Analyses of taste nerve responses with special reference to possible receptor mechanisms of umami taste in the rat.

Umami substances such as monopotassium L-glutamate (MPG) and 5'-inosine monophosphate (IMP) elicit a unique taste called 'umami' in humans. To elucidate the umami receptor mechanism in rats, we examined taste responses of the chorda tympani nerve by using three ionotropic glutamate receptor agonists, NMDA, KA and AMPA, a mGluR4 agonist, L-AP4, and a specific mGluR4 antagonist, MAP4, and an anti-sweet peptide, gurmarin. When IMP was added, synergistic responses were shown only for MPG and L-AP4, but not for NMDA, KA and AMPA. MAP4 enhanced the responses to MPG and L-AP4. Gurmarin suppressed the synergistic responses to mixtures of MPG and IMP or L-AP4 and IMP. These results suggest that glutamate and L-AP4 bind both the sweet-responsive macromolecule and mGluR4, but the synergism occurs only on the macromolecule.