The polymorphism A118G of the human mu-opioid receptor gene decreases the pupil constrictory effect of morphine-6-glucuronide but not that of morphine

Parental history of chronic pain may be associated with impairments in endogenous opioid analgesic systems

A family history of chronic pain has previously been linked to increased incidence of spontaneous acute pain and risk for chronic pain. Mechanisms underlying these associations are unknown, although similar effects on both acute and chronic pain suggest that central endogenous analgesic system differences may be relevant. This study tested whether a positive parental chronic pain history (PH+) was associated with impaired endogenous opioid analgesic responses to acute pain. Seventy-three chronic low back pain patients (LBP) and 46 pain-free controls received opioid blockade (8mg naloxone i.v.) and placebo blockade (saline) in randomized, counterbalanced order in separate sessions. During each, subjects participated in a 1-min finger pressure pain task followed by an ischemic forearm pain task, providing pain intensity ratings during and immediately following each task. To assess opioid analgesic function, blockade effects were derived by subtracting placebo from blockade condition pain responses. Placebo condition analyses indicated that both PH+ subjects and LBP subjects reported greater acute pain sensitivity than respective comparison groups (p's<.05). Multivariate analyses indicated that, beyond any influence of current chronic pain status, PH+ subjects failed to exhibit any endogenous opioid analgesia to acute ischemic pain, whereas PH- subjects elicited effective opioid analgesia (p<.05). A significant multivariate PHxSubject Type interaction (p<.05) indicated that opioid analgesic impairments were most prominent in PH+ LBP subjects. Similar analyses for finger pressure pain blockade effects were nonsignificant (p>.10). The possible heritability of endogenous opioid analgesic dysfunction observed in individuals with a positive parental chronic pain history remains to be investigated.

Targeting treatments for alcohol dependence: the pharmacogenetics of naltrexone

Alcohol dependence is one of the leading causes of morbidity worldwide, yet only a minority of those afflicted engages in treatment. While increasing access to treatment is an important public health approach, increasing the success of treatment is also likely to lead to greater engagement. However, alcohol dependence is a complex disorder likely to consist of several biological subtypes. Recent evidence from a number of different studies has suggested that genetic variation in the mu-opioid receptor has a significant influence on clinical presentation of alcohol problems and response to treatment with an opioid antagonist. Most notably, the A118G polymorphism of the mu-receptor gene has been demonstrated to predict clinical response to naltrexone in alcohol-dependent individuals. This article reviews the biological correlates and outlines a scientific agenda for better understanding the role of opioid neurotransmission in the etiology, maintenance and treatment of alcohol dependence.

Association of mu-opioid receptor gene polymorphism (A118G) with variations in morphine consumption for analgesia after total knee arthroplasty

BACKGROUND

Morphine consumption after a given surgical procedure can vary considerably. Studies show that single nucleotide polymorphism involving the nucleotide position 118 at exon 1 of the mu-opioid receptor gene (OPRM1) may play a role in mediating the effects of opioids. This study was performed to correlate the A118G polymorphism at OPRM1 with morphine consumption in patients undergoing total knee arthroplasty.

METHODS

Post-operative pain was relieved by patient-controlled analgesia (PCA). The analgesic effect was evaluated using a visual analogue scale. Side-effects, such as sedation, nausea and vomiting, and pruritus, were recorded systematically. The genotypes were determined by sequencing polymerase chain reaction-amplified DNA. The differences in demographics and consumed morphine from the PCA device between the different genotypes were tested using one-way analysis of variance. The prevalence of side-effects from morphine and sex distribution were compared using the Kruskal-Wallis test.

RESULTS

One hundred and forty-seven patients were included in the study. Twenty-seven patients who required rescue analgesia were excluded; these patients did not differ demographically or genetically from the 120 who completed the study. Of the latter, 74 were A118 homozygous (AA), 33 were heterozygous (AG) and 13 were G118 homozygous (GG). Group GG consumed significantly more morphine (40.4 +/- 22.0 mg) than group AA (25.3 +/- 15.5 mg) and group AG (25.6 +/- 11.7 mg) during the first 48 h post-operatively. The groups did not differ with respect to reported pain, age, sex, weight and adverse effects.

CONCLUSIONS

G118 homozygotes have a poorer response to morphine for post-operative pain control than A118 homozygotes or heterozygotes. The genotype may thus influence the post-operative response to pain and cause differences in the amounts of analgesic consumed by patients after total knee arthroplasty.

Relevance of frequent mu-opioid receptor polymorphisms for opioid activity in healthy volunteers

Polymorphisms in the mu-opioid receptor gene (OPRM1) are primary candidate sources of clinical variability in opioid therapy. Apart from the 118A>G single nucleotide polymorphism, nothing is known about the role of OPRM1 mutations in opioid therapy. The influence of the OPRM1 mutations on opioid pharmacodynamics was assessed in pooled data from 31 healthy volunteers obtained in previous studies with available plasma concentrations and pupil diameters after intravenous administration of morphine or morphine-6-glucuronide (M6G). A total of 24 candidate ORPM1 mutations were screened for and those found at an allelic frequency of at least 5% in the 31 subjects were analyzed for functional consequences, using population pharmacokinetic-pharmacodynamic modeling of the miotic effects of the opioids as a reliable and sensitive surrogate parameter of the central nervous opioid effects. Polymorphisms at an allelic frequency of > or =5% (n=310) were 118A>G in exon 1 (11.5%), the IVS2-31G>A (8.9%) and IVS2-691C>G (44.5%) SNPs in intron 2. The 118A>G SNP significantly increased the values of EC50 by a factor of more than 2 (non-mutated: EC50,morphine=30 nmol/l, EC50,M6G=750 nmol/l, 118G carriers: EC50,morphine=66 nmol/l, EC50,M6G=1650 nmol/l), whereas the IVS2-691C>G SNP had no effect. Based on morphine and M6G, the present analysis encourages focusing on the 118A>G SNP when investigating the role of OPRM1 mutations for the activity of opioid analgesics. Other OPRM1 mutations are probably less important either owing to low allelic frequency or due to poor indications for functional consequences. This applies to opioid potency in the context of opioid therapy but not to pain processing or substance addiction, in which opioid receptors are involved but other or additional OPRM1 mutations may be important.

