Visualizing vesicular dopamine dynamics in Parkinson's disease

Characteristics and influencing factors of 11C-CFT PET imaging in patients with early and late onset Parkinson's disease

Objective

This study aims to explore the difference between 11C-methyl-N-2β-carbomethoxy-3β-(4-fluorophenyl)-tropanel (11C-CFT) positron emission tomography (PET) imaging in the early-onset Parkinson's disease (EOPD) and late-onset Parkinson's disease (LOPD), and to analyze the correlation between 11C-CFT PET imaging and disease duration, Hoehn & Yahr (H&Y) stage, motor symptoms, and non-motor symptoms in patients with idiopathic Parkinson's disease (PD), so as to explore its application value in assessing the severity of Parkinson's disease.

Materials and methods

A total of 113 patients with idiopathic PD were included in this study. The patients were divided into EOPD and LOPD groups according to the age of 60 years, of which 58 were early-onset and 55 were late-onset. All patients underwent 11C-CFT PET imaging and manually sketched regions of interest (ROI) to delineate the caudate nucleus, anterior putamen, and posterior putamen ROI layer-by-layer, and the corresponding values were recorded. Clinical data [age of onset, disease duration, H&Y stage, total Unified Parkinson's Disease Rating Scale (UPDRS) score, UPDRS III score, tremor score, postural instability/gait difficulty (PIGD) score, rigidity score, bradykinesia score, and Montreal Cognitive Assessment (MoCA) score] were collected from all patients. The differences in striatal 11C-CFT uptake between patients with EOPD and LOPD were compared, and the correlation between striatal 11C-CFT uptake and the clinical data of patients with idiopathic PD was evaluated.

Results

The caudate nucleus 11C-CFT uptake was higher in EOPD than in the LOPD group (t = 3.002, p = 0.003). 11C-CFT uptake in the caudate nucleus in patients with PD was negatively correlated with the age of onset, H&Y stage, disease duration, total UPDRS score, UPDRS III score, rigidity score, and bradykinesia score (p < 0.05). The anterior and posterior putamen 11C-CFT uptake was negatively correlated with H&Y stage, disease duration, total UPDRS score, UPDRS III score, PIGD score, rigidity score, and bradykinesia score (p < 0.05).

Conclusion

11C-CFT PET provides an objective molecular imaging basis for the difference in disease progression rates between patients with EOPD and LOPD. Secondly, 11C-CFT PET can be used as an important objective indicator to assess disease severity and monitor disease progression.

Metal-organic framework-derived porous ternary ZnCo2O4 nanoplate arrays grown on carbon cloth for simultaneous electrochemical determination of ascorbic acid, dopamine, and uric acid

Metal-organic frameworks derived from ternary metal oxide directly grown on the conductive substrate have attracted great interest in electrochemical sensing. In this work, metal-organic framework-derived ternary ZnCo₂O₄ nanoplate arrays that were grown on carbon cloth (ZnCo₂O₄ NA/CC) are fabricated and applied for the electrochemical determination of ascorbic acid (AA), dopamine (DA), and uric acid (UA). Field emission scanning electron microscope (FESEM) reveals that a network-like CC substrate is covered with considerable nanoplate arrays, presenting a large specific area. X-ray photoelectron spectroscopy (XPS) demonstrates the nanoplate arrays to be composed of ZnCo₂O₄. Benefiting from the unique array morphology and ternary element composition, the ZnCo₂O₄ NA/CC shows desirable performances for simultaneous detection of AA, DA, and UA. The individual detection limits are 7.14 μM for AA, 0.25 μM for DA, and 0.33 μM for UA. Additionally, the ZnCo₂O₄ NA/CC is successfully applied for the quantitative determination of AA, DA, and UA in spiked serum samples, showing its great application potential.

Pragmatic Approach on Neuroimaging Techniques for the Differential Diagnosis of Parkinsonisms

Background

Rapid advances in neuroimaging technologies in the exploration of the living human brain also apply to movement disorders. However, the accurate diagnosis of Parkinson's disease (PD) and atypical parkinsonian disorders (APDs) still remains a challenge in daily practice.

Methods

We review the literature and our own experience as the Movement Disorder Society-Neuroimaging Study Group in Movement Disorders with the aim of providing a practical approach to the use of imaging technologies in the clinical setting.

Results

The enormous amount of articles published so far and our increasing recognition of imaging technologies contrast with a lack of imaging protocols and updated algorithms for differential diagnosis. The distinctive pathological involvement in different brain structures and the correlation with imaging findings obtained with magnetic resonance, positron emission tomography, or single-photon emission computed tomography illustrate what qualitative and quantitative measures may be useful in the clinical setting.

