BackgroundPrevious studies have demonstrated that chemokine CXCL12 and its receptor CXCR4 are critical for pain sensitization, but the mechanisms involved are not clear. In this study, we investigated the specific cellular mechanisms of CXCL12/CXCR4 chemokine signaling in the development and maintenance of bone cancer pain after tumor cell implantation (TCI).MethodsTCI in the tibial cavity of rats was used to establish a bone cancer pain model. Mechanical allodynia and thermal hyperalgesia were determined by measuring the paw withdrawal threshold and latency, respectively. The protein expression and cellular localization of CXCL12 and CXCR4 were detected by western blot and immunofluorescence staining. The sensitization of neurons, activation of astrocytes and microglia were examined by observing the immunofluorescence intensity of c-Fos, GFAP and IBA1.ResultsOur results demonstrated that CXCL12 was upregulated in a time-related manner, both in the dorsal root ganglia and spinal cord after TCI. Spinal CXCL12 was predominately expressed in astrocytes, and an intrathecal injection of astrocyte metabolic inhibitor fluorocitrate or selective JNK inhibitor SP600125 abolished TCI-induced CXCL12 production. A single intrathecal injection of a CXCL12 neutralizing antibody (10 μg/10 μl) at day 10 after TCI transiently reversed bone cancer pain in a dose-dependent manner. Whereas repetitive intrathecal administration of a CXCL12 neutralizing antibody (10 μg/10 μl, once a day from day 3 to 5 after TCI) significantly delayed the onset of TCI-induced pain behaviors for nearly five days. Spinal CXCR4 was also upregulated after TCI and colocalized with neurons, astrocytes and microglia. Blocking CXCR4 suppressed TCI-induced activation of neurons, astrocytes and microglia in the spinal cord at day 14. Repeated intrathecal administration of AMD3100 (5 μg/10 μl, once a day for three days) significantly delayed and suppressed the initiation and persistence of bone cancer pain in the early phase (at day 5, 6 and 7 after TCI) and in the late phase (at day 12, 13 and 14 after TCI) of bone cancer, respectively.ConclusionsTaken together, these results demonstrate that CXCL12/CXCR4 signaling contributed to the development and maintenance of bone cancer pain via sensitizing neurons and activating astrocytes and microglia. Additionally, this chemokine signaling may be a potential target for treating bone cancer pain.

Peripheral nerve injury causes neuropathic pain and microglia activation. P2Y12 receptors on microglia are thought to be a key player in the surveillance of the local environment, but whether or how these receptors are engaged in the cross-talk between microglia and neurons of the dorsal horn remain ambiguous. Using a rodent model of nerve injury-induced pain, we investigated the roles of P2Y12 in microglia activation, excitatory synaptic transmission, and nociceptive allodynia. We found that spinal nerve ligation (SNL) significantly increased the level of P2Y12 receptors specifically in the microglia of the ipsilateral dorsal horn. Injections of P2Y12 antagonists (MRS2395 or clopidogrel) attenuated microglia activation and increased the paw withdrawal latency in response to thermal stimuli on the ipsilateral side without affecting the basal threshold on the contralateral side. These effects on pain behaviors were replicated in P2Y12 knockout mice. Patch-clamp recordings further revealed that partial sciatic nerve ligation (PSNL)-induced excessive miniature excitatory postsynaptic currents (mEPSCs) were significantly attenuated in P2Y12 knockout mice. Moreover, we found that SNL activates the GTP-RhoA/ROCK2 signaling pathway and elevates the level of phosphorylated p38 mitogen-activated protein kinase (MAPK), which was inhibited by the P2Y12 antagonist. The phosphorylation of p38 MAPK was inhibited by a ROCK inhibitor, but not vice versa, suggesting that p38 MAPK is downstream of ROCK activation. Our findings suggest that nerve injury engages the P2Y12 receptor-dependent GTP-RhoA/ROCK2 signaling pathway to upregulate excitatory synaptic transmission in the dorsal horn. This cross-talk ultimately participates in the manifestation of nociceptive allodynia, implicating P2Y12 receptor as a potential target for alleviating neuropathic pain.

