Glial-cytokine-neuronal interactions underlying the mechanisms of persistent pain

Forced treadmill running suppresses postincisional pain and inhibits upregulation of substance P and cytokines in rat dorsal root ganglion

Exercise causes a variety of psychophysical effects (eg, alterations in pain sensation). Tissue injury induces mediator releases in the spinal cord resulting in pain hypersensitivity; however, the contribution of the dorsal root ganglion (DRG) is poorly understood. In this study, we tested if forced treadmill running can attenuate postoperative pain and alter substance P (SP) or proinflammatory cytokine level in the DRG by using a rat model of skin/muscle incision and retraction (SMIR). We evaluated mechanical sensitivity to von Frey stimuli (6 and 15 g) and expression of SP, interleukin-1β, and interleukin-6 in the DRG of sham-operated sedentary rats, SMIR sedentary rats, sham-operated rats with forced treadmill running, and SMIR rats with forced treadmill running. At postoperative day 8, trained rats ran for 5 days per week for 4 weeks on a treadmill 70 minutes/d with an intensity of 18 m/min. On postoperative day 6, SMIR sedentary rats displayed a significant mechanical hypersensitivity that persisted until postoperative day 35. By comparison, SMIR-operated rats, which received forced treadmill running, exhibited a quick recovery from mechanical hypersensitivity. SMIR sedentary rats showed an upregulation of SP, interleukin-1β, and interleukin-6 in the DRG at postoperative days 14 and 28, whereas SMIR-operated rats receiving forced treadmill running reversed this upregulation at postoperative day 28. We concluded that forced treadmill running alleviated persistent postincisional pain caused by SMIR surgery. This appears to be protective against postoperative pain, which probably relates to the downturn in excess SP, interleukin-1β, and interleukin-6 in the DRG.

PERSPECTIVE

Controlling the expression of SP, interleukin-6, and interleukin-1β in the DRG can help manage postoperative pain. This finding could potentially help clinicians and physical therapists who seek to examine how exercise may attenuate postsurgical pain and its mechanism.

Mast cells, glia and neuroinflammation: partners in crime?

Glia and microglia in particular elaborate pro-inflammatory molecules that play key roles in central nervous system (CNS) disorders from neuropathic pain and epilepsy to neurodegenerative diseases. Microglia respond also to pro-inflammatory signals released from other non-neuronal cells, mainly those of immune origin such as mast cells. The latter are found in most tissues, are CNS resident, and traverse the blood-spinal cord and blood-brain barriers when barrier compromise results from CNS pathology. Growing evidence of mast cell-glia communication opens new perspectives for the development of therapies targeting neuroinflammation by differentially modulating activation of non-neuronal cells that normally control neuronal sensitization - both peripherally and centrally. Mast cells and glia possess endogenous homeostatic mechanisms/molecules that can be up-regulated as a result of tissue damage or stimulation of inflammatory responses. Such molecules include the N-acylethanolamine family. One such member, N-palmitoylethanolamine is proposed to have a key role in maintenance of cellular homeostasis in the face of external stressors provoking, for example, inflammation. N-Palmitoylethanolamine has proven efficacious in mast-cell-mediated experimental models of acute and neurogenic inflammation. This review will provide an overview of recent progress relating to the pathobiology of neuroinflammation, the role of microglia, neuroimmune interactions involving mast cells and the possibility that mast cell-microglia cross-talk contributes to the exacerbation of acute symptoms of chronic neurodegenerative disease and accelerates disease progression, as well as promoting pain transmission pathways. We will conclude by considering the therapeutic potential of treating systemic inflammation or blockade of signalling pathways from the periphery to the brain in such settings.

Aspirin-triggered Lipoxin A4 attenuates mechanical allodynia in association with inhibiting spinal JAK2/STAT3 signaling in neuropathic pain in rats

Aspirin-triggered Lipoxin A4 (ATL), as a Lipoxin A4 (LXA4) epimer, is endogenously produced by aspirin-acetylated cycloxygenase-2 (COX-2) and plays a vital role in endogenous anti-inflammation via the LXA4 receptor (ALX). Recent investigations have indicated that spinal neuroinflammation and the activation of the Janus Kinase 2 (JAK2)/Signal Transducers and Transcription Activators 3 (STAT3) signaling pathway are involved in neuropathic pain states. However, the effect of ATL on neuroinflammation and JAK2/STAT3 signaling in chronic constriction injury (CCI)-induced neuropathic pain in rats has not been well-studied. The present study demonstrated the anti-inflammatory and analgesic effect of ATL on neuropathic pain and assessed the role of spinal JAK2/STAT3 signaling on the effect of ATL. Intrathecal administration of ATL significantly attenuated mechanical allodynia via spinal ALX and inhibited the upregulation of pro-inflammatory cytokines (IL-1β, IL-6, and TNF-α) on day 7 of CCI surgery. In addition, ATL markedly suppressed the upregulation of p-STAT3 induced by the neuropathic pain. Blockade of JAK2-STAT3 signaling with intrathecal administration of the JAK2 inhibitor AG490 or the STAT3 inhibitor S3I-201 clearly reduced mechanical allodynia and the upregulation of pro-inflammatory cytokines in CCI rats. Interestingly, inhibition of JAK2/STAT3 signaling via ATL or the specific signaling inhibitor (AG49, S3I-201) further promoted the increased expression of suppressor of cytokine signaling 3 (SOCS3) mRNA in the spinal cord induced by CCI surgery. Taken together, our results suggested that the analgesic effect of ATL was mediated by inhibiting spinal JAK2/STAT3 signaling and hence the spinal neuroinflammation in CCI rats.

Neuron-glial communication mediated by TNF-α and glial activation in dorsal root ganglia in visceral inflammatory hypersensitivity

Communication between neurons and glia in the dorsal root ganglia (DRG) and the central nervous system is critical for nociception. Both glial activation and proinflammatory cytokine induction underlie this communication. We investigated whether satellite glial cell (SGC) and tumor necrosis factor-α (TNF-α) activation in DRG participates in a 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced rat model of visceral hyperalgesia. In TNBS-treated rats, TNF-α expression increased in DRG and was colocalized to SGCs enveloping a given neuron. These SGCs were activated as visualized under electron microscopy: they had more elongated processes projecting into the connective tissue space and more gap junctions. When nerves attached to DRG (L6-S1) were stimulated with a series of electrical stimulations, TNF-α were released from DRG in TNBS-treated animals compared with controls. Using a current clamp, we noted that exogenous TNF-α (2.5 ng/ml) increased DRG neuron activity, and visceral pain behavioral responses were reversed by intrathecal administration of anti-TNF-α (10 μg·kg(-1)·day(-1)). Based on our findings, TNF-α and SGC activation in neuron-glial communication are critical in inflammatory visceral hyperalgesia.

