The Chemistry of Cold: Mechanisms of Torpor Regulation in the Siberian Hamster.

Siberian hamsters use spontaneous daily torpor, a state of hypometabolism and hypothermia, to save energy during winter. Multiple neuroendocrine signals set the scene for spontaneous torpor to occur, and several brain areas have been identified as potential sites for torpor regulation. Here, we summarize the known mechanisms of a fascinating physiological state in the Siberian hamster.

Thyroid hormone status affects expression of daily torpor and gene transcription in Djungarian hamsters (Phodopus sungorus).

Thyroid hormones (TH) play a key role in regulation of seasonal as well as acute changes in metabolism. Djungarian hamsters (Phodopus sungorus) adapt to winter by multiple changes in behaviour and physiology including spontaneous daily torpor, a state of hypometabolism and hypothermia. We investigated effects of systemic TH administration and ablation on the torpor behaviour in Djungarian hamsters adapted to short photoperiod. Hyperthyroidism was induced by giving T4 or T3 and hypothyroidism by giving methimazole (MMI) and sodium perchlorate via drinking water. T3 treatment increased water, food intake and body mass, whereas MMI had the opposite effect. Continuous recording of body temperature revealed that low T3 serum concentrations increased torpor incidence, lowered Tb and duration, whereas high T3 serum concentrations inhibited torpor expression. Gene expression of deiodinases (dio) and uncoupling proteins (ucp) were analysed by qPCR in hypothalamus, brown adipose tissue (BAT) and skeletal muscle. Expression of dio2, the enzyme generating T3 by deiodination of T4, and ucps, involved in thermoregulation, indicated a tissue specific response to treatment. Torpor per se decreased dio2 expression irrespective of treatment or tissue, suggesting low intracellular T3 concentrations during torpor. Down regulation of ucp1 and ucp3 during torpor might be a factor for the inhibition of BAT thermogenesis. Hypothalamic gene expression of neuropeptide Y, propopiomelanocortin and somatostatin, involved in feeding behaviour and energy balance, were not affected by treatment. Taken together our data indicate a strong effect of thyroid hormones on torpor, suggesting that lowered intracellular T3 concentrations in peripheral tissues promote torpor.

Torpor expression in juvenile and adult Djungarian hamsters (Phodopus sungorus) differs in frequency, duration and onset in response to a daily cycle in ambient temperature.

In addition to morphological and physiological traits of short-day acclimatisation, Djungarian hamsters (Phodopus sungorus) from Central Asia exhibit spontaneous daily torpor to decrease energy demands during winter. Environmental factors such as food scarcity and low temperatures have been shown to facilitate the use of this temporal reduction in metabolism and body temperature. We investigated the effect of a daily cycle in ambient temperature on short-day acclimation and torpor expression in juvenile and adult Djungarian hamsters. The animals were exposed to a cold dark phase (6°C) and a warmer light phase (18°C) and were compared with control hamsters kept at a constant ambient temperature of 18°C. Under constant conditions, torpor expression did not differ between adult and juvenile hamsters. Although the daily temperature cycle evoked an increased metabolic rate in adult and juvenile hamsters during the dark phase and strengthened the synchronization between torpor entrance and the beginning of the light phase, it did not induce the expected torpor facilitation. In adult hamsters, torpor expression profiles did not differ from those under constant conditions at all. In contrast, juvenile hamsters showed a delayed onset of torpor season, a decreased torpor frequency, depth and duration, as well as an increased number of early torpor terminations coinciding with the rise in ambient temperature after the beginning of the light phase. While the temperature challenge appeared to be of minor importance for energy balance and torpor expression in adult hamsters, it profoundly influenced the overall energy saving strategy of juvenile hamsters, promoting torpor-alleviating active foragers over torpor-prone energy-savers. In addition, our data suggest a more efficient acclimation in juvenile hamsters under additional energy challenges, which reduces the need for torpor expression.

Gene expression analysis and microdialysis suggest hypothalamic triiodothyronine (T3) gates daily torpor in Djungarian hamsters (Phodopus sungorus).

Thyroid hormones play an important role in regulating seasonal adaptations of mammals. Several studies suggested that reduced availability of 3,3',5-triiodothyronine (T3) in the hypothalamus is required for the physiological adaptation to winter in Djungarian hamsters. We have previously shown that T3 is involved in the regulation of daily torpor, but it remains unclear, whether T3 affects torpor by central or peripheral mechanisms. To determine the effect of T3 concentrations within the hypothalamus in regulating daily torpor, we tested the hypothesis that low hypothalamic T3 metabolism would favour torpor and high T3 concentrations would not. In experiment 1 gene expression in torpid hamsters was assessed for transporters carrying thyroid hormones between cerebrospinal fluid and hypothalamic cells and for deiodinases enzymes, activating or inactivating T3 within hypothalamic cells. Gene expression analysis suggests reduced T3 in hypothalamic cells during torpor. In experiment 2, hypothalamic T3 concentrations were altered via microdialysis and torpor behaviour was continuously monitored by implanted body temperature transmitters. Increased T3 concentrations in the hypothalamus reduced expression of torpor as well as torpor bout duration and depth. Subsequent analysis of gene expression in the ependymal layer of the third ventricle showed clear up-regulation of T3 inactivating deiodinase 3 but no changes in several other genes related to photoperiodic adaptations in hamsters. Finally, serum analysis revealed that increased total T3 serum concentrations were not necessary to inhibit torpor expression. Taken together, our results are consistent with the hypothesis that T3 availability within the hypothalamus significantly contributes to the regulation of daily torpor via a central pathway.