LH secretion and ovulation following exposure of Arctic charr to different temperature and photoperiod regimes: responsiveness of females to a gonadotropin-releasing hormone analogue and a dopamine antagonist.

The timing of ovulation and LH plasma levels were investigated in Arctic charr reared at 5 degrees C and 10 degrees C, exposed to the ambient photoperiod, or short or long-day photoperiod regimes during the prespawning period. The effectiveness of sGnRHa alone, or sGnRHa combined with a dopamine antagonist, in stimulating LH secretion and inducing ovulation was also investigated. With the natural photoperiod, ovulation occurred spontaneously at 5 degrees C, but was inhibited at 10 degrees C. A transition from 10 to 5 degrees C soon resulted in suppression of the inhibition. At 5 degrees C, the effectiveness of sGnRHa was similar to that of sGnRHa combined with pimozide in stimulating LH secretion and inducing ovulation. At 10 degrees C, sGnRHa+pimozide was more effective that sGnRHa alone in stimulating LH secretion and inducing a high rate of ovulation, suggesting that dopamine-induced inhibition of LH secretion could occur naturally in Arctic charr at 10 degrees C. Exposure of Arctic charr to a long day (LD) photoperiod regime in fall and winter did not completely inhibit ovulation, but markedly delayed it and prolonged the ensuing ovulation period. The LD photoperiod also reduced LH plasma levels in females while they were ovulating, but did not modify the responsiveness of the pituitary to GnRHa stimulation compared to a control group exposed to a short-day (SD) photoperiod. There was an interval of several weeks after a transition from LD to SD before LH secretion and ovulation were stimulated.

Functional specificity of the rainbow trout (Oncorhynchus mykiss) gonadotropin receptors as assayed in a mammalian cell line.

In vertebrates, gonadotropins (GTHs) (FSH and LH) are two circulating pituitary glycoprotein hormones that play a major role in the regulation of gonadal functions, including gonadal cell proliferation/differentiation and steroidogenesis. In mammals, it is well known that their biological effects are mediated by highly specific membrane-bound receptors expressed preferentially on the somatic cells of the gonads. However, in fish, binding and functional studies have shown that cross-reactivity may occur in GTH receptors depending on the species. To understand the molecular mechanisms involved in GTH actions, functional characterization of trout GTH receptors and their gonadal gene expression pattern has been carried out. The present study describes the presence of two distinct GTH receptors in trout showing similarities with those of higher vertebrates but also differences in their structural determinants. In vitro functional studies demonstrate that rtLH specifically activates its cognate receptor (EC(50) = 117 ng/ml), whereas purified rainbow trout FSH (rtFSH) activates FSHR but also LHR at supraphysiological doses (EC(50) = 38 vs 598 ng/ml for FSHR and LHR respectively). The high doses of rtFSH required to activate LHR put into question the physiological relevance of this interaction. The use of heterologous chinook GTHs confirms the strong preference of each hormone for its cognate receptor. The gonadal expression pattern of the GTH receptor genes suggests that FSH may play an important role in regulating gonadal functions, not only at the early stages but also at the final stages of the male and female reproductive cycles, in addition to the LH pathway.

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.