Epileptogenesis in immature neocortical slices induced by 4-aminopyridine

The laminar site of onset of 4-aminopyridine (4AP)-induced epileptiform discharges in immature neocortical brain slices was localized to layer V using conventional extracellular field electrodes, current source-density analysis (CSD), and subdivided slices. Intracellular patch-electrode recordings in immature layer V neurons confirmed that intrinsically bursting (IB) neurons were not present at the ages studied or with bath application of 50-200 microM 4AP. IB properties were not being masked in the younger animals by the patch electrodes because typical IB neurons in layer V were seen in older rats when the same intracellular techniques were used. These results demonstrate that epileptiform activity can be initiated in the absence of IB neurons, and suggest that other factors are responsible for the preferential onset in layer V.