IMPLICATION OF ATP RECEPTORS IN BRAIN FUNCTIONS

Progressin Neurobiology,Vol. 50, pp. 483 to 492, 1996 Copyright G 1996 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0301-0082/96/$32.00

Pergamon

@

PII: S0301-0082(96)00037-8

IMPLICATION OF ATP RECEPTORS IN BRAIN FUNCTIONS KAZUHIDE

INOUE,*

SCHUICHI

KOIZUMI

and SHINYA UENO

Division of Pharmacology, National Institute of Health Sciences, 1-18-1 Kamiyoga, Setagaya, Tokyo, 158, Japan (Received 28 April 1996; Revised 22 May 1996)

Abstract-The possible implication of P,-purinoceptors in brain functions is reviewed.Involvement of Pz-purinoceptorsin memoryand learning(Section2) is suggestedby ATP releasefrom hippocampalslices ~ieraszko, A., Goldamith, G. and Seyfried,T. N. (1989) Brain Res. 485, 244-250], induction of fast synaptic currents in cultured hippocampal neurons IInoue, K., Nakazawa, K., Fujimori, W. and Takanaka, A. (1992a) Neurosci. Left. 134, 294-299]and long-lasting enhancement of the population spikes ~ieraszko, A. and Seyfried,T. N. (1989) Brain Res. 491, 356359; Nishimura, S., Mohri, M., Okada, Y. and Mori, M. (1990)Brain Res. 525, 165–169; Fujii, S., Kate, H., Furuse, H., Ito, K., Osada, H., Hamaguchi, T. and Kuroda, Y. (1995) Neurosci. Lett. 187, 13G132], as well as ATP release on cellsIInoue,K., Koizumi, glutamate stimulationto evokean increasein intracellularCa2+ in hippocampal S. and Nakazawa, K. (1995) NeuroReport 6, 43740]. Moreover, mRNAs for certain types of P,x-purinoceptorsare present in the hippocampus[Cello, G., North, R. A., Kawashima, E., Merlo-Pich, E., Neidhart, S., Surprenant, A. and Buell, G. (1996)J. Neurosci. 16, 2495–2507]. It is likely, therefore, that ATP may be involvedin modulation of synaptic efficiencyin the hippocampus.The implication of ATP in schizophreniais suggestedby the fact that antipsychoticdrugs inhibit ATP-evokedresponses in PC12 cells IKoizumi,S., Ikeda, M., Nakazawa, K., Inoue, K., Ito, K. and Inoue, K. (1995b)Biochem. Biophys. Res. Commun. 210, 62+630] without blockingthe action of dopamineD, receptors. Involvement of PZ-purinoceptorsin Sections 4 (“Pain and cognition”) and 5 (“Central regulation of the autonomic system”) are also discussed. Copyright Q 1996Ekevier ScienceLtd.

CONTENTS 1. Introduction 2. Memoryand learning 3. Schizophrenia 4. Pain and cognition 5. Centralregulationof the autonomicsystem 6. Conclusion Acknowledgements References

483 483 487 488 489 489 491 491

1995;Tokuyama et al., 1995;P,x: Brake et al., 1994; Valera et al., 1994;Bo et al., 1995;Chen etal., 1995; ATP has joined the growing list of compounds shown Lewis et al., 1995;Buell et al., 1996;S6guela et al., to function as neurotransmitters in various tissues 1996;Cello et aZ., 1996;Soto et al., 1996).Since we including smooth muscle (Burrtstock and Kennedy, now have great knowledge of the molecular 1985), peripheral neurons (Bean and Friel, 1990; fundamentals of the actions by ATP, the functions of Evans et al., 1992)and the central nervous system these receptors in the CNS have been drawing much (CNS) (Edwards et al., 1992; Inoue et al., 1992a; attention from scientists.This reviewaims to discuss Harms et al., 1992; Shen and North, 1993). the implications of ATP receptors in CNS functions. Extracellular ATP evokes responses through two subclasses of P2-purinoceptors,P2Xand P2Y.The Pzx subclass has been shown to be coupled to ligand-gated ion channels whereas the P,, subclassis 2. MEMORY AND LEARNING coupled, via GTP-binding proteins, to the stimuThe hippocampus is well known to be involved in lation of PLCD.Several laboratories have identified P1-purinoceptor receptors by cDNA cloning (P2Y: memory and learning. Glutamate, the major excitWebb et al., 1993;Lustig et al., 1993;Chang et al., atory neurotransmitter in the hippocampus, has been studied extensivelyin relation to neuronal cell death and long-term potentiation (LTP), a phenomenon * Author for correspondence. Tel: + 81-3-3707-6950; which may underlie the process of memory and learning (see reviews by Bliss and Lynch, 1988; Fax: + 81-3-3707-6950;e-mail: [email protected]. 1. INTRODUCTION

