Electrophysiological characterization of the inhibitory effect of a novel peptide gurmarin on the sweet taste response in rats.

The effect of an anti-sweet peptide, gurmarin purified from the leaves of Gymnema sylvestre, was studied electrophysiologically on taste responses of the rat chorda tympani. The action of gurmarin was highly specific to sweet taste so that responses to various sweeteners including sugars, sweet amino acids and an artificial sweetener, saccharin were all suppressed. The most effective pH at which the rat tongue was treated with gurmarin was found to be 4.5, which corresponds to the isoelectric point of the peptide. At this condition about 5 microM of gurmarin was sufficient to reveal maximal effect and this was still significant at 0.5 microM (2 micrograms/ml). Although the suppressed responses required several hours to attain complete recovery, anti-gurmarin serum shortened the recovery time considerably. On the other hand, intravenous injection of gurmarin did not cause any significant effects on taste responses at all. These results suggest that gurmarin acts on the apical side of the taste cell, possibly by binding to the sweet taste receptor protein.