Hyperpolarization-activated cation currents (Ih) appear in the heart and the brain and have a crucial role in controlling electrical pacemaker activity, contributing to biological processes such as heartbeat, sleep-wake cycle and synaptic plasticity.HCN4 is highly expressed in a restricted manner in adult sinoatrial (SA) node, constituting a good molecular marker for the adult cardiac pacemaker and might serve as a unique marker of the developing SA node.mRNA expression of HCN4 is most abundant in medial habenula and anterior and principal relay nuclei of the thalamus
