Amphetamine reverses or blocks the operation of the human noradrenaline transporter depending on its concentration: superfusion studies on transfected cells

Brain Glutamate Dynamics Predict Positive Agency in Healthy Women: Insights from Combined Application of Pharmacological Challenge, Comprehensive Affective Assessment, and Magnetic Resonance Spectroscopy

Contributions of brain glutamate (Glu) to conscious emotion are not well understood. Here, we evaluate the relationship of experimentally induced change in neocortical Glu (ΔGlu) and subjective states in well individuals, using combined application of pharmacological challenge, magnetic resonance spectroscopy (MRS), and comprehensive affective assessment. Drug challenge with d-amphetamine (AMP) (20 mg oral), methamphetamine (MA) (Desoxyn, 20 mg oral), and placebo (PBO) was conducted on three separate test days in a within-subjects double blind design. Proton MRS quantified neurometabolites in the right dorsal anterior cingulate cortex 140-150 min post-drug and PBO. Subjective states were assessed at half hour intervals over 5.5 h on each session, yielding 3792 responses per participant (91,008 responses overall, N = 24 participants), with self-reports reduced by principal components analysis (PCA). PCA produced a primary factor score of AMP- and MA-induced positive agency (ΔPA). MRS indicated drug-induced ΔGlu related positively to ΔPA (ΔGluMA r = +0.44, p < 0.05, N = 21), with large effects in females (ΔGluMA r = +0.52, p < 0.05; ΔGluAMP r = +0.61, p < 0.05, N = 11). Subjective states related to ΔGlu included rise in subjective stimulation, vigor, friendliness, elation, positive mood, positive affect (r's = +0.51 to +0.74, p < 0.05), and alleviation of anxiety in females (r = -0.61, p < 0.05, N = 11). These self-reports correlated with ΔGlu to the extent they loaded on ΔPA (r = 0.95 AMP, p = 5 × 10-10; r = 0.63 MA, p = 0.0015, N = 11), indicating the coherence of ΔGlu effects on emotional states. Timing data indicated Glu shaped positive emotion both concurrently and prospectively, with no relationship with pre-MRS emotion (ΔGluAMP r = +0.59 to +0.65, p's < 0.05; ΔGluMA r = +0.53, p < 0.05, N = 11). Together these findings indicate substantive, mechanistic contributions of neocortical Glu to positive agentic states in healthy individuals, which are most readily observed in women. The findings illustrate the promise of combined application of pharmacological challenge, comprehensive affective assessment, and MRS neuroimaging techniques in basic and clinical studies.

Biochemical Assessment of Pheochromocytoma and Paraganglioma

Pheochromocytoma and paraganglioma (PPGL) require prompt consideration and efficient diagnosis and treatment to minimize associated morbidity and mortality. Once considered, appropriate biochemical testing is key to diagnosis. Advances in understanding catecholamine metabolism have clarified why measurements of the O-methylated catecholamine metabolites rather than the catecholamines themselves are important for effective diagnosis. These metabolites, normetanephrine and metanephrine, produced respectively from norepinephrine and epinephrine, can be measured in plasma or urine, with choice according to available methods or presentation of patients. For patients with signs and symptoms of catecholamine excess, either test will invariably establish the diagnosis, whereas the plasma test provides higher sensitivity than urinary metanephrines for patients screened due to an incidentaloma or genetic predisposition, particularly for small tumors or in patients with an asymptomatic presentation. Additional measurements of plasma methoxytyramine can be important for some tumors, such as paragangliomas, and for surveillance of patients at risk of metastatic disease. Avoidance of false-positive test results is best achieved by plasma measurements with appropriate reference intervals and preanalytical precautions, including sampling blood in the fully supine position. Follow-up of positive results, including optimization of preanalytics for repeat tests or whether to proceed directly to anatomic imaging or confirmatory clonidine tests, depends on the test results, which can also suggest likely size, adrenal vs extra-adrenal location, underlying biology, or even metastatic involvement of a suspected tumor. Modern biochemical testing now makes diagnosis of PPGL relatively simple. Integration of artificial intelligence into the process should make it possible to fine-tune these advances.

Neuroplasticity and Multilevel System of Connections Determine the Integrative Role of Nucleus Accumbens in the Brain Reward System

A growing body of evidence suggests that nucleus accumbens (NAc) plays a significant role not only in the physiological processes associated with reward and satisfaction but also in many diseases of the central nervous system. Summary of the current state of knowledge on the morphological and functional basis of such a diverse function of this structure may be a good starting point for further basic and clinical research. The NAc is a part of the brain reward system (BRS) characterized by multilevel organization, extensive connections, and several neurotransmitter systems. The unique role of NAc in the BRS is a result of: (1) hierarchical connections with the other brain areas, (2) a well-developed morphological and functional plasticity regulating short- and long-term synaptic potentiation and signalling pathways, (3) cooperation among several neurotransmitter systems, and (4) a supportive role of neuroglia involved in both physiological and pathological processes. Understanding the complex function of NAc is possible by combining the results of morphological studies with molecular, genetic, and behavioral data. In this review, we present the current views on the NAc function in physiological conditions, emphasizing the role of its connections, neuroplasticity processes, and neurotransmitter systems.

