Spinal phosphinositide 3-kinase-Akt-mammalian target of rapamycin signaling cascades in inflammation-induced hyperalgesia

Translational control in the spinal cord regulates gene expression and pain hypersensitivity in the chronic phase of neuropathic pain

Sensitization of spinal nociceptive circuits plays a crucial role in neuropathic pain. This sensitization depends on new gene expression that is primarily regulated via transcriptional and translational control mechanisms. The relative roles of these mechanisms in regulating gene expression in the clinically relevant chronic phase of neuropathic pain are not well understood. Here, we show that changes in gene expression in the spinal cord during the chronic phase of neuropathic pain are substantially regulated at the translational level. Downregulating spinal translation at the chronic phase alleviated pain hypersensitivity. Cell-type-specific profiling revealed that spinal inhibitory neurons exhibited greater changes in translation after peripheral nerve injury compared to excitatory neurons. Notably, increasing translation selectively in all inhibitory neurons or parvalbumin-positive (PV+) interneurons, but not excitatory neurons, promoted mechanical pain hypersensitivity. Furthermore, increasing translation in PV+ neurons decreased their intrinsic excitability and spiking activity, whereas reducing translation in spinal PV+ neurons prevented the nerve injury-induced decrease in excitability. Thus, translational control mechanisms in the spinal cord, particularly in inhibitory neurons, play a role in mediating neuropathic pain hypersensitivity.

Small G-Protein Rheb Gates Mammalian Target of Rapamycin Signaling to Regulate Morphine Tolerance in Mice

BACKGROUND

Analgesic tolerance due to long-term use of morphine remains a challenge for pain management. Morphine acts on μ-opioid receptors and downstream of the phosphatidylinositol 3-kinase signaling pathway to activate the mammalian target of rapamycin (mTOR) pathway. Rheb is an important regulator of growth and cell-cycle progression in the central nervous system owing to its critical role in the activation of mTOR. The hypothesis was that signaling via the GTP-binding protein Rheb in the dorsal horn of the spinal cord is involved in morphine-induced tolerance.

METHODS

Male and female wild-type C57BL/6J mice or transgenic mice (6 to 8 weeks old) were injected intrathecally with saline or morphine twice daily at 12-h intervals for 5 consecutive days to establish a tolerance model. Analgesia was assessed 60 min later using the tail-flick assay. After 5 days, the spine was harvested for Western blot or immunofluorescence analysis.

RESULTS

Chronic morphine administration resulted in the upregulation of spinal Rheb by 4.27 ± 0.195-fold (P = 0.0036, n = 6), in turn activating mTOR by targeting rapamycin complex 1 (mTORC1). Genetic overexpression of Rheb impaired morphine analgesia, resulting in a tail-flick latency of 4.65 ± 1.10 s (P < 0.0001, n = 7) in Rheb knock-in mice compared to 10 s in control mice (10 ± 0 s). Additionally, Rheb overexpression in spinal excitatory neurons led to mTORC1 signaling overactivation. Genetic knockout of Rheb or inhibition of mTORC1 signaling by rapamycin potentiated morphine-induced tolerance (maximum possible effect, 52.60 ± 9.56% in the morphine + rapamycin group vs. 16.60 ± 8.54% in the morphine group; P < 0.0001). Moreover, activation of endogenous adenosine 5'-monophosphate-activated protein kinase inhibited Rheb upregulation and retarded the development of morphine-dependent tolerance (maximum possible effect, 39.51 ± 7.40% in morphine + metformin group vs. 15.58 ± 5.79% in morphine group; P < 0.0001).

CONCLUSIONS

This study suggests spinal Rheb as a key molecular factor for regulating mammalian target of rapamycin signaling.

EDITOR’S PERSPECTIVE

4E-BP1-dependent translation in nociceptors controls mechanical hypersensitivity via TRIM32/type I interferon signaling

Activation of the mechanistic target of rapamycin complex 1 (mTORC1) contributes to the development of chronic pain. However, the specific mechanisms by which mTORC1 causes hypersensitivity remain elusive. The eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) is a key mTORC1 downstream effector that represses translation initiation. Here, we show that nociceptor-specific deletion of 4E-BP1, mimicking activation of mTORC1-dependent translation, is sufficient to cause mechanical hypersensitivity. Using translating ribosome affinity purification in nociceptors lacking 4E-BP1, we identified a pronounced translational up-regulation of tripartite motif-containing protein 32 (TRIM32), an E3 ubiquitin ligase that promotes interferon signaling. Down-regulation of TRIM32 in nociceptors or blocking type I interferon signaling reversed the mechanical hypersensitivity in mice lacking 4E-BP1. Furthermore, nociceptor-specific ablation of TRIM32 alleviated mechanical hypersensitivity caused by tissue inflammation. These results show that mTORC1 in nociceptors promotes hypersensitivity via 4E-BP1-dependent up-regulation of TRIM32/interferon signaling and identify TRIM32 as a therapeutic target in inflammatory pain.

Effects of Early Exposure to Isoflurane on Susceptibility to Chronic Pain Are Mediated by Increased Neural Activity Due to Actions of the Mammalian Target of the Rapamycin Pathway

Patients who have undergone surgery in early life may be at elevated risk for suffering neuropathic pain in later life. The risk factors for this susceptibility are not fully understood. Here, we used a mouse chronic pain model to test the hypothesis that early exposure to the general anesthetic (GA) Isoflurane causes cellular and molecular alterations in dorsal spinal cord (DSC) and dorsal root ganglion (DRG) that produces a predisposition to neuropathic pain via an upregulation of the mammalian target of the rapamycin (mTOR) signaling pathway. Mice were exposed to isoflurane at postnatal day 7 (P7) and underwent spared nerve injury at P28 which causes chronic pain. Selected groups were treated with rapamycin, an mTOR inhibitor, for eight weeks. Behavioral tests showed that early isoflurane exposure enhanced susceptibility to chronic pain, and rapamycin treatment improved outcomes. Immunohistochemistry, Western blotting, and q-PCR indicated that isoflurane upregulated mTOR expression and neural activity in DSC and DRG. Accompanying upregulation of mTOR and rapamycin-reversible changes in chronic pain-associated markers, including N-cadherin, cAMP response element-binding protein (CREB), purinergic P2Y12 receptor, glial fibrillary acidic protein (GFAP) in DSC; and connexin 43, phospho-extracellular signal-regulated kinase (p-ERK), GFAP, Iba1 in DRG, were observed. We concluded that early GA exposure, at least with isoflurane, alters the development of pain circuits such that mice are subsequently more vulnerable to chronic neuropathic pain states.

Diabetic Neuropathy: Review on Molecular Mechanisms

Diabetic mellitus is a worldwide endocrine and metabolic disorder with insulin insensitivity or deficiency or both whose prevalence could rise up to 592 million by 2035. Consistent hyperglycemia leads to one of the most common comorbidities like Diabetic Peripheral Neuropathy (DPN). DPN is underlined with unpleasant sensory experience, such as tingling and burning sensation, hyperalgesia, numbness, etc. Globally, 50-60% of the diabetic population is suffering from such symptoms as microvascular complications. Consistent hyperglycemia during DM causes activation/inhibition of various pathways playing important role in the homeostasis of neurons and other cells. Disruption of these pathways results into apoptosis and mitochondrial dysfunctions, causing neuropathy. Among these, pathways like Polyol and PARP are some of the most intensively studied ones whereas those like Wnt pathway, Mitogen activated protein kinase (MAPK), mTOR pathway are comparatively newly discovered. Understanding of these pathways and their role in pathophysiology of DN underlines a few molecules of immense therapeutic value. The inhibitors or activators of these molecules can be of therapeutic importance in the management of DPN. This review, hence, focuses on these underlying molecular mechanisms intending to provide therapeutically effective molecular targets for the treatment of DPN.

mTOR-neuropeptide Y signaling sensitizes nociceptors to drive neuropathic pain

Neuropathic pain is a refractory condition that involves de novo protein synthesis in the nociceptive pathway. The mTOR is a master regulator of protein translation; however, mechanisms underlying its role in neuropathic pain remain elusive. Using the spared nerve injury-induced neuropathic pain model, we found that mTOR was preferentially activated in large-diameter dorsal root ganglion (DRG) neurons and spinal microglia. However, selective ablation of mTOR in DRG neurons, rather than microglia, alleviated acute neuropathic pain in mice. We show that injury-induced mTOR activation promoted the transcriptional induction of neuropeptide Y (Npy), likely via signal transducer and activator of transcription 3 phosphorylation. NPY further acted primarily on Y2 receptors (Y2R) to enhance neuronal excitability. Peripheral replenishment of NPY reversed pain alleviation upon mTOR removal, whereas Y2R antagonists prevented pain restoration. Our findings reveal an unexpected link between mTOR and NPY/Y2R in promoting nociceptor sensitization and neuropathic pain.