Individualizing analgesic prescription Part I: pharmacogenetics of opioid analgesics

The current use of analgesics is based on the empiric administration of a given drug with clinical monitoring for efficacy and toxicity. However, individual responses to drugs are influenced by a combination of pharmacokinetic and pharmacodynamic processes, and each of these components, in addition to pain perception and processing, seem to be regulated by genetic factors. Whereas polymorphic drug-metabolizing enzymes and drug transporters may affect the pharmacokinetics of drugs, polymorphic drug targets and disease-related pathways may influence the pharmacodynamic action of drugs. After usual dose, drug toxicity, as well as inefficacy, can be observed depending on the polymorphism, the analgesic considered and the presence or absence of active metabolites. Thus, cytochrome P450 (CYP)2D6 polymorphism influences codeine and tramadol analgesic effects, CYP2C9 has an impact on the disposition of some nonsteroidal anti-inflammatory drugs, and opioid receptor polymorphism (118A>G) may reduce morphine potency. Moreover, drug interaction mimics genetic deficiency and contributes to the variability in response to analgesics. This two-part review summarizes the available data on the pharmacokinetic-pharmacodynamic consequences of known polymorphisms of drug-metabolizing enzymes (CYP and uridine diphosphate glucuronosyltransferase), drug transporters (multidrug resistance proteins, multidrug resistance-associated proteins, organic anion-transporting polypeptides, and serotonin transporters), relevant drug targets (such as µ-opioid receptor, serotonin receptor and cyclooxygenases) and other nonopioid biological systems, on currently prescribed central and peripheral analgesics.

Association of an Asn40Asp (A118G) polymorphism in the mu-opioid receptor gene with substance dependence: a meta-analysis

INTRODUCTION

The mu-opioid receptor has been implicated in the pathogenesis of dependence on opioids, alcohol, nicotine, and cocaine. Studies examining the association of the mu-opioid receptor gene (genetic locus OPRM1) with substance dependence (SD) have focused on the Asn40Asp (A118G) single nucleotide polymorphism (SNP).

METHOD

We used meta-analysis to examine the literature on the association of Asn40Asp with SD. Twenty-two articles describing 28 distinct samples and over 8000 subjects were included. A variety of factors (i.e., ethnicity, type of SD, rigor with which controls were screened, severity of SD among cases) were examined as potential moderators of the association.

RESULTS

Four studies showed a significantly higher frequency of the Asp40 allele among SD cases, while three studies showed a significantly higher frequency of the Asp40 allele among controls. There was no significant association between Asn40Asp and SD (OR=1.01, 95%CI=0.86-1.19), nor was there substantial evidence of a moderator effect.

CONCLUSION

The Asn40Asp SNP in OPRM1 does not appear to affect risk for SD. Additional research is needed to determine whether these findings reflect no role for OPRM1 in determining risk for SD or whether another polymorphism in the gene influences receptor function and risk for SD.

Genomic variations and transcriptional regulation of the human mu-opioid receptor gene

The mu-opioid receptor (MOR1) is a target of endogenous and exogenous opioids and plays a pivotal role for anesthesia and analgesia. Variations in the 5' flanking sequence of the mu-opioid receptor gene may influence transcriptional regulation and ultimately alter protein expression of MOR1. In the present study we investigated the influence of eight single nucleotide polymorphisms (SNP) within the mu-opioid receptor promoter on promoter activity and evaluated the frequencies of the relevant SNPs in 700 patients under opioid medication. Reporter-gene-constructs were created by means of PCR and site directed mutagenesis, testing eight SNPs previously described. The neuroblastoma cell line SHSY5Y was used for transfection and promoter activity was estimated by luciferase activity. Of the eight reporter gene constructs employed to test genomic variations, two produced a significant change in luciferase activity when compared to wild-type constructs. The G-554A variation located within a known NFkB binding element resulted in a decreased activity whereas the A/G base exchange at position -1320 showed an increased luciferase activity. This particular variant generated a myeloid zinc finger (MZF1) cis-acting element known to impact transcription. The allele frequency of the -1320G variant was 0.21% in 700 Caucasian patients under opioid medication in contrast to 9.1% reported previously in drug addicted African Americans. Because of this unexpected low frequency an association analysis to opioid requirements and effects of mu-opioid receptor agonists was not feasible. In conclusion, transcriptional regulation of MOR1 is modified by two genetic variations at positions -554 and -1320 of the mu-opioid receptor promoter. Individuals presenting these variations may have an altered level of MOR expression. A possible association of these genomic variants on efficacy and side effects of opioid treatment in different ethnic groups has to be elucidated.

Pharmacokinetics of codeine and its metabolite morphine in ultra-rapid metabolizers due to CYP2D6 duplication

Codeine is an analgesic drug acting on mu-opiate receptors predominantly via its metabolite morphine, which is formed almost exclusively by the genetically polymorphic enzyme cytochrome P450 2D6 (CYP2D6). Whereas it is known that individuals lacking CYP2D6 activity (poor metabolizers, PM) suffer from poor analgesia from codeine, ultra-fast metabolizers (UM) due to the CYP2D6 gene duplication may experience exaggerated and even potentially dangerous opioidergic effects and no systematical study has been performed so far on this question. A single dose of 30 mg codeine was administered to 12 UM of CYP2D6 substrates carrying a CYP2D6 gene duplication, 11 extensive metabolizers (EM) and three PM. Genotyping was performed using polymerase chain reaction-restriction fragment length polymorphism methods and a single-base primer extension method for characterization of the gene-duplication alleles. Pharmacokinetics was measured over 24 h after drug intake and codeine and its metabolites in plasma and urine were analyzed by liquid chromatography with tandem mass spectrometry. Significant differences between the EM and UM groups were detected in areas under the plasma concentration versus time curves (AUCs) of morphine with a median (range) AUC of 11 (5-17) microg h l(-1) in EMs and 16 (10-24) microg h l(-1) in UM (P=0.02). In urine collected over 12 h, the metabolic ratios of the codeine+codeine-6-glucuronide divided by the sum of morphine+its glucuronides metabolites were 11 (6-17) in EMs and 9 (6-16) in UM (P=0.05). Ten of the 11 CYP2D6 UMs felt sedation (91%) compared to six (50%) of the 12 EMs (P=0.03). CYP2D6 genotypes predicting ultrarapid metabolism resulted in about 50% higher plasma concentrations of morphine and its glucuronides compared with the EM. No severe adverse effects were seen in the UMs in our study most likely because we used for safety reasons a low dose of only 30 mg. It might be good if physicians would know about the CYP2D6 duplication genotype of their patients before administering codeine.

Genetik, Schmerz und Analgesie

Genomic variations influencing nociceptive sensitivity and susceptibility to pain conditions, as well as responses to pharmacotherapy of pain are currently under investigation. Candidate genes involved in pain perception, pain processing and pain management such as (opioid) receptors, transporters and other targets of pharmacotherapy are discussed. Drug metabolizing enzymes represent a further major target of ongoing research in order to identify associations between an individual's genetic profile and drug response (pharmacogenetics). Polymorphisms of the cytochrome P 450 enzymes influence analgesic efficacy of codeine, tramadol and tricyclic antidepressants (CYP2D6). Blood levels of some non-steroidal anti-inflammatory drugs (NSAIDs) are dependent on CYP2C9 activity, whereas opioid receptor polymorphisms are discussed with respect to differences in opioid-mediated analgesia and side-effects. Pharmacogenetics is seen as a potential diagnostic tool for improving patient therapy and care and will contribute to a more individualized drug treatment in the future.

