Evidences for the regulation of GnRH and GTH expression by GnIH in the goldfish, Carassius auratus

► Three receptors for gonadotrophin-inhibitory hormone were cloned from goldfish brain. ► Three GnIHRs were localized where sGnRH was observed in the hypothalamus of goldfish. ► Two GnIHRs were located to the neurohypophysis instead of the adenohypophysis. ► Injections of GnIH peptides decreased the expression of sGnRH and GtH in vivo. ► GnIH peptides inhibited the GnRH stimulation on GtH synthesis of pituitary cells. Gonadotrophin-inhibitory hormone (GnIH) plays an important role in regulating of reproduction in teleosts. To clarify the mode of action of GnIH on the synthesis of gonadotropin releasing hormone (GnRH) and gonadotrophin (GtH), three GnIHR cDNAs were cloned from the goldfish brain. In situ hybridization results showed that GnIHRs were localized to the hypothalamus and pituitary. In the hypothalamus, GnIHRs were found in the NPP, NPO and NLT, whereas sGnRH neurons were reported to be located, and potentially regulated by GnIH. In the pituitary, only two GnIHRs were observed and they were localized to the PI instead of the adenohypophysis where GtH-expressing cells are localized, suggesting indirect regulation of GtH by GnIH. In vivo, intraperitoneal (i.p.) injections of synthetic goldfish GnIH-II peptide and GnIH-III peptide significantly decreased sGnRH and FSHβ mRNA levels. Only GnIH-II decreased LHβ mRNA levels significantly. In vitro, both GnIH-II and GnIH-III showed no effect on GtH synthesis, but an inhibition of GnRH-stimulated LHβ and FSHβ synthesis was observed when GnIH-III was applied to primary pituitary cells in culture. Thus, GnIH could contribute to the regulation of gonadotropin in the brain and pituitary in teleosts.