Abstracts

Rationale:
SCN8A epileptic encephalopathy (EE) is a severe epilepsy syndrome resulting from de novo mutations in the SCN8A gene, encoding the sodium channel Nav1.6. Patients with SCN8A mutations have seizure onset between birth and 12 months of age and exhibit cognitive and motor dysfunction. Patients also have a notable risk for sudden unexpected death in epilepsy (SUDEP) which increases significantly if seizures are not controlled.
Nav1.6 is expressed in both excitatory and inhibitory neurons, and the function of inhibitory interneurons is critical to constrain activity of excitatory neurons. Parvalbumin-positive interneurons (PV-INs) are among the most numerous inhibitory interneurons and express Nav1.6 in the distal axon initial segment. PV-INs directly inhibit excitatory pyramidal cells (PCs) through highly reliable synaptic connections. Impaired synaptic transmission has been suggested as a contributing factor to various epilepsies, and recent studies in Dravet Syndrome show developmentally regulated impairment of this PV:PC synaptic connection. However, PV-IN function and subsequent synaptic transmission have not yet been studied in the context of SCN8A EE.

Methods: Electrophysiology experiments and seizure confirmation via concurrent video/EEG recording were performed using two mouse models of SCN8A epileptic encephalopathy that express a patient-derived SCN8A mutation globally (Scn8aD/+) or selectively in PV-INs (Scn8aW/+-PV). Brain slices were prepared and recordings of synaptically connected PV:PC pairs were collected using dual whole-cell patch clamp techniques. Synaptic connections were identified by eliciting action potentials in the presynaptic PV-IN and detecting unitary inhibitory postsynaptic currents (uIPSCs) in the postsynaptic PC. Recordings of synaptic connections were obtained from WT, Scn8aD/+, and Scn8aW/+-PV mice.

Results: Our results indicate a significant increase in synaptic transmission failure, specifically at high frequencies. With PV-INs firing reliably at 80 and 120 Hz, we observed a > 20% increase in failure of postsynaptic uIPSCs in both Scn8aD/+ and Scn8aW/+-PV mice. We also note a significant (~25%) increase in synaptic latency in connected PV:PC pairs from both Scn8aD/+ and Scn8aW/+-PV mice, measured from the peak of the presynaptic action potential to the onset of the evoked uIPSC in the pyramidal cell. Additionally, we observed a decrease in the survival of Scn8aW/+-PV mice, which express the R1872W SCN8A mutation selectively in PV-INs.


Conclusions:
These data indicate a distinct impairment of synaptic transmission in SCN8A EE, which will likely result in overall decreased inhibition in the cortical network. Increased failure rates at high frequencies is quite notable due to the fact that PV-INs typically fire at high frequencies in vivo. An increase in synaptic failure rates could suggest impaired action potential propagation, and prolonged synaptic latency could possibly reflect a defect in overall release probability or conduction velocity. In total, further understanding of the PV:PC synapse in SCN8A EE is critical to recognizing the physiological consequences of increased NaV1.6 activity throughout the cortical network.

Funding: NIH RO1 NS103090