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

Blocking ATP-sensitive potassium channel alleviates morphine tolerance by inhibiting HSP70-TLR4-NLRP3-mediated neuroinflammation

BACKGROUND

Long-term use of morphine induces analgesic tolerance, which limits its clinical efficacy. Evidence indicated morphine-evoked neuroinflammation mediated by toll-like receptor 4 (TLR4) - NOD-like receptor protein 3 (NLRP3) inflammasome was important for morphine tolerance. In our study, we investigated whether other existing alternative pathways caused morphine-induced activation of TLR4 in microglia. We focused on heat shock protein 70 (HSP70), a damage-associated molecular pattern (DAMP), which was released from various cells upon stimulations under the control of K_ATP channel and bound with TLR4-inducing inflammation. Glibenclamide, a classic K_ATP channel blocker, can improve neuroinflammation by inhibiting the activation of NLRP3 inflammasome. Our present study investigated the effect and possible mechanism of glibenclamide in improving morphine tolerance via its specific inhibition on the release of HSP70 and activation of NLRP3 inflammasome induced by morphine.

METHODS

CD-1 mice were used for tail-flick test to evaluate morphine tolerance. The microglial cell line BV-2 and neural cell line SH-SY5Y were used to investigate the pharmacological effects and the mechanism of glibenclamide on morphine-induced neuroinflammation. The activation of microglia was accessed by immunofluorescence staining. Neuroinflammation-related cytokines were measured by western blot and real-time PCR. The level of HSP70 and related signaling pathway were evaluated by western blot and immunofluorescence staining.

RESULTS

Morphine induced the release of HSP70 from neurons. The released HSP70 activated microglia and triggered TLR4-mediated inflammatory response, leading to the phosphorylation of p38 mitogen-activated protein kinase (MAPK) and nuclear factor-κB (NF-κB) p65 and the activation of NLRP3 inflammasome. Moreover, anti-HSP70 neutralizing antibody partly attenuated chronic morphine tolerance. The secretion of HSP70 was under the control of MOR/AKT/K_ATP/ERK signal pathway. Glibenclamide as a classic K_ATP channel blocker markedly inhibited the release of HSP70 induced by morphine and suppressed HSP70-TLR4-NLRP3 inflammasome-mediated neuroinflammation, which consequently attenuated morphine tolerance.

CONCLUSIONS

Our study indicated that morphine-induced extracellular HSP70 was an alternative way for the activation of TLR4-NLRP3 in analgesic tolerance. The release of HSP70 was regulated by MOR/AKT/K_ATP/ERK pathway. Our study suggested a promising target, K_ATP channel and a new leading compound, glibenclamide, for treating morphine tolerance.

Inhibition of morphine tolerance by MrgC receptor via modulation of interleukin-1β and matrix metalloproteinase 9 in dorsal root ganglia in rats

Opiate tolerance is a critical issue in pain management. Previous studies show that activation of Mas-related gene (Mrg) C receptor can modulate the development of morphine tolerance. This study was designed to investigate the underlying mechanism(s). Intrathecal (i.t.) administration of morphine (20µg) increased the expression of interleukin-1β (IL-1β) and matrix metalloproteinase-9 (MMP-9) in small- and medium-sized neurons in dorsal root ganglia (DRG). Co-administration of bovine adrenal medulla 8-22 (BAM8-22), a selective MrgC receptor agonist, via i.t. route inhibited the increase of IL-1β and MMP-9 in the DRG. Exposure of DRG cultures to morphine (3.3μM) for 3 or 5 days, but not for 1 day, induced an increase in MMP-9 mRNA expression. The treatment with BAM8-22 (10nM) for 20, 40 or 60min abolished chronic (5 days) morphine-induced increase of MMP-9 mRNA in the cultured DRG. The treatment with BAM8-22 for 1h inhibited chronic morphine-induced increase of MMP-9 and IL-1β mRNA in DRG but these effects were abolished by MrgC receptor antibody. The treatment with BAM8-22 for 24 and 72h respectively inhibited and enhanced morphine-induced expression of MMP-9 and IL-1β mRNA in the cultured DRG. The BAM8-22-induced inhibition and enhancement were abolished by MrgC receptor antibody. The results suggest that the inhibition of IL-1β and MMP-9 expressions in DRG underlain the modulation of morphine tolerance by the acute activation of MrgC receptors. The chronic activation of MrgC receptors can facilitate morphine-induced increase of MMP-9 and IL-1β expressions in DRG.

Potential role of CXCL10/CXCR3 signaling in the development of morphine tolerance in periaqueductal gray

Tolerance to morphine antinociception hinders its long-term use in clinical practice. Interaction between neuron and microglia has been proved to play critical role in the mechanism of morphine tolerance, while CXCL10/CXCR3 signaling has been implicated in neuron-glia signaling and morphine analgesia. This study aims to investigate whether CXCL10/CXCR3 signaling in periaqueductal gray (PAG) contributes to the development of morphine tolerance by modulating neuron-microglia interaction. The results showed that the expressions of CXCR3 and CXCL10 were gradually increased in parallel with repeated morphine administration and activation of microglia. CXCR3 was co-localized with neuronal marker NeuN, while CXCL10 was derived from microglia. Microglia inhibitor minocycline significantly attenuated the expression of CXCL10, besides, both minocycline and CXCR3 inhibitor alleviated the development of morphine tolerance. Taken together, our study provided the evidence that CXCL10/CXCR3 signaling in PAG is involved in the development of morphine analgesic tolerance via neuron-microglia interaction.

Effects of ibudilast on oxycodone-induced analgesia and subjective effects in opioid-dependent volunteers

Opioid-induced glial activation is hypothesized to contribute to the development of tolerance to opioid-induced analgesia. This inpatient, double-blind, placebo-controlled, within-subject and between-groups pilot study investigated the dose-dependent effects of ibudilast, a glial cell modulator, on oxycodone-induced analgesia. Opioid-dependent volunteers were maintained on morphine (30mg, PO, QID) for two weeks and received placebo ibudilast (0mg, PO, BID) during the 1st week (days 1-7). On day 8, participants (N=10/group) were randomized to receive ibudilast (20 or 40mg, PO, BID) or placebo for the remainder of the study. On days 4 (week 1) and 11 (week 2), the analgesic, subjective, and physiological effects of oxycodone (0, 25, 50mg/70kg, PO) were determined. Analgesia was measured using the cold pressor test; participants immersed their hand in cold water (4°C) and pain threshold and pain tolerability were recorded. Oxycodone decreased pain threshold and tolerability in all groups during week 1. During week 2, the placebo group exhibited a blunted analgesic response to oxycodone for pain threshold and subjective pain ratings, whereas the 40mg BID ibudilast group exhibited greater analgesia as measured by subjective pain ratings (p≤0.05). Oxycodone also increased subjective drug effect ratings associated with abuse liability in all groups during week 1 (p≤0.05); ibudilast did not consistently affect these ratings. These findings suggest that ibudilast may enhance opioid-induced analgesia. Investigating higher ibudilast doses may establish the utility of pharmacological modulation of glial activity to maximize the clinical use of opioids.

