Approximately two million adult Americans suffer from disorders of taste and smell. In these individuals, especially the elderly and those undergoing radiation therapy for salivary gland tumors, nutritional status is compromised. More importantly, these individuals lose the protective ability to perceive spoiled and potentially toxic foods. Our limited understanding of taste perception has hindered our ability to address these issues. In particular, taste lags behind other senses with respect to the identification of synaptic mechanisms involved in the peripheral sensory organ. Glutamate has been proposed as a putative neurotransmitter in taste buds but the evidence is far from complete and a detailed functional examination is needed. The experiments outlined in this application introduce a new method to study synaptic transmission in rat taste buds. This method is the electrophoretic injection of dextran-coupled Calcium Green into intact taste buds in situ, followed by the visualization of changes in [Ca2+]i in single taste cells in lingual slices. The applicant will use this new approach to record activation of neurotransmitter receptors in taste cells in situ and test the hypothesis that glutamate is a transmitter in taste buds. Ca2+ imaging will be used and the focus will be on 1) determining whether taste buds express functional synaptic glutamate receptors; 2) identifying which glutamate receptors are present in taste cells; and 3) testing whether sensory innervation provides efferent glutamatergic input to taste cells. This series of experiments will provide a functional identification and characterization of synaptic glutamate receptors in taste cells. Moreover, this application will address the important question of whether glutamate receptors mediate efferent transmission at synapses between taste cells and sensory axons. Evidence for glutamatergic synapses and for efferent modulation in taste buds will modify current models of how sensory information is encoded in the peripheral gustatory system.
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