DEVELOPMENT OF BOTH CONDITIONING AND SENSITIZATION OF THE BEHAVIORAL ACTIVATING EFFECTS OF AMPHETAMINE IS BLOCKED BY THE NONCOMPETITIVE NMDA RECEPTOR ANTAGONIST, MK-801

The non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801, has been shown to block the development of sensitization of the behavioral activating effects of amphetamine. Three experiments were designed to determine in rats whether MK-801 had its effects through interference with long-term changes underlying sensitization, per se, or through interference with the development of conditioning of the drug effect to the environment where the drug was given. In experiment 1, conditioning was promoted by explicitly pairing amphetamine (1.5 mg/kg, IP) with the testing environment. In experiment 2, a random-pairing procedure was used to eliminate the possibility of association between the drug and a specific context. Experiment 3 was carried out to assess the duration of the blockade of sensitization by MK-801. The effect of MK-801 (0.25 mg/kg, IP) during amphetamine pre-exposure was studied in tests for conditioning (following saline injections, experiment 1) and in tests for sensitization (following 0.75 mg/kg amphetamine, experiments 1, 2 and 3). It was found in experiment 1 that MK-801 given with amphetamine during the amphetamine pre-exposure phase blocked the development of both conditioning activity and environment-specific sensitization to amphetamine. The results of experiment 2, showing that sensitization to amphetamine was blocked by MK-801 even when conditioning was prevented, suggest that the two effects of MK-801 are independent, and may implicate different sites of action. Experiment 3 showed that the blockade of sensitization by MK-801 was evident in tests made 10 days after pre-exposure to amphetamine, supporting the view that MK-801 interferes with long-term changes underlying sensitization to amphetamine.