Kisspeptin2 stimulates the HPG axis in immature Nile tilapia (Oreochromis niloticus)

It has been suggested that kisspeptin influences reproduction and onset of puberty in fishes. Unlike mammals, which produce only one kisspeptin (Kiss1), some teleosts have two, Kiss1 and Kiss2, both thought to be involved in the stimulation of gonadotropin (GTH) secretion. In Nile tilapia (Oreochromis niloticus), however, only Kiss2 has been identified so far. The effect of Kiss2 on GTH release varies significantly depending on species and reproductive stage. Furthermore, its physiological function in this species is not clearly defined. In this study, kiss2 gene expression profiles were examined using quantitative real-time PCR (qRT-PCR) in the brain, pituitary, and gonads of Nile tilapia at different reproductive stages (male: immature, pre-spermiation, post-spermiation; female: immature, pre-spawning, post-spawning). The kiss2 mRNA expression profiles of the brain, pituitary, and gonads of both sexes shared a similar pattern their expression was significantly higher at the immature stage than at the mature or post-spawning stages, implying it is involved in early gonadal maturation in this species. To investigate the effect of kisspeptin on the hypothalamus-pituitary-gonad (HPG) axis in vivo, synthetic kisspeptin2 (FNYNPLSLRF) was injected into immature male and female tilapia intraperitoneally (i.p.) at a dose of 200pmol/g body weight. The results showed that synthetic Kiss2 administration increased the expression of GnRH I, fshβ and lhβ mRNA in the brain and increased 17β-estradiol (E₂) and 11-ketotestosterone (11-KT) levels in the plasma. These results suggest that Kiss2 stimulates the expression of GnRH and GTH genes in immature Nile tilapia.