The sour taste channel OTOP1 is located in type III taste bud cells where it transports H+ ions from the extracellular region into the cytosol (reviewed in Liman and Kinnamon 2021). Organic acids such as acetic acid and citric acid are believed to also enter type III taste bud cells by passive diffusion of the protonated (uncharged) form of the acid across the plasma membrane. The increase in cytosolic H+ ions inhibits the KCNJ2 potassium channel and may also open unidentified sodium channels to further depolarize the cell. The resulting depolarization is adequate to generate an action potential which eventually results in release of the neurotransmitters serotonin (5-HT) and gamma-butyric acid (GABA) (inferred from mouse type III cells in Huang et al. 2005, Huang et al. 2009, Huang et al. 2011). Mice lacking both P2x2 and P2x3 ATP receptors do not produce nerve activity in response to sour tastants so ATP may play a role in transmission of sour taste (Eddy et al. 2009).