[3H]Imipramine Binding of Protein Nature in Human Platelets: Inhibition by 5-Hydroxytryptamine and 5-Hydroxytryptamine Uptake Inhibitors

Abstract: The nature of [³H]imipramine binding to human platelets was investigated. Desipramine and 5-hydroxytryptamine (5-HT) displaced the same amount of binding and the binding was sensitive to protease treatment. The nature of pharmacological inhibition of [³H]imipramine binding was investigated in saturation experiments. Increases in K d without changes in B _max were noted with the addition of 5-HT, desipramine, norzimeldine, or 5-methoxytryptoline. Reductions in B _max without alterations in K _D were obtained when citalopram or clomipramine was added. It is concluded that the [³H]imipramine binding site in human platelets is of protein nature and that this binding site contains the substrate recognition site for 5-HT uptake. In addition, [³H]imipramine and other 5-HT uptake inhibitors have bonds to other parts of the 5-HT uptake carrier or to the surrounding lipid membrane. This additional binding outside the substrate recognition site is not one single site but most likely represents sites that are specific for the chemical structure of each uptake inhibitor, respectively.     

Dissociation Kinetics of [3H]Paroxetine Binding to Rat Brain Consistent with a Single-Site Model of the Antidepressant Binding/5-Hydroxytryptamine Uptake Site

The effects of 5-hydroxytryptamine (5-HT) and 5-HT uptake inhibitors on the dissociation of [3H]paroxetine from rat brain membrane binding sites have been investigated. The dissociation induced by 5-HT (100 microM), paroxetine (0.15 microM), clomipramine (1 microM), citalopram (1 microM), imipramine (1 microM), or norzimeldine (1 microM) was consistent with first-order dissociation kinetics with half-life values of dissociation (t1/2) between 130 and 140 min. The dissociation induced by the combination of 5-HT (100 microM) with either citalopram (1 microM) or imipramine (1 microM) was not different from that initiated by either agent alone. These dissociation data, which are at variance with previous data on the 5-HT transporter labeled with [3H]imipramine, support a single-site model of the antidepressant binding/5-HT uptake site.     

Tryptamine, a Substrate for the Serotonin Transporter in Human Platelets, Modifies the Dissociation Kinetics of [3H]Imipramine Binding: Possible Allosteric Interaction

Tricyclic antidepressants and nontricyclic serotonin (5-hydroxytryptamine) uptake blockers monophasically inhibit [3H]imipramine binding in human platelets. Similarly, serotonin and tryptamine inhibit the binding of [3H]imipramine in the low micromolar range and with a pseudo-Hill coefficient near unity. Dissociation of the [3H]imipramine receptor complex in the presence of uptake inhibitors follows first-order kinetics with a half-life of approximately 60 min. Although serotonin and tryptamine do not decrease [3H]imipramine binding when added under equilibrium conditions, simultaneous addition of serotonin or tryptamine with serotonin uptake inhibitors decreases the rate of ligand-receptor dissociation in a concentration-dependent manner. These data suggest a common site of action for serotonin, which is the substrate of the transporter system, and of tryptamine, its nonhydroxylated analog. This hypothesis is supported by the identification of a high-affinity (Km = 0.55 microM), saturable, and temperature-dependent uptake of [3H]tryptamine in human platelets. Uptake of [3H]tryptamine was inhibited potently by imipramine and nontricyclic serotonin uptake inhibitors with a potency similar to that observed for [3H]serotonin uptake. These data support the hypothesis that in platelets, [3H]imipramine, tricyclic, and nontricyclic serotonin uptake inhibitors bind to a common recognition site that is associated with the serotonin transporter but that differs from the substrate recognition site of the carrier through which serotonin and tryptamine exert a heterotropic allosteric modulation on [3H]imipramine binding.     

