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

Paradoxical abatement of striatal dopaminergic transmission by cocaine and methylphenidate

We combined in vitro amperometric, optical analysis of fluorescent false neurotransmitters and microdialysis techniques to unveil that cocaine and methylphenidate induced a marked depression of the synaptic release of dopamine (DA) in mouse striatum. In contrast to the classical dopamine transporter (DAT)-dependent enhancement of the dopaminergic signal observed at concentrations of cocaine lower than 3 μM, the inhibitory effect of cocaine was found at concentrations higher than 3 μM. The paradoxical inhibitory effect of cocaine and methylphenidate was associated with a decrease in synapsin phosphorylation. Interestingly, a cocaine-induced depression of DA release was only present in cocaine-insensitive animals (DAT-CI). Similar effects of cocaine were produced by methylphenidate in both wild-type and DAT-CI mice. On the other hand, nomifensine only enhanced the dopaminergic signal either in wild-type or in DAT-CI mice. Overall, these results indicate that cocaine and methylphenidate can increase or decrease DA neurotransmission by blocking reuptake and reducing the exocytotic release, respectively. The biphasic reshaping of DA neurotransmission could contribute to different behavioral effects of psychostimulants, including the calming ones, in attention deficit hyperactivity disorder.

Differential involvement of the norepinephrine, serotonin and dopamine reuptake transporter proteins in cocaine-induced taste aversion

Despite the impact of cocaine's aversive effects on its abuse potential, the neurochemical basis of these aversive effects remains poorly understood. By blocking the reuptake of the monoamine neurotransmitters dopamine (DA), norepinephrine (NE) and serotonin (5-HT) into the presynaptic terminal, cocaine acts as a potent indirect agonist of each of these systems. The following studies attempted to assess the extent of monoaminergic mediation of cocaine's aversive effects using conditioned taste aversion (CTA) learning [Garcia, J., Kimeldorf, D.J., Koelling, R.A., Conditioned aversion to saccharin resulting from exposure to gamma radiation. Science 1955;122:157-158.]. Specifically, Experiment 1 assessed the ability of selective monoamine transporter inhibitors, e.g., DAT (vanoxerine), NET (nisoxetine) and SERT (fluoxetine), to induce taste aversions (relative to cocaine). Only the NET inhibitor approximated the aversive strength of cocaine. Experiment 2 compared the effects of pretreatment of each of these transport inhibitors on the development of a cocaine-induced CTA. Pretreatment with nisoxetine and fluoxetine both attenuated cocaine-induced aversions in a manner comparable to that produced by cocaine itself. The DAT inhibitor was without effect. Combined, the results of these investigations indicate little or no involvement of dopaminergic systems in cocaine's aversive effects while NE appears to contribute most substantially, with a possible modulatory involvement by serotonin.

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.

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.

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.

Cocaine alters the accessibility of endogenous cysteines in putative extracellular and intracellular loops of the human dopamine transporter

Cocaine and other psychostimulants act by blocking the dopamine transporter. Binding of the cocaine analog, [3H]2-beta-carbomethoxy-3-beta-(4-fluorophenyl) tropane (CFT) to the dopamine transporter is sensitive to polar sulfhydryl-specific derivatives of methanethiosulfonate (MTS). These reagents preferentially react with water-accessible, reduced cysteines. The human dopamine transporter has 13 cysteines. Their topology is not completely determined. We sought to identify those cysteine residues the modification of which affects CFT binding and to determine the topology of these reactive cysteines. We mutated each of the cysteines, one at a time and in various combinations, to residues that preserved binding and transport, and we tested the sensitivity of each of the mutant transporters to the reagents. One construct, X5C, had five mutated cysteines (C90A, C135A, C306A, C319F, and C342A). Using a membrane preparation in which both extracellular and intracellular cysteines could be accessible, we found that CFT binding in X5C, as compared with wild-type transporter, was two orders of magnitude less sensitive to MTS ethylammonium (MTSEA). The wild-type cysteines were substituted back into X5C, one at a time, and these constructs were tested in cells and in membranes. Cys-90 and Cys-306 appear to be extracellular, and Cys-135 and Cys-342 appear to be intracellular. Each of these residues is predicted to be in extramembranous loops. The binding of cocaine increases the rate of reaction of MTSEA and MTS ethyltrimethylammonium with the extracellular Cys-90 and therefore acts by inducing a conformational change. Cocaine decreases the rate of reaction of MTSEA with Cys-135 and Cys-342, acting either directly or indirectly on these intracellular residues.

