The impact of naltrexone and morphine tolerance on mild shock-induced hypoalgesia

We have previously shown that exposure to mild shock elicits a strong hypoalgesia on the tailflick test in rats. The present studies explored the role of endogenous opioids in producing this hypoalgesia. Experiment 1 evaluated the impact offive doses ofnaltrexone (0,0.44,1.75,7, and 28 mg/kg); Experiment 2100ked at the impact ofmorphine tolerance. Both a low dose ofnaltrexone (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 long-Iasting opioid hypoalgesia.