Cold exposure induces vaso-occlusion and pain in sickle mice that depend on complement activation.

Vaso-occlusive pain episodes (VOE) cause severe pain in patients with sickle cell disease (SCD). Vaso-occlusive events promote ischemia/reperfusion pathobiology that activates complement. We hypothesized that complement activation is linked to VOE. We used cold to induce VOE in the Townes sickle homozygous for hemoglobin S (HbSS) mouse model and complement inhibitors to determine whether anaphylatoxin C5a mediates VOE. We used a dorsal skinfold chamber to measure microvascular stasis (vaso-occlusion) and von Frey filaments applied to the plantar surface of the hind paw to assess mechanical hyperalgesia in HbSS and control Townes mice homozygous for hemoglobin A (HbAA) mice after cold exposure at 10°C/50°F for 1 hour. Cold exposure induced more vaso-occlusion in nonhyperalgesic HbSS mice (33%) than in HbAA mice (11%) or HbSS mice left at room temperature (1%). Cold exposure also produced mechanical hyperalgesia as measured by paw withdrawal threshold in HbSS mice compared with that in HbAA mice or HbSS mice left at room temperature. Vaso-occlusion and hyperalgesia were associated with an increase in complement activation fragments Bb and C5a in plasma of HbSS mice after cold exposure. This was accompanied by an increase in proinflammatory NF-κB activation and VCAM-1 and ICAM-1 expression in the liver. Pretreatment of nonhyperalgesic HbSS mice before cold exposure with anti-C5 or anti-C5aR monoclonal antibodies (mAbs) decreased vaso-occlusion, mechanical hyperalgesia, complement activation, and liver inflammatory markers compared with pretreatment with control mAb. Anti-C5 or -C5aR mAb infusion also abrogated mechanical hyperalgesia in HbSS mice with ongoing hyperalgesia at baseline. These findings suggest that C5a promotes vaso-occlusion, pain, and inflammation during VOE and may play a role in chronic pain.

Transient immune activation without loss of intraepidermal innervation and associated Schwann cells in patients with complex regional pain syndrome.

BACKGROUND: Complex regional pain syndrome (CRPS) develops after injury and is characterized by disproportionate pain, oedema, and functional loss. CRPS has clinical signs of neuropathy as well as neurogenic inflammation. Here, we asked whether skin biopsies could be used to differentiate the contribution of these two systems to ultimately guide therapy. To this end, the cutaneous sensory system including nerve fibres and the recently described nociceptive Schwann cells as well as the cutaneous immune system were analysed. METHODS: We systematically deep-phenotyped CRPS patients and immunolabelled glabrous skin biopsies from the affected ipsilateral and non-affected contralateral finger of 19 acute (< 12 months) and 6 chronic (> 12 months after trauma) CRPS patients as well as 25 sex- and age-matched healthy controls (HC). Murine foot pads harvested one week after sham or chronic constriction injury were immunolabelled to assess intraepidermal Schwann cells. RESULTS: Intraepidermal Schwann cells were detected in human skin of the finger-but their density was much lower compared to mice. Acute and chronic CRPS patients suffered from moderate to severe CRPS symptoms and corresponding pain. Most patients had CRPS type I in the warm category. Their cutaneous neuroglial complex was completely unaffected despite sensory plus signs, e.g. allodynia and hyperalgesia. Cutaneous innate sentinel immune cells, e.g. mast cells and Langerhans cells, infiltrated or proliferated ipsilaterally independently of each other-but only in acute CRPS. No additional adaptive immune cells, e.g. T cells and plasma cells, infiltrated the skin. CONCLUSIONS: Diagnostic skin punch biopsies could be used to diagnose individual pathophysiology in a very heterogenous disease like acute CRPS to guide tailored treatment in the future. Since numbers of inflammatory cells and pain did not necessarily correlate, more in-depth analysis of individual patients is necessary.

IL-6 from cerebrospinal fluid causes widespread pain via STAT3-mediated astrocytosis in chronic constriction injury of the infraorbital nerve.

