Lack of allelic association of dopamine D1 and D2 (TaqIA) receptor gene polymorphisms with reduced dopaminergic sensitivity to alcoholism

Evaluation of the Relationship Between Dopamine Receptor D2 Gene TaqIA1 Polymorphism and Alcohol Dependence Risk

Several studies are published, that investigated dopamine receptor 2 (DRD2) gene TaqIA polymorphism as a risk factor for alcohol dependence (AD) with positive and negative associations. To derive a more precise estimation of the relationship, a meta-analysis of case-control studies that examined the association between DRD2 gene Taq1A polymorphism and alcohol dependence was performed. Eligible articles were identified through a search of databases including PubMed, Science Direct, Springer link, and Google Scholar. The association between the DRD2 TaqIA polymorphism and AD susceptibility was conducted using odds ratios (ORs) and 95% confidence intervals (95% CIs) as association measures. A total of 69 studies with 9125 cases and 9123 healthy controls were included in the current meta-analysis. Results of the present analysis showed significant association between DRD2 TaqIA polymorphism and AD risk using five genetic modes (allele contrast model-OR 1.22, 95% CI 1.13-1.32, p < 0.0001; homozygote model-OR 1.35, 95%CI 1.18-1.55; p ≤ 0.0001; dominant model-OR 1.29; 95% CI 1.20-1.39; p < 0.0001; recessive model-OR 1.21; 95% CI 1.08-1.36; p = 0.0006). There was no significant association found in subgroup analysis, TaqIA polymorphism was not significantly associated with AD risk in the Asian population under all genetic models, but in the Caucasian population, TaqIA polymorphism was significantly associated with AD risk. Overall, results support the hypothesis that DRD2 Taq1A polymorphism plays a role in alcohol dependence.

Association of DRD2 (rs 1799732), ANKK1 (rs1800497), DAT (rs28363170), DRD4 (exon 3 - VNTR) gene polymorphisms in the context of relapses in therapy

Introduction: Disorders in the field of reward system neurotransmission are mentioned as one of the most important causes of addiction. Genetic variation is assigned a special role. The literature on the subject mentions primarily the genes of dopamine neurotransmission: DAT (dopamine transporter), DRD2 (dopamine receptor D2), DRD4 (dopamine receptor D4). However, so far there are few literature reports on these genes in the context of innovators in addiction therapy.

The aim: Analysis of the relationship between the variability of specific polymorphisms in the DRD2 (rs1799732), ANKK1 (rs1800497), DAT (rs28363170), DRD4 (exon 3 - VNTR) genes with the occurrence of relapses in people addicted to psychoactive substances.

Material and methods: The research was carried out on a group of 301 people addicted to psychoactive substances staying in an addiction therapy center in Lubuskie and Zachodniopomorskie voivodships in Poland. The control group consisted of 301 people with no diagnosed addiction to psychoactive substances nor mental disorders. The study of polymorphisms DRD2 (rs 1799732), ANKK1 (rs1800497) was performed by real-time PCR method; whereas DAT (rs28363170), DRD4 (exon 3 - VNTR) was genotyped by PCR and the amplified products were visualized using ethidium bromide stained gel electrophoresis (3% agarose) and UV photography.

Results: This study showed that in addicts genotype frequencies of the VNTR polymorphism in the third exon of human DRD4 were as follow: S/L in 33.55%, S/S - 63.12% and L/L 3.32%; while in the control group S/L - 32.56%, S/S - 58.8 % and L/L - 8.6% (χ2 = 7.617, p = 0.022). Significant differences in the frequency of DRD2 gene polymorphism rs1799732 were observed (frequency of alleles; χ2 = 5.48, p = 0.0192) and DRD4 VNTR polymorphism (χ2 = 7.687, p = 0.021) between the addicted to psychoactive substances who have a one-time stay in an inpatient treatment center and the control group.

Assessment of the Association of D2 Dopamine Receptor Gene and Reported Allele Frequencies With Alcohol Use Disorders: A Systematic Review and Meta-analysis

IMPORTANCE

The association between the D2 dopamine receptor gene (DRD2) Taq1A locus (rs1800497) and alcohol use disorder (AUD) is enduring but the subject of long-standing controversy; meta-analysis of studies across 3 decades shows an association between rs1800497 and AUD, but genome-wide analyses have detected no role for rs1800497 in any phenotype. No evidence has emerged that rs1800497, which is located in ANKK1, perturbs the expression or function of DRD2.

OBJECTIVE

To resolve contradictions in previous studies by identifying hidden confounders and assaying for functional effects of rs1800497 and other loci in the DRD2 region.

DATA SOURCES

PubMed (882 studies), Embase (1056 studies), and Web of Science (501 studies) databases were searched through August 2018. Three clinical populations-Finnish, Native American, and African American participants-were genotyped for 208 to 277 informative single-nucleotide polymorphisms (SNPs) across the DRD2 region to test the associations of SNPs in this region with AUD.

STUDY SELECTION

Eligible studies had diagnosis of AUD made by accepted criteria, reliable genotyping methods, sufficient genotype data to calculate odds ratios and 95% CIs, and availability of control allele frequencies or genotype frequencies.

DATA EXTRACTION AND SYNTHESIS

After meta-analysis of 62 studies, metaregression was performed to detect between-study heterogeneity and to explore the effects of moderators, including deviations of cases and controls from allele frequencies in large population databases (ExAC and 1000 Genomes). Linkage to AUD and the effect on gene expression of rs1800497 were evaluated in the context of other SNPs in the DRD2 region. Data analysis was performed from August 2018 to March 2019. This study follows the Preferred Reporting Items for Systematic Reviews and Meta-analyses reporting guideline.

MAIN OUTCOMES AND MEASURES

The effects of rs1800497 and other SNPs in the DRD2 region on gene expression were measured in human postmortem brain samples via differential allelic expression and evaluated in other tissues via publicly available expression quantitative locus data.

RESULTS

A total of 62 studies of DRD2 and AUD with 16 294 participants were meta-analyzed. The rs1800497 SNP was associated with AUD (odds ratio, 1.23; 95% CI, 1.14-1.31; P < .001). However, the association was attributable to spuriously low allele frequencies in controls in positive studies, which also accounted for some between-study heterogeneity (I2 = 43%; 95% CI, 23%-58%; Q61 = 107.20). Differential allelic expression of human postmortem brain and analysis of expression quantitative loci in public data revealed that a cis-acting locus or loci perturb the DRD2 transcript level; however, rs1800497 does not and is not in strong disequilibrium with such a locus. Across the DRD2 region, other SNPs are more strongly associated with AUD than rs1800497, although no DRD2 SNP was significantly associated in these 3 clinical samples.

CONCLUSIONS AND RELEVANCE

In this meta-analysis, the significant association of DRD2 with AUD was reassessed. The DRD2 association was attributable to anomalously low control allele frequencies, not function, in positive studies. For genetic studies, statistical replication is not verification.

Pathological gambling in Parkinson's disease: what are the risk factors and what is the role of impulsivity?

