Strengths and limitations of genetic models of ADHD

The cause and pathophysiology of attention-deficit hyperactivity disorder (ADHD) are unknown, but compelling evidence suggests an involvement of genetic factors. While dopamine is believed to play a major role in ADHD, the role for norepinephrine and serotonin systems has also been indicated. Mutant mice are valuable tools to dissect the contribution of specific neurotransmitter systems to brain dysfunction and particularly useful to decode complex multi-transmitter interaction that is critical to the pathophysiology of ADHD. Genetically altered mice provided also an opportunity to test experimentally the role of novel candidate genes for this disorder identified in genetic clinical studies. While it is clear that no rodent model would be able to recapitulate fully the complex nature of ADHD, certain endophenotypes could be reasonably well mimicked in these models. Multiple studies have reported associations between polymorphisms in dopamine transporter (DAT) gene and ADHD. Although the functional consequences of these associations are still unclear, it is believed that alterations in DAT-mediated processes might contribute to the pathogenesis of ADHD. Mice lacking the dopamine transporter have elevated dopaminergic tone and represent a genetic animal model in which certain endophenotypes of ADHD can be recapitulated. These mutants as well as other mouse models of DAT dysfunction provided an opportunity to investigate the neuronal circuitry and molecular mechanisms involved in the inhibitory action of psychostimulants on hyperactivity. Several additional knockout and transgenic mouse models have been proposed to model ADHD. Strengths and limitations of currently available genetic mouse models of ADHD are discussed.     

Is the treatment with psychostimulants in children and adolescents with attention deficit hyperactivity disorder harmful for the dopaminergic system?

A major concern regarding psychostimulant medication (amphetamine and methylphenidate) in the treatment of children and adolescents with attention deficit/hyperactivity disorder (ADHD) are the potential adverse effects to the developing brain, particularly in regard to dopaminergic brain function. The present review focuses on the pharmacology of these psychostimulants, their mode of action in the human brain and their potential neurotoxic effects to the developing brain in animals, particularly concerning DA brain function. The potential clinical significance of these findings for the treatment of ADHD in children and adolescents is discussed. Studies on sensitization to psychostimulants’ rewarding effects, which is a process expected to increase the risk of substance abuse in humans, are not included. The available findings in non-human primates support the notion that the administration of amphetamine and methylphenidate with procedures simulating clinical treatment conditions does not lead to long-term adverse effects in regard to development, neurobiology or behaviour as related to the central dopaminergic system.     

Transgenic mice expressing a human mutant beta1 thyroid receptor are hyperactive, impulsive, and inattentive

Attention deficit hyperactivity disorder (ADHD) is the most commonly diagnosed childhood psychiatric disorder. We have found that a transgenic mouse bearing a human mutant thyroid receptor (TRbeta1) expresses all of the defining symptoms of ADHD--inattention, hyperactivity, and impulsivity--as well as a 'paradoxical' response to methylphenidate (MPH). As with ADHD, the behavioral phenotypes expressed by the TRbeta transgenic mice are dynamic and sensitive to changes in environmental conditions, stress, and reinforcement. TRbeta transgenic mice are euthyroid except for a brief period during postnatal development, but the behavioral phenotypes, elevated dopamine turnover, and paradoxical response to MPH persist into adulthood. Thus, like the vast majority of children with ADHD, the TRbeta transgenic mice exhibit the symptoms of ADHD in the complete absence of thyroid abnormalities. This suggests that even transient perturbations in developmental thyroid homeostasis can have long-lasting behavioral and cognitive consequences, including producing the full spectrum of symptoms of ADHD.     

