Transduction mechanisms of monosodium glutamate in taste cells of rat fungiform papillae

Monosodium glutamate (MSG) is a potent umami taste stimulus and a widely used flavor enhancer. Previous studies revealed roles of both Na+ and glutamate in umami taste, but transduction mechanisms for MSG are largely unknown. This dissertation focuses on the mechanisms used by taste cells for transducting MSG and initiating sensory signals for umami taste.; Amiloride-sensitive Na+ channels mediate Na+ salt transduction in rodents. Their molecular identity, localization and regulation by aldosterone are not clearly defined. Using subunit-specific antibodies against the epithelial Na+ channel, rENaC, we observed immunoreactivity for α, β, and γ subunits in taste cells. The intensity of labeling for β and γ was significantly lower in vallate than in fungiform papillae. Increasing blood aldosterone levels enhanced the apical immunolabeling for β and γ subunits, concomitant with an increase in number of amiloride-sensitive cells and in the amplitude of amiloride-sensitive current in whole-cell recordings. These results demonstrate that rENaC is the basis for Na+ taste transduction.; Studies of glutamate transduction focused on the identity of receptors involved and their intracellular signaling pathways. In whole-cell recordings with a holding potential of −80 mV, glutamate elicited a decrease in holding current (Type I), an increase in holding current (Type II), and a biphasic response involving a Type II response followed by a Type I response. Type II responses were mimicked by the NMDA receptor agonist NMDA. These responses were potentiated by glycine and suppressed by AP5. NMDA depolarized taste cells and increased the frequency of spontaneous action potentials. Type I responses were suppressed by GDP-β-S, suggesting the involvement of G protein-coupled receptors. These responses were mimicked by the mGluR4 agonist L-AP4 and suppressed by 8-bromo-cAMP and the mGluR4 antagonist CPPG. Some cells were hyperpolarized by L-AP4. These data suggest that both ionotropic and metabotropic glutamate receptors contribute to the transduction of MSG in fungiform taste cells.; Responses to glutamate occurred preferentially in taste cells that express functional rENaCs. Although separate pathways mediate Na+ and glutamate transduction, the co-occurrence of glutamate receptors and rENaCs provides the potential for integration of Na+ and glutamate pathways in single taste cells.