The dopamine D(4) receptor: one decade of research

DRD4 polymorphism moderates the effect of alcohol consumption on social bonding

Development of interpersonal relationships is a fundamental human motivation, and behaviors facilitating social bonding are prized. Some individuals experience enhanced reward from alcohol in social contexts and may be at heightened risk for developing and maintaining problematic drinking. We employed a 3 (group beverage condition) ×2 (genotype) design (N = 422) to test the moderating influence of the dopamine D4 receptor gene (DRD4 VNTR) polymorphism on the effects of alcohol on social bonding. A significant gene x environment interaction showed that carriers of at least one copy of the 7-repeat allele reported higher social bonding in the alcohol, relative to placebo or control conditions, whereas alcohol did not affect ratings of 7-absent allele carriers. Carriers of the 7-repeat allele were especially sensitive to alcohol's effects on social bonding. These data converge with other recent gene-environment interaction findings implicating the DRD4 polymorphism in the development of alcohol use disorders, and results suggest a specific pathway by which social factors may increase risk for problematic drinking among 7-repeat carriers. More generally, our findings highlight the potential utility of employing transdisciplinary methods that integrate genetic methodologies, social psychology, and addiction theory to improve theories of alcohol use and abuse.

Blockade of dopamine d4 receptors attenuates reinstatement of extinguished nicotine-seeking behavior in rats

Since cloning of the dopamine receptor D4 (DRD4), its role in the brain has remained unclear. It has been reported that polymorphism of the DRD4 gene in humans is associated with reactivity to cues related to tobacco smoking. However, the role of DRD4 in animal models of nicotine addiction has seldom been explored. In our study, male Long-Evans rats learned to intravenously self-administer nicotine under a fixed-ratio (FR) schedule of reinforcement. Effects of the selective DRD4 antagonist L-745,870 were evaluated on nicotine self-administration behavior and on reinstatement of extinguished nicotine-seeking behavior induced by nicotine-associated cues or by priming injections of nicotine. L-745,870 was also tested on reinstatement of extinguished food-seeking behavior as a control. In addition, the selective DRD4 agonist PD 168,077 was tested for its ability to reinstate extinguished nicotine-seeking behavior. Finally, L-745,870 was tested in Sprague Dawley rats trained to discriminate administration of 0.4 mg/kg nicotine from vehicle under an FR schedule of food delivery. L-745,870 significantly attenuated reinstatement of nicotine-seeking induced by both nicotine-associated cues and nicotine priming. In contrast, L-745,870 did not affect established nicotine self-administration behavior or reinstatement of food-seeking behavior induced by food cues or food priming. L-745,870 did not produce nicotine-like discriminative-stimulus effects and did not alter discriminative-stimulus effects of nicotine. PD 168,077 did not reinstate extinguished nicotine-seeking behavior. As DRD4 blockade by L-745,870 selectively attenuated both cue- and nicotine-induced reinstatement of nicotine-seeking behavior, without affecting cue- or food-induced reinstatement of food-seeking behavior, DRD4 antagonists are potential therapeutic agents against tobacco smoking relapse.

SOCIAL ADVERSITY, GENETIC VARIATION, STREET CODE, AND AGGRESSION: A GENETICLLY INFORMED MODEL OF VIOLENT BEHAVIOR

Elijah Anderson (1997, 1999) argues that exposure to extreme community disadvantage, residing in "street" families, and persistent discrimination encourage many African Americans to develop an oppositional culture that he labels the "code of the street." Importantly, while the adverse conditions described by Anderson increase the probability of adopting the code of the street, most of those exposed to these adverse conditions do not do so. The present study examines the extent to which genetic variation accounts for these differences. Although the diathesis-stress model guides most genetically informed behavior science, the present study investigates hypotheses derived from the differential susceptibility perspective (Belsky & Pluess, 2009). This model posits that some people are genetically predisposed to be more susceptible to environmental influence than others. An important implication of the model is that those persons most vulnerable to adverse social environments are the same ones who reap the most benefit from environmental support. Using longitudinal data from a sample of several hundred African American males, we examined the manner in which variants in three genes - 5-HTT, DRD4, and MAOA - modulate the effect of community and family adversity on adoption of the street code and aggression. We found strong support for the differential susceptibility perspective. When the social environment was adverse, individuals with these genetic variants manifested more commitment to the street code and aggression than those with other genotypes, whereas when adversity was low they demonstrated less commitment to the street code and aggression than those with other genotypes.

Dopamine transporter (DAT1) and dopamine receptor D4 (DRD4) genotypes differentially impact on electrophysiological correlates of error processing

Recent studies as well as theoretical models of error processing assign fundamental importance to the brain's dopaminergic system. Research about how the electrophysiological correlates of error processing—the error-related negativity (ERN) and the error positivity (Pe)—are influenced by variations of common dopaminergic genes, however, is still relatively scarce. In the present study, we therefore investigated whether polymorphisms in the DAT1 gene and in the DRD4 gene, respectively, lead to interindividual differences in these error processing correlates. One hundred sixty participants completed a version of the Eriksen Flanker Task while a 26-channel EEG was recorded. The task was slightly modified in order to increase error rates. During data analysis, participants were split into two groups depending on their DAT1 and their DRD4 genotypes, respectively. ERN and Pe amplitudes after correct responses and after errors as well as difference amplitudes between errors and correct responses were analyzed. We found a differential effect of DAT1 genotype on the Pe difference amplitude but not on the ERN difference amplitude, while the reverse was true for DRD4 genotype. These findings are in line with predictions from theoretical models of dopaminergic transmission in the brain. They furthermore tie results from clinical investigations of disorders impacting on the dopamine system to genetic variations known to be at-risk genotypes.

