ACQUISITION OF A MORRIS WATER MAZE TASK IS IMPAIRED DURING EARLY BUT NOT LATE WITHDRAWAL FROM MORPHINE

Behavioral changes in male Sprague-Dawley rats during early and late w ithdrawal from morphine were investigated. Morphine-treated subjects ( M) were implanted (SC) with osmotic pumps containing 2.0 ml of a 159 m g/ml morphine sulfate solution while control subjects (C) received sha m implants. Implants were removed after 7 days. M subjects exhibited a significant decrease in body weight during withdrawal that recovered by 21 days after pump removal. Beginning 1 or 21 days following pump r emoval, subjects were tested for 8 days in a Morris water maze (MWM) t ask. M subjects trained in the MWM during early withdrawal exhibited s ignificantly longer escape latencies than C subjects. However, during sequential probe trials, the same subjects exhibited a significant pre ference for the target quadrant of the maze and executed accurate sear ches for the escape platform. Though these subjects failed to locate t he platform as efficiently as controls during training trials, they le arned the location of the escape platform. M rats trained during late withdrawal exhibited no deficits in any measure of MWM performance rel ative to C subjects. The data suggest that a variety of processes invo lved in the acquisition and performance of the MWM task are differenti ally affected during early withdrawal from morphine. Copyright (C) 199 6 Elsevier Science Inc.