1. Please check and comment entries here.
Table of Contents

    Topic review

    Prescription Opioid Misuse

    Submitted by: Maria Carla Gerra


    Prescription opioids are used for some chronic pain conditions. However, generally, long-term therapy has unwanted side effects which may trigger addiction, overdose, and eventually cause deaths. Opioid addiction and chronic pain conditions have both been associated with evidence of genetic and epigenetic alterations. Despite intense research interest, many questions about the contribution of epigenetic changes to this typology of addiction vulnerability and development remain unanswered.

    1. Introduction

    Chronic pain represents significant public health concerns and prescription opioids are a common treatment option for cancer pain management, for end-of-life treatment, in relation to surgery, and for short-term use in severe acute/chronic pain conditions not related to cancer [1]. The non-medical use of opioids and their negative health consequences among people who use drugs have been studied since 2007 after the spread of the opioid crisis. However, in the last few years, we have witnessed a new opioid crisis, even among young people and categories of workers, particularly in North America, the Middle East, Asia, and Africa. This crisis is related to the non-medical use of prescription opioids that can result in opioid misuse, defined as “use contrary to the directed or prescribed pattern of use, regardless of the presence or absence of harm or adverse effects” [2]. Signs of an increase in methadone, buprenorphine, fentanyl, codeine, morphine, tramadol, and oxycodone misuse and the increased prescription rates for opioids for pain management have also been observed in Europe resulting in an increase of vulnerable cohorts of long-term opioid users [3]. The central issue is that long-term opioid therapy is associated with many side effects such as addiction, development of tolerance, and opioid-induced hyperalgesia. In addition, in 2018 more than one-third of overdose deaths involved pharmaceutical opioids with the number of overdose deaths rising from 3442 in 1999 to 17,029 in 2017 [4][5].
    Opioid drugs act not only in nociceptive processes but also in modulating gastrointestinal, endocrine, and autonomic functions, as well as in affecting cognition and reward systems [6]. The relationship between pain states and substance abuse/misuse has been recently examined. Opioids carried an increased susceptibility to abuse even during initial exposure for pain treatment; in particular, when too many opioid drugs are prescribed for conditions not supposed to be treated by opioids or if healthcare systems are not set up to control the number of opioid prescriptions to an individual patient (doctor shopping) [7][8][9].
    Nevertheless, it is important to note that the lack of consistent findings regarding the identification of personal risk factors that may predict opioid misuse in chronic pain patients was recently evidenced in the literature [10]. Among the possible risk factors, the individual genetic variability in conjunction with chronic pain, both affecting stress and reward systems, lead to differential responses to opioids and may determine the transition risk from therapeutic use to opioid addiction. Addiction is a multifactorial condition as both genetics and psychosocial factors can trigger opioid addictive behaviors. Polymorphisms in the μ-opioid receptor 1 (OPRM1), the cytochrome P450 2D6 (CYP2D6), the catechol-O-methyl transferase (COMT) genes and in the ATP-binding cassette family genes have been found to be associated with differences in morphine consumption and metabolization process [11]. The main environmental factors important for developing opioid/substance abuse are described as psychiatric medication prescriptions, mood disorders, specific mental health diagnoses, and adverse childhood experiences [12][13].

    2. Prescription Opioids Pain Relievers

    Opioid receptors are widely distributed both centrally and in the periphery, particularly in the periaqueductal grey, locus ceruleus, rostral ventral medulla, and in the substantia gelatinosa of the dorsal horn. The major mechanism through which opioids relieve pain is the stimulation of descending inhibitory neurons through activation of the μ opioid MOP receptors. Common prescription opioids responsible for this opioid-induced analgesia effect are morphine, codeine, tramadol, fentanyl, hydromorphone, buprenorphine, hydrocodone, oxycodone, oxymorphone, methadone, and tapentadol [14].
    Opioid medications are often prescribed for acute episodes of pain for short-term use or for cancer-related pain. Opioids are also used for chronic non-cancer pain in selected cases when non-opioid and adjuvant therapies have failed and other pain medications have proven ineffective. These drugs are defined as highly effective and safe analgesics when used appropriately and included in a multifaceted strategy by competent clinicians [15].
