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

Dopamine reuptake transporter (DAT) "inverse agonism"--a novel hypothesis to explain the enigmatic pharmacology of cocaine

The long held view is cocaine's pharmacological effects are mediated by monoamine reuptake inhibition. However, drugs with rapid brain penetration like sibutramine, bupropion, mazindol and tesofensine, which are equal to or more potent than cocaine as dopamine reuptake inhibitors, produce no discernable subjective effects such as drug "highs" or euphoria in drug-experienced human volunteers. Moreover they are dysphoric and aversive when given at high doses. In vivo experiments in animals demonstrate that cocaine's monoaminergic pharmacology is profoundly different from that of other prescribed monoamine reuptake inhibitors, with the exception of methylphenidate. These findings led us to conclude that the highly unusual stimulant profile of cocaine and related compounds, eg methylphenidate, is not mediated by monoamine reuptake inhibition alone. We describe the experimental findings which suggest cocaine serves as a negative allosteric modulator to alter the function of the dopamine reuptake transporter (DAT) and reverse its direction of transport. This results in a firing-dependent, retro-transport of dopamine into the synaptic cleft. The proposed mechanism of cocaine is, therefore, different from other small molecule negative allostereric modulators of the monoamine reuptake transporters, eg SoRI-6238, which merely reduce the rate of inward transport. Because the physiological role of DAT is to remove dopamine from the synapse and the action of cocaine is the opposite of this, we have postulated that cocaine's effect is analogous to an inverse agonist. If this hypothesis is validated then cocaine is the prototypical compound that exemplifies a new class of monoaminergic drugs; DAT "inverse agonists". This article is part of the Special Issue entitled 'CNS Stimulants'.

Nonclassical pharmacology of the dopamine transporter: atypical inhibitors, allosteric modulators, and partial substrates

The dopamine transporter (DAT) is a sodium-coupled symporter protein responsible for modulating the concentration of extraneuronal dopamine in the brain. The DAT is a principle target of various psychostimulant, nootropic, and antidepressant drugs, as well as certain drugs used recreationally, including the notoriously addictive stimulant cocaine. DAT ligands have traditionally been divided into two categories: cocaine-like inhibitors and amphetamine-like substrates. Whereas inhibitors block monoamine uptake by the DAT but are not translocated across the membrane, substrates are actively translocated and trigger DAT-mediated release of dopamine by reversal of the translocation cycle. Because both inhibitors and substrates increase extraneuronal dopamine levels, it is often assumed that all DAT ligands possess an addictive liability equivalent to that of cocaine. However, certain recently developed ligands, such as atypical benztropine-like DAT inhibitors with reduced or even a complete lack of cocaine-like rewarding effects, suggest that addictiveness is not a constant property of DAT-affecting compounds. These atypical ligands do not conform to the classic preconception that all DAT inhibitors (or substrates) are functionally and mechanistically alike. Instead, they suggest the possibility that the DAT exhibits some of the ligand-specific pleiotropic functional qualities inherent to G-protein-coupled receptors. That is, ligands with different chemical structures induce specific conformational changes in the transporter protein that can be differentially transduced by the cell, ultimately eliciting unique behavioral and psychological effects. The present overview discusses compounds with conformation-specific activity, useful not only as tools for studying the mechanics of dopamine transport, but also as leads for medication development in addictive disorders.

The atypical stimulant and nootropic modafinil interacts with the dopamine transporter in a different manner than classical cocaine-like inhibitors

