Abstracts

Rationale: The impact of frequent seizures during childhood on the development of inhibitory neurotransmission is unknown. In this study, we sought to characterize the evolution of the structural and functional components of GABAergic signaling in a child with intractable epilepsy who underwent initial surgical resection at 7 months of age and further resection of both intact and previously disconnected cortex at 5 years.Methods: The membrane and cytosolic fractions of resected cortical tissue were isolated and subjected to Western blot analysis. Additional membrane fractions were injected into Xenopus oocytes, resulting in incorporation of the brain membrane vesicles with their associated receptors into the oocyte cellular membrane, allowing two-electrode voltage clamp analysis. Formalin-fixed, paraffin-embedded sections were used for immunohistochemistry studies, which were performed utilizing standard techniques.Results: Infantile spasms developed at 4 weeks, followed by intractable complex partial seizures. Video EEG monitoring demonstrated seizures with independent left temporal or parietooccipital onset. At 7 months she underwent left temporal-occipital resection and disconnection. She became seizure-free for 2 years, after which seizures recurred and escalated despite multiple medication trials. Intracranial EEG monitoring at 5 years recorded electroclinical seizures with parietal onset, in addition to electrographic seizures arising in the disconnected portion of the occipital lobe. Further resection was performed, including parietal lobectomy and removal of previously disconnected occipital cortex. Postoperatively, she has been seizure-free without any new neurologic deficits. Samples from temporal-occipital cortex at 7 months, parietal cortex at 5 years, and occipital cortex disconnected at 7 months then resected at 5 years all demonstrated dyslamination and dysmorphic neurons consistent with focal cortical dysplasia type 2A. The disconnected region also exhibited cortical thinning and neuronal loss. GFAP expression, indicative of cortical gliosis, increased over time, and was highest in the disconnected cortex. The relative density of GABAergic interneurons increased between 7 months and 5 years, as did expression levels of GAD65/67. This was associated with increases in receptor responses to GABA relative to glutamate from intact, but not disconnected, cortex. GABA-A alpha1 subunit expression and pharmacologic responses increased substantially in the disconnected cortex only, whereas GABA-A delta subunit expression and responses were unchanged. Conclusions: This longitudinal study of the structural and functional properties of inhibitory GABAergic signaling in human cortex demonstrates evolution over time in the setting of ongoing seizure activity. Disconnection of epileptogenic cortex from other cortical and subcortical structures alters this evolution. These findings may have implications for anticonvulsant selection in intractable pediatric epilepsy, as seizures arising early in the course of the disease may respond differently to GABAergic agents than do seizures arising later.