Suppression of spinal connexin 43 expression attenuates mechanical hypersensitivity in rats after an L5 spinal nerve injury

Activation of spinal astrocytes may contribute to neuropathic pain. Adjacent astrocytes can make direct communication through gap junctions formed by connexin 43 (Cx43) in the central nervous system. Yet, the role of spinal astroglial gap junctions in neuropathic pain is not fully understood. Since Cx43 is the connexin isoform expressed preferentially in astrocytes in the spinal cord, we used a small interfering RNA (siRNA) approach to examine whether suppression of spinal Cx43 expression inhibits mechanical hypersensitivity in rats after an L5 spinal nerve ligation (SNL). SNL rats were administered intrathecal Cx43 siRNA (3μg/15μl, twice/day) or an equal amount of mismatch siRNA (control) on days 14-17 post-SNL. Cx43 siRNA, but not mismatch siRNA, alleviated mechanical hypersensitivity in SNL rats. Furthermore, Western blot analysis showed that the pain inhibition induced by Cx43 siRNA correlated with downregulation of Cx43 expression, but not that of Cx36 (the neuronal gap junction protein) or glial fibrillary acidic protein (GFAP, a marker for reactive astrocytes) in the spinal cord of SNL rats. Western blot analysis and immunohistochemistry also showed that SNL increased GFAP expression, but decreased Cx43 expression, in spinal cord. Our results provide direct evidence that selective suppression of spinal Cx43 after nerve injury alleviates neuropathic mechanical hypersensitivity. These findings suggest that in the spinal cord, the enhanced function of astroglial gap junctions, especially those formed by Cx43, may be important to neuropathic pain in SNL rats.

Dual orexin receptor antagonist 12 inhibits expression of proteins in neurons and glia implicated in peripheral and central sensitization

Sensitization and activation of trigeminal nociceptors is implicated in prevalent and debilitating orofacial pain conditions including temporomandibular joint (TMJ) disorders. Orexins are excitatory neuropeptides that function to regulate many physiological processes and are reported to modulate nociception. To determine the role of orexins in an inflammatory model of trigeminal activation, the effects of a dual orexin receptor antagonist (DORA-12) on levels of proteins that promote peripheral and central sensitization and changes in nocifensive responses were investigated. In adult male Sprague-Dawley rats, mRNA for orexin receptor 1 (OX₁R) and receptor 2 (OX₂R) were detected in trigeminal ganglia and spinal trigeminal nucleus (STN). OX₁R immunoreactivity was localized primarily in neuronal cell bodies in the V3 region of the ganglion and in laminas I-II of the STN. Animals injected bilaterally with complete Freund's adjuvant (CFA) in the TMJ capsule exhibited increased expression of P-p38, P-ERK, and lba1 in trigeminal ganglia and P-ERK and lba1 in the STN at 2 days post injection. However, levels of each of these proteins in rats receiving daily oral DORA-12 were inhibited to near basal levels. Similarly, administration of DORA-12 on days 3 and 4 post CFA injection in the TMJ effectively inhibited the prolonged stimulated expression of protein kinase A, NFkB, and Iba1 in the STN on day 5 post injection. While injection of CFA mediated a nocifensive response to mechanical stimulation of the orofacial region at 2h and 3 and 5 days post injection, treatment with DORA-12 suppressed the nocifensive response on day 5. Somewhat surprisingly, nocifensive responses were again observed on day 10 post CFA stimulation in the absence of daily DORA-12 administration. Our results provide evidence that DORA-12 can inhibit CFA-induced stimulation of trigeminal sensory neurons by inhibiting expression of proteins associated with sensitization of peripheral and central neurons and nociception.

Inhibiting spinal neuron-astrocytic activation correlates with synergistic analgesia of dexmedetomidine and ropivacaine

Background

This study aims to identify that intrathecal (i.t.) injection of dexmedetomidine (Dex) and ropivacaine (Ropi) induces synergistic analgesia on chronic inflammatory pain and is accompanied with corresponding “neuron-astrocytic” alterations.

Methods

Male, adult Sprague-Dawley rats were randomly divided into sham, control and i.t. medication groups. The analgesia profiles of i.t. Dex, Ropi, and their combination detected by Hargreaves heat test were investigated on the subcutaneous (s.c.) injection of complete Freund adjuvant (CFA) induced chronic pain in rat and their synergistic analgesia was confirmed by using isobolographic analysis. During consecutive daily administration, pain behavior was daily recorded, and immunohistochemical staining was applied to investigate the number of Fos-immunoreactive (Fos-ir) neurons on hour 2 and day 1, 3 and 7, and the expression of glial fibrillary acidic protein (GFAP) within the spinal dorsal horn (SDH) on day 1, 3, 5 and 7 after s.c. injection of CFA, respectively, and then Western blot to examine spinal GFAP and β-actin levels on day 3 and 7.

Results

i.t. Dex or Ropi displayed a short-term analgesia in a dose-dependent manner, and consecutive daily administrations of their combination showed synergistic analgesia and remarkably down-regulated neuronal and astrocytic activations indicated by decreases in the number of Fos-ir neurons and the GFAP expression within the SDH, respectively.

Conclusion

i.t. co-delivery of Dex and Ropi shows synergistic analgesia on the chronic inflammatory pain, in which spinal “neuron-astrocytic activation” mechanism may play an important role.

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.

Mechanisms of acupuncture-electroacupuncture on persistent pain

In the last decade, preclinical investigations of electroacupuncture mechanisms on persistent tissue injury (inflammatory), nerve injury (neuropathic), cancer, and visceral pain have increased. These studies show that electroacupuncture activates the nervous system differently in health than in pain conditions, alleviates both sensory and affective inflammatory pain, and inhibits inflammatory and neuropathic pain more effectively at 2 to 10 Hz than at 100 Hz. Electroacupuncture blocks pain by activating a variety of bioactive chemicals through peripheral, spinal, and supraspinal mechanisms. These include opioids, which desensitize peripheral nociceptors and reduce proinflammatory cytokines peripherally and in the spinal cord, and serotonin and norepinephrine, which decrease spinal N-methyl-D-aspartate receptor subunit GluN1 phosphorylation. Additional studies suggest that electroacupuncture, when combined with low dosages of conventional analgesics, provides effective pain management which can forestall the side effects of often-debilitating pharmaceuticals.

The Molecular and Pharmacological Mechanisms of HIV-Related Neuropathic Pain

Infection of the nervous system with the human immunodeficiency virus (HIV-1) can lead to cognitive, motor and sensory disorders. HIV-related sensory neuropathy (HIV-SN) mainly contains the HIV infection-related distal sensory polyneuropathy (DSP) and antiretroviral toxic neuropathies (ATN). The main pathological features that characterize DSP and ATN include retrograde ("dying back") axonal degeneration of long axons in distal regions of legs or arms, loss of unmyelinated fibers, and variable degree of macrophage infiltration in peripheral nerves and dorsal root ganglia (DRG). One of the most common complaints of HIV-DSP is pain. Unfortunately, many conventional agents utilized as pharmacologic therapy for neuropathic pain are not effective for providing satisfactory analgesia in painful HIV-related distal sensory polyneuropathy, because the molecular mechanisms of the painful HIV-SDP are not clear in detail. The HIV envelope glycoprotein, gp120, appears to contribute to this painful neuropathy. Recently, preclinical studies have shown that glia activation in the spinal cord and DRG has become an attractive target for attenuating chronic pain. Cytokines/chemokines have been implicated in a variety of painful neurological diseases and in animal models of HIV-related neuropathic pain. Mitochondria injured by ATN and/or gp120 may be also involved in the development of HIV-neuropathic pain. This review discusses the neurochemical and pharmacological mechanisms of HIV-related neuropathic pain based on the recent advance in the preclinical studies, providing insights into novel pharmacological targets for future therapy.

