Regulation of μ-opioid type 1 receptors by microRNA134 in dorsal root ganglion neurons following peripheral inflammation

Mechanisms of pain in aging and age-related conditions: Focus on caregivers

Pain is a complex, subjective experience that can significantly impact quality of life, particularly in aging individuals, by adversely affecting physical and emotional well-being. Whereas acute pain usually serves a protective function, chronic pain is a persistent pathological condition that contributes to functional deficits, cognitive decline, and emotional disturbances in the elderly. Despite substantial progress that has been made in characterizing age-related changes in pain, complete mechanistic details of pain processing mechanisms in the aging patient remain unknown. Pain is particularly under-recognized and under-managed in the elderly, especially among patients with Alzheimer's disease (AD), Alzheimer's disease-related dementias (ADRD), and other age-related conditions. Furthermore, difficulties in assessing pain in patients with AD/ADRD and other age-related conditions may contribute to the familial caregiver burden. The purpose of this article is to discuss the mechanisms and risk factors for chronic pain development and persistence, with a particular focus on age-related changes. Our article also highlights the importance of caregivers working with aging chronic pain patients, and emphasizes the urgent need for increased legislative awareness and improved pain management in these populations to substantially alleviate caregiver burden.

Spinal microRNA-134-5p targets glutamate receptor ionotropic kainate 3 to modulate opioid induced hyperalgesia in mice

Background: Fentanyl and its analogs are extensively used for pain relief. However, their paradoxically pronociceptive effects often lead to increased opioids consumption and risk of chronic pain. Compared to other synthetic opioids, remifentanil has been strongly linked to acute opioid hyperalgesia after exposure [remifentanil-induced hyperalgesia (RIH)]. The epigenetic regulation of microRNAs (miRNAs) on targeted mRNAs has emerged as an important pathogenesis in pain. The current research aimed at exploring the significance and contributions of miR-134-5p to the development of RIH. Methods: Both the antinociceptive and pronociceptive effects of two commonly used opioids were assessed, and miRNA expression profiles in the spinal dorsal horn (SDH) of mice acutely exposed to remifentanil and remifentanil equianalgesic dose (RED) sufentanil were screened. Next, the candidate miRNA level, cellular distribution, and function were examined by qPCR, fluorescent in situ hybridization (FISH) and Argonaute-2 immunoprecipitation. Furthermore, bioinformatics analysis, luciferase assays, miRNA overexpression, behavioral tests, golgi staining, electron microscopy, whole-cell patch-clamp recording, and immunoblotting were employed to investigate the potential targets and mechanisms underlying RIH. Results: Remifentanil induced significant pronociceptive effects and a distinct miRNA-profile from sufentanil when compared to saline controls. Among top 30 differentially expressed miRNAs spectrum, spinal miR-134-5p was dramatically downregulated in RIH mice but remained comparative in mice subjected to sufentanil. Moreover, Glutamate Receptor Ionotropic Kainate 3 (Grik3) was a target of miR-134-5p. The overexpression of miR-134-5p attenuated the hyperalgesic phenotype, excessive dendritic spine remodeling, excitatory synaptic structural plasticity, and Kainate receptor-mediated miniature excitatory postsynaptic currents (mEPSCs) in SDH resulting from remifentanil exposure. Besides, intrathecal injection of selective KA-R antagonist was able to reverse the GRIK3 membrane trafficking and relieved RIH. Conclusion: The miR-134-5p contributes to remifentanil-induced pronociceptive features via directly targeting Grik3 to modulate dendritic spine morphology and synaptic plasticity in spinal neurons.

Roles of the Neuron-Restrictive Silencer Factor in the Pathophysiological Process of the Central Nervous System

The neuron-restrictive silencer factor (NRSF), also known as repressor element 1 (RE-1) silencing transcription factor (REST) or X2 box repressor (XBR), is a zinc finger transcription factor that is widely expressed in neuronal and non-neuronal cells. It is a master regulator of the nervous system, and the function of NRSF is the basis of neuronal differentiation, diversity, plasticity, and survival. NRSF can bind to the neuron-restrictive silencer element (NRSE), recruit some co-repressors, and then inhibit transcription of NRSE downstream genes through epigenetic mechanisms. In neurogenesis, NRSF functions not only as a transcriptional silencer that can mediate the transcriptional inhibition of neuron-specific genes in non-neuronal cells and thus give neuron cells specificity, but also as a transcriptional activator to induce neuronal differentiation. Many studies have confirmed the association between NRSF and brain disorders, such as brain injury and neurodegenerative diseases. Overexpression, underexpression, or mutation may lead to neurological disorders. In tumorigenesis, NRSF functions as an oncogene in neuronal tumors, such as neuroblastomas, medulloblastomas, and pheochromocytomas, stimulating their proliferation, which results in poor prognosis. Additionally, NRSF-mediated selective targets gene repression plays an important role in the development and maintenance of neuropathic pain caused by nerve injury, cancer, and diabetes. At present, several compounds that target NRSF or its co-repressors, such as REST-VP16 and X5050, have been shown to be clinically effective against many brain diseases, such as seizures, implying that NRSF and its co-repressors may be potential and promising therapeutic targets for neural disorders. In the present review, we introduced the biological characteristics of NRSF; reviewed the progress to date in understanding the roles of NRSF in the pathophysiological processes of the nervous system, such as neurogenesis, brain disorders, neural tumorigenesis, and neuropathic pain; and suggested new therapeutic approaches to such brain diseases.

DNA Methylation and Non-Coding RNAs during Tissue-Injury Associated Pain

While about half of the population experience persistent pain associated with tissue damages during their lifetime, current symptom-based approaches often fail to reduce such pain to a satisfactory level. To provide better patient care, mechanism-based analgesic approaches must be developed, which necessitates a comprehensive understanding of the nociceptive mechanism leading to tissue injury-associated persistent pain. Epigenetic events leading the altered transcription in the nervous system are pivotal in the maintenance of pain in tissue injury. However, the mechanisms through which those events contribute to the persistence of pain are not fully understood. This review provides a summary and critical evaluation of two epigenetic mechanisms, DNA methylation and non-coding RNA expression, on transcriptional modulation in nociceptive pathways during the development of tissue injury-associated pain. We assess the pre-clinical data and their translational implication and evaluate the potential of controlling DNA methylation and non-coding RNA expression as novel analgesic approaches and/or biomarkers of persistent pain.

