Dopamine D3 receptor binding of (18)F-fallypride: Evaluation using in vitro and in vivo PET imaging studies

Dopamine D3 Receptor in Parkinson Disease: A Prognosis Biomarker and an Intervention Target

Parkinson disease (PD) dementia, pathologically featured as nigrostriatal dopamine (DA) neuronal loss with motor and non-motor manifestations, leads to substantial disability and economic burden. DA therapy targets the DA D3 receptor (D3R) with high affinity and selectivity. The pathological involvement of D3R is evidenced as an effective biomarker for disease progression and DA agnostic interventions, with compensations of increased DA, decreased aggregates of α-synuclein (α-Syn), enhanced secretion of brain-derived neurotrophic factors (BDNF), attenuation of neuroinflammation and oxidative damage, and promoting neurogenesis in the brain. D3R also interacts with D1R to reduce PD-associated motor symptoms and alleviate the side effects of levodopa (L-DOPA) treatment. We recently found that DA D2 receptor (D2R) density decreases in the late-stage PDs, while high D3R or DA D1 receptor (D1R) + D3R densities in the postmortem PD brains correlate with survival advantages. These new essential findings warrant renewed investigations into the understanding of D3R neuron populations and their cross-sectional and longitudinal regulations in PD progression.

Design and Synthesis of Conformationally Flexible Scaffold as Bitopic Ligands for Potent D3-Selective Antagonists

Previous studies have confirmed that the binding of D3 receptor antagonists is competitively inhibited by endogenous dopamine despite excellent binding affinity for D3 receptors. This result urges the development of an alternative scaffold that is capable of competing with dopamine for binding to the D3 receptor. Herein, an SAR study was conducted on metoclopramide that incorporated a flexible scaffold for interaction with the secondary binding site of the D3 receptor. The alteration of benzamide substituents and secondary binding fragments with aryl carboxamides resulted in excellent D3 receptor affinities (Ki = 0.8-13.2 nM) with subtype selectivity to the D2 receptor ranging from 22- to 180-fold. The β-arrestin recruitment assay revealed that 21c with 4-(pyridine-4-yl)benzamide can compete well against dopamine with the highest potency (IC50 = 1.3 nM). Computational studies demonstrated that the high potency of 21c and its analogs was the result of interactions with the secondary binding site of the D3 receptor. These compounds also displayed minimal effects for other GPCRs except moderate affinity for 5-HT3 receptors and TSPO. The results of this study revealed that a new class of selective D3 receptor antagonists should be useful in behavioral pharmacology studies and as lead compounds for PET radiotracer development.

Exercise against cocaine sensitization in mice: a [18F]fallypride micro-PET study

Wheel-running exercise in laboratory rodents (animal model useful to study the neurobiology of aerobic exercise) decreases behavioural markers of vulnerability to addictive properties of various drugs of abuse including cocaine. However, neurobiological mechanisms underpinning this protective effect are far from fully characterized. Here, 28-day-old female C57BL/6J mice were housed with (n = 48) or without (n = 48) a running wheel for 6 weeks before being tested for acute locomotor responsiveness and initiation of locomotor sensitization to intraperitoneal injections of 8 mg/kg cocaine. The long-term expression of sensitization took place 3 weeks after the last session. On the day after, all mice underwent a micro-PET imaging session with [18F]fallypride radiotracer (dopamine 2/3 receptors antagonist). Exercised mice were less sensitive to acute and sensitized cocaine hyperlocomotor effects, such attenuation being particularly well marked for long-term expression of sensitization (η2P = 0.262). Chronic administration of cocaine was associated with a clear-cut increase of [18F]fallypride binding potential in mouse striatum (η2P = 0.170) while wheel-running exercise was associated with a moderate decrease in dopamine 2/3 receptors density in striatum (η2P = 0.075), a mechanism that might contribute to protective properties of exercise against drugs of abuse vulnerability.

