Spinal Microglial Activation Research Articles

Activated Schwann cells in pancreatic cancer are linked to analgesia via suppression of spinal astroglia and microglia

ObjectiveThe impact of glia cells during GI carcinogenesis and in cancer pain is unknown. Here, we demonstrate a novel mechanism how Schwann cells (SCs) become activated in the pancreatic cancer...

Age-Related Differences in Neuropathic Pain Behavior and Spinal Microglial Activity after L5 Spinal Nerve Ligation in Male Rats.

Introduction:Several studies have reported the involvement of age-related changes in the development of neuropathic pain behaviors. However, limited data are available on the role of age in establishing and maintaining chronic neuropathic pain after peripheral nerve injury.Methods:In the present study, we examined age-related neuropathic behavior among rats in 4 age groups: pups (4 weeks old; weight, 60–80 g), juvenile rats (6 weeks old; weight, 120–140 g), and mature rats (10–12 weeks old; weight, 200–250 g). Because the exact contribution of spinal microglia and its association with the development of neuropathic pain remains unknown, we also evaluated the expression of spinal Iba1, a microglial marker, by using western blotting before and 5 days after spinal nerve ligation (SNL) as well as after the daily IP administration of minocycline (30 mg/kg).Results:Our results showed that SNL-induced mechanical allodynia but not thermal hyperalgesia in mature rats but not in pups (P<0.05 and P<0.01, respectively). The expression of spinal Iba1 in the juvenile rats was significantly lower than that in pups and mature rats (P<0.01). Moreover, administration of minocycline decreased the expression of spinal Iba1 in the pup rats more than in juvenile rats (P<0.001) and in the juvenile rats more than in the mature rats (P<0.05).Conclusion:These data suggest that the development of neuropathic behaviors and microglial activation after SNL could be age dependent.

Early Postoperative Nociceptive Threshold and Production of Brain-Derived Neurotrophic Factor Induced by Plantar Incision Are Not Influenced with Minocycline in a Rat: Role of Spinal Microglia

Background: Brain-derived neurotrophic factor (BDNF) from spinal microglia is crucial for aberrant nociceptive signaling in several pathological pain conditions, including postoperative pain. We assess the contribution of spinal microglial activation and associated BDNF overexpression to the early post-incisional nociceptive threshold. Methods: Male Sprague-Dawley rats were implanted with an intrathecal catheter. A postoperative pain model was established by plantar incision. Thermal and mechanical nociceptive responses were assessed by infrared radiant heat and von Frey filaments before and after plantar incision. Rats were injected intrathecally the microglial activation inhibitor minocycline before incision, 24 h after incision, or both. Other groups were subjected to the same treatments and the L4-L5 spinal cord segment removed for immunohistochemical analysis of microglia activation and BNDF expression. Results: Plantar incision reduced both thermal latency and mechanical threshold, indicating thermal hypersensitivity and mechanical allodynia. Minocycline temporally reduced thermal withdrawal latency but had no effect on mechanical withdrawal threshold, spinal microglial activity, or dorsal horn BDNF overexpression during the early post-incision period. Conclusion: These results suggest that spinal microglia does not contribute substantially to post-incisional nociceptive threshold. The BDNF overexpression response that may contribute to postoperative hyperalgesia and allodynia is likely derived from other sources.

The Role of C Fibers in Spinal Microglia Induction and Possible Relation with TRPV3 Expression During Chronic Inflammatory Arthritis in Rats.

