EFFECTS OF NMDA RECEPTOR BLOCKADE ON BEHAVIORS DIFFERENTIALLY AFFECTED BY FIMBRIA FORNIX AND AMYGDALA LESIONS/

We studied the effects of MK-801, a noncompetitive NMDA receptor antag onist, on acquisition and expression of the food conditioned cue prefe rence (CCP) in the eight-arm radial maze. In this task, food-deprived rats are confined on alternate days in one arm of a radial maze with f ood and in another arm with no food. After several such training trial s, they are given a choice between the two arms, both of which are emp ty. Normal rats spend more time in the arm that formerly contained foo d. We previously showed that this CCP is eliminated by lesions of the lateral nucleus of the amygdala. A single 10-min session of preexposur e to the maze with no food retards acquisition of the CCP. This retard ation is eliminated by fornix lesions, suggesting that hippocampus-bas ed learning about the environment during the preexposure session suppr esses subsequent amygdala-based CCP learning. In the present experimen t, rats that received MK-801 before a preexposure session exhibited CC Ps comparable to those for animals that had not experienced preexposur e. We attribute this effect to the prevention of hippocampus-based lea rning during the preexposure due to NMDA receptor blockade. In other g roups of rats, MK-801 given before the training trials or before the t est trial eliminated the CCP in rats that had not received preexposure to the maze. The prevention of both acquisition and expression of the amygdala-based food CCP by MK-801 may distinguish the memory-related function of NMDA receptors in the amygdala from that of similar recept ors in the hippocampus.