Dopamine transporter mRNA: dense expression in ventral midbrain neurons

The Dopaminergic Cells in the Median Raphe Region Regulate Social Behavior in Male Mice

According to previous studies, the median raphe region (MRR) is known to contribute significantly to social behavior. Besides serotonin, there have also been reports of a small population of dopaminergic neurons in this region. Dopamine is linked to reward and locomotion, but very little is known about its role in the MRR. To address that, we first confirmed the presence of dopaminergic cells in the MRR of mice (immunohistochemistry, RT-PCR), and then also in humans (RT-PCR) using healthy donor samples to prove translational relevance. Next, we used chemogenetic technology in mice containing the Cre enzyme under the promoter of the dopamine transporter. With the help of an adeno-associated virus, designer receptors exclusively activated by designer drugs (DREADDs) were expressed in the dopaminergic cells of the MRR to manipulate their activity. Four weeks later, we performed an extensive behavioral characterization 30 min after the injection of the artificial ligand (Clozapine-N-Oxide). Stimulation of the dopaminergic cells in the MRR decreased social interest without influencing aggression and with an increase in social discrimination. Additionally, inhibition of the same cells increased the friendly social behavior during social interaction test. No behavioral changes were detected in anxiety, memory or locomotion. All in all, dopaminergic cells were present in both the mouse and human samples from the MRR, and the manipulation of the dopaminergic neurons in the MRR elicited a specific social response.

Dynamic control of the dopamine transporter in neurotransmission and homeostasis

The dopamine transporter (DAT) transports extracellular dopamine into the intracellular space contributing to the regulation of dopamine neurotransmission. A reduction of DAT density is implicated in Parkinson's disease (PD) by neuroimaging; dopamine turnover is dopamine turnover is elevated in early symptomatic PD and in presymptomatic individuals with monogenic mutations causal for parkinsonism. As an integral plasma membrane protein, DAT surface expression is dynamically regulated through endocytic trafficking, enabling flexible control of dopamine signaling in time and space, which in turn critically modulates movement, motivation and learning behavior. Yet the cellular machinery and functional implications of DAT trafficking remain enigmatic. In this review we summarize mechanisms governing DAT trafficking under normal physiological conditions and discuss how PD-linked mutations may disturb DAT homeostasis. We highlight the complexity of DAT trafficking and reveal DAT dysregulation as a common theme in genetic models of parkinsonism.

Dopamine transporter neuroimaging accurately assesses the maturation of dopamine neurons in a preclinical model of Parkinson's disease

BACKGROUND

Significant developments in stem cell therapy for Parkinson's disease (PD) have already been achieved; however, methods for reliable assessment of dopamine neuron maturation in vivo are lacking. Establishing the efficacy of new cellular therapies using non-invasive methodologies will be critical for future regulatory approval and application. The current study examines the utility of neuroimaging to characterise the in vivo maturation, innervation and functional dopamine release of transplanted human embryonic stem cell-derived midbrain dopaminergic neurons (hESC-mDAs) in a preclinical model of PD.

METHODS

Female NIH RNu rats received a unilateral stereotaxic injection of 6-OHDA into the left medial forebrain bundle to create the PD lesion. hESC-mDA cell and sham transplantations were carried out 1 month post-lesion, with treated animals receiving approximately 4 × 10⁵ cells per transplantation. Behavioural analysis, [¹⁸F]FBCTT and [¹⁸F]fallypride microPET/CT, was conducted at 1, 3 and 6 months post-transplantation and compared with histological characterisation at 6 months.

RESULTS

PET imaging revealed transplant survival and maturation into functional dopaminergic neurons. [¹⁸F]FBCTT-PET/CT dopamine transporter (DAT) imaging demonstrated pre-synaptic restoration and [¹⁸F]fallypride-PET/CT indicated functional dopamine release, whilst amphetamine-induced rotation showed significant behavioural recovery. Moreover, histology revealed that the grafted cells matured differently in vivo producing high- and low-tyrosine hydroxylase (TH) expressing cohorts, and only [¹⁸F]FBCTT uptake was well correlated with differentiation.

CONCLUSIONS

This study provides further evidence for the value of in vivo functional imaging for the assessment of cell therapies and highlights the utility of DAT imaging for the determination of early post-transplant cell maturation and differentiation of hESC-mDAs.

Epigenetic Regulation of the Ontogenic Expression of the Dopamine Transporter

The dopamine transporter (DAT) is a plasma membrane transport protein responsible for regulating the duration and intensity of dopaminergic signaling. Altered expression of DAT is linked to neurodevelopmental disorders, including attention deficit hyperactivity disorder and autism spectrum disorder, and is shown to contribute to the response of psychotropic drugs and neurotoxicants. Although the postnatal levels of DAT have been characterized, there are few data regarding the mechanisms that regulate postnatal DAT expression. Here, we examine the ontogeny of DAT mRNA from postnatal days 0 to 182 in the rat brain and define a role for epigenetic mechanisms regulating DAT expression. DAT mRNA and protein significantly increased between PND 0 and 6 months in rat midbrain and striatum, respectively. The epigenetic modifiers Dnmt1, Dnmt3a, Dnmt3b, and Hdac2 demonstrated age associated decreases in mRNA expression whereas Hdac5 and Hdac8 showed increased mRNA expression with age. Chromatin immunoprecipitation studies revealed increased protein enrichment of acetylated histone 3 at lysines 9 and 14 and the dopaminergic transcription factors Nurr1 and Pitx3 within the DAT promoter in an age-related manner. Together these studies provide evidence for the role of epigenetic modifications in the regulation of DAT during development. The identification of these mechanisms may contribute to potential therapeutic interventions aimed at neurodevelopmental disorders of the dopaminergic system.

MP1104, a mixed kappa-delta opioid receptor agonist has anti-cocaine properties with reduced side-effects in rats

Kappa opioid receptor (KOPr) agonists have preclinical anti-cocaine and antinociceptive effects. However, adverse effects including dysphoria, aversion, sedation, anxiety and depression limit their clinical development. MP1104, an analogue of 3-iodobenzoyl naltrexamine, is a potent dual agonist at KOPr and delta opioid receptor (DOPr), with full agonist efficacy at both these receptors. In this study, we evaluate the ability of MP1104 to modulate cocaine-induced behaviors and side-effects preclinically. In male Sprague-Dawley rats trained to self-administer cocaine, MP1104 (0.3 and 1 mg/kg) reduced cocaine-primed reinstatement of drug-seeking behavior and caused significant downward shift of the dose-response curve in cocaine self-administration tests (0.3 and 0.6 mg/kg). The anti-cocaine effects exerted by MP1104 are in part due to increased dopamine (DA) uptake by the dopamine transporter (DAT) in the dorsal striatum (dStr) and nucleus accumbens (NAc). MP1104 (0.3 and 0.6 mg/kg) showed no significant anxiogenic effects in the elevated plus maze, pro-depressive effects in the forced swim test, or conditioned place aversion. Furthermore, pre-treatment with a DOPr antagonist, led to MP1104 producing aversive effects. This data suggests that the DOPr agonist actions of MP1104 attenuate the KOPr-mediated aversive effects of MP1104. The overall results from this study show that MP1104, modulates DA uptake in the dStr and NAc, and exerts potent anti-cocaine properties in self-administration tests with reduced side-effects compared to pure KOPr agonists. This data supports the therapeutic development of dual KOPr/DOPr agonists to reduce the side-effects of selective KOPr agonists. This article is part of the Special Issue entitled 'Opioid Neuropharmacology: Advances in treating pain and opioid addiction'.

