EPILEPTIC AFTERDISCHARGE IN THE HIPPOCAMPAL-ENTORHINAL SYSTEM - CURRENT SOURCE DENSITY AND UNIT STUDIES

The contribution of the various hippocampal regions to the maintenance of epileptic activity, induced by stimulation of the perforant path o r commissural system, was examined in the awake rat. Combination of mu ltiple-site recordings with silicon probes, current source density ana lysis and unit recordings allowed for a high spatial resolution of the field events. Following perforant path stimulation, seizures began in the dentate gyrus, followed by events in the CA3-CA1 regions. After c ommissural stimulation, rhythmic bursts in the CA3-CA1 circuitry prece ded the activation of the dentate gyrus. Correlation of events in the different subregions indicated that the sustained rhythmic afterdischa rge (2-6 Hz) could not be explained by a cycle-by-cycle excitation of principal cell populations in the hippocampal-entorhinal loop. The pri mary afterdischarge always terminated in the CA1 region, followed by t he dentate gyrus, CA3 region and the entorhinal cortex. The duration a nd pattern of the hippocampal afterdischarge was essentially unaffecte d by removal of the entorhinal cortex. The emergence of large populati on spike bursts coincided with a decreased discharge of interneurons i n both CAI and hilar regions. The majority of hilar interneurons displ ayed a strong amplitude decrement prior to the onset of population spi ke phase of the afterdischarge. These findings suggest that (i) afterd ischarges can independently arise in the CA3-CA1 and entorhinal-dentat e gyrus circuitries, (ii) reverberation of excitation in the hippocamp al-entorhinal loop is not critical for the maintenance of afterdischar ges and (iii) decreased activity of the interneuronal network may rele ase population bursting of principal cells. Copyright (C) 1996 IBRO. P ublished by Elsevier Science Ltd.