Pro- and Anti-inflammatory Cytokine Gene Expression in Rat Sciatic Nerve Chronic Constriction Injury Model of Neuropathic Pain

Role of TNF-alpha during central sensitization in preclinical studies

Tumor necrosis factor-alpha (TNF-α) is a principal mediator in pro-inflammatory processes that involve necrosis, apoptosis and proliferation. Experimental and clinical evidence demonstrate that peripheral nerve injury results in activation and morphological changes of microglial cells in the spinal cord. These adjustments occur in order to initiate an inflammatory cascade in response to the damage. Between the agents involved in this reaction, TNF-α is recognized as a key player in this process as it not only modulates lesion formation, but also because it is suggested to induce nociceptive signals. Nowadays, even though the function of TNF-α in inflammation and pain production seems to be generally accepted, diverse sources of literature point to different pathways and outcomes. In this review, we systematically searched and reviewed original articles from the past 10 years on animal models of peripheral nervous injury describing TNF-α expression in neural tissue and pain behavior.

Cellular and molecular insights into neuropathy-induced pain hypersensitivity for mechanism-based treatment approaches

Neuropathic pain is currently being treated by a range of therapeutic interventions that above all act to lower neuronal activity in the somatosensory system (e.g. using local anesthetics, calcium channel blockers, and opioids). The present review highlights novel and often still largely experimental treatment approaches based on insights into pathological mechanisms, which impact on the spinal nociceptive network, thereby opening the 'gate' to higher brain centers involved in the perception of pain. Cellular and molecular mechanisms such as ectopia, sensitization of nociceptors, phenotypic switching, structural plasticity, disinhibition, and neuroinflammation are discussed in relation to their involvement in pain hypersensitivity following either peripheral neuropathies or spinal cord injury. A mechanism-based treatment approach may prove to be successful in effective treatment of neuropathic pain, but requires more detailed insights into the persistence of cellular and molecular pain mechanisms which renders neuropathic pain unremitting. Subsequently, identification of the therapeutic window-of-opportunities for each specific intervention in the particular peripheral and/or central neuropathy is essential for successful clinical trials. Most of the cellular and molecular pain mechanisms described in the present review suggest pharmacological interference for neuropathic pain management. However, also more invasive treatment approaches belong to current and/or future options such as neuromodulatory interventions (including spinal cord stimulation) and cell or gene therapies, respectively.

Effects of low intensity pulsed ultrasound on rat Schwann cells metabolism

The effects of low intensity pulsed ultrasound to tenocytes and osteocytes are well understood and applied clinically. However, its effects on cultured Schwann cells are still not well elucidated. This study was designed to elucidate the effects of low intensity pulsed ultrasound on cultured Schwann cells and their possible molecular mechanism. Schwann cells were harvested from sciatic nerves of 3-day-old Sprague-Dawley rats. Low intensity pulsed ultrasound stimulator (frequency: 1 MHz, duration: 2 min, duty cycle: 20%, total treatment time: 3 min) was applied to three different culture conditions: regular culture medium containing 0, 5, or 10% fetal bovine serum. The viability, damage, and differentiation of Schwann cells were examined; gene expression was also analyzed. In the presence of 0.3 W/cm(2) pulsed ultrasound stimulation, increases in cell viability and decreases in cell apoptosis were observed in the serum deprivation group; in this culture condition, interleukin-1, tumor necrosis factor-alpha, and protein zero genes expression were downregulated and Desert Hedgehog transcripts gene expression was upregulated. We concluded that intervention with low intensity pulsed ultrasound could promote Schwann cell proliferation, prevent cell death, and keep adequate phenotype presentation for peripheral nerve recovery. The low intensity pulsed ultrasound stimulation to an injured nerve site could be applied as early as possible especially when the microenvironment is almost serum-free to obtain the most benefit.

Influence of depression symptoms on serum tumor necrosis factor-α of patients with chronic low back pain

Introduction

Patients with chronic low back pain (cLBP) have high rates of comorbid psychiatric disorders, mainly depression. Recent evidence suggests that depressive symptoms and pain, as interacting factors, have an effect on the circulating levels of inflammatory markers relevant to coronary artery disease. Our previous work showed a higher serum level of an inflammatory marker tumour necrosis factor-alpha (TNFα) in patients with cLBP, which did not correlate with intensity of low back pain alone. In the present study we investigated the cross-sectional associations of depressive symptoms, low back pain and their interaction with circulating levels of TNFα.

Methods

Each group of 29 patients with cLBP alone or with both cLBP and depression was age-matched and sex-matched with 29 healthy controls. All subjects underwent a blood draw for the assessment of serum TNFα and completed a standardised questionnaire regarding medication, depression scores according to the German version of Centre for Epidemiological Studies Depression Scale (CES-D), pain intensity from a visual analogue scale, and back function using the Roland and Morris questionnaire. The correlations between TNFα level and these clinical parameters were analysed.

Results

There were no differences in TNFα level between cLBP patients with and without depression. Both cLBP patients with (median = 2.51 pg/ml, P = 0.002) and without (median = 2.58 pg/ml, P = 0.004) depression showed significantly higher TNFα serum levels than healthy controls (median = 0 pg/ml). The pain intensity reported by both patient groups was similar, while the patients with depression had higher CES-D scores (P < 0.001) and worse back function (P < 0.001). The variance analysis showed that the interaction between TNFα level and pain intensity, CES-D scores, sex, body mass index and medication was statistically significant.

Conclusions

Depression as a comorbidity to cLBP did not influence the serum TNFα level. It seems that TNFα somehow acts as a mediator in both cLBP and depression, involving similar mechanisms that will be interesting to follow in further studies.

Increased interleukin-6 activity associated with painful chemotherapy-induced peripheral neuropathy in women after breast cancer treatment

Accumulating evidence suggests that neural-immune interactions are involved in the development of painful chemotherapy-induced peripheral neuropathy, particularly through the increased release of proinflammatory cytokines. The purpose of this study was used to evaluate levels of interleukin [IL]-6 and IL-6 receptors in women with breast cancer after the conclusion of chemotherapy who either had painful symptoms of chemotherapy-induced peripheral neuropathy (CIPN group, N = 20) or did not experience CIPN symptoms (Comparison group, N = 20). CIPN participants had significantly higher levels of IL-6 and soluble IL-6R (sIL-6R) compared to women without CIPN symptoms (P < .001 for both). In addition, soluble gp130, which blocks the IL-6/sIL-6R complex from binding to gp130 within the cellular membrane, was significantly lower (P < .01). Circulating concentrations of sIL-6R were inversely correlated with the density of IL-6R on the cell surface of monocytes in the total sample (r = −.614, P = .005). These findings suggest that IL-6 transsignaling may be an important biological mechanism associated with the persistence of painful CIPN symptoms, with potential implications for symptom management and research.