Conclusion

We delineate a pragmatic approach to discuss imaging technologies, updated imaging algorithms, and their implications for differential diagnoses in PD and APDs.

VMAT2 availability in Parkinson's disease with probable REM sleep behaviour disorder

REM sleep behaviour disorder (RBD) can be an early non-motor symptom of Parkinson’s disease (PD) with pathology involving mainly the pontine nuclei. Beyond the brainstem, it is unclear if RBD patients comorbid with PD have more affected striatal dopamine denervation compared to PD patients unaffected by RBD (PD-RBD−). To elucidate this, we evaluated the availability of vesicular monoamine transporter 2 (VMAT2), an index of nigrostriatal dopamine innervation, in 15 PD patients with probable RBD (PD-RBD+), 15 PD-RBD−, and 15 age-matched healthy controls (HC) using [¹¹C]DTBZ PET imaging. This technique measured VMAT2 availability within striatal regions of interest (ROI). A mixed effect model was used to compare the radioligand binding of VMAT2 between the three groups for each striatal ROI, while co-varying for sex, cognitive function and depression scores. Multiple regressions were also computed to predict clinical measures from group condition and VMAT2 binding within all ROIs explored. We observed a significant main effect of group condition on VMAT2 availability within the caudate, putamen, ventral striatum, globus pallidus, substantia nigra, and subthalamus. Specifically, our results revealed that PD-RBD+ had lower VMAT2 availability compared to HC in all these regions except for the subthalamus and substantia nigra, while PD-RBD− was significantly lower than HC in all these regions. PD-RBD− showed a negative relationship between motor severity and VMAT2 availability within the left caudate. Our findings reflect that both PD patient subgroups had similar denervation within the nigrostriatal pathway. There were no significant interactions detected between radioligand binding and clinical scores in PD-RBD+. Taken together, VMAT2 and striatal dopamine denervation in general may not be a significant contributor to the pathophysiology of RBD in PD patients. Future studies are encouraged to explore other underlying neural chemistry mechanisms contributing to RBD in PD patients.

Dopamine Transporter Imaging, Current Status of a Potential Biomarker: A Comprehensive Review

A major goal of current clinical research in Parkinson's disease (PD) is the validation and standardization of biomarkers enabling early diagnosis, predicting outcomes, understanding PD pathophysiology, and demonstrating target engagement in clinical trials. Molecular imaging with specific dopamine-related tracers offers a practical indirect imaging biomarker of PD, serving as a powerful tool to assess the status of presynaptic nigrostriatal terminals. In this review we provide an update on the dopamine transporter (DAT) imaging in PD and translate recent findings to potentially valuable clinical practice applications. The role of DAT imaging as diagnostic, preclinical and predictive biomarker is discussed, especially in view of recent evidence questioning the incontrovertible correlation between striatal DAT binding and nigral cell or axon counts.

Striatal acetylcholine-dopamine imbalance in Parkinson's disease: in vivo neuroimaging study with dual-tracer PET and dopaminergic PET-informed correlational tractography

Previous studies of animal models of Parkinson's disease (PD) suggest an imbalance between striatal acetylcholine (ACh) and dopamine (DA), although other studies have questioned this. To our knowledge, there are no previous in vivo neuroimaging studies examining striatal ACh-DA imbalance in PD patients. Using cholinergic and dopaminergic PET (¹⁸F-FEOBV and ¹¹C-DTBZ, respectively) and correlational tractography, our aim was to investigate the ACh-DA interaction at two levels of dopaminergic loss in PD subjects: integrity loss of the nigrostriatal dopaminergic white matter tract; and loss at the presynaptic-terminal level. Methods: The study involved 45 subjects with mild to moderate PD (36 men, 9 women; mean age, 66.3 ± 6.3 years, disease duration, 5.8 ± 3.6; Hoehn and Yahr stage, 2.2 ± 0.6) and 15 control subjects (9 men, 6 women; mean age, 69.1 ± 8.6 years). PET imaging was performed using standard protocols. We first estimated the integrity of the dopaminergic nigrostriatal white matter tracts in PD subjects by incorporating molecular information from striatal ¹¹C-DTBZ PET into the fiber tracking process using correlational tractography (based on quantitative anisotropy, QA; a measure of tract integrity). Subsequently, we used voxel-based correlation to test the association of the mean QA of the nigrostriatal tract of each cerebral hemisphere with striatal ¹⁸F-FEOBV distribution volume ratio (DVR) in PD subjects. The same analysis was performed for ¹¹C-DTBZ DVR in 12 striatal subregions (presynaptic-terminal level). Results: Unlike ¹¹C-DTBZ DVR in striatal subregions, the mean QA of the nigrostriatal tract of the most affected (MA) hemisphere showed a negative correlation with a striatal cluster of ¹⁸F-FEOBV DVR in PD subjects (p corrected= 0.039). We also found that the mean ¹⁸F-FEOBV DVR within this cluster was higher in the PD group compared to the control group (P = 0.01). Cross-validation analyses confirmed these findings. We also found an increase of bradykinesia ratings associated with increased ACh-DA imbalance in the MA hemisphere (r=0.41, P = 0.006). Conclusion: Our results provide evidence for the existence of striatal ACh-DA imbalance in early PD and may provide an avenue for testing in vivo effects of therapeutic strategies aimed at restoring striatal ACh-DA imbalance in PD.

