Altered Dopamine Transporter Research Articles

Therapeutic Potential of Repeated Intravenous Transplantation of Human Adipose-Derived Stem Cells in Subchronic MPTP-Induced Parkinson's Disease Mouse Model.

Parkinson’s disease (PD) is the second most common neurodegenerative disease, which is clinically and pathologically characterized by motor dysfunction and the loss of dopaminergic neurons in the substantia nigra, respectively. PD treatment with stem cells has long been studied by researchers; however, no adequate treatment strategy has been established. The results of studies so far have suggested that stem cell transplantation can be an effective treatment for PD. However, PD is a progressively deteriorating neurodegenerative disease that requires long-term treatment, and this has been insufficiently studied. Thus, we aimed to investigate the therapeutic potential of human adipose-derived stem cells (hASC) for repeated vein transplantation over long-term in an animal model of PD. In 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD model mice, hASCs were administered on the tail vein six times at two-week intervals. After the last injection of hASCs, motor function significantly improved. The number of dopaminergic neurons present in the nigrostriatal pathway was recovered using hASC transplantation. Moreover, the administration of hASC restored altered dopamine transporter expression and increased neurotrophic factors, such as brain-derived neurotrophic factor (BDNF) and glial cell-derived neurotrophic factor (GDNF), in the striatum. Overall, this study suggests that repeated intravenous transplantation of hASC may exert therapeutic effects on PD by restoring BDNF and GDNF expressions, protecting dopaminergic neurons, and maintaining the nigrostriatal pathway.

Behavioral characterization of DAT-KO rats and evidence of asocial-like phenotypes in DAT-HET rats: The potential involvement of norepinephrine system

Dopamine (DA) is a key neurotransmitter of the central nervous system, whose availability is regulated by the dopamine transporter (DAT). Deletion of DAT gene leading to hyperdopaminergia was previously performed on mouse models. This enabled recapitulation of the core symptoms of Attention-Deficit / Hyper-activity Disorder (ADHD), which include hyperactivity, inattention and cognitive impairment. We used recently developed DAT knockout (DAT-KO) rats to carry out further behavioral profiling on this novel model of hyperdopaminergia.DAT-KO rats display elevated locomotor activity and restless environmental exploration, associated with a transient anxiety profile. Furthermore, these rats show pronounced stereotypy and compulsive-like behavior at the Marble-Burying test. Homozygous DAT-KO rats mantain intact social interaction when tested in a social-preference task, while heterozygous (HET) rats show high inactivity associated with close proximity to the social stimulus. Ex-vivo evaluation of brain catecholamines highlighted increased levels of norepinephrine in the hippocampus and hypothalamus exclusively of heterozygous rats.Taken together, our data present evidence of unexpected asocial tendencies in heterozygous (DAT-HET) rats associated with neurochemical alterations in norepinephrine neurotransmission. We shed light on the behavioral and neurochemical consequences of altered DAT function in a higher, more complex model of hyperdopaminergia. Unraveling the role of DA neurotransmission in DAT-KO rats has very important implications in the understanding of many psychiatric illnesses, including ADHD, where alterations in DA system have been demonstrated.

