A role for proinflammatory cytokines and fractalkine in analgesia, tolerance, and subsequent pain facilitation induced by chronic intrathecal morphine

Spinal CCL2 pronociceptive action is no longer effective in CCR2 receptor antagonist-treated rats

A better understanding of the mechanisms linked to chemokine pronociceptive effects is essential for the development of new strategies to better prevent and treat chronic pain. Among chemokines, MCP-1/CCL2 involvement in neuropathic pain processing is now established. However, the mechanisms by which MCP-1/CCL2 exerts its pronociceptive effects are still poorly understood. In the present study, we demonstrate that MCP-1/CCL2 can alter pain neurotransmission in healthy rats. Using immunohistochemical studies, we first show that CCL2 is constitutively expressed by primary afferent neurons and their processes in the dorsal horn of the spinal cord. We also observe that CCL2 is co-localized with pain-related peptides (SP and CGRP) and capsaicin receptor (VR1). Accordingly, using in vitro superfusion system of lumbar dorsal root ganglion and spinal cord explants of healthy rats, we show that potassium or capsaicin evoke calcium-dependent release of CCL2. In vivo, we demonstrate that intrathecal administration of CCL2 to healthy rats produces both thermal hyperalgesia and sustained mechanical allodynia (up to four consecutive days). These pronociceptive effects of CCL2 are completely prevented by the selective CCR2 antagonist (INCB3344), indicating that CCL2-induced pain facilitation is elicited via direct spinal activation of CCR2 receptor. Therefore, preventing the activation of CCR2 might provide a fruitful strategy for treating pain.

Chronic morphine administration enhances nociceptive sensitivity and local cytokine production after incision

BACKGROUND

The chronic use of opioids prior to surgery leads to lowered pain thresholds and exaggerated pain levels after these procedures. Several mechanisms have been proposed to explain this heightened sensitivity commonly termed opioid-induced hyperalgesia (OIH). Most of these proposed mechanisms involve plastic events in the central or peripheral nervous systems. Alterations in the abundance of peripheral mediators of nociception have not previously been explored.

RESULTS

In these experiments mice were treated with saline (control) or ascending daily doses of morphine to generate a state of OIH followed by hind paw incision. In other experiments morphine treatment was initiated at the time of incision. Both mechanical allodynia and peri-incisional skin cytokine levels were measured. Myeloperoxidase (MPO) assays were used to determine neutrophil activity near the wounds. The cytokine production inhibitor pentoxifylline was used to determine the functional significance of the excess cytokines in previously morphine treated animals. Mice treated chronically treated with morphine prior to incision were found to have enhanced skin levels of IL-1beta, IL-6, G-CSF, KC and TNFalpha after incision at one or more time points compared to saline pretreated controls. The time courses of individual cytokines followed different patterns. There was no discernable effect of chronic morphine treatment on wound area neutrophil infiltration. Pentoxifylline reduced cytokine levels and reversed the excess mechanical sensitization caused by chronic morphine administration prior to incision. Morphine treatment initiated at the time of incision did not lead to a generalized enhancement of cytokine production or nociceptive sensitization in excess of the levels observed after incision alone.

CONCLUSION

The enhanced level of nociceptive sensitization seen after incision in animals chronically exposed to morphine is associated with elevated levels of several cytokines previously reported to be relevant to this incisional pain model. The cytokines may be functional in supporting nociceptive sensitization because pentoxifylline reverses both peri-incisional skin cytokine levels and OIH. Opioid administration beginning at the time of incision does not seem to have the same cytokine enhancing effect. Approaches to postoperative pain control involving a reduction of cytokines may be an effective way to control excessive pain in patients chronically using opioids prior to surgical procedures.

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.

"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.

Morphine treatment accelerates sarcoma-induced bone pain, bone loss, and spontaneous fracture in a murine model of bone cancer

