PET studies of net blood-brain clearance of FDOPA to human brain: age-dependent decline of [18F]fluorodopamine storage capacity

Endocannabinoid levels in plasma and neurotransmitters in the brain: a preliminary report on patients with a psychotic disorder and healthy individuals

BACKGROUND

Interactions between the endocannabinoid system (ECS) and neurotransmitter systems might mediate the risk of developing a schizophrenia spectrum disorder (SSD). Consequently, we investigated in patients with SSD and healthy controls (HC) the relations between (1) plasma concentrations of two prototypical endocannabinoids (N-arachidonoylethanolamine [anandamide] and 2-arachidonoylglycerol [2-AG]) and (2) striatal dopamine synthesis capacity (DSC), and glutamate and y-aminobutyric acid (GABA) levels in the anterior cingulate cortex (ACC). As anandamide and 2-AG might reduce the activity of these neurotransmitters, we hypothesized negative correlations between their plasma levels and the abovementioned neurotransmitters in both groups.

METHODS

Blood samples were obtained from 18 patients and 16 HC to measure anandamide and 2-AG plasma concentrations. For all subjects, we acquired proton magnetic resonance spectroscopy scans to assess Glx (i.e. glutamate plus glutamine) and GABA + (i.e. GABA plus macromolecules) concentrations in the ACC. Ten patients and 14 HC also underwent [18F]F-DOPA positron emission tomography for assessment of striatal DSC. Multiple linear regression analyses were used to investigate the relations between the outcome measures.

RESULTS

A negative association between 2-AG plasma concentration and ACC Glx concentration was found in patients (p = 0.008). We found no evidence of other significant relationships between 2-AG or anandamide plasma concentrations and dopaminergic, glutamatergic, or GABAergic measures in either group.

CONCLUSIONS

Our preliminary results suggest an association between peripheral 2-AG and ACC Glx levels in patients.

Four decades of mapping and quantifying neuroreceptors at work in vivo by positron emission tomography

Decryption of brain images is the basis for the necessary translation of the findings from imaging to information required to meet the demands of clinical intervention. Tools of brain imaging, therefore, must satisfy the conditions dictated by the needs for interpretation in terms of diagnosis and prognosis. In addition, the applications must serve as fundamental research tools that enable the understanding of new therapeutic drugs, including compounds as diverse as antipsychotics, antidepressants, anxiolytics, and drugs serving the relief of symptoms from neurochemical disorders as unrelated as multiple sclerosis, stroke, and dementia. Here we review and explain the kinetics of methods that enable researchers to describe the brain’s work and functions. We focus on methods invented by neurokineticists and expanded upon by practitioners during decades of experimental work and on the methods that are particularly useful to predict possible future approaches to the treatment of neurochemical disorders. We provide an overall description of the basic elements of kinetics and the underlying quantification methods, as well as the mathematics of modeling the recorded brain dynamics embedded in the images we obtain in vivo. The complex presentation to follow is necessary to justify the contribution of modeling to the development of methods and to support the specifications dictated by the proposed use in clinical settings. The quantification and kinetic modeling processes are equally essential to image reconstruction and labeling of brain regions of structural or functional interest. The procedures presented here are essential tools of scientific approaches to all conventional and novel forms of brain imaging. The foundations of the kinetic and quantitative methods are keys to the satisfaction of clinicians that actively engage in treating the neurochemical disorders of mammalian brains in the fields of neurology, neurosurgery, and neuropsychiatry.

Dopaminergic Activity in Antipsychotic-Naïve Patients Assessed With Positron Emission Tomography Before and After Partial Dopamine D2 Receptor Agonist Treatment: Association With Psychotic Symptoms and Treatment Response

BACKGROUND

Dopamine activity has been associated with the response to antipsychotic treatment. Our study used a four-parameter model to test the association between the striatal decarboxylation rate of ¹⁸F-DOPA to ¹⁸F-dopamine (k₃) and the effect of treatment on psychotic symptoms in antipsychotic-naïve patients with first-episode psychosis. We further explored the effect of treatment with a partial dopamine D₂ receptor agonist (aripiprazole) on k₃ and dopamine synthesis capacity (DSC) determined by the four-parameter model and by the conventional tissue reference method.

METHODS

Sixty-two individuals (31 patients and 31 control subjects) underwent ¹⁸F-DOPA positron emission tomography at baseline, and 15 patients were re-examined after 6 weeks. Clinical re-examinations were completed after 6 weeks (n = 28) and 6 months (n = 15). Symptoms were evaluated with the Positive and Negative Syndrome Scale.

RESULTS

High baseline decarboxylation rates (k₃) were associated with more positive symptoms at baseline (p < .001) and with symptom improvement after 6 weeks (p = .006). Subregion analyses showed that baseline k₃ for the putamen (p = .003) and nucleus accumbens (p = .013) and DSC values for the nucleus accumbens (p = .003) were associated with psychotic symptoms. The tissue reference method yielded no associations between DSC and symptoms or symptom improvement. Neither method revealed any effects of group or treatment on average magnitudes of k₃ or DSC, whereas changes in dopamine synthesis were correlated with higher baseline values, implying a potential effect of treatment.

CONCLUSIONS

Striatal decarboxylation rate at baseline was associated with psychotic symptoms and treatment response. The strong association between k₃ and treatment effect potentially implicate on new treatment strategies.

Dynamic planar scintigraphy for the rapid kinetic measurement of myocardial 123I-MIBG turnover can identify Lewy body disease

BACKGROUND

Using two static scans for ¹²³I-meta-iodobenzyl-guanidine (¹²³I-MIBG) myocardial scintigraphy ignores the dynamic response from vesicular trapping in nerve terminals. Moreover, the long pause between scans is impractical for patients with Lewy body diseases (LBDs). Here, we optimized indices that capture norepinephrine kinetics, tested their diagnostic performance, and determined the differences in ¹²³I-MIBG performance among disease groups.

METHODS

We developed a new 30-min protocol for ¹²³I-MIBG dynamic planar imaging for suspected LBD patients. Pharmacokinetic modelling of time-activity curves (TACs) was used to calculate three new indices: unidirectional uptake of ¹²³I-MIBG to vesicular trapping (iUp), rate of myocardial ¹²³I-MIBG loss (iLoss), and non-specific fractional distribution of ¹²³I-MIBG in the interstitial space. We compared the performance of the new and existing indices with regard to discrimination of patients with or without LBDs. Subgroup analysis was performed to examine differences in ¹²³I-MIBG turnover between patients in a dementia with Lewy bodies (DLB) group and two Parkinson's disease (PD) groups, one with and the other without REM sleep behaviour disorder (RBD).

RESULTS

iLoss was highly discriminative, particularly for patients with low myocardial ¹²³I-MIBG trapping, and the new indices outperformed existing ones. ROC analysis revealed that the AUC of iLoss (0.903) was significantly higher than that of early HMR (0.863), while comparable to that of delayed HMR (0.892). The RBD-positive PD group and the DLB group had higher turnover rates than the RBD-negative PD group, indicating a potential association between prognosis and iLoss.

CONCLUSION

¹²³I-MIBG turnover can be quantified in 30 min using a three-parameter model based on ¹²³I-MIBG TACs. The discriminatory performance of the new model-based indices might help explain the neurotoxicity or neurodegeneration that occurs in LBD patients.