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K. Inoue et al.

Malenka et al., 1989). Although the activation of glutamate receptors is an event considered to be necessary for the induction of LTP in the hippocampus, many other factors, such as arachidonic acid (Bliss et al., 1991),nitric oxide and carbon monoxide (Zhuo et al., 1993),are thought to be involved in synaptic plasticity. The ATP is released from brain synaptosomal preparations by stimulation with KC1 (White, 1978) and from Schaffer collateral
hippocampal slices, and can potentate LTP in the hippocampus (Wieraszkoand Ehrlich, 1994).Several mRNAs for certain types of P2x-purinoceptorsare present in the hippocampus (Cello et al., 1996).It is likely, therefore, that ATP may be involved in modulation of synaptic efficiency in the hippocampus. The induction of LTP in postsynaptic neurons requires a rise in intracellular Ca2+ concentration ([Ca]i),which is thought to be mainly mediated by released glutamate via N-methyl-D-aspartate (NMDA) glutamate receptors (seereviewsby Bliss and Lynch, 1988; Malenka et al., 1989). Recently,it has been demonstrated that ATP induces a rise in [Ca]i in cultured hippocampal neurons by activating postsynaptic Pz-purinoceptors (Inoue et al., 1995). Figure 1 shows the average [Ca]i responses to ATP (30 flM) in the presence of tetrodotoxin (TTX; 3 pM) in a buffer solution (solution 1) composed of hexamethonium (C6; 100pM), an acetylcholinenicotinic channel blocker; APV (100 PM) and CNQX (30 pM), glutamate soln 1 with ITX (3+IM) Soo

”

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S300 = z

ATP (30BM) ATP+sursmin (lWVM)

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Fig. 1. Averaged [Ca]i increase evoked by ATP and the effect of suramin in hippocampal neurons. Each symbol shows mean t S.E.M. from 12–15neurons tested. Open circlesand closedcirclesshow the responseevokedby ATP (30 PM) in a buffer solution (solution 1) with tetrodotoxin (TTX, 3 PM) in the absence and presence of suramin (100 PM), respectively.The solution 1 contained CNQX (30 PM), APV (100 PM), Cd’+ (300 PM), C6 (100YM) and bicuculline(10 MM)in a buffer solution composed of 150mM NaCl, 5 mM KC1, 1.8 mM CaCl,, 1.2 MgC12, 1.2 mM NaH,PO,, 10 mM glucoseand 25 mM HEPES(PH adjusted to 7.4 with NaOH).

