Amphetamine induces dopamine efflux through a dopamine transporter channel

Role of the dopamine transporter in the action of psychostimulants, nicotine, and other drugs of abuse

A number of studies over the last two decades have demonstrated the critical importance of dopamine (DA) in the behavioral pharmacology and addictive properties of abused drugs. The DA transporter (DAT) is a major target for drugs of abuse in the category of psychostimulants, and for methylphenidate (MPH), a drug used for treating attention deficit hyperactivity disorder (ADHD), which can also be a psychostimulant drug of abuse. Other drugs of abuse such as nicotine, ethanol, heroin and morphine interact with the DAT in more indirect ways. Despite the different ways in which drugs of abuse can affect DAT function, one evolving theme in all cases is regulation of the DAT at the level of surface expression. DAT function is dynamically regulated by multiple intracellular and extracellular signaling pathways and several protein-protein interactions. In addition, DAT expression is regulated through the removal (internalization) and recycling of the protein from the cell surface. Furthermore, recent studies have demonstrated that individual differences in response to novel environments and psychostimulants can be predicted based on individual basal functional DAT expression. Although current knowledge of multiple factors regulating DAT activity has greatly expanded, many aspects of this regulation remain to be elucidated; these data will enable efforts to identify drugs that might be used therapeutically for drug dependence therapeutics.

Protective actions of the vesicular monoamine transporter 2 (VMAT2) in monoaminergic neurons

Vesicular monoamine transporters (VMATs) are responsible for the packaging of neurotransmitters such as dopamine, serotonin, norepinephrine, and epinephrine into synaptic vesicles. These proteins evolved from precursors in the major facilitator superfamily of transporters and are among the members of the toxin extruding antiporter family. While the primary function of VMATs is to sequester neurotransmitters within vesicles, they can also translocate toxicants away from cytosolic sites of action. In the case of dopamine, this dual role of VMAT2 is combined-dopamine is more readily oxidized in the cytosol where it can cause oxidative stress so packaging into vesicles serves two purposes: neurotransmission and neuroprotection. Furthermore, the deleterious effects of exogenous toxicants on dopamine neurons, such as MPTP, can be attenuated by VMAT2 activity. The active metabolite of MPTP can be kept within vesicles and prevented from disrupting mitochondrial function thereby sparing the dopamine neuron. The highly addictive drug methamphetamine is also neurotoxic to dopamine neurons by using dopamine itself to destroy the axon terminals. Methamphetamine interferes with vesicular sequestration and increases the production of dopamine, escalating the amount in the cytosol and leading to oxidative damage of terminal components. Vesicular transport seems to resist this process by sequestering much of the excess dopamine, which is illustrated by the enhanced methamphetamine neurotoxicity in VMAT2-deficient mice. It is increasingly evident that VMAT2 provides neuroprotection from both endogenous and exogenous toxicants and that while VMAT2 has been adapted by eukaryotes for synaptic transmission, it is derived from phylogenetically ancient proteins that originally evolved for the purpose of cellular protection.

Introducing tetraCys motifs at two different sites results in a functional dopamine transporter

We have introduced tetracysteine motifs into different positions of the dopamine transporter (DAT) for specific FlAsH labeling. Two of the constructs expressed at the cell surface and were functional as determined by [3H] dopamine uptake experiments. The N-terminally modified transporter showed uptake levels comparable to the wild-type DAT, while the construct with tetracysteine motif at position 511 displayed an uptake level about 1/3 of its wild-type counterpart. In addition, these two transporter constructs were visualized on the cell surface following labeling with a fluorescent cocaine analog. YFP introduced into the same N-terminal position was also shown to have surface staining in agreement with activity tests. We propose that these two sites are suitable targets for tetracysteine labeling to be used in FlAsH staining studies, while p134, p342, p427, p433 and p517 sites are not.

Rapid substrate-induced down-regulation in function and surface localization of dopamine transporters: rat dorsal striatum versus nucleus accumbens

The dopamine transporter (DAT) substrates dopamine, d-amphetamine (AMPH), and methamphetamine are known to rapidly and transiently reduce DAT activity and/or surface expression in dorsal striatum and heterologous expression systems. We sought to determine if similar substrate-induced regulation of DATs occurs in rat nucleus accumbens. In dorsal striatum synaptosomes, brief (15-min) in vitro substrate pre-exposure markedly decreased maximal [(3)H]dopamine uptake velocity whereas identical substrate pre-exposure in nucleus accumbens synaptosomes produced a smaller, non-significant reduction. However, 45 min after systemic AMPH administration, maximal ex vivo [(3)H]dopamine uptake velocity was significantly reduced in both brain regions. Protein kinase C inhibition blocked AMPH's down-regulation of DAT activity. DAT synaptosomal surface expression was not modified following either the brief in vitro or in vivo AMPH pre-exposure but was reduced after a longer (1-h) in vitro pre-exposure in both brain regions. Together, our findings suggest that relatively brief substrate exposure results in greater down-regulation of DAT activity in dorsal striatum than in nucleus accumbens. Moreover, exposure to AMPH appears to regulate striatal DATs in a biphasic manner, with an initial protein kinase C-dependent decrease in DAT-mediated uptake velocity and then, with longer exposure, a reduction in DAT surface expression.

Dopamine dysregulation syndrome. Hypothetical application of reward system stimulation for the treatment of anhedonia in Parkinson's disease patients

The management of motor symptoms in Parkinson's disease (PD), although imperfect, has already been standardized. However, patients often spend their time idly despite improvement in the elemental motor symptoms. The main cause of this may be anhedonia. Dopamine dysregulation syndrome (DDS) is a troublesome condition that can occur as a complication of dopamine replacement therapy in PD. As anhedonia and DDS may be converse syndromes in PD patients, it is very important to overcome anhedonia to improve patients' quality of life. In this article, the author proposes the possibility of stimulating patients' desire to participate in physical activity via the incentive of a reward system. Understanding the mechanism of DDS may help in the development of this type of approach.

Amphetamine and methamphetamine differentially affect dopamine transporters in vitro and in vivo

The psychostimulants d-amphetamine (AMPH) and methamphetamine (METH) release excess dopamine (DA) into the synaptic clefts of dopaminergic neurons. Abnormal DA release is thought to occur by reverse transport through the DA transporter (DAT), and it is believed to underlie the severe behavioral effects of these drugs. Here we compare structurally similar AMPH and METH on DAT function in a heterologous expression system and in an animal model. In the in vitro expression system, DAT-mediated whole-cell currents were greater for METH stimulation than for AMPH. At the same voltage and concentration, METH released five times more DA than AMPH and did so at physiological membrane potentials. At maximally effective concentrations, METH released twice as much [Ca(2+)](i) from internal stores compared with AMPH. [Ca(2+)](i) responses to both drugs were independent of membrane voltage but inhibited by DAT antagonists. Intact phosphorylation sites in the N-terminal domain of DAT were required for the AMPH- and METH-induced increase in [Ca(2+)](i) and for the enhanced effects of METH on [Ca(2+)](i) elevation. Calmodulin-dependent protein kinase II and protein kinase C inhibitors alone or in combination also blocked AMPH- or METH-induced Ca(2+) responses. Finally, in the rat nucleus accumbens, in vivo voltammetry showed that systemic application of METH inhibited DAT-mediated DA clearance more efficiently than AMPH, resulting in excess external DA. Together these data demonstrate that METH has a stronger effect on DAT-mediated cell physiology than AMPH, which may contribute to the euphoric and addictive properties of METH compared with AMPH.