Characterization of Serotonin Transporter in Goldfish Retina by the Binding of [3H]Paroxetine and the Uptake of [3H]Serotonin: Modulation by Light

Abstract: The serotonin (5-HT) uptake system of goldfish retina was evaluated by the binding of [³H]paroxetine to membrane preparations and the uptake of [³H]5-HT into isolated cells from goldfish retina. The order of potency of inhibitors of [³H]paroxetine binding was imipramine > 5-methoxy- N,N -dimethyltryptamine > desipramine > fluoxetine > citalopram > 5-HT. The saturation experiments indicated a high-affinity binding site, and positive cooperativity with Hill coefficient higher than unity. The association reached equilibrium at about one hour of incubation and was efficiently displaced by imipramine. The equilibrium dissociation constants calculated by the antilog of the log concentration of ligand giving 50% of occupation, and by the ratio of dissociation/association constants, were similar: 5.84 and 2.34 n M , respectively. The binding was not significantly reduced by decreasing the temperature of incubation and was sodium dependent. The lesion with 5,7-dihydroxytryptamine reduced the binding to 60%. The uptake of [³H]5-HT into isolated cells also showed positive cooperativity. The order of potency of inhibitors was similar to the one obtained for the binding of [³H]paroxetine. Darkness increased the uptake of 5-HT. The allosteric regulation of the 5-HT transporter and the modulation by light could be related to the physiological role of the monoamine, as a neurotransmitter and as a precursor of melatonin synthesis in the retina.     

Complex Inhibition of [3H]Imipramine Binding by Serotonin and Nontricyclic Serotonin Uptake Blockers

Tricyclic antidepressant drugs inhibit [3H]imipramine binding to the rat brain cortex in a competitive manner, giving linear Hofstee plots and Hill coefficients of approximately 1.0. Serotonin, the only neurotransmitter to inhibit [3H]imipramine binding, does so in a complex manner, exhibiting a Hill coefficient of 0.40-0.50. Nontricyclic inhibitors of serotonin uptake such as fluoxetine, paroxetine, norzimelidine, and citalopram inhibit [3H]imipramine binding in the same complex manner as serotonin. These results are interpreted as suggesting that [3H]imipramine binds to a site associated with the serotonin uptake system but different from either the substrate recognition site for serotonin or the site of action of the nontricyclic inhibitors of neuronal uptake of serotonin.     

Species-scanning mutagenesis of the serotonin transporter reveals residues essential in selective, high-affinity recognition of antidepressants

The serotonin transporter (SERT) is a high-affinity 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 profiles 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, fluoxetine, paroxetine and imipramine were several-fold higher at hSERT compared with bSERT. No species selectivity was observed for the antidepressants fluvoxamine, and sertraline or for the psychostimulants cocaine, the cocaine analogue beta-carbomethoxy-3beta-(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 fluoxetine and imipramine. Results were obtained by doing the forward, bovine to human, mutations and confirmed by doing the reverse mutations. Citalopram analogues were used to define 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.     

Characterisation of the zebrafish serotonin transporter functionally links TM10 to the ligand binding site

The selective serotonin reuptake inhibitors and tricyclic antidepressants act by inhibiting pre-synaptic reuptake of serotonin (5-HT) leading to elevated synaptic 5-HT concentrations. However, despite extensive efforts little is known about the protein-ligand interactions of serotonin transporter (SERT) and inhibitors. To identify domains and individual amino acids important for ligand binding, we cloned the serotonin transporter from zebrafish, Danio rerio , (drSERT) and compared its pharmacological profile to that of the human serotonin transporter (hSERT) with respect to inhibition of [³H]5-HT uptake and [³H]-escitalopram binding in transiently transfected human embryonic kidney cells; HEK293-MSR. Residues responsible for altered affinities inhibitors were pinpointed by generating cross-species chimeras and subsequent point mutations by site directed mutagenesis. drSERT has a higher affinity towards compounds of the imipramine class, desipramine in particular, exhibiting a 35-fold increased affinity compared to hSERT. drSERT has a 15–30-fold lower affinity towards cocaine and cocaine analogues. The differences in ligand recognition are shown to be primarily caused by interspecies differences in TM10 and were tracked down to three residues (Ala⁵⁰⁵, Leu⁵⁰⁶ and Ile⁵⁰⁷).     

