VMAT2 knockout mice: heterozygotes display reduced amphetamine-conditioned reward, enhanced amphetamine locomotion, and enhanced MPTP toxicity

Resistance of the golden hamster to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-neurotoxicity is not only related with low levels of cerebral monoamine oxidase-B

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) has been proved to be a potent neurotoxin on dopaminergic neurons inducing most of the symptoms and cerebral lesions observed in the idiopathic Parkinson's disease (PD). Although there is a substantial body of theory and researches about the effects of MPTP on susceptible mice and nonhuman primates, there are only few studies in resistant animals, such as golden hamsters (GH). The low levels of cerebral monoamine oxidase-B (MAO-B) enzyme have been proposed as the cause of the GH insensitivity to MPTP. The aim of this study was to elucidate whether MAO-B is the only factor which confer GH resistance to MPTP. Neither loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) nor cell death in the subventricular zone (SVZ) were found in female GH in response to an acute intraperitoneal (ip) MPTP treatment. To prove the role of MAO-B in the MPTP-resistance, female and male GH was intracerebroventricularly (icv) injected with either MPTP or 1-methyl-4-phenylpyridinum (MPP(+)). Neither depletion in the number of dopaminergic neurons, nor astrogliosis, cell death in the SVZ of female and male GH were registered after an icv treatment with MPTP or MPP(+). Furthermore, we demonstrated that MAO-B is located predominantly within the endothelial cells in the blood brain barrier (BBB), but not in the astroglia. The present results raise the possibility that, in GH, other mechanisms, apart from the low levels of regional MAO-B, confer resistance to MPTP and its metabolites.

Vesicular monoamine transporters: structure-function, pharmacology, and medicinal chemistry

Vesicular monoamine transporters (VMAT) are responsible for the uptake of cytosolic monoamines into synaptic vesicles in monoaminergic neurons. Two closely related VMATs with distinct pharmacological properties and tissue distributions have been characterized. VMAT1 is preferentially expressed in neuroendocrine cells and VMAT2 is primarily expressed in the CNS. The neurotoxicity and addictive properties of various psychostimulants have been attributed, at least partly, to their interference with VMAT2 functions. The quantitative assessment of the VMAT2 density by PET scanning has been clinically useful for early diagnosis and monitoring of the progression of Parkinson's and Alzheimer's diseases and drug addiction. The classical VMAT2 inhibitor, tetrabenazine, has long been used for the treatment of chorea associated with Huntington's disease in the United Kingdom, Canada, and Australia, and recently approved in the United States. The VMAT2 imaging may also be useful for exploiting the onset of diabetes mellitus, as VMAT2 is also expressed in the β-cells of the pancreas. VMAT1 gene SLC18A1 is a locus with strong evidence of linkage with schizophrenia and, thus, the polymorphic forms of the VMAT1 gene may confer susceptibility to schizophrenia. This review summarizes the current understanding of the structure-function relationships of VMAT2, and the role of VMAT2 on addiction and psychostimulant-induced neurotoxicity, and the therapeutic and diagnostic applications of specific VMAT2 ligands. The evidence for the linkage of VMAT1 gene with schizophrenia and bipolar disorder I is also discussed.

The ethical implications of genetic testing in the classroom

The development of classroom experiments where students examine their own DNA is frequently described as an innovative teaching practice. Often these experiences involve students analyzing their genes for various polymorphisms associated with disease states, like an increased risk for developing cancer. Such experiments can muddy the distinction between classroom investigation and medical testing. Although the goals and issues surrounding classroom genotyping do not directly align with those of clinical testing, instructors can use the guidelines and standards established by the medical genetics community when evaluating the ethics of human genotyping. We developed a laboratory investigation and discussion which allowed undergraduate science students to explore current DNA manipulation techniques to isolate their p53 gene, followed by a dialogue probing the ethical implications of examining their sample for various polymorphisms. Students never conducted genotyping on their samples because of the ethical concerns presented in this paper, so the discussion replaced the actual genetic testing in the class. A science faculty member led the laboratory portion, while a genetic counselor facilitated the discussion of the ethical concepts underlying genetic counseling: autonomy, beneficence, confidentiality, and justice. In their final papers, students demonstrated an understanding of the practice guidelines established by the genetics community and acknowledged the ethical considerations inherent in p53 genotyping. Given the burgeoning market for personalized medicine, teaching undergraduates about the psychosocial and ethical dimensions of human genetic testing is important and timely. Moreover, incorporating a genetic counselor in the classroom discussion provided a rich and dynamic discussion of human genetic testing.

VMAT2: a dynamic regulator of brain monoaminergic neuronal function interacting with drugs of abuse

The monoaminergic neuron, in particular the dopaminergic neuron, is central to mediating the hedonic and addictive properties of drugs of abuse. The effects of amphetamine (AMPH) and cocaine (COC), for example, depend on the ability to increase dopamine in the synapse, by effects on either the plasma membrane transporter DAT or the vesicular transporter for monoamine storage, VMAT2. The potential role of DAT as a target for AMPH and COC has been reviewed extensively. Here, we present VMAT2 as a target that enables the rewarding and addictive actions of these drugs, based on imaging, neurochemical, biochemical, cell biological, genetic, and immunohistochemical evidence. The presence of VMAT2 in noradrenergic, serotoninergic, histaminergic, and potentially trace aminergic neurons invites consideration of a wider role for aminergic neurotransmission in AMPH and COC abuse and addiction.

Tetrabenazine inhibition of monoamine uptake and methamphetamine behavioral effects: locomotor activity, drug discrimination and self-administration

Tetrabenazine (TBZ), a benzoquinolizine derivative, binds with high affinity to the vesicular monoamine transporter-2 (VMAT2), inhibiting uptake of cytosolic monoamines. The current study aimed to provide preclinical evidence supporting the potential use of TBZ as a treatment for methamphetamine abuse. Effects of TBZ on function of the dopamine transporter (DAT) and serotonin transporter (SERT) in striatal and hippocampal synaptosomes, respectively, and on VMAT2 function in isolated striatal synaptic vesicles were determined. Effect of TBZ (acute, 0.1-3.0 mg/kg, s.c.; repeated, 1.0 mg/kg for 7 days) on locomotor activity in methamphetamine-sensitized rats was assessed. Ability of TBZ (0.1-3.0 mg/kg; s.c.) or vehicle to decrease the discriminative effect of methamphetamine also was determined. Ability of TBZ (acute, 0.1-1.0 mg/kg, s.c.; repeated, 0.1 or 1.0 mg/kg for 7 days) to specifically decrease methamphetamine self-administration was determined; for comparison, a separate group of rats was assessed for effects of TBZ on food-maintained responding. Results show that TBZ was 11-fold more potent inhibiting DAT than SERT, and 2.5-fold more potent inhibiting VMAT2 than DAT. Results from behavioral studies showed that the lowest dose of TBZ transiently increased methamphetamine self-administration, whereas higher TBZ doses decreased methamphetamine self-administration. Also, TBZ at high doses decreased methamphetamine locomotor sensitization and discriminative stimulus effects, as well as food-maintained responding. Thus, despite acting as a potent VMAT2 inhibitor, these preclinical results indicate that TBZ lacks behavioral specificity as an inhibitor of methamphetamine-induced reinforcement, diminishing its viability as a suitable treatment for methamphetamine abuse.

