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Keywords:

  • Metabotropic glutamate receptors;
  • Neurotransmitters;
  • Synaptic transmission;
  • Excitatory amino acids;
  • Inhibitory amino acids;
  • Monoamines;
  • Neuropeptides

Abstract

  1. Top of page
  2. Abstract
  3. mGlu SUBTYPES, PHARMACOLOGY, AND TRANSDUCTION MECHANISMS
  4. SYNAPTIC LOCALIZATION OF mGlu RECEPTORS
  5. GLUTAMATE/ASPARTATE
  6. GABA
  7. DOPAMINE
  8. SEROTONIN (5-HT)
  9. MISCELLANEOUS
  10. Cholecystokinin (CCK)
  11. CONCLUSIONS AND FUTURE PERSPECTIVES

Abstract: The G protein-coupled metabotropic glutamate (mGlu) receptors are differentially localized at various synapses throughout the brain. Depending on the receptor subtype, they appear to be localized at presynaptic and/or postsynaptic sites, including glial as well as neuronal elements. The heterogeneous distribution of these receptors on glutamate and nonglutamate neurons/cells thus allows modulation of synaptic transmission by a number of different mechanisms. Electrophysiological studies have demonstrated that the activation of mGlu receptors can modulate the activity of Ca2+ or K+ channels, or interfere with release processes downstream of Ca2+ entry, and consequently regulate neuronal synaptic activity. Such changes evoked by mGlu receptors can ultimately regulate transmitter release at both glutamatergic and nonglutamatergic synapses. Increasing neurochemical evidence has emerged, obtained from in vitro and in vivo studies, showing modulation of the release of a variety of transmitters by mGlu receptors. This review addresses the neurochemical evidence for mGlu receptor-mediated regulation of neurotransmitters, such as excitatory and inhibitory amino acids, monoamines, and neuropeptides.

The abundance of glutamatergic pathways and the ubiquitous distribution of glutamate receptors in the CNS are indicative of the critical role glutamate plays in brain function (Nakanishi, 1992, 1994; Conn and Pin, 1997; Nakanishi et al., 1998). Synaptic release of glutamate not only directly mediates glutamatergic function, but can also regulate the activity of neurons comprising other neurotransmitters via excitatory glutamatergic inputs to these neurons. Thus, neuronal signaling can be modified by presynaptic glutamate receptors that directly regulate glutamate release, or by glutamate receptors localized postsynaptically on glutamatergic and nonglutamatergic neurons. Glutamate binds to both ion channel-associated (ionotropic) and G protein-coupled [metabotropic (mGlu)] receptor types (see Nakanishi, 1992, 1994; Conn and Pin, 1997; Nakanishi et al., 1998), which mediate fast excitatory and second messenger-evoked transmission, respectively. This review will consider the roles of mGlu receptors in the modulation of neurotransmitter release.

Although electrophysiological studies have provided a great deal of evidence for the regulation of synaptic transmission by mGlu receptors, this discussion will be principally restricted to neurochemical and biochemical evidence for the modulation of release. Moreover, a comprehensive review of electrophysiological properties of mGlu receptors has been published recently (Anwyl, 1999). However, in the interest of supporting the neuro-chemical studies, some of the electrophysiological actions of the more selective mGlu receptor agonists are mentioned briefly. This discussion will outline pharmacology and synaptic localization of mGlu receptors, and then current evidence for the regulation of various neurotransmitters will be presented.

mGlu SUBTYPES, PHARMACOLOGY, AND TRANSDUCTION MECHANISMS

  1. Top of page
  2. Abstract
  3. mGlu SUBTYPES, PHARMACOLOGY, AND TRANSDUCTION MECHANISMS
  4. SYNAPTIC LOCALIZATION OF mGlu RECEPTORS
  5. GLUTAMATE/ASPARTATE
  6. GABA
  7. DOPAMINE
  8. SEROTONIN (5-HT)
  9. MISCELLANEOUS
  10. Cholecystokinin (CCK)
  11. CONCLUSIONS AND FUTURE PERSPECTIVES

mGlu receptors are classified into three subgroups (groups I-III). This classification was determined by the similarities in coupling mechanisms, molecular structure and homology of sequences, and the pharmacology of the receptors (Nakanishi, 1994) (see Table 1). Group I receptors (mGlu1 and mGlu5, and their splice variants) are positively linked to phospholipase C and, therefore, direct activation of mGlu1/5 receptors results in increased phosphoinositide turnover. Group I mGlu receptors are selectively activated by 3,5-dihydroxyphenylglycine (DHPG) or quisqualate. In contrast, the remaining receptors in the mGlu family are negatively coupled to adenylyl cyclase, in that upon activation they inhibit forskolin-stimulated cyclic AMP formation. These receptors are divided into groups II (mGlu2 and mGlu3) and III (mGlu4, mGlu6, mGlu7, and mGlu8, and their splice variants), and are selectively activated by (1S,2S,5R,6S)-(+)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (LY354740) or (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylic acid (LY379268), and by L-(+)-2-amino-4-phosphonobutyric acid (L-AP4), respectively.

Table 1. Classification of mGlu receptor subtypes
 SubtypeTransduction mechanism
  1. PI, phosphoinositide.

Group ImGlu1[UPWARDS ARROW] PI hydrolysis
 mGlu5[UPWARDS ARROW] PI hydrolysis
Group IImGlu2[DOWNWARDS ARROW] Cyclic AMP
 mGlu3[DOWNWARDS ARROW] Cyclic AMP
Group IIImGlu4[DOWNWARDS ARROW] Cyclic AMP
 mGlu6[DOWNWARDS ARROW] Cyclic AMP
 mGlu7[DOWNWARDS ARROW] Cyclic AMP
 mGlu8[DOWNWARDS ARROW] Cyclic AMP

A brief overview of only the various agonists and antagonists of mGlu receptor subtypes that have been used to study the role of mGlu receptors in neurotransmitter release is presented in Table 2. There are currently other more potent and selective mGlu receptor agents known, but these are not discussed in this article due to the lack of published neurochemical data regarding their effects in neurotransmitter release experiments. However, for detailed reviews of the general pharmacology for these receptors, see the recent articles by Pin et al. (1999) and Schoepp et al. (1999).

Table 2. Potencies of agonists (EC50 values) and antagonists (IC50 values) at mGlu receptor clones (expressed in μM)
 Group IGroup IIGroup III
CompoundmGlu1mGlu5mGlu2mGlu3mGlu4mGlu6mGlu7mGlu8
  1. Data are from Pin et al. (1999) and Schoepp et al. (1999). ant, antagonist activity; weak ag, weak agonist activity; —, not tested. Abbreviations of compounds are as follows: 1S,3R-ACPD, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid; 1S,3S-ACPD, (1S,3S)-1-aminocyclopentane-1,3-dicarboxylic acid; L-CCG-I, (2S,1′S,2′S)-2-(carboxycyclopropyl)glycine; DHPG, 3,5-dihydroxyphenylglycine; LY379268, (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylic acid; LY354740, (1S,2S,5R,6S)-(+)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid; 2R,4R-APDC, (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylic acid; DCG-IV, (2′S,2′R,3′R)-2-(2′,3′-dicarboxycyclopropyl)glycine; L-AP4, L-(+)-amino-4-phosphonobutyric acid; L-SOP, L-serine O-phosphate; MCPG, α-methyl-4-carboxyphenylglycine; AIDA, 1-aminoindan-1,5-dicarboxylic acid; 4CPG, 4-carboxyphenylglycine; L-AP3, L-(+)-2-amino-3-phosphonopropionic acid; MPPG, α-methyl-4-phosphonophenylglycine.

Nonselective agonists
Glutamate1-1841-110.3-200.04-53-385-36>1,0003-11
1S,3R-ACPD 5-2145-1225-569-86012-22018-60>1,00045-166
1S,3S-ACPD 42->30069->3005-2930-5675082
L-CCG-I2-503-170.1-0.80.4-11-500.6-647-2300.4-3
Group I agonists
Quisqualate0.03-170.05-0.3>10010-220>100>1,000>1,000>100
DHPG6-600.7-20>100>100>1,000>1,000>1,000
Group II agonists
LY379268>100>1000.0030.005210.4>1002
LY354740>100>1000.010.02-0.1>100>1002-36
2R,4R-APDC >100>1000.4-100.4-5>100>100>100>100
DCG-IV>1,000(ant)0.1-0.30.1-0.2(ant)(ant)(ant)(ant)
Group III agonists
L-AP4>1,000>1,000>1,000>1,0000.2-10.2-0.9>1000.06-0.9
L-SOP(ant?)(ant?)1-40.4-3>1000.3-2
Antagonists
MCPG40-300100-50015-350∼300 >1,000>100>1,000300-1,000
AIDA3-214>100>1,000>1,000
4CPG4-163(weak ag?)>100>1,000
L-AP3300-1,000∼300 7-11
MPPG>1,000>1,00011-30022-1104-48030020-50

The nonselective agonists (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), (1S,3S)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3S-ACPD), and (2S,1′S,2′S)-2-carboxycyclopropyl)glycine (L-CCG-I) have agonist activity at all three groups, and although the compounds' activities are generally restricted to mGlu receptors, they cannot distinguish between subgroups. Quisqualate, although potent at group I mGlu receptors, also has activity at α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors, but DHPG is very selective for mGlu1 and mGlu5. Although (2S,2′R,3′R)-2-(2′,3′-dicarboxycyclopropyl)glycine (DCG-IV) has high potency at mGlu2/3 receptors, it also has antagonist actions at group III, and acts as an agonist at NMDA receptors. However, (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylic acid (2R,4R-APDC) is very selective for group II with little activity at other mGlu receptors. Moreover, LY354740 and LY379268 are very potent at group II receptors, and although these compounds have some affinity at group III receptors, they are the first available systemically active group II agonists. L-AP4 and L-serine O-phosphate (L-SOP) are both potent and selective compounds at group III receptors. It is interesting that the potency of the agonists described above and that of the endogenous agonist glutamate at mGlu7 receptors are much lower than at the other members of group III. This decreased affinity for mGlu7 receptor may be related to the location of mGlu7 receptors at the synapse (see the next section).

Of the five antagonists shown in Table 2, the potencies at all eight mGlu receptor clones have been published for only α-methyl-4-carboxyphenylglycine (MCPG). MCPG, although widely used, lacks potency and like L-(+)-2-amino-3-phosphonopropionic acid (L-AP3) has antagonist actions at both group I and II receptors, and α-methyl-4-phosphonophenylglycine (MPPG) blocks both group II and III receptors. Although 1-aminoindan-1,5-dicarboxylic acid (AIDA) and 4-carboxyphenylglycine (4CPG) are claimed to be selective antagonists for mGlu1 receptors, the activity of the compounds at all the clones has not yet been reported (Schoepp et al., 1999).

SYNAPTIC LOCALIZATION OF mGlu RECEPTORS

  1. Top of page
  2. Abstract
  3. mGlu SUBTYPES, PHARMACOLOGY, AND TRANSDUCTION MECHANISMS
  4. SYNAPTIC LOCALIZATION OF mGlu RECEPTORS
  5. GLUTAMATE/ASPARTATE
  6. GABA
  7. DOPAMINE
  8. SEROTONIN (5-HT)
  9. MISCELLANEOUS
  10. Cholecystokinin (CCK)
  11. CONCLUSIONS AND FUTURE PERSPECTIVES

Although the precise localization of each subtype of mGlu receptor in specific areas of the brain is beyond the scope of this review, the localization of the receptors at the synapse is pertinent to a discussion of the roles mGlu receptors play in neurotransmitter release (see Ottersen and Landsend, 1997).

Although an early report describing immunoperoxidase localization studies of mGlu1a revealed peroxidase reaction product on postsynaptic specializations (Martin et al., 1992), the majority of subsequent studies have shown that generally group I mGlu receptors are located away from active zones. Immunogold localization of mGlu1a, mGlu1b, mGlu1c, and mGlu5 showed the highest density of the receptors located outside the membrane specializations (Baude et al., 1993; Nusser et al., 1994; Luján et al., 1996, 1997) and appeared to be restricted to postsynaptic terminals (Shigemoto et al., 1997).

Similarly, Petralia et al. (1996) showed that mGlu2/3 staining associated with postsynaptic densities was concentrated on the periphery of the longer specializations, or along the neighboring membrane. Furthermore, work by Shigemoto et al. (1997) supported the perisynaptic localization of group II mGlu receptors and demonstrated that mGlu2 immunoreactivity was located primarily in the presynaptic terminal of the hippocampus. It is interesting that the distribution of mGlu2 receptors in the dendritic plasma membrane of cerebellar Golgi cells was not significantly different from a simulated random distribution and was not closely associated with glutamatergic synapses (Luján et al., 1997). mGlu3 receptors are highly expressed in glial cells (Ohishi et al., 1993, 1994; Mineff and Valtschanoff, 1999), and although the role of these glial receptors is as yet undetermined, given the major contribution of glia to the uptake and synthesis of glutamate, activation of these receptors is likely to result in significant functional effects (see Winder and Conn, 1996).

It is interesting that group III receptors (mGlu4a, mGlu7a, mGlu7b, and mGlu8) seemed to be located in or near presynaptic active zones (Shigemoto et al., 1997). mGlu7 immunoreactivity in the hippocampus was shown to be localized exclusively at asymmetrical synapses (i.e., glutamatergic terminals), whereas mGlu4a was located presynaptically at both asymmetrical and symmetrical (i.e., nonglutamatergic terminals) synapses (Bradley et al., 1996). Therefore, mGlu4 receptors, in addition to regulating glutamatergic transmission, might act as presynaptic heteroceptors, having the ability to regulate directly the release of other neurotransmitters. A role for mGlu4 receptors in this regard is possible due to the comparatively high potency of glutamate at these receptors (generally 3-40 μM; see Table 2); mGlu4 receptors could be activated by relatively low concentrations of glutamate spilling over from glutamatergic synapses. Moreover, the lack of mGlu7 receptors localized on nonglutamatergic synapses is probably to be expected, as it is unlikely that “spillover” glutamate could reach sufficient concentrations to activate mGlu7 receptors (IC50 value of glutamate at mGlu7 receptors is usually >500 μM; see Table 2). It is interesting that Bradley et al. (1996) also identified some postsynaptic localization of mGlu7 receptors. However, this observation is in contrast to the report by Shigemoto et al. (1997) that revealed that immunoreactivity for group III receptors was localized predominantly in the presynaptic elements of the hippocampus. Discrepancies between some of the studies discussed in this section might be explained by methodological differences. The preembedding peroxidase-antiperoxidase technique is a sensitive procedure, but is dependent on the reaction product diffusing uniformly throughout the tissue. In contrast to the immunogold method, there might be significant differences in migration and nonspecific trapping of reaction product, possibly causing extraneous positive results (for review, see Ottersen and Landsend, 1997).

