Sensitization of nociceptors by prostaglandin E2-glycerol contributes to hyperalgesia in mice with sickle cell disease

Arginine Therapy for Pain in Sickle Cell Disease: A Phase-2 Randomized, Placebo-Controlled Trial

We present a prospective randomized, placebo-controlled trial of intravenous arginine in patients 3-21 years hospitalized with sickle cell disease vaso-occlusive pain episodes (SCD-VOE) at two tertiary-care children's hospitals. Participants were randomized into 1 of 3 arms: Standard-dose (SD; 100 mg/kg/dose) every 8 h, Loading-dose (200 mg/kg followed by SD), or Placebo. The primary outcome was total parenteral opioid use (TPO). Secondary outcomes included time-to-crisis-resolution, pain scores, patient-reported outcomes (PROs), arginine bioavailability, and biomarkers of oxidative stress/mitochondrial function. Of 1548 patients screened, 108 were randomized (36 per study-arm; mean 12.6 ± 3.8 years, 52% female, and 65% hemoglobin-SS). This study did not meet its primary endpoint. TPO, time-to-crisis-resolution, pain scores, and PROs at discharge were similar across arms. Post hoc sensitivity analyses of children 5-16 years old demonstrated nearly double TPO utilization in those receiving placebo versus arginine (n = 87, p = 0.056), achieving significance in patients with plasma arginine < 60 μM. Arginine was low at presentation in 79% of patients (mean 50 ± 28 μM), and increased with arginine therapy (p < 0.001). Arginine bioavailability at VOE presentation inversely correlated with time-to-crisis-resolution (r = -0.39, p = 0.01) after placebo, an association eliminated by arginine supplementation (r = -0.04, p = 0.70). A dose-dependent increase in platelet-mitochondrial activity occurred after arginine versus no change after placebo (p < 0.001); plasma protein-carbonyl levels, a measure of oxidative stress, decreased after arginine therapy (p < 0.001) but increased in the placebo group (p = 0.02). SCD-VOE is associated with an acquired arginine deficiency that correlates with worse clinical outcomes. Arginine improved mitochondrial function and decreased oxidative stress compared to placebo, with clinically relevant opioid-sparing becoming significant in children with the lowest arginine concentration. TRIAL REGISTRATION: Registered with ClinicalTrials.gov (NCT02536170) in August 2015.

Targeting sickle cell pathobiology and pain with novel transdermal curcumin

Several comorbidities of sickle cell disease (SCD) originate from red blood cell (RBC) instability, chronic inflammation, and oxidative stress. Development of scalable, cost-effective therapeutics suitable for chronic administration to prevent, attenuate, and perhaps reverse the consequences of RBC instability is needed. Curcumin has many of these attributes as a safe compound with antisickling, antiinflammatory, and antioxidant properties, but its translational potential has been constrained due to limited bioavailability from oral administration. The present study demonstrates the rapid and high bioavailability of a novel topical/transdermal (TD) curcumin gel formulation in the plasma and blood cells and its effectiveness in humanized sickle cell mice in: (i) ameliorating features of sickle cell pain hypersensitivity and axonal injury; (ii) reducing multiple manifestations of RBC instability including evidence of decreased hemolysis (reduced lactate dehydrogenase levels), enhanced RBC ATP levels along with decreased oxidative damage; (iii) decreasing multiple proinflammatory cytokines including interleukin-6, monocyte chemoattractant protein-1, granulocyte-macrophage colony-stimulating factor, and activation, normal T cell expressed and secreted protein in skin secretome; and (iv) reducing mast cell degranulation and activation. Our data suggest that an easy-to-use novel TD curcumin gel formulation has the potential to ameliorate chronic pain, improve RBC stability, and reduce inflammatory consequences of SCD.

