Molecular physiology of norepinephrine and serotonin transporters

Cocaine- and antidepressant-sensitive norepinephrine and serotonin transporters (NETs and SERTs) are closely related members of the Na+/Cl- transporter gene family, whose other members include transporters for inhibitory amino acid transmitters, neuromodulators, osmolytes and nutrients. Availability of cloned NET and SERT cDNAs has permitted rapid progress in the definition of cellular sites of gene expression, the generation of transporter-specific antibodies suitable for biosynthetic and localization studies, the examination of structure-function relationships in heterologous expression systems and a biophysical analysis of transporter function. In situ hybridization and immunocytochemical studies indicate a primary expression of NET and SERT genes in brain by noradrenergic and serotonergic neurons, respectively. Both NET and SERT are synthesized as glycoproteins, with multiple glycosylation states apparent for SERT proteins in the brain and periphery. N-glycosylation of NET and SERT appears to be essential for transporter assembly and surface expression, but not for antagonist binding affinity. Homology cloning efforts have revealed novel NET and SERT homologs in nonmammalian species that are of potential value in the delineation of the precise sites for substrate and antagonist recognition, including a Drosophila melanogaster SERT with NET-like pharmacology. Electrophysiological recording of human NETs and SERTs stably expressed in HEK-293 cells reveals that both transporters move charge across the plasma membrane following the addition of substrates; these currents can be blocked by NET-and SERT-selective antagonists as well as by cocaine.