1. Department of Physiology, Universität des Saarlandes, D-66421 Homburg, Germany
2. Institute for Biological Information Processing, Forschungszentrum Jülich, D-52425 Jülich, Germany
3. Department of Molecular Genetics, Deutsches Institut für Ernährungsforschung, Arthur-Scheunert-Allee 114-116, D-14558 Potsdam-Rehbrücke, Germany
4. Institute for Molecular Immunology, GSF, D-81377 München, Germany

Correspondence to: Bernd Lindemann¹ Correspondence and requests for materials should be addressed to B.L. (e-mail: Email: phblin@uniklinik-saarland.de).

Abstract

Sour taste is initiated by protons acting at receptor proteins or channels. In vertebrates, transduction of this taste quality involves several parallel pathways^{1, 2, 3, 4, 5}. Here we examine the effects of sour stimuli on taste cells in slices of vallate papilla from rat. From a subset of cells, we identified a hyperpolarization-activated current that was enhanced by sour stimulation at the taste pore. This current resembled I_h found in neurons and cardio-myocytes^{6, 7}, a current carried by members of the family of hyperpolarization-activated and cyclic-nucleotide-gated (HCN) channels^{8, 9, 10, 11, 12, 13}. We show by in situ hybridization and immunohistochemistry that HCN1 and HCN4 are expressed in a subset of taste cells. By contrast, gustducin, the G-protein involved in bitter and sweet taste¹⁴, is not expressed in these cells. Lowering extracellular pH causes a dose-dependent flattening of the activation curve of HCN channels and a shift in the voltage of half-maximal activation to more positive voltages. Our results indicate that HCN channels are gated by extracellular protons and may act as receptors for sour taste.