Change Pain: Ever Evolving-An Update for 2016

Role of Microglia in Neuropathic Pain

Microglial cells are specialized macrophage cells of the central nervous system responsible for the innate immunity of the spinal cord and the brain. They protect the brain and spinal cord from invaders, microbes, demyelination, trauma and remove defective cells and neurons. For immune protection, microglial cells possess a significant number of receptors and chemical mediators that allow them to communicate rapidly and specifically with all cells of the nervous tissue. The contribution of microglia in neuropathic pain challenges conventional concepts toward neurons being the only structure responsible for the pathophysiological changes that drive neuropathic pain. The present study is a narrative review focusing on the literature concerning the complex interaction between neurons and microglia in the development of neuropathic pain. Injury in the peripheral or central nervous system may result in maladaptive changes in neurons and microglial cells. In neuropathic pain, microglial cells have an important role in initiating and maintenance of pain and inflammation. The interaction between neural and microglial cells has been proven extremely crucial for chronic pain. The study of individual mechanisms at the level of the spinal cord and the brain is an interesting and groundbreaking research challenge. Elucidation of the mechanisms by which neurons and immune cells interact, could constitute microglial cells a new therapeutic target for the treatment of neuropathic pain.

miR-26a-5p alleviates CFA-induced chronic inflammatory hyperalgesia through Wnt5a/CaMKII/NFAT signaling in mice

BACKGROUND

Inflammation often leads to the occurrence of chronic pain, and many miRNAs have been shown to play a key role in the development of inflammatory pain. However, whether miR-26a-5p relieves pain induced by inflammation and its possible mechanism are still unclear.

METHODS

The complete Freund's adjuvant (CFA)-induced inflammatory pain mouse model was employed. Intrathecal or subcutaneous injection of miR-26a-5p agomir was performed after modeling to study its antinociceptive effect and the comparison of different administration methods. Bioinformatics analysis of miRNAs was performed to study the downstream mechanisms of miR-26a-5p. HE staining, RT-qPCR, Western blotting, and immunofluorescence were used for further validation.

RESULTS

A single intrathecal and subcutaneous injection of miR-26a-5p both reversed mechanical hypersensitivity and thermal latency in the left hind paw of mice with CFA-induced inflammatory pain. HE staining and immunofluorescence studies found that both administrations of miR-26a-5p alleviated inflammation in the periphery and spinal cord. Bioinformatics analysis and dual-luciferase reporter gene analysis identified Wnt5a as a direct downstream target gene of miR-26a-5p. Wnt5a was mainly expressed in neurons and microglia in the spinal cord of mice with inflammatory pain. Intrathecal injection of miR-26a-5p could significantly reduce the expression level of Wnt5a and inhibit the downstream molecules of noncanonical Wnt signaling Camk2/NFAT, inhibiting the release of spinal cord inflammatory factors and alleviating the activation of microglia. In addition, miR-26a-5p could also inhibit lipopolysaccharide (LPS)-stimulated BV2 cell inflammation in vitro through a noncanonical Wnt signaling pathway.

CONCLUSIONS

miR-26a-5p is a promising therapy for CFA-induced inflammatory pain. Both intrathecal and subcutaneous injections provide relief for inflammatory pain. miR-26a-5p regulated noncanonical Wnt signaling to be involved in analgesia partly through antineuroinflammation, suggesting a pain-alleviating effect via noncanonical Wnt signaling pathway in the CFA-induced inflammatory pain model in vivo.

Trifluoro-icaritin alleviates chronic inflammatory pain through α7nAChR-mediated suppression of HMGB1/NF-κB signaling in the spinal cord of rats