Parkinson's disease mouse models in translational research

Animal models with high predictive power are a prerequisite for translational research. The closer the similarity of a model to Parkinson’s disease (PD), the higher is the predictive value for clinical trials. An ideal PD model should present behavioral signs and pathology that resemble the human disease. The increasing understanding of PD stratification and etiology, however, complicates the choice of adequate animal models for preclinical studies. An ultimate mouse model, relevant to address all PD-related questions, is yet to be developed. However, many of the existing models are useful in answering specific questions. An appropriate model should be chosen after considering both the context of the research and the model properties. This review addresses the validity, strengths, and limitations of current PD mouse models for translational research.

Cerebral microglia mediate sleep/wake and neuroinflammatory effects of methamphetamine

Methamphetamine and modafinil exert their wake-promoting effects by elevating monoaminergic tone. The severity of hypersomnolence that occurs subsequent to induced wakefulness differs between these two agents. Microglia detects and modulates CNS reactions to agents such as D-methamphetamine that induce cellular stress. We therefore hypothesized that changes in the sleep/wake cycle that occur subsequent to administration of D-methamphetamine are modulated by cerebral microglia. In CD11b-herpes thymidine kinase transgenic mice (CD11b-TK(mt-30)), activation of the inducible transgene by intracerebroventricular (icv) ganciclovir results in toxicity to CD11b-positive cells (i.e. microglia), thereby reducing cerebral microglial cell counts. CD11b-TK(mt-30)and wild type mice were subjected to chronic icv ganciclovir or vehicle administration with subcutaneous mini-osmotic pumps. D-methamphetamine (1 and 2 mg/kg), modafinil (30 and 100 mg/kg) and vehicle were administered intraperitoneally to these animals. In CD11b-TK(mt-30) mice, but not wild type, icv infusion of ganciclovir reduced the duration of wake produced by D-methamphetamine at 2 mg/kg by nearly 1h. Nitric oxide synthase (NOS) activity, studied ex vivo, and NOS expression were elevated in CD11b-positive cerebral microglia from wild type mice acutely exposed to d-methamphetamine. Additionally, CD11b-positive microglia, but not other cerebral cell populations, exhibited changes in sleep-regulatory cytokine expression in response to d-METH. Finally, CD11b-positive microglia exposed to d-methamphetamine in vitro exhibited increased NOS activity relative to pharmacologically-naïve cells. CD11b-positive microglia from the brains of neuronal NOS (nNOS)-knockout mice failed to exhibit this effect. We propose that the effects of D-METH on sleep/wake cycles are mediated in part by actions on microglia, including possibly nNOS activity and cytokine synthesis.

VMAT2-Deficient Mice Display Nigral and Extranigral Pathology and Motor and Nonmotor Symptoms of Parkinson's Disease

Dopamine is transported into synaptic vesicles by the vesicular monoamine transporter (VMAT2; SLC18A2). Disruption of dopamine storage has been hypothesized to damage the dopamine neurons that are lost in Parkinson's disease. By disrupting vesicular storage of dopamine and other monoamines, we have created a progressive mouse model of PD that exhibits catecholamine neuron loss in the substantia nigra pars compacta and locus coeruleus and motor and nonmotor symptoms. With a 95% reduction in VMAT2 expression, VMAT2-deficient animals have decreased motor function, progressive deficits in olfactory discrimination, shorter latency to behavioral signs of sleep, delayed gastric emptying, anxiety-like behaviors at younger ages, and a progressive depressive-like phenotype. Pathologically, the VMAT2-deficient mice display progressive neurodegeneration in the substantia nigra (SNpc), locus coeruleus (LC), and dorsal raphe (DR) coupled with α-synuclein accumulation. Taken together, these studies demonstrate that reduced vesicular storage of monoamines and the resulting disruption of the cytosolic environment may play a role in the pathogenesis of parkinsonian symptoms and neurodegeneration. The multisystem nature of the VMAT2-deficient mice may be useful in developing therapeutic strategies that go beyond the dopamine system.

Monoamine transporters: vulnerable and vital doorkeepers

Transporters of dopamine, serotonin, and norepinephrine have been empirically used as medication targets for several mental illnesses in the last decades. These protein-targeted medications are effective only for subpopulations of patients with transporter-related brain disorders. Since the cDNA clonings in early 1990s, molecular studies of these transporters have revealed a wealth of information about the transporters' structure-activity relationship (SAR), neuropharmacology, cell biology, biochemistry, pharmacogenetics, and the diseases related to the human genes encoding these transporters among related regulators. Such new information creates a unique opportunity to develop transporter-specific medications based on SAR, mRNA, DNA, and perhaps transporter trafficking regulation for a number of highly relevant diseases including substance abuse, depression, schizophrenia, and Parkinson's disease.

P25α / TPPP expression increases plasma membrane presentation of the dopamine transporter and enhances cellular sensitivity to dopamine toxicity

Parkinson's disease is characterized by preferential degeneration of the dopamine-producing neurons of the brain stem substantia nigra. Imbalances between mechanisms governing dopamine transport across the plasma membrane and cellular storage vesicles increase the level of toxic pro-oxidative cytosolic dopamine. The microtubule-stabilizing protein p25α accumulates in dopaminergic neurons in Parkinson's disease. We hypothesized that p25α modulates the subcellular localization of the dopamine transporter via effects on sorting vesicles, and thereby indirectly affects its cellular activity. Here we show that co-expression of the dopamine transporter with p25α in HEK-293-MSR cells increases dopamine uptake via increased plasma membrane presentation of the transporter. No direct interaction between p25α and the dopamine transporter was demonstrated, but they co-fractionated during subcellular fractionation of brain tissue from striatum, and direct binding of p25α peptides to brain vesicles was demonstrated. Truncations of the p25α peptide revealed that the requirement for stimulating dopamine uptake is located in the central core and were similar to those required for vesicle binding. Co-expression of p25α and the dopamine transporter in HEK-293-MSR cells sensitized them to the toxicity of extracellular dopamine. Neuronal expression of p25α thus holds the potential to sensitize the cells toward dopamine and toxins carried by the dopamine transporter.