[ 18 F]FDG PET/CT Studies in Transgenic Hualpha-Syn (A53T) Parkinson's Disease Mouse Model of α-Synucleinopathy

Transgenic mice line M83 that express the A53T mutant α-synuclein protein at six times the level of endogenous mice α-synuclein are a model of α-synucleinopathy found in Parkinson's disease (PD). This Hualpha-Syn (A53T) PD model is useful in assessing non-motor deficits at earlier stages of onset of PD. We report findings on metabolic changes using [18F]FDG PET/CT in the Hualpha-Syn (A53T) PD mouse model in comparison to non-carrier mice. Whole-body PET/CT imaging of male and female mice were carried out 2 h after [18F]FDG ip administration under 3% isoflurane anesthesia. Brain images were analyzed with PET images coregistered to a mouse brain MRI template. Hualpha-Syn (A53T) mice had significantly lower [18F]FDG uptake in several brain regions compared to the no-carrier mice. Significant hind limb muscle and lower spinal cord [18F]FDG hypometabolism at 9 months of age in A53T PD mice was also indicative of neurodegenerative disease, with a progressive motoric dysfunction leading to death. Significant decrease (up to 30%) in [18F]FDG uptake were observed in 9-month old male and female Hualpha-Syn (A53) mice. This is consistent with the cortical hypometabolism in PD patients. Hualpha-Syn (A53) mice may thus be a suitable model for studies related to PD α-synucleinopathy for the discovery of new biomarkers.

Dopamine receptor density and white mater integrity: 18F-fallypride positron emission tomography and diffusion tensor imaging study in healthy and schizophrenia subjects

Dopaminergic dysfunction and changes in white matter integrity are among the most replicated findings in schizophrenia. A modulating role of dopamine in myelin formation has been proposed in animal models and healthy human brain, but has not yet been systematically explored in schizophrenia. We used diffusion tensor imaging and ¹⁸F-fallypride positron emission tomography in 19 healthy and 25 schizophrenia subjects to assess the relationship between gray matter dopamine D₂/D₃ receptor density and white matter fractional anisotropy in each diagnostic group. AFNI regions of interest were acquired for 42 cortical Brodmann areas and subcortical gray matter structures as well as stereotaxically placed in representative white matter areas implicated in schizophrenia neuroimaging literature. Welch's t-test with permutation-based p value adjustment was used to compare means of z-transformed correlations between fractional anisotropy and ¹⁸F-fallypride binding potentials in hypothesis-driven regions of interest in the diagnostic groups. Healthy subjects displayed an extensive pattern of predominantly negative correlations between ¹⁸F-fallypride binding across a range of cortical and subcortical gray matter regions and fractional anisotropy in rostral white matter regions (internal capsule, frontal lobe, anterior corpus callosum). These patterns were disrupted in subjects with schizophrenia, who displayed significantly weaker overall correlations as well as comparatively scant numbers of significant correlations with the internal capsule and frontal (but not temporal) white matter, especially for dopamine receptor density in thalamic nuclei. Dopamine D₂/D₃ receptor density and white matter integrity appear to be interrelated, and their decreases in schizophrenia may stem from hyperdopaminergia with dysregulation of dopaminergic impact on axonal myelination.

Quantitative Rodent Brain Receptor Imaging

Positron emission tomography (PET) is a non-invasive imaging technology employed to describe metabolic, physiological, and biochemical processes in vivo. These include receptor availability, metabolic changes, neurotransmitter release, and alterations of gene expression in the brain. Since the introduction of dedicated small-animal PET systems along with the development of many novel PET imaging probes, the number of PET studies using rats and mice in basic biomedical research tremendously increased over the last decade. This article reviews challenges and advances of quantitative rodent brain imaging to make the readers aware of its physical limitations, as well as to inspire them for its potential applications in preclinical research. In the first section, we briefly discuss the limitations of small-animal PET systems in terms of spatial resolution and sensitivity and point to possible improvements in detector development. In addition, different acquisition and post-processing methods used in rodent PET studies are summarized. We further discuss factors influencing the test-retest variability in small-animal PET studies, e.g., different receptor quantification methodologies which have been mainly translated from human to rodent receptor studies to determine the binding potential and changes of receptor availability and radioligand affinity. We further review different kinetic modeling approaches to obtain quantitative binding data in rodents and PET studies focusing on the quantification of endogenous neurotransmitter release using pharmacological interventions. While several studies have focused on the dopamine system due to the availability of several PET tracers which are sensitive to dopamine release, other neurotransmitter systems have become more and more into focus and are described in this review, as well. We further provide an overview of latest genome engineering technologies, including the CRISPR/Cas9 and DREADD systems that may advance our understanding of brain disorders and function and how imaging has been successfully applied to animal models of human brain disorders. Finally, we review the strengths and opportunities of simultaneous PET/magnetic resonance imaging systems to study drug-receptor interactions and challenges for the translation of PET results from bench to bedside.