NMDA and non-NMDA receptor antagonists; bicuculline(10 pM), a GABA. receptor blocker; and Cd’+ (300 pM), a calciumchannel blocker. The ATP increased [Ca]ito about 450 nM from basal levelsof about 100nM and the increase was inhibited by suramin (100~M), a P1-purinoceptorblocker. The ADP (30 pM) also evoked a rise in [Ca]i equivalent to that evoked by ATP, however neither adenosine nor AMP (up to 1 mM) evokeda [Ca]iincreaseunder the same conditions. These data strongly suggestthat stimulation of P2-purinoceptorsleads to a rise in [Ca]i in the hippocampus. Figure 2 showsthe changesin [Ca]iin hippocampal cells following various treatments. Many cells, includingNos 1 and 2, responded to ATP in solution 1 in the presence of TTX, suggestingthat these cells have functional P2-purinoceptors[Fig. 2(A)]. However, neither cell No. 1 nor cell No. 2 responded to glutamate in the presence of TTX in a solution (solution 2) composed of APV, C6 and bicuculline [Fig. 2(B)]. Cells Nos 1 and 2, i.e. the “non-glutamate-responders”, did respond to glutamate when cell-to-cellcommunicationwas recoveredby removal of TTX [Fig.2(C)].In addition, the glutamate-evoked rises in [Ca]i in cells Nos 1 and 2 was inhibited by suramin [Fig. 2(D)]. Figure 2(E) showed the time-course of quantitative changes in [Ca]i in cells Nos 1 and 2. These results are interpreted as follows: cells Nos 1 and 2 have no functional glutamate receptors but have Pj-purinoceptors sensitive to suramin. Glutamate cannot evoke a rise in [Ca]i in cellsNos 1and 2 directly,but can produce an indirect rise in [Ca]i by ATP released from “glutamate responders” which innervate cells Nos 1 or 2. Thus, we demonstrated that endogenous ATP released by stimulation of glutamate can act as a transmitter in the interneurons and produce a rise in [Ca]i in the hippocampus. Under physiologicalconditions, Mg2+ blocks the activation of glutamate NMDA receptors. Even under these conditions, ATP can cause a large increase in [Ca]i in the neurons. Moreover, ATP evoked a depolarization (Inoue et al., 1992a)which might remove MgJ+ blocking of the activation of NMDA receptors. These results suggest that ATP can play a very important role in many physiological phenomena through stimulation of [Ca]i increase, and that ATP can facilitate glutamate-mediated transmission when it is co-released with glutamate, thereby resulting in enhancement of synaptic efficiency.The finding that ATP is released from the hippocampus only during the intense stimulation which can induce LTP (Wieraszko et al., 1989)may also support the involvementof ATP in induction of LTP. Zinc ion (Zn’+) is present in high concentrations in the hippocampus(Fredrickson, 1989)where Zn2+ has been shownto be localizedin nerve terminals and to be released during excitatory stimulation (Assaf and Chung, 1984).The Zn2+ also has been shown to enhance dramatically the ATP-evoked responses mediated through P2x-purinoceptors(Brake et al., 1994; Cloues et al., 1993; Koizumi et al., 1995a; Seguela et al., 1996). These reports raise the possibility that Zn2+ may potentate the [Ca]i elevation mediated by Pzx-purinoceptors,leading to

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15s after stimulation

D

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glutamate in soln2 with ITX

ATP in solnl with TTX

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time (rein) Fig. 2. An evidencefor the connectionbetweenglutamatergicneurons and purinergicneurons using[Ca]i increase evoked by ATP and glutamate in hippocampal culture. (A)–(D) Changes in [Ca]i expressedin black/whiteimagesas the scale of the bar on the bottom of (B). The imagesdepict [Ca]ijust before (left) and 15 secafter (right) stimulation.The cellswere stimulatedfor 15 secat intervalsof 2 min first by ATP (10 FM) with 3 #M TTX in solution 1 (A), then by glutamate (10 ~tM)with TTX in a buffer solution 2 (B). The solution 2 had the same composition as the solution 1 except CNQX (30 yM) and Cdz+ (300 PM). Ten minutes later, the cellswerestimulatedagain at 2 min intervalsby glutamate without TTX in solution 2 (C), then glutamate with suramin in solution 2 (D), Note the responses seen in cells Nos I and 2 indicated by black circles in the image at the bottom of (A). (E) The time-courseof the changes in [Ca]i of cells Nos I and 2. 485

. ATP Receptors in Brain Functions

487

changes in synaptic efficiency.These observations open an excitingfield for future studies investigating the physiological role of ATP-mediated synaptic transmission in the hippocampus.

