Chromosomal localization of opioid peptide and receptor genes in the mouse

The Kappa Opioid Receptor: A Promising Therapeutic Target for Multiple Pathologies

Kappa-opioid receptors (KOR) are widely expressed throughout the central nervous system, where they modulate a range of physiological processes depending on their location, including stress, mood, reward, pain, inflammation, and remyelination. However, clinical use of KOR agonists is limited by adverse effects such as dysphoria, aversion, and sedation. Within the drug-development field KOR agonists have been extensively investigated for the treatment of many centrally mediated nociceptive disorders including pruritis and pain. KOR agonists are potential alternatives to mu-opioid receptor (MOR) agonists for the treatment of pain due to their anti-nociceptive effects, lack of abuse potential, and reduced respiratory depressive effects, however, dysphoric side-effects have limited their widespread clinical use. Other diseases for which KOR agonists hold promising therapeutic potential include pruritis, multiple sclerosis, Alzheimer's disease, inflammatory diseases, gastrointestinal diseases, cancer, and ischemia. This review highlights recent drug-development efforts targeting KOR, including the development of G-protein-biased ligands, mixed opioid agonists, and peripherally restricted ligands to reduce side-effects. We also highlight the current KOR agonists that are in preclinical development or undergoing clinical trials.

Exploring Pharmacological Functions of Alternatively Spliced Variants of the Mu Opioid Receptor Gene, Oprm1, via Gene-Targeted Animal Models

The mu opioid receptor has a distinct place in the opioid receptor family, since it mediates the actions of most opioids used clinically (e.g., morphine and fentanyl), as well as drugs of abuse (e.g., heroin). The single-copy mu opioid receptor gene, OPRM1, goes through extensive alternative pre-mRNA splicing to generate numerous splice variants that are conserved from rodents to humans. These OPRM1 splice variants can be classified into three structurally distinct types: (1) full-length 7 transmembrane (TM) carboxyl (C)-terminal variants; (2) truncated 6TM variants; and (3) single TM variants. Distinct pharmacological functions of these splice variants have been demonstrated by both in vitro and in vivo studies, particularly by using several unique gene-targeted mouse models. These studies provide new insights into our understanding of the complex actions of mu opioids with regard to OPRM1 alternative splicing. This review provides an overview of the studies that used these gene-targeted mouse models for exploring the functional importance of Oprm1 splice variants.

Opioids in cancer: The κ‑opioid receptor (Review)

The κ‑opioid receptor (KOR) is one of the primary receptors of opioids and serves a vital role in the regulation of pain, anesthesia, addiction and other pathological and physiological processes. KOR is associated with several types of cancer and may influence cancer progression. It has been proposed that KOR may represent a new tumor molecular marker and provide a novel basis for molecular targeted therapies for cancer. However, the association between KOR and cancer remains to be explored comprehensively. The present review introduces KOR and its association with different types of cancer. Improved understanding of KOR may facilitate development of novel antitumor therapies.

Alternative Pre-mRNA Splicing of the Mu Opioid Receptor Gene, OPRM1: Insight into Complex Mu Opioid Actions

Most opioid analgesics used clinically, including morphine and fentanyl, as well as the recreational drug heroin, act primarily through the mu opioid receptor, a class A Rhodopsin-like G protein-coupled receptor (GPCR). The single-copy mu opioid receptor gene, OPRM1, undergoes extensive alternative splicing, creating multiple splice variants or isoforms via a variety of alternative splicing events. These OPRM1 splice variants can be categorized into three major types based on the receptor structure: (1) full-length 7 transmembrane (TM) C-terminal variants; (2) truncated 6TM variants; and (3) single TM variants. Increasing evidence suggests that these OPRM1 splice variants are pharmacologically important in mediating the distinct actions of various mu opioids. More importantly, the OPRM1 variants can be targeted for development of novel opioid analgesics that are potent against multiple types of pain, but devoid of many side-effects associated with traditional opiates. In this review, we provide an overview of OPRM1 alternative splicing and its functional relevance in opioid pharmacology.

Morphine produces potent antinociception, sedation, and hypothermia in humanized mice expressing human mu-opioid receptor splice variants

Morphine is a strong painkiller acting through mu-opioid receptor (MOR). Full-length 7-transmembrane (TM) variants of MOR share similar amino acid sequences of TM domains in rodents and humans; however, interspecies differences in N- and C-terminal amino acid sequences of MOR splice variants dramatically affect the downstream signaling. Thus, it is essential to develop a mouse model that expresses human MOR splice variants for opioid pharmacological studies. We generated 2 lines of fully humanized MOR mice (hMOR; mMOR mice), line #1 and #2. The novel murine model having human OPRM1 genes and human-specific variants was examined by reverse-transcription polymerase chain reaction and the MinION nanopore sequencing. The differences in the regional distribution of MOR between wild-type and humanized MOR mice brains were detected by RNAscope and radioligand binding assay. hMOR; mMOR mice were characterized in vivo using a tail-flick, charcoal meal, open field, tail suspension, naloxone precipitation tests, and rectal temperature measurement. The data indicated that wild-type and humanized MOR mice exhibited different pharmacology of morphine, including antinociception, tolerance, sedation, and withdrawal syndromes, suggesting the presence of species difference between mouse and human MORs. Therefore, hMOR; mMOR mice could serve as a novel mouse model for pharmacogenetic studies of opioids.

