Calyx I_h was modulated by ZD7288 and cAMP. (A) Representative traces from a wild-type dimorphic terminal in response to voltage steps between −144 mV and −64 mV in 10-mV steps. (B) I_h was blocked by inclusion of 100 µM ZD7288 in the recording pipette. (C) Representative I_h after the addition of 200 µM cAMP in the pipette. (D) G_max and V_1/2 were estimated from activation curves generated as described for Fig. 3 (D–F). The conductance was completely blocked by ZD7288 but unaffected by cAMP. (E) cAMP shifted the voltage of half maximal activation in the positive direction. (F) cAMP increased the activation speed, shown here as a decrease in both fast and slow time constants. The number of samples is shown inside the bars for each condition. **, P < 0.01; ***, P < 0.001. Error bars indicate mean ± SEM. (G) Steady-state I-V plots from calyx terminals with cAMP did not reveal any change in voltage-dependent K⁺ currents.