Spinal CX3CL1/CX3CR1 May Not Directly Participate in the Development of Morphine Tolerance in Rats

CX3CL1 (fractalkine), the sole member of chemokine CX3C family, is implicated in inflammatory and neuropathic pain via activating its receptor CX3CR1 on neural cells in spinal cord. However, it has not been fully elucidated whether CX3CL1 or CX3CR1 contributes to the development of morphine tolerance. In this study, we found that chronic morphine exposure did not alter the expressions of CX3CL1 and CX3CR1 in spinal cord. And neither exogenous CX3CL1 nor CX3CR1 inhibitor could affect the development of morphine tolerance. The cellular localizations of spinal CX3CL1 and CX3CR1 changed from neuron and microglia, respectively, to all the neural cells during the development of morphine tolerance. A microarray profiling revealed that 15 members of chemokine family excluding CX3CL1 and CX3CR1 were up-regulated in morphine-treated rats. Our study provides evidence that spinal CX3CL1 and CX3CR1 may not be involved in the development of morphine tolerance directly.

N-acetyl-cysteine attenuates remifentanil-induced postoperative hyperalgesia via inhibiting matrix metalloproteinase-9 in dorsal root ganglia

Treatment of remifentanil-induced postoperative hyperalgesia (RIH) remains a clinical challenge because the mechanisms are not fully understood. Matrix metalloproteinase-9 (MMP-9) is a key component in neuroinflammation because of its facilitation of pro-inflammatory cytokine maturation. Therefore, inhibition of MMP-9 may represent a novel therapeutic approach to the treatment of RIH. Sprague-Dawley rats were randomly divided into three groups: Control, Incision and Remifentanil. A right plantar surgical incision was performed in Group Incision, and intraoperative remifentanil (0.04 mg/kg, 0.4 ml) was infused subcutaneously for 30 min in Group Remifentanil. The results indicated that intraoperative remifentanil induced an up-regulation and activation of MMP-9 in DRGs but not spinal cords. MMP-9 was expressed primarily in DRG neurons co-expressing mu opioid receptors (MOR), and elicited interleukin-1β (IL-1β) cleavage in DRG neurons and satellite glial cells (SGCs). Intraperitoneal injection of N-acetyl-cysteine (NAC), a broadly used safe drug, significantly attenuated RIH via suppressing the activation of MMP-9 in DRGs. NAC inhibited the cleavage of IL-1β in DRGs, which is a critical substrate of MMP-9, and markedly suppressed glial activation and neuron excitability in spinal dorsal horn induced by remifentanil. These results demonstrated that NAC can effectively alleviate RIH via powerfully inhibiting MMP-9 activation in DRGs.

Effects of Ibudilast on the Subjective, Reinforcing, and Analgesic Effects of Oxycodone in Recently Detoxified Adults with Opioid Dependence

Ibudilast, a nonselective phosphodiesterase inhibitor, is used clinically in Asia for the treatment of asthma and poststroke dizziness. Recent preclinical studies have suggested that it also inhibits glial cell activation in rodents, and may alter opioid-mediated effects, including analgesia and withdrawal symptoms. The effects of ibudilast on the abuse potential of opioids in humans are largely unknown. The present study was designed to examine the influence of ibudilast on subjective (including drug craving), reinforcing, and analgesic effects of oxycodone in human volunteers diagnosed with opioid dependence (equivalent to moderate-severe opioid use disorder). Non-treatment-seeking opioid-dependent male volunteers (n=11) underwent an in-patient detoxification with morphine, followed by maintenance on placebo (0 mg b.i.d.) and active ibudilast (50 mg b.i.d.). Under each maintenance dose, six experimental sample and choice sessions were completed involving oral oxycodone administration (0, 15, and 30 mg/70 kg, p.o.). Subjective effects of oxycodone and drug craving were measured with visual analog scales (VAS) and a Drug Effects Questionnaire. The cold pressor test was used to produce pain, and a modified progressive-ratio choice procedure was used to measure the reinforcing effects of oxycodone. Under the active ibudilast condition compared with the placebo condition, ratings of drug liking following 15 mg of oxycodone were decreased significantly. The mean drug breakpoint value was also significantly lower in the active vs the placebo ibudilast condition under the 15 mg oxycodone condition, but not significantly lower under the 30 mg oxycodone condition. Heroin craving was significantly reduced under active ibudilast vs placebo, and similar effects were observed for tobacco and cocaine craving. Furthermore, mean subjective ratings of pain were lower in the active ibudilast condition. Our data suggest that ibudilast may be useful for treating opioid use disorders and it may enhance the analgesic effects of oxycodone.

Genetic deletion of microglial Panx1 attenuates morphine withdrawal, but not analgesic tolerance or hyperalgesia in mice

Opioids are among the most powerful analgesics for managing pain, yet their repeated use can lead to the development of severe adverse effects. In a recent study, we identified the microglial pannexin-1 channel (Panx1) as a critical substrate for opioid withdrawal. Here, we investigated whether microglial Panx1 contributes to opioid-induced hyperalgesia (OIH) and opioid analgesic tolerance using mice with a tamoxifen-inducible deletion of microglial Panx1. We determined that escalating doses of morphine resulted in thermal pain hypersensitivity in both Panx1-expressing and microglial Panx1-deficient mice. In microglial Panx1-deficient mice, we also found that acute morphine antinociception remained intact, and repeated morphine treatment at a constant dose resulted in a progressive decline in morphine antinociception and a reduction in morphine potency. This reduction in morphine antinociceptive potency was indistinguishable from that observed in Panx1-expressing mice. Notably, morphine tolerant animals displayed increased spinal microglial reactivity, but no change of microglial Panx1 expression. Collectively, our findings indicate microglial Panx1 differentially contributes to opioid withdrawal, but not the development of opioid-induced hyperalgesia or tolerance.

Differentially Severe Cognitive Effects of Compromised Cerebral Blood Flow in Aged Mice: Association with Myelin Degradation and Microglia Activation

Bilateral common carotid artery stenosis (BCAS) models the effects of compromised cerebral blood flow on brain structure and function in mice. We compared the effects of BCAS in aged (21 month) and young adult (3 month) female mice, anticipating a differentially more severe effect in the older mice. Four weeks after surgery there was a significant age by time by treatment interaction on the radial-arm water maze (RAWM; p = 0.014): on the first day of the test, latencies of old mice were longer compared to the latencies of young adult mice, independent of BCAS. However, on the second day of the test, latencies of old BCAS mice were significantly longer than old control mice (p = 0.049), while latencies of old controls were similar to those of the young adult mice, indicating more severe impairment of hippocampal dependent learning and working memory by BCAS in the older mice. Fluorescence staining of myelin basic protein (MBP) showed that old age and BCAS both induced a significant decrease in fluorescence intensity. Evaluation of the number oligodendrocyte precursor cells demonstrated augmented myelin replacement in old BCAS mice (p < 0.05) compared with young adult BCAS and old control mice. While microglia morphology was assessed as normal in young adult control and young adult BCAS mice, microglia of old BCAS mice exhibited striking activation in the area of degraded myelin compared to young adult BCAS (p < 0.01) and old control mice (p < 0.05). These findings show a differentially more severe effect of cerebral hypoperfusion on cognitive function, myelin integrity and inflammatory processes in aged mice. Hypoperfusion may exacerbate degradation initiated by aging, which may induce more severe neuronal and cognitive phenotypes.

