Abstract
Recent molecular studies proposed that various receptors, such as taste mGluR4, heterodimers of T1R1/T1R3, taste mGluR1, and brain-type mGluR4, might underlie umami taste. To date, however, the roles in umami taste of each of these receptors and their downstream signaling molecules have not been made clear. Apparently contradictory data was obtained from two T1R3 knock-out (KO) mouse models: Zhao et al. (2003) showed that umami detection and perception was abolished in their T1R3 KO, while we found that responses to umami compounds were diminished in our T1R3 KO mice. In the present study, we further examined responses to umami compounds at single nerve fiber levels in wild type (WT), T1R3-KO and TRPM5-KO mice. The results indicated that umami-responsive single fibers of the chorda tympani nerve in WT mice could be classified into more than two types, Sucrose-best (S-type) and MPG-best (M-type) fibers. Furthermore, each these fiber types could be classified into 2 groups, one type showing synergistic effect between MPG and IMP (S1, M1) and the other type showing no synergism (S2, M2). In the KO mice, S1-type was absent, but S2 and M2 types still remained. These data suggest that the information from umami receptor candidate T1R1/T1R3 may be mediated by transduction pathway including TRPM5 and carried by sweet taste sensitive nerve, and that specific umami taste information is mediated by other than T1R1/T1R3 and TRPM5 in mice. [J Physiol Sci. 2007;57 Suppl:S103]
