Abstracts

Kv4.2 subunits are thought to compose A-type K⁺ currents (I[sub]A[/sub]) which regulate excitability by dampening back-propagating action potentials in hippocampal dendrites. Kv4.2 expression in CA1 is decreased in the pilocarpine model of epilepsy. Phospho-regulation of Kv4.2 in CA1 via the ERK pathway is also altered in this model. In these studies, we extend the molecular and biochemical findings previously reported using the kainate (KA) model. To determine whether loss of Kv4.2 is associated with increased seizure susceptibility we compared seizure activity in Kv4.2 knockout (-/-), heterozygote (-/+) and wildtype (+/+) littermate mice following KA. KA-induced (15 mg/kg IP) seizure activity in adult rats was assessed with video-EEG. After KA-induced status epilepticus (SE), animals were treated with pentobarbital (30 mg/kg), and allowed to survive. Hippocampal subfields were dissected and prepared for molecular and biochemical studies. Seizures were induced in littermate Kv4.2 (-/-), (-/+), and (+/+) mice with KA (40 mg/kg IP) and the mice were monitored with video-EEG. In epileptic rats there was a significant increase in ERK activation in hippocampal area CA1 (p[lt]0.05) with no change in area CA3 and the dentate gyrus. Total ERK levels were unchanged in area CA1. In CA1 from epileptic rats total Kv4.2 expression was decreased to 50% of control (p[lt]0.001), while total Kv4.2 levels were unchanged in CA3 and dentate gyrus. In line with our findings of ERK activation, in area CA1 we found that the percentage of phospho-Kv4.2 was significantly increased in CA1 (p[lt]0.05), but not in CA3 and dentate gyrus. Kv4.2 mRNA was significantly reduced in area CA1 from epileptic animals (p[lt]0.05). We also evaluated expression of the small-conductance Ca²⁺-activated K⁺ channel subunit, SK2 which together with Kv4.2 functions to compartmentalize action potential propagation in hippocampus. SK2 levels were not significantly different in epileptic animals compared to controls. Following KA treatment 100% (4/4) of Kv4.2 -/- mice developed seizures and SE. In contrast, 80% (4/5) of +/+ mice had seizures and SE and 70% (5/7) of -/+ mice had seizures with 43% (3/7) developing SE. Kv4.2 -/- mice showed a 50% reduction in the latency to the first seizure (p[lt]0.05) and to SE (p[lt]0.05) compared to the +/+ littermates. Our results show region-specific regulation of Kv4.2 channel expression and phosphorylation in hippocampus of epileptic rats. The decreased Kv4.2 expression and phosphorylation in area CA1 in rats with KA-induced epilepsy corroborates the findings previously reported in the pilocarpine model. We also found that Kv4.2 -/- mice exhibit increased seizure susceptibility. Together, these observations suggest that activity-dependent downregulation of Kv4.2 in epilepsy may contribute to hyperexcitability and seizure activity. (Supported by NIH/NINDS and Epilepsy Foundation.)