Optogenetic activation of spinal microglia triggers chronic pain in mice.

Spinal microglia are highly responsive to peripheral nerve injury and are known to be a key player in pain. However, there has not been direct evidence showing that selective microglial activation in vivo is sufficient to induce chronic pain. Here, we used optogenetic approaches in microglia to address this question employing CX3CR1creER/+: R26LSL-ReaChR/+ transgenic mice, in which red-activated channelrhodopsin (ReaChR) is inducibly and specifically expressed in microglia. We found that activation of ReaChR by red light in spinal microglia evoked reliable inward currents and membrane depolarization. In vivo optogenetic activation of microglial ReaChR in the spinal cord triggered chronic pain hypersensitivity in both male and female mice. In addition, activation of microglial ReaChR up-regulated neuronal c-Fos expression and enhanced C-fiber responses. Mechanistically, ReaChR activation led to a reactive microglial phenotype with increased interleukin (IL)-1ß production, which is likely mediated by inflammasome activation and calcium elevation. IL-1 receptor antagonist (IL-1ra) was able to reverse the pain hypersensitivity and neuronal hyperactivity induced by microglial ReaChR activation. Therefore, our work demonstrates that optogenetic activation of spinal microglia is sufficient to trigger chronic pain phenotypes by increasing neuronal activity via IL-1 signaling.

The complement C3-C3aR pathway mediates microglia-astrocyte interaction following status epilepticus.

Gliosis is a histopathological characteristic of epilepsy that comprises activated microglia and astrocytes. It is unclear whether or how crosstalk occurs between microglia and astrocytes in the evolution of epilepsy. Here, we report in a mouse model of status epilepticus, induced by intracerebroventricular injection of kainic acid (KA), sequential activation of microglia and astrocytes and their close spatial interaction in the hippocampal CA3 region. Microglial ablation reduced astrocyte activation and their upregulation of complement C3. When compared to wild-type mice, both C3-/- and C3aR-/- mice had significantly less microglia-astrocyte interaction in response to KA-induced status epilepticus. Additionally, KA-injected C3-/- mice had significantly less histochemical evidence of neurodegeneration. The results suggest that the C3-C3aR pathway contributes to KA-induced neurodegeneration by mediating microglia-astrocyte communication. The C3-C3aR pathway may prove to be a potential therapeutic target for epilepsy treatment.

HDAC1 and HDAC2 regulate anti-inflammatory effects of anesthetic isoflurane in human monocytes.

Pre-exposure to volatile anesthetics inhibits inflammation induced by various stimuli, including surgical procedures and ischemia. We hypothesize that volatile anesthetics may induce anti-inflammatory effects via a mechanism involving regulation of histone deacetylases (HDACs). Pre-exposure of 1.5% isoflurane for 0.5 h induced anti-inflammatory effects [measured by cytokine production of tumor necrosis factor-ɑ, interleukin-8 (IL-8) and IL-1ß] in both human THP-1 cells and primary human peripheral blood monocytes stimulated by lipopolysaccharide. In human THP-1 cells, coadministration of the HDAC inhibitor trichostatin A (TSA) blocked the isoflurane-induced anti-inflammatory effects. TSA also blocked isoflurane-upregulated HDAC1-3 expression and isoflurane-reduced nuclear translocation of p65 and p50 subunits of nuclear factor-κB (NF-κB). The ability of isoflurane to reduce NF-κB nuclear translocation and proinflammatory responses in the cell line was blocked by gene silencing of HDAC1 and HDAC2, but not by gene silencing of HDAC3. A coimmunoprecipitation assay demonstrated that the decreased interaction between HDAC1 and HDAC2 through lipopolysaccharide was restored by isoflurane pretreatment. These findings were validated in primary human peripheral blood monocytes  wherein gene silencing of HDAC1 and HDAC2 resulted in increased cytokine production and NF-κB nuclear translocation induced by isoflurane pre-exposure and lipopolysaccharide stimulation. These results indicate that anti-inflammatory effects of the volatile anesthetic isoflurane in human monocytes involve regulation of HDAC1 and HDAC2.

