Cocaine and GBR 12783 recognize nonidentical, overlapping binding domains on the dopamine neuronal carrier

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.

Methylphenidate and MK-801, an N-methyl-d-aspartate receptor antagonist: shared biological properties

Methylphenidate (MPH), a dopamine reuptake inhibitor, is used increasingly to treat attention deficit and hyperactivity disorders in children. Given that dopaminergic mechanisms, contribute to the structural and functional maturation of brain circuitry, consideration of the potential influence of MPH in disrupting such processes seems warranted. Following a similar logic regarding the relevance of glutamate neurotransmission in mediating aspects of brain maturation, we and others have previously utilized in vivo and in vitro studies of the developing rodent brain to establish that MK-801, an N-methyl-d-aspartate (NMDA) receptor antagonist has both neuroprotective and pro-apoptotic actions. In this study we used a neonatal murine model of excitotoxin-induced cortical injury to compare such actions between MPH and MK-801, and found that MPH shared some biological properties with MK-801. Specifically, both drugs were neuroprotective against excitotoxic challenge resulting in neonatal brain lesions and in vitro neuronal death, but both drugs also exacerbated programmed neural cell death. However, this profile of action was not shared by the dopamine reuptake blocker GBR-12783, a molecule which like MPH binds to and blocks the dopamine transporter, but which is structurally dissimilar to MPH, suggesting that inhibition of dopamine reuptake alone cannot explain the results from our MPH studies. The implications of our findings are that when studied in our developmental mouse model both drugs demonstrate similar capacities to be either neuroprotective or pro-apoptotic, depending on the specific biologic setting in which they act. Additional studies to identify some potential positive as well as negative consequences of exposure to these drugs during brain development in clinical settings are warranted.

Expansion of structure-activity studies of piperidine analogues of 1-[2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)piperazine (GBR 12935) compounds by altering substitutions in the N-benzyl moiety and behavioral pharmacology of selected molecules

A series of substituted N-benzyl analogues of the dopamine transporter (DAT) specific compound, 4-[2-(diphenylmethoxy)ethyl]-1-benzylpiperidine were synthesized and biologically characterized. Different 4'-alkyl, 4'-alkenyl, and 4'-alkynyl substituents were introduced in the phenyl ring of the benzyl moiety along with the replacement of the same phenyl ring by the isomeric alpha- and beta-naphthyl groups. Different polar substitutions at the 3'- and 4'-position were also introduced. Novel compounds were tested for their binding affinity at the dopamine, serotonin, and norepinephrine transporter systems in the brain by competing for [(3)H]WIN 35 428, [(3)H]citalopram, and [(3)H]nisoxetine, respectively. Selected compounds were also evaluated for their activity in inhibiting the uptake of [(3)H]dopamine. Binding results demonstrated that alkenyl and alkynyl substitutions at the 4'-position produced potent compounds in which compound 6 with a vinyl substitution was the most potent. In vivo evaluation of three selected compounds indicated that despite their high potency at the DAT, these compounds stimulated locomotor activity (LMA) less than cocaine when tested across similar dose ranges. In a drug discrimination study procedure, none of these three compounds generalized from cocaine in mice trained to discriminate 10 mg/kg cocaine from vehicle. In a 4 h time course LMA experiment, one of our previous lead piperidine derivatives (1a) showed considerable prolonged action. Thus, in this report, we describe a structure-activity relationship study of novel piperidine analogues assessed by both in vitro transporter assays and in vivo behavioral activity measurements.

