A preliminary study of dopamine D2/3 receptor availability and social status in healthy and cocaine dependent humans imaged with [(11)C](+)PHNO

Chronic social stressors and striatal dopamine functioning in humans: A systematic review of SPECT and PET studies

The dopamine hypothesis of schizophrenia posits that elevated striatal dopamine functioning underlies the development of psychotic symptoms. Chronic exposure to social stressors increases psychosis risk, possibly by upregulating striatal dopamine functioning. Here we systematically review single photon emission computed tomography (SPECT) and positron emission tomography (PET) studies that examined the relationship between chronic social stress exposure and in vivo striatal dopamine functioning in humans. We searched the scientific databases PubMed and PsycINFO from inception to August 2023. The quality of the included studies was evaluated with the ten-item Observational Study Quality Evaluation (PROSPERO: CRD42022308883). Twenty-eight studies were included, which measured different aspects of striatal dopamine functioning including dopamine synthesis capacity (DSC), vesicular monoamine transporter type 2 binding, dopamine release following a pharmacological or behavioral challenge, D2/3 receptor binding, and dopamine transporter binding. We observed preliminary evidence of an association between childhood trauma and increased striatal DSC and dopamine release. However, exposure to low socioeconomic status, stressful life events, or other social stressors was not consistently associated with altered striatal dopamine functioning. The quality of available studies was generally low. In conclusion, there is insufficient evidence that chronic social stressors upregulate striatal dopamine functioning in humans. We propose avenues for future research, in particular to improve the measurement of chronic social stressors and the methodological quality of study designs.

Dopamine mediates a directionally opposite correlation between empathy and the reinforcing effects of amphetamine and gambling in people with gambling disorder vs. healthy controls

Understanding the relationship between empathy, subjective effects of addictive reinforcers and dopamine function in people with gambling disorder (PGD) vs. healthy controls (HCs) may inform GD treatment. The current investigation addressed this issue via retrospective analysis of data from three studies using amphetamine and a slot machine (SLOTS) as reinforcers in PGD and HCs. The Empathy scale of Eysenck's Impulsiveness Questionnaire assessed trait Empathy. The Gamblers Beliefs Questionnaire assessed cognitive distortions. The Eysenck Lie scale assessed socially desirable responding. PET scans quantified dopamine receptor expression and amphetamine-induced dopamine release in Study 1. Pre-treatment with the D2-receptor (D2R)-preferring antagonist, haloperidol or D1R-D2R antagonist, fluphenazine before SLOTS tested the role of D2 autoreceptors and post-synaptic D2R in Study 2. Pre-treatment with the multi-system indirect dopamine agonist, modafinil before SLOTS assessed the reliability of correlations in PGD. Striatal D2R expression predicted greater Empathy and lower amphetamine 'Liking' in HCs, and predicted greater symptom severity in PGD. Empathy predicted lower 'Exciting' effects of SLOTS under placebo in HCs; no correlation emerged under either antagonist. Relative to placebo, haloperidol decreased, whereas fluphenazine increased, the positive correlation between Empathy and Exciting effects of SLOTS in PGD. Modafinil markedly reduced the positive correlation between Empathy and Exciting effects of SLOTS seen under placebo in PGD. Empathy predicted greater cognitive distortions in PGD in all studies. Lie scale variance influenced several primary effects. Prior research linking the insula with Empathy, reactivity to interoceptive signals for risky rewards (uncertainty), and cognitive distortions, provides a parsimonious account for these results.

Clinical correlates of dopamine transporter availability in cross-sectional and longitudinal studies with [18F]FE-PE2I PET: independent validation with new insights

[18F]FE-PE2I PET is a promising alternative to single positron emission computed tomography-based dopamine transporter (DAT) imaging in Parkinson's disease. While the excellent discriminative power of [18F]FE-PE2I PET has been established, so far only one study has reported meaningful associations between motor severity scores and DAT availability. In this study, we use high-resolution (∼3 mm isotropic) PET to provide an independent validation for the clinical correlates of [18F]FE-PE2I imaging in separate cross-sectional (28 participants with Parkinson's disease, Hoehn-Yahr: 2 and 14 healthy individuals) and longitudinal (initial results from 6 participants with Parkinson's disease with 2-year follow-up) cohorts. In the cross-sectional cohort, DAT availability in the putamen and substantia nigra of patients with Parkinson's disease showed a significant negative association with total motor severity (r = -0.59, P = 0.002 for putamen; r = -0.46, P = 0.018 for substantia nigra), but not tremor severity. To our knowledge, this is the first observed association between motor severity in Parkinson's disease and DAT availability in the substantia nigra. The associations with motor severity in most nigrostriatal regions improved if tremor scores were excluded from motor scores. Further, we found significant asymmetry in DAT availability in the putamen (∼28% lower DAT availability within the more-affected side of the putamen), and DAT-based asymmetry index for the putamen was correlated with asymmetry in motor severity (r = -0.60, P = 0.001). In the longitudinal study, [18F]FE-PE2I PET detected significant annual percentage reduction of DAT availability at the individual level in the putamen (9.7 ± 2.6%), caudate (10.5 ± 3.8%) and ventral striatum (5.5 ± 2.7%), but not the substantia nigra. Longitudinal per cent reduction in DAT availability within the putamen was strongly associated with increase in motor severity (r = 0.91, P = 0.011) at follow-up, demonstrating the high sensitivity of [18F]FE-PE2I PET in tracking longitudinal changes. These results provide further evidence for the utility of [18F]FE-PE2I as an important in vivo PET biomarker in future clinical trials of Parkinson's disease.

