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 promotes tetramerization and formation of an active channel. 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 (By similarity). 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. Highly expressed in cerebral cortex, cerebellum, throughout the hippocampus, in medial habenula, anterior dorsal nucleus in the thalamus, tenia tecta, several nuclei of the general motor system and in optic nerve layer. 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.