Comparison of L-monosodium glutamate and L-amino acid taste in rats

T1R2/T1R3 heterodimers are selectively responsive to sweet substances whereas T1R1/T1R3 receptors are selective for umami substances, represented by monosodium glutamate (MSG), and for L-amino acids. If a single receptor is responsible for detection of umami and L-amino acids, then it would be predicted that MSG and L-amino acids elicit similar tastes in rats. The present study compared the taste profile of MSG with four amino acids (glycine, L-proline, L-serine and L-arginine) using conditioned taste aversion, detection threshold, and taste discrimination methods. These experiments were designed to either reduce or neutralize the taste of sodium associated with MSG and the other amino acids. Detection threshold studies showed that rats were most sensitive to L-arginine and least sensitive to L-proline. Glycine and L-serine thresholds were similar to those previously reported for MSG. Like MSG, a conditioned taste aversion to each of the four amino acids generalized to sucrose in the presence of amiloride, a sodium channel blocker. Rats showed moderate generalization of aversion between MSG and L-arginine, suggesting that these two amino acids taste only moderately alike. However, the taste aversion experiments indicated that glycine, L-serine, and L-proline elicit taste sensations similar to MSG when amiloride is present. Discrimination experiments further compared the tastes of these three amino acids with MSG. When the sodium taste associated with MSG was reduced or neutralized, glycine and L-proline elicited tastes very similar but not identical to the taste of MSG. Low (but not higher) concentrations of L-serine were also difficult for rats to discriminate from MSG. While there are taste qualities common to all of these amino acids, the perceptual differences found in this study, combined with previous reports, suggest either multiple taste receptors and/or multiple signaling pathways may be involved in umami and amino acid taste perception in rats.