A subset of taste receptor cells express biocytin‐permeable channels activated by reducing extracellular Ca²⁺ 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. K E Y W O R D Sbiocytin uptake, fungiform papillae, immunohistochemical staining, outwardly rectifying currents, patchclamping 1606 | IWAMOTO eT Al.

Age‐related electrophysiological changes in mouse taste receptor cells

“…Among the 24 cells investigated for voltage‐gated currents, we identified 13 cells with immunohistostaining: type II cells, n = 5; type III cells, n = 3; and non‐IRCs, n = 5. We observed that the electrophysiological features of type II and type III cells in aged mice were similar to those in young mice reported in our previous study (Iwamoto et al., 2020; Kimura et al., 2014). Thus, type II cells expressed mainly TEA‐insensitive and slowly activating outwardly rectifying currents and generated tail currents in repolarization (Figures 2 and 3) and type III cells expressed TEA‐sensitive and faster activating K + currents and did not generate tail currents (Figures 2 and 3).…”

“…Additionally, the sum of these currents, which was the total magnitude of outwardly rectifying currents generated on a depolarization to +50 mV from a holding potential of −70 mV, was similar among the cell types. These cell type‐specific characteristics in aged mice are similar to their respective cell types in neonate and young mice (Iwamoto et al., 2020; Kimura et al., 2014; Ohtubo et al., 2012).…”

“…TEA‐insensitive current channels are probably permeable to ATP. We showed previously that TEA‐insensitive current channels were permeable to a variety of anions with relative molecular mass <1200 and biocytin (Iwamoto et al., 2020; Takeuchi, Seto, Ohtubo, & Yoshii, 2011). The ATP‐permeable hemichannel, which is a TEA‐insensitive channel, is more permeable to ATP than Mg 2+ (Romanov et al., 2008).…”

“…The electrophysiological properties of elongated cells also differ in ∼2‐month‐old mice (Iwamoto, Takashima, & Ohtubo, 2020; Kimura et al., 2014). Most outwardly rectifying currents in type II cells were tetraethylammonium (TEA)‐ and Cs + ‐insensitive currents that pass through voltage‐gated hemichannels and CALHM channels.…”

“…Most outwardly rectifying currents in type II cells were tetraethylammonium (TEA)‐ and Cs + ‐insensitive currents that pass through voltage‐gated hemichannels and CALHM channels. Further, the outwardly rectifying currents in type II cells were slowly activated at positive membrane potentials and type II cells generated tail currents through repolarization (Iwamoto et al., 2020; Kimura et al., 2014). In contrast, most currents in type III cells were TEA‐ and Cs + ‐sensitive currents that passed through voltage‐gated K + channels.…”