Electrophysiological and morphological characterization of neurons within neocortical ectopias

Focal developmental abnormalities in neocortex, including ectopic collectio ns of neurons in layer I (ectopias), have been associated with behavioral a nd neurological deficits. In this study, we used infrared differential inte rference contrast microscopy and whole cell patch-clamp to complete the fir st characterization of neurons within and surrounding neocortical ectopias. Current-clamp recordings revealed that neurons within ectopias display mul tiple types of action potential firing patterns, and biocytin labeling indi cated that similar to 20% of the cells in neocortical ectopias can be class ified as nonpyramidal cells and the rest as atypically oriented pyramidal c ells. All cells had spontaneous excitatory (glutamatergic) and inhibitory ( GABAergic) postsynaptic currents. Exhibitory post-synaptic currents consist ed of both N-methyl-D-aspartate (NMDA) receptor-mediated and AMPA/kainate ( A/K) receptor-mediated currents. The NMDA receptor-mediated component had d ecay time constants of 15.35 +/- 2.2 (SE) ms, while the A/K component had f aster decay kinetics of 7.6 +/- 1.7 ms at -20 mV. GABA(A) receptor-mediated synaptic currents in ectopic cells reversed at potentials near the Cl- equ ilibrium potential and had decay kinetics of 16.65 +/- 1.3 ms at 0 mV. Furt hermore we show that cells within ectopias receive direct excitatory and in hibitory input from adjacent normatopic cortex and can display a form of ep ileptiform activity.