Chemokine signaling involving chemokine (C-C motif) ligand 2 plays a role in descending pain facilitation

Targeting the chemokine ligand 2-chemokine receptor 2 axis provides the possibility of immunotherapy in chronic pain

Chronic pain affects patients' physical and psychological health and quality of life, entailing a tremendous public health challenge. Currently, drugs for chronic pain are usually associated with a large number of side effects and poor efficacy. Chemokines in the neuroimmune interface combine with their receptors to regulate inflammation or mediate neuroinflammation in the peripheral and central nervous system. Targeting chemokines and their receptor-mediated neuroinflammation is an effective means to treat chronic pain. In recent years, growing evidence has shown that the expression of chemokine ligand 2 (CCL2) and its main chemokine receptor 2 (CCR2) is involved in its occurrence, development and maintenance of chronic pain. This paper summarises the relationship between the chemokine system, CCL2/CCR2 axis, and chronic pain, and the CCL2/CCR2 axis changes under different chronic pain conditions. Targeting chemokine CCL2 and its chemokine receptor CCR2 through siRNA, blocking antibodies, or small molecule antagonists may provide new therapeutic possibilities for managing chronic pain.

Effects of prenatal hypoxia-ischemia on male rat periaqueductal gray matter: Hyperalgesia, astrogliosis and nitrergic system impairment

Prenatal hypoxic-ischemic insult (HI) may lead to a variety of neurological consequences that may persist throughout adulthood. In the most severe cases, HI is known to increase pain sensitivity which profoundly impacts quality of life. Periaqueductal gray matter (PAG) is a relevant region of the descending pain pathway and its function may be modulated by a complex network that includes nitrergic neurons and glial response, among other factors. Astrocytes, central players in pain modulation, are known to respond to HI by inducing hyperplasia, hypertrophy and increasing the number of their processes and the staining of glial fibrillary acidic protein (GFAP). In this work we investigated the effects of prenatal HI on touch and pain sensitivity, besides the distribution of the neuronal isoform of Nitric Oxide Synthase (nNOS) and GFAP in the PAG of young and adult male rats. At 18 days of gestation, rats had their uterine arteries clamped for 45 min (HI group). SHAM-operated animals were also generated (SHAM group). At post-natal day 30 (P30) or 90 (P90), the offspring was submitted to the behavioral tests of Von Frey and formalin or histological processing to perform immunohistochemistry for nNOS and GFAP. Although there was no significant difference between the groups concerning touch sensitivity, we observed an increase in pain sensitivity in HI P30 and HI P90. The number of nNOS + cells was reduced in HI adult animals in dlPAG and vlPAG. GFAP immunostaining was increased in HI P90 in dlPAG and dmPAG. Our results demonstrated for the first time an increase in pain sensitivity as a consequence of prenatal HI in an animal model. It reinforces the relevance of this model to mimic the effects of prenatal HI, as hyperalgesia.

The cellular mechanism by which the rostral ventromedial medulla acts on the spinal cord during chronic pain

Clinical therapies for chronic pain are limited. While targeted drugs are promising therapies for chronic pain, they exhibit insufficient efficacy and poor targeting. The occurrence of chronic pain partly results from central changes caused by alterations in neurons in the rostral ventromedial medulla (RVM) in the brainstem regulatory pathway. The RVM, which plays a key role in the descending pain control pathway, greatly contributes to the development and maintenance of pain. However, the exact roles of the RVM in chronic pain remain unclear, making it difficult to develop new drugs targeting the RVM and related pathways. Here, we first discuss the roles of the RVM and related circuits in chronic pain. Then, we analyze synaptic transmission between RVM neurons and spinal cord neurons, specifically focusing on the release of neurotransmitters, to explore the cellular mechanisms by which the RVM regulates chronic pain. Finally, we propose some ideas for the development of drugs targeting the RVM.

