A common polymorphism in the dopamine transporter gene predicts working memory performance and in vivo dopamine integrity in aging

Dopamine (DA) integrity is suggested as a potential cause of individual differences in working memory (WM) performance among older adults. Still, the principal dopaminergic mechanisms giving rise to WM differences remain unspecified. Here, 61 single-nucleotide polymorphisms, located in or adjacent to various dopamine-related genes, were assessed for their links to WM performance in a sample of 1313 adults aged 61-80 years from the Berlin Aging Study II. Least Absolute Shrinkage and Selection Operator (LASSO) regression was conducted to estimate associations between polymorphisms and WM. Rs40184 in the DA transporter gene, SLC6A3, showed allelic group differences in WM, with T-carriers performing better than C homozygotes (p<0.01). This finding was replicated in an independent sample from the Cognition, Brain, and Aging study (COBRA; baseline: n = 181, ages: 64-68 years; 5-year follow up: n = 129). In COBRA, in vivo DA integrity was measured with ¹¹C-raclopride and positron emission tomography. Notably, WM as well as in vivo DA integrity was higher for rs40184 T-carriers at baseline (p<0.05 for WM and caudate and hippocampal D2-receptor availability) and at the 5-year follow-up (p<0.05 for WM and hippocampal D2 availability). Our findings indicate that individual differences in DA transporter function contribute to differences in WM performance in old age, presumably by regulating DA availability.

Parkinson's disease laterality: a 11C-PE2I PET imaging study

Asymmetry of striatal dopaminergic deficits and motor symptoms is a typical characteristic of idiopathic Parkinson's disease (PD). This study aims to characterise the trend of asymmetry in moderate-stage PD. We performed a 19-month longitudinal study in 27 patients with PET-CT imaging and appropriate clinical assessments. 11C-PE2I non-displaceable binding potential (BPND) was calculated bilaterally for the striatum at baseline and follow-up to estimate the in vivo density of striatal dopamine transporters (DAT). Changes in striatal 11C-PE2I BPND over time were more prominent in the ipsilateral as compared to contralateral side. Changes in MDS-UPDRS-III (motor component of the Movement Disorders Society Unified PD Rating Scale) were not different between the clinically most and least affected body sides. Our data support that the asymmetry in striatal dopaminergic degeneration becomes less prominent in moderate-stage PD. In contrast, during the above period, the asymmetry of motor symptoms was maintained between the clinically most and least affected body sides.

11 C-PE2I and 18 F-Dopa PET for assessing progression rate in Parkinson's: A longitudinal study

BACKGROUND

¹⁸ F-dopa PET measuring aromatic l-amino acid decarboxylase activity is regarded as the gold standard for evaluating dopaminergic function in Parkinson's disease. Radioligands for dopamine transporters are also used in clinical trials and for confirming PD diagnosis. Currently, it is not clear which imaging marker is more reliable for assessing clinical severity and rate of progression. The objective of this study was to directly compare ¹⁸ F-dopa with the highly selective dopamine transporter radioligand ¹¹ C-PE2I for the assessment of motor severity and rate of progression in PD.

METHODS

Thirty-three mild-moderate PD patients underwent ¹⁸ F-dopa and ¹¹ C-PE2I PET at baseline. Twenty-three were followed up for 18.8 ± 3.4 months.

RESULTS

Standard multiple regression at baseline indicated that ¹¹ C-PE2I BP_ND predicted UPDRS-III and bradykinesia-rigidity scores (P < 0.05), whereas ¹⁸ F-dopa K_i did not make significant unique explanatory contributions. Voxel-wise analysis showed negative correlations between ¹¹ C-PE2I BP_ND and motor severity across the whole striatum bilaterally. ¹⁸ F-Dopa K_i clusters were restricted to the most affected putamen and caudate. Longitudinally, negative correlations were found between striatal Δ¹¹ C-PE2I BP_ND , ΔUPDRS-III, and Δbradykinesia-rigidity, whereas no significant associations were found for Δ¹⁸ F-dopa K_i . One cluster in the most affected putamen was identified in the longitudinal voxel-wise analysis showing a negative relationship between Δ¹¹ C-PE2I BP_ND and Δbradykinesia-rigidity.