Evaluation of 18F-labeled benzodioxine piperazine-based dopamine D4 receptor ligands: lipophilicity as a determinate of nonspecific binding

Derivatization of the putative neuroleptic 1-(2,3-dihydrobenzo[1,4]dioxin-6-yl)-4-(4-fluorobenzyl)piperazine (3a) led to a series of new dopamine receptor D4 ligands displaying high affinity (Ki=1.1-15 nM) and D2/D4 subtype selectivities of about 800-6700. These ligands were labeled with the short-lived positron emitter fluorine-18 and analyzed for their potential application for imaging studies by positron emission tomography (PET). In vitro autoradiography was used to determine their nonspecific binding behavior as a result of their structural and thus physicochemical properties. The biodistribution, in vivo stability, and brain uptake of the most promising D4 radioligand candidate were determined. This proved to be 1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-4-((6-fluoropyridin-3-yl)methyl)piperazine ([18F]3d), which revealed an excellent binding pattern with a high selectivity and limited nonspecific binding in vitro. This analogue also exhibited a high stability and an extremely high brain uptake in vivo with specific binding in hippocampus, cortex, colliculus, and cerebellum as determined by ex vivo autoradiography. Thus, [18F]3d appears as a suitable D4 radioligand for in vivo imaging, encouraging continued evaluation by PET studies.

Dopaminergic modulation of oscillatory network inhibition in the rat basolateral amygdala depends on initial activity state

The amygdala receives dopaminergic innervation, and dopamine (DA) enhances various activities in cognitive and emotional behaviors. Periodic bursts of spontaneous inhibitory postsynaptic currents (IPSCs) with a low (<1 Hz) inter-event frequency have been observed in projection neurons of the basolateral nucleus of the amygdala (BL). Blockade of ionotropic glutamate receptors or GABA(A) receptors abolishes these oscillatory IPSC bursts in the BL, suggesting that the activity has a network origin. Here, we investigated dopaminergic modulation of the oscillatory network inhibition in rat brain slices. We evaluated the effects of DA receptor agonists and antagonists on the network inhibition; the resultant changes were quantified by integrated power spectral density (0.1-3.0 Hz). DA enhanced the power when its initial activity was low, but reduced it when the activity was initially robust. These changes in the power were accompanied by changes in burst IPSC amplitude. D1-like receptor agonist SKF 38393, or DA together with the D2-like receptor antagonist sulpiride, reproduced DA's facilitatory actions. D2-like receptor agonist quinpirole did not change the periodic IPSC burst activity of the high baseline power, though D(4) receptor agonist PD 168077, or DA together with the D1-like receptor antagonist SCH 23390, reduced its activity. These results suggest that: 1) dopaminergic modulation of the oscillatory network inhibition depends on its initial activity; and 2) facilitatory and suppressing effects of DA in the BL are mediated by D1-like receptors and D(4) receptors, respectively.

DRD2 and DRD4 in relation to regular alcohol and cannabis use among adolescents: does parenting modify the impact of genetic vulnerability? The TRAILS study

AIMS

The aims of the present study were to determine the direct effect of DRD2 and DRD4, as well as their interaction with parenting (i.e. rejection, overprotection and emotional warmth), on the development of regular alcohol and cannabis use in 1192 Dutch adolescents from the general population.

METHODS

Information was obtained by self-report questionnaires. Perceived rejection, overprotection and emotional warmth were assessed at age 10-12. Regular alcohol and cannabis use were determined at age 15-18 and defined as the consumption of alcohol on 10 or more occasions in the past four weeks, and the use of cannabis on 4 or more occasions in the past four weeks. Models were adjusted for age, sex, parental alcohol or cannabis use, and externalizing behavior.

RESULTS

Carrying the A1 allele of the DRD2 TaqIA polymorphism, or the 7 repeat DRD4, was not directly related to regular alcohol or cannabis use. In addition, adolescent carriers of these genetic risk markers were not more susceptible to the influence of less optimal parenting. Main effects for parenting indicated that overprotection increased the risk of regular alcohol use, whereas the risk of cannabis use was enhanced by parental rejection and buffered by emotional warmth.

CONCLUSIONS

Our findings do not support an association between DRD2/DRD4 and regular alcohol and cannabis use in adolescents. Given the substance-specific influences of rejection, overprotection and emotional warmth, these parenting factors might be promising candidates for prevention work.

Psychopathological aspects of dopaminergic gene polymorphisms in adolescence and young adulthood

Dopamine hypotheses of several psychiatric disorders are based upon the clinical benefits of drugs affecting dopamine transporter or receptors, and have prompted intensive candidate gene research within the dopaminergic system during the last two decades. The aim of this review is to survey the most important findings concerning dopaminergic gene polymorphisms in attention deficit hyperactivity disorder (ADHD), Tourette syndrome (TS), obsessive compulsive disorder, and substance abuse. Also, genetic findings of related phenotypes, such as inattention, impulsivity, aggressive behavior, and novelty seeking personality trait are presented, because recent studies have applied quantitative trait measures using questionnaires, symptom scales, or other objective endophenotypes. Unfortunately, genetic variants with minor effects are problematic to detect in these complex inheritance disorders, often leading to contradictory results. The most consistent association findings relate to ADHD and the dopamine transporter and the dopamine D4 receptor genes. Meta-analyses also support the association between substance abuse and the D2 receptor gene. The dopamine catabolizing enzyme genes, such as monoamine oxidase (MAO) A and catechol-O-methyltransferase (COMT) genes, have been linked to aggressive behaviors.