Drosophila vesicular monoamine transporter mutants can adapt to reduced or eliminated vesicular stores of dopamine and serotonin

Physiologic and pathogenic changes in amine release induce dramatic behavioral changes, but the underlying cellular mechanisms remain unclear. To investigate these adaptive processes, we have characterized mutations in the Drosophila vesicular monoamine transporter (dVMAT), which is required for the vesicular storage of dopamine, serotonin, and octopamine. dVMAT mutant larvae show reduced locomotion and decreased electrical activity in motoneurons innervating the neuromuscular junction (NMJ) implicating central amines in the regulation of these activities. A parallel increase in evoked glutamate release by the motoneuron is consistent with a homeostatic adaptation at the NMJ. Despite the importance of aminergic signaling for regulating locomotion and other behaviors, adult dVMAT homozygous null mutants survive under conditions of low population density, thus allowing a phenotypic characterization of adult behavior. Homozygous mutant females are sterile and show defects in both egg retention and development; males also show reduced fertility. Homozygotes show an increased attraction to light but are mildly impaired in geotaxis and escape behaviors. In contrast, heterozygous mutants show an exaggerated escape response. Both hetero- and homozygous mutants demonstrate an altered behavioral response to cocaine. dVMAT mutants define potentially adaptive responses to reduced or eliminated aminergic signaling and will be useful to identify the underlying molecular mechanisms.

Unfaithful neurotransmitter transporters: focus on serotonin uptake and implications for antidepressant efficacy

Biogenic amine transporters for serotonin, norepinephrine and dopamine (SERT, NET and DAT respectively), are the key players terminating transmission of these amines in the central nervous system by their high-affinity uptake. They are also major targets for many antidepressant drugs. Interestingly however, drugs targeted to a specific transporter do not appear to be as clinically efficacious as those that block two or all three of these transporters. A growing body of literature, reviewed here, supports the idea that promiscuity among these transporters (the uptake of multiple amines in addition to their "native" transmitter) may account for improved therapeutic effects of dual and triple uptake blockers. However, even these drugs do not provide effective treatment outcomes for all individuals. An emerging literature suggests that "non-traditional" transporters such as organic cation transporters (OCT) and the plasma membrane monoamine transporter (PMAT) may contribute to the less than hoped for efficacy of currently prescribed uptake inhibitors. OCT and PMAT are capable of clearing biogenic amines from extracellular fluid and may serve to buffer the effects of frontline antidepressants, such as selective serotonin reuptake inhibitors. In addition, polymorphisms that occur in the genes encoding the transporters can lead to variation in transporter expression and function (e.g. the serotonin transporter linked polymorphic region; 5-HTTLPR) and can have profound effects on treatment outcome. This may be accounted for, in part, by compensatory adaptations in other transporters. This review synthesizes the existing literature, focusing on serotonin to illustrate and revive a model for the rationale design of improved antidepressants.

Dopamine transporter/syntaxin 1A interactions regulate transporter channel activity and dopaminergic synaptic transmission

The Caenorhabditis elegans (C. elegans) dopamine (DA) transporter (DAT-1) regulates DA signaling through efficient DA reuptake following synaptic release. In addition to its DA transport function, DAT-1 generates detectable DA-gated currents that may influence neuronal excitability. Previously, we provided evidence that single Cl-channel events underlie DAT-1 currents. In these studies, we identified a distinct population of altered DAT-1 currents arising from DAT-1 transgenic constructs bearing an N-terminal GFP fusion. The presence of these channels suggested disruption of an endogenous regulatory mechanism that modulates occupancy of DAT-1 channel states. A leading candidate for such a regulator is the SNARE protein syntaxin 1A (Syn1A), previously found to interact with homologous transporters through N-terminal interactions. Here we establish that UNC-64 (C. elegans Syn1A homologue) associates with DAT-1 and suppresses transporter channel properties. In contrast, GFP::DAT-1 is unable to form stable transporter/UNC-64 complexes that limit channel states. Although DAT-1 and GFP::DAT-1 expressing DA neurons exhibit comparable DA uptake, GFP::DAT-1 animals exhibit swimming-induced paralysis (SWIP), a phenotype associated with excess synaptic DA release and spillover. We propose that loss of UNC-64/DAT-1 interactions leads to enhanced synaptic DA release, providing a novel mechanism for DA neuron sensitization that may be relevant to mechanisms of DA-associated disorders.

Syntaxin 1A interaction with the dopamine transporter promotes amphetamine-induced dopamine efflux

The soluble N-ethylmaleimide-sensitive factor attachment protein receptor protein syntaxin 1A (SYN1A) interacts with and regulates the function of transmembrane proteins, including ion channels and neurotransmitter transporters. Here, we define the first 33 amino acids of the N terminus of the dopamine (DA) transporter (DAT) as the site of direct interaction with SYN1A. Amphetamine (AMPH) increases the association of SYN1A with human DAT (hDAT) in a heterologous expression system (hDAT cells) and with native DAT in murine striatal synaptosomes. Immunoprecipitation of DAT from the biotinylated fraction shows that the AMPH-induced increase in DAT/SYN1A association occurs at the plasma membrane. In a superfusion assay of DA efflux, cells overexpressing SYN1A exhibited significantly greater AMPH-induced DA release with respect to control cells. By combining the patch-clamp technique with amperometry, we measured DA release under voltage clamp. At -60 mV, a physiological resting potential, AMPH did not induce DA efflux in hDAT cells and DA neurons. In contrast, perfusion of exogenous SYN1A (3 microM) into the cell with the whole-cell pipette enabled AMPH-induced DA efflux at -60 mV in both hDAT cells and DA neurons. It has been shown recently that Ca2+/calmodulin-dependent protein kinase II (CaMKII) is activated by AMPH and regulates AMPH-induced DA efflux. Here, we show that AMPH-induced association between DAT and SYN1A requires CaMKII activity and that inhibition of CaMKII blocks the ability of exogenous SYN1A to promote DA efflux. These data suggest that AMPH activation of CaMKII supports DAT/SYN1A association, resulting in a mode of DAT capable of DA efflux.

PI3K signaling supports amphetamine-induced dopamine efflux

The dopamine (DA) transporter (DAT) is a major molecular target of the psychostimulant amphetamine (AMPH). AMPH, as a result of its ability to reverse DAT-mediated inward transport of DA, induces DA efflux thereby increasing extracellular DA levels. This increase is thought to underlie the behavioral effects of AMPH. We have demonstrated previously that insulin, through phosphatidylinositol 3-kinase (PI3K) signaling, regulates DA clearance by fine-tuning DAT plasma membrane expression. PI3K signaling may represent a novel mechanism for regulating DA efflux evoked by AMPH, since only active DAT at the plasma membrane can efflux DA. Here, we show in both a heterologous expression system and DA neurons that inhibition of PI3K decreases DAT cell surface expression and, as a consequence, AMPH-induced DA efflux.

Identification of Piccolo as a regulator of behavioral plasticity and dopamine transporter internalization

Dopamine transporter (DAT) internalization is a mechanism underlying the decreased dopamine reuptake caused by addictive drugs like methamphetamine (METH). We found that Piccolo, a presynaptic scaffolding protein, was overexpressed in the nucleus accumbens (NAc) of the mice repeatedly administrated with METH. Piccolo downexpression by antisense technique augmented METH-induced behavioral sensitization, conditioned reward and synaptic dopamine accumulation in NAc. Expression of Piccolo C2A domain attenuated METH-induced inhibition of dopamine uptake in PC12 cells expressing human DAT. Consistent with this, it slowed down the accelerated DAT internalization induced by METH, thus maintaining the presentation of plasmalemmal DAT. In immunostaining and structural modeling Piccolo C2A domain displays an unusual feature of sequestering membrane phosphatidylinositol 4,5-bisphosphate, which may underlie its role in modulating DAT internalization. Together, our results indicate that Piccolo upregulation induced by METH represents a homeostatic response in the NAc to excessive dopaminergic transmission. Piccolo C2A domain may act as a cytoskeletal regulator for plasmalemmal DAT internalization, which may underlie its contributions in behavioral plasticity.