BACKGROUND: The spinal inflammatory signal often spreads to distant segments, accompanied by widespread pain symptom under neuropathological conditions. Multiple cytokines are released into the cerebrospinal fluid (CSF), potentially inducing the activation of an inflammatory cascade at remote segments through CSF flow. However, the detailed alteration of CSF in neuropathic pain and its specific role in widespread pain remain obscure. METHODS: A chronic constriction injury of the infraorbital nerve (CCI-ION) model was constructed, and pain-related behavior was observed on the 7th, 14th, 21st, and 28th days post surgery, in both vibrissa pads and hind paws. CSF from CCI-ION rats was transplanted to naïve rats through intracisternal injection, and thermal and mechanical allodynia were measured in hind paws. The alteration of inflammatory cytokines in CCI-ION's CSF was detected using an antibody array and bioinformatic analysis. Pharmacological intervention targeting the changed cytokine in the CSF and downstream signaling was performed to evaluate its role in widespread pain. RESULTS: CCI-ION induced local pain in vibrissa pads together with widespread pain in hind paws. CCI-ION's CSF transplantation, compared with sham CSF, contributed to vibrissa pad pain and hind paw pain in recipient rats. Among the measured cytokines, interleukin-6 (IL-6) and leptin were increased in CCI-ION's CSF, while interleukin-13 (IL-13) was significantly reduced. Furthermore, the concentration of CSF IL-6 was correlated with nerve injury extent, which gated the occurrence of widespread pain. Both astrocytes and microglia were increased in remote segments of the CCI-ION model, while the inhibition of astrocytes in remote segments, but not microglia, significantly alleviated widespread pain. Mechanically, astroglial signal transducer and activator of transcription 3 (STAT3) in remote segments were activated by CSF IL-6, the inhibition of which significantly mitigated widespread pain in CCI-ION. CONCLUSION: IL-6 was induced in the CSF of the CCI-ION model, triggering widespread pain via activating astrocyte STAT3 signal in remote segments. Therapies targeting IL-6/STAT3 signaling might serve as a promising strategy for the widespread pain symptom under neuropathological conditions.

MNK1/2 contributes to periorbital hypersensitivity and hyperalgesic priming in preclinical migraine models.

Migraine is thought to involve sensitization of the trigeminal nociceptive system. In preclinical pain models, activation of MNK-eIF4E signalling contributes to nociceptor sensitization and the development of persistent pain. Despite these observations, the role of MNK signalling in migraine remains unclear. Here, we investigate whether activation of MNK contributes to hypersensitivity in two rodent models of migraine. Female and male wild-type (WT) and MNK1 knock-out mice were subjected to repeated restraint stress or a dural injection of interleukin-6 (IL-6) and tested for periorbital hypersensitivity and grimacing. Upon returning to baseline thresholds, stressed mice were administered a low dose of the nitric oxide donor sodium nitroprusside and mice previously injected with IL-6 were given a second dural injection of pH 7.0 to test for hyperalgesic priming. MNK1 knock-out mice were significantly less hypersensitive than the WT following dural IL-6 and did not prime to pH 7.0 or sodium nitroprusside. Furthermore, treatment with the selective MNK inhibitor, eFT508, in WT mice prevented hypersensitivity caused by dural IL-6 or pH 7.0. Together, these results implicate MNK-eIF4E signalling in the development of pain originating from the dura and strongly suggest that targeting MNK inhibition may have significant therapeutic potential as a treatment for migraine.

Photobiomodulation improves acute restraint stress-induced visceral hyperalgesia in rats.

The purpose of this study is to explore the potential application of photobiomodulation to irritable bowel syndrome. We established the following experimental groups: the Non-Stress + Sham group, which consisted of rats that were not restrained and were only subjected to sham irradiation; the Stress + Sham group, which underwent 1 hour of restraint stress followed by sham irradiation; and the Stress + Laser group, which was subjected to restraint stress and percutaneous laser irradiation bilaterally on the L6 dorsal root ganglia for 5 minutes each. The experiment was conducted twice, with three and two laser conditions examined. Following laser irradiation, a barostat catheter was inserted into the rat's colon. After a 30-minute acclimatization period, the catheter was inflated to a pressure of 60 mmHg, and the number of abdominal muscle contractions was measured over a 5-minute period. The results showed that photobiomodulation significantly suppressed the number of abdominal muscle contractions at average powers of 460, 70, and 18 mW. However, no significant suppression was observed at average powers of 1 W and 3.5 mW. This study suggests that photobiomodulation can alleviate visceral hyperalgesia induced by restraint stress, indicating its potential applicability to irritable bowel syndrome.

Quantitative Assessment of Subjective Symptoms and Corneal Sensitivity in Chronic Orbital Pain Patients.

PURPOSE: To objectively evaluate the subjective symptoms and characteristics of chronic orbital pain as well as to quantify sensitization of peripheral trigeminal nerves. METHODS: In this prospective cohort study, patients who previously showed a response to peripheral trigeminal nerve blocks for unilateral, idiopathic chronic orbital pain and healthy subjects completed validated questionnaires assessing headaches, neuropathic signs and symptoms, photophobia, and pain qualities. Corneal sensitivity was measured in both eyes for all subjects with a Cochet-Bonnet aesthesiometer. For pain patients, the full assessment protocol was repeated 2-4 weeks after the study injection, and corneal sensitivity was also measured 30 minutes postinjection. Outcomes assessed were headache, neuropathic pain, and photophobia scores; pain qualities; and corneal sensitivity. RESULTS: Six female chronic orbital pain patients (mean age 48.2 years) and 11 female controls (mean age 47.5) were included. The mean headache, neuropathic pain, and photophobia questionnaire scores were significantly higher for pain patients than for controls (p < 0.001). On sensory testing, 5 pain patients (83.3%) endorsed allodynia, and all 6 (100%) had hyperalgesia in the ipsilateral frontal nerve dermatome. No controls had allodynia or hyperalgesia. Corneal sensitivity was similar between eyes in pain patients and between groups. Questionnaire scores and corneal sensitivity did not change significantly after the injection. CONCLUSIONS: Chronic orbital pain patients have a measurable reduction in quality of life due to headaches and photophobia. The supraorbital and supratrochlear nerves are sensitized, resulting in cutaneous hypersensitivity in the corresponding dermatome, but corneal nerves have normal sensitivity.

