Ovulatory response and gth ii serum levels in female rainbow trout (oncorhynchus mykiss) with gnrha d-ala(6) induced ovulation

Abstract

The ovulatory response and GtH serum levels were studied in female rainbow trouts induced with GnRHa D-Ala(6) (Syndel). Induced females (group 1, n=50) were injected with 30 mu g/kg of body weight of GnRHa applied in two doses of 15 mu g/kg of body weight each, with and interval of 6 days. Control females (group 2, n=21) were simultaneously injected with a saline placebo. The ovulatory response of induced females, measured as an accumulated percentage of ovulated females post-treatment, was significantly higher than the control group (p <0.01). GtH II serum levels were determined by radioimmunoassay (RIA) in samples collected 6 h after each hormone injection. Of the 18 induced females, 15 showed detectable values: 14.7+/-3.6 ng/ml (n-6) after the first injection, and 16.6+/-3.1 ng/ml (n=9) after the second injection. Only 3 fish showed a value of serum GtH II below the range detected by the assay (>1.94 ng/ml). GtH II serum levels of the control group were undetected after the first injection (n=10) and only one female showed a value of 2.52 ng/ml after the second injection of the hormone. Both, the ovulatory response and the serum GtH II values confirm the capacity of GnRHa D-Ala(6) to stimulate ovulation through and increase in GtH II synthesis and/or secretion from the pituitary gland. Hormonal induction with parenteral injection appears to be an appropriate tool for the synchronization of spawning in cultured rainbow trout.