LOCALIZATION AND CHARACTERIZATION OF GONADOTROPIN-RELEASING HORMONES IN THE BRAIN, GONADS, AND PLASMA OF A DIPNOI (LUNGFISH, PROTOPTERUS ANNECTENS)

Two molecular forms of GnRH (chicken GnRH II and a second variant) are present in the brains of species from all the major vertebrate groups . Their differential distribution in the brain and temporal expression during development suggests they have different functional roles. We investigated the nature of GnRH molecular forms in the brain, plasma, testis, and ovary of adult and juvenile lungfish (Protopterus annecten s), using high performance liquid chromatography and radioimmunoassay with specific GnRH antisera. In the brain of adult and juvenile lungfi sh, two peptides with identical chromatographic and immunologic proper ties to mammalian GnRH and chicken GnRH II were detected. Chicken GnRH II predominated in both the adult and juvenile brain, and the percent age of chicken GnRH II relative to mammalian GnRH was greater in the j uvenile brain. In the plasma, only mammalian GnRH was present. Immunor eactive GnRH was not detected in the testis and ovary. Chicken GnRH II and mammalian GnRH were found in the cells of the preoptic nucleus an d in the ganglion of the nervus terminalis. Fibers were seen in the ve ntral hypothalamus, and chicken GnRH II immunoreactivity was detected within the neural lobe of the pituitary. The finding of chicken GnRH I I in a sarcopterygian fish adds further support to our hypothesis that this ubiquitous structural variant is highly conserved and likely to have an important functional role. Mammalian GnRH, previously describe d in several early-evolved actinopterygian fish, also has a fairly wid espread distribution and early evolutionary origin. The immunocytochem ical distribution of mammalian GnRH and chicken GnRH II fibers in the lungfish brain suggests that both forms are hypophysiotropic. In addit ion, the presence of mammalian GnRH in the plasma of the lungfish sugg ests that this molecular form of GnRH has a hypophysiotropic function reaching target organs (pituitary and gonads) via the general circulat ion.