Temporal dissociation of COX-2-dependent arachidonic acid and 2-arachidonoylglycerol metabolism in RAW264.7 macrophages

Cyclooxygenase-2 converts arachidonic acid to prostaglandins (PGs) and the endocannabinoid, 2-arachidonoylglycerol (2-AG), to PG glyceryl esters (PG-Gs). The physiological function of PG biosynthesis has been extensively studied, but the importance of the more recently discovered PG-G synthetic pathway remains incompletely defined. This disparity is due in part to a lack of knowledge of the physiological conditions under which PG-G biosynthesis occurs. We have discovered that RAW264.7 macrophages stimulated with Kdo2-lipid A (KLA) produce primarily PGs within the first 12 h followed by robust PG-G synthesis between 12 h and 24 h. We suggest that the amount of PG-Gs quantified is less than actually synthesized, because PG-Gs are subject to a significant level of hydrolysis during the time course of synthesis. Inhibition of cytosolic phospholipase A2 by giripladib does not accelerate PG-G synthesis, suggesting the differential time course of PG and PG-G synthesis is not due to the competition between arachidonic acid and 2-AG. The late-phase PG-G formation is accompanied by an increase in the level of 2-AG and a concomitant decrease in 18:0-20:4 diacylglycerol (DAG). Inhibition of DAG lipases by KT-172 decreases the levels of 2-AG and PG-Gs, indicating that the DAG-lipase pathway is involved in delayed 2-AG metabolism/PG-G synthesis. These results demonstrate that physiologically significant levels of PG-Gs are produced by activated RAW264.7 macrophages well after the production of PGs plateaus.

Pathology of pain and its implications for therapeutic interventions

Pain is estimated to affect more than 20% of the global population, imposing incalculable health and economic burdens. Effective pain management is crucial for individuals suffering from pain. However, the current methods for pain assessment and treatment fall short of clinical needs. Benefiting from advances in neuroscience and biotechnology, the neuronal circuits and molecular mechanisms critically involved in pain modulation have been elucidated. These research achievements have incited progress in identifying new diagnostic and therapeutic targets. In this review, we first introduce fundamental knowledge about pain, setting the stage for the subsequent contents. The review next delves into the molecular mechanisms underlying pain disorders, including gene mutation, epigenetic modification, posttranslational modification, inflammasome, signaling pathways and microbiota. To better present a comprehensive view of pain research, two prominent issues, sexual dimorphism and pain comorbidities, are discussed in detail based on current findings. The status quo of pain evaluation and manipulation is summarized. A series of improved and innovative pain management strategies, such as gene therapy, monoclonal antibody, brain-computer interface and microbial intervention, are making strides towards clinical application. We highlight existing limitations and future directions for enhancing the quality of preclinical and clinical research. Efforts to decipher the complexities of pain pathology will be instrumental in translating scientific discoveries into clinical practice, thereby improving pain management from bench to bedside.

Clinical Assessment of Osteoarthritis Pain: Contemporary Scenario, Challenges, and Future Perspectives

The multifaceted nature of osteoarthritis (OA) pain presents a challenge in understanding and managing the condition. The diverse pain experiences, progression rates, individual responses to treatments, and complex disease mechanisms contribute to heterogeneity in the clinical studies outcomes. The lack of a standardized methodology for assessing and classifying OA pain challenges healthcare practitioners. This complicates the establishment of universally applicable protocols or standardized guidelines for treatment. This article explores the heterogeneity observed in clinical studies evaluating OA pain treatments, highlighting the necessity for refined methodologies, personalized patient categorization, and consistent outcome measures. It discusses the role of the multidimensional nature of OA pain, underlying pain mechanisms, and other contributing factors to the heterogeneity in outcome measures. Addressing these variations is crucial to establishing a more consistent framework for evidence-based treatments and advancing care of the patient with OA pain.

Mouse models of sickle cell disease: Imperfect and yet very informative

The root cause of sickle cell disease (SCD) has been known for nearly a century, however, few therapies to treat the disease are available. Over several decades of work, with advances in gene editing technology and after several iterations of mice with differing genotype/phenotype relationships, researchers have developed humanized SCD mouse models. However, while a large body of preclinical studies has led to huge gains in basic science knowledge about SCD in mice, this knowledge has not led to the development of effective therapies to treat SCD-related complications in humans, thus leading to frustration with the paucity of translational progress in the SCD field. The use of mouse models to study human diseases is based on the genetic and phenotypic similarities between mouse and humans (face validity). The Berkeley and Townes SCD mice express only human globin chains and no mouse hemoglobin. With this genetic composition, these models present many phenotypic similarities, but also significant discrepancies that should be considered when interpreting preclinical studies results. Reviewing genetic and phenotypic similarities and discrepancies and examining studies that have translated to humans and those that have not, offer a better perspective of construct, face, and predictive validities of humanized SCD mouse models.

