Hijacking the endogenous opioid system to treat pain: who thought it would be so complicated?

Deciphering nociplastic pain: clinical features, risk factors and potential mechanisms

Nociplastic pain is a mechanistic term used to describe pain that arises or is sustained by altered nociception, despite the absence of tissue damage. Although nociplastic pain has distinct pathophysiology from nociceptive and neuropathic pain, these pain mechanisms often coincide within individuals, which contributes to the intractability of chronic pain. Key symptoms of nociplastic pain include pain in multiple body regions, fatigue, sleep disturbances, cognitive dysfunction, depression and anxiety. Individuals with nociplastic pain are often diffusely tender - indicative of hyperalgesia and/or allodynia - and are often more sensitive than others to non-painful sensory stimuli such as lights, odours and noises. This Review summarizes the risk factors, clinical presentation and treatment of nociplastic pain, and describes how alterations in brain function and structure, immune processing and peripheral factors might contribute to the nociplastic pain phenotype. This article concludes with a discussion of two proposed subtypes of nociplastic pain that reflect distinct neurobiological features and treatment responsivity.

Chronic pain management in sickle cell disease: A systematic scoping review of controlled trials

Existing treatment for chronic pain in sickle cell disease (SCD) is opioid-dependent, which is ineffective and carries risks. We conducted a scoping literature review to assess the size and scope of available literature about controlled trials of therapies for SCD chronic pain and identify research gaps. The search strategy in PubMed and EMBASE utilized keywords for chronic pain and sickle cell and identified seven original articles that met inclusion criteria. Six of the studies recruited from clinics while one recruited from community sources. Cannabis and behavioral modification were associated with improvements in pain scores. However, existing evidence does not represent best practices for assessing chronic pain, and this along with small sample sizes prevents translation to clinical care. The limited evidence concerning treatment for SCD chronic pain highlights the need for larger trials of opioid alternatives and the utilization of chronic pain measures that capture nociplastic pain in SCD.

Questioning the Right to Pain Relief and Its Role in the Opioid Epidemic

The new discipline of palliative care helped to establish the right to pain relief at the end of life and the necessity of using opioids to achieve that goal. Professional pain organizations followed the United Nations' model for universal human rights in their declaration of a universal right to pain management. Both palliative care and pain medicine specialties worked to establish pain as a legitimate focus of medical treatment separate from its association with disease. Pain intensity became the metric used to determine the need for treatment and the success of that treatment. Opioids were favored as the most reliable and feasible means to reduced pain intensity. The Harrison Act of 1914 restricted legitimate opioid use to that prescribed by medical professionals as analgesics. This legislation helped establish opioids as specific painkillers that had a distinct capacity to induce addiction. This understanding of opioids as having distinct and separable analgesic and addictive potential was challenged by the 1970s discovery of an endogenous opioid system, which integrates pain and reward functions to support survival. Our modern pain neurophysiology places the patient with pain in a passive position from which it makes sense to assert a right to pain relief. To prevent future opioid epidemics we need to abandon clinical outpatient use of pain intensity scores and redefine the medical necessity of pain treatment as less about the reduction of pain intensity and more about the capacity to pursue personally valued activities.

Chronic Cancer Pain: Opioids within Tumor Microenvironment Affect Neuroinflammation, Tumor and Pain Evolution

Pain can be a devastating experience for cancer patients, resulting in decreased quality of life. In the last two decades, immunological and pain research have demonstrated that pain persistence is primarily caused by neuroinflammation leading to central sensitization with brain neuroplastic alterations and changes in pain responsiveness (hyperalgesia, and pain behavior). Cancer pain is markedly affected by the tumor microenvironment (TME), a complex ecosystem consisting of different cell types (cancer cells, endothelial and stromal cells, leukocytes, fibroblasts and neurons) that release soluble mediators triggering neuroinflammation. The TME cellular components express opioid receptors (i.e., MOR) that upon engagement by endogenous or exogenous opioids such as morphine, initiate signaling events leading to neuroinflammation. MOR engagement does not only affect pain features and quality, but also influences directly and/or indirectly tumor growth and metastasis. The opioid effects on chronic cancer pain are also clinically characterized by altered opioid responsiveness (tolerance and hyperalgesia), a hallmark of the problematic long-term treatment of non-cancer pain. The significant progress made in understanding the immune-mediated development of chronic pain suggests its exploitation for novel alternative immunotherapeutic approaches.