Modulating the tachykinin: Role of substance P and neurokinin receptor expression in ocular surface disorders

Substance P (SP) is a tachykinin expressed by various cells in the nervous and immune systems. SP is predominantly released by neurons and exerts its biological and immunological effects through the neurokinin receptors, primarily the neurokinin-1 receptor (NK1R). SP is essential for maintaining ocular surface homeostasis, and its reduced levels in disorders like diabetic neuropathy disrupt the corneal tissue. It also plays an essential role in promoting corneal wound healing by promoting the migration of keratocytes. In this review, we briefly discuss the structure, expression, and function of SP and its principal receptor NK1R. In addition, SP induces pro-inflammatory effects through autocrine or paracrine action on the immune cells in various ocular surface pathologies, including dry eye disease, herpes simplex virus keratitis, and Pseudomonas keratitis. We provide an in-depth review of the pathogenic role of SP in various ocular surface diseases and several new approaches developed to counter the immune-mediated effects of SP either through modulating its production or blocking its target receptor.

Different genes may be involved in distal and local sensitisation: a genome-wide gene-based association study and meta-analysis

Background

Neuropathic pain symptoms and signs of increased pain sensitization in osteoarthritis (OA) patients may explain persistent pain after total joint replacement (TJR). Therefore, identifying genetic markers associated with pain sensitization and neuropathic‐like pain phenotypes could be clinically important in identifying targets for early intervention.

Methods

We performed a genome‐wide gene‐based association study (GWGAS) using pressure pain detection thresholds (PPTs) from distal pain‐free sites (anterior tibia), a measure of distal sensitization, and from proximal pain‐affected sites (lateral joint line), a measure of local sensitization, in 320 knee OA participants from the Knee Pain and related health in the Community (KPIC) cohort. We next performed gene‐based fixed‐effects meta‐analysis of PPTs and a neuropathic‐like pain phenotype using genome‐wide association study (GWAS) data from KPIC and from an independent cohort of 613 post‐TJR participants, respectively.

Results

The most significant genes associated with distal and local sensitization were OR5B3 and BRDT, respectively. We also found previously identified neuropathic pain‐associated genes—KCNA1, MTOR, ADORA1 and SCN3B—associated with PPT at the anterior tibia and an inflammatory pain gene—PTAFR—associated with PPT at the lateral joint line. Meta‐analysis results of anterior tibia and neuropathic‐like pain phenotypes revealed genes associated with bone morphogenesis, neuro‐inflammation, obesity, type 2 diabetes, cardiovascular disease and cognitive function.

Conclusions

Overall, our results suggest that different biological processes might be involved in distal and local sensitization, and common genetic mechanisms might be implicated in distal sensitization and neuropathic‐like pain. Future studies are needed to replicate these findings.

Significance

To the best of our knowledge, this is the first GWAS for pain sensitization and the first gene‐based meta‐analysis of pain sensitization and neuropathic‐like pain. Higher pain sensitization and neuropathic pain symptoms are associated with persistent pain after surgery hence, identifying genetic biomarkers and molecular pathways associated with these traits is clinically relevant.

Akt/Aquaporin-4 Signaling Aggravates Neuropathic Pain by Activating Astrocytes after Spinal Nerve Ligation in Rats

Aquaporins (AQPs) play critical physiological roles in water balance in the central nervous system (CNS). Aquaporin-4 (AQP4), the principal aquaporin expressed in the CNS, has been implicated in the processing of sensory and pain transmission. Akt signaling is also involved in pain mediation, such as neuroinflammatory pain and bone cancer pain. Previously, we found that expression of AQP4 and p-Akt was altered in the rat spinal cord after spinal nerve ligation (SNL). Here, we further investigated the effects of the AQP4 and Akt pathways in the spinal dorsal horn (SDH) on the pathogenesis of neuropathic pain (NP). Spinal AQP4 was significantly upregulated after SNL and was primarily expressed in astrocytes in the SDH. Inhibition of AQP4 with TGN-020 attenuated the development and maintenance of NP by inhibiting glial activation and anti-neuroinflammatory mechanisms. Moreover, inhibition of AQP4 suppressed astrocyte activation both in the SDH and in primary cultures. Similar to AQP4, we found that p-Akt was also significantly elevated after SNL. Inhibition of Akt with MK2206 suppressed AQP4 upregulation and astrocyte activation both in vivo and in vitro. Furthermore, Akt blockade with MK2206 alleviated NP in the early and late phases after SNL. These results elucidate the mechanisms involved in the roles of Akt/AQP4 signaling in the development and maintenance of NP. AQP4 is likely to be a novel therapeutic target for NP management.

RETRACTED ARTICLE: Involvement of pro-inflammatory cytokines in diabetic neuropathic pain via central PI3K/Akt/mTOR signal pathway

Retraction statementWe, the Editors and Publisher of Archives of Physiology and Biochemistry, have retracted the following article:Zongming Jiang, Zhonghua Chen, Yonghao Chen, Jing Jiao and Zhifeng WangInvolvement of pro-inflammatory cytokines in diabetic neuropathic pain via central PI3K/Akt/mTOR signal pathway, Archives of Physiology and Biochemistry, Published Online 2019 Aug 9:1-9. DOI: 10.1080/13813455.2019.1651869The article has been retracted following receipt of information from the corresponding author, Zhifeng Wang, on September 11, 2019, informing us that it was realised that inappropriate doses of rapamycin and the corresponding antagonist were used in this study, which may have led to artificial results and misleading interpretations and ultimately do not support the final conclusions drawn by the authors. The article is withdrawn from all print and electronic editions.We have been informed in our decision-making by our policy on publishing ethics and integrity and the COPE guidelines on retractions.The retracted article will remain online to maintain the scholarly record, but it will be digitally watermarked on each page as "Retracted."

Manual Acupuncture at ST37 Modulates TRPV1 in Rats with Acute Visceral Hyperalgesia via Phosphatidylinositol 3-Kinase/Akt Pathway

Acupuncture can significantly ameliorate inflammatory pain in acute visceral hyperalgesia. Hyperalgesia is attenuated by inflammatory mediators that activate transient receptor potential vanilloid 1 (TRPV1), and TRPV1 is regulated by nerve growth factor (NGF)-induced phosphatidylinositol 3-kinase (PI3K)/Akt pathway. However, it is unknown whether NGF-induced PI3K/Akt pathway is associated with manual acupuncture (MA). In this study, the effect and mechanism of MA at Shangjuxu (ST37) and Quchi (LI11) were examined using an acetic acid-induced rat model with visceral hyperalgesia. We demonstrated that MA at ST37 significantly decreased abdominal withdrawal reflex (AWR) scores, proinflammatory cytokine expression (TNF-α, IL-1β, and IL-6), and TRPV1 protein and mRNA expression in rats with acute visceral hyperalgesia compared with the untreated controls, while MA at LI11 showed no effect. The effects of MA at ST37 were reversed after treatment with the PI3K agonist IGF-1 30 min before MA. In rats with visceral hyperalgesia, the upregulation of NGF, tropomyosin-receptor-kinase A (TrkA), PI3K, and phosphorylation-Akt (p-Akt) was decreased by MA at ST37, indicating that TRPV1 regulation via the NGF-induced PI3K/Akt pathway plays a vital role in the effects of MA-mediated amelioration of acute visceral hyperalgesia.

