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The ability to exert control has been widely investigated as a hallmark of adaptive behaviour. Dopamine is recognized as the key neuromodulator mediating various control-related processes. The neural mechanisms underlying the subjective perception of being in control, or Locus of Control (LOC) are however less clear. LOC indicates the subjective tendency to attribute environmental outcomes to one's actions (internal LOC) or instead to external incontrollable factors (external LOC). Here we hypothesized that dopamine levels also relate to LOC. Previous work shows that dopamine signaling mediates learning of action-outcome relationships, outcome predictability, and opportunity cost. Prominent theories propose dopamine dysregulation as the key pathogenetic mechanism in schizophrenia and depression. Critically, external LOC is a risk factor for schizophrenia and depression, and predicts increased vulnerability to stress. However, a direct link between LOC and dopamine levels in healthy control had not been demonstrated. The purpose of our study was to investigate this link. Using [ 11 C]raclopride Positron Emission Tomography we tested the relationship between D2 receptor binding in the striatum and LOC (measured with the Rotter Locus of Control scale) in 15 healthy volunteers. Our results show a large and positive correlation: increased striatal D2 binding was associated with External LOC. This finding opens promising avenues for the study of several psychological impairments that have been associated with both dopamine and LOC, such as addiction, schizophrenia, and depression., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Perceptual changes in shape, size, or color are observed in patients with derealization symptoms; however, the underlying neural and molecular mechanisms are not well understood. The current study explored the relationship between neural activity associated with altered colorfulness perception assessed by fMRI and striatal dopamine D 2 receptor availability measured by [ 11 C]raclopride PET in healthy participants. Inside an fMRI scanner, participants performed the saturation adaptation task, where they rated how much vivid/faded visual objects looked like real/unreal ones using a visual analog scale. We found that participants experienced greater unreality when they perceived fadedness than vividness despite physically identical saturation. The combined fMRI and PET analyses revealed that the faded perception-related activities of the dorsolateral prefrontal and parietal cortex were positively correlated with striatal D 2 receptor availability. This finding may help to understand the neuromolecular mechanisms of faded perception associated with feeling unreal in derealization symptoms.

The changes in the availability of striatal dopamine transporter and dopamine D2 receptor after mild focal ischemia in rats were measured using a small animal positron emission tomography system. Mild focal ischemia was induced by 20-minute middle cerebral artery occlusion. [ 11 C]PE2I binding to dopamine transporter was transiently increased on the ipsilateral side of the striatum at 2 days after middle cerebral artery occlusion. On day 7 and 14 after middle cerebral artery occlusion, [ 11 C]PE2I binding levels were decreased. In contrast, [ 11 C]raclopride binding to dopamine D2 receptor in the ipsilateral striatum had not changed at 2 days after middle cerebral artery occlusion. [ 11 C]Raclopride binding was significantly decreased on the ischemic side of the striatum at 7 and 14 days after middle cerebral artery occlusion. Moreover, on day 1 and 2 after middle cerebral artery occlusion, significant circling behavior to the contralateral direction was induced by amphetamine challenge. This behavior disappeared at 7 days after middle cerebral artery occlusion. At 14 days, circling behavior to the ipsilateral direction (middle cerebral artery occlusion side) was significantly increased, and that to the contralateral direction also appeared again. The present study suggested that amphetamine-induced circling behavior indicated striatal dopaminergic alterations and that dopamine transporter and dopamine D2 receptor binding could be key markers for predicting motor dysfunction after mild focal ischemia.

While several studies have examined how particular personality traits are related to dopamine D2/3 receptor (D2/3R) availability in the striatum of humans, few studies have reported how multiple traits measured in the same persons are differentially related to D2/3R availability in different striatal sub-regions. We examined how personality traits measured with the Karolinska Scales of Personality are related to striatal D2/3R availability measured with [(11)C]-raclopride in 30 healthy humans. Based on previous the literature, five personality traits were hypothesized to be most likely related to D2/3R availability: impulsiveness, monotony avoidance, detachment, social desirability, and socialization. We found self-reported impulsiveness was negatively correlated with D2/3R availability in the ventral striatum and globus pallidus. After controlling for age and gender, monotony avoidance was also negatively correlated with D2/3R availability in the ventral striatum and globus pallidus. Socialization was positively correlated with D2/3R availability in the ventral striatum and putamen. After controlling for age and gender, the relationship between socialization and D2/3R availability in these regions survived correction for multiple comparisons (p-threshold=.003). Thus, within the same persons, different personality traits are differentially related to in vivo D2/3R availability in different striatal sub-regions., (Copyright © 2016 Elsevier B.V. and ECNP. All rights reserved.)

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., (Published by Elsevier Ireland Ltd.)

The neurobiology of anorexia nervosa remains incompletely understood. Here we utilized PET imaging with the radiotracer [(11)C]raclopride to measure striatal dopamine type 2 (D2) receptor availability in patients with anorexia nervosa. 25 women with anorexia nervosa who were receiving treatment in an inpatient program participated, as well as 25 control subjects. Patients were scanned up to two times with the PET tracer [(11)C]raclopride: once while underweight, and once upon weight restoration. Control subjects underwent one PET scan. In the primary analyses, there were no significant differences between underweight patients (n=21) and control subjects (n=25) in striatal D2 receptor binding potential. Analysis of subregions (sensorimotor striatum, associative striatum, limbic striatum) did not reveal differences between groups. In patients completing both scans (n=15), there were no detectable changes in striatal D2 receptor binding potential after weight restoration. In this sample, there were no differences in striatal D2 receptor binding potential between patients with anorexia nervosa and control subjects. Weight restoration was not associated with a change in striatal D2 receptor binding. These findings suggest that disturbances in reward processing in this disorder are not attributable to abnormal D2 receptor characteristics, and that other reward-related neural targets may be of greater relevance., Competing Interests: The other authors report no conflict of interest., (Copyright © 2015 Elsevier Ireland Ltd. All rights reserved.)

We measured the long-term test-retest reliability of [(11)C]raclopride binding in striatal subregions, the thalamus and the cortex using the bolus-plus-infusion method and a high-resolution positron emission scanner. Seven healthy male volunteers underwent two positron emission tomography (PET) [(11)C]raclopride assessments, with a 5-week retest interval. D2/3 receptor availability was quantified as binding potential using the simplified reference tissue model. Absolute variability (VAR) and intraclass correlation coefficient (ICC) values indicated very good reproducibility for the striatum and were 4.5%/0.82, 3.9%/0.83, and 3.9%/0.82, for the caudate nucleus, putamen, and ventral striatum, respectively. Thalamic reliability was also very good, with VAR of 3.7% and ICC of 0.92. Test-retest data for cortical areas showed good to moderate reproducibility (6.1% to 13.1%). Our results are in line with previous test-retest studies of [(11)C]raclopride binding in the striatum. A novel finding is the relatively low variability of [(11)C]raclopride binding, providing suggestive evidence that extrastriatal D2/3 binding can be studied in vivo with [(11)C]raclopride PET to be verified in future studies.

Introduction: Nicotine has long been recognized as a necessary but insufficient component of tobacco cigarettes to maintain a psychophysiological need to smoke. This study examined venous plasma concentrations effects of nicotine in cigarette smoking after overnight abstinence to release striatal dopamine (DA)., Methods: Twenty-two male smokers smoked either denicotinized (denic) or average nicotine (nic) cigarettes under single blind conditions. Each was given [(11)C]raclopride and scanned in a positron emission tomography (PET) facility., Results: Smoking either denic or nic cigarettes released striatal DA. Denic cigarette smoking released DA primarily in the right striatum, whereas nic cigarette smoking released DA in both striata, but especially in the left. Increases in venous plasma nicotine concentrations correlated positively with increased DA release in the left caudate nucleus. Smoking denic cigarettes reduced craving as much as smoking nic cigarettes. Craving reduction after nic tobacco smoking correlated with increases in plasma nicotine., Conclusions: Nonnicotine factors in tobacco smoking produce important right brain effects. Nicotine is a pharmacological factor during tobacco smoking that releases bilateral striatal DA, but more in the left brain.

