Effects of several cations on the neuronal uptake of dopamine and the specific binding of [3H]GBR 12783: attempts to characterize the Na+ dependence of the neuronal transport of dopamine

Implications for Combination Therapy of Selective Monoamine Reuptake Inhibitors on Dopamine Transporters

Monoamine transporters, including dopamine, norepinephrine, and serotonin transporters (DAT, NET, and SERT, respectively), are important therapeutic targets due to their essential roles in the brain. To overcome the slow action of selective monoamine reuptake inhibitors, dual- or triple-acting inhibitors have been developed. Here, to examine whether combination treatments of selective reuptake inhibitors have synergistic effects, the pharmacological properties of DAT, NET, and SERT were investigated using the selective inhibitors of each transporter, which are vanoxerine, nisoxetine, and fluoxetine, respectively. Potencies were determined via fluorescence-based substrate uptake assays in the absence and presence of other inhibitors to test the multi-drug effects on individual transporters, resulting in antagonistic effects on DAT. In detail, fluoxetine resulted in a 1.6-fold increased IC50 value of vanoxerine for DAT, and nisoxetine produced a more drastic increase in the IC50 value by six folds. Furthermore, the effects of different inhibitors, specifically monovalent ions, were tested on DAT inhibition by vanoxerine. Interestingly, these ions also reduced vanoxerine potency in a similar manner. The homology models of DAT suggested a potential secondary inhibitor binding site that affects inhibition in an allosteric manner. These findings imply that the use of combination therapy with monoamine reuptake inhibitors should be approached cautiously, as antagonistic effects may occur.

Structure-Activity Relationships of Dopamine Transporter Pharmacological Chaperones

Mutations in the dopamine transporter gene (SLC6A3) have been implicated in many human diseases. Among these is the infantile parkinsonism-dystonia known as Dopamine Transporter Deficiency Syndrome (DTDS). Afflicted individuals have minimal to no functional dopamine transporter protein. This is primarily due to retention of misfolded disease-causing dopamine transporter variants. This results in a variety of severe motor symptoms in patients and the disease ultimately leads to death in adolescence or young adulthood. Though no treatment is currently available, pharmacological chaperones targeting the dopamine transporter have been shown to rescue select DTDS disease-causing variants. Previous work has identified two DAT pharmacological chaperones with moderate potency and efficacy: bupropion and ibogaine. In this study, we carried out structure-activity relationships (SARs) for bupropion and ibogaine with the goal of identifying the chemical features required for pharmacological chaperone activity. Our results show that the isoquinuclidine substituent of ibogaine and its analogs is an important feature for pharmacological chaperone efficacy. For bupropion, the secondary amine group is essential for pharmacological chaperone activity. Lastly, we describe additional ibogaine and bupropion analogs with varying chemical modifications and variable pharmacological chaperone efficacies at the dopamine transporter. Our results contribute to the design and refinement of future dopamine transporter pharmacological chaperones with improved efficacies and potencies.

Dopamine release mediated by the dopamine transporter, facts and consequences

Spontaneous and/or stimulated neural activity of the nigrostriatal dopamine (DA) pathway makes amines run out from the neurons. This DA dynamic follows a rather complex path, running in or out the terminals, and flushing or diffusing into the extracellular space. The location of this leakage is not limited to the axon terminals; it also occurs from the cell bodies and dendrites. This molecular release mechanism was, for a long time, considered as being produced, in part, by the exocytosis of previously stored vesicles. The DA carrier protein (DAT, DA transporter) embedded in the DA cell membrane is known to clear previously released amines through an inward DA influx. The DAT also appears to be an active vector of amine release. Particular local conditions and the presence of numerous psychostimulant substances are able to trigger an outward efflux of DA through the DAT. This process, delivering slowly large amounts of amine could play a major regulatory role in extracellular DA homeostasis.

The importance of company: Na+ and Cl- influence substrate interaction with SLC6 transporters and other proteins

SLC6 transporters, which include transporters for gamma-aminobutyric acid (GABA), norepinephrine, dopamine, serotonin, glycine, taurine, L-proline, creatine, betaine, and neutral cationic amino acids, require Na+ and Cl- for their function, and this review covers the interaction between transporters of this family with Na+ and Cl- from a structure-function standpoint. Because detailed structure-function information regarding ion interactions with SLC6 transporters is limited, we cover other proteins cotransporting Na+ or Cl- with substrate (SLClA2, PutP, SLC5A1, melB), or ion binding to proteins in general (rhodanese, ATPase, LacY, thermolysine, angiotensin-converting enzyme, halorhodopsin, CFTR). Residues can be involved in directly binding Na+ or Cl-, in coupling ion binding to conformational changes in transporter, in coupling Na+ or Cl- movement to transport, or in conferring ion selectivity. Coordination of ions can involve a number of residues, and portions of the substrate and coupling ion binding sites can be distal in space in the tertiary structure of the transporter, with other portions that are close in space thought to be crucial for the coupling process. The reactivity with methanethiosulfonate reagents of cysteines placed in strategic positions in the transporter provides a readout for conformational changes upon ion or substrate binding. More work is needed to establish the relationships between ion interactions and oligomerization of SLC6 transporters.

Cytotoxicity of 17 tetrahydroisoquinoline derivatives in SH-SY5Y human neuroblastoma cells is related to mitochondrial NADH-ubiquinone oxidoreductase inhibition

Since the first report that 1-methyl-4-phenyl-l,2,3,6-tetrahydropyridine induces parkinsonism, various kinds of low-molecular-weight neurotoxins, such as tetrahydroisoquinoline derivatives, have been identified as possible Parkinson's disease-inducing substances. In the present study, we measured four parameters of 17 tetrahydroisoquinoline derivatives, i.e., cytotoxicity in SH-SY5Y human neuroblastoma cells, inhibitory activity towards mitochondrial NADH-ubiquinone oxidoreductase (complex I), affinity for dopamine transporter, and 1-butanol-H2O partition coefficient (as an index of lipophilicity). Six of the derivatives showed comparatively strong inhibitory activity towards complex I (IC50 values<100 microM) and five of them were cytotoxic to SH-SY5Y cells (TC50 values<200 microM). Some of these compounds are endogenous. We found good correlations between cytotoxicity and complex I inhibitory activity, but not between cytotoxicity and affinity for dopamine transporter. Since cytotoxicity to SH-SY5Y neuroblastoma cells was related to inhibitory activity towards mitochondrial complex I, complex I inhibition is likely to be involved, at least in part, in the mechanism of TIQ derivative-induced cell death. Uptake of most of these compounds seems to be dependent on lipophilicity, rather than active transport via dopamine transporter.

