Early development of voltage-gated ion currents and firing properties in neurons of the mouse cerebral cortex

Voltage- and current-clamp recordings were made from acute slices of mouse cerebral cortex from embryonic day 14 to postnatal day 17. We targeted cells in the migratory population of the embryonic intermediate zone (IZ) and in deep layers of embryonic and postnatal cortical plate (CP). IZ neurons maintain fairly consistent properties through the embryonic period, all expressing high-input resistance, inward Na+ currents and outward K+ currents, and none showing any hyperpolarization-activated currents. In CP neurons, several changes in physiological properties occur in the late embryonic and early postnatal period: inward Na+ current density is strongly upregulated while outward K+ current density remains almost unchanged, input resistance drops dramatically, and a hyperpolarization-activated current resembling I-h appears. As a result of these changes, the action potential becomes larger, shorter in duration, and its threshold shifts to more negative potentials. In addition, CP cells become capable of firing repetitively and an increasing fraction show spontaneous action potentials. This coordinated development of ion channel properties may help to time the occurrence of developmentally relevant spontaneous activity in the immature cortex.