Abstracts

Rationale: Calcineurin (PP2B) is a Ca2+-calmodulin-regulated serine-threonine phosphatase that can be blocked in vivo and in vitro by FK506 (Tacrolimus). Studies demonstrate delayed kindling development in FK506-treated rats; and that FK506 pretreatment both delayed onset of kainate-induced seizures and reduced evoked seizure duration. It has been suggested that calcineurin may contribute to epileptogenesis via dephosphorylation and subsequent endocytosis of GABAA receptors; dephosphorylation and translocation of potassium channels; and dephosphorylation and altered gating of HCN channels. Here we ask whether inhibition of calcineurin (using FK506) prevents or slows the progression of pilocarpine-induced epileptogenesis. Methods: Seizure-induced activation of calcineurin protein expression was characterized at 24 h, 48h, 72 h, 1 week, 30 d, and 60 d after pilocarpine-induced status epilepticus (SE). Rats were injected with FK506 (3 mg/kg) at onset of SE and daily for 2 d. The effect of early calcineurin inhibition on electrographic SE and subsequent spontaneous seizures was examined following FK506 administration. Results: Calcineurin protein levels did not change 24 h, 48 h, 72 h, or 1 week after SE relative to controls. Calcineurin protein levels were significantly lower 30 d after SE compared to controls (p < 0.01) and remained lower than control levels in chronically epileptic rats 60 d after SE. Following pilocarpine-induced status epilepticus (SE) and administration of 3 mg/kg FK506 (n=5) or vehicle (n=7) at SE and for the next 48 h, there was no difference in daily seizure frequency over time. FK506 administration did not delay time to first spontaneous seizure relative to vehicle controls. The percentage of FK506 treated rats remaining seizure free after 2 weeks did not differ from those treated with vehicle. Conclusions: These results argue against the use of calcineurin inhibitors as potential antiepileptogenic drugs. Early administration of FK506 (daily for 3 days), a calcineurin inhibitor with anti-inflammatory and neuroprotective effects, is ineffective at slowing or preventing the progression of epileptogenesis. Decreases in calcineurin protein noted in a subset of chronically epileptic animals may account for some of the adverse affects (i.e., neurotoxicity) noted in patients treated with calcineurin inhibitors and reports that calcineurin is effective at reducing SE, but is not an effective anti-seizure drug. Funding: 5K22NS083722 (HLG)