Expression of membrane currents in rat neocortical neurons in serum-free culture. II. Outward currents.

The timing of expression and properties of outward membrane currents in cultured neocortical pyramidal-shaped neurons have been investigated using the gigaseal whole-cell voltage clamp and single-channel recording techniques. Dissociated primary cultures of synchronized (same cell cycle), growth arrested (G1 phase) and birth-dated cells from fetal rat (E18) were maintained in a serum-free medium. The earliest appearing membrane current in the soma is a voltage-dependent outward current carried by K+. The current consists of two components, one rapidly rising component, resembling those associated with the transient outward current (IA) and the other similar to the delayed rectifier current (IK). The ratio between the peak IA and IK was about 0.3 at all membrane voltages. The magnitude of both IA and IK increased with time in culture but the ratio remained unchanged. Direct measurements of unitary currents showed the presence of two voltage-activated outward conductances, 32 pS and 120 pS. The small conductance channel was sparse. The large conductance channel is K+-selective and was sensitive to both voltage and internal Ca2+.