The Role of Interleukin-6 in Nociception and Pain

Inflammatory substances play a part in the modulation of pain by interfering with nociceptive transduction, conduction, and transmission. This modulation may result from alteration of the transcription rate and/or posttranslational changes in proteins involved in the pain pathway. In this review, an important role is assigned to interleukin-6 (IL-6), an inflammatory cytokine, in the physiology of nociception and the pathophysiology of pain. First, IL-6, its receptor gp80, and its transmembranous signal transducer gp130 are upregulated in peripheral nerves, dorsal root ganglia, and the spinal cord during experimental pain. Second, IL-6 modulates the presence of several extracellular and intracellular mediators that are also known to be active during pain. Third, administration of IL-6 alters the responses to thermal or mechanical stimuli and pain in animals. Fourth, neutralizing IL-6 or changes in the IL-6 pathway alter the perception of pain. Although IL-6 is an important factor in the differentiation and survival of neurons and in nerve regeneration, its role in the cascade of chronic pain can compromise any beneficial effect on the patient’s quality of life. As such, IL-6 can be considered an interesting target in the study of pain. Pain, Inflammatory Agents, and IL-6 Primary- or secondary-order sensory neurons can develop functional, chemical, and structural alterations in response to changes in their environment. These changes can lead to a modification of the transduction, conduction, and transmission functionality of these neurons (1). As a result, the specific role of sensory neurons in mediating normal nociceptive transmission is changed to a new modified condition that contributes to an altered state of sensibility, which is referred to as neural plasticity and pain (1). With respect to molecular cellular biology, neural plasticity may be the result of changes in the number of one or several functional proteins that occur in turn through long-lasting transcription-dependent changes in the nucleus. In the cytoplasm, some properties of functional proteins can be rapidly changed via posttranslational changes. Cytokines, in addition to other inflammatory agents, influence these intracellular modulating processes. After the cytokine binds to its specific membrane-bound receptor, a cascade of phosphorylation of constitutively expressed signal proteins occurs within the cell. These phosphorylated signal proteins migrate through the cytoplasm and, on the condition that they have a nuclear localization sequence or bind to a protein with such sequence, can reach the nucleus. In the nucleus or cytoplasm, they may influence the transcription rate or induce posttranslational changes. Examples of intracellular signal proteins involved in nociception or pain are mitogen-activated protein kinase (MAPK) (2,3), Ras/Raf, c-jun (4), c-fos (4,5), and signal transducer and activator of transcription (STAT) (3). It is interesting to note that these signal proteins are also involved in the intracellular signal pathways of several cytokines, including IL-6 (6) (Fig. 1).Figure 1: Interleukin (IL)-6 signaling. IL-6 binds to its receptor IL-6R (gp80). This binding induces homodimerization of gp130. As a result, Janus kinases (JAKs) are phosphorylated, and subsequently the tails of gp130 are also phosphorylated. This event induces phosphorylation of signal transducers and activators of transcription (STATs), which undergo homodimerization or heterodimerization, which in turn enables these dimers to enter the nucleus. In an alternative pathway, Ras/Raf and mitogen-activated protein kinase (MAPK) influence gene expression through dimerization of two nuclear factor IL-6 molecules (NF-IL6). STATs and NF-IL6 influence the DNA transcription rate of IL-6-dependent proteins by binding at the IL-6-responsive element (IL6-RE), which is located on the DNA. Changes in the concentration of all intracellular signal peptides shown in this figure are observed during pain.The human IL-6 gene is located on Chromosome 7 at p21. After an appropriate challenge to an IL-6-producing cell—due to polymorphism of the DNA and posttranslational and postsecretory modifications—multiple isoforms of IL-6 are formed with molecular weight ranging from 21.5 to 28 kDa (7). Clinical implications of the type of IL-6 produced have been described (8). Most, if not all, nucleated cells have been shown to be capable of synthesizing IL-6 in vitro; however, the expression rate of IL-6 is strongly cell dependent. When IL-6 