Psychostimulant drug effects on glutamate, Glx, and creatine in the anterior cingulate cortex and subjective response in healthy humans

Prescription psychostimulants produce rapid changes in mood, energy, and attention. These drugs are widely used and abused. However, their effects in human neocortex on glutamate and glutamine (pooled as Glx), and key neurometabolites such as N-acetylaspartate (tNAA), creatine (tCr), choline (Cho), and myo-inositol (Ins) are poorly understood. Changes in these compounds could inform the mechanism of action of psychostimulant drugs and their abuse potential in humans. We investigated the acute impact of two FDA-approved psychostimulant drugs on neurometabolites using magnetic resonance spectroscopy (¹H MRS). Single clinically relevant doses of d-amphetamine (AMP, 20 mg oral), methamphetamine (MA, 20 mg oral; Desoxyn®), or placebo were administered to healthy participants (n = 26) on three separate test days in a placebo-controlled, double-blinded, within-subjects crossover design. Each participant experienced all three conditions and thus served as his/her own control. ¹H MRS was conducted in the dorsal anterior cingulate cortex (dACC), an integrative neocortical hub, during the peak period of drug responses (140-150 m post ingestion). D-amphetamine increased the level of Glu (p = .0001), Glx (p = .003), and tCr (p = .0067) in the dACC. Methamphetamine increased Glu in females, producing a significant crossover interaction pattern with gender (p = .02). Drug effects on Glu, tCr, and Glx were positively correlated with subjective drug responses, predicting both the duration of AMP liking (Glu: r = +.49, p = .02; tCr: r = +.41, p = .047) and the magnitude of peak drug high to MA (Glu: r = +.52, p = .016; Glx: r = +.42, p = .049). Neither drug affected the levels of tNAA, Cho, or Ins after correction for multiple comparisons. We conclude that d-amphetamine increased the concentration of glutamate, Glx, and tCr in the dACC in male and female volunteers 2¹/₂ hours after drug consumption. There was evidence that methamphetamine differentially affects dACC Glu levels in women and men. These findings provide the first experimental evidence that specific psychostimulants increase the level of glutamatergic compounds in the human brain, and that glutamatergic changes predict the extent and magnitude of subjective responses to psychostimulants.

The effect of quetiapine (Seroquel™) on conditioned place preference and elevated plus maze tests in rats when administered alone and in combination with (+)-amphetamine

RATIONALE

Recent case reports describe recreational use of quetiapine and drug-seeking behaviour to obtain quetiapine, an atypical antipsychotic.

OBJECTIVE

We examined the hypothesis that quetiapine (10, 20 or 40 mg/kg) alone or co-administered with (+)-amphetamine (0.25, 0.5, 0.75 or 2.0 mg/kg) will affect reward and/or decrease anxiety in rats, as measured by conditioned place preference (CPP) and elevated plus maze (EPM) test, respectively.

RESULTS

Quetiapine (20 mg/kg) produced greater open arm time and entries in the EPM test compared to 10 and 40 mg/kg, and quetiapine (10 mg/kg) significantly increased open arm entries and time when co-administered with (+)-amphetamine (0.5 mg/kg) compared to (+)-amphetamine (0.5 mg/kg) alone, suggesting decreased anxiety. Quetiapine (10, 20 or 40 mg/kg) produced no CPP when administered alone; the lowest dose of quetiapine (10 mg/kg) reduced CPP produced by a low dose of (+)-amphetamine (0.25 mg/kg), but had no significant effect on CPP produced by a higher dose (0.5 mg/kg).

DISCUSSION

The quetiapine-induced anxiolytic effect in the EPM might explain why humans are misusing quetiapine and combining it with (+)-amphetamine. It is possible that humans experience an anxiolytic effect of the combined drugs and relatively unaltered rewarding effects of (+)-amphetamine. The results shed some light on the question of why humans are abusing and misusing quetiapine, despite its dopamine (DA) D2 receptor antagonism; it will be the task of future studies to identify the pharmacological mechanism mediating this behaviour.

The solute carrier 6 family of transporters

The solute carrier 6 (SLC6) family of the human genome comprises transporters for neurotransmitters, amino acids, osmolytes and energy metabolites. Members of this family play critical roles in neurotransmission, cellular and whole body homeostasis. Malfunction or altered expression of these transporters is associated with a variety of diseases. Pharmacological inhibition of the neurotransmitter transporters in this family is an important strategy in the management of neurological and psychiatric disorders. This review provides an overview of the biochemical and pharmacological properties of the SLC6 family transporters.

