Taste buds contain two types of taste receptor cells, inositol 1,4,5-triphosphate receptor type 3-immunoreactive cells (type II cells) and synaptosomal-associating protein-25-immunoreactive cells (type III cells). We investigated their postnatal development in mouse fungiform taste buds immunohistochemically and electrophysiologically. The cell density, i.e. the number of cells per taste bud divided by the maximal area of the horizontal cross-section of the taste bud, of type II cells increased by postnatal day (PD)49, where as that of type III cells was unchanged throughout the postnatal observation period and was equal to that of the adult cells at PD1. The immunoreactivity of taste bud cell subtypes was the same as that of their respective subtypes in adult mice throughout the postnatal observation period. Almost all type II cells were immunoreactive to gustducin at PD1, and then the ratio of gustducin-immunoreactive type II cells to all type II cells decreased to a saturation level, ∼60% of all type II cells, by PD15. Type II and III cells generated voltage-gated currents similar to their respective adult cells even at PD3. These results show that infant taste receptor cells are as excitable as those of adults and propagate in a subtype-dependent manner. The relationship between the ratio of each taste receptor cell subtype to all cells and taste nerve responses are discussed.

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.

No abstract

No abstract