Effects of Passage Number and Differentiation Protocol on the Generation of Dopaminergic Neurons from Rat Bone Marrow-Derived Mesenchymal Stem Cells

Multiple studies have demonstrated the ability of mesenchymal stem cells (MSCs) to differentiate into dopamine-producing cells, in vitro and in vivo, indicating their potential to be used in the treatment of Parkinson’s disease (PD). However, there are discrepancies among studies regarding the optimal time (i.e., passage number) and method for dopaminergic induction, in vitro. In the current study, we compared the ability of early (P4) and later (P40) passaged bone marrow-derived MSCs to differentiate into dopaminergic neurons using two growth-factor-based approaches. A direct dopaminergic induction (DDI) was used to directly convert MSCs into dopaminergic neurons, and an indirect dopaminergic induction (IDI) was used to direct MSCs toward a neuronal lineage prior to terminal dopaminergic differentiation. Results indicate that both early and later passaged MSCs exhibited positive expression of neuronal and dopaminergic markers following either the DDI or IDI protocols. Additionally, both early and later passaged MSCs released dopamine and exhibited spontaneous neuronal activity following either the DDI or IDI. Still, P4 MSCs exhibited significantly higher spiking and bursting frequencies as compared to P40 MSCs. Findings from this study provide evidence that early passaged MSCs, which have undergone the DDI, are more efficient at generating dopaminergic-like cells in vitro, as compared to later passaged MSCs or MSCs that have undergone the IDI.

α-Synuclein-Dependent Calcium Entry Underlies Differential Sensitivity of Cultured SN and VTA Dopaminergic Neurons to a Parkinsonian Neurotoxin

Parkinson's disease (PD) is a debilitating neurodegenerative disease characterized by a loss of dopaminergic neurons in the substantia nigra (SN). Although mitochondrial dysfunction and dysregulated α-synuclein (aSyn) expression are postulated to play a role in PD pathogenesis, it is still debated why neurons of the SN are targeted while neighboring dopaminergic neurons of the ventral tegmental area (VTA) are spared. Using electrochemical and imaging approaches, we investigated metabolic changes in cultured primary mouse midbrain dopaminergic neurons exposed to a parkinsonian neurotoxin, 1-methyl-4-phenylpyridinium (MPP+). We demonstrate that the higher level of neurotoxicity in SN than VTA neurons was due to SN neuron-specific toxin-induced increase in cytosolic dopamine (DA) and Ca2+, followed by an elevation of mitochondrial Ca2+, activation of nitric oxide synthase (NOS), and mitochondrial oxidation. The increase in cytosolic Ca2+ was not caused by MPP+-induced oxidative stress, but rather depended on the activity of both L-type calcium channels and aSyn expression, suggesting that these two established pathogenic factors in PD act in concert.

Dopamine receptor activity participates in hippocampal synaptic plasticity associated with novel object recognition

Physiological and behavioral evidence supports that dopamine (DA) receptor signaling influences hippocampal function. While several recent studies examined how DA influences CA1 plasticity and learning, there are fewer studies investigating the influence of DA signaling to the dentate gyrus. The dentate gyrus receives convergent cortical input through the perforant path fiber tracts and has been conceptualized to detect novelty in spatial memory tasks. To test whether DA-receptor activity influences novelty-detection, we used a novel object recognition (NOR) task where mice remember previously presented objects as an indication of learning. Although DA innervation arises from other sources and the main DA signaling may be from those sources, our molecular approaches verified that midbrain dopaminergic fibers also sparsely innervate the dentate gyrus. During the NOR task, wild-type mice spent significantly more time investigating novel objects rather than previously observed objects. Dentate granule cells in slices cut from those mice showed an increased AMPA/NMDA-receptor current ratio indicative of potentiated synaptic transmission. Post-training injection of a D1-like receptor antagonist not only effectively blocked the preference for the novel objects, but also prevented the increased AMPA/NMDA ratio. Consistent with that finding, neither NOR learning nor the increase in the AMPA/NMDA ratio were observed in DA-receptor KO mice under the same experimental conditions. The results indicate that DA-receptor signaling contributes to the successful completion of the NOR task and to the associated synaptic plasticity of the dentate gyrus that likely contributes to the learning.

Dopamine Regulates Aversive Contextual Learning and Associated In Vivo Synaptic Plasticity in the Hippocampus

Dopamine release during reward-driven behaviors influences synaptic plasticity. However, dopamine innervation and release in the hippocampus and its role during aversive behaviors are controversial. Here, we show that in vivo hippocampal synaptic plasticity in the CA3-CA1 circuit underlies contextual learning during inhibitory avoidance (IA) training. Immunohistochemistry and molecular techniques verified sparse dopaminergic innervation of the hippocampus from the midbrain. The long-term synaptic potentiation (LTP) underlying the learning of IA was assessed with a D1-like dopamine receptor agonist or antagonist in ex vivo hippocampal slices and in vivo in freely moving mice. Inhibition of D1-like dopamine receptors impaired memory of the IA task and prevented the training-induced enhancement of both ex vivo and in vivo LTP induction. The results indicate that dopamine-receptor signaling during an aversive contextual task regulates aversive memory retention and regulates associated synaptic mechanisms in the hippocampus that likely underlie learning.

Amphetamine potency varies with dopamine uptake rate across striatal subregions

Amphetamine is a central nervous system psychostimulant with a high potential for abuse. Recent literature has shown that genetic and drug-induced elevations in dopamine transporter (DAT) expression augment the neurochemical and behavioral potency of psychostimulant releasers. However, it remains to be determined if the well-documented differences in DAT levels across striatal regions drive regionally distinct amphetamine effects within individuals. DAT levels and dopamine uptake rates have been shown to follow a gradient in the striatum, with the highest levels in the dorsal regions and lowest levels in the nucleus accumbens shell; thus, we hypothesized that amphetamine potency would follow this gradient. Using fast scan cyclic voltammetry in mouse brain slices, we examined DAT inhibition and changes in exocytotic dopamine release by amphetamine across four striatal regions (dorsal and ventral caudate-putamen, nucleus accumbens core and shell). Consistent with our hypothesis, amphetamine effects at the DAT and on release decreased across regions from dorsal to ventral, and both measures of potency were highly correlated with dopamine uptake rates. Separate striatal subregions are involved in different aspects of motivated behaviors, such as goal-directed and habitual behaviors, that become dysregulated by drug abuse, making it critically important to understand regional differences in drug potencies.

Restoration of cocaine stimulation and reward by reintroducing wild type dopamine transporter in adult knock-in mice with a cocaine-insensitive dopamine transporter

In previous studies, we generated knock-in mice with a cocaine-insensitive dopamine transporter (DAT-CI mice) and found cocaine does not stimulate locomotion or produce reward in these mice, indicating DAT inhibition is necessary for cocaine stimulation and reward. However, DAT uptake is reduced in DAT-CI mice and thus the lack of cocaine responses could be due to adaptive changes. To test this, we used adeno-associated virus (AAV) to reintroduce the cocaine-sensitive wild type DAT (AAV-DATwt) back into adult DAT-CI mice, which restores cocaine inhibition of DAT in affected brain regions but does not reverse the adaptive changes. In an earlier study we showed that AAV-DATwt injections in regions covering the lateral nucleus accumbens (NAc) and lateral caudate-putamen (CPu) restored cocaine stimulation but not cocaine reward. In the current study, we expanded the AAV-DATwt infected areas to cover the olfactory tubercle (Tu) and the ventral midbrain (vMB) containing the ventral tegmental area (VTA) and substantia nigra (SN) in addition to CPu and NAc with multiple injections. These mice displayed the restoration of both locomotor stimulation and cocaine reward. We further found that AAV-DATwt injection in the vMB alone was sufficient to restore both cocaine stimulation and reward in DAT-CI mice. AAV injected in the VTA and SN resulted in DATwt expression and distribution to the DA terminal regions. In summary, cocaine induced locomotion and reward can be restored in fully developed DAT-CI mice, and cocaine inhibition of DAT expressed in dopaminergic neurons originated from the ventral midbrain mediates cocaine reward and stimulation.

