Post-transcriptional regulation of mouse mu opioid receptor (MOR1) via its 3' untranslated region: a role for microRNA23b

miRNAs and Substances Abuse: Clinical and Forensic Pathological Implications: A Systematic Review

Substance addiction is a chronic and relapsing brain disorder characterized by compulsive seeking and continued substance use, despite adverse consequences. The high prevalence and social burden of addiction are indisputable; however, the available intervention is insufficient. The modulation of gene expression and aberrant adaptation of neural networks are attributed to the changes in brain functions under repeated exposure to addictive substances. Considerable studies have demonstrated that miRNAs are strong modulators of post-transcriptional gene expression in substance addiction. The emerging role of microRNA (miRNA) provides new insights into many biological and pathological processes in the central nervous system: their variable expression in different regions of the brain and tissues may play a key role in regulating the pathophysiological events of addiction. This work provides an overview of the current literature on miRNAs involved in addiction, evaluating their impaired expression and regulatory role in neuroadaptation and synaptic plasticity. Clinical implications of such modulatory capacities will be estimated. Specifically, it will evaluate the potential diagnostic role of miRNAs in the various stages of drug and substance addiction. Future perspectives about miRNAs as potential novel therapeutic targets for substance addiction and abuse will also be provided.

The temporal expression of circulating microRNAs after acute experimental pain in humans

BACKGROUND

MicroRNAs (miRNAs) can modulate several biological systems, including the pain system. This study aimed to evaluate the temporal expression of circulating miRNAs in the plasma of healthy volunteers as a marker for epigenetic changes before and after an acute, experimental, pain provocation by intramuscular hypertonic saline injection.

METHODS

Twenty volunteers were randomly allocated into two groups and received either hypertonic (pain) or isotonic (control) saline injection in the first dorsal interosseous muscle of their dominant hand. Pain intensity was continuously recorded for 20 minutes after injection on a VAS scale from 0 to 100 (0 indicates no pain and 100 the worst imaginable pain). Blood samples were taken at baseline, 30 minutes, 3 hours, and 24 hours post-injection, and plasma was separated. MiRNA extracts were used for RNA sequencing with the Illumina NextSeq platform. MiRNA transcripts were compared between the pain and the no-pain, control group at every time point. Significant differences were considered when folds were >2 and the False Discovery Rate was p < 0.05.

RESULTS

After 30 minutes, 4 miRNAs were significantly altered in the pain group compared to controls, which increased to 24 after 3 hours and to 42 after 24 hours from baseline (p < 0.0001). Two miRNAs were consistently upregulated throughout the experiment. Enrichment analysis showed significant miRNAs involved in brain perception of pain, brain signalling and response to stimuli.

CONCLUSIONS

This exploratory study is the first to report on the temporal expression of circulating miRNAs after an acute, human experimental muscle pain model.

SIGNIFICANCE

This exploratory study evaluated the temporal profile of circulating miRNAs in the plasma of healthy subjects after acute experimental pain. Several miRNAs were altered in subjects at the times of follow-up after the acute pain model when compared to controls. MiRNAs previously associated with pain processes were altered in the pain group. Our results, by showing the fast and prolonged modifications of miRNA elicited by the acute experimental pain model, add new perspectives to the topic of epigenetics and pain.

Association of OPRM1, MIR23B, and MIR107 genetic variability with acute pain, chronic pain and adverse effects after postoperative tramadol and paracetamol treatment in breast cancer

BACKGROUND

Tramadol is an opioid analgesic often used for pain management after breast cancer surgery. Its analgesic activity is due to the activation of the μ-opioid receptor, encoded by the OPRM1 gene. This study investigated the association of genetic variability in OPRM1 and its regulatory miRNA genes with outcomes of tramadol/paracetamol treatment after breast cancer surgery with axillary lymphadenectomy.

PATIENTS AND METHODS

The study included 113 breast cancer patients after breast cancer surgery with axillary lymphadenectomy treated with either 75/650 mg or 37.5/325 mg of tramadol with paracetamol for pain relief within the randomized clinical trial KCT 04/2015-DORETAonko/si at the Institute of Oncology Ljubljana. All patients were genotyped for OPRM1 rs1799971 and rs677830, MIR23B rs1011784, and MIR107 rs2296616 using competitive allele-specific PCR. The association of genetic factors with acute and chronic pain as well as adverse effects of tramadol treatment was evaluated using logistic regression, Fisher's exact test, and Mann-Whitney test.

RESULTS

The investigated OPRM1 related polymorphisms were not associated with acute pain assessed with the VAS scale within four weeks after surgery (all P > 0.05). Carriers of at least one polymorphic OPRM1 rs1799971 allele had a higher risk of constipation in the first four weeks after surgery compared to non-carriers (OR = 4.5, 95% CI = 1.6-12.64, P = 0.004). Carriers of at least one polymorphic OPRM1 rs677830 allele had a higher risk of constipation after third week of tramadol treatment (OR = 3.11, 95% CI = 1.08-8.89, P = 0.035). Furthermore, carriers of two polymorphic MIR23B rs1011784 alleles had a higher risk of nausea after 28 days of tramadol treatment (OR = 7.35, 95% CI = 1.27-42.6, P = 0.026), while heterozygotes for MIR107 rs2296616 allele had a lower risk of nausea after 21 days of tramadol treatment (OR = 0.21, 95% CI = 0.05-0.87, P = 0.031). In carriers of two polymorphic MIR107 rs2296616 alleles, chronic pain was significantly more common than in carriers of two wild-type alleles (P = 0.004). Carriers of at least one polymorphic MIR23B rs1011784 allele experienced more neuropathic pain after adjustment for tramadol dose (OR = 2.85, 95% CI = 1.07-7.59, P = 0.036), while carriers of at least one polymorphic OPRM1 rs677830 allele experienced less neuropathic pain compared to carriers of two wild-type alleles (OR = 0.38, 95% CI = 0.15-0.99, P = 0.047).

CONCLUSIONS

Genetic variability of OPRM1 and genes coding for miRNAs that could affect OPRM1 expression may be associated with adverse effects of tramadol/paracetamol treatment as well as with chronic and neuropathic pain after breast cancer surgery with axillary lymphadenectomy.

