Opioid Receptor Gene Research Articles

Stabilization of remission in patients with opioid dependence with naltrexone implant: a pharmacogenetic approach

To evaluate the effect of opioid receptor genes and dopamine system genes polymorphisms on treatment outcomes of opioid dependence with implantable and oral naltrexone. Authors carried out a randomized double-blind, double-dummy, placebo-controlled clinical trial. Three hundred and six patients with opioid dependence were randomized into 3 equal treatment groups. The first group received implantation of 1000 mg naltrexone every 2 months during 6 months + oral naltrexone placebo; the second group - placebo implant every 2 months + oral naltrexone (50mg/day) and the third group - placebo implant + oral naltrexone placebo. It was genotyped polymorphisms in the following genes: mu-opioid receptor (OPRM1), kappa-opioid receptor (OPRK1), catechol-O-methyltransferase (COMT), dopamine receptors types 2 (DRD2) and 4 (DRD4), dopamine-beta-hydroxylase, and dopamine transporter (DAT1). Regardless of treatment several polymorphisms of these genes were associated with high risk of relapse: an allele L (2R) DRD4 120bp (p=0.05; OR (95% CI)=3.3(1.1-10.1)); an allele С DRD2 NcoI (р=0,051; OR (95% CI)=2,86 (1,09-7,52)); the genotype 9.9 DAT VNTR 40bp (р=0,04; OR (95% CI)=1,4 (1,3-1,5)); on the contrary, (СС+СТ)-(ТТ)) variants of OPRK1-DRD2Ncol increased a chance to complete treatment program (р=0,004; OR (95% CI)=7.4 (1.8-30.4)), Kaplan-Meier survival analysis (р=0,016). The probability of completing treatment program by the carriers of these variants was higher in the oral naltrexone group (p=0.016), lower in the double placebo group (p=0.015), but did not influence on treatment outcomes in the naltrexone-implant group. Naltrexone-implant is a highly effective medication for treatment of opioid dependence and its effectiveness exceeds that of oral naltrexone and placebo. The study has shown the joint influence of opioid receptor genes and genes of dopaminergic system on treatment outcomes of opioid dependence. Genetic analysis is useful for determining potential responders to naltrexone treatment of opioid dependence.

Opioid Receptor Gene A118 G Variants and Persistent Pain Symptoms Among Men and Women Experiencing Motor Vehicle Collision

Opioid Receptor Gene A118 G Variants and Persistent Pain Symptoms Among Men and Women Experiencing Motor Vehicle Collision

Implication of Monoamines, Cortisol, Brain Derived Neurotrophic Factor and Opioid Receptors Genes in the Depressive-like Effect of Isotretinoin in Adult Male Rats: Comparison with Clonidine

of dopamine (DA), norepinephrine (NE), serotonin (5-HT), 5-hydroxyindole acetic acid (5-HIAA) and brain derived neurotrophic factor (BDNF) and activity of monoamine oxidase (MAO) in the hippocampus and frontal cortex of adult male albino rats. The cortisol level was determined in serum. Gene expression of mu (MORs) and Kappa (KORs) opioid receptors genes and DNA damage were measured in the hippocampus-frontal cortex rat brains. The results show that administration of isotretinoin or clonidine caused an increase in DA, NE, 5-HIAA and BDNF concentrations, and a decrease in MAO enzyme activity in both the hippocampus and frontal cortex. In the hippocampus and frontal cortex, the 5-HT concentration increased significantly following clonidine treatment but not significantly changed after isotretinoin treatment. The level of serum cortisol increased following treatment with either isotretinoin or clonidine. Significant increases in MORs and KORs genes expression levels as well as in DNA damage of hippocampusfrontal cortex brain areas were also seen in isotretinoin or clonidine-treated rats compared with control rats. These results provide evidence that the depressive-like effects induced by isotretinoin or clonidine are exerted through alteration in monoamines, brain derived neurotrophic factor and cortisol levels as well as in opioid receptors gene expression.

Effect of naltrexone on neuropathic pain in mice locally transfected with the mutant μ-opioid receptor gene in spinal cord.

