mu-Opioid receptor knockout mice display reduced cocaine conditioned place preference but enhanced sensitization of cocaine-induced locomotion

Role for μ-opioid receptor in antidepressant effects of δ-opioid receptor agonist KNT-127

Previous pharmacological data have shown the possible existence of functional interactions between μ- (MOP), κ- (KOP), and δ-opioid receptors (DOP) in pain and mood disorders. We previously reported that MOP knockout (KO) mice exhibit a lower stress response compared with wildtype (WT) mice. Moreover, DOP agonists have been shown to exert antidepressant-like effects in numerous animal models. In the present study, the tail suspension test (TST) and forced swim test (FST) were used to examine the roles of MOP and DOP in behavioral despair. MOP-KO mice and WT mice were treated with KNT-127 (10 mg/kg), a selective DOP agonist. The results indicated a significant decrease in immobility time in the KNT-127 group compared with the saline group in all genotypes in both tests. In the saline groups, immobility time significantly decreased in MOP-KO mice compared with WT mice in both tests. In female MOP-KO mice, KNT-127 significantly decreased immobility time in the TST compared with WT mice. In male MOP-KO mice, however, no genotypic differences were found in the TST after either KNT-127 or saline treatment. Thus, at least in the FST and TST, the activation of DOP and absence of MOP had additive effects in reducing measures of behavioral despair, suggesting that effects on this behavior by DOP activation occur independently of MOP.

Caenorhabditis Elegans Exhibits Morphine Addiction-like Behavior via the Opioid-like Receptor NPR-17

Addiction has become a profound societal problem worldwide, and few effective treatments are available. The nematode Caenorhabditis elegans (C. elegans) is an excellent invertebrate model to study neurobiological disease states. C. elegans reportedly developed a preference for cues that had previously been paired with addictive drugs, similar to place conditioning findings in rodents. Moreover, several recent studies discovered and reported the existence of an opioid-like system in C. elegans. Still unclear, however, is whether C. elegans exhibits addictive-like behaviors for opioids, such as morphine. In the present study, we found that C. elegans exhibited dose-dependent preference for morphine using the conditioned chemosensory-cue preference (CCP) test. This preference was blocked by co-treatment with the opioid receptor antagonist naloxone. C. elegans also exhibited aversion to naloxone-precipitated withdrawal from chronic morphine exposure. The expression of morphine-induced CCP and morphine withdrawal were abolished in worms that lacked the opioid-like receptor NPR-17. Dopamine-deficient mutant (cat-2 (e1112)) worms also did not exhibit morphine-induced CCP. These results indicate that the addictive function of the opioid system exists in C. elegans, which may serve as a useful model of opioid addiction.

Drug addiction co-morbidity with alcohol: Neurobiological insights

Addiction is a chronic disorder that consists of a three-stage cycle of binge/intoxication, withdrawal/negative affect, and preoccupation/anticipation. These stages involve, respectively, neuroadaptations in brain circuits involved in incentive salience and habit formation, stress surfeit and reward deficit, and executive function. Much research on addiction focuses on the neurobiology underlying single drug use. However, alcohol use disorder (AUD) can be co-morbid with substance use disorder (SUD), called dual dependence. The limited epidemiological data on dual dependence indicates that there is a large population of individuals suffering from addiction who are dependent on more than one drug and/or alcohol, yet dual dependence remains understudied in addiction research. Here, we review neurobiological data on neurotransmitter and neuropeptide systems that are known to contribute to addiction pathology and how the involvement of these systems is consistent or divergent across drug classes. In particular, we highlight the dopamine, opioid, corticotropin-releasing factor, norepinephrine, hypocretin/orexin, glucocorticoid, neuroimmune signaling, endocannabinoid, glutamate, and GABA systems. We also discuss the limited research on these systems in dual dependence. Collectively, these studies demonstrate that the use of multiple drugs can produce neuroadaptations that are distinct from single drug use. Further investigation into the neurobiology of dual dependence is necessary to develop effective treatments for addiction to multiple drugs.

Effects of the monoamine stabilizer, (-)-OSU6162, on cocaine-induced locomotion and conditioned place preference in mice

Cocaine addiction is a severe mental disorder for which few treatment options are available. The underlying mechanisms include facilitation of monoamine-neurotransmission, particularly dopamine. Here, we tested the hypothesis that the monoamine stabilizers, (-)-OSU6162 ((3S)-3-(3-methylsulfonylphenyl)-1-propylpiperidine) and aripiprazole (7-[4-[4-(2,3-dichlorophenyl)piperazin-1-yl]butoxy]-3,4-dihydro-1H-quinolin-2-one), prevent cocaine-induced behaviors. Male Swiss mice received injections of (-)-OSU6162 or aripiprazole and cocaine and were tested for cocaine-induced hyperlocomotion, locomotor sensitization, and acquisition and expression of conditioned place preference (CPP). The increase in the distance traveled induced by cocaine (20 mg/kg) was prevented by pretreatment with aripiprazole (1 and 10 mg/kg), whereas (-)-OSU6162 (3 mg/kg) exerted a minor effect. Aripiprazole, however, also impaired spontaneous locomotion. Neither (-)-OSU6162 nor aripiprazole interfered with the locomotor sensitization and expression of CPP induced by cocaine (15 mg/kg). (-)-OSU6162 (3 mg/kg), but not aripiprazole, prevented the acquisition of CPP induced by cocaine (15 mg/kg). (-)-OSU6162 exerts a minor effect in reducing cocaine-induced stimulatory activity and context-related memories, which are responsible for triggering drug seeking. Further studies are required to establish whether (-)-OSU6162 could be a candidate drug for the treatment of cocaine addiction.

Crosstalk between Opioid and Anti-Opioid Systems: An Overview and Its Possible Therapeutic Significance

Opioid peptides and receptors are broadly expressed throughout peripheral and central nervous systems and have been the subject of intense long-term investigations. Such studies indicate that some endogenous neuropeptides, called anti-opioids, participate in a homeostatic system that tends to reduce the effects of endogenous and exogenous opioids. Anti-opioid properties have been attributed to various peptides, including melanocyte inhibiting factor (MIF)-related peptides, cholecystokinin (CCK), nociceptin/orphanin FQ (N/OFQ), and neuropeptide FF (NPFF). These peptides counteract some of the acute effects of opioids, and therefore, they are involved in the development of opioid tolerance and addiction. In this work, the anti-opioid profile of endogenous peptides was described, mainly taking into account their inhibitory influence on opioid-induced effects. However, the anti-opioid peptides demonstrated complex properties and could show opioid-like as well as anti-opioid effects. The aim of this review is to detail the phenomenon of crosstalk taking place between opioid and anti-opioid systems at the in vivo pharmacological level and to propose a cellular and molecular basis for these interactions. A better knowledge of these mechanisms has potential therapeutic interest for the control of opioid functions, notably for alleviating pain and/or for the treatment of opioid abuse.

Increased cocaine reward in offspring of females exposed to morphine during adolescence

RATIONALE

A growing body of evidence demonstrates that environmental exposures can impact the physiology and behavior of subsequent generations. We have previously demonstrated reduced morphine self-administration in the F1 and F2 offspring of female rats exposed to morphine during adolescence.

OBJECTIVES

The current study was designed to determine whether attenuated self-administration for a substance not in the opioid class is also observed in the F1 progeny of adolescent morphine exposed females.

METHODS

Female adolescent rats were administered morphine at increasing doses for 10 days (P30-39). Females then remained drug free for at least 3 weeks prior to mating with drug-naïve males. As adults, male and female offspring (F1 animals) were tested for cocaine self-administration acquisition, progressive ratio, extinction, and reinstatement. In addition, β-endorphin peptide levels were measured in the nucleus accumbens (NAc) of behaviorally experienced animals following reinstatement and in behaviorally naïve littermates after acute cocaine (0 or 10 mg/kg, i.p.). Proopiomelanocortin, the polypeptide that is cleaved to produce β-endorphin, as well as β-endorphin, was examined in the arcuate nucleus of the hypothalamus and the nucleus accumbens, respectively. Finally, corticosterone was measured following acute cocaine.