Socioenvironmental Factors are Associated With Dopamine Transporter Availability in Healthy Individuals but not in Parkinson's Disease

OBJECTIVE

Social factors can influence the brain's dopaminergic function. This study investigated the relationship between socioenvironmental factors and dopamine transporter (DaT) availability in healthy individuals (n = 74) and those with Parkinson's disease (PD) (n = 240).

METHODS

All single photon emission computed tomography (SPECT) DaT data and clinical data used in this study were obtained from the Parkinson's Progression Markers Initiative (PPMI) dataset. Socioenvironmental data was obtained from Social Explorer analyses of the American Community Survey (2014-2018) using the residential ZIP codes of the subjects available in the PPMI dataset.

RESULTS

Participants resided in 302 ZIP code tabulation areas across 38 U.S. states. In healthy individuals, DaT signals were significant and negatively correlated in the caudate with median household income (r = -0.27, P = 0.02) and educational level of the living area (r = -0.23, P = 0.04), but not significant in the putamen (r = -0.21, P = 0.08; r = -0.11, P = 0.37 respectively). Also, there was a significant positive correlation between DaT signals in caudate and poverty rates (r = 0.29, P = 0.01), but not in the putamen (r = 0.16, P = 0.19) in healthy subjects. No significant associations were observed in the PD group for any variables.

CONCLUSION

The study findings suggest that socioenvironmental factors, such as median household income, education level, and poverty rate, are significantly associated with DaT availability in the caudate of healthy individuals but not in those with PD. This indicates that PD might disrupt the connection between the social environment and dopaminergic function. These results underscore the importance of considering socioenvironmental variables when studying dopaminergic function in the human brain.

Synaptic loss and its association with symptom severity in Parkinson's disease

Parkinson's disease (PD) is the fastest growing neurodegenerative disease, but at present there is no cure, nor any disease-modifying treatments. Synaptic biomarkers from in vivo imaging have shown promise in imaging loss of synapses in PD and other neurodegenerative disorders. Here, we provide new clinical insights from a cross-sectional, high-resolution positron emission tomography (PET) study of 30 PD individuals and 30 age- and sex-matched healthy controls (HC) with the radiotracer [11C]UCB-J, which binds to synaptic vesicle glycoprotein 2A (SV2A), and is therefore, a biomarker of synaptic density in the living brain. We also examined a measure of relative brain perfusion from the early part of the same PET scan. Our results provide evidence for synaptic density loss in the substantia nigra that had been previously reported, but also extend this to other early-Braak stage regions known to be affected in PD (brainstem, caudate, olfactory cortex). Importantly, we also found a direct association between synaptic density loss in the nigra and severity of symptoms in patients. A greater extent and wider distribution of synaptic density loss in PD patients with longer illness duration suggests that [11C]UCB-J PET can be used to measure synapse loss with disease progression. We also demonstrate lower brain perfusion in PD vs. HC groups, with a greater extent of abnormalities in those with longer duration of illness, suggesting that [11C]UCB-J PET can simultaneously provide information on changes in brain perfusion. These results implicate synaptic imaging as a useful PD biomarker for future disease-modifying interventions.

Intense exercise increases dopamine transporter and neuromelanin concentrations in the substantia nigra in Parkinson's disease

Parkinson's disease (PD) is characterized by a progressive loss of dopaminergic neurons. Exercise has been reported to slow the clinical progression of PD. We evaluated the dopaminergic system of patients with mild and early PD before and after a six-month program of intense exercise. Using 18F-FE-PE2I PET imaging, we measured dopamine transporter (DAT) availability in the striatum and substantia nigra. Using NM-MRI, we evaluated the neuromelanin content in the substantia nigra. Exercise reversed the expected decrease in DAT availability into a significant increase in both the substantia nigra and putamen. Exercise also reversed the expected decrease in neuromelanin concentration in the substantia nigra into a significant increase. These findings suggest improved functionality in the remaining dopaminergic neurons after exercise. Further research is needed to validate our findings and to pinpoint the source of any true neuromodulatory and neuroprotective effects of exercise in PD in large clinical trials.

