Locus ceruleus (LC)-supplied norepinephrine (NE) suppresses neuroinflammation in the brain. To elucidate the effect of LC degeneration and subsequent NE deficiency on Alzheimer's disease pathology, we evaluated NE effects on microglial key functions. NE stimulation of mouse microglia suppressed Aβ-induced cytokine and chemokine production and increased microglial migration and phagocytosis of Aβ. Induced degeneration of the locus ceruleus increased expression of inflammatory mediators in APP-transgenic mice and resulted in elevated Aβ deposition. In vivo laser microscopy confirmed a reduced recruitment of microglia to Aβ plaque sites and impaired microglial Aβ phagocytosis in NE-depleted APP-transgenic mice. Supplying the mice the norepinephrine precursor L-threo-DOPS restored microglial functions in NE-depleted mice. This indicates that decrease of NE in locus ceruleus projection areas facilitates the inflammatory reaction of microglial cells in AD and impairs microglial migration and phagocytosis, thereby contributing to reduced Aβ clearance. Consequently, therapies targeting microglial phagocytosis should be tested under NE depletion.neuroinflammation | amyloid beta | neurodegeneration | phagocytosis A lzheimer's disease (AD) is characterized by neocortical and hippocampal atrophy due to neuronal loss, the deposition of Aβ peptides, and the formation of neurofibrillar tangles. In addition, there is a progressive degeneration of cholinergic nuclei in the basal forebrain and of noradrenergic nuclei in the brainstem, most importantly the locus ceruleus (LC). This nucleus is the major source of norepinephrine (NE) supply in the mammalian brain. The LC provides the neurotransmitter via an extensive network of neuronal projections to all major brain regions. These regions include the neocortex and hippocampus, the seat of cognitive functions, learning, and memory.Research dating back to the 1960s implicated LC degeneration in the pathogenesis of AD (1-3). Of particular relevance, several studies show that AD patients present with a prominent loss of LC cells, reaching 70% within the rostral nucleus and causing reduction of cortical and limbic NE levels (4). The drop in NE concentration tightly correlates with the progression and extent of memory dysfunction and cognitive impairment. Degeneration of LC neurons has been observed in patients exhibiting "mild cognitive impairment" (MCI) (5), an early form of AD, with 80% of MCI patients eventually succumbing to full AD (6).Degeneration of LC neurons results in progressive loss of two different types of axons, those with either conventional synaptic contacts or varicosities. Varicosities are believed to release transmitter extrasynaptically into the microenvironment, where it may act on surrounding neurons, glial cells, and blood vessels (7). Locally diffusing NE is thought to execute additional functions apart from its role as a classical neurotransmitter. Indeed, NE negatively regulates transcription of inflammatory genes in astrocytes and microglia (8), both expressi...