Unique behavioural phenotypes of recombinant-inbred CXBK mice: partial deficiency of sensitivity to mu- and kappa-agonists

Reduced supraspinal nociceptive responses and distinct gene expression profile in CXBH recombinant inbred mice

CXBH mice, known as an "opioid receptor-rich" strain, are a recombinant inbred mouse strain established by crossing the C57BL/6By and BALB/cBy strains. In the present study, we investigated nociceptive and antinociceptive sensitivity in CXBH mice and elucidated the underlying molecular mechanisms. CXBH mice exhibited slightly higher morphine-induced antinociception compared with C57BL/6J and BALB/cBy mice in the hot-plate test but not tail-flick test. CXBH mice exhibited a marked reduction of nociceptive sensitivity, regardless of the type of nociceptive stimulus, with the exception of tail stimulation. Changes in gene expression that corresponded to reduced nociceptive sensitivity in the brains of CXBH mice were observed in 62 transcripts, including pain- and analgesia-related transcripts, in a whole-genome expression assay. The total mRNA expression of opioid receptors was higher in CXBH mice than in C57BL/6J and BALB/cBy mice. However, the expression levels of MOR-1 mRNA, a major transcript of the μ opioid receptor gene, were not different among the C57BL/6J, BALB/cBy, and CXBH strains. In conclusion, supraspinal nociceptive responses were reduced in the CXBH mouse strain, and the expression levels of transcripts were altered in the brain of this strain.

PERSPECTIVE

This article presents the nociceptive and antinociceptive properties of CXBH recombinant inbred mice and gene expression differences that may underlie nociceptive tolerance in the strain. The CXBH mouse strain may be a useful animal model to investigate the molecular basis of individual differences in supraspinal pain sensitivity.

Pharmacogenomics of the human µ-opioid receptor

The µ-opioid receptor is a primary target for clinically important opioid analgesics, including morphine, fentanyl and methadone. Many genetic variations have been identified in the human µ-opioid receptor MOP gene (OPRM1), and their implications have been reported in the effects of opioid drugs and susceptibility to drug dependence. Interestingly, agonistic and antagonistic opioid effects are inversely associated with the A118G polymorphism genotype. The A118G polymorphism may also be associated with substance dependence and susceptibility to other disorders, including epilepsy and schizophrenia. The IVS1+A21573G, IVS1-T17286C, and TAA+A5359G polymorphisms in the OPRM1 gene may be associated with alcohol, opioid and tobacco dependence, respectively. However, some studies have failed to confirm the correlations between the polymorphisms and opioid effects and substance dependence. Further studies are needed to elucidate the molecular mechanisms underlying the effects of OPRM1 polymorphisms.

Analgesic requirements after major abdominal surgery are associated with OPRM1 gene polymorphism genotype and haplotype

Aims: The association between SNPs of the human OPRM1 gene encoding the µ-opioid receptor and postoperative analgesic requirements in surgical patients remains controversial. Here, we evaluate whether any of the five tag SNPs (A118G, IVS2+G691C, IVS3+G5953A, IVS3+A8449G and TAA+A2109G) representing the four linkage disequilibrium blocks of the OPRM1 gene influences postoperative analgesic requirements. Materials & methods: We studied 138 adult Japanese patients who underwent major open abdominal surgery under combined general and epidural anesthesia and received continuous postoperative epidural analgesia with opioids. Results: The 118G homozygous (GG) patients required 24-h postoperative analgesics more than 118A homozygous (AA) and heterozygous (AG) patients. Tag SNP haplotypes also were associated with 24-h postoperative analgesic requirements. Conclusions: These results suggest that OPRM1 gene tag SNP genotypes and haplotypes can primarily contribute to prediction of postoperative analgesic requirements in individual patients undergoing major open abdominal surgery.

