Effects of inorganic constituents of saliva on taste responses of the rat chorda tympani nerve

The effects of saliva on the taste responses of the chorda tympani nerve to the 4 standard chemical stimuli (sucrose, NaCl, HCl, and quinine hydrochloride) and water were investigated in anesthetized rats. When the tongue was adapted to pilocarpine-stimulated whole saliva (pH 8.7), the magnitude of neural response to sucrose was about 2 times that obtained when the tongue was adapted to distilled water. Under saliva-adapted conditions, the magnitude of responses to other taste stimuli was reduced by 10–30%, and the water response appeared. These changes were dependent on the concentration of electrolytes (Na⁺, K⁺, Cl⁻, and HCO₃⁻) and on the pH of the saliva. When the tongue was adapted to 10–30 mM NaHCO₃ (pH 8.4–8.6), taste and water responses were similar to those under saliva-adapted conditions. Single fiber analyses revealed that the enhancement of the sucrose response after adaptation to NaHCO₃ was produced by an increased overall activity of sucrose-responsive fibers. The correlation coefficients of the magnitude of the taste responses between the 4 taste stimuli remained unchanged, but the water response showed a high correlation to HCl and quinine hydrochloride responses after adaptation. Possible mechanisms for the effects of saliva on taste and water responses were discussed.