RESULTS

While no differences were observed during the cocaine acquisition phase (FR-1 and FR-5 schedules), under a PR schedule, Mor-F1 animals (both males and females) had increased motivated responding for cocaine. In addition, Mor-F1 males demonstrated enhanced reinstatement compared to Sal-F1 males. In Mor-F1 males, an acute injection of cocaine (10 mg/kg, i.p.) decreased β-endorphin levels in the NAc compared to a saline injection while acute cocaine increased β-endorphin in the NAc in Sal-F1 males compared to saline injection. Following acute cocaine, Mor-F1 males had significantly lower levels of β-endorphin in the Nac compared to Sal-F1 males. Additionally, β-endorphin levels in the nucleus accumbens were negatively correlated with reinstatement behavior only in Mor-F1 males. Levels of POMC in the arcuate nucleus were elevated in Mor-F1 males compared to Sal-F1 males, a main effect driven primarily by POMC levels in the acute cocaine condition. These changes were not observed in Mor-F1 females. Finally, plasma corticosterone was increased in Mor-F1 males regardless of acute injection while Mor-F1 females displayed increased corticosterone in response to acute cocaine.

CONCLUSIONS

These data indicate that morphine prior to conception increases the rewarding effects of cocaine in male and female offspring. In addition, sex-specific alterations in endogenous opioids and hypothalamic physiology were observed.

Regulators of G-protein signaling 2 and 4 differentially regulate cocaine-induced rewarding effects

There is a need to identify new therapeutic targets for the treatment of cocaine addiction due to the rise in cocaine abuse and deaths due to cocaine overdose. Regulator of G protein signaling (RGS) proteins such as RGS2 and RGS4 are widely distributed in brain regions that play a role in drug reward. Importantly, RGS2 and RGS4 negatively regulate G-protein coupled receptor signaling pathways of monoaminergic neurotransmitters that play a role in the rewarding effects of cocaine by enhancing the rate of hydrolysis of Gα-bound guanine nucleotide triphosphate. Thus, the objective of this study was to investigate the effects of cocaine on conditioned place preference (CPP) and locomotor activity in mice that lacked either RGS2 or RGS4 (i.e. knockout (KO) mice) and their wildtype (WT) littermates. Moreover recent studies have reported influence of sex on RGS functioning and hence studies were conducted in both male and female mice. Cocaine-induced CPP was attenuated in male, but not female RGS4 KO mice compared to respective RGS4 WT mice. Cocaine-induced CPP was not influenced by deletion of RGS2 in either male or female mice. Similarly, cocaine-induced locomotor activity was not influenced by deletion of either RGS2 or RGS4 irrespective of sex. Together, the data indicate that the rewarding effects of cocaine were attenuated in the absence of RGS4 expression, but not in the absence of RGS2 expression in a sex-dependent manner. Importantly, these data suggest that RGS4 can serve as a potential target for medications that can be used to treat cocaine addiction.

Enkephalin as a Pivotal Player in Neuroadaptations Related to Psychostimulant Addiction

Enkephalin expression is high in mesocorticolimbic areas associated with psychostimulant-induced behavioral and neurobiological effects, and may also modulate local neurotransmission in this circuit network. Psychostimulant drugs, like amphetamine and cocaine, significantly increase the content of enkephalin in these brain structures, but we do not yet understand the specific significance of this drug-induced adaptation. In this review, we summarize the neurochemical and molecular mechanism of psychostimulant-induced enkephalin activation in mesocorticolimbic brain areas, and the contribution of this opioid peptide in the pivotal neuroadaptations and long-term behavioral changes underlying psychostimulant addiction. There is evidence suggesting that adaptive changes in enkephalin content in the mesocorticolimbic circuit, induced by acute and chronic psychostimulant administration, may represent a key initial step in the long-term behavioral and neuronal plasticity induced by these drugs.

Larger Numbers of Glial and Neuronal Cells in the Periaqueductal Gray Matter of μ-Opioid Receptor Knockout Mice

Background: μ-opioid receptor knockout (MOP-KO) mice display baseline hyperalgesia. We have recently identified changes in tissue volume in the periaqueductal gray matter (PAG) using magnetic resonance imaging voxel-based morphometry. Changes in the structure and connectivity of this region might account for some behavior phenotypes in MOP-KO mice, including hyperalgesia. Methods: Adult male MOP-KO and wild-type (WT) mice were studied. Immunohistochemistry was performed to detect microglia, astrocytes, and neurons in the PAG using specific markers: ionized calcium-binding adaptor molecule 1 (Iba-1) for microglia, glial fibrillary acidic protein (GFAP) for astrocytes, and the neuronal nuclei antigen (NeuN; product of the Rbfox3 gene) for neurons, respectively. Cell counting was performed in the four parallel longitudinal columns of the PAG (dorsomedial, dorsolateral, lateral, and ventrolateral) at three different locations from bregma (-3.5, -4.0, and -4.5 mm). Results: The quantitative analysis showed larger numbers of well-distributed Iba1-IR cells (microglia), NeuN-IR cells (neurons), and GFAP-IR areas (astrocytes) at all the anatomically distinct regions examined, namely, the dorsomedial (DM) PAG, dorsolateral (DL) PAG, lateral (L) PAG, and ventrolateral (VL) PAG, in MOP-KO mice than in control mice. Conclusions: The cellular changes in the PAG identified in this paper may underlie aspects of the behavioral alterations produced by MOP receptor deletion, and suggest that alterations in the cellular structure of the PAG may contribute to hyperalgesic states.

ProSAAS-derived peptides are regulated by cocaine and are required for sensitization to the locomotor effects of cocaine

To identify neuropeptides that are regulated by cocaine, we used a quantitative peptidomic technique to examine the relative levels of neuropeptides in several regions of mouse brain following daily intraperitoneal administration of 10 mg/kg cocaine or saline for 7 days. A total of 102 distinct peptides were identified in one or more of the following brain regions: nucleus accumbens, caudate putamen, frontal cortex, and ventral tegmental area. None of the peptides detected in the caudate putamen or frontal cortex were altered by cocaine administration. Three peptides in the nucleus accumbens and seven peptides in the ventral tegmental area were significantly decreased in cocaine-treated mice. Five of these ten peptides are derived from proSAAS, a secretory pathway protein and neuropeptide precursor. To investigate whether proSAAS peptides contribute to the physiological effects of psychostimulants, we examined acute responses to cocaine and amphetamine in the open field with wild-type (WT) and proSAAS knockout (KO) mice. Locomotion was stimulated more robustly in the WT compared to mutant mice for both psychostimulants. Behavioral sensitization to amphetamine was not maintained in proSAAS KO mice and these mutants failed to sensitize to cocaine. To determine whether the rewarding effects of cocaine were altered, mice were tested in conditioned place preference (CPP). Both WT and proSAAS KO mice showed dose-dependent CPP to cocaine that was not distinguished by genotype. Taken together, these results suggest that proSAAS-derived peptides contribute differentially to the behavioral sensitization to psychostimulants, while the rewarding effects of cocaine appear intact in mice lacking proSAAS.

Genetic variation in the behavioral effects of buprenorphine in female mice derived from a murine model of the OPRM1 A118G polymorphism

Pharmacogenetic studies have identified the non-synonymous single nucleotide polymorphism (A118G) in the human mu opioid receptor (MOR) gene (OPRM1) as a critical genetic variant capable of altering the efficacy of opioid therapeutics. To date few studies have explored the potential impact of the OPRM1 A118G polymorphism on the pharmacological effects of buprenorphine (BPN), a potent MOR partial agonist and kappa opioid receptor antagonist, which is approved by the FDA for the treatment of opioid addiction and chronic pain. The goal of these studies was to determine whether the MOR-mediated behavioral effects of BPN were altered in the Oprm1 A112G mouse model of the human OPRM1 A118G SNP. All studies were conducted in female, AA, AG and GG mice. BPN's maximal analgesic effect in the hot plate test was significantly blunted in AG and GG mice compared to wild type AA mice. Similarly, the BPN-induced reduction of latency to consume food in the novelty induced hypophagia test was blocked entirely in AG and GG mice compared to their AA littermates. In addition, GG mice exhibited marked reductions in psychostimulant hyperlocomotor activity compared to the AA group. In contrast, reduced immobility in the forced swim test, an effect of BPN mediated by kappa opioid receptors, was not affected by genotype. These studies demonstrate the ability of the Oprm1 A112G SNP to attenuate the analgesic, anxiolytic and hyperlocomotor effects of BPN. Overall, these data suggest that the OPRM1 A118G SNP will significantly impact the clinical efficacy of BPN in its therapeutic applications.

