Caffeine promotes wakefulness via dopamine signaling in Drosophila.

Caffeine is the most widely-consumed psychoactive drug in the world, but our understanding of how caffeine affects our brains is relatively incomplete. Most studies focus on effects of caffeine on adenosine receptors, but there is evidence for other, more complex mechanisms. In the fruit fly Drosophila melanogaster, which shows a robust diurnal pattern of sleep/wake activity, caffeine reduces nighttime sleep behavior independently of the one known adenosine receptor. Here, we show that dopamine is required for the wake-promoting effect of caffeine in the fly, and that caffeine likely acts presynaptically to increase dopamine signaling. We identify a cluster of neurons, the paired anterior medial (PAM) cluster of dopaminergic neurons, as the ones relevant for the caffeine response. PAM neurons show increased activity following caffeine administration, and promote wake when activated. Also, inhibition of these neurons abrogates sleep suppression by caffeine. While previous studies have focused on adenosine-receptor mediated mechanisms for caffeine action, we have identified a role for dopaminergic neurons in the arousal-promoting effect of caffeine.

Small-molecule screen in adult Drosophila identifies VMAT as a regulator of sleep.

Sleep is an important physiological state, but its function and regulation remain elusive. In Drosophila melanogaster, a useful model organism for studying sleep, forward genetic screens have identified important sleep-modulating genes and pathways; however, the results of such screens may be limited by developmental abnormalities or lethality associated with mutation of certain genes. To circumvent these limitations, we used a small-molecule screen to identify sleep-modulating genes and pathways. We administered 1280 pharmacologically active small molecules to adult flies and monitored their sleep. We found that administration of reserpine, a small-molecule inhibitor of the vesicular monoamine transporter (VMAT) that repackages monoamines into presynaptic vesicles, resulted in an increase in sleep. Supporting the idea that VMAT is the sleep-relevant target of reserpine, we found that VMAT-null mutants have an increased sleep phenotype, as well as an increased arousal threshold and resistance to the effects of reserpine. However, although the VMAT mutants are consistently resistant to reserpine, other aspects of their sleep phenotype are dependent on genetic background. These findings indicate that small-molecule screens can be used effectively to identify sleep-modulating genes whose phenotypes may be suppressed in traditional genetic screens. Mutations affecting single monoamine pathways did not affect reserpine sensitivity, suggesting that effects of VMAT/reserpine on sleep are mediated by multiple monoamines. Overall, we identify VMAT as an important regulator of sleep in Drosophila and demonstrate that small-molecule screens provide an effective approach to identify genes and pathways that impact adult Drosophila behavior.

Association analysis between polymorphisms in the dopamine D2 receptor (DRD2) and dopamine transporter (DAT1) genes with cocaine dependence.

Genetic research on cocaine dependence (CD) may help clarify our understanding of the disorder as well as provide novel insights for effective treatment. Since dopamine neurotransmission has been shown to be involved in drug reward, related genes are plausible candidates for susceptibility to CD. The dopamine receptor D(2) (DRD2) protein and dopamine transporter (DAT1) protein play regulatory roles in dopamine neurotransmission. The TaqI A single-nucleotide polymorphism (SNP) in the DRD2 gene and the 3' variable number tandem repeat (VNTR) polymorphism in the DAT1 gene have been implicated in psychiatric disorders and drug addictions. In this study, we hypothesize that these polymorphisms contribute to increased risk for CD. Cocaine-dependent individuals (n=347) and unaffected controls (n=257) of African descent were genotyped for the polymorphisms in the DRD2 and DAT1 genes. We observed no statistically significant differences or trends in allele or genotype frequencies between cases and controls for either of the tested polymorphisms. Our study suggests that there is no association between the DRD2 and DAT1 polymorphisms and CD. However, additional studies using larger sample sizes and clinically homogenous populations are necessary before confidently excluding these variants as contributing genetic risk factors for CD.

Association between the catechol-O-methyltransferase Val158Met polymorphism and cocaine dependence.