HIV-1 viral protein R causes peripheral nervous system injury associated with in vivo neuropathic pain

Painful peripheral neuropathy has become the principal neurological disorder in HIV/AIDS patients. Herein, we investigated the effects of a cytotoxic HIV-1 accessory protein, viral protein R (Vpr), on the peripheral nervous system (PNS). Host and viral gene expression was investigated in peripheral nerves from HIV-infected individuals and in HIV-infected human dorsal root ganglion (DRG) cultures by RT-PCR and immunocytochemistry. Cytosolic calcium ([Ca(2+)]) fluxes and neuronal membrane responses were analyzed in cultured DRGs. Neurobehavioral responses and cytokine levels were assessed in a transgenic mouse model in which the vpr transgene was expressed in an immunodeficient background (vpr/RAG1(-/-)). Vpr transcripts and proteins were detected in peripheral nerves and DRGs from HIV-infected patients. Exposure of rat or human cultured DRG neurons to Vpr rapidly increased [Ca(2+)] and action potential frequency while increasing input resistance. HIV infection of human DRG cultures caused neurite retraction (P<0.05), accompanied by induction of interferon-α (IFN-α) transcripts (P<0.05). vpr/RAG1(-/-) mice expressed Vpr together with increased IFN-α (P<0.05) in the PNS and also exhibited mechanical allodynia, unlike their vpr/RAG1(-/-) littermates (P<0.05). Herein, Vpr caused DRG neuronal damage, likely through cytosolic calcium activation and cytokine perturbation, highlighting Vpr's contribution to HIV-associated peripheral neuropathy and ensuing neuropathic pain.

TNF-alpha and neuropathic pain--a review

Tumor necrosis factor alpha (TNF-α) was discovered more than a century ago, and its known roles have extended from within the immune system to include a neuro-inflammatory domain in the nervous system. Neuropathic pain is a recognized type of pathological pain where nociceptive responses persist beyond the resolution of damage to the nerve or its surrounding tissue. Very often, neuropathic pain is disproportionately enhanced in intensity (hyperalgesia) or altered in modality (hyperpathia or allodynia) in relation to the stimuli. At time of this writing, there is as yet no common consensus about the etiology of neuropathic pain - possible mechanisms can be categorized into peripheral sensitization and central sensitization of the nervous system in response to the nociceptive stimuli. Animal models of neuropathic pain based on various types of nerve injuries (peripheral versus spinal nerve, ligation versus chronic constrictive injury) have persistently implicated a pivotal role for TNF-α at both peripheral and central levels of sensitization. Despite a lack of success in clinical trials of anti-TNF-α therapy in alleviating the sciatic type of neuropathic pain, the intricate link of TNF-α with other neuro-inflammatory signaling systems (e.g., chemokines and p38 MAPK) has indeed inspired a systems approach perspective for future drug development in treating neuropathic pain.

Immediate anti-tumor necrosis factor-alpha (etanercept) therapy enhances axonal regeneration after sciatic nerve crush

Peripheral nerve regeneration begins immediately after injury. Understanding the mechanisms by which early modulators of axonal degeneration regulate neurite outgrowth may affect the development of new strategies to promote nerve repair. Tumor necrosis factor-alpha (TNF-alpha) plays a crucial role in the initiation of degenerative cascades after peripheral nerve injury. Here we demonstrate using real-time Taqman quantitative RT-PCR that, during the time course (days 1-60) of sciatic nerve crush, TNF-alpha mRNA expression is induced at 1 day and returned to baseline at 5 days after injury in nerve and the corresponding dorsal root ganglia (DRG). Immediate therapy with the TNF-alpha antagonist etanercept (fusion protein of TNFRII and human IgG), administered systemically (i.p.) and locally (epineurially) after nerve crush injury, enhanced the rate of axonal regeneration, as determined by nerve pinch test and increased number of characteristic clusters of regenerating nerve fibers distal to nerve crush segments. These fibers were immunoreactive for growth associated protein-43 (GAP-43) and etanercept, detected by anti-human IgG immunofluorescence. Increased GAP-43 expression was found in the injured nerve and in the corresponding DRG and ventral spinal cord after systemic etanercept compared with vehicle treatments. This study established that immediate therapy with TNF-alpha antagonist supports axonal regeneration after peripheral nerve injury.

Immunological priming potentiates non-viral anti-inflammatory gene therapy treatment of neuropathic pain

We recently described a non-viral gene therapy paradigm offering long-term resolution of established neuropathic pain in several animal models. Here, the requirements for long-term therapeutic effects are described, and evidence is provided for a mechanism of action based on immunological priming of the intrathecal (i.t.) space. Long-term pain reversal was achieved when two i.t. injections of various naked plasmid DNA doses were separated by 5 h to 3 days. We show that an initial DNA injection, regardless of whether a transgene is included, leads to an accumulation of phagocytic innate immune cells. This accumulation coincides with the time in which subsequent DNA injection efficacy is potentiated. We show the ability of non-coding DNA to induce short-term pain reversal that is dependent on endogenous interleukin-10 (IL-10) signaling. Long-term efficacy requires the inclusion of an IL-10(F129S) transgene in the second injection. Blockade of IL-10, by a neutralizing antibody, either between the two injections or after the second injection induces therapeutic failure. These results show that this gene therapy paradigm uses an initial 'priming' injection of DNA to induce accumulation of phagocytic immune cells, allowing for potentiated efficacy of a subsequent 'therapeutic' DNA injection in a time- and dose-dependent manner.

Involvement of inflammatory mediators in neuropathic pain caused by vincristine

Elucidation of the mechanism of neuropathic pain caused by vincristine is required because long-term treatment with this anticancer agent often causes neuropathic pain. We refer to the involvement of inflammatory mediators in vincristine-induced neuropathic pain in this review. Several reports using rodents have shown that long-lasting neuropathic pain (mechanical allodynia) is caused by repeated systemic injection of vincristine. Vincristine damaged Schwann cells and DRG neurons in this model. Vincristine-induced macrophage infiltration in the peripheral nervous system (PNS) and macrophage-derived IL-6 elicited mechanical allodynia. These findings proved that inhibition of IL-6 function prevented neuropathic pain caused by vincristine. In the central nervous system (CNS), activation of microglia and astrocytes in the spinal cord were demonstrated after long-term vincristine treatment. TNF-alpha was upregulated in activated microglia and astrocytes, and inhibition of TNF-alpha function attenuated neuropathic pain caused by vincristine. These results suggest that vincristine induces macrophage infiltration to the damaged PNS, and that macrophage-derived inflammatory cytokines such as IL-6 elicits neuroinflammation. Signal transduction of pain from the PNS to the CNS activates microglia and astrocytes, and these activated glial cells release inflammatory cytokines such as TNF-alpha. In the CNS, these inflammatory cytokines have an important role in the neuropathic pain caused by vincristine. Immune-modulating agents that prevent activation of immune cells and/or the inhibitory agents of inflammatory cytokines could prevent the neuropathic pain caused by vincristine. These agents could increase the tolerability of vincristine when used for the treatment of leukemia and lymphoma.

Detection of elevatedNε‐carboxymethyllysine levels in muscular tissue and in serum of patients with fibromyalgia

OBJECTIVES

To compare levels of the advanced glycation end product (AGE) N(epsilon)-carboxymethyllysine (CML) present in the muscle tissue and in the serum of patients with fibromyalgia (FM) vs. healthy controls.

METHODS

The serum levels of CML were measured in 41 patients with FM and 81 healthy controls. The presence of CML, nuclear factor kappa B (NF-kappaB), the AGE receptor (RAGE), collagen types I, II, VI, and CD68-positive monocytes/macrophages in muscle tissue of 14 patients with FM was investigated by immunohistochemistry.

RESULTS

Patients with FM showed significantly increased serum levels of CML in comparison to healthy controls. The immunohistochemical investigation revealed a stronger staining for CML and NF-kappaB and more CD68-positive monocytes/macrophages in the muscle of FM patients. The collagens and CML were co-localized, suggesting that the AGE modifications were related to collagen. RAGE was absent in controls but a faint and patchy staining was seen in FM.

