Putative cocaine receptor in striatum is a glycoprotein with active thiol function

Discovery of a macromolecular complex mediating the hunger suppressive actions of cocaine: Structural and functional properties

Cocaine not only increases brain dopamine levels but also activates the sigma1 receptor (σ1 R) that in turn regulates orexigenic receptor function. Identification of interactions involving dopamine D1 (D1 R), ghrelin (GHS-R1a ), and σ1 receptors have been addressed by biophysical techniques and a complementation approach using interfering peptides. The effect of cocaine on receptor functionality was assayed by measuring second messenger, cAMP and Ca2+ , levels. The effect of acute or chronic cocaine administration on receptor complex expression was assayed by in situ proximity ligation assay. In silico procedures were used for molecular model building. σ1 R KO mice were used for confirming involvement of this receptor. Upon identification of protomer interaction and receptor functionality, a unique structural model for the macromolecular complex formed by σ1 R, D1 R, and GHS-R1a is proposed. The functionality of the complex, able to couple to both Gs and Gq proteins, is affected by cocaine binding to the σ1 R, as confirmed using samples from σ1 R-/- mice. The expression of the macromolecular complex was differentially affected upon acute and chronic cocaine administration to rats. The constructed 3D model is consistent with biochemical, biophysical, and available structural data. The σ1 R, D1 R, and GHS-R1a complex constitutes a functional unit that is altered upon cocaine binding to the σ1 R. Remarkably, the heteromer can simultaneously couple to two G proteins, thus allowing dopamine to signal via Ca2+ and ghrelin via cAMP. The anorexic action of cocaine is mediated by such complex whose expression is higher after acute than after chronic administration regimens.

Toxic-Metal-Induced Alteration in miRNA Expression Profile as a Proposed Mechanism for Disease Development

Toxic metals are extensively found in the environment, households, and workplaces and contaminate food and drinking water. The crosstalk between environmental exposure to toxic metals and human diseases has been frequently described. The toxic mechanism of action was classically viewed as the ability to dysregulate the redox status, production of inflammatory mediators and alteration of mitochondrial function. Recently, growing evidence showed that heavy metals might exert their toxicity through microRNAs (miRNA)—short, single-stranded, noncoding molecules that function as positive/negative regulators of gene expression. Aberrant alteration of the endogenous miRNA has been directly implicated in various pathophysiological conditions and signaling pathways, consequently leading to different types of cancer and human diseases. Additionally, the gene-regulatory capacity of miRNAs is particularly valuable in the brain—a complex organ with neurons demonstrating a significant ability to adapt following environmental stimuli. Accordingly, dysregulated miRNAs identified in patients suffering from neurological diseases might serve as biomarkers for the earlier diagnosis and monitoring of disease progression. This review will greatly emphasize the effect of the toxic metals on human miRNA activities and how this contributes to progression of diseases such as cancer and neurodegenerative disorders (NDDs).

Mechanism for cocaine blocking the transport of dopamine: insights from molecular modeling and dynamics simulations

Molecular modeling and dynamics simulations have been performed to study how cocaine inhibits dopamine transporter (DAT) for the transport of dopamine. The computationally determined DAT-ligand binding mode is totally different from the previously proposed overlap binding mode in which cocaine- and dopamine-binding sites are the same (Beuming, T.; et al. Nat. Neurosci. 2008, 11, 780-789). The new cocaine-binding site does not overlap with, but is close to, the dopamine-binding site. Analysis of all results reveals that when cocaine binds to DAT, the initial binding site is likely the one modeled in this study because this binding site can naturally accommodate cocaine. Then cocaine may move to the dopamine-binding site after DAT makes some necessary conformational change and expands the binding site cavity. It has been demonstrated that cocaine may inhibit the transport of dopamine through both blocking the initial DAT-dopamine binding and reducing the kinetic turnover of the transporter following the DAT-dopamine binding. The relative contributions to the phenomenological inhibition of the transport of dopamine from blocking the initial binding and reducing the kinetic turnover can be different in different types of assays. The obtained general structural and mechanistic insights are consistent with available experimental data and could be valuable for guiding future studies toward understanding cocaine's inhibiting of other transporters.

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.