Conserved dorsal-ventral gradient of dopamine release and uptake rate in mice, rats and rhesus macaques

Although the vast majority of research on the dopamine system has been performed in rodents, and it is assumed that this work will inform us about the human condition, there have been very few direct comparisons of presynaptic dopamine terminal function across multiple species. Because it is difficult to query rapid sub-second dopamine signaling in humans using voltammetric methods, we chose to compare dopamine signals across multiple striatal subregions in slices from C57BL/6J mice, Sprague-Dawley rats and rhesus macaques. We found a dorsal to ventral gradient of dopamine uptake rates with highest levels in the dorsal striatum and lowest levels in the nucleus accumbens shell, which is conserved across species. In addition to uptake rates, there was also a dorsal to ventral, high to low, gradient in the magnitude of stimulated DA release observed in monkeys, mice, and rats. These data demonstrate that there is considerable functional homology across striatal regions in non-human primates and rodents, lending support to the use of rodents as model systems to study dopamine-related circuitry and disorders that are clinically relevant to the human population.

Postmortem studies in Parkinson's disease

No animal model to date perfectly replicates Parkinson's disease (PD) etiopathogenesis, and the anatomical organization of the nigrostriatal system differs considerably between species. Human postmortem material therefore remains the gold standard for both formulating hypotheses for subsequent testing in in vitro and in vivo PD models and verifying hypotheses derived from experimental PD models with regard to their validity in the human disease. This article focuses on recent and relevant fields in which human postmortem work has generated significant impact in our understanding of PD. These fields include Lewy body formation, regional vulnerability of dopaminergic neurons, oxidative/nitrative cellular stress, inflammation, apoptosis, infectious and environmental agents, and nondopaminergic lesions.

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.

(18)F-FECNT: validation as PET dopamine transporter ligand in parkinsonism

The positron emission tomography (PET) tracer 2β-carbomethoxy-3β-(4-chlorophenyl)-8-(2-[18F]-fluoroethyl)-nortropane ((18)F-FECNT) is a highly specific ligand for dopamine transporter (DAT) that yields higher peak striatum-to-cerebellum ratios and offers more favorable kinetics than most (18)F-radiolabeled DAT ligands currently available. The goal of this study is to validate the use of (18)F-FECNT as a PET radiotracer to assess the degree of striatal dopamine terminals denervation and midbrain dopaminergic cell loss in MPTP-treated parkinsonian monkeys. Three rhesus monkeys received weekly injections of MPTP (0.2-0.5 mg/kg) for 21 weeks, which resulted in the progressive development of a moderate level of parkinsonism. We carried out (18)F-FECNT PET at baseline (twice; 10 weeks apart) and at week 21 post-MPTP injections. Postmortem stereological cell counts of dopaminergic neurons in the ventral midbrain, and intensity measurements of DAT and tyrosine hydroxylase (TH) immunoreactivity in the striatum were performed and correlated with striatal and ventral midbrain PET data. Three additional monkeys were used as controls for midbrain dopaminergic cell counts, and striatal DAT or TH immunoreactivity measurements. The correlation and coefficient of variance between (18)F-FECNT test-retest specific uptake ratios were 0.99 (R²) and 2.65%, respectively. The (18)F-FECNT binding potential of the ventral midbrain and striatal regions was tightly correlated with postmortem stereological cell counts of nigral dopaminergic neurons (R²=0.91), and striatal DAT (R²=0.83) or TH (R²=0.88) immunoreactivity intensity measurements. These findings demonstrate that (18)F-FECNT is a highly sensitive PET imaging ligand to quantify both striatal dopamine denervation and midbrain dopaminergic cell loss associated with parkinsonism.

On the origin of cortical dopamine: is it a co-transmitter in noradrenergic neurons?

Dopamine (DA) and noradrenaline (NA) in the prefrontal cortex (PFC) modulate superior cognitive functions, and are involved in the aetiology of depressive and psychotic symptoms. Moreover, microdialysis studies in rats have shown how pharmacological treatments that induce modifications of extracellular NA in the medial PFC (mPFC), also produce parallel changes in extracellular DA.To explain the coupling of NA and DA changes, this article reviews the evidence supporting the hypothesis that extracellular DA in the cerebral cortex originates not only from dopaminergic terminals but also from noradrenergic ones, where it acts both as precursor for NA and as a co-transmitter.Accordingly, extracellular DA concentration in the occipital, parietal and cerebellar cortex was found to be much higher than expected in view of the scarce dopaminergic innervation in these areas.Systemic administration or intra-cortical perfusion of alpha(2)-adrenoceptor agonists and antagonists, consistent with their action on noradrenergic neuronal activity, produced concomitant changes not only in extracellular NA but also in DA in the mPFC, occipital and parietal cortex.Chemical modulation of the locus coeruleus by locally applied carbachol, kainate, NMDA or clonidine modified both NA and DA in the mPFC.Electrical stimulation of the locus coeruleus led to an increased efflux of both NA and DA in mPFC, parietal and occipital cortex, while in the striatum, NA efflux alone was enhanced.Atypical antipsychotics, such as clozapine and olanzapine, or antidepressants, including mirtazapine and mianserine, have been found to increase both NA and DA throughout the cerebral cortex, likely through blockade of alpha(2)-adrenoceptors. On the other hand, drugs selectively acting on dopaminergic transmission produced modest changes in extracellular DA in mPFC, and had no effect on the occipital or parietal cortex.Acute administration of morphine did not increase DA levels in the PFC (where NA is diminished), in contrast with augmented dopaminergic neuronal activity; moreover, during morphine withdrawal both DA and NA levels increased, in spite of a diminished dopaminergic activity, both increases being antagonised by clonidine but not quinpirole administration.Extensive 6-hydroxy dopamine lesion of the ventral tegmental area (VTA) decreases below 95% of control both intra- and extracellular DA and DOPAC in the nucleus accumbens, but only partially or not significantly in the mPFC and parietal cortex.The above evidence points to a common origin for NA and DA in the cerebral cortex and suggests the possible utility of noradrenergic system modulation as a target for drugs with potential clinical efficacy on cognitive functions.

Vesicular monoamine transporter 2 and dopamine transporter are molecular targets of Pitx3 in the ventral midbrain dopamine neurons

Midbrain dopamine (mDA) neurons play critical roles in the regulation of voluntary movement and their dysfunction is associated with Parkinson's disease. Pitx3 has been implicated in the proper development of mDA neurons in the substantia nigra pars compacta, which are selectively lost in Parkinson's disease. However, the basic mechanisms underlying its role in mDA neuron development and/or survival are poorly understood. Toward this goal, we sought to identify downstream target genes of Pitx3 by comparing gene expression profiles in mDA neurons of wild-type and Pitx3-deficient aphakia mice. This global gene expression analysis revealed many potential target genes of Pitx3; in particular, the expression of vesicular monoamine transporter 2 and dopamine transporter, responsible for dopamine storage and reuptake, respectively, is greatly reduced in mDA neurons by Pitx3 ablation. In addition, gain-of-function analyses and chromatin immunoprecipitation strongly indicate that Pitx3 may directly activate transcription of vesicular monoamine transporter 2 and dopamine transporter genes, critically contributing to neurotransmission and/or survival of mDA neurons. As the two genes have been known to be regulated by Nurr1, another key dopaminergic transcription factor, we propose that Pitx3 and Nurr1 may coordinately regulate mDA specification and survival, at least in part, through a merging and overlapping downstream pathway.

