Anion size modulates salt taste in rats

Abstract

The purpose of this study was to investigate the influence of anion size and the contribution of the epithelial sodium channel (ENaC) and the transient receptor potential vanilloid-1 (TRPV1) channel on sodium-taste responses in rat chorda tympani (CT) neurons. We recorded multiunit responses from the severed CT nerve and single-cell responses from intact, narrowly tuned and broadly tuned, salt-sensitive neurons in the geniculate ganglion simultaneously with stimulus-evoked summated potentials to signal when the stimulus contacted the lingual epithelium. Artificial saliva served as the rinse and solvent for all stimuli (0.3 M NH(4)Cl, 0.5 M sucrose, 0.03-0.5 M NaCl, 0.01 M citric acid, 0.02 M quinine hydrochloride, 0.1 M KCl, and 0.03-0.5 M Na-gluconate). We used the pharmacological antagonist benzamil to assess NaCl responses mediated by ENaC, and SB-366791 and cetylpyridinium chloride to assess responses mediated by TRPV1. CT nerve responses were greater to NaCl than Na-gluconate at each concentration; this was attributed mostly to broadly tuned, acid-generalist neurons that responded with higher frequency and shorter latency to NaCl than Na-gluconate. In contrast, narrowly tuned NaCl-specialist neurons responded more similarly to the two salts, but with subtle differences in temporal pattern. Benzamil reduced CT nerve and single-cell responses only of narrowly tuned neurons to NaCl. Surprisingly, SB-366791 and cetylpyridinium chloride were without effect on CT nerve or single-cell NaCl responses. Collectively, our data demonstrate the critical role that apical ENaCs in fungiform papillae play in processing information about sodium by peripheral gustatory neurons; the role of TRPV1 channels is an enigma.

Fig. 1.

Raw electrophysiological traces from the chorda tympani (CT) nerve (A) and average responses (B; n = 6) to 0.3 M NH₄Cl, 0.5 M sucrose, 0.03–0.5 M NaCl, 0.01 M citric acid, 0.02 M quinine hydrochloride (QHCl), 0.1 M KCl, and 0.03–0.5 M Na-gluconate. Data are normalized to the average NH₄Cl response. Comparative stimuli (0.3 M NH₄Cl and 0.5 M NaCl) are shown in white, standard taste stimuli are shown in gray, and sodium salt concentration response functions are shown in black. *Significantly different from the proceeding concentration; †significantly different from NaCl at an equimolar concentration. EGG, electrogustogram; RMS, root mean square.

Fig. 2.

Raw electrophysiological traces from the CT nerve (A) and average responses (B; n = 7) to 0.1 M NaCl, 0.1 M NaCl mixed with cetylpyridinium chloride (CPC; 200 μM and 2 mM), SB-366791 (SB; 1 and 5 μM), 0.1% DMSO, and benzamil (Bz; 1 and 5 μM). Data were normalized to the average NH₄Cl response. Comparative stimuli (0.3 M NH₄Cl and 0.5 M NaCl) are shown in white, 0.1 M NaCl applications are shown in gray, and 0.1 M NaCl mixed with pharmacological antagonists of epithelial sodium channel (ENaC) and transient receptor potential vanilloid-1 (TRPV1) are shown in black. Notice that switching rinse channels containing artificial saliva (AS) had no impact on CT nerve responses. Following the 5-s application of 5 μM Bz, 0.1 M NaCl was applied for 60 s to ensure that all Bz was washed from the receptor surface before the final 5-s application of 0.1 M NaCl. *Significantly different from 0.1 M NaCl responses.

Fig. 3.

Raw electrophysiological traces from the CT nerve (A) and average responses (B; n = 4) to 0.1 M NaCl, or 0.1 M NaCl mixed with pharmacological antagonists of ENaC and TRPV1. NaCl 0.1 M was presented alone for 60 s following drug delivery to demonstrate the prolonged, competitive, antagonistic effects of Bz and to ensure its removal from the receptor surface. To measure the efficacy of TRPV1 antagonists on 0.1 M NaCl responses, 5 μM Bz were presented in combination with CPC (200 μM and 2 mM) or SB (1 and 5 μM). Data were normalized to the average 0.1 M NaCl response. Responses to 0.1 M NaCl after AS rinse are shown in white, 0.1 M NaCl mixed with pharmacological antagonists of ENaC and TRPV1 are shown in black, and 0.1 M NaCl applications following drug adaptation are shown in grey (recovery responses). *Significantly different from the proceeding stimulus.

Fig. 4.

Dendrogram of the results from the hierarchical cluster analysis.

Fig. 5.

Raw electrophysiological traces from a NaCl specialist (A) and from an acid generalist (B) in response to 0.5 M sucrose, 0.1 M NaCl, 0.01 M citric acid, 0.02 M QHCl, and 0.1 M KCl.

Fig. 6.

Average responses to all test stimuli (5-s presentations relative to baseline) from NaCl specialists (A) and acid generalists (B). *Significantly different from the proceeding concentration; †significantly different from NaCl at an equimolar concentration.

Fig. 7.

Average response profiles to 0.5 M sucrose, 0.1 M NaCl, 0.01 M citric acid, 0.02 M QHCl, and 0.1 M KCl from NaCl specialists (A) and acid generalists (C), and to 0.1 M NaCl alone or mixed with 1 μM Bz or 1 μM SB from NaCl specialists (B) and acid generalists (D) in 100-ms bins. Vertical line indicates stimulus onset (time 0).

Fig. 8.

Average concentration responses profiles (0.03–0.5 M) of NaCl specialists (A–D) and acid generalists (E–H) to NaCl and to Na-gluconate in 100-ms bins. Vertical line indicates stimulus onset (time 0), → indicates response latency to NaCl, whereas ← indicates response latency to Na-gluconate. †Peak responses to NaCl; ‡peak responses to Na-gluconate.

Fig. 9.

Spikes/100 ms (relative to baseline) to NaCl and Na-gluconate in half-log increments (0.03–0.5) in NaCl specialists and acid generalists for the first 500 ms (A) and 1 s after stimulus onset (B). *Significantly different from the proceeding concentration; †significantly different from NaCl at an equimolar concentration.