Sex and Gender Differences in the Effects of Novel Psychoactive Substances

Sex and gender deeply affect the subjective effects and pharmaco-toxicological responses to drugs. Men are more likely than women to use almost all types of illicit drugs and to present to emergency departments for serious or fatal intoxications. However, women are just as likely as men to develop substance use disorders, and may be more susceptible to craving and relapse. Clinical and preclinical studies have shown important differences between males and females after administration of “classic” drugs of abuse (e.g., Δ9-tetrahydrocannabinol (THC), morphine, cocaine). This scenario has become enormously complicated in the last decade with the overbearing appearance of the new psychoactive substances (NPS) that have emerged as alternatives to regulated drugs. To date, more than 900 NPS have been identified, and can be catalogued in different pharmacological categories including synthetic cannabinoids, synthetic stimulants (cathinones and amphetamine-like), hallucinogenic phenethylamines, synthetic opioids (fentanyls and non-fentanyls), new benzodiazepines and dissociative anesthetics (i.e., methoxetamine and phencyclidine-derivatives). This work collects the little knowledge reached so far on the effects of NPS in male and female animal and human subjects, highlighting how much sex and gender differences in the effects of NPS has yet to be studied and understood.

Emerging Insights into Mu Opioid Pharmacology

Opioid analgesics, most of which act through mu opioid receptors, have long represented valuable therapeutic agents to treat severe pain. Concerted drug development efforts for over a 100 years have resulted in a large variety of opioid analgesics used in the clinic, but all of them continue to exhibit the side effects, especially respiratory depression, that have long plagued the use of morphine. The recent explosion in fatalities resulting from overdose of prescription and synthetic opioids has dramatically increased the need for safer analgesics, but recent developments in mu receptor research have provided new strategies to develop such drugs. This chapter reviews recent advances in developing novel opioid analgesics from an understanding of mu receptor structure and function. This includes a summary of the mechanism of agonist binding deduced from the crystal structure of mu receptors. It will also highlight the development of novel agonist mechanisms, including biased agonists, bivalent ligands, and allosteric modulators of mu receptor function, and describe how receptor phosphorylation modulates these pathways. Finally, it will summarize research on the alternative pre-mRNA splicing mechanisms that produces a multiplicity of mu receptor isoforms. Many of these isoforms exhibit different pharmacological specificities and brain circuitry localization, thus providing an opportunity to develop novel drugs with increased therapeutic windows.

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.

Analysis of candidate genes for morphine preference quantitative trait locus Mop2

Compared to DBA/2J (D2), C57BL/6J (B6) inbred mice exhibit strong morphine preference when tested using a two-bottle choice drinking paradigm. A morphine preference quantitative trait locus (QTL), Mop2, was originally mapped to proximal chromosome (Chr) 10 using a B6xD2 F2 intercross population, confirmed with reciprocal congenic strains and fine mapped with recombinant congenic strains. These efforts identified a ∼ 10-Million base pair (Mbp) interval, underlying Mop2, containing 35 genes. To further reduce the interval, mice from the D2.B6-Mop2-P1 congenic strain were backcrossed to parental D2 mice and two new recombinant strains of interest were generated: D2.B6-Mop2-P1.pD.dB and D2.B6-Mop2-P1.pD.dD. Results obtained from testing these strains in the two-bottle choice drinking paradigm suggest that the gene(s) responsible for the Mop2 QTL is one or more of 22 remaining within the newly defined interval (∼ 7.6 Mbp) which includes Oprm1 and several other genes related to opioid pharmacology. Real-time qRT-PCR analysis of Oprm1 and opioid-related genes Rgs17, Ppp1r14c, Vip, and Iyd revealed both between-strain and within-strain expression differences in comparisons of saline- and morphine-treated B6 and D2 mice. Analysis of Rgs17 protein levels also revealed both between-strain and within-strain differences in comparisons of saline- and morphine-treated B6 and D2 mice. Results suggest that the Mop2 QTL represents the combined influence of multiple genetic variants on morphine preference in these two strains. Relative contributions of each variant remain to be determined.

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.

