Melatonin relieves neuropathic allodynia through spinal MT2-enhanced PP2Ac and downstream HDAC4 shuttling-dependent epigenetic modification of hmgb1 transcription

Pathology of pain and its implications for therapeutic interventions

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

Histone deacetylase as emerging pharmacological therapeutic target for neuropathic pain: From epigenetic to selective drugs

BACKGROUND

Neuropathic pain remains a formidable challenge for modern medicine. The first-line pharmacological therapies exhibit limited efficacy and unfavorable side effect profiles, highlighting an unmet need for effective therapeutic medications. The past decades have witnessed an explosion in efforts to translate epigenetic concepts into pain therapy and shed light on epigenetics as a promising avenue for pain research. Recently, the aberrant activity of histone deacetylase (HDAC) has emerged as a key mechanism contributing to the development and maintenance of neuropathic pain.

AIMS

In this review, we highlight the distinctive role of specific HDAC subtypes in a cell-specific manner in pain nociception, and outline the recent experimental evidence supporting the therapeutic potential of HDACi in neuropathic pain.

METHODS

We have summarized studies of HDAC in neuropathic pain in Pubmed.

RESULTS

HDACs, widely distributed in the neuronal and non-neuronal cells of the dorsal root ganglion and spinal cord, regulate gene expression by deacetylation of histone or non-histone proteins and involving in increased neuronal excitability and neuroinflammation, thus promoting peripheral and central sensitization. Importantly, pharmacological manipulation of aberrant acetylation using HDAC-targeted inhibitors (HDACi) has shown promising pain-relieving properties in various preclinical models of neuropathic pain. Yet, many of which exhibit low-specificity that may induce off-target toxicities, underscoring the necessity for the development of isoform-selective HDACi in pain management.

CONCLUSIONS

Abnormally elevated HDACs promote neuronal excitability and neuroinflammation by epigenetically modulating pivotal gene expression in neuronal and immune cells, contributing to peripheral and central sensitization in the progression of neuropathic pain, and HDACi showed significant efficacy and great potential for alleviating neuropathic pain.

Artesunate Induces Ferroptosis in Hepatic Stellate Cells and Alleviates Liver Fibrosis via the ROCK1/ATF3 Axis

Background and Aims

Development of fibrosis in chronic liver disease requires activation of hepatic stellate cells (HSCs) and leads to a poor outcome. Artesunate (Art) is an ester derivative of artemisinin that can induce ferroptosis in HSCs, and activated transcriptional factor 3 (ATF3) is an ATF/CREB transcription factor that is induced in response to stress. In this study, we examined the role of the Rho-associated protein kinase 1 (ROCK1)/ATF3 axis in Art-induced ferroptosis in HSCs.

Methods

HSC activation and ferroptosis were studied in vitro by western blotting, polymerase chain reaction, immunofluorescence, and other assays. ATF3 electrophoretic mobility and ROCK1 protein stability were assayed by western blotting. Immunoprecipitation was used to detect the interaction of ROCK1 and ATF3, as well as ATF3 phosphorylation. A ubiquitination assay was used to verify ROCK1 degradation. Atf3-interfering and Rock1-overexpressing mice were constructed to validate the anti-hepatic fibrosis activity of Art in vivo.

Results

Art induced ferroptosis in HSCs following glutathione-dependent antioxidant system inactivation resulting from nuclear accumulation of unphosphorylated ATF3 mediated by ROCK1-ubiquitination in vitro. Art also decreased carbon tetrachloride-induced liver fibrosis in mice, which was reversed by interfering with Atf3 or overexpressing Rock1.

Conclusions

The ROCK1/ATF3 axis was involved in liver fibrosis and regulation of ferroptosis, which provides an experimental basis for further study of Art for the treatment of liver fibrosis.

Therapeutic targets and potential delivery systems of melatonin in osteoarthritis

Osteoarthritis (OA) is a highly prevalent age-related musculoskeletal disorder that typically results in chronic pain and disability. OA is a multifactorial disease, with increased oxidative stress, dysregulated inflammatory response, and impaired matrix metabolism contributing to its onset and progression. The neurohormone melatonin, primarily synthesized by the pineal gland, has emerged as a promising therapeutic agent for OA due to its potential to alleviate inflammation, oxidative stress, and chondrocyte death with minimal adverse effects. The present review provides a comprehensive summary of the current understanding regarding melatonin as a promising pharmaceutical agent for the treatment of OA, along with an exploration of various delivery systems that can be utilized for melatonin administration. These findings may provide novel therapeutic strategies and targets for inhibiting the advancement of OA.

Melatonin Relieves Paclitaxel-Induced Neuropathic Pain by Regulating pNEK2-Dependent Epigenetic Pathways in DRG Neurons

The neurohormone melatonin (MLT) demonstrates promising potential in ameliorating neuropathic pain induced by paclitaxel (PTX) chemotherapy. However, little is known about its protective effect on dorsal root ganglion (DRG) neurons in neuropathic pain resulting from the chemotherapeutic drug PTX. Here, PTX-treated rats revealed that intrathecal administration of MLT dose-dependently elevated hind paw withdrawal thresholds and latency, indicating that MLT significantly reversed PTX-induced neuropathic pain. Mechanistically, the analgesic effects of MLT were found to be mediated via melatonin receptor 2 (MT2), as pretreatment with an MT2 receptor antagonist inhibited these effects. Moreover, intrathecal MLT injection reversed the pNEK2-dependent epigenetic program induced by PTX. All of the effects caused by MLT were blocked by pretreatment with an MT2 receptor-selective antagonist, 4P-PDOT. Remarkably, multiple MLT administered during PTX treatment (PTX+MLTs) exhibited not only rapid but also lasting reversal of allodynia/hyperalgesia compared to single-bolus MLT administered after PTX treatment (PTX+MLT). In addition, PTX+MLTs exhibited greater efficacy in reversing PTX-induced alterations in pRSK2, pNEK2, JMJD3, H3K27me3, and TRPV1 expression and interaction in DRG neurons than PTX+MLT. These results indicated that MLT administered during PTX treatment reduced the incidence and/or severity of neuropathy and had a better inhibitory effect on the pNEK2-dependent epigenetic program compared to MLT administered after PTX treatment. In conclusion, MLT/MT2 is a promising therapy for the treatment of pNEK2-dependent painful neuropathy resulting from PTX treatment. MLT administered during PTX chemotherapy may be more effective in the prevention or reduction of PTX-induced neuropathy and maintaining quality.