Hyperbaric oxygen treatment produces an antinociceptive response phase and inhibits astrocyte activation and inflammatory response in a rat model of neuropathic pain

Hyperbaric oxygen (HBO) treatment has been proven to be a promising candidate for protection of the nervous system after acute injury in animal models of neuropathic pain. The purposes of this study were to examine the antinociceptive response phase induced by HBO treatment in a model of neuropathic pain and to determine the dependence of the treatment's mechanism of alleviating neuropathic pain on the inhibition of spinal astrocyte activation. Neuropathic pain was induced in rats by chronic constriction injury of the sciatic nerve. Mechanical threshold and thermal latency were tested preoperatively and for 1 week postoperatively, four times daily at fixed time points. Methane dicarboxylic aldehyde (MDA) and superoxide dismutase (SOD) parameters were used as indices of oxidative stress response and tested before and after the treatment. The inflammatory cytokines interleukin (IL)-1β and IL-10 were assayed in the sciatic nerve were with enzyme-linked immunoassay. Glial fibrillary acidic protein activation in the spinal cord was evaluated immunohistochemically. The rats exhibited temporary allodynia immediately after HBO treatment completion. This transient allodynia was closely associated with changes in MDA and SOD levels. A single HBO treatment caused a short-acting antinociceptive response phase. Repetitive HBO treatment led to a long-acting antinociceptive response phase and inhibited astrocyte activation. These results indicated that HBO treatment played a dual role in the aggravation and alleviation of neuropathic pain, though the aggravated pain effect (transient allodynia) was far less pronounced than the antinociceptive phase. Astrocyte inhibition and anti-inflammation may contribute to the antinociceptive effect of HBO treatment after nerve injury.

Central immune overactivation in the presence of reduced plasma corticosterone contributes to swim stress-induced hyperalgesia

Although it is widely known that immunological, hormonal and nociceptive mechanisms are altered by exposure to repeated stress, the interplaying roles of each function in the development of post-stress hyperalgesia are not completely clear. Thus, we wanted to establish how interleukin 1-beta (IL-1β), corticosterone and microglia interact to contribute in the development of hyperalgesia following repeated forced swim. Rats were subjected to either forced swim, sham swim or non-conditioned. Each group was then treated with minocycline, ketoconazole, or saline. Thermal nociception was measured via the hot plate test, before and after the behavioral conditioning, whereas blood and lumbar spinal cord tissue samples were obtained at the end of the protocol. Serum levels of corticosterone, spinal tissue concentration of IL-1β and spinal OX-42 labeling (microglial marker) were determined. Rats exposed to forced swim stress developed thermal hyperalgesia along with elevated spinal tissue IL-1β, increased OX-42 labeling and relatively diminished serum corticosterone. Pre-treatment with minocycline and ketoconazole prevented the development of thermal hyperalgesia and the increase in IL-1β, without significantly modifying serum corticosterone. These results suggest that the development of forced swim-induced thermal hyperalgesia requires the simultaneous presence of increased spinal IL-1β, microglial activation, and relatively decreased serum corticosterone.

An intricate relationship between pain and depression: clinical correlates, coactivation factors and therapeutic targets

INTRODUCTION

An apparent clinical relationship between pain and depression has long been recognized, which makes an enormous impact on the individual health care. At present, the practical implication of such overlapping symptomatology between pain and depression is not clear, but the prevalence estimates for depression are substantially inflated among patients with chronic pain and vice versa. This interaction has been labeled as the depression-pain syndrome or depression-pain dyad.

AREAS COVERED

This article discusses the neurobiological substrates and neuroanatomical pathways involved in pain-depression dyad along with newer therapeutic targets.

EXPERT OPINION

Several key themes emerged from our review of the relationship between depression and pain. First, the diagnosis of depression in pain or vice versa is particularly challenging, and the development of better diagnostic framework that involves both pain and depression is particularly required. Secondly, the entwined relationship between pain and depression supports the possibility of common coactivating factors that results in their neurophysiological overlap. A broad understanding of the role played by the central nervous system (CNS) in the processing of pain and depression may eventually lead to the introduction of triple reuptake inhibitors, agomelatine, vilazodone and ketamine with novel mechanism of action, hence appear to be of promising potential for pain with depression.

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.

The glial-neuronal GRK2 pathway participates in the development of trigeminal neuropathic pain in rats

This study examined the role of the glial-neuronal G protein-coupled receptor kinase 2 (GRK2) pathway in the development of trigeminal neuropathic pain. Male Sprague Dawley rats, weighing 220 to 240 g, were anesthetized with ketamine (0.2 g/kg) and xylazine (0.02 g/kg). Under anesthesia, the left lower second molar was extracted, followed by the placement of a mini-dental implant to intentionally injure the inferior alveolar nerve. This injury produced mechanical allodynia along with the downregulation of neuronal GRK2 expression in the medullary dorsal horn. On the other hand, early intracisternal treatment with MDL28170, a calpain inhibitor, produced prolonged antiallodynic effects and blocked this downregulation of neuronal GRK2 expression. The intracisternal infusion of minocycline, a microglia inhibitor, and l-α-aminoadipic acid, an astrocytic specific inhibitor, also blocked the induced mechanical allodynia and downregulated neuronal GRK2 expression, respectively. Double immunofluorescence showed that the interleukin (IL)-1β and IL-1R signals colocalize with the astrocytes and neurons, respectively, in the medullary dorsal horn following an inferior alveolar nerve injury. In addition, the intracisternal infusion of an IL-1 receptor antagonist also produced antiallodynic effects and blocked the downregulation of neuronal GRK2 expression. These results suggest that the glial-neuronal GRK2 pathway is a potentially important new target for treating neuropathic pain. Moreover, the IL-1β expressed in astrocytes plays a significant role in modulating this pathway.

PERSPECTIVE

This study showed that the glial-neuronal GRK2 pathway participates in the development of trigeminal neuropathic pain in rats. These results suggest that the glial-neuronal GRK2 pathway is a potentially important new target for the treatment of neuropathic pain.

Proteome of brain glia: the molecular basis of diverse glial phenotypes

Several different types of nonneuronal glial cells with diverse phenotypes are present in the CNS, and all have distinct indispensible functions. Although glial cells primarily provide neurons with metabolic and structural support in the healthy brain, they may switch phenotype from a "resting" to a "reactive" state in response to pathological insults. Furthermore, this reactive gliosis is an invariant feature of the pathogeneses of CNS maladies. The glial proteome serves as a signature of glial phenotype, and not only executes physiological functions, but also acts as a molecular mediator of the reactive glial phenotype. The glial proteome is also involved in intra- and intercellular communications as exemplified by glia-glia and neuron-glia interactions. The utilization of authoritative proteomic tools and the bioinformatic analyses have helped to profile the brain glial proteome and explore the molecular mechanisms of diverse glial phenotypes. Furthermore, technologic innovations have equipped the field of "glioproteomics" with refined tools for studies of the expression, interaction, and function of glial proteins in the healthy and in the diseased CNS. Glioproteomics is expected to contribute to the elucidation of the molecular mechanisms of CNS pathophysiology and to the discovery of biomarkers and theragnostic targets in CNS disorders.