Sensitive determination of dopamine in the presence of uric acid and ascorbic acid using TiO2 nanotubes modified with Pd, Pt and Au nanoparticles

A simple modified TiO(2) nanotubes electrode was fabricated by electrodeposition of Pd, Pt and Au nanoparticles. The TiO(2) nanotubes electrode was prepared using the anodizing method, followed by modifying Pd nanoparticles onto the tubes surface, offering a uniform conductive surface for electrodeposition of Pt and Au. The performance of the modified electrode was characterized by cyclic voltammetry and differential pulse voltammetry methods. The Au/Pt/Pd/TiO(2) NTs modified electrode represented a high sensitivity towards individual detection of dopamine as well as simultaneous detection of dopamine and uric acid using 0.1 M phosphate buffer solution (pH 7.00) as the base solution. In both case, electro-oxidation peak currents of dopamine were linearly related to accumulated concentration over a wide concentration range of 5.0 × 10(-8) to 3.0 × 10(-5) M. However in the same range of dopamine concentration, the sensitivity had a significant loss at Pt/Pd/TiO(2) NTs electrode, suggesting the necessity for Au nanoparticles in modified electrode. The limit of the detection was determined as 3 × 10(-8) M for dopamine at signal-to-noise ratio equal to 3. Furthermore, the Au/Pt/Pd/TiO(2) NTs modified electrode was able to distinguish the oxidation response of dopamine, uric acid and ascorbic acid in mixture solution of different acidity. It was shown that the modified electrode possessed a very good reproducibility and long-term stability. The method was also successfully applied for determination of DA in human urine samples with satisfactory results.

L-DRD4 genotype not associated with sensation seeking, gambling performance and startle reactivity in adolescents: the TRAILS study

The purpose of the present study was to investigate whether a length polymorphism in the dopamine receptor D4 gene (DRD4) was associated with approach related traits in adolescents. Data were used from TRAILS (TRacking Adolescents' Individual Lives Survey), a population based cohort of Dutch adolescents. Sensation seeking, assessed with personality questionnaires from the participants themselves and their biological father and mother (n=1282) was not associated with DRD4 genotype. Gambling performance (n=591) and startle reactivity (n=432) were not associated with DRD4 genotype either. Explanations for the dissociation might be sought in differences in development of the limbic system and the prefrontal cortex, both with high dopamine receptor D4 densities and both involved in approach related behaviours.

The dopamine D(4) receptor, the ultimate disordered protein

The human D4 dopamine receptor is a synaptic neurotransmitter receptor responsible for neuronal signaling in the mesolimbic system of the brain, an area of the brain that regulates emotion and complex behavior. Its structure makes it a very unusual and interesting G protein-coupled receptor (GPCR) as it has several polymorphic variants of its gene in the region encoding the third intracellular loop (IL3). This region contains from two to seven or more similar 48 base pair repeats. These repeats cause this protein to have a very high disorder index and this, in turn, makes it very interactive with other proteins. Among GPCRs in general, the unusually proline-rich IL3 is unique to the D4 receptor (D4R). We believe that, as in the D2R, this region of the receptor plays a role in it's interaction with other receptors.

The neurochemistry of human aggression

Various data from scientific research studies conducted over the past three decades suggest that central neurotransmitters play a key role in the modulation of aggression in all mammalian species, including humans. Specific neurotransmitter systems involved in mammalian aggression include serotonin, dopamine, norepinephrine, GABA, and neuropeptides such as vasopressin and oxytocin. Neurotransmitters not only help to execute basic behavioral components but also serve to modulate these preexisting behavioral states by amplifying or reducing their effects. This chapter reviews the currently available data to present a contemporary view of how central neurotransmitters influence the vulnerability for aggressive behavior and/or initiation of aggressive behavior in social situations. Data reviewed in this chapter include emoiric information from neurochemical, pharmaco-challenge, molecular genetic and neuroimaging studies.

Neurogenetics and pharmacology of learning, motivation, and cognition

Many of the individual differences in cognition, motivation, and learning-and the disruption of these processes in neurological conditions-are influenced by genetic factors. We provide an integrative synthesis across human and animal studies, focusing on a recent spate of evidence implicating a role for genes controlling dopaminergic function in frontostriatal circuitry, including COMT, DARPP-32, DAT1, DRD2, and DRD4. These genetic effects are interpreted within theoretical frameworks developed in the context of the broader cognitive and computational neuroscience literature, constrained by data from pharmacological, neuroimaging, electrophysiological, and patient studies. In this framework, genes modulate the efficacy of particular neural computations, and effects of genetic variation are revealed by assays designed to be maximally sensitive to these computations. We discuss the merits and caveats of this approach and outline a number of novel candidate genes of interest for future study.

Homeostatic regulation of glutamatergic transmission by dopamine D4 receptors

Alterations of synaptic transmission have been considered a core feature of mental disorders; thus, we examined the role of dopamine D(4) receptors, which is highly implicated in attention-deficit hyperactivity disorder and schizophrenia, in regulating synaptic functions of prefrontal cortex, a brain region critical for cognitive and emotional processes. We found that D(4) stimulation caused a profound depression or potentiation of AMPA receptor-mediated excitatory synaptic transmission in prefrontal cortex pyramidal neurons when their activity was elevated or dampened, respectively, which was accompanied by a D(4)-induced decrease or increase of AMPARs at synapses. The dual effects of D(4) on AMPAR trafficking and function was dependent on the D(4)-mediated bidirectional regulation of CaMKII activity via coupling to distinct signaling pathways, which provides a unique mechanism for D(4) receptors to serve as a homeostatic synaptic factor to stabilize cortical excitability.