are believed to be mediated by voltage-gated Ca2+ channels, whereas ATP-evoked responses in PC12 cells are mediated by PZx-purinoceptorsand the contribution of the Ca2+ channelsis negligible(Inoue et al., 1989;Nakazawa and Inoue, 1992).Thus, this discrepancymay be due to differencesin the channels corresponding to each response. An ATP-activated inward current in PC12 cells also was attenuated by 3. SCHIZOPHRENIA the application of haloperidol (Koizumi et al., It is stronglysuggestedthat schizophreniais caused 1995b).This inhibition was reversibleand the current by an extraordinary facilitation of dopaminergic amplitude returned to the control level after a 1 min neurons. The ,findingsthat almost all the drugs which rinse. Changes inthe [Ca]irise evoked by ATP were are clinically effective against schizophrenia have in agreementwith those of the ATP-activatedcurrent. dopamine receptor antagonist activity, and that the Thus, the haloperidol-inducedinhibition appears to clinically effectiveconcentrations of these drugs are be mediated by the suppression of ATP-gated Ca2+ comparable to the concentrations required to block entry. Our previous findings that the ATP-evoked D2-receptors also supports the “dopaminergic hy- increase in [Ca]i correlates with the ATP-activated pothesis” of schizophrenia.The hypothesis,however, current and is attributed to Ca2+-influxthrough the is not wholly accepted, and many other mechanisms ATP-gated channels and not to that through are thought to be involved in the etiology of voltage-gated Ca2+ channels may support this idea schizophrenia. Antipsychotic drugs also inhibit (Nakazawa and Inoue, 1992).The machinery of this voltage-dependent Ca2+ (Fletcher et al., 1994)and inhibitory action remains to be examined. K+ (Nakazawa et al., 1995)channels, besides their well-known antagonism of dopamine D*-receptors. The inhibition of Ca2+ channels was first reported as an action of atypical antipsychotic drugs such as fluspirilene (Gould et al., 1993; Grantham et al., + control 1994) and more recently as that of typical antipsychotic drugs such as haloperidol (Fletcher \ et al., 1994).These reports raise the possibility that ?, ....... HPD30KM $1 \, antipsychotic drugs may act on multiple types of \ $6 cellular molecules, contributing to their anti-schizo‘\ , -----.— phrenic actions. ---—— Inhibition by antipsychotic drugs of ATP-evoked A7P ~TP responses has been reported using PC12 cells c1 ;’; 4 3 (Koizumi et al., 1995b). Figure 3(A) shows the

--\ I

u. TIME ~MIN)

time-course of changes in [Ca]i evoked by 30 PM ATP, and the effects of haloperidol. Haloperidol (10-100 pM) significantly attenuated the ATPevoked [Ca]i rise [Fig. 3(B)]. Chlorpromazine

(30 PM), another typical antipsychotic drug [Fig. 3(B)], and fluspirilene (3-30 ~M, data not shown) exerted similar inhibition. Since these antipsychotic drugs are known as potent antagonists of dopamine D’-receptors, the inhibition of the evoked [Ca]i rise by haloperidol was investigated to determine if the inhibition was due to antagonism of dopamine D2-receptors. The ( – )-sulpiride (1W 100PM), a selectiveantagonist of dopamine D’-receptors, did not affect the [Ca]irise evoked by ATP (Fig. 4). In addition, neither dopamine (O.l– 100PM); ( + )-SKF-38393(1–100PM), a dopamine D,-receptor agonist; nor ( – )-quinpirole (30100pM), a dopamine D,-receptor agonist, had an affect on [Ca]i (Fig. 4). A dopamine D,-receptor antagonist, ( + )-SCH-23390,slightly enhanced the [Ca]i rise at 100PM. It is unlikely, therefore, that the inhibition of the ATP-evoked [Ca]i rise by haloperidol is dependent on dopamine receptor antagonism. Alternatively, Courtney et al. (1991)reported that secretion from PC12 cells evoked by high concentrations of KC1 was enhanced by an antagonist to D2-receptors or haloperidol (at 10 PM), resulting from block of autoinhibition through dopamine D,-receptors. The KC1-evokedresponses generally