Participation of the central p38 and ERK1/2 pathways in IL-1β-induced sensitization of nociception in rats

This study examined the participation of central mitogen-activated protein kinases (MAPKs) in the central sensitization produced by a subcutaneous injection of interleukin-1β (IL-1β) in male Sprague-Dawley rats. Formalin-induced responses were evaluated 24h after an IL-1β injection. A subcutaneous injection of 10ng of IL-1β elevated the formalin-induced scratching response significantly in the second phase compared to the vehicle-treated group. Pretreatment with an IL-1 receptor antagonist reduced the IL-1β-induced sensitization. Pretreatment with IL-1β increased the p-ERK and p-p38 expression induced by the formalin injection. Double immunofluorescence data revealed increases in phospho-extracellular signal-regulated kinase (p-ERK) immunoreactive cells that co-localize with neuronal nuclei (NeuN), a neuronal marker, and in phospho-p38 (p-p38) immunoreactive cells that co-localize with NeuN and OX42, a microglia marker. The intracisternal administration of minocycline (50μg), a microglia inhibitor, attenuated the increased formalin-induced scratching responses in the IL-1β-treated rats. The intracisternal administration of PD98059 (1, 10μg), a MEK inhibitor, and SB203580 (1, 5μg), a p38 inhibitor, also attenuated the number of formalin-induced scratches in the second phase in the IL-1β-treated rats. These results suggest that the IL-1β-induced central sensitization of nociception is mediated by the central MAPK pathways, which are activated differentially in the neurons and microglia under inflammatory pain conditions. Therefore, blockade of the MAPK pathways can be as a potential therapeutic target for the central sensitization of inflammatory pain.

Endogenous interleukin-1β in neuropathic rats enhances glutamate release from the primary afferents in the spinal dorsal horn through coupling with presynaptic N-methyl-D-aspartic acid receptors

Excessive activation of glutamate receptors and overproduction of proinflammatory cytokines, including interleukin-1β (IL-1β) in the spinal dorsal horn, are key mechanisms underlying the development and maintenance of neuropathic pain. In this study, we investigated the mechanisms by which endogenous IL-1β alters glutamatergic synaptic transmission in the spinal dorsal horn in rats with neuropathic pain induced by ligation of the L5 spinal nerve. We demonstrated that endogenous IL-1β in neuropathic rats enhances glutamate release from the primary afferent terminals and non-NMDA glutamate receptor activities in postsynaptic neurons in the spinal dorsal horn. Myeloid differentiation primary response protein 88 (MyD88) is a mediator used by IL-1β to enhance non-NMDA glutamate receptor activities in postsynaptic neurons in the spinal dorsal horn. Presynaptic NMDA receptors are effector receptors used by the endogenous IL-1β to enhance glutamate release from the primary afferents in neuropathic rats. This is further supported by the fact that NMDA currents recorded from small neurons in the dorsal root ganglion of normal rats are potentiated by exogenous IL-1β. Furthermore, we provided evidence that functional coupling between IL-1β receptors and presynaptic NMDA receptors at the primary afferent terminals is mediated by the neutral sphingomyelinase/ceramide signaling pathway. Hence, functional coupling between IL-1β receptors and presynaptic NMDA receptors at the primary afferent terminals is a crucial mechanism leading to enhanced glutamate release and activation of non-NMDA receptors in the spinal dorsal horn neurons in neuropathic pain conditions. Interruption of such functional coupling could be an effective approach for the treatment of neuropathic pain.

Tramadol and propentofylline coadministration exerted synergistic effects on rat spinal nerve ligation-induced neuropathic pain

Neuropathic pain is an intractable clinical problem. Drug treatments such as tramadol have been reported to effectively decrease neuropathic pain by inhibiting the activity of nociceptive neurons. It has also been reported that modulating glial activation could also prevent or reverse neuropathic pain via the administration of a glial modulator or inhibitor, such as propentofylline. Thus far, there has been no clinical strategy incorporating both neuronal and glial participation for treating neuropathic pain. Therefore, the present research study was designed to assess whether coadministration of tramadol and propentofylline, as neuronal and glial activation inhibitors, respectively, would exert a synergistic effect on the reduction of rat spinal nerve ligation (SNL)-induced neuropathic pain. Rats underwent SNL surgery to induce neuropathic pain. Pain behavioral tests were conducted to ascertain the effect of drugs on SNL-induced mechanical allodynia with von-Frey hairs. Proinflammatory factor interleukin-1β (IL-1β) expression was also detected by Real-time RT-PCR. Intrathecal tramadol and propentofylline administered alone relieved SNL-induced mechanical allodynia in a dose-dependent manner. Tramadol and propentofylline coadministration exerted a more potent effect in a synergistic and dose dependent manner than the intrathecal administration of either drug alone. Real-time RT-PCR demonstrated IL-1β up-expression in the ipsilateral spinal dorsal horn after the lesion, which was significantly decreased by tramadol and propentofylline coadministration. Inhibiting proinflammatory factor IL-1β contributed to the synergistic effects of tramadol and propentofylline coadministration on rat peripheral nerve injury-induced neuropathic pain. Thus, our study provided a rationale for utilizing a novel strategy for treating neuropathic pain by blocking the proinflammatory factor related pathways in the central nervous system.

Astrocytes are involved in long-term facilitation of neuronal excitation in the anterior cingulate cortex of mice with inflammatory pain

Neuronal plasticity in the pain-processing pathway is thought to be a mechanism underlying pain hypersensitivity and negative emotions occurring during a pain state. Recent evidence suggests that the activation of astrocytes in the anterior cingulate cortex (ACC) contributes to the development of negative emotions during pain hypersensitivity after peripheral inflammation. However, it is unknown whether these activated astrocytes contribute to neuronal plasticity in the ACC. In this study, by using optical imaging with voltage- and Ca(2+)-sensitive dyes, we examined the long-term facilitation of neuronal excitation induced by high-frequency conditioning stimulation (HFS) in ACC slices of control mice and mice with peripheral inflammation induced by the injection of complete Freund adjuvant (CFA) to the hind paw. Immunoreactivity of glial fibrillary acidic protein in laminae II-III of the ACC in the CFA-injected mice was higher than in the control mice. Neuronal excitation in ACC slices from the CFA-injected mice was gradually increased by HFS, and the magnitude of this long-term facilitation was greater than in the control mice. The long-term facilitation in the CFA-injected mice was inhibited by the astroglial toxin, the N-methyl-d-aspartate (NMDA) receptor antagonist and NMDA receptor glycine binding site antagonist. The increase of intracellular Ca(2+) concentration in astrocytes during HFS was higher in the CFA-injected mice than in the control mice and was inhibited by l-α-aminoadipate (l-α-AA). These results suggest that the activation of astrocytes in the ACC plays a crucial role in the development of negative emotions and LTP during pain hypersensitivity after peripheral inflammation.

Decreased expression and role of GRK6 in spinal cord of rats after chronic constriction injury

Nerve injury and inflammation can both induce neuropathic pain via the production of pro-inflammatory cytokines. In the process, G protein-coupled receptors (GPCRs) were involved in pain signal transduction. GPCR kinase (GRK) 6 is a member of the GRK family that regulates agonist-induced desensitization and signaling of GPCRs. However, its expression and function in neuropathic pain have not been reported. In this study, we performed a chronic constriction injury (CCI) model in adult male rats and investigated the dynamic change of GRK6 expression in spinal cord. GRK6 was predominantly expressed in the superficial layers of the lumbar spinal cord dorsal horn neurons and its expression was decreased bilaterally following induction of CCI. The changes of GRK6 were mainly in IB4 and P substrate positive areas in spinal cord dorsal horn. And over-expression of GRK6 in spinal cord by lentivirus intrathecal injection attenuated the pain response induced by CCI. In addition, the level of TNF-α underwent the negative pattern of GRK6 in spinal cord. And neutralized TNF-α by antibody intrathecal injection up-regulated GRK6 expression and attenuated the mechanical allodynia and heat hyperalgesia in CCI model. All the data indicated that down-regulation of neuronal GRK6 expression induced by cytokine may be a potential mechanism that contributes to increasing neuronal signaling in neuropathic pain.

