MK-801 PROTECTS CONDITIONED-RESPONSES FROM EXTINCTION IN THE RABBIT NICTITATING-MEMBRANE PREPARATION

MK-801 is a noncompetitive antagonist to N-methyl-D-aspartate (NMDA) r eceptors. MK-801 was given (0.05 or 0.10 mg/kg i.v.) to rabbits during extinction of a well-established conditioned response (CR) in the nic titating membrane preparation. The drug administration produced an imm ediate deficit in CR performance during extinction. However, when MK-8 01 administration was suspended, CRs reappeared immediately and showed no evidence of the previous extinction treatment. In addition, we con firmed previous findings that MK-801 impairs both the acquisition and the expression of CRs during CS-US training. A reduction in CS process ing caused by MK-801 is the most parsimonious explanation of the data There are also two-factor alternatives, which contend that MK-801 dire ctly hinders associative formation and either directly blocks CR outpu t or produces a state-dependent generalization decrement.