The relationship between fasting-induced torpor, sleep, and wakefulness in laboratory mice.

Yi-Ge Huang,Russell G Foster,Stuart N Peirson,Vladyslav V Vyazovskiy,Carina A Pothecary,Sarah J Flaherty

doi:10.1093/sleep/zsab093

Abstract

Study ObjectivesTorpor is a regulated and reversible state of metabolic suppression used by many mammalian species to conserve energy. Whereas the relationship between torpor and sleep has been well-studied in seasonal hibernators, less is known about the effects of fasting-induced torpor on states of vigilance and brain activity in laboratory mice.MethodsContinuous monitoring of electroencephalogram (EEG), electromyogram (EMG), and surface body temperature was undertaken in adult, male C57BL/6 mice over consecutive days of scheduled restricted feeding.ResultsAll animals showed bouts of hypothermia that became progressively deeper and longer as fasting progressed. EEG and EMG were markedly affected by hypothermia, although the typical electrophysiological signatures of non-rapid eye movement (NREM) sleep, rapid eye movement (REM) sleep, and wakefulness enabled us to perform vigilance-state classification in all cases. Consistent with previous studies, hypothermic bouts were initiated from a state indistinguishable from NREM sleep, with EEG power decreasing gradually in parallel with decreasing surface body temperature. During deep hypothermia, REM sleep was largely abolished, and we observed shivering-associated intense bursts of muscle activity.ConclusionsOur study highlights important similarities between EEG signatures of fasting-induced torpor in mice, daily torpor in Djungarian hamsters and hibernation in seasonally hibernating species. Future studies are necessary to clarify the effects on fasting-induced torpor on subsequent sleep.

Highlights

States of vigilance in mammals are traditionally defined based on behavioral criteria and brain activity [1,2,3]
We performed a detailed investigation of EEG/EMG defined states of vigilance during hypothermia and torpor induced by restricted feeding in mice
Spectral EEG analysis revealed that cortical slowwave activity (SWA), which, in ET conditions, is a marker of sleep need that increases as a function of prior waking duration and decreases as a function of sleep, was typically high at the beginning of euthermia and declined thereafter. These findings suggested that the preceding torpor bout does not restore sleep need, which seems paradoxical as torpor is comprised predominantly of a state most similar to non-rapid eye movement (NREM) sleep

Summary

Introduction

States of vigilance in mammals are traditionally defined based on behavioral criteria and brain activity [1,2,3]. Sleep corresponds to a state of relative immobility and reduced sensory responsiveness, while wakefulness is characterized by movement and active engagement with the environment. These characteristics of an awake state are thought to be essential for its main functions, including feeding, mating, or defense against predation. Sleep is timed both by an endogenous circadian clock and by a homeostatic drive for sleep which builds during wake These two processes allow the alignment of numerous aspects of behavior and physiology with the occurrence of ecological factors such as light, food availability, ambient temperature, and risk of predation [8,9,10]

Objectives

Methods

Results

Discussion

Conclusion