The Kv3 channel blocker BDS-I blocked a component of the outward current. A Control current and reduced current in the presence of 250 nM BDS-I are shown. Currents were elicited during a voltage step to −18 mV. B BDS-I blocked outward current at potentials above −50 mV in four cells. The peak BDS-sensitive current (IBDS) is shown vs. voltage, different symbols represent different cells. Concentrations of BDS-I were 40 nM (filled circles and unfilled triangles), 200 nM (unfilled squares), and 250 nM (filled triangles). C Control current, current in the presence of 1.3 mM 4-AP (gray trace), and current in the presence of 1.3 mM 4-AP and 130 nM BDS-I (indicated by arrow) are shown. Currents were elicited during a voltage step to +32 mV. Addition of BDS-I did not produce a further block of current than that produced by 4-AP. The cell in this recording was a calyx terminal with a short segment of stalk.