Expression of kisspeptins and their Receptors, gnrh-1/gnrhr-II-1a and gonadotropin genes in the brain of adult male and female European sea bass during different gonadal stages

Abstract

Kisspeptins play a critical role in the control of hypothalamic-gonadotropic function and puberty onset in mammals. Studies in fish have all supported the hypothesis that they might play similar roles in the reproduction of this animal group, however, their physiological relevance in the occurrence of key reproductive events still remains to be determined. This study examines the relative mRNA expression profiles of the duplicate kisspeptin system (kiss1, kiss2, gpr54-1b, and gpr54-2b) in the hypothalamus and pituitary of adult male and female sea bass (Dicentrarchus labrax L.) during different gonadal stages using qRT-PCR. We also report the changes in the expression levels of gnrh-1, gnrhr-II-1a, fshβ, and lhβ and the relationships observed between both kisspeptin and GnRH systems. Our data show clear sex differences in the dynamics of kisspeptin and kisspeptin receptor gene expression in the hypothalamus of sea bass during gonadal development. Overall, all four kisspeptin system genes increased either before or during the advanced stages of oogenesis and declined during atresia, exhibiting profiles that are identical to those observed for gnrhr-II-1a, fshβ, lhβ, and the gonadosomatic index (GSI). While the situation was not as clear in males, the high kiss2 expression levels observed in the hypothalamus during mid recrudescence suggest that it might be playing a role in the neuroendocrine signaling that regulates germ cell proliferation at the testicular level. In this sense, the proposed role attributed to kisspeptins as key factors in the onset of reproduction in fish receives an additional support from the data obtained in the present work. Nevertheless, further research is required to clarify their precise role in sea bass.