Specific binding of [(11)C]raclopride and N-[(3)H]propyl-norapomorphine to dopamine receptors in living mouse striatum: occupancy by endogenous dopamine and guanosine triphosphate-free G protein

Brain Serotonin Release Is Reduced in Patients With Depression: A [11C]Cimbi-36 Positron Emission Tomography Study With a d-Amphetamine Challenge

BACKGROUND

The serotonin hypothesis of depression proposes that diminished serotonergic (5-HT) neurotransmission is causal in the pathophysiology of the disorder. Although the hypothesis is over 50 years old, there is no firm in vivo evidence for diminished 5-HT neurotransmission. We recently demonstrated that the 5-HT_2A receptor agonist positron emission tomography (PET) radioligand [¹¹C]Cimbi-36 is sensitive to increases in extracellular 5-HT induced by an acute d-amphetamine challenge. Here we applied [¹¹C]Cimbi-36 PET to compare brain 5-HT release capacity in patients experiencing a major depressive episode (MDE) to that of healthy control subjects (HCs) without depression.

METHODS

Seventeen antidepressant-free patients with MDE (3 female/14 male, mean age 44 ± 13 years, Hamilton Depression Rating Scale score 21 ± 4 [range 16-30]) and 20 HCs (3 female/17 male, mean age 32 ± 9 years) underwent 90-minute dynamic [¹¹C]Cimbi-36 PET before and 3 hours after a 0.5-mg/kg oral dose of d-amphetamine. Frontal cortex (main region of interest) 5-HT_2A receptor nondisplaceable binding was calculated from kinetic analysis using the multilinear analysis-1 approach with the cerebellum as the reference region.

RESULTS

Following d-amphetamine administration, frontal nondisplaceable binding potential (BP_ND) was significantly reduced in the HC group (1.04 ± 0.31 vs. 0.87 ± 0.24, p < .001) but not in the MDE group (0.97 ± 0.25 vs. 0.92 ± 0.22, not significant). ΔBP_ND of the MDE group was significantly lower than that of the HC group (HC: 15% ± 14% vs. MDE: 6.5% ± 20%, p = .041).

CONCLUSIONS

This first direct assessment of 5-HT release capacity in people with depression provides clear evidence for dysfunctional serotonergic neurotransmission in depression by demonstrating reduced 5-HT release capacity in patients experiencing an MDE.

Molecular and Functional Imaging Studies of Psychedelic Drug Action in Animals and Humans

Hallucinogens are a loosely defined group of compounds including LSD, N,N-dimethyltryptamines, mescaline, psilocybin/psilocin, and 2,5-dimethoxy-4-methamphetamine (DOM), which can evoke intense visual and emotional experiences. We are witnessing a renaissance of research interest in hallucinogens, driven by increasing awareness of their psychotherapeutic potential. As such, we now present a narrative review of the literature on hallucinogen binding in vitro and ex vivo, and the various molecular imaging studies with positron emission tomography (PET) or single photon emission computer tomography (SPECT). In general, molecular imaging can depict the uptake and binding distribution of labelled hallucinogenic compounds or their congeners in the brain, as was shown in an early PET study with N1-([11C]-methyl)-2-bromo-LSD ([11C]-MBL); displacement with the non-radioactive competitor ketanserin confirmed that the majority of [11C]-MBL specific binding was to serotonin 5-HT2A receptors. However, interactions at serotonin 5HT1A and other classes of receptors and pleotropic effects on second messenger pathways may contribute to the particular experiential phenomenologies of LSD and other hallucinogenic compounds. Other salient aspects of hallucinogen action include permeability to the blood-brain barrier, the rates of metabolism and elimination, and the formation of active metabolites. Despite the maturation of radiochemistry and molecular imaging in recent years, there has been only a handful of PET or SPECT studies of radiolabeled hallucinogens, most recently using the 5-HT2A/2C agonist N-(2[11CH3O]-methoxybenzyl)-2,5-dimethoxy- 4-bromophenethylamine ([11C]Cimbi-36). In addition to PET studies of target engagement at neuroreceptors and transporters, there is a small number of studies on the effects of hallucinogenic compounds on cerebral perfusion ([15O]-water) or metabolism ([18F]-fluorodeoxyglucose/FDG). There remains considerable scope for basic imaging research on the sites of interaction of hallucinogens and their cerebrometabolic effects; we expect that hybrid imaging with PET in conjunction with functional magnetic resonance imaging (fMRI) should provide especially useful for the next phase of this research.

The High Affinity Dopamine D2 Receptor Agonist MCL-536: A New Tool for Studying Dopaminergic Contribution to Neurological Disorders

The dopamine D₂ receptor exists in two different states, D₂^high and D_2low; the former is the functional form of the D₂ receptor and associates with intracellular G-proteins. The D₂ agonist [³H]MCL-536 has high affinity for the D₂ receptor (K_d 0.8 nM) and potently displaces the binding of (R-(-)-N-n-propylnorapomorphine (NPA; K_i 0.16 nM) and raclopride (K_i 0.9 nM) in competition binding assays. Here, we further characterize [³H]MCL-536. [³H]MCL-536 was metabolically stable, with about 75% of the compound remaining intact after 1 h incubation with human liver microsomes. Blood-brain barrier penetration in rats was good, attaining at 15 min a % injected dose per gram of wet tissue (%ID/g) of 0.28 in males versus 0.42 in females in the striatum. Specific uptake ratios ([%ID/g striatum]/[%ID/g cerebellum]) were stable in males during the first 60 min and in females up to 15-30 min. The D₂-rich striatum exhibited the highest uptake and slowest washout compared to D₂-poor cortex or cerebellum. In peripheral organs, uptake peaked at 15 min but declined to baseline at 60 min, indicating good clearance from the body. In vitro autoradiography on transaxial and coronal brain sections showed specific binding of [³H]MCL-536, which was abolished by preincubation with D₂/D₃ ligands sulpiride, NPA, and raclopride and in the presence of the stable GTP analogue guanylylimidodiphosphate. In amphetamine-sensitized animals, striatal binding was higher than in controls, indicating specificity for the D₂^high receptor state. [³H]MCL-536's unique properties make it a valuable tool for research on neurological disorders involving the dopaminergic system like Parkinson's disease or schizophrenia.

An open-label, positron emission tomography study of the striatal D2/D3 receptor occupancy and pharmacokinetics of single-dose oral brexpiprazole in healthy participants

Purpose

The aim of this Phase 1, open-label, positron emission tomography (PET) study was to determine the degree of striatal D₂/D₃ receptor occupancy induced by the serotonin–dopamine activity modulator, brexpiprazole, at different single dose levels in the range 0.25–6 mg.

Methods

Occupancy was measured at 4 and 23.5 h post-dose using the D₂/D₃ receptor antagonist [¹¹C]raclopride. The pharmacokinetics, safety and tolerability of brexpiprazole were assessed in parallel.

Results

Fifteen healthy participants were enrolled (mean age 33.9 years; 93.3% male). Mean D₂/D₃ receptor occupancy in the putamen and caudate nucleus increased with brexpiprazole dose, leveled out at 77–88% with brexpiprazole 5 mg and 6 mg at 4 h post-dose, and remained at a similar level at 23.5 h post-dose (74–83%). Estimates of maximum obtainable receptor occupancy (O_max) were 89.2% for the putamen and 95.4% for the caudate nucleus; plasma concentrations predicted to provide 50% of O_max (EC₅₀) were 8.13 ng/mL and 7.75 ng/mL, respectively. Brexpiprazole area under the concentration–time curve (AUC_∞) and maximum plasma concentration (C_max) increased approximately proportional to dose. No notable subjective or objective adverse effects were observed in this cohort.

