Dopamine (D2/3) receptor agonist positron emission tomography radiotracer [11C]-(+)-PHNO is a D3 receptor preferring agonist in vivo

[11C]PHNO is a recently introduced agonist to image DA D2-like receptors with Positron Emission Tomography (PET). In cats and humans, [11C]PHNO revealed an atypical distribution compared to radiolabeled D2-like antagonists (such as [11C]raclopride) or other D2-like agonists (such as [11C]NPA), as it displayed unusual high binding in the globus pallidus (GP). The goal of this study was to assess the pharmacological nature of the binding of [11C]PHNO in the GP in nonhuman primates. As previously reported in humans, [11C]PHNO equilibrium specific to nonspecific equilibrium partition coefficients (V3') in baboons was much higher in GP (3.88 +/- 1.15) than in the dorsal striatum (DST, 2.07 +/- 0.43), whereas the reverse was true for [11C]raclopride (1.48 +/- 0.41 in GP, 2.56 +/- 0.91 in DST) and [11C]NPA (0.87 +/- 0.19 in GP, 1.02 +/- 0.13 in DST). Administration of unlabeled raclopride resulted in similar reductions of [11C] PHNO V3' and [11C]raclopride V3' in both the GP and the DST. This observation demonstrated that the [11C]PHNO binding in the GP was specific to D2-like receptors. To evaluate the respective contribution of D3 and D2 receptors to the binding potential (BP) of [11C]PHNO and [11C]raclopride, experiments were carried out with the selective D3 partial agonist 1-(4(2-Napthoylamino)butyl)-4-(2-methoxyphenyl)-1A-piperazine HCL (BP897). BP897 reduced [11C]raclopride V3' by 29% +/- 9%, 19% +/- 8%, and 10% +/- 7% in GP, VST, and DST, respectively, a result consistent with expectation from postmortem studies (D3/D2 ratio in GP > VST > DST). BP897 reduced [11C]PHNO V3' by 57% +/- 11%, 30% +/- 11%, and 13% +/- 8% in GP, VST, and DST, respectively, indicating that the D3 receptor contribution to [11C]PHNO signal is higher than that of [11C]raclopride. From these experiments we conclude that [11C]PHNO is a D3 preferring agonist, and that this property explains the high GP signal not observed with [11C]raclopride or [11C]NPA. This property might contribute to its higher vulnerability to endogenous DA compared to [11C]raclopride and [11C]NPA.     

Occupancy of dopamine D2/3 receptors in rat brain by endogenous dopamine measured with the agonist positron emission tomography radioligand [11C]MNPA

Estimates of dopamine D(2/3) receptor occupancy by endogenous dopamine using positron emission tomography (PET) in animals have varied almost threefold. This variability may have been caused by incomplete depletion of dopamine or by the use of antagonist radioligands, which appear less sensitive than agonist radioligands to changes in endogenous dopamine. PET scans were performed in rats with the agonist PET radioligand [(11)C]MNPA ([O-methyl-(11)C]2-methoxy-N-propylnorapomorphine). [(11)C]MNPA was injected as a bolus plus constant infusion to achieve steady-state concentration in the body and equilibrium receptor binding in the brain. Radioligand binding was compared at baseline and after treatment with reserpine plus alpha-methyl-para-tyrosine, which cause approximately 95% depletion of endogenous dopamine. Depletion of dopamine increased radioligand binding in striatum but had little effect in cerebellum. Striatal [(11)C]MNPA binding potential was 0.93 +/- 0.12 at baseline and increased to 1.99 +/- 0.25 after dopamine depletion. Occupancy of D(2/3) receptors by endogenous dopamine at baseline was calculated to be approximately 53%. Striatal binding was displaceable with raclopride, but not with BP 897 (a selective D(3) compound), thus confirming the D(2) receptor specificity of [(11)C]MNPA binding. Radioactivity extracted from rat brain contained only 8-10% radiometabolites and was insignificantly altered by administration of reserpine plus alpha-methyl-para-tyrosine. Hence, dopamine depletion did not increase the PET measurements via an effect on radiotracer metabolism. Our in vivo estimate of dopamine's occupancy of D(2/3) receptors at baseline is higher than that previously reported using antagonist radioligands and PET, but is similar to that reported using agonist radioligands and ex vivo measurements.     

