Vasotocin induces final oocyte maturation and ovulation through the production of a maturation-inducing steroid in the catfish Heteropneustes fossilis

The study reports for the first time vasotocin (VT) induction of final oocyte maturation and ovulation through the production of the maturation-inducing steroid 17, 20 beta-dihydroxy-4-pregnen-3-one (MIS, 17, 20 beta-DP). Post-vitellogenic follicles of the catfish Heteropneustes fossilis were incubated with different concentrations of VT (1, 10, 100 and 1000 nM) for different time periods. Germinal vesicle breakdown [GVBD, as a marker of final oocyte maturation (FOM)] and ovulation were scored. In another series of experiments, the follicles were incubated with VT alone or in combination with VT receptor (V-1 and V-2) antagonists, and GVBD and ovulation were increased with progesterone, 17-hydroxy-4-pregnene-3, 20-dione (17-P) and 17, 20 beta-DP levels. VT stimulated both GVBD and ovulation in a concentration and time-dependent manner, and the responses were inhibited to varying degrees in groups incubated with the VT receptor antagonists. The V-1 antagonist inhibited the responses by 2- to 3-fold and more than the V-2 antagonist, and the combination was more potent than the separate incubation. Progestins increased time-dependently in the VT groups and the fold increase was greater for the MIS. The VT-induced steroid stimulation was significantly inhibited to near the control levels in co-incubations with both V-1 and V-2 receptor antagonists, in the order 17, 20 beta-DP > 17-P > P-4. The inhibition by the V-1 receptor antagonist was greater than that with the V-2 blocker, and followed the same order of inhibition described above. The results suggest that VT induces FOM and ovulation mainly through the VI receptors. (C) 2011 Elsevier Inc. All rights reserved.