Relative in vitro seasonal effects of vasotocin and isotocin on ovarian steroid hormone levels in the catfish Heteropneustes fossilis

In the present investigation, catfish (Heteropneustes fossilis) ovarian tissues were incubated in vitro with vasotocin (VT) or isotocin (IT) to demonstrate their effects on estradiol-17β (E2), progesterone (P4), 17α-hydroxy-4-pregnene-3, 20-dione (17-P) and 17α, 20β-hydroxy-4-pregnen-3-one (17, 20β-DP). Parallel incubations with human chorionic gonadotropin (hCG) alone or in combination with VT were used for a comparison. In pre-vitellogenic phase (preparatory phase, GSI—0.48 ± 0.03%), both VT and hCG stimulated E2 significantly, VT in a biphasic manner and hCG in a dose-dependent manner. In pre-spawning (post-vitellogenic, GSI—9.05 ± 0.11%) and spawning (post-vitellogenic, GSI—8.01 ± 0.12%) phases, both hormones decreased E2 levels in a dose- and duration-dependent manner; the VT effect being biphasic in the spawning phase. The co-incubation with VT + hCG stimulated E2 in the preparatory phase but inhibited it in the pre-spawning and spawning phases. The incubations with VT or hCG increased P4 levels in a dose- and duration-dependent manner, the magnitude of the effect was higher in the pre-spawning and spawning phases. The co-incubation with VT + hCG stimulated P4 without any additive effect. The P4 derivatives (17-P and 17, 20β-DP) showed similar changes except 17-P in the spawning phase which decreased at 16 h of the incubation. The incubations with IT produced similar but low responses. In conclusion, like hCG, VT has differential effects on ovarian steroidogenesis and may be involved directly or indirectly in ovarian functions, as a paracrine/autocrine factor or a neurohormone.