Opioid antagonists, such as naloxone and naltrexone, exhibit agonistic properties at the mutated μ receptor, MOR-S196ACSTA. In our previous study, systemic naloxone (10 mg·kg(-1) , s.c.) elicited antinociceptive effect without the induction of tolerance, dependence or rewarding effect in mice 2 weeks after intrathecal administration of double-stranded adeno-associated virus-MOR-S196ACSTA-eGFP. Here, we have investigated if this antinociceptive paradigm would be effective in a mouse model of neuropathic pain. Spinal nerves were ligated in male C57BL/6 mice 3 or 4 weeks after intrathecal injection of the lentivirus encoding the construct of MOR-S196ACSTA-eGFP (LV-MOR-S196ACSTA). Anti-allodynic effects of daily s.c.injections of saline, naltrexone (10 mg·kg(-1) ) or morphine (10 mg·kg(-1) ) were assessed by the von Frey test. After 14 days of treatment with saline, naltrexone or morphine, signs of natural withdrawal were measured at 22 and 46 h after the last injection. To determine the rewarding effects induced by morphine or naltrexone, the conditioned place preference test was carried out. Anti-allodynic effects, as measured by von Frey test, increased after naltrexone or morphine treatment in mice transfected with LV-MOR-S196ACSTA in the spinal cord. Cessation of treatment with morphine, but not naltrexone, induced natural withdrawal and rewarding effects. Systemic injection of naltrexone after the expression of a mutant μ opioid receptor, MOR-S196ACSTA, in the spinal cord may have therapeutic potential for chronic neuropathic pain, without the development of dependence or addiction. 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.

Association between Single Nucleotide Polymorphisms in the OPRM1 Gene and Intraoperative Remifentanil Consumption in Northern Chinese Women

Background: Recent studies have suggested that some single nucleotide polymorphisms (SNPs) in the human -opioid receptor gene (OPRM1) affect the postoperative analgesic efficacy of opioids and their side effects. In this study, we assessed the association between SNPs in the OPRM1 gene and intraoperative remifentanil consumption as well as perioperative side effects during gynecological hysteroscopic surgery in women from Northern China. Methods: We analyzed 178 women undergoing gynecological hysteroscopic surgery. SNP genotyping was performed using the SNaPshot method. The state anxiety index (SAI) and pressure pain threshold (PPT) of all patients were assessed preoperatively. Monitored anesthesia care was maintained by the intravenous infusion of remifentanil. Intraoperative remifentanil usage and perioperative side effects were recorded. Statistical analyses were performed using SPSS software. Results: Patients carrying one or two copies of the minor allele (G allele) of rs558025 required significantly more intraoperative remifentanil than patients without the minor allele (p = 0.001, corrected p = 0.006). There were no significant associations between the six SNPs and various clinical characteristics. No significant associations between the six SNPs and PPT or SAI were found in our study. Conclusions: SNP rs558025 in the OPRM1 gene was associated with intraoperative remifentanil consumption during gynecological hysteroscopic surgery in our subjects. i 2014 S. Karger AG, Basel

Differential expressions of the alternatively spliced variant mRNAs of the µ opioid receptor gene, OPRM1, in brain regions of four inbred mouse strains.

The µ opioid receptor gene, OPRM1, undergoes extensive alternative pre-mRNA splicing in rodents and humans, with dozens of alternatively spliced variants of the OPRM1 gene. The present studies establish a SYBR green quantitative PCR (qPCR) assay to more accurately quantify mouse OPRM1 splice variant mRNAs. Using these qPCR assays, we examined the expression of OPRM1 splice variant mRNAs in selected brain regions of four inbred mouse strains displaying differences in µ opioid-induced tolerance and physical dependence: C56BL/6J, 129P3/J, SJL/J and SWR/J. The complete mRNA expression profiles of the OPRM1 splice variants reveal marked differences of the variant mRNA expression among the brain regions in each mouse strain, suggesting region-specific alternative splicing of the OPRM1 gene. The expression of many variants was also strain-specific, implying a genetic influence on OPRM1 alternative splicing. The expression levels of a number of the variant mRNAs in certain brain regions appear to correlate with strain sensitivities to morphine analgesia, tolerance and physical dependence in four mouse strains.