Rostral ventromedial medulla-mediated descending facilitation following P2X7 receptor activation is involved in the development of chronic post-operative pain.

Chronic postsurgical pain (CPSP) remains a medical problem. Whether the descending modulation of nociceptive transmission from the rostral ventromedial medulla (RVM) plays a role in CPSP induced by skin/muscle incision and retraction (SMIR) in the thigh is still unknown. In this study, we found that SMIR surgery, which induced either bilateral or unilateral mechanical allodynia, activated microglia, and up-regulated interleukin-1ß (IL-1ß), an important cytokine, and 8-hydroxyguanine, an oxidative stress marker in the RVM. In addition, the release of 5-hydroxytryptamine (5-HT) was increased in the ipsilateral and contralateral RVM in rats with either bilateral or unilateral pain following SMIR. The 5-HT level increase, 5-HT3 receptor (5-HT3R) up-regulation, and microglia activation were found bilaterally in SMIR rats with bilateral pain, but only ipsilaterally in SMIR rats with unilateral pain. The intrathecal injection of the 5-HT3R antagonist Y25130 prevented the development of CPSP and the activation of spinal microglia induced by SMIR. Furthermore, P2X7 receptor (P2X7R) was up-regulated in microglia in the RVM. The microinjection of the P2X7R antagonist brilliant blue G (BBG, a non-competitive P2X7R antagonist) into the RVM prevented the development of mechanical allodynia, inhibited the activation of microglia, and decreased the expression of IL-1ß and 8-hydroxyguanine in the RVM following SMIR. Importantly, BBG injected into the RVM also decreased the activation of microglia and the level of 5-HT in the lumbar 3 (L3) spinal cord. The microinjection of the P2X7R agonist BzATP, the NADPH oxidase activator phorbol-12-myristate-13-acetate, or IL-1ß into the RVM induced bilateral mechanical allodynia, microglia activation, and 5-HT release in the L3 spinal dorsal horn. Taken together, P2X7R activation in microglia in the RVM following SMIR might be responsible for the development of CPSP via activating descending serotonergic pathway. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

The role of P2X7R/ERK signaling in dorsal root ganglia satellite glial cells in the development of chronic postsurgical pain induced by skin/muscle incision and retraction (SMIR).

The mechanisms of chronic postsurgical pain remain to be elucidated. We reported here that skin/muscle incision and retraction (SMIR), a rat model of postsurgical pain, phosphorylated the extracellular regulated protein kinases (ERK) signaling components c-Raf, MEK (ERK kinase) and ERK1/2 in lumbar 3 dorsal root ganglion (L3 DRG) in rats. Intrathecal injection of ERK specific inhibitor SCH772984 suppressed the mechanical allodynia induced by SMIR. Furthermore, SMIR upregulated tumor necrosis factor alpha (TNFα) in L3 DRG, which could be inhibited by SCH772984. Intrathecal injection of TNF antagonist Etanercept could also inhibit the mechanical allodynia and the increased ERK phosphorylation in L3 DRG induced by SMIR. In addition, immunofluorescent data showed that P2X7R was located exclusively in GFAP labeled satellite glial cells and was highly colocalized with p-ERK1/2 following SMIR. Pretreatment with P2X7R antagonist Brilliant Blue G (BBG) could also block the mechanical allodynia, inhibited the phosphorylation of c-Raf, MEK, ERK1/2, and decrease the expression of TNF-α. Finally, intrathecal injection of BzATP produced mechanical allodynia and induced ERK phosphorylation in satellite glial cells in L3 DRG. Thus, P2X7R activation in satellite glial cells in L3 DRG, leading to a positive feedback between ERK pathway activation and TNF-α production, is suggested to be involved in the induction of chronic postsurgical pain following SMIR.

Microglia enhance post-anesthesia neuronal activity by shielding inhibitory synapses.

