Analysis of NaCl-LiCl Taste Discrimination Using Electrophysiological and Behavioral Methods

The detection of salty taste stimuli depends on two salt-sensing transduction pathways that involve membrane channels on the surface of fungiform taste bud cells located on the anterior tongue in rats. These two pathways are the transcellular, amiloride-sensitive pathway and paracellular, amiloride-insensitive pathway. The transcellular is specific to NaCl and LiCl. Past studies have shown that LiCl and NaCl generate specific responses from taste nerves and that they are indistinguishable in behavioral tests. To this end, we generated the first dose-response curve comparing various concentrations of NaCl and LiCl and performed brief-access behavioral tests in order to determine if there is differential firing of the chorda tympani nerve to NaCl and LiCl stimulation and whether rats can discriminate between the two salts. We found that CT responses are higher for LiCl and NaCl for most concentrations used. This same trend was found when the salts were adulterated with amiloride. Measurements of the phasic portion of the CT response showed a larger amiloride-sensitive portion to the CT nerve response to NaCl than to LiCl. This difference may be sufficient to mediate LiCl/NaCl discrimination. We also found that rats were able to distinguish 30 mM LiCl from 30 mM NaCl, 30 mM LiCl from water, and 75 mM LiCl from water. These results hint to the possibility that with particular concentrations and particular tests, rats should be able to discriminate between NaCl and LiCl. Our results are similar to past research showing that LiCl responses are greater than NaCl responses. Our findings also suggest that there is differential signaling between NaCl and LiCl at the level of the whole nerve.