Metastatic bone cancer causes severe pain that is primarily treated with opioids. A model of bone cancer pain in which the progression of cancer pain and bone destruction is tightly controlled was used to evaluate the effects of sustained morphine treatment. In cancer-treated mice, morphine enhanced, rather than diminished, spontaneous, and evoked pain; these effects were dose-dependent and naloxone-sensitive. SP and CGRP positive DRG cells did not differ between sarcoma or control mice, but were increased following morphine in both groups. Morphine increased ATF-3 expression only in DRG cells of sarcoma mice. Morphine did not alter tumor growth in vitro or tumor burden in vivo but accelerated sarcoma-induced bone destruction and doubled the incidence of spontaneous fracture in a dose- and naloxone-sensitive manner. Morphine increased osteoclast activity and upregulated IL-1 beta within the femurs of sarcoma-treated mice suggesting enhancement of sarcoma-induced osteolysis. These results indicate that sustained morphine increases pain, osteolysis, bone loss, and spontaneous fracture, as well as markers of neuronal damage in DRG cells and expression of pro-inflammatory cytokines. Morphine treatment may result in "add-on" mechanisms of pain beyond those engaged by sarcoma alone. While it is not known whether the present findings in this model of osteolytic sarcoma will generalize to other cancers or opioids, the data suggest a need for increased understanding of neurobiological consequences of prolonged opioid exposure which may allow improvements in the use of opiates in the effective management of cancer pain.

The narcotic bowel syndrome: clinical features, pathophysiology, and management

Narcotic bowel syndrome (NBS) is a subset of opioid bowel dysfunction that is characterized by chronic or frequently recurring abdominal pain that worsens with continued or escalating dosages of narcotics. This syndrome is underrecognized and may be becoming more prevalent. In the United States this may be the result of increases in using narcotics for chronic nonmalignant painful disorders, and the development of maladaptive therapeutic interactions around its use. NBS can occur in patients with no prior gastrointestinal disorder who receive high dosages of narcotics after surgery or acute painful problems, and among patients with functional gastrointestinal disorders or other chronic gastrointestinal diseases who are managed by physicians who are unaware of the hyperalgesic effects of chronic opioids. The evidence for the enhanced pain perception is based on the following: (1) activation of excitatory antianalgesic pathways within a bimodal opioid regulation system, (2) descending facilitation of pain at the rostral ventral medulla and pain facilitation via dynorphin and cholecystokinin activation, and (3) glial cell activation that produces morphine tolerance and enhances opioid-induced pain. Treatment involves early recognition of the syndrome, an effective physician-patient relationship, graded withdrawal of the narcotic according to a specified withdrawal program, and the institution of medications to reduce withdrawal effects.

Interleukin-1 signaling modulates stress-induced analgesia

Exposure to stressful stimuli is often accompanied by reduced pain sensitivity, termed "stress-induced analgesia" (SIA). In the present study, the hypothesis that interleukin-1 (IL-1) may play a modulatory role in SIA was examined. Two genetic mouse models impaired in IL-1-signaling and their wild-type (WT) controls were employed. Another group of C57 mice was acutely administered with IL-1 receptor antagonist (IL-1ra). Mice were exposed to 2min swim stress at one of three water temperatures: 32 degrees C (mild stress), 20-23 degrees C (moderate stress), or 15 degrees C (severe stress); and then tested for pain sensitivity using the hot-plate test. Corticosterone levels were assessed in separate groups of WT and mutant mice following exposure to the three types of stress. Mild stress induced significant analgesia in the two WT strains and saline-treated mice, but not in the mutant strains or the IL-1ra-treated mice. Similarly, mild stress induced significantly elevated corticosterone levels in WT mice, and blunted corticosterone response in mutant mice. In contrast, both WT and mutant strains, as well as IL-1ra-treated mice, displayed analgesic and corticosterone responses following moderate and severe stress. Interestingly, the analgesic response to moderate stress was markedly potentiated in the mutant strains, as compared with their WT controls. The present results support our previous findings that in the absence of IL-1, stress response to mild stress is noticeably diminished. However, the analgesic response to moderate stress is markedly potentiated in mice with impaired IL-1 signaling, corroborating the anti-analgesic role of IL-1 in several pain modulatory conditions, including SIA.

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.

A novel immune-to-CNS communication pathway: cells of the meninges surrounding the spinal cord CSF space produce proinflammatory cytokines in response to an inflammatory stimulus

