Systemic inflammation can impair cognition with relevance to dementia, delirium and post-operative cognitive dysfunction. Acute episodes of delirium also contribute significantly to rates of long-term cognitive decline, implying that de novo pathology occurs during these acute episodes. Whether systemic inflammation-induced acute dysfunction and acute brain injury occur by overlapping or discrete mechanisms has not been investigated. Here we show that systemic inflammation, induced by bacterial LPS, produces both working memory deficits and acute brain injury in the degenerating brain and that these occur by dissociable IL-1-dependent processes. In normal C57BL/6 mice, LPS (100 μg/kg) did not affect working memory but robustly impaired contextual fear conditioning (CFC). However prior hippocampal synaptic loss left mice selectively vulnerable to LPS-induced working memory deficits. Systemically administered IL-1 receptor antagonist (IL-1RA) was protective against, and systemic IL-1 beta; replicated, these working memory deficits. Although LPS-induced deficits still occured in IL-1RI-/- mice, systemic TNF-alpha; was sufficient to induce similar deficits, indicating redundancy among these cytokines. Dexamethasone abolished systemic cytokine synthesis and was protective against working memory deficits despite failing to block brain IL-1 beta; synthesis. Direct application of IL-1 beta; to ex vivo hippocampal slices induced non-synaptic depolarisation and irrevesible loss of membrane potential in CA1 neurons from diseased animals and systemic LPS increased apoptosis in the degenerating brain, in an IL-1RI-/- dependent-fashion. The data suggest that LPS induces working memory dysfunction via circulating IL-1 beta; but dysfunction leading to neuronal death is mediated by hippocampal IL-1 beta;. The data suggest that acute systemic inflammation produces both reversible cognitive deficits, resembling delirium, and acute brain injury that may lead to long-term cognitive impairment but that these events are mechanistically dissociable. This would have significant implications for management of cognitive dysfunction and decline during acute illness.

Acute transient cognitive dysfunction and acute brain injury induced by systemic inflammation occur by dissociable IL-1-dependent mechanisms