Vasotocin - A new player in the control of oocyte maturation and ovulation in fish

In this article, the physiological role of ovarian vasotocin (VT) on fish final oocyte maturation (FOM) and ovulation is reviewed based on the studies mainly available in the catfish Heteropneustes fossilis. The VT system is characterized in the follicular layer of the oocytes by both immunocytochemical and in situ hybridization techniques. The distribution was confirmed in isolated follicular layer preparations by HPLC characterization and quantification. Three VT receptor subtype genes are identified: Vial and V1a2 subtypes are distributed in the follicular layer and V2 subtype is present along the granulosaoocyte membrane junction. The expression of peptide, VT precursor gene and VT receptor genes shows seasonal and periovulatory changes in the ovary. VT secretion is modulated by E-2 differentially in a season-specific manner, and by progestin steroids positively. VT modulates E-2 in a biphasic manner in early recrudescent phase and induces a steroidogenic shift inhibiting E-2 and stimulating progestin steroid (P-4, 17P(4) and 17,20 beta-DP) pathways in the late recrudescent phase. VT stimulates prostaglandin secretion, germinal vesicle breakdown (GVBD), oocyte hydration and ovulation. VT acts through different receptors to stimulate these processes. It uses the V1 type receptor to stimulate GVBD and ovulation, and the V2 type to stimulate oocyte hydration. VT acts as an important link in the cascade of gonadotropin control of FOM and ovulation. More research is required in other species. (C) 2015 Elsevier Inc. All rights reserved.