Positive association between cerebral grey matter metabolism and dopamine D2/D3 receptor availability in healthy and schizophrenia subjects: An 18F-fluorodeoxyglucose and 18F-fallypride positron emission tomography study

Objectives: Overlapping decreases in extrastriatal dopamine D₂/D₃-receptor availability and glucose metabolism have been reported in subjects with schizophrenia. It remains unknown whether these findings are physiologically related or coincidental.Methods: To ascertain this, we used two consecutive ¹⁸F-fluorodeoxyglucose and ¹⁸F-fallypride positron emission tomography scans in 19 healthy and 25 unmedicated schizophrenia subjects. Matrices of correlations between ¹⁸F-fluorodeoxyglucose uptake and ¹⁸F-fallypride binding in voxels at the same xyz location and AFNI-generated regions of interest were evaluated in both diagnostic groups.Results:¹⁸F-fluorodeoxyglucose uptake and ¹⁸F-fallypride binding potential were predominantly positively correlated across the striatal and extrastriatal grey matter in both healthy and schizophrenia subjects. In comparison to healthy subjects, significantly weaker correlations in subjects with schizophrenia were confirmed in the right cingulate gyrus and thalamus, including the mediodorsal, lateral dorsal, anterior, and midline nuclei. Schizophrenia subjects showed decreased D₂/D₃-receptor availability in the hypothalamus, mamillary bodies, thalamus and several thalamic nuclei, and increased glucose uptake in three lobules of the cerebellar vermis.Conclusions: Dopaminergic system may be involved in modulation of grey matter metabolism and neurometabolic coupling in both healthy human brain and psychopathology. Hyperdopaminergic state in untreated schizophrenia may at least partly account for the corresponding decreases in grey matter metabolism.

Dopamine D3 receptor: A neglected participant in Parkinson Disease pathogenesis and treatment?

Parkinson disease (PD) is a neurodegenerative disorder characterized by motor and non-motor symptoms which relentlessly and progressively lead to substantial disability and economic burden. Pathologically, these symptoms follow the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) associated with abnormal α-synuclein (α-Syn) deposition as cytoplasmic inclusions called Lewy bodies in pigmented brainstem nuclei, and in dystrophic neurons in striatal and cortical regions (Lewy neurites). Pharmacotherapy for PD focuses on improving quality of life and primarily targets dopaminergic pathways. Dopamine acts through two families of receptors, dopamine D1-like and dopamine D2-like; dopamine D3 receptors (D3R) belong to dopamine D2 receptor (D2R) family. Although D3R's precise role in the pathophysiology and treatment of PD has not been determined, we present evidence suggesting an important role for D3R in the early development and occurrence of PD. Agonist activation of D3R increases dopamine concentration, decreases α-Syn accumulation, enhances secretion of brain derived neurotrophic factors (BDNF), ameliorates neuroinflammation, alleviates oxidative stress, promotes neurogenesis in the nigrostriatal pathway, interacts with D1R to reduce PD associated motor symptoms and ameliorates side effects of levodopa (L-DOPA) treatment. Furthermore, D3R mutations can predict PD age of onset and prognosis of PD treatment. The role of D3R in PD merits further research. This review elucidates the potential role of D3R in PD pathogenesis and therapy.

