No abstract

A Na+-and C1-coupled serotonin (5-hydroxytryptamine, SHT) transporter is expressed on human neuronal, platelet, placental, and pulmonary membranes. The brain 5HT transporter appears to be a principal site of action of therapeutic antidepressants and may mediate behavioral and/or toxic effects of cocaine and amphetamines. Oligonucleotides derived from consensus transporter sequences were used to identify human placental cDNAs highly related to the rat brain 5HT carrier. Transfection of one of these cDNAs into HeLa cells yields a high-affinity (Km = 463 nM), Na+-and Cl--dependent 5HT transport activity which can be blocked by selective 5HT transport inhibitors, including paroxetine, fluoxetine, and imipramine, and which is antagonized by cocaine and amphetamine. Sequence analysis reveals a 630-amino acid open reading frame bearing 92% identity to the cloned rat brain 5HT transporter, with identical predicted topological features and conserved sites for posttranslational modifications. Unlike the rodent, where a single mRNA appears to encode 5HT transporters, multiple hybridizing RNAs are observed in human placenta and lung. Somatic cell hybrid and in situ hybridization studies are consistent, however, with a single gene encoding the human 5HT transporter, localized to chromosome 17q11.1-17q12.The endogenous indoleamine serotonin (5-hydroxytryptamine, 5HT) is a neurotransmitter in the central and peripheral nervous system (1, 2). Following release, 5HT is actively cleared from synaptic spaces by a high-affinity (Km 0.5 ,uM), Na+-and Cl--dependent transporter localized in presynaptic neuronal membranes (3-5). Transport of serotonin is exquisitely sensitive to nanomolar concentrations of tricyclic and heterocyclic antidepressants, including imipramine, fluoxetine, and paroxetine (6), which are thought to bind directly to the 5HT carrier (7-10), presumably constituting the initial step in their therapeutic actions. In the periphery, platelet (11), placental (12), and pulmonary (13) plasma membranes exhibit a 5HT transporter whose ionic and pharmacologic sensitivities are highly similar to those described for the neuronal transporter. In particular, plasma membrane vesicle studies reveal central and peripheral 5HT carriers to be dependent upon extracellular Na+ and Cl-, stimulated by intracellular K+, and antagonized competitively by antidepressants (5, 11, 14-17). These properties serve to distinguish the plasma membrane 5HT transporter from the monoamine transporter present in intracellular secretory vesicles, which utilizes a transmembrane H+ gradient in an imipramine-insensitive, but reserpine-sensitive, manner to sequester amines, including 5HT, for release (18).The therapeutic utility of 5HT transport antagonists in the treatment of depression, obsessive-compulsive disorder, and sleep and eating disorders (19) has contributed to an emphasis on central serotonergic dysfunction within the monoamine theory of affective disorders (20). While not a universal finding, reported reductions in platelet or brain 5H...

At most synapses, chemical signalling is terminated by a rapid reaccumulation of neurotransmitter into presynaptic terminals. Uptake systems for the biogenic amines are the initial site of action for therapeutic antidepressants and drugs such as cocaine and the amphetamines. We have isolated a complementary DNA clone encoding a human noradrenaline transporter. The cDNA sequence predicts a protein of 617 amino acids, with 12-13 highly hydrophobic regions compatible with membrane-spanning domains. Expression of the cDNA clone in transfected HeLa cells indicates that noradrenaline transport activity is sodium-dependent and sensitive to selective noradrenaline transport inhibitors. Transporter RNA is localized to the brainstem and the adrenal gland. The predicted protein sequence demonstrates significant amino-acid identity with the Na+/gamma-aminobutyric acid transporter, thus identifying a new gene family for neurotransmitter transporter proteins. Analysis of its structure and function may lead to structure-based drug design for the treatment of human depression and could help determine whether transporter abnormalities underlie affective disorders.