Taste effects of some amino acids and glutamate compounds in the rat

Behavioral responses to five L-amino acids (Gly, Arg, Leu, Ala, Met) and five related L-glutamate compounds (MSG, MKG, MAG, Gln, GluHCl) were measured using 1-min taste reactivity and standard 24-h, two-bottle preference tests. Taste reactivity tests measure the immediate pattern of ingestive and aversive oral motor behavior elicited by direct oral infusion of taste stimuli. By permitting acute observations in non-deprived rats, taste reactivity tests are more sensitive to taste factors than standard long-term tests. Three stimulus concentrations of each compound were selected by behavioral and electrophysiological criteria. Taste reactivity results often conflicted with standard intake results. In taste reactivity tests both Gly and MSG elicit ingestive oral motor responses that increase with stimulus concentration in the absence of aversive behavior. The opposite responses were obtained using long-term intake tests; MSG and Gly preference ratios actually decrease with increasing concentration. These data suggest a reinterpretation of standard, longterm intake tests. Specifically, effects of taste versus post-oral stimuli may be distinguished by contrasting taste reactivity and two-bottle preference tests. Differences in the pattern of oral motor behaviors elicited by the amino acid and glutamate compounds are also discussed.