We have previously shown that exposure to mild shock elicits a strong hypoalgesia on the tail-flick test in rats. The present studies explored the role of endogenous opioids in producing this hypoalgesia. Experiment 1 evaluated the impact of five doses of naltrexone (0,0.44,1.75,7, and 28 mg/kg); Experiment 2 looked at the impact of morphine tolerance. Both a low dose of naltrex-one (1.75 mg/kg) and morphine tolerance attenuated the hypoalgesia observed 6-10 min after mild shock. By contrast, neither morphine tolerance nor a high dose of naltrexone (28 mg/kg) had a significant impact on the hypoalgesia observed 2 min after shock. These findings suggest that mild shock elicits both a transient nonopioid and a longlasting opioid hypoalgesia.
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