Influence of exogenous melatonin on growth, gonadal maturity and kiss/gnrh gene expression patterns in the brain of male sea bass (Dicentrarchus labrax)

Abstract

The influence of long-term melatonin administration, as an implant, on growth performance and reproduction of adult male sea bass was studied in vivo. Two years old male sea bass were organized into two groups: in the first, which acted as a control group, the animals were administered empty implants, whereas in the second, the animals were administered melatonin implants. Our results provided that this hormone had an anorexic action, thus affecting the performance of fish. Melatonin reduced the fish weight and condition factor, with the mean weight of melatonin-treated fish amounting to only 77.7% as compared to controls. Melatonin also affected gonadogenesis following testicular maturation in adult sea bass when administered over an extended period of time throughout their entire reproductive cycle. A decrease in the gonadosomatic index after 150 days of treatment (29% lower than in controls) and a lower percentage of running males during the spermatogenesis and full spermiation stages (January-March) was observed in this species. Exogenous melatonin also resulted in lower plasma levels of testosterone and 11-ketotestosterone during the reproductive period, and showed a significant decrease of circulating Lh and Fsh levels after 30 and 60 days of treatment, respectively. Furthermore, the hypothalamic expression of kiss1 was significantly higher in melatonin-treated fish than in controls after 30 days of treatment, while a significant increase in kiss2 expression was detected on day 90 of treatment. By contrast, melatonin showed a significant decrease in kisspeptin expression in the dorsal brain on day 150 of treatment and also affected the expression of gnrh-1 and gnrh-3 as well as gnrhr-II-1a and 2b and the fshß gene in the pituitary. Taken together, these findings reveal that melatonin evokes seasonal changes in key reproductive hormones that affect testicular maturity. In addition, melatonin might presumably induce the downregulation of kisspeptin-gnrh members on the dorsal brain thus affecting fsh-beta ¿transcription during early gametogenesis that appear to mirror disturbances in spermatogenesis.