Role of NMDA receptors in the emotional memory associated with neuroendocrine responses to conditioned fear stimuli in the rat.

Behavioral experiments have shown that the N-methyl D-aspartate (NMDA) subclass of glutamate receptor plays an important role in acquisition of emotional memory. Exposure of a rat to conditioned fear stimuli suppresses vasopressin (VP) release and augments oxytocin (OT) or prolactin (PRL) release from the pituitary. Present experiments aimed at investigating the effect of intraperitonially administered MK-801, an antagonist of NMDA receptor on the emotional memory associated with the suppressive VP and the augmentative OT or PRL responses to conditioned fear stimuli in male rats. MK-801 injected 30 min before training impaired the VP, OT and PRL responses to the testing fear stimuli. The antagonist injected after training, however, did not block the responses. MK-801 administered before testing impaired the previously acquired VP, OT and PRL responses to conditioned fear stimuli. In the experiments with non-associatively applied fear stimuli, MK-801 did not block the VP, OT or PRL response. In the experiments with novel environmental stimuli, MK-801 did not impair VP, OT or PRL responses. The results suggest that an activation of NMDA receptors are required to acquire and recall but not to consolidate or retain the emotional memory associated with VP, OT and PRL responses to conditioned fear stimuli.