The incidence of pathological gambling in Parkinson's patients is significantly greater than in the general population. A correlation has been observed between dopamine agonist medication and the development of pathological gambling. However, scientists conjecture that the affected patients have underlying risk factors. Studies analysing Parkinson's patients have detected that patients who developed pathological gambling are younger, score higher on novelty‐seeking tests, are more impulsive and are more likely to have a personal or family history of alcohol addiction. In addition, some genetic variations have been associated with the susceptibility of developing pathological gambling, which include mutations of DRD3, 5‐HTTLPR and GRIN2B. Studies focusing on neurofunctional discrepancies between Parkinson's patients with and without pathological gambling have found increased functional activation and dopamine release in regions associated with the mesolimbic reward system. Furthermore, there is also evidence showing increased processing of reward and decreased activation elicited by punishment, suggesting altered learning processes. Furthermore, the role of deep brain stimulation of the nucleus subthalamicus (STN DBS) is controversial. In most Parkinson's patients, pathological gambling resolved after the initiation of the STN DBS, which might be explained by discontinuation or decrease in dopamine agonist medication. However, it has been also shown that some patients are more impulsive while the STN DBS is activated. These differences may depend on the DBS localization in the more limbic or motor part of the STN and their regulative effects on impulsivity. Further research is needed to clarify susceptibility factors for the development of pathological gambling in Parkinson's patients.

A review of pharmacogenetic studies of substance-related disorders

BACKGROUND

Substance-related disorders (SRDs) are a major cause of morbidity and mortality worldwide. Family, twin, and adoption studies have demonstrated the substantial heritability of SRDs. To determine the impact of genetic variation on risk for SRD and the response to treatment, researchers have conducted a number of secondary data analyses and quasi-experimental studies that target one or more candidate gene variants.

METHODS

This review examines studies in which candidate polymorphisms were examined as mediator variables to identify pharmacogenetic effects on subjective responses to drug administration or cues or outcomes of medication trials for SRDs. Efforts to use a meta-analytic approach to quantify these effects are premature because the number of available studies using similar methods and outcomes is limited, so the present review is qualitative.

RESULTS

Findings from these studies provide preliminary evidence of clinically relevant pharmacogenetic effects. However, independent replication of these findings has been sparse.

CONCLUSIONS

Although this growing body of literature has produced conflicting results, improved statistical controls may help to clarify the findings. Additionally, the use of empirically derived sub-phenotypes (i.e., which serve to differentiate distinct groups of affected individuals) may also help to identify genetic mediators of pharmacologic response in relation to SRDs. The identification of genetic mediators can inform clinical care both by identifying risk factors for SRDs and predicting adverse events and therapeutic outcomes associated with specific pharmacotherapies.

Polymorphisms in genes encoding dopamine signalling pathway and risk of alcohol dependence: a systematic review

BACKGROUND

Alcohol dependence (AD) is one of the major elements that significantly influence drinking pattern that provoke the alcohol-induced organ damage. The structural and neurophysiologic abnormalities in the frontal lobes of chronic alcoholics were revealed by magnetic resonance imaging scans. It is well known that candidate genes involved in dopaminergic pathway are of immense interest to the researchers engaged in a wide range of addictive disorders. Dopaminergic pathway gene polymorphisms are being extensively studied with respect to addictive and behavioral disorders.

METHODS

From the broad literature available, the current review summarizes the specific polymorphisms of dopaminergic genes that play a role in alcohol dependence.

RESULTS

No evidence indicating any strong association between AD and polymorphisms of dopamine pathway genes has emerged from the literature.

DISCUSSION

Further studies are warranted, considering a range of alcohol-related traits to determine the genes that influence alcohol dependence.

Computational models of basal-ganglia pathway functions: focus on functional neuroanatomy

Over the past 15 years, computational models have had a considerable impact on basal-ganglia research. Most of these models implement multiple distinct basal-ganglia pathways and assume them to fulfill different functions. As there is now a multitude of different models, it has become complex to keep track of their various, sometimes just marginally different assumptions on pathway functions. Moreover, it has become a challenge to oversee to what extent individual assumptions are corroborated or challenged by empirical data. Focusing on computational, but also considering non-computational models, we review influential concepts of pathway functions and show to what extent they are compatible with or contradict each other. Moreover, we outline how empirical evidence favors or challenges specific model assumptions and propose experiments that allow testing assumptions against each other.

Polymorphisms of genes in neurotransmitter systems were associated with alcohol use disorders in a Tibetan population

Studies of linkage and association in various ethnic populations have revealed many predisposing genes of multiple neurotransmitter systems for alcohol use disorders (AUD). However, evidence often is contradictory regarding the contribution of most candidate genes to the susceptibility of AUD. We, therefore, performed a case-control study to investigate the possible associations of genes selected from multiple neurotransmitter systems with AUD in a homogeneous Tibetan community population in China. AUD cases (N = 281) with an alcohol use disorder identification test (AUDIT) score ≥10, as well as healthy controls (N = 277) with an AUDIT score ≤5, were recruited. All participants were genotyped for 366 single nucleotide polymorphisms (SNPs) of 34 genes selected from those involved in neurotransmitter systems. Association analyses were performed using PLINK version 1.07 software. Allelic analyses before adjustment for multiple tests showed that 15 polymorphisms within seven genes were associated with AUD (p<0.05). After adjustment for the number of SNPs genotyped within each gene, only the association of a single marker (rs10044881) in HTR4 remained statistically significant. Haplotype analysis for two SNPs in HTR4 (rs17777298 and rs10044881) showed that the haplotype AG was significantly associated with the protective effect for AUD. In conclusion, the present study discovered that the HTR4 gene may play a marked role in the pathogenesis of AUD. In addition, this Tibetan population sample marginally replicated previous evidence regarding the associations of six genes in AUD.

A large-scale meta-analysis of the association between the ANKK1/DRD2 Taq1A polymorphism and alcohol dependence

Alcohol dependence (AD) is a common neuropsychiatric disorder with high heritability. A number of studies have analyzed the association between the Taq1A polymorphism (located in the gene cluster ANKK1/DRD2) and AD. In the present study, we conducted a large-scale meta-analysis to confirm the association between the Taq1A polymorphism and the risk for AD in over 18,000 subjects included in 61 case-control studies that were published up to August 2012. Our meta-analysis demonstrated both allelic and genotypic association between the Taq1A polymorphism and AD susceptibility [allelic: P(Z) = 1.1 × 10(-5), OR = 1.19; genotypic: P(Z) = 3.2 × 10(-5), OR = 1.24]. The association remained significant after adjustment for publication bias using the trim and fill method. Sensitivity analysis showed that the effect size of the Taq1A polymorphism on AD risk was moderate and not influenced by any individual study. The pooled odds ratio from published studies decreased with the year of publication, but stabilized after the year 2001. Subgroup analysis indicated that publication bias could be influenced by racial ancestry. In summary, this large-scale meta-analysis confirmed the association between the Taq1A polymorphism and AD. Future studies are required to investigate the functional significance of the ANKK1/DRD2 Taq1A polymorphism in AD.

