Brain dopamine responses to ultra-processed milkshakes are highly variable and not significantly related to adiposity in humans.

Ultra-processed foods high in fat and sugar may be addictive, in part, due to their purported ability to induce an exaggerated postingestive brain dopamine response akin to drugs of abuse. Using standard [11C]raclopride positron emission tomography (PET) displacement methods used to measure brain dopamine responses to addictive drugs, we measured postingestive striatal dopamine responses to an ultra-processed milkshake high in fat and sugar in 50 young, healthy adults over a wide body mass index range (BMI 20-45 kg/m2). Surprisingly, milkshake consumption did not result in significant postingestive dopamine response in the striatum (p=0.62) nor any striatal subregion (p>0.33) and the highly variable interindividual responses were not significantly related to adiposity (BMI: r=0.076, p=0.51; %body fat: r=0.16, p=0.28). Thus, postingestive striatal dopamine responses to an ultra-processed milkshake were likely substantially smaller than many addictive drugs and below the limits of detection using standard PET methods.

The influence of hippocampal dopamine D2 receptor losses on episodic-memory decline across 5 years is moderated by BDNF and KIBRA polymorphisms.

Losses in dopamine (DA) functioning may contribute to aging-related decline in cognition. Hippocampal DA is necessary for successful episodic memory formation. Previously, we reported that higher DA D2 receptor (D2DR) availability in hippocampus is beneficial for episodic memory only in older carriers of more advantageous genotypes of well-established plasticity-related genetic variations, the brain-derived neurotrophic factor (BDNF, rs6265) and the kidney and brain expressed protein (KIBRA, rs17070145) polymorphisms. Extending our observations to the longitudinal level, the current data show that individuals with one or no beneficial BDNF and KIBRA genotype (n = 80) decline more in episodic memory across five years, without any contribution of losses in hippocampal D2DR availability to memory decline. Although carriers of two beneficial genotypes (n = 39) did not decline overall in episodic memory, losses of hippocampal D2DR availability were predictive of episodic-memory decline among these individuals. Our findings have implications for interventions targeting DA modulation to enhance episodic memory in aging, which may not benefit all older individuals.

Effects of deep brain stimulation on dopamine D2 receptor binding in patients with treatment-refractory depression.

BACKGROUND: Depression is a chronic psychiatric disorder related to diminished dopaminergic neurotransmission. Deep brain stimulation (DBS) has shown effectiveness in treating patients with treatment-refractory depression (TRD). This study aimed to evaluate the effect of DBS on dopamine D2 receptor binding in patients with TRD. METHODS: Six patients with TRD were treated with bed nucleus of the stria terminalis (BNST)-nucleus accumbens (NAc) DBS were recruited. Ultra-high sensitivity [11C]raclopride dynamic total-body positron emission tomography (PET) imaging was used to assess the brain D2 receptor binding. Each patient underwent a [11C]raclopride PET scan for 60-min under DBS OFF and DBS ON, respectively. A simplified reference tissue model was used to generate parametric images of binding potential (BPND) with the cerebellum as reference tissue. RESULTS: Depression and anxiety symptoms improved after 3-6 months of DBS treatment. Compared with two-day-nonstimulated conditions, one-day BNST-NAc DBS decreased [11C]raclopride BPND in the amygdala (15.9 %, p < 0.01), caudate nucleus (15.4 %, p < 0.0001) and substantia nigra (10.8 %, p < 0.01). LIMITATIONS: This study was limited to the small sample size and lack of a healthy control group. CONCLUSIONS: Chronic BNST-NAc DBS improved depression and anxiety symptoms, and short-term stimulation decreased D2 receptor binding in the amygdala, caudate nucleus, and substantia nigra. The findings suggest that DBS relieves depression and anxiety symptoms possibly by regulating the dopaminergic system.

Longitudinal support for the correlative triad among aging, dopamine D2-like receptor loss, and memory decline.

