Multiple pathways for signaling glutamate taste in rodents.

L-glutamate, typically as its Na salt (MSG), elicits a taste termed umami. A characteristic feature of umami taste is the synergistic potentiation of glutamate taste by purine nucleotide (inosine, guanosine) monophosphates. This is manifested as an enhanced electrophysiological response from taste receptor cells, as an increase in nerve firing rate, or as increased preference in behavioral assays. Apart from this enhanced intensity, it is not clear whether the addition of nucleotides also leads to a change in the perceived quality of glutamate in animals and humans. The magnitude of nucleotide-potentiation in nerve recordings varies considerably between the chorda tympani (CT) and glossopharyngeal (GL) nerves (Ninomiya et al., 1993). Single-unit recordings further highlight the heterogeneity of umami responses in that nucleotide-potentiated signals are seen in distinct fiber-types (sucrose-best or glutamate-best) in the CT and GL nerves (Ninomiya and Funakoshi, 1989; Yamamoto et al., 1991; Formaker et al., 2004). Gurmarin, a peptide that inhibits sweet taste in rodents, inhibits umami signals differentially across the CT and GL nerves (Ninomiya et al., 1993; Sako and Yamamoto, 1999). Collectively, the nerve recording data suggest that responses to MSG differ significantly between the anterior (CT innervation) and posterior (GL innervation) lingual taste fields. The implication is that umami responses may originate from more than a single type of receptor or receptor combination.