Effects of morphine and nalorphine on kainic acid-induced hypothermia in rats.

Intraventricular administration of kainic acid at the dose of 0.1 microgram induces a significant depression of rectal temperature followed rapidly by its slight elevation. Morphine (40.0 mg.kg-1 IP), which by itself elicited biphasic effect on the body temperature of rats--initially hypothermia followed by hyperthermia--slightly increased the kainic acid-induced hypothermia. Kainic acid did not cause any changes in the hyperthermic effect of low doses of morphine (10.0 mg.kg-1). Pretreatment of rats with nalorphine enhanced the kainic acid-induced hypothermia. On the contrary, nalorphine reversed the hypothermic effect produced by morphine at the dose of 40.0 mg.kg-1. The results suggest that morphine and kainic acid-induced hypothermia are not mediated by the influence on the same type of receptors.