Functional characterization of Type IV basal cells in rat fungiform taste buds.

Taste buds, the end organs of taste, consist of a diverse population of sensory cells that is constantly renewed. Cell differentiation begins with Type IV basal cells, which are ovoid elements located inside the taste bud near its base. These cells are postmitotic precursors that give rise to all other cell types, including glial-like cells (Type I cells) and chemoreceptors (Type II and Type III cells). Despite their critical role in cell turnover, Type IV basal cells are relatively unknown in terms of functional features. Here, we used Lucifer yellow labeling and patch-clamp technique to investigate their electrophysiological properties in the rat fungiform taste buds. All Type IV basal cells showed voltage-gated sodium currents (INa), albeit at a far lower density (17 pA/pF) than chemoreceptors (444 pA/pF), which fire action potentials during sensory transduction. Furthermore, they lacked calcium homeostasis modulator currents, which are required for neurotransmitter release by some chemoreceptor types. Amiloride-sensitive epithelial sodium channel (ENaC) was found to be only present in a subset of Type IV basal cells. Interestingly, Type IV basal cells shared some membrane features with glial-like cells, such as high cell capacitance and low INa density; however, input resistance was greater in Type IV basal cells than in glial-like cells. Thus, although Type IV basal cells may eventually differentiate into distinct cell lineages, our findings indicate that they are quite homogeneous in terms of the electrophysiological characteristics, with the exception of functional ENaCs, which appear to be only expressed in one subset.