The acquisition of new motor skills is essential throughout daily life and involves the processes of learning new motor sequence and encoding elementary aspects of new movement. Although previous animal studies have suggested a functional importance for striatal dopamine release in the learning of new motor sequence, its role in encoding elementary aspects of new movement has not yet been investigated. To elucidate this, we investigated changes in striatal dopamine levels during initial skill-training (Day 1) compared with acquired conditions (Day 2) using (11)C-raclopride positron-emission tomography. Ten volunteers learned to perform brisk contractions using their non-dominant left thumbs with the aid of visual feedback. On Day 1, the mean acceleration of each session was improved through repeated training sessions until performance neared asymptotic levels, while improved motor performance was retained from the beginning on Day 2. The (11)C-raclopride binding potential (BP) in the right putamen was reduced during initial skill-training compared with under acquired conditions. Moreover, voxel-wise analysis revealed that (11)C-raclopride BP was particularly reduced in the right antero-dorsal to the lateral part of the putamen. Based on findings from previous fMRI studies that show a gradual shift of activation within the striatum during the initial processing of motor learning, striatal dopamine may play a role in the dynamic cortico-striatal activation during encoding of new motor memory in skill acquisition.

Updating of working memory has been associated with striato-frontal brain regions and phasic dopaminergic neurotransmission. We assessed raclopride binding to striatal dopamine (DA) D2 receptors during a letter-updating task and a control condition before and after 5 weeks of updating training. Results showed that updating affected DA activity before training and that training further increased striatal DA release during updating. These findings highlight the pivotal role of transient neural processes associated with D2 receptor activity in working memory.

We examined the relationship between antipsychotic-associated mental side effects and dopamine D2 receptor occupancy in striatal subdivisions using high-resolution positron emission tomography with [11C]raclopride to better characterize the neurochemical mechanism underlying these adverse effects. Twenty-one patients with schizophrenia receiving stable doses of antipsychotics and 24 age- and sex-matched healthy controls completed 3-Tesla magnetic resonance imaging and high-resolution positron emission tomography scans with [11C]raclopride to measure D2 receptor binding potential (BP ND) in the striatum. The D2 receptor BP ND was obtained using a Logan plot, and receptor occupancy was calculated as the percentage reduction of receptor BP ND with drug treatment relative to baseline. The data obtained from age- and sex-matched healthy controls were used as an estimate of the patients' baseline, as previously proposed. Antipsychotic-associated mental side effects were measured with the Liverpool University Neuroleptic Side Effect Rating Scale. The whole striatal D2 receptor occupancy ranged from 54% to 95%. The analysis revealed that the Liverpool University Neuroleptic Side Effect Rating Scale score had significant positive associations with D2 occupancy in the precommissural dorsal caudate, postcommissural caudate, and ventral striatum. The results suggest that mental side effects of antipsychotics are associated with D2 receptor blockade in the associative and limbic subdivisions of the striatum, which are considered to play a crucial role in cognition and reward motivation.

Our previous study regarding the changes of D(2) receptor in nigrostriatal dopamine system at an early stage (4 weeks after lesion) indicated a different functional activity of striatal D(2) receptor between two different rat parkinsonian models, lesioning with 6-hydroxydopamine in the striatum and in the medial forebrain bundle (MFB). In the present study, we further examined binding of D(2) receptor as well as pre-synaptic dopamine transporter (DAT) at later stages (6 months after lesion) both in the striatal and MFB lesion models. The D(2) receptor binding in MFB model at 6 months after lesion was significantly lower than that at 4 weeks after lesion, albeit it was still higher than the normal side. The D(2) receptor binding in striatal model was decreased to the same extent at both 4 weeks and 6 months after lesion. DAT binding decreased at 6 months after lesion, more profound in MFB model, and the degree of reduction was not different from that at 4 weeks after lesion. These findings indicated different dynamic processes of the D(2) receptor and DAT during a longer time observation in the striatal and MFB lesion models. The dynamic changes of D(2) receptor activity after lesion should be considered when selecting 6-hydroxydopamine-induced rat parkinsonian models., (Copyright © 2011 Elsevier Ireland Ltd. All rights reserved.)

In vitro binding characteristics of the dopamine D₃/D₂ antagonist [³H]raclopride were compared to the D₃/D₂ agonist [³H](+)-PHNO in membrane preparations from rat striatum, cerebellum Lobules 9 and 10 (CB L9,10), and other cerebellar regions. In striatum, both radioligands labeled a single binding site. [³H](+)-PHNO showed higher affinity, though lower B(max) , compared with [³H]raclopride and was sensitive to inhibition by Gpp(NH)p. [³H](+)-PHNO showed significant specific binding to CB L9,10 membranes with higher affinity compared to striatal membranes. [³H](+)-PHNO binds to a high- and a low-affinity binding site in CB L9,10 membranes; the high-affinity site was not Gpp(NH)p-sensitive. [³H](+)-PHNO did not significantly bind cerebellum left hemisphere membranes. Very low specific binding of [³H]raclopride was found in CB L9,10. The selective dopamine D₃ antagonist SB-277011 did not displace the binding of either ligand to striatal membranes but potently inhibited the binding of [³H](+)-PHNO in CB L9,10 membranes. The highly selective D₂ antagonist SV-156 showed the opposite profile. In vivo experiments were consistent with and supported by in vitro results. In summary, [³H](+)-PHNO and [³H]raclopride mainly label dopamine D₂ receptors in rat striatum, with [³H](+)-PHNO labeling a D₂(High) population. In vitro and in vivo, [³H](+)-PHNO labels CB L9,10 dopamine D₃ receptors that are apparently in a high affinity state whereas [³H]raclopride gave only very low signal in this region. The present approaches appear useful for selectively labeling dopamine D₃ and D₂ receptors in different rat brain regions and offer the possibility to demonstrate D₃ versus D₂ receptor selectivity of compounds using native rat brain tissue., (Copyright © 2010 Wiley-Liss, Inc.)

Background: We used a positron emission tomography paradigm with the D2/3 radiotracer [¹¹C]raclopride and an alcohol challenge to examine the magnitude of alcohol-induced dopamine release and compare it between young men and women., Methods: Twenty-one nonalcohol-dependent young social drinkers completed two positron emission tomography scans on separate days following ingestion of a juice mix containing either ethanol (.75 mg/kg body water) or trace ethanol only. The extent of dopamine released after alcohol was estimated by the percentage difference in [¹¹C]raclopride binding potential (ΔBP(ND)) between days., Results: Alcohol administration significantly displaced [¹¹C]raclopride in all striatal subregions, indicating dopamine release, with the largest effect observed in the ventral striatum. Linear mixed model analysis across all striatal subregions of regional ΔBP(ND) with region of interest as repeated measure showed a highly significant effect of sex (p < .001). Ventrostriatal dopamine release in men, but not in women, showed a significant positive correlation to alcohol-induced measures of subjective activation. Furthermore, we found a significant negative correlation between the frequency of maximum alcohol consumption per 24 hours and ventrostriatal ΔBP(ND) (r = .739, p = .009) in men., Conclusions: This study provides definitive evidence that oral alcohol induces dopamine release in nonalcoholic human subjects and shows sex differences in the magnitude of this effect. The ability of alcohol to stimulate dopamine release may contribute to its rewarding effects and, thereby, to its abuse liability in humans. Our report further suggests several biological mechanisms that may mediate the difference in vulnerability for alcoholism between men and women., (Copyright © 2010 Society of Biological Psychiatry. All rights reserved.)