CONCLUSIONS

In the interstitial connective tissue of fibromyalgic muscles we found a more intensive staining of the AGE CML, activated NF-kappaB, and also higher CML levels in the serum of these patients compared to the controls. RAGE was only present in FM muscle. AGE modification of proteins causes reduced solubility and high resistance to proteolytic digestion of the altered proteins (e.g. AGE-modified collagens). AGEs can stimulate different types of cells by activation of the transcription factor NF-kappaB, mediated by specific receptors of AGEs (e.g. RAGE) on the cell surface. Both mechanisms may contribute to the development, perpetuation, and spreading of pain characteristic in FM patients.

Effects of recombinant human erythropoietin on neuropathic pain and cerebral expressions of cytokines and nuclear factor-kappa B

PURPOSE

The effect of recombinant human erythropoietin (rhEPO) on neuropathic pain remains unclear. This study aimed to determine the effects of preemptive administration of rhEPO on the behavioural changes and neuroinflammatory responses in a rat model of neuropathic pain.

METHODS

Fifty rats were randomly allocated into five groups, sham-operation treated with saline and L5 spinal nerve transection treated with different doses of rhEPO (0 [saline], 1000, 3000, or 5000 U x kg(-1), respectively). The rats were intraperitoneally treated from 1 day before surgery to post-surgery day 7. The mechanical (paw pressure thresholds, PPT) and thermal thresholds (paw withdrawal latencies, PWL) were measured on post-surgery days 1, 3, and 7. The contralateral brain was obtained on post-surgery day 7 to determine the expressions of tumour necrosis factor (TNF-alpha), interleukin (IL)-1beta, IL-6, L-10, and nuclear factor-kappa B (NF-kappaB) activity.

RESULTS

There were significant decreases in PPT and PWL after L5 spinal nerve transection (P < 0.001). Compared with the saline group, the rhEPO 3000 and 5000 U x kg(-1) groups resulted in significant increases in PPT and PWL (P < 0.001) and reduced the cerebral expressions of TNF-alpha, IL-1beta, IL-6, and NF-kappaB activity associated with the increase in IL-10 (rhEPO3000 group, P < 0.05, and rhEPO5000 group, P < 0.001, respectively). Administration of rhEPO 1000 U x kg(-1) had no significant effects on these variables.

CONCLUSIONS

Preemptive rhEPO dose-dependently attenuated the mechanical and thermal hyperalgesia in L5 spinal nerve transection rats, which correlated with the decreased cerebral expressions of TNF-alpha, IL-1beta, and IL-6 via downregulating NF-kappaB activity and the increased expression of IL-10.

Experimental models of peripheral neuropathic pain based on traumatic nerve injuries - an anatomical perspective

Peripheral neuropathic pain (PNP) frequently occurs as a consequence of nerve injury and may differ depending upon the type of insult and the individual patient. Progress in our knowledge of PNP induction mechanisms depends upon the utilization of appropriate experimental models in rodents based on various types of peripheral nerve lesions. In this review, we draw attention to current knowledge on basic cellular and molecular events in various experimental models used to induce the PNP symptoms. Spontaneous ectopic activity of axotomized and non-axotomized primary sensory neurons, the bodies of which are located in the dorsal root ganglion (DRG), seems to be a key mechanism of PNP induction. The primary sensory neurons are directly affected by nerve injury or indirectly by activated satellite glial cells and adjoining immune cells that release a variety of molecules changing the microenvironment of the neurons. Recently, it has become clear that molecules produced during Wallerian degeneration play an important role not only in axon-promoting conditions distal to nerve injury but also in initiation of neuropathic pain. The molecules, transported by the blood, influence afferent neurons and their axons not only in DRG associated, but also those not directly associated with the injured nerve (i.e., in the contralateral DRG or at different spinal segments). Generally, all experimental PNP models based on a partial injury of peripheral nerve segments contain mechanisms initiated by signal molecules of Wallerian degeneration.

Mode of action of cytokines on nociceptive neurons

Cytokines are pluripotent soluble proteins secreted by immune and glial cells and are key elements in the induction and maintenance of pain. They are categorized as pro-inflammatory cytokines, which are mostly algesic, and anti-inflammatory cytokines, which have analgesic properties. Progress has been made in understanding the mechanisms underlying the action of cytokines in pain. To date, several direct and indirect pathways are known that link cytokines with nociception or hyperalgesia. Cytokines may act via specific cytokine receptors inducing downstream signal transduction cascades, which then modulate the function of other receptors like the ionotropic glutamate receptor, the transient vanilloid receptors, or sodium channels. This receptor activation, either through amplification of the inflammatory reaction, or through direct modulation of ion channel currents, then results in pain sensation. Following up on results from animal experiments, cytokine profiles have recently been investigated in human pain states. An imbalance of pro- and anti-inflammatory cytokine expression may be of importance for individual pain susceptibility. Individual cytokine profiles may be of diagnostic importance in chronic pain states, and, in the future, might guide the choice of treatment.

[Cytokine regulation and pain. Results of experimental and clinical research]

Cytokines are soluble peptides and proteins that are predominantly produced and secreted by immune cells. In numerous animal experiments the endogenous increase or application of exogenous pro-inflammatory cytokines is associated with pain behavior. In turn, cytokine inhibitors reduce such pain behavior in inflammatory and neuropathic pain models. Several clinical studies point out that cytokines are also important in different human pain states. Several chronic pain syndromes are associated with systemic pro-inflammatory cytokine profiles. In some pain syndromes these pro-inflammatory profiles are accompanied by a lack of analgesic anti-inflammatory cytokines. Numerous case reports and open clinical studies, but also some controlled trials show successful analgesic treatment using cytokine inhibitors. The following review article summarizes the main data of animal experiments and clinical trials concerning the role of cytokines in pain and the potential importance of cytokine modulation in pain treatment.

Regulation of the trigeminal NR1 subunit expression induced by inflammation of the temporomandibular joint region in rats

Expression of the N-methyl-d-aspartate (NMDA) receptor in trigeminal nuclei has been shown to play a role in the mechanisms of trigeminal pain. Here, we examined the hypothesis that the upregulation of the NR1 subunit of the NMDA receptor (NR1) in the trigeminal subnucleus caudalis (Sp5c) following inflammation of the temporomandibular joint (TMJ) region would be regulated by interleukin-6 (IL-6) and the nuclear factor-kappa B (NF-kappaB). Inflammation of a unilateral TMJ region was produced in rats by injecting 50mul of complete Freund's adjuvant (CFA) into a TMJ and adjacent tissues, which resulted in persistent pain behavior as assessed using algometer before (baseline) and on days 1, 3, and 7 after the CFA injection. The CFA injection also induced a significant upregulation of NR1 and NF-kappaB on days 3 and 7, and of IL-6 on days 1, 3, and 7, within the ipsilateral Sp5c, as compared with the sham TMJ injection group. Once daily intracisternal injection of an IL-6 antiserum or NF-kappaB inhibitor (PDTC) for 6 days, beginning on day 1 immediately after the CFA injection, prevented both the upregulation of NR1 in the ipsilateral Sp5C and pain behavior. Moreover, once daily intracisternal IL-6 administration for 6 days in naïve rats induced the NR1 upregulation and pain behavior similar to that after TMJ inflammation. These results indicate that the upregulation of IL-6 and NF-kappaB after inflammation of the unilateral TMJ region is a critical regulatory mechanism for the expression of NR1 in the ipsilateral Sp5c, which contributed to the development of TMJ pain behavior in rats.