Partial-volume correction increases estimated dopamine D2-like receptor binding potential and reduces adult age differences

The relatively modest spatial resolution of positron emission tomography (PET) increases the likelihood of partial volume effects such that binding potential (BP_ND) may be underestimated. Given structural grey matter losses across adulthood, partial volume effects may be even more problematic in older age leading to overestimation of adult age differences. Here we examined the effects of partial volume correction (PVC) in two studies from different sites using different high-affinity D2-like radioligands (18 F-Fallypride, 11C-FLB457) and different PET camera resolutions (∼5 mm, 2.5 mm). Results across both data sets revealed that PVC increased estimated BP_ND and reduced, though did not eliminate, age effects on BP_ND. As expected, the effects of PVC were smaller in higher compared to lower resolution data. Analyses using uncorrected data that controlled for grey matter volume in each region of interest approximated PVC corrected data for some but not all regions. Overall, the findings suggest that PVC increases estimated BP_ND in general and reduces adult age differences especially when using lower resolution cameras. The findings suggest that the past 30 years of research on dopamine receptor availability, for which very few studies use PVC, may overestimate effects of aging on dopamine receptor availability.

The role of central dopamine and serotonin in human obesity: lessons learned from molecular neuroimaging studies

Obesity results from an imbalance between energy intake and expenditure, and many studies have aimed to determine why obese individuals continue to (over)consume food under conditions of caloric excess. The two major "neurotransmitter hypotheses" of obesity state that increased food intake is partially driven by decreased dopamine-mediated reward and decreased serotonin-mediated homeostatic feedback in response to food intake. Using molecular neuroimaging studies to visualize and quantify aspects of the central dopamine and serotonin systems in vivo, recent PET and SPECT studies have also implicated alterations in these systems in human obesity. The interpretation of these data, however, is more complex than it may appear. Here, we discuss important characteristics and limitations of current radiotracer methods and use this framework to comprehensively review the available human data on central dopamine and serotonin in obesity. On the basis of the available evidence, we conclude that obesity is associated with decreased central dopaminergic and serotonergic signaling and that future research, especially in long-term follow-up and interventional settings, is needed to advance our understanding of the neuronal pathophysiology of obesity in humans.

Synthesis and pre-clinical evaluation of a potential radiotracer for PET imaging of the dopamine D3 receptor

There is considerable interest in using positron emission tomography (PET) imaging to understand the function of dopamine D3 receptors. Due to high sequence homology with D2 receptors, development of D3-selective PET radiotracers has been challenging. In an effort to overcome this issue, we report the radiosynthesis of a new selective D3 ligand with carbon-11 ([11C]1 ), and its initial preclincial evaluation as a potential PET radiotracer for in vivo imaging of D3 receptors. [11C]1 was prepared via [11C]CO2 fixation in 0.1% non-corrected radiochemical yield, good radiochemical purity (>95%) and high specific activity (>2000 Ci mmol-1). [11C]1 exhibited specific binding to D3 receptors using ex vivo autoradiography experiments with rat brain, but only 14-fold selectivity over D2 receptors which is lower than the 1400-fold value reported previously for cell studies. Rodent PET imaging revealed reasonable uptake of the radiotracer in areas of the brain known to be rich in D3 receptors.

Expression of dopamine D2 and D3 receptors in the human retina revealed by positron emission tomography and targeted mass spectrometry