High-Affinity Binding of [3H]Desipramine to Rat Brain: A Presynaptic Marker for Noradrenergic Uptake Sites

Abstract: High-affinity binding sites (apparent K _D= 1.5 nM) for [³H]desipramine have been demonstrated and characterized in membranes prepared from rat brain. The binding of [3H]desipramine was found to be saturable, reversible, heat-sensitive, sodium-dependent, and regionally distributed among various regions of the brain. High concentrations of [³H]desipramine binding sites were found in the septum, cerebral cortex, and hypothalamus, whereas lower concentrations were found in the medulla, cerebellum, and corpus striatum. A very good correlation ( r = 0.81, P < 0.001) was observed between the potencies of a series of drugs in inhibiting high-affinity [³H]desipramine binding and their capacity to block norepinephrine uptake into synaptosomes. In 6-hydroxydopamine-lesioned rats there was a marked decrease in [³H]norepinephrine uptake and [3H]desipramine binding with no significant alterations in either [³H]serotonin uptake or [³H]imipramine binding. These results suggest that the high-affinity binding of [³HJdesipramine to rat brain membranes is pharmacologically and biochemically distinct from the high-affinity binding of [3H]imipramine, and that there is a close relationship between the high-affinity binding site for [³H]desipramine and the uptake site for norepinephrine.     

The brain 5-HT4 receptor binding is down-regulated in the Flinders Sensitive Line depression model and in response to paroxetine administration

The 5-hydroxytryptamine (5-HT₄) receptor may be implicated in depression and is a new potential target for antidepressant treatment. We have investigated the brain 5-HT₄ receptor [³H]SB207145 binding in the Flinders Sensitive Line rat depression model by quantitative receptor autoradiography, and related this to 5-HT transporter ( S )-[ N -methyl-³H]citalopram binding. We also determined the regulation of 5-HT₄ receptor binding by 1, 14, and 21 days of paroxetine administration and subchronic 5-HT depletion, and compared this with changes in 5-HT_2A receptor [³H]MDL100907 binding. In the Flinders Sensitive Line, the 5-HT₄ receptor and 5-HT transporter binding were decreased in the dorsal and ventral hippocampus, and the changes in binding were directly correlated within the dorsal hippocampus. Chronic but not acute paroxetine administration caused a 16–47% down-regulation of 5-HT₄ receptor binding in all regions evaluated including the basal ganglia and hippocampus, while 5-HT depletion increased the 5-HT₄ receptor binding in the dorsal hippocampus, hypothalamus, and lateral globus pallidus. In comparison, the 5-HT_2A receptor binding was decreased in the frontal and cingulate cortices after chronic paroxetine administration, and markedly reduced in several regions after 5-HT depletion. Thus, the 5-HT₄ receptor binding was decreased in the Flinders Sensitive Line depression model and in response to chronic paroxetine administration.     

The Pro-inflammatory Cytokine TNF-α Regulates the Activity and Expression of the Serotonin Transporter (SERT) in Astrocytes

