MK-801-induced disruptions of one-trial inhibitory avoidance are potentiated by stress and reversed by naltrexone.

Five experiments were carried out to investigate opioid and NMDA receptor-mediated responses to one-trial inhibitory avoidance training in CD1 mice. In the first experiment immediate posttraining intraperitoneal administration of the noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist MK-801 impaired the performance of mice. The effects of MK-801 were time-dependent (they were absent in mice injected with the drug starting 120 min after training). No effect was evident in no-foot-shock groups, showing lack of proactive influence of the treatment on performance. In the second experiment preexposure of the mice to the testing apparatus decreased the effects of MK-801. In the the third experiment naltrexone antagonized the effects of MK-801, suggesting an involvement of opioid neurons. In the fourth experiment immediate posttraining immobilization stress exerted a potentiating effect on the performance of MK-801-injected animals. In the fifth experiment the potentiation of the impairing effect of MK-801 induced by immobilization stress was antagonized by naltrexone.