Progressive Brain Damage, Synaptic Reorganization and NMDA Activation in a Model of Epileptogenic Cortical Dysplasia

Francesca Colciaghi,Denise Locatelli,Giada Spigolon,Adele Finardi,Giorgio Stefano Battaglia,Paola Nobili

doi:10.1371/journal.pone.0089898

Abstract

Whether severe epilepsy could be a progressive disorder remains as yet unresolved. We previously demonstrated in a rat model of acquired focal cortical dysplasia, the methylazoxymethanol/pilocarpine - MAM/pilocarpine - rats, that the occurrence of status epilepticus (SE) and subsequent seizures fostered a pathologic process capable of modifying the morphology of cortical pyramidal neurons and NMDA receptor expression/localization. We have here extended our analysis by evaluating neocortical and hippocampal changes in MAM/pilocarpine rats at different epilepsy stages, from few days after onset up to six months of chronic epilepsy. Our findings indicate that the process triggered by SE and subsequent seizures in the malformed brain i) is steadily progressive, deeply altering neocortical and hippocampal morphology, with atrophy of neocortex and CA regions and progressive increase of granule cell layer dispersion; ii) changes dramatically the fine morphology of neurons in neocortex and hippocampus, by increasing cell size and decreasing both dendrite arborization and spine density; iii) induces reorganization of glutamatergic and GABAergic networks in both neocortex and hippocampus, favoring excitatory vs inhibitory input; iv) activates NMDA regulatory subunits. Taken together, our data indicate that, at least in experimental models of brain malformations, severe seizure activity, i.e., SE plus recurrent seizures, may lead to a widespread, steadily progressive architectural, neuronal and synaptic reorganization in the brain. They also suggest the mechanistic relevance of glutamate/NMDA hyper-activation in the seizure-related brain pathologic plasticity.

Highlights

The question whether repeated seizures might be associated with progressive alterations of the brain has been long debated and as yet unresolved [1]
We recently demonstrated that in a rat model of acquired Focal cortical dysplasia (FCD) (MAM/pilocarpine rats) [26] the occurrence of status epilepticus (SE) and spontaneous seizures gives rise to abnormally large cortical pyramidal neurons with neurofilament over-expression and recruitment of NMDA regulatory subunits at the post-synaptic membrane, strictly similar to the hypertrophic/ dysmorphic neurons observed in human FCD [10]
We have here extended the morphologic and molecular analysis of methyl-axozymethanol- acetate (MAM)-pilocarpine rats to both neocortex and hippocampus from few days after epilepsy onset up to six months of recurring seizures to verify whether pathologic brain changes were widespread and progressive over time

Summary

Introduction

The question whether repeated seizures might be associated with progressive alterations of the brain has been long debated and as yet unresolved [1]. Even though recent MRI studies showed that human pharmaco-resistant temporal lobe epilepsy (TLE) was associated with progressive and diffuse cortical atrophy, likely representing seizure-induced damage [2], the causal relationship between seizure activity and brain damage is still controversial [3]. The sequence of events possibly leading to disease progression in drug-resistant epileptic patients and experimental models remains elusive [4,5,6]. Cortical specimens from FCD patients showed increased propensity to generate epileptiform activity, possibly due to enlarged dysmorphic neurons acting as epileptic generators [13], and/or to excessive glutamatergic synaptic input associated with reduced inhibition. Data from surgical specimens of epileptic FCD patients demonstrated increased expression levels of NR2A/B subunits of the NMDA (N-Methyl-D-Aspartate) receptor [14,15,16,17,18] and associated PSD95 protein [19]. The precise mechanisms of the intrinsic hyperexcitability in FCD remain to be fully clarified

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Conclusion