Epileptogenic effects of electrolytic lesions in the hippocampus: role of iron deposition.

Small, anodal electrolytic lesions in the dorsal hippocampus were produced in rats, and continuous electrographic records were made from contralateral hippocampal electrodes for as long as 48 h following the lesion. Intense spontaneous epileptiform activity was observed, which developed from isolated spiking and spindling, to phasic spike trains, recurrent paroxysmal discharges and, in some cases, continuous ictal activity lasting to 7 h. Spontaneous clonic convulsions were observed in two cases. Epileptiform activity usually subsided after 12 h, but electrographic abnormalities such as spontaneous spiking and high-amplitude spindling persisted as long as 12 days. Such effects were reliably produced with iron-depositing lesions using stainless-steel electrodes, but not with platinum electrodes, suggesting that ionic deposition is critical for the lesion-induced seizure development. These results pose interpretive difficulties for the use of ion-depositing lesions in the study of limbic brain function.