The role of glutamate receptors in pentylenetetrazole kindling of rats — A neurochemical study

109

110 PHARMACOLOGICAL CHARACTERISATION OF mGluRs LINKED TO THE INHIBITION OF ADENYLATE CYCLASE ACTIVITY IN RAT STRIATAL SLICES.

COUPLING OF mGluRs TO INWARDLY RECTIFYING POTASSIUM CH.4NiiELS: AN ARTIFACT OR A CLUE TO RECEPTOR CORlPARTMENTALIZATIO;V? JA Saugstad. JM Kink. Vellum

TP Segerson and GL Wrsthroot

Institute.Oregon

Health Sciences University

Portland, OR 97201 USA Metahouopic family

glutamate receptors (mGluRs)

of receptors that utilize

Thus

hou

receptor-spccilic

multiple

actions

H. Schaffhauser, J. Cartmell. and V. Mutel. Pharma Division Preclinical Research, F. Hoffmann-La Ltd. CH-4002 Basel, Switzerland.

constitute a Imgc

transduction pathways.

in

neurons xe

detennirwd

remains a dilemma. Part of the speciticity must an~c from cellular companmentalization. Group I mGluRs presynaptic. mGluR7

For example,

In sirzr hybridisation has sho\vn that mGluR3, mGluR4 and mGluR7 mRNA are present in rat srriatum. Previous immunohistochemical studies have shown the presence of mGluR2 protein in caudate putamen despite the absence of apparent mRNA expression. We have therefore characterised the pharmacological profile of mGluRs negatively linked to adenylyl cyclase in crosschopped slices of adult rat striatum. Among the mGluR agonists tested, DCGIV (I$0 value 0.4 = O.IpM) and L-CCGI (I i 0.2pM) were the most potent inhibitors of fonkolin-stimulated CAMP formation, followed by (I S,3R)-ACPD (7 f 2pM), (1S,3S)ACPD (7 f IpM), (S)-3C4HPG (23 * 6pM), (S)-4C3HPG (39 f 7pM). glutamate (475 f 16pM) and quisqualate (515 f 1IpM). (RS)-DHPG, (S)-DHPG and (S)-3HPG were without effect on the forskolin CAMP response. This rank order of potencies, notably those of L-CCGI and quisqualate, is compatible with a preferential activation of mGluR3. Inhibition of forskolin-stimulated CAMP formation by the group 111agonist L-AP4 was biphasic with I&,0 values of 7 i 2pM and 215 + 4pM. These potencies suggest a sequential recruitment of mGluR4 and mGluR7. Furthermore, we tested the effects of several putative antagonists on the inhibition of forskolin-stimulated CAMP formation by L-CCGI and L-AP4 and the results of these studies will also be presented.

early studic, suggested that

were postsynaptic whereas Group II and 111 were

However,

our

immunohistochemical

studies

of

in the olfactory system have revealed that this Group 111

receptor is postsynaptic as well as presynnptic. Thus the localaxion of mGluRs

nppeors to he considerably more complex than simple

sorting to dendritic vs. uxonal compartments. An

additional

mechanism

for speciticity

restricted interactions of mGluRs many

G

protein

interaction

of by

However,

couple to GIRK.

GIRK.

is o common

there is virtually

a dinxt form

of

no evidence that

WC examined this apparent discrepancy

hy coexpressing two GIRK

subunits with representative mGluRs

2 and 7) in Xenopus ooctyes All the mGluRb activated

(mGluRla, GIRK

from

rectifying

This mechanism. imolving

subunits with

neuronal inhihition. mGluRs

could result

with cffcctors. As an example.

coupled receptors activate inuardly

potassium channels (GIRK).

currents. The coupling of mGluRla

and mGluR2

to GIRK

was tts efficient ns dopeminc D2 rcccplorr. and involved a common pool of

PTX-sensitive

G

proteins.

Thcsc rcsultc suggest that

members of each of the three groups ofmGluRs Whether

mGIuR-GIRK

interactions

have

can activate GIRK. heen

overlooked

in

neurons requires consideration. However it is possible that mGluR~ microdomains. perhaps due lo

arc excluded from GIRK-containing the structurnl differences of mGluRs coupled receptors. S,,p,wre
compared

/>.v N/H

gmmrs

ROIDCOI763(TPS)

lo other G prolcin F32,VSO920O(JAS~

& ROINS26194

(GLIVJ.