    However, since opioid-induced pain relief and addiction do not act through distinct mechanisms in distinct brain areas, opioid drugs could not only have a simple analgesic effect but also affect or compromise the ability to feel pleasure and socialize as well as the reward system [16]. Neural changes were likened between chronic pain and long-term substance abuse; in fact, dysfunctional learning may trigger both these pathological states, producing an extensive reorganization in chronic pain and converting functional rewards into the craving characteristic of addiction [17].
    In light of the fact that the opioid analgesic efficacy often decreases with continuous use and that patients with refractory complex chronic pain are at high risk of abuse, a two-level strategy might be set up to contrast the opioid crisis. The first level would include prescription monitoring programs and dose limitations to prevent abuse/misuse; the second could increase research at the molecular level to identify the central and peripheral mechanisms underlying the drug action and to explore precision medicine options.

    3. Epigenetics and Prescription Opioids

    The term epigenetics refers to the study of heritable changes in the gene function that do not involve changes in the DNA sequence [18]. Epigenetics includes three main mechanisms: DNA methylation and chromatin-related modifications, both affecting the ability of transcriptional machinery to access the DNA tightly packed into chromatin, and non-coding RNAs (ncRNAs).
    DNA methylation, the addition of a methyl group on the 5th carbon of the DNA cytosine, is shown particularly relevant in CpG islands, regions of the genome containing a large number of CpG dinucleotide repeats [19]. It regulates gene expression facilitating the recruiting of proteins involved in the gene repression or inhibiting the binding of transcription factors to DNA [20]. Thus, DNA methylation plays a critical role in the regulation of gene expression. Whole-genome methylation profiling has made it possible to better explore demethylation and de novo methylation in the maternal and paternal genomes during development. This enables highlighting of more complex dynamics within the heterogeneous methylation level at CpG-rich promoters in different cell types. Many unannotated sequences and inactive transposons affected by this epigenetic mark were revealed [21]. Opioids have been demonstrated to stimulate DNA methylation. One study identified the global DNA methylation at LINE-1 as significantly correlated with increased chronic pain. Thus, it was hypothesized that opioid analgesics might be causally associated with increased genome-wide DNA methylation [22].
    The chromatin modifications include those related to histone tails, such as histone methylation, chromatin remodeling, and post-translational modifications affecting electrostatic nucleosome interactions. These changes have been clinically and pre-clinically evidenced to be associated with opioid exposures [23]. Concerning prescription opioids, oxycodone exposure was shown to induce long-term epigenetic consequences in the ventral tegmental area (VTA) of the developing brain with an enrichment of the repressive histone mark, H3K27me3, in prolonged oxycodone withdrawal and with consistent inhibition of the gene regulation [24]. The other chromatin modifications are related to the chromatin structure that is hierarchically folded at different levels in the nucleus with a 3D organization [25]. Previous works evidenced a correlation between the effects of opioids and chromatin remodelers such as CREB, Sox10, and BRG1 [26][27][28]. However, no studies have thoroughly explored prescription opioids’ effect on chromatin structure remodeling.
    The third important regulator of transcriptional activity is represented by ncRNAs [29], i.e., RNA not translated into proteins. ncRNAs act through a variety of mechanisms such as post-transcriptional silencers or activators. Moreover, they are involved in regulating protein-coding and non-coding genes’ expression, in the guide of chromatin-modifying complexes to specific genomic loci, in the modulation of transcriptional programs, and providing molecular scaffolds [30]. ncRNAs have already been correlated with opioid exposure. In particular, morphine, fentanyl, and heroin were found to modulate the expression of specific micro-RNAs [31][32]. Additional studies are required to understand the functional consequences of these epigenetic changes.
    The nociceptive response was demonstrated to activate these epigenetic mechanisms by modulating pain genes and possibly mediating the transition from acute to chronic pain. Studies highlight that also opioids are involved in diverse types of epigenetic regulation and thus they might influence the analgesic effects or the increased risk of continued opioid intake and development of a substance use disorder following long-term opioid therapy [33][34].
    However, the identification of specific factors associated with the individual opioid response or with side effect vulnerability has just been launched. The molecular mechanisms through which some individuals develop negative consequences associated with prolonged prescription opioid use, including hyperalgesia, addiction, sleep problems, hypogonadism, fractures, and surgical failures [35][36] are poorly understood. The following paragraphs provide an overview of the animal and human studies investigating epigenetic changes associated with opioid therapy initiated for pain relief. The studies are illustrated in Table 1; Table 2, respectively.