CONCLUSIONS

Striatal ¹¹ C-PE2I appears to show greater sensitivity for detecting differences in motor severity than ¹⁸ F-dopa. Furthermore, dopamine transporter decline is closely associated with motor progression over time, whereas no such relationship was found with aromatic l-amino acid decarboxylase. ¹¹ C-PE2I may be more effective for evaluating the efficacy of neuroprotective treatments in PD. © 2017 International Parkinson and Movement Disorder Society.

Comparison between visual assessment of dopaminergic degeneration pattern and semi-quantitative ratio calculations in patients with Parkinson's disease and Atypical Parkinsonian syndromes using DaTSCAN® SPECT

OBJECTIVE

To verify if (123)I-FP-CIT, DaTSCAN(®) can differentiate early stages of Parkinson's disease (PD) as well as patients with Atypical Parkinsonian syndromes (APS) from manifest Parkinson's disease.

METHODS

128 consecutive patients were investigated with (123)I-FP-CIT SPECT during a 4-year period. All patients were diagnosed according to the established consensus criteria for diagnosis of PD (n = 53) and APS (n = 19). Remaining patients were grouped early PD (before onset of L-DOPA medication), (n = 20), vascular PD (n = 6), and non-PD syndromes (n = 30) and SWEDD (n = 1). SPECT images were analyzed visually according to a predefined ranking scale of dopaminergic nerve cell degeneration, distinguishing a posterior-anterior degeneration pattern (egg shape) from a more global and severe degeneration pattern (burst striatum). Striatum uptake ratios were quantitatively analyzed with the 3D software, EXINI.

RESULTS

In the group of APS patients, the burst striatum pattern was most frequent and found in 61 % (11/18 patients). In PD patients, the egg shape pattern was dominating, especially in early PD where it was present in 95 % (19/20 patients). The positive predictive value for the egg shape pattern to diagnose PD was 92 % in this material (APS and all PD patients) and the specificity 90 % for the burst striatum pattern to exclude APS. The uptake ratios were reduced in both PD and APS patients and closely related to the image ranking.

CONCLUSION

In this study, we found that in more than half of the patients it was possible to differentiate between PD and APS by visual interpretation only. Similar results were obtained using semi-quantitative uptake ratios. Combining visual assessment with uptake ratios did not add to the discriminating power of DaTSCAN(®) SPECT in this material.

Vesicular integrity in Parkinson's disease

The defining motor characteristics of Parkinson's disease (PD) are mediated by the neurotransmitter dopamine (DA). Dopamine molecules spend most of their lifespan stored in intracellular vesicles awaiting release and very little time in the extracellular space or the cytosol. Without proper packaging of transmitter and trafficking of vesicles to the active zone, dopamine neurotransmission cannot occur. In the cytosol, dopamine is readily oxidized; excessive cytosolic dopamine oxidation may be pathogenic to nigral neurons in PD. Thus, factors that disrupt vesicular function may impair signaling and increase the vulnerability of dopamine neurons. This review outlines the many mechanisms by which disruption of vesicular function may contribute to the pathogenesis of PD. From direct inhibition of dopamine transport into vesicles by pharmacological or toxicological agents to alterations in vesicle trafficking by PD-related gene products, variations in the proper compartmentalization of dopamine can wreak havoc on a functional dopamine pathway. Findings from patient populations, imaging studies, transgenic models, and mechanistic studies will be presented to document the relationship between impaired vesicular function and vulnerability of the nigrostriatal dopamine system. Given the deleterious effects of impaired vesicular function, strategies aimed at enhancing vesicular function may be beneficial in the treatment of PD.