Modafinil is a mild psychostimulant with pro-cognitive and antidepressant effects. Unlike many conventional stimulants, modafinil has little appreciable potential for abuse, making it a promising therapeutic agent for cocaine addiction. The chief molecular target of modafinil is the dopamine transporter (DAT); however, the mechanistic details underlying modafinil's unique effects remain unknown. Recent studies suggest that the conformational effects of a given DAT ligand influence the magnitude of the ligand's reinforcing properties. For example, the atypical DAT inhibitors benztropine and GBR12909 do not share cocaine's notorious addictive liability, despite having greater binding affinity. Here, we show that the binding mechanism of modafinil is different than cocaine and similar to other atypical inhibitors. We previously established two mutations (W84L and D313N) that increase the likelihood that the DAT will adopt an outward-facing conformational state—these mutations increase the affinity of cocaine-like inhibitors considerably, but have little or opposite effect on atypical inhibitor binding. Thus, a compound's WT/mutant affinity ratio can indicate whether the compound preferentially interacts with a more outward- or inward-facing conformational state. Modafinil displayed affinity ratios similar to those of benztropine, GBR12909 and bupropion (which lack cocaine-like effects in humans), but far different than those of cocaine, β-CFT or methylphenidate. Whereas treatment with zinc (known to stabilize an outward-facing transporter state) increased the affinity of cocaine and methylphenidate two-fold, it had little or no effect on the binding of modafinil, benztropine, bupropion or GBR12909. Additionally, computational modeling of inhibitor binding indicated that while β-CFT and methylphenidate stabilize an “open-to-out” conformation, binding of either modafinil or bupropion gives rise to a more closed conformation. Our findings highlight a mechanistic difference between modafinil and cocaine-like stimulants and further demonstrate that the conformational effects of a given DAT inhibitor influence its phenomenological effects.

Rotating disk electrode voltammetric measurements of serotonin transporter kinetics in synaptosomes

Altered serotonin (5-HT) signaling is implicated in several neuropsychiatric disorders, including depression, anxiety, obsessive-compulsive disorder, and autism. The 5-HT transporter (SERT) modulates 5-HT neurotransmission strength and duration. This is the first study using rotating disk electrode voltammetry (RDEV) to measure 5-HT clearance. SERT kinetics were measured in whole brain synaptosomes. Uptake kinetics of exogenous 5-HT were measured using glassy carbon electrodes rotated in 500 μL glass chambers containing synaptosomes from SERT-knockout (-/-), heterozygous (+/-), or wild-type (+/+) mice. RDEV detected 5-HT concentrations of 5nM and higher. Initial velocities were kinetically resolved with K(m) and V(max) values of 99±35 standard error of regression (SER) nM and 181±11 SER fmol/(s×mg protein), respectively in wild-type synaptosomes. The method enables control over drug and chemical concentrations, facilitating interpretation of results. Results are compared in detail to other techniques used to measure SERT kinetics, including tritium labeled assays, chronoamperometry, and fast scan cyclic voltammetry. RDEV exhibits decreased 5-HT detection limits, decreased vulnerability to 5-HT oxidation products that reduce electrode sensitivity, and also overcomes diffusion limitations via forced convection by providing a continuous, kinetically resolved signal. Finally, RDEV distinguishes functional differences between genotypes, notably, between wild-type and heterozygous mice, an experimental problem with other experimental approaches.

Neuropharmacological mechanisms underlying the neuroprotective effects of methylphenidate

Methylphenidate is a psychostimulant that inhibits the neuronal dopamine transporter. In addition, methylphenidate has the intriguing ability to provide neuroprotection from the neurotoxic effects of methamphetamine and perhaps also Parkinson’s disease; both of which may likely involve the abnormal accumulation of cytoplasmic dopamine inside dopaminergic neurons and the resulting formation of dopamine-associated reactive oxygen species. As delineated in this review, the neuroprotective effects of methylphenidate are due, at least in part, to its ability to attenuate or prevent this abnormal cytoplasmic dopamine accumulation through several possible neuropharmacological mechanisms. These may include 1) direct interactions between methylphenidate and the neuronal dopamine transporter which may attenuate or prevent the entry of methamphetamine into dopaminergic neurons and may also decrease the synthesis of cytoplasmic dopamine through a D2 receptor-mediated signal cascade process, and 2) indirect effects upon the functioning of the vesicular monoamine transporter-2 which may increase vesicular dopamine sequestration through both vesicle trafficking and the kinetic upregulation of the vesicular monoamine transporter-2 protein. Understanding these neuropharmacological mechanisms of methylphenidate neuroprotection may provide important insights into the physiologic regulation of dopaminergic systems as well as the pathophysiology of a variety of disorders involving abnormal dopamine disposition ranging from substance abuse to neurodegenerative diseases such as Parkinson’s disease.

