In vivo pharmacological characterization of glycine site ligands of the NMDA receptor complex.

Abstract

N-methyl-D-aspartate (NMDA) receptor agonist recognition site is located on a receptor complex that includes a site sensitive to glycine. The interaction between agonists and antagonists on these two sites has not been well characterized in whole-animal tests. The series of experiments described here were designed to evaluate the ability of glycine site antagonists to modify the convulsive effects of NMDA in mice, and also to study the interaction between glycine site agonists and antagonists using operant behavior in pigeons. The glycine site antagonist (+)3-amino-1-hydroxy-2-pyrrolidone ((+)-HA 966) and 7-chlorokynurenic acid (7CKA) increased the ED$\sb{50}$ of intracerebroventricularly (i.c.v.) administered NMDA in producing convulsive behavior and death in mice. The maximum effect of NMDA was not decreased by these antagonists, but, in contrast to a typical competitive interaction, large doses of (+)-HA 966 did not produce further shifts in the NMDA dose-response curves. This pattern of interaction was consistent with a metaffinoid interaction in which an antagonist produces a decrease in the affinity of an agonist by binding at a different site. Both (+)-HA 966 and 7CKA decreased operant behavior in pigeons in a dose-dependent manner. The potency relationships of the glycine antagonists found in mice and pigeons were consistent with those obtained in vitro. Four glycine site agonists were examined for their effectiveness in reversing the effects of (+)-HA 966 in mice and pigeons. D-serine completely reversed (+)-HA 966's protection of NMDA-induced behavior in mice; 1-aminocyclopropane-1-carboxylic acid (ACC) reversed (+)-HA 966's protection of NMDA-induced saltation, as well as the protection afforded by low but not high doses of (+)-HA 966 agonist NMDA-induced tonic-clonic convulsions and death. D-cycloserine and D-alanine also reversed (+)-HA 966's protection of NMDA-induced saltation, but not tonic-clonic convulsions and death. The partial agonist nature of these latter two drugs was confirmed by the finding that they blocked D-serine's effects in this paradigm. In pigeons, D-serine and ACC were effective, and D-cycloserine and D-alanine were not effective in attenuating the rate-decreasing effects of (+)-HA 966. The rank order of efficacy of these drugs was therefore D-serine $\geq$ ACC $>$ D-cycloserine = D-alanine. These in vivo efficacies are consistent with those obtained in vitro. These results suggest that the behavioral effects of glycine agonists and antagonists are mediated via the glycine site of the NMDA receptor complex. In conclusion, these data confirm the hypothesis that glycine acts as a positive modulator of NMDA at a specific site on the NMDA receptor complex and that this interaction can be revealed in whole-animal tests.