Synthetic glucocorticoid (dexamethasone; DEX) treatment during the neonatal stage is known to affect reproductive activity. However, it is still unknown whether neonatal stress activates gonadotropin-inhibitory hormone (GnIH) synthesizing cells in the dorsomedial hypothalamus (DMH), which could have pronounced suppressive action on gonadotropin-releasing hormone (GnRH) neurons, leading to delayed pubertal onset. This study was designed to determine the effect of neonatal DEX (1.0mg/kg) exposure on reproductive maturation. Therefore, GnRH, GnIH and GnIH receptors, G-protein coupled receptors (GPR) 147 and GPR74 mRNA levels were measured using quantitative real-time PCR in female mice at postnatal (P) days 21, 30 and in estrus stage mice, aged between P45-50. DEX-treated females of P45-50 had delayed vaginal opening, and irregular estrus cycles and lower GnRH expression in the preoptic area (POA) when compared with age-matched controls. The expression levels of GPR147 and GPR74 mRNA in the POA increased significantly in DEX-treated female mice of P21 and P45-50 compared to controls. In addition, GPR147 and GPR74 mRNA expression was observed in laser captured single GnRH neurons in the POA. Although there was no difference in GnIH mRNA expression in the DMH, immunostained GnIH cell numbers in the DMH increased in DEX-treated females of P45-50 compared to controls. Taken together, the results show that the delayed pubertal onset could be due to the inhibition of GnRH gene expression after neonatal DEX treatment, which may be accounted for in part by the inhibitory signals from the up-regulated GnIH-GnIH receptor pathway to the POA.
Stress impairs the hypothalamic-pituitary-gonadal (HPG) axis, probably through its influence on the hypothalamic-pituitary-adrenal (= interrenals in the teleost, HPI) axis leading to reproductive failures. In this study, we investigated the response of hypothalamic neuropeptides, gonadotropin-inhibitory hormone (GnIH), a component of the HPG axis, and corticotropin-releasing hormone (CRH) a component of the HPI axis, to acute social defeat stress in the socially hierarchical male Nile tilapia (Oreochromis niloticus). Localization of GnIH cell bodies, GnIH neuronal processes, and numbers of GnIH cells in the brain during acute social defeat stress was studied using immunohistochemistry. Furthermore, mRNA levels of GnIH and CRH in the brain together with GnIH receptor, gpr147, and adrenocorticotropic hormone (ACTH) in the pituitary were quantified in control and socially defeated fish. Our results show, the number of GnIH-immunoreactive cell bodies and GnIH mRNA levels in the brain and the levels of gpr147 mRNA in the pituitary significantly increased in socially defeated fish. However, CRH and ACTH mRNA levels did not change during social defeat stress. Further, we found glucocorticoid type 2b receptor mRNA in laser captured immunostained GnIH cells. These results show that acute social defeat stress activates GnIH biosynthesis through glucocorticoid receptors type 2b signalling but does not change the CRH and ACTH mRNA expression in the tilapia, which could lead to temporary reproductive dysfunction.