Pro-inflammatory cytokines have been implicated in the precipitation of depression and related disorders, and the antidepressant sensitive serotonin transporter (SERT) may be a major target for immune regulation in these disorders. Here, we focus on astrocytes, a major class of immune competent cells in the brain, to examine the effects of pro-longed treatment with tumor necrosis factor-alpha (TNF-α) on SERT activity. We first established that high-affinity serotonin uptake into C6 glioma cells occurs through a SERT-dependent mechanism. Functional SERT expression is also confirmed for primary astrocytes. In both cell types, exposure to TNF-α resulted in a dose- and time-dependent increase in SERT-mediated 5-HT uptake, which was sustained for at least 48 h post-stimulation. Further analysis in primary astrocytes revealed that TNF-α enhanced the transport capacity (V_max) of SERT-specific 5-HT uptake, suggesting enhanced transporter expression, consistent with our observation of an increase in SERT mRNA levels. We confirmed that in both, primary astrocytes and C6 glioma cells, treatment with TNF-α activates the p38 mitogen-activated protein kinase (MAPK) signaling pathway. Pre-treatment with the p38 MAPK inhibitor SB203580 attenuated the TNF-α mediated stimulation of 5-HT transport in both, C6 glioma and primary astrocytes. In summary, we show that SERT gene expression and activity in astrocytes is subject to regulation by TNF-α, an effect that is at least in part dependent on p38 MAPK activation.     

Temperature Dependence of Drug Interaction with the Platelet 5-Hydroxytryptamine Transporter: A Clue to the Imipramine Selectivity Paradox

Although [3H]imipramine is a selective radioligand for the 5-hydroxytryptamine (5-HT) transporter in human platelets, its affinity for binding to the 5-HT transporter complex at 0 degrees C (0.6 nM) is significantly higher than its potency for inhibition of [3H]5-HT uptake at the physiological temperature of 37 degrees C (Ki = 29 nM). As this apparent discrepancy could be related to the assay temperature, we studied the thermodynamics of drug interaction with the 5-HT transporter at assay temperatures between 0 degrees C and 37 degrees C, using as radioligands [3H]imipramine (0 degrees C and 20 degrees C) and [3H]paroxetine (20 degrees C and 37 degrees C), a newly available probe for the 5-HT transporter. At 20 degrees C, Ki values of 14 tricyclic and nontricyclic drugs for inhibition of [3H]imipramine and [3H]paroxetine binding to human platelet membranes were highly significantly correlated (r = 0.98, p less than 0.001), validating the use of these two radioligands to study the 5-HT transporter over a temperature range larger than was previously possible with [3H]imipramine alone. The affinity of imipramine for the 5-HT transporter is progressively enhanced with decreasing incubation temperature, thus favoring the selectivity of [3H]imipramine for the 5-HT transporter at 0 degrees C. At 37 degrees C, the Ki of imipramine for inhibition of [3H]paroxetine binding is 32 nM, and equals its Ki value for inhibition of 5-HT uptake into human platelets. With the exception of chlorimipramine, other tricyclic 5-HT uptake inhibitors showed a temperature sensitivity in their interaction with the 5-HT transporter similar to that of imipramine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mutational Mapping and Modeling of the Binding Site for (S)-Citalopram in the Human Serotonin Transporter

The serotonin transporter (SERT) regulates extracellular levels of the neurotransmitter serotonin (5-hydroxytryptamine) in the brain by facilitating uptake of released 5-hydroxytryptamine into neuronal cells. SERT is the target for widely used antidepressant drugs, including imipramine, fluoxetine, and (S)-citalopram, which are competitive inhibitors of the transport function. Knowledge of the molecular details of the antidepressant binding sites in SERT has been limited due to lack of structural data on SERT. Here, we present a characterization of the (S)-citalopram binding pocket in human SERT (hSERT) using mutational and computational approaches. Comparative modeling and ligand docking reveal that (S)-citalopram fits into the hSERT substrate binding pocket, where (S)-citalopram can adopt a number of different binding orientations. We find, however, that only one of these binding modes is functionally relevant from studying the effects of 64 point mutations around the putative substrate binding site. The mutational mapping also identify novel hSERT residues that are crucial for (S)-citalopram binding. The model defines the molecular determinants for (S)-citalopram binding to hSERT and demonstrates that the antidepressant binding site overlaps with the substrate binding site.     

