In order to elucidate the taste receptor mechanism for umami tasting substance, monosodium Lglutamate (MSG) and sweet amino acids, the responses of isolated mouse taste receptor cells were investigated using patch-clamp techniques and optical recordings with voltage-sensitive dyes. The effects of the substances that exert synergism with MSG or D-phenylalanine were also investigated. In some taste receptor cells isolated from the circumvallate and foliate papillae, persistent inward currents were induced by MSG (40-85mM) at a holding potential of -60 mV. The inward currents were reversed at about +10 mV and accompanied with slight conductance increase by 14% on average, indicating involvement of non-selective cation channels in the induced currents. In other cells, MSG induced outward currents at a holding potential of -80 mV. Optical recording showed that MSG slowly depolarized several cells both in a clump of taste bud cells isolated from circumvallate papilla and in an intact taste bud of fungiform papilla. 5'-GMP and 5'-IMP (1 mM), the enhancer of MSG response, increased the whole-cell K^+ outward currents. Sweet amino acids, D-phenylalanine (D-Phe, 100 mM) and D-tryptophan (D-Try, 20mM), suppressed the whole-cell outward K^+ currents as well as single K^+ channel activities in a cell-attached patch. When 2 mM saccharin, the synergist, was added to D-Phe, the outward K^+ currents were suppressed more strongly. Optical recordings showed that strong depolarization in some taste bud cells induced by D-Try and D-Phe (20 mM) was eliminated by the addition of K^+ channel blocker to the bath.
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