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We recently characterized two paralogs of the thyrotropin (TSH) beta subunit in Atlantic salmon,  tshβa  and  tshβb , issued from teleost-specific whole genome duplication. The transcript expression of  tshβb , but not of  tshβa , peaks at the time of smoltification, which revealed a specific involvement of  tshβb  paralog in this metamorphic event.  Tshβa  and  tshβb  are expressed by distinct pituitary cells in salmon, likely related to TSH cells from the  pars distalis  and  pars tuberalis , respectively, in mammals and birds. The present study aimed at investigating the neuroendocrine and endocrine factors potentially involved in the differential regulation of  tshβa  and  tshβb  paralogs, using primary cultures of Atlantic salmon pituitary cells. The effects of various neurohormones and endocrine factors potentially involved in the control of development, growth, and metabolism were tested. Transcript levels of  tshβa  and  tshβb  were measured by qPCR, as well as those of growth hormone ( gh ), for comparison and validation. Corticotropin-releasing hormone (CRH) stimulated  tshβa  transcript levels in agreement with its potential role in the thyrotropic axis in teleosts, but had no effect on  tshβb  paralog, while it also stimulated  gh  transcript levels. Thyrotropin-releasing hormone (TRH) had no effect on neither  tshβ  paralogs nor  gh . Somatostatin (SRIH) had no effects on both  tshβ  paralogs, while it exerted a canonical inhibitory effect on  gh  transcript levels. Thyroid hormones [triiodothyronine (T3) and thyroxine (T4)] inhibited transcript levels of both  tshβ  paralogs, as well as  gh , but with a much stronger effect on  tshβa  than on  tshβb  and  gh.  Conversely, cortisol had a stronger inhibitory effect on  tshβb  than  tshβa , while no effect on  gh . Remarkably, insulin-like growth factor 1 (IGF1) dose-dependently stimulated  tshβb  transcript levels, while it had no effect on  tshβa , and a classical inhibitory effect on  gh . This study provides the first data on the neuroendocrine factors involved in the differential regulation of the expression of the two  tshβ  paralogs. It suggests that IGF1 may be involved in triggering the expression peak of the  tshβb  paralog at smoltification, thus representing a potential internal signal in the link between body growth and smoltification metamorphosis.

Differential Regulation of the Expression of the Two Thyrotropin Beta Subunit Paralogs by Salmon Pituitary Cells In Vitro