Morphine regulates expression of μ-opioid receptor MOR-1A, an intron-retention carboxyl terminal splice variant of the μ-opioid receptor (OPRM1) gene via miR-103/miR-107

The μ-opioid receptor (MOR-1) gene OPRM1 undergoes extensive alternative splicing, generating an array of splice variants. Of these variants, MOR-1A, an intron-retention carboxyl terminal splice variant identical to MOR-1 except for the terminal intracellular tail encoded by exon 3b, is quite abundant and conserved from rodent to humans. Increasing evidence indicates that miroRNAs (miRNAs) regulate MOR-1 expression and that μ agonists such as morphine modulate miRNA expression. However, little is known about miRNA regulation of the OPRM1 splice variants. Using 3'-rapid amplification cDNA end and Northern blot analyses, we identified the complete 3'-untranslated region (3'-UTR) for both mouse and human MOR-1A and their conserved polyadenylation site, and defined the role the 3'-UTR in mRNA stability using a luciferase reporter assay. Computer models predicted a conserved miR-103/107 targeting site in the 3'-UTR of both mouse and human MOR-1A. The functional relevance of miR-103/107 in regulating expression of MOR-1A protein through the consensus miR-103/107 binding sites in the 3'-UTR was established by using mutagenesis and a miR-107 inhibitor in transfected human embryonic kidney 293 cells and Be(2)C cells that endogenously express human MOR-1A. Chronic morphine treatment significantly upregulated miR-103 and miR-107 levels, leading to downregulation of polyribosome-associated MOR-1A in both Be(2)C cells and the striatum of a morphine-tolerant mouse, providing a new perspective on understanding the roles of miRNAs and OPRM1 splice variants in modulating the complex actions of morphine in animals and humans.

Mu opioids and their receptors: evolution of a concept

Opiates are among the oldest medications available to manage a number of medical problems. Although pain is the current focus, early use initially focused upon the treatment of dysentery. Opium contains high concentrations of both morphine and codeine, along with thebaine, which is used in the synthesis of a number of semisynthetic opioid analgesics. Thus, it is not surprising that new agents were initially based upon the morphine scaffold. The concept of multiple opioid receptors was first suggested almost 50 years ago (Martin, 1967), opening the possibility of new classes of drugs, but the morphine-like agents have remained the mainstay in the medical management of pain. Termed mu, our understanding of these morphine-like agents and their receptors has undergone an evolution in thinking over the past 35 years. Early pharmacological studies identified three major classes of receptors, helped by the discovery of endogenous opioid peptides and receptor subtypes-primarily through the synthesis of novel agents. These chemical biologic approaches were then eclipsed by the molecular biology revolution, which now reveals a complexity of the morphine-like agents and their receptors that had not been previously appreciated.

Current research on opioid receptor function

The use of opioid analgesics has a long history in clinical settings, although the comprehensive action of opioid receptors is still less understood. Nonetheless, recent studies have generated fresh insights into opioid receptor-mediated functions and their underlying mechanisms. Three major opioid receptors (μ-opioid receptor, MOR; δ-opioid receptor, DOR; and κ-opioid receptor, KOR) have been cloned in many species. Each opioid receptor is functionally sub-classified into several pharmacological subtypes, although, specific gene corresponding each of these receptor subtypes is still unidentified as only a single gene has been isolated for each opioid receptor. In addition to pain modulation and addiction, opioid receptors are widely involved in various physiological and pathophysiological activities, including the regulation of membrane ionic homeostasis, cell proliferation, emotional response, epileptic seizures, immune function, feeding, obesity, respiratory and cardiovascular control as well as some neurodegenerative disorders. In some species, they play an essential role in hibernation. One of the most exciting findings of the past decade is the opioid-receptor, especially DOR, mediated neuroprotection and cardioprotection. The upregulation of DOR expression and DOR activation increase the neuronal tolerance to hypoxic/ischemic stress. The DOR signal triggers (depending on stress duration and severity) different mechanisms at multiple levels to preserve neuronal survival, including the stabilization of homeostasis and increased pro-survival signaling (e.g., PKC-ERK-Bcl 2) and antioxidative capacity. In the heart, PKC and KATP channels are involved in the opioid receptor-mediated cardioprotection. The DOR-mediated neuroprotection and cardioprotection have the potential to significantly alter the clinical pharmacology in terms of prevention and treatment of life-threatening conditions like stroke and myocardial infarction. The main purpose of this article is to review the recent work done on opioids and their receptor functions. It shall provide an informative reference for better understanding the opioid system and further elucidation of the opioid receptor function from a physiological and pharmacological point of view.

Transcriptional and epigenetic regulation of opioid receptor genes: present and future

Three opioid receptors (ORs) are known: μ opioid receptors (MORs), δ opioid receptors (DORs), and κ opioid receptors (KORs). Each is encoded by a distinct gene, and the three OR genes share a highly conserved genomic structure and promoter features, including an absence of TATA boxes and sensitivity to extracellular stimuli and epigenetic regulation. However, each of the genes is differentially expressed. Transcriptional regulation engages both basal and regulated transcriptional machineries and employs activating and silencing mechanisms. In retinoic acid-induced neuronal differentiation, the opioid receptor genes undergo drastically different chromatin remodeling processes and display varied patterns of epigenetic marks. Regulation of KOR expression is distinctly complex, and KOR exerts a unique function in neurite extension, indicating that KOR is not simply a pharmacological cousin of MOR and DOR. As the expression of OR proteins is ultimately controlled by extensive posttranscriptional processing, the pharmacological implication of OR gene regulation at the transcriptional level remains to be determined.

