Ultradian Episodes of Thermogenesis in Mammals: Implications for the Timing of Torpor Entry and Arousal

Abstract

Mammals forage and feed in a periodic manner, and facultative thermogenesis in brown adipose tissue (BAT) contributes to the increases in heat production associated with feeding. Even in the absence of food, BAT thermo- genesis occurs in an episodic manner. In the torpid or hibernating state, mammals do not feed and thermogenesis is substantially suppressed. However, periodic peaks and troughs of metabolic rate, ventilation and heart rate continue to occur at reduced amplitude. We hypothesize that the episodic pattern of thermogenesis in the normothermic and torpid state is part of a centrally mediated ultradian rhythm that serves maintenance of neuronal integrity and alertness. Periodic thermogen- esis may also provide the neuronal and temporal framework for torpor entry and arousal. Therefore, identifying the central pathways that underlie ultradian fluc- tuations in energy metabolism may lead towards understanding and predicting torpor behaviour. C.