Rats that differentially respond to cocaine differ in their dopaminergic storage capacity of the nucleus accumbens

Cocaine (COC) inhibits the re-uptake of dopamine. However, the dopamine response to COC also depends on dopamine inside storage vesicles. The aim of this study was to investigate whether rats that differentially respond to COC differ in their dopaminergic storage capacity of the nucleus accumbens. Total and vesicular levels of accumbal dopamine as well as accumbal vesicular monoamine transporter-2 levels were established in high (HR) and low responders (LR) to novelty rats. Moreover, the effects of reserpine (RES) on the COC-induced increase of extracellular accumbal dopamine were investigated. HR displayed higher accumbal levels of total and vesicular dopamine than LR. Moreover, HR displayed more accumbal vesicular monoamine transporters-2 than LR. COC increased extracellular accumbal dopamine more strongly in HR than in LR. A low dose of RES prevented the COC-induced increase of accumbal dopamine in LR, but not in HR. A higher dose of RES was required to inhibit the COC-induced increase of accumbal dopamine in HR. These data demonstrate that HR were marked by a larger accumbal dopaminergic storage pool than LR. It is hypothesized that HR are more sensitive to COC than LR, because COC can release more dopamine from accumbal storage vesicles in HR than in LR.

J. Neurochem. (2008) 105, 2122–2133.

The reverse operation of Na(+)/Cl(-)-coupled neurotransmitter transporters--why amphetamines take two to tango

Sodium-chloride coupled neurotransmitter transporters achieve reuptake of their physiological substrate by exploiting the pre-existing sodium-gradient across the cellular membrane. This terminates the action of previously released substrate in the synaptic cleft. However, a change of the transmembrane ionic gradients or specific binding of some psychostimulant drugs to these proteins, like amphetamine and its derivatives, induce reverse operation of neurotransmitter:sodium symporters. This effect eventually leads to an increase in the synaptic concentration of non-exocytotically released neurotransmitters [and - in the case of the norepinephrine transporters, underlies the well-known indirect sympathomimetic activity]. While this action has long been appreciated, the underlying mechanistic details have been surprisingly difficult to understand. Some aspects can be resolved by incorporating insights into the oligomeric nature of transporters, into the nature of the accompanying ion fluxes, and changes in protein kinase activities.

A juxtamembrane mutation in the N terminus of the dopamine transporter induces preference for an inward-facing conformation

The human dopamine transporter (hDAT) regulates synaptic dopamine (DA) levels and is the site of action of abused and therapeutic drugs. Here we study the effect of a threonine residue (Thr62 in hDAT) that is highly conserved within a canonical phosphorylation site (RETW) in the juxtamembrane N-terminal region of monoamine transporters. In stably transfected human embryonic kidney 293T cells, expression of T62D-hDAT was reduced compared with hDAT or T62A-hDAT. T62D-hDAT displayed dramatically reduced [(3)H]dopamine up-take but exhibited a higher basal dopamine efflux compared with hDAT or T62A-hDAT, as determined by measurements of [(3)H]dopamine efflux and amperometry. The high constitutive efflux in T62D-hDAT precluded the measurement of amphetamine-stimulated [(3)H]dopamine efflux, but when dopamine was added internally into voltage-clamped T62D-hDAT cells, amphetamine-induced efflux comparable with hDAT was detected by amperometry. In accordance with findings that Zn(2+) can rescue reduced DA uptake in mutant transporters that are predominantly inward-facing, micromolar concentrations of Zn(2+) markedly potentiated [(3)H]dopamine uptake in T62D-hDAT and permitted the measurement of amphetamine-stimulated dopamine efflux. These results suggest that T62D-hDAT prefers an inward-facing conformation in the transition between inward- and outward-facing conformations. For T62A-hDAT, however, the measured 50% reduction in both [(3)H]dopamine uptake and [(3)H]dopamine efflux was consistent with a slowed transition between inward- and outward-facing conformations. The mechanism underlying the important functional role of Thr62 in hDAT activity suggested by these findings is examined in a structural context using dynamic simulations of a three-dimensional molecular model of DAT.

Comparative potencies of 3,4-methylenedioxymethamphetamine (MDMA) analogues as inhibitors of [3H]noradrenaline and [3H]5-HT transport in mammalian cell lines

BACKGROUND AND PURPOSE

Illegal 'ecstasy' tablets frequently contain 3,4-methylenedioxymethamphetamine (MDMA)-like compounds of unknown pharmacological activity. Since monoamine transporters are one of the primary targets of MDMA action in the brain, a number of MDMA analogues have been tested for their ability to inhibit [3H]noradrenaline uptake into rat PC12 cells expressing the noradrenaline transporter (NET) and [3H]5-HT uptake into HEK293 cells stably transfected with the 5-HT transporter (SERT).

