Reproducibility of striatal and thalamic dopamine D2 receptor binding using [11C]raclopride with high-resolution positron emission tomography

Positron emission tomography (PET) imaging of small striatal brain structures such as the ventral striatum (VST) has been hampered by low spatial resolution causing partial-volume effects. The high-resolution research tomograph (HRRT) is a brain-dedicated PET scanner that has considerably better spatial resolution than its predecessors. However, its superior spatial resolution is associated with a lower signal-to-noise ratio. We evaluated the test–retest reliability of the striatal and thalamic dopamine D₂ receptor binding using the HRRT scanner. Seven healthy male volunteers underwent two [¹¹C]raclopride PET scans with a 2.5-hour interval. Dopamine D₂ receptor availability was quantified as binding potential (BP_ND) using the simplified reference tissue model. To evaluate the reproducibility of repeated BP_ND estimations, absolute variability (VAR) and intraclass correlation coefficients (ICCs) were calculated. VAR values indicated fairly good reproducibility and were 3.6% to 4.5% for the caudate nucleus and putamen and 4.5% to 6.4% for the lateral and medial part of the thalamus. In the VST, the VAR value was 5.8% when the definition was made in the coronal plane. However, the ICC values were only moderate, in the range of 0.34 to 0.66, for all regions except the putamen (0.87). Experimental signal processing methods improved neither ICC nor VAR values significantly.     

Measurement of density and affinity for dopamine D2 receptors by a single positron emission tomography scan with multiple injections of [11C]raclopride

Positron emission tomography (PET) with [¹¹C]raclopride has been used to investigate the density (B_max) and affinity (K_d) of dopamine D₂ receptors related to several neurological and psychiatric disorders. However, in assessing the B_max and K_d, multiple PET scans are necessary under variable specific activities of administered [¹¹C]raclopride, resulting in a long study period and unexpected physiological variations. In this paper, we have developed a method of multiple-injection graphical analysis (MI-GA) that provides the B_max and K_d values from a single PET scan with three sequential injections of [¹¹C]raclopride, and we validated the proposed method by performing numerous simulations and PET studies on monkeys. In the simulations, the three-injection protocol was designed according to prior knowledge of the receptor kinetics, and the errors of B_max and K_d estimated by MI-GA were analyzed. Simulations showed that our method could support the calculation of B_max and K_d, despite a slight overestimation compared with the true magnitudes. In monkey studies, we could calculate the B_max and K_d of diseased or normal striatum in a 150 mins scan with the three-injection protocol of [¹¹C]raclopride. Estimated B_max and K_d values of D₂ receptors in normal or partially dopamine-depleted striatum were comparable to the previously reported values.     

Selective serotonin reuptake inhibitor (SSRI) modulation of striatal dopamine measured with [11C]-raclopride and positron emission tomography

The effect of a pharmacologic increase in serotonin concentrations on striatal dopamine (D2) receptor availability has been measured in several studies using positron emission tomography (PET) and the radiotracer [11C]-raclopride as a method for the in vivo imaging of serotonin modulation of striatal dopamine in human subjects. These studies have shown that an acute increase in serotonin concentrations produced a decrease in striatal D2 receptor availability. The current study was undertaken to measure the effects of a more pharmacologically selective serotonergic agent compared to previous studies, the serotonin reuptake inhibitor, citalopram, on striatal D2 receptor availability. Twelve healthy control subjects underwent two PET scans performed on the same day following i.v. administration of saline (Scan 1) and citalopram (Scan 2, 40 mg, i.v.). The [11C]-raclopride data were analyzed with a graphical analysis method using the cerebellum as the input function. Plasma levels of citalopram, cortisol, and prolactin were measured. The citalopram concentrations peaked at the end of infusion (EOI) and remained relatively consistent from 30 min to 3 h postinfusion. An increase in cortisol and prolactin concentrations was observed from the EOI until 60 min after the EOI. A significant decrease in striatal D2 receptor availability was observed after citalopram infusion (-5%), presumably due to an increase in endogenous dopamine concentrations. In summary, i.v. administration of the selective serotonin reuptake inhibitor, citalopram, produced modest reductions in striatal D2 receptor availability, consistent with other human [11C]-raclopride studies using less pharmacologically selective serotonergic agents.     