reaches an IL-6-responsive cell, it binds to its specific receptor IL-6R (gp80). The IL-6/IL-6R complex is responsible for the homodimerization of the transmembranous signal transducer gp130 (Fig. 1). As a result, there is an intracellular cascade of phosphorylation of several signal proteins (6). Two major intracellular cascade systems for IL-6 signaling have been characterized. The classical pathway acts via Janus kinases and STAT factors. A second alternative pathway uses the Ras-dependent MAPK cascade, a pathway that seems to have little relevance to IL-6 signaling under physiological conditions (9). This alternative pathway, however, shares intracellular signal proteins that result in nociceptive potentiation (2–5) (Fig. 1). Members of the “IL-6 cytokine family,” including IL-11, ciliary neurotrophic factor, oncostatin M, and leukemia-inhibiting factor, share the transmembranous signal transducer gp130 (6). These cytokines are redundant: hence, they induce similar physiologic effects. IL-6R is the unique ligand for IL-6. The other members of the IL-6 family use other receptors for binding, but all require gp130 for signaling (Table 1).Table 1: Members of the IL-6 Cytokine Family, Their Receptors, and Type of Dimerization for SignalingSeveral cytokines, including IL-1, tumor necrosis factor (TNF), IL-6, and IL-10, are thought to influence nociception or pain (10). Given the above-mentioned posttranslational or transcriptional alterations of proteins during pain and, further, given the properties of IL-6 in the cellular molecular biology, IL-6 is a good candidate as a mediator in the cascade of pain. Localization of IL-6 in Nervous Tissue— Correlation with Pain Intensity IL-6-like immunoreactivity was found in the peripheral nerves in normal and inflamed human skin (11). IL-6 or IL-6 messenger RNA (mRNA) was hardly detected in normal sciatic nerves (12,13) or dorsal root ganglia (DRG) (14,15) but were discovered to increase with age (16,17). Only small amounts of IL-6 or IL-6 mRNA were observed in the dorsal or ventral horns of the spinal cord (18,19). The peripheral nociceptors lack IL-6R but constitutively express gp130 (20). IL-6R expression was observed to a slight degree in the intact nerve (21), with a predominance in Schwann cells (17). In rat DRG neurons, IL-6R increases markedly with age (17). Within 3 h after a sciatic nerve crush injury, IL-6 was produced distally and proximally to the ipsilateral injured site (12,13) (Table 2). A similar IL-6 increase was found after nerve transsection (12,21). In the distal segment, IL-6 declined rapidly 12 to 24 h after the injury (12,13). In the proximal segment, IL-6 levels were larger and remained so for at least 6 days (12). In the distal segment of transected axons, IL-6 mRNA and IL-6 cannot be transcripted and translated, respectively, because of a lack of DNA. Macrophages or Schwann cells at or near the site of injury were shown to be capable of producing inflammatory cytokines (12,13).Table 2: Localization of the Synthesis of IL-6 or IL-6 mRNA, and IL-6R mRNA After Sciatic Nerve LesionIn rats with different forms of mononeuropathy (chronic constriction, crush injury, axotomy, and sham-operated), Cui et al. (22) found a correlation between postoperative mechanical allodynia and the number of IL-6-positive cells in the sciatic nerve measured 14 days after surgery. Rats without evident allodynia and sham-operated rats had the smallest number of IL-6-upregulated cells. Bolin et al. (13) found that after sciatic nerve injury, the contralateral noninjured but sham-operated sciatic nerve expressed a smaller but detectable amount of IL-6, which declined rapidly, disappearing within 6 h. Constriction of the infraorbital trigeminal nerve also resulted in a bilateral upregulation of IL-6 in these nerves from Day 3 to Day 10 (23). IL-6 receptors also increase after nerve injury. IL-6R mRNA was shown to increase in the distal part soon after sciatic nerve crush, having peaked 2 days after crushing and normalized 28 days after crushing; levels of IL-6R mRNA were five times larger after sciatic nerve transsection but behaved similarly (21). In both cases, the course of IL-6R mRNA was accompanied by an increase in the expression of gp130, but no difference in gp130 mRNA was found after transsection or crush injury of the sciatic nerve (21). The presence of both receptors for IL-6 indicates that IL-6 might play a physiological role under these conditions. The fact that the expression pattern of these receptors differs from that of IL-6 shows that they are regulated