Dopamine transporter binding in rat striatum and nucleus accumbens is unaltered following chronic changes in dopamine levels

Schizophrenia: synthetic strategies and recent advances in drug design

Schizophrenia is a complex and unpredictable mental disorder which affects several domains of cognition and behaviour. It is a heterogeneous illness characterised by positive, negative, and cognitive symptoms, often accompanied by signs of depression. In this tutorial review, we discuss recent progress in understanding the target sites and mechanisms of action of second-generation antipsychotic drugs. Progress in identifying and defining target sites has been accelerated recently by advances in neuroscience, and newly developed agents that regulate signalling by the main excitatory neurotransmitters in the brain are surveyed. Examples of novel molecules for the treatment of schizophrenia in preclinical and clinical development and their industrial sponsors are highlighted.

Unaltered Dopamine Transporter Availability in Drug-Naive Patients With Schizophrenia After 6 Months of Antipsychotics Treatment: A Naturalistic Study

BACKGROUND

Dopaminergic dysfunction, namely, dopamine transporter (DAT) availability variations in patients with drug-naive schizophrenia after long-term treatment, is still not well understood. The aims of the study were to explore (i) whether the DAT availability in patients with drug-naive schizophrenia differed after antipsychotic treatment and (ii) whether treatment with different generations of antipsychotics influenced the DAT availability after follow-up for 6 months.

METHODS

Twenty-four first-episode, drug-naive patients with schizophrenia were divided into first- and second-generation antipsychotic groups naturalistically. After 6 months of follow-up, 7 patients who received first-generation antipsychotic treatment and 17 patients who received second-generation antipsychotic treatment completed the study. The patients underwent premedication and 6-month follow-up measurements using single-photon emission computed tomography with technetium Tc 99m (Tc) TRODAT-1. Psychopathological evaluations and adverse effects were recorded using appropriate scales.

RESULTS

Both of the treatment groups significantly improved according to Positive and Negative Symptoms Scale evaluation. However, no significant difference was noticed between the premedication and 6-month follow-up DAT scans. Nonsignificant differences existed even in the groups of different generations of antipsychotics.

CONCLUSIONS

Improvements in psychotic symptoms in patients with schizophrenia may not be influenced by DAT availability, even under treatment with different antipsychotics for a sufficient treatment period.

Pharmacological treatment with L-DOPA may reduce striatal dopamine transporter binding in in vivo imaging studies

Numerous neurologic and psychiatric conditions are treated with pharmacological compounds, which lead to an increase of synaptic dopamine (DA) levels. One example is the DA precursor L-3,4-dihydroxyphenylalanine (L-DOPA), which is converted to DA in the presynaptic terminal. If the increase of DA concentrations in the synaptic cleft leads to competition with exogenous radioligands for presynaptic binding sites, this may have implications for DA transporter (DAT) imaging studies in patients under DAergic medication. This paper gives an overview on those findings, which, so far, have been obtained on DAT binding in human Parkinson's disease after treatment with L-DOPA. Findings, moreover, are related to results obtained on rats, mice or non-human primates. Results indicate that DAT imaging may be reduced in the striata of healthy animals, in the unlesioned striata of animal models of unilateral Parkinson's disease and in less severly impaired striata of Parkinsonian patients, if animal or human subjects are under acute or subchronic treatment with L-DOPA. If also striatal DAT binding is susceptible to alterations of synaptic DA levels, this may allow to quantify DA reuptake in analogy to DA release by assessing the competition between endogenous DA and the administered exogenous DAT radioligand.

