Differential effects of amphetamine transport vs. dopamine reverse transport on particulate PKC activity in striatal synaptoneurosomes

Overview of the structure and function of the dopamine transporter and its protein interactions

The dopamine transporter (DAT) plays an integral role in dopamine neurotransmission through the clearance of dopamine from the extracellular space. Dysregulation of DAT is central to the pathophysiology of numerous neuropsychiatric disorders and as such is an attractive therapeutic target. DAT belongs to the solute carrier family 6 (SLC6) class of Na⁺/Cl^- dependent transporters that move various cargo into neurons against their concentration gradient. This review focuses on DAT (SCL6A3 protein) while extending the narrative to the closely related transporters for serotonin and norepinephrine where needed for comparison or functional relevance. Cloning and site-directed mutagenesis experiments provided early structural knowledge of DAT but our contemporary understanding was achieved through a combination of crystallization of the related bacterial transporter LeuT, homology modeling, and subsequently the crystallization of drosophila DAT. These seminal findings enabled a better understanding of the conformational states involved in the transport of substrate, subsequently aiding state-specific drug design. Post-translational modifications to DAT such as phosphorylation, palmitoylation, ubiquitination also influence the plasma membrane localization and kinetics. Substrates and drugs can interact with multiple sites within DAT including the primary S1 and S2 sites involved in dopamine binding and novel allosteric sites. Major research has centered around the question what determines the substrate and inhibitor selectivity of DAT in comparison to serotonin and norepinephrine transporters. DAT has been implicated in many neurological disorders and may play a role in the pathology of HIV and Parkinson's disease via direct physical interaction with HIV-1 Tat and α-synuclein proteins respectively.

Neuroprotection in late life attention-deficit/hyperactivity disorder: A review of pharmacotherapy and phenotype across the lifespan

For decades, psychostimulants have been the gold standard pharmaceutical treatment for attention-deficit/hyperactivity disorder (ADHD). In the United States, an astounding 9% of all boys and 4% of girls will be prescribed stimulant drugs at some point during their childhood. Recent meta-analyses have revealed that individuals with ADHD have reduced brain volume loss later in life (>60 y.o.) compared to the normal aging brain, which suggests that either ADHD or its treatment may be neuroprotective. Crucially, these neuroprotective effects were significant in brain regions (e.g., hippocampus, amygdala) where severe volume loss is linked to cognitive impairment and Alzheimer's disease. Historically, the ADHD diagnosis and its pharmacotherapy came about nearly simultaneously, making it difficult to evaluate their effects in isolation. Certain evidence suggests that psychostimulants may normalize structural brain changes typically observed in the ADHD brain. If ADHD itself is neuroprotective, perhaps exercising the brain, then psychostimulants may not be recommended across the lifespan. Alternatively, if stimulant drugs are neuroprotective, then this class of medications may warrant further investigation for their therapeutic effects. Here, we take a bottom-up holistic approach to review the psychopharmacology of ADHD in the context of recent models of attention. We suggest that future studies are greatly needed to better appreciate the interactions amongst an ADHD diagnosis, stimulant treatment across the lifespan, and structure-function alterations in the aging brain.

Methcathinone decreases dopamine transporter function: Role of protein kinase C

Methcathinone (MCAT) is a psychostimulant of abuse that can cause both persistent striatal dopaminergic and serotonergic, as well as hippocampal serotonergic, deficits. Evidence suggests that the rapid effects of stimulants that are structurally and mechanistically similar to MCAT on monoamine transporter function may contribute to the abuse liability and/or persistent monoaminergic deficits caused by these agents. Thus, effects of MCAT on 1) striatal dopamine (DA) transporter (DAT); and 2) striatal and hippocampal serotonin transporter (SERT) function, as determined in tissues from adult male rats, were assessed. As reported previously, a single administration of MCAT rapidly (within 1 hr) decreases striatal [³ H]DA uptake. Similarly, incubation of rat synaptosomes with MCAT at 37℃ (but not 4˚C) decreased striatal [³ H]DA uptake. Incubation with MCAT likewise decreased [³ H]5HT but not vesicular [³ H]DA uptake. MCAT incubation in vitro was without effect on [³ H]DA uptake in striatal synaptosomes prepared from MCAT-treated rats. The decrease in [³ H]DA uptake caused by MCAT incubation: (a) reflected a decrease in V_max , with minimal change in K_m , and (b) was attenuated by co-incubation with the cell-permeable calcium chelator, N,N'-[1,2-ethanediylbis(oxy-2,1-phenylene)]bis[N-[2-[(acetyloxy)methoxy]-2-oxoethyl]-1,1'-bis[(acetyloxy)methyl] ester-glycine (BAPTA-AM), as well as the non-selective protein kinase-C (PKC) inhibitors bisindolylmaleimide-1 (BIM-1) and 2-[1-3(Aminopropyl)indol-3-yl]-3(1-methyl-1H-indol-3-yl)maleimide (or Bisindolylmaleimide VIII; Ro-31-7549). Taken together, these results suggest that in vitro MCAT incubation may model important aspects of MCAT administration in vivo, and that calcium and PKC contribute to the in vitro effects of MCAT on DAT.