112

111

A SIX’ROCHI~MICAL

DIFFERENTIAL TARGETING OF PRESYNAPTIC METABOTROPIC GLUTAMATE RECEPTORS IN RAT CNS

STUD\

Ryuichi Shigcmoto*. Helmut

Schriider

and Awl

Becket

The specific binding

ot’ glulanwte

inositolphoqhatc

lo its receptor

formation

were

subtypes

investigated

on

PTZ-kindled rats. PTZ-kindling induced by I3 intraperitoneal application\ of 3.5 mglkg over a period of 4 week* re\uItcd in a significant long-larting increase of both the reitctirx~ of unin~als to the convulsant end the density of the specific glutamate binding in the hippocampus. The goal of the prcw~t experinwtts hippocampal

XI\

tissue prepxdions

to study

the

sensitivity

of

of different

subtypes

of the

PTZ kindling. The quisqualate- and kainate-semirive glutamate binding sites were significantly incren\cd 2-l hours after last PTZ injection, whereas the NMDA-scn\itive part wax only enhanced in tendency. Furthermore. we investigated the glu~;m~atebindmg site> on metabutropic receptor and lbund a significant increare in the hippocampus following PTZ glutatna~c

receptor

kindling.

In it part of r;~ts we dc~crmincd

stimttlutcd tiatte

inohitolphosphatc

of kindled

is dratically bindmg Front

in the hippocampu~

fornwion.

rat\ the xctwuli~tion

increased

in response

supporting

to

the trans-ACPD In hippocampnl

of inositolphophatc~ the enhanced

mGluR

sites. these

glutwxt~e rats may

findmgs.

it is concluded

that the incraw

in

receptor density in hippocampu~ of PTZ-kindled bc the expression

susceptibility

of glutatnatergic

course of formation

of tt spccilic syztemz

of PTZ-induced

enhnncetnent developing

Hitoshi Ohishi’,

Ayac Kinoshita’.

Eiki

Wada’, Noboru Mi%uno*, J. David B. RobcrtsS. Pctcr SomogyiS. * Dept. Morph. Brain Science, Kyoto Univ., Kyoto 606. Japan.: S MRC Anatomical Neuropharmacology Unit, Oxford, UK. Group II (mGIuR2/3) and group III (mGluR4, 7. and 8) metabouopic glutamate receptors are present in presynaptic elements in various regions of rat CNS (Ohishi et al. Neuron ‘94; NeurosciLett. ‘95; Kinoshita et al. Ncurosci.Lett. ‘96; ‘96) and arc thought to bc autorcccptors. Electron microscopic immunogold localization revealed a differential receptor localization bctwcen the group II and III mGluRs; mGluR2/3 immunoreactivity was found predominantly in prcterminal axons and occasionally in axon tcrtninals but not in prcsynaptic membrane specialization, whereas immunoreactivity for mGluR4, mGluR7, and mGluR8 was abundant in the presynaptic membrane specialization. This result suggests that group II and group III mGluRs may have different cffcctor mechanisms for the regulation of glutamate relctsc. Another type of diffcrcntial targeting of an mGluR was found in single pyramidal cell axons in the hippocampus: mGluR7 density in membrane specialization presynaptic to dcndritcs of mGluR 1apositive interneurons was at least IO times higher than that to spines of pyramidal cells or dendrites of other types of interneurons (Shigemoto et al. Nature ‘96). A similar concentration of other group III mGluRs was also found in synapses on specific neuronal cell types. suggesting that this phenomenon may be a general fcaturc of group III mGluRs. The postsynaptic target-dependent concentration of the presynaptic autoreceptor demonstrates a new principle in neuronal communication, the ability of neurons to regulate the probability of transmitter release differentially at individual synapses under postsynaptic influence.

In\[itute of Phnmmucology and Toxicology, Medical Faculty, University of hlngdehurg. D-39120 Magdeburg. Germany.

and

Roche

of

in the

kindling.

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