EFFECTS OF GONADOTROPIN-RELEASING-HORMONE DURING THE PUBERTAL DEVELOPMENT OF THE MALE AFRICAN CATFISH (CLARIAS-GARIEPINUS) - GONADOTROPIN AND ANDROGEN LEVELS IN PLASMA

The sensitivity of the pituitary to gonadotrophin-releasing hormone (G nRH) and that of the testis to gonadotrophin (GTH) was monitored in Af rican catfish in vivo at different stages of pubertal development (20, 21, 24, 31, 39, 42 and 49 weeks of age). The fish were injected i.p. with chicken GnRH-II (cGnRH-II) or catfish GnRH (cfGnRH), their two en dogenous GnRHs. Blood samples were collected to quantify LH-like GTH-I I and three androgens 11-ketotestosterone (11-KT), testosterone and 11 beta-hydroxyandrostenedione (OHA). The testes of 20- and 21-week-old f ish contained spermatogonia alone, or spermatogonia and spermatocytes, or - in a limited number of specimens - some spermatids as well. Sper matozoa were first observed in the testes of 24-week-old fish and beca me predominant as the fish attained full maturity (49 weeks of age). I n 20- to 24-week-old fish, significantly elevated plasma GTH-II levels were only recorded after treatment with cGnRH-II. In 31- to 49-week-o ld fish, injection of both GnPHs led to increased plasma GTH-II levels , but cGnRH-II was always more effective than cfGnRH. Whereas basal GT H-II plasma levels hardly changed throughout the study, GnRH-stimulate d levels increased with the age of the fish. Plasma concentrations of 11-KT were not different from controls in 20- and 21-week-old males de spite their elevated GTH-II levels following injection of cGnRH-II. Th e first significant increase in levels of 11-KT after cGnRH-II treatme nt was observed in 24-week-old fish and, after cfGnRH treatment, in 39 -week-old fish. Basal and GnRH-stimulated 11-KT plasma levels increase d with the age of the fish. Basal and cGnRH-II-stimulated plasma level s of OHA and testosterone also increased with the age of the fish. How ever, the levels of OHA and testosterone were five- to ten-fold lower than those of 11-KT and, except for OHA in the 49-week-old fish, no in creases were recorded in the cfGnRH-injected fish. Our data show that at the beginning of spermatogenesis the pituitary gland is already sen sitive to GnRH stimuli. However, sensitivity of the testicular steroid ogenic system to GTH-II, sufficient to be reflected in consistently el evated androgen plasma levels, was not observed until 3-4 weeks later. The restricted testicular GTH-II responsiveness at the beginning of s permatogenesis may represent a limiting factor for further pubertal de velopment.