Relevance of assessing drug concentration exposure in pharmacogenetic and imaging studies

Pharmacodynamic differences are difficult to interpret without drug concentration data. In particular, variability in drug exposure may confound the interpretation of pharmacogenetic, therapeutic outcome, and neuroimaging studies. Inter-individual variability in concentrations can be quite high due to variable adherence and pharmacokinetics. For example, clearance may be influenced by genetics, drug interactions, age and illness. We review findings that acute responses to selective serotonin reuptake inhibitors can have a concentration-response relationship using positron emission tomography and neuroendocrine measures. We also present preliminary evidence that the concentration-response relationship for paroxetine is influenced by genotypic differences at the serotonin transporter promoter. In large clinical studies, the accurate assessment of drug exposure can be challenging, with several techniques used to assess exposure. Population pharmacokinetics (Pop PK) is a method that is ideally suited for analysing concentration data from large trials because both patient-specific and population parameters can be determined with only a small number of plasma samples per patient. As opposed to relying on prescribed doses or a single trough level, the ability to determine more accurately exposure with Pop PK reduces the heterogeneity introduced by exposure variability. Pop PK hierarchic Bayesian approaches have been effective for characterizing anticonvulsants, antibiotics, antineoplastics and antiarrhythmics. We have recently successfully incorporated these pop PK analyses into routine assessments of elderly patients in clinical trials of selective serotonin reuptake inhibitors (SSRIs) and second generation antipsychotics. For the design and interpretation of neuroimaging, pharmacogenetic, and behavioural studies, the assessment of drug concentration exposure is therefore feasible and has potentially important ramifications.

Clinical Pharmacology and Pharmacotherapy of Opioid Switching in Cancer Patients

Pain is one of the most common and often most feared symptoms in patients with cancer. Ongoing or progressive pain is physically debilitating and has a marked impact on quality of life. Since a third of the population will die from cancer, and of these, 80% will experience severe pain in their final year of life, effective treatment of cancer-related pain remains both a high priority and an ongoing challenge in clinical practice. Individuals with moderate to severe cancer-related pain require treatment with strong analgesics, namely opioids. There is evidence to support the therapeutic maneuver of opioid switching in clinical practice, but further evidence is needed to elucidate the underlying mechanisms for interindividual differences in response to different opioids. Large, robust clinical trials will be needed if clinical differences among side-effect profiles of different opioids are to be clearly demonstrated. This review discusses candidate genes, which contribute to opioid response; many other genes have also been implicated in "pain" from animal or human studies. In order to continue to evaluate the genetic contributions to both pain susceptibility and analgesic response, further candidate genes need to be considered. Good pain control remains a high priority for clinicians and patients, and there is much work to be done to further individualize analgesic therapy for patients with cancer.

Anger expression and pain: an overview of findings and possible mechanisms

A tendency to manage anger via direct expression (anger-out) is increasingly recognized as influencing responses to pain. Elevated trait anger-out is associated with increased responsiveness to acute experimental and clinical pain stimuli, and is generally related to elevated chronic pain intensity in individuals with diverse pain conditions. Possible mechanisms for these links are explored, including negative affect, psychodynamics, central adipose tissue, symptom specific muscle reactivity, endogenous opioid dysfunction, and genetics. The opioid dysfunction hypothesis has some experimental support, and simultaneously can account for anger-out's effects on both acute and chronic pain. Factors which may moderate the anger-out/pain link are described, including narcotic use, gender, and genetic polymorphisms. Pain exacerbating effects of trait anger-out are contrasted with the apparent pain inhibitory effects of behavioral anger expression exhibited in anger-provoking contexts. Conceptual issues related to the state versus trait effects of expressive anger regulation are discussed.

Morphine-6-Glucuronide: Morphine??s Successor for Postoperative Pain Relief?

In searching for an analgesic with fewer side effects than morphine, examination of morphine's active metabolite, morphine-6-glucuronide (M6G), suggests that M6G is possibly such a drug. In contrast to morphine, M6G is not metabolized but excreted via the kidneys and exhibits enterohepatic cycling, as it is a substrate for multidrug resistance transporter proteins in the liver and intestines. M6G exhibits a delay in its analgesic effect (blood-effect site equilibration half-life 4-8 h), which is partly related to slow passage through the blood-brain barrier and distribution within the brain compartment. In humans, M6G's potency is just half of that of morphine. In clinical studies, M6G is well tolerated and produces adequate and long lasting postoperative analgesia. At analgesic doses, M6G causes similar reduction of the ventilatory response to CO2 as an equianalgesic dose of morphine but significantly less depression of the hypoxic ventilatory response. Preliminary data indicate that M6G is associated less than morphine with nausea and vomiting, causing 50% and 75% less nausea in postoperative and experimental settings, respectively. Although the data from the literature are very promising, we believe that more studies are necessary before we may conclude that M6G is superior to morphine for postoperative analgesia.

Analgesia and local anesthesia during invasive procedures in the neonate

BACKGROUND

Preterm and full-term neonates admitted to the neonatal intensive care unit or elsewhere in the hospital are routinely subjected to invasive procedures that can cause acute pain. Despite published data on the complex behavioral, physiologic, and biochemical responses of these neonates and the detrimental short- and long-term clinical outcomes of exposure to repetitive pain, clinical use of pain-control measures in neonates undergoing invasive procedures remains sporadic and suboptimal. As part of the Newborn Drug Development Initiative, the US Food and Drug Administration and the National Institute of Child Health and Human Development invited a group of international experts to form the Neonatal Pain Control Group to review the therapeutic options for pain management associated with the most commonly performed invasive procedures in neonates and to identify research priorities in this area.

OBJECTIVE

The goal of this article was to review and synthesize the published clinical evidence for the management of pain caused by invasive procedures in preterm and full-term neonates.

METHODS

Clinical studies examining various therapies for procedural pain in neonates were identified by searches of MEDLINE (1980-2004), the Cochrane Controlled Trials Register (The Cochrane Library, Issue 1, 2004), the reference lists of review articles, and personal files. The search terms included specific drug names, infant-newborn, infant-preterm, and pain, using the explode function for each key word. The English-language literature was reviewed, and case reports and small case series were discarded.

RESULTS

The most commonly performed invasive procedures in neonates included heel lancing, venipuncture, IV or arterial cannulation, chest tube placement, tracheal intubation or suctioning, lumbar puncture, circumcision, and SC or IM injection. Various drug classes were examined critically, including opioid analgesics, sedative/hypnotic drugs, nonsteroidal anti-inflammatory drugs and acetaminophen, injectable and topical local anesthetics, and sucrose. Research considerations related to each drug category were identified, potential obstacles to the systematic study of these drugs were discussed, and current gaps in knowledge were enumerated to define future research needs. Discussions relating to the optimal design for and ethical constraints on the study of neonatal pain will be published separately. Well-designed clinical trials investigating currently available and new therapies for acute pain in neonates will provide the scientific framework for effective pain management in neonates undergoing invasive procedures.