Altered dopamine transporter function and immunoregulation in PD macrophages

Parkinson's Disease (PD), the second‐most common neurodegenerative disorder, has three key hallmarks: dysregulated dopamine (DA) signaling in part due to degeneration of Substantia Nigra neurons, intracellular aggregates of a‐synulcein, and neuroinflammation. However, the origin and interplay of these features remains unclear. DA substitution therapy, the standard pharmacological treatment, only alleviates symptoms with limited efficacy. Interestingly, plasma DA levels are altered in PD, and DA can regulate immune cell function in the periphery. Additionally, the dopamine transporter (DAT), a potent regulator of DA tone in the CNS, also exists on leukocytes. However, although it is characterized in the CNS and is a therapeutic target for some neurological disorders, its function in the periphery is largely unknown. Our study asks how DAT regulation of plasma DA modulates the peripheral immune system. To begin to address this hypothesis, we first used biochemical and microscopy approaches to examine DAT distribution and function in the healthy monocyte‐derived human macrophages (MDM). We found DAT constitutively exists in human MDM and is localized to the plasma membrane and in the intracellular space (n=10 biological replicates). We demonstrated the DAT molecules on the human MDM are functional as measured by uptake of IDT307, a fluorescent DAT substrate, and of 3H‐DA (Km=3.178). In addition we found DAT is subjected to canonical trafficking such as PKC‐dependent endocytosis as measured by Total Internal Reflection Florescence Microscopy (TIRF‐M). These data are consistent with the overarching hypothesis that immuno‐activation of human MDM regulates DAT activity and thus dopamine transmission. We discovered lipopolysaccharide (LPS)‐immuno‐activation of human macrophages recruits DAT to the membrane leading to increased surface DAT localization (n= 4 biological replicates P < 0.05). Contrary to our prediction, our preliminary data suggests a decrease in nomifensine‐sensitive, DAT‐mediated substrate uptake. Furthermore, we have also found that basal DAT‐dependent uptake in PD macrophages is elevated (p<0.0001, n=5 biological replicates), and our preliminary data suggests reduced sensitivity to LPS‐induced trafficking. Using simultaneous whole cell patch clamp electrophysiology and amperometry, we are currently testing the hypothesis that LPS‐induced TLR4 activation of macrophages stimulates DAT trafficking to the membrane and stabilizes the transporter in an efflux promoting conformation. We then plan to test the hypothesis that PD macrophages are immunocompromised, exhibiting an altered DAT response to LPS‐stimulation.Support or Funding InformationR21NS103108 R21DA043895This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Dampened Amphetamine-Stimulated Behavior and Altered Dopamine Transporter Function in the Absence of Brain GDNF.

Midbrain dopamine neuron dysfunction contributes to various psychiatric and neurological diseases, including drug addiction and Parkinson's disease. Because of its well established dopaminotrophic effects, the therapeutic potential of glial cell line-derived neurotrophic factor (GDNF) has been studied extensively in various disorders with disturbed dopamine homeostasis. However, the outcomes from preclinical and clinical studies vary, highlighting a need for a better understanding of the physiological role of GDNF on striatal dopaminergic function. Nevertheless, the current lack of appropriate animal models has limited this understanding. Therefore, we have generated novel mouse models to study conditional Gdnf deletion in the CNS during embryonic development and reduction of striatal GDNF levels in adult mice via AAV-Cre delivery. We found that both of these mice have reduced amphetamine-induced locomotor response and striatal dopamine efflux. Embryonic GDNF deletion in the CNS did not affect striatal dopamine levels or dopamine release, but dopamine reuptake was increased due to increased levels of both total and synaptic membrane-associated dopamine transporters. Collectively, these results suggest that endogenous GDNF plays an important role in regulating the function of dopamine transporters in the striatum.SIGNIFICANCE STATEMENT Delivery of ectopic glial cell line-derived neurotrophic factor (GDNF) promotes the function, plasticity, and survival of midbrain dopaminergic neurons, the dysfunction of which contributes to various neurological and psychiatric diseases. However, how the deletion or reduction of GDNF in the CNS affects the function of dopaminergic neurons has remained unknown. Using conditional Gdnf knock-out mice, we found that endogenous GDNF affects striatal dopamine homeostasis and regulates amphetamine-induced behaviors by regulating the level and function of dopamine transporters. These data regarding the physiological role of GDNF are relevant in the context of neurological and neurodegenerative diseases that involve changes in dopamine transporter function.

An acute, epitope-specific modification in the dopamine transporter associated with methamphetamine-induced neurotoxicity.

