Smoltification prepares juvenile Atlantic salmon (Salmo salar) for downstream migration. Dramatic changes characterize this crucial event in the salmon's life cycle, including increased gill Na+/K+-ATPase activity (NKA) and plasma hormone levels. The triggering of smoltification relies on photoperiod and is modulated by temperature. Both provide reliable information, to which fish have adapted for thousands of years, that allows deciphering daily and calendar time. Here we studied the impact of different photoperiod (natural, sustained winter solstice) and temperature (natural, ~ + 4° C) combinations, on gill NKA, plasma free triiodothyronine (T3) and thyroxine (T4), and melatonin (MEL; the time-keeping hormone), throughout smoltification. We also studied the impact of temperature history on pineal gland MEL production in vitro. The spring increase in gill NKA was less pronounced in smolts kept under sustained winter photoperiod and/or elevated temperature. Plasma thyroid hormone levels displayed day-night variations, which were affected by elevated temperature, either independently from photoperiod (decrease in T3 levels) or under natural photoperiod exclusively (increase in T4 nocturnal levels). Nocturnal MEL secretion was potentiated by the elevated temperature, which also altered the MEL profile under sustained winter photoperiod. Temperature also affected pineal MEL production in vitro, a response that depended on previous environmental acclimation of the organ. The results support the view that the salmon pineal is a photoperiod and temperature sensor, highlight the complexity of the interaction of these environmental factors on the endocrine system of S. salar, and indicate that climate change might compromise salmon's time "deciphering" during smoltification, downstream migration and seawater residence.

中文翻译：

---

温度升高对大西洋鲑鱼、鲑鱼熏蒸过程中褪黑激素和甲状腺激素的影响

熏蒸使大西洋鲑鱼幼鱼（Salmo salar）为下游迁移做好准备。鲑鱼生命周期中这一关键事件的显着变化，包括鳃 Na+/K+-ATP 酶活性 (NKA) 和血浆激素水平的增加。冶炼的触发依赖于光周期并受温度调节。两者都提供可靠的信息，鱼类已经适应了数千年，可以破译每日和日历时间。在这里，我们研究了不同光周期（自然、持续冬至）和温度（自然，~+ 4°C）组合对鳃 NKA、血浆游离三碘甲状腺原氨酸 (T3) 和甲状腺素 (T4) 以及褪黑激素 (MEL；计时激素），在整个熔炼过程中。我们还研究了温度历史对体外松果体 MEL 产生的影响。在持续的冬季光周期和/或升高的温度下，幼鱼中鳃 NKA 的春季增加不太明显。血浆甲状腺激素水平显示昼夜变化，受温度升高的影响，无论是独立于光周期（T3 水平降低）还是完全在自然光周期下（T4 夜间水平增加）。夜间 MEL 分泌受升高的温度增强，这也改变了持续冬季光周期下的 MEL 分布。温度还影响体外松果体 MEL 的产生，这种反应取决于器官先前的环境适应情况。结果支持鲑鱼松果体是光周期和温度传感器的观点，突出了这些环境因素相互作用对 S. salar 内分泌系统的复杂性，