Relationships between dopamine D2/3 receptor availability and social-environmental factors in humans

Social factors are associated with psychiatric outcomes and brain function. Relationships between local population data obtained from Social Explorer analyses of the American Community Survey (2014-2018) and dopamine D2/3 receptor (D2/3R) availability were explored in this retrospective analysis of [11C]PHNO positron emission tomography (PET) imaging data (n = 70). Larger local population size and lower percentage of the population with a bachelor's degree or higher were significantly associated with higher striatal D2/3R availability, suggesting that living in a populous area with fewer educational resources may be accompanied by stressors with concomitant dopaminergic changes. Future prospective, collaborative studies are needed to better understand the precise etiology of the observed relationships.

Effects of moderate-intensity exercise on social health and physical and mental health of methamphetamine-dependent individuals: A randomized controlled trial

OBJECTIVE

Methamphetamine (MA)-dependent individuals' health problems are widespread and need to be solved urgently. Exercise is considered a potential treatment for MA dependents. The study aimed to determine the effects of a 12-week aerobic exercise on the social, physical, and mental health of MA-dependent individuals.

MATERIALS AND METHODS

Sixty MA-dependent individuals were randomly assigned into two groups. Subjects in the exercise group (n = 30) received an exercise intervention five days a week for 60 min each for 12 weeks. Subjects in the control group (n = 30) received regular corrective rehabilitation without exercise in the same setting. Outcome measures, including questionnaires [quality of life scale for drug addiction (QOL-DA), self-rating anxiety scale (SAS), self-rating depression scale (SDS), and Pittsburgh sleep quality index (PSQI)] and physical fitness, were arranged the day before the start of the intervention and the day after the end of the intervention. Two-factor repeated measures ANOVA was used to compare the treatment differences between the two groups.

RESULTS

After 12 weeks of the intervention period, social health was significantly improved in the exercise group (P < 0.01), and there was a statistically significant difference in mental health scores between exercise group and control group, with a greater impact in exercise group.(Psychology: P < 0.01; SAS: P < 0.01; SDS: P < 0.01; PSQI: P < 0.01), physical health improved in the exercise group, physiology (P < 0.01), symptom (P < 0.01), heart rate (P < 0.01), systolic blood pressure (P < 0.01), systolic blood pressure (P < 0.01), vital capacity (P < 0.05), grip (P < 0.01), vertical jump (P < 0.001), sit and reach (P < 0.01), 50-meter run (P < 0.01), and reaction time (P < 0.01).

CONCLUSION

Aerobic exercise intervention is an effective treatment for MA-dependent individuals, and the 12-week intervention improved the social, physical, and mental health of MA-dependent individuals. We recommend that future studies focus more on drug-dependent individuals' overall health status rather than just relapse.Clinical trial registration: [https://www.chictr.org.cn/hvshowproject.aspx?id=131048], identifier [ChiCTR2200055348].

D3 Receptors and PET Imaging

This chapter encapsulates a short introduction to positron emission tomography (PET) imaging and the information gained by using this technology to detect changes of the dopamine 3 receptor (D3R) at the molecular level in vivo. We will discuss available D3R radiotracers, emphasizing [11C]PHNO. The focus, however, will be on PET findings in conditions including substance abuse, obesity, traumatic brain injury, schizophrenia, Parkinson's disease, and aging. Finally, there is a discussion about progress in producing next-generation selective D3R radiotracers.

Dopamine dysfunction in stimulant use disorders: mechanistic comparisons and implications for treatment

Dopamine system deficiencies and associated behavioral phenotypes may be a critical barrier to success in treating stimulant use disorders. Similarities in dopamine dysfunction between cocaine and methamphetamine use disorder but also key differences may impact treatment efficacy and outcome. This review will first compare the epidemiology of cocaine and methamphetamine use disorder. A detailed account of the pharmacokinetic and pharmacodynamic properties associated with each drug will then be discussed, with an emphasis on effects on the dopamine system and associated signaling pathways. Lastly, treatment results from pharmacological clinical trials will be summarized along with a more comprehensive review of the involvement of the trace amine-associated receptor on dopamine signaling dysfunction among stimulants and its potential as a therapeutic target.

Dopamine D2/3 receptor availability in cocaine use disorder individuals with obesity as measured by [11C]PHNO PET

BACKGROUND

Positron emission tomography (PET) work with the dopamine D3 receptor (D₃R) preferring ligand [¹¹C]PHNO in obese individuals has demonstrated higher binding and positive correlations with body mass index (BMI) in otherwise healthy individuals. These findings implicated brain reward areas including the substantia nigra/ventral tegmental area (SN/VTA) and pallidum. In cocaine use disorder (CUD), similar SN/VTA binding profiles have been found compared to healthy control subjects. This study investigates whether BMI-[¹¹C]PHNO relationships are similar in individuals with CUD.