In vivo interaction of cocaine with the dopamine transporter as measured by voltammetry

The goal of this review is to describe what the voltammetry technique tells us about cocaine-dopamine transporter (DAT) interactions and the subsequent changes in extracellular dopamine levels in the brain. The primary advantage of voltammetry, in this regard, is the capability for kinetic analysis in situ. Analysis of electrically evoked dynamics suggests that cocaine competitively inhibits dopamine uptake in the caudate-putamen and nucleus accumbens with a similar efficacy. The preferential increase in accumbal dopamine following systemic cocaine administration was found to be related not to a unique cocaine-DAT interaction, but rather to a unique combination of dopamine release and uptake rates. Similar enhancement occurs in sub-regions of the caudate-putamen exhibiting this release and uptake combination. Other factors such as diffusion and whether dopaminergic signaling is tonic or phasic also determine the effects of cocaine on striatal dopamine levels.

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.

L-arginine increases dopamine transporter activity in rat striatum via a nitric oxide synthase-dependent mechanism

Literature reports suggest that nitric oxide (NO) participates in the regulation of dopaminergic neurotransmission, possibly through interaction with cysteine residues of the dopamine transporter (DAT). Rotating disk electrode voltammetry was used to measure dopamine (DA) transport in rat striatum to determine if 1) the nitric oxide synthase (NOS) substrate, L-arginine (L-Arg), could affect DAT activity; 2) L-Arg-dependent effects on DAT activity could be blocked by NOS and guanylate cyclase inhibitors, a NO scavenger, DA, and cocaine; 3) a NO donor could affect DAT activity; and 4) L-Arg could protect the DAT from a sulfhydryl agent. L-Arg increased DAT activity by increasing V(max). NOS inhibitors (S-ethylisothiourea and S-isopropylisothiourea), a NO scavenger (2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide), DA, and cocaine blocked the L-Arg effect. The guanylate cyclase inhibitor, 1H-(1,2,4)-oxadiazolo[4,3a]quinoxalin-1-one, did not. The NO donor, S-nitroso-N-acetylpenicillamine, decreased DAT activity and L-Arg protected the DAT from the effects of the sulfhydryl agent N-ethylmaleimide. These results suggest that L-Arg, via NO, may play a role in regulating DAT activity in rat striatum by increasing the V(max) of DA transport. Furthermore, it is suggested that the effects of L-Arg on DAT activity may be due to modification of the DAT itself, possibly via the NO-mediated modification of DAT cysteine residues. Finally, NO produced from L-Arg may affect the DAT differently than NO from NO donors. These results further the notion that dopaminergic neurotransmission may be regulated by changes in DAT activity caused by L-Arg and NOS.

Covalent and noncovalent chemical modifications of arginine residues decrease dopamine transporter activity

Rotating disk electrode voltammetry was used to measure dopamine (DA) transport in rat striatum and in human embryonic kidney cells expressing the rat dopamine transporter (DAT). The goals of this study were to determine 1) if arginine (Arg) selective agents could alter DA transport, and 2) if DA analogs and DAT inhibitors could attenuate the effects of these agents on the DAT. Phenylglyoxal (PG), Hill coefficient 2.5, and other Arg selective agents decreased DA transport velocities. DA, Hill coefficient 1.0, and its analogs 3-hydroxyphenethylamine and 4-hydroxyphenethylamine attenuated the effects of PG on the DAT while phenethylamine did not. The tropane-based DAT inhibitors cocaine, WIN 35065-2, and WIN 35428 also attenuated the effects of PG. Benztropine, GBR 12935, and GBR 12909 did not. Thus, Arg residues are important for DAT activity and the results suggest that DA and cocaine both interact with Arg residues. Structure-activity studies suggest that DA interacts with Arg through its catechol hydroxyl groups and cocaine through the ester linkage attached to carbon 2 of the tropane ring. The results that 1). DA and cocaine may interact with the same functionally important Arg residue at the DAT, and 2). some members of the tropane and 1,4-dialkylpiperazine classes of DAT inhibitors may interact differently with DAT-derived Arg residue(s) furthers the notion that DAT activity sparing antagonists of cocaine can be designed.

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.

