Amino acid receptor sites in the facial taste system of the sea catfish Arius felis

1. The amino acid sensitivity and specificity of the facial taste system of the marine catfish, Arius felis, is characterized electrophysiologically.
2. The facial taste system of Arius felis responded to all 28 amino acids tested, but was highly sensitive to only a few. In general, acidic amino acids and neutral amino acids with short side chains were more effective than imino, basic and neutral amino acids with long side chains.
3. A reciprocal cross-adaptation protocol used to characterize the receptor sites identified at least some relatively independent receptor sites for L-arginine, L-histidine, L-proline, L-alanine, glycine, D-alanine and L-glutamate.
4. Of the 7 amino acids that were indicated to have relatively independent receptor sites, the median electrophysiological threshold for L-alanine, the most stimulatory, and L-proline, the least stimulatory compounds, were 10 nM and 10,000 nM, respectively. The integrated facial taste response did not saturate at test amino acid concentrations up to 10 mM.
5. The generalized depression in responsiveness to test stimuli observed during amino acid adaptation is proposed to be a result of the co-distribution of sensitivity at the level of single taste cells rather than high cross-reactivity of the respective amino acid receptor sites for the test stimuli.