Functional dynamics of dopamine synthesis during monetary reward and punishment processing

The assessment of dopamine release with the PET competition model is thoroughly validated but entails disadvantages for the investigation of cognitive processes. We introduce a novel approach incorporating 6-[¹⁸F]FDOPA uptake as index of the dynamic regulation of dopamine synthesis enzymes by neuronal firing. The feasibility of this approach is demonstrated by assessing widely described sex differences in dopamine neurotransmission. Reward processing was behaviorally investigated in 36 healthy participants, of whom 16 completed fPET and fMRI during the monetary incentive delay task. A single 50 min fPET acquisition with 6-[¹⁸F]FDOPA served to quantify task-specific changes in dopamine synthesis. In men monetary gain induced stronger increases in ventral striatum dopamine synthesis than loss. Interestingly, the opposite effect was discovered in women. These changes were further associated with reward (men) and punishment sensitivity (women). As expected, fMRI showed robust task-specific neuronal activation but no sex difference. Our findings provide a neurobiological basis for known behavioral sex differences in reward and punishment processing, with important implications in psychiatric disorders showing sex-specific prevalence, altered reward processing and dopamine signaling. The high temporal resolution and magnitude of task-specific changes make fPET a promising tool to investigate functional neurotransmitter dynamics during cognitive processing and in brain disorders.

On the learning of addictive behavior: Sensation-seeking propensity predicts dopamine turnover in dorsal striatum

We asked if sensation-seeking is linked to premorbid personality characteristics in patients with addictive disorders, or the characteristics follow the sensation-seeking activity. We interpreted the former as a state associated with normal rates of dopamine synthesis, and the latter as a trait of individuals with abnormally high rates of synthesis. We previously determined dopaminergic receptor density in striatum, and we now tested the hypothesis that an elevated dopaminergic condition with increased extracellular dopamine and receptor density follows increased dopamine synthesis capacity in highly sensation-seeking individuals, as measured by positron emission tomography of 18 men with tracer fluorodopa (FDOPA). We detected a site in left caudate nucleus where the volume of distribution of FDOPA-derived metabolites correlated negatively with FDOPA metabolite turnover, consistent with decreased metabolite breakdown in highly sensation-seeking subjects. High rates of sensation-seeking attenuated the dopamine turnover in association with a low rate of dopamine recycling, low dopamine oxidation, and elevated extracellular dopamine and receptors in caudate nucleus. In contrast, low rates of sensation-seeking were associated with rapid dopamine recycling, rapid dopamine oxidation, low extracellular dopamine, and low receptor density. We conclude that the modulation of dopaminergic neurotransmission associated with sensation-seeking is a state of sensation-seeking, rather than a trait of personality following abnormal regulation of dopaminergic neurotransmission.

Rapidly (and Successfully) Translating Novel Brain Radiotracers From Animal Research Into Clinical Use

The advent of preclinical research scanners for in vivo imaging of small animals has added confidence into the multi-step decision-making process of radiotracer discovery and development. Furthermore, it has expanded the utility of imaging techniques available to dissect clinical questions, fostering a cyclic interaction between the clinical and the preclinical worlds. Significant efforts from medicinal chemistry have also made available several high-affinity and selective compounds amenable for radiolabeling, that target different receptors, transporters and enzymes in vivo. This substantially increased the range of applications of molecular imaging using positron emission tomography (PET) or single photon emission computed tomography (SPECT). However, the process of developing novel radiotracers for in vivo imaging of the human brain is a multi-step process that has several inherent pitfalls and technical difficulties, which often hampers the successful translation of novel imaging agents from preclinical research into clinical use. In this paper, the process of radiotracer development and its relevance in brain research is discussed; as well as, its pitfalls, technical challenges and future promises. Examples of successful and unsuccessful translation of brain radiotracers will be presented.

TRIMAGE: A dedicated trimodality (PET/MR/EEG) imaging tool for schizophrenia

Simultaneous PET/MR/EEG (Positron Emission Tomography - Magnetic Resonance - Electroencephalography), a new tool for the investigation of neuronal networks in the human brain, is presented here within the framework of the European Union Project TRIMAGE. The trimodal, cost-effective PET/MR/EEG imaging tool makes use of cutting edge technology both in PET and in MR fields. A novel type of magnet (1.5T, non-cryogenic) has been built together with a PET scanner that makes use of the most advanced photodetectors (i.e., SiPM matrices), scintillators matrices (LYSO) and digital electronics. The combined PET/MR/EEG system is dedicated to brain imaging and has an inner diameter of 260 mm and an axial Field-of-View of 160 mm. It enables the acquisition and assessment of molecular metabolic information with high spatial and temporal resolution in a given brain simultaneously. The dopaminergic system and the glutamatergic system in schizophrenic patients are investigated via PET, the same physiological/pathophysiological conditions with regard to functional connectivity, via fMRI, and its electrophysiological signature via EEG. In addition to basic neuroscience questions addressing neurovascular-metabolic coupling, this new methodology lays the foundation for individual physiological and pathological fingerprints for a wide research field addressing healthy aging, gender effects, plasticity and different psychiatric and neurological diseases. The preliminary performances of two components of the imaging tool (PET and MR) are discussed. Initial results of the search of possible candidates for suitable schizophrenia biomarkers are also presented as obtained with PET/MR systems available to the collaboration.

18F-DOPA uptake parameters in glioma: effects of patients' characteristics and prior treatment history

OBJECTIVE

In amino acid positron emission tomography brain tumour imaging, tumour-to-background uptake parameters are often used for treatment monitoring. We studied the effects of patients' characteristics and anticancer treatments on ¹⁸F-fluoro-l-phenylalanine uptake of normal brain and tumour lesions, with particular emphasis on temozolomide (TMZ) chemotherapy.

METHODS

155 studies from 120 patients with glioma were analysed. Average uptake of normal background (standardized uptake value, SUVbckgr) and basal ganglia (SUVbg), as well as tumour-to-brain ratios (TBR) were compared between positron emission tomography/CT studies acquired before (Group A, n = 48), after (Group B, n = 50) or during (Group C, n = 57) TMZ treatment, using analysis of variance.

RESULTS

Overall, mean SUVbckgr and mean SUVbg were 1.06 ± 0.26 and 2.12 ± 0.47, respectively. Female had significantly higher SUVbckgr (p = 0.002) and SUVbg (p = 0.012) than male patients. Age showed a positive correlation with SUVbg (p = 0.001). In the overall cohort, there were significant effects of TMZ on SUVbckgr (p = 0.0237) and TBR (p = 0.0138). In particular, SUVbckgr was lower in Group C than in Group B (1.00 ± 0.25 vs 1.14 ± 0.31, p = 0.0173). Significant variations of SUVbckr could be observed in female only. TBR was significantly higher in Group C than in Group B (2.37 ± 0.54 vs 2.06 ± 0.38, p = 0.010). Variations of SUVbg between groups slightly missed significance (p = 0.0504).

CONCLUSION

Temozolomide chemotherapy and patients' characteristics, including gender and age, affect physiological [¹⁸F]-fluoro-l-phenylalanine uptake and, consequently, the calculation of TBRs. Advances in knowledge: For the first time, the effects of past or concurrent temozolomide chemotherapy on brain physiological amino acid uptake have been investigated. Such effects are relevant and should be taken into account when evaluating tumour-to-background ratios.

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.