Exploring acute-to-chronic neuropathic pain in rats after contusion spinal cord injury

Spinal cord injury (SCI) causes chronic pain in 65% of individuals. Unfortunately, current pain management is inadequate for many SCI patients. Rodent models could help identify how SCI pain develops, explore new treatment strategies, and reveal whether acute post-SCI morphine worsens chronic pain. However, few studies explore or compare SCI-elicited neuropathic pain in rats. Here, we sought to determine how different clinically relevant contusion SCIs in male and female rats affect neuropathic pain, and whether acute morphine worsens later chronic SCI pain. First, female rats received sham surgery, or 150kDyn or 200kDyn midline T9 contusion SCI. These rats displayed modest mechanical allodynia and long-lasting thermal hyperalgesia. Next, a 150kDyn (1s dwell) midline contusion SCI was performed in male and female rats. Interestingly, males, but not females showed SCI-elicited mechanical allodynia; rats of both sexes had thermal hyperalgesia. In this model, acute morphine treatment had no significant effect on chronic neuropathic pain symptoms. Unilateral SCIs can also elicit neuropathic pain that could be exacerbated by morphine, so male rats received unilateral T13 contusion SCI (100kDyn). These rats exhibited significant, transient mechanical allodynia, but not thermal hyperalgesia. Acute morphine did not exacerbate chronic pain. Our data show that specific rat contusion SCI models cause neuropathic pain. Further, chronic neuropathic pain elicited by these contusion SCIs was not amplified by our course of early post-trauma morphine. Using clinically relevant rat models of SCI could help identify novel pain management strategies.

Biphalin, a Dimeric Enkephalin, Alleviates LPS-Induced Activation in Rat Primary Microglial Cultures in Opioid Receptor-Dependent and Receptor-Independent Manners

Neuropathic pain is relatively less responsive to opioids than other types of pain, which is possibly due to a disrupted opioid system partially caused by the profound microglial cell activation that underlines neuroinflammation. We demonstrated that intrathecally injected biphalin, a dimeric enkephalin analog, diminished symptoms of neuropathy in a preclinical model of neuropathic pain in rats (CCI, chronic constriction injury of the sciatic nerve) at day 12 postinjury. Using primary microglial cell cultures, we revealed that biphalin did not influence cell viability but diminished NO production and expression of Iba1 in LPS-stimulated cells. Biphalin also diminished MOP receptor level, as well as pronociceptive mediators (iNOS, IL-1β, and IL-18) in an opioid receptor-dependent manner, and it was correlated with diminished p-NF-κB, p-IκB, p-p38MAPK, and TRIF levels. Biphalin reduced IL-6, IL-10, TNFα, p-STAT3, and p-ERK1/2 and upregulated SOCS3, TLR4, and MyD88; however, this effect was not reversed by naloxone pretreatment. Our study provides evidence that biphalin diminishes neuropathy symptoms, which might be partially related to reduced pronociceptive mediators released by activated microglia. Biphalin may be a putative drug for future pain therapy, especially for the treatment of neuropathic pain, when the lower analgesic effects of morphine are correlated with profound microglial cell activation.

Opioids Resistance in Chronic Pain Management

Chronic pain management represents a serious healthcare problem worldwide. Chronic pain affects approximately 20% of the adult European population and is more frequent in women and older people. Unfortunately, its management in the community remains generally unsatisfactory and rarely under the control of currently available analgesics. Opioids have been used as analgesics for a long history and are among the most used drugs; however, while there is no debate over their short term use for pain management, limited evidence supports their efficacy of long-term treatment for chronic non-cancer pain. Therapy with opioids is hampered by inter-individual variability and serious side effects and some opioids often result ineffective in the treatment of chronic pain and their use is controversial. Accordingly, for a better control of chronic pain a deeper knowledge of the molecular mechanisms underlying resistance to opiates is mandatory.

Opening of the Adenosine Triphosphate-sensitive Potassium Channel Attenuates Morphine Tolerance by Inhibiting JNK and Astrocyte Activation in the Spinal Cord

OBJECTIVES

In the present study, we investigated the role of adenosine triphosphate (ATP)-sensitive potassium (KATP) channels in chronic morphine tolerance.

MATERIALS AND METHODS

Male mice were injected intrathecally with morphine or saline, respectively (each in 10 μL). Different doses of the KATP opener cromakalim (0.3, 1, or 3 μg/10 μL/mouse) were administered 15 minutes before the morphine (10 μg/10 μL/mouse) challenge daily for 7 consecutive days. Half an hour after morphine injection, the tail-flick latency was measured to evaluate the antinociceptive effect of morphine. On the seventh day, mice were euthanized with sodium pentobarbital (100 mg/kg) at 1 hour after morphine injection, and their spinal cords were removed for the assays of Western blot, immunofluorescence, and quantitative real-time polymerase chain reaction.

RESULTS

Opening of the KATP channel attenuates chronic morphine tolerance, suppresses astrocyte activation inhibits the increase in interleukin-1β at the transcriptional and the translational levels, and reduces the upregulation of phosphorylated c-Jun N-terminal kinase mitogen-activated protein kinase in the spinal cord after chronic morphine treatment. Moreover, transcriptional levels of spinal cord astrocyte KATP channel subunits, named the inwardly rectifying potassium (Kir) 6.1 and sulfonylurea receptor 1, are decreased in morphine-tolerant mice.

DISCUSSION

Cromakalim suppresses morphine-induced astrocyte activation significantly by suppressing the c-Jun N-terminal kinase pathway, resulting in a reduced release of interleukin-1β and the attenuation of morphine chronic antinociceptive tolerance.

The RS504393 Influences the Level of Nociceptive Factors and Enhances Opioid Analgesic Potency in Neuropathic Rats

Increasing evidence has indicated that activated glial cells releasing nociceptive factors, such as interleukins and chemokines, are of key importance for neuropathic pain. Significant changes in the production of nociceptive factors are associated with the low effectiveness of opioids in neuropathic pain. Recently, it has been suggested that CCL2/CCR2 signaling is important for nociception. Here, we studied the time course changes in the mRNA/protein level of CD40/Iba-1, CCL2 and CCR2 in the spinal cord/dorsal root ganglia (DRG) in rats following chronic constriction injury (CCI) of the sciatic nerve. Moreover, we examined the influence of intrathecal preemptive and repeated (daily for 7 days) administration of RS504393, CCR2 antagonist, on pain-related behavior and the associated biochemical changes of some nociceptive factors as well as its influence on opioid effectiveness. We observed simultaneous upregulation of Iba-1, CCL2, CCR2 in the spinal cord on 7th day after CCI. Additionally, we demonstrated that repeated administration of RS504393 not only attenuated tactile/thermal hypersensitivity but also enhanced the analgesic properties of morphine and buprenorphine under neuropathy. Our results proof that repeated administration of RS504393 reduced the mRNA and/or protein levels of pronociceptive factors, such as IL-1beta, IL-18, IL-6 and inducible nitric oxide synthase (iNOS), and some of their receptors in the spinal cord and/or DRG. Furthermore, RS504393 elevated the spinal protein level of antinociceptive IL-1alpha and IL-18 binding protein. Our data provide new evidence that CCR2 is a promising target for diminishing neuropathic pain and enhancing the opioid analgesic effects.