Study on the Mechanisms of Glrα3 in Pain Sensitization of Endometriosis.

Endometriosis, often associated with chronic pelvic pain, can lead to anxiety and depression. This study investigates the role and mechanism of Glycine receptor alpha 3 (Glrα3) in the central sensitization of pain in endometriosis, aiming to identify new therapeutic targets. Using a Glrα3 knockout mouse model of endometriosis, we employed behavioral tests, qPCR, immunofluorescence, Nissl staining, MRI, and Western blot to assess the involvement of Glrα3 in central pain sensitization. Our results indicate that endometriosis-induced hyperalgesia and anxiety-depressive-like behaviors are linked to increased Glrα3 expression. Chronic pain in endometriosis leads to gray matter changes in the sensory and insular cortices, with Glrα3 playing a significant role. The inhibition of Glrα3 alleviates pain, reduces neuronal abnormalities, and decreases glial cell activation. The absence of Glrα3 effectively regulates the central sensitization of pain in endometriosis by inhibiting glial cell activation and maintaining neuronal stability. This study offers new therapeutic avenues for the clinical treatment of endometriosis-related pain.

Chemogenetics Modulation of Electroacupuncture Analgesia in Mice Spared Nerve Injury-Induced Neuropathic Pain through TRPV1 Signaling Pathway.

Neuropathic pain, which is initiated by a malfunction of the somatosensory cortex system, elicits inflammation and simultaneously activates glial cells that initiate neuroinflammation. Electroacupuncture (EA) has been shown to have therapeutic effects for neuropathic pain, although with uncertain mechanisms. We suggest that EA can reliably cure neuropathic disease through anti-inflammation and transient receptor potential V1 (TRPV1) signaling pathways from the peripheral to the central nervous system. To explore this, we used EA to treat the mice spared nerve injury (SNI) model and explore the underlying molecular mechanisms through novel chemogenetics techniques. Both mechanical and thermal pain were found in SNI mice at four weeks (mechanical: 3.23 ± 0.29 g; thermal: 4.9 ± 0.14 s). Mechanical hyperalgesia was partially attenuated by 2 Hz EA (mechanical: 4.05 ± 0.19 g), and thermal hyperalgesia was fully reduced (thermal: 6.22 ± 0.26 s) but not with sham EA (mechanical: 3.13 ± 0.23 g; thermal: 4.58 ± 0.37 s), suggesting EA's specificity. In addition, animals with Trpv1 deletion showed partial mechanical hyperalgesia and no significant induction of thermal hyperalgesia in neuropathic pain mice (mechanical: 4.43 ± 0.26 g; thermal: 6.24 ± 0.09 s). Moreover, we found increased levels of inflammatory factors such as interleukin-1 beta (IL1-ß), IL-3, IL-6, IL-12, IL-17, tumor necrosis factor alpha, and interferon gamma after SNI modeling, which decreased in the EA and Trpv1-/- groups rather than the sham group. Western blot and immunofluorescence analysis showed similar tendencies in the dorsal root ganglion, spinal cord dorsal horn, somatosensory cortex (SSC), and anterior cingulate cortex (ACC). In addition, a novel chemogenetics method was used to precisely inhibit SSC to ACC activity, which showed an analgesic effect through the TRPV1 pathway. In summary, our findings indicate a novel mechanism underlying neuropathic pain as a beneficial target for neuropathic pain.

Inhibition of 2-AG hydrolysis alleviates posttraumatic headache attributed to mild traumatic brain injury.