Cellular and molecular mechanisms involved in the neurotoxicity of opioid and psychostimulant drugs

Substance abuse and addiction are the most costly of all the neuropsychiatric disorders. In the last decades, much progress has been achieved in understanding the effects of the drugs of abuse in the brain. However, efficient treatments that prevent relapse have not been developed. Drug addiction is now considered a brain disease, because the abuse of drugs affects several brain functions. Neurological impairments observed in drug addicts may reflect drug-induced neuronal dysfunction and neurotoxicity. The drugs of abuse directly or indirectly affect neurotransmitter systems, particularly dopaminergic and glutamatergic neurons. This review explores the literature reporting cellular and molecular alterations reflecting the cytotoxicity induced by amphetamines, cocaine and opiates in neuronal systems. The neurotoxic effects of drugs of abuse are often associated with oxidative stress, mitochondrial dysfunction, apoptosis and inhibition of neurogenesis, among other mechanisms. Understanding the mechanisms that underlie brain dysfunction observed in drug-addicted individuals may contribute to improve the treatment of drug addiction, which may have social and economic consequences.

Currents in response to rapid concentration jumps of amphetamine uncover novel aspects of human dopamine transporter function

Amphetamine (AMPH) is a widely abused psychostimulant that acts as a substrate for the human dopamine transporter (hDAT). Using a piezoelectric rapid application system, we measured AMPH-induced currents mediated by hDAT. Whole-cell patch-clamp recordings in a heterologous expression system reveal that AMPH induces a rapidly activating and subsequently decaying inward current mediated by hDAT. We hypothesize that this transient inward current reflects a conformational change associated with substrate translocation. The AMPH-induced current strictly depends on extracellular Na+. Elevated intracellular Na+ has no effect on the peak AMPH-induced current amplitude but inhibits the steady-state current. In addition, elevated intracellular Na+ causes an overshoot outward current upon washout of AMPH that reflects hDAT locked in a Na+-exchange mode. Furthermore, elevated intracellular Na+ dramatically accelerates the recovery time from desensitization of the AMPH-induced current, revealing a new role for intracellular Na+ in promoting the transition to the hDAT "outward-facing" conformation. Ion substitution suggests that both extracellular and intracellular Cl- facilitate transporter turnover in contrast to the classical model of Cl- as a cotransported ion. We present an alternating-access model of hDAT function that accurately fits the main features of the experimental data. The model predicts that translocation of substrate occurs within milliseconds of substrate binding but that slow reorientation of the empty transporter is the rate-limiting factor for turnover. The model provides a framework for interpreting perturbations of hDAT activity.

Nonvesicular inhibitory neurotransmission via reversal of the GABA transporter GAT-1

GABA transporters play an important but poorly understood role in neuronal inhibition. They can reverse, but this is widely thought to occur only under pathological conditions. Here we use a heterologous expression system to show that the reversal potential of GAT-1 under physiologically relevant conditions is near the normal resting potential of neurons and that reversal can occur rapidly enough to release GABA during simulated action potentials. We then use paired recordings from cultured hippocampal neurons and show that GABAergic transmission is not prevented by four methods widely used to block vesicular release. This nonvesicular neurotransmission was potently blocked by GAT-1 antagonists and was enhanced by agents that increase cytosolic [GABA] or [Na(+)] (which would increase GAT-1 reversal). We conclude that GAT-1 regulates tonic inhibition by clamping ambient [GABA] at a level high enough to activate high-affinity GABA(A) receptors and that transporter-mediated GABA release can contribute to phasic inhibition.

Secretory vesicle rebound hyperacidification and increased quantal size resulting from prolonged methamphetamine exposure

Acute exposure to amphetamines (AMPHs) collapses secretory vesicle pH gradients, which increases cytosolic catecholamine levels while decreasing the quantal size of catecholamine release during fusion events. AMPH and methamphetamine (METH), however, are retained in tissues over long durations. We used optical and electron microscopic probes to measure the effects of long-term METH exposure on secretory vesicle pH, and amperometry and intracellular patch electrochemistry to observe the effects on neurosecretion and cytosolic catecholamines in cultured rat chromaffin cells. In contrast to acute METH effects, exposure to the drug for 6-48 h at 10 microM and higher concentrations produced a concentration-dependent rebound hyperacidification of secretory vesicles. At 5-10 microM levels, prolonged METH increased the quantal size and reinstated exocytotic catecholamine release, although very high (> 100 microM) levels of the drug, while continuing to produce rebound hyperacidification, did not increase quantal size. Secretory vesicle rebound hyperacidification was temperature dependent with optimal response at approximately 37 degrees C, was not blocked by the transcription inhibitor, puromycin, and appears to be a general compensatory response to prolonged exposure with membranophilic weak bases, including AMPHs, methylphenidate, cocaine, and ammonia. Thus, under some conditions of prolonged exposure, AMPHs and other weak bases can enhance, rather than deplete, the vesicular release of catecholamines via a compensatory response resulting in vesicle acidification.

Characterization of interactions between phencyclidine and amphetamine in rodent prefrontal cortex and striatum: Implications in NMDA/glycine-site-mediated dopaminergic dysregulation and dopamine transporter function

N-Methyl-D-aspartate (NMDA) antagonists induced behavioral and neurochemical changes in rodents that serve as animal models of schizophrenia. Chronic phencyclidine (PCP, 15 mg/(kg day) for 3 weeks via Alzet osmotic pump) administration enhances the amphetamine (AMPH)-induced dopamine (DA) efflux in prefrontal cortex (PFC), similar to that observed in schizophrenia. NMDA/glycine-site agonists, such as glycine (GLY), administered via dietary supplementation, reverse the enhanced effect. The present study investigated mechanisms of glycine-induced reversal of PCP-induced stimulation of AMPH-induced DA release, using simultaneous measurement of DA and AMPH in brain microdialysate, as well as peripheral and tissue AMPH levels. PCP treatment, by itself, increased peripheral and central AMPH levels, presumably via interaction with hepatic enzymes (e.g. cytochrome P450 CYP2C11). GLY (16% diet) had no effect on peripheral AMPH levels in the presence of PCP. Nevertheless, GLY significantly reduced extracellular/tissue AMPH ratios in both PFC and striatum (STR), especially following PCP administration, suggesting a feedback mediated effect on the dopamine transporter. GLY also inhibited acute AMPH (5 mg/kg)-induced DA release in PFC, but not STR. These findings suggest that GLY may modulate DA release in brain by producing feedback regulation of dopamine transporter function, possibly via potentiation of NMDA-stimulated GABA release and presynaptic GABAB receptor activation. The present studies also demonstrate pharmacokinetic interaction between AMPH and PCP, which may be of both clinical and research relevance.

Behavioral genetic contributions to the study of addiction-related amphetamine effects

Amphetamines, including methamphetamine, pose a significant cost to society due to significant numbers of amphetamine-abusing individuals who suffer major health-related consequences. In addition, methamphetamine use is associated with heightened rates of violent and property-related crimes. The current paper reviews the existing literature addressing genetic differences in mice that impact behavioral responses thought to be relevant to the abuse of amphetamine and amphetamine-like drugs. Summarized are studies that used inbred strains, selected lines, single-gene knockouts and transgenics, and quantitative trait locus (QTL) mapping populations. Acute sensitivity, neuroadaptive responses, rewarding and conditioned effects are among those reviewed. Some gene mapping work has been accomplished, and although no amphetamine-related complex trait genes have been definitively identified, translational work leading from results in the mouse to studies performed in humans is beginning to emerge. The majority of genetic investigations have utilized single-gene knockout mice and have concentrated on dopamine- and glutamate-related genes. Genes that code for cell support and signaling molecules are also well-represented. There is a large behavioral genetic literature on responsiveness to amphetamines, but a considerably smaller literature focused on genes that influence the development and acceleration of amphetamine use, withdrawal, relapse, and behavioral toxicity. Also missing are genetic investigations into the effects of amphetamines on social behaviors. This information might help to identify at-risk individuals and in the future to develop treatments that take advantage of individualized genetic information.