Most molecular imaging studies of the dopamine (DA) system performed to date have focused on the striatum, a region receiving dense dopaminergic innervation. In clinical research on the DA D2-receptor, striatal binding has often been regarded as an index of global DA function, based on the underlying assumption of common regulatory mechanisms for receptor expression across brain regions. Recent data has challenged this view, suggesting differences in genetic regulation between striatal and extrastriatal brain regions. The relationship between binding levels in brain regions has, however, not been directly examined in the same sample. In this study, we searched for interregional correlations between DA D2-receptor availability as determined with Positron Emission Tomography in 16 control subjects. The radioligands [11C]raclopride and [11C]FLB 457 were used for measurements of D2-receptor binding in striatal and extrastriatal regions, respectively. No correlation was observed between D2-receptor availability in striatum and any of the extrastriatal regions, as assessed using both region of interest- and voxel-based analyses. Instead, the pattern of correlations was consistent with the model of separate dopaminergic systems as has been originally observed in rodents. These preliminary results encourage approaches searching for individual patterns of receptor binding across the whole brain volume in clinical studies on the dopamine system.

[(11)C]-(+)-PHNO is a new dopamine D(2/3) receptor agonist radiotracer which has been successfully used to measure D(2/3) receptor availability in experimental animals and man. Here we report in vivo evaluation in the rat of the biodistribution, metabolism, specificity, selectivity, and dopamine sensitivity of carbon11-labeled PHNO ([(11)C]-3-PHNO) produced by an alternative radiochemical synthesis method. [(11)C]-3-PHNO showed rapid metabolism and clearance from most peripheral organs and tissues. [(11)C]-3-PHNO, but not its polar metabolite, readily crossed the blood-brain barrier and showed high levels of uptake in the D(2/3)-rich striatum. Pretreatment with unlabeled PHNO and the D(2/3) receptor antagonist raclopride indicated that binding in the striatum was specific and selective to D(2/3) receptors. PET studies in anesthetized rats revealed significant reductions in [(11)C]-3-PHNO binding in the striatum following amphetamine administration, indicating sensitivity to increases in endogenous dopamine concentrations. D(2/3) antagonist pretreatment additionally indicated moderate levels of [(11)C]-3-PHNO specific binding in several extrastriatal brain areas-most notably the olfactory bulbs and tubercles, thalamus, and hypothalamus. Of particular interest, approximately 30% of [(11)C]-3-PHNO signal in the cerebellum-a region often used as a "low-binding" reference region for PET quantification-was attributable to specific signal. These data demonstrate that [(11)C]-3-PHNO shows similar tracer characteristics to [(11)C]-(+)-PHNO, but additionally indicate that radiolabeled PHNO may be used to estimate D(2/3) receptor availability in select extrastriatal brain regions with PET.

The C957T polymorphism of the human dopamine D2 receptor gene (DRD2) regulates DRD2 availability in striatum in vivo. Specifically, the T allele predicts high DRD2 availability in healthy volunteers (T/T>T/C>C/C). However, this finding was unexpected as in vitro the T allele is associated with a decrease in DRD2 mRNA stability and synthesis of the receptor through a putative alteration in the receptor mRNA folding. To elucidate further how changes in DRD2 density (B(max)) and affinity (K(D)) contribute to the differences in DRD2 availability between the C957T genotypes, we studied these parameters separately in a sample of 45 healthy volunteers. The subjects had two PET scans with [(11)C]raclopride (high and low specific radioactivity scans) for the estimation of B(max) and K(D), and were genotyped for the C957T. Moreover, the role of the related and previously studied functional TaqIA polymorphism of ankyrin repeat and kinase domain containing 1 (ANKK1) gene was reassessed for comparative purposes. The results indicate that the C957T increased binding potential by decreasing DRD2 K(D) (C/C>C/T>T/T), while B(max) was not significantly altered. These preliminary findings indicate that the C957T genotype-dependent changes in DRD2 availability are driven by alterations in receptor affinity and putatively in striatal dopamine levels. This mechanism seems to differ from that observed previously for the ANKK1 gene TaqIA polymorphism, where the minor allele (A1) affects DRD2 availability predominantly by changing B(max). The hypothesis that the two SNPs may have independent effects on dopamine neurotransmission needs to be further tested., (Copyright 2009 Wiley-Liss, Inc.)

Intraindividual variability (IIV) reflects lawful but transient within-person changes in performance. Increased IIV in cognition shares systematic associations with numerous conditions characterized by alterations in dopamine (DA) neuromodulation (e.g., old age, ADHD, schizophrenia, and Parkinson's disease). In a group of normal middle-aged adults, we examined links between PET-derived measures of D2 receptor binding in striatum, orbitofrontal cortex (OFC), anterior cingulate cortex (ACC), and hippocampus (HC) and IIV for tasks assessing recognition memory and executive functioning. An index of IIV, the intraindividual standard deviation (ISD), was computed across successful response latency trials for each cognitive outcome. Lower D2 binding in OC, ACC, and HC, but not striatum, was associated with increasing ISDs for the memory and executive measures. Consistent with neurocomputational models, the present findings suggest a role for extrastriatal DA neurotransmission in modulating variability in cognitive functioning.

Objective: To determine whether changes in D(2) receptor availability are present in carriers of genetic mutations for primary dystonia., Methods: Manifesting and nonmanifesting carriers of the DYT1 and DYT6 dystonia mutations were scanned with [(11)C] raclopride (RAC) and PET. Measures of D(2) receptor availability in the caudate nucleus and putamen were determined using an automated region-of-interest approach. Values from mutation carriers and healthy controls were compared using analysis of variance to assess the effects of genotype and phenotype. Additionally, voxel-based whole brain searches were conducted to detect group differences in extrastriatal regions., Results: Significant reductions in caudate and putamen D(2) receptor availability were evident in both groups of mutation carriers relative to healthy controls (p < 0.001). The changes were greater in DYT6 relative to DYT1 carriers (-38.0 +/- 3.0% vs -15.0 +/- 3.0%, p < 0.001). By contrast, there was no significant difference between manifesting and nonmanifesting carriers of either genotype. Voxel-based analysis confirmed these findings and additionally revealed reduced RAC binding in the ventrolateral thalamus of both groups of mutation carriers. As in the striatum, the thalamic binding reductions were more pronounced in DYT6 carriers and were not influenced by the presence of clinical manifestations., Conclusions: Reduced D(2) receptor availability in carriers of dystonia genes is compatible with dysfunction or loss of D(2)-bearing neurons, increased synaptic dopamine levels, or both. These changes, which may be present to different degrees in the DYT1 and DYT6 genotypes, are likely to represent susceptibility factors for the development of clinical manifestations in mutation carriers.

