The contribution of inflammation to the progression of neurodegenerative diseases such as Alzheimer's, Parkinson's, and prion diseases is poorly understood. Brain inflammation in animal models of these diseases is dominated by chronic microglial activation with minimal proinflammatory cytokine expression. However, these inflammatory cells are "primed" to produce exaggerated inflammatory responses to subsequent lipopolysaccharide (LPS) challenges. We show that, using the ME7 model of prion disease, intracerebral challenge with LPS results in dramatic interleukin-1␤ (IL-1␤) expression, neutrophil infiltration, and inducible nitric oxide synthase expression in the brain parenchyma of prion-diseased mice compared with the same challenge in normal mice. Systemic inflammation evoked by LPS also produced greater increases in proinflammatory cytokines, pentraxin 3, and inducible nitric oxide synthase transcription in priondiseased mice than in control mice and induced microglial expression of IL-1␤. These systemic challenges also increased neuronal apoptosis in the brains of ME7 animals. Thus, both central and peripheral inflammation can exacerbate local brain inflammation and neuronal death. The finding that a single acute systemic inflammatory event can induce neuronal death in the CNS has implications for therapy in neurodegenerative diseases.

SummaryBackgroundTidal (12.4 hr) cycles of behavior and physiology adapt intertidal organisms to temporally complex coastal environments, yet their underlying mechanism is unknown. However, the very existence of an independent “circatidal” clock has been disputed, and it has been argued that tidal rhythms arise as a submultiple of a circadian clock, operating in dual oscillators whose outputs are held in antiphase i.e., ∼12.4 hr apart.ResultsWe demonstrate that the intertidal crustacean Eurydice pulchra (Leach) exhibits robust tidal cycles of swimming in parallel to circadian (24 hr) rhythms in behavioral, physiological and molecular phenotypes. Importantly, ∼12.4 hr cycles of swimming are sustained in constant conditions, they can be entrained by suitable stimuli, and they are temperature compensated, thereby meeting the three criteria that define a biological clock. Unexpectedly, tidal rhythms (like circadian rhythms) are sensitive to pharmacological inhibition of Casein kinase 1, suggesting the possibility of shared clock substrates. However, cloning the canonical circadian genes of E. pulchra to provide molecular markers of circadian timing and also reagents to disrupt it by RNAi revealed that environmental and molecular manipulations that confound circadian timing do not affect tidal timing. Thus, competent circadian timing is neither an inevitable nor necessary element of tidal timekeeping.ConclusionsWe demonstrate that tidal rhythms are driven by a dedicated circatidal pacemaker that is distinct from the circadian system of E. pulchra, thereby resolving a long-standing debate regarding the nature of the circatidal mechanism.

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