Dopaminergic brain systems have been documented to have a major role in drug reward, thus making genes involved in these circuits plausible candidates for susceptibility to substance use disorders. The catechol-O-methyltransferase (COMT) is involved in the degradation of catecholamines and a functional polymorphism (Val158Met) has been suggested to influence enzyme activity. In this study we hypothesize that genetic variation in the COMT gene contributes to increased risk for cocaine dependence. Cocaine-dependent individuals (n=330) and screened unaffected normal controls (n=255) were genotyped for three SNPs in the COMT gene (rs737865, rs4680 (Val158Met), rs165599). All cases and controls were of African descent. Genotype and allele frequencies differed significantly for the Val158Met polymorphism between cases (f(Met)=35%) and controls (f(Met)=27%) (p=0.004; corrected p=0.014; OR 1.44; 95% CI 1.12-1.86). Haplotype analysis showed a significant association for a two-marker haplotype rs737865-Val158Met (p=0.005). Results suggest that variation in COMT increases risk for cocaine dependence. The low enzyme activity 158Met allele or haplotypes containing this variant might have functional effects on dopamine-derived reward processes and cortical functions resulting in increased susceptibility for cocaine dependence. Additional studies are required to elucidate the role of COMT in the pathophysiology of substance use disorders.

Association between polymorphisms in the vesicle-associated membrane protein-associated protein A (VAPA) gene on chromosome 18p and bipolar disorder.

Linkage studies in bipolar disorder (BPD) suggest that a susceptibility locus exists on chromosome 18p11. The vesicle-associated membrane protein-associated protein A (VAPA) gene maps to this region. VAPA interacts with presynaptic proteins and is necessary for vesicular neurotransmission. Dysregulation of synaptic neurotransmission might contribute to the pathophysiology of BPD. In this study, we hypothesize that genetic variations in the VAPA gene contribute to BPD. We tested this hypothesis by genotyping 6 SNPs (rs494015; rs29193; rs29162; rs29145; rs29067; rs29066) in BPD patients (n = 570) and healthy controls (n = 730). Genotype and allele frequencies were compared between groups using Chi square contingency analysis. Linkage disequilibrium (LD) between markers was calculated and estimated haplotype frequencies were compared between groups. Single marker analysis revealed an association of rs29067 and rs29066 with BPD; however, after permutation correction, only rs29066 showed a trend towards an allelic association (P = 0.066). Haplotype analysis did not show any significant association with disease after permutation correction. Our results provide suggestive evidence of an association between SNPs in the 3'UTR of the VAPA gene and BPD. Interestingly, these SNPs are in close proximity to the microsatellite marker D18S464, which showed significant signals in previous linkage studies of BPD. Additional studies are necessary to confirm and elucidate the role of VAPA as a susceptibility gene for BPD on chromosome 18p.

Genetic variants in the cocaine- and amphetamine-regulated transcript gene (CARTPT) and cocaine dependence.

Dopaminergic brain systems have been implicated to play a major role in drug reward, thus making genes involved in these circuits plausible candidates for susceptibility to substance use disorders. The cocaine- and amphetamine-regulated transcript peptide (CARTPT) is involved in reward and feeding behavior and has functional characteristics of an endogenous psychostimulant. In this study we tested the hypothesis that variation in the CARTPT gene increases susceptibility to cocaine dependence in individuals of African descent. Genotypes of three HapMap tagging SNPs (rs6894758; rs11575893; rs17358300) across the CARTPT gene region were obtained in cocaine dependent individuals (n=348) and normal controls (n=256). All subjects were of African descent. There were no significant differences in allele, genotype or haplotype frequencies between cases and controls for any of the tested SNPs. Our results do not support an association of the CARTPT gene with cocaine dependence; however, additional studies using larger samples, comprehensive SNP coverage, and different populations are necessary to conclusively rule out CARTPT as a contributing factor in the etiology of cocaine dependence.