Microglia are resident immune cells of the central nervous system and play key roles in brain homeostasis. During anesthesia, microglia increase their dynamic process surveillance and interact more closely with neurons. However, the functional significance of microglial process dynamics and neuronal interaction under anesthesia is largely unknown. Using in vivo two-photon imaging in mice, we show that microglia enhance neuronal activity after the cessation of isoflurane anesthesia. Hyperactive neuron somata are contacted directly by microglial processes, which specifically colocalize with GABAergic boutons. Electron-microscopy-based synaptic reconstruction after two-photon imaging reveals that, during anesthesia, microglial processes enter into the synaptic cleft to shield GABAergic inputs. Microglial ablation or loss of microglial ß2-adrenergic receptors prevents post-anesthesia neuronal hyperactivity. Our study demonstrates a previously unappreciated function of microglial process dynamics, which enable microglia to transiently boost post-anesthesia neuronal activity by physically shielding inhibitory inputs.

SIRT3 Regulates Neuronal Excitability of Alzheimer's Disease Models in an Oxidative Stress-Dependent Manner.

Mitochondrial deacetylase Sirtuin-3 (SIRT3) has been shown to regulate metabolic and antioxidant functions. Previous studies have reported that SIRT3 mediates change of neuronal excitability. However, the underlying mechanism is unclear. Here, we show that SIRT3 deficiency results in neural hyperactivity, decreased survival rate, and increased oxidative stress of culture neurons, while a superoxide dismutase 2 mimetic reduces oxidative stress and suppresses the neuronal hyperactivity. In culture neurons treated with Aß, SIRT3 activator reduces level of reactive oxygen species (ROS) and hyperactivity of neurons while increasing level of ROS restores the neuronal hyperactivity. Utilizing two photon in vivo brain imaging, we show that inhibition of SIRT3 results in elevated neuronal excitatory in an animal model of Alzheimer's disease of early stage, whereas suppression of the ROS level reverses it. These findings demonstrate an oxidative stress-dependent role of SIRT3 in regulation of neuronal excitability in Alzheimer's disease.

The Emerging Role of Microglia in Neuromyelitis Optica.

Neuromyelitis optica (NMO) is an autoantibody-triggered neuro-inflammatory disease which preferentially attacks the spinal cord and optic nerve. Its defining autoantibody is specific for the water channel protein, aquaporin-4 (AQP4), which primarily is localized at the end-feet of astrocytes. Histopathology studies of early NMO lesions demonstrated prominent activation of microglia, the resident immune sentinels of the central nervous system (CNS). Significant microglial reactivity is also observed in NMO animal models induced by introducing AQP4-IgG into the CNS. Here we review the potential roles for microglial activation in human NMO patients as well as different animal models of NMO. We will focus primarily on the molecular mechanisms underlying microglial function and microglia-astrocyte interaction in NMO pathogenesis. Understanding the role of microglia in NMO pathology may yield novel therapeutic approaches for this disease.

Microglial calcium signaling is attuned to neuronal activity in awake mice.

Microglial calcium signaling underlies a number of key physiological and pathological processes in situ, but has not been studied in vivo in awake mice. Using multiple GCaMP6 variants targeted to microglia, we assessed how microglial calcium signaling responds to alterations in neuronal activity across a wide range. We find that only a small subset of microglial somata and processes exhibited spontaneous calcium transients in a chronic window preparation. However, hyperactive shifts in neuronal activity (kainate status epilepticus and CaMKIIa Gq DREADD activation) triggered increased microglial process calcium signaling, often concomitant with process extension. Additionally, hypoactive shifts in neuronal activity (isoflurane anesthesia and CaMKIIa Gi DREADD activation) also increased microglial process calcium signaling. Under hypoactive neuronal conditions, microglia also exhibited process extension and outgrowth with greater calcium signaling. Our work reveals that microglia have highly distinct microdomain signaling, and that processes specifically respond to bi-directional shifts in neuronal activity through increased calcium signaling.

