Ionic currents of Drosophila neurons in embryonic cultures

Drosophila offers a unique opportunity to determine how the genome codes for ionic channels in an organized nervous system. Considerable progress has already been made in studying the molecular biology of Drosophila K channels. In order for similar progress to be made on neuronal voltage-dependent Ca channels, a physiological preparation is needed in which the function of these channels can be directly studied. The patch-clamp studies reported here show that cultures of embryonic Drosophila cells (Seecof and Unanue, 1968) meet this need. These cultures provide the first opportunity to study with voltage-clamp techniques the Ca and Na currents of Drosophila neurons. The focus of these studies is on the Ca current; however, descriptions of the K and Na currents are also given since they help to characterize the cells studied and the quality of the voltage clamp. The voltage-dependent K, Na, and Ca currents of Drosophila neurons are very similar to those of molluscan neurons and other better studied neurons. The K currents are the largest currents in these neurons, averaging over 300 pA at +20 mV. There are 2 classes of Ca-independent K currents, inactivating currents that are 4-AP sensitive, and noninactivating currents that are insensitive to 4-AP. A large fraction of the K currents are located in the somal membrane. The Na currents are TTX sensitive and probably located in the processes. The peak amplitudes of the Ca currents vary from 0 to over 100 pA in these neurons, averaging 40 pA. With 5 mM external Ca2+ or Ba2+, the Ba currents are about twice as large as the Ca currents. Although 100 microM Cd2+ completely blocks the Ca current, organic blockers have very little effect. Variable inactivation characteristics and sensitivity to washout suggest the possibility of multiple types of Ca channels. A search for single-channel Ba currents in the somal membrane was unsuccessful.