Generation and Characterization of the First Construct-Valid Model of ADHD, the DAT Val559 Knock-In Mouse

Abstract

Attention-deficit/hyperactivity disorder (ADHD) is the most commonly diagnosed childhood neuropsychiatric disorder. More than twenty years of genetic, behavioral, and pharmacological research support the hypothesis that compromised brain dopamine (DA) signaling impacts risk for the disorder. To date, however, evidence for this idea is indirect, and we therefore lack a construct-valid animal model for ADHD. To gain direct evidence for changes in DA-regulatory molecules in ADHD, we screened subjects with a DSM-IV diagnosis of the disorder and identified multiple, rare DA transporter (DAT, SLC6A3) coding variants. One of these variants, DAT Val559, induces anomalous, non-vesicular DA release in transfected cells, and efflux that can be blocked by the most common ADHD medications, D-amphetamine (AMPH) and methylphenidate. To pursue the significance of these findings in vivo, we engineered DAT Val559 knock-in mice and, as with heterologous expression studies, found the variant to support expression of wild-type (WT) levels of striatal DAT and tyrosine hydroxylase (TH) protein, as well as DA and DA metabolite levels. However, DAT Val559 mice exhibited a novel, conditional hyperactivity phenotype, “darting”, as well as an impaired sensitivity to the locomotor-activating properties of AMPH. In vivo microdialysis studies in the striatum of DAT Val559 animals demonstrated a pronounced elevation of basal, extracellular DA levels along with a significantly blunted efflux of DA release evoked by locally infused AMPH. Together, our studies confirm a functional impact on extracellular DA homeostasis of the DAT Val559 variant in vivo and establish the first construct-valid animal model of ADHD.