Intrathecal minocycline attenuates peripheral inflammation-induced hyperalgesia by inhibiting p38 MAPK in spinal microglia

Mammalian target of rapamycin in spinal cord neurons mediates hypersensitivity induced by peripheral inflammation

mTOR, the mammalian target of rapamycin, is a serine-threonine kinase known to regulate cell proliferation and growth. mTOR has also been implicated in neuronal synaptic plasticity as well as in pain transmission in models of chemically induced and neuropathic pain. To date, the role of mTOR as a modulator of inflammatory pain has not been examined. In this study, we investigated the role of mTOR in Sprague-Dawley rats using the carrageenan model of inflammatory pain. mRNA of Ras homolog enriched in brain (Rheb), a GTPase that positively regulates mTOR activation, was significantly increased 2 h following carrageenan injection. Four hours after induction of inflammation phosphorylation (p) of p70S6 kinase (S6K), ribosomal protein S6 (S6) and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) was increased, indicating mTOR activation. Inhibition of spinal mTOR with intrathecal (i.t.) injection of rapamycin (0.1-3 microg) led to a dose-dependent decrease in carrageenan-induced thermal hyperalgesia and a reduction of mechanical allodynia. In vitro studies confirmed rapamycin inhibition of the mTOR pathway. Carrageenan-induced activation of the mTOR pathway in rats was localized predominantly to dorsal horn neurons in the superficial lamina. Taken together, these data show that the mTOR pathway is activated in dorsal horn neurons during inflammatory pain, and that inhibition of spinal mTOR attenuates inflammation-induced thermal and tactile hypersensitivity. Hence, our study indicates that spinal mTOR is an important regulator of spinal sensitization and suggests that targeting mTOR may provide a new avenue for pain therapy.

Peripheral formalin injury induces 2 stages of microglial activation in the spinal cord

The formalin test produces 2 well-known acute phases of nociceptive behavior. Recently, we have shown that this same formalin test produces a third phase of nociceptive behavior consisting of prolonged thermal and mechanical hyperalgesia beginning days after formalin injection and lasting for at least 3 weeks. Here we investigated the activity of 3 MAPKs (p38, ERK and JNK) in the spinal dorsal horn following 5% formalin injection into rat hind paw. The p38 MAPK was rapidly activated in the spinal microglia minutes after injection and the activation persisted for 1 hour. In addition, this same injury induced a secondary increase of phospho-p38 expression in spinal microglia that was maximal 3 to 7 days postinjection. Intrathecal administration of p38 inhibitor SB203580 not only inhibited the early acute spontaneous nociceptive behaviors, but also inhibited the long-term formalin injury-induced mechanical hyperalgesia. Our results suggest that peripheral formalin injection induces 2 stages of microglial activation, and p38 activation in spinal microglia plays key roles in central pain modulation in formalin test respectively for the early acute phases and the late secondary long-term pain state as well.

PERSPECTIVE

This article presents unique properties of spinal microglial activation in a pain animal model. This finding could potentially help clinicians to further understand the contributions of spinal microglia to acute and chronic pain state.

Contribution of the chemokine (C-C motif) ligand 2 (CCL2) to mechanical hypersensitivity after surgical incision in rats

BACKGROUND

Neural glial signaling in the spinal cord may underlie pain and sensitization after peripheral injury. The authors test the role of a glial activator, the chemokine (C-C motif) ligand 2 (CCL2), on mechanical hypersensitivity after plantar incision in a rat model of postoperative pain.

METHODS

Twenty-four hours after hind paw incision, rats were intrathecally administered an anti-CCL2 neutralizing antibody (3 and 10 microg) or control immunoglobulin G (10 microg). Mechanical hypersensitivity was assessed acutely and for several days after administration of anti-CCL2 antibody using von Frey filaments. Immunohistochemical analysis was conducted on spinal cord sections to examine the effects of treatment on measures of microglial activation, including levels of ionized calcium-binding adaptor molecule 1 and phosphorylated p38 mitogen-activated protein kinase.

RESULTS

Neutralization of spinal CCL2 acutely reversed mechanical hypersensitivity within 30 min in a dose-dependent manner. A single administration also produced a sustained decrease in mechanical hypersensitivity 48 and 72 h after incision. Anti-CCL2 antibody reduced microglial activation as measured by the levels of ionized calcium-binding adaptor molecule 1 immunoreactivity and the number of microglia containing phosphorylated p38 mitogen-activated protein kinase 48 h after incision but not within 30 min of administration.

CONCLUSIONS

These results provide evidence that CCL2 contributes to the maintenance of mechanical hypersensitivity after plantar incision and establish a role for neural glial signaling in postoperative pain. The long-term effects of anti-CCL2 treatment correlate with reduced microglial activation. Spinal blockade of CCL2 may serve as a useful therapy for the treatment of certain aspects of postoperative pain.

R-flurbiprofen reduces neuropathic pain in rodents by restoring endogenous cannabinoids

Background

R-flurbiprofen, one of the enantiomers of flurbiprofen racemate, is inactive with respect to cyclooxygenase inhibition, but shows analgesic properties without relevant toxicity. Its mode of action is still unclear.

Methodology/Principal Findings

We show that R-flurbiprofen reduces glutamate release in the dorsal horn of the spinal cord evoked by sciatic nerve injury and thereby alleviates pain in sciatic nerve injury models of neuropathic pain in rats and mice. This is mediated by restoring the balance of endocannabinoids (eCB), which is disturbed following peripheral nerve injury in the DRGs, spinal cord and forebrain. The imbalance results from transcriptional adaptations of fatty acid amide hydrolase (FAAH) and NAPE-phospholipase D, i.e. the major enzymes involved in anandamide metabolism and synthesis, respectively. R-flurbiprofen inhibits FAAH activity and normalizes NAPE-PLD expression. As a consequence, R-Flurbiprofen improves endogenous cannabinoid mediated effects, indicated by the reduction of glutamate release, increased activity of the anti-inflammatory transcription factor PPARγ and attenuation of microglia activation. Antinociceptive effects are lost by combined inhibition of CB1 and CB2 receptors and partially abolished in CB1 receptor deficient mice. R-flurbiprofen does however not cause changes of core body temperature which is a typical indicator of central effects of cannabinoid-1 receptor agonists.

Conclusion

Our results suggest that R-flurbiprofen improves the endogenous mechanisms to regain stability after axonal injury and to fend off chronic neuropathic pain by modulating the endocannabinoid system and thus constitutes an attractive, novel therapeutic agent in the treatment of chronic, intractable pain.

Role of spinal p38alpha and beta MAPK in inflammatory hyperalgesia and spinal COX-2 expression

Pharmacological studies indicate that spinal p38 mitogen-activated protein kinase plays a role in the development of hyperalgesia. We investigated whether either the spinal isoform p38alpha or p38beta is involved in peripheral inflammation evoked pain state and increased expression of spinal COX-2. Using intrathecal antisense oligonucleotides, we show that hyperalgesia is prevented by downregulation of p38beta but not p38alpha, whereas increases in spinal COX-2 protein expression at 8 hours are mediated by both p38alpha and beta isoforms. These data suggest that early activation of spinal p38beta isoform may affect acute facilitatory processing, and both p38beta and alpha isoforms mediate temporally delayed upregulation of spinal COX-2.