Role of morphine's metabolites in analgesia: concepts and controversies

The metabolites of morphine, morphine-6-glucuronide (M6G) and morphine-3-glucuronide (M3G), have been extensively studied for their contribution to clinical effects following administration of morphine. Those contributions to both the desired effect (ie, analgesia) and the undesired effects (eg, nausea, respiratory depression) are the subject of clinical controversy. Much attention and effort have been directed at investigating the properties of M6G because of interest in this substance as a possible substitute for morphine. It exhibits increased potency and the possibility of a better side effect profile compared with morphine, although the reported relative benefits vary widely. M3G is not analgesic, but its role in producing side effects, including the development of clinical tolerance, has been proposed. This review is focused on M6G and the factors that contribute to its clinical utility. The formation and distribution of M6G are presented, as are the analgesic effect and the onset of this effect. The impact of genetics, age, and gender on M6G and its effects is also reviewed.

Symbiotic relationship of pharmacogenetics and drugs of abuse

Pharmacogenetics/pharmacogenomics is the study of how genetic variation affects pharmacology, the use of drugs to treat disease. When drug responses are predicted in advance, it is easier to tailor medications to different diseases and individuals. Pharmacogenetics provides the tools required to identify genetic predictors of probable drug response, drug efficacy, and drug-induced adverse events-identifications that would ideally precede treatment decisions. Drug abuse and addiction genetic data have advanced the field of pharmacogenetics in general. Although major findings have emerged, pharmacotherapy remains hindered by issues such as adverse events, time lag to drug efficacy, and heterogeneity of the disorders being treated. The sequencing of the human genome and high-throughput technologies are enabling pharmacogenetics to have greater influence on treatment approaches. This review highlights key studies and identifies important genes in drug abuse pharmacogenetics that provide a basis for better diagnosis and treatment of drug abuse disorders.

Anger Regulation Style, Postoperative Pain, and Relationship to the A118G Mu Opioid Receptor Gene Polymorphism: A Preliminary Study

Greater trait anger-out is associated with elevated pain responsiveness. Previous work suggests this effect may be mediated by deficient endogenous opioid analgesia, possibly reflecting diminished opioid receptor sensitivity. The A118G single nucleotide polymorphism (SNP) of the mu opioid receptor gene influences both opioid receptor sensitivity and clinical responsiveness to opioid analgesics. Therefore, this study tested whether this SNP either mediated or moderated the effects of anger-out on postsurgical pain outcomes. Forty-eight patients undergoing coronary artery bypass graft surgery provided genetic samples, and completed measures of anger-out and postsurgical pain. Postsurgical opioid analgesic use was also recorded. Anger-out was positively associated with postsurgical pain ratings (p < 0.05). Anger-out was not associated with A118G SNP status (p > 0.10), suggesting the latter is unlikely to mediate anger-out's pain-related effects. A significant anger-out x A118G interaction was observed on analgesic use (p < 0.05), due to a much stronger positive relationship between anger-out and analgesic demands in patient with the A118G SNP (b = 0.53) than those with the wild-type receptor (b = 0.07). These results suggest that the A118G SNP may moderate but not mediate the effects of anger-out on postoperative pain responses.

Pharmacogenetics of opioid receptors and addiction

It is generally assumed that combinations of polymorphic alleles of different genes contribute to polygenetic disorders. Variants of the opioid receptors are the obvious candidates underlying addiction. Most research has focused on the coding variation A118G of the mu opioid receptor (MOPr), which replaces asparagine at position 40 by aspartate (Asn40Asp). However, to date, no conclusive evidence exists regarding which physiological effects this mutation may cause. Other rare polymorphisms in the MOPr cause marked effects (e.g. impairment of G-protein coupling) but, due to their low frequency, their role in addiction is questionable. A large number of polymorphic sites have been found in the putative promoter region of the mu opioid receptor. These variations may alter the expression level of the receptors in neurones, but the functional relevance of promoter polymorphisms is hard to predict. In the delta opioid receptor, surprisingly few polymorphisms have been detected within the coding region, and a clear association with addiction has not been shown to date. The kappa opioid receptor contains mainly silent polymorphisms. Nevertheless, there are studies reporting positive associations of silent mutations in the three opioid receptors with drug addiction. The underlying mechanism remains unclear, but examples exist indicating that silent mutations affect mRNA stability. Taken together, the coding polymorphisms, which are rather frequent, reveal no convincing association. The vast number of non-coding, intronic or promoter polymorphisms in the opioid receptors may influence addictive behaviour, but these polymorphisms are far less studied, and their physiological significance remains to be demonstrated.

The mu-opioid receptor polymorphism A118G predicts cortisol responses to naloxone and stress

A polymorphism in the mu-opioid receptor (MOR) (A118G) has been shown to increase beta-endorphin binding affinity, theoretically placing greater inhibitory tone on hypothalamic corticotropin-releasing hormone (CRH) neurons. We hypothesized that the minor allele (G) would predict cortisol responses to both pharmacological (naloxone) and psychological (stress) activation of the hypothalamic-pituitary-adrenal (HPA) axis. Healthy subjects (mean age 25.2 years, SD 9.2 years) completed a naloxone challenge (n=74) and/or the modified Trier Social Stress Test (TSST) (n=86). For the naloxone challenge, two baseline blood samples were obtained. Then, five increasing doses of i.v. naloxone were administered at 30-min intervals and 12 additional blood samples were collected at 15-min intervals. The TSST consisted of 5-min of public speaking and 5-min of mental arithmetic exercises. Three baseline and five post-TSST blood samples were drawn. Both the naloxone and TSST groups had significant adrenocorticotropin (ACTH) and cortisol responses to their respective challenges (P<0.001). There were no differences in baseline ACTH, baseline cortisol, or ACTH response by genotype in either the naloxone or the TSST group. Among subjects expressing a G allele, there was a higher cortisol response to naloxone (P=0.046), but a lower cortisol response to the TSST (P=0.044). In conclusion, the minor allele (G) was associated with a robust cortisol response to naloxone blockade, but a blunted response to psychosocial stress. We speculate that increased opioid avidity of the minor allele receptor contributes to the differential response to naloxone vs stress.

Pharmacogenetics and anesthesiologists

Genetic variation contributes to an individual’s sensitivity and response to a variety of drugs important to anesthetic practice. Early insights into the clinical impact of pharmacogenetics were provided by anesthesiology – investigations into prolonged apnea after succinylcholine administration, thiopental-induced porphyria and malignant hyperthermia contributed to the novel science of pharmacogenetics in the early 1960s. Genetic polymorphisms involved in pharmacokinetics (absorption, distribution, metabolism, and excretion of drugs) and pharmacodynamics (receptors, ion channels and enzymes) can affect an individual’s response to the drugs used in anesthetic practice. In addition, genetic variation in proteins directly unrelated to drug action or metabolism can influence responses to environmental changes that occur during anesthesia. This review will summarize the current knowledge of genetic variation in response to drugs relevant to anesthesia, and how this impacts upon clinical practice.