No antinociceptive synergy between morphine and delta-9-tetrahydrocannabinol in male and female rats with persistent inflammatory pain

Studies have demonstrated antinociceptive synergy between morphine and delta-9-tetrahydrocannabinol (THC) in animals, but whether such synergy occurs against all types of pain and in humans is unclear. Because a majority of chronic pain patients are women, and sex differences in morphine and THC potencies have been observed in rodents, the present study examined sex-specific effects of morphine and THC given alone and in combination, in rats with persistent inflammatory pain. On day 1, baseline mechanical and thermal response thresholds, hindpaw weight-bearing, locomotor activity, and hindpaw thickness were determined. Inflammation was then induced via hindpaw injection of complete Freund's adjuvant (CFA). Three days later, morphine (s.c.), THC (i.p) or a morphine-THC combination (1:1, 3:1 and 1:3 dose ratios) was administered, and behavioral testing was conducted at 30-240 min postinjection. Morphine alone was antiallodynic and antihyperalgesic, with no sex differences, but at some doses increased weight-bearing on the CFA-treated paw more in males than females. THC alone reduced mechanical allodynia with similar potency in both sexes, but reduced thermal hyperalgesia and locomotor activity with greater potency in females than males. All morphine-THC combinations reduced allodynia and hyperalgesia, but isobolographic analysis of mechanical allodynia data showed no significant morphine-THC synergy in either sex. Additionally, whereas morphine alone was antinociceptive at doses that did not suppress locomotion, morphine-THC combinations suppressed locomotion and did not increase weight-bearing on the inflamed paw. These results suggest that THC is unlikely to be a beneficial adjuvant when given in combination with morphine for reducing established inflammatory pain.

Epigenetic changes in headache

INTRODUCTION

Multiple factors, including both genetic and environmental mechanisms, appear to play a role in the aetiology of headache. An interesting area of study is the possible involvement of epigenetic mechanisms in headache development and the transformation to chronic headache, and the potential role of these factors as a therapeutic target.

METHODS

We performed a literature review of the involvement of different epigenetic mechanisms in headache, mainly using the Medline/PubMed database. To this end, we used the following English search terms: headache, migraine, epigenetics, DNA methylation, histones, non-coding RNA, and miRNA.

RESULTS

A total of 15 English-language publications related to the above terms were obtained.

CONCLUSION

There is limited but consistent evidence of the relationship between epigenetics and headache; it is therefore essential to continue research of epigenetic changes in headache. This may help to understand the pathophysiology of headache and even to identify candidate biomarkers and new, more effective, therapeutic targets.

Role of microRNAs in Disorders of Gut-Brain Interactions: Clinical Insights and Therapeutic Alternatives

Disorders of gut–brain interactions (DGBIs) are heterogeneous in nature and intertwine with diverse pathophysiological mechanisms. Regular functioning of the gut requires complex coordinated interplay between a variety of gastrointestinal (GI) cell types and their functions are regulated by multiple mechanisms at the transcriptional, post-transcriptional, translational, and post-translational levels. MicroRNAs (miRNAs) are small non-coding RNA molecules that post-transcriptionally regulate gene expression by binding to specific mRNA targets to repress their translation and/or promote the target mRNA degradation. Dysregulation of miRNAs might impair gut physiological functions leading to DGBIs and gut motility disorders. Studies have shown miRNAs regulate gut functions such as visceral sensation, gut immune response, GI barrier function, enteric neuronal development, and GI motility. These biological processes are highly relevant to the gut where neuroimmune interactions are key contributors in controlling gut homeostasis and functional defects lead to DGBIs. Although extensive research has explored the pathophysiology of DGBIs, further research is warranted to bolster the molecular mechanisms behind these disorders. The therapeutic targeting of miRNAs represents an attractive approach for the treatment of DGBIs because they offer new insights into disease mechanisms and have great potential to be used in the clinic as diagnostic markers and therapeutic targets. Here, we review recent advances regarding the regulation of miRNAs in GI pacemaking cells, immune cells, and enteric neurons modulating pathophysiological mechanisms of DGBIs. This review aims to assess the impacts of miRNAs on the pathophysiological mechanisms of DGBIs, including GI dysmotility, impaired intestinal barrier function, gut immune dysfunction, and visceral hypersensitivity. We also summarize the therapeutic alternatives for gut microbial dysbiosis in DGBIs, highlighting the clinical insights and areas for further exploration. We further discuss the challenges in miRNA therapeutics and promising emerging approaches.

Integrated Analysis of Omics Data Reveal AP-1 as a Potential Regulation Hub in the Inflammation-Induced Hyperalgesia Rat Model

Inflammation-associated chronic pain is a global clinical problem, affecting millions of people worldwide. However, the underlying mechanisms that mediate inflammation-associated chronic pain remain unclear. A rat model of cutaneous inflammation induced by Complete Freund's Adjuvant (CFA) has been widely used as an inflammation-induced pain hypersensitivity model. We present the transcriptomics profile of CFA-induced inflammation in the rat dorsal root ganglion (DRG) via an approach that targets gene expression, DNA methylation, and post-transcriptional regulation. We identified 418 differentially expressed mRNAs, 120 differentially expressed microRNAs (miRNAs), and 2,670 differentially methylated regions (DMRs), which were all highly associated with multiple inflammation-related pathways, including nuclear factor kappa B (NF-κB) and interferon (IFN) signaling pathways. An integrated analysis further demonstrated that the activator protein 1 (AP-1) network, which may act as a regulator of the inflammatory response, is regulated at both the transcriptomic and epigenetic levels. We believe our data will not only provide drug screening targets for the treatment of chronic pain and inflammation but will also shed light on the molecular network associated with inflammation-induced hyperalgesia.

The Life Cycle of the Mu-Opioid Receptor

Opioid receptors (ORs) are undisputed targets for the treatment of pain. Unfortunately, targeting these receptors therapeutically poses significant challenges including addiction, dependence, tolerance, and the appearance of side effects, such as respiratory depression and constipation. Moreover, misuse of prescription and illicit narcotics has resulted in the current opioid crisis. The mu-opioid receptor (MOR) is the cellular mediator of the effects of most commonly used opioids, and is a prototypical G protein-coupled receptor (GPCR) where new pharmacological, signalling and cell biology concepts have been coined. This review summarises the knowledge of the life cycle of this therapeutic target, including its biogenesis, trafficking to and from the plasma membrane, and how the regulation of these processes impacts its function and is related to pathophysiological conditions.