Low-viscosity sodium alginate combined with TiO2 nanoparticles for improving neuroblastoma treatment

Titanium dioxide (TiO₂) nanoparticles have been explored to prevent various cancer developments but it may cause oxidation, inflammation and high cytotoxicity. Alginate has nontoxic, anti-inflammatory, and antioxidant effects. We aimed to explore the effects of alginate-TiO₂ temozolomide (TMZ) nanoparticles on neuroblastoma. A neuroblastoma model was established with neuroblastoma cells and alginate-TiO₂ TMZ nanoparticles were made by spraying low-viscosity sodium alginate (250-360 kDa). The morphology of nanoparticles was observed via scanning electron microscope (SEM). The crystallinity values were analyzed via X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopic study. Neuroblastoma mice were treated with saline solution, TMZ, TiO₂-TMZ and alginate-TiO₂-TMZ nanoparticles. Anti-oxidant, anti-inflammatory, and anti-tumor properties and the mouse survival rates were measured. The spectrometric profiles of alginate-TiO₂ were consistent with those of TiO₂ and alginate. Alginate-TiO₂ TMZ nanoparticles had higher cytotoxicity toward neuroblastoma cells and less inhibitory activity toward normal neuronal cells. The combined nanoparticles increased antioxidant, anti-inflammatory and antitumor activities and prolonged the survival time of the neuroblastoma model (P < 0.05). On the other hand, Alginate-TiO₂ TMZ nanoparticles reduced the levels of mitogen-activated protein kinases (MAPKs) and nuclear factor-κB (NF-κB). The combined nanoparticles improved neuroblastoma treatment by affecting NF-κB and MAPK signals.

The P2Y14 receptor in the trigeminal ganglion contributes to the maintenance of inflammatory pain

P2Y purinergic receptors expressed in neurons and satellite glial cells (SGCs) of the trigeminal ganglion (TG) contribute to inflammatory and neuropathic pain. P2Y₁₄ receptor expression is reported in the spinal cord, dorsal root ganglion (DRG), and TG. In present study, the role of P2Y₁₄ receptor in the TG in inflammatory orofacial pain of Sprague-Dawley (SD) rats was investigated. Peripheral injection of complete Freund's adjuvant (CFA) induced mechanical hyperalgesia with the rapid upregulation of P2Y₁₄ receptor, glial fibrillary acidic protein (GFAP), interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), C-C chemokine CCL2, phosphorylated extracellular signal-regulated kinase 1/2 (p-ERK1/2), and phosphorylated p38 (p-p38) proteins in the TG. Furthermore, immunofluorescence staining confirmed the CFA-induced upregulation of P2Y₁₄ receptor. Double immunostaining showed that P2Y₁₄ receptor colocalized with glutamine synthetase (GS) and neuronal nuclei (NeuN). Finally, trigeminal injection of a selective antagonist (PPTN) of P2Y₁₄ receptor attenuated CFA-induced mechanical hyperalgesia. PPTN also decreased the upregulation of the GFAP, IL-1β, TNF-α, CCL2, p-ERK1/2, and p-p38 proteins. Our findings showed that P2Y₁₄ receptor in TG may contribute to orofacial inflammatory pain via regulating SGCs activation, releasing cytokines (IL-1β, TNF-α, and CCL2), and phosphorylating ERK1/2 and p38.

Crosstalk between NFκB-dependent astrocytic CXCL1 and neuron CXCR2 plays a role in descending pain facilitation

BACKGROUND

Despite accumulating evidence on the role of glial cells and their associated chemicals in mechanisms of pain, few studies have addressed the potential role of chemokines in the descending facilitation of chronic pain. We aimed to study the hypothesis that CXCL1/CXCR2 axis in the periaqueductal gray (PAG), a co-restructure of the descending nociceptive system, is involved in descending pain facilitation.

