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Abstract Vasoactive-intestinal peptide (Vip) is a pleiotropic peptide with a wide range of distribution and functions. Zebrafish possess 2 isoforms of Vip (a and b), in which Vipa is most homologous to the mammalian form. In female zebrafish, Vipa can stimulate LH secretion from the pituitary but is not essential for female reproduction, as vipa−/− females display normal reproduction. In contrast, we have found that vipa−/− males are severely subfertile and sex ratio of offspring is female-biased. By analyzing all aspects of male reproduction with wild-type (WT) males, we show that the testes of vipa−/− are underdeveloped and contain ∼70% less spermatids compared to WT counterparts. The sperm of vipa−/− males displayed reduced potency in terms of fertilization (by ∼80%) and motility span and duration (by ∼50%). In addition, vipa−/− male attraction to WT females was largely nonexistent, indicating decreased sexual motivation. We show that vipa mRNA and protein is present in Leydig cells and in developing germ cells in the testis of WT, raising the possibility that endogenous Vipa contributes to testicular function. Absence of Vipa in vipa−/− males resulted in downregulation of 3 key genes in the androgen synthesis chain in the testis, 3β-hsd, 17β-hsd1, and cyp11c1 (11β-hydrogenase), associated with a pronounced decrease in 11-ketotestosterone production and, in turn, compromised reproductive fitness. Altogether, this study establishes a crucial role for Vipa in the regulation of male reproduction in zebrafish, like in mammals, with the exception that Vipa is also expressed in zebrafish testis.