Inhibition of the Serotonin Transporter Is Altered by Metabolites of Selective Serotonin and Norepinephrine Reuptake Inhibitors and Represents a Caution to Acute or Chronic Treatment Paradigms (original) (raw)
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Journal of Neurochemistry, 2008
The serotonin transporter (SERT) is a high-af®nity sodium/ chloride-dependent neurotransmitter transporter responsible for reuptake of serotonin from the extracellular space. SERT is a selective target of several clinically important antidepressants. In a cross-species analysis comparing human and bovine SERTs, the kinetic parameters for serotonin uptake were found to be similar, however, the pharmacological pro®les of the two transporters differ. Following transient expression in COS-1 cells, IC 50 values were determined for several antidepressants and psychostimulants. The potencies of the antidepressants citalopram,¯uoxetine, paroxetine and imipramine were several-fold higher at hSERT compared with bSERT. No species selectivity was observed for the antidepressants¯uvoxamine, and sertraline or for the psychostimulants cocaine, the cocaine analogue b-carbomethoxy-3b-(4-iodophenyl)tropane, or for 3,4-methylenedioxymethamphetamine (MDMA). Analysis of six hSERT/bSERT chimeras and subsequent species-scanning mutagenesis of each isoform revealed methionine-180, tyrosine-495, and phenylalanine-513 to be responsible for the increase in citalopram and paroxetine potencies at hSERT and methionine-180 and phenylalanine-513 to confer species selectivity at hSERT for¯uoxetine and imipramine. Results were obtained by doing the forward, bovine to human, mutations and con®rmed by doing the reverse mutations. Citalopram analogues were used to de®ne the roles of methionine-180, tyrosine-495, and phenylalanine-513 and to reveal molecular interactions with individual functional groups of citalopram. We suggest that methionine-180 interacts with the heterocyclic nucleus of citalopram or stabilizes the binding pocket and phenylalanine-513 to be a steric blocker of antidepressant recognition.
Serotonin transporters: Implications for antidepressant drug development
The AAPS Journal, 2005
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 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.
Antidepressant-induced internalization of the serotonin transporter in serotonergic neurons
The FASEB Journal, 2008
A deficiency of serotonergic signaling is thought to be involved in the etiology of depression. Thus, drugs blocking the reuptake of serotonin back into the neurons are widely used in treatment of this disease; however, their delayed effect in remission of patients suggests that the clinical response does not rely on simple serotonin uptake inhibition but may include further regulatory mechanisms. We have analyzed cellular serotonin transporter (SERT) expression on exposure to the selective serotonin reuptake inhibitor citalopram in serotonergic neurons expressing the native SERT allele in its natural surroundings. Biotinylation of SERT-expressing HEK293 cells, as well as confocal microscopy analysis in these cells and in serotonergic neurons, revealed that exposure to citalopram time dependently reduces the amount of cell surface-expressed SERT. Furthermore, in serotonergic neurons, longer exposure to citalopram not only caused an internalization of SERT proteins from the cell surface but also induced a redistribution of SERT from neurite extensions into the soma. This process was reversible on drug removal. Microarray analysis performed on citalopram-treated serotonergic neurons revealed that antidepressant treatment does not alter SERT mRNA expression, suggesting that SERT trafficking from and to the cell membrane is regulated on the posttranscriptional level.-Lau, T., Horschitz, S., Berger, S., Bartsch, D., Schloss, P. Antidepressantinduced internalization of the serotonin transporter in serotonergic neurons. FASEB J. 22, 1702-1714 (2008)
Neuropsychopharmacologia Hungarica, 2020
Discovery and development of the selective serotonin reuptake inhibitors mark a milestone in neuro-pharmacology. Drugs from this class alter the functioning of the serotonin system by the potentiation of serotonin through the negative allosteric modulation of its neuronal uptake by the human serotonin transporter. Selective serotonin reuptake inhibitors show few side effects compared to those caused by traditional antidepressants and they vary in the binding interactions formed during binding. Gener-ally, their binding involves three specific regions of the drug structures, each participating in vital inter-actions, such as salt bridge formation and additional hydrophobic interactions with conserved resi-dues in the central binding site of the target protein. Side effects, however, such as the initial lack of response to treatment, or drowsiness, nausea, and sexual dysfunction occasionally may arise. Addi-tional binding studies, furthermore, highlighted the importance of enantioselectivity in the binding of these compounds, raising concerns about the beneficial application of racemate mixtures of some of these compounds. Therefore, additional characterisation of binding and further structural improve-ment of this class of drugs is necessary. The recently synthesized sertraline salts, and functional deriv-atives of fluoxetine and citalopram show promising results in delivering antidepressant activity as well as in effectively overcoming anorexigenic side-effects in rodent models. Hence, despite certain non-desired effects associated with selective serotonin reuptake inhibitor applications, this class of drugs is considered as first-line medication in the management of major depression, and is carrying an excel-lent potential for the development and refinement of the currently available and novel antidepressant therapies.