Preclinical studies demonstrate that repeated, high-dose methamphetamine administrations rapidly decrease plasmalemmal dopamine uptake, which may contribute to aberrant dopamine accumulation, reactive species generation, and long-term dopaminergic deficits. The present study extends these findings by demonstrating a heretofore unreported, epitope-specific modification in the dopamine transporter caused by a methamphetamine regimen that induces these deficits. Specifically, repeated, high-dose methamphetamine injections (4 × 10 mg/kg/injection, 2-h intervals) rapidly decreased immunohistochemical detection of striatal dopamine transporter as assessed 1 h after the final methamphetamine exposure. In contrast, neither a single high dose (1 × 10 mg/kg) nor repeated injections of a lower dose (4 × 2 mg/kg/injection) induced this change. The high-dose regimen-induced alteration was only detected using antibodies directed against the N-terminus. Immunohistochemical staining using antibodies directed against the C-terminus did not reveal any changes. The high-dose regimen also did not alter dopamine transporter expression as assessed using [(125) I]RTI-55 autoradiography. These data suggest that the repeated, high-dose methamphetamine regimen alters the N-terminus of the dopamine transporter. Further, these data may be predictive of persistent dopamine deficits caused by the stimulant. Future studies of the signaling cascades involved should provide novel insight into potential mechanisms underlying the physiological and pathophysiological regulation of the dopamine transporter.

Dopamine Transporter Activity Is Modulated by α-Synuclein

The duration and strength of the dopaminergic signal are regulated by the dopamine transporter (DAT). Drug addiction and neurodegenerative and neuropsychiatric diseases have all been associated with altered DAT activity. The membrane localization and the activity of DAT are regulated by a number of intracellular proteins. α-Synuclein, a protein partner of DAT, is implicated in neurodegenerative disease and drug addiction. Little is known about the regulatory mechanisms of the interaction between DAT and α-synuclein, the cellular location of this interaction, and the functional consequences of this interaction on the basal, amphetamine-induced DAT-mediated dopamine efflux, and membrane microdomain distribution of the transporter. Here, we found that the majority of DAT·α-synuclein protein complexes are found at the plasma membrane of dopaminergic neurons or mammalian cells and that the amphetamine-mediated increase in DAT activity enhances the association of these proteins at the plasma membrane. Further examination of the interaction of DAT and α-synuclein revealed a transient interaction between these two proteins at the plasma membrane. Additionally, we found DAT-induced membrane depolarization enhances plasma membrane localization of α-synuclein, which in turn increases dopamine efflux and enhances DAT localization in cholesterol-rich membrane microdomains.

Dopamine Transporter Activity Is Modulated by α-synuclein.

The duration and strength of the dopaminergic signal is regulated by the dopamine transporter (DAT). Drug addiction, neurodegenerative and neuropsychiatric diseases have all been associated with altered DAT activity. The membrane localization and the activity of DAT are regulated by a number of intracellular proteins. α-synuclein, a protein partner of DAT, is implicated in neurodegenerative disease and drug addiction. Little is known about the regulatory mechanisms of the interaction between DAT and α-synuclein, the cellular location of this interaction, and the functional consequences of this interaction on the basal, amphetamine (AMPH) induced DAT-meditated DA efflux and membrane microdomain distribution of the transporter. Here, we found that the majority of DAT/α-synuclein protein complexes are found at the plasma membrane of dopaminergic neurons or mammalian cells, and that AMPH-mediated increase in DAT activity enhances the association of these proteins at the plasma membrane. Further examination of the interaction of DAT and α-synuclein revealed a transient interaction between these two proteins at the plasma membrane. Additionally, we found DAT-induced membrane depolarization enhances plasma membrane localization of α-synuclein, which in turn increases DA efflux and enhances DAT localization in cholesterol rich membrane microdomains.

Does reverse transport of dopamine play a role in autism?

genetic variants affecting the reverse transport of DA might influence the risk of common, sporadic forms of autism remains open. It is now widely accepted that rare genetic variants could play an important role in susceptibility to common diseases. In light of Cartier et al.'s results, major resequencing efforts of the SLC6A3 and STX1A genes should be undertaken in large cohorts of individuals with autism, with significant insights into disease biology likely from these results. Another interesting implication of the work by Cartier and colleagues lies in unraveling the therapeutic potential of DA reverse transport in autism. Starting from the observation that rare autism-causing variants are associated with inhibition of reverse transport of DA, it could be hypothesized that psychostimulants may serve as a potential therapeutic aid in autism. Notably, not only have psychostimulants been suggested to be effective for ADHD-like symptoms in autism spectrumdisorderindividuals,buttheprevalenceratesoftheiruseinautism

Differential influence of dopamine transport rate on the potencies of cocaine, amphetamine, and methylphenidate.

