Layer V Neocortical Neurons From Individuals With Drug-Resistant Epilepsy Show Multiple Synaptic Alterations but Lack Somatic Hyperexcitability

Abstract

Although neuronal hyperexcitability is the primary mechanism underlying seizure activity in epilepsy, little is known about how different neuronal mechanisms at different organizational levels contribute to network hyperexcitability in the human epileptic brain. In this study, we determined a series of cellular and synaptic properties of layer V pyramidal neurons from neocortical tissue of patients with drug-resistant epilepsy that may contribute the hyperexcitable state associated with epilepsy. Using the whole cell, patch-clamp technique, and extracellular recordings, we determined the passive and active electrophysiological properties of layer V pyramidal neurons with regular spiking phenotypes from temporal, parietal, and frontal neocortices surgically resected from individuals with drug-resistant epilepsy. Also, the glutamatergic strength, the synchronicity between presynaptic volleys and field excitatory postsynaptic potentials, and short-term, frequency-dependent plasticity were determined at the synaptic level. Lastly, biocytin-filled pyramidal neurons were used to perform post hoc digital reconstructions and morphometric analyses. The collected data revealed that pyramidal neurons exhibit minimal spontaneous activity, similar resting membrane potentials, and input resistance values among the temporal, parietal, and frontal neocortices. Although frontal neurons were more hyperexcitable than temporal and parietal neurons, the firing output was comparable to that previously observed in non-pathological human tissue. The digital reconstructions confirmed the identity of pyramidal neurons and revealed alterations in dendritic complexity. In contrast, the analyses of the extracellular recordings uncovered significant desynchronization between presynaptic excitability and postsynaptic activity and loss of short-term depression in response to repetitive stimulation within the gamma range (30 Hz). Our data suggest that neocortical layer V pyramidal neurons from individuals with drug-resistant epilepsy are not necessarily hyperexcitable at the somatic level. Instead, synaptic alterations, such as synaptic desynchronization and a loss of frequency-dependent short-term depression may significantly contribute to the hyperexcitable state observed during seizure activity.