L-Glutamate and 5′-ribonucleotides are known to elicit a unique taste, "umami," that is distinct from the tastes of sweet, salt, sour and bitter. Recent progress in molecular biology has identified several umami receptor candidates, such as the heterodimer T1R1/T1R3, and brain-expressed and taste-expressed type 1 and 4 metabotropic glutamate receptors (brain- and taste-mGluR1 and mGluR4). This paper summarizes recent findings on the receptor system for umami taste. Most of the findings support the idea that multiple receptors exist for umami taste, at least in mice. The accumulating evidence indicates that the potential role of the signal mediated by the transduction pathway involving T1R1/T1R3 may be different from that mediated by the pathway involving mGluRs. The former signal occurs mainly in the anterior tongue, and plays a major role in preference behavior, whereas the latter occurs mainly in the posterior tongue, is active in mice lacking T1R3, Gα-gustducin, IP3R3 or TRPM5, and contributes to behavioral discrimination between umami and other taste compounds. In humans, unlike in mice, T1R1/T1R3 acts as an umami-specific receptor that can discriminate between umami and other tastes, and thus account for umami-linked preferences or discrimination.
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