Seizure activity in vitro: a dual focus model.

Recently, we have reported that the exposure of hippocampal slices in vitro to artificial cerebrospinal fluid (ACSF) containing no added magnesium results in ictal-like (ictaform) activity in area CA3 of the hippocampal formation. Other reports describe such activity in slices of entorhinal cortex (EC) under similar conditions. Because of the close interrelationship between the entorhinal area and the hippocampal formation, we have begun to study, in vitro, brain slices which contain both the entorhinal cortex and the hippocampal formation. In these slices, we have found that, in the magnesium-free (0-Mg2+) model, there is good electrical communication between area CA3 and the EC. Simultaneous recordings of the activity in the EC and CA3 showed that, when the circuitry linking the two areas was intact, the EC tended to initiate the ictaform activity and lead CA3. However, late in the event, CA3 could lead EC. Furthermore, interictal-like spontaneous bursting in CA3 led to a disorganized pattern of ictaform activity in EC. Finally, when the EC was separated from the hippocampal formation, both areas were capable of ictaform activity which was temporally unrelated. This model provides the opportunity to explore the relationship between two epileptogenic areas in vitro, and to compare and contrast the morphology of the ictaform activity present in both structures. As such, it may prove valuable in both pharmacological and physiological studies of seizure disorders.