Antidepressant- and cocaine-sensitive human serotonin transporter: molecular cloning, expression, and chromosomal localization.

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...

Evidence for significant contribution of a newly identified monoamine transporter (PMAT) to serotonin uptake in the human brain

“…We investigated the potential interactions of PMAT with the SSRIs using a stably transfected mammalian cell line. All five commonly marketed SSRIs, including fluoxetine (Prozac), sertraline (Zoloft), citalopram, fluvoxamine and paroxetine, inhibited PMAT with IC 50 values 3-4 orders greater than those of SERT [15,32]. At clinically encountered concentrations in the brain (low nanomolar range) (Table 3) Illustration of positions of PMAT and SERT oligos used in hybrid depletion study.…”

“…0.5 μM) [12,15]. However, PMAT also has a much larger transport capacity (V max ), resulting in roughly comparable uptake efficiencies (V max /K m ) to SERT in heterologous expression systems [12,15]. Consistent with its transport function for monoamine neurotransmitters, PMAT mRNA is most strongly expressed in the human brain, and is widely distributed in the CNS [12].…”

“…In cells stably expressing PMAT and SERT, the apparent affinity of recombinant PMAT for 5-HT is much lower than that of SERT (Km, 114 v.s. 0.5 μM) [12,15]. However, PMAT also has a much larger transport capacity (V max ), resulting in roughly comparable uptake efficiencies (V max /K m ) to SERT in heterologous expression systems [12,15].…”

Enhanced Serotonin Transporter Function during Depression in Seasonal Affective Disorder

“…The 5-HTT is expressed in the central nervous system (CNS) and peripheral tissues, including blood platelets. It is encoded by a single gene (Lesch et al, 1993;Ramamoorthy et al, 1993) regulated by a polymorphic promoter region (5-HTT-promoter polymorphism, 5-HTTLPR Lesch et al, 1996). Brain imaging techniques (positron emission tomography or single photon computed tomography, SPECT) allow for measurement of 5-HTT availability in the living human brain.…”

Serotonin Uptake Is Largely Mediated by Platelets versus Lymphocytes in Peripheral Blood Cells

“…33,34 The gene that is responsible for SERT expression in the brain has been shown to code for SERT in peripheral blood cells. [35][36][37]20 Platelets are small 2−3 μM anucleate cell fragments derived from megakaryocytes that are primarily responsible for hemostasis and blood clotting. 38 Platelets have long been known to express SERT, 38,32 and more recently dopamine transporter (DAT) expression has been identified in platelets.…”