Effect of genetic modifications in the synaptic dopamine clearance systems on addiction-like behaviour in mice

During the last 15 years, genetically modified mouse lines have proved to be a valuable research tool. This review summarizes research that studied addiction-like behaviour in mice that had a targeted mutation in the genes of the synaptic dopamine removal systems, i.e. in the dopamine transporter (DAT), a vesicular monoamine transporter 2 (VMAT2) or two dopamine-metabolizing enzymes (monoamine oxidase, MAO, mainly MAO-A isoenzyme, and catechol-O-methyltransferase, COMT). Majority of the mice are knockouts but also some knock-in and knock down mouse lines are included. Most studies have explored DAT, and it has been shown to be the critical target in addiction to psychostimulants. Its role in the development of addiction-like behaviour to nicotine, opioids or ethanol is less clear. VMAT2 also seems to be linked to psychostimulant addiction. MAO-A and COMT have a minor role in addiction-like behaviour that is further complicated by a sexual dimorphism.

Vulnerability of mesostriatal dopaminergic neurons in Parkinson's disease

The term vulnerability was first associated with the midbrain dopaminergic neurons 85 years ago, before they were identified as monoaminergic neurons, when Foix and Nicolesco (1925) reported the loss of neuromelanin containing neurons in the midbrain of patients with post-encephalitic Parkinson's disease (PD). A few years later, Hassler (1938) showed that degeneration is more intense in the ventral tier of the substantia nigra compacta than in its dorsal tier and the ventral tegmental area (VTA), outlining the concept of differential vulnerability of midbrain dopaminergic (DA-) neurons. Nowadays, we know that other neuronal groups degenerate in PD, but the massive loss of nigral DA-cells is its pathological hallmark, having a pivotal position in the pathophysiology of the disease as it is responsible for the motor symptoms. Data from humans as well as cellular and animal models indicate that DA-cell degeneration is a complex process, probably precipitated by the convergence of different risk factors, mediated by oxidative stress, and involving pathogenic factors arising within the DA-neuron (intrinsic factors), and from its environment and distant interconnected brain regions (extrinsic factors). In light of current data, intrinsic factors seem to be preferentially involved in the first steps of the degenerative process, and extrinsic factors in its progression. A controversial issue is the relative weight of the impairment of common cell functions, such as energy metabolism and proteostasis, and specific dopaminergic functions, such as pacemaking activity and DA handling, in the pathogenesis of DA-cell degeneration. Here we will review the current knowledge about the relevance of these factors at the beginning and during the progression of PD, and in the differential vulnerability of midbrain DA-cells.

Phenyl ring-substituted lobelane analogs: inhibition of [³H]dopamine uptake at the vesicular monoamine transporter-2

Lobeline attenuates the behavioral effects of methamphetamine via inhibition of the vesicular monoamine transporter (VMAT2). To increase selectivity for VMAT2, chemically defunctionalized lobeline analogs, including lobelane, were designed to eliminate nicotinic acetylcholine receptor affinity. The current study evaluated the ability of lobelane analogs to inhibit [³H]dihydrotetrabenazine (DTBZ) binding to VMAT2 and [³H]dopamine (DA) uptake into isolated synaptic vesicles and determined the mechanism of inhibition. Introduction of aromatic substituents in lobelane maintained analog affinity for the [³H]DTBZ binding site on VMAT2 and inhibitory potency in the [³H]DA uptake assay assessing VMAT2 function. The most potent (K(i) = 13-16 nM) analogs in the series included para-methoxyphenyl nor-lobelane (GZ-252B), para-methoxyphenyl lobelane (GZ-252C), and 2,4-dichlorphenyl lobelane (GZ-260C). Affinity of the analogs for the [³H]DTBZ binding site did not correlate with inhibitory potency in the [³H]DA uptake assay. It is noteworthy that the N-benzylindole-, biphenyl-, and indole-bearing meso-analogs 2,6-bis[2-(1-benzyl-1H-indole-3-yl)ethyl]-1-methylpiperidine hemifumarate (AV-1-292C), 2,6-bis(2-(biphenyl-4-yl)ethyl)piperidine hydrochloride (GZ-272B), and 2,6-bis[2-(1H-indole-3-yl)ethyl]-1-methylpiperidine monofumarate (AV-1-294), respectively] inhibited VMAT2 function (K(i) = 73, 127, and 2130 nM, respectively), yet had little to no affinity for the [³H]DTBZ binding site. These results suggest that the analogs interact at an alternate site to DTBZ on VMAT2. Kinetic analyses of [³H]DA uptake revealed a competitive mechanism for 2,6-bis(2-(4-methoxyphenyl)ethyl)piperidine hydrochloride (GZ-252B), 2,6-bis(2-(4-methoxyphenyl)ethyl)-1-methylpiperidine hydrochloride (GZ-252C), 2,6-bis(2-(2,4-dichlorophenyl)ethyl)piperidine hydrochloride (GZ-260C), and GZ-272B. Similar to methamphetamine, these analogs released [³H]DA from the vesicles, but with higher potency. In contrast to methamphetamine, these analogs had higher potency (>100-fold) at VMAT2 than DAT, predicting low abuse liability. Thus, modification of the lobelane molecule affords potent, selective inhibitors of VMAT2 function and reveals two distinct pharmacological targets on VMAT2.

The novel pyrrolidine nor-lobelane analog UKCP-110 [cis-2,5-di-(2-phenethyl)-pyrrolidine hydrochloride] inhibits VMAT2 function, methamphetamine-evoked dopamine release, and methamphetamine self-administration in rats