Role of neurotrophic factors in attention deficit hyperactivity disorder

Neurotrophins (NTs), a family of proteins including nerve growth factor, brain-derived neurotrophic factor (BDNF), neurotrophin-3, and neurotrophin-4, are essential for neural growth, survival, and differentiation, and are therefore crucial for brain development. Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterized by problems of inattention and/or hyperactivity-impulsivity. ADHD is one of the most common childhood onset psychiatric disorders. Studies have suggested that both genetic and environmental factors influence the development of the disorder, although the precise causes of ADHD have not yet been identified. In this review, we assess the role of NTs in the pathophysiology of ADHD. Preclinical evidence indicates that BDNF knockout mice are hyperactive, and an ADHD rodent model exhibited decreased cerebral BDNF levels. Several lines of evidence from clinical studies, including blood level and genetic studies, have suggested that NTs are involved in the pathogenesis of ADHD and in the mechanism of biological treatments for ADHD. Future directions for research are proposed, such as using blood NTs as ADHD biomarkers, optimizing NT genetic studies in ADHD, considering NTs as a link between ADHD and other comorbid mental disorders, and investigating methods for optimally modulating NT signaling to discover novel therapeutics for treating ADHD.     

Unaltered Striatal Dopamine Release Levels in Young Parkin Knockout,Pink1 Knockout,DJ-1 Knockout and LRRK2 R1441G Transgenic Mice

Parkinson''s disease (PD) is one of the most prevalent neurodegenerative brain diseases; it is accompanied by extensive loss of dopamine (DA) neurons of the substantia nigra that project to the putamen, leading to impaired motor functions. Several genes have been associated with hereditary forms of the disease and transgenic mice have been developed by a number of groups to produce animal models of PD and to explore the basic functions of these genes. Surprisingly, most of the various mouse lines generated such as Parkin KO, Pink1 KO, DJ-1 KO and LRRK2 transgenic have been reported to lack degeneration of nigral DA neuron, one of the hallmarks of PD. However, modest impairments of motor behavior have been reported, suggesting the possibility that the models recapitulate at least some of the early stages of PD, including early dysfunction of DA axon terminals. To further evaluate this possibility, here we provide for the first time a systematic comparison of DA release in four different mouse lines, examined at a young age range, prior to potential age-dependent compensations. Using fast scan cyclic voltammetry in striatal sections prepared from young, 6–8 weeks old mice, we examined sub-second DA overflow evoked by single pulses and action potential trains. Unexpectedly, none of the models displayed any dysfunction of DA overflow or reuptake. These results, compatible with the lack of DA neuron loss in these models, suggest that molecular dysfunctions caused by the absence or mutation of these individual genes are not sufficient to perturb the function and survival of mouse DA neurons.     

GIT1 is associated with ADHD in humans and ADHD-like behaviors in mice

Attention deficit hyperactivity disorder (ADHD) is a psychiatric disorder that affects ~5% of school-aged children; however, the mechanisms underlying ADHD remain largely unclear. Here we report a previously unidentified association between G protein-coupled receptor kinase-interacting protein-1 (GIT1) and ADHD in humans. An intronic single-nucleotide polymorphism in GIT1, the minor allele of which causes reduced GIT1 expression, shows a strong association with ADHD susceptibility in humans. Git1-deficient mice show ADHD-like phenotypes, with traits including hyperactivity, enhanced electroencephalogram theta rhythms and impaired learning and memory. Hyperactivity in Git1(-/-) mice is reversed by amphetamine and methylphenidate, psychostimulants commonly used to treat ADHD. In addition, amphetamine normalizes enhanced theta rhythms and impaired memory. GIT1 deficiency in mice leads to decreases in ras-related C3 botulinum toxin substrate-1 (RAC1) signaling and inhibitory presynaptic input; furthermore, it shifts the neuronal excitation-inhibition balance in postsynaptic neurons toward excitation. Our study identifies a previously unknown involvement of GIT1 in human ADHD and shows that GIT1 deficiency in mice causes psychostimulant-responsive ADHD-like phenotypes.     