METHODS

Non-obese CUD subjects (N = 12) were compared to age-matched obese CUD subjects (N = 14). All subjects underwent [¹¹C]PHNO acquisition using a High Resolution Research Tomograph PET scanner. Parametric images were computed using the simplified reference tissue model with cerebellum as the reference region. [¹¹C]PHNO measures of receptor availability were calculated and expressed as non-displaceable binding potential (BP_ND).

RESULTS

In between-group analyses, D2/3R availability in non-obese and obese CUD groups was not significantly different overall. BMI was inversely correlated withBP_ND in the SN/VTA (r = -0.45, p = 0.02 uncorrected) in all subjects.

CONCLUSION

These data suggest that obesity in CUD was not associated with significant differences in D_2/3R availability. This in contrast to previous findings in non-CUD individuals that found increased availability of D₃Rs in the SN/VTA associated with obesity. These findings could potentially reflect dysregulation of D₃R in CUD, impacting how affected individuals respond to natural stimuli such as food.

Separating dopamine D2 and D3 receptor sources of [11C]-(+)-PHNO binding potential: Independent component analysis of competitive binding

Development of medications selective for dopamine D₂ or D₃ receptors is an active area of research in numerous neuropsychiatric disorders including addiction and Parkinson's disease. The positron emission tomography (PET) radiotracer [¹¹C]-(+)-PHNO, an agonist that binds with high affinity to both D₂ and D₃ receptors, has been used to estimate relative receptor subtype occupancy by drugs based on a priori knowledge of regional variation in the expression of D₂ and D₃ receptors. The objective of this work was to use a data-driven independent component analysis (ICA) of receptor blocking scans to separate D₂-and D₃-related signal in [¹¹C]-(+)-PHNO binding data in order to improve the precision of subtype specific measurements of binding and occupancy. Eight healthy volunteers underwent [¹¹C]-(+)-PHNO PET scans at baseline and at two time points following administration of the D₃-preferring antagonist ABT-728 (150-1000 ​mg). Parametric binding potential (BP_ND) images were analyzed as four-dimensional image series using ICA to extract two independent sources of variation in [¹¹C]-(+)-PHNO BP_ND. Spatial source maps for each component were consistent with respective regional patterns of D₂-and D₃-related binding. ICA-derived occupancy estimates from each component were similar to D₂-and D₃-specific occupancy estimated from a region-based approach (intraclass correlation coefficients ​> ​0.95). ICA-derived estimates of D₃ receptor occupancy improved quality of fit to a single site binding model. Furthermore, ICA-derived estimates of the regional fraction of [¹¹C]-(+)-PHNO binding related to D₃ receptors was generated for each subject and values showed good agreement with region-based model estimates and prior literature values. In summary, ICA successfully separated D₂-and D₃-related components of the [¹¹C]-(+)-PHNO binding signal, establishing this approach as a powerful data-driven method to quantify distinct biological features from PET data composed of mixed data sources.

Synaptic Changes in Parkinson Disease Assessed with in vivo Imaging

OBJECTIVE

Parkinson disease is characterized by motor and nonmotor symptoms, reduced striatal dopamine signaling, and loss of dopamine neurons in the substantia nigra. It is now known that the pathological process in Parkinson disease may begin decades before the clinical diagnosis and include a variety of neuronal alterations in addition to the dopamine system.

METHODS

This study examined the density of all synapses with synaptic vesicle glycoprotein 2A (SV2A) in Parkinson disease subjects with mild bilateral disease (n = 12) and matched normal controls (n = 12) using in vivo high-resolution positron emission tomographic imaging as well as postmortem autoradiography in an independent sample with Parkinson disease (n = 15) and normal controls (n = 13) in the substantia nigra and putamen.

RESULTS

A group-by-brain region interaction effect (F10, 22 = 3.52, p = 0.007) was observed in the primary brain areas with in vivo SV2A binding. Post hoc analyses revealed that the Parkinson disease group exhibited lower SV2A in the substantia nigra (-45%; p < 0.001), red nucleus (-31%; p = 0.03), and locus coeruleus (-17%; p = 0.03). Exploratory analyses also revealed lower SV2A binding in clinically relevant cortical areas. Using autoradiography, we confirmed lower SV2A in the substantia nigra (-17%; p < 0.005) and nonsignificant findings in the putamen (-4%; p = 0.06).

INTERPRETATION

This work provides the first evidence of synaptic loss in brainstem nuclei involved in the pathogenesis of Parkinson disease in living patients. SV2A imaging holds promise for understanding synaptic changes central to the disease. Ann Neurol 2020;87:329-338.

Is There a Role for GPCR Agonist Radiotracers in PET Neuroimaging?