Striatal dopamine transporter availability in drug-naive patients with schizophrenia: a case-control SPECT study with [(99m)Tc]-TRODAT-1 and a meta-analysis

Central dopaminergic hyperactivity has been one of the main hypotheses of the pathophysiology of schizophrenia since the 1970s. Excess dopamine (DA) neurotransmission in the striatum is hypothesized to alter the processing of information and result in psychotic symptoms in schizophrenia. Single photon emission computerized tomography (SPECT) provides in vivo indices of DA neurotransmission. Our study aimed to compare dopamine transporter (DAT) availability between drug-naive patients with schizophrenia and controls using SPECT. DAT availability through [(99m)Tc]-TRODAT-1 SPECT was compared between 47 drug-naive patients with recent-onset schizophrenia and 112 healthy controls. We also conducted a random-effects meta-analysis of the available literature synthesizing the results of 6 comparable published articles as well as our current data. The mean specific striatal binding showed a statistical trend for a reduction among the patients compared with controls (estimated difference = 0.071; 95% CI -0.01, 0.15; P = .08). There was an effect of gender, whereby females had a higher ratio of specific striatal binding than males. Age was negatively correlated with the ratio of specific striatal binding, both in patients and controls. The meta-analysis provided a pooled standardized effect size (Cohen's d) of -0.07 (95% CI -0.31, 0.18; P = .60) for the patient vs control comparison in TRODAT binding, with no evidence of heterogeneity between studies or publication bias. Our findings suggest that striatal DAT levels are not altered in the early stages of schizophrenia before medication is introduced. We identified gender differences and aging effects that could have significance for future studies.

Dopamine transporter binding is unaffected by L-DOPA administration in normal and MPTP-treated monkeys

Background

Radiotracer imaging of the presynaptic nigrostriatal dopaminergic system is used to assess disease progression in Parkinson's disease (PD) and may provide a useful adjunct to clinical assessment during therapeutic trials of potential neuroprotective agents. Several clinical trials comparing dopamine agonists to L-DOPA or early vs. late L-DOPA have revealed differences between clinical assessment and imaging of the presynaptic dopaminergic system, hence questioning the comparability of these measures as neuroprotection outcome variables. Thus, results of these studies may have been affected by factors other than the primary biological process investigated.

Methodology/Principal Findings

We tested the possibility that L-DOPA might interfere with DAT binding. Post-mortem DAT binding was conducted in normal and MPTP-treated macaque monkeys that were administered L-DOPA, acutely or chronically. In parallel, DAT SPECT was conducted in MPTP-treated animals that were administered chronic L-DOPA. [99mTc]TRODAT-1 SPECT binding was similarly reduced in all MPTP monkeys regardless of L-DOPA treatment. L-DOPA had no significant effect on post-mortem DAT binding either in saline or in MPTP-lesioned animals.

Conclusions/Significance

These data indicate that L-DOPA does not induce modifications of DAT expression detectable by SPECT of by DAT binding autoradiography, suggesting that differences between clinical assessment and radiotracer imaging in clinical trials may not be specifically related to L-DOPA treatment.

Disease-related and drug-induced changes in dopamine transporter expression might undermine the reliability of imaging studies of disease progression in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder. Standard therapeutic interventions are aimed at replenishment of empty dopamine stores with levodopa or substitution with dopamine receptor agonists. However, in the long term this symptomatic therapy fails. Currently, various neuroprotective agents are being developed, with the intention to slow down the degeneration of dopaminergic neurons. In this context, the early identification of persons at risk to develop the disease as well as the assessment of the effectiveness of putative neuroprotective agents, are critical issues. Dopamine transporter (DAT) scintigraphy with single photon emission computed tomography (SPECT) has been used to assess the dopaminergic function in PD. Initial studies with several radioligands show significant loss of DAT binding in PD patients as compared to controls. In this paper we review the evidence on the utility of DAT imaging with SPECT in early PD detection as well as in monitoring neurprotection.

Dopamine transporter: basic aspects and neuroimaging

The plasma membrane dopamine transporter (DAT) is found exclusively in dopamine neurones and seems to be the defining molecule of the dopamine neurone. It provides effective control over the intensity of dopamine-mediated signalling by recapturing the neurotransmitter released by presynaptic neurones. Positron emission tomography (PET) and single-photon emission computed tomography (SPECT) represent unique techniques for assessing in vivo DAT distribution in humans and offer reliable methods for studying nigrostriatal dopaminergic function in health and disease. The characteristics of different DAT radiotracers, the modifying influences of factors such as age, gender, smoking habit, and dopaminergic drugs on DAT transporters as well as their implication in evaluation of neuroimaging studies are discussed.

