Restraint alters the effects of morphine and heroin on core temperature in the rat

The importance of restraint in determining the magnitude of alteration in the rat's core temperature (Tc) after the administration of morphine sulphate (M) and heroin hydrochloride (H) was investigated. M, in doses of 5, 15 and 30 mg/kg, or H, in doses of 0.1, 1 and 5 mg/kg was administered IP to either the restrained or free-moving rats as Tc was measured. After the administration of 5 mg/kg of H or 30 mg/kg of M to the restrained rat, a marked hypothermia was observed which reached a maximum mean depth of 3.1 and 4.5 degrees C below the baseline Tc, respectively. Conversely, a mean increase in Tc of 1.5 and 1.9 degrees C occurred following the administration of these same doses of M and H in the unrestrained animal. Furthermore, the hypothermic effect of the highest dose of M was not observed when the third of 3 consecutive injections of M, administered at 48-hr intervals, was administered to the restrained rat. On the other hand, when M was repeatedly administered to the free-moving rat, the hyperthermic response was consistently observed. Pretreatment with naloxone hydrochloride (5 mg/kg IP) effectively blocked the opiate-induced hypothermia in the restrained animal, but a total dose of 10 mg/kg was necessary to completely block the hyperthermic response in the free-moving rat. Although the factor of restraint itself did not alter the rat's Tc, it did dramatically alter the action of M and H on the body temperature of the rat.