A common single nucleotide polymorphism A118G of the μ opioid receptor alters its N-glycosylation and protein stability

Alcohol addiction and the mu-opioid receptor

Alcohol addiction is one of the most common and devastating diseases in the world. Given the tremendous heterogeneity of alcohol addicted individuals, it is unlikely that one medication will help nearly all patients. Thus, there is a clear need to develop predictors of response to existing medications. Naltrexone is a mu-opioid receptor antagonist which has been approved in the United States for treatment of alcohol addiction since 1994. It has limited efficacy, in part due to noncompliance, but many patients do not respond despite high levels of compliance. There are reports that a mis-sense single nucleotide polymorphism (rs179919 or A118G) in the mu-opioid receptor gene predicts a favorable response to naltrexone if an individual carries a 'G' allele. This chapter will review the evidence for this hypothesis. The data are promising that the 'G' allele predisposes to a beneficial naltrexone response among alcohol addicted persons, but additional research is needed to prove this hypothesis in prospective clinical trials.

The AC/AG Diplotype for the 118A>G and IVS2 + 691G>C Polymorphisms of OPRM1 Gene is Associated with Sleep Quality Among Opioid-Dependent Patients on Methadone Maintenance Therapy

Introduction

Methadone is a full agonist of the opioid receptor mu 1 which is encoded by the OPRM1 gene. Sleep disorders were frequently reported by opioid-dependent patients during methadone maintenance therapy (MMT). It is possible, therefore, that genetic polymorphisms in OPRM1 influence sleep quality among patients on MMT. This study investigated the association of OPRM1 polymorphisms with sleep quality among opioid-dependent patients on MMT.

Methods

The sleep quality of 165 male opioid-dependent patients receiving MMT was evaluated using the Pittsburgh Sleep Quality Index (PSQI). DNA was extracted from whole blood and subjected to polymerase chain reaction (PCR) genotyping.

Results

Patients with IVS2 + 691 CC genotype had higher PSQI scores [mean (SD) = 5.73 (2.89)] compared to those without the IVS2 + 691 CC genotype (IVS2 + 691 GG/GC genotype) [4.92 (2.31)], but the difference did not reach statistical significance (p = 0.081). Patients with combined 118 AA genotype and IVS2 + 691 GC genotype (AC/AG diplotype) had significantly lower PSQI scores [mean (SD) = 4.25 (2.27)] compared to those without the diplotype [5.68 (2.77)] (p = 0.018).

Conclusion

Our study indicates that the AC/AG diplotype for the 118A>G and IVS2 + 691G>C polymorphisms of OPRM1 gene is associated with better sleep quality among males with opioid dependence on MMT.

Morphine-induced antinociception and reward in "humanized" mice expressing the mu opioid receptor A118G polymorphism

The rewarding and antinociceptive effects of opioids are mediated through the mu-opioid receptor. The A118G single nucleotide polymorphism in this receptor has been implicated in drug addiction and differences in pain response. Clinical and preclinical studies have found that the G allele is associated with increased heroin reward and self-administration, elevated post-operative pain, and reduced analgesic responsiveness to opioids. Male and female mice homozygous for the "humanized" 118AA or 118GG alleles were evaluated to test the hypothesis that 118GG mice are less sensitive to the rewarding and antinociceptive effects of morphine. We found that 118AA and 118GG mice of both genders developed conditioned place preference for morphine. All mice developed tolerance to the antinociceptive and hypothermic effects of morphine. However, morphine tolerance was not different between AA and GG mice. We also examined sensitivity to the antinociceptive and hypothermic effects of cumulative morphine doses. We found that 118GG mice show reduced hypothermic and antinociceptive responses on the hotplate for 10mg/kg morphine. Finally, we examined basal pain response and morphine-induced antinociception in the formalin test for inflammatory pain. We found no gender or genotype differences in either basal pain response or morphine-induced antinociception in the formalin test. Our data suggests that homozygous expression of the GG allele in mice blunts morphine-induced hypothermia and hotplate antinociception but does not alter morphine CPP, morphine tolerance, or basal inflammatory pain response.

Association of single nucleotide polymorphisms of ABCB1, OPRM1 and COMT with pain perception in cancer patients

Pain perception is influenced by multiple factors. The single nucleotide polymorphisms (SNPs) of some genes were found associated with pain perception. This study aimed to examine the association of the genotypes of ABCB1 C3435T, OPRM1 A118G and COMT V108/158M (valine 108/158 methionine) with pain perception in cancer patients. We genotyped 146 cancer pain patients and 139 cancer patients without pain for ABCB1 C3435T (rs1045642), OPRM1 A118G (rs1799971) and COMT V108/158M (rs4680) by the fluorescent dye-terminator cycle sequencing method, and compared the genotype distribution between groups with different pain intensities by chi-square test and pain scores between groups with different genotypes by non-parametric test. The results showed that in these cancer patients, the frequency of variant T allele of ABCB1 C3435T was 40.5%; that of G allele of OPRM1 A118G was 38.5% and that of A allele of COMT V108/158M was 23.3%. No significant difference in the genotype distribution of ABCB1 C3435T (rs1045642) and OPRM1 A118G (rs1799971) was observed between cancer pain group and control group (P=0.364 and 0.578); however, significant difference occurred in the genotype distribution of COMT V108/158M (rs4680) between the two groups (P=0.001). And the difference could not be explained by any other confounding factors. Moreover, we found that the genotypes of COMT V108/158M and ABCB1 C3435T were associated with the intensities of pain in cancer patients. In conclusion, our results indicate that the SNPs of COMT V108/158M and ABCB1 C3435T significantly influence the pain perception in Chinese cancer patients.

