Our results indicate that GABAA
receptors, GAD-positive cells and GABA terminals are differentially affected in Type I and Type II CD. In Type II CD, there were fewer GAD cells in tissue containing cytomegalic neurons, as reported by others in dysplastic regions (Spreafico et al., 2000
; Thom et al., 2003
; Deukmedjian et al., 2004
). This suggests that tissue from Type II CD with cytomegalic cells, corresponding to regions with fewer interneurons could more easily induce or propagate seizures, secondary to decreased GABA neurotransmission. However, we found that regions with decreased numbers of GAD-positive cells had increased GAD cell soma size, consistent with findings from our laboratory showing cytomegalic interneurons (Andre et al., 2007
). In addition, we and others have described a dense plexus of GABAergic fibers surrounding pyramidal cell somata in Type II CD (Spreafico et al., 1998
; Alonso-Nanclares et al., 2005
). These findings support the notion that, while there are fewer GABAergic cells in Type II CD tissue, the remaining GABAergic cells have the ability to support GABA function. However, the subsequent GABAergic network is likely to be abnormal, which could lead to weaker inhibition, synchronize populations of neurons in damaged areas and possibly be epileptogenic.
Although current densities were reduced in cytomegalic neurons, the overall GABA currents might still be increased, because of their large size, especially if they receive significant GABA innervation. The higher frequency of spontaneous IPSCs in both normal and cytomegalic neurons confirms that Type II CD neurons are connected by dense GABA inputs. In addition, slower GABA current kinetics suggest the channels stay open longer, which provides evidence for a stronger effect of GABA. In this case, and even if cytomegalic pyramidal neurons show hyperexcitable properties, an increase in the total number of GABAA
receptors on the soma and slowly desensitizing GABA currents could produce a shunting of excitatory inputs if GABA is inhibitory. Changes in the effect of zolpidem and bretazenil support the concept that GABAA
receptor subunit composition is probably altered in Type II CD, with responses consistent with decreases in the proportion of α1 relative to α3 α4 and α5. The presence of less α1- and more α3 α4 and α5-containing receptors corresponds to GABAA
receptors inducing slow GABA currents described during embryonic development (Araki et al., 1992
; Owens et al., 1999
; Bosman et al., 2002
). This would fit with the hypothesis that in Type II CD, some neurons show characteristics of immature brains. We also showed that cytomegalic neurons displayed less sensitivity to zinc, indicating more γ2 subunits. The presence of more γ2 subunits along with other subunits could render GABAA
receptors more sensitive to some benzodiazepines in severe CD.
Our morphological and electrophysiological data show that GABA cells and GABA function are altered in Type II CD, with evidence showing increased GABA function. Ample GABA release, high GABA concentrations or prolonged GABA application could have a depolarizing effects on Type II CD neurons, especially if they show immature properties and altered intracellular chloride concentrations. In this case, AEDs increasing GABA function might not be beneficial in Type II CD and other options should be considered. In contrast, in Type I CD, we did not find any loss of GAD-labeled cells, any GABA terminal reorganization or any change in IPSC frequency but functionally, there was decreased GABA sensitivity compared to non-CD, which further suggests deficient inhibition in Type I CD. This indicates that AEDs increasing GABA function might be more efficient to control seizures in Type I CD patients.