by independent mechanisms in injured nerves, although in some tissues IL-6 upregulates gp130 expression (24). DRGs contain the nucleus of most peripheral sensory neurons, implying the possibility of de novo synthesis of IL-6 or its receptor. In response to a transsection of the sciatic nerve at its origin, IL-6 and IL-6 mRNA were found in medium to large sensory neurons 2 to 4 days later in the ipsilateral, but not the contralateral, corresponding DRG (14). After nerve transsection, IL-6 persists for <8 days in DRG (14,25). Because the number of macrophages invading the DRG is maximal 8 to 16 days after nerve injury, these cells seemed not to be the initial source of IL-6 after nerve trauma. Mast cell degranulating agents injected into an uninjured nerve upregulated IL-6 mRNA in medium to large neurons of the DRG, whereas agents that stabilized mast cells injected 5 days before nerve injury attenuated the induction of IL-6 mRNA (26). In a nerve constriction model, IL-6 and IL-6 mRNA were found in DRG neurons at lesser concentrations, but both persisted longer than in a nerve transsection model (25). The presence of IL-6 in this nerve constriction model correlated well with the duration of hypersensitivity (25). IL-6 mRNA was also induced in DRG neurons after dorsal spinal nerve root transsection, but in fewer neurons than after peripheral nerve transsection (26). After sciatic nerve injury, IL-6 was found in the corresponding ipsi- and contralateral dorsal and ventral horns, and the increases in IL-6 paralleled pain behaviors over time (18,19,27). Sham-operated animals had no IL-6 increases in the spinal cord. After sciatic cryoneurolysis, IL-6 increased within 3 days in the dorsal and ventral horns, and this IL-6 activity was still present 5 weeks later (18). In the dorsal horn, IL-6 mRNA was found preferentially in the superficial laminae (marginal zone and substantia gelatinosa), where nociceptive fibers terminate (19). The fact that the sciatic nerve is a mixed sensory and motor peripheral nerve explains the increase in IL-6 in both the ventral and the dorsal horn. The fact that mRNA of IL-6 was also found in the spinal neurons contradicts an initial hypothesis (18,28) of retrograde axonal or nonaxonal transport of IL-6 produced by macrophages or Schwann cells in the periphery (19). Both IL-6 and IL-6 mRNA were predominantly found in neurons; however, other cellular sources, such as microglia and astrocytes, were not excluded as far as the production of this cytokine is concerned (18,19). Spinal IL-6 mRNA, spinal IL-6, microglial and astrocyte activation, and pain behavior did not differ in rats that sustained an injury at L5 either proximally or distally to the DRG (29). Adjuvant-induced arthritis in rats also resulted in increased IL-6 and IL-6 mRNA levels in the spinal cord (30). At 3 to 10 days after trigeminus constriction, IL-6 was found bilaterally in the brainstem (23). Little is known about the regulation of expression of IL-6 in neurons. IL-1β, TNFα, and some undefined factors from mast cells have been reported to stimulate IL-6 synthesis in cortical and sensory neurons (26,31). Prostaglandins (PGs) also upregulate IL-6 synthesis in some tissues. The induction of IL-6 in DRG from injured neurons was triggered by a positive signal from the injury site rather than from loss of retrograde inhibition by molecules released from the distal nerve or target tissues (26). The in vitro finding that IL-6 gene transcription was accelerated after membrane depolarization by a yet-undefined calcium-responsive promotor element (32) should help to clarify the transcription and translation of IL-6 in sciatic nerves, DRG, and spinal cord after nerve injury. Influence of IL-6 on Neuronal Functioning The above-mentioned rat studies indicate that, after nerve injury with concomitant neuropathic pain, IL-6 is found in the distal and proximal nerve segments, in DRG, and in the ventral and dorsal horns of the spinal cord. The presence of IL-6 in these places correlates well with pain behavior. In addition, the amount of IL-6R and gp130 on cell membranes increases under this condition, suggesting a physiological role of IL-6. Whatever intracellular pathway of neuromodulation is used, IL-6 might profoundly alter the survival, histological behavior, and functionality of cells that play a part in nociception or pathologic pain. These properties of IL-6 on neurons will now be discussed. Neuronal Survival and Differentiation In Vitro. In vitro, IL-6 induces neurite extension in pheochromocytoma 12 cells (33) and will cause