The metabotropic glutamate receptors: Structure and functions

Vol. 34, No. I, pp. l-26, 1995 Elswier Science Ltd. Printed in Great Britain

Neuropharmacology

002&3908(94)00129-4

Revie-w”f: Neurotransmitter The Metabotropic

receptors I

Glutamate Receptors: Structure and Functions J.-P. PIN’* and R. DUVOISIN2

lMecanismes Moltfculaires des Communications Cellulaires, UPR-CNRS 9023, Centre CNRS-INSERM de Pharmacologic-Endocrinologie, Rue de la Cardonille 34094 Montpellier Cedex 5, France andZCornell University Medical College, Dyson Vision Research Institute, 1300 York Avenue, New York, NY 10021, U.S.A. (Accepted 6 September 1994)

Summary-Glutamate is the main excitatory neurotransmitter in the brain. For many years it has been considered to ac:t only on ligand-gated receptor channels-termed NMDA, AMPA and kainate receptors-involved in the fast excitatory synaptic transmission. Recently, glutamate has been shown to regulate ion channels and enzymes producing second messengers via specific receptors coupled to G-proteins. The existence of these receptors, called metabotropic glutamate receptors, is changing our views on the functioning of fast excitatory s,ynapses. Keywords-mGluR, glutamate, ACPD, G-protein, K+-channels, Ca*+-channels, LTP, LTD.

Most of the excitatory

synapses

in the central

cloning,

alternative

splicing, PLC, adenylyl

cyclase,

(Nicoletti et al., 1986a, b), cultured cerebellar granule cells (Nicoletti et al., 1986~) and cultured astrocytes (Pearce et al., 1986). These results strongly suggested that Glu, like GABA, serotonin and acetylcholine, not only activated ligand-gated channel receptors but also receptors coupled to GTP-binding proteins (G-proteins). The existence of such novel Glu receptors, now called metabotropic Glu receptors (mGluRs), was then confirmed using the Xenopus oocytes model (Sugiyama et al., 1987) and new pharmacological tools (Recasens et al., 1988; Sugiyama et al., 1987). Independently, a Glu analogue, L-2-amino-Cphosphonobutyrate (L-AP4) was recognized as a specific ligand

nervous

(Glu) as a chemical messenger. To mediate fast excitatory transmission, Glu activates ligand-gated cationic channels named N-methyl-D-aspartate (NMDA), cr-amino-.3-hydroxy-5-methyl-isoxazole4-propionate (AMPA) and kainate (KA) receptors (Hollmann and Heinemann, 1994; Monaghan et al., 1989). However, in 1985 it became apparent that Glu had more complex roles since it was reported to stimulate phospholipase C (PLC) in cultured striatal neurons via a receptor that did not belong to the NMDA, AMPA or KA receptor families (Sladeczek et al., 1985). Soon after, a similar effect of Glu was described in hippocampal slices system use glutamate

*To whom correspondence ljhould be addressed. Abbreviations: AMPA: u-amino-3-hydroxy-5-methyl-isoxazole-4-propionate; APV: D-2-amino-5phosphonovalerate; BHK: baby hamster kidney; BMAA: /I-N-methylamino-L-alanine; CHO: Chinese hamster ovary; 3C4HPG: 3-carboxy-4hydroxyphenylglycine; 4C3HPG: 4-carboxy-3-hydroxyphenylglycine; CNQX: 6-cyano-7-nitroquinoxaline-2,3-dione; 4CPG: 4-carboxyphenylglycine; DCG-IV: 2-(2,3-dicarboxycyclopropyl)glycine; DNQX: 6,7-dinitroquinoxaline-2,3-dione; GAMS: 6-D-glutamylaminomethylsulfonate; Glu: glutamate; HCA: homocysteate; HCSA: homocysteine sulfinate; HEK: human embryonic kidney; 3HPG: 3-hydroxyphenylglycine; Ibo: ibotenate; KA: kainate; L-AP4: L-2-amino-4-phosphonobutyrate; L-CCG-I: (2S, 1/&2’S)-2-(carboxycyclopropyl)glycine; L-CCG-II: (2S,l’R,2’R)-2-(carboxycyclopropyl)glycine; L-SOP: L-serineO-phosphate; MCPG: cr-methyl-4-carboxyphenylglycine; mGluR: metabotropic glutamate receptor; MK801: 5-methyl-lo,1 ldihydro-5H-dibenzocyclohepten-5,10-imine maleate; NBQX: 2,3-dihydroxy-6-nitro-7-sulphamoylbenzo(F)-quinoxaline; NMDA: N-methyl-D-aspartate; PBP: penplasmic bacterial protein; PCaRl: parathyroid calcium receptor 1; PLC: phospholipase C; PLA2: phospholipase A2; PTX: pertussis toxin; Quis: quisqualate; ACPD: 1-amino-cyclopentane-1,3dicarboxylate; VSCC: voltage-sensitive Ca*+-channel. tThis is the first in a regular series of review articles on neurotransmitter receptors with a special emphasis on their pharmacology and molecular biology. 1

J.-P. Pin and R. Duvoisin

2

for a new Glu receptor subtype (Foster and Fagg, 1984). It became apparent that L-AP4 acted as an agonist on presynaptic Glu receptors, depressing Glu release at many excitatory synapses (Koerner and Johnson, 1992). A Glu receptor with a similar pharmacology was characterized on ON-bipolar cells in the retina (Nawy and Copenhagen, 1987; Slaughter and Miller, 1985). It was shown to be coupled to G-proteins (Nawy and Jahr, 1990a), and could be another member of the mGluR family. Thanks to molecular biology techniques, and the discovery of a number of specific drugs, our knowledge on this new receptor family is expanding rapidly. Eight genes coding for mGluRs have been cloned, of which several generate different mRNA by alternative splicing (Nakanishi, 1992; Pin et al., 1993; Schoepp and Corm, 1993). Expression studies confirmed the existence of PLC-coupled and L-AP4-sensitive mGluRs, but also revealed mGluRs negatively coupled to adenylyl cyclase. The ongoing discovery of specific agonists and antagonists will help identify these different receptors and reveal their critical roles in regulating fast excitatory synaptic transmission. Metabotropic GluRs have been shown to be involved in many brain functions. For example, they participate in the induction of synaptic plasticity phenomena, such as long-term potentiation and long-term depression that are thought to be at the origin of learning and memory. They are also likely to play a role in modulating Glu-induced neurotoxicity. In this review, we will first describe the mGluR family as defined by the cloned receptors, in terms of diversity, transduction, structurefunction relationship, and pharmacology. We will see that mGluRs form a totally new family of receptor proteins which now include other receptors such as the parathyroid Cazf-sensing receptor 1 (PCaRl), and that a model of their structural domains revealed an interesting hypothesis for their evolution. Then, the effects of mGluRs in the CNS will be described in an attempt to define the exact transduction mechanisms, functional and physiological roles of each cloned mGluRs. It will become apparent that mGluRs are excellent targets for the development of drugs that modulate excitatory synaptic transmission. Such drugs could be useful in treating numerous diseases involving a deregulation of Glu transmission. CLONED mGluRs: CLASSIFICATION droning of Zulus: ~~1tiF~i~it~ due to operas genes and alternative splicing As for many new gene families, the first mGluR was cloned by a functional expression screening procedure (Houamed et al., 1991; Masu et al., 1991). This strategy made use of an endogenous second messenger pathway in Xenopus oocytes that links G-protein activation with Clchannel currents that are detected electrophysiologically. Oocytes were injected with RNA synthesized in vitro prepared from pools of rat cerebellar cDNA clones and

tested for oscillatory Cl- currents in response to Glu stimulation. By screening successive rounds of smaller pools, two laboratories were successful in isolating cDNA clones encoding the metabotropic receptor now generally named mGluRla (Houamed et al., 1991; Masu et al., 1991). To identify related mGluRs, several laboratories have used the mGluRla sequence, either as a probe to screen cDNA libraries by low stringency hybridization, or for the design of degenerate primers for PCR. This has resulted to date in the isolation of seven other related genes and several splice variants (Abe et al., 1992; Minakami et al., 1993; Nakajima et al., 1993; Okamoto et al., 1994; Pin et al., 1992; Saugstad et al., 1994; Tanabe et al., 1992) (Duvoisin, et al., 1995). Because molecular cloning has preceded pharmacological characterization in the identification of novel mGluRs, it has provided the basis for a simple nomenclature of mGluRs: mGluRs are numbered following the order in which their cDNAs have been cloned, to date: mGluR1 through mGluR8. Moreover, because most of this work has been done in a single laboratory, the numbering scheme is consistent and the same number has not been used for different receptors, sadly a source of confusion for other receptor families. Alternative splicing variants have been referred to by

855 ' 910 965 1020 1075 1130 ,185

mGluR1 a Fig. 1. Schematic representation of mGluRla. The grey residues corresponding to the first 20 amino acid residues correspond to the signal peptide. The putative Glu binding domain is indicated as the LIVBP homologous domain. In this region, the hydrophobic segment proposed to form the binding pocket is boxed, and the amino acids involved in Glu binding are indicated with asterisks. The cysteine residues conserved among all mGluRs are indicated by the big black filled circles. The putative intracellular phosphorylation sites are indicated by the small black balls attached to the amino acid chain. The putative glycosylation sites are indicated by (Y). The position of the introns characterized in the mGluR1 gene where alternative splicing can occur are indicated by the vertical black bars. The domains involved in the specificity of G-protein activation are represented with thicker black circles.

Metabotropic 30

40

50

60

70

80

mm potent

GIOllp

PLC

Quisqualate

I

AC

L-CCG-I

II

L-AW

Ill

Transduction

90%

I I I I I I I mGluR5

1+

3:;;:

]

mGluR1

glutamate receptors

agonist

mGluR4 mGluR7 -AC mGluR8 mGluR6

r’-

I

PC‘aRl

+PLC

Cd’+

Fig. 2. Dendrogram and ph.armacological classification of the members of the “mGluR.” family, including the bovine parathyroid Ca2+-sensing receptor (PCaR). The numbers on the top indicate the % amino acid sequence identity between members of this receptor family (at the junction of the horizontal lines). either Greek or lower case Roman characters, although the latter convention is more frequently used, and will be followed here. The deduced amino acid sequence of mGluRs reveals that they are related. These receptors are much larger than all previously identified G-protein-coupled receptors and do not share any sequence homology with members of that gene superfamily (Fig. 1). Metabotropic GluRs define therefore a new family of G-protein coupled receptors. Recently a Ca2+-sensing receptor, isolated from a bovine parathyroid cDNA library, has been found to have about 30% sequence identity with mGluRs (Brown et al., 1993). Th.is receptor is also sensitive to Mg*+ and it is possible that there exist additional ion-sensitive receptors related to mGluRs. Based on their amino acid sequence identity, the 8 mGluRs can be classilied into 3 Groups (Fig. 2) (Nakanishi, 1992). Indeed mGluRs of the same group show about 70% sequence identity whereas between groups this percentage falls to about 45% (Fig. 2). Group-I comprises mGluR1 and mGluR5, Group-II, mGluR2 and mGluR3, and Group-III, all the others.

mCluR5 a b

r

-vlr

I-!

: ‘..,

-.

I---’

mGluR4 a b

c

--y-y

Fig. 3. Schematic representation of the sequence of the mGluR splice variants characterized to date. The coding sequences are represented as white boxes, the 7 TMD correspond to the black squares. The untranslated regions are indicated by horizontal lines. Identical sequences found in the different variants derived from the same gene are joined by dashed lines. Only the introns which may be involved in the generation of these splice variants are presented (V).

3

As mentioned above, splice variants have been found for three mGluRs: mGluR1, mGluR4 and mGluR5 (Fig. 3) (Minakami et al., 1993; Pin et al., 1992; Simoncini et al., 1993; Tanabe et al., 1992). Except for the novel splice variant mGluRle, the splice variants result from the use of alternative acceptor splice sites in an intron whose position is conserved at least between these three genes. In mGluRlb, the insertion of an additional 85 base pair exon, which contains an in frame stop codon, results in the deletion of 318 amino acids from the proposed cytoplasmic carboxy-terminal domain and 20 different residues at the carboxy-terminal tail (Tanabe et al., 1992). In mGluRlc, a distinct insertion results in a similar deletion and 11 other amino acids at the carboxy-terminus (Pin et al., 1992). The mGluR5b variant results from an in frame insertion of 96 bases, and thus 32 amino acids are inserted into the deduced protein (Minakami et al., 1993). For mGluR4, the first cloned mGluR4a possesses an additional exon of 621 bases containing an in frame stop codon; deletion of this exon in mGluR4b results in a longer protein in which the 63 last C-terminal residues are replaced by 136 residues (Simoncini et al., 1993). Recently an additional splice variant of mGluR1, called mGluRle, has been found (Pin, unpublished data). This variant is the result of an additional exon being inserted before the seven transmembrane domain. This exon contains an in frame stop codon and thus the translation product would be truncated to 578 amino acids. The final predicted protein would contain only the extracellular domain and thus could be secreted. Alternatively, it could be attached to the membrane by a post-translational modification, reminiscent of splice variants of adhesion molecules. If this protein is produced and stable, its function and mode of action remain speculative. Interestingly, similar splice variants corresponding to the extracellular portion of receptors have been described for the LH receptor, a G-protein-coupled receptor that also has a large N-terminal extracellular domain (Loosfelt et al., 1989). The short and soluble proteins have been identified in vivo, and have been shown to modulate the efficacy of action of LH (vu Hai-Luu Thi et al., 1992). Since the extracellular domain of mGluRs is supposed to be related to bacterial periplasmic amino acid binding proteins (see next chapter) an important physiological role for mGluRle cannot be excluded. Transduction

mechanisms

of cloned mGZuRs

As already discussed, mGluRs can be subdivided into three groups according to the level of conservation of their amino acid sequences. This classification is also supported by their respective transduction mechanism (Fig. 2) as revealed after expression in Xenopus oocytes, Chinese hamster ovary (CHO) cells, baby hamster kidney (BHK) cells or human embryonic kidney 293 (HEK) cells. In every expression system examined, Group-I receptors, including their splice variants, stimulate phospholipase C as revealed by an increase in phosphoinositide turnover, and Ca*+ release from

4

J.-P. Pin and R. Duvoisin

internal stores. Some differences in the kinetic of Ca*+ release induced by mGluRla, b and c have been noticed in Xenopus oocytes and in transfected mammalian cells (Pickering et al., 1993; Pin et al., 1992; Simoncini et al., 1993). Indeed, the Ca2+ response induced by mGluRlb and c is slower but longer in duration than that induced by mGluRla. The reason for this difference is not known but may reflect a difference in the affinity of the receptors for the G-protein. The G-proteins involved in the activation of PLC by Group-I mGluRs have not been clearly identified. However, stimulation of PLC by mGluRla is found to be partly sensitive to pertussis toxin (PTX) in Xenopus oocytes, in CHO, BHK and HEK cells, indicating that G-proteins of the Gi-Go family are involved (Aramori and Nakanishi, 1992; Gabellini et al., 1993; Houamed et al., 1991; Masu et al., 1991; Pickering et al., 1993). Since a large component of the response mediated by mGluRla is not inhibited by high concentration of this toxin, it is likely mediated by a G-protein of the PLC-activating PTX-insensitive, Gq family. In the case of mGluR5, mGluRlb and c, no PTX-sensitivity has been observed in mammalian cells, suggesting that these receptors may be more specifically coupled to Gq-like G-proteins (Abe et al., 1992; Pickering et al., 1993). However, responses generated by these receptors expressed in Xenopus oocytes are partly PTX-sensitive suggesting that these receptors can also be coupled to Gi-Go proteins (Pin et al., 1992). Among the Group-I receptors, mGluRla has also been shown to activate adenylyl cyclase when expressed in CHO or BHK cells (Aramori and Nakanishi, 1992; Thomsen et al., 1993b). Although a direct coupling through Gs-protein has not been clearly demonstrated, it is likely to be the case for two reasons. First, mGluR5 does not activate adenylyl cyclase even though it stimulates PLC as well as mGluRla in CHO cells, suggesting that the activation of adenylyl cyclase is not a consequence of a stimulation of PLC (Abe et aE., 1992). Second, although PLC stimulation by mGluRla is inhibited by phorbol esters and PTX, stimulation of CAMP production is not affected by phorbol esters and is potentiated by PTX (Aramori and Nakanishi, 1992). However, since it is not possible to precisely determine the exact receptor density in the transfected cells due to the lack of high affinity radioactive ligand, non-specific coupling of mGluRla to Gs due to a too high expression level cannot be excluded. Group-II and -111receptors have been transfected into CHO and BHK cells. In these fibroblast-derived cell lines, these receptors are coupled to the inhibition of adenylyl cyclase. In the case of Group-II mGluRs, strong inhibition of forskolin stimulation of CAMP production was observed (Tanabe et al., 1992, 1993). In contrast, depending on the investigators, the receptor examined and the cell line, the maximal inhibition obtained with Group-III mGluRs was always less than 50% (Nakajima et al., 1993; Okamoto et al., 1994; Saugstad et al., 1994; Tanabe et al., 1993; Thomsen et al., 1992). Aside from technical problems, this could be due to an inappropriate

coupling of these receptors to this transduction pathway. However, in every case was this transduction totally inhibited by PTX, suggesting that, as in the case of other receptors inhibiting adenylyl cyclase, the G-protein involved in this coupling is of the Gi family. Pharmacology of cloned mGluRs Among excitatory amino acids (EAA) that have been shown to stimulate mGluRs, several are also active on ionotropic GluRs, such as quisqualate (Quis), Ibotenate (Ibo) and, of course, the most likely endogenous neurotransmitter, Glu. Quis is in fact more potent on Group-I mGluRs than on the ionotropic AMPA-type GluRs for which it used to serve as a specific agonist. NMDA, AMPA and kainate, which are potent agonists at NMDA and non-NMDA ionotropic receptors respectively have no effect on all cloned mGluRs. Similarly, ionotropic Glu receptor antagonists 5-methyl10,ll -dihydro-SH-dibenzocyclohepten-5, lo-imine maleate (MKSOl), D-2-amino-4-phosphonovalerate (APV), Joro spider toxin, y-D-glutamylamino-methylsulfonate (GAMS), 6-cyano-7-nitroquinoxaline-2,3dione (CNQX) and 2,3-dihydroxy-6-nitro-7-sulphamoylbenzo-(F)-quinoxaline (NBQX) are devoid of activity on mGluRla in their active concentration range on ionotropic GluRs. Some Glu analogues are specific for mGluRs and, since they are not equally active on all mGluRs, they can serve to pharmacologically distinguish the different receptors. These are l-aminocyclopentane1,3-dicarboxylate (ACPD), L-AP4, 2-(carboxycyclopropyl)glycine (CCG) with the (2S,l’S,2’S) and (2S, l’R,2’R) isomers (L-CCG-I and L-CCG-II respectively) being the most active (Hayashi et al., 1992), 2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) and L-serine-O-phosphate (L-SOP). Additional compounds that have been reported to specifically affect mGluRs are phenylglycine derivatives such as (+)+methyl-Ccarboxyphenylglycine (( +)MCPG), (S)-3-hydroxyphenylglycine ((S)3HPG), (S)-Ccarboxy-3-hydroxyphenylglycine ((S)4C3HPG), (S)-3-carboxy-4-hydroxyphenylglycine ((S)3C4HPG) and (S)-4-carboxy-phenylglycine ((594CPG). The R form of 4CPG and 3C4HPG are antagonists on AMPA and NMDA receptors. Ideally, the availability of cloned receptors should allow their expression in pure form in heterologous systems and thus the determination of an unambiguous pharmacological profile for each receptor (Table 1). However, at this time the paucity of specific agonists and antagonists does not permit such a definite characterization. Here we will first review the activities of several compounds on cloned mGluRs and, in the next chapters, attempt to identify the mGluRs involved in particular physiological responses by their pharmacological properties. The pharmacological profiles of Group-I receptors in all expression systems are comparable with the following potency rank order for agonists: Quis > Glu > Ibo > LCCG-I > ACPD (see footnote to Table 1 for references).

