Neuronal responses of the nucleus tractus solitarius to oral stimulation with umami substances.

In order to investigate coding mechanisms of special taste modality (umami), responses of neurons within the nucleus tractus solitarius (NTS) to oral stimulation with monosodium glutamate, disodium 5'-inosinate (IMP) or their mixture were recorded in the conventional electrophysiological method. Results obtained were as follows: Neither MSG-best nor IMP-best neuron was recorded within the NTS as in the primary taste afferents. Some of the sucrose-best neurons, NaCl-best neurons and HCl-best neurons responded to oral stimulation with MSG or IMP. A remarkable synergistic effect was observed in all of the sucrose-best neurons and in some of the NaCl-best neurons but not in all of the HCl-best neurons, when the mixed solution of MSG and IMP was applied into the oral cavity. As to the sucrose-best neurons, potency of the synergism was positively correlated with the responsiveness to sucrose. No correlation was recognized between them in the case of NaCl-best neurons. These results suggest a view that the sucrose-best neurons and the NaCl-best neurons which show the synergism may participate in coding umami taste.