DRD4 and substance withdrawal syndrome: It cannot be excluded that pharmacological activation of the DRD4 might also be a useful approach to treat drug addiction, considering that the DRD4 agonist PD-168,077 (>400-fold selectivity over the DRD2 and >300-fold selectivity versus DRD3 but affinity also for α-adrenergic receptors as well as 5-HT1A and 5-HT1B receptors [135,136]) was able to decrease morphine-induced hyperlocomotion, reward and withdrawal syndrome, without interfering with the analgesic properties [137].