The uptake inhibitors cocaine and benztropine differentially alter the conformation of the human dopamine transporter

The binding affinity of the cocaine analog [(3)H]2 beta-carbomethoxy-3beta-(4-fluorophenyl) tropane (WIN) for the dopamine transporter (DAT) is increased by the reaction of Cys-90, at the extracellular end of the first transmembrane segment, with methanethiosulfonate (MTS) reagents. Cocaine enhances the reaction of Cys-90 with the sulfhydryl reagents, thereby augmenting the increase in binding. In contrast, cocaine decreases the reaction of Cys-135 and Cys-342, endogenous cysteines in cytoplasmic loops, with MTS reagents. Because this reaction inhibits [(3)H]WIN binding, cocaine protects against the loss of binding caused by reaction of these cysteines. In the present work, we compare the abilities of DAT inhibitors and substrates to affect the reaction of Cys-90, Cys-135, and Cys-342 with MTS ethyltrimethylammonium (MTSET). The results indicate that the different abilities of compounds to protect against the MTSET-induced inhibition of binding are attributable to differences in their abilities to attenuate the inhibitory effects of modification of Cys-135 and Cys-342 as well as to enhance the reaction with Cys-90 and the resulting potentiation of binding. The inhibitor benztropine was unique in its inability to protect Cys-135. Moreover, whereas cocaine, WIN, mazindol, and dopamine enhanced the reaction of Cys-90 with MTSET, benztropine had no effect on this reaction. These two features combine to give benztropine its weak potency in protecting ligand binding to wild-type DAT from MTSET. These results indicate that different inhibitors of DAT, such as cocaine and benztropine, produce different conformational changes in the transporter. There are differences in the psychomotor stimulant-like effects of these compounds, and it is possible that the different behavioral effects of these DAT inhibitors stem from their different molecular actions on 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.

Reaction of oxidized dopamine with endogenous cysteine residues in the human dopamine transporter

There is evidence to suggest that dopamine (DA) oxidizes to form dopamine ortho-quinone (DAQ), which binds covalently to nucleophilic sulfhydryl groups on protein cysteinyl residues. This reaction has been shown to inhibit dopamine uptake, as well as other biological processes. We have identified specific cysteine residues in the human dopamine transporter (hDAT) that are modified by this electron-deficient substrate analog. DAQ reactivity was inferred from its effects on the binding of [(3)H]2-beta-carbomethoxy-3-beta-(4-fluorophenyl)tropane (beta-CFT) to hDAT cysteine mutant constructs. One construct, X5C, had four cysteines mutated to alanine and one to phenylalanine (Cys(90)A, Cys(135)A, C306A, C319F and Cys(342)A). In membrane preparations 1 mM DAQ did not affect [(3)H]beta-CFT binding to X5C hDAT, in contrast to its effect in wild-type hDAT in which it reduced the B:(max) value by more than half. Wild-type cysteines were substituted back into X5C, one at a time, and the ability of DAQ to inhibit [(3)H]beta-CFT binding was assessed. Reactivity of DAQ with Cys(90) increased the affinity of [(3)H]beta-CFT for the transporter, whereas reactivity with Cys(135) decreased the affinity of [(3)H]beta-CFT. DAQ did not change the K:(D) for [(3)H]beta-CFT binding to wild-type. The reactivity of DAQ at Cys(342) decreased B:(max) to the same degree as wild-type. The latter result suggests that Cys(342) is the wild-type residue most responsible for DAQ-induced inhibition of [(3)H]beta-CFT binding.

Transport-dependent accessibility of a cytoplasmic loop cysteine in the human dopamine transporter

The effect of covalent sulfhydryl modification on dopamine uptake by the human dopamine transporter was determined by rotating disc electrode voltammetry. A transporter construct, X5C, with five mutated cysteines (C90A, C135A, C306A, C319F, and C342A) and the constructs into which the wild-type cysteines were substituted back into X5C, one at a time, all showed nearly normal binding affinity for [(3)H]CFT and for cocaine, but they displayed significant reductions in K(m) and V(max) for DA uptake. Reaction of Cys-90 or Cys-306 with impermeant methanethiosulfonate derivatives enhanced dopamine uptake to a similar extent as the previously observed enhancement of [(3)H]CFT binding caused by the same reaction, suggesting that cocaine may bind preferentially to a conformation in the transport cycle. m-Tyramine increased the rate of reaction of (2-aminoethyl)methanethiosulfonate (MTSEA) with X-A342C, the construct with a cytoplasmic loop residue Cys-342 restored. This m-tyramine-induced increase in reactivity appeared to require the inward transport rather than the outward transport or external binding of m-tyramine, and it was prevented by cocaine. Thus, inward translocation of substrates may involve structural rearrangement of hDAT, which likely exposes Cys-342 to reaction with MTSEA, and Cys-342 may be located on a part of the transporter associated with cytoplasmic gating.

