Vesicular Dopamine Uptake Research Articles

The catecholaldehyde hypothesis: where MAO fits in.

Monoamine oxidase (MAO) plays a central role in the metabolism of the neurotransmitters dopamine, norepinephrine, and serotonin. This brief review focuses on 3,4-dihydroxyphenylacetaldehyde (DOPAL), which is the immediate product of MAO acting on cytoplasmic dopamine. DOPAL is toxic; however, normally DOPAL is converted via aldehyde dehydrogenase (ALDH) to 3,4-dihydroxyphenylacetic acid (DOPAC), which rapidly exits the neurons. In addition to vesicular uptake of dopamine via the vesicular monoamine transporter (VMAT), the two-enzyme sequence of MAO and ALDH keeps cytoplasmic dopamine levels low. Dopamine oxidizes readily to form toxic products that could threaten neuronal homeostasis. The catecholaldehyde hypothesis posits that diseases featuring catecholaminergic neurodegeneration result from harmful interactions between DOPAL and the protein alpha-synuclein, a major component of Lewy bodies in diseases such as Parkinson disease, dementia with Lewy bodies, and pure autonomic failure. DOPAL potently oligomerizes alpha-synuclein, and alpha-synuclein oligomers impede vesicular functions, shifting the fate of cytoplasmic dopamine toward MAO-catalyzed formation of DOPAL-a vicious cycle. When MAO deaminates dopamine to form DOPAL, hydrogen peroxide is generated; and DOPAL, hydrogen peroxide, and divalent metal cations react to form hydroxyl radicals, which peroxidate lipid membranes. Lipid peroxidation products in turn inhibit ALDH, causing DOPAL to accumulate-another vicious cycle. MAO inhibition decreases DOPAL formation but concurrently increases the spontaneous oxidation of dopamine, potentially trading off one form of toxicity for another. These considerations rationalize a neuroprotection strategy based on concurrent treatment with an MAO inhibitor and an anti-oxidant.

Age-dependent dopamine transporter dysfunction and Serine129 phospho-\u03b1-synuclein overload in G2019S LRRK2 mice

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most common genetic cause of Parkinson’s disease. Here, we investigated whether the G2019S LRRK2 mutation causes morphological and/or functional changes at nigro-striatal dopamine neurons. Density of striatal dopaminergic terminals, nigral cell counts, tyrosine hydroxylase protein levels as well as exocytotic dopamine release measured in striatal synaptosomes, or striatal extracellular dopamine levels monitored by in vivo microdialysis were similar between ≥12-month-old G2019S knock-in mice and wild-type controls. In vivo striatal dopamine release was insensitive to the LRRK2 inhibitor Nov-LRRK2-11, and was elevated by the membrane dopamine transporter blocker GBR-12783. However, G2019S knock-in mice showed a blunted neurochemical and motor activation response to GBR-12783 compared to wild-type controls. Western blot and dopamine uptake analysis revealed an increase in dopamine transporter levels and activity in the striatum of 12-month-old G2019S KI mice. This phenotype correlated with a reduction in vesicular monoamine transporter 2 levels and an enhancement of vesicular dopamine uptake, which was consistent with greater resistance to reserpine-induced hypolocomotion. These changes were not observed in 3-month-old mice. Finally, Western blot analysis revealed no genotype difference in striatal levels of endogenous α-synuclein or α-synuclein bound to DOPAL (a toxic metabolite of dopamine). However, Serine129-phosphorylated α-synuclein levels were higher in 12-month-old G2019S knock-in mice. Immunohistochemistry confirmed this finding, also showing no genotype difference in 3-month-old mice. We conclude that the G2019S mutation causes progressive dysfunctions of dopamine transporters, along with Serine129-phosphorylated α-synuclein overload, at striatal dopaminergic terminals, which are not associated with dopamine homeostasis dysregulation or neuron loss but might contribute to intrinsic dopaminergic terminal vulnerability. We propose G2019S knock-in mice as a presymptomatic Parkinson’s disease model, useful to investigate the pathogenic interaction among genetics, aging, and internal or environmental factors leading to the disease.

