Morphine reduces local cytokine expression and neutrophil infiltration after incision

Focal Adhesion Kinase Inhibition Ameliorates Burn Injury-Induced Chronic Pain in Rats

Burn injury-induced pain (BIP) is a significant global health concern, affecting diverse populations including children, military veterans, and accident victims. Current pharmacotherapeutics for the management of BIP are associated with severe side effects including drug addiction, respiratory depression, sedation, and constipation posing significant barrier to their clinical utility. In the present study, we have investigated the potential role of focal adhesion kinase (p-FAK) for the very first time in BIP and elucidated the associated underlying mechanisms. Defactinib (DFT), a potent p-FAK inhibitor, administered at doses of 5, 10, and 20 mg/kg via intraperitoneal injection, demonstrates significant efficacy in reducing both evoked and spontaneous pain without causing addiction or other central nervous system toxicities. Burn injury triggers p-FAK-mediated phosphorylation of Erk1/2 and NR2B signaling in the DRG, resulting in heightened hypersensitivity through microglial activation, neuropeptide release, and elevated proinflammatory cytokines. Defactinib (DFT) counteracts these effects by reducing NR2B upregulation, lowering substance P levels, inhibiting microglial activation, and restoring IL-10 levels while leaving CGRP levels unchanged. These findings provide valuable insights into the pivotal role of p-FAK in regulating BIP and highlight the potential for developing novel therapeutics for burn injury-induced pain with minimal side effects.

Increased LPS-Induced Fever and Sickness Behavior in Adult Male and Female Rats Perinatally Exposed to Morphine

As a result of the current opioid crisis, the rate of children born exposed to opioids has skyrocketed. Later in life, these children have an increased risk for hospitalization and infection, raising concerns about potential immunocompromise, as is common with chronic opioid use. Opioids can act directly on immune cells or indirectly via the central nervous system to decrease immune system activity, leading to increased susceptibility, morbidity, and mortality to infection. However, it is currently unknown how perinatal opioid exposure (POE) alters immune function. Using a clinically relevant and translatable model of POE, we have investigated how baseline immune function and the reaction to an immune stimulator, lipopolysaccharide, is influenced by in utero opioid exposure in adult male and female rats. We report here that POE potentiates the febrile and neuroinflammatory response to lipopolysaccharide, likely as a consequence of suppressed immune function at baseline (including reduced antibody production). This suggests that POE increases susceptibility to infection by manipulating immune system development, consistent with the clinical literature. Investigation of the mechanisms whereby POE increases susceptibility to pathogens is critical for the development of potential interventions for immunosuppressed children exposed to opioids in utero.

Neonatal injury modulates incisional pain sensitivity in adulthood: An animal study

Neonatal pain experiences including traumatic injury influences negatively on development of nociceptive circuit developments, resulting in persistent pain hypersensitivity in adults. However, the detailed mechanism is not yet well understood. In the present study, to clarify the pathogenesis of orofacial pain hypersensitivity associated with neonatal injury, the involvement of the voltage-gated sodium channel (Na_v) 1.8 and the C-C chemokine ligand 2 (CCL2)/C-C chemokine receptor 2 (CCR2) signaling in the trigeminal ganglion (TG) in facial skin incisional pain hypersensitivity was examined in 190 neonatal facial-injured and sham male rats. The whisker pad skin was incised on postnatal day 4 and week 7 (Incision-Incision group). Compared to the group without neonatal incision (Sham-Incision group), mechanical hypersensitivity in the whisker pad skin was enhanced in Incision-Incision group. The number of Na_v1.8-immunoreactive TG neurons and the amount of CCL2 expressed in the macrophages and satellite glial cells in the TG were increased on day 14 after re-incision in the Incision-Incision group, compared with Sham-Incision group. Blockages of Na_v1.8 in the incised region and CCR2 in the TG suppressed the enhancement of mechanical hypersensitivity in the Incision-Incision group. Administration of CCL2 into the TG enhanced mechanical hypersensitivity in the Sham-Sham, Incision-Sham and Sham-Incision group. Our results suggest that neonatal facial injury accelerates the TG neuronal hyperexcitability following orofacial skin injury in adult in association with Na_v1.8 overexpression via CCL2 signaling, resulting in the enhancement of orofacial incisional pain hypersensitivity in the adulthood.

Role of leukotriene B4 (LTB4)-LTB4 receptor 1 signaling in post-incisional nociceptive sensitization and local inflammation in mice

Leukotriene B4 (LTB4) is a potent lipid mediator involved in the recruitment and activation of neutrophils, which is an important feature of tissue injury and inflammation. The biological effects of LTB4 are primarily mediated through the high-affinity LTB4 receptor, BLT1. Postoperative incisional pain is characterized by persistent acute pain at the site of tissue injury and is associated with local inflammation. Here, we compared the role of LTB4-BLT1 signaling in postoperative incisional pain between BLT1-knockout (BLT1KO) and wild-type (BLT1WT) mice. A planter incision model was developed, and mechanical pain hypersensitivity was determined using the von Frey test before and after incision. Local infiltration of neutrophils and inflammatory monocytes was quantified by flow cytometry. Inflammatory cytokine levels in the incised tissue were also determined. Mechanical pain hypersensitivity was significantly reduced in BLT1KO mice compared to BLT1WT mice at 2, 3, and 4 days after incision. LTB4 levels in the tissue at the incision site peaked 3 hours after the incision. Infiltrated neutrophils peaked 1 day after the incision in both BLT1KO and BLT1WT mice. The accumulation of inflammatory monocytes increased 1-3 days after the incision and was significantly more reduced in BLT1KO mice than in BLT1WT mice. In BLT1KO mice, Interleukin-1β and Tumor Necrosis Factor-α levels 1 day after the incision were significantly lower than those of BLT1WT mice. Our data suggest that LTB4 is produced and activates its receptor BLT1 in the very early phase of tissue injury, and that LTB4-BLT1 signaling exacerbates pain responses by promoting local infiltration of inflammatory monocytes and cytokine production. Thus, LTB4-BLT1 signaling is a potential target for therapeutic intervention of acute and persistent pain induced by tissue injury.

Emerging roles of keratinocytes in nociceptive transduction and regulation

Keratinocytes are the predominant block-building cells in the epidermis. Emerging evidence has elucidated the roles of keratinocytes in a wide range of pathophysiological processes including cutaneous nociception, pruritus, and inflammation. Intraepidermal free nerve endings are entirely enwrapped within the gutters of keratinocyte cytoplasm and form en passant synaptic-like contacts with keratinocytes. Keratinocytes can detect thermal, mechanical, and chemical stimuli through transient receptor potential ion channels and other sensory receptors. The activated keratinocytes elicit calcium influx and release ATP, which binds to P2 receptors on free nerve endings and excites sensory neurons. This process is modulated by the endogenous opioid system and endothelin. Keratinocytes also express neurotransmitter receptors of adrenaline, acetylcholine, glutamate, and γ-aminobutyric acid, which are involved in regulating the activation and migration, of keratinocytes. Furthermore, keratinocytes serve as both sources and targets of neurotrophic factors, pro-inflammatory cytokines, and neuropeptides. The autocrine and/or paracrine mechanisms of these mediators create a bidirectional feedback loop that amplifies neuroinflammation and contributes to peripheral sensitization.

Opioidergic Signaling-A Neglected, Yet Potentially Important Player in Atopic Dermatitis

Atopic dermatitis (AD) is one of the most common skin diseases, the prevalence of which is especially high among children. Although our understanding about its pathogenesis has substantially grown in recent years, and hence, several novel therapeutic targets have been successfully exploited in the management of the disease, we still lack curative treatments for it. Thus, there is an unmet societal demand to identify further details of its pathogenesis to thereby pave the way for novel therapeutic approaches with favorable side effect profiles. It is commonly accepted that dysfunction of the complex cutaneous barrier plays a central role in the development of AD; therefore, the signaling pathways involved in the regulation of this quite complex process are likely to be involved in the pathogenesis of the disease and can provide novel, promising, yet unexplored therapeutic targets. Thus, in the current review, we aim to summarize the available potentially AD-relevant data regarding one such signaling pathway, namely cutaneous opioidergic signaling.

Dose-Dependent Early Postoperative Opioid Use Is Associated with Periprosthetic Joint Infection and Other Complications in Primary TJA

BACKGROUND

Opioids are commonly prescribed for postoperative pain following total joint arthroplasty. Despite widespread use, few studies have examined the dose-dependent effect of perioperative opioid use on postoperative complications following total hip arthroplasty (THA) and total knee arthroplasty (TKA). Therefore, we examined the dose-dependent relationship between opioid use and postoperative complications following primary THA and TKA.