Phenotype, compartmental organization and differential vulnerability of nigral dopaminergic neurons

The degeneration of nigral dopaminergic (DA-) neurons is the histopathologic hallmark of Parkinson's disease (PD), but not all nigral DA-cells show the same susceptibility to degeneration. This starts in DA-cells in the ventrolateral and caudal regions of the susbtantia nigra (SN) and progresses to DA-cells in the dorsomedial and rostral regions of the SN and the ventral tegmental area, where many neurons remain intact until the final stages of the disease. This fact indicates a relationship between the topographic distribution of midbrain DA-cells and their differential vulnerability, and the possibility that this differential vulnerability is associated with phenotypic differences between different subpopulations of nigral DA-cells. Studies carried out during the last two decades have contributed to establishing the existence of different compartments of nigral DA-cells according to their neurochemical profile, and a possible relationship between the expression of some factors and the relative vulnerability or resistance of DA-cell subpopulations to degeneration. These aspects are reviewed and discussed here.

The comparative distributions of the monoamine transporters in the rodent, monkey, and human amygdala

The monoamines in the amygdala modulate multiple aspects of emotional processing in the mammalian brain, and organic or pharmacological dysregulation of these systems can result in affective pathologies. Knowledge of the normal distribution of these neurotransmitters, therefore, is central to our understanding of both the normal processes regulated by the amygdala and the pathological conditions associated with monoaminergic dysregulation. The monoaminergic transporters have proven to be accurate and reliable markers of the distributions of their substrates. The purpose of this review was twofold: First, to briefly recount the functional relevance of dopamine, serotonin, and norepinephrine transmission in the amygdala, and second, to describe and compare the distributions of the monoamine transporters in the rodent, monkey, and human brain. The transporters were found to be heterogeneously distributed in the amygdala. The dopamine transporter (DAT) is consistently found to be extremely sparsely distributed, however the various accounts of its subregional topography are inconsistent, making any cross-species comparisons difficult. The serotonin transporter (SERT) had the greatest overall degree of labeling of the three markers, and was characterized by substantial inter-species variability in its relative distribution. The norepinephrine transporter (NET) was shown to possess an intermediate level of labeling, and like the SERT, its distribution is not consistent across the three species. The results of these comparisons indicate that caution should be exercised when using animal models to investigate the complex processes modulated by the monoamines in the amygdala, as their relative contributions to these functions may differ across species.

Imaging genetics in ADHD: a focus on cognitive control

This paper evaluates neuroimaging of cognitive control as an endophenotype for investigating the role of dopamine genes in ADHD. First, this paper reviews both data-driven and theory-driven approaches from genetics and neuroimaging. Several viable candidate genes have been implicated in ADHD, including the dopamine DRD4 and DAT1 genes. Neuroimaging studies have resulted in a good understanding of the neurobiological basis of deficits in cognitive control in this disorder. Second, this paper discusses imaging genetics in ADHD. Papers to date have taken one of two approaches: whereas early papers investigated the effects of one or two candidate genes on many brain areas, later papers constrained regions of interest by gene expression patterns. These papers have largely focused on cognitive control and the dopamine circuits involved in this ability. The results show that neuroimaging of cognitive control is useful as an endophenotype in investigating dopamine gene effects in ADHD. Other avenues of investigation are suggested by a combination of data- and theory-driven approaches in both genetics and neuroimaging.

Relationship between VNTR polymorphisms of the human dopamine transporter gene and expression in post-mortem midbrain tissue

Attention deficit hyperactivity disorder (ADHD) is currently one of the most prevalent childhood behavioral disorders. The disorder is found to be highly heritable, suggesting a large genetic component. Association studies have repeatedly implicated the dopamine transporter (DAT1) gene, and in particular the 10-repeat allele of a variable number tandem repeat (VNTR) polymorphism located in the 3'UTR of the gene. Inconclusive data has been generated from several earlier studies on the functional effects of this polymorphism. Therefore, there is call for further investigation and thus the focus on data described here from TaqMan RT-PCR assays. The expression levels of the DAT1 gene from post-mortem midbrain tissue was measured in relation to the polymorphism present at the 3'UTR VNTR, together with a further VNTR marker located within intron 8 of the gene (Int8 VNTR). The findings suggest that the presence of the 10-repeat allele of the 3'UTR VNTR, the 3-repeat of the intron 8 VNTR and both VNTR markers are correlated with increased levels of the DAT1 transcript in midbrain post-mortem tissue. Further work using linear regression (LR) shows agreement with the correlation analysis, and either nominal significance or a trend in that direction. Given the small sample size, bootstrapping-derived confidence intervals were calculated for the LR. These empirical analyses suggest that the 3'UTR VNTR to show a significant main effect on relative DAT1 expression. Furthermore, a significant effect was found for the combined model (3'UTR and Int8 VNTR markers) on expression. These data provide further evidence on the plausible molecular mechanism underlying the aetiology of the disorder.

The medial and lateral substantia nigra in Parkinson's disease: mRNA profiles associated with higher brain tissue vulnerability

Sporadic Parkinson's disease (PD) is characterized by progressive death of dopaminergic neurons within the substantia nigra. However, pathological cell death within this nucleus is not uniform. In PD, the lateral tier of the substantia nigra (SNl) degenerates earlier and more severely than the more medial nigral component (SNm). The cause of this brain regional vulnerability remains unknown. We have used DNA oligonucleotide microarrays to compare gene expression profiles from the SNl to those of the SNm in both PD and control cases. Genes expressed more highly in the PD SNl included the cell death gene, p53 effector related to PMP22, the tumour necrosis factor (TNF) receptor gene, TNF receptor superfamily, member 21, and the mitochondrial complex I gene, NADH dehydrogenase (ubiquinone) 1beta subcomplex, 3, 12 kDa (NDUFbeta3). Genes that were more highly expressed in PD SNm included the dopamine cell signalling gene, cyclic adenosine monophosphate-regulated phosphoprotein, 21 kDa, the activated macrophage gene, stabilin 1, and two glutathione peroxidase (GPX) genes, GPX1 and GPX3. Thus, there is increased expression of genes encoding pro-inflammatory cytokines and subunits of the mitochondrial electron transport chain, and there is a decreased expression of several glutathione-related genes in the SNl suggesting a molecular basis for pathoclisis. Importantly, some of the genes that are differentially regulated in the SNl are known to be expressed highly or predominantely in glial cells. These findings support the view that glial cells can be primarily affected in PD emphasizing the importance of using a whole tissue approach when investigating degenerative CNS disease.

Survival of DA neurons is independent of CREM upregulation in absence of CREB

cAMP response element binding protein (CREB) and the related factors CREM (cAMP response element modulator) and ATF1 (activation transcription factor 1) are bZIP-domain-containing transcription factors activated through cAMP and other signaling pathways. The disruption of CREB function in developing and mature neurons affects their development and survival when associated with loss of CREM. Since dopaminergic (DA) neurons are affected in several neurological diseases, we generated CREB conditional mutants in DA neurons by using a newly generated transgenic Cre line targeting the dopaminergic system (DATCre). Here we report the generation and analysis of mutant mice lacking CREB in DA neurons (CREB(DATCre) mutants). During adulthood, lack of CREB leads to a partial loss of DA neurons. Since CREM is upregulated in absence of CREB, we have introduced this mutation in a CREM-/- genetic background to assess a compensatory role of CREM. Additional inactivation of CREM does not lead to a more severe phenotype.