[3H]Citalopram Binding to Brain and Platelet Membranes of Human and Rat

Citalopram, a selective serotonin (5-HT) uptake inhibitor with antidepressant properties, was found to bind with high affinity to the 5-HT transporter from human neuronal and platelet membranes. At 20 degrees C, KD was about 1.5 nM in both tissues. [3H]Citalopram bound to rat neuronal membranes with higher affinity than to human neuronal and platelet membranes; at 20 degrees C KD was about 0.7 nM. The Bmax value for the binding of [3H]citalopram to platelet membranes was close to that found using the 5-HT uptake inhibitors [3H]imipramine and [3H]paroxetine, suggesting that all three 5-HT uptake inhibitors bind to the 5-HT transporter. The dissociation rate of [3H]citalopram increased twofold with each 4-5 degree C increase in temperature in both human and rat membranes, although at any given temperature, the dissociation rate was about four times faster in the human neuronal and platelet membranes than in rat neuronal membranes.     

Characterization of [3H]Paroxetine Binding in Rat Brain

The binding of the 5-hydroxytryptamine (5-HT, serotonin) uptake inhibitor [3H]paroxetine to rat cortical homogenates has been characterized. The effect of tissue concentration was examined and, with 0.75 mg wet weight tissue/ml in a total volume of 1,600 microliter, the binding was optimized with an apparent dissociation constant (KD) of 0.03-0.05 nM. Competition experiments with 5-HT, citalopram, norzimeldine, and desipramine revealed a high (90%) proportion of displaceable binding that fitted a single-site binding model. Fluoxetine and imipramine revealed, in addition to a high-affinity (nanomolar) site, also a low-affinity (micromolar) site representing approximately 10% of the displaceable binding. The specificity of the [3H]paroxetine binding was emphasized by the fact that 5-HT was the only active neurotransmitter bound and that the serotonin S1 and S2 antagonist methysergide was without effect on the binding. Both 5-HT- and fluoxetine-sensitive [3H]paroxetine binding was completely abolished after protease treatment, suggesting that the binding site is of protein nature. Saturation studies with 5-HT (100 microM) sensitive [3H]paroxetine binding were also consistent with a single-site binding model, and the binding was competitively inhibited by 5-HT and imipramine. The number of binding sites (Bmax) for 5-HT-sensitive [3H]paroxetine and [3H]imipramine binding was the same, indicating that the radioligands bind to the same sites. Lesion experiments with p-chloroamphetamine resulted in a binding in frontal and parietal cortices becoming undetectable and a greater than 60% reduction in the striatum and hypothalamus, indicating a selective localization on 5-HT terminals. Together these findings suggest that [3H]paroxetine specifically and selectively labels the substrate recognition site for 5-HT uptake in rat brain.     

Serotonin (5-HT) transport in human platelets is modulated by Src-catalysed Tyr-phosphorylation of the plasma membrane transporter SERT.

BACKGROUND/AIM: platelets possess tightly regulated systems for serotonin (5-HT) transport. This study analysed whether the 5-HT transport mediated by the plasma-membrane transporter SERT is regulated by its Tyr-phosphorylation. METHODS: 5-HT transport was determined by filtration techniques, while immunoblotting procedures were adopted for detecting the Tyr-phosphorylation of SERT in human platelet fractions. RESULTS: 5-HT accumulation in platelets pre-treated with reserpine, which prevents the neurotransmitter transport into the dense granules, decreased upon cellular exposure to PP2 and SU6656, two structurally unrelated inhibitors of Src-kinases. By contrast, the protein Tyr-phosphatase inhibitor pervanadate increased the 5-HT accumulation. Anti-SERT immunostaining of the platelet fractions showed a major band displaying an apparent molecular mass of 50 kappaDa, indicating that, during the analytical procedure, SERT underwent proteolysis, which was counteracted by addition of 4 M urea in the cellular disrupting medium. The Tyr-phosphorylation degree of SERT immunoprecipitated from membrane extracts decreased by platelet treatment with SU6656 or PP2, and enhanced upon pervanadate treatment. The anti-SERT immunoprecipitates displayed anti-Src immunostaining and in vitro kinase activity towards a Src-specific peptide-substrate. Platelet treatment with PP2 or SU6656 also caused a decrease in the imipramine binding to platelets. It was concluded that the Src-mediated SERT Tyr-phosphorylation regulates the 5-HT transport by affecting the neurotransmitter binding sites.     