Metabotropic

glutamate receptors

5

Other are: L-CCG-II agonists at mGluRla (ECSo= 200 PM), homocysteate (HCA; partial agonist) homocysteine sulfinate (HCSA: EC& = 300 ,uM) and the neurotoxin /I-N-methylamino+alanine (BMAA; ECSo= 480 PM) (Aramori and Nakanishi, 1992; Hayashi et al., 1992; Thomsen et al., 1993b, 1994). At this time it appears difficult to pharmacologically distinguish mGluR1 and mGluR5, although mGluR5 might be more sensitive to ACPD than mGluR1. The effect of removing the long carboxy-terminal tail of mGluRla by alternative splicing appears to slightly affect the potency, but not the rank order of these agonists for mGluR1 b (Pickering et al., 1993). However, no such difference has been noticed for mGluRlc (Pin et al., 1992). Most phenylglycines are antagonists of mGluRla, with 4C3HPG and 4CPG being the most potent and specific for at least Group-I mGluRs (Hayashi et al., 1994; Thomsen et al., 1994). (S)-3HPG is however a potent but partial agonist at mGluRla (Hayashi et al., 1994; Thomsen et al., 1994). The rank order of potency of the Group-II receptors, mGluR2 and mGluR3 is: DCG-IV2 L-CCGI > Glu > ACPD > Ibo > Quis (see footnote to Table 1 for references). Thus, compared to Group-I mGluRs, the relative order of ACPD and Quis is reversed, and cycloglycine derivatives appear to be relatively specific agonists of this mGluR group. The sensitivity of mGluR2 and mGluR3 to Quis is quite different with an EC,, of 1 mM for mGluR2 and 40 PM for mGluR3, a difference which is big enough to be useful in determining the exact mGluR subtype involved in a physiological response (Table 1). The only antagonist known for mGluR2 is MCPG (Hayashi et aZ., 1994; Thomsen et aZ., 1994) but it is weak and blocks mGluRla in the same concentration range. Interestingly, the other phenylglycine derivatives 4CPG, 4C3HPG and 3C4HPG which are antagonists at mGluR1, are agonists at mGluR2 (Table I). However, Thomsen et al. (1994) described 4CPG as an antagonist on mGluR2. Recently, a-methyl-L-CCG-I has been proposed as a selective Group-II mGluR antagonist, based on electrophysiological recordings in the rat spinal cord (Jane et al., 1994). Group-III receptors are distinctive in their sensitivity to L-AP4, and insensitivity to ACPD (Table 1). They also respond to L-SOP stimulation and, with a lower potency, to L-Glu. mGluR4 can be identified by its high sensitivity for Ibo (Tanabe et al., 1993), whereas mGluR7 is characterizeud by its very low affinity for L-AP4 and L-Glu (Okamoto et al., 1994; Saugstad et al., 1994). There are no known antagonists which have been characterized on cloned Group-III mGluRs. However, a-methyl-L-AP4 has been proposed as a selective Group-III mGluR antagonist, based on electrophysiological recordings in the rat spinal cord (Jane et al., 1994). These data indicate that Group-I, Group-II and Group-III mGluRs which have different transduction mechanisms, also have different pharmacology. mGluR pharmacology is however clearly independent from their transduction mechanism as revealed by the construction

J.-P. Pin and R. Duvoisin

6

and expression of chimeric receptors (Pin et al., 1994; Takahashi et al., 1993). STRUCTURE AND FUNCTIONAL DOMAINS OF CLONED mGluRs

General structure A comparison of the aligned deduced amino acid sequence of mGluRs and PCaRl reveals characteristic features of these receptors (Figs 1 and 4). Each has a putative signal peptide, indicating that the amino-terminal domain of the receptor is likely to be extracellular (Fig. 1). Seven closely located hydrophobic segments are predicted to form membrane spanning segments; this region is often referred to as the seven transmembrane domain. The carboxy-terminal domain, presumably intracellular, is variable in length and not as conserved between members of this receptor family. The most conserved regions are an additional hydrophobic domain in the extracellular domain, postulated to form the ligand binding domain (see below) and segments surrounding this region, and the first (il) and third (i3) intracellular loops, possibly involved in G-protein coupling. Several putative transmembrane segments, especially the sixth, are very conserved between mGluRs but not as much with PCaRl. Twenty-one cysteine residues are conserved in all mGluRs (Figs 1 and 4). All but one, in TM6, are also present in PCaRl and except for one in TMS, the remaining 19 are located in the predicted extracellular domain and extracellular loops. Nine cysteines are closely located in the carboxy-terminal portion of the extracellular domain and this region has been called the cysteine-rich region and compared to similar regions in receptor tyrosine kinases (O’Hara et al., 1993). The glutamate binding site The agonist binding site of G-protein coupled receptors has been extensively studied. In the case of small ligands, such as catecholamines, it is located in a pocket formed by the seven transmembrane domain segments (Ostrowski et al., 1992; Savarese and Fraser, 1992; Trumpp-Kallmeyer et al., 1992). Two studies aimed to identify the agonist binding site in mGluRs (O’Hara et aI, 1993; Takahashi et al., 1993). Takahashi et al. (1993) took advantage of the different pharmacological profile of mGluR1 and mGluR2 to map the regions involved in the agonist selectivity. By exchanging parts of the extracellular domains of these two receptors, they showed that the N-te~inal two-thirds of this domain are suilicient to convert the pharmacological profile of mGluR1 into that of mGluR2. In another approach, the weak sequence similarity of the extra~ellnlar domain of mGluRs with bacterial periplasmic binding proteins (PBP) was used to construct a model of the tertiary structure of this mGluR domain (O’Hara et al., 1993). This model was constructed

using the reported 3-D structures of several PBPs and predicts that the extracellular domain of mGluRs is made up of two globular domains with a hinge region. The Glu binding site is proposed to be equivalent to the known amino acid binding sites of PBPs. In PBPs this site is located within a cleft formed between two globular domains. After an initial binding to one domain the hinge is proposed to fold and trap the amino acid in a pocket. To support this model of the mGluR tertiary structure, several mutants of the proposed Glu binding site were constructed in vitro (see Fig. 1). Two of these mutations strikingly alter the functional affinities of mGluRla for Glu and Quis (indicated by X in Fig. 4). Because of the homology between mGluRs and PCaRl, the finding of mutations in the human Ca’+-sensing receptor gene in patients with two related diseases, familial hypo~lciuric hypercalcemia and neonatal severe hype~arathyroidism is interesting (Pollak et aZ., 1993). Two of these mutations occur in the extracellular domain and one in the third intracellular loop (i3). Consistent with the tertiary structure model proposed by O’Hara et al. (1993), one of the extracellular domain mutations (indicated by o in Fig. 4) is located very close to the proposed ligand binding pocket. The mutation in i3 might interfere with normal signal transduction. G-protein coapIing The regions involved in the activation of G-protein by other G-protein-coupled receptors have been extensively studied (Ostrowski et al., 1992; Savarese and Fraser, 1992). mGluRs are interesting in that they couple to the same G-proteins, but do not share any sequence similarity with other G-protein-coupled receptors. By constructing chimera between mGluRlc and mGluR3, Pin et al. (1994) have shown that the less conserved second intracellular loop (i2) as well as the amino portion of the carboxy-te~inal tail (i4), determine the specific coupling to PLC. Both regions are rich in basic residues and could form amphipathic a-helices. Similar elements are present in the domains interacting with G-proteins in other G-protein-coupled receptors, indicating that even though amino acid sequences are not conserved, common structural features are involved in G-protein coupling. However, G-protein activation is a complicated proteinprotein interaction and much more needs to be understood about the mechanisms and specificity of mGluR-G-protein coupling. As already discussed, the sequence of if and i3 is highly conserved among all members of this receptor family including the PCaRl, strongly suggesting that these domains play an important role in G-protein activation. Indeed, a mutation in the human PCaRl i3 that prevents the receptor from activating PLC has been found in FHH patients (Pollak et al., 1993). It is at a position conserved in all members of this receptor family (Fig. 4).

Metabotropic

rnGlUR7 PC.sRl

TGl”R6 XGl”R7 PCaRl

PIN PVH

MKMLTRLQILMLALFSKGF LLSLGDHNFMRREIKI MSGKGGWAWWWARLPLCLLLSLYAPWVPSSLGKPKGHPHMNSIRI

AQSSERRVVAHM MVLLLILSVLLLKEDVRGS MGRLP"LLLWLAwwLSQAGIACGAGS"RL AARGQEHYAPHSIRI M"QLGKLLR"LTLMKFPCC"LEVLLCVLAA MALYSCCWILLAFSTWCTSAYGPDQRAQK

* rnGlUR1 rnGlUR2 rnGlUR3 lllGl”Rl rnGlUR5

7

receptors

SVH P””

MVRLLLIFFPMIFLEHSILPRHPDRKVLLAGASSPRSVA~M LWGA”AEGPAKK”LTL MESLLGFLALLL

XlGl”Rl lGl”R2

rnGl"R3 rnGlURl rnGlLlR5 rnGlUR6

glutamate

SVH PVH PVH PIHFGVA

x

EI HI HI

wriss

RI EI RL RI RI

SRDT WHSA SRDT SRDT NTVS

x “KR

SKDT SRDT

"QALIRGRGDGDEASV

PDGSYATHSDAPTA

VT

pDGSYnI*ENIpLL

IA

PGGVPPLRSAPPER

vv

LRF LRF VRA "KR "RA VKA IEY

l rnGlUR1 rnGlUR2 lnGlUR3 rnGlUR4

SDKIYSNAGEKS FDRLL SEKVGRAMSRAA FEGVV YDSVI AEKVGRSNIRKS SVKIPREP KTGEFDKII SYKIYSNAGEQS FDKLL SIKIPREP KPGEFHKVIR SVRIPQERKDRTIDFDRIIK SELISQYSDEE KIQQVVE

KELA AOEG ELEAR ARN EQEAR LRN *QKSRENGG KDNS AKEG VQISREAGG TQISKEAGG REEA EERD

Y

F F L Y L L F

EGMTVRGLL RSEDARELL RSDDSRELI NEDDIRRVL EGMTVRGLL NEDDIRRVL NDEDIKQIL SGPDLEPLI

ERLPK *K PS QK PN ET SN SHLPK ET PN DT PN N ST

RRLGWG *RLNAS NRVNAS RRANQTG RRLGLAG RQANLTG KRADQVG 1 "RRNITGRI

DRD EVIEGYE” ALE SVVAGSER AQE SIVKGSEH SKS APVLRLEE DRY DVTDGYQR SK* SPILNLEE SKI NPLHQHED SSSILIAMPEYFHtiG

SLI Twv TW" FWM LL LWV LW" WL l

* rnGlUR1 mGluR2 mGluR3 rnGlUR4 nlGlUR5 nlGlUR6

'LPVDTFLRGHEEGGARI

DAPFRPADTDDE

KENEF QPYEY EPYEY TGYQY KENEY DGYRF DGYQY PDGEYSDE

LKGQ LQNE APNE QPGE EKGQ GPGE KPGQ LAGT

rnGlUR1

rnGlUP.2 mGluR3 rnGl"Rl IllGl"R5 rnGl"R6 lllGl"R7 PC3Rl

" R L Q " Q Q

LENPNFKK QRD NKRNHRQ LKKGSHIK QENSKYNK GQSDDSTR SKKEDTDR SNSPTAFRP

GNESL EENYti AH SL R AVPF KHLAI D SSNY DSAY NRERIGQ RTHH SSLTL DSAY GEERIGQ GQERIGK DSNY GEENISS "ETPYMDY

NIDDYKIQMNKSG

DIMNLQYTE ANRY NIFTYL RAGSGRY NVFN L QQTGGKY DIYQYQLRN GSA EIHNFKEMG KDYF DIFQYQATNGSASSG DIFQYQTTN TTNP SIINWHL SPEDGSI

M”

TLDTSFIPWASPSAGPLP SRNSVP SLDVDSIHW

EA

DE \KKGE

KMDDDEVWSKKNN RLDMEVLRWSGDPHE QLNIEDMQWGKGVRE FINDEKILWSGFSRE

II VP Ip VP

SDIESIIAIAFSC GDAWAVGPVTIAC

SSPWAALPLLLA"

rnGlUR1 ltGl”R2 lllGl”R3 dl”R4 rnGl”R5 mGluR6 lllGl”R7 PCa.Rl

mGluR2 rnGlUR3 rnGlURQ mGluR5 rnGlUR6 rnGl"R7 PC.?lRl

NLGVVAP" DASMLGSL DSSMLISL DLSLICLL NLGVVTPL DLSLIGCL DLQIICSL SLMALGFLI

rnGlUR1

“GDGK

rnGlURl

--•

rnGl"R2 lXGl"P.3 ll?Gl"Rl rnGl"R5 lllGl"R6 mGluR7

PCciR.1

YIIIA HIILF "IYLF YIILF YIILA YVILF YIIIF YIILF

YKI CFA" FGSN CVS" SDYRVQT YVTS SDYRVQT CIS" YVTS TVSV FGTSQSADKLYIQT YKI CFSV FGSN LTVSL FGTAQSAEKIYIQT FFGTAQSAEKLYIQT iii TISM YASTYGKFVSAVEVIAILAASF 13

SAFTTSDWRMH s T K SAFTTSTVVRMH K K EEVRCSTAAHAF

VI-

TAVIKPLTKSYQGSGKSLTFSDASTKTLYNVEEEEDNTPSAHFSPP LPCRSNTFLNIFRRKKPGAGNANSNGKSVSWSEPGGRQAPKGQHWlQRLSVHVKTNETACNQ HRAPTSRFGSAAPRASANLGQGSGSQFVPTVCNGREVVDSTTSSL TATTYSQSSASTYVPTVCNGREVLDSTTSSL HRLHLNRFSVSG RKRSLKAV"TAATMSNXFTQKGNFRPNOEAKSELCENLETpALATKQT~"T~T~"A~ TAVIKPFPKSTENRGPGAAAGGSGPGVAGAONAGAGNAGCTATGGPEPP KSTRGQHLWQRLSVHINKKENP NQ "GDGKSSSAASRSSSLVNLWKRRGSSGETLSSNGKSVTWAQNE PPQNENAEDAK RKRSLKKTSTMAA RKRSFKAVVTAATMSSRLSHKPSDRPNGEAKTELCENVDPNSPAAKKKYVSYNNLVI KVAARATLRRSNVSRQRSSSLGGSTGSTPSSSISSKSNSEDPFPQQQPKRQKQPQPLALSPHNAQQPQPRPPSTPQPQPQSQQPPRCKQKVIFGSGTVTFSLSFDEPQKTAVAKRNST

mGluR5 PCslRl

AGPGTPGNSLRSLYPPPPPPQHL LADS "1PKGLPPPLpQQQPQQpPpQQPPQQpKSLHDQLQG""TNFGSGIPDF"A"L SSPSMVVHRRGPPVATTPFLPPHLTAEETPLF SLMEQISS""TRFTANISELNSMMLSTAATPGPPGTPICSSYL*PKEIQ DAGPKALYDVAEAEESFPPAARPRSPSPISTLSHLAGSAGRTDDDAPSLHSETAARSSSSQG HQTSLEAQKNNDALTKHQALLPLQCGETDSELTSQETGLQGPVGEDHQLEMEDPEEMSPALWSNSRSFVISGGGSTVTENMLRS

XlGl"Rl 87lGlURS

QMLPLHLSTFQEESISPPGEDIDDDSERFK;LPEFVYEREGNTK~DSLSKKKDL~TASKLTP~DSPALTP~SP~~DSVASGSS~~SSP~SKSVLCTPPN"T~ASV~L~D~KQSSSTL EEL"ALTPPSPFRDS"DSGSTTPNSPVSESALClPSSPKYDTL~I~D~TQSSSSL ATGVSPAQETPTGAESAPGKPDL LPTTMTTFAEIQPLPAIEVTGGAQG

mGluR1

Fig. 4. Multiple alignment of the mGluR family and PCaRl. Residues conserved among all members of this receptor family are underliwd in black. Those conserved in all mGluRs only are boxed. Conserved cysteines are indicated by a star (*). Residu.es affecting Glu affinity are indicated by an X. The position of mutations found in the human PCaRl from patients are indicated by (a).

H H H

G PTX sens.

Activation of cGMP PDE ON bipolar cells

G PTX and CTX

+

+ N.E.