Overview of Genetic Analysis of Human Opioid Receptors

The human μ-opioid receptor gene (OPRM1 ), due to its genetic and structural variation, has been a target of interest in several pharmacogenetic studies. The μ-opioid receptor (MOR ), encoded by OPRM1 , contributes to regulate the analgesic response to pain and also controls the rewarding effects of many drugs of abuse, including opioids, nicotine, and alcohol. Genetic polymorphisms of opioid receptors are candidates for the variability of clinical opioid effects. The non-synonymous polymorphism A118G of the OPRM1 has been repeatedly associated with the efficacy of treatments for pain and various types of dependence. Genetic analysis of human opioid receptors has evidenced the presence of numerous polymorphisms either in exonic or in intronic sequences as well as the presence of synonymous coding variants that may have important effects on transcription, mRNA stability, and splicing, thus affecting gene function despite not directly disrupting any specific residue. Genotyping of opioid receptors is still in its infancy and a relevant progress in this field can be achieved by using advanced gene sequencing techniques described in this review that allow researchers to obtain vast quantities of data on human genomes and transcriptomes in a brief period of time and with affordable costs.

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.

Morphine modulates the expression of mu-opioid receptor exon 5-associated full-length C-terminal splice variants by upregulating miR-378a-3p

The mu-opioid receptor gene, OPRM1, undergoes extensive alternative splicing, creating an array of splice variants that are conserved from rodent to human. Both mouse and human OPRM1 have five exon 5-associated seven transmembrane full-length carboxyl terminal variants, MOR-1B1, MOR-1B2, MOR-1B3, MOR-1B4, and MOR-1B5, all of which are derived from alternative 3' splicing from exon 3 to alternative sites within exon 5. The functional relevance of these exon 5-associated MOR-1Bs has been demonstrated in mu agonist-induced G protein coupling, adenylyl cyclase activity, receptor internalization and desensitization, and post-endocytic sorting, as well as region-specific expression at the mRNA level. In the present study, we mapped a polyadenylation site for both mouse and human MOR-1Bs that defines the 3'-untranslated regions (3'-UTR) of MOR-1Bs and stabilizes mMOR-1Bs mRNAs. We identified a conserved miR378a-3p sequence in the 3'-UTR of both mouse and human MOR-1B_S transcripts through which miR-378a-3p can regulate the expression of MOR-1Bs at the mRNA level. Chronic morphine treatment significantly increased the miR-378-3p level in Be(2)C cells and the brainstem of the morphine tolerant mice, contributing to the decreased expression of the mouse and human MOR-1B3 and MOR-1B4. Our study provides new insights into the role of miRNAs and Oprm1 splice variants in morphine tolerance.

Decreased Level of Blood MicroRNA-133b in Men with Opioid Use Disorder on Methadone Maintenance Therapy

Although previous animal studies have indicated that certain micro ribonucleic acids (microRNAs) play a part in the pathway of opioid addiction, whether such findings extend to human models is yet unknown. This study aims to investigate the important microRNA expressions in patients with opioid use disorder (OUD) on methadone maintenance treatment (MMT) compared to healthy controls and analyze the correlation between microRNAs and opioid characteristics among the patients. We recruited 50 patients and 25 controls, and both groups were matched regarding gender, age, and body mass index. Serum microRNAs (miR-133b, miR-23b, miR-190, miR-206, miR-210, and miR-21) were measured. The age of OUD onset, duration of MMT participation, and recent daily methadone dosage were considered the opioid characteristics. We adopted the t-test to compare the difference between patients and controls and Pearson's correlation to evaluate the association between microRNAs and opioid profiles. Only the level of miR-133b in OUD patients on MMT was significantly lower than that in healthy controls. We did not detect differences of any other microRNA expressions between the two groups. Furthermore, we found no evidence to support the association between microRNAs and opioid characteristics. This study indicates that miR-133b values may be decreased in OUD patients on MMT.

Novel biomarkers to assess in utero effects of maternal opioid use: First steps toward understanding short- and long-term neurodevelopmental sequelae

Maternal opioid use disorder is common, resulting in significant neonatal morbidity and cost. Currently, it is not possible to predict which opioid-exposed newborns will require pharmacotherapy for neonatal abstinence syndrome. Further, little is known regarding the effects of maternal opioid use disorder on the developing human brain. We hypothesized that novel methodologies utilizing fetal central nervous system-derived extracellular vesicles isolated from maternal blood can address these gaps in knowledge. Plasma from opioid users and controls between 9 and 21 weeks was precipitated and extracellular vesicles were isolated. Mu opioid and cannabinoid receptor levels were quantified. Label-free proteomics studies and unbiased small RNA next generation sequencing was performed in paired fetal brain tissue. Maternal opioid use disorder increased mu opioid receptor protein levels in extracellular vesicles independent of opioid equivalent dose. Moreover, cannabinoid receptor levels in extracellular vesicles were upregulated with opioid exposure indicating cross talk with endocannabinoids. Maternal opioid use disorder was associated with significant changes in extracellular vesicle protein cargo and fetal brain micro RNA expression, especially in male fetuses. Many of the altered cargo molecules and micro RNAs identified are associated with adverse clinical neurodevelopmental outcomes. Our data suggest that assays relying on extracellular vesicles isolated from maternal blood extracellular vesicles may provide information regarding fetal response to opioids in the setting of maternal opioid use disorder. Prospective clinical studies are needed to evaluate the association between extracellular vesicle biomarkers, risk of neonatal abstinence syndrome and neurodevelopmental outcomes.

The Emerging Perspective of Morphine Tolerance: MicroRNAs

Morphine has unfavorable side effects including analgesic tolerance. Morphine tolerance counteracts analgesic efficacy and drives dose escalation. The mechanisms underlying morphine tolerance remain disputed, which has prevented the development of therapies to maximize and sustain analgesic efficacy. Morphine tolerance is an adaptive process induced by chronic morphine that has been shown to result from complex alterations at the molecular level with μ opioid receptors (MORs), as well as at the synaptic, cellular, and circuit levels. MicroRNAs are noncoding RNAs that have been proposed to regulate gene expression and degradation at the posttranscriptional level, including the MOR, as well as synaptic plasticity and neuroplasticity, in both the peripheral and central nervous systems. This review covers some of the most striking microRNA functions involved in morphine tolerance and presents limitations on our knowledge of their physiological roles.

Emerging roles of microRNAs in morphine tolerance

Morphine is commonly used in clinical management to alleviate moderate-to-severe pain. However, prolonged and repeated use of morphine leads to tolerance. Morphine tolerance is a challenging clinical problem that limits its clinical application in pain treatment. The mechanisms underlying morphine tolerance are still not completely understood. MicroRNAs (miRNAs) are small noncoding RNAs containing 18~22 nucleotides that modulate gene expression in a post-transcriptional manner, and their dysregulation causes various diseases. miRNAs bind to the 3ʹ-UTR (untranslated region) of target gene mRNA, inhibiting or destabilizing translation of the transcripts. Morphine causes differential miRNA upregulation or downregulation. This review will present evidence for the contribution of miRNAs to tolerance of the antinociception effect of opioids.