EXPERIMENTAL APPROACH

Concentration-response curves for the following compounds at both NET and SERT were determined under saturating substrate conditions: 4-hydroxy-3-methoxyamphetamine (HMA), 4-hydroxy-3-methoxymethamphetamine (HMMA), 3,4-methylenedioxy-N-hydroxyamphetamine (MDOH), 2,5-dimethoxy-4-bromophenylethylamine (2CB), 3,4-dimethoxymethamphetamine (DMMA), 3,4-methylenedioxyphenyl-2-butanamine (BDB), 3,4-methylenedioxyphenyl-N-methyl-2-butanamine (MBDB) and 2,3-methylenedioxymethamphetamine (2,3-MDMA).

KEY RESULTS

2,3-MDMA was significantly less potent than MDMA at SERT, but equipotent with MDMA at NET. 2CB and BDB were both significantly less potent than MDMA at NET, but equipotent with MDMA at SERT. MBDB, DMMA, MDOH and the MDMA metabolites HMA and HMMA, were all significantly less potent than MDMA at both NET and SERT.

CONCLUSIONS AND IMPLICATIONS

This study provides an important insight into the structural requirements of MDMA analogue affinity at both NET and SERT. It is anticipated that these results will facilitate understanding of the likely pharmacological actions of structural analogues of MDMA.

Difference in magnitude of psychostimulant-induced extracellular norepinephrine in the ventral tegmental area contributes to discrepant prefrontal dopamine outflow

The dopamine (DA) efflux in the medial prefrontal cortex (mPFC) can be modulated by the interaction between afferent norepinephrine (NE) and somatodendritic DA in the ventral tegmental area (VTA). However, it is unclear how locally administered amphetamine (AMPH) or cocaine in the VTA results in discrepant response of DA efflux in the mPFC. In this study, intra-VTA infusion of AMPH (1000 microM) or cocaine (200 microM) in anesthetized rats was employed to study the concurrent profile of extracellular DA level in the VTA and mPFC. In addition, the extracellular NE levels during the intra-VTA infusion of these two psychostimulants were analyzed to compare their effects on prefrontal DA efflux. During the intra-VTA infusion of AMPH, both extracellular DA and NE increased significantly in the VTA (270 +/- 12% and 819 +/- 40%, respectively). Meanwhile, the DA efflux in the mPFC elevated significantly. During the intra-VTA infusion of cocaine, the extracellular DA and NE in the VTA also increased (271 +/- 21% and 150 +/- 15%, respectively). However, the DA efflux decreased significantly in the mPFC. Noteworthy, the increase of extracellular NE in the VTA was much more robust via AMPH infusion, as compared with cocaine. It is suggested that AMPH and cocaine enhance the extracellular NE concentrations in the VTA in different magnitudes, which in turn contribute to discrepant profiles of distal DA efflux in the mPFC.

Catecholamine mapping within nucleus accumbens: differences in basal and amphetamine-stimulated efflux of norepinephrine and dopamine in shell and core

The nucleus accumbens is believed to play a critical role in mediating the behavioral responses to rewarding stimuli. Although most studies of the accumbens focus on dopamine, it receives afferents from many other nuclei, including noradrenergic cell groups in the brainstem. We used in vivo microdialysis to measure extracellular levels of both norepinephrine and dopamine in the accumbens shell and core. Regional analysis of shell and core and border regions demonstrated that norepinephrine was high in shell and decreased from medial shell to lateral core, where baseline levels were low or undetectable. Conversely, extracellular dopamine in core was twice the level seen in shell. Both catecholamines increased following a single injection of amphetamine (2 mg/kg, i.p.). The norepinephrine response was greater and long-lasting in shell compared with core. The maximal dopamine response was higher in core than in shell, but the duration of the effect was comparable in both regions. The distinct neurochemical characteristics of shell and core are likely to contribute to the functional heterogeneity of the two subregions. Furthermore, norepinephrine may be involved in many of the functions generally attributed to the accumbens, either directly or indirectly via modulation of extracellular dopamine.

Blockade of noradrenaline transport abolishes 4-methylthioamphetamine-induced contraction of the rat aorta in vitro

