Effect of teleost GnRH on reinitiation of oocyte meiosis in goldfish, in vitro

The effects of native and an agonist analogue of teleost gonadotropin-releasing hormone (GnRH) [(D-Arg6, Trp7, Leu8, Pro9-NHEt)-GnRH; tGnRH-A] on the hormone-induced meiotic maturation of goldfish oocytes were investigated. Incubation of follicle-enclosed goldfish oocytes with either carp gonadotropin (GtH) or meiogenic progestogens, including 17 alpha-hydroxyprogesterone (HP) and 17 alpha,20 beta-dihydroxyprogesterone (DHP), reinitiated oocyte meiosis in vitro, as indicated by germinal vesicle dissolution (GVD). Addition of tGnRH-A (10(-7) M) significantly attenuated the GVD response to GtH, HP, and DHP in a dose-related fashion; tGnRH-A reduced the magnitude of progestogen-induced GVD response without significantly affecting the sensitivity of the oocytes to the meiogenic steroids. Similarly, administration of native teleost GnRH (tGnRH) significantly attenuated GtH-induced GVD response, although the 50% maximal response of the native peptide was significantly higher than that of tGnRH-A (380.9 +/- 125 and 2.72 +/- 0.94 nM, respectively). The present results provide the first demonstration of an inhibitory action of a GnRH agonist on the process of progestogen-induced oocyte meiosis and support the hypothesis that GnRH or a GnRH-like peptide might play a modulating role in the control of oocyte meiosis at the ovarian level in goldfish.