eIF4E phosphorylation regulates ongoing pain, independently of inflammation, and hyperalgesic priming in the mouse CFA model

ephrin-B2 promotes nociceptive plasticity and hyperalgesic priming through EphB2-MNK-eIF4E signaling in both mice and humans

Ephrin-B-EphB signaling can promote pain through ligand-receptor interactions between peripheral cells, like immune cells expressing ephrin-Bs, and EphB receptors expressed by DRG neurons. Previous studies have shown increased ephrin-B2 expression in peripheral tissues like synovium of rheumatoid and osteoarthritis patients, indicating the clinical significance of this signaling. The primary goal of this study was to understand how ephrin-B2 acts on mouse and human DRG neurons, which express EphB receptors, to promote pain and nociceptor plasticity. We hypothesized that ephrin-B2 would promote nociceptor plasticity and hyperalgesic priming through MNK-eIF4E signaling, a critical mechanism for nociceptive plasticity induced by growth factors, cytokines and nerve injury. Both male and female mice developed dose-dependent mechanical hypersensitivity in response to ephrin-B2, and both sexes showed hyperalgesic priming when challenged with PGE2 injection either to the paw or the cranial dura. Acute nociceptive behaviors and hyperalgesic priming were blocked in mice lacking MNK1 (Mknk1 knockout mice) and by eFT508, a specific MNK inhibitor. Sensory neuron-specific knockout of EphB2 using Pirt-Cre demonstrated that ephrin-B2 actions require this receptor. In Ca2+-imaging experiments on cultured DRG neurons, ephrin-B2 treatment enhanced Ca2+ transients in response to PGE2 and these effects were absent in DRG neurons from MNK1-/- and EphB2-PirtCre mice. In experiments on human DRG neurons, ephrin-B2 increased eIF4E phosphorylation and enhanced Ca2+ responses to PGE2 treatment, both blocked by eFT508. We conclude that ephrin-B2 acts directly on mouse and human sensory neurons to induce nociceptor plasticity via MNK-eIF4E signaling, offering new insight into how ephrin-B signaling promotes pain.

RNA-binding proteins in pain

RNAs are meticulously controlled by proteins. Through direct and indirect associations, every facet in the brief life of an mRNA is subject to regulation. RNA-binding proteins (RBPs) permeate biology. Here, we focus on their roles in pain. Chronic pain is among the largest challenges facing medicine and requires new strategies. Mounting pharmacologic and genetic evidence obtained in pre-clinical models suggests fundamental roles for a broad array of RBPs. We describe their diverse roles that span RNA modification, splicing, stability, translation, and decay. Finally, we highlight opportunities to expand our understanding of regulatory interactions that contribute to pain signaling. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications Translation > Regulation RNA in Disease and Development > RNA in Disease.

Vinorelbine causes a neuropathic pain-like state in mice via STING and MNK1 signaling associated with type I interferon induction

Type I interferons (IFNs) increase the excitability of dorsal root ganglia (DRGs) neurons via MNK-eIF4E signaling to promote pain sensitization in mice. Activation of stimulator of interferon response cGAMP interactor 1 (STING) signaling is pivotal for type I IFN induction. We hypothesized that vinorelbine, a chemotherapeutic and activator of STING, would cause a neuropathic pain-like state in mice via STING signaling in DRG neurons associated with IFN production. Vinorelbine caused tactile allodynia and grimacing in wild-type (WT) mice and increased p-IRF3, type I IFNs, and p-eIF4E in peripheral nerves. Supporting our hypothesis, vinorelbine failed to induce IRF3-IFNs-MNK-eIF4E in StingGt/Gt mice and, subsequently, failed to cause pain. The vinorelbine-elicited increase of p-eIF4E was not observed in Mknk1-/- (MNK1 knockout) mice in peripheral nerves consistent with the attenuated pro-nociceptive effect of vinorelbine in these mice. Our findings show that activation of STING signaling in the periphery causes a neuropathic pain-like state through type I IFN signaling to DRG nociceptors.

eIF4E plays the role of a pathogenic gene in psoriasis, and the inhibition of eIF4E phosphorylation ameliorates psoriasis-like skin damage

Psoriasis is a complex inflammatory skin disease with uncertain pathogenesis. eIF4E (eukaryotic translation initiation factor 4E) and its phosphorylation state p-eIF4E are highly expressed in psoriatic tissues. However, the role eIF4E played in psoriasis is still unclear. To investigate the function of eIF4E and p-eIF4E in psoriasis and to figure out whether eFT-508 (Tomivosertib, eIF4E phosphorylation inhibitor) can relieve the disease severity and become a promising candidate for the psoriasis treatment. We first verified the expression of eIF4E and p-eIF4E in psoriasis patients' lesional skin. Then, we demonstrated the effect of eIF4E and p-eIF4E on the abnormal proliferation and inflammatory state of keratinocytes by using eIF4E-specific small interfering RNA (si-eIF4E) and eFT-508. In this study, all cell experiments were performed under the psoriasis-model condition. Moreover, the external application of eFT-508 on imiquimod (IMQ)-induced psoriasis mice was performed to explore its potential clinical value. Results showed that eIF4E and p-eIF4E were significantly overexpressed in skin lesions of psoriasis patients. Knocking down eIF4E or adding eFT-508 can relieve the abnormal proliferation and the excessive inflammatory state of keratinocytes by reducing the expression of cyclin D1, IL-1β, CXCL10, IL23, Wnt 5a, NBS1 and p-AKT from mRNA or protein levels. Furthermore, these results were consistent with those obtained from the in vitro experiments. Then, we conclude that eIF4E plays the role of the pathogenic gene in psoriasis, and eFT-508 may be a promising candidate for anti-prosoriasis drugs.