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Fig. 3. Effectsofantipsychotic drugs on the increasein [Ca]i evoked by 30 PM ATP in PC12 cells. (A) The time-course of ATP-evoked rise in [Ca]i and effects of haloperidol (HPD) on it. The ATP was appliedto the cellstwicefor 15 s separated by 2 min (filledhorizontal bars) and the second ATP-applicationwas performed in the absence @lain line) and presence(broken line) of haloperidol. Haloperidol was applied to the cells 1 min before and during the second ATP-application(dotted horizontal bar). (B) The effectsof haloperidol (HPD) and chlorpromazine (CPZ) on the ATP-evoked[Ca]i rise. Data are mean+- S.E.M. of 16-56 cells tested. Values show S2/S1 ratios. Asterisks show significantdifferencefrom the ratio obtained by 30 PM ATP alone (*P < 0.05; **P < 0.01).

488

—— 1 K. Inoue et al,

agonist

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L

antagonist

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0.1110100 1 10 10Q ?0l~30Q1030100 (pM) 30100 (+)-SCH-23390~ ‘E (+)-SKF-33393 — (-)-SULPIRIDE (-)-QUINPIROLE

Fig. 4. The effectsof agonistsand antagonistsofdopamine receptors on the ATP-evokedincreasein [Ca]i. Valuesindicate S2/S1ratios. The ( + )-SKF-38393,( – )-quinpirole,( + )-SCH-23390and ( – )-sulpiride were applied by the same method for HPD as shown in Fig. 3. Data are mean + S.E.M. of 19–55cells tested.

Antipsychotic drugs, including haloperidol and chlorpromazine, are known to inhibit the activity of calmodulin (Prozialeck and Weiss, 1982).However, the effectiveconcentration of this action is significantly different from that of the inhibition of ATP-evoked increase in [Ca]i. It is unlikely, therefore, that the blocking action of antipsychotic drugs on calmodulin contributes to the inhibition of ATP-receptor/channels. A similar proposal has already been presented by Fletcher et al. (1994)to account for the blocking action of antipsychotic drugs on the voltage-gated Ca2+ channels in hippocampal neurons. Taken together, various antipsychotic drugs, i.e. haloperidol, chlorpromazine and fluspirilene, inhibited the ATP-evoked responses mediated by P2x-purinoceptors. ATP induces various responses in many regions of brain (Edwards et al., 1992; Chen et al., 1994; Inoue et al., 1992a; Shen and North, 1993; Ueno et al., 1992).The existence of mRNAs for P2x-purinoceptors (Brake et al., 1994; Buell et al., 1996; Chen et al., 1995;Lewis et al., 1995;Seguela et al., 1996) and u,/3-methylene ATP binding sites (Bo and Burnstock, 1994) has been reported already. Furthermore, Zhang et al. (1995) demonstrated that exogenously applied ATP produced a release of dopamine via P2-purinoceptor-mediatedmechanisms in rat brain. Judging from these reports, it is highly probable that ATP may have a facilitator role for dopaminergic neurons, and that various antipsychotic drugs may express their therapeutic effectsby suppression of dopaminergic hyperactivity through the inhibition of P2x-purinoceptors.Thus, there is currently great interest in whether the inhibition by haloperidol and other antipsychotic drugs of the ATP-evoked responses in PC12 cells also occurs in the human brain.

