VOLTAGE IMAGING OF EPILEPTIFORM ACTIVITY IN SLICES FROM RAT PIRIFORM CORTEX - ONSET AND PROPAGATION

The piriform cortex is a temporal lobe structure with a very high seiz ure susceptibility. To investigate the spatiotemporal characteristics of epileptiform activity, slices of piriform cortex were examined by i maging electrical activity with a voltage-sensitive fluorescent dye. D ischarge activity was studied for different sites of stimulation and d ifferent planes of slicing along the anterior-posterior axis. Epilepti form behavior was elicited either by disinhibition with a gamma-aminob utyric acid-a receptor antagonist or by induction with a transient per iod of spontaneous bursting in low-chloride medium. Control activity r ecorded with fluorescent dye had the same pharmacological and temporal characteristics as control activity reported previously with microele ctrodes. Simultaneous optical and extracellular microelectrode recordi ngs of epileptiform discharges showed the same duration, latency, and all-or-none character as described previously with microelectrodes. Un der all conditions examined, threshold electrical stimulation applied throughout the piriform cortex evoked all-or-none epileptiform dischar ges originating in a site that included the endopiriform nucleus, a pr eviously identified site of discharge onset. In induced slices, but no t disinhibited slices, the site of onset also included layer VI of the adjoining agranular insular cortex and perirhinal cortex, in slices f rom anterior and posterior piriform cortex, respectively. These locati ons had not been identified previously as sites of discharge onset. Th us like the endopiriform nucleus, the deep agranular insular cortex an d perirhinal cortex have a very low seizure threshold. Additional subt le differences were noted between the induced and disinhibited models of epileptogenesis. Velocity was determined for discharges after onset , as they propagated outward to the overlying piriform cortex. Propaga tion in other directions was examined as well. In most cases, velociti es were below that for action potential conduction, suggesting that re current excitation and/or ephaptic interactions play a role in dischar ge propagation. Future investigations of the cellular and organization al properties of regions identified in this study should help clarify the neurobiological basis of high seizure susceptibility.