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Elucidating the active δ-opioid receptor crystal structure with peptide and small-molecule agonists.
Selective activation of the δ-opioid receptor (DOP) has great potential for the treatment of chronic pain, benefitting from ancillary anxiolytic and antidepressant-like effects. Moreover, DOP agonists show reduced adverse effects as compared to μ-opioid receptor (MOP) agonists that are in the spotlight of the current "opioid crisis." Here, we report the first crystal structures of the DOP in an activated state, in complex with two relevant and structurally diverse agonists: the potent opioid agonist peptide KGCHM07 and the small-molecule agonist DPI-287 at 2.8 and 3.3 Å resolution, respectively. Our study identifies key determinants for agonist recognition, receptor activation, and DOP selectivity, revealing crucial differences between both agonist scaffolds. Our findings provide the first investigation into atomic-scale agonist binding at the DOP, supported by site-directed mutagenesis and pharmacological characterization. These structures will underpin the future structure-based development of DOP agonists for an improved pain treatment with fewer adverse effects.
Learn More >Evidence supports but is inconclusive that sensitization contributes to chronic pain in some adults with sickle cell disease (SCD). We determined the prevalence of pain sensitization among adults with SCD pain compared with pain-free healthy adults. In a cross sectional, single session study of 186 African American outpatients with SCD pain (age 18-74 years, 59% female) and 124 healthy age, gender, and race matched control subjects (age 18-69 years, 49% female), we compared responses to standard thermal (Medoc TSA II) and mechanical stimuli (von Frey filaments). Although we observed no significant differences in thermal thresholds between controls and patients, patients with SCD had lower pain thresholds to mechanical stimuli and reported higher pain intensity scores to all thermal and mechanical stimuli at a non-painful body site. Compared with controls, about twice as many patients with SCD showed sensitization: 12% versus 23% at the anterior forearm site (p=.02), and 16% versus 32% across three tested sites (p=.004). Among patients with SCD, 18% exhibited some element of central sensitization. Findings indicate that persistent allodynia and hyperalgesia can be part of the SCD pain experience and should be considered when selecting therapies for SCD pain. Perspective: Compared with matched healthy controls, quantitative sensory testing in adults with pain and sickle cell disease (SCD) demonstrates higher prevalence of sensitization, including central sensitization. The findings of allodynia and hyperalgesia may indicate neuropathic pain and could contribute to a paradigm shift in assessment and treatment of SCD pain.
Learn More >Patients with chronic pain demonstrate interpretational bias to pain and models of pain suggest interpretational bias affects subsequent pain experience. This study developed an interpretation bias modification for pain (IBM-P) and examined its efficacy. A total of 48 patients with chronic pain were recruited and randomly assigned to either the training group (n = 24) or the control group (n = 24). Interpretational bias, negative emotions, and attentional bias to pain-related stimuli were assessed before and after conducting IBM-P. The main results indicated that the training group showed less interpretational bias and negative emotions after IBM-P than the control group. The training group also gazed at neutral words longer than at "new" affective pain words after IBM-P than they did prior to the intervention. Furthermore, significant mediating effects of post-interpretational bias were found in the relationship between the group type and post-negative emotions and post-dwell time bias. These results suggest that IBM-P can modify interpretational bias which leads to changes in negative emotions and attentional bias. Future research is needed to confirm the effect of modifying interpretational bias and its clinical utility in the field of pain management. Perspective: This article investigated the efficacy of IBM-P and suggested that modifying interpretational bias is followed by changes in negative emotions and attentional bias. These findings may help health professionals understand the role of interpretational bias in chronic pain and encourage the potential use of IBM in pain management.
Learn More >Headache disorders are highly prevalent and debilitating, with limited treatment options. Previous studies indicate that many pro-inflammatory immune cells contribute to headache pathophysiology. Given the well-recognized role of regulatory T (Treg) cells in maintaining immune homeostasis, we hypothesized that enhancing Treg function may be effective to treat multiple headache disorders. In a mouse model of chronic migraine, we observed that repeated nitroglycerin (NTG, a reliable trigger of migraine in patients) administration doubled the number of CD3 T cells in the trigeminal ganglia without altering the number of Treg cells, suggesting a deficiency in Treg-mediated immune homeostasis. We treated mice with low-dose interleukin-2 (ld-IL2) to preferentially expand and activate endogenous Treg cells. This not only prevented the development of NTG-induced persistent sensitization, but also completely reversed the established facial skin hyper-sensitivity resulting from repeated NTG administration. The effect of ld-IL2 was independent of mouse sex and/or strain. Importantly, ld-IL2 treatment did not alter basal nociceptive responses, and repeated usage did not induce tolerance. The therapeutic effect of ld-IL2 was abolished by Treg depletion and was recapitulated by Treg adoptive transfer. Furthermore, treating mice with ld-IL2 1-7 days after mild traumatic brain injury effectively prevented as well as reversed the development of behaviors related to acute and chronic post-traumatic headache. In a model of medication overuse headache, Ld-IL2 completely reversed the cutaneous hyper-sensitivity induced by repeated administration of sumatriptan. Collectively, the present study identifies ld-IL2 as a promising prophylactic for multiple headache disorders with a mechanism distinct from the existing treatment options.