Pain is enhanced in response to elevations of proinflammatory cytokines in spinal cerebrospinal fluid (CSF), following either intrathecal injection of these cytokines or intrathecal immune challenge with HIV-1 gp120 that induces cytokine release. Spinal cord glia have been assumed to be the source of endogenous proinflammatory cytokines that enhance pain. However, assuming that spinal cord glia are the sole source of CSF cytokines may be an underestimate, as the cellular composition of the meninges surrounding the spinal cord CSF space includes several cell types known to produce proinflammatory cytokines. The present experiments provide the first investigation of the immunocompetent nature of the spinal cord meninges. Here, we explore whether rat meninges are responsive to intrathecal gp120. These studies demonstrate that: (a) intrathecal gp120 upregulates meningeal gene expression of proinflammatory signals, including tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), interleukin 6 (IL-6), and inducible nitric oxide synthase (iNOS), and (b) intrathecal gp120 induces meningeal release of TNF-alpha, IL-1beta, and IL-6. In addition, stimulation of isolated meninges in vitro with gp120 induced the release of TNF-alpha and IL-1beta, indicating that the resident cells of the meninges are able to respond without immune cell recruitment. Taken together, these data document that the meninges are responsive to immunogenic stimuli in the CSF and that the meninges may be a source of immune products detected in CSF. The ability of the meninges to release to proinflammatory signals suggests a potential role in the modulation of pain.

Ibudilast (AV-411)

The treatment of neuropathic pain is a major unresolved medical challenge. Present pharmacotherapies only have modest efficacy and numerous side effects. The use of opioid analgesics is additionally coupled with dependence and withdrawal syndromes. Ibudilast (AV-411) is a non-selective phosphodiesterase inhibitor that is also known to suppress glial cell activation. It has been used clinically for other indications with a good safety profile. As glial cell activation is considered to crucially contribute to neuropathic pain as well as opioid dependence and withdrawal, the authors conceived that ibudilast may be useful for treating these conditions. Preclinical data indicate that ibudilast crosses the blood-brain barrier, is well tolerated, is active on oral administration, reduces glial activation and attenuates pain symptoms in diverse rat models of neuropathic pain. In addition, it enhances acute morphine analgesia and attenuates morphine tolerance and withdrawal. Thus ibudilast may improve opioid efficacy and is a promising therapeutic candidate for neuropathic pain, with a novel mechanism of action.

Amitriptyline preserves morphine’s antinociceptive effect by regulating the glutamate transporter GLAST and GLT-1 trafficking and excitatory amino acids concentration in morphine-tolerant rats

The present study was undertaken to examine the effect of amitriptyline on the antinociceptive effect of morphine and its underlying mechanisms in regulating glutamate transporters trafficking in morphine-tolerant rats. Long-term morphine infusion induced antinociceptive tolerance and down-regulation of glutamate transporters (GTs), GLAST, GLT-1, and EAAC1, expression in the rat spinal cord dorsal horn. Acute amitriptyline treatment potentiated morphine's antinociceptive effect, with a 5.3-fold leftward shift of morphine's dose-response curve in morphine-tolerant rats, and this was associated with GLAST and GLT-1 trafficking onto the cell surface. Similar to our previous studies, morphine challenge (10 microg/10 microl, i.t.) significant by increased the excitatory amino acids (EAAs) aspartate and glutamate level in the CSF dialysates of morphine-tolerant rats. Acute amitriptyline treatment not only suppressed this morphine-evoked EAA release, but further reduced the EAA concentration than baseline level. Furthermore, long-term morphine infusion up-regulated PKA and PKC protein expression in the spinal cord dorsal horn, while amitriptyline inhibited the increase in expression of phospho-PKA, PKCalpha, PKCbetaII, and PKCgamma. In morphine-tolerant rats, acute treatment with PKA inhibitor H89 and PKC inhibitor Gö6805 attenuated morphine tolerance and the morphine-induced CSF glutamate and aspartate elevation, and induced trafficking of GLAST and GLT-1 from cytosol onto the cell surface. These results show that acute amitriptyline treatment preserved morphine's antinociceptive effect in morphine-tolerant rats; the mechanisms may be involved in inhibition of phospho-PKA and PKC expression, and thus inducing the GLAST and GLT-1 trafficking onto glial cell surface which enhances the EAA uptake from the synaptic cleft and reduces EAA concentration in the spinal CSF.