Peter Riederer "70th birthday" neurobiological foundations of modern addiction treatment

Alcohol dependence is caused by complex interactions of multiple susceptibility genes with little effect each and environmental factors. Candidate genes influence metabolism of alcohol, such as alcohol dehydrogenase and aldehyde dehydrogenase, and modulatory transmitter systems, such as the dopaminergic, serotonergic, acetylcholinergic, gamma-aminobutyric acidergic, and various neuropeptidergic systems. Dysfunctional behavioral choices, learning, and memory are involved in the etiology of alcohol dependence. Systematic promotion and maintenance of motivation is a lifetime challenge in the treatment of alcohol use disorders. The second step of treatment management is the discontinuation of alcohol consumption. Withdrawal symptoms can be treated with gamma-aminobutyric acidergic substances such as benzodiazepines. Long-term relapse prevention is another challenge. Multimodal treatment can include naltrexone, a non-selective opioid receptor antagonist, or acamprosate, an N-methyl-D-aspartate receptor modulator, which are first line for pharmacological treatment on the basis of recent Cochrane analyses. Due to the complexity of etiology with both psychological and neurobiological factors, future treatment management of alcoholism may include the combination of individualized disorder-specific psychotherapy and drugs acting on different neuronal pathways, on the basis of individual vulnerability. However, the question remains unsolved whether an individualized approach is feasible and how subgroups should be defined.

Influence of DRD2 and ANKK1 genotypes on apomorphine-induced growth hormone (GH) response in alcohol-dependent patients

BACKGROUND

D(2) receptor function can be assessed by growth hormone (GH) response to apomorphine. Several association studies between dopamine receptor polymorphisms and results of the apomorphine challenge test with normal and alcohol-dependent subjects yielded inconsistent results. In this pilot study, we tested polymorphisms from the DRD2 region for GH response to apomorphine challenge in more detail.

METHODS

Apomorphine challenge tests measuring GH responses on 5 time points were performed on day 1 of alcohol detoxification in 43 patients with alcohol dependence; patients were genotyped for 11 polymorphisms including DRD2, ANKK1, NCAM1 and TTC12.

RESULTS

Associations (p<0.05) were found for ANKK1 (rs11604671, rs1800497) and DRD2 (rs6276, rs1076560), which are located on adjacent chromosomal positions. Consistent with PET studies suggesting a reduced D(2) receptor availability in patients carrying the ANKK1 rs1800497 T polymorphism (formerly known as DRD2 TaqI A1) we found a reduced GH response to apomorphine in those subjects.

CONCLUSION

This has been the first study showing significant associations between apomorphine-induced GH response and SNPs in DRD2 and ANKK1 in alcohol-dependent patients. In this respect, our preliminary results are in line with other reports which suggested that DRD2 and ANKK1 polymorphisms influence D(2) receptor availability and signal transduction in the dopaminergic pathways. Small sample size in our study limits the generalizability of our results.

DRD1 5'UTR variation, sex and early infant stress influence ethanol consumption in rhesus macaques

The mesolimbic dopamine system plays an important role in mediating a variety of behaviors and is involved in mediating the reinforcing effects of ethanol. Genes encoding dopamine receptor subtypes are thus good candidate loci for understanding the genetic etiologies of susceptibility to alcohol dependence and its antecedent behavioral phenotypes. We tested whether variation in DRD1 influences alcohol consumption in rhesus macaques and whether its influence is mediated by sex and early rearing experience. We genotyped a single nucleotide polymorphism (-111 G/T) in the 5'UTR of DRD1 in 96 subjects raised with their mothers until 6 months of age (n = 43) or in peer-only groups (n = 53). As young adults they underwent a 7-week voluntary ethanol consumption experiment. anova revealed a significant main effect of sex (F(1,95) = 6.3, P = 0.014) and an interaction between genotype, sex and rearing on ethanol consumption (F(7,95) = 4.63, P = 0.0002). Maternally deprived males heterozygous for the T allele consumed significantly more ethanol (P > t <or= 0.0001) than the other subgroups. Maternal deprivation can produce individuals that are anxious and impulsive, both of which are known risk factors for alcohol dependence. Our work demonstrates a potential role for the dopamine D1 receptor gene in modulating alcohol consumption, especially in the context of early environmental stress.

Gender effect on association between DRD2 polymorphism and substance dependence in a Spanish sample

Our aim was to examine a possible association between substance dependence and the TaqIA polymorphism of the D2 dopamine receptor (DRD2), a single nucleotide polymorphism (SNP) located at the 3' UTR region of the DRD2 gene. A case-control design stratified by gender was used to analyze the genotypes of this SNP in a sample of 125 substance-dependent patients according to DSM-IV and 203 blood donors recruited as controls in two general city hospitals in Madrid, Spain. Genomic DNA from peripheral blood samples was amplified through PCR to identify the variants of the SNP in the DRD2 gene. Analyses performed with Chi(2) tests revealed that the A1 allele (A1/A1 and A1/A2 genotypes) of the Taq 1A SNP of the DRD2 gene was significantly associated with substance dependence in males, but not in the whole sample. Male patients had significantly higher rates of the A1-containing genotypes than male controls. The finding of an association between substance dependence and the DRD2 gene TaqIA SNP only in males suggests the existence of gender-specific differences in the genetic underpinnings of substance dependence.

Genetic contributions to avoidance-based decisions: striatal D2 receptor polymorphisms

Individuals differ in their tendencies to seek positive decision outcomes or to avoid negative ones. At the neurobiological level, our model suggests that phasic changes in dopamine support learning to reinforce good decisions via striatal D1 receptors, and to avoid maladaptive choices via striatal D2 receptors. Accordingly, in a previous study individual differences in positive and negative learning were strongly modulated by two genetic polymorphisms factors related to striatal D1 and D2 function, respectively. Nevertheless, whereas the role for dopamine in positive learning is relatively well accepted, that in learning to avoid negative outcomes is more controversial. Here we further explore D2-receptor-related genetic contributions to probabilistic avoidance in humans, in light of recent data showing that particular DRD2 polymorphisms are associated with functional modulation of receptor expression [Zhang Y, Bertolino A, Fazio L, Blasi G, Rampino A, Romano R, Lee M-LT, Xiao T, Papp A, Wang D, Sadée W (2007) Polymorphisms in human dopamine d2 receptor gene affect gene expression, splicing, and neuronal activity during working memory. Proc Natl Acad Sci U S A 104(51):20552-20557]. We find that a promoter polymorphism rs12364283 associated with transcription and D2 receptor density was strongly and selectively predictive of avoidance-based decisions. Two further polymorphisms (rs2283265 and rs1076560) associated with relatively reduced presynaptic relative to postsynaptic D2 receptor expression were predictive of relative impairments in negative compared to positive decisions. These previously undocumented effects of DRD2 polymorphisms were largely independent of those we reported previously for the C957T polymorphism (rs6277) associated with striatal D2 density. In contrast, effects of the commonly studied Taq1A polymorphism on reinforcement-based decisions were due to indirect association with C957T. Taken together these findings suggest multiple D2-dependent genetic mechanisms contributing to avoidance. We discuss these effects in the context of neurocomputational models of reinforcement leaning in the basal ganglia.

