The paradoxic effects of these agents, however, led researchers to hypothesize that abnormal dopaminergic signaling causes ADHD and to search for an association between a polymorphism at the dopamine transporter locus (DAT1) and ADHD [12]. The findings of hypothesis-driven studies focusing on the genes involved in catecholaminergic systems suggest various genes potentially involved in

the pathogenesis of ADHD. Meta-analyses of the hypothesis-driven research support significant associations of several candidate genes, including DAT1, DRD2, DRD4, DRD5, 5HTT, HTR1B, and SNAP25 13 and 14]. These learn more studies, however, also revealed modest odds ratios (<1.33) for all of the significant polymorphisms, suggesting that each gene has only a small effect and supporting a multifactorial and polygenic etiology of ADHD. The polygenic etiology is further supported by hypothesis-free genome-wide scan studies. These studies implicate multiple loci, thus diluting the significance of the classic candidate genes involved in catecholaminergic signaling, and suggest the potential involvement of genes for ‘new’ neurotransmission and cell-cell communication systems,

including T-cadherin [15]. A recent Fulvestrant genome-wide copy number variation study provided evidence for an association of metabotropic glutamate receptors and their interacting molecules with ADHD [16••]. Taken together, human genetic studies have established a complex etiology of ADHD, similar to that of other psychiatric disorders. Thus, different types of model animals are needed and proposed [17]. This article focuses on the mouse genetic models. DAT is expressed on axon terminals and regulates dopamine (DA) signaling by transporting DA from the synaptic cleft back into the presynaptic terminal. Multiple lines of evidence from genetic, pharmacologic, and imaging studies suggest that DAT1 is a strong candidate gene involved in the pathogenesis of ADHD. The behavioral phenotypes of mutant mice generated by gene-targeting methods support this notion. Dat1-knockout (KO) mice exhibit hyperactivity and deficits in

learning and memory [18]. The mice also show attention deficits in an auditory prepulse inhibition ROS1 (PPI) test [19]. Hyperactivity and PPI deficits in Dat1-KO mice are ameliorated by methylphenidate 18 and 20]. A recent study revealed that Dat1-KO mice with a mixed genetic background of C57BL/6J and 129Sv/J were impaired in a cliff avoidance reaction (CAR) test based on their inability to remain on an elevated small round platform without falling, suggesting impulsivity [21]. Methylphenidate or nisoxetine ameliorated the cliff avoidance reaction impairment in the Dat1-KO mice [21]. Dat1-knockdown mice also exhibited hyperactivity and risk-taking behavior in a mouse version of the Iowa gambling test [22], reflecting impulsivity.