    Table 1. Epigenetic changes after prescribed opioid exposure in experimental models.
    Opioids Tissues/Sample Epigenetic Methods Change Animals Findings PMID Authors
    Morphine Brain tissues
    (PAG, PFC, temporal lobe, and ventral striatum)
    Microarray gene expression profiling and pattern matching Gene expression Adult male mice The development of tolerance is influenced by a region in OPRM1 gene. The genes epigenetically modified by chronic morphine administration are mainly related to neuroadaptation. 19386926 Tapocik et al., 2009 [37]
    Morphine NAc Chromatin immunoprecipitation followed by massive parallel sequencing H3K9me2 distribution in NAc in the absence and presence of chronic morphine 9 to 11-week-old C57BL/6J male mice or G9afl/fl mice Chronic morphine decreases G9a expression and H3K9me2 at global level and in specific loci in mouse NAc. 23197736 Sun et al., 2012 [38]
    Morphine Central nucleus of amygdala Chromatin immunoprecipitation Gene and protein expression Female mice with persistent and acute pain Persistent pain and repeated morphine upregulate the transcriptional regulator MeCP2. MeCP2 enhances BDNF expression and represses G9a action and its repressive mark H3K9me2 in CeA. 24990928 Zhang et al., 2014 [39]
    Morphine Central nucleus of amygdala Chromatin immunoprecipitation Gene expression Rat model of morphine self-administration The repression of GluA1 function by MeCp2 is proposed as a mechanism for morphine-seeking behavior in pain experience. 25716866 Hou et al., 2015 [40]
    Morphine Dorsal root ganglia and spinal cord tissues Quantitative RT-PCR, Western Immunoblotting and ChIP-PCR Gene and protein expression, histone modifications analysis Male Sprague-Dawley rats SNL (spinal nerve ligation) model G9a contributes to transcriptional repression of MORs in primary sensory neurons in neuropathic pain. G9a inhibitors: potential treatment of chronic neuropathic pain 26917724 Zhang et al., 2016 [41]
    Morphine Dorsal root ganglia Quantitative RT-PCR and Western Blot Gene and protein expression Adult male CD-1 mice Neuropathic pain increases C/EBPβ expression. C/EPBβ activates the G9a gene, that epigenetically silences Kv1.2 and MOR genes. Blocking the induced increase in C/EBPβ in the DRG, morphine analgesia after CCI is improved. 28698219 Li et al., 2017 [42]
    Morphine Basolateral amygdala Quantitative RT-PCR and Western Blot Gene and protein expression Male Sprague–Dawley Increase in H3K14ac together with upregulation of the BDNF and FosB; and CREB activation. 24829091 Wang et al., 2015 [43]
    Morphine Rat brain regions Pyrosequencing DNA methylation (5mC) and global DNA 5-hydroxymethylation (5hmC) Male Wistar rats Acute and chronic exposure is associated with significantly decreased/increased 5mC at specific genes (BDNF, IL1B, IL6, NR3C1, COMT). Global 5hmC levels increase in the cerebral cortex, hippocampus, and hypothalamus, but decrease in the midbrain. 29111854 Barrow et al., 2017 [44]
    Morphine, phentayl Hippocampus RNAseq Gene and protein expression Mice chronically treated with μ-opioid agonists The increased expression of MiR-339-3p inhibits intracellular MOR biosynthesis and acts as a negative feedback modulator of MOR signals. 23085997 Wu et al., 2013 [31]
    Morphine Dorsal root ganglia Quantitative RT-PCR and Western Blot Gene and protein expression Male CD-1 mice treated with morphine to establish systemic chronic tolerance to morphine anti-nociception MiR-219 contributes to the development of chronic tolerance to morphine analgesia by targeting CaMKIIγ and enhancing CaMKIIγ-dependent brain-derived neurotrophic factor expression. 27599867 Hu et al., 2016 [45]
    Morphine Dorsal root ganglia Quantitative RT-PCR and Western Blot Gene and protein expression Male CD-1 mice injected with morphine to elicit morphine tolerance The increased BDNF expression is regulated by the miR-375 and JAK2/STAT3 pathway. Inhibition of this pathway decreases BDNF production, and thus, attenuated morphine tolerance. 28603428 Li et al., 2017 [46]
    Oxycodone Ventral tegmental area of the developing brain Quantitative RT-PCR and chromatin immunoprecipitation Gene expression and histone modifications analysis Male offspring of C57Bl/6NTac mice Adolescent oxycodone exposure increases the repressive mark H3K27me3, at key dopamine-related genes. 33325096 Carpenter et al., 2020 [24]
    Oxycodone Striatum (NAc and CPu) RNAseq Gene expression Mice following extended 14-day oxycodone self-administration Alterations in the expression of
    heterodimer receptor, integrins, semaphorins, semaphorin receptors, plexins, selective axon guidance genes.
