The ovarian regulation of ovulation in teleost fish

Abstract

Ovulation is the release of a mature oocyte from its follicle wall enclosure in the ovary. This process requires the separation of the oocyte from the granulosa layer, the rupture of the follicle wall and the active expulsion of the oocyte through the rupture site. Results of experiments on various vertebrates, including fish, have shown that the control of these processes may involve the cooperative action of a number of ovarian regulators including proteases, protease inhibitors, progestational steroids, eicosanoids, catecholamines and vasoactive peptides. We have used two teleost models, the brook trout (Salvelinus fontinalis) and the yellow perch (Perca flavescens) to study the mechanism and control of ovulation in fish. Using subtractive cDNA cloning, a family of ovarian and ovulation specific mRNAs (“TOPs” = trout ovulatory proteins) was isolated from the brook trout ovary. These mRNAs have not previously been observed in the ovary of any vertebrate species; however, the proteins they code for have significant sequence homology to a group of mammalian protease inhibitors called antileukoproteinases. These inhibitors have been isolated from several mammalian mucosal tissues and their function may be to protect the mucosal lining from nonspecific degradation by proteases released from infiltrating leukocytes. The ovarian proteins encoded by the TOP mRNAs have now been characterized by Western blotting using antibodies derived against recombinant TOPs. Given the similarity of TOPs to antileukoproteinases, one function of TOPs may be to regulate proteolysis at the time of ovulation. In yellow perch, the maturational steroid, 17α,20β-dihydroxy-4-pregnen-3-one (17,20-PG), stimulates both germinal vesicle breakdown and ovulation in vitro. The stimulation of ovulation can be blocked by indomethacin, a prostaglandin endoperoxide synthase inhibitor. Thus, it appears that 17,20-PG acts through the production of an eicosanoid that is most likely a primary prostaglandin. This hypothesis is further supported by the observations that (1) a direct correlation exists between indomethacin levels that block ovulation and those that block primary prostaglandin synthesis in the ovary; (2) ovulation can be restored in indomethacin-blocked incubates with primary prostaglandins; (3) PGF levels increase at the time of ovulation in incubations of yellow perch follicles stimulated with 17,20-PG; and (4) the stimulation of PGF by steroids in the ovary is specific for 17,20-PG. Finally, 17,20-PG-stimulated ovulation and follicular prostaglandin synthesis requires the close interaction of extrafollicular tissue and other follicle wall layers.