METHODS

We queried the Premier Healthcare Database to identify adult patients who underwent primary elective THA or TKA from 2004 to 2014, and quantified opioid consumption within the first 3 postoperative days. Opioid consumption was standardized to morphine milligram equivalents (MMEs). Patients were divided into quintiles on the basis of MME exposure: <54, 54 to 82, 83 to 116, 117 to 172, and >172 MMEs. Primary outcomes included postoperative periprosthetic joint infection, pulmonary embolism, deep venous thrombosis, and pulmonary complications. Secondary outcomes included wound infection, wound dehiscence, and readmission within 30 and 90 days postoperatively. Univariate and multivariate analyses were performed to compare differences between groups and to account for confounders.

RESULTS

A total of 1,525,985 patients were identified. The mean age was 65.7 ± 10.8 years, 598,320 patients (39.2%) were male, and 1,174,314 patients (77.0%) were Caucasian. On multiple logistic regression analysis, increasing MME exposure was associated with a dose-dependent increased risk of postoperative complications. Compared with patients receiving <54 MMEs, exposure to >172 MMEs was associated with greater odds of periprosthetic joint infection (adjusted odds ratio [aOR], 1.37; 95% confidence interval [CI], 1.33 to 1.42), deep venous thromboembolism (aOR, 1.34; 95% CI, 1.30 to 1.38), pulmonary embolism (aOR, 1.29; 95% CI, 1.25 to 1.34), and pulmonary complications (aOR, 1.06; 95% CI, 1.05 to 1.08). Exposure to >172 MMEs was associated with increased risk of wound infection (aOR, 1.37; 95% CI, 1.33 to 1.41), wound dehiscence (aOR, 1.24; 95% CI, 1.19 to 1.31), and readmission within 30 (aOR, 1.21; 95% CI, 1.20 to 1.22) and 90 days (aOR, 1.20; 95% CI, 1.19 to 1.21).

CONCLUSIONS

Increasing opioid use within the early postoperative period following THA or TKA was associated with a dose-dependent increased risk of periprosthetic joint infection and venous thromboembolic events.

LEVEL OF EVIDENCE

Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

Calming the Storm: Natural Immunosuppressants as Adjuvants to Target the Cytokine Storm in COVID-19

The COVID-19 pandemic has caused a global health crisis, with no specific antiviral to treat the infection and the absence of a suitable vaccine to prevent it. While some individuals contracting the SARS-CoV-2 infection exhibit a well coordinated immune response and recover, others display a dysfunctional immune response leading to serious complications including ARDS, sepsis, MOF; associated with morbidity and mortality. Studies revealed that in patients with a dysfunctional immune response, there is a massive cytokine and chemokine release, referred to as the 'cytokine storm'. As a result, such patients exhibit higher levels of pro-inflammatory/modulatory cytokines and chemokines like TNFα, INFγ, IL-1β, IL-2, IL-4, IL-6, IL-7, IL-9, IL-10, IL-12, IL-13, IL-17, G-CSF, GM-CSF, MCSF, HGF and chemokines CXCL8, MCP1, IP10, MIP1α and MIP1β. Targeting this cytokine storm is a novel, promising treatment strategy to alleviate this excess influx of cytokines observed at the site of infection and their subsequent disastrous consequences. Natural immunosuppressant compounds, derived from plant sources like curcumin, luteolin, piperine, resveratrol are known to inhibit the production and release of pro-inflammatory cytokines and chemokines. This inhibitory effect is mediated by altering signal pathways like NF-κB, JAK/STAT, MAPK/ERK that are involved in the production and release of cytokines and chemokines. The use of these natural immunosuppressants as adjuvants to ameliorate the cytokine storm; in combination with antiviral agents and other treatment drugs currently in use presents a novel, synergistic approach for the treatment and effective cure of COVID-19. This review briefly describes the immunopathogenesis of the cytokine storm observed in SARS-CoV-2 infection and details some natural immunosuppressants that can be used as adjuvants in treating COVID-19 disease.

Investigation of the effects of systemic meperidine administration on fascia healing in an experimental rat model

Purpose:

To evaluate the effects of meperidine on fascial healing.

Methods:

Seventy adult male Sprague-Dawley rats divided into 7 groups with 10 rats in each group. One of these groups was determined as the sham group, 3 of the remaining 6 groups as meperidine groups, and 3 as control groups. These were grouped as 1st, 2nd, and 6th weeks. In the anterior abdominal wall of the rat, the skin was detached and a wound model including the peritoneum was created with a median incision. Mice in the meperidine group were injected with meperidine intraperitoneally (IP) 3 × 20 mg/kg meperidine on postoperative days 0, 1 and 2, and 2 × 20 mg/kg meperidine on postoperative days 3, 4, 5, and 6 after surgical intervention. Similar to the control group, an equal volume of saline was administered, corresponding to the doses. After sacrifice, the midline fascia was used for facial tensile strength measurement, and the other for histopathological analysis.

Results:

When compared, the meperidine and control groups inflammatory cell density was higher in the 1st week (p < 0.05) in the meperidine group, fibroplasia density was found to be higher at the 2nd week in the meperidine group than the control group (p < 0.05) When the tensile strength in the meperidine and control groups were compared, there was no significant difference (p > 0.05) at each of the three weeks.

Conclusion:

The application of postoperative systemic meperidine affects positively wound healing in the inflammatory stage and fibroplasia without changing the resistance to traction.

Wound Pain and Wound Healing Biomarkers From Wound Exudate: A Scoping Review

PURPOSE

Effective management of wound pain is essential for optimal wound healing. Nevertheless, the outcomes of wound pain interventions are based on subjective measures, which can prove problematic in patients with cognitive impairment. Identification of biomarkers associated with wound pain and wound healing can be used to more objectively estimate wound pain and contribute to the development of precise management options to reduce wound pain and promote wound healing. This scoping review aimed to identify wound pain and wound healing biomarkers from wound exudates and to describe different wound collection methods to identify these biomarkers.

METHODS

We searched the literature (PROSPERO database registration number: CRD42018103843) via a scoping review.

SEARCH STRATEGY

The PubMed database was searched for articles that explored relationships between cutaneous wound pain, wound healing, and biomolecules. Inclusion criteria were articles that reported original data, used adult human samples, and were published in English.

FINDINGS

Twenty-one articles were retrieved: 17 investigated molecules from wound exudate associated with wound healing status, and 4 reported molecules associated with wound pain. The most frequently observed wound pain biomarkers were proinflammatory cytokines; the most frequently observed wound healing biomarkers were proteases including those in the matrix metalloproteinase family. Six wound exudate collection methods were identified to extract potential wound pain and wound healing biomarkers from wound exudate.

IMPLICATIONS

The results can guide future wound exudate research to validate these wound pain and wound healing biomarkers and to develop therapies targeting these biomarkers to reduce wound pain and promote wound healing.

The Protective Effects of Pre- and Post-Administration of Micronized Palmitoylethanolamide Formulation on Postoperative Pain in Rats

Background: Postoperative pain (PO) is a common form of acute pain. Inadequate PO treatment is an important health problem, as it leads to worse outcomes, such as chronic post-surgical pain. Therefore, it is necessary to acquire new knowledge on PO mechanisms to develop therapeutic options with greater efficacy than those available today and to lower the risk of adverse effects. For this reason, we evaluated the ability of micronized palmitoylethanolamide (PEA-m) to resolve the pain and inflammatory processes activated after incision of the hind paw in an animal model of PO. Methods: The animals were subjected to surgical paw incision and randomized into different groups. PEA-m was administered orally at 10 mg/kg at different time points before or after incision. Results: Our research demonstrated that the pre- and post-treatment with PEA-m reduced the activation of mast cells at the incision site and the expression of its algogenic mediator nerve growth factor (NGF) in the lumbar spinal cord. Furthermore, again at the spinal level, it was able to decrease the activation of phospho-extracellular signal-regulated kinases (p-ERK), ionized calcium binding adaptor molecule 1 (Iba1), glial fibrillary acidic protein (GFAP), and the expression of brain-derived neurotrophic factor (BDNF). PEA-m also reduced the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) spinal pathway, showing a protective effect in a rat model of PO. Conclusion: The results obtained reinforce the idea that PEA-m may be a potential treatment for the control of pain and inflammatory processes associated with PO. In addition, pre- and post-treatment with PEA-m is more effective than treatment alone after the surgery and this limits the time of taking the compound and the abuse of analgesics.

