Molecular and physiological evidence for glutamate (umami) taste transduction via a G protein-coupled receptor

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 (NMDA) and L-AP4. We speculate upon the significance of two glutamate receptor pathways in taste buds.