The depolarization-induced outflow of D-[3H]aspartate from rat brain slices is modulated by metabotropic glutamate receptors

The gray area between synapse structure and function-Gray's synapse types I and II revisited

On the basis of ultrastructural parameters, the concept was formulated that asymmetric Type I and symmetric Type II synapses are excitatory and inhibitory, respectively. This "functional Gray synapses concept" received strong support from the demonstration of the excitatory neurotransmitter glutamate in Type I synapses and of the inhibitory neurotransmitter γ-aminobutyric acid in Type II synapses, and is still frequently used in modern literature. However, morphological and functional evidence has accumulated that the concept is less tenable. Typical features of synapses like shape and size of presynaptic vesicles and synaptic cleft and presence of a postsynaptic density (PsD) do not always fit the postulated (excitatory/inhibitory) function of Gray's synapses. Furthermore, synapse function depends on postsynaptic receptors and associated signal transduction mechanisms rather than on presynaptic morphology and neurotransmitter type. Moreover, the notion that many synapses are difficult to classify as either asymmetric or symmetric has cast doubt on the assumption that the presence of a PsD is a sign of excitatory synaptic transmission. In view of the morphological similarities of the PsD in asymmetric synapses with membrane junctional structures such as the zonula adherens and the desmosome, asymmetric synapses may play a role as links between the postsynaptic and presynaptic membrane, thus ensuring long-term maintenance of interneuronal communication. Symmetric synapses, on the other hand, might be sites of transient communication as takes place during development, learning, memory formation, and pathogenesis of brain disorders. Confirmation of this idea might help to return the functional Gray synapse concept its central place in neuroscience.

Presynaptic metabotropic glutamate and GABAB receptors

Glutamate and GABA, the two most abundant neurotransmitters in the mammalian central nervous system, can act on metabotropic receptors that are structurally quite dissimilar from those targeted by most other neurotransmitters/modulators. Accordingly, metabotropic glutamate receptors (mGluRs) and GABA(B) receptors (GABA(B)Rs) are classified as members of family 3 (or family C) of G protein-coupled receptors. On the other hand, mGluRs and GABA(B)Rs exhibit pronounced and partly unresolved differences between each other. The most intriguing difference is that mGluRs exist as multiple pharmacologically as well as structurally distinct subtypes, whereas, in the case of GABA(B)Rs, molecular biologists have so far identified only one structurally distinct heterodimeric complex whose few variants seem unable to explain the pharmacological heterogeneity of GABA(B)Rs observed in many functional studies. Both mGluRs and GABA(B)Rs can be localized on axon terminals of different neuronal systems as presynaptic autoreceptors and heteroreceptors modulating the exocytosis of various transmitters.

The group II metabotropic glutamate receptor agonist, LY379268, inhibits both cocaine- and food-seeking behavior in rats

RATIONALE

Group II metabotropic glutamate receptor (mGluR2/3) agonists are proposed to serve as potential treatment for addiction.

OBJECTIVES

The present study examined the hypothesis that mGluR2/3 agonists exert inhibitory effects on cocaine-induced reinstatement of cocaine-seeking.

METHODS

Rats were trained to self-administer either cocaine or control reinforcer (food), then responding on the reinforcer-paired lever was extinguished. Reinstatement of responding was induced by a noncontingent presentation of the self-administered reinforcer (10 mg/kg cocaine, i.p. or 765 mg of food). In one experiment, rats were systemically pretreated with vehicle (Veh) or the mGluR2/3 agonist LY379268 (0.3, 1, or 3 mg/kg, i.p.) 30 min before the reinstatement test session. In a second experiment, Veh or LY379268 (0.05, 0.5, or 5 nmol/side) was microinjected into the nucleus accumbens core (NAc core) 5 min before the reinstatement test session. The effects of LY379268 on cocaine- and food-induced reinstatement on reward seeking were assessed.

RESULTS

Both systemic and intra-NAc core pretreatment with LY379268 inhibited both cocaine- and food-seeking behavior. However, the effect of LY379268 appeared somewhat more effective for cocaine-seeking than food-seeking.

CONCLUSIONS

These results support a potential therapeutic role for mGluR2/3 agonists on relapse of cocaine-seeking. However, doses that inhibited cocaine-seeking were only threefold lower than those inhibiting food-seeking, indicating possible unacceptable nonspecific effects. In addition, the NAc core is one site of action where the mGluR2/3 agonists elicit effects on reward-seeking behavior.

Dopamine-glutamate reciprocal modulation of release and motor responses in the rat caudate-putamen and nucleus accumbens of "intact" animals

Functional interactions between dopaminergic neurotransmission and glutamatergic neurotransmission are well known to play a crucial integrative role in the striatum, the major input structure of the basal ganglia now widely recognized to contribute to the control of motor activity and movements but also to the processing of cognitive and limbic functions. However, the nature of these interactions is still a matter of debate and controversy. This review (1) summarizes anatomical data on the distribution of dopaminergic and glutamatergic receptors in the striatum-accumbens complex, (2) focuses on the dopamine-glutamate interactions in the modulation of each other's release in the striatum-accumbens complex, and (3) examines the dopamine-glutamate interactions in the entire striatum involved in the control of locomotor activity. The effects of dopaminergic and glutamatergic receptor selective agonists and antagonists on dopamine and glutamate release as well on motor responses are analyzed in the entire striatum, by reviewing both in vitro and in vivo data. Regarding in vivo data, only findings from focal injections studies in the nucleus accumbens or the caudate-putamen of "intact" animals are reviewed. Altogether, the available data demonstrate that dopamine and glutamate do not uniformly interact to modulate each others' release and postsynaptic modulation of striatal output neurons. Depending on the receptor subtypes involved, interactions between dopaminergic and glutamatergic transmission vary as a multiple and complex combination of tonic, phasic, facilitatory, and inhibitory properties.