A polymorphism in the OPRM1 3'-untranslated region is associated with methadone efficacy in treating opioid dependence

The mu-opioid receptor (MOR) is the primary target of methadone and buprenorphine. The primary neuronal transcript of the OPRM1 gene, MOR-1, contains a ~13kb 3′ untranslated region with five common haplotypes in European-Americans. We analyzed the effects of these haplotypes on the percentage of opioid positive urine tests in European-Americans (n = 582) during a 24-week, randomized, open-label trial of methadone or buprenorphine/naloxone (Suboxone) for the treatment of opioid dependence. A single haplotype, tagged by rs10485058, was significantly associated with patient urinalysis data in the methadone treatment group. Methadone patients with the A/A genotype at rs10485058 were less likely to have opioid-positive urine drug screens than those in the combined A/G and G/G genotypes group (Relative Risk = 0.76, 95% confidence intervals = 0.73–0.80, p = 0.0064). Genotype at rs10485058 also predicted self-reported relapse rates in an independent population of Australian patients of European descent (n = 1215) who were receiving opioid substitution therapy (p = 0.003). In silico analysis predicted that miR-95-3p would interact with the G, but not the A allele of rs10485058. Luciferase assays indicated miR-95-3p decreased reporter activity of constructs containing the G, but not the A allele of rs10485058, suggesting a potential mechanism for the observed pharmacogenetic effect. These findings suggest that selection of a medication for opioid dependence based on rs10485058 genotype might improve outcomes in this ethnic group.

Responses to drugs of abuse and non-drug rewards in leptin deficient ob/ob mice

RATIONALE

Although leptin receptors are found in hypothalamic nuclei classically associated with homeostatic feeding mechanisms, they are also present in brain regions known to regulate hedonic-based feeding, natural reward processing, and responses to drugs of abuse. The ob/ob mouse is deficient in leptin signaling, and previous work has found altered mesolimbic dopamine signaling and sensitivity to the locomotor activating effects of amphetamine in these mice.

OBJECTIVES

We directly assessed responses to three drugs of abuse and non-drug rewards in the leptin-deficient ob/ob mouse.

METHODS

Ob/ob mice were tested in assays of sweet preference, novelty seeking, and drug reward/reinforcement.

RESULTS

In assays of novelty seeking, novel open field activity and operant sensation seeking were reduced in ob/ob mice, although novel object interaction and novel environment preference were comparable to wild types. We also found that ob/ob mice had specific phenotypes in regard to cocaine: conditioned place preference for 2.5 mg/kg was increased, while the locomotor response to 10 mg/kg was reduced, and cocaine self-administration was the same as wild types. Ob/ob mice also acquired self-administration of the potent opioid remifentanil, but breakpoints for the drug were significantly reduced. Finally, we found significant differences in ethanol drinking in ob/ob mice that correlated negatively with body weight and positively with operant sensation seeking.

CONCLUSIONS

In conclusion, ob/ob mice displayed task-specific deficits in novelty seeking and dissociable differences in reward/reinforcement associated with cocaine, remifentanil, and ethanol.

Female Japanese quail with high levels of estradiol demonstrate cocaine-induced conditioned place preference

Preclinical research has indicated that females may be more sensitive to the rewarding properties of cocaine. However, the majority of this research has been done in rodent species. Environmental cues associated with human drug-taking behavior tend to be visual. Because rodents do not rely on the visual system as their primary sense modality, the use of a visually oriented species may add to our understanding of cue-elicited drug cravings and relapse. The present study examined the potential role of the steroid hormone, estradiol, in the rewarding properties of cocaine in female Japanese quail using a conditioned place preference (CPP) procedure. In the current experiment, female quail were housed on either an 8L:16D (light:dark) or 16L:8D (light:dark) cycle for 21 days to induce photoregression or photostimulation, respectively. They then received 10, 20, or 30 mg/kg cocaine, or saline during conditioning. Conditioning trials were carried out for 8 days, once per day for 30 min, for a total of 4 cocaine and 4 saline alternating conditioning trials. Results indicated that female quail housed in long-light conditions (16L:8D) had significantly higher levels of estradiol than short-cycle females. Additionally, photostimulated female quail developed a CPP to 10 and 20 mg/kg cocaine. Short-cycle females did not show cocaine-induced CPP to any dose tested. Results indicate that cocaine is dose-dependently rewarding to photostimulated female Japanese quail. Furthermore, the current findings suggest that estradiol may enhance the rewarding properties of cocaine in female quail. (PsycINFO Database Record

The opioid receptors as targets for drug abuse medication

The endogenous opioid system is largely expressed in the brain, and both endogenous opioid peptides and receptors are present in areas associated with reward and motivation. It is well known that this endogenous system plays a key role in many aspects of addictive behaviours. The present review summarizes the modifications of the opioid system induced by chronic treatment with drugs of abuse reported in preclinical and clinical studies, as well as the action of opioid antagonists and agonists on the reinforcing effects of drugs of abuse, with therapeutic perspectives. We have focused on the effects of chronic psychostimulants, alcohol and nicotine exposure. Taken together, the changes in both opioid peptides and opioid receptors in different brain structures following acute or chronic exposure to these drugs of abuse clearly identify the opioid system as a potential target for the development of effective pharmacotherapy for the treatment of addiction and the prevention of relapse.

State-dependent μ-opioid modulation of social motivation

Social mammals engage in affiliative interactions both when seeking relief from negative affect and when searching for pleasure and joy. These two motivational states are both modulated by μ-opioid transmission. The μ-opioid receptor (MOR) system in the brain mediates pain relief and reward behaviors, and is implicated in social reward processing and affiliative bonding across mammalian species. However, pharmacological manipulation of the μ-opioid system has yielded opposite effects on rodents and primates: in rodents, social motivation is generally increased by MOR agonists and reduced by antagonists, whereas the opposite pattern has been shown in primates. Here, we address this paradox by taking into account differences in motivational state. We first review evidence for μ-opioid mediation of reward processing, emotion regulation, and affiliation in humans, non-human primates, rodents and other species. Based on the consistent cross-species similarities in opioid functioning, we propose a unified, state-dependent model for μ-opioid modulation of affiliation across the mammalian species. Finally, we show that this state-dependent model is supported by evidence from both rodent and primate studies, when species and age differences in social separation response are taken into account.

Genome-wide association study identifies a potent locus associated with human opioid sensitivity

Opioids, such as morphine and fentanyl, are widely used as effective analgesics for the treatment of acute and chronic pain. In addition, the opioid system has a key role in the rewarding effects of morphine, ethanol, cocaine and various other drugs. Although opioid sensitivity is well known to vary widely among individual subjects, several candidate genetic polymorphisms reported so far are not sufficient for fully understanding the wide range of interindividual differences in human opioid sensitivity. By conducting a multistage genome-wide association study (GWAS) in healthy subjects, we found that genetic polymorphisms within a linkage disequilibrium block that spans 2q33.3-2q34 were strongly associated with the requirements for postoperative opioid analgesics after painful cosmetic surgery. The C allele of the best candidate single-nucleotide polymorphism (SNP), rs2952768, was associated with more analgesic requirements, and consistent results were obtained in patients who underwent abdominal surgery. In addition, carriers of the C allele in this SNP exhibited less vulnerability to severe drug dependence in patients with methamphetamine dependence, alcohol dependence, and eating disorders and a lower 'Reward Dependence' score on a personality questionnaire in healthy subjects. Furthermore, the C/C genotype of this SNP was significantly associated with the elevated expression of a neighboring gene, CREB1. These results show that SNPs in this locus are the most potent genetic factors associated with human opioid sensitivity known to date, affecting both the efficacy of opioid analgesics and liability to severe substance dependence. Our findings provide valuable information for the personalized treatment of pain and drug dependence.