METHODS

Intramedullary injection of Walker 256 mammary gland carcinoma cells of adult female Sprague Dawley rats was used to establish a bone cancer pain (BCP) model. RT-PCR, Western blot, and immunohistochemistry were performed to detect pNfkb, Cxcl1, and Cxcr2 and their protein expression in the ventrolateral PAG (vlPAG). Immunohistochemical co-staining with NeuN, GFAP, and CD11 were used to examine the cellular location of pNFκB, CXCL1, and CXCR2. The effects of NFκB and CXCR2 antagonists and CXCL1 neutralizing antibody on pain hypersensitivity were evaluated by behavioral testing.

RESULTS

BCP induced cortical bone damage and persistent mechanical allodynia and increased the expression of pNFκB, CXCL1, and CXCR2 in vlPAG. The induced phosphorylation of NFκB was co-localized with GFAP and NeuN, but not with CD11. Micro-injection of BAY11-7082 attenuated BCP and reduced CXCL1 increase in the spinal cord. The expression level of CXCL1 in vlPAG showed co-localization with GFAP, but not with CD11 and NeuN. Micro-administration of CXCL1 neutralizing antibody from 6 to 9 days after inoculation attenuated mechanical allodynia. Furthermore, vlPAG application of CXCL1 elicited pain hypersensitivity in normal rats. Interestingly, CXCR2 was upregulated in vlPAG neurons (not with CD11 and GFAP) after BCP. CXCR2 antagonist SB225002 completely blocked the CXCL1-induced mechanical allodynia and attenuated BCP-induced pain hypersensitivity.

CONCLUSION

The NFκB-dependent CXCL1-CXCR2 signaling cascade played a role in glial-neuron interactions and in descending facilitation of BCP.

CC chemokine ligand 2 (CCL2) enhances TTX-sensitive sodium channel activity of primary afferent neurons in the complete Freud adjuvant-induced inflammatory pain model

CC chemokine ligand 2 (CCL2) has been implicated in pathological pain, but the mechanism underlying the pronociceptive effect of CCL2 is not fully understood. Voltage-gated sodium (Nav) channels are important determinants of the excitability of sensory neurons. Hence we tested the hypothesis that CCL2 contributes to inflammatory pain via modulating Nav channel activity of primary afferent neurons. Chronic inflammatory pain was induced in rats by intraplantar injection of the complete Freud adjuvant (CFA) to one of the hind paws. Control rats received intraplantar injection of equal volume of saline. A significant increase of CCL2 mRNA and CCL2 receptor (CCR2) protein expression was detected in the ipsilateral dorsal root ganglion (DRG) in CFA-treated rats. Intraplantar injection of CCL2 protein in the control rats had minimal effect on the paw withdrawal threshold (PWT) in response to mechanical stimulation. However, in CFA-treated rats, intraplantar CCL2 led to an increase in pain responses. Patch-clamp recording of acutely dissociated DRG neurons revealed that CCL2 had minimum effect on the excitability of sensory neurons from control rats. However, CCL2 directly depolarized a large proportion of small to medium-sized sensory neurons from CFA-treated rats. In addition, CCL2 was found to enhance whole-cell TTX-sensitive sodium currents without significantly affecting the TTX-resistant sodium currents and the potassium currents. These results are in agreement with previous reports concerning the involvement of CCL2-CCR2 signaling in inflammatory hyperalgesia and further indicate that enhanced TTX-sensitive channel activity may partly underlie the pronociceptive effects of CCL2.