1,4-Diphenalkylpiperidines: A new scaffold for the design of potent inhibitors of the vesicular monoamine transporter-2

A series of 1,4-diphenalkylpiperidine analogs were synthesized and evaluated for their affinity and inhibitory potency at the [3H]dihydrotetrabenazine (DTBZ) binding site and [3H]dopamine (DA) uptake site on the vesicular monoamine transporter-2 (VMAT2). Results revealed that translocation of the phenethyl side chains of lobelane from C2 and C6 to C1 and C4 around the central piperidine ring slightly reduces affinity and inhibitory potency at VMAT2 with respect to lobelane. However, methoxy and fluoro-substitution of either phenyl ring of these 1,4-diphenethyl analogs afforded VMAT2 inhibition comparable or higher (5-fold) affinity at the DTBZ binding and DA uptake sites relative to lobelane, whereas replacement of the 4-phenethyl moiety in these analogs with a 4-phenmethyl moiety markedly reduced affinity for the DTBZ binding and DA uptake sites by 3- and 5-fold, respectively. Among the twenty five 1,4-diphenethylpiperidine analogs evaluated, compounds containing a 4-(2-methoxyphenethyl) moiety exhibited the most potent inhibition of DTBZ binding and vesicular DA uptake. From this subgroup, analogs 8h, 8j and 8m exhibited Ki values of 9.3nM, 13nM and 13nM, respectively, for inhibition of [3H]DA uptake by VMAT2, and represent some of the most potent inhibitors of VMAT2 function reported thus far.

Deletion of SHATI/NAT8L increases dopamine D1 receptor on the cell surface in the nucleus accumbens, accelerating methamphetamine dependence

In a previous report, we identified a novel molecule, SHATI/NAT8L, having an inhibitory effect on methamphetamine (METH)-induced hyperlocomotion, sensitization, and conditioned place preference (CPP). SHATI/NAT8L attenuates the METH-induced increase in dopamine overflow in the nucleus accumbens (NAc) by promoting plasmalemmal and vesicular dopamine uptake. However, the biological functions of the protein remain unclear. In this study, we explored NAT8L-binding proteins using pull-down assays and identified a number of components of the adaptor protein (AP)-2 complex, which is a multimeric protein localized to the plasma membrane that functions to internalize cargo during clathrin-mediated endocytosis. To investigate whether NAT8L regulates the receptor localization to the cell surface, cell-surface dopamine D1 receptor in the NAc of Nat8l knockout (KO) mice was quantified. We found that dopamine D1 receptor on the cell surface was increased in the NAc of Nat8l KO mice compared with the wild type (WT) animals. Consistent with this finding, Nat8l KO mice showed higher basal locomotor activity and heightened sensitivity to D1 agonist compared with WT mice. In addition, METH-induced sensitization and CPP were enhanced in Nat8l KO mice. These results suggest that NAT8L might regulate the localization of cell-surface dopamine D1 receptor, thereby controlling basal behaviour and sensitivity to METH. Furthermore, we observed a single nucleotide polymorphism (SNP) in the human NAT8L gene related to reward dependence, a personality trait, and grey matter volume in the caudate nucleus in healthy subjects, suggesting that NAT8L might also affect human personality.

Determinants of buildup of the toxic dopamine metabolite DOPAL in Parkinson's disease

Intra-neuronal metabolism of dopamine (DA) begins with production of 3,4-dihydroxyphenylacetaldehyde (DOPAL),which is toxic. According to the 'catecholaldehyde hypothesis', DOPAL destroys nigrostriatal DA terminals and contributes to the profound putamen DA deficiency that characterizes Parkinson’s disease (PD). We tested the feasibility of using post-mortem patterns of putamen tissue catechols to examine contributions of altered activities of the type 2 vesicular monoamine transporter (VMAT2) and aldehyde dehydrogenase(ALDH) to the increased DOPAL levels found in PD. Theoretically, the DA : DOPA concentration ratio indicates vesicular uptake, and the 3,4-dihydroxyphenylacetic acid: DOPAL ratio indicates ALDH activity. We validated these indices in transgenic mice with very low vesicular uptake VMAT2-Lo) or with knockouts of the genes encoding ALDH1A1 and ALDH2 (ALDH1A1,2 KO), applied these indices in PD putamen, and estimated the percent decreases in vesicular uptake and ALDH activity in PD. VMAT2-Lo mice had markedly decreased DA:DOPA (50 vs. 1377, p < 0.0001),and ALDH1A1,2 KO mice had decreased 3,4-dihydroxyphenylacetic acid:DOPAL (1.0 vs. 11.2, p < 0.0001). In PD putamen, vesicular uptake was estimated to be decreased by 89% and ALDH activity by 70%. Elevated DOPAL levels in PD putamen reflect a combination of decreased vesicular uptake of cytosolic DA and decreased DOPAL detoxification by ALDH.

