Kora-Mareen Bühler

Common single nucleotide variants underlying drug addiction: more than a decade of research.

Drug‐related phenotypes are common complex and highly heritable traits. In the last few years, candidate gene (CGAS) and genome‐wide association studies (GWAS) have identified a huge number of single nucleotide polymorphisms (SNPs) associated with drug use, abuse or dependence, mainly related to alcohol or nicotine. Nevertheless, few of these associations have been replicated in independent studies. The aim of this study was to provide a review of the SNPs that have been most significantly associated with alcohol‐, nicotine‐, cannabis‐ and cocaine‐related phenotypes in humans between the years of 2000 and 2012. To this end, we selected CGAS, GWAS, family‐based association and case‐only studies published in peer‐reviewed international scientific journals (using the PubMed/MEDLINE and Addiction GWAS Resource databases) in which a significant association was reported. A total of 371 studies fit the search criteria. We then filtered SNPs with at least one replication study and performed meta‐analysis of the significance of the associations. SNPs in the alcohol metabolizing genes, in the cholinergic gene cluster CHRNA5‐CHRNA3‐CHRNB4, and in the DRD2 and ANNK1 genes, are, to date, the most replicated and significant gene variants associated with alcohol‐ and nicotine‐related phenotypes. In the case of cannabis and cocaine, a far fewer number of studies and replications have been reported, indicating either a need for further investigation or that the genetics of cannabis/cocaine addiction are more elusive. This review brings a global state‐of‐the‐art vision of the behavioral genetics of addiction and collaborates on formulation of new hypothesis to guide future work.

The genetic basis of the endocannabinoid system and drug addiction in humans

The cannabinoid receptor (CNR1) and the fatty acid amide hydrolase (FAAH) genes are located on chromosomes 6 and 1 in the 6q15 and 1p33 cytogenetic bands, respectively. CNR1 encodes a seven-transmembrane domain protein of 472 amino acids, whereas FAAH encodes one transmembrane domain of 579 amino acids. Several mutations found in these genes lead to altered mRNA stability and transcription rate or a reduction of the activity of the encoded protein. Increasing evidence shows that these functional mutations are related to dependence upon cocaine, alcohol, marijuana, heroin, nicotine and other drugs. One of the most compelling associations is with the C385A single nucleotide polymorphism (SNP), which is found in the FAAH gene. For all of the genetic polymorphisms reviewed here, it is difficult to form overall conclusions due to the high diversity of population samples being studied, ethnicity, the use of volunteers, heterogeneity of the recruitment criteria and the drug addiction phenotype studied. Care should be taken when generalizing the results from different studies. However, many works have repeatedly associated polymorphisms in the CNR1 and FAAH genes with drug-related behaviours; this suggests that these genes should be examined in further genetic studies focusing on drug addiction and other psychiatric disorders.

C957T polymorphism of the dopamine D2 receptor gene is associated with motor learning and heart rate

Genetic variants that are related to the dopaminergic system have been frequently found to be associated with various neurological and mental disorders. Here, we studied the relationships between some of these genetic variants and some cognitive and psychophysiological processes that are implicated in such disorders. Two single nucleotide polymorphisms were chosen: one in the dopamine D2 receptor gene (rs6277‐C957T) and one in the catechol‐O‐methyltransferase gene (rs4680‐Val158Met), which is involved in the metabolic degradation of dopamine. The performance of participants on two long‐term memory tasks was assessed: free recall (declarative memory) and mirror drawing (procedural motor learning). Heart rate (HR) was also monitored during the initial trials of the mirror‐drawing task, which is considered to be a laboratory middle‐stress generator (moderate stress), and during a rest period (low stress). Data were collected from 213 healthy Caucasian university students. The C957T C homozygous participants showed more rapid learning than the T allele carriers in the procedural motor learning task and smaller differences in HR between the moderate‐ and the low‐stress conditions. These results provide useful information regarding phenotypic variance in both healthy individuals and patients.

Cocaine reverses the naltrexone-induced reduction in operant ethanol self-administration: The effects on immediate-early gene expression in the rat prefrontal cortex

Naltrexone is a clinically approved medication for alcoholism. We aimed to investigate the effectiveness of naltrexone co-administered with cocaine and the association of these substances with immediate-early gene expression in the rat prefrontal cortex. We used chronic operant ethanol self-administration and oral treatments prescribed for alcoholism and available in pharmacies to maximise the predictive validity in humans. We performed real-time PCR analysis to determine gene expression levels in the prefrontal cortex. Only the highest dose of naltrexone (1, 3, and 10 mg/kg, p.o.) reduced the response to ethanol. Cocaine increased ethanol self-administration in a dose-dependent manner (2.5, 10, 20 mg/kg, i.p.) and reversed the naltrexone-induced reduction. Naltrexone failed to prevent the cocaine-induced increase in locomotor activity observed in these animals. Chronic self-administration of ethanol reduced the expression of the C-fos gene 4- to 12-fold and increased expression of the COX-2 (up to 4-fold) and Homer1a genes in the rat prefrontal cortex. Chronic ethanol self-administration is prevented by naltrexone, but cocaine fully reverses this effect. This result suggests that cocaine may overcome naltrexones effectiveness as a treatment for alcoholism. The ethanol-induced reduction in C-fos gene expression in the prefrontal cortex reveals an abnormal activity of these neurons, which may be relevant in the compulsive consumption of ethanol, the control of reward-related areas and the behavioural phenotype of ethanol addiction.