Based on current knowledge, Fig. 1 shows the hypothetical localizations of mGlu receptor subtypes at a theoretical synapse containing all the currently identified mGlu receptors. However, it is unlikely that all mGlu receptors are present at the same synapse in vivo (also see Shigemoto et al., 1997), as the expression of presynaptic mGlu receptors can vary considerably between terminals, even those from the same axon. A number of reports have indicated that the presynaptic mGlu receptor distribution can depend on the identity of the postsynaptic target. Shigemoto et al. (1996, 1997) showed that the density of mGlu7 labeling in the active zone of the presynaptic membrane was much higher in those terminals that made synapses with mGlu1a-immunopositive dendritic shafts. Moreover, Scanziani et al. (1998) demonstrated that the pharmacological and physiological properties of Schaffer collateral terminals from the same axon are different, and seem to depend on the target cells that they innervate. Specifically, L-AP4 reduced transmitter release from Schaffer collateral terminals that made synapses with interneurons, but not those terminals sharing a synapse with CA1 pyramidal cells (Scanziani et al., 1998). Therefore, it appears that the presynaptic properties of receptors modulating transmitter release are also determined by the target cell and not exclusively by the presynaptic neuron.

image

Figure 1. Synaptic localization of glutamate receptors at a theoretical CNS synapse as shown by immunocytochemical studies. Although group II mGlu receptors (particularly mGlu2) are located on both sides of the synaptic cleft, group I (mGlu1 and mGlu5) and group III (mGlu4, mGlu7, and mGlu8) receptors are localized post-and presynaptically, respectively. Only mGlu7 receptors are situated in the active zone of the presynaptic terminal. In contrast, the other receptor subtypes, in particular mGlu2, are located away from the site of release. Note that some biochemical and electrophysiological studies indicate that group I mGlu receptors might also be located presynaptically (see Table 3). The localization of mGlu3 can be questioned as most antibodies also labeled mGlu2.

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The perisynaptic location of mGlu2/3 receptors in comparison with the location of mGlu7 receptors at the active zone of the terminal might go some way to explaining the vast differences in the affinities of these receptors for their endogenous ligand. Glutamate is highly potent at group II receptors (generally in the range of 0.3-20 μM; see Table 2), but the EC50 value of glutamate at mGlu7 receptors has been reported to be >500 μM (see Table 2). However, it seems that under conditions favoring glutamate release from the terminal, concentrations of glutamate at the site of release would be sufficient to activate mGlu7 receptors located in the same area. The preterminal localization of mGlu2 receptors at the hippocampal mossy fiber synapse observed by Yokoi et al. (1996) might predict that the receptor would be activated only under conditions of high neurotransmitter release, and indeed this has been supported by data from Scanziani et al. (1997). In this regard, the mGlu receptor antagonist MCPG was without effect on electrically evoked transmitter release at hippocampal mossy fiber synapses, unless release was evoked by high-frequency stimulation. Only under these conditions of enhanced release (or during uptake blockade), glutamate concentrations in the synapse were sufficiently high to spread from the site of release and activate mGlu2 receptors located perisynaptically, leading to an inhibition of release. Therefore, it seems that although perisynaptic mGlu2 receptors are possibly inactive under normal conditions, under high-frequency stimulation, activation of these receptors might prevent pathologically high levels of glutamate from accumulating in the synaptic cleft (Fig. 2; see also review by Forsythe and Barnes-Davies, 1997).

image

Figure 2. Activation of perisynaptically localized mGlu2 receptors during conditions of high neuronal stimulation. During normal synaptic activity, glutamate concentrations do not reach sufficient levels for diffusion from the site of release to activate mGlu2 located away from the active zone. However, under conditions of greatly enhanced release, glutamate is able to diffuse to perisynaptic sites and activate mGlu2 receptors, ultimately resulting in an inhibition of glutamate release back to “normal” physiological levels.

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Localization of group I mGlu receptors on the postsynaptic terminal seems to be under the regulation of a small family of “Homer” proteins that contains a “PDZ-like” protein-interaction domain (Brakeman et al., 1997). Ciruela et al. (1999) showed that coexpression of mGlu1a and Homer-1a in human embryonic kidney 293 cells increased both the level of receptor expression at the cell surface and the hydrolysis of phosphoinositides in response to agonist stimulation. The Homer family binds selectively to mGlu1 and mGlu5 receptors and appears to regulate the positioning of these receptors close to second messengers (specifically, the inositol phosphate pathway) necessary for signal transduction in the postsynaptic terminal (Ciruela et al., 1999; Worley, 1999).

GLUTAMATE/ASPARTATE

  1. Top of page
  2. Abstract
  3. mGlu SUBTYPES, PHARMACOLOGY, AND TRANSDUCTION MECHANISMS
  4. SYNAPTIC LOCALIZATION OF mGlu RECEPTORS
  5. GLUTAMATE/ASPARTATE
  6. GABA
  7. DOPAMINE
  8. SEROTONIN (5-HT)
  9. MISCELLANEOUS
  10. Cholecystokinin (CCK)
  11. CONCLUSIONS AND FUTURE PERSPECTIVES

The high level of expression of mGlu receptors at various excitatory synapses throughout the CNS implies that these receptors play an important role in the release of glutamate. Indeed, all three groups of mGlu receptors have been implicated in the inhibition of excitatory synaptic transmission at specific synapses. The selective group II agonist DCG-IV reduced excitatory synaptic transmission in the medial (Kilbride et al., 1998) and lateral (Bushell et al., 1996) perforant path of the dentate gyrus, striatum (Lovinger and McCool, 1995), olfactory bulb (Hayashi et al., 1993), and spinal cord motoneurons (Ishida et al., 1993). Furthermore, LY354740 also reduced glutamatergic transmission in the medial (Kilbride et al., 1998) and lateral (Bushell et al., 1996) perforant path of the dentate gyrus and in the medial prefrontal cortex (Marek et al., 2000). The inhibition of transmission at glutamatergic synapses by L-AP4, a selective agonist for group III mGlu receptors, has been well characterized. Inhibitory effects have been observed in the hippocampal CA1 region (Baskys and Malenka, 1991; Gereau and Conn, 1995), the medial and lateral perforant path of the dentate gyrus (Koerner and Cotman, 1981), striatum (Pisani et al., 1997), nucleus tractus solitarius (Glaum and Miller, 1993), olfactory bulb (Trombley and Westbrook, 1992), nucleus accumbens (Manzoni et al., 1997), and locus coeruleus (Dubé and Marshall, 1997). Although the majority of studies have observed inhibitory effects of group II and III receptor agonists, some electrophysiological studies have also shown that group I agonists can reduce excitatory transmission in the CA1 area of the hippocampus (Baskys and Malenka, 1991; Gereau and Conn, 1995; Manzoni and Bockaert, 1995).

Much of the electrophysiological evidence for mGlu receptor control of excitatory synaptic transmission has demonstrated an inhibitory effect of mGlu receptor activation. However, as discussed below, other studies using electrophysiology and biochemistry have shown that group I mGlu receptors can facilitate the release of glutamate.

Synaptosomes/brain slices

The augmentation of excitatory synaptic transmission by group I mGlu receptors has been demonstrated in studies using preparations of nerve terminals (synaptosomes). In the presence of low concentrations of arachidonic acid, the nonselective mGlu receptor agonist, 1S,3R-ACPD enhanced glutamate release from cortical synaptosomes evoked by the K+-channel blocker 4-aminopyridine (4-AP), which is thought to result in a more physiological depolarization (Herrero et al., 1992; Reid et al., 1999). However, 1S,3R-ACPD had no effect following suboptimal “clamped” depolarization caused by 10 mM KCl (Herrero et al., 1992). In contrast to some of the earlier studies, a recent report has shown that the facilitation of 4-AP-evoked glutamate release in cortical synaptosomes by group I mGlu receptors is not necessarily dependent on the presence of exogenously applied arachidonic acid (Reid et al., 1999). Although the increase of the 4-AP response by 1S,3R-ACPD occurred only in the presence of arachidonic acid, DHPG augmented 4-AP (200 μM)-evoked glutamate release in the absence of arachidonate. Furthermore, in contrast to the actions of 1S,3R-ACPD, DHPG also increased glutamate release evoked by the clamped depolarization caused by 30 or 50 mM KCl. The effects of 1S,3R-ACPD might be due to the nonselectivity of the compound, in that it has agonist activity at all three subgroups of mGlu receptors. In support of this, in the absence of arachidonic acid, the selective group II and III agonists DCG-IV and L-AP4 inhibited 4-AP-evoked glutamate release (Reid et al., 1999).

It is interesting that the receptor mediating the facilitation of glutamate release undergoes a rapid desensitization. The group I mGlu receptor agonist DHPG increased 4-AP (50 μM)-evoked glutamate release from cortical or hippocampal synaptosomes by approximately twofold, but had no effect on the response to 1 mM 4-AP (Herrero et al., 1998; Rodriguez-Moreno et al., 1998). When a pulse of DHPG was given 5 min later, the compound had no effect on the response to the submaximal concentration of 4-AP, but inhibited glutamate release stimulated by 1 mM 4-AP. This reduction in glutamate release might occur via a membrane-delimited block of voltage-dependent Ca2+ channel (VDCC) activity. The facilitatory effect was recovered 30 min later, but at this time the inhibitory response was lost, indicating that the desensitization of the receptor mediating the augmentation of glutamate release was attenuated.

These experiments have been verified by electrophysiological studies using brain slices. Excitatory postsynaptic currents evoked by electrical stimulation of Schaffer collaterals and recorded from CA1 pyramidal cells were inhibited by DHPG (Rodriguez-Moreno et al., 1998). However, when the tonic actions of endogenous glutamate were prevented, mGlu activation by DHPG resulted in a facilitation of synaptic transmission followed by an inhibition. It was concluded that mGlu receptors have a dual action on glutamate release, switching from facilitatory to inhibitory upon receptor desensitization that could be triggered by low concentrations of glutamate. This process could prevent excessive glutamate accumulation and, therefore, limit possible excitotoxic effects of high concentrations of glutamate.

The identity of the group I receptor mediating this response is unknown as immunocytochemical studies have failed to detect the localization of mGlu1 or mGlu5 receptors presynaptically in the CA1 area of the hippocampus (Baude et al., 1993; Luján et al., 1996; Shigemoto et al., 1997). Furthermore, Sistiaga et al. (1998) have demonstrated that the enhancement of glutamate release by DHPG and the switch from facilitation to inhibition were still observed in cortical synaptosomes prepared from mGlu1 knock-out mice. This might suggest that mGlu5 receptors mediate the facilitatory response, but studies in which a clear mGlu5 postsynaptic labeling was observed failed to locate mGlu5 presynaptically (Martin et al., 1992; Baude et al., 1993; Nusser et al., 1994; Luján et al., 1996; Shigemoto et al., 1996, 1997). In spite of the fact that the receptor mediating the facilitation of glutamate release is, as yet, unknown, many electrophysiological data support the existence of presynaptic mGlu receptors that are activated by selective group I receptor agonists (also see Anwyl, 1999). For example, DHPG inhibited transmission at Schaffer collateral-CA1 (Gereau and Conn, 1995) and CA3-CA1 pyramidal cell synapses (Manzoni and Bockaert, 1995) in slices of hippocampus, by an apparent presynaptic mechanism. As yet, the identity of the receptors mediating these responses is unknown.

Nevertheless, there is an abundance of functional evidence confirming the roles of group II and III mGlu receptors that are localized at presynaptic terminals. Allen et al. (1999) recently showed that 100 nM LY354740 attenuated injury-induced increases in glutamate levels 2 h after injury in cortical neuronal—glia cultures. This decrease was associated with attenuated injury-induced neuronal cell death (Allen et al., 1999), further implicating mGlu receptors as targets for neuro-protection (Bond et al., 1998).

Perhaps the most well characterized effect of selective mGlu receptor agonists on glutamate release is the inhibition of release mediated by group III mGlu receptors. The selective group III agonist L-AP4 inhibited glutamate release from hippocampal nerve terminals, prepared from 3-week-old rats, maximally stimulated by 1 mM 4-AP with an IC50 value of 190 μM (Rodriguez-Moreno et al., 1998). These data possibly implicate the involvement of mGlu7 receptors at which L-AP4 has a potency in the high micromolar range (see Table 2). An inhibitory effect of L-AP4 has also been reported in adult rat striatal synaptosomes (East et al., 1995). A reduction of 4-AP (2 mM)-stimulated glutamate release by L-AP4 reached a maximum of 60% inhibition with a potency (IC50 value of 0.2 μM) suggesting that the effect might be mediated via mGlu4 or mGlu8 receptors that can be activated by low micromolar concentrations of L-AP4 (East et al., 1995). Vazquez and Sanchez-Prieto (1997) showed that L-AP4 also reduced glutamate release from synaptosomes evoked by 30 mM KCl, showing that the effect could be mediated via regulation of VDCC activity.

In addition to the effects on glutamate release, the actions of mGlu receptor activation on the release of other excitatory amino acid neurotransmitters have also been determined using synaptosomal preparations. Studies by Attwell et al. (1995, 1998a,b) using preloaded cerebrocortical synaptosomes showed that 1S,3S-ACPD and the selective mGlu2/3 agonists DCG-IV and 2R,4R-APDC inhibited veratridine (50 μM)-stimulated D-[3H]-aspartate release.

A number of reports have claimed that D-[3H]aspartate release from preloaded brain slices is also modulated by mGlu receptor activation. The nonspecific mGlu agonists L-CCG-I and 1S,3R-ACPD and the group I selective agonist quisqualate inhibited KCl (30 mM)-induced output of D-[3H]aspartate from adult rat striatal slices (Lombardi et al., 1993). Such an inhibitory effect of 1S,3R-ACPD on neurotransmitter release supports electrophysiological studies demonstrating a reduction of synaptic transmission between corticostriatal glutamatergic afferents and postsynaptic caudate neurons (Lovinger, 1991; Calabresi et al., 1992).

Work by the same authors has shown that, in contrast to the inhibition evoked by 1S,3R-ACPD of D-[3H]aspartate release in the striatum, 1S,3R-ACPD potentiated KCl (30 mM)-evoked release of [3H]glutamate and [3H]aspartate by ∼50-60% over basal in cortical slices. It is probable that these contrasting effects of 1S,3R-ACPD reflect the activation of group I and II mGlu receptors in the cortex and striatum, respectively. The augmentation of the KCl response seemed to be dependent on the presence of endogenous fatty acids, as the potentiation was prevented by inhibitors of fatty acid formation and phospholipase A2 (Lombardi et al., 1994, 1996). Although these results at first seem to concur with the studies showing augmentation of glutamate release in synaptosomal preparations in the presence of low concentrations of arachidonic acid, different stimuli were used by the two groups to evoke neurotransmitter release. Although Lombardi et al. (1994, 1996) used 30 mM KCl in their study, this clamped depolarization was found to be unresponsive to either 1S,3R-ACPD or protein kinase C manipulation in the Herrero et al. (1992) study using synaptosomes. However, this discrepancy might be due to time-course or methodological differences, such as measurement of prelabeled versus endogenous amino acids or the use of different preparations (Lombardi et al., 1993, 1994, 1996). In particular, there is likely to be a considerable involvement of glial cells in the responses in brain slices, but not the synaptosome preparations (Winder and Conn, 1996).

