Presence and activity of compounds with GnRH-like activity in the ovary of seabream Sparus aurata

The binding and activity of gonadotropin-releasing hormone (GnRH) were characterized in the mature gilthead seabream (Sparus aurata) ovary by use of an analogue of salmon GnRH([D-Arg(6),Trp(7),Leu(8),Pro(9)-N(Et)] GnRH, sGnRH-A) as labeled ligand. The binding of I-125-sGnRH-A to the seabream ovarian membrane preparation was saturable, displaceable, reversible, and dependent on time, temperature, and tissue concentration. Addition of unlabeled s-GnRH-A displaced the radioligand in a dose-related manner, indicating the presence of one class of high-affinity binding sites with an equilibrium dissociation constant of 45.5 +/- 6.2 nM. Addition of other GnRH peptides, including salmon GnRH ([Trp(7),Leu(8)]GnRH) and chicken GnRH-II ([His(5),Trp(7),Tyr(8)] GnRH), also displaced I-125-sGnRH-A; all these peptides bound with lower affinities than sGnRH-A to the seabream ovarian binding site. In this study, we also demonstrated the presence of compounds with GnRH-like activity in the ovary of seabream. Seabream ovarian extract stimulated pituitary gonadotropin release from the goldfish pituitary and displaced I-125-sGnRH-A binding in the seabream ovary. Furthermore, addition of sGnRH-A to cultured seabream oocytes directly stimulated reinitiation of oocyte meiosis, as indicated by germinal vesicle breakdown. Overall, the present study characterizes GnRH-binding sites in the seabream ovary and supports the hypothesis that GnRH or compounds with GnRH-like activity play an autocrine/paracrine role in the regulation of ovarian function in the seabream ovary.