Urotensin-I suppresses the hypothalamo-hypophyseal-ovarian axis in the Mozambique tilapia, Oreochromis mossambicus

Urotensin-I (UI) is a peptide belonging to the corticotropin-releasing hormone family, produced in the urophysis of teleosts. This investigation was undertaken to examine the effect of UI on the hypothalamo-hypophyseal-ovarian axis in the Mozambique tilapia, a cichlid fish. In the first experiment, administration of UI (2 μg/100 μL saline) for 22 days caused a significant reduction in the number of follicles at stages I, II, III (previtellogenic), IV (vitellogenic), and V (preovulatory), concomitant with a significant increase in the percent atresia of follicles at stages II, III, and IV than in controls. In the pituitary, gonadotropin-releasing hormone (GnRH) - positive fibres were detected in the proximal pars distalis (PPD) in both controls and UI-treated fish; however, there was a significant reduction in the intensity and percent area of GnRH-immunoreactive fibres in fish treated with UI compared with controls. The serum luteinizing hormone (LH) level was significantly lower, whereas a significant rise in the serum level of cortisol was noticed in UI-treated fish compared with controls. In the second experiment, treatment with a low dose of UI (10 mM) did not affect the number of ACTH-secreting cells in vitro, but the number of these cells was significantly higher in high-dose UI (50 mM)-treated pituitaries compared to controls. Together, the present investigation reveals for the first time that the treatment of UI affects the follicular development process through the downregulation of the GnRH-LH axis and activation of the pituitary-interrenal pathway in teleosts.