Reduced dopamine receptors and transporters but not synthesis capacity in normal aging adults: a meta-analysis

Many theories of cognitive aging are based on evidence that dopamine (DA) declines with age. Here, we performed a systematic meta-analysis of cross-sectional positron emission tomography and single-photon emission-computed tomography studies on the average effects of age on distinct DA targets (receptors, transporters, or relevant enzymes) in healthy adults (N = 95 studies including 2611 participants). Results revealed significant moderate to large, negative effects of age on DA transporters and receptors. Age had a significantly larger effect on D1- than D2-like receptors. In contrast, there was no significant effect of age on DA synthesis capacity. The average age reductions across the DA system were 3.7%-14.0% per decade. A meta-regression found only DA target as a significant moderator of the age effect. This study precisely quantifies prior claims of reduced DA functionality with age. It also identifies presynaptic mechanisms (spared synthesis capacity and reduced DA transporters) that may partially account for previously unexplained phenomena whereby older adults appear to use dopaminergic resources effectively. Recommendations for future studies including minimum required samples sizes are provided.

Pathway-Specific Dopamine Abnormalities in Schizophrenia

In light of the clinical evidence implicating dopamine in schizophrenia and the prominent hypotheses put forth regarding alterations in dopaminergic transmission in this disease, molecular imaging has been used to examine multiple aspects of the dopaminergic system. We review the imaging methods used and compare the findings across the different molecular targets. Findings have converged to suggest early dysregulation in the striatum, especially in the rostral caudate, manifesting as excess synthesis and release. Recent data showed deficit extending to most cortical regions and even to other extrastriatal subcortical regions not previously considered to be "hypodopaminergic" in schizophrenia. These findings yield a new topography for the dopaminergic dysregulation in schizophrenia. We discuss the dopaminergic innervation within the individual projection fields to provide a topographical map of this dual dysregulation and explore potential cellular and circuit-based mechanisms for brain region-dependent alterations in dopaminergic parameters. This refined knowledge is essential to better guide translational studies and efforts in early drug development.

Effects of Smoking Cessation on Presynaptic Dopamine Function of Addicted Male Smokers

BACKGROUND

There is evidence of abnormal cerebral dopamine transmission in nicotine-dependent smokers, but it is unclear whether dopaminergic abnormalities are due to acute nicotine abuse or whether they persist with abstinence. We addressed this question by conducting longitudinal positron emission tomography (PET) examination of smokers before and after 3 months of abstinence.

METHODS

We obtained baseline 6-[(18)F]fluoro-L-DOPA (FDOPA)-PET scans in 15 nonsmokers and 30 nicotine-dependent smokers, who either smoked as per their usual habit or were in acute withdrawal. All smokers then underwent cessation treatment, and successful abstainers were re-examined by FDOPA-PET after 3 months of abstinence (n = 15). Uptake of FDOPA was analyzed using a steady-state model yielding estimates of the dopamine synthesis capacity (K); the turnover of tracer dopamine formed in living brain (kloss); and the tracer distribution volume (Vd), which is an index of dopamine storage capacity.

RESULTS

Compared with nonsmokers, K was 15% to 20% lower in the caudate nuclei of consuming smokers. Intraindividual comparisons of consumption and long-term abstinence revealed significant increases in K in the right dorsal and left ventral caudate nuclei. Relative to acute withdrawal, Vd significantly decreased in the right ventral and dorsal caudate after prolonged abstinence. Severity of nicotine dependence significantly correlated with dopamine synthesis capacity and dopamine turnover in the bilateral ventral putamen of consuming smokers.

CONCLUSIONS

The results suggest a lower dopamine synthesis capacity in nicotine-dependent smokers that appears to normalize with abstinence. Further investigations are needed to clarify the role of dopamine in nicotine addiction to help develop smoking prevention and cessation treatments.

Evaluation of 6-11C-Methyl-m-Tyrosine as a PET Probe for Presynaptic Dopaminergic Activity: A Comparison PET Study with β-11C-l-DOPA and 18F-FDOPA in Parkinson Disease Monkeys

We recently developed a novel PET probe, 6-(11)C-methyl-m-tyrosine ((11)C-6MemTyr), for quantitative imaging of presynaptic dopamine synthesis in the living brain. In the present study, (11)C-6MemTyr was compared with β-(11)C-l-DOPA and 6-(18)F-fluoro-l-dopa ((18)F-FDOPA) in the brains of normal and Parkinson disease (PD) model monkeys (Macaca fascicularis).

METHODS

PD model monkeys were prepared by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration, and (11)C-β-CFT was applied to assess neuronal damage as dopamine transporter (DAT) availability. (11)C-6MemTyr, β-(11)C-l-DOPA, or (18)F-FDOPA was injected with and without carbidopa, a specific inhibitor of peripheral aromatic L-amino acid decarboxylase. In normal and PD monkeys, the dopamine synthesis rates calculated using PET probes were analyzed by the correlation plot with DAT availability in the striatum.

RESULTS

In normal monkeys, whole-brain uptake of β-(11)C-l-DOPA and (18)F-FDOPA were significantly increased by carbidopa at the clinical dose of 5 mg/kg by mouth. In contrast, (11)C-6MemTyr was not affected by carbidopa at this dose, and the metabolic constant value of (11)C-6MemTyr in the striatum was significantly higher than those of the other 2 PET probes. Significant reduction of the presynaptic DAT availability in the striatum was detected in MPTP monkeys, and correlation analyses demonstrated that (11)C-6MemTyr could detect dopaminergic damage in the striatum with much more sensitivity than the other PET probes.

CONCLUSION

(11)C-6MemTyr is a potential PET probe for quantitative imaging of presynaptic dopamine activity in the living brain with PET.

Generalized whole-body Patlak parametric imaging for enhanced quantification in clinical PET

We recently developed a dynamic multi-bed PET data acquisition framework to translate the quantitative benefits of Patlak voxel-wise analysis to the domain of routine clinical whole-body (WB) imaging. The standard Patlak (sPatlak) linear graphical analysis assumes irreversible PET tracer uptake, ignoring the effect of FDG dephosphorylation, which has been suggested by a number of PET studies. In this work: (i) a non-linear generalized Patlak (gPatlak) model is utilized, including a net efflux rate constant kloss, and (ii) a hybrid (s/g)Patlak (hPatlak) imaging technique is introduced to enhance contrast to noise ratios (CNRs) of uptake rate Ki images. Representative set of kinetic parameter values and the XCAT phantom were employed to generate realistic 4D simulation PET data, and the proposed methods were additionally evaluated on 11 WB dynamic PET patient studies. Quantitative analysis on the simulated Ki images over 2 groups of regions-of-interest (ROIs), with low (ROI A) or high (ROI B) true kloss relative to Ki, suggested superior accuracy for gPatlak. Bias of sPatlak was found to be 16-18% and 20-40% poorer than gPatlak for ROIs A and B, respectively. By contrast, gPatlak exhibited, on average, 10% higher noise than sPatlak. Meanwhile, the bias and noise levels for hPatlak always ranged between the other two methods. In general, hPatlak was seen to outperform all methods in terms of target-to-background ratio (TBR) and CNR for all ROIs. Validation on patient datasets demonstrated clinical feasibility for all Patlak methods, while TBR and CNR evaluations confirmed our simulation findings, and suggested presence of non-negligible kloss reversibility in clinical data. As such, we recommend gPatlak for highly quantitative imaging tasks, while, for tasks emphasizing lesion detectability (e.g. TBR, CNR) over quantification, or for high levels of noise, hPatlak is instead preferred. Finally, gPatlak and hPatlak CNR was systematically higher compared to routine SUV values.

