Microglia dynamically survey the brain parenchyma. Microglial processes interact with neuronal elements; however, what role neuronal network activity plays in regulating microglial dynamics is not entirely clear. Most studies of microglial dynamics use either slice preparations or in vivo imaging in anesthetized mice. Here we demonstrate that microglia in awake mice have a relatively reduced process area and surveillance territory and that reduced neuronal activity under general anesthesia increases microglial process velocity, extension and territory surveillance. Similarly, reductions in local neuronal activity through sensory deprivation or optogenetic inhibition increase microglial process surveillance. Using pharmacological and chemogenetic approaches, we demonstrate that reduced norepinephrine signaling is necessary for these increases in microglial process surveillance. These findings indicate that under basal physiological conditions, noradrenergic tone in awake mice suppresses microglial process surveillance. Our results emphasize the importance of awake imaging for studying microglia–neuron interactions and demonstrate how neuronal activity influences microglial process dynamics.
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This work is supported by the National Institutes of Health (R01NS088627, R21DE025689 and R01NS112144 to L.-J.W.) and by a postdoctoral fellowship from the Mayo Clinic Center for Multiple Sclerosis and Autoimmune Neurology (to T.C). The authors thank M. Mattson (National Institute on Aging) for critical reading of the paper and members of the Wu Lab at the Mayo Clinic for insightful discussions.
The authors declare no competing interests.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Supplementary Figs. 1–12.
Microglial process surveillance increases after anesthesia. Time-lapse imaging of microglia (50–64 µm in depth) in the somatosensory cortex before and after isoflurane anesthesia. Experiments were repeated three times independently with similar results.
Neuronal network activity in the somatosensory cortex immediately decreases after isoflurane anesthesia. Calcium activity in excitatory CamKII-positive soma (150 µm in depth, left) or dendrites (50 µm, right) before and after isoflurane induction. Experiments were repeated three times independently with similar results.
Neuronal network activity in the barrel cortex decreases after contralateral whisker trimming. Calcium activity in CamKII neuronal dendrites (50 µm in depth, right) of the contralateral barrel cortex before and after whisker trimming. Experiments were repeated three times independently with similar results.
Microglial process surveillance increases after contralateral whisker trimming. Time-lapse imaging of microglia (50–64 µm in depth) in the contralateral barrel cortex before and after whisker trimming. Experiments were repeated three times independently with similar results.
Intracerebral administration of muscimol (870 µM) reduces neuronal network activity. Calcium activity of CamKII neuronal somas (150 µm in depth) in the somatosensory cortex before and after muscimol administration. Experiments were repeated three times independently with similar results.
Microglial process surveillance increases after intracerebral administration of muscimol (870 µM). Time-lapse imaging of microglia (50–64 µm depth) in somatosensory cortex before and after muscimol administration. Experiments were repeated three times independently with similar results.
Microglial process surveillance increases after optogenetic stimulation of VGAT-positive inhibitory neurons. Time-lapse imaging of microglia (50–64 µm depth) in the somatosensory cortex before and after optogenetic stimulation (10 Hz, 1 ms pulse, 10 min of stimulation) of ChR2-expressing VGAT-positive interneurons. Experiments were repeated three times independently with similar results.
In vivo imaging dense noradrenergic neuronal projections from the locus coeruleus. Z-stack movie showing td-Tomato-labeled axons, which project from noradrenergic neurons in the LC. The video progresses from the pial surface down to a 100 µm in depth in the somatosensory cortex. Experiments were repeated three times independently with similar results.
Neuronal network activity in acute brain slices exposed to glutamate. Calcium activity in excitatory CamKII-positive soma in three cortical slices exposed to vehicle, 50 µM glutamate or 1 mM glutamate. Experiments were repeated three times independently with similar results.
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Liu, Y.U., Ying, Y., Li, Y. et al. Neuronal network activity controls microglial process surveillance in awake mice via norepinephrine signaling. Nat Neurosci 22, 1771–1781 (2019). https://doi.org/10.1038/s41593-019-0511-3
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