INFLUENCE OF SHOCK SEVERITY ON THE ACTIVATION OF ANALGESIC SYSTEMS IN PENTOBARBITAL TREATED RATS. J.W. Grau*, P.A. Illich, K.D. Burks and M.W. Meagher. Dept. of Psychology, Texas A&M Univ., College Station, TX 77843.

Terman et al. (Science, 226, 1270) showed that exposure to severe shock can elicit "analgesia" on the tail-flick test in pento-baribital anesthetized rats. Their least severe shock parameters generated a naltrexone reversible ("opioid") analgesia and the most severe elicited a naltrexone-insensitive ("nonopioid") analgesia. They proposed that a coulometric relation (shock intensity x duration) predicts whether the opioid or nonopioid system is engaged. The present experiments tested this hypothesis. Experiment 1 looked at the minimum shock intensity and duration that generates analgesia on the tail-flick test in pento-barbital (40 mg/kg) anesthetized rats. After baseline testing, subjects were exposed to three tailshocks and pain reactivity was tested 5 more times. We found 1.5-s/1.0mA, 25-s/1.0mA, & 25-s/0.5mA shocks elicited analgesia, and 1.5-s/0.5mA, 3.0-s/0.5mA, & 25-s/0.3mA did not. The results fail to support the coulometric relation but can be accommodated by a relation derived from Steven's power law: shock duration x intensity**3.5. Experiment 2 tested the impact of naltrexone (14 mg/kg) on theanalgesia observed in pentobarbital anesthetized rats. In contrast to Terman et al., naltrexone did not attenuate the analgesia observed after the least severe shock schedules (e.g., 1.5-s/1.0 mA & 25-s/0.5 mA), but did attenuate the analgesia observed after more severe shocks (e.g., 2-s/3.0mA & 25-s/1.5mA). Very severe shocks (75-s/3.0mA) elicited a naltrexone-insensitive analgesia.

Published in Society for Neuroscience Abstracts, 18, 1991, 686.

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