Effects of topiramate on ethanol‐cocaine interactions and DNA methyltransferase gene expression in the rat prefrontal cortex

Recent and ongoing clinical studies have indicated that topiramate (Topamax®) could be effective in treating ethanol or cocaine abuse. However, the effects of topiramate on the co‐administration of ethanol and cocaine remain largely unknown.

Nalmefene is effective at reducing alcohol seeking, treating alcohol-cocaine interactions and reducing alcohol-induced histone deacetylases gene expression in blood.

The opioid antagonist nalmefene (selincro®) was approved for alcohol‐related disorders by the European Medicines Agency in 2013. However, there have been no studies regarding the effectiveness of nalmefene when alcohol is used in combination with cocaine.

Red Bull® energy drink increases consumption of higher concentrations of alcohol

Mixing alcohol with caffeinated energy drinks is a common practice, especially among young people. In humans, the research on this issue has mainly focused on the use of the mass‐marketed energy drinks themselves, whereas in animal models, it has focused on the individual effects of their active ingredients (i.e. caffeine). Here, we have characterized how Red Bull®, one of the most consumed caffeinated energy drink worldwide, modulates operant alcohol self‐administration in Wistar rats. We found that animals readily and steadily responded for Red Bull (mean: 90 responses, 30 minutes and fixed‐ratio 1), which was accompanied by locomotor stimulating effects (26 percent increase). The higher the concentration of alcohol (3–20 percent), the higher the consumption of alcohol (g/kg) and associated blood alcohol levels (91.76 percent) in the mixed Red Bull–alcohol group (60 percent increase). Blood caffeine levels in the Red Bull group were 4.69 μg/ml and 1.31 μg/ml in the Red Bull–alcohol group after the 30‐minute session. Because Red Bull also contains 11 percent sucrose, we examined the time course of blood glucose as well as insulin and corticosterone. The correlation between intake of Red Bull and blood glucose levels was higher at 90 minutes than 5 minutes after its consumption, and there was no relationship with blood insulin or blood corticosterone levels. Red Bull did not alter extinction and reacquisition of responding for alcohol nor did it affect relapse‐like drinking. Overall, our results suggest that Red Bull might be a vulnerability factor to develop alcoholism given that it intensifies the consumption of higher concentrations of alcohol.

PPARα/CB1 receptor dual ligands as a novel therapy for alcohol use disorder: Evaluation of a novel oleic acid conjugate in preclinical rat models

Graphical abstract Figure. No Caption available. ABSTRACT Recent studies have demonstrated the utility of drugs modulating the endogenous cannabinoid system to control excessive alcohol intake. Among them, drugs interacting with acylethanolamide receptors including cannabinoid CB1 receptor antagonists/inverse agonists, peroxisome proliferator‐activated receptor alpha (PPAR&agr;) agonists or peroxisome proliferator‐activated receptor gamma (PPAR&ggr;) agonists have demonstrated utility in the reduction of alcohol intake in animal models. However, few studies have addressed the potential utility of combining these classes of drugs, especially because of expected safety problems. In the present work we took the advantage of the availability of two novel dual ligands for these receptors, to test the hypothesis that these types of drugs might reproduce and even improve the pharmacological profile of those drugs interacting with single targets. To this end we tested (R)‐3‐[(4‐Benzyl‐2‐oxooxazolidin‐3‐yl)methyl]‐N‐[4‐(dodecylcarbamoyl)phenyl]benzamide (NF 10–360), a dual PPAR&agr;/&ggr; agonist, and N‐[1‐(3,4‐dihydroxyphenyl)propan‐2‐yl]oleamide (OLHHA), a dual CB1 receptor antagonist/PPAR&agr; agonist, in animal models of alcohol consumption. Both drugs were effective in reducing alcohol intake and alcohol self‐administration, being OLHHA a very potent alcohol intake inhibitor (EC50 0.2 mg/kg). OLHHA also reduced self‐administration of the opioid oxycodone. OLHHA actions on alcohol self‐administration were replicated in alcohol‐preferring Marchigian‐Sardinian msP rats. Repeated administration of OLHHA did result neither in tolerance nor in toxicological or deleterious metabolic changes in the liver of msP rats. These data support the feasibility of developing novel dual ligands interacting with cannabinoid targets to treat alcohol use disorder in humans.