Abstracts

Rationale: Cortical malformations are a leading cause of intractable epilepsy in children. Because of their resistance to pharmacological treatment, patients often require surgical resection of the affected tissue to adequately treat seizures. Despite the prevalence of intractable epilepsy the underlying cellular and network defects that lead to hyperexcitability in these patients are not well understood. Abnormalities in neuronal migration and position are associated with malformations but it is unknown how defects in early development contributes to hyperexcitable circuitry. We hypothesize that both abnormal neuronal structure and migration of cortical pyramidal cells in dysplastic tissue results in defective connectivity and membrane properties that predispose the tissue to hyperexcitability.Methods: In this study, we used whole-cell patch clamp techniques combined with post-hoc morphological analysis of physiologically characterized cells to investigate the state of inhibitory GABAergic circuits and intrinsic properties in cortical pyramidal cells located in upper cortical layers of acutely resected dysplastic tissue from pediatric patients undergoing surgical resection for intractable epilepsy. Control tissue was acquired from the temporal tip or supratemporal gyrus from patients undergoing mesial temporal resection. Both spontaneous post-synaptic currents (sIPSCs) as well as evoked potentials were recorded in Layer II/III pyramidal cells in both groups. Intracellular solutions included 0.5% biocytin. For evoked potentials, we applied electrical stimulation (2-12 Volts) to neighboring cortical columns in the same cortical layer as the recorded cell. Cells were analyzed for passive and active instrinsic properties and correlated with post-hoc morphology (biocytin) for identification and analysis of abnormalities in dysplastic tissue.Results: We observed decreased spontaneous inhibitory postsynaptic currents (sIPSCs) in layer II/III pyramidal neurons compared with normal cortex indicative of compromised GABAergic circuitry. In addition, extracellular stimulation revealed a high percentage of intrinsically bursting neurons similar to those observed in hippocampal CA1 pyramidal cells in a rodent model of temporal lobe epilepsy (Sanabria et al., 2001 J PHYSIOL 532). In this model, intractable bursting is related to a redistribution of voltage gated calcium channels on apical dendrites (Yaari et al., 2007 J PHYSIOL 580). Morphologically we observed several unevenly distributed pyramidal cells with malformed apical dendrites.Conclusions: Abnormal apical dendrites may contribute to improper positioning and connectivity within the cortex and also to altered calcium channel expression. Therefore these data suggest a morphological correlate to multiple mechanisms underlying hyperexcitable circuitry in malformations. Abnormal GABAergic circuitry combined with altered intrinsic properties of principal neurons could represent a common feature of malformation-related seizures and provide novel, efficacious targets for treatment of these disorders.