HCN3 and Alzheimer disease: Immunohistochemistry showed elevated HCN channels in the cornu ammonis (CA) 1 and CA3 hippocampal regions in the AD brain, with statistically significant increases in the percentages of HCN1+, HCN2+, and HCN3+ cells in CA3, and in HCN1+ and HCN2+ cells in the CA1 region (Figure 1A‐C).