Abstracts

Rationale: Dravet syndrome (DS) is the most common developmental and epileptic encephalopathy and is caused primarily by mutations in SCN1A which codes for the voltage gated sodium (Na+) channel α subunit Nav1.1. Nav1.1 is prominently expressed in GABAergic interneurons of the cerebral cortex, and it is therefore hypothesized that selective dysfunction of interneurons leads to impaired inhibition in the developing brain, which in turn underlies the epilepsy and other cognitive comorbidities found in DS. Both parvalbumin (PV) and somatostatin (SST) expressing interneurons have been shown to be impaired in DS; however, the function of the third major group of interneurons – the vasoactive intestinal peptide (VIP) expressing interneurons – has not been specifically investigated. Methods: Here, we used Scn1a+/- mice crossed to VIP-Cre.tdTomato reporter mice to perform targeted whole cell recordings from VIP interneurons in layer 2/3 primary somatosensory and visual cortex of acute brain slices prepared from male and female mice across development. We measured VIP interneuron intrinsic properties and excitability in Scn1a+/- mice and used Nav subtype specific pharmacology with the Nav1.1 selective toxin Hm1a. We additionally performed detailed immunohistochemistry with confocal microscopy to detect the presence of Nav1.1 on VIP interneurons. Results: We demonstrated that VIP interneurons have impaired excitability, with depolarized action potential thresholds, reduced steady state firing frequencies, and prominent spike height attenuation in Scn1a+/- mice at both post-natal day (P) 18-21 and P35-56 age groups relative to age-matched wildtype littermate controls. Immunohistochemistry confirmed expression of Nav1.1 on VIP interneuron axons, and selectively activating Nav1.1 with the toxin Hm1a was able to rescue the cellular phenotype of VIP cells in Scn1a+/- mice. These deficits were found to primarily affect a large (50% of the total) subgroup of VIP interneurons which show a prominent irregular spiking (IS) firing pattern determined by presence of M current. The KCNQ channel activator retigabine converted continuous adapting (CA) VIP interneurons to IS, while the KCNQ antagonist linopiridine converted IS VIP interneurons to CA. We further explored the interaction between loss of Nav1.1 and the presence of M current with a biophysical modelling approach. Conclusions: Our results indicate that VIP-positive interneurons express Nav1.1 and, along with PV and SST interneurons, are dysfunctional in DS. IS VIP interneurons are selectively dysfunctional due to the synergistic effects of Scn1a loss of function and the suppressive influence of M current. As canonical VIP interneuron microcircuits are disinhibitory, these findings suggest that the prominent and durable VIP cell dysfunction identified here could relate to non-epilepsy comorbid conditions in DS. Funding: This work was supported by NIH NINDS K08 NS097633, NIH NINDS R01 NS110869, and the Burroughs Wellcome Fund Career Award for Medical Scientists to E.M.G.