Taste Cells of the Type III Employ CASR to Maintain Steady Serotonin Exocytosis at Variable Ca2+ in the Extracellular Medium

Abstract

Type III taste cells are the only taste bud cells which express voltage-gated (VG) Ca²⁺ channels and employ Ca²⁺-dependent exocytosis to release neurotransmitters, particularly serotonin. The taste bud is a tightly packed cell population, wherein extracellular Ca²⁺ is expected to fluctuate markedly due to the electrical activity of taste cells. It is currently unclear whether the Ca²⁺ entry-driven synapse in type III cells could be reliable enough at unsteady extracellular Ca². Here we assayed depolarization-induced Ca²⁺ signals and associated serotonin release in isolated type III cells at varied extracellular Ca²⁺. It turned out that the same depolarizing stimulus elicited invariant Ca²⁺ signals in type III cells irrespective of bath Ca²⁺ varied within 0.5-5 mM. The serotonin release from type III cells was assayed with the biosensor approach by using HEK-293 cells co-expressing the recombinant 5-HT4 receptor and genetically encoded cAMP sensor Pink Flamindo. Consistently with the weak Ca²⁺ dependence of intracellular Ca²⁺ transients produced by VG Ca²⁺ entry, depolarization-triggered serotonin secretion varied negligibly with bath Ca²⁺. The evidence implicated the extracellular Ca²⁺-sensing receptor in mediating the negative feedback mechanism that regulates VG Ca²⁺ entry and levels off serotonin release in type III cells at deviating Ca²⁺ in the extracellular medium.

Keywords: extracellular Ca2+-sensing receptor; serotonin biosensor; serotonin secretion; taste cells; voltage-gated Ca2+ channels.