Radiosynthesis, ex vivo and in vivo evaluation of [11C]preclamol as a partial dopamine D2 agonist radioligand for positron emission tomography

Dopamine D2 partial agonists have been successfully used as schizophrenia therapeutics. Radiolabeled D2 partial agonists may have application in elucidating dopaminergic transmission. It was the goal of this work to radiolabel (S)-(-)propyl-3-(3-hydroxyphenyl)piperidine (preclamol; (-)3-PPP), a partial dopamine D2 agonist with carbon-11 (half-life=20.4 min) and to evaluate this novel radiopharmaceutical for dopaminergic imaging in rodent models. [11C]Preclamol was synthesized by acylation of (S)-3-(3-hydroxyphenyl)piperidine hydrochloride with [11C]propionyl chloride, followed by LiAlH4 reduction, and HPLC purification. Male Sprague-Dawley rats were injected in the tail vein with a saline solution of [11C]preclamol (1.1 mug/kg) and sacrificed at 5, 15, 30 and 60 min postinjection. Brain regions were excised, weighed, and measured for radioactivity. In vivo binding kinetics of [11C]preclamol were determined with beta-sensitive microprobes implanted into the striatum and cerebellum of an anesthetized rat. A full production of [11C]preclamol resulted in 34 mCi ready for injection (corresponding to 4% uncorrected radiochemical yield, based on starting [11C]CO2) with specific activity of 535 mCi/micromol. The total synthesis time was 45 min and resulted in chemically and radiochemically pure [11C]preclamol (>99%; n=3). High levels of radioactivity were observed in rat brain indicating good blood-brain barrier penetration of [11C]preclamol, with 0.5 to 0.7% injected dose per gram of wet tissue present in all brain regions at 5 minutes postinjection. Unfortunately, [11C]preclamol displayed minimal preferential uptake in dopaminergic brain regions. A low striatal specific binding (SB) ratio of 0.32 was determined ex vivo at 60 min postinjection and was in close agreement with the microprobe study over 60 min (peaked at 27 min postinjection; SB ratio=0.6). The binding potential value was only 0.34 over a 1 hour time course, suggesting that [11C]preclamol is not suitable for cerebral PET studies.     

Evaluation of d-amphetamine effects on the binding of dopamine D-2 receptor radioligand, 18F-fallypride in nonhuman primates using positron emission tomography

We have investigated the ability of dopamine to compete with the binding of the high affinity dopamine D₂ receptor positron emission tomography (PET) radioligand, ¹⁸F-fallypride. In vitro dissociation of ¹⁸F-fallypride with dopamine in rat striatal homogenates exhibited a dissociation rate, k_off, of 1.76 × 10⁻² min⁻¹ while the association rate constant, k_on, was found to be 5.30 × 10⁸ M⁻¹ min⁻¹. This resulted in a dissociation constant, K_D of 33 pM for ¹⁸F-fallypride. For in vivo studies, we investigated the effects of reserpine and d-amphetamine treatment on ¹⁸F-fallypride in an attempt to study competition of endogenous dopamine with the radioligand at the receptor sites in rats and monkeys. PET experiments with ¹⁸F-fallypride in two male rhesus monkeys were carried out in a PETT VI scanner. In control experiments, rapid specific uptake of ¹⁸F-fallypride in the striata was observed (0.05–0.06% injected dose (ID)/g) while nonspecifically bound tracer cleared from other parts of the brain. Striata/cerebellum ratios for ¹⁸F-fallypride were approximately 8 at 80 min postinjection, respectively. The monkeys received various doses (0.25 to 1.50 mg/kg) of d-amphetamine (AMPH) pre- and postinjection of the radioligand. There was a decrease of specifically bound ¹⁸F-fallypride as well as evidence of an enhanced clearance of specifically bound ¹⁸F-fallypride after administering AMPH in the two monkeys. The dissociation rates, k_off, of ¹⁸F-fallypride without AMPH was <10⁻⁴ min⁻¹ but after 25 min preadministration of AMPH (1 mg/kg), it was 4.1 × 10⁻³ min⁻¹ and after 17, 45 and 90 min postadministration of AMPH (1 mg/kg) it was 3.6 × 10⁻³ to 4.0 × 10⁻³ min⁻¹. Lower doses of AMPH (0.25 mg/kg) had a reduced effect on the binding of ¹⁸F-fallypride. No effect was seen until about 30 minutes after the injection of AMPH. Studies with various doses indicated that ¹⁸F-fallypride has a maximum response at doses of 0.75–1.50 mg/kg, with an approximately 16%/hour reduction in binding. These results indicate that AMPH stimulated release of endogenous dopamine reduces the specific binding of ¹⁸F-fallypride. Synapse 27:1–13, 1997. © 1997 Wiley-Liss, Inc.     

