Encyclopedia
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.
- epigenetics
- prescription opioids
- pain
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].
The aim of this review was to report the collected data from published studies on the identified epigenetic modifications associated with opioid prescription and misuse in patients who have initiated prescribed opioids for pain.
2. Specifics
All the publications collected in the literature search phase underwent abstract screening to determine eligibility for full-text data extraction. To be eligible for data extraction, studies met the following inclusion criteria: (1) the selected publications were in English language only; (2) the samples were composed of animal models to study chronic pain or human subjects with chronic noncancer pain (persistent pain lasting longer than 3 months), (3) participants were exposed to or were using prescription opioids, (4) the abstract listed one or more of the following terms in reference to potential epigenetic changes: DNA methylation, histone, chromatin, non-coding RNAs, microRNAs, gene or protein expression. Studies related to genetic polymorphisms, which are changes in the DNA sequence, were excluded from the review.
Finally, to identify potentially relevant studies not detected through the main screening, a few articles were selected from other reviews.
A thorough screening of the collected papers was conducted based on the inclusion criteria and quality of the experiments (methods typology to detect the epigenetic changes, adequate description of study methodology, sample size, and inclusion/exclusion criteria). Analyzing the identified papers, three main categories were evidenced and grouped in Table 1 , Table 2 and Table 3 , respectively: (i) studies investigating epigenetic changes in animal models (16), (ii) studies involving human subjects (3), and (iii) studies related to intergenerational or transgenerational prescribed opioid effects (5). Among the three studies on human subjects, one was related to cancer pain patients [35]. Nevertheless, it was included because considered relevant.
| 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 [17] |
| 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 [18] |
| 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 [19] |
| 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 [20] |
| 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 [21] |
| 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 [22] |
| 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 [23] |
| 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 [24] |
| 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 [25] |
| 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 [26] |
| 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 [27] |
| 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 [28] |
| 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 [29] |
| 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 [30] |
| 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 [31] |
| 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 [32] |
| 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 [33] |
| 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 [34] |
| 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 [35] |
| Opioids | Tissues/Sample | Epigenetic Methods | Change | Organism | Findings | PMID | Authors |
|---|---|---|---|---|---|---|---|
| Oxycodone | Rat brains | Quantitative RT-PCR | Gene expression | Timed pregnant Sprague-Dawley rats | Exposed rat pups have lower birth weight and postnatal weight gain and greater congenital malformations. Endothelin B receptor expression is altered in the brain of oxycodone-treated rat pups indicating a possible delay in CNS(central nervous system) development. | 26676852 | Devarapalli et al., 2016 [36] |
| Methadone or buprenorphine | Cord blood or saliva | Sequencing of bisulfite-treated DNA | Determination of Percent DNA Methylation | 86 infants with Neonatal Abstinence Syndrome from mothers receiving methadone or buprenorphine during pregnancy | High methylation of three specific CpG sites of the OPRM1 promoter identified is associated with a worse outcome for Neonatal Abstinence Syndrome. | 24996986 | Wachman et al., 2014 [37] |
| Oxycodone | Brain-derived extracellular vesicle | RNA sequencing | MicroRNA expression | Male and female Sprague Dawley rats | Distinct miRNA signatures are identified in brain-derived extracellular vesicle at a key stage of brain development in offspring that were in utero and postnatal oxycodone-exposed. | 31861723 | Shahjin et al. 2019 [38] |
| Morphine | NAc | Quantitative RT-PCR | Gene expression | Female Sprague-Dawley rats | Increased expression of KOR (K opioid receptor gene) and DRD2 (Dopamine 2 receptor gene) in response to repeated administration of quinpirole (a D2/D3 receptors agonist) in the progeny of females exposed to opiates during adolescence (also observed in the F2 generations). | 23314440 | Byrnes et al., 2013 [39] |
| Morphine self-administration | NAc | RNA deep sequencing and qPCR | Gene expression | Female adolescent Sprague Dawley rats | Genes related to synaptic plasticity and the myelin basic protein are dysregulated; some effects persisted into the subsequent generation. | 27729240 | Vassoler et al., 2017 [40] |
Moreover, within the three subgroups, the articles were listed in the tables by the type of opioids (morphine, oxycodone, etc.). It thus became clear that the identified studies primarily investigated the morphine exposure effect on gene expression and chromatin modifications. Among the studies exploring oxycodone exposure in animal models, experiments measuring DNA methylation and DNA hydroxymethylation were selected. The few studies that considered these effects on human subjects were focused on DNA methylation. Interestingly, the screening revealed a group of articles related to the intergenerational and transgenerational effects of the epigenetic marks identified. Hence, the related remarkable information was described in a dedicated paragraph. In light of the few human studies identified, a paragraph about the epigenetic research related to heroin users was included to represent how these modifications have been studied in human subjects and to give future directions for the prescription opioid research related to different pain conditions.
3. Results
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 [57,58].
Other studies focused on the rewarding effects of the widely abused prescription drug oxycodone, exploring related gene expression and DNA methylation changes after a period of chronic oxycodone self-administration. In particular, RNA-sequencing in the NAc and caudate-putamen (CPu) of mice following extended 14-day oxycodone self-administration revealed differential expression of the axon guidance molecule integrins, semaphorins, and ephrins, which may correspond to alterations in the axon-target connections and synaptogenesis and thus to the oxycodone-induced neuroadaptations [29]. In addition, levels of numerous glial-specific and immune cell-specific genes were also found to be altered in the CPu and NAc consistent with a previous study identifying altered expression of genes related to the inflammation and immune functions [30]. Furthermore, two interesting studies in the rat VTA and hippocampus, respectively, demonstrated that the oxycodone induction of CPP acquisition was associated with changes in gene expression and DNA methylation. Specifically, a down-regulation of DNMT1 and up-regulation of TET1-3 were observed in the VTA leading to a decrease in global DNA methylation levels and differential demethylation of the SYN and PSD95 genes with consistent higher expression of these synaptic proteins and the synaptic density [32]. In the hippocampus, global hypomethylation, higher synaptic density, and increased expression of specific synaptic genes, SYNAPSIN, SHANK2 , and GAP4 , were observed, too [31]. In addition, these studies suggested the therapeutic potential of oxytocin to prevent oxycodone addiction. In fact, oxytocin pretreatment markedly inhibited the acquisition of oxycodone CPP and restrained stress-induced reinstatement of oxycodone. Consistently, the synaptic density and the global DNA hypomethylation induced by oxycodone were normalized and the transcription of synaptic genes and DNA methylation enzyme genes was restored [31,32].
The studies investigating intergenerational and transgenerational epigenetic changes following prescription opioid exposure are reported in Table 3 .
The role of miRNAs, 18-25 nucleotide non-coding sequences with a wide range of regulatory roles in gene expression and neuronal functions, was also investigated in heroin-seeking behavior. MiR-218 was found to regulate many neuroplasticity-related genes and target the methyl CpG-binding protein 2 (Mecp2), thus inhibiting heroin-induced reinforcement [56].
This entry is adapted from the peer-reviewed paper 10.3390/genes12081226
