A new method for assessing salt taste; Maturation of the salt sensing mechanism

The tongue is the main sensory element in taste. However, the actual perception of taste is complex and involves input from other systems as well as complex processing in the brain. This and animal habituation complicates the interpretation of behavioral experiments aimed at understanding single mode taste, e.g., salt. To circumvent these issues and to test the role of the Epithelial Na+ Channel in salt taste, we developed an isolated tongue chamber that allows measurement of electrical parameters across the short-circuited and intact lingual epithelium. Using impedance analysis we demonstrate that the lingual epithelium is polarized and electrically coupled. Two time constants are observed in the Nyquist impedance spectra consistent with that expected from a series arrangement of an apical and a basolateral membrane. Cellular resistances averaged 2993±272 Ωcm2 (n=37). A series resistance of 707±25 Ωcm2 (n=37) was also observed. This series parameter was purely resistive consistent with that expected from the non-coupled and non-polarized underlying muscle and connective tissue. Using this approach, we examined the response of the epithelium to [NaCl] in the range of 100-500 mM. We report that both Isc and Rcell are sensitive to apical NaCl with 500 mM eliciting amiloride sensitive changes to these parameters indicating a role of an amiloride blockable epithelial Na+ Channel in this process. To determine the role of ENaC in salt taste development, we examined the responses of epithelia from animals in the first 23 days of life as well as adult mother. We report that the effect of external Na+ on Isc and Rcell varied during the early period of life with responses approaching adult levels after day 17. These changes were accompanied by increased expression and proteolytic processing of alpha and gamma ENaC, along with increased amiloride sensitivity. All together, our results support a hypothesis in which the entire lingual epithelium participates in sensing external salt. These data further indicate that biochemical and functional changes of ENaC expression and activity as an underlying process to salt taste development.