Tricyclic and tetracyclic antidepressants have long been used in the clinic for the treatment of depression in humans, thus studies involving positron emission tomography (PET) imaging could be conducted to investigate if the VMAT2 protein upregulation phenomenon could be observed in patients who are taking tricyclic or tetracyclic antidepressants. The gene discussed is SLC18A2; the disease is major depressive disorder.