PI3Kγ/AKT Signaling in High Molecular Weight Hyaluronan (HMWH)-Induced Anti-Hyperalgesia and Reversal of Nociceptor Sensitization

High molecular weight hyaluronan (HMWH), a well-established treatment for osteoarthritis pain, is anti-hyperalgesic in preclinical models of inflammatory and neuropathic pain. HMWH-induced anti-hyperalgesia is mediated by its action at cluster of differentiation 44 (CD44), the cognate hyaluronan receptor, which can signal via phosphoinositide 3-kinase (PI3K), a large family of kinases involved in diverse cell functions. We demonstrate that intrathecal administration of an oligodeoxynucleotide (ODN) antisense to mRNA for PI3Kγ (a Class I PI3K isoform) expressed in dorsal root ganglia (DRGs), and intradermal administration of a PI3Kγ-selective inhibitor (AS605240), markedly attenuates HMWH-induced anti-prostaglandin E₂ (PGE₂) hyperalgesia, in male and female rats. Intradermal administration of inhibitors of mammalian target of rapamycin (mTOR; rapamycin) and protein kinase B (AKT; AKT Inhibitor IV), signaling molecules downstream of PI3Kγ, also attenuates HMWH-induced anti-hyperalgesia. In vitro patch-clamp electrophysiology experiments on cultured nociceptors from male rats demonstrate that some HMWH-induced changes in generation of action potentials (APs) in nociceptors sensitized by PGE₂ are PI3Kγ dependent (reduction in AP firing rate, increase in latency to first AP and increase in slope of current ramp required to induce AP) and some are PI3Kγ independent [reduction in recovery rate of AP afterhyperpolarization (AHP)]. Our demonstration of a role of PI3Kγ in HMWH-induced anti-hyperalgesia and reversal of nociceptor sensitization opens a novel line of research into molecular targets for the treatment of diverse pain syndromes.SIGNIFICANCE STATEMENT We have previously demonstrated that high molecular weight hyaluronan (HMWH) attenuates inflammatory hyperalgesia, an effect mediated by its action at cluster of differentiation 44 (CD44), the cognate hyaluronan receptor, and activation of its downstream signaling pathway, in nociceptors. In the present study, we demonstrate that phosphoinositide 3-kinase (PI3K)γ and downstream signaling pathway, protein kinase B (AKT) and mammalian target of rapamycin (mTOR), are crucial for HMWH to induce anti-hyperalgesia.

Rapamycin reduces orofacial nociceptive responses and microglial p38 mitogen-activated protein kinase phosphorylation in trigeminal nucleus caudalis in mouse orofacial formalin model

The mammalian target of rapamycin (mTOR) plays a role in various cellular phenomena, including autophagy, cell proliferation, and differentiation. Although recent studies have reported its involvement in nociceptive responses in several pain models, whether mTOR is involved in orofacial pain processing is currently unexplored. This study determined whether rapamycin, an mTOR inhibitor, reduces nociceptive responses and the number of Fos-immunoreactive (Fos-ir) cells in the trigeminal nucleus caudalis (TNC) in a mouse orofacial formalin model. We also examined whether the glial cell expression and phosphorylated p38 (p-p38) mitogen-activated protein kinases (MAPKs) in the TNC are affected by rapamycin. Mice were intraperitoneally given rapamycin (0.1, 0.3, or 1.0 mg/kg); then, 30 min after, 5% formalin (10 µl) was subcutaneously injected into the right upper lip. The rubbing responses with the ipsilateral forepaw or hindpaw were counted for 45 min. High-dose rapamycin (1.0 mg/kg) produced significant antinociceptive effects in both the first and second phases of formalin test. The number of Fos-ir cells in the ipsilateral TNC was also reduced by high-dose rapamycin compared with vehicle-treated animals. Furthermore, the number of p-p38-ir cells the in ipsilateral TNC was significantly decreased in animals treated with high-dose rapamycin; p-p38 expression was co-localized in microglia, but not neurons and astrocytes. Therefore, the mTOR inhibitor, rapamycin, reduces orofacial nociception and Fos expression in the TNC, and its antinociceptive action on orofacial pain may be associated with the inhibition of p-p38 MAPK in the microglia.

Mechanistic insight into toxicity of phthalates, the involved receptors, and the role of Nrf2, NF-κB, and PI3K/AKT signaling pathways

The wide use of phthalates, as phthalates are used in the manufacturing of not only plastics but also many others goods, has become a main concern in the current century because of their potency to induce deleterious effects on organism health. The toxic effects of phthalates such as reproductive toxicity, cardiotoxicity, hepatotoxicity, nephrotoxicity, teratogenicity, and tumor development have been widely indicated by previous experimental studies. Some of the important mechanisms of toxicity by phthalates are the induction and promotion of inflammation, oxidative stress, and apoptosis. Awareness of the involved molecular pathways of these mechanisms will permit the detection of exact molecular targets of phthalates to protect or treat their toxicity. Up to now, various transcription factors and signaling pathways have been associated with phthalate-induced toxicity which by influencing on nuclear surface and the expression of different genes can alter cell hemostasis. In different studies, the role of nuclear factor erythroid 2-related factor 2 (Nrf2), nuclear factor-κB (NF-κB), and phosphatidylinositol-3-kinase (PI3K)/AKT signaling pathways in processes of oxidative stress, inflammation, apoptosis, and cancer has been shown following exposure to phthalates. In the present review, we aim to survey experimental studies (in vitro and in vivo) in order to show firstly the most involved receptors and also the importance and the role of the mentioned signaling pathways in phthalate-induced toxicity, and with considering this point, the future studies can focus on these molecular targets as a strategic method to reduce environmental chemicals-induced toxicity especially phthalates toxic effects.

Modulation of Pathological Pain by Epidermal Growth Factor Receptor

Chronic pain has been widely recognized as a major public health problem that impacts multiple aspects of patient quality of life. Unfortunately, chronic pain is often resistant to conventional analgesics, which are further limited by their various side effects. New therapeutic strategies and targets are needed to better serve the millions of people suffering from this devastating disease. To this end, recent clinical and preclinical studies have implicated the epidermal growth factor receptor signaling pathway in chronic pain states. EGFR is one of four members of the ErbB family of receptor tyrosine kinases that have key roles in development and the progression of many cancers. EGFR functions by activating many intracellular signaling pathways following binding of various ligands to the receptor. Several of these signaling pathways, such as phosphatidylinositol 3-kinase, are known mediators of pain. EGFR inhibitors are known for their use as cancer therapeutics but given recent evidence in pilot clinical and preclinical investigations, may have clinical use for treating chronic pain. Here, we review the clinical and preclinical evidence implicating EGFR in pathological pain states and provide an overview of EGFR signaling highlighting how EGFR and its ligands drive pain hypersensitivity and interact with important pain pathways such as the opioid system.

Inflammatory cytokines in midbrain periaqueductal gray contribute to diabetic induced pain hypersensitivity through phosphoinositide 3-kinase/protein kinase B/mammalian target of rapamycin signaling pathway

Background

Diabetes-related neuropathic pain frequently occurs, and the underpinning mechanism remains elusive. The periaqueductal gray (PAG) exhibits descending inhibitory effects on central pain transmission. The current work aimed to examine whether inflammatory cytokines regulate mechanical allodynia and thermal hyperalgesia induced by diabetes through the phosphoinositide 3-kinase (PI3K)-mammalian target of rapamycin (mTOR) pathway in the PAG.

Methods

Streptozotocin (STZ) was administered intraperitoneally to mimic allodynia and hyperalgesia evoked by diabetes in rats. Behavioral assays were carried out for determining mechanical pain and thermal hypersensitivity. Immunoblot and ELISA were performed to examine PAG protein amounts of interleukin-1β (IL-1β), IL-6, and tumor necrosis factor-α (TNF-α), as well as their corresponding receptors in STZ rats, and the expression of PI3K/protein kinase B (Akt)/mTOR signaling effectors.

Results

Increased PAG p-PI3K/p-Akt/p-mTOR protein amounts were observed in STZ-induced animals, a PI3K-mTOR pathway inhibition in the PAG attenuated neuropathic pain responses. Moreover, the PAG concentrations of IL-1β, IL-6, and TNF-α and their receptors (namely, IL-1R, IL-6R, and tumor necrosis factor receptor [TNFR] subtype TNFR1, respectively) were increased in the STZ rats. Additionally, inhibiting IL-1R, IL-6R, and TNFR1 ameliorated mechanical allodynia and thermal hyperalgesia in STZ rats, alongside the downregulation of PI3K-mTOR signaling.

Conclusions

Overall, the current study suggests that upregulated proinflammatory cytokines and their receptors in the PAG activate PI3K-mTOR signaling, thereby producing a de-inhibition effect on descending pathways in modulating pain transmission, and eventually contributing to neuropathic pain.

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.

[Escin alleviates chemotherapy-induced peripheral neuropathic pain by inducing autophagy in the spinal cord of rats]

OBJECTIVE

To investigate the effect of escin in relieving chemotherapy-induced peripheral neuropathic pain in rats and explore and the underlying mechanism.