Abolished thermal and mechanical antinociception but retained visceral chemical antinociception induced by butorphanol in mu-opioid receptor knockout mice

Butorphanol is hypothesized to induce analgesia via opioid pathways, although the precise mechanisms for its effects remain unknown. In this study, we investigated the role of the mu-opioid receptor (MOP) in thermal, mechanical, and visceral chemical antinociception induced by butorphanol using MOP knockout (KO) mice. Butorphanol-induced thermal antinociception, assessed by the hot-plate and tail-flick tests, was significantly reduced in heterozygous and abolished in homozygous MOP-KO mice compared with wildtype mice. The results obtained from our butorphanol-induced mechanical antinociception experiments, assessed by the Randall-Selitto test, were similar to the results obtained from the thermal antinociception experiments in these mice. Interestingly, however, butorphanol retained its ability to induce significant visceral chemical antinociception, assessed by the writhing test, in homozygous MOP-KO mice. The butorphanol-induced visceral chemical antinociception that was retained in homozygous MOP-KO mice was completely blocked by pretreatment with nor-binaltorphimine, a kappa-opioid receptor (KOP) antagonist. In vitro binding and cyclic adenosine monophosphate assays also showed that butorphanol possessed higher affinity for KOPs and MOPs than for delta-opioid receptors. These results molecular pharmacologically confirmed previous studies implicating MOPs, and partially KOPs, in mediating butorphanol-induced analgesia.

Acute and chronic heroin dependence in mice: contribution of opioid and excitatory amino acid receptors

Opioid and excitatory amino acid receptors contribute to morphine dependence, but there are no studies of their role in heroin dependence. Thus, mice injected with acute or chronic heroin doses in the present study were pretreated with one of the following selective antagonists: 7-benzylidenenaltrexone (BNTX), naltriben (NTB), nor-binaltorphimine (nor-BNI; delta1, delta2, and kappa opioid receptors, respectively), MK-801, or LY293558 (NMDA and AMPA excitatory amino acid receptors, respectively). Naloxone-precipitated withdrawal jumping frequency, shown here to be a reliable index of heroin dependence magnitude, was reduced by BNTX or NTB in mice injected with both acute and chronic heroin doses. In contrast, nor-BNI did not alter jumping frequencies in mice injected with an acute heroin dose but significantly increased them in mice receiving chronic heroin injections. Continuous MK-801 or LY293558 infusion, but not injection, reduced jumping frequencies during withdrawal from acute heroin treatment. Their delivery by injection was nonetheless effective against chronic heroin dependence, suggesting mechanisms not simply attributable to NMDA or AMPA blockade. These data indicate that whereas delta1, delta2, NMDA, and AMPA receptors enable acute and chronic heroin dependence, kappa receptor activity limits the dependence liability of chronic heroin. With the exception of delta1 receptors, the apparent role of these receptors to heroin dependence is consistent with their contribution to morphine dependence, indicating that there is substantial physiological commonality underlying dependence to both heroin and morphine. The ability of kappa receptor blockade to differentially alter acute and chronic dependence supports previous assertions from studies with other opioids that acute and chronic opioid dependence are, at least in part, mechanistically distinct. Elucidating the substrates contributing to heroin dependence, and identifying their similarities and differences with those of other opioids such as morphine, may yield effective treatment strategies to the problem of heroin dependency.

Is there genetic polymorphism evidence for individual human sensitivity to opiates?

Opiate analgesics have been widely used for severe acute pain and chronic cancer-related pain. Individual differences in the effectiveness of opiates and their side effects limit the clinical benefits and increase risks of drug abuse. Genetic factors might affect variations of opiate sensitivity. The mu opioid peptide receptor (MOP) is the principal site of pharmacologic actions for most clinically important opiate drugs. Recent studies using various knockout mice and recombinant-inbred strain CXBK mice have indicated that the analgesic effect of morphine is dependent on the amount of the MOP. There are more than 100 polymorphisms identified in the human MOP (OPRM1) gene. These polymorphisms might be correlated with OPRM1 mRNA stability and opiate sensitivity, including opiate analgesia, tolerance, and dependence. More precise studies on the relationship between gene polymorphisms and opiate sensitivity will enable realization of personalized pain treatment by predicting opiate sensitivity and requirement for each patient.