Opioid neuroscience for addiction medicine: From animal models to FDA approval for alcohol addiction

Alcohol addiction is one of the most common and devastating diseases in the world. Given the tremendous heterogeneity of alcohol-addicted individuals, it is unlikely that one medication will help nearly all patients. Thus, there is a clear need to develop predictors of response to existing medications. Naltrexone is a mu opioid receptor antagonist which has been approved in the United States for treatment of alcohol addiction since 1994. It has limited efficacy, in part due to noncompliance, but many patients do not respond despite high levels of compliance. There are reports that a mis-sense single-nucleotide polymorphism (rs179919 or A118G) in the mu opioid receptor gene predicts a favorable response to naltrexone if an individual carries a "G" allele. This chapter will review the evidence for this hypothesis. The data suggest that the "G" allele has a complex role in alcohol addiction, increasing the rewarding valence of alcohol. Whether the G allele increases risk for alcoholism and whether it predisposes to a beneficial naltrexone response among alcohol-addicted persons must await additional research with large sample sizes of multiple ethnicities in prospective clinical trials.

Receptor Reserve Moderates Mesolimbic Responses to Opioids in a Humanized Mouse Model of the OPRM1 A118G Polymorphism

The OPRM1 A118G polymorphism is the most widely studied μ-opioid receptor (MOR) variant. Although its involvement in acute alcohol effects is well characterized, less is known about the extent to which it alters responses to opioids. Prior work has shown that both electrophysiological and analgesic responses to morphine but not to fentanyl are moderated by OPRM1 A118G variation, but the mechanism behind this dissociation is not known. Here we found that humanized mice carrying the 118GG allele (h/mOPRM1-118GG) were less sensitive than h/mOPRM1-118AA littermates to the rewarding effects of morphine and hydrocodone but not those of other opioids measured with intracranial self-stimulation. Reduced morphine reward in 118GG mice was associated with decreased dopamine release in the nucleus accumbens and reduced effects on GABA release in the ventral tegmental area that were not due to changes in drug potency or efficacy in vitro or receptor-binding affinity. Fewer MOR-binding sites were observed in h/mOPRM1-118GG mice, and pharmacological reduction of MOR availability unmasked genotypic differences in fentanyl sensitivity. These findings suggest that the OPRM1 A118G polymorphism decreases sensitivity to low-potency agonists by decreasing receptor reserve without significantly altering receptor function.

Mouse model of OPRM1 (A118G) polymorphism increases sociability and dominance and confers resilience to social defeat

A single nucleotide polymorphism (SNP) in the human μ-opioid receptor gene (OPRM1 A118G) has been widely studied for its association in drug addiction, pain sensitivity, and, more recently, social behavior. The endogenous opioid system has been shown to regulate social distress and reward in a variety of animal models. However, mechanisms underlying the associations between the OPRM1 A118G SNP and these behaviors have not been clarified. We used a mouse model possessing the human equivalent nucleotide/amino acid substitution to study social affiliation and social defeat behaviors. In mice with the Oprm1 A112G SNP, we demonstrate that the G allele is associated with an increase in home-cage dominance and increased motivation for nonaggressive social interactions, similar to what is reported in human populations. When challenged by a resident aggressor, G-allele carriers expressed less submissive behavior and exhibited resilience to social defeat, demonstrated by a lack of subsequent social avoidance and reductions in anhedonia as measured by intracranial self-stimulation. Protection from social defeat in G-allele carriers was associated with a greater induction of c-fos in a resilience circuit comprising the nucleus accumbens and periaqueductal gray. These findings led us to test the role of endogenous opioids in the A112G mice. We demonstrate that the increase in social affiliation in G carriers is blocked by pretreatment with naloxone. Together, these data suggest a mechanism involving altered hedonic state and neural activation as well as altered endogenous opioid tone in the differential response to aversive and rewarding social stimuli in G-allele carriers.

The mu opioid receptor: A new target for cancer therapy?

Mu opioids are among the most widely used drugs for patients with cancer with both acute and chronic pain as well as in the perioperative period. Several retrospective studies have suggested that opioid use might promote tumor progression and as a result negatively impact survival in patients with advanced cancer; however, in the absence of appropriate prospective validation, any changes in recommendations for opioid use are not warranted. In this review, the authors present preclinical and clinical data that support their hypothesis that the mu opioid receptor is a potential target for cancer therapy because of its plausible role in tumor progression. The authors also propose the hypothesis that peripheral opioid antagonists such as methylnaltrexone, which reverses the peripheral effects of mu opioids but maintains centrally mediated analgesia and is approved by the US Food and Drug Administration for the treatment of opioid-induced constipation, can be used to target the mu opioid receptor.

Regional brain [(11)C]carfentanil binding following tobacco smoking

OBJECTIVE

To determine if overnight tobacco abstinent carriers of the AG or GG (*G) vs. the AA variant of the human mu opioid receptor (OPRM1) A118G polymorphism (rs1799971) differ in [(11)C]carfentanil binding after tobacco smoking.

METHODS

Twenty healthy American male smokers who abstained from tobacco overnight were genotyped and completed positron emission tomography (PET) scans with the mu opioid receptor agonist, [(11)C]carfentanil. They smoked deniconized (denic) and average nicotine (avnic) cigarettes during the PET scans.

