Epileptic sensitization. Spontaneous and induced activity of unsensitized and sensitized acutely isolated cerebral cortex.

UMMARY Subpial isolation of slabs of cerebral cortex was performed in cats and rabbits under Nembutal® anaesthesia, care being taken not to interfere with the vascular supply of the isolated cortex. During the same experimental session both the isolated and the surrounding cortex were treated with topical application of strychnine and with electrical stimulation. Electrical activity was recorded with silver-tipped electrodes placed on the cortical surface. Isolated cortex showed spontaneous activity, which did not change after undercutting surrounding cortex in spite of marked decrease in the amplitude of the record from the latter. Strychnine and electrical stimulation of the isolated cortex produced spikes and electrical after-discharges respectively, which remained localized to the isolated slab. When similar stimulation was applied to the surrounding cortex, discharges were recorded from the isolated cortex, but the discharge was synchronous with that of the surrounding area and terminated when the latter stopped discharging. Other characteristics of the epileptiform discharge in the isolated slab further suggested that it propagated across the neural gap from the surrounding cortex by volume conduction. In five rabbits epileptogenic lesions were made by freezing a small cortical area with ethyl chloride under aseptic conditions. Forty-eight hours later both hemispheres were exposed, ECoGs were recorded from the area of the lesion and from homotopic area of the opposite hemisphere, and these areas were then isolated. Isolated and surrounding cortex were then electrically stimulated and recording was performed in the same way as in the non-epileptic animals. Recording from contralateral cortex homotopic to the epileptogenic lesion made 48 hours earlier showed the presence of an actively firing spike focus, which persisted after isolation of the primary epileptogenic cortex. When the cortex containing the mirror focus was isolated, firing stopped. In this instance, however, when an after-discharge was initiated in the surrounding cortex, it propagated to the slab but was not synchronous with the originating discharge and the after-discharge in the isolated area outlasted that from the surrounding cortex. It is concluded that propagation of epileptiform activity across a neural gap into isolated cortex probably constitutes a manifestation of volume conduction. However, if the isolated slab is previously sensitized by epileptigonic bombardment, and even when it itself no longer produces spontaneous spikes, electrical after-discharges propagated from surrounding cortex generate an independent, self-sustained after-discharge in the sensitized isolated cortex. Epileptic sensitization has thus made possible the production of self-sustained epileptiform discharge triggered by a volume-conducted after-discharge. RESUME Chez des chats et des lapins sous anesthesie au Nembutal, on a isole par section sous-piale des ilots de cortex cerebral en prenant soin de respecter la vascularisation du cortex isole. Pendant la meme seance experimentale, on a fait des applications locales de strychnine et des stimulations electriques a la fois au niveau du cortex isole et au niveau du cortex avoisinant. L'activiteelectrique a ete enregistree avec des electrodes en argent placees sur le cortex. Le cortex isole avait une activite spontanee qui n'etait pas modifiee apres section en dessous du cortex avoisinant, bien que l'on observât â ce niveau une nette diminution d'amplitude de l'activiteelectrique. L'application de strychnine et la stimulation electrique du cortex isole provoquaient respectivement des pointes et des post-decharges electriques qui restaient localisees dans le cortex isole. Lorsque des stimulations analogues etaient appliquees sur le cortex avoisinant, on enregistrait des decharges dans le cortex isole, mais ces decharges etaient synchrones de celles des regions avoisinantes et s'arretaient avec la fin des decharges dans ces regions avoisinantes. D'autres caracteristiques de la decharge epileptique dans le cortex isole suggeraient aussi qu'il s'agissait d'une decharge du cortex avoisinant transmise par ← volume conduction →. Chez cinq lapins, on a cree aseptiquement des lesions epileptiques par congelation d'une aire corticale limitee, par l'ethylchloride. 48 h apres, les deux hemispheres ont ete exposes et, apres enregistrement de la region lesee et de la region homologue sur l'hemisphere oppose, on a isole ces regions. La region isolee et le cortex avoisinant ont ete ensuite stimules electriquement et l'enregistrement a ete fait de la meme maniere que chez les animaux non epileptiques. L'enregistrement du cortex homologue contro-lateral a la lesion epileptique (pratiquee 48 h avant) mettait en evidence la presence d'un foyer de pointes tres actif qui persistait apres isolement du cortex epileptogene primaire. Lorsque le cortex contenant le foyer en miroir etait isole, les pointes s'arretaient. Dans cette eventualite, lorsque une post-decharge commencait dans le cortex avoisinant, elle se propageait au cortex isole mais n'etait pas synchrone et y persistait plus longtemps que la decharge dans le cortex avoisinant. On conclut que la propagation de l'activiteepileptique dans le cortex isole est probablement un phenomene de ← volume conduction →. Cependant, si le cortex isole est d'abord sensibilise par des volees afferentielles epileptogenes, et meme quand il ne produit plus lui meme de pointes spontanees apres l'isolement, les post-decharges electriques propagees du cortex avoisinant font naitre une post-decharge independante, auto-entretenue, dans le cortex isole sensibilise. La sensibilisation epileptique a ainsi rendu possible la production d'une decharge epileptiforme auto-entretenue, declenchee par une post-decharge propagee par ← volume conduction →.