Confirmation of the association between a polymorphism in the promoter region of the prodynorphin gene and cocaine dependence

Epigenetic and Transcriptional Control of the Opioid Prodynorphine Gene: In-Depth Analysis in the Human Brain

Neuropeptides serve as neurohormones and local paracrine regulators that control neural networks regulating behavior, endocrine system and sensorimotor functions. Their expression is characterized by exceptionally restricted profiles. Circuit-specific and adaptive expression of neuropeptide genes may be defined by transcriptional and epigenetic mechanisms controlled by cell type and subtype sequence-specific transcription factors, insulators and silencers. The opioid peptide dynorphins play a critical role in neurological and psychiatric disorders, pain processing and stress, while their mutations cause profound neurodegeneration in the human brain. In this review, we focus on the prodynorphin gene as a model for the in-depth epigenetic and transcriptional analysis of expression of the neuropeptide genes. Prodynorphin studies may provide a framework for analysis of mechanisms relevant for regulation of neuropeptide genes in normal and pathological human brain.

The Streetlight Effect: Reappraising the Study of Addiction in Light of the Findings of Genome-wide Association Studies

Drug dependence has long been thought to have a genetic component. Research seeking to identify the genetic basis of addiction has gone through important transitions over its history, in part based upon the emergence of new technologies, but also as the result of changing perspectives. Early research approaches were largely dictated by available technology, with technological advancements having highly transformative effects on genetic research, but the limitations of technology also affected modes of thinking about the genetic causes of disease. This review explores these transitions in thinking about the genetic causes of addiction in terms of the "streetlight effect," which is a type of observational bias whereby people search for something only where it is easiest to search. In this way, the genes that were initially studied in the field of addiction genetics were chosen because they were the most "obvious," and formed current understanding of the biological mechanisms underlying the actions of drugs of abuse and drug dependence. The problem with this emphasis is that prior to the genomic era the vast majority of genes and proteins had yet to be identified, much less studied. This review considers how these initial choices, as well as subsequent choices that were also driven by technological limitations, shaped the study of the genetic basis of drug dependence. While genome-wide approaches overcame the initial biases regarding which genes to choose to study inherent in candidate gene studies and other approaches, genome-wide approaches necessitated other assumptions. These included additive genetic causation and limited allelic heterogeneity, which both appear to be incorrect. Thus, the next stage of advancement in this field must overcome these shortcomings through approaches that allow the examination of complex interactive effects, both gene × gene and gene × environment interactions. Techniques for these sorts of studies have recently been developed and represent the next step in our understanding of the genetic basis of drug dependence.

Association of variants of prodynorphin promoter 68-bp repeats in caucasians with opioid dependence diagnosis: Effect on age trajectory of heroin use

The dynorphin/kappa opioid receptor (Dyn/KOR) system is involved in reward processing and dysphoria/anhedonia. Exposure to mu-opioid receptor agonists such as heroin increases expression of the prodynorphin gene (PDYN) in the brain. In this study in a Caucasian cohort, we examined the association of the functional PDYN 68-bp repeat polymorphism with opioid use disorders. In this case-control study, 554 subjects with Caucasian ancestry (142 healthy controls, 153 opioid-exposed, but never opioid dependent, NOD, and 259 with an opioid dependence diagnosis, OD) were examined for association of the PDYN 68-bp repeats with the diagnosis of opioid dependence (DSM-IV criteria), with a dimensional measure of heroin exposure (KMSK scale), and age trajectory parameters of heroin use (age of heroin first use, and age of onset of heaviest use). The PDYN 68-bp repeat genotype (classified as: "short-short" [SS], "long-long" [LL], and "short-long" [SL], based on the number of repeats) was not associated with categorical opioid dependence diagnoses. However, the LL genotype was associated with later age of first heroin use than the SS + SL genotype (19 versus 18 years; p < 0.01). This was also confirmed by a significant positive correlation between the number of repeats and the age of first use of heroin, in volunteers with OD (Spearman r = 0.16; p = 0.01). This suggests that the functional PDYN 68-bp repeat genotype is associated with the age of first use of heroin in Caucasians diagnosed with opioid dependence.