Pre-filtering improves reliability of Affymetrix GeneChips results when used to analyze gene expression in complex tissues

Affymetrix GeneChip represents a very reliable and standardized technology for genome-wide gene expression screening. However, in experiments carried out on complex biological samples (e.g. brain tissues composed of several diverse cell types), significant noise can arise due to important transcripts being expressed in a relatively small number of cells. This noise results in many observations coming from unreliable hybridization reactions. Here we propose a method for pre-filtering Affymetrix data according to measures of hybridization reliability. We used our pre-filtering method on a microarray dataset obtained from the brains of rats chronically treated with a psychostimulant drug. Our pre-filter protocol facilitates selection of biologically relevant candidate genes, which could be validated by real-time PCR with a rate of 98%.

Sex differences in K+-evoked striatal dopamine output from superfused striatal tissue fragments of reserpine-treated CD-1 mice

Reserpine inhibits vesicular monoamine transporter-2 (VMAT-2) function and thereby impairs vesicular dopamine (DA) storage within nerve terminals. The present report compared the effects of reserpine treatment upon the striatal dopaminergic system in male and female mice as a means to assess potential sex differences in VMAT-2/DA storage function. After treatment with reserpine, male mice showed significantly greater striatal DA concentrations and K+ -evoked DA output from the striatum compared to females. By contrast, no statistically significant sex differences were observed in methamphetamine-evoked DA output in reserpine-treated mice. These results demonstrate a clear sex difference in the striatal dopaminergic responses to reserpine and suggest that females possess a more active VMAT-2/DA storage capacity, as indicated by the greater degree of deficits observed when VMAT-2/DA storage function is inhibited by reserpine. Such findings have important implications for understanding some of the bases for sex differences in neurotoxicity and neurodegeneration of the nigrostriatal dopaminergic system.

Specification of a dopaminergic phenotype from adult human mesenchymal stem cells

Dopamine (DA) neurons derived from stem cells are a valuable source for cell replacement therapy in Parkinson disease, to study the molecular mechanisms of DA neuron development, and for screening pharmaceutical compounds that target DA disorders. Compared with other stem cells, MSCs derived from the adult human bone marrow (BM) have significant advantages and greater potential for immediate clinical application. We report the identification of in vitro conditions for inducing adult human MSCs into DA cells. Using a cocktail that includes sonic hedgehog and fibroblast growth factors, human BM-derived MSCs were induced in vitro to become DA cells in 12 days. Based on tyrosine hydroxylase (TH) expression, the efficiency of induction was determined to be approximately 67%. The cells develop a neuronal morphology expressing the neuronal markers NeuN and beta III tubulin, but not glial markers, glial fibrillary acidic protein and Olig2. As the cells acquire a postmitotic neuronal fate, they downregulate cell cycle activator proteins cyclin B, cyclin-dependent kinase 2, and proliferating cell nuclear antigen. Molecular characterization revealed the expression of DA-specific genes such as TH, Pitx3, Nurr1, DA transporter, and vesicular monoamine transporter 2. The induced MSCs also synthesize and secrete DA in a depolarization-independent manner. The latter observation is consistent with the low expression of voltage gated Na(+) and Ca(2+) channels in the induced MSCs and suggests that the cells are at an immature stage of development likely representing DA neuronal progenitors. Taken together, the results demonstrate the ability of adult human BM-derived MSCs to form DA cells in vitro.

Simple transporter trafficking model for amphetamine-induced dopamine efflux

Amphetamine and its derivatives are important drugs of abuse causing both short-term excitatory and long-term addictive effects. The short-term excitatory effects are linked to amphetamine's ability to maintain high levels of dopamine (DA) outside the cell both by inhibiting DA reuptake after synaptic transmission and by enhancing the efflux of DA from the dopaminergic cells. The molecular mechanisms by which amphetamine elicits the efflux of DA and similar monoamines are still unclear. Recent literature data suggest that trafficking of the monoamine transporters is a phenomenon that underlies observed changes in amphetamine-induced monoamine reuptake and efflux. We develop an ordinary differential equation model incorporating the diverse mechanistic details behind amphetamine-induced DA efflux and demonstrate its utility in describing our experimental data. We also demonstrate an experimental method to track the time-varying concentration of membrane-bound transporter molecules from the DA efflux data. The good fit between our model and the experimental data supports the hypothesis that amphetamine-induced transporter trafficking is necessary to produce extended efflux of DA. This model can explain the relative significance of different processes associated with DA efflux at different times and at different concentration ranges of amphetamine and DA.

Perturbation of dopamine metabolism by 3-amino-2-(4'-halophenyl)propenes leads to increased oxidative stress and apoptotic SH-SY5Y cell death

We have recently characterized a series of 3-amino-2-phenyl-propene (APP) derivatives as reversible inhibitors for the bovine adrenal chromaffin granule vesicular monoamine transporter (VMAT) that have been previously characterized as potent irreversible dopamine-beta-monooxygenase (DbetaM) and monoamine oxidase (MAO) inhibitors. Halogen substitution on the 4'-position of the aromatic ring gradually increases VMAT inhibition potency from 4'-F to 4'-I, parallel to the hydrophobicity of the halogen. We show that these derivatives are taken up into both neuronal and non-neuronal cells, and into resealed chromaffin granule ghosts efficiently through passive diffusion. Uptake rates increased according to the hydrophobicity of the 4'-substituent. More importantly, these derivatives are highly toxic to human neuroblastoma SH-SY5Y but not toxic to M-1, Hep G2, or human embryonic kidney 293 non-neuronal cells at similar concentrations. They drastically perturb dopamine (DA) uptake and metabolism in SH-SY5Y cells under sublethal conditions and are able to deplete both vesicular and cytosolic catecholamines in a manner similar to that of amphetamines. In addition, 4'-IAPP treatment significantly increases intracellular reactive oxygen species (ROS) and decreases glutathione (GSH) levels in SH-SY5Y cells, and cell death is significantly attenuated by the common antioxidants alpha-tocopherol, N-acetyl-l-cysteine and GSH, but not by the nonspecific caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone. DNA fragmentation analysis further supports that cell death is probably due to a caspase-independent ROS-mediated apoptotic pathway. Based on these and other findings, we propose that drastic perturbation of DA metabolism in SH-SY5Y cells by 4'-halo APP derivatives causes increased oxidative stress, leading to apoptotic cell death.

Currents in neurotransmitter transporters

Traditionally, substrate translocation by neurotransmitter transporters has been described by the alternate access model. Recent structural data obtained with three distantly related transporters have also been interpreted as supportive of this model, because conformational correlates were visualized (inward-facing conformation, occluded state). However, the experimental evidence is overwhelmingly in favour of a more complex mode of operation: Transporters also exist in conformations that do not seal the permeation pathway. These conformations support a channel-like activity, including random permeation of substrate and co-substrate ions in a single-file mode. It is likely that the channel-like activity is modified by the interaction of the transporters with accessory proteins and regulatory kinases. Finally, channel-like activity is instrumental to understand the mechanism of action of amphetamines.

New insights into the mechanism of action of amphetamines

Amphetamine is a psychostimulant commonly used to treat several disorders, including attention deficit, narcolepsy, and obesity. Plasmalemmal and vesicular monoamine transporters, such as the neuronal dopamine transporter and the vesicular monoamine transporter-2, are two of its principal targets. This review focuses on new insights, obtained from both in vivo and in vitro studies, into the molecular mechanisms whereby amphetamine, and the closely related compounds methamphetamine and methylenedioxymethamphetamine, cause monoamine, and particularly dopamine, release. These mechanisms include amphetamine-induced exchange diffusion, reverse transport, and channel-like transport phenomena as well as the weak base properties of amphetamine. Additionally, amphetamine analogs may affect monoamine transporters through phosphorylation, transporter trafficking, and the production of reactive oxygen and nitrogen species. All of these mechanisms have potential implications for both amphetamine- and methamphetamine-induced neurotoxicity, as well as dopaminergic neurodegenerative diseases.

Adderall produces increased striatal dopamine release and a prolonged time course compared to amphetamine isomers

RATIONALE

Adderall is currently used for the treatment of Attention-Deficit Hyperactivity Disorder (ADHD) and is composed of a novel mixture of approximately 24% L-amphetamine and 76% D-amphetamine salts. There are, however, no investigations of the pharmacological effects of this combination in vivo.