Pain and Opioid Use After Thoracic Surgery: Where We Are and Where We Need To Go

As many as one third of patients undergoing minimally invasive thoracic surgery and one half undergoing thoracotomy will have chronic pain, defined as pain lasting 2 to 3 months. There is limited information regarding predictors of chronic pain and even less is known about its impact on health-related quality of life, known as pain interference. Currently, there is a focus on decreased opioid prescribing after surgery. Interestingly, thoracic surgical patients are the least likely to be receiving opioids before surgery and have the highest rate of new persistent opioid use after surgery compared with other surgical cohorts. These studies of opioid use have identified important predictors of new persistent opioid use, but their findings are limited by failing to correlate opioid use with pain. The objectives of this invited review are to present the findings of pertinent studies of chronic pain and opioid use after thoracic surgery, "where we are," and to discuss gaps in our knowledge of these topics and opportunities for research to fill those gaps, "where we need to go."

High-Voltage-Activated Calcium Channel in the Afferent Pain Pathway: An Important Target of Pain Therapies

High-voltage-activated (HVA) Ca2+ channels are widely expressed in the nervous system. They play an important role in pain conduction by participating in various physiological processes such as synaptic transmission, changes in synaptic plasticity, and neuronal excitability. Available evidence suggests that the HVA channel is an important therapeutic target for pain management. In this review, we summarize the changes in different subtypes of HVA channel during pain and present the currently available evidence from the clinical application of HVA channel blockers. We also review novel drugs in various phases of development. Moreover, we discuss the future prospects of HVA channel blockers in order to promote "bench-to-bedside" translation.

High-Frequency Medical Cannabis Use Is Associated With Worse Pain Among Individuals With Chronic Pain

Cannabis is widely used for chronic pain. However, there is some evidence of an inverse dose-response relationship between cannabis effects and pain relief that may negatively affect analgesic outcomes. In this cross-sectional survey, we examined whether daily cannabis use frequency was associated with pain severity and interference, quality of life measures relevant to pain (eg, anxiety and depressive symptoms), and cannabis use preferences (administration routes and cannabinoid ratio). Our analysis included 989 adults who used cannabis every day for chronic pain. Participant use was designated as light, moderate, and heavy (1-2, 3-4, and 5 or more cannabis uses per day, respectively). The sample was also subgrouped by self-reported medical-only use (designated MED, n = 531, 54%) versus medical use concomitant with a past-year history of recreational use (designated MEDREC, n = 458, 46%). In the whole sample, increased frequency of use was significantly associated with worse pain intensity and interference, and worse negative affect, although high-frequency users also reported improved positive affect. Subgroup analyses showed that these effects were driven by MED participants. Heavy MED participant consumption patterns showed greater preference for smoking, vaporizing, and high tetrahydrocannabinol products. In contrast, light MED participants had greater preference for tinctures and high cannabidiol products. Selection bias, our focus on chronic pain, and our cross-sectional design likely limit the generalizability of our results. Our findings suggest that lower daily cannabis use frequency is associated with better clinical profile as well as lower risk cannabis use behaviors among MED participants. Future longitudinal studies are needed to examine how high frequency of cannabis use interacts with potential therapeutic benefits. PERSPECTIVE: Our findings suggest that lower daily cannabis use frequency is associated with better clinical profile as well as safer use behaviors (eg, preference for cannabidiol and noninhalation administration routes). These trends highlight the need for developing cannabis use guidelines for clinicians to better protect patients using cannabis.

The Endogenous Opioid System: Role and Dysfunction Caused by Opioid Therapy

The endogenous opioid system is comprised of a wide array of receptors and ligands that are present throughout the central and peripheral nervous system, the gastrointestinal tract, and the immune system. This explains the multitude of physiological functions it is responsible for including analgesia, mood regulation, and modulation of the stress response. It also plays a pivotal role in modulating the brain's reward center with behavioral and social implications on mood disorders and addiction. Exogenous opioid therapy hijacks the endogenous system and alters its functions contributing to an imbalance that is responsible for the pathogenesis of several disease states.

From pain treatment to opioid dependence: a qualitative study of the environmental influence on codeine use in UK adults

OBJECTIVES

To investigate the views and experiences of people who use codeine in order to describe the 'risk environment' capable of producing and reducing harm.

DESIGN

This was a qualitative interview study. Psychological dependence on codeine was measured using the Severity of Dependence Scale. A cut-off score of 5 or higher indicates probable codeine dependence.

SETTING

Participants were recruited from an online survey and one residential rehabilitation service.

PARTICIPANTS

16 adults (13 women and 3 men) from the UK who had used codeine in the last 12 months other than as directed or as indicated. All participants began using codeine to treat physical pain. Mean age was 32.7 years (SD=10.1) and mean period of codeine use was 9.1 years (SD=7.6).

RESULTS

Participants' experiences indicated that they became dependent on codeine as a result of various environmental factors present in a risk environment. Supporting environments to reduce risk included: medicine review of repeat prescribing of codeine, well-managed dose tapering to reduce codeine consumption, support from social structures in form of friends and online and access to addiction treatment. Environments capable of producing harm included: unsupervised and long-term codeine prescribing, poor access to non-pharmacological pain treatments, barriers to provision of risk education of codeine related harm and breakdown in structures to reduce the use of over the counter codeine other than as indicated.