Genetic association analyses and meta-analysis of Dynorphin-Kappa Opioid system potential functional variants with heroin dependence

Prodynorphin (PDYN) binds to k-opioid receptors (KOPr; encoded by OPRK1) and is known to regulate dopaminergic tone, making this system important for drugs addiction. Dynorphin (Dyn)/KORr system are powerful effectors of stress-induced alterations in reward processing and dysphoric states. Thus, We identified 11 potential functional SNPs and one variable number of tandem repeat (VNTR) in this system, performed a case-control association analysis, investigated particular disease phenotypes, assessed the joint effect of variants in two genes, carried out a meta-analysis to analyze the association between this VNTR and Heroin dependence (HD) risk. Eleven single-nucleotide polymorphisms (SNPs) were genotyped using SNaPshot SNP technology. Participants included 566 healthy controls and 541 patients with HD. We found that PDYN polymorphisms modulate the susceptibility to HD. An increased risk of HD was significantly associated with H alleles of PDYN VNTR (χ² = 10.824, p = 0.001, OR = 1.419, 95% CI = 1.151-1.748). In addition, the results revealed the patients with the HH genotype showed greater number of withdrawal instances (F(2538) = 7.987, p = 0.0004) compared to the patients with the LL genotype. The Meta-analysis showed the pooled effect of the H allele at this locus is a risk factor for HD in Chinese Han. Gene-gene interaction analysis indicated strong interactions between PDYN rs3830064, 68-bp VNTR and OPRK1 rs16918842, rs3802279. These findings support the important role of PDYN polymorphism in HD, and may guide future studies to identify genetic risk factors for HD.

Evaluation of prodynorphin gene polymorphisms and their association with heroin addiction in a sample of the southeast Iranian population

Genetic factors are supposed to account for about 30-50% of the predisposition to cocaine and heroin addiction. This study aims at investigating the effect of rs2281285, rs2235749, rs910080 and 68bp VNTR polymorphisms of prodynorphin (PDYN) gene on heroin dependence risk in a sample of the southeast Iranian population. This case-control study was done on 216 heroin dependence subjects and 219 healthy subjects. Genomic DNA was extracted from peripheral blood cells using salting out method. Genotyping of PDYN polymorphisms were performed using polymerase chain reaction (PCR) or PCR-RFLP method. The findings showed that PDYN rs910080 T>C variant significantly increased the risk of heroin dependence (OR=7.91, 95%CI=3.36-18.61, P<0.0001, CC vs TT; OR=7.53, 95%CI=3.30-17.16, P<0.0001, CC vs TT+TC; OR=1.75, 95%CI=1.33-2.32, p<0.0001, C vs T). The rs2235749 C>T, rs2281285 A>G and 68bp VNTR variants of PDYN gene were not associated with heroin dependence. Altogether, our results provide an association between rs910080 polymorphism of PDYN gene and risk of heroin dependence in a sample of the southeast Iranian population.

Gender-specific association of functional prodynorphin 68 bp repeats with cannabis exposure in an African American cohort

BACKGROUND

Cannabis use disorders (CUDs) cause substantial neuropsychiatric morbidity and comorbidity. There is evidence for gender-based differences in CUDs, for instance, a greater prevalence in males than in females. The main active component of cannabis is delta 9-tetrahydrocannabinol (delta 9-THC), a partial agonist of the cannabinoid type 1 receptor. Preclinical studies show that genetic or pharmacological manipulation of the kappa opioid receptor/dynorphin system modulates the effects of delta 9-THC.

METHODS

In this case-control study of adult African Americans (n=476; 206 females, 270 males), we examined the association of the functional prodynorphin 68 bp (PDYN 68 bp) promoter repeats with categorical diagnoses of cannabis dependence (Diagnostic and Statistical Manual of Mental Disorders-IV criteria), as well as with a rapid dimensional measure of maximum lifetime cannabis exposure (the Kreek-McHugh-Schluger-Kellogg cannabis scale).

RESULTS

The PDYN 68 bp genotype (examined as short-short [SS], short-long [SL], or long-long [LL], based on the number of repeats) was not significantly associated with categorical cannabis-dependence diagnoses, either in males or in females. However, in males, the PDYN 68 bp SS+SL genotype was associated with both greater odds of any use of cannabis (p<0.05) and earlier age of first cannabis use, compared to the LL genotype (ie, 15 versus 16.5 years of age; p<0.045). Males in the SS+SL group also had greater odds of high lifetime exposure to cannabis, compared to the LL group (p<0.045). Of interest, none of the aforementioned genetic associations were significant in females.

CONCLUSION

This study provides the first data on how the PDYN 68 bp genotype is associated with gender-specific patterns of exposure to cannabis. Overall, this study shows that PDYN 68 bp polymorphisms affect behaviors involved in early stages of nonmedical cannabis use and potentially lead to increasing self-exposure. These data may eventually lead to improvements in personalized medicine for the prevention and treatment of highly prevalent CUDs and neuropsychiatric comorbidities.

Poststress block of kappa opioid receptors rescues long-term potentiation of inhibitory synapses and prevents reinstatement of cocaine seeking

BACKGROUND

Dopaminergic neurons in the ventral tegmental area of the brain are an important site of convergence of drugs and stress. We previously identified a form of long-term potentiation of gamma-aminobutyric acid (GABA)ergic synapses on these neurons (LTPGABA). Our studies have shown that exposure to acute stress blocks this LTP and that reversal of the block of LTPGABA is correlated with prevention of stress-induced reinstatement of cocaine-seeking behavior.

