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 sal ty of all salts, but even this stimulus tastes sweet at low concentrat ions and somewhat sour at mid-range intensities. Other salts taste sig nificantly sour or bitter in addition to salty. Previous studies have shown that the saltiness of simple halide salts is reduced by adaptati on to NaCl, suggesting that a single mechanism might be responsible fo r 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 heav ily dependent on this amiloride-sensitive transduction component than NaCl, and are generally less salty and more sour. In order to investig ate the relationship between NaCl saltiness and that evoked by other s alts, we adapted the tongue to distilled H2O and to 0.1 M NaCl and obt ained 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, a nd eliminated the saltiness of all other salts. The magnitude estimate s of the bitterness and sourness of many salts increased after NaCl ad aptation. Since recent biophysical data suggest that adaptation in tas te receptors may involve whole-cell mechanisms, we propose that saltin ess 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 mode l proposes, for example, that KCl has a salty component because it sti mulates some of the same receptor cells as NaCl, even though the trans duction mechanisms for KCl are different than those engaged by NaCl. A daptation to NaCl blocks the saltiness of KCl and other salts because they stimulate NaCl-sensitive receptor cells.