    29946272 Yuferov et al., 2018 [47]
    Oxycodone Dorsal striatum and ventral striatum RNAseq Gene expression Adult male C57BL/6J mice underwent a 14-day oxycodone self-administration Inflammation/immune genes have altered expression during chronic self-administration of oxycodone 28653080 Zhang et al., 2017 [48]
    Oxycodone Hippocampus DNA ELISA Kit for total 5mC; quantitative RT-PCR Global 5mC levels and gene expression Male Sprague-Dawley rats The global DNA hypomethylation induced by oxycodone can be reversed through oxytocin and could significantly attenuate the oxycodone rewarding effects. 31526808 Fan et al., 2019 [49]
    Oxycodone Ventral tegmental area DNA ELISA Kit for total 5mC and OneStep qMethyl™ kit for gene-specific 5mC, quantitative RT-PCR, Western blotting Global and specific 5mC levels and gene expression Sprague-Dawley rats Down-regulation of DNMT1 and up-regulation of TET1-3 lead to a decrease in global 5mC levels and differential demethylation at exon 1 of SYN and exon 2 of PSD95. 31735530 Fan et al., 2019 [50]
    Table 2. Epigenetic changes after prescribed opioid exposure in humans.
    Opioids Tissues Epigenetic Methods Change Sample Findings PMID Authors
    Opioids Whole blood Bisulfite modification and Array-based genome-wide DNA methylation assay DNA methylation at specific CpG sites 140 opioid dependence cases and 80 opioid-exposed controls Three genome-wide significant differentially methylated CpGs map to genes involved in chromatin remodeling, DNA binding, cell survival, and cell projection (PARG, RERE, and CFAP77 genes). 31801960 Montalvo-Ortiz et al., 2019 [51]
    Opioid medication self-administration (hydrocodone, oxycodone, and codeine: 5–30 mg) Saliva collected at 3 time points Genome-wide DNA methylation assay and candidate approach DNA methylation at OPRM1 gene promoter 33 opioid-naïve participants who underwent standard dental surgery Hypermethylation of the OPRM1 promoter is measured in response to opioid use, and such epigenetic restructuring can be induced even by short-term use of therapeutic opioids. 32493461 Sandoval-Sierra et al., 2020 [52]
    Remifentanil, oxycodone, codeine Whole blood Pyrosequencing at specific CpG sites and LINE1 (global genome-wide DNA methylation assay) DNA methylation 140 women with persistent pain after breast cancer surgery The global DNA methylation is shown to be a pain predictive biomarker, providing useful information to allocate the patients to either a “persistent pain” or “non-persistent pain” phenotype. 31775878 Kringel et al., 2019 [53]

    The entry is from 10.3390/genes12081226


    1. Dowell, D.; Haegerich, T.M.; Chou, R. CDC Guideline for Prescribing Opioids for Chronic Pain—United States, 2016. Recomm. Rep. 2016, 65, 1–49.
    2. Vowles, K.E.; McEntee, M.L.; Julnes, P.S.; Frohe, T.; Ney, J.P.; van der Goes, D.N. Rates of opioid misuse, abuse, and addiction in chronic pain: A systematic review and data synthesis. Pain 2015, 156, 569–576.
    3. UNODC. World Drug Report 2020 (United Nations Publication, Sales No. E.20.XI.6); UNODC: Vienna, Austria, 2020.
    4. Fishbain, D.A.; Cole, B.; Lewis, J.; Rosomoff, H.L.; Rosomoff, R.S. What percentage of chronic nonmalignant pain patients exposed to chronic opioid analgesic therapy develop abuse/addiction and/or aberrant drug-related behaviors? A structured evidence-based review. Pain Med. 2008, 9, 444–459.