15 years of genetic approaches in vivo for addiction research: Opioid receptor and peptide gene knockout in mouse models of drug abuse

The endogenous opioid system is expressed throughout the brain reinforcement circuitry, and plays a major role in reward processing, mood control and the development of addiction. This neuromodulator system is composed of three receptors, mu, delta and kappa, interacting with a family of opioid peptides derived from POMC (β-endorphin), preproenkephalin (pEnk) and preprodynorphin (pDyn) precursors. Knockout mice targeting each gene of the opioid system have been created almost two decades ago. Extending classical pharmacology, these mutant mice represent unique tools to tease apart the specific role of each opioid receptor and peptide in vivo, and a powerful approach to understand how the opioid system modulates behavioral effects of drugs of abuse. The present review summarizes these studies, with a focus on major drugs of abuse including morphine/heroin, cannabinoids, psychostimulants, nicotine or alcohol. Genetic data, altogether, set the mu receptor as the primary target for morphine and heroin. In addition, this receptor is essential to mediate rewarding properties of non-opioid drugs of abuse, with a demonstrated implication of β-endorphin for cocaine and nicotine. Delta receptor activity reduces levels of anxiety and depressive-like behaviors, and facilitates morphine-context association. pEnk is involved in these processes and delta/pEnk signaling likely regulates alcohol intake. The kappa receptor mainly interacts with pDyn peptides to limit drug reward, and mediate dysphoric effects of cannabinoids and nicotine. Kappa/dynorphin activity also increases sensitivity to cocaine reward under stressful conditions. The opioid system remains a prime candidate to develop successful therapies in addicted individuals, and understanding opioid-mediated processes at systems level, through emerging genetic and imaging technologies, represents the next challenging goal and a promising avenue in addiction research. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Implications of genome wide association studies for addiction: are our a priori assumptions all wrong?

Substantial genetic contributions to addiction vulnerability are supported by data from twin studies, linkage studies, candidate gene association studies and, more recently, Genome Wide Association Studies (GWAS). Parallel to this work, animal studies have attempted to identify the genes that may contribute to responses to addictive drugs and addiction liability, initially focusing upon genes for the targets of the major drugs of abuse. These studies identified genes/proteins that affect responses to drugs of abuse; however, this does not necessarily mean that variation in these genes contributes to the genetic component of addiction liability. One of the major problems with initial linkage and candidate gene studies was an a priori focus on the genes thought to be involved in addiction based upon the known contributions of those proteins to drug actions, making the identification of novel genes unlikely. The GWAS approach is systematic and agnostic to such a priori assumptions. From the numerous GWAS now completed several conclusions may be drawn: (1) addiction is highly polygenic; each allelic variant contributing in a small, additive fashion to addiction vulnerability; (2) unexpected, compared to our a priori assumptions, classes of genes are most important in explaining addiction vulnerability; (3) although substantial genetic heterogeneity exists, there is substantial convergence of GWAS signals on particular genes. This review traces the history of this research; from initial transgenic mouse models based upon candidate gene and linkage studies, through the progression of GWAS for addiction and nicotine cessation, to the current human and transgenic mouse studies post-GWAS.

Case-control association study of WLS variants in opioid and cocaine addicted populations

The opioid receptor family is involved in the development and maintenance of drug addiction. The mu-opioid receptor (MOR) mediates the rewarding effects of multiple drugs, including opiates and cocaine. A number of proteins interact with MOR, potentially modulating MOR function and altering the physiological consequences of drug use. These mu-opioid receptor interacting proteins (MORIPs) are potential therapeutic targets for the treatment of addiction. The Wntless (WLS) protein was recently identified as a MORIP in a yeast two-hybrid screen. In this study, we conducted a case-control association analysis of 16 WLS genetic variants in opioid and cocaine addicted individuals of both African-American (opioid n=336, cocaine n=908) and European-American (opioid n=335, cocaine n=336) ancestry. Of the analyzed SNPs, three were nominally associated with opioid addiction and four were nominally associated with cocaine addiction. None of these associations were significant following multiple testing correction. These data suggest that the common variants of WLS analyzed in this study are not associated with opioid or cocaine addiction. However, this study does not exclude the possibilities that rare variants in WLS may affect susceptibility to drug addiction, or that common variants with small effect size may fall below the detection level of our analysis.

Low frequency genetic variants in the μ-opioid receptor (OPRM1) affect risk for addiction to heroin and cocaine

The μ-opioid receptor (MOR) binds exogenous and endogenous opioids and is known to mediate the rewarding effects of drugs of abuse. Numerous genetic studies have sought to identify common genetic variation in the gene encoding MOR (OPRM1) that affects risk for drug addiction. The purpose of this study was to examine the contribution of rare coding variants in OPRM1 to the risk for addiction. Rare and low frequency variants were selected using the National Heart Lung and Blood Institute - Exome Sequencing Project (NHLBI-ESP) database, which has screened the exomes of over 6500 individuals. Two SNPs (rs62638690 and rs17174794) were selected for genotyping in 1377 European American individuals addicted to heroin and/or cocaine. Two different SNPs (rs1799971 and rs17174801) were genotyped in 1238 African American individuals addicted to heroin and/or cocaine. Using the minor allele frequencies from the NHLBI-ESP dataset as a comparison group, case-control association analyses were performed. Results revealed an association between rs62638690 and cocaine and heroin addiction in European Americans (p=0.02; 95% C.I. 0.47 [0.24-0.92]). This study suggests a potential role for rare OPRM1 variants in addiction disorders and highlights an area worthy of future study.

Case-control association analysis of polymorphisms in the δ-opioid receptor, OPRD1, with cocaine and opioid addicted populations

BACKGROUND

Addiction susceptibility and treatment responsiveness are greatly influenced by genetic factors. Sequence variation in genes involved in the mechanisms of drug action have the potential to influence addiction risk and treatment outcome. The opioid receptor system is involved in mediating the rewarding effects of cocaine and opioids. The μ-opioid receptor (MOR) has traditionally been considered the primary target for opioid addiction. The MOR, however, interacts with and is regulated by many known MOR interacting proteins (MORIPs), including the δ-opioid receptor (DOR).

METHODS

The present study evaluated the contribution of OPRD1, the gene encoding the DOR, to the risk of addiction to opioids and cocaine. The association of OPRD1 polymorphisms with both opioid addiction (OA) and cocaine addiction (CA) was analyzed in African American (OA n=336, CA n=503) and European American (OA n=1007, CA n=336) populations.

RESULTS

The primary finding of this study is an association of rs678849 with cocaine addiction in African Americans (allelic p=0.0086). For replication purposes, this SNP was analyzed in a larger independent population of cocaine addicted African Americans and controls and the association was confirmed (allelic p=4.53 × 10(-5); n=993). By performing a meta-analysis on the expanded populations, the statistical evidence for an association was substantially increased (allelic p=8.5 × 10(-7)) (p-values non-FDR corrected).

CONCLUSION

The present study suggests that polymorphisms in OPRD1 are relevant for cocaine addiction in the African American population and provides additional support for a broad role for OPRD1 variants in drug dependence.

The endogenous opioid system in cocaine addiction: what lessons have opioid peptide and receptor knockout mice taught us?

Cocaine addiction has become a major concern in the UK as Britain tops the European 'league table' for cocaine abuse. Despite its devastating health and socio-economic consequences, no effective pharmacotherapy for treating cocaine addiction is available. Identifying neurochemical changes induced by repeated drug exposure is critical not only for understanding the transition from recreational drug use towards compulsive drug abuse but also for the development of novel targets for the treatment of the disease and especially for relapse prevention. This article focuses on the effects of chronic cocaine exposure and withdrawal on each of the endogenous opioid peptides and receptors in rodent models. In addition, we review the studies that utilized opioid peptide or receptor knockout mice in order to identify and/or clarify the role of different components of the opioid system in cocaine-addictive behaviours and in cocaine-induced alterations of brain neurochemistry. The review of these studies indicates a region-specific activation of the µ-opioid receptor system following chronic cocaine exposure, which may contribute towards the rewarding effect of the drug and possibly towards cocaine craving during withdrawal followed by relapse. Cocaine also causes a region-specific activation of the κ-opioid receptor/dynorphin system, which may antagonize the rewarding effect of the drug, and at the same time, contribute to the stress-inducing properties of the drug and the triggering of relapse. These conclusions have important implications for the development of effective pharmacotherapy for the treatment of cocaine addiction and the prevention of relapse.

