Distribution of trigeminal fibers in the primary facial gustatory center of channel catfish, Ictalurus punctatus

Previous studies in several fishes including catfish, have shown that prima ry trigeminal nerve (NV) axons terminate not only in the principal and spin al trigeminal nuclei, but in the facial (gustatory) lobes. The present stud y was undertaken to determine the extent and distribution of trigeminal ter minations within the facial lobe (FL) and principal trigeminal nucleus (nVp r) in the channel catfish, Ictalurus punctatus. In order to reveal the dist ribution of trigeminal fibers, the carbocyanine dye, diI, was applied to th e central cut stump of the trigeminal root in isolated, paraformaldehyde-fi xed brains. After a diffusion period of 10-90 days, the brains were seriall y sectioned on a vibratome and examined with epifluorescence. The trigemina l motor nucleus (nVm) and principal sensory nucleus lie near the level of e ntrance of NV. The majority of primary trigeminal fibers, however, sweep ca udally after entering into the brain to form the descending root. At the le vel of the caudal third of the FL, collaterals emitted by the descending ro ot fibers turn medially and dorsally to terminate in the FL. The trigeminal fibers are coarser than the facial nerve (NVII) fibers which terminate wit hin the same structure. The trigeminal fibers terminate throughout the FL e xcept for the lateral-most lobule which contains the representation of tast e buds innervated by the recurrent branch of NVII, i.e., those over the tru nk and tail of the animal. These results show that in catfish, the trigemin al input to the primary gustatory complex is restricted to those portions o f the nucleus receiving chemosensory inputs from the face and barbels, i.e. , the trigeminally innervated sensory fields. (C) 1999 Elsevier Science B.V . All rights reserved.