Figure 3. HCN channels control resting conductance and voltage attenuation in the apical dendritic tuft. A, C, HCN channels regulate the amplitude and spread of voltage responses in the apical dendritic tuft. Traces illustrate voltage responses simultaneously recorded from the apical dendritic tuft (red traces, 143 μm from nexus electrode) and the nexus of the apical dendritic trunk (black traces), generated in response to current steps (bottom traces) delivered to either the tuft (A) or nexus (C) under control (top traces) and in ZD7288 (10 μm, middle traces). The inset shows a line reconstruction of the apical dendritic tuft and the placement of recording electrodes. B, D, Pooled data illustrating the steady-state amplitude of voltage responses at the nexus (B) or apical dendritic tuft (D) when 200 pA positive or negative currents steps were delivered to the apical dendritic tuft (B) or nexus sites (D) under control conditions (open symbols) and in ZD7288 (10 μm, filled symbols). The continuous lines represent exponential fits to the data. E, Impact of blocking HCN channels on the decremental transfer of voltage from tuft to nexus (top graph) and nexus to tuft (bottom graph) under control conditions (open symbols) and in ZD7288 (10 μm, filled symbols). Voltage responses were generated in response to −200 pA current steps. F, Impact of blocking HCN channels (ZD) on steady-state input resistance (Rin) at apical dendritic tuft (top graph) and nexus sites (bottom graph). G, Blocking HCN channels unmasks tuft electrogenesis. Traces illustrate simultaneously recorded tuft (159 μm from nexus, red traces) and nexus (black traces) voltage responses evoked by a 400 pA current step injected through the tuft electrode, under control (top traces) and in ZD7288 (10 μm, bottom traces). Note the generation of a local apical dendritic tuft spike in ZD7288, when the resting membrane was repolarized to control values (−61 mV). H, Pooled data demonstrating the augmentation of local apical dendritic spike initiation by blocking HCN channels (ZD) at the tuft site of generation (top graph), and the decremental spread to the nexus (bottom graph).