The aim of this study was to characterize the effects of 4-methylthioamphetamine (4-MTA) on contractility and noradrenaline (NA) transport and release in the isolated rat aorta. Descending thoracic aortic rings were isolated from male Wistar rats (220-240 g) and the effect of 4-MTA on contractility was measured by isometric force displacement. 4-MTA (0.1 microm-1 mm) induced a concentration-dependent contraction of aortic rings, with a pD(2) of 4.40 +/- 0.38, and an E(max) of 0.80 +/- 0.05 g tension. The alpha(1)-adrenoceptor antagonist, prazosin (1 microm) and alpha(2) antagonist, yohimbine (1 microm) inhibited maximal contraction to 100 microm 4-MTA by 45.0 +/- 6.7% and 53.5 +/- 7.1% of control values respectively, whereas the 5-hydroxytryptamine (5-HT) antagonist, ketanserin (100 nm) had no effect on the 4-MTA-mediated contraction. The specific NA transport inhibitor, nisoxetine (1 microm) abolished contraction of the aorta by 4-MTA. 4 Nisoxetine-sensitive [(3)H]-NA transport in aortic rings was measured over a concentration range of 0-5 microm [(3)H]-NA, and had a maximal rate of transport (V(max)) of 0.77 +/- 0.07 pmol [(3)H]-NA min(-1) mg(-1) protein and a Michaelis affinity constant (K(M)) of 2.3 +/- 0.5 microm. 4-MTA inhibited nisoxetine-sensitive [(3)H]-NA transport with a pIC(50) of 6.16 +/- 0.18 and the pIC(50) for inhibition of nisoxetine-sensitive [(3)H]-NA transport by 3,4-methylenedioxymethamphetamine (MDMA) was 6.83 +/- 0.13. 4-MTA (1-100 microm) significantly stimulated release of pre-loaded [(3)H]-NA from aortic rings and 4-MTA-induced [(3)H]-NA release was inhibited by 1 microm nisoxetine. These data suggest that 4-MTA causes contraction of the rat aorta in vitro by a mechanism that is consistent with an ability to cause release of NA at the level of the NA transporter. It is concluded that 4-MTA has the potential to increase the extracellular concentration of NA peripherally as well as centrally, and that this may cause adverse cardiovascular effects in its users.

Distribution of norepinephrine transporters in the non-human primate brain

Noradrenergic terminals in the central nervous system are widespread; as such this system plays a role in varying functions such as stress responses, sympathetic regulation, attention, and memory processing, and its dysregulation has been linked to several pathologies. In particular, the norepinephrine transporter is a target in the brain of many therapeutic and abused drugs. We used the selective ligand [(3)H]nisoxetine, therefore, to describe autoradiographically the normal regional distribution of the norepinephrine transporter in the non-human primate central nervous system, thereby providing a baseline to which alterations due to pathological conditions can be compared. The norepinephrine transporter in the monkey brain was distributed heterogeneously, with highest levels occurring in the locus coeruleus complex and raphe nuclei, and moderate binding density in the hypothalamus, midline thalamic nuclei, bed nucleus of the stria terminalis, central nucleus of the amygdala, and brainstem nuclei such as the dorsal motor nucleus of the vagus and nucleus of the solitary tract. Low levels of binding to the norepinephrine transporter were measured in basolateral amygdala and cortical, hippocampal, and striatal regions. The distribution of the norepinephrine transporter in the non-human primate brain was comparable overall to that described in other species, however disparities exist between the rodent and the monkey in brain regions that play a role in such critical processes as memory and learning. The differences in such areas point to the possibility of important functional differences in noradrenergic information processing across species, and suggest the use of caution in applying findings made in the rodent to the human condition.

Mechanisms of neurotransmitter release by amphetamines: a review

Amphetamine and substituted amphetamines, including methamphetamine, methylphenidate (Ritalin), methylenedioxymethamphetamine (ecstasy), and the herbs khat and ephedra, encompass the only widely administered class of drugs that predominantly release neurotransmitter, in this case principally catecholamines, by a non-exocytic mechanism. These drugs play important medicinal and social roles in many cultures, exert profound effects on mental function and behavior, and can produce neurodegeneration and addiction. Numerous questions remain regarding the unusual molecular mechanisms by which these compounds induce catecholamine release. We review current issues on the two apparent primary mechanisms--the redistribution of catecholamines from synaptic vesicles to the cytosol, and induction of reverse transport of transmitter through plasma membrane uptake carriers--and on additional drug effects that affect extracellular catecholamine levels, including uptake inhibition, effects on exocytosis, neurotransmitter synthesis, and metabolism.

Mesolimbic dopamine super-sensitivity in melanin-concentrating hormone-1 receptor-deficient mice