Positron emission tomography studies in drug-addicted patients have shown that exposure to drug-related cues increases striatal dopamine, which displaces binding of the D(2) ligand, [(11)C]-raclopride. However, it is not known if animals will also show cue-induced displacement of [(11)C]-raclopride binding. In this study, we use [(11)C]-raclopride imaging in awake rodents to capture cue-induced changes in dopamine release associated with the conditioned place preference model of drug craving. Ten animals were conditioned to receive cocaine in a contextually distinct environment from where they received saline. Following conditioning, each animal was tested for preference and then received two separate [(11)C]-raclopride scans. For each scan, animals were confined to the cocaine and/or the saline-paired environment for the first 25 min of uptake, after which they were anesthetized and scanned. [(11)C]-raclopride uptake in the saline-paired environment served as a within-animal control for uptake in the cocaine-paired environment. Cocaine produced a significant place preference (p = 0.004) and exposure to the cocaine-paired environment decreased [(11)C]-raclopride binding relative to the saline-paired environment in both the dorsal (20%; p < 0.002) and ventral striatum (22%; p < 0.05). The change in [(11)C]-raclopride binding correlated with preference in the ventral striatum (R(2) = -0.87; p = 0.003). In this region, animals who showed little or no preference exhibited little or no change in [(11)C]-raclopride binding in the cocaine-paired environment. This noninvasive procedure of monitoring neurochemical events in freely moving, behaving animals advances preclinical molecular imaging by interrogating the degree to which animal models reflect the human condition on multiple dimensions, both biological and behavioral.

The D(3) dopamine (DA) receptor is a member of the D(2)-like DA receptor family. While the D(2) receptor is abundant especially in motor-regions of the striatum, the D(3) receptor shows a relative abundance in limbic regions and globus pallidus. This receptor is of current interest in neurology because of its potential involvement in psychiatric and motor complications in Parkinson's disease and the possibility that dopamine D(3)-preferring agonist therapy might delay progression of the disorder. Preclinical data indicate that striatal levels of the D(3) (but not the D(2)) DA receptor are decreased following lesion of nigrostriatal DA neurons; at present, there are no in vivo data on this receptor subtype in Parkinson's disease. The objective of this positron emission tomography study was to compare [(11)C]-(+)-PHNO (D(3) versus D(2) preferring) and [(11)C]raclopride (D(3) = D(2)) binding in brain of non-depressed, non-demented, dopaminergic drug-naïve patients with early-stage Parkinson's disease (n = 10), relative to matched-controls (n = 9). Parkinson's disease was associated with a trend for bilaterally decreased [(11)C]-(+)-PHNO (but not [(11)C]raclopride) binding in the D(3)-rich ventral striatum (-11%, P = 0.07) and significantly decreased binding in globus pallidus (-42%, P = 0.02). In contrast, in the primarily D(2)-populated putamen, both [(11)C]-(+)-PHNO (25%, P = 0.02) and [(11)C]raclopride (25%, P < 0.01) binding were similarly increased, especially on the side contra-lateral to the symptoms. In the midbrain, presumably containing D(3) receptors localized to the substantia nigra, [(11)C]-(+)-PHNO binding was normal. Decreased [(11)C]-(+)-PHNO to [(11)C]raclopride ratio correlated with motor deficits and lowered-mood (P < 0.02). Our imaging data suggest that brain DA neuron loss in the human causes region-specific differential changes in DA D(2) and D(3) receptors with D(3) receptor 'downregulation' possibly related to some motor and mood problems in Parkinson disease. D(3) receptor levels might be a determinant vulnerability factor underlying side-effects associated with treatment; hence, these initial findings provide valuable baseline information to understand the role of D(3) receptors in response to Parkinson's disease medication.

Unlabelled: The purpose of this study was to evaluate the mechanisms of age-related decline of dopamine transporter (DAT) and dopamine D(2)-like receptor (D2R) densities in the human striatum by focusing on regional difference., Methods: Positron emission tomography (PET) with [(11)C]CFT and [(11)C]raclopride for measuring DATs and D2Rs, respectively, was performed on 16 healthy volunteers ranging from 21 to 74 years in age. To evaluate in detail the regional difference within the striatum, in addition to the conventional region-of-interest-based analysis, we created a parametric image that enabled us to visualize the regional decline rate on a voxel-by-voxel basis, mapping the slope of the regression line between the age and uptake index of each tracer., Results: The decreasing rates corresponded to 6.1, 5.5, and 5.6% per decade for DATs and 5.8, 4.9, and 4.8% per decade for D(2)Rs in the caudate nucleus, anterior putamen, and posterior putamen, respectively. The caudate nucleus for both DATs and D(2)Rs were the fastest among the striatum, and the regional difference of the decreasing rate for DATs was consistently associated with that for D2Rs. Meanwhile, previous histological studies have shown that age-related cell loss in the substantia nigra is likely to preferentially affect its dorsomedial part, which projects to the caudate nucleus., Conclusions: These results suggested that neuronal cell loss in the substantia nigra may be associated with the age-related DAT decline, and DAT decline may be associated functionally with age-related D2R decline., (Copyright 2008 Wiley-Liss, Inc.)

Intoxication induced by MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) in mice results in a significant loss of nigrostriatal dopamine (DA) neurons. This is accompanied by a change in behavioural phenotype that can be reversed by L-DOPA (3,4-dihydroxy-L-phenylalanine) treatment. Here, we examined the extracellular levels of DA, behavioural deficits and the response to L-DOPA treatment in severely intoxicated mice. The MPTP intoxication produced more than a 90% reduction in tissue DA and a 65% decline in extracellular DA levels. In-vivo binding did not show any increased raclopride binding to the D2 receptor. Administration of L-DOPA, 5 or 20 mg/kg (subcutaneously), significantly increased dialysate DA levels and both doses of L-DOPA reversed the behavioural deficit. Interestingly, only 5 mg/kg L-DOPA normalized DA levels to 56% of controls showing that only a minor increase in DA levels is sufficient to yield motor recovery.

The influence of cannabis on mental health receives growing scientific and political attention. An increasing demand for treatment of cannabis dependence has refueled the discussion about the addictive potential of cannabis. A key feature of all addictive drugs is the ability to increase synaptic dopamine levels in the striatum, a mechanism involved in their rewarding and motivating effects. However, it is currently unknown if cannabis can stimulate striatal dopamine neurotransmission in humans. Here we show that Delta 9-tetrahydrocannabinol (THC), the main psychoactive component in cannabis, induces dopamine release in the human striatum. Using the dopamine D(2)/D(3) receptor tracer [(11)C]raclopride and positron emission tomography in seven healthy subjects, we demonstrate that THC inhalation reduces [(11)C]raclopride binding in the ventral striatum and the precommissural dorsal putamen but not in other striatal subregions. This is consistent with an increase in dopamine levels in these regions. These results suggest that THC shares a potentially addictive property with other drugs of abuse. Further, it implies that the endogenous cannabinoid system is involved in regulating striatal dopamine release. This allows new directions in research on the effects of THC in neuropsychiatric disorders, such as schizophrenia.

Long-term occupancy of dopamine D(2)-receptors, as achieved by chronic treatment with antipsychotics, leads to D(2)-receptor upregulation, and this upregulation is thought to be responsible for loss of efficacy and development of tardive dyskinesia. However, little is known about the parameters of D(2)-receptor blockade (duration and percentage of blockade) that lead to upregulation. In this study, we investigated the effects of different degrees (60 vs >80%) and durations (a transient peak vs 24 h/day) of D(2)-receptor blockade on inducing this upregulation. These different patterns of D(2)-receptor occupancy kinetics were produced in cats using bolus vs constant infusion of haloperidol for 4 weeks. D(2)-receptors were measured using positron emission tomography and Scatchard analyses of [(11)C]raclopride binding, before and after withdrawal of treatment. Continuously high (80% for 24 h/day) D(2)-receptor blockade led to a robust upregulation of striatal D(2)-receptors that was maximal at 1-week withdrawal (35+/-5%) and still detectable at 2-week withdrawal (20+/-3%). This pattern of D(2)-receptor blockade also induced behavioral tolerance to the effect of haloperidol on spontaneous locomotor activity. Continuously moderate (60% for 24 h/day) or transiently high (80% for a few hours/day) D(2)-receptor blockade did not produce any of these effects. The long-term effect of haloperidol on D(2)-receptor density and behavioral tolerance thus appears to be dependent not only on a critical threshold of D(2)-receptor blockade but also on the daily duration of D(2)-receptors blockade. This suggests that as far as antipsychotics are concerned, not only dose but disbursment throughout the day have an impact on eventual pharmacodynamic and behavioral outcomes.

