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The Journal of Neurophysiology Vol. 80 No. 4 October 1998,
pp. 1852-1859
Copyright ©1998 The American Physiological Society
Department of Physiology, Nagasaki University School of Dentistry, Nagasaki 852-8588, Japan
Miyamoto, Takenori, Rie Fujiyama, Yukio Okada, and Toshihide Sato. Sour transduction involves activation of NPPB-sensitive conductance in mouse taste cells. J. Neurophysiol. 80: 1852-1859, 1998. We examined the sour taste transduction mechanism in the mouse by applying whole cell patch-clamp technique to nondissociated taste cells from the fungiform papillae. Localized stimulation with 0.5 M NaCl and 25 mM citric acid (pH 3.0) of the apical membrane enabled us to obtain responses from single taste cells under a quasi-natural condition. Of 28 taste cells examined, 11 cells (39%) responded to 0.5 M NaCl alone and 2 cells (7%) responded to 25 mM citric acid alone, indicating the presence of salty- and sour-specific taste cells. Ten cells (36%) responded to both NaCl and citric acid and 5 cells (18%) responded to neither salt nor citric acid. Amiloride reversibly suppressed NaCl-induced responses in mouse taste cells but not citric acid-induced responses. On the other hand, a Cl
channel blocker, 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB), reversibly suppressed all the citric-acid-induced responses. Most of the NaCl-induced current responses displayed an inwardly rectifying property, whereas all the citric-acid-induced responses displayed an outwardly rectifying property. The reversal potential for NPPB-sensitive component in citric-acid-induced current responses was 
2 ± 7 mV (mean ± SE, n = 4), which was close to the equilibrium potential of Cl (ECl), whereas the reversal potential for NPPB-insensitive component was 34 ± 8 mV (n = 4). The reversal potential of citric-acid-induced current responses (19 ± 8 mV, n = 4) was mostly present at the middle point between reversal potentials of NPPB-sensitive and -insensitive current components. In some taste cells, an inorganic cation channel blocker, Cd2+, suppressed citric-acid-induced responses, but an inorganic stretch-activated cation channel blocker, Gd3+, did not affect these responses. These results suggest that salt- and acid-induced responses were mediated by differential transduction mechanisms in mouse taste cells and that NPPB-sensitive Cl channels play a more important role to sour taste transduction rather than amiloride-sensitive Na+ channels. However, the fact that the reversal potentials of citric-acid-induced responses had more positive than ECl suggests that Ca2+ or H+ permeable and poorly selective cation channels, which should be amiloride insensitive, may be activated by citric acid.
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