Dopamine transporter (DAT) levels vary across brain regions and individuals, and are altered by drug history and disease states; however, the impact of altered DAT expression on psychostimulant effects in brain has not been systematically explored. Using fast scan cyclic voltammetry, we measured the effects of elevated DAT levels on presynaptic dopamine parameters as well as the uptake inhibition potency of the blockers cocaine and methylphenidate (MPH) and the releaser amphetamine (AMPH) in the nucleus accumbens core. Here we found that increases in DAT levels, resulting from either genetic overexpression or MPH self-administration, caused markedly increased maximal rates of uptake (Vmax) that were positively correlated with the uptake inhibition potency of AMPH and MPH, but not cocaine. AMPH and MPH were particularly sensitive to DAT changes, with a 100% increase in Vmax resulting in a 200% increase in potency. The relationship between Vmax and MPH potency was the same as that for AMPH, but was different from that for cocaine, indicating that MPH more closely resembles a releaser with regard to uptake inhibition. Conversely, the effects of MPH on stimulated dopamine release were similar to those of cocaine, with inverted U-shaped increases in release over a concentration–response curve. This was strikingly different from the release profile of AMPH, which showed only reductions at high concentrations, indicating that MPH is not a pure releaser. These data indicate that although MPH is a DAT blocker, its uptake-inhibitory actions are affected by DAT changes in a similar manner to releasers. Together, these data show that fluctuations in DAT levels alter the potency of releasers and MPH but not blockers and suggest an integral role of the DAT in the addictive potential of AMPH and related compounds.

Amphetamine and methamphetamine reduce striatal dopamine transporter function without concurrent dopamine transporter relocalization

Amphetamine (AMPH) and methamphetamine (METH) alter dopamine transporter (DAT) function. In vitro heterologous cell line and synaptosome studies demonstrate AMPH-induced DAT internalization, implicating relocalization in reduced DAT uptake following drug exposure. However, few studies have evaluated DAT localization following in vivo AMPH/METH administration. To determine DAT subcellular localization following drug administration, a centrifugation technique was developed to isolate striatal synaptosomal membrane and vesicle fractions. DAT was distributed between the synaptosomal membrane (60%) and endosomal vesicles (40%), and in vitro application of the protein kinase C activator phorbol 12-myristate 13-acetate to striatal synaptosomes caused DAT internalization into the vesicle fractions. In contrast, neither single nor repeated in vivo AMPH and/or METH administrations altered DAT localization 5, 15, 30, or 60 min post-treatment, despite reduced DAT uptake. Importantly, repeated METH injections uniformly decreased total DAT immunoreactivity within all fractions 7 days post-treatment. These findings suggest that factors other than internalization can contribute to the observed acute and persistent DAT dysfunction and dopaminergic deficits following in vivo AMPH or METH administration.

Quantification of reserve pool dopamine in methionine sulfoxide reductase A null mice

Methionine sulfoxide reductase A knockout (MsrA−/−) mice, which serve as a potential model for neurodegeneration, suffer from increased oxidative stress and have previously been found to have chronically elevated brain dopamine (DA) content levels relative to control mice. Additionally, these high levels parallel the increased presynaptic DA release. In this study, fast-scan cyclic voltammetry (FSCV) at carbon-fiber microelectrodes was used to quantify striatal reserve pool DA in knockout mice and wild-type control mice. Reserve pool DA efflux, induced by amphetamine (AMPH), was measured in brain slices from knockout and wild type (WT) mice in the presence of α-methyl-p-tyrosine, a DA synthesis inhibitor. Additionally, the stimulated release of reserve pool DA, mobilized by cocaine (COC), was measured. Both efflux and stimulated release measurements were enhanced in slices from knockout mice, suggesting that these mice have greater reserve pool DA stores than wild-type and that these stores are effectively mobilized. Moreover, dopamine transporter (DAT) labeling data indicate that the difference in measured DA efflux was likely not caused by altered DAT protein expression. Additionally, slices from MsrA−/− and wild-type mice were equally responsive to increasing extracellular calcium concentrations, suggesting that potential differences in either calcium entry or intracellular calcium handling are not responsible for increased reserve pool DA release. Collectively, these results demonstrate that MsrA−/− knockout mice maintain a larger DA reserve pool than wild-type control mice, and that this pool is readily mobilized.