Vinorelbine causes a neuropathic pain-like state in mice via STING and MNK1 signaling associated with type I interferon induction

Type I interferons (IFNs) increase the excitability of dorsal root ganglion (DRG) neurons via activation of MNK-eIF4E translation signaling to promote pain sensitization in mice. Activation of STING signaling is a key component of type I IFN induction. Manipulation of STING signaling is an active area of investigation in cancer and other therapeutic areas. Vinorelbine is a chemotherapeutic that activates STING and has been shown to cause pain and neuropathy in oncology clinical trials in patients. There are conflicting reports on whether STING signaling promotes or inhibits pain in mice. We hypothesized that vinorelbine would cause a neuropathic pain-like state in mice via STING and signaling pathways in DRG neurons associated with type I IFN induction. Vinorelbine (10 mg/kg, i.v.) induced tactile allodynia and grimacing in WT male and female mice and increased p-IRF3 and type I IFN protein in peripheral nerves. In support of our hypothesis, vinorelbine-mediated pain was absent in male and female StingGt/Gt mice. Vinorelbine also failed to induce IRF3 and type I IFN signaling in these mice. Since type I IFNs engage translational control via MNK1-eIF4E in DRG nociceptors, we assessed vinorelbine-mediated p-eIF4E changes. Vinorelbine increased p-eIF4E in DRG in WT animals but not in StingGt/Gt or Mknk1-/- (MNK1 KO) mice. Consistent with these biochemical findings, vinorelbine had an attenuated pro-nociceptive effect in male and female MNK1 KO mice. Our findings support the conclusion that activation of STING signaling in the peripheral nervous system causes a neuropathic pain-like state that is mediated by type I IFN signaling to DRG nociceptors.

MNK1 and MNK2 Expression in the Human Dorsal Root and Trigeminal Ganglion

Mitogen activated protein kinase interacting kinases (MNK) 1 and 2 are serine/threonine protein kinases that play an important role in translation of mRNAs through their phosphorylation of the RNA 5'-cap binding protein, eukaryotic translation initiation factor (eIF) 4E. These kinases are downstream targets for mitogen activated protein kinases (MAPKs), extracellular activity regulated protein kinase (ERK) and p38. MNKs have been implicated in the sensitization of peripheral nociceptors of the dorsal root and trigeminal ganglion (DRG and TG) using transgenic mouse lines and through the use of specific inhibitors of MNK1 and MNK2. While specific knockout of the Mknk1 gene suggests that it is the key isoform for regulation of nociceptor excitability and nociceptive behaviors in mice, both MKNK1 and MKNK2 genes are expressed in the DRG and TG of mice and humans based on RNA sequencing experiments. Single cell sequencing in mice suggests that Mknk1 and Mknk2 may be expressed in different populations of nociceptors. We sought to characterize mRNA expression in human DRG and TG (N = 3 ganglia for both DRG and TG) for both MNK1 and MNK2. Our results show that both genes are expressed by nearly all neurons in both human ganglia with expression in other cell types as well. Our findings provide evidence that MNK1 and MNK2 are expressed by human nociceptors of males and females and suggest that efforts to pharmacologically target MNKs for pain would likely be translatable due its conserved expression in both species.

Age and sex drive differential behavioral and neuroimmune phenotypes during postoperative pain

Surgical procedures in the geriatric population are steadily increasing, driven by improved healthcare technologies and longer lifespans. However, effective postoperative pain treatments are lacking, and this diminishes quality of life and recovery. Here we present one of the first preclinical studies to pursue sex- and age-specific differences in postoperative neuroimmune phenotypes and pain. We found that aged males, but not females, had a delayed onset of mechanical hypersensitivity post-surgery and faster resolution than young counterparts. This sex-specific age effect was accompanied by decreased paw innervation and increased local inflammation. Additionally, we find evidence of an age-dependent decrease in hyperalgesic priming and perioperative changes in nociceptor populations and spinal microglia in the aged. These findings suggest that impaired neuronal function and maladaptive inflammatory mechanisms influence postoperative pain development in advanced age. Elucidation of these neuroimmune phenotypes across age and sex enables the development of novel therapies that can be tailored for improved pain relief.

Design, synthesis, and mechanism of action of novel μ-conotoxin KIIIA analogues for inhibition of the voltage-gated sodium channel Nav1.7

μ-Conotoxin KIIIA, a selective blocker of sodium channels, has strong inhibitory activity against several Na_v isoforms, including Na_v1.7, and has potent analgesic effects, but it contains three pairs of disulfide bonds, making structural modification difficult and synthesis complex. To circumvent these difficulties, we designed and synthesized three KIIIA analogues with one disulfide bond deleted. The most active analogue, KIIIA-1, was further analyzed, and its binding pattern to hNa_v1.7 was determined by molecular dynamics simulations. Guided by the molecular dynamics computational model, we designed and tested 32 second-generation and 6 third-generation analogues of KIIIA-1 on hNa_v1.7 expressed in HEK293 cells. Several analogues showed significantly improved inhibitory activity on hNa_v1.7, and the most potent peptide, 37, was approximately 4-fold more potent than the KIIIA Isomer I and 8-fold more potent than the wildtype (WT) KIIIA in inhibiting hNa_v1.7 current. Intraperitoneally injected 37 exhibited potent in vivo analgesic activity in a formalin-induced inflammatory pain model, with activity reaching ∼350-fold of the positive control drug morphine. Overall, peptide 37 has a simplified disulfide-bond framework and exhibits potent in vivo analgesic effects and has promising potential for development as a pain therapy in the future.

MNK1 and MNK2 expression in the human dorsal root and trigeminal ganglion

Mitogen activated protein kinase interacting kinases (MNK) 1 and 2 are serine/threonine protein kinases that play an important role in translation of mRNAs through their phosphorylation of the RNA 5’-cap binding protein, eukaryotic translation initiation factor (eIF) 4E. These kinases are downstream targets for mitogen activated protein kinases (MAPKs), extracellular activity regulated protein kinase (ERK) and p38. MNKs have been implicated in the sensitization of peripheral nociceptors of the dorsal root and trigeminal ganglion (DRG and TG) using transgenic mouse lines and through the use of specific inhibitors of MNK1 and MNK2. While specific knockout of the Mknk1 gene suggests that it is the key isoform for regulation of nociceptor excitability and nociceptive behaviors in mice, both MKNK1 and MKNK2 genes are expressed in the DRG and TG of mice and humans based on RNA sequencing experiments. Single cell sequencing in mice suggests that Mknk1 and Mknk2 may be expressed in different populations of nociceptors. We sought to characterize mRNA expression in human DRG and TG for both MNK1 and MNK2. Our results show that both genes are expressed by nearly all neurons in both human ganglia with expression in other cell types as well. Our findings provide evidence that MNK1 and MNK2 are expressed by human nociceptors and suggest that efforts to pharmacologically target MNKs for pain would likely be translatable due its conserved expression in both species.