Dopamine decline is suggested to underlie aging-related cognitive decline, but longitudinal examinations of this link are currently missing. We analyzed 5-year longitudinal data for a sample of healthy, older adults (baseline: n = 181, age: 64-68 years; 5-year follow-up: n = 129) who underwent positron emission tomography with 11C-raclopride to assess dopamine D2-like receptor (DRD2) availability, magnetic resonance imaging to evaluate structural brain measures, and cognitive tests. Health, lifestyle, and genetic data were also collected. A data-driven approach (k-means cluster analysis) identified groups that differed maximally in DRD2 decline rates in age-sensitive brain regions. One group (n = 47) had DRD2 decline exclusively in the caudate and no cognitive decline. A second group (n = 72) had more wide-ranged DRD2 decline in putamen and nucleus accumbens and also in extrastriatal regions. The latter group showed significant 5-year working memory decline that correlated with putamen DRD2 decline, along with higher dementia and cardiovascular risk and a faster biological pace of aging. Taken together, for individuals with more extensive DRD2 decline, dopamine decline is associated with memory decline in aging.

Vectorial-based analysis of dual-tracer PET imaging: A proof of concept.

BACKGROUND: The diagnosis of neurological diseases is complicated since they often share similar symptoms and occur in different severity levels. Imaging techniques such as PET molecular imaging are helpful for an early and accurate diagnosis and, staging allowing a noninvasive evaluation of the disease. The combination of two radioligands in the same patient could be valuable to achieve these diagnostic goals; nevertheless, the imaging data obtained with two radioligands is commonly interpreted independently. This novel approach to combine the PET data of two radiopharmaceuticals, separately acquired in the same subject, is to obtain new quantitative metrics. PET images of patients with Parkinson's disease (PD) and healthy controls (HC) were analyzed. Voxel-by-voxel uptake is compared by combining the imaging data. Dual-tracer PET imaging analysis was tested with [11C]DTBZ-[11C]Raclopride as proof of concept. RESULTS: The new proposed metric based on a resultant vector is capable of efficiently discriminating healthy controls from PD patients (p < 0.0001) allowing the detection of slight changes in patients undergoing therapeutic approaches. Significant differences were found between HC and PD patients for the evaluated radiotracers. CONCLUSIONS: The resultant vector appears to deliver useful information that could be helpful to evaluate PD patients under treatment and to improve differential diagnoses.

Striatal dopamine tone is positively associated with body mass index in humans as determined by PET using dual dopamine type-2 receptor antagonist tracers.

The relationship between adiposity and dopamine type-2 receptor binding potential (D2BP) in the human brain has been repeatedly studied for >20 years with highly discrepant results, likely due to variable methodologies and differing study populations. We conducted a controlled inpatient feeding study to measure D2BP in the striatum using positron emission tomography with both [18F]fallypride and [11C]raclopride in pseudo-random order in 54 young adults with a wide range of body mass index (BMI 20-44 kg/m2). Within-subject D2BP measurements using the two tracers were moderately correlated (r=0.47, p<0.001). D2BP was negatively correlated with BMI as measured by [11C]raclopride (r= -0.51; p<0.0001) but not [18F]fallypride (r=-0.01; p=0.92) and these correlation coefficients were significantly different from each other (p<0.001). Given that [18F]fallypride has greater binding affinity to dopamine type-2 receptors than [11C]raclopride, which is more easily displaced by endogenous dopamine, our results suggest that adiposity is positively associated with increased striatal dopamine tone.

Striatal dopamine D2 receptor availability as a predictor of subsequent alcohol use in social drinkers.