Supradural inflammatory soup in awake and freely moving rats induces facial allodynia that is blocked by putative immune modulators

Facial allodynia is a migraine symptom that is generally considered to represent a pivotal point in migraine progression. Treatment before development of facial allodynia tends to be more successful than treatment afterwards. As such, understanding the underlying mechanisms of facial allodynia may lead to a better understanding of the mechanisms underlying migraine. Migraine facial allodynia is modeled by applying inflammatory soup (histamine, bradykinin, serotonin, prostaglandin E2) over the dura. Whether glial and/or immune activation contributes to such pain is unknown. Here we tested if trigeminal nucleus caudalis (Sp5C) glial and/or immune cells are activated following supradural inflammatory soup, and if putative glial/immune inhibitors suppress the consequent facial allodynia. Inflammatory soup was administered via bilateral indwelling supradural catheters in freely moving rats, inducing robust and reliable facial allodynia. Gene expression for microglial/macrophage activation markers, interleukin-1β, and tumor necrosis factor-α increased following inflammatory soup along with robust expression of facial allodynia. This provided the basis for pursuing studies of the behavioral effects of 3 diverse immunomodulatory drugs on facial allodynia. Pretreatment with either of two compounds broadly used as putative glial/immune inhibitors (minocycline, ibudilast) prevented the development of facial allodynia, as did treatment after supradural inflammatory soup but prior to the expression of facial allodynia. Lastly, the toll-like receptor 4 (TLR4) antagonist (+)-naltrexone likewise blocked development of facial allodynia after supradural inflammatory soup. Taken together, these exploratory data support that activated glia and/or immune cells may drive the development of facial allodynia in response to supradural inflammatory soup in unanesthetized male rats.

Regulation of Physical Microglia-Neuron Interactions by Fractalkine Signaling after Status Epilepticus

Microglia, the resident immune cells of the brain, perform elaborate surveillance in which they physically interact with neuronal elements. A novel form of microglia–neuron interaction named microglial process convergence (MPC) toward neuronal axons and dendrites has recently been described. However, the molecular regulators and pathological relevance of MPC have not been explored. Here, using high-resolution two-photon imaging in vivo and ex vivo, we observed a dramatic increase in MPCs after kainic acid– or pilocarpine-induced experimental seizures that was reconstituted after glutamate treatment in slices from mice. Interestingly, a deficiency of the fractalkine receptor (CX3CR1) decreased MPCs, whereas fractalkine (CX3CL1) treatment increased MPCs, suggesting that fractalkine signaling is a critical regulator of these microglia–neuron interactions. Furthermore, we found that interleukin-1β was necessary and sufficient to trigger CX3CR1-dependent MPCs. Finally, we show that a deficiency in fractalkine signaling corresponds with increased seizure phenotypes. Together, our results identify the neuroglial CX3CL1–CX3CR1 communication axis as a modulator of potentially neuroprotective microglia–neuron physical interactions during conditions of neuronal hyperactivity.

Cytokine and neuropeptide levels are associated with pain relief in patients with chronically painful total knee arthroplasty: a pilot study

Background

There are few studies with an assessment of the levels of cytokines or neuropeptides as correlates of pain and pain relief in patients with painful joint diseases. Our objective was to assess whether improvements from baseline to 2-months in serum cytokine, chemokine and substance P levels were associated with clinically meaningful pain relief at 2-months post-injection in patients with painful total knee arthroplasty (TKA).

Methods

Using data from randomized trial of 60 TKAs, we assessed the association of change in cytokine/chemokine/Substance P levels with primary study outcome, clinically important improvement in Western Ontario McMaster Osteoarthritis Index (WOMAC) pain subscale at 2-months post-injection using Student’s t-tests and Spearman’s correlation coefficient (non-parametric). Patients were categorized as pain responders (20-point reduction or more on 0-100 WOMAC pain) vs. pain non-responders. Sensitivity analysis used 0–10 daytime pain numeric rating scale (NRS) instead of WOMAC pain subscale.

Results

In a pilot study, compared to non-responders (n = 23) on WOMAC pain scale at 2-months, pain responders (n = 12) had significantly greater increase in serum levels of IL-7, IL-10, IL-12, eotaxin, interferon gamma and TNF-α from baseline to 2-months post-injection (p < 0.05 for all). Change in several cytokine/chemokine and substance P levels from pre-injection to 2-month follow-up correlated significantly with change in WOMAC pain with correlation coefficients ranging −0.37 to −0.51: IL-2, IL-7, IL-8, IL-9, IL-16, IL-12p, GCSF, IFN gamma, IP-10, MCP, MIP1b, TNF-α and VEGF (n = 35). Sensitivity analysis showed that substance P decreased significantly more from baseline to 2-months in the pain responders (0.54 ± 0.53; n = 10) than in the pain non-responders (0.48 ± 1.18; n = 9; p = 0.023) and that this change in serum substance P correlated significantly with change in daytime NRS pain, correlation coefficient was 0.53 (p = 0.021; n = 19). Findings should be interpreted with caution, since cytokine analyses were performed for a sub-group of the entire trial population.

Conclusion

Serum cytokine, chemokine and Substance P levels correlated with pain response in patients with painful TKA after an intra-articular injection in a randomized trial.

Metformin reduces morphine tolerance by inhibiting microglial-mediated neuroinflammation

Background

Tolerance seriously impedes the application of morphine in clinical medicine. Thus, it is necessary to investigate the exact mechanisms and efficient treatment. Microglial activation and neuroinflammation in the spinal cord are thought to play pivotal roles on the genesis and maintaining of morphine tolerance. Activation of adenosine monophosphate-activated kinase (AMPK) has been associated with the inhibition of inflammatory nociception. Metformin, a biguanide class of antidiabetic drugs and activator of AMPK, has a potential anti-inflammatory effect. The present study evaluated the effects and potential mechanisms of metformin in inhibiting microglial activation and alleviating the antinociceptive tolerance of morphine.

Methods

The microglial cell line BV-2 cells and mouse brain-derived endothelial cell line bEnd3 cells were used. Cytokine expression was measured using quantitative polymerase chain reaction. Cell signaling was assayed by western blot and immunohistochemistry. The antinociception and morphine tolerance were assessed in CD-1 mice using tail-flick tests.

Results

We found that morphine-activated BV-2 cells, including the upregulation of p38 mitogen-activated protein kinase (p38 MAPK) phosphorylation, pro-inflammatory cytokines, and Toll-like receptor-4 (TLR-4) mRNA expression, which was inhibited by metformin. Metformin suppressed morphine-induced BV-2 cells activation through increasing AMPK phosphorylation, which was reversed by the AMPK inhibitor compound C. Additionally, in BV-2 cells, morphine did not affect the cell viability and the mRNA expression of anti-inflammatory cytokines. In bEnd3 cells, morphine did not affect the mRNA expression of interleukin-1β (IL-1β), but increased IL-6 and tumor necrosis factor-α (TNF-α) mRNA expression; the effect was inhibited by metformin. Morphine also did not affect the mRNA expression of TLR-4 and chemokine ligand 2 (CCL2). Furthermore, systemic administration of metformin significantly blocked morphine-induced microglial activation in the spinal cord and then attenuated the development of chronic morphine tolerance in mice.