5-Methoxytryptoline, a Competitive Endocoid Acting at [3H]Imipramine Recognition Sites in Human Platelets

5-Methoxytryptoline potently inhibits [3H]imipramine binding to membranes from the cerebral cortex and platelets. Since 5-methoxytryptoline, which appears to occur endogenously with particularly high levels in the human pineal gland, also inhibits 5-hydroxytryptamine (5-HT, serotonin) uptake, it should be considered as a putative endogenous ligand modulating 5-HT transport. As the 5-HT transporter complex comprises the imipramine and the substrate recognition sites, which interact allosterically, it was essential to define the mechanism of inhibition of [3H]imipramine binding by 5-methoxytryptoline. Human platelets show an active and saturable uptake of 5-HT and tryptamine. The uptake of both substrates appears to be mediated by the same carrier and it is inhibited by 5-methoxytryptoline at submicromolar concentrations. 5-HT and tryptamine inhibit [3H]imipramine binding in human platelets with a Hill slope for inhibition close to unity and IC50 values of 3,265 and 3,475 nM, respectively. This inhibition is, however, not competitive because both 5-HT and tryptamine significantly decrease the rate of [3H]imipramine-receptor dissociation. Although 5-methoxytryptoline potently inhibits [3H]imipramine binding (IC50 = 44 nM) in human platelets with a Hill slope of unity, it does not affect the receptor-ligand dissociation rate of [3H]imipramine even at concentrations up to 100 microM. The present experiments show that 5-methoxytryptoline, in spite of its chemical similarity to the indoleamine transporter substrates, interacts with the imipramine receptor through a mechanism of competitive inhibition. This conclusion is supported by a selective effect of 5-methoxytryptoline on the Kd of [3H]imipramine binding.(ABSTRACT TRUNCATED AT 250 WORDS)     

Substrate binding and translocation of the serotonin transporter studied by docking and molecular dynamics simulations

The serotonin (5-HT) transporter (SERT) plays an important role in the termination of 5-HT-mediated neurotransmission by transporting 5-HT away from the synaptic cleft and into the presynaptic neuron. In addition, SERT is the main target for antidepressant drugs, including the selective serotonin reuptake inhibitors (SSRIs). The three-dimensional (3D) structure of SERT has not yet been determined, and little is known about the molecular mechanisms of substrate binding and transport, though such information is very important for the development of new antidepressant drugs. In this study, a homology model of SERT was constructed based on the 3D structure of a prokaryotic homologous leucine transporter (LeuT) (PDB id: 2A65). Eleven tryptamine derivates (including 5-HT) and the SSRI (S)-citalopram were docked into the putative substrate binding site, and two possible binding modes of the ligands were found. To study the conformational effect that ligand binding may have on SERT, two SERT–5-HT and two SERT–(S)-citalopram complexes, as well as the SERT apo structure, were embedded in POPC lipid bilayers and comparative molecular dynamics (MD) simulations were performed. Our results show that 5-HT in the SERT–5-HT^B complex induced larger conformational changes in the cytoplasmic parts of the transmembrane helices of SERT than any of the other ligands. Based on these results, we suggest that the formation and breakage of ionic interactions with amino acids in transmembrane helices 6 and 8 and intracellular loop 1 may be of importance for substrate translocation.     

