Gustatory specificity for amino acids in the facial taste system of the carp,Cyprinus carpio L.

Abstract

1. The stimulatory effectiveness of amino acids and analogs on the external gustatory receptors of the carp (Cyprinus carpio L.) was investigated with extracellular electrophysiological techniques. 2. Amino acids are highly effective stimuli for the carp gustatory system. Threshold for L-proline, the most effective stimulus tested, averaged 10−8.5±0.9 (SD) mol/l. Thresholds for the other stimulatory chemicals ranged between 10−4–10−8 mol/l (Table 1). Averaged gustatory neural activity for L-proline and L-alanine increased with logarithmic increase in stimulus concentration over a wide concentration range (approximately 6–7 log units) and showed a tendency for saturation at concentrations ≧ 10−3 mol/l (Figs. 5 and 6). 3. The external taste system of the carp responded to neutral and acidic amino acids, but not to basic ones. 4. Taste responses to amino acids were highly stereospecific; the L-isomer of an amino acid was stimulatory, but the enantiomer was not (Table 2). The gustatory receptors were highly responsive to amino acids containing 3 to 4 carbon atoms having unbranched and uncharged side chains. Further, this study indicated that the α-amino group and the α-hydrogen were essential for maximal effectiveness, and that for some amino acids on ionically charged primary carboxyl group was generally unnecessary for a maximal receptor response (Table 3). 5. The molecular requirements of the neutral amino acid receptor sites for the external gustatory system in the carp appeared to be similar in general to those described in the channel catfish (Caprio 1978). 6. Due to the limited number of stimulatory amino acids, the taste receptor in the carp shows a more narrowly-tuned chemical spectrum than does the olfactory system in the same species. Additionally, the taste system of the carp is more sensitive to amino acids than the olfactory system in the same species (Table 1; Goh and Tamura 1978).