The contribution of genetics to addiction therapy approaches

Addictions, including alcohol dependence, which is the focus of this article, are complex genetic diseases. Recently, several individual genes that contribute to the risk for alcohol dependence have been identified, and more are expected to be in the near future. Among these are genes encoding alcohol and aldehyde dehydrogenases and GABA(A) receptor subunits. These reveal pathways of vulnerability and provide targets for rational drug design. It is likely that response to particular therapies is also a complex trait influenced by genetics, but studies to explore this are just beginning. We discuss some studies on bromocriptine, naltrexone, and serotonergic agents. Adding a genetic component to treatment trials could greatly help to understand the biological basis of variations in the efficacy of therapies and, in the future, could lead to individualized choices of therapy.

Sind μ-Opioidrezeptorpolymorphismen wichtig für die Opioidtherapie?

Polymorphisms in the mu-opioid receptor gene may potentially alter the clinical effects of opioid analgesics. A common mu-opioid receptor polymorphism occurring at an allelic frequency of 12% decreases the potency of opioid analgesics in humans. Interestingly, in carriers of this mutation, it appears to be possible to reach analgesia by increasing the opioid dose but side effects appear to occur less often despite the higher opioid dose. This suggests a broadened therapeutic range of the opioids. Other mutations of the mu-opioid receptor, for example three mutations within the third intracellular loop of the receptor, impair receptor signaling, but they are too rare to greatly affect pain therapy or have not yet been investigated in the context of pain therapy.

The Val158Met polymorphism of the human catechol-O-methyltransferase (COMT) gene may influence morphine requirements in cancer pain patients

Catechol-O-methyltransferase (COMT) inactivates dopamine, epinephrine and norepinephrine in the nervous system. A common functional polymorphism (Val158Met) leads to a three- to-four-fold variation in the COMT enzyme activity, the Met form displaying lower enzymatic activity. The Val158Met polymorphism affects pain perception, and subjects with the Met/Met genotype have the most pronounced response to experimental pain. Based on this information we analyzed the influence from the COMT Val158Met polymorphism on the efficacy of morphine in a cohort of patients suffering from cancer pain. We genotyped 207 Caucasian cancer patients on morphine treatment with respect to the Val158Met polymorphism and compared the morphine doses, serum concentrations of morphine and morphine metabolites between the genotype groups. Patients with the Val/Val genotype (n=44) needed more morphine (155+/-160 mg/24 h) when compared to the Val/Met (117+/-100 mg/24 h; n=96) and the Met/Met genotype (95+/-99 mg/24 h; n=67) groups (P=0.025). This difference was not explained by other factors such as duration of morphine treatment, performance status, time since diagnosis, perceived pain intensity, adverse symptoms, or time until death. These results suggest that genetic variation in the COMT gene may contribute to variability in the efficacy of morphine in cancer pain treatment.

Clinical response to morphine in cancer patients and genetic variation in candidate genes

Morphine is the analgesic of choice for moderate to severe cancer pain; however, 10-30% of patients do not tolerate morphine. This study evaluated genetic variation in the mu-opioid receptor, betaarrestin2, stat6 and uridine diphosphate-glucuronysltransferase 2B7 (UGT2B7) genes, in patients who responded to morphine vs those who were switched to alternative opioids. We prospectively recruited and genotyped 162 Caucasian patients (117 controls, 39 switchers). Switchers, were more likely to carry the common allele at 1182 G/A, 5864 G/A, 8622T/C and 11143 G/A in the betaarrestin2 gene (P = 0.021, 0.043, 0.013, 0.043, respectively). Switchers had increased carriage of the T allele (-1714 C/T) and a significant difference in the allelic frequency at 9065 C/T (chi(2) = 3.86, P = 0.049) in the stat6 gene. No differences were seen in genotype or allele frequencies of SNPs in the mu-opioid receptor gene or UGT2B7 gene. This study presents novel data suggesting that variation in genes involved in mu-opioid receptor signalling influence clinical response to morphine.

Genetic variability and clinical efficacy of morphine

The individual variability of opioid pharmacology suggests that the patients' genetic disposition influences the response to opioids. Given the complexity of morphine pharmacology, variability may be caused by several genes. We review data which shows that variability in genes coding the enzyme metabolizing morphine (UGT2B7 gene), mu-opioid receptors (OPRM gene) and blood-brain barrier (BBB) transport of morphine by multidrug resistance transporters (MDR1 gene) influences the clinical efficacy of morphine. Furthermore, variability in an enzyme degrading catecholamines (COMT gene) alters the efficacy of morphine demonstrating that genetic variability in non-opioid systems may indirectly influence the clinical efficacy from morphine. Thus, results obtained so far strongly argue that opioid efficacy is partly related to inborn properties caused by genetic variability.

The A118G single nucleotide polymorphism of the mu-opioid receptor gene (OPRM1) is associated with pressure pain sensitivity in humans

Responses to painful stimuli are characterized by tremendous interindividual variability, and genetic factors likely account for some proportion of this variability. However, few studies have identified genetic contributions to experimental pain perception in humans. This experiment investigated whether the A118G single nucleotide polymorphism of the mu-opioid receptor gene ( OPRM1 ) was associated with responses to three different experimental pain modalities in a sample of 167 healthy volunteers (96 female, 71 male). Responses to thermal, mechanical, and ischemic pain were assessed in all subjects, and genotyping of OPRM1 was performed, which revealed that the rare A118G allele occurred in 24 females (25%) and 12 males (17%). Statistical analyses indicated that subjects with a rare allele had significantly higher pressure pain thresholds than those homozygous for the common allele. Also, a sex by genotype interaction emerged for heat pain ratings at 49 degrees C, such that the rare allele was associated with lower pain ratings among men but higher pain ratings among women. These data indicate an association of a common single nucleotide polymorphism of OPRM1 with mechanical pain responses and that this genotype may be associated with heat pain perception in a sex-dependent manner. This study examines the association of the A118G SNP of OPRM1 to experimental pain sensitivity. The results indicate that the rare allele is associated with higher pressure pain thresholds. These results support previous contentions that OPRM1 may be a pain-relevant gene; however, replication of these findings is needed.

How individual sensitivity to opiates can be predicted by gene analyses

Opiate analgesics are widely used and abused drugs. Individual differences in opiate sensitivity can hamper effective pain treatments and increase risks of drug abuse. Although genetic factors might affect individual differences in opiate sensitivity, scientific evidence for specific genetic mechanisms that underlie these differences has been sparse. Recent studies using inbred and knockout mice have revealed that the mu opioid peptide (MOP) receptor encoded by the Oprm1 gene has a mandatory role in the analgesic and addictive properties of opiate drugs. Increasing evidence suggests that differences in Oprm1 gene sequences affect the amount of Oprm1 mRNA and sensitivity to opiates, and >100 polymorphisms have been identified in the human OPRM1 gene, some of which are related to vulnerability to drug dependence in some populations. Rapid advances in this research field are leading to improved understanding of the relationships between gene polymorphisms and opiate sensitivities that will enable more-accurate prediction of the opiate sensitivity and opiate requirements in individual patients.