Inhibition of non-vesicular glutamate release by group III metabotropic glutamate receptors in the nucleus accumbens

Previous in vitro studies have shown that group III metabotropic glutamate receptors (mGluRs) regulate synaptic glutamate release. The present study used microdialysis to characterize this regulation in vivo in rat nucleus accumbens. Reverse dialysis of the group III mGluR agonist l-(+)-2-amino-4-phosphonobutyric acid (L-AP4) decreased, whereas the antagonist (R,S)-alpha-methylserine-O-phosphate (MSOP) increased the extracellular level of glutamate. The decrease by L-AP4 or the increase by MSOP was antagonized by co-administration of MSOP or L-AP4, respectively. Activation of mGluR4a by (1S,3R,4S)-1-aminocyclopentane-1,2,4-tricarboxylic acid or mGluR6 by 2-amino-4-(3-hydroxy-5-methylisoxazol-4-yl)butyric acid had no effect on extracellular glutamate. (R,S)-4-Phosphonophenylglycine (PPG), another group III agonist with high affinity for mGluR4/6/8, reduced extracellular glutamate only at high concentrations capable of binding to mGluR7. The increase in extracellular glutamate by MSOP was tetrodotoxin-independent, and resistant to both the L-type and N-type Ca2+ channel blockers. L-AP4 failed to block 30 mm K+-induced vesicular glutamate release. Blockade of glutamate uptake by d,l-threo-beta-benzyloxyaspartate caused a Ca2+-independent elevation in extracellular glutamate that was reversed by L-AP4. Finally, (S)-4-carboxyphenylglycine, an inhibitor of cystine-glutamate antiporters, attenuated the L-AP4-induced reduction in extracellular glutamate. Together, these data indicate that group III mGluRs regulate in vivo extracellular glutamate in the nucleus accumbens by inhibiting non-vesicular glutamate release.

Interactions of carbon monoxide and metabotropic glutamate receptor groups in the nucleus tractus solitarii of rats

Carbon monoxide has been shown to act as a neurotransmitter and neuronal messenger in the brain. Heme oxygenase catalyzes the conversion of heme to carbon monoxide and biliverdin. We have recently reported that carbon monoxide was involved in central cardiovascular regulation. Carbon monoxide modulated the baroreflex and may affect glutamatergic neurotransmission. In addition, metabotropic glutamate receptors may be coupled to the activation of heme oxygenase in the nucleus tractus solitarii of rats. The present study was designed to investigate the possible interactions of carbon monoxide and metabotropic glutamate receptor groups in the nucleus tractus solitarii. Unilateral microinjection of several agonists for metabotropic glutamate receptor groups such as (R,S)-3,5-dihydroxyphenylglycine (DHPG) (group I) (0.03 nmol), 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (APDC) (group II) (0.3 nmol), and l-(+)-2-amino-4-phosphonobutyric acid (l-AP4) (group III) (0.3 nmol) produced a significant decrease in blood pressure and heart rate. Among the metabotropic glutamate receptor agonists, prior administration of zinc protoporphyrin IX, an inhibitor of heme oxygenase activity, significantly attenuated the cardiovascular effects of APDC and l-AP4, and failed to attenuate the cardiovascular responses of DHPG. These results indicated interactions between carbon monoxide and group II and III metabotropic glutamate receptors in central cardiovascular regulation.

Characterisation of the actions of group I metabotropic glutamate receptor subtype selective ligands on excitatory amino acid release and sodium-dependent re-uptake in rat cerebrocortical minislices

