Department of Arctic and Marine Biology, University of Tromsø, Tromsø, Norway.
PRC, INRA, CNRS, IFCE, Université de Tours, Nouzilly, France.
J Neuroendocrinol. 2018 Sep;30(9):e12631. doi: 10.1111/jne.12631. Epub 2018 Aug 7.
In mammals, melatonin is the hormone responsible for synchronisation of seasonal physiological cycles of physiology to the solar year. Melatonin is secreted by the pineal gland with a profile reflecting the duration of the night and acts via melatonin-responsive cells in the pituitary pars tuberalis (PT), which in turn modulate hypothalamic thyroid hormone status. Recent models suggest that the actions of melatonin in the PT depend critically on day length-dependent changes in the expression of eyes absent 3 (Eya3), which is a coactivator for thyrotrophin β-subunit (Tshβ) gene transcription. According to this model, short photoperiods suppress Eya3 and hence Tshβ expression, whereas long photoperiods produce the inverse effect. Studies underpinning this model have relied on step changes in photoperiod (from 8 to 16 hours of light/24 hours) and have not compared the sensitive ranges of photoperiods for changes in Eya3 and Tshβ expression with those for relevant downstream molecular and endocrine responses. We therefore performed a "critical day length" experiment in Soay sheep, in which animals acclimated to 8 hours of light/24 hours (SP) were exposed to a range of increased photoperiods spanning the range 11.75 to 16 hours (LP) and then responses at the level of the PT, hypothalamus and hormonal output were assessed. Although Eya3 and Tshβ both showed the predicted SP vs LP differences, they responded quite differently to intermediate photoperiods within this range and, at the individual animal level, no clear Eya3-Tshβ relationship could be seen. This result is inconsistent with a simple coactivator model for EYA3 action in the PT. Further downstream layers of nonlinearity were also seen in terms of the Tshβ-dio2 and the dio2-testosterone relationships. We conclude that the transduction of progressive changes in photoperiod into transitions in endocrine output is an emergent property of a multistep signalling cascade within the mammalian neuroendocrine system.
在哺乳动物中，褪黑素是一种激素，负责将季节性生理周期与太阳年同步。褪黑素由松果体分泌，其分泌模式反映了夜晚的持续时间，并通过垂体中间部的褪黑素反应细胞（PT）发挥作用，进而调节下丘脑甲状腺激素状态。最近的模型表明，褪黑素在 PT 中的作用取决于眼睛缺失 3（Eya3）表达的日长依赖性变化，Eya3 是促甲状腺素β亚基（Tshβ）基因转录的共激活因子。根据该模型，短光照周期会抑制 Eya3 从而抑制 Tshβ 的表达，而长光照周期则产生相反的效果。支持该模型的研究依赖于光照周期的阶跃变化（从 8 小时到 16 小时光照/24 小时），并且没有比较 Eya3 和 Tshβ 表达变化的敏感光照周期范围与相关下游分子和内分泌反应的敏感光照周期范围。因此，我们在斯旺绵羊中进行了“关键日长”实验，其中适应 8 小时光照/24 小时（SP）的动物暴露于 11.75 至 16 小时（LP）的一系列增加的光照周期中，然后评估 PT、下丘脑和激素输出水平的反应。尽管 Eya3 和 Tshβ 都表现出预期的 SP 与 LP 差异，但它们对该范围内的中间光照周期的反应非常不同，并且在个体动物水平上，无法看到明显的 Eya3-Tshβ 关系。这一结果与 EYA3 在 PT 中的共激活因子模型不一致。在 Tshβ-dio2 和 dio2-睾酮关系方面，也出现了更下游的非线性层次。我们得出的结论是，将光照周期的渐进变化转化为内分泌输出的转变是哺乳动物神经内分泌系统中多步骤信号级联的一个涌现特性。
