Connections of the basal forebrain of the weakly electric fish, Eigenmannia virescens

The organization of the ventral nucleus of the ventral telencephalon (Vv) was examined in the weakly electric fish, Eigenmannia virescens. This nucleus, which is considered the teleost homologue to the basal forebrain nuclei of other vertebrates, was subdivided into dorsal and ventral subdivisions, based upon cytoarchitectonic, immunohistochemical, and connectional criteria. Afferent projections were observed from the medial olfactory bulb as well as the terminal nerve ganglion. Telencephalic afferents to the Vv were very restricted, consisting of the supracommissural and the dorsal intermediate nuclei of the ventral telencephalon, the nucleus taenia, and the medial region of the posterior nucleus of the dorsal telencephalon. However, the major afferents to the Vv were diencephalic, particularly those originating from the rostral preoptic area and other hypothalamic nuclei. Additional afferents included the posterior tubercular nucleus, the locus coeruleus, the medial perilemniscal nucleus, and the periventricular nucleus of the posterior tuberculum. Relatively weak projections were observed from the ventral thalamus and the dorsal posterior thalamic nucleus. As described previously, the diencephalic complex of the central posterior thalamic nucleus/prepacemaker nucleus (CP/PPn), which also has cells that innervate the pacemaker circuitry controlling the production of an electric organ discharge, projects to the Vv. Terminal fields of the Vv were observed to be coextensive with afferent cell groups in the preoptic area, lateral and caudal hypothalamic nuclei, and thalamus. An additional efferent target of the Vv was the pretectal nucleus electrosensorius. That many cell groups that are connected with the Vv are also connected with the CP/PPn, particularly the preoptic and hypothalamic nuclei, suggests that the electrocommunicatory system is intimately linked with basal forebrain limbic pathways.