Involvement of the melanocortin-1 receptor in acute pain and pain of inflammatory but not neuropathic origin

Background

Response to painful stimuli is susceptible to genetic variation. Numerous loci have been identified which contribute to this variation, one of which, MC1R, is better known as a gene involved in mammalian hair colour. MC1R is a G protein-coupled receptor expressed in melanocytes and elsewhere and mice lacking MC1R have yellow hair, whilst humans with variant MC1R protein have red hair. Previous work has found differences in acute pain perception, and response to analgesia in mice and humans with mutations or variants in MC1R.

Methodology and Principal Findings

We have tested responses to noxious and non-noxious stimuli in mutant mice which lack MC1R, or which overexpress an endogenous antagonist of the receptor, as well as controls. We have also examined the response of these mice to inflammatory pain, assessing the hyperalgesia and allodynia associated with persistent inflammation, and their response to neuropathic pain. Finally we tested by a paired preference paradigm their aversion to oral administration of capsaicin, which activates the noxious heat receptor TRPV1. Female mice lacking MC1R showed increased tolerance to noxious heat and no alteration in their response to non-noxious mechanical stimuli. MC1R mutant females, and females overexpressing the endogenous MC1R antagonist, agouti signalling protein, had a reduced formalin-induced inflammatory pain response, and a delayed development of inflammation-induced hyperalgesia and allodynia. In addition they had a decreased aversion to capsaicin at moderate concentrations. Male mutant mice showed no difference from their respective controls. Mice of either sex did not show any effect of mutant genotype on neuropathic pain.

Conclusions

We demonstrate a sex-specific role for MC1R in acute noxious thermal responses and pain of inflammatory origin.

Fine mapping of a major QTL influencing morphine preference in C57BL/6 and DBA/2 mice using congenic strains

C57BL/6J (B6) and DBA/2J (D2) mice differ in behaviors related to substance abuse, including voluntary morphine consumption and preference in a two-bottle choice paradigm. Two major quantitative trait loci (QTL) for morphine consumption and preference exist between these strains on chromosomes (Chrs.) 6 and 10 when the two-bottle choice involves morphine in saccharin vs quinine in saccharin. Here, we report the refinement of the Chr. 10 QTL in subcongenic strains of D2.B6-Mop2 congenic mice described previously. With these subcongenic mouse strains, we have divided the introgressed region of Chr. 10 containing the QTL gene(s) into two segments, one between the acromere and Stxbp5 (in D2.B6-Mop2-P1 mice) and the other between marker D10Mit211 and marker D10Mit51 (in D2.B6-Mop2-D1 mice). We find that, similar to B6 mice, the D2.B6-Mop2-P1 congenic mice exhibit a strong preference for morphine over quinine, whereas D2.B6-Mop2-D1 congenic mice avoid morphine (similar to D2 mice). We have also created a line of double congenic mice, B6.D2-Mop2.Qui, which contains both Chr. 10 and Chr. 6 QTL. We find that they are intermediate in their morphine preference scores when compared with B6 and D2 animals. Overall, these data suggest that the gene(s) involved in morphine preference in the morphine-quinine two-bottle choice paradigm are contained within the proximal region of Chr. 10 (which harbors Oprm1) between the acromere and Stxbp5, as well as on distal Chr. 6 between marker D6Mit10 and the telomere.

Morphine and heroin differentially modulate in vivo hippocampal LTP in opiate-dependent rat

Addictive drugs have been shown to severely influence many neuronal functions, which are considered as the underlying mechanisms for physiological and psychological dependences. We previously showed that in vivo LTP in rat hippocampal CA1 region is significantly reduced during withdrawal following chronic opiates treatment, and the reduced LTP can be restored by re-exposure of animals to corresponding drugs. Here, we further demonstrated that during opiates withdrawal, the re-exposure of morphine either systemically (subcutaneously) or locally (intracerebroventricularly) could restore the reduced LTP in heroin-dependent rats, but heroin could not restore the reduced LTP, in morphine-dependent rats, indicating differential modulations of hippocampal functions by those two opiates. In contrast, DAMGO, a mu-opioid receptor (MOR) agonist, could restore the reduced LTP, and CTOP, a MOR antagonist, could block the restoration in rats dependent on both opiates, showing that MOR is functional under such conditions. However, the upregulation of hippocampal PKA activity during morphine withdrawal could be suppressed by re-exposure of morphine but not that of heroin, suggesting a likely underlying mechanism of the differential modulation of LTP by two opiates. Taken together, our study clearly demonstrates that chronic abuse of opiates inevitably leads to severe alteration of hippocampal LTP, and reveals the interesting differences between morphine and heroin in their effects on the differential modulation of hippocampal synaptic plasticity.