Dopamine D₂ receptors (D₂R) are expressed in the human retina and play an important role in the modulation of neural responses to light-adaptation. However, it is unknown whether dopamine D₃ receptors (D₃R) are expressed in the human retina. Using positron emission tomography (PET), we have observed significant uptake of the D₃R-preferring agonist radiotracer [¹¹C]-(+)-PHNO into the retina of humans in vivo. This led us to examine whether [¹¹C]-(+)-PHNO binding in the retina was quantifiable using reference tissue methods and if D₃R are expressed in human post-mortem retinal tissue. [¹¹C]-(+)-PHNO data from 49 healthy controls (mean age: 39.96 ± 14.36; 16 female) and 12 antipsychotic-naïve patients with schizophrenia (mean age: 25.75 ± 6.25; 4 female) were analyzed. We observed no differences in [¹¹C]-(+)-PHNO binding in the retina between first-episode, drug-naïve patients with schizophrenia and healthy controls. Post-mortem retinal tissues from four healthy persons (mean age: 59.75 ± 9.11; 2 female) and four patients with schizophrenia (mean age: 54 ± 17.11; 2 female) were analyzed using a targeted mass spectrometry technique: parallel reaction monitoring (PRM) analysis. Using targeted mass spectrometry, we confirmed that D₃R are expressed in human retinal tissue ex vivo. Notably, there was far greater expression of D₂R relative to D₃R in the healthy human retina (∼12:1). Moreover, PRM analysis revealed reduced D₂R, but not D₃R, expression in the retinas of non-first episode patients with schizophrenia compared to healthy controls. We confirm that D₃R are expressed in the human retina. Future studies are needed to determine what proportion of the [¹¹C]-(+)-PHNO signal in the human retina in vivo is due to binding to D₃R versus D₂R. Knowledge that both D₂R and D₃R are expressed in the human retina, and potentially quantifiable in vivo using [¹¹C]-(+)-PHNO, poses new research avenues for better understanding the role of retinal dopamine in human vision. This work may have important implications for elucidating pathophysiological and antipsychotic induced visual deficits in schizophrenia.

[18F]fallypride-PET/CT Analysis of the Dopamine D₂/D₃ Receptor in the Hemiparkinsonian Rat Brain Following Intrastriatal Botulinum Neurotoxin A Injection

Intrastriatal injection of botulinum neurotoxin A (BoNT-A) results in improved motor behavior of hemiparkinsonian (hemi-PD) rats, an animal model for Parkinson’s disease. The caudate–putamen (CPu), as the main input nucleus of the basal ganglia loop, is fundamentally involved in motor function and directly interacts with the dopaminergic system. To determine receptor-mediated explanations for the BoNT-A effect, we analyzed the dopamine D₂/D₃ receptor (D₂/D₃R) in the CPu of 6-hydroxydopamine (6-OHDA)-induced hemi-PD rats by [¹⁸F]fallypride-PET/CT scans one, three, and six months post-BoNT-A or -sham-BoNT-A injection. Male Wistar rats were assigned to three different groups: controls, sham-injected hemi-PD rats, and BoNT-A-injected hemi-PD rats. Disease-specific motor impairment was verified by apomorphine and amphetamine rotation testing. Animal-specific magnetic resonance imaging was performed for co-registration and anatomical reference. PET quantification was achieved using PMOD software with the simplified reference tissue model 2. Hemi-PD rats exhibited a constant increase of 23% in D₂/D₃R availability in the CPu, which was almost normalized by intrastriatal application of BoNT-A. Importantly, the BoNT-A effect on striatal D₂/D₃R significantly correlated with behavioral results in the apomorphine rotation test. Our results suggest a therapeutic effect of BoNT-A on the impaired motor behavior of hemi-PD rats by reducing interhemispheric changes of striatal D₂/D₃R.

Low Striatal Dopamine D2-type Receptor Availability is Linked to Simulated Drug Choice in Methamphetamine Users

Individuals with drug use disorders seek drugs over other rewarding activities, and exhibit neurochemical deficits related to dopamine, which is involved in value-based learning and decision-making. Thus, a dopaminergic disturbance may underpin drug-biased choice in addiction. Classical drug-choice assessments, which offer drug-consumption opportunities, are inappropriate for addicted individuals seeking treatment or abstaining. Fifteen recently abstinent methamphetamine users and 15 healthy controls completed two laboratory paradigms of 'simulated' drug choice (choice for drug-related vs affectively pleasant, unpleasant, and neutral images), and underwent positron emission tomography measurements of dopamine D2-type receptor availability, indicated by binding potential (BP_ND) for [¹⁸F]fallypride. Thirteen of the methamphetamine users and 10 controls also underwent [¹¹C]NNC112 PET scans to measure dopamine D1-type receptor availability. Group analyses showed that, compared with controls, methamphetamine users chose to view more methamphetamine-related images on one task, with a similar trend on the second task. Regression analyses showed that, on both tasks, the more methamphetamine users chose to view methamphetamine images, specifically vs pleasant images (the most frequently chosen images across all participants), the lower was their D2-type BP_ND in the lateral orbitofrontal cortex, an important region in value-based choice. No associations were observed with D2-type BP_ND in striatal regions, or with D1-type BP_ND in any region. These results identify a neurochemical correlate for a laboratory drug-seeking paradigm that can be administered to treatment-seeking and abstaining drug-addicted individuals. More broadly, these results refine the central hypothesis that dopamine-system deficits contribute to drug-biased decision-making in addiction, here showing a role for the orbitofrontal cortex.

