Taste provides a model system for the study of regeneration because of the continual turnover of receptor cells and their trophic dependence on innervation by gustatory nerves. Studies on this project have shown that several cell-surface molecules are differentially expressed on taste cells and that this expression depends upon innervation by gustatory nerve fibers. The first Specific Aim is to determine the relationship between the expression of these molecules and taste cell type and whether this expression is determined by the innervating nerve or the gustatory epithelium. EM immunocytochemistry will address the relation between NCAM, L1, several blood group antigens, and other carbohydrates to taste cell type. The roles of the epithelium and the innervating nerve In determining the taste cell phenotype will be assessed by characterization of vallate and fungiform taste buds following cross-reinnervation of the posterior tongue by the chorda tympani (CT) nerve or the anterior tongue by the IXth nerve. The second Specific Aim is to determine the plasticity of central gustatory connections in the nucleus of the solitary tract (NST) following denervation and reinnervation of the taste buds. The first experiments will assess CNS responses to denervation. Preliminary studies show intense glial reactions in the NST following gustatory nerve injury as well as a significant constitutive expression of GAP-43 and astroglial and microglial markers in the NST. This suggests continual reorganization in the CNS accompanying normal taste cell turnover. The extent and time course of terminal degeneration and reactive gliosis following peripheral nerve injury will be studied by the cupric-silver degeneration method and antibodies against microglia (OX42) and astroglia (GFAP). Regeneration will be assessed using antibodies against regenerating axons (GAP43 and CDA-1). Golgi methods will be used to characterize the effects of taste nerve damage on NST cells. A second series of studies will examine the terminal field reorganization of regenerated IXth nerve axons following (a) IXth nerve crush, (b) IXth nerve crush combined with geniculate ganglion destruction, and (c) cross- anastomosis of proximal IXth to distal CT. Regenerated fibers will be labeled with HRP and their terminal fields mapped. Functional studies will localize responsive NST cells following the above three manipulations by mapping c-fos protein expression in response to electrical stimulation of the IXth nerve and by single-cell neurophysiological recording from the NST in response to gustatory, tactile, and thermal stimulation of the tongue.
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