Exploring the Validity of Proposed Transgenic Animal Models of Attention-Deficit Hyperactivity Disorder (ADHD)

Attention-deficit/hyperactivity disorder (ADHD) is a common, behavioral, and heterogeneous neurodevelopmental condition characterized by hyperactivity, impulsivity, and inattention. Symptoms of this disorder are managed by treatment with methylphenidate, amphetamine, and/or atomoxetine. The cause of ADHD is unknown, but substantial evidence indicates that this disorder has a significant genetic component. Transgenic animals have become an essential tool in uncovering the genetic factors underlying ADHD. Although they cannot accurately reflect the human condition, they can provide insights into the disorder that cannot be obtained from human studies due to various limitations. An ideal animal model of ADHD must have face (similarity in symptoms), predictive (similarity in response to treatment or medications), and construct (similarity in etiology or underlying pathophysiological mechanism) validity. As the exact etiology of ADHD remains unclear, the construct validity of animal models of ADHD would always be limited. The proposed transgenic animal models of ADHD have substantially increased and diversified over the years. In this paper, we compiled and explored the validity of proposed transgenic animal models of ADHD. Each of the reviewed transgenic animal models has strengths and limitations. Some fulfill most of the validity criteria of an animal model of ADHD and have been extensively used, while there are others that require further validation. Nevertheless, these transgenic animal models of ADHD have provided and will continue to provide valuable insights into the genetic underpinnings of this complex disorder.     

Paradoxical striatal cellular signaling responses to psychostimulants in hyperactive mice

Recent investigations have shown that three major striatal-signaling pathways (protein kinase A/DARPP-32, Akt/glycogen synthase kinase 3, and ERK) are involved in the regulation of locomotor activity by the monoaminergic neurotransmitter dopamine. Here we used dopamine transporter knock-out mice to examine which particular changes in the regulation of these cell signaling mechanisms are associated with distinct behavioral responses to psychostimulants. In normal animals, amphetamine and methylphenidate increase extracellular levels of dopamine, leading to an enhancement of locomotor activity. However, in dopamine transporter knock-out mice that display a hyperactivity phenotype resulting from a persistent hyperdopaminergic state, these drugs antagonize hyperactivity. Under basal conditions, dopamine transporter knock-out mice show enhanced striatal DARPP-32 phosphorylation, activation of ERK, and inactivation of Akt as compared with wild-type littermates. However, administration of amphetamine or methylphenidate to these mice reveals that inhibition of ERK signaling is a common determinant for the ability of these drugs to antagonize hyperactivity. In contrast, psychostimulants activate ERK and induce hyperactivity in normal animals. In hyperactive mice psychostimulant-mediated behavioral inhibition and ERK regulation are also mimicked by the serotonergic drugs fluoxetine and 5-carboxamidotryptamine, thereby revealing the involvement of serotonin-dependent inhibition of striatal ERK signaling. Furthermore, direct inhibition of the ERK signaling cascade in vivo using the MEK inhibitor SL327 recapitulates the actions of psychostimulants in hyperactive mice and prevents the locomotor-enhancing effects of amphetamine in normal animals. These data suggest that the inhibitory action of psychostimulants on dopamine-dependent hyperactivity results from altered regulation of striatal ERK signaling. In addition, these results illustrate how altered homeostatic state of neurotransmission can influence in vivo signaling responses and biological actions of pharmacological agents used to manage psychiatric conditions such as Attention Deficit Hyperactivity Disorder (ADHD).     

Effects of lisdexamfetamine alone and in combination with s‐citalopram on acetylcholine and histamine efflux in the rat pre‐frontal cortex and ventral hippocampus

Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterized by poor attention, impulse control and hyperactivity. A significant proportion of ADHD patients are also co‐morbid for other psychiatric problems including mood disorders and these patients may be managed with a combination of psychostimulants and anti‐depressants. While it is generally accepted that enhanced catecholamine signalling via the action of psychostimulants is likely responsible for the cognitive improvement in ADHD, other neurotransmitters including acetylcholine and histamine may be involved. In the present study, we have examined the effect of lisdexamfetamine dimesylate (LDX), an amphetamine pro‐drug that is approved for the treatment of ADHD on acetylcholine and histamine efflux in pre‐frontal cortex and hippocampus alone and in combination with the anti‐depressant s‐citalopram. LDX increased cortical acetylcholine efflux, an effect that was not significantly altered by co‐administration of s‐citalopram. Cortical and hippocampal histamine were markedly increased by LDX, an effect that was attenuated in the hippocampus but not in pre‐frontal cortex when co‐administered with s‐citalopram. Taken together, these results suggest that efflux of acetylcholine and histamine may be involved in the therapeutic effects of LDX and are differentially influenced by the co‐administration of s‐citalopram.