Lack of association of OPRM1 genotype and smoking cessation

INTRODUCTION

Previous studies have reported an association between μ-opioid receptor (OPRM1) genotype and smoking cessation, with some evidence that the strength of this association depends on dose of nicotine replacement therapy (NRT). We examined whether a single-nucleotide polymorphism in the OPRM1 gene is associated with cessation and whether this variant moderates the effects of higher doses of NRT on abstinence.

METHODS

Participants were recruited from the practices of primary care physicians in the United Kingdom. Patients smoking an average of at least 10 cigarettes a day, who wanted to quit and were 18 years or older were eligible for inclusion. A total of N = 633 participants were recruited into the original trial, of whom complete data for pharmacogenetic analyses were available on n = 598. Logistic regression was used to test for the effects of OPRM1 genotype and NRT dose, including the genotype × dose interaction term, on smoking status at 4-week, and 26-week follow-up. Analyses were adjusted for potential confounders.

RESULTS

There was no evidence of a genotype effect at either follow-up, and no evidence of a genotype × dose interaction effect.

CONCLUSIONS

OPRM1 genotype may not affect the likelihood of smoking cessation, and it may not influence response to high- versus low-dose NRT. OPRM1 may have at most only a modest role in explaining cigarette smoking and cessation.

Disturbances in behavior and cortical enkephalin gene expression during the anticipation of ethanol in rats characterized as high drinkers

The process of ethanol anticipation is a particularly important phenomenon that can determine subsequent drug-taking behavior. Recent studies suggest that systems within the medial prefrontal cortex (mPFC), during anticipation, may contribute to the goal-directed seeking of ethanol. The current investigation examined the possibility that the opioid peptide enkephalin (ENK), known to mediate some of the reinforcing properties of ethanol, may function in the mPFC during the anticipation of ethanol access. Using a limited access (3 h/d) paradigm for 10 days with 20% ethanol, Sprague-Dawley rats were first identified either as low drinkers (LD, <1.0 g/kg/3 h) or as high drinkers (HD, >2.0 g/kg/3 h) that exhibited a long-term phenotype of high ethanol consumption and a significant ethanol deprivation effect. During the anticipation period immediately preceding daily ethanol access, the HD rats compared to LD or Control animals with ad libitum ethanol access exhibited increased anticipatory behaviors, including greater exploratory behavior in a novel open field as revealed by significantly more time spent in the rearing position (+53-65%, p < 0.05) and increased number of rears made (+33-44%, p < 0.05) and greater novelty-seeking behavior in a hole-board apparatus revealed by an increase in total (+50-52%, p < 0.05) and novel nose pokes (+45-48%, p < 0.05). In the HD rats, analysis of the mPFC using real-time quantitative PCR showed significantly greater mRNA levels of ENK (p < 0.05) and the mu-opioid receptor (MOR) (p < 0.05), but not delta-opioid receptor (DOR), and this increase in ENK expression was found, using in situ hybridization, to occur specifically in the prelimbic (PrL) subregion of the mPFC. When injected into the PrL during the anticipation period, a MOR agonist but not DOR agonist significantly increased consumption of 20% ethanol (p < 0.05). These findings support the role of ENK, acting through MOR within the PrL to promote the anticipation and excessive consumption of ethanol.

Genetic association of GABA-A receptor alpha-2 and mu opioid receptor with cocaine cue-reactivity: evidence for inhibitory synaptic neurotransmission involvement in cocaine dependence

BACKGROUND

This pilot feasibility study examined the role of genetics in laboratory-induced cocaine craving.

METHODS

Thirty-four African American, cocaine-depend- ent male subjects underwent a baseline assessment, cue-exposure session, and genetic analysis. Subjects were classified as either cue-reactive or nonreactive.

RESULTS

Among single nucleotide polymorphism markers in 13 candidate genes examined for association with cocaine cue-reactivity, two were statistically significant: GABRA2 (coding for GABA-A receptor alpha-2 subunit; rs11503014, nominal p= .001) and OPRM1 (coding for mu opioid receptor; rs2236256, nominal p= .03).

CONCLUSIONS

These pilot results suggest that cocaine craving shows variability among cocaine-dependent subjects, and that GABRA2 and OPRM1 polymorphisms have differential influences on cocaine cue-reactivity, warranting studies in future research.

Association study of the β-arrestin 2 gene (ARRB2) with opioid and cocaine dependence in a European-American population

The rewarding properties of drugs of abuse are mediated by the mu-opioid receptor (MOR). Genetic variations in MOR and MOR interacting proteins (MORIPs) involved in MOR signaling may increase the risk for drug dependence. The MORIP β-arrestin plays an important role in the regulation of MOR trafficking, thereby highlighting it as a candidate gene for addiction phenotypes. In this case-control association study, DNA samples from cocaine-dependent (n=336) and opioid-dependent (n=335) patients and controls (n=656) were genotyped for seven single nucleotide polymorphisms (rs11868227, rs3786047, rs4522461, rs1045280, rs2271167, rs2036657, and rs4790694) across ARRB2, the gene encoding the β-arrestin 2 protein. No significant differences were observed in genotype or allele frequency between drug-dependent and control individuals for any of the single nucleotide polymorphisms analyzed. Haplotype analysis was similarly negative. Further studies are needed to determine whether variations in ARRB2 (or other MORIPs) are relevant to cocaine or opioid dependence in different ethnic populations or whether they confer a risk that is specific to dependence on other drugs of abuse.

Decreased cocaine motor sensitization and self-administration in mice overexpressing cannabinoid CB₂ receptors

The potential involvement of the cannabinoid CB₂ receptors (CB₂r) in the adaptive responses induced by cocaine was studied in transgenic mice overexpressing the CB₂r (CB₂xP) and in wild-type (WT) littermates. For this purpose, the acute and sensitized locomotor responses to cocaine, conditioned place preference, and cocaine intravenous self-administration were evaluated. In addition, we assessed whether CB₂r were localized in neurons and/or astrocytes, and whether they colocalized with dopamine D1 and D2 receptors (D1Dr and D2Dr). Dopamine (DA) extracellular levels in the nucleus accumbens (NAcc), and gene expression of tyrosine hydroxylase (TH) and DA transporter (DAT) in the ventral tegmental area (VTA), and μ-opioid and cannabinoid CB₁ receptors in the NAcc were also studied in both genotypes. CB₂xP mice showed decreased motor response to acute administration of cocaine (10-20 mg/kg) and cocaine-induced motor sensitization compared with WT mice. CB₂xP mice presented cocaine-induced conditioned place aversion and self-administered less cocaine than WT mice. CB₂r were found in neurons and astrocytes and colocalized with D2Dr in the VTA and NAcc. No significant differences in extracellular DA levels in the NAcc were observed between genotypes after cocaine administration. Under baseline conditions, TH and DAT gene expression was higher and μ-opioid receptor gene expression was lower in CB₂xP than in WT mice. However, both genotypes showed similar changes in TH and μ-opioid receptor gene expression after cocaine challenge independently of the pretreatment received. Importantly, the cocaine challenge decreased DAT gene expression to a lesser extent in cocaine-pretreated CB₂xP than in cocaine-pretreated WT mice. These results revealed that CB₂r are involved in cocaine motor responses and cocaine self-administration, suggesting that this receptor could represent a promising target to develop novel treatments for cocaine addiction.