Cocaine upregulates the dopamine transporter in fetal rhesus monkey brain

Cocaine is a highly addictive drug that binds to the dopamine transporter (DAT), inhibits the reuptake of dopamine, and initiates multiple actions within midbrain dopaminergic systems. Using the rhesus monkey, we have investigated the consequences of in utero cocaine exposure on the expression of DAT in the fetal brain. By using the selective DAT ligand [125I]RTI-121 and tyrosine hydroxylase (TH) immunocytochemistry, we found that DAT binding sites are highly developed by day 70 of gestation and show a distribution pattern similar to TH. The rank order of specific 3beta-(4-[125I]iodophenyl)tropane-2beta-carboxylic acid isopropyl ester ([125I]RTI-121) binding densities was substantia nigra-ventral tegmental area > putamen > caudate > lateral hypothalamus > accumbens > linear/interfascicular nuclei >/= globus pallidus > prefrontal cortex. Furthermore, we observed that DAT mRNA was differentially expressed within fetal midbrain dopamine neurons with the highest levels detected in the ventral tier of the substantia nigra pars compacta, and the lowest levels in the ventral tegmental area and the linear/interfascicular nuclei. In utero cocaine exposure between days 22 and 70 significantly increased DAT mRNA expression, and the density of [125I]RTI-121 binding sites within midbrain dopamine neurons in the 70-d-old fetus. This increased DAT expression is accompanied by other presynaptic and postsynaptic neuronal changes, which collectively suggest that midbrain dopamine neurons are hypoactive after prolonged cocaine exposure, a state that may be a contributing factor in the development of attention deficit disorders observed in subjects exposed prenatally to cocaine.

Design, synthesis, and biological evaluation of novel non-piperazine analogues of 1-[2-(diphenylmethoxy)ethyl]- and 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazines as dopamine transporter inhibitors

A series of novel diamine, amine-amide, and piperazinone analogues of N-[2-(bisarylmethoxy)ethyl]-N'-(phenylpropyl)piperazines, GBR 12909 and 12935, were synthesized and evaluated as inhibitors of presynaptic monoamine neurotransmitter transporters. The primary objective of the study was to determine the structural requirements for selectivity of ligand binding and potency for neurotransmitter reuptake inhibition. In general, the target compounds retained transporter affinity; however, structural variations produced significant effects on reuptake inhibition and transporter selectivity. For example, analogues prepared by replacing the piperazine ring in the GBR structure with an N, N'-dimethylpropyldiamine moiety displayed enhanced selectivity for binding and reuptake inhibition at the norepinephrine (NE) transporter site (e.g. 4 and 5). Congeners in which the amide nitrogen atom was attached to the aralkyl moiety of the GBR molecule showed moderate affinity (K(i) = 51-61 nM) and selectivity for the dopamine transporter (DAT) site. In contrast, introduction of a carbonyl group adjacent to either nitrogen atom of the piperazine ring (e.g. 25 and 27) was not well tolerated. From the compounds prepared, analogue 16 was selected for further evaluation. With this congener, locomotor activity induced by cocaine at a dose of 20 mg/kg was attenuated with an AD(50) (dose attenuating cocaine-induced stimulation by 50%) of 60.0 +/- 3.6 mg/kg.

