Transmission of Taste Signals in Taste Buds

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Taste receptor cells transduce chemical signals into a neural message transmitted to the gustatory nerve fibers that innervate taste buds. This proposal describes experiments designed to test the hypothesis that in response to stimulation, taste cells release ATP which then activates ionotropic purinergic receptors (P2X2 and P2X3) on the gustatory nerve fibers. Previous studies have demonstrated the presence of these two receptors on intragemmal gustatory fibers. Our preliminary data show that gustatory nerves in P2X2/P2X3 double-knockout mice do not respond to any tastants applied to the oral cavity although the nerves give robust responses to touch or thermal stimuli. Similarly, the P2X2/3 KO mice do not respond behaviorally to many tastants including sweeteners, and many bitter substances although they do respond aversively to citric acid and other bitter substances. This spectrum of responsiveness matches that of the superior laryngeal nerve which innervates non-taste bud chemoreceptors on the larynx. The proposed experiments fall into 3 aims: first to test whether P2X receptors are necessary for transmission of taste information. For these studies we will characterize the system behaviorally and electrophysiologically to test the extent of function of the lingual gustatory system. We will also test whether laryngeal chemoreception accounts for remaining behavioral capabilities of these KO mice. Second, we will utilize anatomical and physiological measures to test whether taste buds in the knockout mice are normal both in terms of structure and function. We will assess structure and function of taste buds in P2X2/3 KO mice to test whether the receptor cells are normal and that no overt changes in the receptor epithelium may underlie the loss of taste function in the P2X2/3 KO mice. In the final aim, we will test whether taste buds in both wildtype and knockout mice release ATP upon stimulation. These functional studies will be carried out in vitro both by using the luciferin- luciferase assay and by ATP-sensing biological probes (so-called "sniffer-cells"). In addition, we will utilize quinacrine loading to assess vesicular uptake and release of ATP by taste cells. Taken together, the proposed studies will test the role of ATP and ionotropic purinergic receptors in transmission of taste information.