Identification of five mouse mu-opioid receptor (MOR) gene (Oprm1) splice variants containing a newly identified alternatively spliced exon

The mouse mu-opioid receptor gene, Oprm1, currently contains 18 recognized alternatively spliced exons [Doyle, G.A., Sheng, X.R., Lin, S.S.J., Press, D.M., Grice, D.E., Buono, R.J., Ferraro, T.N., Berrettini, W.H., 2007. Identification of three mouse mu-opioid receptor (MOR) gene (Oprm1) splice variants containing a newly identified alternatively spliced exon. Gene 388 (1-2) 135-147, in press (doi:10.1016/j.gene.2006.10.017). Electronic publication 2006 November 1] that generate 27 splice variants encoding at least 11 morphine-binding isoforms of the receptor. Here, we identify five MOR variants that contain an as yet undescribed exon (exon 19) of the gene, and we provide evidence that these MOR splice variants are expressed in the C57BL/6 and DBA/2 mouse strains. Three splice variants, MOR-1Eii, MOR-1Eiii and MOR-1Eiv, encode the MOR-1E isoform and contain the newly identified exon 19 in their 3' untranslated regions. The fourth splice variant encodes a novel mu-opioid receptor isoform, MOR-1U, and contains exon 19 in its coding region. The cytoplasmic tail of the putative MOR-1U isoform contains a putative nuclear localization signal encoded by the sequence of exon 19. Exon 19 appears to be conserved in the rat, but not in humans. In mouse and rat Oprm1, exon 19 is located between described exons 7 and 8. We also report the cloning of the "full-length" MOR-1T splice variant [Kvam, T.-M., Baar, C., Rakvag, T.T., Kaasa, S., Krokan, H.E., Skorpen, F., 2004. Genetic analysis of the murine mu-opioid receptor: increased complexity of Oprm1 gene splicing, J. Mol. Med. 82 (4) 250-255] that encodes MOR-1 and contains the newly identified exon in its 3' UTR. RT-PCR analysis suggests that splice variants MOR-1Eii, MOR-1Eiii, MOR-1Eiv, MOR-1T and MOR-1U are expressed in all brain regions analyzed (cortex, cerebellum, hypothalamus, thalamus and striatum). These exon 19-containing splice variants add to the growing complexity of the mouse Oprm1 gene.

Identification of three mouse mu-opioid receptor (MOR) gene (Oprm1) splice variants containing a newly identified alternatively spliced exon

The mouse mu-opioid receptor gene, Oprm1, is recognized currently to contain 17 alternatively spliced exons that generate 24 splice variants encoding at least 11 morphine-binding isoforms of the receptor. Here, we identify three new MOR splice variants that contain a previously undescribed exon, exon 18, and provide evidence that they are expressed in two mouse strains. The transcripts containing the newly identified exon 18 encode two new putative mu-opioid receptor isoforms, MOR-1V and MOR-1W. In mouse Oprm1, exon 18 is located between the described exons 10 and 6. Exon 18 appears to be conserved in the rat genome between exons 4 and 7. A BLAST search of the non-redundant GenBank database suggests that human OPRM1 may also contain exon 18. Analysis of mouse brain mRNA by RT-PCR suggests that MOR-1Vii transcripts are expressed in all areas of the brain analyzed, whereas expression of MOR-1Vi transcripts was restricted to thalamus and striatum. MOR-1W transcripts are expressed most highly in the hypothalamus, thalamus and striatum. In summary, we have identified three brain expressed, alternatively spliced mouse MOR splice variants containing a novel exon and encoding new putative MOR isoforms, MOR-1V and MOR-1W.

Identification and functional significance of polymorphisms in the μ-opioid receptor gene (Oprm) promoter of C57BL/6 and DBA/2 mice

C57BL/6J and DBA/2J mice demonstrate differences in morphine preference when tested in a two-bottle choice paradigm. Quantitative trait loci (QTL) mapping suggested the proximal region of chromosome 10 was responsible for 41% of the observed genetic variance. The mu-opioid receptor (MOR) gene (Oprm) maps to this region and is a prime candidate for explaining the QTL. We hypothesized that variations in Oprm between these strains are responsible for differences in morphine preference. We identify five single nucleotide polymorphisms (SNPs) in the Oprm promoter; three within or near putative transcription factor binding sites. Promoter fragments were amplified from genomic DNA by polymerase chain reaction (PCR) and subcloned into luciferase reporter vectors. A significant difference in basal Oprm promoter activity was seen with C57BL/6 and DBA/2 approximately 1675 constructs in MOR-positive BE(2)-C cells, but not in MOR-negative Neuro-2a cells. In BE(2)-C cells, average DBA/2 approximately 1675 construct activity was 1.3-2.0x greater than average C57BL/6 activity suggesting that the SNPs might alter MOR expression in these two mouse strains. Significant differences in promoter activities between the two cell lines suggest that cell-type-specific transcription factors are involved. No significant differences in construct activity were found between untreated and morphine-treated BE(2)-C or Neuro-2a cells, suggesting that morphine does not regulate transcription of Oprm.

