THE ENHANCING ACTION OF 5'-RIBONUCLEOTIDE ON RAT GUSTATORY NERVE FIBER RESPONSE TO MONOSODIUM GLUTAMATE

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Responses of single chorda tympani fibers of rats to gustatory stimulations by monosodium glutamate (MSG) of varying concentrations with or without the presence of a fixed amount of sodium 5'-guanylate (5'-GMP) were recorded. The effect of addition of 5'-GMP to MSG differs according to the response characteristics of the unit. In units predominantly responsive to NaCl, little enhancement was observed in the response magnitude with the addition of 5'-GMP, but in units responding well to sucrose a marked enhancement occurred. In a majority of these units, not only a synergistic enhancement in the response magnitude but also a marked lowering of threshold was obtained after the addition of 5'-GMP. The relative magnitude of enhancement was greater at a lower concentration of MSG, and decreased with increasing concentration. Analysis of the concentration-response magnitude relationships suggests that the synergistic action of MSG and 5'-GMP on gustatory receptors is a mechanism known as ‘competitive and noncompetitive’ sensitization.