The impact of morphine after a spinal cord injury

Nociceptive stimulation, at an intensity that elicits pain-related behavior, attenuates recovery of locomotor and bladder functions, and increases tissue loss after a contusion injury. These data imply that nociceptive input (e.g., from tissue damage) can enhance the loss of function after injury, and that potential clinical treatments, such as pretreatment with an analgesic, may protect the damaged system from further secondary injury. The current study examined this hypothesis and showed that a potential treatment (morphine) did not have a protective effect. In fact, morphine appeared to exacerbate the effects of nociceptive stimulation. Experiment 1 showed that after spinal cord injury 20mg/kg of systemic morphine was necessary to induce strong antinociception and block behavioral reactivity to shock treatment, a dose that was much higher than that needed for sham controls. In Experiment 2, contused rats were given one of three doses of morphine (Vehicle, 10, 20mg/kg) prior to exposure to uncontrollable electrical stimulation or restraint alone. Despite decreasing nociceptive reactivity, morphine did not attenuate the long-term consequences of shock. Rats treated with morphine and shock had higher mortality rates, and displayed allodynic responses to innocuous sensory stimuli three weeks later. Independent of shock, morphine per se undermined recovery of sensory function. Rats treated with morphine alone also had significantly larger lesions than those treated with saline. These results suggest that nociceptive stimulation affects recovery despite a blockade of pain-elicited behavior. The results are clinically important because they suggest that opiate treatment may adversely affect the recovery of function after injury.

The chemokine CX3CL1/fractalkine interferes with the antinociceptive effect induced by opioid agonists in the periaqueductal grey of rats

We have reported that there is heterologous interaction between the mu, delta or kappa opioid receptors and the receptors for the chemokines CCL5/RANTES or CXCL12/SDF-1 in the regulation of antinociception in rats. CX3CL1/fractalkine, a chemokine that exclusively binds to CX3CR1, has been found to affect morphine analgesia and tolerance in the spinal cord. The purpose of the present study was to see if the interaction between the chemokine CX3CL1/fractalkine receptor and mu, delta or kappa opioid receptors occurs in the periaqueductal grey (PAG) of adult male S-D rats. The cold-water tail-flick (CWT) test was used to measure antinociception. The results showed that intra-PAG injection of 100 ng CX3CL1/fractalkine 30 min before administration of 400 ng DAMGO, 100 ng DPDPE or 20 microg dynorphin significantly reduced the antinociception induced by each of these peptides. These results demonstrate that activation of the CX3CL1 receptor diminishes the effect of mu, delta and kappa opioid agonists on their receptors in the PAG of rats.

Peri-sciatic administration of recombinant rat TNF-alpha induces mechanical allodynia via upregulation of TNF-alpha in dorsal root ganglia and in spinal dorsal horn: the role of NF-kappa B pathway

Previous studies have shown that tumor necrosis factor-alpha (TNF-alpha) and TNF receptor 1 (TNFR1) in dorsal root ganglia (DRG) and in spinal dorsal horn are upregulated after nerve injury and that many TNF-alpha-containing neurons overexpress TNFR1. In the present study, we found that peri-sciatic administration of rat recombinant TNF-alpha (rrTNF) at the concentrations of 10, 100 and 1000 pg/ml (daily for 2 days) induced mechanical allodynia in bilateral hindpaws, lasting for about 20 days. The immunoreactivity (IR) of TNF-alpha and TNFR1 in the ipsilateral (but not in the contralateral) L4 and L5 DRGs increased significantly on day 1 and day 3 after administration of rrTNF, respectively. Double immunofluorescence staining revealed that in DRGs the increased TNF-alpha-IR was mainly in neuronal cells and with a lesser extent in satellite glial cells, while the upregulation of TNFR1-IR was almost restricted at neuronal cells. TNF-alpha-IR but not TNFR1-IR also increased in bilateral lumbar spinal dorsal horn from day 3 to day 14, which was observed in astrocytes, microglias and neurons. In addition, a progressive infiltration of monocyte/macrophages and T lymphocytes in the ipsilateral L5 DRG and sciatic nerve was observed, starting on day 2 following administration of rrTNF. Intrathecal delivery of PDTC (8.2 ng in 10 microl volume), a nuclear factor-kappa B (NF-kappaB) inhibitor, 30 min before each rrTNF administration blocked mechanical allodynia completely and inhibited the upregulation of TNF-alpha-IR and TNFR1-IR substantially. The results suggest that peri-sciatic administration of rrTNF may induce mechanical allodynia by an autocrine mechanism via activation of the NF-kappaB pathway.

