Abstract

The ability of repeated morphine administration to produce hyperalgesia was studied in rats. The hot plate procedure was used to measure changes in the animal's response latency. The response latency was measured twice at 30-minute intervals, before the injection of the experimental agent and again at 30, 60, 90, 120, 180 and 240 minutes after drug administration. Morphine (5 mg/kg) or saline (2 ml/kg) was given s.c. for varying periods of time. It was found that chronic morphine administration produced hyperalgesia between 60 and 120 minutes after the drug. This hyperalgesia was found not to be the result of a morphine-test interaction. Acute administration of nalorphine, 5 mg/kg, delayed the onset of the hyperahgesic response produced by morphine. Studies designed to delineate the responsible mechanism(s) for this phenomenon may be useful in understanding the analgesic effect of morphine.