Introduction:Stimulation of peptidergic fibers activates microglia in the dorsal horn. Microglia activation causes fractalkine (FKN) release, a neuron-glia signal, which enhances pain. The transient vanilloid receptor 1 (TRPV1) mediates the release of neuropeptides, which can subsequently activate glia. TRPV1 and TRPV2 are generally expressed on C and Aδ fibers, respectively. Expression of both proteins is upregulated during inflammation, but expression of TRPV3 after induction of inflammation is unclear.Methods:Adult male Wistar rats were used in all experiments. Arthritis was induced in them by single subcutaneous injection of complete Freund’s adjuvant (CFA) in their right hindpaws. Resiniferatoxin (RTX) was used to eliminate peptidergic fibers. We examined the relation between FKN and TRPV3 expression by administration of anti-FKN antibody.Results:Our study findings indicated that 1) spinal TRPV3 was mostly expressed on nonpeptidergic fibers, 2) expression of spinal TRPV3 increased following inflammation, 3) elimination of peptidergic fibers decreased spinal TRPV3 expression, 4) alteration of hyperalgesia was compatible with TRPV3 changes in RTX-treated rat, and 5) anti-FKN antibody reduced spinal TRPV3 expression.Discussion:It seems that the hyperalgesia variation during different phases of CFA-induced arthritis correlates with spinal TRPV3 expression variation on peptidergic fibers. Moreover, spinal microglial activation during CFA inflammation is involved in TRPV3 expression changes via FKN signaling.

Functional inhibition of chemokine receptor CCR2 by dicer-substrate-siRNA prevents pain development.

BackgroundAccumulating evidence suggests that the C-C chemokine ligand 2 (CCL2, or monocyte chemoattractant protein 1) acts as a neuromodulator in the central nervous system through its binding to the C-C chemokine receptor 2 (CCR2). Notably, it is well established that the CCL2/CCR2 axis plays a key role in neuron-glia communication as well as in spinal nociceptive transmission. Gene silencing through RNA interference has recently emerged as a promising avenue in research and drug development, including therapeutic management of chronic pain. In the present study, we used 27-mer Dicer-substrate small interfering RNA (DsiRNA) targeting CCR2 and assessed their ability to reverse the nociceptive behaviors induced by spinal CCL2 injection or following intraplantar injection of complete Freund’s adjuvant.ResultsTo this end, we first developed high-potency DsiRNAs designed to target different sequences distributed across the rat CCR2 (rCCR2) messenger RNA. For optimization, methyl groups were added to the two most potent DsiRNA candidates (Evader and M7 2′-O-methyl modified duplexes) in order to improve in vivo duplex stability and to reduce potential immunostimulatory activity. Our results demonstrated that all modified candidates formulated with the cell-penetrating peptide reagent Transductin showed strong RNAi activity following intrathecal delivery, exhibiting >50% rCCR2 knockdown in lumbar dorsal root ganglia. Accordingly, we found that these DsiRNA duplexes were able to reduce spinal microglia activation and were effective at blocking CCL2-induced mechanical hypersensitivity. Along with similar reductions of rCCR2 messenger RNA, both sequences and methylation patterns were similarly effective in inhibiting the CCL2 nociceptive action for the whole seven days testing period, compared to mismatch DsiRNA. DsiRNAs against CCR2 also reversed the hypernociceptive responses observed in the complete Freund’s adjuvant-induced inflammatory chronic pain model.ConclusionAltogether, these results validate CCR2 as a an appropriate molecular target for pain control and demonstrate that RNAi-based gene therapy represent an highly specific alternative to classical pharmacological approaches to treat central pathologies such as chronic pain.

Disruption of Spinal Noradrenergic Activation Delays Recovery of Acute Incision-Induced Hypersensitivity and Increases Spinal Glial Activation in the Rat

Results of clinical studies suggest that descending inhibitory controls from the brainstem are important for speeding recovery from pain after surgery. We examined the effects of destroying spinally projecting noradrenergic neurons via intrathecally administered antibody to dopamine β-hydroxylase conjugated to saporin (DβH-saporin) on recovery in an acute incisional pain model. Mechanical and thermal paw withdrawal thresholds and nonevoked spontaneous guarding scores were tested for several weeks postoperatively and analyzed using mixed effects growth curve modeling. DβH-saporin treatment resulted in a significant prolongation in the duration of mechanical and to a lesser degree thermal hypersensitivity in the ipsilateral paw of incised rats but did not increase the duration of spontaneous guarding. DβH-saporin treatment was also associated with increased microglial and astrocyte activation in the ipsilateral spinal cord 21 days after incision compared with immunoglobulin G-saporin treated controls. Chronic intrathecal administration of the α2 adrenergic receptor antagonist atipamezole (50–200 μg/d) produced similar effects. These data suggest that spinally projecting noradrenergic pathways and spinal α2 adrenergic receptor activation are important for speeding recovery from hypersensitivity after surgical incision possibly by reducing spinal glial activation. Interventions that augment the noradrenergic system might be important to speed recovery from pain after surgery. PerspectiveEndogenous descending spinal noradrenergic activation promotes resolution of incision-induced hypersensitivity and inhibits spinal microglial and astrocyte activation in part through α2 adrenergic receptors.

