title: Umami taste in mice uses multiple receptors and transduction pathways.
abstract: The distinctive umami taste elicited by l-glutamate and some other amino acids is thought to be initiated by G-protein-coupled receptors. Proposed umami receptors include heteromers of taste receptor type 1, members 1 and 3 (T1R1+T1R3), and metabotropic glutamate receptors 1 and 4 (mGluR1 and mGluR4). Multiple lines of evidence support the involvement of T1R1+T1R3 in umami responses of mice. Although several studies suggest the involvement of receptors other than T1R1+T1R3 in umami, the identity of those receptors remains unclear. Here, we examined taste responsiveness of umami-sensitive chorda tympani nerve fibres from wild-type mice and mice genetically lacking T1R3 or its downstream transduction molecule, the ion channel TRPM5. Our results indicate that single umami-sensitive fibres in wild-type mice fall into two major groups: sucrose-best (S-type) and monopotassium glutamate (MPG)-best (M-type). Each fibre type has two subtypes; one shows synergism between MPG and inosine monophosphate (S1, M1) and the other shows no synergism (S2, M2). In both T1R3 and TRPM5 null mice, S1-type fibres were absent, whereas S2-, M1- and M2-types remained. Lingual application of mGluR antagonists selectively suppressed MPG responses of M1- and M2-type fibres. These data suggest the existence of multiple receptors and transduction pathways for umami responses in mice. Information initiated from T1R3-containing receptors may be mediated by a transduction pathway including TRPM5 and conveyed by sweet-best fibres, whereas umami information from mGluRs may be mediated by TRPM5-independent pathway(s) and conveyed by glutamate-best fibres.
authors.affiliation: Section of Oral Neuroscience, Graduate School of Dental Sciences, Kyushu University, 3-1-1 Maidashi, Higashi- ku, Fukuoka 812-8582, Japan.
authors.forename: Keiko
authors.initials: K
authors.lastname: Yasumatsu
authors.forename: Yoko
authors.initials: Y
authors.lastname: Ogiwara
authors.forename: Shingo
authors.initials: S
authors.lastname: Takai
authors.forename: Ryusuke
authors.initials: R
authors.lastname: Yoshida
authors.forename: Ken
authors.initials: K
authors.lastname: Iwatsuki
authors.forename: Kunio
authors.initials: K
authors.lastname: Torii
authors.forename: Robert F
authors.initials: RF
authors.lastname: Margolskee
authors.forename: Yuzo
authors.initials: Y
authors.lastname: Ninomiya
mesh_terms: D000596:Amino Acids / Q000502:physiology*; D000818:Animals; D002814:Chorda Tympani Nerve / Q000502:physiology*; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D005260:Female; D005998:Glycine / Q000031:analogs & derivatives / Q000494:pharmacology; D066298:In Vitro Techniques; D007189:Indans / Q000494:pharmacology; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009412:Nerve Fibers / Q000502:physiology; D043562:Receptors, G-Protein-Coupled / Q000502:physiology*; D018094:Receptors, Metabotropic Glutamate / Q000037:antagonists & inhibitors / Q000502:physiology*; D015398:Signal Transduction; D050053:TRPM Cation Channels / Q000502:physiology*; D013649:Taste; D014059:Tongue / Q000502:physiology
chemical_list: C095756:1-aminoindan-1,5-dicarboxylic acid; D000596:Amino Acids; D018691:Excitatory Amino Acid Antagonists; D007189:Indans; D043562:Receptors, G-Protein-Coupled; D018094:Receptors, Metabotropic Glutamate; D050053:TRPM Cation Channels; C498617:Trpm5 protein, mouse; C104331:cyclopropyl-4-phosphonophenylglycine; C431926:taste receptors, type 1; D005998:Glycine