Responses of single facial taste fibers in the channel catfish, Ictalurus punctatus, to amino acids

Abstract

SUMMARYAND CONCLUSIONS 1. Amino acids and nucleotides stimulate taste receptors of teleosts. In this report, responses to these compounds of 105 facial taste fibers (79 fully characterized) that innervate maxillary barbel taste buds of the channel catfish (Ictalurus punctatus) were analyzed.2. The fully characterized facial taste fibers that responded to amino acids ( y2 = 68) were generally poorly responsive to nucleotides and related substances (NRS), whereas the fibers responsive to NRS (~2 = 11) were poorly responsive to amino acids. Spike discharge of the amino acid-responsive fibers to the most potent amino acid stimulus tested per fiber increased 44-fold from a mean spontaneous activity of 2.1 t 3.5 to 92.1 t 42.4 (SD) spikes/3 s. Spike activity of the NRS-responsive fibers to NRS increased 11 S-fold from a mean spontaneous activity of 3.4 t 5.9 to 39.1 t 27.4 spikes/3 s. There was no significant difference between the spontaneous rates, but stimulus evoked spike rates for the amino acid-responsive fibers were significantly greater (P < 0.05; Mann-Whitney test) than those for the NRS-responsive fibers.3. Hierarchical cluster analysis based on the 3-s response time identified three major groups of neurons. The identified clusters comprised neurons that were highly responsive to either L-alanine (i.e., Ala cluster; y1 = 39) L-arginine (i.e., Arg cluster; yt = 29), or NRS (NRS cluster; y1 = 11) . Fibers comprising the Arg cluster were more narrowly tuned than those within the Ala cluster. This report further characterizes the responses to amino acids of the individual facial taste fibers comprising the Ala and Arg clusters.4. Subclusters were evident within both of the amino acid-responsive clusters. The Arg cluster was divisible into two subclusters dependent on the response to 1 mM L-proline. Twelve neurons that were significantly (P < 0.05; Mann-Whitney test) more responsive to L-proline than the remaining 17 neurons within the Arg cluster formed the Arg/ Pro subcluster; these latter 17 neurons comprised the Arg subcluster. However, there was no significant difference (Mann-Whitney test) in the response to L-arginine between fibers within either subcluster across four different response times analyzed. Fibers within the Ala cluster were generally poorly responsive to L-proline. Four alanine subclusters were suggested on the basis of their relative responses to L-alanine, D-alanine, L-arginine, and the NRS; however of the 39 fibers comprising the alanine cluster, two alanine subclusters comprised only two fibers each, and the third subcluster consisted of four fibers. Neuron classification into clusters and subclusters was little influenced by the four different response times analyzed.5. Single-fiber dose-response relationships indicate that the Lproline gustatory receptor is of relatively low affinity in comparison to the receptors for L-alanine and L-arginine. Electrophysiological thresholds of single fibers within the Ala and Arg clusters were approximately nanomolar for L-alanine and L-arginine, respe...

Bile salts are effective taste stimuli in channel catfish

“…TCA was a relatively poor stimulus compared both with TCDC and with the two additional bile salts tested, CDC and GCDC, that are not produced by channel catfish. The data indicate that thresholds to bile salts are lower than those for amino acids in the facial taste system in the same species (Kohbara et al, 1992) and are 1-2 log units higher than those recorded for TCA in the rainbow trout (Yamashita et al, 2006). Although the thresholds to bile salts were lower than those to amino acids in the facial taste system, amino acids elicited greater integrated response magnitudes at equivalent stimulus concentrations.…”

“…effectiveness of bile salts relative to that for amino acids -the more well-established tastants for channel catfish (Caprio, 1978;Kohbara et al, 1992). Only prominent phasic responses were evident for both classes of stimuli (Fig.…”

“…The test stimuli included L-amino Alanine, arginine and proline were shown previously to be highly potent taste stimuli for the channel catfish (Caprio, 1978;Kohbara et al, 1992). The four bile salts tested in this study were selected to include two produced by the channel catfish [TCDC and TCA (Kellogg, 1975)] and two bile salts (GCDC and CDC) that have a close structural resemblance to TCDC and TCA (Fig.…”

The Physiology of Taste in Fish: Potential Implications for Feeding Stimulation and Gut Chemical Sensing

“…Nevertheless, 5 0 -nucleotides are well known and powerful gustatory stimulants on their own (Hara, 1994;Kubitza et al, 1997;Li and Gatlin, 2006). For instance, facial taste fibers in channel catfish that respond to AAs have a low responsiveness to nucleotides and vice versa (Kohbara et al, 1992).…”

“…Also, peptides tested so far are less effective than their AA residues (Kasumyan and Døving, 2003). Furthermore, the existence of different independent receptor mechanisms (not necessarily stimulatory) has been proposed for L-Ala, L-Arg, and L-Pro, based on electrophysiological studies in channel catfish (Kanwal and Caprio, 1983;Kohbara et al, 1992;Kumazawa et al, 1998), and it was additionally suggested that L-Ala and other short-chain neutral AA activate GPCRs that indirectly gate membrane ion channels via second messengers while L-Arg and L-Pro directly activate cation ion channels (Kohbara et al, 1992;Kumazawa et al, 1998). The induction of ion channel activity by L-Arg (and its blocking by D-Arg) binding to ligand-gated ion channel receptors from catfish barbel epithelium was later clearly demonstrated by Grosvenor et al (2004).…”