Daniel J. Hall Mitchell S Fleming Gersende Maugers Bjorn Bjornsson Sylvie Dufour Katsuhisa Uchida Stephen D. McCormick Shotaro Irachi 2021 <div id="abstracts" class="Abstracts u-font-serif"><div id="abs0010" class="abstract author" lang="en"><div id="abssec0010"><p id="abspara0010">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 (<i>Salmo salar</i>) 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<span>&nbsp;</span><i>tshβb</i><span>&nbsp;</span>mRNA levels after 10 days and were sustained for at least 20 days.<span>&nbsp;</span><i>tshβb</i><span>&nbsp;</span>mRNA levels in the saccus vasculosus were low and not influenced by photoperiod. Increased daylength resulted in significant increases in<span>&nbsp;</span><i>dio2b</i><span>&nbsp;</span>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.</p></div></div></div> application/pdf 10.1016/j.mce.2020.111056 en Elsevier Photoperiodic regulation of pituitary thyroid-stimulating hormone and brain deiodinase in Atlantic salmon article