Glial cells

In addition to release via exocytosis, other sources of increased neurotransmitter could include glial stores or the reversal of glial and/or neuronal transporters that are responsible for the removal of the neurotransmitter from the synaptic cleft. As mentioned earlier, mGlu3 receptors are expressed by glial cells, and their activation might have significant effects on the release of glutamate from glial stores (also see Winder and Conn, 1996).

Bezzi et al. (1998) showed that 1S,3R-ACPD and DHPG increased glutamate release from cultured cortical astrocytes, but that L-CCG-I and L-AP4 were much less effective, implicating group I receptors in this increase. Furthermore, coactivation of group I mGlu and AMPA receptors resulted in a potentiation of glutamate release, apparently via a mechanism involving the release of prostaglandins (Bezzi et al., 1998).

Another recent study demonstrated that mGlu receptors regulate glutamate release from primary cultures of hippocampal astrocytes (Ye and Sontheimer, 1999). Although 1S,3R-ACPD, L-CCG-I, and quisqualate reduced extracellular basal glutamate levels in these cultures, the selective agonists, DHPG, DCG-IV, and L-AP4 were without effect. Furthermore, the effects of 1S,3R-ACPD persisted even in the absence of the agonist. Ion-replacement studies and the use of transport inhibitors indicated that the effects of 1S,3R-ACPD were not mediated via changes in intracellular glutamate levels or by the inhibition of glutamate transport. However, one possible explanation, particularly as the effects of ACPD were slow in onset and long-lasting, might be that 1S,3R-ACPD is taken up during application and subsequently leaks or is slowly released into the extracellular medium. Similar uptake effects have been shown elsewhere for quisqualate and L-CCG-I and its analogues (Ishida and Shinozaki, 1999). This might explain the lack of effect of the more selective agonists, because DHPG, DCG-IV, and L-AP4 are not taken up into the cells. Nevertheless, the uptake and release of glutamate from glia play a fundamental role in controlling extracellular concentrations of glutamate and thus neuronal synaptic activity. The presence of mGlu receptors on glia may reflect a mechanism with which to control indirectly the activity of neurons.

In vivo extracellular sampling

Microdialysis is a method of in vivo extracellular sampling from awake, freely moving rats and can reflect more physiological changes in the brain environment in comparison with in vitro methods. However, in vivo microdialysis can be associated with problems of local drug administration as application of compounds into specific brain areas via reverse dialysis does not ensure the precise concentration of the drug administered. Furthermore, in contrast to monoamines, microdialysis sampling of amino acids such as glutamate and GABA is also associated with difficulties in the interpretation of basal extracellular levels (Timmerman and Westerink, 1997). Given the abundance of glial cells in comparison with neurons in the brain, and the compartmentalization of glutamate in glial cells, specific sampling of neuronal stores of glutamate by microdialysis is limited. Furthermore, as glutamate is involved in a number of metabolic processes within a cell in addition to neurotransmission, neuronal glutamate levels consist of both metabolic and vesicular pools of glutamate. In addition, basal extracellular glutamate levels are usually insensitive to tetrodotoxin (TTX) or to the removal of Ca2+ from the perfusion buffer, demonstrating that the source of basal glutamate is probably not neuronally derived. Thus, it is difficult to ascertain the precise source of glutamate present in basal dialysate samples. However, although basal levels of glutamate are not usually sensitive to TTX, reports of pharmacologically enhanced glutamate levels suggest that these increases can be TTX-sensitive.

Administration of the nonselective mGlu receptor agonists 1S,3R-ACPD or L-CCG-I by reverse dialysis in the caudate produced a limited (20-30%) reduction in basal extracellular glutamate levels (Cozzi et al., 1997). Conversely, the mGlu receptor antagonist MCPG increased basal glutamate levels up to two- to fourfold, suggesting that mGlu receptors tonically inhibit glutamate levels in the caudate. Moroni et al. (1998) recently showed that local application of 1S,3R-ACPD or DHPG in the parietal cortex caused dose-dependent increases in basal extracellular glutamate levels (three- and twofold increases evoked by 1S,3R-ACPD and DHPG, respectively). The effect of 1S,3R-ACPD was completely prevented by perfusion with the group I-selective antagonist AIDA. However, in contrast to the Cozzi et al. (1997) study, basal levels were not changed by AIDA, implying that basal levels of extracellular glutamate in the parietal cortex are not under tonic control of group I mGlu receptors. Nevertheless, as there was no indication of Ca2+ or TTX dependency of basal or stimulated glutamate levels in these studies, it is difficult to determine exactly what these changes reflect. The complexity of basal excitatory amino acid levels is illustrated further by the work of Lada et al. (1998) using a novel high-resolution method of measuring on-line dialysate samples every 5 s. In contrast to the decrease in basal glutamate and aspartate levels that might be expected, TTX was without effect on basal glutamate, but caused an approximately twofold increase in striatal aspartate levels.

In addition to the studies by Cozzi et al. (1997) and Moroni et al. (1998), increases in basal extracellular glutamate levels by a locally applied mGlu receptor agonist were also reported by Jones et al. (1998a). However, the agonist used in this study was of low potency and active across all the mGlu subgroups: 30 μM trans-(±)-1-aminocyclopentane-1,3-dicarboxylic acid (t-ACPD) increased basal glutamate and aspartate levels in the nucleus tractus solitarius of the medulla by 190 and 500%, respectively. Concurrent with an impulse flow-dependent mechanism of action, these concentration-dependent increases were sensitive to Ca2+ and TTX. The increases were also reversed by MCPG and 4CPG, although these antagonists were without effect on basal levels.

In spite of the fact that basal levels of glutamate and aspartate may come from a variety of sources, depolarization-stimulated levels are often more likely to be derived from increased neuronal activity. However, it should be noted that this is not always the case as stimulated increases in extracellular glutamate could also originate from specific leakage from glial cells or the reversal of uptake carriers. The study by Lada et al. (1998), as mentioned above, also examined the effects of mGlu receptor activation on depolarization-stimulated excitatory amino acid levels. Application of a 10-s train of depolarizing pulses to the prefrontal cortex caused a rapid increase in striatal glutamate and aspartate levels (200-300% of basal). These increases were completely blocked by TTX or by Ca2+ depletion, demonstrating that the augmentation was impulse flow-dependent, and were also suppressed by 1S,3R-ACPD. The effect of 1S,3R-ACPD was reversed by MCPG, although the antagonist alone was without effect on the depolarizationevoked increase per se. It is interesting that this study also supports previous electrophysiological data regarding the activation of presynaptic mGlu2 receptors by high synaptic concentrations of glutamate (Scanziani et al., 1997). During infusion with the glutamate transport inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid, electrically evoked overflow was unchanged relative to control, because the increase in glutamate concentration following decreased uptake was probably offset by an autoreceptor-mediated reduction in release. However, in the presence of the mGlu receptor antagonist MCPG, autoinhibition of release was blocked, resulting in a large increase in electrically evoked glutamate release due to the lack of uptake and the unrestrained release (Lada et al., 1998). In concurrence with this finding, Battaglia et al. (1997, 1998) observed no effects of selective mGlu2/3 receptor agonists 2R,4R-APDC or LY354740 on striatal basal extracellular levels of glutamate and aspartate. However, when levels of glutamate and aspartate were increased by a 3-min pulse of 100 μM veratridine into the striatum, these increases were blocked by local administration of TTX or 2R,4R-APDC, and by systemic injection of LY354740 (10 mg/kg i.p.). In the latter study (Battaglia et al., 1997), LY354740 was also measured when suppression of evoked release was observed, and it was found at levels in the striatal extracellular fluid that exceeded those needed to activate mGlu2/3 receptors selectively.

Extracellular glutamate levels can also be increased via the activation of limbic neuronal circuits by NMDA receptor antagonists such as ketamine and phencyclidine (PCP) (Moghaddam et al., 1997). In the study by Moghaddam and Adams (1998), glutamate levels in the prefrontal cortex of rats were increased by approximately twofold following systemic injection of PCP (5 mg/kg). The increase evoked by PCP was reduced by preadministration of the mGlu2/3 receptor-selective agonist LY354740 (10 mg/kg). LY354740 also attenuated some of the behaviors associated with administration of NMDA receptor antagonists. These data implicate mGlu receptors in the treatment of psychiatric diseases such as schizophrenia.

In general, biochemical studies discussed above and shown in Table 3 consistently indicate that group II and III mGlu receptor activation leads to suppression of excitatory amino acid release in a number of preparations, whereas group I activation enhances glutamate release. Nonselective agents, such as ACPD, appear capable of either enhancing or suppressing glutamate/aspartate release. This might reflect the ability of ACPD to activate certain group I, II, or III receptor subtypes coupled to the predominant mGlu receptor subtype expressing itself in that preparation or condition.

Table 3. Effects of mGlu receptor agonists and antagonists on the tissue release of various neurotransmitters
 mGlu ligandEffectStimulation conditionsSubstance measuredTissue preparation/procedureReferences
  1. AA, arachidonic acid; ACh, acetylcholine; n. accumbens, nucleus accumbens; NTS, nucleus tractus solitarius; PAG, periaqueductal gray; t-PDC, L-trans-pyrrolidine-2,4-dicarboxylic acid; PFC, prefrontal cortex; VTA, ventral tegmental area.