Differences in interactions with the dopamine transporter as revealed by diminishment of Na+ gradient and membrane potential: Dopamine versus other substrates

In heterologous cells expressing the dopamine transporter (DAT), simultaneous elevation of intracellular Na(+) and depolarization of the membrane with gramicidin reduced the potency of various DAT substrates, including dopamine, d-amphetamine, beta-phenethylamine, p-tyramine, and MPP(+), in inhibiting binding of the cocaine analog [(3)H]CFT, with the greatest reduction observed for d-amphetamine. In rat striatal synaptosomes, gramicidin exerted similar effects; in addition, the potency of d-amphetamine was reduced by the Na(+)-channel activator veratridine. The latter effect was counteracted by the Na(+)-channel blocker tetrodotoxin. In broken membranes, where, as the situation with gramicidin, both sides of the non-polarized membrane were exposed to 130 mM Na(+), gramicidin was ineffective. Dopamine had a potency for membrane preparations that was not significantly different from that for control cells or synaptosomes, while other substrates had potencies for membrane preparations that were reduced to a level similar to those observed in gramicidin-treated cells or synaptosomes. These results suggest that diminishing Na(+) gradient and membrane potential may convert DAT to a conformational state that dopamine could easily bind to when gaining free access to its intracellular portion. In contrast, non-dopamine substrates may not be able to readily interact with this state from either side of the membrane.

Interactions of cations and anions with the binding of uptake blockers to the dopamine transporter

Uptake blockers and substrates are likely to recognise a common binding domain on the dopamine neuronal transporter (DAT). Among cations that form ionic gradients at the level of the cellular plasma membrane, Na+ is the only one that can stimulate their binding. The binding stimulation appears over Na+ concentrations ranging from 0 to 10-60 mM; at higher Na+ concentrations, binding reaches a plateau or decreases, according to the uptake blocker that is studied. The majority of the other cations, including K+, Ca2+, Mg2+ and Tris+, inhibit the binding of uptake blockers. Several metals impair binding to the DAT and/or the dopamine transport, but, under specific conditions, some of them, and chiefly Zn2+, stimulate binding. The complex relationships between cations, uptake blockers and the DAT suggest that cations recognise at least three different sites: the first one, site 1, is for cation-induced binding inhibition; the second one, site 2, is for Na+-induced binding stimulation; and the third one, site 3, is for Zn2+-induced binding stimulation. Modelling of the interactions between Na+, K+ and radioligands allows a better understanding of the effects of cations at sites 1 and 2, and of uptake blockers at site 1. Some anions also facilitate the binding of uptake blockers to the DAT, as far as they are associated with Na+. The dependence of the binding of dopamine on ions could be involved in its preferential inward transport and used by uptake blockers for their own binding to the DAT.

The Role of N-Glycosylation in Function and Surface Trafficking of the Human Dopamine Transporter

The present study addressed the role of N-linked glycosylation of the human dopamine transporter (DAT) in its function with the help of mutants, in which canonical N-glycosylation sites have been removed (N181Q, N181Q,N188Q, and N181Q,N188Q,N205Q), expressed in human embryonic kidney-293 cells. Removal of canonical sites produced lower molecular weight species as did enzymatic deglycosylation or blockade of glycosylation, and all three canonical sites were found to carry sugars. Prevention of N-glycosylation reduced both surface and intracellular DAT. Although partially or non-glycosylated DAT was somewhat less represented at the surface, no evidence was found for preferential exclusion of such material from the plasma membrane, indicating that glycosylation is not essential for DAT expression. Non-glycosylated DAT was less stable at the surface as revealed by apparently enhanced endocytosis, consonant with weaker DAT immunofluorescence at the cell surface and stronger presence in cytosol in confocal analysis of the double and triple mutant. Non-glycosylated DAT did not transport dopamine as efficiently as wild-type DAT as judged from the sharp reduction in uptake V(max), and prevention of N-glycosylation enhanced the potency of cocaine-like drugs in inhibiting dopamine uptake into intact cells without changing their affinity for DAT when measured in membrane preparations prepared from these cells. Thus, non-glycosylated DAT at the cell surface displays appreciably reduced catalytic activity and altered inhibitor sensitivity compared with wild type.

Binding of cocaine-like radioligands to the dopamine transporter at 37°C: effect of Na+ and substrates

Although dopamine (DA) translocation by the DA transporter (DAT) requires Na+, a role for Na+ in the DA recognition step in the translocation cycle has been questioned. Thus, when binding techniques were used to indirectly measure the affinity of DA for DAT via its potency in inhibiting cocaine analog binding, no stimulation of DA binding was observed when assay temperature was at or below room temperature. The present work describes the use of 3H-labeled cocaine analogs for assays at 37 degrees C. When there is sufficient Na+ in the medium (> or =25 mM), [3H]2beta-carbomethoxy-3beta-(4-iodophenyl)tropane ([3H]CIT) is an excellent radioligand to label human DAT with high affinity in membrane preparations of HEK-293 cells expressing the transporter. However, at 0 and 5 mM of Na+, appreciable binding of [3H]CIT occurs to proteins other than DAT, hampering accurate assessment of DAT-associated binding. No such problems occur with the binding of the 4-fluoro homolog of [3H]CIT, [3H]CFT at 37 degrees C, and this radioligand can be used at low [Na+], provided enough protein is present in the assay. The application of these assays show that, in contrast to the strong Na+ dependency of the binding of CFT, the substrates DA, D-amphetamine, p-tyramine, and DL-octopamine are not stimulated by Na+. This demonstrates that lack of Na+ stimulation of binding of substrates, including DA to DAT, in membrane preparations at room temperature is not caused by the reduced fluidity of the frozen state of the hydrocarbon membrane interior at this temperature as compared with the liquid-expanded state at 37 degrees C.

Rapid regulation of dopamine transporter function by substrates, blockers and presynaptic receptor ligands

The extracellular actions of dopamine are terminated primarily through its binding to dopamine transporters and translocation back into dopamine neurons. The transporter thereby serves as an optimal target to regulate dopamine neurotransmission. Although acute pharmacological blockade of dopamine transporters is known to reversibly inhibit transporter function by preventing the binding of its endogenous substrate dopamine, it recently has become clear that dopamine transporter substrates, such as amphetamines, and blockers, such as cocaine, also have the ability to rapidly and persistently regulate transporter function after their direct pharmacological effect has subsided. Presynaptic receptor ligands can also regulate dopamine transporter function. This has been investigated most extensively for dopamine D2 receptors, but there is also evidence for regulation by gamma-aminobutyric acid (GABA) GABAB receptors, metabotropic glutamate, nicotinic acetylcholine, serotonin, sigma2- and kappa-opioid receptors. The focus of this review is the rapid, typically reversible, regulation of dopamine transporter velocity by substrates, blockers and presynaptic receptor ligands. The research discussed here suggests that a common mechanism through which these different classes of compounds regulate transporter activity is by altering the cell surface expression of dopamine transporters.

