Ascending general visceral sensory pathways from the brainstem to the forebrain in a cichlid fish, Oreochromis (Tilapia) niloticus.

Fiber connections of the general visceral sensory centers in the brainstem were studied with tract-tracing methods in a percomorph teleost, tilapia Oreochromis niloticus. General visceral afferents of the vagal nerve from abdominal viscera terminated bilaterally in the commissural nucleus of Cajal (NCC) and area postrema (AP). The NCC and AP projected bilaterally to the secondary general visceral nucleus (SVN), four diencephalic nuclei (the preglomerular general visceral nucleus [pVN], nucleus of the lateral recess, posterior thalamic nucleus, and lateral tuberal area), preoptic area, and ventral telencephalon (supracommissural, dorsal, and ventral parts) in addition to the glossopharyngeal and vagal lobes and medullary reticular formation. Injections to the SVN resulted in labeled terminals in the forebrain structures that receive fibers from the primary centers and additionally in the diffuse nucleus of the inferior lobe, lateral torus, and inferior subdivision of lateral torus. The present study suggests that the ascending general visceral projections arising from the brainstem centers in teleosts are quite similar to those in mammals and birds. Descending pathways were also notable. In addition to descending projections from the SVN and medullary structures to the primary centers, long descending pathways to the SVN, NCC, and AP were found to originate from the pVN, nucleus of the lateral recess, posterior thalamic nucleus, and preoptic area. The SVN was found to receive fibers from the ventral telencephalon as well. Therefore, the present study indicates that most of the general visceral structures in the forebrain are reciprocally connected with the brainstem centers.