Activation of Astrocytes and Microglial Cells and CCL2/CCR2 Upregulation in the Dorsolateral and Ventrolateral Nuclei of Periaqueductal Gray and Rostral Ventromedial Medulla Following Different Types of Sciatic Nerve Injury

Peripheral nerve injuries (PNIs) may result in cellular and molecular changes in supraspinal structures possibly involved in neuropathic pain (NPP) maintenance. Activated glial cells in specific supraspinal subregions may affect the facilitatory role of descending pathways. Sterile chronic compression injury (sCCI) and complete sciatic nerve transection (CSNT) in rats were used as NPP models to study the activation of glial cells in the subregions of periaqueductal gray (PAG) and rostral ventromedial medulla (RVM). Molecular markers for activated astrocytes (glial fibrillary acidic protein, GFAP) and microglial cells (OX42) were assessed by quantitative immunohistochemistry and western blotting. The cellular distribution of CCL2/CCR2 was monitored using immunofluorescence. sCCI induced both mechanical and thermal hypersensitivity from day 1 up to 3 weeks post-injury. Unilateral sCCI or CSNT for 3 weeks induced significant activation of astrocytes bilaterally in both dorsolateral (dlPAG) and ventrolateral PAG (vlPAG) compared to naïve or sham-operated rats. More extensive astrocyte activation by CSNT compared to sCCI was induced bilaterally in dlPAG and ipsilaterally in vlPAG. Significantly more extensive activation of astrocytes was also found in RVM after CSNT than sCCI. The CD11b immunopositive region, indicating activated microglial cells, was remarkably larger in dlPAG and vlPAG of both sides from sCCI- and CSNT-operated rats compared to naïve or sham-operated controls. No significant differences in microglial activation were detected in dlPAG or vlPAG after CSNT compared to sCCI. Both nerve injury models induced no significant differences in microglial activation in the RVM. Neurons and activated GFAP+ astrocytes displayed CCL2-immunoreaction, while activated OX42+ microglial cells were CCR2-immunopositive in both PAG and RVM after sCCI and CSNT. Overall, while CSNT induced robust astrogliosis in both PAG and RVM, microglial cell activation was similar in the supraspinal structures in both injury nerve models. Activated astrocytes in PAG and RVM may sustain facilitation of the descending system maintaining NPP, while microglial activation may be associated with a reaction to long-lasting peripheral injury. Microglial activation via CCR2 may be due to neuronal and astrocytal release of CCL2 in PAG and RVM following injury.

Platelet-Rich Plasma Injection in Burn Scar Areas Alleviates Neuropathic Scar Pain

Objective: No effective treatments have yet been developed for burn-induced neuropathic pain. Platelet-rich plasma (PRP) has been reported to ameliorate various types of inflammation pain. However, the effect of PRP on burn-induced neuropathic pain is unclear. Methods: Burn-induced neuropathic pain Sprague-Dawley rat model was confirmed using a mechanical response test 4 weeks after the burn injuries were sustained, following which PRP was injected in the scar area. The rats were divided into four groups (n = 6) as following: Group A, Sham; Group B, Sham + PRP; Group C, Burn; and Group D, Burn + PRP. Four weeks after the PRP injection, the animals were subjected to behavior tests and then sacrificed; specimens were collected for inflammation tests, Masson's trichrome stain and chromosome 10 (PTEN) in the injured skin; and PTEN, phosphorylated mammalian target of rapamycin (p-mTOR), p38, nuclear factor κB (NFκB), chemokine (CC motif) ligand 2 (CCL2), and CCL2 cognate receptor (CCR2) in spinal cord dorsal horns through immunohistochemistry and immunofluorescence staining. Results: PRP significantly alleviated allodynia in burn-induced neuropathic pain 4 weeks after treatment, and PTEN expression in the skin and spinal cord were significantly increased in group D compared with the group C. p-PTEN, p-mTOR, and CCL2 expression in neuron cells; p-p38 and p-NFκB expression in microglia; and p-JNK and p-NFκB activation in spinal astrocytes decreased significantly in the group D compared with the group C. Conclusions: PRP is effective in treating burn-induced neuropathic pain and may be used in clinical practice.