Genetics of dopamine receptors and drug addiction: a comprehensive review

Drug dependence is a chronic, relapsing disorder in which compulsive drug-seeking and drug-taking behaviours persist despite serious negative consequences. Addictive substances, such as opioids, ethanol, psychostimulants and nicotine, induce pleasant states or relieve distress, effects that contribute to their recreational use. Dopamine is critically involved in drug addiction processes. However, the role of the various dopaminergic receptor subtypes has been difficult to delineate. Here, we will review the information collected implicating the receptors of the D1 family (DRD1 and DRD5) and of the D2 family (DRD2, DRD3 and DRD4) in drug addiction. We will summarize the distribution of these receptors in the brain, the preclinical experiments carried out with pharmacological and transgenic approaches and the genetic studies carried out linking genetic variants of these receptors to drug addiction phenotypes. A meta-analysis of the studies carried out evaluating DRD2 and alcohol dependence is also provided, which indicates a significant association. Overall, this review indicates that different aspects of the addiction phenotype are critically influenced by dopaminergic receptors and that variants of those genes seem to influence some addiction phenotypes in humans.

Association between Val66Met brain-derived neurotrophic factor (BDNF) gene polymorphism and post-treatment relapse in alcohol dependence

BACKGROUND

The purpose of this study was to examine relationships between genetic markers of central serotonin (5-HT) and dopamine function, and risk for post-treatment relapse, in a sample of alcohol-dependent patients.

METHODS

The study included 154 patients from addiction treatment programs in Poland, who met DSM-IV criteria for alcohol dependence. After assessing demographics, severity of alcohol use, suicidality, impulsivity, depression, hopelessness, and severity of alcohol use at baseline, patients were followed for approximately 1 year to evaluate treatment outcomes. Genetic polymorphisms in several genes (TPH2, SLC6A4, HTR1A, HTR2A, COMT, and BDNF) were tested as predictors of relapse (defined as any drinking during follow-up) while controlling for baseline measures.

RESULTS

Of 154 eligible patients, 123 (80%) completed follow-up and 48% (n = 59) of these individuals relapsed. Patients with the Val allele in the Val66Met BDNF polymorphism and the Met allele in the Val158Met COMT polymorphism were more likely to relapse. Only the BDNF Val/Val genotype predicted post-treatment relapse [odds ratio (OR) = 2.62; p = 0.019], and time to relapse (OR = 2.57; p = 0.002), after adjusting for baseline measures and other significant genetic markers. When the analysis was restricted to patients with a family history of alcohol dependence (n = 73), the associations between the BDNF Val/Val genotype and relapse (OR = 5.76, p = 0.0045) and time to relapse (hazard ratio = 4.93, p = 0.001) were even stronger.

CONCLUSIONS

The Val66Met BDNF gene polymorphism was associated with a higher risk and earlier occurrence of relapse among patients treated for alcohol dependence. The study suggests a relationship between genetic markers and treatment outcomes in alcohol dependence. Because a large number of statistical tests were conducted for this study and the literature on genetics and relapse is so novel, the results should be considered as hypothesis generating and need to be replicated in independent studies.

Association between the Stin2 VNTR polymorphism of the serotonin transporter gene and treatment outcome in alcohol-dependent patients

AIMS

The aim of this study was to investigate the potential association between functional polymorphisms of dopaminergic [dopamine receptor D2 (DRD2), dopamine receptor D3 (DRD3) and dopamine transporter (SLC6A3)] and serotonergic [serotonin 2A receptor (HTR2A) and serotonin transporter (SLC6A4)] genes and treatment outcome in alcohol-dependent patients.

METHODS

A total of 90 Spanish Caucasian alcohol-dependent outpatients (ICD-10 criteria) were enrolled in the study. The association between genotypes and drinking outcomes was measured over 6 months of treatment. Biomarkers of alcohol consumption, as well as alcohol consumption and its consequences, craving, disability and quality of life, were assessed. Based on those measures, we created a composite secondary measure to globally assess treatment outcome in alcoholism.

RESULTS

No association was found between DRD2, DRD3, SLC6A3 or HTR2A gene variants and treatment outcome. However, SLC6A4 STin2 12/12 carriers showed poor 6-month time point treatment outcome [32.8% in the good outcome group versus 64.0% in the poor outcome group, chi(2) (df) = 7.20 (1), corrected P = 0.042, OR (95% CI) = 0.27 (0.10-0.72)]. Nevertheless, independent analysis of each treatment group reveals that the excess of 12/12 carriers in the poor outcome group was only found in the naltrexone-treated group [24.1% versus 64.7% chi(2) (df) = 7.41 (1), corrected P = 0.042, OR (95% CI) = 0.17 (0.05-0.64)]. In the whole sample, the L-10 repeats haplotype (5-HTTLPR-STin2 VNTR) is associated with good outcome (LRT = 3.88, df = 1, P = 0.049).

CONCLUSIONS

Our findings suggest that functional polymorphism of the SLC6A4 gene may have an influence on treatment outcome in alcohol-dependent patients.

Meta-analysis of the association of the Taq1A polymorphism with the risk of alcohol dependency: a HuGE gene-disease association review

The human dopamine 2 receptor Taq1A allele has been implicated as a vulnerability factor for alcohol dependence in a number of studies and reviews. To determine whether this allele is associated with alcoholism, the authors conducted a Human Genome Epidemiology review and meta-analysis. Forty-four studies with 9,382 participants were included. An odds ratio of 1.38 (95% confidence interval: 1.20, 1.58; heterogeneity, 50.5%) was found for the A1A1 + A1A2 versus the A2A2 genotype. Sensitivity analyses suggested lack of ethnic matching as a possible source of heterogeneity; a small, significant association was detected in studies with ethnic-matched controls (odds ratio = 1.26, 95% confidence interval: 1.02, 1.56; heterogeneity, 37%). Significant associations were also found in analyses restricted to studies reporting use of blinding and those with adequate screening of controls for alcohol dependency. For the A1A1 versus the A1A2 + A2A2 genotype, the odds ratio was 1.22 (95% confidence interval: 1.05, 1.43; heterogeneity, 0%). Sensitivity analyses on groups of studies reporting use of ethnic-matched controls and those that screened controls for alcohol dependency still showed significant associations. The relatively small effect for the association of the A1 allele, or another genetic variant linked to it, with alcohol dependence indicates a multigene causality for this complex disorder.

