Identification, expression, and physiological functions of Siberian hamster gonadotropin inhibitory hormone.
Gonadotropin inhibitory hormone (GnIH) is a hypothalamic neuropeptide that inhibits gonadotropin secretion in birds and mammals. To further understand its physiological roles in mammalian reproduction, we identified its precursor cDNA and endogenous mature peptides in the Siberian hamster brain. The Siberian hamster GnIH precursor cDNA encoded two RFamide related peptide (RFRP) sequences. SPAPANKVPHSAANLPLRF NH(2) (Siberian hamster RFRP 1) and TLSRVPSLPQRF NH(2) (Siberian hamster RFRP 3) were confirmed as mature endogenous peptides by mass spectrometry from brain samples purified by immunoaffinity chromatography. GnIH mRNA expression was higher in long days (LD) compared with short days (SD) GnIH mRNA was also highly expressed in SD plus pinealectomized animals, whereas expression was suppressed by melatonin, a nocturnal pineal hormone, administration. GnIH immunoreactive ir) neurons were localized to the dorsomedial region of the hypothalamus, and GnIH ir fibers projected to hypothalamic and limbic structures. The density of GnIH ir perikarya and fibers were higher in LD and SD plus pinealectomized hamsters than in LD plus melatonin or SD animals. The percentage of GnRH neurons receiving close appositions from GnIH ir fiber terminals was also higher in LD than SD, and GnIH receptor was expressed in GnRH ir neurons. Finally, central administration of hamster RFRP 1 or RFRP 3 inhibited LH release 5 and 30 min after administration in LD. In sharp contrast, both peptides stimulated LH release 30 min after administration in SD. These results suggest that GnIH peptides fine tune LH levels via its receptor expressed in GnRH ir neurons in an opposing fashion across the seasons in Siberian hamsters.
Endocrinology