Pyrrolidine analogs of GZ-793A: Synthesis and evaluation as inhibitors of the vesicular monoamine transporter-2 (VMAT2)

Central heterocyclic ring size reduction from piperidinyl to pyrrolidinyl in the vesicular monoamine transporter-2 (VMAT2) inhibitor GZ-793A and its analogs resulted in novel N-propane-1,2(R)-diol analogs 11a–i. These compounds were evaluated for their affinity for the dihydrotetrabenazine (DTBZ) binding site on VMAT2 and for their ability to inhibit vesicular dopamine (DA) uptake. The 4-difluoromethoxyphenethyl analog 11f was the most potent inhibitor of [3H]-DTBZ binding (Ki=560nM), with 15-fold greater affinity for this site than GZ-793A (Ki=8.29μM). Analog 11f also showed similar potency of inhibition of [3H]-DA uptake into vesicles (Ki=45nM) compared to that for GZ-793A (Ki=29 nM). Thus, 11f represents a new water-soluble inhibitor of VMAT function.

P.1.f.004 Behavioural and neurodevelopmental effects of ACTH and glucocorticoids in neonatal rats

This chapter highlights how studies of the giant dopamine neuron (GDN) located in the left pedal ganglion of the pond snail Planorbis corneus have provided direct demonstration of reverse transport from a neuron in real time. The GDN is advantageous for these studies because it has nomifensine-sensitive plasma membrane dopamine (DA) uptake, reserpine-sensitive vesicular DA uptake, it metabolizes DA to dihydroxyphenylacetic acid, and lacks ascorbic acid, facilitating the unambiguous amperometric detection of DA. Exocytic quantal release events can easily be distinguished from release by reverse transport using carbon fiber electrodes. The large size of the neuron allows the estimation of cytosolic levels of DA with intracellular carbon electrodes. The large size of the cell, the presence of the DA uptake system, and presence of many synaptic vesicles provides an enormous transmitter pool, proving the means to detect reverse transport from a single cell.

Ephedrine decreases vesicular monoamine transporter‐2 function

Ephedrine decreases vesicular monoamine transporter‐2 function

Methamphetamine alters vesicular monoamine transporter‐2 function and potassium‐stimulated dopamine release

This report demonstrates that a repeated 'challenge' high-dose methamphetamine (METH) injection regimen rapidly decreases striatal K(+)-stimulated dopamine (DA) release concurrent with decreases in both synaptosomal membrane-associated (referred to herein as membrane-associated) and previously reported decreases in non-synaptosomal membrane-associated (presumably cytoplasmic) vesicular DA uptake and content. Resembling previously reported effects involving cytoplasmic vesicles wherein uptake was decreased 48 h after treatment, the decrease in membrane-associated uptake persisted 72 h. Cytoplasmic and membrane-associated vesicular DA uptakes were decreased 7 days after the challenge regimen. A single METH injection also rapidly decreased K(+)-stimulated DA release, membrane-associated DA content, and membrane-associated DA uptake; however, unlike after the challenge regimen, the decrease in uptake recovered by 24 h. Pre-treatment with the D(2) receptor antagonist, eticlopride, did not attenuate the decrease in membrane-associated uptake as assessed 1 h after either a single or challenge treatment. However, eticlopride attenuated the decrease in membrane-associated uptake caused by the challenge regimen as assessed 24 h later. These data reveal complex effects of METH on vesicular function that vary according to the vesicle population under study, dosing regimen, and time after treatment. These may contribute to both the decrease in K(+)-stimulated DA release and the persistent dopaminergic deficits caused by METH.

The effects of a novel anti-Parkisonian drug on dopamine syshesis and vesicular dopamine uptake in dopaminergic cells

The effects of a novel anti-Parkisonian drug on dopamine syshesis and vesicular dopamine uptake in dopaminergic cells

Alteration of dopamine uptake into rat striatal vesicles and synaptosomes caused by an in vitro exposure to atrazine and some of its metabolites

