Structure-activity relationships for substrate recognition by the human dopamine transporter

Novelty-induced hyperactivity and suppressed cocaine induced locomotor activation in mice lacking threonine 53 phosphorylation of dopamine transporter

Dopamine (DA) transporter (DAT) is dynamically regulated by several protein kinases and the Thr53 phosphorylation of DAT (pT53-DAT) is documented in heterologous cell models and in rat brain. However, the role of endogenous pT53-DAT in living animals has never been addressed. Here we generated and studied the pT53-lacking DAT mouse model (DAT-Ala53) by CRISPR/Cas9 technology. DAT-Ala53 mice showed normal growth, body weight, body temperature, grip strength, and sucrose preference while pT53-DAT was completely absent. However, DAT-Ala53 mice showed hyperlocomotion, pronounced vertical exploratory behavior, and stereotypy in a novel environment compared to wild-type littermates (WT). DAT-Ala53 mice displayed unaltered levels of monoamines, glutamate, and GABA in the striatum compared to WT. There were also no significant differences between DAT-Ala53 mice and WT in tyrosine hydroxylase (TH) and phospho-TH levels, or in total and surface DAT levels, or in DA-transport kinetic parameters V_max and K_m. Immunohistochemical and colocalization analyses of TH and DAT in caudate-putamen and nucleus accumbens revealed no significant differences between DAT-Ala53 and WT mice. Interestingly, cocaine's potency to inhibit striatal DA transport and cocaine-induced locomotor activation were significantly reduced in the DAT-Ala53 mice. Also, ERK1/2 inhibitors completely failed to inhibit striatal DA uptake in DAT-Ala53 mice. Collectively, our findings reveal that the mice lacking pT53-DAT display novelty-induced hyperactive phenotype despite having normal transporter protein expression, DA-transport kinetics and DA-linked markers. The results also reveal that the lack of endogenous pT53-DAT renders DAT resistant to ERK1/2 inhibition and also less susceptible to cocaine inhibition and cocaine-evoked locomotor stimulation.

Uliginosin B, a phloroglucinol derivative from Hypericum polyanthemum: a promising new molecular pattern for the development of antidepressant drugs

In this study we have demonstrated that cyclohexane extract of Hypericum polyanthemum (POL) and its main phloroglucinol derivative uliginosin B (ULI) present antidepressant-like activity in rodent forced swimming test (FST). The involvement of monoaminergic neurotransmission on the antidepressant-like activity of ULI was evaluated in vivo and in vitro. POL 90 mg/kg (p.o.) and ULI 10 mg/kg (p.o.) reduced the immobility time in the mice FST without altering locomotion activity in the open-field test. The combination of sub-effective doses of POL (45 mg/kg, p.o.) and ULI (5 mg/kg, p.o.) with sub-effective doses of imipramine (10 mg/kg, p.o.), bupropion (3 mg/kg, p.o.) and fluoxetine (15 mg/kg, p.o.) induced a significant reduction on immobility time in FST. The pretreatment with SCH 23390 (15 μg/kg, s.c., dopamine D1 receptor antagonist), sulpiride (50 mg/kg, i.p., dopamine D2 receptor antagonist), prazosin (1mg/kg, i.p., α1-adrenoceptor antagonist), yohimbine (1mg/kg, i.p., α2-adrenoceptor antagonist) and pCPA (100 mg/kg/day, i.p., p-chlorophenilalanine methyl ester, inhibitor of serotonin synthesis, for four consecutive days) before ULI administration (10 mg/kg, p.o.) significantly prevented the anti-immobility effect in FST. ULI was able to inhibit synaptosomal uptake of dopamine (IC50 = 90 ± 38 nM), serotonin (IC50 = 252 ± 13 nM) and noradrenaline (280 ± 48 nM), but it did not bind to any of the monoamine transporters. These data firstly demonstrated the antidepressant-like effect of POL and ULI, which depends on the activation of the monoaminergic neurotransmission in a different manner from the most antidepressants.

