Mapping the Pattern of Essential Neuroendocrine Cells Related to Puberty and VA Opsin Expression Provides Further Insight in the Photoreceptive Regulation of the Brain-Pituitary-Gonadal Axis in Atlantic Salmon (Salmo salar).

In Atlantic salmon (Salmo salar), seasonal photoperiod is shown to regulate the onset of sexual maturation, yet which brain region(s) is involved, and how light information impacts the neuroendocrine system are still not fully understood in teleosts. Detailed knowledge about the photoperiodic regulation of maturation in fish is still missing. In birds, it is shown that gonadotropin-releasing hormone (Gnrh) is located in the same neurons as vertebrate ancient (VA) opsin, suggesting a direct photoreceptive regulation for the onset of sexual maturity.

Photoperiodic regulation of pituitary thyroid-stimulating hormone and brain deiodinase in Atlantic salmon.

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 (T). 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.

Functional divergence of thyrotropin beta-subunit paralogs gives new insights into salmon smoltification metamorphosis.

Smoltification is a metamorphic event in salmon life history, which initiates downstream migration and pre-adapts juvenile salmon for seawater entry. While a number of reports concern thyroid hormones and smoltification, few and inconclusive studies have addressed the potential role of thyrotropin (TSH). TSH is composed of a α-subunit common to gonadotropins, and a β-subunit conferring hormone specificity.