Neuronal network activity controls microglial process surveillance in awake mice via norepinephrine signaling.

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.

Administration of Dexmedetomidine inhibited NLRP3 inflammasome and microglial cell activities in hippocampus of traumatic brain injury rats.

The abnormally high nucleotide-binding oligomerization domain (NOD)-like receptor family pyrin domain containing 3 (NLRP3) inflammasome activity is a typical characteristic of traumatic brain injury (TBI). Dexmedetomidine (Dex) is a highly selective α-2 adrenergic receptor agonist that inhibits the activation of NLRP3. Thus, it was hypothesized that Dex could attenuate TBI by inhibiting NLRP3 inflammasome activity in hippocampus. Rats were subjected to controlled cortical impact method to induce TBI, and treated with Dex. The effect of Dex treatment on the cognitive function, NLRP3 activity, and microglial activation in rat brain tissues was assessed. The administration of Dex improved performance of TBI rats in Morris water maze (MWM) test, which was associated with the increased neurone viability and suppressed microglia activity. Moreover, the administration of Dex inhibited the neuroinflammation in brain tissue as well as the expressions of NLRP3 and caspase-1. Additionally, Dex and NLRP3 inhibitor, BAY-11-7082 had a synergistic effect in inhibiting NLRP3/caspase-1 axis activity and improving TBI. The findings outlined in the current study indicated that the improvement effect of Dex on TBI was related to its effect on NLRP3 activity.

Crutchlike Incision Along the Mastoid Groove and Above the Occipital Artery Protects the Lesser Occipital Nerve and Occipital Artery in Microvascular Decompression Surgery.

BACKGROUND: Patients who have had microvascular decompression (MVD) surgery often report sensory discomfort around the surgical area. In most cases, injury of the lesser occipital nerve (LON) is responsible for this postoperative complication. This study aimed to explore an effective method to protect the LON and reduce postoperative discomfort. METHODS: Doppler was used to determine the course of the occipital artery (OA) and the incision. Direct LON identification and a novel crutchlike incision were performed from January 1, 2016, to February 1, 2017, to reduce postoperative sensory disturbance. Postoperative sensory disturbance was evaluated and retrospectively analyzed compared with previous linear incision cases (from January 1, 2015, to December 31, 2015). Anatomic information at the lateral occiput was measured and recorded. RESULTS: The difference in the amount of postoperative sensory disturbance at 3-month follow-up was significant (P = 0.008). Sensory disturbance was significantly lower in patients who had crutchlike incision (P = 0.002) and patients with direct LON identification (P = 0.035) compared with the previous linear incision cases. The distance from OA to the projection of the transverse sinus was 3.2 ± 0.6 cm at the mastoid groove and 2.5 ± 0.4 cm at a site 0.5-1.0 cm from the mastoid groove. CONCLUSIONS: A crutchlike incision at the mastoid groove superior to the OA reduced the incidence of postoperative sensory disturbance and OA injury. The mastoid groove and OA are simple landmarks for determination of the incision in microvascular decompression.

Intranasal Dexmedetomidine as a Sedative Premedication for Patients Undergoing Suspension Laryngoscopy: A Randomized Double-Blind Study.

BACKGROUND: Intranasal dexmedetomidine, a well-tolerated and convenient treatment option, has been shown to induce a favorable perioperative anxiolysis in children. We investigate intranasal dexmedetomidine as a sedative premedication for anesthesia recovery in an adult population. METHODS: A prospective randomized controlled trial; 81 adult patients scheduled for elective suspension laryngoscopy received intranasal dexmedetomidine (1 µg∙kg-1) or a placebo 45-60 min before anesthetic induction. Extubation time was used as the primary outcome measure. Secondary variables included the levels of sedation (Observer's Assessment of Alertness/Sedation scale, OAA/S) and anxiety (4-point anxiety score), anesthetic and analgesic requirements, hemodynamic fluctuations, and anesthesia recovery as well as side effects. RESULTS: The levels of sedation and anxiety differed significantly between the two groups at anesthesia pre-induction (p < 0.001 and = 0.001, respectively). Repeated-measure general linear model determined no significant interaction effect between group and time on the targeted concentration of propofol (F = 1.635, p = 0.200), but a significant main effect of group existed (F = 6.880, p = 0.010). A moderate but significant decrease in the heart rate was recorded in the dexmedetomidine group at pre-induction. Episodes of tachycardia and hypertension after tracheal intubation and extubation were more frequent in the placebo group. CONCLUSIONS: Intranasal dexmedetomidine as a sedative premedication induced a favorable perioperative anxiolysis without prolongation in anesthesia recovery; the hemodynamic effect was modest. TRIAL REGISTRATION: ClinicalTrials.gov NCT 02108171.