Na+ and the substrate permeation pathway in dopamine transporters

Advances have been made in characterizing the relationship between Na+ and the substrate permeation pathway in the dopamine transporter. This review covers the role of Na+ in co-transport with dopamine as well as in the recognition of dopamine. Apparent recognition depends on the preparation studied: it differs between intact cells heterologously expressing the dopamine transporter and membranes prepared from these cells. In our search for amino acid residues in the transporter involved in Na+ action, W84 and D313 were found to play a special role in cation interaction, with evidence for regulation of both Na+ and H+ sensitivity. Mutation of D313 to N appeared to decrease the affinity for the dopamine transporter in intact cells, not by altering recognition per se. A model is proposed in which access of dopamine, not recognition itself, is regulated by D313 and Na+. Thus, the role of external Na+ in intact cell preparations is to turn dopamine transporters to the externally facing form, allowing access of dopamine to its binding site.

A preliminary, controlled investigation of magnesium L-aspartate hydrochloride for illicit cocaine and opiate use in methadone-maintained patients

Based on pre-clinical studies suggesting that magnesium (Mg) reduces cocaine self-administration and potentiates the antinociceptive effects of morphine, we conducted a preliminary randomized clinical trial investigating Mg for the treatment of illicit cocaine and opiate use. Eighteen methadone-maintained patients who used illicit opiates and cocaine received either Mg (732 mg/day) or placebo for 12 weeks. Overall, findings showed that the percentage of urine screens testing positive for opiates in the Mg group (22.6%) was half that of the placebo group (46.4%), p = .04; the difference was even greater in the "medication compliant" sample (Mg: 16.3%, placebo: 47.9%), p = .02. Cocaine craving was lower in the Mg compared to the placebo group, but there was no difference between groups in cocaine use. These preliminary findings suggest that Mg may have a beneficial effect for reducing illicit opiate use. It is possible that a higher dose of Mg than was used in this study may be needed to decrease cocaine use.

Na+ stimulates binding of dopamine to the dopamine transporter in cells but not in cell-free preparations

Although Na+ is crucial for the function of the dopamine (DA) transporter (DAT), its role in the substrate binding step has been questioned. To address this issue, we investigated the effect of Na+ on DA binding by measuring the potency of DA in inhibiting the binding of the cocaine analogue [3H]2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane (CFT) in intact cells expressing DAT in their plasma membranes and in membranes isolated from these cells. In cells, Na+ substantially enhanced the potency of DA in inhibiting CFT binding. This effect of Na+ was independent of buffer compositions and substitutes (sucrose vs. NMDG), more pronounced at 4 degrees C than 25 degrees C, and correlated with its stimulatory effect on DA uptake Km. Removing extracellular Na+ had little effect on intracellular concentrations of Na+ and K+, or on membrane potential. These data suggest that extracellular Na+ most likely acts at the transporter level to enhance the binding of external DA during the transport cycle. In contrast, in cell-free membrane preparations the Na+ stimulation was abolished without impairment of the potency of DA in inhibiting CFT binding, regardless of whether sucrose was used to maintain the buffer osmolarity. The difference in Na+ dependence for DA to inhibit CFT binding between plasma membranes of intact cells and isolated membranes raises the possibility that intracellular ion environment, alone or in combination with other cellular factors, plays a critical role in determining DA-DAT interaction and the integration of Na+ modulation in this interaction.

An investigation into the role of calcium in the modulation of rat synaptosomal D-[3H]aspartate transport by docosahexaenoic acid

The effect of the polyunsaturated fatty acid cis-4,7,10,13,16,19-docosahexaenoic acid (DHA) on the high-affinity, sodium-dependent uptake of D-[3H]aspartate into purified rat brain synaptosomes was examined. Incubation of the synaptosomes with 20 microM DHA caused over 50% inhibition of the maximum velocity (V(max)) of D-[3H]aspartate transport. This inhibition was significantly potentiated by pre-exposure of the synaptosomes to the fatty acid for 10 min prior to the start of the transport assay. Less highly unsaturated fatty acids such as arachidonic acid (cis-5,8,11,14-eicosatetraenoic acid), linolenic acid (cis-9,12,15-octadecatrienoic acid) and oleic acid (cis-9-octadecenoic acid) were significantly less potent than DHA. Removal of extracellular calcium, or reduction of the intracellular calcium concentration using the intracellular calcium chelator BAPTA/AM (10 microM), did not reduce the inhibition caused by DHA. On the other hand, an increase in the concentration of intracellular calcium mediated by thapsigargin (25 microM) or the calcium ionophore A23187 (10 or 100 nM) led to a reduction in the rate of D-[3H]aspartate transport in the absence of DHA. The CaM kinase II inhibitor, KN-93, reduced D-[3H]aspartate uptake independently of whether DHA was also present, but had no effect on the inhibition of D-[3H]aspartate uptake by either A23187 or thapsigargin. We conclude that whereas DHA inhibits synaptosomal D-[3H]aspartate uptake in a calcium-independent manner, a calcium-based mechanism exists that can also modulate glutamate transporter activity.

The functioning neuronal transporter for dopamine: kinetic mechanisms and effects of amphetamines, cocaine and methylphenidate

The dopamine transporter (DAT) is a transmembrane spanning protein that catalyzes the transport of dopamine across the neuronal membrane to concentrate the neurotransmitter inside the cell. Although the uptake of dopamine has been studied since the 1960s, more recent advances in knowledge of the protein itself and in making kinetically resolved measurements of its action have led to more insights into its mechanism and pharmacology. The literature of the kinetics of transporters and kinetic measurements of DAT activity is reviewed to provide an overview of the multisubstrate mechanism of DAT activity, its pharmacology with regard to amphetamine, cocaine and methylphenidate, and correlations of DAT activity with some behavioral outputs.