Role for dopamine neurons of the rostral linear nucleus and periaqueductal gray in the rewarding and sensitizing properties of heroin

There is a mesencephalic dopaminergic network outside the ventral tegmental area (VTA), including structures such as the rostral linear nucleus (RLi) and periaqueductal gray (PAG). These nuclei project to neural areas implicated in reinforcing effects of drugs, indicating that they could participate in opiate reward. The objectives were to study the morphological characteristics of the dopamine network of the RLi/PAG region, and to discern its role on rewarding and sensitizing effects of heroin in rats, following dopamine depletion or local injection of dopaminergic antagonists. The findings indicated that this network is composed of small cells in the RLi/ventral PAG, large multipolar dopamine PAG neurons, and periaqueductal PAG neurons. Following repeated heroin, large PAG neurons and small RLi/ventral PAG cells (not periaqueductal neurons) were activated, since tyrosine-hydroxylase was adaptively induced, without changes in protein kinase Aalpha. After dopamine depletion, small RLi/ventral PAG neurons and large cells of the PAG (not periaqueductal ones) were selectively affected by the neurotoxin. Dopamine neurons of the nearby VTA and dorsal raphe were not affected, as revealed by cell counting. After lesion, 'anxiety-like' responses and basal locomotion were not altered. However, conditioned place preference to heroin was found to be abolished, as well as heroin-induced motor sensitization. Following infusions of dopaminergic antagonists into RLi/PAG, D(2) (not D(1)) receptor blocking dose-dependently abolished heroin-induced reward. The present study provides evidence that dopamine neurons of the RLi/PAG region (excluding PAG periaqueductal cells) show adaptive biochemical changes after heroin, and mediate the rewarding and sensitizing effects of this drug. D(2) dopamine receptors within the RLi/PAG region participate in these effects.

Striatal expression of GDNF and differential vulnerability of midbrain dopaminergic cells

Glial cell line-derived neurotrophic factor (GDNF) is a member of the transforming growth factor-beta superfamily that when exogenously administrated exerts a potent trophic action on dopaminergic (DA) cells. Although we know a lot about its signalling mechanisms and pharmacological effects, physiological actions of GDNF on the adult brain remain unclear. Here, we have used morphological and molecular techniques, and an experimental model of Parkinson's disease in rats, to investigate whether GDNF constitutively expressed in the adult mesostriatal system plays a neuroprotective role on midbrain DA cells. We found that although all midbrain DA cells express both receptor components of GDNF (GFRalpha1 and Ret), those in the ventral tegmental area (VTA) and rostromedial substantia nigra (SNrm) also contain GDNF but not GDNFmRNA. The levels of GDNFmRNA are significantly higher in the ventral striatum (vSt), the target region of VTA and SNrm cells, than in the dorsal striatum (dSt), the target region of DA cells in the caudoventral substantia nigra (SNcv). After fluoro-gold injection in striatum, VTA and SNrm DA cells show triple labelling for tyrosine hydroxylase, GDNF and fluoro-gold, and after colchicine injection in the lateral ventricle, they become GDNF-immunonegative, suggesting that GDNF in DA somata comes from their striatal target. As DA cells in VTA and SNrm are more resistant than those in SNcv to intracerebroventricular injection of 6-OHDA, as occurs in Parkinson's disease, we can suggest that the fact that they project to vSt, where GDNF expression is significantly higher than in the dSt, is a neuroprotective factor involved in the differential vulnerability of midbrain DA neurons.

Expression of dopamine and vesicular monoamine transporters and differential vulnerability of mesostriatal dopaminergic neurons

Numerous studies suggest that the dopamine transporter (DAT), responsible for dopamine reuptake, may act as a vulnerability factor in the pathogenesis of Parkinson's disease (PD) and the vesicular monoamine transporter (VMAT2), responsible for its vesicular storage, as a neuroprotective factor. However, the relevance of each on the differential vulnerability of midbrain DA cells remains unknown. Here we studied the relationship between the expression pattern (mRNA and protein) of both transporters and the differential vulnerability of midbrain DA cells in a model of PD (intracerebroventricular injection of 6-OHDA in rats) and in monkey and human midbrain. Our results revealed that the expression patterns for VMAT2 mRNA and protein and DAT mRNA are similar, with the highest levels in the rostromedial region of substantia nigra (SNrm), followed by the caudoventral region of SN (SNcv), the ventral tegmental area and pigmented parabrabraquial nucleus (VTA/PBP), and finally the linear and interfascicular nuclei (Li/IF). In contrast, the expression of DAT protein in rats, monkeys, and humans followed a caudoventrolateral-to-rostrodorsomedial decreasing gradient (SNcv > SNrm > VTA/PBP > Li/IF), matching the degeneration profile observed after intracerebroventricular injection of 6-OHDA and in PD. In addition, DAT blockade made all midbrain DA cells equally resistant to 6-OHDA. These data indicate that DAT protein levels, but not DAT mRNA levels, are closely related to the differential vulnerability of midbrain DA cells and that this relationship is unaffected by the relative levels of VMAT2. Furthermore, the difference between DAT mRNA and protein profiles suggests internuclear differences in its posttransductional regulation.

The somatodendritic release of dopamine in the ventral tegmental area and its regulation by afferent transmitter systems

The release of dopamine in the ventral tegmental area (VTA) plays an important role in the autoinhibition of the dopamine neurons of the mesocorticolimbic system through the activation of somatodendritic dopamine D2 autoreceptors. Accordingly, the intra-VTA application of dopamine D2 receptor agonists reduces the firing rate and release of dopamine in the VTA, and this control appears to possess a tonic nature because the corresponding antagonists enhance the somatodendritic release of the transmitter. In addition, the release of dopamine in the VTA is increased by potassium or veratridine depolarization and abolished by tetrodotoxin and calcium omission. Overall, it appears that the somatodendritic release of dopamine is consistently lower than that in nerve endings. Apart from intrinsic dopaminergic mechanisms, other transmitter systems such as serotonin, noradrenaline, acetylcholine, GABA and glutamate play a role in the control of the activity of dopaminergic neurons of the VTA, although the final action depends on the particular receptor involved as well as the neuronal type where it is localized. Given the involvement of the mesocorticolimbic dopaminergic systems in the pathogenesis of severe neuropsychiatric disorders such as schizophrenia, the knowledge of the factors that regulate the release of dopamine in the VTA could provide new insight into the ethiogenesis of the disease as well as its implication on the mechanisms of action of therapeutic drugs.

Ultrastructural localization of the norepinephrine transporter in superficial and deep layers of the rat prelimbic prefrontal cortex and its spatial relationship to probable dopamine terminals

The prefrontal cortex (PFC) is a likely site of action for the therapeutic efficacy of antidepressants that inhibit norepinephrine (NE) reuptake. Moreover, drugs that block the NE transporter (NET) increase extracellular levels of both NE and dopamine (DA), an interaction that may contribute to their therapeutic properties. To examine the subcellular localization of NET and to investigate the spatial relationships between presumed NE and DA axons within the rat prelimbic PFC, we combined immunogold-silver localization of NET with immunoperoxidase staining for the catecholamine synthetic enzyme tyrosine hydroxylase (TH). An additional aim was to quantify the proportion of profiles dually labeled for NET and TH to test the common observation that TH immunolabeling is relatively selective for DA axons. NET-immunoreactive (NET-ir) axonal profiles were typically unmyelinated and occasionally were observed to form symmetric axodendritic synapses. The majority of immunogold NET labeling was unexpectedly observed in the cytoplasm rather than on the plasma membrane. Furthermore, in tissue dually labeled for both NET and TH, only 8-10% of profiles contained both markers. Unlike observations for singly labeled profiles, gold-silver particles for NET in dually labeled axons were localized primarily to the plasmalemma. A systematic survey of terminals labeled only for TH revealed that they were typically separated by at least 1.2 mum from NET-ir varicosities, and the two profile types were not seen to contact common targets. These results suggest that, in the rat PFC, NE axons (1) contain predominantly cytoplasmic NET, (2) infrequently contain TH immunolabeling, and (3) may interact with probable DA afferents by means of extrasynaptic mechanisms.