Intracisternal A-particle element in the 3' noncoding region of the mu-opioid receptor gene in CXBK mice: a new genetic mechanism underlying differences in opioid sensitivity

OBJECTIVES

CXBK mice, recombinant inbred mice derived from C57BL/6By and BALB/cBy progenitors, display reduced morphine-induced analgesia. Earlier we reported that CXBK mice expressed a reduced amount of the major transcript, MOR-1 mRNA, of the mu-opioid receptor gene. The CXBK MOR-1 mRNA contains a normal coding region and an abnormally long untranslated region.

METHODS AND RESULTS

To identify the nucleotide-sequence difference between the CXBK MOR-1 mRNA and that of the progenitors, we first characterized the 3' untranslated region of the MOR-1 mRNA, which was largely unknown. A 3' rapid amplification of cDNA ends-PCR analysis revealed that the 3' untranslated region of the C57BL/6By MOR-1 mRNA was 10 181 nucleotides transcribed from an exon. Next, we compared the MOR-1 genes in C57BL/6By, CXBK, and BALB/cBy mice, and found a 5293 nucleotide insertion only in CXBK mice. The inserted sequence was a variant of the intracisternal A-particle elements that exist in the mouse genome at approximately 1000 sites. Reverse transcription-PCR analyses revealed that the intracisternal A-particle element was transcribed as a part of the CXBK MOR-1 mRNA. No other differences were found in the MOR-1 mRNA between CXBK and BALB/cBy mice, whereas 100 nucleotides differed between C57BL/6By and CXBK mice aside from the intracisternal A-particle insertion. Finally, CXBK mice displayed reduced morphine responses compared with BALB/cBy mice.

CONCLUSIONS

Our data suggest that differences in the MOR-1 3' untranslated region appear to cause the CXBK phenotype. This genetic mechanism underlying the CXBK phenotype may provide good insight into the possible genetic mechanisms underlying individual differences in opioid sensitivity in humans.

Linkage disequilibrium and association with methamphetamine dependence/psychosis of mu-opioid receptor gene polymorphisms

Several studies indicate that the mu-opioid receptor plays a role in addiction not only to opiate drugs but also to alcohol and non-opiate addictive drugs. Our studies aim to reveal the associations between gene polymorphisms and methamphetamine (MAP) dependence/psychosis. We newly identified several polymorphisms and four substantial linkage disequilibrium (LD) blocks in the mu-opioid receptor (OPRM1) gene. We found significant differences in both genotype and allele frequencies of the single-nucleotide polymorphism (SNP) IVS2+G691C between control (n=232) and MAP-dependent/psychotic patients (n=128). There was also a significant association between IVS2+G691C and patients with transient psychosis. These results suggest that the OPRM1 gene variations may be a factor in development and prognosis of MAP psychosis.

Mu opioid receptor-dependent and independent components in effects of tramadol

Tramadol is thought to induce analgesia via both opioid and non-opioid pathways, although the precise mechanisms remain to be elucidated. In this study, we investigated the roles of the mu-opioid receptor (MOP) in analgesic and rewarding effects of tramadol by using MOP knockout (KO) mice. Tramadol-induced antinociception, assessed by hot-plate and tail-flick tests, was significantly reduced in heterozygous and homozygous MOP-KO mice when compared with that in wild-type mice. Interestingly, however, tramadol retained its ability to induce significant antinociception in homozygous MOP-KO mice. The tramadol-induced antinociception remaining in homozygous MOP-KO mice was not significantly affected by methysergide, a serotonin receptor antagonist, but was partially blocked by yohimbine, an adrenaline alpha2 receptor antagonist, and both naloxone, a non-selective opioid receptor antagonist, and yohimbine. In addition, antinociceptive effects of an active tramadol metabolite M1 were abolished or remarkably reduced in MOP-KO mice. On the other hand, neither wild-type nor homozygous MOP-KO mice showed significant place preference for tramadol in a conditioned place preference test, although there were slight tendencies toward preference in wild-type mice and avoidance in homozygous MOP-KO mice. These results strongly support the idea suggested in the previous pharmacological studies that MOP and the adrenaline alpha2 receptor mediate most of the analgesic properties of tramadol.

