Transition from interictal to ictal activity in limbic networks in vitro.

The transition from brief bursts of synchronous population activity characteristic of interictal epileptiform discharges (IEDs) to more prolonged epochs of population activity characteristic of seizures (ictal-like activity) was recorded in juvenile rat hippocampal-entorhinal cortex slices and hippocampal slices using multiple-site extracellular electrodes. Epileptiform activity was elicited by either increased extracellular potassium or 4-AP. IEDs originated in the CA3 a-b region and spread bidirectionally into CA1 and CA3c dentate gyrus. The transition from IEDs to ictal-like sustained epileptiform activity was reliably preceded by (1) increase in IED propagation velocity, (2) increase in IED secondary afterdischarges and their reverberation between CA3a and CA3c, and (3) shift in the IED initiation area from CA3 a-b to CA3c. Ictal-like sustained network oscillations (10-20 Hz) originated in CA3c and spread to CA1. The pattern of hippocampal ictal-like activity was unaffected by removal of the entorhinal cortex. These findings indicate that interictal and ictal activity can originate in the same neural network, and that the transition from interictal to ictal-like-sustained activity is preceded by predictable alterations in the origin and spread of IEDs. These findings elucidate new targets for investigating the proximate causes, prediction, and treatment of seizures.