The medial prefrontal cortex in the rat: evidence for a dorso-ventral distinction based upon functional and anatomical characteristics

The prefrontal cortex in rats can be distinguished anatomically from other frontal cortical areas both in terms of cytoarchitectonic characteristics and neural connectivity, and it can be further subdivided into subterritories on the basis of such criteria. Functionally, the prefrontal cortex of rats has been implicated in working memory, attention, response initiation and management of autonomic control and emotion. In humans, dysfunction of prefrontal cortical areas with which the medial prefrontal cortex of the rat is most likely comparable is related to psychopathology including schizophrenia, sociopathy, obsessive-compulsive disorder, depression, and drug abuse. Recent literature points to the relevance of conducting a functional analysis of prefrontal subregions and supports the idea that the area of the medial prefrontal cortex in rats is characterized by its own functional heterogeneity, which may be related to neuroanatomical and neurochemical dissociations. The present review covers recent findings with the intent of correlating these distinct functional differences in the dorso-ventral axis of the rat medial prefrontal cortex with anatomical and neurochemical patterns.

Exercise induces behavioral recovery and attenuates neurochemical deficits in rodent models of Parkinson's disease

Exercise is thought to improve motor function and emotional well-being in patients with Parkinson's disease (PD). However, it is not clear if the improvements are due to neurochemical alterations within the affected nigrostriatal region or result from a more general effect of exercise on affect and motivation. In this study we show that motorized treadmill running improves the neurochemical and behavioral outcomes in two rodent models of PD: the unilateral 6-hydroxydopamine (6-OHDA) rat model and bilateral 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model in aged C57bl mice. Exposure to the dopamine (DA) toxins 6-OHDA or MPTP resulted in permanent behavioral and neurochemical loss. In contrast, when lesioned animals were exposed to treadmill activity two times a day for the first 10 days post-lesion they displayed no behavioral deficits across testing days and had significant sparing of striatal DA, its metabolites, tyrosine hydroxylase, vesicular monoamine transporter, and DA transporter levels compared to lesion sedentary animals. These results demonstrate that exercise following nigrostriatal damage ameliorates related motor symptoms and neurochemical deficits in rodent models of PD.

Strain differences in the distribution of dopamine transporter sites in rat brain

The Wistar Kyoto (WKY) rat has long been proposed as an animal model of depressive behavior. Exposure to stress produces symptoms such as anhedonia, psychomotor retardation, ambivalence, and negative memory bias. Autoradiographic studies have revealed significant differences in the density of norepinephrine transporter (NET) and serotonin transporter (5-HTT) sites in several brain regions in WKY rats compared to Sprague-Dawley (S-D) rats. Since the mesolimbic dopamine (DA) system is involved in cognitive, emotional, and motivational behaviors, this study examined the distribution of dopamine transporter (DAT) sites in the brains of WKY compared to Wistar (WIS) and S-D rats. DAT sites were labeled with [3H]-GBR12935 (1 nM), and mazindol (50 microM) was used to define nonspecific binding. Quantitative analysis of the specific binding indicated that WKY rats exhibited significant differences in DAT binding sites in the cell body as well as mesolimbic areas in comparison to WIS and S-D rats. While the binding of [3H]-GBR to DAT sites was significantly decreased in the nucleus accumbens (NAc), the amygdala, the ventral tegmental area (VTA), and the reticular part of the substantia nigra (P<.05), the binding was significantly increased in the hippocampal subregions and the hypothalamus (P<.05) in WKY rats compared to the other two strains. In contrast, no strain differences were found in the caudate-putamen. The observed differences in the density and distribution of DAT sites in WKY rats may lead to altered modulation of synaptic DA levels in the cell body and mesolimbic regions, thereby contributing to the noted depression-like behaviors reported in this rat strain.

1-Methyl-4-phenylpyridinium accumulates in cerebellar granule neurons via organic cation transporter 3

1-Methyl-4-phenylpyridinium (MPP+), the toxic metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, induces apoptosis in cerebellar granule neurons (CGNs). We have tested the hypothesis that organic cation transporter (OCT) 3 mediates the accumulation and, hence, the toxicity of MPP+ in CGNs. CGNs in primary culture express OCT3 but do not express mRNA for OCT1, OCT2 or the dopamine transporter. Cerebellar astrocytes are negative for OCT3 protein by immunocytochemistry. [3H]MPP+ accumulation by CGNs exhibits first-order kinetics, and a Kt value of 5.3 +/- 1.2 micro m and a Tmax of 0.32 +/- 0.02 pmol per min per 106 cells. [3H]MPP+ accumulation is inhibited by corticosterone, beta-estradiol and decynium 22 with Ki values of 0.25 micro m, 0.17 micro m and 4.0 nm respectively. [3H]MPP+ accumulation is also inhibited by desipramine, dopamine, serotonin and norepinephrine, but is not affected by carnitine (10 mm), mazindol (9 micro m) or GBR 12909 (1 micro m). MPP+-induced caspase-3-like activation and cell death are prevented by pretreatment with 5 micro mbeta-estradiol. In contrast, the neurotoxic effects of rotenone are unaffected by beta-estradiol. Interestingly, GBR 12909 protects CGNs from both MPP+ and rotenone toxicity. In summary, CGNs accumulate MPP+ in manner that is consistent with uptake via OCT3 and the presence of this protein in CGNs explains their sensitivity to MPP+ toxicity.

Regional and subcellular compartmentation of the dopamine transporter and tyrosine hydroxylase in the rat ventral pallidum

The ventral pallidum (VP) is a major intermediary in the prefrontal cortical circuitry regulating sensorimotor gating and locomotor behavior, both of which are potently modulated by catecholamines. The VP catecholaminergic innervation is derived from midbrain dopaminergic neurons that differ in expression levels of the dopamine transporter (DAT) and from brainstem noradrenergic neurons without DAT. The preferentially low level of DAT in dopaminergic terminals in the prefrontal cortex and in striatal regions projecting more extensively to the VP medial (VPm) compared with VP lateral (VPl) compartment suggests possible region-specific differences in VP axonal distribution of DAT. To test this hypothesis, we examined the electron microscopic localization of DAT and the catecholamine-synthesizing enzyme, tyrosine hydroxylase (TH), in the VPm and VPl of rat brain. In both regions, DAT and TH were localized primarily in small unmyelinated axons and morphologically heterogeneous axon terminals. DAT-immunogold particles were few in number, but mostly located on the plasma membrane. In contrast, TH immunoreactivity was distributed in the cytoplasm of individual profiles, many of which were without detectable DAT. In comparison with TH, the mean area density of DAT-labeled axons was low throughout the VP. The mean area density of DAT-immunogold axon terminals, however, was significantly higher in VPl than in VPm, whereas that of TH-labeled axons was higher in VPm than in VPl. This dissociation suggests that, compared to the VPl, the VPm receives the greatest input from catecholaminergic afferents that are either nondopaminergic or characterized by having low levels or less terminal distributions of DAT.