Pain and aging: A unique challenge in neuroinflammation and behavior

Chronic pain is one of the most common, costly, and potentially debilitating health issues facing older adults, with attributable costs exceeding $600 billion annually. The prevalence of pain in humans increases with advancing age. Yet, the contributions of sex differences, age-related chronic inflammation, and changes in neuroplasticity to the overall experience of pain are less clear, given that opposing processes in aging interact. This review article examines and summarizes pre-clinical research and clinical data on chronic pain among older adults to identify knowledge gaps and provide the base for future research and clinical practice. We provide evidence to suggest that neurodegenerative conditions engender a loss of neural plasticity involved in pain response, whereas low-grade inflammation in aging increases CNS sensitization but decreases PNS sensitivity. Insights from preclinical studies are needed to answer mechanistic questions. However, the selection of appropriate aging models presents a challenge that has resulted in conflicting data regarding pain processing and behavioral outcomes that are difficult to translate to humans.

Local translation in primary afferents and its contribution to pain

ABSTRACT

Chronic pain remains a significant problem due to its prevalence, impact, and limited therapeutic options. Progress in addressing chronic pain is dependent on a better understanding of underlying mechanisms. Although the available evidence suggests that changes within the central nervous system contribute to the initiation and maintenance of chronic pain, it also suggests that the primary afferent plays a critical role in all phases of the manifestation of chronic pain in most of those who suffer. Most notable among the changes in primary afferents is an increase in excitability or sensitization. A number of mechanisms have been identified that contribute to primary afferent sensitization with evidence for both increases in pronociceptive signaling molecules, such as voltage-gated sodium channels, and decreases in antinociceptive signaling molecules, such as voltage-dependent or calcium-dependent potassium channels. Furthermore, these changes in signaling molecules seem to reflect changes in gene expression as well as posttranslational processing. A mechanism of sensitization that has received far less attention, however, is the local or axonal translation of these signaling molecules. A growing body of evidence indicates that this process not only is dynamically regulated but also contributes to the initiation and maintenance of chronic pain. Here, we review the biology of local translation in primary afferents and its relevance to pain pathobiology.

Enhanced nociceptive behavior and expansion of associated primary afferents in a rabbit model of cerebral palsy

Spastic cerebral palsy (CP) is a movement disorder marked by hypertonia and hyperreflexia; the most prevalent comorbidity is pain. Since spinal nociceptive afferents contribute to both the sensation of painful stimuli as well as reflex circuits involved in movement, we investigated the relationship between prenatal hypoxia-ischemia (HI) injury which can cause CP, and possible changes in spinal nociceptive circuitry. To do this, we examined nociceptive afferents and mechanical and thermal sensitivity of New Zealand White rabbit kits after prenatal HI or a sham surgical procedure. As described previously, a range of motor deficits similar to spastic CP was observed in kits born naturally after HI (40 min at ~70%-80% gestation). We found that HI caused an expansion of peptidergic afferents (marked by expression of calcitonin gene-related peptide) in both the superficial and deep dorsal horn at postnatal day (P)5. Non-peptidergic nociceptive afferent arborization (labeled by isolectin B4) was unaltered in HI kits, but overlap of the two populations (peptidergic and non-peptidergic nociceptors) was increased by HI. Density of glial fibrillary acidic protein was unchanged within spinal cord white matter regions important in nociceptive transmission at P5. We found that mechanical and thermal nociception was enhanced in HI kits even in the absence of motor deficits. These findings suggest that prenatal HI injury impacts spinal sensory pathways in addition to the more well-established disruptions to descending motor circuits. In conclusion, changes to spinal nociceptive circuitry could disrupt spinal reflexes and contribute to pain experienced by individuals with CP.

The cannabinoid agonist CB-13 produces peripherally mediated analgesia in mice but elicits tolerance and signs of central nervous system activity with repeated dosing

ABSTRACT

Activation of cannabinoid receptor type 1 (CB 1 ) produces analgesia in a variety of preclinical models of pain; however, engagement of central CB 1 receptors is accompanied by unwanted side effects, such as psychoactivity, tolerance, and dependence. Therefore, some efforts to develop novel analgesics have focused on targeting peripheral CB 1 receptors to circumvent central CB 1 -related side effects. In the present study, we evaluated the effects of acute and repeated dosing with the peripherally selective CB 1 -preferring agonist CB-13 on nociception and central CB 1 -related phenotypes in a model of inflammatory pain in mice. We also evaluated cellular mechanisms underlying CB-13-induced antinociception in vitro using cultured mouse dorsal root ganglion neurons. CB-13 reduced inflammation-induced mechanical allodynia in male and female mice in a peripheral CB 1 -receptor-dependent manner and relieved inflammatory thermal hyperalgesia. In cultured mouse dorsal root ganglion neurons, CB-13 reduced TRPV1 sensitization and neuronal hyperexcitability induced by the inflammatory mediator prostaglandin E 2 , providing potential mechanistic explanations for the analgesic actions of peripheral CB 1 receptor activation. With acute dosing, phenotypes associated with central CB 1 receptor activation occurred only at a dose of CB-13 approximately 10-fold the ED 50 for reducing allodynia. Strikingly, repeated dosing resulted in both analgesic tolerance and CB 1 receptor dependence, even at a dose that did not produce central CB 1 -receptor-mediated phenotypes on acute dosing. This suggests that repeated CB-13 dosing leads to increased CNS exposure and unwanted engagement of central CB 1 receptors. Thus, caution is warranted regarding therapeutic use of CB-13 with the goal of avoiding CNS side effects. Nonetheless, the clear analgesic effect of acute peripheral CB 1 receptor activation suggests that peripherally restricted cannabinoids are a viable target for novel analgesic development.

Neuroinflammation in Tinnitus

Purpose of Review

The current review aims to explore recent studies that have illustrated a link between neuroinflammation and tinnitus and the consequential effect on neuronal functioning. We explore parallels amongst pain and tinnitus pathologies and a novel treatment option.

Recent Findings

Genetic and pharmacological blockage of pro-inflammatory cytokines mitigates the physiological and behavioral tinnitus phenotype in acute rodent models. In addition, recent pain studies target a signaling pathway to prevent the transition from acute to chronic neuropathic pain, which could translate to tinnitus.

Summary

Neuroinflammation likely mediates hyperexcitability of the auditory pathway, driving the development of acute tinnitus. In chronic tinnitus, we believe translational regulation plays a role in maintaining persistent tinnitus signaling. We therefore propose this pathway as a potential therapeutic strategy.