Ruboxistaurin Reduces Cocaine-Stimulated Increases in Extracellular Dopamine by Modifying Dopamine-Autoreceptor Activity

Cocaine is a highly abused drug, and cocaine addiction affects millions of individuals worldwide. Cocaine blocks normal uptake function at the dopamine transporter (DAT), thus increasing extracellular dopamine. Currently, no chemical therapies are available to treat cocaine abuse. Previous works showed that the selective inhibitors of protein kinase Cβ (PKCβ), enzastaurin and ruboxistaurin, attenuate dopamine overflow and locomotion stimulated by another psychostimulant drug, amphetamine. We now test if ruboxistaurin similarly affects cocaine action. Perfusion of 1 μM ruboxistaurin directly into the core of the nucleus accumbens via retrodialysis reduced cocaine-stimulated increases in dopamine overflow, measured using microdialysis sampling, with simultaneous reductions in locomotor behavior. Because cocaine activity is highly regulated by dopamine autoreceptors, we examined whether ruboxistaurin was acting at the level of the D2 autoreceptor. Perfusion of 5 μM raclopride, a selective D2-like receptor antagonist, before addition of ruboxistaurin, abrogated the effect of ruboxistaurin on cocaine-stimulated dopamine overflow and hyperlocomotion. Further, ruboxistaurin was inactive against cocaine-stimulated locomotor activity in mice with a genetic deletion in D2 receptors as compared to wild-type mice. In contrast, blockade or deletion of dopamine D2 receptors did not abolish the attenuating effect of ruboxistaurin on amphetamine-stimulated activities. Therefore, the inhibition of PKCβ reduces dopamine overflow and locomotor activity stimulated by both cocaine and amphetamine, but the mechanism of action differs for each stimulant. These data suggest that inhibition of PKCβ would serve as a target to reduce the abuse of either amphetamine or cocaine.

Effects of MDPV on dopamine transporter regulation in male rats. Comparison with cocaine

RATIONALE

MDPV (3,4-methylenedioxypyrovalerone) is a synthetic cathinone present in bath salts. It is a powerful psychostimulant and blocker of the dopamine transporter (DAT), like cocaine. It is known that acute exposure to psychostimulants induces rapid changes in DAT function.

OBJECTIVES

To investigate the effects of MDPV on DAT function comparing with cocaine.

METHODS

Binding of [³H]WIN 35428 was performed on PC 12 cells treated with MDPV and washed. Rat striatal synaptosomes were incubated with MDPV or cocaine (1 μM) for 1 h and [³H]dopamine (DA) uptake was performed. Also, different treatments with MDPV or cocaine were performed in Sprague-Dawley rats to assess locomotor activity and ex vivo [³H]DA uptake.

RESULTS

MDPV increased surface [³H]WIN 35428 binding on PC 12 cells. In vitro incubation of synaptosomes with MDPV produced significant increases in V_max and K_M for [³H]DA uptake. In synaptosomes from MDPV- (1.5 mg/kg, s.c.) and cocaine- (30 mg/kg, i.p.) treated rats, there was a significantly higher and more persistent increase in [³H]DA uptake in the case of MDPV than cocaine. Repeated doses of MDPV developed tolerance to this DAT upregulation and 24 h after the 5-day treatment with MDPV, [³H]DA uptake was reduced. However, a challenge with the same drugs after withdrawal recovered the DAT upregulation by both drugs and showed an increased response to MDPV vs the first dose. At the same time, animals were sensitized to the stereotypies induced by both psychostimulants.