METHODS

Eighteen SD rats were randomly divided into 3 groups (n=6), including an escin preconditioning group (treated with 4 mg/kg escin on days 1-7 and then with 2 mg/kg taxol on days 8, 10, 12, and 14), an escin postconditioning group (treated with 2 mg/kg taxol on days 1, 3, 5, and 7 and then with 4 mg/mg escin on days 8-14) and control group (treated with 2 mg/kg taxol on days 1, 3, 5, and 7 and then with saline on days 8-14). Mechanical allodynia and thermal hyperalgesia of the mice were tested on days 4, 7, 10 and 14, and the expression levels of LC3II and p62 in the spinal cord of the rats were detected using Western blotting.

RESULTS

The rats in both the escin preconditioning group and escin postconditioning group showed obviously increased thresholds of mechanical allodynia and thermal hyperalgesia as compared with those in the control group (P < 0.01). Western blotting showed that the expression level of LC3II was significantly increased while p62 expression was lowered in escin preconditioning group as compared with those in the control group (P < 0.05). The escin postconditioning group also showed significantly higher LC3II expression and lower p62 expression levels than the control group (P < 0.05).

CONCLUSIONS

Escin can alleviate chemotherapy-induced peripheral neuropathic pain in rats possibly by upregulating the expressions of autophagy-related proteins in the spinal cord.

The analgesic effects of pioglitazone in the bone cancer pain rats via regulating the PPARγ/PTEN/mTOR signaling pathway in the spinal dorsal horn

BACKGROUND AND OBJECTIVES

Bone cancer pain (BCP) remains a difficult clinical problem. This study examined whether pioglitazone, a peroxisome proliferator-activated receptor gamma (PPARγ) agonist, is effective for attenuating BCP, and investigated the interaction between activation of PPARγ and phosphatase and tensin homolog deleted from chromosome 10 (PTEN) / mammalian target of rapamycin (mTOR) signal in the spinal dorsal horn (SDH) of BCP rats.

METHODS

We tested the effects of intrathecal (i.t.) injection of adenovirus-mediated PTEN (Ad-PTEN), PTEN antisense oligonucleotide (Ad-antisense PTEN), mTOR inhibitor rapamycin, pioglitazone and PPARγ antagonist GW9662 on bone cancer-induced mechanical allodynia by measuring the paw withdrawal threshold (PWT). Western blot or immunofluorescence examined the expression of spinal PPARγ, PTEN, mTOR, p-mTOR and p-S6K1.

RESULTS

Bone cancer did not alter total mTOR expression but caused significant downregulation of PTEN and upregulation of p-mTOR and p-S6K1 in spinal neurons. Rapamycin markedly reduced BCP. Upregulation of spinal PTEN by i.t. Ad-PTEN significantly relieved BCP and downregulated p-mTOR and p-S6K1; while i.t. Ad-antisense PTEN led to the opposite effects of Ad-PTEN. Spinal PPARγ expression increased in BCP rats, co-localizing mainly with neurons and a few astrocytes, but not in microglia. Pioglitazone (500 μg/day i.t. for one week, from 7 days after surgery) attenuated BCP, further increased expression of PPARγ, and inhibited downregulation of PTEN and upregulation of p-mTOR and p-S6K1 in the SDH. Pioglitazone's analgesic effect was enhanced by Ad-PTEN and attenuated by Ad-antisense PTEN. Blockade of PPARγ with GW9662 (300 μg i.t. 15 min prior to pioglitazone) reversed the effects of pioglitazone on BCP and regulations of PPARγ/PTEN/mTOR signal.

CONCLUSIONS

Intrathecal pioglitazone administration alleviates BCP by regulating the PPARγ/PTEN/mTOR signal in the SDH. Our data provided new insight in the therapeutic strategy in BCP management.

Effect of lycopene on pain facilitation and the SIRT1/mTOR pathway in the dorsal horn of burn injury rats

To explore the effect of intrathecal injection of lycopene on pain facilitation, glial activation, and the SIRT1/mTOR pathway in the dorsal horn of rats with burn injury pain (BIP). Here we found that the mechanical pain threshold increased in the lycopene group compared with that of the control group, (P < 0.05). Compared with expression in the sham group, mTOR, pS6, p4EBP, GFAP, and Iba-1 decreased and SIRT1 increased in the lycopene group (P < 0.01). Glial activation in the spinal dorsal horn of BIP rats was alleviated by lycopene (P < 0.01). The SIRT1 and mTOR were mainly distributed in neurons in the spinal dorsal horn in the BIP model. Intrathecal injection of 3-MA (a mTOR agonist) or EX-527 (an inhibitor of Sirt1) partially antagonized lycopene-induced analgesia. Intrathecal injection of rapamycin (an mTOR inhibitor) or SRT1720 (an agonist of Sirt1) induced analgesia in BIP rats. 3-MA abrogated the SRT1720-induced analgesic effects. The present data indicated that the SIRT1/mTOR pathway changed in the spinal dorsal horn of BIP rats; Lycopene alleviated the pain sensitization of BIP rats by regulating the SIRT1/mTOR pathway and glial activation in the spinal dorsal horn.

Role of Spinal Cord Akt-mTOR Signaling Pathways in Postoperative Hyperalgesia Induced by Plantar Incision in Mice

Poor postoperative pain (POP) control increases perioperative morbidity, prolongs hospitalization days, and causes chronic pain. However, the specific mechanism(s) underlying POP is unclear and the identification of optimal perioperative treatment remains elusive. Akt and mammalian target of rapamycin (mTOR) are expressed in the spinal cord, dorsal root ganglion, and sensory axons. In this study, we explored the role of Akt and mTOR in pain-related behaviors induced by plantar incision in mice. Plantar incision activated spinal Akt and mTOR in a dose-dependent manner. Pre-treatment with Akt inhibitors intrathecally prevented the activation of mTOR dose-dependently. In addition, blocking the Akt-mTOR signaling cascade attenuated pain-related behaviors and spinal Fos protein expression induced by plantar incision. Our observations demonstrate that Akt-mTOR might be a potential therapeutic target for the treatment of POP.

<p>Sonic Hedgehog Signaling Contributes to Chronic Post-Thoracotomy Pain via Activating BDNF/TrkB Pathway in Rats</p>

Purpose

Some patients undergoing thoracotomy may suffer from chronic post-thoracotomy pain (CPTP). Treatment of CPTP has been a clinical challenge and the underlying mechanisms of CPTP remain elusive. Recently, sonic hedgehog (Shh) signaling has been shown to be associated with various pain states but its role in the pathogenesis of CPTP is still unclear.

Methods

CPTP was induced in rats by thoracotomy. Rats were divided into CPTP group and non-CPTP group based on the mechanical withdrawal threshold (MWT). Rats were administered with Shh signaling inhibitor cyclopamine and activator smoothened agonist (SAG), and then evaluated by MWT and cold allodynia testing. The expressions of Shh signaling (Shh ligand, patched and smoothened receptor, Gli transcription factors), brain-derived neurotrophic factor (BDNF), tropomyosin-related kinase receptor B (Trk-B), phosphatidylinositol 3-kinase (PI3K) and protein kinase B (Akt) in rat T4-5 spinal cord dorsal horn (SDH) were detected by Western blotting and immunohistochemistry.

Results

The expression of Shh signaling significantly increased and the BDNF/TrkB pathway was activated in T4-5 SDH of CPTP rats. Cyclopamine attenuated hyperalgesia and down-regulated the expressions of Gil1, BDNF, p-TrkB, p-PI3K and p-Akt in CPTP rats. SAG induced hyperalgesia in non-CPTP rats and elevated the expressions of Gil1, BDNF, p-TrkB, p-PI3K and p-Akt.

Conclusion

Shh signaling may contribute to CPTP via activating BDNF/TrkB signaling pathway, and inhibition of Shh signaling may effectively alleviate CPTP.

