Gustatory Behavior of Channel Catfish to Amino Acids

Abstract

Electrophysiological recordings from both nerve twigs (Caprio,1978; Davenport and Caprio, 1982; Kanwal and Caprio, 1983) and single taste fibers (Kohbara et al., 1990) provided evidence that the taste system of the channel catfish is highly responsive to the amino acids, L-alanine, L-arginine and L-proline. Dose-response relations determined electrophysiologically indicate that the taste system of the channel catfish is most sensitive to L-alanine and L-arginine (Caprio, 1975), whereas L-proline becomes highly stimulatory at stimulus concentrations >10⁻³M (Kanwal and Caprio, 1983; Wegert and Caprio, 1991). Electrophysiological cross-adaptation (Davenport and Caprio, 1982; Kanwal and Caprio, 1983; Wegert and Caprio, 1991) and biochemical competition (Cagan, 1986) experiments indicated relatively independent receptor sites for these three amino acids. Recent patch clamp experiments (Teeter at al., 1990; Brand et al. 1991) directly confirmed the existence of independent receptor sites for the L-isomers of alanine, arginine and proline and additionally provided insight into the respective transduction mechanisms associated with these receptor sites. Current evidence indicates that the receptor sites for L-arginine and L-proline are direct ligand-operated, whereas those for L-alanine involve GTP-binding proteins and the generation of second messengers. Interestingly, the majority of amino acid taste information from the direct ligand-operated receptor sites is transmitted to the central nervous system by narrowly-tuned taste fibers that are highly responsive to L-arginine, whereas the majority of amino acid taste information involving second messenger generation is transmitted centrally by relatively broadly-tuned taste fibers that are most stimulated by L-alanine (Kohbara et al., 1990).