The rewarding action of acute cocaine is reduced in β-endorphin deficient but not in μ opioid receptor knockout mice

We have previously shown that β-endorphin plays a functional role in the rewarding effect of acute cocaine. Considering that β-endorphin has high affinity for the μ opioid receptor, we determined the role of this receptor in the rewarding action of acute cocaine. For comparison, we assessed the role of the μ opioid receptor in the rewarding effect of acute morphine. We also examined the effect of intracerebroventricular (i.c.v.) administration of β-funaltrexamine (β-FNA), an irreversible μ opioid receptor antagonist, on the rewarding action of acute cocaine as well as that of morphine. Using the conditioned place preference (CPP) paradigm as an animal model of reward, we first assessed the rewarding action of cocaine in mice lacking β-endorphin or the μ opioid receptor and their respective wild-type littermates/controls. Mice were tested for preconditioning place preference on day 1, conditioned once daily with saline/cocaine (30mg/kg, i.p.) or cocaine/saline on days 2 and 3, and then tested for postconditioning place preference on day 4. We next studied the rewarding action of acute morphine in μ knockout mice and their wild-type controls. The CPP was induced by single alternate-day saline/morphine (10mg/kg, s.c.) or morphine/saline conditioning. We finally determined the effect of β-FNA on CPP induced by cocaine or morphine in wild-type mice, in which mice were treated with saline or β-FNA (9ug/3μl; i.c.v.) a day prior to the preconditioning test day. Our results revealed that morphine induced a robust CPP in wild-type mice but not in mice lacking the μ opioid receptor or in wild-type mice treated with β-FNA. In contrast, cocaine induced CPP in μ knockout mice as well as in wild-type mice treated with β-FNA. On the other hand, cocaine failed to induce CPP in mice lacking β-endorphin. These results illustrate that β-endorphin is essential for the rewarding action of acute cocaine, but the μ opioid receptor may not mediate the regulatory action of endogenous β-endorphin.

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.

OPRM1 gene variants modulate amphetamine-induced euphoria in humans

The μ-opioid receptor is involved in the rewarding effects of not only opioids like morphine but also psychostimulants like amphetamine. This study aimed to investigate associations between subjective response to amphetamine and genetic polymorphisms and haplotypes in the μ-opioid receptor including the exonic variant rs1799971 (Asp40Asn). One hundred and sixty-two Caucasian volunteers participated in three sessions receiving either placebo or d-amphetamine (10 and 20 mg). Associations between levels of self-reported Euphoria, Energy and Stimulation [Addiction Research Center Inventory 49-item questionnaire (ARCI-49)] after d-amphetamine ingestion and polymorphisms in OPRM1 were investigated. The intronic single nucleotide polymorphisms (SNPs) rs510769 and rs2281617 were associated with significantly higher ratings of Euphoria, Energy and Stimulation after 10 mg amphetamine. Feelings of Euphoria, Energy and Stimulation were also found to be associated with a two-SNP haplotype formed with rs1799971 and rs510769 and a three-SNP haplotype formed with rs1918760, rs2281617 and rs1998220. These results support the hypothesis that genetic variability in the μ-opioid receptor gene influences the subjective effects of amphetamine and may suggest new strategies for prevention and treatment of psychostimulant abuse.

The endogenous opioid system: a common substrate in drug addiction

Drug addiction is a chronic brain disorder leading to complex adaptive changes within the brain reward circuits that involve several neurotransmitters. One of the neurochemical systems that plays a pivotal role in different aspects of addiction is the endogenous opioid system (EOS). Opioid receptors and endogenous opioid peptides are largely distributed in the mesolimbic system and modulate dopaminergic activity within these reward circuits. Chronic exposure to the different prototypical drugs of abuse, including opioids, alcohol, nicotine, psychostimulants and cannabinoids has been reported to produce significant alterations within the EOS, which seem to play an important role in the development of the addictive process. In this review, we will describe the adaptive changes produced by different drugs of abuse on the EOS, and the current knowledge about the contribution of each component of this neurobiological system to their addictive properties.

Dietary restriction mitigates cocaine-induced alterations of olfactory bulb cellular plasticity and gene expression, and behavior

Because the olfactory system plays a major role in food consumption, and because 'food addiction' and associated morbidities have reached epidemic proportions, we tested the hypothesis that dietary energy restriction can modify adverse effects of cocaine on behavior and olfactory cellular and molecular plasticity. Mice maintained on an alternate day fasting (ADF) diet exhibited increased baseline locomotion and increased cocaine-sensitized locomotion during cocaine conditioning, despite no change in cocaine conditioned place preference, compared with mice fed ad libitum. Levels of dopamine and its metabolites in the olfactory bulb (OB) were suppressed in mice on the ADF diet compared with mice on the control diet, independent of acute or chronic cocaine treatment. The expression of several enzymes involved in dopamine metabolism including tyrosine hydroxylase, monoamine oxidases A and B, and catechol-O-methyltransferase were significantly reduced in OBs of mice on the ADF diet. Both acute and chronic administration of cocaine suppressed the production of new OB cells, and this effect of cocaine was attenuated in mice on the ADF diet. Cocaine administration to mice on the control diet resulted in up-regulation of OB genes involved in mitochondrial energy metabolism, synaptic plasticity, cellular stress responses, and calcium- and cAMP-mediated signaling, whereas multiple olfactory receptor genes were down-regulated by cocaine treatment. ADF abolished many of the effects of cocaine on OB gene expression. Our findings reveal that dietary energy intake modifies the neural substrates underlying some of the behavioral and physiological responses to repeated cocaine treatment, and also suggest novel roles for the olfactory system in addiction. The data further suggest that modification of dietary energy intake could provide a novel potential approach to addiction treatments.

Reward processing by the opioid system in the brain

The opioid system consists of three receptors, mu, delta, and kappa, which are activated by endogenous opioid peptides processed from three protein precursors, proopiomelanocortin, proenkephalin, and prodynorphin. Opioid receptors are recruited in response to natural rewarding stimuli and drugs of abuse, and both endogenous opioids and their receptors are modified as addiction develops. Mechanisms whereby aberrant activation and modifications of the opioid system contribute to drug craving and relapse remain to be clarified. This review summarizes our present knowledge on brain sites where the endogenous opioid system controls hedonic responses and is modified in response to drugs of abuse in the rodent brain. We review 1) the latest data on the anatomy of the opioid system, 2) the consequences of local intracerebral pharmacological manipulation of the opioid system on reinforced behaviors, 3) the consequences of gene knockout on reinforced behaviors and drug dependence, and 4) the consequences of chronic exposure to drugs of abuse on expression levels of opioid system genes. Future studies will establish key molecular actors of the system and neural sites where opioid peptides and receptors contribute to the onset of addictive disorders. Combined with data from human and nonhuman primate (not reviewed here), research in this extremely active field has implications both for our understanding of the biology of addiction and for therapeutic interventions to treat the disorder.

A role for calmodulin-stimulated adenylyl cyclases in cocaine sensitization

Cocaine sensitization is produced by repeated exposure to the drug and is thought to reflect neuroadaptations that contribute to addiction. Here, we identify the Ca(2+)/calmodulin-stimulated adenylyl cyclases, type 1 (AC1) and type 8 (AC8), as novel regulators of this behavioral plasticity. We show that, whereas AC1 and AC8 single knock-out mice (AC1(-/-) and AC8(-/-)) exhibit Ca(2+)-stimulated adenylyl cyclase activity in striatal membrane fractions, AC1/8 double-knock-out (DKO) mice do not. Furthermore, DKO mice are acutely supersensitive to low doses of cocaine and fail to display locomotor sensitization after chronic cocaine treatment. Because of the known role for the extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase signaling pathway in cocaine-induced behavioral plasticity and its coupling to calcium-stimulated cAMP signaling in the hippocampus, we measured phosphorylated ERK (pERK) levels in the striatum. Under basal conditions, pERK is upregulated in choline acetyltransferase-positive interneurons in DKO mice relative to wild-type (WT) controls. After acute cocaine treatment, pERK signaling is significantly suppressed in medium spiny neurons (MSNs) of DKO mice relative to WT mice. In addition to the lack of striatal ERK activation by acute cocaine, signaling machinery downstream of ERK is uncoupled in DKO mice. We demonstrate that AC1 and AC8 are necessary for the phosphorylation of mitogen and stress-activated kinase-1 (pMSK1) at Ser376 and Thr581 and cAMP response element-binding protein (pCREB) at Ser133 after acute cocaine treatment. Our results demonstrate that the Ca(2+)-stimulated adenylyl cyclases regulate long-lasting cocaine-induced behavioral plasticity via activation of the ERK/MSK1/CREB signaling pathway in striatonigral MSNs.