Interleukin-10 reduces hyperalgesia and the level of Interleukin-1β in BALB/c mice infected with Leishmania major with no major effect on the level of Interleukin-6

Infection with a high dose of Leishmania major has been shown to induce hyperalgesia in BALB/c mice accompanied by a sustained upregulation of Interleukin-1beta (IL-1beta) and an early upregulation of Interleukin-6 (IL-6). On the other hand, Interleukin 10 (IL-10) has been demonstrated to be hypoalgesic in other models such as rats exposed to UV rays. In this study, we injected BALB/c mice with a high dose of Leishmania major and treated them with IL-10 (15 ng/animal) for six consecutive days. Hyperalgesia was assessed using thermal pain tests and the levels of IL-1beta and IL-6 were also assessed at different post-infection days. Our results show that IL-10 can reduce the Leishmania major-induced hyperalgesia during the treatment period through a direct effect on the levels of IL-1beta which seems to play an important role in this hyperalgesia induction since its level was reduced during the period of IL-10 injection and was increased again when this treatment was stopped. On the contrary IL-10 has no direct effect on the levels IL-6 which seems to have no direct role in the induced hyperalgesia.

Norman Cousins Lecture. Glia as the "bad guys": implications for improving clinical pain control and the clinical utility of opioids

Within the past decade, there has been increasing recognition that glia are far more than simply "housekeepers" for neurons. This review explores two recently recognized roles of glia (microglia and astrocytes) in: (a) creating and maintaining enhanced pain states such as neuropathic pain, and (b) compromising the efficacy of morphine and other opioids for pain control. While glia have little-to-no role in pain under basal conditions, pain is amplified when glia become activated, inducing the release of proinflammatory products, especially proinflammatory cytokines. How glia are triggered to become activated is a key issue, and appears to involve a number of neuron-to-glia signals including neuronal chemokines, neurotransmitters, and substances released by damaged, dying and dead neurons. In addition, glia become increasingly activated in response to repeated administration of opioids. Products of activated glia increase neuronal excitability via numerous mechanisms, including direct receptor-mediated actions, upregulation of excitatory amino acid receptor function, downregulation of GABA receptor function, and so on. These downstream effects of glial activation amplify pain, suppress acute opioid analgesia, contribute to the apparent loss of opioid analgesia upon repeated opioid administration (tolerance), and contribute to the development of opioid dependence. The potential implications of such glial regulation of pain and opioid actions are vast, suggestive that targeting glia and their proinflammatory products may provide a novel and effective therapy for controlling clinical pain syndromes and increasing the clinical utility of analgesic drugs.

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.

Inhibition of neuronal nitric oxide synthase antagonizes morphine antinociceptive tolerance by decreasing activation of p38 MAPK in the spinal microglia

We have demonstrated that the activation of p38 mitogen-activated protein kinase (MAPK) in the spinal microglia played an essential role in the development of morphine antinociceptive tolerance. The aim of this study was to investigate whether inhibition of neuronal nitric oxide synthase (nNOS) attenuated tolerance to morphine analgesia by modulating p38 activation in the spinal microglia. It was shown that the selective inhibitor of nNOS, 7-NINA (7-Nitroindazole, sodium salt) (25 microg, i.t.) attenuated not only the development of morphine antinociceptive tolerance, but also the activation of p38 MAPK in the spinal microglia induced by chronic intrathecal administration of morphine. Our results suggest that neuronal NO signals to microglia, leading to the upregulation of microglial phospho-p38 MAPK. Such p38 MAPK activation in microglia is consistent with a potential role in the development of morphine antinociceptive tolerance. We demonstrated for the first time that the inhibition of nNOS attenuated morphine antinociceptive tolerance by reducing p38 MAPK activation in the spinal microglia.

Rofecoxib modulates multiple gene expression pathways in a clinical model of acute inflammatory pain

New insights into the biological properties of cyclooxygenase-2 (COX-2) and its response pathway challenge the hypothesis that COX-2 is simply pro-inflammatory and inhibition of COX-2 solely prevents the development of inflammation and ameliorates inflammatory pain. The present study performed a comprehensive analysis of gene/protein expression induced by a selective inhibitor of COX-2, rofecoxib, compared with a non-selective COX inhibitor, ibuprofen, and placebo in a clinical model of acute inflammatory pain (the surgical extraction of impacted third molars) using microarray analysis followed by quantitative RT-PCR verification and Western blotting. Inhibition of COX-2 modulated gene expression related to inflammation and pain, the arachidonic acid pathway, apoptosis/angiogenesis, cell adhesion and signal transduction. Compared to placebo, rofecoxib treatment increased the gene expression of ANXA3 (annexin 3), SOD2 (superoxide dismutase 2), SOCS3 (suppressor of cytokine signaling 3) and IL1RN (IL1 receptor antagonist) which are associated with inhibition of phospholipase A(2) and suppression of cytokine signaling cascades, respectively. Both rofecoxib and ibuprofen treatment increased the gene expression of the pro-inflammatory mediators, IL6 and CCL2 (chemokine C-C motif ligand 2), following tissue injury compared to the placebo treatment. These results indicate a complex role for COX-2 in the inflammatory cascade in addition to the well-characterized COX-dependent pathway, as multiple pathways are also involved in rofecoxib-induced anti-inflammatory and analgesic effects at the gene expression level. These findings may also suggest an alternative hypothesis for the adverse effects attributed to selective inhibition of COX-2.