Recovery of dopamine neuronal transporter but lack of change of its mRNA in substantia nigra after inactivation by a new irreversible inhibitor characterized in vitro and ex vivo in the rat

1. In vitro, the ability of DEEP-NCS {1-[2-(diphenylmethoxy)ethyl]-4-[2-(4-isothiocyanatophenyl)ethyl]- piperazine} to inhibit [3H]-dopamine uptake by rat striatal synaptosomes was concentration-dependent and inversely related to the protein concentration. This inhibition was irreversible and resulted from changes in Vmax and KM. DEEP-NCS was less potent on noradrenaline, serotonin and choline transport. 2. One day after intrastriatal injections of DEEP-NCS (100 and 1000 pmol) in 20% dimethylsulphoxide, moderate decreases in the ex vivo dopamine uptake were observed in synaptosomes obtained from striatum injected with DEEP-NCS or solvent, and the contralateral uninjected striatum. 3. In similar conditions, 300 pmol DEEP-NCS in 45% 2 hydroxypropyl-gamma-cyclodextrin - 0.5% dimethylsulphoxide solution sub-totally reduced ex vivo dopamine uptake and mazindol binding, and moderately decreased choline and serotonin transport. These reductions were specific to DEEP-NCS-injected striata. A clomipramine pretreatment (16 mg kg-1 i.p. 1 h before) was performed in following experiments, since it reduced the DEEP-NCS-elicited decrease in serotonin uptake without affecting other indices. 4. One day after intrastriatal injection, DEEP-NCS elicited similar dose-dependent decreases in ex vivo dopamine uptake and mazindol binding (ID50=6.9-8 ng striatum-1). Changes in KM and Vmax for ex vivo dopamine transport produced by DEEP-NCS disappeared according to similar time-courses. 5. The t(1/2) for transporter recovery was 6. 1 days. This value should correspond to its actual turnover rate in vivo, since no change in transporter mRNA level was observed in substantia nigra ipsilateral to 300 pmol DEEP-NCS-injected striatum. 6. The results indicate that DEEP-NCS behaves as a potent, quite selective, irreversible inhibitor of the DAT, in vitro and in vivo. Its use in vivo suggests that the physiological half-life of the rat striatal DAT is close to 6 days.

Continuous infusion of selective dopamine uptake inhibitors or cocaine produces time-dependent changes in rat locomotor activity

Chronic continuous cocaine treatment produces a unique pattern of locomotor activation over time. An initial, progressive increase in locomotion is indicative of sensitization. Unlike intermittent cocaine, this increase is subsequently reversed during the continuous exposure, and activity returns to pre-sensitization levels within days. To study the pharmacological mechanisms that underlie this phenomenon, osmotic minipumps containing cocaine or selective uptake inhibitors of dopamine (GBR 12909 or RTI-117), serotonin (fluoxetine), or norepinephrine (nisoxetine) were implanted into rats. Locomotor activity was measured for 1 h each day, beginning 4 h after pumps were implanted. In the cocaine group, activity was significantly elevated on the first day, peaked between the second and third days, then decreased to a plateau which remained significantly above control levels through 14 days. Peak activity in the GBR 12909 and RTI-117 animals occurred on the first day, followed by a significant decrease 24-48 h later, but not complete tolerance. Neither fluoxetine nor nisoxetine altered locomotor activity. The selective dopamine uptake inhibitors produced some of the effects of cocaine. The possibilities that cocaine interacts with the dopamine transporter in a qualitatively different manner from that of these selective dopamine uptake inhibitors, or that other monoamine systems are involved, are discussed.

Evidence for the sequential formation of two complexes between an uptake inhibitor, GBR 12783 [1-[2-(diphenylmethoxy)ethyl]-4-(3-phenyl-2-propenyl)piperazine], and the neuronal transporter of dopamine

