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First published online August 22, 2008
Journal of Experimental Biology 211, 2786-2791 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.018648
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Bile salts are effective taste stimuli in channel catfish

S. H. Rolen* and J. Caprio

Department of Biological Sciences, Louisiana State University, Life Sciences Building Room 107 Baton Rouge, LA 70803, USA


Figure 1
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  		Fig. 1. The molecular structures of the bile salts tested. Molecular features, designated by R1 and R2, of each bile salt tested vary at carbon positions C12 of the steroid backbone and C24 of the side-chain, respectively. The stimuli included different classes of bile salts based on the specific molecular feature (R2) attached to C24 [glycine-conjugated (GBS), taurine-conjugated (TBS), non-conjugated (NBS)]. All of the bile salts are 3{alpha}, 5β, 7{alpha} and 12{alpha} isomers. An asterisk indicates those bile salts produced by the channel catfish (Kellogg, 1975Go).

 

Figure 2
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  		Fig. 2. Integrated whole-nerve taste responses to (A) 10–6 mol l–1 L-alanine and (B) 10–5 mol l–1 TCDC. Each compound was tested at two stimulus durations, 2 s and 5 min. Note that each compound evoked only phasic responses regardless of the stimulus duration.

 

Figure 3
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  		Fig. 3. Integrated taste responses to 10–11 to 10–4 mol l–1 TCDC recorded from the entire branch of the facial–trigeminal complex that innervates the caudal portion of the maxillary barbel. Responses to CFTW (charcoal-filtered tap water; C) control and 10–6 mol l–1 L-alanine are shown to allow comparisons.

 

Figure 4
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  		Fig. 4. Dose–response plots of integrated taste responses to bile salts standardized to the response to 10–6 mol l–1 L-alanine. The number of fish tested (N) is provided in the key. The averaged control magnitude value was subtracted from the averaged stimulus magnitude response at each concentration. Data points and error bars, means ± s.e.m.

 

Figure 5
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  		Fig. 5. Representative cross-adaptation experiments illustrating the integrated taste activity recorded (1) before, (2) during and (3) after adaptation to (A) 10–5 mol l–1 TCDC and (B) a mixture of 10–6 mol l–1 L-amino acids (Ala, Arg and Pro; AA). The adapting solution is underlined.

 

Figure 6
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  		Fig. 6. Results of cross-adaptation experiments. (A) Adaptation to 10–5 mol l–1 TCDC; (B) adaptation to a mixture of 10–6 mol l–1 amino acids (Ala, Arg and Pro). Bars indicate the percentage of the unadapted response (means ± s.d.). Responses significantly greater than control responses: (A) Pro, Ala and Arg; (B) TCDC, GCDC and CDC (one-way ANOVA; Tukey's post hoc test, P<0.05). N=3 fish tested.

 

Figure 7
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  		Fig. 7. Integrated multiunit taste recordings from three separate facial nerve twigs (A–C) innervating the maxillary barbel in a single fish showing the variability of the magnitude of the integrated responses to bile salts with respect to amino acids. (A) Nerve twig lacking a bile-salt response but showing a large-magnitude amino acid response. (B) Nerve twig with a significant bile-salt response and an even-greater-magnitude amino acid response. (C) Nerve twig responding approximately equally to the bile salt and amino acid mixtures. C, CFTW control; BS, a mixture of 10–5 mol l–1 TCDC, TCA, GCDC and CDC; AA, a mixture of 10–6 mol l–1 L-alanine, L-arginine and L-proline.

 

Figure 8
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  		Fig. 8. Responses of single facial taste fibers. (A) A single fiber responding only to the bile salt (BS) mixture (10–5 mol l–1 TCDC, TCA, GCDC and CDC); (B) a single fiber responding only to the amino acid (AA) mixture (10–6 mol l–1 Ala, Arg and Pro); (C) a single fiber responding to both stimulus mixtures. A and B were recorded simultaneously from different fibers in the same preparation, whereas unit no. 3 was recorded from a different animal. Clusters of individual spike trains are shown in insets I and II to indicate that the action potentials recorded in A–C were evoked by single fibers. Ct, CFTW. Horizontal bars indicate 2 s stimulus applications.

 

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© The Company of Biologists Ltd 2008