Studies have shown that both in vivo and in vitro exposure to the herbicide atrazine (ATR) results in dopaminergic neurotoxicity manifested by decreased striatal dopamine (DA) levels. However, the mechanism behind this reduction is largely unknown. A decrease in striatal DA could be due to ATR exposure affecting vesicular and/or synaptosomal uptake resulting in disrupted vesicular storage and/or cellular uptake of DA. Hence, we investigated the effects of in vitro ATR exposure on DA uptake into isolated rat striatal synaptosomes and synaptic vesicles. In addition to ATR, effects of its major mammalian metabolites, didealkyl atrazine (DACT), desethyl atrazine (DE) and desiopropyl atrazine (DIP) were investigated. ATR (1–250 μM) inhibited DA uptake into synaptic vesicles in a dose-dependent manner. Of the three ATR metabolites tested, DACT did not affect vesicular DA uptake. DE and DIP, on the other hand, significantly decreased vesicular DA uptake with the effect of 100 μM DE/DIP being similar to the effect of the same concentration of ATR. Kinetic analysis of vesicular DA uptake indicated that ATR significantly decreased the V max while the K m value was not affected. Contrary to the inhibitory effects on vesicular DA uptake, synaptosomal DA uptake was marginally (6–13%) increased by ATR and DE, but not by DACT and DIP, at concentrations of ≤100 μM. As a result, ATR, DIP and DE increased the synaptosomal/vesicular (DAT/VMAT-2) uptake ratio. Collectively, results from this study suggest that ATR and two of its metabolites, DIP and DE, but not its major mammalian metabolite, DACT, decrease striatal DA levels, at least in part, by increasing cytosolic DA, which is prone to oxidative breakdown.

A Novel Molecule “Shati” Is Involved in Methamphetamine-Induced Hyperlocomotion, Sensitization, and Conditioned Place Preference

Drug addiction places an enormous burden on society through its repercussions on crime rate and healthcare. Repeated exposure to drugs of abuse causes cellular adaptations in specific neuronal populations that ultimately can lead to a state of addiction. In the present study, we have identified a novel molecule "shati" from the nucleus accumbens (NAc) of mice treated with methamphetamine (METH) using the PCR-select complementary DNA subtraction method. Moreover, we investigated whether shati is involved in METH-induced hyperlocomotion, sensitization, and conditioned place preference (CPP). METH induced expression of shati mRNA dose dependently via dopamine (DA) receptors. We prepared antibodies against shati and, using them, found shati to be expressed in neuronal cells of the mouse brain. Treatment with the shati antisense oligonucleotide (shati-AS), which significantly inhibited the expression of shati mRNA, enhanced the acute METH response, METH-induced behavioral sensitization, and CPP. Blockage of shati mRNA by shati-AS potentiated the METH-induced increase of DA overflow in the NAc and the METH-induced decrease in synaptosomal and vesicular DA uptake in the midbrain. These results suggest that a novel molecule shati is involved in the development of METH-induced hyperlocomotion, sensitization, and CPP. The functional roles of shati in METH-regulated behavioral alternations are likely to be mediated by its inhibitory effects on the METH-induced increase of DA overflow in the NAc and the METH-induced decrease in DA uptake in the midbrain.

Methamphetamine Administration Reduces Hippocampal Vesicular Monoamine Transporter-2 Uptake

Repeated high-dose injections of methamphetamine (METH) rapidly decrease dopamine uptake by the vesicular monoamine transporter-2 (VMAT-2) associated with dopaminergic nerve terminals, as assessed in nonmembrane-associated vesicles purified from striata of treated rats. The purpose of this study was to determine whether METH similarly affects vesicular uptake in the hippocampus; a region innervated by both serotonergic and noradrenergic neurons and profoundly affected by METH treatment. Results revealed that repeated high-dose METH administrations rapidly (within 1 h) reduced hippocampal vesicular dopamine uptake, as assessed in vesicles purified from treated rats. This reduction was likely associated with serotonergic nerve terminals because METH did not further reduce vesicular monoamine uptake in para-chloroamphetamine-lesioned animals. Pretreatment with the serotonin transporter inhibitor fluoxetine blocked both this acute effect on VMAT-2 and the decrease in serotonin content observed 7 days after METH treatment. In contrast, there was no conclusive evidence that METH affected vesicular dopamine uptake in noradrenergic neurons or caused persistent noradrenergic deficits. These findings suggest a link between METH-induced alterations in serotonergic hippocampal vesicular uptake and the persistent hippocampal serotonergic deficits induced by the stimulant.