Thiamine Deficiency in Cultured Neuroblastoma Cells: Effect on Mitochondrial Function and Peripheral Benzodiazepine Receptors

Abstract: When neuroblastoma cells were transferred to a medium of low (6 n M ) thiamine concentration, a 16-fold decrease in total intracellular thiamine content occurred within 8 days. Respiration and ATP levels were only slightly affected, but addition of a thiamine transport inhibitor (amprolium) decreased ATP content and increased lactate production. Oxygen consumption became low and insensitive to oligomycin and uncouplers. At least 25% of mitochondria were swollen and electron translucent. Cell mortality increased to 75% within 5 days. [³H]PK 11195, a specific ligand of peripheral benzodiazepine receptors (located in the outer mitochondrial membrane) binds to the cells with high affinity ( K _D = 1.4 ± 0.2 n M ). Thiamine deficiency leads to an increase in both B _max and K _D. Changes in binding parameters for peripheral benzodiazepine receptors may be related to structural or permeability changes in mitochondrial outer membranes. In addition to the high-affinity (nanomolar range) binding site for peripheral benzodiazepine ligands, there is a low-affinity (micromolar range) saturable binding for PK 11195. At micromolar concentrations, peripheral benzodiazepines inhibit thiamine uptake by the cells. Altogether, our results suggest that impairment of oxidative metabolism, followed by mitochondrial swelling and disorganization of cristae, is the main cause of cell mortality in severely thiamine-deficient neuroblastoma cells.     

Phosphorylation of threonine residue 276 is required for acute regulation of serotonin transporter by cyclic GMP

Cellular protein kinases, phosphatases, and other serotonin transporter (SERT) interacting proteins participate in several signaling mechanisms regulating SERT activity. The molecular mechanisms of protein kinase G (PKG)-mediated SERT regulation and the site of transporter phosphorylation were investigated. Treatment of rat midbrain synaptosomes with 8-bromo-cGMP increased SERT activity, and the increase was selectively blocked by PKG inhibitors. The V(max) value for serotonin (5-HT) transport increased following cGMP treatment. However, surface biotinylation studies showed no change in SERT surface abundance following PKG activation. (32)P metabolic labeling experiments showed increased SERT phosphorylation in the presence of cGMP that was abolished by selectively inhibiting PKG. Phosphoamino acid analysis revealed that cGMP-stimulated native SERT phosphorylation occurred only on threonine residues. When added to CHO-1 cells expressing SERT, 8-bromo-cGMP stimulated 5-HT transport and SERT phosphorylation. Mutation of SERT threonine 276 to alanine completely abolished cGMP-mediated stimulation of 5-HT transport and SERT phosphorylation. Although the T276A mutation had no significant effect on 5-HT transport or SERT protein expression, mutation to aspartate (T276D) increased the level of 5-HT uptake to that of cGMP-stimulated 5-HT uptake in wild-type SERT-expressing cells and was no longer sensitive to cGMP. These findings provide the first identification of a phosphorylation site in SERT and demonstrate that phosphorylation of Thr-276 is required for cGMP-mediated SERT regulation. They also constitute the first evidence that in the central nervous system PKG activation stimulates endogenous SERT activity by a trafficking-independent mechanism.     

Binding of [3H]Imipramine to Human Platelet Membranes with Compensation for Saturable Binding to Filters and Its Implication for Binding Studies with Brain Membranes

Abstract: Apparent specific binding of [³H]imipramine to human platelet membranes at high concentrations of imipramine showed deviation from that expected of a single binding site, a result consistent with a low-affinity binding site. The deviation was due to displaceable, saturable binding to the glass fibre filters used in the assays. Imipramine, chloripramine, desipramine, and fluoxetine inhibited binding to filters whereas 5-hydroxytryptamine and ethanol were ineffective. Experimental conditions were developed that eliminated filter binding, allowing assay of high and low-affinity binding to membranes. Failure to correct for filter binding may lead to overestimation of binding parameters, B_max and K_D for high-affinity binding to membranes, and may also be misinterpreted as indicating a low-affinity binding component in both platelet and brain membranes. Low-affinity binding ( K_D < 2 μ M ) of imipramine to human platelet membranes was demonstrated and its significance discussed.