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) axis, the endocrine system that regulates reproductive processes. Two main concepts 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 gonadostat concept, which refers to the negative feedback sex steroids exert on the brain in many prepubertal mammals, is unlikely to be of relevance in fish. Instead, sex steroids are considered as (part of) a missing link needed to establish the physiological competence of the BPG axis as a functional unit. In male African catfish, a strong activation of the pituitary's gonadotropic function is observed shortly after the beginning of meiosis. This activation appears to be mediated largely by oestradiol-17 beta (E2), which is produced in the pituitary by aromatization of testosterone (T) of testicular origin. However, the quantitatively dominating steroids produced by teleost testes are Ii-oxygenated androgens, amongst which 11-ketotestosterone (KT) is the major circulating androgen in most gonochoristic species. In juvenile male African catfish, treatment with KT inhibited, though not completely, the puberty-associated increase in pituitary LH content. At the testicular 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 critical to the puberty-associated activation of the pituitary-testis axis in African catfish. (C) 1999 Elsevier Science B.V. All rights reserved.