RESULTS

Smoking avnic cigarette decreased the binding potential (BPND) of [(11)C]carfentanil in the right medial prefrontal cortex (mPfc; 6, 56, 18), left anterior medial prefrontal cortex (amPfc; -2, 46, 44), right ventral striatum (vStr; 16, 3, -10), left insula (Ins; -42, 10, -12), right hippocampus (Hippo; 18, -6, -14) and left cerebellum (Cbl; -10, -88, -34), and increased the BPND in left amygdala (Amy; -20, 0, -22), left putamen (Put; -22, 10, -6) and left nucleus accumbens (NAcc; -10, 12, -8). In the AA allele carriers, avnic cigarette smoking significantly changed the BPND compared to after denic smoking in most brain areas listed above. However in the *G carriers the significant BPND changes were confirmed in only amPfc and vStr. Free mu opioid receptor availability was significantly less in the *G than the AA carriers in the Amy and NAcc.

CONCLUSION

The present study demonstrates that BPND changes induced by avnic smoking in OPRM1 *G carriers were blunted compared to the AA carriers. Also *G smokers had less free mu opioid receptor availability in Amy and NAcc.

Buprenorphine signalling is compromised at the N40D polymorphism of the human μ opioid receptor in vitro

BACKGROUND AND PURPOSE

There is significant variation in individual response to opioid drugs, which may result in inappropriate opioid therapy. Polymorphisms of the μ opioid receptor (MOP receptor) may contribute to individual variation in opioid response by affecting receptor function, and the effect may be ligand-specific. We sought to determine functional differences in MOP receptor signalling at several signalling pathways using a range of structurally distinct opioid ligands in cells expressing wild-type MOP receptors (MOPr-WT) and the commonly occurring MOP receptor variant, N40D.

EXPERIMENTAL APPROACH

MOPr-WT and MOPr-N40D were stably expressed in CHO cells and in AtT-20 cells. Assays of AC inhibition and ERK1/2 phosphorylation were performed on CHO cells, and assays of K activation were performed on AtT-20 cells. Signalling profiles for each ligand were compared between variants.

KEY RESULTS

Buprenorphine efficacy was reduced by over 50% at MOPr-N40D for AC inhibition and ERK1/2 phosphorylation. Buprenorphine potency was reduced threefold at MOPr-N40D for K channel activation. Pentazocine efficacy was reduced by 50% for G-protein-gated inwardly rectifying K channel activation at MOPr-N40D. No other differences were observed for any other ligands tested.

CONCLUSIONS AND IMPLICATIONS

The N40D variant is present in 10-50% of the population. Buprenorphine is a commonly prescribed opioid analgesic, and many individuals do not respond to buprenorphine therapy. This study demonstrates that buprenorphine signalling to several effectors via the N40D variant of MOP receptors is impaired, and this may have important consequences in a clinical setting for individuals carrying the N40D allele.

Functional mu opioid receptor polymorphism (OPRM1 A(118) G) associated with heroin use outcomes in Caucasian males: A pilot study

BACKGROUND

Heroin's analgesic, euphoric and dependence-producing effects are primarily mediated by the mu opioid receptor (MOR). A single gene, OPRM1, encodes the MOR. The functional polymorphism A(118)G, located in exon 1 of the OPRM1 gene, results in anatomically-specific reductions in MOR expression, which may alter an individual's response to heroin. In prior studies 118G (rare allele) carriers demonstrated significantly greater opioid tolerance, overdose vulnerability, and pain sensitivity than 118AA homozygotes. Those findings suggest OPRM1 genotype may impact characteristics of heroin use.

METHODS

The present pilot study characterized the impact of OPRM1 genotype (rs1799971, 118G allele carriers vs. 118AA homozygotes) on heroin-use phenotypes associated with heroin dependence severity in a sample of male, Caucasian chronic heroin users (n = 86).

RESULTS

Results indicate that 118G allele carriers reported significantly more heroin use-related consequences and heroin-quit attempts, and were more likely to have sought treatment for their heroin use than 118AA homozygotes.

CONCLUSIONS

These preliminary findings, consistent with extant data, illustrate a role for OPRM1 allelic variation on heroin use characteristics, and provide support for considering genotype in heroin treatment and relapse prevention.

Detection of mu opioid receptor (MOPR) and its glycosylation in rat and mouse brains by western blot with anti-μC, an affinity-purified polyclonal anti-MOPR antibody

Our experience demonstrates that it is difficult to identify MOPR in rat and mouse brains by western blot, in part due to low abundance of the receptor and a wide relative molecular mass (Mr) range of the receptor associated with its heterogeneous glycosylation states. Here, we describe generation and purification of anti-μC (a rabbit polyclonal anti-MOPR antibody), characterization of its specificity in immunoblotting of HA-tagged MOPR expressed in a cell line, and ultimately, unequivocal detection of the MOPR in brain tissues by western blot with multiple rigorous controls. In particular, using brain tissues from MOPR knockout (K/O) mice as the negative controls allowed unambiguous identification of the MOPR band, since the anti-MOPR antibody, even after affinity purification, recognizes nonspecific protein bands. The MOPR was resolved as a faint, broad, and diffuse band with a wide Mr range of 58-84 kDa depending on brain regions and species. Upon deglycosylation to remove N-linked glycans by PNGase F (but not Endo H), the MOPR became a dense and sharp band with Mr of ~43 kDa, close to the theoretical Mr of its deduced amino acid sequences. Thus, MOPRs in rodent brains are differentially glycosylated by complex type of N-linked glycans in brain region- and species-specific manners. Furthermore, we characterized the MOPR in an A112G/N38D-MOPR knockin mouse model that possesses the equivalent substitution of the A118G/N40D SNP in the human MOPR gene. The substitution removes one of the four and five N-linked consensus glycosylation sites of the mouse and human MOPR, respectively. We demonstrated that the Mr of the MOPR in A112G mouse brains was lower than that in wild-type mouse brains, and that the difference was due to lower degrees of N-linked glycosylation.

