Since the discovery of GnRH in mammals (mGnRH), a family of homologous decapeptides has been characterized in lower vertebrates. One form (cGnRH-II) is present in all classes of gnathostomes, and coexists with another form exhibiting species-specific molecular variations. The highest diversity has been observed in fishes, where cGnRH-11 coexists with dfGnRH in chondrichthyes, with mGnRH in primitive osteichthyes (including some primitive teleosts, the eels), with sGnRH in most teleosts, and with cfGnRH in catfishes; in addition, a 3rd form (sbGnRH) coexists with cGnRH-11 and sGnRH in recent teleosts, the perciforms. Immunocytochemistry using highly specific antibodies or in situ hybridization demonstrates a differential distribution of GnRH peptides in teleosts as in other vertebrates: cGnRH-11 is located in neurons of the midbrain tegmentum while the other GnRH forms are located in neurons of the anterior brain (olfactory bulbs, telence-phalon, and diencephalon). The coexistence of 2 GnRH systems raises the question of their respective physiological roles, particularly in regard to hypophysiotropic function. In teleosts, the adenohypophysis is directly innervated by hypophysiotropic neurons; GnRH from the anterior system (mGnRH, sGnRH, cfGnRH or sbGnRH) is the major molecular form innervating the pituitary. Also, depending on the species of teleost, cGnRH-1i fibers contributing to the innervation of the pituitary may be absent, few or numerous, suggesting important differences between species in the hypophysiotropic role of cGnRH-11. Apart from their hypophysiotropic roles, GnRHs may also act as brain neuromediators in teleosts, as in all other vertebrates. Clues to the respective roles of the GnRH forms can also be obtained by comparing how they are regulated. The 2 GnRH forms showed opposite responses in eels subjected to experimental maturation or steroid treatments, with a large stimulation of mGnRH (as well as pituitary gonadotropin) levels, but a decrease in cGnRH-11 levels. All these results suggest a major role for the anterior brain GnRH system in the hypophysiotropic control of gonadotropin.
Evolution, Neuropeptide, Teleost