Inflammatory pain is a chronic, persistent and serious disease that greatly impacts public health, which is often accompanied by allodynia, hyperalgesia, and spontaneous pain. It is evident that α7 nicotinic acetylcholine receptor (α7nAChR) plays a key role in cholinergic anti-inflammatory pathway and exhibits the inhibition of neuroinflammation in chronic pain. Trifluoro-icaritin (ICTF), a derivative of icaritin from the extract of a genus of Epimedium plant, is identified to possess profound anti-inflammatory activity. However, whether ICTF has anti-nociceptive effect on inflammatory pain and its potential mechanisms remain poorly elucidated. Intraperitoneal injection (i.p.) of ICTF to complete Freund's adjuvant (CFA)-induced inflammatory pain rats once daily for 21 consecutive days. Pain-related behaviors were evaluated with paw withdrawal threshold (PWT), paw withdrawal latency (PWL), and CatWalk gait analysis. Expression of pain-related signaling molecules in the spinal cord were detected using qRT-PCR, western blot assay, and immunofluorescence staining. This results showed that ICTF (3.0mg/kg, i.p.) effectively alleviated mechanical allodynia and thermal hyperalgesia not 0.3 and 1.0mg/kg in CFA rats. Subsequently, we further observed that ICTF (3.0mg/kg) dramatically decreased the mRNA and protein levels of HMGB1, NF-κB p65, and IL-1β but markedly enhanced α7nAChR and IL-10 expression in the spinal cord of CFA rats, and Immunofluorescence staining also showed that ICTF (3.0mg/kg) significantly increased the expression of α7nAChR and reduced IBA1 in the spinal cord of CFA rats, along with suppressing the alterations of gait parameters induced by CFA. Moreover, Intrathecal injection (i.t.) of α7nAChR antagonist alpha-bungarotoxin (α-Bgtx, 1.0μg/kg) not only reversed the anti-nociceptive effect of ICTF on pain hypersensitivity, but also inhibited the down-regulation of HMGB1, NF-κB p65, and IL-1β as well as the up-regulation of α7nAChR and IL-10 protein expression induced by ICTF treatment. Altogether, our results illustrate that ICTF enables to ameliorate CFA-induced inflammatory pain through α7nAChR-mediated inhibition of HMGB1/NF-κB signaling pathway in the spinal cord of rats, suggesting that ICTF may be exploited as a potential painkiller against chronic inflammatory pain.

Astaxanthin alleviates inflammatory pain by regulating the p38 mitogen-activated protein kinase and nuclear factor-erythroid factor 2-related factor/heme oxygenase-1 pathways in mice

Inflammatory pain is a complex process that has a substantial negative impact on post-injury quality of life. Astaxanthin (AST), which is a lipid-soluble red-orange carotenoid that is found in lobsters, inhibits the development and maintenance of inflammation in mice via its antioxidant and anti-inflammatory activities. However, the specific mechanisms underlying these effects remain unclear. In this study, we aimed to elucidate the mechanism by which astaxanthin alleviated inflammation using a mouse model with Complete Freund's adjuvant (CFA)-induced inflammatory pain. Mechanical allodynia and thermal hyperalgesia were observed on days 1-14 post CFA injection. Expression of p38 mitogen-activated protein kinase (MAPK) in the left paw and L4-6 dorsal root ganglia (DRG) were upregulated in the CFA-induced mice. Expression of the nuclear factor erythroid 2-related factor 2 (Nrf2)/heme oxygenase-1 (HO-1) pathways were also increased. Astaxanthin relieved mechanical allodynia and thermal hyperalgesia induced by CFA and inhibited the inflammatory response (e.g., infiltration of inflammatory cells and production of inflammatory factors) in the ipsilateral paw and DRG. Additionally, AST  inhibited p38 MAPK and enhanced Nrf2/HO-1 contents in the left paw and DRG, and reversed the pain induced by p38 MAPK agonist and Nrf2 inhibitors. These findings suggest that AST exerts anti-inflammatory effects and regulates p38 MAPK and Nrf2/HO-1 to alleviate inflammatory pain. AST may be a potential therapeutic agent for relieving inflammation.

Common Clinical Practice for Low Back Pain Treatment: A Modified Delphi Study

INTRODUCTION

Low back pain (LBP) is a common reason for adults to seek medical care and is associated with important functional limitation and patient burden. Yet, heterogeneity in the causes and presentation of LBP and a lack of standardization in its management impede effective prevention and treatment.

METHODS

We conducted a modified Delphi study to generate consensus statements for the diagnosis, management, and prognosis of LBP. A panel of five experts proposed 19 statements that were subsequently evaluated by physicians who treat LBP in their everyday clinical practice. Physicians were asked to validate statements in the form of a web survey assessing level of agreement on a five-point Likert-like scale.

RESULTS

Consensus (≥ 70% agreement) was obtained for all 19 statements. Strength of agreement and physician comments highlighted the importance of pain management, but also strategies to ameliorate functional limitation and prevent future LBP episodes. Respondents favored multidisciplinary approaches and multimodal management for LBP, although there was some ambiguity as to how multidisciplinary strategies could be feasibly incorporated into daily practice. Finally, the results indicated some conflict regarding the use of imaging for the diagnosis of LBP and how to classify LBP for targeted treatment.

CONCLUSION

The results of this study provide a summary of favored clinical practice for the management of chronic LBP. While the consensus statements were generally agreeable to survey respondents, some areas of ambiguity, including how to increase the feasibility of multidisciplinary strategies, when and how to use diagnostic imaging in LBP, and LBP classification, necessitate clarification in future studies and guidelines.