Positron emission tomography (PET) is a molecular imaging modality that enables in vivo exploration of metabolic processes and especially the pharmacology of neuroreceptors. G protein-coupled receptors (GPCRs) play an important role in numerous pathophysiologic disorders of the central nervous system. Thus, they are targets of choice in PET imaging to bring proof concept of change in density in pathological conditions or in pharmacological challenge. At present, most radiotracers are antagonist ligands. In vitro data suggest that properties differ between GPCR agonists and antagonists: antagonists bind to receptors with a single affinity, whereas agonists are characterized by two different affinities: high affinity for receptors that undergo functional coupling to G-proteins, and low affinity for those that are not coupled. In this context, agonist radiotracers may be useful tools to give functional images of GPCRs in the brain, with high sensitivity to neurotransmitter release. Here, we review all existing PET radiotracers used from animals to humans and their role for understanding the ligand-receptor paradigm of GPCR in comparison with corresponding antagonist radiotracers.

Social status and demographic effects of the kappa opioid receptor: a PET imaging study with a novel agonist radiotracer in healthy volunteers

Kappa opioid receptors (KORs) have been characterized as an aversive system in the brain and implicated in social behavior in preclinical models. This work investigated the effect of social status on the KOR system in humans using positron emission tomography (PET) imaging with the novel KOR agonist radiotracer [¹¹C]EKAP. Eighteen healthy participants (mean age 41.2 ± 9.3) completed the Barratt Simplified Measure of Social Status (BSMSS), an MRI and an [¹¹C]EKAP PET scan on the High Resolution Research Tomograph. Arterial blood sampling and metabolite analysis were conducted to obtain the input function. Regions of interest were based upon an MR template and included the reward/aversion areas of the brain. The multilinear analysis-1 (MA1) method was applied to the regional time-activity curves (TACs) to calculate [¹¹C]EKAP regional volume of distribution (V_T). Mixed models and Pearson correlation coefficients were used for body mass index (BMI), gender and age, with age being dropped in subsequent analyses because of nonsignificance. An overall effect of primary ROIs (F_7, 112 7.43, p < 0.0001), BSMSS score (F_1, 13 7.45, p = 0.02), BMI (F_1, 13 23.5, p < 0.001), and gender (F_1, 13 23.75, p < 0.001), but not age (F_1, 13 1.12, p = 0.35) was observed. Regional [¹¹C]EKAP V_T and BSMSS were found to be negatively correlated in the amygdala (r = -0.69, p < 0.01), anterior cingulate cortex (r = -0.56, p = 0.02), caudate (r = -0.66, p < 0.01), frontal cortex (r = -0.52, p = 0.04), hippocampus (r = -0.60, p = 0.01), pallidum (r = -0.59, p = 0.02), putamen (r = -0.62, p = 0.01), and ventral striatum (r = -0.66, p < 0.01). In secondary (non-reward) regions, correlations of [¹¹C]EKAP V_T and BSMSS were nonsignificant with the exception of the insula. There was an inverse correlation between social status and KOR levels that was largely specific to the reward/aversion (e.g., saliency) areas of the brain. This finding suggests the KOR system may act as a mediator for the negative effects of social behaviors in humans.

Effects of stimulant drug use on the dopaminergic system: A systematic review and meta-analysis of in vivo neuroimaging studies

BACKGROUND

Stimulant drugs can cause persistent changes in the brain. Imaging studies show that these changes are most apparent in dopamine transporter (DAT) or receptor availability within the striatum.

METHODS

This work focuses on influences of stimulant use on dopaminergic function assessed using nuclear-medicine imaging (PET/SPECT). Included are 39 studies on 655 cocaine, amphetamine, methamphetamine or nicotine users, as well as 690 healthy controls. Metaanalyses were conducted separately for D2/D3 receptors and dopamine transporters of the entire striatum, its subregions caudate and putamen respectively.

RESULTS

Meta-analyses results regarding nicotine did not show significant effects between smokers and nonsmokers. In cocaine users there was a significant decrease in dopamine receptor availability in all regions. The striatal DAT availability was significantly increased in cocaine users. Methamphetamine users showed a significantly decreased dopamine receptor and transporter density in all regions. Significant results also indicate a lower transporter availability in all regions. Amphetamine users showed reduced DAT availability in the striatum, as well as in the sub regions.

CONCLUSION

This meta-analysis provides evidence that there are ongoing changes in the dopaminergic system associated with the use of stimulants. Especially the results of cocaine, methamphetamine and amphetamine use mainly showed a downregulation. In addition, this meta-analysis is the first to include nicotine. This subset of studies showed evidence for a decreased receptor and DAT availability but no significant results were found in the metaanalyses.