CONCLUSIONS

MDPV induces a rapid and reversible functional upregulation of DAT more powerfully and lasting than cocaine.

Molecular Mechanisms of Amphetamines

There is a plethora of amphetamine derivatives exerting stimulant, euphoric, anti-fatigue, and hallucinogenic effects; all structural properties allowing these effects are contained within the amphetamine structure. In the first part of this review, the interaction of amphetamine with the dopamine transporter (DAT), crucially involved in its behavioral effects, is covered, as well as the role of dopamine synthesis, the vesicular monoamine transporter VMAT2, and organic cation 3 transporter (OCT3). The second part deals with requirements in amphetamine's effect on the kinases PKC, CaMKII, and ERK, whereas the third part focuses on where we are in developing anti-amphetamine therapeutics. Thus, treatments are discussed that target DAT, VMAT2, PKC, CaMKII, and OCT3. As is generally true for the development of therapeutics for substance use disorder, there are multiple preclinically promising specific compounds against (meth)amphetamine, for which further development and clinical trials are badly needed.

Phosphorylation of the Amino Terminus of the Dopamine Transporter: Regulatory Mechanisms and Implications for Amphetamine Action

Amphetamines (AMPHs) are potent psychostimulants that are widely used and abused, with profound medical and societal impact. Their actions at dopaminergic neurons are thought to mediate their therapeutic efficacy as well as their liability for abuse and dependence. AMPHs target the dopamine transporter (DAT), the plasmalemmal membrane protein that mediates the inactivation of released dopamine (DA) through its reuptake. AMPHs act as substrates for DAT and are known to cause mobilization of dopamine (DA) to the cell exterior via DAT-mediated reverse transport (efflux). It has become increasingly evident that the mechanisms that regulate AMPH-induced DA efflux are distinct from those that regulate DA uptake. Central to these mechanisms is the phosphorylation of the DAT amino (N)-terminus, which has been repeatedly demonstrated to facilitate DAT-mediated DA efflux, without impacting other aspects of DAT physiology. This review aims to summarize the current status of knowledge regarding DAT N-terminal phosphorylation and its regulation by protein modulators and the membrane microenvironment. A better understanding of these mechanisms may lead to the identification of novel therapeutic approaches that interfere selectively with the pharmacological effects of AMPHs without altering the physiological function of DAT.

PKCβ Inhibitors Attenuate Amphetamine-Stimulated Dopamine Efflux

Amphetamine abuse afflicts over 13 million people, and there is currently no universally accepted treatment for amphetamine addiction. Amphetamine serves as a substrate for the dopamine transporter and reverses the transporter to cause an increase in extracellular dopamine. Activation of the beta subunit of protein kinase C (PKCβ) enhances extracellular dopamine in the presence of amphetamine by facilitating the reverse transport of dopamine and internalizing the D2 autoreceptor. We previously demonstrated that PKCβ inhibitors block amphetamine-stimulated dopamine efflux in synaptosomes from rat striatum in vitro. In this study, we utilized in vivo microdialysis in live, behaving rats to assess the effect of the PKCβ inhibitors, enzastaurin and ruboxistaurin, on amphetamine-stimulated locomotion and increases in monoamines and their metabolites. A 30 min perfusion of the nucleus accumbens core with 1 μM enzastaurin or 1 μM ruboxistaurin reduced efflux of dopamine and its metabolite 3-methoxytyramine induced by amphetamine by approximately 50%. The inhibitors also significantly reduced amphetamine-stimulated extracellular levels of norepinephrine. The stimulation of locomotor behavior by amphetamine, measured simultaneously with the analytes, was comparably reduced by the PKCβ inhibitors. Using a stable isotope label retrodialysis procedure, we determined that ruboxistaurin had no effect on basal levels of dopamine, norepinephrine, glutamate, or GABA. In addition, normal uptake function through the dopamine transporter was unaltered by the PKCβ inhibitors, as measured in rat synaptosomes. Our results support the utility of using PKCβ inhibitors to reduce the effects of amphetamine.