In this study we have tested the effects of a wide range of metabotropic glutamate receptor ligands on (i) depolarisation-evoked efflux of pre-accumulated d-[3H]aspartic acid (d-[3H]asp) from rapidly superfused rat cerebrocortical minislices, and (ii) Na+-dependent uptake of d-[3H]asp into cerebrocortical tissue. Transient elevations in extracellular K+ produced concentration-dependent increases in d-[3H]asp efflux. A submaximally effective concentration (50 mm) was used in all subsequent experiments. The broad-spectrum mGlu receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD; EC50 17.8 microm], the group I mGlu-selective agonist (S)-3,5-dihydroxyphenylglycine [(S)-3,5-DHPG; EC50 0.5 microm] and the mGlu5 receptor subtype-selective agonist (RS)-2-chloro-5-hydroxyphenylglycine [(RS)-CHPG; EC50 7.3 microm] all concentration-dependently potentiated high K+-evoked d-[3H]asp efflux in the absence of effects on basal outflow of radiolabel. At concentrations selective for mGlu1 receptors, the antagonists (RS)-1-aminoindan-1,5-dicarboxylic acid [(RS)-AIDA; 10-300 microm]; (+)-2-methyl-4-carboxyphenylglycine [LY367385; 1-100 microm] and 7-hydroxyiminocyclopropan[b]chromen-1a-carboxylate ethyl ester [CPCCOEt, 1-30 microm] all failed to inhibit responses to (S)-3,5-DHPG. However, the broad-spectrum mGlu receptor antagonist (S)-alpha-methyl-4-carboxyphenylglycine [(S)-MCPG; IC50 88.5 microm] together with the recently described mGlu5-selective antagonists, 2-methyl-6-(phenylethynyl)-pyridine (MPEP; IC50 0.6 microm), 6-methyl-2-(phenyl-azo)-3-pyridinol (SIB-1757; IC50 4.4 microm) and (E)-2-methyl-6-(2-phenylethenyl)pyridine (SIB-1893; IC50 3.1 microm), at mGlu5-selective concentrations, all powerfully and concentration-dependently inhibited (S)-3,5-DHPG-evoked responses. Two selective excitatory amino acid (EAA) uptake inhibitors, l-trans-2,4-pyrrolidine dicarboxylate (l-trans-2,4-PDC; IC50 229 microm) and dl-threo-beta-benzyloxyaspartate (dl-TBOA; IC50 665 microm) both inhibited the Na+-dependent uptake of d-[3H]asp into cerebrocortical minislices. Importantly, none of the mGlu ligands utilized in the present study significantly inhibited d-[3H]asp uptake at concentrations shown to potentiate K+-evoked efflux. These data demonstrate for the first time that mGlu5 ligands modulate extracellular EAA concentrations by a direct effect on mGlu5-type autoreceptors on EAA nerve terminals as they evoke clear changes in EAA release in the absence of any effects on EAA uptake. Selective mGlu5 receptor antagonists that show high potency and good central bioavailability may provide novel classes of neuroprotective agents for the treatment of brain disorders associated with abnormal EAAergic neurotransmission.

Actions of Group I and Group II metabotropic glutamate receptor ligands on 5-hydroxytryptamine release in the rat cerebral cortex in vivo: differential roles in the regulation of central serotonergic neurotransmission

We have previously shown that the release of central neurotransmitters can be modulated by the activation of Group I and Group II subtypes of G-protein-linked metabotropic glutamate (mGlu) receptors. To date, however, very little is known about the regulation of serotonergic neurotransmission by these receptor subtypes. In the present study, we have utilized in vivo intracerebral microdialysis to elucidate the roles of Group I and Group II mGlu receptors in the regulation of neuronal 5-hydroxytryptamine (5-HT) release in the frontal cortex of conscious, freely moving rats. Dialysate 5-HT was of neuronal origin with basal release showing strong calcium dependency and tetrodotoxin sensitivity and marked elevation following K(+)-induced depolarization. The broad-spectrum mGlu receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD; 1-3 mM] did not significantly modify basal cerebrocortical 5-HT release. Similarly, the Group I mGlu receptor-specific agonist (RS)-3,5-dihydroxyphenylglycine [(RS)-3,5-DHPG; 1-3 mM] showed no marked effect on cortical dialysate 5-HT levels. To eliminate the possibility that these findings were the result of receptor desensitization, the effects of lower concentrations of (RS)-DHPG (100-300 microM) and shorter ligand exposure time (15 min) were also evaluated. Dialysate 5-HT levels remained unmodified by these manipulations. In comparison, the Group II mGlu receptor agonist, (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-1; 500 microM), evoked a marked facilitation of release (approximately 150% of basal) which was fully reversed by the Group I/II antagonist, (S)-alpha-methyl-4-carboxyphenylglycine [(S)-MCPG; 3 mM]. The modulatory action of L-CCG-1 showed a bell-shaped concentration-response relationship. (S)-MCPG (3 mM) and the potent and selective mGlu(5) receptor antagonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP; 100 microM), when given alone, did not significantly modify 5-HT levels.The current data provide strong evidence to suggest that while the release of neuronal 5-HT in the rat frontal cortex is not subject to regulation by facilitatory Group I mGlu receptors, it may be positively modulated by activation of Group II mGlu receptors. Taken together with data from other studies, the present investigation lends emphasis to the notion that neuromodulation by mGlu receptors is a region-specific phenomenon and also proposes that the heterogeneous distribution of these receptors is neurone-specific in its complexity. The failure of (S)-MCPG alone to modify cortical 5-HT release suggests that Group II mGlu receptors do not tonically modulate serotonergic neurotransmission in the cerebral cortex but this does not preclude an important functional role for these receptors during pathological conditions when endogenous neurotransmitter levels become excessively elevated. The strategic development of new subtype-specific mGlu receptor ligands may provide novel therapeutic agents for the treatment of a range of neurological and psychiatric disorders.