Cold exposure induces vaso-occlusion and pain in sickle mice that depend on complement activation

Vaso-occlusive pain episodes (VOE) cause severe pain in patients with sickle cell disease (SCD). Vaso-occlusive events promote ischemia/reperfusion pathobiology that activates complement. We hypothesized that complement activation is linked to VOE. We used cold to induce VOE in the Townes sickle homozygous for hemoglobin S (HbSS) mouse model and complement inhibitors to determine whether anaphylatoxin C5a mediates VOE. We used a dorsal skinfold chamber to measure microvascular stasis (vaso-occlusion) and von Frey filaments applied to the plantar surface of the hind paw to assess mechanical hyperalgesia in HbSS and control Townes mice homozygous for hemoglobin A (HbAA) mice after cold exposure at 10°C/50°F for 1 hour. Cold exposure induced more vaso-occlusion in nonhyperalgesic HbSS mice (33%) than in HbAA mice (11%) or HbSS mice left at room temperature (1%). Cold exposure also produced mechanical hyperalgesia as measured by paw withdrawal threshold in HbSS mice compared with that in HbAA mice or HbSS mice left at room temperature. Vaso-occlusion and hyperalgesia were associated with an increase in complement activation fragments Bb and C5a in plasma of HbSS mice after cold exposure. This was accompanied by an increase in proinflammatory NF-κB activation and VCAM-1 and ICAM-1 expression in the liver. Pretreatment of nonhyperalgesic HbSS mice before cold exposure with anti-C5 or anti-C5aR monoclonal antibodies (mAbs) decreased vaso-occlusion, mechanical hyperalgesia, complement activation, and liver inflammatory markers compared with pretreatment with control mAb. Anti-C5 or -C5aR mAb infusion also abrogated mechanical hyperalgesia in HbSS mice with ongoing hyperalgesia at baseline. These findings suggest that C5a promotes vaso-occlusion, pain, and inflammation during VOE and may play a role in chronic pain.

Set up and validation of a sensitive method to quantify prostaglandins, prostaglandin-glycerol esters and prostaglandin-ethanolamides, as well as their respective precursors

Arachidonic acid-derived prostaglandins are widely studied for their role in inflammation. However, besides arachidonic acid, other arachidonic moiety-containing lipids can be metabolized by COX-2. Indeed, the endocannabinoids 2-arachidonoylglycerol (2-AG) and N-arachidonoylethanolamine (anandamide, AEA) can follow the same biochemical pathways than arachidonic acid leading to the formation of prostaglandin-glycerol esters (PG-G) and prostaglandin-ethanolamides (or prostamides, PG-EA), respectively. The data reported so far support the interest of these bioactive lipids in inflammatory conditions. However, there is only a handful of methods described for their quantification in biological matrices. Moreover, given the shared biochemical pathways for arachidonic acid, 2-AG and AEA, a method allowing for the quantification of these precursors and the corresponding prostaglandin derivatives appears as largely needed. Thus, we report here the development and validation of a single run UPLC-MS/MS quantification method allowing the quantification of these endocannabinoids-derived mediators together with the classical prostaglandin. Moreover, we applied the method to the quantification of these lipids in vitro (using lipopolysaccharides-activated J774 macrophage cells) and in vivo in several tissues from DSS-induced colitis mice. This femtomole-range method should improve the understanding of the interaction between these lipid mediators and inflammation.

