Abstract
The Siberian hamster (Phodopus sungorus) is a seasonal mammal responding to the annual cycle in photoperiod with anticipatory physiological adaptations. This includes a reduction in food intake and body weight during the autumn in anticipation of seasonally reduced food availability. In the laboratory, short-day induction of body weight loss can be reversed or prevented by voluntary exercise undertaken when a running wheel is introduced into the home cage. The mechanism by which exercise prevents or reverses body weight reduction is unknown, but one hypothesis is a reversal of short-day photoperiod induced gene expression changes in the hypothalamus that underpin body weight regulation. Alternatively, we postulate an exercise-related anabolic effect involving the growth hormone axis. To test these hypotheses we established photoperiod-running wheel experiments of 8 to 16 weeks duration assessing body weight, food intake, organ mass, lean and fat mass by magnetic resonance, circulating hormones FGF21 and insulin and hypothalamic gene expression. In response to running wheel activity, short-day housed hamsters increased body weight. Compared to short-day housed sedentary hamsters the body weight increase was accompanied by higher food intake, maintenance of tissue mass of key organs such as the liver, maintenance of lean and fat mass and hormonal profiles indicative of long day housed hamsters but there was no overall reversal of hypothalamic gene expression regulated by photoperiod. Therefore the mechanism by which activity induces body weight gain is likely to act largely independently of photoperiod regulated gene expression in the hypothalamus.
Highlights
The Siberian hamster, Phodopus sungorus, is an exemplar of responsiveness to seasonal photoperiod
The Siberian hamster can lose up to 40% of body weight over a 12–16 week interval when transferred from a long day photoperiod (LD; 16 h light:8 h dark) to a short day photoperiod (SD; 8 h light:16 h dark), but this can be reversed by switching hamsters back into LD or occurs in extended periods of SD, known as the photorefractory response [9,10,11,12,13]
The mechanistic basis of the effect of exercise on physiology is unknown, but we hypothesised that the effect may be due to a reversal of key gene expression changes in the hypothalamus of the Siberian hamster involved in photoperiod-induced adaptation of body weight physiology
Summary
The Siberian hamster, Phodopus sungorus ( known as the Djungarian hamster), is an exemplar of responsiveness to seasonal photoperiod. The Siberian hamster can lose up to 40% of body weight over a 12–16 week interval when transferred from a long day photoperiod (LD; 16 h light: h dark) to a short day photoperiod (SD; 8 h light: h dark), but this can be reversed by switching hamsters back into LD or occurs in extended periods of SD, known as the photorefractory response [9,10,11,12,13] This model of reversible body weight regulation offers opportunities to identify new components or structures in the brain which are involved in the regulation of body weight
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