Development of three distinct GnRH neuron populations expressing two different GnRH forms in the brain of the African catfish (Clarias gariepinus)

The early development of both the catfish gonadotropin-releasing hormone (c fGnRH)and the chicken GnRH-II (cGnRH-II) system was investigated in African catfish by immunocytochemistry by using antibodies against the GnRH-associ ated peptide (GAP) of the respective preprohormones. Weakly cfGnRH-immunore active Qr) neurons and fibers were present at 2 weeks after hatching (ph) b ut only in the ventral telencephalon and pituitary. Two weeks later, cfGnRH fibers and neurons were also observed in more rostral and in more caudal b rain areas, mainly in the preoptic area and hypothalamus. Based on differen ces in temporal, spatial, and morphologic appearance, two distinct cfGnRH p opulations were identified in the ventral forebrain: a population innervati ng the pituitary (ventral forebrain system) and a so-called terminal nerve (TN) population. DiI tracing studies revealed that the TN population has no neuronal connections with the pituitary. The cGnRH-II system is present fr om 2 weeks ph onward in the midbrain tegmentum and only their size and stai ning intensity increased during development. Based on the comparison of GnR H systems amongst vertebrates, we hypothesize that during fish evolution, t hree different GnRH systems evolved, each expressing their own molecular fo rm: the cGnRH-II system in the midbrain, a hypophysiotropic GnRH system in the hypothalamus with a species-specific GnRH form, and a salmon GnRH-expre ssing TN population. This hypothesis is supported by phylogenetic analysis of known GnRH precursor amino acid sequences. We hypothesize, because the A frican catfish is a less advanced teleost species, that it contains the cfG nRH form both in the ventral forebrain system and in the TN population. J. Comp. Neurol. 437: 308-320,2001. (C) 2001 Wiley-Liss, Inc.