Intrahippocampal infusion of the NMDA receptor antagonist AP5 impairs retention of an inhibitory avoidance task: protection from impairment by pretraining or preexposure to the task apparatus.

The purpose of the present research was to further evaluate the role of hippocampal N-methyl-D-aspartate (NMDA) receptors in inhibitory avoidance task consolidation. Adult male Wistar rats were trained and tested in a step-down inhibitory avoidance task (0.4 mA footshock, 24 h training-test interval). Immediately after training, animals received a 0.5-microl intrahippocampal infusion of the NMDA receptor antagonist aminophosphonopentanoic acid (AP5) (5.0 microg) or vehicle (phosphate buffer in saline, pH 7.4). The infusion of AP5 impaired retention test performance. Both pretraining with a low footshock intensity (0.2 mA) or preexposure to the inhibitory avoidance box 24 h before training prevented the amnestic effect of AP5. The results suggest that hippocampal NMDA receptors are critical for neither the enhancement of retention induced by an additional training session nor the inhibitory avoidance retention in animals that have previously learned about the task environment by preexposure to the apparatus.