Kv3.4-mediated K⁺ currents reduce membrane excitability in the axonal growth cones. A, An outside-out patch, isolated from growth cones of cultured dorsal spinal neurons, was held at −70 mV and hyperpolarized to −90 mV for 1.5 s before test pulses ranging from −70 to 70 mV (800 ms, 20 mV increments). Representative total outward K⁺ currents are shown. B, C, Plot of 20%–80% rise time (B) or decay time constant (C) versus test pulse voltage. Number in parentheses indicates the total number of neurons analyzed. D, Normalized peak conductance (G/G_max)-voltage relationship (n = 3). The solid curve is given by Boltzmann function raised to the fourth power: [1/(1 + exp [−(V − V_h)/k])]⁴, where V is the membrane potential, V_h is the potential at which the Boltzmann function is 0.5, and k is the slope factor. V_h is −35.4 mV, and the slope factor is 23.9 mV. This function reaches a midpoint at a value of 1.8 ± 6.6 mV. E, BDSII-sensitive K⁺ current (red) is obtained by subtracting the sustained component (green) from the total current (control, black). F, Summary of the effect of 0.5 μm BDSII on peak current amplitude (n = 6). G, H, Summary of 20%–80% rise time and decay time constant between the control and BDSII-sensitive currents (n = 4). I, J, Whole-cell current-clamp recording was performed in growth cones of cultured dorsal spinal neurons. Membrane potential (V_m) was measured before and after focal application of BDSII (n = 4) (I). Representative action potentials were elicited with 200 ms current pulse of 50 pA in the growth cone before (black trace) and after BDSII (red trace) (J). Peak is defined from the initial rising phase (blue arrow) to the first valley (J, black or red arrow). K–O, Properties of action potential before and after BDSII (n = 6). Data are mean ± SEM. n.s., no significant difference. *p < 0.05, compared with control (paired-samples t test). ***p < 0.001, compared with control (paired-samples t test).