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

Taar1 gene variants have a causal role in methamphetamine intake and response and interact with Oprm1

We identified a locus on mouse chromosome 10 that accounts for 60% of the genetic variance in methamphetamine intake in mice selectively bred for high versus low methamphetamine consumption. We nominated the trace amine-associated receptor 1 gene, Taar1, as the strongest candidate and identified regulation of the mu-opioid receptor 1 gene, Oprm1, as another contributor. This study exploited CRISPR-Cas9 to test the causal role of Taar1 in methamphetamine intake and a genetically-associated thermal response to methamphetamine. The methamphetamine-related traits were rescued, converting them to levels found in methamphetamine-avoiding animals. We used a family of recombinant inbred mouse strains for interval mapping and to examine independent and epistatic effects of Taar1 and Oprm1. Both methamphetamine intake and the thermal response mapped to Taar1 and the independent effect of Taar1 was dependent on genotype at Oprm1. Our findings encourage investigation of the contribution of Taar1 and Oprm1 variants to human methamphetamine addiction.

Differential regulation of mouse and human Mu opioid receptor gene depends on the single stranded DNA structure of its promoter and α-complex protein 1

The Mu opioid receptor (MOR) mediates various functions of opioid-induced analgesia, euphoria and respiratory depression, and is a major target of opioid analgesics. Understanding of MOR gene expression among species is important for understanding its analgesic function in humans. In the current study, the polypyrimidine/polypurine (PPy/u) region, a key element of MOR gene expression, was compared in humans and mice. The mouse PPy/u element is highly homologous to its human element (84%), and the mouse MOR (mMOR) reporter drives luciferase activity 35-fold more effectively than the human MOR (hMOR) reporter. The structural study of reporter plasmids using S1 nuclease indicates that the mouse PPy/u element has a particular conformational structure, namely a single-stranded DNA (ssDNA) region that promotes strong promoter activity. DNA electrophoretic mobility shift assays demonstrated that the recombinant α-complex protein 1 (α-CP1) is capable of binding to a single-stranded mouse PPy/u sequence. Furthermore, plasmid-expressing α-CP1 activated the expression of a luciferase reporter when cotransfected with a single-stranded (p336/306) construct. In addition, the α-CP1 gene induced the mMOR gene in mouse neuronal cells and did not induce the human neuronal MOR gene. The current study demonstrates that α-CP1 functions as a transcriptional activator in the mMOR gene, but does not function in the hMOR gene due to species-specific structural differences. The differences in human and mouse MOR gene expression are based on α-CP1 and the ssDNA structure of the MOR promoter. The MOR gene is species-specifically regulated, as the PPy/u element adopts a unique species-specific conformation and α-CP1 recruitment.

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.

Opioid sensitivity in mice selectively bred to consume or not consume methamphetamine

There has been little investigation of genetic factors and associated mechanisms that influence risk for development of methamphetamine (MA) dependence. Selectively bred mouse lines that exhibit high (MAHDR) or low (MALDR) levels of MA intake in a two-bottle choice MA drinking (MADR) procedure provide a genetic tool for this purpose. These lines were used to determine whether opioid sensitivity and MA intake are genetically associated, because opioid-mediated pathways influence some effects of MA. Sensitivity to the analgesic effects of the μ-opioid receptor (MOP-r) agonist fentanyl (0.05, 0.1, 0.2, 0.4 mg/kg) was examined using two acute thermal tests (hot plate and tail flick) and one chronic pain test (magnesium sulfate abdominal constriction). Locomotor stimulant responses to fentanyl (0.05, 0.1, 0.2, 0.4 mg/kg) and morphine (10, 20, 30 mg/kg) were also examined. In addition, MADR was measured in the progenitor strains [(C57BL/6J (B6), DBA/2J (D2)] of the F2 population from which the selected lines were generated. The MADR lines did not differ in sensitivity to the analgesic effects of fentanyl; however, MALDR mice exhibited greater locomotor activation than MAHDR mice to both fentanyl and morphine. D2 mice consumed more MA than B6 mice. The line differences for MA consumption and morphine activation recapitulated B6 and D2 strain differences for these two traits, but not strain differences previously found for opioid analgesic responses. These results support a negative genetic correlation between MA consumption and sensitivity to the stimulant effects of opioids and suggest the involvement of MOP-r regulated systems in MA intake.

Opioids and their receptors: Are we there yet?

