Organization of galanin-like immunoreactive neuronal systems in weakly electric fish (Apteronotus leptorhynchus)

Immunohistochemical procedures were used to investigate the distribution of galanin-like immunoreactive neuronal somata, fiber pathways and apparent termination fields in the gymnotiform brain. Immunoreactive somata were observed only in the hypothalamus and were confined to preoptic, lateral and caudal hypothalamic regions. Within these areas, positive cells tended to be most concentrated in juxtaventricular nuclei. Dense immunoreactive fiber systems originating from hypothalamic regions were seen to project in separate or coalescing fiber bundles to the basal telencephalan, thalamus/tuberal diencephalon, midbrain and brainstem. The density of positive axons and boutons was quite variable, but regions which displayed the most massive network of axons included structures within the hypothalamus itself (anterior periventricular preoptic nucleus, caudal and lateral hypothalamus), ventral telencephalon (superior and ventral subdivisions), thalamic/tuberal areas (central posterior nucleus and tuberal neuropil within the ventral territory of the prepacemaker nucleus) and brainstem nuclei (dorsal reticular nucleus and the medial paralemniscal nucleus). Within these areas axons appeared more randomly distributed and varicose than along fiber tracs, and in counterstained sections were occasionally seen in apposition to unstained neuronal cell bodies and dendrites. In addition, a system of fibers was seen in the neurointermediate lobe of the pituitary. It is concluded that galanin-like immunoreactive neurons in the gymnotiform brain have a more restricted distribution than those in mammals, and that the major fiber systems emanating from the hypothalamus resemble the diverse projections of the tuberomammillary nucleus of higher vertebrates. The anatomy of galanin-like immunoreactive systems in the apteronotid brain suggests a role in neuroendocrine regulation and an involvement with anatomical areas controlling aggressive and courtship behaviour.