No differential regulation of dopamine transporter (DAT) and vesicular monoamine transporter 2 (VMAT2) binding in a primate model of Parkinson disease

Radioligands for DAT and VMAT2 are widely used presynaptic markers for assessing dopamine (DA) nerve terminals in Parkinson disease (PD). Previous in vivo imaging and postmortem studies suggest that these transporter sites may be regulated as the numbers of nigrostriatal neurons change in pathologic conditions. To investigate this issue, we used in vitro quantitative autoradioradiography to measure striatal DAT and VMAT2 specific binding in postmortem brain from 14 monkeys after unilateral internal carotid artery infusion of 1-Methyl-4-Phenyl-1,2,3,6-tetrahydropyridine (MPTP) with doses varying from 0 to 0.31 mg/kg. Quantitative estimates of the number of tyrosine hydroxylase (TH)-immunoreactive (ir) neurons in substantia nigra (SN) were determined with unbiased stereology, and quantitative autoradiography was used to measure DAT and VMAT2 striatal specific binding. Striatal VMAT2 and DAT binding correlated with striatal DA (r_s = 0.83, r_s = 0.80, respectively, both with n = 14, p<0.001) but only with nigra TH-ir cells when nigral cell loss was 50% or less (r = 0.93, n = 8, p = 0.001 and r = 0.91, n = 8, p = 0.002 respectively). Reduction of VMAT2 and DAT striatal specific binding sites strongly correlated with each other (r = 0.93, n = 14, p<0.0005). These similar changes in DAT and VMAT2 binding sites in the striatal terminal fields of the surviving nigrostriatal neurons demonstrate that there is no differential regulation of these two sites at 2 months after MPTP infusion.

Enhancing aromatic L-amino acid decarboxylase activity: implications for L-DOPA treatment in Parkinson's disease

Aromatic L-amino acid decarboxylase (AAAD) is an essential enzyme for the formation of catecholamines, indolamines, and trace amines. Moreover, it is a required enzyme for converting L-DOPA to dopamine when treating patients with Parkinson's disease (PD). There is now substantial evidence that the activity of AAAD in striatum is regulated by activation and induction, and second messengers play a role. Enzyme activity can be modulated by drugs acting on a number of neurotransmitter receptors including dopamine (D1-4), glutamate (NMDA), serotonin (5-HT(1A), 5-HT(2A)) and nicotinic acetylcholine receptors. Generally, antagonists enhance AAAD activity; while, agonists may diminish it. Enhancement of AAAD activity is functional, as the formation of dopamine from exogenous L-DOPA mirrors activity. Following a lesion of nigrostriatal dopaminergic neurons, AAAD in striatum responds more robustly to pharmacological manipulations, and this is true for the decarboxylation of exogenous L-DOPA as well. We review the evidence for parallel modulation of AAAD activity and L-DOPA decarboxylation and propose that this knowledge can be exploited to optimize the formation of dopamine from exogenous L-DOPA. This information can be used as a blue print for the design of novel L-DOPA treatment adjuvants to benefit patients with PD.

Dihydrotetrabenazine positron emission tomography imaging in early, untreated Parkinson's disease

OBJECTIVE

To determine the sensitivity of positron emission tomography with 11C-labeled dihydrotetrabenazine (DTBZ) to the nigrostriatal changes associated with early, untreated Parkinson's disease (PD), and to determine the correlation between any regionally reduced DTBZ binding and the major motor features of PD.

METHODS

Untreated patients with early PD (n = 27) and age-matched control subjects (n = 33) underwent DTBZ/positron emission tomography scanning to measure binding to the presynaptic type 2 vesicular monoamine transporter site in dopaminergic neurons in basal ganglia regions. Clinical symptoms were rated with the Unified Parkinson's Disease Rating Scale.

