Effects of naltrexone on morphine-induced tolerance and physical dependence and changes in cellular immune function in mice.

The effects of naltrexone on tolerance/dependence, as well as alterations in cellular immune function induced by morphine administration, were determined. Mice were rendered tolerant to and physically dependent on morphine by subcutaneous implantation of pellets containing 75 mg of morphine. Implantation of naltrexone pellets (10 mg) blocked the development of tolerance to the analgesic action of morphine, as well as the development of physical dependence. Morphine suppressed lymphoid organ weights and cellularities, and this suppression was blocked by naltrexone. B-Cell proliferation was suppressed in morphine-tolerant but not in morphine-abstinent mice, and this suppression was exacerbated by naltrexone. Morphine tolerance and abstinence were associated with suppression of IL-2 production, which was completely blocked by naltrexone. NK cell activity was not significantly affected by either morphine or naltrexone exposure. The results suggest that the effects of morphine on the immune system are at least partially mediated through opioid receptors.