Development of gonadotropin-releasing hormone systems in the male African catfish, Clarias gariepinus

Abstract

Reproductive processes are mainly regulated by the brain-pituitary-gonad axis (BPG-axis). Gonadotropin-releasing hormone (GnRH) neurons localized in the brain release their hormone GnRH, which allows the release of gonadotropic hormone by gonadotropic cells in the pituitary. Gonadotropic hormone, in turn, regulates the production of sex steroids and germ cells in the gonads. The steroids complete the dynamic BPG-axis by exerting feedback effects at the level of the brain, pituitary and gonads. The transition period of a juvenile, inactive axis towards a mature, functional system is known as puberty. The prevailing "missing-link" concept assumes that one or more components of the axis are absent or not functional before puberty. For our animal model, the African catfish, it has been proposed that sex steroids initiate and/or accelerate pubertal development. However, based on earlier findings, it was suggested that the "missing link" could also be localized at a higher level of the BPG-axis, i.e. at the GnRH system in the brain. In order to test the hypothesis that sex steroids and/or a functional GnRH system are important determinants in the onset of puberty in the African catfish, we studied the normal development of the GnRH system in the brain and the effects of certain steroids on this development. The present thesis shows that specific sex steroids are important for the development of GnRH neurons. Thus, in the African catfish, steroids are a serious candidate for the "missing-link", since it is required for activating the GnRH neurons in the preparation for the onset of puberty. Another candidate for the "missing-link" is the actual released amount of GnRH that is available for the gonadotropic cells at the onset of puberty. We hypothesize that functional contact between the first GnRH terminals in the pituitary and the gonadotropic cells is required for the initiation of puberty. Once this switch is turned on, the three levels of the BPG-axis simultaneously display their maturational processes: innervation of the pituitary by GnRH fibers, development of gonadotropic cells and the first wave of spermatogenesis.