Conclusion

By extrapolating the observed single-dose D₂/D₃ receptor occupancy data in healthy participants, multiple doses of brexpiprazole 2 mg/day and above are expected to result in an efficacious brexpiprazole concentration, consistent with clinically active doses in schizophrenia and major depressive disorder.

Trial registration

ClinicalTrials.gov NCT00805454 December 9, 2008.

Electronic supplementary material

The online version of this article (10.1007/s00228-020-03021-9) contains supplementary material, which is available to authorized users.

Molecular imaging of schizophrenia: Neurochemical findings in a heterogeneous and evolving disorder

The past four decades have seen enormous efforts placed on a search for molecular markers of schizophrenia using positron emission tomography (PET) and single photon emission computed tomography (SPECT). In this narrative review, we cast a broad net to define and summarize what researchers have learned about schizophrenia from molecular imaging studies. Some PET studies of brain energy metabolism with the glucose analogue FDGhave have shown a hypofrontality defect in patients with schizophrenia, but more generally indicate a loss of metabolic coherence between different brain regions. An early finding of significantly increased striatal trapping of the dopamine synthesis tracer FDOPA has survived a meta-analysis of many replications, but the increase is not pathognomonic of the disorder, since one half of patients have entirely normal dopamine synthesis capacity. Similarly, competition SPECT studies show greater basal and amphetamine-evoked dopamine occupancy at post-synaptic dopamine D_2/3 receptors in patients with schizophrenia, but the difference is likewise not pathognomonic. We thus propose that molecular imaging studies of brain dopamine indicate neurochemical heterogeneity within the diagnostic entity of schizophrenia. Occupancy studies have established the relevant target engagement by antipsychotic medications at dopamine D_2/3 receptors in living brain. There is evidence for elevated frontal cortical dopamine D₁ receptors, especially in relation to cognitive deficits in schizophrenia. There is a general lack of consistent findings of abnormalities in serotonin markers, but some evidence for decreased levels of nicotinic receptors in patients. There are sparse and somewhat inconsistent findings of reduced binding of muscarinic, glutamate, and opioid receptors ligands, inconsistent findings of microglial activation, and very recently, evidence of globally reduced levels of synaptic proteins in brain of patients. One study reports a decline in histone acetylase binding that is confined to the dorsolateral prefrontal cortex. In most contexts, the phase of the disease and effects of past or present medication can obscure or confound PET and SPECT findings in schizophrenia.

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

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

Hunting for the high-affinity state of G-protein-coupled receptors with agonist tracers: Theoretical and practical considerations for positron emission tomography imaging

The concept of the high‐affinity state postulates that a certain subset of G‐protein‐coupled receptors is primarily responsible for receptor signaling in the living brain. Assessing the abundance of this subset is thus potentially highly relevant for studies concerning the responses of neurotransmission to pharmacological or physiological stimuli and the dysregulation of neurotransmission in neurological or psychiatric disorders. The high‐affinity state is preferentially recognized by agonists in vitro. For this reason, agonist tracers have been developed as tools for the noninvasive imaging of the high‐affinity state with positron emission tomography (PET). This review provides an overview of agonist tracers that have been developed for PET imaging of the brain, and the experimental paradigms that have been developed for the estimation of the relative abundance of receptors configured in the high‐affinity state. Agonist tracers appear to be more sensitive to endogenous neurotransmitter challenge than antagonists, as was originally expected. However, other expectations regarding agonist tracers have not been fulfilled. Potential reasons for difficulties in detecting the high‐affinity state in vivo are discussed.

Marmoset Serotonin 5-HT1A Receptor Mapping with a Biased Agonist PET Probe 18F-F13714: Comparison with an Antagonist Tracer 18F-MPPF in Awake and Anesthetized States

BACKGROUND

In vivo mapping by positron emission tomography of the serotonin 1A receptors has been hindered by the lack of suitable agonist positron emission tomography probes. ¹⁸F-labeled F13714 is a recently developed biased agonist positron emission tomography probe that preferentially targets subpopulations of serotonin 1A receptors in their "active state," but its brain labeling pattern in nonhuman primate has not been described. In addition, a potential confound in the translatability of PET data between nonhuman animal and human arise from the use of anesthetics that may modify the binding profiles of target receptors.

METHODS

Positron emission tomography scans were conducted in a cohort of common marmosets (n=4) using the serotonin 1A receptor biased agonist radiotracer, ¹⁸F-F13714, compared with a well-characterized ¹⁸F-labeled antagonist radiotracer, ¹⁸F-MPPF. Experiments on each animal were performed under both consciousness and isoflurane-anesthesia conditions.

RESULTS

¹⁸F-F13714 binding distribution in marmosets by positron emission tomography differs markedly from that of the ¹⁸F-MPPF. Whereas ¹⁸F-MPPF showed highest binding in hippocampus and amygdala, ¹⁸F-F13714 showed highest labeling in other regions, including insular and cingulate cortex, thalamus, raphe, caudate nucleus, and putamen. The binding potential values of ¹⁸F-F13714 were about one-third of those observed with ¹⁸F-MPPF, with marked individual- and region-specific differences under isoflurane-anesthetized vs conscious conditions.

CONCLUSIONS

These findings highlight the importance of investigating the brain imaging of serotonin 1A receptors using agonist probes such as ¹⁸F-F13714, which may preferentially target subpopulations of serotonin 1A receptors in specific brain regions of nonhuman primate as a biased agonist.

Serotonin 2A receptor agonist binding in the human brain with [(11)C]Cimbi-36: Test-retest reproducibility and head-to-head comparison with the antagonist [(18)F]altanserin

INTRODUCTION

[(11)C]Cimbi-36 is a recently developed serotonin 2A (5-HT2A) receptor agonist positron emission tomography (PET) radioligand that has been successfully applied for human neuroimaging. Here, we investigate the test-retest variability of cerebral [(11)C]Cimbi-36 PET and compare [(11)C]Cimbi-36 and the 5-HT2A receptor antagonist [(18)F]altanserin.

METHODS

Sixteen healthy volunteers (mean age 23.9 ± 6.4years, 6 males) were scanned twice with a high resolution research tomography PET scanner. All subjects were scanned after a bolus of [(11)C]Cimbi-36; eight were scanned twice to determine test-retest variability in [(11)C]Cimbi-36 binding measures, and another eight were scanned after a bolus plus constant infusion with [(18)F]altanserin. Regional differences in the brain distribution of [(11)C]Cimbi-36 and [(18)F]altanserin were assessed with a correlation of regional binding measures and with voxel-based analysis.

RESULTS

Test-retest variability of [(11)C]Cimbi-36 non-displaceable binding potential (BPND) was consistently <5% in high-binding regions and lower for reference tissue models as compared to a 2-tissue compartment model. We found a highly significant correlation between regional BPNDs measured with [(11)C]Cimbi-36 and [(18)F]altanserin (mean Pearson's r: 0.95 ± 0.04) suggesting similar cortical binding of the radioligands. Relatively higher binding with [(11)C]Cimbi-36 as compared to [(18)F]altanserin was found in the choroid plexus and hippocampus in the human brain.

