Morphine effects upon discriminated approach and discriminated avoidance in rats: antagonism by naloxone

The effects of various doses of morphine (5, 10 and 20 mg/kg) alone or in combination with a constant dose of naloxone (1.25 mg/kg) were examined in rats trained on a discriminated approach schedule (in which bar pressing in the presence of a stimulus light produced food), or on a discriminated avoidance (in which the same response produced stimulus-shock termination). Since the performance of rats in the discriminated avoidance varied widely, drug effects were examined separately in groups of good, intermediate or poor performer rats. Comparable patterns of responding in the presence of light were found in the approach group and in the good performer avoidance group. Morphine induced a dose-related decrease of this responding which was identical in both cases. Other effects of morphine were a dose-related increase of escape failures in all the avoidance groups and stimulatory or depressant effects upon bar presses performed during the no light periods. All the effects of morphine were antagonized by naloxone. The data suggest that comparable patterns of responding maintained by different reinforcements can be similarly affected by morphine.