Role of IL-15 in spinal cord and sciatic nerve after chronic constriction injury: regulation of macrophage and T-cell infiltration

The release of inflammatory mediators from immune and glial cells either in the peripheral or CNS may have an important role in the development of physiopathological processes such as neuropathic pain. Microglial, then astrocytic activation in the spinal cord, lead to chronic inflammation, alteration of neuronal physiology and neuropathic pain. Standard experimental models of neuropathic pain include an important peripheral inflammatory component, which involves prominent immune cell activation and infiltration. Among potential immunomodulators, the T-cell cytokine interleukin-15 (IL-15) has a key role in regulating immune cell activation and glial reactivity after CNS injury. Here we show, using the model of chronic constriction of the sciatic nerve (CCI), that IL-15 is essential for the development of the early inflammatory events in the spinal cord after a peripheral lesion that generates neuropathic pain. IL-15 expression in the spinal cord was identified in both astroglial and microglial cells and was present during the initial gliotic and inflammatory (NFkappaB) response to injury. The expression of IL-15 was also identified as a cue for macrophage and T-cell activation and infiltration in the sciatic nerve, as shown by intraneural injection of the cytokine and activity blockage approaches. We conclude that the regulation of IL-15 and hence the initial events following its expression after peripheral nerve injury could have a future therapeutic potential in the reduction of neuroinflammation.

The effects of thalidomide and minocycline on taxol-induced hyperalgesia in rats

Chemotherapy-induced pain is the most common treatment-limiting complication encountered by cancer patients receiving taxane-, vinca alkaloid- or platin-based chemotherapy. Several lines of evidence indicate that activation of pro-inflammatory cascades involving the release of cytokines including tumor necrosis factor-alpha (TNF-alpha), interleukin-1 beta (IL-1beta) and interleukin-6 (IL-6) as well as various growth factors are key events in the pathogenesis of many types of nerve-injury related pain. Similar mechanisms might also be involved in the etiology of chemotherapy-induced pain. Thalidomide and minocycline have profound immunomodulatory actions in addition to their originally intended pharmacological actions. These compounds were evaluated here for effects in preventing the development of taxol-induced mechanical and thermal hyperalgesia in rats. Thalidomide (50.0 mg/kg) reduced taxol-induced mechanical allodynia and hyperalgesia whereas minocycline (20.0 mg/kg) reduced taxol-induced mechanical hyperalgesia and allodynia as well as taxol-induced thermal hyperalgesia. These results suggest that immunomodulatory agents may provide a treatment option for the protection or reversal of chemotherapy-related pain.

Interleukin-10 reduces scarring and enhances regeneration at a site of sciatic nerve repair

Axonal regeneration at a site of peripheral nerve repair can be impeded by the formation of scar tissue, which creates a mechanical barrier and initiates the development of multiple branched axonal sprouts that form a neuroma. We have investigated the hypothesis that the application of a scar-reducing agent to the nerve repair site would permit better axonal regeneration. In anaesthetised C57 Black-6 mice, the left sciatic nerve was sectioned and immediately re-approximated using four epineurial sutures. In five groups of eight mice, we injected transforming growth factor-beta3 (50 or 500 ng), interleukin-10 (IL-10) (125 or 500 ng), or saline into and around the repair site, both before and after the nerve section. Another group of eight animals acted as sham-operated controls. After 6 weeks, the outcome was assessed by recording compound action potentials (CAPs), measuring collagen levels using picrosirius red staining, and counting the number of myelinated axons proximal and distal to the repair. CAPs evoked by electrical stimulation distal to the repair were significantly smaller in all repair groups except for the low-dose IL-10 group, where they were not significantly different from that in controls. The area of staining for collagen had significantly increased in all repair groups except for the low-dose IL-10 group, which was not significantly different from that in controls. The myelinated fibre counts were always higher distal to the repair site, but there were no significant differences between groups. We conclude that administration of a low-dose of IL-10 to a site of sciatic nerve repair reduces scar formation and permits better regeneration of the damaged axons.

Cytokine regulation in animal models of neuropathic pain and in human diseases

Cytokines are soluble proteins secreted mainly by immune cells and are key players in the induction and maintenance of pain. Pro-inflammatory cytokines are mostly algesic, while anti-inflammatory cytokines have analgesic properties. After the role of cytokines was shown in diverse animal models of pain, interest arose in the systemic and local regulation of cytokines in human pain states. Most clinical studies give evidence for an imbalance between pro- and anti-inflammatory cytokines in neuropathic and other pain states with pronounced pro-inflammatory cytokine profiles. Anti-cytokine treatment gives encouraging preliminary results and supports the notion of a crucial role of cytokines also in human pain states. Further research is needed for a better understanding of the mechanisms linking altered cytokine profiles to the sensation of pain.

Anticytokine therapy in neuropathic pain management

Cytokine activation or dysregulation is implied in a variety of painful disease states. Numerous experimental studies provide evidence that proinflammatory cytokines induce or facilitate neuropathic pain. Cytokine levels are rapidly and markedly upregulated in the peripheral nerves, dorsal root ganglia, spinal cord and in particular regions of the brain, after peripheral nerve injuries. Direct receptor-mediated actions on afferent nerve fibers as well as cytokine effects involving further mediators have been reported. Whereas direct application of exogenous proinflammatory cytokines induces pain, blockade of these cytokines or application of anti-inflammatory cytokines reduces pain behavior in most experimental paradigms. Cytokine measurements may identify patients at risk of developing chronic pain associated with their neuropathic conditions, as in the examples of peripheral neuropathies and postherpetic neuralgia. Anticytokine agents currently on the market are effective for the treatment of mostly inflammatory pain conditions, and are starting to be introduced for neuropathic pain states; however, their use is limited by potential life-threatening complications. Owing to the pleiotropy and redundancy of the cytokine system, the successful approach may not be inhibition of one particular cytokine but strategies shifting the balance between pro- and anti-inflammatory cytokines in properly selected patients. Agents that specifically target downstream signaling molecules may provide hope for safer and more specific therapies.

"Listening" and "talking" to neurons: implications of immune activation for pain control and increasing the efficacy of opioids

It is recently become clear that activated immune cells and immune-like glial cells can dramatically alter neuronal function. By increasing neuronal excitability, these non-neuronal cells are now implicated in the creation and maintenance of pathological pain, such as occurs in response to peripheral nerve injury. Such effects are exerted at multiple sites along the pain pathway, including at peripheral nerves, dorsal root ganglia, and spinal cord. In addition, activated glial cells are now recognized as disrupting the pain suppressive effects of opioid drugs and contributing to opioid tolerance and opioid dependence/withdrawal. While this review focuses on regulation of pain and opioid actions, such immune-neuronal interactions are broad in their implications. Such changes in neuronal function would be expected to occur wherever immune-derived substances come in close contact with neurons.

Pathophysiology of peripheral neuropathic pain: immune cells and molecules

Damage to the peripheral nervous system often leads to chronic neuropathic pain characterized by spontaneous pain and an exaggerated response to painful and/or innocuous stimuli. This pain condition is extremely debilitating and usually difficult to treat. Although inflammatory and neuropathic pain syndromes are often considered distinct entities, emerging evidence belies this strict dichotomy. Inflammation is a well-characterized phenomenon, which involves a cascade of different immune cell types, such as mast cells, neutrophils, macrophages, and T lymphocytes. In addition, these cells release numerous compounds that contribute to pain. Recent evidence suggests that immune cells play a role in neuropathic pain in the periphery. In this review we identify the different immune cell types that contribute to neuropathic pain in the periphery and release factors that are crucial in this particular condition.