Brain neuroreceptor density and personality traits: towards dimensional biomarkers for psychiatric disorders

Positron emission tomography has, for 30 years, been used in numerous case-control studies searching for hypothesized differences in the density of neuroreceptor or transporter proteins in psychiatric disorders such as schizophrenia and depression. In most cases, the results have not been conclusive. One reason could be the sizeable interindividual variability in biochemical markers, which in twin studies have shown to emanate from both environmental and genetic factors, leading to low statistical power for the detection of group effects. On the other hand, the same interindividual variability has served as an opportunity for correlative studies on the biological underpinning of behaviour. Using this approach, a series of studies has linked markers for the dopamine and serotonin system to personality traits associated with psychiatric conditions. Based on increasing evidence for the view that many psychopathological states represent extremes of a continuum rather than distinct categories, this research strategy may lead to new biological insights about the vulnerability to and pathophysiology of major psychiatric disorders.This article is part of the theme issue 'Diverse perspectives on diversity: multi-disciplinary approaches to taxonomies of individual differences'.

Neural and Endocrine Correlates of Early Life Abuse in Youth With Depression and Obesity

Depression and insulin resistance are becoming increasingly prevalent in younger populations. The origin and consequence of insulin resistance in depressed youth may, in part, be rooted in exposure to environmental stressors, such as early life abuse, that may lead to aberrant brain motivational networks mediating maladaptive food-seeking behaviors and insipient insulin resistance. In this paper, we aimed to investigate the impact of early life abuse on the development of insulin resistance in depressed and overweight youth aged 9 to 17 years. We hypothesized that youth with the greatest burden of early life abuse would have the highest levels of insulin resistance and corresponding aberrant reward network connectivities. To test this hypothesis, we evaluated sixty-nine depressed and overweight youth aged 9 to 17, using multimodal assessments of early life abuse, food-seeking behavior, and insulin resistance. Based on results of the Childhood Trauma Questionnaire (CTQ), we separated our study participants into two groups: 35 youth who reported high levels of the sum of emotional, physical, or sexual abuse and 34 youth who reported insignificant or no levels of any abuse. Results of an oral glucose tolerance test (OGTT) and resting state functional connectivity (RSFC), using the amygdala, insula, and nucleus accumbens (NAcc) as seed-based reward network regions of interest, were analyzed for group differences between high abuse and low abuse groups. High abuse youth exhibited differences from low abuse youth in amygdala-precuneus, NAcc-paracingulate gyrus, and NAcc-prefrontal cortex connectivities, that correlated with levels of abuse experienced. The more different their connectivity from of that of low abuse youth, the higher were their fasting glucose and glucose at OGTT endpoint. Importantly, level of abuse moderated the relation between reward network connectivity and OGTT glucose response. In contrast, low abuse youth showed hyperinsulinemia and more insulin resistance than high abuse youth, and their higher OGTT insulin areas under the curve correlated with more negative insula-precuneus connectivity. Our findings suggest distinct neural and endocrine profiles of youth with depression and obesity based on their histories of early life abuse.

Use of carbon-11 labelled tool compounds in support of drug development

The pharmaceutical industry is facing key challenges to improve return on R&D investment. Positron emission tomography (PET), by itself or in combination with complementary technologies such as magnetic resonance imaging (MRI), provides a unique opportunity to confirm a candidate's ability to meet the so-called 'three pillars' of drug development. Positive confirmation provides confidence for go/no-go decision making at an early stage of the development process and enables informed clinical progression. Whereas fluorine-18 has probably gained wider use in the community, there are benefits to using carbon-11 given the greater flexibility the use of this isotope permits in adaptive clinical study design. This review explores the scope of available carbon-11 chemistries and provides clinical examples to highlight its value in PET studies in support of drug development.

Investigating the effects of norepinephrine α1 receptor blockade on dopamine levels: A pilot PET study with [11 C]-(+)-PHNO in controls

Interest in a role for norepinephrine (NE) in substance use disorders has increased over recent years. In particular, its interaction with dopamine (DA) is of importance. In this study, positron emission tomography (PET) was used to explore the impact of prazosin (an alpha 1 NE antagonist) on DA levels. Healthy volunteers were administered prazosin for approximately 4 weeks at the daily dose of 15 mg to reach steady state. Participants were scanned with PET imaging and the [¹¹ C]-(+)-PHNO tracer at baseline (before prazosin), at steady state, and after a wash out period. Prazosin administration was associated with an increase of [¹¹ C]-(+)-PHNO binding potential in the dorsal caudate relative to baseline, which corresponds to a decrease in DA levels. This study is the first to demonstrate interactions between DA and NE in healthy humans.