Iron chelation down-regulates dopamine transporter expression by decreasing mRNA stability and increasing endocytosis in N2a cells

Cell surface expression of the dopamine transporter (DAT) is determined by the relative rates of its internalization and recycling. Changes in the cellular labile iron pool (LIP) affect many cellular mechanisms including those that regulate DAT trafficking. In this study, we analyzed DAT expression and posttranslational modifications in response to changes in cellular iron in transfected neuroblastoma cells (N2a). Iron chelation by desferrioxamine (DFO) altered DAT protein levels by decreasing the stability of DAT mRNA. Increased phosphorylation and ubiquitination of this transporter protein following DFO treatment were also observed. Cellular iron depletion elevated protein levels of the early endosomal marker Rab5. Moreover, confocal microscopy studies showed increased localization of DAT into the endosomal compartment in DFO-treated cells compared to control. Together, these findings suggest that cellular iron depletion regulates DAT expression through reducing mRNA stability as well as an increasing in endocytosis.

Effect of zinc and paraquat co-exposure on neurodegeneration: Modulation of oxidative stress and expression of metallothioneins, toxicant responsive and transporter genes in rats

Oxidative stress is implicated in Parkinson's disease (PD). Metallothioneins (MT), cytochrome P450 IIE1 (CYP2E1) and glutathione S-transferases alpha4-4 (GSTA4-4) are involved in oxidative stress-mediated damage. Altered dopamine transporter (DAT) and vesicular monoamine transporter-2 (VMAT-2) are also documented in PD. The present study was undertaken to investigate the effect of Zn and PQ co-exposure on neurodegeneration in rats. A significant reduction was observed in spontaneous locomotor activity (SLA), striatal dopamine (DA) levels, tyrosine hydroxylase (TH) immunoreactivity, glutathione reductase (GR) and catalase activity along with increased lipid peroxidation (LPO) and glutathione peroxidase (GPx) activity after Zn and/or PQ exposure. Zn and/or PQ exposure increased gene expression of DAT, CYP2E1, GSTA4-4, MT-I and MT-II, but reduced the expression of VMAT-2. Protein expression analysis of TH, VMAT-2 and DAT showed results similar to those obtained with gene expression study. Zn and PQ co-exposure caused a more pronounced effect than that of individual exposure. The results obtained in this study suggest that, similar to PQ, Zn induced neurodegeneration via alterations in oxidative stress and expression of the above-mentioned genes. However, the effect of Zn+PQ was only slightly higher than that of alone, indicating that probably Zn and PQ follow some different molecular events leading to neurodegeneration.

Function of dopamine transporter is compromised in DYT1 transgenic animal model in vivo

