MOLECULAR PHYSIOLOGY OF GLUTAMATE IN TASTE

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Unlike vision and olfaction, taste has not been examined extensively with the modern tools of molecular biology. This Program Project brings a synthesis of molecular biology, cell biology and physiology into the field of taste by analyzing glutamate receptors in taste buds. Glutamate is an important taste stimulus (e.g. monosodium glutamate, MSG). Responses to glutamate have been recorded in sensory fibers and from the brain. Psychophysical and hedonic aspects of glutamate have been researched at great length. Yet, the initial events in glutamate taste, namely the interaction of glutamate with membrane-bound receptors, remains relatively unstudied. Consequently, critical information about glutamate receptors in taste buds--their molecular structure, their localization in taste cells, their function and modulation--is missing. The experiments outlined in this Program Project will provide these important data and will identify the first specific receptor for a taste stimulus. In the long term, this will pave the way for a comprehensive definition of the entire peripheral sequence of events in taste reception--from ligand binding to signal generation in the sensory nerves. The findings will also increase our understanding and awareness of the controversial food additive, MSG. The unifying premise underlying our Program Project is that receptors that transduce the taste of glutamate in taste cells are similar to glutamate receptors in the brain. Brain glutamate receptors form 2 large extended families of ionotropic and metabotropic receptors. We propose to take advantage of recent information about the molecular biology of these receptors to investigate glutamate receptors in taste buds. We will conduct PCR (polymerase chain reaction) with degenerate primers based on brain receptors to search for novel glutamate receptors in taste buds. Selected PCR products from lingual tissue will serve as probes to isolate full-length cDNAs encoding taste-specific glutamate receptors (Chaudhari). We will use in situ hybridization to localize mRNAs to specific taste cells and immunocytochemistry to localize receptor proteins (Roper). We will conduct functional studies of glutamate receptors endogenous to taste cells using microelectrode recording techniques. We will also express cloned glutamate receptors in oocytes and in heterologous mammalian cells and use patch clamp techniques to measure currents elicited by glutamate under different experimental conditions (Kinnamon). Lastly, we will use conditioned taste aversion to investigate the significance for taste of pharmacologically distinct receptors (Roper). By integrating these three approaches in a small, focused Program Project, we will take full advantage of the expertise and enthusiasm of 3 independent, productive, and highly interactive principal investigators. The collective insights gained on the molecular physiology of glutamate in taste will be much greater than could be achieved from the individual efforts of these researchers.