Melanin-concentrating hormone (MCH) neurons and MCH-1 receptors (MCH1r) densely populate mesolimbic dopaminergic brain regions such as the nucleus accumbens (NAc). The regulation of dopamine by MCH1r was suggested to be an important mechanism underlying the hyperactive phenotype of MCH1r knock-out (ko) mice. However, MCH1r modulation of monoamine neurotransmission has yet to be examined. We tested whether dopamine, norepinephrine, and serotonin function is dysregulated in MCH1r ko and wild-type (wt) mice. MCH1r ko mice exhibited robust hyperactivity in a novel or familiar environment and were super-sensitive to the locomotor activating effects of d-amphetamine and the D1 agonist 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benazepine HCl. The D2 agonist, quinpirole, decreased locomotion similarly in both ko and wt mice. Tissue contents of dopamine within the NAc and caudate-putamen were not significantly different in ko compared with wt mice. Basal and amphetamine-evoked NAc dopamine, norepinephrine, and serotonin efflux, as measured using in vivo microdialysis, were not significantly different between genotypes. In contrast, D1-like and D2-like receptor binding were significantly higher within the olfactory tubercle, ventral tegmental area, and NAc core and shell of ko mice. Norepinephrine transporter (NET) binding was significantly elevated within the NAc shell and globus pallidus of ko mice, whereas serotonin transporter binding was decreased in the NAc shell. Thus, deletion of MCH1r results in an upregulation of mesolimbic dopamine receptors and NET, indicating that MCH1r may negatively modulate mesolimbic monoamine function. MCH1r may be an important therapeutic target for neuropsychiatric disorders involving dysregulation of limbic monoamine systems.

Regulation of dopamine transporter function and plasma membrane expression by dopamine, amphetamine, and cocaine

Pharmacological alterations in dopamine transporter (DAT) function not only modulate dopamine reuptake, but they can induce rapid changes in the plasmalemmal expression of the transporter. By modifying transporter membrane expression, drugs may alter the maximum rate of neurotransmitter clearance, shifting cellular transport capacity and disrupting normal receptor stimulation. DAT-interacting drugs include the illicit and highly abused psychostimulants amphetamine and cocaine. Regulation of transporter activity and plasma membrane expression by these drugs has been implicated in the long-term processes of reward and addiction. This review summarizes the regulation of DAT by transporter substrates and blockers with particular emphasis on the modulation of DAT cell surface expression by acute exposure to amphetamine and cocaine.

Dopamine transporter: basic science and human variation of a key molecule for dopaminergic function, locomotion, and parkinsonism

We review the basic science of the dopamine transporter (DAT), a key neurotransmitter for locomotor control and reward systems, including those lost or deranged in Parkinson's disease (PD). Physiology, pharmaceutical features, expression, cDNA, protein structure/function relationships, and phosphorylation and regulation are discussed. The localization of DAT provides the best marker for the integrity of just the pre-synaptic dopaminergic systems that are most affected in PD. Its function is key for the actions of several toxins that provide some of the best current models for idiopathic parkinsonism, and its variation can clearly alter movement. The wealth of information about this interesting molecule that has been developed over the last 12 years has led to increased interest in DAT among workers interested in both normal and abnormal movement.

Transporter-mediated actions of R-(−)-1-(benzofuran-2-yl)-2-propylaminopentane

R-(-)-1-(Benzofuran-2-yl)-2-propylaminopentane [(-)-BPAP] is a catecholaminergic and serotonergic activity enhancer that increases impulse-evoked catecholamine and serotonin release from nerve terminals, and is a candidate for symptomatic treatment of early Parkinson's disease. We now report the catecholamine and serotonin transporter-mediated actions of (-)-BPAP. The effects of (-)-BPAP on inhibition of neurotransmitter uptake and radioligand binding were assessed using human embryonic kidney 293 cells (HEK 293 cells) expressing cDNA for the human dopamine transporter (hDAT), norepinephrine transporter (hNET), and serotonin transporter (hSERT). The IC(50) values for the effects of (-)-BPAP on [3H]dopamine, [3H]norepinephrine, and [3H]serotonin uptake were 42+/-9, 52+/-19, and 640+/-120 nM, respectively. The IC(50) values for the effects of (-)-BPAP on [125I]3 beta-(4-iodophenyl)tropane-2 beta-carboxylic acid methyl ester ([125I]RTI-55) binding to hDAT, hNET, and hSERT were 16+/-2, 211+/-61, and 638+/-63 nM, respectively. The effects of (-)-BPAP on spontaneous and tyramine-induced norepinephrine and dopamine release from rat brain synaptosomes using a superfusion system were also assessed. Tyramine but not (-)-BPAP potentiated norepinephrine release. Furthermore, (-)-BPAP inhibited tyramine-induced norepinephrine release. Thus, (-)-BPAP may block tyramine-induced adverse effects such as hypertensive crisis. The actions of (-)-BPAP on the spontaneous and tyramine-induced dopamine release resembled its effects on norepinephrine release. We conclude that (-)-BPAP is not only catecholaminergic and serotonergic activity enhancer, but also a norepinephrine and dopamine uptake inhibitor and a weak serotonin uptake inhibitor that does not possess a tyramine-like action on catecholamine release, and is an inhibitor of tyramine-induced release of norepinephrine.