CONCLUSIONS

Excellent test-retest reproducibility highlights the potential of [(11)C]Cimbi-36 for PET imaging of 5-HT2A receptor agonist binding in vivo. Our data suggest that Cimbi-36 and altanserin both bind to 5-HT2A receptors, but in regions with high 5-HT2C receptor density, choroid plexus and hippocampus, the [(11)C]Cimbi-36 binding likely represents binding to both 5-HT2A and 5-HT2C receptors.

Synthesis and evaluation in rats of homologous series of [(18)F]-labeled dopamine D 2/3 receptor agonists based on the 2-aminomethylchroman scaffold as potential PET tracers

Background

Agonist positron emission tomography (PET) tracers for dopamine D_2/3 receptors (D_2/3Rs) offer greater sensitivity to changes in endogenous dopamine levels than D_2/3R antagonist tracers. D_2/3R agonist tracers currently available for clinical research are labeled with the short-lived isotope carbon-11, which limits their use. We aimed to develop high-affinity D₂R agonists amenable for labeling with the longer-living fluorine-18. Here, we report the evaluation as potential PET tracers of two homologous series of [¹⁸F]fluorinated tracers based on the 2-aminomethylchroman-7-ol (AMC) scaffold: (R)-2-((4-(2-fluoroalkoxy)benzylamino)methyl)chroman-7-ols (AMC13 homologues) and (R)-2-((2-(4-(4-(fluoroalkoxy)phenyl)piperazin-1-yl)ethylamino)methyl)chroman-7-ols (AMC15 homologues). We varied the length of the ¹⁸F-fluoroalkyl chain in these structures to balance brain penetration and non-specific binding of the radioligands by adjusting their lipophilicity.

Methods

The tracers were evaluated in brain slices of Sprague-Dawley rats by in vitro autoradiography and in living rats by microPET imaging and ex vivo autoradiography. PET data were analyzed with one- and two-tissue compartmental models (1TCM/2TCM), simplified reference tissue model (SRTM), and Logan graphical analysis. Specificity of binding was tested by blocking D_2/3R with raclopride.

Results

Homologues with a shorter fluoroalkyl chain consistently showed greater D_2/3R-specific-to-total binding ratios in the striatum than those with longer chains. The fluoroethoxy homologue of AMC13 ([¹⁸F]FEt-AMC13) demonstrated the highest degree of D_2/3R-specific binding among the evaluated tracers: mean striatum-to-cerebellum uptake ratio reached 4.4 in vitro and 2.1/2.8 in vivo/ex vivo (PET/autoradiography). Striatal binding potential (BP_ND) relative to cerebellum was 0.51–0.63 depending on the estimation method. Radiometabolites of [¹⁸F]FEt-AMC13 did not enter the brain. In vitro, application of 10 μmol/L raclopride reduced D_2/3R-specific binding of [¹⁸F]FEt-AMC13 in the striatum by 81 %. In vivo, pre-treatment with 1 mg/kg (2.9 μmol/kg) raclopride led to 17–39 % decrease in D_2/3R-specific binding in the striatum.

Conclusions

Varying the length of the [¹⁸F]fluoroalkyl chain helped improve the characteristics of the original candidate tracers. Further modifications of the current lead [¹⁸F]FEt-AMC13 can provide an agonist radiopharmaceutical suitable for D_2/3R imaging by PET.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-015-0119-x) contains supplementary material, which is available to authorized users.

Agonist high- and low-affinity states of dopamine D₂ receptors: methods of detection and clinical implications

Dopamine D(2) receptors, similar to other G-protein-coupled receptors, exist in a high- and low-affinity state for agonists. Based upon a review of the methods for detecting D(2) receptor agonist high-affinity states, we discuss alterations of such states in animal models of disease and the implications of such alterations for their labelling with positron emission tomography (PET) and single-photon emission computed tomography (SPECT) tracers. The classic approach of detecting agonist high-affinity states compares agonist competition for antagonist radioligands, in most cases using [(3)H]-spiperone as the radioligand; alternative approaches and radioligands have been proposed, but their claimed advantages have not been substantiated by other investigators. In view of the advantages and disadvantages of various techniques, we critically have reviewed reported findings on the detection of D(2) receptor agonist high-affinity states in a variety of animal models. These data are compared to the less numerous findings from human in vivo studies based on PET and SPECT tracers; they are interpreted in light of the finding that D(2) receptor agonist high-affinity states under control conditions may differ between rodent and human brain. The potential advantages of agonist ligands in studies of pathophysiology and as diagnostics are being discussed.

Isoflurane anaesthesia differentially affects the amphetamine sensitivity of agonist and antagonist D2/D3 positron emission tomography radiotracers: implications for in vivo imaging of dopamine release

PURPOSE

Using positron emission tomography in isoflurane-anaesthetised cat, we recently demonstrated that the effect of D-amphetamine (AMPH) was greater on the binding potential (BP(ND)) of the agonist dopamine D2/D3 radiotracer (+)-4-[(11)C]propyl-3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b][1, 4]oxazin-9-ol ([(11)C]-(+)-PHNO) than on that of the antagonist [(11)C]-raclopride, a finding that we were unable to replicate in conscious rat. Herein we tested whether isoflurane differentially affects the AMPH sensitivity of [(11)C]-(+)-PHNO and [(3)H]-raclopride.

PROCEDURES

Conscious or isoflurane-anaesthetised rats pretreated intravenously (i.v.) with saline or 4 mg/kg AMPH were co-injected i.v. with [(11)C]-(+)-PHNO/[(3)H]-raclopride or [(3)H]-(+)-PHNO/[(11)C]-(-)-N-propyl-norapomorphine ([(11)C]-(-)-NPA) and euthanised 2, 10, 20, 30, 40 or 60 min following [(11)C]-(+)-PHNO/[(3)H]-raclopride or 60 min following [(3)H]-(+)-PHNO/[(11)C]-(-)-NPA. Striatal binding at 60 min, estimated by the specific binding ratio (SBR) and the binding potential with respect to non-displaceable binding (BP(ND)) for pseudodynamic data, was calculated using the simplified reference tissue model.

RESULTS

Isoflurane increased [(11)C]-(+)-PHNO, [(3)H]-(+)-PHNO and [(11)C]-(-)-NPA SBR (mean ± SD) by 80 ± 30%, 170 ± 50% and 120 ± 40%, and doubled the effect of AMPH on the SBR of these radiotracers to -61 ± 9%, -69 ± 12% and -60 ± 12%, respectively. Neither effect was seen for [(3)H]-raclopride SBR. Similar results were observed for [(11)C]-(+)-PHNO and [(3)H]-raclopride BP(ND).

CONCLUSIONS

Isoflurane differentially increases the binding and AMPH sensitivity of [(11)C]-(+)-PHNO and [(11)C]-(-)-NPA relative to [(3)H]-raclopride, suggesting that agonist radiotracers will prove no more effective for imaging dopaminergic activity in human than antagonist radiotracers.

Imaging the high-affinity state of the dopamine D2 receptor in vivo: fact or fiction?

