Immunohistochemical localization of three GnRH systems in brain and pituitary of Japanese flounder

ABSTRACT:   To clarify the possible roles of gonadotropin-releasing hormone (GnRH) in the reproduction of Japanese flounder Paralichthys olivaceus , localization of salmon GnRH (sGnRH), chicken GnRH-II (cGnRH-II), and sea bream GnRH (sbGnRH) immunoreactive (ir) cell bodies and fibers in the brain and pituitary were examined together with follicle stimulating hormone (FSH) and luteinizing hormone (LH)-ir cells in the pituitary by immunohistochemistry. sGnRH-ir cell bodies were localized in the ventromedial part of the rostral olfactory bulb and cGnRH-II-ir cell bodies were restricted to the midbrain tegmentum, while sbGnRH-ir cell bodies were evident in the preoptic area. sGnRH-ir fibers were distributed throughout the brain, especially abundant in the forebrain. cGnRH-II-ir fibers were also scattered in many areas of the brain with abundance in the midbrain, but sbGnRH-ir fibers were observed in the preoptic–hypothalamic area and innervated the pituitary. In the pituitary, neither sGnRH-ir fibers nor cGnRH-II-ir fibers were found, but sbGnRH-ir fibers were profuse in the neurohypophysis and invaded the proximal pars distalis, targeting FSH and LH cells. These results suggest that three GnRH systems can play different physiological roles in the brain of Japanese flounder. Among them, sbGnRH is considered to be involved in reproduction by stimulating gonadotropin secretion, while sGnRH and cGnRH-II can function as a neurotransmitter and/or neuromodulator within the brain in this species.