Spontaneous inward opening of the dopamine transporter is triggered by PIP2-regulated dynamics of the N-terminus

We present the dynamic mechanism of concerted motions in a full-length molecular model of the human dopamine transporter (hDAT), a member of the neurotransmitter/sodium symporter (NSS) family, involved in state-to-state transitions underlying function. The findings result from an analysis of unbiased atomistic molecular dynamics simulation trajectories (totaling >14 μs) of the hDAT molecule immersed in lipid membrane environments with or without phosphatidylinositol 4,5-biphosphate (PIP₂) lipids. The N-terminal region of hDAT (N-term) is shown to have an essential mechanistic role in correlated rearrangements of specific structural motifs relevant to state-to-state transitions in the hDAT. The mechanism involves PIP₂-mediated electrostatic interactions between the N-term and the intracellular loops of the transporter molecule. Quantitative analyses of collective motions in the trajectories reveal that these interactions correlate with the inward-opening dynamics of hDAT and are allosterically coupled to the known functional sites of the transporter. The observed large-scale motions are enabled by specific reconfiguration of the network of ionic interactions at the intracellular end of the protein. The isomerization to the inward-facing state in hDAT is accompanied by concomitant movements in the extracellular vestibule and results in the release of an Na⁺ ion from the Na2 site and destabilization of the substrate dopamine in the primary substrate binding S1 site. The dynamic mechanism emerging from the findings highlights the involvement of the PIP₂-regulated interactions between the N-term and the intracellular loop 4 in the functionally relevant conformational transitions that are also similar to those found to underlie state-to-state transitions in the leucine transporter (LeuT), a prototypical bacterial homologue of the NSS.

Computational modeling of the N-terminus of the human dopamine transporter and its interaction with PIP2 -containing membranes

The dopamine transporter (DAT) is a transmembrane protein belonging to the family of neurotransmitter:sodium symporters (NSS). Members of the NSS are responsible for the clearance of neurotransmitters from the synaptic cleft, and for their translocation back into the presynaptic nerve terminal. The DAT contains long intracellular N- and C-terminal domains that are strongly implicated in the transporter function. The N-terminus (N-term), in particular, regulates the reverse transport (efflux) of the substrate through DAT. Currently, the molecular mechanisms of the efflux remain elusive in large part due to lack of structural information on the N-terminal segment. Here we report a computational model of the N-term of the human DAT (hDAT), obtained through an ab initio structure prediction, in combination with extensive atomistic molecular dynamics (MD) simulations in the context of a lipid membrane. Our analysis reveals that whereas the N-term is a highly dynamic domain, it contains secondary structure elements that remain stable in the long MD trajectories of interactions with the bilayer (totaling >2.2 μs). Combining MD simulations with continuum mean-field modeling we found that the N-term engages with lipid membranes through electrostatic interactions with the charged lipids PIP2 (phosphatidylinositol 4,5-Biphosphate) or PS (phosphatidylserine) that are present in these bilayers. We identify specific motifs along the N-term implicated in such interactions and show that differential modes of N-term/membrane association result in differential positioning of the structured segments on the membrane surface. These results will inform future structure-based studies that will elucidate the mechanistic role of the N-term in DAT function.

Dopamine transporter phosphorylation site threonine 53 regulates substrate reuptake and amphetamine-stimulated efflux

In the central nervous system, levels of extraneuronal dopamine are controlled primarily by the action of the dopamine transporter (DAT). Multiple signaling pathways regulate transport activity, substrate efflux, and other DAT functions through currently unknown mechanisms. DAT is phosphorylated by protein kinase C within a serine cluster at the distal end of the cytoplasmic N terminus, whereas recent work in model cells revealed proline-directed phosphorylation of rat DAT at membrane-proximal residue Thr(53). In this report, we use mass spectrometry and a newly developed phospho-specific antibody to positively identify DAT phosphorylation at Thr(53) in rodent striatal tissue and heterologous expression systems. Basal phosphorylation of Thr(53) occurred with a stoichiometry of ~50% and was strongly increased by phorbol esters and protein phosphatase inhibitors, demonstrating modulation of the site by signaling pathways that impact DAT activity. Mutations of Thr(53) to prevent phosphorylation led to reduced dopamine transport V(max) and total apparent loss of amphetamine-stimulated substrate efflux, supporting a major role for this residue in the transport kinetic mechanism.

