Reduced dendritic arborization and hyperexcitability of pyramidal neurons in a Scn1b-based model of Dravet syndrome

Dravet syndrome is an epileptic encephalopathy associated with mutations in the sodium channel subunits SCN1A or SCN1B. Although interneuron pathology is implicated in SCN1A-based Dravet syndrome, Reid et al. obtain evidence of excitatory neuron pathology in mice homozygous for the SCN1B mutation. This convergence of disease pathways may have therapeutic implications.Epileptic encephalopathies, including Dravet syndrome, are severe treatment-resistant epilepsies with developmental regression. We examined a mouse model based on a human beta 1 sodium channel subunit (Scn1b) mutation. Homozygous mutant mice shared phenotypic features and pharmaco-sensitivity with Dravet syndrome. Patch-clamp analysis showed that mutant subicular and layer 2/3 pyramidal neurons had increased action potential firing rates, presumably as a consequence of their increased input resistance. These changes were not seen in L5 or CA1 pyramidal neurons. This raised the concept of a regional seizure mechanism that was supported by data showing increased spontaneous synaptic activity in the subiculum but not CA1. Importantly, no changes in firing or synaptic properties of gamma-aminobutyric acidergic interneurons from mutant mice were observed, which is in contrast with Scn1a-based models of Dravet syndrome. Morphological analysis of subicular pyramidal neurons revealed reduced dendritic arborization. The antiepileptic drug retigabine, a K+ channel opener that reduces input resistance, dampened action potential firing and protected mutant mice from thermal seizures. These results suggest a novel mechanism of disease genesis in genetic epilepsy and demonstrate an effective mechanism-based treatment of the disease.