Institute of Arctic Biology, University of Alaska Fairbanks, 2140 Koyukuk Drive, Fairbanks, AK 99775, USA.
Rocky Mountain Research Station, United States Forest Service, 800 E. Beckwith Missoula, MT 59801, USA.
Integr Comp Biol. 2022 Oct 29;62(4):1012-1021. doi: 10.1093/icb/icac112.
Many animals adjust the timing of seasonal events, such as reproduction, molt, migration, and hibernation, in response to interannual variation and directional climate-driven changes in temperature. However, the mechanisms by which temperature influences seasonal timing are relatively under-explored. Seasonal timing involves retrograde signaling in which thyrotropin (TSH) in the pars tuberalis (PT) alters expression of thyroid hormone (TH) deiodinases (Dio2/Dio3) in tanycyte cells lining the third ventricle of the hypothalamus. This, in turn, affects the availability of triiodothyronine (T3) within the mediobasal hypothalamus-increased hypothalamic T3 restores a summer phenotype and activates the reproductive axis in long-day breeders. Recently, we showed that retrograde TH signaling is activated during late hibernation in arctic ground squirrels (Urocitellus parryii) held in constant darkness and constant ambient temperature. Sensitivity of seasonal pathways to nonphotic cues, such as temperature, is likely particularly important to hibernating species that are sequestered in hibernacula during spring. To address this issue, we exposed captive arctic ground squirrels of both sexes to an ecologically relevant increase in ambient temperature (from -6 to -1°C) late in hibernation and examined the effects of warming on the seasonal retrograde TSH/Dio/T3 signaling pathway, as well as downstream elements of the reproductive axis. We found that warmed males tended to have higher PT TSHβ expression and significantly heavier testis mass whereas the TSH/Dio/T3 signaling pathway was unaffected by warming in females, although warmed females exhibited a slight decrease in ovarian mass. Our findings suggest that temperature could have different effects on gonadal growth in male and female arctic ground squirrels, which could lead to mismatched timing in response to rapid climate change.
许多动物通过调整季节性事件的时间，例如繁殖、换羽、迁徙和冬眠，来应对年际变化和温度的方向性气候驱动变化。然而，温度影响季节性时间的机制相对较少被探索。季节性时间涉及逆行信号，其中垂体柄（PT）中的促甲状腺素（TSH）改变下丘脑第三脑室衬里的甲状腺激素（TH）脱碘酶（Dio2/Dio3）的表达。这反过来又影响了中脑基底部三碘甲状腺原氨酸（T3）的可用性——增加的下丘脑 T3 恢复了夏季表型并激活了长日繁殖者的生殖轴。最近，我们表明，逆行 TH 信号在北极地松鼠（Urocitellus parryii）的晚期冬眠中被激活，这些北极地松鼠处于持续黑暗和恒定环境温度中。季节性途径对非光线索（如温度）的敏感性可能对在春季被隔离在冬眠地的冬眠物种尤其重要。为了解决这个问题，我们让处于冬眠晚期的雌雄北极地松鼠暴露在环境温度（从-6°C 到-1°C）的生态相关增加中，并研究了变暖对季节性逆行 TSH/Dio/T3 信号通路以及生殖轴下游元件的影响。我们发现，变暖的雄性往往具有更高的 PT TSHβ 表达和显著更重的睾丸质量，而雌性的 TSH/Dio/T3 信号通路不受变暖的影响，尽管变暖的雌性卵巢质量略有下降。我们的发现表明，温度可能对雄性和雌性北极地松鼠的性腺生长产生不同的影响，这可能导致对快速气候变化的时间不匹配。