Microglia and spinal cord synaptic plasticity in persistent pain

Microglia are regarded as macrophages in the central nervous system (CNS) and play an important role in neuroinflammation in the CNS. Microglial activation has been strongly implicated in neurodegeneration in the brain. Increasing evidence also suggests an important role of spinal cord microglia in the genesis of persistent pain, by releasing the proinflammatory cytokines tumor necrosis factor-alpha (TNFα), Interleukine-1beta (IL-1β), and brain derived neurotrophic factor (BDNF). In this review, we discuss the recent findings illustrating the importance of microglial mediators in regulating synaptic plasticity of the excitatory and inhibitory pain circuits in the spinal cord, leading to enhanced pain states. Insights into microglial-neuronal interactions in the spinal cord dorsal horn will not only further our understanding of neural plasticity but may also lead to novel therapeutics for chronic pain management.

Pathobiology and management of prostate cancer-induced bone pain: recent insights and future treatments

Prostate cancer (PCa) has a high propensity for metastasis to bone. Despite the availability of multiple treatment options for relief of PCa-induced bone pain (PCIBP), satisfactory relief of intractable pain in patients with advanced bony metastases is challenging for the clinicians because currently available analgesic drugs are often limited by poor efficacy and/or dose-limiting side effects. Rodent models developed in the past decade show that the pathobiology of PCIBP comprises elements of inflammatory, neuropathic and ischemic pain arising from ectopic sprouting and sensitization of sensory nerve fibres within PCa-invaded bones. In addition, at the cellular level, PCIBP is underpinned by dynamic cross talk between metastatic PCa cells, cellular components of the bone matrix, factors associated with the bone microenvironment as well as peripheral components of the somatosensory system. These insights are aligned with the clinical management of PCIBP involving use of a multimodal treatment approach comprising analgesic agents (opioids, NSAIDs), radiotherapy, radioisotopes, cancer chemotherapy agents and bisphosphonates. However, a major drawback of most rodent models of PCIBP is their short-term applicability due to ethical concerns. Thus, it has been difficult to gain insight into the mal(adaptive) neuroplastic changes occurring at multiple levels of the somatosensory system that likely contribute to intractable pain at the advanced stages of metastatic disease. Specifically, the functional responsiveness of noxious circuitry as well as the neurochemical signature of a broad array of pro-hyperalgesic mediators in the dorsal root ganglia and spinal cord of rodent models of PCIBP is relatively poorly characterized. Hence, recent work from our laboratory to develop a protocol for an optimized rat model of PCIBP will enable these knowledge gaps to be addressed as well as identification of novel targets for drug discovery programs aimed at producing new analgesics for the improved relief of intractable PCIBP.

Implication of the chemokine CCL2 in trigeminal nociception and traumatic neuropathic orofacial pain

BACKGROUND

Chemokine (C-C motif) ligand 2 (CCL2) participates in different mechanisms contributing to the spinal cord inflammation and pain development after sciatic nerve injury. Recent data also support its role in orofacial thermal hypersensitivity, although its implication in different phases of trigeminal pain emergence is unclear. We assessed the importance of CCL2 signalling in biochemical and behavioural alterations during the early and late stages following chronic constriction injury of infraorbital nerve (ION-CCI), a model of peripheral traumatic trigeminal pain.

METHODS

After evaluating the consequences of CCL2 intracisternal injection in naïve rats, we determined the expression changes for CCL2, inflammatory and glia activation markers in the somatosensory trigeminal complex (STC) and trigeminal ganglia (TG) after ION-CCI. The role of CCL2 signalling was assessed using pre-emptive or 'curative' intracisternal treatment with specific CCL2 receptor antagonist - INCB3344.

RESULTS

Exogenous CCL2 evoked spontaneous behaviour reminiscent of orofacial pain and marked mechanical hypersensitivity, associated with increased expression of proinflammatory cytokines and glial markers in STC and TG. CCL2-evoked changes were prevented by the co-administration of INCB3344. Two weeks after ION-CCI, mRNA for CCL2, glial and inflammatory markers were up-regulated, and CCL2-immunoreactivity accumulated in central and ganglionic tissues. At this time, repeated intracisternal administration of INCB3344 did not attenuate the ION-CCI-associated behavioural nor biochemical changes. By contrast, pre-emptive INCB3344 treatment delayed the emergence of trigeminal mechanical allodynia and associated biochemical alterations.

CONCLUSIONS

Our data suggest that CCL2 is involved principally in the early events accompanying the ION lesion rather than in long-term alterations and the maintenance of trigeminal mechanical hypersensitivity.

Astrocytes are involved in trigeminal dynamic mechanical allodynia: potential role of D-serine

Trigeminal neuropathic pain affects millions of people worldwide. Despite decades of study on the neuronal processing of pain, mechanisms underlying enhanced pain states after injury remain unclear. N-methyl-D-aspartate (NMDA) receptor-dependent changes play a critical role in triggering central sensitization in neuropathic pain. These receptors are regulated at the glycine site through a mandatory endogenous co-agonist D-serine, which is synthesized by astrocytes. Therefore, the present study was carried out to determine whether astrocytes are involved, through D-serine secretion, in dynamic mechanical allodynia (DMA) obtained after chronic constriction of the infraorbital nerve (CCI-IoN) in rats. Two weeks after CCI-IoN, an important reaction of astrocytes was present in the medullary dorsal horn (MDH), as revealed by an up-regulation of glial fibrillary acidic protein (GFAP) in allodynic rats. In parallel, an increase in D-serine synthesis, which co-localized with its synthesis enzyme serine racemase, was strictly observed in astrocytes. Blocking astrocyte metabolism by intracisternal delivery of fluorocitrate alleviated DMA. Furthermore, the administration of D-amino-acid oxidase (DAAO), a D-serine-degrading enzyme, or that of L-serine O-sulfate (LSOS), a serine racemase inhibitor, significantly decreased pain behavior in allodynic rats. These results demonstrate that astrocytes are involved in the modulation of orofacial post-traumatic neuropathic pain via the release of the gliotransmitter D-serine.

Spinal astrocyte gap junctions contribute to oxaliplatin-induced mechanical hypersensitivity

Spinal glial cells contribute to the development of many types of inflammatory and neuropathic pain. Here the contribution of spinal astrocytes and astrocyte gap junctions to oxaliplatin-induced mechanical hypersensitivity was explored. The expression of glial fibrillary acidic protein (GFAP) in spinal dorsal horn was significantly increased at day 7 but recovered at day 14 after oxaliplatin treatment, suggesting a transient activation of spinal astrocytes by chemotherapy. Astrocyte-specific gap junction protein connexin 43 (Cx43) was significantly increased in dorsal horn at both day 7 and day 14 following chemotherapy, but neuronal (connexin 36 [Cx36]) and oligodendrocyte (connexin 32 [Cx32]) gap junction proteins did not show any change. Blockade of astrocyte gap junction with carbenoxolone (CBX) prevented oxaliplatin-induced mechanical hypersensitivity in a dose-dependent manner and the increase of spinal GFAP expression, but had no effect once the mechanical hypersensitivity induced by oxaliplatin had fully developed. These results suggest that oxaliplatin chemotherapy induces the activation of spinal astrocytes and this is accompanied by increased expression of astrocyte-astrocyte gap junction connections via Cx43. These alterations in spinal astrocytes appear to contribute to the induction but not the maintenance of oxaliplatin-induced mechanical hypersensitivity. Combined, these results suggest that targeting spinal astrocyte/astrocyte-specific gap junction could be a new therapeutic strategy to prevent oxaliplatin-induced neuropathy.