Reference tissue models in the assessment of 11 C-DTBZ binding to the VMAT2 in rat striatum: A test-retest reproducibility study

Dopaminergic PET imaging is a useful tool to assess the dopaminergic integrity and to follow-up longitudinal studies. The aim of this study was to evaluate the reliability and reproducibility of different reference tissue-based methods to determine the non-displaceable binding potential (BP_ND ) as a quantitative measure of ¹¹ C-DTBZ binding to the VMAT2 in rat striatum using cerebellum as reference region. Eight healthy Wistar rats underwent two microPET scans at the age of 12 (test) and 20 weeks (retest). BP_ND was determined using the simplified reference tissue model, Logan reference tissue model, and multilinear reference tissue models (MRTMo and MRTM2). Additionally, a striatal-to-cerebellar-ratio (SCR) analysis was performed. The reproducibility between the two scans was assessed using the interclass correlation coefficients (ICC) and the variability index. Repeatability indices showed acceptable ICC = 0.66 (SCR) to excellent ICC = 0.98 (MRTM2) reliability for this study and a variability ranging from 12.26% (SCR) to 3.28% (MRTM2). To the best of our knowledge, this is the first report on longitudinal studies for ¹¹ C-DTBZ in rats using reference tissue methods. Excellent intersubject and intrasubject reproducibility was obtained with the multilinear reference MRTM2, suggesting this as the best method to compare longitudinal studies, whereas the SCR method had poor reliability. Logan method, however, is a method simple to compute that shows accurate reproducibility with a reasonable level of inter- and intra-subject variability allowing crossover studies to follow-up the uptake of ¹¹ C-DTBZ in rat striatum.

Performing multi-step chemical reactions in microliter-sized droplets by leveraging a simple passive transport mechanism

Despite the increasing importance of positron emission tomography (PET) imaging in research and clinical management of disease, access to myriad new radioactive tracers is severely limited due to their short half-lives (which requires daily production) and the high cost and complexity of tracer production. The application of droplet microfluidics based on electrowetting-on-dielectric (EWOD) to the field of radiochemistry can significantly reduce the amount of radiation shielding necessary for safety and the amount of precursor and other reagents needed for the synthesis. Furthermore, significant improvements in the molar activity of the tracers have been observed. However, widespread use of this technology is currently hindered in part by the high cost of prototype chips and the operating complexity. To address these issues, we developed a novel microfluidic device based on patterned wettability for multi-step radiochemical reactions in microliter droplets and implemented automated systems for reagent loading and collection of the crude product after synthesis. In this paper, we describe a simple and inexpensive method for fabricating the chips, demonstrate the feasibility of prototype chips for performing multi-step radiochemical reactions to produce the PET tracers [18F]fallypride and [18F]FDG, and further show that synthesized [18F]fallypride can be used for in vivo mouse imaging.