Design and synthesis of an irreversible dopamine-sparing cocaine antagonist

Cocaine is a powerful reinforcer and stimulant that binds to specific recognition sites associated with monoamine transporters in the mammalian brain. The search for a functional antagonist to the addictive properties of cocaine has focused on the discovery of a molecule that can inhibit cocaine binding to the dopamine transporter (DAT) but continue to allow dopamine transport by the DAT. No such dopamine-sparing cocaine antagonist has been reported and it is becoming evident that dopamine-sparing antagonism of the pharmacological effects of cocaine by a classical antagonist may not be possible. Herein we present a new concept for the design of dopamine-sparing cocaine antagonists. A unique approach is utilized to deliver an inhibitor that binds irreversibly to the DAT, then cleaves and leaves behind a small fragment attached to the DAT that blocks access by cocaine but permits dopamine transport. The design of these compounds takes advantage of a cysteinyl sulfhydryl group in the DAT. This group is hypothesized to attack the incoming inhibitor and lead to selective inhibition of the cocaine binding site while sparing dopamine transport. This concept of a mechanism based irreversible dopamine-sparing cocaine antagonist has now been demonstrated to be viable and, as example, the unsaturated 6 showed inhibition of cocaine (63%) at the DAT after 24h incubation, while at that point considerably less inhibition of dopamine is manifested (23%). In contrast, the epoxide 7 showed a greater inhibition of dopamine reuptake than cocaine binding at 24h (68% versus 18%).

Determination of release and uptake parameters from electrically evoked dopamine dynamics measured by real-time voltammetry

Quantifying mechanisms underlying extracellular signaling by the neurotransmitter dopamine (DA) is a difficult task, particularly in the complex extracellular microenvironment of the intact brain. In this study, two methods for evaluating release and uptake from DA dynamics monitored by real-time voltammetry are described. Both are based on a neurochemical model characterizing electrically evoked levels of DA as a balance between these opposing mechanisms. The theoretical basis of what is called here nonlinear regression and single curve analyses is given. Fitting simulated data tests the reliability of the methods. The two analyses are also compared with an experimental data set describing the effects of pharmacologically inhibiting the DA transporter in the caudate-putamen (CP) and nucleus accumbens (NAc). The results indicate that nonlinear regression and single curve analyses are suitable for quantifying release and uptake mechanisms underlying DA neurotransmission. Additionally, the most important experimental finding of this technical study was the independent confirmation of high affinity (approximately 0.2 microM) DA uptake in the intact striatum.

Preferential increases in nucleus accumbens dopamine after systemic cocaine administration are caused by unique characteristics of dopamine neurotransmission

In vivo voltammetry was used to investigate the preferential increase of extracellular dopamine in the nucleus accumbens relative to the caudate-putamen after systemic cocaine administration. In the first part of this study, cocaine (40 mg/kg, i.p.) was compared with two other blockers of dopamine uptake, nomifensine (10 mg/kg, i.p.) and 3beta-(p-chlorophenyl)tropan-2beta-carboxylic acid p-isothiocyanatophenylmethyl ester hydrochloride (RTI-76; 100 nmol, i.c.v.), to assess whether the inhibitory mechanism of cocaine differed in the two regions. All three drugs robustly increased electrically evoked levels of dopamine, and cocaine elevated dopamine signals to a greater extent in the nucleus accumbens. However, kinetic analysis of the evoked dopamine signals indicated that cocaine and nomifensine increased the K(m) for dopamine uptake whereas the dominant effect of RTI-76 was a decrease in V(max). Under the present in vivo conditions, therefore, cocaine is a competitive inhibitor of dopamine uptake in both the nucleus accumbens and caudate-putamen. Whether the preferential effect of cocaine was mediated by regional differences in the presynaptic control of extracellular DA that are described by rates for DA uptake and release was examined next by a correlation analysis. The lower rates for dopamine release and uptake measured in the nucleus accumbens were found to underlie the preferential increase in extracellular dopamine after cocaine. This relationship explains the paradox that cocaine more effectively increases accumbal dopamine despite identical effects on the dopamine transporter in the two regions. The mechanism proposed for the preferential actions of cocaine may also mediate the differential effects of psychostimulant in extrastriatal regions and other uptake inhibitors in the striatum.

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.

