Regulation of the gut-brain-pituitary-gonad axis in tilapia: Evidence for the involvement of orexin.

The neuropeptide orexin is known as the regulator of appetite and is implicated in many physiological functions in vertebrates. Nevertheless, the physiological importance of this peptide within the gut-brain-reproductive axis remains poorly understood in teleosts. This work aimed at assessing the response of orexin to starvation and its impact on food consumption and the reproductive axis in tilapia. In the first experiment, the fish subjected to 21 days of starvation showed a greater increase in the intensity of orexin-A immunoreactivity in the hypothalamus and pituitary gland compared to controls. The administration of either 0.1 or 1 mg orexin antagonist lemborexant (LBX) for 21 days led to a dose-dependent significant reduction in food intake, whereas a significant increase in the numbers of spermatogenic cells was observed in a high-dose LBX-treated group compared to the control fish. Moreover, a high-dose of LBX significantly enhanced the intensity of androgen receptor protein immunolabelling in the testis, percent area of gonadotropin-releasing hormone (GnRH) immunolabelling in the proximal pars distalis of the pituitary gland (PPD), and the serum levels of luteinizing hormone (LH) and 11-ketotestosterone (11-KT). In addition, the in vitro testicular concentration of 11-KT was significantly increased following a high-dose LBX treatment. Overall, these findings suggest that blockade of orexin receptors by LBX promotes the spermatogenesis process. This effect might be mediated via the stimulation of 11-KT and androgen receptors at the testicular level and the GnRH-LH pathway at the hypothalamic-pituitary level in tilapia.