Amino acids as taste stimuli. I. Neural and behavioral attributes

The physiological importance of L-amino acids implies that the gustatory system should be sensitive to their presence in the environment. This notion is reinforced by research showing that rats are able to self-regulate their dietary consumption of amino acids and other important nutrients. This paper describes two studies which begin a systematic evaluation of the neural and behavioral effects of L-amino acids on gustation in the rat. The first experiment defined the neural thresholds, intensity-response functions and time courses of whole chorda tympani nerve responses to 12 stimuli. Thresholds varied from 10(-4) M (arginine) to 8 X 10(-2) M (proline), with pH-related amino acids being low and those reported by humans to be sweet being high. Maximum neural effectiveness, relative to the resting level of neural activity, ranged from 108% (2.0 M lysine-hydrochloride) to 8% (0.1 M leucine), with pH-related stimuli evoking the greatest activity. Neural time courses were varied, and often did not include well-defined phasic and tonic components. The second experiment measured the rat's preferences in 24-h two-bottle tests. Preference thresholds were typically lower than neural thresholds, presumably due to extra-chorda tympani input involved in the perception of flavor. They ranged from 2 X 10(-6) M (methionine) to 6 X 10(-2) m (leucine) and were in good agreement with psychophysically defined humar thresholds. Seven stimuli were preferred at low concentrations, but only two (alanine, leucine) continued to be appetitive as intensity increased. Preferences were predictable for 10 of the 12 stimuli from psychophysical reports. Rats also rejected amino acids in direct proportion to their toxicity, demonstrating the protective function of the taste-hedonic system.