Monosodium glutamate (MSG) elicits a unique taste in humans called 'umami' that is potentiated synergistically by the 5′-ribonucleotides IMP and GMP.Recent studies suggest that several mechanisms are involved in the transduction of umami taste. We have previously shown that MSG (10 mM) induced three different responses in mouse taste cells under whole-cell voltage-clamp in Na+ free Ringer solution; transient inward current, sustained inward current and outward current. Two, and occasionally all three types of currents, were observed in the same cell. We report here responses of mouse taste cells to the metabotropic glutamate receptor agonist, L-AP4 (1 mM), a mixture of MSG (10 mM) and IMP (0.5 mM), and a mixture of L-AP4 (1 mM) and IMP (0.5 mM) under whole-cell voltage-clamp. The mixture of MSG and IMP induced three different types of responses, similar to MSG alone. But the amplitudes of both the transient and sustained inward currents were larger than observed with MSG alone. The outward currents were similar in amplitude to those elicited by MSG alone. L-AP4 induced only outward currents in taste cells, while the mixture of L-AP4 and IMP induced both sustained inward currents and outward currents. These results suggest that umami taste reception may involve the coactivation of mGluR4 and one or more types of ionotropic glutamate receptors. Presumably the net transduction current resulting from activation of these receptors mediates the umami taste response, while an additional receptor or receptor site which is related to sustained inward current response, is involved in the synergistic effect of ribonucleotides.