Relationship between brain gonadotrophin-releasing hormone (GnRH) and seasonal reproductive cycle of "caribe colorado," Pygocentrus notatus.

Immunoreactive gonadotrophic hormone-releasing hormone (ir-GnRH) was detected in hypothalamic and telencephalic extracts of the Venezuelan freshwater fish "caribe colorado," Pygocentrus notatus. Hypothalamic ir-GnRH from female fish demonstrated displacement curves parallel to those of synthetic mammalian luteinizing hormone-releasing hormone (LHRH). The content of hypothalamic and telencephalic ir-GnRH from female fish was more than four-fold greater than that of male animals. Also, fluctuations that depended on the reproductive state and environmental conditions (rainfall) occurred in females but not in males. Thus, ir-GnRH levels were higher in hypothalamic and telencephalic extracts from sexually mature females than in those from fish sampled outside the climatically determined breeding season.