Effect of melatonin on GnIH precursor gene expression in Nile tilapia,Oreochromis niloticus

Sexual maturation and gonadal development of fish is greatly influenced by photic information, an external environmental factor, and melatonin mediates this information to regulate gonadotropin (GTH) secretion and gonadal activation. The relationship between gonadotropin inhibiting hormone (GnIH) and melatonin in fish, however, has not been studied to date. Here, the GnIH expression pattern and daily change of melatonin levels were compared to each other in mature tilapia (body length 16.1 ± 0.2 cm, body weight 77.7 ± 3.43 g), and the effect of melatonin injection on GnIH gene expression was investigated. GnIH gene expression increased at night when the secretion of melatonin increased, whereas gene expression decreased during the day when melatonin secretion decreased. Injecting tilapia intraperitoneally with melatonin increased GnIH gene expression and decreased the expression of gonadotropin releasing hormone (GnRH) and GTH. Furthermore, the injection decreased the 11-KT concentration in male tilapia. These results indicate that melatonin is likely to suppress the hypothalamus-pituitary-gonad (HPG) axis via the action of GnIH in this species.