Beta-endorphin and drug-induced reward and reinforcement

Although drugs of abuse have different acute mechanisms of action, their brain pathways of reward exhibit common functional effects upon both acute and chronic administration. Long known for its analgesic effect, the opioid beta-endorphin is now shown to induce euphoria, and to have rewarding and reinforcing properties. In this review, we will summarize the present neurobiological and behavioral evidences that support involvement of beta-endorphin in drug-induced reward and reinforcement. Currently, evidence supports a prominent role for beta-endorphin in the reward pathways of cocaine and alcohol. The existing information indicating the importance of beta-endorphin neurotransmission in mediating the reward pathways of nicotine and THC, is thus far circumstantial. The studies described herein employed diverse techniques, such as biochemical measurements of beta-endorphin in various brain sites and plasma, and behavioral measurements, conducted following elimination (via administration of anti-beta-endorphin antibodies or using mutant mice) or augmentation (by intracerebral administration) of beta-endorphin. We suggest that the reward pathways for different addictive drugs converge to a common pathway in which beta-endorphin is a modulating element. Beta-endorphin is involved also with distress. However, reviewing the data collected so far implies a discrete role, beyond that of a stress response, for beta-endorphin in mediating the substance of abuse reward pathway. This may occur via interacting with the mesolimbic dopaminergic system and also by its interesting effects on learning and memory. The functional meaning of beta-endorphin in the process of drug-seeking behavior is discussed.

A neuropeptide-centric view of psychostimulant addiction

Drugs of abuse all share common properties classically observed in human beings and laboratory animals. They enhance neural firing and dopamine tone within the nucleus accumbens and produce progressively greater drug-induced motor responses defined as behavioural sensitization. They produce conditioned place preference, a behavioural model of incentive motivation, which highlights the role of environmental cues in drug addiction. They increase brain reward function as seen by a lowering of intracranial self-stimulation thresholds. And last but not least, they are self-administered, and sometimes even abused, and can trigger reinstatement of drug-seeking behaviour in animals extinguished from drug self-administration. It has long been considered that the reinforcing properties of virtually all drugs of abuse, more specifically psychostimulants, are primarily dependent on activation of the mesolimbic dopamine system. However, recent evidence raises the importance of dopamine-independent mechanisms in reward-related behaviours. The overwhelming body of evidence that indicates a critical role for the mesolimbic dopamine system in the reinforcing effect of psychostimulants should not mask the key contribution of other modulatory systems in the brain. This review summarizes the complex and subtle role of several neuropeptidergic systems in various aspects of addictive behaviours observed in laboratory animals exposed to psychostimulants. A special emphasis is given to the cannabinoid, opioid, nociceptin/orphanin FQ, corticotropin-releasing factor and hypocretin/orexin systems. The relevance of these systems viewed as potential therapeutic targets for drug addiction is discussed in the light of their narrow pharmacological profile and their effectiveness in preventing drug addiction at doses usually not accompanied by severe side effects.

Endogenous nociceptin (orphanin FQ) suppresses basal hedonic state and acute reward responses to methamphetamine and ethanol, but facilitates chronic responses

The opioid peptide nociceptin (orphanin FQ) suppresses drug reward, drug self-administration, and impedes some of the processes believed to underlie the transition to addiction. As virtually all previous studies have used administration of nociceptin receptor agonists to evaluate the role of nociceptin on addiction-like behavior, the current study used a pharmacological (nociceptin receptor antagonist) and genetic (nociceptin receptor knockout mice) approach to elucidate the role of endogenous nociceptin. The nociceptin receptor antagonist UFP-101 induced a modest place preference, and enhanced the conditioned place preference induced by methamphetamine. In agreement with this, nociceptin receptor knockout mice had slightly enhanced methamphetamine and ethanol conditioned place preferences compared to wild-type mice. This effect did not appear to depend on differences in learning ability, as nociceptin receptor knockout mice had slightly weaker-conditioned place aversions to lithium chloride, the kappa-opioid receptor agonist, U50488H, and the general opiate antagonist, naloxone. The development of behavioral sensitization to methamphetamine was lower in nociceptin receptor knockout mice, and attenuated by UFP-101 administration to wild-type mice. Additionally, ethanol consumption and preference in a two-bottle choice test was lower in nociceptin receptor knockout mice, though ethanol-stimulated locomotion was stronger. Whereas the rewarding effect of methamphetamine and ethanol following chronic treatment, as measured by place conditioning, strengthened in wild-type mice, this effect was absent in nociceptin receptor knockout mice. These results suggest that endogenous N/OFQ suppresses basal and drug-stimulated increases in hedonic state, and plays either a permissive or facilitatory role in the development of addiction.

Increased adiposity on normal diet, but decreased susceptibility to diet-induced obesity in mu-opioid receptor-deficient mice

The mu-opioid receptor encoded by the Oprm1 gene plays a crucial role in the mediation of food reward and drug-induced positive reinforcement, but its genetic deletion has been shown to provide food intake-independent, partial protection from diet-induced obesity. We hypothesized that mu-opioid receptor-deficient mice would show an even greater, intake-dependent, resistance to high-fat diet-induced obesity if the diet comprises a sweet component. We generated an F2 population by crossing the heterozygous offspring of homozygous female Oprm1(-/-) mice (on a mixed C57BL/6 and BALB/c genetic background) with male inbred C57BL/6 mice. Groups of genotyped wild-type (WT) and homozygous mutant (KO) males and females were fed either control chow or a high caloric palatable diet consisting of sweet, liquid chocolate-flavored Ensure together with a solid high-fat diet. Food intake, body weight, and body composition was measured over a period of 16 weeks. Unexpectedly, male, and to a lesser extent female, KO mice fed chow for the entire period showed progressively increased body weight and adiposity while eating significantly more chow. In contrast, when exposed to the sweet plus high-fat diet, male, and to a lesser extent female, KO mice gained significantly less body weight and fat mass compared to WT mice when using chow fed counterparts for reference values. Male KO mice consumed 33% less of the sweet liquid diet but increased intake of high-fat pellets, so that total calorie intake was not different from WT animals. These results demonstrate a dissociation of the role of mu-opioid receptors in the control of adiposity for different diets and sex. On a bland diet, normal receptor function appears to confer a slightly catabolic predisposition, but on a highly palatable diet, it confers an anabolic metabolic profile, favoring fat accretion. Because of the complexity of mu-opioid gene regulation and tissue distribution, more selective and targeted approaches will be necessary to fully understand the underlying mechanisms.

A role for mu opioid receptors in cocaine-induced activity, sensitization, and reward in the rat

RATIONALE

Considerable evidence suggests that the endogenous opioid system plays a role in mediating the behavioral effects of psychostimulants. Opioidergic drugs have been shown to have profound effects on cocaine-induced behavioral sensitization and conditioned reward. However, the role specifically of the mu opioid receptor in this regard is unclear as most previous pharmacological studies have used nonselective opioid receptor ligands.

OBJECTIVES

The objective of this series of experiments was to elucidate the role of mu opioid receptors in the behavioral effects of cocaine in the rat.

MATERIALS AND METHODS

Adult male rats were used to assess the effects of the selective mu opioid receptor antagonist D: -Phe-Cys-Tyr-D: -Trp-Arg-Thr-Pen-Thr (CTAP) on acute hyperactivity, locomotor sensitization, and conditioned place preference induced by cocaine. Intracerebroventricular administration of CTAP, 4 microg, was paired with peripheral injections of cocaine, 10-15 mg/kg.

RESULTS

Mu receptor blockade significantly attenuated cocaine-induced hyperactivity, as well as the development of behavioral sensitization. Pretreatment with CTAP also prevented the development of conditioned place preference to cocaine. Administration of CTAP alone had neither effect on locomotor activity nor did it demonstrate aversive or rewarding properties.