Which idea is better with regard to immune response? Opioid anesthesia or opioid free anesthesia

The stress of surgery is characterized by an inflammatory response with immune suppression resulting from many factors, including the type of surgery and the kind of anesthesia, linked with the drugs that are used and the underlying disease of the patient. The trauma of surgery triggers a cascade of reactions involving the immune response and nociception. As strong analgesics, opioids provide the analgesic component of general anesthesia with bi-directional effect on the immune system. Opioids influence almost all aspects of the immune response in regards to leukocytes, macrophages, mast cells, lymphocytes, and NK cells. The suppressive effect of opioids on the immune system is limiting their use, especially in patients with impaired immune response, so the possibility of using multimodal anesthesia without opioids, known as opioid-free anesthesia (OFA), is gaining more and more sympathizers. The idea of OFA is to eliminate opioid analgesia in the treatment of acute pain and to replace it with drugs from other groups that are assumed to have a comparable analgesic effect without affecting the immune system. Here, we present a review on the impact of anesthesia, with and without the use of opioids, on the immune response to surgical stress.

Dual-Acting Peripherally Restricted Delta/Kappa Opioid (CAV1001) Produces Antinociception in Animal Models of Sub-Acute and Chronic Pain

Background

The development of highly efficacious alternatives to mu-opioid analgesics represents an urgent unmet medical and public health need. In the presence of inflammation both delta- and kappa-opioid agonists, acting on peripheral sensory neurons, mediate analgesia. The dual-acting, peripherally restricted kappa/delta-opioid agonist, CAV1001, was tested in four rodent pain models.

Methods

Experiment 1 – Formalin testing in mice. Three doses (1–10 mg/kg) of CAV1001 or ICI204448 at 30 minutes were tested after formalin injection. Spontaneous nocifensive responses were video recorded. Experiment 2 – Complete Freund’s Adjuvant (CFA)-induced arthritis. CFA was injected into the ankle joint of rats. Joint compression thresholds (JCT) were measured. CAV1001 was compared to celecoxib. Experiment 3 – Spinal nerve ligation (SNL) in rats. Paw compression thresholds (PCT) were measured. CAV1001 was compared to gabapentin. Experiment 4 – MMRT-1 bone cancer implantation into the rat tibia. Weight-bearing was assessed. CAV1001 was compared to morphine.

Results

In Phase 2 of the formalin model, CAV1001 (1 mg/kg) significantly reduced pain behaviors to a degree comparable to the peripherally restricted kappa-opioid agonist, ICI204448 (10 mg/kg). CAV1001 (10 mg/kg) effectively eliminated pain behaviors associated with phase 2. In the CFA-induced arthritis model, a significant increase in JCTs, similar to the comparator celecoxib, was observed with CAV1001 at 1 mg/kg at 2 hours; CAV1001 (10 mg/kg) was effective at 1 hour. In the SNL model, both the comparator gabapentin and CAV1001 (5 mg/kg) significantly reduced PCT at 2 hours, but at 4 hours, the CAV1001 thresholds improved to baseline. CAV1001 10 mg/kg significantly improved weight bearing at 4-hour post-dosing compared to baseline following MMRT-1 implantation.

Conclusion

CAV1001 demonstrated efficacy in several different preclinical pain models. Time- and dose-dependent differences in the efficacy of CAV1001 amongst these rodent pain models parallel the degree of underlying inflammation.

Renilla Luciferase Reporter Assay to Study 3'-UTR-Driven Posttranscriptional Regulation of OPRM1

MOR expression levels at a specific cell type or tissue significantly contribute to its role in pain transmission and in other responses involving opioid receptors. Therefore, molecular processes regulating MOR levels have gained more and more interest. Recently, posttranscriptional regulation mechanisms have been shown to play a relevant role in influencing MOR expression levels, with polymorphisms and mutations within OPRM1 3'-UTR region impacting the differential opioid-mediated response observed within individuals. Here we report a Renilla luciferase reporter assay format suitable for dissecting the contribution of different and distinct OPRM1 3'-UTR elements to MOR expression levels in a model of glial cells, both under basal conditions and following specific treatments.

Granulocyte Colony Stimulating Factor (GCSF) Can Attenuate Neuropathic Pain by Suppressing Monocyte Chemoattractant Protein-1 (MCP-1) Expression, through Upregulating the Early MicroRNA-122 Expression in the Dorsal Root Ganglia

Our previous animal studies and several human clinical trials have shown that granulocyte-colony stimulating factor (GCSF) can attenuate neuropathic pain through various mechanisms. GCSF itself is also a multipotent cytokine that can modulate microribonucleic acid (microRNA) expression profiles in vitro. In this study, we used the NanoString nCounter analysis system to screen the expression of different rodent microRNAs at early stage after nerve injury and studied the expression of related cytokines/chemokines in the dorsal root ganglia (DRGs) of rats that underwent chronic constriction injury (CCI) to explore the underlying mechanisms of the analgesic effects of GCSF. We found that microRNA-122 expression was downregulated by CCI; in contrast, GCSF treatment significantly upregulated microRNA-122 expression in the DRGs of CCI rats on the 1st day after nerve injury. We further studied the expression of different cytokines/chemokines (IL-1β, IL-6, and monocyte chemoattractant protein-1 (MCP-1)) that were modulated by microRNA-122. MCP-1 has been reported to participate in neuropathic pain development, and its expression on the DRGs of vehicle-treated CCI rats was significantly higher than that on the DRGs of sham-operated rats; in contrast, GCSF-treated rats exhibited significantly lower MCP-1 expression in the DRG than vehicle-treated rats on the 7th day after nerve injury. An early GCSF treatment can suppress MCP-1 expressions, through upregulating microRNA-122 expressions in the DRGs of CCI rats at an earlier stage, thus indirectly attenuating neuropathic pain development.

Non-coding RNAs in neuropathic pain

Neuro-immune alterations in the peripheral and central nervous system play a role in the pathophysiology of chronic pain in general, and members of the non-coding RNA (ncRNA) family, specifically the short, 22 nucleotide microRNAs (miRNAs) and the long non-coding RNAs (lncRNAs) act as master switches orchestrating both immune as well as neuronal processes. Several chronic disorders reveal unique ncRNA expression signatures, which recently generated big hopes for new perspectives for the development of diagnostic applications. lncRNAs may offer perspectives as candidates indicative of neuropathic pain in liquid biopsies. Numerous studies have provided novel mechanistic insight into the role of miRNAs in the molecular sequelae involved in the pathogenesis of neuropathic pain along the entire pain pathway. Specific processes within neurons, immune cells, and glia as the cellular components of the neuropathic pain triad and the communication paths between them are controlled by specific miRNAs. Therefore, nucleotide sequences mimicking or antagonizing miRNA actions can provide novel therapeutic strategies for pain treatment, provided their human homologues serve the same or similar functions. Increasing evidence also sheds light on the function of lncRNAs, which converge so far mainly on purinergic signalling pathways both in neurons and glia, and possibly even other ncRNA species that have not been explored so far.