Both lobeline and lobelane attenuate methamphetamine self-administration in rats by decreasing methamphetamine-induced dopamine release via interaction with vesicular monoamine transporter-2 (VMAT2). A novel derivative of nor-lobelane, cis-2,5-di-(2-phenethyl)-pyrrolidine hydrochloride (UKCP-110), and its trans-isomers, (2R,5R)-trans-di-(2-phenethyl)-pyrrolidine hydrochloride (UKCP-111) and (2S,5S)-trans-di-(2-phenethyl)-pyrrolidine hydrochloride (UKCP-112), were evaluated for inhibition of [(3)H]dihydrotetrabenazine binding and [(3)H]dopamine uptake by using a rat synaptic vesicle preparation to assess VMAT2 interaction. Compounds were evaluated for inhibition of [(3)H]nicotine and [(3)H]methyllycaconitine binding to assess interaction with the major nicotinic receptor subtypes. In addition, compounds were evaluated for inhibition of methamphetamine-evoked endogenous dopamine release by using striatal slices. The most promising compound, UKCP-110, was evaluated for its ability to decrease methamphetamine self-administration and methamphetamine discriminative stimulus cues and for its effect on food-maintained operant responding. UKCP-110, UKCP-111, and UKCP-112 inhibited [(3)H]dihydrotetrabenazine binding (K(i) = 2.66 ± 0.37, 1.05 ± 0.10, and 3.80 ± 0.31 μM, respectively) and had high potency inhibiting [(3)H]dopamine uptake (K(i) = 0.028 ± 0.001, 0.046 ± 0.008, 0.043 ± 0.004 μM, respectively), but lacked affinity at nicotinic receptors. Although the trans-isomers did not alter methamphetamine-evoked dopamine release, UKCP-110 inhibited (IC(50) = 1.8 ± 0.2 μM; I(max) = 67.18 ± 6.11 μM) methamphetamine-evoked dopamine release. At high concentrations, UKCP-110 also increased extracellular dihydroxyphenylacetic acid. It is noteworthy that UKCP-110 decreased the number of methamphetamine self-infusions, while having no effect on food-reinforced behavior or the methamphetamine stimulus cue. Thus, UKCP-110 represents a new lead in the development of novel pharmacotherapies for the treatment of methamphetamine abuse.

Expression of neurotransmitter transporters for structural and biochemical studies

Neurotransmitter transporters play essential roles in the process of neurotransmission. Vesicular neurotransmitter transporters mediate storage inside secretory vesicles in a process that involves the exchange of lumenal H(+) for cytoplasmic transmitter. Retrieval of the neurotransmitter from the synaptic cleft catalyzed by sodium-coupled transporters is critical for the termination of the synaptic actions of the released neurotransmitter. Our current understanding of the mechanism of these transporters is based on functional and biochemical characterization but is lacking high-resolution structural information. Very few structures of membrane transport systems from mammalian origin have been solved to atomic resolution, mainly because of the difficulty in obtaining large amounts of purified protein. Development of high yield heterologous expression systems suitable for mammalian neurotransmitter transporters is essential to enable the production of purified protein for structural studies. Such a system makes possible also the production of mutants that can be used in biochemical and biophysical studies. We describe here a screen for the expression of the vesicular monoamine transporter 2 (VMAT2) in cell-free and baculovirus expression systems and discuss the expression of VMAT2 in other systems as well (bacterial, yeast and mammalian cell lines). After screening and optimization, we achieved high yield (2-2.5 mg/l) expression of functional VMAT2 in insect cells. The system was also used for the expression of three additional plasma membrane neurotransmitter transporters. All were functional and expressed to high levels. Our results demonstrate the advantages of the baculovirus expression system for the expression of mammalian neurotransmitter transporters in a functional state.

The Drosophila vesicular monoamine transporter reduces pesticide-induced loss of dopaminergic neurons

Dopamine is cytotoxic and may play a role in the development of Parkinson's disease. However, its interaction with environmental risk factors such as pesticides remains poorly understood. The vesicular monoamine transporter (VMAT) regulates intracellular dopamine content, and we have tested the neuroprotective effects of VMAT in vivo using the model organism Drosophila melanogaster. We find that Drosophila VMAT (dVMAT) mutants contain fewer dopaminergic neurons than wild type, consistent with a developmental effect, and that dopaminergic cell loss in the mutant is exacerbated by the pesticides rotenone and paraquat. Overexpression of DVMAT protein does not increase the survival of animals exposed to rotenone, but blocks the loss of dopaminergic neurons caused by this pesticide. These results are the first to demonstrate an interaction between a VMAT and pesticides in vivo, and provide an important model to investigate the mechanisms by which pesticides and cellular DA may interact to kill dopaminergic cells.

Transgenic mice in the study of drug addiction and the effects of psychostimulant drugs

The first transgenic models used to study addiction were based upon a priori assumptions about the importance of particular genes in addiction, including the main target molecules of morphine, amphetamine, and cocaine. This consequently emphasized the importance of monoamine transporters, opioid receptors, and monoamine receptors in addiction. Although the effects of opiates were largely eliminated by mu opioid receptor gene knockout, the case for psychostimulants was much more complex. Research using transgenic models supported the idea of a polygenic basis for psychostimulant effects and has associated particular genes with different behavioral consequences of psychostimulants. Phenotypic analysis of transgenic mice, especially gene knockout mice, has been instrumental in identifying the role of specific molecular targets of addictive drugs in their actions. In this article, we summarize studies that have provided insight into the polygenic determination of drug addiction phenotypes in ways that are not possible with other methods, emphasizing research into the effects of psychostimulant drugs in gene knockouts of the monoamine transporters and monoamine receptors.

Preparation and Characterization of Tetrabenazine Enantiomers against Vesicular Monoamine Transporter 2

As a clinical medication for the treatment of hyperkinetic movement disorders, in conditions such as Huntington's disease, tetrabenazine (TBZ) has been always used in its racemic form. To establish whether or not its beneficial therapeutic actions are enantiospecific, a practical total synthetic route was developed to yield each enantiomeric form to allow their chemical and pharmacological characterization. We briefly summarize the total synthesis of TBZ and report a detailed procedure for resolution of TBZ into its enantiomers, (+)-TBZ and (-)-TBZ. This allowed determination of the optical rotation and absolute configurations of each TBZ enantiomer, based on X-ray crystallographic analysis, together with characterization of their inhibitory action at the vesicular monoamine transporter 2, where (+)-TBZ proved three-fold more active than (-)-TBZ.

High regulatability favors genetic selection in SLC18A2, a vesicular monoamine transporter essential for life

SLC18A2 encodes the vesicular monoamine transporter 2 protein that regulates neurotransmission and reduces cytosolic toxicity of monoamines. Deletion of this gene causes lethality in mice, and DNA sequence variation in this gene is associated with alcoholism and Parkinson's disease, among other disorders. The Caucasian SLC18A2 promoter has at least 20 haplotypes (A-T), with A representing two-thirds of 1460 chromosomes. It is not known why A is selected in the human lineage. To understand the selection, here we took a functional approach by investigating the regulations of 4 representative haplotypes (A, C, G, and T) by 17 agents. We show that 76.5% of the agents were able to regulate A but only 11.8-23.5% of them regulated the 3 other infrequent ones, observing a positive correlation between haplotype frequency and regulatability. Pathway and molecular analyses revealed five signaling hubs that regulate the four haplotypes differentially, probably through targeting the polymorphic core promoter region. These findings suggest that greater diversity of transcriptional regulations is the driving force for the haplotype selection in SLC18A2.