How individual sensitivity to opiates can be predicted by gene analyses

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

A Possible Genetic Mechanism Underlying Individual and Interstrain Differences in Opioid Actions: Focus on the Mu Opioid Receptor Gene

Individual differences in responses to opioids limit effective pain treatment with these drugs. Identifying the mechanism could help to improve the analgesic effects of them. Since the molecular cloning of the mu opioid receptor (muOR) gene, substantial advances in opioid research have been made, including the discoveries that muOR plays a mandatory role in the analgesic effects of opioids and that the sequence of the muOR gene varies from one individual to another. It is conceivable that the differences in the muOR gene cause individual differences in opioid actions. The present review summarizes the recent advances made in research on human and mouse muOR genes and proposes that the variances in the 3' untranslated region (39-UTR) of the muOR gene might participate in the variability of the opioid response.

Buprenorphine antinociception is abolished, but naloxone-sensitive reward is retained, in mu-opioid receptor knockout mice

Buprenorphine is a relatively nonselective opioid receptor partial agonist that is used in the management of both pain and addiction. To improve understanding of the opioid receptor subtypes important for buprenorphine effects, we now report the results of our investigation on the roles of mu-, delta-, and kappa-opioid receptors in antinociceptive responses and place preferences induced by buprenorphine. Buprenorphine antinociception, assessed by hot-plate and tail-flick tests, was significantly reduced in heterozygous mu-opioid receptor knockout (MOR-KO) mice and abolished in homozygous MOR-KO mice. In contrast, buprenorphine retained its ability to establish a conditioned place preference (CPP) in homozygous MOR-KO, although the magnitude of place preference was reduced as the number of copies of wild-type mu-opioid receptor genes was reduced. The remaining CPP of buprenorphine was abolished by pretreatment with the nonselective opioid antagonist naloxone, but only partially blocked by pretreatment with either the delta-selective opioid antagonist naltrindole or the kappa-selective opioid antagonist norbinaltorphimine. These data, and biochemical confirmation of buprenorphine actions as a partial delta-, mu-, and kappa-agonist, support the ideas that mu-opioid receptors mediate most of analgesic properties of buprenorphine, but that mu- and delta- and/or kappa-opioid receptors are each involved in the rewarding effects of this drug.

Differential sensitivities of mouse strains to morphine and [Dmt1]DALDA analgesia

[Dmt(1)]DALDA (H-Dmt-D-Arg-Phe-Lys-NH(2)), is a highly potent and selective mu-opioid agonist. Nevertheless, systemic [Dmt(1)]DALDA retained its analgesic actions in MOR-1 knockout animals and CXBK mice despite the inactivity of morphine in these mice. [Dmt(1)]DALDA was 6-fold less potent in C57BL/6J mice than in CD-1 mice, whereas morphine potency did not differ between the two strains. Thus, [Dmt(1)]DALDA is a highly selective mu-opioid analgesic with significant pharmacological differences with the prototypic mu-opioid morphine.

Quantitative assessment and characterization of visceral nociception and hyperalgesia in mice

Colorectal distension (CRD) is a well-characterized model of visceral nociception, which we adapted to the mouse. CRD reproducibly evoked contractions of the abdominal musculature [visceromotor response (VMR)], which was graded to stimulus intensity. The magnitude of VMR was greater in male C57BL6 and female 129S6 mice than in male 129S6 and B6.129 mice. In 129S6, C57BL6, and B6.129 mice strains, VMR was reduced dose dependently by morphine (1-10 mg/kg) and by the kappa-opioid agonist U-69593 (0.2-2 mg/kg), although U-69593 was significantly less potent in C57BL6 mice. In additional experiments, the VMR was recorded from adult male 129S6 mice before and after intracolonic administration of various irritants. Only 30% ethanol significantly enhanced responses to CRD. The colon hyperalgesia persisted for 14 days and was associated with a significant shift of the morphine dose-response function to the left. We believe this will be a useful model for study of visceral nociception and hyperalgesia, including studies of transgenic mice with mutations relevant to pain.

Molecular mechanisms of analgesia induced by opioids and ethanol: is the GIRK channel one of the keys?