Interleukin-6 mediates low-threshold mechanical allodynia induced by intrathecal HIV-1 envelope glycoprotein gp120

Spinal cord glia (microglia and astrocytes) contribute to enhanced pain states. One model that has been used to study this phenomenon is intrathecal (i.t.) administration of gp120, an envelope glycoprotein of HIV-1 known to activate spinal cord glia and thereby induce low-threshold mechanical allodynia, a pain symptom where normally innocuous (non-painful) stimuli are perceived as painful. Previous studies have shown that i.t. gp120-induced allodynia is mediated via the release of the glial pro-inflammatory cytokines, tumor necrosis factor-alpha (TNF), and interleukin-1beta (IL-1). As we have recently reported that i.t. gp120 induces the release of interleukin-6 (IL-6), in addition to IL-1 and TNF, the present study tested whether this IL-6 release in spinal cord contributes to gp120-induced mechanical allodynia and/or to gp120-induced increases in TNF and IL-1. An i.t. anti-rat IL-6 neutralizing antibody was used to block IL-6 actions upon its release by i.t. gp120. This IL-6 blockade abolished gp120-induced mechanical allodynia. While the literature predominantly documents the cascade of pro-inflammatory cytokines as beginning with TNF, followed by the stimulation of IL-1, and finally TNF plus IL-1 stimulating the release of IL-6, the present findings indicate that a blockade of IL-6 inhibits the gp120-induced elevations of TNF, IL-1, and IL-6 mRNA in dorsal spinal cord, elevation of IL-1 protein in lumbar dorsal spinal cord, and TNF and IL-1 protein release into the surrounding lumbosacral cerebrospinal fluid. These results would suggest that IL-6 induces pain facilitation, and may do so in part by stimulating the production and release of other pro-inflammatory cytokines.

Targeting invading macrophage-derived PGE2, IL-6 and calcitonin gene-related peptide in injured nerve to treat neuropathic pain

Immune and inflammatory responses occurring in an injured nerve have been generally believed to contribute to the generation and maintenance of neuropathic pain. In this review, the authors demonstrate the upregulation of COX-2/prostaglandin E2, IL-6 and calcitonin gene-related peptide in invading macrophages and discuss possible mechanisms involved in their upregulation and how they contribute to the maintenance of neuropathic pain. By acting on nociceptors in dorsal root ganglion and local inflammatory cells via autocrine or paracrine pathways, these inflammatory mediators facilitate spontaneous ectopic activity and sustain nociceptive responses, an important mechanism underlying both ongoing and evoked neuropathic pain state. Targeting these mediators in injured nerve may provide novel therapeutic avenues to more successfully treat nerve injury-associated neuropathic pain.

Pentoxifylline attenuates the development of hyperalgesia in a rat model of neuropathic pain

Pentoxifylline, a non-specific cytokine inhibitor, has shown to be beneficial in inflammatory pain in both experimental and clinical studies. The present study demonstrates for the first time, to our knowledge, the antihyperalgesic effect of pentoxifylline in the neuropathic pain using L5 spinal nerve transection rat model. In a preventive paradigm, pentoxifylline (12.5, 25, 50, or 100mg/kg intraperitoneally) was administered systemically daily, beginning 1h prior to nerve transection. Pentoxifylline (50, or 100mg/kgi.p.) produced significant decrease in the mechanical and thermal hyperalgesia. However, pentoxifylline (100mg/kgi.p.) did not influence the paw pressure thresholds and paw withdrawal latency in sham-operated rats. In order to understand the possible antinocicieptive effect of pentoxifylline in neuropathic pain, we examined the level of TNFalpha, IL-1beta, IL-6 and IL-10 protein in the contralateral brain on day 7 post-transection. Pentoxifylline administration resulted in a dose-dependent reduction of the production of proinflammatory cytokines like TNFalpha, IL-1beta and IL-6, and enhancement of IL-10. Furthermore, we investigated the activity of nuclear factor kappa B (NF-kappaB) in the contralateral brain on days 7 after surgery. In accordance with the change of proinflammatory cytokines, Pentoxifylline (50 or 100mg/kg) significantly inhibited the activation of NF-kappaB in the brain. This research supports a growing body of literature emphasizing the importance of neuroinflammation and neuroimmune activation in the development of neuropathic pain states, and the potential preventive value of pentoxifylline in the treatment of neuropathic pain.

Mechanisms of neuropathic pain

Neuropathic pain refers to pain that originates from pathology of the nervous system. Diabetes, infection (herpes zoster), nerve compression, nerve trauma, "channelopathies," and autoimmune disease are examples of diseases that may cause neuropathic pain. The development of both animal models and newer pharmacological strategies has led to an explosion of interest in the underlying mechanisms. Neuropathic pain reflects both peripheral and central sensitization mechanisms. Abnormal signals arise not only from injured axons but also from the intact nociceptors that share the innervation territory of the injured nerve. This review focuses on how both human studies and animal models are helping to elucidate the mechanisms underlying these surprisingly common disorders. The rapid gain in knowledge about abnormal signaling promises breakthroughs in the treatment of these often debilitating disorders.

Neurotrophin-3 attenuates galanin expression in the chronic constriction injury model of neuropathic pain

We have recently shown that exogenous neurotrophin-3 (NT-3) acts antagonistically to nerve growth factor (NGF) in regulation of nociceptor phenotype in intact neurons and suppresses thermal hyperalgesia and expression of molecules complicit in this behavioral response induced by chronic constriction injury (CCI) of the sciatic nerve. The present study examines whether there is a global influence of NT-3 in mitigating alterations in peptide and NGF receptor expression; molecules believed to also contribute to CCI-associated pain. Thus, the influence of NT-3 on phenotypic changes in dorsal root ganglion (DRG) neurons in rats coincident with CCI was examined using in situ hybridization. Seven days following injury, the incidence of expression of the neuropeptides galanin and pituitary adenylate cyclase-activating polypeptide (PACAP) was increased in L5 sensory neurons ipsilateral to the injury from 12% to 60% and 16% to 37% respectively, in addition to an increased level of expression. In contrast, there was no consistent significant change in tropomyosin-related kinase A (trkA) expression following CCI. Intrathecal infusion of NT-3 globally mitigated both the increased incidence and elevated levels of galanin messenger RNA (mRNA) expression observed following CCI, reducing the former from 60% to 39%. NT-3 infusion resulted in a limited reduction in the incidence and level of neuronal PACAP in medium to large size, but not small size, DRG neurons. NT-3 had no significant net effect on CCI-induced alterations in trkA mRNA expression.

Increased calcitonin gene-related peptide in neuroma and invading macrophages is involved in the up-regulation of interleukin-6 and thermal hyperalgesia in a rat model of mononeuropathy

The pain related peptide, calcitonin gene-related peptide (CGRP), plays an important role in inflammatory pain and immune responses. However, its role in neuropathic pain is not established. Following nerve injury, CGRP and pro-inflammatory interleukin-6 (IL-6) are increased in injured nerves. The aim of this study was to determine if CGRP in injured nerves is involved in the up-regulation of IL-6 and in the maintenance of neuropathic pain. Perineural injection of a neutralizing IL-6 antiserum or CGRP receptor antagonists (CGRP8-37 and BIBN4096BS) effectively attenuated thermal hyperalgesia 4 weeks after partial sciatic nerve ligation. Perineural CGRP antagonists also dramatically reduced IL-6 level in injured nerves. CGRP release from injured sites was dramatically increased and CGRP immunoreactivity was localized in both neuroma and invading macrophages. CGRP receptor markers (CRLR and RAMP1) were expressed in invading macrophages. Both CGRP antagonists significantly reduced IL-6 release from injured nerve explants. In cell cultures derived from injured nerves, CGRP concentration-dependently increased IL-6 release, an effect also blocked by CGRP antagonists. Taken together, these data show that increased levels of CGRP in injured neuroma and invading macrophages are involved in the up-regulation of IL-6 in macrophages as well as in the maintenance of neuropathic pain.

