Polymorphic alleles of the human dopamine D-4 receptor gene (DRD4) have been consistently associated with individual differences in personality traits and neuropsychiatric disorders, particularly between the gene encoding dopamine D-4.7 receptor variant and attention deficit hyperactivity disorder (ADHD). The alpha(2A) adrenoceptor gene has also been associated with ADHD. In fact, drugs targeting the alpha(2A) adrenoceptor (alpha R-2A), such as guanfacine, are commonly used in ADHD treatment. In view of the involvement of dopamine D-4 receptor (D4R) and alpha R-2A in ADHD and impulsivity, their concurrent localization in cortical pyramidal neurons and the demonstrated ability of D4R to form functional heteromers with other G protein-coupled receptors, in this study we evaluate whether the alpha R-2A forms functional heteromers with D4R and weather these heteromers show different properties depending on the D4R variant involved. Using cortical brain slices from hD(4.7)R knock-in and wild-type mice, here, we demonstrate that alpha R-2A and D4R heteromerize and constitute a significant functional population of cortical alpha R-2A and D4R. Moreover, in cortical slices from wild-type mice and in cells transfected with alpha R-2A and D4.4R, we detect a negative crosstalk within the heteromer. This negative crosstalk is lost in cortex from hD(4.7)R knock-in mice and in cells expressing the D4.7R polymorphic variant. We also show a lack of efficacy of D4R ligands to promote G protein activation and signaling only within the alpha R-2A-D4.7R heteromer. Taken together, our results suggest that alpha R-2A-D4R heteromers play a pivotal role in catecholaminergic signaling in the brain cortex and are likely targets for ADHD pharmacotherapy.