HCN channels are important determinants of intrinsic excitability and the downstream targets of 5-HT-induced response. A, B, Representative traces showing that bath application of 2 mm Cs+ or 10 μm ZD7288 induced an outward shift of holding current in fusiform cells. C, Response to 10 μm 5-HT was blocked by HCN channel blockers 2 mm Cs+ or 10 μm ZD7288, (top and bottom traces, respectively). D, Summary of the effects of Cs+ (n = 5) and ZD7288 (n = 7) on the 5-HT evoked current. Open symbols represent 5-HT currents of individual neurons, and filled symbols represent the mean of 5-HT currents. E, Under current-clamp, representative traces from a fusiform cell showing 5-HT increased the spike firing (left trace: control; middle trace: 5-HT application), and application of Cs+ produces a hyperpolarization of membrane potential and also suppressed the increase in the spike firing by 5-HT (right trace). F, Similarly, representative traces from another fusiform cell showing that 5-HT increased the spike firing (left trace), and application of ZD7288 produced a hyperpolarization of membrane potential and also suppressed the increase in the spike firing by 5-HT (right trace). Error bars are ± SEM.