Proceedings of The Physiological Society

Sleep Sleep and Circadian Rhythms (London, UK) (2018) Proc Physiol Soc 42, C24

Poster Communications

The relationship between fasting-induced torpor and sleep in mice

Y. Huang1, V. Vyazovskiy1

1. Physiology, Anatomy and Genetics, University of Oxford, Oxford, United Kingdom.

Torpor is a regulated and reversible state of metabolic suppression employed by many animals mainly to conserve energy. Torpor can be induced by fasting or by changes in photoperiod (i.e. seasonal torpor, which includes hibernation and daily torpor). Previous studies on seasonal torpor revealed notable similarities and differences between torpor and sleep. Behaviourally, both states are associated with immobility and reduced responsiveness. However, both hibernation and daily torpor induced by shortening of photoperiod appear to be sleep-depriving states. Specifically, animals emerging from torpor usually enter sleep characterised by high encephalographic (EEG) slow-wave activity, an established marker of homeostatic sleep pressure. Much less is known about the relationship between sleep regulation and fasting-induced torpor, which can be readily induced in laboratory mice. In this study, we established a model of fasting-induced torpor in C57BL/6J mice (n=8, male; 12-weeks old; mean weight 26.9 g), and performed continuous electrophysiological and surface body temperature (Tsurface) recording (via infra-red cameras) across successive days of food restriction. The animals were implanted with epidural EEG electrodes above the frontal and occipital cortices and electromyogram (EMG) electrodes in the nuchal muscle, and allowed to recover for at least 7 days after surgery prior to starting the experiment. Mice were kept at a 12:12 light-dark cycle throughout the experiment and provided with approximately 1 g of food daily between ZT6 and ZT9. Ambient temperature was kept at 22 to 24 °C, and body weight was carefully monitored to ensure that it remained above 85% of ad lib feeding weight. Our preliminary analyses revealed that all animals entered torpor bouts (defined as Tsurface<28 °C for at least 1 hour) within 5 days of food restriction. Torpor bouts were invariably initiated via a state that, based on EEG and EMG signals, resembles NREM sleep, but EEG amplitude subsequently showed a prominent and progressive reduction during entrance into torpor, in some cases reaching below 10% of the values during euthermic NREM sleep. We did not observe REM sleep or extended spontaneous wakefulness periods during torpor bouts. However, in all animals the torpor bouts were punctuated by prominent EMG bursts, associated with a transient EEG activation, at a regular periodicity of <5-10 min. Upon spontaneous return to euthermia, the torpor bouts were typically followed by periods of wakefulness and often further torpor bouts until the animals were fed. The animals entered deep sleep with high EEG SWA shortly after feeding. Our study tentatively suggests that fasting-induced torpor and sleep are closely related yet distinct neurophysiological states. It remains to be determined whether fasting-induced torpor is a sleep-depriving state functionally similar to seasonal torpor.

Where applicable, experiments conform with Society ethical requirements