Conclusions

Metformin significantly attenuated morphine antinociceptive tolerance by suppressing morphine-induced microglial activation through increasing AMPK phosphorylation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0754-9) contains supplementary material, which is available to authorized users.

Nor-Binaltorphimine Blocks the Adverse Effects of Morphine after Spinal Cord Injury

Opioids are frequently used for the treatment of pain following spinal cord injury (SCI). Unfortunately, we have shown that morphine administered in the acute phase of SCI results in significant, adverse secondary consequences including compromised locomotor and sensory recovery. Similarly, we showed that selective activation of the κ-opioid receptor (KOR), even at a dose 32-fold lower than morphine, is sufficient to attenuate recovery of locomotor function. In the current study, we tested whether activation of the KOR is necessary to produce morphine's adverse effects using nor-Binaltorphimine (norBNI), a selective KOR antagonist. Rats received a moderate spinal contusion (T12) and 24 h later, baseline locomotor function and nociceptive reactivity were assessed. Rats were then administered norBNI (0, 0.02, 0.08, or 0.32 μmol) followed by morphine (0 or 0.32 μmol). Nociception was reassessed 30 min after drug treatment, and recovery was evaluated for 21 days. The effects of norBNI on morphine-induced attenuation of recovery were dose dependent. At higher doses, norBNI blocked the adverse effects of morphine on locomotor recovery, but analgesia was also significantly decreased. Conversely, at low doses, analgesia was maintained, but the adverse effects on recovery persisted. A moderate dose of norBNI, however, adequately protected against morphine's adverse effects without eliminating its analgesic efficacy. This suggests that activation of the KOR system plays a significant role in the morphine-induced attenuation of recovery. Our research suggests that morphine, and other opioid analgesics, may be contraindicated for the SCI population. Blocking KOR activity may be a viable strategy for improving the safety of clinical opioid use.

Opioids and GI Motility-Friend or Foe?

OPINION STATEMENT

The use of opioids for the treatment of chronic non-cancer pain is growing at an alarming rate. Opioid-induced bowel dysfunction (OBD) is a common adverse effect of long-term opioid treatment manifesting as constipation, nausea, and vomiting. These effects are primarily mediated by peripheral μ-opioid receptors with resultant altered GI motility and function. As a result, patients may present with opioid-induced constipation (OIC), opioid-induced nausea and vomiting (OINV), and/or narcotic bowel syndrome (NBS). This often leads to decreased quality of life and in many cases, discontinuation of opioid therapy. There is limited evidence to support the use of traditional anti-emetics and laxatives in the treatment of OBD. Tapering the dose of opioids, switching to transdermal application, opioid rotation, or dual-action opioids, such as tapentadol, may be helpful in the treatment of OBD. Novel agents, such as peripherally acting μ-opioid receptor antagonists which target the cause of OIC, show promise in the treatment of OBD and should be considered when conventional laxatives fail. This chapter will review the pathophysiology of OBD, including OINV and OIC, and treatment options available.

Neurobiological Effects of Morphine after Spinal Cord Injury

Opioids and non-steroidal anti-inflammatory drugs are used commonly to manage pain in the early phase of spinal cord injury (SCI). Despite its analgesic efficacy, however, our studies suggest that intrathecal morphine undermines locomotor recovery and increases lesion size in a rodent model of SCI. Similarly, intravenous (IV) morphine attenuates locomotor recovery. The current study explores whether IV morphine also increases lesion size after a spinal contusion (T12) injury and quantifies the cell types that are affected by early opioid administration. Using an experimenter-administered escalating dose of IV morphine across the first seven days post-injury, we quantified the expression of neuron, astrocyte, and microglial markers at the injury site. SCI decreased NeuN expression relative to shams. In subjects with SCI treated with IV morphine, virtually no NeuN⁺ cells remained across the rostral-caudal extent of the lesion. Further, whereas SCI per se increased the expression of astrocyte and microglial markers (glial fibrillary acidic protein and OX-42, respectively), morphine treatment decreased the expression of these markers. These cellular changes were accompanied by attenuation of locomotor recovery (Basso, Beattie, Bresnahan scores), decreased weight gain, and the development of opioid-induced hyperalgesia (increased tactile reactivity) in morphine-treated subjects. These data suggest that morphine use is contraindicated in the acute phase of a spinal injury. Faced with a lifetime of intractable pain, however, simply removing any effective analgesic for the management of SCI pain is not an ideal option. Instead, these data underscore the critical need for further understanding of the molecular pathways engaged by conventional medications within the pathophysiological context of an injury.

Allostatic Mechanisms of Opioid Tolerance Beyond Desensitization and Downregulation

Mechanisms of opioid tolerance have focused on adaptive modifications within cells containing opioid receptors, defined here as cellular allostasis, emphasizing regulation of the opioid receptor signalosome. We review additional regulatory and opponent processes involved in behavioral tolerance, and include mechanistic differences both between agonists (agonist bias), and between μ- and δ-opioid receptors. In a process we will refer to as pass-forward allostasis, cells modified directly by opioid drugs impute allostatic changes to downstream circuitry. Because of the broad distribution of opioid systems, every brain cell may be touched by pass-forward allostasis in the opioid-dependent/tolerant state. We will implicate neurons and microglia as interactive contributors to the cumulative allostatic processes creating analgesic and hedonic tolerance to opioid drugs.

The dark side of opioids in pain management: basic science explains clinical observation

Although there is no doubt about opioids' ability to relieve pain in the short term, it is not always clear why longer-term analgesic efficacy seems to be impaired. Tolerance and hyperalgesia have been suggested as mechanisms for opioid analgesic deterioration. But could there also be an effect of opioids on pain itself?

Introduction:

In the past 2 decades, opioids have been used increasingly for the treatment of persistent pain, and doses have tended to creep up. As basic science elucidates mechanisms of pain and analgesia, the cross talk between central pain and opioid actions becomes clearer.

Objectives:

We aimed to examine the published literature on basic science explaining pronociceptive opioid actions, and apply this knowledge to clinical observation.

Methods:

We reviewed the existing literature on the pronociceptive actions of opioids, both preclinical and clinical studies.

Results:

Basic science provides a rationale for the clinical observation that opioids sometimes increase rather than decrease pain. Central sensitization (hyperalgesia) underlies pain chronification, but can also be produced by high dose and high potency opioids. Many of the same mechanisms account for both central pain and opioid hyperalgesia.

Conclusion:

Newly revealed basic mechanisms suggest possible avenues for drug development and new drug therapies that could alter pain sensitization through endogenous and exogenous opioid mechanisms. Recent changes in practice such as the introduction of titration-to-effect for opioids have resulted in higher doses used in the clinic setting than ever seen previously. New basic science knowledge hints that these newer dosing practices may need to be reexamined. When pain worsens in a patient taking opioids, can we be assured that this is not because of the opioids, and can we alter this negative effect of opioids through different dosing strategies or new drug intervention?

A new pharmacological role for thalidomide: Attenuation of morphine-induced tolerance in rats

OBJECTIVE

Tolerance to the analgesic effect is the main side effect of chronic administration of opioids. Several drugs have been studied to try to find agents to prevent the development of this phenomenon. In the present study we aimed to evaluate the effect of thalidomide on morphine-induced tolerance to the analgesic effect.