Pharmacogenetics and Human Molecular Genetics of Opiate and Cocaine Addictions and Their Treatments

Opiate and cocaine addictions are major social and medical problems that impose a significant burden on society. Despite the size and scope of these problems, there are few effective treatments for these addictions. Methadone maintenance is an effective and most widely used treatment for opiate addiction, allowing normalization of many physiological abnormalities caused by chronic use of short-acting opiates. There are no pharmacological treatments for cocaine addiction. Epidemiological, linkage, and association studies have demonstrated a significant contribution of genetic factors to the addictive diseases. This article reviews the molecular genetics and pharmacogenetics of opiate and cocaine addictions, focusing primarily on genes of the opioid and monoaminergic systems that have been associated with or have evidence for linkage to opiate or cocaine addiction. This evidence has been marshalled either through identification of variant alleles that lead to functional alterations of gene products, altered gene expression, or findings of linkage or association studies. Studies of polymorphisms in the mu opioid receptor gene, which encodes the receptor target of some endogenous opioids, heroin, morphine, and synthetic opioids, have contributed substantially to knowledge of genetic influences on opiate and cocaine addiction. Other genes of the endogenous opioid and monoaminergic systems, particularly genes encoding dopamine beta-hydroxylase, and the dopamine, serotonin, and norepinephrine transporters have also been implicated. Variants in genes encoding proteins involved in metabolism or biotransformation of drugs of abuse and also of treatment agents are reviewed.

Are μ-opioid receptor polymorphisms important for clinical opioid therapy?

Mutations in the mu-opioid receptor--the primary site of action of opioid analgesics--are candidates for the variability of clinical opioid effects. This has been substantiated by recent advances in genetic research. A common mu-opioid receptor polymorphism was associated with higher demands for alfentanil or morphine for pain relief. It also decreased the potency of morphine for pupil constriction and experimental analgesia, but its molecular mechanisms are unclear. Another opioid receptor mutation greatly impaired receptor signalling in vitro, but is very rare. The accumulated evidence provides a solid basis for continuing research that should address the underlying molecular mechanisms and the role and benefits of OPRM1 genotyping for clinical pain therapy.

Morphine-6-glucuronide: Actions and mechanisms

Morphine-6-glucuronide (M6G) appears to show equivalent analgesia to morphine but to have a superior side-effect profile in terms of reduced liability to induce nausea and vomiting and respiratory depression. The purpose of this review is to examine the evidence behind this statement and to identify the possible reasons that may contribute to the profile of M6G. The vast majority of available data supports the notion that both M6G and morphine mediate their effects by activating the micro-opioid receptor. The differences for which there is a reasonable consensus in the literature can be summarized as: (1) Morphine has a slightly higher affinity for the micro-opioid receptor than M6G, (2) M6G shows a slightly higher efficacy at the micro-opioid receptor, (3) M6G has a lower affinity for the kappa-opioid receptor than morphine, and (4) M6G has a very different absorption, distribution, metabolism, and excretion (ADME) profile from morphine. However, none of these are adequate alone to explain the clinical differences between M6G and morphine. The ADME differences are perhaps most likely to explain some of the differences but seem unlikely to be the whole story. Further work is required to examine further the profile of M6G, notably whether M6G penetrates differentially to areas of the brain involved in pain and those involved in nausea, vomiting, and respiratory control or whether micro-opioid receptors in these brain areas differ in either their regulation or pharmacology.

Contribution to variability in response to opioids

Opioids are the oldest and most effective agents for the short- and long-term control of severe pain, particularly chronic cancer pain palliation. However, morphine and other opioids display wide variations in pharmacological efficacy and tolerability, and a significant number of patients are unable to achieve adequately controlled pain at doses that do not produce intolerable adverse effects. This article reviews factors that affect the efficacy and tolerability of opioid analgesics and clinical strategies for successful pain mangement.

The 118 A > G polymorphism in the human mu-opioid receptor gene may increase morphine requirements in patients with pain caused by malignant disease

BACKGROUND

Dispositions for genes encoding opioid receptors may explain some variability in morphine efficacy. Experimental studies show that morphine and morphine-6-glucuronide are less effective in individuals carrying variant alleles caused by the 118 A > G polymorphism in the mu-opioid receptor gene (OPRM1). The purpose of the study was to investigate whether this and other genetic polymorphisms in OPRM1 influence the efficacy of morphine in cancer pain patients.

METHODS

We screened 207 cancer pain patients on oral morphine treatment for four frequent OPRM1 gene polymorphisms. The polymorphisms were the -172 G > T polymorphism in the 5'untranslated region of exon 1, the 118 A > G polymorphism in exon 1, and the IVS2 + 31 G > A and IVS2 + 691 G > C polymorphisms, both in intron 2. Ninety-nine patients with adequately controlled pain were included in an analysis comparing morphine doses and serum concentrations of morphine and morphine metabolites in the different genotypes for the OPRM1 polymorphisms.

RESULTS

No differences related to the -172 G > T, the IVS2 + 31 G > A and the IVS2 + 691 G > C polymorphisms were observed. Patients homozygous for the variant G allele of the 118 A > G polymorphism (n = 4) needed more morphine to achieve pain control, compared to heterozygous (n = 17) and homozygous wild-type (n = 78) individuals. This difference was not explained by other factors such as duration of morphine treatment, performance status, time since diagnosis, time until death, or adverse symptoms.

CONCLUSION

Patients homozygous for the 118 G allele of the mu-opioid receptor need higher morphine doses to achieve pain control. Thus, genetic variation at the gene encoding the mu-opioid receptor contributes to variability in patients' responses to morphine.

Morphine metabolites as novel analgesic drugs?

PURPOSE OF REVIEW

Morphine metabolites have attracted continuing interest for their contribution to the desired and unwanted effects of morphine. Among the metabolites of morphine, morphine-6-glucuronide has been given most scientific attention. It accounts for 10% of the morphine metabolism, acts as an agonist at mu-opioid receptors and exerts antinociceptive effects. This review summarizes the recent findings on morphine-6-glucuronide and discusses its potential use as an analgesic.

RECENT FINDINGS

Morphine-6-glucuronide has a very long delay between the time course of its plasma concentrations and the time course of its central nervous effects, with 6-8 h probably the longest transfer half-life between plasma and effect site of all opioids administered in humans. This complicates the control of morphine-6-glucuronide therapy when used as an intravenous analgesic, and the long duration of action confers no advantage over other opioids because long-lasting opioid analgesia can be readily obtained with sustained release formulations of other opioids. During acute treatment, however, morphine-6-glucuronide appears to be sufficiently potent to exert peripheral analgesic effects, without exerting major central nervous opioid side effects for a short period of time. The side effects profile does not clearly separate morphine-6-glucuronide from morphine, with reports of similar side effects. There are contrasting reports, however, about similar or less respiratory depression and other side effects compared with morphine after systemic injection.

