Hyperthermia evoked by the intracerebral injection of morphine sulphate in the rat: the effect of restraint.

Morphine sulphate (M) was microinjected intracerebrally into 26 sites in the free-moving and 11 sites in the restrained rat. The injections were made into the preoptic-anterior hypothalamic (POAH) region as core temperature (Tc) was measured. A marked increase in Tc was evoked from the free-moving rat following doses of M of 1 (n = 10); 10 (n = 19); 20 (n = 7); or 50 microgram (n = 6). On the other hand, no increase in Tc significantly above the control level was observed in the restrained rat following the intracerebral administration of M in doses of 10 (n = 8), 20 (n = 9), 50 (n = 10), or 70 microgram (n = 4). The control injection of the artificial cerebrospinal fluid (5-ion), used as the drug vehicle, caused a mild hyperthermia about 1.0 degrees C above the baseline level in both the free-moving and restrained rat. We conclude that morphine evokes a hyperthermia by its action on POAH neurons, but the M-induced hyperthermia can be attenuated by restraining the rat.