N.E. L

:

+

L

L

+

L

+

H/L

+

+

+

+

ACPD

H

+

+ + +

+

Ibo

L +

H

H

Quis

G f PTX sens.

Intermediate

Increase in Ca2+i Cultured neurones IP3 stores Dorsal horn neurons vscc Cultured astrocytes stores Stimulation of CAMP production Brain slices adenosine Inhibition of CAMP production Cultured striatal neurons G PTX sens. Cultured cortical neurons G PTX sens. Cultured cerebellar granule cells G PTX sens. Cultured striatal astrocytes G PTX sens. Cultured cortical astrocytes Brain slices G PTX sens.

IP3 production Cultured striatal and cortical neur. Cultured cerebeller granule cells Cultured astrocytes Brain slices

Transduction

+

L

N.E.

N.E.

H

H

+

N.E.

N.E.

N.E.

N.E.

~-At'4

L

DCG-IV

+

-

L-SOP

mechanisms

+

CCG-I

Table 2. Transduction

+

N.E.

P.A.

+

P.A.

L-AP3

of mGluRs

-

MCPG

+

P.A.

4C3HPG

3C4HPG

-

4CPG

28 15, 29, 30, 31, 32, 33

mGluR3 2 and G-III

34, 35, 36

27

unknown

mGluR6

27

2 and G-III

27

26, 21 mGluR3 3 and G-III

23, 24, 25

1, 16, 17, 18, 19 20 21,22

13, 14, 15

P 6. g.

3, 4, 5 6, 7, 8 9, 10, 11, 12,

References

G-I or II

G-I G-I mGluR5

1 (or 5) 5 (or 1) G-I

G-I

mGluR?

+J cd E’ m

H H

G PTX sens.

G noPTX

Cai

+

-

+

-

cGMP

N.E.

-

N.E.

N.E.

+

+

+

N.E.

cGMP

+

+

+

+

+

-

-

41 42

G-I and III mGluR1

45, 46, 49, 50 51

G-I G-I

G-I or II

G-I

20, 53

20, 46, 52

46, 52

44,47, 48

G-I

G-I

44,45, 46

G-I

43

39, 4

G-I

G-II

31, 38

G-I

(+) indicates that this compound is an agonist, and (-) an antagonist. N.E.: means no effect. H: the compound is very potent as an agonist; L, it has a low potency. G-I, G-II and G-III means Group-I, Group-II and Group-III respectively. tort., cortical; DH, dorsal horn; Hipp., hippocampal; LGNd, dorso lateral geniculate nucleus; NTS, nucleus of the tractus sohtarius; OB, olfactory bulb; PDE; phosphodiesterase. References are as follows: 1 (Manzoni et al., 1991); 2 (Pate1 et al., 1990); 3 (Nicoletti et al., 1986~); 4 (Aronica et al., 1993); 5 (Suzdak et al., 1993); 6 (Milani et al., 1989); 7 (Stella et al., 1994); 8 (Nicoletti et al., 1990); 9 (Schoepp et al., 1990); 10 (Vecil et al., 1992); 11 (Nakagawa et al., 1990); 12 (Pin et al., 1993); 13 (Schoepp and Conn, 1993); 14 (Birse et al., 1993); 15 (Cartmell et al., 1994); 16 (Murphy and Miller, 1988); 17 (Murphy and Miller, 1989); 18 (Yuzaki and Mikoshiba, 1992); 19 (Milani et al., 1993); 20 (Bleakman et al., 1992); 21 (Glaum et al., 1990); 22 (De Barry et al., 1991); 23 (Winder et al., 1993); 24 (Schoepp and Johnson, 1993); 25 (Winder and Conn, 1993); 26 (Prezeau et al., 1992); 27 (Prezeau et al., 1994); 28 (Baba et al., 1993); 29 (Schoepp et al., 1992); 30 (Casabona et al., 1992); 31 (Kemp et al., 1994); 32 (Cartmell et al., 1993); 33 (Lombardi et al., 1993); 34 (Nawy and Jahr, 1990a); 35 (Shiells and Falk, 1990); 36 (Shiells and Falk, 1992a); 37 (Swartz and Bean, 1992); 38 (Swartz et al., 1993); 39 (Lester and Jahr, 1990); 40 (Sayer er al., 1992); 41 (Trombley and Westbrook, 1992); 42 (Chavis et al., 1994c); 43 (Chavis et al., 1994b); 44 (Charpak et al., 1990); 45 (Glaum and Miller, 1992); 46 (Glaum et al., 1993); 47 (Baskys et al., 1990); 48 (Gerber et al., 1992); 49 (McCormick and Krosigk, 1992); 50 (Guerineau et al., 1994); 51 (Fagni et al., 1991); 52 (Glaum and Miller, 1993a); 53 (Aniksztejn ef al., 1992).

neurons (+) AMPA in NTS and DH neurons (+) NMDA in CA1 and DH neur.

H

H

not PKCjPKA

G

H

not PKCjPKA

N.E.

H

G/Ca

G PTX sens.

H

G no PTXjPKC

Action on ionotropic receptors (-) GABA-A in NTS

CA3 neur. (-) IKAHP in hippocampal neur. (-) IKleak in CA3, NTS, LGNd neur. ( +irrn:t cerebellar

Regulation of K-channels (-) IKM in NTS and

pyramidal neur. (-) L-type in Hipp. and tort. neur. (-) N or L-type in OB neur. (+) L-type in cerebellar granule (-) L-type in cerebellar granule

Regulation of Ca-channels (-) N-type in

10

J.-P. Pin and R. Duvoisin

The role of the large intracellular C-terminal domain of mGluRla and mGluR5 is not known. As discussed earlier, natural deletion of this domain by alternative splicing generates receptors which are still capable of stimulating PLC. However, the shorter receptors mGluRlb and mGluRlc generate slower Ca2+ responses, do not activate adenylyl cyclase (Prezeau and Pin, in preparation), and seem to activate PLC mainly via PTX-insensitive G-proteins. These results suggest that the long C-terminal domain play some role in the transduction mechanism of these receptors. However, this may not be the main role of this 350 amino acid-long domain. The presence of numerous phosphorylation sites and of numerous threonine and serine residues at the C-terminus suggest that it could be the target of several types of kinases that could regulate receptor activity. TRANSDUCTION MECHANISMS OF NATIVE mGluRs In this chapter, the transduction mechanisms of mGluRs in neurons and glial cells will be presented. Based on their pharmacological profiles, the possible involvement of specific cloned mGluRs will be discussed. Activation

of PLC and release of CaZf from internal stores

The pharmacology of PLC stimulation by Glu analogues in neurons and glial cells has already been reviewed (Pin et al., 1993; Schoepp et al., 1990; Schoepp and Conn, 1993). In most cases, Quis is the most potent agonist followed by Glu, Ibo and lS,3R-ACPD (Table 2) in agreement with the involvement of Group-I mGluRs. Moreover, and in agreement with their effects on mGluRla, 3HPG is a potent and selective partial agonist for PLC-coupled mGluRs (Birse et al., 1993) and the two isomers L-CCG-I and L-CCG-II stimulate PLC with a low potency (Nakagawa et al., 1990). Finally, the Group-I mGluR antagonists 4C3HPG, 4CPG and MCPG competitively antagonize IP3 production stimulated by ACPD in brain slices with a relatively low potency (Birse et al., 1993) (Table 2). However, there are some differences between the pharmacology of mGluR-stimulated IP3 production in brain cells and that of Group-I mGluRs. First, L-SOP, L-AP4 and L-AP3 have been proposed as possible antagonists for PLC-coupled mGluRs in brain slices (Schoepp et al., 1990), whereas these compounds are almost inactive at mGluR1 or 5 (Table 1). Second, Ibo is more efficacious on PLC-coupled mGluRs in hippocampal slices, but not on Group-I mGluRs. Third, 4C3HPG seems to be a partial agonist at PLC-coupled mGluRs (Birse et al., 1993), but is a full and competitive antagonist at mGluRla (Table 1). These differences may be explained by the presence of additional PLC-coupled mGluRs, or alternatively, by the lack of specificity of these drugs. L-AP3 is clearly not a competitive antagonist at PLC-coupled mGluRs (Birse et al., 1993; Desai and Conn, 1990; Lonart et al., 1992; Manzoni et al., 1991; Sortino et al., 1991; Vecil et al., 1992) and have

non-specific effects (Batchelor and Garthwaite, 1993; Salt and Eaton, 1991); Ibo not only activates mGluRs, but also NMDA receptors. The G-proteins involved in the activation of PLC by mGluRs in neurons and glial cells are not clearly defined. However, as in the case of Group-I mGluRs expressed in different cell lines, the IP3 production induced by mGluR agonists can be either not sensitive (Sladeczek et al., 1985), or sensitive to PTX such as in cerebellar granule cells (Nicoletti et al., 1988; Suzdak et al., 1993) and in striatal neurons (Ambrosini and Meldolesi, 1989). This indicates that either Gi/Go or Gq proteins may couple mGluRs to PLC in their natural environment. Activation of PLC leads not only to the formation of IP3 but also to that of diacylglycerol, which in turn activates PKC. Accordingly, mGluR agonists activate PKC in striatal neurons (Manzoni et al., 1990; Weiss et al., 1989) and in cortical astrocytes (Nicoletti et al., 1990). With a pharmacology identical to that reported for PLC activation (Table 2), Glu acting on mGluRs increases Cat+ concentrations in neurons (Courtney et al., 1990; Llano et al., 1991; Manzoni et al., 1991; Murphy and Miller, 1988, 1989; Yuzaki and Mikoshiba, 1992) astrocytes (De Barry et al., 1991; Glaum et al., 1990; Jensen and Chiu, 1991) and brain synaptosomes (Brammer et al., 1991; Guiramand et al., 1991). In many cases, oscillations of CaF+ have been described, and in astrocytes Ca2+-waves propagating from cell to cell are probably generated by mGluRs (Charles et al., 1991; Cornell-Bell et al., 1990; Glaum et al., 1990; Jensen and Chiu, 1990). These effects may result from the release of Ca2+ from intracellular pools since they are also observed in the absence of extracellular Ca2+. However, in some cases such as in the dendrites of cerebellar Purkinje cells, no increase in Caf+has been observed under particular conditions even though the presence of active PLCcoupled receptors has been clearly demonstrated (Vranesic et al., 1991; Yuzaki and Mikoshiba, 1992). Subtle mechanisms may therefore regulate this mGluR response. In cerebellar granule neurons, Ca*+ release from internal stores by mGluRs depends on a slight rise in intracellular Ca2+ induced by K+ depolarization or activation of KA or NMDA receptors (Courtney et al., 1990; Irving et al., 1992). Taken together, these results indicate that Group-I mGluRs are coupled to PLC, activate PKC and stimulate Ca2+ release from internal stores, and suggest that additional PLC-coupled mGluRs may exist. More work is however necessary to help define the exact roles of mGluR1 and mGluR5, and their different splice variants. Stimulation

of adenylyl

cyclase

Several Glu analogues were found to stimulate CAMP formation in brain slices (Casabona et al., 1992; Schoepp and Johnson, 1993; Winder and Conn, 1992, 1993). Consistent with the involvement of PLC-coupled mGluRs, Quis is a relatively potent agonist, although far

Metabotropic glutamate receptors less efficacious than lS,3R-ACPD (Schoepp and Johnson, 1993) and L-CCG-I has a low potency (Cartmell et al., 1994). Moreover, ACPD stimulation of CAMP production is stronger in neonatal brain slices than in adult slices, as is the PLC activation by mGluRs (Table 2) (Casabona et al., 1992; Schoepp and Conn, 1993). Some results, however, are not consistent with the involvement of PLC-coupled mGluR in the stimulation of adenylyl cyclase. For example, lS,3S-ACPD which stimulates IP3 formation with a very low potency, but activates Group-II-mGluRs, potently stimulates CAMP formation (Cartmell et al., 1993; Winder and Conn, 1993). L-SOP, an agonist at Group-III mGluRs competitively inhibits the ACPD effect (Winder et al., 1993). These results suggest that the stimulation of CAMP formation could be mediated by a new mGluR subtype. ACPD-stimulation of CAMP formation does not result from the direct coupling of an mGluR to the G-protein stimulating adenylyl cyclase, Gs, but from a potentiation of the stimulation of adenylyl cyclase by endogenous adenosine acting on A2 receptors (Schoepp and Johnson, 1993; Winder and Corm: 1993). Moreover, ACPD and Glu have been shown to potentiate the effect of other Gs-coupled receptors on adenylyl cyclase activity (Cartmell et al., 1993, 1994; Winder and Conn, 1993; Winder et al., 1993). Such an effect can be explained by the potentiating effect of by subunits on the stimulation of adenylyl cyclase by Gcrs (Sternweis, 1994). Therefore, by releasing /?y subunits, any G-protein coupled receptor, including Gi-coupled (Uezono et al., 1993), can potentiate adenylyl cyclase activity. This could explain the complex pharmacological profile described for this mGluR-mediated effect. Whatever the exact mGluR subtype and mechanism involved, mGluR potentiation of CAMP production regulates the excitatory synaptic transmission, as recently reported in the CA1 field of the hippocampus (Gereau and Conn, 1994a, b). A direct activation of Gs by mGluRs in neurons or astrocytes, although not yet demonstrated, cannot be excluded. Experiments in brain slices are quite complicated to interpret due to the multitude of putative indirect effects. Moreover, ACPD which is often used in these studies, is very potent on mGluRs negatively coupled to adenylyl cyclase (Table 1, and the following paragraphs) and therefore precludes any possible observation of adenylyl cyclase stimulation. Inhibition of adenylyl cyclase Since the discovery by Nakanishi’s group that mGluR2 inhibits adenylyl cyclase in CHO cells (Tanabe et al., 1992) several groups have reported inhibition of adenylyl cyclase by mGluR agonists in brain slices, cultured neurons and cultured astrocytes (Table 2). This Glu response is antagonized by PTX, indicating that a G-protein of the Gi family is involved as in the case of other receptors inhibiting adenylyl cyclase. The demonstration that Glu receptors directly coupled to adenylyl cyclase via inhibitory G-proteins exist in neurons is given

11

by the observation that Glu inhibits adenylyl cyclase in plasma membranes (Prezeau et al., 1992, 1994). The pharmacology of this Glu effect is variable (Table 2). In cultured cortical neurons, cortical astrocytes, cerebellar granule cells and striatal neurons, lS,3RACPD and L-CCG-I potently inhibit CAMP formation indicating that Group-II mGluRs are involved (Table 2). Quis is a relatively good agonist in cortical and striatal neurons, and in cortical astrocytes, suggesting that mGluR3 is involved. In contrast, Quis is almost inactive in cerebellar granule cells in agreement with its very low affinity on mGluR2 (Prezeau et al., 1992, 1994). The effect of L-AP4, which has been proposed as a selective agonist at Group-III mGluRs, on the inhibition of CAMP formation has been examined in cultured neurons and in cerebral cortex slices. In cerebellar granule cells (Prtzeau et al., 1994) and in rat cortical slices (Kemp et al., 1994) L-AP4 potently inhibits CAMP formation, suggesting the involvement of Group-III mGluR. However, in rat cortical slices, this L-AP4 effect is antagonized by MCPG which does not antagonize mGluR4 and mGluR7 (Table 1). In cerebellar granule cells, the maximal effect of L-AP4 is very small and unrelated with the amount of mGluR4 mRNA found in these cells (Prezeau et al., 1994; Tanabe et al., 1993). Moreover, it was impossible to detect any inhibition of adenylyl cyclase with L-AP4 on cerebellar granule cell membranes. Thus, the exact transduction mechanism of mGluR4 in neurons remains to be established. mGluR4, like mGluR6, may be possibly coupled to other G-proteins that regulate different effecters like cGMP phosphodiesterases. In cortical neurons (Prezeau et al., 1994) and in guinea pig cortical slices (Cartmell et al., 1993, 1994), L-AP4 inhibits CAMP formation with a low potency, which could be in agreement with the involvement of mGluR7. However, more work is necessary to establish the exact transduction mechanism of mGluR7 in vivo. In cultured striatal astrocytes, the pharmacology of the Glu inhibition of CAMP formation is totally original since none of the mGluR agonists Quis, lS,3R-ACPD and L-AP4 are active (Prezeau et al., 1994). This suggests the existence of additional mGluRs negatively coupled to adenylyl cyclase. However, the difficulty to demonstrate that Glu inhibits adenylyl cyclase activity in striatal astrocyte membranes does not permit to conclude that a direct coupling exists between this receptor and adenylyl cyclase. Activation of a cGMP phosphodiesterase The L-AP4 sensitive mGluR located on ON-bipolar cells of the retina has been proposed to activate a cGMP-phosphodiesterase (Nawy and Jahr, 1990a; Shiells and Falk, 1990). The proposed transduction cascade is therefore identical to that following the activation of rhodopsin in photoreceptor cells: the activated G-protein stimulates a cGMP phosphodiesterase, leading to a decrease in cGMP concentration and the closure of

12

J.-P.