Differences in the mechanisms that increase noradrenaline efflux after administration of d-amphetamine: a dual-probe microdialysis study in rat frontal cortex and hypothalamus

1. The extent to which impulse-independent release of noradrenaline and/or inhibition of its reuptake contribute to the response to d-amphetamine in vivo is unclear. Here, dual-probe microdialysis was used to investigate this question in the rat frontal cortex and hypothalamus. 2. After systemic administration of d-amphetamine (10 mg kg(-1)), or its local infusion (10 micro M), the increase in noradrenaline efflux in the hypothalamus was greater than in the frontal cortex. 3. In contrast, during local infusion of the noradrenaline reuptake inhibitor, BTS 54 354 (50 micro M), the noradrenaline response was similar in the frontal cortex and hypothalamus, even after systemic administration of the alpha(2)-antagonist, atipamezole, to block presynaptic inhibition of transmitter release and neuronal firing. 4. In the frontal cortex, but not the hypothalamus, the noradrenaline response to 10 micro M d-amphetamine was constrained by activation of alpha(2)-adrenoceptors. This suggests that, at this concentration, inhibition of reuptake of noradrenaline, following its impulse-dependent release, is evident in the frontal cortex, but that the noradrenaline response in the hypothalamus derives mostly from impulse-independent release (retrotransport). 5. Atipamezole did not affect the noradrenaline response to 100 micro M d-amphetamine in either brain region possibly because, at this higher concentration, retrotransport of noradrenaline masks any compensatory reduction in impulse-evoked release. 6. It is concluded that inhibition of reuptake and retrotransport make different contributions to the noradrenaline response to d-amphetamine in the frontal cortex and hypothalamus and that retrotransport increases with the concentration of d-amphetamine.

Characterization of a functional bacterial homologue of sodium-dependent neurotransmitter transporters

The tnaT gene of Symbiobacterium thermophilum encodes a protein homologous to sodium-dependent neurotransmitter transporters. Expression of the tnaT gene product in Escherichia coli conferred the ability to accumulate tryptophan from the medium and the ability to grow on tryptophan as a sole source of carbon. Transport was Na(+)-dependent and highly selective. The K(m) for tryptophan was approximately 145 nm, and tryptophan transport was unchanged in the presence of 100 microM concentrations of other amino acids. Tryptamine and serotonin were weak inhibitors with K(I) values of 200 and 440 microM, respectively. By using a T7 promoter-based system, TnaT with an N-terminal His(6) tag was expressed at high levels in the membrane and was purified to near-homogeneity in high yield.

Amphetamine-induced dopamine efflux. A voltage-sensitive and intracellular Na+-dependent mechanism

Amphetamine (AMPH) elicits its behavioral effects by acting on the dopamine (DA) transporter (DAT) to induce DA overflow into the synaptic cleft. Facilitated exchange diffusion is the classical model used to describe AMPH-induced DA efflux. This model hypothesizes that AMPH-induced DA efflux is mediated by DAT and results from the transport of AMPH into the cell followed by a counter movement of DA out to the extracellular compartment. To further characterize the action of AMPH, we used the patch clamp technique in the whole-cell configuration combined with amperometry on human embryonic kidney HEK-293 cells stably transfected with the human DAT (DAT cells). In DAT cells, AMPH-induced DAT-mediated currents were blocked by cocaine. We demonstrate that DA efflux mediated by DAT is voltage-dependent, electrogenic, and dependent on intracellular Na(+) concentration in the recording electrode. Intracellular Na(+) fluorescence, as measured by confocal microscopy using a Na(+)-sensitive dye, was enhanced by AMPH application. Furthermore, the ability of AMPH to induce DA efflux was regulated by intracellular Na(+) concentration and correlated with the size of the DAT-mediated, AMPH-induced ion flux across the plasma membrane. In the absence of intracellular Na(+) but the presence of high intracellular Cl(-), AMPH-induced inward currents elicited DA efflux proportionally to their dimension and duration. Thus, we propose that AMPH-induced DA efflux depends on two correlated transporter processes. First, AMPH binds to the DAT and is transported, thereby causing an inward current. Second, because of this AMPH-induced inward current, Na(+) becomes more available intracellularly to the DAT, thereby enhancing DAT-mediated reverse transport of DA.

Binding and transport in norepinephrine transporters. Real-time, spatially resolved analysis in single cells using a fluorescent substrate