Spinal CCL2 Promotes Central Sensitization, Long-Term Potentiation, and Inflammatory Pain via CCR2: Further Insights into Molecular, Synaptic, and Cellular Mechanisms

Mounting evidence supports an important role of chemokines, produced by spinal cord astrocytes, in promoting central sensitization and chronic pain. In particular, CCL2 (C-C motif chemokine ligand 2) has been shown to enhance N-methyl-D-aspartate (NMDA)-induced currents in spinal outer lamina II (IIo) neurons. However, the exact molecular, synaptic, and cellular mechanisms by which CCL2 modulates central sensitization are still unclear. We found that spinal injection of the CCR2 antagonist RS504393 attenuated CCL2- and inflammation-induced hyperalgesia. Single-cell RT-PCR revealed CCR2 expression in excitatory vesicular glutamate transporter subtype 2-positive (VGLUT2⁺) neurons. CCL2 increased NMDA-induced currents in CCR2⁺/VGLUT2⁺ neurons in lamina IIo; it also enhanced the synaptic NMDA currents evoked by dorsal root stimulation; and furthermore, it increased the total and synaptic NMDA currents in somatostatin-expressing excitatory neurons. Finally, intrathecal RS504393 reversed the long-term potentiation evoked in the spinal cord by C-fiber stimulation. Our findings suggest that CCL2 directly modulates synaptic plasticity in CCR2-expressing excitatory neurons in spinal lamina IIo, and this underlies the generation of central sensitization in pathological pain.

Are the emergence of affective disturbances in neuropathic pain states contingent on supraspinal neuroinflammation?

Neuro-immune interactions contribute to the pathogenesis of neuropathic pain due to peripheral nerve injury. A large body of preclinical evidence supports the idea that the immune system acts to modulate the sensory symptoms of neuropathy at both peripheral and central nervous system sites. The potential involvement of neuro-immune interactions in the highly debilitating affective disturbances of neuropathic pain, such as depression, anhedonia, impaired cognition and reduced motivation has received little attention. This is surprising given the widely accepted view that sickness behaviour, depression, cognitive impairment and other neuropsychiatric conditions can arise from inflammatory mechanisms. Moreover, there is a set of well-described immune-to-brain transmission mechanisms that explain how peripheral inflammation can lead to supraspinal neuroinflammation. In the last 5years increasing evidence has emerged that peripheral nerve injury induces supraspinal changes in cytokine or chemokine expression and alters glial cell activity. In this systematic review, based on strong preclinical evidence, we advance the argument that the emergence of affective disturbances in neuropathic pain states are contingent on pro-inflammatory mediators in the interconnected hippocampal-medial prefrontal circuitry that subserve affective behaviours. We explore how dysregulation of inflammatory mediators in these networks may result in affective disturbances through a wide variety of neuromodulatory mechanisms. There are also promising results from clinical trials showing that anti-inflammatory agents have efficacy in the treatment of a variety of neuropsychiatric conditions including depression and appear suited to sub-groups of patients with elevated pro-inflammatory profiles. Thus, although further research is required, aggressively targeting supraspinal pro-inflammatory mediators at critical time-points in appropriate clinical populations is likely to be a novel avenue to treat debilitating affective disturbances in neuropathic conditions.

ATP-sensitive potassium channels alleviate postoperative pain through JNK-dependent MCP-1 expression in spinal cord

