SEX-DIFFERENCES IN NALOXONE-MIF-1-INDUCED AND TYR-MIF-1-INDUCED HYPOALGESIA

Reproductive adult male and female deer mice that received daily (7 da ys) injections of either the prototypic exogenous opiate antagonist, n aloxone (1.0 mg/kg), or the endogenous putative antiopioid tetrapeptid e, Tyr-MIF-1 (Tyr-Prol-Leu-Gly amide; 1.0 and 10 mg/kg), followed by d eterminations of thermal nociceptive sensitivity (hot-plate response) developed hypoalgesia. There were significant sex differences in this opioid blockade-induced or associated analgesia, with male mice displa ying significantly greater hypoalgesia than females. Mice that receive d daily injections of either naloxone or Tyr-MIF-1 for 7 days without any accompanying determinations of nociceptive sensitivity (days 2-6 o f treatment) failed to show any hypoalgesia on day 7 when they receive d the antagonist followed by a measurement of nociception. These resul ts indicate that there are sex differences in both naloxone- and Tyr-M IF-1-induced hypoalgesia, and suggest that this pattern may be associa ted with sexually dimorphic opioid modulation of aversive conditioning .