Targeted Endoplasmic Reticulum Localization of Storage Protein mRNAs Requires the RNA-Binding Protein RBP-L

The transport and targeting of glutelin and prolamine mRNAs to distinct subdomains of the cortical endoplasmic reticulum is a model for mRNA localization in plants. This process requires a number of RNA-binding proteins (RBPs) that recognize and bind to mRNA cis-localization (zipcode) elements to form messenger ribonucleoprotein complexes, which then transport the RNAs to their destination sites at the cortical endoplasmic reticulum. Here, we present evidence that the rice (Oryza sativa) RNA-binding protein, RBP-L, like its interacting RBP-P partner, specifically binds to glutelin and prolamine zipcode RNA sequences and is required for proper mRNA localization in rice endosperm cells. A transfer DNA insertion in the 3' untranslated region resulted in reduced expression of the RBP-L gene to 10% to 25% of that in the wild-type. Reduced amounts of RBP-L caused partial mislocalization of glutelin and prolamine RNAs and conferred other general growth defects, including dwarfism, late flowering, and smaller seeds. Transcriptome analysis showed that RBP-L knockdown greatly affected the expression of prolamine family genes and several classes of transcription factors. Collectively, these results indicate that RBP-L, like RBP-P, is a key RBP involved in mRNA localization in rice endosperm cells. Moreover, distinct from RBP-P, RBP-L exhibits additional regulatory roles in development, either directly through its binding to corresponding RNAs or indirectly through its effect on transcription factors.

Modulation of miR-139-5p on chronic morphine-induced, naloxone-precipitated cAMP overshoot in vitro

Chronic exposure to morphine can produce tolerance, dependence and addiction, but the underlying neurobiological basis is still incompletely understood. c-Jun, as an important component of the activator protein-1 transcription factor, is supposed to take part in regulating gene expression in AC/cAMP/PKA signaling. MicroRNA (miRNA) has emerged as a critical regulator of neuronal functions. Although a number of miRNAs have been reported to regulate the μ-opioid receptor expression, there has been no report about miRNAs to regulate chronic morphine-induced, naloxone-precipitated cAMP overshoot. Our results showed that chronic morphine pretreatment induced naloxone-precipitated cAMP overshoot in concentration- and time-dependent manners in HEK 293/μ cells. Chronic morphine pretreatment alone elevated both c-Jun protein and miR-139-5p expression levels, while dramatically artificial elevation of miR-139-5p inhibited c-Jun at the translational level. Furthermore, dramatically artificial upregulation of intracellular miR-139-5p limited chronic morphine-induced, naloxone-precipitated cAMP overshoot. These findings suggested that miR-139-5p was involved in regulating chronic morphine-induced, naloxone-precipitated cAMP overshoot in a negative feedback manner through its target c-Jun, which extends our understanding of neurobiological mechanisms underlying morphine dependence and addiction.

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.

Phosphorylation of poly(rC) binding protein 1 (PCBP1) contributes to stabilization of mu opioid receptor (MOR) mRNA via interaction with AU-rich element RNA-binding protein 1 (AUF1) and poly A binding protein (PABP)

Gene regulation at the post-transcriptional level is frequently based on cis- and trans-acting factors on target mRNAs. We found a C-rich element (CRE) in mu-opioid receptor (MOR) 3'-untranslated region (UTR) to which poly (rC) binding protein 1 (PCBP1) binds, resulting in MOR mRNA stabilization. RNA immunoprecipitation and RNA EMSA revealed the formation of PCBP1-RNA complexes at the element. Knockdown of PCBP1 decreased MOR mRNA half-life and protein expression. Stimulation by forskolin increased cytoplasmic localization of PCBP1 and PCBP1/MOR 3'-UTR interactions via increased serine phosphorylation that was blocked by protein kinase A (PKA) or (phosphatidyl inositol-3) PI3-kinase inhibitors. The forskolin treatment also enhanced serine- and tyrosine-phosphorylation of AU-rich element binding protein (AUF1), concurrent with its increased binding to the CRE, and led to an increased interaction of poly A binding protein (PABP) with the CRE and poly(A) sites. AUF1 phosphorylation also led to an increased interaction with PCBP1. These findings suggest that a single co-regulator, PCBP1, plays a crucial role in stabilizing MOR mRNA, and is induced by PKA signaling by conforming to AUF1 and PABP.

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.

Decoding the ubiquitous role of microRNAs in neurogenesis

Neurogenesis generates fledgling neurons that mature to form an intricate neuronal circuitry. The delusion on adult neurogenesis was far resolved in the past decade and became one of the largely explored domains to identify multifaceted mechanisms bridging neurodevelopment and neuropathology. Neurogenesis encompasses multiple processes including neural stem cell proliferation, neuronal differentiation, and cell fate determination. Each neurogenic process is specifically governed by manifold signaling pathways, several growth factors, coding, and non-coding RNAs. A class of small non-coding RNAs, microRNAs (miRNAs), is ubiquitously expressed in the brain and has emerged to be potent regulators of neurogenesis. It functions by fine-tuning the expression of specific neurogenic gene targets at the post-transcriptional level and modulates the development of mature neurons from neural progenitor cells. Besides the commonly discussed intrinsic factors, the neuronal morphogenesis is also under the control of several extrinsic temporal cues, which in turn are regulated by miRNAs. This review enlightens on dicer controlled switch from neurogenesis to gliogenesis, miRNA regulation of neuronal maturation and the differential expression of miRNAs in response to various extrinsic cues affecting neurogenesis.

Interaction of drugs of abuse and microRNA with HIV: a brief review

MicroRNAs (miRNAs), the post-transcriptional regulators of gene expression, play key roles in modulating many cellular processes. The changes in the expression profiles of several specific miRNAs affect the interactions between miRNA and their targets in various illnesses, including addiction, HIV, cancer etc. The presence of anti-HIV-1 microRNAs (which regulate the level of infectivity of HIV-1) have been validated in the cells which are the primary targets of HIV infection. Drugs of abuse impair the intracellular innate anti-HIV mechanism(s) in monocytes, contributing to cell susceptibility to HIV infection. Emerging evidence has implicated miRNAs are differentially expressed in response to chronic morphine treatment. Activation of mu opioid receptors (MOR) by morphine is shown to down regulate the expression of anti-HIV miRNAs. In this review, we summarize the results which demonstrate that several drugs of abuse related miRNAs have roles in the mechanisms that define addiction, and how they interact with HIV.

