Abstracts

Rationale: Cortical dysplasia is a common cause of intractable epilepsy in infants and children, producing seizures that are frequently resistant to GABAergic anticonvulsant medications. In this study, we sought to better understand the cellular mechanisms of this resistance through analysis of responses to agents that modulate GABA-A receptor function in resected dysplastic cortex as compared with age-matched control cortex. Methods: Cortical tissue from children undergoing epilepsy surgery at Seattle Children s Hospital was snap frozen in liquid nitrogen and stored at -80 C. Frozen control neocortical specimens were obtained from the NICHD Brain and Tissue Bank for Developmental Disorders. The membrane fractions of the frozen tissues 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 of GABA-A receptor currents. An additional membrane fraction was isolated and subjected to Western blot analysis. Results: Fourteen control and eleven focal cortical dysplasia (FCD) type 2A specimens were analyzed. In the control specimens, maturation of GABA-A receptor pharmacologic properties was noted, as older children (age > 3 yrs) displayed decreased receptor affinity, increased response to the alpha 1 subunit-selective benzodiazepine site agonist zolpidem versus the nonselective benzodiazepine diazepam, and increased degrees of current enhancement by the barbiturate pentobarbital and by the neurosteroid 5?-pregnane-3?-ol-20-one. In contrast, the pharmacologic profiles of the epileptogenic FCD 2A specimens obtained from both infants and older children were most comparable to those seen in immature control cortex. The maturation of GABAergic pharmacologic properties in control children, as well as the dysmaturity present in the FCD 2A specimens, was directly related to the level of GABA-A alpha 1 subunit expression and inversely related to the level of alpha 4 subunit expression. Conclusions: Epileptogenic regions of focal cortical dysplasia type 2A demonstrate immature responses to GABAergic pharmacologic agents, which may contribute to resistance to current anticonvulsant therapies and suggest alternative treatment approaches.