Metabotropic glutamate receptor subtype agonists facilitate long-term potentiation within a distinct time window in the dentate gyrus in vivo.

Trans-azetidine-2,4-dicarboxylic acid (ADA) is a putative selective agonist of group 1 metabotropic glutamate receptors. It has been shown previously that application of ADA prior to a short-term potentiation-inducing high-frequency tetanus facilitates long-term potentiation in vivo. In order to examine the role of metabotropic glutamate receptors in this response, we studied the effect of ADA in the dentate gyrus of the rat when applied after high-frequency tetanus to the perforant path. A comparison was made with the effects of the metabotropic glutamate receptor group 1 agonist 3,5-dihydroxyphenylglycine. Drugs were applied via a cannula implanted in the lateral cerebral ventricle. Both population spike amplitude and field excitatory postsynaptic potential were measured. Weak tetanization produced a short-term potentiation of field excitatory postsynaptic potential and population spike which decayed to baseline values by 90 min, and was unaffected by vehicle injections. Application of ADA (20 mM/5 microliters) or 3,5-dihydroxyphenylglycine (4mM/5 microliters) 5 min after high-frequency tetanus facilitated short-term potentiation into a long-term potentiation which lasted over 24 h. (R,S)-alpha-Methyl-4-carboxyphenylglycine (200 mM/5 microliters), a metabotropic glutamate receptor antagonist, when applied prior to high-frequency tetanus and ADA or 3,5-dihydroxyphenylglycine, completely inhibited this effect. ADA applied 10,15,20 and 25 min after high-frequency tetanus also facilitated short-term potentiation into long-term potentiation, but the magnitude of long-term potentiation was smaller than than produced by ADA given 5 min after tetanus. Similar effects were seen with 3,5-dihydroxyphenylglycine applied 25 min after high-frequency tetanus. When (R,S)-alpha-methyl-4-carboxyphenylglycine was applied prior to high-frequency tetanus and ADA or 3,5-dihydroxyphenylglycine applied 30 min after high-frequency tetanus, or after short-term potentiation decay, elicited no facilitation of long-term potentiation. These results indicate that a distinct time window for the enhancement by ADA and 3,5-dihydroxyphenylglycine of short-term potentiation into long-term potentiation occurs in the dentate gyrus in vivo. This suggests that metabotropic glutamate receptor activation in long-term potentiation occurs within a finite period of time and may be mediated by group 1 metabotropic glutamate receptors. Furthermore, it suggests that metabotropic glutamate receptor modulation of N-methyl-D-aspartate receptors does not account for the role of metabotropic glutamate receptors in long-term potentiation.