A heroin addiction severity-associated intronic single nucleotide polymorphism modulates alternative pre-mRNA splicing of the μ opioid receptor gene OPRM1 via hnRNPH interactions

Single nucleotide polymorphisms (SNPs) in the OPRM1 gene have been associated with vulnerability to opioid dependence. The current study identifies an association of an intronic SNP (rs9479757) with the severity of heroin addiction among Han-Chinese male heroin addicts. Individual SNP analysis and haplotype-based analysis with additional SNPs in the OPRM1 locus showed that mild heroin addiction was associated with the AG genotype, whereas severe heroin addiction was associated with the GG genotype. In vitro studies such as electrophoretic mobility shift assay, minigene, siRNA, and antisense morpholino oligonucleotide studies have identified heterogeneous nuclear ribonucleoprotein H (hnRNPH) as the major binding partner for the G-containing SNP site. The G-to-A transition weakens hnRNPH binding and facilitates exon 2 skipping, leading to altered expressions of OPRM1 splice-variant mRNAs and hMOR-1 proteins. Similar changes in splicing and hMOR-1 proteins were observed in human postmortem prefrontal cortex with the AG genotype of this SNP when compared with the GG genotype. Interestingly, the altered splicing led to an increase in hMOR-1 protein levels despite decreased hMOR-1 mRNA levels, which is likely contributed by a concurrent increase in single transmembrane domain variants that have a chaperone-like function on MOR-1 protein stability. Our studies delineate the role of this SNP as a modifier of OPRM1 alternative splicing via hnRNPH interactions, and suggest a functional link between an SNP-containing splicing modifier and the severity of heroin addiction.

Endogenous opioid system influences depressive reactions to socially painful targeted rejection life events

Although exposure to a recent major life event is one of the strongest known risk factors for depression, many people who experience such stress do not become depressed. Moreover, the biological mechanisms underlying differential emotional reactions to social adversity remain largely unknown. To investigate this issue, we examined whether the endogenous opioid system, which is known to influence sensitivity to physical pain, is also implicated in differential risk for depression following socially painful targeted rejection versus non-targeted rejection life events. Adolescents (n=420) enrolled in a large longitudinal birth cohort study had their recent stress exposure and current mental health status assessed using self-report and interview-based methods. Participants were also genotyped for the A118G polymorphism in the μ-opioid receptor gene (OPRM1, rs1799971), which has been found to influence neural and psychological responses to rejection, likely by affecting opioid receptor expression and signaling efficiency. As hypothesized, G allele carriers, who are known to exhibit less opioid receptor expression and signaling efficiency, were more severely depressed and twice as likely to meet criteria for major depressive disorder following a recent targeted rejection major life event (e.g., being broken up with, getting fired) relative to A/A homozygotes who experienced such stress. However, A118G genotype did not moderate the effects of other similarly severe major life events on depression. These data thus elucidate a biological pathway that may specifically influence sensitivity to social pain and rejection, which in turn has implications for understanding differential risk for depression and several other social stress-related disorders.

Association of OPRM1 A118G variant with risk of morphine-induced respiratory depression following spine fusion in adolescents

The μ1 opioid receptor (OPRM1) genetic variant A118G results in decreased μ-receptor binding potential in the brain and increases morphine requirement. We hypothesized that OPRM1 A118G polymorphism will affect morphine-induced respiratory depression (MIRD) risk in children receiving morphine. A prospective genotype-blinded study was conducted in 88 healthy adolescents (11-18 years; 67% female, 85% Caucasian) who underwent spine fusion for scoliosis. They were followed for 48 h postoperatively for MIRD, pain scores, morphine consumption and use of analgesic adjuvants. Patients were genotyped for OPRM1 A118G variant-76% were wild type (AA) and 24% heterozygous/homozygous for variant (AG/GG). Multivariable logistic regression showed that the risk of MIRD in patients with AA genotype was significantly higher (odds ratio 5.6, 95% CI: 1.4-37.2, P=0.030). Presence of G allele was associated with higher pain scores (effect size 0.73, P=0.045). This novel association is an important step toward predicting MIRD susceptibility and personalizing morphine use.

Cellular signalling of non-synonymous single-nucleotide polymorphisms of the human μ-opioid receptor (OPRM1)

There is significant variability in individual responses to opioid drugs, which is likely to have a significant genetic component. A number of non-synonymous single-nucleotide polymorphisms (SNPs) in the coding regions of the μ-opioid receptor gene (OPRM1) have been postulated to contribute to this variability. Although many studies have investigated the clinical influences of these μ-opioid receptor variants, the outcomes are reported in the context of thousands of other genes and environmental factors, and we are no closer to being able to predict individual response to opioids based on genotype. Investigation of how μ-opioid receptor SNPs affect their expression, coupling to second messengers, desensitization and regulation is necessary to understand how subtle changes in receptor structure can impact individual responses to opioids. To date, the few functional studies that have investigated the consequences of SNPs on the signalling profile of the μ-opioid receptor in vitro have shown that the common N40D variant has altered functional responses to some opioids, while other, rarer, variants display altered signalling or agonist-dependent regulation. Here, we review the data available on the effects of μ-opioid receptor polymorphisms on receptor function, expression and regulation in vitro, and discuss the limitations of the studies to date. Whether or not μ-opioid receptor SNPs contribute to individual variability in opioid responses remains an open question, in large part because we have relatively little good data about how the amino acid changes affect μ-opioid receptor function.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Subjective health complaints in patients with lumbar radicular pain and disc herniation are associated with a sex - OPRM1 A118G polymorphism interaction: a prospective 1-year observational study

Background

Earlier observations show that development of persistent pain may be associated with the genetic variability in the gene encoding for the μ-opioid receptor 1, the OPRM1 A118G (rs1799971). The aim of this study was to investigate the association between OPRM1 genotype and subjective health complaints in patients with radicular pain and disc herniation.