Effects of intrathecal injection of rapamycin on pain threshold and spinal cord glial activation in rats with neuropathic pain.

OBJECTIVE: To evaluate the effects of intrathecal injection of rapamycin on pain threshold and spinal cord glial activation in rats with neuropathic pain. METHODS: Healthy 30 male Sprague Dawley (SD) rats were randomly divided into six groups (n = 5 in each group): (1) control group without any treatments; (2) chronic constriction injury (CCI) group; (3) Early-rapamycin group with intrathecal injection of rapamycin 4 hours after CCI days; (4) Early-vehicle group with intrathecal injection of DMSO; (5) Late-rapamycin group with intrathecal injection of rapamycin 7 days after CCI; (6) Late-vehicle group with intrathecal injection of DMSO 7 days after CCI. Rapamycin or DMSO was injected for 3 consecutive days. Mechanical and thermal threshold were tested before and after the CCI operation. Lumbar segment of spinal cords was tested for glial fibrillary acidic protein (GFAP) by immunohistochemistry on 14th day after operation. RESULTS: Mechanical and thermal hyperalgesia emerged on fourth day were maintained till fourteenth day after operation. After intrathecal injection of rapamycin 4 hours or 7 days after CCI, mechanical and thermal threshold significantly increased compared to injection of DMSO. The area of GFAP positive and the mean density of GFAP positive area in the dorsal horn of the ipsilateral side greatly increased in rapamycin-treated groups. CONCLUSIONS: Intrathecal injection of rapamycin may attenuate CCI-induced hyperalgesia and inhibit the activation of astrocyte.

Over-expression of P2X7 receptors in spinal glial cells contributes to the development of chronic postsurgical pain induced by skin/muscle incision and retraction (SMIR) in rats.

Many patients suffer from chronic postsurgical pain (CPSP) following surgery, and the underlying mechanisms are poorly understood. In the present work, with use of the skin/muscle incision and retraction (SMIR) model, the role of P2X7 receptors (P2X7Rs) in spinal glial cells in the development of CPSP was evaluated. Consistent with previous reports, we found that SMIR decreased the ipsilateral 50% paw withdrawal threshold (PWT), lasting for at least 2weeks. No injury was done to L3 dorsal root ganglia (DRG) neurons and no axonal or Schwann cell damage at the retraction site in the saphenous nerve was observed 7days after SMIR. The results of immunofluorescence showed that both microglia and astrocytes were activated in the spinal dorsal horn following SMIR. In addition, both P2X7Rs and tumor necrosis factor-alpha (TNF-α) were up-regulated following SMIR. Double immunofluorescence staining revealed that the up-regulated P2X7R immunoreactivity was mainly located in microglia, and to a lesser extent in astrocytes, but not in neurons. Intrathecal delivery of specific P2X7R antagonist BBG (10µM in 10µl volume) or A438079 (10µM in 10µl volume), started 30min before the surgery and once daily thereafter for 7days, prevented the mechanical allodynia. Intrathecal injection of BBG inhibited the activation of microglia and astrocytes, and the up-regulation of TNF-α induced by SMIR. These data suggest that P2X7Rs in the spinal dorsal horn might mediate the development of CPSP via activation of glial cells and up-regulation of TNF-α.