A subset of taste receptor cells express biocytin‐permeable channels activated by reducing extracellular Ca 2+ concentration

Abstract Taste receptor cells (type II cells) transmit taste information to taste nerve fibres via ATP‐permeable channels, including calcium homeostasis modulator (CALHM), connexin and/or pannexin1 channels, via the paracrine release of adenosine triphosphate (ATP) as a predominant transmitter. In the present study, we demonstrate that extracellular Ca 2+ ‐dependent biocytin‐permeable channels are present in a subset of type II cells in mouse fungiform taste buds using biocytin uptake, immunohistochemistry and in situ whole‐cell recordings. Type II cells were labelled with biocytin in an extracellular Ca 2+ concentration ([Ca 2+ ] out )‐sensitive manner. We found that the ratio of biocytin‐labelled type II cells to type II cells per taste bud was approximately 20% in 2 mM Ca 2+ saline, and this ratio increased to approximately 50% in nominally Ca 2+ ‐free saline. The addition of 300 µM GdCl 3 , which inhibits various channels including CALHM1 channels, significantly inhibited biocytin labelling in nominally Ca 2+ ‐free saline, whereas the addition of 20 µM ruthenium red did not. Moreover, Cs + ‐insensitive currents increased in nominally Ca 2+ ‐free saline in approximately 40% of type II cells. These increased currents appeared at a potential of above −35 mV, reversed at approximately +10 mV and increased with depolarization. These results suggest that biocytin labels type II cells via ion channels activated by [Ca 2+ ] out reduction, probably “CALHM‐like” channels, on the basolateral membrane and that taste receptor cells can be categorized into two groups based on differences in the expression levels of [Ca 2+ ] out ‐dependent biocytin‐permeable channels. These data indicate electrophysiological and pharmacologically relevant properties of biocytin‐permeable channels and suggest their contributions to taste signal transduction.