Pin and R. Duvoisin

cGMP-gated channels (Nawy and Jahr, 1990a, b; Shiells and Falk, 1992a, b; Yamashita and Wassle, 1991). The parallelism between the rhodopsin and L-AP4 receptor transduction mechanisms is reinforced by their sensitivity to both PTX and cholera toxin (Shields and Falk, 1992a). The pharmacology of this L-AP4 receptor is identical of that of mGluR6 (Tables 1 and 2) (Nakajima et al., 1993, Nawy and Copenhagen, 1987; Slaughter and Miller, 198 1, 1985). Consistent with this L-AP4 effect being mediated by mGluR6 is the exclusive expression of this receptor in retinal bipolar cells (Nomura et al., 1994). Whether such a transduction mechanism exists for other mGluRs in the brain remains to be examined. Activation of PLA2 and PLD Glu stimulates arachidonic acid (ArAc) release from cultured striatal neurons as a consequence of PLA2 activation (Dumuis et al., 1990). A mGluR having a pharmacology identical to that of a PLC-coupled mGluR is involved in this response, however, the release of ArAc is observed only when the cells are also depolarized, either with AMPA or veratridine. The reversal of the Na/Ca exchanger has been proposed as a key step in this co-operative effect of different subtypes of Glu receptors (Dumuis et al., 1993). Glu has also been shown to stimulate ArAc release in cultured striatal astrocytes with a pharmacology different from that of any known receptor (Stella et al., 1994). This indicates that either a new Glu receptor exists, or that a complex cascade of events involving different Glu receptors subtypes is responsible for this PLA2 activation. The activation of ArAc-selective PLA2 by hormone or neurotransmitters requires an increase in Caf+, but &subunits of G-proteins have been shown to also be involved (Corda, 1993). Although mGluRla activates PLA2 in CHO cells (Aramori and Nakanishi, 1992) this mechanism of mGluRla action may be subject to complex regulations in neurons. The possible coupling of metabotropic receptors to PLD has been examined in hippocampal slices (Boss and Conn, 1992). The pharmacology of this response appears quite different from that of any known Glu receptors. Although lS,3R-ACPD but not 1R,3SACPD stimulates PLD activity, Glu is without effect (Boss and Conn, 1992) and L-cysteine sulfinate is the only endogenous compound able to mimic ACPD (Boss et al., 1994). Reputation of ~a~~-channei~ Agonists of mGluRs have been shown to inhibit voltage-sensitive Ca*+-channels (VSCC) in cultured hippocampal, olfactory and cerebellar granule neurons (Table 2). Several subtypes of mGluRs are involved in this effect. In pyramidal hippocampal and cortical neurons (Lester and Jahr, 1990; Sayer et al., 1992; Swartz and Bean, 1992; Swartz et al., 1993), the receptor involved has a pharmacology almost identical to that of Group-I mGluRs. Quis has the highest potency, followed by Glu,

Ibo and ACPD. L-AP4 is inactive (Table 2). Quis displays also a high potency in inhibiting Caz+-channels in olfactory neurons, also suggesting the involvement of Group-I mGluRs (Table 2). However, L-AP4 is also highly potent in inhibiting Caz+ -channels in these neurons indicating that Group-III mGluRs can also inhibit VSCC (Trombley and Westbrook, 1992). Finally, in cortical and cerebellar granule neurons, L-CCG-I was found to have a high affinity at mGluRs inhibiting VSCC whereas Quis was inactive indicating the involvement of Group-II mGluRs (Chavis et al., 1994b; Lovinger et al., 1994a). Both L- (Chavis et al., 1994b; Lester and Jahr, 1990; Sayer ef aE., 1992) and N-type Ca2+-channels (Chavis et al., 1994a; Ikeda et al., 1994; Swartz and Bean, 1992; Swartz et al., 1993) can be inhibited by mGluRs, but the mechanism involved in these effects is not characterized. The inhibition of L-type Ca’+-channels by Group-II and III mGluRs results from the activation of a PTX-sensitive G-protein (Chavis et al., 1994b; Trombley and Westbrook, 1992), and that mediated by Group-I mGluRs is Ca2+-dependent, G-protein mediated, and does not involve protein kinases (Lester and Jahr, 1990; Sayer et al., 1992). The inhibition of N-type VSCC is mediated by a G-protein and is blocked after activation of PKC by phorbol esters (Swartz and Bean, 1992; Swartz et al., 1993). Although a direct coupling of mGluRs to VSCC via G-proteins is possible, these effects are also observed with the cell-attached configuration of the patch-clamp technique, indicating that the signal between the receptor and the channel is able to pass the membrane sealed to the pipette. Surprisingly, Ca2+-channel opening can also be induced by mGluRs that have the pha~acolo~cal profile of Group-I mGluRs (Chavis et al., 1994~). This effect is observed in cerebellar granule cells that are maintained depolarized. It is not sensitive to PTX and does not involve protein kinases or phosphatases. In summary, all three subfamilies of mGluRs can inhibit L-type Ca2+-channels, probably directly via a Go-protein as has been demonstrated for other receptors (Kleuss et al., 1991) (Fig. 5). Group-I and Group-II mGluRs also inhibit N-type Caz+-channels. Moreover, Group-I mGluRs can also activate these L-type channels under particular conditions and thus play a complex role in regulating Caz+ concentrations.

PKC

Fig. 5. Schematic representation of the modulation of N- and L-type VSCCs by mGluRs.

Metabotropic glutamate receptors

13

Table 3. Summary of the possible transduction mechanisms and physiological roles of mGluRs Group-1 /“PLC (k PTX) /“PKC /“Cai release ( f PTX) /PLA2 /“A.denylyl cyclase \L & N-VSCC (G) /L-VSCC (Ca) /“NMDA (PKC) /“A.MPA \GABA-A jIIKn($kak, IKAHP Slow exck (IK, Na/Ca) Very slow hyperpol. (BK) /Release (needs AA) \Release (VSCC) /“LTP /=LTD /“Neurotoxicity

Group-II

Group-III /“cGMP-PDE

/Adenylyl \L-VSCC

cyclase (+ PTX)

(PTX, CTX)

\Adenylyl cyclase \L or N-VSCC (+ PTX)

/“IK Hyperpolarization \Release

(VSCC)

\Release /“LTP

(VSCC)

\Neurotoxicity

Regulation of K+-channelr Several subtypes of K+-channels are regulated by mGluR agonists (Table 2) (Baskys, 1992). Inhibition of the voltage-dependent K+-current IKM and the Ca*+activated K+-current IKAHP has been observed in hippocampal neurons (Baskys et al., 1990; Charpak et al., 1990; Gerber et al., 1992) and neurons of the nucleus of the tractus solitarius (Glaum and Miller, 1992). IKM is a voltage-dependent current which is active at the resting membrane potential and which slowly inactivates upon depolarization. IKAHP k commonly observed as a voltage-dependent slow outward current and is a major contributor of the slow afterhyperpolarization that follows an action potential. It is partly mediated by the apamin-sensitive Ca*+-dependent K+-channels (SK channels; 615 pS), but also by apamin-insensitive channels. The pharmacology of the mGluRs inhibiting these two channels resemble that of Group-I mGluRs. In both cases, Quis is the more potent agonist, followed by Ibo and ACPD. However, the phenylglycine derivatives 4C3HPG, 4CPG and MCPG are without effect on the ACPD-induced K+-channel inhibition in neurons of the nucleus of the tractus solitarius (Glaum et al., 1993). The exact mechanism involved in this control of K+-channel activity by mGluRs has not been clearly established. It is clear however, that Ca*+-activated K+-current IKAHP can be inhibited without any change in Ca’+ (Charpak et al., 1990). Although PKC activation is apparently involved in the inhibition of IKAHP in dentate granule neurons (Baskys et al., 1990) neither PKC nor PKA are involved in this effect in CA3 pyramidal neurons (Gerber et al., 1992). Resting K+-currents, called IKleak, can also be inhibited by lS,3R-ACPD in CA3 hippocampal (Guerineau et al., 1994), tractus solitarius nucleus (Glaum and Miller, 1992) and dorsal lateral geniculate nucleus (McCormick and Krosigk, 1992) neurons. The pharmacological characterization of this effect is relatively poor.

However, since L-AP4 is inactive, a role of Group-III mGluRs is unlikely. The mechanism involved in this K+-channel regulation is also unknown. The activation of K+-channels by mGluRs has also been observed. In cerebellar granule cells, a PLC-coupledlike mGluR activates the Ca*+-activated K+-channel called BK as a consequence of the increase in Caf+ (Fagni et al., 1991). Regulation of ionotropic receptors The major ionotropic receptors involved in fast synaptic transmission, GABA-A, NMDA and AMPA receptors can also be regulated by mGluRs (Table 2). In neurons of the tractus solitarius nucleus, lS,3R-ACPD inhibits GABA-A receptor-mediated currents and potentiates AMPA-mediated responses (Glaum and Miller, 1993a). These ACPD effects are antagonized by phenylglycine derivatives (Glaum et al., 1993) indicating that a Group-I mGluR is involved. In spinal cord neurons, ACPD potentiates both NMDA and AMPAmediated currents (Bleakman et al., 1992). In CA1 pyramidal neurons, ACPD potentiates only NMDA-mediated currents, but not those activated by AMPA (Aniksztejn et al., 1992; Harvey and Collingridge, 1993). This ACPD effect on NMDA responses is prevented by intracellular injection of selective PKC inhibitors (Aniksztejn et al., 1992). Similarly, mGluR-mediated inhibition of NMDA currents in Xenopus oocytes injected with rat brain RNA, also results from PKC activation (Kelso et al., 1992). Accordingly, PKC has been reported to reduce the Mg *+-block of NMDA-receptor channels (Chen and Huang, 1992) and to potentiate NMDAinduced currents in cells expressing NMDA receptor subunit (1 alone (Yamazaki et al., 1992) or in combination with E subunits (Kutsuwada et al., 1992; Meguro et al., 1992). However, the activation of PKC may not be the only mechanism by which mGluRs modulate NMDA receptors, since PKC inhibitors do not

14

J.-P. Pin and R. Duvoisin

prevent this effect in some preparations (Harvey and Collingridge, 1993). Inhibition of NMDA-mediated increase in Ca’+ by mGluR agonists have also been observed when measured in the absence of Mg2+ to relieve the Mg2+-block of the NMDA-associated channel (Courtney and Nicholls, 1992). This effect has also been proposed to involve PKC (Courtney and Nicholls, 1992). PHYSIOLOGICAL

ROLES OF mGluRs

Excitatory ejkts ACPD and other mGluR agonists induce a slowly developing depolarization and inward current associated with an increase in cell firing in many neurons including those of the hippocampus, dorsolateral septal nucleus, geniculate thalamus, nucleus of the solitary tract, cortex and cerebellum (for reviews, see Baskys, 1992; Pin et al., 1993; Schoepp and Conn, 1993). ACPD also shifts the firing mode from a rhythmic burst to single spikes, as shown in dorsal lateral geniculate thalamic (McCormick and Krosigk, 1992) and layer V neocortical (Wang and McCormick, 1991) neurons. Most of these effects likely result from the inhibition of K+-channels described above. The depolarizing effect may be due to an inhibition of K+-channels such as IKleak or IKM which control the resting membrane potential of neurons, whereas the increase in firing rate may be due to the inhibition of the Ca2+-activated K+-current IKAHP which participates in the afterhyperpolarization that follows an action potential. However, the ACPD-induced depolarization of Purkinje neurons, which is faster than the depolarization observed in other neurons, is not mediated by the inhibition of K+-channels. An inward current mediated by a Na+/Ca’+ exchanger resulting from an increase in Caf+ is apparently responsible for this depolarization (Glaum et al., 1992; Staub et al., 1992). IKAHP and IKM are also involved in a phenomenon called accommodation. This phenomenon is observed as a reduction in the action potential firing frequency during a long-lasting depolarizing step. Consistent with an mGluR-induced blockade of IKAHP and IKM, mGluR agonists block the accommodation of action potential firing in hippocampal (Charpak et al., 1990; Hu and Storm, 1991; Pacelli and Kelso, 1991; Stratton et al., 1989, 1990), septal (Zheng and Gallagher, 1992a), ventrobasal (McLennan and Wheal, 1978) and thalamic (Salt and Eaton, 1991) neurons. Few studies report a pharmacological characterization of this excitatory effect. Low concentrations of Quis, in the presence of ionotropic Glu receptor antagonists, mimic the ACPD effect (Charpak et al., 1990; Constanti and Libri, 1992; Greene et al., 1992; Hu and Storm, 1991; Stratton et al., 1989), and different derivatives of phenylglycine antagonize ACPD-induced excitation (Bashir et al., 1993; Birse et al., 1993; Eaton et al., 1993b; Lingenhohl et al., 1993; Manzoni et al., 1994). L-AP3 never inhibited the ACPD induced excitation. These results are consistent with the involvement of Group-I

mGluRs. However, two studies report an absence of antagonism by MCPG of the ACPD-induced inhibition of afterhyperpolarization in CA1 neurons (Chinestra et al., 1993) and neurons in the nucleus of the tractus solitarius (Glaum et al., 1993). All these effects were observed by applying exogenous mGluR agonists. Few studies aimed to demonstrate that these excitatory effects of mGluRs could also be induced by the natural transmitter during synaptic transmission. In hippocampal slice cultures, stimulation of the mossy fiber input to CA3 pyramidal neurons induced an excitation that resulted from the blockade of K+-channels, and this effect is mimicked by ACPD (Charpak and Gahwiler, 199 1). In the cerebellum, repetitive stimulation of the parallel fibers in the presence of ionotropic receptor antagonist reveals a slowly generating excitatory response which could also be reproduced by ACPD (Batchelor and Garthwaite, 1993). However, the definite demonstration of the involvement of Group-I mGluRs in postsynaptic excitation came with the use of the new antagonists (S)-4C3HPG and (S)-4CPG. In ventro basal thalamic neurons, both lS,3R-ACPD and noxious stimuli induced excitations were antagonized by these phenylglycine derivatives (Eaton et al., 1993a). Inhibitory effects Inhibitory actions of Glu were first characterized in the retina. Upon release from the photoreceptor cells in the dark, Glu depolarizes OFF-bipolar cells by acting on AMPA receptors, and hyperpolarizes ON-bipolar cells. L-AP4 and then L-SOP were shown to mimic the effect of Glu on ON-bipolar cells (Nawy and Copenhagen, 1987; Slaughter and Miller, 198 1, 1985). As discussed above, in rat (Yamashita and Wlssle, 1991), goldfish (Nawy and Jahr, 1990a; Scott and Jahr, 199 1) and dogfish (Shiells and Falk, 1990, 1992a, b) retina, this hyperpolarizing effect of Glu results from the closure of a non-selective cGMP-activated cation channel. In tiger salamander solitary-bipolar cells, however, the hyperpolarizing effect of r_-AP4 receptors is due to the opening of a K+- channel (Hirano and MacLeish, 1991). Hyperpolarizing effects, probably mediated by mGluRs, have also been observed in cerebellar Purkinje cells. In these cells, the slow depolarization induced by ACPD is often followed by a very slow hyperpolarization (Batchelor and Garthwaite, 1993; Glaum et al., 1992; Staub et al., 1992; Vranesic et al., 1993). Similar effects were reported with Quis, but not with the ionotropic selective agonist AMPA (Joels et al., 1989; Takagi et al., 1992), suggesting the involvement of Group-I mGluRs. In agreement with this conclusion, this late hyperpolarization likely results from the opening of Ca2+-activated K+-channels following an increase in Gaff (Glaum et al., 1992; Joels et al., 1989; Takagi et al., 1992). Accordingly, and as already discussed, Ca’+-dependent activation of K+-channels by ACPD or Quis-sensitive mGluRs has been described in cerebellar granule cells (Fagni et al., 1991). Alternatively, inhibition of a tonic Ca2+-channel

Metabotropic glutamate receptors

15

concentrations (in the PM range), whereas others, such as at the Schaffer collateral-CA1 pyramidal neurons (Koerner and Cotman, 1982) and at the lateral olfactory tract-cortical neurons (Anson and Collins, 1987) synapses, are observed at higher concentrations. These results suggest that several subtypes of L-AP4 mGluRs having different affinities for this agonist may exert these effects. Interestingly, mGluR4 and mGluR7 have a high and a low affinity for L-AP4 respectively (Table 1). However, MCPG which does not antagonize mGluR4 and mGluR7, inhibits a possible presynaptic effect of L-AP4 on neonatal motorneurons (Pook et al., 1993). Different data indicate that Group-II mGluRs mediate the ACPD presynaptic inhibition. First, many presynPresynaptic effects aptic receptors known to inhibit synaptic transmission, Presynaptic inhibition. Since 198 1, numerous reports such as the aZadrenergic or Al-adenosine receptors have described inhibitions of Glu-ergic transmission by strongly inhibit adenylyl cyclase. Furthermore, ACPD L-AP4 on motorneurons in the spinal cord, in the mediated inhibition of synaptic transmission in the striatum occurs at concentrations which do not induce basolateral amygdala, in the olfactory bulb, in the cortex, at the lateral perforant path-granule cells synapses in the excitation (Calabresi et al., 1992) and accordingly Group-II mGluRs have a 10 times higher affinity for dentate gyrus, and at mossy fiber-CA3 pyramidal ACPD than Group-I mGluRs (Table 1). Finally, the neurons and Schaffer collateral-CA1 pyramidal neurons involvement of Group-II mGluRs in presynaptic synapses in the hippocampus (for reviews, see Koerner and Johnson, 1992; Pin et al., 1993). The absence of effect inhibition has clearly been demonstrated for the granule of L-AP4 on responses induced by a direct application of to mitral cell synapses in the accessory olfactory bulb ionotropic Glu receptor agonists, plus the analysis of the (Hayashi et al., 1993), and at the excitatory synapses on amplitude fluctuation of evoked mono-synaptic excit- motorneurons (Ishida et al., 1993), by using the selective atory currents on hippocampal neurons (Cotman et al., Group-II agonist DCG-IV. Electron microscopic localiz1986; Forsythe and Clements, 1990) indicate that this ation of mGluR2 at the presynaptic level in the granule neurons of the accessory olfactory bulb definitively effect is mediated by presynaptic L-AP4 receptors. proves that this specific mGluR is indeed involved in the Similar techniques also revealed that ACPD presynaptically depresses excitatory synaptic transmission on inhibition of GABA release at these synapses (Hayashi et al., 1993). However, low concentrations of Quis which do striatal, hippocampal CA 1 pyramidal, basolateral amygdala, Purkinje, nucleus of the solitari tract and visual not activate Group-II mGluRs have also been reported to depress synaptic transmission (Baskys and Malenka, cortex neurons (for a review, see Pin et al., 1993). ACPD receptors also depress inhibitory synaptic transmission in 1991; Calabresi et al., 1992), suggesting that Group-I mGluRs may also inhibit neurotransmitter release at the striatum (Calabresi et al., 1992) and in the accessory olfactory bulb (Hayashi et al., 1993). This latter effect has some synapses. In agreement with this hypothesis, the presence of mGluR1 proteins in striatal nerve terminals recently been shown to be involved in olfactory memory has been reported (Fotuhi et al., 1993). (Kaba et al., 1994). The mechanisms involved in this effect are not known. The ACPD and L-AP4 presynaptic effects are likely mediated by two different mGluRs. In the striatum for However, as discussed previously, the three subfamilies of mGluRs can inhibit voltage-sensitive Caz+-channels. If example, ACPD inhibits EPSPs but L-AP4 is without effect (Lovinger, 1991; Lovinger et al., 1993). In the this is also true in nerve terminals, this may be a way by release. In basolateral amygdala, the ACPD and L-AP4 effects are which mGluRs inhibit neurotransmitter additives (Rainnie and Shinnick-Gallagher, 1992), and in agreement with this hypothesis, the inhibition of both the spinal cord, the presynaptic effects of ACPD and Ca*+-channels and EPSPs by L-AP4 receptors in the olfactory bulb neurons are sensitive to PTX (Trombley L-AP4 can be selectively inhibited by cr-methyl-L-CCG-I and cr-methyl-L-AP4 respectively (Jane et al., 1994). The and Westbrook, 1992). Similarly, presynaptic inhibition ACPD effect is mimicked by L-CCG-I, DCG-IV and by ACPD in the striatum is sensitive to PKC activation, as is the mGluR-inhibition of N-type Ca’+-channels lS,3S-ACPD, three agonists of Group-II mGluRs, strongly suggesting the involvement of mGluR2 or (Swartz et al., 1993). However, in this case, inhibition of N-type Ca*+-channels with o-conotoxin GVIA does not mGluR3 (Hayashi et al., 11993;Jane et al., 1994; Lovinger totally prevent ACPD to depress synaptic transmission, et al., 1994a). The L-AP4 effect is mimicked by L-SOP (Cotman et al., 1986; Harris and Cotman, 1983; indicating that other mechanisms are involved (Lovinger et al., 1994b). Lanthorm et al., 1984) indicating that it likely results Recently it has been reported that a low frequency from the activation of mGluR4 or 7. Interestingly, some of the L-AP4 effects are observed at very low stimulation of the tractus solitarius projections to the

has been proposed as a plossible mechanism responsible for this ACPD induced lam hyperpolarization in Purkinje neurons (Vranesic et al., 1993). This hyperpolarizing elfect, which as been observed by directly applying mGluR agonists, can also be observed upon repetitive stimulation of the parallel fibers in the presence of ionotropic receptor antagonists (Batchelor and Garthwaite, 1993), indicating that Glu released from parallel fiber terminals can activate mGluRs under these conditions. The exact physiological role of such an effect has not yet been examined but it is tempting to speculate that it could prevent an excess excitation of Purkinje neurons and therefore protect them from excitotoxicity.