SUMMARY

Morphine-6-glucuronide might qualify as an analgesic but it has several pharmacological properties that make it far from ideal for therapeutic use. Whether it will be a useful addition to the currently established analgesics has yet to be demonstrated.

Genes associated with addiction: alcoholism, opiate, and cocaine addiction

Drug addiction is a complex disorder that has a large spectrum of causes. Vulnerability to addiction has been shown in twin studies to have a robust genetic component. This genetic basis for addiction has general and specific components for each drug abused. Although many genes have been implicated in drug addiction, only a handful have either been replicated to have an association or to have an identified functional mechanism related to specific effects of abused drugs. A few selected genetic variants that currently look promising for the study of alcohol, opiate, and cocaine addiction are discussed in this article.

Effect of the A118G polymorphism on binding affinity, potency and agonist-mediated endocytosis, desensitization, and resensitization of the human mu-opioid receptor

The most prevalent single-nucleotide polymorphism (SNP) A118G in the human mu-opioid receptor gene predicts an amino acid change from an asparagine residue to an aspartatic residue in amino acid position 40. This N40D mutation, which has been implicated in the development of opioid addiction, was previously reported to result in an increased beta-endorphin binding affinity and a decreased potency of morphine-6-glucuronide. Therefore, in the present study we have investigated whether this mutation might affect the binding affinity, potency, and/or the agonist-induced desensitization, internalization and resensitization of the human mu-opioid receptor stably expressed in human embryonic kidney 293 cells. With the exception of a reduced expression level of N40D compared to human mu-opioid receptor (hMOR) in HEK293 cells, our analyses revealed no marked functional differences between N40D and wild-type receptor. Morphine, morphine-6-glucuronide and beta-endorphin revealed similar binding affinities and potencies for both receptors. Both the N40D-variant receptor and hMOR exhibited robust receptor internalization in the presence of the opioid peptide [d-Ala(2),N-MePhe(4),Glyol(5)]enkephalin (DAMGO) and beta-endorphin but not in response to morphine or morphine-6-glucuronide. After prolonged treatment with morphine, morphine-6-glucuronide or beta-endorphin both receptors showed similiar desensitization time courses. In addition, the receptor resensitization rates were nearly identical for both receptor types.

Opioids and alcoholism

Although far from conclusive, evidence implicating the endogenous opioid system in the development and maintenance of alcoholism is growing. Currently available data suggest that ethanol increases opioid neurotransmission and that this activation is part of the mechanism responsible for its reinforcing effects. Findings from preclinical research indicate that ethanol consumption and ethanol-induced dopamine (DA) release are both reduced by opioid antagonists. Individual differences in endogenous opioid activity have been linked to inherited risks for alcoholism in studies comparing ethanol-preferring and nonpreferring rats, as well as in studies using targeted gene mutation (knockout) strategies. To a large extent, findings from human studies have paralleled those from the preclinical work. Persons who differ in family history of alcoholism have been shown to also differ in basal beta-endorphin activity, beta-endorphin response to alcohol, and subjective and HPA axis hormonal response to opioid antagonists. Findings from clinical trials indicate that opioid antagonists may reduce ethanol consumption in alcoholics, particularly in persons who have resumed drinking. Nevertheless, many questions remain unanswered about the use of opioid antagonists in alcoholism treatment and about the exact role of the opioid system in ethanol preference and reward. The progression of knowledge in this field suggests that many of these questions are imminently answerable, as our ability to characterize relationships between opioid activity and human behavior continues to develop. This paper summarizes both the progress that has been made and the gaps that remain in our understanding of the interactions between the endogenous opioid system and risk for alcoholism.

Functional analysis of MOR-1 splice variants of the mouse mu opioid receptor gene Oprm

A series of mu opioid receptor gene Oprm splice variants have been reported that differ only at their C-terminus. These variants all contain exons 1, 2, and 3 of the gene, the exons responsible for coding all seven transmembrane domains. Whereas MOR-1 also has exon 4 that encodes for an additional 12 amino acids at the tip of the C-terminus, the other MOR-1 variants have unique amino acid sequences distinct from those in MOR-1 due to alternative splicing. All these variants are mu-selective in binding assays. The current study explored the ability of these variants to stimulate [35S]GTPgammaS binding to assess them functionally. Only mu opioids stimulated [35S]GTPgammaS binding. Among the mu opioids we noted marked differences in their maximal stimulation among the clones. This was most prominent with beta-endorphin, which stimulated [35S]GTPgammaS binding in the MOR-1E expressing cells to a greater degree than [D-Ala2,MePhe4,Gly(ol)5]enkephalin (DAMGO; 130%) and was far less effective than DAMGO in MOR-1C cells (44%). The rank order of maximal stimulation of the drugs varied among the clones as well. Dynorphin A, beta-endorphin and morphine were most effective in stimulating [35S]GTPgammaS binding in MOR-1E, while M6G and fentanyl were most effective in MOR-1 expressing cells. The potency (EC50) of some of the drugs also varied extensively among the clones, with a poor correlation between the potency of the drugs to stimulate [35S]GTPgammaS binding and their binding affinity. Together, these findings reveal marked functional differences among the variants that only can be explained by their structural differences at the tip of their C-terminus.

β-Phenylethylamines and the isoquinoline alkaloids

This review covers beta-phenylethylamines and isoquinoline alkaloids derived from them, including further products of oxidation. condensation with formaldehyde and rearrangement, some of which do not contain an isoquinoline system, together with naphthylisoquinoline alkaloids, which have a different biogenetic origin. The occurrence of the alkaloids, with the structures of new bases, together with their reactions, syntheses and biological activities are reported. The literature from July 2002 to June 2003 is reviewed, with 568 references cited.

Genetic Predictors of the Clinical Response to Opioid Analgesics

This review uses a candidate gene approach to identify possible pharmacogenetic modulators of opioid therapy, and discusses these modulators together with demonstrated genetic causes for the variability in clinical effects of opioids. Genetically caused inactivity of cytochrome P450 (CYP) 2D6 renders codeine ineffective (lack of morphine formation), slightly decreases the efficacy of tramadol (lack of formation of the active O-desmethyl-tramadol) and slightly decreases the clearance of methadone. MDR1 mutations often demonstrate pharmacogenetic consequences, and since opioids are among the P-glycoprotein substrates, opioid pharmacology may be affected by MDR1 mutations. The single nucleotide polymorphism A118G of the mu opioid receptor gene has been associated with decreased potency of morphine and morphine-6-glucuronide, and with decreased analgesic effects and higher alfentanil dose demands in carriers of the mutated G118 allele. Genetic causes may also trigger or modify drug interactions, which in turn can alter the clinical response to opioid therapy. For example, by inhibiting CYP2D6, paroxetine increases the steady-state plasma concentrations of (R)-methadone in extensive but not in poor metabolisers of debrisoquine/sparteine. So far, the clinical consequences of the pharmacogenetics of opioids are limited to codeine, which should not be administered to poor metabolisers of debrisoquine/sparteine. Genetically precipitated drug interactions might render a standard opioid dose toxic and should, therefore, be taken into consideration. Mutations affecting opioid receptors and pain perception/processing are of interest for the study of opioid actions, but with modern practice of on-demand administration of opioids their utility may be limited to explaining why some patients need higher opioid doses; however, the adverse effects profile may be modified by these mutations. Nonetheless, at a limited level, pharmacogenetics can be expected to facilitate individualised opioid therapy.