Oral D-amphetamine causes prolonged displacement of [11C]raclopride as measured by PET

Parenterally administered D-amphetamine has been used as a challenge drug to release dopamine, which in turns inhibits [11C]raclopride binding to dopaminergic D2 receptors as measured using positron emission tomography (PET) techniques. The primary objective of this study was to determine whether orally administered D-amphetamine would inhibit [11C]raclopride binding in a manner similar to that produced by intravenously administered D-amphetamine. The secondary objective was to assess the timeline of these effects. Twelve healthy human volunteers participated in this study. Subjects were scanned at baseline and 2 h after D-amphetamine administration (n = 5); at baseline, 2 and 6 h postdrug (n = 4); or at baseline, 2 and 24 h postdrug (n = 3). Orally administered D-amphetamine caused a significant decrease in [11C]raclopride binding at 2 h (13% +/- 5%). Receptor availability was still decreased at 6 h (18% +/- 6%), even though physiological effects had completely returned to baseline. [11C]Raclopride binding returned to baseline at 24 h. The percentage of [11C]raclopride displacement was not correlated with plasma D-amphetamine concentrations. In conclusion, orally administered D-amphetamine caused a reliable and prolonged [11C]raclopride displacement, the magnitude of which is similar to that observed after intravenous administration. Possible mechanisms for the observed prolonged displacement may include persistence of intrasynaptic dopamine and/or receptor internalization.     

Imaging dopamine D4 receptors in the living primate brain: A positron emission tomography study using the novel D1/ D4 antagonist [11C]SDZ GLC 756

The dopamine D₄ receptor has lately attracted interest since it has been hypothesized to be involved in the pathogenesis and pharmacotherapy of neuropsychiatric diseases. The present study provides first in vivo evidence of dopamine D₄ receptors in primate brain using a [¹¹C]benzo[g]quinoline, the novel radioligand [¹¹C]SDZ GLC 756 ([¹¹C]GLC: in vitro dissociation constants at human receptor clones [nM]: 1.10 at D₁; 0.40 at D₂; 25 at D₃; 0.18 at D_4.2; 6.03 at D₅). Dynamic positron emission tomography scans were performed on healthy baboons (Papio hamadryas, n = 3). Specific receptor binding (SB) was calculated for striatum and neocortex (frontal, temporal, parietal, and occipital) based on the differences between the regional and the cerebellar concentration of [¹¹C]. Blockade of D₁ and D₅ receptors by SCH23390 (1.7 μmol/ kg) diminished SB in the striatum by 55 ± 4% (mean ± standard deviation, P < 0.05) and in the frontal cortex by 13 ± 8% (P < 0.05) when compared to SB in the unblocked state (SB_D1–D5). In the presence of the dopamine antagonists SCH23390 (1.7 μmol/ kg) and raclopride (5.7 μmol/ kg)—which mask the D₁, D₂, D₃, and D₅ subtypes—SB of [¹¹C]GLC to D₄ receptors (SB_D4) was demonstrated in the striatum and all cortical regions of interest. In the striatum, the ratio of SB_D4/ SB_D1–D5 was 0.13 ± 0.07. In the neocortex, SB_D4/ SB_D1–D5 was notably higher (0.77 ±0.29; mean of all cortical regions of interest). The widespread distribution of dopamine D₄ receptors suggests a basic functional role of this receptor subtype in the modulation of cortical and subcortical neuronal activity. Synapse 30:341–350, 1998. © 1998 Wiley-Liss, Inc.     

Psychiatric wandering behaviour in dementia patients correlated with increased striatal dopamine D2 receptor as shown by [11C]YM-09151–2 and positron emission tomography