16

J.-P. Pin and R. Duvoisin

nucleus tractus solitarii is sufficient to release enough Glu to presynaptically depress EPSC (Glaum and Miller, 1993b), indicating that presynaptic Glu autoreceptors may be activated under particular physiological conditions. Presynapticpotentiation. In the presence of arachidonic acid, lS,3R-ACPD greatly potentiates Glu release from synaptosomes induced by the K+-channel blocker, 4-aminopyridine (Herrero et al., 1992). This ACPD effect is mimicked by Ibo and low concentrations of Quis suggesting that a PLC-coupled mGluR is responsible for this effect. Moreover, this ACPD effect involves the activation of PKC, and PKC activity rapidly desensitizes this ACPD receptor (Herrero et al., 1994) as in the case of PLC-coupled mGluRs (Manzoni et al., 1990). Accordingly, and as previously presented, mGluR1 proteins have been observed in presynaptic nerve terminals (Fotuhi et al., 1993). However, both L-AP3 and L-AP4 inhibit this effect. It is possible that this inhibition results from the activation of other mGluR subtypes inhibiting Glu release. It is proposed that the stimulation of a presynaptic PLC-coupled mGluR leads to the formation of diacylglycerol which will activate PKC only in the presence of arachidonic acid. PKC would then inhibit K+-channels and therefore increase the depolarization of nerve terminals leading to an enhancement of Glu release. Another possibility would be that the depolarization induced by 4-aminopyridine plus arachidonic acid allows the mGluR to directly activate VSCCs as reported in depolarized cerebellar granule cells (Chavis et al., 1994b). However, whereas the former effect is PTX-sensitive, the latter is not. Whatever the exact mechanism, a presynaptic facilitation of neurotransmitter release by mGluRs is supported by the increase in Ca’+ concentration observed upon stimulation of synaptosomes with low concentrations of Quis (Adamson et al., 1990). Roles of mGluRs in synaptic plasticity Plasticity phenomena at Glu-ergic synapses are characterized by long-term changes in the synaptic efficacy. Both long-term potentiation (LTP) and long-term depression (LTD) of Glu-ergic synapses are observed (Baudry and Davis, 1991; Bliss and Collingridge, 1993; Ito, 1989; Siegelbaum and Kandel, 1991). Since they can last for hours in vitro and for weeks in freely moving animals, they are taken as models for the study of cellular processes that may underlie learning and memory. For example, spatial learning is thought to involve hippocampal LTP, and the LTD of parallel fibers to the Purkinje neurons may be implicated in the establishment of the vestibulo-ocular reflex. Considering the roles played by mGluRs in the Glu-ergic transmission it is not surprising that they also participate in these synaptic plasticity phenomena. Long-term potentiation. The mechanisms involved in the induction of LTP in the CA1 field of the hippocampus have been extensively studied (for a review see Bliss and

Collingridge, 1993). High frequency stimulation of the Schaffer collaterals, or the concomitant activation of this neuronal pathway with the depolarization of a pyramidal neuron, leads to an association between NMDA receptor activation and postsynaptic depolarization. This allows the removal of the Mg2+-block of the NMDA-receptorassociated channel. The Ca2+-influx resulting from the opening of the NMDA receptor channels activates postsynaptic Ca2+-sensitive enzymes which leads to the potentiation of the synaptic efficacy by either pre- or postsynaptic modifications. The voltage-sensitivity of the Mg2+-block of the NMDA channel is therefore critical for the induction of LTP. Transmitter receptors, such as PLC-coupled mGluRs that enhance NMDA receptor activity, depolarize neurons, and increase the Ca’+ concentration, are expected to facilitate the induction of LTP, or even to induce LTP. Accordingly, application of ACPD during tetanic stimulation enhances LTP (Behnisch and Reymann, 1993; McGuinness et al., 1991). Moreover, the administration of ACPD combined with a subthreshold tetanic stimulation (Aniksztejn et al., 1992; Otani and Ben-Ari, 1991) or the co-application of NMDA and ACPD (Musgrave et al., 1993) induces LTP in hippocampal slices. This metabotropic form of LTP is blocked by NMDA antagonists and PKC inhibitors (Aniksztejn et al., 1992) confirming the involvement of the PKC/NMDA receptor interaction in the induction of LTP (Ben-Ari et al., 1992). Such a mechanism may also be involved in the induction of olfactory cortical LTP by lS,3R-ACPD since it requires low frequency stimulation of the afferent fibers, and is blocked by NMDA receptor antagonists (Collins, 1994). The involvement of mGluR in the tetanus-induced LTP has been examined using the mGluR antagonist MCPG. In the CA1 field of the hippocampus, MCPG inhibits tetanus-induced LTP but not short-term potentiation (STP) (Bashir et al., 1993). Such an effect of MCPG has however not been observed by other investigators (Chinestra et al., 1993; Manzoni et al., 1994). MCPG also completely blocks a potentiation at perforant path/dentate gyrus synapses in vivo (Riedel et al., 1994). Recently, it has been shown that after a first LTP has been induced by a tetanus or ACPD, MCPG is no more able to inhibit tetanus-induced LTP (Bortolotto et al., 1994). This suggests that mGluRs activate a molecular switch which, once turned on during the induction of a first LTP, becomes no longer necessary. This observation may help understand the apparent discrepancies on the effect of MCPG on LTP induction observed in different laboratories. In some cases, mGluR stimulation seems to be sufficient to induce LTP, even in the absence of any stimulation, in the CA1 field of the hippocampus (Bortolotto and Collingridge, 1992, 1993) and dorsolatera1 septal nucleus (Zheng and Gallagher, 1992b). In the CAl, the activation of postsynaptic mGluRs may, however, not be sufficient, since it is dependent on an

Metabotropic glutamate receptors

17

intact connection between the CA3 and CA1 fields of the LTD can also be induced when AMPA receptors and hippocampus (Bortolotto and Collingridge, 1993). The ACPD-sensitive mGluRs are co-activated (Ito and ACPD-induced LTP has a slow onset, suggesting that it Karachot, 1990a, b; Linden et al., 199 1). Interestingly and occurs without STP (Bortolotto and Collingridge, 1993; as discussed above, simultaneous activation of AMPA Zheng and Gallagher, 199:2b). This is in agreement with and ACPD receptors was required for the release of the absence of effect of MCPG on tetanus-induced STP arachidonic acid in striatal neurons (Dumuis et al., 1990). (Bashir et al., 1993), and indicates that mGluRs may be Nevertheless, there is still no obvious evidence that arachidonic acid or some of its metabolites would be involved in the induction of the late phase LTP but not involved in LTD. in STP. In the dorsolateral septal nucleus, LTP induced The involvement of PLC-coupled mGluRs in the by tetanic stimulation is blocked by mGluR antagonists, and intracellular injection of BAPTA or GDPBS, but not induction of LTD is suggested not only by the high level by NMDA antagonists, fsuggesting that mGluR acti- of mGluR1 in the Purkinje cells, but also because the intracellular messengers involved in the generation of vation is both necessary and sufficient for the induction LTD are produced by these receptors. For example, an of this LTP (Zheng and Giallagher, 1992b). The exact mechanisms involved in LTP induction or increase in Ca’+ concentration is essential for LTD induction (Daniel et al., 1992; Ito and Karachot, 1990a; potentiation by mGluRs are not known. It is likely that Linden et al., 1991; Sakurai, 1990). Moreover, PKC this effect results from a release of Ca*+ from internal stores, as revealed by the inhibition caused by the Ca2+ which can be activated by mGluRs (Manzoni et al., 1990) inhibitor release thapsigargin (Bortolotto and seems to play an important role in the induction of LTD, at least in cultured Purkinje cells (Crepe1 and Jaillard, Collingridge, 1993; Collins, 1994) and a PKC activation, 1990; Crepe1 and Krupa, 1988; Linden and Connor, as indicated by the use of PKC inhibitors (Aniksztejn 1991). Finally, the Ca2+-activated enzyme NO-synthase et al., 1992; Bortolotto and Collingridge, 1993). Although the activation of PKC and the resulting potentiation of which can also be activated by mGluRs, has been proposed by some (Crepe1 and Jaillard, 1990; Shibuki and NMDA responses (O’Connor et al., 1994) may explain the facilitatory role played by mGluRs in the Okasa, 1991), but not by other authors (Linden and Connor, 1992), to be involved in the induction of LTD. NMDA-mediated induction of LTP, this action may not be sufficient for a pure mGluR-mediated induction of Recently, antibodies directed against N-terminal extracellular segments of mGluR1 were shown to specifically LTP. It is possible that arachidonic acid release induced by the association of mGluR activation and Na+ entry inhibit the action of Glu on mGluR1 (Shigemoto et al., (Dumuis et al., 1990, 1993) facilitates a potentiation of 1994). These antibodies were shown to also inhibit LTD Glu release by presynaptic mGluRs as described using ,induction in cultured Purkinje neurons, indicating that brain synaptosomes (Herrero et al., 1994, 1992). The mGluR1 and not other mGluRs plays a critical role in this permissive role of arachidonic acid in this form of LTP is plasticity phenomena (Shigemoto et al., 1994). A role for ACPD-sensitive mGluRs in the induction of supported by the facilitatory effect of this metabolite observed in the LTP induction in dentate gyrus (Williams LTD at other synapses has also been reported (Bolshakov and Siegelbaum, 1994; Kato, 1993). et al., 1989). Alternatively, an increase in CAMP induced by mGluRs may be involved (Gereau and Conn, 1994a; Role of mGluRs in neuronal death Glu is a main neurotoxic agent in the brain, and is Musgrave et al., 1993). Accordingly, activation of PKA responsible for the damage observed after ischemia, plays a crucial role in the induction of the late phase of LTP, but not in STP (Fre:y et al., 1993), as shown using hypoglycemia and anoxia (Choi, 1988). Because of their high Ca2+-permeability, NMDA receptors play a major the membrane permeable CAMP analogues Rp-CAMPS and Sp-CAMPS, which act as competitive inhibitors and role in these toxic effects. However, most neurons do not die during a short-term ischemia, or an acute NMDA activators of PKA respectively. exposure, but rather die within 24 hr after such accidents. Long-term depression. LTD of Glu-ergic synapses have been described in many different brain areas This delayed toxicity is probably due to Glu-induced Glu (Linden, 1994; Siegelbaum and Kandel, 1991). The best release as recently demonstrated using tetanus toxin, an inhibitor of this release process (Monyer et al., 1992). characterized example is the LTD of the parallel According to the previously described effects of mGluRs fibers-Purkinje cell synapses (Ito, 1989) which is induced receptors as well as on Glu-ergic by the co-activation of climbing and parallel fibers, which on NMDA neurotransmission, two opposite effects of mGluRs are are the two main excitatory inputs of these neurons. Although Quis application induces LTD when therefore expected in Glu toxicity: a potentiating effect associated with a Purkinje cell depolarization (Kano and due to the neuronal excitation and the potentiation of NMDA receptors mediated by Group-I mGluRs; and a Kato, 1987), the specific activation of AMPA or ACPD protective effect due to a presynaptic inhibition of Glu receptors, even accompanied by Purkinje cell depolarizrelease. ation, is not sufficient to induce a parallel fiber LTD. Accordingly, a facilitation of the NMDA but not However, activation of ACPD receptors together with the activation of VSCC induces LTD (Daniel et al., 1992). AMPA-mediated neurotoxicity by subtoxic doses of

J.-P. Pin and R. Duvoisin

18

ACPD has been observed (McDonald and Schoepp, 1992). Moreover, injection of higher doses of lS,3RACPD in ~ppocamp~ results in the induction of limbic seizures and a loss of dentate granule cells, as well as of hippocampal CA1 and CA2 pyramidal cells (Sacaan and Schoepp, 1992). Systemic administration of high doses of lS,3R-ACPD in neonatal rats also induces convulsion and brain damage (McDonald et al., 1993). These were not inhibited by NMDA and AMPA receptor antagonists, indicating that activation of mGluRs may be, per se, sufficient to kill neurons. Neuroprotective effects of mGluR agonists have also been reported. For example, ACPD (20 mg/kg ip) reduces by 34% the infarct volume of cerebral cortex observed following middle cerebral artery occlusion (Chiamulera et al., 1992). In the retina (Siliprandi et al., 1992) and in cultured cortical neurons (Birrell et al., 1993; Bruno et al., 1994; Koh et al., 199la, b), IS,3R-ACPD is not neurotoxic and reduces the excitotoxic effects induced by NMDA. Recently, DCG-IV and L-CCG-I were shown to potently inhibit kainate and NMDA-induced neurotoxicity in cortical neurons (Bruno et al,, 1994) indicating that Group-II mGluRs are responsible for this neuroprotective effect. There is no evidence yet for a neuroprotective effect of Group-III mGluRs although such an effect would be expected from these receptors because of their presynaptic inhibitory action on glutamatergic terminals. Another study reports that ACPD neither protects neurons from NMDA-induced neurotoxicity nor is toxic for neurons (Thomsen et al., 1993a). Aside from technical problems or differences in the culture conditions, this may be explained by a combination of the opposite effects of this drug on neuronal survival described above. CONCLUSION In the last two years, the full complexity of G-protein-coupled Glu receptors as well as their multiple roles in the brain, has been revealed. These receptors were found to have a great potential in modulating the fast excitatory neurotransmission in the CNS, either as presynaptic autoreceptors, or as an element of postsynaptic transmission. As expected from the multiplicity of transduction mechanisms associated with these different mGluRs, these modulator effects can either be a potentiation or an inhibition of Glu-mediated transmission, and the consequences can be as diverse as neuronal death, neuronal development, synaptic plasticity, spatial learning (Riedel et al., 1994), olfactory memory (Kaba et al., 1994), neuronal development (Dudek and Bear, 1989) central control of cardiac activity (Pawloski-Dahm and Gordon, 1992), waking (McCormick and Krosigk, 1992), control of movements (Sacaan et al., 1991,1992), control of the vestibulo ocular reflex (Ito, 1989). For many years, a number of scientists and pharmaceutical companies have tried to develop drugs that could help control Glu-induced effects via the

modulation of ionotropic AMPA, KA or NMDA receptors. Such compounds were expected to be useful to prevent neuronal death, diminish epileptic seizure or ameliorate learning and memory. However, such compounds were found to have multiple side effects, probably because of the ubiquity of the ionotropic Glu receptor actions. The discovery of the multiplicity of mGluRs and their crucial role in the modulation of Glu-ergic neurotransmission, appears therefore as a great hope in the development of new drugs able to control excitatory transmission in the CNS. The development of drugs specific for each of these receptor types will first be useful to further define the respective roles of each of these mGluRs, and may allow the development of new therapeutic agents. authors wish to thank Drs L. Fagni and T. Kniipfel for critical reading of the manuscript and access to unpublished results, and Drs J. Conn and D. M. Lovinger for

Acknowledgements-The

sending us manuscripts in press. The work of J.-P. Pin in Joel Bockaert’s laboratory was supported by grants from CNRS, CEE (BI02-CT93-0243), the Human Frontiers Science Program (RG 5792B). This work has been done as part of the BioAvenir program supported by Rhine-Poulenc with the participation of the French Ministry of Research and the French Ministry of Industry. Bayer company (Germany) and R.M.D. is the recipient of a Research to Prevent Blindness Career Development Award, and is supported by NIH grant (EY09534).

REFERENCES Abe T., Sugihara H., Nawa H., Shigemoto R., Mizuno N. and Nakanishi S. (1992) Molecular characterization of a novel metabotropic glutamate receptor mGluR5 coupled to inositol phosphate/Ca2+ signal transduction. J. Bid. Chem. 26% 13,361-13,368. Adamson P., Hajimohammadreza I., Brammer M. J., Cambell I. C. and Meldrum B. S. (1990) Presynaptic glutamate/ quisqualate receptors: effects on synaptosomal free calcium con~ntrations. J. ~e~rochem. 55: 185&1854. Ambrosini A. and Meldolesi J. (1989) Muscarinic and quisqualate receptor-induced phosphoinositide hydrolysis in primary cultures of striatal and hippocampal neurons. Evidence for differential mechanisms of activation. J. ~eurochem. 53: 825-833.

