Abstracts

Rationale: Early onset epileptic encephalopathies (EOEE), such as Dravet syndrome, are a severe group of epilepsies with developmental regression for which multiple genetic causes are emerging. Previously EOEE has been reported in a SCN1B functional null patient and to further elucidate the pathogenic mechanisms we investigated a Scn1b(C121W) homozygous mouse model and examined behavioural and neuronal phenotypes. Methods: Thermal seizure susceptibility was determined by placing P14-17 mice in a chamber heated to 41°C, and the time to first tonic-clonic seizure was recorded. Gait was assessed using Digigait (Mouse Specifics Inc., Boston, MA, USA) when mice were P17. Whole-cell patch clamp was used to compare neuronal properties in homozygous and control subicular pyramidal neurons from P14-16 mice. Current clamp mode was used to measure input-output relationships, and cellular passive properties were also recorded. Neuronal tracing and analysis was performed using the Neurolucida and Neurolucida Explorer software package (MBF Biosciences, Willison, VT). Results: Homozygous mice exhibited heightened susceptibility to thermally triggered seizures, possessed abnormal gait, died prematurely at around P21 and, similar to human Dravet patients, were responsive to stiripentol and unresponsive to lamotrigine. Neurons from homozygous mice had left-shifted action potential (AP) input-output curves (n=10, p<0.05), suggesting increased excitability. While the voltage threshold for AP firing was unchanged, neurons from homozygous mice had a significantly higher input resistance (R_in) (WT 131.92±8.45 MΩ, n=25; Hom 216.98±17.8 MΩ, n=18), which could readily account for this hyperexcitability. Neuron morphological analysis revealed that the average distance to the first dendritic branch point was significantly smaller in homozygous animals compared with WT (WT 25.64±3.35 microns, n=10; Hom 13.99±1.41 microns, n=11; p<0.01). Membrane capacitance values recorded during whole-cell recordings were significantly reduced in homozygous (95.48±5.95 pF, n=18) compared to WT (116.5±7.26 pF, n=25) neurons. We used retigabine, a voltage-gated potassium channel activator, as a way to reduce R_in to test the idea of a disease mechanism specific therapy. Retigabine effectively lowered R_in in pyramidal neurons from homozygous mice and significantly decreased AP firing (n=4, p<0.05). Importantly, retigabine treated mice displayed a profound decrease in sensitivity to thermally triggered seizures (n=9, p<0.05). Conclusions: These data suggest that altered membrane passive properties enhance membrane excitability in neurons from Scn1b(C121W) homozygous mice, presumably as a consequence of altered neuronal morphology, demonstrating a new disease mechanism in genetic epilepsy. Further, drugs that decrease input resistance, and thereby specifically target the functional deficit, dampen hyperexcitability of homozygous neurons and reduce thermal seizure susceptibility in mice. These findings highlight the potential therapeutic utility in targeting disease specific mechanisms in epilepsy.