Quantitative evaluation of sweet-responsive type II cells in mouse taste buds via biocytin uptake

Sweet taste is mediated by type II taste bud cells (TBCs), which express the heterodimeric taste receptor composed of type 1 members 2 and 3, a G protein-coupled receptor. Activating this receptor triggers phospholipase Cβ2 (PLCβ2)-dependent signaling, depolarizes cell membrane, and leads to ATP release via calcium homeostasis modulator 1 and 3 channels. However, the number of sweet-responsive cells within individual fungiform taste buds remains poorly understood. To quantify the number of sweet-responsive TBCs, we developed a novel method using biocytin uptake as an indicator of membrane depolarization. The apical side of peeled mouse lingual epithelia was stimulated with 1 M sucrose or 30 mM saccharin, while biocytin was applied to the basolateral side. Sweet stimulation significantly increased the number of biocytin-labeled cells compared to deionized-water controls. Biocytin labeling was observed primarily in PLCβ2-positive type II cells, with additional labeling in PLCβ2 and synaptosomal-associated protein 25-negative cells, suggesting the involvement of type II and, likely, type I cells. On average, 11% of type II cells per taste bud were sweet-responsive; however, this proportion varied substantially across individual taste buds. These results indicate that sweet-responsive cells form a subset of type II cells and are distributed heterogeneously among fungiform taste buds. Such heterogeneity may reflect divergent tuning properties and contribute to robust sweet taste perception. Given the short lifespan and continuous turnover of TBCs, asynchronous renewal of sweet-responsive cells across taste buds may help maintain sweet sensitivity by ensuring that some sweet-sensitive cells are consistently present.