Abstracts

Rationale: 30-70% of individuals with Temporal Lobe Epilepsy (TLE) have a history of febrile status epilepticus (Cendes et al., 1993). In a rodent model, we found a causal relationship of FSE and TLE-like epilepsy (Dub et al., 2010). High-field, high-resolution MR-imaging 2-4 hours following FSE uncovered changes in some FSE rats that predicted later development of spontaneous limbic seizures. The nature of the MRI changes was consistent with enhanced oxygen uptake (augmented metabolism) in amygdala and hippocampus. We reasoned that many inflammatory processes are metabolically demanding, and we have already shown inflammation following FSE. Thus, we searched for early inflammatory processes as a source of the MRI signal, and as an early marker of epileptogenesis that follow FSE. Methods: FSE was induced in P10 Sprague Dawley rats as previously described (Dub et al., 2010). Rats were MR-imaged 2 hours later then perfused for immunohistochemical analysis for early inflammatory markers. We focused on high mobility group box 1 (Maroso, et al., 2010), a known early inflammatory mediator. To establish a time course of HMGB1 translocation, additional rats were sacrificed at 1,3,8 or 24 hours post FSE and assessed for translocation. We focused on areas CA1 and CA3c of dorsal hippocampus and basal amygdala because febrile seizures are limbic in nature and lead to limbic epilepsy. Translocation was defined as the ratio of cytoplasmic HMGB1 expressing cells versus total cells in limbic regions. To evaluate the cell type expressing HMGB1, we employed dual immunocytochemistry with NeuN (neurons), GFAP (astrocytes) and CD11b (microglia). Results: HMGB1 translocation to the cytoplasm was apparent as early as an hour after the end of FSE. At this time translocation of HMGB1 to the cytoplasm was exclusive to neurons. By 3 hours, both neurons and activated microglia harbored HMGB1 in their cytoplasm. HMGB1 cytoplasmic expression was no longer apparent at 8 and 24 hours following FSE. The presence and degree of HMGB1 translocation in basal amygdala of individual rats correlated significantly with the amygdala MRI-T2 signal (found at 2 hours post FSE) in the same rats. This correlation was unique to the basal amygdala.Conclusions: FSE induce transient HMGB1 translocation in neurons followed by glia. This translocation likely induces a cascade of inflammatory processes that is enduring. We have previously detected interleukin-1 (an inflammatory cytokine that is downstream of HMGB1) 4-72 hours following the FSE, as well as in animals that became epileptic. There appears to be a temporal and spatial correlation between HMGB1 translocation and the predicitve MRI-T2 signal changes. Both take place in amygdala (and to a lesser extent in hippocmapus), and within the time window of 1-4 hours. This correlations sugges that the neurobiological basis of the epilepsy-predictive MRI signal is the onset of inflammation. If so, then targeting HMGB1 post-FSE may be a selective way to block inflammation and potentially block epileptogenesis..