Descending Facilitation of Nociceptive Transmission From the Rostral Ventromedial Medulla Contributes to Hyperalgesia in Mice with Sickle Cell Disease

Sickle cell disease (SCD) is an inherited blood disorder that is associated with acute episodic and chronic pain. Mice with SCD have robust hyperalgesia mediated, in part, by sensitization of spinal dorsal horn neurons. However, underlying mechanisms are not fully understood. Since the rostral ventromedial medulla (RVM) is a major component of descending circuitry that modulates nociceptive transmission in the spinal cord, we examined if the RVM contributes to hyperalgesia in mice with SCD. Injection of lidocaine, but not vehicle, into the RVM eliminated mechanical and heat hyperalgesia in sickle (HbSS-BERK) mice without altering mechanical and heat sensitivity in naïve C57B mice. These data indicate that the RVM contributes to the maintenance of hyperalgesia in mice with SCD. In electrophysiological studies, we determined the changes in response properties of RVM neurons that might contribute to hyperalgesia in sickle mice. Recordings were made from single ON, OFF, and Neutral cells in the RVM of sickle and control (HbAA-BERK) mice. Spontaneous activity and responses of ON, OFF and Neutral cells evoked by heat (50 °C) and mechanical (26 g) stimuli applied to the hind paw were compared between sickle and control mice. Although there were no differences in the proportions of functionally-identified neurons or spontaneous activity between sickle and control mice, evoked responses of ON cells to heat and mechanical stimuli were increased approximately 3-fold in sickle mice as compared to control mice. Thus, the RVM contributes to hyperalgesia in sickle mice via a specific ON cell-dependent descending facilitation of nociceptive transmission.

Peripheral transient receptor potential vanilloid type 4 hypersensitivity contributes to chronic sickle cell disease pain

ABSTRACT

Debilitating pain affects the lives of patients with sickle cell disease (SCD). Current pain treatment for patients with SCD fail to completely resolve acute or chronic SCD pain. Previous research indicates that the cation channel transient receptor potential vanilloid type 4 (TRPV4) mediates peripheral hypersensitivity in various inflammatory and neuropathic pain conditions that may share similar pathophysiology with SCD, but this channel's role in chronic SCD pain remains unknown. Thus, the current experiments examined whether TRPV4 regulates hyperalgesia in transgenic mouse models of SCD. Acute blockade of TRPV4 alleviated evoked behavioral hypersensitivity to punctate, but not dynamic, mechanical stimuli in mice with SCD. TRPV4 blockade also reduced the mechanical sensitivity of small, but not large, dorsal root ganglia neurons from mice with SCD. Furthermore, keratinocytes from mice with SCD showed sensitized TRPV4-dependent calcium responses. These results shed new light on the role of TRPV4 in SCD chronic pain and are the first to suggest a role for epidermal keratinocytes in the heightened sensitivity observed in SCD.

Identification of key factors driving inflammation-induced sensitization of muscle sensory neurons

Sensory neurons embedded in muscle tissue that initiate pain sensations, i.e., nociceptors, are temporarily sensitized by inflammatory mediators during musculoskeletal trauma. These neurons transduce peripheral noxious stimuli into an electrical signal [i.e., an action potential (AP)] and, when sensitized, demonstrate lower activation thresholds and a heightened AP response. We still do not understand the relative contributions of the various transmembrane proteins and intracellular signaling processes that drive the inflammation-induced hyperexcitability of nociceptors. In this study, we used computational analysis to identify key proteins that could regulate the inflammation-induced increase in the magnitude of AP firing in mechanosensitive muscle nociceptors. First, we extended a previously validated model of a mechanosensitive mouse muscle nociceptor to incorporate two inflammation-activated G protein-coupled receptor (GPCR) signaling pathways and validated the model simulations of inflammation-induced nociceptor sensitization using literature data. Then, by performing global sensitivity analyses that simulated thousands of inflammation-induced nociceptor sensitization scenarios, we identified three ion channels and four molecular processes (from the 17 modeled transmembrane proteins and 28 intracellular signaling components) as potential regulators of the inflammation-induced increase in AP firing in response to mechanical forces. Moreover, we found that simulating single knockouts of transient receptor potential ankyrin 1 (TRPA1) and reducing the rates of G_αq-coupled receptor phosphorylation and G_αq subunit activation considerably altered the excitability of nociceptors (i.e., each modification increased or decreased the inflammation-induced fold change in the number of triggered APs compared to when all channels were present). These results suggest that altering the expression of TRPA1 or the concentration of intracellular G_αq might regulate the inflammation-induced increase in AP response of mechanosensitive muscle nociceptors.