Long-lasting neuropathic pain induced by brachial plexus injury in mice: Role triggered by the pro-inflammatory cytokine, tumour necrosis factor α

Brachial plexus avulsion (BPA) resulted in a marked and long-lasting mechanical hypernociception (up to 80 days) in comparison to a sham-operated group, as assessed by Von Frey filaments, in both Swiss and C57/BL6 mice. In the tail-flick test, both Swiss and C57/BL6 mice submitted to BPA showed a significant thermal hypernociception, which persisted for 10 days. Both mechanical and thermal hypernociception following BPA were abolished in tumour necrosis factor alpha (TNFalpha) p55 receptor knockout mice. Moreover, the mechanical hypernociception caused by BPA was inhibited by the local application of the anti-TNFalpha (10 and 100 ng/site) antibody at the time of the surgery or by the intravenous administration (100 microg/kg) of this antibody at the time of the surgery or 4 days after the BPA. A similar inhibition of the mechanical hypernociception was observed when treating mice with the TNFalpha synthesis inhibitor thalidomide (50 mg/kg, s.c.), either at the time of the surgery or 4 days after. The results suggest that the persistent thermal, and especially the persistent mechanical, hypernociception observed following BPA in mice is largely dependent on the generation of TNFalpha. Based on these results, it is possible to suggest that therapeutic strategies for blocking TNFalpha could represent a valuable approach for the treatment of persistent neuropathic pain.

Reduced nerve injury-induced neuropathic pain in kinin B1 receptor knock-out mice

Injury to peripheral nerves often results in a persistent neuropathic pain condition that is characterized by spontaneous pain, allodynia, and hyperalgesia. Nerve injury is accompanied by a local inflammatory reaction in which nerve-associated and immune cells release several pronociceptive mediators. Kinin B1 receptors are rarely expressed in nontraumatized tissues, but they can be expressed after tissue injury. Because B1 receptors mediate chronic inflammatory painful processes, we studied their participation in neuropathic pain using receptor gene-deleted mice. In the absence of neuropathy, we found no difference in the paw-withdrawal responses to thermal or mechanical stimulation between B1 receptor knock-out mice and 129/J wild-type mice. Partial ligation of the sciatic nerve in the wild-type mouse produced a profound and long-lasting decrease in thermal and mechanical thresholds in the paw ipsilateral to nerve lesion. Threshold changed neither in the sham-operated animals nor in the paw contralateral to lesion. Ablation of the gene for the B1 receptor resulted in a significant reduction in early stages of mechanical allodynia and thermal hyperalgesia. Furthermore, systemic treatment with the B1 selective receptor antagonist des-Arg9-[Leu8]-bradykinin reduced the established mechanical allodynia observed 7-28 d after nerve lesion in wild-type mice. Partial sciatic nerve ligation induced an upregulation in B1 receptor mRNA in ipsilateral paw, sciatic nerve, and spinal cord of wild-type mice. Together, kinin B1 receptor activation seems to be essential to neuropathic pain development, suggesting that an oral-selective B1 receptor antagonist might have therapeutic potential in the management of chronic pain.

Genetic impairment of interleukin-1 signaling attenuates neuropathic pain, autotomy, and spontaneous ectopic neuronal activity, following nerve injury in mice

Peripheral nerve injury may lead to neuropathic pain, which is often associated with mechanical and thermal allodynia, ectopic discharge of from injured nerves and from the dorsal root ganglion neurons, and elevated levels of proinflammatory cytokines, particularly interleukin-1 (IL-1). In the present study, we tested the role of IL-1 in neuropathic pain models using two mouse strains impaired in IL-1 signaling: Deletion of the IL-1 receptor type I (IL-1rKO) and transgenic over-expression of the IL-1 receptor antagonist (IL-1raTG). Neuropathy was induced by cutting the L5 spinal nerve on one side, following which mechanical and thermal pain sensitivity was measured. Wild-type (WT) mice and the parent strains developed significant allodynia and hyperalgesia in the hind-paw ipsilateral to the injury compared with the contralateral hind-paw. The mutant strains, however, did not display decreased pain threshold in either hind-paw. Pain behavior was also assessed by cutting the sciatic and saphenous nerves and measuring autotomy scores. WT mice developed progressive autotomy, beginning at 7 days post-injury, whereas the mutant strains displayed delayed onset of autotomy and markedly reduced severity of the autotomy score. Electrophysiological assessment revealed that in WT mice a significant proportion of the dorsal root axons exhibited spontaneous ectopic activity at 1, 3, and 7 days following spinal nerve injury, whereas in IL-1rKO and IL-1raTG mice only a minimal number of axons exhibited such activity. Taken together, these results suggest that IL-1 signaling plays an important role in neuropathic pain and in the altered neuronal activity that underlies its development.

Effect of betamethasone on neuropathic pain and cerebral expression of NF-kappaB and cytokines

Glucocorticoids have been used to treat neuropathic pain for many years, but the underlying mechanisms are still unknown. Recent studies indicate that pathological pain states may be mediated by cytokines. We, therefore, examined the effect of betamethasone on neuropathic pain and the relationship between pain behavior and the expression of cytokines in the brain. Rats were given epidural injections of betamethasone (Diprospan) after L5 spinal nerve transection. Mechanical allodynia and thermal hyperalgesia were evaluated on post-operative days 1, 3, 7, 14 and 21 with von Frey and Hargreaves tests. Cerebral expression of NF-kappaB, TNFalpha, IL-1beta and IL-10 was quantified using electrophoretic mobility shift assay (EMSA) or enzyme-linked immunosorbent assay (ELISA). We found that spinal nerve injury caused long-lasting mechanical and thermal hyperalgesia in the hind paw and stimulated the expression of NF-kappaB, TNFalpha, IL-1beta and IL-10 in the brain. A single epidural injection of betamethasone at the time of nerve injury partially inhibited the development of neuropathic hyperalgesia and reduced the subsequent elevated levels of pro-inflammatory cytokines in the brain, while stimulating the expression of the anti-inflammatory cytokine IL-10. These data support our hypothesis that pro-inflammatory cytokines in the brain may mediate the hyperalgesic effects of spinal nerve injury and that the long-acting anti-hyperalgesic effects of epidural glucocorticoid treatment are due to an inhibitory effect on pro-inflammatory cytokine levels and a stimulatory effect on anti-inflammatory cytokine levels in the brain.

Immune and inflammatory mechanisms in neuropathic pain

Tissue damage, inflammation or injury of the nervous system may result in chronic neuropathic pain characterised by increased sensitivity to painful stimuli (hyperalgesia), the perception of innocuous stimuli as painful (allodynia) and spontaneous pain. Neuropathic pain has been described in about 1% of the US population, is often severely debilitating and largely resistant to treatment. Animal models of peripheral neuropathic pain are now available in which the mechanisms underlying hyperalgesia and allodynia due to nerve injury or nerve inflammation can be analysed. Recently, it has become clear that inflammatory and immune mechanisms both in the periphery and the central nervous system play an important role in neuropathic pain. Infiltration of inflammatory cells, as well as activation of resident immune cells in response to nervous system damage, leads to subsequent production and secretion of various inflammatory mediators. These mediators promote neuroimmune activation and can sensitise primary afferent neurones and contribute to pain hypersensitivity. Inflammatory cells such as mast cells, neutrophils, macrophages and T lymphocytes have all been implicated, as have immune-like glial cells such as microglia and astrocytes. In addition, the immune response plays an important role in demyelinating neuropathies such as multiple sclerosis (MS), in which pain is a common symptom, and an animal model of MS-related pain has recently been demonstrated. Here, we will briefly review some of the milestones in research that have led to an increased awareness of the contribution of immune and inflammatory systems to neuropathic pain and then review in more detail the role of immune cells and inflammatory mediators.