It is known that some demented patients show psychiatric symptoms and behavioural abnormalities such as wandering. However, how such behavioural abnormalities are correlated with brain function is unclear. In this study, the relationship between the dopaminergic receptor and psychiatric wandering behaviour in dementia was examined. Nine probable Alzheimer's disease (AD) patients with the NINCDS–ADRDA criteria, nine vascular dementia (VD) patients with the NINDS–AIREN criteria and six age-matched controls were studied. Four of each of the dementia group showed psychiatric wandering behaviour. The dopamine D₂ receptor was assessed using the radiolabeled D₂ antagonist [¹¹C]YM-09151–2, and positron emission tomography (PET). A three-compartment analysis using the metabolite-corrected (with HPLC) plasma radioactivity curve was performed for the calculation of receptor kinetics. Time–activity curves of the striatum were obtained. Two-way analysis of variance showed that there was a significant difference in the k₃/k₄ value between the groups (normal, VD and AD) considering the effect of wandering behaviour (wanderers or not). Post hoc test indicated that the values for AD and VD were greater than those of the controls. Since the k₃/k₄ value was equal to the binding potential under the tracer condition, the result indicated that psychiatric wandering behaviour of dementia was correlated with increased dopamine D₂ receptors.     

Increased dopamine D2/D3 receptor binding after recovery from anorexia nervosa measured by positron emission tomography and [11c]raclopride.

BACKGROUND: Several lines of evidence support the possibility that disturbances of dopamine (DA) function could contribute to alterations of weight, feeding, motor activity, and reward in anorexia nervosa (AN). METHODS: To assess possibly trait-related disturbances but avoid confounding effects of malnutrition, 10 women who were recovered from AN (REC AN) were compared with 12 healthy control women (CW). Positron emission tomography with [(11)C]raclopride was used to assess DA D2/D3 receptor binding. RESULTS: The women who were recovered from AN had significantly higher [(11)C]raclopride binding potential in the antero-ventral striatum than CW. For REC AN, [(11)C]raclopride binding potential was positively related to harm avoidance in the dorsal caudate and dorsal putamen. CONCLUSIONS: These data lend support for the possibility that decreased intrasynaptic DA concentration or increased D2/D3 receptor density or affinity is associated with AN and might contribute to the characteristic harm avoidance or increased physical activity found in AN. Most intriguing is the possibility that individuals with AN might have a DA related disturbance of reward mechanisms contributing to altered hedonics of feeding behavior and their ascetic, anhedonic temperament.     

Unchanged striatal dopamine transporter availability in narcolepsy: a PET study with [11C]-CFT

OBJECTIVE: To investigate dopamine reuptake sites (dopamine transporter) in the caudate nucleus and putamen in narcolepsy. PATIENTS AND METHODS: Ten patients with narcolepsy and 15 controls were studied with positron emission tomography. A cocaine analogue [11C]-CFT was used as a radioligand. RESULTS: The uptake of[11C]-CFT was within normal limits (89% of age-adjusted control mean in the caudate nucleus and 91% in the putamen) in patients with narcolepsy. CONCLUSIONS: No evidence of altered striataldopamine transporter availability was found in narcolepsy.     

Temporal characterisation of amphetamine-induced dopamine release assessed with [11C]raclopride in anaesthetised rodents

Competition between endogenous neurotransmitters and radiolabelled tracers, as measured by positron emission tomography (PET), may provide a measure of endogenous neurotransmitter flux in vivo. For example, carbon-11 labelled raclopride has been effectively used to monitor dopamine release following pharmacological and behavioural manipulations. The current study describes a rodent model of amphetamine-induced [11C]raclopride reduction, which allowed the characterisation of the dose-response and temporal dynamics of this reduction over a 24-h time course. Over the range studied, a monotonic dose-response relationship between amphetamine dose and [11C]raclopride reduction was observed. When compared with previously published microdialysis data, an approximate 16% reduction in [11C]raclopride binding potential was associated with a approximately 25-fold increase in extracellular dopamine. A reduction of 20-30% in raclopride binding was observed 30 min after amphetamine injection (4 mg/kg i.p.). This reduction in [11C]raclopride binding persisted for 4 h but returned to baseline by 8 h. The data suggest a persistent amphetamine-induced raclopride displacement in rodents and reinforce findings from nonhuman primates that a simple competitive occupancy model may not adequately explain the temporal characteristics of the amphetamine-induced decrease in radiotracer binding.     