Melatonin Improves Mitochondrial Dysfunction and Attenuates Neuropathic Pain by Regulating SIRT1 in Dorsal Root Ganglions

Neuropathic pain is a debilitating chronic pain condition and is refractory to the currently available treatments. Emerging evidence suggests that melatonin exerts analgesic effects in rodent models of neuropathic pain. Nevertheless, the exact underlying mechanisms of the analgesic effects of melatonin on neuropathic pain are largely unknown. Here, we observed that spinal nerve ligation (SNL) in rats L5 and L6 induced an obvious decrease in the 50% paw withdrawal threshold (PWT) and paw withdrawal latency (PWL), indicating the induction of mechanical allodynia and the hyperalgesia, and melatonin prevented the genesis and maintenance of mechanical allodynia and the hyperalgesia. Notably, the inhibitory action of melatonin on SNL-induced mechanical allodynia and heat hypersensitivity was inhibited by a SIRT1 inhibitor (EX527). Melatonin treatment increased the expression of neuronal sirtuin1 (SIRT1) in DRGs following nerve injury. Furthermore, melatonin treatment restored the injury-dependent decrease in mitochondrial membrane potential and peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) and reduced the injury-dependent increase in hydrogen peroxide and 8-hydroxy-2-deoxyguanosine (8-OHdG), which was inhibited by EX527. In addition, we found that EX527 impeded the inhibitory effects of melatonin on the SNL-induced increased expression of cytokines tumor necrosis factor-alpha (TNF-α) and interleukin-1 beta (IL-1β). In conclusion, the above data demonstrated that melatonin alleviated mechanical allodynia and hyperalgesia induced by peripheral nerve injury via SIRT1 activation. Melatonin resolved mitochondrial dysfunction-oxidative stress-dependent and neuroinflammation mechanisms that were driven by SIRT1 after nerve injury.

Supraspinal melatonin MT2 receptor agonism alleviates pain via a neural circuit that recruits mu opioid receptors

Melatonin, through its G protein-coupled receptor (GPCR) (MTNR1B gene) MT₂ , is implicated in analgesia, but the relationship between MT₂ receptors and the opioid system remains elusive. In a model of rodent neuropathic pain (spared nerve injured [SNI]), the selective melatonin MT₂ agonist UCM924 reversed the allodynia (a pain response to a non-noxious stimulus), and this effect was nullified by the pharmacological blockade or genetic inactivation of the mu opioid receptor (MOR), but not the delta opioid receptor (DOR). Indeed, SNI MOR, but not DOR knockout mice, did not respond to the antiallodynic effects of the UCM924. Similarly, the nonselective opioid antagonist naloxone and the selective MOR antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) blocked the effects of UCM924 in SNI rats, but not the DOR antagonist naltrindole (NTI). Electrophysiological recordings in the rostral-ventromedial medulla (RVM) revealed that the typical reduction of the firing activity of pronociceptive ON-cells, and the enhancement of the firing of the antinociceptive OFF-cells, induced by the microinjection of the MT₂ agonist UCM924 into the ventrolateral periaqueductal gray (vlPAG) were blocked by MOR, but not DOR, antagonism. Immunohistochemistry studies showed that MT₂ receptors are expressed in both excitatory (CaMKIIα⁺ ) and inhibitory (GAD65⁺ ) neuronal cell bodies in the vlPAG (~2.16% total), but not RVM. Only 0.20% of vlPAG neurons coexpressed MOR and MT₂ receptors. Finally, UCM924 treatment induced an increase in the enkephalin precursor gene (PENK) in the PAG of SNI mice. Collectively, the melatonin MT₂ receptor agonism requires MORs to exert its antiallodynic effects, mostly through an interneuronal circuit involving MOR and MT₂ receptors.

HMGB1 in the mPFC governs comorbid anxiety in neuropathic pain

Background

Whether neuroinflammation causes comorbid mood disorders in neuropathic pain remains elusive. Here we investigated the role of high mobility group box 1 protein (HMGB1), a proinflammatory cytokine, in the medial prefrontal cortex (mPFC) in anxiety comorbidity of neuropathic pain.

Methods

Neuropathic pain was induced by partial transection of the infraorbital nerve (p-IONX) or partial sciatic nerve ligation (PSL) in mice and evaluated by measuring nociceptive thresholds to mechanical and heat stimulation. Anxiety-like behaviors were assessed by elevated plus maze, light dark box and open field tests. Aversive or anti-aversive effect was detected by conditioned place preference test. Neuronal activity was evaluated by single-unit and patch clamp recordings. The contribution of mPFC pyramidal neurons to anxiety was further examined by selectively inhibiting them by optogenetics. HMGB1 expression was measured by immunohistochemistry and western blotting. Antagonism of HMGB1 was achieved by injecting anti-HMGB1 monoclonal antibody (mAb) intracerebrally or intraperitoneally.

Results

Anxiety-like behaviors were presented earlier after p-IONX than after PSL. HMGB1 expression was upregulated in the mPFC temporally in parallel to anxiety onset, rather than in other regions associated with anxiety. The upregulation of HMGB1 expression and its translocation from the nucleus to cytoplasm in the mPFC occurred predominantly in neurons and were accompanied with activation of microglia and astrocytes. Infusion of anti-HMGB1 mAb into the mPFC during the early and late phases after either p-IONX or PSL alleviated anxiety-like behaviors and aversion without changing pain sensitization, while local infusion of exogenous ds-HMGB1, the proinflammatory form of HMGB1, into the mPFC induced anxiety and aversion but not pain sensitization in naïve mice. In addition to reversing established pain sensitization and anxiety simultaneously, intraperitoneal injection of anti-HMGB1 mAb reduced HMGB1 upregulation and suppressed the hyperexcitability of layer 2/3 pyramidal neurons in the mPFC after p-IONX. Moreover, optogenetic inhibition of mPFC pyramidal neurons alleviated anxiety in p-IONX mice.

Conclusion

These results demonstrate that HMGB1 in the mPFC drives and maintains anxiety comorbidity in neuropathic pain by increasing the excitability of layer 2/3 pyramidal neurons, and justify antagonism of HMGB1, e.g., neutralization by mAb, as a promising therapeutic strategy for neuropathic pain with anxiety comorbidity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s10194-022-01475-z.

PP2A and cancer epigenetics: a therapeutic opportunity waiting to happen

The epigenetic state of chromatin is altered by regulators which influence gene expression in response to environmental stimuli. While several post-translational modifications contribute to chromatin accessibility and transcriptional programs, our understanding of the role that specific phosphorylation sites play is limited. In cancer, kinases and phosphatases are commonly deregulated resulting in increased oncogenic signaling and loss of epigenetic regulation. Aberrant epigenetic states are known to promote cellular plasticity and the development of therapeutic resistance in many cancer types, highlighting the importance of these mechanisms to cancer cell phenotypes. Protein Phosphatase 2A (PP2A) is a heterotrimeric holoenzyme that targets a diverse array of cellular proteins. The composition of the PP2A complex influences its cellular targets and activity. For this reason, PP2A can be tumor suppressive or oncogenic depending on cellular context. Understanding the nuances of PP2A regulation and its effect on epigenetic alterations can lead to new therapeutic avenues that afford more specificity and contribute to the growth of personalized medicine in the oncology field. In this review, we summarize the known PP2A-regulated substrates and potential phosphorylation sites that contribute to cancer cell epigenetics and possible strategies to therapeutically leverage this phosphatase to suppress tumor growth.