Behavioral characterization, potential clinical relevance and mechanisms of latent pain sensitization

Chronic pain is a debilitating disorder that can occur as painful episodes that alternates with bouts of remission and occurs despite healing of the primary insult. Those episodes are often triggered by stressful events. In the last decades, a similar situation has been evidenced in a wide variety of rodent models (including inflammatory pain, neuropathy and opioid-induced hyperalgesia) where animals develop a chronic latent hyperalgesia that silently persists after behavioral signs of pain resolution. This state, referred as latent pain sensitization, is due to the compensatory activation of antinociceptive systems, such as the opioid system or NPY and its receptors. A transitory phase of hyperalgesia can then be reinstated by pharmacological or genetic blockade of these antinociceptive systems or by submitting animals to acute stress. Those observations reveal that there is a constant endogenous analgesia responsible for chronic pain inhibition that might paradoxically contribute to maintain this maladaptive state and could then participate to the transition from acute to chronic pain. Thus, demonstration of the existence of this phenomenon in humans and a better understanding of the mechanisms by which latent pain sensitization develops and maintains over long periods of time will be of particular interest to help identifying new therapeutic strategies and targets for chronic pain treatment. The present review aims to recapitulate behavioral expression, potential clinical relevance, cellular mechanisms and intracellular signaling pathways involved so far in latent pain sensitization.

Update on the Development of MNK Inhibitors as Therapeutic Agents

Mitogen-activated protein kinase-interacting kinases 1 and 2 (MNK1/2) represent a central class of enzymes that are activated by extracellular signal-regulated kinase (ERK) or p38 mitogen-activated protein (MAP) kinases. MNK1 and MNK2 coordinate cellular signaling, control production of inflammatory chemokines, and regulate cell proliferation and survival. MNK1/2 are referred to as serine/threonine kinases as they phosphorylate serine or threonine residues on their substrates. Upon activation, MNK1/2 phosphorylate eukaryotic translation initiation factor 4E (eIF4E) at Ser209, which in turn initiates ribosome assembly and protein translation. Deleterious overexpression of MNK1/2 and/or eIF4E have been reported in several diseases including cancers, neurological disorders, autism, and inflammation. Recently, there have been intense efforts toward the development of potent and selective inhibitors of MNK1/2 in both academia and industry. Herein, we review the current understanding of the structural and biological aspects of MNK1/2 and provide an update of pharmacological inhibitors of MNK1/2 including candidates in clinical trials.

Investigating the Role of Ly6G+ Neutrophils in Incisional and Inflammatory Pain by Multidimensional Pain-Related Behavioral Assessments: Bridging the Translational Gap

In recent years, preclinical pain research has failed to develop genuinely new analgesics for clinical use. This fact is reflected by a high number of patients, limited drug efficacy accompanied by side effects, and a long-term opioid intake. Two main aspects have been addressed, which hinder translation: the use of non-relevant pain models and a mismatch between pain-related outcomes in preclinical and clinical studies. Conversely, disease-specific pain models that mirror more closely the clinical situation and multidimensional behavioral outcome measures that objectively and reproducibly assess relevant pain-related symptoms in a preclinical setting could improve translation. Mechanistically, a matter of debate is the role of Ly6G⁺ neutrophil granulocytes (NGs) for pain. NGs are essential to eliminate pathogens and promote the wound healing process. For this purpose, there is a need to release various pro- and anti-inflammatory mediators, some of which could ameliorate or enhance pain. However, the contribution of NGs to different pain entities is contradictory for reflex-based tests, and completely unknown in the context of non-evoked pain (NEP) and movement-evoked pain (MEP). First, we combined withdrawal reflex-based assays with novel video-based assessments for NEP- and MEP-related behavior in two mouse pain models. The pain models utilized in this study were incision (INC) and pathogen/adjuvant-induced inflammation (CFA), translating well to postsurgical and inflammatory pain entities. Second, we depleted NGs and applied a set of behavioral assessments to investigate the role of NG migration in different pain modalities. Our comprehensive behavioral approach identified pain-related behaviors in mice that resemble (NEP) or differentiate (MEP) behavioral trajectories in comparison to mechanical and heat hypersensitivity, thereby indicating modality-dependent mechanisms. Further, we show that injury-induced accumulation of NGs minimally affects pain-related behaviors in both pain models. In conclusion, we report a novel assessment to detect NEP in mice after unilateral injuries using a more unbiased approach. Additionally, we are capable of detecting an antalgic gait for both pain entities with unique trajectories. The different trajectories between MEP and other pain modalities suggest that the underlying mechanisms differ. We further conclude that NGs play a subordinate role in pain-related behaviors in incisional and inflammatory pain.

Sensory Neurons, Neuroimmunity, and Pain Modulation by Sex Hormones

The inclusion of women in preclinical pain studies has become more commonplace in the last decade as the National Institutes of Health (NIH) released its "Sex as a Biological Variable" mandate. Presumably, basic researchers have not had a comprehensive understanding about neuroimmune interactions in half of the population and how hormones play a role in this. To date, we have learned that sex hormones contribute to sexual differentiation of the nervous system and sex differences in behavior throughout the lifespan; however, the cycling of sex hormones does not always explain these differences. Here, we highlight recent advances in our understanding of sex differences and how hormones and immune interactions influence sensory neuron activity to contribute to physiology and pain. Neuroimmune mechanisms may be mediated by different cell types in each sex, as the actions of immune cells are sexually dimorphic. Unfortunately, the majority of studies assessing neuronal contributions to immune function have been limited to males, so it is unclear if the mechanisms are similar in females. Finally, pathways that control cellular metabolism, like nuclear receptors, have been shown to play a regulatory role both in pain and inflammation. Overall, communication between the neuroimmune and endocrine systems modulate pain signaling in a sex-dependent manner, but more research is needed to reveal nuances of these mechanisms.