The DRD4 gene and severity of tics and comorbid symptoms: main effects and interactions with delivery complications

In this study, we investigated the role of the dopamine receptor D4 (DRD4) 48-base pairs (bp) variable number of tandem repeats (VNTR) and perinatal adversities regarding severity of tics and comorbid symptoms in children with tic disorders. We genotyped 110 children with tics with regard to the 48-bp VNTR and assessed presence of prenatal smoking exposure, and pregnancy and delivery complications by parent questionnaires. We examined associations between 2, 3, 4, and 7 repeat (R) alleles and severity of tics and comorbid obsessive-compulsive, depressive, anxious, and autistic symptoms. Through linear regressions, we investigated whether perinatal adversities and the 2R, 3R, 4R, and 7R alleles would interact with severity ratings of tics or comorbid symptoms as outcome. Presence of a 2R allele was related to more severe obsessive-compulsive symptoms, and presence of a 3R allele to increased severity of autistic features. Pregnancy complications were associated with decreased obsessive-compulsive symptom severity, and prenatal smoking exposure to more severe depressive and autistic symptoms. In children without a 3R allele delivery complications were associated with more severe tics, but in children with a 3R variant an inverse relation between delivery complications and tic severity was found. Moreover, the relation between delivery complications and internalizing symptom severity appeared to be most pronounced in children with a 2R allele. In conclusion, this study provides evidence for a role of the 48-bp VNTR in the etiology of tic and associated disorders, and for interactions with delivery complications regarding severity of tics and co-occurring internalizing symptoms.

Cognitive and emotional processing associated with the Season of Birth and dopamine D4 receptor gene

The 7-repeats variant of the dopamine D4 receptor (7R) VNTR polymorphism has been associated with higher novelty seeking (NS) and disadvantageous decision making in the Iowa Gambling Task (IGT). Season of Birth (SOB) is a significant determinant of NS. SOB and L-DRD4 genetic polymorphism may independently and interactively influence similar behaviors through their common effects on the dopaminergic system. Two hundred and twenty-seven healthy males grouped in summer-born/4-repeats (4R) (n=75), winter-born/4R (n=90), summer-born/7R (n=31) and winter-born/7R (n=31) groups, completed multimodal assessment for personality, planning for problem solving and decision making. Winter-born/7R subjects had significantly worse IGT performance throughout the task compared to 4R individuals, while summer-born 7R subjects had intermediate, although not significantly different performance. Moreover, winter-born/7R subjects had increased behavioral approach to reward without parallel reduction in sensitivity to fear or to social approval cues. The DRD4-by-SOB groups did not differ in planning for problem solving. These results suggest that a DRD4-by-SOB interaction is associated with increased behavioral approach to reward and risk taking but efficient problem solving. In addition, these results further support the hypothesis that SOB modifies the behavioral expression of dopaminergic genetic polymorphism. SOB should be included in future studies of risky behaviors and behavioral genetic studies of the dopamine system.

Regulation of inhibitory synapses by presynaptic D₄ dopamine receptors in thalamus

Dopamine (DA) receptors are the principal targets of drugs used in the treatment of schizophrenia. Among the five DA receptor subtypes, the D(4) subtype is of particular interest because of the relatively high affinity of the atypical neuropleptic clozapine for D(4) compared with D(2) receptors. GABA-containing neurons in the thalamic reticular nucleus (TRN) and globus pallidus (GP) express D(4) receptors. TRN neurons receive GABAergic afferents from globus pallidus (GP), substantia nigra pars reticulata (SNr), and basal forebrain as well as neighboring TRN neuron collaterals. In addition, TRN receives dopaminergic innervations from substantia nigra pars compacta (SNc); however, the role of D(4) receptors in neuronal signaling at inhibitory synapses is unknown. Using whole cell recordings from in vitro pallido-thalamic slices, we demonstrate that DA selectively suppresses GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSCs) evoked by GP stimulation. The D(2)-like receptor (D(2,3,4)) agonist, quinpirole, and selective D(4) receptor agonist, PD168077, mimicked the actions of DA. The suppressive actions of DA and its agonists were associated with alterations in paired pulse ratio and a decrease in the frequency of miniature IPSCs, suggesting a presynaptic site of action. GABA(A) receptor agonist, muscimol, induced postsynaptic currents in TRN neurons were unaltered by DA or quinpirole, consistent with the presynaptic site of action. Finally, DA agonists did not alter intra-TRN inhibitory signaling. Our data demonstrate that the activation of presynaptic D(4) receptors regulates GABA release from GP efferents but not TRN collaterals. This novel and selective action of D(4) receptor activation on GP-mediated inhibition may provide insight to potential functional significance of atypical antipsychotic agents. These findings suggest a potential heightened TRN neuron activity in certain neurological conditions, such as schizophrenia and attention deficit hyperactive disorders.

The DRD4 receptor Exon 3 VNTR and 5' SNP variants and mRNA expression in human post-mortem brain tissue

Genetic variation within the dopamine D4 receptor (DRD4) gene has been implicated in many neuropsychiatric disorders and behavioral traits. This variation includes the extensively studied exon 3 variably numbered tandem repeat (VNTR), and several 5' polymorphisms including a120-bp duplication and two single-nucleotide polymorphisms at -521 C/T (rs1800955) and -616 C/G (rs747302). Several reports have provided evidence for a functional role for some of these variants using in vitro techniques. This study investigated the functionality of these polymorphisms in 28 human post-mortem brain tissue samples by quantifying DRD4 mRNA expression in relation to genotype. No statistically significant relationship between genotype and mRNA expression levels was found for these four polymorphisms although a weak trend toward the 7-repeat of the exon 3 VNTR reducing DRD4 mRNA expression was found. Employing post-mortem brain tissue, rather than using in vitro techniques may provide a more relevant paradigm to study functional effects of reported risk alleles.