SKF-83566, a D1-dopamine receptor antagonist, inhibits the dopamine transporter

Dopamine (DA) is an important transmitter in both motor and limbic pathways. We sought to investigate the role of D(1)-receptor activation in axonal DA release regulation in dorsal striatum using a D(1)-receptor antagonist, SKF-83566. Evoked DA release was monitored in rat striatal slices using fast-scan cyclic voltammetry. SKF-83566 caused a concentration-dependent increase in peak single-pulse evoked extracellular DA concentration, with a maximum increase of ∼ 65% in 5 μM SKF-83566. This was accompanied by a concentration-dependent increase in extracellular DA concentration clearance time. Both effects were occluded by nomifensine (1 μM), a dopamine transporter (DAT) inhibitor, suggesting that SKF-83566 acted via the DAT. We tested this by examining [(3)H]DA uptake into LLc-PK cells expressing rat DAT, and confirmed that SKF-83566 is a competitive DAT inhibitor with an IC(50) of 5.7 μM. Binding studies with [(3)H]CFT, a cocaine analog, showed even more potent action of SKF-83566 at the DAT cocaine binding site (IC(50) = 0.51 μM). Thus, data obtained using SKF-83566 as a D(1) DA-receptor antagonist may be confounded by concurrent DAT inhibition. More positively, however, SKF-83566 might be a candidate to attenuate cocaine effects in vivo because of the greater potency of this drug at the cocaine versus DA binding site of the DAT.

Design and synthesis of a photocleavable biotin-linker for the photoisolation of ligand-receptor complexes based on the photolysis of 8-quinolinyl sulfonates in aqueous solution

The ability of avidin (Avn) to form strong complex with biotin (Btn) is frequently used in the detection and isolation of biomolecules in biochemical, analytical, and medicinal research. The fact that the binding is nealy irreversible, however, constitutes a drawback in term of the isolation and purification of intact biomolecules. We recently found that 8-quinolinyl esters of aromatic or aliphatic sulfonic acids undergo photolysis when irradiated at 300-330 nm in aqueous solution at neutral pH. In this work, a biotin-dopamine (BD) conjugate containing a photocleavable 8-quinolinyl benzenesulfonate (QB) linker, BDQB, was designed and synthesized for use in the efficient recovery of dopamine-protein (e.g., antibody) complexes from an Avn-Btn system. The complexation of BDQB with a primary anti-dopamine antibody (anti-dopamine IgG(1) from mouse) on an Avn-coated plate was confirmed by an enzyme-linked immunosorbent assay (ELISA) utilizing a secondary antibody (anti-IgG(1) antibody) conjugated with horseradish peroxidase (HRP). Upon the photoirradiation (at 313 nm) of the BDQB-IgG(1) complex, the release of dopamine-IgG(1) complex was confirmed by ELISA. Characterization of the resulting photoreleased dopamine-anti-dopamine IgG(1) complex was performed by SDS-PAGE and Western blot.

Interaction of catechol and non-catechol substrates with externally or internally facing dopamine transporters

Our previous work suggested that collapsing the Na(+) gradient and membrane potential converts the dopamine (DA) transporter (DAT) to an inward-facing conformation with a different substrate binding profile. Here, DAT expressing human embryonic kidney 293 cells were permeabilized with digitonin, disrupting ion/voltage gradients and allowing passage of DAT substrates. The potency of p-tyramine and other non-catechols (d-amphetamine, beta-phenethylamine, MPP(+)) in inhibiting cocaine analog binding to DAT in digitonin-treated cells was markedly weakened to a level similar to that observed in cell-free membranes. In contrast, the potency of DA and another catechol, norepinephrine, was not significantly changed by the same treatment, whereas epinephrine showed only a modest reduction. These findings suggest that catechol substrates interact symmetrically with both sides of DAT and non-catechol substrates, favoring binding to outward-facing transporter. In the cocaine analog binding assay, the mutant W84L displayed enhanced intrinsic binding affinity for substrates in interacting with both outward- and inward-facing states; D313N showed wild-type-like symmetric binding; but D267L and E428Q showed an apparent improvement in the permeation pathway from the external face towards the substrate site. Thus, the structure of both substrate and transporter play a role in the sidedness and mode of interaction between them.

chi-Conotoxin and tricyclic antidepressant interactions at the norepinephrine transporter define a new transporter model

Monoamine neurotransmitter transporters for norepinephrine (NE), dopamine and serotonin are important targets for antidepressants and analgesics. The conopeptide chi-MrIA is a noncompetitive and highly selective inhibitor of the NE transporter (NET) and is being developed as a novel intrathecal analgesic. We used site-directed mutagenesis to generate a suite of mutated transporters to identify two amino acids (Leu(469) and Glu(382)) that affected the affinity of chi-MrIA to inhibit [(3)H]NE uptake through human NET. Residues that increased the K(d) of a tricyclic antidepressant (nisoxetine) were also identified (Phe(207), Ser(225), His(296), Thr(381), and Asp(473)). Phe(207), Ser(225), His(296), and Thr(381) also affected the rate of NE transport without affecting NE K(m). In a new model of NET constructed from the bLeuT crystal structure, chi-MrIA-interacting residues were located at the mouth of the transporter near residues affecting the binding of small molecule inhibitors.