Regional and source-based patterns of [11C]-(+)-PHNO binding potential reveal concurrent alterations in dopamine D2 and D3 receptor availability in cocaine-use disorder

Dopamine type 2 and type 3 receptors (D₂R/D₃R) appear critical to addictive disorders. Cocaine-use disorder (CUD) is associated with lower D₂R availability and greater D₃R availability in regions primarily expressing D₂R or D₃R concentrations, respectively. However, these CUD-related alterations in D₂R and D₃R have not been concurrently detected using available dopaminergic radioligands. Furthermore, receptor availability in regions of mixed D₂R/D₃R concentration in CUD remains unclear. The current study aimed to extend investigations of CUD-related alterations in D₂R and D₃R availability using regional and source-based analyses of [¹¹C]-(+)-PHNO positron emission tomography (PET) of 26 individuals with CUD and 26 matched healthy comparison (HC) participants. Regional analysis detected greater binding potential (BP_ND) in CUD in the midbrain, consistent with prior [¹¹C]-(+)-PHNO research, and lower BP_ND in CUD in the dorsal striatum, consistent with research using non-selective D₂R/D₃R radiotracers. Exploratory independent component analysis (ICA) identified three sources of BP_ND (striatopallidal, pallidonigral, and mesoaccumbens sources) that represent systems of brain regions displaying coherent variation in receptor availability. The striatopallidal source was associated with estimates of regional D₂R-related proportions of BP_ND (calculated using independent reports of [¹¹C]-(+)-PHNO receptor binding fractions), was lower in intensity in CUD and negatively associated with years of cocaine use. By comparison, the pallidonigral source was associated with estimates of regional D₃R distribution, was greater in intensity in CUD and positively associated with years of cocaine use. The current study extends previous D₂R/D₃R research in CUD, demonstrating both lower BP_ND in the D₂R-rich dorsal striatum and greater BP_ND in the D₃R-rich midbrain using a single radiotracer. In addition, exploratory ICA identified sources of [¹¹C]-(+)-PHNO BP_ND that were correlated with regional estimates of D₂R-related and D₃R-related proportions of BP_ND, were consistent with regional differences in CUD, and suggest receptor alterations in CUD may also be present in regions of mixed D₂R/D₃R concentration.

Histidine Decarboxylase Knockout Mice as a Model of the Pathophysiology of Tourette Syndrome and Related Conditions

While the normal functions of histamine (HA) in the central nervous system have gradually come into focus over the past 30 years, the relationship of abnormalities in neurotransmitter HA to human disease has been slower to emerge. New insight came with the 2010 description of a rare nonsense mutation in the biosynthetic enzyme histidine decarboxylase (Hdc) that was associated with Tourette syndrome (TS) and related conditions in a single family pedigree. Subsequent genetic work has provided further support for abnormalities of HA signaling in sporadic TS. As a result of this genetic work, Hdc knockout mice, which were generated more than 15 years ago, have been reexamined as a model of the pathophysiology of TS and related conditions. Parallel work in these KO mice and in human carriers of the Hdc mutation has revealed abnormalities in the basal ganglia system and its modulation by dopamine (DA) and has confirmed the etiologic, face, and predictive validity of the model. The Hdc-KO model thus serves as a unique platform to probe the pathophysiology of TS and related conditions, and to generate specific hypotheses for subsequent testing in humans. This chapter summarizes the development and validation of this model and recent and ongoing work using it to further investigate pathophysiological changes that may contribute to these disorders.

Elevated Dopamine D2/3 Receptor Availability in Obese Individuals: A PET Imaging Study with [11C](+)PHNO

Most prior work with positron emission tomography (PET) dopamine subtype 2/3 receptor (D_2/3R) non-selective antagonist tracers suggests that obese (OB) individuals exhibit lower D_2/3Rs when compared with normal weight (NW) individuals. A D₃-preferring D_2/3R agonist tracer, [¹¹C](+)PHNO, has demonstrated that body mass index (BMI) was positively associated with D_2/3R availability within striatal reward regions. To date, OB individuals have not been studied with [¹¹C](+)PHNO. We assessed D_2/3R availability in striatal and extrastriatal reward regions in 14 OB and 14 age- and gender-matched NW individuals with [¹¹C](+)PHNO PET utilizing a high-resolution research tomograph. Additionally, in regions where group D_2/3R differences were observed, secondary analyses of 42 individuals that constituted an overweight cohort was done to study the linear association between BMI and D_2/3R availability in those respective regions. A group-by-brain region interaction effect (F_{7, 182}=2.08, p=0.047) was observed. Post hoc analyses revealed that OB individuals exhibited higher tracer binding in D₃-rich regions: the substantia nigra/ventral tegmental area (SN/VTA) (+20%; p=0.02), ventral striatum (VST) (+14%; p<0.01), and pallidum (+11%; p=0.02). BMI was also positively associated with D_2/3R availability in the SN/VTA (r=0.34, p=0.03), VST (r=0.36, p=0.02), and pallidum (r=0.30, p=0.05) across all subjects. These data suggest that individuals who are obese have higher D_2/3R availability in brain reward regions densely populated with D₃Rs, potentially identifying a novel pharmacologic target for the treatment of obesity.