BACKGROUND AND AIMS: Whereas striatal dopamine D2 receptor (D2R) availability has shown to be altered in individuals with alcohol use disorder (AUD) and in healthy individuals with a family history of AUD, the role of D2R in the development of AUD is unknown. In this positron emission tomography (PET) study, we measured whether D2R availability is associated with subsequent alcohol use and alcohol-related factors, at a follow-up 8 to 16 years post-PET scan, in social drinkers. DESIGN: Longitudinal study investigating the association between PET data and later self-report measures in healthy individuals. SETTING: Academic research imaging centre in Stockholm, Sweden. PARTICIPANTS: There were 71 individuals (68 of whom had evaluable PET data, 5 females, 42.0 years mean age) from a series of previous PET studies. MEASUREMENTS: One PET examination with the D2R antagonist radioligand [11 C]raclopride at baseline and self-report measures assessing alcohol use, drug use, impulsivity, reward sensitivity and family history of alcohol or substance use disorder at follow-up. FINDINGS: We found no evidence for an association between D2R availability and later alcohol use (B = -0.019, B 95% CI = -0.043 to -0.006, P = 0.147) nor for the majority of the alcohol-related factors (B 95% CI = -0.034 to 0.004, P = 0.273-0.288). A negative association with a small effect size was found between D2R availability and later impulsivity (B = -0.017, B 95% CI = -0.034 to -0.001, P = 0.046). CONCLUSIONS: Low striatal dopamine D2 receptor availability may not be a strong predictor in the development of alcohol use disorder.

Measurement of Striatal Dopamine Release Induced by Neuropsychological Stimulation in Positron Emission Tomography With Dual Injections of [11C]Raclopride.

Objectives: Positron emission tomography (PET) with [11C]raclopride has been applied to measure changes in the concentration of endogenous dopamine induced by pharmacological challenge or neuropsychological stimulation by evaluating the binding potential (BP) between the baseline and activated state. Recently, to reliably estimate BP in the activated state, a new approach with dual-bolus injections in a single PET scan was developed. In this study, we investigated the feasibility of applying this dual-bolus injection approach to measure changes in endogenous dopamine levels induced by cognitive tasks in humans. Methods: First, the reproducibility of BP estimation using the dual-bolus injection approach was evaluated using PET scans without stimulation in nine healthy volunteers. A 90-min scan was performed with bolus injections of [11C]raclopride administered at the beginning of the scan and 45 min after the first injection. BPs in the striatum for the first injection (BP1) and second injection (BP2) were estimated using an extended simplified reference tissue model, and the mean absolute difference (MAD) between the two BPs was calculated. The MAD was also compared with the conventional bolus-plus-continuous infusion approach. Next, PET studies with a cognitive reinforcement learning task were performed on 10 healthy volunteers using the dual-bolus injection approach. The BP1 at baseline and BP2 at the activated state were estimated, and the reduction in BP was evaluated. Results: In the PET scans without stimulation, the dual-bolus injection approach showed a smaller MAD (<2%) between BP1 and BP2 than the bolus-plus-continuous infusion approach, demonstrating good reproducibility of this approach. In the PET scans with the cognitive task performance, the reduction in BP was not observed in the striatum by either approach, showing that the changes in dopamine level induced by the cognitive tasks performed in this study were not sufficient to be detected by PET. Conclusion: Our results indicate that the cognitive task-induced changes in dopamine-related systems may be complex and difficult to measure accurately using PET scans. However, the proposed dual-bolus injection approach provided reliable BP estimates with high reproducibility, suggesting that it has the potential to improve the accuracy of PET scans for measuring changes in dopamine concentrations.

Binding of the Dual-Action Anti-Parkinsonian Drug AG-0029 to Dopamine D2 and Histamine H3 Receptors: A PET Study in Healthy Rats.

