Fasting-induced torpor in mice: effects on sleep and behaviour

Food restriction, fasting or scheduled feeding in mice are routinely used in many laboratories worldwide in the context of behavioural neuroscience, studies on metabolism and circadian neurobiology. Hunger is associated with a reduction in blood glucose, which triggers wakefulness and food-seeking behaviour. An alternative strategy that many species employ when facing food scarcity is energy conservation, which can be achieved by entering torpor. Torpor is a unique adaptation, characterised by a profound attenuation of physiological functions, wherein body temperature can drop to within a few degrees of ambient temperature. It has been proposed that fasting blocks cold-induced thermogenesis while triggering a decrease in energy expenditure and body temperature, resulting in torpor.

Surprisingly few researchers are aware that laboratory mice are a facultative heterothermic species that readily display torpor bouts in response to food deprivation at the temperature levels commonly used in animal facilities. While torpor itself is an adaptive response to limited food supply, and is easily reversible, little is known about the lasting consequences of torpor on physiology, sleep and brain function, and thus it may be a potential source of significant biological variation in scientific data. Studies suggest that torpor and hypothermia lead to a substantial loss of synapses across the brain including the hippocampus, cortex, and thalamus, suggesting that it will likely have fundamental influences on behaviour and learning. Evidence also suggests that torpor is associated with profound changes in metabolic rates and changes in hormones such as leptin. Therefore, since most researchers are unaware that fasted mice may undergo prolonged periods of torpor, it is essential to investigate this process in the context of commonly used food restriction paradigms to determine the influence of torpor on subsequent data.

This project will investigate the effects of commonly used food restriction protocols on torpor in mice, the effects of torpor on subsequent behavioural performance and sleep, and will develop a standardized approach to detect the occurrence of torpor during food restriction. Our project will inform behavioural and physiological studies in mice, where food restriction is widely used. Our preliminary study suggests that even a short period of food restriction can induce torpor, which is associated with profound effects on sleep and waking. Given that sleep is crucially important for cognitive functions and energy homeostasis, the occurrence of torpor will therefore have a marked influence on animal’s subsequent performance in behavioural tasks and on many physiological variables. Our project will therefore inform the experimental design of future studies to ensure that fasting is sufficient without inducing torpor. This will provide a major 3Rs impact, as it will allow food restriction protocols to be optimised whilst avoiding the major metabolic disturbances associated with torpor, which have the potential to disrupt behaviour and learning (Refinement). Moreover, we predict that this will also decrease variance in the data reducing the number of animals required for such studies (Reduction). Scientifically, as well as improving reproducibility and avoiding confounds, this model offers a unique opportunity to understand torpor as a third fundamental state of vigilance, in addition to wake and sleep.

• Early Career Engagement Award: Inaugural award scheme supports NC3Rs-funded junior researchers