Enhancement of naloxone-induced analgesia by pretreatment with morphine.

Recently there have been demonstrations of a form of analgesia in rats that depends on the repeated administration of an opiate antagonist for its occurrence. The mechanism of this naloxone-induced analgesia (NIA) is not clear. This experiment tested the hypothesis that the relationship between behavioral effects of previous experience with opiate agonists and antagonists would be reciprocal with respect to analgesia. Consistent with such an hypothesis, prior exposure to morphine increased sensitivity to the effect of naloxone as measured by the rate of acquisition of NIA. Although receptor functions were not measured, reciprocal changes in the regulation of opiate receptors by opiate receptors by opiate agonists and antagonists may underlie the behavioral effects observed in this experiment.