Glutamate/aspartate      
Nonselective agonistst-ACPD [UPWARDS ARROW]BasalGlutamate/aspartateMicrodialysis: rat NTSJones et al., 1998a
 t-ACPD [UPWARDS ARROW]BasalGlutamate/aspartateMicrodialysis: rat striatumLiu and Moghaddam, 1995
 t-ACPD [UPWARDS ARROW]Basal (+ AA) [3H]Glutamate Rat hippocampal synaptosomesMcGahon and Lynch, 1994, 1996a,b
 t-ACPD [UPWARDS ARROW]+ AdenosineGlutamateRat cortical synaptosomesBudd and Nicholls, 1995, 1998
  1S,3R-ACPD [UPWARDS ARROW]Basal (+ AMPA)GlutamateCultured rat cortical astrocytes and hippocampal slicesBezzi et al., 1998
  1S,3R-ACPD [DOWNWARDS ARROW]BasalGlutamateCultured hippocampal astrocytesYe and Sontheimer, 1999
  1S,3R-ACPD [DOWNWARDS ARROW]BasalGlutamateMicrodialysis: rat caudateCozzi et al., 1997
  1S,3R-ACPD [UPWARDS ARROW]BasalGlutamateMicrodialysis: rat parietal cortexMoroni et al., 1998
  1S,3R-ACPD [DOWNWARDS ARROW]Depolarizing pulseGlutamate/asparateMicrodialysis: rat PFCLada et al., 1998
  1S,3R-ACPD [DOWNWARDS ARROW]Depolarizing pulse [14C]Glutamate Rat hippocampal slicesDi Iorio et al., 1995, 1996
  1S,3R-ACPD [UPWARDS ARROW]50 μM 4-AP (+ AA)GlutamateRat cortical synaptosomesHerrero et al., 1992, 1996; Vazquez et al., 1994
  1S,3R-ACPD [UPWARDS ARROW] 30 mM KCl [3H]Glutamate Rat cortical slicesLombardi et al., 1996
  1S,3R-ACPD [UPWARDS ARROW] / [DOWNWARDS ARROW]200 μM 4-AP (+ AA) [3H]Glutamate Rat cortical synaptosomesReid et al., 1999
  1S,3R-ACPD [UPWARDS ARROW] 30 mM KCl D-[3H]Aspartate Rat NTS slicesJones et al., 1998b
  1S,3R-ACPD [DOWNWARDS ARROW] 30 mM KCl D-[3H]Aspartate Rat striatal slicesLombardi et al., 1993
  1S,3R-ACPD [UPWARDS ARROW] 30 mM KCl D-[3H]Aspartate Rat cortical slicesLombardi et al., 1994, 1996
  1S,3S-ACPD [DOWNWARDS ARROW]50 μM veratridine [3H]Glutamate Rat cortical synaptosomesAttwell et al., 1995
  1S,3S-ACPD [DOWNWARDS ARROW] 2 mM 4-APGlutamateRat striatal synaptosomesEast et al., 1995
  1S,3S-ACPD [DOWNWARDS ARROW]50 μM veratridine D-[3H]Aspartate Rat cortical synaptosomesAttwell et al., 1995
 L-CCG-I[DOWNWARDS ARROW]BasalGlutamateMicrodialysis: rat caudateCozzi et al., 1997
 L-CCG-I[DOWNWARDS ARROW]200 μM 4-AP [3H]Glutamate Rat cortical synaptosomesReid et al., 1999
 L-CCG-I[UPWARDS ARROW] 100 mM KClAspartateMicrodialysis: rat striatumSamuel et al., 1996
 L-CCG-I[DOWNWARDS ARROW] 30 mM KCl D-[3H]Aspartate Rat striatal slicesLombardi et al., 1993
Group I selective agonistsQuisqualate[DOWNWARDS ARROW] 30 mM KCl D-[3H]Aspartate Rat striatal slicesLombardi et al., 1993
 DHPG[UPWARDS ARROW]BasalGlutamateMicrodialysis: rat parietal cortexMoroni et al., 1998
 DHPG[UPWARDS ARROW]BasalGlutamateCultured rat cortical astrocytes and hippocampal slicesBezzi et al., 1998
 DHPG[UPWARDS ARROW]50 μM 4-APGlutamateRat cortical synaptosomesHerrero et al., 1998
 DHPG[UPWARDS ARROW]50 μM 4-APGlutamateRat hippocampal synaptosomesRodriguez-Moreno et al., 1998
 DHPG[UPWARDS ARROW] 0.2-10 mM 4-AP [3H]Glutamate Rat cortical synaptosomesReid et al., 1999
 DHPG[UPWARDS ARROW] 30 mM KCl D-[3H]Aspartate Rat NTS slicesJones et al., 1998b
Group II selective agonistsDCG-IV[DOWNWARDS ARROW]200 μM 4-AP [3H]Glutamate Rat cortical synaptosomesReid et al., 1999
 DCG-IV[DOWNWARDS ARROW]Depolarizing pulse [14C]Glutamate Rat hippocampal slicesDi Iorio et al., 1995, 1996
 DCG-IV[DOWNWARDS ARROW]50 μM veratridine D-[3H]Aspartate Rat cortical synaptosomesAttwell et al., 1998b
  2R,4R-APDC [DOWNWARDS ARROW]100 μM veratridineGlutamate/aspartateMicrodialysis: rat striatumBattaglia et al., 1998
  2R,4R-APDC [DOWNWARDS ARROW]50 μM veratridine D-[3H]Aspartate Rat cortical synaptosomesAttwell et al., 1998a
  2R,4R-APDC [DOWNWARDS ARROW] 30 mM KCl D-[3H]Aspartate Rat NTS slicesJones et al., 1998b
 LY354740[DOWNWARDS ARROW]100 μM veratridineGlutamate/aspartateMicrodialysis: rat striatumBattaglia et al., 1997
 LY354740[DOWNWARDS ARROW]5 mg/kg PCPGlutamateMicrodialysis: rat medial PFCMoghaddam and Adams, 1998
 LY354740[DOWNWARDS ARROW]Traumatic injuryGlutamateRat cortical neuronal-glialAllen et al., 1999
Group III selective agonistsL-AP4[DOWNWARDS ARROW] 30 mM KClGlutamateRat cortical synaptosomesVazquez and Sanchez-Prieto, 1997; Vazquez et al., 1995a,b
 L-AP4[DOWNWARDS ARROW] 2 mM 4-APGlutamateRat striatal synaptosomesEast et al., 1995
 L-AP4[DOWNWARDS ARROW]200 μM 4-AP [3H]Glutamate Rat cortical synaptosomesReid et al., 1999
 L-AP4[DOWNWARDS ARROW] 30 mM KClGlutamateRat cortical synaptosomesHerrero et al., 1996
 L-AP4[DOWNWARDS ARROW]50 μM veratridine D-[3H]Aspartate Rat cerebrocortical synaptosomesAbdul-Ghani et al., 1997
Nonselective antagonistsMCPG[UPWARDS ARROW]BasalGlutamateMicrodialysis: rat caudateCozzi et al., 1997
 MCPG[DOWNWARDS ARROW] 100 mM KClGlutamateMicrodialysis: rat striatumSamuel et al., 1996
 MCPG[UPWARDS ARROW]t-PDC presence GlutamateMicrodialysis: rat PFCLada et al., 1998
 L-AP3[DOWNWARDS ARROW] 35 mM KClGlutamateRat cerebellar slicesDickie et al., 1994
GABA      
Nonselective agonistst-ACPD [DOWNWARDS ARROW] 1 mM 4-APGABARat hippocampal synaptosomesBreukel et al., 1998
  1S,3R-ACPD [UPWARDS ARROW] 30 mM KCl [14C]GABA Rat NTS slicesJones et al., 1998b
  1S,3R-ACPD [UPWARDS ARROW]NMDA [14C]GABA Rat striatal slicesHanania and Johnson, 1999
Group I selective agonistsDHPG[UPWARDS ARROW] 30 mM KCl [14C]GABA Rat NTS slicesJones et al., 1998b
 Quisqualate[UPWARDS ARROW]BasalEndogenous GABARat striatal slicesWang et al., 1996
 Quisqualate[UPWARDS ARROW]Basal [3H]GABA Rat hippocampal slicesJanáky et al., 1994
 Quisqualate[UPWARDS ARROW]300 μM NMDA [14C]GABA Rat striatal slicesHanania and Johnson, 1999
Group II selective agonistsDCG-IV[DOWNWARDS ARROW] 30 mM KCl [3H]GABA Cultured rat cortical neuronsSchaffhauser et al., 1998
  2R,4R-APDC [DOWNWARDS ARROW] 30 mM KCl [14C]GABA Rat NTS slicesJones et al., 1998b
  2R,4R-APDC [DOWNWARDS ARROW]300 μM NMDA [14C]GABA Rat striatal slicesHanania and Johnson, 1999
 LY354740[DOWNWARDS ARROW] 30 mM KCl [3H]GABA Cultured rat cortical neuronsSchaffhauser et al., 1998
 LY379268[UPWARDS ARROW]Basal/ischemiaGABAMicrodialysis: gerbil hippocampusGreenslade et al., 1999
Group III selective agonistsL-AP4[DOWNWARDS ARROW] 30 mM KCl [3H]GABA Cultured rat cortical neuronsSchaffhauser et al., 1998
 L-AP4[DOWNWARDS ARROW]Basal/100 μM NMDAGABACultured rat striatal neuronsLafon-Cazal et al., 1999
 L-SOP[DOWNWARDS ARROW] 30 mM KCl [3H]GABA Cultured rat cortical neuronsSchaffhauser et al., 1998
Dopamine      
Nonselective agonistst-ACPD [UPWARDS ARROW]BasalDopamineMicrodialysis: rat striatumVerma and Moghaddam, 1998
 t-ACPD [UPWARDS ARROW] 40 mM KClDopamineMicrodialysis: rat striatumVerma and Moghaddam, 1998
 t-ACPD [DOWNWARDS ARROW]Light stimulationDopamineXenopus laevis retinaBoatright et al., 1994
  1S,3R-ACPD [UPWARDS ARROW]BasalDopamineTissue levels: rat striatumSacaan et al., 1992
  1S,3R-ACPD [UPWARDS ARROW]BasalDopamineMicrodialysis: rat striatumBruton et al., 1999
  1S,3R-ACPD [UPWARDS ARROW]Basal (methamphetamine-sensitized rats)DopamineMicrodialysis: rat striatumArai et al., 1996
  1S,3R-ACPD [UPWARDS ARROW]Basal (methamphetamine-sensitized rats)DopamineRat striatal slicesArai et al., 1997
  1S,3R-ACPD [UPWARDS ARROW] / [DOWNWARDS ARROW]BasalDopamineMicrodialysis: rat n. accumbensTaber and Fibiger, 1995
  1S,3R-ACPD [UPWARDS ARROW]BasalDopamineMicrodialysis: rat n. accumbensOhno and Watanabe, 1995
  1S,3R-ACPD [UPWARDS ARROW]BasalDopamineMicrodialysis: frontal cortex from aged ratsPintor et al., 1998
  1S,3R-ACPD [DOWNWARDS ARROW]PFC or VTA stimulationDopamineMicrodialysis: rat n. accumbensTaber and Fibiger, 1995
  1S,3R-ACPD [DOWNWARDS ARROW]Stress induced [UPWARDS ARROW]DopamineMicrodialysis: rat n. accumbensFeenstra et al., 1998
Group I selective agonistsDHPG[UPWARDS ARROW]BasalDopamineMicrodialysis: rat striatumBruton et al., 1999
Group II selective agonistsDCG-IV[UPWARDS ARROW]BasalDopamineMicrodialysis: rat striatumBruton et al., 1999
 DCG-IV[DOWNWARDS ARROW]BasalDopamineMicrodialysis: rat n. accumbensHu et al., 1999
Group III selective agonistsL-AP4[DOWNWARDS ARROW]BasalDopamineMicrodialysis: rat n. accumbensHu et al., 1999
 L-AP4[DOWNWARDS ARROW]Light stimulationDopamineXenopus laevis retinaBoatright et al., 1994
Nonselective antagonistsMPPG[UPWARDS ARROW]BasalDopamineMicrodialysis: rat n. accumbensHu et al., 1999
 MCPG[DOWNWARDS ARROW]Hippocampal stimulationDopamine oxidation currentIn vivo rat n. accumbensBlaha et al., 1997
Serotonin (5-HT)      
Nonselective agonists 1S,3R-ACPD [UPWARDS ARROW]Basal5-HTMicrodialysis: rat PAGMaione et al., 1998
 L-CCG-I[UPWARDS ARROW]Basal5-HTMicrodialysis: rat PAGMaione et al., 1998
Group III selective agonistsL-SOP[UPWARDS ARROW]Basal5-HTMicrodialysis: rat PAGMaione et al., 1998
Acetylcholine      
Nonselective agonists 1S,3R-ACPD [UPWARDS ARROW]BasalEndogenous AChTissue levels: rat striatumSacaan et al., 1992
Group II selective agonists 2R,4R-APDC [DOWNWARDS ARROW]300 μM NMDA [3H]ACh Rat striatal slicesHanania and Johnson, 1999
Group III selective agonistsL-AP4[DOWNWARDS ARROW] 25 mM KCl [3H]ACh Cultured chick amacrine-like cellsCaramelo et al., 1999
Purines      
Nonselective agonists 1S,3R-ACPD [DOWNWARDS ARROW]Depolarizing pulse [3H]Purines Cultured rat cortical astrocytesDi Iorio et al., 1995, 1996
  1S,3R-ACPD [DOWNWARDS ARROW]Depolarizing pulse [3H]Purines Rat hippocampal slicesDi Iorio et al., 1995, 1996
Group II selective agonistsDCG-IV[DOWNWARDS ARROW]Depolarizing pulse [3H]Purines Cultured rat cortical astrocytesDi Iorio et al., 1995, 1996
 DCG-IV[DOWNWARDS ARROW]Depolarizing pulse [3H]Purines Rat hippocampal slicesDi Iorio et al., 1995, 1996
Cholecystokinin (CCK)      
Nonselective agonistst-ACPD [DOWNWARDS ARROW] 1 mM 4-APCCK-8 radioimmunoreactivityRat hippocampal synaptosomesBreukel et al., 1998
Substance P      
Group I selective agonistsDHPG[DOWNWARDS ARROW]CapsaicinSubstance P radioimmunoreactivityRat trigeminal nucleus slicesCuesta et al., 1999
Group III selective agonistsL-AP4[DOWNWARDS ARROW]CapsaicinSubstance P radioimmunoreactivityRat trigeminal nucleus slicesCuesta et al., 1999
Taurine      
Group I selective agonistsQuisqualate[UPWARDS ARROW]BasalTaurineMouse hippocampal slicesSaransaari and Oja, 1999
 DHPG[UPWARDS ARROW]BasalTaurineMouse hippocampal slicesSaransaari and Oja, 1999
Group II selective agonistsDCG-IV[UPWARDS ARROW] 50 mM KClTaurineMouse hippocampal slicesSaransaari and Oja, 1999

GABA

  1. Top of page
  2. Abstract
  3. mGlu SUBTYPES, PHARMACOLOGY, AND TRANSDUCTION MECHANISMS
  4. SYNAPTIC LOCALIZATION OF mGlu RECEPTORS
  5. GLUTAMATE/ASPARTATE
  6. GABA
  7. DOPAMINE
  8. SEROTONIN (5-HT)
  9. MISCELLANEOUS
  10. Cholecystokinin (CCK)
  11. CONCLUSIONS AND FUTURE PERSPECTIVES

Electrophysiological reports suggest that mGlu receptors regulate the amino acid γ-aminobutyric acid (GABA) release from interneurons of the hippocampus, thalamus, and accessory olfactory bulb. Studies using the nonselective agonist 1S,3R-ACPD showed a reduction in the amplitude of GABA-mediated inhibitory postsynaptic currents in the CA1 region of the hippocampus (Liu et al., 1993; Jouvenceau et al., 1995) and in the cerebellum (Llano and Marty, 1995). Furthermore, Gereau and Conn (1995) showed that the more selective group I agonist DHPG reduced inhibitory synaptic transmission in area CA1. Extensive characterization of mGlu receptor-mediated responses in the thalamus has also been carried out using a variety of subgroup-selective agonists. Thalamic GABAergic inhibitory transmission, evoked by either electrical stimulation of the somatosensory cortex or stimulation of the vibrissae by a jet of air, was suppressed by 2R,4R-APDC and L-AP4, but not DHPG (Salt and Eaton, 1995; Salt and Turner, 1998).

Hayashi et al. (1993) proposed a specific role for mGlu2 receptors in suppression of GABAergic transmission in the accessory olfactory bulb. DCG-IV suppressed GABA-mediated inhibitory postsynaptic currents in mitral cells. The synapses between the granule and mitral cells appear to mediate reciprocal transmission in which the granule cell, which is excited by glutamate from the mitral cell, can exert a GABA-mediated inhibition onto the mitral cell, resulting in hyperpolarization.

Biochemical studies have also suggested that mGlu receptors suppress GABA release. Inhibition of KCl (30 mM)-evoked [3H]GABA release in primary cortical cultures has been shown to be sensitive to both group II and III mGlu receptors (Schaffhauser et al., 1998). LY354740, DCG-IV, and L-AP4 all potently inhibited KCl (30 mM)-stimulated release by ∼30-50%. Occlusion of these responses by selective VDCC blockers indicated that LY354740- and L-AP4-mediated effects might involve regulation of L- and N-type VDCCs, respectively. However, as L-AP4 was ∼1,000-fold less potent at inhibiting VDCCs in these cultures than suppressing [3H]GABA release, the mechanism of the L-AP4-evoked effects was possibly mediated down-stream of VDCC activation. Furthermore, Lafon-Cazal et al. (1999) showed that in GABAergic neuron-rich mouse striatal cultures, L-AP4 inhibited both basal and NMDA-evoked GABA release, probably via activation of mGlu7 receptors. These inhibitions of GABA release might be responsible for the increases in L-AP4-evoked basal and NMDA-stimulated cell death (Lafon-Cazal et al., 1999).

In contrast to the reductions in GABA release described above, facilitatory effects of mGlu receptor activation have also been reported. An augmentation of KCl (15 mM)-evoked endogenous GABA release by 1S,3R-ACPD was observed in slices of rat striatum (Wang and Johnson, 1995). Although the effect of 1S,3R-ACPD on stimulated GABA levels was not statistically significant, costimulation with the dopamine D1 receptor agonist SKF38393 and 1S,3R-ACPD significantly potentiated KCl (15 mM)-evoked GABA release by 270% over control levels. It was proposed that increased intracellular Ca2+ levels mobilized by activation of group I mGlu receptors together with elevation of cyclic AMP levels mediated via D1 receptors might potentiate depolarization-induced GABA release (Wang and Johnson, 1995). The same authors showed an increase in GABA release in striatal slices by the selective group I agonist quisqualate (Wang et al., 1996). This effect was blocked by a combination of the AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione and by the mGlu receptor antagonist L-AP3, indicating that both AMPA and mGlu receptors were contributing to this effect. Similarly, the quisqualate-evoked release of [3H]GABA from rat coronal hippocampal slices was also reduced, in Ca2+-free medium, by the antagonist L-AP3 (Janáky et al., 1994).

In addition to their effects on basal GABA levels, 1S,3R-ACPD and the selective group I agonist DHPG also increased K+ (30 mM)-evoked [14C]GABA release in superfused slices of rat nucleus tractus solitarius (Jones et al., 1998b). In contrast, 2R,4R-APDC decreased K+ (30 mM)-evoked [14C]GABA outflow, but L-AP4 was without effect. Therefore, group I and II mGlu receptors appear to mediate augmentation or suppression, respectively, of depolarization-evoked [14C]GABA release in the nucleus tractus solitarius, supporting other studies that implicate mGlu receptors as potential modulators in this region (see previous section on glutamate and aspartate). Furthermore, a recent report by Matsumura et al. (1999) has shown that all three groups of mGlu receptors participate in cardiovascular regulation in the nucleus tractus solitarius.

Also, both 1S,3R-ACPD and quisqualate potentiated increases in [14C]GABA release stimulated by NMDA in striatal slices, demonstrating a role for group I mGlu receptors in this effect (Hanania and Johnson, 1999). Conversely, the selective group II agonist 2R,4R-APDC inhibited NMDA-evoked [14C]GABA release in the same preparation, reflecting differential roles for distinct mGlu receptor subgroups.