Early onset torsion dystonia (DYT1), the most common form of hereditary primary dystonia, is caused by a mutation in the TOR1A gene, which codes for the protein, torsinA. We previously examined the effect of the human mutant torsinA on striatal dopaminergic function in a conventional transgenic mouse model of DYT1 dystonia (hMT1), in which human mutant torsinA is expressed under the cytomegalovirus promotor. Systemic administration of amphetamine did not increase dopamine (DA) release as efficiently in these mice as compared with wild-type transgenic and non-transgenic mice. We, now, studied the contribution of the DA transporter (DAT) to amphetamine-induced DA release in hMT1 transgenic mice using in vivo no-net flux microdialysis. This method applies different concentrations of DA through the microdialysis probe and measures DA concentration at the output of the probe following an equilibrium period. The slope (extraction fraction) is the measure of the DAT activity in vivo. The slope for hMT1 transgenic mice was 0.58 +/- 0.07 and for non-transgenic animals, 0.87 +/- 0.06 (p < 0.05). We further investigated the efficacy of nomifensine (a specific DAT inhibitor) in inhibiting amphetamine-induced DA release. Local application of nomifensine 80 min before the systemic application of amphetamine inhibited DA release in both transgenic mice and their non-transgenic littermates. The efficiency of the inhibition appeared to be different, with mean values of 48% for hMT1 transgenic mice versus 84% for non-transgenic littermates. Moreover, we have evaluated basal and amphetamine-induced locomotion in hMT1 transgenic mice compared with their non-transgenic littermates, using an O-maze behavioral chamber. Basal levels of locomotion in the hMT1 transgenic mice showed that they moved much less than their non-transgenic littermates (0.9 +/- 0.3 m for transgenic mice vs. 2.4 +/- 0.7 m for non-transgenic littermates, p < 0.05). This relative reduction in locomotion was also observed following amphetamine administration (48.5 +/- 6.7 m for transgenics vs. 73.7 +/- 9.8 m for non-transgenics, p < 0.05). These results support the finding that there are altered dynamics of DA release and reuptake in hMT1 transgenic mice in vivo, with DAT activity is reduced in the presence of mutant torsinA, which is consistent with behavioral consequences such as reduced locomotion and (previously described) abnormal motor phenotypes such as increased hind-base width and impaired performance on the raised-beam task. These data implies that altered DAT function may contribute to impaired DA neurotransmission and clinical symptoms in human DYT1 dystonia.

Altered dopamine transporter function and phosphorylation following chronic cocaine self-administration and extinction in rats

Cocaine binds with the dopamine transporter (DAT), an effect that has been extensively implicated in its reinforcing effects. However, persisting adaptations in DAT regulation after cocaine self-administration have not been extensively investigated. Here, we determined the changes in molecular mechanisms of DAT regulation in the caudate-putamen (CPu) and nucleus accumbens (NAcc) of rats with a history of cocaine self-administration, followed by 3 weeks of withdrawal under extinction conditions (i.e., no cocaine available). DA uptake was significantly higher in the CPu of cocaine-experienced animals as compared to saline-yoked controls. DAT V max was elevated in the CPu without changes in apparent affinity for DA. In spite of elevated CPu DAT activity, total and surface DAT density and DAT-PP2Ac (protein phosphatase 2A catalytic subunit) interaction remained unaltered, although p-Ser- DAT phosphorylation was elevated. In contrast to the CPu, there were no differences between cocaine and saline rats in the levels of DA uptake, DAT V max and K m values, total and surface DAT, p-Ser-DAT phosphorylation, or DAT-PP2Ac interactions in the NAcc. These results show that chronic cocaine self-administration leads to lasting, regionally specific alterations in striatal DA uptake and DAT-Ser phosphorylation. Such changes may be related to habitual patterns of cocaine-seeking observed during relapse.

Preweaning manganese exposure causes hyperactivity, disinhibition, and spatial learning and memory deficits associated with altered dopamine receptor and transporter levels

Epidemiological studies in children have reported associations between elevated dietary manganese (Mn) exposure and neurobehavioral and neurocognitive deficits. To better understand the relationship between early Mn exposure and neurobehavioral deficits, we treated neonate rats with oral Mn doses of 0, 25, or 50 mg Mn/kg/day over postnatal day (PND) 1-21, and evaluated behavioral performance using open arena (PND 23), elevated plus maze (PND 23), and 8-arm radial maze (PND 33-46) paradigms. Brain dopamine D1 and D2-like receptors, and dopamine transporter (DAT) densities were determined on PND 24, and blood and brain Mn levels were measured to coincide with behavioral testing (PND 24, PND 36). Preweaning Mn exposure caused hyperactivity and behavioral disinhibition in the open arena, but no altered behavior in the elevated plus maze. Manganese-exposed males committed significantly more reference and marginally more working errors in the radial arm maze compared to controls. Fewer Mn exposed males achieved the radial maze learning criterion, and they required more session days to reach it compared to controls. Manganese-exposed animals also exhibited a greater frequency of stereotypic response strategy in searching for the baited arms in the maze. These behavioral and learning deficits were associated with altered expression of the dopamine D1 and D2 receptors and the DAT in prefrontal cortex, nucleus accumbens, and dorsal striatum. These data corroborate epidemiological studies in children, and suggest that exposure to Mn during neurodevelopment significantly alters dopaminergic synaptic environments in brain nuclei that mediate control of executive function behaviors, such as reactivity and cognitive flexibility.

