The first is whether endogenously produced anandamide and 2-AG participate in the modulation of specific disease states. Drugs that block endocannabinoid inactivation should magnify this adaptive function in the same way as serotonin reuptake or monoaminooxidase inhibitors heighten the mood-regulating actions of endogenous biogenic amines. The second question is whether inhibiting endocannabinoid clearance provides a therapeutic advantage over direct activation of cannabinoid receptors with agonist drugs. The latter approach has been generally favored thus far, and several classes of subtype-selective cannabinoid agonists are already available for preclinical use . Thus, demonstrating that inhibitors of endocannabinoid inactivation possess a unique pharmacological profile is essential to justify the substantial efforts associated with the development of a new class of drugs. In the following sections, we illustrate with some examples the endocannabinoids’ rolein pathology and discuss the potential therapeutic value of drugs that target endocannabinoid inactivation. Pain. For example, in vivo microdialysis experiments have shown that peripheral injections of the chemical irritant formalin are accompanied by increases in anandamide outflow within the PAG, a brain region intimately involved in pain processing . Since activation of CB1 receptors in the PAG causes profound analgesia, it has been argued that inhibitors of anandamide inactivation “may form the basis of a modern pharmacotherapy of pain, particularly in instances where opiates are ineffective” . The fact that the endocannabinoid transport inhibitor AM404 has no antinociceptive effect in models of acute pain seems to contradict this possibility . It should be noted, however,4×4 grow table that neither AM404 nor any other inhibitor of anandamide clearance has yet been tested in animal models that are directly relevant to pathological pain states in humans.
In models that mimic such states , the CB1 receptor antagonist SR141716A exacerbates pain when administered alone, suggesting that inflammation and nerve injury may be associated with compensatory increases in cannabinergic activity . If this hypothesis is correct, one would expect endocannabinoid inactivation inhibitors to alleviate inflammatory or neuropathic pain. This possibility has not yet been tested, however. Hypotensive Shock. During hemorrhagic and septic shock, anandamide and 2-AG may be released from macrophages and platelets, activate CB1-type receptors on the surface of vascular smooth muscle cells, and produce vasodilatation . The physiological significance of this response is still unclear. Nevertheless, the fact that a CB1 antagonist reduces survival time in “shocked” rats suggests that activation of the endocannabinoid system may have beneficial effects, possibly by redistributing cardiac output to or improving microcirculation in vital organs such as the kidneys . If this is true, inhibitors of endocannabinoid inactivation that do not appear to exert direct vasoactive effects could be used to prolong life expectancy in hemorrhagic and septic shock. Disorders of Dopamine Transmission. Functional interactions between dopamine and endocannabinoids are well documented. CB1 receptors are highly expressed in CNS regions that are innervated by dopamine-releasing neurons . In one of these regions, the striatum, anandamide release is stimulated by activation of dopamine D2-family receptors . Furthermore, the CB1 antagonist SR141716A, which has no effect on motor activity when administered alone, enhances the motor hyperactivity elicited by D2-family agonists . These findings suggest that one role of the endocannabinoid system in the CNS may be to act as an inhibitory feedback mechanism countering dopamine-induced facilitation of psychomotor activity . A corollary of this idea is that drugs that prevent endocannabinoid clearance should antagonize dopamine-mediated responses.
As a test of this hypothesis, the endocannabinoid transport inhibitor AM404 was injected into the cerebral ventricles of rats that were then systemically treated with the mixed D1/D2 dopamine agonist apomorphine or the selective D2-family agonist quinpirole. AM404 blocked the yawning evoked by apomorphine and reduced the motor stimulation elicited by quinpirole. By contrast, when administered alone, AM404 produced only a mild hypokinesia, not other cannabinoid actions such as catalepsy . The effects of AM404 were also studied in juvenile spontaneously hypertensive rats . Juvenile SHR are not yet hypertensive but are hyperactive and show a number of attention deficits, which have been linked to alterations in mesocorticolimbic dopamine transmission and dopamine receptor expression . Systemic administration of AM404 normalizes the behavior of juvenile SHR without affecting that of control rats . These findings suggest that inhibitors of endocannabinoid inactivation may be used to alleviate certain symptoms of dopamine dysfunction. Clinical data showing that 9 -tetrahydrocannabinol ameliorates tics in Tourette’s syndrome patients lend further support to this possibility . Future Challenges. In conclusion, three major challenges lie before the pharmacologist interested in the mechanisms of endocannabinoid inactivation from the perspective of drug discovery. The first is the need for a deeper molecular understanding of these mechanisms. Considerable insight has been gained in the last few years on the structure and catalytic properties of AAH, but many questions remain unanswered, including the identity of the putative endocannabinoid transporter and the existence of additional hydrolytic enzymes for anandamide and 2-AG. The second challenge lies in the development of potent and selective inhibitors of endocannabinoid inactivation. Future AAH inhibitors should combine the potency of those currently available with greater pharmacological selectivity and biological availability. A second generation of endocannabinoid transport blockers that overcome the limitations of AM404 and its congeners is also needed.