Innovative Molecular Imaging for Clinical Research, Therapeutic Stratification, and Nosography in Neuroscience

Over the past few decades, several radiotracers have been developed for neuroimaging applications, especially in PET. Because of their low steric hindrance, PET radionuclides can be used to label molecules that are small enough to cross the blood brain barrier, without modifying their biological properties. As the use of 11C is limited by its short physical half-life (20 min), there has been an increasing focus on developing tracers labeled with 18F for clinical use. The first such tracers allowed cerebral blood flow and glucose metabolism to be measured, and the development of molecular imaging has since enabled to focus more closely on specific targets such as receptors, neurotransmitter transporters, and other proteins. Hence, PET and SPECT biomarkers have become indispensable for innovative clinical research. Currently, the treatment options for a number of pathologies, notably neurodegenerative diseases, remain only supportive and symptomatic. Treatments that slow down or reverse disease progression are therefore the subject of numerous studies, in which molecular imaging is proving to be a powerful tool. PET and SPECT biomarkers already make it possible to diagnose several neurological diseases in vivo and at preclinical stages, yielding topographic, and quantitative data about the target. As a result, they can be used for assessing patients' eligibility for new treatments, or for treatment follow-up. The aim of the present review was to map major innovative radiotracers used in neuroscience, and explain their contribution to clinical research. We categorized them according to their target: dopaminergic, cholinergic or serotoninergic systems, β-amyloid plaques, tau protein, neuroinflammation, glutamate or GABA receptors, or α-synuclein. Most neurological disorders, and indeed mental disorders, involve the dysfunction of one or more of these targets. Combinations of molecular imaging biomarkers can afford us a better understanding of the mechanisms underlying disease development over time, and contribute to early detection/screening, diagnosis, therapy delivery/monitoring, and treatment follow-up in both research and clinical settings.

Effect of STN DBS on vesicular monoamine transporter 2 and glucose metabolism in Parkinson's disease

INTRODUCTION

Deep brain stimulation (DBS) is an established treatment for Parkinson's Disease (PD). Despite the improvement of motor symptoms in most patients by sub-thalamic nucleus (STN) DBS and its widespread use, the neurobiological mechanisms are not completely understood. The objective of the present study was to elucidate the effects of subthalamic nucleus (STN) DBS in PD on the dopamine system and neural circuitry, employing high-resolution positron emission tomography (PET) imaging. The hypotheses tested were that STN DBS would decrease the striatal vesicular monoamine transporter (VMAT2), secondary to an increase in dopamine concentrations, and would decrease striatal cerebral metabolism and increase cortical cerebral metabolism.

METHODS

PET imaging of the vesicular monoamine transporter (VMAT2) and cerebral glucose metabolism was performed prior to DBS surgery and after 4-6 months of STN stimulation in seven PD patients (mean age 67 ± 7).

RESULTS

The patients demonstrated significant improvement in motor and neuropsychiatric symptoms after STN DBS. Decreased VMAT2 was observed in the caudate, putamen and associative striatum and in extra-striatal, cortical and limbic regions. Cerebral glucose metabolism was decreased in striatal sub-regions and increased in temporal and parietal cortices and the cerebellum. Decreased striatal VMAT2 was correlated with decreased striatal and increased cortical and limbic metabolism. Improvement of depressive symptoms was correlated with decreased VMAT2 in striatal and extra-striatal regions and with striatal decreases and cortical increases in metabolism.

CONCLUSIONS

The present results support further investigation of the role of VMAT2, and associated changes in neural circuitry in the improvement of motor and non-motor symptoms with STN DBS in PD.