Age-dependent differences in dopamine transporter and vesicular monoamine transporter-2 function and their implications for methamphetamine neurotoxicity

The abuse of methamphetamine (METH) is a serious public health problem because METH can cause persistent dopaminergic deficits in the brains of both animal models and humans. Surprisingly, adolescent postnatal day (PND)40 rats are resistant to these METH-induced deficits, whereas young adult PND90 rats are not. Studies described in this report used rotating disk electrode voltammetry and western blotting techniques to investigate whether there are age-dependent differences in monoamine transporter function in PND38-42 and PND88-92 rats that could contribute to this phenomenon. The initial velocities of dopamine (DA) transport into, METH-induced DA efflux from, and DA transporter (DAT) immunoreactivity in striatal suspensions are greater in PND38-42 rats than in PND88-92 rats. DA transport velocities into vesicles that cofractionate with synaptosomal membranes after osmotic lysis are also greater in PND38-42 rats. However, there is no difference in vesicular monoamine transporter-2 (VMAT-2) immunoreactivity between the two age groups in this fraction. This suggests that younger rats have a greater capacity to sequester cytoplasmic DA into membrane-associated vesicles due to kinetically upregulated VMAT-2 and also have increased levels of functionally active DAT. In the presence of METH, these may provide additional routes of cellular efflux for DA that is released from vesicles into the cytoplasm and thereby prevent cytoplasmic DA concentrations in younger rats from rising to neurotoxic levels after drug administration. These findings provide novel insight into the age-dependent physiological regulation of neuronal DA sequestration and may advance the treatment of disorders involving abnormal DA disposition including substance abuse and Parkinson's disease.

Methylphenidate administration alters vesicular monoamine transporter-2 function in cytoplasmic and membrane-associated vesicles

In vivo methylphenidate (MPD) administration increases vesicular monoamine transporter-2 (VMAT-2) immunoreactivity, VMAT-2-mediated dopamine (DA) transport, and DA content in a nonmembrane-associated (referred to herein as cytoplasmic) vesicular subcellular fraction purified from rat striatum: a phenomenon attributed to a redistribution of VMAT-2-associated vesicles within nerve terminals. In contrast, the present study elucidated the nature of, and the impact of MPD on, VMAT-2-associated vesicles that cofractionate with synaptosomal membranes after osmotic lysis (referred to herein as membrane-associated vesicles). Results revealed that, in striking contrast to the cytoplasmic vesicles, DA transport velocity versus substrate concentration curves in the membrane-associated vesicles were sigmoidal, suggesting positive cooperativity with respect to DA transport. Additionally, DA transport into membrane-associated vesicles was greater in total capacity in the presence of high DA concentrations than transport into cytoplasmic vesicles. Of potential therapeutic relevance, MPD increased DA transport into the membrane-associated vesicles despite rapidly decreasing (presumably by redistributing) VMAT-2 immunoreactivity in this fraction. Functional relevance was suggested by findings that MPD treatment increased both the DA content of the membrane-associated vesicle fraction and K(+)-stimulated DA release from striatal suspensions. In summary, the present data demonstrate the existence of a previously uncharacterized pool of membrane-associated VMAT-2-containing vesicles that displays novel transport kinetics, has a large sequestration capacity, and responds to in vivo pharmacological manipulation. These findings provide insight into both the regulation of vesicular DA sequestration and the mechanism of action of MPD, and they may have implications regarding treatment of disorders involving abnormal DA disposition, including Parkinson's disease and substance abuse.