Opioid-Mediated HIV-1 Immunopathogenesis

Despite the ability of combination antiretroviral therapy to dramatically suppress viremia, the brain continues to be a reservoir of HIV-1 low-level replication. Adding further complexity to this is the comorbidity of drug abuse with HIV-1 associated neurocognitive disorders and neuroHIV. Among several abused drugs, the use of opiates is highly prevalent in HIV-1 infected individuals, both as an abused drug as well as for pain management. Opioids and their receptors have attained notable attention owing to their ability to modulate immune functions, in turn, impacting disease progression. Various cell culture, animal and human studies have implicated the role of opioids and their receptors in modulating viral replication and virus-mediated pathology both positively and negatively. Further, the combinatorial effects of HIV-1/HIV-1 proteins and morphine have demonstrated activation of inflammatory signaling in the host system. Herein, we summarized the current knowledge on the role of opioids on peripheral immunopathogenesis, viral immunopathogenesis, epigenetic profiles of the host and viral genome, neuropathogenesis of SIV/SHIV-infected non-human primates, blood-brain-barrier, HIV-1 viral latency, and viral rebound. Overall, this review provides recent insights into the role of opioids in HIV-1 immunopathogenesis. Graphical abstract.

Effect of Preperitoneal Versus Epidural Analgesia on Postoperative Inflammatory Response and Pain Following Radical Cystectomy: A Prospective, Randomized Trial

OBJECTIVES

Continuous wound infiltration of local anesthetics has been proposed as an alternative to epidural analgesia during abdominal surgery. Cytokines have a major role in inflammatory changes caused by surgery. This study aimed to compare the effects of continuous preperitoneal versus epidural analgesia on inflammatory cytokines postoperatively.

MATERIALS AND METHODS

Forty patients scheduled for radical cystectomy were included in this observer-blinded, randomized trial; patients were randomly assigned into 2 groups to receive; continuous preperitoneal wound infiltration (PPB) or epidural analgesia (EDB). Serum levels of interleukins (IL1β, IL6, IL10, and tumor necrosis factor α) were measured at baseline (before induction of anesthesia), preinfusion (before the start of local anesthetic infusion), 6 and 24 hours postoperatively. Visual Analog Scale at rest/movement (VAS-R/M), time to the first request of analgesia, total morphine consumption, sedation score, hemodynamics, and side effects were observed 24 hours postoperatively.

RESULTS

There was a significant reduction in IL6, IL1β and increase in IL10 in PPB compared with EDB at 6 and 24 hours postoperatively and compared with preinfusion levels (P≤0.001). In EDB, a significant increase in IL1β, IL10, and tumor necrosis factor α at 6 hours compared with preinfusion levels (P≤0.002). VAS-R/M was significantly decreased at 2, 4, 6, 8, and 12 hours in EDB compared with PPB (P≤0.014), with no significant difference in the mean time to the first request of analgesia and total morphine consumption between the 2 groups.

CONCLUSION

Continuous preperitoneal analgesia better attenuated postoperative inflammatory response and provided a comparable overall analgesia to that with continuous epidural analgesia following radical cystectomy.

The effect of adjuvant oral application of honey in the management of postoperative pain after tonsillectomy in adults: A pilot study

Objective

To analyze the effect of adjuvant oral application of honey for treating postoperative pain after tonsillectomy.

Design

Single centre prospective cohort study.

Setting

Two cohorts of patients after tonsillectomy.

Participants

56 patients treated with honey 8 times per day (honey group), 18 patients treated without honey (control group); baseline analgesia were non-steroidal anti-inflammatory drugs (NSAID) or coxibs; opioids were used as pro re nata (PRN) medication; mean age 34.4 ± 13.4 years; 36% women.

Main outcome measures

On first to fifth postoperative day, patients rated their pain using the validated questionnaire of the German-wide project Quality Improvement in Postoperative Pain Treatment (QUIPS) including a numeric rating scale (NRS, 0–10) for determination of patient's pain. QUIPS allows standardized assessment of patients' characteristics andpain-associated patient-reported outcomes (PROs). The influence of preoperative and postoperative parameters on patients' postoperative pain were estimated by univariate and multivariate statistical analysis.

Results

Average pain in activity in the control group was greater than 4 (NRS 4.4 ± 2.4) during the first five postoperative days, with a renewed increase in pain intensity on the fifth day (4.3 ± 2.5). In the honey group, the pain in activity decreased without any further pain increase and was only higher than 4 on the first three postoperative days (4.3 ± 2.1, all p>0.05). However; neither minimal nor maximal pain were significantly different between both groups on the first postoperative day (p = 0.217, p = 0.980). Over the five postoperative days, the minimal and maximal pain in the honey group decreased continuously and faster than in the control group. With regard to pain-related impairments on the first day, the honey group reported less pain-related sleep disturbance (p = 0.026), as well as significantly fewer episodes of postoperative oral bleeding (p = 0.028) than the control group. Patients without honey consumption had on the first and fifth postoperative day a higher risk of increased minimal pain (OR = -2.424, CI = -4.075 –-0.385). Gender was an independent factor for compliance of honey consumption on the second postoperative day (p = 0.037). Men had a lower probability for compliance of honey consumption (OR = -0.288, CI = -2.863 –-0.090).

Conclusion

There was a trend of reduced postoperative pain after oral honey application. Honey also seems to reduce pain-related impairments. The need for additional opioids on the first day could be reduced. A larger controlled trial is now needed to varify the effect of honey on pain after tonsillectomy.

Clinical trial registration number

German Clinical Trials Register DRKS00006153. The authors confirm that all ongoing and related trials for this drug/intervention are registered.

The Role of Opioid Receptors in Immune System Function

Research on the effects of opioids on immune responses was stimulated in the 1980s by the intersection of use of intravenous heroin and HIV infection, to determine if opioids were enhancing HIV progression. The majority of experiments administering opioid alkaloids (morphine and heroin) in vivo, or adding these drugs to cell cultures in vitro, showed that they were immunosuppressive. Immunosuppression was reported as down-regulation: of Natural Killer cell activity; of responses of T and B cells to mitogens; of antibody formation in vivo and in vitro; of depression of phagocytic and microbicidal activity of neutrophils and macrophages; of cytokine and chemokine production by macrophages, microglia, and astrocytes; by sensitization to various infections using animal models; and by enhanced replication of HIV in vitro. The specificity of the receptor involved in the immunosuppression was shown to be the mu opioid receptor (MOR) by using pharmacological antagonists and mice genetically deficient in MOR. Beginning with a paper published in 2005, evidence was presented that morphine is immune-stimulating via binding to MD2, a molecule associated with Toll-like Receptor 4 (TLR4), the receptor for bacterial lipopolysaccharide (LPS). This concept was pursued to implicate inflammation as a mechanism for the psychoactive effects of the opioid. This review considers the validity of this hypothesis and concludes that it is hard to sustain. The experiments demonstrating immunosuppression were carried out in vivo in rodent strains with normal levels of TLR4, or involved use of cells taken from animals that were wild-type for expression of TLR4. Since engagement of TLR4 is universally accepted to result in immune activation by up-regulation of NF-κB, if morphine were binding to TLR4, it would be predicted that opioids would have been found to be pro-inflammatory, which they were not. Further, morphine is immunosuppressive in mice with a defective TLR4 receptor. Morphine and morphine withdrawal have been shown to permit leakage of Gram-negative bacteria and LPS from the intestinal lumen. LPS is the major ligand for TLR4. It is proposed that an occult variable in experiments where morphine is being proposed to activate TLR4 is actually underlying sepsis induced by the opioid.

A Review of the Effects of Pain and Analgesia on Immune System Function and Inflammation: Relevance for Preclinical Studies

One of the most significant challenges facing investigators, laboratory animal veterinarians, and IACUCs, is how to balance appropriate analgesic use, animal welfare, and analgesic impact on experimental results. This is particularly true for in vivo studies on immune system function and inflammatory disease. Often times the effects of analgesic drugs on a particular immune function or model are incomplete or don't exist. Further complicating the picture is evidence of the very tight integration and bidirectional functionality between the immune system and branches of the nervous system involved in nociception and pain. These relationships have advanced the concept of understanding pain as a protective neuroimmune function and recognizing pathologic pain as a neuroimmune disease. This review strives to summarize extant literature on the effects of pain and analgesia on immune system function and inflammation in the context of preclinical in vivo studies. The authors hope this work will help to guide selection of analgesics for preclinical studies of inflammatory disease and immune system function.

