Distinct alpha (α)-subunits of salmon glycoprotein hormones: production sites in the pituitary with sexual maturity

Salmon pituitary glands contain two structurally distinct α-subunit proteins (α1 and α2) of glycoprotein hormones: the α2-subunit is common to all salmon gonadotropins (GTH I and GTH II), whereas the α1-subunit is present in only some GTH I molecules. GTH I is predominant in the pituitary gland and plasma during gametogenesis of salmon, but the roles of the 2 GTHs in gametogenesis remain unclear. To understand the roles of GTH I, it is important to clarify patterns of α1- and α2-subunit production with sexual maturity. Thus, we produced antisera that recognized the α1- or α2-subunit, and then immunohistochemically examined the production sites of these subunits in the trout pituitary gland during ovarian development. In all pituitary glands examined, the immunoreactivity of both the α1- and α2-subunits was strong in the GTH II-producing cells, although salmon GTH II, both α1- and IIβ-subunits, has not been detected. However, GTH I-producing cells showed a less dense immunoreactivity for α1- and α2-subunits, whereas the Iβ-subunit was abundant. On the other hand, TSH cells, reacted with α2 but not with α1.