PET radiotracer development for imaging high-affinity state of dopamine D2 and D3 receptors: Binding studies of fluorine-18 labeled aminotetralins in rodents

Imaging the high-affinity, functional state (HA) of dopamine D2 and D3 receptors has been pursued in PET imaging studies of various brain functions. We report further evaluation of ¹⁸ F-5-OH-FPPAT, and the newer ¹⁸ F-5-OH-FHXPAT and ¹⁸ F-7-OH-FHXPAT. Syntheses of ¹⁸ F-5-OH-FHXPAT and ¹⁸ F-7-OH-FHXPAT were improved by modifications of our previously reported procedures. Brain slices and brain homogenates from male Sprague-Dawley rats were used with the 3 radiotracers (74-111 kBq/cc). Competition with dopamine (1-100 nM) and Gpp(NH)p (10-50 µM) were carried out to demonstrate binding to dopamine D2 and D3 HA-states and binding kinetics of ¹⁸ F-5-OH-FPPAT measured. Ex vivo brain slice autoradiography was carried out on rats administered with ¹⁸ F-5-OH-FHXPAT to ascertain HA-state binding. PET/CT imaging in rats and wild type (WT) and D2 knock-out mice were carried out using ¹⁸ F-7-OH-FHXPAT (2-37 MBq). Striatum was clearly visualized by the three radiotracers in brain slices and dopamine displaced more than 80% of binding, with dissociation rate in homogenates of 2.2 × 10^-2 min^-1 for ¹⁸ F-5-OH-FPPAT. Treatment with Gpp(NH)p significantly reduced 50-80% striatal binding with faster dissociation rates (5.0 × 10^-2 min^-1 ), suggesting HA-state binding of ¹⁸ F-5-OH-FPPAT and ¹⁸ F-5-OH-FHXPAT. Striatal binding of ¹⁸ F-5-OH-FHXPAT in ex vivo brain slices were sensitive to Gpp(NH)p, suggesting HA-state binding in vivo. PET binding ratios of ¹⁸ F-7-OH-FHXPAT in rat brain were ventral striatum/cerebellum = 2.09 and dorsal striatum/cerebellum = 1.65; similar binding ratios were found in the D2 WT mice. These results suggest that in vivo PET measures of agonists in the brain at least in part reflect binding to the membrane-bound HA-state of the dopamine receptor.

Sex Differences in Midbrain Dopamine D2-Type Receptor Availability and Association with Nicotine Dependence

Women differ from men in smoking-related behaviors, among them a greater difficulty in quitting smoking. Unlike female smokers, male smokers have lower striatal dopamine D2-type receptor availability (binding potential, BPND) than nonsmokers and exhibit greater smoking-induced striatal dopamine release. Because dopamine D2-type autoreceptors in the midbrain influence striatal dopamine release, a function that has been linked to addiction, we tested for sex differences in midbrain dopamine D2-type receptor BPND and in relationships between midbrain BPND, nicotine dependence and striatal dopamine D2-type receptor BPND. Positron emission tomography was used with [¹⁸F]fallypride to measure BPND in a midbrain region, encompassing the substantia nigra and ventral tegmental area, in 18 daily smokers (7 women, 11 men) and 19 nonsmokers (10 women, 9 men). A significant sex-by-group interaction reflected greater midbrain BPND in female but not male smokers than in corresponding nonsmokers (F_{1, 32}=5.089, p=0.03). Midbrain BPND was positively correlated with BPND in the caudate nucleus and putamen in nonsmokers and female smokers but not in male smokers and with nicotine dependence in female but not in male smokers. Striatal BPND was correlated negatively with nicotine dependence and smoking exposure. These findings extend observations on dopamine D2-type receptors in smokers and suggest a sex difference in how midbrain dopamine D2-type autoreceptors influence nicotine dependence.

Cigarette Use and Striatal Dopamine D2/3 Receptors: Possible Role in the Link between Smoking and Nicotine Dependence

BACKGROUND

Cigarette smoking induces dopamine release in the striatum, and smoking- or nicotine-induced ventral striatal dopamine release is correlated with nicotine dependence. Smokers also exhibit lower dopamine D2/3 receptor availability in the dorsal striatum than nonsmokers. Negative correlations of striatal dopamine D2/3 receptor availability with smoking exposure and nicotine dependence, therefore, might be expected but have not been tested.