In Vitro Assessment of Dopamine Uptake and Methamphetamine‐Induced Dopamine Efflux at the Vesicular Monoamine Transporter‐2

The vesicular monoamine transporter-2 (VMAT-2) is a vesicular transporter protein that utilizes proton electrochemical gradients to sequester cytoplasmic dopamine (DA) into vesicles for storage and subsequent release. Herein, we report the first use of rotating disk electrode (RDE) voltammetry to measure DA uptake and efflux by the VMAT-2 in small synaptic vesicles. DA uptake profiles followed mixed order kinetics with apparent zero order kinetics for the first 25 s and apparent first order kinetics thereafter. Vesicular DA uptake was temperature- and ATP-dependent and was blocked by the VMAT-2 inhibitor tetrabenazine. Initial velocities of DA uptake followed Michaelis-Menten kinetics with a Km and Vmax of 289 ± 59 nM and 1.9 ± 0.2 fmol / (s * μg protein), respectively. The VMAT-2 substrate analog, methamphetamine, caused DA efflux while the VMAT-2 inhibitor, tetrabenazine, did not. Methamphetamine-induced DA efflux had an initial velocity of 0.54 ± 0.08 fmol / (s * μg protein) and was blocked by tetrabenazine. These results suggest that RDE voltammetric measurements are selective for VMAT-2 uptake activity in preparations of small synaptic vesicles and will allow the design of experiments that could reveal important information about the kinetics of VMAT-2 activity and its inhibition. Additionally, methamphetamine-induced DA efflux in these preparations is not diffusive but rather is mediated by the VMAT-2. This work was supported by grants DA 00689, DA 04222, DA 13367, DA 11389, DA 019447, and DA 00378 from the National Institute on Drug Abuse.

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

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

Methyllycaconitine Prevents Methamphetamine-Induced Effects in Mouse Striatum: Involvement of α7 Nicotinic Receptors

In a previous study, we demonstrated that in rat striatal synaptosomes, methamphetamine (METH)-induced reactive oxygen species (ROS) production was prevented by methyllycaconitine (MLA), a specific antagonist of alpha7 neuronal nicotinic acetylcholine receptors (alpha7 nAChR). The aim of this study was to test the influence of MLA on acute METH effects and neurotoxicity in mice, using both in vivo and in vitro models. MLA inhibited METH-induced climbing behavior by 50%. Acute effects after 30-min preincubation with 1 microM METH also included a decrease in striatal synaptosome dopamine (DA) uptake, which was prevented by MLA. METH-induced neurotoxicity was assessed in vivo in terms of loss of striatal dopaminergic terminals (73%) and of tyrosine hydroxylase levels (by 90%) at 72 h post-treatment, which was significantly attenuated by MLA. Microglial activation [measured as 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide binding] was also present at 24 h post-treatment and was fully prevented by MLA, tending to confirm its neuroprotective activity. MLA had no effect on METH-induced hyperthermia. Additionally, flow cytometry assays showed that METH-induced ROS generation occurs inside synaptosomes from mouse striatum. This effect implied release of vesicular DA and was calcium-, neuronal nitric-oxide synthase-, and protein kinase C-dependent. MLA and alpha-bungarotoxin, but not dihydro-beta-erythroidine (an antagonist that blocks nAChR-containing beta2 subunits), fully prevented METH-induced ROS production without affecting vesicular DA uptake. The importance of this study lies not only in the neuroprotective effect elicited by the blockade of the alpha7 nicotinic receptors by MLA but also in that it proposes a new mechanism with which to study METH-induced acute and long-term effects.

Bupropion increases striatal vesicular monoamine transport

The vesicular monoamine transporter-2 (VMAT-2) is principally involved in regulating cytoplasmic dopamine (DA) concentrations within terminals by sequestering free DA into synaptic vesicles. This laboratory previously identified a correlation between striatal vesicular DA uptake through VMAT-2 and inhibition of the DA transporter (DAT). For example, administration of methylphenidate (MPD), a DAT inhibitor, increases vesicular DA uptake through VMAT-2 in a purified vesicular preparation; an effect associated with a redistribution of VMAT-2 protein within DA terminals. The purpose of this study was to determine if other DAT inhibitors, including bupropion, similarly affect VMAT-2. Results revealed bupropion rapidly, reversibly, and dose-dependently increased vesicular DA uptake; an effect also associated with VMAT-2 protein redistribution. The bupropion-induced increase in vesicular DA uptake was prevented by pretreatment with eticlopride, a DA D2 receptor antagonist, but not by SCH23390, a DA D1 receptor antagonist. We previously reported that MPD post-treatment prevents persistent DA deficits associated with multiple methamphetamine (METH) administrations. Although bupropion attenuated the METH-induced reduction in VMAT-2 activity acutely, it did not prevent the long-term dopaminergic toxicity or the METH-induced redistribution of VMAT-2 protein. The findings from this study demonstrate similarities and differences in the mechanism by which MPD and bupropion affect striatal dopaminergic nerve terminals.