It is interesting that preliminary data using microdialysis sampling in the gerbil dorsal hippocampus showed that the selective mGlu receptor agonist LY379268 administered systemically (10 mg/kg) produced a small, but significant, augmentation of both basal GABA levels and the increase of GABA evoked by 5-min bilateral carotid occlusion (Greenslade et al., 1999). As LY379268 has been shown previously to be neuroprotective (Kingston et al., 1999), it was proposed that an increase in GABA levels might underlie the neuroprotective actions of this compound. However, systemic pretreatment with bicuculline did not reduce the neuroprotective effects of LY379268. Although enhanced GABA function does not appear to be the basis for the novel neuroprotective effects of LY379268, further investigation of these increases in GABA levels evoked by the mGlu2/3 receptor agonist per se is warranted.

Although GABA release in one of the studies mentioned above was sensitive to Ca2+ depletion (Schaffhauser et al., 1998), the effect of TTX does not appear to have been examined. However, the Ca2+ or TTX sensitivity of extracellular GABA levels is controversial (Timmerman and Westerink, 1997). In those studies that showed a reduction in GABA levels by TTX, the decrease, in contrast to monoamines, was gradual and somewhat limited, possibly suggesting that the effect was secondary to other mechanisms and might not be indicative of vesicular release. Therefore, as with glutamate, it is difficult to determine the source of basal GABA levels. However, disinhibition of GABAergic function represents a way by which group II and III mGlu receptors might increase neuronal excitability, and conversely, an increase in activity of GABAergic inputs by group I mGlu receptors could ultimately result in decreased neurotransmission.

DOPAMINE

  1. Top of page
  2. Abstract
  3. mGlu SUBTYPES, PHARMACOLOGY, AND TRANSDUCTION MECHANISMS
  4. SYNAPTIC LOCALIZATION OF mGlu RECEPTORS
  5. GLUTAMATE/ASPARTATE
  6. GABA
  7. DOPAMINE
  8. SEROTONIN (5-HT)
  9. MISCELLANEOUS
  10. Cholecystokinin (CCK)
  11. CONCLUSIONS AND FUTURE PERSPECTIVES

Evidence for the modulation of dopaminergic neuronal activity by mGlu receptors has been provided by a number of studies using a variety of techniques. All three groups of mGlu receptors have been implicated in the regulation of electrically evoked glutamate release onto dopamine neurons (Mercuri et al., 1993; Bonci et al., 1997; Fiorillo and Williams, 1998; Wigmore and Lacey, 1998).

The modulation of striatal dopaminergic function by mGlu receptors was first shown by Sacaan et al. (1992), using the nonselective agonist 1S,3R-ACPD. Unilateral intrastriatal injection of 1S,3R-ACPD (1 μmol/2 μl) produced contralateral turning in rats, which was temporally correlated to increases in tissue levels of dopamine and its metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid up to 300% of basal levels. Furthermore, depletion of dopamine with α-methyl-DL-p-tyrosine prevented the behavioral actions of 1S,3R-ACPD. These effects of 1S,3R-ACPD on the striatal dopamine system have been extended recently by the results of in vivo microdialysis studies.

In vivo extracellular sampling

In comparison with the amino acids glutamate and GABA, basal levels of dopamine sampled by in vivo microdialysis are generally more sensitive to TTX, indicating that basal dopamine levels might be more representative of the neuronal dopamine pool (Westerink et al., 1987).

In support of the early work by Sacaan et al. (1992), local administration of t-ACPD increased extracellular levels of dopamine in the striatum in a concentration-dependent manner (Verma and Moghaddam, 1998). However, an increase by t-ACPD was still observed in the presence of 2 μM TTX, indicating that this effect was independent of impulse flow. It is interesting that t-ACPD also markedly reduced the fourfold increase in dopamine levels evoked by 40 mM KCl. Thus, it appears that the effects of t-ACPD on extracellular dopamine levels in the striatum depend on whether levels have been augmented by a depolarizing agent. More recently, the roles of specific receptor subgroups have been examined by the use of the selective mGlu receptor agonists DHPG and DCG-IV. In contrast to the study by Verma and Moghaddam (1998), Bruton et al. (1999) found that striatal dopamine levels were reduced in the presence of TTX, and when 1S,3R-ACPD was infused via reverse dialysis, extracellular dopamine increased to ∼400% of basal levels. This effect was also mimicked by DHPG, which produced an increase of basal levels of >200%. The increases by both 1S,3R-ACPD and DHPG were blocked by MCPG. It is interesting that a more modest increase in dopamine levels was also produced by the group II selective agonist DCG-IV. However, the DCG-IV concentration used (50 μM in the probe) would probably correspond to a tissue concentration of ∼5 μM, at which NMDA receptors are also activated (Hayashi et al., 1993). This is supported by previous reports showing NMDA-evoked increases in extracellular dopamine levels in the striatum (Carrozza et al., 1992). It is interesting that the combined response of DHPG and DCG-IV was abolished completely in the presence of MCPG, indicating that the increase in dopamine release by DCG-IV was mediated via mGlu receptors (Bruton et al., 1999) and thus suggesting the involvement of other nongroup II mGlu receptors.

Arai et al. (1996) also reported that perfusion of 1S,3R-ACPD increased striatal dopamine levels, and this increase was attenuated by MCPG. It is interesting that the increase evoked by 1S,3R-ACPD (130% of basal levels) was enhanced to 180% of basal levels in rats systemically treated with 1 mg/kg methamphetamine once daily for 6 consecutive days and followed by a 6-day withdrawal period, further implicating mGlu receptors in mechanisms of drug dependence and withdrawal (also see Helton et al., 1997; Vandergriff and Rasmussen, 1999). Likewise, the actions of 1S,3R-ACPD in vivo appear to be direct, as similar increases in dopamine release from sensitized animals were found in vitro (Arai et al., 1997).

In addition to mGlu receptor-mediated increases in striatal dopamine, regulation of extracellular dopamine levels of the nucleus accumbens by mGlu receptors has also been observed. Taber and Fibiger (1995) demonstrated that although local application of 1 mM 1S,3R-ACPD increased basal dopamine levels by around two-fold, 100 μM 1S,3R-ACPD reduced baseline dopamine by ∼30%. It is thought that these distinct actions of 1S,3R-ACPD might reflect the recruitment of different mGlu receptor subtypes. Application of 100 μM 1S,3R-ACPD via reverse dialysis probably results in a tissue concentration of ∼10 μM, sufficient to activate group II mGlu receptors, but a 10-fold increase in concentration (i.e., application of 1 mM 1S,3R-ACPD via reverse dialysis) would presumably also activate group I receptors.

Taber and Fibiger (1995) also showed that bilateral stimulation of the prefrontal cortex increased dopamine levels in the nucleus accumbens, and this was prevented by infusion of TTX in the accumbens. Local application of 100 μM 1S,3R-ACPD in the nucleus accumbens also blocked the prefrontal cortex-stimulated (100 μA) increase in dopamine levels (Taber and Fibiger, 1995). Similarly, 1S,3R-ACPD blocked the stimulation-induced increase in accumbens dopamine when electrical stimulation was applied to the ventral tegmental area, suggesting that this effect may be mediated via the projections from the cortex to the ventral tegmental area. In support of this, Murase et al. (1993) showed that injection of glutamate into the prefrontal cortex increased the burst firing of dopaminergic neurons in the ventral tegmental area and enhanced dopamine release in the nucleus accumbens.

Ohno and Watanabe (1995) also investigated the effects of local administration of 1S,3R-ACPD on dopamine levels in the nucleus accumbens. At a probe concentration of 1 mM, 1S,3R-ACPD increased extracellular accumbal dopamine levels by approximately threefold. The 1S,3R-ACPD-evoked increase in dopamine was attenuated by MCPG, which had no effect alone on dopamine levels. Later studies, using more selective compounds, further characterized the effect of mGlu receptor activation in the nucleus accumbens. In a recent report, 1S,3R-ACPD and DHPG (up to 500 and 100 μM, respectively) were without effect on extracellular dopamine levels of the nucleus accumbens (Hu et al., 1999). At first this might appear surprising, given the data by Taber and Fibiger (1995) described above; however, the concentration of 1S,3R-ACPD in the Hu et al. (1999) study was between those that inhibited basal dopamine release (100 μM) and those that enhanced basal dopamine levels (1 mM) in the previous report. Hu et al. (1999) also showed that basal dopamine levels in the accumbens were reduced by the group III agonist L-AP4. Furthermore, the group II/III mGlu receptor antagonist MPPG increased dopamine levels, an effect that was blocked by an L-type VDCC blocker or by L-AP4. This indicates that there is significant glutamatergic tone on group II/III receptors to inhibit dopamine release presynaptically in the nucleus accumbens. DCG-IV evoked a biphasic effect with respect to concentration, 1 μM DCG-IV caused a 35% reduction, but concentrations of 0.1 and 10 μM were without significant effect. This is possibly due to the higher concentration of DCG-IV also activating NMDA receptors (Hayashi et al., 1993), and as these receptors have been shown to increase dopamine release, this could mask a possible inhibitory effect mediated via group II mGlu receptors. It is interesting that systemic administration of the selective group II mGlu receptor agonist LY354740 (10 mg/kg) was without effect on dopamine levels in the nucleus accumbens or prefrontal cortex (Moghaddam and Adams, 1998). Given the effects of DCG-IV on dopamine levels in the nucleus accumbens, the lack of effect of LY354740 is somewhat surprising. However, it is feasible that the systemic administration of the compound evokes effects elsewhere in the brain, which could mask possible effects that could occur when the compound was applied locally to the accumbens or prefrontal cortex.

Pintor et al. (1998) recently demonstrated an age-dependent increase in dopamine levels of the prefrontal cortex. Although basal levels of dopamine in prefrontal cortex dialysate from young (3 months) rats were around twice the levels in aged (24 months) rats, local infusion with 1S,3R-ACPD increased dopamine levels in old rats by approximately sixfold. However, 1S,3R-ACPD was without effect on prefrontal cortex dopamine levels in young rats. Whether this is indicative of age-dependent changes in mGlu receptor expression (Catania et al., 1994) or in some other part of the mechanism underlying the dopamine increase is yet to be elucidated.

The mGlu receptor-mediated regulation of dopaminergic function in the prefrontal cortex, nucleus accumbens, and striatum has numerous therapeutic implications. The mesocortical dopamine pathway has been proposed as a potential target for the treatment of schizophrenia. Therefore, modulation of prefrontal cortical dopamine levels might result in the control of the negative symptoms of schizophrenia that have been associated with reduced dopaminergic function in this area. Furthermore, regulation of nigrostriatal pathways by mGlu receptors could be effective in the treatment of Parkinson's disease and other movement disorders (also see Konieczny et al., 1998; Conn et al., 1999; Marino et al., 1999). Recently, the nonselective agonist 1S,3R-ACPD was shown to prevent stress-induced increases in nucleus accumbens, but not prefrontal cortex dopamine release (Feenstra et al., 1998). This study indicates clear brain regional differences in mGlu receptor modulation of stress-induced neurotransmitter release. It would be important for the future to understand regional differences in dopamine modulation as they relate to these different therapeutic modalities.

SEROTONIN (5-HT)

  1. Top of page
  2. Abstract
  3. mGlu SUBTYPES, PHARMACOLOGY, AND TRANSDUCTION MECHANISMS
  4. SYNAPTIC LOCALIZATION OF mGlu RECEPTORS
  5. GLUTAMATE/ASPARTATE
  6. GABA
  7. DOPAMINE
  8. SEROTONIN (5-HT)
  9. MISCELLANEOUS
  10. Cholecystokinin (CCK)
  11. CONCLUSIONS AND FUTURE PERSPECTIVES

In contrast to the abundance of reports on the regulation of dopaminergic function by mGlu receptors, very few studies have investigated interactions between 5-HT and these receptors. A recent study by Marek et al. (2000) demonstrated a colocalization of 5-HT2A and group II mGlu receptors in the medial prefrontal cortex. Furthermore, the selective mGlu2/3 receptor antagonist LY341495 increased 5-HT-induced excitatory postsynaptic potentials in layer V pyramidal cells and reversed the inhibition of electrically and 5-HT-evoked excitatory postsynaptic potentials produced by LY354740. To date, there are no published biochemical studies on the actions of mGlu ligands on 5-HT release in the prefrontal cortex. Nevertheless, these group II receptor-mediated electrophysiological effects further support the potential for mGlu receptors as therapeutic agents in neuropsychiatric diseases, and further studies are warranted.

It is interesting that in vivo microdialysis sampling from the periaqueductal gray matter (PAG) indicated that group II mGlu receptors modulate 5-HT release in this area (Maione et al., 1998). Although local application, by reverse dialysis, of the selective group I agonist DHPG was without effect on extracellular 5-HT, 1S,3R-ACPD or L-CCG-I enhanced extracellular levels of 5-HT in the PAG, by around twofold, suggesting an involvement of group II mGlu receptors. Furthermore, the increase produced by 1S,3R-ACPD was attenuated by the nonselective mGlu receptor antagonist MCPG, but not by the group I selective antagonist AIDA. An increase in PAG 5-HT levels was also produced by L-SOP, implicating group III mGlu receptors in the modulation of 5-HT. Although the mGlu receptor antagonists tested in this study were without effect on basal 5-HT levels, the GABA receptor antagonist bicuculline increased basal levels, suggesting that basal 5-HT levels are under tonic control of GABA neurons. Therefore, the authors speculate that regulation of extracellular 5-HT levels in the PAG by group II and III mGlu receptors is not mediated by a direct effect, but possibly via the inactivation of tonically active GABAergic neurons. However, as the authors acknowledge, extracellular levels of 5-HT observed in this study were particularly high, and as no indication of TTX or Ca2+ sensitivity of either basal or stimulated 5-HT levels was discussed, the mechanism mediating the effect is difficult to elucidate. Although increases in 5-HT in the amygdala has anxiogenic effects, the reverse is true for the PAG, where an increase in 5-HT produces anxiolytic effects (for review, see Graeff et al., 1993). Therefore, if the study can be verified, it seems that one mechanism by which activation of group II mGlu receptors might produce the anxiolytic effects observed with LY354740 (see Helton et al., 1998) could be via increases in serotonergic transmission in the PAG. In addition, a preliminary report from Maione et al. (1999) indicated that local injection of mGlu receptor agonists into the PAG decreased the nociceptive response in the formalin test for persistent pain. These data further implicate mGlu receptors in nociceptive responses and suggest that a mechanism of action involving the PAG might underlie the reported analgesic properties of some mGlu receptor agonists (Fisher and Coderre, 1996).

MISCELLANEOUS

  1. Top of page
  2. Abstract
  3. mGlu SUBTYPES, PHARMACOLOGY, AND TRANSDUCTION MECHANISMS
  4. SYNAPTIC LOCALIZATION OF mGlu RECEPTORS
  5. GLUTAMATE/ASPARTATE
  6. GABA
  7. DOPAMINE
  8. SEROTONIN (5-HT)
  9. MISCELLANEOUS
  10. Cholecystokinin (CCK)
  11. CONCLUSIONS AND FUTURE PERSPECTIVES

Acetylcholine

The first observation of mGlu receptor-mediated effects on modulation of acetylcholine was reported by Sacaan et al. (1992). As described above, microinjection of 1S,3R-ACPD (1 μmol/2 μl) into rat striatum increased levels of dopamine, 3,4-dihydroxyphenylacetic acid, and homovanillic acid, and this was accompanied by a significant increase in tissue acetylcholine by ∼150% of basal levels, 3 h after administration. However the mechanism underlying the increase in striatal acetylcholine levels produced by 1S,3R-ACPD and its relevance to the behavior effects (turning) has not been elucidated. In this regard, Sacaan et al. (1992) speculated that the increase was possibly secondary to an increase in dopamine.