Opioids have an important place in pharmacology. While their clinical use as analgesics is fundamental in medicine, their use is constrained by their side-effects and abuse potential. Pharmacologists have sought analgesics lacking side-effects and the abuse liability of the current agents. The identification of the opioid receptors in 1973 marked the beginning of our understanding of the molecular mechanisms of these agents. The isolation of the opioid peptides quickly followed, along with the classification of three families of opioid receptors. Clinicians have long been aware of subtle differences among the mu opioids that were not easily reconciled with a single receptor and selective antagonists implied two subdivisions of mu receptors. However, the cloning of the mu opioid receptor MOR-1 has led to the realization of the extensive complexity of the mu opioid receptor gene and its vast array of splice variants. Many of these splice variants are truncated and do not conform to the structure of traditional G-protein coupled receptors. Yet, evidence now shows that they are quite important and may prove valuable targets in the development of potent analgesics lacking the undesirable properties of current opioids. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Differential gene expression activity among species-specific polypyrimidine/polypurine motifs in mu opioid receptor gene promoters

The mu opioid receptor (MOR) is the principle molecular target of opioid analgesics. An appropriate understanding of MOR gene expression across species is critical for understanding its analgesic functions in humans. Here, we undertake a cross-species analysis of the polymorphic polypyrimidine/polypurine (PPy/u) motif, a key enhancer of MOR gene expression. The mouse PPy/u motif is highly homologous to those of rat (67%) and human (83%), but drives reporter gene expression tenfold and fivefold more effectively than those of rat and human, respectively. Circular dichroism profiles of PPy/u oligonucleotides from different species showed that they are primarily different in structure. Conformational studies of reporter plasmids using confocal Raman spectra, S1 nuclease and restriction enzymes demonstrated that the structural difference is the result of changes in the phosphodiester backbone. Furthermore, these conformational disparities produce differences in torsional stress, as shown by topoisomerase II relaxation and activation of different levels of gene expression under hypertonic conditions. This study demonstrates that homologous PPy/u motifs adopt unique species-specific conformations with different mechanisms and activities for gene expression. We further discuss how structural aspects of transcription regulatory elements, rather than the sequence itself, are significant when studying functional gene expression regulatory elements.

Mouse strain differences in locomotor, sensitisation and rewarding effect of heroin; association with alterations in MOP-r activation and dopamine transporter binding

There is growing agreement that genetic factors play an important role in the risk to develop heroin addiction, and comparisons of heroin addiction vulnerability in inbred strains of mice could provide useful information on the question of individual vulnerability to heroin addiction. This study examined the rewarding and locomotor-stimulating effects of heroin in male C57BL/6J and DBA/2J mice. Heroin induced locomotion and sensitisation in C57BL/6J but not in DBA/2J mice. C57BL/6J mice developed conditioned place preference (CPP) to the highest doses of heroin, while DBA/2J showed CPP to only the lowest heroin doses, indicating a higher sensitivity of DBA/2J mice to the rewarding properties of heroin vs C57BL/6J mice. In order to investigate the neurobiological substrate underlying some of these differences, the effect of chronic 'intermittent' escalating dose heroin administration on the opioid, dopaminergic and stress systems was explored. Twofold higher mu-opioid receptor (MOP-r)-stimulated [35S]GTPgammaS binding was observed in the nucleus accumbens and caudate of saline-treated C57BL/6J mice compared with DBA/2J. Heroin decreased MOP-r density in brain regions of C57BL/6J mice, but not in DBA/2J. A higher density of dopamine transporters (DAT) was observed in nucleus accumbens shell and caudate of heroin-treated DBA/2J mice compared with heroin-treated C57BL/6J. There were no effects on D1 and D2 binding. Chronic heroin administration decreased corticosterone levels in both strains with no effect of strain. These results suggest that genetic differences in MOP-r activation and DAT expression may be responsible for individual differences in vulnerability to heroin addiction.

Bowman-Birk inhibitor affects pathways associated with energy metabolism in Drosophila melanogaster

Bowman-Birk inhibitor (BBI) is toxic when fed to certain insects, including the fruit fly, Drosophila melanogaster. Dietary BBI has been demonstrated to slow growth and increase insect mortality by inhibiting the digestive enzymes trypsin and chymotrypsin, resulting in a reduced supply of amino acids. In mammals, BBI influences cellular energy metabolism. Therefore, we tested the hypothesis that dietary BBI affects energy-associated pathways in the D. melanogaster midgut. Through microarray and metabolomic analyses, we show that dietary BBI affects energy utilization pathways in the midgut cells of D. melanogaster. In addition, ultrastructure studies indicate that microvilli are significantly shortened in BBI-fed larvae. These data provide further insights into the complex cellular response of insects to dietary protease inhibitors.