Effects of metabotropic glutamate receptor agonists and antagonists on D-aspartate release from mouse cerebral cortical and striatal slices

The cytosolic release of L-glutamate has been held to be responsible for the increase in extracellular glutamate to toxic levels in the brain. The mechanism and regulation of this release was now studied in cerebral cortical and striatal slices with D-[3H]aspartate, a non-metabolized analogue of L-glutamate and a poor substrate for vesicular uptake. L-Glutamate and D-aspartate strongly stimulated the release in a concentration-dependent manner. Of the ionotropic glutamate receptor agonists, only kainate enhanced the basal release in the striatum. Of the metabotropic glutamate receptor ligands, the group I agonist (S)-3,5-dihydroxyphenylglycine (S-DHPG) failed to affect the basal release but inhibited the D-aspartate-evoked release in the striatum. The group I antagonist (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA) had no effect on the basal release in either preparation but enhanced the L-glutamate-evoked release and inhibited the D-aspartate-evoked release in the striatum, not however in the cerebral cortex. The group II agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) and the group II antagonist (2S)-2-ethylglutamate (EGLU) were without effect on the basal, D-aspartate- and L-glutamate-evoked releases of D-[3H]aspartate in either preparation. The group III agonist L-serine-O-phosphate (L-SOP) failed to affect the basal release but reduced the D-aspartate-evoked release in the striatum. The group III antagonist (RS)alpha-methylserine-O-phosphate (MSOP) failed to affect the basal release but increased the glutamate-evoked release and inhibited the D-aspartate-evoked release in the striatum. Both L-trans-pyrrolidine-2,4-dicarboxylate (L-trans-PDC) and (2S,1'S,2'R)-2-carboxycyclopropyl)glycine (L-CCG-III), transportable inhibitors of the high-affinity glutamate uptake, enhanced the basal release, more strongly in the striatum than in the cerebral cortex. L-CCG-III also increased the L-glutamate-evoked release in the striatum. Nontransportable dihydrokainate enhanced the basal release much less and failed to affect the glutamate-evoked release. The results indicate that the release of glutamate from cytosolic pools is carrier-mediated via homoexchange. This process is regulated in the striatum by metabotropic group I and group III receptors in a manner different from the regulation of the vesicular release of glutamate from presynaptic terminals.

Endogenous sulphur-containing amino acids: potent agonists at presynaptic metabotropic glutamate autoreceptors in the rat central nervous system

We have recently demonstrated that presynaptically located metabotropic glutamate (mGlu) autoreceptors regulate synaptic glutamate release both in vitro and in vivo. We now report a positive modulatory action of the sulphur-containing amino acids (SCAAs), L-cysteic acid (CA) and L-cysteine sulphinic acid (CSA), at presynaptic group I mGlu receptors, specifically of the mGlu5 subtype, acting to enhance synaptic glutamate release from the rat forebrain in vitro. Neuronal glutamate release was monitored using electrically-evoked efflux of preloaded [(3)H]-D-aspartate from rat forebrain hemisections. Both CA (3 - 100 muM) and CSA (1 - 100 microM), in addition to the selective group I mGlu receptor agonist, (S)-3,5-dihydroxyphenylglycine ((S)-DHPG), concentration-dependently enhanced electrically-stimulated efflux of [(3)H]-D-aspartate from the rat forebrain slices. Basal efflux of label remained unchanged. The inhibitory activity of the broad spectrum mGlu receptor antagonist, (+/-)-alpha-methyl-4-carboxyphenylglycine ((+/-)-MCPG; 200 microM), coupled with the inactivity of the selective mGlu1 receptor antagonists, (R,S)-1-aminoindan-1,5-dicarboxylic acid ((R,S)-AIDA; 100 - 500 microM) and the more potent (+)-2-methyl-4-carboxyphenylglycine (LY367385; 10 microM) against these responses, indicates an action of the SCAAs at the mGlu5 receptor subtype. This proposal is supported by the potent inhibition of these responses by the selective, non-competitive mGlu5 receptor antagonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP; 10 microM). The observed enhancement of the responses to high concentrations of CA by the selective mGlu5 receptor desensitization inhibitor, cyclothiazide (CYZ; 10 microM), is also consistent with this concept. Administration of the agonists in the presence of bovine serum albumin (BSA; 5 - 15 mg ml(-1)) markedly attenuated the positive modulatory responses observed, strongly supporting a role for arachidonic acid in the expression of these mGlu5 receptor-mediated responses. The regulatory actions of SCAAs on synaptic glutamate release demonstrated in the present study may provide a physiological function for these putative neurotransmitter amino acids in the mammalian brain. These central actions of the SCAAs may have wide-ranging implications for a range of neurological and neuropsychiatric disease states and their treatment.

Metabotropic glutamate receptors modulate GABA release from mouse hippocampal slices

The effects of metabotropic glutamate receptor agonists on the basal and potassium (50 mM K+)-stimulated release of [3H]GABA from mouse hippocampal slices were investigated using a superfusion system. The group I agonist (1+/-)-1-aminocyclopentane-trans-1,3-dicarboxylate enhanced the basal GABA release and reduced the K+-evoked release by a mechanism antagonized by (RS)-1-aminoindan-1,5-dicarboxylate in both cases. The group II agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine failed to have any effect on the basal release, but inhibited the stimulated release. This inhibition was not affected by the antagonist (2S)-2-ethylglutamate. The group III agonists L(+)-amino-4-phosphonobutyrate and O-phospho-L-serine inhibited the basal GABA release, which effects were blocked by the antagonist (RS)-2-cyclopropyl-4-phosphonophenylglycine. Moreover, the suppression of the K+-evoked release by L(+)2-amino-4-phosphonobutyrate was apparently receptor-mediated, being blocked by (RS)-2-cyclopropyl-4-phosphonophenylglycine. The results show that activation of metabotropic glutamate receptors of group I is able to potentiate the basal release of GABA, whereas activation of groups I and III receptors reduce K+-stimulated release in mouse hippocampal slices.

