A 40-bp variable number (3-13) of tandem repeats (VNTR) polymorphism exists in the 3' untranslated region (UTR) of the dopamine transporter (DAT) gene (dat1). The 10-repeat VNTR (hDAT10), has been associated with attention deficit hyperactivity disorder (ADHD) and nonresponse of ADHD to methylphenidate (MPH). The mechanisms underlying these genotype-phenotype correlations are unknown. This exploratory R21 proposal seeks to use in vitro molecular genetic studies to guide in vivo positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) approaches to clarifying the role of this genetic polymorphism in the pathophysiology and pharmacotherapy of ADHD. Specifically, in vitro gene transfection/expression experiments would define the influence of dat1 VNTR genotype (hDATI0, hDAT9, hDAT0 and hDAT) on DAT binding and function, and pharmacological regulation by MPH. In vivo [18F]FECNT PET studies would compare brain regional DAT binding potential and MPH dose-dependent DAT occupancy in sib pairs discordant for ADHD and dat1 VNTR genotype. Finally, in vivo fMRI studies would define the effect of hDAT10 on the neural correlates of the effect of MPH administration on cognitive impairments associated with ADHD. These studies would test the following specific hypotheses: that the 10- repeat VNTR of dat1 is associated with (1) increases in DAT number and/or function, and attenuated MPH inhibition of DAT in in vitro model cell systems; with (2) increases in the in vivo regional brain DAT binding potential and diminished MPH dose-dependent occupancy of human brain DAT; and, with (3) a blunted effect of MPH administration on the cognitive task-related activation of a prefrontal-parietal cortical circuit. This exploratory proposal would novelly use in vivo imaging approaches to explore genotype-phenotype correlations and partner in vitro and in vivo approaches, and is thus well suited to the goals of the R21 mechanism to foster the exploration of the use of approaches new to a area. By uniquely defining the impact of the dat1 VNTR polymorphism on DAT, these findings will generate knowledge of how genes confer vulnerability to ADHD and medication nonresponse.