Omega-3 fatty acids are able to modulate the painful symptoms associated to cyclophosphamide-induced-hemorrhagic cystitis in mice

This study investigated the effects of the long-term dietary fish oil supplementation or the acute administration of the omega-3 fatty acid docosahexaenoic acid (DHA) in the mouse hemorrhagic cystitis (HC) induced by the anticancer drug cyclophosphamide (CYP). HC was induced in mice by a single CYP injection (300mg/kg ip). Animals received four different diets containing 10% and 20% of corn or fish oil, during 21days. Separated groups received DHA by ip (1μmol/kg) or intrathecal (i.t.; 10μg/site) routes, 1h or 15min before CYP. The behavioral tests (spontaneous nociception and mechanical allodynia) were carried out from 1h to 6h following CYP injection. Bladder inflammatory changes, blood cell counts and serum cytokines were evaluated after euthanasia (at 6h). Immunohistochemistry analysis was performed for assessing spinal astrocyte and microglia activation or GPR40/FFAR1 expression. Either fish oil supplementation or DHA treatment (ip and i.t.) markedly prevented visceral pain, without affecting CYP-evoked bladder inflammatory changes. Moreover, systemic DHA significantly prevented the neutrophilia/lymphopenia caused by CYP, whereas this fatty acid did not significantly affect serum cytokines. DHA also modulated the spinal astrocyte activation and the GPR40/FFAR1 expression. The supplementation with fish oil enriched in omega-3 fatty acids or parenteral DHA might be interesting nutritional approaches for cancer patients under chemotherapy schemes with CYP.

CX3CR1 Mediates Nicotine Withdrawal-Induced Hyperalgesia via Microglial P38 MAPK Signaling.

Previously, we reported that nicotine withdrawal (NT) significantly increased pain sensitivity in rats. Recent reports suggest that fractalkine is involved in the spinal cord neuron-to-microglia activation via CX3CR1 signaling. However, its contribution to NT-induced hyperalgesia and the underlying mechanisms have yet to be elucidated. In the present study, a rat model of NT was used to test the changes in CX3CR1 expression in the spinal cord. We also evaluated the effect of the CX3CR1 neutralizing antibody on spinal microglial activity, the expression of phosphorylated p38-mitogen-activated protein kinase (p-p38-MAPK) and heat-induced pain responses. We established a NT model via subcutaneous injection of pure nicotine (3mg/kg), three times daily for 7days. The expression of CX3CR1 was studied by Western blot and immunofluorescence staining. Following NT, the rats received daily intrathecal injections of CX3CR1 neutralizing antibody for 3days. The change in paw withdrawal latency (PWL) was observed. The activation of microglia and the expression of p-p38-MAPK were investigated by Western blot and immunofluorescence staining. The expression of CX3CR1 was significantly increased after NT and co-localized with IBA-1. NT rats treated with CX3CR1 neutralizing antibody showed significantly increased PWL on day 4 after NT. Furthermore, the activation of microglia and the expression of p-p38-MAPK in the spinal cord were suppressed. These results indicate that microglial CX3CR1/p38MAPK pathway is critical for the development of pain hypersensitivity after NT.

Substance P spinal signaling induces glial activation and nociceptive sensitization after fracture