Regulation of neurotransmitter release by metabotropic glutamate receptors

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 Ca(2+) or K(+) channels, or interfere with release processes downstream of Ca(2+) 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.

Metabotropic glutamate autoreceptors of the mGlu(5) subtype positively modulate neuronal glutamate release in the rat forebrain in vitro

In the present study we have examined the role of presynaptic group I metabotropic glutamate (mGlu) receptors in the control of neuronal glutamate release using rat forebrain slices pre-loaded with [(3)H]D-aspartate. We have also addressed the question of which group I mGlu receptor subtype, mGlu(1) or mGlu(5), mediates the facilitatory response observed by the use of a range of established and some more novel agonists and antagonists showing selectivity for these receptors. The electrically-stimulated release of pre-loaded [(3)H]D-aspartate from rat forebrain slices was markedly potentiated by the potent group I mGlu receptor agonist, L-quisqualic acid (L-QUIS), in a concentration-dependent manner (EC(50) 17.31 microM). This response was inhibited by the mGlu receptor antagonists (S)-MCPG (100 microM) and (RS)-MTPG (100 microM) but not by the AMPA-type ionotropic glutamate receptor antagonist, NBQX (100 microM). The selective group I mGlu receptor agonist (S)-3, 5-dihydroxyphenylglycine ((S)-DHPG) also enhanced electrically-stimulated efflux of label, although responses diminished with high (10-100 microM) concentrations of the agonist. Maximum responses were fully restored when (S)-DHPG (10 microM) was applied in the presence of the proposed mGlu(5) receptor desensitization inhibitor, cyclothiazide (10 microM). The positive modulatory response to (S)-DHPG (1 microM) was powerfully inhibited by (S)-MCPG (IC(50) 0.08 microM) but was resistant to the mGlu(1) receptor antagonists, (RS)-AIDA (1-500 microM), CPCCOEt (0.1-100 microM) and (+)-2-methyl-4-carboxyphenylglycine (LY367385) (0.1-10 microM). The recently developed, selective mGlu(5) receptor agonist (RS)-2-chloro-5-hydroxyphenylglycine ((RS)-CHPG) enhanced electrically-stimulated [(3)H]D-aspartate efflux from rat forebrain slices with a similar concentration-response profile to that of (S)-DHPG. Responses to this receptor subtype-selective agonist were also blocked by (S)-MCPG (IC(50) 1.13 microM) but were unaffected by (RS)-AIDA (500 microM), CPCCOEt (100 microM) or LY367385 (10 microM). These results indicate that the positive modulation of neuronal glutamate release seen in the rat forebrain in the presence of group I mGlu receptor agonists is mediated by presynaptically located mGlu(5) glutamate autoreceptors. The pharmacological profile of these receptors appears to be distinct from that of postsynaptic mGlu receptors. Novel antagonists acting at these presynaptic receptors may provide new drugs for the experimental therapy of a range of acute or chronic neurodegenerative disorders.

1-Aminoindan-1,5-dicarboxylic acid and (S)-(+)-2-(3'-carboxybicyclo[1.1.1] pentyl)-glycine, two mGlu1 receptor-preferring antagonists, reduce neuronal death in in vitro and in vivo models of cerebral ischaemia

Metabotropic glutamate (mGlu) receptors have been implicated in a number of physiological and pathological responses to glutamate, but the exact role of group I mGlu receptors in causing postischaemic injury is not yet clear. In this study, we examined whether the recently-characterized and relatively selective mGlu1 receptor antagonists 1-aminoindan-1,5-dicarboxylic acid (AIDA) and (S)-(+)-2-(3'-carboxybicyclo[1.1.1]pentyl)-glycine (CBPG) could reduce neuronal death in vitro, following oxygen-glucose deprivation (OGD) in murine cortical cell and rat organotypic hippocampal cultures, and in vivo, after global ischaemia in gerbils. When present in the incubation medium during the OGD insult and the subsequent 24 h recovery period, AIDA and CBPG significantly reduced neuronal death in vitro. The extent of protection was similar to that observed with the nonselective mGlu receptor antagonist (+)-alpha-methyl-4-carboxyphenylglycine [(+)MCPG] and with typical ionotropic glutamate (iGlu) receptor antagonists. Neuroprotection was also observed when AIDA or CBPG were added only after the OGD insult was terminated. Neuronal injury was not attenuated by the inactive isomer (-)MCPG, but was significantly enhanced by the nonselective mGlu receptor agonist (1S,3R)-1-aminocyclopentane-1, 3-dicarboxylic acid [(1S,3R)-ACPD] and the group I mGlu receptor agonist 3,5-dihydroxyphenylglycine (3,5-DHPG). The antagonists (+)MCPG, AIDA and CBPG were also neuroprotective in vivo, because i. c.v. administration reduced CA1 pyramidal cell degeneration examined 7 days following transient carotid occlusion in gerbils. Our results point to a role of mGlu1 receptors in the pathological mechanisms responsible for postischaemic neuronal death and propose a new target for neuroprotection.