Whole-genome expression profile in zebrafish embryos after chronic exposure to morphine: identification of new genes associated with neuronal function and mu opioid receptor expression.

BackgroundA great number of studies have investigated changes induced by morphine exposure in gene expression using several experimental models. In this study, we examined gene expression changes during chronic exposure to morphine during maturation and differentiation of zebrafish CNS.ResultsMicroarray analysis showed 254 genes whose expression was identified as different by at least 1.3 fold change following chronic morphine exposure as compared to controls. Of these, several novel genes (grb2, copb2, otpb, magi1b, grik-l, bnip4 and sox19b) have been detected for the first time in an experimental animal model treated with morphine. We have also identified a subset of genes (dao.1, wls, bnip4 and camk1γb) differentially expressed by chronic morphine exposure whose expression is related to mu opioid receptor gene expression. Altered expression of copb2, bnip4, sox19b, otpb, dao.1, grik-l and wls is indicative of modified neuronal development, CNS patterning processes, differentiation and dopaminergic neurotransmission, serotonergic signaling pathway, and glutamatergic neurotransmission. The deregulation of camk1γb signaling genes suggests an activation of axonogenesis and dendritogenesis.ConclusionsOur study identified different functional classes of genes and individual candidates involved in the mechanisms underlying susceptibility to morphine actions related to CNS development. These results open new lines to study the treatment of pain and the molecular mechanisms involved in addiction. We also found a set of zebrafish-specific morphine-induced genes, which may be putative targets in human models for addiction and pain processes.Electronic supplementary materialThe online version of this article (doi:10.1186/1471-2164-15-874) contains supplementary material, which is available to authorized users.

P.1.g.046 Neuroprotective effect of fluoxetine associated with Bcl-2 expression in an experimental model of ischemic stroke in mice

Methamphetamine has powerful stimulant and euphoric effects that are experienced as rewarding and encourage use. Methamphetamine addiction is associated with debilitating illnesses, destroyed relationships, child neglect, violence, and crime; but after many years of research, broadly effective medications have not been identified. Individual differences that may impact not only risk for developing a methamphetamine use disorder but also affect treatment response have not been fully considered. Human studies have identified candidate genes that may be relevant, but lack of control over drug history, the common use or coabuse of multiple addictive drugs, and restrictions on the types of data that can be collected in humans are barriers to progress. To overcome some of these issues, a genetic animal model comprised of lines of mice selectively bred for high and low voluntary methamphetamine intake was developed to identify risk and protective alleles for methamphetamine consumption, and identify therapeutic targets. The mu opioid receptor gene was supported as a target for genes within a top-ranked transcription factor network associated with level of methamphetamine intake. In addition, mice that consume high levels of methamphetamine were found to possess a nonfunctional form of the trace amine-associated receptor 1 (TAAR1). The Taar1 gene is within a mouse chromosome 10 quantitative trait locus for methamphetamine consumption, and TAAR1 function determines sensitivity to aversive effects of methamphetamine that may curb intake. The genes, gene interaction partners, and protein products identified in this genetic mouse model represent treatment target candidates for methamphetamine addiction.

Skin cytokine expression in patients with fibromyalgia syndrome is not different from controls

BackgroundFibromyalgia syndrome (FMS) is a chronic pain syndrome of unknown etiology. There is increasing evidence for small nerve fiber impairment in a subgroup of patients with FMS. We investigated whether skin cytokine and delta opioid receptor (DOR) gene expression in FMS patients differs from controls as one potential contributor to small nerve fiber sensitization.MethodsWe investigated skin punch biopsies of 25 FMS patients, ten patients with monopolar depression but no pain, and 35 healthy controls. Biopsies were obtained from the lateral upper thigh and lower calf. Gene expression of the pro-inflammatory cytokines tumor necrosis factor-alpha (TNF), interleukin (IL)-6, and IL-8 and of the anti-inflammatory cytokine IL-10 was analyzed using quantitative real-time PCR and normalizing data to 18sRNA as housekeeping gene. Additionally, we assessed DOR gene expression.ResultsAll cytokines and DOR were detectable in skin samples of FMS patients, patients with depression, and healthy controls without intergroup difference. Also, gene expression was not different in skin of the upper and lower leg within and between the groups and in FMS patient subgroups.ConclusionsSkin cytokine and DOR gene expression does not differ between patients with FMS and controls. Our results do not support a role of the investigated cytokines in sensitization of peripheral nerve fibers as a potential mechanism of small fiber pathology in FMS.

