1. Enhancement of the gustatory responses in single chorda tympani fibers of rats, produced by the addition of 5'-ribonucleotides to MSG, was investigated.
2. No fibers were found which were exclusively responsive to MSG or one of 5'-ribonucleotides. Fibers responsive to MSG also gave responses to 5'-ribonucleotides. Marked enhancement of the magnitude of response by replacing one-tenth of MSG with 5'-GMP, 5'-IMP or 5'-AMP was observed in fibers responding well to sucrose but not in those predominantly sensitive to NaCl, the magnitude of potentiation produced being 5'-AMP=5'-GMP>5'-IMP. The enhancement by 5'-UMP and 5'-CMP was small.
3. A highly significant correlation was observed between the magnitude of response to the mixture of MSG with 5'-GMP, 5'-IMP or 5'-AMP and that for sucrose and saccharin, thus showing the similarity of the quality of taste between the two. On the other hand, the magnitude of response to the mixture of MSG with 5'-UMP is significantly correlated with that for NaCI but not with the response magnitude for sucrose.
4. Fibers predominantly sensitive to sucrose elicited rhythmic bursts of impulses in response to, not only, sucrose and saccharin, but also to the mixture of MSG with 5'-GMP, 5'-IMP or 5'-AMP, the period of the bursts being several hundred msec.
5. A highly significant correlation was observed between pairs of responses to MSG4-51-GMP, MSG+5'-IMP and MSG+5'-AMP, indicating similarity of quality of taste among each other. However, correlations between the former three and MSG+5'-UMP are small, suggesting a difference in taste quality.
6. Equations were calculated for expressing the neural response magnitude for MSG, 5'-ribonucleotides and their mixtures by those for the four basic gustatory stimuli.
7. Based on the results of the experiments it has been postulated that glutamate ions combine with sucrose-sensitive receptor-molecules and that 5'-GMP, 5'-IMP and 5'-AMP facilitate binding of the latter with the former.