Cationic interactions at the human dopamine transporter reveal binding conformations for dopamine distinguishable from those for the cocaine analog 2 alpha-carbomethoxy-3 alpha-(4-fluorophenyl)tropane

In membrane preparations, CFT, a phenyltropane cocaine analog, and dopamine (DA) interact with the recombinant human dopamine transporter (hDAT) in Na+ -free medium. Na+ markedly increased the transporter's affinity for CFT, but had little or no effect on DA potency for inhibiting CFT binding. Raising [Na+ ] from 20 to 155 mm reduced Li+ -induced increase in DA K (i), but not CFT K (d). The presence of 155 mm Na+ enhanced the tolerance to low pH of CFT Kd but not DA Ki. Leucine substitution for tryptophan 84 (W84L) in transmembrane domain (TM) 1 or asparagine substitution for aspartate 313 (D313N) in TM 6 did not or only modestly enhance the affinity of Na+ -independent CFT binding, and retained the near normal ability of DA, Li+, K+, or H+ to inhibit this binding. However, the mutations significantly enhanced the Na+ stimulation of CFT binding as well as the Na+ antagonism against Li+ and H+ inhibition of CFT binding. In contrast, the mutations neither changed the Na+ -insensitive feature of DA Ki nor enhanced the Na+ protection of DA Ki against Li+ 's inhibitory effect, though they caused Na+ protection of DA Ki against H+ 's inhibitory action. These results are consistent with the existence of binding conformations for DA that are distinguishable from those for CFT, and with a differential association of cation interactions with DA and CFT binding. The mutations likely alter Na+ -bound state(s) of hDAT, preferentially strengthening the positive allosteric coupling between Na+ and CFT binding, and reducing the impact of Li+ or H+ on the CFT binding.

Involvement of the NH2 terminal domain of catecholamine transporters in the Na(2+) and Cl(-)-dependence of a [3H]-dopamine uptake

The ionic dependence of the [3H]-dopamine uptake was studied in transfected cells expressing the human neuronal transporter for dopamine (hDAT) or noradrenaline (hNET), and chimeric transporters resulting from the symmetrical exchange of the region from the NH2 terminal through the first two transmembrane domains (cassette I). Chimera A is formed by hDAT comprising cassette I from hNET, whereas chimera B corresponds to the reverse construct. The appearance or the intensity of a Cl(-)-independent component of transport was linked to the presence of the COOH terminal part of hNET in both monoclonal and polyclonal Ltk(-) cells (Cl(-) substituted by isethionate and NO3(-), respectively), and in transiently transfected COS-7 cells. Cassette I was also involved in the Cl(-)-dependence because the transport activity of polyclonal Ltk(-) cells expressing A was partly Cl(-)-independent and because Ltk(-) cells expressing transporters containing cassette I of hDAT displayed higher K(mCl)- values than cells expressing the reverse constructs. In monoclonal Ltk(-) cell lines, K(mNa)+ values and biphasic vs monophasic dependence upon Na(+) concentrations differentiate transporters containing cassette I of hNET from those containing cassette I of hDAT. In COS-7 cells, the exchange of cassette I produced a significant change in Hill number values. In Na(+)-dependence studies, exchange of the COOH terminal part significantly modified Hill number values in both Ltk(-) and COS-7 cells. Hill number values close to two were found for hNET and hDAT when sucrose was used as substitute for NaCl. The NH2 terminal part of the transporters bears some of the differences in the Na(+) and Cl(-)-dependence of the uptake that are observed between hDAT and hNET. Present results also support a role of the COOH terminal part in the ionic dependence.

Differential sensitivity to NaCl for inhibitors and substrates that recognize mutually exclusive binding sites on the neuronal transporter of dopamine in rat striatal membranes

Addition of NaCl (90--290 mM) to a 10 mM Na(+) medium did not significantly modify B(max) and K(d) values for [3H]mazindol binding to the dopamine neuronal transporter (DAT) studied on rat striatal membranes at 20 degrees C. Addition of NaCl differentially affected the ability of other uptake inhibitors and substrates to block the [3H]mazindol binding. Ratios of 50% inhibiting concentrations calculated for 290 and 90 mM NaCl allowed to distinguish three groups of agents: substrates which were more potent in the presence of 290 mM NaCl (group 1; ratio < 1) and two groups of uptake inhibitors displaying ratio values either ranging around two (group 2: WIN 35,428, cocaine, methylphenidate, pyrovalerone) or close to unity (group 3: BTCP, mazindol, benztropine, nomifensine). However, agents from these three groups recognize mutually exclusive binding sites since in interaction studies the presence of WIN 35,428 (group 2) or mazindol (group 3) increased the 50% inhibiting concentrations of D-amphetamine (group 1) and WIN 35,428 on the [3H]mazindol binding to theoretical values expected for a competition of all of these compounds for the same binding domain on the DAT.

Structural domains of chimeric dopamine-noradrenaline human transporters involved in the Na(+)- and Cl(-)-dependence of dopamine transport

Catecholamine transporters constitute the biological targets for several important drugs, including antidepressants, cocaine, and related compounds. Some information exists about discrete domains of these transporters that are involved in substrate translocation and uptake blockade, but delineation of domains mediating the ionic dependence of the transport remains to be defined. In the present study, human neuronal transporters for dopamine and noradrenaline (hDAT and hNET) and a series of six functional chimeras were transiently expressed in LLC-PK1 cells. Substitution of Cl(-) by isethionate reveals that cassette IV (i.e., the region of the transporter encompassing transmembrane domain 9 through the COOH terminal) plays an important role in the Cl(-)- dependence of the uptake. Substitutions of Na(+) and NaCl by Tris(+) and sucrose, respectively, demonstrate that three different segments scattered across the transporter are involved in the Na(+)- dependence of the transport activity: cassette I (i.e., the region from the amino terminus through the first two transmembrane domains), cassette IV, and junction between transmembrane domains 3 to 5 and 6 to 8. Results of the present work also suggest that the use of Tris(+) as a substitute for Na(+) results in a biased estimate of the Hill number value for hDAT. This study provides useful clues for identifying specific residues involved in the uptake function of the catecholamine transporters.

Dopamine transporter and catechol-O-methyltransferase activities are required for the toxicity of 1-(3',4'-dihydroxybenzyl)-1,2,3, 4-tetrahydroisoquinoline

1-(3',4'-Dihydroxybenzyl)-1,2,3,4-tetrahydroisoquinoline [3', 4'DHBnTIQ (1)] is an endogenous parkinsonism-inducing substance. It is taken up into dopaminergic neurons via the dopamine transporter, inhibits mitochondrial respiration, and induces parkinsonism in mice. We synthesized four derivatives [aromatized, N-methylated, N-methyl-aromatized, and O-methylated (2-5, respectively)] and studied the cellular uptake and cytotoxicity of 1-5, as well as the metabolism of 1. All except the O-methyl derivative (5) were specifically taken up by the dopamine transporter, but 1 was taken up most efficiently. Relative to 1, oxidation reduced v(max), N-methylation markedly increased K(m), and O-methylation eliminated the uptake activity. The cytotoxicity of 1-5 was examined in a mesencephalic cell primary culture. Compound 1 reduced cell viability by nearly 80% at 100 microM, but the other compounds had little or no effect on cell viability. In vivo and in vitro studies revealed that 1 was O-methylated by soluble catechol-O-methyltransferase (COMT). Aromatization and N-methylation of 1 were not observed. We found that dopamine transporter inhibitors and a COMT inhibitor each blocked the cytotoxicity of 1, indicating that uptake and O-methylation are both necessary for neurotoxicity. Thus, we consider that 1 is taken up into dopaminergic neurons via the dopamine transporter and then converted by COMT to 5, which has cytotoxic and parkinsonism-inducing activities.