BACKGROUND: Posttraumatic headache (PTH) is a common and debilitating symptom following repetitive mild traumatic brain injury (rmTBI), and it mainly resembles a migraine-like phenotype. While modulation of the endocannabinoid system (ECS) is effective in treating TBI and various types of pain including migraine, the role of augmentation of endocannabinoids in treating PTH has not been investigated. METHODS: Repetitive mild TBI was induced in male C57BL/6J mice using the non-invasive close-head impact model of engineered rotational acceleration (CHIMERA). Periorbital allodynia was assessed using von Frey filaments and determined by the "Up-Down" method. Immunofluorescence staining was employed to investigate glial cell activation and calcitonin gene-related peptide (CGRP) expression in the trigeminal ganglion (TG) and trigeminal nucleus caudalis (TNC) of the rmTBI mice. Levels of 2-arachidonoyl glycerol (2-AG), anandamide (AEA), and arachidonic acid (AA) in the TG, medulla (including TNC), and periaqueductal gray (PAG) were measured by mass spectrometry. The therapeutic effect of endocannabinoid modulation on PTH was also assessed. RESULTS: The rmTBI mice exhibited significantly increased cephalic pain hypersensitivity compared to the sham controls. MJN110, a potent and selective inhibitor of the 2-AG hydrolytic enzyme monoacylglycerol lipase (MAGL), dose-dependently attenuated periorbital allodynia in the rmTBI animals. Administration of CGRP at 0.01 mg/kg reinstated periorbital allodynia in the rmTBI animals on days 33 and 45 post-injury but had no effect in the sham and MJN110 treatment groups. Activation of glial cells along with increased production of CGRP in the TG and TNC at 7 and 14 days post-rmTBI were attenuated by MJN110 treatment. The anti-inflammatory and anti-nociceptive effects of MJN110 were partially mediated by cannabinoid receptor activation, and the pain-suppressive effect of MJN110 was completely blocked by co-administration of DO34, an inhibitor of 2-AG synthase. The levels of 2-AG in TG, TNC and PAG were decreased in TBI animals, significantly elevated and further reduced by the selective inhibitors of 2-AG hydrolytic and synthetic enzymes, respectively. CONCLUSION: Enhancing endogenous levels of 2-AG appears to be an effective strategy for the treatment of PTH by attenuating pain initiation and transmission in the trigeminal pathway and facilitating descending pain inhibitory modulation.

Inhibition of Tumor-Derived C-C Motif Chemokine Ligand 2 Expression Attenuates Tactile Allodynia in NCTC 2472 Fibrosarcoma-Inoculated Mice.

Neuropathic pain associated with cancers is caused by tumor growth compressing and damaging nerves, which would also be enhanced by inflammatory factors through sensitizing nociceptor neurons. A troublesome hallmark symptom of neuropathic pain is hypersensitivity to innocuous stimuli, a condition known as "tactile allodynia", which is often refractory to NSAIDs and opioids. The involvement of chemokine CCL2 (monocyte chemoattractant protein-1) in cancer-evoked neuropathic pain is well established, but opinions remain divided as to whether CCL2 is involved in the production of tactile allodynia with tumor growth. In this study, we constructed Ccl2 knockout NCTC 2472 (Ccl2-KO NCTC) fibrosarcoma cells and conducted pain behavioral test using Ccl2-KO NCTC-implanted mice. Implantation of naïve NCTC cells around the sciatic nerves of mice produced tactile allodynia in the inoculated paw. Although the growth of Ccl2 KO NCTC-formed tumors was comparable to that of naïve NCTC-formed tumors, Ccl2-KO NCTC-bearing mice failed to show tactile pain hypersensitivity, suggesting the involvement of CCL2 in cancer-induced allodynia. Subcutaneous administration of controlled-release nanoparticles containing the CCL2 expression inhibitor NS-3-008 (1-benzyl-3-hexylguanidine) significantly attenuated tactile allodynia in naïve NCTC-bearing mice accompanied by a reduction of CCL2 content in tumor masses. Our present findings suggest that inhibition of CCL2 expression in cancer cells is a useful strategy to attenuate tactile allodynia induced by tumor growth. Development of a controlled-release system of CCL2 expression inhibitor may be a preventative option for the treatment of cancer-evoked neuropathic pain. SIGNIFICANCE STATEMENT: The blockade of chemokine/receptor signaling, particularly for C-C motif chemokine ligand 2 (CCL2) and its high-affinity receptor C-C chemokine receptor type 2 (CCR2), has been implicated to attenuate cancer-induced inflammatory and nociceptive pain. This study demonstrated that continuous inhibition of CCL2 production from cancer cells also prevents the development of tactile allodynia associated with tumor growth. Development of a controlled-release system of CCL2 expression inhibitor may be a preventative option for management of cancer-evoked tactile allodynia.

Resolvin D1 attenuates mechanical allodynia after burn injury: Involvement of spinal glia, p38 mitogen-activated protein kinase, and brain-derived neurotrophic factor/tropomyosin-related kinase B signaling.