Aniksztejn L., Otani S. and Ben-Ari Y. (1992) Quisqualate metabotropic receptors modulate NMDA currents and facilitate induction of long-term potentiation through protein kinase C. Eur. J. Neurosci. 4: 500-505. Anson J. and Collins G. G. S. (1987) Possible presynaptic actions of 2-amino~-phosphonobutyrate in rat olfactory cortex. Br. J. Pharmac. 91: ‘7.53-761. Aramori I. and Nakanishi S. (1992) Signal transduction and pharmacological characteristics of a metabotropic glutamate receptor, mGluR1, in transfected CHO cells. Neuron 8: 757-765.

Aronica E., Condorelli D. F., Nicoletti F., Del~Albani P., Amico C. and Balazs R. (1993) Metabotropic glutamate receptors in cultured cerebellar granule cells: developmental profile. J. Neurochem. 60: 559-565.

Metabotropic

glutamate receptors

Baba A., Saga H. and Hashimoto H. {1993) Inhibitory glutamate response on cyclic AMP formation in cultured astrocytes. Neurosci. Lett. 149: 182-I 84. Bashir Z. I., Bortolotto Z. A., Davies C. H., Berreta N., Irving A. J., Seal A. J., Henley .J. M., Jane D. E., Watkins J. C. and Collingridge G. L. (1993) Induction of LTP in the ~ppocampus needs synaptic activation of glutamate metabotropic receptors. Nature 363: 347-350. Baskys A. (1992) Metabotropic receptors and “slow” excitatory actions of glutamate agonists in the hippocampus. Trends Pharmac. Sci. 15: 92-96. Baskys A. and Malenka R. C. (1991) Agonists at the metabotropic glutamate receptors presynaptically inhibit EPSCs in neonatal rat hippocampus. J. Physiol., Lond. 444: 687-701.

Baskys A., Bernstein N. K., Barolet A. W. and Carlen P. L. (1990) NMDA and quisqualate reduce a Ca2+-dependent KC current by a protein kinase-mediated mechanism. Neurosci. Lett. 112: 76-81. Batchelor A. M. and Garthwaite J. (1993) Novel synaptic potentials in cerebellar Purkinje cells: probable mediation by metabotropic glutamate receptors. Neuropharmacology 32: 1 l-20.

Baudry M. and Davis J. (1991) Long-term potentiation: a debate on current issues. The MIT press, Cambridge U.S.A., 450 p. Behnisch T. and Reymann K. G. (1993) Co-activation of metabotropic glutamate and N-methyl-D-aspartate receptors is involved in mechanisms of long-term potentiation maintenance in rat ~pp~ampal CA 1 neurons. N~rosc~ence 54: 3747. Ben-Ari Y., Aniksztejn L. and Bregestovski P. (1992) Protein kinase C modulation of NMDA currents: an important link for LTP induction. Trends Neurosci. 9: 333-339. Birrell G. J., Gordon M. P. and Marcoux F. W. (1993) (1S,3R)- I-aminocyclopemane-1,3-dicarboxylic acid attenuates N-methyl-D-aspartate-induced neuronal cell death in cortical cultures via a reduction in delayed Ca2+ accumulation. Neuropharmacolog,y 32: 1351-1358. Birse E. F., Eaton S. A., Jane D. E., Jones P. L. S. J., Porter R. H. P., Pook P. C.-K., Sunter D. C., Udvarhelyi P. M., Wharton B., Roberts P. J., Salt T. E. and Watkins J. C. (1993) Phenylglycine derivatives as new pharmacological tools for investigating thle role of metabotropic glutamate receptors in the central nervous system. Neuroscience 52: 481488. Bleakman D., Rusin K. I.., Chard P. S., Glaum S. R. and Miller R. J. (1992) Metabotropic glutamate receptors potentiate ionotropic glutamate responses in the rat dorsal horn. Molec. Pharmac. 4% 192-196. Bliss T. V. P. and Collingridge G. (1993) A synaptic model of memory: long-term potentiation in the hippocampus. Nature 361: 31-39. Bolshakov V. Y. and Siegelbaum S. A. (1994) Postsynaptic induction and presynaptic expression of hippocampal long-term depression. Science 264: 1148-l 152. Bortolotto 2. A. and Collingridge G. L. (1992) Activation of glutamate metabotropic receptors induces long-term potentiation. Eur. J. Pharmac. 214: 297-298. Bortolotto Z. A. and Collingridge G. L. (1993) Characterization of LTP induced by the activation of glutamate metabotropic receptors in area CA1 01’the hippocampus. Neuropharmacology 32: 1-9.

19

Bortolotto Z, A., Bashir Z. I., Davies C. H. and Collingridge G. L. (1994) A molecular switch activated by metabotropic glutamate receptors regulates induction of long-term potentiation. Nature 368: 740-743. Boss V. and Conn P. J. (1992) Metabotropic excitatory amino acid receptor activation stimulates phospholipase D in hippoc~pal slices. J. Neurochem. 59: 2340-2343. Boss V., Nutt K. M. and Conn P. J. (1994) L-systein sulfinic acid as an endogenous agonist of a novel metabotropic receptor coupled to stimulation of phospholipase D activity. Molec. Pharmac. 45: 1177-l 182. Brammer M. J., Richmond S., Xiang J. L., Adamson P., Hajimohammadreza I., Silva M. A. and Cambell I. C. (1991) Kainate and quisqualate effects on presynaptic cortical receptors are metabotropic and non-additive. Neurosci. Lett. 128: 23 l-234.

Brown E. M., Gamba G., Riccardi D., Lombardi M., Butters R., Kifor O., Sun A., Hediger M. A., Lytton J. and Hebert S. C. (1993) Cloning and characterization of an extracellular Ca2+-sensing receptor from bovine parathyroid. Nature 366: 575-580.

Bruno V., Copani A., Battaglia G., Raffaele R., Shinozaki II. and Nicoletti F. (1994) Protective effect of the metabotropic glutamate receptor agonist, DCG-IV, against excitotoxic neuronal death. Eur. .I. Pharmac. 256: 109-I 12. Calabresi P., Mercuri N. B. and Bernardi G. (1992) Activation of quisqualate metabotropic receptors reduces glutamate and GABA-mediated synaptic potentials in the rat striatum. Neurosci. Lktt. 139: 41-44. Cartmell J., Curtis A. R., Kemp J. A., Kendall D. A. and Alexander S. P. H. (1993) Subtypes of metabotropic excitatory amino acid receptor distinguished by stereoisomers of the rigid glutamate analogue, l-aminocyclopentane-1,3dicarboxylate. Neurosci. Lett. 153: 107-l 10. Cartmell J., Kemp J. A., Alexander S. P. H., Shinozaki H. and Kendall D. A. (1994) Modulation of cyclic AMP formation by putative metabotropic receptor agonists. Br. J. Pharmac. 111: 364-369. Casabona G., Genazzani A. A., DiStefano M., Sortino M. A. and Nicoletti F. (1992) Developmental changes in the modulation of cyclic AMP fo~ation by the metabotropic glutamate receptor agonist 1S,3R-aminocyclopentane-1,3-d& carboxylic acid in brain slices. J. Neurochem. 59: 1161-I 163. Charles A. C., Merrill J. E., Dirksen E. R. and Sanderson M. J. (1991) Intercellular signaling in glial cells: calcium waves and oscillations in response to mechanical stimulation and glutamate. Neuron 6: 983-992. Charpak S. and Gahwiler B. H. (1991) Glutamate mediates a slow synaptic response in hippocampal slice cultures. Proc. R. Sot. Lond. B243: 221-226. Charpak S., Gghwiler B. H., Do K. Q. and Knopfel T. (1990) Potassium ~onductan~s in ~ppocampal neurons blocked by excitatory amino-acid transmitters. Nature 347: 765-767.

Chavis P., Fagni L., Bockaert J. and Lansman J. B. (1994a) Metabotropic glutamate receptors increase L-type calcium current in cerebellar granule cells. Sot. Neurosci. Abst. 20: 904.

Chavis P., Shinozaki H., Bockaert J. and Fagni L. (1994b) The metabotropic glutamate receptor types 213 inhibit L-type calcium channels via a Pertussis toxin-sensitive G-protein in cultured cerebellar granule cells. J. Neurosci. 14: 7067-7076.

J.-P. Pin and R. Duvoisin

20

Chavis P., Nooney J. M., Bockaert J., Fagni L., Feltz A. and Bossu J.-L. (1994c) Facilitator coupling between a glutamate metabotropic receptor and dihydropyridine-sensitive calcium channels in cultured cerebellar granule cells. J. Neurosci. 15: 135-143. Chen L. and Huang L.-Y. M. (1992) Protein kinase C reduces Mg2+ block of NMDA-receptor channels as a mechanism of mod~ation. Nature 356: 521-523. Chiamulera C., Albertini P., Valerio E. and Reggiani A. (1992) Activation of metabotropic receptors shows a neuroprotective effect in a rodent model of focal ischemia. Eur. J. Pharmac. 216: 335-336.

Chinestra P., Aniksztejn L., Diabira D. and Ben-Ari Y. (1993) (RS)-~-Methyl-4-c~boxyphenylglyc~e neither prevents induction of LTP nor antagonizes metabotropic glutamate receptors in CA1 hippocampal neurons. J. Neurophysiol. 70: 26842689.

Choi D. W. (1988) Calcium-mediated neurotoxicity: relationship to specific channel types and role in ischemic damage. Trends Neurosci. 11: 465-468.

Collins G. G. S. (1994) The characteristics and pharmacology of olfactory cortical LTP induced by Theta-burst high frequency stimulation and 15,3 R-ACPD. Neuropharmacology 33: 87-95.

Constanti A. and Libri V. (1992) Trans-ACPD induces a slow post-stimulus inward tail current (IADP) in Guinea-pig olfactory cortex neurones in vitro Eur. J. Pharmac. 214: 105-106.

Corda D. (1993) Hormonal regulation of phospholipid metabolism via G-proteins II: PLA2 and inhibitory regulation of PLC. In: GTPases in Biology I?(Dickey B. F. and Birnbaumer L., Eds). pp. 387-400. Springer-Verlag, Berlin. Cornell-Bell A. H., Finkbeiner S. M., Cooper M. S. and Smith S. J. (1990) Glutamate induces calcium waves in cultured astrocytes: long-range glial signaling. Science 247: 470-473. Cotman C. W., Flatman J. A., Ganong A. H. and Perkins M. N. (1986) Effects of excitatory amino acid antagonists on evoked and spontaneous excitatory potentials in guinea-pig hippocampus. J. Physiol., Lond. 378: 403415. Courtney M. J. and Nicholls D. G. (1992) Interaction between phopholipase C-coupled and N-methyl-D-aspartate receptors in cultured cerebellar granule cells: protein kinase C mediated inhibition of N-methyl-D-aspartate responses. J. Neuroc~em. 59: 983-992.

Courtney M. J., Lambert J. J. and Nicholls D. G. (1990) The interaction between plasma membrane depolarization and glutamate receptor activation in the regulation of cytoplasmic free calcium in cultured cerebellar granule cells. J. Neurosci. lo: 3873-3879.

Crepe1 F. and Jaillard D. (1990) Protein kinases, nitric oxide and long-term depression of synapses in the cerebellum. NeuroReport 1: 133-136.

Crepe1 F. and Krupa M. (1988) Activation of protein kinase C induces a long-term depression of glutamate sensitivity of cerebellar Purkinje cells. Brain Res. 458: 397-401. Daniel H., Hemart N., Jaillard D. and Crepe1 F. (1992) Co-activation of metabotropic glutamate receptors and of voltage-gated calcmm channels induces long-term depression in cerebellar Purkinje cells in vitro. Exp. Brain Res. 90: 327-331.

De Barry J., Ogura A. and Kudo Y. (1991) Ca mobilization in cultured rat cerebellar cells: astrocytes are activated by trans-ACPD. Eur. J. Neurosci. 3: 11461154.

Desai M. A. and Conn P. J. (1990) Selective activation of phosphoinositide hydrolysis by a rigid analogue of glutamate. Neurosci. Lett. 109: 157-162.

Dudek S. M. and Bear M. F. (1989) A biochemical correlate of the critical period for synaptic modification in the visual cortex. Science 246: 6733675. Dumuis A., Pin J. P., Oomagari K., Sebben M. and Bockaert J. (1990) Arac~donic acid released from striatal neurons by joint stimulation of ionotropic and metabotropic quisqualate receptors. Nature 347: 182-184. Dumuis A., Sebben M., Fagni L., Prezeau L., Manzoni O., Cragoe Jr E. J. and Bockaert J. (1993) Stimulation of arachidonic acid release by glutamate receptors depends on Na+/Ca2+ exchanger in neuronal cells. Molec. Pharmac. 43: 976-98 1. Duvoisin R. M., Zhang C. and Ramonell K. (1995) A novel metabotropic glutamate receptor expressed in the retina and olfactory bulb. J. Neurosci. In press. Eaton S. A., Birse E. F., Wharton B., Sunter D. C., Udvarhelyi P. M., Watkins J. C. and Salt T. E. (1993a) Mediation of thalamic sensory responses in vivo by ACPDactivated excitatory amino acid receptors. Eur. J. Neurosci. 5: 186-189.

Eaton S. A., Jane D. E., Jones P. L. S. J., Porter R. H. P., Pook P. C.-K., Sunter D. C., Udvarhelyi P. M., Roberts P. J., Salt T. E. and Watkins J. C. (1993b) Competitive antagonism at metabotropic glutamate receptors by (S)4-carboxyphenylglycine and (RS)-a-methyl-4-carboxyphenylglycine. Eur. J. Pharmac 244: 195-197. Fagni L., Bossu J.-L. and Bockaert J. (1991) Activation of a large-conductance Ca 2+-dependent K+-channel by stimulation of glutamate phosphoinositide-coupled receptors in cultured cerebellar granule cells. Eur. J. Neurosci. 3: 778-789. Forsythe I. D. and Clements J. D. (1990) Presynaptic glutamate receptors depress excitatory monosynaptic transmission between mouse hippocampal neurones. J. Physiol., Lond. 429: l-16.

Foster A. C. and Fagg G. E. (1984) Acidic amino acid binding sites in mammalian neuronal membranes: their characteristics and relationship to synaptic receptors. Brain Res. Rev. 7: 103-164.

Fotuhi M., Sharp A. H., Glatt C. E., Hwang P. M., Von Krosigk M., Snyder S. H. and Dawson T. M. (1993) Differential localization of phosphoinositide-linked metabotropic glutamate receptor (mGluR1) and the inositol 1,4,5-triphosphate receptor in rat brain. J. Neurosci. 13: 2001-2012. Frey U., Huang Y .-Y. and Kandel E. R. (1993) Effects of CAMP simulate a late stage of LTP in hippocampal CA1 neurons. Science 260: 1661-1664. Gabellini N., Manev R. M., Candeo P., Favaron M. and Manev H. (1993) Carboxyl domain of glutamate receptor directs its coupling to metabolic pathways. NeuroReport 4: 53 1-534.

Gerber U., Sim J. A. and GHhwiler B. H. (1992) Reduction of potassium conductances mediated by metabotropic glutamate receptors in rat CA3 pyramidal cells does not require protein kinase C or protein kinase A. Eur. J. Neurosci. 4: 792-797.

Gereau R. W. and Conn P. J. (1994a) A cyclic AMPdependent form of associative synaptic plasticity induced by coactivation of fi-adrenergic receptors and metabotropic glutamate receptors in rat hippocampus. J. Neurosci. 14: 331@-3318.

Metabotropic

glu.tamate receptors

Gereau R. W. and Corm P. J. (1994b) Potentiation of CAMP responses by metabotropic glutamate receptors depresses excitatory synaptic transmission by a kinase-independent mechanism. Neuron 12: 1121-l 129. Glaum S. R., Holzwarth J. A. and Miller R. J. (1990) Glutamate receptors activate Ca2+ mobi~zation and Ca2+ influx into astrocytes. Proc. Natn. Acad. Set’. U.S.A. 87: 3454-3458.

Glaum S. R. and Miller R. J. (1992) Metabotropic glutamate receptors mediate excitatory transmission in the nucleus of the solitary tract. J. Neurosci. 12: 2251-2258. Glaum S. R. and Miller R. J. (1993a) Activation of metabotropic glutamate receptors produces reciprocal regulation of ionotropic glutamate and GABA responses in the nucleus of the tractus solitarius of the rat. J. Neurosci. 13: 1636-1641. Glaum S. R. and Miller R. J. (1993b) Metabotropic glutamate receptors depress afferent excitatory transmission in the rat nucleus-tractus-solitarii. J. Neurophysiol. 70: 2669-2672. Glaum S. R., Slater N. T., Rossi D. J. and Miller R. J. (1992) Role of metabotropic glutamate (ACPD) receptors at the parallel fiber-Purkinje cell synapse. 1. Neurophysiol. 68: 1453-1462.

Glaum S. R., Sunter D. C., Udvarhelyi P. M., Watkins J. C. and Miller R. J. (1993) The actions of phenyigly~ne derived metabotropic glutamate receptor antagonists on multiple (lS,3R)-ACPD responses in the rat nucleus of the tractus solitarius. Neuropharmacology 32: 1419-1425. Greene C., Schwindt P. and Crill W. (1992) Metabotropic receptor mediated after depolarization in neocortical neurons. Eur. J. Pharmac. 226: 279-280. Guerineau N. C., Gahwiler B. H. and Gerber U. (1994) Reduction of resting K’ current by metabotropic glutamate and muscarinic receptors in rat CA3 ceils-mediation by G-proteins, J. Physiol., Lond. 474: 27-33. Guiramand J., Vignes M. and Recasens M. (1991) A specific transduction mechanism for the glutamate action on phosphoinositide metabolism via the quisqualate metabotropic receptor in rat brain synaptoneurosomes. II: Calcium cadmium inhibition. J. Neurochem. 57: dependency, 1501-1509.

