Voltage-gated currents, dye and electrical coupling in the embryonic mouse neocortex

We measured dye coupling, electrical coupling, and voltage-gated currents using whole-cell voltage clamp in slices of mouse sensorimotor cortex at embryonic day 14 (E14). As in rat ventricular zone (VZ), cells of the VZ were extensively dye coupled, often in clusters of >100 cells. In mouse VZ, however, cells were much less electrically coupled, making measurement of voltage-gated currents more accurate. All VZ cells expressed delayed K+ currents (I-K), and 30%, including morphologically identified radial glia, also expressed inward Na+ currents (I-Na). This fraction is consistent with I-Na expression being an early event following cell cycle exit. Intermediate zone (IZ) cells also expressed I-K and I-Na. Na+ current amplitude distributions indicated three populations of IZ cells: those without I-Na, those with I-Na similar in amplitude to VZ cells, and those with I-Na being almost 10 times larger than in VZ cells. Cells of the cortical plate (CP) expressed both I-K and I-Na, with I-Na being almost 10-fold larger than in VZ cells. No cell in any zone expressed detectable hyperpolarization-activated currents. Our data suggest that the distribution and density of I-Na may be related to early events of cell cycle exit and migration.