Paradoxical Effect of NMDA Receptor Blockade in Chicks on Learning and Memory in Passive Avoidance Model.

Activity of NMDA receptors is a prerequisite for numerous but not all forms of neuronal plasticity and learning. The present study examined the role of NMDA receptors in standard, weak, and repeated passive avoidance training in young chicks. Injection of MK-801, an antagonist of NMDA receptor, prior to strong training episode impaired subsequent memory recall. Moreover, repeated training did not restore the lost memory. In the double weak training protocol, the impairing effect of MK-801 was observed only when it was injected prior to the second but not to the first training episode. These results suggest that activation of NMDA receptors is not a necessary stage for memory acquisition in the weak training task. In contrast, the mechanisms of strong training depending on activation of NMDA receptors can be probably involved into the second training episode performed against the background of existing NMDA receptor-independent memory about the first training episode.