    5. Centers for Disease Control and Prevention (CDC) Wide-Ranging Online Data for Epidemiologic Research (WONDER). National Center for Health Statistics. Available online: http://wonder.cdc.gov (accessed on 30 September 2020).
    6. Trescot, A.M.; Datta, S.; Lee, M.; Hansen, H. Opioid pharmacology. Pain Physician 2008, 11, S133–S153.
    7. Manhapra, A.; Becker, W.C. Pain and Addiction: An Integrative Therapeutic Approach. Med. Clin. N. Am. 2018, 102, 745–763.
    8. Biernikiewicz, M.; Taieb, V.; Toumi, M. Characteristics of doctor-shoppers: A systematic literature review. J. Mark. Access Health Policy 2019, 7, 1595953.
    9. Volkow, N.; Benveniste, H.; McLellan, A.T. Use and Misuse of Opioids in Chronic Pain. Annu. Rev. Med. 2018, 69, 451–465.
    10. Voon, P.; Karamouzian, M.; Kerr, T. Chronic pain and opioid misuse: A review of reviews. Subst. Abuse Treat. Prev. Policy 2017, 12, 36.
    11. Vieira, C.M.P.; Fragoso, R.M.; Pereira, D.; Medeiros, R. Pain polymorphisms and opioids: An evidence based review. Mol. Med. Rep. 2019, 19, 1423–1434.
    12. Klimas, J.; Gorfinkel, L.; Fairbairn, N.; Amato, L.; Ahamad, K.; Nolan, S.; Simel, D.L.; Wood, E. Strategies to Identify Patient Risks of Prescription Opioid Addiction When Initiating Opioids for Pain: A Systematic Review. JAMA Netw. Open 2019, 2, e193365.
    13. Merrick, M.T.; Ford, D.C.; Haegerich, T.M.; Simon, T. Adverse Childhood Experiences Increase Risk for Prescription Opioid Misuse. J. Prim. Prev. 2020, 41, 139–152.
    14. Rosenblum, A.; Marsch, L.A.; Joseph, H.; Portenoy, R.K. Opioids and the treatment of chronic pain: Controversies, current status, and future directions. Exp. Clin. Psychopharmacol. 2008, 16, 405–416.
    15. O’Brien, T.; Christrup, L.L.; Drewes, A.M.; Fallon, M.T.; Kress, H.G.; McQuay, H.J.; Mikus, G.; Morlion, B.J.; Perez-Cajaraville, J.; Pogatzki-Zahn, E.; et al. European Pain Federation position paper on appropriate opioid use in chronic pain management. Eur. J. Pain 2017, 21, 3–19.
    16. Elman, I.; Borsook, D. Common Brain Mechanisms of Chronic Pain and Addiction. Neuron 2016, 89, 11–36.
    17. Ballantyne, J.C. Opioids for the Treatment of Chronic Pain: Mistakes Made, Lessons Learned, and Future Directions. Anesth. Analg. 2017, 125, 1769–1778.
    18. Deans, C.; Maggert, K.A. What do you mean, “epigenetic”? Genetics 2015, 199, 887–896.
    19. Kundu, T.K.; Rao, M.R. CpG islands in chromatin organization and gene expression. J. Biochem. 1999, 125, 217–222.
    20. Moore, L.D.; Le, T.; Fan, G. DNA methylation and its basic function. Neuropsychopharmacology 2013, 38, 23–38.
    21. Edwards, J.R.; Yarychkivska, O.; Boulard, M.; Bestor, T.H. DNA methylation and DNA methyltransferases. Epigenet. Chromatin 2017, 10, 23.
    22. Doehring, A.; Oertel, B.G.; Sittl, R.; Lötsch, J. Chronic opioid use is associated with increased DNA methylation correlating with increased clinical pain. Pain 2013, 154, 15–23.
    23. Browne, C.J.; Godino, A.; Salery, M.; Nestler, E.J. Epigenetic Mechanisms of Opioid Addiction. Biol. Psychiatry 2020, 87, 22–33.
    24. Carpenter, M.D.; Manners, M.T.; Heller, E.A.; Blendy, J.A. Adolescent oxycodone exposure inhibits withdrawal-induced expression of genes associated with the dopamine transmission. Addict. Biol. 2020, e12994.
    25. Zheng, H.; Xie, W. The role of 3D genome organization in development and cell differentiation. Nat. Rev. Mol. Cell Biol. 2019, 20, 535–550.