METHODS

Groups of male rats were randomly rendered and received daily morphine in combination with thalidomide vehicle or thalidomide (2.5 mg/kg, 5 mg/kg, or 10 mg/kg, intraperitoneally). Nociception was measured using the plantar test apparatus. Latency time was recorded when the animal reacted to the light stimulus; licking or raising its hind paw. Treatments and evaluations continued until completion of tolerance to the analgesic effect of morphine.

RESULTS

Our findings indicated that tolerance was achieved following 11 days of morphine administration, while thalidomide postponed the day of tolerance completion for 4 days (2.5 mg/kg and 5 mg/kg thalidomide) or 10 days (10 mg/kg thalidomide). Moreover, thalidomide prevented the morphine-induced shift to the right of the ED50 in the dose-response curve.

CONCLUSION

It was concluded that thalidomide attenuated the morphine-induced tolerance to the analgesic effect.

Microglial P2Y12 receptors regulate microglial activation and surveillance during neuropathic pain

Microglial cells are critical in the pathogenesis of neuropathic pain and several microglial receptors have been proposed to mediate this process. Of these receptors, the P2Y12 receptor is a unique purinergic receptor that is exclusively expressed by microglia in the central nervous system (CNS). In this study, we set forth to investigate the role of P2Y12 receptors in microglial electrophysiological and morphological (static and dynamic) activation during spinal nerve transection (SNT)-induced neuropathic pain in mice. First, we found that a genetic deficiency of the P2Y12 receptor (P2Y12(-/-) mice) ameliorated pain hypersensitivities during the initiation phase of neuropathic pain. Next, we characterised both the electrophysiological and morphological properties of microglia in the superficial spinal cord dorsal horn following SNT injury. We show dramatic alterations including a peak at 3days post injury in microglial electrophysiology while high resolution two-photon imaging revealed significant changes of both static and dynamic microglial morphological properties by 7days post injury. Finally, in P2Y12(-/-) mice, these electrophysiological and morphological changes were ameliorated suggesting roles for P2Y12 receptors in SNT-induced microglial activation. Our results therefore indicate that P2Y12 receptors regulate microglial electrophysiological as well as static and dynamic microglial properties after peripheral nerve injury, suggesting that the microglial P2Y12 receptor could be a potential therapeutic target for the treatment of neuropathic pain.

The effects of ibudilast, a glial activation inhibitor, on opioid withdrawal symptoms in opioid-dependent volunteers

Glial activation is hypothesized to contribute directly to opioid withdrawal. This study investigated the dose-dependent effects of a glial cell modulator, ibudilast, on withdrawal symptoms in opioid-dependent volunteers after abrupt discontinuation of morphine administration. Non-treatment-seeking heroin-dependent volunteers (n = 31) completed the in-patient, double-blind, placebo-controlled, within-subject and between-group study. Volunteers were maintained on morphine (30 mg, QID) for 14 days and placebo (0 mg, QID) for the last 7 days of the 3-week study. Volunteers also received placebo (0 mg, PO, BID) capsules on days 1-7. On days 8-21, volunteers were randomized to receive ibudilast (20 or 40 mg, PO, BID) or placebo capsules. Subjective and clinical ratings of withdrawal symptoms were completed daily using daily using the Subjective Opioid Withdrawal Scale (SOWS) and Clinical Opioid Withdrawal Scale (COWS). Medication side effects were also monitored. Relative to the first 2 weeks, all groups exhibited withdrawal during the third week as assessed by the SOWS and COWS (P ≤ 0.0001). Although overall SOWS scores did not differ between groups, exploratory analyses pooling the two ibudilast groups demonstrated that they had lower ratings of withdrawal symptoms on SOWS items ('anxious,' 'perspiring,' 'restless,' 'stomach cramps') during detoxification relative to the placebo group. Ibudilast was well tolerated; no serious adverse events occurred during the study. Pharmacological modulation of glial activity with ibudilast decreased some subjective ratings of opioid withdrawal symptoms. These exploratory findings are the first to demonstrate the potential clinical utility of glial modulators for treating opioid withdrawal in humans.

Plasma levels of eight different mediators and their potential as biomarkers of various clinical malaria conditions in African children

BACKGROUND

Plasmodium falciparum infection can lead to several clinical manifestations ranging from asymptomatic infections (AM) and uncomplicated malaria (UM) to potentially fatal severe malaria (SM), including cerebral malaria (CM). Factors implicated in the progression towards severe disease are not fully understood.

METHODS

In the present study, an enzyme-linked immunosorbent assay (ELISA) method was used to investigate the plasma content of several biomarkers of the immune response, namely Neopterin, sCD163, suPAR, Pentraxin 3 (PTX3), sCD14, Fractalkine (CX3CL1), sTREM-1 and MIG (CXCL9), in patients with distinct clinical manifestations of malaria. The goal of this study was to determine the relative involvement of these inflammatory mediators in the pathogenesis of malaria and test their relevance as biomarkers of disease severity.

RESULTS

ROC curve analysis show that children with AM were characterized by high levels of Fractalkine and sCD163 whereas children with UM were distinguishable by the presence of PTX3 in their plasma. Furthermore, principal component analysis indicated that the combination of Fractalkine, MIG, and Neopterin was the best predictor of AM condition, while suPAR, PTX3 and sTREM-1 combination was the best indicator of UM when compared to AM. The association of Neopterin, suPAR and Fractalkine was strongly predictive of SM or CM compared to UM.

CONCLUSIONS

The results indicate that the simultaneous evaluation of these bioactive molecules as quantifiable blood parameters may be helpful to get a better insight into the clinical syndromes in children with malaria.

Opioid-induced hyperalgesia: Cellular and molecular mechanisms

Opioids produce strong analgesia but their use is limited by a paradoxical hypersensitivity named opioid-induced hyperalgesia (OIH) that may be associated to analgesic tolerance. In the last decades, a significant number of preclinical studies have investigated the factors that modulate OIH development as well as the cellular and molecular mechanisms underlying OIH. Several factors have been shown to influence OIH including the genetic background and sex differences of experimental animals as well as the opioid regimen. Mu opioid receptor (MOR) variants and interactions of MOR with different proteins were shown important. Furthermore, at the cellular level, both neurons and glia play a major role in OIH development. Several neuronal processes contribute to OIH, like activation of neuroexcitatory mechanisms, long-term potentiation (LTP) and descending pain facilitation. Increased nociception is also mediated by neuroinflammation induced by the activation of microglia and astrocytes. Neurons and glial cells exert synergistic effects, which contribute to OIH. The molecular actors identified include the Toll-like receptor 4 and the anti-opioid systems as well as some other excitatory molecules, receptors, channels, chemokines, pro-inflammatory cytokines or lipids. This review summarizes the intracellular and intercellular pathways involved in OIH and highlights some mechanisms that may be challenged to limit OIH in the future.

The narcotic bowel syndrome: a recent update

OBJECTIVES

The paradoxical development of chronic abdominal pain is an underrecognized side effect of opioid use. Narcotic bowel syndrome (NBS), occurring in a small proportion of chronic opioid users, consists of chronic or intermittent abdominal pain, which often increases in severity despite continued or escalating dosages of opioids prescribed to relieve pain.

