Influence of leucine-enkephalin on pituitary-ovary axis of the cichlid fish Oreochromis mossambicus

The present investigation was conducted to elucidate the influence of an opioid peptide, leucine-enkephalin (L-ENK), on the reproductive axis of the tilapia Oreochromis mossambicus. In the first experiment, administration (i.p.) of 25, 100, and 300 mu g L-ENK to the stripped female tilapia, for a period of 22 days, resulted in a significantly higher number of stage I follicles compared to those of initial controls and experimental controls, whereas the mean number of stage II and III follicles and serum levels of E-2 did not significantly differ among different experimental groups. A significant increase in the number of stage V (fully ripened) follicles was concomitant with significant reduction in the follicular diameter in 25 or 100 mu g L-ENK-treated fish compared to those of experimental controls. However, significant reduction in the mean number and diameter of these follicles was observed in 300 mu g L-ENK-treated fish compared to those of experimental controls and 25 or 100 mu g L-ENK-treated fish. In the second experiment, the stimulatory effect of 25 mu g L-ENK on the ovary was abolished in combination with gonadotropin-releasing hormone antagonist (GnRH-A). In conclusion, these results suggest that L-ENK exerts stimulatory as well as inhibitory effects on the ovary in a dose-dependent manner, and that these effects are possibly mediated through the GnRH, for the first time in fish.