    26. McDaid, J.; Dallimore, J.E.; Mackie, A.R.; Napier, T.C. Changes in accumbal and pallidal pCREB and deltaFosB in morphine-sensitized rats: Correlations with receptor-evoked electrophysiological measures in the ventral pallidum. Neuropsychopharmacology 2006, 31, 1212–1226.
    27. Hwang, C.K.; Kim, C.S.; Kim, D.K.; Law, P.-Y.; Wei, L.-N.; Loh, H.H. Up-regulation of the mu-opioid receptor gene is mediated through chromatin remodeling and transcriptional factors in differentiated neuronal cells. Mol. Pharmacol. 2010, 78, 58–68.
    28. Martin, J.A.; Caccamise, A.; Werner, C.T.; Viswanathan, R.; Polanco, J.J.; Stewart, A.F.; Thomas, S.A.; Sim, F.J.; Dietz, D.M. A Novel Role for Oligodendrocyte Precursor Cells (OPCs) and Sox10 in Mediating Cellular and Behavioral Responses to Heroin. Neuropsychopharmacology 2018, 43, 1385–1394.
    29. Nestler, E.J. Epigenetic mechanisms of drug addiction. Neuropharmacology 2014, 76 Pt B, 259–268.
    30. DiStefano, J.K. The Emerging Role of Long Noncoding RNAs in Human Disease. Methods Mol. Biol. 2018, 1706, 91–110.
    31. Wu, Q.; Hwang, C.K.; Zheng, H.; Wagley, Y.; Lin, H.-Y.; Kim, D.K.; Law, P.-Y.; Loh, H.H.; Wei, L.-N. MicroRNA 339 down-regulates μ-opioid receptor at the post-transcriptional level in response to opioid treatment. FASEB J. 2013, 27, 522–535.
    32. Yan, B.; Hu, Z.; Yao, W.; Le, Q.; Xu, B.; Liu, X.; Ma, L. MiR-218 targets MeCP2 and inhibits heroin seeking behavior. Sci. Rep. 2017, 7, 40413.
    33. Niederberger, E.; Resch, E.; Parnham, M.J.; Geisslinger, G. Drugging the pain epigenome. Nat. Rev. Neurol. 2017, 13, 434–447.
    34. Banerjee, G.; Edelman, E.J.; Barry, D.T.; Becker, W.C.; Cerdá, M.; Crystal, S.; Gaither, J.R.; Gordon, A.J.; Gordon, K.S.; Kerns, R.D.; et al. Non-medical use of prescription opioids is associated with heroin initiation among US veterans: A prospective cohort study. Addiction 2016, 111, 2021–2031.
    35. Krashin, D.; Murinova, N.; Sullivan, M. Challenges to Treatment of Chronic Pain and Addiction During the “Opioid Crisis”. Curr. Pain Headache Rep. 2016, 20, 65.
    36. Chou, R.; Turner, J.A.; Devine, E.B.; Hansen, R.N.; Sullivan, S.D.; Blazina, I.; Dana, T.; Bougatsos, C.; Deyo, R.A. The effectiveness and risks of long-term opioid therapy for chronic pain: A systematic review for a National Institutes of Health Pathways to Prevention Workshop. Ann. Intern. Med. 2015, 162, 276–286.
    37. Tapocik, J.D.; Letwin, N.; Mayo, C.L.; Frank, B.; Luu, T.; Achinike, O.; House, C.; Williams, R.; Elmer, G.I.; Lee, N.H. Identification of candidate genes and gene networks specifically associated with analgesic tolerance to morphine. J. Neurosci. 2009, 29, 5295–5307.
    38. Sun, H.; Maze, I.; Dietz, D.M.; Scobie, K.N.; Kennedy, P.J.; Damez-Werno, D.; Neve, R.L.; Zachariou, V.; Shen, L.; Nestler, E.J. Morphine epigenomically regulates behavior through alterations in histone H3 lysine 9 dimethylation in the nucleus accumbens. J. Neurosci. 2012, 32, 17454–17464.
    39. Zhang, Z.; Tao, W.; Hou, Y.-Y.; Wang, W.; Kenny, P.J.; Pan, Z.Z. MeCP2 repression of G9a in regulation of pain and morphine reward. J. Neurosci. 2014, 34, 9076–9087.