Protection with metabotropic glutamate 1 receptor antagonists in models of ischemic neuronal death: time-course and mechanisms

In order to study the role of metabotropic glutamate 1 (mGlu1) receptors in ischemic neuronal death, we examined the effects of the recently characterized and relatively selective mGlu1 receptor antagonists 1-aminoindan-1,5-dicarboxylic acid (AIDA) and (S)-(+)-2-(3'-carboxybicyclo[1.1.1]pentyl)-glycine (CBPG) in murine cortical cell cultures and rat organotypic hippocampal slices exposed to oxygen glucose deprivation (OGD) and in vivo, following transient global ischemia in gerbils. AIDA and CBPG significantly reduced neuronal death when added to the incubation medium during the OGD insult and the subsequent recovery period. Neuroprotection was observed even when these compounds were added up to 60 min (in cortical neurons) or 30 min (in hippocampal slices) after OGD. In vivo, i.c.v. administration of AIDA and CBPG reduced hippocampal CA1 pyramidal cell injury following transient global ischemia. Neuroprotection was also observed when AIDA was added to the hippocampal perfusion fluid in microdialysis experiments, and this effect was associated with an increase in the basal output of GABA. These findings demonstrate that AIDA and CBPG are neuroprotective when administered during the maturation of ischemic damage and that different mechanisms are likely to be involved in mediating their effects following blockade of mGlu1 receptors in cortical and hippocampal neurons.

Involvement of metabotropic glutamate receptors in taurine release in the adult and developing mouse hippocampus

The inhibitory amino acid taurine has been held to function as an osmoregulator and modulator of neural activity, being particularly important in the immature brain. Ionotropic glutamate receptor agonists are known markedly to potentiate taurine release. The effects of different metabotropic glutamate receptor (mGluR) agonists and antagonists on the basal and K(+)-stimulated release of [3H]taurine from hippocampal slices from 3-month-old (adult) and 7-day-old mice were now investigated using a superfusion system. Of group I metabotropic glutamate receptor agonists, quisqualate potentiated basal taurine release in both age groups, more markedly in the immature hippocampus. This action was not antagonized by the specific antagonists of group I but by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione (NBQX), which would suggest an involvement of ionotropic glutamate receptors. (S)-3,5-dihydroxyphenylglycine (DHPG) potentiated the basal release by a receptor-mediated mechanism in the immature hippocampus. The group II agonist (2S, 2'R, 3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) markedly potentiated basal taurine release at both ages. These effects were antagonized by dizocilpine, indicating again the participation of ionotropic receptors. Group III agonists slightly potentiated basal taurine release, as did several antagonists of the three metabotropic receptor groups. Potassium-stimulated (50 mM K+) taurine release was generally significantly reduced by mGluR agents, mainly by group I and II compounds. This may be harmful to neurons in hyperexcitatory states. On the other hand, the potentiation by mGluRs of basal taurine release, particularly in the immature hippocampus, together with the earlier demonstrated pronounced enhancement by activation of ionotropic glutamate receptors, may protect neurons against excitotoxicity.

Type I and II metabotropic glutamate receptors regulate the outflow of [3H]D-aspartate and [14C]gamma-aminobutyric acid in rat solitary nucleus

Metabotropic glutamate (mGlu) receptors modulating amino acid outflow were examined in a model system in order to further characterize the pharmacological nature of the mGlu receptors involved in viscerosensory processing in the nucleus tractus solitarii. The actions of a number of subtype-selective mGlu receptor agonists and antagonists were monitored on the K+-evoked outflow of [3H]D-aspartate and [14C]gamma-aminobutyric acid (GABA) from superfused slices of rat nucleus tractus solitarii. (+/-)1S,3R-1-Amino-cyclopentane-1,3-dicarboxylate (10-300 microM), produced a concentration-dependent increase in outflow, which was attenuated by a number of phenylglycine antagonists. (2S,3S,4S)-alpha-(Carboxycyclopropyl)-glycine (30-300 microM) had mixed effects on outflow. The type I-selective agonist (RS)-3,5-dihydroxyphenylglycine (300 microM) also increased outflow and these effects were reversed by the type I antagonist (RS)-1-aminoindan-1,5-dicarboxylate (100 microM). Activation of type II mGlu receptors with (2R,4R)-aminopyrrolidine-2,4-dicarboxylate (300 microM), however, decreased outflow, and this effect was antagonized by the type II antagonist LY307452 (200 microM). Interestingly, LY307452 (200 microM) alone, enhanced outflow of [3H]D-aspartate, but not [14C]GABA. Type III mGlu receptors may not be involved in outflow of [3H]D-aspartate and [14C]GABA in the nucleus tractus solitarii, as L-2-amino-4-phosphonobutyrate (30-300 microM) had no effect under the present experimental conditions. These in vitro studies provide new evidence for roles for Type I and II mGlu receptors in viscerosensory processing in nucleus tractus solitarii.