Dopamine D4 receptors modulate brain metabolic activity in the prefrontal cortex and cerebellum at rest and in response to methylphenidate

Methylphenidate (MP) is widely used to treat attention deficit hyperactivity disorder (ADHD). Variable number of tandem repeats polymorphisms in the dopamine D4 receptor (D(4)) gene have been implicated in vulnerability to ADHD and the response to MP. Here we examined the contribution of dopamine D4 receptors (D4Rs) to baseline brain glucose metabolism and to the regional metabolic responses to MP. We compared brain glucose metabolism (measured with micro-positron emission tomography and [(18)F]2-fluoro-2-deoxy-D-glucose) at baseline and after MP (10 mg/kg, i.p.) administration in mice with genetic deletion of the D(4). Images were analyzed using a novel automated image registration procedure. Baseline D(4)(-/-) mice had lower metabolism in the prefrontal cortex (PFC) and greater metabolism in the cerebellar vermis (CBV) than D(4)(+/+) and D(4)(+/-) mice; when given MP, D(4)(-/-) mice increased metabolism in the PFC and decreased it in the CBV, whereas in D(4)(+/+) and D(4)(+/-) mice, MP decreased metabolism in the PFC and increased it in the CBV. These findings provide evidence that D4Rs modulate not only the PFC, which may reflect the activation by dopamine of D4Rs located in this region, but also the CBV, which may reflect an indirect modulation as D4Rs are minimally expressed in this region. As individuals with ADHD show structural and/or functional abnormalities in these brain regions, the association of ADHD with D4Rs may reflect its modulation of these brain regions. The differential response to MP as a function of genotype could explain differences in brain functional responses to MP between patients with ADHD and healthy controls and between patients with ADHD with different D(4) polymorphisms.

The dopamine D4 receptor: biochemical and signalling properties

Dopamine is an important neurotransmitter that regulates several key functions in the brain, such as motor output, motivation and reward, learning and memory, and endocrine regulation. Dopamine does not mediate fast synaptic transmission, but rather modulates it by triggering slow-acting effects through the activation of dopamine receptors, which belong to the G-protein-coupled receptor superfamily. Besides activating different effectors through G-protein coupling, dopamine receptors also signal through interaction with a variety of proteins, collectively termed dopamine receptor-interacting proteins. We focus on the dopamine D4 receptor, which contains an important polymorphism in its third intracellular loop. This polymorphism has been the subject of numerous studies investigating links with several brain disorders, such as attention-deficit hyperactivity disorder and schizophrenia. We provide an overview of the structure, signalling properties and regulation of dopamine D4 receptors, and briefly discuss their physiological and pathophysiological role in the brain.

DRD4 and DAT1 in ADHD: Functional neurobiology to pharmacogenetics

Attention deficit/hyperactivity disorder (ADHD) is a common and potentially very impairing neuropsychiatric disorder of childhood. Statistical genetic studies of twins have shown ADHD to be highly heritable, with the combination of genes and gene by environment interactions accounting for around 80% of phenotypic variance. The initial molecular genetic studies where candidates were selected because of the efficacy of dopaminergic compounds in the treatment of ADHD were remarkably successful and provided strong evidence for the role of DRD4 and DAT1 variants in the pathogenesis of ADHD. However, the recent application of non-candidate gene strategies (eg, genome-wide association scans) has failed to identify additional genes with substantial genetic main effects, and the effects for DRD4 and DAT1 have not been replicated. This is the usual pattern observed for most other physical and mental disorders evaluated with current state-of-the-art methods. In this paper we discuss future strategies for genetic studies in ADHD, highlighting both the pitfalls and possible solutions relating to candidate gene studies, genome-wide studies, defining the phenotype, and statistical approaches.

Monitoring antisaccades: inter-individual differences in cognitive control and the influence of COMT and DRD4 genotype variations

Conscious monitoring of behavior is an essential control function for adaptation and learning. Antisaccade performance was investigated in a large sample of young healthy men in two tasks, one that required conscious error monitoring and one that did not. Conscious error monitoring did not lead to changes in error rate between the two tasks, while other antisaccade parameters were significantly modulated. Application of signal detection theory showed a large inter-individual variability in error detection sensitivity: the majority of individuals were unable to monitor antisaccade errors (chance error detection group), while a minority that successfully monitored their errors (non-chance error detection group) were worse in antisaccade performance in both tasks. These results were explained by the hypothesis of two modes of antisaccade processing favored by each one of the two groups: a mode of conscious cortical cognitive control leading to error monitoring, worse performance and no post-error adaptation and a mode of non-conscious subcortical control leading to chance error monitoring, post-error slowing and better performance of the antisaccade task. This hypothesis was corroborated by the results of the genotype analysis. Error-monitoring sensitivity in the non-chance error detection group was modulated by COMT genotype variations that in turn did not have an effect on error rate. On the other hand, DRD4 genotype variations were related to differences in antisaccade error rate while not affecting error-monitoring sensitivity.

Association of VNTR polymorphisms in the MAOA promoter and DRD4 exon 3 with heroin dependence in male Chinese addicts

OBJECTIVE

To explore the involvement of variable number tandem repeat (VNTR) polymorphisms in the monoamine oxidase A (MAOA) promoter and exon 3 of the dopamine D4 receptor (DRD4) gene in heroin addiction modulate the vulnerability of individuals to heroin addiction.

METHODS

Eight hundred and ninety-four male heroin addicts without other psychiatric disorders, were recruited as subjects. Another community 180 males were selected randomly as controls.

RESULTS

The geno-distribution of the DRD4 exon 3 VNTR polymorphism in controls was in Hardy-Weinberg equilibrium (HWEchi(2)=0.925), but the distribution in heroin addicts was not (HWEchi(2)=28.35). The long-repeat alleles of the DRD4 exon 3 VNTR polymorphism were found more frequently in the heroin addicts (P=0.019). However, the long-repeat alleles of the MAOA promoter VNTR polymorphism were not (P=0.828). No interaction between these two VNTR polymorphisms was found by using multiple logistic regression analysis (P=0.261).

CONCLUSION

The long-repeat allelic variants (>4-repeats) and 2-repeat allele of the DRD4 exon 3 VNTR polymorphism might be risk alleles for individual vulnerability to heroin addiction in Chinese men, but the MAOA promoter VNTR polymorphism does not mean that the partial dominant inherited mode might involved in the genetics of heroin dependence.