Simulation-based partial volume correction for dopaminergic PET imaging: Impact of segmentation accuracy

AIM

Partial volume correction (PVC) is an essential step for quantitative positron emission tomography (PET). In the present study, PVELab, a freely available software, is evaluated for PVC in (18)F-FDOPA brain-PET, with a special focus on the accuracy degradation introduced by various MR-based segmentation approaches.

METHODS

Four PVC algorithms (M-PVC; MG-PVC; mMG-PVC; and R-PVC) were analyzed on simulated (18)F-FDOPA brain-PET images. MR image segmentation was carried out using FSL (FMRIB Software Library) and SPM (Statistical Parametric Mapping) packages, including additional adaptation for subcortical regions (SPML). Different PVC and segmentation combinations were compared with respect to deviations in regional activity values and time-activity curves (TACs) of the occipital cortex (OCC), caudate nucleus (CN), and putamen (PUT). Additionally, the PVC impact on the determination of the influx constant (Ki) was assessed.

RESULTS

Main differences between tissue-maps returned by three segmentation algorithms were found in the subcortical region, especially at PUT. Average misclassification errors in combination with volume reduction was found to be lowest for SPML (PUT < 30%) and highest for FSL (PUT > 70%). Accurate recovery of activity data at OCC is achieved by M-PVC (apparent recovery coefficient varies between 0.99 and 1.10). The other three evaluated PVC algorithms have demonstrated to be more suitable for subcortical regions with MG-PVC and mMG-PVC being less prone to the largest tissue misclassification error simulated in this study. Except for M-PVC, quantification accuracy of Ki for CN and PUT was clearly improved by PVC.

CONCLUSIONS

The regional activity value of PUT was appreciably overcorrected by most of the PVC approaches employing FSL or SPM segmentation, revealing the importance of accurate MR image segmentation for the presented PVC framework. The selection of a PVC approach should be adapted to the anatomical structure of interest. Caution is recommended in subsequent interpretation of Ki values. The possible different change of activity concentrations due to PVC in both target and reference regions tends to alter the corresponding TACs, introducing bias to Ki determination. The accuracy of quantitative analysis was improved by PVC but at the expense of precision reduction, indicating the potential impropriety of applying the presented framework for group comparison studies.

Influence of O-methylated metabolite penetrating the blood-brain barrier to estimation of dopamine synthesis capacity in human L-[β-(11)C]DOPA PET

O-methyl metabolite (L-[β-(11)C]OMD) of (11)C-labeled L-3,4-dihydroxyphenylalanine (L-[β-(11)C]DOPA) can penetrate into brain tissue through the blood-brain barrier, and can complicate the estimation of dopamine synthesis capacity by positron emission tomography (PET) study with L-[β-(11)C]DOPA. We evaluated the impact of L-[β-(11)C]OMD on the estimation of the dopamine synthesis capacity in a human L-[β-(11)C]DOPA PET study. The metabolite correction with mathematical modeling of L-[β-(11)C]OMD kinetics in a reference region without decarboxylation and further metabolism, proposed by a previous [(18)F]FDOPA PET study, were implemented to estimate radioactivity of tissue L-[β-(11)C]OMD in 10 normal volunteers. The component of L-[β-(11)C]OMD in tissue time-activity curves (TACs) in 10 regions were subtracted by the estimated radioactivity of L-[β-(11)C]OMD. To evaluate the influence of omitting blood sampling and metabolite correction, relative dopamine synthesis rate (kref) was estimated by Gjedde-Patlak analysis with reference tissue input function, as well as the net dopamine synthesis rate (Ki) by Gjedde-Patlak analysis with the arterial input function and TAC without and with metabolite correction. Overestimation of Ki was observed without metabolite correction. However, the kref and Ki with metabolite correction were significantly correlated. These data suggest that the influence of L-[β-(11)C]OMD is minimal for the estimation of kref as dopamine synthesis capacity.

The impact of dopamine on aggression: an [18F]-FDOPA PET Study in healthy males

Cerebral dopamine (DA) transmission is thought to be an important modulator for the development and occurrence of aggressive behavior. However, the link between aggression and DA transmission in humans has not been investigated using molecular imaging and standardized behavioral tasks. We investigated aggression as a function of DA transmission in a group of (N = 21) healthy male volunteers undergoing 6-[18F]-fluoro-L-DOPA (FDOPA)-positron emission tomography (PET) and a modified version of the Point Subtraction Aggression Paradigm (PSAP). This task measures aggressive behavior during a monetary reward-related paradigm, where a putative adversary habitually tries to cheat. The participant can react in three ways (i.e., money substraction of the putative opponent [aggressive punishment], pressing a defense button, or continuing his money-making behavior). FDOPA-PET was analyzed using a steady-state model yielding estimates of the DA-synthesis capacity (K), the turnover of tracer DA formed in living brain (kloss), and the tracer distribution volume (Vd), which is an index of DA storage capacity. Significant negative correlations between PSAP aggressive responses and the DA-synthesis capacity were present in several regions, most prominently in the midbrain (r = -0.640; p = 0.002). Lower degrees of aggressive responses were associated with higher DA storage capacity in the striatum and midbrain. Additionally, there was a significant positive correlation between the investment into monetary incentive responses on the PSAP and DA-synthesis capacity, notably in the midbrain (r = +0.618, p = 0.003). The results suggest that individuals with low DA transmission capacity are more vulnerable to reactive/impulsive aggression in response to provocation.

Increased turnover of dopamine in caudate nucleus of detoxified alcoholic patients

A previous study of the DOPA decarboxylase substrate 6-[¹⁸F]fluoro-L-DOPA (FDOPA) with positron emission tomography (PET) detected no difference of the net blood-brain transfer rate (K_in^app) between detoxified alcoholic patients and healthy controls. Instead, the study revealed an inverse correlation between K_in^app in left ventral striatum and alcohol craving scores. To resolve the influx and efflux phases of radiolabeled molecules, we independently estimated the unidirectional blood-brain FDOPA clearance rate (K) and the washout rate of [¹⁸F]fluorodopamine and its deaminated metabolites (k_loss), and we also calculated the total distribution volume of decarboxylated metabolites and unmetabolized FDOPA as a steady-state index of the dopamine storage capacity (V_d) in brain. The craving scores in the 12 alcoholics correlated positively with the rate of loss (k_loss) in the left ventral striatum. We conclude that craving is most pronounced in the individuals with relatively rapid dopamine turnover in the left ventral striatum. The blood-brain clearance rate (K), corrected for subsequent loss of radiolabeled molecules from brain, was completely normal throughout the brain of the alcoholics, in whom the volume of distribution (V_d) was found to be significantly lower in the left caudate nucleus. The magnitude of V_d in the left caudate head was reduced by 43% relative to the 16 controls, consistent with a 58% increase of k_loss. We interpret the findings as indicating that a trait for rapid dopamine turnover in the ventral striatum subserves craving and reward-dependence, leading to an acquired state of increased dopamine turnover in the dorsal striatum of detoxified alcoholic patients.