Although adenosine triphosphate-sensitive potassium (KATP) channels have been proven to be involved in regulating postoperative pain, the underlying mechanism remains to be investigated. In this study, we aimed to determine the role of spinal KATP channels in the control of mechanical hypersensitivity in a rat pain model, in which rats were subjected to skin/muscle incision and retraction (SMIR) surgery, as well as in LPS-stimulated astrocytes. The results showed that KATP channel subunits Kir6.1, SUR1 and SUR2 were normally expressed in the spinal cord and significantly downregulated after SMIR. SMIR caused a marked increase in monocyte chemoattractant protein-1 (MCP-1) mRNA expression and in the protein level of p-JNK in the spinal cord. Intrathecal administration of a KATP channel opener pinacidil (Pina) suppressed mechanical allodynia after SMIR and significantly downregulated the MCP-1 mRNA expression and the protein level of p-JNK induced by SMIR. Inverted fluorescence microscopy showed that Kir6.1 was co-localized with astrocytes only and SUR2 was co-localized primarily with neurons, in a small amount with astrocytes. Furthermore, in vitro studies showed that following incubation with LPS, the astrocytic MCP-1 mRNA expression and p-JNK content were markedly increased, whereas the mRNA levels of Kir6.1 and SUR2 were significantly downregulated in astrocytes. KATP channel opener pinacidil inhibited the LPS-triggered MCP-1 and p-JNK elevation in rat primary astrocytes. The results suggested that KATP channel opener treatment is an effective therapy for postoperative pain in animals, through the activation of the JNK/MCP-1 pathway in astrocytes.

Emerging targets in neuroinflammation-driven chronic pain

Current analgesics predominately modulate pain transduction and transmission in neurons and have limited success in controlling disease progression. Accumulating evidence suggests that neuroinflammation, which is characterized by infiltration of immune cells, activation of glial cells and production of inflammatory mediators in the peripheral and central nervous system, has an important role in the induction and maintenance of chronic pain. This Review focuses on emerging targets - such as chemokines, proteases and the WNT pathway - that promote spinal cord neuroinflammation and chronic pain. It also highlights the anti-inflammatory and pro-resolution lipid mediators that act on immune cells, glial cells and neurons to resolve neuroinflammation, synaptic plasticity and pain. Targeting excessive neuroinflammation could offer new therapeutic opportunities for chronic pain and related neurological and psychiatric disorders.

Chemokine CXCL1 enhances inflammatory pain and increases NMDA receptor activity and COX-2 expression in spinal cord neurons via activation of CXCR2

Recent studies have shown that CXCL1 upregulation in spinal astrocytes is involved in the maintenance of neuropathic pain. However, whether and how CXCL1 regulates inflammatory pain remains unknown. Here we show that intraplantar injection of CFA increased mRNA and protein expressions of CXCL1 and its major receptor CXCR2 in the spinal cord at 6h and 3days after the injection. Immunofluorescence double staining showed that CXCL1 and CXCR2 were expressed in spinal astrocytes and neurons, respectively. Intrathecal injection of CXCL1 neutralizing antibody or CXCR2 antagonist SB225002 attenuated CFA-induced mechanical and heat hypersensitivity on post-CFA day 3. Patch-clamp recordings showed that CXCL1 potentiated NMDA-induced currents in lamina II neurons via CXCR2, and this potentiation was further increased in CFA-treated mice. Furthermore, intrathecal injection of CXCL1 increased COX-2 expression in dorsal horn neurons, which was blocked by pretreatment with SB225002 or MEK (ERK kinase) inhibitor PD98059. Finally, pretreatment with SB225002 or PD98059 decreased CFA-induced heat hyperalgesia and COX-2 mRNA/protein expression and ERK activation in the spinal cord. Taken together, our data suggest that CXCL1, upregulated and released by spinal astrocytes after inflammation, acts on CXCR2-expressing spinal neurons to increase ERK activation, synaptic transmission and COX-2 expression in dorsal horn neurons and contributes to the pathogenesis of inflammatory pain.