Slowing Parkinson's disease progression: recent dopamine agonist trials

In recent clinical trials, chronic treatment of patients with PD with pramipexole or ropinirole was associated with a slower decline of imaged striatal dopaminergic signal, compared to levodopa monotherapy. Although this could reflect slowed progression of PD, equally plausible is a pharmacologic effect on proteins that interact with the imaging radioligands. To date, there is no compelling evidence favoring dopamine agonists over levodopa; either is an appropriate choice for initial treatment of PD.

Selective modifications in the nucleus accumbens of dopamine synaptic transmission in rats exposed to chronic stress

Stressful events are accompanied by modifications in dopaminergic transmission in distinct brain regions. As the activity of the neuronal dopamine (DA) transporter (DAT) is considered to be a critical mechanism for determining the extent of DA receptor activation, we investigated whether a 3-week exposure to unavoidable stress, which produces a reduction in DA output in the nucleus accumbens shell (NAcS) and medial prefrontal cortex (mPFC), would affect DAT density and DA D1 receptor complex activity in the NAcS, mPFC and caudate-putamen (CPu). Rats exposed to unavoidable stress showed a decreased DA output in the NAcS accompanied by a decrease in the number of DAT binding sites, and an increase in the number of DA D1 binding sites and Vmax of SKF 38393-stimulated adenylyl cyclase. In the mPFC, stress exposure produced a decrease in DA output with no modification in DAT binding or in DA D1 receptor complex activity. Moreover, in the CPu stress exposure induced no changes in DA output or in the other neurochemical variables examined. This study shows that exposure to a chronic unavoidable stress that produces a decrease in DA output in frontomesolimbic areas induced several adaptive neurochemical modifications selectively in the nucleus accumbens.

Tyrosine hydroxylase and dopamine transporter expression in residual dopaminergic neurons: potential contributors to spontaneous recovery from experimental Parkinsonism

1-Methyl-4-phenyl-1,2,3,6-tetrahyrdropyridine (MPTP)-exposed cats develop severe Parkinsonism that spontaneously resolves in 4-6 weeks. The present study examined the extent to which compensatory changes in tyrosine hydroxylase (TH) and dopamine transporter (DAT) gene and protein expression may underlie this behavioral recovery. In normal cats, TH and DAT protein levels were higher in the dorsal vs. ventral striatum. Expression of DAT and TH mRNA was higher in substantia nigra pars compacta (SNc) than in the ventral tegmental area (VTA). In symptomatic parkinsonian animals, DAT and TH protein levels were significantly decreased in all striatal areas studied. TH and DAT mRNA expression in residual SNc neurons were decreased a mean 32% and 38%, respectively. DAT gene expression in residual VTA neurons in symptomatic animals was decreased 30% whereas TH gene expression was unaffected. In spontaneously recovered cats, TH protein levels were significantly higher than the levels in symptomatic cats only in the ventral striatum, whereas no increase in DAT protein levels were observed in any striatal area. Residual neurons in most ventral mesencephalic regions of recovered cats had increased TH mRNA expression but not increased DAT gene expression, compared with symptomatic animals. Thus, increased TH protein and mRNA and suppression of DAT protein and mRNA expression in the striatum and ventral mesencephalon were associated with functional recovery from MPTP-induced parkinsonism.