Monoamine transporters, the molecular targets for drugs of abuse and antidepressants, clear norepinephrine, dopamine, or serotonin from the synaptic cleft. Neurotransmitters, amphetamines, and neurotoxins bind before being transported, whereas cocaine and antidepressants bind to block transport. Although binding is crucial to transport, few assays separate binding from transport, nor do they provide adequate temporal or spatial resolution to describe real-time kinetics or localize sites of active uptake. Here, we report a new method that distinguishes substrate binding from substrate transport using single-cell, space-resolved, real-time fluorescence microscopy. For these studies we use a fluorescent analogue of 1-methyl-4-phenylpyridinium, a neurotoxic metabolite and known substrate of monoamine transporters, to assess binding and transport with 50-ms, sub-micron resolution. We show that ASP(+) (4-(4-(dimethylamino)styrl)-N-methylpyridinium) has micromolar potency for the human norepinephrine transporter, that ASP(+) accumulation is Na(+)-, Cl(-)-, cocaine-, and desipramine-sensitive and temperature-dependent, and that ASP(+) competes with norepinephrine uptake. Using this method we demonstrate that norepinephrine transporters are efficient buffers for substrate, with binding rates exceeding transport rates by 100-fold. Furthermore, substrates bind deep within the transporter, isolated from both the bath and the lipid bilayer. Although transport per se depends on Na(+) and Cl(-), binding is independent of Na(+) and actually increases in low Cl(-). We further demonstrate that ASP(+) interacts with transporters not only in transfected cells but in cultured neurons. ASP(+) is also a substrate for dopamine and serotonin transporters and therefore represents a powerful new technique for studying the biophysical properties of monoamine transporters, an approach also amenable to high throughput assays for drug discovery.

Amphetamine enhances Ca2+ entry and catecholamine release via nicotinic receptor activation in bovine adrenal chromaffin cells

Amphetamine, a psychostimulant, has been shown to act as a channel blocker of muscle nicotinic receptors and to induce a Ca(2+)-dependent secretion from adrenal chromaffin cells. In this study, the relationship between amphetamine and nicotinic receptors was studied using bovine adrenal chromaffin cells as a model system. Our results show that D-amphetamine sulfate alone induced an increase in the cytosolic Ca(2+) concentration ([Ca(2+)](c)) and [3H]norepinephrine release in a dose-dependent and extracellular Ca(2+)-dependent manner. Two common nicotinic receptor antagonists, hexamethonium and mecamylamine, suppressed the D-amphetamine sulfate-induced [Ca(2+)](c) rise and [3H]norepinephrine release. In addition, D-amphetamine sulfate inhibited the 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP)-induced [Ca(2+)](c) rise and [3H]norepinephrine release, but not the high K(+)- or veratridine-induced [Ca(2+)](c) increase and [3H]norepinephrine release. Antagonists, including alpha-bungarotoxin and choline, that are more specific for alpha7 nicotinic receptors were capable of inhibiting the D-amphetamine sulfate-induced [Ca(2+)](c) rise, while D-amphetamine sulfate was found to be capable of inhibiting the [Ca(2+)](c) rise induced by the alpha7-nicotinic receptor agonists, epibatidine and choline. Moreover, D-amphetamine sulfate dose-dependently suppressed [3H]nicotine binding to chromaffin cells. We, therefore, conclude that D-amphetamine sulfate acts as a nicotinic receptor agonist to induce [Ca(2+)](c) increase and [3H]norepinephrine release in bovine adrenal chromaffin cells.

Bi-directional transport of GABA in human embryonic kidney (HEK-293) cells stably expressing the rat GABA transporter GAT-1

1. Bi-directional GABA-transport was studied by performing uptake and superfusion experiments in human embryonic kidney 293 cells stably expressing the rat GABA transporter rGAT-1. 2. K(M) and V(max) values for [(3)H]-GABA uptake were 11.7+/-1.8 microM and 403+/-55 pmol min(-1) 10(-6) cells (n=9), respectively. 3. Kinetic analysis of outward transport was performed by pre-labelling the cells with increasing concentrations of [(3)H]-GABA and triggering outward transport with 333 microM GABA. Approximate apparent K(M) and V(max) values were 12 mM and 50 pmol min(-1) 10(-6) cells, respectively. 4. GABA re-uptake inhibitors (RI; e.g. tiagabine), as well as, substrates of the rGAT-1 (e.g. GABA, nipecotic acid) concentration dependently decreased [(3)H]-GABA uptake and increased efflux of [(3)H]-GABA from pre-labelled cells. The IC(50) values for inhibiting uptake and the EC(50) values for increasing efflux were significantly correlated (r(2)=0.99). 5. On superfusion, RI antagonized the efflux-enhancing effect of the substrates. The effect of the latter was markedly augmented in the presence of ouabain (100 microM), whereas the effect of RI remained unchanged. The most likely explanation for the release enhancing effect of RI is interruption of ongoing re-uptake. 6. The structural GABA-analogue 2,4-diamino-n-butyric acid (DABA) exhibited a bell-shaped concentration response curve on [(3)H]-GABA efflux with the maximum at 1 mM, and displayed a deviation from the sigmoidal inhibition curve in uptake experiments in the same concentration range. At concentrations below 1 mM, DABA inhibited [(3)H]-GABA uptake non-competitively, while at 1 mM and above the inhibition of uptake followed a competitive manner. 7. The results provide information of GABA inward and outward transport, and document a complex interaction of the rGAT-1 with its substrate DABA.