Hyperpolarization-activated channels HCN1 and HCN4   mediate responses to sour stimuli.
Sour taste is initiated by protons acting at receptor proteins or channels. In vertebrates, transduction of this taste quality involves several parallel pathways. 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 Ih found in neurons and cardio-myocytes, a current carried by members of the family of hyperpolarization-activated and cyclic-nucleotide-gated (HCN) channels. 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 taste, 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.
Stevens
D R
DR
Department of Physiology, Universität des Saarlandes, Homburg, Germany.
Seifert
R
R
Bufe
B
B
Müller
F
F
Kremmer
E
E
Gauss
R
R
Meyerhof
W
W
Kaupp
U B
UB
Lindemann
B
B
D000818:Animals; D002460:Cell Line; D054815:Cyclic Nucleotide-Gated Cation Channels; D018076:DNA, Complementary; D006801:Humans; D006863:Hydrogen-Ion Concentration; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D007150:Immunohistochemistry; D017403:In Situ Hybridization; D066298:In Vitro Techniques; D015640:Ion Channel Gating; D007473:Ion Channels / Q000235:genetics / Q000502:physiology*; D051379:Mice; D009124:Muscle Proteins; D009419:Nerve Tissue Proteins; D015221:Potassium Channels; D012333:RNA, Messenger / Q000032:analysis; D051381:Rats; D017208:Rats, Wistar; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013649:Taste / Q000502:physiology*; D013650:Taste Buds / Q000502:physiology*; D015293:Transducin / Q000378:metabolism
D054815:Cyclic Nucleotide-Gated Cation Channels; D018076:DNA, Complementary; C578665:HCN1 protein, human; C119587:HCN4 protein, human; C578666:Hcn1 protein, mouse; C578667:Hcn1 protein, rat; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D007473:Ion Channels; D009124:Muscle Proteins; D009419:Nerve Tissue Proteins; D015221:Potassium Channels; D012333:RNA, Messenger; C075192:gustducin; D015293:Transducin