Morphine modulates mouse hippocampal progenitor cell lineages by upregulating miR-181a level

The mechanism by which addictive drugs such as morphine regulate adult neurogenesis remains elusive. We now demonstrate that morphine can regulate neurogenesis by control of miR-181a and subsequent hippocampal neural progenitor cell (hNPC) lineages. In the presence of morphine, hNPCs preferentially differentiated into astrocytes, an effect blocked by the specific μ-opioid receptor antagonist, Cys(2)-Tyr(3)-Orn(5)-Pen(7)-amide. This effect was mediated by the Prox1/Notch1 pathway as demonstrated by an increase in Notch1 level in the morphine- but not fentanyl-treated hNPCs and blocked by overexpression of Notch1 siRNA. Overexpression of Prox1 siRNA upregulated Notch1 level and potentiated the morphine-induced lineage changes. Prox1 transcript level was regulated by direct interaction between miR-181a and its 3'-UTR sequence. In vitro and in vivo treatment with morphine resulted in an increase in miR-181a level in hNPCs and mouse hippocampi, respectively. Overexpression of miR-181a mimics reduced Prox1 levels, increased Notch1 levels, and enhanced hNPCs differentiation into astrocytes. Meanwhile, overexpression of the miR-181a inhibitor raised Prox1 levels, decreased Notch1 levels, and subsequently blocked the morphine-induced lineage changes. Thus, by modulating Prox1/Notch1 activities via miR-181a, morphine influences the fate of differentiating hNPCs differentiation and therefore the ultimate quantities of mature neurons and astrocytes.

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.

Overview of genetic analysis of human opioid receptors

The human μ-opioid receptor gene (OPRM1), due to its genetic and structural variation, has been a target of interest in several pharmacogenetic studies. The μ-opioid receptor (MOR), encoded by OPRM1, contributes to regulate the analgesic response to pain and also controls the rewarding effects of many drugs of abuse, including opioids, nicotine, and alcohol. Genetic polymorphisms of opioid receptors are candidates for the variability of clinical opioid effects. The non-synonymous polymorphism A118G of the OPRM1 has been repeatedly associated with the efficacy of opioid treatments for pain and various types of dependence. Genetic analysis of human opioid receptors has evidenced the presence of numerous polymorphisms either in exonic or in intronic sequences as well as the presence of synonymous coding variants that may have important effects on transcription, mRNA stability, and splicing, thus affecting gene function despite not directly disrupting any specific residue. Genotyping of opioid receptors is still in its infancy and a relevant progress in this field can be achieved by using advanced gene sequencing techniques described in this review that allow the researchers to obtain vast quantities of data on human genomes and transcriptomes in a brief period of time and with affordable costs.

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.

MicroRNAs as modulators and biomarkers of inflammatory and neuropathic pain conditions

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.

Morphine regulates expression of μ-opioid receptor MOR-1A, an intron-retention carboxyl terminal splice variant of the μ-opioid receptor (OPRM1) gene via miR-103/miR-107

The μ-opioid receptor (MOR-1) gene OPRM1 undergoes extensive alternative splicing, generating an array of splice variants. Of these variants, MOR-1A, an intron-retention carboxyl terminal splice variant identical to MOR-1 except for the terminal intracellular tail encoded by exon 3b, is quite abundant and conserved from rodent to humans. Increasing evidence indicates that miroRNAs (miRNAs) regulate MOR-1 expression and that μ agonists such as morphine modulate miRNA expression. However, little is known about miRNA regulation of the OPRM1 splice variants. Using 3'-rapid amplification cDNA end and Northern blot analyses, we identified the complete 3'-untranslated region (3'-UTR) for both mouse and human MOR-1A and their conserved polyadenylation site, and defined the role the 3'-UTR in mRNA stability using a luciferase reporter assay. Computer models predicted a conserved miR-103/107 targeting site in the 3'-UTR of both mouse and human MOR-1A. The functional relevance of miR-103/107 in regulating expression of MOR-1A protein through the consensus miR-103/107 binding sites in the 3'-UTR was established by using mutagenesis and a miR-107 inhibitor in transfected human embryonic kidney 293 cells and Be(2)C cells that endogenously express human MOR-1A. Chronic morphine treatment significantly upregulated miR-103 and miR-107 levels, leading to downregulation of polyribosome-associated MOR-1A in both Be(2)C cells and the striatum of a morphine-tolerant mouse, providing a new perspective on understanding the roles of miRNAs and OPRM1 splice variants in modulating the complex actions of morphine in animals and humans.

Pharmacogenetics of OPRM1

Pharmacogenetic research has the potential to explain the variation in treatment efficacy within patient populations. Understanding the interaction between genetic variation and medications may provide a method for matching patients to the most effective therapeutic options and improving overall patient outcomes. The OPRM1 gene has been a target of interest in a large number of pharmacogenetic studies due to its genetic and structural variation, as well as the role of opioid receptors in a variety of disorders. The mu-opioid receptor (MOR), encoded by OPRM1, naturally regulates the analgesic response to pain and also controls the rewarding effects of many drugs of abuse, including opioids, nicotine, and alcohol. Genetic variants in OPRM1, particularly the non-synonymous polymorphism A118G, have been repeatedly associated with the efficacy of treatments for pain and various types of dependence. This review focuses on the current understanding of the pharmacogenetic impact of OPRM1, primarily with regard to the treatment of pain and addiction.

Neuroplasticity, axonal guidance and micro-RNA genes are associated with morphine self-administration behavior

Neuroadaptations in the ventral striatum (VS) and ventral midbrain (VMB) following chronic opioid administration are thought to contribute to the pathogenesis and persistence of opiate addiction. In order to identify candidate genes involved in these neuroadaptations, we utilized a behavior-genetics strategy designed to associate contingent intravenous drug self-administration with specific patterns of gene expression in inbred mice differentially predisposed to the rewarding effects of morphine. In a Yoked-control paradigm, C57BL/6J mice showed clear morphine-reinforced behavior, whereas DBA/2J mice did not. Moreover, the Yoked-control paradigm revealed the powerful consequences of self-administration versus passive administration at the level of gene expression. Morphine self-administration in the C57BL/6J mice uniquely up- or down-regulated 237 genes in the VS and 131 genes in the VMB. Interestingly, only a handful of the C57BL/6J self-administration genes (<3%) exhibited a similar expression pattern in the DBA/2J mice. Hence, specific sets of genes could be confidently assigned to regional effects of morphine in a contingent- and genotype-dependent manner. Bioinformatics analysis revealed that neuroplasticity, axonal guidance and micro-RNAs (miRNAs) were among the key themes associated with drug self-administration. Noteworthy were the primary miRNA genes H19 and micro-RNA containing gene (Mirg), processed, respectively, to mature miRNAs miR-675 and miR-154, because they are prime candidates to mediate network-like changes in responses to chronic drug administration. These miRNAs have postulated roles in dopaminergic neuron differentiation and mu-opioid receptor regulation. The strategic approach designed to focus on reinforcement-associated genes provides new insight into the role of neuroplasticity pathways and miRNAs in drug addiction.

