Amino acids as taste stimuli. II. Quality coding.
Two experiments were performed in rats to evaluate the relative taste qualities of 12 L-amino acids, each at a concentration which evoked half the maximum response for that chemical. The first study involved recording the activity of 40 individual chorda tympani fibers to the stimulus series. Only 34% of the evoked responses resembled the short latency phasic-tonic activity which characterizes gustatory responses to inorganic salts and acids. 32% had latencies exceeding 1 s; another 27% consisted of only a phasic burst lasting less than 1 s. The remaining 7% were inhibitory. Both long latency and purely phasic activity were stimulus selective: 61% of the former were in response to Gly or Pro while 69% of the latter were evoked by Cys-HCl, Lys-HCl or His. Response inhibition was not associated with either specific fibers or stimuli. Thus amino acids, which to humans represent a class of perceptually complex stimuli, show a corresponding complexity of evoked neural properties in the rat. The second study employed a conditioned taste aversion paradigm to assess the qualitative similarity of each amino acid to the others and to the 4 prototypical taste stimuli; NaCl, HCl, quinine-HCl and sucrose. Some amino acids showed strong generalization to a single gustatory prototype (Pro and Gly to sucrose; Cys-HCl to HCl); others generalized well to multiple prototypes (e.g. Arg to sucrose and NaCl). Several showed poor generalization to all 4 prototypical tastes, calling into question the assumption that these 4 totally encompass the gustatory domain.
D000818:Animals; D001362:Avoidance Learning / Q000502:physiology; D001369:Axons / Q000502:physiology; D002814:Chorda Tympani Nerve / Q000502:physiology*; D003214:Conditioning, Classical / Q000502:physiology; D005073:Evoked Potentials, Somatosensory; D005795:Generalization, Stimulus / Q000502:physiology; D008297:Male; D009433:Neural Inhibition; D009474:Neurons / Q000502:physiology; D051381:Rats; D011919:Rats, Inbred Strains; D011930:Reaction Time / Q000502:physiology; D009435:Synaptic Transmission; D013649:Taste / Q000502:physiology*; D013652:Taste Threshold / Q000502:physiology