Development of spontaneous seizures over extended electrical kindling - I. Electrographic, behavioral, and transfer kindling correlates

The present study was aimed at evaluating an extended kindling model of spontaneous epilepsy. Behavioral and electrographic responses to repeated kindling of either the perforant path or amygdala were monitored for up to 300 trials. Kindling initially led to generalized convulsions equivalent to the level 5 seizure on the rating scale developed by Racine. The evoked seizures became progressively more complex with additional kindling, which was described by a 10-stage classification system. The highest stage (stage 10) was achieved when the kindling stimulation evoked two or more bouts of level 5 seizures combined with running and jumping fits. These more complex seizures developed over the course of amygdala, but not perforant path kindling. Electrographic seizures from both the amygdala and dentate gyrus increased in duration and amplitude during the early phase of kindling, but did not correlate with motor seizure development beyond level 5. During the late phase of kindling, the dentate gyrus afterdischarge amplitude decreased and became dissociated from the behavioral seizures. Manifestations of spontaneously recurring seizures were seen in the majority of animals, but spontaneous seizures of level 4 or greater were observed in only five rats. The second part of this study examined kindling transfer effects, the efficacy of kindling a new site after the completion of the initial (in this case extended) kindling protocol. The effect depended on both primary and secondary site location. When the amygdala served as primary site, perforant path transfer was complete in some animals but absent in others. No transfer occurred in the opposite direction, from the perforant path to the amygdala. Finally, transfer effects in the dentate gyrus, which was tested as tertiary site, were complete. Previous studies have found weaker transfer effects in the dentate when kindling to the standard stage 5 level. (C) 1998 Elsevier Science B.V. All rights reserved.