Knocking down the transcript of protein kinase C-lambda modulates hypothalamic glutathione peroxidase, melanocortin receptor and neuropeptide Y gene expression in amphetamine-treated rats

It has been reported that neuropeptide Y (NPY) contributes to the behavioral response of amphetamine (AMPH), a psychostimulant. The present study examined whether protein kinase C (PKC)-λ signaling was involved in this action. Moreover, possible roles of glutathione peroxidase (GP) and melanocortin receptor 4 (MC4R) were also examined. Rats were given AMPH daily for 4 days. Hypothalamic NPY, PKCλ, GP and MC4R were determined and compared. Pretreatment with α-methyl-para-tyrosine could block AMPH-induced anorexia, revealing that endogenous catecholamine was involved in regulating AMPH anorexia. PKCλ, GP and MC4R were increased with maximal response on Day 2 during AMPH treatment, which were concomitant with the decreases in NPY. cAMP response element binding protein (CREB) DNA binding activity was increased during AMPH treatment, revealing the involvement of CREB-dependent gene transcription. An interruption of cerebral PKCλ transcript could partly block AMPH-induced anorexia and partly reverse NPY, MC4R and GP mRNA levels to normal. These results suggest that PKCλ participates in regulating AMPH-induced anorexia via a modulation of hypothalamic NPY gene expression and that increases of GP and MC4R may contribute to this modulation. Our results provided molecular evidence for the regulation of AMPH-induced behavioral response.

Neuronal Nicotinic Receptors as New Targets for Amphetamine-Induced Oxidative Damage and Neurotoxicity

Amphetamine derivatives such as methamphetamine (METH) and 3,4-methylenedioxymethamphetamine (MDMA, “ecstasy”) are widely abused drugs in a recreational context. This has led to concern because of the evidence that they are neurotoxic in animal models and cognitive impairments have been described in heavy abusers. The main targets of these drugs are plasmalemmal and vesicular monoamine transporters, leading to reverse transport and increased monoamine efflux to the synapse. As far as neurotoxicity is concerned, increased reactive oxygen species (ROS) production seems to be one of the main causes. Recent research has demonstrated that blockade of α7 nicotinic acetylcholine receptors (nAChR) inhibits METH- and MDMA-induced ROS production in striatal synaptosomes which is dependent on calcium and on NO-synthase activation. Moreover, α7 nAChR antagonists (methyllycaconitine and memantine) attenuated in vivo the neurotoxicity induced by METH and MDMA, and memantine prevented the cognitive impairment induced by these drugs. Radioligand binding experiments demonstrated that both drugs have affinity to α7 and heteromeric nAChR, with MDMA showing lower K_i values, while fluorescence calcium experiments indicated that MDMA behaves as a partial agonist on α7 and as an antagonist on heteromeric nAChR. Sustained Ca increase led to calpain and caspase-3 activation. In addition, modulatory effects of MDMA on α7 and heteromeric nAChR populations have been found.

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.

Pharmacological and neurotoxicological actions mediated by bupropion and diethylpropion

The antiappetite agent diethylpropion (DEP), and the antidepressant and antismoking aid compound bupropion (BP), not only share the same structural motif but also present similar mechanisms of action in the CNS. For example, both drugs induce the release as well as inhibit the reuptake of neurotransmitters such as a dopamine (DA) and norepinephrine (NE). In general, they produce mild side effects, including reversible psychomotor alterations mostly in geriatric patients (by BP), or moderate changes in neurotransmitter contents linked to oxidative damage (by DEP). Therefore, attention must be paid during any therapeutic use of these agents. Regarding the interaction of BP with the DA transporter, residues S359, located in the middle of TM7, and A279, located close to the extracellular end of TM5, contribute to the binding and blockade of translocation mediated by BP, respectively. Additional mechanisms of action have also been determined for each compound. For example, BP is a noncompetitive antagonist (NCA) of several nicotinic acetylcholine receptors (AChRs). Based on this evidence, the dual antidepressant and antinicotinic activity of BP is currently considered to be mediated by its stimulatory action on DA and NE systems as well as its inhibitory action on AChRs. Considering the results obtained in the archetypical mouse muscle AChR, a sequential mechanism can be hypothesized to explain the inhibitory action of BP on neuronal AChRs: (1) BP first binds to AChRs in the resting state, decreasing the probability of ion channel opening, (2) the remnant fraction of open ion channels is subsequently decreased by accelerating the desensitization process, and finally (3) BP interacts with a binding domain located between the serine (position 9') and valine (position 13') rings that is shared with the NCA phencyclidine and other tricyclic antidepressants. The homologous location in the alpha3beta4 AChR is between the serine and valine/phenylalanine rings. This new evidence opens a window for further investigation using AChRs as targets for the action of safer antidepressants and novel antiaddictive compounds.