Pain mechanisms in sickle cell disease. Are we closer to a breakthrough?

Not available.

Inhibition of DAGLβ as a therapeutic target for pain in sickle cell disease

Sickle cell disease (SCD) is the most common inherited disease. Pain is a key morbidity of SCD and opioids are the main treatment but their side effects emphasize the need for new analgesic approaches. Humanized transgenic mouse models have been instructive in understanding the pathobiology of SCD and mechanisms of pain. Homozygous (HbSS) Berkley mice express >99% human sickle hemoglobin and several features of clinical SCD including hyperalgesia. Previously, we reported that the endocannabinoid 2-arachidonoylglycerol (2-AG) is a precursor of the pro-nociceptive mediator prostaglandin E2-glyceryl ester (PGE2-G) which contributes to hyperalgesia in SCD. We now demonstrate the causal role of 2-AG in hyperalgesia in sickle mice. Hyperalgesia in HbSS mice correlated with elevated levels of 2-AG in plasma, its synthesizing enzyme diacylglycerol lipase β (DAGLβ) in blood cells, and with elevated levels of PGE2 and PGE2-G, pronociceptive derivatives of 2-AG. A single intravenous injection of 2-AG produced hyperalgesia in non-hyperalgesic HbSS mice, but not in control (HbAA) mice expressing normal human HbA. JZL184, an inhibitor of 2-AG hydrolysis, also produced hyperalgesia in non-hyperalgesic HbSS or hemizygous (HbAS) mice, but did not influence hyperalgesia in hyperalgesic HbSS mice. Systemic and intraplantar administration of KT109, an inhibitor of DAGLβ, decreased mechanical and heat hyperalgesia in HbSS mice. The decrease in hyperalgesia was accompanied by reductions in 2-AG, PGE2 and PGE2-G in the blood. These results indicate that maintaining the physiological level of 2-AG in the blood by targeting DAGLβ may be a novel and effective approach to treat pain in SCD.

Sciatic Nerve Block Combined with Flurbiprofen Inhibits Spinal Cord Inflammation and Improves Postoperative Pain in Rats with Plantar Incision

Background and Purpose

Peripheral nerve block is often used to relieve postoperative pain. But the effect of nerve block on inflammatory response is not fully understood. Spinal cord is the primary center of pain processing. This study is to investigate the effect of single sciatic nerve block on the inflammatory response of the spinal cord in rats with plantar incision and the combined effect with flurbiprofen.

Methods

The plantar incision was used to establish a postoperative pain model. Single sciatic nerve block, intravenous flurbiprofen or the combination of both were used for intervention. The sensory and motor functions after nerve block and incision were evaluated. The changes of IL-1β, IL-6, TNF-α, microglia and astrocytes in the spinal cord were examined by qPCR and immunofluorescence respectively.

Results

Sciatic nerve block with 0.5% ropivacaine in rats induced sensory block for 2h and motor block for 1.5h. In the rats with plantar incision, the single sciatic nerve block did not alleviate postoperative pain or inhibit the activation of spinal microglia and astrocytes, but the levels of IL-1β and IL-6 in spinal cord were decreased when the nerve block wore off. The combined effect of a single sciatic nerve block and intravenous flurbiprofen not only decreased the levels of IL-1β, IL-6, and TNF-α, but also relieved the pain and alleviated the activation of microglia and astrocytes.

Conclusion

The single sciatic nerve block cannot improve postoperative pain or inhibit the activation of spinal cord glial cells, but can reduce the expression of spinal inflammatory factors. Nerve block combined with flurbiprofen can inhibit spinal cord inflammation and improve postoperative pain. This study provides a reference for rational clinical application of nerve block.