Melatonin Moderates the Triangle of Chronic Pain, Sleep Architecture and Immunometabolic Traffic

Preclinical as well as human studies indicate that melatonin is essential for a physiological sleep state, promotes analgesia and is involved in immunometabolic signaling by regulating neuroinflammatory pathways. Experimental and clinical neuromodulation studies for chronic pain treatment suggest that neurostimulation therapies such as spinal cord stimulation, vagus nerve stimulation and dorsal root ganglion stimulation have an impact on circulating inflammatory mediators in blood, cerebrospinal fluid and saliva. Herein, we provide an overview of current literature relevant for the shared pathways of sleep, pain and immunometabolism and elaborate the impact of melatonin on the crossroad of sleep, chronic pain and immunometabolism. Furthermore, we discuss the potential of melatonin as an adjunct to neurostimulation therapies. In this narrative review, we addressed these questions using the following search terms: melatonin, sleep, immunometabolism, obesity, chronic pain, neuromodulation, neurostimulation, neuroinflammation, molecular inflammatory phenotyping. So far, the majority of the published literature is derived from experimental studies and studies specifically assessing these relationships in context to neurostimulation are sparse. Thus, the adjunct potential of melatonin in clinical neurostimulation has not been evaluated under the umbrella of randomized-controlled trials and deserves increased attention as melatonin interacts and shares pathways relevant for noninvasive and invasive neurostimulation therapies.

Melatonin Abates TMJOA Chronic Pain by MT2R in Trigeminal Ganglion Neurons

Temporomandibular joint osteoarthritis (TMJOA) is one of the most common diseases causing chronic pain in the oral and maxillofacial region. So far, there are few ways to relieve the pain of TMJOA. Melatonin (MT) has a good analgesic effect in many diseases, including fibromyalgia, neuropathic pain, chronic headache, and burn pain, with very low acute toxicity and side effects. This study was to investigate the role and mechanism of MT in TMJOA chronic pain. In rats TMJOA chronic pain occurred at day 14 after an intra-temporomandibular joint injection of monosodium iodoacetate, which we previously reported. The enzyme-linked immunosorbent assay results showed that MT levels were higher in the synovial fluid from patients and rats with TMJOA as compared with those from control. Fluorescent retrograde tracing (Dil) identified that upregulation of MT type 2 receptor (MT₂R) in trigeminal ganglion (TG) neurons innervating rat temporomandibular joints was accompanied by TMJOA chronic pain. Nociceptive behavior as assessed by von Frey and the Rat Grimace Scale demonstrated that exogenous administration of MT relieved chronic pain in TMJOA rats, whereas blocking MT₂R with 4P-PDOT reversed the analgesic effect of MT. Immunofluorescence analysis also confirmed that MT inhibited CGRP and IB4 expression of TG neurons, and this inhibition was reversed by administering the MT₂R antagonist in TMJOA rats. By using Fluo-3 AM-based calcium imaging in vitro, MT elicited calcium transients in Dil⁺ TG neurons, which were significantly abolished by 4P-PDOT. Collectively, this study suggested that MT relieves the TMJOA chronic pain of rats through downregulation of sensitized CGRP⁺ and IB4⁺ neurons in TG via MT₂R. This will be helpful for health care professionals utilizing MT as an option against TMJOA chronic pain.

Inhibiting MLL1-WDR5 interaction ameliorates neuropathic allodynia by attenuating histone H3 lysine 4 trimethylation-dependent spinal mGluR5 transcription

ABSTRACT

Mixed lineage leukemia 1 (MLL1)-mediated histone H3 lysine 4 trimethylation (H3K4me3) of a subset of genes has been linked to the transcriptional activation critical for synaptic plasticity, but its potential contribution to neuropathic allodynia development remains poorly explored. Here, we show that MLL1, which is induced in dorsal horn neuron after spinal nerve ligation (SNL), is responsible for mechanical allodynia and increased H3K4me3 at metabotropic glutamate receptor subtype 5 (mGluR5) promoter. Moreover, SNL induced WD (Trp-Asp) repeat domain 5 subunit (WDR5) expression as well as the MLL1-WDR5 interaction accompany with H3K4me3 enrichment and transcription of mGluR5 gene in the dorsal horn in neuropathic allodynia progression. Conversely, WDR5-0103, a novel inhibitor of the MLL1-WDR5 interaction, reversed SNL-induced allodynia and inhibited SNL-enhanced mGluR5 transcription/expression as well as MLL1, WDR5, and H3K4me3 at the mGluR5 promoter in the dorsal horn. Furthermore, disrupting the expression of MLL1 or WDR5 using small interfering RNA attenuated mechanical allodynia and reversed protein transcription/expression and complex localizing at mGluR5 promoter in the dorsal horn induced by SNL. This finding revealed that MLL1-WDR5 complex integrity regulates MLL1 and WDR5 recruitment to H3K4me3 enrichment at mGluR5 promoter in the dorsal horn underlying neuropathic allodynia. Collectively, our findings indicated that SNL enhances the MLL1-WDR5 complex, which facilitates MLL1 and WDR5 recruitment to H3K4me3 enrichment at mGluR5 promoter in spinal plasticity contributing to neuropathic allodynia pathogenesis.

Blocking the Spinal Fbxo3/CARM1/K+ Channel Epigenetic Silencing Pathway as a Strategy for Neuropathic Pain Relief

Many epigenetic regulators are involved in pain-associated spinal plasticity. Coactivator-associated arginine methyltransferase 1 (CARM1), an epigenetic regulator of histone arginine methylation, is a highly interesting target in neuroplasticity. However, its potential contribution to spinal plasticity-associated neuropathic pain development remains poorly explored. Here, we report that nerve injury decreased the expression of spinal CARM1 and induced allodynia. Moreover, decreasing spinal CARM1 expression by Fbxo3-mediated CARM1 ubiquitination promoted H3R17me2 decrement at the K+ channel promoter, thereby causing K+ channel epigenetic silencing and the development of neuropathic pain. Remarkably, in naïve rats, decreasing spinal CARM1 using CARM1 siRNA or a CARM1 inhibitor resulted in similar epigenetic signaling and allodynia. Furthermore, intrathecal administration of BC-1215 (a novel Fbxo3 inhibitor) prevented CARM1 ubiquitination to block K+ channel gene silencing and ameliorate allodynia after nerve injury. Collectively, the results reveal that this newly identified spinal Fbxo3-CARM1-K+ channel gene functional axis promotes neuropathic pain. These findings provide essential insights that will aid in the development of more efficient and specific therapies against neuropathic pain.