Dexmedetomidine blocks thermal hyperalgesia and spinal glial activation in rat model of monoarthritis

AIM

To investigate the effect of systemic administration dexmedetomidine, a selective alpha 2 adrenergic receptor (alpha(2)AR) agonist, on thermal hyperalgesia and spinal glial activation evoked by monoarthritis (MA).

METHODS

MA was induced by an intra-articular injection of complete Freund's adjuvant (CFA). Thermal hyperalgesia was measured by Hargreaves' test. The spinal glial activation status was analyzed by GFAP (an astrocytic marker) and Iba-1 (a microglial marker) immunohistochemistry or immunoblotting.

RESULTS

Unilateral intra-articular injection of CFA produced a robust glial activation of astrocytes and microglia in the spinal cord, which was associated with the development and maintenance of thermal hyperalgesia. Intraperitoneal (ip) injection of dexmedetomidine (2.5 and 10 microg/kg) was repeatedly given once daily for 5 days with the first injection 60 min before intra-articular CFA. At the dose of 10 microg/kg, dexmedetomidine significantly attenuated MA-induced ipsilateral hyperalgesia from day 2 to day 5. MA-induced up-regulation of GFAP expression on both sides of the spinal dorsal horn was significantly suppressed by day 5 post-MA following dexmedetomidine application, whereas MA-induced Iba-1 up-regulation was only partially suppressed.

CONCLUSION

Systemic dexmedetomidine inhibits the activation of spinal glia, which is possibly associated with its antihyperalgesia in monoarthritic rats.

Systemic minocycline differentially influences changes in spinal microglial markers following formalin-induced nociception

In the present study, intraperitoneal administration of minocycline attenuated enhancing nociceptive behaviors in those rats receiving dual formalin injections (5% formalin followed at 7 days later by 1% formalin). The minocycline treatment did not prevent the increase in OX-42 and MHC class I labeling and morphological changes, but significantly attenuated upregulation of phospho-p38 in activated microglia. These results suggest that the later days of microglial activation with upregulated immune markers in the spinal cord contributes to enhancing long-term pain response by a pathway of p38 activation in microglia.

Chemokines, neuronal-glial interactions, and central processing of neuropathic pain

Millions of people worldwide suffer from neuropathic pain as a result of damage to or dysfunction of the nervous system under various disease conditions. Development of effective therapeutic strategies requires a better understanding of molecular and cellular mechanisms underlying the pathogenesis of neuropathic pain. It has been increasingly recognized that spinal cord glial cells such as microglia and astrocytes play a critical role in the induction and maintenance of neuropathic pain by releasing powerful neuromodulators such as proinflammatory cytokines and chemokines. Recent evidence reveals chemokines as new players in pain control. In this article, we review evidence for chemokine modulation of pain via neuronal-glial interactions by focusing on the central role of two chemokines, CX3CL1 (fractalkine) and CCL2 (MCP-1), because they differentially regulate neuronal-glial interactions. Release of CX3CL1 from neurons is ideal to mediate neuronal-to-microglial signaling, since the sole receptor of this chemokine, CX3CR1, is expressed in spinal microglia and activation of the receptor leads to phosphorylation of p38 MAP kinase in microglia. Although CCL2 was implicated in neuronal-to-microglial signaling, a recent study shows a novel role of CCL2 in astroglial-to-neuronal signaling after nerve injury. In particular, CCL2 rapidly induces central sensitization by increasing the activity of NMDA receptors in dorsal horn neurons. Insights into the role of chemokines in neuronal-glial interactions after nerve injury will identify new targets for therapeutic intervention of neuropathic pain.

Spinal glia modulate both adaptive and pathological processes

Recent research indicates that glial cells control complex functions within the nervous system. For example, it has been shown that glial cells contribute to the development of pathological pain, the process of long-term potentiation, and the formation of memories. These data suggest that glial cell activation exerts both adaptive and pathological effects within the CNS. To extend this line of work, the present study investigated the role of glia in spinal learning and spinal learning deficits using the spinal instrumental learning paradigm. In this paradigm rats are transected at the second thoracic vertebra (T2) and given shock to one hind limb whenever the limb is extended (controllable shock). Over time these subjects exhibit an increase in flexion duration that reduces net shock exposure. However, when spinalized rats are exposed to uncontrollable shock or inflammatory stimuli prior to testing with controllable shock, they exhibit a learning deficit. To examine the role of glial in this paradigm, spinal glial cells were pharmacologically inhibited through the use of fluorocitrate. Our results indicate that glia are involved in the acquisition, but not maintenance, of spinal learning. Furthermore, the data indicate that glial cells are involved in the development of both shock and inflammation-induced learning deficits. These findings are consistent with prior research indicating that glial cells are involved in both adaptive and pathological processes within the spinal cord.

Activation of p-38alpha MAPK contributes to neuronal hyperexcitability in caudal regions remote from spinal cord injury

In the present study, we examined whether activation of p-38alpha MAPK modulates mechanical allodynia and neuronal hyperexcitability, and if propentofylline (PPF, a glial modulator) modulates specifically localized activated p-38alpha MAPK expression in caudal regions remote from a low thoracic hemisection injury in rats. T13 spinal hemisection produces bilateral mechanical allodynia in hindpaws with evoked (in response to mechanical stimuli) neuronal hyperexcitability in lumbar spinal wide dynamic range (WDR) neurons compared to sham controls. The mechanical allodynia and the evoked activity of WDR neurons is attenuated by intrathecal and topical administration of SB203580, an inhibitor of p-38 MAPK activation, dose dependently (p<0.05); however, the spontaneous activity showed no significant differences compared to sham controls. After T13 spinal hemisection, significantly increased phosphorylated (activated form) p-38alpha MAPK expression was present in both superficial and deep dorsal horn neurons as well as in microglia, but not in astrocytes, in the lumbar spinal cord compared to sham controls (p<0.05). Intrathecal application of PPF significantly attenuated the expression of phosphorylated p-38alpha MAPK in superficial dorsal horn neurons (10 mM) and in microglia (1 and 10 mM) in the lumbar spinal cord compared to the hemisection group (p<0.05). In conclusion, our present data demonstrate that activated neuronal and microglial, but not astrocytic, p-38alpha MAPK contributes to the maintenance of neuronal hyperexcitability in caudal regions following spinal cord injury.

Mitochondria and calcium flux as targets of neuroprotection caused by minocycline in cerebellar granule cells

Minocycline, an antibiotic of the tetracycline family, has attracted considerable interest for its theoretical therapeutic applications in neurodegenerative diseases. However, the mechanism of action underlying its effect remains elusive. Here we have studied the effect of minocycline under excitotoxic conditions. Fluorescence and bioluminescence imaging studies in rat cerebellar granular neuron cultures using fura2/AM and mitochondria-targeted aequorin revealed that minocycline, at concentrations higher than those shown to block inflammation and inflammation-induced neuronal death, inhibited NMDA-induced cytosolic and mitochondrial rises in Ca(2+) concentrations in a reversible manner. Moreover, minocycline added in the course of NMDA stimulation decreased Ca(2+) intracellular levels, but not when induced by depolarization with a high K(+) medium. We also found that minocycline, at the same concentrations, partially depolarized mitochondria by about 5-30 mV, prevented mitochondrial Ca(2+) uptake under conditions of environmental stress, and abrogated NMDA-induced reactive oxygen species (ROS) formation. Consistently, minocycline also abrogates the rise in ROS induced by 75 microM Ca(2+) in isolated brain mitochondria. In search for the mechanism of mitochondrial depolarization, we found that minocycline markedly inhibited state 3 respiration of rat brain mitochondria, although distinctly increased oxygen uptake in state 4. Minocycline inhibited NADH-cytochrome c reductase and cytochrome c oxidase activities, whereas the activity of succinate-cytochrome c reductase was not modified, suggesting selective inhibition of complexes I and IV. Finally, minocycline affected activity of voltage-dependent anion channel (VDAC) as determined in the reconstituted system. Taken together, our results indicate that mitochondria are a critical factor in minocycline-mediated neuroprotection.