Opioids and ethanol have been used since ancient times for pain relief. Opioid signaling is mediated by various effectors, including G protein-activated inwardly rectifying potassium (GIRK) channels, adenylyl cyclases, voltage-dependent calcium channels, phospholipase Cbeta(PLCbeta), and mitogen-activated protein kinases, although it has been unclear which effector mediates the analgesic effects of opioids. Ethanol induces a variety of physiological phenomena via various proteins, including GIRK channels rather than via membrane lipids. GIRK channel activation by either G proteins or ethanol is impaired in weaver mutant mice. The mutant mice may therefore serve as a useful animal model for studying the role of GIRK channels in vivo. Reduced analgesia by using either opioids or ethanol in weaver mutant mice suggests that GIRK channels are important effectors in both opioid- and ethanol-induced analgesia. This hypothesis is supported by similar findings in GIRK2 knockout mice. Among the various effectors coupled with opioid receptors and various targets of ethanol, GIRK channels are the only molecules whose involvement in opioid- and ethanol-induced analgesia has been demonstrated in vivo. The GIRK channel is potentially one of the key molecules in furthering the understanding of the pain control system and in developing advanced analgesics with fewer adverse effects.

The untranslated region of (mu)-opioid receptor mRNA contributes to reduced opioid sensitivity in CXBK mice

It is well known that there are individual differences in a sensitivity to analgesics. Several lines of evidence have suggested that the level of opioid-induced analgesia is dependent on the level of expression of the mu-opioid receptor (mu-OR). However, the molecular mechanisms underlying the diversity of the level of the opioid receptor and the opioid sensitivity among individuals remain to be elucidated. In the present study, we analyzed the opioid-receptor genes of CXBK recombinant-inbred mice, which show reduced sensitivity to opioids. Northern blotting, nucleotide sequencing, and in situ hybridization histochemical analyses demonstrated that CXBK mice possessed mu-OR mRNA with a normal coding region but an abnormally long untranslated region (UTR). In addition, the mu-OR mRNA level in CXBK mice was less than in the control mice. Next, we produced littermate mice that had inherited two copies of the wild-type mu-OR gene, had inherited two copies of the CXBK mu-OR gene, and had inherited both copies of the mu-OR genes. In these mice, inheritance of the CXBK mu-OR gene was well correlated with less mu-OR mRNA and reduced opioid effects on nociception and locomotor activity. We conclude that the CXBK mu-OR gene is responsible for the CXBK phenotypes. Because UTR differences are known to affect the level of the corresponding mRNA and protein and because UTRs are more divergent among individuals than coding regions, the present findings suggest that opioid sensitivity may vary, depending on different mu-OR levels attributable to divergent UTR of mu-OR mRNA.

Endogenous opiates: 1999

This paper is the twenty-second installment of the annual review of research concerning the opiate system. It summarizes papers published during 1999 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunologic responses.

Involvement of G-protein-activated inwardly rectifying K (GIRK) channels in opioid-induced analgesia

To investigate the role of G-protein-activated inwardly rectifying K+ (GIRK) channels in opioid-induced analgesia, we compared the effects of opioids in wild-type and weaver mutant mice having mutant GIRK channels. In the tail-flick and hot-plate tests, weaver mutant mice displayed significantly lower analgesia after either morphine or (-)-U-50488 administration. These findings suggest that GIRK channel activation is important in the induction of analgesia by opioids.

Ethanol opens G-protein-activated inwardly rectifying K+ channels

Ethanol affects many functions of the brain and peripheral organs. Here we show that ethanol opens G-protein-activated, inwardly rectifying K + (GIRK) channels, which has important implications for inhibitory regulation of neuronal excitability and heart rate. At pharmacologically relevant concentrations, ethanol activated both brain-type GIRK1/2 and cardiac-type GIRK1/4 channels without interaction with G proteins or second messengers. Moreover, weaver mutant mice, which have a missense mutation in the GIRK2 channel, showed a loss of ethanol-induced analgesia. These results suggest that the GIRK channels in the brain and heart are important target sites for ethanol.