Evaluation of a specific gonadotropin-releasing hormone binding protein in the serum of goldfish: A study on the influence of sex, season, GnRH injection and estradiol treatment in vivo

Abstract

A gonadotropin releasing hormone (GnRH) binding protein (GnRH-BP) from goldfish serum was isolated and characterized. In the present studies, the differences in serum titer of GnRH-BP between male and female goldfish, and between goldfish at different stages of gonadal development, were investigated. The effects of estradiol (E 2) treatment or multiple injections of [ d-Arg 6-Pro 9-NEt]-salmon GnRH (sGnRH-A) treatment on the serum titer of GnRH-BP were also studied. GnRH-BP in individual serum samples was quantified with a ligand binding assay using 125I-sGnRH-A as tracer; GnRH-BP-tracer complex was separated from free tracer by gel filtration using Sephadex G-50 mini-columns. GnRH-BP was detected in every individual sample; however, no differences between males and females, nor seasonal changes in either sex, were detected. Serum total protein content doubled with E 2 treatment, but no effects of E 2 on GnRH-BP titer were detected. Long-term treatment with sGnRH-A stimulated an increase in serum gonadotropin levels; the long-term treatment with sGnRH-A did not alter the serum titer of GnRH-BP. Although the GnRH-BP is present in excess relative to the low concentration of GnRH in goldfish serum, it is not a major component of serum proteins. It is concluded that the serum titer of GnRH-BP was not fine tuned by GnRH, gonadotropin, or estradiol. Therefore physiological regulators have yet to be discovered.