Analysis of epigenetic mechanisms regulating opioid receptor gene transcription.

Opioid drugs are generally used for moderate and severe pain reductions which act through opioid receptors. Studies on transcriptional regulation of opioid receptors are still invaluable because not only transcription is the first step to produce protein products in cells, but the receptor transcription levels also affect the pain reduction by opioids, as observed in studies of heterozygous opioid receptor knockout mice.There are growing evidences that epigenetic regulation has played significant roles in transcriptional regulation of genes, including opioid receptors. In general, epigenetic mechanisms include three main regulatory factors: DNA methylation, chromatin modification, and noncoding RNAs (such as microRNA). From previous studies of ours and others on opioid receptors, those epigenetic factors were clearly involved in regulating opioid receptor expression in vivo and in vitro. In this chapter, among those three techniques we describe more details of DNA methylation methods because of emerging concepts of DNA methylation with the recent discovery of 5-hydroxymethylcytosine converting enzyme, TET1. Another analytical method of the epigenetic factors, chromatin modification, will be described briefly and information of analyzing noncoding RNAs is briefly mentioned in Subheading 1.

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.

Comparison of allele frequencies in opioid receptor gene OPRM1 (6q24-25, A118G) among different racial population groups, and the effects of the genetic polymorphism on μ-opioid receptor agonist requirements

Comparison of allele frequencies in opioid receptor gene OPRM1 (6q24-25, A118G) among different racial population groups, and the effects of the genetic polymorphism on μ-opioid receptor agonist requirements

Effects of topiramate on ethanol-cocaine interactions and DNA methyltransferase gene expression in the rat prefrontal cortex

Recent and ongoing clinical studies have indicated that topiramate (Topamax®) could be effective in treating ethanol or cocaine abuse. However, the effects of topiramate on the co-administration of ethanol and cocaine remain largely unknown. We studied the effects of topiramate, in Wistar rats, on operant ethanol self-administration with the co-administration of cocaine (i.p.). The psychomotor effects of topiramate were examined before ethanol self-administration and cocaine exposure. Blood samples were collected to analyse ethanol and cocaine metabolism (blood ethanol levels and benzoylecgonine). Quantitative real-time PCR was used to characterize the gene expression in the prefrontal cortex. Topiramate prevented the cocaine-induced increased response to ethanol in a dose-dependent manner without causing any motor impairment by itself. This effect was observed when topiramate was administered before ethanol access, but not when topiramate was administered before the cocaine injection. Topiramate did not block cocaine-induced psychomotor stimulation. Topiramate reduced blood ethanol levels but did not affect cocaine metabolism. Ethanol increased the gene expression of DNA methyltransferases (Dnmt1 and Dnmt3a), the corepressor Dnmt1-associated protein 1 (Dmap1), and the RNA methyltransferase Trdmt1. These effects were prevented by topiramate or cocaine. Gene expression of histone deacetylase-2 and glutamate receptor kainate-1 were only increased by cocaine treatment. Topiramate and cocaine co-administration caused an up-regulation of dopamine (Drd1, Th) and opioid (Oprm1) receptor genes. Topiramate showed a tendency to alter episodic-like memory. Topiramate is an effective inhibitor of the cocaine-induced increase in operant ethanol self-administration.