METHODS

Sprague-Dawley rats were subjected to cold-water swim stress. Midbrain slices were prepared following stress, and whole-cell patch clamp recordings of inhibitory postsynaptic currents were performed from ventral tegmental area dopamine neurons. Antagonists of glucocorticoid receptors and kappa opioid receptors (κORs) were administered at varying time points after stress. Additionally, the ability of a kappa antagonist administered following stress to block forced swim stress-induced reinstatement of cocaine self-administration was tested.

RESULTS

We found that an acute stressor blocks LTPGABA for 5 days after stress through a transient activation of glucocorticoid receptors and more lasting contribution of κORs. Even pharmacological block of κORs beginning 4 days after stress has occurred reversed the block of LTPGABA. Administration of a κORs antagonist following stress prevents reinstatement of cocaine-seeking behavior.

CONCLUSIONS

A brief stressor produces changes in the reward circuitry lasting several days. Our findings reveal roles for glucocorticoid receptors and κORs as mediators of the lasting effects of stress on synaptic plasticity. κORs antagonists reverse the neuroadaptations underlying stress-induced drug-seeking behavior and may be useful in the treatment of cocaine addiction.

A genetic polymorphism of the endogenous opioid dynorphin modulates monetary reward anticipation in the corticostriatal loop

The dynorphin/κ-opioid receptor (KOP-R) system has been shown to play a role in different types of behavior regulation, including reward-related behavior and drug craving. It has been shown that alleles with 3 or 4 repeats (HH genotype) of the variable nucleotide tandem repeat (68-bp VNTR) functional polymorphism of the prodynorphin (PDYN) gene are associated with higher levels of dynorphin peptides than alleles with 1 or 2 repeats (LL genotype). We used fMRI on N = 71 prescreened healthy participants to investigate the effect of this polymorphism on cerebral activation in the limbic-corticostriatal loop during reward anticipation. Individuals with the HH genotype showed higher activation than those with the LL genotype in the medial orbitofrontal cortex (mOFC) when anticipating a possible monetary reward. In addition, the HH genotype showed stronger functional coupling (as assessed by effective connectivity analyses) of mOFC with VMPFC, subgenual anterior cingulate cortex, and ventral striatum during reward anticipation. This hints at a larger sensitivity for upcoming rewards in individuals with the HH genotype, resulting in a higher motivation to attain these rewards. These findings provide first evidence in humans that the PDYN polymorphism modulates neural processes associated with the anticipation of rewards, which ultimately may help to explain differences between genotypes with respect to addiction and drug abuse.

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.

Polymorphisms of the kappa opioid receptor and prodynorphin genes: HIV risk and HIV natural history

OBJECTIVE

Studies indicate cross-desensitization between opioid receptors (eg, kappa opioid receptor, OPRK1) and chemokine receptors (eg, CXCR4) involved in HIV infection. Whether gene variants of OPRK1 and its ligand, prodynorphin (PDYN), influence the outcome of HIV therapy was tested.

METHODS

Three study points, admission to the Women's Interagency HIV Study, initiation of highly active antiretroviral therapy (HAART), and the most recent visit, were chosen for analysis as crucial events in the clinical history of the HIV patients. Regression analyses of 17 variants of OPRK1 and 11 variants of PDYN with change of viral load (VL) and CD4 count between admission and initiation of HAART and initiation of HAART to the most recent visit to Women's Interagency HIV Study were performed in 598 HIV+ subjects, including African Americans, Hispanics, and Whites. Association with HIV status was done in 1009 subjects.

RESULTS

Before HAART, greater VL decline (improvement) in carriers of PDYN IVS3+189C>T and greater increase of CD4 count (improvement) in carriers of OPRK -72C>T were found in African Americans. Also, greater increase of CD4 count in carriers of OPRK1 IVS2+7886A>G and greater decline of CD4 count (deterioration) in carriers of OPRK1 -1205G>A were found in Whites. After HAART, greater decline of VL in carriers of OPRK1 IVS2+2225G>A and greater increase of VL in carriers of OPRK1 IVS2+10658G>T and IVS2+10963A>G were found in Whites. Also, a lesser increase of CD4 count was found in Hispanic carriers of OPRK1 IVS2+2225G>A.

CONCLUSIONS

OPRK1 and PDYN polymorphisms may alter severity of HIV infection and response to treatment.