Presynaptic mGlu1 type receptors potentiate transmitter output in the rat cortex

In the present study we used freely moving rats with a microdialysis probe placed in their parietal cortex to study the effects of local application of agonists and antagonists of metabotropic glutamate (mGlu) receptors on glutamate release. (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD; 0.1-1 mM), a non-selective agonist of metabotropic glutamate (mGlu) receptors, increased glutamate concentration in the dialysate up to 3-fold. A significant increase in glutamate output in cortical dialysates was also obtained with (RS)-3,5-dihydroxyphenylglycine (DHPG; 0.5-1 mM), a group 1-selective mGlu receptor agonist, suggesting the involvement of group 1 mGlu receptors in 1S,3R-ACPD effects. S-4-carboxyphenylglycine (S-4CPG; 0.3 microM), a mGlu1 receptor antagonist with a mild agonist action on mGlu2 receptors, antagonised, in a surmountable manner, the effects of 1S,3 R-ACPD. Similarly, 1-aminoindan-1,5-dicarboxylic acid (AIDA; 0.03-1 mM) a selective group 1 antagonist with a preferential action on mGlu1 type receptors, antagonised the effects of 1S,3R-ACPD. Finally, (S)-(+)-2-(3'-Carboxybicyclo[1.1.1]pentyl)-glycine (UPF596; 30-300 microM), a potent mGlu1 antagonist with modest agonist activity on mGlu5, antagonised 1S,3R-ACPD-induced glutamate release. In conclusion, our data showed that 1S,3R-ACPD-induced glutamate release in the parietal cortex is mediated by mGlu1 receptors and that, under basal conditions, these receptors are not tonically activated.

Multiple pathways for regulation of the KCl-induced [3H]-GABA release by metabotropic glutamate receptors, in primary rat cortical cultures

In rat cortical primary cultures, group II- and III-metabotropic glutamate receptor-selective agonists concentration-dependently reduced KCl-induced [3H]GABA release, with IC50 values of 11 nM for LY354740, 80 nM for L(+)-2-amino-4-phosphonobutyric acid (L-AP4), 180 nM for DCG-IV, and 330 nM for L-SOP. The group II antagonists, LY341495 and EGLU, reversed the effect of LY354740, and the group III antagonist MTPG reversed the effect of L-AP4. In the presence of omega-conotoxin GVIA, LY354740 inhibited the remaining [3H]GABA release, whereas L-AP4 was inactive. In contrast, in the presence of nifedipine, L-AP4 inhibited the remaining [3H]GABA release, but LY354740 was no longer active. The PKA inhibitor, H89, blocked the effects of both L-AP4 and LY354740, whereas the PKC inhibitor Ro 31-8220 blocked only the effect of LY354740. Both Ro 31-8220 and H89 reduced the [3H]GABA release to 60% of control. In whole-cell, voltage-clamp experiments, LY354740 and L-AP4 inhibited voltage-gated calcium channel currents with IC50 values of 28 nM and 22 microM, respectively. The results suggest that, in these cells, KCl-induced [3H]GABA release is modulated by two different mechanisms, one involving group II receptors and a direct control of the Ca2+ channel activity, and the other mediated by group III receptors and possibly involving a regulation located downstream of the Ca2+ channel activation.

Evidence for a role of presynaptic AMPA receptors in the control of neuronal glutamate release in the rat forebrain

The role of presynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in controlling the neuronal release of excitatory amino acids has been investigated. Stimulation of presynaptic AMPA receptors by the endogenous agonist L-glutamate, or by (R,S)-AMPA, dose-dependently enhanced the Ca(2+)-dependent, tetrodotoxin-insensitive, electrically-stimulated release of [3H]D-aspartate from rat forebrain slices. This AMPA receptor-mediated response showed marked stereoselectivity with the activity residing solely in the (S)-isomer. (R)-AMPA was inactive in this respect. AMPA-evoked responses were significantly enhanced in the presence of the AMPA receptor desensitization inhibitor, cyclothiazide (10 microM). Moreover, responses to both AMPA and glutamate were inhibited by competitive (NBQX) and non-competitive (GYKI 52466) AMPA receptor-selective antagonists in a dose-dependent manner. These results provide strong support for the existence of presynaptic AMPA receptors acting to enhance the synaptic release of excitatory amino acids in the mammalian forebrain. Such a positive feedback system may play an important functional role in physiological (e.g., long-term potentiation) and/or pathological (e.g., epileptogenesis) processes in the mammalian central nervous system. AMPA-type autoreceptors may provide new targets for drug action.

