Restraint alters the thermic response to morphine by postural interference.

The effects of morphine on body temperature have been shown to be altered by restraint. The purpose of this study was to determine how the type of restraint alters body temperature measurements and whether restraint alters the effects of morphine on body temperature by interfering with the ability of the rats to adjust their posture. The thermic effects of 5 doses of morphine (3.8 to 45 mg/kg) were compared in two types of restraint and confinement to a 13 X 20 X 20 cm pan without restraint. In unrestrained rats, morphine caused predominantly hyperthermia, but with restraint morphine caused hyperthermia at low doses and hypothermia at higher doses. Morphine hypothermia was greater in rats restrained in a wire-mesh restrainer which prevented heat and humidity build-up than in the commonly used plastic restrainer. In the unrestrained rats, morphine treatment was associated with a posture characterized by exophthalmos, immobility, a hunched position and increased muscle tone. Restrained rats could not assume a compact posture. These results suggest that restraint alters the thermic effect of morphine mainly by interfering with postural mechanisms which reduce heat loss.