MULTIFOCAL SPONTANEOUS EPILEPTIC ACTIVITY-INDUCED BY RESTRICTED BICUCULLINE EJECTION IN THE PIRIFORM CORTEX OF THE ISOLATED GUINEA-PIG BRAIN

1. We investigated the development of epileptic activity in the limbic lobe of the in vitro isolated brain preparation after induction of a restricted epileptic focus in the rostral portion of the anterior piri form cortex (APC) by means of local bicuculline ejection. 2. The modif ications of the local responses evoked by stimulation of the lateral o lfactory tract were consistent with an increase in excitability of lay er II pyramidal neurons. The abnormal discharge output of the epilepti c focus propagated throughout the piriform cortex (PC) and generated l ate synaptic stimulus-evoked epileptiform potentials in layer Ib, wher e corticocortical associative fibers contact the distal dendrites of p yramidal cells. 3. Spontaneous epileptiform potentials (SEPs) originat ed in the primary focus at the bicuculline ejection site 10-15 min aft er drug application and propagated via associative fibers to the poste rior PC(PPC). 4. Autonomous secondary foci generating SEPs independent ly from the primary anterior focus developed in the PPC within 2 h aft er the bicuculline application. 5. Ictal events seldom occurred sponta neously in PC but could be induced by tetanic afferent stimulation. Ev oked ictal events in PC had the characteristics of afterdischarges, be ing short in duration and localized in space and showing a progressive ly decreasing firing rate. 6. Self-sustained spontaneous ictal dischar ges were observed in the entorhinal cortex and hippocampus but not in PC after activation of the APC focus. 7. We conclude that in an acute pharmacological model of focal epilepsy, associative corticocortical c onnections hold a primary role I) in the propagation of interictal epi leptiform activity throughout the limbic lobe and 2) in the generation of secondary epileptic foci. We also demonstrate that self-sustained ictal events are not spontaneously generated in PC, although paroxysma l seizure-like discharges occur in hippocampus. We discuss the relevan ce of the rostral-to-caudal-directed associative fiber system in gener ation of limbic seizures.