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

Anatomical, biochemical and electrophysiological studies have previously shown that cortico-striatal terminals contain abundant presynaptic group 2 metabotropic glutamate (mGlu) receptors. Using brain slices we have previously shown that these receptors inhibit depolarization-induced transmitter release. Using microdialysis in freely moving rats, we now report the effects of group 2 mGlu receptor agonists and antagonists on glutamate concentration in the caudate extracellular fluid. A mild decrease (20-30%) in glutamate concentration in caudate dialysates was observed when 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid or (2S,3S,4S)-alpha-carboxycyclopropyl-glycine (L-CCG-1), mGlu receptor agonists, was locally administered. On the contrary, alpha-methyl-4-carboxyphenylglycine, an antagonist of type 1 and type 2 mGlu receptors, increased the glutamate concentration in dialysates by up to 3.5-fold, and its effects were prevented by the simultaneous administration of L-CCG-1, a preferential type 2 mGlu receptor agonist. A significant increase of glutamate output in striatal dialysate was also found after local administration of (2S,1'S,2'S,3'R)-2-(2'-carboxy-3'-phenylcyclopropyl)glycine, another structurally unrelated, relatively selective and potent type 2 mGlu receptor antagonist. The results suggest that type 2 mGlu receptors tonically inhibit transmitter release from cortico-striatal terminals. Since the cortico-striatal pathway profoundly affects the function of a large percentage of caudate neurons, it is reasonable to predict that the use of selective type 2 mGlu receptor agents will be helpful for scientific and therapeutic studies on the physiopathology of basal ganglion disorders.

Activation of multiple metabotropic glutamate receptor subtypes prevents NMDA-induced excitotoxicity in rat hippocampal slices

Metabotropic glutamate receptors (mGluRs) belong to a relative large receptor family consisting of multiple members with important roles in a number of brain functions. We report here that activation of mGluRs prevents the neurotoxic effect induced by N-methyl-D-aspartate (NMDA) in slices from the rat hippocampus. Neuroprotection was elicited when slices were simultaneously exposed to both the selective mGluR agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (tACPD) and NMDA. Persisting stimulation of mGluRs after the toxic exposure did not improve the survival of pyramidal or granular cells. The neuroprotection elicited by tACPD toxic exposure did not improve the survival of pyramidal or granular cells. The neuroprotection elicited by tACPD was also evoked by its active isomer, (1S, 3R)-ACPD, and was prevented by the selective mGluR antagonist (+)-alpha-methyl-4-carboxyphenyl-glycine (500 microM), confirming that mGluR activation is involved in the mechanism of action of tACPD. The effect of 100 microM tACPD was reproduced by 100 microM quisqualate, an agonist of mGluR2 and mGluR3 subtypes. No neuroprotection was induced by L-2-amino-4-phosphonobutyrate, a selective agonist for mGluR4, mGluR6, mGluR7 and mGluR8, at 500 microM. Since the NMDA-mediated cell death in hippocampal slices is considered relevant to ischaemia-induced brain injury, these results indicate that mGluRs may be important safety devices used by neurons to decrease their sensitivity to excitotoxic stimuli and increase their chance of survival.

Metabotropic glutamate receptors, transmitter output and fatty acids: studies in rat brain slices

1. The effects of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), a non-selective agonist of the metabotropic glutamate receptors (mGluRs), have been studied in rat cortical and striatal slices by measuring the depolarization-induced output of D-[3H]-aspartate (D-[3H]-Asp) and of [3H]-glutamate ([3H]-Glu), neosynthesized from [3H]-glutamine. 2. In cortical slices, 1S,3R-ACPD potentiated the depolarization-induced (KCl, 30 mM) output of both D-[3H]-Asp and [3H]-Glu. The potentiation, obtained at 300 microM 1S,3R-ACPD was 65 +/- 6% for D-[3H]-Asp and 56 +/- 10% for [3H]-Glu. Conversely, in striatal slices, 1S,3R-ACPD reduced the depolarization-induced transmitter output. The reduction, obtained at 300 microM of the agonist, was 60 +/- 8% for D-[3H]-Asp and 50 +/- 5% for neosynthesized [3H]-Glu. 3. Bovine serum albumin (BSA, 15 microM), which is able to bind locally produced fatty acids, completely eliminated the potentiating effect 1S,3R-ACPD had on D-[3H]-Asp output from cortical slices. Low concentrations of arachidonic acid (1-10 microM) or of oleic acid (1-10 microM) added to BSA-containing perfusion medium, restored this potentiating effect. BSA, however, had no effect on the inhibitory action of 1S,3R-ACPD in striatal slices. 4. Bromophenacyl bromide (100 microM), an inhibitor of phospholipase A2, and RG80267 (100 microM), an inhibitor of diacylglycerol lipase, have been shown to inhibit fatty acid production. These compounds prevented the potentiating effect of 1S,3R-ACPD on D-[3H]-Asp-output in cortical slices. 5. Indomethacin (100 microM), an inhibitor of cyclo-oxygenases, plus nordihydroguaiaretic acid (100 microM), an inhibitor of lipoxygenases, increased D-[3H]-Asp output in cortical slices perfused with BSA-containing medium. 6. These experiments suggest that the mGluR-mediated potentiation of transmitter output requires the availability of unsaturated fatty acids, such as arachidonic or oleic acids, in cortical slices. In contrast, the mGluR-induced inhibition of transmitter output is not dependent upon fatty acid availability in striatal slices. The requirement of both unsaturated fatty acids and 1S,3R-ACPD in the facilitation of transmitter exocytosis may play an important role in the regulation of synaptic plasticity.