Age-Dependent Methamphetamine-Induced Alterations in Vesicular Monoamine Transporter-2 Function: Implications for Neurotoxicity

Tens of thousands of adolescents and young adults have used illicit methamphetamine. This is of concern since its high-dose administration causes persistent dopaminergic deficits in adult animal models. The effects in adolescents are less studied. In adult rodents, toxic effects of methamphetamine may result partly from aberrant cytosolic dopamine accumulation and subsequent reactive oxygen species formation. The vesicular monoamine transporter-2 (VMAT-2) sequesters cytoplasmic dopamine into synaptic vesicles for storage and perhaps protection against dopamine-associated oxidative consequences. Accordingly, aberrant VMAT-2 function may contribute to the methamphetamine-induced persistent dopaminergic deficits. Hence, this study examined effects of methamphetamine on VMAT-2 in adolescent (postnatal day 40) and young adult (postnatal day 90) rats. Results revealed that high-dose methamphetamine treatment caused greater acute (within 1 h) decreases in vesicular dopamine uptake in postnatal day 90 versus 40 rats, as determined in a nonmembrane-associated subcellular fraction. Greater basal levels of VMAT-2 at postnatal day 90 versus 40 in this purified fraction seemed to contribute to the larger effect. Basal tissue dopamine content was also greater in postnatal day 90 versus 40 rats. In addition, postnatal day 90 rats were more susceptible to methamphetamine-induced persistent dopaminergic deficits as assessed by measuring VMAT-2 activity and dopamine content 7 days after treatment, even if drug doses were adjusted for age-related pharmacokinetic differences. Together, these data demonstrate dynamic changes in VMAT-2 susceptibility to methamphetamine as a function of development. Implications with regard to methamphetamine-induced dopaminergic deficits, as well as dopamine-associated neurodegenerative disorders such as Parkinson's disease, are discussed.

Dopamine D2 receptor activation increases vesicular dopamine uptake and redistributes vesicular monoamine transporter-2 protein

Recent studies demonstrate that multiple dopamine receptor subtypes contribute to the regulation of vesicular monoamine transporter-2 (VMAT-2) activity. The present studies extend these findings by demonstrating that administration of the nonselective dopamine D2 receptor family agonist, quinpirole, rapidly increased vesicular dopamine uptake in purified rat striatal vesicles. This effect occurred in both postnatal day 40 and 90 rats, and was associated with redistribution of the vesicular monoamine transporter-2 (VMAT-2) within nerve terminals. Neither a full nor a partial dopamine D1 receptor family agonist (SKF81297 nor SKF38393, respectively) affected vesicular dopamine uptake per se, nor the effect of quinpirole. Neither the dopamine D3 nor the D4 receptor antagonists, NGB2904 and clozapine, respectively, altered the quinpirole-mediated increase in uptake. However, the nonselective dopamine D2 receptor family antagonist, eticlopride, prevented the quinpirole-induced increase. Taken together, these data demonstrate that dopamine D2 receptor subtype activation increases vesicular dopamine uptake. Implications of this phenomenon with regard to the treatment of Parkinson's disease will be discussed.

Apomorphine increases vesicular monoamine transporter-2 function: implications for neurodegeneration

Apomorphine is a nonselective dopamine D1/D2 receptor agonist used in Europe to treat symptoms resulting from the dopaminergic degeneration associated with Parkinson's disease. In addition, neuroprotective effects of this agent in rodent models have been reported. Recent studies indicate that treatments that alter vesicular monoamine transporter-2 (VMAT-2) function may be protective in models of dopaminergic degeneration. Hence, the purpose of the present study was to examine the effect of apomorphine on VMAT-2 function. Results revealed that apomorphine rapidly and reversibly increased vesicular dopamine uptake, as determined in purified striatal vesicles obtained from treated rats. This increase occurred in both postnatal day 40 and postnatal day 90 rats, and was associated with a redistribution of VMAT-2 protein within nerve terminals. This effect of apomorphine on vesicular dopamine uptake was blocked by pretreating with eticlopride, a dopamine D2 receptor antagonist. The implications of these findings relevant to the treatment of neurodegeneration are discussed.