METHODS

Twenty smokers had positron emission tomography scans with [¹⁸F]fallypride to measure dopamine D2/3 receptor availability in ventral and dorsal regions of the striatum and provided self-report measures of recent and lifetime smoking and of nicotine dependence.

RESULTS

As reported before, lifetime smoking was correlated with nicotine dependence. New findings were that ventral striatal dopamine D2/3 receptor availability was negatively correlated with recent and lifetime smoking and also with nicotine dependence.

CONCLUSION

The results suggest an effect of smoking on ventral striatal D2/3 dopamine receptors that may contribute to nicotine dependence.

In Vitro and In Vivo Characterization of Selected Fluorine-18 Labeled Radioligands for PET Imaging of the Dopamine D3 Receptor

Cerebral dopamine D3 receptors seem to play a key role in the control of drug-seeking behavior. The imaging of their regional density with positron emission tomography (PET) could thus help in the exploration of the molecular basis of drug addiction. A fluorine-18 labeled D3 subtype selective radioligand would be beneficial for this purpose; however, as yet, there is no such tracer available. The three candidates [¹⁸F]1, [¹⁸F]2a and [¹⁸F]2b were chosen for in vitro and in vivo characterization as radioligands suitable for selective PET imaging of the D3 receptor. Their evaluation included the analysis of radiometabolites and the assessment of non-specific binding by in vitro rat brain autoradiography. While [¹⁸F]1 and [¹⁸F]2a revealed high non-specific uptake in in vitro rat brain autoradiography, the D3 receptor density was successfully determined on rat brain sections (n = 4) with the candidate [¹⁸F]2b offering a B_max of 20.38 ± 2.67 pmol/g for the islands of Calleja, 19.54 ± 1.85 pmol/g for the nucleus accumbens and 16.58 ± 1.63 pmol/g for the caudate putamen. In PET imaging studies, the carboxamide 1 revealed low signal/background ratios in the rat brain and relatively low uptake in the pituitary gland, while the azocarboxamides [¹⁸F]2a and [¹⁸F]2b showed binding that was blockable by the D3 receptor ligand BP897 in the ventricular system and the pituitary gland in PET imaging studies in living rats.

Occupancy of pramipexole (Sifrol) at cerebral dopamine D2/3 receptors in Parkinson's disease patients

Whereas positron emission tomography (PET) with the antagonist ligand [¹⁸F]fallypride reveals the composite of dopamine D2 and D3 receptors in brain, treatment of Parkinson's disease (PD) patients with the D3-prefering agonist pramipexole should result in preferential occupancy in the nucleus accumbens, where the D3-subtype is most abundant. To test this prediction we obtained pairs of [¹⁸F]fallypride PET recordings in a group of nine PD patients, first in a condition of treatment as usual with pramipexole (ON-Sifrol; 3 × 0.7 mg p.d.), and again at a later date, after withholding pramipexole 48–72 h (OFF-Sifrol); in that condition the serum pramipexole concentration had declined by 90% and prolactin levels had increased four-fold, in conjunction with a small but significant worsening of PD motor symptoms. Exploratory comparison with historical control material showed 14% higher dopamine D2/3 availability in the more-affected putamen of patients OFF medication. On-Sifrol there was significant (p ˂ 0.01) occupancy at [¹⁸F]fallypride binding sites in globus pallidus (8%) thalamus (9%) and substantia nigra (19%), as well as marginally significant occupancy in frontal and temporal cortex of patients. Contrary to expectation, comparison of ON- and OFF-Sifrol results did not reveal any discernible occupancy in nucleus accumbens, or elsewhere in the extended striatum; present methods should be sensitive to a 10% change in dopamine D2/3 receptor availability in striatum; the significant findings elsewhere in the basal ganglia and in cerebral cortex are consistent with a predominance of D3 receptors in those structures, especially in substantia nigra, and imply that therapeutic effects of pramipexole may be obtained at sites outside the extended striatum.

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Fallypride PET recordings in nine PD patients, scanned on- and off medication with pramipexole

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No occupancy in the striatum, despite improved motor symptoms

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Substantial occupancy in substantia nigra, thalamus and globus pallidus