MicroRNA134 of Ventral Hippocampus Is Involved in Cocaine Extinction-Induced Anxiety-like and Depression-like Behaviors in Mice

We previously found that cocaine abuse could increase microRNA134 (miR134) levels in the hippocampus; yet the roles of miR134 in cocaine-related abnormal psychiatric outcomes remain unknown. In this study, using the cocaine-induced conditioned place preference (CPP) mice model, we found that mice exhibit enhanced anxiety-like and depression-like behaviors during the cocaine extinction (CE) period of CPP, accompanied by obviously increased miR134 levels and decreased levels of 19 genes that are associated with synaptic plasticity, glia activity, and neurochemical microenvironments, in the ventral hippocampus (vHP). Knockdown of miR134 in vHP in vivo reversed the changes in 15 of 19 potential gene targets of miR134 and rescued the abnormal anxiety-like and depression-like behavioral outcomes in CE mice. In parallel, knockdown of miR134 reversed CE-induced changes in dendritic spines and synaptic proteins and increased the field excitatory postsynaptic potential (fEPSP) of CA1 pyramidal neurons in the vHP of CE mice. In addition, knockdown of miR134 suppressed the CE-enhanced microglia activity, inflammatory, apoptotic, and oxidative stress statuses in the vHP. With the data taken together, miR134 may be involved in cocaine-associated psychiatric problems, potentially via regulating the expressions of its gene targets that are related to synaptic plasticity and neurochemical microenvironments.

Comparison of MicroRNA Expression in Tears of Normal Subjects and Sjögren Syndrome Patients

Purpose

Deregulated expression of several microRNAs (miRNAs) in sera or salivary glands of patients with Sjögren syndrome (SS) has been reported. However, none have investigated miRNAs in samples that can represent lacrimal glands. We compared the miRNAs expression in the tears of SS patients and healthy controls. Moreover, we investigated the correlation between miRNAs expression and ocular staining score (OSS).

Methods

Individual tear samples were collected from 18 SS patients and 8 age-matched controls. Clinical ophthalmologic assessments included Schirmer I test, tear film breakup time (tBUT), and OSS. The expression of 43 different miRNAs in tears was measured using real-time polymerase chain reaction, and compared between the SS patients and controls. And we also compared between the three groups of control, primary SS, and secondary SS patients. The correlation between the miRNA expression and OSS was evaluated.

Results

The expression levels of miR-16-5p, miR-34a-5p, miR-142-3p, and miR-223-3p were significantly upregulated in patients with SS when compared with those in the control group (P < 0.05). The expression of 10 miRNAs (miR-30b-5p, miR-30c-5p, miR-30d-5p, miR-92a-3p, miR-134-5p, miR-137, miR-302d-5p, miR-365b-3p, miR-374c-5p, miR-487b-3p) was significantly downregulated in the SS patients (P < 0.05). Eight miRNAs showed statistically significant differences between the three groups of control, primary SS and secondary SS. All 14 miRNAs with significant differences in SS patients and control group were not significantly correlated with OSSs.

Conclusions

The 14 differentially expressed miRNAs may be involved in the pathogenesis of SS, in particular, related to autoimmunity and neuropathy.

Retracted Article: Upregulation of miR-26b alleviates morphine tolerance by inhibiting BDNF via Wnt/β-catenin pathway in rats

Background: Morphine is a commonly used analgesic drug. However, long-term use of morphine will cause tolerance which limits its clinical application in pain treatment. MicroRNAs (miRNAs) have been reported to be involved in the morphine tolerance, but the underlying mechanism is still poorly understood. Methods: Tail flick test was used to measure the maximum possible effect (MPE). Quantitative real-time PCR was employed to detect miR-26b, BDNF, and Wnt5a expression in rat dorsal root ganglia (DRG). Luciferase report assay was introduced to verify the binding relationship between miR-26b and Wnt5a. BDNF, Wnt5a and β-catenin protein level were tested by western blotting. Results: MiR-26b was down-regulated during the development of morphine tolerance while BDNF was upregulated. Overexpression of miR-26b or BDNF inhibition alleviated morphine tolerance. Wnt5a was directly targeted and inhibited by miR-26b via binding to the 3'-UTR of Wnt5a. The Wnt/β-catenin pathway was active in morphine tolerant rats. Moreover, overexpression of Wnt5a could partially enhance miR-26 mimic-mediated morphine tolerance, while a Wnt5a inhibitor could attenuate the tolerance. Conclusion: The present study demonstrated that miR-26b overexpression alleviated morphine tolerance by inhibiting BDNF via the Wnt/β-catenin pathway in rats, highlighting a promising target for the treatment of morphine tolerance.

Small RNA sequencing reveals microRNAs related to neuropathic pain in rats

The present study aimed to identify microRNAs (miRNAs) that are involved in neuropathic pain and predict their corresponding roles in the pathogenesis and development process of neuropathic pain. The rat model of neuropathic pain caused by spared nerve injury (SNI) was established in Sprague-Dawley male rats, followed by small RNA sequencing of the L3–L6 dorsal root ganglion. Real-time PCR was performed to validate the differently expressed miRNAs. Functional verification was performed by intrathecally injecting the animals with miRNA agomir. A total of 72 differentially expressed miRNAs were identified in the SNI rats, including 33 upregulated and 39 downregulated miRNAs. The results of qPCR further verified the expression levels of rno-miR-6215 (P=0.015), rno-miR-1224 (P=0.030), rno-miR-1249 (P=0.038), and rno-miR-488-3p (P=0.048), which were all significantly downregulated in the SNI rats compared to the control ones. The majority of differentially expressed miRNAs were associated with phosphorylation, intracellular signal transduction, and cell death. Target prediction, Gene Ontology, and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses suggested that these differentially expressed miRNAs targeted genes that are related to axon guidance, focal adhesion, and Ras and Wnt signaling pathways. Moreover, miR-1224 agomir significantly alleviated SNI-induced neuropathic pain. The current findings provide new insights into the role of miRNAs in the pathogenesis of neuropathic pain.