Involvement of nicotinic receptors in methamphetamine- and MDMA-induced neurotoxicity: pharmacological implications

During the last years, we have focused on the study of the neurotoxic effects of 3,4-methylenedioxymethamphetamine (MDMA) and methamphetamine (METH) on the central nervous system (CNS) and their pharmacological prevention methods. In the process of this research, we have used a semipurified synaptosomal preparation from striatum of mice or rats as a reliable in vitro model to study reactive oxygen species (ROS) production by these amphetamine derivatives, which is well-correlated with their dopaminergic injury in in vivo models. Using this preparation, we have demonstrated that blockade of alpha7 nicotinic receptors with methyllycaconitine (MLA) prevents ROS production induced by MDMA and METH. Consequently, in vivo, MLA significantly prevents MDMA- and METH-induced neurotoxicity at dopaminergic level (mouse striatum), without affecting hyperthermia induced by these amphetamines. Additionally, when neuroprotection was assayed with memantine (MEM), a dual antagonist of NMDA and alpha7 receptors, an effective neuroprotection was obtained also ahead of serotonergic injury induced by MDMA in rats. MEM also prevents MDMA effect on serotonin transporter functionality and METH effect on dopamine transporter (DAT), suggesting that behavioral effects of these psychostimulants can also be modulated by MEM. Finally, we have demonstrated that MEM prevents the impaired memory function induced by MDMA, and also, using binding studies with radioligands, we have characterized the interaction of these substances with nicotinic receptors. Studies at molecular level showed that both MDMA and METH displaced competitively the binding of radioligands with homomeric alpha7 and heteromeric nicotinic acetylcholine receptors (nAChRs), indicating that they can directly interact with them. In all the cases, MDMA displayed higher affinity than METH and it was higher for heteromeric than for alpha7 subtype. Pre-incubation of differentiated PC12 cells with MDMA or METH induces nAChR upregulation in a concentration- and time-dependent manner, as many nicotinic ligands do, supporting their functional interaction with nAChRs. Such interaction expands the pharmacological profile of amphetamines and can account for some of their effects.

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.

Repeated administration of d-amphetamine induces loss of [123I]FP-CIT binding to striatal dopamine transporters in rat brain: a validation study

In recent years, several PET and SPECT studies have shown loss of striatal dopamine transporter (DAT) binding in amphetamine (AMPH) users. However, the use of DAT SPECT tracers to detect AMPH-induced changes in DAT binding has not been validated. We therefore examined if repeated administration of D-AMPH or methamphetamine (METH) may induce loss of binding to striatal DATs in rats by using an experimental biodistribution study design and a SPECT tracer for the DAT ([123I]FP-CIT).

METHODS

Groups of male rats (n = 10 per group) were treated with D-AMPH (10 mg/kg body weight), METH (10 mg/kg body weight), or saline, twice a day for 5 consecutive days. Five days later, [123I]FP-CIT was injected intravenously, and 2 h later, the rats were sacrificed and radioactivity was assayed.

RESULTS

In d-AMPH but not METH-treated rats, striatal [123I]FP-CIT uptake was significantly lower (approximately 17%) than in the control group.

CONCLUSION

These data show that [123I]FP-CIT can be used to detect AMPH-induced changes in DAT binding and may validate the use of DAT radiotracers to study AMPH-induced changes in striatal DAT binding in vivo.

Repeated Amphetamine Couples Norepinephrine Transporter and Calcium Channel Activities in PC12 Cells

Repeated intermittent amphetamine enhances efflux of dopamine through the dopamine transporter in rat basal ganglia and through the norepinephrine transporter in rat pheochromocytoma PC12 cells. Extracellular Ca2+ is required for the detection of this enhancement in the rat. In this study, we examined the role of Ca2+ and Ca2+ channels in the enhanced amphetamine-induced dopamine efflux that develops in PC12 cells following repeated intermittent amphetamine. Repeated pretreatment of PC12 cells with 1 microM amphetamine followed by a drug-free period increased amphetamine-induced efflux of dopamine compared with controls. The enhancement in amphetamine-induced dopamine efflux depended upon the presence of extracellular Ca2+ and was inhibited by the blockade of N-type and L-type Ca2+ channels. The enhanced dopamine efflux was not altered by tetanus toxin or reserpine, treatments that abrogate synaptic vesicle-mediated, exocytotic dopamine efflux. Measurement of intracellular Ca2+ concentrations using fura-2/acetoxymethyl ester revealed that amphetamine increased intracellular Ca2+ by a transporter-dependent mechanism. In amphetamine-pretreated cells, amphetamine elicited a greater increase in intracellular Ca2+; this increase depended upon the presence of extracellular Ca2+ and N- and L-type Ca2+ channel activity. The enhanced amphetamine-induced dopamine efflux requires Ca2+/calmodulin kinase activity. In vehicle-treated cells, 1 microM amphetamine inhibited the calmodulin kinase activity although it did not in amphetamine-pretreated cells. This study suggests that repeated intermittent amphetamine couples norepinephrine transporter activity and Ca2+ signaling.

