The role of gonadotropin-inhibitory hormone (GnIH) and its receptor on chicken reproduction

Gonadotropin-inhibitory hormone (GnIH), a hypothalamic peptide, has been shown to inhibit pituitary LH release in both avian and mammalian species. The objectives of the present study were to clone and characterize expression of GnIH and its receptor (GnIHR) in the chicken. GnIH precursor protein cDNA was cloned and sequenced and found that GnIH cDNA encodes for GnIH and two GnIH related peptides (GnIH-RP1 and GnIH-RP2). By RT-PCR, GnIH mRNA was detected in the chicken diencephalon but not in the pituitary gland or ovary. Using anti-chicken GnIH antibody, GnIH immunoreactive (ir) neuronal soma and axonal fibers were localized throughout the chicken hypothalamus including median eminence. Real-time quantitative PCR (qRT-PCR) analyses revealed that hypothalamic GnIH mRNA quantity was not different in laying chickens compared with nonlaying hens. Treatment of pituitaries with synthetic GnIH peptide reduced the release of LH in sexually immature birds in vitro but not in sexually mature chickens.;GnIHR precursor protein cDNA was cloned and detected mRNA expression in the chicken diencephalon, pituitary gland, testis and ovary and in particular theca and granulosa cell layers. The anti-chicken GnIHR antibody was generated against synthetic GnIHR peptide and identified GnIHR-immunoreactive cells in cephalic and caudal lobes of the pituitary gland. qRT-PCR studies revealed greater GnIHR mRNA abundance in diencephalon compared to that of pituitary and ovary. GnIHR mRNA quantity was decreased (p<0.05) in pituitary glands and ovaries of sexually mature chickens compared with sexually immature chickens. qRT-PCR analysis revealed higher GnIHR mRNA quantity in granulosa cells compared to theca cells in both preovulatory and prehierarchical follicles. Estradiol and/or progesterone treatment of sexually immature chickens decreased (p<0.05) pituitary gland and ovarian GnIHR mRNA abundance. Treatment of prehierarchical follicular granulosa cells in vitro with chicken GnIH peptide decreased (p<0.05) basal but not FSH-stimulated cellular viability. GnIH treatment had no effect on progesterone release from granulosa cells dispersed from F1 follicles in vitro. Collectively, pituitary and ovarian GnIHR gene expression is regulated by sexual maturation as well as gonadal steroids. In addition, presence of GnIHR ir cells in the pituitary gland and ovary suggest direct effects of GnIH on gonadotropins and follicular development in the chicken.