Abstracts

Rationale: Kainic acid (KA)-induced status epilepticus (SE) in adult rats is often used to initiate a condition that simulates acquired temporal lobe epilepsy (TLE). It is often assumed that the hippocampus is the most important area of the brain to study, and moreover, that it is the hippocampus where the process of epileptogenesis probably starts. In this study, our goal was to test the hypothesis that this may not necessarily be true. Therefore, we examined the relative timing of EEG activity recorded by neocortical and hippocampal electrodes immediately after systemic KA administration and as SE developed. We asked whether the first seizure activity occurred in hippocampal or neocortical sites, and whether this pattern changed in the minutes before or during the start of SE. Methods: Adult male and female Sprague-Dawley rats (n = 12) were deeply anesthetized with isoflurane and electrodes were implanted with stereotaxic methods. For neocortical electrodes, a subdural screw was implanted over the left frontal and right frontal cortex (-1.0mm Anterior-Posterior (AP), ± 3.0mm Medial-Lateral (ML) relative to Bregma). Other screws were placed over the left and right occipital cortices (-6.0mm AP, ± 3.0mm ML). For hippocampal electrodes, a bipolar electrode (78 µm diam.) was implanted in each hippocampus (-4.5mm AP, ± 3.5mm ML, -3.5mm Ventral (V) from surface). A reference electrode was placed over the cerebellum (-8.5mm AP, +2.2mm ML, -2.5mm V); the ground was over the olfactory bulb (+1.7mm AP, -1.7mm ML). After a >2-week recovery period, rats were placed in a translucent cage with low light and no bedding and video-recorded. Simultaneous video-EEG was acquired with a digital telemetry system (Pearce etal., Epilepsy Beh., 2014). Animals were recorded for >10 min prior to KA to capture both exploratory and restful behavioral states. Then animals were injected with KA and recordings continued undisturbed for >3 hrs. KA (12mg/ml stock solution Milestone Pharm.) was diluted to a final concentration of 12 mg/kg and injected s.c. Anticonvulsants were administered = 30 min after SE and comparisons of the EEG stopped at this time. SE was defined as the time when the EEG showed seizure activity at all recording sites (generalized) and continued for = 3 min without a return to normal EEG. Results: All rats showed SE following KA administration. The first seizure activity occurred shortly after KA (mean ± sem: 8.83 ± 1.53 min; n=12). Seizure activity before SE was either a train of spikes (1-10Hz, >3 sec-duration) or other rhythmic activity with higher frequency spiking (>10Hz, >3sec). Remarkably, the first seizure activity typically occurred at one or more neocortical sites (n=10/12 rats). In these rats, 1-5 spike trains often occurred before hippocampal seizure activity was recruited. The first generalized electrographic seizure occurred 21.16 ± 8.93 min(n=12) after KA administration. Conclusions: In summary, the first seizure activity after systemic KA injection often appeared at neocortical electrodes and eventually the hippocampus as the seizures progressed. These findings support the hypothesis that KA and TLE have significant extrahippocampal components and possibly neocortical involvement. Furthermore, the data are consistent with the idea that initially the hippocampus has innate inhibitory mechanisms to resist seizures, but repeated cortical seizure activity ultimately weakens these mechanisms. The breakdown of cortical-to-hippocampal inhibition may be an important factor that allows an insult/injury to lead to epilepsy. Funding: NIH, Savoy Foundation for Epilepsy Research