Neuroimmune Consequences of eIF4E Phosphorylation on Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a major dose-limiting side effect that occurs in up to 63% of patients and has no known effective treatment. A majority of studies do not effectively assess sex differences in the onset and persistence of CIPN. Here we investigated the onset of CIPN, a point of therapeutic intervention where we may limit, or even prevent the development of CIPN. We hypothesized that cap-dependent translation mechanisms are important in early CIPN development and the bi-directional crosstalk between immune cells and nociceptors plays a complementary role to CIPN establishment and sex differences observed. In this study, we used wild type and eIF4E-mutant mice of both sexes to investigate the role of cap-dependent translation and the contribution of immune cells and nociceptors in the periphery and glia in the spinal cord during paclitaxel-induced peripheral neuropathy. We found that systemically administered paclitaxel induces pain-like behaviors in both sexes, increases helper T-lymphocytes, downregulates cytotoxic T-lymphocytes, and increases mitochondrial dysfunction in dorsal root ganglia neurons; all of which is eIF4E-dependent in both sexes. We identified a robust paclitaxel-induced, eIF4E-dependent increase in spinal astrocyte immunoreactivity in males, but not females. Taken together, our data reveals that cap-dependent translation may be a key pathway that presents relevant therapeutic targets during the early phase of CIPN. By targeting the eIF4E complex, we may reduce or reverse the negative effects associated with chemotherapeutic treatments.

Progress in developing MNK inhibitors

The MNKs (mitogen-activated protein kinase-interacting protein kinases) phosphorylate eIF4E (eukaryotic initiation factor 4 E) at serine 209; eIF4E plays an important role in the translation of cytoplasmic mRNAs, all of which possess a 5' 'cap' structure to which eIF4E binds. Elevated levels of eIF4E, p-eIF4E and/or the MNK protein kinases have been found in many types of cancer, including solid tumors and leukemia. MNKs also play a role in metabolic disease. Regulation of the activities of MNKs (MNK1 and MNK2), control the phosphorylation of eIF4E, which in turn has a close relationship with the processes of tumor development, cell migration and invasion, and energy metabolism. MNK knock-out mice display no adverse effects on normal cells or phenotypes suggesting that MNK may be a potentially safe targets for the treatment of various cancers. Several MNK inhibitors or 'degraders' have been identified. Initially, some of the inhibitors were developed from natural products or based on other protein kinase inhibitors which inhibit multiple kinases. Subsequently, more potent and selective inhibitors for MNK1/2 have been designed and synthesized. Currently, three inhibitors (BAY1143269, eFT508 and ETC-206) are in various stages of clinical trials for the treatment of solid cancers or leukemia, either alone or combined with inhibitors of other protein kinase. In this review, we summarize the diverse MNK inhibitors that have been reported in patents and other literature, including those with activities in vitro and/or in vivo.

Pharmacological Manipulation of Translation as a Therapeutic Target for Chronic Pain

Dysfunction in regulation of mRNA translation is an increasingly recognized characteristic of many diseases and disorders, including cancer, diabetes, autoimmunity, neurodegeneration, and chronic pain. Approximately 50 million adults in the United States experience chronic pain. This economic burden is greater than annual costs associated with heart disease, cancer, and diabetes combined. Treatment options for chronic pain are inadequately efficacious and riddled with adverse side effects. There is thus an urgent unmet need for novel approaches to treating chronic pain. Sensitization of neurons along the nociceptive pathway causes chronic pain states driving symptoms that include spontaneous pain and mechanical and thermal hypersensitivity. More than a decade of preclinical research demonstrates that translational mechanisms regulate the changes in gene expression that are required for ongoing sensitization of nociceptive sensory neurons. This review will describe how key translation regulation signaling pathways, including the integrated stress response, mammalian target of rapamycin, AMP-activated protein kinase (AMPK), and mitogen-activated protein kinase-interacting kinases, impact the translation of different subsets of mRNAs. We then place these mechanisms of translation regulation in the context of chronic pain states, evaluate currently available therapies, and examine the potential for developing novel drugs. Considering the large body of evidence now published in this area, we propose that pharmacologically manipulating specific aspects of the translational machinery may reverse key neuronal phenotypic changes causing different chronic pain conditions. Therapeutics targeting these pathways could eventually be first-line drugs used to treat chronic pain disorders. SIGNIFICANCE STATEMENT: Translational mechanisms regulating protein synthesis underlie phenotypic changes in the sensory nervous system that drive chronic pain states. This review highlights regulatory mechanisms that control translation initiation and how to exploit them in treating persistent pain conditions. We explore the role of mammalian/mechanistic target of rapamycin and mitogen-activated protein kinase-interacting kinase inhibitors and AMPK activators in alleviating pain hypersensitivity. Modulation of eukaryotic initiation factor 2α phosphorylation is also discussed as a potential therapy. Targeting specific translation regulation mechanisms may reverse changes in neuronal hyperexcitability associated with painful conditions.

Neonatal complete Freund's adjuvant-induced inflammation does not induce or alter hyperalgesic priming or alter adult distributions of C-fibre dorsal horn innervation

Introduction:

Inflammation during the neonatal period can exacerbate pain severity following reinjury in adulthood. This is driven by alterations in the postnatal development of spinal and supraspinal nociceptive circuitry. However, the contribution of alterations in peripheral nociceptor function remains underexplored.

Objectives:

We examined whether neonatal complete Freund's adjuvant (CFA)-induced inflammation induced or altered adult development of hyperalgesic priming (inflammation-induced plasticity in nonpeptidergic C fibres) or altered postnatal reorganization of calcitonin gene-related peptide (CGRP)-expressing and isolectin B4 (IB4)-binding C fibres in the spinal dorsal horn (DH).

Methods:

After intraplantar injection of CFA at postnatal day (P) 1, we assessed mechanical thresholds in adult (P60) rats before and after intraplantar carrageenan. One week later, intraplantar PGE₂-induced hypersensitivity persisting for 4 hours was deemed indicative of hyperalgesic priming. CGRP expression and IB4 binding were examined in adult rat DH after CFA.

Results:

P1 CFA did not alter baseline adult mechanical thresholds, nor did it change the extent or duration of carrageenan-induced hypersensitivity. However, this was slower to resolve in female than in male rats. Rats that previously received carrageenan but not saline were primed, but P1 hind paw CFA did not induce or alter hyperalgesic priming responses to PGE₂. In addition, CFA on P1 or P10 did not alter intensity or patterns of CGRP or IB4 staining in the adult DH.

Conclusion:

Complete Freund's adjuvant-induced inflammation during a critical period of vulnerability to injury during early postnatal development does not induce or exacerbate hyperalgesic priming or alter the broad distribution of CGRP-expressing or IB4-binding afferent terminals in the adult dorsal horn.