CONCLUSION

The study identified microenvironments and macroenvironments capable of producing dependence on codeine, including repeat prescribing and unsupervised use over a longer time period. The economic environment was important in its influence on the available resources for holistic pain therapy in primary care in order to offer alternative treatments to codeine. Overall, the goal is to create an environment that reduces risk of harm by promoting safe use of codeine for treatment of pain, while providing effective care for those developing withdrawal and dependence.

TRPV1 promotes opioid analgesia during inflammation

Pain and inflammation are inherently linked responses to injury, infection, or chronic diseases. Given that acute inflammation in humans or mice enhances the analgesic properties of opioids, there is much interest in determining the inflammatory transducers that prime opioid receptor signaling in primary afferent nociceptors. Here, we found that activation of the transient receptor potential vanilloid type 1 (TRPV1) channel stimulated a mitogen-activated protein kinase (MAPK) signaling pathway that was accompanied by the shuttling of the scaffold protein β-arrestin2 to the nucleus. The nuclear translocation of β-arrestin2 in turn prevented its recruitment to the μ-opioid receptor (MOR), the subsequent internalization of agonist-bound MOR, and the suppression of MOR activity that occurs upon receptor desensitization. Using the complete Freund's adjuvant (CFA) inflammatory pain model to examine the role of TRPV1 in regulating endogenous opioid analgesia in mice, we found that naloxone methiodide (Nal-M), a peripherally restricted, nonselective, and competitive opioid receptor antagonist, slowed the recovery from CFA-induced hypersensitivity in wild-type, but not TRPV1-deficient, mice. Furthermore, we showed that inflammation prolonged morphine-induced antinociception in a mouse model of opioid receptor desensitization, a process that depended on TRPV1. Together, our data reveal a TRPV1-mediated signaling pathway that serves as an endogenous pain-resolution mechanism by promoting the nuclear translocation of β-arrestin2 to minimize MOR desensitization. This previously uncharacterized mechanism may underlie the peripheral opioid control of inflammatory pain. Dysregulation of the TRPV1-β-arrestin2 axis may thus contribute to the transition from acute to chronic pain.

Characteristics of opioid-maintained clients smoking fentanyl patches: The importance of confirmatory drug analysis illustrated by a case series and mini-review

The increase in opioid prescribing in many European countries over the last decade has raised concerns about associated diversion, overdose, and mortality. Fentanyl is one of these synthetic opioids that is typically prescribed as a transdermal patch for pain that requires continuous pain relief and has been the focus of investigation due to reports of overdose and death. We report a case series of 14 drug addiction treatment entrants, who entered treatment in a service located in the region of Southern Denmark from August 2015 to December 2015 for smoking fentanyl patches. Clients presented with difficulties breathing and pains in the lungs. The clients had a history of past opioid use, including heroin. Relapses resulted in treatment disengagement. Immunoassays for fentanyl were used in the service. In some cases, false negative results occurred. Clients' urine samples were subsequently analysed in a collaborating laboratory. Seven clients tested positive for fentanyl. One client was positive for both fentanyl and heroin. Analyses were also positive for other opioids and metabolites in 6 clients, predominantly codeine and oxycodone. Results from confirmatory analysis contributed to clearer insights into clients' drug histories, which facilitated personalised care plans consisting of opioid agonist therapy informed by confirmed drug use. In Denmark, prescription levels of fentanyl are high, which has been accompanied by observations of diversion and smoking in a smaller population. In addition to revision of inappropriate prescribing to reduce diversion, we recommend increased reliance upon confirmatory drug analysis in the addiction treatment sector in Denmark.

Targeting immune-driven opioid analgesia by sigma-1 receptors: Opening the door to novel perspectives for the analgesic use of sigma-1 antagonists

Immune cells have a known role in pronociception, since they release a myriad of inflammatory algogens which interact with neurons to facilitate pain signaling. However, these cells also produce endogenous opioid peptides with analgesic potential. The sigma-1 receptor is a ligand-operated chaperone that modulates neurotransmission by interacting with multiple protein partners, including the μ-opioid receptor. We recently found that sigma-1 antagonists are able to induce opioid analgesia by enhancing the action of endogenous opioid peptides of immune origin during inflammation. This opioid analgesia is seen only at the inflamed site, where immune cells naturally accumulate. In this article we review the difficulties of targeting the opioid system for selective pain relief, and discuss the dual role of immune cells in pain and analgesia. Our discussion creates perspectives for possible novel therapeutic uses of sigma-1 antagonists as agents able to maximize the analgesic potential of the immune system.