Approach to the genetics of alcoholism: A review based on pathophysiology

Alcohol dependence is a common disorder with a heterogenous etiology. The results of family, twin and adoption studies on alcoholism are reviewed. These studies have revealed a heritability of alcoholism of over 50%. After evaluating the results, it was epidemiologically stated that alcoholism is heterogenous complex disorder with a multiple genetic background. Modern molecular genetic techniques allow examining specific genes involved in the pathophysiology of complex diseases such as alcoholism. Strategies for gene identification are introduced to the reader, including family-based and association studies. The susceptibility genes that are in the focus of this article have been chosen because they are known to encode for underlying mechanisms that are linked to the pathophysiology of alcoholism or that are important for the pharmacotherapeutic approaches in the treatment of alcohol dependence. Postulated candidate genes of the metabolism of alcohol and of the involved neurotransmitter systems are introduced. Genetic studies on alcoholism examining the metabolism of alcohol and the dopaminergic, GABAergic, glutamatergic, opioid, cholinergic and serotonergic neurotransmitter systems as well as the neuropeptide Y are presented. The results are critically discussed followed by a discussion of possible consequences.

Association of the DRD2 gene Taq1A polymorphism and alcoholism: a meta-analysis of case-control studies and evidence of publication bias

We investigated the association of the dopamine D2 receptor (DRD2) Taq1A polymorphism and alcoholism, using meta-analytic techniques, and specifically undertook an investigation of possible publication bias. Potential publication bias represents a genuine risk to the integrity of published research, but its impact has rarely been documented. We observed a small effect of the DRD2 Taq1A polymorphism on risk of alcoholism, indicating increased alcoholism in individuals possessing the A1 allele of the Taq1A polymorphism (OR=1.21, 95% CI 1.13-1.30, P<0.001). This association remained significant when data from samples of European and East Asian ancestry were analyzed separately. We did not find evidence for association in high-severity alcoholism compared to low-severity alcoholism. Removing the first published study significantly reduced the magnitude of the pooled effect size estimate, although the association remained significant. In addition, we observed evidence for possible publication bias and for the strength of individual study effect size to be inversely related to year of publication. These results support the association of the DRD2 Taq1A polymorphism with alcoholism. This conclusion is qualified by the possibility of publication bias in the literature and the observed between-study heterogeneity, which indicates that the observed association may differ in strength between populations or may not exist at all in some populations.

Striatal dopamine D1 receptors in type 1 and 2 alcoholics measured with human whole hemisphere autoradiography

A considerable number of human and animal studies have implied the importance of dopamine system and alterations in dopamine receptors in the context of alcoholism. However, it has remained unclear if the alcohol-abuse related dopaminergic deficit is specifically associated with certain receptor subtype. The aim of this study was to compare putative alterations of dopamine D(1) receptors in caudate and putamen of nine type 1 alcoholics, eight type 2 alcoholics and 10 healthy controls by using [(3)H]SCH 23390 as a radioligand in postmortem human whole hemisphere autoradiography. In addition, we compared the present results to our earlier studies on dopamine transporters and dopamine D(2) receptors in these same subjects and evaluated the putative correlations of dopamine D(1) receptor densities between the nucleus accumbens and the above-mentioned structures. Our results show that alcoholics do not have significantly different striatal dopamine D(1) receptor densities compared to controls. Neither were there any significant correlations between the dopamine D(1) receptors and the two other dopamine binding sites. However, the correlations of the dopamine D(1) receptors between nucleus accumbens and dorsal striatal structures were consistently and mostly statistically significantly positive in alcoholics, but not in controls, which may suggest some pathology related to addiction. In addition, considering the facts that dopamine D(1) receptors were more abundant in the mesolimbic nucleus accumbens than in the caudate or putamen and that there was a strong tendency towards lower binding among type 1 alcoholics may suggest the importance of dopamine D(1) receptors in reward and/or alcoholism.

Dopamine and alcoholism: neurobiological basis of ethanol abuse

The role of the dopamine (DA) system in brain reward mechanisms and the development of substance abuse has been well established. We review earlier animal and human studies on DA and alcoholism with some relevant issues relating to those studies. The present animal and human data suggest several alterations in the DA system in the context of alcoholism. Receptor studies imply that DA D(2) receptor density and function are lower at least among type 1 alcoholics, which suggests that they could benefit from drugs that enhance DAergic activity, such as partial DA agonists. These drugs could help to restore suboptimal levels of DAergic activity by reducing both the craving for alcohol in abstinence and the euphoria subsequent to alcohol's release of DA in the nucleus accumbens (NAC), thus providing negative reinforcement for relapse.

Advances in molecular genetics and the prevention and treatment of substance misuse: Implications of association studies of the A1 allele of the D2 dopamine receptor gene

Substance misuse is influenced by multiple genetic and environmental factors. Recent research has identified a number of potential genetic markers of risk and those associated with drug reward substrates show particular promise. The current study reexamines the extant published data of the association between the D2 dopamine receptor (DRD2) gene minor Taq 1A (A1) allele and substance misuse risk. A series of meta-analyses was performed on 64 studies examining DRD2 A1+ allelic status and substance misuse. In addition, personality was examined as a possible endophenotype. Significant association was found between the A1 allele and severe substance dependence in both Caucasian and non-Caucasian groups. The data did not support a significant association between the A1 allele and personality features. While the specific mechanism underlying these associations requires further elucidation, this genetic marker shows promise as a marker of brain reinforcement processes. Possible ways of utilising the A1 allele to inform prevention and treatment initiatives are discussed.

Catalog of 178 variations in the Japanese population among eight human genes encoding G protein-coupled receptors (GPCRs)

We screened DNAs from 48 Japanese individuals for single-nucleotide polymorphisms (SNPs) in eight genes encoding G protein-coupled receptors (GPCRs) by directly sequencing the entire relevant genomic regions except for repetitive-sequence elements. This approach identified 147 SNPs and 31 insertion/deletion polymorphisms among the eight GPCR genes. On average, we identified one SNP in every 584 nucleotides. Of the 147 SNPs, 69 were identified in AGTR1, 12 in AGTR2, nine in AGTRL1, 20 in AVPR1A, nine in AVPR2, 16 in DRD1, six in ITGA2B, and six in PTGIR. Twenty-one SNPs were located in 5' flanking regions, 76 in introns, 32 in exons, and 18 in 3' flanking regions. These variants should contribute to investigations of possible correlations between genotypes and phenotypes as regards susceptibility to disease or responsiveness to drug therapy.

Dopamine receptors and transporters in the brain reward circuits of type 1 and 2 alcoholics measured with human whole hemisphere autoradiography

The role of the dopamine system in brain reward mechanisms and development of substance abuse is well-established with nucleus accumbens as a key structure in mediating these effects. Several studies on alcoholism have indicated defects in dopaminergic neurotransmission and alterations in dopamine receptor densities. However, it has remained unclear if the substance abuse-related dopaminergic defect is specifically associated with a certain receptor subtype. The aim of this study was to compare putative alterations of dopamine D(1,) D(2), and D(3) receptors in nucleus accumbens, amygdala, and substantia nigra among alcoholics and controls. We studied the densities of dopamine D(1) and D(3) receptors in brains of 9 type 1 alcoholics, 8 type 2 alcoholics, and 10 healthy controls by using postmortem human whole hemisphere autoradiography. The mean densities of dopamine D(1) and D(3) receptors were at the same level in all study groups. Combining these with our previous results, our data suggest that among type 1 alcoholics dopamine transporters are lower in nucleus accumbens and dopamine D(2), but not D(1) or D(3) receptors in nucleus accumbens and amygdala. Further, the densities of all these dopamine-binding sites among type 2 alcoholics are at the level of healthy controls. The results suggest that lower dopamine receptor density is specific for D(2) receptor and for type 1 alcoholism, which supports Cloninger's neurogenetic model of two alcoholic subtypes, and indicates the importance of classifying these subgroups separately when issues related to dopaminergic activity are studied.

