Effect of periaqueductal morphine injection on thermal response in rats.

The effect of morphine injection into the midbrain periaqueductal gray (PAG) was examined on thermal response in rats. Rectal temperature (Tre) was recorded in unanesthetized and unrestrained animals before and after PAG morphine injection of 5 or 10 micrograms in cold (10 degrees C), neutral (22 degrees C), and hot (34 degrees C) environments. Both doses of morphine caused hyperthermia. Sixty to 105 min after the injection, Tre rose by 1.0-1.5 degrees C regardless of ambient temperature. Naloxone (2 mg/kg, i.p.) blocked the hyperthermic effects of morphine. Injection sites producing hyperthermia were distributed mostly in the ventral PAG and its ventral environs. The analgesic effect of morphine was examined by the tail-flick test. Locations of morphine injection effective for producing analgesia were restricted to the ventral area of the region responsible for hyperthermia. Magnitude of the hyperthermia did not significantly differ between animals with or without analgesia. The effect of PAG morphine (5 micrograms) was tested on tail vasomotor response to hypothalamic and scrotal thermal stimulations in urethane anesthetized (1.0 g/kg) animals. Threshold hypothalamic temperature for the vasodilation was lower at a scrotal temperature of 40 degrees C than at 30 degrees C. Following PAG morphine, threshold hypothalamic temperature rose and the difference in threshold hypothalamic temperature at the two scrotal temperatures disappeared.