OBJECTIVES

The technique of high-speed chronoamperometry using Nafion-coated single carbon-fiber microelectrodes was used to study amphetamine-evoked dopamine (DA) release produced by Adderall, D-amphetamine, or D,L-amphetamine in the striatum of anesthetized male Fischer 344 (F344) rats. The amphetamine solutions were locally applied from micropipettes by pressure ejection.

RESULTS

Local applications of Adderall resulted in significantly greater DA release signal amplitudes with prolonged time course of dopamine release and re-uptake as compared to D-amphetamine and D,L-amphetamine.

CONCLUSIONS

These data support the hypothesis that the combination of amphetamine enantiomers and salts in Adderall has effects on DA release, which result in increased and prolonged DA release, compared to D- and D,L-amphetamine.

Voltammetric characterization of the effect of monoamine uptake inhibitors and releasers on dopamine and serotonin uptake in mouse caudate-putamen and substantia nigra slices

Fast scan cyclic voltammetry is an electrochemical technique used to measure dynamics of transporter-mediated monoamine uptake in real time and provides a tool to evaluate the detailed effects of monoamine uptake inhibitors and releasers on dopamine and serotonin transporter function. We measured the effects of cocaine, methylphenidate, 2beta-propanoyl-3beta-(4tolyl) tropane (PTT), fluoxetine, amphetamine, methamphetamine, 3,4-methylenedioxymethamphetamine (MDMA), phentermine and fenfluramine on dopamine and serotonin uptake following electrically stimulated release in mouse caudate-putamen and substantia nigra pars reticulata slices. We determined rank orders of uptake inhibition effects based on two variables; increases in apparent K(m) for dopamine and serotonin uptake and inhibition constant (K(i)) values. For example, the rank order of uptake inhibition based on apparent K(m) values at the dopamine transporter was amphetamine>or=PTT>or=methylphenidate>>methamphetamine=phentermine=MDMA>cocaine>>fluoxetine=fenfluramine, and at the serotonin transporter was fluoxetine=methamphetamine=fenfluramine=MDMA > amphetamine=cocaine=PTT>or=methylphenidate>phentermine. Additionally, changes in electrically stimulated release were documented. This is the first study using voltammetry to measure the effects of a wide range of monoamine uptake inhibitors and releasers on dopamine and serotonin uptake in mouse brain slices. These studies also highlight methodological considerations for comparison of effects between heterogeneous brain regions.

Transporters as Channels

This review investigates some key aspects of transport mechanisms and recent advances in our understanding of this ubiquitous cellular process. The prevailing model of cotransport is the alternating access model, which suggests that large conformational changes in the transporter protein accompany cotransport. This model rests on decades of research and has received substantial support because many transporter characteristics are explained using its premises. New experiments, however, have revealed the existence of channels in transporters, an idea that is in conflict with traditional models. The alternating access model is the subject of previous detailed reviews. Here we concentrate on the relatively recent data that document primarily the channel properties of transporters. In some cases, namely, the observation of single-transporter currents, the evidence is direct. In other cases the evidence--for example, from fluctuation analysis or transporter currents too large to be described as anything other than channel-like--is indirect. Although the existence of channels in transporters is not in doubt, we are far from understanding the significance of this property. In the online Supplemental Material , we review some pertinent aspects of ion channel theory and cotransport physiology to provide background for the channels and transporters presented here. We discuss the existence of channels in transporters, and we speculate on the biological significance of this newly unveiled property of transport proteins.

Mapping the amphetamine-evoked changes in [11C]raclopride binding in living rat using small animal PET: Modulation by MAO-inhibition

The performance of small animal PET for neuroreceptor studies in a psychopharmacological challenge paradigm is not yet well-described. Therefore, we used microPET and [(11)C]raclopride to map the availability of dopamine D(2/3) receptors in brain of anesthetized rats, first in a baseline condition, and again after challenge with saline or d-amphetamine. Parametric maps of the specific binding (binding potential, pB) were calculated using a reference tissue input from cerebellum, and spatially normalized to a digitized stereotaxic coordinate system for rat brain. In volumes of interest (VOIs), the mean baseline pB (n=6) was 2.05 in dorsal striatum (caudate-putamen), and 1.34 in ventral striatum (nucleus accumbens), and did not significantly differ upon retest 2 h later. The availability of [(11)C]raclopride binding sites at baseline was 8% higher in the right striatum. Challenge with amphetamine sulfate (1 mg/kg, i.v., n=4) decreased pB by 19% in both ventral and dorsal striatum. We have earlier predicted that blockade of monoamine oxidase (MAO) should potentiate the amphetamine-evoked dopamine release, thus enhancing the displacement of [(11)C]raclopride binding in vivo. However, pretreatment of rats with pargyline hydrochloride (4 mg/kg, n=4; 20 mg/kg, n=4) 1 day prior to PET did not potentiate the amphetamine-evoked reduction in dopamine receptor availability within the extended striatum. We conclude that small animal PET can be used to investigate stimulant-induced dopamine release, but that the spatial resolution is insufficient to detect differences between relative changes in dorsal vs. ventral divisions of the rat striatum. Furthermore, the present results do not reveal potentiation of the amphetamine-evoked release of dopamine in rats with MAO inhibition.

Distribution of the dopamine innervation in the macaque and human thalamus

We recently defined the thalamic dopaminergic system in primates; it arises from numerous dopaminergic cell groups and selectively targets numerous thalamic nuclei. Given the central position of the thalamus in subcortical and cortical interplay, and the functional relevance of dopamine neuromodulation in the brain, detailing dopamine distribution in the thalamus should supply important information. To this end we performed immunohistochemistry for dopamine and the dopamine transporter in the thalamus of macaque monkeys and humans to generate maps, in the stereotaxic coronal plane, of the distribution of dopaminergic axons. The dopamine innervation of the thalamus follows the same pattern in both species and is most dense in midline limbic nuclei, the mediodorsal and lateral posterior association nuclei, and in the ventral lateral and ventral anterior motor nuclei. This distribution suggests that thalamic dopamine has a prominent role in emotion, attention, cognition and complex somatosensory and visual processing, as well as in motor control. Most thalamic dopaminergic axons are thin and varicose and target both the neuropil and small blood vessels, suggesting that, besides neuronal modulation, thalamic dopamine may have a direct influence on microcirculation. The maps provided here should be a useful reference in future experimental and neuroimaging studies aiming at clarifying the role of the thalamic dopaminergic system in health and in conditions involving brain dopamine, including Parkinson's disease, drug addiction and schizophrenia.

The role of the plasmalemmal dopamine and vesicular monoamine transporters in methamphetamine‐induced dopaminergic deficits

Amphetamine (AMPH) and methamphetamine (METH) are members of a collection of phenethylamine psychostimulants that are commonly referred to collectively as "amphetamines." Amphetamines exert their effects, in part, by affecting neuronal dopamine transport. This review thus focuses on the effects of AMPH and METH on the plasmalemmal dopamine transporter and the vesicular monoamine transporter-2 in animal models with a particular emphasis on how these effects, which may vary for the different stereoisomers, contribute to persistent dopaminergic deficits.

