Abstract

The strychnine-insensitive glycine site on the N-methyl-D-aspartate (NMDA) receptor complex is a target for development of a host of therapeutic agents including anxiolytics, antidepressants, antiepileptics, anti-ischemics and cognitive enhancers. In the present experiments, the discriminative stimulus effects of (+)-HA-966 [R-(+)-3-amino-1-hydroxypyrrolid-2-one], a low-efficacy partial agonist of the glycine site, was explored. Male, Swiss-Webster mice were trained to discriminate (+)-HA-966 (170 mg/kg i.p.) from saline in a T-maze under which behavior was controlled by food. Other glycine partial agonists, 1-amino-1-cyclopropanecarboxilic acid and D-cycloserine, fully substituted for the discriminative stimulus effects of (+)-HA-966 despite known differences in other pharmacological effects of these compounds. The glycine site antagonist, 7-chlorkynurenic acid, did not substitute for (+)-HA-966. Likewise other functional NMDA antagonists acting at nonglycine sites of the NMDA receptor also did not substitute: neither the high (dizocilpine) or low affinity (ibogaine) ion-channel blocker, the competitive antagonist, NPC 17742 [2R,4R,5S-2-amino-4,5-(1, 2-cyclohexyl)-7-phosphonoheptanoic acid], nor the polyamine antagonist, ifenprodil, substituted for (+)-HA-966. Although the full agonist, glycine, did not substitute, this compound fully blocked the discriminative stimulus effects of (+)-HA-966. In a separate group of mice trained to discriminate 0.17 mg/kg of dizocilpine from saline, (+)-HA-966 produced a maximum of only 50% dizoclipine-appropriate responses. These data suggest that the discriminative stimulus effects of (+)-HA-966 are based upon its partial agonist actions at the strychnine-insensitive glycine site.(ABSTRACT TRUNCATED AT 250 WORDS)