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Immunohistochemical localization of three different prepro‐GnRHs in the brain and pituitary of the European sea bass (Dicentrarchus labrax) using antibodies to the corresponding GnRH‐associated peptides

The distribution of the cells expressing three prepro‐gonadotrophin‐releasing hormones (GnRH), corresponding to salmon GnRH (sGnRH), seabream GnRH (sbGnRH), and chicken GnRH‐II (cGnRH‐II) forms, was studied in the brain and pituitary of the sea bass (Dicentrarchus labrax) by using immunohistochemistry. To circumvent the cross‐reactivity problems of antibodies raised to GnRH decapeptides, we used specific antibodies generated against the different sea bass GnRH‐associated peptides (GAP): salmon GAP (sGAP), seabream GAP (sbGAP), and chicken‐II GAP (cIIGAP). The salmon GAP immunostaining was mostly detected in terminal nerve neurons but also in ventral telencephalic and preoptic perikarya. Salmon GAP‐immunoreactive (ir) fibers were observed mainly in the forebrain, although sGAP‐ir projections were also evident in the optic tectum, mesencephalic tegmentum, and ventral rhombencephalon. The pituitary only receives a few sGAP‐ir fibers. The seabream GAP‐ir cells were mainly detected in the preoptic area. Nevertheless, sbGAP‐ir neurons were also found in olfactory bulbs, ventral telencephalon, and ventrolateral hypothalamus. The sbGAP‐ir fibers were only observed in the ventral forebrain, innervating strongly the pituitary gland. Finally, chicken‐II GAP immunoreactivity was only detected in large synencephalic cells, which are the origin of a profuse innervation reaching the telencephalon, preoptic area, hypothalamus, thalamus, pretectum, posterior tuberculum, mesencephalic tectum and tegmentum, cerebellum, and rhombencephalon. However, no cIIGAP‐ir fibers were detected in the hypophysis. These results corroborate the overlapping of sGAP‐ and sbGAP‐expressing cells in the forebrain of the sea bass, and provide, for the first time, unambiguous information on the distribution of projections of the three different GnRH forms expressed in the brain of a single species. J. Comp. Neurol. 446:95–113, 2002. © 2002 Wiley‐Liss, Inc.