Mu-Opioid Receptors in Ganglia, But Not in Muscle, Mediate Peripheral Analgesia in Rat Muscle Pain

BACKGROUND

Previous studies have demonstrated the participation of peripheral μ-opioid receptors (MOR) in the antinociceptive effect of systemically administered morphine and loperamide in an orofacial muscle pain model, induced by hypertonic saline, but not in a spinally innervated one, in rats. In this study, we determine whether this peripheral antinociceptive effect is due to the activation of MOR localized in the muscle, ganglia, or both.

METHODS

To determine the local antinociceptive effect of morphine and loperamide, 2 models of acute muscle pain (trigeminal and spinal) were used. Also, to study the MOR expression, protein quantification was performed in the trigeminal and spinal ganglia, and in the muscles.

RESULTS

The behavioral results show that the intramuscular injection of morphine and loperamide did not exert an antinociceptive effect in either muscle (morphine: P = .63, loperamide: P = .9). On the other hand, MOR expression was found in the ganglia but not in the muscles. This expression was on average 44% higher (95% confidence interval, 33.3-53.9) in the trigeminal ganglia than in the spinal one.

CONCLUSIONS

The peripheral antinociceptive effect of systemically administered opioids may be due to the activation of MOR in ganglia. The greater expression of MOR in trigeminal ganglia could explain the higher antinociceptive effect of opioids in orofacial muscle pain than in spinal muscle pain. Therefore, peripheral opioids could represent a promising approach for the treatment of orofacial pain.

Novel Roles of Non-Coding RNAs in Opioid Signaling and Cardioprotection

Cardiovascular disease (CVD) is a significant cause of morbidity and mortality across the world. A large proportion of CVD deaths are secondary to coronary artery disease (CAD) and myocardial infarction (MI). Even though prevention is the best strategy to reduce risk factors associated with MI, the use of cardioprotective interventions aimed at improving patient outcomes is of great interest. Opioid conditioning has been shown to be effective in reducing myocardial ischemia-reperfusion injury (IRI) and cardiomyocyte death. However, the molecular mechanisms behind these effects are under investigation and could provide the basis for the development of novel therapeutic approaches in the treatment of CVD. Non-coding RNAs (ncRNAs), which are functional RNA molecules that do not translate into proteins, are critical modulators of cardiac gene expression during heart development and disease. Moreover, ncRNAs such as microRNAs (miRNAs) and long non-coding RNAs (lncRNAs) are known to be induced by opioid receptor activation and regulate opioid signaling pathways. Recent advances in experimental and computational tools have accelerated the discovery and functional characterization of ncRNAs. In this study, we review the current understanding of the role of ncRNAs in opioid signaling and opioid-induced cardioprotection.

The Regulatory Mechanisms and Therapeutic Potential of MicroRNAs: From Chronic Pain to Morphine Tolerance

Chronic pain, including cancer-related pain, is a pain condition often caused by inflammation or dysfunctional nerves. Chronic pain treatment poses a significant health care challenge, where opioids especially morphine are widely used and patients often develop tolerance over time with aggravated pain. microRNA (miRNA) is known to play important roles in regulating gene expressions in the nervous system to affect neuronal network plasticity related to algogenesis and the developing of morphine tolerance. In this article, we reviewed studies conducted in rodent animal models investigating the mechanisms of miRNAs regulation in chronic pain with different phenotypes and morphine tolerance. In addition, the potential of targeting miRNAs for chronic pain and morphine tolerance treatment is also reviewed. Finally, we point out the directions of the future research in chronic pain and morphine tolerance.

Epigenetic changes in headache

INTRODUCTION

Multiple factors, including both genetic and environmental mechanisms, appear to play a role in the aetiology of headache. An interesting area of study is the possible involvement of epigenetic mechanisms in headache development and the transformation to chronic headache, and the potential role of these factors as a therapeutic target.

METHODS

We performed a literature review of the involvement of different epigenetic mechanisms in headache, mainly using the Medline/PubMed database. To this end, we used the following English search terms: headache, migraine, epigenetics, DNA methylation, histones, non-coding RNA, and miRNA.

RESULTS

A total of 15 English-language publications related to the above terms were obtained.

CONCLUSION

There is limited but consistent evidence of the relationship between epigenetics and headache; it is therefore essential to continue research of epigenetic changes in headache. This may help to understand the pathophysiology of headache and even to identify candidate biomarkers and new, more effective, therapeutic targets.

Exosomal cargo including microRNA regulates sensory neuron to macrophage communication after nerve trauma

Following peripheral axon injury, dysregulation of non-coding microRNAs (miRs) occurs in dorsal root ganglia (DRG) sensory neurons. Here we show that DRG neuron cell bodies release extracellular vesicles, including exosomes containing miRs, upon activity. We demonstrate that miR-21-5p is released in the exosomal fraction of cultured DRG following capsaicin activation of TRPV1 receptors. Pure sensory neuron-derived exosomes released by capsaicin are readily phagocytosed by macrophages in which an increase in miR-21-5p expression promotes a pro-inflammatory phenotype. After nerve injury in mice, miR-21-5p is upregulated in DRG neurons and both intrathecal delivery of a miR-21-5p antagomir and conditional deletion of miR-21 in sensory neurons reduce neuropathic hypersensitivity as well as the extent of inflammatory macrophage recruitment in the DRG. We suggest that upregulation and release of miR-21 contribute to sensory neuron–macrophage communication after damage to the peripheral nerve.

Exosomes are known to contain microRNAs (miRs). Here the authors show that dorsal root ganglion neurons release exosomes containing miR-21-5p, which contributes to inflammatory cell recruitment following peripheral nerve injury.

Long-lasting antinociceptive effects of green light in acute and chronic pain in rats

Treatments for chronic pain are inadequate, and new options are needed. Nonpharmaceutical approaches are especially attractive with many potential advantages including safety. Light therapy has been suggested to be beneficial in certain medical conditions such as depression, but this approach remains to be explored for modulation of pain. We investigated the effects of light-emitting diodes (LEDs), in the visible spectrum, on acute sensory thresholds in naive rats as well as in experimental neuropathic pain. Rats receiving green LED light (wavelength 525 nm, 8 h/d) showed significantly increased paw withdrawal latency to a noxious thermal stimulus; this antinociceptive effect persisted for 4 days after termination of last exposure without development of tolerance. No apparent side effects were noted and motor performance was not impaired. Despite LED exposure, opaque contact lenses prevented antinociception. Rats fitted with green contact lenses exposed to room light exhibited antinociception arguing for a role of the visual system. Antinociception was not due to stress/anxiety but likely due to increased enkephalins expression in the spinal cord. Naloxone reversed the antinociception, suggesting involvement of central opioid circuits. Rostral ventromedial medulla inactivation prevented expression of light-induced antinociception suggesting engagement of descending inhibition. Green LED exposure also reversed thermal and mechanical hyperalgesia in rats with spinal nerve ligation. Pharmacological and proteomic profiling of dorsal root ganglion neurons from green LED-exposed rats identified changes in calcium channel activity, including a decrease in the N-type (CaV2.2) channel, a primary analgesic target. Thus, green LED therapy may represent a novel, nonpharmacological approach for managing pain.

