Molecular and Physiological Evidence for Glutamate (Umami) Taste Transduction via a G Protein‐Coupled Receptor a

ABSTRACT: Recent molecular analyses have demonstrated that a metabotropic glutamate receptor, mGluR4, is expressed in taste buds from rat circumvallate and foliate papillae. Behavioral studies demonstrated that l(+)‐2‐amino‐4‐phosphonobutyric acid (L‐AP4), an agonist for mGluR4 and related receptors, mimics the taste of monosodium glutamate (MSG) in rats. mGluR4 is known to signal through inhibition of the cyclic adenosine‐5′,3′‐monophosphate (cAMP) cascade. Circumvallate and foliate taste buds exhibit decreases of cAMP levels following stimulation with MSG, and the response is potentiated by 5′‐inosine monophosphate, suggesting that it is related to umami taste. Further, experiments on mice with the mGluR4 gene knocked out support the interpretation that mGluR4 is a key component in glutamate taste. Glutamate may also stimulate taste buds through an ionotropic receptor pathway. In patch‐clamp studies, glutamate evokes two types of currents, similar to those elicited by N‐methyl‐d‐aspartate (N MDA) and L‐AP4. We speculate upon the significance of two glutamate receptor pathways in taste buds.

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