PERSPECTIVE

Spinal astrocytes but not microglia were recently shown to be recruited in paclitaxel-related chemoneuropathy. Here, spinal astrocyte gap junctions are shown to play an important role in the induction of oxaliplatin neuropathy.

Fractalkine mediates inflammatory pain through activation of satellite glial cells

The activation of the satellite glial cells (SGCs) surrounding the dorsal root ganglion (DRG) neurons appears to play a role in pathological pain. We tested the hypothesis that fractalkine, which is constitutively expressed by primary nociceptive neurons, is the link between peripheral inflammation and the activation of SGCs and is thus responsible for the genesis of the inflammatory pain. The injection of carrageenin into the rat hind paw induced a decrease in the mechanical nociceptive threshold (hypernociception), which was associated with an increase in mRNA and GFAP protein expression in the DRG. Both events were inhibited by anti-fractalkine antibody administered directly into the DRG (L5) [intraganglionar (i.gl.)]. The administration of fractalkine into the DRG (L5) produced mechanical hypernociception in a dose-, time-, and CX3C receptor-1 (CX3CR1)-dependent manner. Fractalkine's hypernociceptive effect appears to be indirect, as it was reduced by local treatment with anti-TNF-α antibody, IL-1-receptor antagonist, or indomethacin. Accordingly, the in vitro incubation of isolated and cultured SGC with fractalkine induced the production/release of TNF-α, IL-1β, and prostaglandin E2. Finally, treatment with i.gl. fluorocitrate blocked fractalkine (i.gl.)- and carrageenin (paw)-induced hypernociception. Overall, these results suggest that, during peripheral inflammation, fractalkine is released in the DRG and contributes to the genesis of inflammatory hypernociception. Fractalkine's effect appears to be dependent on the activation of the SGCs, leading to the production of TNFα, IL-1β, and prostanoids, which are likely responsible for the maintenance of inflammatory pain. Thus, these results indicate that the inhibition of fractalkine/CX3CR1 signaling in SGCs may serve as a target to control inflammatory pain.

CCL-1 in the spinal cord contributes to neuropathic pain induced by nerve injury

Cytokines such as interleukins are known to be involved in the development of neuropathic pain through activation of neuroglia. However, the role of chemokine (C-C motif) ligand 1 (CCL-1), a well-characterized chemokine secreted by activated T cells, in the nociceptive transmission remains unclear. We found that CCL-1 was upregulated in the spinal dorsal horn after partial sciatic nerve ligation. Therefore, we examined actions of recombinant CCL-1 on behavioural pain score, synaptic transmission, glial cell function and cytokine production in the spinal dorsal horn. Here we show that CCL-1 is one of the key mediators involved in the development of neuropathic pain. Expression of CCL-1 mRNA was mainly detected in the ipsilateral dorsal root ganglion, and the expression of specific CCL-1 receptor CCR-8 was upregulated in the superficial dorsal horn. Increased expression of CCR-8 was observed not only in neurons but also in microglia and astrocytes in the ipsilateral side. Recombinant CCL-1 injected intrathecally (i.t.) to naive mice induced allodynia, which was prevented by the supplemental addition of N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801. Patch-clamp recordings from spinal cord slices revealed that application of CCL-1 transiently enhanced excitatory synaptic transmission in the substantia gelatinosa (lamina II). In the long term, i.t. injection of CCL-1 induced phosphorylation of NMDA receptor subunit, NR1 and NR2B, in the spinal cord. Injection of CCL-1 also upregulated mRNA level of glial cell markers and proinflammatory cytokines (IL-1β, TNF-α and IL-6). The tactile allodynia induced by nerve ligation was attenuated by prophylactic and chronic administration of neutralizing antibody against CCL-1 and by knocking down of CCR-8. Our results indicate that CCL-1 is one of the key molecules in pathogenesis, and CCL-1/CCR-8 signaling system can be a potential target for drug development in the treatment for neuropathic pain.

Interleukin-1β increased the expression of protease-activated receptor 4 mRNA and protein in dorsal root ganglion neurons

Protease-activated receptor-4 (PAR4) is localized in primary sensory neurons and is believed to implicate in the modulation of nociceptive mechanisms. The pro-inflammatory cytokine interleukin-1β (IL-1β) is involved in the generation of hyperalgesia in pathological states such as neuropathy and inflammation. Previous studies have shown that IL-1β enhances the expression of PAR4 in many cell types but the effect of this cytokine on primary sensory neuron PAR4 expression is less clear. In the present study, we evaluated in rat dorsal root ganglion (DRG) neurons the influence of IL-1β on PAR4 mRNA and protein levels after IL-1β intraplantar injection into the hind-paw or treatment of cultured DRG neurons. The expression of PAR4 in cultured DRG neurons was also assessed after treatment with IL-1β with pre-addition of phorbol-12-myristate 13-acetate (PMA, a PKC activator) or chelerythrine chloride (a PKC inhibitor). We found that IL-1β intraplantar injection into the hind-paw or long-term exposure of cultured DRG neurons to IL-1β significantly increased the proportion of DRG neurons expressing PAR4 immunoreactivity. Real-time PCR and western blotting showed that IL-1β treatment also significantly elevated PAR4 mRNA and protein levels in DRG neurons. This IL-1β effect was enhanced in DRG neurons when DRG cultures were pre-treatment with the PMA. But pre-incubation with chelerythrine chloride strongly inhibited the IL-1β-induced increase of PAR4 mRNA and protein levels. These results demonstrate that the expression of PAR4 mRNA and protein induced by IL-1β is PKC signaling pathway dependent.

Neurovascular unit in chronic pain

Chronic pain is a debilitating condition with major socioeconomic impact, whose neurobiological basis is still not clear. An involvement of the neurovascular unit (NVU) has been recently proposed. In particular, the blood-brain barrier (BBB) and blood-spinal cord barrier (BSCB), two NVU key players, may be affected during the development of chronic pain; in particular, transient permeabilization of the barrier is suggested by several inflammatory- and nerve-injury-based pain models, and we argue that the clarification of molecular BBB/BSCB permeabilization events will shed new light in understanding chronic pain mechanisms. Possible biases in experiments supporting this theory and its translational potentials are discussed. Moving beyond an exclusive focus on the role of the endothelium, we propose that our understanding of the mechanisms subserving chronic pain will benefit from the extension of research efforts to the NVU as a whole. In this view, the available evidence on the interaction between analgesic drugs and the NVU is here reviewed. Chronic pain comorbidities, such as neuroinflammatory and neurodegenerative diseases, are also discussed in view of NVU changes, together with innovative pharmacological solutions targeting NVU components in chronic pain treatment.