Identification and characterization of six new alternatively spliced variants of the human mu opioid receptor gene, Oprm

The mu opioid receptor plays an important role in mediating the actions of morphine and morphine-like drugs. Receptor binding and a wide range of pharmacological studies have proposed several mu receptor subtypes, but only one mu opioid receptor (Oprm) gene has been isolated. Like the mouse and rat, the human Oprm gene undergoes alternative splicing. In the present studies, we have identified and characterized six new splice variants from the human Oprm gene using a reverse transcription-polymerase chain reaction strategy, yielding a total of 10 human splice variants of the mu opioid receptor MOR-1. All the variants identified contained exons 1, 2 and 3, but differed from MOR-1 itself and each other by splicing downstream from exon 3, resulting in different amino acid sequences. Northern blot analysis demonstrated expression of the variant mRNAs. Receptor binding assays established that these variants belonged to the mu opioid receptor family with limited differences in mu opioid ligand affinities and selectivity. However, adenylyl cyclase and [35S]GTPgammaS binding assays revealed major differences in both potency and efficacy among these variants. The dissociation between binding affinity, potency and efficacy for the opioids among these variants may provide insights into the wide range of opioid responses among these agents observed clinically and opens new avenues in designing selective drugs based upon their efficacy and potency rather simple binding affinity.

Confirmation of a major QTL influencing oral morphine intake in C57 and DBA mice using reciprocal congenic strains

C57BL/6 (B6) and DBA/2 (D2) mice exhibit disparate behavior when tested for voluntary morphine intake in a two-bottle choice drinking paradigm with B6 mice consuming 10 times more drug than D2 mice. Previous genetic mapping studies identified a locus, Mop2, on the proximal part of chromosome 10 that explained over half of the genetic variance in this mouse model of opioid self-administration. We constructed a set of reciprocal congenic strains between B6 and D2 mice in which the proximal portion of chromosome 10 has been introgressed from one strain onto the background of the other. We tested mice from this pair of reciprocal strains together with progenitor B6 and D2 mice in a two-bottle choice drinking paradigm with morphine and quinine. The results showed that introgression of chromosome 10 alleles from the B6 strain onto a D2 genetic background increased voluntary morphine intake four-fold compared to progenitor D2 mice. Preference for morphine was also increased significantly in D2.B6-Mop2 mice compared to progenitor D2 mice. Conversely, introgression of chromosome 10 alleles from the D2 strain onto a B6 genetic background decreased morphine intake by half compared to progenitor B6 mice in B6.D2 -Mop2 mice; however, high morphine preference was maintained in this congenic strain most likely due to strong quinine aversion. When quinine was eliminated from the control bottle, morphine preference in B6.D2-Mop2 mice was decreased significantly relative to B6 and D2.B6-Mop2 mice. Overall, these data confirm the existence of a gene(s) on chromosome 10 proximal to D10Mit124 that has a strong influence on the difference in morphine drinking behavior between B6 and D2 mice.

Genetic analysis of the murine mu opioid receptor: increased complexity of Oprm gene splicing

Evidence exists that mu analgesics such as morphine, methadone and fentanyl may act through distinct mu opioid receptor mechanisms. It has been proposed that the functional diversity of mu opioid receptors may be related to alternative splicing of the Oprm gene. Although a number of mu opioid receptor mRNA splice variants have been reported, their biological relevance has been controversial, due in part to their very low abundance and a general lack of validation from independent laboratories. We have identified 11 of 17 proposed exons as well as the majority of exon combinations used to make 21 differentially spliced Oprm mRNAs from mouse whole brain cDNA, using polymerase chain reaction (PCR) conditions different from those used by the single other group that has reported multiple splice forms. Alternative splicing was shown to occur at both the 5' and 3' termini. Moreover, verification of a short variant, containing exons 1 and 4 only, suggests that splicing also occurs directly between 5' and 3' exons. Notably, a novel splice variant, MOR-1T, demonstrates for the first time that exon 4 can be used in combination with further downstream exons to make the 3'-end of MOR-1 splice variants. The putative protein encoded by MOR-1T is predicted to be identical to that of MOR-1, implying that the MOR-1 protein can be generated from at least five differentially spliced mRNAs. Our results support the view that the Oprm gene undergoes extensive alternative splicing, as a likely major contributor to the diversity of mu opioid receptors.

The Wheels mutation in the mouse causes vascular, hindbrain, and inner ear defects