Kappa opioid receptors regulate stress-induced cocaine seeking and synaptic plasticity

Stress facilitates reinstatement of addictive drug seeking in animals and promotes relapse in humans. Acute stress has marked and long-lasting effects on plasticity at both inhibitory and excitatory synapses on dopamine neurons in the ventral tegmental area (VTA), a key region necessary for drug reinforcement. Stress blocks long-term potentiation at GABAergic synapses on dopamine neurons in the VTA (LTPGABA), potentially removing a normal brake on activity. Here we show that blocking kappa opioid receptors (KORs) prior to forced-swim stress rescues LTPGABA. In contrast, blocking KORs does not prevent stress-induced potentiation of excitatory synapses nor morphine-induced block of LTPGABA. Using a kappa receptor antagonist as a selective tool to test the role of LTPGABA in vivo, we find that blocking KORs within the VTA prior to forced-swim stress prevents reinstatement of cocaine seeking. These results suggest that KORs may represent a useful therapeutic target for treatment of stress-triggered relapse in substance abuse.

κ-opioid receptor/dynorphin system: genetic and pharmacotherapeutic implications for addiction

Addictions to cocaine or heroin/prescription opioids [short-acting μ-opioid receptor (MOPr) agonists] involve relapsing cycles, with experimentation/escalating use, withdrawal/abstinence, and relapse/re-escalation. κ-Opioid receptors (KOPr; encoded by OPRK1), and their endogenous agonists, the dynorphins (encoded by PDYN), have counter-modulatory effects on reward caused by cocaine or MOPr agonist exposure, and exhibit plasticity in addictive-like states. KOPr/dynorphin activation is implicated in depression/anxiety, often comorbid with addictions. In this opinion article we propose that particular stages of the addiction cycle are differentially affected by KOPr/dynorphin systems. Vulnerability and resilience can be due to pre-existing (e.g., genetic) factors, or epigenetic modifications of the OPRK1 or PDYN genes during the addiction cycle. Pharmacotherapeutic approaches limiting changes in KOPr/dynorphin tone, especially with KOPr partial agonists, may hold potential for the treatment of specific drug addictions and psychiatric comorbidity.

Utility of genetically modified mice for understanding the neurobiology of substance use disorders

Advances in our ability to modify the mouse genome have enhanced our understanding of the genetic and neurobiological mechanisms contributing to addiction-related behaviors underlying substance use and abuse. These experimentally induced manipulations permit greater spatial and temporal specificity for modification of gene expression within specific cellular populations and during select developmental time periods. In this review, we consider the current mouse genetic model systems that have been employed to understand aspects of addiction and highlight significant conceptual advances achieved related to substance use and abuse. The mouse models reviewed herein include conventional knock-out and knock-in, conditional knockout, transgenic, inducible transgenic, mice suitable for optogenetic control of discrete neuronal populations, and phenotype-selected mice. By establishing a reciprocal investigatory relationship between genetic findings in humans and genomic manipulations in mice, a far better understanding of the discrete neuromechanisms underlying addiction can be achieved, which is likely to provide a strong foundation for developing and validating novel therapeutics for the treatment of substance abuse disorders.

The genetics of the opioid system and specific drug addictions

Addiction to drugs is a chronic, relapsing brain disease that has major medical, social, and economic complications. It has been established that genetic factors contribute to the vulnerability to develop drug addiction and to the effectiveness of its treatment. Identification of these factors may increase our understanding of the disorders, help in the development of new treatments and advance personalized medicine. In this review, we will describe the genetics of the major genes of the opioid system (opioid receptors and their endogenous ligands) in connection to addiction to opioids, cocaine, alcohol and methamphetamines. Particular emphasis is given to association and functional studies of specific variants. We will provide information on the sample populations and the size of each study, as well as a list of the variants implicated in association with addiction-related phenotypes, and with the effectiveness of pharmacotherapy for addiction.

Genetic association analyses of PDYN polymorphisms with heroin and cocaine addiction

Genetic factors are believed to account for 30-50% of the risk for cocaine and heroin addiction. Dynorphin peptides, derived from the prodynorphin (PDYN) precursor, bind to opioid receptors, preferentially the kappa-opioid receptor, and may mediate the aversive effects of drugs of abuse. Dynorphin peptides produce place aversion in animals and produce dysphoria in humans. Cocaine and heroin have both been shown to increase expression of PDYN in brain regions relevant for drug reward and use. Polymorphisms in PDYN are therefore hypothesized to increase risk for addiction to drugs of abuse. In this study, 3 polymorphisms in PDYN (rs1022563, rs910080 and rs1997794) were genotyped in opioid-addicted [248 African Americans (AAs) and 1040 European Americans (EAs)], cocaine-addicted (1248 AAs and 336 EAs) and control individuals (674 AAs and 656 EAs). Sex-specific analyses were also performed as a previous study identified PDYN polymorphisms to be more significantly associated with female opioid addicts. We found rs1022563 to be significantly associated with opioid addiction in EAs [P = 0.03, odds ratio (OR) = 1.31; false discovery rate (FDR) corrected q-value]; however, when we performed female-specific association analyses, the OR increased from 1.31 to 1.51. Increased ORs were observed for rs910080 and rs199774 in female opioid addicts also in EAs. No statistically significant associations were observed with cocaine or opioid addiction in AAs. These data show that polymorphisms in PDYN are associated with opioid addiction in EAs and provide further evidence that these risk variants may be more relevant in females.

