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

Abstract

Study objectives Torpor 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. Methods Continuous 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. Results All 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 NREM sleep, 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. Conclusions Our 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.

The burst of electrophysiological signals in the suprachiasmatic nucleus of mouse during the arousal detected by microelectrode arrays

“…The results showed that LFPs experienced first decreasing, then increasing and then falling from P1 to P4 ( Figure 4B ). The trend of power changes in the LFPs of the SCN experienced changes in activity decline and recovery ( Figure 4A ), which was consistent with existing EEG (Electroencephalogram) studies entering the torpor and arousal ( Heller and Ruby, 2004 ; Huang et al, 2021 ). In addition, the dramatic increase in the power of LFPs would be a novel finding in the arousal, which mined that the SCN in the arousal needed a huge power to start.…”

Re-examining extreme sleep duration in bats: implications for sleep phylogeny, ecology, and function

“…Hence bats would be expected to switch between strategies depending on whether biological investment (sleep) or energy conservation (torpor) is the prevailing requirement. Nevertheless, it is possible for these states to coincide ([ 69 , 81 , 82 ] see Box 2 ). If sleep and torpor did coincide in Brebbia and Pyne’s [ 12 ] recordings, this may render their findings incompatible with sleep recordings from other animals for two reasons.…”

“…Sleep at 19–21°C was characterized by an increase in the ratio of NREM:REM sleep and a reduction in the voltage of electrical activity in the brain which could indicate shallow torpor. The induction of shivering at this temperature range could indicate bursts of maintenance and rewarming thermogenesis during torpor-arousal cycles [ 82 ]; however, important details of the time course of shivering are absent. Below 19°C an isoelectric EEG lacking sleep rhythms suggests a deepening of torpor, which the authors recognized as a “unique state of consciousness...at hypothermic extreme” [ 12 ].…”

Turn it off and on again: characteristics and control of torpor

“… We are not familiar with the earliest work on MSG and ARC neurons this but we’ve wondered since reading this original story how selective MSG actually is. RE torpor and sleep: Kindly add to discussion the recent story: “The relationship between fasting-induced torpor, sleep, and wakefulness in laboratory mice” 5 . The data presented in the studies of Hrvatin, Takahashi, and Zhang represent significant advances in our understanding of torpor.…”

“…RE torpor and sleep: Kindly add to discussion the recent story: “The relationship between fasting-induced torpor, sleep, and wakefulness in laboratory mice” 5 .…”