Neural Control of Mammalian Hibernation

Abstract

Hibernation is the most profound, nonpathological change of behavioral state occurring in mammals. It requires dramatic adjustments in ail physiological systems (Lyman et al. 1982). We might study any one of a number of physiological control systems to understand how homeostasis is maintained and, therefore, how animals survive throughout the hibernation cycle. To understand the mechanisms of induction and control of the hibernation cycle, however, we should focus on those physiological control systems which appear to be prime movers in the generation of hibernation. These appear to be the neural systems controlling arousal state changes and regulating body temperature. We now know that the entrance is accompanied by a progressively lowered hypothalamic set point for Tb regulation and that a set point is continuously present during a hibernation bout (Heller et al. 1978). The set point is also lower during euthermic slow wave sleep (SWS) than during wakefulness (Glotzbach and Heller 1976), suggesting possible homology between torpor and SWS. Electrophysiological studies support this contention as hibernation is entered through sleep and a bout of torpor consists mostly of SWS with REM sleep being virtually absent when Tb falls below 27 {\textdegree}C (Walker et al. 1977). Therefore, it seems entirely likely that shallow torpor and, subsequently, deep hibernation could have evolved as a result of selective pressures favoring the energy savings from a lowering of the regulated Tb during SWS (Heller et al. 1978). Hibernation is, thus, a valuable model system for the study of mechanisms controlling mammalian arousal states and the physiological alterations which accompany them.