4. PAIN AND COGNITION It has been shown that ATP-gated cation channels exist in particular neuron groups in the peripheral and central nervous systems (Kennedy and Leff, 1995).In addition, it has been confirmed that ATP acts as a neurotransmitter or co-transmitter in the peripheral and central nervous systems. The ATP is released from nerve ending with noradrenaline or ACh in the autonomic nervous system, spinal cord and also the brain (Burnstock, 1990; Evans et al., 1992; Edwards et al., 1992). Electrophysiological data and detection of ATP release in the spinal cord suggested the possibility that ATP-gated cation channels play a role in pain transmission (Krishtal et al., 1983; Jahr and Jessell, 1983; Salt and Hill, 1983). Recently, P2Xreceptors (Lewis et al., 1995; Chen et al., 1995) have been cloned and the distribution of these receptors strongly supports the idea that ATP acts as a neurotransmitter for nociception. The first-order neurons carrying pain, typically sensory neurons in the dorsal root ganglia (DRG), have free endings in many tissues. Several chemicalmediators, such as protons, 5-HT and ATP, act on receptors on the peripheral terminals of these first-order neurons, initiating a pain signal. This theory agrees with the observation that exogenous ATP and other ATP analogues induced a slow onset pain in the human blister base preparation (Bleehen and Keele, 1977).Pain impulses are transmitted to the spinal cord via fast-conductingAs type fibres and slow-conductingC fibres. The ATP and purines are involvedmainly in the latter conduction. The axons entering the spinal cord from DRG neurons synapse with cells of the substantialgelatinosa in the dorsal horn of the spinal cord. There is accumulating evidence that ATP and its analogues can modulate

ATP Receptors in Brain Functions

pain processing, especially at the spinal level. The ATP application has an excitatory effect on sensory neurons and on a subpopulation of dorsal horn neurons which are known to be nociceptive(Krishtal et al., 1983;Jahr and Jessell, 1983).In biochemical experiments,it has been shown that ATP is released from spinal cord synaptosomes by depolarization with K+ and capsaicin (White et al., 1985;Sweeney et al., 1989).Experiments to assess ATP effectsand modulation by opioids or other pain-related agents at the spinal level have shown various results, some of them controversial or biphasic (Fyffeand Perl, 1984; Salter and Henry, 1985; Doi et al., 1987). These resultsmay be explainedby the fact that releasedATP is converted rapidly to adenosine, which can evoke feedback inhibition of the neural activity, especially in vivo. The ATP-gated cation channels have been cloned and designated as Pzxl–PjxG(Valera et al., 1994; Brake et al., 1994; Lewis et aZ., 1995; Buell et al., 1996).The Pzx~subclass is expressedin spinal neurons and DRG neurons which are known to be nociceptive neurons (Lewis et al., 1995;Chen et al., 1995).The mRNA for P,x, and P,x, is distributed in various organs such as urinary bladder smooth muscle, neuronal tissue, lung and thymus (Valera et al., 1994;Brake et al., 1994).In neuronal tissue,the mRNA for Pjx purinoceptors generally is found in sensory neurons. However, that of Pzxjis localized selectivelyin the dorsal root and trigeminal ganglia. In addition, the Pzxqreceptor is expressedin a subset of rat dorsal root ganglion neurons, which are responsible for nociception (Chen et al., 1995); therefore, P2x~ receptors might be involved in nociceptive transmission. There is also supporting evidencethat P*-purinoceptorantagonists provide an antinociceptive effect (Driessen et al., 1994). The intrathecal injection of PzX-purinoceptorantagonists caused antinociception, while that of PJXagonists enhanced the response to nociceptive stimuli in the tail-flickassay.The Pzxdand Pzxesubtypes,whichwere the latest to be cloned, are present predominantly in the CNS and these subunits showed insensitivity to P,x antagonists (Buell et al., 1996;Cello et al., 1996). There is still no evidenceof whether Pzxare involved in nociception at higher levelsin the brain.

