Hyperpolarization-activated ion channel exhibiting weak selectivity for potassium over sodium ions. Contributes to the native pacemaker currents in heart (If) and in neurons (Ih). May mediate responses to sour stimuli. Activated by cAMP, and at 10-100 times higher concentrations, also by cGMP. cAMP binding causes a conformation change that leads to the assembly of an active tetramer and channel opening. Compared to other family members, cAMP has less stimulatory effect on HCN1 because part of the molecules already contain bound cAMP and form homotetramers when cAMP levels are low. Homotetramer. Heterotetramer with HCN2. The potassium channel is composed of a homo- or heterotetrameric complex of pore-forming subunits. Interacts with KCNE2. Interacts with the SH3 domain of CSK (PubMed:9405696). Predominantly expressed in brain. Highly expressed in apical dendrites of pyramidal neurons in the cortex, in the layer corresponding to the stratum lacunosum-moleculare in the hippocampus and in axons of basket cells in the cerebellum. Expressed 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. N-glycosylated. Inhibited by extracellular cesium ions. Belongs to the potassium channel HCN family.