Ex vivo [11C]‐(+)‐PHNO binding is unchanged in animal models displaying increased high‐affinity states of the D2 receptor in vitro

Dopamine (DA) D₂ receptor supersensitivity has been linked to an increase in the density of the D₂ high‐affinity state as measured in vitro. The two‐ affinity‐state model of the D₂ receptor predicts that the ex vivo specific binding of [¹¹C]‐(+)‐PHNO, an agonist radiotracer thought to bind selectively to the high‐affinity state in vivo, should be increased in animal models that display in vitro increases in the proportion of receptors in the D₂ high‐affinity state. Here, we test this hypotheses by comparing the ex vivo SBR of [¹¹C]‐(+)‐PHNO with that of the antagonist radiotracer [³H]‐raclopride in three dopaminergically supersensitive rat models—AMPH‐sensitized rats, rats withdrawn from chronic ethanol, and unilaterally 6‐OHDA‐lesioned rats—using ex vivo dual‐radiotracer biodistribution studies. We find that in AMPH‐sensitized rats and rats withdrawn from chronic ethanol treatment, models that exhibited ～4‐fold increases in the D₂ high‐affinity state in vitro, the SBRs of [¹¹C]‐(+)‐PHNO and [³H]‐raclopride are unchanged relative to control rats. In unilaterally 6‐OHDA‐lesioned rats, we find that the increase in [¹¹C]‐(+)‐PHNO SBR is no different than that observed for the antagonist radiotracer [³H]‐raclopride (54% ± 16% and 52% ± 14%, respectively). In addition, the effect of acute AMPH pretreatment (4 mg/kg, i.v.) on the SBRs of [¹¹C]‐(+)‐PHNO and [³H]‐raclopride is equivalent in AMPH‐sensitized (?38% ± 12% and ?36% ± 8%, respectively) and in control rats (?40% ± 11% and ?38% ± 7%). These data emphasize a significant discrepancy between in vitro and in vivo measures of D₂ agonist binding, indicating that the two‐affinity‐state model of the D₂ receptor may not apply veridically to living systems. The potential implications of this discrepancy are discussed. Synapse 63:998–1009, 2009. © 2009 Wiley‐Liss, Inc.     

Characterization of in vivo pharmacokinetic properties of the dopamine D1 receptor agonist DAR-0100A in nonhuman primates using PET with [11C] NNC112 and [11C] raclopride

DAR-0100A, the active enantiomer of dihydrexidine, is a potent dopamine D1 agonist under investigation for treatment of cognitive impairment and negative symptoms of schizophrenia. We measured the dose–occupancy relationship for DAR-0100A at D1 receptors using positron emission tomography (PET) imaging in baboons with [¹¹C] NNC112 and its binding to D2 with [¹¹C] raclopride. Two baboons were scanned with [¹¹C] NNC112 at baseline and after three different doses of DAR-0100A. Two baboons were scanned with [¹¹C] raclopride at baseline and after one dose of DAR-0100A. Occupancy (ΔBP_ND) was computed in the striatum and cortex. A clear relationship was observed between plasma concentration of DAR-0100A and ΔBP_ND. ΔBP_ND was larger in the striatum than in the cortex, consistent with reports showing that 25% of [¹¹C] NNC112 BP_ND in the cortex is attributed to 5-HT_2A. Plasma EC₅₀ estimates ranged from 150 to 550 ng/mL according to the constraints on the model. There was no detectable effect of DAR-0100A on [¹¹C] raclopride BP_ND. These data suggest that at doses likely to be administered to patients, occupancy will not be detectable with [¹¹C] NNC112 PET and binding of DAR-0100A to D2 will be negligible. This is the first demonstration with PET of a significant occupancy by a full D1 agonist in vivo.     

Striatal binding of11C-NMSP studied with positron emission tomography in patients with persistent tardive dyskinesia: no evidence for altered dopamine D2 receptor binding

Summary Dopamine D₂ receptor binding characteristics were studied by positron emission tomography (PET) using N-¹¹C-methyl spiperone as receptor ligand in patients on longterm treatment with neuroleptic drugs and in control subjects. Eight of the patients had symptoms of tardive dyskinesia whereas three patients did not have any symptoms. Control subjects comprised 5 healthy volunteers and 7 patients with pituitary tumors. All patients had been free of neuroleptic drugs for at least 4 weeks. The time dependent regional radioactivity in the striatum was measured and the receptor binding rate, k₃, proportional to receptor number, B_max and association rate for the receptor was calculated in relation to the cerebellum. The lack in difference in k₃ values between TD patients, neuroleptic treated patients without TD and control subjects throws doubt on the hypothesis that changes in striatal D₂ dopamin receptor number or binding affinity is an etiological mechanism for persistent TD.     