Incubation of a crude synaptosomal fraction from rat striatum with GBR 12783 at 37 degrees C produced an inhibition of the specific uptake of [3H]dopamine that increased with time. The inhibition increased when GBR 12783 was present during preincubation and incubation (IC50 = 1.85+/-0.1 nM) instead of incubation alone (IC50 = 25+/-3.5 nM). Time-course studies of uptake inhibition demonstrated that a first collision transporter-inhibitor complex (TI) was formed immediately after addition of GBR 12783 so that the initial uptake velocity (V0) decreased for increasing concentrations of inhibitor (Ki > or = 20 nM). TI slowly isomerized to a more stable complex TI* (Ki* < or = 5 nM) with a value of t1/2 = 20-270 s. Fits of data to model 2 in which the steady-state uptake (VS) is set to zero were generally preferred, suggesting that formation of TI* could tend to irreversibility, as a consequence of a very low reverse isomerization. As expected, k, V0, and VS tended to steady-state values in an asymptotic manner for high concentrations of GBR 12783. GBR 12783 at 2.5 nM produced a mixed inhibition of the uptake, with an increase in KM and a decrease in Vmax; these effects were improved for 10 nM GBR 12783 and at 20 degrees C. These results are discussed in relation to previous data concerning [3H]GBR 12783 binding. The present work gives the first experimental demonstration that dopamine uptake blockers can act according to a two-step mechanism of inhibition; this is of great interest, because these inhibitors can oppose the effects of cocaine or amphetamine on the transporter according to a reaction that is partly nondependent on the concentration of the abused agent.

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.

Regulation of the functional activity of the human dopamine transporter by protein kinase C

The role of protein kinase C (PKC) was examined in the regulation of dopamine transport in C6 glioma cells stably expressing the human dopamine transporter. The PKC activating phorbol esters phorbol 12-myristate 13-acetate (PMA) and 4 beta-12,13-dibutyrate phorbol-ester (PDBu) inhibited [3H]dopamine uptake concentration dependently. These effects were attenuated by the PKC inhibitor staurosporine but were unaltered by another inhibitor, chelerythrine, or the phosphatase inhibitor okadaic acid. The potency of PMA in inhibiting [3H]dopamine uptake was similar to that in inhibiting the binding of 2 beta-carbomethoxy-3 beta-(4-fluorophenyl)tropane ([3H]WIN 35,428), and again staurosporine, but not chelerythrine, weakened the effect of PMA. The reduction in dopamine transporter activity by PMA was caused by a decrease in the Vmax value of [3H]dopamine uptake, opposed by a smaller reduction in the Km value, whereas the effect of PMA on [3H]WIN 35,428 binding was caused by a reduction in the Bmax value without a change in the Kd value. The lower Km value in the presence of PMA was accompanied by a higher IC50 of dopamine in inhibiting [3H]WIN 35,428 binding; the latter effect was attenuated by the co-presence of staurosporine. The results are discussed in the context of transporter loss from the cell surface, or a model with phosphorylation affecting the shared dopamine and WIN 35,428 binding domain on the transporter as well as affecting a part of the dopamine binding domain lying outside that for WIN 35,428.

Monoamine transporters and the neurobehavioral teratology of cocaine

Prenatal cocaine exposure has been associated with various postnatal behavioral abnormalities in human infants, and also with changes in locomotor activity, learning deficits, and altered responses to drug challenge in nonhuman offspring. Several studies have further demonstrated that cocaine exerts an activating effect on fetal behavior. A variety of mechanisms have been proposed to account for the neurobehavioral teratogenic effects of developmental cocaine treatment, including inhibition of fetal brain neurotransmitter uptake and fetal hypoxemia secondary to constriction of the uteroplacental vascular bed. In support of the hypothesis that cocaine effects may be mediated partly by monoamine uptake inhibition, we and other investigators have recently demonstrated the presence of functional dopamine (DA), serotonin (5-HT), and norepinephrine (NE) transporters in the fetal rat brain. Transporter-related cocaine recognition sites are found in a number of fetal brain areas and could mediate the acute effects of cocaine on these areas as well as developmental changes that are manifested postnatally. For example, DA transporter blockade may underlie the above-mentioned activational effects of cocaine on fetal behavior. Time-dependent changes in DA or 5-HT transporter expression produced by prenatal cocaine exposure could likewise play an important role in some of the behavioral effects observed when offspring are tested postnatally.

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.