Introduction: Parkinson's disease (PD) is a neurodegenerative disorder characterized by motor dysfunction and a diverse range of nonmotor symptoms. Functional relationships between the dopaminergic and histaminergic systems suggest that dual-action pharmaceuticals like AG-0029 (D2/D3 agonist/H3 antagonist) could ameliorate both the motor and cognitive symptoms of PD. The current study aimed to demonstrate the interaction of AG-0029 with its intended targets in the mammalian brain using positron emission tomography (PET). Methods: Healthy male Wistar rats were scanned with a small-animal PET camera, using either the dopamine D2/D3 receptor ligand [11C]raclopride or the histamine H3 receptor ligand [11C]GSK-189254, before and after treatment with an intravenous, acute, single dose of AG-0029. Dynamic [11C]raclopride PET data (60 min duration) were analyzed using the simplified reference tissue model 2 (SRTM2) with cerebellum as reference tissue and the nondisplaceable binding potential as the outcome parameter. Data from dynamic [11C]GSK-189254 scans (60 min duration) with arterial blood sampling were analyzed using Logan graphical analysis with the volume of distribution (VT) as the outcome parameter. Receptor occupancy was estimated using a Lassen plot. Results: Dopamine D2/3 receptor occupancies in the striatum were 22.6 ± 18.0 and 84.0 ± 3.5% (mean ± SD) after administration of 0.1 and 1 mg/kg AG-0029, respectively. In several brain regions, the VT values of [11C]GSK-189254 were significantly reduced after pretreatment of rats with 1 or 10 mg/kg AG-0029. The H3 receptor occupancies were 11.9 ± 8.5 and 40.3 ± 11.3% for the 1 and 10 mg/kg doses of AG-0029, respectively. Conclusions: Target engagement of AG-0029 as an agonist at dopamine D2/D3 receptors and an antagonist at histamine H3 receptors could be demonstrated in the rat brain with [11C]raclopride and [11C]GSK-189254 PET, respectively. The measured occupancy values reflect the previously reported high (subnanomolar) affinity of AG-0029 to D2/D3 and moderate (submicromolar) affinity to H3 receptors.

Atlas of type 2 dopamine receptors in the human brain: Age and sex dependent variability in a large PET cohort.

BACKGROUND: The dopamine system contributes to a multitude of functions ranging from reward and motivation to learning and movement control, making it a key component in goal-directed behavior. Altered dopaminergic function is observed in neurological and psychiatric conditions. Numerous factors have been proposed to influence dopamine function, but due to small sample sizes and heterogeneous data analysis methods in previous studies their specific and joint contributions remain unresolved. METHODS: In this cross-sectional register-based study we investigated how age, sex, body mass index (BMI), as well as cerebral hemisphere and regional volume influence striatal type 2 dopamine receptor (D2R) availability in the human brain. We analyzed a large historical dataset (n=156, 120 males and 36 females) of [11C]raclopride PET scans performed between 2004 and 2018. RESULTS: Striatal D2R availability decreased through age for both sexes (2-5 % in striatal ROIs per 10 years) and was higher in females versus males throughout age (7-8% in putamen). BMI and striatal D2R availability were weakly associated. There was no consistent lateralization of striatal D2R. The observed effects were independent of regional volumes. These results were validated using two different spatial normalization methods, and the age and sex effects also replicated in an independent sample (n=135). CONCLUSIONS: D2R availability is dependent on age and sex, which may contribute to the vulnerability of neurological and psychiatric conditions involving altering D2R expression.

Harmonization of [11C]raclopride brain PET images from the HR+ and HRRT: method development and validation in human subjects.