Oncostatin M induces hyperalgesic priming and amplifies signaling of cAMP to ERK by RapGEF2 and PKA

Hyperalgesic priming is characterized by enhanced nociceptor sensitization by pronociceptive mediators, prototypically PGE₂ . Priming has gained interest as a mechanism underlying the transition to chronic pain. Which stimuli induce priming and what cellular mechanisms are employed remains incompletely understood. In adult male rats, we present the cytokine Oncostatin M (OSM), a member of the IL-6 family, as an inducer of priming by a novel mechanism. We used a high content microscopy based approach to quantify the activation of endogenous PKA-II and ERK of thousands sensory neurons in culture. Incubation with OSM increased and prolonged ERK activation by agents that increase cAMP production such as PGE₂ , forskolin, and cAMP analogs. These changes were specific to IB4/CaMKIIα positive neurons, required protein translation, and increased cAMP-to-ERK signaling. In both, control and OSM-treated neurons, cAMP/ERK signaling involved RapGEF2 and PKA but not Epac. Similar enhancement of cAMP-to-ERK signaling could be induced by GDNF, which acts mostly on IB4/CaMKIIα-positive neurons, but not by NGF, which acts mostly on IB4/CaMKIIα-negative neurons. In vitro, OSM pretreatment rendered baseline TTX-R currents ERK-dependent and switched forskolin-increased currents from partial to full ERK-dependence in small/medium sized neurons. In summary, priming induced by OSM uses a novel mechanism to enhance and prolong coupling of cAMP/PKA to ERK1/2 signaling without changing the overall pathway structure.

Phosphorylation of the mRNA cap-binding protein eIF4E and cancer

Dysregulated protein synthesis is frequently involved in oncogenesis and cancer progression. Translation initiation is thought to be the rate-limiting step in protein synthesis, and the mRNA 5' cap-binding protein eukaryotic translation initiation factor 4E (eIF4E) is a pivotal factor that initiates translation. The activities of eIF4E are regulated at multiple levels, one of which is through its phosphorylation at Serine 209 by the mitogen-activated protein kinase-interacting kinases (MNKs, including MNK1 and MNK2). Benefiting from novel mouse genetic tools and pharmacological MNK inhibitors, our understanding of a role for eIF4E phosphorylation in tumor biology and cancer therapy has greatly evolved in recent years. Importantly, recent studies have found that the level of eIF4E phosphorylation is frequently upregulated in a wide variety of human cancer types, and phosphorylation of eIF4E drives a number of important processes in cancer biology, including cell transformation, proliferation, apoptosis, metastasis and angiogenesis. The MNK-eIF4E axis is being assessed as a therapeutic target either alone or in combination with other therapies in different cancer models. As novel MNK inhibitors are being developed, experimental studies bring new hope to cure human cancers that are not responsive to traditional therapies. Herein we review recent progress on our understanding of a mechanistic role for phosphorylation of eIF4E in cancer biology and therapy.

Heat shock protein 90 inhibitors block the antinociceptive effects of opioids in mouse chemotherapy-induced neuropathy and cancer bone pain models

Heat shock protein 90 (Hsp90) is a ubiquitous signal transduction regulator, and Hsp90 inhibitors are in clinical development as cancer therapeutics. However, there have been very few studies on the impact of Hsp90 inhibitors on pain or analgesia, a serious concern for cancer patients. We previously found that Hsp90 inhibitors injected into the brain block opioid-induced antinociception in tail flick, paw incision, and HIV neuropathy pain. This study extended from that initial work to test the cancer-related clinical impact of Hsp90 inhibitors on opioid antinociception in cancer-induced bone pain in female BALB/c mice and chemotherapy-induced peripheral neuropathy in male and female CD-1 mice. Mice were treated with Hsp90 inhibitors (17-AAG, KU-32) by the intracerebroventricular, intrathecal, or intraperitoneal routes, and after 24 hours, pain behaviors were evaluated after analgesic drug treatment. Heat shock protein 90 inhibition in the brain or systemically completely blocked morphine and oxymorphone antinociception in chemotherapy-induced peripheral neuropathy; this effect was partly mediated by decreased ERK and JNK MAPK activation and by increased protein translation, was not altered by chronic treatment, and Hsp90 inhibition had no effect on gabapentin antinociception. We also found that the Hsp90 isoform Hsp90α and the cochaperone Cdc37 were responsible for the observed changes in opioid antinociception. By contrast, Hsp90 inhibition in the spinal cord or systemically partially reduced opioid antinociception in cancer-induced bone pain. These results demonstrate that Hsp90 inhibitors block opioid antinociception in cancer-related pain, suggesting that Hsp90 inhibitors for cancer therapy could decrease opioid treatment efficacy.

eIF4E phosphorylation modulates pain and neuroinflammation in the aged

The aged population has a higher probability of developing chronic pain from acute insults because of age-associated low-grade inflammation. Several emerging studies have shown a crucial role of cap-dependent translation in the development of chronic pain in young adult animals; however, its role in the aged has never been reported. Acute and chronic inflammatory responses, including pain, are altered over age, and understanding how cap-dependent translation can represent an important and druggable pathway is imperative for understanding its therapeutic potential. Here we have tested how an inflammatory stimulus, complete Freund's adjuvant (CFA), affects spontaneous and evoked pain, as well as inflammation in young versus aged mice that lack functional cap-dependent translation machinery (eukaryotic translation initiation factor 4E (eIF4E)) compared with age-matched wild-type (WT) mice. Interestingly, we found that CFA-induced acute pain and inflammation are modulated by eIF4E phosphorylation in aged but not young animals. Aged transgenic animals showed attenuated paw temperature and inflammation, as well as a mitigation in the onset and quicker resolution in mechanical and thermal hypersensitivity. We found that levels of interleukin (IL)-1β and tumor necrosis factor (TNF)-α are elevated in dorsal root ganglia in aged WT and eIF4E transgenic groups, despite faster resolution of acute inflammation and pain in the aged eIF4E transgenic animals. We propose that these cytokines are important in mediating the observed behavioral responses in the young and represent an alternate pathway in the development of age-associated inflammation and behavioral consequences. These findings demonstrate that eIF4E phosphorylation can be a key target for treating inflammatory pain in the aged.