Interaction of Na+, K+, and Cl- with the binding of amphetamine, octopamine, and tyramine to the human dopamine transporter

Little information is available on the role of Na+, K+, and Cl- in the initial event of uptake of substrates by the dopamine transporter, i.e., the recognition step. In this study, substrate recognition was studied via the inhibition of binding of [3H]WIN 35,428 [2beta-carbomethoxy-3beta-(4-fluorophenyl)[3H]tropane], a cocaine analogue, to the human dopamine transporter in human embryonic kidney 293 cells. D-Amphetamine was the most potent inhibitor, followed by p-tyramine and, finally, dl-octopamine; respective affinities at 150 mM Na+ and 140 mM Cl- were 5.5, 26, and 220 microM. For each substrate, the decrease in the affinity with increasing [K+] could be fitted to a competitive model involving the same inhibitory cation site (site 1) overlapping with the substrate domain as reported by us previously for dopamine. K+ binds to this site with an apparent affinity, averaged across substrates, of 9, 24, 66, 99, and 134 mM at 2, 10, 60, 150, and 300 mM Na+, respectively. In general, increasing [Na+] attenuated the inhibitory effect of K+ in a manner that deviated from linearity, which could be modeled by a distal site for Na+, linked to site 1 by negative allosterism. The presence of Cl- did not affect the binding of K+ to site 1. Models assuming low binding of substrate in the absence of Na+ did not provide fits as good as models in which substrate binds in the absence of Na+ with appreciable affinity. The binding of dl-octopamine and p-tyramine was strongly inhibited by Na+, and stimulated by Cl- only at high [Na+] (300 mM), consonant with a stimulatory action of Cl- occurring through Na+ disinhibition.

The in vivo modulation of dopamine synthesis by calcium ions: influences on the calcium independent release

To investigate the contribution of the dopamine (DA) synthesis to both the calcium-dependent and the carrier-mediated, mechanisms of DA release in the striatum, anaesthetized rats were locally superfused in the striatum with a push pull cannula supplied with an artificial CSF containing tritiated tyrosine. DA, dihydroxyphenylacetic acid (DOPAC) and their respective specific activity were measured in effluent and used to evaluate changes in the DA synthesizing rate. Excluding calcium ions from the CSF only partially reduced spontaneous DA release (70%) still leaving a possible carrier-mediated DA release. This effect was not additive with a local superfusion with 0.1 mM a-methyl-p-tyrosine, a blocker of DA synthesis, suggesting that synthesis could already be reduced by calcium-free superfusion. Local superfusion with 100 microM cadmium in the presence or not of calcium ions, increased the DA release (220 and 350%, respectively), simultaneously reducing DA synthesis. Local application of 1 microM calcium ionophore (A23187) was without effect on the basal release of DA but enhanced DA synthesis and increased the amphetamine-evoked and carrier-mediated amine release. We conclude that DA synthesis can be a modulatory process of the firing-independent and carrier-mediated amine release while it weakly affects the classical calcium-dependent release.

Multisubtrate mechanism for the inward transport of dopamine by the human dopamine transporter expressed in HEK cells and its inhibition by cocaine

Rotating disk electrode voltammetry was used to measure the time-resolved inward transport of dopamine into human embryonic kidney cells expressing the human transporter for dopamine and a kinetic mechanism of transport is hypothesized. Dopamine transport in this preparation was highly concentrative, with a 10(6)-10(7) inward bias, first order in dopamine and the K(m) and V(max) were found to be 1.6 microM and 18 pmol/sec x 10(6) cells), respectively. The hDAT turnover was estimated to be approximately 18 s(-1) and the second order rate constant of association of dopamine with hDAT was approximately 10(7) M(-1)s(-1). Dopamine transport was found to have a second order dependence on Na(+) (K(Na) approximately 100 mM) and a first order dependence on Cl(-) (K(Cl) approximately 12 mM). Multisubstrate analyses suggested that hDAT operates with an ordered kinetic mechanism in which Na(+) binds first to the transporter protein, dopamine second, and Cl(-) last before translocation of dopamine into or across the membrane. Cocaine competitively inhibited dopamine transport (reaction order of unity and K(i) approximately 0.34 microM) with no discernible effect at the Na(+) and Cl(-) binding sites. These results differ from those of previous studies conducted in preparations of the striatum and nucleus accumbens. Comparisons of the variant results are made and an analysis of the differing apparent kinetic mechanisms is presented.

Modeling of the interaction of Na+ and K+ with the binding of dopamine and [3H]WIN 35,428 to the human dopamine transporter

Although much is known about the effects of Na+, K+, and Cl- on the functional activity of the neuronal dopamine transporter, little information is available on their role in the initial event in dopamine uptake, i.e., the recognition step. This was addressed here by studying the inhibition by dopamine of the binding of [3H]WIN 35,428 [2beta-carbomethoxy-3beta-(4-fluorophenyl)[3H]tropane], a phenyltropane analogue of cocaine, to the cloned human dopamine transporter expressed in HEK-293 cells. The decrease in the affinity of dopamine (or WIN 35,428) binding affinity with increasing [K+] could be fitted to a competitive model involving an inhibitory cation site (1) overlapping with the dopamine (or WIN 35,428) domain. The K+ IC50 for inhibiting dopamine or WIN 35,428 binding increased linearly with [Na+], indicating a K(D,Na+) of 30-44 mM and a K(D,K+) of 13-16 mM for this cation site. A second Na+ site (2), distal from the WIN 35,428 domain but linked by positive allosterism, was indicated by model fitting of the WIN 35,428 binding affinities as a function of [Na+]. No strong evidence for this second site was obtained for dopamine binding in the absence or presence of low (20 mM) Cl- and could not be acquired for high [Cl-] because of the lack of a suitable substitute ion for Na+. The K(D) but not Bmax of [3H]WIN 35,428 binding increased as a function of the [K+]/[Na+] ratio regardless of total [Cl-] or ion tonicity. A similar plot was obtained for the Ki of dopamine binding, with Cl- at > or = 140 mM decreasing the Ki. At 290 mM Cl- and 300 mM Na+ the potency of K+ in inhibiting dopamine binding was enhanced as compared with the absence of Cl- in contrast to the lack of effect of Cl- up to 140 mM (Na up to 150 mM). The results indicate that Cl- at its extracellular level enhances dopamine binding through a mechanism not involving site 1. The observed correspondence between the WIN 35,428 and dopamine domains in their inclusion of the inhibitory cation site explains why many of the previously reported interrelated effects of Na+ and K+ on the binding site of radiolabeled blockers to the dopamine transporter are applicable to dopamine uptake in which dopamine recognition is the first step.