Role of G protein-coupled receptors-microRNA interactions in gastrointestinal pathophysiology

G protein-coupled receptors (GPCRs) make up the largest transmembrane receptor superfamily in the human genome and are expressed in nearly all gastrointestinal cell types. Coupling of GPCRs and their respective ligands activates various phosphotransferases in the cytoplasm, and, thus, activation of GPCR signaling in intestine regulates many cellular and physiological processes. Studies in microRNAs (miRNAs) demonstrate that they represent critical epigenetic regulators of different pathophysiological responses in different organs and cell types in humans and animals. Here, we reviewed recent research on GPCR-miRNA interactions related to gastrointestinal pathophysiology, such as inflammatory bowel diseases, irritable bowel syndrome, and gastrointestinal cancers. Given that the presence of different types of cells in the gastrointestinal tract suggests the importance of cell-cell interactions in maintaining gastrointestinal homeostasis, we also discuss how GPCR-miRNA interactions regulate gene expression at the cellular level and subsequently modulate gastrointestinal pathophysiology through molecular regulatory circuits and cell-cell interactions. These studies helped identify novel molecular pathways leading to the discovery of potential biomarkers for gastrointestinal diseases.

MicroRNAs Are Involved in the Development of Morphine-Induced Analgesic Tolerance and Regulate Functionally Relevant Changes in Serpini1

Long-term opioid treatment results in reduced therapeutic efficacy and in turn leads to an increase in the dose required to produce equivalent pain relief and alleviate break-through or insurmountable pain. Altered gene expression is a likely means for inducing long-term neuroadaptations responsible for tolerance. Studies conducted by our laboratory (Tapocik et al., 2009) revealed a network of gene expression changes occurring in canonical pathways involved in neuroplasticity, and uncovered miRNA processing as a potential mechanism. In particular, the mRNA coding the protein responsible for processing miRNAs, Dicer1, was positively correlated with the development of analgesic tolerance. The purpose of the present study was to test the hypothesis that miRNAs play a significant role in the development of analgesic tolerance as measured by thermal nociception. Dicer1 knockdown, miRNA profiling, bioinformatics, and confirmation of high value targets were used to test the proposition. Regionally targeted Dicer1 knockdown (via shRNA) had the anticipated consequence of eliminating the development of tolerance in C57BL/6J (B6) mice, thus supporting the involvement of miRNAs in the development of tolerance. MiRNA expression profiling identified a core set of chronic morphine-regulated miRNAs (miR's 27a, 9, 483, 505, 146b, 202). Bioinformatics approaches were implemented to identify and prioritize their predicted target mRNAs. We focused our attention on miR27a and its predicted target serpin peptidase inhibitor clade I (Serpini1) mRNA, a transcript known to be intricately involved in dendritic spine density regulation in a manner consistent with chronic morphine's consequences and previously found to be correlated with the development of analgesic tolerance. In vitro reporter assay confirmed the targeting of the Serpini1 3′-untranslated region by miR27a. Interestingly miR27a was found to positively regulate Serpini1 mRNA and protein levels in multiple neuronal cell lines. Lastly, Serpini1 knockout mice developed analgesic tolerance at a slower rate than wild-type mice thus confirming a role for the protein in analgesic tolerance. Overall, these results provide evidence to support a specific role for miR27a and Serpini1 in the behavioral response to chronic opioid administration (COA) and suggest that miRNA expression and mRNA targeting may underlie the neuroadaptations that mediate tolerance to the analgesic effects of morphine.

MicroRNA-Based Biomarkers in Pain

Biomarkers are measurable characteristics reflective of the physiological or diseased state and a crucial feature in rendering personalized medicine more precise. Dysregulated expression of circulating microRNAs (miRNAs) in bodily fluids is being explored as noninvasive clinical biomarker for a variety of disorders including chronic pain. High-precision qPCR-based signal amplification of these miRNAs enables the detection of small changes making them ideal biomarker candidates. Presence of circulating miRNAs in exosomes, small vesicles that mediate intercellular communication, opens up novel avenues for target intervention and biomarker discovery. miRNA signatures specific to different pain conditions, and their reversal on treatment in patients and animal models can be beneficial in patient stratification, prognosis, and in bridging preclinical and clinical results. Identification of multiple miRNAs as opposed to reliance on one specific molecule as a biomarker could improve treatment efficacies in an extremely heterogeneous pain patient population. Additionally, owing to the stability of miRNAs, retrospective studies could be performed using banked samples from completed clinical trials. Irrespective of the phase and outcome, these studies can provide insights on molecular underpinnings influencing treatment outcome, or specific therapeutic intervention. Identification of miRNAs altered in chronic pain states will have a significant impact on the identification of right leads, targets, doses, and patients. Effective implementation of miRNA-based biomarkers would provide treatment guidance for clinicians, better clinical trial designs for pharmaceutical companies, all leading to individualized care and better treatment outcome for chronic pain patients.

Selective repression of gene expression in neuropathic pain by the neuron-restrictive silencing factor/repressor element-1 silencing transcription (NRSF/REST)

Neuropathic pain often develops following nerve injury as a result of maladaptive changes that occur in the injured nerve and along the nociceptive pathways of the peripheral and central nervous systems. Multiple cellular and molecular mechanisms likely account for these changes; however, the exact nature of these mechanisms remain largely unknown. A growing number of studies suggest that alteration in gene expression is an important step in the progression from acute to chronic pain states and epigenetic regulation has been proposed to drive this change in gene expression. In this review, we discuss recent evidence that the DNA-binding protein neuron-restrictive silencing factor/repressor element-1 silencing transcription factor (NRSF/REST) is an important component in the development and maintenance of neuropathic pain through its role as a transcriptional regulator for a select subset of genes that it normally represses during development.