The third challenge is the validation of endocannabinoid mechanisms as targets for therapeutic drugs. This task is intertwined, of course, with that of understanding the endocannabinoids’ roles in normal physiology, one on which much research is currently focused.Alcohol abuse is a global problem, constituting the seventh leading risk factor for death and disability . Worldwide, over 100 million people had an alcohol use disorder in 2016. Statistics from the National Survey on Drug Use and Health show that >85% of adults in the United States report ever having consumed alcohol, with >25% reporting binge drinking in the past month . The proportion of adults in the United States with an AUD is estimated to be 6.2% . Alcohol use behaviors are complex, and how and why people drink is partially influenced by genetic factors. However, identifying the genetic factors that increase the risk for harmful drinking has been challenging, partially because patterns of alcohol use are dynamic across the lifespan. The terms used to describe alcohol use and abuse are as diverse as the behaviors themselves. Hazardous drinking describes heavy drinking that places an individual at risk for future harm. Harmful drinking and alcohol abuse are defined as drinking that causes mental or physical damage to the individual. These descriptive terms were devised to identify individuals who would benefit from brief interventions and are assessed using screening questionnaires such as the Alcohol Use Disorders Identification Test . Alcohol dependence was, until recently, defined according to the DSM-IV and required the presence of 3 or more of 7 criteria in a 12-month period. The DSM-IV made a distinction between alcohol abuse and dependence that was removed under DSM-V and replaced with ‘mild’ to ‘severe’ definitions of AUD. Genetic studies encompass the wide range of alcohol use phenotypes; in this review we mirror the language used in the original studies. AUD can be viewed as the end point of a series of transitions ,cannabis drying system which begin with the initiation of use, continue with the escalation to hazardous drinking and culminate in compulsive harmful use that persists despite negative consequences. Genome-wide association studies have been instrumental in discovering novel genetic loci associated with multiple psychiatric conditions. In the field of AUD genetics, studies have mostly focused on either levels of consumption or AUD diagnosis. Recent GWAS have now begun to identify hundreds of genome-wide significant variants, and provide evidence that the components of alcohol use behavior have a distinct genetic architecture.In this review, we provide an overview of recent molecular genetic findings of alcohol use behaviors from the largest GWAS performed to date. Other reviews have elegantly summarized findings from twin and family studies of heritability, linkage, candidate gene and GWAS [e.g. ], and we extend on recent reviews of the molecular genetics of AUD by including additional GWAS of alcohol use behaviors that identify genome-wide significant hits . In addition, we discuss the application of polygenic methods, which provide mounting evidence that alcohol use and misuse are partially distinct. Finally, we delineate future directions to investigate the different etiologic sources that underlie the life course of alcohol use behaviors. For decades, candidate gene studies were used to determine the contribution of specific genes that increase risk for AUD.
Candidate gene studies tended to focus on genes that influenced pharmacokinetic and pharmacodynamic factors. Larger genetic studies have generally not replicated the findings from candidate gene studies . One exception to this are the genes encoding ethanol metabolizing enzymes, particularly alcohol dehydrogenase and aldehyde dehydrogenase , which have repeatedly been shown to have the largest impact on alcohol consumption and risk for AUD . As study designs have evolved to incorporate GWAS, researchers have been able to scan the whole genome without any hypotheses about the underlying biology of alcohol use behaviors. Initial efforts focused on collecting clinically defined cases of AUD, but these ascertainment strategies could not amass the large sample sizes required for GWAS . Accordingly, multi-ethnic and clinically defined samples have been combined through the Psychiatric Genomic Consortium of Substance Use Disorders working group. The efforts of the PGC-SUD have led to a trans-ancestral meta-analysis consisting of almost 15,000 AD cases and almost 38,000 controls from 28 independent cohorts , identifying a single locus , which was robustly associated with AD. More recently, using information from electronic health records to infer AUD status, a GWAS of 274,424 multi-ethnic individuals from the Million Veterans Program cohort identified 10 loci associated with AUD . Kranzler et al showed that alcohol consumption and AUD were genetically correlated but distinct, thus allowing them to adjust for consumption in the AUD GWAS and for AUD in the GWAS of consumption. In parallel with these efforts, which have focused on clinical diagnoses, other GWAS have incorporated continuous measures of alcohol use. These include self reported weekly alcohol intake or the scores from screening questionnaires such as the AUDIT . The AUDIT can be decomposed to provide a measure of alcohol use from the first 3 questions and misuse from questions 4-10 . These quantitative measures are available in large population-based cohorts such as the UK Biobank , MVP and 23andMe. The GWAS meta-analysis of AUDIT identified 10 associated risk loci . Large consortia were also formed to collate quantitative measures of alcohol use, including AlcGen and the GWAS & Sequencing Consortium of Alcohol and Nicotine Use . GSCAN have recently identified nearly 100 loci associated with alcohol consumption . The MVP study also examined alcohol consumption, allowing for an explicit comparison between AUD and consumption in a single population; of the 18 loci detected in that study, 5 were common to both AUD diagnosis and alcohol consumption. As the prior two paragraphs make clear, population based cohorts have provided larger sample sizes, which are critical for obtaining adequate power for GWAS. Their use can come at the cost of missing more severe alcohol use phenotypes. For example, the frequency of AUD in the UKB is lower than the population average [7% ], indicating that certain population studies may be under powered to detect genetic effects specific to dependence . The frequency of AUD in the MVP, on the contrary, was much higher [20%, ]. Despite these limitations, population based cohorts provide a cost-effective strategy for obtaining very large samples, compared to traditional study designs that require obtaining a diagnosis from clinically trained staff. Table 1 summarizes the most recent GWAS of alcohol use behaviors ; Figure 2 provides an overview of the chronology of these studies. Figure 3 shows that the list of genes identified by these studies is highly heterogeneous. These data suggest incomplete genetic overlap between measures of alcohol use behaviors , though ascertainment bias and limited power are likely to be additional contributing factors. The 4q23 region, which contains the genes for several alcohol metabolizing enzymes, has been associated with multiple alcohol use behaviors. This association is one of the most consistently replicated findings in the field of psychiatric genetics, although the effects are clearly ancestry-specific . There appear to be multiple signals in this region, including ADH1C , ADH4 , ADH5 and the METAP1/EIF4E region .