Hypericum caprifoliatum (Guttiferae) Cham. & Schltdl.: a species native to South Brazil with antidepressant-like activity

In this work, previously published and unpublished results on biological activity of Hypericum caprifoliatum, a native species to South Brazil, are presented. Lipophilic extracts obtained from this species showed an antidepressant-like activity in mice and rat forced swimming test. Results from in vivo experiments suggest an effect on the dopaminergic transmission. Besides that, in vitro experiments demonstrated that the extract and its main component (a phloroglucinol derivative) inhibit monoamine uptake in a concentration-dependent manner, more potently to dopamine, but this effect is not related to direct binding at the uptake sites. It was also observed that a 3-day treatment with lipophilic extract prevents stress-induced corticosterone rise in mice frontal cortex but not in plasma. The lipophilic and methanolic H. caprifoliatum extracts also demonstrated antinociceptive effect, which seems to be indirectly mediated by the opioid system. These results indicate that H. caprifoliatum presents a promising antidepressant-like effect in rodents which seems to be related to a mechanism different from that of other classes of antidepressants.

Role of dopamine transporter (DAT) in dopamine transport across the nasal mucosa

Dopamine is a catecholamine neurotransmitter necessary for motor functions. Its deficiency has been observed in several neurological disorders, but replacement of endogenous dopamine via oral or parenteral delivery is limited by poor absorption, rapid metabolism and the inability of dopamine to cross the blood-brain barrier. The intranasal administration of dopamine, however, has resulted in improved central nervous system (CNS) bioavailability compared to that obtained following intravenous delivery. Portions of the nasal mucosa are innervated by olfactory neurons expressing dopamine transporter (DAT) which is responsible for the uptake of dopamine within the central nervous system. The objective of these studies was to study the role of DAT in dopamine transport across the bovine olfactory and nasal respiratory mucosa. Western blotting studies demonstrated the expression of DAT and immunohistochemistry revealed its epithelial and submucosal localization within the nasal mucosa. Bidirectional transport studies over a 0.1-1 mM dopamine concentration range were carried out in the mucosal-submucosal and submucosal-mucosal directions to quantify DAT activity, and additional transport studies investigating the ability of GBR 12909, a DAT inhibitor, to decrease dopamine transport were conducted. Dopamine transport in the mucosal-submucosal direction was saturable and was decreased in the presence of GBR 12909. These studies demonstrate the activity of DAT in the nasal mucosa and provide evidence that DAT-mediated dopamine uptake plays a role in the absorption and distribution of dopamine following intranasal administration.

Synthesis and monoamine transporter affinity of 3-aryl substituted trop-2-enes

A series of new 3-aryl-tropanes have been synthesized, and their affinities and selectivities were evaluated for monoamine transporters. (1RS)-3-(Fluoren-2-yl)-8-methyl-8-azabicyclo[3.2.1]oct-2-ene exhibited the highest affinity for the human serotonin transporter (IC(50)=14.5nM). It is also 52-fold and 230-fold selective over human dopamine and norepinephrine transporters, respectively.

Pharmacological characterization of ecstasy synthesis byproducts with recombinant human monoamine transporters