The role of LAT1 in (18)F-DOPA uptake in malignant gliomas

Positron emission tomography (PET) imaging with the amino acid tracer 6-(18)F-fluoro-L-3,4-dihydroxy-phenylalanine ((18)F-DOPA) may provide better spatial and functional information in human gliomas than CT or MRI alone. The L-type amino acid transporter 1 (LAT1) is responsible for membrane transport of large neutral amino acids in normal cells. This study assessed the relationship between LAT1 expression and (18)F-DOPA uptake in human astrocytomas. Endogenous LAT1 expression was measured in established glioblastoma (GBM) cell lines and primary GBM xenografts using Western blotting and quantitative reverse transcription polymerase chain reaction (qRT-PCR). Uptake of (18)F-DOPA was approximated in vitro using (3)H-L-DOPA as an analog. Uptake of (3)H-L-DOPA was assessed in cells expressing LAT1 shRNA or LAT1 siRNA and compared to non-targeted (NT) control shRNA or siRNA sequences, respectively. To demonstrate the clinical relevance of these findings, LAT1 immunofluorescence staining was compared with corresponding regions of (18)F-DOPA PET uptake in patients with newly diagnosed astrocytomas. LAT1 mRNA and protein expression varies in GBM, and the extent of (3)H-L-DOPA uptake was positively correlated with endogenous LAT1 expression. Stable shRNA-mediated LAT1 knockdown in T98 and GBM28 reduced (3)H-L-DOPA uptake relative to NT shRNA by 57 (P < 0.0001) and 52 % (P < 0.001), respectively. Transient siRNA-mediated LAT1 knockdown in T98 reduced (3)H-L-DOPA uptake relative to NT siRNA up to 68 % (P < 0.01). In clinical samples, LAT1 expression positively correlated with (18)F-DOPA PET uptake (P = 0.04). Expression of LAT1 is strongly associated with (3)H-L-DOPA uptake in vitro and (18)F-DOPA uptake in patient biopsy samples. These results define LAT1 as a key determinant of (18)F-DOPA accumulation in GBM.

Alzheimer's disease - not an exaggeration of healthy aging

The world population is becoming older now. The boom of the elderly population comes from public health efforts to improve living conditions and prevent disease, and from improved medical interventions. People more than 65-year-old who are representing 12.9% of the population now is expected to grow to be 19% of the population by 2030. Very few numbers of diseases will have such socioeconomic burden on society in the newer world. Although Alzheimer's disease (AD) has been studied very well recently, still its exact etiopathogenesis is unknown. Currently there are no available tests for the definitive diagnosis of AD. So the clinical diagnosis of AD remains a diagnosis of exclusion. This limits the potential for early intervention. The difference between normal degenerative processes of brain and preclinical changes of AD is a gray zone and there is no particular way to distinguish between the two. Now several modalities like functional magnetic resonance imaging (fMRI), positron emission tomography (PET) scan, electrophysiological tests and cerebrospinal fluid (CSF) biomarkers for tauopathy and Aβ have shown to be promising in the development of early diagnostic tools for neurodegenerative changes and help us to differentiate between healthy aging and pathological aging. In this article we tried to discuss about the differences between pathological and physiological aging process from radiological, pathological, biochemical, and electrophysiological point of view. However, differentiating between physiological and pathological dementia still remains a challenge.

Sensitivity of kinetic macro parameters to changes in dopamine synthesis, storage, and metabolism: a simulation study for [¹⁸F]FDOPA PET by a model with detailed dopamine pathway

Quantitative interpretation of brain [¹⁸F]FDOPA PET data has been made possible by several kinetic modeling approaches, which are based on different assumptions about complex [¹⁸F]FDOPA metabolic pathways in brain tissue. Simple kinetic macro parameters are often utilized to quantitatively evaluate metabolic and physiological processes of interest, which may include DDC activity, vesicular storage, and catabolism from (18) F-labeled dopamine to DOPAC and HVA. A macro parameter most sensitive to the changes of these processes would be potentially beneficial to identify impaired processes in a neurodegenerative disorder such as Parkinson's disease. The purpose of this study is a systematic comparison of several [¹⁸F]FDOPA macro parameters in terms of sensitivities to process-specific changes in simulated time-activity curve (TAC) data of [¹⁸F]FDOPA PET. We introduced a multiple-compartment kinetic model to simulate PET TACs with physiological changes in the dopamine pathway. TACs in the alteration of dopamine synthesis, storage, and metabolism were simulated with a plasma input function obtained by a non-human primate [¹⁸F]FDOPA PET study. Kinetic macro parameters were calculated using three conventional linear approaches (Gjedde-Patlak, Logan, and Kumakura methods). For simulated changes in dopamine storage and metabolism, the slow clearance rate (k(loss) ) as calculated by the Kumakura method showed the highest sensitivity to these changes. Although k(loss) performed well at typical ROI noise levels, there was large bias at high noise level. In contrast, for simulated changes in DDC activity it was found that K(i) and V(T), estimated by Gjedde-Patlak and Logan method respectively, have better performance than k(loss).

PET studies of cerebral levodopa metabolism: a review of clinical findings and modeling approaches

[(18)F]Fluoro-3,4-dihydroxyphenyl-L-alanine (FDOPA) was one of the first successful tracers for molecular imaging by positron emission tomography (PET), and has proven immensely valuable for studies of Parkinson's disease. Following intravenous FDOPA injection, the decarboxylated metabolite [(18)F] fluorodopamine is formed and trapped within terminals of the nigrostriatal dopamine neurons; reduction in the simple ratio between striatum and cerebellum is indicative of nigrostriatal degeneration. However, the kinetic analysis of dynamic FDOPA-PET recordings is formidably complex due to the entry into brain of the plasma metabolite O-methyl-FDOPA and due to the eventual washout of decarboxylated metabolites. Linear graphical analysis relative to a reference tissue input function is popular and convenient for routine clinical studies in which serial arterial blood samples are unavailable. This simplified approach has facilitated longitudinal studies in large patient cohorts. Linear graphical analysis relative to the metabolite-corrected arterial FDOPA input yields a more physiological index of FDOPA utilization, the net blood-brain clearance. Using a constrained compartmental model, FDOPA-PET recordings can be used to calculate the relative activity of the enzyme DOPA decarboxylase in living brain. We have extended this approach so as to obtain an index of steady-state trapping of [( 18)F]fluorodopamine in synaptic vesicles. Although simple methods of image analysis are sufficient for the purposes of routine clinical studies, the more complex approaches have revealed hidden aspects of brain dopamine in personality, healthy aging, and in the pathophysiologies of Parkinson's disease and schizophrenia.

Elevated [(18)F]FDOPA utilization in the periaqueductal gray and medial nucleus accumbens of patients with early Parkinson's disease

PET studies with the DOPA decarboxylase substrate 6-[(18)F]fluoro-l-DOPA (FDOPA) reveal the storage of [(18)F]-fluorodopamine within synaptic vesicles, mainly of dopamine fibres. As such, FDOPA PET is a sensitive indicator of the integrity of the nigrostriatal dopamine innervation. Nonetheless, there have been several reports of focal elevations of FDOPA utilization in brain of patients with Parkinson's disease (PD), all based on reference tissue methods. To investigate this phenomenon further, we used voxel-wise steady-state kinetic analysis to search for regions of elevated FDOPA utilization (K; ml g(-1) min(-1)) and steady-state trapping (V(d); ml g(-1)) in a group of well-characterized patients with early, asymmetric PD, who were contrasted with an age-matched control group. Subtraction of the population mean parametric maps revealed foci of increased FDOPA utilization K (+25%) in the bilateral medial nucleus accumbens, whereas the expected declines in the trapping of FDOPA were seen in the caudate and putamen. This observation suggests hyperfunction of catecholamine fibres innervating specifically the limbic striatum, which could guide the design of future prospective FDOPA-PET studies of the impulse control disorders occurring in some PD patients under treatment with dopamine agonists. A focus of increased FDOPA influx and also V(d) was detected in the periaqueductal grey, consistent with some earlier reports based on reference tissue analysis. Increased FDOPA trapping in the periaqueductal grey of PD patients seems consistent with recent reports of increased activity of serotonin neurons in a rat model of parkinsonism.

