Region Specificity in Endogenous Opioid Peptides and Mu-opioid Receptor Gene Expression in Rat Brain Areas Involved in Addiction After Frequent Morphine Treatment

Backgrounds: Molecular mechanisms involved in adverse effects of morphine, including tolerance and dependence, have remained elusive. We examined possible alterations in the gene expression of proenkephalin (Penk), prodynorphin (Pdyn), and mu-opioid receptor (Oprm1) in reward brain areas following frequent morphine treatment. Methods: Two groups of male Wistar rats were used. The groups received either saline (1 mL/kg) or morphine (10 mg/kg) twice daily for eight days. On day 8, rats were decapitated, brain areas involved in addiction were dissected, including the midbrain, striatum, prefrontal cortex (PFC), hippocampus, and hypothalamus, and gene expression was evaluated with real-time PCR. Results: Prolonged morphine treatment decreased Penk, Pdyn, and Oprm1 gene expressions in the midbrain but upregulated them in the striatum compared to the control group treated with saline. Significant increases in Pdyn and Oprm1 gene expressions were detected in the PFC, but there was no significant difference in Penk gene expression between the two groups. Besides, Pdyn gene expression was decreased in the hippocampus and hypothalamus; however, no significant differences in Penk and Oprm1 gene expressions were detected between the groups in these areas. Conclusions: The expression of endogenous opioid peptides and receptors after frequent use of morphine follows a region specificity in brain areas involved in addiction. These alterations may result in new physiological setpoints outside the normal range, which need to be considered when using morphine in medicine.

MeCP2 Epigenetic Silencing of Oprm1 Gene in Primary Sensory Neurons Under Neuropathic Pain Conditions

Opioids are the last option for the pharmacological treatment of neuropathic pain, but their antinociceptive effects are limited. Decreased mu opioid receptor (MOR) expression in the peripheral nervous system may contribute to this. Here, we showed that nerve injury induced hypermethylation of the Oprm1 gene promoter and an increased expression of methyl-CpG binding protein 2 (MeCP2) in injured dorsal root ganglion (DRG). The downregulation of MOR in the DRG is closely related to the augmentation of MeCP2, an epigenetic repressor, which could recruit HDAC1 and bind to the methylated regions of the Oprm1 gene promoter. MeCP2 knockdown restored the expression of MOR in injured DRG and enhanced the analgesic effect of morphine, while the mimicking of this increase via the intrathecal infusion of viral vector-mediated MeCP2 was sufficient to reduce MOR in the DRG. Moreover, HDAC1 inhibition with suberoylanilide hydroxamic acid, an HDAC inhibitor, also prevented MOR reduction in the DRG of neuropathic pain mice, contributing to the augmentation of morphine analgesia effects. Mechanistically, upregulated MeCP2 promotes the binding of a high level of HDCA1 to hypermethylated regions of the Oprm1 gene promoter, reduces the acetylation of histone H3 (acH3) levels of the Oprm1 gene promoter, and attenuates Oprm1 transcription in injured DRG. Thus, upregulated MeCP2 and HDAC1 in Oprm1 gene promoter sites, negatively regulates MOR expression in injured DRG, mitigating the analgesic effect of the opioids. Targeting MeCP2/HDAC1 may thus provide a new solution for improving the therapeutic effect of opioids in a clinical setting.

Pharmacological Treatment for Neonatal Abstinence Syndrome is Associated with Altered DNA Methylation and Neurobehavior

Objective: To determine whether pharmacological treatment for neonatal abstinence syndrome (NAS) is associated with changes in DNA methylation (DNAm) of the mu-opioid receptor gene (OPRM1) and improvements in neonatal neurobehavior.Study Design: Buccal swabs were collected from 37 neonates before and after morphine treatment for NAS. Genomic DNA was extracted and DNAm was examined at four CpG sites within the OPRM1 gene. The NICU Network Neurobehavioral Scales (NNNS) was also performed before and after NAS treatment. Changes in DNAm (DNAmpost-tx – DNAmpre-tx) and NNNS summary scores (NNNSpost-tx – NNNSpre-tx) were then calculated. Path analysis was used to examine associations among pharmacologic treatment (length of treatment and total dose of morphine), changes in DNAm, and changes in NNNS summary scores. Results: DNAm significantly decreased from pre- to post-treatment at 1 of 4 CpG sites within the OPRM1 gene. Neonates also demonstrated decreased excitability, hypertonia, lethargy, signs of stress and abstinence, and increased quality of movement and regulation from pre- to post-treatment. Increased length of treatment and higher morphine doses were associated with greater decreases in DNAm; greater decreases in DNAm were associated with greater decreases in excitability and hypertonia on the NNNS.Conclusions: Pharmacological treatment of NAS is associated with decreased DNAm of the OPRM1 gene and improved neonatal neurobehavior. Epigenetic changes may play a role in these changes in neonatal neurobehavior.