Analysis of epigenetic mechanisms regulating opioid receptor gene transcription

Opioid drugs are generally used for moderate and severe pain reductions which act through opioid receptors. Studies on transcriptional regulation of opioid receptors are still invaluable because not only transcription is the first step to produce protein products in cells, but the receptor transcription levels also affect the pain reduction by opioids, as observed in studies of heterozygous opioid receptor knockout mice.There are growing evidences that epigenetic regulation has played significant roles in transcriptional regulation of genes, including opioid receptors. In general, epigenetic mechanisms include three main regulatory factors: DNA methylation, chromatin modification, and noncoding RNAs (such as microRNA). From previous studies of ours and others on opioid receptors, those epigenetic factors were clearly involved in regulating opioid receptor expression in vivo and in vitro. In this chapter, among those three techniques we describe more details of DNA methylation methods because of emerging concepts of DNA methylation with the recent discovery of 5-hydroxymethylcytosine converting enzyme, TET1. Another analytical method of the epigenetic factors, chromatin modification, will be described briefly and information of analyzing noncoding RNAs is briefly mentioned in Subheading 1.

Role of MicroRNA in Visceral Pain

The long-lasting nociceptive transmission under various visceral pain conditions involves transcriptional and/or translational alteration in neurotransmitter and receptor expression as well as modification of neuronal function, morphology and synaptic connections. Although it is largely unknown how such changes in posttranscriptional expression induce visceral pain, recent evidence strongly suggests an important role for microRNAs (miRNAs, small non-coding RNAs) in the cellular plasticity underlying chronic visceral pain. MicroRNAs are small noncoding RNA endogenously produced in our body and act as a major regulator of gene expression by either through cleavage or translational repression of the target gene. This regulation is essential for the normal physiological function but when disturbed can result in pathological conditions. Usually one miRNA has multiple targets and target mRNAs are regulated in a combinatorial fashion by multiple miRNAs. In recent years, many studies have been performed to delineate the posttranscriptional regulatory role of miRNAs in different tissues under various nociceptive stimuli. In this review, we intend to discuss the recent development in miRNA research with special emphases on miRNAs and their targets responsible for long term sensitization in chronic pain conditions. In addition, we review miRNAs expression and function data for different animal pain models and also the recent progress in research on miRNA-based therapeutic targets for the treatment of chronic pain.

Epigenetic mechanisms of chronic pain

Neuropathic and inflammatory pain promote a large number of persisting adaptations at the cellular and molecular level, allowing even transient tissue or nerve damage to elicit changes in cells that contribute to the development of chronic pain and associated symptoms. There is evidence that injury-induced changes in chromatin structure drive stable changes in gene expression and neural function, which may cause several symptoms, including allodynia, hyperalgesia, anxiety, and depression. Recent findings on epigenetic changes in the spinal cord and brain during chronic pain may guide fundamental advances in new treatments. Here, we provide a brief overview of epigenetic regulation in the nervous system and then discuss the still-limited literature that directly implicates epigenetic modifications in chronic pain syndromes.

MicroRNA biology and pain

Endogenously expressed small noncoding microRNAs (miRNAs) play an important role in posttranscriptionally regulating gene expression by binding to mRNAs with complementary sequences. miRNA-mRNA interactions allow for cellular flexibility to fine-tune gene expression by controlling translation in response to a multitude of signaling events. Disease states or perturbations in cellular homeostasis can lead to aberrant miRNA expression. The discovery of stable miRNAs in circulation generated enormous interest in exploring their utility as potential noninvasive biomarkers. Additionally, selectively inhibiting or supplementing an miRNA contributing to pathogenesis is being pursued as a therapeutic strategy for a variety of disorders. Studies from rodent models of pain and patients have now implicated a role for miRNAs in mediating various aspects of pain processing. These noncoding RNAs can provide mechanistic insights into the pathways modulated and could serve as therapeutic targets. Here, we discuss the challenges associated with miRNA research and the promises ahead in this vastly unexplored avenue in pain biology.

Epigenetic regulation of persistent pain

Persistent or chronic pain is tightly associated with various environmental changes and linked to abnormal gene expression within cells processing nociceptive signaling. Epigenetic regulation governs gene expression in response to environmental cues. Recent animal model and clinical studies indicate that epigenetic regulation plays an important role in the development or maintenance of persistent pain and possibly the transition of acute pain to chronic pain, thus shedding light in a direction for development of new therapeutics for persistent pain.

microRNA and Pain

Pain is an important protective system that alerts organisms to actual or possible tissue damage. However, a variety of pathologies can lead to chronic pain that is no longer beneficial. Lesions or diseases of the somatosensory nervous system cause intractable neuropathic pain that occasionally lasts even after the original pathology subsides. Chronic inflammatory diseases like arthritis are also associated with severe pain. Because conventional analgesics such as non-steroidal anti-inflammatory drugs and opioids have limited efficacy and/or severe adverse events associated with long-term use, chronic pain remains a major problem in clinical practice. Recently, causal roles of microRNAs in chronic pain and their therapeutic potential have been emerging. microRNA expressions are altered not only at the primary origin of pain, but also along the somatosensory pathways. Notably, microRNA expressions are differentially affected depending on the causes of chronic pain. This chapter summarizes current insights into the roles of microRNAs in pain based on the underlying pathologies.

Renilla luciferase reporter assay to study 3'UTR-driven posttranscriptional regulations of OPRM1

The regulation of MOR expression at the level of mRNA is relevant for its role in pain transmission and in other functions involving opioid receptors. Recently, the role of the 3'UTR in the posttranscriptional regulation of MOR expression has been highlighted. Here we describe a Renilla luciferase reporter assay for the study of the effect of any selective treatment on the 3'UTR-dependent regulation of OPRM1 in a model of glial cells.

Endogenous opiates and behavior: 2013

This paper is the thirty-sixth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2013 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Epigenetic modification of spinal miR-219 expression regulates chronic inflammation pain by targeting CaMKIIγ

Emerging evidence has shown that miRNA-mediated gene expression modulation contributes to chronic pain, but its functional regulatory mechanism remains unknown. Here, we found that complete Freund's adjuvant (CFA)-induced chronic inflammation pain significantly reduced miRNA-219 (miR-219) expression in mice spinal neurons. Furthermore, the expression of spinal CaMKIIγ, an experimentally validated target of miR-219, was increased in CFA mice. Overexpression of spinal miR-219 prevented and reversed thermal hyperalgesia and mechanical allodynia and spinal neuronal sensitization induced by CFA. Concurrently, increased expression of spinal CaMKIIγ was reversed by miR-219 overexpression. Downregulation of spinal miR-219 in naive mice induced pain-responsive behaviors and increased p-NMDAR1 expression, which could be inhibited by knockdown of CaMKIIγ. Bisulfite sequencing showed that CFA induced the hypermethylation of CpG islands in the miR-219 promoter. Treatment with demethylation agent 5'-aza-2'-deoxycytidine markedly attenuated pain behavior and spinal neuronal sensitization, which was accompanied with the increase of spinal miR-219 and decrease of CaMKIIγ expression. Together, we conclude that methylation-mediated epigenetic modification of spinal miR-219 expression regulates chronic inflammatory pain by targeting CaMKIIγ.