Theta burst stimulation‐induced inhibition of dorsolateral prefrontal cortex reveals hemispheric asymmetry in striatal dopamine release during a set‐shifting task – a TMS–[11C]raclopride PET study

The prefrontostriatal network is considered to play a key role in executive functions. Previous neuroimaging studies have shown that executive processes tested with card‐sorting tasks requiring planning and set‐shifting [e.g. Montreal‐card‐sorting‐task (MCST)] may engage the dorsolateral prefrontal cortex (DLPFC) while inducing dopamine release in the striatum. However, functional imaging studies can only provide neuronal correlates of cognitive performance and cannot establish a causal relation between observed brain activity and task performance. In order to investigate the contribution of the DLPFC during set‐shifting and its effect on the striatal dopaminergic system, we applied continuous theta burst stimulation (cTBS) to left and right DLPFC. Our aim was to transiently disrupt its function and to measure MCST performance and striatal dopamine release during [¹¹C]raclopride PET. A significant hemispheric asymmetry was observed. cTBS of the left DLPFC impaired MCST performance and dopamine release in the ipsilateral caudate–anterior putamen and contralateral caudate nucleus, as compared to cTBS of the vertex (control). These effects appeared to be limited only to left DLPFC stimulation while right DLPFC stimulation did not influence task performance or [¹¹C]raclopride binding potential in the striatum. This is the first study showing that cTBS, by disrupting left prefrontal function, may indirectly affect striatal dopamine neurotransmission during performance of executive tasks. This cTBS‐induced regional prefrontal effect and modulation of the frontostriatal network may be important for understanding the contribution of hemisphere laterality and its neural bases with regard to executive functions, as well as for revealing the neurochemical substrate underlying cognitive deficits.     

Pharmacological MRI responses of raclopride in rats: The relationship with D2 receptor occupancy and cataleptic behavior

Pharmacological magnetic resonance imaging (phMRI) allows the visualization of brain pharmacological effects of drugs using functional MRI (fMRI). phMRI can help us facilitate central nervous system (CNS) drug development. However, there have been few studies demonstrating the dose relationship of the fMRI response induced by CNS drugs to underlying target engagement or behavioral efficacy. To clarify these relationships, we examined receptor occupancy measurements using positron emission tomography (PET) (n = 3~5), fMRI (n = 5~8) and a cataleptic behavior (n = 6) with raclopride, a dopamine D2 receptor antagonist (8, 20, and 200 μg/kg) on Wistar rats. Dopamine D2 receptor occupancy was increased dose dependently by raclopride (41.8 ± 2.7%, 8 μg/kg; 64.9 ± 2.8%, 20 μg/kg; 83.1 ± 3.0%, 200 μg/kg). phMRI study revealed significant positive responses to raclopride at 200 μg/kg specifically in the striatum and nucleus accumbens, related to dopaminergic system. Slight fMRI responses were observed at 20 μg/kg in some areas corresponding to the striatum and nucleus accumbens. There were no noticeable fMRI responses at 8 μg/kg raclopride administration. Raclopride at 200 μg/kg significantly increased the cataleptic score, although, at 8 and 20 μg/kg, raclopride had no significant effects. These findings showed that raclopride-induced fMRI responses were observed at doses inducing cataleptic behavior and high D2 receptor occupancy, suggesting that phMRI can be useful for dose selection in clinical trial as an evaluation method of brain activity, which reflects behavioral responses induced by target engagements.     