Flurbiprofen inhibits heme induced NLRP3 inflammasome in Berkeley sickle cell disease mice

Sickle cell disease (SCD) is accompanied by several complications, which emanate from the sickling of erythrocytes due to a point mutation in the β-globin chain of hemoglobin. Sickled erythrocytes are unable to move smoothly through small blood capillaries and therefore, cause vaso occlusion and severe pain. Apart from pain, continuous lysis of fragile sickled erythrocytes leads to the release of heme, which is a strong activator of the NLRP3 inflammasome, thus producing chronic inflammation in sickle cell disease. In this study, we identified flurbiprofen among other COX-2 inhibitors to be a potent inhibitor of heme-induced NLRP3 inflammasome. We found that apart from being a nociceptive agent, flurbiprofen exerts a strong anti-inflammatory effect by suppressing NF-κB signaling, which was evidenced by reduced levels of TNF-α and IL-6 in wild-type and sickle cell disease Berkeley mice models. Our data further demonstrated the protective effect of flurbiprofen on liver, lungs, and spleen in Berkeley mice. The current sickle cell disease pain management regime relies mainly on opiate drugs, which is accompanied by several side effects without modifying the sickle cell disease-related pathology. Considering the potent role of flurbiprofen in inhibiting NLRP3 inflammasome and other inflammatory cytokines in sickle cell disease, our data suggests that it can be explored further for better sickle cell disease pain management along with the possibility of disease modification.

Potential Therapeutic Strategies for Skeletal Muscle Atrophy

The maintenance of muscle homeostasis is vital for life and health. Skeletal muscle atrophy not only seriously reduces people's quality of life and increases morbidity and mortality, but also causes a huge socioeconomic burden. To date, no effective treatment has been developed for skeletal muscle atrophy owing to an incomplete understanding of its molecular mechanisms. Exercise therapy is the most effective treatment for skeletal muscle atrophy. Unfortunately, it is not suitable for all patients, such as fractured patients and bedridden patients with nerve damage. Therefore, understanding the molecular mechanism of skeletal muscle atrophy is crucial for developing new therapies for skeletal muscle atrophy. In this review, PubMed was systematically screened for articles that appeared in the past 5 years about potential therapeutic strategies for skeletal muscle atrophy. Herein, we summarize the roles of inflammation, oxidative stress, ubiquitin-proteasome system, autophagic-lysosomal pathway, caspases, and calpains in skeletal muscle atrophy and systematically expound the potential drug targets and therapeutic progress against skeletal muscle atrophy. This review focuses on current treatments and strategies for skeletal muscle atrophy, including drug treatment (active substances of traditional Chinese medicine, chemical drugs, antioxidants, enzyme and enzyme inhibitors, hormone drugs, etc.), gene therapy, stem cell and exosome therapy (muscle-derived stem cells, non-myogenic stem cells, and exosomes), cytokine therapy, physical therapy (electroacupuncture, electrical stimulation, optogenetic technology, heat therapy, and low-level laser therapy), nutrition support (protein, essential amino acids, creatine, β-hydroxy-β-methylbutyrate, and vitamin D), and other therapies (biomaterial adjuvant therapy, intestinal microbial regulation, and oxygen supplementation). Considering many treatments have been developed for skeletal muscle atrophy, we propose a combination of proper treatments for individual needs, which may yield better treatment outcomes.

A model of painful vaso-occlusive crisis in mice with sickle cell disease

In order to better understand mechanisms underlying acute pain during vaso-occlusive crises (VOCs) in patients with sickle cell disease, Khasabova et al report on a clinically relevant model in mice where VOC is stimulated by exposure to cold. Cold exposure produces robust hyperalgesia, stasis, hypoxia, elevated heart rate, and increased levels of the endocannabinoid 2-AG and its synthesizing enzyme, DAGLβ, in plasma and blood cells, respectively. Blocking DAGLβ prevents the development of hyperalgesia. Collectively, these data point to 2-AG signaling as a targetable mediator of VOC pain.

