Abstracts

Rationale: Interictal epileptiform discharges (IEDs) originate from synaptic connectivity changes leading to neuronal hypersynchrony. Theories on IEDs mechanisms are based on recordings of single cells and of field potentials with limited number of electrodes: the spatial distribution of IED field potentials has not been studied extensively. It remains unclear which cells in which cortical layers generate IEDs and how large cortical areas are recruited. We have previously found that human epileptic neocortex and neocortex of rats with IEDs activate key signaling pathways in the neocortex superficial layers. We aim at a spatially detailed neurophysiological characterization of IED-related field potentials to correlate IED neurophysiology with the expression of genes activated in epileptogenesis. Methods: We used multielectrode arrays to record extracellular IEDs from neocortical slices from adult animals stimulated with zero magnesium and several concentrations of 4-Aminopyridine (4-AP). Preparation of viable acute slices from adult rats has been optimized allowing brains to cool down to 2-3 C for prolonged time intervals and by maintaining the vibratome chamber temperature constantly at 3-5 C. Through this protocol more than 90% of adult rat neocortical slices showed epileptiform activity. Electrodes were localized to specific cortical layers and those exhibiting discharges were mapped to the corresponding zone in the cortical slice. Results: There was a 4-AP dose-dependent increase of the frequency of IEDs. IEDs were observed over several cortical layers though more often over layers I and II/III. IEDs occurred in a mosaic of multiple, circumscribed, microscopic foci. In the same slice IEDs of distinct microscopic foci were often independent from one another. At times in each slice the discharge of one microscopic focus appeared to recruit epileptiform activity in a distinct focus that was otherwise firing independently. Superficial layers could recruit deep layers but also deep layers initiated IEDs subsequently spreading to superficial layers. In long duration recordings, through time there was often a slow and gradual increase in synchronicity between these spatially distinct foci. Ictal patterns were observed more rarely and appeared over superficial layers or over deeper cortical layers but more over deeper layers. Conclusions: IED are prevalent over the superficial cortical layers, in the same location of MAP-kinase activation: this biochemical pathway may participate to IED generation over Layers I and II/III. IEDs over the deep layers may be generated by a different pathway. Epileptogenesis may present as a mosaic of independent microscopic foci of neuronal hyper-synchrony. The gradual increase in synchronicity between different foci might result from synaptic plasticity triggered by IEDs themselves. Epileptogenesis may consist in a progressive cascade recruitment of foci generating IEDs over surfaces sufficiently large to ultimately disrupt cortical function.