Gonadotropin-releasing hormone fibers innervate the pituitary of the male African catfish (Clarias gariepinus) during puberty

The development of the catfish gonadotropin-releasing hormone (cfGnRH) fibe r network in the pituitary of male African catfish (Clarias gariepinus) was investigated in relation to puberty. Double immunolabeling studied by conf ocal laser scanning microscopy revealed a concomitant development of gonado tropes and of pituitary cfGnRH innervation during the first wave of spermat ogenesis. Catfish GnRH-immunoreactive fibers in the proximal pars distalis (PPD) of the pituitary were initially observed at the age of 10 weeks (onse t of spermatogonial proliferation) and gradually reached the adult pattern at the age of 20 weeks (spermatozoa present in the testis). The content of cfGnRH-associated peptide (cfGAP, part of the prohormone) in the pituitary similarly increased during puberty. At the electron microscopical level, fi bers containing cfGAP-ir granules came into close proximity of the gonadotr opes at 18 weeks of age. In vitro studies indicated a progressively increas ing basal and cfGnRH-stimulated luteinizing hormone (LH) secretion during p ubertal development. The LH secretion patterns were similar in response to exogenous cfGnRH (0.1 muM) or to endogenous cfGnRH, the release of which wa s induced by forskolin (1 muM). Castration experiments demonstrated that th e innervation of the pituitary with cfGnRH fibers continued after surgery, accompanied by an increase in the cfGAP levels. However, gonadotrope develo pment was retarded, suggesting a differential regulation of the two maturat ional processes. Since testosterone stimulates both processes, other testic ular factors may also be involved. Puberty-associated changes in LH release patterns appear to reflect changes in the GnRH sensitivity and in the pool of releasable LH, while availability of cfGnRH does not appear to be a lim iting factor. Copyright (C) 2000 S. Karger AG, Basel.