Sirtuin 1-dependent regulation of high mobility box 1 in hypoxia-reoxygenated brain microvascular endothelial cells: roles in neuronal amyloidogenesis

Hypoxia-reperfusion injury is one of the major risk factors for neurodegeneration. However, it is unclear whether ischaemic damage in brain microvascular endothelial cells plays roles in neurodegeneration, particularly in the amyloidogenic changes contributing to the development of Alzheimer's disease (AD) pathologies. Therefore, we investigated the roles of hypoxia-reoxygenation (H/R)-induced release of high mobility group box protein 1 (HMGB1), a risk molecule for AD pathogenesis in the ischaemic damaged brain, from human brain microvascular endothelial cells (HBMVECs) in neuronal amyloid-beta (Aβ) production. H/R increased nuclear-cytosolic translocation and secretion of HMGB1 in HBMVECs, along with increased permeability and HMGB1-dependent p-c-Jun activation. In addition, H/R increased the expression of Sirtuin 1 (Sirt1), coincident with an increase of intracellular Sirt1-HMGB1 binding in HBMVECs. H/R increased the acetylation of HMGB1 and extracellular secretion, which was significantly inhibited by Sirt1 overexpression. Furthermore, Sirt1 contributed to autophagy-mediated endogenous HMGB1 degradation. More importantly, treatment of neuronal cells with conditioned medium from H/R-stimulated HBMVECs (H/R-CM) activated their amyloidogenic pathways. The neuronal amyloidogenic changes (i.e. increased levels of extracellular Aβ40 and Aβ42) by H/R-CM from HBMVECs were further increased by Sirt1 inhibition, which was significantly suppressed by neutralization of the HMGB1 in H/R-CM. Collectively, our results suggest that HMGB1 derived from H/R-stimulated HBMVECs contributes to amyloidogenic pathways in neurons playing roles in the pathogenesis of AD, which are regulated by endothelial Sirt1.

Melatonin Rescues the Dendrite Collapse Induced by the Pro-Oxidant Toxin Okadaic Acid in Organotypic Cultures of Rat Hilar Hippocampus

The pro-oxidant compound okadaic acid (OKA) mimics alterations found in Alzheimer's disease (AD) as oxidative stress and tau hyperphosphorylation, leading to neurodegeneration and cognitive decline. Although loss of dendrite complexity occurs in AD, the study of this post-synaptic domain in chemical-induced models remains unexplored. Moreover, there is a growing expectation for therapeutic adjuvants to counteract these brain dysfunctions. Melatonin, a free-radical scavenger, inhibits tau hyperphosphorylation, modulates phosphatases, and strengthens dendritic arbors. Thus, we determined if OKA alters the dendritic arbors of hilar hippocampal neurons and whether melatonin prevents, counteracts, or reverses these damages. Rat organotypic cultures were incubated with vehicle, OKA, melatonin, and combined treatments with melatonin either before, simultaneously, or after OKA. DNA breaks were assessed by TUNEL assay and nuclei were counterstained with DAPI. Additionally, MAP2 was immunostained to assess the dendritic arbor properties by the Sholl method. In hippocampal hilus, OKA increased DNA fragmentation and reduced the number of MAP2(+) cells, whereas melatonin protected against oxidation and apoptosis. Additionally, OKA decreased the dendritic arbor complexity and melatonin not only counteracted, but also prevented and reversed the dendritic arbor retraction, highlighting its role in post-synaptic domain integrity preservation against neurodegenerative events in hippocampal neurons.

Nociceptive responses in melatonin MT2 receptor knockout mice compared to MT1 and double MT1 /MT2 receptor knockout mice

Melatonin, a neurohormone that binds to two G protein-coupled receptors MT₁ and MT_2, is involved in pain regulation, but the distinct role of each receptor has yet to be defined. We characterized the nociceptive responses of mice with genetic inactivation of melatonin MT₁ (MT₁ ^-/- ), or MT₂ (MT₂ ^-/- ), or both MT₁ /MT₂ (MT₁ ^-/- /MT₂ ^-/- ) receptors in the hot plate test (HPT), and the formalin test (FT). In HPT and FT, MT₁ ^-/- display no differences compared to their wild-type littermates (CTL), whereas both MT₂ ^-/- and MT₁ ^-/- /MT₂ ^-/- mice showed a reduced thermal sensitivity and a decreased tonic nocifensive behavior during phase 2 of the FT in the light phase. The MT₂ partial agonist UCM924 induced an antinociceptive effect in MT₁ ^-/- but not in MT₂ ^-/- and MT₁ ^-/- /MT₂ ^-/- mice. Also, the competitive opioid antagonist naloxone had no effects in CTL, whereas it induced a decrease of nociceptive thresholds in MT₂ ^-/- mice. Our results show that the genetic inactivation of melatonin MT₂ , but not MT₁ receptors, produces a distinct effect on nociceptive threshold, suggesting that the melatonin MT₂ receptor subtype is selectively involved in the regulation of pain responses.

Role of Melatonin in the Regulation of Pain

Melatonin is a pleiotropic hormone synthesized and secreted mainly by the pineal gland in vertebrates. Melatonin is an endogenous regulator of circadian and seasonal rhythms. Melatonin is involved in many physiological and pathophysiological processes demonstrating antioxidant, antineoplastic, anti-inflammatory, and immunomodulatory properties. Accumulating evidence has revealed that melatonin plays an important role in pain modulation through multiple mechanisms. In this review, we examine recent evidence for melatonin on pain regulation in various animal models and patients with pain syndromes, and the potential cellular mechanisms.

Advances in Characterizing Recently-Identified Molecular Actions of Melatonin: Clinical Implications

Melatonin, N-acetyl-5-methoxy-tryptamine, was discovered to be a product of serotonin metabolism in the mammalian pineal gland where its synthesis is under control of the light:dark cycle. Besides its regulatory pathway involving ganglion cells in the retina, the neural connections between the eyes and the pineal gland include the master circadian clock, the suprachiasmatic nuclei, and the central and peripheral nervous systems. Since pineal melatonin is released into the blood and into the cerebrospinal fluid, it has access to every cell in an organism and it mediates system-wide effects. Subsequently, melatonin was found in several extrapineal organs and, more recently, perhaps in every cell of every organ. In contrast to the pinealocytes, non-pineal cells do not discharge melatonin into the blood; rather it is used locally in an intracrine, autocrine, or paracrine manner. Melatonin levels in non-pineal cells do not exhibit a circadian rhythm and do not depend on circulating melatonin concentrations although when animals are treated with exogenous melatonin it is taken up by presumably all cells. Mitochondria are the presumed site of melatonin synthesis in all cells; the enzymatic machinery for melatonin synthesis has been identified in mitochondria. The association of melatonin with mitochondria, because of its ability to inhibit oxidative stress, is very fortuitous since these organelles are a major site of damaging reactive oxygen species generation. In this review, some of the actions of non-pineal-derived melatonin are discussed in terms of cellular and subcellular physiology.

[Trichostatin A suppresses up-regulation of histone deacetylase 4 and reverses differential expressions of miRNAs in the spinal cord of rats with chronic constrictive injury]

OBJECTIVE

To explore the analgesic mechanism of intrathecal trichostatin A (TSA) injection in a rat model of neuropathic pain induced by chronic constrictive injury (CCI).

METHODS

Male SD rats were randomized into sham operation+ DMSO group (group S), CCI +DMSO group (group C), CCI +10 μg TSA group (group T), and in the latter two groups, rat models of neuropathic pain were established induced by CCI. The rats were given intrathecal injections of 10 μL 5% DMSO or 10 μg TSA (in 5% DMSO) once a day on days 7 to 9 after CCI or sham operation. The rats were euthanized after behavioral tests on day 10, and the lumbar segment of the spinal cord was sampled to determine the expression of histone deacetylase 4 (HDAC4) protein and mRNA and detect the differentially expressed miRNAs using a miRNA chip. MiR-190b-5p and miR-142-3p were selected for validation of the results using RT-qPCR.

