Metabolic anatomy of the focal epilepsy produced by cessation of chronic intracortical GABA infusion in the rat

Abstract

Cessation of chronic (5 days), unilateral infusion of GABA into the somatomotor cortex of rats induces focal epileptic spikes which remain limited to the infused site and never evolve into generalized seizures. We have considered this finding as a new model of focal epilepsy and named it “GABA withdrawal syndrome”. In the present study, we have measured local cerebral glucose utilization in order to map the cortical and subcortical regions involved in the GABA withdrawal syndrome. Local cerebral glucose utilization increased two- to three-fold in a 1–1.5 mm diameter area, involving all the cortical layers at the GABA-infusion site. This hypermetabolic area contained a central (1–2 mm diameter) hypometabolic zone showing neuronal depopulation in some animals. Except for the epileptic focus, the hemisphere ipsilateral to the infusion site was slightly hypometabolic. However, there was a large increase (three- to five-fold) in some ipsilateral thalamic nuclei (posterior oralis, ventralis postero-lateralis, centralis lateralis, ventralis lateralis and reticularis thalami nucleus). The local cerebral glucose utilization of the controlateral cortex and thalamus were unchanged. The present results confirm the focal nature of the epileptogenic syndrome produced by stopping chronic, intracortical GABA infusion. These results are markedly different from those described in the penicillin focal epilepsy model. Our data also show that specific ipsilateral thalamic relays may, by an as yet unknown mechanism, play a role in maintaining paroxysmal activity during the GABA withdrawal syndrome.