Objectives: The purpose of this study was to investigate whether dopamine D(2) receptor binding was altered in the striatum of essential blepharospasm patients., Methods: Striatal dopamine D(2) receptor binding was measured with positron emission tomography and [(11)C]raclopride. We studied eight drug-naive patients with bilateral blepharospasm and eight age-matched normal controls., Results: The uptake indices in the blepharospasm group were significantly reduced by 11.7% in the caudate (P < 0.005), 11.6% in the anterior putamen (P < 0.0001), and 10.3% in the posterior putamen (P < 0.005) relative to the control group., Conclusions: This study indicates decreased dopamine D(2) receptor binding in the entire striatal region of blepharospasm patients. The findings suggest that decreased dopamine D(2) receptor binding might be one of the predisposing factors that leads to the dysfunction of the motor circuit, resulting in the loss of broad inhibition of unwanted movements during an intended movement in blepharospasm patients.

In humans, behavioral dopamine supersensitivity occurs in schizophrenia and in Parkinson's disease. In animals, behavioral dopamine supersensitivity is consistently associated with increased dopamine D2(High) receptors in homogenized striata in vitro. Because D2(High) receptors have not yet been detected in intact cells, we used [(3)H]domperidone to detect D2(High) sites in intact rat anterior pituitary adenoma culture cells. Although [(3)H]raclopride and [(3)H]spiperone did not detect D2(High) receptors in intact cells or in rat fresh striatal slices, [(3)H]domperidone readily detected D2(High) receptors, warranting an in vivo search for D2(High) variations in human diseases.

The D(2) receptors exist in either the high- or low-affinity state with respect to agonists, and while agonists bind preferentially to the high-affinity state, antagonists do not distinguish between the two states. [(11)C]-(+)-PHNO is a PET D(2) agonist radioligand and therefore provides a preferential measure of the D(2) (high) receptors. In contrast, [(11)C]raclopride is an antagonist radioligand and thus binds with equal affinity to the D(2) high- and low-affinity states. The aim was to compare the brain uptake, distribution and binding characteristics between [(11)C]-(+)-PHNO and [(11)C]raclopride in volunteers using a within-subject design. Both radioligands accumulated in brain areas rich in D(2)/D(3)-receptors. However, [(11)C]-(+)-PHNO showed preferential uptake in the ventral striatum and globus pallidus, while [(11)C]raclopride showed preferential uptake in the dorsal striatum. Mean binding potentials were higher in the putamen (4.3 vs. 2.8) and caudate (3.4 vs 2.1) for [(11)C]raclopride, equal in the ventral-striatum (3.4 vs. 3.3), and higher in the globus pallidus for [(11)C]-(+)-PHNO (1.8 vs. 3.3). Moreover [(11)C]-(+)-PHNO kinetics in the globus pallidus showed a slower washout than other regions. One explanation for the preferential binding of [(11)C]-(+)-PHNO in the globus pallidus and ventral-striatum could be the presence of a greater proportion of high- vs. low-affinity receptors in these areas. Alternatively, the observed distribution could also be explained by a preferential binding of D(3)-over-D(2) with [(11)C]-(+)-PHNO. This differential binding of agonist vs. antagonist radioligand, especially in the critically important region of the limbic striatum/pallidum, offers new avenues to investigate the role of the dopamine system in health and disease., ((c) 2007 Wiley-Liss, Inc.)

Normal aging is associated with a decrease in dopaminergic function and a reduced ability to form new motor memories with training. This study examined the link between both phenomena. We hypothesized that levodopa would (a) ameliorate aging-dependent deficits in motor memory formation, and (b) increase dopamine availability at the dopamine type 2-like (D2) receptor during training in task-relevant brain structures. The effects of training plus levodopa (100mg, plus 25mg carbidopa) on motor memory formation and striatal dopamine availability were measured with [(11)C]raclopride (RAC) positron emission tomography (PET). We found that levodopa did not alter RAC-binding potential at rest but it enhanced training effects on motor memory formation as well as dopamine release in the dorsal caudate nucleus. Motor memory formation during training correlated with the increase of dopamine release in the caudate nucleus. These results demonstrate that levodopa may ameliorate dopamine deficiencies in the elderly by replenishing dopaminergic presynaptic stores, actively engaged in phasic dopamine release during motor training.

Objective: The aim of this study was to characterize the in vitro binding of [(11)C]raclopride with ultrahigh specific activity (SA) in the striatum and cerebral cortex of rat brain., Methods: [(11)C]Raclopride, a dopamine D(2) receptor ligand, with an ultrahigh SA of 4880+/-2360 GBq/micromol (132+/-64 Ci/micromol, n=25) was synthesized. In vitro binding experiment was performed using brain homogenate assay and autoradiography (ARG)., Results: In vitro homogenate assay demonstrated that high SA [(11)C]raclopride (2520-6340 GBq/micromol; 68-171 Ci/micromol) had two-affinity (high and low) binding sites in the striatum and cerebral cortex of rat brain. In the striatum, K(d,high) and B(max,high) values were 0.005+/-0.002 nM and 0.19+/-0.04 fmol/mg tissue, respectively, while K(d,low) and B(max,low) values were 2.2+/-1.0 nM and 35.8+/-16.4 fmol/mg tissue, respectively. In the cerebral cortex, K(d,high) and B(max,high) values were 0.061+/-0.087 nM and 0.2+/-0.2 fmol/mg tissue, respectively, while K(d,low) and B(max,low) values were 2.5+/-3.2 nM and 5.5+/-4.8 fmol/mg tissue, respectively. On the other hand, only one binding site was found in the striatum and no binding site was identified in the cerebral cortex using low SA [(11)C]raclopride (44 GBq/micromol; 1.2 Ci/micromol). In vitro ARG for the rat brain using high SA [(11)C]raclopride (6212 GBq/micromol; 168 Ci/micromol) gave a coronal image of the striatum and cerebral cortex with a higher signal/noise ratio than using low SA [(11)C]raclopride (40 GBq/micromol; 1.1 Ci/micromol)., Conclusion: Using ultrahigh SA [(11)C]raclopride for the in vitro homogenate assay, we succeeded in detecting two-affinity binding sites of [(11)C]raclopride, not only in the striatum but also in the cerebral cortex of rat brain.

In all, 19 research subjects, with current histories of frequent cocaine use, were exposed to cocaine-related cues to elicit drug craving. We measured the change of occupancy of dopamine at D2-like receptors with positron emission tomography (PET) and inferred a change of intrasynaptic dopamine (endogenous dopamine release), based on the displacement of radiotracer [(11)C]raclopride. Receptor occupancy by dopamine increased significantly in putamen of participants who reported cue-elicited craving compared to those who did not. Further, the intensity of craving was positively correlated with the increase in dopamine receptor occupancy in the putamen. These results provide direct evidence that occupancy of dopamine receptors in human dorsal striatum increased in proportion to subjective craving, presumably because of increased release of intrasynaptic dopamine.

