Abstracts

Rationale: Dravet syndrome (DS) is an epileptic disorder that affects infants during the first year of age and has devastating consequences on the patient s cognitive development. This disorder has been associated with autosomal dominant mutations of the SCN1a gene, coding for a voltage gated sodium channel, Nav 1.1, that is mainly localized in GABAergic interneurons and not pyramidal cells. It is widely believed that the presence of seizures during development is the main cause of cognitive impairment in this syndrome. However, because interneurons play a critical role in driving neuronal processing, it appears likely that SCN1a mutations result in abnormal information processing, independently of seizures.Methods: To control the onset of SCN1A deficit and begin to elucidate its contribution to cognition and behavior, we used a siRNA-mediated RNA interference approach to knock down expression of SCN1A. Nine adult (P60-P90) Sprague-Dawley rats were implanted with an injection cannula into the left lateral ventricle and a custom EEG recording electrode into the right dorsal hippocampus CA1 region. Rats were injected with siRNA complexes once daily for 4 days (4 control, 5 SCN1A). Each day rats were placed in a circular arena with 3 objects at the perimeter, and EEG and position data were recorded during 10 minute sessions. On the last day, rats were assessed for performance in an object recognition task. The rat was habituated to the 3 objects in the arena for 10 minutes. In the first test for spatial memory, one object was moved to a new location, and the rat was allowed to explore this novel spatial configuration for 10 minutes. In the second test for novel object recognition, one object was replaced with a new object, and the rat was allowed to explore for an additional 10 minutes. The percent time spent near each object was measured.Results: SCN1A-targeted siRNA treatment in rat neuroblastoma cells resulted in a mean 61% (t(2)=-8.008, p=0.0152) suppression of SCN1A expression after 24 hours (Figure 1A). After four days of intraventricular administration of siRNA complexes, rats treated with SCN1A-targeted siRNA were impaired on a spatial task of object recognition. While rats that were delivered control siRNA spent more time exploring the moved object (F=9.03, p=0.0155), the SCN1A group showed no preference (F=2.19, p=0.155; Figure 1C). The SCN1A group showed no significant impairment on novel object recognition, but there was a trend for a weaker preference to the novel object compared to the control group (Figure 1C). In support of this difference in a task of spatial memory, we found a trend toward reduced theta power in the dorsal hippocampus (Figure 1B). Seizure activity was not observed during any recordings.Conclusions: These results suggest that SCN1A-targeted siRNA treatment in vivo is sufficient to cause spatial cognitive impairments, independently of seizures. In consequence, it could be argued that seizures may not be the only contributors for the cognitive impairments in this syndrome. This work was supported by NIH Grants RO1NS056170, RO1NS041595 and R21MH086833-01A2.