Group II and III mGlu receptors have been implicated in the inhibitory modulation of [3H]acetylcholine release. In a report by Hanania and Johnson (1999), the selective group II agonist 2R,4R-APDC inhibited [3H]acetylcholine release stimulated by NMDA in striatal slices. Likewise, Caramelo et al. (1999) showed that L-AP4 inhibited KCl (25 mM)-evoked [3H]acetylcholine release from cultured chick amacrine-like neurons, in a pertussis toxin-sensitive manner, with an EC50 value of 4 μM, implicating activation of mGlu4, mGlu6, or mGlu8. L-AP4 was without effect on KCl- or forskolin-stimulated cyclic AMP formation in these cells, and the inhibition of release by L-AP4 appeared to be mediated via a direct modulation of L-type and/or N-type VDCCs. Although no effects of either DHPG or DCG-IV on [3H]acetylcholine release per se were observed, both compounds reversed the inhibition evoked by L-AP4, reflecting a modulation of group III mGlu receptor responses by the other members of the mGlu family.

Purines

The release of cyclic AMP and/or its metabolites, including adenosine, has been implicated in the depression of synaptic transmission resulting from the coactivation of mGlu and β-adrenergic receptors (Winder et al., 1996). The effects of mGlu receptor activation on the release of purines (primarily adenosine, inosine, hypoxanthine, AMP, ADP, and ATP) has been examined in astrocyte cultures and slices of hippocampus prelabeled with [3H]adenosine (Di Iorio et al., 1995, 1996). [3H]Purine release following application of an electrical field stimulation to cultures of rat cortical astrocytes was suppressed by 1S,3R-ACPD. DCG-IV also reduced electrically evoked [3H]purine release from these cultures, but only at high concentrations; the response was inhibited by 18 and 34% by 3 and 6 μM DCG-IV, respectively (Di Iorio et al., 1995). The same group also showed similar effects of 1S,3R-ACPD and DCG-IV in hippocampal slices and found that L-AP4 was without effect (Di Iorio et al., 1996). In the slice preparation, DCG-IV inhibited release of [3H]purines by 60%, but with an IC50 value of ∼3 μM. The high concentrations of DCG-IV used in these studies might reflect an involvement of NMDA receptors, as DCG-IV has activity at these receptors at similar concentrations (Hayashi et al., 1993). However, the authors report that the noncompetitive NMDA receptor antagonist MK-801 (dizocilpine maleate) did not preclude the effect of 3 μM DCG-IV. Nevertheless, it is unlikely that the actions of DCG-IV on [3H]purine release are mediated specifically via group II mGlu receptors, due to the large differences in potency observed in this study and the low nanomolar potency of DCG-IV at mGlu2/3 receptor clones (generally between 100 and 300 nM; see Table 2). However, DCG-IV is reported to have antagonist actions at other mGlu receptor subtypes (see Table 2). It is interesting that the potency of 1S,3R-ACPD in this report was more representative of an effect mediated via group I mGlu receptors. The use of more selective compounds is necessary to determine definitively the receptors involved in this response.

Cholecystokinin (CCK)

  1. Top of page
  2. Abstract
  3. mGlu SUBTYPES, PHARMACOLOGY, AND TRANSDUCTION MECHANISMS
  4. SYNAPTIC LOCALIZATION OF mGlu RECEPTORS
  5. GLUTAMATE/ASPARTATE
  6. GABA
  7. DOPAMINE
  8. SEROTONIN (5-HT)
  9. MISCELLANEOUS
  10. Cholecystokinin (CCK)
  11. CONCLUSIONS AND FUTURE PERSPECTIVES

The nonselective mGlu receptor agonist t-ACPD elicited an inhibitory effect on 4-AP (1 mM)-evoked release of the octapeptide CCK (CCK-8) in rat hippocampal synaptosomes (Breukel et al., 1998). In this study, protein kinase C activation by the phorbol ester 4β-phorbol 12,13-dibutyrate dose-dependently increased 4-AP (1 mM)-evoked CCK-8 release, in a Ca2+-dependent manner (Breukel et al., 1998). Because previous studies had shown a similar activation of protein kinase C mediated by mGlu receptors (Coffey et al., 1993), the effect of the broad spectrum mGlu receptor agonist t-ACPD on CCK release was examined. In contrast to the expected results, 10 μMt-ACPD maximally suppressed CCK release by ∼30-40%. However, it is likely that this low concentration of t-ACPD is activating only group II mGlu receptors, although 100 μM produced no different effects than the lower concentration. It is possible that concentrations might have to be increased further to see any group I-mediated effects.

CCK is capable of inducing anxiogenic effects and panic in humans and laboratory animals, whereas antagonists of CCK receptors can be anxiolytic (for reviews, see Harro et al., 1993; Bradwejn and Koszycki, 1994). With this in mind, it is possible that group II mGlu receptors might mediate their anxiolytic effects in animals (see Helton et al., 1998) via a suppression of CCK release. Clearly, in vivo experiments demonstrating the involvement of mGlu receptors in the modulation of CCK release are necessary before such conclusions can be drawn.

Substance P

A recent report by Cuesta et al. (1999) demonstrated that group I and III mGlu receptors are able to modulate substance P release produced by capsaicin from rat trigeminal nucleus slices. DHPG and L-AP4 both concentration-dependently inhibited capsaicin-evoked substance P release by ∼60%. Although it is not known whether these effects result from activation of mGlu receptors on substance P-containing neurons or via an indirect mechanism involving other neurotransmitters, these data further implicate mGlu receptors in nociception. However, although previous studies have supported an antinociception role for group III receptors, others have demonstrated that group I mGlu receptors are pronociceptive (Fisher and Coderre, 1996). These apparently contrary data might be the result of the stimulation of pre- or postsynaptic group I receptors, which could result in opposing actions. Further insight into the effects of mGlu receptor activation on substance P release will be gained by additional studies, and therefore precise mGlu receptor targets for pain relief can be determined.

Taurine

The inhibitory amino acid taurine has been implicated as a modulator of neuronal function, particularly in the immature brain. Recently, Saransaari and Oja (1999) have shown potentiation of basal taurine release by group I agonists, such as quisqualate and DHPG. The effects of DHPG were reversed by AIDA and MCPG, further indicating mGlu receptor involvement. It is interesting that these actions were most pronounced in immature rat tissue. In contrast, K+-stimulated release was selectively enhanced by DCG-IV, indicating a different mechanism for modulation by group II receptors.

CONCLUSIONS AND FUTURE PERSPECTIVES

  1. Top of page
  2. Abstract
  3. mGlu SUBTYPES, PHARMACOLOGY, AND TRANSDUCTION MECHANISMS
  4. SYNAPTIC LOCALIZATION OF mGlu RECEPTORS
  5. GLUTAMATE/ASPARTATE
  6. GABA
  7. DOPAMINE
  8. SEROTONIN (5-HT)
  9. MISCELLANEOUS
  10. Cholecystokinin (CCK)
  11. CONCLUSIONS AND FUTURE PERSPECTIVES

The direct modulation of excitatory neurotransmission by mGlu receptors has extensive implications for the treatment of a number of pathological conditions, including pain, neurological insults such as ischemia and epilepsy, and psychiatric diseases (e.g., anxiety and psychosis). These receptors might provide a mechanism by which to “fine tune” neuronal activity, rather than completely shut down excitatory neurotransmission. Moreover, mGlu receptor-mediated modulation of nonglutamatergic neurotransmitters (directly or indirectly) might underlie therapeutic uses of mGlu receptor ligands for these and other disease states. However, detailed characterization of the potential role of mGlu receptors in the regulation of these other neurotransmitter systems (e.g., serotonergic and noradrenergic pathways) has yet to be determined.

To gain more insight into the role of mGlu receptors in neurotransmitter release, the precise role of each of the subtypes of the receptors requires investigation. Although the use of knock-out mice would significantly enhance current knowledge, potential problems with receptor/pathway adaptation and species specificity are possible. One of the more accessible ways to study the roles of individual receptors would be the use of subtype-specific drugs. Although newer, more selective compounds are being developed and characterized (see Pin et al., 1999; Schoepp et al., 1999), these are yet to be tested in many of the models discussed in this review.

The mechanism of action underlying mGlu receptor-evoked actions on neurotransmitter release also needs to be examined further. Although many of the effects observed might be due to regulation of Ca2+ or K+ currents, some actions are attributed to effects downstream of Ca2+ entry. Therefore, the modulation of specific elements of the neurotransmitter release cascade requires extensive investigation.

Despite the existence of the data described in this review demonstrating modulation of the synaptic release of numerous neurotransmitters by mGlu receptors, work in this area is still preliminary. In addition, the methods used in the studies reviewed in this article have potential drawbacks. Of particular note and as discussed here are the issues of tissue concentrations when applying drugs locally in microdialysis studies, and the extrapolation of such results in the absence of any comparison with systemically administered compounds. Furthermore, the interpretation of the results of studies using radiolabeled amino acids is made more complex by the fact that these effects may not be mediated via vesicular pools of transmitters (e.g., D-aspartate) or might involve the reversal of transporters (see Nicholls and Attwell, 1990; Ruzicka and Jhamandas, 1993).

Due to the relatively recent introduction of selective mGlu receptor compounds, we are only just beginning to clarify the roles of these receptors in neurotransmitter release. However, the continued development of more potent and selective compounds, coupled with neurochemical, electrophysiological, and molecular biological techniques, should further accelerate this relatively new area of mGlu receptor research.

