Abstracts

Rationale: The dentate gyrus (DG) filters perforant path input to the hippocampus, and a breakdown of this filter is hypothesized to be involved in the pathogenesis of temporal lobe epilepsy (TLE). Prior work in Dravet syndrome (Scn1a+/-) mice has revealed impairments of DG GABAergic inhibitory interneurons, with hippocampal-onset seizures and increased post-seizure c-Fos staining in DG. However, data linking across cellular and macro-level scales is lacking. We hypothesized that inhibitory dysfunction in Scn1a+/- mice would manifest at the circuit level as impaired DG filtering of perforant path input, similar to that observed in models of acquired TLE. Methods: We use 2-photon calcium imaging to determine the response of DG granule cells (GCs) to perforant path input. We introduced a genetically-encoded calcium indicator, GCaMP7s, into the mouse DG via an adeno-associated viral vector. In acute brain slices, we stimulated the perforant path while performing large-scale imaging of GC activity. We then calculated the fluorescent calcium signal in response to stimulation and quantified (1) percentage of all cells activated across a range of stimulus intensity and frequency, and (2) the stimulation intensity resulting in a half-maximal activation of recruitable cells. Results: Perforant path input activated a larger proportion of DG GCs in Scn1a+/- mice vs. controls (4 pulses: Scn1a+/-, 76%; wild-type (WT), 60%; p = 0.0015. 1 pulse: Scn1a+/-, 63%; WT, 43%; p = 0.0092; n = 689 cells from Scn1a+/- and 446 cells from WT mice). The stimulation intensity for half-maximal activation was lower in Scn1a+/- mice vs. controls (4 pulses: Scn1a+/-, 137µA; WT, 201 µA; p < 0.0001. 1 pulse: Scn1a+/-, 225µA; WT, 331µA; p = 0.0002; n = 509 cells from Scn1a+/- and 263 cells from WT mice). Conclusions: This work uses dynamic multicellular imaging in Scn1a+/- mice to, for the first time, link across scales and demonstrate a circuit-level abnormality in a well-validated and established preclinical experimental model of Dravet Syndrome. This finding suggests that abnormal cortico-hippocampal circuitry may be a common finding between genetic and acquired mechanisms of TLE. In future studies, this basic approach will be leveraged to identify novel circuit manipulations capable of reconstituting normal cortico-hippocampal function that can then be implemented in the intact animal to attempt to treat or prevent epilepsy. Funding: This work was supported by the NIH NINDS Research Education Grant (R25) (J.M.) and by the NIH NINDS K08 NS097633, NIH NINDS R01 NS110869, and the Burroughs Wellcome Fund Career Award for Medical Scientists (E.M.G.)