The perception of saltiness is eliminated by NaCl adaptation: implications for gustatory transduction and coding

The tastes of salts to humans are complex. NaCl is the most purely salty of all salts, but even this stimulus tastes sweet at low concentrations and somewhat sour at mid-range intensities. Other salts taste significantly sour or bitter in addition to salty. Previous studies have shown that the saltiness of simple halide salts is reduced by adaptation to NaCl, suggesting that a single mechanism might be responsible for the salty taste of these stimuli. In electrophysiological studies in rodents, the response to NaCl is reduced by application to the tongue of the Na(+)-channel blocker amiloride. Organic Na+ salts are more heavily dependent on this amiloride-sensitive transduction component than NaCl, and are generally less salty and more sour. In order to investigate the relationship between NaCl saltiness and that evoked by other salts, we adapted the tongue to distilled H2O and to 0.1 M NaCl and obtained direct magnitude estimates of the taste intensity of 15 organic and inorganic Na+, Li+, K+ and Ca2+ salts, matched for total intensity. Subjects divided these magnitude estimates among the component taste qualities. Adaptation to NaCl abolished the taste of NaCl and LiCl, and eliminated the saltiness of all other salts. The magnitude estimates of the bitterness and sourness of many salts increased after NaCl adaptation. Since recent biophysical data suggest that adaptation in taste receptors may involve whole-cell mechanisms, we propose that saltiness is reduced by NaCl adaptation because it originates in the subset of taste receptors responsive to NaCl. This implies that saltiness is coded within the CNS in cells whose receptive fields include the NaCl-sensitive receptor cells and that the degree to which any salt tastes salty is determined by its ability to drive these receptors. This model proposes, for example, that KCl has a salty component because it stimulates some of the same receptor cells as NaCl, even though the transduction mechanisms for KCl are different than those engaged by NaCl. Adaptation to NaCl blocks the saltiness of KCl and other salts because they stimulate NaCl-sensitive receptor cells.