    40. Hou, Y.-Y.; Cai, Y.-Q.; Pan, Z.Z. Persistent pain maintains morphine-seeking behavior after morphine withdrawal through reduced MeCP2 repression of GluA1 in rat central amygdala. J. Neurosci. 2015, 35, 3689–3700.
    41. Zhang, Y.; Chen, S.-R.; Laumet, G.; Chen, H.; Pan, H.-L. Nerve Injury Diminishes Opioid Analgesia through Lysine Methyltransferase-mediated Transcriptional Repression of μ-Opioid Receptors in Primary Sensory Neurons. J. Biol. Chem. 2016, 291, 8475–8485.
    42. Li, Z.; Mao, Y.; Liang, L.; Wu, S.; Yuan, J.; Mo, K.; Cai, W.; Mao, Q.; Cao, J.; Bekker, A.; et al. The transcription factor C/EBPβ in the dorsal root ganglion contributes to peripheral nerve trauma-induced nociceptive hypersensitivity. Sci. Signal. 2017, 10.
    43. Wang, Y.; Lai, J.; Cui, H.; Zhu, Y.; Zhao, B.; Wang, W.; Wei, S. Inhibition of histone deacetylase in the basolateral amygdala facilitates morphine context-associated memory formation in rats. J. Mol. Neurosci. 2015, 55, 269–278.
    44. Barrow, T.M.; Byun, H.-M.; Li, X.; Smart, C.; Wang, Y.-X.; Zhang, Y.; Baccarelli, A.A.; Guo, L. The effect of morphine upon DNA methylation in ten regions of the rat brain. Epigenetics 2017, 12, 1038–1047.
    45. Hu, X.-M.; Cao, S.-B.; Zhang, H.-L.; Lyu, D.-M.; Chen, L.-P.; Xu, H.; Pan, Z.-Q.; Shen, W. Downregulation of miR-219 enhances brain-derived neurotrophic factor production in mouse dorsal root ganglia to mediate morphine analgesic tolerance by upregulating CaMKIIγ. Mol. Pain 2016, 12, 1744806916666283.
    46. Li, H.; Tao, R.; Wang, J.; Xia, L. Upregulation of miR-375 level ameliorates morphine analgesic tolerance in mouse dorsal root ganglia by inhibiting the JAK2/STAT3 pathway. J. Pain Res. 2017, 10, 1279–1287.
    47. Yuferov, V.; Zhang, Y.; Liang, Y.; Zhao, C.; Randesi, M.; Kreek, M.J. Oxycodone Self-Administration Induces Alterations in Expression of Integrin, Semaphorin and Ephrin Genes in the Mouse Striatum. Front. Psychiatry 2018, 9, 257.
    48. Zhang, Y.; Liang, Y.; Levran, O.; Randesi, M.; Yuferov, V.; Zhao, C.; Kreek, M.J. Alterations of expression of inflammation/immune-related genes in the dorsal and ventral striatum of adult C57BL/6J mice following chronic oxycodone self-administration: A RNA sequencing study. Psychopharmacology 2017, 234, 2259–2275.
    49. Fan, X.-Y.; Shi, G.; Zhao, P. Reversal of oxycodone conditioned place preference by oxytocin: Promoting global DNA methylation in the hippocampus. Neuropharmacology 2019, 160, 107778.
    50. Fan, X.-Y.; Shi, G.; Zhao, P. Methylation in Syn and Psd95 genes underlie the inhibitory effect of oxytocin on oxycodone-induced conditioned place preference. Eur. Neuropsychopharmacol. 2019, 29, 1464–1475.
    51. Montalvo-Ortiz, J.L.; Cheng, Z.; Kranzler, H.R.; Zhang, H.; Gelernter, J. Author Correction: Genomewide Study of Epigenetic Biomarkers of Opioid Dependence in European- American Women. Sci. Rep. 2019, 9, 18774.
    52. Sandoval-Sierra, J.V.; Salgado García, F.I.; Brooks, J.H.; Derefinko, K.J.; Mozhui, K. Effect of short-term prescription opioids on DNA methylation of the OPRM1 promoter. Clin. Epigenet. 2020, 12, 76.
    53. Kringel, D.; Kaunisto, M.A.; Kalso, E.; Lötsch, J. Machine-learned analysis of global and glial/opioid intersection-related DNA methylation in patients with persistent pain after breast cancer surgery. Clin. Epigenet. 2019, 11, 167.