Fractalkine and minocycline alter neuronal activity in the spinal cord dorsal horn

Fractalkine (FKN) evokes nociceptive behavior in nai ve rats, whereas minocycline attenuates pain acutely after neuronal injury. We show that, in nai ve rats, FKN causes hyperresponsiveness of lumbar wide dynamic range neurons to brush, pressure and pinch applied to the hindpaw. One day after spinal nerve ligation (SNL), minocycline attenuates after-discharge and responses to brush and pressure. In contrast, minocycline does not alter evoked neuronal responses 10 days after SNL or sciatic constriction, but increases spontaneous discharge. We speculate that microglia rapidly alter sensory neuronal activity in nai ve and neuropathic rats acutely, but not chronically, after injury.

Amitriptyline suppresses neuroinflammation and up-regulates glutamate transporters in morphine-tolerant rats

The present study was performed to evaluate the effects of the tricyclic antidepressant amitriptyline on morphine tolerance in rats. Male Wistar rats were implanted with two intrathecal (i.t.) catheters with or without a microdialysis probe, then received a continuous i.t. infusion of saline (control) or morphine (15 microg/h) and/or amitriptyline (15 microg/h) for 5 days. The results showed that amitriptyline alone did not produce an antinociceptive effect, while morphine alone induced antinociceptive tolerance and down-regulation of spinal glutamate transporters (GLAST, GLT-1, and EAAC1) in the rat spinal cord dorsal horn. Co-administration of amitriptyline with morphine attenuated morphine tolerance and up-regulated GLAST and GLT-1 expression. On day 5, morphine challenge (10 microg/10 microl) resulted in a significant increase in levels of the excitatory amino acids (EAAs), aspartate and glutamate, in CSF dialysates in morphine-tolerant rats. Amitriptyline co-infusion not only markedly suppressed this morphine-evoked EAA release, but also preserved the antinociceptive effect of acute morphine challenge at the end of infusion. Glial cells activation and increased cytokine expression (TNFalpha, IL-1beta, and IL-6) in the rat spinal cord were induced by the 5-day morphine infusion and these neuroimmune responses were also prevented by amitriptyline co-infusion. These results show that amitriptyline not only attenuates morphine tolerance, but also preserves its antinociceptive effect. The mechanisms involved may include: (a) inhibition of pro-inflammatory cytokine expression, (b) prevention of glutamate transporter down-regulation, and even up-regulation of glial GTs GLAST and GLT-1 expression, with (c) attenuation of morphine-evoked EAA release following continuous long-term morphine infusion.

Expression of fractalkine and fractalkine receptor in urinary bladder after cyclophosphamide (CYP)-induced cystitis

Alterations in the expression of the chemokine, fractalkine (CX3CL1), were examined in the urinary bladder after cyclophosphamide (CYP)-induced cystitis of varying duration: acute (4 h or 48 h), or chronic (10 day). CYP-induced cystitis significantly (p<or=0.01) increased fractalkine protein expression in the urinary bladder with acute (48 h) and chronic CYP treatment. Western blot analysis also demonstrated significantly (p<or=0.01) increased fractalkine expression in the whole urinary bladder with acute (1.5-2.2-fold) and chronic (3-fold) CYP-induced cystitis. Immunohistochemistry for fractalkine-immunoreactivity revealed little fractalkine-IR in control or acute (4 h) CYP-treated rat urinary bladders except in a vascular bed but showed no colocalization with nerve fibers in the suburothelial plexus in any experimental group. However, expression was significantly (p<or=0.001) upregulated in the urothelium with 48 h or chronic CYP treatment. Similarly, fractalkine receptor (CX3CR1)-IR was significantly (p<or=0.001) upregulated in the urothelium with 48 h or chronic CYP treatment. These studies demonstrated upregulation of the chemokine, fractalkine, in the urinary bladder and specifically in the urothelium with CYP-induced cystitis. Chemokines, and specifically, fractalkine, may be another class of neuromodulatory agents upregulated in the urinary bladder that can affect micturition function and sensory processing with cystitis and may represent novel, drug targets for cystitis.