METHODS

A PubMed search was conducted using terms such as "narcotic bowel syndrome" and "opioid hyperalgesia" through January 2014.

RESULTS

Abdominal pain is the defining symptom of NBS and is thought to be mediated by central nervous system dysfunction; it should be distinguished from the peripheral side effects of opioids, such as nausea, bloating, intermittent vomiting, abdominal distension, and constipation. This latter cluster of symptoms is called opioid bowel dysfunction, although it may co-occur with NBS. Hypothesized mechanisms of the central effects of opioids on nociception in NBS include spinal cord inflammation and dysfunction in opioid receptor activity and related neuroanatomical substrates. With continued use, ∼6% of patients taking narcotics chronically will develop NBS, with profound consequences in terms of daily function. The primary management paradigm for NBS is a structured opioid withdrawal program accompanied by centrally acting adjunctive therapy comprising antidepressants, benzodiazepines, and clonidine to target pain, anxiety, and depression, and prevent withdrawal effects, in addition to peripherally acting agents such as laxatives (e.g., osmotic laxatives and chloride channel activators) to control transient constipation. Such structured withdrawal programs have been prospectively evaluated in small clinical trials and have met with considerable success in the short term.

CONCLUSIONS

Because rates of NBS are likely to rise, integrated intensive pharmacotherapy and psychosocial interventions are needed to help patients with NBS go off and stay off opioids. These programs will likely also reduce comorbid psychopathology and lead to adequate pain control and improved quality of life.

Elevated Fractalkine (CX3CL1) Levels in the Trigeminal Ganglion Mechanically Sensitize Temporalis Muscle Nociceptors

It has been proposed that after nerve injury or tissue inflammation, fractalkine (CX3CL1) released from dorsal root ganglion neurons acts on satellite glial cells (SGCs) through CX3C receptor 1 (CX3CR1) to induce neuroplastic changes. The existence and importance of fractalkine/CX3CR1 signaling in the trigeminal ganglia has not yet been clarified. This study investigated (1) whether trigeminal ganglion neurons that innervate temporalis muscle and their associated SGCs contain fractalkine and/or express CX3CR1, (2) if intraganglionic injection of fractalkine increases the mechanical sensitivity of temporalis muscle afferent fibers, (3) whether complete Freund's adjuvant (CFA)-induced inflammation of the temporalis muscle alters the expression of fractalkine or its receptor in the trigeminal ganglion, and (4) if intraganglionic administration of CX3CR1 antibodies alters afferent mechanical sensitivity. Immunohistochemistry and in vivo electrophysiological recordings in male and female rats were used to address these questions. It was found that ∼50 % of temporalis ganglion neurons and ∼25 % of their associated SGCs express CX3CR1, while only neurons expressed fractalkine. Temporalis muscle inflammation increased the expression of fractalkine, but only in male rats. Intraganglionic injection of fractalkine (25 g/ml; 3 μl) induced prolonged afferent mechanical sensitization. Intraganglionic injection of CX3CR1 antibody increased afferent mechanical threshold, but this effect was greater in controls than in rats with CFA-induced muscle inflammation. These findings raise the possibility that basal fractalkine signalling within the trigeminal ganglion plays an important role in mechanical sensitivity of masticatory muscle sensory afferent fibers and that inhibition of CX3CR1 signaling within the trigeminal ganglia may induce analgesia through a peripheral mechanism.

Evaluation of the effects of specific opioid receptor agonists in a rodent model of spinal cord injury

Objective

The current study aimed to evaluate the contribution(s) of specific opioid receptor systems to the analgesic and detrimental effects of morphine, observed after spinal cord injury in prior studies.

Study Design

We used specific opioid receptor agonists to assess the effects of µ- (DAMGO), δ- (DPDPE), and κ- (GR89696) opioid receptor activation on locomotor (BBB, tapered beam, ladder tests) and sensory (girdle, tactile, and tail-flick tests) recovery in a rodent contusion model (T12). We also tested the contribution of non-classic opioid binding using [+]- morphine.

Methods

First, a dose-response curve for analgesic efficacy was generated for each opioid agonist. Baseline locomotor and sensory reactivity was assessed 24 h after injury. Subjects were then treated with an intrathecal dose of a specific agonist and re-tested after 30 min. To evaluate effects on recovery, subjects were treated with a single dose of an agonist and both locomotor and sensory function were monitored for 21 d.

Results

All agonists for the classic opioid receptors, but not the [+]- morphine enantiomer, produced antinociception at a concentration equivalent to a dose of morphine previously shown to produce strong analgesic effects (0.32 μmol). DAMGO and [+]- morphine did not affect long-term recovery. GR89696, however, significantly undermined recovery of locomotor function at all doses tested.

Conclusions

Based on these data, we hypothesize that the analgesic efficacy of morphine is primarily mediated by binding to the classic μ-opioid receptor. Conversely, the adverse effects of morphine may be linked to activation of the κ-opioid receptor. Ultimately, elucidating the molecular mechanisms underlying the effects of morphine is imperative in order to develop safe and effective pharmacological interventions in a clinical setting.

Setting

USA

Contribution of adrenomedullin to the switch of G protein-coupled μ-opioid receptors from Gi to Gs in the spinal dorsal horn following chronic morphine exposure in rats

BACKGROUND AND PURPOSE

Chronic exposure to morphine increases spinal adrenomedullin (AM) bioactivity resulting in the development and maintenance of morphine tolerance. This study investigated the possible involvement of AM in morphine-evoked alteration in μ-opioid receptor-coupled G proteins.

EXPERIMENTAL APPROACH

Agents were administered intrathecally (i.t.) in rats. Nociceptive behaviours and cumulative dose-response of morphine analgesia were assessed. Neurochemicals in the spinal dorsal horn were assayed by immunoprecipitation, Western blot analysis and ELISA.

KEY RESULTS

Intrathecal injection of AM (8 μg) for 9 days decreased and increased the levels of μ receptor-coupled Gi and Gs proteins respectively. Morphine stimulation (5 μg) after chronic treatment with AM also induced an increase in cAMP production in the spinal dorsal horn. Co-administration of the selective AM receptor antagonist AM22-52 inhibited chronic morphine-evoked switch of G protein-coupled μ receptor from Gi to Gs. Chronic exposure to AM increased the phosphorylation of cAMP-responsive element-binding protein (CREB) and ERK. Co-administration of the PKA inhibitor H-89 (5 μg) or MEK1 inhibitor PD98059 (1 μg) reversed the AM-induced thermal/mechanical hypersensitivity, decline in morphine analgesic potency, switch of G protein-coupled μ receptor and increase in cAMP.

CONCLUSIONS AND IMPLICATIONS

The present study supports the hypothesis that an increase in AM activity in the spinal dorsal horn contributes to the switch of the μ receptor-coupled G protein from Gi to Gs protein via the activation of cAMP/PKA/CREB and ERK signalling pathways in chronic morphine use.