Mapping the binding sites of UDP and prostaglandin E2 glyceryl ester in the nucleotide receptor P2Y6

Cyclooxygenase‐2 catalyzes the biosynthesis of prostaglandins from arachidonic acid and the biosynthesis of prostaglandin glycerol esters (PG‐Gs) from 2‐arachidonoylglycerol. PG‐Gs are mediators of several biological actions such as macrophage activation, hyperalgesia, synaptic plasticity, and intraocular pressure. Recently, the human UDP receptor P2Y₆ was identified as a target for the prostaglandin E2 glycerol ester (PGE₂‐G). Here, we show that UDP and PGE₂‐G are evolutionary conserved endogenous agonists at vertebrate P2Y₆ orthologs. Using sequence comparison of P2Y₆ orthologs, homology modeling, and ligand docking studies, we proposed several receptor positions participating in agonist binding. Site‐directed mutagenesis and functional analysis of these P2Y₆ mutants revealed that both UDP and PGE₂‐G share in parts one ligand‐binding site. Thus, the convergent signaling of these two chemically very different agonists has already been manifested in the evolutionary design of the ligand‐binding pocket.

Nociceptors protect sickle cell disease mice from vaso-occlusive episodes and chronic organ damage

Sickle cell disease (SCD) is a common hereditary hematologic disorder. SCD patients suffer from acute vaso-occlusive episodes (VOEs), chronic organ damage, and premature death, with few therapeutic options. Although severe pain is a major clinical manifestation of SCD, it remains unknown whether nociception plays a role in SCD pathogenesis. To address this question, we generated nociceptor-deficient SCD mice and found, unexpectedly, that the absence of nociception led to more severe and more lethal VOE, indicating that somatosensory nerves protect SCD mice from VOE. Mechanistically, the beneficial effects of sensory nerves were induced by the neuropeptide calcitonin gene–related peptide (CGRP), which acted on hematopoietic cells. Additionally, oral capsaicin consumption, which can activate somatosensory nerves by binding to TRPV1, dramatically alleviated acute VOE and significantly prevented chronic liver and kidney damage in SCD mice. Thus, the manipulation of nociception may provide a promising approach to treat SCD.

Role of innate immunity in chemotherapy-induced peripheral neuropathy

It has become increasingly clear that the innate immune system plays an essential role in the generation of many types of neuropathic pain including that which accompanies cancer treatment. In this article we review current findings of the role of the innate immune system in contributing to cancer treatment pain at the distal endings of peripheral nerve, in the nerve trunk, in the dorsal root ganglion and in the spinal dorsal horn.

Effects of R-flurbiprofen and the oxygenated metabolites of endocannabinoids in inflammatory pain mice models

Pain is one of the cardinal signs accompanying inflammation. The prostaglandins (PGs), synthetized from arachidonic acid by cyclooxygenase (COX)-2, are major bioactive lipids implicated in inflammation and pain. However, COX-2 is also able to metabolize other lipids, including the endocannabinoids 2-arachidonoylglycerol (2-AG) and anandamide (AEA), to give glycerol ester (PG-G) and ethanolamide (PG-EA) derivatives of the PGs. Consequently, COX-2 can be considered as a hub not only controlling PG synthesis, but also PG-G and PG-EA synthesis. As they were more recently characterized, these endocannabinoid metabolites are less studied in nociception compared to PGs. Interestingly R-profens, previously considered as inactive enantiomers of nonsteroidal anti-inflammatory drugs (NSAIDs), are substrate-selective COX inhibitors. Indeed, R-flurbiprofen can selectively block PG-G and PG-EA production, without affecting PG synthesis from COX-2. Therefore, we compared the effect of R-flurbiprofen and S-flurbiprofen in models of inflammatory pain triggered by local administration of lipopolysaccharides (LPS) and carrageenan in mice. Remarkably, the effects of flurbiprofen enantiomers on mechanical hyperalgesia seem to depend on (i) the inflammatory stimuli, (ii) the route of administration, and (iii) the timing of administration. We also assessed the effect of administration of the PG-Gs, PG-EAs, and PGs on LPS-induced mechanical hyperalgesia. Our data support the interest of studying the nonhydrolytic endocannabinoid metabolism in the context of inflammatory pain.