these cells to differentiate into the neuronal phenotype. IL-6 increases neuronal survival (16,34) and neurite outgrowth (28,35), especially if IL-6R is added to the neuronal cultures. It has been observed that IL-6 supports the survival of embryonic rat sensory neurons via the production of brain-derived neurotrophic factor (36). Pretreatment of cultured hippocampal neurons with IL-6 protects these cells against glutamate-induced cell death. IL-6 attenuates the neurotoxic effects of N-methyl-d-aspartate (NMDA) on rat striatal cholinergic neurons (37). These effects of IL-6 are thought to develop through the interference of IL-6 with the calcium transport systems (16). Nerve Regeneration and Neuronal Survival In Vivo. Sensory axonal regeneration is attenuated in IL-6 knockout mice (38). Systemically administered IL-6 retarded the loss of motoneurons after sciatic nerve transsection (39). Anti-IL-6R retarded the regeneration of axotomized hypoglossal nerves, whereas accelerated regeneration was observed in mice that constitutively expressed IL-6 and IL-6R In IL-6 knockout neuronal loss in DRG after nerve injury was than in mice (25). These studies indicate a positive effect of IL-6 on neuronal survival and nerve regeneration in In addition, the quality of nerve was IL-6 because nerves had in IL-6 knockout This is to the smaller of the with respect to the It was that IL-6 induces microglial during regeneration and that this is for regeneration to occur of IL-6 on cytokines are involved in plasticity and as a result of their to with IL-6 or from mice for IL-6 potentiation and potentiation The effects of IL-6 on potentiation are thought to require an of or other receptors and to an increase in intracellular calcium was at larger of IL-6, suggesting that IL-6 also acts on the of potentiation is to the of from the The concomitant increase in levels and inhibition of MAPK production in neuronal by IL-6 had a role in changes in plasticity It is not known these can be to neurons located in DRG or the spinal cord. IL-6 and with or IL-6 increases production in the modulates the effect of receptor and the synthesis of and nerve factor induces an upregulation of the IL-6R substances are with IL-6 by neurons in of IL-6 to DRG or into the intact sciatic nerve in the induction of the With to IL-6 and its effect on or it is thought that IL-6 increases production or via A positive effect of IL-6 on production in cells was found but was not by IL-6 and the IL-6 knockout mice had receptors in the larger levels of and a response to or to the administration of cells found in inflamed contain and and their that these proteins are by these cells In inflamed the administration of IL-6 was and this effect was by that IL-6 induced or from these inflammatory cells it was that IL-6 is involved in the responses to nociceptive stimuli and appropriate modulation of the pathway IL-6 and the IL-6 and IL-6 mRNA were also shown to be produced by neurons IL-6 administered with IL-6R increased neuronal survival and induced synthesis of several and mRNA by these cells IL-6 and Pain these in vitro and in indicate the possibility of a modulating role for IL-6 in nociception or pain. This was by IL-6 or neutralizing against IL-6 or in normal or that were to pain. IL-6. study has been found that with the effect of or administered IL-6 on In there is some that an increase in IL-6 has no effect on pain whereas other that such an increase has a effect IL-6. IL-6, on the condition that it was injected in with nociceptors to of IL-6 in a rat induced mechanical in both although the effect of smaller was in the injected This which was maximal with of human IL-6, a between 2 and 3 h after persisted for at least 6 and to within 24 h. with but not profoundly the an with but not with The the in the contralateral to a of the injected cytokine IL-6 a role in pain was shown during inflammatory pain by a into the with resulted in a of the in this pain model whereas had no effect on nociception IL-6 administered after of into a resulted in which was by on the condition that IL-6 was injected but not with respect to the This for sustained inflammatory pain was to a of peptides by cells after IL-6 challenge In IL-6 knockout in with the nociceptive response to mechanical and thermal was and to was was a with to the response to sciatic nerve in the that IL-6 knockout mice a of neuropathic pain, to a than knockout or mice It be to all these to IL-6 because in this study mice with different were In IL-6 knockout mechanisms are to the IL-6 which also the responses to pain. the