Daily intermittent fasting in mice enhances morphine-induced antinociception while mitigating reward, tolerance, and constipation

The opioid epidemic has plagued the United States with high levels of abuse and poor quality of life for chronic pain patients requiring continuous use of opioids. New drug discovery efforts have been implemented to mitigate this epidemic; however, new medications are still limited by low efficacy and/or high side effect and abuse potential. Intermittent fasting (IF) has recently been shown to improve a variety of pathological states, including stroke and neuroinflammation. Numerous animal and human studies have shown the benefits of IF in these disease states, but not in pain and opioid treatment. We thus subjected male and female CD-1 mice to 18-hour fasting intervals followed by 6-hour feed periods with standard chow for 1 week. Mice that underwent this diet displayed an enhanced antinociceptive response to morphine both in efficacy and duration using thermal tail-flick and postoperative paw incision pain models. While showing enhanced antinociception, IF mice also demonstrated no morphine reward and reduced tolerance and constipation. Seeking a mechanism for these improvements, we found that the mu-opioid receptor showed enhanced efficacy and reduced tolerance in the spinal cord and periaqueductal gray, respectively, from IF mice using a S-GTPγS coupling assay. These improvements in receptor function were not due to changes in mu-opioid receptor protein expression. These data suggest that a daily IF diet may improve the therapeutic index of acute and chronic opioid therapies for pain patients in the clinic, providing a novel tool to improve patient therapy and reduce potential abuse.

Reversal of peripheral nerve injury-induced neuropathic pain and cognitive dysfunction via genetic and tomivosertib targeting of MNK

Neuropathic pain caused by nerve injury presents with severe spontaneous pain and a variety of comorbidities, including deficits in higher executive functions. None of these clinical problems are adequately treated with current analgesics. Targeting of the mitogen-activated protein kinase-interacting kinase (MNK1/2) and its phosphorylation target, the mRNA cap binding protein eIF4E, attenuates many types of nociceptive plasticity induced by inflammatory mediators and chemotherapeutic drugs but inhibiting this pathway does not alter nerve injury-induced mechanical allodynia. We used genetic manipulations and pharmacology to inhibit MNK-eIF4E activity in animals with spared nerve injury, a model of peripheral nerve injury (PNI)-induced neuropathic pain. We assessed the presence of spontaneous pain using conditioned place preference. We also tested performance in a medial prefrontal cortex (mPFC)-dependent rule-shifting task. WT neuropathic animals showed signs of spontaneous pain and were significantly impaired in the rule-shifting task while genetic and pharmacological inhibition of the MNK-eIF4E signaling axis protected against and reversed spontaneous pain and PNI-mediated cognitive impairment. Additionally, pharmacological and genetic inhibition of MNK-eIF4E signaling completely blocked and reversed maladaptive shortening in the length of axon initial segments (AIS) in the mPFC of PNI mice. Surprisingly, these striking positive outcomes on neuropathic pain occurred in the absence of any effect on mechanical allodynia, a standard test for neuropathic pain efficacy. Our results illustrate new testing paradigms for determining preclinical neuropathic pain efficacy and point to the MNK inhibitor tomivosertib (eFT508) as an important drug candidate for neuropathic pain treatment.

MNK-eIF4E signalling is a highly conserved mechanism for sensory neuron axonal plasticity: evidence from Aplysia californica

Injury to sensory neurons causes an increase in the excitability of these cells leading to enhanced action potential generation and a lowering of spike threshold. This type of sensory neuron plasticity occurs across vertebrate and invertebrate species and has been linked to the development of both acute and persistent pain. Injury-induced plasticity in sensory neurons relies on localized changes in gene expression that occur at the level of mRNA translation. Many different translation regulation signalling events have been defined and these signalling events are thought to selectively target subsets of mRNAs. Recent evidence from mice suggests that the key signalling event for nociceptor plasticity is mitogen-activated protein kinase-interacting kinase (MNK) -mediated phosphorylation of eukaryotic translation initiation factor (eIF) 4E. To test the degree to which this is conserved in other species, we used a previously described sensory neuron plasticity model in Aplysia californica. We find, using a variety of pharmacological tools, that MNK signalling is crucial for axonal hyperexcitability in sensory neurons from Aplysia. We propose that MNK-eIF4E signalling is a core, evolutionarily conserved, signalling module that controls nociceptor plasticity. This finding has important implications for the therapeutic potential of this target, and it provides interesting clues about the evolutionary origins of mechanisms important for pain-related plasticity. This article is part of the Theo Murphy meeting issue 'Evolution of mechanisms and behaviour important for pain'.

Dural Calcitonin Gene-Related Peptide Produces Female-Specific Responses in Rodent Migraine Models

Migraine is the second leading cause for disability worldwide and the most common neurological disorder. It is also three times more common in women; reasons for this sex difference are not known. Using preclinical behavioral models of migraine, we show that application of calcitonin gene-related peptide (CGRP) to the rat dura mater produces cutaneous periorbital hypersensitivity. Surprisingly, this response was observed only in females; dural CGRP at doses from 1 pg to 3.8 μg produce no responses in males. In females, dural CGRP causes priming to a pH 7.0 solution after animals recover from the initial CGRP-induced allodynia. Dural application of interleukin-6 causes acute responses in males and females but only causes priming to subthreshold dural CGRP (0.1 pg) in females. Intracisternal application of BDNF also causes similar acute hypersensitivity responses in males and females but only priming to subthreshold dural CGRP (0.1 pg) in females. Females were additionally primed to a subthreshold dose of the NO-donor sodium nitroprusside (0.1 mg/kg) following dural CGRP. Finally, the sexually dimorphic responses to dural CGRP were not specific to rats as similar female-specific hypersensitivity responses were seen in mice, where increased grimace responses were also observed. These data are the first to demonstrate that CGRP-induced headache-like behavioral responses at doses up to 3.8 μg are female-specific both acutely and following central and peripheral priming. These data further implicate dural CGRP signaling in the pathophysiology of migraine and propose a model where dural CGRP-based mechanisms contribute to the sexual disparity of this female-biased disorder.SIGNIFICANCE STATEMENT Calcitonin gene-related peptide (CGRP) has long been implicated in the pathophysiology of migraine, and CGRP-based therapeutics are efficacious for the treatment of migraine in humans. However, the location of action for CGRP in migraine remains unclear. We show here that application of CGRP to the cranial meninges causes behavioral responses consistent with headache in preclinical rodent models. Surprisingly, however, these responses are only observed in females. Acute responses to meningeal CGRP are female-specific and sensitization to CGRP after two distinct stimuli are also female-specific. These data implicate the dura mater as a primary location of action for CGRP in migraine and suggest that female-specific mechanisms downstream of CGRP receptor activation contribute to the higher prevalence of migraine in women.