Polymorphisms in dopamine system genes are associated with individual differences in attention in infancy

Knowledge about the functional status of the frontal cortex in infancy is limited. This study investigated the effects of polymorphisms in four dopamine system genes on performance in a task developed to assess such functioning, the Freeze-Frame task, at 9 months of age. Polymorphisms in the catechol-O-methyltransferase (COMT) and the dopamine D4 receptor (DRD4) genes are likely to impact directly on the functioning of the frontal cortex, whereas polymorphisms in the dopamine D2 receptor (DRD2) and dopamine transporter (DAT1) genes might influence frontal cortex functioning indirectly via strong frontostriatal connections. A significant effect of the COMT valine(1)methionine (Val 158 Met) polymorphism was found. Infants with the Met/Met genotype were significantly less distractible than infants with the Val/Val genotype in Freeze-Frame trials presenting an engaging central stimulus. In addition, there was an interaction with the DAT1 3; variable number of tandem repeats polymorphism; the COMT effect was present only in infants who did not have two copies of the DAT1 10-repeat allele. These findings indicate that dopaminergic polymorphisms affect selective aspects of attention as early as infancy and further validate the Freeze-Frame task as a frontal cortex task.

Dopamine and serotonin systems modify environmental effects on human behavior: a review

The relative influences of genetic and environmental factors in the development of human behavior have been a long-term topic for an intense debate. Recent behavioral genetic studies suggest focusing on the joint effect of genes and environment, and especially on the life-course developmental interplay between nature and nurture. Vulnerability to environmental adversities and sensitivity to its benefits may be conditional on genetic background, and regarding psychological outcomes, these kinds of gene x environment interactions may be of higher importance than direct gene-trait associations. In our recent series of studies, we have shown that different variants of serotonergic and dopaminergic genes may moderate the influence of environmental conditions on a range of psychological outcomes, i.e. temperament, depression, hostility, and educational attainment. These studies suggest that depending on their genotype, people may be differentially sensitive to the environmental conditions they encounter. In the light of these results it seems highly plausible that the effects of genes may become evident only when studied in the context of environmental factors.

Polymorphisms of the mu-opioid receptor and dopamine D4 receptor genes and subjective responses to alcohol in the natural environment

Polymorphisms of the mu-opioid receptor (OPRM1) and dopamine D4 receptor (DRD4) genes are associated with subjective responses to alcohol and urge to drink under laboratory conditions. This study examined these associations in the natural environment using ecological momentary assessment. Participants were non-treatment-seeking heavy drinkers (n = 112, 52% female, 61% alcohol dependent) who enrolled in a study of naltrexone effects on craving and drinking in the natural environment. Data were culled from 5 consecutive days of drinking reports prior to medication randomization. Analyses revealed that, after drinking, carriers of the Asp40 allele of the OPRM1 gene reported higher overall levels of vigor and lower levels negative mood, as compared to homozygotes for the Asn40 variant. Carriers of the long allele (i.e., >or=7 tandem repeats) of the DRD4 endorsed greater urge to drink than homozygotes for the short allele. Effects of OPRM1 and DRD4 variable-number-of-tandem-repeats genotypes appear to be alcohol dose-dependent. Specifically, carriers of the DRD4-L allele reported slight decreases in urge to drink at higher levels of estimated blood alcohol concentration (eBAC), and Asp40 carriers reported decreases in vigor and increases in negative mood as eBAC rose, as compared to carriers of the major allele for each gene. Self-reported vigor and urge to drink were positively associated with alcohol consumption within the same drinking episode. This study extends findings on subjective intoxication, urge to drink, and their genetic bases from controlled laboratory to naturalistic settings.

Thyroid hormone and adrenergic signaling interact to control pineal expression of the dopamine receptor D4 gene (Drd4)

Dopamine plays diverse and important roles in vertebrate biology, impacting behavior and physiology through actions mediated by specific G-protein-coupled receptors, one of which is the dopamine receptor D4 (Drd4). Here we present studies on the >100-fold daily rhythm in rat pineal Drd4 expression. Our studies indicate that Drd4 is the dominant dopamine receptor gene expressed in the pineal gland. The gene is expressed in pinealocytes at levels which are approximately 100-fold greater than in other tissues, except the retina, in which transcript levels are similar. Pineal Drd4 expression is circadian in nature and under photoneural control. Whereas most rhythmically expressed genes in the pineal are controlled by adrenergic/cAMP signaling, Drd4 expression also requires thyroid hormone. This advance raises the questions of whether Drd4 expression is regulated by this mechanism in other systems and whether thyroid hormone controls expression of other genes in the pineal gland.

Association between dopaminergic polymorphisms and borderline personality traits among at-risk young adults and psychiatric inpatients

BACKGROUND

In the development of borderline personality disorder (BPD) both genetic and environmental factors have important roles. The characteristic affective disturbance and impulsive aggression are linked to imbalances in the central serotonin system, and most of the genetic association studies focused on serotonergic candidate genes. However, the efficacy of dopamine D2 receptor (DRD2) blocking antipsychotic drugs in BPD treatment also suggests involvement of the dopamine system in the neurobiology of BPD.

METHODS

In the present study we tested the dopamine dysfunction hypothesis of impulsive self- and other-damaging behaviors: borderline and antisocial traits were assessed by Structured Clinical Interview for Diagnosis (SCID) for DSM-IV in a community-based US sample of 99 young adults from low-to-moderate income families. For the BPD trait analyses a second, independent group was used consisting of 136 Hungarian patients with bipolar or major depressive disorder filling out self-report SCID-II Screen questionnaire. In the genetic association analyses the previously indicated polymorphisms of the catechol-O-methyl-transferase (COMT Val158Met) and dopamine transporter (DAT1 40 bp VNTR) were studied. In addition, candidate polymorphisms of the DRD2 and DRD4 dopamine receptor genes were selected from the impulsive behavior literature.

