forward.

1. Rakvic. V, 4 G. Burnstock. I=. ikepum far princs iurd mrirn- idinea. P h a m d . R ~ K S(t4134Q2-

4 Bumtock. G. 1976. some w v e mlki rekass more l h n uac I ~ ~ 4 d W c u ~ i c n w 1 :23'KM8.

5. Bomstoek C. [Guca Ed.h 1998 Puriimgic ~lmdransniiwion. Scrrrin M W J ~ ~ . 8: 17 1-257.

6. BumGak, G. I-. Purinqic mdrunsmisxjon. Bdzrin Refib RsrJt, 5QU4-337.

7. Emdmk, €I. 1995. Noradmalint AT: Cacmmir~a and nwromatlulmm I Phhvsid. Plba~vrrrrwL 4L:96S3MM

8. Abbreechia, M. P, and G. B u m ~ ~ k . 1998. Buriamgic liignallimg: Psthaphysibid roleti. Jpa. J, Pfic i8u~c'~i . 78:113-145.

9. Burnaaock. G. XKXl, P2X leaplm in mwry w m e c BE J. Amgth. W:47w8&.

10. Duan. P. M., Y Zhg , md G. Bunrstmk. 2001. PZX nwepmrs in periphd na~rtmcs. Pnw. &ti-. (25: 1W-134.

11. BumlacL C. 1W9. hmi w w a d prim'gi signalling in rhc nwv~ras sykru . Pmg. Bmim IPcs. I20:3-10-