Coadministration of Chemokine Receptor Antagonists with Morphine Potentiates Morphine's Analgesic Effect on Incisional Pain in Rats

Crossdesensitization between opioid and chemokine receptors and involvement of chemokines in pain modulation are well established. We investigated if coadministration of chemokine receptor antagonists (CRAs) with morphine would enhance the analgesic potency of morphine on incisional pain in rats. Animals underwent incisional surgery on the left hind paw and pain responses were evaluated using von Frey filaments at various time points postsurgery between 15 and 360 minutes and daily between 24 and 72 hours. Dose-response curves for morphine, maraviroc (a CCR5 antagonist), and AMD3100 (a CXCR4 antagonist) alone were established. While morphine significantly reduced pain in a time- and dose-dependent manner, maraviroc and AMD3100 had no effect by themselves. Coadministration of either maraviroc or AMD3100 with morphine significantly increased morphine's analgesic effect on incisional pain, shifting the dose-response curve to the left 2.3- and 1.8-fold, respectively. Coadministration of both CRAs with morphine significantly shifted further the morphine dose-response curve to the left 3.3-fold. The effect of treatments on mRNA levels in the draining popliteal lymph node for a panel of chemokines and cytokines showed that message for many of these mediators was upregulated by the incision, and the combination of morphine with the CRAs markedly downregulated them. The data show that combining morphine with CRAs potentiates morphine's analgesic effect on incisional pain. Thus, the same analgesic effect of morphine alone can be achieved with lower doses of morphine when combined with CRAs. Using morphine in lower doses could reduce unwanted side effects and possibly block development of tolerance and dependence.

Kappa Opioid Receptor Distribution and Function in Primary Afferents

Primary afferents are known to be inhibited by kappa opioid receptor (KOR) signaling. However, the specific types of somatosensory neurons that express KOR remain unclear. Here, using a newly developed KOR-cre knockin allele, viral tracing, single-cell RT-PCR, and ex vivo recordings, we show that KOR is expressed in several populations of primary afferents: a subset of peptidergic sensory neurons, as well as low-threshold mechanoreceptors that form lanceolate or circumferential endings around hair follicles. We find that KOR acts centrally to inhibit excitatory neurotransmission from KOR-cre afferents in laminae I and III, and this effect is likely due to KOR-mediated inhibition of Ca2+ influx, which we observed in sensory neurons from both mouse and human. In the periphery, KOR signaling inhibits neurogenic inflammation and nociceptor sensitization by inflammatory mediators. Finally, peripherally restricted KOR agonists selectively reduce pain and itch behaviors, as well as mechanical hypersensitivity associated with a surgical incision. These experiments provide a rationale for the use of peripherally restricted KOR agonists for therapeutic treatment.

Modulation of anti-tumor immunity by the brain's reward system

Regulating immunity is a leading target for cancer therapy. Here, we show that the anti-tumor immune response can be modulated by the brain's reward system, a key circuitry in emotional processes. Activation of the reward system in tumor-bearing mice (Lewis lung carcinoma (LLC) and B16 melanoma) using chemogenetics (DREADDs), resulted in reduced tumor weight. This effect was mediated via the sympathetic nervous system (SNS), manifested by an attenuated noradrenergic input to a major immunological site, the bone marrow. Myeloid derived suppressor cells (MDSCs), which develop in the bone marrow, became less immunosuppressive following reward system activation. By depleting or adoptively transferring the MDSCs, we demonstrated that these cells are both necessary and sufficient to mediate reward system effects on tumor growth. Given the central role of the reward system in positive emotions, these findings introduce a physiological mechanism whereby the patient's psychological state can impact anti-tumor immunity and cancer progression.

Adenosine Monophosphate-activated Protein Kinase (AMPK) Activators For the Prevention, Treatment and Potential Reversal of Pathological Pain

Pathological pain is an enormous medical problem that places a significant burden on patients and can result from an injury that has long since healed or be due to an unidentifiable cause. Although treatments exist, they often either lack efficacy or have intolerable side effects. More importantly, they do not reverse the changes in the nervous system mediating pathological pain, and thus symptoms often return when therapies are discontinued. Consequently, novel therapies are urgently needed that have both improved efficacy and disease-modifying properties. Here we highlight an emerging target for novel pain therapies, adenosine monophosphate-activated protein kinase (AMPK). AMPK is capable of regulating a variety of cellular processes including protein translation, activity of other kinases, and mitochondrial metabolism, many of which are thought to contribute to pathological pain. Consistent with these properties, preclinical studies show positive, and in some cases disease-modifying effects of either pharmacological activation or genetic regulation of AMPK in models of nerve injury, chemotherapy-induced peripheral neuropathy (CIPN), postsurgical pain, inflammatory pain, and diabetic neuropathy. Given the AMPK-activating ability of metformin, a widely prescribed and well-tolerated drug, these preclinical studies provide a strong rationale for both retrospective and prospective human pain trials with this drug. They also argue for the development of novel AMPK activators, whether orthosteric, allosteric, or modulators of events upstream of the kinase. Together, this review will present the case for AMPK as a novel therapeutic target for pain and will discuss future challenges in the path toward development of AMPK-based pain therapeutics.

Circadian control of pain and neuroinflammation

The importance of a neuroinflammatory response to the development and maintenance of inflammatory and neuropathic pain have been highlighted in recent years. Inflammatory cells contributing to this response include circulating immune cells such as monocytes, T and B lymphocytes, and neutrophils, as well as microglia in the central nervous system. Pain signals are transmitted via sensory neurons in the peripheral nervous system, which express various receptors and channels that respond to mediators secreted from these inflammatory cells. Chronobiological rhythms, which include the 24-hr circadian cycle, have recently been shown to regulate both nervous and immune cell activity and function. This review examines the current literature on chronobiological control of neuroinflammatory processes, with a focus on inflammatory and neuropathic pain states. While the majority of this work has stemmed from observational studies in humans, recent advances in using animal models have highlighted distinct mechanisms underlying these interactions. Better understanding interactions between the circadian and neuroimmune systems can help guide the development of new treatments and provide improved care for patients suffering from acute and chronic pain.

Effect of low-dose ketamine on post-operative serum IL-6 production among elective surgical patients: a randomized clinical trial

BACKGROUND

Surgery and Anesthesia cause an excessive pro-inflammatory response. Mulago Hospital is faced with staff shortage making post-operative pain management difficult.Interleukin-6 (IL-6) drives inflammatory pain, endothelial cell dysfunction and fibrogenesis. Ketamine is cheap and, readily available. We hypothesized that its attenuation of serum IL-6 was a surrogate for clinical benefit.

MATERIALS AND METHODS

Institutional Review Board's approval was sought and RCT was registered at clinical trials.gov (identifier number: NCT01339065). Consenting patients were randomized to receive pre-incision intravenous ketamine - 0.5mg/kg or 0.9% saline placebo in weighted dosing. Blood samples were collected and laboratory analyzed at baseline, post-operatively in PACU, 24 and 48 hours respectively.

RESULTS

We recruited 39 patients of whom 18 were randomized to the ketamine arm and 21 in the placebo arm with follow up at 24 and 48 hours. Serum IL-6 and IL-1β levels were analyzed using ELIZA assay of pre-coated micro wells. Ketamine suppressed serum IL-6 at PACU with reduced increase at 24 hours. There was no reaction in 98% of IL-1β assayed.

CONCLUSION

Low-dose ketamine attenuated early serum IL-6 levels due to surgical response with reduced 24 hour increase, but the difference was not statistically significant and we recommend more studies.

Quercetin inhibits gout arthritis in mice: induction of an opioid-dependent regulation of inflammasome

We investigated the anti-inflammatory and analgesic effects of quercetin in monosodium urate crystals (MSU)-induced gout arthritis, and the sensitivity of quercetin effects to naloxone, an opioid receptor antagonist. Mice were treated with quercetin, and mechanical hyperalgesia was assessed at 1-24 h after MSU injection. In vivo, leukocyte recruitment, cytokine levels, oxidative stress, NFκB activation, and gp91phox and inflammasome components (NLRP3, ASC, Pro-caspase-1, and Pro-IL-1β) mRNA expression by qPCR were determined in the knee joints at 24 h after MSU injection. Inflammasome activation was determined, in vitro, in lipopolysaccharide-primed macrophages challenged with MSU. Quercetin inhibited MSU-induced mechanical hyperalgesia, leukocyte recruitment, TNFα and IL-1β production, superoxide anion production, inflammasome activation, decrease of antioxidants levels, NFκB activation, and inflammasome components mRNA expression. Naloxone pre-treatment prevented all the inhibitory effects of quercetin over MSU-induced gout arthritis. These results demonstrate that quercetin exerts analgesic and anti-inflammatory effect in the MSU-induced arthritis in a naloxone-sensitive manner.

Effects of fentanyl on pain and motor behaviors following a collagenase-induced intracerebral hemorrhage in rats

Purpose

Intracerebral hemorrhage (IH) and cephalalgia are common consequences of traumatic brain injury. One of the primary obstacles for patient recovery is the paucity of treatments to support an appropriate analgesic protocol. The present study aimed to assess pain and motor behaviors following different doses of fentanyl on a rat model of IH.