Identification of candidate genes and gene networks specifically associated with analgesic tolerance to morphine

Chronic morphine administration may alter the expression of hundreds to thousands of genes. However, only a subset of these genes is likely involved in analgesic tolerance. In this report, we used a behavior genetics strategy to identify candidate genes specifically linked to the development of morphine tolerance. Two inbred genotypes [C57BL/6J (B6), DBA2/J (D2)] and two reciprocal congenic genotypes (B6D2, D2B6) with the proximal region of chromosome 10 (Chr10) introgressed into opposing backgrounds served as the behavior genetic filter. Tolerance after therapeutically relevant doses of morphine developed most rapidly in the B6 followed by the B6D2 genotype and did not develop in the D2 mice and only slightly in the D2B6 animals indicating a strong influence of the proximal region of Chr10 in the development of tolerance. Gene expression profiling and pattern matching identified 64, 53, 86, and 123 predisposition genes and 81, 96, 106, and 82 tolerance genes in the periaqueductal gray (PAG), prefrontal cortex, temporal lobe, and ventral striatum, respectively. A potential gene network was identified in the PAG in which 19 of the 34 genes were strongly associated with tolerance. Eleven of the network genes were found to reside in quantitative trait loci previously associated with morphine-related behaviors, whereas seven were predictive of tolerance (morphine-naive condition). Overall, the genes modified by chronic morphine administration show a strong presence in canonical pathways representative of neuroadaptation. A potentially significant role for the micro-RNA and epigenetic mechanisms in response to chronic administration of pharmacologically relevant doses of morphine was highlighted by candidate genes Dicer and H19.

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.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 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 neurological 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).

Involvement of the cerebellar dorsal vermis in vergence eye movements in monkeys

Frontal-eyed primates use both smooth pursuit in frontoparallel planes (frontal pursuit) and pursuit-in-depth (vergence pursuit) to track objects moving slowly in 3-dimensional (3D) space. To understand how 3D-pursuit signals represented in frontal eye fields are processed further by downstream pathways, monkeys were trained to pursue a spot moving in 3D virtual space. We characterized pursuit signals in Purkinje (P) cells in the cerebellar dorsal vermis and their discharge during vergence pursuit. In 41% of pursuit P-cells, 3D-pursuit signals were observed. However, the majority of vermal-pursuit P-cells (59%) discharged either for vergence pursuit (43%) or for frontal pursuit (16%). Moreover, the majority (74%) of vergence-related P-cells carried convergence signals, displaying both vergence eye position and velocity sensitivity during sinusoidal and step vergence eye movements. Preferred frontal-pursuit directions of vergence + frontal-pursuit P-cells were distributed in all directions. Most pursuit P-cells (73%) discharged before the onset of vergence eye movements; the median lead time was 16 ms. Muscimol infusion into the sites where convergence P-cells were recorded resulted in a reduction of peak convergence eye velocity, of initial convergence eye acceleration, and of frontal-pursuit eye velocity. These results suggest involvement of the dorsal vermis in conversion of 3D-pursuit signals and in convergence eye movements.

Gene-environment interactions in vulnerability to cocaine intravenous self-administration: a brief social experience affects intake in DBA/2J but not in C57BL/6J mice

RATIONALE

Individual differences in cocaine-taking behavior and liability to develop abuse are clearly observed, but underlying mechanisms are still poorly understood. A role for gene-environment interactions has been proposed but remains hypothetical.

OBJECTIVES

We investigated whether gene-environment interactions influence intravenous cocaine self-administration (SA) in mice. We tested the effect of a past short group housing experience on cocaine SA in two inbred strains of mice, the C57BL/6J (C57) and DBA/2J (DBA).

METHODS

Adult C57 and DBA mice were individually housed upon arrival in the laboratory. After 3 weeks, half of the animals of each strain were group housed for 19 days. One week after the end of group housing, cocaine SA or measurement of brain cocaine levels took place.

RESULTS

Individually and ex-group-housed C57 mice did not differ for cocaine SA. On the contrary, the ex-group-housed DBA mice showed an upward shift in the dose-response curve as compared to individually housed DBA. Differences in brain cocaine levels could not account for the observed behavioral differences.

CONCLUSIONS

These results demonstrate that vulnerability to cocaine reinforcing effects can be affected by gene-environment interactions. We propose a mouse model for the characterization of gene-environment interactions in the vulnerability to cocaine-taking behavior.