Resolvin D1 (RvD1) suppresses inflammatory, postoperative, and neuropathic pain. The present study assessed the roles and mechanisms of RvD1 in mechanical allodynia after burn injury. A rat model of burn injury was established for analyses, and RvD1 was injected intraperitoneally. Pain behavior and the expression levels of spinal dorsal horn Iba-1 (microglia marker), GFAP (astrocyte marker), p-p38 mitogen-activated protein kinase (MAPK), brain-derived neurotrophic factor (BDNF), and tropomyosin-related kinase B (TrkB) were detected by behavioral and immunocytochemical assays. The results showed that RvD1 attenuated mechanical allodynia after burn injury, prevented microglial and astroglial activation, and downregulated p-p38 MAPK in microglia and BDNF/TrkB following burn injury. Similarly, inhibition of p38 MAPK and BDNF/TrkB signaling attenuated mechanical allodynia after burn injury. In addition, inhibition of p38 MAPK prevented spinal microglial activation and downregulated BDNF/TrkB following burn injury. Furthermore, inhibition of BDNF/TrkB signaling prevented spinal microglial activation and downregulated p-p38 MAPK within spinal microglia. Taken together, this study demonstrated that RvD1 might attenuate mechanical allodynia after burn injury by inhibiting spinal cord glial activation, microglial p38 MAPK, and BDNF/TrkB signaling in the spinal dorsal horn.

Rab27a-mediated exosome secretion in anterior cingulate cortex contributes to colorectal visceral pain in adult mice with neonatal maternal deprivation.

Chronic visceral pain is a common symptom of irritable bowel syndrome (IBS). Exosomes are involved in the development of pain. Rab27a can mediate the release of exosomes. The purpose of this study is to investigate how Rab27a-mediated exosome secretion in the anterior cingulate cortex (ACC) regulates visceral hyperalgesia induced with neonatal maternal deprivation (NMD) in adult mice. The colorectal distension method was adopted to measure visceral pain. The BCA protein assay kit was applied to detect the exosome protein concentration. Western blotting, quantitative PCR, and immunofluorescence technique were adopted to detect the expression of Rab27a and the markers of exosomes. Exosomes extracted from ACC were more in NMD mice than in control (CON) mice. Injection of the exosome-specific inhibitor GW4869 in ACC attenuated colorectal visceral pain of NMD mice. Injection of NMD-derived exosomes produced colorectal visceral pain in CON mice. Rab27a was upregulated in ACC of NMD mice. Rab27a was highly expressed in ACC neurons of NMD mice, rather than astrocytes and microglia. Injection of Rab27a-siRNA reduced the release of exosomes and attenuated the colorectal visceral pain in NMD mice. This study suggested that overexpression of Rab27a increased exosome secretion in ACC neurons, thus contributing to visceral hyperalgesia in NMD mice.NEW & NOTEWORTHY This work demonstrated that the expression of Rab27a in the anterior cingulate cortex was upregulated, which mediated multivesicular bodies trafficking to the plasma membrane and led to the increased release of neuronal exosomes, thus contributing to colorectal visceral pain in neonatal maternal deprivation (NMD) mice. Blocking the release of exosomes or downregulation of Rab27a could alleviate colorectal visceral pain in NMD mice. These data may provide a promising strategy for the treatment of visceral pain in irritable bowel syndrome patients.

TSLP in DRG neurons causes the development of neuropathic pain through T cells.

BACKGROUND: Peripheral nerve injury to dorsal root ganglion (DRG) neurons develops intractable neuropathic pain via induction of neuroinflammation. However, neuropathic pain is rare in the early life of rodents. Here, we aimed to identify a novel therapeutic target for neuropathic pain in adults by comprehensively analyzing the difference of gene expression changes between infant and adult rats after nerve injury. METHODS: A neuropathic pain model was produced in neonatal and young adult rats by spared nerve injury. Nerve injury-induced gene expression changes in the dorsal root ganglion (DRG) were examined using RNA sequencing. Thymic stromal lymphopoietin (TSLP) and its siRNA were intrathecally injected. T cells were examined using immunofluorescence and were reduced by systemic administration of FTY720. RESULTS: Differences in changes in the transcriptome in injured DRG between infant and adult rats were most associated with immunological functions. Notably, TSLP was markedly upregulated in DRG neurons in adult rats, but not in infant rats. TSLP caused mechanical allodynia in adult rats, whereas TSLP knockdown suppressed the development of neuropathic pain. TSLP promoted the infiltration of T cells into the injured DRG and organized the expressions of multiple factors that regulate T cells. Accordingly, TSLP caused mechanical allodynia through T cells in the DRG. CONCLUSION: This study demonstrated that TSLP is causally involved in the development of neuropathic pain through T cell recruitment.

Repeated closed-head mild traumatic brain injury-induced inflammation is associated with nociceptive sensitization.