Toward Personalized Medicine in the Pharmacotherapy of Alcohol Use Disorder: Targeting Patient Genes and Patient Goals

Toward Personalized Medicine in the Pharmacotherapy of Alcohol Use Disorder: Targeting Patient Genes and Patient Goals

Treatment with steroid hormones and morphine alters general activity, sexual behavior, and opioid gene expression in female rats

AimsPrevious studies have shown that brain opioid peptides exert an inhibitory influence on gonadotropin secretion. Different types of brain opioids, such as β-endorphin, enkephalin, and dynorphin, exert their actions by binding to specific opioid receptors (i.e., μ, δ, and κ, respectively). The present study determined the effects of chronic treatment with morphine in female rats with pharmacologically induced estrus on behavior and opioid receptor gene and protein expression in the hypothalamus, striatum, and periaqueductal gray. Main methodsFemale ovariectomized rats treated with estrogen+progesterone received 3.5mg/kg morphine once per day for 6days. We evaluated general activity, sexual behavior, Oprm1, Oprd1, and Oprk1 gene expression, and μ opioid receptor (MOR), δ opioid receptor (DOR), and κ opioid receptor (KOR) protein expression in the hypothalamus, striatum, and periaqueductal gray in adult virgin female ovariectomized rats. Key findingsChronic morphine treatment increased locomotion and grooming behavior, decreased immobility time, decreased sexual behavior, and decreased the lordosis quotient. The molecular biology results showed that morphine treatment increased Oprm1 gene and MOR protein expression in the striatum and decreased KOR protein expression in the hypothalamus in animals that were assessed for general activity. The animals that were evaluated for sexual behavior exhibited an increase in Oprm1 expression in the periaqueductal gray and increase in KOR expression in the striatum. SignificanceThese results suggest that both opioid system activation and sex hormones alter behavioral and molecular patterns in ovariectomized rats within a relatively short period of time.

Effect of OPRM1 and stressful life events on symptoms of major depression in African American adolescents

BackgroundIn a community sample of low-income African American adolescents, we tested the interactive effects of variation in the mu 1 opioid receptor (OPRM1) gene and the occurrence of stressful life events on symptoms of depression. MethodInteractive effects of 24 OPRM1 simple nucleotide polymorphisms (SNP) and adolescent report of stressful life events on depression were tested using multilevel regressions. SNPs were dummy coded to test both additive and dominate forms of coding. ResultsFive OPRM1 SNPs showed significant evidence of interaction with stressful life events to alter depression risk (or symptoms) after adjusting for multiple testing and the correlated nature of the SNPs. Follow-up analyses showed significant differences based on OPRM1 genotype at both lower and higher frequencies of stressful life events, suggesting that participants with a copy of the minor allele on OPRM1 SNPs rs524731, rs9478503, rs3778157, rs10485057, and rs511420 have fewer symptoms in low stress conditions but more symptoms in high stress conditions compared to major allele homozygotes. LimitationsThe genetic variants associated with depression in African American adolescents may not translate to other ethnic groups. This study is also limited in that only one gene that functions within a complex biological system is addressed. ConclusionsThis current study is the first to find an interaction between OPRM1 and life stress that is associated with depression. It also addressed an understudied population within the behavioral genetics literature. Further research should test additional genes involved in the opioid system and expand the current findings to more diverse samples.

Autistic-Like Syndrome in Mu Opioid Receptor Null Mice is Relieved by Facilitated mGluR4 Activity

The etiology of Autism Spectrum Disorders (ASDs) remains largely unknown. Identifying vulnerability genes for autism represents a major challenge in the field and allows the development of animal models for translational research. Mice lacking the mu opioid receptor gene (Oprm1(-/-)) were recently proposed as a monogenic mouse model of autism, based on severe deficits in social behavior and communication skills. We confirm this hypothesis by showing that adult Oprm1(-/-) animals recapitulate core and multiple comorbid behavioral symptoms of autism and also display anatomical, neurochemical, and genetic landmarks of the disease. Chronic facilitation of mGluR4 signaling, which we identified as a novel pharmacological target in ASDs in these mice, was more efficient in alleviating behavioral deficits than the reference molecule risperidone. Altogether, our data provide first evidence that disrupted mu opioid receptor signaling is sufficient to trigger a comprehensive autistic syndrome, maybe through blunted social reward processes, and this mouse model opens promising avenues for therapeutic innovation.

