Current-source-density profiles associated with sharp waves in human epileptic neocortical tissue

In human neocortical slices obtained during epilepsy surgery, sharp waves h ave been described to appear spontaneously, the shape of which met all crit eria of epileptiform field potentials. In the present investigation, the cu rrent sinks and sources underlying these potentials were analysed. The cort ical tissue used in the present study was a small portion of the tissue blo cks excised for treatment of pharmacoresistant focal epilepsy. The tissue c ame from the temporal (n = 26), frontal (n = 1) and parietal (n= 1) lobes. Slices of 500 mu m thickness were cut in the frontal plane perpendicular to the pial surface. Field potentials were recorded using a linear array of e ight wire electrodes (diameter: 33 mu m) with interelectrode distances of 3 00 mu m. To scan a slice for sharp field potentials, this array was placed perpendicular to the pial surface at the midsection of each preparation, an d consecutively at the respective midsections of the resulting halves of th e slice. Each of these locations was termed a recording line. Depending on the appearance of spontaneous potentials, recording lines and slices were c lassified as either spontaneous or non-spontaneous. With both spontaneous a nd zero Mg2+-induced interictal discharges, in spontaneous slices, current sinks were preferentially located in layers II and III. In non-spontaneous slices, current sinks associated with interictal potentials could be found throughout all cortical laminae. With zero Mg2+-induced ictal activity, in spontaneous slices, the initial sinks were preferentially located in cortic al laminae II and ma, and were shifted to lower ones after additional appli cation of bicuculline. In non-spontaneous slices, no ictal-type discharges could be induced with omission of Mg2+ from the superfusate. Only addition of bicuculline elicited ictal-type activity, and sinks associated with this were preferentially located in layers II and IIIa. The results suggest that the supragranular layers, especially layer II, cha nge qualitatively in functional organization in slices showing spontaneous discharges. We think that this special feature represents the function of t he upper layers and can be blocked by bicuculline. This interpretation is s upported by the observation that ictal discharges normally started in the u pper layers in spontaneous and non-spontaneous slices, except for spontaneo us slices with bicuculline, where the zone initiating discharges was transl ocated to deeper layers. (C) 1999 IBRO. Published by Elsevier Science Ltd.