Functional polymorphisms of UCP2 and UCP3 are associated with a reduced prevalence of diabetic neuropathy in patients with type 1 diabetes

OBJECTIVE

We studied the association between polymorphisms in the UCP genes and diabetes complications in patients with type 1 diabetes.

RESEARCH DESIGN AND METHODS

We analyzed 227 patients with type 1 diabetes using PCR and subsequent cleavage by restriction endonucleases for the promoter variants A-3826G in the UCP1 gene, G-866A in the UCP2 gene, and C-55T in the UCP3 gene.

RESULTS

No effect of the A-3826G polymorphism in the UCP1 gene on diabetes complications was found. Patients who were heterozygous or homozygous for the G-866A polymorphism in the UCP2 gene or the C-55T polymorphism in the UCP3 gene had a significantly reduced prevalence of diabetic neuropathy (UCP2: odds ratio 0.44 [95% CI 0.24-0.79], P = 0.007; UCP3: 0.48 [0.25-0.92], P = 0.031), whereas there was no association with other diabetes complications. This effect was stronger when G-866A and C-55T occurred in a cosegregatory manner (UCP2 and UCP3: 0.28 [0.12-0.65], P = 0.002). Furthermore, a multiple logistic regression model showed an age- and diabetes duration-independent effect of the cosegregated polymorphisms on the prevalence of diabetic neuropathy (P = 0.013).

CONCLUSIONS

Our data indicate that both the G-866A polymorphism in the UCP2 gene and the C-55T polymorphism in the UCP3 gene are associated with a reduced risk of diabetic neuropathy in type 1 diabetes. Thus, the results presented here support the hypothesis that higher expression of uncoupling protein might prevent mitochondria-mediated neuronal injury and, ultimately, diabetic neuropathy.

Increase in inflammatory cytokines in median nerves in a rat model of repetitive motion injury

We examined cytokines in rat median nerves following performance of a high repetition reaching and grasping task at a rate of 8 reaches/min for up to 8 weeks. IL-1alpha, IL-1beta, TNF-alpha, IL-6 and IL-10 were analyzed by immunohistochemistry. Double-labeling immunohistochemistry for ED1, a marker of phagocytic macrophages, was also performed. We found increased immunoexpression of IL-6 by week 3, increases in all 5 cytokines by week 5. This response was transient as all cytokines returned to control levels by 8 weeks of performance of a high repetition negligible force task. Cytokine sources included Schwann cells, fibroblasts and phagocytic macrophages (ED1-immunopositive). These findings suggest that cytokines are involved in the pathophysiology of repetitive motion injuries in peripheral nerves.

Cytokine responses during chronic denervation

Background

The aim of the present study was to examine inflammatory responses during Wallerian degeneration in rat peripheral nerve when the regrowth of axons was prevented by suturing.

Methods

Transected rat sciatic nerve was sutured and ligated to prevent reinnervation. The samples were collected from the left sciatic nerve distally and proximally from the point of transection. The endoneurium was separated from the surrounding epi- and perineurium to examine the expression of cytokines in both of these compartments. Macrophage invasion into endoneurium was investigated and Schwann cell proliferation was followed as well as the expression of cytokines IL-1β, IL-10, IFN-γ and TNF-α mRNA. The samples were collected from 1 day up to 5 weeks after the primary operation.

Results

At days 1 to 3 after injury in the epi-/perineurium of the proximal and distal stump, a marked expression of the pro-inflammatory cytokines TNF-α and IL-1β and of the anti-inflammatory cytokine IL-10 was observed. Concurrently, numerous macrophages started to gather into the epineurium of both proximal and distal stumps. At day 7 the number of macrophages decreased in the perineurium and increased markedly in the endoneurium of both stumps. At this time point marked expression of TNF-α and IFN-γ mRNA was observed in the endo- and epi-/perineurium of the proximal stump. At day 14 a marked increase in the expression of IL-1β could be noted in the proximal stump epi-/perineurium and in the distal stump endoneurium. At that time point many macrophages were observed in the longitudinally sectioned epineurium of the proximal 2 area as well as in the cross-section slides from the distal stump. At day 35 TNF-α, IL-1β and IL-10 mRNA appeared abundantly in the proximal epi-/perineurium together with macrophages.

Conclusion

The present studies show that even during chronic denervation there is a cyclic expression pattern for the studied cytokines. Contrary to the previous findings on reinnervating nerves the studied cytokines show increased expression up to 35 days. The high expressions of pro-inflammatory and anti-inflammatory cytokines in the proximal epi-/perineurial area at day 35 may be involved in the formation of fibrosis due to irreversible nerve injury and thus may have relevance to the formation of traumatic neuroma.

The fusion of bone-marrow-derived proinsulin-expressing cells with nerve cells underlies diabetic neuropathy

Diabetic neuropathy is the most common microvascular complication of diabetes. Here we show that, in streptozotocin-induced diabetic rodents with neuropathy, a subpopulation of bone-marrow-derived cells marked by proinsulin expression migrates to and fuses with neurons in the sciatic nerve and dorsal root ganglion (DRG), resulting in neuronal dysfunction and accelerated apoptosis. The absence or presence of proinsulin expression, which identifies the fusion cells, and not the disease state (nondiabetic vs. diabetic) of the rats from which the DRG neurons are isolated determines whether the DRG neurons show normal or abnormal calcium homeostasis and apoptosis. These results suggest that bone-marrow-derived cells may play an important role in the pathogenesis of diabetic complications.

Central glucocorticoid receptors modulate the expression and function of spinal NMDA receptors after peripheral nerve injury

Central glucocorticoid receptors (GRs) and NMDA receptors (NMDARs) have been shown to play a significant role in the mechanisms of neuropathic pain after peripheral nerve injury; however, how central GRs and NMDARs interact in this process remains unknown. Here we show that the expression and function of spinal NMDARs after peripheral nerve injury were modulated by central GRs. Chronic constriction nerve injury (CCI) in rats induced a time-dependent upregulation of NR1 and NR2 subunits of the NMDAR within the spinal cord dorsal horn ipsilateral to CCI. The upregulation of NMDARs was significantly diminished by intrathecal administration (twice daily for postoperative days 1-6) of either the GR antagonist RU38486 or an antisense oligonucleotide against GRs. Moreover, this CCI-induced expression of NMDARs was significantly attenuated in rats receiving intrathecal treatment with an interleukin-6 (IL-6) antiserum and in mice with protein kinase Cgamma (PKCgamma) knock-out. Because IL-6 and PKCgamma mediated the upregulation of central GRs after CCI as demonstrated previously, the results suggest that IL-6 and PKCgamma served as cellular mediators contributing to the GR-mediated expression of NMDARs after CCI. Functionally, nociceptive behaviors induced by NMDAR activation and CCI were reversed by a single intrathecal administration of the GR antagonist RU38486. Conversely, a single intrathecal injection with the noncompetitive NMDAR antagonist MK-801 reversed neuropathic pain behaviors exacerbated by the GR agonist dexamethasone in CCI rats. These data suggest that interactions between central GRs and NMDARs through genomic and nongenomic regulation may be an important mechanism critical to neuropathic pain behaviors in rats.