Methods

A prospective, 1-year observational study was conducted at a hospital back clinic, including 118 Caucasian patients with lumbar radicular pain and MRI confirmed disc herniation. Single nucleotide polymorphism genotyping regarding the OPRM1 A118G was performed. The data of individuals with AA versus AG or GG were analysed separately by linear mixed models. The Subjective Health Complaints Inventory (0-81) including 27 common complaints experienced the previous month on a scale from not at all (0) to severe (3) was used as outcome. Pain, prior duration of leg pain, age, smoking status, and lumbar disc surgery were considered as covariates.

Results

In total 23 of 118 patients were carriers of the OPRM1 G-allele. All patients except female carriers of the G-allele reported a decrease in pain from baseline to 1 year. Female carriers of the G-allele reported significantly higher subjective health complaints score during the study time span than male carriers of the G-allele when controlling for pain and pain duration.

Conclusion

The present data indicate that, when controlling for pain intensity and duration, subjective health complaints are associated with a sex - OPRM1 A118G polymorphism interaction in patients with radicular pain.

Solubilization and reconstitution of the mu-opioid receptor expressed in human neuronal SH-SY5Y and CHO cells

The zwitterionic detergent CHAPS was used to solubilize the human mu-opioid receptor (hMOR) from SH-SY5Y neuroblastoma cells and recombinant hMOR-CHO (CHO-T7-hMOR) and hMOR-SH-SY5Y (SH-SY5Y-T7-hMOR) cell membranes. Agonist stimulation and G-protein activation by the mu-selective opioid agonist DAMGO ([D-Ala2, N-MePhe4, Gly-ol]-enkephalin) were recovered after removing of CHAPS after polyethylene glycol (PEG) precipitation. Binding assays show that hMOR solubilized and reconstituted this way was functional and able to interact with both agonist peptides and with G-protein. The effective solubilization and reconstitution of hMOR from mammalian cells, without truncation and extensive modification, represent an essential step toward the purification of a receptor bearing important post-translational modifications.

Pharmacogenomics of methadone maintenance treatment

Methadone is the major opioid substitution therapy for opioid dependence. Dosage is highly variable and is often controlled by the patient and prescriber according to local and national policy and guidelines. Nevertheless many genetic factors have been investigated including those affecting its metabolism (CYP2B6-consistent results), efflux transport (P-gp-inconsistent results), target μ-opioid receptor (μ-opioid receptor-inconsistent results) and a host of other receptors (DRD2) and signaling elements (GIRK2 and ARRB2; not replicated). None by themselves have been able to substantially explain dosage variation (the major but not sole end point). When multiple genes have been combined such as ABCB1, CYP2B6, OPRM1 and DRD2 a greater contribution to dosage variation was found but not as yet replicated. As stabilization of dosage needs to be made rapidly, it is imperative that larger internationally based studies be instigated so that genetic contribution to dosage can be properly assessed, which may or may not tailor to different ethnic groups and each country’s policy towards an outcome that benefits all.

Brain region- and sex-specific alterations in DAMGO-stimulated [(35) S]GTPγS binding in mice with Oprm1 A112G

The A118G single nucleotide polymorphism (SNP) of the human μ-opioid receptor (MOPR) gene (OPRM1) was associated with heightened dopamine release by alcohol intake, better treatment outcome for nicotine and alcohol addiction, and reduced analgesic responses to morphine. A mouse model that possesses the equivalent substitution (A112G) in the mouse MOPR gene (OPRM1) was generated to delineate the mechanisms of the impact of the SNP. Mice homozygous for the G112 allele (G/G) displayed lower morphine-induced antinociception than mice homozygous for the A112 allele (A/A), similar to the results in humans. In this study, we examined whether A112G SNP affected MOPR-mediated G protein activation in the mouse model. We compared A/A and G/G mice in the MOPR-selective agonist [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO)-stimulated [(35) S]GTPγS binding in brain regions by autoradiography. When the data of males and females were combined, G/G mice exhibited lower DAMGO-stimulated [(35) S]GTPγS binding in the ventral tegmental area than A/A mice, in accord with the previously reported reduced morphine-induced hyperactivity and locomotor sensitization in G/G mice. In the nucleus accumbens (NAc) core, female G/G mice displayed lower DAMGO-stimulated [(35) S]GTPγS binding than female A/A mice, which is consistent with the previously reported deficiency in morphine-induced conditioned place preference in female G/G mice. In G/G mice, males showed higher DAMGO-stimulated [(35) S]GTPγS binding than females in the cingulate cortex, caudate putamen, NAc core, thalamus and amygdala. Thus, A112G SNP affects DAMGO-stimulated [(35) S]GTPγS binding in region- and sex-specific manners.