Up-regulation of interleukin-23 induces persistent allodynia via CX3CL1 and interleukin-18 signaling in the rat spinal cord after tetanic sciatic stimulation

Tetanic stimulation of the sciatic nerve (TSS) induces sciatic nerve injury and long-lasting pain hypersensitivity in rats, and spinal glial activation and proinflammatory cytokines releases are involved. In the present study, we showed that spinal interleukin (IL)-23 and its receptor, IL-23R, are crucial for the development of mechanical allodynia after TSS. In the spinal dorsal horn, both IL-23 and IL-23R are expressed in astrocytes, and this expression is substantially increased after TSS. Inhibition of IL-23 signaling attenuated TSS-induced allodynia and decreased the induction of glial fibrillary acidic protein (GFAP, an astrocytic marker). Conversely, intrathecally delivered IL-23 induced a persistent allodynia. Similar to IL-23 signaling, an increase in IL-18 and its receptor, IL-18R, as well as CX3CL1 and its receptor, CX3CR1, was simultaneously observed in the spinal dorsal horn after TSS. Interestingly, IL-18 and CX3CR1 were exclusively expressed in microglia, while IL-18R was mainly localized in astrocytes. In contrast, CX3CL1 was predominately expressed in neurons and secondarily in astrocytes. The functional inhibition of CX3CL1 and IL-18 signaling attenuated TSS-induced allodynia and suppressed IL-23 and IL-23R upregulation. Activation of CX3CR1 and IL-18R induced similar behavioral and biochemical changes to those observed after TSS. These results indicate that the interaction among CX3CL1, IL-18 and IL-23 signaling in the spinal cord plays a critical role in the development of allodynia. Thus, interrupting this chemokine-cytokine network might provide a novel therapeutic strategy for neuropathic pain.

Monocyte chemoattractant protein-1 and the blood-brain barrier

The blood-brain barrier (BBB) is a dynamic structure that maintains the homeostasis of the brain and thus proper neurological functions. BBB compromise has been found in many pathological conditions, including neuroinflammation. Monocyte chemoattractant protein-1 (MCP1), a chemokine that is transiently and significantly up-regulated during inflammation, is able to disrupt the integrity of BBB and modulate the progression of various diseases, including excitotoxic injury and hemorrhage. In this review, we first introduce the biochemistry and biology of MCP1, and then summarize the effects of MCP1 on BBB integrity as well as individual BBB components.

Glia and pain: is chronic pain a gliopathy?

Activation of glial cells and neuro-glial interactions are emerging as key mechanisms underlying chronic pain. Accumulating evidence has implicated 3 types of glial cells in the development and maintenance of chronic pain: microglia and astrocytes of the central nervous system (CNS), and satellite glial cells of the dorsal root and trigeminal ganglia. Painful syndromes are associated with different glial activation states: (1) glial reaction (ie, upregulation of glial markers such as IBA1 and glial fibrillary acidic protein (GFAP) and/or morphological changes, including hypertrophy, proliferation, and modifications of glial networks); (2) phosphorylation of mitogen-activated protein kinase signaling pathways; (3) upregulation of adenosine triphosphate and chemokine receptors and hemichannels and downregulation of glutamate transporters; and (4) synthesis and release of glial mediators (eg, cytokines, chemokines, growth factors, and proteases) to the extracellular space. Although widely detected in chronic pain resulting from nerve trauma, inflammation, cancer, and chemotherapy in rodents, and more recently, human immunodeficiency virus-associated neuropathy in human beings, glial reaction (activation state 1) is not thought to mediate pain sensitivity directly. Instead, activation states 2 to 4 have been demonstrated to enhance pain sensitivity via a number of synergistic neuro-glial interactions. Glial mediators have been shown to powerfully modulate excitatory and inhibitory synaptic transmission at presynaptic, postsynaptic, and extrasynaptic sites. Glial activation also occurs in acute pain conditions, and acute opioid treatment activates peripheral glia to mask opioid analgesia. Thus, chronic pain could be a result of "gliopathy," that is, dysregulation of glial functions in the central and peripheral nervous system. In this review, we provide an update on recent advances and discuss remaining questions.