RESULTS

Mean striatal DTBZ binding values in the patient group were decreased as compared with control subjects (p < 0.001) in all regions examined. The difference between patients and control subjects was most marked in the midputamen, where only one patient had DTBZ binding within 3 standard deviations of the control mean. Bradykinesia and rigidity scores correlated with DTBZ binding in the contralateral midputamen region, particularly for the clinically least affected limbs. Tremor scores showed no significant correlation.

INTERPRETATION

Reduced striatal binding of DTBZ is associated with early PD. Tremor appears to be only partially related to presynaptic dopaminergic function and may have a mechanism differing from that of symptoms such as bradykinesia. The method appears to be most sensitive in mildly affected individuals with a possible "floor" effect that may limit the degree of additional change occurring once more severe clinical symptoms are evident.

18F-FP-CIT PET imaging and SPM analysis of dopamine transporters in Parkinson's disease in various Hoehn & Yahr stages

To investigate the usefulness of 18F-FP-CIT PET for assessing the severity of Parkinson's disease (PD) at various clinical stages, 41 patients with PD were divided into early (Hoehn&Yahr I-II, n = 23) and advanced (Hoehn & Yahr III-IV, n = 18) subgroups. 18F-FP-CIT PET was performed in these patients and 12 normal subjects. 18F-FP-CIT uptake in striatal subregions and its correlation with UPDRS were first evaluated by ROI analysis, and between-group differences were also analyzed by Statistical Parametric Mapping (SPM). Our results showed that striatal 18F-FP-CIT binding were significantly reduced to 70.9% (caudate), 46.8% (anterior putamen) and 24.0% (posterior putamen) in early PD compared with that of the control, and to 52.0%, 34.5% and 16.5% correspondingly in advanced PD, respectively. There was significant negative correlation between total motor UPDRS score of all parkinsonian patients and 18F-FP-CIT uptake in caudate nucleus (r = -0.53, p < 0.001), anterior putamen (r = -0.53, p < 0.001) and posterior putamen (r = -0.61, p < 0.001). SPM comparison of 18F-FP-CIT uptake between early or advanced PD and the control group showed significant decline in striatum, predominantly localized on the contralateral side and in the dorsal-posterior putamen. These results indicate that 18F-FP-CIT PET can serve as a suitable biomarker to represent the severity of PD in early and advanced stages.

Positron emission tomography of monoaminergic vesicular binding in aging and Parkinson disease

The type-2 vesicular monoamine transporter (VMAT2) might serve as an objective biomarker of Parkinson disease (PD) severity. Thirty-one subjects with early-stage PD and 75 normal subjects underwent continuous intravenous infusion of (+)-[(11)C]dihydrotetrabenazine (DTBZ) and positron emission tomography (PET) imaging to estimate the striatal VMAT2 binding site density with equilibrium tracer modeling. Parkinson disease patients were evaluated clinically in the practically defined 'off' state with the Unified Parkinson Disease Rating Scale (UPDRS), the Hoehn and Yahr Scale (HY), and the Schwab and England Activities of Daily Living Scale (SE). In normal subjects there was age-related decline in striatal DTBZ binding, approximating 0.5% per year. In PD subjects, specific DTBZ binding was reduced in the caudate nucleus (CD; -44%), anterior putamen (-68%), and posterior putamen (PP; -77%). The PP-to-CD ratio of binding was reduced significantly in PD subjects. Dihydrotetrabenazine binding was also reduced by approximately 50% in the PD substantia nigra. Striatal binding reductions correlated significantly with PD duration and SE scores, but not with HY stage or with UPDRS motor subscale (UPDRS(III)) scores. Striatal and midbrain DTBZ binding was asymmetric in PD subjects, with greatest reductions contralateral to the most clinically affected limbs. There was significant correlation between asymmetry of DTBZ binding and clinical asymmetry measured with the UPDRS(III). In HY stage 1 and 1.5 subjects (n=16), PP DTBZ binding contralateral to the clinically unaffected body side was reduced by 73%, indicating substantial preclinical nigrostriatal pathology in PD. We conclude that (+)-[(11)C]DTBZ-PET imaging displays many properties necessary of a PD biomarker.

