Accn2 is a cation channel with high affinity for sodium, which is gated by extracellular protons and inhibited by the diuretic amiloride. It also permeable for Ca2+, Li+ and K+. It generates a biphasic current with a fast inactivating and a slow sustained phase and mediates glutamate-independent Ca2+ entry into neurons upon acidosis. This Ca2+ overloading is toxic for cortical neurons and may be in part responsible for ischemic brain injury. Heteromeric channel assembly seems to modulate channel properties. Accn2 functions as a postsynaptic proton receptor that influences intracellular Ca2+ concentration and calmodulin-dependent protein kinase II phosphorylation and thereby the density of dendritic spines. It modulates activity in the circuits underlying innate fear.