Age-related changes in midbrain dopaminergic regulation of the human reward system

The dopamine system, which plays a crucial role in reward processing, is particularly vulnerable to aging. Significant losses over a normal lifespan have been reported for dopamine receptors and transporters, but very little is known about the neurofunctional consequences of this age-related dopaminergic decline. In animals, a substantial body of data indicates that dopamine activity in the midbrain is tightly associated with reward processing. In humans, although indirect evidence from pharmacological and clinical studies also supports such an association, there has been no direct demonstration of a link between midbrain dopamine and reward-related neural response. Moreover, there are no in vivo data for alterations in this relationship in older humans. Here, by using 6-[(18)F]FluoroDOPA (FDOPA) positron emission tomography (PET) and event-related 3T functional magnetic resonance imaging (fMRI) in the same subjects, we directly demonstrate a link between midbrain dopamine synthesis and reward-related prefrontal activity in humans, show that healthy aging induces functional alterations in the reward system, and identify an age-related change in the direction of the relationship (from a positive to a negative correlation) between midbrain dopamine synthesis and prefrontal activity. These results indicate an age-dependent dopaminergic tuning mechanism for cortical reward processing and provide system-level information about alteration of a key neural circuit in healthy aging. Taken together, our findings provide an important characterization of the interactions between midbrain dopamine function and the reward system in healthy young humans and older subjects, and identify the changes in this regulatory circuit that accompany aging.

Age-dependent decline of steady state dopamine storage capacity of human brain: an FDOPA PET study

Conventional indices of the utilization of FDOPA in living human brain have not consistently revealed important declines in dopamine function with normal aging. However, most methods of kinetic analysis have assumed irreversible trapping of decarboxylated FDOPA metabolites in brain, an assumption that is violated even in PET recordings of short duration. Therefore, we have developed methods for the calculation of steady-state storage of FDOPA together with its decarboxylated metabolites (V(d), mlg(-1)), based upon improved kinetic analysis of 120-min emission recordings. In a group of 28 normal male subjects, of age ranging from 23 to 73 years, the magnitude of V(d) in the striatum and in extrastriatal regions declined by approximately 10% with each decade. The utilization of FDOPA was also calculated by several conventional methods assuming irreversible trapping, i.e. the net blood brain clearance (K(in)(app), mlg(-1)min(-1)), the DOPA decarboxylase activity relative to a reference tissue input (k(3)(S), min(-1)), and relative to the arterial input (k(3)(D), min(-1)). None of these methods revealed an age-related decline in FDOPA utilization in the extended striatum, although the magnitude of K(in)(app) did decline in cerebral cortex. Thus, the capacity to synthesize [(18)F]fluorodopamine remained largely intact in striatum of the elderly subjects, but in the presence of a substantially increased rate of washout (k(loss)), which was evident in all brain regions examined. Consequently, the magnitude of V(d) declined with healthy aging, possibly reflecting impaired vesicular storage capacity, resulting in enhanced exposure of cytosolic [(18)F]fluorodopamine to monoamine oxidase.

Dopaminergic dysfunction in attention deficit hyperactivity disorder (ADHD), differences between pharmacologically treated and never treated young adults: a 3,4-dihdroxy-6-[18F]fluorophenyl-l-alanine PET study

The dopaminergic system plays a key role in attention-deficit/hyperactivity disorder (ADHD). Methylphenidate (MP), a dopamine (DA) reuptake inhibitor, is a drug of first choice for treating ADHD. This cross-over study investigated alterations in DA metabolism in young males with ADHD who had never been pharmacologically treated and MP-treated patients in comparison to healthy subjects. Dynamic 3,4-dihdroxy-6-[18F]fluorophenyl-L-alanine (FDOPA) PET scans were carried out on 20 male patients with ADHD and 18 healthy men. Eight ADHD patients had never been treated with psychostimulants, the rest had received MP. Based on the tissue-slope-intercept plot parametric images of FDOPA influx rate constant (Ki) were generated for each subject from dynamic 3D FDOPA datasets and transformed into standard stereotactic space. First a volume of interest analysis was performed on each single subject. In a second step data were introduced to a SPM2 analysis to detect significant changes in mean voxel Ki values between the normal control group and each patient group. In comparison to controls, ADHD patients as a group (irrespective of treatment status) showed a lower Ki in bilateral putamen, amygdala and dorsal midbrain. There was a lower Ki in the left putamen, right amygdala and right dorsal midbrain in untreated patients compared to controls together with a relative higher influx in the left amygdala and right anterior cingulate cortex. In contrast, methylphenidate treatment was associated with a significantly lower Ki in the striatum and amygdala bilaterally, and in the right dorsal midbrain. Untreated young adult ADHD patients showed a dopamine dysfunction that might be partly due to compensatory mechanisms. MP seems to down-regulate dopamine turnover. This effect might be one component in the mechanism of action of this drug in ADHD treatment.

Baseline [18F]-FDOPA kinetics are predictive of haloperidol-induced changes in dopamine turnover and cognitive performance: A positron emission tomography study in healthy subjects

The telencephalic dopamine innervations contribute to the modulation of cognitive processing. However, the relationship between cognitive effects of D(2/3)-receptor antagonism and dopamine transmission is not described in healthy subjects. We therefore tested effects of acute haloperidol (5 mg/d over 3 days) on continuous performance task (CPT) performance and 6-[(18)F]-fluoro-l-DOPA (FDOPA) PET parameters. Nine physically and mentally healthy male men performed two FDOPA-PET scans including arterial plasma withdrawal. Over 3 days before the second scan, all subjects were treated with 5 mg/d haloperidol orally. Using our novel steady-state analysis, we calculated the intrinsic rate of the cerebral FDOPA utilization (K), the turnover of [(18)F]fluorodopamine formed in brain (k(loss)) and the storage for FDOPA and its brain metabolites (V(d)). Furthermore, a ds-CPT and EPS-screening was performed before every PET scan. We found that FDOPA kinetics in those normal subjects with relatively high baseline K showed a more pronounced sensitivity to haloperidol treatment, manifesting in reduced storage capacity and elevated turnover of [(18)F]fluorodopamine, whereas subjects with lower K showed the opposite pattern of responses. Furthermore, low baseline K predicted improvements in the CPT task after haloperidol, whereas participants with higher baseline K showed a decline in cognitive performance. We conclude that the initial increase of [(18)F]fluorodopamine turnover after acute haloperidol challenge is associated with an over-stimulation in individuals with initially more pharmacologically responsive dopamine systems, but optimizes cognitive performance in those with lower normal FDOPA utilization at baseline. We hypothesize that these effects may be driven by D(1)-receptor mediated transmission during D(2) blockade.