Origin of 4-hydroxynonenal incubation-induced inhibition of dopamine transporter and Na+/K+ adenosine triphosphate in rat striatal synaptosomes

Previous experiments reported that an incubation of striatal synaptosomes with 4-hydroxynonenal (4-HNE) resulted in an inhibition of dopamine (DA) uptake and Na+/K+ adenosine triphosphate (ATPase) activity. The present work investigated whether theses inhibitions are related to a 4-HNE binding to the DA transporter (DAT) and the Na+/K+ ATPase. The number of specific [125I]-PE21 binding sites on the DAT was significantly reduced after incubation with 4-HNE. The Na+/K+ ATPase activity decrease induced by 4-HNE was partially reversed, in a dose-dependent manner, by veratridine, a pump stimulator agent. Our previous data (Morel, P., Tallineau, C., Pontcharraud, R., Piriou, A. and Huguet, F., Effects of 4-hydroxynonenal, a lipid peroxidation product, on dopamine transport and Na+/K+ ATPase in rat striatal synaptosomes. Neurochem. Int., 33 (1999) 531-540) combining with the data observed in this study suggest that changes in DA uptake in striatal synaptosomes are directly related to 4-HNE binding to the DAT, whereas the decrease in Na+/K+ ATPase activity resulted only partially from 4-HNE binding to the pump and is mainly secondary to membrane lipid disruption.

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.

Oxygen radicals differentially affect Na+/Cl(-)-dependent transporters

Incubation with the oxygen radical-generating enzyme, xanthine oxidase, dramatically reduced striatal dopamine transporter activity, but was unexpectedly without effect on rat hippocampal norepinephrine uptake. To determine whether environmental differences between the striatum and hippocampus contributed to this lack of oxidative effect on norepinephrine transporters, synaptosomal gamma-aminobutyric acid (GABA) uptake was assessed in both regions. Xanthine oxidase similarly decreased [3H]GABA uptake in both the striatum and hippocampus, supporting the conclusion that environmental differences did not account for the lack of effect on norepinephrine transport. These data suggest that norepinephrine transporters are less vulnerable than other Na+/Cl(-)-dependent transporters to oxidative inactivation.

Effects of 4-hydroxynonenal, a lipid peroxidation product, on dopamine transport and Na+/K+ ATPase in rat striatal synaptosomes

Incubation of rat striatal synaptosomes in ascorbic acid induced the production of thiobarbituric acid reactive substances, a marker of lipid peroxidation, and 4-hydroxynonenal (4-HNE), a lipid peroxidation aldehydic product. Incubations with 4-HNE, used at a range of concentrations comparable to those obtained during peroxidation, induced a simultaneous, dose-dependent decrease of dopamine (DA) uptake and Na+/K+ ATPase activity and a loss of sulfhydryl (SH) groups. Similar results were observed in a previous study when lipid peroxidation was induced after incubation of synaptosomes in ascorbic acid. Taken together, these data suggest that 4-HNE is an important mediator of oxidative stress and may alter DA uptake after binding to SH groups of the DA transporter and to Na+/K+ ATPase. These toxic events may contribute to the onset and progression of Parkinson's disease.

Inhibitory effects of ascorbic acid on dopamine uptake by rat striatal synaptosomes: relationship to lipid peroxidation and oxidation of protein sulfhydryl groups

Ascorbic acid is frequently added in the incubation medium to prevent oxidation of dopamine (DA) during uptake assays. However, a preliminary study showed that the presence of ascorbic acid induced a decrease of DA uptake after prolonged incubation. The purpose of this study was to determine the mechanism underlying ascorbic acid-induced alterations of DA uptake in rat striatal synaptosomes. In this context, the effects of physiological concentrations of ascorbic acid (100-500 microM) on DA uptake and Na+/K+ ATPase activity (which is essential for DA transporter function) were assessed in synaptosomes before and after incubation at 37 degrees C. The capacity of synaptosomes to take up DA was significantly decreased after incubation owing to a reduction in DA transporters (but with no modification of their affinity for DA). This partial inhibition was associated with a decrease of Na+/K+ ATPase activity, a production of thiobarbituric acid reactive substances (TBARS) and malonaldehyde (MDA), and a loss of sulfhydryl group content. Addition of Trolox C to the medium prevented the reduction of DA uptake, the inhibition of Na+/K+ ATPase activity, the decrease in sulfhydryl group content and the production of TBARS and MDA. These results suggest that ascorbic acid in the presence of contaminant ferrous ions induced a decrease in functional DA transporters, probably through a lipid peroxidation process involving oxidation of sulfhydryl groups and at least in part through a decrease of Na+/K+ ATPase activity.

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.