Glial activation in the rostroventromedial medulla promotes descending facilitation to mediate inflammatory hypersensitivity

Substantial evidence shows that activation of glial cells in the spinal cord may promote central sensitization and pain. Descending facilitation from the rostroventromedial medulla (RVM) is a critical component in the maintenance of chronic pain states, although the precise mechanisms through which facilitation maintains pain are unclear. Here, we investigated the possibility that glial activation in the RVM could promote descending facilitation from the RVM in states of inflammatory pain. Peripheral inflammation was induced with carrageenan injected into the hindpaws of male Sprague-Dawley rats, and behavioral responses to noxious thermal and light tactile stimuli were determined. Microinjection of the glial inhibitors minocycline or fluorocitrate, or of the p38 mitogen-activated protein kinase (MAPK) inhibitor SB 203580, produced a significant and time-related attenuation of behavioral hypersensitivity resulting from hindpaw inflammation. Carrageenan-induced inflammation increased immunolabeling for microglia and astrocytes in the RVM, as well as for phosphorylated p38 MAPK. Phosphorylated p38 MAPK was found in microglia and neurons of the RVM. Inflammation-induced microglial and astrocytic activation in the RVM were attenuated by RVM microinjection of the glial inhibitors. The data show that inflammatory pain is associated with glial activation in the RVM that probably participates in driving descending pain facilitation. These findings reveal a novel site of glial modulation of inflammatory pain.

Spinal translocator protein (TSPO) modulates pain behavior in rats with CFA-induced monoarthritis

Translocator protein 18 kDa (TSPO), previously known as the peripheral benzodiazepine receptor (PBR), is predominantly located in the mitochondrial outer membrane and plays an important role in steroidogenesis, immunomodulation, cell survival and proliferation. Previous studies have shown an increased expression of TSPO centrally in neuropathology, as well as in injured nerves. TSPO has also been implicated in modulation of nociception. In the present study, we examined the hypothesis that TSPO is involved in the initiation and maintenance of inflammatory pain using a rat model of Complete Freund's Adjuvant (CFA)-induced monoarthritis of the tibio-tarsal joint. Immunohistochemistry was performed using Iba-1 (microglia), NeuN (neurons), anti-Glial Fibrillary Acidic Protein, GFAP (astrocytes) and anti-PBR (TSPO) on Days 1, 7 and 14 after CFA-induced arthritis. Rats with CFA-induced monoarthritis showed mechanical allodynia and thermal hyperalgesia on the ipsilateral hindpaw, which correlated with the increased TSPO expression in ipsilateral laminae I-II on all experimental days. Iba-1 expression in the ipsilateral dorsal horn was also increased on Days 7 and 14. Moreover, TSPO was colocalized with Iba-1, GFAP and NeuN within the spinal cord dorsal horn. The TSPO agonist Ro5-4864, given intrathecally, dose-dependently retarded or prevented the development of mechanical allodynia and thermal hyperalgesia in rats with CFA-induced monoarthritis. These findings provide evidence that spinal TSPO is involved in the development and maintenance of inflammatory pain behaviors in rats. Thus, spinal TSPO may present a central target as a complementary therapy to reduce inflammatory pain.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Intrathecal delivery of PDGF produces tactile allodynia through its receptors in spinal microglia

Neuropathic pain is a debilitating pain condition that occurs after nerve damage. Such pain is considered to be a reflection of the aberrant excitability of dorsal horn neurons. Emerging lines of evidence indicate that spinal microglia play a crucial role in neuronal excitability and the pathogenesis of neuropathic pain, but the mechanisms underlying neuron-microglia communications in the dorsal horn remain to be fully elucidated. A recent study has demonstrated that platelet-derived growth factor (PDGF) expressed in dorsal horn neurons contributes to neuropathic pain after nerve injury, yet how PDGF produces pain hypersensitivity remains unknown. Here we report an involvement of spinal microglia in PDGF-induced tactile allodynia. A single intrathecal delivery of PDGF B-chain homodimer (PDGF-BB) to naive rats produced a robust and long-lasting decrease in paw withdrawal threshold in a dose-dependent manner. Following PDGF administration, the immunofluorescence for phosphorylated PDGF β-receptor (p-PDGFRβ), an activated form, was markedly increased in the spinal dorsal horn. Interestingly, almost all p-PDGFRβ-positive cells were double-labeled with an antibody for the microglia marker OX-42, but not with antibodies for other markers of neurons, astrocytes and oligodendrocytes. PDGF-stimulated microglia in vivo transformed into a modest activated state in terms of their cell number and morphology. Furthermore, PDGF-BB-induced tactile allodynia was prevented by a daily intrathecal administration of minocycline, which is known to inhibit microglia activation. Moreover, in rats with an injury to the fifth lumbar spinal nerve (an animal model of neuropathic pain), the immunofluorescence for p-PDGFRβ was markedly enhanced exclusively in microglia in the ipsilateral dorsal horn. Together, our findings suggest that spinal microglia critically contribute to PDGF-induced tactile allodynia, and it is also assumed that microglial PDGF signaling may have a role in the pathogenesis of neuropathic pain.

Role of spinal microglia in visceral hyperalgesia and NK1R up-regulation in a rat model of chronic stress

BACKGROUND & AIMS

Chronic psychological stress is associated with visceral hyperalgesia and increased expression of spinal NK1 receptors (NK1Rs). We aimed to identify the role of spinal microglia in this process.

METHODS

Male Wistar rats were exposed to water avoidance (WA) or sham stress 1 hour each day for 10 days and given daily injections of minocycline, the p38 inhibitor SB203580, or saline. Phosphorylation levels of the kinase p38 (P-p38), the microglia marker OX42, NK1R, and IkappaBalpha were assessed by immunoblotting and/or immunostaining of spinal samples collected at day 11. The visceromotor response to colorectal distention at baseline and following WA were also assayed in rats given injections of minocycline, SB203580, or vehicle. The effects of fractalkine were assessed on the visceromotor response in rats exposed to minocycline or vehicle.

RESULTS

P-p38 protein levels and immunoreactivity were increased in stressed rats and colocalized with OX42-positive cells and neurons in the dorsal horn. This increase was reversed by minocycline or SB203580 exposure. Stress-induced increased NK1R expression was blocked by minocycline but not SB203580. WA-induced decreased IkappaBalpha expression was blocked by minocycline and SB203580. WA-induced hyperalgesia was blocked by minocycline and SB203580 intrathecally. Fractalkine-induced hyperalgesia was blocked by minocycline.