Dopaminergic Disturbances in Tourette Syndrome: An Integrative Account

Tourette syndrome (TS) is thought to involve dopaminergic disturbances, but the nature of those disturbances remains controversial. Existing hypotheses suggest that TS involves 1) supersensitive dopamine receptors, 2) overactive dopamine transporters that cause low tonic but high phasic dopamine, 3) presynaptic dysfunction in dopamine neurons, or 4) dopaminergic hyperinnervation. We review evidence that contradicts the first two hypotheses; we also note that the last two hypotheses have traditionally been considered too narrowly, explaining only small subsets of findings. We review all studies that have used positron emission tomography and single-photon emission computerized tomography to investigate the dopaminergic system in TS. The seemingly diverse findings from those studies have typically been interpreted as pointing to distinct mechanisms, as evidenced by the various hypotheses concerning the nature of dopaminergic disturbances in TS. We show, however, that the hyperinnervation hypothesis provides a simple, parsimonious explanation for all such seemingly diverse findings. Dopaminergic hyperinnervation likely causes increased tonic and phasic dopamine. We have previously shown, using a computational model of the role of dopamine in basal ganglia, that increased tonic dopamine and increased phasic dopamine likely increase the propensities to express and learn tics, respectively. There is therefore a plausible mechanistic link between dopaminergic hyperinnervation and TS via increased tonic and phasic dopamine. To further bolster this argument, we review evidence showing that all medications that are effective for TS reduce signaling by tonic dopamine, phasic dopamine, or both.

Update on Molecular Imaging in Parkinson's Disease

Advances in radionuclide tracers have allowed for more accurate imaging that reflects the actions of numerous neurotransmitters, energy metabolism utilization, inflammation, and pathological protein accumulation. All of these achievements in molecular brain imaging have broadened our understanding of brain function in Parkinson's disease (PD). The implementation of molecular imaging has supported more accurate PD diagnosis as well as assessment of therapeutic outcome and disease progression. Moreover, molecular imaging is well suited for the detection of preclinical or prodromal PD cases. Despite these advances, future frontiers of research in this area will focus on using multi-modalities combining positron emission tomography and magnetic resonance imaging along with causal modeling with complex algorithms.

Exploring the use of shape and texture descriptors of positron emission tomography tracer distribution in imaging studies of neurodegenerative disease

Positron emission tomography (PET) data related to neurodegeneration are most often quantified using methods based on tracer kinetic modeling. In contrast, here we investigate the ability of geometry and texture-based metrics that are independent of kinetic modeling to convey useful information on disease state. The study was performed using data from Parkinson's disease subjects imaged with (11)C-dihydrotetrabenazine and (11)C-raclopride. The pattern of the radiotracer distribution in the striatum was quantified using image-based metrics evaluated over multiple regions of interest that were defined on co-registered PET and MRI images. Regression analysis showed a significant degree of correlation between several investigated metrics and clinical evaluations of the disease (p < 0.01). The best results were obtained with the first-order moment invariant of the radioactivity concentration values estimated over the full structural extent of the region as defined by MRI (R(2 )= 0.94). These results demonstrate that there is clinically relevant quantitative information in the tracer distribution pattern that can be captured using geometric and texture descriptors. Such metrics may provide an alternate and complementary data analysis approach to traditional kinetic modeling.

PET imaging in neurology: Alzheimer's and Parkinson's diseases

PET studies play an important role in the early detection of Alzheimer's and Parkinson's diseases (AD and PD). Fluorine-18 fluorodeoxyglucose (F-FDG) PET imaging of regional cerebral glucose metabolism and PET amyloid imaging are the two major PET studies for AD. F-FDG PET is highly sensitive for the early diagnosis of AD, in predicting conversion from mild cognitive impairment to AD, and in differentiating AD from other dementias. PET amyloid imaging is positive in the majority of patients with AD. Negative amyloid PET reduces the likelihood of AD. The main limitations of PET amyloid imaging is its high positivity in the normal elderly population and in other medical conditions with amyloid pathologies. Various PET tracers are available to assess motor and cognitive dysfunctions in PD. PET tracers targeting presynaptic dopaminergic function (F-DOPA, radiolabeled PET tracers assessing the availability of dopamine transporters and vesicular monoamine transporters) and postsynaptic dopamine receptors are used to assess motor dysfunction in PD. PET tracers, particularly dopamine transporters, are highly sensitive in the early diagnosis of PD. Uptake of PET tracers in the striatum is inversely correlated with disease severity. PET is valuable in differentiating PD from other movement disorders. PET studies, particularly F-FDG PET, help to evaluate cortical metabolism in PD patients with cognitive dysfunction and dementia.