'Prefrontal' cognitive performance of healthy subjects positively correlates with cerebral FDOPA influx: an exploratory [18F]-fluoro-L-DOPA-PET investigation

Dopamine neurotransmission influences those cognitive processes, which are generally regarded as prefrontal cortical functions. In previous positron-emission-tomography (PET) studies, net blood-brain clearance of [18F]-fluoro-l-DOPA (FDOPA) correlated with impaired cognitive performance in patients with Parkinson's disease or schizophrenia. We hypothesized that FDOPA influx also correlates with performance of cognitive tasks associated with prefrontal functioning in healthy volunteers. The net blood-brain clearance of FDOPA (K(in)(app)) was mapped in a group of 11 healthy volunteers and calculated in striatal volumes-of-interest. The Wisconsin-Card-Sorting-Test (WCST), Stroop-Test, Trail-Making-Test (TMT-A/B), and Continuous-Performance-Test (CPT-M) had been administered previously to the same subjects. No correlation of K(in) (app) with perseverative errors in WCST or age could be found. However, there were significant positive correlations between the magnitude of K(in)(app) in caudate nucleus, putamen, and midbrain with performance of the TMT-B, CPT-M, and the Stroop test. Highest correlations were found between the time needed to perform the Stroop interference task and the K(in)(app) of striatal areas (Caudate nucleus: -0.780, P = 0.005; putamen: -0.870, P < 0. 001). Thus, the present findings reveal a strong correlation between dopamine synthesis capacity in striatum of healthy volunteers and performance of cognitive tasks linked to the prefrontal cortex.

Elevated [18F]fluorodopamine turnover in brain of patients with schizophrenia: an [18F]fluorodopa/positron emission tomography study

Previous positron emission tomography (PET) studies with levodopa analogs have revealed a modestly increased capacity for dopamine synthesis in the striatum of patients with schizophrenia compared with healthy age-matched control subjects. We hypothesized that not just the synthesis but also the turnover of radiolabeled dopamine is elevated in patients. To test the hypothesis, we reanalyzed 2-h-long [18F]fluorodopa (FDOPA)/PET recordings from eight unmedicated patients with schizophrenia and 15 healthy age-matched control subjects, using new methods for the quantification of [18F]fluorodopamine steady-state kinetics. The fractional rate constant for the catabolism and elimination of [18F]fluorodopamine was elevated nearly twofold in striatum, the largest biochemical difference in brain of schizophrenics yet reported. The magnitude of the intrinsic blood-brain FDOPA clearance with correction for this loss of [18F]fluorodopamine metabolites was increased by 20% in caudate and putamen and by 50% in amygdala and midbrain of the patients. However, the magnitude of the steady-state storage of FDOPA and its decarboxylated metabolites (V(d)) was reduced by one-third in the caudate nucleus and amygdala of the schizophrenic group. Thus, reduced steady-state storage of [18F]fluorodopamine occurs in the midst of accelerated synthesis in brain of untreated patients. Positive scores of the positive and negative syndrome scale correlated inversely with the magnitude of V(d) in amygdala, suggesting an association between positive symptoms and impaired steady-state storage of FDOPA metabolites in that structure.

Dopaminergic function in depressed patients with affective flattening or with impulsivity: [18F]fluoro-L-dopa positron emission tomography study with voxel-based analysis

A decreased striatal presynaptic dopaminergic function has been reported in depressed patients with affective flattening and psychomotor retardation, using (18)F-fluorodopa positron emission tomography and regions-of-interest. The present study aimed to investigate regional ;[(18)F]dopa uptake in mesolimbic and mesocortical dopaminergic projections with the hypothesis that there should be a decrease in mesolimbic [(18)F]dopa uptake associated with affective flattening and psychomotor retardation. [(18)F]Dopa-positron emission tomography and anatomical magnetic resonance imaging datasets from 12 screened depressed patients with either marked affective flattening and psychomotor retardation (n=6) or with marked impulsivity (n=6), and from eight healthy subjects, were analyzed using a voxel-based approach. Regional differences in [(18)F]dopa uptake rate constant (K(i)) values between the healthy group and the two depression subgroups were compared using both statistical parametric mapping and cluster-based regions-of-interest. Patients with affective flattening and psychomotor retardation had [(18)F]dopa K(i) decreases in the left caudate, bilateral putamen and nucleus accumbens, left parahippocampus and dorsal brainstem. Impulsive depressives had [(18)F]dopa K(i) decreases in the anterior cingulate and hypothalamus, and an increase in the right parahippocampal gyrus. These findings support distinct regional dysfunctions of monoamines depending on the depressive symptomatology.

Longitudinal genetic analysis of executive function in elderly men

The objective of this study was to characterize the relative contribution of genetic and environmental influences to individual differences in longitudinal performance and decline of executive function (EF) using a population-based prospective study of male, WWII veteran twins (NHLBI twin study). Three tests of EF were administered when the twins were 59-70 years old, with 9- and 13-year follow-up. APOE epsilon4 allele status was incorporated in the genetic models to determine its contribution to longitudinal genetic variability. Mean EF performance significantly worsened over time. EF performance was highly genetically correlated across repeat assessment. There were significant genetic influences on 9- and 13-year decline in digit symbol performance. For all tasks decline over the last 4-year follow-up was influenced by individual-specific environmental effects. Controlling for APOE epsilon4 allele presence did not appreciably change the magnitude of genetic effects. These results suggest that common genetic factors underlie longitudinal EF task performance. Genetic influences on EF decline, however, appear to be evident at longer time intervals between assessments.

Quantitative analysis of dopamine synthesis in human brain using positron emission tomography with L-[β-11C]DOPA

BACKGROUND AND OBJECTIVES

To estimate the presynaptic function of the central dopaminergic system, the rate of endogenous dopamine synthesis has been measured by using L-[beta-C]DOPA or 6-[F]fluoro-L-DOPA with positron emission tomography. However, the regional kinetics of L-[beta-C]DOPA in human brain have not been investigated in detail. In the present study, the regional kinetics of L-[beta-C]DOPA in normal human brain and the accuracy of the method for quantifying L-[beta-C]DOPA kinetics, employing reference regions, were investigated.

METHODS

After intravenous injection of L-[beta-C]DOPA, dynamic scanning was performed on ten healthy subjects for 89 min. The overall uptake rate constant K was calculated by the kinetic and graphical approaches, in which the occipital cortex was used as a reference brain region.

RESULTS

Regional distribution of K was similar to those of dopamine D2 receptor. A significant negative correlation was observed between the neutral amino acid concentration in plasma and the influx rate constant through the blood-brain barrier (K1). The K values calculated by graphical approach were in good agreement with the values calculated by kinetic approach for both experimental and simulated data.

CONCLUSIONS

The regional distribution of K corresponds to that of the nigrostriatal and mesolimbic dopaminergic system. Negative correlation between neutral amino acid concentration and K1 supports the suggestion that L-DOPA is transported in a competitive fashion via the same carrier system as neutral amino acids at the blood-brain barrier. Because the graphical approach can obviate the need for an arterial input function, it is useful for investigating the rate of regional dopamine synthesis in neuropsychiatric and neurodegenerative diseases.