Learn More >Human endocannabinoid systems modulate multiple physiological processes mainly through the activation of cannabinoid receptors CB1 and CB2. Their high sequence similarity, low agonist selectivity, and lack of activation and G protein-coupling knowledge have hindered the development of therapeutic applications. Importantly, missing structural information has significantly held back the development of promising CB2-selective agonist drugs for treating inflammatory and neuropathic pain without the psychoactivity of CB1. Here, we report the cryoelectron microscopy structures of synthetic cannabinoid-bound CB2 and CB1 in complex with G, as well as agonist-bound CB2 crystal structure. Of important scientific and therapeutic benefit, our results reveal a diverse activation and signaling mechanism, the structural basis of CB2-selective agonists design, and the unexpected interaction of cholesterol with CB1, suggestive of its endogenous allosteric modulating role.
Learn More >Visceral pain is commonly associated with acute or remitting inflammatory bowel disease (IBD). In marked contrast, chronic IBD is often painless, even in the presence of active inflammation. This suggests that inflammation in itself is insufficient to sustain altered nociceptive signaling and raises the possibility that there is an endogenous analgesic system in effect in chronic disease. A new study by Basso et al. published in this issue of Neurogastroenterology & Motility provides additional support for an immune-mediated mechanism that suppresses visceral hypersensitivity. The authors examined visceral pain in the IL-10-piroxicam model of chronic colitis, which differs from other experimental IBD models in that it involves immune suppression. During active inflammation, responses by these mice to graded increases in colorectal distension were equivalent to healthy controls, consistent with normal afferent signaling. However, treatment with a peripherally restricted opioid receptor antagonist resulted in marked visceral hypersensitivity to the same stimuli. This effect was attributed to the production of endogenous opioids by colitogenic CD4 T cells present in the mucosa. This mini-review provides a brief overview of analgesia by immune-derived opioids under inflammatory conditions and highlights how the work of Basso et al. contributes to this area of research. Potential pharmacological approaches to harness or mimic this system are provided. These strategies may prove to be an effective means through which targeted and sustained relief of IBD pain may be achieved.
Learn More >The United States is in the midst of an opioid overdose epidemic, with a significant portion of the burden associated with prescription opioids. In response, the CDC released a Guideline for Prescribing Opioids for Chronic Pain, which promotes access to treatment for opioid use disorder. Decades of research have linked childhood adversity to negative health and risk behavior outcomes, including substance misuse. Our present study builds upon this work to examine the relationship between adverse childhood experiences (ACEs) and prescription opioid misuse. We compiled data from the Behavioral Risk Factor Surveillance System implemented by Montana and Florida in 2010 and 2011, respectively. Logistic regressions (run in 2017) tested the associations between ACEs and subsequent prescription pain medicine/opioid misuse outcomes in adulthood. ACEs were prevalent, with 62.7% of respondents in Montana and 50% in Florida reporting at least one ACE. The presence of ACEs was positively associated with prescription opioid misuse across both samples. Respondents reporting three or more ACEs had increased odds of taking opioids more than prescribed, without a prescription, and for the feeling they cause. Our results support a strong link between ACEs and prescription opioid misuse. Opportunities to prevent opioid misuse start with assuring safe, stable, nurturing relationships and environments in childhood and across the lifespan to prevent ACEs from occurring, and intervening appropriately when they do occur. Substance use prevention programs for adolescents, appropriate pain management and opioid prescribing protocols, and treatments for opioid use disorder can address ACEs by enhancing treatment safety and effectiveness and can reduce the intergenerational continuity of early adversity.
Learn More >Despite the high prevalence of neck pain, few studies have addressed the pharmacological treatment of this condition. We evaluated the effectiveness of tapentadol prolonged-release (PR) in patients with or without a neuropathic pain component, with a focus on functional movements, disability and Quality of Life (QoL). Observational, retrospective study. Ninety-four adult patients with severe neck pain not responsive to opioid step III treatment. The primary endpoint was a ≥30% improvement of pain intensity at 4 weeks (W4). Several secondary outcomes were evaluated, including neck disability index (NDI), range of motion (ROM), and QoL. Patients received tapentadol PR at the starting dose of 100 mg/day. Dose titration was allowed in 50 mg increments, up to 500 mg daily. At W4, the primary endpoint of ≥30% improvement of pain was reported in 70% (n = 35; 95% confidence interval [CI]: 55-82%) of patients with a neuropathic pain component and in 69% (n = 20; 95% CI: 49-85%) of those without a neuropathic component. The percentage of patients reporting a neuropathic pain component significantly decreased from baseline (64.2%) to W4 (27.8%). NDI significantly improved in both groups at W12. ROM significantly improved in all three planes of motion (P < 0.01), with no difference between the two groups. Interference of pain with sleep and QoL also improved. The reduction in pain provided by tapentadol is associated with functional recovery, which may in turn be linked to an improvement in QoL.
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