Rapid Recovery of Vesicular Dopamine Levels in Methamphetamine Users in Early Abstinence

We previously reported very low levels of dopamine in post-mortem striatum of chronic methamphetamine users, raising the possibility that restoration of normal dopamine levels could help in this addiction and perhaps prevent early relapse. To establish relevance of this finding to the living brain, we tested whether striatal [(11)C]-(+)-dihydrotetrabenazine binding, a vesicular monoamine transporter probe sensitive to changes in (stored) vesicular dopamine, is elevated in methamphetamine users. Chronic methamphetamine users underwent [(11)C]-(+)-dihydrotetrabenazine positron emission tomography scans during early (mean 2.6 days) and later (~10 days) abstinence. Striatal [(11)C]-(+)-dihydrotetrabenazine binding was elevated (suggesting low stored dopamine) in methamphetamine users (n=28; 2.6 days after last use) relative to controls (n=22) (+28%, p<0.0001) and correlated with severity and recency of drug use and with cognitive impairment and withdrawal symptoms. Mean [(11)C]-(+)-dihydrotetrabenazine binding levels in the subgroup of methamphetamine users who could remain abstinent ~10 days following last use (n=17) were normal at the follow-up scan. Our imaging data support post-mortem findings and suggest that chronic methamphetamine users have low brain levels of stored dopamine during very early abstinence from MA, which could contribute to behavioral and cognitive deficits. Findings also suggest a rapid recovery of stored dopamine in some methamphetamine users who become abstinent and who therefore might not benefit from dopamine replacement medication (eg, levodopa). Further study is necessary to establish whether those users who could not maintain abstinence for the second scan might have a more severe and persistent dopamine deficiency and who could benefit from this medication.

Structural, functional and spectroscopic MRI studies of methamphetamine addiction

This chapter reviews selected neuroimaging findings related to long-term amphetamine and methamphetamine (MA) use. An overview of structural and functional (fMRI) MR studies, Diffusion Tensor Imaging (DTI), Magnetic Resonance Spectroscopy (MRS) and Positron Emission Tomography (PET) studies conducted in long-term MA abusers is presented. The focus of this chapter is to present the relevant studies as tools to understand brain changes following drug abstinence and recovery from addiction. The behavioral relevance of these neuroimaging studies is discussed as they relate to clinical symptoms and treatment. Within each imaging section this chapter includes a discussion of the relevant imaging studies as they relate to patterns of drug use (i.e., duration of MA use, cumulative lifetime dose and time MA abstinent) as well as an overview of studies that link the imaging findings to cognitive measures. In our conclusion we discuss some of the future directions of neuroimaging as it relates to the pathophysiology of addiction.

In vivo detection of monoaminergic degeneration in early Parkinson disease by (18)F-9-fluoropropyl-(+)-dihydrotetrabenzazine PET

PET with (18)F-9-fluoropropyl-(+)-dihydrotetrabenzazine ((18)F-DTBZ), a novel radiotracer targeting vesicular monoamine transporter type 2 (VMAT2), has been proven as a useful imaging marker to measure dopaminergic integrity.

METHODS

The aim of this study was to evaluate the capability of (18)F-DTBZ PET in detecting the monoaminergic degeneration in early Parkinson disease (PD) in vivo. Seventeen age-matched healthy subjects and 30 PD patients at early stage of disease (duration of disease ≤ 5 y) with mild and unilateral motor symptoms underwent (18)F-DTBZ PET scans. The severity of disease, including Unified Parkinson Disease Rating Scale and modified Hoehn and Yahr Stage (mHY), were recorded at off-medication states. The standardized volumes of interest were applied to the spatial normalized image for quantification analysis. The specific uptake ratios (SURs) were calculated according to the formula (specific volumes-of-interest counts/occipital cortex counts) - 1. SUR measurements were summarized for each brain region.

RESULTS

The mean duration of disease in the PD group was 3.2 ± 2.1 y (range, 0.5-5 y). The mean mHY was 1.0 ± 0.1 (range, 1-1.5). The SURs of bilateral caudate, anterior putamen, posterior putamen, substantia nigra, and nucleus accumbens were significantly lower in PD patients than those of healthy subjects. The reduction of SURs was most severe in the contralateral (the brain regions that are located opposite to the symptomatic side) posterior putamen (-81%), followed by the ipsilateral posterior putamen (-67%). Receiver-operating-characteristic curve analysis showed that the SURs of the bilateral posterior putamen and contralateral anterior putamen had a sensitivity of 100% and specificity of 100% in differentiating PD patients from healthy subjects.

CONCLUSION

(18)F-DTBZ PET was as an excellent tool for the early diagnosis of PD. The obvious decline of (18)F-DTBZ uptake in the ipsilateral (asymptomatic) striatum suggested that (18)F-DTBZ PET might serve as an in vivo biomarker to detect the monoaminergic degeneration in the premotor phase of PD.