Functional domains in dorsal striatum of the nonhuman primate are defined by the dynamic behavior of dopamine

The dorsal striatum comprises a continuum of distinct functional domains, limbic, associative, and sensorimotor. In the primate it exclusively subdivides further into two nuclei, the putamen and caudate. Dopamine (DA) transmission is differentially affected between these nuclei in neurodegenerative diseases such as Parkinson's and by psychostimulants such as cocaine. Because rodent systems can offer only limited insight into DA systems of the human brain, a fuller appreciation of DA transmission and its role in dysfunction requires direct study in primates. DA behavior was explored in the major functional domains of the caudate nucleus and compared with the putamen, using fast-scan cyclic voltammetry in striatal sections from the marmoset (Callithrix jacchus). There was domain-specific variation in extracellular DA transients [i.e., concentration ([DA](o)) released by a single stimulus and the rate maximum of DA uptake, V(max)]. Across nuclei, functional rather than anatomical regions were differentiated by these dynamics. The largest, fastest DA transients were at motor-associated loci. Evoked [DA](o) at physiological frequencies was differently frequency-sensitive between functional domains but not between anatomical nuclei. In contrast, presynaptic depression was not an index of regional differentiation, recovering with similar kinetics at all loci. Within a given functional domain of dorsal striatum, the dynamics of DA release and uptake are similar for the putamen and the caudate nucleus. Conversely, distinct functional domains are defined by these DA dynamics, in a manner more marked in primates than in rodents. These data from the primate brain highlight differences in DA availability that may be central to DA function and dysfunction in the human.

Autoradiographic studies using L-[(14)C]DOPA and L-DOPA reveal regional Na(+)-dependent uptake of the neurotransmitter candidate L-DOPA in the CNS

We previously proposed that L-3,4-dihydroxyphenylalanine (L-DOPA) is a neurotransmitter in the CNS. Receptor and transporter molecules for L-DOPA, however, have not been determined. In the present study, in order to localize the uptake sites of L-DOPA in the CNS, we performed autoradiographic uptake studies using L-[14C]DOPA and L-[3H]DOPA in the uptake study on rat brain slice preparations, and further analyzed the properties of L-DOPA uptake. Image analysis of the L-[14C]DOPA autoradiogram showed a unique heterogeneous distribution of uptake sites in the brain. The intensity was relatively high in the cerebral cortex, the hypothalamus, the cerebellum and the hippocampus, while the density was moderate or even low in the striatum and the substantia nigra. L-DOPA and phenylalanine, but not dopamine (10mM) were able to almost completely inhibit the uptake of L-[14C]DOPA to basal levels. Microautoradiographic studies using L-[3H]DOPA revealed accumulation of dense grains in the median eminence, the supraoptic nucleus of the hypothalamus, the cerebral cortex (layer I) and the hippocampus. In the cerebellum, grains formed in clusters surrounding the Purkinje cells. This grain accumulation was concluded to be in Bergmann glial cells, since the morphological pattern of grain accumulation was similar to that of the immunoreactivity of the glutamate aspartate transporter, a marker protein for Bergmann glial cells. In the hippocampus, the grain density significantly decreased under Na(+)-free conditions. In addition, grain density also decreased in the absence of Cl(-). In contrast, grains in the choroid plexus and the ependymal cell layer, were not affected by the absence of Na(+). These findings indicated that the uptake of L-DOPA occurs via various types of large neutral amino acid transport mechanisms. It appears that neuronal and/or glial cells, which take up L-DOPA in a Na(+)-dependent manner, exist in the CNS. Our finding further supports the concept that L-DOPA itself may act as a neurotransmitter or neuromodulator.

Dopamine transporter expression distinguishes dopaminergic neurons from other catecholaminergic neurons in the developing zebrafish embryo

To characterize the formation of the dopaminergic system in the developing zebrafish CNS, we cloned cDNAs encoding tyrosine hydroxylase (th), an enzyme in dopamine synthesis, and the dopamine transporter (dat), a membrane transport protein which terminates dopamine action by re-uptake. Dopaminergic neurons are first detected between 18 and 19 h post-fertilization in a cluster of cells in the ventral diencephalon. Subsequently, th and dat detection identifies dopaminergic neurons in the olfactory bulb, the pretectum, the retina and the locus coeruleus. Neurons expressing th but not dat are adrenergic or noradrenergic, and are found in the locus coeruleus, the medulla, the likely analog of the carotid body, and precursors of the enteric and sympathetic nervous system.

Expression of sensitization to amphetamine and dynamics of dopamine neurotransmission in different laminae of the rat medial prefrontal cortex

The present study investigated the effect of acute and repeated administrations of amphetamine (AMPH) on dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) in the two main cytoarchitectonic subterritories of the medial prefrontal cortex (mPFC) (anterior cingulate and dorsocaudal prelimbic cortices vs ventral prelimbic and rostral infralimbic cortices). Both the acute locomotor effects of AMPH and the expression of behavioral sensitization following its repeated administration were also simultaneously assessed. The repeated, intermittent administration of AMPH over five consecutive days led to a significant sensitized locomotor response to a subsequent challenge that occurred following a 48-h withdrawal period. Basal dialysate DA levels were higher in the ventral mPFC compared with its dorsal counterpart in naive animals, that is prior to the acute administration of AMPH. However, the inverse relationship was observed in animals that had developed sensitization: basal dialysate DA levels were significantly lower in the ventral mPFC compared with the dorsal mPFC. In naïve animals, AMPH produced a significant decrease in DA levels in both the ventral and dorsal subregions of the mPFC. However, the inverse relationship was observed in animals that had developed sensitization: dialysate DA levels in response to AMPH remained significantly decreased in the dorsal mPFC, whereas DA went back to baseline levels in the ventral mPFC. Given that a critical concentration of DA is required for normal function of the mPFC, our results suggest that AMPH-induced changes in DA levels in different subregions of the mPFC are critical for both the acute effects of the drug and the expression of behavioral sensitization to its repeated administration by producing either less or more selectivity or sharpening of stimuli to cortico-cortical dendrites and subcortical synaptic afferents to the pyramidal cells located in the dorso-ventral axis of the mPFC.

Dopamine release and uptake dynamics within nonhuman primate striatum in vitro

The putamen of the human striatum is a heterogeneous nucleus that contains the primary site of loss of dopamine (DA) in Parkinson's disease (PD). Furthermore, different functional domains of the putamen are heterogeneously susceptible to DA loss, and yet the dynamic regulation of extracellular DA concentration ([DA](o)) and comparison between domains has not been explored in the primate brain. In these studies, DA was measured in real time using fast-scan cyclic voltammetry at a carbon-fiber microelectrode in vitro in striatal sections from the common marmoset (Callithrix jacchus). [DA](o) released by a single stimulus pulse varied threefold along a ventromedial-dorsolateral axis. DA uptake was via the DA transporter (GBR12909 sensitive, desipramine insensitive). On the basis of data modeling with simulations of Michaelis-Menten kinetics, rate maximum, V(max), varied with region: both [DA](o) and V(max) were greatest in regions most vulnerable in PD. These differences were reflected in part by regional variation in DA content. [DA](o), V(max), and regional variation were two- to threefold greater than in rodent caudatoputamen. In addition, steady-state [DA](o) at physiological firing rates in primate striatum was controlled by depolarization frequency, uptake, and presynaptic autoreceptors. Furthermore, regulation of [DA](o) by these mechanisms differed significantly between limbic- and motor-associated domains. These data indicate interspecies heterogeneity in striatal DA dynamics that must be considered when extrapolating behavioral and drug responses from rodent to the primate brain. Moreover, the heterogeneity demonstrated within the primate putamen in the availability and dynamic regulation of DA may be central to understanding DA function in health, cocaine abuse, and disease.

