Differential effects of morphine on core temperature in stressed and non-stressed rats

The effects of morphine on body temperature were studied in rats in two different states - stressed and non-stressed. Morphine injected subcutaneously (s.c.) produced a dual action on body temperature in non-stressed rats. Hyperthermia occurred at lower doses (2.5-10 mg/kg) while hypothermia was produced with a higher dose (20 mg/kg). Both of these effects of morphine were reversed by naloxone (0.1-5.0 mg/kg). Stressing the rats (immobilization with wire mesh) produced slight hypothermia which was markedly potentiated by morphine (5-20 mg/kg) in a dose-dependent manner. Enhancement of hypothermia by morphine in the stressed animals was antagonized by pretreatment with naloxone (0.1-5.0 mg/kg). When rats were treated with morphine (10 mg/kg) 1 h before stress, and were then exposed to immobilization stress, the hyperthermia exhibited in the non-stressed state changed to hypothermia in the stressed state. When the rats which were treated with morphine and then stressed for 1 h were released from stress, the hypothermia observed in the stressed state progressively changed to hyperthermia. Furthermore, these morphine effects, i.e. hyper- and hypothermia in the non-stressed and stressed states, respectively, were reversed but not eliminated by naloxone. These results suggest that the effects of morphine on core temperature in rats are altered depending upon the state of the animals. That is, morphine appears to have a dual action, hyperthermia in the non-stressed state and hypothermia in the stressed state. It also appears that these actions are mediated via opiate receptors.