Differential effects of agents acting at various sites of the NMDA receptor complex in a place preference conditioning model.

A conditioned place preference paradigm was used to assess the potential rewarding properties of the uncompetitive NMDA receptor antagonist, MK-801 (dizolcipine), the two competitive NMDA receptor antagonists, CGP 37849 (DL-(E)-2-amino-4-methyl-5-phosphono-3-pentonoic acid) and its (R)-enantiomer CGP 40116, as well as the partial agonist at strychnine-insensitive glycine receptors, ACPC (1-aminocyclopropanecarboxylic acid). MK-801 (0.3 mg/kg), CGP 37849 (1.25-10 mg/kg) and CGP 40116 (1.25-10 mg/kg), administered in association with either the initially non-preferred or initially preferred side of the two-arm chamber, caused a significant increase in the time spent on that side in a post-conditioning test. In contrast, ACPC did not support the conditioned place preference. Thus, the time spent on the drug-associated side following conditioning with ACPC (50-400 mg/kg) did not significantly differ from that measured in the pre-conditioning test, irrespective of whether it was associated with the initially non-preferred black side or the initially preferred white side. These results are consistent with both clinical and pre-clinical data demonstrating differences in psychopharmacological properties among compounds acting at the multiple, allosteric regulatory sites on the NMDA receptor complex. Moreover, these results indicate that the abuse potential of ACPC, which acts as a functional NMDA receptor antagonist, may be lower than that of either uncompetitive or competitive NMDA receptor antagonists.