Effects of morphine on body temperature of squirrel monkeys of various ages

Increased sensitivity to certain drugs is believed to contribute to dysthermia in the elderly. To learn whether the temperature-altering effects of an opiate are increased in aged primates, injections of morphine sulfate (0.5-4 mg/kg) were given SC in randomly assigned order to squirrel monkeys ranging in age from 3.5 to over 17 years. Hyperthermia was the predominant response with no clear relationship to age, although hypothermic and biphasic responses also occurred, most commonly after the highest dose. Lateral cerebral ventricular injections of 0.625 and 1.25 micrograms morphine sulfate evoked hyperthermia in monkeys over 8 years of age but did not affect the temperature of animals less than 5 years old. Doses of 2.5 and 5 micrograms usually elicited hyperthermia regardless of age, but 10 micrograms induced hypothermia in a majority of monkeys. Naloxone was given intraventricularly to several monkeys to limit the degree of hypothermia after high doses of morphine given peripherally or centrally. Thus in these primates, as in other species such as the rat, lower doses of morphine usually evoked hyperthermia, but sufficiently high doses caused body temperature to fall. Unlike the case in the squirrel monkey with diazepam and with endogenous substances such as leukocytic pyrogen and taurine, there was not a strong or consistent relationship between age and morphine-induced temperature changes.