Object: The "wearing-off" phenomenon often hampers the treatment of Parkinson disease (PD). Although deep brain stimulation (DBS) of the subthalamic nucleus (STN) is known to ameliorate the wearing-off phenomenon, the mechanism by which it does this remains unclear. As part of an inquiry into the mechanism of STN DBS, the authors measured synaptic dopamine levels in the striatum by performing positron emission tomography (PET) with [11C]raclopride., Methods: Three patients with PD who were experiencing the wearing-off phenomenon underwent PET scanning before and after DBS of the STN. The clinical features in these patients were evaluated by applying the Hoehn and Yahr, United Parkinson's Disease Rating, and Schwab and England Activities of Daily Living Scales. Before and after surgery, PET scans were obtained using [11C]raclopride prior to and 1 hour following an oral administration of levodopa. Regions of interest for the [11C]raclopride binding potential (RacloBP) were set in the bilateral putamen and the caudate nucleus. All clinical scores were dramatically improved postoperatively. Deep brain stimulation of the STN reduced the baseline RacloBP in both the putamen and caudate nucleus, but the differences between the pre- and postoperative levels were insignificant. Before DBS of the STN, the levodopa administration significantly reduced RacloBP in the putamen (p < 0.0001). Postoperatively the drug-induced reduction in RacloBP became statistically insignificant. The drug-induced increase in synaptic dopamine concentrations in the putamen preoperatively was estimated to be approximately four times higher than that after surgery (p < 0.01). The drug-induced RacloBP change in the caudate nucleus was similar to that in the putamen, although the magnitude of the change was lower (p < 0.005). The drug-induced increase in the caudate nucleus was also reduced postoperatively (p < 0.05)., Conclusions: Deep brain stimulation of the STN induces the stabilization of synaptic dopamine concentrations in the striatum and may attribute to the alleviation of levodopa-related motor fluctuations.

Behavioral response to novelty in rats has been linked both to dopamine transmission in the ventral striatum, and to propensity to self-administer psychostimulant drugs. In order to probe the relationship between behavioral response to novelty and dopamine systems we have developed a behavioral model for correlation with positron emission tomography (PET) of dopamine transmission in brain of Göttingen minipigs. In the present study, we measured exploration of a novel object by recording the number of contacts, and duration of contact with a novel object, in groups of six male and six female adult minipigs. We hypothesized that these novelty scores would correlate with the amphetamine-evoked dopamine release in ventral striatum, measured 2 weeks later in a PET study of the availability of binding sites for the dopamine D2/3 antagonist [11C]raclopride. There were significant correlations between duration of contact with a novel object and the amphetamine-evoked reductions in binding potential (DeltapB) in the left ventral striatum of the 12 animals; Comparison of results by gender revealed that the correlation was driven mainly by the male group, and was not present in the female group. We interpret these results to show that propensity to explore an unfamiliar object is relatively elevated in pigs with low basal occupancy of dopamine D2/3 receptors by endogenous dopamine, and with high amphetamine-induced occupancy of released dopamine in the male pigs.

Among 27 preclinical carriers of the Huntington disease mutation (PMC), the authors found normal striatal values for MRI volumetry in 88% and for fluorodesoxyglucose PET metabolic index in 67%. Raclopride PET binding potential (RAC-BP) was decreased in 50% and correlated with increases in the product of age and CAG repeat length (p < 0.0005). Dopamine D2 receptor availability measured by RAC-BP seems the most sensitive indicator of early neuronal impairment in PMC.

5-HT1A receptor agonists consistently reduce neuroleptic induced catalepsy in rats. A serotonin-dopamine interaction has been proposed to underlie this effect. Specifically, 5-HT1A receptor agonists may reduce the activity of serotonergic projections that inhibit dopaminergic nigrostriatal neurones, therefore increasing dorsal striatal dopamine levels and partially overcoming the neuroleptic blockade of D2 receptors. We tested the hypothesis that 5-HT1A receptor agonists increase striatal dopamine release in man using PET scanning with the selective D2 receptor radioligand [11C]raclopride, which is sensitive to endogenous dopamine levels. Six healthy volunteers received two PET scans, one after placebo, the other after 1 mg flesinoxan, a selective 5-HT1A receptor agonist. Binding potential values for striatal subdivisions were determined using a simplified reference tissue model. We did not find any difference in striatal [11C]raclopride binding between conditions, even though flesinoxan lead to typical 5-HT1A receptor agonist side effects and produced elevation of growth hormone in five of the six subjects. Our results suggest that the anticataleptic effect of 5-HT1A receptor agonists is not mediated by striatal dopamine release, and indicates a need for further research with other suitable 5-HT1A receptor agonists., (Copyright (c) 2005 Wiley-Liss, Inc.)

The distributions of dopamine D1 and D2/3 binding sites in living brain have not been compared directly using positron emission tomography (PET). To map these binding sites, we first optimized methods for the assay of dopamine receptors in brain of Göttingen miniature pigs. The binding potentials (pB) of [11C]NNC 112 for dopamine D1 receptors and [11C]raclopride for dopamine D2/3 receptors were calculated in pig striatum volumes of interest using metabolite corrected arterial inputs or using cerebellum as a non-binding reference region. Depending upon the method for quantitation, the pB for [11C]NNC 112 was 1.2-5.1 in pig striatum, whereas the pB for [11C]raclopride was 1.0-1.8. We used the reference tissue method of Logan to calculate pB maps for the two tracers. The maps were co-registered to the common stereotaxic space for the pig brain and normalized to a global mean for pB in striatum; t-maps showed that dopamine D1 binding was relatively more abundant in the ventral-anterior striatum of the pig, while dopamine D2/3 binding was greater in the dorsal striatum. Similar comparisons were made for the pBs of [11C]Sch 23390 for dopamine D1 receptors and for [11C]raclopride in the brain of six rhesus monkeys. The magnitudes of pB for both binding sites in monkey brain were close to those in the pig. Consistent with the pig results, there were distinct gradients in the distributions of the two binding sites in monkey brain: D1 binding predominated in the ventral striatum, whereas D2/3 binding was relatively greater in the dorsal-posterior striatum. Gradients of dopamine receptor concentration within the striatum may be a general phenomenon of mammalian brain., (Copyright 2004 Elsevier Inc.)

In this study, we evaluated the potential of using a new beta-sensitive microprobe system for in vivo quantification of [11C]raclopride binding and for in vivo determination of drug-induced receptor occupancy in the rat striatum. To validate this system, an ex vivo tissue dissection method was used to corroborate in vivo beta-microprobe measurements. Our data showed that the beta-microprobe-derived [11C]raclopride binding kinetics in striatum could be quantified using a tissue compartmental model with a cerebellar reference region. Haloperidol (0.001-0.1 mg/kg; i.v.) induced a dose-dependent decrease in [11C]raclopride binding in striatum as measured using the beta-microprobe with an ED50 value of 0.013 mg/kg. Highly significant relationships (P < 0.0001) were observed, within the same animals, between in vivo and ex vivo measures of haloperidol-induced D2-receptor occupancy (r = 0.98) as well as between in vivo and ex vivo measures of [11C]raclopride binding potentials (r = 0.99). Results from pretreatment and displacement studies with unlabeled raclopride and amphetamine conformed to the effect of these drugs as observed in humans using [11C]raclopride and PET and allowed estimation of the in vivo k(off) value of raclopride to 0.025 +/- 0.004 min(-1). However, allowing the system to stabilize before measurements and shielding the photomultiplier tubes were critical for obtaining these consistent results. This study demonstrates that the beta-microprobe provides reliable measurements of [11C]raclopride binding kinetics in rodents, allows for quantitative in vivo measurements of antipsychotic drug action in brain, and represents a valid and cost-effective alternative to positron emission tomography imaging in small animals., (Copyright 2004 Wiley-Liss, Inc.)