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.     

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.     

Binding characteristics of high-affinity dopamine D2/D3 receptor agonists, 11C-PPHT and 11C-ZYY-339 in rodents and imaging in non-human primates by PET

We have evaluated the in vitro autoradiographic binding characteristics and in vivo brain distribution of two high-affinity dopamine D2/D3 receptor agonists, (+/-)-2-(N-phenethyl-N-1'-11C-propyl)amino-5-hydroxytetralin (11C-PPHT) and (+/-)-2-(N-cyclohexylethyl-N-1'-11C-propyl)amino-5-hydroxytetralin (11C-ZYY-339) in rodents and in monkeys using positron emission tomography (PET). In vitro autoradiograms in rat brain slices with (11)C-PPHT and 11C-ZYY-339 revealed binding to dopaminergic regions in the striata, which was substantially (>90%) displaced by 10 microM sulpiride. Striatal binding was also removed in the presence of 5-guanylylimidophosphate (Gpp(NH)p), indicative of binding of these radiotracers to the high-affinity (HA) state. The results of in vivo studies in rats exhibited binding of the two radiotracers to the striata (striata/cerebellum approached 2 in 30 min). The regional selectivity to the striata was reduced by preadministration of haloperidol. PET studies in male rhesus monkeys using an ECAT EXACT HR+ scanner indicated localization of 11C-PPHT and 11C-ZYY-339 in the striata and thalamus. Striata to cerebellum and thalamus to cerebellum ratios were low (1.5 and 1.3, respectively, at 30 min postinjection) for both 11C-PPHT and 11C-ZYY-339, apparently due to the slower nonspecific clearance from cerebellum. These findings with 11C-PPHT and 11C-ZYY-339 indicate the possibility of in vivo imaging of high-affinity state of dopamine D2/D3 receptors in both the striata and the thalamus.     

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.     

Similar striatal D2/D3 dopamine receptor availability in adults with Tourette syndrome compared with healthy controls: A [11C]‐(+)‐PHNO and [11C]raclopride positron emission tomography imaging study

Pharmacological and anatomical evidence implicates striatal dopamine receptors in Tourette syndrome (TS). Nevertheless, results of positron emission tomography (PET) studies of the dopamine system in TS have been inconsistent. We investigated striatal D2/3 dopamine receptors in TS using the radioligands [¹¹C]raclopride and [¹¹C]‐(+)‐PHNO, an agonist that binds preferentially to D3 receptors, thus allowing higher sensitivity and measurement of receptors in a high affinity state. Eleven adults with TS and 11 matched healthy control (HC) participants underwent [¹¹C]raclopride and [¹¹C]‐(+)‐PHNO PET scans. General linear model was used for voxelwise contrasts of striatal binding potentials (BP_ND) between TS and HC participants. Analysis of variance was performed to investigate main effect of radioligand. In addition, BP_ND values were extracted for ventral, motor, and associative striatum. Finally, we examined the relationship between BP_ND measures and symptom severity in TS participants. Main effects analyses showed that [¹¹C]‐(+)‐PHNO BP_ND was higher in ventral striatum, whereas [¹¹C]raclopride BP_ND was higher in motor and associative striatum. There were no significant group differences between TS and HC. Furthermore, TS and HC participants had similar [¹¹C]‐(+)‐PHNO and [¹¹C]raclopride BP_ND in the three striatal subregions. Moreover, there was no significant correlation between BP_ND and symptom severity. TS and HC participants had similar striatal D2/3 receptor availability measures. These results challenge the assumption that striatal dopamine receptors have a major role in the pathophysiology of TS. Consistent with previous findings, [¹¹C]‐(+)‐PHNO localized preferentially to ventral striatal, D3 receptor‐rich regions, in contrast to [¹¹C]raclopride, which localized preferentially in the dorsal striatum. Hum Brain Mapp 36:2592–2601, 2015. © 2015 Wiley Periodicals, Inc .     

