Discriminative stimulus effects of morphine and oxycodone in the absence and presence of acetic acid in male and female C57Bl/6 mice

The use of prescription opioids for clinical management of pain remains problematic because of concerns about addiction associated with opioid use. Another difficulty in pain management is the increasing evidence for sex differences in pain behavior and opioid-induced behavioral effects. However, few studies have documented the abuse potential of prescription opioids as a function of pain in rodents, with significant gaps in the literature pertaining to sex differences in the interaction between pain and opioid effects. The present study evaluated the effects of an experimentally induced acute pain state (acetic acid injections) on the potency of morphine and oxycodone to produce discriminative stimulus effects in male and female C57Bl/6 mice trained to discriminate 3.2 mg/kg morphine from saline. Acetic acid injections attenuated the stimulus potency of morphine by 2.2-fold but not the stimulus potency of oxycodone in male mice. Acetic acid injections did not alter the discriminative stimulus effects of either morphine or oxycodone in female mice. The antinociceptive effects of the 2 opioids were evaluated using the acetic acid-induced stretching test. For antinociceptive effects, morphine was 2.0-fold less potent relative to oxycodone in male mice, whereas morphine and oxycodone were equipotent in female mice. Taken together, these results indicate that acetic acid-induced acute pain differentially modulates the discriminative stimulus effects of morphine in male and female mice and that this change may be related to the variable antinociceptive effectiveness of these opioids across sexes.