Long-lasting microencephaly following exposure to cocaine during the brain growth spurt in the rat

In utero exposure to cocaine has been shown to produce somatic and behavioral effects. As microencephaly is often present in children born from cocaine-addicted mothers, aim of the present study was to develop an animal model for cocaine-induced microencephaly. Rats were treated with cocaine (20, 30 or 50 mg/kg/day, s.c., each dose divided in two equal doses given 3 h apart) from postnatal day 4 through 10. None of the doses had any effect on growth, however, at 50 mg/kg, cocaine caused a significant decrease in brain weight, measured on day 12. The effect of cocaine was similar in male and female rats, and microencephaly was still present in 45-day-old animals. When the same dose of cocaine was given as a single daily injection, long-lasting microencephaly was also present, but it was accompanied by a decrease in body weight and significant toxicity. Ethanol (4 g/kg), used as a positive control, also caused microencephaly without affecting body weight, but, differently from cocaine, its effect was more pronounced in female animals. Blood and brain levels of cocaine and its metabolites norcocaine and benzoylecgonine were measured by HPLC during treatment (postnatal day 8). After administration of the 50 mg/kg dose, concentrations of cocaine were 1.92 micrograms/g in brain and 0.94 microgram/ml in blood. These levels are encountered in cases of cocaine overdoses and have been found in meconium of newborns from crack-addicted mothers.(ABSTRACT TRUNCATED AT 250 WORDS)

Translocation of dopamine and binding of 2 beta-carbomethoxy-3 beta-(4-fluorophenyl) tropane (WIN 35,428) measured under identical conditions in rat striatal synaptosomal preparations. Inhibition by various blockers

Translocation of [3H]dopamine and binding of 2 beta-carbomethoxy-3 beta-(4-fluorophenyl)[3H]-tropane ([3H]WIN 35,428) were measured in crude synaptosomal preparations from rat striatum under identical conditions of assay buffer (phosphate-Krebs) and temperature (25 degrees). [3H]Dopamine uptake as a function of time was close to linear for at least 8 min, whereas [3H]WIN 35,428 binding had reached equilibrium within 1 min and remained at its plateau value for at least 20 min. The following inhibitors were tested in uptake and binding assays run in parallel with the same synaptosomal preparation: cocaine, WIN 35,428, benztropine, nomifensine, mazindol, methylphenidate, N-[1-(2-benzo[b]-thiophenyl)cyclohexyl]piperidine (BTCP), Lu 19-005 (Indatraline), 1-(2-(di(4-fluorophenyl)-methoxy)-ethyl)-4-(3-phenyl-2-propyl)piperazine (GBR 12909), 1-(2-(diphenylmethoxy)-ethyl)-4-(3-phenyl-2-propyl)piperazine (GBR 12935) and 7-trifluoromethyl-4-(4-methyl-1-piperazinyl)-pyrrolo [1,2-a]quinoxaline (CGS 12066B). When present together with [3H]dopamine or [3H]WIN 35,428 for 8 min, the observed binding IC50 values were generally higher (average 1.4-fold) than the uptake IC50 values, with a significant y-axis intercept in linear regression analysis of binding on uptake IC50. For slowly equilibrating inhibitors, estimates of uptake IC50 values were overestimates, and relatively lower values were obtained by monitoring [3H]dopamine uptake for 1 min only during the last minute of the 8-min presence of inhibitor; under these conditions, binding over uptake IC50 ratios were on the average 2.3. Kinetic calculations, taking into account both radioligand and inhibitor equilibration kinetics, indicated that the latter comparison between binding and uptake measurements was most relevant, and suggested the involvement of complexities beyond simple competitive inhibition of dopamine transport, such as different binding domains for substrate and blocker recognition, or spare receptors for blockers. The present data indicate that binding over uptake IC50 ratios should be interpreted with caution, depending on the experimental conditions used to measure these ratios.

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

In incubation medium containing Na+ as the only cation, the specific binding of [3H]cocaine to a membrane preparation obtained from rat striatum reached a maximal level for 10 mM Na+, whereas higher concentrations decreased its affinity. The specific binding of [3H]cocaine was inhibited monophasically by GBR 12783, mazindol, nomifensine and substrates of the transporter; in saturation experiments, GBR 12783 competitively blocked the [3H]cocaine specific binding and vice versa. Treatment of the striatal membranes with N-ethylmaleimide resulted in a concentration-dependent reduction of the specific binding of [3H]GBR 12783 (1-[2-(diphenylmethoxy)ethyl]4-(3-phenyl-2-[1-3H]propenyl)-piperaz ine) which was significantly more marked than that of the specific binding of [3H]cocaine, the nonspecific binding of [3H]cocaine being measured with either cocaine or dopamine. Addition of substrates or pure uptake inhibitors to the treatment medium afforded protection against the N-ethylmaleimide-induced reduction in both bindings. In particular, cocaine offered protection for [3H]GBR 12783 binding and vice versa. All results are consistent with a model in which pure uptake blockers and substrates recognize nonidentical but overlapping binding domains on the neuronal carrier of dopamine.

