Fig. 3. Different levels of transient K⁺ currents. Typical examples of voltage-sensitive currents of a Kenyon cell (left) and a projection neuron (right) are shown. (A) In the presence of tetrodotoxin (TTX) and Cd²⁺ in the external saline to block currents through voltage-sensitive Na⁺ and Ca²⁺ channels, voltage-sensitive outward currents were isolated. Activation protocols for the experiments: cells were held at -80 mV. To remove channel inactivation, a long conditioning pulse to -120 mV (1 s) (cf. Pelz et al., 1999) preceded depolarizing voltage commands (potentials from -100 to +120 mV, 10 mV increments, duration 100 ms). Under these conditions Kenyon cells expressed a prominent inactivating K⁺ current (I_Peak), which was less pronounced in projection neurons, where a sustained K⁺ current (I_Sust) dominated. Arrows indicate time points where currents were measured in the other figures and they point to the peak current (I_Peak) and to the sustained current (I_Sust) at the end of the voltage pulse. (B) Following an inactivating prepulse to -20 mV (1 s), the transient K⁺ current of Kenyon cells was completely inactivated during depolarising pulses to various command potentials, whereas the outward currents of projection neurons remained relatively unaffected. (C) Subtracting trace B from trace A gave the amount of inactivating K⁺ current in the two neurons. Note the different scale bars for Kenyon cells and projection neurons.