Abstracts

Rationale: The dentate gyrus (DG) is a critical entry point for regulating function of the hippocampus. Integral to this role are the sparse, selective activation characteristics of the principal cells of the DG, dentate granule cells (DGCs). This sparse activation is important both in cognitive processing and in regulation of pathological activity in disease states such as epilepsy. In both humans with epilepsy and animal models, there are multiple disruptions within local DG circuitry, including mossy fiber spouting, alterations to local inhibition, and aberrant neurogenesis. Given these circuit changes, we hypothesized that network activation properties of DGCs would be corrupted in epileptic mice.Methods: We utilized calcium imaging by fast swept-field confocal microscopy in hippocampal slices prepared from male C57-B6 mice using a pilocarpine status epilepticus (SE) model of epilepsy to understand if and how the development of chronic epilepsy alters circuit function of the DG in chronic epilepsy.Results: We assessed proportional activation of DGCs responding to perforant path stimulation and found that control slices showed relatively sparse activation with only 3.8 +/- 4.2% of DGCs responding to half-maximal stimulation and 19.9 +/- 2.1% responding to supra-maximal stimulation. Slices prepared from epileptic mice 2 months post-status epilepticus displayed a significant increase in DGC activation, with 50.4 +/- 14.8% (p = 0.039) and 64.4 +/- 12.6% (p = 0.014) responding to half and supra-maximal stimulation respectively. We also assayed the firing latencies of responsive DGCs using cell-attached patch recording. Control DGCs activated 7.3 +/- 0.49 ms following supra-maximal stimulation. Picrotoxin significantly decreased firing latency to 5.18 ms (p = 0.001), suggesting that feedforward inhibition may regulate granule cell activation. Epileptic DGCs had significantly faster activation latencies than controls (5.7 +/- 1.4 ms, p = 0.016), and picrotoxin increased activation latency to 6.85 +/- .32 ms (p= .0183), which is consistent with an excitatory action of GABA mediated by elevated intracellular chloride. We assessed inhibitory function in DGCs from epileptic mice using patch recordings and found a significant, 32% reduction in mIPSC amplitude compared to controls (p < 0.0001). Mimicking this reduction in inhibitory function in control slices with 5 uM picrotoxin was sufficient to elevate DGC activation to 52.4 +/- 9.7%, and demonstrates that disinhibition is a likely mechanism for the collapse in sparse activation observed in epileptic mice.Conclusions: Given that the ensemble activation properties of DGCs are critical determinants in both hippocampal dependent cognitive function and control of aberrant excitation in the limbic system, erosion in these properties may play a critical role both in cognitive comorbidities and seizure propensity in epilepsy. This research was supported by grants from NIH-NINDS to D.A.C. and C.G.D. and from EF to C.G.D.