Abstracts

Rationale: Status epilepticus is associated with hippocampal injury and with activation and accumulation of microglial cells. During and after SE microglial cells undergo morphological and biochemical changes that may be associated with their phagocytic or inflammatory functions. Microglia remodel their actin cytoskeleton and change from highly ramified to hypertrophied/amoeboid in order to engulf and phagocytose their targets. Also, microglia produce and release a number of cytokines in response to injury. Recently, we mapped the temporal progression of microglial morphological changes and inflammatory markers after SE, and found that the highest levels of inflammatory cytokines correlated with the presence of hypertrophied microglia. Interestingly, maximal accumulation of amoeboid microglia at two weeks after SE was not associated with significant changes in inflammatory markers determined from whole hippocampal homogenates. In this study we sought to determine whether the SE-induced expression of inflammatory cytokines is associated with specific morphological shapes of microglia. Methods: SE was induced in rats using pilocarpine and stopped with diazepam. Tissue was collected at 14-days after SE for immunohistochemistry. Antibodies against IBA1 or CD11b were used to identify microglial cells in the hippocampus. Microglial morphology was classified using a 1-5 scale as follows: 1-ramified, 2-bushy/reactive, 3-hypertrophied, 4-amoeboid and 5-rod. To identify the biochemical properties associated with these morphologies co-localization was done with antibodies against cytokines such as IL-1߬ TNFa, IL-6, and with CD68 and MHCII. Results: We found a significant increase in the total number of microglia after SE compared to controls (p < 0.05). In controls, a higher number of ramified microglia was evident when compared to the other morphologies (p < 0.05). In contrast, SE hippocampi showed a significant increase in the numbers of amoeboid and rod-shaped microglia when compared to the control group (p < 0.05). We found little to no cytokine or MHCII expression in ramified microglia. MHCII was strong in hypertrophied and amoeboid microglia. In contrast, TNFa was expressed in hypertrophied but not amoeboid microglia. Different cytokine profiles for IL6 and IL-1ߠwere evident in hypertrophied and rod-shaped microglia. Conclusions: Our data showed that SE-induced activation of microglia is associated with different morphological and cytokine profiles that follow specific combinations. This would suggest that after SE-induced injury the shifts in the microglia morphology are associated with different functions such as inflammatory or phagocytic. This is important because phagocytic microglia may be contributing to the neuropathology of SE and epilepsy. Future studies will investigate this possibility. Funding: NA