Harris E. W. and Cotman C. W. (1983) Effects of acidic amino acid antagonists on paired-pulse potentiation at the lateral perforant path. Exp. Bra& Res. 52: 455-460. Harvey J. and Colling~dge G. L. (1993) Signal tran~uction pathways involved in the acute potentiation of NMDA responses by lS,3R-ACPD in rat hippocampdl slices. Br. J. Pharmac. 109: 1085-1090. Hayashi Y., Tanabe Y., Aramori I., Masu M., Shimamoto K., Ohfune Y. and Nakanishi S. (1992) Agonist analysis of 2-(carboxycyclopropyl)glycine isomers for cloned metabotropic glutamate receptor subtypes expressed in Chinese hamster ovary cells. Br. J. Pharmac. 107: 539-543. Hayashi Y., Momiyama A., Takahashi T., Ohishi H., Ogawa-Meguro R., Shigemoto R., Mizuno N. and Nakanishi S. (1993) Role of a metabotropic glutamate receptor in synaptic modulation in the accessory olfactory bulb. Nature 366: 687-690. Hayashi Y., Sekiyama N., Nakanishi S., Jane D. E., Sunter D. C., Birse E. F., Udvarhelyi P. M. and Watkins J. C. (1994) Analysis of agonist and antagonist activities of phenylglycine derivatives for different cloned metabotropic glutamate receptor subtypes. J. Neurosci. 14: 3370-3377.

21

Herrero I., Miras-Portugal T. and Sanchez-Prieto J. (1992) Positive feedback of ~u~mate exocytosis by metabotropic presynaptic receptor stimulation. Nature 360: 163-166. Herrero I., Miras-Portugal M. T. and Sanchez-Prieto J. (1994) Rapid desensitization of the metabotropic glutamate receptor that facilitates glutamate release in rat cerebrocortical Nerve terminals. Eur. J. Neurosci. 6: 115-120. Hirano A. A. and MacLeish P. R. (1991) Glutamate and 2-amino-4-phosphonobutyrate evoke an increase in potassium conductance in retinal bipolar cells. Proc. Natn. Acad. Sci. U.S.A. 88: 805-809.

Hollmann M. and Heinemann S. (1994) Cloned glutamate receptors. A. Rev. Neurosci. 17: 31-108. Houamed K. M., Kuijper J. L., Gilbert T. L,, Haldeman B. A., O’Hara P. J., Mulvihill E. R., Almers W. and Hagen F. S. (1991) Cloning, expression, and gene structure of a G-protein-coupled glutamate receptor from rat brain. Science 252: 1318-1321. Hu G. Y. and Storm J. F. (1991) Excitatory amino acids acting on metabotropic glutamate receptors broaden the action potential in hippocampal neurons. Brain Res. 568: 339-344. Ikeda S. R., Lovinger D. M., MeCool B. A. and Lewis D. L. (1994) Heterologous expression and functional coupling of a metabotropic glutamate receptor (mGluR2) to N-type Ca2 + channels in adult rat sympathetic neurons. Sot. Neurosci. Abst. 20: 904.

Irving A. J., Collingridge G. L. and Schofield J. G. (1992) L-glutamate and acetylcholine mobilise Ca*+ from the same intracellular pool in cerebellar granule cells using transduction mechanisms with different Ca2+ sensitivities. Cell Calcium 13: 293-301. Ishida M., Saitoh T., Shimamoto K., Ohfune Y. and Shinozaki H. (1993) A novel metabotropic glutamate receptor agonist: marked depression of monosynaptic excitation in the newborn rat isolated spinal cord. Br. J. Pharmac. 109:1169-l 177. Ito M. (1989) Long-term depression. A. Rev. Neurosci. 12: 85-102. Ito M. and Karachot L. (1990a) Messengers mediating long-term desensitization in cerebellar Purkinje cells. NemoReport 1: 129-132.

Ito M. and Karachot L. (1990b) Receptor subtypes involved in, and time course of, the long-term desensitization of glutamate receptors in cerebellar Purkinje cells. Neurosci. Res. 8: 303-307.

Jane D. E., Jones P. L. S. J., Pook P. C.-K., Tse H.-W. and Watkins J. C. (1994) Actions of two new antagonists showing selectivity for different sub-types of metabotropic glutamate receptor in the neonatal rat spinal cord. Br. J. Pharmac. 112: 809-816.

Jensen A. M. and Chiu S. Y. (1990) Fluorescence measurement of changes in intracellular calcium induced by excitatory amino acids in cultured cortical astrocytes. J. Neurosci. 10: 1165-1175. Jensen A. M. and Chiu S. Y. (1991) Differential intracellular calcium responses to glutamate in type 1 and type 2 cultured brain astrocytes. J, Neurosci. 11: 1674-1684. Joels M., Yool A, J. and Gruol D. L. (1989) Unique properties of non-N-methyl-D-aspartate excitatory responses in cultured Purkinje neurons. Proc. Natn. Acad. Sci. U.S.A. 86: 34043408.

Kaba H., Hayashi Y., Higuchi T. and Nakanishi S. (1994) Induction of an olfactory memory by the activation of a metabotropic glutamate receptor. Science 265: 262-264.

J.-P. Pin and R. Duvoisin

22

Kano M. and Kato M. (1987) Quisqualate receptors are specifically involved in cerebellar synaptic plasticity. Nature 325: 276-279. Kato N. (1993) Dependence of long-term depression on postsynaptic metabotropic glutamate receptors in visual cortex. Proc. Natn. Acad. Sci. U.S.A. 90: 3650-3654. Kelso S. R., Nelson T. E. and Leonard J. P. (1992) Protein kinase C-mediated enhancement of NMDA currents by metabotropic glutamate receptors in Xenopus oocytes. J. Physiol., Lond. 449: 705-718.

Kemp M., Roberts P., Pook P., Jane D., Jones A., Jones P., Sunter D., Udvarhelyi P. and Watkins J. (1994) Antagonism of presynaptically mediated depressant responses and cyclic AMP-coupled metabotropic glutamate receptors. Eur. J. Pharmac. Molec. Pharmac. Sec. 266: 187-192. Kleuss C., Hescheler J., Ewe1 C., Rosenthal W., Schultz G. and Wittig B. (1991) Assignment of G-protein subtypes to specific receptors inducing inhibition of calcium currents. Nature 353: 43-48. Koerner J. F. and Cotman C. W. (1982) Responses of Schaffer collateral-CA1 pyramidal cell synapses of the hippocampus to analogues of acidic amino acids. Brain Res. 251: 105-l 1.5. Koerner J. F. and Johnson R. L. (1992) L-AP4 receptor ligands. In: Excitatory Amino Acid Receptors; Design of Agonists and Antagonists (Krogsgaard-Larsen P. and Hansen J. J., Eds), pp. 308-330. Ellis Horwood Limited, West Sussex, U.K. Koh J.-Y., Palmer E. and Cotman C. W. (1991a) Activation of the metabotropic glutamate receptor attenuates N-methyl-Daspartate neurotoxicity in cortical cultures. Proc. Natn. Acad. Sci. U.S.A. 88: 9431-9435. Koh J.-Y., Palmer E., Lin A. and Cotman C. W. (1991b) A metabotropic glutamate receptor agonist does not mediate neuronal degeneration in cortical cultures. Brain Res. 561: 338-343.

Kutsuwada T., Kashiwabuchi N., Mori H., Sakimura K., Kushiya E., Araki K., Meguro H., Masaki H., Kumanishi T., Arakawa M. and Mishina M. (1992) Molecular diversity of the NMDA receptor channel. Nature 358: 36-41. Lanthorm T. H., Ganong A. H. and Cotman C. W. (1984) 2-amino-4-phosphonobutyrate selectively blocks mossy fiber -CA3 responses in guinea pig but not rat hippocampus. Brain Res. 290: 174178.

Lester R. A. J. and Jahr C. E. (1990) Quisqualate receptor-mediated depression of calcium currents in hippocampal neurons. Neuron 4: 741-749. Linden D. J. (1994) Long-term synaptic depression in the mammalian brain. Neuron 12: 457472. Linden D. J. and Connor J. A. (1991) Participation of postsynaptic PKC in cerebellar long-term depression in culture. Science 254: 16561659. Linden D. J. and Connor J. A. (1992) Long-term depression of glutamate currents in cultured cerebellar Purkinje neurons does not require nitric oxide signalling. Eur. J. Neurosci. 4: 10-15.

Linden D. J., Dickinson M. H., Smeyne M. and Connor J. A. (1991) A long-term depression of AMPA currents in cultured cerebellar Purkinje neurons. Neuron 7: 81-89. Lingenhohl K., Olpe H. R., Bendali N. and Kniipfel T. (1993) Phenylglycine derivatives antagonize the excitatory response of Purkinje cells to lS,3R-ACPD-an in vivo and in vitro study. Neurosci. Res. 18: 229-234.

Llano I., Dreesen J., Kano M. and Konnerth A. (1991) Intradendritic release of calcium induced by glutamate in cerebellar Purkinje cells. Neuron 7: 577-583. Lombardi G., Alesiani M., Leonardi P., Cherici G., Pellicciari R. and Moroni F. (1993) Pharmacological characterization of the metabotropic glutamate receptor inhibiting D-[(3)H]aspartate output in rat striatum. Br. J. Pharmac. 110: 1407-1412. Lonart G., Alagarsamy S., Ravula R., Wang J. and Johnson K. M. (1992) Inhibition of the phospholipase C-linked metabotropic glutamate receptor by 2-amino-3-phosphonopropionate is dependent on extracellular calcium. J. Neurochem.

59: 772-775.

Loosfelt H., Misrahi M., Atger M., Salesse R., Hai-Luu Thi M. T. V., Jolivet A., Guiochon-Mantel A., Sar S., Jallal B., Garnier J. and Milgrom E. (1989) Cloning and sequencing of porcine LH-hCG receptor cDNA: variants lacking transmembrane domain. Science 24% 525-528. Lovinger D. M. (199 1) Trans- 1-aminocyclopentane- 1,3-dicarboxylic acid (t-ACPD) decreases synaptic excitation in rat striatal slices through a presynaptic action. Neurosci. Lett. 129: 17-21.

Lovinger D. M., Tyler E., Fidler S. and Merritt A. (1993) Properties of a presynaptic metabotropic glutamate receptor in rat neostriatal slices. J. Neurophysiol. 69: 12361244.

Lovinger D. M., Choi S. and McCool B. A. (1994a) Pharmacological properties of metabotropic glutamate receptors which inhibit synaptic transmission and voltagegated calcium currents. Sot. Neurosci. Abs. 20: 278. Lovinger D. M., Merritt A. and Reyes D. (1994b) Involvement of N- and non-N-type calcium channels in synaptic transmission at corticostriatal synapses. Neuroscience. 62: 31-40.

Manzoni 0. J. J., Finiels-Marlier F., Sassetti I., Bockaert J., LePeuch C. and Sladeczek F. A. J. (1990) The glutamate receptor of the Qp-type activates protein kinase-C and is regulated by protein kinase-C. Neurosci. Left. 109: 146151. Manzoni 0. J. J., Poulat F., Do E., Sahuquet A., Sassetti I., Bockaert J. and Sladeczek F. A. J. (1991) Pharmacological characterization of the quisqualate receptor coupled to phospholipase C (Qp) in striatal neurons. Eur. J. Pharmac. Molec. Pharmac.

Sec. 207: 23 1-241.

Manzoni 0. J., Weisskop M. G. and Nicoll R. A. (1994) MCPG antagonizes metabotropic glutamate receptors but not long-term potentiation in the hippocampus. Eur. J. Neurosci. 6: 105&1054.

Masu M., Tanabe Y., Tsuchida K., Shigemoto R. and Nakanishi S. (1991) Sequence and expression of a metabotropic glutamate receptor. Nature 349: 760-765. McCormick D. A. and Krosigk M. V. (1992) Corticothalamic activation modulates thalamic firing through glutamate “metabotropic” receptors. Proc. Natn. Acad. Sci. U.S.A. 89: 27742778.

McDonald J. W. and Schoepp D. D. (1992) The metabotropic excitatory amino acid receptor agonist lS,3R-ACPD selectively potentiates N-methyl-D-aspartate-induced brain injury. Eur. J. Pharmac. 215: 353-354. McDonald J. W., Fix A. S., Tizzano J. P. and Schoepp D. D. (1993) Seizures and brain injury in neonatal rats induced by lS,SR-ACPD, a metabotropic glutamate receptor agonist. J. Neurosci. 13: 44454455.

Metabotropic

glutamate receptors

McGuinness N., Anwyl R. and Rowan M. (1991) Trans-ACPD enhances long-term potentiation in the hippocampus. Eur. J. Pharmac. 197:231-232. McLennan H. and Wheal H. V. (1978) A synthetic conformationally restricted analogue of L-glutamic acid which acts as a powerful neuronal excitant. Neurosci. Lett. 8: 5 l-54.

Meguro H., Mori H., Araki K., Kushiya E., Kutsuwada T., Yamazaki M., Kumanishi T., Arakawa M., Sakimura K. and Mishina M. (1992) Functional characterization of a heteromeric NMDA receptor channel expressed from cloned cDNAs. Nature 357: 7&14. Milani D., Facci L., Guidol.in D., Leon A. and Skaper S. D. (1989) Activation of polyphosphoinositide metabolism as a signal-transducing system coupled to excitatory amino acid receptors in astroglial cells. Glia 2: 161-169. Milani D., Candeo P., Favaron M., Blackstone C. D. and Manev H. (1993) A subpopulation of cerebellar granule neurons in culture expresses a functional mGluR1 metabotropic glutamate receptor-effect of depolarizing growing conditions. Recept. Channel. 1: 243-250. Minakami R., Katsuki F. and Sugiyama H. (1993) A variant of metabotropic glutamate receptor subtype 5: an evolutionally conserved insertion with no termination codon. Biochem. Biophys. Res. Commun. 194: 622-627.

Monaghan D. T., Bridges R. J. and Cotman C. W. (1989) The excitatory amino acid receptors: their classes, pharmacology and distinct properties in the function of the central nervous system. A. Rev. Pharmac. Toxic. 29: 365402. Monyer H., Giffard R. G., Hartley D. M., Dugan L. L., Goldberg M. P. and Choi D. W. (1992) Oxygen or glucose deprivation-induced neuronal injury in cortical cell cultures is reduced by Tetanus toxin. Neuron 8: 967-973. Murphy S. N. and Miller IR. J. (1988) A glutamate receptor regulates CaZ+ mobilization in hippocampal neurons. Proc. Natn. Acad. Sci. U.S.A. 85: 8737-8741.

Murphy S. W. and Miller R. J. (1989) Two quisqualate receptors regulate Ca 2+ homeostass in hippocampal neurons. Molec. Pharmac. 35: 671-680.

Musgrave M. A., Ballyk B. A. and Goh J. W. (1993) Co-activation of metabotropic and NMDA receptors is required for LTP inducti’on. NeuroReport 4: 171-174. Nakagawa Y., Saitoh K., Ishihara T., Ishida M. and Shinozaki H. (1990) (2S,3S,4S)cc-(carboxycyclopropyl)glycine is a novel agonist of metabotropic glutamate receptors. Eur. J. Pharmac. 184:205-206.

Nakajima Y., Iwakabe H., Akazawa C., Nawa H., Shigemoto R., Mizuno N. and Nakanishi S. (1993) Molecular characterization of a novel retinal metabotropic glutamate receptor mGluR6 with a high agonist selectivity for L-2-amino-4-phosphonobutyrate. J. Biol. Chem. 268: 11,86811,873. Nakanishi S. (1992) Molecular diversity of glutamate receptors and implications for brain function. Science 258: 597603. Nawy S. and Copenhagen D. R. (1987) Multiple classes of glutamate receptor on depolarizing bipolar cells in retina.

23

Nicoletti F., Iadarola M. J., Wroblewski J. T. and Costa E. (1986a) Excitatory amino acid recognition sites coupled with inositol phospholipid metabolism: developmental changes and interaction with ul-adrenoreceptors. Proc. Natn. Acad. Sci. U.S.A. 83: 1931-1935.

Nicoletti F., Meek J. L., Iadarola M. J., Chuang D. M., Roth B. L. and Costa E. (1986b) Coupling of inositol phospholipid metabolism with excitatory amino acid recognition sites in rat hippocampus. J. Neurochem. 46: 4&46. Nicoletti F., Wroblewski J. T., Novelli A., Alho H., Guidotti A. and Costa E. (1986~) The activation of inositol phospholipid metabolism as a signal-transduction system for excitatory amino acids in primary cultures of cerebellar granule cells. J. Neurosci. 6: 1905-1911. Nicoletti F., Wroblewski J. T., Fadda E. and Costa E. (1988) Pertussis toxin inhibits signal transduction at a specific metabotropic glutamate receptor in primary cultures of cerebellar granule cells. Neuropharmacology 27: 55 l-556.

Nicoletti F., Magri G., Ingrao F., Bruno V., Catania M. V., Dell’Albani P., Condorelli D. F. and Avola R. (1990) Excitatory amino acids stimulate inositol phospholipid hydrolysis and reduce proliferation in cultured astrocytes. J. Neurochem. 54: 771-777.

Nomura A., Shigemoto R., Nakamura Y., Okamoto N., Mizuno N. and Nakanishi S. (1994) Developmentally regulated postsynaptic localization of a metabotropic glutamate receptor in rat rod bipolar cells. Cell 77: 361-369. O’Connor J. J., Rowan M. J. and Anwyl R. (1994) Long-lasting enhancement of NMDA receptor-mediated synaptic transmission by metabotropic glutamate receptor activation. Nature 367: 557-559.

O’Hara P. J., Sheppard P. O., Thogersen H., Venezia D., Haldeman B. A., McGrane V., Houamed K. M., Thomsen C., Gilbert T. L. and Mulvihill E. R. (1993) The ligand-binding domain in metabotropic glutamate receptors is related to bacterial periplasmic binding proteins. Neuron 11: 41-52. Okamoto N., Hori S., Akazawa C., Hayashi Y., Shigemoto R., Mizuno N. and Nakanishi S. (1994) Molecular characterization of a new metabotropic glutamate receptor mGluR7 coupled to inhibitory cyclic AMP signal transduction. J. Biol. Chem. 269: 1231-1236. Ostrowski J., Kjelsberg M. A., Caron M. G. and Lelkowitz R. J. (1992) Mutagenesis of the bZadrenergic receptor: how structure elucidates function. A. Rev. Pharmac. Toxic 32: 167-183. Otani S. and Ben-Ari Y. (1991) Metabotropic receptor-mediated long-term potentiation in rat hippocampal slices. Eur. J. Pharmac. 205: 325-326.

