Olfaction and taste in the channel catfish: An electrophysiological study of the responses to amino acids and derivatives

Abstract

1. Peripheral olfactory and gustatory responses of the channel catfishIctalurus punctatus to amino acids and derivatives have been studied using extracellular electrophysiological recording techniques. 2. Amino acids are highly effective stimuli for both chemosensory systems with taste more sensitive than smell. Threshold for L-alanine, the most effective gustatory stimulus tested, averaged 10−11.5±1.7 M, the lowest electrophysiological threshold reported for taste in any vertebrate. Threshold for L-cysteine, the most effective olfactory stimulus tested, averaged 10−8.8±0.8 M. In both systems the L-isomer of anα-amino acid was always more stimulatory than its enantiomer. 3. Averaged olfactory and gustatory neural activity or underwater electro-olfactogram (EOG) recordings increased exponentially with logarithmic increase of stimulus concentration from threshold to 10−2M (Figs. 1, 2, 5). 4. The olfactory receptors were highly responsive to amino acids containing 5 to 6 carbon atoms having unbranched (Table 2) and uncharged (Table 3) side chains; a major exception was the large response to the 3 carbon amino acid L-cysteine. 5. The gustatory receptors were highly responsive to amino acids containing 3 to 4 carbon atoms having unbranched and uncharged side chains; a major exception was the large response to L-arginine, a 6-carbon positively charged amino acid (Table 5). 6. Esterification of the primary carboxyl group of particular amino acids resulted without loss of stimulatory activity indicating for these cases an ionically charged carboxyl group is unnecessary for receptor response (Tables 4, 5). 7. The molecular requirements of the amino acid receptor sites for olfaction and taste in the channel catfish appear to be similar in part to those described for other amino acid chemoreceptive system, in bacteria fish and animal.