Cesium (A) and ZD 7288 (B) are potent blockers of theI_h current amplitude. A,B, Amplitude of the I_hcurrent is plotted against the membrane potential under control conditions (filled squares) and after bath application of cesium (A, open circles) or ZD 7288 (B, open circles). All voltage steps were applied from a holding potential of −60 mV.C, Histogram showing the percentage of theI_h current that remained unblocked during bath application of cesium (middle gray bar) or ZD 7288 (black bar). The values were determined from voltage steps from a holding potential of −40 mV to a potential of −120 mV. The numbers of slices used for these experiments are indicated on each bar. The control bar summarizes all experiments for cesium and ZD 7288.D, E, Effect of 5 mm cesium on potassium outward currents recorded in respiratory neurons.D, Hyperpolarizing and depolarizing potentials were applied in 10 mV steps from −80 to +40 mV in control conditions (left) and under cesium (right). The holding potential was −70 mV. E, TheI–V curve is based on four recordings from respiratory neurons using the same protocol as described in D. The potassium current amplitude was obtained at the end of the pulse (steady state) and normalized to the maximal amplitude under control conditions (100%).