NMDA RECEPTOR ANTAGONISTS PREVENT CONDITIONED ACTIVATION OF INTRACRANIAL SELF-STIMULATION IN RATS

Rats with bipolar electrodes implanted unilaterally into the ventral t egmental area were trained to lever-press for response contingent elec trical stimulation (continuous reinforcement). After preliminary lever -press training, two types of daily sessions were held on 10 consecuti ve days: type T+, during which current intensity was set at the Thresh old level and each response was accompanied by the visual signal (stim ulus lights above the lever briefly went off). and type ST-, during wh ich current was set at the SubThreshold level and there were no visual stimuli. On day 11, combination of the subthreshold current intensiti es and stimulus lights previously associated with the threshold stimul ation (session type ST+) resulted in significantly elevated response r ates compared to the performance under the subthreshold current withou t visual stimuli (session type ST-). This effect was dose dependently blocked by competitive NMDA receptor antagonist (+/-)-CPP -)-3-(2-carb oxypiperazin-4-yl)-propyl-1-phosphonic acid) and CGS 19755 (cis-4-(pho sphonomethyl) piperidine-2-carboxylic acid). The present findings sugg est that the activation of intracranial self-stimulation induced by a conditioned visual stimulus is dependent on the NMDA receptor function ing.