Acquisition in the Morris swim task is impaired by a benzodiazepine but not an NMDA antagonist: A new procedure for distinguishing acquisition and performance effects

Abstract

The problem of learning versus performance continues to challenge researchers interested in drug effects on place learning and memory. To address this problem, the present study adapted the repeated acquisition/performance (RAP) procedure to the Morris swim task. The procedure involved training subjects to swim to a hidden platform that was always in the same location in one pool and in a new location each day in another pool. The advantages of this procedure are that acquisitions of new place responses and performances of previously learned place responses can be directly compared in individual subjects within single sessions, and dose- response functions can be determined for each individual subject. Experiment 1 demonstrated the ability of rats to learn new platform locations in one pool while maintaining stable performance on a previously learned place response in another pool within individual sessions. Experiment 2 used the RAP adaptation of the Morris swim task to study effects of chlordiazepoxide (CDZ), a benzodiazepine, and dizocilpine (DZP; a noncompetitive N-methyl-D- aspartate antagonist), on place learning and performance. Both drugs impaired performance in a dose-dependent manner. More interestingly, a moderate dose (10 mg/kg) of CDZ caused a small but reliable acquisition impairment without affecting performance of the previously learned place response. Doses of DZP that were substantially greater than doses previously shown to disrupt induction of NMDA-dependent long-term potentiation in the hippocampus did not impair acquisition in this procedure. Acquisition was affected only at DZP doses that also impaired the performance measure.