Which form of dopamine is the substrate for the human dopamine transporter: the cationic or the uncharged species?

The question of which is the active form of dopamine for the neuronal dopamine transporter is addressed in HEK-293 cells expressing the human dopamine transporter. The Km value for [3H]dopamine uptake fell sharply when the pH was increased from 6.0 to 7.4 and then changed less between pH 7.4 and 8.2. The KI for dopamine in inhibiting the cocaine analog [3H]2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane binding displayed an identical pH dependence, suggesting that changes in uptake result from changes in dopamine recognition. Dopamine can exist in the anionic, neutral, cationic, or zwitterionic form, and the contribution of each form was calculated. The contribution of the anion is extremely low (</=0.1%), and its pH dependence differs radically from that of dopamine binding. The increase in the neutral form upon raising the pH can model the results only when the pKa1 (equilibrium neutral-charged) is set to a much lower value (6.8) than reported for dopamine in solution (8.86). The sum of cationic and zwitterionic dopamine concentrations remained constant over the entire pH range studied. These forms are the likely transporter substrates with pH-dependent changes occurring in their interaction with the transporter. The binding of dopamine, a hydroxylated phenylethylamine derivative, displays the same pH dependence as guanethidine, a heptamethyleniminoethyl- guanidine derivative fully protonated under our conditions. An ionizable residue in the transporter could be involved that does not interact with or impact the binding of bretylium, a quaternary ammonium phenylmethylamine derivative that is always positively charged and shows only a minor reduction in KI upon increasing pH.

Modulation of the discriminative stimulus and rate-altering effects of cocaine by competitive and noncompetitive N-methyl-D-aspartate antagonists

The purpose of this study was to determine the extent to which N-methyl-D-aspartic acid (NMDA) antagonists modified the discriminative stimulus effects of cocaine in rats trained to discriminate 5 mg/kg cocaine from vehicle on a fixed-ratio schedule of food presentation as well as the rate-altering effects of cocaine in rats maintained on a fixed-interval schedule of food presentation. NMDA-associated ion channel blockers (dizocilpine, phencyclidine, and magnesium chloride) and competitive NMDA antagonists (NPC 17742 and CGP 37849) displayed similar behavioral effects when administered alone: each drug engendered intermediate levels of cocaine-appropriate responses and rate-dependent effects on food-reinforced operant responding. Selected doses of dizocilpine, magnesium chloride, and phencyclidine given in combination with 1 mg/kg cocaine produced more cocaine-appropriate responses than this dose of cocaine alone. In addition, dizocilpine and magnesium chloride each attenuated the discriminative stimulus effects of higher doses of cocaine. The competitive NMDA antagonists did not appreciably modify the discriminative stimulus effects of any dose of cocaine. Under the fixed-interval schedule, each NMDA antagonist attenuated the effects of 3 mg/kg cocaine, which normally produced maximal increases in response rate. Attenuation of the rate-decreasing effects of the highest dose of cocaine (30 mg/kg) also were observed after pretreatment with dizocilpine and magnesium chloride. These findings demonstrated differences in the way that NMDA-associated ion channel blockers and competitive NMDA antagonists interact with cocaine, and suggest that some NMDA-associated ion channel blockers may either enhance or antagonize the effects of cocaine, depending on the dose and type of behavioral procedure.

Potential misconceptions in dopamine transporter assays arising from the binding of [125I]RTI-121 to filters: effect of ions and cocaine

Binding of the cocaine analog 3 beta-(4-[125I]iodophenyl)tropane-2 beta-carboxylic acid isopropyl ester ([125I]RTI-121) to filters was studied in order to assess its contribution to labeling dopamine transporters on rat striatal synaptosomal membranes in filtration assays. Filter binding (FB) decreased with increasing Na+. Cocaine (30 and 100 microM) substantially reduced the FB at low Na+ with much less of an effect at higher Na+. Similar results were observed with K+. At 10 mM Na+, RTI-121 (1 microM) displaced the FB to the same degree as cocaine (100 microM); mazindol (10 microM), BTCP (1 microM), and dopamine (1 mM) did so to a lesser degree; and GBR12935 (1 microM) did not. If the specific binding was calculated without deducting the FB displaced with cocaine (DFB), the DFB accounted for 15-19% of the 'specific binding' at 10 mM Na+ in the assay. This additional binding population resulted in an upward curvilinear Scatchard plot and incorrect estimation of equilibrium binding parameters and ion potencies. At 10 mM Na+, without deduction of DFB, the high-affinity component had a Kd of 3.4 nM and Bmax of 2.4 pmol/mg protein, and the respective values for the low-affinity component were 84 nM and 16 pmol/mg protein; when DFB was deducted, one component was observed with a Kd of 4.4 nM and Bmax of 3.3 pmol/mg protein. The presence of higher Na+ in the assay diminished these artifacts. Thus, at 150 mM Na+, without deduction of DFB, there was one binding component with a Kd of 3.9 nM and Bmax of 4.6 pmol/mg protein; these values became 3.3 nM and 3.8 pmol/mg protein when DFB was deducted.