BACKGROUND: There has been an ongoing need to compare and combine the results of new PET imaging studies conducted with [11C]raclopride with older data. This typically means harmonizing data across different scanners. Previous harmonization studies have utilized either phantoms or human subjects, but the use of both phantoms and humans in one harmonization study is not common. The purpose herein was (1) to use phantom images to develop an inter-scanner harmonization technique and (2) to test the harmonization technique in human subjects. METHODS: To develop the harmonization technique (Experiment 1), the Iida brain phantom was filled with F-18 solution and scanned on the two scanners in question (HRRT, HR+, Siemens/CTI). Phantom images were used to determine the optimal isotropic Gaussian filter to harmonize HRRT and HR+ images. To evaluate the harmonization on human images (Experiment 2), inter-scanner variability was calculated using [11C]raclopride scans of 3 human subjects on both the HRRT and HR+ using percent difference (PD) in striatal non-displaceable binding potential (BPND) between HR+ and HRRT (with and without Gaussian smoothing). Finally, (Experiment 3), PDT/RT was calculated for test-retest (T/RT) variability of striatal BPND for 8 human subjects scanned twice on the HR+. RESULTS: Experiment 1 identified the optimal filter as a Gaussian with a 4.5 mm FWHM. Experiment 2 resulted in 13.9% PD for unfiltered HRRT and 3.71% for HRRT filtered with 4.5 mm. Experiment 3 yielded 5.24% PDT/RT for HR+. CONCLUSIONS: The PD results show that the variability of harmonized HRRT is less than the T/RT variability of the HR+. The harmonization technique makes it possible for BPND estimates from the HRRT to be compared to (and/or combined with) those from the HR+ without adding to overall variability. Our approach is applicable to all pairs of scanners still in service.

Assessment of motion and model bias on the detection of dopamine response to behavioral challenge.

Compartmental modeling analysis of 11C-raclopride (RAC) PET data can be used to measure the dopaminergic response to intra-scan behavioral tasks. Bias in estimates of binding potential (BPND) and its dynamic changes (ΔBPND) can arise both when head motion is present and when the compartmental model used for parameter estimation deviates from the underlying biology. The purpose of this study was to characterize the effects of motion and model bias within the context of a behavioral task challenge, examining the impacts of different mitigation strategies. Seventy healthy adults were administered bolus plus constant infusion RAC during a simultaneous PET/magnetic resonance (MR) scan with a reward task experiment. BPND and ΔBPND were estimated using an extension of the Multilinear Reference Tissue Model (E-MRTM2) and a new method (DE-MRTM2) was proposed to selectively discount the contribution of the initial uptake period. Motion was effectively corrected with a standard frame-based approach, which performed equivalently to a more complex reconstruction-based approach. DE-MRTM2 produced estimates of ΔBPND in putamen and nucleus accumbens that were significantly different from those estimated from E-MRTM2, while also decoupling ΔBPND values from first-pass k2' estimation and removing skew in the spatial bias distribution of parametric ΔBPND estimates within the striatum.

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.

Assessment of transient dopamine responses to smoked cannabis.

BACKGROUND: Dopaminergic mechanisms that may underlie cannabis' reinforcing effects are not well elucidated in humans. This positron emission tomography (PET) imaging study used the dopamine D2/3 receptor antagonist [11C]raclopride and kinetic modelling testing for transient changes in radiotracer uptake to assess the striatal dopamine response to smoked cannabis in a preliminary sample. METHODS: PET emission data were acquired from regular cannabis users (n = 14; 7 M/7 F; 19-32 years old) over 90 min immediately after [11C]raclopride administration (584 ± 95 MBq) as bolus followed by constant infusion (Kbol = 105 min). Participants smoked a cannabis cigarette, using a paced puff protocol, 35 min after scan start. Plasma concentrations of Δ9-THC and metabolites and ratings of subjective "high" were collected during imaging. Striatal dopamine responses were assessed voxelwise with a kinetic model testing for transient reductions in [11C]raclopride binding, linear-parametric neurotransmitter PET (lp-ntPET) (cerebellum as a reference region). RESULTS: Cannabis smoking increased plasma Δ9-THC levels (peak: 0-10 min) and subjective high (peak: 0-30 min). Significant clusters (>16 voxels) modeled by transient reductions in [11C]raclopride binding were identified for all 12 analyzed scans. In total, 26 clusters of significant responses to cannabis were detected, of which 16 were located in the ventral striatum, including at least one ventral striatum cluster in 11 of the 12 analyzed scans. CONCLUSIONS: These preliminary data support the sensitivity of [11C]raclopride PET with analysis of transient changes in radiotracer uptake to detect cannabis smoking-induced dopamine responses. This approach shows future promise to further elucidate roles of mesolimbic dopaminergic signaling in chronic cannabis use. ClinicalTrials.gov Identifier: NCT02817698.

