Gonadotropin-releasing hormone (GnRH) innervation of the pituitary in a cichlid fish, oreochromis niloticus: A brain lesion study

In most vertebrates, multiple gonadotropin-releasing hormone (GnRH) neuronal groups have been reported. In tilapia three GnRH neuronal groups (terminal nerve, preoptic, midbrain) have been reported. Which of the three GnRH cell groups regulate the pituitary is not well known. We performed brain lesions of each neuronal group and studied immunocytochemically the changes of GnRH fiber distribution in the pituitary. Lesions of the preoptic cell group resulted in almost complete absence of GnRH fibers in the neurohypophysis of the proximal pars distalis. After lesions of the terminal nerve GnRH cell group, no changes were observed in the distribution of GnRH fibers in the pituitary. Lesions of the midbrain cell group were unsuccessful because of high mortality. The present study indicates that the preoptic GnRH cell group is the main contributor of the pituitary innervation.