Differential effects of Zn2+ on the kinetics and cocaine inhibition of dopamine transport by the human and rat dopamine transporters

Zn2+ may play a major role in the modulation of neurotransmission because it modulates membrane receptors and channels. Recent literature has shown Zn2+ inhibits dopamine transport by the dopamine transporter (DAT), the main target of cocaine and some other drugs of abuse. Cocaine inhibits DAT and modulation of the DAT by Zn2+ may alter effects of cocaine on dopamine neurotransmission. This study investigates how Zn2+ changes DAT kinetics and its inhibition by cocaine. Steady-state and pre-steady-state kinetics of DAT activity were investigated using rotating disk electrode voltammetry. Values of KM and Vmax in hDAT and effects of cocaine match those in the literature. Zn2+ allosterically inhibited transport in the human DAT (hDAT) with a KI=7.9+/-0.42 microM. Removal of endogenous Zn2+ with penicillamine in hDAT increased transport values. In contrast, Zn2+ did not alter transport by rat DAT (rDAT), with KM and Vmax values of 1.2+/-0.49 microM and 15.7+/-2.57 pmol/(sx10(6) cells), respectively, and removal of Zn2+ did not increase dopamine transport values. Zn2+ allosterically reduced the inhibition by cocaine in hDAT. Results of pre-steady-state studies demonstrated that Zn2+ increases the second order binding rate constant for dopamine to hDAT (3.5 fold to 19.2x10(6) M-1 s-1 for hDAT). In rat striatal homogenates Zn2+ increased initial dopamine transport velocity and decreased cocaine inhibition providing evidence for differences in sensitivity to Zn2+ between the three different preparations. Modulation of the DAT by Zn2+ needs to be assessed further in development of cocaine antagonists.

Measurement of kinetically resolved vesicular dopamine uptake and efflux using rotating disk electrode voltammetry

The vesicular monoamine transporter-2 (VMAT-2) sequesters cytoplasmic dopamine (DA) into vesicles for storage and subsequent release. VMAT-2 activity has traditionally been measured in small synaptic vesicles isolated from rat striatum by monitoring [3H] DA uptake and in cellular expression systems using fast scan cyclic voltammetry. This is the first report using rotating disk electrode (RDE) voltammetry to measure VMAT-2 DA uptake and efflux in small synaptic vesicles. DA uptake profiles followed mixed order kinetics with apparent zero order kinetics for the first 25 s and apparent first order kinetics thereafter. Vesicular DA uptake was temperature- and ATP-dependent and was blocked by the VMAT-2 inhibitor tetrabenazine. Initial velocities of DA uptake were kinetically resolved and displayed Michaelis-Menten kinetics with a Km and Vmax of 289 +/- 59 nM and 1.9 +/- 0.2 fmol/(s microg protein), respectively. Methamphetamine-induced DA efflux was blocked by tetrabenazine and kinetically resolved with an initial velocity of 0.54 +/- 0.08 fmol/(s microg protein). These results suggest that RDE voltammetry can be used to make kinetically resolved measurements of vesicular DA uptake and efflux and will allow the design of experiments that could reveal important information about the kinetics of VMAT-2 activity and its inhibition.

Kinetic analysis of developmental changes in vesicular monoamine transporter-2 function