Effects of ABCB1 (multidrug resistance transporter) gene mutations on disposition and central nervous effects of loperamide in healthy volunteers

OBJECTIVE

Mutations in the ABCB1 gene have been associated with decreased expression and net function of P-glycoprotein (P-gp). We investigated the modulation of the central nervous effects of loperamide resulting from ABCB1 genetic variants.

METHODS

On two occasions, 20 healthy volunteers received 24 mg loperamide suspension orally and, in a double-blind randomized two-way crossover fashion, 800 mg quinidine or placebo orally 1 h before loperamide. Pupil size was measured for 5 h following loperamide administration, and plasma concentrations of loperamide and quinidine were measured for 6 h. Single nucleotide polymorphisms and haplotypes including G2677T(A) (exon 21) and C3435T (exon 26) were analysed for their relation to plasma concentrations of quinidine and loperamide and to the miotic effects of loperamide.

RESULTS

Loperamide plasma concentrations with quinidine co-administration were about twice as high as those without quinidine. The ABCB1 haplotype G2677/T3435 was associated with the highest loperamide plasma concentrations, which were about 1.5 times higher than in non-carriers of this haplotype. Plasma concentrations of quinidine did not differ among carriers and non-carriers of genetic variants. When quinidine was co-administered with loperamide, pupil size decreased. Without quinidine it changed only minimally. The ABCB1 TT3435 genotype was associated with the most pronounced increase of the miotic effects of loperamide when quinidine was co-administered. This was accompanied by a tendency toward higher plasma loperamide in TT3435 carriers.

CONCLUSIONS

Our data support a functional importance of the ABCB1 mutations for plasma concentrations and central nervous actions of the opioid loperamide.

Association between human mu-opioid receptor gene polymorphism, pain tolerance, and opioid addiction

Central to both pain responses and opioid addiction is activity at the micro -opioid receptor. To explore the role of the micro -opioid receptor gene (OPRM) in human pain tolerance and opioid addiction, we examined the relationships among OPRM genotype and experimental pain tolerance in opioid addicts in methadone treatment (n = 50) and healthy normal controls (n = 59). Pain phenotype (pain tolerant vs. pain intolerant) was operationalized as tolerance to a standardized noxious stimulus (either thermal or mechanical), and dichotomized based on distribution. One microsatellite and two single nucleotide polymorphisms, A118G and C17T, in exon 1 were typed to study the OPRM gene. Although the established relationship between the phenotypes of opioid addiction and pain intolerance was validated (P = 0.02), genotype differed neither between addict-affected vs. control, nor pain tolerant vs. intolerant subjects. The variant A118G was absent in all individuals and the C17T polymorphism appeared in only three African-American individuals (two addicts and one control). The absence of this polymorphism, the small sample size and the heterogeneous ethnic backgrounds of participants in the pilot study allow only tentative conclusions based on the results, thus the role of the opioid receptor in pain and opioid reward response remains uncertain.

Endogenous opiates and behavior: 2002

This paper is the twenty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2002 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).

Haplotypes at the OPRM1 locus are associated with susceptibility to substance dependence in European-Americans

Our objective was to investigate the relationship between the gene encoding the mu-opioid receptor (OPRM1) and susceptibility to substance dependence in European-American (EA) and African-American (AA) subjects. Eight single nucleotide polymorphisms (SNPs) at the OPRM1 locus, i.e., -2044C/A, -1793T/A, -1699insT, -1469T/C, -1320A/G, -111C/T, +17C/T (Ala6Val), and +118A/G (Asn40Asp) were genotyped in 676 subjects: 318 EA subjects and 124 AA subjects with substance dependence, and 179 EA normal controls, and 55 AA normal controls. Affection status was defined by each unique combination of alcohol, cocaine, and opioid dependence and analysis of association examined in relation to the possible combinations. We used a newly implemented permutation method to evaluate statistical significance. In EAs, a significant difference in haplotype frequency distributions was found between controls and "alcohol + opioid" dependent patients (P = 0.0036). This finding is also supported by logistic regression analysis and a simulation method. The frequencies of allele -2044A and haplotypes including -2044A are higher in these patients than in controls. In AAs, no allele, haplotype, or genotype frequencies were significantly different between cases and controls. There were highly significant differences in the allele, haplotype, and genotype frequencies between EA and AA controls. Four of the variants [-1793T/A, -1699insT, -1320A/G, and -111C/T] are in virtually complete linkage disequilibrium (LD) to compose a sequence pattern, which does not associate with any of the seven categories of substance dependence. In EAs, allele -2044A and haplotypes that include -2044A are the susceptibility allele and haplotypes for substance dependence. These findings suggest that OPRM1 may play a role in the pathophysiology of substance dependence and this role is population- and diagnosis-specific.

Peripheral opioid analgesia in experimental human pain models

This placebo-controlled, double-blind crossover study assessed whether exclusive activation of peripheral opioid receptors results in significant pain reduction. To achieve opioid activity restricted to the periphery, we used a short-term (2 h) low dose infusion of morphine-6-beta-glucuronide (M6G) because M6G does not pass the blood-brain barrier during this time in amounts sufficient to induce CNS effects. The lack of central opioid effects of M6G was confirmed by a lack of change of the pupil size and absence of other opioid-related CNS effects. As a positive control, morphine was infused at a dosage that definitely produced CNS effects. This was evident by a rapid decrease of the pupil size and by other typical opioid-related side effects including nausea, vomiting, itchiness, hiccup and sedation. Three different pain models were employed to evaluate the analgesic effects: (i) cutaneous inflammatory hyperalgesia induced by briefly freezing a small skin area to -30 degrees C ('freeze lesion'); (ii) muscle hyperalgesia induced by a series of concentric and eccentric muscle contractions (DOMS model; delayed onset of muscle soreness); and (iii) pain induced by electrical current (5 Hz sinus stimuli of 0-10 mA). M6G significantly reduced cutaneous hyperalgesia in the 'freeze lesion' model as assessed with von Frey hairs. It also reduced muscle hyperalgesia in the DOMS model. Electrical pain, however, was not affected by M6G. Morphine was significantly more active in the 'freeze lesion' and DOMS model, and also significantly increased the electrical pain threshold and tolerance. Subcutaneous tissue concentrations of M6G and morphine as assessed with microdialysis were about half those of the respective plasma concentrations. The results of the study indicate that M6G has antihyperalgesic effects in inflammatory pain through activation of peripheral opioid receptors. Since this occurs at concentrations that do not cause central opioid effects, M6G might be useful as a peripheral opioid analgesic.