Sex differences in thermoregulation after acute and chronic morphine administration in mice

Sex differences in thermoregulation have been reported following acute morp hine administration in rats only. This study assessed whether male and fema le mice also differ in thermoregulatory responses following acute and chron ic morphine administration. Females displayed significantly higher baseline colorectal temperature and greater morphine (24 mg/kg, s.c.) hypothermia ( 2.5-fold) on day 1. Two additional days of morphine treatment did not alter baseline temperature readings on Day 4 in either sex, but significantly re duced the morphine hypothermia relative to Day 1 in a sex-dependent manner. Whereas the morphine hypothermia was completely abolished in males, signif icant hypothermia was still observed in females. Acute and chronic saline i njections had no effect on colorectal temperature. The data demonstrate sex differences in the thermoregulatory responses to acute and chronic morphin e administration in mice. (C) 2000 Published by Elsevier Science Ireland Lt d.