DRD2 and obesity due to melanocortin 4 receptor deficiency: Another interesting effect is the decreased expression of the dopamine D2 receptor within the mesocorticolimbic system in rats and humans with obesity [99], reduced DA concentration in the striatum in rats with alimentary obesity [100], and suppressed expression of the D1 receptor in the nucleus accumbens in rats genetically predisposed to excess weight gain [101], indicating an important regulatory role of decreased DA levels in the brain during the development of alimentary forms of obesity.