CONCLUSIONS

This is the first demonstration that stress-induced activation of spinal microglia has a key role in visceral hyperalgesia and associated spinal NK1R up-regulation.

Peripheral formalin injection induces unique spinal cord microglial phenotypic changes

Microglia are resident immune cells of brain and activated by peripheral tissue injury. In the present study, we investigated the possible induction of several microglial surface immunomolecules in the spinal cord, including leukocyte common antigen (LCA/CD45), MHC class I antigen, MHC class II antigen, Fc receptor, and CD11c following formalin injection into the rat's hind paw. CD45 and MHC class I were upregulated in the activated microglia, which was evident on day 3 with the peak expression on day 7 following peripheral formalin injection. There was a very low basal expression of MHC class II, CD11c, and the Fc receptor, which did not change after the formalin injection. These results, for the first time, indicate that peripheral formalin injection can induce phenotypic changes of microglia with distinct upregulation of CD45 and MHC class I antigen. The data suggest that phenotypic changes of the activated microglia may be a unique pattern of central changes following peripheral tissue injury.

Gliopathy ensures persistent inflammation and chronic pain after spinal cord injury

Research focused on improving recovery of function, including the reduction of central neuropathic pain (CNP) after spinal cord injury (SCI) is essential. After SCI, regional neuropathic pain syndromes above, at and below the level or spinal injury develop and are thought to have different mechanisms, but may share common dysfunctional glial mechanisms. Detloff et al., [Detloff, M.R., Fisher, L.C., McGaughy, V., Longbrake, E.E., Popovich, P.G., Basso, D.M., Remote activation of microglia and pro-inflammatory cytokines predict the onset and severity of below-level neuropathic pain after spinal cord injury in rats. Exp. Neurol. (2008), doi: 10.1016/j.expneurol.2008.04.009.] describe events in the lumbar region of the spinal cord after a midthoracic SCI injury, the so called "below-level" pain and compares the findings to peripheral nerve lesion findings. This commentary briefly reviews glial contributions and intracellular signaling mechanisms, both neuronal and glial, that provide the substrate for CNP after SCI, including the persistent glial production of factors that can maintain sensitization of dorsal horn neurons in segments remote from the spinal injury; ie. dorsal horn hyperexcitability to formerly non noxious stimuli that become noxious after SCI resulting in allodynia. The term "gliopathy" is proposed to describe the dysfunctional and maladaptive response of glial cells, specifically astrocytes and microglia, to neural injury that is initiated by the sudden injury induced increase in extracellular concentrations of glutamate and concomitant production of several proinflammatory molecules. It is important to understand the roles that different glia play in "gliopathy", a condition that appears to persist after SCI. Furthermore, targeted treatment of gliopathy will attenuate mechanical allodynia in both central and peripheral neuropathic pain syndromes.

Activation of spinal microglia in a murine model of peripheral inflammation-induced, long-lasting contralateral allodynia

Increased sensitivity contralateral to an injury has been described in humans and in various models of neuropathic pain in rats. The mechanism underlying contralateral hypersensitivity is as yet unclear, although previous studies have implicated involvement of both spinal neurons and glia. We describe the development of a temporally delayed, robust and long-lasting contralateral allodynia in mice after hindpaw injection with 4% carrageenan. Both ipsilateral and contralateral allodynia could be inhibited temporarily by intrathecally administered morphine, clonidine, or neostigmine. The delayed development of contralateral allodynia correlated with an increase in OX-42, but not GFAP immunoreactivity in the contralateral dorsal horn. Furthermore, intrathecal treatment with minocycline inhibited the development of contralateral allodynia, suggesting that microglial activation plays a key role in contralateralization, and may be a potential target for clinical intervention after injury or inflammation has occurred, to eliminate the subsequent development of extraterritorial pain.

A comparison of spinal Iba1 and GFAP expression in rodent models of acute and chronic pain

The treatment of acute and chronic pain is still deficient. The modulation of glial cells may provide novel targets to treat pain. We hypothesize that astrocytes and microglia participate in the initiation and maintenance of both, acute surgical and chronic neuropathic pain. Rats underwent paw incision, L5 nerve exposure or L5 nerve transection surgery. Behavioral mechanical allodynia was assessed using von Frey filaments. Immunohistochemistry was performed using anti-ionized calcium binding adaptor protein, Iba-1 (microglia), and anti-Glial Fibrillary Acidic Protein, GFAP (astrocytes) on day 1, 4 and 7 after surgery. Following paw incision and at spinal L5 segment GFAP expression was increased in laminae I-II and Iba1 in deep laminae on day 1, in the entire dorsal horn on day 4 and dissipated on day 7 after paw incision in parallel with the allodynia. L5 nerve transection induced mechanical allodynia from day 1 to 7 which correlated with Iba-1 increases on day 1, 4 (entire dorsal horn) and day 7 after nerve injury (deep laminae of the dorsal horn) at spinal L5 segment. Conversely, GFAP increased at later time points from day 4 (deep laminae) and on day 7 (entire dorsal horn). Our data demonstrates that astrocytes (GFAP expression) play a role in the initiation of acute pain and the maintenance of chronic pain while Iba-1 increases closely correlated with the early phase of neuropathic pain. Iba1 and GFAP increased rostrally, at L3 segment, after paw incision (day 4) and only Iba1 increased following L5 nerve transection (day 7).

Expression of interleukin-16 in sciatic nerves, spinal roots and spinal cords of experimental autoimmune neuritis rats

Experimental autoimmune neuritis (EAN) is a well-known animal model of Guillain-Barré Syndrome. In this study, we studied the spatiotemporal expression of interleukin-16 (IL-16) in the nervous system of EAN rats and pharmacological effects of minocycline on IL-16 expressions in EAN rats. In sciatic nerves and dorsal/ventral roots of EAN rats, IL-16+ cells, identified as macrophages and T cells, were mainly found to concentrate around blood vessels. However, in spinal cords, IL-16+ microglial cells were mainly found in lumbar dorsal horns. Massive IL-16+ cell accumulation in sciatic nerves and spinal roots was temporally correlated with severity of neurological signs of EAN. Furthermore, a strong correlation of IL-16+ cell accumulation with local demyelination in perivascular areas of sciatic nerves, and significant reduction of IL-16+ cell numbers in sciatic nerves and spinal cords by minocycline suggested a pathological contribution of IL-16+ cells in EAN. Taken together, robust IL-16+ cell accumulation in the nervous system and its temporal correlation with severity of neurological signs in EAN might suggest a pathological role of IL-16 in EAN, which makes IL-16 a potential pharmacological target.

Differential activation of p38 and extracellular signal-regulated kinase in spinal cord in a model of bee venom-induced inflammation and hyperalgesia

Background

Honeybee's sting on human skin can induce ongoing pain, hyperalgesia and inflammation. Injection of bee venom (BV) into the intraplantar surface of the rat hindpaw induces an early onset of spontaneous pain followed by a lasting thermal and mechanical hypersensitivity in the affected paw. The underlying mechanisms of BV-induced thermal and mechanical hypersensitivity are, however, poorly understood. In the present study, we investigated the role of mitogen-activated protein kinase (MAPK) in the generation of BV-induced pain hypersensitivity.