Pain characteristics and biomarkers in treatment approaches for osteoarthritis pain

Osteoarthritis (OA) is a progressive disease and OA pain intensity is related to ongoing pathophysiological changes. However, OA pain is complex and multimodal; its characteristics, including severity, localization and the stimuli that elicit it, can change as the disease progresses and differ greatly among patients. Understanding mechanisms underlying specific pain characteristics may help guide clinicians in choosing appropriate treatments, targeting treatments to those patients most likely to benefit. Associations have been demonstrated between biomarkers and some characteristics of OA pain, and to processes linked to the shift in pain characteristics over the course of OA. This article examines how understanding OA pain characteristics and their relation to the disease process could inform treatment choice when applying well-established treatment guidelines.

Harmaline Analogs as Substrate-Selective Cyclooxygenase-2 Inhibitors

We report the design, synthesis, and evaluation of a series of harmaline analogs as selective inhibitors of 2-arachidonylglycerol (2-AG) oxygenation over arachidonic acid (AA) oxygenation by purified cyclooxygenase-2 (COX-2). A fused tricyclic harmaline analog containing a CH₃O substituent at C-6 and a CH₃ group at the C-1 position of 4,9-dihydro-3H-pyrido[3,4-b]indole (compound 3) was the best substrate-selective COX-2 inhibitor of those evaluated, exhibiting a 2AG-selective COX-2 inhibitory IC₅₀ of 0.022 μM as compared to >1 μM for AA. The 2.66 Å resolution crystal complex of COX-2 with compound 3 revealed that this series of tricyclic indoles binds in the cyclooxygenase channel by flipping the side chain of L531 toward the dimer interface. This novel tricyclic indole series provides the foundation for the development of promising substrate-selective molecules capable of increasing endocannabinoid (EC) levels in the brain to offer new treatments for a variety of diseases, from pain and inflammation to stress and anxiety disorders.

Mechanisms of pain in sickle cell disease

Objectives

The hallmark of sickle cell disease (SCD) is acute and chronic pain, and the pain dominates the clinical characteristics of SCD patients. Although pharmacological treatments of SCD targeting the disease mechanisms have been improved, many SCD patients suffer from pain. To overcome the pain of the disease, there have been renewed requirements to understand the novel molecular mechanisms of the pain in SCD.

Methods

We concisely summarized the molecular mechanisms of SCD-related acute and chronic pain, focusing on potential drug targets to treat pain.

Results

Acute pain of SCD is caused by vaso-occulusive crisis (VOC), impaired oxygen supply or infarction-reperfusion tissue injuries. In VOC, inflammatory cytokines include tryptase activate nociceptors and transient receptor potential vanilloid type 1. In tissue injury, the secondary inflammatory response is triggered and causes further tissue injuries. Tissue injury generates cytokines and pain mediators including bradykinin, and they activate nociceptive afferent nerves and trigger pain. The main causes of chronic pain are from extended hyperalgesia after a VOC and central sensitization. Neuropathic pain could be due to central or peripheral nerve injury, and protein kinase C might be associated with the pain. In central sensitization, neuroplasticity in the brain and the activation of glial cells may be related with the pain.

Discussion

In this review, we summarized the molecular mechanisms of SCD-related acute and chronic pain. The novel treatments targeting the disease mechanisms would interrupt complications of SCD and reduce the pain of the SCD patients.

Endocannabinoid and Prostanoid Crosstalk in Pain

Interfering with endocannabinoid (eCB) metabolism to increase their levels is a proven anti-nociception strategy. However, because the eCB and prostanoid systems are intertwined, interfering with eCB metabolism will affect the prostanoid system and inversely. Key to this connection is the production of the cyclooxygenase (COX) substrate arachidonic acid upon eCB hydrolysis as well as the ability of COX to metabolize the eCBs anandamide (AEA) and 2-arachidonoylglycerol (2-AG) into prostaglandin-ethanolamides (PG-EA) and prostaglandin-glycerol esters (PG-G), respectively. Recent studies shed light on the role of PG-Gs and PG-EAs in nociception and inflammation. Here, we discuss the role of these complex systems in nociception and new opportunities to alleviate pain by interacting with them.