Abstracts

Rationale: Malformations of Cortical Development (MCDs) are developmental brain abnormalities frequently associated with drug resistant focal epilepsy. Among MCDs, type IIB Focal Cortical Dysplasia (FCD IIB) possess distinctive neuro-pathologic and clinical features, and affected patients show severe epilepsy course. To better understand the molecular and cellular mechanisms underscoring the origin and recurrence of seizures in human FCDs, we have undertaken a two-fold approach in both human and experimental settings. Methods: We have analyzed on one side human pediatric and adult subjects with type IIB FCD surgically treated for drug-resistant focal epilepsy with a combined morphologic and molecular analysis of NMDA receptor complex composition. In addition, we have generated and characterized an experimental double-hit animal model by first inducing cortical malformations with prenatal exposure to methylazoxymethanol acetate (MAM) and then triggering, in adulthood, spontaneous recurrent seizures by means of pilocarpine. Results: Our results on human subjects revealed increased expression of NMDA regulatory subunits in the post-synaptic membranes of dysmorphic neurons, which was evident in both pediatric and adult FCD patients (see Fig 1), thus indicating that abnormalities of NMDA receptor complex are consistently associated with, and may sustain epileptogenesis in FCD IIB patients. The behavioral and EEG data in our experimental model clearly demonstrated higher severity of epilepsy in MAM-pilocarpine (MAM-PILO) than na ve rats made epileptic with pilocarpine. Furthermore, the combined morphologic and molecular analysis demonstrated that chronic epilepsy worsened cortical architectural and NMDA abnormalities induced by MAM administration. Indeed, MAM-PILO rats were characterized by decreased cortical thickness and larger dysplastic pyramidal neurons with recruitment of NMDA regulatory subunits to the post-synaptic membrane, impressively resembling dysmorphic neurons of human FCD IIB (see Fig. 2). The observed abnormalities in chronic epileptic MAM-PILO rats were not due to direct pilocarpine effects or more severe epileptic status. Indeed, FluoroJade and thionine staining demonstrated more widespread cell degeneration and more evident edema in the cerebral cortex of na ve rats treated with pilocarpine alone, thus supporting that cellular/molecular abnormalities observed in the chronic epileptic MAM-PILO rats were determined by the recurrence of seizures. Conclusions: The present data indicate that in the malformed brain a seizure-induced, cellular/molecular pathologic plasticity may play as a key actor in establishing a pathological circuitry further affecting the propensity of generating seizures.