Expression dynamics of gonadotropin-releasing hormone-I and its mutual regulation with luteinizing hormone in chicken ovary and follicles.

Gonadotropin-releasing hormone-I (GnRH-I) has been identified in the ovaries of vertebrate species, and this decapeptide is a key regulator of reproductive functions. However, its biological action and regulatory mechanism in the chicken ovary remain to be characterized. In this study, the expression of GnRH-I gene in chicken hypothalamus and ovaries at different developmental stages and different sizes of follicles was investigated, and the effect of GnRH-I mRNA on chicken follicular cells was analyzed in vitro. The results showed that the expression of GnRH-I was dramatically decreased in the hen ovary compared to that in the hypothalamus after sexual maturation. In the mature ovarian follicles, GnRH-I mRNA levels were significantly higher in theca cells than that in granulosa cells. Overexpression of GnRH-I decreased the expression of luteinizing hormone receptor (LHR) mRNA in theca cells from preovulatory follicles but had no effect on granulosa cells. Treatment of theca cells with different concentrations of luteinizing hormone (LH) significantly increased GnRH-I mRNA expression at low doses (50 ng/ml) but significantly decreased it at higher doses (200 ng/ml). Furthermore, GnRH-I inhibited LH-induced LHR expression at the lower dose of LH (50 ng/ml). These findings provide strong evidence indicating that GnRH-I is an important regulator in the chicken ovary.