NALOXONE DOES NOT ALTER THE REGULATED DECREASE IN CORE TEMPERATURE DURING HYPOXEMIA IN GUINEA-PIGS

In newborns and adults of a number of species, exposure to acute hypox emia produces a ''regulated'' decrease in core temperature, the mechan ism of which is unknown. The present experiments were carried out in c hronically instrumented newborn (5-10 days of age; n = 59) and older ( 25-30 days of age; n = 61) guinea pigs to test the hypothesis that the endogenous opioids mediate this regulated decrease in core temperatur e. During an experiment, core temperature, oxygen consumption, and sel ected ambient temperature were measured in a thermocline (linear tempe rature gradient of 10-40 degrees C) during a control period of normoxe mia, an experimental period of normoxemia or hypoxemia (inspired oxyge n fraction 0.10), and during a recovery period of normoxemia following an intraperitoneal injection of naloxone hydrochloride (a nonspecific opioid antagonist; 1, 2, or 4 mg/kg) or vehicle. Naloxone did not sig nificantly alter basal core temperature or the core temperature respon se to acute hypoxemia in newborn or older guinea pigs. Naloxone did, h owever, decrease basal oxygen consumption in newborn and older guinea pigs and altered the thermoregulatory effector mechanism used to decre ase core temperature during hypoxemia in the newborn guinea pigs. Our data do not, support the hypothesis that the endogenous opioids mediat e the regulated decrease in core temperature that occurs in newborn an d older guinea pigs during exposure to acute hypoxemia.