title: The importance of the presence of a 5'-ribonucleotide and the contribution of the T1R1 + T1R3 heterodimer and an additional low-affinity receptor in the taste detection of L-glutamate as assessed psychophysically.
abstract: The molecular receptors underlying the purported "umami" taste quality commonly associated with l-glutamate have been controversial. Evidence supports the involvement of the T1R1 + T1R3 heterodimer, a GPCR broadly tuned to l-amino acids, but variants of two mGluRs expressed in taste buds have also been implicated. Using a rigorous psychophysical taste-testing paradigm, we demonstrated impaired, if not eliminated, detection of MSG in WT and T1R1, T1R2, T1R3, and T1R2 + T1R3 KO mice when the contribution of sodium was minimized by the epithelial sodium channel blocker amiloride. When inosine 5'-monophosphate (IMP), a ribonucleotide that potentiates the l-glutamate signal through the T1R1 + T1R3 heterodimer, was added, the WT and T1R2 KO mice were able to detect the compound stimulus across all MSG (+amiloride) concentrations due, in part, to the taste of IMP. In contrast, mice lacking T1R1 or T1R3 could not detect IMP alone, yet some were able to detect MSG + amiloride + IMP, but only at the higher MSG concentrations. Interestingly, the sensitivity of T1R1 KO mice to another l-amino acid, lysine, was unimpaired, suggesting that some l-amino acids can be detected through T1R1 + T1R3-independent receptors without sensitivity loss. Given that IMP is not thought to affect mGluRs, behavioral detection of l-glutamate appears to require the contribution of the T1R1 + T1R3 receptor. However, the partial competence observed in some T1R1 and T1R3 KO mice when MSG + amiloride + IMP was tested suggests that a T1R1 or T1R3 homodimer or an unidentified protein, perhaps in conjunction with T1R1 or T1R3, can serve as a low-affinity taste receptor for l-glutamate in the presence of IMP.
authors.affiliation: Department of Psychology and Program in Neuroscience, Florida State University, Tallahassee, Florida 32306-4301.
authors.forename: Kimberly R
authors.initials: KR
authors.lastname: Smith
authors.affiliation: Department of Psychology and Program in Neuroscience, Florida State University, Tallahassee, Florida 32306-4301 spector@psy.fsu.edu.
authors.forename: Alan C
authors.initials: AC
authors.lastname: Spector
mesh_terms: D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D005260:Female; D006185:Habituation, Psychophysiologic; D007291:Inosine Monophosphate / Q000494:pharmacology*; D008239:Lysine / Q000494:pharmacology; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D055503:Protein Multimerization; D021122:Protein Subunits / Q000819:agonists / Q000235:genetics / Q000378:metabolism; D043562:Receptors, G-Protein-Coupled / Q000235:genetics / Q000378:metabolism*; D026941:Sodium Channel Blockers / Q000494:pharmacology; D012970:Sodium Glutamate / Q000494:pharmacology*; D013649:Taste; D013650:Taste Buds / Q000378:metabolism / Q000502:physiology; D055697:Taste Perception; D000098706:Taste Receptors, Type 2
chemical_list: D021122:Protein Subunits; D043562:Receptors, G-Protein-Coupled; D026941:Sodium Channel Blockers; C431926:taste receptors, type 1; D000098706:Taste Receptors, Type 2; D007291:Inosine Monophosphate; D000584:Amiloride; D008239:Lysine; D012970:Sodium Glutamate
keywords: amino acids; gustatory; psychophysics; taste receptors; taste sensitivity