MicroRNAs as modulators and biomarkers of inflammatory and neuropathic pain conditions. Neurobiol Dis. 2014;71:159-168. [PMID: 25119878 DOI: 10.1016/j.nbd.2014.08.003]

The post-transcriptional regulator molecules, microRNAs, have emerged as important biomarkers and modulators of numerous pathophysiological processes including oncogenesis and cardiovascular diseases. Recently, a significant number of dysregulations in microRNAs have been reported in patients suffering from painful disorders such as complex regional pain syndrome, cystitis-induced chronic pain and irritable bowel disorder, in both affected tissues and the circulation. Moreover, microRNAs are known to be involved in pain processing based on several recent findings in animal models of inflammatory and neuropathic pain. The basis of this review was to cover and summarize available articles in English encompassing "microRNA and pain". In animal pain models widespread microRNA modulation is present and manifests on multiple levels i.e.: the dorsal root ganglia, the spinal dorsal horn and the brain. Numerous functional in vivo studies have found that dysregulated microRNAs are involved in the post-transcriptional modulation of genes implicated in pain generation and maintenance. Lastly, a few animal studies have delivered promising results as to the possibility of applying microRNAs as therapeutics to alleviate established pain and several clinical studies have highlighted the potential in applying microRNAs as biomarkers in painful conditions such as complex regional pain syndrome and fibromyalgia. This review briefly introduces the basics of microRNAs, their biogenesis and function, and mainly focuses on the recent advances made in understanding the role of microRNAs in relation to pain processing and painful conditions. It also provides an overview of widely diverse methodological approaches and results with a potential for future implications of microRNAs in the diagnosis and treatment of pain.

Noncoding RNAs: key molecules in understanding and treating pain

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A comprehensive understanding of diverse ncRNAs in modulating pain is lacking.

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Among ncRNAs, miRNAs have been relatively well studied in pain regulation.

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lncRNAs also hold large potential for pain regulation.

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ncRNAs offer potential therapeutic options for treating chronic pain.

Although noncoding RNAs (ncRNAs) were initially considered to be transcriptional byproducts, recent technological advances have led to a steady increase in our understanding of their importance in gene regulation and disease pathogenesis. In keeping with these developments, pain research is also experiencing rapid growth in the investigation of links between ncRNAs and pathological pain. Although the initial focus was on analyzing expression and dysregulation of candidate miRNAs, elucidation of other ncRNAs and ncRNA-mediated functional mechanisms in pain modulation has just commenced. Here we review the major ncRNA literature available to date with respect to pain modulation and discuss tools and opportunities available for testing the impact of other types of ncRNA on pain.

Brain microRNAs and insights into biological functions and therapeutic potential of brain enriched miRNA-128

MicroRNAs, the non-coding single-stranded RNA of 19–25 nucleotides are emerging as robust players of gene regulation. Plethora of evidences support that the ability of microRNAs to regulate several genes of a pathway or even multiple cross talking pathways have significant impact on a complex regulatory network and ultimately the physiological processes and diseases. Brain being a complex organ with several cell types, expresses more distinct miRNAs than any other tissues. This review aims to discuss about the microRNAs in brain development, function and their dysfunction in brain tumors. We also provide a comprehensive summary of targets of brain specific and brain enriched miRNAs that contribute to the diversity and plasticity of the brain. In particular, we uncover recent findings on miRNA-128, a brain-enriched microRNA that is induced during neuronal differentiation and whose aberrant expression has been reported in several cancers. This review describes the wide spectrum of targets of miRNA-128 that have been identified till date with potential roles in apoptosis, angiogenesis, proliferation, cholesterol metabolism, self renewal, invasion and cancer progression and how this knowledge might be exploited for the development of future miRNA-128 based therapies for the treatment of cancer as well as metabolic diseases.

MicroRNA-based therapy in pain medicine: Current progress and future prospects

MicroRNAs (miRNAs) are small noncoding RNA molecules of 18-25 nucleotides in length that regulate gene expression involved in fundamental cell processes. The induction and chronification of pain is associated with many expressional changes in pain-related proteins. miRNA has the potential to regulate gene and protein expression associated with the induction and chronification of pain. Thus, miRNAs might have promise in therapy and as a diagnostic and prognostic biomarker in pain medicine. The application of miRNA has been an emerging field in pain research in recent years. Many studies focusing on the regulation of miRNAs under different tissue and nociceptive stimuli have been performed in recent years. In this review, we intend to introduce the most recent research in the field of miRNA related with pain medicine such as the expression and function of miRNA in different animal pain model, the challenge of application and delivery of miRNA in vivo, the potential toxic effects of miRNA and future problems in clinical application that need to be resolved. This review focuses on the results of miRNA in animal studies and the prospect for future success.

microRNAs in nociceptive circuits as predictors of future clinical applications

Neuro-immune alterations in the peripheral and central nervous system play a role in the pathophysiology of chronic pain, and non-coding RNAs – and microRNAs (miRNAs) in particular – regulate both immune and neuronal processes. Specifically, miRNAs control macromolecular complexes in neurons, glia and immune cells and regulate signals used for neuro-immune communication in the pain pathway. Therefore, miRNAs may be hypothesized as critically important master switches modulating chronic pain. In particular, understanding the concerted function of miRNA in the regulation of nociception and endogenous analgesia and defining the importance of miRNAs in the circuitries and cognitive, emotional and behavioral components involved in pain is expected to shed new light on the enigmatic pathophysiology of neuropathic pain, migraine and complex regional pain syndrome. Specific miRNAs may evolve as new druggable molecular targets for pain prevention and relief. Furthermore, predisposing miRNA expression patterns and inter-individual variations and polymorphisms in miRNAs and/or their binding sites may serve as biomarkers for pain and help to predict individual risks for certain types of pain and responsiveness to analgesic drugs. miRNA-based diagnostics are expected to develop into hands-on tools that allow better patient stratification, improved mechanism-based treatment, and targeted prevention strategies for high risk individuals.