Hyperpolarization-activated cation currents (Ih) appear in the heart and the brain and play a crucial role in controlling electrical pacemaker activity, contributing to biological processes such as heartbeat, sleep-wake cycle and synaptic plasticity.nThe Ih currents are generated by the Hyperpolarization-activated Cyclic Nucleotide-gated channel family (HCN), which comprises of four homologous members, named HCN1-4. Each HCN subunit consist of six transmembrane domains (TM), a pore region between TM5-TM6 and a binding domain to cyclic nucleotides (CNBD) in the cytoplasmic C-terminus.nThe HCN subunits can form functional homomers and can also co-assemble into functionally heteromers.nThe channels are closely related to each other and share homology of about 60%. However, they differ from each other in their cytoplasmic N- and the C-terminus. HCN1-4 mainly differ from each other with regard to their speed of activation and the extent by which they are modulated by cAMP. HCN1 is the fastest channel, followed by HCN2, HCN3 and HCN4.nThe HCN3 channels are abundant in the brain where high expression of HCN3 mRNA was found in the hypothalamus.