Alcohol dependence, disinhibited behavior and variation in the prodynorphin gene

The results of the current analyses present preliminary evidence of an association between putatively functional variation in the prodynorphin (PDYN) gene and a dimensional measure of disinhibited behavior. A 68bp sequence in the core promoter region of the PDYN gene was genotyped in a community sample of 1021 adults aged 30-54. Participants were interviewed for lifetime history of DSM-IV alcohol dependence and completed two self-report measures of sensation seeking and impulsiveness. Fifteen percent (n=151) of the sample met DSM-IV criteria for alcohol dependence and while results did not support an association between the PDYN polymorphism and the diagnosis of alcohol dependence, we did observe an association between the "low" expressing L allele of the PDYN gene and a preference for engaging in disinhibited behavior. Additionally, people who had both a history of alcohol dependence and higher scores on this Disinhibited Behavior scale were most likely to carry an L allele. These results indicate that variation in the PDYN gene is associated with a dimensional trait or intermediate phenotype that reflects a preference for heavy drinking and engaging in related risky behaviors (e.g., drug use, sexual activity).

Cell-specific effects of variants of the 68-base pair tandem repeat on prodynorphin gene promoter activity

A polymorphic 68-bp tandem repeat has been identified within the promoter of the human prodynorphin (PDYN) gene. We found that this 68-bp repeat in the PDYN promoter occurs naturally up to five times. We studied the effect of the number of 68-bp repeats, and of a SNP (rs61761346) found within the repeat on PDYN gene promoter activity. Thirteen promoter forms, different naturally occurring combinations of repeats and the internal SNP, were cloned upstream of the luciferase reporter gene, transfected into human SK-N-SH, H69, or HEK293 cells. Cells were then stimulated with TPA or caffeine. We found cell-specific effects of the number of 68-bp repeats on the transcriptional activity of the PDYN promoter. In SK-N-SH and H69 cells, three or four repeats led to lower expression of luciferase than did one or two repeats. The opposite effect was found in HEK293 cells. The SNP also had an effect on PDYN gene expression in both SK-N-SH and H69 cells; promoter forms with the A allele had significantly higher expression than promoter forms with the G allele. These results further our understanding of the complex transcriptional regulation of the PDYN gene promoter.

Association between heroin dependence and prodynorphin gene polymorphisms

Dynorphin peptides and k-opioid receptor are important in the rewarding effects of drugs of abuse such as heroin. This study examined potential association between heroin dependence and four single nucleotide polymorphisms (SNPs) of prodynorphin (PDYN) gene (rs35286281 in promoter region and rs1022563, rs2235749, rs910080 in 3'UTR). Participants included 304 heroin-dependent subjects and 300 healthy controls. Genotype, allele frequencies and difference between groups were analyzed by HaploView 4.0 and SPSS 11.5 software. The analysis indicated a significant higher frequency of the PDYN 68bp VNTR (rs35286281) H allele in heroin-dependent subjects than in controls (p=0.002 after Bonferroni correction). Strong linkage disequilibrium was observed between rs1022563, rs2235749 and rs910080 polymorphism (D'>0.9). Significantly more TCT haplotypes were found in heroin-dependent patients than in the controls (p=0.006 after Bonferroni correction). We found significant pointwise correlation of these three variants (rs1022563, rs2235749 and rs910080) with heroin dependence. These findings support the important role of PDYN polymorphism in heroin dependence, and may guide future studies to identify genetic risk factors for heroin dependence.

Search for genetic markers and functional variants involved in the development of opiate and cocaine addiction and treatment

Addiction to opiates and illicit use of psychostimulants is a chronic, relapsing brain disease that, if left untreated, can cause major medical, social, and economic problems. This article reviews recent progress in studies of association of gene variants with vulnerability to develop opiate and cocaine addictions, focusing primarily on genes of the opioid and monoaminergic systems. In addition, we provide the first evidence of a cis-acting polymorphism and a functional haplotype in the PDYN gene, of significantly higher DNA methylation rate of the OPRM1 gene in the lymphocytes of heroin addicts, and significant differences in genotype frequencies of three single-nucleotide polymorphisms of the P-glycoprotein gene (ABCB1) between "higher" and "lower" methadone doses in methadone-maintained patients. In genomewide and multigene association studies, we found association of several new genes and new variants of known genes with heroin addiction. Finally, we describe the development and application of a novel technique: molecular haplotyping for studies in genetics of drug addiction.

