Due to the complexity of the disease, several hypotheses exist to explain the etiology of depression. The monoamine theory of depression suggests that disruptions in the serotonergic and noradrenergic systems result in depressive symptoms. Therefore, the serotonin transporter (SERT) has become a pharmacological target for treating these symptoms. This review will discuss what is known about the molecular interactions of antidepressants with SERT. The effects of antidepressants on SERT regulation and expression in addition to the receptors that may be involved in mediating these effects will be addressed. Specifically, how changes to SERT expression following chronic antidepressant treatment may contribute to the therapeutic benefits of antidepressants will be discussed. Furthermore, the effects of SERT gene polymorphisms on antidepressant efficacy will be examined. Finally, a brief overview of other hypotheses of depression will be addressed as well as factors that must be considered for future antidepressant development.

The human and Drosophila serotonin transporters (hSERT and dSERT, respectively) were used to explore differences in substrate properties. hSERT and dSERT showed similar K m values for 5-hydroxytryptamine (5-HT; serotonin) transport (1.2 and 0.9 M, respectively), suggesting similar recognition of 5-HT by the two species variants. Although dSERT cell surface expression was approximately 8-fold lower than that of hSERT, dSERT does appear to have a 2-fold faster turnover number for inward transport of 5-HT. Interestingly, another substrate, N-methyl-4-phenylpyridinium (MPP ϩ ), was transported only by hSERT. However, MPP ϩ inhibited 5-HT uptake in both species variants with similar potencies. Two cross-species chimeras, H 1-118 D 119 -627 and H 1-281 D 282-476 H 477-638 , were also unable to transport MPP ϩ , implicating the role of transmembrane domains V to IX in the substrate permeation pathway. Based on exchange experiments, certain substituted-amphetamines also appear to be poor substrates at dSERT. Two-electrode voltage-clamp studies in oocytes confirmed that the amphetamines do not possess substrate-like properties for dSERT. Our data suggest distinct molecular recognition among SERT substrate classes that influence translocation mechanisms.

A novel substituted cysteine accessibility method (SCAM) reagent was developed for monoamine uptake transporters. The new reagent, MTS-MPP + , was a derivative of the neurotoxin and transporter substrate MPP + . MTS-MPP + labeled cysteine residues introduced into the serotonin transporter protein. Although it did not prove to be a substrate, as is MPP + , it appears to label cysteine residues lining the permeation pore of the transporter more readily than currently-available nonspecific SCAM reagents.

The human serotonin transporter (hSERT) regulates neurotransmission by removing released serotonin (5-HT) from the synapse. Previous studies identified residues in SERT transmembrane helices (TMHs) I and III as interaction sites for substrates and antagonists. Despite an abundance of data supporting a 12-TMH topology, the arrangement of the TMHs in SERT and other biogenic amine transporters remains undetermined. A high-resolution structure of a bacterial leucine transporter that demonstrates homology with SERT has been reported, thus providing the basis for the development of a SERT model. Zn 2+ -binding sites have been utilized in transporters and receptors to define experimentally TMH proximity. Focusing on residues near the extracellular ends of hSERT TMHs I and III, we engineered potential Zn 2+ -binding sites between V102 or W103 (TMH I) and I179-L184 (TMH III). Residues were mutated to either histidine or cysteine. TMH I/III double mutants were constructed from functional TMH I mutants, and Zn 2+ sensitivity was assessed. Dose-response assays suggest an approximately twofold increase in sensitivity to Zn 2+ inhibition at the hSERT V102C/M180C and approximately fourfold at the V102C/I179C mutant compared to the hSERT V102C single mutant. We propose that the increased sensitivity to Zn 2+ confirms the proximity and the orientation of TMHs I and III in the membrane. Homology modeling of the proposed Zn 2+ -binding sites using the coordinates of the Aquifex aeolicus leucine transporter structure provided a structural basis for interpreting the results and developing conclusions.

A BSTRACTDue to the complexity of the disease, several hypotheses exist to explain the etiology of depression. The monoamine theory of depression suggests that disruptions in the sero tonergic and noradrenergic systems result in depressive symptoms. Therefore, the serotonin transporter (SERT) has become a pharmacological target for treating these symptoms. This review will discuss what is known about the molecular interactions of antidepressants with SERT. The effects of antidepressants on SERT regulation and expression in addition to the receptors that may be involved in mediating these effects will be addressed. Specifically, how changes to SERT expression following chronic antidepressant treatment may contribute to the therapeutic benefi ts of antidepressants will be discussed. Furthermore, the effects of SERT gene polymorphisms on antidepressant effi cacy will be examined. Finally, a brief overview of other hypotheses of depression will be addressed as well as factors that must be considered for future antidepressant development. K EYWORDS:SSRIs , antidepressant , serotonin transporter , depression , reuptake MONOAMINE THEORY OF DEPRESSIONFunctional defi ciencies in serotonin (5-hydroytryptamine, 5-HT) and norepinephrine (NE) have been implicated in the pathophysiology of depressive syndromes, and restoring the normal function of 5-HT-and NE-associated signaling pathway has been the target of antidepressants. Restoration of monoamine defi ciencies to normal levels as a therapeutic strategy is based on the monoamine hypothesis of depression. 1 The oldest antidepressants, the monoamine oxidase inhibitors (MAOIs), increase synaptic levels of 5-HT and NE by inhibiting the enzymatic degradation of these neurotransmitters. The tricyclic antidepressants (TCAs), as well as newer selective 5-HT reuptake inhibitors (SSRIs) and 5-HT/ NE reuptake inhibitors (SNRIs), all increase synaptic levels of 5-HT or NE by inhibiting reuptake via the 5-HT transporter (SERT) or NE transporter (NET), respectively. Emerging evidence indicates that the monoamine hypothesis of 5-HT and NE modulation fails to explain the whole mechanism of antidepressants. Other hypotheses, including the cytokine hypothesis of depression, the hypothalamic-pituitary-thyroid hypothesis of depression, as well as the role of brain-derived neurotrophic factor and cyclic AMP response element binding protein will be considered later in this review.