Ghrelinergic system in fish ovaries and ghrelin inhibition of germinal vesicle breakdown in zebrafish oocytes

Ghrelin, the only known orexigenic gut hormone has been proposed to integrate energy balance and reproduction in mammals. There is a large set of data available on the orexigenic and LH stimulatory roles of ghrelin in fish. Ghrelin and ghrelin receptor mRNAs are expressed in the gonads of several fishes. However, the direct roles of ghrelin on fish gonads remain unclear. Our objective was to identify the ghrelinergic system in fish ovaries, at the protein level in the cross sections of paraffin fixed ovaries, and test the direct effects of ghrelin on oocyte maturation. Both ghrelin and ghrelin receptor like immunoreactivity was detected in the follicle cells in the cross section of goldfish and zebrafish ovaries. This agrees with the mRNA expression data and further confirms the presence of the ghrelinergic system in fish ovaries. We found that native ghrelin at 10, 50 and 100 ng/mL concentrations inhibited both basal and maturation-inducing hormone stimulated stage IV germinal vesicle breakdown (GVBD) of zebrafish oocyte maturation in vitro. This result indicates that ghrelin acts directly on zebrafish follicles. When zebrafish follicles were co-incubated with ghrelin and a well-characterized ghrelin receptor antagonist, D-lys(3)-GHRP-6, the inhibitory effects of ghrelin on stage IV GVBD was abolished. This result indicates that ghrelin inhibits stage IV GVBD via its receptor(s). Collectively, our results for the first time indicate a direct role for ghrelin in the ovarian physiology of fish. (C) 2012 Elsevier Inc. All rights reserved.