Current concepts in the therapy of the spondyloarthritides

Recent success in the treatment of patients with the more severe forms of spondyloarthritides (SpA) has dramatically changed old paradigms. There is evidence that anti-tumor necrosis factor (TNF)-alpha therapy is highly effective in SpA, especially in ankylosing spondylitis (AS) and psoriatic arthritis. Based on recent data on more than 1000 patients with AS and psoriatic arthritis, this treatment seems to be even more effective than in rheumatoid arthritis (RA). The currently available anti-TNFalpha agents, infliximab, etanercept, and adalimumab, are approved for the treatment of RA in the US and in Europe. TNFalpha blockers may even be considered as a first-line treatment in patients with active AS whose condition is not sufficiently controlled with NSAIDs, as in the case of axial disease. There is preliminary evidence that both agents also work in other SpA, such as undifferentiated SpA. There is hope that ankylosis may be preventable, but it remains to be shown whether patients benefit from long-term anti-TNFalpha therapy and whether radiologic progression and ankylosis can be stopped. Furthermore, it seems that anti-TNFalpha therapy can also improve clinical manifestations of other inflammatory spinal disorders, such as sciatica and back pain caused by disc herniation, or possibly even intermittent inflammatory states of degenerative disc disease. Severe adverse events from treatment with anti-TNFalpha continue to be rare. Tuberculosis can be largely prevented by appropriate screening. As it stands now, the benefits of anti-TNFalpha therapy in AS seem to outweigh the shortcomings.

Expression of central glucocorticoid receptors after peripheral nerve injury contributes to neuropathic pain behaviors in rats

Peripheral glucocorticoid receptors (GRs) play a significant role in the anti-inflammatory effects of glucocorticoids; however, the role of central GRs in nociceptive behaviors after peripheral nerve injury (neuropathic pain behaviors) remains unknown. Here we show that the development of neuropathic pain behaviors (thermal hyperalgesia and mechanical allodynia) induced by chronic constriction nerve injury (CCI) in rats was attenuated by either the GR antagonist RU38486 (4 = 2 > 1 = 0.5 microg) or a GR antisense oligonucleotide administered intrathecally twice daily for postoperative days 1-6. The development of thermal hyperalgesia and mechanical allodynia after CCI also was prevented in adrenalectomized rats, whereas the GR agonist dexamethasone (100 microg/kg) given subcutaneously twice daily for postoperative day 1-6 restored CCI-induced neuropathic pain behaviors in the adrenalectomized rats. Mechanistically, CCI induced a time-dependent and region-specific expression of neuronal GRs primarily within the spinal cord dorsal horn ipsilateral to nerve injury, which showed a time course parallel to that of the development of neuropathic pain behaviors. Moreover, the expression of neuronal GR after CCI was mediated in part through an elevated spinal level of interleukin-6 (IL-6) and protein kinase Cgamma (PKCgamma), because intrathecal treatment with an IL-6 antiserum, a PKC inhibitor (cheryrithrine), or PKCgamma knock-out substantially reduced the expression of neuronal GRs as well as neuropathic pain behaviors after CCI. These findings indicate a central role of neuronal GRs in the mechanisms of neuropathic pain behaviors in rats and suggest a potential role for GR antagonists in clinical management of neuropathic pain.

Fast modulation of heat-activated ionic current by proinflammatory interleukin 6 in rat sensory neurons

The pro-inflammatory cytokine interleukin-6 (IL-6) together with its soluble receptor (sIL-6R) induces and maintains thermal hyperalgesia. It facilitates the heat-induced release of calcitonin gene-related peptide from rat cutaneous nociceptors in vivo and in vitro. Here we report that exposure of nociceptive neurons to the IL-6-sIL-6R complex or the gp130-stimulating designer IL-6-sIL-6R fusion protein Hyper-IL-6 (HIL-6) resulted in a potentiation of heat-activated inward currents (I(heat)) and a shift of activation thresholds towards lower temperatures without affecting intracellular calcium levels. The Janus tyrosine kinase inhibitor AG490, the selective protein kinase C (PKC) inhibitor, bisindolylmaleimide 1 (BIM1), as well as rottlerin, a selective blocker of the PKCdelta isoform, but not the cyclooxygenase inhibitor indomethacin, effectively reduced the effect. Reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization revealed expression of mRNA for the signal-transducing beta subunit of the receptor gp130 in neuronal somata, rather than satellite cells in rat dorsal root ganglia. Together, the results suggest that IL-6-sIL-6R acts directly on sensory neurons. It increases their susceptibility to noxious heat via the gp130/Jak/PKCdelta signalling pathway.

Proteomic identification of brainstem cytosolic proteins in a neuropathic pain model

Neuropathic pain involves co-regulation of many genes and their translational products in both peripheral and central nervous system. We used proteomics approaches to investigate expressional changes in cytosolic protein levels in rat brainstem tissues following ligation of lumbar 5 and 6 (L5, L6) spinal nerves, which generates a model of peripheral neuropathic pain (NP). Proteins from brainstem tissue homogenates of NP and SHAM animals were fractionated by two-dimensional (2-DE) gel electrophoresis to produce a high-resolution map of the brainstem soluble proteins. Proteins showing altered expression levels between NP and SHAM were selected. Isolated proteins were in-gel trypsin-digested and the resulting peptides were analyzed by matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry. Using the mass spectrometric data, we were able to identify 17 proteins of interest through searches of the Swiss-Prot and NCBi nonredundant protein sequence database. Several of the identified proteins, including fatty acid binding protein-brain (FABP-B), major histocompatibility complex (MHC) class 1, T-cell receptor (TCR) alpha chain, and interleukin-1 (IL-1), showed significantly higher levels in the NP rat brainstem. Proteomic analysis has identified several proteins with differential expression levels in NP as compared to SHAM. However, the function of the proteins identified is postulated; therefore, further experiments are required to determine the true role of each protein in NP.

Loss of pain perception in diabetes is dependent on a receptor of the immunoglobulin superfamily

Molecular events that result in loss of pain perception are poorly understood in diabetic neuropathy. Our results show that the receptor for advanced glycation end products (RAGE), a receptor associated with sustained NF-kappaB activation in the diabetic microenvironment, has a central role in sensory neuronal dysfunction. In sural nerve biopsies, ligands of RAGE, the receptor itself, activated NF-kappaBp65, and IL-6 colocalized in the microvasculature of patients with diabetic neuropathy. Activation of NF-kappaB and NF-kappaB-dependent gene expression was upregulated in peripheral nerves of diabetic mice, induced by advanced glycation end products, and prevented by RAGE blockade. NF-kappaB activation was blunted in RAGE-null (RAGE(-/-)) mice compared with robust enhancement in strain-matched controls, even 6 months after diabetes induction. Loss of pain perception, indicative of long-standing diabetic neuropathy, was reversed in WT mice treated with soluble RAGE. Most importantly, loss of pain perception was largely prevented in RAGE(-/-) mice, although they were not protected from diabetes-induced loss of PGP9.5-positive plantar nerve fibers. These data demonstrate, for the first time to our knowledge, that the RAGE-NF-kappaB axis operates in diabetic neuropathy, by mediating functional sensory deficits, and that its inhibition may provide new therapeutic approaches.