Amiloride-Insensitive Salt Taste Is Mediated by Two Populations of Type III Taste Cells with Distinct Transduction Mechanisms

Abstract

Responses in the amiloride-insensitive (AI) pathway, one of the two pathways mediating salty taste in mammals, are modulated by the size of the anion of a salt. This "anion effect" has been hypothesized to result from inhibitory transepithelial potentials (TPs) generated across the lingual epithelium as cations permeate through tight junctions and leave their larger and less permeable anions behind (Ye et al., 1991). We tested directly the necessity of TPs for the anion effect by measuring responses to NaCl and Na-gluconate (small and large anion sodium salts, respectively) in isolated taste cells from mouse circumvallate papillae. Using calcium imaging, we identified AI salt-responsive type III taste cells and demonstrated that they compose a subpopulation of acid-responsive taste cells. Even in the absence of TPs, many (66%) AI salt-responsive type III taste cells still exhibited the anion effect, demonstrating that some component of the transduction machinery for salty taste in type III cells is sensitive to anion size. We hypothesized that osmotic responses could explain why a minority of type III cells (34%) had AI salt responses but lacked anion sensitivity. All AI type III cells had osmotic responses to cellobiose, which were significantly modulated by extracellular sodium concentration, suggesting the presence of a sodium-conducting osmotically sensitive ion channel. However, these responses were significantly larger in AI type III cells that did not exhibit the anion effect. These findings indicate that multiple mechanisms could underlie AI salt responses in type III taste cells, one of which may contribute to the anion effect.

The Perceptual Characteristics of Sodium Chloride to Sodium-Depleted Rats

“…However, because NaCl also activates an amiloride-insensitive response in gustatory afferent nerves (Breza and Contreras 2012;Formaker and Hill 1988;Hettinger and Frank 1990;Ninomiya and Funakoshi 1988), additional transduction mechanisms exist. These additional mechanisms may show roughly equal affinity for sodium and potassium salts, but appear to be anion dependent, such that salts such as sodium gluconate may have lower affinity for these additional mechanisms (Ye et al 1991;Simon 1992;Rehnberg et al 1993;Ye et al 1993;Elliott and Simon 1990;Breza and Contreras 2012;Lewandowski et al 2016). Thus, our Experiment 2 was designed to probe the role of amiloride-sensitive and amiloride-insensitive transduction pathways in the expression of a sodium appetite, both by mixing NaCl with amiloride and by using salts with relatively exclusive potency for amiloride-sensitive mechanisms (sodium gluconate) or amilorideinsensitive mechanisms (KCl).…”

“…John and Smith 2000a;Wu et al 2015), this result is consistent with the suggestion (Oka et al 2013) that amiloride-insensitive pathways communicate an aversive signal at high concentrations of salt. However, this conclusion hinges on whether sodium gluconate truly activates the amiloride-sensitive pathway more or less exclusively; recent data suggest this may not be the case (St. John and Smith 2000b;Breza and Contreras 2012;Lewandowski et al 2016).…”

“…Likewise, the suggestion that ENaC-elicited activity contributes to a positive hedonic evaluation and non-EnaC-elicited activity contributes to a negative hedonic evaluation (Oka et al 2013) is consistent with the greater licking response to high concentrations of sodium gluconate over NaCl in our Experiment 2. However, this conclusion depends on the assumption that sodium gluconate does not activate non-ENaC transduction mechanisms that are activated by NaCl, a conclusion that is at odds with some electrophysiological data (St. John and Smith 2000b;Breza and Contreras 2012;Lewandowski et al 2016). This conclusion is also at odds with the observation that, while reduced, licking for NaCl was not completely eliminated by 300 μM amiloride.…”

“…ENaCs in taste receptor cells are one of multiple transduction pathways for sodium salts and may be particularly important for the discrimination of sodium from nonsodium salts (Hill et al 1990;Spector et al 1996;Kopka et al 2000) and for the recognition of sodium in sodium appetite (Bernstein and Hennessy 1987;McCutcheon 1991). Sodium and lithium salts (but less so, other nonsodium salts) with large anions, like sodium gluconate, are thought to be transduced primarily through ENaCs, whereas NaCl is transduced via multiple mechanisms including those shared by nonsodium salts like KCl (Heck et al 1984;DeSimone and Ferrell 1985;Ninomiya and Funakoshi 1988;Elliott and Simon 1990;Hettinger and Frank 1990;Ye et al 1991;Simon 1992;Ye et al 1993;DeSimone et al 2001;Breza and Contreras 2012;Lewandowski et al 2016).…”

Epithelial cell behaviours during neurosensory organ formation

“…They include Type I (support), Type II (taste receptor, sensing bitter, sweet, umami) and Type III [5-hydroxytryptamine (5-HT)/sour and amiloride-insensitive salt sensing] cells (Chandrashekar et al, 2006(Chandrashekar et al, , 2009(Chandrashekar et al, , 2010Chaudhari and Roper, 2010;Lewandowski et al, 2016;Oka et al, 2013;Roper, 2013Roper, , 2015. In the majority of vertebrates, taste buds are located in the oropharyngeal cavity and, together with teeth, contribute to food assessment and processing.…”