Results

We found that BV injection resulted in a quick activation of p38, predominantly in the L4/L5 spinal dorsal horn ipsilateral to the inflammation from 1 hr to 7 d post-injection. Phosphorylated p38 (p-p38) was expressed in both neurons and microglia, but not in astrocytes. Intrathecal administration of the p38 inhibitor, SB203580, prevented BV-induced thermal hypersensitivity from 1 hr to 3 d, but had no effect on mechanical hypersensitivity. Activated ERK1/2 was observed exclusively in neurons in the L4/L5 dorsal horn from 2 min to 1 d, peaking at 2 min after BV injection. Intrathecal administration of the MEK inhibitor, U0126, prevented both mechanical and thermal hypersensitivity from 1 hr to 2 d. p-ERK1/2 and p-p38 were expressed in neurons in distinct regions of the L4/L5 dorsal horn; p-ERK1/2 was mainly in lamina I, while p-p38 was mainly in lamina II of the dorsal horn.

Conclusion

The results indicate that differential activation of p38 and ERK1/2 in the dorsal horn may contribute to the generation and development of BV-induced pain hypersensitivity by different mechanisms.

Locomotor dysfunction and pain: the scylla and charybdis of fiber sprouting after spinal cord injury

Injury to the spinal cord (SCI) can produce a constellation of problems including chronic pain, autonomic dysreflexia, and motor dysfunction. Neuroplasticity in the form of fiber sprouting or the lack thereof is an important phenomenon that can contribute to the deleterious effects of SCI. Aberrant sprouting of primary afferent fibers and synaptogenesis within incorrect dorsal horn laminae leads to the development and maintenance of chronic pain as well as autonomic dysreflexia. At the same time, interruption of connections between supraspinal motor control centers and spinal cord output cells, due to lack of successful regenerative sprouting of injured descending fiber tracts, contributes to motor deficits. Similarities in the molecular control of axonal growth of motor and sensory fibers have made the development of cogent therapies difficult. In this study, we discuss recent findings related to the degradation of inhibitory barriers and promotion of sprouting of motor fibers as a strategy for the restoration of motor function and note that this may induce primary afferent fiber sprouting that can contribute to chronic pain. We highlight the importance of careful attentiveness to off-target molecular- and circuit-level modulation of nociceptive processing while moving forward with the development of therapies that will restore motor function after SCI.

Alarm or curse? The pain of neuroinflammation

The nociceptive nervous system and the immune system serve to defend and alarm the host of imminent or actual damage. However, persistent or recurring exposure of neurons to activated immune cells is associated with an increase in painful behavior following experimental neuropathic injuries. Our understanding of the functional consequences of immune cell-neuron interaction is still incomplete. The purpose of this review is to focus on a seriously detrimental consequence of chronic activation of these two systems, by discussing the contributions of microglia and polymorphonuclear neutrophils to neuropathic pain following experimental spinal cord injury or peripheral nerve injury. Identification of molecules mediating pro-nociceptive signaling between immune cells and neurons, as well as the distinction between neuroprotective versus neuroexcitatory effects of activated immune cells, may be useful in the development of pharmacotherapy for the management of chronic pain and restoration of the beneficial alarm function of pain.

P2Y12 receptor upregulation in activated microglia is a gateway of p38 signaling and neuropathic pain

Microglia in the spinal cord may play an important role in the development and maintenance of neuropathic pain. A metabotropic ATP receptor, P2Y(12), has been shown to be expressed in spinal microglia constitutively and be involved in chemotaxis. Activation of p38 mitogen-activated protein kinase (MAPK) occurs in spinal microglia after nerve injury and may be related to the production of cytokines and other mediators, resulting in neuropathic pain. However, it remains unknown whether any type of P2Y receptor in microglia is involved in the activation of p38 MAPK and the pain behaviors after nerve injury. Using the partial sciatic nerve ligation (PSNL) model in the rat, we found that P2Y(12) mRNA and protein increased in the spinal cord and peaked at 3 d after PSNL. Double labeling studies revealed that cells expressing increased P2Y(12) mRNA and protein after nerve injury were exclusively microglia. Both pharmacological blockades by intrathecal administration of P2Y(12) antagonist and antisense knockdown of P2Y(12) expression suppressed the development of pain behaviors and the phosphorylation of p38 MAPK in spinal microglia after PSNL. The intrathecal infusion of the P2Y(12) agonist 2-(methythio) adenosine 5'-diphosphate trisodium salt into naive rats mimicked the nerve injury-induced activation of p38 in microglia and elevated pain behaviors. These data suggest a new mechanism of neuropathic pain, in which the increased P2Y(12) works as a gateway of the following events in microglia after nerve injury. Activation of this receptor by released ATP or the hydrolyzed products activate p38 MAPK pathway and may play a crucial role in the generation of neuropathic pain.

A novel role of minocycline: attenuating morphine antinociceptive tolerance by inhibition of p38 MAPK in the activated spinal microglia

We have previously demonstrated that activation of p38 mitogen-activated protein kinase (p38 MAPK) in the spinal microglia mediates morphine antinociceptive tolerance. Minocycline, a selective inhibitor of microglia activation, has been reported to attenuate peripheral inflammation-induced hyperalgesia by depressing p38 MAPK in the spinal microglia. The aim of the present study is to explore the effect of intrathecal minocycline on the development of morphine antinociceptive tolerance and p38 activation in the spinal microglia induced by chronic morphine treatment. Minocycline (20, 50 and 100 microg) was given intrathecally 30 min before each morphine (15 microg) administration for consecutive 7 days. It was shown that minocycline attenuated tolerance to morphine analgesia in a dose-dependent manner. Minocycline administration (50 microg) which was initiated on day 4 followed by another 4 days administration partially reversed the established morphine antinociceptive tolerance. However, minocycline treatment which was started on day 8 followed by its administration for 4 more days failed to reverse the established morphine tolerance. Immunohistochemical analysis showed that chronic intrathecal morphine-induced activation of p38 MAPK in the spinal microglia. Minocycline at a dose that was shown to antagonize tolerance to morphine analgesia significantly inhibited the increase in p38 MAPK activation in the spinal microglia. To our knowledge, this is the first study to demonstrate that minocycline antagonizes morphine antinociceptive tolerance, possibly due to the inhibition of p38 activation in the spinal microglia.

Localization of S100A8 and S100A9 expressing neutrophils to spinal cord during peripheral tissue inflammation

Investigation of hyperalgesia at the spinal transcriptome level indicated that carrageenan-induced inflammation of rat hind paws leads to a rapid but sustained increase in S100A8 and S100A9 expression, two genes implicated in the pathology of numerous inflammatory diseases including rheumatoid arthritis and gout. In situ hybridization revealed that the elevation occurred in neutrophils that migrate to the spinal cord vasculature during peripheral inflammation, not in spinal neurons or glial cells. Immunohistochemical analysis suggests, but does not prove, that these neutrophils abundantly release S100A8 and S100A9. Consistent with this, we detected an increase in ICAM and VCAM, both indicators of endothelial activation, a known trigger for secretion of S100A8 and S100A9. Migration of S100A8- and S100A9-expressing neutrophils to spinal cord is selective, since MCP-1- and CD68-expressing leukocytes do not increase in spinal cord vasculature during hind paw inflammation. Examination of many neutrophil granule mediators in spinal cord indicated that they are not regulated to the same degree as S100A8 and S100A9. Neutrophil migration also occurs in the vasculature of brain and pituitary gland during peripheral inflammation. Together, these findings suggest an interaction between a subpopulation of leukocytes and the CNS during peripheral tissue inflammation, as implied by an apparent release and possible diffusion of S100A8 and S100A9 through the endothelial blood-brain barrier. Although the present findings do not establish the neurophysiological or behavioral relevance of these observations to nociceptive processing, the data raise the possibility that selective populations of leukocytes may communicate the presence of disease or tissue damage from the periphery to cells in the central nervous system.