Positron Emission Tomography (PET) has been used for more than three decades to image and quantify dopamine D2 receptors (D2R) in vivo with antagonist radioligands but in the recent years agonist radioligands have also been employed. In vitro competition studies have demonstrated that agonists bind to both a high and a low-affinity state of the D2Rs, of which the high affinity state reflects receptors that are coupled to G-proteins and the low-affinity state reflects receptors uncoupled from G-proteins. In contrast, antagonists bind with uniform affinity to the total pool of receptors. Results of these studies led to the proposal that D2Rs exist in high and low-affinity states for agonists in vivo and sparked the development and use of agonist radioligands for PET imaging with the primary purpose of measuring the proportion of receptors in the high-affinity (activating) state. Although several lines of research support the presence of high and low-affinity states of D2Rs and their detection by in vivo imaging paradigms, a growing body of controversial data has now called this into question. These include both in vivo and ex vivo studies of anesthesia effects, rodent models with increased proportions of high-affinity state D2Rs as well as the molecular evidence for stable receptor-G-protein complexes. In this commentary we review these data and discuss the evidence for the in vivo existence of D2Rs configured in high and low-affinity states and whether or not the high-affinity state of the D2R can, in fact, be imaged in vivo with agonist radioligands.

What have positron emission tomography and 'Zippy' told us about the neuropharmacology of drug addiction?

Translational molecular imaging with positron emission tomography (PET) and allied technologies offer unrivalled applications in the discovery of biomarkers and aetiological mechanisms relevant to human disease. Foremost among clinical PET findings during the past two decades of addiction research is the seminal discovery of reduced dopamine D(2/3) receptor expression in the striatum of drug addicts, which could indicate a predisposing factor and/or compensatory reaction to the chronic abuse of stimulant drugs. In parallel, recent years have witnessed significant improvements in the performance of small animal tomographs (microPET) and a refinement of animal models of addiction based on clinically relevant diagnostic criteria. This review surveys the utility of PET in the elucidation of neuropharmacological mechanisms underlying drug addiction. It considers the consequences of chronic drug exposure on regional brain metabolism and neurotransmitter function and identifies those areas where further research is needed, especially concerning the implementation of PET tracers targeting neurotransmitter systems other than dopamine, which increasingly have been implicated in the pathophysiology of drug addiction. In addition, this review considers the causal effects of behavioural traits such as impulsivity and novelty/sensation-seeking on the emergence of compulsive drug-taking. Previous research indicates that spontaneously high-impulsive rats--as exemplified by 'Zippy'--are pre-disposed to escalate intravenous cocaine self-administration, and subsequently to develop compulsive drug taking tendencies that endure despite concurrent adverse consequences of such behaviour, just as in human addiction. The discovery using microPET of pre-existing differences in dopamine D(2/3) receptor expression in the striatum of high-impulsive rats suggests a neural endophenotype that may likewise pre-dispose to stimulant addiction in humans.

Ex vivo evaluation of the serotonin 1A receptor partial agonist [³H]CUMI-101 in awake rats

[³H]CUMI-101 is a 5-HT(1A) partial agonist, which has been evaluated for use as a positron emission tracer in baboon and humans. We sought to evaluate the properties of [³H]CUMI-101 ex vivo in awake rats and determine if [³H]CUMI-101 can measure changes in synaptic levels of serotonin after different challenge paradigms. [³H]CUMI-101 shows good uptake and good specific binding ratio (SBR) in frontal cortex 5.18 and in hippocampus 3.18. Binding was inhibited in a one-binding-site fashion by WAY100635 and unlabeled CUMI-101. The ex vivo B(max) of [³H]CUMI-101 in frontal cortex (98.7 fmol/mg) and hippocampus (131 fmol/kg) agree with the ex vivo B(max) of [³H]MPPF in frontal cortex (147.1 fmol/mg) and hippocampus (72.1 fmol/mg) and with in vitro values reported with 8-OH-DPAT. Challenges with citalopram, a selective serotonin reuptake inhibitor, fenfluramine, a serotonin releaser, and 4-chloro-DL-phenylalanine, a serotonin synthesis inhibitor, did not show any effect on the standardized uptake values (SUVs) in any region. Citalopram did alter SBR, but this was due to changes in cerebellar SUVs. Our results indicate that [³H]CUMI-101 is a good radioligand for imaging 5-HT(1A) high-density regions in rats; however, the results from pharmacological challenges remain inconclusive.

In vivo serotonin-sensitive binding of [11C]CUMI-101: a serotonin 1A receptor agonist positron emission tomography radiotracer

Positron emission tomography studies of 5-hydroxytryptamine (5-HT)(1A) receptors have hitherto been limited to antagonist radiotracers. Antagonists do not distinguish high/low-affinity conformations of G protein-coupled receptors and are less likely to be sensitive to intrasynaptic serotonin levels. We developed a novel 5-HT(1A) agonist radiotracer [(11)C]CUMI-101. This study evaluates the sensitivity of [(11)C]CUMI-101 binding to increases in intrasynaptic serotonin induced by intravenous citalopram and fenfluramine. Two Papio anubis were scanned, using [(11)C]CUMI-101 intravenous bolus of 4.5 ± 1.5 mCi. Binding potential (BP(F)=B(avail)/K(D)) was measured before (n=10) and 20 minutes after elevation of intrasynaptic serotonin by intravenous citalopram (2 mg/kg, n=3; 4 mg/kg, n=3) and fenfluramine (2.5 mg/kg, n=3) using a metabolite-corrected arterial input function. Occupancy was also estimated by the Lassen graphical approach. Both citalopram and fenfluramine effects were significant for BP(F) (P=0.031, P=0.049, respectively). The Lassen approach estimated 15.0, 30.4, and 23.7% average occupancy after citalopram 2 mg/kg, 4 mg/kg, and fenfluramine 2.5 mg/kg, respectively. [(11)C]CUMI-101 binding is sensitive to a large increase in intrasynaptic serotonin in response to robust pharmacological challenges. These modest changes in BP(F) may make it unlikely that this ligand will detect changes in intrasynaptic 5-HT under physiologic conditions; future work will focus on evaluating its utility in measuring the responsiveness of the 5-HT system to pharmacological challenges.

Dopamine beta-hydroxylase-deficient mice have normal densities of D(2) dopamine receptors in the high-affinity state based on in vivo PET imaging and in vitro radioligand binding

In vitro, D(2) dopamine receptors (DAR) can exist in low- and high-affinity states for agonists and increases of D(2) receptors in high-affinity state have been proposed to underlie DA receptor supersensitivity in vivo. Deletion of the gene for dopamine beta-hydroxylase (DBH) causes mice to become hypersensitive to the effects of psychostimulants, and in vitro radioligand binding results suggest an increased percentage of D(2) receptors in a high-affinity state. To determine whether DBH knockout mice display an increase of high-affinity state D(2) receptors in vivo, we scanned DBH knockout and control mice with the agonist PET radioligand [(11)C]MNPA, which is thought to bind preferentially to the high-affinity state of the D(2) receptor. In addition, we performed in vitro binding experiments on striatal homogenates with [(3)H]methylspiperone to measure B(max) values and the percentages of high- and low-affinity states of the D(2) receptor. We found that the in vivo striatal binding of [(11)C]MNPA was similar in DBH knockout mice and heterozygous controls and the in vitro B(max) values and percentages of D(2) receptors in the high-affinity state, were not significantly different between these two groups. In summary, our results suggest that DBH knockout mice have normal levels of D(2) receptors in the high-affinity state and that additional mechanisms contribute to their behavioral sensitivity to psychostimulants.

