NMDA receptor blockers facilitate and impair learning via different mechanisms.

The NMDA receptor blockers CGP 37849 (competitive) and MK 801 (noncompetitive) improve retention performance in the step-down passive avoidance situation but impair it in the step-through dark avoidance. It is suggested that the drugs in the two tasks act on different underlying processes. Whereas the memory facilitating effects of both drugs can be suppressed by pretreatment with either aldosterone or corticosterone, their negative effects on retention were steroid-insensitive. This set of results suggests that the mechanisms by which NMDA blockers improve memory are different from the mechanisms by which they block memory.