Early Treatment With Metformin in a Mice Model of Complex Regional Pain Syndrome Reduces Pain and Edema

BACKGROUND

Metformin, an adenosine monophosphate (AMP)-activated protein kinase activator, as well as a common drug for type 2 diabetes, has previously been shown to decrease mechanical allodynia in mice with neuropathic pain. The objective of this study is to determine if treatment with metformin during the first 3 weeks after fracture would produce a long-term decrease in mechanical allodynia and improve a complex behavioral task (burrowing) in a mouse tibia fracture model with signs of complex regional pain syndrome.

METHODS

Mice were allocated into distal tibia fracture or nonfracture groups (n = 12 per group). The fracture was stabilized with intramedullary pinning and external casting for 21 days. Animals were then randomized into 4 groups (n = 6 per group): (1) fracture, metformin treated, (2) fracture, saline treated, (3) nonfracture, metformin treated, and (4) nonfracture, saline treated. Mice received daily intraperitoneal injections of metformin 200 mg/kg or saline between days 14 and 21. After cast removal, von Frey force withdrawal (every 3 days) and burrowing (every 7 days) were tested between 25 and 56 days. Paw width was measured for 14 days after cast removal. AMP-activated protein kinase downregulation at 4 weeks after tibia fracture in the dorsal root ganglia was examined by immunohistochemistry for changes in the AMP-activated protein kinase pathway.

RESULTS

Metformin injections elevated von Frey thresholds (reduced mechanical allodynia) in complex regional pain syndrome mice versus saline-treated fracture mice between days 25 and 56 (difference of mean area under the curve, 42.5 g·d; 95% CI of the difference, 21.0-63.9; P < .001). Metformin also reversed burrowing deficits compared to saline-treated tibial fracture mice (difference of mean area under the curve, 546 g·d; 95% CI of the difference, 68-1024; P < .022). Paw width (edema) was reduced in metformin-treated fracture mice. After tibia fracture, AMP-activated protein kinase was downregulated in dorsal root ganglia neurons, and mechanistic target of rapamycin, ribosomal S6 protein, and eukaryotic initiation factor 2α were upregulated.

CONCLUSIONS

The important finding of this study was that early treatment with metformin reduces mechanical allodynia in a complex regional pain syndrome model in mice. Our findings suggest that AMP-activated protein kinase activators may be a viable therapeutic target for the treatment of pain associated with complex regional pain syndrome.

eIF4E-Dependent Translational Control: A Central Mechanism for Regulation of Pain Plasticity

Translational control of gene expression has emerged as a key mechanism in regulating different forms of long-lasting neuronal plasticity. Maladaptive plastic reorganization of peripheral and spinal nociceptive circuits underlies many chronic pain states and relies on new gene expression. Accordingly, downregulation of mRNA translation in primary afferents and spinal dorsal horn neurons inhibits tissue injury-induced sensitization of nociceptive pathways, supporting a central role for translation dysregulation in the development of persistent pain. Translation is primarily regulated at the initiation stage via the coordinated activity of translation initiation factors. The mRNA cap-binding protein, eukaryotic translation initiation factor 4E (eIF4E), is involved in the recruitment of the ribosome to the mRNA cap structure, playing a central role in the regulation of translation initiation. eIF4E integrates inputs from the mTOR and ERK signaling pathways, both of which are activated in numerous painful conditions to regulate the translation of a subset of mRNAs. Many of these mRNAs are involved in the control of cell growth, proliferation, and neuroplasticity. However, the full repertoire of eIF4E-dependent mRNAs in the nervous system and their translation regulatory mechanisms remain largely unknown. In this review, we summarize the current evidence for the role of eIF4E-dependent translational control in the sensitization of pain circuits and present pharmacological approaches to target these mechanisms. Understanding eIF4E-dependent translational control mechanisms and their roles in aberrant plasticity of nociceptive circuits might reveal novel therapeutic targets to treat persistent pain states.

Temporal and sex differences in the role of BDNF/TrkB signaling in hyperalgesic priming in mice and rats

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The effect of TrkB-Fc on hyperalgesic priming is sexually dimorphic in mice.

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The effect of TrkB-Fc on hyperalgesic priming is equivalent in male and female rats.

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Microglial BDNF does not contribute to hyperalgesic priming in mice.

Brain-derived neurotrophic factor (BDNF) signaling through its cognate receptor, TrkB, is a well-known promoter of synaptic plasticity at nociceptive synapses in the dorsal horn of the spinal cord. Existing evidence suggests that BDNF/TrkB signaling in neuropathic pain is sex dependent. We tested the hypothesis that the effects of BDNF/TrkB signaling in hyperalgesic priming might also be sexually dimorphic. Using the incision postsurgical pain model in male mice, we show that BDNF sequestration with TrkB-Fc administered at the time of surgery blocks the initiation and maintenance of hyperalgesic priming. However, when BDNF signaling was blocked prior to the precipitation of hyperalgesic priming with prostaglandin E₂ (PGE₂), priming was not reversed. This result is in contrast to our findings in male mice with interleukin-6 (IL6) as the priming stimulus where TrkB-Fc was effective in reversing the maintenance of hyperalgesic priming. Furthermore, in IL6-induced hyperalgesic priming, the BDNF sequestering agent, TrkB-fc, was effective in reversing the maintenance of hyperalgesic priming in male mice; however, when this experiment was conducted in female mice, we did not observe any effect of TrkB-fc. This markedly sexual dimorphic effect in mice is consistent with recent studies showing a similar effect in neuropathic pain models. We tested whether the sexual dimorphic role for BDNF was consistent across species. Importantly, we find that this sexual dimorphism does not occur in rats where TrkB-fc reverses hyperalgesic priming fully in both sexes. Finally, to determine the source of BDNF in hyperalgesic priming in mice, we used transgenic mice (Cx3cr1^CreER × Bdnf^flx/flx mice) with BDNF eliminated from microglia. From these experiments we conclude that BDNF from microglia does not contribute to hyperalgesic priming and that the key source of BDNF for hyperalgesic priming is likely nociceptors in the dorsal root ganglion. These experiments demonstrate the importance of testing mechanistic hypotheses in both sexes in multiple species to gain insight into complex biology underlying chronic pain.