Binding of the [125I]3 beta-(iodophenyl)tropan-2 beta-carboxylic acid isopropyl ester to the dopamine transporter at a physiologically relevant temperature: mutually exclusive binding and different ionic requirements for various uptake blockers and substrates

Binding of 3H-WIN-35,428 and 125I-RTI-121 to Human Platelet Membranes

The dopamine transporter (DAT) is a protein regulating dopamine concentration in the synaptic cleft through the re-uptake mechanism. The DAT is the main target of psychostimulants and seems to play a pivotal role in neuronal degeneration and different neuropsychiatric disorders involving the dopamine system. Exhaustive research, however, regarding the presence of this protein in human platelets is still inconclusive, although it is thought that it might provide a peripheral tool to serve as a mean of exploring the same structure present in the brain. Therefore, we assessed some binding assays in platelets derived from healthy human subjects by means of 3H-WIN 35,428, a compound which is considered a selective ligand for the labelling of this protein, and by means of 125I-RTI-121, another compound with high specificity for DAT. The results showed that the binding of 3H-WIN-35,428 was too low to enable the detection of any structure; the binding of 125I-RTI-121, on the other hand, revealed the presence of two binding sites with pharmacological profiles similar to that of the serotonin transporter (SERT). In conclusions, therefore, platelets would not seem to be a useful model for exploring the DAT, given the prevalence therein of the SERT and the difficulty of labelling the DAT with the currently available ligands.

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.

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.

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.

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.

The neurochemical basis for the behavioral effects of triadimefon

Administration of triadimefon (TDF) [1-(4-chlorophenoxy)-3,3-dimethyl-1-(1-H-1,2,4-triazol-1-yl)-2-butanone] in rodents incites heightened locomotor and stereotypy response, primarily through potentiation of dopaminergic activity. In the present study, 8 male Sprague-Dawley rats received repeated injections, on alternate days (100 mg/kg, 14 days) of TDF or corn oil. Enhanced locomotor and stereotypy behavioral patterns occurred in response to TDF injections, lasting until the 12th day of injection. Tolerance to this effect was evident by a lack of response to TDF injection by the 14th day. Similarly, a withdrawal period and challenge dose of TDF (5 days, 25 mg/kg) was ineffective in espousing locomotor or stereotypy behavioral changes. Cross sensitization to cocaine was also evident, since withdrawal and a challenge dose (8 days, 5 mg/kg) was also ineffective. Adaptative biochemical changes in dopaminergic function were examined after both acute (100 mg/kg) and repeated administration of TDF (100 mg/kg, 14 days) by examining [3H]dopamine (DA) uptake and DA release in both striatal (ST) and nucleus accumbens (NA) tissue. In corroboration with behavioral pattern indicating development of tolerance, there were significant changes in dopaminergic function. Repeated TDF exposure in vivo resulted in significant attenuation of ST and NA DA uptake in response to TDF (10(-4) to 10(-7) M) or cocaine (10(-5) to 10(-8) M) in vitro. Acute exposure to TDF in vivo also attenuated ST DA inhibitory effects of cocaine and TDF. Repetitive administration of TDF in vivo had no effect on in vitro TDF- or amphetamine-stimulated release of ST or NA DA. However, there was a significant reduction in amphetamine-stimulated DA release in animals after acute exposure to TDF in vivo. It was concluded from this study that the effects of chronic TDF exposure may primarily involve effects on DA uptake in the ST and NA.

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.

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.

Pharmacology and regulation of the neuronal dopamine transporter

The dopamine transporter, a member of the family of Na+,Cl(-)-dependent transporters, mediates uptake of dopamine into dopaminergic neurons by an electrogenic, Na(+)- and Cl(-)-transport-coupled mechanism. Dopamine and blockers of uptake such as cocaine probably bind to both shared and separate domains on the transporter, which can be influenced dramatically by the presence of cations. Regulation of the dopamine transporter occurs both by chronic occupancy with blocker and by acute effects of D2 dopamine receptors or second messengers such as diacylglycerol (protein kinase C) and arachidonic acid. The dopamine transporter is involved in the uptake of toxins generating Parkinson's disease; it is also an important target for psychostimulant drugs, ligands for in vivo imaging and medications used for neurologic diseases involving changes in the dopamine system.