Effects of hCG and ovarian steroid hormones on vasotocin levels in the female catfish Heteropneustes fossilis

Effects of hCG, ovariectomy and estradiol replacement on brain, plasma and/or ovarian vasotocin in vivo, and estradiol, progesterone, 17α, 20β-hydroxy-4-pregnen-3-one and hCG on ovarian vasotocin in vitro were investigated in the catfish. A 100 IU/fish of hCG induced ovulation and elicited both periovulatory and post-ovulatory changes in vasotocin concentrations with a significant increase up to 8 h in the brain and up to 16 h in both plasma and ovary. After stripping the fish at 16 h, the peptide concentration decreased significantly with time, up to 4 days. Ovariectomy in early pre-spawning phase resulted in a duration-dependent significant reduction of both brain and plasma vasotocin. Estradiol replacement in 3-week ovariectomized fish produced dosage-dependent biphasic effects: the lower dosage (0.1 μg/g) restored the vasotocin level while the higher dosage (0.5 μg/g) decreased it significantly below the control level. In vitro incubation of ovarian tissues with estradiol produced season-dependent effects on vasotocin. The incubation of pre-vitellogenic ovarian pieces with estradiol (1, 10, and 100 ng/ml) elevated vasotocin level in a dose- and duration-dependent manner while that of post-vitellogenic follicles resulted in a significant decrease. The incubation of intact post-vitellogenic follicles or follicular envelope with progesterone and 17α, 20β-hydroxy-4-pregnen-3-one (1 μg/ml) or hCG (20 IU/ml) for 8 and 16 h significantly increased vasotocin in a duration-dependent manner. The results show that both gonadotropin and ovarian steroids modulate vasotocin titer, which may influence follicular growth, ovulation and spawning in the catfish.