Cellular morphology and physiology of the perinatal rat cerebral cortex

The cellular morphology and electrophysiology of the rat neocortex between embryonic day (E) 18 and postnatal day (P) 3 was studied in vitro by extracellular biocytin injections and whole-cell recordings, respectively. Most neurons were characterized by a small number of short-range dendrites and a main axon that was directed towards the white matter. Biocytin injections into the marginal zone and the cortical plate labeled far-reaching connections extending up to 2 mm in horizontal direction, indicating the existence of a dense network of long-range intrinsic projections in the neonatal cortex. Action potentials could be elicited as early as E18 and repetitive firing could first be observed at P0. Electrical stimulation of the immature cortex at various positions elicited polyphasic and long-lasting (up to 1 s) excitatory postsynaptic potentials and currents, which were significantly reduced in amplitude by a selective N-methyl-D-aspartate receptor antagonist. Our data indicate that the perinatal cortex manifests the structural and functional conditions for powerful excitatory interactions, which increase the likelihood for the generation of epileptiform activity during this developmental period.