Abstracts

Rationale: Changes in ion channel expression following brain insult and seizure activity can result in both adaptive and maladaptive changes in excitability. Large conductance voltage- and calcium-activated potassium (BK) channels contribute to early spike timing in hippocampal neurons, and several studies have indicated that the BK channel plays a pathological role in promoting increased excitability early after a seizure. However, it is currently unknown whether there are changes in BK channels properties after a seizure, and how those changes affect excitability. Methods: We have investigated changes in BK channels and their accessory ߴ subunit in dentate gyrus granule neurons of the hippocampus, key neurons that regulate excitability of the hippocampus circuit. We used the single channel recording technique to investigate changes in BK channel biophysical properties and pharmacology 48 hours after pilocarpine or kainic acid induced seizures. We complemented these studies with RT-PCR, immunostaining and patch clamp recording of dentate gyrus granule neurons. Results: We have found that the predominant effect on BK channels is a downregulation of the BK ߴ (Kcnmb4) accessory subunit mRNA. Consistent with downregulated expression, single channel recording and pharmacology of dentate gyrus hippocampus granule neurons indicate a switch in the relative subtype of BK channels expressed; from iberiotoxin-resistant, type II BK channels (BK a/ߴ) that have relatively higher channel open probability and slow gating, to iberiotoxin-sensitive type I channels (BK a alone) with lower open probability and fast gating. These findings were corroberated by RT-PCR and immunostaining studies for a BK beta4 subunit transcriptional reporter. In control neurons, the ߴ subunit inhibits BK channel opening during the repolarization phase of action potentials as revealed by little effect of BK blocker paxilline on the action potential width. However, following pilocarpine treatment, current clamp recordings revealed a loss-of-function of ߴ as evidenced by an increase effect of paxilline on the action potential width. Conclusions: We conclude that seizure-induced neuronal downregulation of Kcnmb4 is an activity dependent mechanism that alters the excitability of hippocampal granule cells. These novel findings indicate that BK channel subtypes are not only defined by cell-specific expression, but can also be plastic depending on the recent history of neuronal excitability. Funding: National Science Foundation grant 1456862