Mechanism of fasting heterothermia and re-feeding normothermia in mice

Mice subjected to caloric restriction (CR) typically display heterothermia characterized by hypothermia in the daytime and normothermia at night. The possible thermoregulatory mechanisms that mediate the CR-induced daytime hypothermia or the recovery of core temperature upon re-feeding are not well understood. In the present study drugs that inhibit three different pathways of thermogenesis were applied before, during and after CR in mice, while core temperature was monitored by biotelemetry. The time course of core temperature during complete CR and re-feeding was not modified by administration of the postganglionic adrenergic blocker guanethidine (10 mg/kg/day). Administration of the centrally acting muscle relaxant mephenesin (42 mg/kg/day) exacerbated the CR-induced hypothermia. Administration of the nonspecific opioid antagonist naloxone (20 mg/kg/day) also exacerbated the CR-induced hypothermia. None of the drugs had any effect on the rate of the rise in core temperature during re-feeding after a fast. It is concluded that mice may rely on the heat from motor activity to remain normothermic during the first night of complete fasting. Shivering thermogenesis appears to be critical in thermoregulation during fasting. Furthermore, opiate-dependent thermogenesis may also contribute to the regulation of body temperature during the second night of fasting.