FIGURE 7.

Mechanism of transduction of sour taste. OTOP1 is recognized as the sour receptor and conducts the H⁺ ions (protons) from acids into the cell cytosol. The influx of cations causes the membrane potential to change direction, and the change in intracellular pH blocks KIR2.1 K⁺ channels, which further depolarizes the membrane potential. With sufficient depolarization, voltage-gated Na⁺ channels open causing a train of action potentials that open voltage-gated calcium channels and lead to release of the neurotransmitter 5-HT. 5-HT released at the synapses activates the afferent nerve fibers via excitatory 5-HT₃ receptors (R). Image created with BioRender.com, with permission.