RESULTS

Compared with those in group S, the rats in group C showed significantly decreased paw withdrawal mechanical threshold (PWMT) from day 3 to day 10 after CCI (P < 0.05); intrathecal injection of TSA significantly reversed the reduction of PWMT following CCI (P < 0.05). Positive HDAC4 expression was detected mainly in the cytoplasm of the neurons in the gray matter of the spinal cord, and was obviously up-regulated after CCI (Ρ < 0.05). Intrathecal injection of TSA significantly suppressed CCI-induced up-regulation of HDAC4 at 10 days after the operation (P < 0.05). Compared with the miRNA profile in group S, miRNA profiling identified 83 differentially expressed miRNAs in group C (fold change ≥2 or ≤0.5, P < 0.05); TSA treatment reversed the expressions of 58 of the differentially expressed miRNAs following CCI, including 41 miRNAs that were decreased after CCI but up-regulated following TSA treatment. The results of real-time PCR validated the changes in the expressions of miR-190b-5p and miR-142-3p.

CONCLUSIONS

TSA suppresses CCI-induced up-regulation of HDAC4 and reverses differential expressions of miRNAs in the spinal cord of rats, which may contribute to the analgesic effect of TSA on neuropathic pain.

Network-based transcriptomic analysis reveals novel melatonin-sensitive genes in cardiovascular system

PURPOSE

Heart disease is a major cause of mortality and disability worldwide. Melatonin is a neuroendocrine hormone and has been found to be protective in heart disease. However, the molecular basis underlying this cardioprotective effect is not fully understood. Here we aim to investigate melatonin-sensitive genes in cardiovascular system using public gene expression databases.

METHODS

An innovative genomic analysis method, the weighted gene co-expression network analysis (WGCNA) combined with differential gene expression analysis, was used in this study. The algorithm was implemented in R/Bioconductor.

RESULTS

Using this method, we provide a comprehensive characterization of transcriptional profiles associated with melatonin treatment. We found that 357 differentially expressed genes (DEGs) were highly sensitive to melatonin in mouse myocardium. Enrichment analysis showed that these 357 genes were mostly related to GO:0051984 (positive regulation of chromosome segregation), GO:0016605 (PML body) and GO:0006281 (DNA repair). We further obtained 5 hub genes from the 357 DEGs, including Set, Dhx40, Scaf11, Cfh, and Nup43.

CONCLUSIONS

We identified numerous melatonin-sensitive genes and further identified five hub genes. The five novel genes are possibly associated with the myocardial benefits of melatonin.

Dose-Dependent Effect of Hyperbaric Oxygen Treatment on Burn-Induced Neuropathic Pain in Rats

Hyperbaric oxygen treatment (HBOT) has been used to reduce neuropathic pain. Melatonin and opioid receptors are involved in neuropathic pain, but it is not known if HBOT works through these pathways to achieve its antinociceptive effect. We divided anesthetized rats into two treatment and three sham groups. The two treatment groups received third-degree burns on their right hind paws, one treated in a hyperbaric chamber for a week and the other for two weeks. We evaluated the mechanical paw-withdrawal threshold (MWT) and expression of melatonin receptor 1 (MT1), melatonin receptor 2 (MT2), μ (MOR) and κ (KOR) opioid receptor, brain-derived neurotrophic factor (BDNF), Substance P, and calcitonin gene-related peptide (CGRP) in cuneate nucleus, dorsal horn, and hind paw skin by immunohistochemical, immunofluorescence assays and real-time quantitative polymerase chain reaction (RT-PCR). The group receiving one-week HBOT had increased expressions of MT1, MT2, MOR and KOR and decreased expressions of BDNF, Substance P, and CGRP. Their mechanically measured pain levels returned to normal within a week and lasted three weeks. This anti-allodynia effect lasted twice as long in those treated for two weeks. Our findings suggest that increasing the duration of HBOT can reduce burn-induced mechanical allodynia for an extended period of time in rats. The upregulation of melatonin and opioid receptors observed after one week of HBOT suggests they may be partly involved in attenuation of the mechanical allodynia. Downregulation of BDNF, substance P and CGRP may have also contributed to the overall beneficial effect of HBOT.

Normalizing HDAC2 Levels in the Spinal Cord Alleviates Thermal and Mechanical Hyperalgesia After Peripheral Nerve Injury and Promotes GAD65 and KCC2 Expression

Neuropathic pain is a worldwide health concern with poor treatment outcomes. Accumulating evidence suggests that histone hypoacetylation is involved in development and maintenance of neuropathic pain. Thus, many natural and synthetic histone deacetylase (HDACs) inhibitors were tested and exhibited a remarkable analgesic effect against neuropathic pain in animals. However, studies evaluating specific subtypes of HDACs contributing to neuropathic pain are limited. In this study, using the chronic constriction injury (CCI) rat model, we found that mRNA and protein levels of HDAC2 were increased in the lumbar spinal cord of rats after sciatic nerve injury. Intrathecal injection of TSA, a pan-HDAC inhibitor, suppressed the increase in HDAC2 protein but not mRNA, and showed a dose-dependent pain-relieving effect. By introducing HDAC2-specific shRNA into the spinal cord via a lentivirus vector, we confirmed that HDAC2 mediates mechanical and thermal hyperalgesia after nerve injury. Further examination found two essential participants in neuropathic pain in the inhibitory circuit of the central nervous system: GAD65 and KCC2 were increased in the spinal cord of CCI rats after HDAC2 knockdown. Thus, our research confirmed that HDAC2 was involved in mechanical and thermal hyperalgesia induced by peripheral nerve injury. Furthermore, GAD65 and KCC2 were the possible downstream targets of HDAC2 in pain modulation pathways.

Melatonin and their analogs as a potential use in the management of Neuropathic pain

Melatonin (N-acetyl-5-methoxytryptamine), secreted by the pineal gland is known to perform multiple functions including, antioxidant, anti-hypertensive, anti-cancerous, immunomodulatory, sedative and tranquilizing functions. Melatonin is also known to be involved in the regulation of body mass index, control the gastrointestinal system and play an important role in cardioprotection, thermoregulation, and reproduction. Recently, several studies have reported the efficacy of Melatonin in treating various pain syndromes. The current paper reviews the studies on Melatonin and its analogs, particularly in Neuropathic pain. Here, we first briefly summarized research in preclinical studies showing the possible mechanisms through which Melatonin and its analogs induce analgesia in Neuropathic pain. Second, we reviewed research indicating the role of Melatonin in attenuating analgesic tolerance. Finally, we discussed the recent studies that reported novel Melatonin agonists, which were proven to be effective in treating Neuropathic pain.