Epigenetics: a promising paradigm for better understanding and managing pain

Epigenetic regulation of gene expression is a rapidly growing area of research. Considering the longevity and plasticity of neurons, the studies on epigenetic pathways in the nervous system should be of special interest for both epigeneticists and neuroscientists. Activation or inactivation of different epigenetic pathways becomes more pronounced when the cells experience rapid changes in their environment, and such changes can be easily caused by injury and inflammation, resulting in pain perception or distortion of pain perception (eg, hyperalgesia). Therefore, in this regard, the field of pain is at an advantage to study the epigenetic pathways. More importantly, understanding pain from an epigenetics point of view would provide a new paradigm for developing drugs or strategies for pain management. In this review, we introduce basic concepts of epigenetics, including chromatin dynamics, histone modifications, DNA methylation, and RNA-induced gene silencing. In addition, we provide evidence from published studies suggesting wide implication of different epigenetic pathways within pain pathways.

PERSPECTIVE

This article provides a brief overview of epigenetic pathways for gene regulation and highlights their involvement in pain. Our goal is to expose the readers to these concepts so that pain-related phenotypes can be investigated from the epigenetic point of view.

A single gene defect causing claustrophobia

Claustrophobia, the well-known fear of being trapped in narrow/closed spaces, is often considered a conditioned response to traumatic experience. Surprisingly, we found that mutations affecting a single gene, encoding a stress-regulated neuronal protein, can cause claustrophobia. Gpm6a-deficient mice develop normally and lack obvious behavioral abnormalities. However, when mildly stressed by single-housing, these mice develop a striking claustrophobia-like phenotype, which is not inducible in wild-type controls, even by severe stress. The human GPM6A gene is located on chromosome 4q32-q34, a region linked to panic disorder. Sequence analysis of 115 claustrophobic and non-claustrophobic subjects identified nine variants in the noncoding region of the gene that are more frequent in affected individuals (P=0.028). One variant in the 3'untranslated region was linked to claustrophobia in two small pedigrees. This mutant mRNA is functional but cannot be silenced by neuronal miR124 derived itself from a stress-regulated transcript. We suggest that loosing dynamic regulation of neuronal GPM6A expression poses a genetic risk for claustrophobia.

MicroRNAs in opioid pharmacology

MicroRNAs (miRNA), a class of ~22-nucleotide RNA molecules, are important gene regulators that bind to the target sites of mRNAs to inhibit the gene expressions either through translational inhibition or mRNA destabilization. There are growing evidences that miRNAs have played several regulatory roles in opioid pharmacology. Like other research fields such as cancer biology, the area where numerous miRNAs are found to be involved in gene regulation, we assume that in opioid studies including research fields of drug additions and opioid receptor regulation, there may be more miRNAs waiting to be discovered. This review will summarize our current knowledge of miRNA functions on opioids biology and briefly describe future research directions of miRNAs related to opioids.

Cocaine modulates the expression of opioid receptors and miR-let-7d in zebrafish embryos

Prenatal exposure to cocaine, in mammals, has been shown to interfere with the expression of opioid receptors, which can have repercussions in its activity. Likewise, microRNAs, such as let-7, have been shown to regulate the expression of opioid receptors and hence their functions in mammals and in vitro experiments. In light of this, using the zebrafish embryos as a model our aim here was to evaluate the actions of cocaine in the expression of opioid receptors and let-7d miRNA during embryogenesis. In order to determine the effects produced by cocaine on the opioid receptors (zfmor, zfdor1 and zfdor2) and let-7d miRNA (dre-let-7d) and its precursors (dre-let-7d-1 and dre-let-7d-2), embryos were exposed to 1.5 µM cocaine hydrochloride (HCl). Our results revealed that cocaine upregulated dre-let-7d and its precursors, and also increased the expression of zfmor, zfdor1 and zfdor2 during early developmental stages and decreased them in late embryonic stages. The changes observed in the expression of opioid receptors might occur through dre-let-7d, since DNA sequences and the morpholinos of opioid receptors microinjections altered the expression of dre-let-7d and its precursors. Likewise, opioid receptors and dre-let-7d showed similar distributions in the central nervous system (CNS) and at the periphery, pointing to a possible interrelationship between them.In conclusion, the silencing and overexpression of opioid receptors altered the expression of dre-let-7d, which points to the notion that cocaine via dre-let-7 can modulate the expression of opioid receptors. Our study provides new insights into the actions of cocaine during zebrafish embryogenesis, indicating a role of miRNAs, let-7d, in development and its relationship with gene expression of opioid receptors, related to pain and addiction process.

Morphine and microRNA Activity: Is There a Relation with Addiction?

When we talk about drug addiction, we are really dealing with an extremely complex system in which there still remain many unknowns and where many empty spaces or missing links are still present. Recent studies have identified changes in the expression profiles of several specific miRNAs which affect the interactions between these molecules and their targets in various illnesses, including addiction, and which may serve as valuable targets for more efficient therapies. In this review, we summarize results which clearly demonstrate that several morphine-related miRNAs have roles in the mechanisms that define addiction. In this regard, morphine has been shown to have an important role in the regulation of different miRNAs, such as miR-let-7 [which works as a mediator of the movement of the mu opioid receptor (MOR) mRNA into P-bodies, leading to translational repression], miR-23b (involved in linking MOR expression and morphine treatment at the post-transcriptional level), and miR-190 (a key post-transcriptional repressor of neurogenic differentiation, NeuroD). Fentanyl increases NeuroD levels by reducing the amount of miR-190, but morphine does not affect the levels of NeuroD. We also discuss the relationship between morphine, miRNAs, and the immune system, based on the discovery that morphine treatment of monocytes led to a decrease in several anti-HIV miRNAs (mir-28, 125b, 150, and 382). This review is centered on miR-133b and its possible involvement in addiction through the effects of morphine. We establish the importance of miR-133b as a regulatory factor by summarizing its activity in different pathological processes, especially cancer. Using the zebrafish as a research model, we discuss the relationship between mir-133b, the dopaminergic system, and morphine, considering: (1) that morphine modulates the expression of miR-133b and of its target transcript Pitx3, (2) the role of the zebrafish mu opioid receptor (zfMOR) in morphine-induced regulation of miR-133b, which depends on ERK1/2, (3) that morphine regulates miR-133b in hippocampal neurons, and (4) the role of delta opioid receptors in morphine-induced regulation of miR-133b. We conclude that the control of miR-133b levels may be a mechanism for the development of addiction to morphine, or other drugs of abuse that increase dopaminergic levels in the extracellular space. These results show that miR-133b is a possible new target for the design of new treatments against addictive disorders.

