Needham Katie B, Bergeon Burns Christy, Graham Jessica L, Bauer Carolyn M, Kittilson Jeffrey D, Ketterson Ellen D, Hahn Thomas, Greives Timothy J
Department of Biological Sciences, North Dakota State University, Fargo, ND, USA; Environmental and Conservation Sciences Program, North Dakota State University, Fargo, ND, USA.
Department of Biology, Indiana University, Bloomington, IN, USA.
Gen Comp Endocrinol. 2019 Jan 1;270:103-112. doi: 10.1016/j.ygcen.2018.10.012. Epub 2018 Oct 16.
Mechanisms related to seasonal reproductive timing in vertebrates have received far more study in males than in females, despite the fact that female timing decisions dictate when rearing of offspring will occur. Production and release of gonadotropin-releasing hormone (GnRH) by the hypothalamus stimulates the pituitary to secrete gonadotropins, initiating the beginning stages of gonadal recrudescence and production of the sex steroids, testosterone and estradiol, which are necessary to prime the liver for secretion of yolk precursors in breeding female birds. While stimulation by the hypothalamus can occur during the pre-breeding period, egg development itself is likely regulated downstream of the hypothalamus. We used GnRH challenges to examine variation in breeding-stage-specific patterns of pituitary and ovarian responsiveness in free-living female dark-eyed juncos (Junco hyemalis) and also examined the ovary and liver for variation in mRNA expression of candidate genes. Baseline LH levels increased during the transition from pre-breeding to egg-development, however no significant difference was observed in post-GnRH injection levels for LH or sex steroids (testosterone and estradiol). Interestingly, a stage by time-point interaction was observed, with post-GnRH LH levels increasing over baseline during the pre-breeding stage, but not during the egg-development stage. We observed a decrease in liver mRNA expression of estradiol receptor-alpha, and glucocorticoid and mineralocorticoid receptors and a decrease in glucocorticoid receptor expression levels in the ovary. A decline in FSH receptor expression across stages was also observed in the ovary. Combined, our data suggest seasonal variation in female's sensitivity to signals of HPG activity and energetic or stress signals. These data provide additional insight into the physiological mechanisms regulating onset of clutch initiation.
尽管雌性的繁殖时间决定了后代养育的时间，但与脊椎动物季节性繁殖时间相关的机制在雄性中得到的研究远比在雌性中多。下丘脑产生并释放促性腺激素释放激素（GnRH）会刺激垂体分泌促性腺激素，从而启动性腺再发育的初始阶段以及性类固醇睾酮和雌二醇的产生，这些对于促使繁殖期雌性鸟类的肝脏分泌卵黄前体是必需的。虽然下丘脑的刺激可在繁殖前期发生，但卵子发育本身可能在下丘脑下游受到调节。我们使用GnRH刺激来研究自由生活的雌性暗眼灯草雀（Junco hyemalis）垂体和卵巢在繁殖阶段特异性反应模式的变化，并且还检查了卵巢和肝脏中候选基因mRNA表达的变化。从繁殖前期到卵子发育阶段，促黄体生成素（LH）的基线水平升高，然而，注射GnRH后LH或性类固醇（睾酮和雌二醇）的水平未观察到显著差异。有趣的是，观察到一个阶段与时间点的相互作用，即在繁殖前期，注射GnRH后的LH水平高于基线，但在卵子发育阶段则不然。我们观察到肝脏中雌二醇受体α、糖皮质激素受体和盐皮质激素受体的mRNA表达下降，以及卵巢中糖皮质激素受体表达水平下降。在卵巢中还观察到不同阶段促卵泡激素（FSH）受体表达的下降。综合来看，我们的数据表明雌性对下丘脑 - 垂体 - 性腺（HPG）活动信号以及能量或应激信号的敏感性存在季节性变化。这些数据为调节产卵起始的生理机制提供了更多见解。