Age-dependent dopaminergic dysfunction in Nurr1 knockout mice

The Nurr1 gene, which codes for a transcriptional factor in the nuclear receptor superfamily, plays an important role in the development of the mesencephalic dopaminergic (DAergic) system. To study the age-dependent effects of Nurr1 expression in maintaining mature nigrostriatal DAergic neuronal function, we examined motor behaviors, determined nigrostriatal dopamine (DA) levels and the number of nigral DAergic neurons, and measured the expression of several DAergic neuron-associated genes in heterozygous Nurr1-deficient (Nurr1+/-) and wild-type mice of different ages. In contrast to the same-aged, wild-type mice, old Nurr1+/- mice (>15 months) had a significant decrease in both rotarod performance and locomotor activities, suggesting a motor impairment that is analogous to parkinsonian deficit. Furthermore, the abnormal motor behaviors in old Nurr1+/- mice were associated with decreased DA levels in the striatum, decreased number of DAergic neurons in the nigra, and reduced expression of Nurr1 and DA transporter in the nigra. Our data indicate that Nurr1 plays an important role in the functional maintenance and survival of nigral DAergic neurons and suggest that the Nurr1+/- mouse is a useful animal model to study the pathogenesis of Parkinson disease (PD) and to explore disease-modifying strategies.

Differential effects of scopolamine on in vivo binding of dopamine transporter and vesicular monoamine transporter radioligands in rat brain

The in vivo equilibrium specific binding of d-threo-[3H]methylphenidate, a radioligand for the dopamine transporter (DAT), and +-alpha-[3H]dihydrotetrabenazine, a radioligand for the vesicular monoamine transporter (VMAT2), were examined in rat brain with and without prior administration of 5 mg/kg scopolamine. Drug-treated animals exhibited a 30% increase in d-threo-[3H]methylphenidate binding to the DAT in the striatum relative to controls. No changes in specific binding of +-alpha-[3H]dihydrotetrabenazine were observed in any brain region following scopolamine pretreatment. Cholinergic drugs thus differentially affect in vivo specific binding of DAT and VMAT2 radioligands, suggesting this should be a consideration in selection of in vivo markers for imaging studies of dopaminergic terminals in the brain of animals and humans.

Measuring the rate of progression of Parkinson's disease over a 5-year period with beta-CIT SPECT

Recent imaging studies suggest a rapid degeneration of the dopaminergic system in early Parkinson's disease (PD), followed by a slowing of the degenerative process in advanced disease. In the present study, a group of early-stage PD patients underwent three sequential [123I]beta-CIT SPECT studies to assess the decline of striatal dopamine transporter binding over a 5-year period. Twenty-one of a cohort of 24 early PD patients who participated in an earlier longitudinal beta-CIT SPECT imaging study [Mov Disord 2002;17:45-53] were included. Scan intervals were 26 +/- 11 months (scan 1-2) and 38 +/- 15 months (scan 2-3), respectively. The relative annual rate of decline of striatal beta-CIT binding from age-expected normal values at the time of Scan 1 was used as primary outcome variable. The relative annual decline of striatal binding from Scan 1 to Scan 2 (4.5 +/- 4.6%) and from Scan 2 to Scan 3 (3.0 +/- 3.0%) was not significantly different. The non-significant difference in progression rate was due mainly to the rapid early decline of striatal binding in 1 patient who subsequently developed a severe dysexecutive dementia syndrome. These data are not suggestive of substantial change in the course of dopaminergic degeneration in PD within the first 5 to 7 years after symptom onset.

