Acid-sensing ion channel-2a (ASIC2a) is a proton-gated cation channel, probably contributing to sour-taste detection in rat taste cells. We found the transcripts of ASIC2b in the circumvallate, foliate and fungiform papillae. Immunohistochemical analyses also revealed that ASIC2b, as well as ASIC2a, was expressed in a subpopulation of taste cells, and some of the cells displayed both ASIC2a and ASIC2b immunoreactivities. Coimmunoprecipitation studies with circumvallate papillae extracts indicated that ASIC2b associated with ASIC2a to form assemblies. Oocyte electrophysiology demonstrated that the ASIC2a/ASIC2b channel generated amiloride-insensitive currents at pH 2.0. These findings provide persuasive explanations for the amiloride insensitivity of acid-induced responses of rat taste cells. Acid-sensing ion channels (ASICs) also mediate acid-induced nociception in mammals. In our psychophysical experiments, direct infusion of acidic solutions (pH > or = 6.0) into human skin caused localized pain, which was blocked by amiloride, an inhibitor of ASICs, but not by capsazepine, an inhibitor of TRPV1. Under more severe acidification (pH 5.0) amiloride was less effective in reducing acid-evoked pain. In addition, capsazepine had a partial blocking effect under these conditions. Amiloride itself did not block capsaicin-evoked localized pain. Our results suggest that ASICs (ASIC1a and ASIC3) are leading acid sensors in human nociceptors and that TRPV1 participates in the nociception mainly under extremely acidic conditions.