Puberty in male fish: concepts and recent developments with special reference to the African catfish (Clarias gariepinus)

Pubertal maturation of the male reproductive system is associated with the development to functional competence of the brain-pituitary-gonad (BPG) axi s, the endocrine system that regulates reproductive processes. Two main con cepts can be discerned as regards the endocrine regulation of puberty, the 'gonadostat' and the 'missing link' concept. In juvenile fish, sex steroids stimulate the development of all levels of the BPG axis. Hence, the gonado stat concept, which refers to the negative feedback sex steroids exert on t he brain in many prepubertal mammals, is unlikely to be of relevance in fis h. Instead, sex steroids are considered as (part of) a missing link needed to establish the physiological competence of the BPG axis as a functional u nit. In male African catfish, a strong activation of the pituitary's gonado tropic function is observed shortly after the beginning of meiosis. This ac tivation appears to be mediated largely by oestradiol-17 beta (E2), which i s produced in the pituitary by aromatization of testosterone (T) of testicu lar origin. However, the quantitatively dominating steroids produced by tel eost testes are Ii-oxygenated androgens, amongst which 11-ketotestosterone (KT) is the major circulating androgen in most gonochoristic species. In ju venile male African catfish, treatment with KT inhibited, though not comple tely, the puberty-associated increase in pituitary LH content. At the testi cular level, on the other hand, KT but not T, stimulated spermatogenesis, T even inhibiting KT-induced spermatogenesis. It is concluded that a tightly balanced production of Ii-oxygenated and of aromatizable androgens is crit ical to the puberty-associated activation of the pituitary-testis axis in A frican catfish. (C) 1999 Elsevier Science B.V. All rights reserved.