Imaging of dopamine D2/3 agonist binding in cocaine dependence: a [11C]NPA positron emission tomography study

Objective: Positron emission tomography (PET) studies performed with [¹¹C]raclopride have consistently reported lower binding to D_2/3 receptors and lower amphetamine‐induced dopamine (DA) release in cocaine abusers relative to healthy controls. A limitation of these studies that were performed with D_2/3 antagonist radiotracers such as [¹¹C]raclopride is the failure to provide information that is specific to D_2/3 receptors configured in a state of high affinity for the agonists (i.e., D_2/3 receptors coupled to G‐proteins, D_{2/3 HIGH}). As the endogenous agonist DA binds with preference to D_{2/3 HIGH} relative to D_{2/3 LOW} receptors (i.e., D_2/3 receptors uncoupled to G‐proteins) it is critical to understand the in vivo status of D_{2/3 HIGH} receptors in cocaine dependence. Thus, we measured the available fraction of D_{2/3 HIGH} receptors in 10 recently abstinent cocaine abusers (CD) and matched healthy controls (HC) with the D_2/3 antagonist and agonist PET radiotracers [¹¹C]raclopride and [¹¹C]NPA. Methods: [¹¹C]raclopride and [¹¹C]NPA binding potential (BP) (BP_ND) in the striatum were measured with kinetic analysis using the arterial input function. The available fraction of D_{2/3 HIGH} receptors, i.e., % R_HIGH available = D_{2/3 HIGH}/(D_{2/3 HIGH} + D_{2/3 LOW}) was then computed as the ratio of [¹¹C]NPA BP_ND/[¹¹C]raclopride BP_ND. Results: No differences in striatal [¹¹C]NPA BP_ND (HC = 1.00 ± 0.17; CD = 0.97 ± 0.17, P = 0.67) or available % R_HIGH (HC = 39% ± 5%; CD = 41% ± 5%, P = 0.50) was observed between cocaine abusers and matched controls. Conclusions: The results of this [¹¹C]NPA PET study do not support alterations in D_{2/3 HIGH} binding in the striatum in cocaine dependence. Synapse, 2011. © 2011 Wiley‐Liss, Inc.     

Estimating the effect of endogenous dopamine on baseline [11C]‐(+)‐PHNO binding in the human brain

Endogenous dopamine (DA) levels at dopamine D_2/3 receptors (D_2/3R) have been quantified in the living human brain using the agonist radiotracer [¹¹C]‐(+)‐PHNO. As an agonist radiotracer, [¹¹C]‐(+)‐PHNO is more sensitive to endogenous DA levels than antagonist radiotracers. We sought to determine the proportion of the variance in baseline [¹¹C]‐(+)‐PHNO binding to D_2/3Rs which can be accounted for by variation in endogenous DA levels. This was done by computing the Pearson's coefficient for the correlation between baseline binding potential (BP_ND) and the change in BP_ND after acute DA depletion, using previously published data. All correlations were inverse, and the proportion of the variance in baseline [¹¹C]‐(+)‐PHNO BP_ND that can be accounted for by variation in endogenous DA levels across the striatal subregions ranged from 42‐59%. These results indicate that lower baseline values of [¹¹C]‐(+)‐PHNO BP_ND reflect greater stimulation by endogenous DA. To further validate this interpretation, we sought to examine whether these data could be used to estimate the dissociation constant (Kd) of DA at D_2/3R. In line with previous in vitro work, we estimated the in vivo Kd of DA to be around 20 nM. In summary, the agonist radiotracer [¹¹C]‐(+)‐PHNO can detect the impact of endogenous DA levels at D_2/3R in the living human brain from a single baseline scan, and may be more sensitive to this impact than other commonly employed radiotracers.     

Differential effects of stress on [11C]raclopride and [11C]MNPA binding to striatal D2/D3 dopamine receptors: A PET study in conscious monkeys

It has been reported that stress and facilitation of dopamine neuronal system are closely related. In the present study, the effects of stress on the binding of antagonist‐based [¹¹C]raclopride and agonist‐based (R)‐2‐CH3O‐N‐n‐ propylnorapomorphine ([¹¹C]MNPA) to D₂/D₃ receptors were evaluated in the striatum of conscious monkey brain. The stress state assessed from plasma cortisol level was negatively correlated with [¹¹C]raclopride binding as expected. It was noteworthy that [¹¹C]MNPA binding exhibited a positive correlation with stress state; thus, the animals with higher cortisol levels showed higher binding to D₂/D₃ receptors. Synapse 65:84–89, 2011. © 2010 Wiley‐Liss, Inc.