Automatic classification of idiopathic Parkinson's disease and atypical Parkinsonian syndromes combining [11C]raclopride PET uptake and MRI grey matter morphometry.

Objective.To explore the viability of developing a computer-aided diagnostic system for Parkinsonian syndromes using dynamic [11C]raclopride positron emission tomography (PET) and T1-weighted magnetic resonance imaging (MRI) data.Approach.The biological heterogeneity of Parkinsonian syndromes renders their statistical classification a challenge. The unique combination of structural and molecular imaging data allowed different classifier designs to be tested. Datasets from dynamic [11C]raclopride PET and T1-weighted MRI scans were acquired from six groups of participants. There were healthy controls (CTRLn= 15), patients with Parkinson's disease (PDn= 27), multiple system atrophy (MSAn= 8), corticobasal degeneration (CBDn= 6), and dementia with Lewy bodies (DLBn= 5). MSA, CBD, and DLB patients were classified into one category designated as atypical Parkinsonism (AP). The distribution volume ratio (DVR) kinetic parameters obtained from the PET data were used to quantify the reversible tracer binding to D2/D3 receptors in the subcortical regions of interest (ROI). The grey matter (GM) volumes obtained from the MRI data were used to quantify GM atrophy across cortical, subcortical, and cerebellar ROI.Results.The classifiers CTRL vs PD and CTRL vs AP achieved the highest balanced accuracy combining DVR and GM (DVR-GM) features (96.7%, 92.1%, respectively), followed by the classifiers designed with DVR features (93.3%, 88.8%, respectively), and GM features (69.6%, 86.1%, respectively). In contrast, the classifier PD vs AP showed the highest balanced accuracy (78.9%) using DVR features only. The integration of DVR-GM (77.9%) and GM features (72.7%) produced inferior performances. The classifier CTRL vs PD vs AP showed high weighted balanced accuracy when DVR (80.5%) or DVR-GM features (79.9%) were integrated. GM features revealed poorer performance (59.5%).Significance.This work was unique in its combination of structural and molecular imaging features in binary and triple category classifications. We were able to demonstrate improved binary classification of healthy/diseased status (concerning both PD and AP) and equate performance to DVR features in multiclass classifications.

The rate of dasotraline brain entry is slow following intravenous administration.

RATIONALE: Drugs that rapidly increase dopamine levels have an increased risk of abuse. Dasotraline (DAS) is a dopamine and norepinephrine reuptake inhibitor characterized by slow oral absorption with low potential for abuse. However, it remains unclear whether intravenous (i.v.) administration would facilitate the rapid elevation of dopamine levels associated with stimulant drugs. OBJECTIVE: To assess the kinetics of DAS across the blood-brain barrier and time to onset of dopamine transporters (DAT) inhibition. METHODS: We compared the onset of DAT occupancy and the associated elevation of synaptic dopamine levels in rhesus monkey following i.v. administration of DAS or methylphenidate (MPH) using positron emission tomography (PET). Brain entry times were estimated by reductions in [18F]-FE-PE2I binding to DAT in rhesus monkeys. Elevations of synaptic dopamine were estimated by reductions in [11C]-Raclopride binding to D2 receptors. RESULTS: Intravenous administration of DAS (0.1 and 0.2 mg/kg) resulted in striatal DAT occupancies of 54% and 68%, respectively; i.v. administered MPH (0.1 and 0.5 mg/kg) achieved occupancies of 69% and 88% respectively. Brain entry times of DAS (22 and 15 min, respectively) were longer than for MPH (3 and 2 min). Elevations in synaptic dopamine were similar for both DAS and MPH however the time for half-maximal displacement by MPH (t = 23 min) was 4-fold more rapid than for DAS (t = 88 min). CONCLUSIONS: These results demonstrate that the pharmacodynamics effects of DAS on DAT occupancy and synaptic dopamine levels are more gradual in onset than those of MPH even with i.v. administration that is favored by recreational drug abusers.

