ADORA2A and attention deficit-hyperactivity disorder: In view of the involvement of D4R and α2AR in impulsivity and ADHD (see next section), their concurrent localization in frontal cortical P neurons (see below) and the demonstrated ability of D4R to form functional heteromers with other adrenoceptors in the pineal gland (see above), we investigated the possible existence of functionally significant α2AR-D4.4R and α2AR-D4.7R heteromers in the brain.