MALAT-1, a non protein-coding RNA is upregulated in the cerebellum, hippocampus and brain stem of human alcoholics

Chronic alcohol intake induces neurochemical adaptative changes in the brain characterised by altered gene expression. A role for non-coding RNAs in alcoholism is beginning to emerge. PCR-differential display using total RNA extracted from brain material of human alcoholics and control cases identified a cDNA fragment corresponding to a section of a known non protein-coding RNA (ncRNA), MALAT-1, (also known as NEAT2). Comparison of mRNA levels of MALAT-1 was performed by northern and dot blot experiments using different regions of brain from human alcoholics and rats chronically treated with ethanol vapours and following withdrawal. A massive increase of MALAT-1 transcripts was detected in cerebellum of human alcoholics and increases were also noted in hippocampus and brain stem, while no significant increase of MALAT-1 expression was noted in frontal or motor cortices. In the rat no significant difference of MALAT-1 ortholog mRNA could be detected in cerebellum. In addition, similarly to humans, no significant increase of MALAT-1 expression was detected in cortex of alcohol-treated rats, however, after 24 h alcohol withdrawal, a significant upregulation of MALAT-1 expression was observed in rat cortex. MALAT-1 is upregulated in specific regions of the human alcoholic brain and following alcohol withdrawal in the rat. As MALAT-1 regulates RNA processing, this suggests that alcohol-induced upregulation of MALAT-1 represents an important novel mechanism for alcohol actions in the CNS.

MicroRNA 339 down-regulates μ-opioid receptor at the post-transcriptional level in response to opioid treatment

μ-Opioid receptor (MOR) level is directly related to the function of opioid drugs, such as morphine and fentanyl. Although agonist treatment generally does not affect transcription of mor, previous studies suggest that morphine can affect the translation efficiency of MOR transcript via microRNAs (miRNAs). On the basis of miRNA microarray analyses of the hippocampal total RNA isolated from mice chronically treated with μ-opioid agonists, we found a miRNA (miR-339-3p) that was consistently and specifically increased by morphine (2-fold) and by fentanyl (3.8-fold). miR-339-3p bound to the MOR 3'-UTR and specifically suppressed reporter activity. Suppression was blunted by adding miR-339-3p inhibitor or mutating the miR-339-3p target site. In cells endogenously expressing MOR, miR-339-3p inhibited the production of MOR protein by destabilizing MOR mRNA. Up-regulation of miR-339-3p by fentanyl (EC(50)=0.75 nM) resulted from an increase in primary miRNA transcript. Mapping of the miR-339-3p primary RNA and its promoter revealed that the primary miR-339-3p was embedded in a noncoding 3'-UTR region of an unknown host gene and was coregulated by the host promoter. The identified promoter was activated by opioid agonist treatment (10 nM fentanyl or 10 μM morphine), a specific effect blocked by the opioid antagonist naloxone (10 μM). Taken together, these results suggest that miR-339-3p may serve as a negative feedback modulator of MOR signals by regulating intracellular MOR biosynthesis.

MicroRNAs in learning, memory, and neurological diseases

MicroRNAs (miRNAs) represent a class of small regulatory noncoding RNAs ∼22 bp in length that mediate post-transcriptional silencing of gene expression via the recognition of specific sequences in target messenger (m)RNAs. The current body of literature suggests that miRNAs are fine-tuning regulators of gene expression profiles in a wide range of biological processes, from development to cancer. Many miRNAs are highly expressed in the adult nervous system in a spatially and temporally controlled manner in normal physiology, as well as in certain pathological conditions. These findings emphasize that gene regulation networks based on miRNA activities may be particularly important to brain function, and that perturbation of these networks may result in abnormal brain function. Indeed, miRNAs have been implicated in various aspects of dendrite remodeling and synaptic plasticity, as well as in experience-dependent adaptive changes of neural circuits in the postnatal developmental and adult brain. Recent advances in methods of next-generation sequencing, such as RNA-seq, offer the means to quantitatively evaluate the functions of miRNAs in a genome-wide manner in large cohorts of samples. These new technologies have already yielded valuable information and are expanding our understanding of miRNA-based mechanisms in higher-order brain processing, including learning and memory and cognition, as well as in neuropsychiatric disorders.

Epigenetics of µ-opioid receptors: intersection with HIV-1 infection of the central nervous system

The abuse of intravenous drugs, such as heroin, has become a major public health concern due to the increased risk of HIV-1 infection. Opioids such as heroin were originally identified and subsequently abused for their analgesic effects. However, many investigations have found additional effects of opioids, including regulation of the immune system. As such, chronic opioid abuse has been shown to promote HIV-1 pathogenesis and facilitate HIV-1-associated neurocognitive dysfunction. Clinical opioids, such as morphine and methadone, as well as illicit opioids, such as heroin, exert their effects primarily through interactions with the µ-opioid receptor (MOR). However, the mechanisms by which opioids enhance neurocognitive dysfunction through MOR-mediated signaling pathways are not completely understood. New findings in the regulation of MOR expression, particularly epigenetic and transcriptional regulation as well as alternative splicing, sheds new insights into possible mechanisms of HIV-1 and opiate synergy. In this review, we identify mechanisms regulating MOR expression and propose novel mechanisms by which opioids and HIV-1 may modulate this regulation. Additionally, we suggest that differential regulation of newly identified MOR isoforms by opioids and HIV-1 has functional consequence in enhancing HIV-1 neurocognitive dysfunction.

