Abstracts

The studies presented here evaluate ERK activation and phosphorylation of the K⁺ channel Kv4.2 in the kainate model of epilelpsy. Kv4.2 subunits are thought to compose hippocampal A-type K⁺ channels, which regulate dendritic excitability in hippocampus. ERK activation via PKA and PKC downregulates dendritic A-type K⁺ currents, leading to an increase in post-synaptic excitability. Hence, Kv4.2 is an intriguing effector protein for the ERK pathway in epilepsy. We previously demonstrated a dramatic increase in ERK activation and phosphorylation of Kv4.2 acutely following kainate seizures. In the studies presented here we further characterize the changes in ERK activation and Kv4.2 phosphorylation by ERK during epileptogenesis.
Seizures were induced in adult rats by kainate (KA)(15 mg/kg IP). Seizure activity was monitored and scored based on video-EEG recordings. After 1-hr of status epilepticus (SE) animals were treated with pentobarbital (30mg/kg IP) and allowed to survive. Animals were sacrificed at various time points after kainate and hippocampal tissue was prepared for biochemistry studies.
Kainate-induced seizures elicited a significant increase in ERK activation in area CA3 (p[lt]0.05) that was evident at the onset of electrographic seizures and persisted to SE. Significant ERK activation in CA1 and dentate (p[lt]0.05 and p[lt]0.01, respectively) occurred slightly later, but prior to onset of SE. Increased ERK phosphorylation of Kv4.2 coincided with onset of SE (CA1 and den p[lt]0.01, CA3 p[lt]0.05). Total ERK and Kv4.2 protein levels were unchanged during SE. At the 24-hour time point following SE, a downregulation of phospho-ERK was evident in CA1 and dentate (p[lt]0.05) with a trend toward a decrease in CA3. Total ERK levels were unchanged. A similar pattern of down-regulation was observed with phospho-Kv4.2 (p[lt]0.001 CA1 and CA3). Interestingly, a downregulation in total Kv4.2 protein levels (p[lt]0.01 CA1 and p[lt]0.05 CA3) was present in hippocampus at 24-hours after SE. By the 3 months following SE an increase in ERK activation was evident in CA1 (p[lt]0.05) with no change in total ERK. At this time point total Kv4.2 in CA1 was decreased (p[lt]0.01), while the percent phosphorylated Kv4.2 was increased (p[lt]0.05). Studies are underway to evaluate changes in CA3 and dentate gyrus at the 3-month time point.
Our results demonstrate altered patterns of ERK activation and Kv4.2 phosphorylation in hippocampus during kainate-induced epileptogenesis. While the specific role(s) of the ERK pathway in epilepsy is unclear, our findings demonstrate that Kv4.2 is a candidate effector of ERK in the kainate model. Indirect functional evidence suggests that through this effector ERK could contribute to increases in excitability. Downregulation of total Kv4.2 protein is an additional candidate mechanism for increased hippocampal excitability in epilepsy.
[Supported by: NIH/NINDS.]