Meeting Abstract
Seasonal reproduction is a strategy used by temperate zone animals to maximise the propagation of the species. The hypothalamus in birds and mammals is responsible for timing seasonal breeding. Seasonal epigenetic modifications in the hypothalamus have been demonstrated to regulate long-term timing of reproduction and energy balance in several photoperiodic species. Enzymes involved in de novo DNA methylation (Dnmt3a/b) are expressed within the hypothalamic-gonadal axis and exhibit seasonal variation. The objectives of my research were to investigate the effect of thyrotrophin-stimulating hormone (TSH) and triiodothyronine (T3) on the photoperiodic regulation of DNA methyltransferase enzyme expression. I tested the hypothesis that TSH or T3 in short day hamsters would stimulate hypothalamic expression in Siberian hamsters. Short winter-like day lengths induced a significant reduction in Dnmt3a/b in the hypothalamus. Male hamster hypothalamic dnmt3a nor dnmt3b expression did not vary in response to TSH. Then, I examined the impact of photoperiod and daily T3 injections or saline on hypothalamic dnmt3a/b expression and female reproductive physiology. SD photoperiods were observed to reduce body weight and uterine weight. Unlike previous reports in male hamsters, daily T3 injections in SD females were ineffective to stimulate gonadal recrudescence. Hypothalamic Dnmt3a and dnmt3b expression was reduced in SD, independent of T3. These data suggest that an alternative hormonal signal regulates dnmt3a and dnmt3b expression or that cyclical dnmt3a/b expression reflect an endogenous circannual timing system. Ongoing work seeks to identify whether long days reduce hypothalamic dnmt3a/b expression during the Japanese quail photoperiodic response. These data indicate that reduced neuroendocrine DNA methylation permits seasonal gonadal recrudescence.
