Sodium/chloride-dependent neurotransmitter cotransporters, which constitute a gene superfamily (SLC6), are crucial for limiting neurotransmitter activity (Hertting and Axelrod 1961; Iversen 1971). At the plasma membrane, substrate influx is directly coupled to transmembrane ion gradients that provide the energy for transport against a concentration gradient. On the basis of their remarkable amino acid identity and their properties as targets of tri- and heterocyclic antidepressants, as well as of psychostimulants including amphetamine, cocaine, and their analogs, the carriers for the monoamines norepinephrine (NE), dopamine (DA), and serotonin (5-hydroxytryptamine, 5HT) comprise a distinct subfamily (Lesch et al. 1996; Amara and Sonders 1998).