Tibia fracture in rodents induces substance P (SP)-dependent keratinocyte activation and inflammatory changes in the hindlimb, similar to those seen in complex regional pain syndrome (CRPS). In animal pain models spinal glial cell activation results in nociceptive sensitization. This study tested the hypothesis that limb fracture triggers afferent C-fiber SP release in the dorsal horn, resulting in chronic glial activation and central sensitization. At 4weeks after tibia fracture and casting in rats, the cast was removed and hind paw allodynia, unweighting, warmth, and edema were measured, then the antinociceptive effects of microglia (minocycline) or astrocyte (L-2-aminoadipic acid (LAA)) inhibitors or an SP receptor antagonist (LY303870) were tested. Immunohistochemistry and PCR were used to evaluate microglial and astrocyte activation in the dorsal horn. Similar experiments were performed in intact rats after brief sciatic nerve electric stimulation at C-fiber intensity. Microglia and astrocytes were chronically activated at 4weeks after fracture and contributed to the maintenance of hind paw allodynia and unweighting. Furthermore, LY303870 treatment initiated at 4weeks after fracture partially reversed both spinal glial activation and nociceptive sensitization. Similarly, persistent spinal microglial activation and hind paw nociceptive sensitization were observed at 48h after sciatic nerve C-fiber stimulation and this effect was inhibited by treatment with minocycline, LAA, or LY303870. These data support the hypothesis that C-fiber afferent SP signaling chronically supports spinal neuroglial activation after limb fracture and that glial activation contributes to the maintenance of central nociceptive sensitization in CRPS. Treatments inhibiting glial activation and spinal inflammation may be therapeutic for CRPS.

Treatment with a carbon monoxide-releasing molecule (CORM-2) inhibits neuropathic pain and enhances opioid effectiveness in rats

Experiments were conducted to evaluate the contribution of P2X4 receptors to the modulation of neuropathy and their ability to amplify opioid effectiveness. The study consisted of behavioral and biochemical analysis of the effect of a carbon monoxide donor - CORM-2, on the development of neuropathic pain in a rat model of chronic constriction injury (CCI) to the sciatic nerve. Here, we exam if chronic intraperitoneal or intrathecal administration of CORM-2 influences CCI-induced allodynia and hyperalgesia. In parallel, changes of spinal microglial and/or astroglial activation were studied. CORM-2 was administered intrathecally [20μg/5μl] or intraperitoneally [10mg/kg]. Here, we report that intraperitoneal or intrathecal chronic administration of the carbon monoxide donor CORM-2 significantly reduced the allodynia/hyperalgesia induced by CCI, with a parallel reduction of spinal microglial and/or astroglial activation. Furthermore, even a single intraperitoneal administration of CORM-2 had antiallodynic potency and moreover, increased morphine/buprenorphine analgesia compared to the effects of these drugs alone, completely eliminating the neuropathic pain symptoms. When CORM-2 was administered for 7 consecutive days, the antinociceptive effect of CORM-2 after CCI was stronger on day 7 than on day 2, which indicates that this effect built up over time. We are the first to demonstrate that even a single intraperitoneal injection of CORM-2 potentiates the antihyperalgesic and antiallodynic properties of morphine/buprenorphine in a CCI rat model. Our data suggest that P2X4 receptors play a significant role in neuropathic pain development, suggesting that their blockade may have potential therapeutic utility.

Spinal Heme Oxygenase-1 (HO-1) Exerts Antinociceptive Effects Against Neuropathic Pain in a Mouse Model of L5 Spinal Nerve Ligation.

Heme oxygenase-1 (HO-1) exerts protective effects against ischemia and inflammation in the central nervous system. However, its role in neuropathic pain is still unclear. This study was undertaken to explore the distribution and possible mechanism of HO-1 in a mouse model of peripheral nerve injury. The experiment was conducted using a mouse model of L5 spinal nerve ligation (SNL). Mice received repeated intraperitoneal injection of Carbon monoxide-releasing molecule-2 (CO-RM-2), HO-1 inducer cobalt protoporphyrin IX (CoPP) or single intraspinal injection of lentivirus (LV) over-expressing HO-1. The behavior analyses were conducted. The distribution and expression of HO-1 in the spinal cord were analyzed. HO-1 but not HO-2 was upregulated in spinal cord microglia cells after nerve injury, and the repeated intraperitoneal administration of CORM-2 (10 mg/kg/d) or CoPP (5 mg/kg/d) both significantly reduced the mechanical allodynia and thermal hyperalgesia induced by SNL (P < 0.01). Intraspinal injection of LV-HO-1 persistently suppresses SNL-induced neuropathic pain (P < 0.01 or P < 0.05), significantly induced the spinal HO-1 protein content (P < 0.01) and inhibited the microglia activation (P < 0.01) 7 days after SNL. HO-1 upregulation could elicit potent analgesic effects against neuropathic pain, which might partly be attributed to inhibition of spinal microglia activation. HO-1 signaling pathway may present a novel strategy for the treatment of neuropathic pain.

