IndoUS Center for Biological Timing, Department of Zoology, University of Delhi, Delhi, India.
IndoUS Center for Biological Timing, Department of Zoology, University of Lucknow, Lucknow, India.
J Neuroendocrinol. 2018 Sep;30(9):e12627. doi: 10.1111/jne.12627. Epub 2018 Aug 6.
Circannual rhythm regulates the annual timing of reproduction in spotted munia, with sex differences in its relationship with the external photoperiod environment. Interestingly, munia show an atypical photosensitivity and exhibit gonadal maturation when acutely exposed to an unnatural short photoperiod (eg 3 hours of light per day; ie a long scotoperiod). The proximate mechanisms regulating scotoperiod-induced hypothalamic-pituitary-gonadal (HPG) activation are unclear. Because thyroid hormone signalling plays a central role in photoperiodic induction, we hypothesised the involvement of similar mechanism, comprising alterations in hypothalamic deiodinases, under long scotoperiod-induced HPG activation. To test this, several endpoints of cellular and molecular correlates were assayed in male and female munias after 1 and 4 weeks of exposure to an 3:21 hour light/dark cycle (3L:21D), with controls on a 21:3 hour light/dark cycle (21L:3D). We measured the hypothalamic expression of mRNA and protein of light-sensitive (neuropsin, OPN5) and reproductive (vasoactive intestinal peptide [VIP], neuropeptide Y [NPY], gonadotrophin-releasing hormone [GnRH], gonadotrophin-inhibiting hormone [GnIH]) neuropeptides by quantitative polymerase chain reaction (PCR) and immunohistochemistry, respectively. In addition, we also measured mRNA expression of types 2 (DIO2) and 3 (DIO3) deiodinases that regulate triiodothyronine-mediated GnRH release and gonadal maturation in photoperiodic species. The quantitative PCR and immunohistochemistry results were consistent. Higher OPN5 levels under 21L:3D than under 3L:21D suggested its role in sensing the length of the light period. Similarly, low VIP and high NPY expression under 3L:21D than under 21L:3D were consistent with their roles as cellular correlates of photic and nonphotic environment, respectively. High GnRH-I/low GnIH levels and gonadal recrudescence under 3L:21D, and an inverse pattern under 21L:3D, confirmed the scotostimulation of HPG axis in spotted munia. However, DIO2 and DIO3 mRNA levels did not differ between 2 scotoperiods, in contrast to their reciprocal expression pattern found during long-day photostimulation. We demonstrate for the first time sex-dependent scotostimulation of reproductive neural pathways and suggest the involvement of molecules other than hypothalamic deiodinases in the regulation of gonad development cycle in 'nonphotoperiodic' seasonally breeding vertebrates.
年周期节律调节斑胸草雀的年度繁殖时间，其与外部光周期环境的关系存在性别差异。有趣的是，斑胸草雀表现出非典型的光敏感性，并且在急性暴露于非自然短光周期（例如每天 3 小时光照；即长暗期）时表现出性腺成熟。调节暗期诱导的下丘脑-垂体-性腺（HPG）激活的近因机制尚不清楚。由于甲状腺激素信号在光周期诱导中起核心作用，我们假设在长暗期诱导的 HPG 激活下，存在类似的机制，包括下丘脑脱碘酶的改变。为了验证这一点，我们在雄性和雌性斑胸草雀暴露于 3:21 小时光照/黑暗循环（3L:21D）1 周和 4 周后，以及对照 21:3 小时光照/黑暗循环（21L:3D）时，检测了细胞和分子相关性的几个终点。我们通过定量聚合酶链反应（PCR）分别测量了光敏感（神经丝蛋白、OPN5）和生殖（血管活性肠肽 [VIP]、神经肽 Y [NPY]、促性腺激素释放激素 [GnRH]、促性腺激素抑制激素 [GnIH]）神经肽的下丘脑 mRNA 和蛋白质表达，和免疫组织化学。此外，我们还测量了调节光周期物种中三碘甲状腺原氨酸介导的 GnRH 释放和性腺成熟的 2 型（DIO2）和 3 型（DIO3）脱碘酶的 mRNA 表达。定量 PCR 和免疫组织化学结果一致。21L:3D 下的 OPN5 水平高于 3L:21D 表明其在感知光期长度中的作用。同样，3L:21D 下的 VIP 表达低而 NPY 表达高与它们分别作为光和非光环境的细胞相关性一致。3L:21D 下的 GnRH-I/低 GnIH 水平和性腺复育，以及 21L:3D 下的相反模式，证实了斑胸草雀 HPG 轴的暗刺激。然而，与长日光照刺激下发现的相反表达模式不同，2 个暗周期之间的 DIO2 和 DIO3 mRNA 水平没有差异。我们首次证明了生殖神经通路的性别依赖性暗刺激，并表明在“非光周期”季节性繁殖脊椎动物中，调节性腺发育周期的分子不仅是下丘脑脱碘酶。
