Effects of NMDA antagonists on memory processes in a novel two-trial swimming test.

Rats were placed in a circular swimming pool for two 3-min swims separated by 3 days. In the first swim control rats swam initially around the perimeter of the pool and later spent more time in the central region and swam more slowly. The time spent in the centre during min 1 was much higher in the second swim than in the first. This alteration of behaviour by the previous experience suggests that a memory of the first swim was formed. The NMDA antagonists, CPP (10-20mg/kg) or MK-801 (50-100µg/kg), administered before the first swim attenuated or reversed the decline in swimming speed and attenuated the intra-trial increase in the time spent in the centre. The alteration of swimming pattern between the two swims was also reduced. Administering the antagonists immediately after the first swim did not have this effect. Drug administration shortly before the second swim prevented the within-trial decline of swimming speed, but did not significantly reduce the high proportion of time spent in the central region during min 1. The results suggest that in this paradigm NMDA receptors are involved in within-trial habituation of swimming activity and in the initial stages of long-term memory formation, but are not involved in memory consolidation or retrieval.