Abstracts

Rationale: We have recently generated a rat model of chronic epilepsy associated with cortical malformations, i.e., the methylazoxymethanol-pilocarpine rat (MAM-PILO; Colciaghi et al, 2011), which shows similarities with human epileptogenic cortical malformations. In this model, status epilepticus (SE) and subsequent seizures set in motion a pathologic process capable of modifying the morphology of cortical epileptic neurons and expression and subcellular localization of NMDA receptors. We have here verified whether the extent of the observed abnormalities might be related with the duration and severity of epilepsy.Methods: MAM rats treated with pilocarpine and diazepam (MAM-DZP-PILO), not experiencing SE, were used as controls. MAM-PILO rats were analyzed at 18h after SE, i.e., in the acute stage; 3-5 days after epilepsy onset, the early chronic stage (EC); and 3 and 6 months after epilepsy onset, the chronic epilepsy stages. Spontaneous recurrent seizures of epileptic rats were quantified with EEG and video-monitoring. In all groups of rats morphologic and molecular analysis of brain cortical areas were performed at different time-points after SE induction.Results: The analysis of thionine-stained coronal sections revealed a progressive, time-dependent decrease of cortical thickness, related to epilepsy duration, particularly pronounced at caudal cerebral levels (Fig. 1). Overall, the neocortical thickness of chronic epileptic MAM-PILO rats after 3 and 6 months of epilepsy was reduced to 20% and 35%, respectively, when compared with control MAM-DZP-PILO rats. No significant differences were observed in the acute and EC stages when compared to non-epileptic MAM brains. We also detected an epilepsy-dependent increase in the density of abnormally large cortical pyramidal neurons with neurofilaments over-expression (EC vs 3 months epilepsy, *p<0.05; EC vs 6 months epilepsy, ***p<0.001; 3 months vs 6 months epilepsy, **p<0.01). Further, the quantification of pyramidal neurons with increased recruitment of NR2A/B subunits to the post-synaptic membrane revealed that the density of these neurons was progressively and significantly higher in chronic epileptic MAM-PILO vs EC MAM-PILO rats (p<0.01**). Interestingly, in EC MAM-PILO rats (and in 2-3 months epileptic rats) the NR2AB positive neurons were concentrated in the heterotopic cortical areas (corresponding to the sensorimotor cortex), whereas after 6 months of epilepsy they were also detectable in more rostral and caudal cortical areas, suggesting a progressively more widespread pathologic event.Conclusions: Our data further indicate that in MAM-PILO rats the presence and the extent of structural and NMDA abnormalities of cortical pyramidal neurons are positively related to the length of the epilepsy process. Even if it still remains to be established whether these changes might further favor chronic neuronal hyperexcitability, they represent a relevant read-out to evaluate epilepsy progression and the efficacy of anti-epileptic treatments.