Intracellular Ca2+ regulates amphetamine-induced dopamine efflux and currents mediated by the human dopamine transporter

Although it is clear that amphetamine-induced dopamine (DA) release mediated by the dopamine transporter (DAT) is integral to the behavioral actions of this psychostimulant, the mechanism of this release is not clear. In this study, we explored the requirement for intracellular Ca(2+) in amphetamine-induced DA efflux and currents mediated by the human DAT. The patch-clamp technique in the whole-cell configuration was used on Na(+) and DA-preloaded human embryonic kidney 293 cells stably transfected with the human DAT (hDAT cells). Chelation of intracellular Ca(2+) by inclusion of 50 microM BAPTA in the whole-cell pipette reduced the voltage-dependent amphetamine-induced hDAT current, with the greatest effect seen at positive voltages. Likewise, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) reduced amphetamine-induced DA efflux as measured by amperometry. Furthermore, preincubation of the cells with 50 microM BAPTA acetoxy methyl ester (AM) or thapsigargin also blocked amphetamine-induced release of preloaded N-methyl-4-[(3)H]phenylpyridinium from superfused hDAT cells. BAPTA-AM also reduced DA release from striatal synaptosomes. Amphetamine also led to an increase in intracellular Ca(2+) that was blocked by prior treatment with 5 microM thapsigargin or 10 microM cocaine. These studies demonstrate that amphetamine-induced DAT-mediated currents and substrate efflux require internal Ca(2+) and that amphetamine can stimulate dopamine efflux by regulating cytoplasmic Ca(2+) levels through its interaction with DAT.

N-terminal phosphorylation of the dopamine transporter is required for amphetamine-induced efflux

Amphetamine (AMPH) elicits its behavioral effects by acting on the dopamine (DA) transporter (DAT) to induce DA efflux into the synaptic cleft. We previously demonstrated that a human DAT construct in which the first 22 amino acids were truncated was not phosphorylated by activation of protein kinase C, in contrast to wild-type (WT) DAT, which was phosphorylated. Nonetheless, in all functions tested to date, which include uptake, inhibitor binding, oligomerization, and redistribution away from the cell surface in response to protein kinase C activation, the truncated DAT was indistinguishable from the full-length WT DAT. Here, however, we show that in HEK-293 cells stably expressing an N-terminal-truncated DAT (del-22 DAT), AMPH-induced DA efflux is reduced by approximately 80%, whether measured by superfusion of a population of cells or by amperometry combined with the patch-clamp technique in the whole cell configuration. We further demonstrate in a full-length DAT construct that simultaneous mutation of the five N-terminal serine residues to alanine (S/A) produces the same phenotype as del-22-normal uptake but dramatically impaired efflux. In contrast, simultaneous mutation of these same five serines to aspartate (S/D) to simulate phosphorylation results in normal AMPH-induced DA efflux and uptake. In the S/A background, the single mutation to Asp of residue 7 or residue 12 restored a significant fraction of WT efflux, whereas mutation to Asp of residues 2, 4, or 13 was without significant effect on efflux. We propose that phosphorylation of one or more serines in the N-terminus of human DAT, most likely Ser7 or Ser12, is essential for AMPH-induced DAT-mediated DA efflux. Quite surprisingly, N-terminal phosphorylation shifts DAT from a "reluctant" state to a "willing" state for AMPH-induced DA efflux, without affecting inward transport. These data raise the therapeutic possibility of interfering selectively with AMPH-induced DA efflux without altering physiological DA uptake.