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Interleukin‐33 (IL‐33) and its receptor ST2 contribute to spinal glial activation and chronic pain. A recent study showed that peripheral IL‐33 plays a pivotal role in the pathogenesis of chronic itch induced by poison ivy. However, how IL‐33/ST2 signaling in the spinal cord potentially mediates chronic itch remains elusive. Here, we determined that St2−/− substantially reduced scratching behaviors in 2,4‐dinitrofluorobenzene (DNFB)‐induced allergic contact dermatitis (ACD) as well as acetone and diethylether followed by water‐induced dry skin in mice. Intrathecal administration of the neutralizing anti‐ST2 or anti‐IL‐33 antibody remarkably decreased the scratching response in DNFB‐induced ACD mice. Expression of spinal IL‐33 and ST2 significantly increased in ACD mice, as evidenced by increased mRNA and protein levels. Immunofluorescence and in situ hybridization demonstrated that increased expression of spinal IL‐33 was predominant in oligodendrocytes and astrocytes, whereas ST2 was mainly expressed in astrocytes. Further studies showed that in ACD mice, the activation of astrocytes and increased phosphorylation of signal transducer and activator of transcription 3 (STAT3) were markedly attenuated by St2−/−. Intrathecal injection of Janus Kinase 2 Inhibitor AG490 significantly alleviated scratching behaviors in ACD mice. rIL‐33 pretreatment exacerbated gastrin‐releasing peptide (GRP)‐evoked scratching behaviors. This increased gastrin‐releasing peptide receptor (GRPR) expression was abolished by St2−/−. Tnf‐α upregulation was suppressed by St2−/−. Our results indicate that the spinal IL‐33/ST2 signaling pathway contributes to chronic itch via astrocytic JAK2‐STAT3 cascade activation, promoting TNF‐α release to regulate the GRP/GRPR signaling‐related itch response. Thus, these findings provide a potential therapeutic option for treating chronic pruritus.

The activation of MAPK pathways in spinal cord and subsequent production of proinflammatory cytokines in glial cells contribute to the development of spinal central sensitization, the basic mechanism underlying bone cancer pain (BCP). Our previous study showed that spinal CXCL12 from astrocytes mediates BCP generation by binding to CXCR4 in both astrocyters and microglia. Here, we verified that CXCL12/CXCR4 signaling contributed to BCP through a MAPK-mediated mechanism. In na€ ıve rats, a single intrathecal administration of CXCL12 considerably induced pain hyperalgesia and phosphorylation expression of spinal MAPK members (including extracellular signal-regulated kinase, p38, and c-Jun N-terminal kinase), which could be partially prevented by pre-treatment with CXCR4 inhibitor AMD3100. This CXCL12-induced hyperalgesia was also reduced by MAPK inhibitors. In bone cancer rats, tumor cell inoculation into the tibial cavity caused prominent and persistent pain hyperalgesia, and associated with up-regulation of CXCL12 and CXCR4, activation of glial cells, phosphorylation of MAPKs, and production of proinflammatory cytokines in the spinal cord. These tumor cell inoculation-induced behavioral and neurochemical alterations were all suppressed by blocking CXCL12/CXCR4 signaling or MAPK pathways. Taken together, these results demonstrate that spinal MAPK pathways mediated CXCL12/CXCR4-induced pain hypersensitivity in bone cancer rats, which could be druggable targets for alleviating BCP and glia-derived neuroinflammation. Keywords: astrocytes, bone cancer pain, chemokine, mitogenactivated protein kinase, microglia, neuroinflammation. Bone cancer pain (BCP) caused by primary tumors or bone metastases is the most common source of moderate and severe cancer pain, which not only makes patients less confident to receive therapeutic plan but also discourages tumor-burdened people from desiring to live (Knopp et al. 2011). With effective treatment strategies depending on a better understanding of BCP, intensive studies of underlying pathogenic mechanisms of BCP and development of novel analgesic targets are highly anticipated in the basic and clinical research communities.Following local inoculation of primary or metastatic bone tumor in the bone microenvironment, cellular and neurochemical alterations take place abundantly in the spinal cord, triggering hyperalgesic behaviors (Medhurst et al. 2002). In this complex pathophysiologic process, glial cells (especially astrocytes and microglia) react and release various proinflammatory cytokines, including tumor necrosis factor a (TNF-a), interleukin 1b (IL-1b) and IL-6, which enhance central sensitization and thus evoking pain hypersensitivity (Falk and Dickenson 2014). Indeed, inhibiting functional activation of astrocytes and microglia at the spinal level is sufficient to suppress BCP and these glia-derived mediators Moreover, numerous studies have shown that all three MAPK members are prone to be phosphorylated following the activation of certain GPCRs, especially che...