Early treatment with UR13870, a novel inhibitor of p38α mitogenous activated protein kinase, prevents hyperreflexia and anxiety behaviors, in the spared nerve injury model of neuropathic pain

Microglia cell activation plays a role in the development of neuropathic pain partly due to the activation of the p38α MAPK signaling pathway after nerve injury. In this study we assessed the effect of UR13870, a p38α MAPK inhibitor, in the “spared nerve injury” (SNI) model, to study its effects on modulation of spinal microglial activation and to test behavioral hyperreflexia responses and cerebral-mediated pain behavior. The effect of daily administration of UR13870 (10mg/kg p.o.) and Pregabalin (50mg/kg p.o.) on reflex hypersensitivity to mechanical and cold test stimuli and on affective related pain responses measured with the place escape avoidance paradigm and the open field-induced anxiety test, were evaluated after SNI in Sprague Dawley rats. Microglial reactivity in the ipsilateral lumbar laminae I/II dorsal horn was evaluated with OX-42 immunohistochemistry. UR13870 treatment significantly decreased hindlimb hyperreflexia to both mechanical and cold stimuli after SNI without loss of general motor function, in addition to a reduction in pain-related anxiety behavior at day 21 after SNI, accompanied by normalization of OX-42 immunoreactivity within the ipsilateral lumbar dorsal horn. Pregabalin treatment only reduced mechanical hyperreflexia and affected general motor function. Oral administration of the p38α MAPK inhibitor, UR13870, mediates antinociception to both mechanical and cold stimuli, and significantly restored inner-zone exploration in the open field test, accompanied by normalization in dorsal horn microglial activation in the SNI model.

P2X7 Receptor Mediates Spinal Microglia Activation of Visceral Hyperalgesia in a Rat Model of Chronic Pancreatitis.

Background & AimsMolecular mechanisms underlying the activated spinal microglia in association with the pain in chronic pancreatitis (CP) remain unknown. We tested whether P2X7R on spinal microglia mediates the pathogenesis of visceral pain using a CP rat model.MethodsThe CP model was induced via intraductal injection of 2% trinitrobenzene sulfonic acid into male Sprague-Dawley rats. Hyperalgesia was assessed based on the mechanical sensitivity to Von-Frey filaments (VFFs), and nocifensive behaviors were measured in response to electrical stimulation of the pancreas. Three weeks after CP induction, spinal cord samples were harvested for immunostaining, immunoblot, and real-time polymerase chain reaction analyses of the P2X7R. Changes in nocifensive behaviors and associated molecular effectors were assessed by blocking spinal cord P2X7R pharmacologically using the selective P2X7R antagonist brilliant blue G (BBG) or genetically using short interfering RNA (siRNA).ResultsCP induced a significant up-regulation of spinal P2X7R expression, which colocalized with a microglial marker (OX-42). Intrathecal administration of BBG significantly attenuated CP-related visceral hyperalgesia in response to VFF-mediated or electrical stimulation of the pancreas, which was associated with suppressed spinal expression of P2X7R and inhibited activation of spinal microglia. Intrathecal injection of siRNA to knock down P2X7R expression in the spinal cord would suppress the nociceptive behaviors in CP rats.ConclusionsSpinal microglia P2X7R mediates central sensitization of chronic visceral pain in CP. BBG may represent an effective drug for the treatment of chronic pain in CP patients.

