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Chronic pain is a debilitating condition that often emerges as a clinical symptom of inflammatory diseases. It has therefore been widely accepted that the immune system critically ...contributes to the pathology of chronic pain. Microglia, a type of immune cell in the central nervous system, has attracted researchers’ attention because in rodent models of neuropathic pain that develop strong mechanical and thermal hypersensitivity, histologically activated microglia are seen in the dorsal horn of spinal cord. Several kinds of cytokines are generated by damaged peripheral neurons and contribute to microglial activation at the distal site of the injury where damaged neurons send their projections. Microglia are known as key players in the surveillance of the local environment in the central nervous system and have a significant role of circuit remodeling by physical contact to synapses. Key molecules for the pathology of neuropathic pain exist in the activated microglia, but the factors driving pain-inducible microglial activation remain unclear. Therefore, to find the key molecules inducing activation of spinal microglia and to figure out the precise mechanism of how microglia modulate neuronal circuits in the spinal cord to form chronic pain state is a critical step for developing effective treatment of neuropathic pain.
Neuropathic pain, a highly debilitating chronic pain following nerve damage, is a reflection of the aberrant functioning of a pathologically altered nervous system. Previous studies have implicated ...activated microglia in the spinal dorsal horn (SDH) as key cellular intermediaries in neuropathic pain. Microgliosis is among the dramatic cellular alterations that occur in the SDH in models of neuropathic pain established by peripheral nerve injury (PNI), but detailed characterization of SDH microgliosis has yet to be realized. In the present study, we performed a short-pulse labeling of proliferating cells with ethynyldeoxyuridine (EdU), a marker of the cell cycle S-phase, and found that EdU+ microglia in the SDH were rarely observed 32 h after PNI, but rapidly increased to the peak level at 40 h post-PNI. Numerous EdU+ microglia persisted for the next 20 h (60 h post-PNI) and decreased to the baseline on day 7. These results demonstrate a narrow time window for rapidly inducing a proliferation burst of SDH microglia after PNI, and these temporally restricted kinetics of microglial proliferation may help identify the molecule that causes microglial activation in the SDH, which is crucial for understanding and managing neuropathic pain.
Neuropathic pain is a highly debilitating chronic pain state that is a consequence of nerve injury or of diseases such as diabetes, cancer, infection, autoimmune disease, or trauma. Neuropathic pain ...is often resistant to currently available analgesics. There is a rapidly growing body of evidence indicating that signalings from spinal microglia play crucial roles in the pathogenesis of neuropathic pain. After peripheral nerve injury, microglia transform to reactive states through the expression of various genes such as cell-surface receptors (including purinergic receptors) and proinflammatory cytokines that enhance synaptic transmission in dorsal horn neurons. Inhibiting function or expression of these microglial molecules strongly suppresses pain hypersensitivity to innocuous mechanical stimuli (tactile allodynia), a hallmark symptom of neuropathic pain. A recent study also reveals that the transcription factor IRF8 (interferon regulatory factor 8) is a critical regulator of the nerve injury–induced gene expression in microglia. The present review article highlights the recent advances in our understanding of spinal microglia in neuropathic pain.
Microglia, which are pathological effectors and amplifiers in the central nervous system, undergo various forms of activation. A well‐studied microglial‐induced pathological paradigm, spinal ...microglial activation following peripheral nerve injury (PNI), is a key event for the development of neuropathic pain but the transcription factors contributing to microglial activation are less understood. Herein, we demonstrate that MafB, a dominant transcriptional regulator of mature microglia, is involved in the pathology of a mouse model of neuropathic pain. PNI caused a rapid and marked increase of MafB expression selectively in spinal microglia but not in neurons. We also found that the microRNA mir‐152 in the spinal cord which targets MafB expression decreased after PNI, and intrathecal administration of mir‐152 mimic suppressed the development of neuropathic pain. Reduced MafB expression using heterozygous Mafb deficient mice and by intrathecal administration of siRNA alleviated the development of PNI‐induced mechanical hypersensitivity. Furthermore, we found that intrathecal transfer of Mafb deficient microglia did not induce mechanical hypersensitivity and that conditional Mafb knockout mice did not develop neuropathic pain after PNI. We propose that MafB is a key mediator of the PNI‐induced phenotypic alteration of spinal microglia and neuropathic pain development.
Main Points
Activated spinal microglia upregulates MafB protein which is essential for the development of neuropathic pain after peripheral nerve injury.
Intrathecal administration of mir‐152 mimic or siRNA for MafB can suppress the development neuropathic pain.
