Three gonadotropin-releasing hormone neuronal groups with special reference to teleosts

Gonadotropin-releasing hormone (GnRH) is a decapeptide, which has been isolated from the hypothalamus as a releasing hormone of gonadotropins from the pituitary. However, subsequent morphological studies have demonstrated the presence of multiple GnRH neuronal groups outside the hypothalamus and preoptic area. In most vertebrate lineages studied to date, GnRH neuronal groups are present along the terminal nerve and in the midbrain tegmentum, in addition to a population in the preoptico-hypothalamic areas. The presence of GnRH fibers in extrahypothalamic areas has also been demonstrated, indicating a significance for GnRH neurons in functions other than those that are purely hypophysiotropic. Among vertebrate lineages, GnRH neurons have been most extensively studied in teleost fish through morphological, electrophysiological, behavioral and molecular approaches. To date, studies on differential roles of GnRH neuronal groups have been mostly restricted to teleosts. In the present review, the anatomy and functions of each GnRH neuronal group are reconsidered, based mainly on knowledge from teleosts. Recent findings in teleosts indicate that the preoptico-hypothalamic GnRH neurons are hypophysiotropic and that GnRH neurons of the terminal nerve and midbrain tegmentum regulate neural activities in various regions, including extrahypothalamic areas. The latter populations presumably serve as neuromodulatory systems to control aspects of neural functions such as reproductive behavior. Similar functional differentiation may be generalized to other vertebrate lineages as well.