Nitric oxide inhibits [3H]dopamine uptake

Sodium nitroprusside (SNP), a generator of nitric oxide (NO), decreased [3H]dopamine uptake into rat striatal synaptosomal preparations in a dose-, time- and temperature-dependent fashion. Reduced hemoglobin, a substance that binds NO, prevented the SNP-induced decrease in uptake. Potassium ferri- and ferrocyanide, compounds similar to SNP that do not produce NO, were without effect on uptake. SNP inhibited [3H]dopamine uptake in synaptosomes from nucleus accumbens and olfactory tubercle as well but with a lower potency. SNP inhibited [3H]serotonin and [3H]glutamate uptake but had no effect on [3H]norepinephrine uptake. S-Nitroso-N-acetylpenicillamine (SNAP), another generator of NO, had effects similar to those of SNP. The SNP-induced decrease in [3H]dopamine uptake was due to a Vmax decrease at 100 microM SNP and to both a Vmax and Km change at 300 microM SNP. Depletion of calcium by omission of calcium from buffers and addition of EGTA increased the potency of SNP in inhibiting uptake. There was no change in [3H]WIN 35428 binding to the dopamine transporter with doses of SNP that inhibited uptake. These data indicate that NO can decrease [3H]dopamine transporter function.

Striatal transporter for dopamine: catechol structure-activity studies and susceptibility to chemical modification

The apparent second-order association rate constant of dopamine binding to the striatal transporter (approximately 1 x 10(6) M-1 s-1) as well as the transporter turnover number (approximately 1.5 s-1) was estimated using rotating disk electrode voltammetry to monitor apparent zero trans entry of dopamine into striatal suspensions. The substrate specificity of the transporter was also assessed using catechol derivatives. Dopamine and norepinephrine were transported, whereas epinephrine and the acidic metabolites of dopamine were not transported. The metabolite, 3-methoxytyramine, was transported with a Km seven times greater than and a Vmax close to that for dopamine. 4-Methoxytyramine was transported more facilely than the 3-methoxy derivative. N-Alkylation of the amine side chain of dopamine reduced transport dramatically. 4-Ethylcatechol and 3,4-dihydroxybenzylamine were transported with velocities 79 and 91% less than that for dopamine, respectively. The rigid analogue 6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene was transported with a greater velocity than the 5,7-dihydroxy derivative. Finally, the apparent Km values for 4-ethylcatechol, 1-amino-2-phenylethane, tyramine, and m-tyramine as cosubstrates with dopamine were 1.1, 11, 17, and 2.6 microM, respectively. Pretreatments of striatal suspensions with chloroethylnorapomorphine, N-ethylmaleimide, Hg2+, 4,5-dihydroxy-4,5-dioxo-1H-pyrrolo[2,3-f] quinoline-2,7,9-tricarboxylic acid (a redox modulator of receptors in neuronal as well as other tissues), and neuraminidase reduced the velocity of transport of dopamine, whereas N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline had no effect. Thus, the dopamine transporter requires an intact catechol with a primary ethylamine side chain for optimal activity relative to shorter side chain derivatives (side chains longer than two carbons were not tested), the 3-hydroxyl group of dopamine is the more critical hydroxyl group, and the beta rotamer of the extended conformation of dopamine is transported preferentially. The catechol appears to mediate the recognition of the substrate, whereas the amine side chain apparently facilitates the conformational change of the transporter that results in movement of dopamine into or across the membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

Mercuric chloride and p-chloromercuriphenylsulfonate exert a biphasic effect on the binding of the stimulant [3H]methylphenidate to the dopamine transporter