Endogenous competition against binding of [(18)F]DMFP and [(18)F]fallypride to dopamine D(2/3) receptors in brain of living mouse

AIM

Molecular imaging studies with benzamide radioligands can reveal competition from endogenous binding at D(2/3)-receptors in living brain. However, single photon emission computed tomography (SPECT) methods suffer from limited spatial resolution, and [(11)C]-labeled ligands are only available at positron emission tomography (PET) research sites with cyclotron-radiochemistry facilities, whereas [(18)F] can be transported, due to its longer physical half-life. Therefore, we endeavored to characterize the vulnerabilities of the benzamide antagonist [(18)F]desmethoxyfallypride (DMFP) and its high-affinity congener [(18)F]fallypride (FP) to competition from endogenous dopamine in living mouse brain.

METHODS

Groups of awake mice were pretreated with saline, amphetamine (10 mg/kg), or reserpine (5 mg/kg), followed by i.v. tracer injections. Mice were killed at 2.5-90 min (DMFP) or 2.5-180 min (FP) circulation times. Brains were dissected and regional radioactivity concentration measured by gamma counting. Other groups of mice were anesthetized for dynamic microPET recordings with DMFP or FP. Binding potentials (BP(ND)) were calculated using cerebellum as reference region.

RESULTS

With 90-min circulation, DMFP BP(ND) in striatum was 2.4 by dissection and 2.2 by microPET, which showed a 62% decrease in response to amphetamine-evoked dopamine release and a 33% increase after reserpine-evoked dopamine depletion. With 120-min circulation, FP BP(ND) in striatum was 24.1 by dissection and 9.2 by microPET, which showed a 31% decrease in the amphetamine group, but no effect of reserpine. Dissection showed similar sensitivities for FP binding, but only a 29% amphetamine-evoked reduction for DMFP.

CONCLUSIONS

Relative to gold standard ex vivo results, microPET estimates of DMFP BP(ND) were unbiased, whereas FP BP(ND) in striatum was substantially underestimated. Both tracers proved suitable for revealing pharmacologically evoked changes in competition at D(2/3)-receptors in striatum of living mice.

D2 dopamine receptor internalization prolongs the decrease of radioligand binding after amphetamine: a PET study in a receptor internalization-deficient mouse model

Dopamine released by amphetamine decreases the in vivo binding of PET radioligands to the dopamine D(2) receptor. Although concentrations of extracellular dopamine largely return to baseline within 1 to 2 h after amphetamine treatment, radioligand binding remains decreased for several hours. The purpose of this study was to determine whether the prolonged decrease of radioligand binding after amphetamine administration is caused by receptor internalization. To distinguish dopamine displacement from receptor internalization, we used wild-type and arrestin3 (arr3) knockout mice, which are incapable of internalizing D(2) receptors. In addition, we used both the D(2) selective agonist [(11)C]MNPA (which is thought to bind to the high affinity state of the receptor) and the D(2) selective antagonist [(18)F]fallypride (which does not differentiate between high and low affinity state). After an initial baseline scan, animals were divided in three groups for a second scan: either 30 min or 4 h after amphetamine administration (3 mg/kg, i.p.) or as retest. At 30 min, [(11)C]MNPA showed greater displacement than [(18)F]fallypride, but each radioligand gave similar displacement in knockout and wild-type mice. At 4 h, the binding of both radioligands returned to baseline in arr3 knockout mice, but remained decreased in wild-type mice. Radioligand binding was unaltered on retest scanning. Our results suggest that the prolonged decrease of radioligand binding after amphetamine is mainly due to internalization of the D(2) receptor rather than dopamine displacement. In addition, this study demonstrates the utility of small animal PET to study receptor trafficking in vivo in genetically modified mice.

Agonist binding fraction of dopamine D2/3 receptors in rat brain: a quantitative autoradiographic study

There has arisen considerable interest in the study of dopamine D(2/3) agonist binding sites by positron emission tomography (PET), based on the claim that agonist sites represent a functional subset of the total number of sites labeled by more conventional antagonist ligands. To test the basis of this claim, we used quantitative autoradiography to measure the abundance of binding sites of a dopamine D(2/3) agonist ([(3)H]NPA) and an antagonist ([(3)H]raclopride) in cryosections of rat brain. Saturation binding studies revealed that the B(max) for [(3)H]NPA was nearly identical to that of [(3)H]raclopride in dorsal brain regions, but was 25% less in the ventral striatum and 56% less in the olfactory tubercle. We also tested the displacement of the two ligands by the hallucinogen LSD, which is known to have dopamine agonist properties. Whereas displacement of [(3)H]raclopride by increasing LSD concentrations was monophasic, displacement of [(3)H]NPA was biphasic, suggesting an action of LSD via a subset of dopamine D(2/3) agonist binding sites. Addition of the stable GTP analogue Gpp(NH)p to the medium abolished 90% of the [(3)H]NPA binding, and increased [(3)H]raclopride binding by 10%, with a shift to the right in the LSD competition curve, suggesting retention of endogenous dopamine in washed cryostat sections. Thus [(3)H]NPA and [(3)H]raclopride binding sites have nearly identical abundances in rat dorsal striatum, but are distinct in the ventral striatum, and with respect to their displacement by LSD.

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 approximately 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.

Radiosynthesis and ex vivo evaluation of (R)-(-)-2-chloro-N-[1-11C-propyl]n-propylnorapomorphine

INTRODUCTION

Several dopamine D(2) agonist radioligands have been used with positron emission tomography (PET), including [(11)C-]-(-)-MNPA, [(11)C-]-(-)-NPA and [(11)C]-(+)-PHNO. These radioligands are considered particularly powerful for detection of endogenous dopamine release, but they either provide PET brain images with limited contrast or have affinity for both D(2) and D(3) receptors. We here present the carbon-11 radiolabeling and ex vivo evaluation of 2-Cl-(-)-NPA, a novel PET-tracer candidate with high in vitro D(2)/D(3) selectivity.

METHODS

2-Cl-[(11)C]-(-)-NPA and [(11)C]-(-)-NPA were synthesized by a two step N-acylation-reduction process using [(11)C]-propionyl chloride. Awake rats were injected with either tracer, via the tail vein. The rats were decapitated at various times, the brains were removed and quickly dissected, and plasma metabolites were measured. Radioligand specificity, and P-glycoprotein involvement in brain uptake, was also assessed.

RESULTS

2-Cl-[(11)C]-(-)-NPA and [(11)C]-(-)-NPA were produced in high specific activity and purity. 2-Cl-[(11)C]-(-)-NPA accumulated slower in the striatum than [(11)C]-(-)-NPA, reaching maximum concentrations after 30 min. The maximal striatal uptake of 2-Cl-[(11)C]-(-)-NPA (standard uptake value 0.72+/-0.24) was approximately half that of [(11)C]-(-)-NPA (standard uptake value 1.37+/-0.18). Nonspecific uptake was similar for the two compounds. 2-Cl-[(11)C]-(-)-NPA was metabolized quickly, leaving only 17% of the parent compound in the plasma after 30 min. The specific binding of 2-Cl-[(11)C]-(-)-NPA was completely blocked and inhibition of P-glycoprotein did not alter the brain uptake.