Imaging of Dopamine in PD and Implications for Motor and Neuropsychiatric Manifestations of PD

Parkinson’s disease (PD) is characterized by dopamine depletion in the putamen, which leads to motor dysfunction. As the disease progresses, a substantial degree of dopamine depletion also occurs in caudate and nucleus accumbens. This may explain a number of neuropsychiatric manifestations, including depression, apathy, and cognitive decline. Dopamine replacement therapy partially restores motor function but long-term treatment is often associated with motor complications (motor fluctuations and dyskinesias). Positron emission tomography (PET) studies suggest that the dopamine release rate is substantially higher in PD subjects with motor complications compared to stable responders. Notably, this differential pattern of dopamine release is already present in the early stages of the disease, before motor complications become clinically apparent. Converging evidence suggests that striatal dopamine depletion in PD leads to reduced plasticity in the primary motor cortex and, presumably, in non-motor cortical areas as well. Although dopamine replacement therapy tends to restore physiological plasticity, treatment-induced motor, and neuropsychiatric complications could be related to abnormalities in corticostriatal synaptic plasticity.

Positron emission tomography imaging in neurological disorders

Positron emission tomography (PET) is a powerful tool for in vivo imaging investigations of human brain function. It provides non-invasive quantification of brain metabolism, receptor binding of various neurotransmitter systems, and alterations in regional blood flow. The use of PET in a clinical setting is still limited due to the high costs of cyclotrons and radiochemical laboratories. However, once these limitations can be bypassed, PET could aid clinical practice by providing a useful imaging technique for the diagnosis, the planning of treatment, and the prediction outcome in various neurological diseases.This review aims to explain the PET imaging technique and its applications in neurological disorders such as Parkinson’s disease, Huntington’s disease, multiple sclerosis, and dementias.

Longitudinal evolution of compensatory changes in striatal dopamine processing in Parkinson's disease

Parkinson's disease is a relentlessly progressive neurodegenerative disease. Breakdown of compensatory mechanisms influencing putaminal dopamine processing could contribute to the progressive motor symptoms. We studied a cohort of 78 subjects (at baseline) with sporadic Parkinson's disease and 35 healthy controls with multi-tracer positron emission tomography scans to investigate the evolution of adaptive mechanisms influencing striatal dopamine processing in Parkinson's disease progression. Presynaptic dopaminergic integrity was assessed with three radioligands: (i) [(11)C](±)dihydrotetrabenazine, to estimate the density of vesicular monoamine transporter type 2; (ii) [(11)C]d-threo-methylphenidate, to label the dopamine transporter; and (iii) 6-[(18)F]fluoro-L-DOPA, to assess the activity of aromatic amino acid decarboxylase and storage of 6-[(18)F]-fluorodopamine in synaptic vesicles. The subjects with Parkinson's disease and the healthy controls underwent positron emission tomography scans at the initial visit and after 4 and 8 years of follow-up. Non-linear multivariate regression analyses with random effects were utilized to model the longitudinal changes in tracer values in the putamen standardized relative to normal controls. We found evidence for possible upregulation of dopamine synthesis and downregulation of dopamine transporter in the more severely affected putamen in the early stage of Parkinson's disease. The standardized 6-[(18)F]fluoro-L-DOPA and [(11)C]d-threo-methylphenidate values tended to approach [(11)C](±)dihydrotetrabenazine values in the putamen in later stages of disease (i.e. for [(11)C](±)dihydrotetrabenazine values <25% of normal), when the rates of decline in the positron emission tomography measurements were similar for all the markers. Our data suggest that compensatory mechanisms decline as Parkinson's disease progresses. This breakdown of compensatory strategies in the putamen could contribute to the progression of motor symptoms in advanced disease.

Advances in imaging in Parkinson's disease

Advances in imaging have made it possible to detect functional and, increasingly, structural changes in Parkinson's disease. Although imaging is not yet routinely used for diagnosis, such an application is becoming increasingly feasible. Of potentially greater interest, however, is the use of imaging as a biomarker to detect premotor disease and disease progression. Imaging also provides insights into complications of Parkinson's disease and its long-term treatment, and the role of dopamine in the normal brain. Furthermore, these techniques can be applied to animal models, to help validate these models and allow their use in the study of potential disease-modifying therapies.