Forebrain PENK and PDYN gene expression levels in three inbred strains of mice and their relationship to genotype-dependent morphine reward sensitivity

RATIONALE

Vulnerability to drug abuse disorders is determined not only by environmental but also by genetic factors. A body of evidence suggests that endogenous opioid peptide systems may influence rewarding effects of addictive substances, and thus, their individual expression levels may contribute to drug abuse liability.

OBJECTIVES

The aim of our study was to assess whether basal genotype-dependent brain expression of opioid propeptides genes can influence sensitivity to morphine reward.

METHODS

Experiments were performed on inbred mouse strains C57BL/6J, DBA/2J, and SWR/J, which differ markedly in responses to morphine administration: DBA/2J and SWR/J show low and C57BL/6J high sensitivity to opioid reward. Proenkephalin (PENK) and prodynorphin (PDYN) gene expression was measured by in situ hybridization in brain regions implicated in addiction. The influence of the kappa opioid receptor antagonist nor-binaltorphimine (nor-BNI), which attenuates effects of endogenous PDYN-derived peptides, on rewarding actions of morphine was studied using the conditioned place preference (CPP) paradigm.

RESULTS

DBA/2J and SWR/J mice showed higher levels of PDYN and lower levels of PENK messenger RNA in the nucleus accumbens than the C57BL/6J strain. Pretreatment with nor-BNI enhanced morphine-induced CPP in the opioid-insensitive DBA/2J and SWR/J strains.

CONCLUSIONS

Our results demonstrate that inter-strain differences in PENK and PDYN genes expression in the nucleus accumbens parallel sensitivity of the selected mouse strains to rewarding effects of morphine. They suggest that high expression of PDYN may protect against drug abuse by limiting drug-produced reward, which may be due to dynorphin-mediated modulation of dopamine release in the nucleus accumbens.

A functional haplotype implicated in vulnerability to develop cocaine dependence is associated with reduced PDYN expression in human brain

Dynorphin peptides and the kappa-opioid receptor are important in the rewarding properties of cocaine, heroin, and alcohol. We tested polymorphisms of the prodynorphin gene (PDYN) for association with cocaine dependence and cocaine/alcohol codependence. We genotyped six single nucleotide polymorphisms (SNPs), located in the promoter region, exon 4 coding, and 3' untranslated region, in 106 Caucasians and 204 African Americans who were cocaine dependent, cocaine/alcohol codependent, or controls. In Caucasians, we found point-wise significant associations of 3'UTR SNPs (rs910080, rs910079, and rs2235749) with cocaine dependence and cocaine/alcohol codependence. These SNPs are in high linkage disequilibrium, comprising a haplotype block. The haplotype CCT was significantly experiment-wise associated with cocaine dependence and with combined cocaine dependence and cocaine/alcohol codependence (false discovery rate, q=0.04 and 0.03, respectively). We investigated allele-specific gene expression of PDYN, using SNP rs910079 as a reporter, in postmortem human brains from eight heterozygous subjects, using SNaPshot assay. There was significantly lower expression for C allele (rs910079), with ratios ranging from 0.48 to 0.78, indicating lower expression of the CCT haplotype of PDYN in both the caudate and nucleus accumbens. Analysis of total PDYN expression in 43 postmortem brains also showed significantly lower levels of preprodynorphin mRNA in subjects having the risk CCT haplotype. This study provides evidence that a 3'UTR PDYN haplotype, implicated in vulnerability to develop cocaine addiction and/or cocaine/alcohol codependence, is related to lower mRNA expression of the PDYN gene in human dorsal and ventral striatum.

Association of variants in MANEA with cocaine-related behaviors

CONTEXT

Cocaine dependence (CD) and related behaviors are highly heritable, but no genetic association has been consistently demonstrated. A recent genome-wide study of drug dependence identified an association between cocaine-induced paranoia (CIP) and a single-nucleotide polymorphism (SNP) in the alpha-endomannosidase (MANEA) locus in a family-based sample of European Americans and African Americans.

OBJECTIVE

To conduct a comprehensive genetic association study of the MANEA locus with CD and CIP.

DESIGN

Genome-wide association study.

SETTING

Four university hospitals.

PARTICIPANTS

A total of 3992 individuals from 2 family-based and 2 case-control samples.

INTERVENTION

Participants were classified as having CD or CIP or as a control using the Semi-Structured Assessment for Drug Dependence and Alcoholism. They were genotyped for 11 SNPs spanning MANEA and its surrounding region.

MAIN OUTCOME MEASURE

Association of CD and CIP with individual SNPs and haplotypes.