Na+, Cl-, and pH dependence of the human choline transporter (hCHT) in Xenopus oocytes: the proton inactivation hypothesis of hCHT in synaptic vesicles

The recent cloning of the human choline transporter (hCHT) has allowed its expression in Xenopus laevis oocytes and the simultaneous measurement of choline transport and choline-induced current under voltage clamp. hCHT currents and choline transport are evident in cRNA-injected oocytes and significantly enhanced by the hCHT trafficking mutant L530A/V531A. The charge/choline ratio of hCHT varies from 10e/choline at -80 mV to 3e/choline at -20 mV, in contrast with the reported fixed stoichiometry of the Na+-coupled glucose transporter in the same gene family. Ion substitution shows that the choline uptake and choline-induced current are Na+ and Cl- dependent; however, the reversal potential of the induced current suggests a Na+-selective mechanism, consigning Cl- to a regulatory role rather than a coupled, cotransported-ion role. The hCHT-specific inhibitor hemicholinium-3 (HC-3) blocks choline uptake and choline-induced current; in addition, HC-3 alone reveals a constitutive, depolarizing leak current through hCHT. We show that external protons reduce hCHT current, transport, and binding with a similar pKa of 7.4, suggesting proton titration of residue(s) that support choline binding and transport. Given the localization of the choline transporter to synaptic vesicles, we propose that proton inactivation of hCHT prevents acetylcholine and proton leakage from the acidic interior of cholinergic synaptic vesicles. This mechanism would allow cholinergic, activity-triggered delivery of silent choline transporters to the plasma membrane, in which normal pH would reactivate the transporters for choline uptake and subsequent acetylcholine synthesis.

Modafinil inhibits rat midbrain dopaminergic neurons through D2-like receptors

Modafinil is a well-tolerated medication for excessive sleepiness, attention-deficit disorder, cocaine dependence and as an adjunct to antidepressants with low propensity for abuse. We investigated the modafinil action on identified dopaminergic and GABAergic neurons in the ventral tegmental area (VTA) and substantia nigra (SN) of rat brain slices. Modafinil (20 microM) inhibited the firing of dopaminergic, but not GABAergic neurons. This inhibition was maintained in the presence of tetrodotoxin and was accompanied by hyperpolarization. Sulpiride (10 microM), a D2-receptor antagonist, but not prazosine (20 microM, an alpha1-adrenoreceptor blocker) abolished the modafinil action. Inhibition of dopamine reuptake with a low dose of nomifensine (1 microM) reduced the firing of DA neurons in a sulpiride-dependent manner and blunted the effect of modafinil. On acutely isolated neurons, modafinil evoked D2-receptor-mediated outward currents in tyrosine-hydroxylase positive cells, identified by single-cell RT-PCR, which reversed polarity near the K(+) equilibrium potential and were unchanged in the presence of nomifensine. Thus modafinil directly inhibits DA neurons through D2 receptors.

Synthesis, protein levels, activity, and phosphorylation state of tyrosine hydroxylase in mesoaccumbens and nigrostriatal dopamine pathways of chronically food-restricted rats

Chronic food restriction (FR) enhances the rewarding and motor-activating effects of abused drugs, and is accompanied by changes in dopamine (DA) dynamics and increased D-1 DA receptor-mediated cell signaling and transcriptional responses in nucleus accumbens (NAc). However, little is known about effects of FR on DA synthetic activity in the mesoaccumbens and nigrostriatal pathways. In Experiment 1 of the present study, tyrosine hydroxylase (TH) gene expression was measured in ventral tegmental area and substantia nigra, using real-time RT-PCR and in situ hybridization; no differences were observed between FR and ad libitum fed (AL) rats. In Experiment 2, TH protein levels, determined by Western blot, were found to be elevated in NAc and caudate-putamen (CPu) of FR relative to AL rats. In the absence of increased transcription, this may reflect a slowing of TH degradation. In Experiments 3 and 4, DA synthetic activity was assessed by Western blot measurement of TH phosphorylation at Ser40, and HPLC measurement of in vivo tyrosine hydroxylation rate, as reflected by DOPA accumulation following administration of a decarboxylase inhibitor (NSD-1015; 100 mg/kg, i.p.). Basal phospho-(Ser40)-TH levels did not differ between groups but DOPA accumulation was decreased by FR. Decreased DOPA synthesis, despite increased levels of TH protein, may reflect the inhibitory effect of increased DA binding to TH protein or decreased concentrations of cofactor tetrahydrobiopterin. Finally, in response to D-amphetamine (0.5 and 5.0 mg/kg, i.p.), phospho-(Ser40)-TH was selectively decreased in NAc of FR rats. This suggests increased feedback inhibition of DA synthesis-a possible consequence of postsynaptic receptor hypersensitivity, or increased extracellular DA concentration. These results indicate that FR increases TH protein levels, but may decrease the capacity for DA synthesis by decreasing TH activity. According to this scheme, the previously observed upregulation of striatal cell signaling and transcriptional responses to DA receptor agonist administration may include compensatory neuroadaptations.

Calmodulin kinase II interacts with the dopamine transporter C terminus to regulate amphetamine-induced reverse transport

Efflux of dopamine through the dopamine transporter (DAT) is critical for the psychostimulatory properties of amphetamines, but the underlying mechanism is unclear. Here we show that Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) plays a key role in this efflux. CaMKIIalpha bound to the distal C terminus of DAT and colocalized with DAT in dopaminergic neurons. CaMKIIalpha stimulated dopamine efflux via DAT in response to amphetamine in heterologous cells and in dopaminergic neurons. CaMKIIalpha phosphorylated serines in the distal N terminus of DAT in vitro, and mutation of these serines eliminated the stimulatory effects of CaMKIIalpha. A mutation of the DAT C terminus impairing CaMKIIalpha binding also impaired amphetamine-induced dopamine efflux. An in vivo role for CaMKII was supported by chronoamperometry measurements showing reduced amphetamine-induced dopamine efflux in response to the CaMKII inhibitor KN93. Our data suggest that CaMKIIalpha binding to the DAT C terminus facilitates phosphorylation of the DAT N terminus and mediates amphetamine-induced dopamine efflux.

Sex differences in methamphetamine-evoked striatal dopamine of mice are reversed by nomifensine

Male mice show more severe striatal dopamine depletions to the psychostimulant, methamphetamine (MA). To gain some understanding for this sex difference, we examined MA-evoked dopamine (DA) responses from superfused striatal tissue fragments of male and female mice under conditions of a dopamine transporter which was either unaltered (Experiment 1) or inhibited, with use of the drug, nomifensine (Experiment 2). In Experiment 1, MA-evoked DA was significantly greater in male versus female mice. In Experiment 2, diminished, albeit statistically significant, DA responses to MA infusion in the presence of nomifensine were obtained from striatal tissue of female, but not male, mice. In Experiment 3, potassium-evoked DA responses and sex differences were abolished in the presence of nomifensine. These data demonstrate a clear sex difference in DA responses to MA. Interestingly, under conditions where dopamine transporter function is inhibited, MA retains its ability to evoke DA. However, this capacity was only observed within striatal tissue fragments of female mice and not under conditions of potassium-evoked DA. These results indicate an additional component for the bases of sex differences in nigrostriatal dopaminergic function in health and in disease. In particular, the present findings have important implications in suggesting an alternative, non-traditional, mechanism for MA effects and indicate that such a function is limited to females.

Cocaine-but not methamphetamine-associated memory requires de novo protein synthesis

Context-induced drug craving and continuous drug use manifest the critical roles of specific memory episodes associated with the drug use experiences. Drug-induced conditioned place preference (CPP) in C57BL/6J mouse model, in this regard, is an appropriate behavioral paradigm to study such drug use-associated memories. Requirement of protein synthesis in various forms of long-term memory formation and storage has been phylogenetically demonstrated. This study was undertaken to study the requirement of protein synthesis in the learning and memory aspect of the conditioned place preference induced by cocaine and methamphetamine, two abused drugs of choice in local area. Since pCREB has been documented as a candidate substrate for mediating the drug-induced neuroadaptation, the pCREB level in hippocampus, nucleus accumbens, and prefrontal cortex was examined for its potential participation in the formation of CPP caused by these psychostimulants. We found that cocaine (2.5 and 5.0 mg/kg/dose)-induced CPP was abolished by the pretreatment of anisomycin (50 mg/kg/dose), a protein synthesis inhibitor, whereas methamphetamine (0.5 or 1.0 mg/kg/dose)-induced CPP was not affected by the anisomycin pretreatment. Likewise, cocaine-induced CPP was mitigated by another protein synthesis inhibitor, cycloheximide (15 mg/kg/injection) pretreatment, whereas methamphetamine-induced CPP remained intact by such pretreatment. Moreover, anisomycin treatment 2h after each drug-place pairing disrupted the cocaine-induced CPP, whereas the same treatment did not affect methamphetamine-induced CPP. An increase of accumbal pCREB level was found to associate with the learning phase of cocaine, but not with the learning phase of methamphetamine. We further found that intraaccumbal CREB antisense oligodeoxynucleotide infusion diminished cocaine-induced CPP, whereas did not affect the methamphetamine-induced CPP. Taken together, these data suggest that protein synthesis and accumbal CREB phosphorylation are essential for the learning and consolidation of the cocaine-induced CPP, whereas methamphetamine-induced CPP may be unrelated to the synthesis of new proteins.