Dopamine transporter development in postnatal rat striatum: an autoradiographic study with [3H]WIN 35,428

The dopamine transporter mediates the reinforcing effects of cocaine, thus playing a central role in human cocaine addiction, and perhaps providing the mechanism for inducing the effects of prenatal cocaine exposure. This possibility has stimulated growing interest in the normal and abnormal development of this transporter. [3H]WIN 35,428 is a cocaine analog that is useful for studying the distribution and density of the dopamine transporter in striatum and other brain regions. The postnatal development of the dopamine transporter in the rat striatum was measured by quantitative autoradiography with [3H]WIN 35,428. Dopamine transporter levels were low at birth, increased through day 15, followed by much more rapid growth in late postnatal development. The majority of the transporter sites appeared after day 15. Lateral to medial and anterior to posterior gradients in transporter density were established early during development, and there was also an early concentration of transporter in striosomes that became difficult to identify by day 15. Differences between the developmental patterns described here and studies using other ligands for the dopamine transporter suggest there are significant differences in the transporter binding sites for these drugs. These differences in transporter ligand binding characteristics may reflect developmental changes in post-translational modification of the transporter and/or changes in the functional activity rather than simply the presence of the transporter.

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.

Differential reinforcing effects of cocaine and GBR-12909: biochemical evidence for divergent neuroadaptive changes in the mesolimbic dopaminergic system

The dopamine (DA) transporter is thought to be the primary mediator of reinforcing effects of cocaine. In the present study, an intravenous drug self-administration procedure, in vitro autoradiography, and HPLC methods were used to investigate possible differences in reinforcing and neuroadaptive responses to cocaine versus GBR-12909, a selective inhibitor of the DA transporter with a postulated therapeutic use in cocaine abuse. Drug-naive rats readily acquired and subsequently maintained cocaine self-administration behavior during 2 hr daily sessions over a prolonged period. In contrast, although GBR-12909 was initially self-administered, both cocaine-naive and cocaine-trained rats failed to maintain self-administration behavior for GBR-12909 over prolonged periods of time. After self-administration responding decreased with GBR-12909, rats showed a delay of 6.6 +/- 1.3 sessions in reacquiring consistent cocaine self-administration. Moreover, when GBR-12909 was again substituted for cocaine, they failed to self-administer GBR-12909, even during the initial days of testing. In contrast, after extinction of self-administration responding by water substitution, rats readily self-administered both cocaine and GBR-12909. Cocaine self-administration upregulated DA transporters, whereas water-substituted cocaine withdrawal upregulated both DA transporters and D1 receptors. Unlike cocaine, GBR-12909 self-administration by itself altered neither DA transporters nor D1 or D2 receptors. Nevertheless, substitution of GBR-12909 for cocaine reversed the cocaine-induced upregulation of DA transporters and reduced DA and dihydroxyphenylacetic acid levels in the mesolimbic system. These data suggest that cocaine and GBR-12909 differentially affect dopaminergic systems and also cause different reinforcing and neuroadaptive effects. GBR-12909-like compounds may be useful pharmacotherapeutic agents for cocaine addiction. Upregulation of DA transporters and D1 receptors might play important roles in the neuroadaptive cascade that leads to cocaine addiction and withdrawal.

Binding domains for blockers and substrates on the cloned human dopamine transporter studied by protection against N-ethylmaleimide-induced reduction of 2 beta-carbomethoxy-3 beta-(4-fluorophenyl)[3H]tropane ([3H]WIN 35,428) binding

Binding sites for 2 beta-carbomethoxy-3 beta-(4-fluorophenyl)[3H]tropane ([3H]WIN 35,428) on the human dopamine transporter expressed in C6 glioma cells were alkylated with N-ethylmaleimide (NEM), and the protective potency of the blockers cocaine, N[1-(2-benzo[b]thiophenyl) cyclohexyl]piperidine (BTCP), and benztropine, and of the substrates dopamine, d-amphetamine, and norepinephrine was measured. In general, the protective potency was lower (at least 4-5 times) than the potency in inhibiting [3H]WIN 35,428 binding with the compounds present under the same experimental conditions used for the NEM alkylation. However, the disparity was substantially greater for all substrates tested (23- to 44-fold) than for the blockers (4- to 11-fold), especially cocaine (5-fold) and BTCP (4-fold). Benztropine took an intermediate place (11-fold) between cocaine (5-fold) and BTCP (4-fold), on the one hand, and dopamine (23-fold), on the other hand. [3H]WIN 35,428 binding was best described by a one-site model under the present conditions. The results are discussed in terms of models involving blocker-induced conformational changes and overlapping nonidentical binding domains for blockers and substrates.