Chronic psychosocial stress reduces the density of dopamine transporters

The effect of different types of physical stress on brain dopaminergic function has been well established in rodents; however, the role of the dopaminergic system in more naturalistic stress situations is poorly understood. Therefore, the aim of the current study was to investigate the effect of chronic psychosocial stress on the dopamine transporter, which is an important component in the regulation of dopaminergic neurotransmission. For this purpose, we used the well-characterized paradigm of subordination stress in male tree shrews (Tupaia belangeri). In the present study, the animals were subjected to psychosocial stress for 28 days. Animals were daily videotaped and locomotor activity was quantified. In subordinate animals, urinary cortisol and noradrenaline, as well as adrenal weight, were increased, whereas body weight, locomotor activity and testicular function were decreased. Brain dopamine transporter binding sites were quantified by in vitro autoradiography using [3H] WIN 35,428 as ligand. Chronic stress reduced the number of binding sites (Bmax) in the caudate nucleus and the putamen without affecting the affinity (Kd). Stress did not influence the binding parameters in the nucleus accumbens, the substantia nigra or the ventral tegmental area. Furthermore, we found a positive correlation between locomotor activity and the Bmax values for [3H] WIN 35,428 binding in the caudate nucleus, the putamen and the nucleus accumbens. The present study shows that a naturalistic stressor, such as chronic psychosocial conflict, decreases dopamine transporter binding sites in motor-related brain areas, suggesting that the reduction in locomotor activity in subordinate tree shrews is related to the downregulation of dopamine transporter binding sites.

The effect of dopamine agonist therapy on dopamine transporter imaging in Parkinson's disease

Single-photon emission computed tomography (SPECT) imaging with the dopamine transporter ligand, [123I] beta-CIT (2beta-carboxymethoxy-3beta-[4-iodophenyl] tropane), has been proposed as a means of measuring Parkinson's disease (PD) progression. To be useful in this role, however, [123I] beta-CIT imaging should not be influenced by the medications used to treat PD, including the dopamine agonist drugs such as pergolide. We assessed the effect of adjunctive pergolide administration on [123I] beta-CIT uptake in 12 patients with PD, who were being treated with levodopa, initiating pergolide therapy for motor fluctuations. Patients underwent [123I] beta-CIT imaging at baseline, subsequently while on pergolide therapy (6 weeks), and again 4 weeks after pergolide wash-out. Uptake in the striatum was averaged for the two sides and expressed as (striatum - occipital)/occipital, with similar calculations for putamen and caudate. Consistent with PD, the patients' mean striatal and putamen uptake ratios at baseline were significantly less (p <0.001) than the mean values from 26 normal control subjects of similar age. During pergolide treatment, the striatal and putamen [123I] beta-CIT uptake ratios were each statistically similar to baseline, although there was a slight trend toward an increased striatal value (8% higher on pergolide; p = 0.105). Caudate [123I] beta-CIT uptake was 11% higher on pergolide therapy (nominal p = 0.042, but not significant when adjusted for multiple comparisons: p = 0.126). After pergolide wash-out, the striatal, putamen, and caudate uptake ratios did not differ from baseline. Therefore, we found that pergolide therapy did not significantly affect [123I] beta-CIT SPECT imaging but we cannot exclude a small influence.

Cocaine upregulates the dopamine transporter in fetal rhesus monkey brain

Cocaine is a highly addictive drug that binds to the dopamine transporter (DAT), inhibits the reuptake of dopamine, and initiates multiple actions within midbrain dopaminergic systems. Using the rhesus monkey, we have investigated the consequences of in utero cocaine exposure on the expression of DAT in the fetal brain. By using the selective DAT ligand [125I]RTI-121 and tyrosine hydroxylase (TH) immunocytochemistry, we found that DAT binding sites are highly developed by day 70 of gestation and show a distribution pattern similar to TH. The rank order of specific 3beta-(4-[125I]iodophenyl)tropane-2beta-carboxylic acid isopropyl ester ([125I]RTI-121) binding densities was substantia nigra-ventral tegmental area > putamen > caudate > lateral hypothalamus > accumbens > linear/interfascicular nuclei >/= globus pallidus > prefrontal cortex. Furthermore, we observed that DAT mRNA was differentially expressed within fetal midbrain dopamine neurons with the highest levels detected in the ventral tier of the substantia nigra pars compacta, and the lowest levels in the ventral tegmental area and the linear/interfascicular nuclei. In utero cocaine exposure between days 22 and 70 significantly increased DAT mRNA expression, and the density of [125I]RTI-121 binding sites within midbrain dopamine neurons in the 70-d-old fetus. This increased DAT expression is accompanied by other presynaptic and postsynaptic neuronal changes, which collectively suggest that midbrain dopamine neurons are hypoactive after prolonged cocaine exposure, a state that may be a contributing factor in the development of attention deficit disorders observed in subjects exposed prenatally to cocaine.