Regulation of the dopamine transporter by phosphorylation

The dopamine transporter (DAT) is a neuronal phosphoprotein and target for psychoactive drugs that plays a critical role in terminating dopaminergic transmission by reuptake of dopamine from the synaptic space. Control of DAT activity and plasma membrane expression are therefore central to drug actions and the spatial and temporal regulation of synaptic dopamine levels. DATs rapidly traffic between the plasma membrane and endosomal compartments in both constitutive and protein kinase C-dependent manners. Kinase activators, phosphatase inhibitors, and transported substrates modulate DAT phosphorylation and activity, but the underlying mechanisms and role of phosphorylation in these processes are poorly understood. Complex adaptive changes in DAT function potentially related to these processes are also induced by psychostimulant and therapeutic transport blockers such as cocaine and methylphenidate. This chapter provides an overview of the current state of knowledge regarding DAT phosphorylation and its relationship to transporter activity and trafficking. A better understanding of how dopaminergic neurons regulate DAT function and the role of phosphorylation may lead to the identification of novel therapeutic targets for the treatment and prevention of dopaminergic disorders.

Prenatal cadmium and ethanol increase amphetamine-evoked dopamine release in rat striatum

To explore interactive deleterious effects of the teratogens ethanol and cadmium, pregnant rats were given cadmium (CdCl(2), 50 ppm) and/or ethanol (10%), or tap water (controls) in the drinking water for the entire 21 days of pregnancy. At 3 months after birth, in vivo microdialysis was used to determine that there was a 4000% evoked release of DA by AMPH (AMPH, 4.0 mg/kg i.p.) in the striatum of rats exposed prenatally to both ethanol and cadmium, vs. a 2000% evoked release by AMPH in rats exposed prenatally to only ethanol or cadmium or tap water. Haloperidol (HAL)-evoked DA release was suppressed in groups exposed prenatally to ethanol, while HAL-evoked DOPAC and HVA release was greatest after co-exposure to prenatal cadmium and ethanol. These in vivo microdialysis results indicate that ontogenetic co-exposure to cadmium, and ethanol produces a long-lived suppressive effect on HAL-evoked DA release and a long-lived enhancing effect on AMPH-evoked DA release in rat striatum. These findings clearly demonstrate that there is marked alteration in dopaminergic regulation after ontogenetic cadmium and ethanol co-exposure, which in this regard resembles the reaction of the striatonigral pathway on AMPH-evoked DA release in rats with behavioral sensitization.

Nigral injection of antisense oligonucleotides to synaptotagmin I using HVJ-liposome vectors causes disruption of dopamine release in the striatum and impaired skill learning

To produce an animal model of a dopa-responsive motor disorder with depletion of dopamine (DA) release in the striatum by dysfunction of the transmitter release machinery of the nigrostriatal DA system, we performed an intra-nigral injection of an HVJ-liposome gene transfer vector containing antisense oligodeoxynucleotides (ODNs) against synaptotagmin I (SytI), a key regulator of Ca(2+)-dependent exocytosis and endocytosis in adult rats. A unilateral intra-nigral injection of HVJ-liposome vectors containing antisense ODNs against SytI (syt-AS) caused a moderate disruption of methamphetamine-induced release of DA in the treated side of the striatum, while the syt-AS treatment did not affect physiological release of DA in the treated striatum. A bilateral intra-nigral injection of HVJ-liposome vectors containing syt-AS induced an impairment of the striatal DA-mediated acquisition of skilled behavior in a rotarod task without any deficits in general motor functions, such as spontaneous locomotor activity, motor adjusting steps, equilibrium function, or muscle strength. These findings suggest that an intra-nigral treatment with HVJ-liposome vectors containing syt-AS may cause a long-lasting nigral knockdown of SytI which, in turn, leads to a moderate dysfunction of the DA release machinery in the terminals of the nigrostriatal DA system and a subsequent mild depletion of DA release in the striatum.

Impact of basic FGF expression in astrocytes on dopamine neuron synaptic function and development

Behavioural sensitization to amphetamine (AMPH) requires action of the drug in the ventral midbrain where dopamine (DA) neurons are located. In vivo studies suggest that AMPH sensitization requires enhanced expression of basic fibroblast growth factor (bFGF) in the nucleus of midbrain astrocytes. One idea is that the AMPH-induced increase in bFGF expression in astrocytes leads to enhanced secretion of this peptide and to long-term plasticity in DA neurons. To study directly the effects of astrocytic expression of bFGF on DA neurons, we established a cell-culture model of mesencephalic astrocytes and DA neurons. Immunolabelling showed that even in the absence of a pharmacological stimulus, the majority of mesencephalic astrocytes in culture express bFGF at a nuclear level. Arguing against the idea that bFGF was secreted, bFGF was undetectable in the extracellular medium (below 10 pg/mL). However, supplementing culture medium with exogenous bFGF at standard concentrations (20 ng/mL) led to a dramatic change in the morphology of astrocytes, increased spontaneous DA release, and inhibited synapse formation by individual DA neurons. RNA interference (siRNA) against bFGF mRNA, caused a reduction in DA release but produced no change in synaptic development. Together these data demonstrate that under basal conditions (in the absence of a pharmacological stimulus such as amphetamine) bFGF is not secreted even though there is abundant nuclear expression in astrocytes. The effects of bFGF seen here on DA neurons are thus likely to be mediated through more indirect glial-neuronal interactions, leading to enhanced DA release without a necessary change in synapse number.

Safety of obesity drugs

The safety of obesity drugs has historically been poor. This and the stigmatisation of obesity in society ensured that a higher standard of safety for obesity drugs must be met. The authors review the safety disasters of obesity drugs that were withdrawn. The authors then review the safety of presently available drugs--benzphetamine, phendimetrazine, diethylpropion, phentermine, sibutramine and orlistat. The safety of rimonabant, a drug with a pending new drug application that has an independent effect on metabolic syndrome, is also reviewed. The authors compare the stage of obesity drug development to that of hypertension in the 1950s. As new and safer drugs with more downstream mechanisms are developed that have independent effects on the cardiovascular risks associated with obesity, third party reimbursement for obesity medicine is likely to improve. This may lead to obesity being treated like hypertension and other chronic diseases with long-term medication. With improved technological tools, the authors believe this process will be more rapid for obesity than it was for hypertension.

Getting the Message Across: A Recent Transporter Structure Shows the Way

Efforts to define the mechanisms governing neurotransmitter uptake and drug action have moved into high gear with the publication of a high-resolution structure of a leucine transporter from Aquifex aeolicus, a bacterial member of the SLC6 transporter family. Solved with the substrate leucine bound, the new structure corroborates extensive biochemical and mutagenesis studies performed with related mammalian neurotransmitter transporters and provides exciting suggestions as to how coupling arises between ions and substrates to permit efficient neurotransmitter clearance.