RESULTS

The DRD2 TaqI B1-allele and A1-allele were associated with borderline traits in the young adult sample (p = 0.001, and p = 0.005, respectively). Also, the DRD4 -616 CC genotype appeared as a risk factor (p = 0.02). With severity of abuse accounted for in the model, genetic effects of the DRD2 and DRD4 polymorphisms were still significant (DRD2 TaqIB: p = 0.001, DRD2 TaqIA: p = 0.008, DRD4 -616 C/G: p = 0.002). Only the DRD4 promoter finding was replicated in the independent sample of psychiatric inpatients (p = 0.007). No association was found with the COMT and DAT1 polymorphisms.

CONCLUSIONS

Our results of the two independent samples suggest a possible involvement of the DRD4 -616 C/G promoter variant in the development of BPD traits. In addition, an association of the DRD2 genetic polymorphisms with impulsive self-damaging behaviors was also demonstrated.

ADHD candidate gene (DRD4 exon III) affects inhibitory control in a healthy sample

BACKGROUND

Dopamine is believed to be a key neurotransmitter in the development of attention-deficit/hyperactivity disorder (ADHD). Several recent studies point to an association of the dopamine D4 receptor (DRD4) gene and this condition. More specifically, the 7 repeat variant of a variable number of tandem repeats (VNTR) polymorphism in exon III of this gene is suggested to bear a higher risk for ADHD. In the present study, we investigated the role of this polymorphism in the modulation of neurophysiological correlates of response inhibition (Go/Nogo task) in a healthy, high-functioning sample.

RESULTS

Homozygous 7 repeat carriers showed a tendency for more accurate behavior in the Go/Nogo task compared to homozygous 4 repeat carriers. Moreover, 7 repeat carriers presented an increased nogo-related theta band response together with a reduced go-related beta decrease.

CONCLUSIONS

These data point to improved cognitive functions and prefrontal control in the 7 repeat carriers, probably due to the D4 receptor's modulatory role in prefrontal areas. The results are discussed with respect to previous behavioral data on this polymorphism and animal studies on the impact of the D4 receptor on cognitive functions.

Dopamine D4 receptor (D4R) deletion in mice does not affect operant responding for food or cocaine

In this study we examined the genetic contribution of the D4R in food and cocaine self-administration using D4R mice. Mice were examined for operant responding to food pellets or intravenous cocaine. Compared to wild-type mice (D4R(+/+)), both heterozygous (D4R(+/-)) and knockout (D4R(-/-)) mice showed no difference in responding for food or cocaine. Our findings suggest that the D4R is not directly involved in mediating operant response behaviors for food or cocaine.

Environmental and genetic influences on early attachment

Attachment theory predicts and subsequent empirical research has amply demonstrated that individual variations in patterns of early attachment behaviour are primarily influenced by differences in sensitive responsiveness of caregivers. However, meta-analyses have shown that parenting behaviour accounts for about one third of the variance in attachment security or disorganisation. The exclusively environmental explanation has been challenged by results demonstrating some, albeit inconclusive, evidence of the effect of infant temperament. In this paper, after reviewing briefly the well-demonstrated familial and wider environmental influences, the evidence is reviewed for genetic and gene-environment interaction effects on developing early attachment relationships. Studies investigating the interaction of genes of monoamine neurotransmission with parenting environment in the course of early relationship development suggest that children's differential susceptibility to the rearing environment depends partly on genetic differences. In addition to the overview of environmental and genetic contributions to infant attachment, and especially to disorganised attachment relevant to mental health issues, the few existing studies of gene-attachment interaction effects on development of childhood behavioural problems are also reviewed. A short account of the most important methodological problems to be overcome in molecular genetic studies of psychological and psychiatric phenotypes is also given. Finally, animal research focusing on brain-structural aspects related to early care and the new, conceptually important direction of studying environmental programming of early development through epigenetic modification of gene functioning is examined in brief.

Understanding genes, environment and their interaction in attention-deficit hyperactivity disorder: is there a role for neuroimaging?

Attention-deficit hyperactivity disorder (ADHD) has an established heritable component, but identifying the genes involved has proven difficult. To date, the two most investigated risk genes in ADHD are the DRD4 and DAT1-genes. However, individual risk genes have only explained up to 1% of the variance in the phenotype, suggesting that they represent only relatively small risk factors for ADHD. As such, the role of environmental factors, gene-gene and gene-environment interactions are being investigated. However, studies have not always been able to address the neurobiological mechanisms by which environmental factors and interactions with genes exert their effect on the ADHD-phenotype. Neuroimaging is being used as a tool to investigate the neurobiological effects of individual risk genes. We suggest it could also be applied to investigate the mechanisms involved in environmental effects and interactions between genetic and environmental factors.

Genetic variability in the dopamine system (dopamine receptor D4, catechol-O-methyltransferase) modulates neurophysiological responses to gains and losses

BACKGROUND

Interindividual variability in the processing of reward might be partially explained by genetic differences in the dopamine system. Here, we study whether brain responses (event-related potentials [ERPs], oscillatory activity) to monetary gains and losses in normal human subjects are modulated as a function of two dopaminergic polymorphisms (catechol-O-methyltransferase [COMT] valine [Val]158methionine [Met], dopamine receptor D4 [DRD4] single nucleotide polymorphism [SNP] -521).

METHODS

Forty participants homozygous for the different alleles of both polymorphisms were selected from a larger population to assess the main effects and interactions. Based on the phasic/tonic dopamine hypothesis, we expected increased brain responses to losses and gains in participants homozygous for the Val/Val variant of the COMT polymorphism (related to higher enzyme activity).

RESULTS

The medial frontal negativity (MFN) of the ERP and the increase in beta power for gains were enhanced for participants homozygous for the COMT ValVal allele when compared with homozygous MetMet participants. In contrast, no modulations in gain- and loss-related brain activity were found to be a function of the DRD4 SNP -521 polymorphism.

