Abstract

Morphine-HCl (50 mg/kg s.c.) produced a fall in the rectal tenmperature of rats maintained in an ambient temperature of 20°C. The administratioin of p-chlorophenylalanine (p-CPA, 320 mg/kg i.p.) 48 hours before the morphine injection prevented the fall in body temperature. When rats pretreated with p-CPA were given 5-hydroxytryptophan (100 mg/kg s.c.) 30 minutes before morphine administration, the hypothermic response to morphine was restored. Thus, the hypothermic response of nontolerant rats to morphine may depend on the release of 5-hydroxytryptamine by morphine. The administration of morphine to morphine-tolerant rats produced an elevation of rectal temperature. The magnitude of the elevation was reduced by pretreatment of the morphinetolerant rats with cholinergic blocking drugs; the reduction by scopolamine was greater than that by methscopolamine or atropine. It appears that both central and peripheral cholinergic mecimanisms participate in the elevation of body temperature induceci by the administration of morphine to morphine-tolerant rats. The administration of naloxone-HCl (1 mg/kg s.c.) to morphine-tolerant rats induced a hypothermie response which was not inhibited by pretreatment of the animal with p-CPA, eholinergic antagonists or α-methyltyrosine. Raising the ambient temperature from 20-30°C partially prevented the hypotimermic response of morphine-tolerant rats to naloxone.