GONADOTROPIN-RELEASING HORMONES (GNRH) IN FISHES - EVOLUTIONARY DATA ON THEIR STRUCTURE, LOCALIZATION, REGULATION, AND FUNCTION

Since the discovery of GnRH in mammals (mGnRH), a family of homologous decapeptides has been characterized in lower vertebrates. One form (c GnRH-II) is present in all classes of gnathostomes, and coexists with another form exhibiting species-specific molecular variations. The hig hest diversity has been observed in fishes, where cGnRH-II coexists wi th dfGnRH in chondrichthyes, with mGnRH in primitive osteichthyes (inc luding some primitive teleosts, the eels), with sGnRH in most teleosts , and with cfGnRH in catfishes; in addition, a 3rd form (sbGnRH) coexi sts with cGnRH-II and sGnRH in recent teleosts, the perciforms. Immuno cytochemistry using highly specific antibodies or in situ hybridizatio n demonstrates a differential distribution of GnRH peptides in teleost s as in other vertebrates: cGnRH-II is located in neurons of the midbr ain tegmentum while the other GnRH forms are located in neurons of the anterior brain (olfactory bulbs, telencephalon, and diencephalon). Th e coexistence of 2 GnRH systems raises the question of their respectiv e physiological roles, particularly in regard to hypophysiotropic func tion. In teleosts, the adenohypophysis is directly innervated by hypop hysiotropic neurons; GnRH from the anterior system (mGnRH, sGnRH, cfGn RH or sbGnRH) is the major molecular form innervating the pituitary. A lso, depending on the species of teleost, cGnRH-II fibers contributing to the innervation of the pituitary may be absent, few or numerous, s uggesting important differences between species in the hypophysiotropi c role of cGnRH-II. Apart from their hypophysiotropic roles, GnRHs may also act as brain neuromediators in teleosts, as in all other vertebr ates. Clues to the respective roles of the GnRH forms can also be obta ined by comparing how they are regulated. The 2 GnRH forms showed oppo site responses in eels subjected to experimental maturation or steroid treatments, with a large stimulation of mGnRH (as well as pituitary g onadotropin) levels, but a decrease in cGnRH-II levels. All these resu lts suggest a major role for the anterior brain GnRH system in the hyp ophysiotropic control of gonadotropin.