Methods

Twenty-one male Sprague Dawley rats underwent a stereotaxic surgery to produce a collagenase-induced IH in the right caudoputamen nucleus. The control group (n=6) received saline subcutaneously (SC), and experimental groups received either 5 (n=6), 10 (n=6), or 20 (n=3) µg/kg of fentanyl SC, 2 hours following surgery and on 2 subsequent days. Only 3 animals received 20 µg/kg because this dose caused catalepsy for 15–20 minutes following the injection. The rat grimace scale, a neurological examination, balance beam test, and rotarod test were performed for 5 consecutive days postoperatively to evaluate pain and motor performance. At the end of the experimentation, the brains were evaluated to determine hematoma volume, and the number of reactive astrocytes and necrotic neurons.

Results

When compared to controls, the grimace scale showed that 5 µg/kg fentanyl significantly alleviated pain on day 2 only (P<0.01) and that 10 µg/kg alleviated pain on days 1 (P<0.01), 2 (P<0.001), and 3 (P<0.01). For the rotarod test, only the 10 µg/kg group showed significant decreases in performance on days 5 (P<0.05) and 6 (P<0.02). The neurological examination was not significantly different between the groups, but only the hopping test showed poor recuperation for the 5 and 10 µg/kg fentanyl group when compared to saline (P<0.01). No differences were found between the groups for the balance beam test, the histopathological results.

Conclusion

Fentanyl, at a dose of 10 µg/kg SC, provides substantial analgesia following a collagenase-induced IH in rats; however, it can alter motor performance following analgesic treatments.

Targeting AMPK for the Alleviation of Pathological Pain

Chronic pain is a major clinical problem that is poorly treated with available therapeutics. Adenosine monophosphate-activated protein kinase (AMPK) has recently emerged as a novel target for the treatment of pain with the exciting potential for disease modification. AMPK activators inhibit signaling pathways that are known to promote changes in the function and phenotype of peripheral nociceptive neurons and promote chronic pain. AMPK activators also reduce the excitability of these cells suggesting that AMPK activators may be efficacious for the treatment of chronic pain disorders, like neuropathic pain, where changes in the excitability of nociceptors is thought to be an underlying cause. In agreement with this, AMPK activators have now been shown to alleviate pain in a broad variety of preclinical pain models indicating that this mechanism might be engaged for the treatment of many types of pain in the clinic. A key feature of the effect of AMPK activators in these models is that they can lead to a long-lasting reversal of pain hypersensitivity even long after treatment cessation, indicative of disease modification. Here, we review the evidence supporting AMPK as a novel pain target pointing out opportunities for further discovery that are likely to have an impact on drug discovery efforts centered around potent and specific allosteric activators of AMPK for chronic pain treatment.

Linking traits based on their shared molecular mechanisms

There is growing recognition that co-morbidity and co-occurrence of disease traits are often determined by shared genetic and molecular mechanisms. In most cases, however, the specific mechanisms that lead to such trait-trait relationships are yet unknown. Here we present an analysis of a broad spectrum of behavioral and physiological traits together with gene-expression measurements across genetically diverse mouse strains. We develop an unbiased methodology that constructs potentially overlapping groups of traits and resolves their underlying combination of genetic loci and molecular mechanisms. For example, our method predicts that genetic variation in the Klf7 gene may influence gene transcripts in bone marrow-derived myeloid cells, which in turn affect 17 behavioral traits following morphine injection; this predicted effect of Klf7 is consistent with an in vitro perturbation of Klf7 in bone marrow cells. Our analysis demonstrates the utility of studying hidden causative mechanisms that lead to relationships between complex traits.

Translational control of chronic pain

Pain is a crucial physiological response to injury and pathologies. The development and maintenance of pain requires the expression of novel genes. The expression of such genes occurs in highly regulated and orchestrated manner where protein translation provides an exquisite temporal and spatial fidelity within the axons and dendrites of neurons. Signaling pathways that regulate local translation are activated by cytokines, neurotrophic factors, or neurotransmitters, which are released either due to tissue damage or neuronal activity. In recent years, the ERK and mTOR pathways have been demonstrated to be central in regulating local translation in neurons of both the peripheral and central nervous systems in diverse models of chronic pain. The ERK and mTOR pathways converge onto the cap-dependent translational machinery that regulates genes essential for the development of nociceptive sensitization. Moreover, inhibition of these pathways has proved to be effective in normalizing the biochemical changes and the associated pain in various preclinical models.

Evaluation of the effects of methadone and tramadol on postoperative analgesia and serum interleukin-6 in dogs undergoing orthopaedic surgery

BACKGROUND

Acute postsurgical pain is of great interest due to potential risk of becoming chronic if not treated properly, worsening patient's recovery and quality of life. Twenty-eight dogs with ruptured cruciate ligaments were divided into three groups that received intramuscular injections of 4 mg/kg of tramadol (TRA), 0.5 mg/kg of methadone (MET0.5), or 0.7 mg/kg of methadone (MET0.7). Physiological parameters (heart and respiratory rates and blood pressure) were evaluated at specified times: baseline (TBL), 1 (T1), 2 (T2), 4 (T4), 6 (T6), and 24 (T24) hours after premedication. Pain scores were described by visual analogue scale (VAS), modified Glasgow Composite, and Colorado University Acute Pain scales. Blood samples for measurement of interleukin (IL)-6 were collected at TBL, T1, T6, and T24. This was a prospective, randomised investigation to evaluate the efficacy of tramadol and methadone as premedications in dogs undergoing osteotomies.

RESULTS

There were no statistically significant differences between groups with respect to age, weight, gender, surgery time, and time to extubation. Heart rate, respiratory rate, and blood pressure values were maintained within acceptable ranges, and a reduction was observed at T2 in MET0.5 and MET0.7 compared with TBL. Increases in VAS scores were observed in TRA at T4 compared with TBL, T1, and T24 and between T1 and T6 (p < 0.001). In MET0.5, there was significant increase in VAS score at T4 compared with T1 (p < 0.001). TRA and MET0.5 showed significantly higher mean ± SD VAS scores (3.4 ± 2.5 and 2.5 ± 2.6, respectively) than MET0.7 (1.1 ± 1.5) at T4 (p < 0.001). TRA showed greater demand of rescue analgesia (four animals in T4 and two in T6) (p < 0.037). There were no statistically significant differences in sedation scores, Colorado Scale scores, or interleukin levels between groups and time points.

CONCLUSIONS

Methadone given as premedication in doses of 0.7 mg/kg was better at controlling pain compared with lower doses and tramadol. However, dosage increases, administered as rescue analgesia, promoted adequate pain control even in tramadol group. Influence of these analgesics on IL-6 release could not be demonstrated, but significant levels were not found.

Opioids enhance CXCL1 expression and function after incision in mice

Chronic opioid consumption increases postoperative pain. Epigenetic changes related to chronic opioid use and surgical incision may be partially responsible for this enhancement. The CXCL1/CXCR2 signaling pathway, implicated in several pain models, is known to be epigenetically regulated via histone acetylation. The current study was designed to investigate the role of CXCL1/CXCR2 signaling in opioid-enhanced incisional sensitization and to elucidate the possible epigenetic mechanism underlying CXCL1/CXCR2 pathway-mediated regulation of nociceptive sensitization in mice. Chronic morphine treatment generated mechanical and thermal nociceptive sensitization and also significantly exacerbated incision-induced mechanical allodynia. Peripheral but not central messenger RNA levels of CXCL1 and CXCR2 were increased after incision. The source of peripheral CXCL1 appeared to be wound area neutrophils. Histone H3 subunit acetylated at the lysine 9 position (AcH3K9) was increased in infiltrating dermal neutrophils after incision and was further increased in mice with chronic morphine treatment. The association of AcH3K9 with the promoter region of CXCL1 was enhanced in mice after chronic morphine treatment. The increase in CXCL1 near wounds caused by chronic morphine pretreatment was mimicked by pharmacologic inhibition of histone deacetylation. Finally, local injection of CXCL1 induced mechanical sensitivity in naive mice, whereas blocking CXCR2 reversed mechanical hypersensitivity after hind paw incision.

PERSPECTIVE

Peripheral CXCL1/CXCR2 signaling helps to control nociceptive sensitization after incision, and epigenetic regulation of CXCL1 expression explains in part opioid-enhanced incisional allodynia in mice. These results suggest that targeting CXCL1/CXCR2 signaling may be useful in treating nociceptive sensitization, particularly for postoperative pain in chronic opioid-consuming patients.