Dissociation of spinal microglia morphological activation and peripheral inflammation in inflammatory pain models

We compared the effects of peripheral Freund's Complete Adjuvant (CFA) and formalin injection on spinal microglia activation. Both qualitative and quantitative analyses showed signs of microglia activation on the ipsilateral side of the lumbar dorsal horn on day 3, day 7 and day 14 after formalin injection. However, significant microglia morphological alteration was not found in the CFA model. At the injection site in the paw, CFA injection induced considerably more inflammation than formalin injection. Although spinal microglia might be activated in inflammatory pain models, morphologically, spinal microglia activation was not closely correlated with peripheral inflammation.

Inhibition of spinal constitutive NOS-2 by 1400W attenuates tissue injury and inflammation-induced hyperalgesia and spinal p38 activation

Nitric oxide (NO) and its synthesizing enzymes, including NO synthase-2 (NOS-2, also called inducible NOS, iNOS), have been implicated in spinal nociception. 1400W is a highly selective NOS-2 inhibitor, as compared with either NOS-1 (neuronal NOS, nNOS) or NOS-3 (endothelial NOS). Here we examined the anti-nociceptive effects of intrathecal (IT) administration of 1400W in two experimental models of hyperalgesia (formalin and carrageenan models), in addition to the effect of 1400W on stimulation-induced activation of spinal p38 mitogen-activated protein kinase (p38). IT treatment of rats with 1400W produced a dose-dependent inhibition of paw formalin-induced phase II flinches, and attenuated carrageenan-induced thermal hyperalgesia. In contrast, IT injection of a selective inhibitor of NOS-1, nNOS inhibitor-I, had no effect in either model. Furthermore, 1400W at a dose that suppressed formalin-induced flinching behavior also blocked formalin-evoked p38 phosphorylation (activation) in the spinal cord, while nNOS inhibitor-I displayed no activity. The prompt effects of IT 1400W suggest involvement of constitutively expressed NOS-2 in spinal nociception. The NOS-2 protein in rat spinal cords was undetectable by Western blotting. However, when the protein was immunoprecipitated prior to Western blotting, NOS-2-immunoreactive bands were detected in the tissues, including naïve spinal cords. The presence of constitutive spinal NOS-2 was further confirmed by reverse transcriptase-polymerase chain reaction. Taken together, the present studies suggest that constitutively expressed spinal NOS-2 mediates tissue injury and inflammation-induced hyperalgesia, and that activation of p38 is one of the downstream factors in NO-mediated signaling in the initial processing of spinal nociception.

Modulation of thalamic nociceptive processing after spinal cord injury through remote activation of thalamic microglia by cysteine cysteine chemokine ligand 21

Spinal cord injury (SCI) results in the generation and amplification of pain caused in part by injury-induced changes in neuronal excitability at multiple levels along the sensory neuraxis. We have previously shown that activated microglia, through an ERK (extracellular signal-regulated kinase)-regulated PGE(2) (prostaglandin E(2)) signaling mechanism, maintain neuronal hyperexcitability in the lumbar dorsal horn. Here, we examined whether microglial cells in the thalamus contribute to the modulation of chronic pain after SCI, and whether microglial activation is governed by spinally mediated increases in the microglial activator cysteine-cysteine chemokine ligand 21 (CCL21). We report that CCL21 is upregulated in dorsal horn neurons, that tissue levels are increased in the dorsal horn and ventral posterolateral (VPL) nucleus of the thalamus 4 weeks after SCI, and that the increase can be differentially reduced by spinal blockade at T1 or L1. In intact animals, electrical stimulation of the spinothalamic tract induces increases in thalamic CCL21 levels. Recombinant CCL21 injected into the VPL of intact animals transiently activates microglia and induces pain-related behaviors, effects that could be blocked with minocycline. After SCI, intra-VPL antibody-mediated neutralization of CCL21 decreases microglial activation and evoked hyperexcitability of VPL neurons, and restores nociceptive thresholds to near-normal levels. These data identify a novel pathway by which SCI triggers upregulation of the neuroimmune modulator CCL21 in the thalamus, which induces microglial activation in association with pain phenomena.

Inhibition of fatty acid amide hydrolase produces analgesia by multiple mechanisms

1 The reversible fatty acid amide hydrolase (FAAH) inhibitor OL135 reverses mechanical allodynia in the spinal nerve ligation (SNL) and mild thermal injury (MTI) models in the rat. The purpose of this study was to investigate the role of the cannabinoid and opioid systems in mediating this analgesic effect. 2 Elevated brain concentrations of anandamide (350 pmol g(-1) of tissue vs 60 pmol g(-1) in vehicle-treated controls) were found in brains of rats given OL135 (20 mg kg(-1)) i.p. 15 min prior to 20 mg kg(-1) i.p. anandamide. 3 Predosing rats with OL135 (2-60 mg kg(-1) i.p.) 30 min before administration of an irreversible FAAH inhibitor (URB597: 0.3 mg kg(-1) intracardiac) was found to protect brain FAAH from irreversible inactivation. The level of enzyme protection was correlated with the OL135 concentrations in the same brains. 4 OL135 (100 mg kg(-1) i.p.) reduced by 50% of the maximum possible efficacy (MPE) mechanical allodynia induced by MTI in FAAH(+/+)mice (von Frey filament measurement) 30 min after dosing, but was without effect in FAAH(-/-) mice. 5 OL135 given i.p. resulted in a dose-responsive reversal of mechanical allodynia in both MTI and SNL models in the rat with an ED(50) between 6 and 9 mg kg(-1). The plasma concentration at the ED(50) in both models was 0.7 microM (240 ng ml(-1)). 6 In the rat SNL model, coadministration of the selective CB(2) receptor antagonist SR144528 (5 mg kg(-1) i.p.), with 20 mg kg(-1) OL135 blocked the OL135-induced reversal of mechanical allodynia, but the selective CB(1) antagonist SR141716A (5 mg kg(-1) i.p.) was without effect. 7 In the rat MTI model neither SR141716A or SR144528 (both at 5 mg kg(-1) i.p.), or a combination of both antagonists coadministered with OL135 (20 mg kg(-1)) blocked reversal of mechanical allodynia assessed 30 min after dosing. 8 In both the MTI model and SNL models in rats, naloxone (1 mg kg(-1), i.p. 30 min after OL135) reversed the analgesia (to 15% of control levels in the MTI model, to zero in the SNL) produced by OL135.

Do glial cells control pain?