[Mechanisms in the development of pain. Key issue in the periphery]

The interaction between neurons, tissue cells and inflammatory cells is of major importance for the development of pain in the periphery. In this interaction a variety of inhibitory and activating circuits has been identified in recent years. In addition to the receptors for classical inflammatory mediators on the sensory terminals like bradykinin, axonal ion channels have been identified as major modulators of pain and sensitization apart from their traditional role in conduction of action potentials. Sensitization of spinal nociceptive processing is crucial for the expansion of pain beyond the initially injured site and contributes to chronic pain. Learning processes and extinction of aversive memory are of major importance for the development, but also the therapy of chronic pain states.

Neonatal infection-induced memory impairment after lipopolysaccharide in adulthood is prevented via caspase-1 inhibition

We have reported that neonatal infection leads to memory impairment after an immune challenge in adulthood. Here we explored whether events occurring as a result of early infection alter the response to a subsequent immune challenge in adult rats, which may then impair memory. In experiment 1, peripheral infection with Escherichia coli on postnatal day 4 increased cytokines and corticosterone in the periphery, and cytokine and microglial cell marker gene expression in the hippocampus of neonate pups. Next, rats treated neonatally with E. coli or PBS were injected in adulthood with lipopolysaccharide (LPS) or saline and killed 1-24 h later. Microglial cell marker mRNA was elevated in hippocampus in saline controls infected as neonates. Furthermore, LPS induced a greater increase in glial cell marker mRNA in hippocampus of neonatally infected rats, and this increase remained elevated at 24 h versus controls. After LPS, neonatally infected rats exhibited faster increases in interleukin-1beta (IL-1beta) within the hippocampus and cortex and a prolonged response within the cortex. There were no group differences in peripheral cytokines or corticosterone. In experiment 2, rats treated neonatally with E. coli or PBS received as adults either saline or a centrally administered caspase-1 inhibitor, which specifically prevents the synthesis of IL-1beta, 1 h before a learning event and subsequent LPS challenge. Caspase-1 inhibition completely prevented LPS-induced memory impairment in neonatally infected rats. These data implicate IL-1beta in the set of immune/inflammatory events that occur in the brain as a result of neonatal infection, which likely contribute to cognitive alterations in adulthood.

Activation of p38 mitogen-activated protein kinase in spinal microglia mediates morphine antinociceptive tolerance

Compelling evidence has suggested that spinal glial cells were activated by chronic morphine treatment and involved in the development of morphine tolerance. However, the mechanisms of glial activation were still largely unknown in morphine tolerance. In present study, we investigated the role of p38 mitogen-activated protein kinase (p38 MAPK) in the spinal cord in the development of chronic morphine antinociceptive tolerance. We found that intrathecal administration of morphine (15 microg) daily for 7 consecutive days significantly induced an increase in number of phospho-p38 (p-p38) immunoreactive cells in the spinal cord compared with chronic saline or acute morphine treated rats. Double immunofluorescence staining revealed that p-p38 immunoreactivity was exclusively restricted in the activated spinal microglia, not in astrocytes or neurons. Repeated intrathecal administration of 4-(4-fluorophenyl)-2-(4-methylsulfonylphenyl)-5-(4-pyridyl)-1H-imidazole (SB203580) (10 microg or 2 microg), a specific p38 inhibitor, 30 min before each morphine injection for 7 consecutive days significantly attenuated tolerance to morphine analgesia assessed by tail flick test. However, a single intrathecal administration of SB203580 (10 microg) did not antagonize the established tolerance to morphine analgesia. Taken together, these findings suggested that p38 MAPK activation in the spinal microglia was involved in the development of morphine antinociceptive tolerance. Inhibition of p38 MAPK by SB203580 in the spinal cord attenuated but not reversed the tolerance to morphine analgesia. The present study provides the first evidence that p38 activation in spinal microglia played an important role in the development of tolerance to morphine analgesia.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

An initial investigation of spinal mechanisms underlying pain enhancement induced by fractalkine, a neuronally released chemokine