A logistic equation to determine the validity of tramadol from related gene polymorphisms and psychological factors

Aim: This study was performed to develop an algorithm using polymorphisms of CYP2D6, p-gp, OPRM1, COMT and psychological variables to predict tramadol response in Chinese patients recovering from upper limb fracture internal fixation surgery. Methods: A total of 250 Han Chinese patients recovering from fracture in the upper limb were enrolled. CYP2D6*10, p-gp G2677T, p-gp C3435T, OPRM1 A118G and COMT Val158Met were detected by the ligase detection reaction (LDR) method. The algorithm was developed with binary logistic regression in cohort 1 (200 patients) and assessed with Wilcoxon signed-rank test in cohort 2 (50 patients). Results: According to cohort 1, the predictive equation was calculated with the following logistic regression parameters: Logit (1) = 2.304–4.841 × (anxiety I) – 23.709 × (anxiety II) + 2.823 × (p-gp 3435CT) + 5.737 × (p-gp 3435 TT) – 1.586 × (CYP2D6*10 CT) – 4.542 × (CYP2D6*10 TT). The cutoff point for the prediction was defined as a probability value ≥0.5. The equation’s positive predictive value is 90%. When applied to a new sample, the equation’s positive predictive value is 86%. The Nagelkerke R2 of the model is 0.819, the results of the Hosmer and Leme test show a value of 0.981. The nonparametric correlations between predicted and observed response showed significant correlation (coefficient = 0.879; p < 0.001). Conclusion: The algorithm we have developed might predict tramadol response in Chinese upper limb fracture patients.

Original submitted 24 April 2013; Revision submitted 24 January 2014

Targeting opioid receptors with pharmacological chaperones

G protein-coupled receptors (GPCRs) are polytopic membrane proteins that have a pivotal role in cellular signaling. Like other membrane proteins, they fold in the endoplasmic reticulum (ER) before they are transported to the plasma membrane. The ER quality control monitors the folding process and misfolded proteins and slowly folding intermediates are targeted to degradation in the cytosol via the ubiquitin-proteasome pathway. The high efficiency of the quality control machinery may lead to the disposal of potentially functional receptors. This is the major underlying course for loss-of-function conformational diseases, such as retinitis pigmentosa, nephrogenic diabetes insipidus and early onset obesity, which involve mutant GPCRs. During the past decade, it has become increasingly evident that small-molecular lipophilic and pharmacologically selective receptor ligands, called pharmacological chaperones (PCs), can rescue these mutant receptors from degradation by stabilizing newly synthesized receptors in the ER and enhancing their transport to the cell surface. This has raised the interesting prospect that PCs might have therapeutic value for the treatment of conformational diseases. At the same time, accumulating evidence has indicated that wild-type receptors might also be targeted by PCs, widening their therapeutic potential. This review focuses on one GPCR subfamily, opioid receptors that have been useful models to unravel the mechanism of action of PCs. In contrast to most other GPCRs, compounds that act as PCs for opioid receptors, including widely used opioid drugs, target wild-type receptors and their common natural variants.

Endogenous opiates and behavior: 2012

This paper is the thirty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2012 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).

Functional characteristics of the naked mole rat μ-opioid receptor

While humans and most animals respond to µ-opioid receptor (MOR) agonists with analgesia and decreased aggression, in the naked mole rat (NMR) opioids induce hyperalgesia and severe aggression. Single nucleotide polymorphisms in the human mu-opioid receptor gene (OPRM1) can underlie altered behavioral responses to opioids. Therefore, we hypothesized that the primary structure of the NMR MOR may differ from other species. Sequencing of the NMR oprm1 revealed strong homology to other mammals, but exposed three unique amino acids that might affect receptor-ligand interactions. The NMR and rat oprm1 sequences were cloned into mammalian expression vectors and transfected into HEK293 cells. Radioligand binding and 3'-5'-cyclic adenosine monophosphate (cAMP) enzyme immunoassays were used to compare opioid binding and opioid-mediated cAMP inhibition. At normalized opioid receptor protein levels we detected significantly lower [3H]DAMGO binding to NMR compared to rat MOR, but no significant difference in DAMGO-induced cAMP inhibition. Strong DAMGO-induced MOR internalization was detectable using radioligand binding and confocal imaging in HEK293 cells expressing rat or NMR receptor, while morphine showed weak or no effects. In summary, we found minor functional differences between rat and NMR MOR suggesting that other differences e.g. in anatomical distribution of MOR underlie the NMR's extreme reaction to opioids.

A G protein-coupled receptor (GPCR) in red: live cell imaging of the kappa opioid receptor-tdTomato fusion protein (KOPR-tdT) in neuronal cells

INTRODUCTION

In contrast to green fluorescent protein and variants (GFPs), red fluorescent proteins (RFPs) have rarely been employed for the generation of GPCR fusion proteins, likely because of formation of aggregates and cell toxicity of some RFPs. Among all the RFPs, tdTomato (tdT), one of the non-aggregating RFP, has the highest brightness score (about 3 times that of eGFP) and unsurpassed photostability.

METHODS

We fused tdT to the KOPR C-terminus. The KOPR-tdT cDNA construct was transfected into a Neuro2A mouse neuroblastoma cell line (Neuro2A cells) and rat cortical primary neurons for characterization of pharmacological properties and imaging studies on KOPR trafficking.

RESULTS

KOPR-tdT retained KOPR properties (cell surface expression, ligand binding, agonist-induced signaling and internalization) when expressed in Neuro2A cells and rat primary cortical neurons. Live cell imaging of KOPR-tdT enables visualization of the time course of agonist-induced internalization of KOPR in real time for 60 min, without photobleaching and apparent cell toxicity. U50,488H-induced KOPR internalization occurred as early as 4min and plateaued at about 30 min. A unique pattern of internalized KOPR in processes of primary neurons was induced by U50,488H.

DISCUSSION

tdT is an alternative to, or even a better tool than, GFPs for fusion to GPCR for trafficking studies, because tdT has higher brightness and thus better resolution and less photobleaching problems due to the reduced laser power used. It also has advantages associated with its longer-wavelength emission including spectral separation from autofluorescence and GFPs, reduced cell toxicity that the laser may impose, and greater tissue penetration. These advantages of tdT over GPFs may be critical for live cell imaging studies of GPCRs in vitro and for studying GPCRs in vivo because of their low abundance.