Dysregulated TNFα promotes cytokine proteome profile increases and bilateral orofacial hypersensitivity

BackgroundTumor necrosis factor alpha (TNFα) is increased in patients with headache, neuropathic pain, periodontal and temporomandibular disease. This study and others have utilized TNF receptor 1/2 (TNFR1/2) knockout (KO) animals to investigate the effect of TNFα dysregulation in generation and maintenance of chronic neuropathic pain. The present study determined the impact of TNFα dysregulation in a trigeminal inflammatory compression (TIC) nerve injury model comparing wild-type (WT) and TNFR1/2 KO mice. MethodsChromic gut suture was inserted adjacent to the infraorbital nerve to induce the TIC model mechanical hypersensitivity. Cytokine proteome profiles demonstrated serology, and morphology explored microglial activation in trigeminal nucleus 10weeks post. ResultsTIC injury induced ipsilateral whisker pad mechanical allodynia persisting throughout the 10-week study in both TNFR1/2 KO and WT mice. Delayed mechanical allodynia developed on the contralateral whisker pad in TNFR1/2 KO mice but not in WT mice. Proteomic profiling 10weeks after chronic TIC injury revealed TNFα, interleukin-1alpha (IL-1α), interleukin-5 (IL-5), interleukin-23 (IL-23), macrophage inflammatory protein-1β (MIP-1β), and granulocyte–macrophage colony-stimulating factor (GM-CSF) were increased more than 2-fold in TNFR1/2 KO mice compared to WT mice with TIC. Bilateral microglial activation in spinal trigeminal nucleus was detected only in TNFR1/2 KO mice. p38 mitogen-activated protein kinase (MAPK) inhibitor and microglial inhibitor minocycline reduced hypersensitivity. ConclusionsThe results suggest the dysregulated serum cytokine proteome profile and bilateral spinal trigeminal nucleus microglial activation are contributory to the bilateral mechanical hypersensitization in this chronic trigeminal neuropathic pain model in the mice with TNFα dysregulation. Data support involvement of both neurogenic and humoral influences in chronic neuropathic pain.

Delayed activation of spinal microglia contributes to the maintenance of bone cancer pain in female Wistar rats via P2X7 receptor and IL-18.

Accumulating evidence suggests that activation of spinal microglia contributes to the development of inflammatory and neuropathic pain. However, the role of spinal microglia in the maintenance of chronic pain remains controversial. Bone cancer pain shares features of inflammatory and neuropathic pain, but the temporal activation of microglia and astrocytes in this model is not well defined. Here, we report an unconventional role of spinal microglia in the maintenance of advanced-phase bone cancer pain in a female rat model. Bone cancer elicited delayed and persistent microglial activation in the spinal dorsal horn on days 14 and 21, but not on day 7. In contrast, bone cancer induced rapid and persistent astrocytic activation on days 7-21. Spinal inhibition of microglia by minocycline at 14 d effectively reduced bone cancer-induced allodynia and hyperalgesia. However, pretreatment of minocycline in the first week did not affect the development of cancer pain. Bone cancer increased ATP levels in CSF, and upregulated P2X7 receptor, phosphorylated p38, and IL-18 in spinal microglia. Spinal inhibition of P2X7/p-38/IL-18 pathway reduced advanced-phase bone cancer pain and suppressed hyperactivity of spinal wide dynamic range (WDR) neurons. IL-18 induced allodynia and hyperalgesia after intrathecal injection, elicited mechanical hyperactivity of WDR neurons in vivo, and increased the frequency of mEPSCs in spinal lamina IIo nociceptive synapses in spinal cord slices. Together, our findings demonstrate a novel role of microglia in maintaining advanced phase cancer pain in females via producing the proinflammatory cytokine IL-18 to enhance synaptic transmission of spinal cord nociceptive neurons.

Minocycline attenuates pain by inhibiting spinal microglia activation in diabetic rats.

The mechanisms associated with diabetes-induced neuropathic pain are complex and poorly understood. In order to understand the involvement of spinal microglia activity in diabetic pain, the present study investigated whether minocycline treatment is able to attenuate diabetic pain using a rat model. Diabetes was induced using a single intraperitoneal injection of streptozotocin (STZ). Minocycline was then intrathecally administered to the rats. Paw withdrawal threshold (PWT) and paw withdrawal latency (PWL) were tested weekly. The expression of OX-42, Iba-1, phospho-p38 mitogen-activated protein kinase (MAPK), tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and inducible nitric oxide synthase (iNOS), were examined in the spinal cord in order to evaluate the activation of microglia. The present study demonstrated that rats with STZ-induced diabetes exhibited increased mean plasma glucose concentration, decreased mean body weight and significant pain hypersensitivity compared with control rats. PWT and PWL values of rats with STZ-induced diabetes increased following treatment with minocycline. No differences were observed in expression levels of the microglial activity markers (OX-42, Iba-1 and phospho-p38 MAPK) between rats with STZ-induced diabetes and control rats. However, TNF-α, IL-1β and iNOS expression levels were higher in rats with STZ-induced diabetes compared with control rats. Following treatment with minocycline markers of microglial activation, including cytokines and iNOS, were downregulated in rats with STZ-induced diabetes. The results of the present study indicated that minocycline treatment may inhibit spinal microglial activation and attenuate diabetic pain in rats with STZ-induced diabetes.

