Abstracts

Rationale: De novo mutations in the voltage-gated sodium channel gene SCN8A, encoding Nav1.6, are linked to early-infantile epileptic encephalopathies (EIEE). A heterozygous knock-in mouse model carrying SCN8A patient mutation p.Asn1768Asp (Scn8aN1768D/+) recapitulates several key features of EIEE including spontaneous convulsive seizures and sudden unexpected death in epilepsy (SUDEP). Expression of Nav1.6-N1768D in a heterologous system resulted in impaired channel inactivation, but its in vivo effects on neuronal firing have not been investigated.Methods: We examined the passive and evoked membrane electrical properties of cortical layer II/III and hippocampal CA1 and CA3 pyramidal cells in acute brain slices prepared from Scn8aN1768D/+ mice and wild-type littermate controls using the whole-cell patch clamp recording technique.Results: No significant differences in resting membrane potential, input resistance, threshold potential, time constant (tau) of the rising phase of the action potential (AP), or peak amplitude of the AP were observed between mutant and WT neurons. The input/output relationships and maximum AP firing frequencies were not different between genotypes. In contrast, Scn8aN1768D/+ neurons showed significantly slower AP decay than WT, leading to a significant increase in the area under the AP waveform (n=12, P<0.05). Early afterdepolarization (EAD)-like hyperexcitability was observed in the late phase of AP decay in the majority of CA1 pyramidal neurons examined. Interestingly, this EAD-like waveform was observed in CA1 neurons in response to both short- and long-pulse depolarizing current injections, but not in CA3 or cortical layer II/III pyramidal cells, suggesting region-specific changes in neuronal excitability. Moreover, spontaneous AP firing occurred in a subset of both CA1 and CA3 pyramidal neurons from Scn8aN178D/+, but not WT, brain slices. Some CA1 neurons displayed EAD-like AP waveforms during spontaneous firing, suggesting an intrinsic change in neuronal excitability.Conclusions: Taken together, these data suggest that the Scn8aN1768D/+ mutation may selectively affect hippocampal neurons to induce abnormal AP firing activity and hyperexcitability. Supported by NIH grants R01-NS-076752 to LLI, U01-NS-090364 to JP and LLI, and R01-NS-034509 to MHM.