Using in Vivo Electrochemistry To Study the Physiological Effects of Cocaine and Other Stimulants on the Drosophila melanogaster Dopamine Transporter

Dopamine neurotransmission is thought to play a critical role in addiction reinforcing mechanisms of drugs of abuse. Electrochemical techniques have been employed extensively for monitoring in vivo dopamine changes in the brains of model organisms including rats, mice, and primates. Here, we investigated the effects of several stimulants on dopamine clearance using recently developed microanalytical tools for in vivo electrochemical measurements of dopamine in the central nervous system of Drosophila melanogaster. A cylindrical carbon-fiber microelectrode was placed in the protocerebral anterior medial region of the Drosophila brain (an area dense with dopamine neurons) while a micropipette injector was positioned to exogenously apply dopamine. Background-subtracted fast-scan cyclic voltammetry was carried out to quantify changes in dopamine concentration in the adult fly brain. Clearance of exogenously applied dopamine was significantly decreased in the protocerebral anterior medial area of the wild-type fly following treatment with cocaine, amphetamine, methamphetamine, or methylphenidate. In contrast, dopamine uptake remained unchanged when identical treatments were employed in fumin mutant flies that lack functional dopamine transporters. Our in vivo results support in vitro binding affinity studies that predict these four stimulants effectively block normal Drosophila dopamine transporter function. Furthermore, we found 10 muM to be a sufficient physiological cocaine concentration to significantly alter dopamine transporter uptake in the Drosophila central nervous system. Taken together, these data indicate dopamine uptake in the Drosophila brain is decreased by psychostimulants as observed in mammals. This validates the use of Drosophila as a model system for future studies into the cellular and molecular mechanisms underlying drug addiction in humans.

Homeostatic mechanisms in dopamine synthesis and release: a mathematical model.

BackgroundDopamine is a catecholamine that is used as a neurotransmitter both in the periphery and in the central nervous system. Dysfunction in various dopaminergic systems is known to be associated with various disorders, including schizophrenia, Parkinson's disease, and Tourette's syndrome. Furthermore, microdialysis studies have shown that addictive drugs increase extracellular dopamine and brain imaging has shown a correlation between euphoria and psycho-stimulant-induced increases in extracellular dopamine [1]. These consequences of dopamine dysfunction indicate the importance of maintaining dopamine functionality through homeostatic mechanisms that have been attributed to the delicate balance between synthesis, storage, release, metabolism, and reuptake.MethodsWe construct a mathematical model of dopamine synthesis, release, and reuptake and use it to study homeostasis in single dopaminergic neuron terminals. We investigate the substrate inhibition of tyrosine hydroxylase by tyrosine, the consequences of the rapid uptake of extracellular dopamine by the dopamine transporters, and the effects of the autoreceoptors on dopaminergic function. The main focus is to understand the regulation and control of synthesis and release and to explicate and interpret experimental findings.ResultsWe show that the substrate inhibition of tyrosine hydroxylase by tyrosine stabilizes cytosolic and vesicular dopamine against changes in tyrosine availability due to meals. We find that the autoreceptors dampen the fluctuations in extracellular dopamine caused by changes in tyrosine hydroxylase expression and changes in the rate of firing. We show that short bursts of action potentials create significant dopamine signals against the background of tonic firing. We explain the observed time courses of extracellular dopamine responses to stimulation in wild type mice and mice that have genetically altered dopamine transporter densities and the observed half-lives of extracellular dopamine under various treatment protocols.ConclusionDopaminergic systems must respond robustly to important biological signals such as bursts, while at the same time maintaining homeostasis in the face of normal biological fluctuations in inputs, expression levels, and firing rates. This is accomplished through the cooperative effect of many different homeostatic mechanisms including special properties of tyrosine hydroxylase, the dopamine transporters, and the dopamine autoreceptors.