Astrocytes are critical regulators of CNS function and are proposed to be heterogeneous in the developing brain and spinal cord. Here we identify a population of astrocytes located in the superficial ...laminae of the spinal dorsal horn (SDH) in adults that is genetically defined by Hes5. In vivo imaging revealed that noxious stimulation by intraplantar capsaicin injection activated Hes5
SDH astrocytes via α
-adrenoceptors (α
-ARs) through descending noradrenergic signaling from the locus coeruleus. Intrathecal norepinephrine induced mechanical pain hypersensitivity via α
-ARs in Hes5
astrocytes, and chemogenetic stimulation of Hes5
SDH astrocytes was sufficient to produce the hypersensitivity. Furthermore, capsaicin-induced mechanical hypersensitivity was prevented by the inhibition of descending locus coeruleus-noradrenergic signaling onto Hes5
astrocytes. Moreover, in a model of chronic pain, α
-ARs in Hes5
astrocytes were critical regulators for determining an analgesic effect of duloxetine. Our findings identify a superficial SDH-selective astrocyte population that gates descending noradrenergic control of mechanosensory behavior.
J. Neurochem. (2010) 114, 810-819. Microglia plays an important role in many neurodegenerative conditions. ATP leaked or released by damaged cells triggers microglial activation through P2 receptors, ...and stimulates the release of oxygen radicals, proinflammatory cytokines and chemokines from activated microglia. However, little is known about mechanisms underlying ATP-induced chemokine release from microglia. In this study, we found that a high concentration of ATP induces the mRNA expression and release of CXCL2 from microglia. A similar effect was observed following treatment of microglia with a P2X7 receptor (P2X7R) agonist, 2′-and 3′-O-(4-benzoylbenzoyl) ATP, and this was inhibited by pre-treatment with a P2X7R antagonist, Brilliant Blue G. ATP induced both activation of nuclear factor of activated T cells (NFAT) and MAPKs (p38, ERK, and JNK) through P2X7R. ATP-induced mRNA expression of CXCL2 was inhibited by INCA-6 (an NFAT inhibitor), SB203580 (a p38 inhibitor), U0126 (a MEK-ERK inhibitor) and JNK inhibitor II (a JNK inhibitor). However, MAPK inhibitors did not inhibit activation of NFAT. In addition, protein kinase C inhibitors suppressed ATP-induced ERK and JNK activation, and also inhibited ATP-induced CXCL2 expression in microglia. These results suggest that ATP increased CXCL2 production via both NFAT and protein kinase C/MAPK signaling pathways through P2X7 receptor stimulation in microglia.
Up‐regulation of P2X4 receptors in spinal cord microglia is crucial for tactile allodynia, an untreatable pathological pain reaction occurring after peripheral nerve injury. How nerve injury in the ...periphery leads to this microglia reaction in the dorsal horn of the spinal cord is not yet understood. It is shown here that CCL21 was rapidly expressed in injured small‐sized primary sensory neurons and transported to their central terminals in the dorsal horn. Intrathecal administration of a CCL21‐blocking antibody diminished tactile allodynia development in wild‐type animals. Mice deficient for CCL21 did not develop any signs of tactile allodynia and failed to up‐regulate microglial P2X4 receptor expression. Microglia P2X4 expression was enhanced by CCL21 application in vitro and in vivo. A single intrathecal injection of CCL21 to nerve‐injured CCL21‐deficient mice induced long‐lasting allodynia that was undistinguishable from the wild‐type response. This effect of CCL21 injection was strictly dependent on P2X4 receptor function. Since neuronal CCL21 is the earliest yet identified factor in the cascade leading to tactile allodynia, these findings may lead to a preventive therapy in neuropathic pain.
This study identifies the chemokine CCL21 as a critical upstream factor in the signalling cascade leading to tactile allodynia—pain hypersensitivity after nerve injury—and analyses the pathway by which it acts.
Microglia are implicated as a source of diverse proinflammatory factors in the CNS. Extracellular nucleotides are well known to be potent activators of glial cells and trigger the release of ...cytokines from microglia through purinergic receptors. However, little is known about the role of purinoceptors in microglial chemokine release. In this study, we found that high concentrations of ATP evoked release of CC-chemokine ligand 3 (CCL3)/macrophage inflammatory protein-1α from MG-5 cells, a mouse microglial cell line, and rapid up-regulation of CCL3 mRNA was elicited within 30 min of ATP stimulation. The release of CCL3 was also stimulated by 2'- and 3'-O-(4-benzoylbenzoyl) ATP, an agonist of P2X₇ receptors. Brilliant Blue G, an antagonist of P2X₇ receptors, strongly inhibited this ATP-induced CCL3 release. Similar pharmacological profile was observed in primary microglia. In MG-5 cells, ATP caused de-phosphorylation and nuclear translocation of the transcription factor nuclear factor of activated T cells (NFAT). ATP-induced NFAT de-phosphorylation was also dependent on P2X₇ receptor activation. Furthermore, ATP-induced CCL3 release and production were prevented by a selective inhibitor of NFAT. Taken together, the results of this study demonstrate an involvement of NFAT in the mechanism underlying P2X₇ receptor-mediated CCL3 release.
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