Rheb, an activator of target of rapamycin, in the blackback land crab, Gecarcinus lateralis: cloning and effects of molting and unweighting on expression in skeletal muscle

Molt-induced claw muscle atrophy in decapod crustaceans facilitates exuviation and is coordinated by ecdysteroid hormones. There is a 4-fold reduction in mass accompanied by remodeling of the contractile apparatus, which is associated with an 11-fold increase in myofibrillar protein synthesis by the end of the premolt period. Loss of a walking limb or claw causes a loss of mass in the associated thoracic musculature; this unweighting atrophy occurs in intermolt and is ecdysteroid independent. Myostatin (Mstn) is a negative regulator of muscle growth in mammals; it suppresses protein synthesis, in part, by inhibiting the insulin/metazoan target of rapamycin (mTOR) signaling pathway. Signaling via mTOR activates translation by phosphorylating ribosomal S6 kinase (s6k) and 4E-binding protein 1. Rheb (Ras homolog enriched in brain), a GTP-binding protein, is a key activator of mTOR and is inhibited by Rheb-GTPase-activating protein (GAP). Akt protein kinase inactivates Rheb-GAP, thus slowing Rheb-GTPase activity and maintaining mTOR in the active state. We hypothesized that the large increase in global protein synthesis in claw muscle was due to regulation of mTOR activity by ecdysteroids, caused either directly or indirectly via Mstn. In the blackback land crab, Gecarcinus lateralis, a Mstn-like gene (Gl-Mstn) is downregulated as much as 17-fold in claw muscle during premolt and upregulated 3-fold in unweighted thoracic muscle during intermolt. Gl-Mstn expression in claw muscle is negatively correlated with hemolymph ecdysteroid level. Full-length cDNAs encoding Rheb orthologs from three crustacean species (G. lateralis, Carcinus maenas and Homarus americanus), as well as partial cDNAs encoding Akt (Gl-Akt), mTOR (Gl-mTOR) and s6k (Gl-s6k) from G. lateralis, were cloned. The effects of molting on insulin/mTOR signaling components were quantified in claw closer, weighted thoracic and unweighted thoracic muscles using quantitative polymerase chain reaction. Gl-Rheb mRNA levels increased 3.4-fold and 3.9-fold during premolt in claw muscles from animals induced to molt by eyestalk ablation (ESA) and multiple leg autotomy (MLA), respectively, and mRNA levels were positively correlated with hemolymph ecdysteroids. There was little or no effect of molting on Gl-Rheb expression in weighted thoracic muscle and no correlation of Gl-Rheb mRNA with ecdysteroid titer. There were significant changes in Gl-Akt, Gl-mTOR and Gl-s6k expression with molt stage. These changes were transient and were not correlated with hemolymph ecdysteroids. The two muscles differed in terms of the relationship between Gl-Rheb and Gl-Mstn expression. In thoracic muscle, Gl-Rheb mRNA was positively correlated with Gl-Mstn mRNA in both ESA and MLA animals. By contrast, Gl-Rheb mRNA in claw muscle was negatively correlated with Gl-Mstn mRNA in ESA animals, and no correlation was observed in MLA animals. Unweighting increased Gl-Rheb expression in thoracic muscle at all molt stages; the greatest difference (2.2-fold) was observed in intermolt animals. There was also a 1.3-fold increase in Gl-s6k mRNA level in unweighted thoracic muscle. These data indicate that the mTOR pathway is upregulated in atrophic muscles. Gl-Rheb, in particular, appears to play a role in the molt-induced increase in protein synthesis in the claw muscle.

Let-7 microRNAs and Opioid Tolerance

This chapter will focus on the role of microRNAs (miRs) in regulating the actions of opioid drugs through the opioid receptors. Opioids, such as morphine, are analgesics that are used for treating many forms of acute and chronic pain. However, their chronic use is limited by undesirable effects such as opioid tolerance. The μ opioid receptor (MOR) is the primary receptor responsible for opioids' analgesia and antinociceptive tolerance. The long 3'-untranslated region (3'-UTR) of MOR mRNA is of great interest since this region may contain elements for the post-transcriptional regulation of receptor expression, such as altering the stability of mRNA, influencing translational efficiency, and controlling mRNA transport. MicroRNAs are small non-coding RNA molecules that exert their functions through base-pairing with partially complementary sequences in the 3'-UTR of target mRNAs, resulting in decreased polypeptide formation from those mRNAs. Since the discovery of the first miR, lin-4 in Caenorhabditis elegans, hundreds of miRs have been identified from humans to viruses, which have provided a crucial and pervasive layer of post-transcriptional gene regulation. The nervous system is a rich source of miR expression, with a diversity of miR functions in fundamental neurobiological processes including neuronal development, plasticity, metabolism, and apoptosis. Recently, the let-7 family of miRs is found to be a critical regulator of MOR function in opioid tolerance. Let-7 is the first identified human miR. Its family members are highly conserved across species in sequence and function. In the review, we will present a brief review of the opioid receptors, their regulation, and opioid tolerance as well as an overview of miRs and a perspective how miRs may interact with MOR and serve as a regulator of opioid tolerance.

Non-Coding RNAs Regulating Morphine Function: With Emphasis on the In vivo and In vitro Functions of miR-190

Non-coding RNAs (ncRNAs), especially microRNAs, are reported to be involved in a variety of biological processes, including several processes related to drug addiction. It has been suggested that the biological functions of opioids, one typical type of addictive drugs, are regulated by ncRNAs. In the current review, we examine a variety of mechanisms through which ncRNAs could regulate μ-opioid receptor (OPRM1) activities and thereby contribute to the development of opioid addiction. Using miR-23b as an example, we present the possible ways in which ncRNA-mediated regulation of OPRM1 expression could impact opioid addiction. Using miR-190 as an example, we demonstrate the critical roles played by ncRNAs in the signal cascade from receptor to systemic responses, including the possible modulation of adult neurogenesis and in vivo contextual memory. After discussing the possible targets of ncRNAs involved in the development of opioid addiction, we summarize the mechanisms underlying the interaction between ncRNAs and opioid addiction and present suggestions for further study.

MicroRNAs in neuronal function and dysfunction

MicroRNAs (miRNAs) are small noncoding RNA transcripts expressed throughout the brain that can regulate neuronal gene expression at the post-transcriptional level. Here, we provide an overview of the role for miRNAs in brain development and function, and review evidence suggesting that dysfunction in miRNA signaling contributes to neurodevelopment disorders such as Rett and fragile X syndromes, as well as complex behavioral disorders including schizophrenia, depression and drug addiction. A better understanding of how miRNAs influence the development of neuropsychiatric disorders may reveal fundamental insights into the causes of these devastating illnesses and offer novel targets for therapeutic development.