Curcumin treatment attenuates pain and enhances functional recovery after incision

BACKGROUND

Acute pain after surgery remains moderate to severe for 20% to 30% of patients despite advancements in the use of opioids, adjuvant drugs, and regional anesthesia. Depending on the type of surgery, 10% to 50% of patients experience persistent pain postoperatively, and there are no established methods for its prevention. Curcumin (diferuloylmethane) is one of the phenolic constituents of turmeric that has been used in Eastern traditional medicine as an antiseptic, antioxidant, anti-inflammatory, and analgesic agent. It may be effective for treating postoperative pain.

METHODS

We used the hindpaw incision model with C57BL/6 mice. Sensitization to mechanical and thermal stimuli as well as effects on edema and temperature were measured up to 7 days after surgery. Spontaneous pain after incision was assessed by using conditioned place preference (CPP), and alterations in gait function were assessed using multiparameter digital gait analysis.

RESULTS

Curcumin (50 mg/kg) significantly reduced the intensity of mechanical and heat sensitization after hindpaw incision in mice. No effects of curcumin on baseline nociceptive thresholds were observed. Curcumin also reduced hindpaw swelling after incision, suggesting an anti-inflammatory effect. In addition, perioperative curcumin treatment attenuated hyperalgesic priming due to incision when mice were subsequently challenged with hindpaw prostaglandin E2 application. Furthermore, while vehicle-treated mice had evidence of spontaneous pain 48 hours after incision in the CPP paradigm, no evidence of ongoing pain was observed in the mice treated with curcumin. Likewise, hindpaw incision caused changes in several gait-related indices, but most of these were normalized in the curcumin-treated animals. The peri-incisional levels of several pronociceptive immune mediators including interleukin (IL)-1β, IL-6, tumor necrosis factor α, and macrophage inflammatory protein-1α were either not reduced or were even augmented 1 and 3 days after incision in curcumin-treated mice. The anti-inflammatory cytokine IL-10 was unchanged, while transforming growth factor-β levels were enhanced under the same conditions.

CONCLUSIONS

Our studies suggest that curcumin treatment is effective in alleviating incision-induced inflammation, nociceptive sensitization, spontaneous pain, and functional gait abnormalities. Augmented transforming growth factor-β production provides one possible mechanism. These preclinical findings demonstrate curcumin's potential as a preventative strategy in postoperative pain treatment.

Peripheral administration of morphine attenuates postincisional pain by regulating macrophage polarization through COX-2-dependent pathway

BACKGROUND

Macrophage infiltration to inflammatory sites promotes wound repair and may be involved in pain hypersensitivity after surgical incision. We recently reported that the development of hyperalgesia during chronic inflammation is regulated by macrophage polarity, often referred to as proinflammatory (M1) or anti-inflammatory (M2) macrophages. Although opioids such as morphine are known to alter the inflammatory milieu of incisional wounds through interactions with immunocytes, the macrophage-mediated effects of morphine on the development of postincisional pain have not been well investigated. In this study, we examined how morphine alters pain hypersensitivity through phenotypic shifts in local macrophages during the course of incision-induced inflammation.

RESULTS

Local administration of morphine in the early phase, but not in the late phase alleviated mechanical hyperalgesia, and this effect was reversed by clodronate-induced peripheral depletion of local macrophages. At the morphine-injected incisional sites, the number of pro-inflammatory F4/80+iNOS+M1 macrophages was decreased during the course of pain development whereas increased infiltration of wound healing F4/80+CD206+M2 macrophages was observed during the early phase. Morphine increased the gene expression of endogenous opioid, proenkephalin, and decreased the pronociceptive cytokine, interleukin-1β. Heme oxygenase (HO)-1 promotes the differentiation of macrophages to the M2 phenotype. An inhibitor of HO-1, tin protoporphyrin reversed morphine-induced analgesic effects and the changes in macrophage phenotype. However, local expression levels of HO-1 were not altered by morphine. Conversely, cyclooxygenase (COX)-2, primarily produced from peripheral macrophages in acute inflammation states, was up-regulated in the early phase at morphine-injected sites. In addition, the analgesic effects and a phenotype switching of infiltrated macrophages by morphine was reversed by local administration of a COX inhibitor, indomethacin.

CONCLUSIONS

Local administration of morphine alleviated the development of postincisional pain, possibly by altering macrophage polarity at the incisional sites. A morphine-induced shift in macrophage phenotype may be mediated by a COX-2-dependent mechanism. Therefore, μ-opioid receptor signaling in macrophages may be a potential therapeutic target during the early phase of postincisional pain development.

Tephrosia sinapou ethyl acetate extract inhibits inflammatory pain in mice: opioid receptor dependent inhibition of TNFα and IL-1β production

CONTEXT. Tephrosia toxicaria is currently known as Tephrosia sinapou (Buc'hoz) A. Chev. (Fabaceae) and is a source of compounds such as flavonoids that inhibit inflammatory pain.

OBJECTIVE

To investigate the analgesic effect and mechanisms of the ethyl acetate extract of T. sinapou in inflammatory pain in mice.

MATERIALS AND METHODS

Behavioral responses were evaluated using mechanical (1-24 h) and thermal hyperalgesia (0.5-5 h), writhing response (20 min) and rota-rod (1-5 h) tests. Neutrophil recruitment (myeloperoxidase activity), cytokines (tumor necrosis factor [TNF]α and interleukin [IL]-1β), aspartate aminotransferase (AST) and alanine aminotransferase (ALT) serum levels were determined by colorimetric assays. Pharmacological treatments were opioid receptor antagonist (naloxone, 0.1-1 mg/kg) and control opioid (morphine, 5 mg/kg). Inflammatory stimuli were carrageenin (100 µg/paw), complete Freund's adjuvant (CFA, 10 µl/paw), prostaglandin E2 (PGE2, 100 ng/paw) and acetic acid (0.8%).

RESULTS

The intraperitoneal pre-treatment with extract inhibited in a dose-dependent (30-300 mg/kg) dependent manner the mechanical hyperalgesia induced by carrageenin (up to 93% inhibition). The post-treatment (100 mg/kg) inhibited CFA-induced hyperalgesia (up to 63% inhibition). Naloxone (1 mg/kg) prevented the inhibitory effect of the extract over carrageenin-induced mechanical (100%) and thermal (100%) hyperalgesia, neutrophil recruitment (52%) and TNFα (63%) and IL-1β (98%) production, thermal threshold in naïve mice (99%), PGE2-induced mechanical hyperalgesia (88%) and acetic acid-induced writhing response (49%). There was no significant alteration in the rota-rod test, and AST and ALT serum levels by extract treatment. Discussion and conclusion. Tephrosia sinapou ethyl acetate extract reduces inflammatory pain by activating an opioid receptor-dependent mechanism.

Morphine prevents lipopolysaccharide-induced TNF secretion in mast cells blocking IκB kinase activation and SNAP-23 phosphorylation: correlation with the formation of a β-arrestin/TRAF6 complex

We have previously shown that morphine pretreatment inhibits mast cell-dependent TNF production after LPS injection in the murine peritoneal cavity. In this study, we used bone marrow-derived mast cells (BMMCs) to investigate the molecular mechanisms of that inhibition. We found that morphine prevented LPS-induced TNF secretion in these cells. The observed inhibition was not due to morphine-induced TLR4 internalization and it was related to the blockage of preformed TNF secretion. LPS-induced TNF exocytosis in BMMCs was dependent on tetanus toxin-insensitive vesicle-associated membrane proteins and calcium mobilization, as well as PI3K, MAPK, and IκB kinase (IKK) activation. TNF secretion was also associated to the phosphorylation of synaptosomal-associated protein 23 (SNAP-23), which was found forming a complex with IKK in LPS-activated BMMCs. Morphine pretreatment prevented TLR4-dependent ERK and IKK phosphorylation. Analyzing the signaling events upstream of IKK activation, we found diminished TGF-β-activated kinase 1 (TAK1) phosphorylation and TNFR-associated factor (TRAF) 6 ubiquitination in BMMCs pretreated with morphine and stimulated with LPS. Morphine pretreatment provoked a marked increase in the formation of a molecular complex composed of TRAF6 and β-arrestin-2. Naloxone and a combination of μ and δ opioid receptor antagonists prevented morphine inhibitory actions. In conclusion, our results show that activation of μ and δ opioid receptors with morphine suppresses TLR4-induced TNF release in mast cells, preventing the IKK-dependent phosphorylation of SNAP-23, which is necessary for TNF exocytosis, and this inhibition correlates with the formation of a β-arrestin-2/TRAF6 complex. To our knowledge, these findings constitute the first evidence of molecular crosstalk between opioid receptors and the TLR4 signal transduction system in mast cells.

