A novel role of microglial NADPH oxidase in mediating extra-synaptic function of norepinephrine in regulating brain immune homeostasis

Although the peripheral anti‐inflammatory effect of norepinephrine (NE) is well documented, the mechanism by which this neurotransmitter functions as an anti‐inflammatory/neuroprotective agent in the central nervous system (CNS) is unclear. This article aimed to determine the anti‐inflammatory/neuroprotective effects and underlying mechanisms of NE in inflammation‐based dopaminergic neurotoxicity models. In mice, NE‐depleting toxin N‐(2‐chloroethyl)‐N‐ethyl‐2‐bromobenzylamine (DSP‐4) was injected at 6 months of lipopolysaccharide (LPS)‐induced neuroinflammation. It was found that NE depletion enhanced LPS‐induced dopaminergic neuron loss in the substantia nigra. This piece of in vivo data prompted us to conduct a series of studies in an effort to elucidate the mechanism as to how NE affects dopamine neuron survival by using primary midbrain neuron/glia cultures. Results showed that submicromolar concentrations of NE dose‐dependently protected dopaminergic neurons from LPS‐induced neurotoxicity by inhibiting microglia activation and subsequent release of pro‐inflammatory factors. However, NE‐elicited neuroprotection was not totally abolished in cultures from β2‐adrenergic receptor (β2‐AR)‐deficient mice, suggesting that novel pathways other than β2‐AR are involved. To this end, It was found that submicromolar NE dose‐dependently inhibited NADPH oxidase (NOX2)‐generated superoxide, which contributes to the anti‐inflammatory and neuroprotective effects of NE. This novel mechanism was indeed adrenergic receptors independent since both (+) and (−) optic isomers of NE displayed the same potency. We further demonstrated that NE inhibited LPS‐induced NOX2 activation by blocking the translocation of its cytosolic subunit to plasma membranes. In summary, we revealed a potential physiological role of NE in maintaining brain immune homeostasis and protecting neurons via a novel mechanism. GLIA 2015;63:1057–1072 Main Points NE depletion enhanced dopaminergic neurodegeneration in an inflammation‐4 based mouse model; Extra‐synaptic level of NE mediates anti‐inflammatory/neuroprotective 5 effects through NOX2 independent of adrenergic receptors.