Abstracts

Rationale: An episode of status epilepticus (SE) is associated with the development of unprovoked seizures later in life. At the cellular level, SE is associated with activation and proliferation of microglia (microgliosis), and hyperactivation of the mammalian target of rapamycin (mTOR) pathway in the hippocampus. Recent studies have shown that LPS- and cytokine-induced activation of microglia is mTOR dependent, and consequently microgliosis is suppressed with the mTOR pathway inhibitor rapamycin. In fact, rapamycin is a potent immunosuppressant that has been shown to decrease microgliosis following traumatic brain injury. However, whether rapamycin reduces SE-induced microgliosis has not been evaluated. Therefore, we tested the hypothesis that the SE-induced mTOR pathway dysregulation occurs in microglial cells, and that rapamycin suppresses the associated microgliosis. Methods: Pilocarpine was used to induce SE (1hr). Two weeks later, SE and sham rats were treated with rapamycin (Rap) or vehicle (Veh) for 1 week. Immunohistochemistry was performed in different cohorts of rats 2 and 3 weeks after SE, before and after the treatments. Antibodies against the phosphorylated ribosomal S6 protein (P-S6), IBA1 and NeuN were used to assess mTOR pathway activation, and to stain microglia and neurons, respectively. Immunohistochemical analyses focused on the hippocampal formation. Results: Increased P-S6 and IBA1 staining was evident 2 and 3 weeks after SE. The SE-induced increase in P-S6 signal was apparent throughout the hippocampus in the principal cell layers and in cells scattered throughout CA1 strata radiatum and lacunosum moleculare. In hippocampi from Sham+Veh and SE+Veh rats, P-S6 signal co-localized with NeuN, indicating neuronal activation of mTOR. However, relatively more P-S6 staining was co-localized with IBA1 in the SE+Veh group, where microglial cells were hypertrophied (an indication of microglial activation). Rapamycin decreased the P-S6 signal below basal levels in both sham and SE groups, and dramatically reduced IBA1 staining and reversed the microglial hypertrophy in the SE+Rap group. Conclusions: Our data indicate that mTOR dysregulation occurs in neurons and reactive microglia, and that rapamycin suppresses microgliosis. These data suggest that SE-induced mTOR dysregulation may alter neuronal and glial properties, which together may contribute to the molecular and behavioral phenotypes associated with epileptogenesis.