G protein-coupled receptors in the hypothalamic paraventricular and supraoptic nuclei--serpentine gateways to neuroendocrine homeostasis

G protein-coupled receptors (GPCRs) are the largest family of transmembrane receptors in the mammalian genome. They are activated by a multitude of different ligands that elicit rapid intracellular responses to regulate cell function. Unsurprisingly, a large proportion of therapeutic agents target these receptors. The paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus are important mediators in homeostatic control. Many modulators of PVN/SON activity, including neurotransmitters and hormones act via GPCRs--in fact over 100 non-chemosensory GPCRs have been detected in either the PVN or SON. This review provides a comprehensive summary of the expression of GPCRs within the PVN/SON, including data from recent transcriptomic studies that potentially expand the repertoire of GPCRs that may have functional roles in these hypothalamic nuclei. We also present some aspects of the regulation and known roles of GPCRs in PVN/SON, which are likely complemented by the activity of 'orphan' GPCRs.

Up-regulation of microRNAs in brain of human alcoholics

BACKGROUND

MicroRNAs (miRNAs) are small, noncoding oligonucleotides with an important role in posttranscriptional regulation of gene expression at the level of translation and mRNA degradation. Recent studies have revealed that miRNAs play important roles in a variety of biological processes, such as cell proliferation, neuronal differentiation, developmental timing, synapse function, and neurogenesis. A single miRNA can target hundreds of mRNA transcripts for either translation repression or degradation, but the function of many human miRNAs is not known.

METHODS

miRNA array analysis was performed on the prefrontal cortex of 27 individual human cases (14 alcoholics and 13 matched controls). Target genes for differentially expressed miRNAs were predicted using multiple target prediction algorithms and a consensus approach, and predicted targets were matched against differentially expressed mRNAs from the same samples. Over- and under-representation analysis was performed using hypergeometric probability and z-score tests.

RESULTS

Approximately 35 miRNAs were significantly up-regulated in the alcoholic group compared with controls. Target prediction showed a large degree of overlap with our published cDNA microarray data. Functional classification of the predicted target genes of the regulated miRNAs includes apoptosis, cell cycle, cell adhesion, nervous system development, and cell-cell signaling.

CONCLUSIONS

These data suggest that the reduced expression of genes in human alcoholic cases may be because of the up-regulated miRNAs. Cellular processes fundamental to neuronal plasticity appear to represent major targets of the suggested miRNA regulation.

MicroRNA function in the nervous system

MicroRNAs (miRNAs) are an extensive class of small noncoding RNAs that control posttranscriptional gene expression. miRNAs are highly expressed in neurons where they play key roles during neuronal differentiation, synaptogenesis, and plasticity. It is also becoming increasingly evident that miRNAs have a profound impact on higher cognitive functions and are involved in the etiology of several neurological diseases and disorders. In this chapter, we summarize our current knowledge of miRNA functions during neuronal development, physiology, and dysfunction.

Regulation of opioid tolerance by let-7 family microRNA targeting the mu opioid receptor

MicroRNA has emerged as a critical regulator of neuronal functions. This study aimed to test whether let-7 microRNAs can regulate the mu opioid receptor (MOR) and opioid tolerance. Employing bioinformatics, we identified a let-7 binding site in the 3'-untranslated region (UTR) of MOR mRNA, which was experimentally confirmed as a direct target of let-7. The repressive regulation of MOR by let-7 was revealed using a LNA-let-7 inhibitor to knockdown let-7 in SH-SY5Y cells. Conversely, morphine significantly upregulated let-7 expression in SH-SY5Y cells and in a mouse model of opioid tolerance. The LNA-let-7 inhibitor decreased brain let-7 levels and partially attenuated opioid antinociceptive tolerance in mice. Although chronic morphine treatment did not change overall MOR transcript, polysome-associated mRNA declined in a let-7-dependent manner. let-7 was identified as a mediator translocating and sequestering MOR mRNA to P-bodies, leading to translation repression. These results suggest that let-7 plays an integral role in opioid tolerance.

MicroRNAs: master regulators of ethanol abuse and toxicity?

Ethanol exerts complex effects on human physiology and health. Ethanol is not only addictive, but it is also a fetal teratogen, an adult neurotoxin, and an etiologic agent in hepatic and cardiovascular disease, inflammation, bone loss, and fracture susceptibility. A large number of genes and signaling mechanisms have been implicated in ethanol's deleterious effects leading to the suggestion that ethanol is a "dirty drug." An important question is, are there cellular "master-switches" that can explain these pleiotropic effects of ethanol? MicroRNAs (miRNAs) have been recently identified as master regulators of the cellular transcriptome and proteome. miRNAs play an increasingly appreciated and crucial role in shaping the differentiation and function of tissues and organs in both health and disease. This critical review discusses new evidence showing that ethanol-sensitive miRNAs are indeed regulatory master-switches. More specifically, miRNAs control the development of tolerance, a crucial component of ethanol addiction. Other drugs of abuse also target some ethanol-sensitive miRNAs suggesting that common biochemical mechanisms underlie addiction. This review also discusses evidence that miRNAs mediate several ethanol pathologies, including disruption of neural stem cell proliferation and differentiation in the exposed fetus, gut leakiness that contributes to endotoxemia and alcoholic liver disease, and possibly also hepatocellular carcinomas and other gastrointestinal cancers. Finally, this review provides a perspective on emerging investigations into potential roles of miRNAs as mediators of ethanol's effects on inflammation and fracture healing, as well as the potential for miRNAs as diagnostic biomarkers and as targets for therapeutic interventions for alcohol-related disorders.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 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 (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Long-term morphine treatment decreases the association of mu-opioid receptor (MOR1) mRNA with polysomes through miRNA23b

Mu-opioid receptor (MOR) mediates most of the pharmacological effects of opioid drugs. The expression of MOR is temporarily and spatially regulated at both the transcriptional and post-transcriptional levels. Long-term morphine treatment that induces tolerance does not alter MOR mRNA expression, suggesting no direct link between agonist treatment and MOR gene transcription. We previously identified the 3'-untranslated region (3'-UTR) of the major transcript of mu-opioid receptor (MOR1) and revealed a novel trans-acting factor, miRNA23b, that binds to the K box motif in the 3'-UTR. The interaction between miRNA23b with the MOR1 3'-UTR suppressed receptor translation by inhibiting polysome-mRNA association. In this report, we demonstrate that long-term morphine treatment increases miRNA23b expression in a dose- and time-dependent manner and represses the association of MOR1 mRNA with polysomes through the MOR1 3'-UTR. The translational luciferase reporter assay shows a suppression effect of morphine on reporter activity that requires the MOR1 3'-UTR. This suggests a potential link between MOR expression and morphine treatment at the post-transcriptional level in which a specific miRNA, miRNA23b, is involved.