BACKGROUND: Individuals who have experienced mild traumatic brain injuries (mTBIs) suffer from several comorbidities, including chronic pain. Despite extensive studies investigating the underlying mechanisms of mTBI-associated chronic pain, the role of inflammation in long-term pain after mTBIs is not fully elucidated. Given the shifting dynamics of inflammation, it is important to understand the spatial-longitudinal changes in inflammatory processes following mTBIs and their effects on TBI-related pain. METHODS: We utilized a recently developed transgenic caspase-1 luciferase reporter mouse model to monitor caspase-1 activation through a thinned skull window in the in vivo setting following three closed-head mTBI events. Organotypic coronal brain slice cultures and acutely dissociated dorsal root ganglion (DRG) cells provided tissue-relevant context of inflammation signal. Mechanical allodynia was assessed by mechanical withdrawal threshold to von Frey and thermal hyperalgesia withdrawal latency to radiant heat. Mouse grimace scale (MGS) was used to detect spontaneous or non-evoked pain. In some experiments, mice were prophylactically treated with MCC950, a potent small molecule inhibitor of NLRP3 inflammasome assembly to inhibit injury-induced inflammatory signaling. Bioluminescence spatiotemporal dynamics were quantified in the head and hind paws, and caspase-1 activation was confirmed by immunoblot. Immunofluorescence staining was used to monitor the progression of astrogliosis and microglial activation in ex vivo brain tissue following repetitive closed-head mTBIs. RESULTS: Mice with repetitive closed-head mTBIs exhibited significant increases of the bioluminescence signals within the brain and paws in vivo for at least one week after each injury. Consistently, immunoblotting and immunofluorescence experiments confirmed that mTBIs led to caspase-1 activation, astrogliosis, and microgliosis. Persistent changes in MGS and hind paw withdrawal thresholds, indicative of pain states, were observed post-injury in the same mTBI animals in vivo. We also observed enhanced inflammatory responses in ex vivo brain slice preparations and DRG for at least 3 days following mTBIs. In vivo treatment with MCC950 significantly reduced caspase-1 activation-associated bioluminescent signals in vivo and decreased stimulus-evoked and non-stimulus evoked nociception. CONCLUSIONS: Our findings suggest that the inflammatory states in the brain and peripheral nervous system following repeated mTBIs are coincidental with the development of nociceptive sensitization, and that these events can be significantly reduced by inhibition of NLRP3 inflammasome activation.

Metabotropic glutamate receptor 5-mediated inhibition of inward-rectifying K+ channel 4.1 contributes to orofacial ectopic mechanical allodynia following inferior alveolar nerve transection in male mice.

Inward-rectifying K+ channel 4.1 (Kir4.1), which regulates the electrophysiological properties of neurons and glia by affecting K+ homeostasis, plays a critical role in neuropathic pain. Metabotropic glutamate receptor 5 (mGluR5) regulates the expression of Kir4.1 in retinal Müller cells. However, the role of Kir4.1 and its expressional regulatory mechanisms underlying orofacial ectopic allodynia remain unclear. This study aimed to investigate the biological roles of Kir4.1 and mGluR5 in the trigeminal ganglion (TG) in orofacial ectopic mechanical allodynia and the role of mGluR5 in Kir4.1 regulation. An animal model of nerve injury was established via inferior alveolar nerve transection (IANX) in male C57BL/6J mice. Behavioral tests indicated that mechanical allodynia in the ipsilateral whisker pad lasted at least 14 days after IANX surgery and was alleviated by the overexpression of Kir4.1 in the TG, as well as intraganglionic injection of an mGluR5 antagonist (MPEP hydrochloride) or a protein kinase C (PKC) inhibitor (chelerythrine chloride); Conditional knockdown of the Kir4.1 gene downregulated mechanical thresholds in the whisker pad. Double immunostaining revealed that Kir4.1 and mGluR5 were co-expressed in satellite glial cells in the TG. IANX downregulated Kir4.1 and upregulated mGluR5 and phosphorylated PKC (p-PKC) in the TG; Inhibition of mGluR5 reversed the changes in Kir4.1 and p-PKC that were induced by IANX; Inhibition of PKC activation reversed the downregulation of Kir4.1 expression caused by IANX (p < .05). In conclusion, activation of mGluR5 in the TG after IANX contributed to orofacial ectopic mechanical allodynia by suppressing Kir4.1 via the PKC signaling pathway.

HBO treatment enhances motor function and modulates pain development after sciatic nerve injury via protection the mitochondrial function.