Quercetin attenuates diabetic neuropathic pain by inhibiting mTOR/p70S6K pathway-mediated changes of synaptic morphology and synaptic protein levels in spinal dorsal horn of db/db mice

Numerous studies indicate that the changes of synaptic morphology and synaptic protein levels in spinal dorsal horn neurons contributes to the development and maintenance of neuropathic pain. Quercetin, a bioflavonoid compound, has been shown to have analgesic effect in several pain models. However, the underlying mechanism for quercetin to allieviate pain is unclear. Therefore, in this study, we observed the effect of quercetin on diabetic neuropathic pain in db/db mice and explored the underlying mechanisms. Our results showed that chronic quercetin treatment alleviated thermal hyperalgesia in db/db mice. Moreover, quercetin administration significantly reduced the total dendritic length, the number of dendritic branches, and the dendritic spine density in the spinal dorsal horn neurons of db/db mice. Meanwhile, the up-regulated expressions of synaptic plasticity-associated proteins postsynaptic density protein 95 (PSD-95) and synaptophysin in spinal dorsal horn of db/db mice were decreased by quercetin treatment. In addition, quercetin treatment reduced the phosphorylated levels of mammalian target of rapamycin (mTOR) and p70 ribosomal S6 kinase (p70S6K) in spinal dorsal horn of db/db mice. These results demonstrate that quercetin may alleviate diabetic neuropathic pain by inhibiting mTOR/p70S6K pathway-mediated changes of synaptic morphology and synaptic protein levels in spinal dorsal horn neurons of db/db mice. These findings suggest that quercetin may be a promising therapeutic drug in neuropathic pain.

Exercise intervention attenuates neuropathic pain in diabetes via mechanisms of mammalian target of rapamycin (mTOR)

This study was to examine the role of exercise intervention in modulating neuropathic pain induced by diabetes. Diabetes was induced by streptozotocin (STZ, i.p.) in rats and mechanical hyperalgesia was observed three weeks after STZ. Mechanical withdrawal thresholds were increased after four to five weeks of exercise in STZ rats. We also examined the role of signal of mammalian target of rapamycin (mTOR) in regulating neuropathic pain. Inhibition of neuropathic pain by exercise in STZ rats was accompanied with decreases of p-mTOR, p-S6K1, and p-4E-BP1 in sensory nerves. Blocking mTOR also elevated mechanical withdrawal thresholds in STZ rats. Furthermore, pro-inflammatory IL-6 was greater in sensory nerves of STZ rats. Inhibition of IL-6 decreased mTOR and increased mechanical withdrawal thresholds in STZ rats. Overall, our data suggest the role played by exercise in improving neuropathic pain after STZ and that IL-6-mTOR signal is a part of mechanisms engaged in the effects of exercise.

Progesterone receptor-mediated actions and the treatment of central nervous system disorders: An up-date of the known and the challenge of the unknown

Progesterone has been shown to exert a wide range of remarkable protective actions in experimental models of central nervous system injury or disease. However, the intimate mechanisms involved in each of these beneficial effects are not fully depicted. In this review, we intend to give the readers a thorough revision on what is known about the participation of diverse receptors and signaling pathways in progesterone-mediated neuroprotective, pro-myelinating and anti-inflammatory outcomes, as well as point out to novel regulatory mechanisms that could open new perspectives in steroid-based therapies.

Effects of Early Exposure of Isoflurane on Chronic Pain via the Mammalian Target of Rapamycin Signal Pathway

Persistent post-surgical pain (PPSP) is a chronic pain condition, often with neuropathic features, that occurs in approximately 20% of children who undergo surgery. The biological basis of PPSP has not been elucidated. Anesthetic drugs can have lasting effects on the developing nervous system, although the clinical impact of this phenomenon is unknown. Here, we used a mouse model to test the hypothesis that early developmental exposure to isoflurane causes cellular and molecular alteration in the pain perception circuitry that causes a predisposition to chronic, neuropathic pain via a pathologic upregulation of the mammalian target of the rapamycin (mTOR) signaling pathway. Mice were exposed to isoflurane at postnatal day 7 and select cohorts were treated with rapamycin, an mTOR pathway inhibitor. Behavioral tests conducted 2 months later showed increased evidence of neuropathic pain, which did not occur in rapamycin-treated animals. Immunohistochemistry showed neuronal activity was chronically increased in the insular cortex, anterior cingulate cortex, and spinal dorsal horn, and activity was attenuated by rapamycin. Immunohistochemistry and western blotting (WB) showed a co-incident chronic, abnormal upregulation in mTOR activity. We conclude that early isoflurane exposure alters the development of pain circuits and has the potential to contribute to PPSP and/or other pain syndromes.

A novel role of oxytocin: Oxytocin-induced well-being in humans

We review the involvement of a small molecule, oxytocin, in various effects of physical stimulation of somatosensory organs, mindfulness meditation, emotion and fragrance on humans, and then propose a hypothesis that complex human states and behaviors, such as well-being, social bonding, and emotional behavior, are explained by oxytocin. We previously reported that oxytocin can induce pain relief and described the possibility how oxytocin in the dorsal horn and/or the dorsal root ganglion relieves joint and muscle pain. In the present article, we expand our research target from the physical analgesic effects of oxytocin to its psychologic effects to upregulate well-being and downregulate stress and anxiety. For this purpose, we propose a “hypothalamic-pituitary-adrenal (HPA) axis-oxytocin model” to explain why mindfulness meditation, placebo, and fragrance can reduce stress and anxiety, resulting in contentment. This new proposed model of HPA axis-oxytocin in the brain also provides a target to address other questions regarding emotional behaviors, learning and memory, and excess food intake leading to obesity, aimed at promoting a healthy life.

Post-surgical inhibition of phosphatidylinositol 3-kinase attenuates the plantar incision-induced postoperative pain behavior via spinal Akt activation in male mice

Background

Postoperative pain (POP) is a severe acute pain encountered in patients suffering from an operation, and is less than adequately controlled by the currently available analgesics. Phosphatidylinositol 3-kinase (PI3K) has been reported to have an important role in neuropathic and inflammatory pain. Our previous research revealed that pre-surgical inhibition of spinal PI3K alleviated the pain behavior induced by plantar incision in mice. The aim of this study was to clarify whether post-surgical inhibition of PI3K would attenuate the POP and the underlying mechanisms.

Methods

A POP model was established by plantar incision in Kunming mice. A behavioral test was performed to determine mechanical allodynia, thermal hyperalgesia, and cumulative pain scores. The spinal Fos was detected by immunohistochemistry. The spinal expression of protein kinase B (Akt) or phosphorylated Akt (pAkt) was explored using western blot. The cellular location of pAkt was determined by immunofluorescence.

Results

Post-surgical inhibition of PI3K attenuated mechanical allodynia, thermal hyperalgesia, and cumulative pain scores induced by plantar incision significantly in male mice, and mildly in female mice. Post-surgical inhibition of PI3K attenuated the expression of spinal Fos in male mice. Plantar incision induced a time-dependent expression of spinal pAkt in male mice, which was primarily expressed in the spinal dorsal horn, and localized with the neuron and microglia’s marker. Post-surgical inhibition of PI3K attenuated the activation of Akt induced by plantar incision in male mice as well.

Conclusions

We concluded that post-surgical inhibition of PI3K could attenuate the pain-related behaviors induced by plantar incision, by suppressing the activation of spinal Akt in male mice. This finding might be used in clinical studies to reach a better understanding of POP mechanisms and optimal treatment.

Neuroinflammation and central PI3K/Akt/mTOR signal pathway contribute to bone cancer pain

BACKGROUND

Pain is one of the most common and distressing symptoms suffered by patients with progression of cancer; however, the mechanisms responsible for hyperalgesia are not well understood. Since the midbrain periaqueductal gray is an important component of the descending inhibitory pathway controlling on central pain transmission, in this study, we examined the role for pro-inflammatory cytokines of the periaqueductal gray in regulating mechanical and thermal hyperalgesia evoked by bone cancer via phosphatidylinositide 3-kinase (PI3K)-mammalian target of rapamycin (mTOR) signals.

METHODS

Breast sarcocarcinoma Walker 256 cells were implanted into the tibia bone cavity of rats to induce mechanical and thermal hyperalgesia. Western blot analysis and ELISA were used to examine PI3K/protein kinase B (Akt)/mTOR and pro-inflammatory cytokine receptors and the levels of interleukin (IL)-1β, IL-6, and tumor necrosis factor-alpha (TNF-α).

RESULTS

Protein expression levels of p-PI3K/p-Akt/p-mTOR were amplified in the periaqueductal gray of bone cancer rats, and blocking PI3K-mTOR pathways in the periaqueductal gray attenuated hyperalgesia responses. In addition, IL-1β, IL-6, and TNF-α were elevated in the periaqueductal gray of bone cancer rats, and expression of their respective receptors (namely, IL-1R, IL-6R, and tumor necrosis factor receptor (TNFR) subtype TNFR1) was upregulated. Inhibition of IL-1R, IL-6R, and TNFR1 alleviated mechanical and thermal hyperalgesia in bone cancer rats, accompanied with downregulated PI3K-mTOR.

