Effects of morphine and heroin on discrimination learning and consolidation in mice.

Morphine and heroin were administered to mice learning to swim toward a light source (L procedure) or toward the dark (D procedure), in a Y water maze, under pre and post-trial drug treatment conditions. In the pre-trial experiments a clear disrupting effect on performance with the two procedures followed administration of both drugs, but for the L performance never fell below the 50% level of correct choices. Analysis of the performance within each session demonstrated a disruption in the long term memory consolidation mechanism. The administration of naloxone, or alternatively, discontinuation of the treatment, was followed by a gradual improvement, in performance by the treated animals. In both procedures, a performance disruption also followed the administration of the drugs immediately after each experimental session.