     

Appropriate tissue- and cell-specific expression of a single copy human angiotensinogen transgene specifically targeted upstream of the HPRT locus by homologous recombination

Development of experimental models by genetic manipulation in mice has proven to be very useful in determining the significance of particular genes in the development of or susceptibility to hypertension. Advances in molecular genetics, transgenic mouse technology, and physiological measurements in mice provided an opportunity to go a step further and develop models to analyze the physiological significance of specific gene variants potentially causing hypertension. In this report, we describe the development of a human angiotensinogen transgenic mouse model generated by targeting the human angiotensinogen gene upstream of the mouse HPRT locus by homologous recombination. The main benefit of this transgenic mouse model is that the human angiotensinogen gene is inserted into the mouse genome as a single copy at a predefined locus and in a specific orientation-a process that can be repeated utilizing other variants of this gene. We establish the validity of this approach by showing that the hAGT(hprt) mice have normal tissue- and cell-specific expression of the human angiotensinogen gene and normally produce and process the hAGT protein at physiological levels.     

Role of Dopamine Receptors in ADHD: A Systematic Meta-analysis

The dopaminergic system plays a pivotal role in the central nervous system via its five diverse receptors (D1-D5). Dysfunction of dopaminergic system is implicated in many neuropsychological diseases, including attention deficit hyperactivity disorder (ADHD), a common mental disorder that prevalent in childhood. Understanding the relationship of five different dopamine (DA) receptors with ADHD will help us to elucidate different roles of these receptors and to develop therapeutic approaches of ADHD. This review summarized the ongoing research of DA receptor genes in ADHD pathogenesis and gathered the past published data with meta-analysis and revealed the high risk of DRD5, DRD2, and DRD4 polymorphisms in ADHD.     

Inflammation: good or bad for ADHD?

Attention deficit hyperactivity disorder (ADHD) is characterised by the typical behavioural core symptoms of inattentiveness, hyperactivity and impulsiveness. ADHD is a usually chronic health conditions, mostly diagnosed in childhood, creating a significant challenge for youth, their families and professionals who treat it. This disorder requires long-term treatments, including psychotherapeutic and pharmacological interventions, which in some cases may lead to adverse effects. Understanding the mechanism by which ADHD risk factors affect the biochemical processes in the human brain and consequentially the behaviour will help to identify novel targets for the development of therapeutics with less adverse results and better efficacy including higher responder rates. Although inflammatory responses in the brain have been recognised for years as critical in neurodegeneration and behaviour in a number of neurological and psychiatric disorders, their role for the development, treatment and prevention of ADHD has been so far largely overlooked, although historically, ADHD symptoms were initially observed in patients who survived an ONJ infection, i.e. inflammation. In this review, we discuss the interrelationship between different ADHD risk factors and inflammation with respect to the triggered molecular mechanisms and the contribution they are likely to have to this disorder. This paper provides a rationale for future studies on ADHD with an intent to inspiring the development of new agents for a more efficient management of this disorder.     

