Abstracts

Rationale: Epilepsy is associated with marked alterations in the function of several different ion channels in the hippocampus, a key structure for epileptogenesis. As the modulatory gate-keeper and filter for the hippocampus, the dentate gyrus (DG) has inhibitory control over excitatory inputs from the entorhinal cortex and other brain regions. Extrasynaptic GABA-A receptors play an important inhibitory role, but their impact on epileptogenesis remains poorly understood. Voltage-gated, M-type potassium channels, composed of KCNQ2-5 subunits, play a dominant role in neuronal excitability. M channels have become a promising target in epilepsy treatment and M-channel openers are novel anti-epileptic drugs. In this study, we determined the role of ion-channel-mediated tonic and M currents in neuronal excitability in DG granule cells. We hypothesize complementary mechanisms whereby extrasynaptic GABA-A receptors and M-channels provide control over epileptogenic pathways.Methods: GABAergic currents and M-currents were recorded from patch voltage-clamp from DG granule cells in brain-slice. Tonic currents were quantified before and after application of gabazine. Hippocampus epileptogenesis via the kindling model was conducted in wild-type and δ-subunit knockout mice, and electrographic and motor seizures recorded. To induce acute seizures, single-dose pilocarpine (200 mg/kg) was administered to wild-type mice by i.p. injection. Pilocarpine-induced seizures terminated naturally after 3-4 hours. M current was assayed in DG 72 hours after pilocarpine-induced seizures. M-currents were elicited by holding the cell at -20 mV, and deactivating relaxation quantified during 700 ms hyperpolarizing step to -60 mV.Results: GABAergic currents were highly sensitive to allopregnanolone, but were ablated in neurons from δ-knockout animals. In the hippocampal kindling seizure model, δ-knockout mice achieved stage 5 motor seizures at a faster rate and displayed prolonged electrographic seizures compared to wild-type animals. M currents recorded from DG granule cells had mean amplitudes of 52 ± 20 pA at -60 mV that were sensitive to the M-channel blocker XE-991. We found no significant difference between M current amplitudes recorded from sham and seizure-experienced mice.Conclusions: Extrasynaptic GABA-A receptors, which are sensitive to neurosteroids, mediate tonic inhibition in the DG and play a role in epileptogenesis. We also report that M current, which is mediated by M-channels within the DG, is not up-regulated following a chemoconvulsant-induced seizures. Further in situ hybridizations and brain-slice recordings may uncover their role in hippocampus epilepsy. **Supported by NIH grant NS051398 (to D.S.R); NS 04339 and a SOM IIMS award (to M.S.S.)**