Effects of estradiol or testosterone treatment on expression of gonadotropin subunit mRNAs and proteins in the pituitary of juvenile brown hagfish, Paramyxine atami.

A single functional gonadotropin (GTH) comprising two subunits, α and β, was recently identified in the pituitary of brown hagfish (Paramyxine atami). Little is known about the feedback mechanisms that regulate these GTH subunits by sex steroids in the hagfish. The present study was designed to examine feedback effects of estradiol and testosterone on mRNA expression and protein expression of both GTHα and GTHβ subunits in the pituitary of the juvenile P. atami. Intraperitoneal administration of estradiol over the course of 27days resulted in substantial accumulation of immunoreactive (ir)-GTHα and ir-GTHβ in the adenohypophysis, but testosterone treatments over 27days had no effects on ir-GTHα or ir-GTHβ. Estradiol treatment for 1, 2, 4 or 14days had no effect on GTHα mRNA levels. In contrast, after 2days of estradiol treatment, GTHβ mRNA levels had increased significantly from baseline, while these levels were not affected after treatment over 1, 4, or 14days. After 14days of testosterone treatment, both GTHα and GTHβ mRNA levels had decreased significantly from baseline levels. These results indicate that estradiol acted primarily to suppress the secretion of GTH, and hence resulted in the accumulations of ir-GTHα and ir-GTHβ in the pituitary. On the other hand, testosterone appeared to suppress both the synthesis and the secretion of GTH. Thus, estradiol and testosterone probably differ in their effects on the regulation of pituitary GTH synthesis and secretion in juvenile hagfish.