Management of chronic pain is a real challenge, and current treatments that focus on blocking neurotransmission in the pain pathway have resulted in limited success. Activation of glial cells has been widely implicated in neuroinflammation in the CNS, leading to neurodegeneration in conditions such as Alzheimer's disease and multiple sclerosis. The inflammatory mediators released by activated glial cells, such as tumor necrosis factor-a and interleukin-1b not only cause neurodegeneration in these disease conditions, but also cause abnormal pain by acting on spinal cord dorsal horn neurons in injury conditions. Pain can also be potentiated by growth factors such as brain-derived growth factor and basic fibroblast growth factor, which are produced by glia to protect neurons. Thus, glial cells can powerfully control pain when they are activated to produce various pain mediators. We review accumulating evidence that supports an important role for microglial cells in the spinal cord for pain control under injury conditions (e.g. nerve injury). We also discuss possible signaling mechanisms, in particular mitogen-activated protein kinase pathways that are crucial for glial-mediated control of pain.Investigating signaling mechanisms in microglia might lead to more effective management of devastating chronic pain.

Characterization of the antinociceptive and anti-inflammatory activities of doxycycline and minocycline in different experimental models

Tetracyclines induce anti-inflammatory effects unrelated to their antimicrobial activities. We investigated the effect induced by minocycline and doxycycline in models of nociceptive and inflammatory pain, edema, fever, cell migration and formation of fibrovascular tissue, as these effects have not been fully investigated. Tetracyclines were administered via intraperitoneal route 1 h before the tests. Minocycline and doxycycline (100 mg/kg) inhibited the second phase of the formalin-induced nociceptive response in mice. Doxycycline (100 mg/kg) also inhibited the first phase. The nociceptive response induced by phorbol 12,13-didecanoate (PDD) in mice was inhibited by doxycycline (100 mg/kg). Furthermore, carrageenan-induced mechanical allodynia in rats was inhibited by doxycycline and minocycline (50 or 100 mg/kg). However, they did not enhance the latency in the hot-plate test. It is unlikely that antinociception resulted from motor incoordination or muscle relaxing effect, as both tetracyclines (100 mg/kg) did not impair the motor activity of mice in the rota-rod test. Doxycycline (50 or 100 mg/kg) or minocycline (50 or 100 mg/kg) inhibited carrageenan-induced paw edema in rats. However, only minocycline (100 mg/kg) inhibited PDD-induced edema. Carrageenan-induced leukocyte migration into the peritoneal cavity of rats was inhibited by both tetracyclines (100 mg/kg). Endotoxin-induced fever in rats was also inhibited by doxycycline (50 or 100 mg/kg) or minocycline (100 mg/kg). Finally, formation of fibrovascular tissue induced by subcutaneous implant of a cotton pellet in mice was inhibited by a 6-day administration of both tetracyclines (50 or 100 mg/kg day). Concluding, this study clearly shows the antinociceptive and anti-inflammatory activities of these second-generation tetracyclines.

Role of the CX3CR1/p38 MAPK pathway in spinal microglia for the development of neuropathic pain following nerve injury-induced cleavage of fractalkine

Accumulating evidence suggests that microglial cells in the spinal cord play an important role in the development of neuropathic pain. However, it remains largely unknown how glia interact with neurons in the spinal cord after peripheral nerve injury. Recent studies suggest that the chemokine fractalkine may mediate neural/microglial interaction via its sole receptor CX3CR1. We have examined how fractalkine activates microglia in a neuropathic pain condition produced by spinal nerve ligation (SNL). SNL induced an upregulation of CX3CR1 in spinal microglia that began on day 1, peaked on day 3, and maintained on day 10. Intrathecal injection of a neutralizing antibody against CX3CR1 suppressed not only mechanical allodynia but also the activation of p38 MAPK in spinal microglia following SNL. Conversely, intrathecal infusion of fractalkine produced a marked p38 activation and mechanical allodynia. SNL also induced a dramatic reduction of the membrane-bound fractalkine in the dorsal root ganglion, suggesting a cleavage and release of this chemokine after nerve injury. Finally, application of fractalkine to spinal slices did not produce acute facilitation of excitatory synaptic transmission in lamina II dorsal horn neurons, arguing against a direct action of fractalkine on spinal neurons. Collectively, our data suggest that (a) fractalkine cleavage (release) after nerve injury may play an important role in neural-glial interaction, and (b) microglial CX3CR1/p38 MAPK pathway is critical for the development of neuropathic pain.

Involvement of glia in central sensitization in trigeminal subnucleus caudalis (medullary dorsal horn)

Central sensitization is a crucial mechanism underlying the increased excitability of nociceptive pathways following peripheral tissue injury and inflammation. We have previously demonstrated that the small-fiber excitant and inflammatory irritant mustard oil (MO) applied to the tooth pulp produces glutamatergic- and purinergic-dependent central sensitization in brainstem nociceptive neurons of trigeminal subnucleus caudalis (Vc). Recent studies have implicated both astrocytes and microglia in spinal nociceptive mechanisms, showing, for example, that inhibition of spinal astroglial metabolism or spinal microglial p38MAPK activation can attenuate hyperalgesia in inflammatory pain models but have not tested effects of glial inhibitors on central sensitization in functionally identified spinal nociceptive neurons. The aim of the present study was to determine whether glial cells are involved in the MO-induced central sensitization in Vc nociceptive neurons, by examining the effects of intrathecally applied SB203580 (SB), an inhibitor of p38MAPK, and fluoroacetate (FA), an inhibitor of the astroglial metabolic enzyme aconitase. During continuous superfusion of phosphate-buffered saline over Vc, MO application to the pulp-induced central sensitization in Vc nociceptive neurons reflected in significant increases in cutaneous mechanoreceptive field (RF) size and responses to noxious mechanical stimuli and a decrease in mechanical activation threshold. The i.t. application of SB or FA markedly attenuated the MO-induced increases in pinch RF size and responses to noxious stimuli and the decrease in activation threshold. Neither SB nor FA application significantly affected the baseline (i.e., pre-MO application) RF and response properties. These results suggest that glial metabolic processes are important in the development of Vc central sensitization.

Extracellular signal-regulated kinase-regulated microglia-neuron signaling by prostaglandin E2 contributes to pain after spinal cord injury

Many patients with traumatic spinal cord injury (SCI) report pain that persists indefinitely and is resistant to available therapeutic approaches. We recently showed that microglia become activated after experimental SCI and dynamically maintain hyperresponsiveness of spinal cord nociceptive neurons and pain-related behaviors. Mechanisms of signaling between microglia and neurons that help to maintain abnormal pain processing are unknown. In this study, adult male Sprague Dawley rats underwent T9 spinal cord contusion injury. Four weeks after injury when lumbar dorsal horn multireceptive neurons became hyperresponsive and when behavioral nociceptive thresholds to mechanical and thermal stimuli were decreased, we tested the hypothesis that prostaglandin E2 (PGE2) contributes to signaling between microglia and neurons. Immunohistochemical data showed specific localization of phosphorylated extracellular signal-regulated kinase 1/2 (pERK1/2), an upstream regulator of PGE2 release, to microglial cells and a neuronal localization of the PGE2 receptor E-prostanoid 2 (EP2). Enzyme immunoassay analysis showed that PGE2 release was dependent on microglial activation and ERK1/2 phosphorylation. Pharmacological antagonism of PGE2 release was achieved with the mitogen-activated protein kinase kinase 1/2 (MEK1/2) inhibitor PD98059 [2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one] and the microglial inhibitor minocycline. Cyclooxygenase-2 expression in microglia was similarly reduced by MEK1/2 inhibition. PD98059 and EP2 receptor blockade with AH6809 (6-isopropoxy-9-oxoxanthene-2-carboxylic acid) resulted in a decrease in hyperresponsiveness of dorsal horn neurons and partial restoration of behavioral nociceptive thresholds. Selective targeting of dorsal horn microglia with the Mac-1-SAP immunotoxin, a chemical conjugate of mouse monoclonal antibody to CD11b and the ribosome-inactivating protein saporin, resulted in reduced microglia staining, reduction in PGE2 levels, and reversed pain-related behaviors [corrected]. On the basis of these observations, we propose a PGE2-dependent, ERK1/2-regulated microglia-neuron signaling pathway that mediates the microglial component of pain maintenance after injury to the spinal cord.

