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.