Abstract
The availability of genetic, molecular, and biophysical techniques makes Drosophila an ideal system for the study of ion channel function. We have used the patch-clamp technique to characterize voltage-gated K+ channels in cultured larval Drosophila CNS neurons. Whole-cell currents from different cells vary in current kinetics and magnitude. Most of the cells contain a transient A-type 4-AP-sensitive current. In addition, many cells also have a more slowly inactivating TEA-sensitive component and/or a sustained component. No clear correlation between cell morphology and whole-cell current kinetics was observed. Single-channel analysis in cell-free patches revealed that 3 types of channels, named A2, KD, and K1 can account for the whole-cell currents. None of these channels requires elevated intracellular calcium concentration for activation. The A2 channels have a conductance of 6-8 pS and underlie the whole-cell A current. They turn on rapidly, inactivate in response to depolarizing voltage steps, and are completely inactivated by prepulses to -50 mV. The KD (delayed) channels have a conductance of 10-16 pS and can account, in part, for the more slowly inactivating component of whole-cell current. They have longer open times and activate and inactivate more slowly than the A2 channels. The K1 channels have a slope conductance, measured between 0 and +40 mV, of 20-40 pS. These channels do not inactivate during 500 msec voltage steps and thus can contribute to the sustained component of current. They exhibit complex gating behavior with increased probability of being open at higher voltages. Although the K1 channels are sufficient to account for the noninactivating component of whole-cell current, we have observed several other channel types that have a similar voltage dependence and average kinetics.