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

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/100μL 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 experi-ment,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.