Effects of morphine, buprenorphine, pentazocine and nalorphine on acquisition and extinction of active avoidance responses in rats.

The effects of subcutaneous administration of morphine, buprenorphine, pentazocine and nalorphine were studied at two dose levels in rats (low dose x 10 and high dose x 20 of equivalent human dose) on the performance of active avoidance responses using a shuttle box. Pretraining injections of both doses of pentazocine and low dose nalorphine impaired acquisition on day 1 and day 2. Morphine and buprenorphine (at both dose levels) and high dose nalorphine did not affect the acquisition process. Post-training administration of morphine (high dose) and buprenorphine (both doses) delayed extinction of active avoidance responses. Low dose of morphine, high dose of pentazocine and both doses of nalorphine did not appreciably affect the extinction process. Mu opioid receptor agonists probably act as reinforcers to facilitate memory.