Targeting the Microglial Signaling Pathways: New Insights in the Modulation of Neuropathic Pain

The microglia, once thought only to be supporting cells of the central nervous system (CNS), are now recognized to play essential roles in many pathologies. Many studies within the last decades indicated that the neuro-immune interaction underlies the generation and maintenance of neuropathic pain. Through a large number of receptors and signaling pathways, the microglial cells communicate with neurons, astrocytes and other cells, including those of the immune system. A disturbance or loss of CNS homeostasis causes rapid responses of the microglia, which undergo a multistage activation process. The activated microglia change their cell shapes and gene expression profiles, which induce proliferation, migration, and the production of pro- or antinociceptive factors. The cells release a large number of mediators that can act in a manner detrimental or beneficial to the surrounding cells and can indirectly alter the nociceptive signals. This review discusses the most important microglial intracellular signaling cascades (MAPKs, NF-kB, JAK/STAT, PI3K/Akt) that are essential for neuropathic pain development and maintenance. Our objective was to identify new molecular targets that may result in the development of powerful tools to control the signaling associated with neuropathic pain.

Endomorphin analog analgesics with reduced abuse liability, respiratory depression, motor impairment, tolerance, and glial activation relative to morphine

Opioids acting at the mu opioid receptor (MOR) are the most effective analgesics, however adverse side effects severely limit their use. Of particular importance, abuse liability results in major medical, societal, and economic problems, respiratory depression is the cause of fatal overdoses, and tolerance complicates treatment and increases the risk of side effects. Motor and cognitive impairment are especially problematic for older adults. Despite the host of negative side effects, opioids such as morphine are commonly used for acute and chronic pain conditions. Separation of analgesia from unwanted effects has long been an unmet goal of opioid research. Novel MOR agonist structures may prove critical for greater success. Here we tested metabolically stable analogs of the endomorphins, endogenous opioids highly selective for the MOR. Compared to morphine, the analogs showed dramatically improved analgesia-to-side-effect ratios. At doses providing equal or greater antinociception than morphine in the rat, the analogs showed reduced a) respiratory depression, b) impairment of motor coordination, c) tolerance and hyperalgesia, d) glial p38/CGRP/P2X7 receptor signaling, and e) reward/abuse potential in both conditioned place preference and self-administration tests. Differential effects on glial activation indicate a mechanism for the relative lack of side effects by the analogs compared to morphine. The results suggest that endomorphin analogs described here could provide gold standard pain relief mediated by selective MOR activation, but with remarkably safer side effect profiles compared to opioids like morphine.

Treatment of Cancer Pain by Targeting Cytokines

Inflammation is one of the most important causes of the majority of cancer symptoms, including pain, fatigue, cachexia, and anorexia. Cancer pain affects 17 million people worldwide and can be caused by different mediators which act in primary efferent neurons directly or indirectly. Cytokines can be aberrantly produced by cancer and immune system cells and are of particular relevance in pain. Currently, there are very few strategies to control the release of cytokines that seems to be related to cancer pain. Nevertheless, in some cases, targeted drugs are available and in use for other diseases. In this paper, we aim to review the importance of cytokines in cancer pain and targeted strategies that can have an impact on controlling this symptom.

The effects of pioglitazone, a PPARγ receptor agonist, on the abuse liability of oxycodone among nondependent opioid users

AIMS

Activation of PPARγ by pioglitazone (PIO) has shown some efficacy in attenuating addictive-like responses in laboratory animals. The ability of PIO to alter the effects of opioids in humans has not been characterized in a controlled laboratory setting. The proposed investigation sought to examine the effects of PIO on the subjective, analgesic, physiological and cognitive effects of oxycodone (OXY).

METHODS

During this investigation, nondependent prescription opioid abusers (N=17 completers) were maintained for 2-3weeks on ascending daily doses of PIO (0mg, 15mg, 45mg) prior to completing a laboratory session assessing the aforementioned effects of OXY [using a within-session cumulative dosing procedure (0, 10, and 20mg, cumulative dose=30mg)].

RESULTS

OXY produced typical mu opioid agonist effects: miosis, decreased pain perception, and decreased respiratory rate. OXY also produced dose-dependent increases in positive subjective responses. Yet, ratings such as: drug "liking," "high," and "good drug effect," were not significantly altered as a function of PIO maintenance dose.

DISCUSSION

These data suggest that PIO may not be useful for reducing the abuse liability of OXY. These data were obtained with a sample of nondependent opioid users and therefore may not be applicable to dependent populations or to other opioids. Although PIO failed to alter the abuse liability of OXY, the interaction between glia and opioid receptors is not well understood so the possibility remains that medications that interact with glia in other ways may show more promise.

Chronic morphine treatment increased the expression of myeloid differentiation primary response protein 88 in rat spinal cord

Chronic morphine exposure leads to tolerance, which limits the clinical use of this potent analgesic in the treatment of severe and chronic pain. Compelling evidence suggest that neuro-immune activation (pro-inflammatory cytokines including IL-1β, IL-6 and TNF) as well as neuro-inflammation have been shown to mediate the development of morphine analgesic tolerance. Toll-like receptors (TLRs), especially TLR-4, have also been reported to contribute to the development of morphine analgesic tolerance. Besides, mitogen-activated protein kinases (MAPKs; especially p38 MAPK and c-Jun N -terminal kinase), as well as nuclear factor-κB (NF-κB) modulate the development of morphine antinociceptive tolerance. Hence, we hypothesis the possible involvement of myeloid differentiation primary response protein 88 (MyD88), a key adaptor protein for the TLR and IL-1R families, in the development of tolerance to morphine-induced analgesia. Our study demonstrated that chronic intrathecal morphine injection led to a robust increase of MyD88 expression in rat spinal cord. Sustained elevation of MyD88 may play a role in modulating the development of morphine antinociceptive tolerance.

CX3CR1 is an important surface molecule for respiratory syncytial virus infection in human airway epithelial cells

Respiratory syncytial virus (RSV) is a major cause of severe pneumonia and bronchiolitis in infants and young children, and causes disease throughout life. Understanding the biology of infection, including virus binding to the cell surface, should help develop antiviral drugs or vaccines. The RSV F and G glycoproteins bind cell surface heparin sulfate proteoglycans (HSPGs) through heparin-binding domains. The G protein also has a CX3C chemokine motif which binds to the fractalkine receptor CX3CR1. G protein binding to CX3CR1 is not important for infection of immortalized cell lines, but reportedly is so for primary human airway epithelial cells (HAECs), the primary site for human infection. We studied the role of CX3CR1 in RSV infection with CX3CR1-transfected cell lines and HAECs with variable percentages of CX3CR1-expressing cells, and the effect of anti-CX3CR1 antibodies or a mutation in the RSV CX3C motif. Immortalized cells lacking HSPGs had low RSV binding and infection, which was increased markedly by CX3CR1 transfection. CX3CR1 was expressed primarily on ciliated cells, and ∼50 % of RSV-infected cells in HAECs were CX3CR1+. HAECs with more CX3CR1-expressing cells had a proportional increase in RSV infection. Blocking G binding to CX3CR1 with anti-CX3CR1 antibody or a mutation in the CX3C motif significantly decreased RSV infection in HAECs. The kinetics of cytokine production suggested that the RSV/CX3CR1 interaction induced RANTES (regulated on activation normal T-cell expressed and secreted protein), IL-8 and fractalkine production, whilst it downregulated IL-15, IL1-RA and monocyte chemotactic protein-1. Thus, the RSV G protein/CX3CR1 interaction is likely important in infection and infection-induced responses of the airway epithelium, the primary site of human infection.