Antagonism between the suppressive effects of NH3 and CO2 on bullfrog taste nerve responses to quinine.

The suppression mechanism of NH3 and CO2 on bullfrog taste nerve responses to 0.1 mM quinine was investigated by applying them directly on the tongue surface (surface application) or at the tongue interstices through the lingual artery (interstitial application). The surface application of NH3 and CO2 reversibly suppressed the taste nerve responses with IC50 values of 0.37 mM and 2.2 mM, respectively, whereas their hydrates were ineffective. The interstitial application of NH3 reversibly suppressed the taste nerve responses. The surface application of CO2 recovered the suppressed responses to quinine. The 4 s application of CO2 prior to that of 0.1 mM quinine had the maximum effect. These results show that NH3-induced alkalosis and CO2-induced acidosis of taste cells, taste nerve endings, or tongue interstices surrounding them suppressed the taste nerve responses, and that the neutralization of their intracellular pH recovered these responses. The time-dependent recovery suggests that the neutralization at a layer inside taste cells, taste nerve endings, or the interstices surrounding them is critical in taste transduction.