Modulation of serine/threonine phosphatases by melatonin: therapeutic approaches in neurodegenerative diseases

Melatonin is an endogenous hormone produced by the pineal gland as well as many other tissues and organs. The natural decline in melatonin levels with ageing contributes significantly to the development of neurodegenerative disorders. Neurodegenerative diseases share common mechanisms of toxicity such as proteinopathy, mitochondrial dysfunction, metal dyshomeostasis, oxidative stress, neuroinflammation and an imbalance in the phosphorylation/dephosphorylation ratio. Several reports have proved the usefulness of melatonin in counteracting the events that lead to a neurodegenerative scenario. In this review, we have focused on the fact that melatonin could rectify the altered phosphorylation/dephosphorylation rate found in some neurodegenerative diseases by influencing the activity of phosphoprotein phosphatases. We analyse whether melatonin offers any protective activity towards these enzymes through a direct interaction. LINKED ARTICLES: This article is part of a themed section on Recent Developments in Research of Melatonin and its Potential Therapeutic Applications. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.16/issuetoc.

Melatonin protects against defects induced by deoxynivalenol during mouse oocyte maturation

Deoxynivalenol (DON) is one of the most prevalent fusarium mycotoxins in feedstuff and food. DON causes detrimental effects on human and animal reproductive systems by inducing oxidative stress and apoptosis. However, melatonin is a multifunctional endogenous hormone that plays crucial roles in the development of animal germ cells and embryos as a robust deoxidizer. In this study, we explored the effects of melatonin on the DON exposure mouse oocytes. Our in vitro and in vivo results showed that DON adversely affected mouse oocyte maturation and early embryo cleavage, while melatonin administration ameliorated the toxic effects of DON. DON exposure disrupted the meiotic spindle formation and kinetochore-microtubule attachment, which induced aneuploidy in oocytes. This might be through DON effects on the acetylated tubulin level. Moreover, we found that DON exposure caused the alteration of DNA and histone methylation level, which might affect early embryo cleavage. The toxic effects of DON on oocytes might be through its induction of oxidative stress-mediated early apoptosis, while the treatment with melatonin significantly ameliorated these phenotypes in DON-exposed mouse oocytes. Collectively, our results indicated the protection effects of melatonin against defects induced by DON during mouse oocyte meiotic maturation.

Spinal TNF-α impedes Fbxo45-dependent Munc13-1 ubiquitination to mediate neuropathic allodynia in rats

Presynaptic active zone proteins play a crucial role in regulating synaptic plasticity. Although the ubiquitin–proteasome system underlying the degradation of the presynaptic active zone protein is well established, the contribution of this machinery to regulating spinal plasticity during neuropathic pain development remains unclear. Here, using male Sprague Dawley rats, we demonstrated along with behavioral allodynia, neuropathic injury induced a marked elevation in the expression levels of an active zone protein Munc13-1 in the homogenate and synaptic plasma membrane of the ipsilateral dorsal horn. Moreover, nerve injury-increased Munc13-1 expression was associated with an increase in the frequency and amplitude of miniature excitatory postsynaptic currents (mEPSCs) in ipsilateral dorsal horn neurons. This neuropathic injury-induced accumulation of Munc13-1 colocalized with synaptophysin but not homer1 in the dorsal horn. Focal knockdown of spinal Munc13-1 expression attenuated behavioral allodynia and the increased frequency, not the amplitude, of mEPSCs in neuropathic rats. Remarkably, neuropathic injury decreased spinal Fbxo45 expression, Fbxo45-Munc13-1 co-precipitation, and Munc13-1 ubiquitination in the ipsilateral dorsal horn. Conversely, focal knockdown of spinal Fbxo45 expression in naive animals resulted in behavioral allodynia in association with similar protein expression and ubiquitination in the dorsal horn as observed with neuropathic injury rats. Furthermore, both neuropathic insults and intrathecal injection of tumor necrosis factor-α (TNF-α) impeded spinal Fbxo45-dependent Munc13-1 ubiquitination, which was reversed by intrathecal TNF-α-neutralizing antibody. Our data revealed that spinal TNF-α impedes Fbxo45-dependent Munc13-1 ubiquitination that accumulates Munc13-1 in the presynaptic area and hence facilitates the synaptic excitability of nociceptive neurotransmission underlying neuropathic pain.

Melatonin impedes Tet1-dependent mGluR5 promoter demethylation to relieve pain

Melatonin (N-acetyl-5-methoxytryptamine)/MT2 receptor-dependent epigenetic modification represents a novel pathway in the treatment of neuropathic pain. Because spinal ten-eleven translocation methylcytosine dioxygenase 1 (Tet1)-dependent epigenetic demethylation has recently been linked to pain hypersensitivity, we hypothesized that melatonin/MT2-dependent analgesia involves spinal Tet1-dependent demethylation. Here, we showed that spinal Tet1 gene transfer by intrathecal delivery of Tet1-encoding vectors to naïve rats produced profound and long-lasting nociceptive hypersensitivity. In addition, enhanced Tet1 expression, Tet1-metabotropic glutamate receptor subtype 5 (mGluR5) promoter coupling, demethylation at the mGluR5 promoter, and mGluR5 expression in dorsal horn neurons were observed. Rats subjected to spinal nerve ligation and intraplantar complete Freund's adjuvant injection displayed tactile allodynia and behavioral hyperalgesia associated with similar changes in the dorsal horn. Notably, intrathecal melatonin injection reversed the protein expression, protein-promoter coupling, promoter demethylation, and pain hypersensitivity induced by Tet1 gene transfer, spinal nerve ligation, and intraplantar complete Freund's adjuvant injection. All the effects caused by melatonin were blocked by pretreatment with a MT2 receptor-selective antagonist. In conclusion, melatonin relieves pain by impeding Tet1-dependent demethylation of mGluR5 in dorsal horn neurons through the MT2 receptor. Our findings link melatonin/MT2 signaling to Tet1-dependent epigenetic demethylation of nociceptive genes for the first time and suggest melatonin as a promising therapy for the treatment of pain.

Bromodomain-containing Protein 4 Activates Voltage-gated Sodium Channel 1.7 Transcription in Dorsal Root Ganglia Neurons to Mediate Thermal Hyperalgesia in Rats

Background

Bromodomain-containing protein 4 binds acetylated promoter histones and promotes transcription; however, the role of bromodomain-containing protein 4 in inflammatory hyperalgesia remains unclear.

Methods

Male Sprague–Dawley rats received hind paw injections of complete Freund’s adjuvant to induce hyperalgesia. The dorsal root ganglia were examined to detect changes in bromodomain-containing protein 4 expression and the activation of genes involved in the expression of voltage-gated sodium channel 1.7, which is a key pain-related ion channel.