CONCLUSIONS

Our data suggest that upregulation of pro-inflammatory cytokine signal in the periaqueductal gray of cancer rats amplifies PI3K-mTOR signal in this brain region and alters the descending pathways in regulating pain transmission, and this thereby contributes to the development of bone cancer-induced pain.

Nicotine inhibits rapamycin-induced pain through activating mTORC1/S6K/IRS-1-related feedback inhibition loop

Mammalian target of rapamycin complex 1 (mTORC1) inhibitors increase the incidence of pain in patients, and this finding has been replicated in animal models. However, reports on possible analgesics for this condition are scant. Accumulating evidence finds that nicotinic acetylcholine receptors (nAChRs) are involved in mediating pain. However, whether nicotine, a full agonist of nAChRs, alleviates mTORC1 inhibition-induced pain and its underlying mechanisms remain unknown. In this study, pain was induced in naïve male C57BL/6J mice by intraperitoneally injecting rapamycin acutely or repeatedly. Subsequently, pain thresholds, including mechanical and thermal pain, were measured. The involving signaling pathway was tested using western blot analysis and immunofluorescent assay. Changes in neuronal excitability caused by different treatments were also analyzed using whole-cell recording. Microinjection into the anterior cingulate cortex (ACC) was used to test the role of nAChRs containing the α4β2 or α7 subtype in this brain region in pain modulation. Our results showed that nicotine significantly reduced hyperalgesia in mice that received acute or repeated rapamycin injections, and reversed the effects of rapamycin on the phosphorylation of S6K, 4E-BP1, insulin receptor substrate-1 (IRS-1) at Ser636/639, AKT at Ser473, and ERK at Thr202/Tyr204. Whole-cell recording results showed that nicotine reduced the firing rates of pyramidal neurons in the ACC, and a pharmacological blockade of nAChRs containing the α4β2 or α7 subtype in ACC inhibited the antinociceptive effects of nicotine in mice with rapamycin-induced pain. Our findings indicate that analgesics targeting nAChRs can be developed to help patients with rapamycin-induced pain.

Reciprocal regulation among TRPV1 channels and phosphoinositide 3-kinase in response to nerve growth factor

Although it has been known for over a decade that the inflammatory mediator NGF sensitizes pain-receptor neurons through increased trafficking of TRPV1 channels to the plasma membrane, the mechanism by which this occurs remains mysterious. NGF activates phosphoinositide 3-kinase (PI3K), the enzyme that generates PI(3,4)P₂ and PIP₃, and PI3K activity is required for sensitization. One tantalizing hint came from the finding that the N-terminal region of TRPV1 interacts directly with PI3K. Using two-color total internal reflection fluorescence microscopy, we show that TRPV1 potentiates NGF-induced PI3K activity. A soluble TRPV1 fragment corresponding to the N-terminal Ankyrin repeats domain (ARD) was sufficient to produce this potentiation, indicating that allosteric regulation was involved. Further, other TRPV channels with conserved ARDs also potentiated NGF-induced PI3K activity. Our data demonstrate a novel reciprocal regulation of PI3K signaling by the ARD of TRPV channels.

Translational Control Mechanisms in Persistent Pain

Persistent pain, which is poorly treated and estimated to afflict one third of the world's population, is largely mediated by the sensitization of nociceptive neurons. This sensitization involves de novo gene expression to support biochemical and structural changes required to maintain amplified pain signaling that frequently persists even after injury to tissue resolves. While transcription-dependent changes in gene expression are important, recent work demonstrates that activity-dependent regulation of mRNA translation is key to controlling the cellular proteome and the development and maintenance of persistent pain. In this review, we highlight recent advances in translational regulation of gene expression in nociceptive circuits, with a focus on key signaling pathways and mRNA targets that may be tractable for the creation of next-generation pain therapeutics.

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.

Beneficial Effects of Electroacupuncture on Neuropathic Pain Evoked by Spinal Cord Injury and Involvement of PI3K-mTOR Mechanisms

The purpose of this study was to examine the beneficial effects of electroacupuncture (EA) on neuropathic pain evoked by spinal cord injury (SCI) and determine the underlying molecular mechanisms of these effects. SCI was induced in rats. Behavioral tests were performed to examine pain responses induced by mechanical and thermal stimulation. Western blot analysis was used to measure the protein expression of phosphorylated mammalian target of rapamycin (p-mTOR), mTOR-mediated phosphorylated ribosomal protein S6 kinase beta-1 (p-S6K1), and phosphorylated eukaryotic translation initiation factor 4E-binding protein 1 (p-4E-BP1) in the superficial dorsal horn of the spinal cord. We showed that SCI increased the expression of p-mTOR, p-S6K1, and p-4E-BP1. The EA intervention attenuated the upregulation of mTOR signaling and alleviated mechanical and thermal pain responses in SCI rats. Blocking spinal mTOR by intrathecal injection of rapamycin also inhibited mechanical and thermal pain. In addition, blocking spinal phosphorylated phosphatidylinositide 3-kinase (p-PI3K) pathway attenuated p-mTOR pathways and mechanical and thermal hyperalgesia in SCI rats. EA also decreased the enhanced p-PI3K in the superficial dorsal horn of SCI rats. In conclusion, findings revealed specific signaling pathways that lead to neuropathic pain in response to SCI, including activation of PI3K-mTOR signaling. Further, results link the beneficial role of EA in alleviating SCI-induced neuropathic pain to its effect on these molecular mechanisms.

Rapamycin alleviates proinflammatory cytokines and nociceptive behavior induced by chemotherapeutic paclitaxel

Background/Aims: Paclitaxel is largely used as a chemotherapeutic agent for the treatment of several types of cancers. However, one of the significant limiting complications of paclitaxel is painful peripheral neuropathy during its therapy. The purposes of this study were to examine (1) the effects of blocking mammalian target of rapamycin (mTOR) on mechanical and thermal hypersensitivity evoked by paclitaxel; and (2) the underlying mechanisms responsible for the role of mTOR in regulating paclitaxel-induced neuropathic pain. Methods: Behavioral test was performed to determine mechanical and thermal sensitivity in rats. ELISA was used to examine the levels of proinflammatory cytokines (PICs including IL-1β, IL-6, and TNF-α) and substance P and calcitonin gene-related peptide (CGRP) in the dorsal root ganglion (DGR); and Western blot analysis was used to examine expression of mTOR signal pathway. Results: Paclitaxel increased mechanical and thermal sensitivity as compared with vehicle control animals (P < 0.05 vs. controls). Paclitaxel also amplified the expression of p-mTOR, mTOR-mediated phosphorylation of p70 ribosomal S6 protein kinase 1 (p-S6K1), 4E-binding protein 1 (p-4E-BP1) in the DRG. Blocking mTOR using rapamycin attenuated peripheral painful neuropathy observed in paclitaxel rats (P < 0.05 vs. without rapamycin). This inhibitory effect was accompanied with decreases of IL-1β, IL-6, and TNF-α as well as substance P and CGRP. In addition, inhibition of phosphatidylinositide 3-kinase (p-PI3K) attenuated expression of p-mTOR and PICs/substance P/CGRP in paclitaxel rats and this further attenuated mechanical and thermal hypersensitivity. Conclusions: The data revealed specific signaling pathways leading to paclitaxel-induced peripheral neuropathic pain, including the activation of PI3K-mTOR, PIC signal, and substance P and CGRP. Inhibition of these pathways alleviates neuropathic pain. Targeting one or more of these molecular mediators may present new opportunities for treatment and management of neuropathic pain observed during chemotherapeutic application of paclitaxel.

Histone deacetylase inhibition inhibits brachial plexus avulsion-induced neuropathic pain

INTRODUCTION

Neuropathic pain induced by brachial plexus avulsion (BPA) is a pathological condition. We hypothesized that inhibition of histone deacetylase (HDAC) could suppress BPA-induced neuropathic pain through inhibition of transient reception potential (TRP) overexpression and protein kinase B (Akt)-mediated mammalian target of rapamycin (mTOR) activation.

METHODS

We generated a rat BPA model; administered HDAC inhibitor tricostatin A (TSA) for 7 days postsurgery; and assessed the effects on HDAC expression, Akt phosphorylation, neuroinflammation, and mTOR activation.

RESULTS

TSA treatment alleviated BPA-induced mechanical hyperalgesia, suppressed Akt phosphorylation, and increased HDAC. We found suppressed proinflammatory cytokine levels, TRPV1 and TRPM8 expression, and mTOR activity in TSA-treated BPA rats.