CONCLUSION

Ex vivo experiments showed, despite a favorable D(2)/D(3) selectivity, that 2-Cl-[(11)C]-(-)-NPA is inferior to [(11)C]-(-)-NPA as a PET tracer in rat, because of slower brain uptake and lower specific to nonspecific binding ratio.

Pharmacological characterization of 2-methoxy-N-propylnorapomorphine's interactions with D2 and D3 dopamine receptors

Dopaminergic signaling pathways have been extensively investigated using PET imaging, primarily with antagonist radioligands of D(2) and D(3) dopamine receptors (DARs). Recently, agonist radioligands of D(2)/D(3) DARs have begun to be developed and employed. One such agonist is (R)-2-(11)CH(3)O-N-n-propylnorapomorphine (MNPA). Here, we perform a pharmacological characterization of MNPA using recombinant D(2) and D(3) DARs expressed in HEK293 cells. MNPA was found to robustly inhibit forskolin-stimulated cAMP accumulation to the same extent as dopamine in D(2) or D(3) DAR-transfected cells, indicating that it is a full agonist at both receptors. MNPA is approximately 50-fold more potent than dopamine at the D(2) DAR, but equally potent as dopamine at the D(3) DAR. MNPA competition binding curves in membrane preparations expressing D(2) DARs revealed two binding states of high and low-affinity. In the presence of GTP, only one binding state of low affinity was observed. Direct saturation binding assays using [(3)H]MNPA revealed similar results as with the competition experiments leading to the conclusion that MNPA binds to the D(2) DAR in an agonist-specific fashion. In contrast to membrane preparations, using intact cell binding assays, only one site of low affinity was observed for MNPA and other agonists binding to the D(2) DAR. MNPA was also found to induce D(2) DAR internalization to an even greater extent than dopamine as determined using both cell surface receptor binding assays and confocal fluorescence microscopy. Taken together, our data indicate that the PET tracer, MNPA, is a full and potent agonist at both D(2) and D(3) receptors.

Dopamine D(2/3) 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 D(2) 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. ID(50) 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.

The dopaminergic basis of human behaviors: A review of molecular imaging studies

This systematic review describes human molecular imaging studies which have investigated alterations in extracellular DA levels during performance of behavioral tasks. Whilst heterogeneity in experimental methods limits meta-analysis, we describe the advantages and limitations of different methodological approaches. Interpretation of experimental results may be limited by regional cerebral blood flow (rCBF) changes, head movement and choice of control conditions. We revisit our original study of striatal DA release during video-game playing [Koepp, M.J., Gunn, R.N., Lawrence, A.D., Cunningham, V.J., Dagher, A., Jones, T., Brooks, D.J., Bench, C.J., Grasby, P.M., 1998. Evidence for striatal dopamine release during a video game. Nature 393, 266-268] to illustrate the potentially confounding influences of head movement and alterations in rCBF. Changes in [(11)C]raclopride binding may be detected in extrastriatal as well as striatal brain regions-however we review evidence which suggests that extrastriatal changes may not be clearly interpreted in terms of DA release. Whilst several investigations have detected increases in striatal extracellular DA concentrations during task components such as motor learning and execution, reward-related processes, stress and cognitive performance, the presence of potentially biasing factors should be carefully considered (and, where possible, accounted for) when designing and interpreting future studies.

Dopamine D2High receptors measured ex vivo are elevated in amphetamine-sensitized animals

Although dopamine supersensitivity is a fundamental aspect of diseases such as schizophrenia and Parkinson's disease, the molecular basis of dopamine supersensitivity is not known. Because behavioral dopamine supersensitivity is associated with a marked elevation of striatal dopamine D2(High) receptors in vitro, it is important to develop methods to measure D2(High) receptors in vivo. The present ex vivo study found that the dopamine agonist NPA ([-]-N-propyl-norapomorphine) inhibited the binding of the agonist [(3)H](+)PHNO to rat striatal D2 receptors significantly more than the D2 antagonist [(3)H]raclopride, when NPA was coinjected i.v. with each radioligand. These results suggest that the greater sensitivity of [(3)H](+)PHNO to inhibition by the coinjected NPA reflects in vivo competition at D2(High) receptors. Using rats that had been sensitized to amphetamine, this ex vivo method found that the specific binding of [(3)H](+)PHNO that was displaced by 10 microg/kg of NPA was 2.4-fold higher than that for control rats. These data agree with in vitro data showing a marked increase in D2(High) sites after amphetamine sensitization. Therefore, it is recommended that this method of co-injecting the D2 radioligand and the dopamine agonist displacer be used in human positron tomography to detect D2(High) receptors in health and disease.

Neuroimaging and physiological evidence for involvement of glutamatergic transmission in regulation of the striatal dopaminergic system

Aberrant neurotransmissions via glutamate and dopamine receptors have been the focus of biomedical research on the molecular basis of psychiatric disorders, but the mode of their interaction is yet to be uncovered. In this study, we demonstrated the pharmacological reversal of methamphetamine-stimulated dopaminergic overflow by suppression of group I metabotropic glutamate (mGlu) receptor in living primates and rodents. In vivo positron emission tomography (PET) was conducted on cynomolgus monkeys and rats using a full agonistic tracer for dopamine D(2/3) receptor, [(11)C]MNPA [(R)-2-(11)CH(3)O-N-n-propylnorapomorphine], and fluctuation of kinetic data resulting from anesthesia was avoided by scanning awake subjects. Excessive release of dopamine induced by methamphetamine and abolishment of this alteration by treatment with an antagonist of group I mGlu receptors, 2-methyl-6-(phenylethynyl)pyridine (MPEP), were measured in both species as decreased binding potential because of increased dopamine and its recovery to baseline levels, respectively. Counteraction of MPEP to the methamphetamine-induced dopamine spillover was also supported neurochemically by microdialysis of unanesthetized rat striatum. Moreover, patch-clamp electrophysiological assays using acute brain slices prepared from rats indicated that direct targets of MPEP mechanistically involved in the effects of methamphetamine are present locally within the striatum. Because MPEP alone did not markedly alter the baseline dopaminergic neurotransmission according to our PET and electrophysiological data, the present findings collectively extend the insights on dopamine-glutamate cross talk from extrastriatal localization of responsible mGlu receptors to intrastriatal synergy and support therapeutic interventions in case of disordered striatal dopaminergic status using group I mGlu receptor antagonists assessable by in vivo imaging techniques.

Modeling considerations for 11C-CUMI-101, an agonist radiotracer for imaging serotonin 1A receptor in vivo with PET

Several lines of evidence demonstrate involvement of serotonin 1A receptors (5-HT1ARs) in the pathophysiology of neuropsychiatric disorders such as depression, suicidal behavior, schizophrenia, and Alzheimer's disease. We recently published the synthesis and initial evaluation of [O-methyl-11C]2-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butyl)-4-methyl-1,2,4-triazine-3,5(2H,4H)dione (11C-MMP), a 5-HT1AR agonist. Here we determine the optimal modeling parameters for 11C-MMP under its new name, 11C-CUMI-101, in Papio anubis.

