Abstract

Malformations of cortical development are increasingly recognized in association with severe epileptic syndromes, neuropsychological disorders and mental retardation. Several clinical and experimental studies suggest that functional consequences of cortical dysplasias are not restricted to the area of the dysplastic lesion but also involve remote brain regions. In the present study cortical malformations were induced in newborn rats at day of birth by intracerebral injection of the glutamatergic agonist ibotenate. The resulting cytoarchitectonic lesion associates neuronal depopulation of deep cortical layers, ectopic neurons in superficial layers and sulcus formation, mimicking human polymicrogyria and migration disorders. Electrophysiological recordings of evoked field potentials in slice preparations of adult animals reveal hyperexcitability in widespread cortical regions surrounding the dysplasia. Low-intensity stimulation induced epileptiform activity consisting of long-lasting, multiphasic and N-methyl-D-aspartate-dependent field responses. They appeared with high variability as all-or-none events. These widespread changes in excitability were not observed in sham-operated animals with small superficial ectopias but intact deep cortical layers, indicating that focal loss of these layers induces extended alterations in cortical connectivity and imbalance of excitation and inhibition. Restricted zones of increased excitability were also found in the forelimb and hindlimb representation cortex in sham-operated and control animals, demonstrating that this activity has to be considered as an intrinsic property of specific cortical areas. Deoxyglucose autoradiography showed that the widespread hyperexcitability in ibotenate-injected animals was not accompanied by alterations in glucose metabolism, although in the area of structural abnormality a typical metabolic pattern was found, revealing an increased glucose uptake in layer I. Hypometabolism as described for many types of human dysplastic lesions was not observed. This difference between the experimental and clinical data may be due to the absence of behavioral seizures in this model. However, it can be hypothesized that in patients with developmental malformations, additional pathogenic factors contribute to the manifestation of seizure disorders.