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.