Involvement of CD137L in peripheral nerve injury-induced neuropathic pain behaviors (CAM1P.161)

Abstract CD137L, also known as 4-1BBL, is a co-stimulatory molecule involved in many immune functions. CD137L-mediated signaling can promote monocyte/macrophage proliferation, migration, and production of proinflammatory factors; however, CD137L-mediated microglial responses have not been studied extensively. Here, we sought to investigate the role of microglial CD137L in the development of peripheral nerve injury-induced neuropathic pain using a murine model, spinal nerve L5 transection (L5Tx). First, compared to wild type (WT) controls, B6_CD137L knockout (KO) mice displayed significantly decreased L5Tx-induced mechanical and heat hypersensitivity starting at day 3 post-L5Tx until day 35 post-L5Tx when the experiment was terminated. To further determine the role of microglial CD137L, an anti-CD137L neutralizing antibody (Ab, clone TKS-1) was intrathecally injected into WT BALB/c mice daily from day 0 (before surgery) to day 7 post-L5Tx. The neutralizing Ab significantly reduced L5Tx-induced mechanical hypersensitivity but not the heat hypersensitivity. Our data indicate that spinal cord CD137L contributes to the maintenance of peripheral nerve injury-induced neuropathic pain-like behaviors. Future study will examine the involvement of microglial CD137L in L5Tx-induced lumbar spinal cord microglial and astrocyte activation.

Electroacupuncture attenuates spinal nerve ligation-induced microglial activation mediated by p38 mitogen-activated protein kinase

To investigate whether analgesic effect of electroacupuncture (EA) is affected by p38 mitogen-activated protein kinase (p38 MAPK) on microglia. There were two experiments. The experiment 1: 40 male Sprague-Dawley (SD) rats were randomly divided into the normal, surgery, EA and sham EA groups, and the L5 spinal nerve ligation (SNL) on the right side was used to establish neuropathic pain model. EA was applied to bilateral Zusanli (ST36) and Kunlun (BL60) at 24, 48 and 72 h after SNL for 30 min, once per day. The paw withdrawal thresholds (PWTs) were measured before surgery (as base) and at 24, 25, 49 and 73 h after surgery. Phospho-p38 MAPK (p-p38 MAPK), oxycocin-42 (OX-42, marker of microglia), and glial fibrillary acidic protein (GFAP, marker of astrocyte) in bilateral spinal cord dorsal horn (SCDH) were detected by immunofluorescence, respectively. The experiment 2: 40 male SD rats were cannulated for SNL-induced neuropathic pain, and then were randomly divided into the dimethyl sulfoxide (DMSO), EA plus DMSO, 4-(4-fluorophenyl)-2-(4-methylsulfonylpheny)-5-(4-pyridyl)-1H-imidazole (SB203580) and EA plus SB203580 groups. SB203580 (30 nmol/L) was administered 5 min prior to EA treatment. The PWTs and OX-42 in bilateral SCDH were measured as mentioned above. SNL-induced neuropathic pain reduced PWTs and increased the expression of p-p38 MAPK and OX-42 in bilateral lumbar SCDH of rats (P<0.01). Spinal p-p38 MAPK was only co-localized with OX-42 in our study. EA treatment significantly alleviated SNL-mediated mechanical hyperalgesia, and suppressed the expression of p-p38 MAPK and OX-42 in lumbar SCDH (P<0.05 or P<0.01). Intrathecal injection of low dose SB203580 had no influence on PWTs (P>0.05), but significantly inhibited the expression of OX-42 positive cells in bilateral SCDH (P<0.01 or P<0.05). EA plus SB203580 synergistically increased PWTs, and reduced the expression of bilateral spinal OX-42 (P<0.01 or P<0.05). The central mechanism of EA-induced anti-hyperalgesia may be partially associated with the reduced expression of p-p38 MAPK, and subsequently reducing the activation of OX-42 in neuropathic pain. Therefore, EA may be a new complementary and alternative therapy for neuropathic pain.