The dopamine transporter carboxyl-terminal tail. Truncation/substitution mutants selectively confer high affinity dopamine uptake while attenuating recognition of the ligand binding domain

In order to delineate structural motifs regulating substrate affinity and recognition for the human dopamine transporter (DAT), we assessed [3H]dopamine uptake kinetics and [3H]CFT binding characteristics of COS-7 cells transiently expressing mutant DATs in which the COOH terminus was truncated or substituted. Complete truncation of the carboxyl tail from Ser582 allowed for the expression of biphasic [3H]dopamine uptake kinetics displaying both a low capacity (Vmax approximately 0.4 pmol/10(5) cells/min) high affinity (Km approximately 300 nM) component and one exhibiting low affinity (Km approximately 15 microM] and high capacity (Vmax approximately 5 pmol/10(5)cells/min) with a concomitant 40% decrease in overall apparent Vmax relative to wild type (WT) DAT. Truncation of the last 22 amino acids or substitution of the DAT-COOH tail with sequences encoding the intracellular carboxyl-terminal of either dopamine D1 or D5 receptors produced results that were identical to those with the fully truncated DAT, suggesting that the induction of biphasic dopamine uptake kinetics is likely conferred by removal of DAT-specific sequence motifs distal to Pro597. The attenuation of WT transport activity, either by lowering levels of DAT expression or by pretreatment of cells with phorbol 12-myristate 13-acetate (1 microM), did not affect the kinetics of [3H]dopamine transport. The estimated affinity of dopamine (Ki approximately 180 nM) for all truncated/substituted DAT mutants was 10-fold lower than that of WT DAT (approximately 2000 nM) and appears selective for the endogenous substrate, since the estimated inhibitory constants for numerous putative substrates or uptake inhibitors were virtually identical to those obtained for WT DATs. In marked contrast, DAT truncation/substitution mutants displayed significantly reduced high affinity [3H]CFT binding interactions with estimated Ki values for dopamine and numerous other substrates and inhibitors tested from 10-100-fold lower than that observed for WT DAT. Moreover, co-expression of truncated and/or substituted DATs with WT transporter failed to reconstitute functional or pharmacological activities associated with both transporters. Instead, complete restoration of uniphasic low affinity [3H]dopamine uptake kinetics (Km approximately 2000 nM) and high affinity substrate and inhibitor [3H]CFT binding interactions attributable to WT DATs were evident. These data clearly suggest the functional independence and differential regulation of the dopamine translocation process from the characteristics exhibited by its ligand binding domain. The lack of functional phenotypic expression of mutant DAT activities in cells co-expressing WT transporter is consistent with the contention that native DATs may exist as multisubunit complexes, the formation and maintenance of which is dependent upon sequences encoded within the carboxyl tail.

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.

Synthesis and pharmacology of potential cocaine antagonists. 2. Structure-activity relationship studies of aromatic ring-substituted methylphenidate analogs

As part of a program to develop medications which can block the binding of cocaine to the dopamine transporter, yet spare dopamine uptake, a series of aromatic ring-substituted methylphenidate derivatives was synthesized and tested for inhibitory potency in [3H]WIN 35,428 binding and [3H]dopamine uptake assays using rat striatal tissue. Synthesis was accomplished by alkylation of 2-bromopyridine with anions derived from various substituted phenylacetonitriles. In most cases, erythro compounds were markedly less potent than the corresponding (+/-)-threo-methylphenidate (TMP; Ritalin) derivatives. The ortho-substituted compounds were much less potent than the corresponding meta- and/or para-substituted derivatives. The most potent compound against [3H]WIN 35,428 binding, m-bromo-TMP, was 20-fold more potent than the parent compound, whereas the most potent compound against [3H]dopamine uptake, m,p-dichloro-TMP, was 32-fold more potent. Threo derivatives with m- or p-halo substituents were more potent than TMP, while electron-donating substituents caused little change or small loss of potency. All of the derivatives had Hill coefficients approaching unity, except m,p-dichloro-TMP, which had an nH of 2.0. Although the potency of the (+/-)-methylphenidate derivatives in the two assays was highly correlated (R2 = 0.986), the compounds m-chloro-,m-methyl-, and p-iodo-TMP were 4-5-fold more potent at inhibiting [3H]-WIN 35,428 binding than [3H]dopamine uptake (cocaine has a ratio of 2.3). These and other compounds may be promising candidates for further testing as potential partial agonists or antagonists of cocaine.

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.