BACKGROUND: Peripheral nerve injury can cause neuroinflammation and neuromodulation that lead to mitochondrial dysfunction and neuronal apoptosis in the dorsal root ganglion (DRG) and spinal cord, contributing to neuropathic pain and motor dysfunction. Hyperbaric oxygen therapy (HBOT) has been suggested as a potential therapeutic tool for neuropathic pain and nerve injury. However, the specific cellular and molecular mechanism by which HBOT modulates the development of neuropathic pain and motor dysfunction through mitochondrial protection is still unclear. METHODS: Mechanical and thermal allodynia and motor function were measured in rats following sciatic nerve crush (SNC). The HBO treatment (2.5 ATA) was performed 4 h after SNC and twice daily (12 h intervals) for seven consecutive days. To assess mitochondrial function in the spinal cord (L2-L6), high-resolution respirometry was measured on day 7 using the OROBOROS-O2k. In addition, RT-PCR and Immunohistochemistry were performed at the end of the experiment to assess neuroinflammation, neuromodulation, and apoptosis in the DRG (L3-L6) and spinal cord (L2-L6). RESULTS: HBOT during the early phase of the SNC alleviates mechanical and thermal hypersensitivity and motor dysfunction. Moreover, HBOT modulates neuroinflammation, neuromodulation, mitochondrial stress, and apoptosis in the DRG and spinal cord. Thus, we found a significant reduction in the presence of macrophages/microglia and MMP-9 expression, as well as the transcription of pro-inflammatory cytokines (TNFa, IL-6, IL-1b) in the DRG and (IL6) in the spinal cord of the SNC group that was treated with HBOT compared to the untreated group. Notable, the overexpression of the TRPV1 channel, which has a high Ca2+ permeability, was reduced along with the apoptosis marker (cleaved-Caspase3) and mitochondrial stress marker (TSPO) in the DRG and spinal cord of the HBOT group. Additionally, HBOT prevents the reduction in mitochondrial respiration, including non-phosphorylation state, ATP-linked respiration, and maximal mitochondrial respiration in the spinal cord after SNC. CONCLUSION: Mitochondrial dysfunction in peripheral neuropathic pain was found to be mediated by neuroinflammation and neuromodulation. Strikingly, our findings indicate that HBOT during the critical period of the nerve injury modulates the transition from acute to chronic pain via reducing neuroinflammation and protecting mitochondrial function, consequently preventing neuronal apoptosis in the DRG and spinal cord.

HSV-1 infection-induced herpetic neuralgia involves a CCL5/CCR5-mediated inflammation mechanism.

Herpetic-related neuralgia (HN) caused by varicella-zoster virus (VZV) infection is one of the most typical and common neuropathic pain in the clinic. However, the potential mechanisms and therapeutic approaches for the prevention and treatment of HN are still unclear. This study aims to provide a comprehensive understanding of the molecular mechanisms and potential therapeutic targets of HN. We used an HSV-1 infection-induced HN mouse model and screened the differentially expressed genes (DEGs) in the DRG and spinal cord using an RNAseq technique. Moreover, bioinformatics methods were used to figure out the signaling pathways and expression regulation patterns of the DEGs enriched. In addition, quantitative real-time RT-PCR and western blot were carried out to further confirm the expression of DEGs. HSV-1 inoculation in mice resulted in mechanical allodynia, thermal hyperalgesia, and cold allodynia, following the infection of HSV-1 in both DRG and spinal cord. Besides, HSV-1 inoculation induced an up-regulation of ATF3, CGRP, and GAL in DRG and activation of astrocytes and microglia in the spinal cord. Moreover, 639 genes were upregulated, 249 genes were downregulated in DRG, whereas 534 genes were upregulated and 12 genes were downregulated in the spinal cord of mice 7 days after HSV-1 inoculation. GO and KEGG enrichment analysis suggested that immune responses and cytokine-cytokine receptor interaction are involved in DRG and spinal cord neurons in mice after HSV-1 infection. In addition, CCL5 and its receptor CCR5 were significantly upregulated in DRG and spinal cord upon HSV-1 infection in mice. And blockade of CCR5 exhibited a significant analgesic effect and suppressed the upregulation of inflammatory cytokines in DRG and spinal cord induced by HSV-1 infection in mice. HSV-1 infection-induced allodynia and hyperalgesia in mice through dysregulation of immune response and cytokine-cytokine receptor interaction mechanism. Blockade of CCR5 alleviated allodynia and hyperalgesia probably through the suppression of inflammatory cytokines. Therefore, CCR5 could be a therapeutic target for the alleviation of HSV-1 infection-induced HN.

Inhibition of DAGLß as a therapeutic target for pain in sickle cell disease.