DISCUSSION

Our results suggest that altered HDAC and Akt signaling are involved in BPA-induced neuropathic pain and that inhibition of HDAC could be an effective therapeutic approach in reducing neuropathic pain. Muscle Nerve 58: 434-440, 2018.

Characterization of changes of pain behavior and signal transduction system in food-deprived mice

Fasting in general causes several metabolic changes. In the present study, we examined the possible changes of several types of nociception during the food deprivation were investigated in mice. After the mice were forced into the fasting for 12, 24, or 48 h, the changes of nociception were measured by the tail-flick, writhing, formalin or von-frey tests. We found that the nociceptive behavior induced by intraperitoneally (i.p.) administered acetic acid (writhing response) or intraplantar injection of 5% formalin into the hind-paw were reduced in fasted group. In addition, the tail-flick response and threshold for nociception in mechanical von-frey test were also elevated in fasted group. Moreover, the p-CREB and p-ERK levels in the dorsal root ganglia (DRG) and the spinal cord were reduced in food-deprived group. Furthermore, p-AMPKα₁ expressions in DRG and the spinal cord were up-regulated, whereas p-mTOR in DRG and the spinal cord was down-regulated in food-deprived group. Our results suggest that the chemical, mechanical, and thermal nociceptions appear to be reduced in a food-deprived mouse group. Additionally, reduction of nociception in food-deprived group appears to be closely associated with the expressions of several signal transduction molecules such as ERK, CREB, AMPKα₁ and mTOR proteins in DRG and the spinal cord.

Translation regulation in the spinal dorsal horn - A key mechanism for development of chronic pain

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Spinal sensitization shares molecular mechanisms with hippocampal LTP and memory.

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Changes in mRNA translation are observed in many chronic pain conditions.

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Targeting translational control mechanisms is a promising strategy to inhibit pain.

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Targeting spinal reconsolidation can reverse established hypersensitivity.

Chronic pain is a pathological condition characterized by long-lasting pain after damaged tissue has healed. Chronic pain can be caused and maintained by changes in various components of the pain pathway, including sensory neurons, spinal cord and higher brain centers. Exaggerated sensitivity and responsiveness of spinal nociceptive circuits, representing maladaptive plasticity, play key roles in the amplification of peripheral signals in chronic pain conditions. This spinal amplification mechanism profoundly contributes to the development and maintenance of chronic pain hypersensitivity in response to peripheral injury, and in some cases occurs independently of the peripheral stimulus. Long-lasting changes in the activity of spinal neurons are caused by alterations in their cellular proteome, which relies on de novo gene expression. Recent evidence indicates that translational control of gene expression plays a major role in determining protein levels, and is intricately involved in different forms of intrinsic and synaptic plasticity. In this review, we summarize findings supporting a key role for translational control in spinal cord-dependent mechanisms of chronic pain, and present recent approaches to reverse persistent pain by targeting these mechanisms.

Involvement of pro-inflammation signal pathway in inhibitory effects of rapamycin on oxaliplatin-induced neuropathic pain

Background

Oxaliplatin is a third-generation chemotherapeutic agent that is commonly used to treat metastatic digestive tumors; however, one of the main limiting complications of oxaliplatin is painful peripheral neuropathy. The purpose of this study was to examine the underlying mechanisms by which mammalian target of rapamycin (mTOR) and its signal are responsible for oxaliplatin-evoked neuropathic pain.

Methods

Neuropathic pain was induced by intraperitoneal injection of oxaliplatin in rats. ELISA and Western blot analysis were used to examine the levels of pro-inflammatory cytokines (including interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α) and the expression of mTOR signal pathway.

Results

Oxaliplatin increased mechanical and cold sensitivity as compared with control animals (P < 0.05 vs. control rats). Oxaliplatin also amplified the expression of p-mTOR and mTOR-mediated phosphorylation of p70 ribosomal S6 protein kinase 1 and 4E-binding protein 1 in the lumbar dorsal root ganglion. Blocking mTOR using rapamycin attenuated peripheral painful neuropathy observed in oxaliplatin rats (P < 0.05 vs. vehicle control). This inhibitory effect was accompanied with decreases of IL-1β, IL-6, and TNF-α. In addition, inhibition of phosphatidylinositide 3-kinase (p-PI3K) attenuated the expression of p-mTOR and the levels of pro-inflammatory cytokines in oxaliplatin rats, and this further attenuated mechanical and cold hypersensitivity.

Conclusions

The data revealed specific signaling pathways leading to oxaliplatin-induced peripheral neuropathic pain, including the activation of PI3K-mTOR and pro-inflammatory cytokine signal. Inhibition of these pathways alleviates neuropathic pain. Targeting one or more of these molecular mediators may present new opportunities for treatment and management of neuropathic pain observed during chemotherapeutic application of oxaliplatin.

Urinary bladder organ hypertrophy is partially regulated by Akt1-mediated protein synthesis pathway

AIMS

The present study aims to investigate the role of Akt in the regulation of urinary bladder organ hypertrophy caused by partial bladder outlet obstruction (pBOO).

MAIN METHODS

Male rats were surgically induced for pBOO. Real-time PCR and western blot were used to examine the levels of mRNA and protein. A phosphoinositide 3-kinase (PI3K) inhibitor LY294002 was used to inhibit the activity of endogenous Akt.

KEY FINDINGS

The urinary bladder developed hypertrophy at 2 weeks of pBOO. The protein but not mRNA levels of type I collagen and α-smooth muscle actin (αSMA) were increased in pBOO bladder when compared to sham control. The phosphorylation (activation) levels of Akt1 (p-Ser⁴⁷³), mammalian target of rapamycin (mTOR), p70S6 kinase (p70S6K), and 4E-BP1 were also increased in pBOO bladder. LY294002 treatment reduced the phosphorylation levels of Akt1 and 4E-BP1, and the protein levels of type I collagen and αSMA in pBOO bladder. The mRNA and protein levels of proliferating cell nuclear antigen (PCNA) were increased in pBOO bladder, and PCNA up-regulation occurred in urothelial not muscular layer. LY294002 treatment had no effect on the mRNA and protein levels of PCNA in pBOO bladder. LY294002 treatment partially reduced the bladder weight caused by pBOO.

SIGNIFICANCE

pBOO-induced urinary bladder hypertrophy is attributable to fibrosis, smooth muscle cellular hypertrophy, and urothelium cell hyper-proliferation. Akt1-mediated protein synthesis in pBOO bladder contributes to type I collagen and αSMA but not PCNA up-regulation. Target of Akt1 is necessary but not sufficient in treatment of urinary bladder hypertrophy following pBOO.

Revealing brain mechanisms of mTOR-mediated translational regulation: Implications for chronic pain

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mTOR is a major regulator of protein translation.

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mTOR serves an important role in neural plasticity.

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mTOR signalling in the brain as a pathology for neurological disorder is known.

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mTOR signalling in the brain as a chronic pain mechanism is understudied.

In the spinal cord, altered protein transcription and translation have received a lot of recent attention for their role in neural plasticity, a major mechanism leading to the development of chronic pain. However, changes in brain plasticity are also associated with the maintenance of pain symptoms, but these cellular mechanisms remain less clear. The mechanistic/mammalian target of rapamycin (mTOR) is a master regulator of protein synthesis, and controls several neuronal functions, including neural plasticity. While aberrant changes in mTOR signaling are associated with sensitization of the pain pathway (sensory neurons and spinal cord), there are various nervous system diseases that have pain as a comorbidity and altered mTOR activity in the brain. Here, we provide a brief review of mTOR changes in the brain that are associated with some neurological disorders and focus on how these changes may be relevant to the pain of the underlying condition and chronic pain itself.

Therapeutic opportunities for pain medicines via targeting of specific translation signaling mechanisms

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A common underlying cause of chronic pain is a phenotypic change in nociceptors in the peripheral nervous system.

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Translation regulation signaling pathways control gene expression changes that drive chronic pain.

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We focus on developments in pharmacology around translation regulation signaling that may yield new pain therapeutics.