Opioid pharmacogenetics of alcohol addiction

Alcohol addiction is one of the most common and devastating diseases in the world. Given the tremendous heterogeneity of alcohol-addicted individuals, it is unlikely that one medication will help nearly all patients. Thus, there is a clear need to develop predictors of response to existing medications. Naltrexone is a μ-opioid receptor antagonist, which has been approved in the United States for treatment of alcohol addiction since 1994. It has limited efficacy, in part because of noncompliance, but many patients do not respond despite high levels of compliance. There are reports that a missense single nucleotide polymorphism (rs179919 or A118G) in the μ-opioid receptor gene predicts a favorable response to naltrexone if an individual carries a "G" allele. This work will review the evidence for this hypothesis. The data are promising that the "G" allele predisposes to a beneficial naltrexone response among alcohol-addicted persons, but additional research is needed to prove this hypothesis in prospective clinical trials.

Consequences of the 118A>G polymorphism in the OPRM1 gene: translation from bench to bedside?

The 118A>G single nucleotide polymorphism (SNP) in the μ-opioid receptor (OPRM1) gene has been the most described variant in pharmacogenetic studies regarding opioid drugs. Despite evidence for an altered biological function encoded by this variant, this knowledge is not yet utilized clinically. The aim of the present review was to collect and discuss the available information on the 118A>G SNP in the OPRM1 gene, at the molecular level and in its clinical manifestations. In vitro biochemical and molecular assays have shown that the variant receptor has higher binding affinity for β-endorphins, that it has altered signal transduction cascade, and that it has a lower expression compared with wild-type OPRM1. Studies using animal models for 118A>G have revealed a double effect of the variant receptor, with an apparent gain of function with respect to the response to endogenous opioids but a loss of function with exogenous administered opioid drugs. Although patients with this variant have shown a lower pain threshold and a higher drug consumption in order to achieve the analgesic effect, clinical experiences have demonstrated that patients carrying the variant allele are not affected by the increased opioid consumption in terms of side effects.

Influence of OPRM1 Asn40Asp variant (A118G) on [11C]carfentanil binding potential: preliminary findings in human subjects

The Asn40Asp variant (A118G) of the μ opioid receptor (OPRM1) gene is thought to contribute to the development and treatment of alcohol dependence. Employing positron emission tomography (PET), we first examined whether the single nucleotide polymorphism (SNP) modifies binding potential (BP(ND)) of the μ-selective ligand [(11)C]carfentanil in healthy control (Con) and 5-d abstinent alcohol-dependent (AD) subjects (unblocked basal scan). Second, we examined whether the allelic variants were associated with differences in OPRM1 occupancy by naltrexone (50 mg) in AD subjects. Con and AD carriers of the G allele (AG) had lower global BP(ND) at the basal scan than subjects homozygous for the A allele (AA). In AD subjects, naltrexone occupancy was slightly higher in AG subjects (98.9%) compared to AA subjects (93.1%), but this was not significant. We are the first to demonstrate using PET in healthy normal and AD subjects that the A118G SNP alters OPRM1 availability.

Pain intensity the first year after lumbar disc herniation is associated with the A118G polymorphism in the opioid receptor mu 1 gene: evidence of a sex and genotype interaction

Earlier studies have shown that the single nucleotide polymorphism (SNP) A118G (rs1799971) in the opioid receptor mu 1 (OPRM1) gene may affect pain sensitivity. In the present study we investigated whether the A118G SNP could predict clinical outcome regarding progression of pain intensity and disability in patients with low back pain and sciatica after lumbar disc herniation. Patients (n = 258) with lumbar disc herniation and sciatic pain, all European-Caucasian, were recruited from two hospitals in Norway. Pain and disability were rated on a visual analog scale (VAS), by McGill Sensory Questionnaire and by Oswestry Disability Index (ODI) over a 12 months period. The data revealed a significant interaction between sex and A118G genotype regarding the pain intensity during the 12 months (VAS, p = 0.002; McGill, p = 0.021; ODI, p = 0.205, repeated-measures ANOVA). We found that */G women had a slower recovery rate than the */G men. Actually, the */G women had 2.3 times as much pain as the */G men 12 months after the disc herniation (VAS, p = 0.043, one-way ANOVA; p = 0.035, Tukey HSD). In contrast, the A/A women and A/A men seemed to have almost exactly the same recovery rate. The present data suggest that OPRM1 G allele increases the pain intensity in women, but has a protective effect in men the first year after disc herniation.

Potentiation of dopamine D1-like receptor signaling by concomitant activation of δ- and μ-opioid receptors in mouse medial prefrontal cortex

Opioid receptors located in the ventral tegmental area are known to regulate dopamine (DA) release from mesocortical afferents to medial prefrontal cortex (mPFC) but little is known on whether in this cortical region activation of opioid receptors affect DA receptor signaling. In the present study we show that in mouse mPFC concomitant activation of either δ- or μ-opioid receptors, but not κ-opioid receptors, potentiated DA D1-like receptor-induced stimulation of adenylyl cyclase activity through a G protein βγ subunit-dependent mechanism. In tissue slices of mPFC, the combined addition of the opioid agonist leu-enkephalin and the DA D1-like receptor agonist SKF 81297 produced more than additive increase in the phosphorylation state of AMPA and NMDA receptor subunits GluR1 and NR1, respectively. Moreover, in primary cultures of mouse frontal cortex neurons, DA D1-like receptor-induced Ser133 phosphorylation of the transcription factor cyclic AMP responsive element binding protein was potentiated by concurrent stimulation of opioid receptors. Double immunofluorescence analysis of cultured cortical cells indicated that a large percentage of DA D1 receptor positive cells expressed either δ- or μ-opioid receptor immunoreactivity. These data indicate that in mouse mPFC activation of μ- and δ-opioid receptors enhances DA D1-like receptor signaling likely through converging regulatory inputs on βγ-stimulated adenylyl cyclase isoforms. This previously unrecognized synergistic interaction may selectively affect DA D1 transmission at specific postsynaptic sites where the receptors are co-localized and may play a role in prefrontal DA D1 regulation of opioid addiction.