Tyrosine hydroxylase immunoreactivity and [3H]WIN 35,428 binding to the dopamine transporter in a hamster model of idiopathic paroxysmal dystonia

Recent pharmacological studies and receptor analyses have suggested that dopamine neurotransmission is enhanced in mutant dystonic hamsters (dt(sz)), a model of idiopathic paroxysmal dystonia which displays attacks of generalized dystonia in response to mild stress. In order to further characterize the nature of dopamine alterations, the present study investigated possible changes in the number of dopaminergic neurons, as defined by tyrosine hydroxylase immunohistochemistry, as well as binding to the dopamine transporter labelled with [3H]WIN 35,428 in dystonic hamsters. No differences in the number of tyrosine hydroxylase-immunoreactive neurons were found within the substantia nigra and ventral tegmental area of mutant hamsters compared to non-dystonic control hamsters. Similarly, under basal conditions, i.e. in the absence of a dystonic episode, no significant changes in [3H]WIN 35,428 binding were detected in dystonic brains. However, in animals killed during the expression of severe dystonia, significant decreases in dopamine transporter binding became evident in the nucleus accumbens and ventral tegmental area in comparison to controls exposed to the same external stimulation. Since stimulation tended to increase [3H]WIN 35,428 binding in control brains, the observed decrease in the ventral tegmental area appeared to be due primarily to the fact that binding was increased less in dystonic brains than in similarly stimulated control animals. This finding could reflect a diminished ability of the dopamine transporter to undergo adaptive changes in response to external stressful stimulation in mutant hamsters. The selective dopamine uptake inhibitor GBR 12909 (20 mg/kg) aggravated dystonia in mutant hamsters, further suggesting that acute alterations in dopamine transporter function during stimulation may be an important component of dystonia in this model.

Effect of low-dose treatment with selegiline on dopamine transporter (DAT) expression and amphetamine-induced dopamine release in vivo

1. Chronic treatment with low doses of the selective monoamine oxidase (MAO) type B inhibitors selegiline [(-)-deprenyl] and rasagiline, causes elevation in extracellular level of 3,4-dihydroxyphenylethylamine (dopamine) in the rat striatum in vivo (Lamensdorf et al., 1996). The present study was carried out to determine whether this effect of selegiline could be the result of an inhibition of the high-affinity dopamine neuronal transport process. 2. Changes in activity of the dopamine transporter (DAT) in vivo following selegiline treatment were evaluated indirectly by microdialysis technique in the rat, from the change in striatal dopamine extracellular concentration following systemic amphetamine administration (4 mg kg(-1), i.p.). Striatal levels of the DAT molecule were determined by immunoblotting. Uptake of [3H]-dopamine was determined in synaptosomes from selegiline-treated animals. 3. Amphetamine-induced increase in striatal extracellular dopamine level was attenuated by one day and by chronic (21 days) treatment with selegiline (0.25 mg kg(-1), s.c.). 4. Striatal levels of DAT were elevated after 1 and 21 days treatment with selegiline, but were not affected by clorgyline, rasagiline, nomifensine or amphetamine. 5. The increase in DAT expression, and attenuation of amphetamine-induced dopamine release, were not accompanied by a change in [3H]-dopamine uptake in synaptosomes of selegiline-treated animals. 6. The results suggest that a reversible inhibition of dopamine uptake occurs following chronic low dose selegiline treatment in vivo which may be mediated by an increase in endogenous MAO-B substrates such as 2-phenylethylamine, rather than by the inhibitor molecule or its metabolites. Increased DAT expression appears to be a special property of the selegiline molecule, since it occurs after one low dose of selegiline, and is not seen with other inhibitors of MAO-A or MAO-B. The new DAT molecules formed following selegiline treatment appear not to be functionally active.