489

5. CENTRAL REGULATION OF THE AUTONOMIC SYSTEM Now it is well known that ATP is released as a co-transmitter with noradrenaline or ACh from sympathetic and parasympathetic neurons respectively in the peripheral nervous system (Burnstock, 1990).Electrophysiologicalstudieshave revealedthat ganglion neurons possess both ATP-evoked and ACh-evokedexcitatoryresponses(Bean, 1990;Fieber and Adams, 1991).From this analogy, Pqxreceptors might be expected to be located on CNS neurons which contain catecholamine or have nicotinic ACh receptors. In fact, there are several reports which support this expectation (Edwards et al., 1992;Shen and North, 1993; Nabekura et al., 1995). These neurons are involved in autonomic regulation or relay of its information. Binding studies using [3H]ct,~-methylene ATP also support the distribution of Pm receptors in the brain (Bo and Burnstock, 1994).Binding and in situ hybridization studies have revealedthat many structures in the brain expressPzx receptors (Buell et al., 1996),suggestingthat the P2X receptor may be involved in unknown functions in the brain. The interaction of ACh and ATP has been studied. The ATP facilitates the sensitivity of nicotinic ACh receptors at cholinergic synapses (Akasu and Koketu, 1985), inhibits the release of ACh from ganglionic synapses and regulates ACh synthesis(Silinskyand Ginsborg, 1983;Silinsky and Gerzanich, 1993).In addition, there is an observation that the currents evoked by ATP and ACh are not independent, suggesting that P2X channels and nicotinic ACh channels may interact with each other (Nakazawa et al., 1991).

6. CONCLUSION The investigation of the molecular biology of purinoceptors has grown rapidly in recent years, resulting in significant new information. Tables 1 and 2 are summaries of recent advances in the molecular biology of purinoceptors, taken from the Purinoceptor update newsheet (Burnstock and King, 1996), with minor modifications. Figure 5 and

Table 1. Characterization of Pzx-PurinoceptorSubclass* Pz-purinoceptor subtype P*X, P2X2 P>x, P*X4

*1’2x4

(tentative) P,x, P2X6

Tissue

Activity

Vas deferens rat) PC12 cells (rat) DRG cells (rat) DRG cells (rat) Hippocampus (rat) SCG Wk (rat) Neurons (rat) SCG cells (rat) SCG cells (rat)

2-MeSATP> ATP > a,&meATP 2-MeSATP> ATP c@-meATPinactive 2-MeSATP> ATP > a,&meATP ATP > 2-MeSATP> a,~-meATP ATP > 2-MeSATP> a,~-meATP ATP active a,/3-meATPinactive ATP > > 2-MeSATP> CTP > a,&meATP > dATP ATP active cqfl-meATPinactive ATP active a,/3-meATPinactive

Channel properties

References

GENBANK/ EMBL accession No.

1~.,~,c. Valera et al. (1994) I~,,~,c. Brake et al. (1994)

X80477 U14414

INa,K 1~.,~,ca 1~.,~ INa,K,ca

X91167

Chen et al. (1995) Lewis et al. (1995) Bo er al. (1995) Buell et al. (1995)

INa,K,ca Soto et al. (1996) I~,,~,c.

Cello er al. (1996)

1~.,~,c, Cello et al. (1996)

*Based on Purinoceptor update newsheeiby G. Burnstock and B. King.

— X93565 —

K. Inoue et al.

490

Table 2. Characterization of P,Y-PurinoceptorSubclass

Pz-purinoceptor subtype Tissue P*Y,

Activity

Coupling

2-MeSATP> ATP > ADP UTP inactive 2-MeSATP> ATP > ATP UTP inactive (ND’)

Brain

(chick) Brain (turkey) Insulinoma cells (mouse) Insulinoma cells

PLCo/IP3/Ca2i Webb et al. (1993)

X73268

PLCt/IP3/Ca2+ Filtz et al. (1994)

U09842

(ND)

2-MeSATP> 2-CIATP > ATPa,~-meATP inactive (ND) 2-MeSATP> ATP > > UTP

References

GENBANK/ EMBL accession No.