Keratinocytes express cytokines and nerve growth factor in response to neuropeptide activation of the ERK1/2 and JNK MAPK transcription pathways

Sensory neurons innervating the skin can release neuropeptides that are believed to modulate cellular proliferation, wound healing, pigmentation, and keratinocyte innate immune responses. While the ability of neuropeptides to stimulate keratinocyte production of inflammatory mediators has been demonstrated, there is no information concerning the mechanisms by which neuropeptide activation of keratinocyte cell surface receptors ultimately leads to the up-regulation of mediator production. In this study we used a keratinocyte cell line to identify the presence of substance P (SP) and calcitonin gene-related peptide (CGRP) receptors on keratinocytes and examined the effects of SP and CGRP stimulation on keratinocyte neuropeptide signaling, cell proliferation, and interleukin-1β (IL-1β), interleukin-6 (IL-6), tumor necrosis factor α (TNF-α), and nerve growth factor (NGF) expression. Neuropeptide stimulation caused an up-regulation of neuropeptide receptor expression in keratinocytes and a dramatic increase in keratinocyte secretion of SP and CGRP, suggesting possible autocrine or paracrine stimulatory effects and amplification of neuropeptide signaling. Both SP and CGRP concentration-dependently stimulated cellular proliferation and the expression and secretion of inflammatory cytokines and NGF in keratinocytes. SP also activated all 3 families of mitogen activated protein kinase (MAPK) and nuclear factor κB (NFκB) in keratinocytes, while CGRP only activated p38 and extracellular signal related kinase1/2 (ERK1/2) MAPKs. Neuropeptide stimulated inflammatory mediatory production in keratinocytes was reversed by ERK1/2 and JNK inhibitors. The current study is the first to observe; 1) that CGRP stimulates keratinocyte expression of CGRP and its receptor complex, 2) that SP and CGRP stimulate IL-6 and TNF-α secretion in keratinocytes, 3) that SP activated all three MAPK families and the NFκB transcriptional signaling pathway in keratinocytes, and 4) that SP and CGRP stimulated inflammatory mediator production in keratinocytes is dependent on ERK1/2 and JNK activation. These studies provide evidence suggesting that disruption of ERK1/2 and JNK signaling may potentially be an effective therapy for inflammatory skin diseases and pain syndromes mediated by exaggerated sensory neuron-keratinocyte signaling.

AMPK: An emerging target for modification of injury-induced pain plasticity

Chronic pain is a critical medical problem afflicting hundreds of millions of people worldwide with costly effects on society and health care systems. Novel therapeutic avenues for the treatment of pain are needed that are directly targeted to the molecular mechanisms that promote and maintain chronic pain states. Recent evidence suggests that peripheral pain plasticity is promoted and potentially maintained via changes in translation control that are mediated by mTORC1 and MAPK pathways. While these pathways can be targeted individually, stimulating the AMPK pathway with direct or indirect activators achieves inhibition of these pathways via engagement of a single kinase. Here we review the form, function and pharmacology of AMPK with special attention to its emerging role as a potential target for pain therapeutics. We present the existing evidence supporting a role of AMPK activation in alleviating symptoms of peripheral nerve injury- and incision-induced pain plasticity and the blockade of the development of chronic pain following surgery. We argue that these preclinical findings support a strong rationale for clinical trials of currently available AMPK activators and further development of novel pharmacological strategies for more potent and efficacious manipulation of AMPK in the clinical setting. Finally, we posit that AMPK represents a unique opportunity for drug development in the kinase area for pain because it is pharmacologically manipulated via activation rather than inhibition potentially offering a wider therapeutic window with interesting additional pharmacological opportunities. Altogether, the physiology, pharmacology and therapeutic opportunities surrounding AMPK make it an attractive target for novel intervention for chronic pain and its prevention.

Preprotachykinin-A gene disruption attenuates nociceptive sensitivity after opioid administration and incision by peripheral and spinal mechanisms in mice

The preprotachykinin A gene (ppt-A) codes for Substance P (SP), supports nociceptive sensitization, and modulates inflammatory responses after incision. Repeated opioid use produces paradoxical pain sensitization-termed opioid-induced hyperalgesia (OIH) -which can exacerbate pain after incision. Here the contribution of SP to peri-incisional nociceptive sensitization and nociceptive mediator production after opioid treatment was examined utilizing ppt-A knockout (-/-) mice and the neurokinin (NK1) receptor antagonist LY303870. Less mechanical allodynia was observed in ppt-A(-/-) mice compared to wild types (wt) after morphine treatment both before and after incision. Moreover, LY303870 administered with morphine reduced incisional hyperalgesia in wt mice. Incision after saline or escalating morphine treatment upregulated skin IL-1β, IL-6, G-CSF and MIP-1α levels in ppt-A(-/-) and wt mice similarly. However, chronic morphine treatment greatly exacerbated increases in skin nerve growth factor levels after incision, an effect entirely dependent upon intact SP signaling. Additionally, SP dependent upregulation of prodynorphin, NMDA1 and NK1 receptor expression in spinal cord was seen after morphine treatment and incision. A similar pattern was seen for 5-HT3 receptor expression in tissue from dorsal root ganglia. Therefore, SP may work at both central and peripheral sites to enhance nociceptive sensitization after morphine treatment and incision.

PERSPECTIVE

These studies show that SP signaling modulates enhanced nerve growth factor production and changes in neuronal gene expression seen after incision in mice previously exposed to morphine.

Neutrophils recruited by CXCR1/2 signalling mediate post-incisional pain

BACKGROUND

Neutrophil recruitment mediated by the CXCL1/KC chemokine and its receptors CXCR1/CXCR2 plays a critical role in inflammatory diseases. Recently, neutrophil migration and activation triggered by CXCL1-CXCR1/2 signalling was implicated in inflammatory nociception; however, their role in post-surgical pain has not been elucidated. In this study, we addressed the function of neutrophils in the genesis of post-incisional pain in an experimental model of post-surgical pain.

METHODS

Mechanical hyperalgesia was determined with an electronic von Frey test in a mouse hindpaw incisional model. Neutrophil accumulation and the level of CXCL1/KC in the plantar tissue were determined by myeloperoxidase activity assay and enzyme-linked immunosorbent assay, respectively.

RESULTS

An incision in the mouse hindpaw produces long-lasting mechanical hyperalgesia that persists for at least 72 h after surgery. Following surgery, there was an increase in both neutrophil accumulation and the CXCL1/KC level in the incised paws. The depletion of the mouse neutrophils by vinblastine sulphate or anti-neutrophil antibody treatments reduced the mechanical hyperalgesia after paw incision. Furthermore, the treatment of mice with ladarixin, an orally acting CXCR1/2 antagonist, also reduced both the mechanical hyperalgesia and the infiltration of neutrophils in the incised paws.

CONCLUSION

In conclusion, it appears that after surgical processes, neutrophils are recruited by CXCL1-CXCR1/2 signalling and participate in the cascade of events, leading to mechanical hyperalgesia. These results suggest that blocking neutrophil migration through the inhibition of CXCL1-CXCR1/2 signalling might be a target to control post-surgical pain.

miR-203 regulates nociceptive sensitization after incision by controlling phospholipase A2 activating protein expression

BACKGROUND

After incision keratinocytes in the epidermis become activated to produce a range of pain-related mediators. microRNA 203 (miR-203) is known to be involved in keratinocyte growth, differentiation, and skin inflammation. We hypothesized that one or more of these mediators might be under the control of miR-203.

METHODS

The expression of miR-203 and its target gene, phospholipase A2 activating protein (PLAA), were examined after hind paw incision in mice. We investigated the local effect of intraplantar PLAA peptide injection in normal mice and the effects of a selective secretory phospholipase A2 inhibitor (HK064) on PLAA or incision-induced mechanical allodynia. Last, we investigated the role of substance P signaling in regulating miR-203 and PLAA expression in vitro and in vivo.

RESULTS

Levels of miR-203 were strongly down-regulated in keratinocytes after incision. Informatics-based approaches identified PLAA as a likely candidate for regulation by miR-203. PLAA caused mechanical allodynia and conditioned place aversion but not thermal sensitization. HK064 reduced mechanical allodynia after incision and after intraplantar injection of PLAA. Using preprotachykinin gene knockout mice or with neurokinin-1 selective antagonist LY303870 treatment, we observed that substance P-mediated signaling was also required for miR-203 and PLAA regulation after incision. Finally, using the rat epidermal keratinocyte cell line, we observed that a miR-203 mimic molecule could block the substance P-induced increase in PLAA expression observed under control conditions.

CONCLUSIONS

miR-203 may regulate expression of the novel nociceptive mediator PLAA after incision. Furthermore, the regulation of miR-203 and PLAA levels is reliant upon intact substance P signaling.