Huntington's Disease: A Review of the Known PET Imaging Biomarkers and Targeting Radiotracers.

Huntington's disease (HD) is a fatal neurodegenerative disease caused by a CAG expansion mutation in the huntingtin gene. As a result, intranuclear inclusions of mutant huntingtin protein are formed, which damage striatal medium spiny neurons (MSNs). A review of Positron Emission Tomography (PET) studies relating to HD was performed, including clinical and preclinical data. PET is a powerful tool for visualisation of the HD pathology by non-invasive imaging of specific radiopharmaceuticals, which provide a detailed molecular snapshot of complex mechanistic pathways within the brain. Nowadays, radiochemists are equipped with an impressive arsenal of radioligands to accurately recognise particular receptors of interest. These include key biomarkers of HD: adenosine, cannabinoid, dopaminergic and glutamateric receptors, microglial activation, phosphodiesterase 10 A and synaptic vesicle proteins. This review aims to provide a radiochemical picture of the recent developments in the field of HD PET, with significant attention devoted to radiosynthetic routes towards the tracers relevant to this disease.

Balance between Transmitter Availability and Dopamine D2 Receptors in Prefrontal Cortex Influences Memory Functioning.

Insufficient or excessive dopaminergic tone impairs cognitive performance. We examine whether the balance between transmitter availability and dopamine (DA) D2 receptors (D2DRs) is important for successful memory performance in a large sample of adults (n = 175, 64-68 years). The Catechol-O-Methyltransferase polymorphism served as genetic proxy for endogenous prefrontal DA availability, and D2DRs in dorsolateral prefrontal cortex (dlPFC) were measured with [11C]raclopride-PET. Individuals for whom D2DR status matched DA availability showed higher levels of episodic and working-memory performance than individuals with insufficient or excessive DA availability relative to the number of receptors. A similar pattern restricted to episodic memory was observed for D2DRs in caudate. Functional magnetic resonance imaging data acquired during working-memory performance confirmed the importance of a balanced DA system for load-dependent brain activity in dlPFC. Our data suggest that the inverted-U-shaped function relating DA signaling to cognition is modulated by a dynamic association between DA availability and receptor status.

High long-term test-retest reliability for extrastriatal 11C-raclopride binding in healthy older adults.

In vivo dopamine D2-receptor availability is frequently assessed with 11C-raclopride and positron emission tomography. Due to low signal-to-noise ratios for 11C-raclopride in areas with low D2 receptor densities, the ligand has been considered unreliable for measurements outside the dopamine-dense striatum. Intriguingly, recent studies show that extrastriatal 11C-raclopride binding potential (BPND) values are (i) reliably higher than in the cerebellum (where D2-receptor levels are negligible), (ii) correlate with behavior in the expected direction, and (iii) showed good test-retest reliability in a sample of younger adults. The present work demonstrates high seven-month test-retest reliability of striatal and extrastriatal 11C-raclopride BPND values in healthy, older adults (n = 27, age: 64-78 years). Mean 11C-raclopride BPND values were stable between test sessions in subcortical nuclei, and in frontal and temporal cortices (p > 0.05). Across all structures analyzed, intraclass correlation coefficients were high (0.85-0.96), absolute variability was low (mean: 4-8%), and coefficients of variance ranged between 9 and 25%. Furthermore, regional 11C-raclopride BPND values correlated with previously determined 18F-fallypride BPND values (ρ = 0.97 and 0.92 in correlations with and without striatal values, respectively, p < 0.01) and postmortem determined D2-receptor densities (including striatum: ρ = 0.92; p < 0.001; excluding striatum: ρ = 0.75; p = 0.067). These observations suggest that extrastriatal 11C-raclopride measurements represent a true D2 signal.