The Journal of Neurophysiology Vol. 77 No. 3 March 1997, pp. 1393-1406
Copyright ©1997 The American Physiological Society

Chorda Tympani Responses Under Lingual Voltage Clamp: Implications for NH₄ Salt Taste Transduction

Mamoun A. Kloub, Gerard L. Heck, and John A. Desimone

Department of Physiology, Virginia Commonwealth University, Richmond, Virginia 23298-0551

Kloub, Mamoun A., Gerard L. Heck, and John A. DeSimone. Chorda tympani responses under lingual voltage clamp: implications for NH₄ salt taste transduction. J. Neurophysiol. 77: 1393-1406, 1997. Rat chorda tympani (CT) responses to NH₄Cl, ammonium acetate (NH₄Ac), and ammonium hippurate (NH₄Hp) were obtained during simultaneous current and voltage clamping of the lingual field potential. Although functional and developmental similarities for gustation have been reported for NH⁺₄ and K⁺ salts, we report here that significant differences are discernible in the CT responses to both salts. Unlike neural responses to KCl, those to NH₄Cl are voltage sensitive, enhanced by submucosa negative and suppressed by positive voltage clamp. In this regard, NH₄Cl responses are qualitatively similar to NaCl responses; however, the magnitude of NH₄Cl voltage sensitivity is significantly less than that of NaCl. The concentration dependence of the CT response to NH₄Cl manifests a biphasic nonlinear relationship not observed with KCl or NaCl. Below 0.3 M, the CT response increases as if to approach a saturation value. However, beyond 0.3 M an inflection appears in the CT-concentration curve because of an abrupt increase in CT responses. This kinetic profile is Cl dependent and is correlated with an increase in transepithelial conductance that displays similar NH₄Cl concentration dependence. The biphasic relation to salt concentration is not observed when acetate or hippurate is substituted for Cl. As with Na⁺ and K⁺ salts, less mobile anions than Cl (Ac and Hp) lower the CT responses. However, like Na⁺ salts, but in contrast to K⁺ salts, the onset kinetics of CT responses to NH₄Ac or NH₄Hp remain rapid, even under positive voltage-clamp conditions. Amiloride (100 µM) partially suppresses CT responses within the concentration range of 0.05-0.3 M (48-20% suppression). Amiloride also suppresses the voltage sensitivity of NH₄Cl CT responses, but does not eliminate that sensitivity as it does for Na⁺ salts. In conclusion, the data suggest that taste transduction for NH₄ salts is mediated over two NH⁺₄ conduction pathways in the taste bud. This is especially evident with NH₄Cl, where the CT-concentration curves show two distinct kinetic regimes. Below 0.3 M the saturation with increasing concentration, clamp voltage response dependence, and amiloride sensitivity suggest an apical membrane transduction conductance. Above 0.3 M, the high anion dependence of the response and its amiloride insensitivity indicate participation of the paracellular pathway in transduction.

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