astrocyte and microglial to and is in these IL-6 knockout mice When mice were used, these were not (25). In this in which chronic constriction of the sciatic nerve was IL-6 knockout mice a lesser degree of thermal and mechanical and no were found between these mice and animals (25). It is that, at the IL-6 induces a via the pathway. via can be in the of inflammatory pain. IL-6 in A in and a in sensory survival in DRGs after sciatic nerve was found in IL-6 knockout with to their which is by is to the of hypersensitivity to mechanical and thermal but not thermal was profoundly attenuated but not in these IL-6 knockout at least for the 10 days after injury With to the effect of IL-6 in DRG, rats with allodynia that after sciatic nerve injury IL-6 expression in the DRG than rats with sustained allodynia after the (18). these a role for IL-6 in DRG during neuropathic pain. or IL-6. IL-6 administration in 10 produced but not thermal allodynia in normal rats and produced thermal but no in rats that had sustained sciatic (18). In the IL-6 induced in the contralateral that had not a but not in animals. This finding that IL-6 has a nociceptive effect at the spinal in a spinal cord a effect that be to (18). of IL-6 induced an thermal for h in which was on synthesis administration of neutralizing IL-6 in rats before and after L5 spinal nerve mechanical allodynia The of all these in of a role for IL-6 in through the spinal cord. of the In and IL-6 an initial beneficial effect after nerve injury, including against neuronal cell of and against by the nerve injury (19). In this an of neurons with such as Schwann cells and microglia via IL-6 and its receptors seems and sensory effects the IL-6 but on the role by IL-6 in the modulation of nociception is It should be that IL-6 is not the that modulates nociceptive pathways (10). members of the “IL-6 cytokine (Table share the transmembranous signal transducer gp130, in (21). In addition, IL-1, IL-10, and are other cytokines that might influence pain. other such as and have a specific role in nociception and pain. Only of the effect of a specific factor at a well can the physiological role that the factor at that As that IL-6 has modulating effects in the cascade of pain at several IL-6 will an important target in the study of pain. IL-6 can be by the use of neutralizing against IL-6 or In addition, several of modulating IL-6 signaling have been Changes in the number of IL-6R present on the cell membranes can the of a cell to IL-6. The presence of gp130, the of intracellular of some phosphorylation and the of agents that binding of intracellular signal proteins to other factors can all IL-6 signaling. Clinical the of IL-6 on Pain in is no that IL-6 a role in the physiology of pain in however, the possibility that IL-6 has a modulating effect on human nociception or pain. injury by or about well pain. This pain is sustained after the initial injury, implying that substances are produced to pain. IL-6 is produced in at the site of a IL-6 the where its concentration correlates with the of and, with the of the injury. At 24 to h after the levels of IL-6 in the reach because its production is pain or postoperative pain correlates with the of injury and days a this can be by the described or induced effect of IL-6. is a that increases levels in in a inhibition of cytokine synthesis The increases the nociceptive for mechanical stimuli in animals it is given before smaller IL-6 levels and smaller produced larger of IL-6, in addition to other inflammatory than into with the and L5 nerve in the rat resulted in an increased IL-6 concentration in that nerve root and DRG to 4 was observed 3 to 14 days after suggesting a role for IL-6 in the of sciatic pain with had larger IL-6 levels 8 after and this was accompanied by a and is as to the of cytokines in and chronic pain (10). In this cytokines can influence transduction, conduction, and transmission of the nociceptive in or signaling to the in the of a or IL-6 is an interesting target in the study of pain because this cytokine is after nerve injury in the peripheral nerves, in and in the spinal cord. of IL-6 in the skin pain, and experimental pain increases if IL-6 is injected in the In addition to its beneficial effect on neuronal survival and regeneration, this cytokine might alter the properties of neurons via or changes in proteins involved in The modulating effect of IL-6 on pain is in and no are on the of the the beneficial effect of IL-6 not be against for the This fact that is to the role IL-6 in pain after forms of or nerve injury.