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In vitro formation of a secondary epileptogenic mirror focus by interhippocampal propagation of seizures

Abstract

We have determined whether seizures generate an epileptogenic focus in distal structures using an in vitro preparation composed of three independent chambers that accommodate two intact hippocampi and their connecting commissures. This enabled us to apply a convulsive agent to one hippocampus, allow the propagation of a given number of seizures to the other side and block the connections reversibly by applying tetrodotoxin (TTX) to the commissural chamber. The propagation of seizures from the kainate-treated side to the naive side transformed the latter into an independent epileptogenic focus that was capable of generating spontaneous and evoked seizures. The induction mechanism required activation of NMDA receptors and the epileptogenic transformation was associated with long-term alterations in GABAergic synapses, which became excitatory because of a shift in the chloride reversal potential, ECl. These data indicate that the excitatory actions of GABA may be a fundamental property of epileptogenic structures.

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Figure 1: Interhemispheric propagation of epileptiform activity.
Figure 2: The naive side becomes epileptogenic after repeated seizures.
Figure 3: Chronic epileptogenesis in the secondary focus.
Figure 4: Activation of NMDA receptors is required for the formation of a secondary focus.
Figure 5: Different effects of bicuculline in control and epileptic slices.
Figure 6: Shift in ECl and in the excitatory actions of GABA in the secondary focus.

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Acknowledgements

We are indebted for financial support to the Institut de la Santé et de la Recherche Médicale (INSERM), the French foundation of medial research, the Cino Del Duca and the Electricité de France foundations. I.K. received financial support from the French Federation of Research in Epilepsies, INSERM and International Brain Research Organization. We thank R. Khazipov for his suggestions and criticism.

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Correspondence to Yehezkel Ben-Ari.

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Supplementary information

Supplementary Fig. 1.

Kainate was applied repetitively to one hippocampus and CNQX was applied continuously to the contralateral hippocampus to fully block AMPA and kainate receptors. (a) Field recordings in both hippocampi as shown in the schematic illustration. Note that kainate generated seizure in the treated hippocampus but this did not propagate to the contralateral CNQX treated hippocampus. (b) After repeated applications of kainate, TTX was applied to the commissural chamber. Note that the kainate treated hippocampus but not the contralateral hippocampus generated both spontaneous and evoked EAs. (c) The two hippocampi were separated and kept in vitro for 24 hours. Note that electrical stimulation evoked an EA in the treated but only a field EPSP in the CNQX treated side. (GIF 23 kb)

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Khalilov, I., Holmes, G. & Ben-Ari, Y. In vitro formation of a secondary epileptogenic mirror focus by interhippocampal propagation of seizures. Nat Neurosci 6, 1079–1085 (2003). https://doi.org/10.1038/nn1125

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