In a screen for mouse mutations with dominant behavioral anomalies, we identified Wheels, a mutation associated with circling and hyperactivity in heterozygotes and embryonic lethality in homozygotes. Mutant Wheels embryos die at E10.5-E11.5 and exhibit a host of morphological anomalies which include growth retardation and anomalies in vascular and hindbrain development. The latter includes perturbation of rhombomeric boundaries as detected by Krox20 and Hoxb1. PECAM-1 staining of embryos revealed normal formation of the primary vascular plexus. However, subsequent stages of branching and remodeling do not proceed normally in the yolk sac and in the embryo proper. To obtain insights into the circling behavior, we examined development of the inner ear by paint-filling of membranous labyrinths of Whl/+ embryos. This analysis revealed smaller posterior and lateral semicircular canal primordia and a delay in the canal fusion process at E12.5. By E13.5, the lateral canal was truncated and the posterior canal was small or absent altogether. Marker analysis revealed an early molecular phenotype in heterozygous embryos characterized by perturbed expression of Bmp4 and Msx1 in prospective lateral and posterior cristae at E11.5. We have constructed a genetic and radiation hybrid map of the centromeric portion of mouse Chromosome 4 across the Wheels region and refined the position of the Wheels locus to the approximately 1.1-cM region between D4Mit104 and D4Mit181. We have placed the locus encoding Epha7, in the Wheels candidate region; however, further analysis showed no mutations in the Epha7-coding region and no detectable changes in mRNA expression pattern. In summary, our findings indicate that Wheels, a gene which is essential for the survival of the embryo, may link diverse processes involved in vascular, hindbrain, and inner ear development.

Comparative immunohistochemical distributions of carboxy terminus epitopes from the mu-opioid receptor splice variants MOR-1D, MOR-1 and MOR-1C in the mouse and rat CNS

The present study examined immunohistochemically the CNS distributions of a splice variant of the mu-opioid receptor, MOR-1D, in both rats and mice. In MOR-1D, exon 4 of MOR-1 is replaced by two additional exons that code for seven amino acids. Using rabbit antisera, we compared immunohistochemically the regional distribution of a C-terminal epitope of MOR-1D to that of a C-terminal epitope from MOR-1 and a C-terminal epitope from another splice variant, MOR-1C. The general distribution of MOR-1D-like immunoreactivity was similar in both mouse and rat. MOR-1D-like immunoreactivity was seen in the dentate gyrus and in the mossy fibers of the hippocampal formation, the nucleus of the solitary tract and the area postrema, the inferior olivary nucleus, the nucleus ambiguous, the spinal trigeminal nucleus and the spinal cord. MOR-1D-like immunoreactivity was not observed in some regions containing dense MOR-1-like immunoreactivity, such as the striatum or the locus coeruleus. In regions containing MOR-1, MOR-1C and MOR-1D, the pattern of each variant was unique.MOR-1D and MOR-1C are splice variants of the cloned mu-opioid receptor MOR-1. Although they differ only at the tip of the carboxy terminus, they show marked differences in their regional distributions, as determined immunohistochemically by epitopes in their unique carboxy termini. Since the splice variants are derived from the same gene, these differences in regional distribution imply region-specific messenger RNA processing.

The opioid receptor system and alcoholism: a genetic perspective

Over the past decade, mounting evidence has implicated the endogenous opioid receptor system as a central player in the etiology of alcohol drinking behavior in animals and alcoholism in humans. Much of this work is a product of a pharmacological approach, where differences in opioid receptor pharmacology have been found to predict drinking behavior in animal models of alcoholism, including rats and mice selectively bred for alcohol preference and avoidance. This review considers the opioid receptor system and alcoholism from a genetic standpoint, and discusses investigation into opioid receptor pharmacology in animal models of alcoholism as work that paved the way for the more recent molecular genetic studies implicating the delta-, and particularly, the mu opioid receptors as genetically linked to alcoholism-associated phenotypes in animal models of the disease. These genetic studies are set within the broader context of the candidate gene approach for alcoholism, where opioid receptor genes are taken to be partial, rather than complete, risk factors for alcoholism. Building upon these findings, the recent genetic association between alcoholism and the mu opioid receptor gene in humans is discussed. Finally, the translation of such genetic association studies between opioid receptor genes and alcoholism to a pharmacogenetic approach, allowing for the evaluation of putative relationships between genotype and pharmacological response profiles, is suggested to address the etiological question of what the molecular mechanism is underlying opioid receptor genetic risk for alcoholism phenotypes.

Differential distribution in rat brain of mu opioid receptor carboxy terminal splice variants MOR-1C-like and MOR-1-like immunoreactivity: evidence for region-specific processing

The present study examined immunohistochemically the regional distribution of the mu opioid receptor splice variant MOR-1C by using a rabbit antisera generated against the C-terminal peptide sequences and compared it with MOR-1. Overall, the distribution of MOR-1C-like immunoreactivity (-LI) differed from MOR-1-LI. Both MOR-1C-LI and MOR-1-LI were prominent in a few central nervous system regions, including the lateral parabrachial nucleus, the periaqueductal gray, and laminae I-II of the spinal trigeminal nuclei and the spinal cord. In the striatum, hippocampal formation, presubiculum and parasubiculum, amygdaloid nuclei, thalamic nuclei, locus coeruleus, and nucleus ambiguous MOR-1-LI predominated, whereas MOR-1C-LI was absent or sparse. Conversely, MOR-1C-LI exceeded MOR-1-LI in the lateral septum, the deep laminae of the spinal cord, and most hypothalamic nuclei such as the median eminence, periventricular, suprachiasmatic, supraoptic, arcuate, paraventricular, ventromedial, and dorsomedial nuclei. Double-labeling studies showed colocalization of the two receptors in neurons of the lateral septum, but not in the median eminence or in the arcuate nucleus, even though both MOR-1 isoforms were expressed. Because both MOR-1 and MOR-1C are derived from the same gene, these differences in regional distribution represent region-specific mRNA processing. The regional distributions reported in this study involve the epitope seen by the combinations of exons 7, 8, and 9. However, if other MOR-1 variants containing exons 7, 8, and 9 exist, the antisera would not distinguish between them and MOR-1C.