  • 1
    Abdul-Ghani A., Attwell P.J.E., Kent N.S., Bradford H.F., Croucher M.J., Jane D.E. (1997) Anti-epileptogenic and anticonvulsant activity of L-2-amino-4-phosphonobutyrate, a presynaptic glutamate receptor agonist.Brain Res. 755 202212.
  • 2
    Allen J.W., Ivanova S.A., Fan L., Espey M.G., Basile A.S., Faden A.I. (1999) Group II metabotropic glutamate receptor activation attenuates traumatic neuronal injury and improves neurological recovery after traumatic brain injury.J. Pharmacol. Exp. Ther. 290 112120.
  • 3
    Anwyl R. (1999) Metabotropic glutamate receptors: electrophysiological properties and role in plasticity.Brain Res. Rev. 29 83120.
  • 4
    Arai I., Shimazoe T., Shibata S., Inoue H., Yoshimatsu A., Watanabe S. (1996) Enhancement of dopamine release from the striatum through metabotropic glutamate receptor activation in methamphetamine sensitized rats.Brain. Res. 729 277280.
  • 5
    Arai I., Shimazoe T., Shibata S., Inoue H., Yoshimatsu A., Watanabe S. (1997) Methamphetamine-induced sensitization of dopamine release via a metabotropic glutamate receptor mediated pathway in rat striatal slices.Jpn. J. Pharmacol. 73 243246.
  • 6
    Attwell P.J.E., Kaura S., Sigala G., Bradford H.F., Croucher M.J., Jane D.E., Watkins J.C. (1995) Blockade of both epileptogenesis and glutamate release by (1S,3S)-ACPD, a presynaptic glutamate receptor agonist. Brain Res. 698 155162.
  • 7
    Attwell P.J.E., Koumentaki A., Croucher M.J., Bradford H.F. (1998a) Specific group II metabotropic glutamate receptor activation inhibits the development of kindled epilepsy in rats.Brain Res. 787 286291.
  • 8
    Attwell P.J.E., Singh Kent N., Jane D.E., Croucher M.J., Bradford H.F. (1998b) Anticonvulsant and glutamate release-inhibiting properties of the highly potent metabotropic glutamate receptor agonist (2S,2′R,3′R)-2-(2′,3′-dicarboxycyclopropyl)glycine (DCG-IV). Brain Res. 805 138143.
  • 9
    Baskys A. & Malenka R.C. (1991) Agonists at metabotropic glutamate receptors presynaptically inhibit EPSCs in neonatal rat hippocampus.J. Physiol (Lond.) 444 687701.
  • 10
    Battaglia G., Monn J.A., Schoepp D.D. (1997) In vivo inhibition of veratridine-evoked release of striatal excitatory amino acids by the group II metabotropic glutamate receptor agonist LY354740 in rats.Neurosci. Lett. 229 161164.
  • 11
    Battaglia G., Perry K.W., Monn J.A., Schoepp D.D. (1998) Inhibition of glutamate and aspartate release in vivo by 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate, a selective group II mGluR agonist, in Metabotropic Glutamate Receptors and Brain Function (Moroni F., Nicoletti F., and Pellegrini-Giampietro D. E., eds), pp. 235248. Portland Press Ltd., London.
  • 12
    Baude A., Nusser Z., Roberts J.D.B., Mulvihill E., McIlhinney R.A.J., Somogyi P. (1993) The metabotropic glutamate receptor (mGluR1α) is concentrated at perisynaptic membrane of neuronal subpopulations as detected by immunogold reaction. Neuron 11 771787.
  • 13
    Bezzi P., Carmignoto G., Pasti L., Vesce S., Rossi D., Lodi Rizzini B., Pozzan T., Volterra A. (1998) Prostaglandins stimulate calcium-dependent glutamate release in astrocytes.Nature 391 281285.
  • 14
    Blaha C.D., Yang C.R., Floresco S.B., Barr A.M., Phillips A.G. (1997) Stimulation of the ventral subiculum of the hippocampus evokes glutamate receptor-mediated changes in dopamine efflux in the rat nucleus accumbens.Eur. J. Neurosci. 9 902911.
  • 15
    Boatright J.H., Gordon J.R., Iuvone P.M. (1994) Inhibition of endogenous dopamine release in amphibian retina by L-2-amino-4-phosphonobutyric acid (L-AP4) and trans-2-aminocyclopentane-1,3-dicarboxylic acid (ACPD). Brain Res. 649 339342.
  • 16
    Bonci A., Grillner P., Siniscalchi A., Mercuri N.B., Bernardi G. (1997) Glutamate metabotropic receptor agonists depress excitatory and inhibitory transmission on rat mesencephalic principal neurons.Eur. J. Neurosci. 9 23592369.
  • 17
    Bond A., O'Neill M.J., Hicks C.A., Monn J.A., Lodge D. (1998) Neuroprotective effects of a systemically active group II metabotropic glutamate receptor agonist LY354740 in a gerbil model of global ischaemia.Neuroreport 9 11911193.
  • 18
    Bradley S.R., Levey A.I., Hersch S.M., Conn P.J. (1996) Immunocytochemical localization of group III metabotropic glutamate receptors in the hippocampus with subtype-specific antibodies.J. Neurosci. 16 20442056.
  • 19
    Bradwejn J. & Koszycki D. (1994) The cholecystokinin hypothesis of anxiety and panic disorder.Ann. NY Acad. Sci. 713 273282.
  • 20
    Brakeman P.R., Lanahan A.A., O'Brien R., Roche K., Barnes C.A., Huganir R.L., Worley P.F. (1997) Homer: a protein that selectively binds metabotropic glutamate receptors.Nature 386 284288.
  • 21
    Breukel A.I.M., Weigant V.M., Lopes da Silva F.H., Ghijsen W.E.J.M. (1998) Presynaptic modulation of cholecystokinin release by protein kinase C in the rat hippocampus.J. Neurochem. 70 341348.
  • 22
    Bruton R.K., Ge J., Barnes N.M. (1999) Group I mGlu receptor modulation of dopamine release in the rat striatum in vivo.Eur. J. Pharmacol. 369 175181.
  • 23
    Budd D.C. & Nicholls D.G. (1995) Protein kinase C-mediated suppression of the presynaptic adenosine A1 receptor by a facilitatory metabotropic glutamate receptor.J. Neurochem. 65 615621.
  • 24
    Budd D.C. & Nicholls D.G. (1998) Arachidonic acid potentiates the duration of the metabotropic, protein kinase C-mediated, suppression of the inhibitory adenosine A1 receptor pathway in glutamatergic nerve terminals from rat cerebral cortex.Neurosci. Lett. 244 133136.
  • 25
    Bushell T.J., Jane D.E., Tse H., Watkins J.C., Garthwaite J., Collingridge G.L. (1996) Pharmacological antagonism of the actions of group II and III mGluR agonists in the lateral perforant path of rat hippocampal slices.Br. J. Pharmacol. 117 14571462.
  • 26
    Calabresi P., Mercuri N., Bernardi G. (1992) Activation of quisqualate metabotropic receptors reduces glutamate and GABA-mediated synaptic potentials in the rat striatum.Neurosci. Lett. 139 4144.
  • 27
    Caramelo O.L., Santos P.F., Carvalho A.P., Duarte C.B. (1999) Metabotropic glutamate receptors modulate [3H]acetylcholine release from cultured amacrine-like neurons. J. Neurosci. Res. 58 505514.
  • 28
    Carrozza D.P., Ferrero T.N., Golden G.T., Reyes P.F., Hare T.A. (1992) In vivo modulation of excitatory amino acid receptors: microdialysis studies on methyl-D-aspartate-evoked striatal dopamine release and effects of antagonists.Brain Res. 574 4248.
  • 29
    Catania M.V., Landwehrmeyer G.B., Testa C.M., Standaert D.G., Penney J.B., Young A.B. (1994) Metabotropic glutamate receptors are differentially regulated during development.Neuroscience 61 481495.
  • 30
    Ciruela F., Soloviev M.M., McIlhinney R.A.J. (1999) Co-expression of metabotropic glutamate receptor type 1α with Homer-1a/Vesl-1S increases the cell surface expression of the receptor.Biochem. J. 341 795803.
  • 31
    Coffey E.T., Sihra T.S., Nicholls D.G. (1993) Protein kinase C and the regulation of glutamate exocytosis from cerebrocortical synaptosomes.J. Biol. Chem. 268 2106021065.
  • 32
    Conn P.J. & Pin J. -P. (1997) Pharmacology and functions of metabotropic glutamate receptors.Annu. Rev. Pharmacol. Toxicol. 37 205237.
  • 33
    Conn P.J., Bradley S.R., Marino M.J., Wittman M., Rouse S.T. (1999) Physiological role of multiple mGluR subtypes in rat basal ganglia.Neuropharmacology 38 A12.
  • 34
    Cozzi A., Attucci S., Peruginelli F., Marinozzi M., Luneia R., Pelliciari R., Moroni F. (1997) Type 2 metabotropic glutamate (mGlu) receptors tonically inhibit transmitter release in rat caudate nucleus: in vivo studies with (2S,1′S,2′S,3′R)-2-(2′-carboxy-3′-phenylcyclopropyl)glycine, a new potent and selective antagonist. Eur. J. Neurosci. 9 13501355.
  • 35
    Cuesta M.C., Arcaya J.L., Cano G., Sanchez L., Maixner W., Suarez-Roca H. (1999) Opposite modulation of capsaicin-evoked substance P release by glutamate receptors.Neurochem. Int. 35 471478.
  • 36
    Dickie B.G., Annels S.J., Davies J.A. (1994) Effects of cyclo-oxygenase inhibition upon glutamate release from rat cerebellum.Neuroreport 5 393396.
  • 37
    Di Iorio P., Giuliani P., Ciccarelli R., Ballerini P., Battaglia G., Nicoletti F., Caciagli F. (1995) Effect of DCG-IV-sensitive metabotropic glutamate receptors on purine and glutamate release from rat hippocampal slices and cultured astrocytes.Res. Commun. Mol. Pathol. Pharmacol. 87 5758.
  • 38
    Di Iorio P., Battaglia G., Ciccarelli R., Ballerini P., Giuliani P., Poli A., Nicoletti F., Caciagli F. (1996) Interaction between A1 adenosine and class II metabotropic glutamate receptors in the regulation of purine and glutamate release from rat hippocampal slices.J. Neurochem. 67 302309.
  • 39
    Dubé G.R. & Marshall K.C. (1997) Modulation of excitatory synaptic transmission in locus coeruleus by multiple presynaptic metabotropic glutamate receptors.Neuroscience 80 511521.
  • 40
    East S.J., Hill M.O., Brotchie J.M. (1995) Metabotropic glutamate receptor agonists inhibit endogenous glutamate release from rat striatal synaptosomes.Eur. J. Pharmacol. 277 117121.
  • 41
    Feenstra M.G.P., Botterblom M.H.A., Van Uum J.F.M. (1998) Local activation of metabotropic glutamate receptors inhibits the handling-induced increased release of dopamine in the nucleus accumbens but not that of dopamine or noradrenaline in the prefrontal cortex: comparison with inhibition of ionotropic receptors.J. Neurochem. 70 11041113.
  • 42
    Fiorillo C.D. & Williams J.T. (1998) Glutamate mediates an inhibitory postsynaptic potential in dopamine neurons.Nature 394 7882.
  • 43
    Fisher K. & Coderre T.J. (1996) The contribution of metabotropic glutamate receptors (mGluRs) to formalin-induced nociception.Pain 68 255263.
  • 44
    Forsythe I.D. & Barnes-Davies M. (1997) Synaptic transmission: well-placed modulators.Curr. Biol. 7 R362R365.
  • 45
    Gereau R.W. & Conn P.J. (1995) Multiple presynaptic metabotropic glutamate receptors modulate excitatory and inhibitory synaptic transmission in hippocampal area CA1.J. Neurosci. 15 68796889.
  • 46
    Glaum S.R. & Miller R.J. (1993) 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 16361641.
  • 47
    Graeff F.G., Silveira M.C.L., Nogueira R.L., Audi E.A., Oliveira R.M.W. (1993) Role of the amygdala and periaqueductal gray in anxiety and panic.Behav. Brain Res. 58 123131.
  • 48
    Greenslade R.G., Woodhouse S., O'Neill M.J., Bond A., Ward M.A., Mitchell S.N. (1999) Is there a role for GABA in the neuroprotective effects of LY379268, a group II mGluR agonist, in global cerebral ischaemia?Proceedings of the 8th International Conference on In Vivo Methods. June 19-23, 1999, New York, NY, U.S.A.
  • 49
    Hanania T. & Johnson K.M. (1999) Regulation of NMDA-stimulated [14C]GABA and [3H]acetylcholine release by striatal glutamate and dopamine receptors. Brain Res. 844 106117.
  • 50
    Harro J., Vasar E., Bradwejn J. (1993) CCK in animal and human research on anxiety.Trends Pharmacol. Sci. 14 244249.
  • 51
    Hayashi Y., Momiyama A., Takahashi T., Ohishi H., Ogawa-Meguro R., Shigemoto R., Mizuno N., Nakanishi S. (1993) Role of metabotropic glutamate receptor in synaptic modulation in the accessory olfactory bulb.Nature 366 687690.
  • 52
    Helton D.R., Tizzano J.P., Monn J.A., Schoepp D.D., Kallman M.J. (1997) LY354740: a metabotropic glutamate receptor agonist which ameliorates symptoms of nicotine withdrawal in rats.Neuropharmacology 36 15111516.
  • 53
    Helton D., Tizzano J., Monn J., Schoepp D.D., Kallman M. (1998) Anxiolytic and side effect profile of LY354740: a potent, highly selective, orally active agonist for group II metabotropic glutamate receptors.J. Pharmacol. Exp. Ther. 284 651660.
  • 54
    Herrero I., Miras-Portugal M.T., Sanchez-Prieto J. (1992) Positive feedback of glutamate exocytosis by metabotropic presynaptic receptor stimulation.Nature 360 163166.
  • 55
    Herrero I., Castro E., Miras-Portugal M.T., Sanchez-Prieto J. (1996) Two components of glutamate exocytosis differentially affected by presynaptic modulation.J. Neurochem. 67 23462354.
  • 56
    Herrero I., Miras-Portugal M.T., Sanchez-Prieto J. (1998) Functional switch from facilitation to inhibition in the control of glutamate release by metabotropic glutamate receptors.J. Biol. Chem. 273 19511958.
  • 57
    Hu G., Duffy P., Swanson C., Ghasemzadeh M.B., Kalivas P.W. (1999) The regulation of dopamine transmission by metabotropic glutamate receptors.J. Pharmacol Exp. Ther. 289 412416.
  • 58
    Ishida M. & Shinozaki H. (1999) Inhibition of uptake and release of a novel mGluR agonist (L-F2CCG-I) by anion transport blockers in the rat spinal cord.Neuropharmacology 38 15311541.
  • 59
    Ishida M., Saitoh T., Shimamoto K., Ohfune Y., Shinozaki H. (1993) A novel metabotropic glutamate receptor agonist: marked depression of monosynaptic excitation in the newborn rat isolated spinal cord.Br. J. Pharmacol. 109 11691177.
  • 60
    Janáky R., Varga V., Saransaari P., Oja S.S. (1994) Glutamate agonists and [3H]GABA release from rat hippocampal slices: involvement of metabotropic glutamate receptors in the quisqualate-evoked release. Neurochem. Res. 19 729734.
  • 61
    Jones N.M., Lawrence A.J., Beart P.M. (1998a) In vivo microdialysis reveals facilitatory metabotropic glutamate receptors regulating excitatory amino acid release in rat nucleus tractus solitarius.Neurochem. Int. 32 3138.
  • 62
    Jones N.M., Monn J.A., Beart P.M. (1998b) Type I and II metabotropic glutamate receptors regulate the outflow of [3H]D-aspartate and [14C]γ-aminobutyric acid in rat solitary nucleus. Eur. J. Pharmacol. 353 4351.
  • 63
    Jouvenceau A., Dutar P., Billard J. -M. (1995) Presynaptic depression of inhibitory postsynaptic potentials by metabotropic glutamate receptors in rat hippocampal CA1 pyramidal cells.Eur. J. Pharmacol. 281 131139.
  • 64
    Kilbride J., Huang L.Q., Rowan M.J., Anwyl R. (1998) Presynaptic inhibitory action of the group II metabotropic glutamate receptor agonists, LY354740 and DCG-IV.Eur. J. Pharmacol. 356 149157.
  • 65
    Kingston A.E., O'Neill M.J., Lam A., Bales K.R., Monn J.A., Schoepp D.D. (1999) Neuroprotection by metabotropic glutamate receptor agonists: LY354740, LY379268, and LY389795.Eur. J. Pharmacol. 377 155165.
  • 66
    Koerner J.F. & Cotman C.W. (1981) Micromolar L-2-amino-phosphonobutyric acid selectively inhibits perforant path synapses from lateral entorhinal cortex.Brain Res. 216 192198.
  • 67
    Konieczny J., Ossowska K., Wolfarth S., Pilc A. (1998) LY354740, a group II metabotropic glutamate receptor agonist with potential antiparkinsonian properties in rats.Naunyn Schmiedebergs Arch. Pharmacol. 358 500502.
  • 68
    Lada M.W., Vickroy T.W., Kennedy R.T. (1998) Evidence for the neuronal origin and metabotropic receptor-mediated regulation of extracellular glutamate and aspartate in rat striatum in vivo following electrical stimulation of the prefrontal cortex.J. Neurochem. 70 617625.
  • 69
    Lafon-Cazal M., Viennois G., Kuhn R., Malitschek B., Pin J., Shigemoto R., Bockaert J. (1999) mGluR7-like receptor and GABAB receptor activation enhance neurotoxic effects of N-methyl-D-aspartate in cultured mouse striatal GABAergic neurones. Neuropharmacology 38 16311640.