Lipoxins and aspirin-triggered lipoxin inhibit inflammatory pain processing

Inflammatory conditions can lead to debilitating and persistent pain. This hyperalgesia reflects sensitization of peripheral terminals and facilitation of pain signaling at the spinal level. Studies of peripheral systems show that tissue injury triggers not only inflammation but also a well-orchestrated series of events that leads to reversal of the inflammatory state. In this regard, lipoxins represent a unique class of lipid mediators that promote resolution of inflammation. The antiinflammatory role of peripheral lipoxins raises the hypothesis that similar neuraxial systems may also down-regulate injury-induced spinal facilitation of pain processing. We report that the lipoxin A₄ receptor is expressed on spinal astrocytes both in vivo and in vitro and that spinal delivery of lipoxin A₄, as well as stable analogues, attenuates inflammation-induced pain. Furthermore, activation of extracellular signal-regulated kinase and c-Jun N-terminal kinase in astrocytes, which has been indicated to play an important role in spinal pain processing, was attenuated in the presence of lipoxins. This linkage opens the possibility that lipoxins regulate spinal nociceptive processing though their actions upon astrocytic activation. Targeting mechanisms that counterregulate the spinal consequences of persistent peripheral inflammation provide a novel endogenous mechanism by which chronic pain may be controlled.

Spinal cord glia and interleukin-1 do not appear to mediate persistent allodynia induced by intramuscular acidic saline in rats

Spinal glial activation and consequent interleukin-1 (IL-1) release are implicated in pain facilitation induced by inflammation/damage to skin and peripheral nerves. It is unclear whether pain facilitation induced at deep tissue sites also depends on these. We investigated whether spinal IL-1 and/or glial activation mediates bilateral allodynia induced by repeated unilateral intramuscular injections of acidic saline to rats. Given the prominent role of spinal IL-1 in various bilateral pain models, we predicted that intrathecal IL-1 receptor antagonist (IL-1ra) would suppress bilateral allodynia in this model as well. Surprisingly, neither single nor repeated intrathecal injections of IL-1ra affected allodynia, measured by the von Frey test, induced by prior intramuscular acidic saline compared with vehicle-injected controls. In addition, we tested the effect of 2 additional intrathecal manipulations that are broadly efficacious in suppressing glially mediated pain facilitation: (1) a glial metabolic inhibitor (fluorocitrate) and (2) the anti-inflammatory cytokine, interleukin-10 (IL-10). Like IL-1ra, fluorocitrate and IL-10 each failed to reverse allodynia. Finally, we observed no significant activation of glial cells, as assessed by immunohistochemistry of glial activation markers, in the lumbar spinal cord in response to intramuscular acidic saline. Taken together, the present data suggest that acidic saline-induced bilateral allodynia is created independently of glial activation.

PERSPECTIVE

From converging lines of evidence, the current studies suggest that persistent bilateral allodynia induced by repeated intramuscular acidic saline is not mediated by spinal IL-1 and/or spinal glial activation. As such, this might represent the first evidence for pain facilitation occurring in the absence of glial involvement.

Activated microglia contribute to the maintenance of chronic pain after spinal cord injury

Traumatic spinal cord injury (SCI) results not only in motor impairment but also in chronic central pain, which can be refractory to conventional treatment approaches. It has been shown recently that in models of peripheral nerve injury, spinal cord microglia can become activated and contribute to development of pain. Considering their role in pain after peripheral injury, and because microglia are known to become activated after SCI, we tested the hypothesis that activated microglia contribute to chronic pain after SCI. In this study, adult male Sprague Dawley rats underwent T9 spinal cord contusion injury. Four weeks after injury, when lumbar dorsal horn multireceptive neurons became hyperresponsive and when behavioral nociceptive thresholds were decreased to both mechanical and thermal stimuli, intrathecal infusions of the microglial inhibitor minocycline were initiated. Electrophysiological experiments showed that minocycline rapidly attenuated hyperresponsiveness of lumbar dorsal horn neurons. Behavioral data showed that minocycline restored nociceptive thresholds, at which time spinal microglial cells assumed a quiescent morphological phenotype. Levels of phosphorylated-p38 were decreased in SCI animals receiving minocycline. Cessation of delivery of minocycline resulted in an immediate return of pain-related phenomena. These results suggest an important role for activated microglia in the maintenance of chronic central below-level pain after SCI and support the newly emerging role of non-neuronal immune cells as a contributing factor in post-SCI pain.

Suppression of interleukin-6 by minocycline in a rat model of neuropathic pain

Inflammatory mediators produced in the injured nerve have been proposed as contributing factors in the development of neuropathic pain. In this regard an important role is assigned to interleukin-6. The present study, evaluated the effect of pretreatment with minocycline, on pain behavior (hyperalgesia and allodynia) and serum level of interleukin-6 in chronic constriction injury (CCI) model of neuropathic pain in rat. Minocycline (5, 10, 20 and 40 mg/kg, i.p.) was injected 1 h before surgery and continued daily to day 14 post-ligation. Behavioral tests were recorded before surgery and on postoperative days 1, 3, 5, 7, 9, 10, 14, and the serum concentration of interleukin-6 was determined at day 14. We observed that minocycline which was reported to have a neuroprotective effect in some neurodegenerative diseases, reversed hyperalgesia and allodynia due to sciatic nerve ligation and inhibited the interleukin-6 production. It seems that minocycline could have an anti-inflammatory and analgesic effect in some chronic pain states.

Role of spinal microglia in rat models of peripheral nerve injury and inflammation

Mounting evidence supports the hypothesis that spinal microglia modulate the development and maintenance of some chronic pain states. Here we examined the role of spinal microglia following both peripheral inflammatory insult and peripheral nerve injury. We observed significant ipsilateral dorsal horn microglia activation 2 weeks after injury and bilateral activation 50 days following nerve injury as well as 24 h following intraplantar zymosan but not intraplantar complete Freund's adjuvant (CFA). Ipsilateral but not contralateral microglia activation was associated with hind paw mechanical hyperalgesia. Spinal injection of the glial metabolic inactivator fluorocitrate attenuated ipsilateral hyperalgesia and bilateral spinal microglia activation after peripheral nerve injury. Intrathecal fluorocitrate reversed hyperalgesia after intraplantar zymosan and produced no reversal of CFA-induced hyperalgesia. These data suggest a role for spinal glia in the persistence of mechanical hyperalgesia following peripheral nerve injury. However, activation of spinal microglia contralaterally did not correlate to nociception. Furthermore, it would appear that the time course of microglia activation and their contribution to inflammatory pain is dependent on the inflammatory stimulus administered.