Computational analysis of determinants of dopamine (DA) dysfunction in DA nerve terminals

Dopamine signaling is involved in a number of brain pathways, and its disruption has been suggested to be involved in the several disease states, including Parkinson's disease (PD), schizophrenia, and attention deficit hyperactivity disorder (ADHD). It has been hypothesized that altered storage, release, and reuptake of dopamine contributes to both the hypo- and hyperdopaminergic states that exist in various diseases. Here, we use our recently described mathematical model of dopamine metabolism, combined with a comprehensive Monte Carlo simulation analysis, to identify key determinants of dopamine metabolism associated with the dysregulation of dopamine homeostasis that may contribute to the pathogenesis of dopamine-based disorders. Our model reveals that the dopamine transporter (DAT), the vesicular monoamine transporter (VMAT2), and the enzyme monoamine oxidase (MAO) are the most influential components controlling the synaptic level of dopamine and the formation of toxic intracellular metabolites. The results are consistent with experimental observations and point to metabolic processes and combinations of processes that may be biochemical drivers of dopamine neuron degeneration. Since many of the identified components can be targeted therapeutically, the model may aid in the design of combined therapeutic regimens aimed at restoring proper dopamine signaling with toxic intermediates under control.

The effect of electroaucpuncture for 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced proteomic changes in the mouse striatum

Using a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) Parkinson's disease mouse model, we investigated protein expression changes associated with the action of electroacupuncture (EA) in the mouse striatum. Twelve-week-old male C57BL/6 mice were injected intraperitoneally with 30 mg/kg of MPTP at 24-h intervals for 5 days, and the 100-Hz EA stimulation was performed at GB34 and GB39 once a day for 12 days consecutively from the first injection. With the EA, the MPTP-induced dopaminergic neuronal destruction was reduced. Of the 13 proteins that were differentially expressed between control and MPTP treated mice, cytosolic malate dehydrogenase, munc18-1, and hydroxyacylglutathione hydrolase, which were increased by MPTP, and cytochrome c oxidase subunit Vb, which was decreased by MPTP, were restored to the level of the saline group after EA treatment. These proteins are likely related to cellular metabolism. Altogether, we propose that the EA may exert neuroprotective effects in mice striatum through reducing MPTP-induced toxicity such as oxidative stress.

Dose-dependent attenuation of heroin self-administration with lobeline

Behavioural studies have yielded results that show lobeline has the ability to attenuate d-methamphetamine self-administration. Further in vivo and in vitro studies have demonstrated a blockade of mu-opioid receptors with lobeline. The present investigation examined the ability of lobeline to attenuate heroin intravenous (i.v.) self-administration when administered prior to testing. Male Sprague-Dawley rats were surgically implanted with jugular catheters and trained to lever press for i.v. heroin infusions (18 microg/kg) under a fixed ratio-2 schedule wherein two active lever presses resulted in heroin delivery. Rats then were tested for heroin self-administration after pretreatment with subcutaneous lobeline injections (0.3, 1.0, or 3.0 mg/kg, 15 min prior to testing sessions). At doses of 1.0 and 3.0 mg/kg, lobeline attenuated self-administration of heroin. The results suggest a potential for lobeline to be used in pharmacotherapy for opioid abuse.

Lobelane inhibits methamphetamine-evoked dopamine release via inhibition of the vesicular monoamine transporter-2

Lobeline is currently being evaluated in clinical trials as a methamphetamine abuse treatment. Lobeline interacts with nicotinic receptor subtypes, dopamine transporters (DATs), and vesicular monoamine transporters (VMAT2s). Methamphetamine inhibits VMAT2 and promotes dopamine (DA) release from synaptic vesicles, resulting ultimately in increased extracellular DA. The present study generated structure-activity relationships by defunctionalizing the lobeline molecule and determining effects on [(3)H]dihydrotetrabenazine binding, inhibition of [(3)H]DA uptake into striatal synaptic vesicles and synaptosomes, the mechanism of VMAT2 inhibition, and inhibition of methamphetamine-evoked DA release. Compared with lobeline, the analogs exhibited greater potency inhibiting DA transporter (DAT) function. Saturated analogs, lobelane and nor-lobelane, exhibited high potency (K(i) = 45 nM) inhibiting vesicular [(3)H]DA uptake, and lobelane competitively inhibited VMAT2 function. Lobeline and lobelane exhibited 67- and 35-fold greater potency, respectively, in inhibiting VMAT2 function compared to DAT function. Lobelane potently decreased (IC(50) = 0.65 microM; I(max) = 73%) methamphetamine-evoked DA overflow, and with a greater maximal effect compared with lobeline (IC(50) = 0.42 microM, I(max) = 56.1%). These results provide support for VMAT2 as a target for inhibition of methamphetamine effects. Both trans-isomers and demethylated analogs of lobelane had reduced or unaltered potency inhibiting VMAT2 function and lower maximal inhibition of methamphetamine-evoked DA release compared with lobelane. Thus, defunctionalization, cis-stereochemistry of the side chains, and presence of the piperidino N-methyl are structural features that afford greatest inhibition of methamphetamine-evoked DA release and enhancement of selectivity for VMAT2. The current results reveal that lobelane, a selective VMAT2 inhibitor, inhibits methamphetamine-evoked DA release and is a promising lead for the development of a pharmacotherapeutic for methamphetamine abuse.

Constitutive Ret signaling is protective for dopaminergic cell bodies but not for axonal terminals

Ret is the canonical signaling receptor for glial cell line-derived neurotrophic factor (GDNF), which has been shown to have neuroprotective effects when administered prior to neurotoxic challenge. A missense Meth918Thr mutation causes the constitutive activation of Ret, resulting in multiple endocrine neoplasia type 2 B (MEN2B). To clarify the role of Ret signaling in neuroprotection, we studied the effects of the neurotoxins 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 6-hydroxydopamine (6-OHDA) on the dopaminergic system of mice carrying the MEN2B mutation. We found that MEN2B mice were significantly more resistant to nigral tyrosine hydroxylase (TH)-positive cell loss induced by unilateral striatal 6-OHDA than Wt mice. However, 6-OHDA caused profound dopamine (DA) depletion in the striatum of both MEN2B and Wt mice. Systemic MPTP caused similar DA depletion and a decrease in TH-immunostaining in the striatum of MEN2B and Wt mice. Neither neurotoxin induced a compensatory increase in striatal metabolite/DA ratios in the MEN2B mice, possibly contributing to an increased amphetamine-induced turning behavior observed in behavioral assessments of these mice. Thus, our data suggest that activated Ret protects DA cell bodies in the substantia nigra pars compacta, but does not protect DA axons in the striatum.