Results

The intraplantar complete Freund’s adjuvant injections resulted in thermal hyperalgesia (4.0 ± 1.5 s; n = 7). The immunohistochemistry and immunoblotting results demonstrated an increase in the bromodomain-containing protein 4–expressing dorsal root ganglia neurons (3.78 ± 0.38 fold; n = 7) and bromodomain-containing protein 4 protein levels (2.62 ± 0.39 fold; n = 6). After the complete Freund’s adjuvant injection, histone H3 protein acetylation was enhanced in the voltage-gated sodium channel 1.7 promoter, and cyclin-dependent kinase 9 and phosphorylation of RNA polymerase II were recruited to this area. Furthermore, the voltage-gated sodium channel 1.7–mediated currents were enhanced in neurons of the complete Freund’s adjuvant rats (55 ± 11 vs. 19 ± 9 pA/pF; n = 4 to 6 neurons). Using bromodomain-containing protein 4–targeted antisense small interfering RNA to the complete Freund’s adjuvant–treated rats, the authors demonstrated a reduction in the expression of bromodomain-containing protein 4 (0.68 ± 0.16 fold; n = 7), a reduction in thermal hyperalgesia (7.5 ± 1.5 s; n = 7), and a reduction in the increased voltage-gated sodium channel 1.7 currents (21 ± 4 pA/pF; n = 4 to 6 neurons).

Conclusions

Complete Freund’s adjuvant triggers enhanced bromodomain-containing protein 4 expression, ultimately leading to the enhanced excitability of nociceptive neurons and thermal hyperalgesia. This effect is likely mediated by the enhanced expression of voltage-gated sodium channel 1.7.

MicroRNA-142-3p relieves neuropathic pain by targeting high mobility group box 1

MicroRNA (miRNA) are emerging as critical regulators of neuropathic pain development. Neuroinflammation contributes to the development of neuropathic pain. miR‑142‑3p has been characterized as an inflammation‑related miRNA in various pathological processes. However, little is known about the role of miR‑142‑3p in neuroinflammation and neuropathic pain. The present study aimed to investigate the function of miR‑142‑3p in neuropathic pain by creating a murine model using spinal nerve ligation (SNL). A significant reduction in miR‑142‑3p expression was observed in the dorsal root ganglion of mice with SNL (P<0.05) compared with control mice. Overexpression of miR‑142‑3p significantly inhibited neuropathic pain and neuroinflammation in mice with SNL (P<0.05). High mobility group box 1 (HMGB1) was identified as a direct target gene of miR‑142‑3p by bioinformatic analysis and dual‑luciferase reporter assays. Overexpression of miR‑142‑3p significantly reduced the mRNA and protein expression levels of HMGB1 in vitro and in vivo (P<0.05). In addition, HMGB1 mRNA expression and miR‑142‑3p expression were inversely correlated in mice with SNL. Furthermore, overexpression of HMGB1 significantly reversed the inhibitory effect of miR‑142‑3p on neuroinflammation and neuropathic pain development (P<0.05). Overall, these results suggest that miR‑142‑3p functions as a negative regulator of neuropathic pain development through the downregulation of HMGB1, indicating that miR‑142‑3p may serve as a potential therapeutic target for neuropathic pain.

Single Administration of Melatonin Modulates the Nitroxidergic System at the Peripheral Level and Reduces Thermal Nociceptive Hypersensitivity in Neuropathic Rats

Neuropathic pain is a severe condition with unsatisfactory treatments. Melatonin, an indolamine, seems to be a promising molecule suitable for this purpose due to its well-known anti-inflammatory, analgesic, and antioxidant effects, as well as its modulation of the nitroxidergic system. Nevertheless, the data on its mechanism of action and potentialities are currently insufficient in this pathology, especially at the peripheral level. Thus, this work evaluated the effect of a single administration of melatonin in an established mononeuropathy pain model that monitors the behaviour and the changes in the nitroxidergic system in dorsal root ganglia and skin, which are affected by nervous impairment. Experiments were carried out on Sprague Dawley rats subdivided into the sham operated (control) and the chronic constriction injured animals, a model of peripheral neuropathic pain on sciatic nerve. Single administrations of melatonin (5–10 mg/kg) or vehicle were injected intraperitoneally on the 14th day after surgery, when the mononeuropathy was established. The animals were behaviourally tested for thermal hyperalgesia. The dorsal root ganglia and the plantar skin of the hind-paws were removed and processed for the immunohistochemical detection of neuronal and inducible nitric oxide synthases. The behavioural results showed an increase of withdrawal latency during the plantar test as early as 30 min after melatonin administration. The immunohistochemical results indicated a modulation of the nitroxidergic system both at dorsal root ganglia and skin level, permitting speculate on a possible mechanism of action. We showed that melatonin may be a possible therapeutic strategy in neuropathic pain.

Exogenous melatonin in the treatment of pain: a systematic review and meta-analysis

Melatonin is an important hormone for regulating mammalian circadian biology and cellular homeostasis. Recent evidence has shown that melatonin exerts anti-nociception effects in both animals and humans. However, according to clinical trials, the anti-nociception effects of melatonin are still controversial. The aim of this meta-analysis was to investigate the anti-nociception effects of melatonin premedication. The primary outcome was the effects of melatonin on pain intensity. The secondary outcomes included the number of patients with analgesic requirements, total analgesic consumption, and brain-derived neurotrophic factor (BDNF) levels. In total, 19 studies were included in the current meta-analysis. The pooling data show that melatonin significantly decreased the pain intensity, as evidenced by the pain scores. Moreover, melatonin administration also reduced the proportion of patients with analgesic requirements and BDNF levels. However, the effects of melatonin on total analgesic consumption still require further confirmation. Collectively, the current meta-analysis supports the use of melatonin for anti-nociception.

Spinal Fbxo3-Dependent Fbxl2 Ubiquitination of Active Zone Protein RIM1α Mediates Neuropathic Allodynia through CaV2.2 Activation

Spinal plasticity, a key process mediating neuropathic pain development, requires ubiquitination-dependent protein turnover. Presynaptic active zone proteins have a crucial role in regulating vesicle exocytosis, which is essential for synaptic plasticity. Nevertheless, the mechanism for ubiquitination-regulated turnover of presynaptic active zone proteins in the progression of spinal plasticity-associated neuropathic pain remains unclear. Here, after research involving Sprague Dawley rats, we reported that spinal nerve ligation (SNL), in addition to causing allodynia, enhances the Rab3-interactive molecule-1α (RIM1α), a major active zone protein presumed to regulate neural plasticity, specifically in the synaptic plasma membranes (SPMs) of the ipsilateral dorsal horn. Spinal RIM1α-associated allodynia was mediated by Fbxo3, which abates Fbxl2-dependent RIM1α ubiquitination. Subsequently, following deubiquitination, enhanced RIM1α directly binds to CaV2.2, resulting in increased CaV2.2 expression in the SPMs of the dorsal horn. While exhibiting no effect on Fbxo3/Fbxl2 signaling, the focal knockdown of spinal RIM1α expression reversed the SNL-induced allodynia and increased spontaneous EPSC (sEPSC) frequency by suppressing RIM1α-facilitated CaV2.2 expression in the dorsal horn. Intrathecal applications of BC-1215 (a Fbxo3 activity inhibitor), Fbxl2 mRNA-targeting small-interfering RNA, and ω-conotoxin GVIA (a CaV2.2 blocker) attenuated RIM1α upregulation, enhanced RIM1α expression, and exhibited no effect on RIM1α expression, respectively. These results confirm the prediction that spinal presynaptic Fbxo3-dependent Fbxl2 ubiquitination promotes the subsequent RIM1α/CaV2.2 cascade in SNL-induced neuropathic pain. Our findings identify a role of the presynaptic active zone protein in pain-associated plasticity. That is, RIM1α-facilitated CaV2.2 expression plays a role in the downstream signaling of Fbxo3-dependent Fbxl2 ubiquitination/degradation to promote spinal plasticity underlying the progression of nociceptive hypersensitivity following neuropathic injury.