Changes in the control of dopaminergic neurotransmission by noradrenergic locus coeruleus (LC) projections has been implicated in such disorders as depression, drug addiction, and Parkinson's disease. In the present study, the effect of DSP-4, a neurotoxin highly selective for LC projections, on D(2) receptor abundance as assessed by [3H]-raclopride binding in the striatum was studied in rats after administration in doses of 10 and 50 mg/kg either 3 days or 1 month before decapitation. Three days after DSP-4 the levels of noradrenaline in the frontal cortex were dose-dependently reduced; after 1 month, noradrenaline levels were lowered only by the higher dose. DOPAC levels were dose-dependently reduced in the frontal cortex and striatum 3 days but not 1 month after DSP-4 treatment. Cortical 5-HIAA levels were reduced 3 days but not 1 month after DSP-4. The apparent number of D(2) receptor binding sites in the striatum was higher 1 month after either dose of DSP-4. DSP-4 treatment had no effect on [3H]-raclopride binding affinity, the ability of dopamine (DA) to compete with [3H]-raclopride binding and to activate [35S]GTPgammaS binding or on the binding affinities of GDP and [35S]GTPgammaS for corresponding G proteins 1 month after administration of the neurotoxin. These data suggest that after administration of DSP-4, short-term reduction in DA and 5-HT metabolism occurs. Subsequently, an upregulation of D(2) receptor binding sites develops in the striatum even after a minor denervation of the LC projections. Thus, alterations in the LC projection systems elicit lasting adaptive changes in DA-ergic neurotransmission that can serve as a substrate for psychiatric disorders.

Animal studies indicate that mu-opioids indirectly modulate neurotransmission in the nigrostriatal dopaminergic pathway. We used positron emission tomography (PET) to study the effects of alfentanil (a mu-opioid receptor agonist) on striatal dopamine D2 receptor binding in eight healthy male volunteers. D2 receptor binding was determined by using [(11)C]raclopride as radioligand. Each subject underwent two PET sessions on the same day, the first without the drug (control) and the second during alfentanil infusion. Alfentanil was administered as target-controlled infusion to maintain pseudo steady-state plasma concentration of 80 ng/ml throughout the PET session. A freeze lesion model was used for pain testing at the end of both PET sessions. A mechanical pain stimulus of 5 N was rated by the subjects using a visual analog scale. Regions of interest for the putamen, caudate nucleus, and cerebellum were drawn on MRI images and transferred to PET images. Alfentanil increased the binding potential of [(11)C]raclopride in the putamen by 6.0% (P = 0.04) and in the caudate nucleus by 7.4% (P = 0.008). Alfentanil caused a small reduction in respiratory rate (P = 0.046) and oxygen saturation (P < 0.001), and a moderate consistent increase in end-tidal CO(2) (P < 0.001). Pain scores were significantly smaller after alfentanil PET scan (median VAS 9 (0-42) vs. 23.5 (15-52), P = 0.008). These results indicate that pharmacologically relevant concentrations of alfentanil increase D2 dopamine receptor binding in the striatum in man. This increase is assumed to reflect reduced dopamine release., (Copyright 2002 Wiley-Liss, Inc.)

According to the ternary complex model of G-protein linkage to receptors, agonists increase the affinity of the receptors for the G protein. The model predicts that an endogenous agonist's constant of inhibition toward an agonist radioligand is lower than that toward an antagonistic radioligand. The authors hypothesized that competition from endogenous dopamine in striatum of living mice should have a greater effect on the binding of the D2,3 partial agonist N-[3H]propylnorapomorphine than on the binding of the D2,3 antagonist [(11)C]raclopride. The baseline binding potential (pB(0)), defined as the ratio of bound-to-unbound ligand in the absence of competition from endogenous dopamine, was simultaneously measured in mouse striatum for [(11)C]raclopride (pB(0) = 8.5) and N-[(3)H]propylnorapomorphine (p'B(0) = 5.3). The baseline was established by treatment with alpha-methyl-p-tyrosine and reserpine. Relative to these baseline values in saline-treated mice, the pB of N-[(3)H]propylnorapomorphine decreased 52% whereas the pB of [(11)C]raclopride decreased only 30%, indicating greater sensitivity of the former compound to inhibition by synaptic dopamine. Furthermore, amphetamine decreased the pB of N-[(3)H]propylnorapomorphine to a greater extent (73%) than that of [(11)C]raclopride (43%) relative to the reserpine condition. For both radioligands, the occupancy of the dopamine receptors by endogenous agonist obeyed Michaelis-Menten kinetics over a wide range of agonist concentrations established by the pharmacologic treatments. The apparent inhibition constant of endogenous dopamine depended on the dopamine occupancy and decreased to a value 1.66 times greater for N-[(3)H]propylnorapomorphine than for [(11)C]raclopride at its highest occupancies. The results are consistent with the hypothesis that agonist binding is more sensitive than antagonist binding to competition from endogenous dopamine. Therefore, dopamine agonist ligands may be superior to benzamide antagonist ligands for the estimation of dopamine receptor occupancy by endogenous synaptic dopamine. The analysis of the effect of dopamine occupancy on the inhibition of N-[(3)H]propylnorapomorphine binding indicated a limited supply of G protein with a maximum ternary complex fraction of 40% of maximum agonist binding capacity.

To clarify age-related changes in dopamine D1-like and D2-like receptor binding in the striatum, positron emission tomography (PET) and in vitro receptor autoradiography (in vitro ARG) were performed using F344/N rats of various ages (6, 12, 18, and 24 months). In the PET study, [11C]SCH23390 and [11C]raclopride were used to image dopamine D1-like receptors and dopamine D2-like receptors, respectively, while [3H]SCH23390 and [3H]raclopride were used for the in vitro ARG study. With PET, we calculated the binding potential (= k3/k4, Bmax/Kd) of [11C]SCH23390 and [11C]raclopride in the striatum according to the curve fitting (CF) and the Logan plot (LP) methods. The binding potential of [11C]SCH23390 in the striatum demonstrated significant decrease as a function of age (max. decrease -26%) by the LP method, while this was not observed in the data analyzed by the CF method. In contrast, the binding potential of [11C]raclopride in the striatum decreased significantly with age by both the CF (max. decrease -28%) and the LP (max. decrease -36%) methods. However, no significant difference by either method was observed in rats between 6 and 12 months old using either ligand. In the in vitro ARG study, the specific binding (fmol/mg tissue) of [3H]SCH23390 and [3H]raclopride in the striatum were determined. Both [3H]SCH23390 and [3H]raclopride binding declined considerably with age as noted by comparing 12, 18, and 24-month-old rats against those 6 months old (max. decrease -29% and -31%, respectively). The substantial difference in binding shown in 12-month-olds in comparison with 6-month-olds using either ligand with in vitro ARG was in contrast with the PET results. These distinctions between the PET and the in vitro ARG studies may be attributed to the receptor microenvironment created under these experimental conditions. The results indicate that PET with LP analysis is useful in obtaining age-related changes of D1-like and D2-like receptor binding in the striatum of living rats., (Copyright 2001 Wiley-Liss, Inc.)

The power of placebos has long been recognized for improving numerous medical conditions such as Parkinson's disease (PD). Little is known, however, about the mechanism underlying the placebo effect. Using the ability of endogenous dopamine to compete for [11C]raclopride binding as measured by positron emission tomography, we provide in vivo evidence for substantial release of endogenous dopamine in the striatum of PD patients in response to placebo. Our findings indicate that the placebo effect in PD is powerful and is mediated through activation of the damaged nigrostriatal dopamine system.