Localization and dynamic regulation of biogenic amine transporters in the mammalian central nervous system

The monoamines, serotonin, dopamine, norepinephrine, epinephrine and histamine, play a critical role in the function of the hypothalamic-pituitary-adrenal axis and in the integration of information in sensory, limbic, and motor systems. The primary mechanism for termination of monoaminergic neurotransmission is through reuptake of released neurotransmitter by Na+, CI-dependent plasma membrane transporters. A second family of transporters packages monoamines into synaptic and secretory vesicles by exchange of protons. Identification of those cells which express these two families of neurotransmitter transporters is an initial step in understanding what adaptive strategies cells expressing monoamine transporters use to establish the appropriate level of transport activity and thus attain the appropriate efficiency of monoamine storage and clearance. The most recent advances in this field have yielded several surprises about their function, cellular and subcellular localization, and regulation, suggesting that these molecules are not static and most likely are the most important determinants of extracellular levels of monoamines. Here, information on the localization of mRNAs for these transporters in rodent and human brain is summarized along with immunohistochemical information at the light and electron microscopic levels. Regulation of transporters at the mRNA level by manipulation in rodents and differences in transporter site densities by tomographic techniques as an index of regulation in human disease and addictive states are also reviewed. These studies have highlighted the presence of monoamine neurotransmitter transporters in neurons but not in glia in situ. The norepinephrine transporter is present in all cells which are both tyrosine hydroxylase (TH)- and dopamine beta-hydroxylase-positive but not in those cells which are TH- and phenyl-N-methyltransferase-positive, suggesting that epinephrine cells may have their own, unique transporter. In most dopaminergic cells, dopamine transporter mRNA completely overlaps with TH mRNA-positive neurons. However, there are areas in which there is a lack of one to one correspondence. The serotonin transporter (5-HTT) mRNA is found in all raphe nuclei and in the hypothalamic dorsomedial nucleus where the 5-HTT mRNA is dramatically reduced following immobilization stress. The vesicular monoamine transporter 2 (VMAT2) is present in all monoaminergic neurons including epinephrine- and histamine-synthesizing cells. Immunohistochemistry demonstrates that the plasma membrane transporters are present along axons, soma, and dendrites. Subcellular localization of DAT by electron microscopy suggests that these transporters are not at the synaptic density but are confined to perisynaptic areas, implying that dopamine diffuses away from the synapse and that contribution of diffusion to dopamine signalling may vary between brain regions. Interestingly, the presence of VMAT2 in vesicles underlying dendrites, axons, and soma suggests that monoamines may be released at these cellular domains. An understanding of the regulation of transporter function may have important therapeutic consequences for neuroendocrine function in stress and psychiatric disorders.

Evidence for synthesis and release of catecholamines by human amniotic epithelial cells

The present study investigated the presence, possible synthesis and release of catecholamines (CA) by human amniotic epithelial cells (HAEC) using HPLC with electrochemical detection. The presence of CA was indicated by the detection of norepinephrine (NE), dopamine (DA) and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in extracts of cultured HAEC. Incubation of HAE cells in medium supplemented with 1-tyrosine (CA precursor) and tetrahydrobiopterin (tyrosine hydroxylase cofactor) significantly increased the production of catecholamines, suggesting CA synthesis by HAEC. In contrast, pharmacological inhibition of tyrosine hydroxylase by alpha-methyl-p-tyrosine (MPT) significantly reduced CA production, further confirming CA synthesis by HAEC. Catecholamines were also detected in the cell incubation media, demonstrating the ability of HAEC to spontaneously secrete CA. Moreover, incubation of cells with 50 mM K+ for 10 min increased the amount of CA released into the medium. Additionally, the detection of DOPAC, a primary metabolite of DA, in HAEC strongly indicates that these cells contain DA metabolizing enzymes. The present results suggest that HAEC synthesize and release CA. These cells may be a possible candidate for transplantation therapy of neurodegenerative diseases such as Parkinson's disease and also may serve as a model to study the aspects of catecholaminergic activity.