Seasonal timing is important for many critical life history events of vertebrates, and photoperiod is often used as a reliable seasonal cue. In mammals and birds, it has been established that a photoperiod-driven seasonal clock resides in the brain and pituitary, and is driven by increased levels of pituitary thyroid stimulating hormone (TSH) and brain type 2 iodothyronine deiodinase (DIO2), which leads to local increases in triiodothyronine (T₃). In order to determine if a similar mechanism occurs in fish, we conducted photoperiod manipulations in anadromous (migratory) Atlantic salmon (Salmo salar) that use photoperiod to time the preparatory development of salinity tolerance which accompanies downstream migration in spring. Changing daylength from short days (light:dark (LD) 10:14) to long days (LD 16:8) for 20 days increased gill Na⁺/K⁺-ATPase (NKA) activity, gill NKAα1b abundance and plasma growth hormone (GH) levels that normally accompany increased salinity tolerance of salmon in spring. Long-day exposure resulted in five-fold increases in pituitary tshβb mRNA levels after 10 days and were sustained for at least 20 days. tshβb mRNA levels in the saccus vasculosus were low and not influenced by photoperiod. Increased daylength resulted in significant increases in dio2b mRNA levels in the hypothalamus and midbrain/optic tectum regions of the brain. The results are consistent with the presence of a photoperiod-driven seasonal clock in fish which involves pituitary TSH, brain DIO2 and the subsequent production of T₃, supporting the hypothesis that this is a common feature of photoperiodic regulation of seasonality in vertebrates.

季节性时机对于脊椎动物的许多重要生命史事件都很重要，并且光周期通常被用作可靠的季节性线索。在哺乳动物和鸟类中，已经确定光周期驱动的季节性时钟驻留在大脑和垂体中，并且由垂体甲状腺刺激激素（TSH）和脑2型碘甲状腺素脱碘酶（DIO2）水平升高所驱动。三碘甲状腺素（T ₃）局部升高。为了确定鱼中是否存在类似的机制，我们在无定形（迁徙）大西洋鲑鱼（Salmo salar）中进行了光周期操作），利用光周期来计时盐分耐受性的准备发展，该过程伴随着春季的下游迁移。将日长从短日（浅色：黑暗（LD）10:14）更改为长日（LD 16：8），持续20天，增加g Na ⁺ / K ⁺ -ATPase（NKA）活性，gNKAα1b丰度和血浆生长激素（ GH）水平通常会在春季增加鲑鱼的耐盐性。长期接触会导致10天后垂体tshβbmRNA水平增加五倍，并持续至少20天。saccus vasculosus中的tshβbmRNA水平较低，不受光周期的影响。日长的增加导致dio2b的显着增加下丘脑和大脑中脑/视神经顶盖区域的mRNA水平。结果与鱼类中存在光周期驱动的季节性时钟一致，该时钟涉及垂体TSH，脑DIO2和随后的T _3产生，支持这一假设，这是脊椎动物光周期调节季节性的共同特征。