ORGANIZATION OF GNRH AND FMRFAMIDE SYSTEMS IN 2 PRIMITIVE BONY FISHES(ORDER POLYPTERIFORMES)

To investigate the evolution of the neural organization of gonadotropi n-releasing hormone (GnRH), we have examined GnRH-immunoreactivity in two brachiopterygian fishes (Polypterus palmas and Calamoichthys calab aricus). Distal regions of the terminal nerve (TN) within the medial o lfactory nerve contained clusters of GnRH-immunoreactive (ir) perikary a (<10 mu m). More proximal, isolated GnRH-ir neurons were present amo ng TN fascicles as they penetrated the ventral forebrain, and a few ir neurons were observed accompanying GnRH-ir fibers in the rostromedial telencephalon. GnRH-ir neurons were not observed in the preoptic area or ventral hypothalamus. In contrast, a small group of GnRH-ir neuron s was localized in the periventricular nucleus of the posterior tuberc ulum. GnRH-ir fibers were present in widespread areas of the brain, in cluding the olfactory bulb, telencephalon, optic nerve, hypothalamus, thalamus, habenula, optic tectum, tegmentum, pituitary and spinal cord . To further characterize projections of TN neurons, we utilized antis erum to FMRF-amide, a small peptide produced by TN cells in other vert ebrates. Perikarya that were FMRF-amide-ir within the TN were similar in distribution to GnRH-TN neurons, and the distribution of FMRF-amide -ir fibers overlapped those of GnRH-ir fibers, thus providing a useful marker for identifying TN projections. An additional population of FM RF-amide-ir neurons was present in the periventricular hypothalamus, O ur results suggest that in the polypteriformes, GnRH and FMRF-amide ne urons of the TN are similar to those observed in other vertebrates; ho wever, the paucity of GnRH cells in the basal forebrain may be unique to primitive actinopterygians and elasmobranchs, and may result from t he lack of migration of GnRH neurons into the forebrain, a phenomenon that likely occurs in all other vertebrate classes. Finally, the ident ification of GnRH-ir neurons in the posterior tuberculum is consistent with similar, and perhaps homologous, GnRH neurons present in nearly all other vertebrate classes.