MPTP-induced neuroblast apoptosis in the subventricular zone is not regulated by dopamine or other monoamine transporters

For 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to exert neurotoxicity on dopaminergic neurons, 1-methyl-4-phenylpyridinium (MPP+), a metabolite of MPTP, must be taken up into the dopaminergic neuron via the dopamine transporter (DAT). Previous reports have shown that MPTP also causes neuroblast apoptosis in the subventricular zone (SVZ) of adult mice. The aim of this study is to elucidate the role of DAT and other monoamine transporters including vesicular monoamine transporter 2 (VMAT2), the serotonin transporter (SERT), and the norepinephrine transporter (NET) on the neuroblast apoptosis induced by MPTP administration. There were no DAT-positive neuroblasts in the SVZ, whereas some neuroblasts were immunopositive for VMAT2 and SERT. To examine whether these transporters are involved in MPTP-induced neuroblast apoptosis in the SVZ, terminal deoxynucleotidyl transferase-mediated dUTP endlabeling (TUNEL)-positive cells were semiquantitatively analyzed after the injection of GBR12909 (GBR), a DAT inhibitor; tetrabenazine (TBZ), a VMAT2 inhibitor; fluoxetine (FLU), a SERT inhibitor, or desipramine (DES), a NET inhibitor, prior to MPTP injection. However, the injection of these transporter inhibitors had no influence on the MPTP-induced neuroblast apoptosis in the SVZ. It is likely that neither DAT nor other monoamine transporters are involved in MPTP-induced neuroblast apoptosis. The present findings suggest that the neurotoxicity of MPTP to neuroblasts in the SVZ does not require DAT or other monoamine transporters, and the apoptosis it induces may be executed through other unknown pathways.

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

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

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

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

The effect of alpha-synuclein knockdown on MPP+ toxicity in models of human neurons

The protein alpha-synuclein is central to the pathophysiology of Parkinson's disease (PD) but its role in the development of neurodegeneration remains unclear. alpha-Synuclein-knockout mice develop without gross abnormality and are resistant to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a mitochondrial inhibitor widely used to model parkinsonism. Here we show that differentiated human dopaminergic neuron-like cells also have increased resistance to 1-methyl-4-phenylpyridine (MPP+), the active metabolite of MPTP, when alpha-synuclein is knocked down using RNA interference. In attempting to understand how this occurred we found that lowering alpha-synuclein levels caused changes to intracellular vesicles, dopamine transporter (DAT) and vesicular monoamine transporter (VMAT2), each of which is known to be an important component of the early events leading to MPP+ toxicity. Knockdown of alpha-synuclein reduced the availability of DAT on the neuronal surface by 50%, decreased the total number of intracellular vesicles by 37% but increased the density of VMAT2 molecules per vesicle by 2.8-fold. However, these changes were not associated with any reduction in MPP+ -induced superoxide production, suggesting that alpha-synuclein knockdown may have other downstream effects which are important. We then showed that alpha-synuclein knockdown prevented MPP+ -induced activation of nitric oxide synthase (NOS). Activation of NOS is an essential step in MPTP toxicity and increasing evidence points to nitrosative stress as being important in neurodegeneration. Overall, these results show that as well as having a number of effects on cellular events upstream of mitochondrial dysfunction alpha-synuclein affects pathways downstream of superoxide production, possibly involving regulation of NOS activity.

Deglycosylation and subcellular redistribution of VMAT2 in the mesostriatal system during normal aging

The vesicular monoamine transporter type 2 (VMAT2) is a transmembrane glycoprotein responsible for the vesicular monoamine uptake in the brain. This function declines in the dopaminergic mesostriatal system during normal aging, but the mechanisms responsible for this deficit are unknown. We investigated possible age-related changes in the expression and subcellular distribution of VMAT2 in the rat mesostriatal system. VMAT2 is constitutively expressed as glycosylated (75 kDa), partially glycosylated (55 kDa) and native (45 kDa) forms, they are all present in both synaptosomal compartments (synaptosomal membrane and synaptic vesicle-enriched fractions) of the striatal terminals in young rats. In aged rats, no changes were found in midbrain VMAT2mRNA and VMAT2 total protein levels in whole striatal extracts. However, its subcellular distribution and glycosylation pattern were severely modified. The three VMAT2 forms virtually disappeared from the synaptic vesicle-enriched fraction, while the 55 kDa form was accumulated in the soluble compartment. These changes may be responsible for the loss of VMAT2 activity during aging and may contribute to the high susceptibility of aged midbrain dopaminergic cells to degeneration.

Reduced vesicular storage of dopamine exacerbates methamphetamine-induced neurodegeneration and astrogliosis

The vesicular monoamine transporter 2 (VMAT2) controls the loading of dopamine (DA) into vesicles and therefore determines synaptic properties such as quantal size, receptor sensitivity, and vesicular and cytosolic DA concentration. Impairment of proper DA compartmentalization is postulated to underlie the sensitivity of DA neurons to oxidative damage and degeneration. It is known that DA can auto-oxidize in the cytosol to form quinones and other oxidative species and that this production of oxidative stress is thought to be a critical factor in DA terminal loss after methamphetamine (METH) exposure. Using a mutant strain of mice (VMAT2 LO), which have only 5-10% of the VMAT2 expressed by wild-type animals, we show that VMAT2 is a major determinant of METH toxicity in the striatum. Subsequent to METH exposure, the VMAT2 LO mice show an exacerbated loss of dopamine transporter and tyrosine hydroxylase (TH), as well as enhanced astrogliosis and protein carbonyl formation. More importantly, VMAT2 LO mice show massive argyrophilic deposits in the striatum after METH, indicating that VMAT2 is a regulator of METH-induced neurodegeneration. The increased METH neurotoxicity in VMAT2 LO occurs in the absence of any significant difference in basal temperature or METH-induced hyperthermia. Furthermore, primary midbrain cultures from VMAT2 LO mice show more oxidative stress generation and a greater loss of TH positive processes than wild-type cultures after METH exposure. Elevated markers of neurotoxicity in VMAT2 LO mice and cultures suggest that the capacity to store DA determines the amount of oxidative stress and neurodegeneration after METH administration.

Deletion of Go2alpha abolishes cocaine-induced behavioral sensitization by disturbing the striatal dopamine system

The alpha-subunits of the trimeric Go class of GTPases, comprising the splice variants Go1alpha and Go2alpha, are abundantly expressed in brain and reside on both plasma membrane and synaptic vesicles. Go2alpha is involved in the vesicular storage of monoamines but its physiological relevance is still obscure. We now show that genetic depletion of Go2alpha reduces motor activity induced by dopamine-enhancing drugs like cocaine, as repeated injections of cocaine fail to provoke behavioral sensitization in Go2alpha(-/-) mice. In Go2alpha(-/-) mice, D1 receptor signaling in the striatum is attenuated due to a reduced expression of Golf alpha and Gs alpha. Following cocaine treatment, Go2alpha(-/-) mice have lower D1 and higher D2 receptor amounts compared to wild-type mice. The lack of behavioral sensitization correlates with reduced dopamine levels in the striatum and decreased expression of tyrosine hydroxylase. One reason for the neurochemical changes may be a reduced uptake of monoamines by synaptic vesicles from Go2alpha(-/-) mice as a consequence of a lowered set point for filling. We conclude that Go2alpha optimizes vesicular filling which is instrumental for normal dopamine functioning and for the development of drug-induced behavioral sensitization.

