Interneurons, GABAA currents, and subunit composition of the GABAA receptor in type I and type II cortical dysplasia

Abstract

Interneurons, gamma-aminobutyric acid (GABA)(A) receptor density, and subunit composition determine inhibitory function in pyramidal neurons and control excitability in cortex. Abnormalities in GABAergic cells or GABA(A) receptors could contribute to seizures in malformations of cortical development. Herein we review data obtained in resected cortex from pediatric epilepsy surgery patients with type I and type II cortical dysplasia (CD) and non-CD pathologies. Our studies found fewer interneurons immunolabeled for glutamic acid decarboxylase (GAD) in type II CD, whereas there were no changes in tissue from type I CD. GAD-labeled neurons had larger somata, and GABA transporter (VGAT and GAT1) staining showed a dense plexus surrounding cytomegalic neurons in type II CD. Functionally, neurons from type I CD tissue showed GABA currents with increased half maximal effective concentration compared to cells from the other groups. In type II CD, cytomegalic pyramidal neurons showed alterations in GABA currents, decreased sensitivity to zolpidem and zinc, and increased sensitivity to bretazenil. In addition, pyramidal neurons from type II CD displayed higher frequency of spontaneous inhibitory post synaptic currents. The GABAergic system is therefore, altered differently in cortex from type I and type II CD patients. Alterations in zolpidem, zinc, and bretazenil sensitivity and spontaneous inhibitory postsynaptic currents (IPSCs) suggest that type II CD neurons have altered GABA(A) receptor subunit composition and receive dense GABA inputs. These findings support the hypothesis that patients with type I and type II CD will respond differently to GABA receptor-mediated antiepileptic drugs and that cytomegalic neurons have features similar to immature neurons.

Figure 1

A. The percent of GAD-labeled neurons was decreased in tissue from Type II CD, but not in Type I CD. In regions containing cytomegalic neurons, the loss was more pronounced. GAD-labeled cell somatic area was larger in Type II CD tissue containing cytomegalic neurons. B. Staining for the vesicular glutamic acid transporter (VGAT) showed dense GABA terminal innervation around cytomegalic pyramidal neurons. C. GAD staining shows abnormally large cell somas in Type II CD in regions with cytomegalic pyramidal neurons. *: P<0.05, ***: P<0.001 (compared to non-CD).

Figure 2

A. Traces show responses to increasing GABA concentrations. GABA currents were larger in cytomegalic pyramidal neurons B. Because of their large capacitance, current densities for GABA currents were smaller in cytomegalic neurons compared to all other groups. C. GABA EC₅₀ values were higher in Type I compared to all other groups. D. Zolpidem modulation of GABA currents was smaller in Type II CD compared to non-CD cells. E. Bretazenil effect was larger in Type II CD cells. F. Zinc block was smaller in Type II CD, in cytomegalic neurons only. Number of cells in each group is indicated in parentheses. *: P<0.05, **: P<0.01, ***: P<0.001 (compared to non-CD).

Figure 3

A. Traces show spontaneous IPSCs in the four groups. B. Cumulative inter-event interval histogram shows Type II cells (normal and cytomegalic) have higher IPSC frequencies than non-CD and Type I CD cells. C. Overall frequency of spontaneous IPSCs is higher in Type II cells compared to Type I CD and non CD. In addition, cytomegalic neurons showed higher IPSC frequency than normal pyramidal neurons from Type II CD. Number of cells in each group is indicated in parentheses. D. Cumulative amplitude histogram shows Type II cells (normal and cytomegalic) have larger IPSCs than non-CD and Type I CD cells. E. Traces are averages of spontaneous GABA synaptic currents. F. G. Decay time constant and area under the curve are larger in cytomegalic neurons compared to all other groups. *: P<0.05, **: P<0.01, ***: P<0.001 (compared to non-CD).