Symptom covariance accounts for behavioral approach associations across impulse control disorders

Behavioral approach system (BAS) dysfunction has been identified as a correlate of and a potential mechanism for attention-deficit/hyperactivity disorder (ADHD) and comorbid disorders. This study examined the role of symptom covariation in the relations among BAS dysfunction, ADHD symptoms, and comorbid impulsive personality disorder features. Undergraduates (N = 207) completed measures of BAS functioning, ADHD symptoms, and borderline and antisocial personality disorder symptoms, and associated features (i.e., relational aggression). Hierarchical regression suggested that age, impulsive ADHD symptoms, and relational aggression were associated with BAS functioning. Adding other ADHD symptom dimensions (inattention, hyperactivity) and antisocial and borderline scores to the model did not increase variance accounted for beyond that accounted for by ADHD impulsivity scores. Results highlight a role of symptom covariance in the previously demonstrated relation between BAS, impulsive presentations of ADHD, and comorbid impulsive personality pathology. Implications for etiological models of ADHD and its co-occurrence with other disorders are discussed.     

The Role of the Dopamine Transporter in the Effects of Amphetamine on Sleep and Sleep Architecture in Drosophila

The dopamine transporter (DAT) mediates the inactivation of released dopamine (DA) through its reuptake, and thereby plays an important homeostatic role in dopaminergic neurotransmission. Amphetamines exert their stimulant effects by targeting DAT and inducing the reverse transport of DA, leading to a dramatic increase of extracellular DA. Animal models have proven critical to investigating the molecular and cellular mechanisms underlying transporter function and its modulation by psychostimulants such as amphetamine. Here we establish a behavioral model for amphetamine action using adult Drosophila melanogaster. We use it to characterize the effects of amphetamine on sleep and sleep architecture. Our data show that amphetamine induces hyperactivity and disrupts sleep in a DA-dependent manner. Flies that do not express a functional DAT (dDAT null mutants) have been shown to be hyperactive and to exhibit significantly reduced sleep at baseline. Our data show that, in contrast to its action in control flies, amphetamine decreases the locomotor activity of dDAT null mutants and restores their sleep by modulating distinct aspects of sleep structure. To begin to explore the circuitry involved in the actions of amphetamine on sleep, we also describe the localization of dDAT throughout the fly brain, particularly in neuropils known to regulate sleep. Together, our data establish Drosophila as a robust model for studying the regulatory mechanisms that govern DAT function and psychostimulant action.

     

Behavioral phenotypes of genetic mouse models of autism

More than a hundred de novo single gene mutations and copy‐number variants have been implicated in autism, each occurring in a small subset of cases. Mutant mouse models with syntenic mutations offer research tools to gain an understanding of the role of each gene in modulating biological and behavioral phenotypes relevant to autism. Knockout, knockin and transgenic mice incorporating risk gene mutations detected in autism spectrum disorder and comorbid neurodevelopmental disorders are now widely available. At present, autism spectrum disorder is diagnosed solely by behavioral criteria. We developed a constellation of mouse behavioral assays designed to maximize face validity to the types of social deficits and repetitive behaviors that are central to an autism diagnosis. Mouse behavioral assays for associated symptoms of autism, which include cognitive inflexibility, anxiety, hyperactivity, and unusual reactivity to sensory stimuli, are frequently included in the phenotypic analyses. Over the past 10 years, we and many other laboratories around the world have employed these and additional behavioral tests to phenotype a large number of mutant mouse models of autism. In this review, we highlight mouse models with mutations in genes that have been identified as risk genes for autism, which work through synaptic mechanisms and through the mTOR signaling pathway. Robust, replicated autism‐relevant behavioral outcomes in a genetic mouse model lend credence to a causal role for specific gene contributions and downstream biological mechanisms in the etiology of autism.     

7th Biennial Meeting of the Asian Pacific Society for Neurochemistry, APSN 2006, Singapore, 2-5 July 2006. Abstracts

