Effects of dose on effector mechanisms in morphine-induced hyperthermia and poikilothermia

The effect of a variety of morphine doses on thermoregulatory effector systems was examined in ambient temperatures of 27.0 degrees C and 4.0 degrees C. Rats were given saline or morphine sulfate (5, 15, or 25 mg/kg); their core temperature, oxygen consumption, and activity were monitored for 4 or 6 h post-injection. The results suggest two distinct actions of morphine, possibly mediated by two opiate receptors. Low doses of morphine produce hyperthermia that is the result of a direct activation of activity and whole body heat production. High doses produce effects dependent on ambient temperature: hypermetabolism and hyperthermia in the 27.0 degrees C environment; hypometabolism, vasodilation, and hypothermia in the 4.0 degrees C environment. The findings suggest limitations in current set-point theories of morphine's thermic actions.