Milestones in neuroimaging

In the last 25 years there have been enormous advances in brain imaging. In addition to utility in diagnosis, these have led to novel insights into the pathogenesis of basal ganglia disease and the role of dopamine and the basal ganglia in normal health. The authors review highlights of this work, with a focus on advances in Parkinson's disease, the dystonias, Huntington's disease, and the role of dopamine in cognition and reward signaling. Emerging areas for future development include studies of functional connectivity, the analysis of default mode networks, studies of novel neurochemical pathways, methods to study disease pathogenesis, and the application of imaging techniques to investigate animal models of disease.

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

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

Functional neuroimaging in Parkinson's disease

INTRODUCTION

Functional neuroimaging techniques have greatly contributed to improving our understanding of Parkinson's disease (PD) neurodegeneration and related compensatory mechanisms.

AREAS COVERED

In this paper, the authors analyze the role of functional neuroimaging as a diagnostic tool in PD and review functional neuroimaging studies on PD progression and compensatory adaptations. Through this, the article provides the reader with sensible approaches for the use of functional neuroimaging in the diagnosis of PD. The reader is also provided with knowledge on the time course of nigrostriatal dopamine dysfunction in PD as well as an overview of the potential beneficial and deleterious effects of increased dopamine turnover. Finally, the reader is provided with a critical discussion of the differential effects of levodopa and dopamine agonists on presynaptic dopamine markers and the implications for the interpretation of clinical trials.

EXPERT OPINION

Functional neuroimaging probably plays a limited role in the diagnosis of PD. Parkinson's disease pathology leads to an exponential decline in nigrostriatal dopamine function and a compensatory increase in dopamine turnover, which may help delay symptom onset. On the negative side, increased dopamine turnover contributes to the development of treatment-related motor complications. Presynaptic markers of dopamine function are subject to regulatory changes, compromising the direct interpretation of neuroimaging results in trials of neuroprotective therapies for PD.

Imaging of compensatory mechanisms in Parkinson's disease

PURPOSE OF REVIEW

The on-going quest for potentially disease-modifying therapies in Parkinson's disease has prompted the development of methods that can differentiate direct disease effects from compensatory processes.

RECENT FINDINGS

PET studies have suggested a number of changes at the synaptic level to maintain integrity of dopaminergic systems. Functional MRI studies support the long-held belief that relatively intact cerebellar circuits may compensate for impaired basal ganglia function. Altered connectivity and increased spatial extent of activation also appear to be mechanisms through which motor and cognitive performance can be maintained.

SUMMARY

Ascertaining which changes in brain activation in Parkinson's disease are, in fact, compensatory represents a serious challenge. Compensatory mechanisms have been demonstrated from the microscopic, synaptic level to the macroscopic, system level. Augmentation of compensatory mechanisms, in addition to ameliorating the loss of dopaminergic neurons, may represent a joint strategy for overall minimization of disability.

Age-specific progression of nigrostriatal dysfunction in Parkinson's disease

OBJECTIVE

To investigate in vivo the impact of age on nigrostriatal dopamine dysfunction in Parkinson's disease (PD).

METHODS

PD patients (n = 78) and healthy control subjects (n = 35) underwent longitudinal positron emission tomography assessments using 3 presynaptic dopamine markers: (1) [¹¹C](±)dihydrotetrabenazine (DTBZ), to estimate the density of the vesicular monoamine transporter type 2; (2) [¹¹C]d-threo-methylphenidate, to estimate the density of the plasma membrane dopamine transporter; and (3) 6-[¹⁸F]-fluoro-L-dopa, to estimate the activity of the enzyme dopa-decarboxylase.

RESULTS

The study comprised 438 PD scans and 241 control scans (679 scans in total). At symptom onset, the loss of putamen DTBZ binding was substantially greater in younger compared to older PD patients (p = 0.015). Remarkably, however, the rate of progression of DTBZ binding loss was significantly slower in younger patients (p < 0.05). The estimated presymptomatic phase of the disease spanned more than 2 decades in younger patients, compared to 1 decade in older patients.

INTERPRETATION

Our results suggest that, compared to older patients, younger PD patients progress more slowly and are able to endure more damage to the dopaminergic system before the first motor symptoms appear. These observations suggest that younger PD patients have more efficient compensatory mechanisms.

Influence of a low dose of amphetamine on vesicular monoamine transporter binding: a PET (+)[11C]DTBZ study in humans

We previously reported increased binding of (+)[11C]DTBZ (dihydrotetrabenazine), the vesicular monoamine transporter (VMAT2) positron emission tomography (PET) radioligand, in striatum of some methamphetamine users. This finding might be explained by stimulant-induced vesicular DA depletion resulting in decreased DA (+)[11C]DTBZ competition at VMAT2. In a prospective PET study, we now find that administration of an acute oral dose of amphetamine (0.4 mg/kg) to humans does not cause increased striatal (+)[11C]DTBZ binding but a slight 5% decrease. Our data suggest that a low amphetamine dose is unlikely to cause sufficient DA depletion to detect increased (+)[11C]DTBZ binding and that a higher dose might be required.