Loss of neurons in rostral ventromedial medulla that express neurokinin-1 receptors decreases the development of hyperalgesia

It is well known that neurons in the rostral ventromedial medulla (RVM) are involved in descending modulation of nociceptive transmission in the spinal cord. It has been shown that activation of neurokinin-1 receptors (NK-1Rs) in the RVM, which are presumably located on pain facilitating ON cells, produces hyperalgesia whereas blockade of NK-1Rs attenuates hyperalgesia. To obtain a better understanding of the functions of NK-1R expressing neurons in the RVM, we selectively ablated these neurons by injecting the stable analog of substance P (SP), Sar(9),Met(O2)(11)-Substance P, conjugated to the ribosomal toxin saporin (SSP-SAP) into the RVM. Rats received injections of SSP-SAP (1 μM) or an equal volume of 1 μM of saporin conjugated to artificial peptide (Blank-SAP). Stereological analysis of NK-1R- and NeuN-labeled neurons in the RVM was determined 21-24 days after treatment. Withdrawal responses to mechanical and heat stimuli applied to the plantar hindpaw were determined 5-28 days after treatment. Withdrawal responses were also determined before and after intraplantar injection of capsaicin (acute hyperalgesia) or complete Freund's adjuvant (CFA) (prolonged hyperalgesia). The proportion of NK-1R-labeled neurons in the RVM was 8.8 ± 1.3% in naïve rats and 8.1 ± 0.8% in rats treated with Blank-SAP. However, injection of SSP-SAP into the RVM resulted in a 90% decrease in NK-1R-labeled neurons. SSP-SAP did not alter withdrawal responses to mechanical or heat stimuli under normal conditions, and did not alter analgesia produced by morphine administered into the RVM. In contrast, the duration of nocifensive behaviors produced by capsaicin and mechanical and heat hyperalgesia produced by capsaicin and CFA were decreased in rats pretreated with SSP-SAP as compared to those that received Blank-SAP. These data support our earlier studies using NK-1R antagonists in the RVM and demonstrate that RVM neurons that possess the NK-1R do not play a significant role in modulating acute pain or morphine analgesia, but rather are involved in pain facilitation and the development and maintenance of hyperalgesia.

Induction of monocyte chemoattractant protein-1 (MCP-1) and its receptor CCR2 in primary sensory neurons contributes to paclitaxel-induced peripheral neuropathy

The use of paclitaxel (Taxol), a microtubule stabilizer, for cancer treatment is often limited by its associated peripheral neuropathy (chemotherapy-induced peripheral neuropathy [CIPN]), which predominantly results in sensory dysfunction, including chronic pain. Here we show that paclitaxel CIPN was associated with induction of chemokine monocyte chemoattractant protein-1 (MCP-1) and its cognate receptor CCR2 in primary sensory neurons of dorsal root ganglia. Immunostaining revealed that MCP-1 was mainly expressed in small nociceptive neurons whereas CCR2 was expressed in large and medium-sized myelinated neurons. Direct application of MCP-1 consistently induced intracellular calcium increases in dorsal root ganglia large and medium-sized neurons but not in small neurons mainly dissociated from paclitaxel-treated but not vehicle-treated animals. Paclitaxel also induced increased expression of MCP-1 in spinal astrocytes, but no CCR2 signal was detected in the spinal cord. Local blockade of MCP-1/CCR2 signaling by anti-MCP-1 antibody or CCR2 antisense oligodeoxynucleotides significantly attenuated paclitaxel CIPN phenotypes including mechanical hypersensitivity and loss of intraepidermal nerve fibers in hindpaw glabrous skin. These results suggest that activation of paracrine MCP-1/CCR2 signaling between dorsal root ganglion neurons plays a critical role in the development of paclitaxel CIPN, and targeting MCP-1/CCR2 signaling could be a novel therapeutic approach.

PERSPECTIVE

CIPN is a severe side effect accompanying paclitaxel chemotherapy and lacks effective treatments. The current study suggests that blocking MCP-1/CCR2 signaling could be a new therapeutic strategy to prevent or reverse paclitaxel CIPN. This preclinical evidence encourages future clinical evaluation of this strategy.