METHODS

PET scans were performed on 2 adult male P. anubis; 166.5 MBq +/- 43.0 (4.50 +/- 1.16 mCi) of 11C-CUMI-101 were injected as an intravenous bolus, and emission data were collected for 120 min in 3-dimensional mode. We evaluated 4 different models (1- and 2-tissue compartment iterative and noniterative kinetic models, basis pursuit, and likelihood estimation in graphical analysis [LEGA]), using binding potential (BPF = Bmax/Kd) (Bmax = maximum number of binding sites; Kd = dissociation constant) as the outcome measure. Arterial blood sampling and metabolite-corrected arterial input function were used for full quantification of BPF. To assess the performance of each model, we compared results using 6 different metrics (percentage difference, within-subject mean sum of squares [WSMSS] for reproducibility; variance across subjects, intraclass correlation coefficient [ICC] for reliability; identifiability based on bootstrap resampling of residuals; and time stability analysis to determine minimal required scanning time) at each of 6 different scanning durations. Models were also evaluated on scans acquired after injecting the 5-HT1A antagonist [N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl) cyclohexane carboxamide] [WAY100635] 0.5 mg/kg, intravenous) and the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin) [8-OH-DPAT] 2 mg/kg, intravenous).

RESULTS

All metabolites are more polar than 11C-CUMI-101, and no significant change in metabolites was observed in the blocking studies. The free fraction is 59% +/- 3%. We determined that 100 min of scanning time is adequate and that for the region-of-interest (ROI)-level analysis, the LEGA model gives the best results. The median test-retest percentage difference for BPF is 11.15% +/- 4.82% across all regions, WSMSS = 2.66, variance = 6.07, ICC = 0.43, and bootstrap identifiability = 0.59. Preadministration of WAY100635 and 8-OH-DPAT resulted in 87% and 76% average reductions in BPF values, respectively, across ROIs.

CONCLUSION

On the basis of the measurable free fraction, high affinity and selectivity, adequate blood-brain permeability, and favorable plasma and brain kinetics, 11C-CUMI-101 is an excellent candidate for imaging high-affinity 5-HT1ARs in humans.

Dopamine D2 receptor radiotracers [(11)C](+)-PHNO and [(3)H]raclopride are indistinguishably inhibited by D2 agonists and antagonists ex vivo

INTRODUCTION

In vitro, the dopamine D2 receptor exists in two states, with high and low affinity for agonists. The high-affinity state is the physiologically active state thought to be involved in dopaminergic illnesses such as schizophrenia. The positron emission tomography radiotracer [(11)C](+)-PHNO ([(11)C](+)-4-propyl-3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b][1,4]oxazin-9-ol), being a D2 agonist, should selectively label the high-affinity state at tracer dose and therefore be more susceptible to competition by agonist as compared to the antagonist [(3)H]raclopride, which binds to both affinity states.

METHODS

We tested this prediction using ex vivo dual-radiotracer experiments in conscious rats. D2 antagonists (haloperidol or clozapine), a partial agonist (aripiprazole), a full agonist [(-)-NPA] or the dopamine-releasing drug amphetamine (AMPH) were administered to rats prior to an intravenous coinjection of [(11)C](+)-PHNO and [(3)H]raclopride. Rats were sacrificed 60 min after radiotracer injection. Striatum, cerebellum and plasma samples were counted for (11)C and (3)H. The specific binding ratio {SBR, i.e., [%ID/g (striatum)-%ID/g (cerebellum)]/(%ID/g (cerebellum)} was used as the outcome measure.

RESULTS

In response to D2 antagonists, partial agonist or full agonist, [(11)C](+)-PHNO and [(3)H]raclopride SBRs responded indistinguishably in terms of both ED(50) and Hill slope (e.g., (-)-NPA ED(50) values are 0.027 and 0.023 mg/kg for [(11)C](+)-PHNO and [(3)H]raclopride, respectively). In response to AMPH challenge, [(11)C](+)-PHNO and [(3)H]raclopride SBRs were inhibited to the same degree.

CONCLUSIONS

We have shown that the SBRs of [(11)C](+)-PHNO- and [(3)H]raclopride do not differ in their response to agonist challenge. These results do not support predictions of the in vivo binding behavior of a D2 agonist radiotracer and cast some doubt on the in vivo applicability of the D2 two-state model, as described by in vitro binding experiments.

Comparison of the binding distribution of agonist and antagonist ligands for histamine H3 receptors in pig brain by quantitative autoradiography

The relationship between the abundances of agonist and antagonist-binding sites for monoamine receptors is poorly established. Therefore, we used quantitative autoradiography to investigate the distribution and concentration of binding sites for histamine H(3) receptor ligands in cryostat sections of pig brain. As in other species, binding of the histamine H(3) receptor agonist [(3)H]N(alpha)-methylhistamine was highly heterogeneous in the pig brain, with highest B(max) in the substantia nigra, followed by the nucleus accumbens and caudate, intermediate binding in frontal cortex, diencephalon, and mesencephalon, and absent specific binding in cerebellum: the affinity of [(3)H]N(alpha)-methylhistamine was close to 1 nM in all regions of pig brain. Thus, the saturation binding parameters for this H(3) receptor agonist in pig brain were similar to the earlier reports in rat, guinea pig, and human. The distribution of histamine H(3) receptors labeled with the receptor antagonist [(125)I]iodophenpropit in adjacent cryostat sections from the same group of pigs was very similar to that of [(3)H]N(alpha)-methylhistamine. However, the B(max) of the receptor antagonist was 40% higher in the basal ganglia than was the B(max) of the receptor agonist. The K(d) for the receptor antagonist ligand was close to 0.9 nM in all regions. These results suggest that histamine H(3) receptor agonist-binding sites, i.e. those linked to intracellular G-protein, comprise a subset of the total receptor antagonist-binding sites in the basal ganglia, as has been reported for dopamine D(2) receptors.

Mapping the amphetamine-evoked changes in [11C]raclopride binding in living rat using small animal PET: Modulation by MAO-inhibition

The performance of small animal PET for neuroreceptor studies in a psychopharmacological challenge paradigm is not yet well-described. Therefore, we used microPET and [(11)C]raclopride to map the availability of dopamine D(2/3) receptors in brain of anesthetized rats, first in a baseline condition, and again after challenge with saline or d-amphetamine. Parametric maps of the specific binding (binding potential, pB) were calculated using a reference tissue input from cerebellum, and spatially normalized to a digitized stereotaxic coordinate system for rat brain. In volumes of interest (VOIs), the mean baseline pB (n=6) was 2.05 in dorsal striatum (caudate-putamen), and 1.34 in ventral striatum (nucleus accumbens), and did not significantly differ upon retest 2 h later. The availability of [(11)C]raclopride binding sites at baseline was 8% higher in the right striatum. Challenge with amphetamine sulfate (1 mg/kg, i.v., n=4) decreased pB by 19% in both ventral and dorsal striatum. We have earlier predicted that blockade of monoamine oxidase (MAO) should potentiate the amphetamine-evoked dopamine release, thus enhancing the displacement of [(11)C]raclopride binding in vivo. However, pretreatment of rats with pargyline hydrochloride (4 mg/kg, n=4; 20 mg/kg, n=4) 1 day prior to PET did not potentiate the amphetamine-evoked reduction in dopamine receptor availability within the extended striatum. We conclude that small animal PET can be used to investigate stimulant-induced dopamine release, but that the spatial resolution is insufficient to detect differences between relative changes in dorsal vs. ventral divisions of the rat striatum. Furthermore, the present results do not reveal potentiation of the amphetamine-evoked release of dopamine in rats with MAO inhibition.

