Anion Size and Acetic Acid Modulate Salt Taste in Rats

For my dissertation, I used whole nerve and single-cell recording methods to investigate the influence of organic anions and organic acids on sodium responses by the chorda tympani nerve (CT) that innervates taste buds on the anterior tongue. For single cell recording, I focused on the responses of NaCl-specialist and acid generalist neurons, the two major neuron types of the chorda tympani with robust salt sensitivity. To assess the influence of organic anions on sodium responses (Chapter 2), I examined the responses of the whole nerve and both neuron types to a broad concentration range of NaCl (an inorganic salt) and Na gluconate (an organic salt). The CT nerve responded better to NaCl than to Na gluconate at each concentration; however both salts were differentially represented by NaCl-specialist and acid-generalist neuron types. In particular, I showed that NaCl specialists responded to sodium salts with short-response latencies and high-response frequencies--spike latency and spike frequency was virtually unaffected by the large organic anion, gluconate. Responses to NaCl in NaCl specialists were attenuated by application of benzamil, indicating that they respond to sodium via apical epithelial sodium channels (ENaCs). In contrast, acid generalists responded to sodium salts with longer-response latencies and lower-response frequencies than their NaCl specialist counterparts--gluconate severely attenuated spike frequency and increased spike latency. NaCl responses in acid generalists were unaffected by benzamil or SB366791, indicating that they do not respond to sodium via ENaCs or via transient receptor potential vanilloid receptor 1 (TRPV1) channels, respectively. Interestingly, humans report that Na gluconate tastes less salty and they recognize it more slowly than NaCl, similar to the response characteristics of acid-generalist neurons. It is possible that the receptor mechanism(s) of acid generalists is functionally conserved in humans but this awaits future investigation. To assess the influence of an organic acid (Chapter 3) on sodium responses, I examined the responses by the whole nerve and by both neuron types to a broad concentration range of acetic acid, NaCl, and mixtures of NaCl with acetic acid. I showed that the rat CT nerve and acid-generalist neurons responded in a concentration-dependent manner across the entire range of acetic-acid concentrations. Acid-generalist neurons responded with progressively increasing spike frequency and decreasing spike latency as acetic acid concentration increased. NaCl-specialist neurons were virtually unresponsive to acetic acid. Interestingly, CT nerve responses to acetic acid/NaCl mixtures were less than the sum of responses to each stimulus component, indicating that either acetic acid or NaCl decreased the responses of the other. In NaCl specialists, responses to NaCl/acetic acid mixtures were less than the sum of responses to each component, but there was no apparent concentration dependent effect of acetic acid evoked inhibition. In contrast, responses to acetic acid/NaCl mixtures in acid generalists were additive, equal to the sum of responses to each stimulus component. Together, results from both studies indicate that NaCl specialists respond to sodium salts through apical ENaCs and are attenuated with acetic acid, whereas acid-generalists responded to sodium salts and acetic acid through separate mechanisms whose origins are still largely unknown. The pronounced "anion effect" in acid generalists indicates that this unknown receptor site is not apical, rather it is most likely located deeper within the epithelium--below the tight-junctional barrier.