Pharmacology and behavioral pharmacology of the mesocortical dopamine system

The prefrontal cortex (PFC) has long been known to be involved in the mediation of complex behavioral responses. Considerable research efforts are directed towards refining the knowledge about the function of this brain area and the role it plays in cognitive performance and behavioral output. In the first part, this review provides, from a pharmacological perspective, an overview of anatomical, electrophysiological and neurochemical aspects of the function of the PFC, with an emphasis on the mesocortical dopamine system. Anatomy of the mesocortical system, basic physiological and pharmacological properties of neurotransmission within the PFC, and interactions between dopamine and glutamate as well as other transmitters within the mesocorticolimbic circuit are included. The coverage of these data is largely restricted to what is relevant for the second part of the review which focuses on behavioral studies that have examined the role of the PFC in a variety of phenomena, behaviors and paradigms. These include reward and addiction, locomotor activity and sensitization, learning, cognition, and schizophrenia. Although the focus of this review is on the mesocortical dopamine system, given the intricate interactions of dopamine with other transmitter systems within the PFC and the importance of the PFC as a source of glutamate in subcortical areas, these aspects are also covered in some detail where appropriate. Naturally, a topic as complex as this cannot be covered comprehensively in its entirety. Therefore this review is largely limited to data derived from studies using rats, and it is also specifically restricted to data concerning the medial PFC (mPFC). Since in several fields of research the findings concerning the function or role of the mPFC are relatively inconsistent, the question is addressed whether these inconsistencies might, at least in part, be related to the anatomical and functional heterogeneity of this brain area.

Dopaminergic mRNA expression in the intact substantia nigra of unilaterally 6-OHDA-lesioned and grafted rats: an in situ hybridization study

The present study was performed to investigate the influence of intrastriatal fetal mesencephalic grafts on dopaminergic mRNA expression in the non-lesioned substantia nigra pars compacta of unilaterally 6-hydroxydopamine-lesioned rats. The expression of dopamine transporter mRNA, synaptic vesicular monoamine transporter mRNA and tyrosine hydroxylase mRNA was assessed in adjacent cryostat sections using in situ hybridization. Rotational behavior induced by apomorphine and amphetamine as well as hybridization of striatal sections cut at the grafting coordinates were used to prove the functional recovery and the presence of grafted cells, respectively. After grafting, the number of rotations was decreased and hybridization signals overlying cells in the grafted striatum were detected. Mean grain densities overlying labeled neurons in the substantia nigra pars compacta of grafted rats were compared to those of shamgrafted rats and revealed differential expression of dopamine transporter mRNA, whereas synaptic vesicular monoamine transporter mRNA and tyrosine hydroxylase mRNA expression showed no difference. The results will be discussed in relation to previous in vitro and in vivo studies suggesting a reduction of functional dopamine transporter molecules in the contralateral striatum.

Novel in vivo electrophysiological assay for the effects of cocaine and putative "cocaine antagonists" on dopamine transporter activity of substantia nigra and ventral tegmental area dopamine neurons

The aim of these studies was to establish a rapid in vivo assay for evaluating potential "cocaine antagonists," i.e., drugs postulated to block cocaine binding to the dopamine transporter (DAT) without corresponding blockade of dopamine reuptake. The assay is based on the ability of dopamine, and drugs that elevate synaptic dopamine levels, to inhibit the extracellular single unit activities of midbrain dopamine neurons in chloral hydrate-anesthetized rats. As expected, cocaine itself (0.06-16 mg/kg, i.v.) caused a dose-dependent inhibition of firing of both substantia nigra and ventral tegmental area (VTA) dopamine neurons, but had a significantly higher potency on VTA than nigral dopamine cells (ED(50)'s 1.2 and 8.8 mg/kg, respectively). VTA cells were also inhibited to a greater extent (to 4.7 +/- 4.5% vs. 41.3 +/- 6.3% of baseline rates at 16 mg/kg, respectively). We next evaluated GBR12909, a piperazine analog promoted as a "cocaine antagonist" because of its ability to bind with high affinity to the DAT, while only modestly elevating extracellular dopamine levels. The agonist- and antagonist-like properties of GBR12909 were evaluated on only VTA dopamine cells since these neurons were more fully inhibited by cocaine and have been implicated in its rewarding effects. Given alone, GBR12909 exhibited modest "cocaine-like" activity insofar as it partially inhibited VTA dopamine neurons (to 59.0 +/- 4.6% of baseline at 8 mg/kg). However, consistent with an antagonist profile, pretreatment with a low (0.5 mg/kg) dose of GBR12909, which depressed firing only slightly, resulted in a >2-fold rightward shift in the dose-response curve to cocaine (ED(50) 2.6 mg/kg). We conclude that electrophysiological testing of putative "anti-cocaine" drugs for their abilities to inhibit the firing of VTA dopamine neurons, and to block their inhibitory responses to cocaine, may provide a rapid in vivo screen for compounds expected to behave as functional cocaine antagonists in the dopamine reward system.

Dopaminergic phenotype of hNT cells in vitro

We investigated the catecholaminergic nature of cultured hNT neurons previously treated either for 4 or 5 weeks with retinoic acid (RA). There were significantly more tyrosine hydroxylase (TH)-positive neurons (60%) in cultures treated for 4 weeks with RA compared to 5 week-treated cultures (</=15%). Furthermore, numerous TH-positive hNT cells were also immunoreactive to dopamine transporter (DAT), dopamine receptor (D2) and aldehyde dehydrogenase (AHD-2), an enzyme exclusively expressed by dopaminergic (DA) ventral mesencephalic (VM) precursors. Thus this cell line has all the necessary cellular machinery to produce functional DA neurons and therefore is a good alternative tissue source to fetal VM.

Perinatal asphyxia induces region-specific long-term changes in mRNA levels of tyrosine hydroxylase and dopamine D(1) and D(2) receptors in rat brain

To study the effects of neonatal asphyxia on gene expression of the dopaminergic systems, we determined quantitatively the mRNA levels of tyrosine hydroxylase, dopamine transporter, dopamine D(1) and D(2) receptors in substantia nigra/ventral tegmental area, striatum and limbic area. The mRNA levels were determined at one and 4 weeks after asphyxia by a quantitative reverse transcription polymerase chain reaction method. Spontaneously and Caesarean section born rats showed similar mRNA levels with the exception of an increase of tyrosine hydroxylase mRNA levels in the limbic area of 4-week-old animals. Five min of asphyxia did not change the mRNA levels in any region compared to that in the spontaneously born rats. Fifteen and twenty min of asphyxia induced region-specific alterations in mRNA levels. In SN/VTA an increase of tyrosine hydroxylase mRNA levels in the 1-week-old rats and in striatum an increase of D(1) and D(2) dopamine receptor mRNA levels in the 4-week-old rats were observed. Fifteen min of asphyxia induced a selective increase of D(1) and D(2) dopamine receptor mRNA levels in the limbic area of 4-week-old rats. These observations indicate that neonatal asphyxia triggers a cascade of gene expressions for tyrosine hydroxylase and D(1) and D(2) dopamine receptors. In 1-week-old rats, the gene expression of tyrosine hydroxylase increased in the cell body region substantia nigra/ventral tegmental area. This change may increase the D(1) and D(2) dopamine receptor expression in the target regions striatum and limbic area during further development.