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Cellular physiology of the neonatal rat cerebral cortex: Intrinsic membrane properties, sodium and calcium currents

The cellular physiology of the primary somatosensory cortex was studied in postnatal day (P) 0 to P5 rats using whole‐cell patch‐clamp recordings. Visually identified Cajal‐Retzius, subplate, bifurcated pyramidal, and immature, putatively migrating neurons showed resting membrane potentials between –44 and –50 mV and TTX‐sensitive action potentials. Immature pyramidal neurons with the smallest surface area (∼1,600 μm2) revealed the largest input resistance (∼1.8 GΩ), and subplate cells with the largest surface area (∼6,200 μm2) showed an input resistance of ∼1 GΩ. Ontogenetically older Cajal‐Retzius and subplate cells revealed shorter and larger action potentials compared to bifurcated and immature pyramidal neurons. Whereas Cajal‐Retzius and subplate cells responded to injection of depolarizing current pulses with a repetitive nonadapting and fast spiking firing pattern, immature pyramidal neurons showed strong adaptation. Subplate cells revealed the fastest action potentials, largest sodium current amplitude (–714 pA), and highest sodium current density (–38 μA/cm2), enabling these cells to transmit afferent activity faithfully to postsynaptic neurons. Whereas all cell types expressed a high‐voltage‐activated (HVA) calcium current, none of them showed a significant low‐voltage‐activated calcium current. The largest peak (–25.5 μA/cm2) and steady‐state (–7.6 μA/cm2) HVA calcium current density could be observed in immature presumed migrating neurons. In contrast, Cajal‐Retzius and subplate neurons showed a significantly lower peak (−4.9 μA/cm2) and steady‐state (<−3.3 μA/cm2) HVA calcium current density. Whereas a large HVA calcium current may promote neuronal migration of immature neurons, low intracellular calcium levels may provoke apoptosis in Cajal‐Retzius and subplate cells. J. Neurosci. Res. 62:574–584, 2000. © 2000 Wiley‐Liss, Inc.