Vagal afferent projections from the pharyngeal jaw of the cichlid Nile tilapia (<i>Oreochromis niloticus</i>)

We identified vagal innervation in the pharyngeal tooth and jawbones of Nile tilapia through macroscopic observations and immunohistochemistry. We also revealed the apposition of the nerve and osteoclasts in the pharyngeal jaw, suggesting the possibility of neuronal regulation for bone remodeling. However, the central projection from the vagal nerve, which innervates the pharyngeal jaws, remains unknown. To determine the projection of the vagus nerve in the brain, we applied carbocyanine dye (DiI) into the vagus nerve, revealing DiI-labeled neurons in the caudal vagal ganglion. The labeled fibers of the neurons were then traced to the vagal lobe, revealing that they branched and ran dorsally before terminating in a band-like pattern. Meanwhile, the labeled fibers running ventral to the vagal lobe were directed toward the dorsal motor nucleus of the vagus and did not have a definite terminal structure. The vagus nerve innervates the pharyngeal jaw, mainly projects to the vagal lobe, where it receives gustatory information. Pharyngeal tooth-derived sensory information might occur during occlusion and be processed precisely for determining the regurgitation and swallowing of prey.