Expression of prepro-GnRH and GnRH receptor messengers in rainbow trout ovary depends on the stage of ovarian follicular development.

Gonadotropin-Releasing Hormones (GnRHs) are decapeptides well known to regulate the reproductive cycle. They are expressed not only in the brain, but also in other tissues including the gonads. It is believed that they may be involved in the endocrine and paracrine regulation of the reproductive cycle. To date, two forms of GnRH have been identified in salmonids: salmon (sGnRH) and chicken II (cGnRH-II). In the present study, the temporal expression of sGnRH-1, sGnRH-2, cGnRH-II, and rtGnRH receptor genes was studied in rainbow trout ovary during the reproductive cycle according to the stages of follicular development. Using RT-PCR coupled with Southern-blot hybridization, sGnRH-1, sGnRH-2, cGnRH-II, and rtGnRH-R transcripts were detected in morphologically nondifferentiated ovaries as early as 55-65 days post-fertilization and throughout all stages of vitellogenesis. Using Northern blot analysis, cGnRH-II mRNA was detected only in immature previtellogenic ovary, whereas sGnRH mRNA was detected also during early and mid-exogenous vitellogenesis. No sGnRH mRNA was detected at the end of vitellogenesis. In maturing pre-ovulated ovary, sGnRH transiently reappeared before germinal vesicle breakdown (GVBD) and decreased thereafter. A few days after ovulation, a strong sGnRH mRNA expression was found in ovarian tissue as the eggs were kept in the body cavity of females. However, in females stripped just after ovulation, sGnRH mRNA levels remained low in ovary during several weeks. Fully spliced sGnRH-1 and sGnRH-2 messengers were mostly expressed during the reproductive cycle; however different sGnRH-1 and sGnRH-2 splicing variants containing intronic sequences were also detected. Some of these messengers may encode prepro-GnRH precursors with truncated GnRH-associated peptides. The stage-dependent expression and different cell localization of sGnRH, cGnRH-II, and rtGnRH-R transcripts suggest that GnRH-like peptides may have different roles in the paracrine regulation of ovarian follicular development. Copyright 2002 Wiley-Liss, Inc.