Possible involvement of calmodulin-dependent kinases in Ca(2+)-dependent enhancement of [3H]5-hydroxytryptamine uptake in rat cortex

Effects of Ca2+ on [3H]5-hydroxytryptamine (5-HT) uptake into rat cortical synaptosomes were studied. The uptake was enhanced in the presence of Ca2+ in Krebs-Ringer medium and the uptake at 0.3-5 mM Ca2+ was 2.4-2.7 times greater than that observed in the absence of Ca2+. The maximal increase at the concentration of 1 mM Ca2+ was achieved after 2 min preincubation. Ca(2+)-dependent enhancement of the [3H]5-HT uptake reflected an increase in Vmax of the uptake process. However, Kd and Bmax values for [3H]paroxetine were not significantly changed in the presence of 1 mM Ca2+ compared with Ca(2+)-free condition. On the other hand, uptake was still enhanced after synaptosomes were washed with Ca(2+)-free after preincubation with 1 mM Ca2+. Staurosporine (a protein kinase C inhibitor) and wortmannin (a myosin light chain kinase inhibitor) did not affect Ca(2+)-dependent enhancement of the uptake, whereas 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazin e (KN-62, an inhibitor of Ca2+ /calmodulin-dependent kinase II) and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7, a calmodulin antagonist) significantly reduced it. Moreover, L-type, but not P- or N-type, voltage-dependent Ca(2+)-channel blockers suppressed enhancement of the uptake. These results indicate that Ca(2+)-dependent enhancement of [3H]5-HT uptake is mediated by activation of calmodulin-dependent protein kinases, suggesting a possibility of calmodulin-dependent regulation of in vivo 5-HT uptake.

Cation requirements of basal and ATP-regulated dopamine transport in rat pheochromocytoma cells

The transport of dopamine into presynaptic nerve terminals is the primary mechanism for the termination of dopaminergic neurotransmission. This transport process has recently been found to be composed of two components, a basal dopamine transport pathway which exists in the absence of extracellular ATP and an ATP-regulated moiety which comprises approximately 66% of the total transport system [Cao C. J. et al. (1990) Biochem. Pharmac. 39, R9-R14; Cao C. J. et al. (1989) Biochemistry 8, 207-220; Dunigan C. D. and Shamoo A. E. (1995) Neuroscience 65, 1-4; Eshleman A. et al. (1995) Life Sci. 56, 1613-1621]. Using a rat pheochromocytoma cell line and a Krebs bicarbonate buffering system, the present study examined the effect of several cations on both basal and ATP-regulated dopamine transport. In the absence of extracellular ATP, dopamine transport had an absolute dependence on the presence of Na+, but exhibited no requirement for Mg2+. Kinetically, the addition of 120 mM NaCl increased the Vmax of basal dopamine transport by approximately 150%. In contrast, the ATP-regulated dopamine transport pathway displayed a different sensitivity to Na+ and was completely dependent upon the presence of Mg2+. The addition of 1.2 mM MgSO4 increased the Vmax of transport in the presence of 0.7 mM extracellular ATP by 222%. Both basal and ATP-regulated transport were unaffected by the removal of either Ca2+ or K+ from the assay buffer. When the effects of ouabain, a potent inhibitor of Na+, K(+)-ATPase, were tested in the rat pheochromocytoma cell model, it was found that concentrations of ouabain as high as 1 mM were ineffective at inhibiting either the basal or ATP-regulated dopamine transport components. These results imply that the Na+ gradient supplied by Na+, K(+)-ATPase is not the sole provider of energy needed to drive either transport process. The ionic requirements of the basal and ATP-regulated dopamine transport pathways demonstrate the distinction between the two transport processes. In addition, the ionic dependency profile of the ATP-regulated moiety has provided some mechanistic insights into ATP-regulated catecholamine uptake, as the absolute Mg2+ requirement and the ineffectiveness of Ca2+ argues against the involvement of either purinergic receptors or a Ca(2+)-dependent, Mg(2+)-independent ectokinase in the ATP-regulated transport system.

Effect of low concentrations of K+ and Cl- on the Na(+)-dependent neuronal uptake of [3H] dopamine

The specific uptake of [3H] dopamine (DA) was studied using a crude synaptosomal fraction obtained from rat striatum. In a medium containing a 10 mM NaHCO3/NaH2PO4 buffer and no added K+ ions, addition of NaCl elicited an increase in DA uptake for Na+ concentrations from 10 to 60 mM, and then a decrease of uptake for Na+ concentrations up to 130 mM. These data confirm that rather low NaCl concentrations produce a maximal DA uptake. This biphasic curve of uptake resulted from significant changes in the Vmax of the DA uptake. Except for 10 mM Na+, this curve was not significantly modified when 9 mM NaHCO3/NaH2PO4 were replaced by 9 mM NaCl. This result indicates that the Cl- dependence of the DA uptake is mainly secondary to the Na+ dependence. Addition of KCl up to 3 mM did not modify the ascending part of the NaCl-dependent uptake curve. In contrast, the reduction in uptake produced by high Na+ concentrations was prevented in a concentration-dependent manner by KCl; this effect resulted from a decrease in the Km and an increase in the Vmax for the uptake. Measurements of membrane potential, with the help of the fluorescent probe 3, 3'-diethylthiadicarbocyanine iodide [DiSC2(5)] and purified synaptosomes prepared from rat striatum and cerebral cortex, revealed that addition of 3 mM KCl to a medium containing a high Na+ concentration and no K+ ions produced a marked and stable decrease in the fluorescence level. This decrease which corresponds to an increase in membrane polarization was blocked by 0.1 mM ouabain. These data suggest that low K+ concentrations are likely to prevent the decrease in uptake elicited by high Na+ concentrations by restoration, via a Na+/K+ ATPase-mediated mechanism, of the membrane potential and/or a transmembrane electrochemical Na+ gradient more favourable to DA uptake.

Complex ionic control of [3H]GBR 12783 binding to the dopamine neuronal carrier

At 20 degrees C, [3H]GBR 12783, {1-[2-(diphenylmethoxy)ethyl]4-(3-phenyl-2-([1-3H]propenyl)-pip era zine} dissociated from the dopamine neuronal carrier present in rat striatal membranes with a t1/2 value of 27 min. At this temperature, KCI, CaCl2 and MgCl2 increased the binding dissociation, revealing that they recognize a binding site which is not mutually exclusive with that of [3H]GBR 12783. The comparison of the ability of KCl to increase the binding dissociation (by 160% at 30 mM KCl) with its potency as a binding inhibitor (Ki-2.6 +/- 0.3 mM) suggests an involvement of two recognition sites for K+ in binding inhibition, a not mutually exclusive site and another, mutually exclusive, site. Divalent cations mainly inhibited the binding via a mutually exclusive site since 3 mM Ca2+ and 10 mM Mg2+ increased the binding dissociation by 90% at 20 degrees C whereas their Ki values were 0.049 +/- 0.006 and 0.141 +/- 0.035 mM, respectively. Involvement of this mutually exclusive site was also supported by the persistence of the binding inhibition elicited by Ca2+ and Mg2+ at 0 degree C, a temperature at which they reduced the binding dissociation. At 20 degrees C, 100 mM NaCl did not modify [3H]GBR 12783 binding but it antagonized the binding dissociation elicited by inhibitory cations. Ca2+ reduced the off-rate of [3H]GBR 12783 binding at 0 degree C and in the presence of 100 mM Na+. Finally, [3H]GBR 12783-binding dissociation was increased by high 'cytosolic' K+ while 'synaptic' concentrations of Na+, K+, Ca2+, Mg2+ and Cl- were ineffective. A reduction of H2PO4-/HCO3- from 10 to 5 mM and a substitution of 5 mM H2PO4-/HCO3- by 5 mM Cl- increased the binding dissociation, suggesting that an anion-binding site could also regulate the binding.