Tokuyama et al. (1995) U22829

PLCJIP3/Ca2+ Tokuyama et al. (1995) U22830

(rat) Placenta (ND) Leon et al. (1995) (human) Endothelium PLCP/1P3/Ca2+ Henderson et al. (1995) (bovine) P2Y2 NG108-15 Cdk ATP = UTP > > 2-MeSATP PLCfl/IP3/Ca2+ Lustig et al. (1993) (mouse) ATP = UTP > > 2-MeSATP PLCJIP3/Ca’+ Parr et al. (1994) CT/43 cells (human) Lung ATP = UTP PLCP/IP3/Ca2+ Rice et al. (1995) (rat) Bone (ND) (ND) Bowler et al. (1995) (human) Pituitary (ND) (ND) Chen et al. (rat) ADP > UTP > ATP = UDP P*Y3 Brain PLCfl/IP3/Ca2+ Webb et al. (1995a) (chick) Webb et al. (1995b) Plancenta UTP = UDP > ATP = UDP P,y, PLCJIP3/Ca2+ Communi et al. (1995) (human) Brain (ND) (ND) Webb et al. (1995c) (rat) ‘ADP> ATP > UTP (ND) P2Y5 HEL cells Kunapuli et al. (1995) (humap) UTP > ADP = 2-MeSATP> ATP PLC#P3/Ca2+ Chang et al. (1996) P2Y6 Aortic SM (rat) . .. *Based on Purinocepior uP&ti newsheet by G. Burnstockand B. King.

2.49205 — L14751 U07225 U09402

L46865

—

U4107O D63665

‘ND,not determin&d. Table 3 are examples of arrangements to distinguish various subclasses (Pzx,P2Y)amongst the characterized Pj-purinoceptors.In brain, the P2Y,,PzYl,and PzYd receptor subclasses have been reported to be purinoceptors of the GTP-binding type. Receptors P2X,and P,x, have been reported to be monotropic CHARACTERIZATION OF Pm SUBCLASSES FromCdloet d., 19% STRONG

........A,.

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~

WEAK lNSENSi7VE

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j .. ;“ .......

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JP f &rrin ~ ~s~n~~i~~ti~n o$-meATP PPADS —by ECW

purinergic receptors and appear to co-localize extensivelyin many brain areas. Furthermorej they are very similar in their sensitivity to antagonists (Cello et al., 1996). The purinoceptor subclass nomenclature is temporary and needs to be reconsidered since we can expect that many subclasses of purinoceptors will be detected in mRNAs from brain. For example, a novel Pzx subclasshas been detected in the brain and named as et al., 1996).Ueno et al. (1992) reported P,x~(S&gu61a in dissociated rat nucleus solitarii neurons that ATP evoked currents through a novel Pzx which was

by ICm

Fig. 5. Characterization of P,x subclasses based on the sensitivity of agonist and antagonist, and strength of desensitization. Data from Cello et al. (1996).

Table 3. Potency Orders on P,, Subclass Agonist subtype Ply, P2Y2 P2Y, P*Y4 P*Y5 P2Y6

2-Mestp +++ N+D ND ND ++

ATP

ADP

UTP

UDP

++ +++

+– ND +++ ++ +++ ++

+++ ++ +++ + +++

ND* ND + +++ ND ND

+++ ++ +

Symbols: + ++, strong; ++, moderate; +, weak; –, inactive. *ND, not determined.

ATP Receptors in Brain Functions

sensitiveto suramin, whereas Pzxqand P2XC receptors are know to be insensitiveto suramin (Cello et al., 1996). We have attempted to reveal the possible implicationsof Pz-purinoceptorsin brain functionsin this review.Much, however,remains to be examined, since we have relatively little information on this matter. The evidencethat one type of Pzxreceptor is regulated by established neurotransmitters, i.e. dopamine (Inoue et al., 1992b; Nakazawa et al., 1993), serotonin (Nakazawa et aZ., 1994; Koizumi et al., 1995c) and adenosine (Inoue et al., 1994; Koizumi et al., 1994) suggests that synaptic transmission through Pz-purinoceptors may have serious effectson the functions of the central nervous system.These reports, as wellas molecular biological data, encourage us to examine the important implications of these receptors for brain function. Acknowledgements—We are grateful to Dr

James

G.

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