RESULTS

Cocaine-induced paranoia was associated with 6 SNPs in the European American families and 9 SNPs in the African American families. The strongest evidence in the total sample of families was observed in 3 markers located in the promoter and 3' untranslated regions (P < .001). The association of MANEA SNPs with CD in both family samples was much weaker. In the African American case-control sample, multiple markers were significantly associated with CIP and CD; CIP and CD were also significantly associated with a 2-SNP haplotype in the European American case-control sample. The A allele of the 3' untranslated region SNP rs9387522 was associated with increased risk of CIP in all 4 data sets.

CONCLUSIONS

Our findings suggest that CD and associated behaviors may involve biological pathways not typically thought to be associated with brain metabolism.

Opiate and cocaine addiction: from bench to clinic and back to the bench

This review primarily focuses on our recent findings in bidirectional translational research on opiate and cocaine addictions. First, we present neurobiological and molecular studies on endogenous opioid systems (e.g. proopiomelanocortin, mu opioid receptor, dynorphin, and kappa opioid receptor), brain stress-responsive systems (e.g. orexin, arginine vasopressin, V1b receptor, and corticotropin-releasing factor), hypothalamic-pituitary-adrenal axis, and neurotransmitters (especially dopamine), in response to both chronic cocaine or opiate exposure and to drug withdrawal, using several newly developed animal models and molecular approaches. The second aspect is human molecular genetic association investigations including hypothesis-driven studies and genome-wide array studies, to define particular systems involved in vulnerability to develop specific addictions, and response to pharmacotherapy.

Role of a functional human gene polymorphism in stress responsivity and addictions

Over the past decade, enormous progress has been made in both the technical approaches and the scientific information available for studying the human genome. Therefore, increasingly, scientists have begun to address not just single-gene disorders but complex disorders. The limiting factor in most of such studies remains appropriate, well-focused detailed phenotyping of the complex disorders under study, with careful ascertainment of subjects with the specific disorder, as well as healthy control subjects.

Opioid neuropeptide genotypes in relation to heroin abuse: dopamine tone contributes to reversed mesolimbic proenkephalin expression

Striatal enkephalin and dynorphin opioid systems mediate reward and negative affect, respectively, relevant to addiction disorders. We examined polymorphisms of proenkephalin (PENK) and prodynorphin (PDYN) genes in relation to heroin abuse and gene expression in the human striatum and the relevance of genetic dopaminergic tone, critical for drug reward and striatal function. Heroin abuse was significantly associated with PENK polymorphic 3' UTR dinucleotide (CA) repeats; 79% of subjects homozygous for the 79-bp allele were heroin abusers. Such individuals tended to express higher PENK mRNA than the 81-bp homozygotes, but PENK levels within the nucleus accumbens (NAc) shell were most strongly correlated to catecholamine-O-methyltransferase (COMT) genotype. Control Met/Met individuals expressed lower PENK mRNA than Val carriers, a pattern reversed in heroin users. Up-regulation of NAc PENK in Met/Met heroin abusers was accompanied by impaired tyrosine hydroxylase (TH) mRNA expression in mesolimbic dopamine neurons. In contrast to PENK, no association was detected between PDYN genotype (68-bp repeat element containing one to four copies of AP-1 binding sites in the promoter region) and heroin abuse, although there was a clear functional association with striatal PDYN mRNA expression: an increased number of inducible repeats (three and four) correlated with higher PDYN levels than adult or fetal subjects with noninducible (one and two) alleles. Moreover, PDYN expression was not related to COMT genotype. Altogether, the data suggest that dysfunction of the opioid reward system is significantly linked to opiate abuse vulnerability and that heroin use alters the apparent influence of heritable dopamine tone on mesolimbic PENK and TH function.

Dynorphin and the pathophysiology of drug addiction

Drug addiction is a chronic relapsing disease in which drug administration becomes the primary stimulus that drives behavior regardless of the adverse consequence that may ensue. As drug use becomes more compulsive, motivation for natural rewards that normally drive behavior decreases. The discontinuation of drug use is associated with somatic signs of withdrawal, dysphoria, anxiety, and anhedonia. These consequences of drug use are thought to contribute to the maintenance of drug use and to the reinstatement of compulsive drug use that occurs during the early phase of abstinence. Even, however, after prolonged periods of abstinence, 80-90% of human addicts relapse to addiction, suggesting that repeated drug use produces enduring changes in brain circuits that subserve incentive motivation and stimulus-response (habit) learning. A major goal of addiction research is the identification of the neural mechanisms by which drugs of abuse produce these effects. This article will review data showing that the dynorphin/kappa-opioid receptor (KOPr) system serves an essential function in opposing alterations in behavior and brain neurochemistry that occur as a consequence of repeated drug use and that aberrant activity of this system may not only contribute to the dysregulation of behavior that characterizes addiction but to individual differences in vulnerability to the pharmacological actions of cocaine and alcohol. We will provide evidence that the repeated administration of cocaine and alcohol up-regulates the dynorphin/KOPr system and that pharmacological treatments that target this system may prove effective in the treatment of drug addiction.

