Sustained plateau activity precedes and can generate ictal-like dischargesin low-Cl- medium in slices from rat piriform cortex

Interictal and ictal discharges represent two different forms of abnormal b rain activity associated with epilepsy. Ictal discharges closely parallel s eizure activity, but depending on the form of epilepsy, interictal discharg es may or may not be correlated with the frequency, severity, and location of seizures. Recent voltage-imaging studies in slices of piriform cortex in dicated that interictal-like discharges are generated in a two-stage proces s. The first stage consists of a sustained, low-amplitude depolarization (p lateau activity) lasting the entire latent period prior to discharge onset. Plateau activity takes place at a site distinct from the site of discharge onset and serves to sustain and amplify activity initiated by an electrica l stimulus. In the second stage a rapidly accelerating depolarization begin s at the onset site and then spreads over a wide region. Here, we asked whe ther ictal-like discharges can be generated in a similar two-stage process. As with interictal-like activity, the first sign of an impending ictal-lik e discharge is a sustained depolarization with a plateau-like time course. The rapidly accelerating depolarization that signals the start of the actua l discharge develops later at a separate onset site. As found previously wi th interictal-like discharges, local application of kynurenic acid to the p lateau site blocked ictal-like discharges throughout the entire slice. Howe ver, in marked contrast to interictal-like activity, blockade of synaptic t ransmission at the onset site failed to block the ictal-like discharge. Thi s indicates that interictal- and ictal-like discharges share a common pathw ay in the earliest stage of their generation and that their mechanisms subs equently diverge.