  • 70
    Liu J. & Moghaddam B. (1995) Regulation of glutamate efflux by excitatory amino acid receptors: evidence for tonic inhibitory and phasic excitatory regulation.J. Pharmacol. Exp. Ther. 274 12091215.
  • 71
    Liu Y., Disterhoft J.F., Slater N.T. (1993) Activation of metabotropic glutamate receptors induces long-term depression of GABAergic inhibition in hippocampus.J. Neurophysiol. 69 10001004.
  • 72
    Llano I. & Marty A. (1995) Presynaptic metabotropic glutamatergic regulation of inhibitory synapses in rat cerebellar slices.J. Physiol. (Lond.) 486 163176.
  • 73
    Lombardi G., Alesiani M., Leonardi P., Cherici G., Pelliciari R., Moroni F. (1993) Pharmacological characterization of the metabotropic glutamate receptor inhibiting D-[3H]-aspartate output in rat striatum. Br. J. Pharmacol. 110 14071412.
  • 74
    Lombardi G., Pellegrini-Giampietro D.E., Leonardi P., Cherici G., Pellicciari R., Moroni F. (1994) The depolarization-induced outflow of D-[3H]aspartate from rat brain slices is modulated by metabotropic glutamate receptors. Neurochem. Int. 24 525532.
  • 75
    Lombardi G., Leonardi P., Moroni F. (1996) Metabotropic glutamate receptors, transmitter output and fatty acids: studies in rat brain slices.Br. J. Pharmacol. 117 189195.
  • 76
    Lovinger D.M. (1991) trans-1-Aminocyclopentane-1,3-dicarboxylic acid (t-ACPD) decreases synaptic excitation in rat striatal slices through a presynaptic action. Neurosci. Lett. 129 1721.
  • 77
    Lovinger D.M. & McCool B.A. (1995) Metabotropic glutamate receptor-mediated presynaptic depression at corticostriatal synapses involves mGluR2 or 3.J. Neurophysiol. 73 10761083.
  • 78
    Luján R., Nusser Z., Roberts J.D.B., Shigemoto R., Somogyi P. (1996) Perisynaptic location of metabotropic glutamate receptors mGluR1 and mGluR5 on dendrites and dendritic spines in the rat hippocampus.Eur. J. Neurosci. 8 14881500.
  • 79
    Luján R., Roberts J.D.B., Shigemoto R., Ohishi H., Somogyi P. (1997) Differential plasma distribution of metabotropic glutamate receptors mGluR1α, mGluR2 and mGluR5, relative to neurotransmitter release sites. J. Comp. Neuroanat. 13 219241.
  • 80
    Maione S., Palazzo E., De Novellis V., Stella L., Leyva J., Rossi F. (1998) Metabotropic glutamate receptors modulate serotonin release in the rat periaqueductal gray matter.Naunyn Schmiedebergs Arch. Pharmacol. 358 411417.
  • 81
    Maione S., Palazzo E., Oliva P., Marabese I., Liguori G., Rossi F., Riccio M.R., Rossi F. (1999) Role of metabotropic glutamate receptors in nociception.Neuropharmacology 38 A48.
  • 82
    Manzoni O. & Bockaert J. (1995) Metabotropic glutamate receptors inhibiting excitatory synapses in the CA1 area of rat hippocampus.Eur. J. Neurosci. 7 25182523.
  • 83
    Manzoni O., Michel J., Bockaert J. (1997) Metabotropic glutamate receptors in the rat nucleus accumbens.Eur. J. Neurosci. 9 15141523.
  • 84
    Marek G.J., Wright R.A., Schoepp D.D., Monn J.A., Aghajanian G.K. (2000) Physiological antagonism between 5-hydroxytryptamine2A and group II metabotropic glutamate receptors in prefrontal cortex.J. Pharmacol. Exp. Ther. 292 7687.
  • 85
    Marino M.J., Bradley S.R., Wittmann M., Rouse S.T., Levey A.I., Conn P.J. (1999) Potential antiparkinsonian actions on metabotropic glutamate receptors in the substantia nigra pars reticulate.Neuropharmacology 38 A29.
  • 86
    Martin L.J., Blackstone C.D., Huganir R.L., Price D.L. (1992) Cellular localization of a metabotropic glutamate receptor.Neuron 9 259270.
  • 87
    Matsumura K., Tsuchihashi T., Kagiyama S., Abe I., Fujishima M. (1999) Subtypes of metabotropic glutamate receptors in the nucleus of the solitary tract of rats.Brain Res. 842 461468.
  • 88
    McGahon B. & Lynch M.A. (1994) A study of the synergism between metabotropic glutamate receptor activation and arachidonic acid in the rat hippocampus.Neuroreport 5 23532357.
  • 89
    McGahon B. & Lynch M.A. (1996a) The synergism between metabotropic glutamate receptor activation and arachidonic acid on glutamate release is occluded by induction of long-term potentiation in the dentate gyrus.Neuroscience 72 847855.
  • 90
    McGahon B. & Lynch M.A. (1996b) The synergism between ACPD and arachidonic acid on glutamate release in hippocampus is age-dependent.Eur. J. Pharmacol. 309 323326.
  • 91
    Mercuri N.B., Stratta F., Calabresi P., Bonci A., Bernardi G. (1993) Activation of metabotropic glutamate receptors induces an inward current in rat dopamine mesencephalic neurons.Neuroscience 56 399407.
  • 92
    Mineff E. & Valtschanoff J. (1999) Metabotropic glutamate receptors 2 and 3 expressed by astrocytes in rat ventrobasal thalamus.Neurosci. Lett. 270 9598.
  • 93
    Moghaddam B. & Adams B.K. (1998) Reversal of phencyclidine effects by a group II metabotropic receptor agonist in rats.Science 281 13491351.
  • 94
    Moghaddam B., Adams B., Verma A., Daly D. (1997) Activation of glutamatergic neurotransmission by ketamine: a novel step in the pathway from NMDA receptor blockade to dopaminergic and cognitive disruptions associated with the prefrontal cortex.J. Neurosci. 17 29212927.
  • 95
    Moroni F., Cozzi A., Lombardi G., Sourtcheva S., Leonardi P., Carfi M., Pellicciari R. (1998) Presynaptic mGlul type receptors potentiate transmitter output in the rat cortex.Eur. J. Pharmacol. 347 189195.
  • 96
    Murase S., Grenhoff J., Chouvet G., Gonon F.G., Svensson T.H. (1993) Prefrontal cortex regulates burst firing and transmitter release in rat mesolimbic dopamine neurons studied in vivo.Neurosci. Lett. 157 5356.
  • 97
    Nakanishi S. (1992) Molecular diversity of glutamate receptors and implications for brain function.Science 258 597603.
  • 98
    Nakanishi S. (1994) Metabotropic glutamate receptors: synaptic transmission, modulation, and plasticity.Neuron 13 10311037.
  • 99
    Nakanishi S., Nakajima Y., Masu M., Ueda Y., Nakahara K., Watanabe D., Yamaguchi S., Kawabata S., Okada M. (1998) Glutamate receptors: brain function and signal transduction.Brain Res. Rev. 26 230235.
  • 100
    Nicholls D. & Attwell D. (1990) The release and uptake of excitatory amino acids.Trends Pharmacol. Sci. 11 462468.
  • 101
    Nusser Z., Mulvihill E., Streit P., Somogyi P. (1994) Subsynaptic segregation of metabotropic and ionotropic glutamate receptors as revealed by immunogold localization.Neuroscience 61 421427.
  • 102
    Ohishi H., Shigemoto R., Nakanishi S., Mizuno N. (1993) Distribution of the mRNA for a metabotropic glutamate receptor (mGluR3) in the rat brain: an in situ hybridization study.J. Comp. Neurol. 335 252266.
  • 103
    Ohishi H., Ogawa-Meguro R., Shigemoto R., Kaneko T., Nakanishi S., Mizuno N. (1994) Immunohistochemical localization of metabotropic glutamate receptors, mGluR2 and mGluR3, in rat cerebellar cortex.Neuron 13 5566.
  • 104
    Ohno M. & Watanabe S. (1995) Persistent increase in dopamine release following activation of metabotropic glutamate receptors in the rat nucleus accumbens.Neurosci. Lett. 200 113116.
  • 105
    Ottersen O.P. & Landsend A.S. (1997) Organization of glutamate receptors at the synapse.Eur. J. Neurosci. 9 22192224.
  • 106
    Petralia R.S., Wang, Y., Niedzielski A.S., Wenthold R.J. (1996) The metabotropic glutamate receptors, mGluR2 and mGluR3, show unique postsynaptic, presynaptic and glial localizations.Neuroscience 71 949976.
  • 107
    Pin J., De Colle C., Bessis A., Ascher F. (1999) New perspectives for the development of selective metabotropic glutamate receptor ligands.Eur. J. Pharmacol. 375 277294.
  • 108
    Pintor A., Tiburzi F., Pezzola A., Volpe M.T. (1998) Metabotropic glutamate receptor agonist (1S,3R-ACPD) increased frontal complex dopamine release in aged but not in young rats. Eur. J. Pharmacol. 359 139142.
  • 109
    Pisani A., Calabresi P., Centonze D., Bernardi G. (1997) Activation of group III metabotropic glutamate receptors depresses glutamatergic transmission at corticostriatal synapse.Neuropharmacology 36 845851.
  • 110
    Reid M.E., Toms N.J., Bedingfield J.S., Roberts P.J. (1999) Group I mGlu receptors potentiate synaptosomal [3H]glutamate release independently of exogenously applied arachidonic acid. Neuropharmacology 38 477485.
  • 111
    Rodriguez-Moreno A., Sistiaga A., Lerma J., Sanchez-Prieto J. (1998) Switch from facilitation to inhibition of excitatory synaptic transmission by group I mGluR desensitization.Neuron 21 14771486.
  • 112
    Ruzicka B.B. & Jhamandas K.H. (1993) Excitatory amino acid action on the release of brain neurotransmitters and neuromodulators: biochemical studies.Prog. Neurobiol. 40 223247.
  • 113
    Sacaan A.I., Bymaster F.P., Schoepp D.D. (1992) Metabotropic glutamate receptor activation produces extrapyramidal motor system activation that is mediated by striatal dopamine.J. Neurochem. 59 245251.
  • 114
    Salt T.E. & Eaton S.A. (1995) Distinct presynaptic metabotropic receptors for L-AP4 and CCG-1 on GABAergic terminals: pharmacological evidence using novel α-methyl derivative mGluR antagonists. MAP4 and MCCG, in the rat thalamus in vivo. Neuroscience 65 513.
  • 115
    Salt T.E. & Turner J.P. (1998) Modulation of sensory inhibition in the ventrobasal thalamus via activation of group II metabotropic glutamate receptors by 2R,4R-aminopyrrolidine-2,4-dicarboxylate. Exp. Brain Res. 121 181185.
  • 116
    Samuel D., Pisano P., Forni C., Nieoullon A., Kerkerian-Le Goff L. (1996) Involvement of the glutamatergic metabotropic receptors in the regulation of glutamate uptake and extracellular excitatory amino acid levels in the striatum of chloral hydrate-anesthetized rats.Brain Res. 739 156162.
  • 117
    Saransaari P. & Oja S.S. (1999) Involvement of metabotropic glutamate receptors in taurine release in the adult and developing mouse hippocampus.Amino Acids 16 165179.
  • 118
    Scanziani M., Salin P.A., Vogt K.E., Malenka R.C., Nicoll R.A. (1997) Use-dependent increases in glutamate concentration activate presynaptic metabotropic glutamate receptors.Nature 385 630634.
  • 119
    Scanziani M., Gahwiler B.H., Charpak S. (1998) Target cell-specific modulation of transmitter release at terminals from a single axon.Proc. Natl. Acad. Sci. USA 95 1200412009.
  • 120
    Schaffhauser H., Knoflach K., Pink J.R., Bleuel Z., Cartmell J., Goepfert F., Kemp J.A., Richards J.G., Adam G., Mutel V. (1998) Multiple pathways for regulation of the KCl-induced [3H]-GABA release by metabotropic glutamate receptors, in primary rat cortical cultures. Brain Res. 782 91104.
  • 121
    Schoepp D.D., Jane D.E., Monn J.A. (1999) Pharmacological agents acting at subtypes of metabotropic glutamate receptors.Neuropharmacology 38 14311476.
  • 122
    Shigemoto R., Kulik A., Roberts J.D.B., Ohishi H., Nusser Z., Kaneko T., Somogyi P. (1996) Target-cell-specific concentration of a metabotropic glutamate receptor in the presynaptic active zone.Nature 381 523525.
  • 123
    Shigemoto R., Kinoshita A., Wada E., Nimura S., Ohishi H., Takada M., Flor P.J., Neki A., Abe T., Nakanishi S., Mizuno N. (1997) Differential presynaptic localization of metabotropic glutamate receptor subtypes in rat hippocampus.J. Neurosci. 17 75037522.
  • 124
    Sistiaga A., Herrero I., Conquet F., Sanchez-Prieto J. (1998) The metabotropic glutamate receptor 1 is not involved in the facilitation of glutamate release in cerebrocortical nerve terminals.Neuropharmacology 37 14851492.
  • 125
    Taber M.T. & Fibiger H.C. (1995) Electrical stimulation of the prefrontal cortex increases dopamine release in the nucleus accumbens of the rat: modulation by metabotropic glutamate receptors.J. Neurosci. 15 38963904.
  • 126
    Timmerman W. & Westerink B.H.C. (1997) Brain microdialysis of GABA and glutamate: what does it signify?Synapse 27 242261.
  • 127
    Trombley P.Q. & Westbrook G.L. (1992) L-AP4 inhibits calcium currents and synaptic transmission via a G-protein-coupled glutamate receptor.J. Neurosci. 12 20432050.
  • 128
    Vandergriff J. & Rasmussen K. (1999) The selective mGlu2/3 receptor agonist LY354740 attenuates morphine-withdrawal-induced activation of locus coeruleus neurons and behavioral signs of morphine withdrawal.Neuropharmacology 38 217222.
  • 129
    Vazquez E. & Sanchez-Prieto J. (1997) Presynaptic modulation of glutamate release targets different calcium channels in rat cerebrocortical nerve terminals.Eur. J. Neurosci. 9 20092018.
  • 130
    Vazquez E., Herrero I., Miras-Portugal M.T., Sanchez-Prieto J. (1994) Facilitation of glutamate release by metabotropic glutamate receptors in hippocampal nerve terminals.Neurosci. Res. Commun. 15 187194.
  • 131
    Vazquez E., Budd D.C., Herrero I., Nicholls D.G., Sanchez-Prieto J. (1995a) Co-existence and interaction between facilitatory and inhibitory metabotropic glutamate receptors and the inhibitory adenosine A1 receptor in cerebrocortical nerve terminals.Neuropharmacology 34 919927.
  • 132
    Vazquez E., Herrero I., Miras-Portugal M.T., Sanchez-Prieto J. (1995b) Developmental change from inhibition to facilitation in the presynaptic control of glutamate exocytosis by metabotropic glutamate receptors.Neuroscience 68 117124.
  • 133
    Verma A. & Moghaddam B. (1998) Regulation of striatal dopamine release by metabotropic glutamate receptors.Synapse 28 220226.
  • 134
    Wang J. & Johnson K.M. (1995) Regulation of striatal cyclic-3′,5′-adenosine monophosphate accumulation and GABA release by glutamate metabotropic and dopamine D1 receptors.J. Pharmacol. Exp. Ther. 275 877884.
  • 135
    Wang J., Lonart G., Johnson K.M. (1996) Glutamate receptor activation induces carrier mediated release of endogenous GABA from rat striatal slices.J. Neural Transm. 103 3143.
  • 136
    Westerink B., Tuntler J., Damama G., Rollema H., De Vries J. (1987) The use of tetrodotoxin for the characterization of drug-enhanced dopamine release in conscious rats by brain dialysis.Naunyn Schmiedebergs Arch. Pharmacol. 336 502507.
  • 137
    Wigmore M.A. & Lacey M.G. (1998) Metabotropic glutamate receptors depress glutamate-mediated synaptic input to rat midbrain dopamine neurones in vitro.Br. J. Pharmacol. 123 667674.
  • 138
    Winder D.G. & Conn P.J. (1996) Roles of metabotropic glutamate receptors in glial function and glial-neuronal communication.J. Neurosci. Res. 46 131137.
  • 139
    Winder D.G., Ritch P.S., Gereau R.W., Conn P.J. (1996) Novel glial-neuronal signalling by co-activation of metabotropic glutamate and β-adrenergic receptors in rat hippocampus. J. Physiol. (Lond.) 494 743755.
  • 140
    Worley P.F. (1999) Homer signaling complex links group I metabotropic and IP3 receptors. (Abstr.)J. Neurochem. 73 (Suppl.),S103.
  • 141
    Ye Z. & Sontheimer H. (1999) Metabotropic glutamate receptor agonists reduce glutamate release from cultured astrocytes.Glia 25 270281.
  • 142
    Yokoi M., Kobayashi K., Manabe T., Takahashi T., Sakaguchi I., Katsuura G., Shigemoto R., Ohishi H., Nomura S., Nakamura K., Nakao K., Katsuki M., Nakanishi S. (1996) Impairment of hippocampal mossy fiber LTD in mice lacking mGluR2.Science 273 645647.