The role of dopamine in cognition

Functional imaging provides a sensitive means of studying the dopaminergic system in the brain. Both pre-synaptic and post-synaptic aspects of this system can be explored, and recent technical advances even allow the estimation of the synaptic level of dopamine in the striatum. However, only a few studies have used functional imaging to identify the role of dopamine in cognition. This paper reviews recent evidence provided by studies in healthy individuals and patients with Parkinson's disease or schizophrenia supporting the role of dopamine in normal and pathological cognitive processes.

Organization of midbrain dopamine systems and the pathophysiology of Parkinson's disease

Understanding of the pathophysiology of Parkinson disease (PD) has advanced rapidly over the last two decades through basic and clinical studies using modern neuroanatomical, clinical assessment, neuropathological and functional brain imaging methods. Two interacting processes determine the development of functional impairment, neuronal degeneration with selective denervation of specific regions and compensatory responses, which oppose the effects of denervation. The clinical manifestations of PD, at least in early stages, reflect selective degeneration of dopamine neurons in the substantia nigra projecting through the nigrostriatal pathway to the caudal putamen with compensatory changes in this and related systems. Positron emission tomography with specific ligands for the dopamine system is a powerful tool for analysis of both degenerative and compensatory processes in the pathophysiology of Parkinson disease in vivo and can be used to confirm the diagnosis of dopamine deficient Parkinson disease.

The feasibility of statistical parametric mapping for the analysis of positron emission tomography studies using 11C-2-beta-carbomethoxy-3-beta-(4-fluorophenyl)-tropane in patients with movement disorders

Movement disorders, including Parkinson's disease and parkinsonian syndromes, e.g. progressive supranuclear palsy, multiple system atrophy, and Lewy body dementia, may be difficult to differentiate among each other at an early stage, since they may share similar clinical features and response to dopaminergic drugs. As new tracers for imaging the dopamine transporters become available, the use of positron emission tomography (PET) for the differential diagnosis of movement disorders is gaining clinical relevance. Visual interpretation is generally used for PET image analysis. However, the use of some form of less subjective analysis is desirable in order to detect subtle changes that may be difficult to identify by visual interpretation and to achieve an operator independent analysis. To this end this study was aimed at assessing the feasibility of using statistical parametric mapping (SPM) for the clinical evaluation of single PET scans performed with 2-beta-carbomethoxy-3-beta-(4-fluorophenyl)-tropane ( C-beta-CIT-FE). Eleven healthy volunteers and five patients with movement disorders (Parkinson's disease, essential tremor, PSP and Lewy body dementia) were included in this study. Each subject underwent a PET study after i.v. injection of C-beta-CIT-FE. The PET images of C-beta-CIT-FE distribution acquired between 60 and 90 min were spatially fitted into the Talairach and Tournoux space. A template of normal C-beta-CIT-FE distribution was derived from studies in the 11 normal control subjects. Different patterns of reduction of the uptake of the tracer were detected in the basal ganglia of the five patients, in relation to each pathological condition. The patterns of distribution were all consistent with the severity and type of disease. The results of this study demonstrate the feasibility of differentiating among different states of dopaminergic impairment, due to Parkinson's disease and parkinsonian syndromes, by using PET scans with C-beta-CIT-FE and by using the SPM procedure for analysis of the data.

PET studies and motor complications in Parkinson's disease

Parkinson's disease (PD) patients with motor complications show a greater reduction in putamen [18F]dopa uptake on positron emission tomography (PET) compared with sustained responders to L-dopa, although individual ranges overlap considerably. This implies that, although loss of putamen dopamine storage predisposes motor complications in PD, it cannot be the only factor determining timing of onset. Additional PET studies suggest that loss of striatal dopamine storage capacity along with pulsatile exposure to exogenous L-dopa results in pathologically raised synaptic dopamine levels and deranged basal ganglia opioid transmission.This, rather than altered dopamine receptor binding, then causes inappropriate overactivity of basal ganglia-frontal projections, resulting in breakthrough involuntary movements.