Expression of membrane currents in rat neocortical neurons in serum-free culture. I. Inward currents.

The gigaseal whole-cell voltage clamp technique has been used to investigate the timing of expression and properties of voltage-dependent inward currents in cultured neocortical pyramidal-shaped neurons. The dissociated primary cultures of synchronized (same cell cycle), growth arrested (G1-phase) and birth-dated neurons from fetal rat (E18) were maintained in a serum-free medium. The earliest inward current is expressed within 24 h. This current is carried by Na+ and the channels are located in distal neurites at discrete sites. The Na+ channels near the cell body are expressed after 5 days in culture, at which time the neuritic Na+ current persists. The magnitude of the current near the soma increases with age of the neuron. The Na+ current is blocked by both tetrodotoxin (TTX) and nitrendipine. The sensitivity to nitrendipine changes with age of the culture. The results suggest that Na channels expressed early during neuronal development have some structural component common also to Ca2+ channels.