Dopamine (DA) uptake through the neuronal plasma membrane DA transporter (DAT) is essential for the maintenance of normal DA homeostasis in the brain. The DAT‐mediated re‐uptake system limits not only the intensity but also the duration of DA actions at presynaptic and postsynaptic receptors. This protein is the primary target for cocaine and amphetamine, both highly addictive and major substances of abuse worldwide. DAT is also the molecular target for therapeutic agents used in the treatment of mental disorders, such as attention deficit hyperactivity disorder and depression. Given the role played by the DAT in regulation of DA neurotransmission and its contribution to the abuse potential of psychostimulants, it becomes not only important but also necessary to understand the functional regulation of this protein. To investigate the cellular and molecular mechanisms associated with DAT function and regulation, our laboratory and others have embarked on a systematic search for DAT protein–protein interactions. Recently, a growing number of proteins have been shown to interact with DAT. These novel interactions might be important in the assembly, targeting, trafficking and/or regulation of transporter function. In this review, I summarize the main findings obtained from the characterization of DAT‐interacting proteins and discuss the functional implications of these novel interactions. Based on these new data, I propose to use the term DAT proteome to explain how interacting proteins regulate DAT function. These novel interactions might help define new mechanisms associated with the function of the transporter.     

Paradoxical Abatement of Striatal Dopaminergic Transmission by Cocaine and Methylphenidate

We combined in vitro amperometric, optical analysis of fluorescent false neurotransmitters and microdialysis techniques to unveil that cocaine and methylphenidate induced a marked depression of the synaptic release of dopamine (DA) in mouse striatum. In contrast to the classical dopamine transporter (DAT)-dependent enhancement of the dopaminergic signal observed at concentrations of cocaine lower than 3 μm, the inhibitory effect of cocaine was found at concentrations higher than 3 μm. The paradoxical inhibitory effect of cocaine and methylphenidate was associated with a decrease in synapsin phosphorylation. Interestingly, a cocaine-induced depression of DA release was only present in cocaine-insensitive animals (DAT-CI). Similar effects of cocaine were produced by methylphenidate in both wild-type and DAT-CI mice. On the other hand, nomifensine only enhanced the dopaminergic signal either in wild-type or in DAT-CI mice. Overall, these results indicate that cocaine and methylphenidate can increase or decrease DA neurotransmission by blocking reuptake and reducing the exocytotic release, respectively. The biphasic reshaping of DA neurotransmission could contribute to different behavioral effects of psychostimulants, including the calming ones, in attention deficit hyperactivity disorder.     

Prevalence of sexual dysfunctions and other sexual disorders in adults with attention-deficit/hyperactivity disorder compared to the general population

Attention-deficit/hyperactivity disorder (ADHD) is characterized by symptoms of inattention and/or hyperactivity and impulsivity that lead to dysfunctioning in daily life. One of the affected areas of life that has so far not been studied in ADHD is sexual functioning. The goal of this study was to assess prevalence of sexual dysfunctions and other sexual disorders among adults with ADHD. A total of n = 136 adult patients treated in a Dutch outpatient ADHD clinic filled out two questionnaires to screen for sexual dysfunctions and other sexual disorders. We compared the prevalence of sexual dysfunctions and other sexual disorders in our ADHD patient population to results from two large surveys among the general Dutch population. We found that 39% of the male and 43% of the female ADHD patients had symptoms of a sexual dysfunction, and 17% of the male and 5% of the female ADHD patients had symptoms of any other sexual disorder. Only one male patient had received a diagnosis of a sexual disorder at this clinic prior to study participation. In conclusion, sexual dysfunctions and other sexual disorders are highly prevalent in adults with ADHD. Screening for sexual disorders should be therefore standard procedure during diagnostic assessment.     

Gene targeting in the mouse

Mice with alterations to specific endogenous genes can be produced by gene targeting in embryonic stem cells. The field has developed rapidly over the past decade, so that large numbers of mice with different gene deficiencies have been generated. Knockout mice provide an ideal opportunity to analyse the function of individual mammalian genes and to model a range of human inherited disorders. This powerful approach has also identified numerous examples of gene redundancy and has highlighted the need to consider metabolic differences between man and mouse in disease modelling. More sophisticated gene-targeting methods are now being used to introduce subtle gene alterations. In the future, more refined genetic analysis and genome, rather than individual gene, alterations will be achieved by incorporating site-specific recombination into targeting strategies. Gene targeting could also make a contribution to improved protocols for gene therapy.