Mercuric chloride was found to have a biphasic effect on the binding of the radiolabeled stimulant [3H]methylphenidate to membranes from a crude synaptosomal preparation of rat striatal tissue. Binding was enhanced at low concentrations of HgCl2, reaching a maximum of 62% above control values at 2.5 microM HgCl2. It was inhibited in a dose-dependent manner at concentrations greater than 5 microM HgCl2, with an IC50 of 7.2 microM. The increase in binding observed at the low concentrations of HgCl2 was shown by Scatchard analysis to be due to an increase in the affinity of [3H]methylphenidate for its binding site on the dopamine transporter, while a decrease in both affinity and Bmax accompanied the reduction of [3H]methylphenidate binding observed at the higher concentrations of the inorganic mercury compound. The sodium salt of the organic mercurial p-chloromercuriphenylsulfonic acid also caused an increase in [3H]methylphenidate binding (159% above controls at 2,000 microM), followed by an immediate decrease in binding at higher concentrations of the reagent. Because both of these mercury-containing compounds have a high propensity for interacting with sulfhydryl groups, these data suggest that the cysteine residues in the dopamine transporter molecule may play an important role in the regulation of stimulant binding to the uptake complex.

Can stimulant binding and dopamine transport be differentiated? Studies with GBR 12783 derivatives

Both 3- and 4-substituted GBR 12783 derivatives were synthesized in an effort to create site-directed cocaine antagonists. The potencies of these compounds to inhibit stimulant ([3H]WIN 35,428) binding and synaptosomal [3H]dopamine uptake were determined and compared with a large number of compounds assayed under identical experimental conditions. Three groups of compounds were identified which affected stimulant binding and dopamine transport to varying degrees. The 3-substituted GBR 12783 derivatives resembled the known non-amphetamine stimulant agents, in that they were approximately equipotent in inhibiting [3H]WIN 35,428 binding and [3H]dopamine transport. The 4-substituted GBR 12783 derivatives formed a second group that was approximately seven times more potent in the inhibition of [3H]dopamine uptake compared to [3H]WIN 35,428 binding. The third group, approximately one hundred-fold more potent at inhibiting [3H]dopamine uptake than stimulant binding, consisted mainly of substrates for the dopamine transporter. Although these GBR derivatives did not meet the criteria for potential cocaine antagonists, the results demonstrate that slight modifications of the molecular structure of a stimulant drug can differentially affect binding and transport. This finding holds great promise for the eventual development of a true pharmacological antagonist of cocaine.

Cocaine and dopamine differentially protect [3H]mazindol binding sites from alkylation by N-ethylmaleimide

The binding of cocaine, d-amphetamine and dopamine to the site on the dopamine transporter labeled by [3H]mazindol was investigated in rat striatal membranes. N-Ethylmaleimide inhibited about 95% of the specific binding of 5 nM [3H]mazindol in a concentration-dependent manner. The effect of 10 mM N-ethylmaleimide was completely prevented by cocaine (EC50 of 3 microM), but neither 300 microM dopamine nor d-amphetamine afforded any significant protection. On the other hand, high concentrations of cocaine, d-amphetamine and dopamine provided similar protection against inhibition by 0.1 mM N-ethylmaleimide. Taken together these data support the hypothesis that a significant portion of the cocaine binding domain on the transporter is distinct from that of either dopamine or amphetamine. This distinction may be sufficient to allow properly designed drugs to prevent cocaine binding without inhibiting DA transport.

Synthesis and pharmacology of irreversible affinity labels as potential cocaine antagonists: aryl 1,4-dialkylpiperazines related to GBR-12783

As part of a program aimed at designing irreversible antagonists of the stimulant and reinforcing properties of cocaine, derivatives of GBR-12783 containing electrophilic substituents were synthesized. GBR-12783, a potent and selective inhibitor of both stimulant binding and dopamine transport, was modified to incorporate either isothiocyanate or maleimido groups at the meta- or para-positions in one phenyl ring of the geminal diphenyl portion of the molecule. The effect of these compounds, as well as their respective amino- or nitro-substituted precursors, on stimulant binding to rat striatal tissue was studied using the [3H]methylphenidate radioreceptor assay. Under the assay conditions used, the compounds were found to have IC50s (nM) ranging from 11.9 (m-nitro) to 1677 (p-maleimido); the parent compound, GBR-12783, had an IC50 of 12.0. Using a washout technique (repeated washing with 100 mM KCl) which completely removed the tightly bound, but reversible GBR-12783, both the m- and p-isothiocyanate compounds were found to irreversibly inhibit binding of [3H]methylphenidate to the stimulant recognition site. The m-maleimido derivative also irreversibly inhibited binding, albeit with lower efficacy than was observed with the isothiocyanate compounds. Neither the p-maleimido, nor the amino or nitro intermediates, were capable of irreversible inhibition.