Isolation and expression of a novel alternatively spliced mu opioid receptor isoform, MOR-1F

The MOR-1 gene is large, with a recent study reporting nine exons spanning 250 kb which combine to yield six different mu opioid receptor splice variants. We now report the isolation of exon 10, which is contained within yet another splice variant, MOR-1F, which is composed of exons 1, 2, 3, 10, 6, 7, 8 and 9. Exon 10 comprises 186 bp which predict a unique 58 amino acid sequence extending beyond exon 3. It has been mapped between exons 4 and 6 and has flanking consensus splice sequences. On Northern blot analysis, the MOR-1F mRNA is smaller than the other MOR-1 mRNAs. When expressed in CHO cells, MOR-1F binds the mu opioid radioligand [3H]DAMGO with high affinity (K(D) = 1.04+/-0.03 nM). Competition studies demonstrated the selectivity of the variant for mu opioid ligands, supporting its classification within the mu opioid receptor family.

Isolation and structural and genetic analysis of the mouse enkephalin gene and its d(AC/TG)n repeats

Enkephalins, the endogenous opioids, mediate a wide variety of intercellular communications through ontogeny and their involvement has been suggested in drug addiction and alcohol abuse as well as in various neuropsychiatric disorders. In order to generate a genetic model, we have isolated the mouse enkephalin (mENK) gene, analyzed its regulatory region and compared its structure to the well characterized rat ENK (rENK) gene. We analyzed 2600 bp and found 3 highly homologous regions: The highest level (98%) of positional and sequence homology between mice and rats was in the TATA/proximal regulatory region. This region contains all the inducible regulatory elements (enkCRE1, NF1, AP-2, NFkappaB, etc.) and also an octamer-like element at -543 bp. This high homology is interrupted in both mice and rats by the typically polymorphic d(AC/TG)n and d(TC/GA)n dinucleotide repeats positioned between nucleotides -670 and -950. The position and orientation of these repetitive elements differ substantially in the two species. Genomic PCR analysis of the d(AC/TG)n repeat in various mouse strains, including aberrant behavioral or neurological phenotypes, showed lack of polymorphism at this repeat. The positional and sequence homologies between the rat and the mouse ENK genes decrease in more upstream regions due to the presence of nonhomologues repetititve DNA sequences.

Identification and characterization of three new alternatively spliced mu-opioid receptor isoforms

We have identified four new mu-opiod receptor (MOR)-1 exons, indicating that the gene now contains at least nine exons spanning more than 200 kilobases. Replacement of exon 4 by combinations of the new exons yields three new receptors. When expressed in Chinese hamster ovary cells, all three variants displayed high affinity for mu-opioid ligands, but kappa and delta drugs were inactive. However, there were subtle, but significant, differences in the binding profiles of the three variants among themselves and from MOR-1. Immunohistochemically, the major variant, MOR-1C, displayed a regional distribution quite distinct from that of MOR-1. Region-specific processing also was seen at the mRNA level. Antisense mapping revealed that the four new exons were all involved in morphine analgesia. Together with two other variants generated from alternative splicing of exon 4, there are now six distinct MOR-1 receptors.

The effects of repeated morphine exposure on mu opioid receptor number and affinity in C57BL/6J and DBA/2J mice

C57BL/6J (B6) mice self-administer substantial quantities of morphine compared to DBA/2J (D2) mice, and most of the genetic component of this strain difference has been attributed to a locus on chromosome 10 in the vicinity of the mu opioid receptor gene. To compare binding characteristics of mu opioid receptor populations between the two strains, mice were given single daily injections of a long-acting preparation of morphine sulfate (80 mg/kg, s.c.) or saline for a period of seven days, and euthanatized six hours after the last injection. Brains were removed and dissected into specific regions. Receptor binding studies were performed on frontal cortex and striatum. Data were analyzed using non-linear regression, and Kd and Bmax comparisons made between strains and treatments. Specific [3H]DAMGO binding in striatum indicates that the density of mu opioid receptors in saline-treated B6 mice and saline-treated D2 mice does not differ significantly. After repeated morphine injection, B6 mice exhibited a decrease in striatal [3H]DAMGO binding, indicating a downregulation of receptor density by approximately 45% (p=.0003 vs saline-treated B6), a phenomenon not observed in D2 mice. In frontal cortex, no differences in [3H]DAMGO binding were observed between strains or treatment groups. These results demonstrate a significant difference between mu opioid receptor regulation in B6 and D2 mice, and may underlie well documented strain differences in specific opioid-related behaviors.