Photoperiod-induced changes in hypothalamic de novo DNA methyltransferase expression are independent of triiodothyronine in female Siberian hamsters (Phodopus sungorus)

•Short days induce uterine involution and body mass loss in female hamsters.•Daily triiodothyronine injections did not stimulate uterine growth.•Triiodothyronine reduced pituitary thyrotropin-stimulating hormone mRNA.•Triiodothyronine did not affect hypothalamic Gnrh or Gnih mRNA.•Short days stimulated hypothalamic de novo DNA methyltransferase mRNA. Many temperate zone animals engage in seasonal reproductive physiology and behavior as a strategy to maximise the propagation of the species. The hypothalamus integrates environmental cues and hormonal signalling to optimize the timing of reproduction. Recent work has revealed that epigenetic modifications, such as DNA methylation, vary across seasonal reproductive states. Multiple hormones act in the hypothalamus to permit or inhibit reproductive physiology, and the increase in thyroid hormone triiodothyronine (T3) has been implicated in the initiation of breeding in many species. The objective of this study was to examine the effect of T3 on the photoperiod-dependent regulation of reproductive physiology and hypothalamic DNA methyltransferase enzyme expression in female Siberian hamsters (Phodopus sungorus). We tested the hypothesis that T3 in short days (SD) would stimulate hypothalamic Rfrp3 and de novo DNA methyltransferase (Dnmt) expression in female Siberian hamsters. 10 weeks of SD lengths induced a decrease in body and uterine mass. Hamsters maintained in SD were found to express lower levels of GnRH, Rfrp3, Dnmt3a and Dnmt3b. Two weeks of daily T3 injections did not affect body mass, uterine mass, Gnrh, Rfrp3, Dnmt3a or Dnmt3b expression in neuroendocrine tissues. SD significantly lowered Tshβ mRNA expression and T3 reduced Tshβ in LD hamsters. Our data indicate sex-dependent effects of T3 for the neuroendocrine regulation of seasonal reproduction in hamsters.