SIGNIFICANCE STATEMENT

Ubiquitination is a well known process required for protein degradation. Studies investigating pain pathology have demonstrated that ubiquitination contributes to chronic pain by regulating the turnover of synaptic proteins. Here, we found that the spinal presynaptic active zone protein Rab3-interactive molecule-1α (RIM1α) participates in neuropathic pain development by binding to and upregulating the expression of CaV2.2. In addition, Fbxo3 modifies this pathway by inhibiting Fbxl2-mediated RIM1α ubiquitination, suggesting that presynaptic protein ubiquitination makes a crucial contribution to the development of neuropathic pain. Research in this area, now in its infancy, could potentially provide a novel therapeutic strategy for pain relief.

Growth Arrest and DNA-damage–inducible Protein 45β-mediated DNA Demethylation of Voltage-dependent T-type Calcium Channel 3.2 Subunit Enhances Neuropathic Allodynia after Nerve Injury in Rats

Background

Growth arrest and DNA-damage–inducible protein 45β reactivates methylation-silenced neural plasticity-associated genes through DNA demethylation. However, growth arrest and DNA-damage–inducible protein 45β–dependent demethylation contributes to neuropathic allodynia-associated spinal plasticity remains unclear.

Methods

Adult male Sprague–Dawley rats (654 out of 659) received a spinal nerve ligation or a sham operation with or without intrathecal application of one of the following: growth arrest and DNA-damage–inducible protein 45β messenger RNA–targeted small interfering RNA, lentiviral vector expressing growth arrest and DNA-damage–inducible protein 45β, Ro 25–6981 (an NR2B-bearing N-methyl-d-aspartate receptor antagonist), or KN-93 (a calmodulin-dependent protein kinase II antagonist) were used for behavioral measurements, Western blotting, immunofluorescence, dot blots, detection of unmodified cytosine enrichment at cytosine-phosphate-guanine site, chromatin immunoprecipitation quantitative polymerase chain reaction analysis, and slice recordings.

Results

Nerve ligation-enhanced growth arrest and DNA-damage–inducible protein 45β expression (n = 6) in ipsilateral dorsal horn neurons accompanied with behavioral allodynia (n = 7). Focal knockdown of growth arrest and DNA-damage–inducible protein 45β expression attenuated ligation-induced allodynia (n = 7) by reducing the binding of growth arrest and DNA-damage–inducible protein 45β to the voltage-dependent T-type calcium channel 3.2 subunit promoter (n = 6) that decreased expression of and current mediated by the voltage-dependent T-type calcium channel 3.2 subunit (both n = 6). In addition, NR2B-bearing N-methyl-d-aspartate receptors and calmodulin-dependent protein kinase II act in an upstream cascade to increase growth arrest and DNA-damage–inducible protein 45β expression, hence enhancing demethylation at the voltage-dependent T-type calcium channel 3.2 subunit promoter and up-regulating voltage-dependent T-type calcium channel 3.2 subunit expression. Intrathecal administration of Ro 25–6981, KN-93, or a growth arrest and DNA-damage–inducible protein 45β–targeting small interfering RNA (n = 6) reversed the ligation-induced enrichment of unmodified cytosine at the voltage-dependent T-type calcium channel 3.2 subunit promoter by increasing the associated 5-formylcytosine and 5-carboxylcytosine levels.

Conclusions

By converting 5-formylcytosine or 5-carboxylcytosine to unmodified cytosine, the NR2B-bearing N-methyl-d-aspartate receptor, calmodulin-dependent protein kinase II, or growth arrest and DNA-damage–inducible protein 45β pathway facilitates voltage-dependent T-type calcium channel 3.2 subunit gene demethylation to mediate neuropathic allodynia.

Melatonin Suppresses Neuropathic Pain via MT2-Dependent and -Independent Pathways in Dorsal Root Ganglia Neurons of Mice

Melatonin (Mel) and its receptors (MT1 and MT2) have a well-documented efficacy in treating different pain conditions. However, the anti-nociceptive effects of Mel and Mel receptors in neuropathic pain (NP) are poorly understood. To elucidate this process, pain behaviors were measured in a dorsal root ganglia (DRG)-friendly sciatic nerve cuffing model. We detected up-regulation of MT2 expression in the DRGs of cuff-implanted mice and its activation by the agonist 8-M-PDOT (8MP). Also, Mel attenuated the mechanical and thermal allodynia induced by cuff implantation. Immunohistochemical analysis demonstrated the expression of MT2 in the DRG neurons, while MT1 was expressed in the satellite cells. In cultured primary neurons, microarray analysis and gene knockdown experiments demonstrated that MT2 activation by 8MP or Mel suppressed calcium signaling pathways via MAPK1, which were blocked by RAR-related orphan receptor alpha (RORα) activation with a high dose of Mel. Furthermore, expression of nitric oxide synthase 1 (NOS1) was down-regulated upon Mel treatment regardless of MT2 or RORα. Application of Mel or 8MP in cuff-implanted models inhibited the activation of peptidergic neurons and neuro-inflammation in the DRGs by down-regulating c-fos, calcitonin gene-related peptide [CGRP], and tumor necrosis factor-1α [TNF-1α] and interleukin-1β [IL-1β]. Addition of the MT2 antagonist luzindole blocked the effects of 8MP but not those of Mel. In conclusion, only MT2 was expressed in the DRG neurons and up-regulated upon cuff implantation. The analgesic effects of Mel in cuff-implanted mice were closely associated with both MT2-dependent (MAPK-calcium channels) and MT2-independent (NOS1) pathways in the DRG.

The Emerging Role of HMGB1 in Neuropathic Pain: A Potential Therapeutic Target for Neuroinflammation

Neuropathic pain (NPP) is intolerable, persistent, and specific type of long-term pain. It is considered to be a direct consequence of pathological changes affecting the somatosensory system and can be debilitating for affected patients. Despite recent progress and growing interest in understanding the pathogenesis of the disease, NPP still presents a major diagnostic and therapeutic challenge. High mobility group box 1 (HMGB1) mediates inflammatory and immune reactions in nervous system and emerging evidence reveals that HMGB1 plays an essential role in neuroinflammation through receptors such as Toll-like receptors (TLR), receptor for advanced glycation end products (RAGE), C-X-X motif chemokines receptor 4 (CXCR4), and N-methyl-D-aspartate (NMDA) receptor. In this review, we present evidence from studies that address the role of HMGB1 in NPP. First, we review studies aimed at determining the role of HMGB1 in NPP and discuss the possible mechanisms underlying HMGB1-mediated NPP progression where receptors for HMGB1 are involved. Then we review studies that address HMGB1 as a potential therapeutic target for NPP.