Effects of fenfluramine and antidepressants on protein kinase C activity in rat cortical synaptoneurosomes

Fenfluramine releases serotonin (5-HT) via the 5-HT transporter (SERT). Previous work has shown that amphetamine increases particulate protein kinase C (PKC) activity in striatal synaptoneurosomes. The increased PKC activity is linked to the outward transport of dopamine, and when release is diminished, the inward transport of amphetamine inhibits PKC instead. Since there is homology among monoamine transporters, this study was undertaken to determine if D-fenfluramine has similar effects on PKC. The role of 5-HT receptors and endogenous 5-HT were also examined. Naive rats and rats pretreated with p-chlorophenylalanine (PCPA), a 5-HT synthesis inhibitor, were sacrificed. Cortical synaptoneurosomes were prepared and incubated with fenfluramine. PKC activity was determined by thiophosphorylation of endogenous substrates. It was found that 5-HT, D/L-fenfluramine, and D-fenfluramine increased PKC activity in a time- and dose-dependent manner. The 5-HT-mediated increase in PKC activity was attenuated by pretreatment with the 5-HT(2) antagonist ketanserin, but not with the SERT inhibitor fluoxetine. The D-fenfluramine-induced increase in PKC activity was completely prevented, however, by pretreatment with SERT inhibitors and partially with ketanserin. It was also attenuated by pretreatment with PCPA, resulting in a dose-dependent inhibition of PKC instead. Thus, when 5-HT release was diminished the uptake of D-fenfluramine inhibited PKC. Similar effects have been observed with amphetamine. Unlike D-fenfluramine, the D/L-fenfluramine-induced increase in PKC activity was partially resistant to PCPA pretreatment but was attenuated with bupropion, a dopamine transporter (DAT) inhibitor. SERT inhibitors (sertraline, paroxetine, citalopram, and fluoxetine) also increased PKC activity. Nefazodone and bupropion increased PKC activity, but mirtazapine was relatively inactive. The SERT inhibitor-induced increase in PKC was unaffected by pretreatment with PCPA but was inhibited by calcium. Similar effects on PKC activity have been observed with DAT inhibitors. These results, showing that D-fenfluramine altered PKC activity similar to D-amphetamine, suggest that the topographic homology between DAT and SERT may extend to their effects on PKC activity.

Mechanisms underlying the effects of amphetamine on particulate PKC activity

Amphetamine stimulates particulate protein kinase C (PKC) activity that is associated with the outward-transport of dopamine (DA) (Giambalvo [2003] Synapse 49:125-133). This stimulatory effect requires intracellular calcium ([Ca](i)) and endogenous DA and when DA release is diminished, the inward-transport of amphetamine inhibits PKC activity. This study examines the mechanisms involved. It was found that synaptoneurosomes incubated with amphetamine showed a dose-dependent increase in phospholipase C and A(2) activities. Furthermore, pretreatments with the phospholipase C inhibitor D609 or the phospholipase A(2) inhibitors quinacrine or p-bromophenacylbromide attenuated the amphetamine-induced increase in PKC activity. This suggests that both phospholipases were essential for the amphetamine-induced increase in PKC activity. The Na/Ca antiporter was also involved, since pretreatment with amiloride or benzamil attenuated the amphetamine-induced increase in PKC activity. Since these drugs by themselves increased PKC activity, the return to basal activity after addition of amphetamine suggests that, in the absence of Na/Ca exchange, amphetamine had an inhibitory effect on PKC activity. This inhibitory effect might be due to the activation of phospholipase A(2) through an increase in intracellular pH induced by amphetamine. This was supported by the finding that pretreatment with dimethylamiloride, an inhibitor of the Na/H antiporter that increases intracellular [H(+)], attenuated the effects of amphetamine on PKC activity. Other drugs that decrease intracellular [H(+)] (ammonia, monensin) also inhibited PKC activity without Ca. In contrast to amphetamine, monensin had no effect on PKC activity with Ca. This could be related to its large differential effects on phospholipase A(2) vs. phospholipase C activity. Thus, the monensin-mediated decrease in PKC activity seen without Ca was partially attenuated by pretreatment with quinacrine. Furthermore, when Na/Ca antiporter was inhibited with benzamil, monensin inhibited PKC activity. These results suggest that amphetamine, as well as monensin, may have dual effects on PKC activity, a Ca-dependent stimulatory effect via phospholipase C, and an inhibitory effect via phospholipase A(2).