D2 dopamine receptor gene in psychiatric and neurologic disorders and its phenotypes

The D2 dopamine receptor (DRD2) has been one of the most extensively investigated gene in neuropsychiatric disorders. After the first association of the TaqI A DRD2 minor (A1) allele with severe alcoholism in 1990, a large number of international studies have followed. A meta-analysis of these studies of Caucasians showed a significantly higher DRD2 A1 allelic frequency and prevalence in alcoholics when compared to controls. Variants of the DRD2 gene have also been associated with other addictive disorders including cocaine, nicotine and opioid dependence and obesity. It is hypothesized that the DRD2 is a reinforcement or reward gene. The DRD2 gene has also been implicated in schizophrenia, posttraumatic stress disorder, movement disorders and migraine. Phenotypic differences have been associated with DRD2 variants. These include reduced D2 dopamine receptor numbers and diminished glucose metabolism in brains of subjects who carry the DRD2 A1 allele. In addition, pleiotropic effects of DRD2 variants have been observed in neurophysiologic, neuropsychologic, stress response, personality and treatment outcome characteristics. The involvement of the DRD2 gene in certain neuropsychiatric disorders opens up the potential of a targeted pharmacogenomic approach to the treatment of these disorders.

Association of seven polymorphisms of the D2 dopamine receptor gene with brain receptor-binding characteristics

Association of alleles at the Taq1 A, Taq1 B, intron 6, Taq1 D, exon 7, exon 8, and promoter-141C sites of the D2 dopamine receptor gene with D2 dopamine receptor binding characteristics in the caudate nucleus of Caucasian alcoholic and nonalcoholic subjects was determined. For the Taq1 D, exon 7, exon 8, and promoter- 141C sites there were no significant allelic differences in Bmax (number of binding sites) or Kd (binding affinity) of the D2 dopamine receptors. However, subjects having the minor alleles at the Taq1 A, Taq1 B, and intron 6 sites had significantly lower Bmax than subjects not having them. None of these three polymorphisms had any significant effect on Kd. Highly significant linkage disequilibria were observed among the Taq1 A, Taq1 B, and intron 6 polymorphic sites, but linkage disequilibria between these three sites and each of the Taq1 D, exon 7, exon 8, and promoter-141C sites were of lesser or of no significance. Taken together, these findings suggest that the Taq1 A, Taq1 B, and intron 6 polymorphisms, but not the Taq1 D, exon 7, exon 8, and promoter-141C polymorphisms, are in linkage disequilibrium with a functional allelic variant that affects D2 dopamine receptor expression.

Effects of alcohol detoxification on dopamine D2 receptors in alcoholics: a preliminary study

Imaging studies in patients with Type II alcohol dependence have revealed significant reductions in dopamine (DA) D2 receptor availability. Here we assessed the effects of alcohol detoxification in DA D2 receptors in alcoholic subjects. We evaluated 14 patients with Type II alcohol dependence tested within 6 weeks of detoxification and then re-tested 1-4 months later while alcohol free. The comparison group comprised 11 healthy controls. PET was used with [11C]raclopride to measure DA D2 receptors. Eight alcoholics and all control subjects were tested with a CTI 931 PET scanner and six alcoholics with a Siemens HR+ PET scanner. Data were analyzed separately for the studies done in the different scanners. Comparisons between early and late alcohol detoxification showed no significant changes in DA D2 receptor availability (B(max)/K(d)) for the studies done with the CTI and the HR+ scanners. Comparison with controls showed lower DA D2 receptor levels in caudate and putamen in alcoholics tested during early detoxification and in caudate during late detoxification. These studies replicate previous findings of lower striatal DA D2 receptors in alcoholics than in controls and absence of significant recovery during alcohol detoxification. These findings suggest that low DA D2 receptor availability in alcoholics is not due to alcohol withdrawal and may reflect a predisposing factor.

Dopaminergic dysfunction in alcoholism and schizophrenia--psychopathological and behavioral correlates

Dysfunction of central dopaminergic neurotransmission has been implicated in the pathogenesis of schizophrenia as well as drug and alcohol dependence. Different drugs of abuse stimulate dopamine release in the ventral striatum and thus reinforce drug consumption. Increased subcortical dopamine release has also been associated with the pathogenesis of positive symptoms in schizophrenia and may be driven by a prefrontal dopaminergic dysfunction. These seemingly heterogeneous findings may be explained by recent research in non-human primates. According to these studies, reward anticipation but not anticipated reward consumption is accompanied by a phasic dopamine release in the striatum and prefrontal cortex. In the striatum, phasic dopamine release primarily affects motivation, psychomotor activation and reward craving, while in the prefrontal cortex, dopaminergic stimulation is involved in the activation of working memory and reward anticipation. In alcoholism, previously neutral stimuli that have been associated with alcohol intake can become conditioned cues which activate phasic dopamine release and reward craving. In schizophrenia, stress-induced or chaotic activation of dopamine release may attribute incentive salience to otherwise irrelevant stimuli and thus be involved in the pathogenesis of delusional mood and other positive symptoms. Studies in humans and non-human primates emphasize the role of dopaminergic neurotransmission in reward anticipation and its dysfunction in different neuropsychiatric diseases.

Imaging neurochemical endophenotypes: promises and pitfalls

A large number of polymorphisms in genes coding for neurotransmitter receptors and transporters have been associated with neuropsychiatric conditions, although few of these associations have been consistently replicated. These proteins are critical targets of psychoactive drugs and the clarification of the functional significance of these polymorphisms might offer important leads for drug development and therapeutic applications. Brain imaging techniques such as single photon emission computed tomography (SPECT) and positron emission tomography (PET) provide the means to monitor the expression and function of many of these proteins in the living human brain. This paper reviews brain imaging studies designed to evaluate the significance of polymorphisms in genes coding for important drug targets (e.g., the serotonin transporter [SERT], the dopamine transporter [DAT] and the dopamine D(2) receptor) in terms of expression or function. These studies illustrate the unique opportunities, as well as the pitfalls, generated by combining genetic analysis with brain imaging studies.

