Methyl xanthines enhance taste: evidence for modulation of taste by adenosine receptor.

The methyl xanthines (MX), theophylline, caffeine, and theobromine, are potent antagonists of adenosine receptors. Adaptation of the human tongue to methyl xanthines at concentrations ranging from 10(-5) M to 10(-2) M was found to potentiate taste. The artificial sweetener acesulfam-K, which has a bitter component, was potentiated the most by MX, i.e., approximately 100%. This increase in perceived intensity for acesulfam-K occurred at 10(-5) M MX, a concentration known to inhibit adenosine receptors but below that required to inhibit phosphodiesterase. Increasing the concentration of MX as high as 10(-2) M did not increase the degree of enhancement appreciably. Taste enhancement was found for NaCl and quinine hydrochloride as well. When 10(-5) M adenosine was added to the MX, the potentiation was reversed. The human results were confirmed by animal studies in which single unit extracellular recordings were made from the nucleus of the solitary tract. These results suggest that the inhibitory A1 adenosine receptor plays an important local role in taste perception.