Ephaptic Coupling Contributes to the Propagation of Paroxysmal Depolarization Shifts In Vitro

Introduction: Paroxysmal depolarization shifts (PDSs), correlated with interictal epileptiform discharges, involve significant membrane potential changes and action potentials. While synchronicity is crucial in paroxysmal activity, the precise function of PDSs and their propagation mechanisms, especially non-synaptic pathways like ephaptic coupling, remains poorly understood. This study investigates the role of ephaptic coupling in PDS propagation in hippocampal cultures, focusing on voltage-gated calcium channel (VGCC) subtypes. Methods: PDSs were induced in hippocampal neurone-glial cultures using bicuculline. The outside-out patch-clamp technique was used to record PDS activity at varying distances from the neuronal network. The effects of L-type (nifedipine) and T-type (ML-218) VGCC inhibitors on PDS amplitude and frequency were assessed. Membrane capacitance and resistance were monitored to verify the outside-out configuration. Results: PDSs could be recorded up to 16 µm from the network, with amplitude decreasing exponentially with distance. PDS frequency remained constant. Blocking L-type VGCCs completely abolished PDS activity at a distance, while T-type VGCC inhibition significantly reduced PDS amplitude. The transition from whole-cell to outside-out configuration was confirmed by a significant decrease in membrane capacitance. Discussion: The findings suggest that ephaptic coupling contributes to PDS propagation in vitro, with L-type VGCCs playing a critical role in field-mediated signal transmission. Constant PDS frequency with varying amplitude at a distance highlights a potential synchronization mechanism during epileptiform activity. Further research should investigate the interplay between ion channels and the extracellular environment during ephaptic coupling, paving the way for brain stimulation-based therapies. Conclusion: Research demonstrates that ephaptic coupling can propagate PDSs in hippocampal neurone-glial cultures, highlighting a promising mechanism for understanding epileptiform foci. This finding is critical for comprehending how these foci form and expand, and it also opens avenues for developing brain stimulation-based therapies.