Pathophysiologic Approach to Pain Therapy for Complex Pain Entities: A Narrative Review

Pain management is challenging for both clinicians and patients. In fact, pain patients are frequently undertreated or even completely untreated. Optimal treatment is based on targeting the underlying mechanisms of pain and tailoring the management modality for each patient using a personalized approach. This narrative review deals with pain conditions that have a complex underlying mechanism and need an individualized and frequently multifactorial approach to pain management. The research is based on previously conducted studies, and does not contain any studies with human participants or animals performed by any of the authors. This is not an exhaustive review of the current evidence. However, it provides the clinician with a perspective on pain therapy targeting the underlying pain mechanism(s). When dealing with complex pain conditions, the prudent physician benefits from having a deep knowledge of various underlying pain mechanisms in order to provide a plan for optimal pharmacological pain relief to patients.

Towards an Effective and Safe Treatment of Inflammatory Pain: A Delphi-Guided Expert Consensus

OBJECTIVE

The clinical management of inflammatory pain requires an optimal balance between effective analgesia and associated safety risks. To date, mechanisms associated with inflammatory pain are not completely understood because of their complex nature and the involvement of both peripheral and central mechanisms. This Expert Consensus document is intended to update clinicians about evolving areas of clinical practice and/or available treatment options for the management of patients with inflammatory pain.

METHOD

An international group of experts in pain management covering the pharmacology, neurology and rheumatology fields carried out an independent qualitative systematic literature search using MEDLINE, EMBASE and the Cochrane Central Register of Controlled Trials.

RESULTS

Existing guidelines for pain management provide recommendations that do not satisfactorily address the complex nature of pain. To achieve optimal outcomes, drug choices should be individualized to guarantee the best match between the characteristics of the patient and the properties of the medication. NSAIDs represent an important prescribing choice in the management of inflammatory pain, and the recent results on paracetamol question its appropriate use in clinical practice, raising the need for re-evaluation of the recommendations in the clinical practice guidelines.

CONCLUSIONS

Increasing clinicians' knowledge of the available pharmacologic options to treat different pain mechanisms offers the potential for safe, individualized treatment decisions. We hope that it will help implement the needed changes in the management of inflammatory pain by providing the best strategies and new insights to achieve the ultimate goal of managing the disease and obtaining optimal benefits for patients.

FUNDING

Dompé Farmaceutici SPA and Paolo Procacci Foundation.

[Therapy of herpes zoster and postherpetic neuralgia]

Herpes zoster (HZ) is caused by the reactivation of varicella zoster virus. The incidence of herpes zoster and associated problems increases with age. With a life-long prevalence of 30%, every second 85-year-old person experiences HZ once in his lifetime. Three therapeutic columns are based on antiviral, topical and analgetic therapies. An extreme handicap is acute and persistent pain which can develop into postherpetic neuralgia (PHN). Those pain symptoms are predominantly neuropathic. The management of acute and chronic manifestation of pain may be challenging. HZ vaccination represents a substantial improvement in terms of prevention of herpes zoster and reduction of long-term complications, such as PHN. The permanent vaccination commission of the Robert Koch Institute recommends vaccination with dead virus for all persons over the age of 60 years. Risk groups like immunosuppressed patients are advised to be vaccinated starting at the age of 50 years.

Stopping or Decreasing Opioid Therapy in Patients on Chronic Opioid Therapy

With the rising concerns about long-term opioid use, particularly in patients with chronic noncancer pain, more and more patients are being considered for decreased doses or discontinuation of opioid therapy. This is a challenging clinical situation for both patient and clinician and should be presented in a shared decision-making model so that the patient understands the risks of opioid therapy and how the therapy will be discontinued. The patient should be aware of the long-range plan and its milestones. It is imperative that alternate pain control treatments be made available to the patient early, and that the patient never feels abandoned by the healthcare team. There can be many barriers in shared decision-making and multiple discussions between patient and provider may be required. Opioid use should not be decreased sharply or discontinued abruptly, but should be gradually decremented in a process known as tapering. Tapering should be systematic and planned in advance with the patient knowing the steps. Slow tapers (over months) are more comfortable for the patients but may not always be appropriate. There is guidance for planning the taper and the patient should be closely monitored throughout this process. If withdrawal symptoms occur, they can be managed, for example, with lofexidine. Patients should get full support as they explore new pain control options. For patients who have opioid use disorder, addiction counseling may be appropriate. Navigating opioid discontinuation can be slow work, but optimal results occur when the healthcare team works together and respectfully with the patient.