Computational systems analysis of dopamine metabolism

A prominent feature of Parkinson's disease (PD) is the loss of dopamine in the striatum, and many therapeutic interventions for the disease are aimed at restoring dopamine signaling. Dopamine signaling includes the synthesis, storage, release, and recycling of dopamine in the presynaptic terminal and activation of pre- and post-synaptic receptors and various downstream signaling cascades. As an aid that might facilitate our understanding of dopamine dynamics in the pathogenesis and treatment in PD, we have begun to merge currently available information and expert knowledge regarding presynaptic dopamine homeostasis into a computational model, following the guidelines of biochemical systems theory. After subjecting our model to mathematical diagnosis and analysis, we made direct comparisons between model predictions and experimental observations and found that the model exhibited a high degree of predictive capacity with respect to genetic and pharmacological changes in gene expression or function. Our results suggest potential approaches to restoring the dopamine imbalance and the associated generation of oxidative stress. While the proposed model of dopamine metabolism is preliminary, future extensions and refinements may eventually serve as an in silico platform for prescreening potential therapeutics, identifying immediate side effects, screening for biomarkers, and assessing the impact of risk factors of the disease.

Mechanisms by which amphetamine redistributes dopamine out of vesicles: a computational study

Computer simulations of dopamine (DA) and amphetamine interactions associated with dopaminergic storage vesicles were developed in order to better explain how amphetamine causes redistribution of DA out of the vesicles. In the model, DA can be transported into vesicles via the vesicular monoamine transporter. Amphetamine competitively inhibits DA uptake either as a substrate for the transporter or by interference with DA binding to the transporter. Both of the amines can passively diffuse across the membrane in both directions, but only the neutral species can cross the membrane in this manner. The abundance of neutral and positive moieties of the amines is governed by the Henderson-Hasselbalch equation. The model reproduces experimentally observed steady-state DA levels in vesicles, vesicles emptying faster after a change of pH inside the vesicle than after a change in access of DA for the vesicular monoamine transporter, and the impact of amphetamine on DA uptake and efflux in a variety of experimental paradigms. The simulations indicate that a small percentage of DA is constantly diffusing out of vesicles and is replaced by DA entering the vesicle by the vesicular monoamine transporter. Low doses of amphetamine cause DA redistribution out of vesicles primarily by inhibiting DA storage while an enhancement of efflux rates as a result of a change in vesicle pH is added at higher concentrations of amphetamine.

Trafficking of vesicular neurotransmitter transporters

Vesicular neurotransmitter transporters are required for the storage of all classical and amino acid neurotransmitters in secretory vesicles. Transporter expression can influence neurotransmitter storage and release, and trafficking targets the transporters to different types of secretory vesicles. Vesicular transporters traffic to synaptic vesicles (SVs) as well as large dense core vesicles and are recycled to SVs at the nerve terminal. Some of the intrinsic signals for these trafficking events have been defined and include a dileucine motif present in multiple transporter subtypes, an acidic cluster in the neural isoform of the vesicular monoamine transporter (VMAT) 2 and a polyproline motif in the vesicular glutamate transporter (VGLUT) 1. The sorting of VMAT2 and the vesicular acetylcholine transporter to secretory vesicles is regulated by phosphorylation. In addition, VGLUT1 uses alternative endocytic pathways for recycling back to SVs following exocytosis. Regulation of these sorting events has the potential to influence synaptic transmission and behavior.

Expression and function of the rat vesicular monoamine transporter 2

The vesicular monoamine transporters (VMATs) are essential proteins, involved in the storage of monoamines in the central nervous system and in endocrine cells, in a process that involves exchange of 2H+with one substrate molecule. The VMATs interact with various native substrates and clinically relevant drugs and display the pharmacological profile of multidrug transporters. Vesicular transporters suffer from a lack of biochemical and structural data due to the difficulties in their expression. In this work we present the high-level expression of rat VMAT2 (rVMAT2) in a stable a human embryonic kidney cell line (HEK293), generated using the resistance to the neurotoxin 1-methyl-4-phenylpyridinium (MPP+) conferred by the protein. In addition, we describe novel procedures for the solubilization and purification of active protein, and its reconstitution into proteoliposomes. The partially purified protein in detergent binds the inhibitor tetrabenazine and, after reconstitution, displays high levels of ΔμH+-driven electrogenic transport of serotonin. The reconstituted purified rVMAT2 has wild-type affinity for serotonin, and its turnover rate is ∼0.4 substrate molecule/s.

Experimenting with spirituality: analyzing The God Gene in a nonmajors laboratory course

References linking genes to complex human traits, such as personality type or disease susceptibility, abound in the news media and popular culture. In his book The God Gene: How Faith is Hardwired into Our Genes, Dean Hamer argues that a variation in the VMAT2 gene plays a role in one's openness to spiritual experiences. In a nonmajors class, we read and discussed The God Gene and conducted on a small scale an extension of the study it describes. Students used polymerase chain reaction to replicate a portion of their VMAT2 genes, and they analyzed three polymorphic sites in the sequence of these products. Associations between particular VMAT2 alleles and scores on a personality test were assessed by t test. The course, of which this project was a major part, stimulated student learning; scores on a test covering basic genetic concepts, causation/correlation, and laboratory methodology improved after completion of the course. In a survey, students reported the laboratory project aided their learning, especially in the areas of statistics and the linking of genes to behaviors. They reported high levels of engagement with the project, citing in particular its personal nature as motivating their interest.

A metabolomic study of brain tissues from aged mice with low expression of the vesicular monoamine transporter 2 (VMAT2) gene

The vesicular monoamine transporter 2 (VMAT2) sequesters monoamines into synaptic vesicles in preparation for neurotransmission. Samples of cerebellum, cortex, hippocampus, substantia nigra and striatum from VMAT2-deficient mice were compared to age-matched control mice. Multivariate statistical analyses of (1)H NMR spectral profiles separated VMAT2-deficient mice from controls for all five brain regions. Although the data show that metabolic alterations are region- and age-specific, in general, analyses indicated decreases in the concentrations of taurine and creatine/phosphocreatine and increases in glutamate and N-acetyl aspartate in VMAT2-deficient mouse brain tissues. This study demonstrates the efficacy of metabolomics as a functional genomics phenotyping tool for mouse models of neurological disorders, and indicates that mild reductions in the expression of VMAT2 affect normal brain metabolism.

