Victor Echeverry-Alzate

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.

Phosphodiesterase7 Inhibition Activates Adult Neurogenesis in Hippocampus and Subventricular Zone In Vitro and In Vivo.

The phosphodiesterase 7 (PDE7) enzyme is one of the enzymes responsible for controlling intracellular levels of cyclic adenosine 3′,5′‐monophosphate in the immune and central nervous system. We have previously shown that inhibitors of this enzyme are potent neuroprotective and anti‐inflammatory agents. In addition, we also demonstrated that PDE7 inhibition induces endogenous neuroregenerative processes toward a dopaminergic phenotype. Here, we show that PDE7 inhibition controls stem cell expansion in the subgranular zone of the dentate gyrus of the hippocampus (SGZ) and the subventricular zone (SVZ) in the adult rat brain. Neurospheres cultures obtained from SGZ and SVZ of adult rats treated with PDE7 inhibitors presented an increased proliferation and neuronal differentiation compared to control cultures. PDE7 inhibitors treatment of neurospheres cultures also resulted in an increase of the levels of phosphorylated cAMP response element binding protein, suggesting that their effects were indeed mediated through the activation of the cAMP/PKA signaling pathway. In addition, adult rats orally treated with S14, a specific inhibitor of PDE7, presented elevated numbers of proliferating progenitor cells, and migrating precursors in the SGZ and the SVZ. Moreover, long‐term treatment with this PDE7 inhibitor shows a significant increase in newly generated neurons in the olfactory bulb and the hippocampus. Also a better performance in memory tests was observed in S14 treated rats, suggesting a functional relevance for the S14‐induced increase in SGZ neurogenesis. Taken together, our results indicate for the first time that inhibition of PDE7 directly regulates proliferation, migration and differentiation of neural stem cells, improving spatial learning and memory tasks. Stem Cells 2017;35:458–472

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.

A spontaneous deletion of α-synuclein is associated with an increase in CB1 mRNA transcript and receptor expression in the hippocampus and amygdala: effects on alcohol consumption.

α‐Synuclein (α‐syn) protein and endocannabinoid CB1 receptors are primarily located in presynaptic terminals. An association between α‐syn and CB1 receptors has recently been established in Parkinsons disease, but it is completely unknown whether there is an association between these two proteins in alcohol addiction. Therefore, we aimed to examine the α‐syn mRNA transcript and protein expression levels in the prefrontal cortex, striatum, amygdala and hippocampus. These brain regions are the most frequently implicated in alcohol and other drug addiction. In these studies, we used C57BL/6 mice carrying a spontaneous deletion of the α‐syn gene (C57BL/6Snca‐/‐) and their respective controls (C57BL/6Snca+/+). These animals were monitored for spontaneous alcohol consumption (3–10%) and their response to a hypnotic‐sedative dose of alcohol (3 g kg−1) was also assessed. Compared with the C57BL/6Snca+/+ mice, we found that the C57BL/6Snca‐/‐ mice exhibited a higher expression level of the CB1 mRNA transcript and CB1 receptor in the hippocampus and amygdala. Furthermore, C57BL/6Snca‐/‐ mice showed an increase in alcohol consumption when offered a 10% alcohol solution. There was no significant difference in sleep time after the injection of 3 g/kg alcohol. These results are the first to reveal an association between α‐syn and the CB1 receptor in the brain regions that are most frequently implicated in alcohol and other drug addictions. Synapse 00:000–000, 2013.

Developmentally-induced hypothyroidism alters the expression of Egr-1 and Arc genes and the sensitivity to cannabinoid agonists in the hippocampus. Possible implications for memory and learning

We analyzed the role of the cannabinoid system in the cognitive deficits caused by developmentally-induced hypothyroidism. We studied in control and hypothyroid rats the effect of a cannabinoid agonist on spatial memory, hippocampal phosphorylation of CREB and expression of early genes. Our results show that, 1-basal hippocampal expression of early genes and spatial learning are decreased in hypothyroid rats; 2-hypothyroid rats are very sensitive to cannabinoid agonists. Low dose of cannabinoid agonist ineffective in controls altered spatial memory, CREBs phosphorylation and early gene expression in hypothyroids. These effects are not due a change in CB1 receptor (CB1R) content. 3-Treatment of hypothyroid rats with thyroid hormones normalized the biochemical and behavioral responses to cannabinoid agonists but did not correct the low basal levels of early gene transcripts and the deficits in spatial learning. All these data suggest that the hippocampal deregulation of early genes expression could play an important role in the basal cognitive deficits of hypothyroid rats.

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.