The opioid peptide dynorphin suppresses pituitary-ovary axis in the tilapia Oreochromis mossambicus.

The opioid peptides are implicated in regulation of neuroendocrine functions in vertebrates. However, influence of an opioid peptide, dynorphin A (DYN) on reproduction is understudied in fish. The aim of the present investigation was to study the influence of DYN on the pituitary-ovary axis in the fish Oreochromis mossambicus. Daily injections (i.p.) of 250 μg DYN kg-1 body weight for 22 days during the ovarian cycle caused reduction in the intensity and the percent area of luteinizing hormone (LH) immunoreactive content in the proximal pars distalis region of the pituitary gland compared to an intense immunostaining in time-matched controls. In the ovary, DYN treatment caused a decrease in the number of stage I (previtellogenic) follicles compared with time-matched controls. No difference was noticed in the number of stage IV (vitellogenic) follicles among different experimental groups, whereas the number of stage II and III follicles (previtellogenic) was higher in DYN-treated fish than in time-matched controls. However, there was a reduction in the number of stage V (preovulatory) follicles in DYN treated fish compared with time-matched controls. Taken together, these results indicate that DYN exerts inhibitory effect on follicular recruitment at late vitellogenic stage, through the suppression of LH secretion in fish. This article is protected by copyright. All rights reserved.