Sickle cell disease (SCD) is the most common inherited disease. Pain is a key morbidity of SCD and opioids are the main treatment but their side effects emphasize the need for new analgesic approaches. Humanized transgenic mouse models have been instructive in understanding the pathobiology of SCD and mechanisms of pain. Homozygous (HbSS) Berkley mice express >99% human sickle hemoglobin and several features of clinical SCD including hyperalgesia. Previously, we reported that the endocannabinoid 2-arachidonoylglycerol (2-AG) is a precursor of the pro-nociceptive mediator prostaglandin E2-glyceryl ester (PGE2-G) which contributes to hyperalgesia in SCD. We now demonstrate the causal role of 2-AG in hyperalgesia in sickle mice. Hyperalgesia in HbSS mice correlated with elevated levels of 2-AG in plasma, its synthesizing enzyme diacylglycerol lipase ß (DAGLß) in blood cells, and with elevated levels of PGE2 and PGE2-G, pronociceptive derivatives of 2-AG. A single intravenous injection of 2-AG produced hyperalgesia in non-hyperalgesic HbSS mice, but not in control (HbAA) mice expressing normal human HbA. JZL184, an inhibitor of 2-AG hydrolysis, also produced hyperalgesia in non-hyperalgesic HbSS or hemizygous (HbAS) mice, but did not influence hyperalgesia in hyperalgesic HbSS mice. Systemic and intraplantar administration of KT109, an inhibitor of DAGLß, decreased mechanical and heat hyperalgesia in HbSS mice. The decrease in hyperalgesia was accompanied by reductions in 2-AG, PGE2 and PGE2-G in the blood. These results indicate that maintaining the physiological level of 2-AG in the blood by targeting DAGLß may be a novel and effective approach to treat pain in SCD.

Activation of the STING pathway induces peripheral sensitization via neuroinflammation in a rat model of bone cancer pain.

BACKGROUND: Neuroinflammation in the peripheral nervous system has been linked to cancer metastasis-induced bone pain. The stimulator of interferon genes (STING), an innate immune sensor for cytosolic DNA, plays an important role in inflammation and cancer metastasis and is reported to be a critical regulator of nociception. Here, we examined the role of STING in primary nociceptive neurons and chronic pain to determine if it could be a new target for treating bone cancer pain (BCP). METHODS: Walker 256 cancer cells were injected intratibially to induce bone cancer pain in rats. STING and its downstream inflammatory factors in dorsal root ganglia (DRG) were detected using western blotting and immunofluorescent staining. Transmission electron microscopy and the BCL2-associated X (Bax) expression were used to detect the mitochondrial stress in DRG neurons. C-176, a specific inhibitor of STING, was used to block STING activation and to test the pain behavior. RESULTS: Mechanical hyperalgesia and spontaneous pain were observed in BCP rats, accompanied by the upregulation of the STING expression in the ipsilateral L4-5 DRG neurons which showed significant mitochondrion stress. The STING/TANK-binding kinase 1 (TBK1)/nuclear factor-kappa B (NF-κB) pathway activation was observed in the DRGs of BCP rats as well as increased IL-1ß, IL-6, and TNF-α expression. C-176 alleviated bone cancer pain and reduced the STING and its downstream inflammatory pathway. CONCLUSION: We provide evidence that STING pathway activation leads to neuroinflammation and peripheral sensitization. Pharmacological blockade of STING may be a promising novel strategy for preventing BCP.

Spontaneous activity of specific C-nociceptor subtypes from diabetic patients and mice: Involvement of reactive dicarbonyl compounds and (sensitized) transient receptor potential channel A1.

BACKGROUND: Diabetic metabolism causes changes of the chemical milieu including accumulation of reactive carbonyl species, for example, methylglyoxal (MGO). MGO activates chemosensitive TRPA1 on nociceptors, but the contribution to neuronal pathophysiology causing pain and hyperalgesia in diabetic neuropathy is not fully understood. METHODS: We employed single-nerve-fiber recordings in type 2 diabetes patients with (spDN) and without cutaneous pain (DN) and in streptozotocin-diabetic and healthy mice. In mice, we measured Ca++ transients in cultured DRG neurons and stimulated CGRP release from hairy skin. RESULTS: In diabetic patients, we recorded a large proportion of pathologically altered nerve C-fibers (79%). In spDN patients we found a higher percentage (72%) of spontaneously active C-nociceptors than in DN patients (15%). The proportion of spontaneous activity was highest among pathological fibers with mechanoinsensitive fiber properties which are particularly sensitive to MGO in contrast to mechanosensitive fibers. Mouse polymodal nociceptors, in contrast to purely mechanosensitive C-fibers, showed highest prevalence of TRPA1-related chemosensitivity. In diabetic mice about 37% of polymodal nociceptors developed spontaneous activity and exhibited significantly greater MGO responses, indicating sensitized TRPA1 receptors. Low-threshold mechanosensitive Aδ-fibers were vigorously activated by MGO but independently of TRPA1 activation. INTERPRETATION: Our translational findings suggest that TRPA1-expressing C-nociceptors, which in human correspond to mechanoinsensitive and in mice to polymodal nociceptors, are especially vulnerable to develop spontaneous activity. Those two different nociceptor classes might share the functional role as dicarbonyl-sensitive chemosensors and represent the critical nociceptor population that support the development of pain and hyperalgesia in diabetic neuropathy.