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The sensitivity of the pituitary to gonadotrophin-releasing hormone (GnRH) and that of the testis to gonadotrophin (GTH) was monitored in African 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 endogenous GnRHs. Blood samples were collected to quantify LH-like GTH-II and three androgens 11-ketotestosterone (11-KT), testosterone and 11β-hydroxyandrostenedione (OHA). The testes of 20- and 21-week-old fish contained spermatogonia alone, or spermatogonia and spermatocytes, or – in a limited number of specimens – some spermatids as well. Spermatozoa were first observed in the testes of 24-week-old fish and became predominant as the fish attained full maturity (49 weeks of age). In 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-old fish, injection of both GnRHs led to increased plasma GTH-II levels, but cGnRH-II was always more effective than cfGnRH. Whereas basal GTH-II plasma levels hardly changed throughout the study, GnRH-stimulated 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 despite their elevated GTH-II levels following injection of cGnRH-II. The first significant increase in levels of 11-KT after cGnRH-II treatment was observed in 24-week-old fish and, after cfGnRH treatment, in 39-week-old fish. Basal and GnRH-stimulated 11-KT plasma levels increased with the age of the fish. Basal and cGnRH-II-stimulated plasma levels of OHA and testosterone also increased with the age of the fish. However, the levels of OHA and testosterone were five- to tenfold lower than those of 11-KT and, except for OHA in the 49-week-old fish, no increases were recorded in the cfGnRH-injected fish. Our data show that at the beginning of spermatogenesis the pituitary gland is already sensitive to GnRH stimuli. However, sensitivity of the testicular steroidogenic system to GTH-II, sufficient to be reflected in consistently elevated androgen plasma levels, was not observed until 3–4 weeks later. The restricted testicular GTH-II responsiveness at the beginning of spermatogenesis may represent a limiting factor for further pubertal development.

Journal of Endocrinology (1994) 140, 265–273

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