Comparison of interval timing behaviour in mice following dorsal or ventral hippocampal lesions with mice having δ -opioid receptor gene deletion

Mice with cytotoxic lesions of the dorsal hippocampus (DH) underestimated 15 s and 45 s target durations in a bi-peak procedure as evidenced by proportional leftward shifts of the peak functions that emerged during training as a result of decreases in both 'start' and 'stop' times. In contrast, mice with lesions of the ventral hippocampus (VH) displayed rightward shifts that were immediately present and were largely limited to increases in the 'stop' time for the 45 s target duration. Moreover, the effects of the DH lesions were congruent with the scalar property of interval timing in that the 15 s and 45 s functions superimposed when plotted on a relative timescale, whereas the effects of the VH lesions violated the scalar property. Mice with DH lesions also showed enhanced reversal learning in comparison to control and VH lesioned mice. These results are compared with the timing distortions observed in mice lacking δ-opioid receptors (Oprd1(-/-)) which were similar to mice with DH lesions. Taken together, these results suggest a balance between hippocampal-striatal interactions for interval timing and demonstrate possible functional dissociations along the septotemporal axis of the hippocampus in terms of motivation, timed response thresholds and encoding in temporal memory.

Opioid Receptors and Their Ligands in the Musculoskeletal System and Relevance for Pain Control

Interest in opioid drugs like morphine, as the oldest and most potent pain-killing agents known, has been maintained through the years. One of the most frequent chronic pain sensations people experience is associated with pathological conditions of the musculoskeletal system. Chronic musculoskeletal pain is a major health problem, and an adequate management requires understanding of both peripheral and central components, with more attention drawn to the former. Intense experimental and clinical research activities resulted in important knowledge on the mechanisms and functions of the endogenous opioid system located in the periphery. This review describes the occurrence and distribution of endogenous opioids and their receptors in the musculoskeletal system, and their role in pain control in musculoskeletal disorders, such as rheumatoid arthritis and osteoarthritis. Using different techniques, including immunohistochemistry, electron microscopy or radioimmunoassay, expression of enkephalins, dynorphin, β-endorphin, and endomorphins was demonstrated in musculoskeletal tissues of animals and humans. Localization of opioid peptides was found in synovial membrane, periosteum, bone and bone marrow, loose connective tissue, the paratenon and musculotendinous junction of the achilles tendon. Animal and human studies have also demonstrated expression of µ, δ and κ opioid receptor proteins in musculoskeletal tissues using radioligand binding assays, autoradiography, electrophysiology, immunohistochemistry and Western blotting. Opioid receptor gene expression was reported based on polymerase chain reaction and in situ hybridization techniques. Combining morphological and quantitative approaches, important evidence that the musculoskeletal apparatus is equipped with a peripheral opioid system is provided. Demonstration of the occurrence of an endogenous opioid system in bone and joint tissues represents an essential step for defining novel pharmacological strategies to attain peripheral control of pain in musculoskeletal disorders.

Isolating and characterizing three alternatively spliced mu opioid receptor variants: mMOR-1A, mMOR-1O, and mMOR-1P

Extensive alternative pre-mRNA splicing of the mu opioid receptor gene, OPRM1, has demonstrated an array of splice variants in mice, rats and humans. Three classes of splice variants have been identified: full-length seven transmembrane (TM) domain variants with C-terminal splicing, truncated 6TM variants and single TM variants. The current studies isolates and characterizes an additional three full-length C-terminal splice variants generated from the mouse OPRM1 gene: mMOR-1A, mMOR-1O, and mMOR-1P. Using RT-qPCR, we demonstrated differential expression of these variants' mRNAs among selected brain regions, supporting region-specific alternative splicing. When expressed in Chinese Hamster Ovary cells, all the variants displayed high mu binding affinity and selectivity with subtle differences in the affinities toward some agonists. [³⁵S]γGTP binding assays revealed marked differences in agonist-induced G protein activation in both potency and efficacy among the variants. Together with the previous studies of mu agonist-induced phosphorylation and internalization in several carboxyl terminal splice variants, the current studies further suggest the existence of biased signaling of various agonists within each individual variant and/or among different variants.