CONCLUSIONS

The results demonstrate the role of the COMT Val/Met polymorphism in the processing of reward, consistent with theoretical explanations that suggest the possible role of dopamine in the MFN and beta power increase generation. In addition, the present results might agree with the phasic/tonic dopamine theory that predicts higher phasic dopamine responses in ValVal participants.

F15063, a potential antipsychotic with dopamine D2/D3 receptor antagonist, 5-HT1A receptor agonist and dopamine D4 receptor partial agonist properties: influence on neuronal firing and neurotransmitter release

F15063 (N-[(2,2-dimethyl-2,3-dihydro-benzofuran-7-yloxy)-ethyl]-(3-cyclopenten-1-yl-benzyl)-amine) is a potential antipsychotic with dopamine D2/D3 receptor antagonist, 5-HT1A receptor agonist and dopamine D4 receptor partial agonist properties. Herein, we compared its effects on rat ventral tegmental area dopamine and dorsal raphe serotonin electrical activity with those of the dopamine D2 receptor partial agonist/5-HT1A receptor agonist, SSR181507. Further, we investigated the modulation of extracellular dopamine and noradrenaline in the medial prefrontal cortex and serotonin in the hippocampus of freely moving rats by F15063 using in vivo microdialysis. In the ventral tegmental area, F15063 (200-700 microg/kg, i.v.) did not alter the electrical activity of dopamine neurons whereas SSR181507 (250-1000 microg/kg, i.v.) partially inhibited it, consistent with dopamine D2 receptor partial agonism. Both compounds reduced the inhibition of firing rate induced by the full agonist apomorphine. In the dorsal raphe, both ligands suppressed firing activity, consistent with agonism at 5-HT1A autoreceptors, although SSR181507 (25-75 microg/kg, i.v.) was more potent than F15063 (100-300 microg/kg, i.v.). F15063 (0.63-40 mg/kg, i.p.) dose-dependently increased dopamine levels in the prefrontal cortex and decreased hippocampal 5-HT. These effects were reversed by the selective 5-HT1A receptor antagonist WAY100635 (0.16 mg/kg, s.c.), indicating that they were mediated by 5-HT1A receptors (at post- and pre-synaptic levels, respectively). In the medial prefrontal cortex, noradrenaline levels were moderately but significantly increased by F15063 at 2.5 mg/kg. In conclusion, whereas SSR181507 exhibits (partial) agonism at dopamine D2 and 5-HT1A receptors, F15063 blocks dopamine D2-like receptors whilst activating 5-HT1A receptors. Such a profile distinguishes F15063 from SSR181507 and currently available antipsychotic drugs.

Genetic association studies of performance monitoring and learning from feedback: the role of dopamine and serotonin

Performance monitoring is essential for optimization of action outcomes. Research consistently implicates the posterior medial frontal cortex, particularly the rostral cingulate zone, in monitoring for unfavorable action outcomes, signaling the need for adjustments and learning from feedback. Current theories suggest that phasic dopaminergic signals coding unexpected positive or negative outcomes play a major role in this function. Here, I review EEG, neuroimaging and computational modeling studies making use of polymorphisms of candidate genes affecting neurotransmission, with a specific focus on dopamine. Although the evidence is still rather sparse, findings speak for a prominent role of dopamine in performance monitoring. However, the exact function in cortical areas underlying this function, such as the rostral cingulate zone, remains to be determined. Different hypotheses on the interaction of the rostral cingulate zone, the striatum, and the mesencephalic dopaminergic nuclei are discussed.

The dopamine D Receptor (DRD4) gene exon III polymorphism, problematic alcohol use and novelty seeking: direct and mediated genetic effects

The present study sought to integrate convergent lines of research on the associations among the dopamine D(4) receptor (DRD4) gene, novelty seeking and drinking behaviors with the overall goal of elucidating genetic influences on problematic drinking in young adulthood. Specifically, this study tested a model in which novelty seeking mediated the relationship between DRD4 variable number of tandem repeats (VNTR) genotype and problematic alcohol use. Participants (n = 90, 40 females) were heavy-drinking college students. Analyses using a structural equation modeling framework suggested that the significant direct path between DRD4 VNTR genotype and problematic alcohol use was reduced to a trend level in the context of a model that included novelty seeking as a mediator, thereby suggesting that the effects of DRD4 VNTR genotype on problematic alcohol use among heavy-drinking young adults were partially mediated by novelty seeking. Cross-group comparisons indicated that the relationships among the model variables were not significantly different in models for men versus women. These results extend recent findings of the association between this polymorphism of the DRD4 receptor gene, problematic alcohol use and novelty seeking. These findings may also help elucidate the specific pathways of risk associated with genetic influences on alcohol use and abuse phenotypes.

The DRD4 exon 3 VNTR polymorphism and addiction-related phenotypes: a review

In addition to the large literatures on associations of the DRD4 VNTR polymorphism with ADHD and personality traits, there is an emerging literature linking this variant to addiction and addiction-related phenotypes. When only diagnosis-based studies are considered, an inconsistent picture emerges raising doubts as to the relevance of this polymorphism to addiction. However the use of multiple levels of analysis in examining the importance of this polymorphism has raised the possibility of an urge-related "intermediate phenotype" that puts one at risk for developing addiction but may not be found in all persons with an addiction diagnosis. From cellular assays through neuroimaging and behavioral phenotypes, these studies highlight the power of the "intermediate phenotype" approach and suggest a possible explanation of the mixed findings when diagnosis is used as the phenotype. Strengths and weaknesses of alternative DRD4 VNTR genotype grouping strategies are discussed. In sum, converging evidence across multiple methodologies supports the possibility of a robust relationship between the DRD4 exon 3 VNTR polymorphism and urge for addictive substances.