As the population of the world ages and as more and more people survive diseases that used to be primary causes of mortality, the incidence of severe chronic pain in most of the world has risen dramatically. This type of pain is very difficult to treat and the opioid overdose epidemic that has become a leading cause of death in the United States and other parts of the world highlights the urgent need to develop new pain therapeutics. A common underlying cause of severe chronic pain is a phenotypic change in pain-sensing neurons in the peripheral nervous system called nociceptors. These neurons play a vital role in detecting potentially injurious stimuli, but when these neurons start to detect very low levels of inflammatory meditators or become spontaneously active, they send spurious pain signals to the brain that are significant drivers of chronic pain. An important question is what drives this phenotypic shift in nociceptors from quiescence under most conditions to sensitization to a broad variety of stimuli and spontaneous activity. The goal of this review is to discuss the critical role that specific translation regulation signaling pathways play in controlling gene expression changes that drive nociceptor sensitization and may underlie the development of spontaneous activity. The focus will be on advances in technologies that allow for identification of such targets and on developments in pharmacology around translation regulation signaling that may yield new pain therapeutics. A key advantage of pharmacological manipulation of these signaling events is that they may reverse phenotypic shifts in nociceptors that drive chronic pain thereby creating the first generation of disease modifying drugs for chronic pain.

Research progress of mechanisms and drug therapy for neuropathic pain

Neuropathic pain is maladaptive pain caused by injury or dysfunction in peripheral and central nervous system, and remains a worldwide thorny problem leading to decreases in physical and mental quality of people's life. Currently, drug therapy is the main treatment regimen for resolving pain, while effective drugs are still unmet in medical need, and commonly used drugs such as anticonvulsants and antidepressants often make patients experience adverse drug reactions like dizziness, somnolence, severe headache, and high blood pressure. Thus, in this review we overview the anatomical physiology, underlying mechanisms of neuropathic pain to provide a better understanding in the initiation, development, maintenance, and modulation of this pervasive disease, and inspire research in the unclear mechanisms as well as potential targets. Furthermore, we summarized the existing drug therapies and new compounds that have shown antalgic effects in laboratory studies to be helpful for rational regimens in clinical treatment and promotion in novel drug discovery.

Adenosine Monophosphate-activated Protein Kinase (AMPK) Activators For the Prevention, Treatment and Potential Reversal of Pathological Pain

Pathological pain is an enormous medical problem that places a significant burden on patients and can result from an injury that has long since healed or be due to an unidentifiable cause. Although treatments exist, they often either lack efficacy or have intolerable side effects. More importantly, they do not reverse the changes in the nervous system mediating pathological pain, and thus symptoms often return when therapies are discontinued. Consequently, novel therapies are urgently needed that have both improved efficacy and disease-modifying properties. Here we highlight an emerging target for novel pain therapies, adenosine monophosphate-activated protein kinase (AMPK). AMPK is capable of regulating a variety of cellular processes including protein translation, activity of other kinases, and mitochondrial metabolism, many of which are thought to contribute to pathological pain. Consistent with these properties, preclinical studies show positive, and in some cases disease-modifying effects of either pharmacological activation or genetic regulation of AMPK in models of nerve injury, chemotherapy-induced peripheral neuropathy (CIPN), postsurgical pain, inflammatory pain, and diabetic neuropathy. Given the AMPK-activating ability of metformin, a widely prescribed and well-tolerated drug, these preclinical studies provide a strong rationale for both retrospective and prospective human pain trials with this drug. They also argue for the development of novel AMPK activators, whether orthosteric, allosteric, or modulators of events upstream of the kinase. Together, this review will present the case for AMPK as a novel therapeutic target for pain and will discuss future challenges in the path toward development of AMPK-based pain therapeutics.

Epiregulin and EGFR interactions are involved in pain processing

The EGFR belongs to the well-studied ErbB family of receptor tyrosine kinases. EGFR is activated by numerous endogenous ligands that promote cellular growth, proliferation, and tissue regeneration. In the present study, we have demonstrated a role for EGFR and its natural ligand, epiregulin (EREG), in pain processing. We show that inhibition of EGFR with clinically available compounds strongly reduced nocifensive behavior in mouse models of inflammatory and chronic pain. EREG-mediated activation of EGFR enhanced nociception through a mechanism involving the PI3K/AKT/mTOR pathway and matrix metalloproteinase-9. Moreover, EREG application potentiated capsaicin-induced calcium influx in a subset of sensory neurons. Both the EGFR and EREG genes displayed a genetic association with the development of chronic pain in several clinical cohorts of temporomandibular disorder. Thus, EGFR and EREG may be suitable therapeutic targets for persistent pain conditions.

Regulation of transient receptor potential cation channel subfamily V1 protein synthesis by the phosphoinositide 3-kinase/Akt pathway in colonic hypersensitivity

The transient receptor potential cation channel subfamily V member 1 (TRPV1), also known as the capsaicin receptor or vanilloid receptor 1 (VR1), is expressed in nociceptive neurons in the dorsal root ganglia (DRG) and participates in the transmission of pain. The present study investigated the underlying molecular mechanisms by which TRPV1 was regulated by nerve growth factor (NGF) signaling pathways in colonic hypersensitivity in response to colitis. We found that during colitis TRPV1 protein levels were significantly increased in specifically labeled colonic afferent neurons in both L1 and S1 DRGs. TRPV1 protein up-regulation in DRG was also enhanced by NGF treatment. We then found that TRPV1 protein up-regulation in DRG was regulated by activation of the phosphoinositide 3-kinase (PI3K)/Akt pathway both in vivo and in vitro. Suppression of endogenous PI3K/Akt activity during colitis or NGF treatment with a specific PI3K inhibitor LY294002 reduced TRPV1 protein production in DRG neurons, and also reduced colitis-evoked TRPV1-mediated visceral hypersensitivity tested by hyper-responsiveness to colorectal distention (CRD) and von Frey filament stimulation of abdomen. Further studies showed that TRPV1 mRNA levels in the DRG were not regulated by either colitis or NGF. We then found that an up-regulation of the protein synthesis pathway was involved by which both colitis and NGF caused a PI3K-dependent increase in the phosphorylation level of eukaryotic translation initiation factor 4E-binding protein (4E-BP)1. These results suggest a novel mechanism in colonic hypersensitivity which involves PI3K/Akt-mediated TRPV1 protein, not mRNA, up-regulation in primary afferent neurons, likely through activation of the protein synthesis pathways.

Inflammation-induced GluA1 trafficking and membrane insertion of Ca2+ permeable AMPA receptors in dorsal horn neurons is dependent on spinal tumor necrosis factor, PI3 kinase and protein kinase A

Peripheral inflammation induces sensitization of nociceptive spinal cord neurons. Both spinal tumor necrosis factor (TNF) and neuronal membrane insertion of Ca²⁺ permeable AMPA receptor (AMPAr) contribute to spinal sensitization and resultant pain behavior, molecular mechanisms connecting these two events have not been studied in detail. Intrathecal (i.t.) injection of TNF-blockers attenuated paw carrageenan-induced mechanical and thermal hypersensitivity. Levels of GluA1 and GluA4 from dorsal spinal membrane fractions increased in carrageenan-injected rats compared to controls. In the same tissue, GluA2 levels were not altered. Inflammation-induced increases in membrane GluA1 were prevented by i.t. pre-treatment with antagonists to TNF, PI3K, PKA and NMDA. Interestingly, administration of TNF or PI3K inhibitors followed by carrageenan caused a marked reduction in plasma membrane GluA2 levels, despite the fact that membrane GluA2 levels were stable following inhibitor administration in the absence of carrageenan. TNF pre-incubation induced increased numbers of Co²⁺ labeled dorsal horn neurons, indicating more neurons with Ca²⁺ permeable AMPAr. In parallel to Western blot results, this increase was blocked by antagonism of PI3K and PKA. In addition, spinal slices from GluA1 transgenic mice, which had a single alanine replacement at GluA1 ser 845 or ser 831 that prevented phosphorylation, were resistant to TNF-induced increases in Co²⁺ labeling. However, behavioral responses following intraplantar carrageenan and formalin in the mutant mice were no different from littermate controls, suggesting a more complex regulation of nociception. Co-localization of GluA1, GluA2 and GluA4 with synaptophysin on identified spinoparabrachial neurons and their relative ratios were used to assess inflammation-induced trafficking of AMPAr to synapses. Inflammation induced an increase in synaptic GluA1, but not GluA2. Although total GluA4 also increased with inflammation, co-localization of GluA4 with synaptophysin, fell short of significance. Taken together these data suggest that peripheral inflammation induces a PI3K and PKA dependent TNFR1 activated pathway that culminates with trafficking of calcium permeable AMPAr into synapses of nociceptive dorsal horn projection neurons.