Investigator Workshop 8:30 a.m.-5:30 p.m.

Abstract

Abstract 1 Veronique M. Andre, 2 My Huynh, 1 Michael S. Levine, 2 Gary W. Mathern, and 1 Carlos Cepeda ( 1 Mental Retardation Research Center, David Geffen School of Medicine at UCLA ; and 2 Department of Neurosurgery, David Geffen School of Medicine, Los Angeles, CA ) Rationale: In pediatric cortical dysplasia (CD), we previously found more GABA relative to glutamate spontaneous synaptic activity (Cepeda et al., 2005; Dev. Neurosci., In Press). In this study, we determined the proportion of GABAergic neurons and their morphology in non-CD and CD patients. In addition, we examined functional properties of GABAA receptor signaling in normal-appearing and cytomegalic neurons. Methods: Double immunofluorescence stainings and cell counts for NeuN and for GAD were performed on non-CD and CD patients. Additional brain sections were processed for parvalbumin, calretinin and GAT-1. We separated the CD tissue into CD normal (containing only normal-appearing neurons) and CD cytomegalic (containing some cytomegalic neurons). Electrophysiological recordings were performed on acutely dissociated pyramidal neurons from non-CD and CD patients. Patch electrodes were filled with N-methyl-D-glucamine for whole-cell voltage clamp recordings. Different concentrations of GABA were applied to the cells. Results: The percent of GAD-labeled cells was similar in non-CD and CD normal tissue. In contrast, there were fewer GAD-labeled cells in the CD tissue containing cytomegalic neurons. However, a dense plexus of GABAergic fibers was observed in CD tissue, mostly around cytomegalic neurons, but not in non-CD cases. We also noticed the presence of abnormally large neurons positive for GAD that could be cytomegalic interneurons. GABA-induced currents were recorded in pyramidal neurons from non-CD and CD cases and in cytomegalic neurons. GABA peak current amplitudes were similar in normal-appearing pyramidal neurons from the non-CD and CD groups while they were larger in cytomegalic neurons. However, when the currents were normalized by cell capacitance, GABA peak current densities were smaller in CD cytomegalic neurons. Normal-appearing pyramidal neurons from non-CD and CD tissue showed the same desensitization rate while it was significantly slower in the cytomegalic neurons. Conclusions: We found that the number of GABAergic cells was similar in non-CD and CD tissue containing only normal-appearing neurons, whereas it was reduced in regions containing cytomegalic neurons. However, we also observed dense networks of GABAergic fibers around cytomegalic pyramidal-shaped neurons and the presence of putative cytomegalic interneurons. These findings suggest that some GABAergic inputs originate outside the most affected areas and/or that important synaptic reorganization occurs within these regions, including dense innervation from cytomegalic GABAergic interneurons. These findings could account for the higher frequency of GABA relative to glutamate spontaneous synaptic activity shown by previous electrophysiological studies in slices. Finally, data from dissociated cells indicate that GABAA receptor subunit composition is also altered in cytomegalic neurons. (Supported by NIH NS38992.)