Trials of neuroprotective therapies for Parkinson's disease: problems and limitations

Since the initial results of the DATATOP study, considerable effort has been devoted over the past 20 years to test neuroprotective therapies for Parkinson's disease (PD). Two trials (CALM-PD-CIT and REAL-PET studies) used neuroimaging dopamine changes as a surrogate marker for PD progression, and concluded that pramipexole and ropinirole could have a neuroprotective effect compared to levodopa. However, it should be recognized that all the presynaptic dopamine markers currently available for SPECT and PET studies are potentially subject to regulatory changes. Consequently, the results of these two trials can also be interpreted in terms of drug-related differences in dopamine regulation. More recently, the delayed-start design was applied to test whether rasagiline could have a neuroprotective effect in PD (ADAGIO study). Unfortunately, a major limitation of the delayed-start design is that, whenever the active treatment has a symptomatic effect, the blinding can be broken. This can lead to unequally-distributed placebo responses during phase 2, and is also a potential source of raters' biases. None of the trials on neuroprotective therapies for PD has yet provided solid evidence for neuroprotection.

Progression of dopaminergic depletion in a model of MPTP-induced Parkinsonism in non-human primates. An (18)F-DOPA and (11)C-DTBZ PET study

Dopaminergic depletion in the nigrostriatal system is the neurochemical hallmark of Parkinson's disease (PD). Although numerous efforts have been made to determine the evolution of dopaminergic depletion in PD, "in vivo" data concerning the stages of this process are still scarce. We evaluated 6-[18F]-fluoro-l-DOPA ((18)F-DOPA) and 11C-(+)-alpha-dihydrotetrabenazine ((11)C-DTBZ) using PET in a model of chronically MPTP-induced parkinsonism in non-human primates.

METHODS

Sixty-seven cynomolgus monkeys (Macacafascicularis) were included in the study. Progressive parkinsonism was induced by repeated administration of small doses of MPTP (iv) over several months. Animals were classified as controls, asymptomatic, recovered (having exhibited parkinsonian features transiently) and stable parkinsonian, according to their motor status. Analysis of striatal dopaminergic activity was conducted by regions of interest (ROI) and statistical parametric mapping (SPM) over normalized parametric images.

RESULTS

A progressive loss of striatal uptake was evident among groups for both radiotracers, which correlated significantly with the clinical motor status. Changes occurred earlier, i.e. in the less affected stages, with (11)C-DTBZ. Similar results were achieved by ROI and SPM analysis. Uptake was similar with both radiotracers for the asymptomatic and recovered groups.

CONCLUSIONS

Serial assessment with (18)F-DOPA and (11)C-DTBZ PETs provides an effective approach to evaluate evolution of dopaminergic depletion in monkeys with MPTP-induced parkinsonism. This approach could be useful to perform studies aiming to test the effect of early therapeutic intervention and putative neuroprotective treatments.

In vivo [11C]dihydrotetrabenazine binding in rat striatum: sensitivity to dopamine concentrations

INTRODUCTION

The sensitivity of the in vivo binding of [(11)C]dihydrotetrabenazine ([(11)C]DTBZ) and [(11)C]methylphenidate ([(11)C]MPH) to their respective targets - vesicular monoamine transporter type 2 (VMAT2) and neuronal membrane dopamine transporter - after alterations in endogenous levels of dopamine was examined in the rat brain.

METHODS

In vivo binding of [(11)C]DTBZ and [(11)C]MPH was determined using a bolus+infusion protocol. The in vitro number of VMAT2 binding sites was determined by autoradiography.

RESULTS

Repeated dosing with alpha-methyl-p-tyrosine (AMPT) at doses that significantly (-75%) depleted brain tissue dopamine levels resulted in increased (+36%) in vivo [(11)C]DTBZ binding to VMAT2 in the striatum. The increase in binding could be completely reversed via treatment with L-DOPA/benserazide to restore dopamine levels. There were no changes in the total number of VMAT2 binding sites, as measured using in vitro autoradiography. No changes were observed for in vivo [(11)C]MPH binding to the dopamine transporter in the striatum following AMPT pretreatment.

CONCLUSION

These results indicate that large reductions in dopamine concentrations in the rat brain can produce modest but significant changes in the binding of radioligands to VMAT2, which can be reversed by replenishment of dopamine using exogenous L-DOPA.