Opiate addiction and cocaine addiction: underlying molecular neurobiology and genetics

Addictive diseases, including addiction to heroin, prescription opioids, or cocaine, pose massive personal and public health costs. Addictions are chronic relapsing diseases of the brain caused by drug-induced direct effects and persisting neuroadaptations at the epigenetic, mRNA, neuropeptide, neurotransmitter, or protein levels. These neuroadaptations, which can be specific to drug type, and their resultant behaviors are modified by various internal and external environmental factors, including stress responsivity, addict mindset, and social setting. Specific gene variants, including variants encoding pharmacological target proteins or genes mediating neuroadaptations, also modify vulnerability at particular stages of addiction. Greater understanding of these interacting factors through laboratory-based and translational studies have the potential to optimize early interventions for the therapy of chronic addictive diseases and to reduce the burden of relapse. Here, we review the molecular neurobiology and genetics of opiate addiction, including heroin and prescription opioids, and cocaine addiction.

Cellulose degradation by Sulfolobus solfataricus requires a cell-anchored endo-β-1-4-glucanase

A sequence encoding a putative extracellular endoglucanase (sso1354) was identified in the complete genome sequence of Sulfolobus solfataricus. The encoded protein shares signature motifs with members of glycoside hydrolases family 12. After an unsuccessful first attempt at cloning the full-length coding sequences in Escherichia coli, an active but unstable recombinant enzyme lacking a 27-residue N-terminal sequence was generated. This 27-amino-acid sequence shows significant similarity with corresponding regions in the sugar binding proteins AraS, GlcS, and TreS of S. solfataricus that are responsible for anchoring them to the plasma membrane. A strategy based on an effective vector/host genetic system for Sulfolobus and on expression control by the promoter of the S. solfataricus gene which encodes the glucose binding protein allowed production of the enzyme in sufficient quantities for study. In fact, the enzyme expressed in S. solfataricus was stable and highly thermoresistant and showed optimal activity at low pH and high temperature. The protein was detected mainly in the plasma membrane fraction, confirming the structural similarity to the sugar binding proteins. The results of the protein expression in the two different hosts showed that the SSO1354 enzyme is endowed with an endo-β-1-4-glucanase activity and specifically hydrolyzes cellulose. Moreover, it also shows significant but distinguishable specificity toward several other sugar polymers, such as lichenan, xylan, debranched arabinan, pachyman, and curdlan.

Influence of the OPRM1 A118G polymorphism on alcohol-induced euphoria, risk for alcoholism and the clinical efficacy of naltrexone

Alcohol-use disorders are thought to be heterogeneous in etiology, pathophysiology and response to treatment. One hypothesized contributor to this variability is the common A118G polymorphism of the µ-opioid receptor gene, OPRM1. This article critically evaluates the evidence that the A118G substitution affects subjective, behavioral and neurobiological responses to alcohol and the opioid receptor antagonist, naltrexone. Although screening of patients in a clinical setting remains premature, results suggest the A118G substitution may influence one etiological pathway to alcoholism, for which naltrexone pharmacotherapy is more effective.

Reduced expression of the μ opioid receptor in some, but not all, brain regions in mice with OPRM1 A112G

OPRM1 A118G is a common single nucleotide polymorphism (SNP) in the coding region of the human mu opioid receptor (MOPR) gene OPRM1. This SNP is associated with higher morphine doses required for postoperative analgesia as well as a variety of drug addiction phenotypes. A mouse model possessing the equivalent substitution (A112G) in the Oprm1 gene was generated to facilitate mechanistic studies. Mice homozygous for the G112 allele (G/G) displayed lower antinociception to morphine compared with those homozygous for A112 allele (A/A), similar to humans, suggesting that the mice are a good model to further characterize underlying factors contributing to phenotypes associated with this SNP. Here, we compared [³H]DAMGO binding to the MOPR in the brains of A/A and G/G mice using quantitative in vitro autoradiography. A/A mice exhibited higher [³H]DAMGO binding than G/G in the cingulate, motor, and insular cortices, nucleus accumbens core and shell, hypothalamus, thalamus, amygdala, periaqueductal gray, superficial gray of superior colliculus, and ventral tegmental area. No genotype differences were observed in somatosensory cortex, caudate putamen, and hippocampus. When males and females were examined separately, A/A mice showed higher [³H]DAMGO binding than G/G mice in more brain regions in males than in females. Radioligand binding using brain membranes also showed higher [³H]DAMGO binding in the cortex and thalamus in A/A mice than G/G mice but no genotype differences in the caudate putamen or hippocampus. Thus, the A112G SNP is associated with reduced MOPR expression in some, but not all, brain regions, and appears to have some sex differences. The elevated MOPR expression in periaqueductal gray and thalamus in A/A mice are consistent with their higher antinociceptive responses to morphine. The higher MOPR levels in nucleus accumbens and/or ventral tegmental area of A/A mice is consistent with the higher morphine-induced hyperactivity and locomotor sensitization observed in these mice. Thus, these results provide some insights into the observed decreased clinical opioid potency in humans with the A118G SNP.