Ž . Comparative Biochemistry and Physiology Part B 129 2001 381387 Gonadal steroids and the maturation of the species-specific gonadotropin-releasing hormone system in brain and pituitary of the male African ž / catfish Clarias gariepinus E.A. Dubois  , S. Slob, M.A. Zandbergen, J. Peute, H.J.Th. Goos Comparati  e Endocrinology, Faculty of Biology, Graduate School of De  elopmental Biology, Uni  ersity of Utrecht, Utrecht, The Netherlands Received 4 August 2000; received in revised form 3 December 2000; accepted 8 December 2000 Abstract Ž. Ž . Ž . The effect of testosterone T , 11-ketotestosterone KT and estradiol E on the development of the catfish 2 Ž . Ž . gonadotropin-releasing hormone system cfGnRH of male African catfish Clarias gariepinus , at the onset of puberty  Ž . 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 Ž . peptide cfGAP in the pituitary were measured by RIA. Steroid treatments did not significantly alter the numbers of immunoreactive GnRH neurons. However, T and E caused an increase in the amount of GnRH, demonstrated by the 2 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 Ž 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 E , however, are still able to 2 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.  2001 Elsevier Science Inc. All rights reserved. Keywords: Brain; Fish; GnRH; Immunocytochemistry; Maturation; Pituitary; Puberty; RIA; Steroids 1. Introduction To date 16 different forms of gonadotropin Ž . releasing hormone GnRH have been identified Ž Powell et al., 1996; Carolsfeld et al., 2000; Yoo et  Corresponding author. Padualaan 8 3584 CH Utrecht. Tel.: 31-30-2532947; fax: 31-30-2532837. Ž . E-mail address: E.A.Dubois@bio.uu.nl E.A. Dubois . al., 2000; Okubo et al., 2000; Montaner et al., . 2000 . In most species two forms are expressed in the brain, but it was recently shown that a third Ž GnRH exists in modern teleost species Fernald and White, 1999; Amano et al., 1997; Parhar et . Ž . al., 1998 . The two or three forms not only differ in amino acid sequence, but also in embryonic origin, localization and function. The species- Ž Ž specific GnRH also indicated as GnRH1 Fernald . and White, 1999 is localized in the ventral fore- 1096-495901$ - see front matter  2001 Elsevier Science Inc. All rights reserved. Ž . PII: S 1 0 9 6 - 4 9 5 9 01 00328-1