The vesicular monoamine transporter-2 (VMAT-2) sequesters cytoplasmic dopamine (DA) into vesicles and may thus protect neurons from DA-associated oxidative damage. VMAT-2 function, as assessed by measuring [(3)H]DA uptake, is less in adolescent when compared with young adult rats and VMAT-2 susceptibility to methamphetamine also changes with age. Thus, developmental alterations in VMAT-2 function warrant further investigation. The current study extends these findings by showing that the initial velocities of both DA uptake and methamphetamine-induced DA efflux are less in adolescent postnatal day (PND 38-42) vs. young adult (PND 88-92) rats as assessed in nonmembrane associated (presumably cytoplasmic) vesicles purified from rat striatal synaptosomes. The decrease in DA uptake velocities is due to a decrease in the V(max) of DA uptake with no change in the K(m). The density of kinetically active VMAT-2 and VMAT-2 immunoreactivity are less in adolescent vs. young adult rats while both the turnover number (2.4-2.8 s(-) (1)) and rate constant for the association of DA with VMAT-2 ( 1 x 10(7) M(-) (1) s(-) (1)) are similar in these age groups. These results suggest that the kinetics of DA binding and translocation across the membrane are unaltered in the vesicles of PND 38-42 vs. PND 88-92 rats. However, decreased VMAT-2 density in PND 38-42 rats reduces V(max), which in turn lowers DA uptake.

Acute Ethanol Decreases Dopamine Transporter Velocity in Rat Striatum: In Vivo and In Vitro Electrochemical Measurements

BACKGROUND

Ethanol increases dopamine transporter (DAT) velocity when measured in cell expression systems, but its effects in vivo are mixed. The present experiments examined the effect of acute ethanol on dopamine transmission, particularly DAT velocity, in anesthetized animals as well as rat striatal suspensions.

METHODS

To determine the effect of acute ethanol on DAT function in vivo, we measured dopamine uptake in real time using fast-scan cyclic voltammetry and constant potential amperometry in the olfactory tubercle of anesthetized rats. Dopamine fibers were electrically stimulated, and the resulting transient dopamine signals were analyzed to describe the release and uptake kinetics. We also measured the effect of ethanol on DAT velocity in vitro in striatal tissue suspensions using rotating disk electrode voltammetry.

RESULTS

Ethanol (2.5 and 4 g/kg, intraperitoneally) decreased the electrically stimulated dopamine signal in the olfactory tubercle by 35-55%. The slope of the clearance curve of dopamine was 40% shallower after both doses of ethanol, indicating slower uptake. Modeling the data using Michaelis-Menten uptake kinetics showed that the slower uptake was due to a decrease in DAT V(max). These results were confirmed in vitro, because ethanol decreased the velocity of dopamine uptake by 35% in striatal tissue suspensions.

CONCLUSIONS

These results indicate that acute ethanol decreases DAT function in rat dorsal and ventral striatum in anesthetized rats and tissue suspensions, in contrast to its effects on human DAT expressed in single cells. Given the variety of molecular targets of ethanol in the brain, including the DAT itself, it is likely that several mechanisms converge to produce a net effect on DAT regulation and function that could very well be different in intact tissue versus single cells.

Unraveling neuronal dopamine transporter mechanisms with rotating disk electrode voltammetry

Herein we describe how the rotating disk electrode voltammetric technique can be used to examine the mechanism(s) of the inward transport of dopamine by the neuronal transporter for dopamine (DAT). The usefulness of making measurements kinetically resolving dopamine transport, interpretations of changes in Km and Vmax, approaches to defining pre-steady-state binding of dopamine to DAT, interactions between competing inhibitors, chemical modification of functional groups within DAT, and a presentation of a hypothetical multi-state model of dopamine transport are presented and discussed.

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.

Methylphenidate analogs with behavioral differences interact differently with arginine residues on the dopamine transporter in rat striatum

The methylphenidate analogs N-methyl-4-methyl-methylphenidate and N-benzylmethylphenidate are believed to interact differently with the dopamine transporter (DAT) in vitro and in vivo. Herein, we report that methylphenidate and N-methyl-4-methyl-methylphenidate, but not N-benzylmethylphenidate, protect the rat striatal DAT from the arginine-selective chemical modifying agent, phenylglyoxal. This suggests that methylphenidate and N-methyl-4-methyl-methylphenidate, but not N-benzylmethylphenidate, interact with the guanidine groups of arginine residues in the DAT of rat striatum. This differential interaction may, at least in part, explain the in vitro and in vivo differences between N-methyl-4-methyl-methylphenidate and N-benzylmethylphenidate.

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.