DRD2 and obesity due to melanocortin 4 receptor deficiency: Dopamine subtype 2 receptor (D2DR) knockdown mice fail to gain weight or exhibit elevated appetitive motivation in comparison with the wild-type mice within standard environments yet in enriched environments incorporating voluntary exercise facilities, these D2DR knockdown mice expressed markedly lower activity with a rapid increase in obesity compared with the wild-type mice without being receptive of the protective benefit from exercise contingencies [14].