Responses of the hamster chorda tympani nerve to sucrose+acid and sucrose+citrate taste mixtures

Abstract

Studies of taste receptor cells, chorda tympani (CT) neurons, and brainstem neurons show stimulus interactions in the form of inhibition or enhancement of the effectiveness of sucrose when mixed with acids or citrate salts, respectively. To investigate further the effects of acids and the trivalent citrate anion on sucrose responses in hamsters (Mesocricetus auratus), we recorded multifiber CT responses to 100 mM sucrose; a concentration series of HCl, citric acid, acetic acid, sodium citrate (with and without amiloride added), potassium citrate, and all binary combinations of acids and salts with 100 mM sucrose. Compared with response additivity, sucrose responses were increasingly suppressed in acid + sucrose mixtures with increases in titratable acidity, but HCl and citric acid were more effective suppressors than acetic acid. Citrate salts suppressed sucrose responses and baseline CT neural activity to a similar degree. Citrate salts also elicited prolonged, concentration-dependent, water-rinse responses. The specific loss in sucrose effectiveness as a CT stimulus with increasing titratable acidity was confirmed; however, no increase in sucrose effectiveness was found with the addition of citrate. Further study is needed to define the chemical basis for effects of acids and salts in taste mixtures.

Figure 1

Integrated multifiber CT responses to anterior tongue stimulation with single components and binary mixtures of sucrose with HCl or citric acid. (A) Component response to 3 mM HCl alone (smaller, red trace) superimposed on the mixture response to 3 mM HCl + 100 mM sucrose (larger, green trace). (B) Component response to 10 mM citric acid alone (red trace) superimposed on the mixture response to 10 mM citric acid + 100 mM sucrose (green trace). (C, D) Component responses to sucrose alone, beneath each mixture, respectively. Dotted lines = average baseline voltage; downward arrows = water rinses.

Figure 2

Mean (±standard error of the mean) relative CT responses to each of the 3 acids used in phase 1, (A) 10 mM citric acid, (B) 3 mM HCl, and (C) 10 mM acetic acid. Component responses summed were obtained by adding the responses to each of the mixture components presented separately. The response to the sucrose + citric acid mixture was smaller than that predicted by the component responses summed implying sucrose-response suppression. In contrast, responses to sucrose + HCl or sucrose + acetic acid were similar to the component responses summed implying modality independent responses; **P < 0.001. This figure appears in color in the online version of Chemical Senses.

Figure 3

Mean (±standard error of the mean) CT responses to acid + sucrose mixtures (squares) plotted as a function of titratable acidity. As titratable acidity increased, responses to each of the sucrose + acid mixtures deviated from the component responses summed (open diamonds). Filled circles illustrate responses to the acid components alone. Single square, with horizontal dashed line, shows the average response to 100 mM sucrose for each acid + sucrose mixture. These results indicate a suppressive interaction between the acidic stimuli and sucrose. (*P < 0.05, **P < 0.01, mixture response vs. component responses summed.) This figure appears in color in the online version of Chemical Senses.

Figure 4

(A) Integrated multifiber CT responses to anterior tongue stimulation with 1 mM K₃Citrate alone (smaller, red trace) superimposed on the response to a mixture of 1 mM K₃Citrate + 100 mM sucrose (larger, green trace). K₃Citrate had a small inhibitory effect on the CT; release of K₃Citrate inhibition is evident in the prolonged water-rinse response. (B) The response to the 100 mM sucrose component presented alone. Dotted lines = average baseline voltage; upward arrow = stimulus onset; downward arrows = water rinse.

Figure 5

(A) Mean (±standard error of the mean [SEM]) CT responses to citrate salts plotted as a function of stimulus concentration. Citrate salts represent the combined mean of the responses to Na₃Citrate mixed with amiloride and K₃Citrate. See text for an explanation. Average mixture responses with citrate salts at 1 and 3 mM were reliably smaller (*P < 0.05, **P < 0.01) than the average response to 100 mM sucrose alone and less than the component responses summed (open triangles) at every concentration (P < 0.01). (B) Mean (±SEM) CT responses to sucrose + Na₃Citrate (without amiloride present) plotted as a function of stimulus concentration. Mixture responses were equivalent to the component responses summed at all concentrations except 10 mM, where the mixture response was still greater than either component presented alone but less than the component responses summed (**P < 0.01 mixture response vs. component responses summed). This figure appears in color in the online version of Chemical Senses.

Figure 6

Mean (±standard error of the mean) rinse responses to citrate containing stimuli plotted as a function of stimulus concentration. Rinse responses to each sucrose + citrate salt mixture were averaged with the rinse responses to the citrate component alone. When averaged across the three concentrations, K₃Citrate water rinses ≥ Na₃Citrate water rinses > Na₃Citrate amiloride rinses (P < 0.05). This figure appears in color in the online version of Chemical Senses.