Induction of thermal hyperalgesia and synaptic long-term potentiation in the spinal cord lamina I by TNF-α and IL-1β is mediated by glial cells

Long-term potentiation (LTP) of synaptic strength in nociceptive pathways is a cellular model of hyperalgesia. The emerging literature suggests a role for cytokines released by spinal glial cells for both LTP and hyperalgesia. However, the underlying mechanisms are still not fully understood. In rat lumbar spinal cord slices, we now demonstrate that conditioning high-frequency stimulation of primary afferents activated spinal microglia within <30 min and spinal astrocytes within ~2 s. Activation of spinal glia was indispensible for LTP induction at C-fiber synapses with spinal lamina I neurons. The cytokines interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α), which are both released by activated glial cells, were individually sufficient and necessary for LTP induction via redundant pathways. They differentially amplified 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)-propanoic acid receptor-mediated and N-methyl-D-aspartic acid receptor-mediated synaptic currents in lamina I neurons. Unexpectedly, the synaptic effects by IL-1β and TNF-α were not mediated directly via activation of neuronal cytokine receptors, but rather, indirectly via IL-1 receptors and TNF receptors being expressed on glial cells in superficial spinal dorsal horn. Bath application of IL-1β or TNF-α led to the release profiles of pro-inflammatory and anti-inflammatory cytokines, chemokines, and growth factors, which overlapped only partially. Heat hyperalgesia induced by spinal application of either IL-1β or TNF-α in naive animals also required activation of spinal glial cells. These results reveal a novel, decisive role of spinal glial cells for the synaptic effects of IL-1β and TNF-α and for some forms of hyperalgesia.

Inclusion of cocoa as a dietary supplement represses expression of inflammatory proteins in spinal trigeminal nucleus in response to chronic trigeminal nerve stimulation

SCOPE

Central sensitization is implicated in the pathology of temporomandibular joint disorder and other types of orofacial pain. We investigated the effects of dietary cocoa on expression of proteins involved in the development of central sensitization in the spinal trigeminal nucleus (STN) in response to inflammatory stimulation of trigeminal nerves.

METHODS AND RESULTS

Male Sprague-Dawley rats were fed either a control diet or an isocaloric diet consisting of 10% cocoa powder 14 days prior to bilateral injection of complete Freund's adjuvant (CFA) into the temporomandibular joint to promote prolonged activation of trigeminal ganglion neurons and glia. While dietary cocoa stimulated basal expression of glutamate-aspartate transporter and mitogen-activated protein kinase phosphatase-1 when compared to animals on a normal diet, cocoa suppressed basal calcitonin gene-related peptide levels in the STN. CFA-stimulated levels of protein kinase A, P2X3 , P-p38, glial fibrillary-associated protein, and OX-42, whose elevated levels in the STN are implicated in central sensitization, were repressed to near control levels in animals on a cocoa-enriched diet. Similarly, dietary cocoa repressed CFA-stimulated inflammatory cytokine expression.

CONCLUSION

Based on our findings, we speculate that cocoa-enriched diets could be beneficial as a natural therapeutic option for temporomandibular joint disorder and other chronic orofacial pain conditions.

Tissue plasminogen activator contributes to morphine tolerance and induces mechanical allodynia via astrocytic IL-1β and ERK signaling in the spinal cord of mice

Accumulating evidence indicates that activation of spinal cord astrocytes contributes importantly to nerve injury and inflammation-induced persistent pain and chronic opioid-induced antinociceptive tolerance. Phosphorylation of extracellular signal-regulated kinase (pERK) and induction of interleukin-1 beta (IL-1β) in spinal astrocytes have been implicated in astrocytes-mediated pain. Tissue plasminogen activator (tPA) is a serine protease that has been extensively used to treat stroke. We examined the potential involvement of tPA in chronic opioid-induced antinociceptive tolerance and activation of spinal astrocytes using tPA knockout (tPA(-/-)) mice and astrocyte cultures. tPA(-/-) mice exhibited unaltered nociceptive pain and morphine-induced acute analgesia. However, the antinociceptive tolerance, induced by chronic morphine (10mg/kg/day, s.c.), is abrogated in tPA(-/-) mice. Chronic morphine induces tPA expression in glial fibrillary acidic protein (GFAP)-expressing spinal cord astrocytes. Chronic morphine also increases IL-1β expression in GFAP-expressing astrocytes, which is abolished in tPA-deficient mice. In cultured astrocytes, morphine treatment increases tPA, IL-1β, and pERK expression, and the increased IL-1β and pERK expression is abolished in tPA-deficient astrocytes. tPA is also sufficient to induce IL-1β and pERK expression in astrocyte cultures. Intrathecal injection of tPA results in up-regulation of GFAP and pERK in spinal astrocytes but not up-regulation of ionized calcium binding adapter molecule 1 in spinal microglia. Finally, intrathecal tPA elicits persistent mechanical allodynia, which is inhibited by the astroglial toxin alpha-amino adipate and the MEK (ERK kinase) inhibitor U0126. Collectively, these data suggest an important role of tPA in regulating astrocytic signaling, pain hypersensitivity, and morphine tolerance.

Central sensitization and MAPKs are involved in occlusal interference-induced facial pain in rats

We previously developed a rat dental occlusal interference model of facial pain that was produced by bonding a crown onto the right maxillary first molar and was reflected in sustained facial hypersensitivity that was suggestive of the involvement of central sensitization mechanisms. The aim of the present study was to investigate potential central mechanisms involved in the occlusal interference-induced facial hypersensitivity. A combination of behavioral, immunohistochemical, Western blot, and electrophysiological recording procedures was used in 98 male adult Sprague Dawley rats that either received the occlusal interference or were sham-operated or naive rats. Immunohistochemically labeled astrocytes and microglia in trigeminal subnucleus caudalis (Vc) showed morphological changes indicative of astrocyte and microglial activation after the occlusal interference. Prolonged upregulation of p38 mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase (ERK) was also documented in Vc after placement of the occlusal interference and was expressed in both neurons and glial cells at time points when rats showed peak mechanical facial hypersensitivity. The intrathecal administration of the p38 MAPK inhibitor SB203580 to the medulla significantly inhibited the occlusal interference-induced hypersensitivity, and the ERK inhibitor PD98059 produced an even stronger effect. Central sensitization of functionally identified Vc nociceptive neurons following placement of the occlusal interference was also documented by extracellular electrophysiological recordings, and intrathecal administration of PD98059 could reverse the neuronal central sensitization. These novel findings suggest that central mechanisms including central sensitization of trigeminal nociceptive neurons and non-neuronal processes involving MAPKs play significant roles in the production of occlusal interference-induced facial pain.

PERSPECTIVE

Central mechanisms including trigeminal nociceptive neuronal sensitization, non-neuronal processes involving glial activation, and MAPKs play significant roles in occlusal interference-induced facial pain. These mechanisms may be involved in clinical manifestations of facial pain that have been reported in patients with an occlusal interference.

