Cell‐type‐dependent action potentials and voltage‐gated currents in mouse fungiform taste buds

Abstract

Taste receptor cells fire action potentials in response to taste substances to trigger non-exocytotic neurotransmitter release in type II cells and exocytotic release in type III cells. We investigated possible differences between these action potentials fired by mouse taste receptor cells using in situ whole-cell recordings, and subsequently we identified their cell types immunologically with cell-type markers, an IP3 receptor (IP3 R3) for type II cells and a SNARE protein (SNAP-25) for type III cells. Cells not immunoreactive to these antibodies were examined as non-IRCs. Here, we show that type II cells and type III cells fire action potentials using different ionic mechanisms, and that non-IRCs also fire action potentials with either of the ionic mechanisms. The width of action potentials was significantly narrower and their afterhyperpolarization was deeper in type III cells than in type II cells. Na(+) current density was similar in type II cells and type III cells, but it was significantly smaller in non-IRCs than in the others. Although outwardly rectifying current density was similar between type II cells and type III cells, tetraethylammonium (TEA) preferentially suppressed the density in type III cells and the majority of non-IRCs. Our mathematical model revealed that the shape of action potentials depended on the ratio of TEA-sensitive current density and TEA-insensitive current one. The action potentials of type II cells and type III cells under physiological conditions are discussed.

A subset of taste receptor cells express biocytin‐permeable channels activated by reducing extracellular Ca2+ concentration

“…In the presence of type II cells expressing biocytin‐permeable channels, enhanced channel currents are anticipated by reducing [Ca 2+ ] out . To confirm this, we used Cs + ‐containing pipette solution because the principal component of outwardly rectifying channel currents in type II cells was insensitive to the K channel blockers Cs + and TEA (Kimura et al, ).…”

“…Voltage‐gated currents of fungiform TBCs were assessed under in situ whole‐cell clamp conditions as previously described (Higure et al, ; Kimura et al, ; Ohtubo, Iwamoto, & Yoshii, ). Briefly, peeled epithelia were mounted on a recording platform and then were examined under a microscope (BX50; Olympus corporation, Tokyo, Japan) equipped with a 60 × water‐immersion objective.…”

“…After biocytin uptake or patch‐clamp experiments, we stained the peeled epithelium with cell‐type specific antibodies to identify cell types among the biocytin‐labelled or electrophysiologically recorded TBCs as previously described (Kimura et al, ; Mori, Eguchi, Yoshii, & Ohtubo, ; Takeuchi et al, ). In these experiments, we used the cell‐type markers PLCβ2 or IP 3 R3 for type II cells and synaptosomal‐associated protein 25 (SNAP‐25) for type III cells according to previous reports (Clapp, Stone, Margolskee, & Kinnamon, ; Clapp et al, ; Hayato, Ohtubo, & Yoshii, ; Ohtubo & Yoshii, ; Yang, Crowley, Rock, & Kinnamon, ).…”

“…In the present study, we employed biocytin (MW, 372; net charge, 0) because it does not modify voltage‐gated channels and has been widely used to study uptake mechanisms in cells (Baur, Suormala, & Baumgartner, ) and to label neurons or TBCs examined with patch‐clamp experiments (Kawaguchi, ; Kimura et al, ; Wellis & Kauer, ). To investigate the electrophysiological and pharmacologically relevant properties of a large pore‐forming ion channel that are expressed on type II cells for ATP secretion, we performed biocytin uptake, immunohistochemical and electrophysiological analyses.…”

Selective expression of muscarinic acetylcholine receptor subtype M3 by mouse type III taste bud cells

“…We removed their tongues and hypodermically injected an elastase solution into the tongue, before peeling the lingual epithelia containing fungiform taste buds. The peeled epithelium was mounted on a recording platform with the basolateral membrane side facing upward and then placed under a 60× water immersion objective (Fluor-60×, Olympus, Tokyo, Japan), as described previously [8, 10, 20]. The basolateral membrane side of the peeled epithelium was irrigated with either a control bathing solution or a test solution by exchanging the composition of the water column between the water immersion objective and the basolateral membrane side.…”