Network dynamics during the progression of seizure-like events in the hippocampal-parahippocampal regions

Seizure patterns in temporal lobe epilepsies have been described both in humans and in animal models. The involvement of specific hippocampal- parahippocampal subregions in the initiation and progression of temporal lobe seizures is not defined yet. We analyzed limbic network dynamics during seizures induced by 3-min arterial perfusion of 50M bicuculline in the in vitro isolated guinea pig brain preparation. As for human and animal temporal lobe epilepsies, 2 seizure types characterized at onset by either fast activity (FA) or hypersynchronous activity (HSA) were observed in our acute model. Simultaneous extracellular recordings were performed from ventral hippocampal-parahippocampal subregions with multichannel electrodes, and laminar analysis and propagation directions were computed to define reciprocal interactions during seizures. FA seizures started with fast oscillations generated in CA1-subiculum and entorhinal cortex, followed by irregular spikes and progressively regular bursts well defined in all subfields, with the exception of pre- and parasubiculum that do not participate in seizure activity. Dentate gyrus was not involved at FA seizure onset and became prominent during the transition to bursting in both FA and HSA patterns. HSA seizures were similar to FA events, but lacked initial FA. During seizures, reliable and steady propagation within the intra-hippocampal re-entrant loop was observed.