Ecstasy samples often contain byproducts of the illegal, uncontrolled synthesis of N-methyl-3,4-methylenedioxy-amphetamine or 3,4-methylenedioxymethamphetamine (MDMA). MDMA and eight chemically defined byproducts of MDMA synthesis were investigated for their interaction with the primary sites of action of MDMA, namely the human plasmalemmal monamine transporters for norepinephrine, serotonin, and dopamine [(norepinephrine transporter (NET), serotonin transporter (SERT), and dopamine transporter (DAT)]. SK-N-MC neuroblastoma and human embryonic kidney cells stably transfected with the transporter cDNA were used for uptake and release experiments. Two of the eight compounds, 1,3-bis (3,4-methylenedioxyphenyl)-2-propanamine (12) and N-formyl-1,3-bis (3,4-methylenedioxyphenyl)-prop-2-yl-amine (13) had uptake inhibitory potencies with IC50 values in the low micromolar range similar to MDMA. Compounds with nitro instead of amino groups and a phenylethenyl instead of a phenylethyl structure or a formamide or acetamide modification had IC50 values beyond 100 microM. MDMA, 12, and 13 were examined for induction of carrier-mediated release by superfusion of transporter expressing cells preloaded with the metabolically inert transporter substrate [3H]1-methyl-4-phenylpyridinium. MDMA induced release mediated by NET, SERT, or DAT with EC50 values of 0.64, 1.12, and 3.24 microM, respectively. 12 weakly released from NET- and SERT-expressing cells with maximum effects less than one-tenth of that of MDMA and did not release from DAT cells. 13 had no releasing activity. 12 and 13 inhibited release induced by MDMA, and the concentration dependence of this effect correlated with their uptake inhibitory potency at the various transporters. These results do not support a neurotoxic potential of the examined ecstasy synthesis byproducts and provide interesting structure-activity relationships on the transporters.

Time-dependent changes in receptor/G-protein coupling in rat brain following chronic monoamine transporter blockade

The potent tropane analog, WF-23 [2beta-propanoyl-3beta-(2-naphthyl) tropane], blocks dopamine, serotonin, and norepinephrine transporters with high affinity in vitro and blocks transporters for at least 2 days following a single in vivo administration. Previous studies demonstrated desensitization of monoamine receptor-coupled G-proteins in brain following chronic treatment of rats with WF-23. The current study sought to determine the time course of this desensitization and the behavioral effects of receptor desensitization. Rats were treated with 1 mg/kg WF-23 and injected i.p. every 48 h for 1 to 21 days. Receptor activation of G-proteins was determined by guanosine 5'-O-(3-[(35)S]thiotriphosphate) ([(35)S]GTPgammaS) binding in brain sections for monoamine receptors, as well as mu opioid receptors as a nonmonoamine receptor control. Chronic treatment with WF-23 produced significant reductions in D(2), 5-hydroxytryptamine 1A, and alpha(2)-adrenergic receptor-stimulated [(35)S]GTPgammaS binding; however, the time course of desensitization varied with different receptors. There was no effect of WF-23 treatment on mu opioid-stimulated [(35)S]GTPgammaS binding at any time point. Consistent with previous studies, there was no effect of WF-23 treatment on D(2) receptor binding, as determined by [(3)H]spiperone autoradiography. Locomotor activity was significantly increased for up to 48 h following acute administration of WF-23, demonstrated by increased photocell beam interruptions. WF-23-induced increases in locomotor activity occurred following repeated administration, as above, for up to 7 days. Following 7 days of treatment, there was a significant decrease in WF-23-increased locomotor activity. This reduction occurred at the same time point as the decrease in D(2) receptor/G-protein coupling, suggesting a role of D(2) desensitization in producing tolerance to WF-23-mediated behavior.

A comprehensive atlas of the topography of functional groups of the dopamine transporter

The neuronal dopamine transporter (DAT) is a transmembrane transporter that clears DA from the synaptic cleft. Knowledge of DAT functional group topography is a prerequisite for understanding the molecular basis of transporter function, the actions of psychostimulant drugs, and mechanisms of dopaminergic neurodegeneration. Information concerning the molecular interactions of drugs of abuse (such as cocaine, amphetamine, and methamphetamine) with the DAT at the functional group level may also aid in the development of compounds useful as therapeutic agents for the treatment of drug abuse. This review will provide a cumulative and comprehensive focus on the amino acid functional group topography of the rat and human DATs, as revealed by protein chemical modification and the techniques of site-directed mutagenesis. The results from these studies, represented mostly by site-directed mutagenesis, can be classified into several main categories: modifications without substantial affects on substrate transport, DAT membrane expression, or cocaine analog binding; those modifications which alter both substrate transport and cocaine analog binding; and those that affect DAT membrane expression. Finally, some modifications can selectively affect either substrate transport or cocaine analog binding. Taken together, these literature results show that domains for substrates and cocaine analogs are formed by interactions with multiple and sometimes distinct DAT functional groups.