The N-terminus of the norepinephrine transporter regulates the magnitude and selectivity of the transporter-associated leak current

The norepinephrine (NE) transporter (NET) mediates the removal of NE from synaptic spaces and is a major target for antidepressants, amphetamine and cocaine. Previously, we have shown that syntaxin 1A (SYN 1A) supports human NET (hNET) cell surface expression, that hNET/SYN 1A interactions are direct and mediated by the hNET N-terminus, and that the hNET/SYN 1A association limits substrate-induced hNET-associated currents [Sung, U., Apparsundaram, S., Galli, A., Kahlig, K.M., Savchenko, V., Schroeter, S., Quick, M.W., Blakely, R.D., 2003. A regulated interaction of syntaxin 1A with the antidepressant-sensitive norepinephrine transporter establishes catecholamine clearance capacity. J. Neurosci. 23, 1697-1709]. These data raise the possibility that the hNET N-terminus, and potentially its interaction with SYN 1A, might regulate other hNET conductance states, including the hNET-mediated leak current. Importantly for monoamine transporters, the leak conductance has been shown to play a critical role in regulating cell membrane potential and possibly neuronal excitability [Quick, M.W., 2003. Regulating the conducting states of a mammalian serotonin transporter. Neuron 40, 537-549]. Here we demonstrate that deletion of the binding domain for SYN 1A in the NET N-terminus robustly enhances the NET-mediated leak current as well as its selectivity for Cl- permeation under particular intracellular ionic compositions. In addition, we show that the NET N-terminus coordinates the ability of intracellular Na+ and Cl- to regulate the leak conductance. These data suggest that the NET N-terminus regulates and defines the ionic specificity of the NET-mediated leak current.

The primate thalamus is a key target for brain dopamine

The thalamus relays information to the cerebral cortex from subcortical centers or other cortices; in addition, it projects to the striatum and amygdala. The thalamic relay function is subject to modulation, so the flow of information to the target regions may change depending on behavioral demands. Modulation of thalamic relay by dopamine is not currently acknowledged, perhaps because dopamine innervation is reportedly scant in the rodent thalamus. We show that dopaminergic axons profusely target the human and macaque monkey thalamus using immunolabeling with three markers of the dopaminergic phenotype (tyrosine hydroxylase, dopamine, and the dopamine transporter). The dopamine innervation is especially prominent in specific association, limbic, and motor thalamic nuclei, where the densities of dopaminergic axons are as high as or higher than in the cortical area with the densest dopamine innervation. We also identified the dopaminergic neurons projecting to the macaque thalamus using retrograde tract-tracing combined with immunohistochemistry. The origin of thalamic dopamine is multiple, and thus more complex, than in any other dopaminergic system defined to date: dopaminergic neurons of the hypothalamus, periaqueductal gray matter, ventral mesencephalon, and the lateral parabrachial nucleus project bilaterally to the monkey thalamus. We propose a novel dopaminergic system that targets the primate thalamus and is independent from the previously defined nigrostriatal, mesocortical, and mesolimbic dopaminergic systems. Investigating this "thalamic dopaminergic system" should further our understanding of higher brain functions and conditions such as Parkinson's disease, schizophrenia, and drug addiction.

The dopamine transporter: a vigilant border control for psychostimulant action

Neurotransmission within the mesocorticolimbic dopamine system has remained the central focus of investigation into the molecular, cellular and behavioral properties of psychostimulants for nearly three decades. The primary means by which dopamine transmission in the synapse is terminated is via the dopamine transporter (DAT), the presynaptic plasmalemmal protein that is responsible for the reuptake of released dopamine. Numerous abused as well as clinically important drugs have important pharmacological interactions with DAT. In general, these compounds fall into two categories: those that block dopamine transport (e.g., cocaine, methylphenidate) and those that serve as substrates for transport [e.g., dopamine, amphetamine and 3,4-methylenedioxymethamphetamine (MDMA or "ecstasy")]. Recent data from in vitro and in vivo studies have suggested that DAT, like other biogenic amine transporters, share several characteristics with classical ligand-gated ion channels. In addition, substrates for transport promote redistribution of DAT away from the plasma membrane, while transport inhibitors such as cocaine disrupt this process. In addition, presynaptic autoreceptors for dopamine have been implicated in the modulation of DAT surface expression and function. The present chapter summarizes some of the recent discoveries pertaining to the electrogenic properties of DAT and their potential relevance to the effects of amphetamine-like stimulants on DAT function. Although there are a number of intracellular and extracellular modulatory influences on dopamine clearance that may play particular roles in psychostimulant action, we specifically focus on the differential direct modulation of DAT function by transport substrates and inhibitors, and we also discusses the role of presynaptic D2 receptors in transport regulation.

Role of monoamine transporters in mediating psychostimulant effects

Monoamine transporters such as the dopamine (DA) transporter (DAT) and the vesicular monoamine transporter-2 (VMAT-2) are critical regulators of DA disposition within the brain. Alterations in DA disposition can lead to conditions such as drug addiction, Parkinson's disease, and schizophrenia, a fact that underscores the importance of understanding DAergic signaling. Psychostimulants alter DAergic signaling by influencing both DAT and VMAT-2, and although the effects of these drugs result in increased levels of synaptic DA, the mechanisms by which this occurs and the effects that these drugs exert on DAT and VMAT-2 vary. Many psychostimulants can be classified as releasers (ie, amphetamine analogs) or uptake blockers (ie, cocaine-like drugs) based on the mechanism of their acute effects on neurotransmitter flux through the DAT. Releasers and uptake blockers differentially modulate the activity and subcellular distribution of monoamine transporters, a phenomenon likely related to the neurotoxic potential of these drugs to DAergic neurons. This article will review some of the recent findings whereby releasers and uptake blockers alter DAT and VMAT-2 activity and how these alterations may be involved in neurotoxicity, thus providing insight on the neurodegeneration observed in Parkinson's disease.

Serotonin-transporter mediated efflux: A pharmacological analysis of amphetamines and non-amphetamines

The physiological function of neurotransmitter transporter proteins like the serotonin transporter (SERT) is reuptake of neurotransmitter that terminates synaptic serotoninergic transmission. SERT can operate in reverse direction and be induced by SERT substrates including 5-HT, tyramine and the positively charged methyl-phenylpyridinium (MPP(+)), as well as the amphetamine derivatives para-chloroamphetamine (pCA) and methylene-dioxy-methamphetamine (MDMA). These substrates also induce inwardly directed sodium currents that are predominantly carried by sodium ions. Efflux via SERT depends on this sodium flux that is believed to be a prerequisite for outward transport. However, in recent studies, it has been suggested that substrates may be distinct in their properties to induce efflux. Therefore, the aim of the present study was a pharmacological characterization of different SERT substrates in uptake experiments, their abilities to induce transporter-mediated efflux and currents. In conclusion, the rank order of affinities in uptake and electrophysiological experiments correlate well, while the potencies of the amphetamine derivatives for the induction of efflux are clearly higher than those of the other substrates. These discrepancies can be only explained by mechanisms that can be induced by amphetamines. Therefore, based on our pharmacological observations, we conclude that amphetamines distinctly differ from non-amphetamine SERT substrates.

Differences in interactions with the dopamine transporter as revealed by diminishment of Na+ gradient and membrane potential: Dopamine versus other substrates

In heterologous cells expressing the dopamine transporter (DAT), simultaneous elevation of intracellular Na(+) and depolarization of the membrane with gramicidin reduced the potency of various DAT substrates, including dopamine, d-amphetamine, beta-phenethylamine, p-tyramine, and MPP(+), in inhibiting binding of the cocaine analog [(3)H]CFT, with the greatest reduction observed for d-amphetamine. In rat striatal synaptosomes, gramicidin exerted similar effects; in addition, the potency of d-amphetamine was reduced by the Na(+)-channel activator veratridine. The latter effect was counteracted by the Na(+)-channel blocker tetrodotoxin. In broken membranes, where, as the situation with gramicidin, both sides of the non-polarized membrane were exposed to 130 mM Na(+), gramicidin was ineffective. Dopamine had a potency for membrane preparations that was not significantly different from that for control cells or synaptosomes, while other substrates had potencies for membrane preparations that were reduced to a level similar to those observed in gramicidin-treated cells or synaptosomes. These results suggest that diminishing Na(+) gradient and membrane potential may convert DAT to a conformational state that dopamine could easily bind to when gaining free access to its intracellular portion. In contrast, non-dopamine substrates may not be able to readily interact with this state from either side of the membrane.