Electroencephalographic characterization of an adult rat model of radiation-induced cortical dysplasia.

PURPOSE: Cortical dysplasia (CD) is a frequent cause of medically intractable focal epilepsy. The mechanisms of CD-induced epileptogenicity remain unknown. The difficulty in obtaining and testing human tissue warrants the identification and characterization of animal model(s) of CD that share most of the clinical, electroencephalographic (EEG), and histopathologic characteristics of human CD. In this study, we report on the in vivo EEG characterization of the radiation-induced model of CD. METHODS: Timed-pregnant Sprague-Dawley rats were irradiated on E17 using a single dose of 145 cGy or left untreated. Their litters were identified and implanted with bifrontal epidural and hippocampal depth electrodes for prolonged continuous EEG recordings. After prolonged EEG monitoring, animals were killed and their brains sectioned and stained for histologic studies. RESULTS: In utero-irradiated rats showed frequent spontaneous interictal epileptiform spikes and spontaneous seizures arising independently from the hippocampal or the frontal neocortical structures. No epileptiform or seizure activities were recorded from age-matched control rats. Histologic studies showed the presence of multiple cortical areas of neuronal clustering and disorganization. Moreover, pyramidal cell dispersion was seen in the CA1>CA3 areas of the hippocampal formations. CONCLUSIONS: Our results further characterize the in vivo EEG characteristics of the in utero radiation model of CD using long-term EEG monitoring. This model may be used to study the molecular and cellular changes in epileptogenic CD and to test the efficacy of newer antiepileptic medications.

Brain plasticity and cellular mechanisms of epileptogenesis in human and experimental cortical dysplasia.

PURPOSE: The cellular mechanisms that may contribute to epilepsy in resected human cortical dysplasia (CD) were compared with the in utero radiated rat CD model. In human and rat focal hippocampal epilepsy, postsynaptic N-methyl-D-aspartate receptors are up-regulated and presynaptic axon collaterals hyperinnervate them. We hypothesized that in both human and rat CD: (a) the N-methyl-D-aspartate receptor subunits NR1 and NR2A/B would be increased and coassembled, and (b) aberrant axons would be in regions of CD. METHODS: Tests for presynaptic and postsynaptic changes in human and rat CD included the following: (a) cytology, (b) immunocytochemistry, (c) coimmunoprecipitation, (d) double-labeled immunofluorescence, and (e) Timm histochemistry of hippocampal mossy fibers. Within-patient comparisons were made between epileptic tissue, identified by subdural electro-encephalographic seizure onsets, and nonepileptic tissue remote from the focus but within the therapeutic resection. Rats were radiated at embryonic day 17, and offspring were studied postnatally. Statistical comparisons were made against normal rats matched for age and tissue processing. RESULTS: In focal CD patients, NR2A/B subunits and their coassemblies with NR1 were increased significantly more than for the remote nonepileptic cortex. Confocal microscopy showed that NR1-NR2A/B colabeled single dysplastic neurons in both human and rat. In CD rats, mossy fibers innervated the anomalously oriented hippocampal neurons. CONCLUSIONS: Human epileptic CD exhibits a spectrum of abnormal cell orientations and laminations that must require plastic axodendritic changes during development. These altered circuits and receptors could account for the seizures and cognitive deficits found in patients with CD. The radiated rat CD model with cortical dyslaminations and NR2A/B subunit increases would allow the development and testing of drugs targeted at only the NR2A/B subunit or at decoupling the NR1-NR2 coassembly, which could provide a specific antiepileptic drug for dysplastic circuits without inducing general depression of all brain neurons.

NMDA-receptors 1 and 2A/B coassembly increased in human epileptic focal cortical dysplasia.

PURPOSE: This study was designed to quantify the relation between expressions of NMDA receptor (NMDAR) subunits (1 and 2A/B) and the epileptogenicity in human focal cortical dysplasia. METHODS: Immunoblotting and immunoprecipitation were used to quantify these receptor subunits in tissue resected from EEG-verified epileptic and distal nonepileptic frontal cortical areas in each of three patients as determined by chronic subdural electrode recordings. In each patient, adjacent sections were immunostained to verify that the numbers of dysplastic neurons were greater in epileptic than in nonepileptic cortex. RESULTS: In all patients, NMDAR2A/B expressions and their coassemblies with NMDAR1 were increased in epileptic dysplastic cortex compared with the relatively normal appearing nonepileptic cortex. For all three patients, there were no significant differences in NMDAR1 protein expressions between the two EEG groups. CONCLUSIONS: These results suggest that increased NMDAR1-NMDAR2A/B coassembly contributes to hyperexcitability in dysplastic cortical neurons and focal seizure onsets.