Hyperpolarization-activated ion channel with very slow activation and inactivation exhibiting weak selectivity for potassium over sodium ions. May contribute to the native pacemaker currents in heart (If) that regulate the rhythm of heart beat. May contribute to the native pacemaker currents in neurons (Ih) (By similarity). May mediate responses to sour stimuli. Activated by cAMP. cAMP binding causes a conformation change that leads to the assembly of an active tetramer and channel opening (By similarity). Homotetramer. The potassium channel is composed of a homo- or heterotetrameric complex of pore-forming subunits (By similarity). Highly expressed in pyramidal and granule layer of the hippocampus, in thalamus anterior nucleus, in the supraoptic nucleus in hypothalamus, in cerebellum, and in trapezoid nuclei and superior olivary complex in the auditory system. Detected in a subset of elongated cells in taste buds. The segment S4 is probably the voltage-sensor and is characterized by a series of positively charged amino acids at every third position. Belongs to the potassium channel HCN family.