Age-related decline of dopamine synthesis in the living human brain measured by positron emission tomography with L-[beta-11C]DOPA

Loss of dopamine synthesis in the striatum with normal human aging has been observed in the postmortem brain. To investigate whether there is age-associated change in dopamine synthesis in the extrastriatal brain regions similar to that in the striatum, positron emission tomography studies with (11)C-labelled l-DOPA were performed on 21 normal healthy male subjects (age range 20-67 years). Decline in the tissue fraction of gray matter per region of interest was also investigated. The overall uptake rate constant for each region of interest was quantified by the Patlak plot method using the occipital cortex as reference region. Regions of interest were set on the dorsolateral prefrontal cortex, lateral temporal cortex, medial temporal cortex, occipital cortex, parietal cortex, anterior cingulate, thalamus, midbrain, caudate nucleus, and putamen. Test-retest analysis indicated good reproducibility of the overall uptake rate constant. Significant age-related declines of dopamine synthesis were observed in the striatum and extrastriatal regions except midbrain. The decline in the overall uptake rate constant was more prominent than in the tissue fraction of gray matter. These results indicate that the previously demonstrated age-related decline in striatal dopamine synthesis extends to several extrastriatal regions in normal human brain.

Healthy and pathological processes in adult development: new evidence from neuroimaging of the aging brain

PURPOSE OF REVIEW

Recent research has revealed that the population of older adults is composed not only of individuals who are either healthy or have an age-related disease, most commonly Alzheimer's disease, but also individuals with mild cognitive impairment who are at-risk for or already in the prodromal stage of Alzheimer's disease. These variations in cognitive aging can be related to their neural bases via structural and functional neuroimaging methods.

RECENT FINDINGS

Healthy aging appears to primarily affect a frontal-striatal system that undergirds executive control of cognition, while minimally affecting medial temporal lobe structures. Functional imaging studies suggest that enhanced prefrontal engagement may offer compensatory plasticity that minimizes age-related cognitive losses. Mild cognitive impairment appears to affect the entorhinal cortex in particular, with functional consequences in other brain regions. Alzheimer's disease is characterized by severe hippocampal injury, although early-stage Alzheimer's disease may relatively spare some cortical regions.

SUMMARY

Advances in in-vivo imaging methods are providing the tools for identifying different trajectories of neurocognitive aging, and knowledge about these brain changes may promote opportunities for treatment.

Effect of age on markers for monoaminergic neurons of normal and MPTP-lesioned rhesus monkeys: A multi-tracer PET study

The binding of three tracers for monoaminergic terminals was mapped in the brain of healthy young (N=6) and healthy old rhesus monkeys (N=4), aged monkeys with mild unilateral intracarotid MPTP lesions (N=3), and monkeys of intermediate age with severe systemic MPTP lesions (N=6). The ligand for monoaminergic vesicles (+)-[(11)C]dihydrotetrabenazine (+DTBZ) had a mean binding potential (pB) of 1.4 in striatum of the healthy young monkeys, which was reduced by 20% in putamen of the old monkeys. The catecholamine transporter ligand (+)-[(11)C]methylphenidate (+MP) had a mean pB of 1.3 in striatum of the young monkeys, which was reduced by 40% in caudate and putamen of the old monkeys. The DOPA decarboxylase substrate [(18)F]fluoro-l-DOPA (FDOPA) had a mean decarboxylation coefficient (k(3)(S)) of 0.4 h(-1) in striatum of the young group, and was not significantly reduced in the aged group. Of the three ligands, only +DTBZ pB was significantly reduced in striatum of the small group of animals with mild unilateral lesions. In the group with systemic MPTP lesions, the mean reduction of the binding of the three ligands was 80% in the caudate and putamen. However, the decline in +MP pB in the ventral striatum (-75%) exceeded the declines of +DTBZ pB and FDOPA k(3)(S) in that region (-65%), suggesting that compensatory down-modulation of uptake sites may occur in the striatal regions with the least dopamine depletion. Binding of all three ligands was reduced by 50% in the anterior cingulate cortex and in the thalamus, suggesting toxicity of MPTP for extrastriatal catecholamine innervations. +DTBZ binding in the hypothalamus, presumably mainly in serotonin fibers, was unaffected by systemic MPTP treatment. Of the three tracers, +DTBZ was most sensitive for detecting MPTP-induced dopamine depletion in monkey striatum.

Dopamine storage capacity in caudate and putamen of patients with early Parkinson's disease: correlation with asymmetry of motor symptoms

Conventional graphical analysis of positron emission tomography (PET) recordings of the cerebral uptake of the DOPA decarboxylase substrate [(18)F]fluorodopa (FDOPA) assumes irreversible trapping of [(18)F]fluorodopamine formed in the brain. However, 4-h long PET recordings allow the estimation of a rate constant for elimination of [(18)F]fluorodopamine from the brain (k(loss)), from which can be calculated an effective distribution volume (EDV(1)), which is an index of [(18)F]fluorodopamine storage capacity. We earlier developed a method employing 2-h long FDOPA recordings for the estimation of k(loss) and EDV, here defined as EDV(2). This method is based on subtraction of the calculated brain concentrations of the FDOPA metabolite O-methyl-FDOPA, rather than the subtraction of the entire radioactivity in a reference region. We now extend this method for the parametric mapping of these parameters in the brain of healthy aged volunteers and patients with Parkinson's disease (PD), with asymmetry of motor symptoms. For parametric mapping, we use a novel application of a multilinear solution for the two-tissue compartment FDOPA model. We also test a new application of the Logan graphical analysis for mapping of the FDOPA distribution volume at equilibrium. The estimates of k(loss) and EDV(2) were more sensitive for the discrimination of biochemical abnormality in the putamen of patients with early PD relative to healthy aged subjects, than was the conventional net influx estimate. Of the several methods, multilinear estimates of EDV(2) were most sensitive for discrimination of PD and normal putamen. However, k(loss) was most sensitive for detecting biochemical asymmetry in the putamen of PD patients, and only k(loss) also detected in the caudate of PD patients a decline in the retention of [(18)F]fluorodopamine relative to healthy aged control subjects.

Age-related differences in levodopa dynamics in Parkinson's: implications for motor complications

Treatment-related motor complications in Parkinson's disease have been previously linked to disease-induced pre-synaptic alterations: dopaminergic denervation and changes in dopamine (DA) release patterns. The occurrence of such complications is also known to be partly dependent on the age of disease onset, occurring more frequently in patients with disease onset at a younger age. Using positron emission tomography (PET) and 4-h-long 18F-fluorodopa (FD) scans we have investigated in vivo an age dependence of disease-induced changes in DA turnover as a possible contributing factor to the age-related differences in treatment-related motor complications. We evaluated the relative changes in DA turnover (measured by its direct inverse, effective DA distribution volume--EDV) and DA synthesis and vesicular storage capacity (quantified by the plasma input uptake rate constant Ki) in Parkinson's disease patients as a function of age (n = 27, age range 38-79 years). After correcting for disease severity, a significant negative correlation was found between age and magnitude of disease-induced decrease in EDV and in Ki in the putamen (P < 0.001, P = 0.02, respectively). However, the difference between the disease-induced decrease in EDV and that in Ki also exhibited an age dependence (P < 0.001), indicating a relatively higher disease-induced increase in DA turnover (inverse of EDV) compared with the decrease in DA synthesis and storage rate in patients of younger age compared with older patients. This finding implies that DA turnover in younger-onset patients undergoes a relatively greater alteration and thus likely contributes to a greater imbalance between DA synthesis, storage and release, which could lead to larger swings in synaptic DA levels. It has indeed been suggested on theoretical grounds that such imbalance may contribute to the greater propensity for motor fluctuations. These results provide one possible explanation for the age-dependent occurrence of complications and support the existence of a pre-synaptic contribution to the occurrence of motor complications.