In vivo mesolimbic D2/3 receptor binding predicts posttherapeutic clinical responses in restless legs syndrome: a positron emission tomography study

Although D2/3 agonists have been used as a first-line medication for idiopathic restless legs syndrome (iRLS), findings on D2/3 receptors have been inconsistent. Here, we aimed to clarify the contribution of D2/3 receptor function to the clinical symptoms of iRLS by comparing the binding potential (BP_ND) of [¹¹C]raclopride with clinical improvements after D2/3 stimulation by pramipexole. Eight drug-naïve, iRLS patients and eight age-matched healthy subjects were scanned with positron emission tomography (PET). After PET scans, all patients received pramipexole (0.125 mg) orally for 2 weeks. Patients were evaluated every day with several standardized clinical tests. The BP_ND values were compared using regions of interest and voxel-based methods. Results showed that the mean magnitude of [¹¹C]raclopride BP_ND in the mesolimbic dopamine region (nucleus accumbens (NA) and caudate) was significantly lower in the iRLS group. No significant differences between groups were observed in the putamen. The NA [¹¹C]raclopride BP_ND levels correlated negatively with clinical severity scores and positively with the degree of posttreatment improvement in iRLS. The present results suggest that alterations in mesolimbic D2/3 receptor function reflect the pathophysiology of iRLS, and the baseline availability of D2/3 receptors may predict the clinical outcome after D2/3 agonist treatment.     

Electroconvulsive shock decreases binding to 5-HT2 receptors in nonhuman primates: an in vivo positron emission tomography study with [18F]setoperone.

BACKGROUND: Dysfunction within the serotonin (5-HT) system plays a major role in the etiology of human depression, and treatment with antidepressant drugs downregulates 5-HT(2) receptors in rodents and humans. The consequences of another effective antidepressant treatment, electroconvulsive therapy (ECT), on 5-HT(2) receptors are less established. METHODS: We studied the effects of a course of electroconvulsive shock (ECS) on 5-HT(2) receptor binding in nonhuman primates in vivo using positron emission tomography (PET) and the radiotracer [(18)F]setoperone. Seven adult male rhesus monkeys received two bilateral ECS treatments per week for 3 weeks; PET scans were performed before treatment, and 24 hours, 1 week, and 4-6 weeks after completion of the course of ECS. Regions of interest were placed throughout the cortex, and the data analyzed as the ratio of specific:nonspecific radioactivity accumulation, with the cerebellum used as a measure of nonspecific binding. RESULTS: Serotonin 5-HT(2) binding was significantly decreased at 24 hours and 1 week post-ECS, but returned to baseline 4-6 weeks posttreatment. CONCLUSIONS: These results show for the first time in a primate species that chronic ECS decreases binding to 5-HT(2) receptors and indicate that 5-HT(2) receptor downregulation may be a common effect of both pharmacologic and nonpharmacologic antidepressant treatments.     

[11C]β-CIT-FE,a radioligand for quantitation of the dopamine transporter in the living brain using positron emission tomography

The cocaine analogue β-CIT-FE (N-(2-fluoroethyl)-2β-carbomethoxy-3β-(4-iodophenyl)nortropane) was labeled with ¹¹C for positron emission tomography (PET) studies of the dopamine transporter. After intravenous administration to a cynomolgus monkey, [¹¹C]β-CIT-FE accumulated in the striatum with a striatum-to-cerebellum ratio of about 9 after 60 min. Pseudoequilibrium of specific [¹¹C]β-CIT-FE binding in the striatum was obtained within 30–50 min. The radioactivity ratios of the thalamus to the cerebellum and the neocortex to the cerebellum were about 2 and 1.5, respectively. In displacement and pretreatment experiments, radioactivity in the striatum but not in the cerebellum was reduced after injection of β-CIT or the dopamine transporter inhibitor GBR 12909, indicating that striatal radioactivity following injection of [¹¹C]β-CIT-FE represents reversible binding to dopamine transporter sites. After displacement or pretreatment with cocaine there was a marked effect not only in the striatum but also in the thalamus and neocortex. [¹¹C]β-CIT-FE has potential as a useful PET radioligand for quantitation of the dopamine transporter in the primate brain in vivo. © 1996 Wiley-Liss, Inc.     

