The Projections of Trigeminal Nerve Fibers to the Medullary Taste Center in Some Teleosts

Abstract

In order to reveal whether trigeminal fibers project directly onto the medullary taste center of fish, the fluorescent carbocyanine dye DiI was applied to the stump of the trigeminal nerve root in isolated, fixed brains of 6 species of fish (Halichoerus poecilopterus, Pagrus major, Stephanolepis cirrhifer, Platycephalus indicus, Cyprinus carpio and Carassius auratus). After a diffusion period, the brains were serially sectioned on a vibratome and examined on an Olympus epifluorescence microscope, In each species the entire trigeminal root was clearly labeled and the whole central projections of the trigeminal root were easily traced. The afferent fibers of the trigeminal nerve turn caudally after their entrance to the brain to form the descending root. Through the descending root trigeminal fibers were found to project onto the medullary taste center as well as other trigeminal targets in the medulla and spinal cord in all species of fish examined. The area of the medullary taste center, which receives the trigeminal projections, was found to be a ventromedial portion of the intermediate facial lobe (FL) in Cyprinus carpio and Carassius auratus, and an anterior portion of the longitudinal taste center in the other species examined. This anterior portion corresponds to the FL of the cyprinids. These results suggest that direct projection of trigeminal fibers onto the FL is a general feature in teleosts although the amount of projection varies considerably among species.