Gonadal steroids and the maturation of the species-specific gonadotropin-releasing hormone system in brain and pituitary of the male African catfish (Clarias gariepinus)

Abstract

The effect of testosterone (T), 11-ketotestosterone (KT) and estradiol (E2) on the development of the catfish gonadotropin-releasing hormone system (cfGnRH) of male African catfish (Clarias gariepinus), at the onset of puberty [between 10 and 12 weeks post hatching (ph)] was investigated. The cfGnRH neurons, located in the ventral forebrain, were visualized by immunofluorescence and their numbers were determined and the amounts of cfGnRH-associated peptide (cfGAP) in the pituitary were measured by RIA. Steroid treatments did not significantly alter the numbers of immunoreactive GnRH neurons. However, T and E2 caused an increase in the amount of GnRH, demonstrated by the intensity of the immunostaining of GnRH neurons and fibers in the brain and the amount of cfGAP in the pituitary. Treatment with KT, the main circulating androgen in adult male catfish, neither changed the number of cfGnRH neurons, nor elevated the cfGnRH content in the pituitary. In previous experiments with younger, prepubertal fish (2–6 weeks ph), T caused an elevation of the number of cfGnRH neurons to the same level as present in pubertal fish of 12–14 weeks. We conclude that the onset of puberty in the male African catfish coincides with the completion of the steroid-dependent structural maturation of the cfGnRH system in the brain. T and/or E2, however, are still able to exert a positive influence on the amounts of cfGnRH during the later stages of pubertal development, thus still playing a role in the control of the cfGnRH system.