Psychopharmacology
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Exercise stimulates the release of endogenous opioid peptides and increases nociceptive (i.e. pain) thresholds in both human and animal subjects. During chronic, long-term exercise, sensitivity to the effects of morphine and other mu opioids decreases, leading some investigators to propose that exercise may lead to the development of cross tolerance to exogenously administered opioid agonists. ⋯ These data suggest that chronic exercise leads to the development of mu-opioid tolerance and physical dependence, and that these effects are similar to those produced by chronic opioid administration.