Exploring the relationship between social attachment and dopamine D2/3 receptor availability in the brains of healthy humans using [11C]-(+)-PHNO

Differences in striatal dopamine (DA) function may be related to differences in the degree of social attachment to others. Using positron emission tomography (PET), socially detached persons demonstrate reduced DA D_2/3 receptor (D_2/3R) availability in the striatum. However, previous PET studies have only used antagonist radiotracers for D_2/3R and have not specifically examined regions of interest (ROIs) such as the ventral striatum (VS). In 32 healthy persons, we investigated the relationship between self-reported attachment and DA D_2/3R availability in striatal and extrastriatal ROIs as measured using the agonist radiotracer [¹¹C]-(+)-PHNO. Surprisingly, more social attachment-as measured by the attachment subscale of the temperament and character inventory-was related to less [¹¹C]-(+)-PHNO binding in the VS (r(30) = -.43, p = .01). This relationship held in a subsample who also completed the detachment subscale of the Karolinska Scales of Personality (r(10) = .62, p = .03). However, no relationships were observed with BP_ND in the dorsal striatum or D₃R-specific ROIs. One potential explanation for these findings is that persons who are more socially detached have less endogenous DA occupying D_2/3R in the VS. This interpretation warrants investigation by future research. These findings may help us better understand the neurochemical basis of attachment.

Age-related changes in binding of the D2/3 receptor radioligand [(11)C](+)PHNO in healthy volunteers

OBJECTIVE

Previous imaging studies with positron emission tomography (PET) have reliably demonstrated an age-associated decline in the dopamine system. Most of these studies have focused on the densities of dopamine receptor subtypes D2/3R (D2R family) in the striatum using antagonist radiotracers that are largely nonselective for D2R vs. D3R subtypes. Therefore, less is known about any possible age effects in D3-rich extrastriatal areas such as the substantia nigra/ventral tegmental area (SN/VTA) and hypothalamus. This study sought to investigate whether the receptor availability measured with [(11)C](+)PHNO, a D3R-preferring agonist radiotracer, also declines with age.

METHODS

Forty-two healthy control subjects (9 females, 33 males; age range 19-55 years) were scanned with [(11)C](+)PHNO using a High Resolution Research Tomograph (HRRT). Parametric images were computed using the simplified reference tissue model (SRTM2) with cerebellum as the reference region. Binding potentials (BPND) were calculated for the amygdala, caudate, hypothalamus, pallidum, putamen, SN/VTA, thalamus, and ventral striatum and then confirmed at the voxel level with whole-brain parametric images.

RESULTS

Regional [(11)C](+)PHNO BPND displayed a negative correlation between receptor availability and age in the caudate (r=-0.56, corrected p=0.0008) and putamen (r=-0.45, corrected p=0.02) in healthy subjects (respectively 8% and 5% lower per decade). No significant correlations with age were found between age and other regions (including the hypothalamus and SN/VTA). Secondary whole-brain voxel-wise analysis confirmed these ROI findings of negative associations and further identified a positive correlation in midbrain (SN/VTA) regions.

CONCLUSION

In accordance with previous studies, the striatum (an area rich in D2R) is associated with age-related declines of the dopamine system. We did not initially find evidence of changes with age in the SN/VTA and hypothalamus, areas previously found to have a predominantly D3R signal as measured with [(11)C](+)PHNO. A secondary analysis did find a significant positive correlation in midbrain (SN/VTA) regions, indicating that there may be differential effects of aging, whereby D2R receptor availability decreases with age while D3R availability stays unchanged or is increased.

Socioeconomic status is associated with striatal dopamine D2/D3 receptors in healthy volunteers but not in cocaine abusers

Positron emission tomography (PET) studies in animals and humans have shown that social status is associated with striatal dopamine D2/D3 receptor (D2/D3R) availability. That is, higher social hierarchy and higher scores on questionnaires assessing social status correlated positively with striatal D2/D3R availability in animals and humans respectively. Furthermore, subordinate monkeys were vulnerable to cocaine self-administration, suggesting that alternations in social hierarchy can change D2/D3R availability and vulnerability to cocaine use. Here, we investigated whether socioeconomic status (SES) measured with the Hollingshead scale is associated with striatal D2D/3R availability using [(11)C]raclopride PET in 38 cocaine abusers and 42 healthy controls matched for age and education. Compared to controls, cocaine abusers showed lower D2/D3R availability in the caudate, putamen and ventral striatum (all p≤0.001). Despite matching groups for education, SES scores were lower in cocaine abusers than controls (p<0.001). In the control group only, SES scores significantly correlated with D2/D3R in caudate (r=0.35, p=0.024) and putamen (r=0.39, p=0.011) but not in ventral striatum (p=0.61); all corrected for age. The study confirms that SES is associated with striatal D2/D3R availability in healthy human volunteers. However, reductions in D2/D3R availability in cocaine abusers may be driven by factors other than SES such as chronic cocaine exposure.