Synthesis and in vivo evaluation of a novel 5-HT1A receptor agonist radioligand [O-methyl-11C]2-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butyl)-4-methyl-1,2,4-triazine-3,5(2H,4H)dione in nonhuman primates

PURPOSE

Serotonin1A (5-HT1A) receptors exist in high- and low-affinity states, and agonist ligands bind preferentially to the high-affinity state of the receptor and provide a measure of functional 5-HT1A receptors. Although the antagonist tracers are established PET ligands in clinical studies, a successful 5-HT1A receptor agonist radiotracer in living brain has not been reported. [11C]MPT, our first-generation agonist radiotracer, shows in vivo specificity in baboons; however, its utility is limited owing to slow washout and immeasurable plasma free fraction. Hence we performed structure-activity relationship studies of MPT to optimize a radiotracer that will permit valid quantification of 5-HT1A receptor binding. We now report the synthesis and evaluation of [11C]MMP as an agonist PET tracer for 5-HT1A receptors in baboons.

METHODS

In vitro binding assays were performed in bovine hippocampal membranes and membranes of CHO cells expressing 5-HT1A receptors. [11C] labeling of MMP was performed by reacting desmethyl-MMP with [11C]CH(3)OTf. In vivo studies were performed in baboons, and blocking studies were conducted by pretreatment with 5-HT1A receptor ligands WAY-100635 and (+/-)-8-OH-DPAT.

RESULTS

MMP is a selective 5-HT1A receptor agonist (Ki 0.15 nM). Radiosynthesis of [11C]MMP was achieved in 30 +/- 5% (n = 15) yield at EOS with a specific activity of 2,600 +/- 500 Ci/mmol (n = 12). PET studies in baboons demonstrated specific binding of [11C]MMP to 5-HT1A receptor-enriched brain regions, as confirmed by blockade with WAY-100635 and (+/-)-8-OH-DPAT.

CONCLUSION

We identified [11C]MMP as an optimal agonist PET tracer that shows quantifiable, specific binding in vivo to 5-HT1A receptors in baboons.

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 fenfluramine-induced increases in serotonin release on [18F]MPPF binding: a continuous infusion PET study in conscious monkeys

[(18)F]MPPF is a selective and reversible antagonist to the serotonin-1A (5-HT(1A)) receptor. The aim of the present study was to investigate whether the binding of [(18)F]MPPF is sensitive to increases in 5-HT levels. We used the 5-HT releasing agent and reuptake inhibitor fenfluramine (FEN) to increase the concentration of 5-HT. [(18)F]MPPF binding was assessed using positron emission tomography (PET) in conscious monkeys. Possible effects of blood flow on ligand binding were excluded by using a bolus-infusion paradigm. Control scans were obtained to assess the state of ligand equilibrium. FEN (5 or 10 mg/kg, i.v.) was administered between 90 and 130 min after the start of the [(18)F]MPPF infusion. The binding potential (BP) was calculated for an early interval (30 min preceding FEN administration) and late interval (20-50 min after administration of FEN). Microdialyses results showed a 20- and 35-fold increase in extracellular 5-HT levels in the prefrontal cortex after injection of FEN at a dose of 5 mg/kg and 10 mg/kg respectively. However, despite these large increases in 5-HT levels, no differences in BP were found between the control and FEN scans. These results may imply that the majority of 5-HT(1A) receptors is in the low affinity state in the living brain.

Synthesis and in Vivo Validation of [O-Methyl-11C]2-{4-[4-(7-methoxynaphthalen-1-yl)piperazin- 1-yl]butyl}-4-methyl-2H-[1,2,4]triazine-3,5-dione: A Novel 5-HT1A Receptor Agonist Positron Emission Tomography Ligand

Antagonist 5-HT(1A) PET ligands are available, but an agonist ligand would give more information about signal transduction capacity. Synthesis and in vivo evaluation of [O-methyl-(11)C]2-{4-[4-(7-methoxynaphthalen-1-yl)piperazin-1-yl]butyl}-4-methyl-2H-[1,2,4]triazine-3,5-dione (10), a potential high affinity (K(i) = 1.36 nM) 5-HT(1A) agonist PET tracer is described. Piperazine 10 is a 5-HT(1A) agonist with an EC(50) comparable to serotonin, based on cAMP formation and GTP(gamma)S binding assays. Radiosynthesis of [(11)C]10 has been achieved by reacting 2-{4-[4-(7-hydroxynaphthalen-1-yl)piperazin-1-yl]butyl}-4-methyl-2H-[1,2,4]triazine-3,5-dione (9) and [(11)C]CH(3)OTf in 25 +/- 5% (n = 15) yield at the end of synthesis (EOS). The chemical and radiochemical purities of [(11)C]10 were >99% with a specific activity 1500 +/- 300 Ci/mmol (n =15). PET studies in anesthetized baboon demonstrate [(11)C]10 specific binding in brain regions rich in 5-HT(1A) receptors. Binding of [(11)C]10 was blocked by WAY100635 and 8-OH-DPAT. The regional brain volumes of distribution (V(T)) of [(11)C]10 in baboon correlate with [(11)C]WAY100635 V(T) in baboons. These data provide evidence that [(11)C]10 is the first promising agonist PET tracer for the 5-HT(1A) receptors.

Behavioral response to novelty correlates with dopamine receptor availability in striatum of Göttingen minipigs

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.

Lack of effect of reserpine-induced dopamine depletion on the binding of the dopamine-D3 selective radioligand, [11C]RGH-1756

The effect of reserpine induced dopamine depletion on the binding of the putative dopamine-D3 receptor ligand, [(11)C]RGH-1756 was examined in the monkey brain with positron emission tomography (PET). In a previous series of experiments, we have made an attempt to selectively label D3 receptors in the monkey brain using [(11)C]RGH-1756. Despite high selectivity and affinity of RGH-1756 in vitro, [(11)C]RGH-1756 displayed only low specific binding to D3 receptors in vivo. The aim of the present study was to examine whether low specific binding of [(11)C]RGH-1756 is caused by insufficient in vivo affinity of the ligand, or by high physiological occupancy of D3 receptors by endogenous dopamine (DA). PET experiments were performed in three monkeys under baseline conditions and after administration of reserpine (0.5 mg/kg). The results of the baseline measurements corresponded well to our earlier observations with [(11)C]RGH-1756. Reserpine caused no evident change in the regional distribution of [(11)C]RGH-1756 in the monkey brain, and no conspicuous regional accumulation of activity could be observed. After reserpine treatment there was no evident increase of specific binding and binding potential (BP) of [(11)C]RGH-1756. The lack of increased [(11)C]RGH-1756 binding after reserpine treatment indicates that competition with endogenous DA is not the predominant reason for the failure of the radioligand to label D3 receptors. Therefore, the low binding of [(11)C]RGH-1756 could largely be explained by the need for very high affinity of radioligand for D3 receptors in vivo, to obtain a suitable signal for the minute densities of D3 receptors expressed in the primate brain.