Interaction of two sulfhydryl reagents with a cation recognition site on the neuronal dopamine carrier evidences small differences between [3H]GBR 12783 and [3H]cocaine binding sites

We have compared the effect of treating rat striatal cell membranes with ionic hydrophilic sulfhydryl reagents on the specific bindings of [3H]cocaine and of [3H]GBR 12783 (1-[2-(diphenylmethoxy)ethyl]4-(3-phenyl-2-[1-3H]propenyl)-piperaz ine) to the neuronal transporter of dopamine. Treatment with 1 mmol/l 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) resulted in similar time- and concentration-dependent reductions of the specific binding of both radioligands. None of the uptake blockers tested afforded any protection against 1 mmol/l DTNB. Addition of (sub)millimolar concentrations of CaCl2 or MgCl2, or 250 mmol/l KCl to a treatment medium containing 10 mmol/l Na+ significantly increased the DTNB-induced reduction of the specific binding of both radioligands. Cations were likely to be responsible for this effect since ions in combination with DTNB induced similar reductions in binding when either 1 mmol/l CaCl2 or 50-250 mmol/NaCl were added. Effects of cations on the DTNB-induced inhibition of binding were generally more marked on [3H]GBR 12783 than on [3H]cocaine binding. When added to a medium containing 10 mmol/l Na+ 1 mmol/l DTNB induced a reduction in the Bmax of the specific binding of both radioligands. Addition of 1 mmol/l Ca2+ maintained or increased this Bmax reduction and elicited a decrease in affinity which was significant for [3H]GBR 12783 binding. Treatment of membranes with the sodium salt of p-hydroxymercurybenzenesulfonate (pHMBS) induced time- and concentration-dependent decreases in [3H]GBR 12783 binding which were significantly greater than decreases in [3H]cocaine binding. However, 50 mumol/l pHMBS produced a similar decrease in the Bmax of the specific binding of both radioligands. The pHMBS-induced reduction of [3H]GBR 12783 binding was not reversed by drugs whose action is purely that of uptake inhibition or by substrates of the dopamine carrier. Some of these drugs (100 mumol/l dopamine, 1 mumol/l mazindol or 100 mumol/l cocaine) protected the specific binding of [3H]cocaine against the effects of pHMBS, whereas 1 mmol/l p-tyramine, 10 mumol/l nomifensine and 10 nmol/l GBR 12783 were ineffective. Addition of 120 mmol/l Na+, 1 mmol/l Ca2+ or 10 mmol/l Mg2+ to a treatment medium containing 10 mmol/l Na+ significantly reduced the effects of pHMBS on the specific binding of both radioligands. When striatal cell membranes were treated in a medium containing 130 mmol/l Na+, there was a general decrease in the effects of ions on the reductions of specific binding produced by DTNB or pHMBS.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence that pure uptake inhibitors including cocaine interact slowly with the dopamine neuronal carrier

We have studied the ability of various uptake blockers to protect the dopamine neuronal carrier labeled with [3H]GBR 12783 (1-[2-(diphenylmethoxy)ethyl]-4-(3-phenyl-2-(propenyl)-piperazine) against N-ethylmaleimide-induced alkylation, using membrane preparations obtained from rat striatum. Pure uptake inhibitors such as mazindol, pyrovalerone, nomifensine and methylphenidate, and substrates (dopamine, d-amphetamine, m-tyramine) protected the [3H]GBR 12783 binding site in a concentration-dependent manner. Preincubation of the membranes with these agents prior to N-ethylmaleimide treatment did not modify the protecting ability of substrates, whereas it significantly improved that of pure uptake inhibitors including cocaine. When the preincubation was omitted, the concentration dependence of the protection observed with pure uptake inhibitors decreased and a maximal 40% protection was observed for 10 microM to 1 mM cocaine concentrations. Effective protecting concentrations of blockers are correlated with their Ki determined in standard binding studies. These results reveal that all pure uptake inhibitors bind slowly to the dopamine neuronal carrier whereas substrates interact with it rapidly.

Specific binding of [3H]GBR 12783 to the dopamine neuronal carrier included in polarized membranes

We have compared the properties of the binding to the neuronal dopamine carrier located either in polarized membranes of synaptosomes or in non polarized, classical membranes. Non-polarized membranes were prepared by sonication of the partially purified synaptosomal fraction obtained from rat striatum which was used as the source of polarized membranes. Binding experiments were carried out at 37 degrees C in Krebs Ringer related media. [3H]GBR 12783 (1-[2-(diphenylmethoxy)ethyl]4-(3-phenyl-2-[1- 3H]propenyl)piperazine) specifically bound with a nanomolar affinity to a homogeneous population of site (maximal binding site concentration: 8-10 pmol/mg protein). Pure uptake inhibitors, but not substrates, competed for the [3H]GBR 12783 binding site located in polarized membranes of synaptosomes at concentrations effective against dopamine neuronal transport. Except for [3H]GBR 12783, the replacement of Cl- by isethionate- did not result in significant change in the ability of pure uptake inhibitors to compete for the specific binding site. A reduction in the Na+ concentration from 135 to 10 mM induced a significant decrease in the inhibitory potency of GBR 12783, mazindol, nomifensine and methylphenidate. This decrease was likely to result from the presence of K+, Mg2+ and Ca2+, whose inhibitory effects were modified and/or increased by decreasing the Na+ concentration. These data indicate that the membrane polarity is not clearly involved in the binding of pure uptake inhibitors to the dopamine neuronal carrier; furthermore they underline the critical role of Na+ and K+ transmembrane gradients in both the recognition of the carrier by dopamine and its inward transport.

Neurotransmitter transporters: three distinct gene families

Our understanding of the plasma membrane and vesicular transport systems that mediate neurotransmitter re-uptake has been greatly enhanced in the past year by the cloning and characterization of two additional gene families involved in this process, the excitatory amino acid transporters and the vesicular amine transporters. Additional members of the previously defined family of Na+/Cl(-)-dependent transporters continue to be identified.