Roles of Gr-1+ leukocytes in postincisional nociceptive sensitization and inflammation

BACKGROUND

Neutrophils are one of the predominant immune cells initially migrating to surgical wound edges. They produce mediators both associated with supporting (interleukin [IL]-1β, C5a) and reducing (opioid peptides) pain. Studies demonstrate neutrophil depletion/blockade reduces nociceptive sensitization after nerve injury and carrageenan administration, but enhance sensitization in complete Freund's adjuvant inflammation. This research identifies the contribution of infiltrating neutrophils to incisional pain and inflammation.

METHODS

Antibody-mediated Gr1 neutrophil depletion preceded hind paw incisions. Sensitization to mechanical and thermal stimuli, effects on edema and local levels of IL-1β and C5a were measured. Local effects of C5a or IL-1 receptor antagonists PMX-53 and anakinra on sensitization after neutrophil depletion were examined. Groups of 4-8 mice were used.

RESULTS

Anti-Gr1 antibody depleted more than 90% of circulating and infiltrating skin neutrophils after incision. Neutrophil depletion did not change magnitude or duration of mechanical hypersensitivity in incised mice. However, paw edema was significantly reduced and heat hypersensitivity was slightly increased in depleted animals. In depleted animals IL-1β levels were half of controls 24 h after incision, whereas C5a levels were increased in both. Prominent IL-1β immunohistochemical staining of epidermis was seen in both groups. PMX-53 and anakinra reduced incisional mechanical and heat nociceptive sensitization to the same extent, regardless of neutrophil depletion.

CONCLUSIONS

Neutrophil-derived IL-1β and C5a do not appear to contribute critically to peri-incisional nociceptive signaling. Other sources of mediators, such as epidermal cells, may need to be considered. Controlling inflammatory activation of resident cells in epidermis/deeper structures may show therapeutic efficacy in reducing pain from surgical incisions.

Collecting and measuring wound exudate biochemical mediators in surgical wounds

We describe a methodology by which we are able to collect and measure biochemical inflammatory and nociceptive mediators at the surgical wound site. Collecting site-specific biochemical markers is important to understand the relationship between levels in serum and surgical wound, determine any associations between mediator release, pain, analgesic use and other outcomes of interest, and evaluate the effect of systemic and peripheral drug administration on surgical wound biochemistry. This methodology has been applied to healthy women undergoing elective cesarean delivery with spinal anesthesia. We have measured wound exudate and serum mediators at the same time intervals as patient's pain scores and analgesics consumption for up to 48 hours post-cesarean delivery. Using this methodology we have been able to detect various biochemical mediators including nerve growth factor (NGF), prostaglandin E2 (PG-E2) substance P, IL-1β, IL-2, IL-4, IL-6, IL-7, IL-8, IL-10, IL-12, IL-13, IL-17, TNFα, INFγ, G-CSF, GM-CSF, MCP-1 and MIP-1β. Studies applying this human surgical wound bioassay have found no correlations between wound and serum cytokine concentrations or their time-release profile (J Pain. 2008; 9(7):650-7).(1) We also documented the utility of the technique to identify drug-mediated changes in wound cytokine content.

Tonsillectomy Healing

Objectives:

We performed a prospective observation study in an outpatient surgical and office setting to compare human post-tonsillectomy healing to human cutaneous wound healing and to established animal models of oral healing.

Methods:

Fourteen teenaged patients underwent planned tonsillectomy. Intraoral digital photographs were collected at the time of tonsillectomy, during the management of complications, and at postoperative office visits. Serial intraoral photographs of one patient were taken at 48-hour intervals from the time of surgery until postoperative day 17.

Results:

Intraoral photographs from the days after tonsillectomy revealed a pattern of inflammation and healing that closely paralleled that in human skin and in canine and porcine oral wound models.

Conclusions:

Edema and pain are greatest immediately after surgery, probably as a result of thermal effects and expression of inflammatory mediators that stimulate pharyngeal nociceptors. Pain gradually decreases over time, with an increase in analog pain measures on postoperative days 3 to 5 corresponding to the maximal wound inflammation documented in experimental models. Epithelial ingrowth beneath a fibrin clot begins shortly after wounding. Separation of the fibrin clot about 7 days after surgery exposes vascular stroma. Involution of the vascular stroma and completion of epithelial coverage correlate with decreased pain levels and a lessened risk of bleeding.

Transdermal opioid patches for pain treatment in ancient Greece

Pain treatment in ancient Greece, and through the middle ages in Europe, was to a great extent based on the expertise of the Greek physician Galen (c. 129-200 A.D.). Galen makes particular reference to "Olympic Victor's Dark Ointment" (OVDO), which is listed with a number of collyria. Galen states that OVDO can be useful for treating extreme pain and swellings, forming one of the best eye salves. Olympic Victor's Dark Ointment, an opium-based treatment, forms a "patch" when applied externally as an ointment, because it quickly dries to cover a localized region but still retains its elastic properties. This study has recreated OVDO and applied the ointment to abdominal mouse skin, in vitro. To assess the efficacy of OVDO, the transdermal transfer of morphine was measured when given as OVDO and compared to morphine administered in the form of a solution of Opium + PBS (ringer). Olympic Victor's Dark Ointment showed a transdermal transfer of morphine over time comparable to 25% of the most efficient modern transdermal opioid patches, while hardly any morphine was able to penetrate the skin when applied mixed in PBS. We conclude that OVDO is very efficient in its composition and may carry some forgotten abilities in terms of drug delivery, which could be transferred to modern medicine. Indeed, this may lead to a better choice of morphine use and controlled management in individual patient cases, taking both pain relief and anti-inflammatory aspects into account.

Direct gene transfer with compacted DNA nanoparticles in retinal pigment epithelial cells: expression, repeat delivery and lack of toxicity

AIM

To evaluate the safety of compacted DNA nanoparticles (NPs) in retinal pigment epithelial (RPE) cells.

MATERIALS & METHODS

Enhanced GFP expression cassettes controlled by the RPE-specific vitelloform macular dystrophy promoter were constructed with and without a bacterial backbone and compacted into NPs formulated with polyethylene glycol-substituted lysine 30-mers. Single or double subretinal injections were administered in adult BALB/c mice. Expression levels of enhanced GFP, proinflammatory cytokines and neutrophil/macrophage mediators, and retinal function by electroretinogram were evaluated at different time-points postinjection.

RESULTS

Immunohistochemistry and real-time PCR demonstrated that NPs specifically transfect RPE cells at a higher efficiency than naked DNA and similar results were observed after the second injection. At 6 h postinjections, a transient inflammatory response was observed in all cohorts, including saline, indicating an adverse effect to the injection procedure. Subsequently, no inflammation was detected in all experimental groups.

CONCLUSION

This study demonstrates the safety and efficacy of NP-mediated RPE gene transfer therapy following multiple subretinal administrations.

Sensitization of dural afferents underlies migraine-related behavior following meningeal application of interleukin-6 (IL-6)

BACKGROUND

Migraine headache is one of the most common neurological disorders, but the pathophysiology contributing to migraine is poorly understood. Intracranial interleukin-6 (IL-6) levels have been shown to be elevated during migraine attacks, suggesting that this cytokine may facilitate pain signaling from the meninges and contribute to the development of headache.

METHODS

Cutaneous allodynia was measured in rats following stimulation of the dura with IL-6 alone or in combination with the MEK inhibitor, U0126. The number of action potentials and latency to the first action potential peak in response to a ramp current stimulus as well as current threshold were measured in retrogradely-labeled dural afferents using patch-clamp electrophysiology. These recordings were performed in the presence of IL-6 alone or in combination with U0126. Association between ERK1 and Nav1.7 following IL-6 treatment was also measured by co-immunoprecipitation.

RESULTS

Here we report that in awake animals, direct application of IL-6 to the dura produced dose-dependent facial and hindpaw allodynia. The MEK inhibitor U0126 blocked IL-6-induced allodynia indicating that IL-6 produced this behavioral effect through the MAP kinase pathway. In trigeminal neurons retrogradely labeled from the dura, IL-6 application decreased the current threshold for action potential firing. In response to a ramp current stimulus, cells treated with IL-6 showed an increase in the numbers of action potentials and a decrease in latency to the first spike, an effect consistent with phosphorylation of the sodium channel Nav1.7. Pretreatment with U0126 reversed hyperexcitability following IL-6 treatment. Moreover, co-immunoprecipitation experiments demonstrated an increased association between ERK1 and Nav1.7 following IL-6 treatment.

CONCLUSIONS

Our results indicate that IL-6 enhances the excitability of dural afferents likely via ERK-mediated modulation of Nav1.7 and these responses contribute to migraine-related pain behavior in vivo. These data provide a cellular mechanism by which IL-6 in the meninges causes sensitization of dural afferents therefore contributing to the pathogenesis of migraine headache.