Changes in plasma and pituitary GtH-II levels in vitro and in vivo in female blue gourami during the end of vitellogenesis and final oocyte maturation

Abstract

Gonadotropin II (GtH-II), synthesized in the pituitary of fish under the stimulatory action of the gonadotropin-releasing hormone (GnRH), is the key hormone in the regulation of the final stages of reproduction. Changes in GtH-II levels were monitored in the plasma and pituitary of female blue gourami (Trichogaster trichopterus, Anabantidae) during the late stages of vitellogenesis and final oocyte maturation (FOM). In addition, the effect of the two native forms of GnRH present in the brain of anabantid fish, salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II), and a third synthetic form, mammalian GnRH analogue (GnRHa), were studied in vivo and in vitro in female blue gourami. In females with a high percentage of oocytes in vitellogenesis, injection with GnRHa, cGnRH-II, and sGnRH stimulated oocyte maturation in vivo. The maximum effect was observed in treatment with cGnRH-II, followed by GnRHa and sGnRH. In vitro, all three forms of GnRH stimulated GtH-II secretion from pituitary cell culture in a dose-dependent manner. Responses to treatment with GnRHa were faster (1 h vs. 4–8 h) but lower than with the native forms. Maximum stimulation of GtH-II release was effected by cGnRH-II. In maturing females, the transition from late vitellogenic stages to FOM was associated with a rise in plasma GtH-II levels, whereas GtH-II content in the pituitary did not change during this period. The variations in GtH-II levels in females and the differing responses to different GnRH forms suggest an important role for GtH-II in the control of FOM in blue gourami and possible differences in the role of various GnRHs in this control FOM and in GtH-II synthesis and release. J. Exp. Zool. 279:377–385, 1997. © 1997 Wiley-Liss, Inc.