Genotype effects on neurodegeneration and neuroadaptation in monoaminergic neurotransmitter systems

Neuroadaptation and neurodegeneration in central dopaminergic and serotonergic systems are central to vulnerability, process and consequences of addictive behavior. Serotonergic dysfunction has been associated with behavior disinhibition and negative mood states that may predispose to excessive alcohol intake, while alcohol-induced stimulation of dopaminergic neurotransmission may encode the reinforcing properties of alcohol consumption. Chronic alcohol intake induces neuroadaptive reductions in striatal dopamine transporter (DAT) and D2 receptor availability, which were reversible during early abstinence. A polymorphism of the DAT gene (SLC6A3) was associated with the in vivo transporter availability in the putamen of abstinent alcoholics and control subjects. The same genotype was associated with severity of alcohol withdrawal symptoms, hypothetically due to interactions of genotype and alcohol-induced neuroadaptation. Reduction in raphe serotonin transporter (5-HTT) availability was observed in abstinent male alcoholics and it may be the result of neurodegeneration rather than reversible neuroadaptation. Neurotoxic reduction in 5-HTT protein expression seems to be limited to homozygous carriers of a long, more transcriptionally active allele of a promoter repeat polymorphism of the 5-HTT gene (SCL6A4). This genotype was also associated with a low level of acute unpleasant effects of alcohol consumption, a factor predisposing to excessive alcohol intake. The time course of neuroadaptation and recovery of monoaminergic neurotransmission in alcohol intake and withdrawal imply that monoamine transporter genotype could profoundly influence alcohol-induced reinforcement and, perhaps, contribute to neurochemical changes which are long lasting or permanent.

Reward deficiency syndrome: a biogenetic model for the diagnosis and treatment of impulsive, addictive, and compulsive behaviors

The dopaminergic system, and in particular the dopamine D2 receptor, has been implicated in reward mechanisms. The net effect of neurotransmitter interaction at the mesolimbic brain region induces "reward" when dopamine (DA) is released from the neuron at the nucleus accumbens and interacts with a dopamine D2 receptor. "The reward cascade" involves the release of serotonin, which in turn at the hypothalmus stimulates enkephalin, which in turn inhibits GABA at the substania nigra, which in turn fine tunes the amount of DA released at the nucleus accumbens or "reward site." It is well known that under normal conditions in the reward site DA works to maintain our normal drives. In fact, DA has become to be known as the "pleasure molecule" and/or the "antistress molecule." When DA is released into the synapse, it stimulates a number a DA receptors (D1-D5) which results in increased feelings of well-being and stress reduction. A consensus of the literature suggests that when there is a dysfunction in the brain reward cascade, which could be caused by certain genetic variants (polygenic), especially in the DA system causing a hypodopaminergic trait, the brain of that person requires a DA fix to feel good. This trait leads to multiple drug-seeking behavior. This is so because alcohol, cocaine, heroin, marijuana, nicotine, and glucose all cause activation and neuronal release of brain DA, which could heal the abnormal cravings. Certainly after ten years of study we could say with confidence that carriers of the DAD2 receptor A1 allele have compromised D2 receptors. Therefore lack of D2 receptors causes individuals to have a high risk for multiple addictive, impulsive and compulsive behavioral propensities, such as severe alcoholism, cocaine, heroin, marijuana and nicotine use, glucose bingeing, pathological gambling, sex addiction, ADHD, Tourette's Syndrome, autism, chronic violence, posttraumatic stress disorder, schizoid/avoidant cluster, conduct disorder and antisocial behavior. In order to explain the breakdown of the reward cascade due to both multiple genes and environmental stimuli (pleiotropism) and resultant aberrant behaviors, Blum united this hypodopaminergic trait under the rubric of a reward deficiency syndrome.

The DRD2 gene in psychiatric and neurological disorders and its phenotypes

The TaqIA D2 dopamine receptor (DRD2) minor (A1) allele was first associated with severe alcoholism a decade ago. Since then, studies both confirming and not confirmnning this finding were reported. However, a meta-analysis of a large number of Caucasian alcoholics (both more severe and less severe) and controls (both assessed and unassessed for substance use disorders) revealed a significantly higher frequency (p < 10(-6)) and prevalence (p < 10(-8)) of the DRD2 A1 allele in the alcoholics. Further analysis showed that the more severe alcoholics had a 3-fold higher prevalence of the DRD2 A1 allele than the assessed controls (p < 10(-10)), whereas no difference was found between the less severe alcoholics and the unassessed controls. DRD2 exonic or promoter mutations have not yet been associated with alcoholism, although two intronic variants at the TaqIB and intron 6 sites, which are in linkage disequilibrium with the TaqIA site, were associated with this disorder. Variants of the DRD2 gene have also been associated with cocaine, nicotine and opioid dependence, obesity and gambling. It is hypothesised that the DRD2 is a reinforcement or reward gene. Although less intensively studied than substance use disorders, the DRD2 gene has been implicated in Tourette's syndrome (TS), post-traumatic stress disorder (PTSD) and certain symptoms associated with affective disorders and schizophrenia. Further, DRD2 variants have been implicated in Parkinson's disease (PD) and in iatrogenically-induced movement disorders, as well as in certain migraineurs. Phenotypic differences have been associated with DRD2 variants. These include reduced D2 dopamine receptor numbers and diminished glucose metabolism in the brain of subjects who carry the DRD2 A1 allele. In addition, phenotypic differences have been found in neurocognitive and personality characteristics, and in treatment outcome of DRD2 variants. The involvement of the DRD2 gene in certain neuropsychiatric disorders opens up the potential of a targeted pharmacogenomic approach to the prevention and treatment of these disorders.

Association study between genetic polymorphisms in the 14-3-3 eta chain and dopamine D4 receptor genes and alcoholism

BACKGROUND

The dopaminergic system may be involved in the development of alcoholism. As part of our ongoing studies on the association between alcoholism and dopaminergic genes, we report herein a mutation analysis of the 14-3-3 eta chain gene (YWHAH) and an association study between alcoholism and the YWHAH and dopamine D4 receptor gene (DRD4) polymorphisms.

METHODS

Nucleotide mutations were investigated using single-strand conformation polymorphism methods. Associations were analyzed using a case-control design involving 185 Japanese alcoholics and 286 Japanese controls.

RESULTS

Five polymorphisms, -147G>A, -134(GCCTGCA)2-4, IVS1+31(G)7-8, IVS1+73-74ins(G), and 753A>G, were detected on the YWHAH, and three of them were novel. No significant associations were found between alcoholism and these polymorphisms or two additional polymorphisms on DRD4 exon III and DRD4 -521C/T.

CONCLUSIONS

YWHAH and DRD4 do not appear to play a major role in the development of alcoholism.

Genetics of alcohol withdrawal

Alcohol withdrawal is a clinically and etiologically heterogeneous syndrome caused by a complex interaction of environmental (e.g., amount of ethanol) and genetic factors. Multiple genes are considered to be involved in various components of the syndrome, each of them contributing only modestly to withdrawal vulnerability. Association studies using candidate genes of the dopamine, serotonin, gabaergic and opioidergic systems are reviewed and methodological limitations are discussed.