Association of the OPRM1 A118G polymorphism and Pavlovian-to-instrumental transfer: Clinical relevance for alcohol dependence

Background: Pavlovian-to-instrumental transfer (PIT) quantifies the extent to which a stimulus that has been associated with reward or punishment alters operant behaviour. In alcohol dependence (AD), the PIT effect serves as a paradigmatic model of cue-induced relapse. Preclinical studies have suggested a critical role of the opioid system in modulating Pavlovian–instrumental interactions. The A118G polymorphism of the OPRM1 gene affects opioid receptor availability and function. Furthermore, this polymorphism interacts with cue-induced approach behaviour and is a potential biomarker for pharmacological treatment response in AD. In this study, we tested whether the OPRM1 polymorphism is associated with the PIT effect and relapse in AD. Methods: Using a PIT task, we examined three independent samples: young healthy subjects ( N = 161), detoxified alcohol-dependent patients ( N = 186) and age-matched healthy controls ( N = 105). We used data from a larger study designed to assess the role of learning mechanisms in the development and maintenance of AD. Subjects were genotyped for the A118G (rs1799971) polymorphism of the OPRM1 gene. Relapse was assessed after three months. Results: In all three samples, participants with the minor OPRM1 G-Allele (G+ carriers) showed increased expression of the PIT effect in the absence of learning differences. Relapse was not associated with the OPRM1 polymorphism. Instead, G+ carriers displaying increased PIT effects were particularly prone to relapse. Conclusion: These results support a role for the opioid system in incentive salience motivation. Furthermore, they inform a mechanistic model of aberrant salience processing and are in line with the pharmacological potential of opioid receptor targets in the treatment of AD.

The impact of genetic polymorphism in pain mechanisms

Background:Variations in genes codifying target structures in the nociceptive pathway can result in pain attenuation or increase.Objective:Investigate the genetic polymorphism influence in the individual pain threshold. Methods: Search on PubMed with the terms “genetic”, “pain” and its synonyms published in the last 10 years. Results:The subjective and individual mechanisms of pain aren’t completely understood, but genetic susceptibility is one of the hypothesis to explain these differences.The KCNK18 gene influences the synaptic transmission by producing potassium channel protein that equalizes resting membrane potential, calcineurin activated and inhibited by arachidonic acid. This gene was found more frequently in migraine individuals. The COMT gene increase the sensibility to pain by met-enkephalins reduction and/or catecholamine elevation. Its activity’s reduced in fibromyalgia patients. However, the OPRM1 gene, an opioid receptor, was found in individuals with a higher pain threshold.Furthermore, studies with human cell culture shows the analgesic role of the gene A118G, by its greater binding affinity for β-endorphin.It is associated with more effective endorphinergic endogenous pain inhibition. Conclusion:Researches indicates a striking participation of genetic polymorphism in pain mechanisms. The knowledge about genetic variables on pain perception can contribute to the development of individualized analgesic protocols and therapeutic strategies, accordantly to the patient genetic profile. This evolution becomes fundamental in a population that tend to the indiscriminate use of analgesics.

Analysis of Single Nucleotide Polymorphism A118g Gene Opioid Receptor Mu-1 (OPRM1) In Opioid Analgesic Patients in Indonesia

Introduction: Opioid Receptor Mu-1 or Opioid Receptor Mu-1 (OPRM1) is a prime candidate for pharmacogenetic studies related to the variability of patient response to opioids. This variant is high frequency in Asian populations that is associated with differences in sensitivity and increased need for opioid analgesics in the treatment of pain. Therefore, this study was conducted with the aim of analyzing OPRM1 in patients receiving opioid analgesics in Indonesia, especially in the city of Surabaya. Method: The method used is the Polymerase Chain Reaction–Restriction Fragment Length Polymorphism (PCR-RFLP), where the data collected is recorded and tabulated and then analyzed using SPSS version 20.0. Result: The results showed that the frequency of SNP A118G of the OPRM1 gene was found to be quite high, namely 58 (72.5%) out of a total of 80 subjects. In 40 male subjects, the GG genotype frequency was found to be the highest at 19 (47.5%). While the genotype frequencies of AA, AG and GG were found to be almost comparable in women. Conclusion: Therefore, it can be concluded that the frequency of SNP A118G of the OPRM1 gene was found to be high in the Indonesian population and had a high tendency in the male population, although it was not statistically significant.

Naltrexone Use in Treating Hypersexuality Induced by Dopamine Replacement Therapy: Impact of OPRM1 A/G Polymorphism on Its Effectiveness

Hypersexuality is a well-known adverse side effect of dopamine replacement therapy (DRT), and anti-craving drugs could be an effective therapeutic option. Our aim was to update the knowledge on this issue, particularly on the influence of an Opioid Receptor Mu 1 (OPRM1) genetic polymorphism. A systematic review was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement. We also analyzed a case of iatrogenic hypersexuality that occurred in a patient treated with DRT. An analysis of the OPRM1 gene was performed on said patient. Our search identified 597 publications, of which only 7 were included in the final data synthesis. All seven publications involved naltrexone use. Five of them were case reports. None of the publications mentioned DRT side effects, nor did they report genetic data. Regarding our case report, the introduction of naltrexone corresponded with the resolution of the patient’s hypersexuality. Moreover, the patient carried the A/G genotype, which has been reported to be associated with a stronger response to naltrexone for patients with an alcohol use disorder. Although studies are inconclusive so far, naltrexone could be an interesting therapeutic option for resistant hypersexuality due to DRT. Carrying the A/G genotype could help explain a good response to treatment.