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.     

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.     

Dopamine D2/3 receptor occupancy of apomorphine in the nonhuman primate brain—A comparative PET study with [11C]raclopride and [11C]MNPA

Binding studies in vitro have demonstrated that the dopamine D₂ receptor may exist in two affinity states for agonists. The high affinity state is thought to represent the functional state of the receptor and proportions might alter during disease. In vitro studies further indicate that agonists induce measurable D₂ receptor occupancy at clinically relevant concentrations but only when measured at the high affinity state. Recently developed PET‐radioligands, such as [¹¹C]MNPA, have now made it possible to directly study agonist binding in vivo. The aim of this study was to compare the inhibition by apomorphine of agonist and antagonist radioligand binding to D_2/3 receptors in vivo. A total of 36 PET measurements were performed with the D_2/3 antagonist [¹¹C]raclopride or the D_2/3 agonist [¹¹C]MNPA in two cynomolgus monkeys. On each study day, a baseline measurement was followed by two consecutive pretreatment studies with rising doses of apomorphine (0.01, 0.05, 0.15, 0.5, 1.0, and 3.0 mg/kg). Binding potential (BP_ND) values were calculated for the striatum with cerebellum as reference region. Apomorphine inhibited [¹¹C]raclopride and [¹¹C]MNPA binding in a dose‐dependent manner and to a similar extent. ID₅₀ and K_i values were 0.26 mg/kg and 29 ng/ml for [¹¹C]raclopride and 0.50 mg/kg and 31 ng/ml for [¹¹C]MNPA. The present observations do not support the existence of two affinity states in vivo. It might thus be speculated that all D_2/3 receptors are in the high affinity state at in vivo conditions. Synapse 63:378–389, 2009. © 2009 Wiley‐Liss, Inc.     

Effect of endogenous dopamine on endogenous dopamine on extrastriated [11C]FLB 457 binding measured by PET

Central dopaminergic systems are known to be implicated in the pathophysiology of schizophrenia and recent in vivo dopamine receptor imaging studies have focused on the measurement of extrastriatal dopamine receptor. However, there are only a limited number of ligands that can measure the low-density D2 receptor in extrastriatal regions and their sensitivity to endogenous dopamine in extrastriatal regions has not yet been fully examined. In this study, the effect of endogenous dopamine on the extrastriatal binding of [(11)C]FLB 457 was examined in the rhesus monkey after facilitation with 1 mg/kg of methamphetamine (MAP) and was compared with the effect on the striatal binding of [(11)C]raclopride. The indices of receptor binding were obtained by four methods using cerebellum as a reference region. The bindings of [(11)C]FLB 457 in the frontal cortex, temporal cortex, and thalamus were not significantly changed after MAP treatment, while the striatal binding of [(11)C]raclopride was decreased by more than 20%. These results suggest that [(11)C]FLB 457 is not sensitive to endogenous dopamine in the extrastriatal regions of rhesus monkeys, despite a sufficient dose of MAP to decrease the binding of [(11)C]raclopride in the striatum.     