Addiction and its reward process through polymorphisms of the D2 dopamine receptor gene: a review

Since 1990, association studies have amassed strong evidence implicating the D(2) dopamine receptor (DRD2) gene in alcoholism. Specifically, the TaqI A minor (A1) allele of the DRD2 gene has been associated with alcoholism. The DRD2 gene has also been found to be involved in other substance use disorders including cocaine, nicotine and opioid dependence, and obesity. Beyond association studies, pharmacologic studies have shown reduced brain D(2) dopamine receptor numbers in A1(+) allele carriers (A1A1 and A1A2 genotypes) compared to A1(-) allele carriers (A2A2 genotype). Through a number of other approaches, different phenotypes have also been identified in subjects with the A1(+) and A1(-) alleles. These include metabolic, neurophysiological, neuropsychological, personality, stress and treatment studies. It is hypothesized that in an effort to compensate for deficiencies in the dopaminergic system, substance abusers may seek to stimulate the mesocorticolimbic circuits of the brain, long thought to be important in behavioral reward and reinforcement. In effect, one form of the DRD2 gene, the A1 allele, renders the dopaminergic system inefficient and rewards substance abuse that increases brain dopamine levels.

Genome polymorphism and alcoholism

Different gene variants have been identified as risk or protective factors in alcoholism. The genes coding for dopamine receptors, serotonin transporters, and dehydrogenases represent susceptibility loci for addictive behaviour. However, alcoholism represents a complex psychiatric symptomatology which is caused by multiple factors, both genetic and environmental. Furthermore, there are probably different subtypes of alcoholism each with a distinct pathophysiology, and thus a different genetic background. Genetic research can help to identify such subtypes, which may require different therapeutic approaches. However, gene polymorphisms are not only responsible for a predisposition to alcoholism, but also for personality traits which influence the likelihood of developing addictive behaviour. Moreover, genetic polymorphisms are probably involved in the way an individual responds to treatment. Also, the severity of secondary diseases resulting from chronic alcohol uptake may depend on the genetic makeup of an individual. New treatment strategies focusing on genes contributing towards drug and alcohol dependence (such as gene therapy) are already under examination in animal models. However, further research is required before these developments will considerably change today's clinical handling of alcoholism.

mu-opioid receptor variants and dopaminergic sensitivity in alcohol withdrawal

OBJECT

The endogenous opioid system plays an important role in the reinforcing properties of alcohol by an interconnected activation of the mesolimbic dopamine system. The Asn40Asp substitution polymorphism of the human mu-opioid-receptor (OPRM) influences binding of opioids and signal transduction and may, thereby, contribute to the development of alcoholism. The present study tested whether the Asn40Asp substitution polymorphism of the OPRM gene is associated with a variation in central dopaminergic sensitivity during alcohol withdrawal in alcoholics.

METHOD

Sensitivity of central dopamine receptors was assessed by apomorphine-induced growth hormone (GH) secretion in 97 alcohol-dependent patients before and 1 week after alcohol cessation, and in a subgroup of 19 alcoholics after 3 months of abstinence. GH response was defined as area under the hormone/time curve. Comparisons of the GH response were conducted between alcoholics carrying the Asn40Asp genotype versus those with the Asn40Asn genotype using U-test statistics.

RESULTS

Marginal differences in apomorphine-induced GH response were found between both genotype groups before detoxification (P = 0.799 (n = 97)/P = 0.459 (n = 19)) and after 3 months of abstinence (P = 0.331 (n = 19)). In contrast, the GH response measured seven days after alcohol withdrawal was significantly increased in alcoholics with the Asn40Asp genotype compared with those carrying the Asn40Asn genotype (P = 0.013 (n = 97)/P = 0.026 (n = 19)).

CONCLUSION

Our results suggest that genetic variation of the mu-opioid receptor modulates the central dopaminergic sensitivity during acute alcohol withdrawal.

Alcohol and genetics: new animal models

In recent years, it has become increasingly evident that there is a genetic component to alcoholism. Attempts to isolate alcoholism genes have met with modest success, in part because alcoholism is a multigenic trait. Recently, experimental animal models and novel genetic manipulations have provided several clues as to the specific genes involved in alcoholism, and extensive research has identified many genes that might influence responses to alcohol. Although not all of these might be proven to influence drug sensitivity, research has provided evidence for the involvement of a few genes. Ultimately, findings from animal models that investigate the function of specific genes could aid the development of pharmacotherapies to treat alcohol dependence.

Alcohol, slow wave sleep, and the somatotropic axis

When alcohol is a large proportion of daily nutrient energy, the network of signals for energy homeostasis appears to adapt with abnormal patterns of sleep and growth hormone (GH) release along with gradual acquisition of an addictive physical dependency on alcohol. Early relapse during treatment of alcoholism is associated with a lower GH response to challenge, perhaps reflecting an altered balance of somatostatin (SS) to somatropin releasing hormone (GHRH) that also affects slow wave sleep (SWS) in dependent patients. Normal patterns of sleep have progressively shorter SWS episodes and longer rapid eye movement (REM) episodes during the overall sleep period, but the early sleep cycles of alcoholics have truncated or non-existent SWS episodes, and the longer REM episodes occur in early cycles. During SWS delta wave activity, the hypothalamus releases GHRH, which causes the pituitary to release GH. Alcohol-dependent patients have lower levels of SWS power and GH release than normal subjects, and efforts to understand the molecular basis for this maladaptation and its relation to continued alcohol dependence merit encouragement. More needs to be learned about the possibility of decreasing alcohol dependency by increasing SWS or enhancing GHRH action.

The D2 dopamine receptor gene: a review of association studies in alcoholism and phenotypes

The role of the D2 dopamine receptor (DRD2) gene in alcoholism and other substance use disorders has come under intense investigation since the minor TaqI A (A1) allele of the DRD2 gene was first reported to be associated with alcoholism. In a meta-analysis of 15 US and international studies of European (non-Hispanic) Caucasians, consisting of 1015 alcoholics (more severe and less severe) and 898 controls (unassessed and assessed for alcoholism), alcoholics had a higher prevalence (p < 10(-7)) and frequency (p < 10(-5)) of the A1 allele than controls. The prevalence of the A1 allele was 1.5-fold higher in more severe than less severe alcoholics (p < 10(-4)), whereas unassessed controls had a twofold higher prevalence of the A1 allele than assessed controls (p < 10(-4)). Whereas more severe alcoholics had a threefold higher A1 allelic prevalence than assessed controls (p < 10(-10)), A1 allelic prevalence was virtually identical in less severe alcoholics and in unassessed controls. The A1 allele has also been associated with other drug problems including cocaine, nicotine, and polysubstance abuse. Furthermore, the minor TaqI B (B1) allele of the DRD2 gene has been associated with alcoholism and psychostimulant (cocaine, amphetamine) abuse. Beyond association studies, phenotypic differences exist between genotypes containing the TaqI A minor (A1A1 and A1A2) and major (A2A2) alleles of the DRD2. These different phenotypes have been identified through a number of approaches, including pharmacological, neurophysiological, neuropsychological, stress, personality, metabolic, and treatment studies. In conclusion, the present review suggests that the type of alcoholics and the nature of controls used are among critical factors in DRD2 association studies in alcoholism. Intronic mutations in both the 3'(TaqI A) and 5'(TaqI B) regions of the DRD2 associate with alcoholism and other drug use disorders. The identification of phenotypes of DRD2 genotypes suggests that the observed intronic DRD2 mutations may have functional consequences that predispose individuals to a variety of substance use disorders.