NERVE-TARGET CELL INTERACTION IN TASTE BUD MAINTENANCE

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The peripheral gustatory system exhibits remarkable receptoneural plasticity. Taste bud receptor cells are continuously renewed and interact dynamically with nerves. Moreover, taste nerves respond to injury by regenerating to the tongue epithelium with great effectiveness. While structural features of receptoneural plasticity in taste are known, its neurochemical substrates are poorly understood. Recently, a few molecular factors have been associated with gustatory neurotrophism. The hamster is a critical model in this approach because fungiform buds atrophy, but the buds do not disappear after denervation, allowing the residual bud cells to be assessed for their differences relative to innervated cells as well as during reinnervation. In contrast, hamster foliate buds do disappear after denervation, allowing comparison with persistent fungiform buds. Hamster fungiform buds express NCAM, NSE and CGRP in centrally located gemmal cells. In denervated buds, these cells are dramatically reduced. The identifying features of immunopositive cells in innervated and denervated buds are not known, and the process by which bud cell become reinnervated, not understood. Electron microscopy (EM) combined with immunocytochemistry (IC) in normal buds will identify NCAM-, NSE- and CGRP-immunopositive cells with respect to their type, e.g., dark or light cells, and whether immunoreactive cells synapse with nerve fibers. In chronically-denervated buds, and buds with regenerating nerves, EM similarly will identify immunopositive gemmal cell types and any receptoneural contacts. A new tetramethylrhodamine dextran amine (TRDA) iontophoretic method for labeling single taste buds and their innervation will evaluate the timing and quantity of reinnervation relative to bud IC. Recent unexpected observations of sparse CGRP-immunopositive nerve fibers near some sensory-denervated buds raise.questions about the source of these fibers and their possible trophic influence upon residual bud cells. The cell bodies of oigin of these fibers will be determined by TRDA labeling in chronic sensory denervated animals. To better characterize the molecular nature of fungiform taste buds during degeneration and reinnervation, markers that have proven useful in other sensory systems for evaluating cell-cell recognition, differentiating receptor cells and growing axons, will be used in addition to anti-NCAM, -NSE and -CGRP. Use of these markers will be extended to include study of hamster foliate buds after glossopharyngeal nerve section. This will permit intraspecies comparison of anterior and posterior lingual taste bud fields which have responses to denervation that are dramatically different. The possibility that this difference relates to the different embryological origin of the two taste bud fields, i.e. as ectoderm vs. endoderm derivatives, will enhance the data interpretation.