Synaptic Dysregulation Drives Hyperexcitability in Pyramidal Neurons Surrounding Freeze-Induced Neocortical Malformations in Rats.

Focal cortical dysplasia (FCD) is a leading cause of drug-resistant epilepsy; however, the mechanisms underlying hyperexcitability in the affected cortical regions remain poorly understood. In this study, we employed a freeze-induced neocortical malformation model in rats to investigate the electrophysiological properties of pyramidal neurons surrounding the microgyrus and to evaluate changes in synaptic transmission. Using whole-cell patch-clamp recordings, we analyzed passive and active membrane properties, synaptic responses, and epileptiform activity in brain slices from rats with FCD and sham-operated controls. Our results revealed that while the intrinsic biophysical properties of neurons remained largely unchanged, the summation of excitatory and inhibitory inputs was significantly enhanced. Notably, the balance of inhibitory and excitatory synaptic currents was shifted toward excitation, making the perilesional cortex more susceptible to seizure generation. In a model of epileptiform activity induced by GABA(A) receptor blockade and reduced Mg(2+) concentration, we observed early ictal activity originating in the microgyrus and spreading to adjacent regions. These findings demonstrate that synaptic perturbations, rather than alterations in intrinsic neuronal properties, are the primary drivers of hyperexcitability in this model. Our study highlights the importance of synaptic dysregulation in FCD-related epilepsy and suggests that targeting synaptic transmission may offer a promising therapeutic strategy for controlling seizures in patients with cortical malformations.