SLC6A3 and major depressive disorder: Data from photon emission computerized tomography (SPECT) evaluating the uptake of a high-affinity dopamine transporter specific radioligand in the basal ganglia of depressed patients and healthy controls, suggested that the primary dysfunction of the dopaminergic system in depression is related to the up-regulation of dopamine transporter density, causing a more effective re-uptake of dopamine into the presynaptic neurons, thus resulting in a lower concentration of dopamine in the synaptic gap and a consequent decrease in the dopaminergic neurotransmission (Laasonen-Balk et al., 1999).