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Submitted on May 8, 2006
Accepted on July 14, 2006
Département de Neurobiologie des Rythmes, CNRS UMR-7168/LC2, Institut des Neurosciences Cellulaires et Intégratives, Université Louis Pasteur - IFR des Neurosciences, 5 rue Blaise Pascal, 67084 Strasbourg Cedex, France; Department of Functional Neuroanatomy and Biomarkers, NeuroSearch A/S, Ballerup, Denmark
* To whom correspondence should be addressed. E-mail: simonneaux{at}neurochem.u-strasbg.fr.
In seasonal species, photoperiod organizes various physiological processes, including reproduction. Recent data indicate that the expression of type 2 iodothyronine deiodinase (Dio2) is modulated by photoperiod in the mediobasal hypothalamus of some seasonal species. Dio2 is believed to control the local synthesis of bioactive triiodothyronine to regulate gonadal response. Here, we used in situ hybridization to study Dio2 expression in the hypothalamus of a photoperiodic rodent, the Syrian hamster. Dio2 was highly expressed in reproductively active hamsters in LD, while it was dramatically reduced in sexually inhibited hamsters maintained in SD. This contrasted with the laboratory rat, a non-photoperiodic species, where no evidence for Dio2 photoperiodic modulation was seen. We also demonstrate that photoperiodic variations of Dio2 expression in hamsters are independent from secondary changes in gonadal steroids. Studies in pinealectomized hamsters showed that the photoperiodic variation of Dio2 expression is melatonin-dependent, and injections of LD hamsters with melatonin for only 7 days were sufficient to inhibit Dio2 expression to that of SD levels. Finally, because in some seasonal species thyroid hormones are involved in photorefractoriness, we examined Dio2 expression in SD-refractory hamsters, and found that Dio2 mRNA levels remained low despite full reproductive recrudescence. Altogether, these results demonstrate that in the Syrian hamster Dio2 is photoperiodically modulated via a melatonin-dependent process. Furthermore, refractoriness to photoperiod in hamsters appears to occur independently of Dio2. These results raise new perspectives for understanding how thyroid hormones are involved in the control of photoperiodic neuroendocrine processes.
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