Abstracts

Rationale: Although EEG electrode implantation is essential to evaluate seizures in rodent models of epilepsy, the inflammatory consequences of this procedure are unclear. Both tissue damage and prolonged seizures increase immune cell activation and infiltration, suggesting that electrode implantation may interfere with inflammation inherent to epilepsy development. Depending on the outcome measures tested, traditional, untreated surgical controls may not reveal confounding inflammatory effects of electrode implantation.Methods: We quantified hippocampal inflammation as the green fluorescent signal in the CA3 region of CX3CR1GFP/+ transgenic C57BL/6 mice that express a green fluorescent protein in lymphocytes and monocytes. Untreated, surgical control mice underwent cortical electrode implantation or sham surgery on P25 and were perfused on P28. Treated mice underwent cortical electrode implantation or sham surgery on P25, kainic acid (KA) induced status epilepticus (SE) on P28, and perfusions on P29. The KA dose required to induce SE was titrated through serial intraperitoneal injections and served as a surrogate for seizure susceptibility. Seizure susceptibility after P25 electrode implantation was then evaluated as the latency to KA induced seizures in a “two-hit” model of temporal lobe epilepsy: brief, recurrent febrile seizures (FS) on P14 followed by KA-SE on P28.Results: There was no significant difference in hippocampal inflammation (p>0.05) between untreated, electrode-implanted, control mice (n=6) and shams (n=6). KA-treated, electrode-implanted mice (n=6) had significantly more hippocampal inflammation (p<0.0001) than KA-treated shams (n=6), indicating a priming effect of electrode implantation on immune cell activation and infiltration. Although not significant at this sample size, electrode-implanted mice tended to require a smaller average dose of KA to reach SE (15.8 mg/kg) than did shams (19.2 mg/kg), suggesting increased seizure susceptibility in electrode-implanted mice. In the two-hit model, the KA dose was reduced to 10 mg/kg because higher doses were fatal in electrode-implanted mice. This sensitization interfered with comparisons of seizure latencies across groups depending on early life FS and electrode implantation.Conclusions: The inflammatory effects of electrode implantation were not evident in traditional, untreated surgical controls. Inflammatory priming by electrode implantation was only apparent after interacting with KA treatment. In the two-hit model, the sensitization to KA in electrode-implanted mice hindered our evaluation of the long term effects of mild FS. Electrode implantation surgery should be seriously considered as a contributor to neuroinflammation with the potential to aggravate inflammatory responses to experimental treatments.