Effect of partial reinforcement on tolerance to morphine-induced analgesia and weight loss in the rat

The effect of partial reinforcement on the development of tolerance to morphine-induced analgesia and weight loss was examined. Groups of rats were presented a distinctive set of environmental cues on several occasions. For one group of rats, morphine (40 mg/kg) was injected each time the cues were presented (morphine--continuous reinforcement). For a second group of rats, morphine was injected only following one of every four cue presentations (morphine--partial reinforcement). Two additional groups were injected with only saline, one on the continuous reinforcement schedule and one on the partial reinforcement schedule. Results demonstrated less tolerance to morphine in the partially reinforced morphine-injected rats than in continuously reinforced morphine-injected rats. Unlike other demonstrations of a tolerance-retarding effect of partial reinforcement, the present results could not have resulted from nonassociative factors related to differential novelty, stress, or practice. Clinical implications for the tolerance-retarding effect of partial reinforcement are discussed.