Responses to umami substances in taste bud cells innervated by the chorda tympani and glossopharyngeal nerves

The chorda tympani (CT) and glossopharyngeal (GL) nerves of several mammalian species respond differently to umami substances (US) such as monosodium glutamate (MSG), disodium 5'-inosinate (IMP) and disodium 5'- guanylate (GMP). In mice and rhesus monkeys, responses to US are greater in the GL than the CT nerve, with the GL nerve containing larger numbers of MSG- sensitive fibers. Gurmarin, a sweet response inhibitor, suppresses the mouse CT responses to the mixture of MSG and IMP to ~65% of control levels but not to the metabotropic and ionotropic glutamate agonists 2-amino-4- phophonobutyrate and N-methyl-D-aspartate. Gurmarin does not inhibit any taste responses in the GL. In mice, CT responses to MSG may be masked by their greater sensitivity to sodium ions. Calcium imaging studies demonstrate that some mouse taste cells isolated from the fungiform papilla innervated by the CT respond selectively (as indicated by a rise in intracellular Ca²⁺ concentrations) to MSG and/or IMP or GMP. These MSG responses are not suppressed notably by reducing the Ca²⁺ concentration of the stimulus solution, suggesting that the observed Ca²⁺ release is from intracellular stores. Measurements of second messengers in the mouse fungiform papilla have revealed consistently that MSG elicits increases in both inositol 1,4,5- trisphosphate and adenosine 3',5'-cyclic monophosphate levels. Together, these results suggest that US may stimulate two different transduction mechanisms in the fungiform papilla. They also suggest that gurmarin- insensitive components of receptors for US, including metabotropic and ionotropic glutamate receptors, may be commonly involved in transduction for umami taste in taste cells on both anterior and posterior parts of the tongue.