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

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

A small dopamine permeability of storage vesicle membranes and end product inhibition of tyrosine hydroxylase are sufficient to explain changes occurring in dopamine synthesis and storage after inhibition of neuron firing

Computer simulations of dopamine (DA) regulation at a striatal varicosity were developed to determine basic principles that explain the pattern of changes in level of neurotransmitter and its rate of synthesis and metabolism when DA neuron firing is inhibited. The models suggest that DA synthesis is normally at a slower rate because of end-product inhibition of tyrosine hydroxylase (TH) by cytosolic DA. The vast majority of DA in the cytosol arrives there via "recycling"--DA that was released during an exocytotic event is moved into the cytosol via the dopamine transporter (DAT). When neuronal firing is inhibited, the amount of cytosolic DA markedly decreases as there is no recycling. The rate of DA synthesis then increases because of the loss of end-product inhibition of TH. The newly synthesized DA is stored in vesicles, thus increasing the total amount of DA in the vesicles. A small amount of DA is continually leaking out of vesicles, and the amount leaking out increases proportionally to the amount of DA in vesicles. When the amount of DA leaking out balances the amount being stored by the vesicular monoamine transporter, DA accumulates in the cytosol. The accumulating DA inhibits TH activity, and the system enters a steady state condition characterized by approximately double the normal amount of DA in vesicles and approximately normal rate of DA synthesis and metabolism.

Visualization of monoaminergic neurons and neurotoxicity of MPTP in live transgenic zebrafish

We describe an enhancer trap transgenic zebrafish line, ETvmat2:GFP, in which most monoaminergic neurons are labeled by green fluorescent protein (GFP) during embryonic development. The reporter gene of ETvmat2:GFP was inserted into the second intron of vesicular monoamine transporter 2 (vmat2) gene, and the GFP expression pattern recapitulates that of the vmat2 gene. The GFP positive neurons include the large and pear-shaped tyrosine hydroxylase positive neurons (TH populations 2 and 4) in the posterior tuberculum of ventral diencephalon (PT neurons), which are thought to be equivalent to the midbrain dopamine neurons in mammals. We found that these PT neurons and two other GFP labeled non-TH type neuronal groups, one in the paraventricular organ of the posterior tuberculum and the other in the hypothalamus, were significantly reduced after exposure to MPTP, while the rest of GFP-positive neuronal clusters, including those in telencephalon, pretectum, raphe nuclei and locus coeruleus, remain largely unchanged. Furthermore, we showed that the effects of hedgehog signaling pathway inhibition on the development of monoaminergic neurons can be easily visualized in individual living ETvmat2:GFP embryos. This enhancer trap line should be useful for genetic and pharmacological analyses of monoaminergic neuron development and processes underlying Parkinson's disease.

Disruption of dopamine homeostasis underlies selective neurodegeneration mediated by alpha-synuclein

A key challenge in Parkinson's disease research is to understand mechanisms underlying selective degeneration of dopaminergic neurons mediated by genetic factors such as alpha-synuclein (alpha-Syn). The present study examined whether dopamine (DA)-dependent oxidative stress underlies alpha-Syn-mediated neurodegeneration using Drosophila primary neuronal cultures. Green fluorescent protein (GFP) was used to identify live dopaminergic neurons in primary cultures prepared on a marked photoetched coverslip, which allowed us to repeatedly access preidentified dopaminergic neurons at different time points in a non-invasive manner. This live tracking of GFP-marked dopaminergic neurons revealed age-dependent neurodegeneration mediated by a mutant human alpha-Syn (A30P). Degeneration was rescued when alpha-Syn neuronal cultures were incubated with 1 mm glutathione from Day 3 after culturing. Furthermore, depletion of cytoplasmic DA by 100 microm alpha-methyl-p-tyrosine completely rescued the early stage of alpha-Syn-mediated dopaminergic cell loss, demonstrating that DA plays a major role in oxidative stress-dependent neurodegeneration mediated by alpha-Syn. In contrast, overexpression of a Drosophila tyrosine hydroxylase gene (dTH1) alone caused DA neurodegeneration by enhanced DA synthesis in the cytoplasm. Age-dependent dopaminergic cell loss was comparable in alpha-Syn vs dTH1-overexpressed neuronal cultures, indicating that increased DA levels in the cytoplasm is a critical change downstream of mutant alpha-Syn function. Finally, overexpression of a Drosophila vesicular monoamine transporter rescued alpha-Syn-mediated neurodegeneration through enhanced sequestration of cytoplasmic DA into synaptic vesicles, further indicating that a main cause of selective neurodegeneration is alpha-Syn-induced disruption of DA homeostasis. All of these results demonstrate that elevated cytoplasmic DA is a main factor underlying the early stage of alpha-Syn-mediated neurodegeneration.

Vmat2 heterozygous mutant mice display a depressive-like phenotype

The vesicular monoamine transporter 2 (VMAT2) is localized primarily within the CNS and is responsible for transporting monoamines from the cytoplasm into secretory vesicles. Because reserpine (a VMAT inhibitor) can precipitate depressive-like symptoms in humans, we investigated whether Vmat2 heterozygous (HET) mice present with depressive-like behaviors. The mutants showed locomotor and rearing retardation in the open field and appeared anhedonic to 1 and 1.5% sucrose solutions. Immobility times for Vmat2 heterozygotes were prolonged in forced swim and imipramine normalized this behavior. HET animals also showed enhanced immobility in tail suspension and this response was alleviated by fluoxetine, reboxetine, and bupropion. Stimulated GTPgammaS binding indicated that alpha2-adrenergic receptors in HET hippocampus were more sensitive to UK 14,304 (5-bromo-N-(4,5-dihydro-1-H-imidazol-2-yl)-6-quinoxalinamine) stimulation than in wild type (WT) mice. In learned helplessness, mice were exposed to a shuttle box for 4 d or were given inescapable foot-shocks for the same time period. On day 5, all animals were tested in shock escape. Failure rates and the latency to escape were similar for WT and HET mice that were only pre-exposed to the test apparatus. In foot-shock groups, learned helplessness was more robust in heterozygotes than in WT controls. Basal secretion of serum corticosterone was not distinguished by genotype; however, corticosterone levels in mutants were more responsive to stress. Anxiety-like responses of WT and HET animals in the open field, light-dark exploration, zero maze, and novelty-suppressed feeding tests were indistinguishable. Collectively, these findings suggest that Vmat2 heterozygotes display a depressive-like phenotype that is devoid of anxiety-like behavior.