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Submitted on March 7, 2007
Accepted on April 20, 2007
Rowett Research Institute and Aberdeen Centre for Energy Regulation and Obesity, Bucksburn, Aberdeen AB21 9SB, United Kingdom, School of Biomedical Sciences, University of Nottingham Medical School, Nottingham, NG7 2UH, United Kingdom, Department of Endocrinology, Metabolism, F5-165, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands., Erasmus University, Medical Center, Department of Internal Medicine, Rotterdam, NL-3015 GE Netherlands
* To whom correspondence should be addressed. E-mail: P.Barrett{at}Rowett.ac.uk.
Seasonal adaptations in physiology exhibited by many animals involve an interface between biological timing and specific neuroendocrine systems, but the molecular basis of this interface is unknown. In this study of Siberian hamsters we show that the availability of thyroid hormone within the hypothalamus is a key determinant of seasonal transitions. The expression of the gene encoding type III deiodinase (Dio3) and Dio3 activity in vivo (catabolism of T4 and T3), are dynamically and temporally regulated by photoperiod, consistent with the loss of hypothalamic T3 concentrations under short photoperiods. Chronic replacement of T3 in the hypothalamus of male hamsters exposed to short photoperiods, thus bypassing synthetic or catabolic deiodinase enzymes located in cells of the ependyma of the third ventricle, prevented the onset of short day physiology: hamsters maintained a long day body weight phenotype and failed to undergo testicular and epididymal regression. However, pelage moult to a winter coat was not affected. Type II deiodinase gene expression was not regulated by photoperiod in these hamsters. Collectively these data point to a pivotal role for hypothalamic DIO3 and T3 catabolism in seasonal cycles of body weight and reproduction in mammals.
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