Preoptic gonadotropin-releasing hormone (GnRH) neurons innervate the pituitary in teleosts

In most teleosts, there are three groups of gonadotropin-releasing hormone (GnRH) neurons. In this study we addressed the question of GnRH neuronal innervation of the pituitary in the dwarf gourami and the tilapia using immunocytochemistry combined with biocytin tract tracing. Biocytin was applied to the pituitary attached to the brain in vitro. Similar results were obtained in both species. GnRH neurons retrogradely labeled with biocytin were observed only in the preoptic area. These results indicate that preoptic GnRH neurons innervate the pituitary. Negative labeling of biocytin in the terminal-nerve and midbrain GnRH neurons suggests that these two GnRH neuronal populations do not project to the pituitary. Biocytin-positive but GnRH-negative neurons were also observed in the preoptic area and the ventromedial parts of the hypothalamus, suggesting neuropeptidergic and aminergic innervation of the pituitary besides GnRH.