Putative functional mechanisms of dopaminergic polymorphisms. Putative functional mechanisms of dopaminergic polymorphisms. A, COMT Val158Met polymorphism: genotype modulates activity of the COMT enzyme that represents the major dopamine breakdown mechanism in prefrontal cortex; lower enzyme activity results in higher prefrontal dopamine activity. B, SLC6A3 3′-UTR-VNTR polymorphism: genotype modulates availability of the DAT that terminates striatal dopamine activity by fast reuptake from the synaptic cleft; lower availability of the transporter results in higher striatal dopamine activity. Jutta Billino et al. eNeuro 2016;3:ENEURO.0190-16.2016 ©2016 by Society for Neuroscience