Spinal astrocytic activation contributes to mechanical allodynia in a rat chemotherapy-induced neuropathic pain model

Chemotherapy-induced neuropathic pain (CNP) is the major dose-limiting factor in cancer chemotherapy. However, the neural mechanisms underlying CNP remain enigmatic. Accumulating evidence implicates the involvement of spinal glia in some neuropathic pain models. In this study, using a vincristine-evoked CNP rat model with obvious mechanical allodynia, we found that spinal astrocyte rather than microglia was dramatically activated. The mechanical allodynia was dose-dependently attenuated by intrathecal administratration of L-α-aminoadipate (astrocytic specific inhibitor); whereas minocycline (microglial specific inhibitor) had no such effect, indicating that spinal astrocytic activation contributes to allodynia in CNP rat. Furthermore, oxidative stress mediated the development of spinal astrocytic activation, and activated astrocytes dramatically increased interleukin-1β expression which induced N-methyl-D-aspartic acid receptor (NMDAR) phosphorylation in spinal neurons to strengthen pain transmission. Taken together, our findings suggest that spinal activated astrocytes may be a crucial component of the pathophysiology of CNP and “Astrocyte-Cytokine-NMDAR-neuron” pathway may be one detailed neural mechanisms underlying CNP. Thus, inhibiting spinal astrocytic activation may represent a novel therapeutic strategy for treating CNP.

Gap junction channels and hemichannels in the CNS: regulation by signaling molecules

Coordinated interaction among cells is critical to develop the extremely complex and dynamic tasks performed by the central nervous system (CNS). Cell synchronization is in part mediated by connexins and pannexins; two different protein families that form gap junction channels and hemichannels. Whereas gap junction channels connect the cytoplasm of contacting cells and coordinate electric and metabolic activities, hemichannels communicate intra- and extra-cellular compartments and serve as diffusional pathways for ions and small molecules. Cells in the CNS depend on paracrine/autocrine communication via several extracellular signaling molecules, such as, cytokines, growth factors, transmitters and free radical species to sense changes in microenvironment as well as to adapt to them. These signaling molecules modulate crucial processes of the CNS, including, cellular migration and differentiation, synaptic transmission and plasticity, glial activation, cell viability and microvascular blood flow. Gap junction channels and hemichannels are affected by different signaling transduction pathways triggered by these paracrine/autocrine signaling molecules. Most of the modulatory effects induced by these signaling molecules are specific to the cell type and the connexin and pannexin subtype expressed in different brain areas. In this review, we summarized and discussed most of the relevant and recently published information on the effects of signaling molecules on connexin or pannexin based channels and their possible relevance in CNS physiology and pathology. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

Diminished neurokinin-1 receptor availability in patients with two forms of chronic visceral pain

Central sensitization and dysregulation of peripheral substance P and neurokinin-1 receptor (NK-1R) signaling are associated with chronic abdominal pain in inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS). Although positron emission tomography (PET) has demonstrated that patients with injury-related chronic pain have diminished NK-1R availability in the brain, it is unknown whether these deficits are present in IBD and IBS patients, who have etiologically distinct forms of non-injury-related chronic pain. This study's aim was to determine if patients with IBD or IBS exhibit deficits in brain expression of NK-1Rs relative to healthy controls (HCs), the extent to which expression patterns differ across patient populations, and if these patterns differentially relate to clinical parameters. PET with [(18)F]SPA-RQ was used to measure NK-1R availability by quantifying binding potential (BP) in the 3 groups. Exploratory correlation analyses were performed to detect associations between NK-1R BP and physical symptoms. Compared to HCs, IBD patients had NK-1R BP deficits across a widespread network of cortical and subcortical regions. IBS patients had similar, but less pronounced deficits. BP in a subset of these regions was robustly related to discrete clinical parameters in each patient population. Widespread deficits in NK-1R BP occur in IBD and, to a lesser extent, IBS; however, discrete clinical parameters relate to NK-1R BP in each patient population. This suggests that potential pharmacological interventions that target NK-1R signaling may be most effective for treating distinct symptoms in IBD and IBS.

Characterization of frequency-dependent responses of the vascular system to repetitive vibration

OBJECTIVE

Occupational exposure to hand-transmitted vibration can result in damage to nerves and sensory loss. The goal of this study was to assess the frequency-dependent effects of repeated bouts of vibration on sensory nerve function and associated changes in nerves.

METHODS

The tails of rats were exposed to vibration at 62.5, 125, or 250 Hz (constant acceleration of 49 m/s2) for 10 days. The effects on sensory nerve function, nerve morphology, and transcript expression in ventral tail nerves were measured.

RESULTS

Vibration at all frequencies had effects on nerve function and physiology. However, the effects tended to be more prominent with exposure at 250 Hz.

CONCLUSION

Exposure to vibration has detrimental effects on sensory nerve function and physiology. However, many of these changes are more prominent at 250-Hz exposure than at lower frequencies.

Transplantation of human umbilical cord blood or amniotic epithelial stem cells alleviates mechanical allodynia after spinal cord injury in rats

Stem cell therapy is a potential treatment for spinal cord injury (SCI), and a variety of different stem cell types have been grafted into humans suffering from spinal cord trauma or into animal models of spinal injury. Although several studies have reported functional motor improvement after transplantation of stem cells into injured spinal cord, the benefit of these cells for treating SCI-induced neuropathic pain is not clear. In this study, we investigated the therapeutic effect of transplanting human umbilical cord blood-derived mesenchymal stem cells (hUCB-MSCs) or amniotic epithelial stem cells (hAESCs) on SCI-induced mechanical allodynia (MA) and thermal hyperalgesia (TH) in T13 spinal cord hemisected rats. Two weeks after SCI, hUCB-MSCs or hAESCs were transplanted around the spinal cord lesion site, and behavioral tests were performed to evaluate changes in SCI-induced MA and TH. Immunohistochemical and Western blot analyses were also performed to evaluate possible therapeutic effects on SCI-induced inflammation and the nociceptive-related phosphorylation of the NMDA NR1 receptor subunit. While transplantation of hUCB-MSCs showed a tendency to reduce MA, transplantation of hAESCs significantly reduced MA. Neither hUCB-MSC nor hAESC transplantation had any effect on SCI-induced TH. Transplantation of hAESCs also significantly reduced the SCI-induced increase in NMDA receptor NR1 subunit phosphorylation (pNR1) expression in the spinal cord. Both hUCB-MSCs and hAESCs reduced the SCI-induced increase in spinal cord expression of the microglial marker, F4/80, but not the increased expression of GFAP or iNOS. Taken together, these findings demonstrate that the transplantation of hAESCs into the injured spinal cord can suppress mechanical allodynia, and this effect seems to be closely associated with the modulation of spinal cord microglia activity and NR1 phosphorylation.

Emerging role of Toll-like receptors in the control of pain and itch

Toll-like receptors (TLRs) are germline-encoded pattern-recognition receptors that initiate innate immune responses by recognizing molecular structures shared by a wide range of pathogens, known as pathogen-associated molecular patterns (PAMPs). After tissue injury or cellular stress, TLRs also detect endogenous ligands known as danger-associated molecular patterns (DAMPs). TLRs are expressed in both non-neuronal and neuronal cell types in the central nervous system (CNS) and contribute to both infectious and non-infectious disorders in the CNS. Following tissue insult and nerve injury, TLRs (such as TLR2, TLR3, and TLR4) induce the activation of microglia and astrocytes and the production of the proinflammatory cytokines in the spinal cord, leading to the development and maintenance of inflammatory pain and neuropathic pain. In particular, primary sensory neurons, such as nociceptors, express TLRs (e.g., TLR4 and TLR7) to sense exogenous PAMPs and endogenous DAMPs released after tissue injury and cellular stress. These neuronal TLRs are new players in the processing of pain and itch by increasing the excitability of primary sensory neurons. Given the prevalence of chronic pain and itch and the suffering of affected people, insights into TLR signaling in the nervous system will open a new avenue for the management of clinical pain and itch.