CONCLUSIONS

These results suggest that activation of mu opioid receptors by endogenous opioids is an important contributor to cocaine-induced hyperactivity and the development of behavioral sensitization and conditioned reward.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

Association of the mu-opioid receptor gene with smoking cessation

We investigated the association of the OPRM1 genotype with long-term smoking cessation and change in body mass index (BMI) following a smoking cessation attempt among smokers who attempted to quit using the nicotine replacement therapy (NRT) patch or placebo in a randomized controlled trial, and were followed-up over an 8-year period following their initial cessation attempt. We also investigated possible sex differences in these relationships, given evidence for sex differences in smoking cessation and central opioid mechanisms, as well as some evidence for sex differences in response to NRT. Our results indicate that OPRM1 genotype may moderate the effect of transdermal nicotine patch compared to placebo during active treatment, with a benefit of active NRT treatment evident in the OPRM1 AA genotype group only and those carrying one or more copies of the G allele demonstrating no benefit of active NRT versus placebo patch. Our results also indicate a sex difference in change in BMI at 8-year follow-up following a smoking cessation attempt, with ex-smokers demonstrating an increase in BMI, and this increase being greater in female subjects than in male subjects. We did not observe any association of OPRM1 genotype with change in BMI, although there was a trend for genotype to influence the observed sex difference in change in BMI over time. Future studies should attempt to replicate these findings, and investigate the relationship between both short- and long-term weight gain and smoking cessation and investigate possible mechanisms that may underlie these processes. Future studies should also investigate the role of OPRM1 genotype and smoking cessation on other appetitive and reward behaviours such as alcohol consumption.

Association of mu-opioid receptor gene polymorphism A118G with alcohol dependence in a Japanese population

Ethanol is considered to activate the brain reward system by increasing the release of an endogenous opioid receptor ligand, beta-endorphin. The polymorphism A118G in the mu-opioid receptor gene (OPRM1) causes the amino acid change Asn40Asp and has been reported to affect the affinity of the ligand for the receptor. The association of this polymorphism with the vulnerability to alcohol dependence has been studied in many populations, but not yet in Japanese people. In the present study, we compared the frequencies of the polymorphism OPRM1 A118G between patients with alcohol dependence and healthy control subjects living in a Japanese provincial prefecture. We also genotyped a polymorphism, G1510A, in the acetaldehyde dehydrogenase 2 gene (ALDH2), in which the A allele causes poor metabolism of acetaldehyde, a major metabolite of alcohol. Both OPRM1 118G and ALDH2 1510G were significantly associated with alcohol dependence. These results suggest that OPRM1 118G in addition to ALDH2 1510G might be one of the risk factors for alcohol dependence in Japanese people.

An antisense oligodeoxynucleotide to the mu opioid receptor attenuates cocaine-induced behavioral sensitization and reward in mice

Numerous studies support a role for the endogenous opioid system in cocaine-influenced behavior. Few of these studies, however, selectively delineate a role for the mu opioid receptor (MOR) in this regard. This investigation examined if the MOR modulates cocaine-induced behavior in mice using a 17-base antisense oligodeoxynucleotide (AS ODN) directed against the MOR coding sequence 16-32. Specifically, cocaine-induced behavioral sensitization and conditioned reward were investigated. For the sensitization study, C57BL/6J mice received eight intermittent i.c.v. infusions of saline, mismatch oligodeoxynucleotide (ODN) (20 microg/4 microl) or AS ODN (20 microg/4 microl) over 20 days. Mice also received concomitant once daily i.p. injections of saline (4 ml/kg) or cocaine (15 mg/kg) for 10 days. There was a 7-day withdrawal period, after which all mice were challenged with cocaine (15 mg/kg) to test for behavioral sensitization. For the conditioned place preference (CPP) study, mice received five i.c.v. infusions of mismatch ODN or MOR AS ODN (days 1-5). An unbiased counterbalanced conditioning procedure was used where mice were conditioned with saline (4 ml/kg, i.p.) and cocaine (15 mg/kg, i.p.) on alternate days for four sessions (days 3-6). Mice were tested on day 7 for CPP. Immediately following testing, [3H]DAMGO (D-Ala2, N-Me-Phe4, Gly-ol5-enkephalin) receptor binding to brain homogenates was conducted. MOR AS attenuated cocaine-induced behavioral sensitization and conditioned reward. MOR AS ODN also reduced [3H]DAMGO binding. Collectively, these findings implicate the MOR as playing an important neuromodulatory role in the behavioral effects of cocaine in mice.

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.

Association analysis of delta-opioid receptor gene polymorphisms in methamphetamine dependence/psychosis

The role of the delta-opioid receptor (OPRD1) in methamphetamine (MAP) addiction was investigated using association analysis between OPRD1 gene polymorphisms and MAP dependence/psychosis. DNA samples from Japanese patients with MAP dependence/psychosis were analyzed to find polymorphisms in OPRD1 gene exons and exon-intron boundaries. One novel single nucleotide polymorphism (SNP) in intron 1 and two SNPs in exon 3 were identified. The two SNPs in exon 3 were in linkage disequilibrium. No significant difference was observed in either genotypic or allelic frequencies of these SNPs between controls (n = 260) and MAP dependent/psychotic patients (n = 170). Global analyses using the three SNPs and subcategory analyses on clinical parameters also showed no significant differences. These results suggest that the OPRD1 gene variants may not be a factor in vulnerability to MAP dependence/psychosis.

Reduced psychostimulant effects on dopamine dynamics in the nucleus accumbens of mu-opioid receptor knockout mice

Dopamine neurotransmission in the nucleus accumbens plays a pivotal role in the reinforcing properties of drugs of abuse. Two interacting processes regulate nucleus accumbens dopamine overflow: release of dopamine from presynaptic terminals and the subsequent reuptake by dopamine transporters. Opioid neurotransmission, primarily through mu-opioid receptors has also been strongly implicated in drug reward. We have previously shown that mice lacking the mu-opioid receptor display decreased cocaine self-administration. In addition, we found decreased impulse activity of midbrain dopaminergic neurons and an increased GABAergic input to these neurons in mu-opioid receptor knockout mice. In the present study we investigated whether these changes in dopaminergic cell bodies are accompanied by altered dopamine dynamics at the terminal level. To that aim, we measured nucleus accumbens dopamine overflow using fast scan cyclic voltammetry. Our data demonstrate that in mu-opioid receptor knockout mice 1) the reuptake of dopamine in the nucleus accumbens is slower, and 2) the relative effect of cocaine and amphetamine on the reuptake of dopamine is smaller compared with wild type mice. These data provide a mechanism for the decreased reinforcing properties of cocaine observed in mu-opioid receptor knockout mice.

NrCAM in addiction vulnerability: positional cloning, drug-regulation, haplotype-specific expression, and altered drug reward in knockout mice

Several lines of evidence support roles for the cell adhesion molecule NrCAM in addictions. Fine mapping within a chromosome 7 region that contains previously linked and associated genomic markers identifies NrCAM haplotypes that are associated with substance abuse vulnerabilities in four samples of abusers and controls. Differential display identifies NrCAM as a drug regulated gene. NrCAM is expressed in neurons linked to reward and memory. NrCAM displays haplotype-specific gene expression in human post-mortem brain samples. Knockout mice display reduced opiate- and stimulant-conditioned place preferences. These observations support NrCAM as a positionally cloned and drug-regulated gene whose variants are likely to change expression and alter substance abuse vulnerabilities in human addictions and animal models of drug reward.

Attenuation of methamphetamine-induced behavioral sensitization in mice by systemic administration of naltrexone

Repeated intermittent exposure to psychostimulants was found to produce behavioral sensitization. The present study was designed to establish a mouse model and by which to investigate whether opioidergic system plays a role in methamphetamine-induced behavioral sensitization. Mice injected with 2.5 mg/kg of methamphetamine once a day for 7 consecutive days showed behavioral sensitization after challenge with 0.3125 mg/kg of the drug on day 11, whereas mice injected with a lower daily dose (1.25 mg/kg) did not. Mice received daily injections with either 1.25 or 2.5 mg/kg of methamphetamine showed behavioral sensitization after challenge with 1.25 mg/kg of the drug on days 11, 21, and 28. To investigate the role of opioidergic system in the induction and expression of behavioral sensitization, long-acting but non-selective opioid antagonist naltrexone was administrated prior to the daily injections of and challenge with methamphetamine, respectively. Our results show that the expressions of behavioral sensitization were attenuated by pretreatment with 10 or 20 mg/kg of naltrexone either during the induction period or before methamphetamine challenge when they were tested on days 11 and 21. These results indicate that repeated injection with methamphetamine dose-dependently induced behavioral sensitization in mice, and suggest the involvement of opioid receptors in the induction and expression of methamphetamine-induced behavioral sensitization.