No latency to dentate granule cell epileptogenesis in experimental temporal lobe epilepsy with hippocampal sclerosis

OBJECTIVE: To determine when spontaneous granule cell epileptiform discharges first occur after hippocampal injury, and to identify the postinjury "latent" period as either a "silent" gestational state of epileptogenesis or a subtle epileptic state in gradual transition to a more obvious epileptic state. METHODS: Nonconvulsive status epilepticus evoked by perforant path stimulation in urethane-sedated rats produced selective and extensive hippocampal injury and a "latent" period that preceded the onset of the first clinically obvious epileptic seizures. Continuous granule cell layer depth recording and video monitoring assessed the time course of granule cell hyperexcitability and the onset/offset times of spontaneous epileptiform discharges and behavioral seizures. RESULTS: One day postinjury, granule cells in awake rats were hyperexcitable to afferent input, and continuously generated spontaneous population spikes. During the ~2-4 week "latent" period, granule cell epileptiform discharges lasting ~30 seconds caused subtle focal seizures characterized by immobilization and facial automatisms that were undetected by behavioral assessment alone but identified post hoc. Granule cell layer epileptiform discharge duration eventually tripled, which caused the first clinically obvious seizure, ending the "latent" period. Behavioral seizure duration was linked tightly to spontaneous granule cell layer events. Granule cell epileptiform discharges preceded all behavioral seizure onsets, and clonic behaviors ended abruptly within seconds of the termination of each granule cell epileptiform discharge. Noninjurious hippocampal excitation produced no evidence of granule cell hyperexcitability or epileptogenesis. SIGNIFICANCE: The latent period in this model is a subtle epileptic state in transition to a more clinically obvious epileptic state, not a seizure-free "gestational" state when an unidentified epileptogenic mechanism gradually develops. Based on the onset/offset times of electrographic and behavioral events, granule cell behavior may be the prime determinant of seizure onset, phenotype, duration, and offset in this model of hippocampal-onset epilepsy. Extensive hippocampal neuron loss could be the primary epileptogenic mechanism.