Subtype-specific involvement of metabotropic glutamate receptors in two forms of long-term potentiation in the dentate gyrus of freely moving rats

Abstract

In this study, the role of metabotropic glutamate receptors in N-methyl- d-aspartate receptor-dependent and voltage-gated calcium channel-dependent long-term potentiation in the dentate gyrus of freely moving rats was investigated. Antagonists for group 1 metabotropic glutamate receptors ((S)-4-carboxyphenylglycine), group 1/2 metabotropic glutamate receptors ((RS)- α-methyl-4-carboxyphenylglycine) and group 2 metabotropic glutamate receptors ((RS)- α-methylserine O-phosphate monophenylester) were used. The N-methyl- d-aspartate receptor antagonist, d(−)-2-amino-5-phosphonopentanoic acid, and the L-type voltage-gated calcium channel antagonist, methoxyverapamil were used to investigate the N-methyl- d-aspartate receptor and voltage-gated calcium channel contribution to the long-term potentiation recorded. Field excitatory postsynaptic potential slope function and population spike amplitude were measured. Drugs were applied, prior to tetanus, via a cannula implanted into the lateral cerebral ventricle. 200 Hz tetanization produces an long-term potentiation which is inhibited by application of d(−)-2-amino-5-phosphonopentanoic acid and (RS)- α-methyl-4-carboxyphenylglycine. In this study, a dose-dependent inhibition of 200 Hz long-term potentiation expression was obtained with (S)-4-carboxyphenylglycine. Long-term potentiation induced by 400 Hz tetanization was not inhibited by d(−)-2-amino-5-phosphonopentanoic acid, although the amplitude of short-term potentiation was reduced. (RS)- α-methyl-4-carboxyphenylglycine and (S)-4-carboxyphenylglycine, both in the presence and absence of d(−)-2-amino-5-phosphonopentanoic acid, inhibited the development of 400 Hz long-term potentiation. (RS)- α-methylserine O-phosphate monophenylester had no significant effect on long-term potentiation induced by either 200 or 400 Hz tetanization. Application of methoxyverapamil significantly inhibited 400 Hz long-term potentiation, but had no effect on 200 Hz long-term potentiation. These data suggest that 400 Hz long-term potentiation, induced in the presence of d(−)-2-amino-5-phosphonopentanoic acid, requires activation of L-type calcium channels. Furthermore, these results strongly support a critical role for group 1 metabotropic glutamate receptors in both N-methyl- d-aspartate receptor- and voltage-gated calcium channel-dependent long-term potentiation.