Pacelli G. J. and Kelso S. R. (1991) Trans-ACPD reduces multiple components of synaptic transmission in the rat hippocampus. Neurosci. Lett. 132: 267-269. Pate1 J., Moore W. C., Thompson C., Keith R. A. and Salama A. I. (1990) Characterization of the quisqualate receptor linked to phosphoinositide hydrolysis in neurocortical cultures. J. Neurochem. 54: 1461-1466. Pawloski-Dahm C. and Gordon F. J. (1992) Evidence for a Nature 325: 56-58. kinurenate-insensitive glutamate receptor in the nucleus Nawy S. and Jahr C. E. (1990a) Suppression by glutamate of cGMP-activated conductance in retina bipolar cells. Nature tractus solitarius. Am. J. Physiol. 363: 1611-1615. Pearce B., Albrecht J., Morrow C. and Murphy S. (1986) 346: 269-27 1. Astrocyte glutamate receptor activation promotes inositol Nawy S. and Jahr C. E. (1!)90b) Time-dependent reduction of phospholipid turnover and calcium flux. Neurosci. Lett. 72: glutamate current in retinal Bipolar cells. Neurosci. Lett. 108~ 279-283.

335-340.

J.-P. Pin and R. Duvoisin

24

Pickering D. S., Thomsen C., Suzdak P. D., Fletcher E. J., Robitaille R., Salter M. W., MacDonald J. F., Huang X. and Hampson D. R. (1993) A comparison of two alternatively spliced forms of a metabotropic glutamate receptor coupled to phosphoinositides turnover. J. Neurochem. 61: 85-92. Pin J.-P., Bockaert J. and Rtcasens M. (1984) The Caz+/C1--dependent L[3H]-glutamate binding: a new receptor or a particular transport process? FEBS Lett. 175: 31-36. Pin J.-P., Waeber C., Prezeau L., Bockaert J. and Heinemann S. F. (1992) Alternative splicing generates metabotropic glutamate receptors inducing different patterns of calcium release in Xenopus oocytes. Proc. Natn. Acad. Sci. U.S.A. 89: 10,331-10,335.

Pin J.-P., Fagni L. and Bockaert J. (1993) The metabotropic glutamate receptors: targets for new neuropharmacologically active drugs. Curr. Drugs: Neurodegeneratiue Disorders 1: 111-137. Pin J.-P., Joly C., Heinemann S. F. and Bockaert J. (1994) Domains involved in the specificity of G protein activation in phospholipase C coupled metabotropic glutamate receptor. EMBO J. 13: 342-348.

Pollak M. R., Brown E. M., Chou Y.-H. W., Hebert S. C., Marx S. J., Steinmann B., Levi T., Seidman C. E. and Seidman J. G. (1993) Mutations in the human Caz+-sensing receptor gene cause familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism. Cell 75: 1297-1303. Pook P., Birse E. F., Jane D. E., Jones A. W., Jones P. L. S., Mewett K. N., Sunter D. C., Udvarhelyi P., Wharton B. and Watkins J. C. (1993) Differential actions of the metabotropic glutamate receptor antagonists 4C-PG and aM4C-PG at L-APClike receptors in neonatal rat spinal cord. Br. J. Pharmac. 108: 87P. Prezeau L., Manzoni O., Hornburger V., Sladeczek F., Curry K. and Bockaert J. (1992) Characterization of a metabotropic glutamate receptor: direct negative coupling to adenylate cyclase and involvement of a pertussis toxin-sensitive G protein. Proc. Natn. Acad. Sci. U.S.A. 89: 804&8044. Prezeau L., Carette J., Helpap B., Curry K., Pin J.-P. and Bockaert J. (1994) Pharmacological characterization of metabotropic glutamate receptors in several types of brain cells in primary cultures. Molec. Pharmac. 45: 570-577. Rainnie D. G. and Shinnick-Gallagher P. (1992) Trans-ACPD and L-APB presynaptically inhibit excitatory glutamatergic transmission in the basolateral amygdala (BLA). Neurosci. Lett. 139: 87-91.

Recasens M., Guiramand J., Nourigat A., Sassetti I. and Devilliers G. (1988) A new quisqualate receptor subtype (sAA2) responsible for the glutamate-induced inositol phosphate formation in rat brain synaptoneurosomes. Neurochem. Int. 13: 463-467.

K. G. (1994) Riedel G., Wetzel W. and Reymann (R,S)-alpha-methyl-4-carboxyphenylglycine (MCPG) blocks spatial learning in rats and long-term potentiation in the dentate gyrus in vivo. Neurosci. L&t. 167: 141-144. Sacaan A. I. and Schoepp D. D. (1992) Activation of hippocampal metabotropic excitatory amino acid receptors leads to seizures and neuronal damage. Neurosci. Lett. 139: 77-82.

Sacaan A. I., Monn J. A. and Schoepp D. D. (1991) Intrastriatal injection of a selective metabotropic excitatory amino acid receptor agonist induces contralateral turning in the rat. J. Pharmac. Exp. Ther. 259: 1366-1370.

Sacaan A. I., Bymaster F. P. and Schoepp D. D. (1992)

Metabotropic glutamate receptor activation produces extrapyramidal motor system activation that is mediated by striatal dopamine. J. Neurochem. 59: 245-25 1. Sakurai M. (1990) Calcium is an intracellular mediator of the climbing fiber in induction of cerebellar long-term depression. Proc. Natn. Acad. Sci. U.S.A. 87: 383-385.

Salt T. E. and Eaton S. A. (1991) Excitatory action of the metabotropic excitatory amino acid receptor agonist, trans- 1-aminocyclopentane- 1,3-dicarboxylic acid (transACPD) in rat thalamic neurons. Eur. J. Neurosci. 3: 1104-1111. Saugstad J. A., Kinzie J. M., Mulvihill E. R., Segerson T. P. and Westbrook G. L. (1994) Cloning an expression of a new member of the L-2-amino-4-phosphonobutyric acid-class of metabotropic glutamate receptors. Molec. Pharmac. 45: 367-372.

Savarese T. M. and Fraser C. M. (1992) In vitro mutagenesis and the search for structure-function relationships among G protein-coupled receptors. Biochem. J. 283: 1-19. Sayer R. J., Schwindt P. C. and Crill W. E. (1992) Metabotropic glutamate receptor-mediated suppression of L-type calcium current in acutely isolated neocortical neurons. J. Neurophysiol. 68: 833-842.

Schoepp D. D. and Conn P. J. (1993) Metabotropic glutamate receptors in brain function and pathology. Trends Pharmac. 14: 13-20. Schoepp D. D. and Johnson B. G. (1993) Metabotropic glutamate receptor modulation of CAMP accumulation in the neonatal rat hippocampus. Neuropharmacology 32: 1359-1365.

Schoepp D. D., Bockaert J. and Sladeczek F. (1990) Pharmacological and functional characteristics of metabotropic excitatory amino acid receptors. Trends Pharmac. Sci. 11: 508-515. Schoepp D. D., Johnson B. G. and Monn J. A. (1992) Inhibition of cyclic AMP formation by a selective metabotropic glutamate receptor agonist. J. Neurochem. 58: 1184-l 186. Scott N. and Jahr C. E. (1991) cGMP-gated conductance in retinal bipolar cells is suppressed by the photoreceptor transmitter. Neuron 7: 677-683. Shibuki K. and Okase D. (1991) Endogenous nitric oxide release required for long-term depression in the cerebellum. Nature 349: 326328.

Shiells R. A. and Falk G. (1990) Glutamate receptors of rod bipolar cells are linked to a cyclic GMP cascade via a G-Protein. Proc. R. Sot. Lond., Biol242: 91-94. Shiells R. A. and Falk G. (1992a) The glutamate-receptor linked cGMP cascade of retinal On-bipolar cells is Pertussis and cholera toxin-sensitive. Proc. R. Sot. Lond., Biol. 247: 17-20. Shiells R. A. and Falk G. (1992b) Properties of the cGMP-activated channel of retinal on-bipolar cells. Proc. R. Sot. Lond., Biol. 247: 21-25.

Shigemoto R., Abe T., Nomura S., Nakanishi S. and Hirano T. (1994) Antibodies inactivating mGluR1 metabotropic glutamate receptor block long-term depression in cultured Purkinje cells. Neuron 12: 12451255. Siegelbaum S. A. and Kandel E. R. (1991) Learning related synaptic plasticity: LTP and LTD. Curr. Op. Neurobiol. 1: 113-120.

Siliprandi R., Lipartiti M., Fadda E., Sauter J. and Manev H. (1992) Activation of the glutamate metabotropic receptor protects retina against N-methyl-D-aspartate toxicity. Eur. J. Pharmac. 219: 173-l 74.

Metabotropic

glutamate receptors

Simoncini L., Haldeman B. A., Yamagiwa T. and Mulvihill E. (1993) Functional characterization of metabotropic glutamate receptor subtypes. 13iophy.s.J. 64: A84. Sladeczek F., Pin J.-P., Rtcasens M., Bockaert J. and Weiss S. (1985) Glutamate stimulates inositol phosphate formation in striatal neurones. Nature 317: 717-719. Slaughter M. M. and Miller R. F. (1981) 2-amino-4-phosphonobutyric acid: a new pharmacological tool for retina research. Science 211: 182185. Slaughter M. M. and Miller R. F. (1985) Identification of a distinct synaptic glutamate receptor on horizontal cells in mudpuppy retina. Nature 314: 9697. Sortino M. A., Nicoletti F. and Canonico P. L. (1991) Metabotropic glutamate receptors in rat hypothalamus: characterization and developmental profile. Deu. Brain Res. 61: 169-172.

Staub C., Vranesic I. and Knijpfel T. (1992) Responses to metabotropic glutamate receptor activation in cerebellar Purkinje cells: induction of an inward current. Eur. J. Neurosci. 4: 832-839.

Stella N., Tence M., Glowinski J. and Premont D. (1994) Glutamate-evoked release of arachidonic acid from Mouse brain astrocytes. J. Neurosci 14: 568-575. Sternweis P. C. (1994) The active role of fly in signal transduction. Curr. Cell Biol. 6: 198-203. Stratton K. R., Worley I’. F. and Baraban J. M. (1989) Excitation of hippocampal neurons by stimulation of glutamate Qp receptors. Eur. J. Pharmac. 173: 235-237. Stratton K. R., Worley I’. F. and Baraban J. M. (1990) Pharmacological characterization of phosphoinositidelinked glutamate receptor excitation of hippocampal neurons. Eur. J. Pharma~c. 186: 357-361. Sugiyama H., Ito I. and Hirono C. (1987) A new type of glutamate receptor linked to inositol phospholipid metabolism. Nature 325: 531-533. Suzdak P. D., Sheardown M. J. and Honore T. (1993) Characterization of the metabotropic glutamate receptor in mouse cerebellar granule cells: lack of effect of 2,3-dihydroxy6-nitro-7-sulphamoylbenzo(F)-quinoxaline (NBQX). Eur. J. Pharmac. Molec. Pharmltc. Sec. 245: 215-220.

Swartz K. .I. and Bean B. P. (1992) Inhibition of calcium channels in rat CA3 pyramidal neurons by a metabotropic glutamate receptor. J. Neurosci. 12: 43584371. Swartz K. J., Merrit A., Be.an B. P. and Lovinger D. M. (1993) Protein kinase C modulates glutamate receptor inhibition of Ca2+ channels and synaptic transmission. Nature 361: 165-168.

Takagi H., Takimizu H., Yoshioka T., Suzuki N., Ito E. and Kudo Y. (1992) Delayed appearance of G-protein coupled signal transduction system in developing cerebellar Purkinje cell dendrites. Neurosci. Res. 15: 206212. Takahashi K., Tsuchida K., Tanabe Y., Masu M. and Nakanishi S. (1993) Role of the large extracellular domain of metabotropic glutamate receptors in agonist selectivity determination. J. Biol. Chem. 268: 19,341-19,345. Tanabe Y., Masu M., I&ii T., Shigemoto R. and Nakanishi S. (1992) A family of metabotropic glutamate receptors. Neuron 8: 169-179.

Tanabe Y., Nomura A., Masu M., Shigemoto R., Mizuno N. and Nakanishi S. (1993) Signal transduction, pharmacological properties, and expression patterns of two rat metabotropic glutamate receptors, mGluR3 and mGluR4. J. Neurosci. 13: 1372-1378.

25

Thomsen C., Boel E. and Suzdak P. D. (1994) Action of phenylglycine analogs at subtypes of the metabotropic glutamate receptor family. Eur. J. Pharmac., Molec. Pharmac. Sec. 267: 77-84.

Thomsen C. and Suzdak P. D. (1993a) 4-Carboxy-3-hydroxyphenylglycine, an antagonist at type I metabotropic glutamate receptor. Eur. J. Pharmac., Molec. Pharmac. Sec. 245: 299-301.

Thomsen C. and Suzdak P. D. (1993b) Serine-O-phosphate has affinity for type IV, but not type I, metabotropic glutamate receptor. NeuroReport 4: 1099-l 101. Thomsen C., Kristensen P., Mulvihill E., Haldeman B. and Suzdak P. D. (1992) L-2-amino-4-phosphonobutyrate (L-AP4) is an agonist at the type IV metabotropic glutamate receptor which is negatively coupled to adenylate cyclase. Eur. J. Pharmac. 227: 361-362.

Thomsen C., Frandsen A., Suzdak P. D., Andersen C. F. and Schousboe A. (1993a) Effects of t-ACPD on neural survival and second messengers in cultured cerebral cortical neurones. NeuroReport 4: 1255-1258.

Thomsen C., Mulvihill E. R., Haldeman B., Pickering D. S., Hampson D. R. and Suzdak P. D. (1993b) A pharmacological characterization of the mGluR1cc subtype of the metabotropic glutamate receptor expressed in a cloned baby hamster kidney cell line. Brain Res. 619: 22-28. Trombley P. Q. and Westbrook G. L. (1992) L-AP4 inhibits calcium currents and synaptic transmission via a G-protein-coupled glutamate receptor. J. Neurosci. 12: 2043-2050. Trumpp-Kallmeyer S., Hoflack J., Bruinvels A. and Hibert M. (1992) Modeling of G-protein-coupled receptors: application to dopamine, adrenaline, serotonine, acetylcholine, and mammalian opsin receptors. J. Med. Chem. 35: 3448-3462. Uezono Y., Bradley J., Min C., McCarty N. A., Quick M., Riordan J. R., Chavkin C., Zinn K., Lester H. A. and Davidson N. (1993) Receptors that couple to two classes of G-proteins increase CAMP and activate CFTR expressed in Xenopus-oocytes. Recept. Channel 1: 233-241. Vecil G. G., Li P. P. and Warsh J. J. (1992) Evidence for metabotropic excitatory amino acid receptor heterogeneity: developmental and brain regional studies. J. Neurochem. 59: 252-258.

Vranesic I., Batchelor A., Gahwiler B. H., Garthwaite J., Staub C. and Knopfel T. (1991) Trans-ACPD-induced Ca2+ signals in cerebellar Purkinje cells. NeuroReport 2: 759-762.

Vranesic I., Staub C. and Kniipfel T. (1993) Activation of metabotropic glutamate receptors induces an outward current which is potentiated by methylxanthines in rat cerebellar Purkinje cells. Neurosci. Res. 16: 209-216. Vu Hai-Luu Thi M. T., Misrahi M., Houiller A., Jolivet A. and Milgrom E. (1992) Variant forms of the pig Lutropin/Choriogonadotropin receptor. Biochemistry 31: 8377-8383. Wang Z. and McCormick D. A. (1991) Activation of glutamate metabotropic muscarinic and alpha1 receptors controls firing mode in neocortical bursting neuron. Sot. Neurosci. Abs. 17: 254.

Weiss S., Ellis J., Hendley D. D. and Lenox R. H. (1989) Translocation and activation of protein kinase C in striatal neurons in primary culture: relationship to phorbol dibutyrate actions on the inositol phosphate generating system and neurotransmitter release. J. Neurochem. 52: 53&536.

26

J.-P. Pin and R. Duvoisin

Williams J., Errington M., Lynch M. and Bliss T. (1989) Arachidonic acid induces a long-term activity-dependent enhancement of synaptic transmission. Nature 341:739-742. Winder D. G. and Conn P. J. (1992) Activation of metabotropic glutamate receptors in the hippocampus increases cyclic AMP accumulation. J. Neurochem. 59: 375-378. Winder D. G. and Conn P. J. (1993) Activation of metabotropic glutamate receptors increases CAMP accumulation in hippocampus by potentiating responses to endogenous adenosine. J. Neurosci. 13: 384. Winder D. G., Smith T. S. and Conn P. J. (1993) Pharmacological differentiation of metabotropic glutamate receptors coupled to potentiation of CAMP responses and phosphoinositides hydrolysis. J. Pharmac. Exp. Ther. 266: 518-525.

Yamashita M. and WHssle H. (1991) Responses of rod bipolar

cells isolated from the rat retina to the glutamate agonist 2-amino-4-phosphonobutyric acid (APB). J. Neurosci. 11: 2372-2382.

Yamazaki M., Mori H., Araki K., Mori K. and Mishina M. (1992) Cloning, expression and modulation of mouse NMDA receptor subunit. FEBS Lett. 300: 39-45. Yuzaki M. and Mikoshiba K. (1992) Pharmacological and immunocytochemical characterization of metabotropic glutamate receptors in cultured Purkinje cells. J. Neurosci. 12: 42534263.

Zheng F. and Gallagher J. P. (1992a) Metabotropic glutamate receptor agonists potentiate a slow depolarization in CNS neurons. NeuroReport 3: 622-624. Zheng F. and Gallagher J. P. (1992b) Metabotropic glutamate receptors are required for the induction of long-term potentiation. Neuron 9: 163-l 72.