To verify the hypothesis according to which dopaminergic transmission is impacted in early AD, we characterized in the midbrain and the striatum the markers of dopaminergic system (density of D2/3R, TH and DAT, dopamine transporter), glial reactivity (astrocytes and microglia) and behavioral effects (locomotion and locomotor response to stimulation of D2R) in heterozygous 3xTg-AD mice at an age when pathological markers are weakly expressed (Aβ and Tau). This evidence concerns the gene SLC6A3 and Alzheimer disease.