AN INVITRO STUDY OF FOCAL EPILEPTOGENESIS IN COMBINED HIPPOCAMPAL-PARAHIPPOCAMPAL SLICES

Brain slices from adult rats that included ventral hippocampus and adj acent parahippocampal areas were studied at various sites for the appe arance of epileptiform discharges as extracellular potassium ([K+]o) w as systematically varied over the range of 3 mM to 10 mM. The developm ent of evoked epileptiform discharges (EEDs) and spontaneous epileptif orm discharges (SEDs) was monitored in areas CA1, CA3, the hilus of th e dentate gyrus (dentate hilus), the granule cell layer of the dentate gyrus (dentate gyrus), subiculum and entorhinal cortex. Increasing [K +]o elicited EEDs in a concentration-dependent manner. The entorhinal cortex and CA1 areas were most susceptible to this effect; the dentate gyrus was least susceptible. Increasing [K+]o also caused an increase in SEDs in all hippocampal areas. Separating entorhinal cortex from t he rest of the slice by transections did not abolish discharges in any location. Lesions of the Schaeffer collaterals abolished discharges i n CA1 but not in CA3. These findings demonstrate that changes in the e xtracellular milieu that are associated with epileptiform discharges i n vivo are capable of eliciting paroxysmal discharges at multiple loci in the hippocampus and adjacent regions. Further, these findings demo nstrate for the first time the intrinsic capacity of the entorhinal co rtex to generate epileptiform discharges under in vitro ionic conditio ns known to occur in the intact brain during seizures.