Human kappa opioid receptor gene (OPRK1) polymorphism is associated with opiate addiction

Variants of the opioid receptors are the obvious candidates underlying addiction. The kappa opioid receptor (KOR) system seems to play a role in stress responsivity, opiate withdrawal and responses to psycho-stimulants, inhibiting mesolimbic dopamine. KOR gene polymorphisms have been reported to contribute to predisposition to voluntary alcohol-drinking behavior in experimental animals. In humans, the 36G > T single nucleotide polymorphism (SNP) on KOR gene, that was recently identified, has been found associate with substance dependence, with inconclusive findings. In the present study, 106 heroin addicts (West European, Caucasians) and 70 healthy control subjects matched for race and gender, with no history of substance use disorder, have been genotyped. The frequency of KOR 36G > T SNP was significantly higher among heroin-dependent individuals compared with control subjects (Fisher's exact = 0.044; Pearson chi(2) = 4.2734, P = 0.039; likelihood ratio chi(2) tests = 4.6156, P = 0.032). Although KOR silent polymorphisms may apparently have no consequences on mRNA transcription, post-transcriptional mechanisms, such as mRNA stability, translation efficiency, and regulability may impair the function of kappa receptors system, with increased risk for substance use disorders. In specific, the neurobiological changes induced by mu-kappa opioid imbalance could underlie vulnerable personality traits and risk behavior.

Prodynorphin gene promoter repeat associated with cocaine/alcohol codependence

There is strong evidence for a genetic contribution to individual differences in vulnerability to drug addictions. Studies have shown that the 68-base pair repeat polymorphism in the promoter region of the human prodynorphin gene contains a putative AP-1 binding site, and that three or four repeat copies result in greater transcriptional activation. Here, we report on a separate cohort of 302 subjects ascertained and characterized extensively by Diagnostic and Statistical Manual of Mental Disorders-Fourth Edition and Addiction Severity Index criteria as: (1) a control group of 127 subjects with no history of alcohol or drug abuse or dependence; (3) a case group of 82 with cocaine dependence only; and (3) a case group of 93 with cocaine and alcohol codependence. The promoter region of the prodynorphin gene containing the repeat was amplified from genomic DNA by polymerase chain reaction and analyzed via gel electrophoresis. Statistical tests were performed with data stratified by the three major ethnic groups studied: African American, Caucasian and Hispanic. For analyses, genotypes were grouped into short (1,1; 1,2; 2,2), short/long (1,3; 2,3; 1,4; 2,4) and long (3,3; 3,4; 4,4) repeats. Deviation from Hardy-Weinberg Equilibrium in the African American control group necessitated testing for association using grouped genotypes rather than grouped alleles. In controls, a significant difference was found in grouped genotype distribution among ethnicities. We found a point-wise, but not experiment-wise across-ethnicities, significant difference in grouped genotype frequency between the cocaine/alcohol-codependent group and the controls in African Americans, with genotypes containing longer alleles found at higher frequency in the codependent group.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

Perspectives on current directions in the neurobiology of addiction disorders relevant to genetic risk factors

There is a significant heritability of drug addiction disorders, but potential genes that may underlie such vulnerability have not been clearly identified. Common neurobiological candidates for drug abuse include genes related to dopamine, opioid neuropeptide, and glutamate transmission that play important roles in drug reward and inhibitory control. This article provides an overview of genetic polymorphisms linked to these neurobiological systems, particularly in relation to psychostimulant- and opioid-addiction vulnerability.

No association between polymorphisms in the prostate apoptosis factor-4 gene and cocaine dependence

OBJECTIVE

A number of studies have investigated the genes underlying dopamine, serotonin, and glutamine neurotransmitter systems in order to find a genetic basis for the pathology of cocaine dependence. The gene that encodes the prostate apoptosis factor-4 (Par-4) protein is located in the 12q21 region and has been shown to directly interact with the D2 dopamine receptor and through such interaction is thought to directly affect the activity of D2 receptors. The aim of this study is to investigate whether polymorphisms in the human Par-4 gene contribute to the etiology of cocaine dependence.

METHODS

To test this hypothesis, we used a case-control design in which the genotype and allele frequencies for five single nucleotide polymorphisms in the human Par-4 gene were compared between cocaine-dependent individuals (n=172) and controls (n=92) of African descent.

RESULTS

The genotype results failed to detect any associations between polymorphisms in the Par-4 gene and the cocaine-dependent phenotype.

CONCLUSIONS

The results of this study suggest that variations in the human Par-4 gene are unlikely to play a major role in the pathophysiology of cocaine dependence. This study, however, should be repeated in larger cocaine-dependent and control populations to determine that this is indeed the case.