Postnatal manganese exposure alters dopamine transporter function in adult rats: Potential impact on nonassociative and associative processes.

In the present study, we examined whether exposing rats to a high-dose regimen of manganese chloride (Mn) during the postnatal period would depress presynaptic dopamine functioning and alter nonassociative and associative behaviors. To this end, rats were given oral supplements of Mn (750 microg/day) on postnatal days (PD) 1-21. On PD 90, dopamine transporter (DAT) immunoreactivity and [3H]dopamine uptake were assayed in the striatum and nucleus accumbens, while in vivo microdialysis was used to measure dopamine efflux in the same brain regions. The effects of postnatal Mn exposure on nigrostriatal functioning were evaluated by assessing rotorod performance and amphetamine-induced stereotypy in adulthood. In terms of associative processes, both cocaine-induced conditioned place preference (CPP) and sucrose-reinforced operant responding were examined. Results showed that postnatal Mn exposure caused persistent declines in DAT protein expression and [3H]dopamine uptake in the striatum and nucleus accumbens, as well as long-term reductions in striatal dopamine efflux. Rotorod performance did not differ according to exposure condition, however Mn-exposed rats did exhibit substantially more amphetamine-induced stereotypy than vehicle controls. Mn exposure did not alter performance on any aspect of the CPP task (preference, extinction, or reinstatement testing), nor did Mn affect progressive ratio responding (a measure of motivation). Interestingly, acquisition of a fixed ratio task was impaired in Mn-exposed rats, suggesting a deficit in procedural learning. In sum, these results indicate that postnatal Mn exposure causes persistent declines in various indices of presynaptic dopaminergic functioning. Mn-induced alterations in striatal functioning may have long-term impact on associative and nonassociative behavior.

P.1.30 Inflammation down-regulates β2-adrenoceptor expression on primary glia: reversal by the glucocorticoid dexamethasone

GARCÍA-GIL, L., J. A. RAMOS, T. RUBINO, D. PAROLARO AND J. J. FERNÁNDEZ-RUIZ. Perinatal Δ9-tetrahydrocannabinol exposure did not alter dopamine transporter and tyrosine hydroxylase mRNA levels in midbrain dopaminergic neurons of adult male and female rats. NEUROTOXICOL TERATOL 20(5) 549–553, 1998.—We have recently demonstrated that the magnitude of L-3,4-dihydroxyphenylacetic acid (DOPAC) lowering effect caused by amphetamine in midbrain dopaminergic neurons of adult rats was lesser in animals that had been perinatally exposed to Δ9-tetrahydrocannabinol (Δ9-THC) than controls. In the present study, we have examined whether this loss in the responsiveness to amphetamine might be due to changes at the level of dopamine transporter (DAT), the main molecular site for the action of amphetamine, following the perinatal exposure to Δ9-THC. To this end, we have analyzed DAT mRNA levels, by using in situ hybridization, in the substantia nigra and ventral tegmental area, the areas where cell bodies of DAT-containing midbrain neurons are located, of adult male and female rats that had been perinatally exposed to Δ9-THC. In addition, we also analyzed mRNA levels of tyrosine hydroxylase (TH), the rate-limiting enzyme in DA synthesis. Results were as follows. Both adult male and female rats that had been perinatally exposed to Δ9-THC exhibited similar mRNA levels to controls for both DAT and TH in the substantia nigra as well as in the ventral tegmental area. This observation makes it difficult to support the idea that the differences found in adulthood after pharmacological challenges were caused by irreversible changes at the level of gene expression for these two key proteins.