Effects of Mammalian and Salmon Gonadotropin-Releaslng Hormones and Analogues on Plasma Gonadotropin Levels and Ovulation in Coho Salmon (Oncorhynchus kisutch)

The effects of intraperitoneal injections of mammalian gonadotropin-releasing hormone (mGnRH) and salmon gonadotropin-releasing hormone (sGnRH; [Trp7, Leu8]-mGnRH) as well as analogues of each peptide on plasma gonadotropin levels and ovulation in coho salmon (Oncorhynchus kisutch) were investigated. The native peptides had similar potencies in terms of the magnitude and duration of the gonadotropin release response. Analogues including the D-Ala6 and (imbz1) D-His6 derivatives of [Pro9-NEt]-mGnRH and the D-Arg6 and D-Ala6 derivatives of [Pro9-NEt]-sGnRH stimulate a more prolonged increase in plasma gonadotropin levels than native forms of these peptides. Each of the analogue peptides at a dosage of 0.2 mg/kg body wt induced a high rate of ovulation; the native salmon peptide at the same dosage was also effective although the time of ovulation was delayed compared with the response to analogue peptides. A higher dosage of the native salmon peptide (1.0 mg/kg body wt) was less effective in stimulating gonadotropin release and failed to induce a high rate of ovulation. Our studies demonstrate that several analogues of mGnRH and sGnRH have superactive agonist activity in coho salmon and are effective in inducing ovulation in this species.