Abstracts

Rationale: We have been studying the role of hyperpolarization-activated (HCN) ion channels in epilepsy and the mechanisms underlying their chronic downregulation. We published findings (Jung et al., J. Neurosci., 2010) that downregulation of HCN channel gating in chronic epilepsy was associated in part with decreased activity of p38 mitogen-activated kinase (p38 MAPK). Although this phosphorylation pathway affects numerous cellular processes, we asked whether pharmacological reversal of the p38 MAPK activation deficit in vivo might produce an antiepileptic benefit in chronically epileptic animals.Methods: We studied rats induced with the pilocarpine protocol at 1 month post-pilocarpine, using continuous video-EEG monitoring for 7-10 days and drug or vehicle administration via intracerebroventricular (ICV) cannula and osmotic pump. We calculated the change in seizure frequency in within-subject comparisons. We harvested hippocampal tissue at the end of monitoring for biochemical measurement of kinase activity.Results: Administration of the p38 MAPK inhibitor, SB203580, worsened Racine grade 3-5 convulsive seizure frequency (176% of baseline, n=9, p<.01), consistent with our prior results. However, administration of anisomycin, a non-specific activator of p38 MAPK, dramatically worsened seizure frequency (277% of baseline, n=4, p<.05). We hypothesized that this unexpected finding resulted from the known activation by anisomycin of a related MAPK, c-Jun-N-terminal kinase (JNK), an entity that mediates apoptotic neuronal death, and is associated with neurodegenerative diseases. Administration of the JNK antagonist SP600125 produced a decrease in seizure frequency to 62% of baseline (n=6, p<.05). Measurement of the phosphorylation state of the canonical JNK target, c-Jun, from our drug-treated animals found a decrease in phospho-c-Jun to 71% of control, demonstrating modest JNK inhibition by SP600125. Finally, we found an increase in JNK activation in untreated, chronically epileptic animals (136% of that in naive animals, n=5, p<.05), suggesting chronic upregulation of this pathway in epilepsy. Interestingly, JNK activity does not alter HCN channel properties, at least in naive hippocampal tissue (Poolos et al., J. Neurosci., 2006).Conclusions: These serendipitous findings demonstrate that JNK is a novel, pathologically altered signaling pathway in a refractory epilepsy model. JNK inhibition produces an antiepileptic action against convulsive seizures. Because JNK hyperactivity has been widely implicated in neurodegeneration, pharmacologic JNK inhibition might be effective in refractory convulsive epilepsy syndromes that produce progressive neuronal loss, both as an antiepileptic and possibly antiepileptogenic therapy.