The spinal anti-inflammatory mechanism of motor cortex stimulation: cause of success and refractoriness in neuropathic pain?

BackgroundMotor cortex stimulation (MCS) is an effective treatment in neuropathic pain refractory to pharmacological management. However, analgesia is not satisfactorily obtained in one third of patients. Given the importance of understanding the mechanisms to overcome therapeutic limitations, we addressed the question: what mechanisms can explain both MCS effectiveness and refractoriness? Considering the crucial role of spinal neuroimmune activation in neuropathic pain pathophysiology, we hypothesized that modulation of spinal astrocyte and microglia activity is one of the mechanisms of action of MCS.MethodsRats with peripheral neuropathy (chronic nerve injury model) underwent MCS and were evaluated with a nociceptive test. Following the test, these animals were divided into two groups: MCS-responsive and MCS-refractory. We also evaluated a group of neuropathic rats not stimulated and a group of sham-operated rats. Some assays included rats with peripheral neuropathy that were treated with AM251 (a cannabinoid antagonist/inverse agonist) or saline before MCS. Finally, we performed immunohistochemical analyses of glial cells (microglia and astrocytes), cytokines (TNF-α and IL-1β), cannabinoid type 2 (CB2), μ-opioid (MOR), and purinergic P2X4 receptors in the dorsal horn of the spinal cord (DHSC).FindingsMCS reversed mechanical hyperalgesia, inhibited astrocyte and microglial activity, decreased proinflammatory cytokine staining, enhanced CB2 staining, and downregulated P2X4 receptors in the DHSC ipsilateral to sciatic injury. Spinal MOR staining was also inhibited upon MCS. Pre-treatment with AM251 blocked the effects of MCS, including the inhibitory mechanism on cells. Finally, MCS-refractory animals showed similar CB2, but higher P2X4 and MOR staining intensity in the DHSC in comparison to MCS-responsive rats.ConclusionsThese results indicate that MCS induces analgesia through a spinal anti-neuroinflammatory effect and the activation of the cannabinoid and opioid systems via descending inhibitory pathways. As a possible explanation for MCS refractoriness, we propose that CB2 activation is compromised, leading to cannabinoid resistance and consequently to the perpetuation of neuroinflammation and opioid inefficacy.Electronic supplementary materialThe online version of this article (doi:10.1186/s12974-014-0216-1) contains supplementary material, which is available to authorized users.

Microglia are Involved in Pruritus Induced by DNFB via the CX3CR1/p38 MAPK Pathway

Background/Aims: Pruritus, also known as itch, is a common, unpleasant sensation that can be difficult to treat. Frequently, chronic itch is associated with the development of neuropathic pain resulting from nerve injury or insult. Previous studies have shown the involvement of spinal microglia in the development of neuropathic pain, but their role in chronic pruritus is unclear. Methods: For this study, we constructed a model of chronic pruritus in mice using repeated applications of 2, 4-dinitrofluorobenzene (DNFB) and showed prolonged scratching behavior in treated mice that continued for at least 7 d after the final DNFB treatment. Results: Scratching was accompanied by activation of spinal microglia and both were reduced by an inhibitor of microglial activity. We also showed that microglial activation entailed increased signaling in the p38 MAPK pathway, and treatment with a p38 inhibitor reduced scratching in DNFB-treated mice. We also examined the role of fractalkine/CX3CR1 signaling in the development of DNFB-induced pruritus and showed that intrathecal administration of antiserum against either CX3CR1or FKN inhibited p38 activity and decreased scratching. Conclusion: Our results suggest that microglia are involved in pruritus induced by DNFB via FKN/CX3CR1/p38MAPK pathways similar to those participating in the development of neuropathic pain.