Levels of the native forms of GnRH in the pituitary of the gilthead seabream, Sparus aurata, at several characteristic stages of the gonadal cycle

Brains of the gilthead seabream, Sparus aurata, contain three different for ms of gonadotropin-releasing hormone (GnRH): seabream (sb) GnRH, chicken (c ) GnRH-II, and salmon (s) GnRH. In the present study, we developed three sp ecific enzyme-linked-immunosorbent assays (ELISA) for sbGnRH, cGnRH-II, and sGnRH and used them to measure the levels of each GnRH form in the pituita ry of male and female seabream at different stages of gametogenesis. The se nsitivity was 6 pg/well for the sbGnRH assay 7 pg/well for the cGnRH-II ass ay and 2 pg/well for the sGnRH assay. Levels of each of the three GnRH form s were measured in pituitaries from fish sampled at the beginning of gonada l recrudescence and during the spawning season. Of the three forms, only sb GnRH and cGnRH-II were detected in the pituitary, irrespective of reproduct ive state or sex. Recrudescent fish had similar levels of sbGnRH and cGnRH- II in the pituitary. In sexually mature fish, the levels of sbGnRH were hig her than those in recrudescent fish while pituitary cGnRH-II content remain ed unchanged. Consequently, sbGnRH levels were 3- to 17-fold higher than cG nRH-II levels in mature fish. Positive correlations also existed between pi tuitary sbGnRH content and pituitary and plasma gonadotropin (GtH) II level s. Surprisingly, mature 1-year-old males had significantly higher levels of sbGnRH in the pituitary than mature 3-year-old males, while pituitary and plasma GtH II levels were similar between these two groups. Although the re ason for this difference in sbGnRH levels is unclear, a possible role of sb GnRH in the processes of puberty or sex-inversion is implied. Based on the present results, it can be suggested that in the gilthead seabream, sbGnRH is the most relevant form of GnRH in the control of reproduction. (C) 1998 Academic Press.