MOLECULAR PHYSIOLOGY OF GLUTAMATE IN TASTE

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L-Glutamate, in the form of its monosodium salt (MSG), is a distinctive and potent taste stimulus found naturally in many foods. The goal of the first funding period for this Program Project was to determine if glutamate receptors (GluRs) resembling those at synapses in the brain are found in taste receptor cells and might transduce MSG taste. This goal was approached using molecular biological (Project #1, cell biological metabotropic and one or more ionotropic GluRs are expressed in rat taste bud cells. These receptors are similar to CNS GluRs but have important sequence and functional differences that may imply a role in taste transduction. Lastly, we obtained critical molecular and functional evidence for the role of a novel taste-specific metabotropic glutamate receptor (taste-mGluR4) in transducing the taste of MSG. In the next period, we propose more stringent tests to discriminate whether taste-mGluR4 is necessary and sufficient for MSG taste transduction, or whether additional GluRs might also be involved. This question will be addressed through molecular and functional analysis of taste bud cells and heterologous cells transfected with GluRs cloned from taste buds. We will obtain detailed pharmacological profiles of cloned receptors (Project #1) and compare these data with the responses to pharmacological agents used as taste stimulate in isolated tissues (Project #3) and intact tongues (Project #2). Functional studies will include biochemical measurements of second messengers (Project #1), patch- clamp and calcium imaging of taste receptor cells (Project #2), electrophysiological recordings of gustatory afferent fiber activity (Project #2), and animal behavioral tests (Project #2). This converted multi-disciplinary effort from molecular and cellular levels to animals behavior will serve to elucidate the transduction of MSG s a taste stimulus. The long term of this Program Project is to provide a comprehensive analysis of taste transduction for an important gustatory stimulus.