2-BFI (2-(2-benzofuranyl)-2-imidazoline) is a prototypical I2-imidazoline receptor ligand. In vivo, however, 2-BFI (1-20 mg/kg) decreased the synthesis of dopa/dopamine (DA) in rat striatum through mechanisms not related to interaction with I2-imidazoline receptors or to inhibition of the enzyme monoamine oxidase. The aim of this study was to unravel the mechanism underlying this potent effect of 2-BFI in brain. In vitro 2-BFI showed very low affinity for D2-dopamine receptors (K(i)=47 microM), and in vivo the drug (7 mg/kg) decreased the synthesis of striatal dopa/DA similarly in control rats (43%) and in rats pre-treated with alpha-methyl-para-tyrosine (50%) or cocaine (51%), indicating that this effect was not the result of D2-dopamine autoreceptor direct stimulation, inhibition of the enzyme tyrosine hydroxylase or blockade of neuronal DA reuptake. In DA-depleted (reserpine-treated) rats, however, 2-BFI did not inhibit significantly (11%), the synthesis of dopa/DA in the striatum, indicating that the effect of 2-BFI was indirectly mediated by endogenous DA through the activation of D2-dopamine autoreceptors. In conclusion, the I2-imidazoline receptor ligand 2-BFI is also a DA releasing agent in brain, and consequently a DA indirect agonist in vivo.

The in vivo binding of positron emission tomography (PET) and single photon emission computer tomography (SPECT) radiotracers to dopamine D2 receptors in the striatum can be influenced by competition with endogenous dopamine. The present study was undertaken to determine if a similar inhibition of radiotracer binding to dopamine receptors could be observed following pharmacologically-evoked dopamine release in rat brain striatal slices. Striatal slices were incubated in a large volume of oxygenated Krebs saline and exposed to amphetamine or methamphetamine to evoke dopamine release within the slice. Amphetamine and methamphetamine, at concentrations up to 30 microM, reduced [3H]raclopride binding in the slices by 77% and 86%, respectively, with 50% inhibition at 1.6 microM amphetamine or 3.0 microM methamphetamine. Neither drug produced a significant effect on binding of [3H]SCH 23390 in the slices. This suggests that dopamine was able to interfere with radiotracer binding to D2 but not D1 receptors. The dopamine uptake blockers, cocaine and methylphenidate, had relatively little effect by themselves on [3H]raclopride binding but, by inhibiting amphetamine-induced dopamine release, significantly reduced inhibition of [3H]raclopride binding by a low (3 microM) amphetamine concentration. At a higher (30 microM) amphetamine concentration the inhibition of [3H]raclopride binding was not antagonized by uptake blockers and data obtained from homogenate binding experiments indicated a direct displacement of [3H]raclopride binding by amphetamine at this concentration. In conclusion the data obtained in the present study demonstrate that the effects of amphetamine on striatal radiotracer accumulation observed in PET and SPECT can also be observed in brain slices in vitro and, at least at low amphetamine concentrations, are mediated by competition with released dopamine.

Studies of dopamine (DA) receptor binding in early parkinsonian patients, or in models of Parkinson's disease, have revealed a supersensitivity of the D2-like receptor subtype as compared to age-matched controls. The lack of upregulation in advanced patients is often attributed to the effects of prolonged antiparkinsonian therapy, but the impact of therapy vs. intrinsic mechanisms in untreated patients or animals with long-term lesions of the DA nigrostriatal pathway has been difficult to address. We studied, in vivo, by PET using the DA D2 receptor ligand raclopride, the status of the DA receptors in normal rhesus monkeys and those with acute (3 months) or long-term (10 years) MPTP-induced nigrostriatal lesions. Compared to age-matched controls, there was no change in raclopride binding in MPTP-treated animals without parkinsonian symptoms. There was a significant increase in raclopride binding in the putamen (but not caudate nucleus) of all the animals displaying rigidity, hypo- and bradykinesia. This increase was greater in the animals with acute lesions (32%) than with established, long-term lesions (18%). There was no correlation between the postmortem striatal DA concentrations and in vivo raclopride binding but there was a correlation between PET raclopride binding and [(3)H]raclopride binding in vitro. Complex changes in D2 receptor binding occur in various stages of parkinsonism. Antiparkinsonian therapy is unlikely to be solely responsible for the lack of upregulation found in advanced parkinsonian patients but may be a contributing factor., (Copyright 2000 Wiley-Liss, Inc.)

The role of G-proteins in D2 receptor supersensitivity was studied in striatal membranes from rats with unilateral 6-hydroxydopamine (6-OHDA) induced lesions of the nigral dopamine (DA) system. Thirteen months after the lesion the number of [3H]raclopride binding sites was increased in the DA denervated striatum, but no changes in ligand binding affinities and in proportion of high-affinity agonist binding sites could be detected. The affinity of [35S]GTPgammaS binding was unaltered after the striatal DA denervation, whereas the binding affinity of GDP was decreased in the DA denervated as compared to the intact striatum. It is proposed that the decrease in GDP binding affinity to D2 DA receptor-coupled G proteins is an important factor in the D2 receptor supersensitivity following degeneration of the striatal DA terminals.

The in vivo binding of dopamine (DA) radioligands to D2 receptors can be affected by competition with endogenous dopamine. In the present study, we used a brain slice preparation that provides more controlled conditions than in vivo preparations in order to examine the relationship between synaptic DA and the binding of [3H]raclopride to D2 receptors. We also estimated the synaptic DA concentration in rat striatal slices by determining the changes in [3H]raclopride binding. To correlate the changes in [3H]raclopride binding with the concentration of synaptic DA, the kinetic parameters were determined. [3H]Raclopride reached equilibrium binding conditions within two hours. The Ki value for DA in inhibiting [3H]raclopride binding was about 2.2 nM. The increase in synaptic DA evoked by electrical stimulation decreased the striatal binding of [3H]raclopride in a frequency-dependent manner. Increases in the DA concentration evoked by amphetamine (AMPH) or cocaine decreased [3H]raclopride binding by 74% or 20%, respectively, corresponding to increases in the synaptic DA concentrations of 1.6 nM or 0.6 nM, respectively. Pargyline also decreased [3H]raclopride binding by 36% corresponding at a concentration of 1.2 nM. In contrast, the depletion of synaptic DA by alpha-methyl-p-tyrosine (alpha-MpT) increased the specific binding of [3H]raclopride by 43% when the DA concentration was decreased to 0.7 nM. The changes in the DA concentration at the synapse were responsible for the changes in the striatal binding of [3H]raclopride. The values calculated in this study may therefore approximate the changes in the synaptic DA concentration in rat striatal slices following manipulation.

The effects of ketamine, a noncompetitive antagonist of NMDA receptors, on the striatal dopaminergic system were evaluated multiparametrically in the monkey brain using high-resolution positron emission tomography (PET) in combination with microdialysis. L-[beta-(11)C]DOPA, [(11)C]raclopride, and [(11)C]beta-CFT were used to evaluate dopamine synthesis rate, D(2) receptor binding, and transporter availability, respectively, in conscious and ketamine-anesthetized animals. Dopamine concentrations in the striatal extracellular fluid (ECF) were simultaneously measured by PET. Thirty minutes prior to PET scan, intravenous administration of ketamine was started by continuous infusion at a rate of 3 or 10 mg/kg/h. Ketamine infusion dose-dependently decreased [(11)C]raclopride binding, but induced no significant changes in dopamine concentration in the striatal ECF as measured by microdialysis at any dose used. In contrast, ketamine increased both dopamine synthesis and DAT availability as measured by L-[beta-(11)C]DOPA and [(11)C]beta-CFT, respectively, in a dose-dependent manner. These results suggest that the inhibition of glutamatergic neuronal activity modulates dopamine turnover in the striatum by simultaneous enhancement of the dynamics of dopamine synthesis and DAT availability to the same extent, resulting in no apparent changes in ECF dopamine concentration as measured by microdialysis. It also suggests that the alteration of [(11)C]raclopride binding in vivo as measured by PET might not simply be modulated by the static synaptic concentration of dopamine., (Copyright 2000 Wiley-Liss, Inc.)