Thyroid hormones affect the torpor response of the Djungarian hamsters

Djungarian hamsters (Phodopus sungorus) live in a seasonally changing environment. To cope with harsh winter conditions they undergo multiple changes in behaviour and physiology including spontaneous daily torpor, a state of hypometabolism and hypothermia. Thyroid hormones play a key role in regulation of seasonal as well as acute changes in metabolism. In our study we investigated effects of thyroid hormone manipulation on the spontaneous torpor response. Thyroid hormone levels were increased by giving T4, T3 or decreased by methimazole via drinking water. Body temperature was recorded during the entire experiment and gene expression was analysed by qPCR in hypothalamus, brown adipose tissue (BAT) and muscle. High or low serum T3 levels had reciprocal effects on torpor bout frequency and duration as well as body temperature during torpor bouts and active periods, food intake and body weight. Gene expression of type 2 deiodinase (dio2) involved in T3 activation indicated a tissue specific response to treatment. Torpor per se affected dio2 expression irrespective of treatment or tissue, suggesting down regulation of T3 production during hypometabolism. Uncoupling proteins, target genes of T3 that are involved in thermoregulation, were affected by treatment and torpor. Taken together our data indicate a strong effect of thyroid hormones on torpor. Understanding sites and mechanisms of thyroid hormone action in naturally occurring torpor will help to understand general regulatory mechanisms of metabolism and thermoregulation.