Antidepressants Targeting the Serotonin Reuptake Transporter Act via a Competitive Mechanism
Journal of Pharmacology and Experimental Therapeutics, 2008
Although several antidepressants (including fluoxetine, imipramine, citalopram, venlafaxine and duloxetine) are known to inhibit the serotonin transporter (SERT), whether or not these molecules compete with serotonin (5-HT) for binding to SERT has remained controversial. We have performed radioligand competition binding experiments and found that all data can be fitted via a simple competitive interaction model, employing Cheng-Prusoff analysis (Cheng & Prusoff, 1973). Two different SERTselective radioligands, [ 3 H]DASB (N,N-dimethyl-2-(2-amino-4-cyanophenyl thio)-benzylamine) and [ 3 H]S-citalopram were used to probe competitive binding to recombinantly expressed human SERT (hSERT), or native SERT in rat cortical membranes. All the SERT inhibitors that we tested were able to inhibit [ 3 H]DASB and [ 3 H]S-citalopram binding in a concentration-dependent manner with unity Hill coefficient. In accordance with the Cheng-Prusoff relationship for a competitive interaction, we observed that test compound concentrations associated with 50% maximal inhibition of radiotracer binding (IC 50) increased linearly with increasing radioligand concentration for all ligands: 5-HT, S-citalopram, Rcitalopram, paroxetine, clomipramine, fluvoxamine, imipramine venlafaxine, duloxetine, indatraline, cocaine and 2-beta-carboxy-3-beta-(4-iodophenyl)tropane (β-CIT). The equilibrium dissociation constant of 5-HT and SERT inhibitors were also derived using Scatchard analysis of the dataset, and were found to be comparable to the data obtained using the Cheng-Prusoff relationship. Our studies establish a reference framework that will contribute to ongoing efforts to understand ligand binding modes at SERT by demonstrating that 5-HT and the SERT inhibitors tested bind to the serotonin transporter in a competitive manner. This article has not been copyedited and formatted. The final version may differ from this version.
The serotonin transporter from human brain: Purification and partial characterization
Neurochemistry International, 1996
The serotonin (5-HT) transporter from hhman striatum was solubilized by digitonin and purified by affinity chromatography. The native protein-detergent complex had a molecular mass of 205 kDa, as estimated by gel-exclusion chromatography of the eluates obtained from affinity chromatography. The purified 5-HT transporter migrated as a single band of 67 kDa in SDS-PAGE. To clarify the spatial relationships between the binding sites of the tricyclic antidepressants, as [3H]-imipramine, and of the selective serotonin reuptake inhibitors, as [3H]-paroxetine, on the 5-HT transporter, both radioligands were used to label it in the purification steps.
Pharmacological profile of antidepressants and related compounds at human monoamine transporters
European Journal of Pharmacology, 1997
Using radioligand binding assays, we determined the equilibrium dissociation constants (KD's) for 37 antidepressants, three of their metabolites (desmethylcitalopram, desmethylsertraline, and norfluoxetine), some mood stabilizers, and assorted other compounds (some antiepileptics, Ca2+ channel antagonists, benzodiazepines, psychostimulants, antihistamines, and monoamines) for the human serotonin, norepinephrine, and dopamine transporters. Among the compounds that we tested, mazindol was the most potent at the human norepinephrine and dopamine transporters with KD's of 0.45 +/- 0.03 nM and 8.1 +/- 0.4 nM, respectively. Sertraline (KD = 25 +/- 2 nM) and nomifensine (56 +/- 3 nM) were the two most potent antidepressants at the human dopamine transporter. We showed significant correlations for antidepressant affinities at binding to serotonin (R = 0.93), norepinephrine (R = 0.97), and dopamine (R = 0.87) transporters in comparison to their respective values for inhibiting uptake of monoamines into rat brain synaptosomes. These data are useful in predicting some possible adverse effects and drug-drug interactions of antidepressants and related compounds.
Regulated phosphorylation and trafficking of antidepressant-sensitive serotonin transporter proteins
Biological Psychiatry, 1998
Presynaptic serotonin (5-hydroxytryptamine, 5-HT) transporters (SERTs) mediate antidepressant-sensitive clearance of 5-HT following release. Although we have been aware for decades that SERT-mediated 5-HT clearance can be modulated by exogenous agents including serotonin-selective reuptake inhibitors, amphetamines, and cocaine, we have had little reason to speculate that SERT activity was actively controlled through endogenous pathways. Recent studies indicate that SERTs are likely to be trafficked to specific plasma membrane subdomains to achieve localized clearance of 5-HT, and that the number of SERTs resident in the plasma membrane is controlled through kinase-and phosphatase-linked pathways. In particular, roles for protein kinase C and phosphatase 2A become apparent through studies with enzyme activators and inhibitors in SERT-transfected cells, where SERT proteins are rapidly phosphorylated in parallel with transporter redistribution and loss of functional uptake capacity. Based on our findings, and the studies of others in native tissues and transfected cells, we propose a model whereby SERTs are organized in a macromolecular complex in the plasma membrane that may serve to locate reuptake activity near release sites. Although many elements of this model remain hypothetical, our findings suggest a much more dynamic picture of transportermediated 5-HT reuptake than typically described and suggest opportunities both for the development of new SERT regulatory agents and for the identification of regulatory pathways that may be compromised in mental illness.
The Journal of neuroscience : the official journal of the Society for Neuroscience, 1995
Serotonin (5HT) transporters (SERTs) are responsible for clearance of synaptic and plasma 5HT and are molecular targets for multiple therapeutic and addictive compounds. Recently brain and peripheral SERT cDNAs have been cloned and characterized functionally in transfected cells. Antipeptide (S365) and anti-fusion protein (CT-2) antibodies, directed at epitopes poorly conserved among other Na+/Cl- cotransporters, have been prepared to facilitate the identification and characterization of SERT proteins in native and transfected cells. Immunoprecipitations and immunoblots of rat/human SERT-transfected HeLa cells reveal specific SERT-immunoreactive glycoproteins absent from extracts of vector-transfected cells and absent when incubations were conducted using peptide- or fusion protein-absorbed antibody. In SDS-PAGE of membranes prepared from rat midbrain and cortex, SERTs migrate as single 76 kDa polypeptides with a relative abundance consistent with the known distribution of 5HT neuro...