Fractalkine is a chemokine that is tethered to the extracellular surface of neurons. Fractalkine can be released, forming a diffusible signal. Spinal fractalkine (CX3CL1) is expressed by sensory afferents and intrinsic neurons, whereas its receptor (CX3CR1) is predominantly expressed by microglia. Pain enhancement occurs in response both to intrathecally administered fractalkine and to spinal fractalkine endogenously released by peripheral neuropathy. The present experiments examine whether fractalkine-induced pain enhancement is altered by a microglial inhibitor (minocycline) and/or by antagonists/inhibitors of three putative glial products implicated in pain enhancement: interleukin-1 (IL1), interleukin-6 (IL6) and nitric oxide (NO). In addition, it extends a prior study that demonstrated that intrathecal fractalkine-induced mechanical allodynia is blocked by a neutralizing antibody to the rat fractalkine receptor, CX3CR1. Here, intrathecal anti-CX3CR1 also blocked fractalkine-induced thermal hyperalgesia. Furthermore, blockade of microglial activation with minocycline prevented both fractalkine-induced mechanical allodynia (von Frey test) and thermal hyperalgesia (Hargreaves test). Microglial activation appears to lead to the release of IL1, given that pretreatment with IL1 receptor antagonist blocked both fractalkine-induced mechanical allodynia and thermal hyperalgesia. IL1 is not the only proinflammatory cytokine implicated, as a neutralizing antibody to rat IL6 also blocked fractalkine-induced pain facilitation. Lastly, NO appears to be importantly involved, as l-NAME, a broad-spectrum NO synthase inhibitor, also blocked fractalkine-induced effects. Taken together, these data support that neuronally released fractalkine enhances pain via activation of spinal cord glia. Thus, fractalkine may be a neuron-to-glia signal triggering pain facilitation.

Chemokines: integrators of pain and inflammation

Chronic (neuropathic) pain is one of the most widespread and intractable of human complaints, as well as being one of the most difficult syndromes to treat successfully with drugs or surgery. The development of new therapeutic approaches to the treatment of painful neuropathies requires a better understanding of the mechanisms that underlie the development of these chronic pain syndromes. It is clear that inflammatory responses often accompany the development of neuropathic pain, and here we discuss the idea that chemokines might be key to integrating the development of pain and inflammation and could furnish new leads in the search for effective analgesic agents for the treatment of painful neuropathies.

Glia: novel counter-regulators of opioid analgesia

Development of analgesic tolerance and withdrawal-induced pain enhancement present serious difficulties for the use of opioids for pain control. Although neuronal mechanisms to account for these phenomena have been sought for many decades, their bases remain unresolved. Within the past four years, a novel non-neuronal candidate has been uncovered that opposes acute opioid analgesia and contributes to development of opioid tolerance and tolerance-associated pain enhancement. This novel candidate is spinal cord glia. Glia are important contributors to the creation of enhanced pain states via the release of neuroexcitatory substances. New data suggest that glia also release neuroexcitatory substances in response to morphine, thereby opposing its effects. Controlling glial activation could therefore increase the clinical utility of analgesic drugs.

Interleukin-1 antagonizes morphine analgesia and underlies morphine tolerance

Pain sensitivity reflects a balance between pain facilitatory and inhibitory systems. To characterize the relationships between these systems we examined the interactions between the analgesic effects of morphine and the anti-analgesic effects of the pro-inflammatory cytokine interleukin-1 (IL-1). We report that administration of a neutral dose of IL-1beta abolished morphine analgesia in mice, whereas acute or chronic blockade of IL-1 signaling by various IL-1 blockers (IL-1 receptor antagonist (IL-1ra), alpha-melanocyte-stimulating hormone, or IL-1 tri-peptide antagonist) significantly prolonged and potentiated morphine analgesia. Morphine-induced analgesia was also extended in strains of mice genetically impaired in IL-1 signaling (mice with transgenic over-expression of IL-1 receptor antagonist, deletion of the IL-1 receptor type I, or deletion of the IL-1 receptor accessory protein). The finding that IL-1 produces a marked anti-analgesic effect, suggests that it may also be involved in the development of opiate tolerance. Indeed, genetic or pharmacological blockade of IL-1 signaling prevented the development of tolerance following repeated morphine administration. Moreover, acute administration of IL-1ra in wild type mice, either immediately following the cessation of acute morphine analgesia, or following the development of chronic morphine tolerance, re-instated the analgesia, suggesting that blockade of the IL-1 system unmasks the analgesic effect of morphine. These findings suggest that morphine produces an IL-1-mediated homeostatic response, which serves to limit the duration and extent of morphine analgesia and which underlies the development of tolerance.