Effect of amphetamine on dopamine D2 receptor binding in nonhuman primate brain: a comparison of the agonist radioligand [11C]MNPA and antagonist [11C]raclopride

PET measurements of stimulant-induced dopamine (DA) release are typically performed with antagonist radioligands that bind to both the high- and low-affinity state of the receptor. In contrast, an agonist radioligand binds preferentially to the high-affinity state and is expected to have greater sensitivity to DA, which is the endogenous agonist. [(11)C]MNPA, (R)-2-CH(3)O-N-n-propylnorapomorphine (MNPA), is a D(2) agonist radioligand with subnanomolar affinity to the D(2) receptor. The aim of the present study is to assess and compare the sensitivity of the agonist radioligand [(11)C]MNPA and antagonist radioligand [(11)C]raclopride to synaptic DA levels. Four cynomolgus monkeys were examined with [(11)C]MNPA and [(11)C]raclopride (16 PET measurements with each tracer) at baseline and after pretreatment with various doses of amphetamine. The effect of amphetamine was calculated by the change in binding potential (BP) analyzed with the multilinear reference tissue model (MRTM2). Amphetamine caused a reduction in [(11)C]MNPA BP of 4% at 0.1, 23% at 0.2, 25% at 0.5, and 46% at 1.0 mg/kg. [(11)C]Raclopride BP was reduced to a lesser extent by 2% at 0.1, 16% at 0.2, 15% at 0.5, and 23% at 1.0 mg/kg. The data were used to estimate the in vivo percentage of high-affinity state receptors to be approximately 60%. These results demonstrate that [(11)C]MNPA is more sensitive than [(11)C]raclopride to displacement by endogenous DA, and that it may provide additional information about the functional state of the D(2) receptor in illnesses such as schizophrenia and Parkinson's disease.     

Effect of reduction in endogenous dopamine on extrastriatal binding of [11C]FLB 457 in rat brain--an ex vivo study

Carbon-11 labeled FLB 457 has been used successfully as a selective, high affinity PET ligand for the quantification of extrastriatal D2-like receptors in man. This study was carried out in rats to investigate regional values for maximal binding and ED50 (a measure of apparent K(d)) for the radioligand in vivo in control animals and in a group pretreated with the neuronal impulse flow inhibitor, gamma-butyrolactone. The aims were to obtain further information regarding the specific activity needed to ensure tracer kinetics and to investigate baseline occupancy by dopamine (DA), each relevant to optimal clinical use of the radioligand. Regional B(max) values were consistent with the distribution of D2-like receptors in rat brain. Of interest, 60% of the binding in cerebellum, often used as a low-binding "reference region" for PET quantification, was saturable, with B(max) only 2- to 3-fold less than that in neocortex, hippocampus, and thalamus. ED50 values were in the range 2-3 nmol/kg, confirming minimal receptor occupancy by the tracer in human PET, using high but achievable specific activities. In the majority of extrastriatal tissues, reduction in synaptic DA did not significantly decrease the apparent K(d), except in cortical regions, where the extent of the effect suggested a low ( approximately 10%), but measurable baseline receptor occupancy by DA.     

First visualization of adenosine A(2A) receptors in the human brain by positron emission tomography with [11C]TMSX

[11C]TMSX is a new positron emission tomography (PET) radioligand that provides visualization of adenosine A(2A) receptors (A(2A)Rs) in the brain, heart and skeletal muscle. Here we report on the first visualization of the A(2A)Rs in the human brain by PET and [11C]TMSX in a male healthy volunteer, compared with the adenosine A1 receptors (A1Rs) and dopamine D2 receptors (D2Rs) which were measured by PET with [11C]MPDX and [11C]raclopride, respectively. The distribution volume (DV) of [11C]TMSX in the baseline was relatively high in the head of caudate nucleus, putamen, and thalamus and relatively low in the cortical regions. Infusion of theophylline, a nonselective A(2A)R antagonist (Ki for A(2A)Rs = 16000 nM for theophylline vs 5.9 nM for TMSX), slightly reduced the DVs in the head of caudate nucleus (8.0% reduction) and putamen (4.5% reduction), but not in the other regions having much lower levels of A(2A)Rs, demonstrating the A(2A)R-specific binding of [11C]TMSX. On the other hand, the A1Rs were widely distributed in the whole brain except for the cerebellum, while the binding potential of [11C]raclopride was predominantly high in the striatum. We concluded that [11C]TMSX is an applicable PET ligand for mapping the A(2A)Rs in the caudate nucleus and putamen in clinical studies because of no availability of other radioligands until now. The [11C]TMSX PET is of great interest for studying the pathophysiology of neurological and psychiatric disorders together with the [11C]raclopride PET for D2Rs evaluation and/or the [11C]MPDX PET for A1Rs evaluation.