[3H]4-(dimethylamino)-N-(4-(4-(2-methoxyphenyl)piperazin-1-yl) butyl)benzamide: a selective radioligand for dopamine D(3) receptors. II. Quantitative analysis of dopamine D(3) and D(2) receptor density ratio in the caudate-putamen

In vitro characterization of [125I]HY-3-24, a selective ligand for the dopamine D3 receptor

Introduction

Dopamine D3 receptor (D3R) ligands have been studied for the possible treatment of neurological and neuropsychiatric disorders. However, selective D3R radioligands for in vitro binding studies have been challenging to identify due to the high structural similarity between the D2R and D3R. In a prior study, we reported a new conformationally-flexible benzamide scaffold having a high affinity for D3R and excellent selectivity vs. D2R. In the current study, we characterized the in vitro binding properties of a new radioiodinated ligand, [125I]HY-3-24.

Methods

In vitro binding studies were conducted in cell lines expressing D3 receptors, rat striatal homogenates, and rat and non-human primate (NHP) brain tissues to measure regional brain distribution of this radioligand.

Results

HY-3-24 showed high potency at D3R (Ki = 0.67 ± 0.11 nM, IC50 = 1.5 ± 0.58 nM) compared to other D2-like dopamine receptor subtypes (D2R Ki = 86.7 ± 11.9 nM and D4R Ki > 1,000). The Kd (0.34 ± 0.22 nM) and Bmax (38.91 ± 2.39 fmol/mg) values of [125I]HY-3-24 were determined. In vitro binding studies in rat striatal homogenates using selective D2R and D3R antagonists confirmed the D3R selectivity of [125I]HY-3-24. Autoradiography results demonstrated that [125I]HY-3-24 specifically binds to D3Rs in the nucleus accumbens, islands of Calleja, and caudate putamen in rat and NHP brain sections.

Conclusion

These results suggest that [125I]HY-3-24 appears to be a novel radioligand that exhibits high affinity binding at D3R, with low binding to other D2-like dopamine receptors. It is anticipated that [125I]HY-3-24 can be used as the specific D3R radioligand.

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

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

Dopamine D1 + D3 receptor density may correlate with parkinson disease clinical features

Objective

Dopamine D2‐like receptors – mainly dopamine D2 receptors (D2R) and dopamine D3 receptors (D3R) – are believed to be greatly involved in the pathology of Parkinson disease (PD) progression. However, these receptors have not been precisely examined in PD patients. Our aim was to quantitatively calculate the exact densities of dopamine D1 receptors (D1R), D2R, and D3R in control, Alzheimer disease (AD), and Lewy body disease (LBD) patients (including PD, Dementia with Lewy bodies, and Parkinson disease dementia); and analyze the relationship between dopamine receptors and clinical PD manifestations.

Methods

We analyzed the densities of D1R, D2R, and D3R in the striatum and substantia nigra (SN) using a novel quantitative autoradiography procedure previously developed by our group. We also examined the expression of D2R and D3R mRNA in the striatum by in situ hybridization.

Results

The results showed that although no differences of striatal D1R were found among all groups; D2R was significantly decreased in the striatum of PD patients when compared with control and AD patients. Some clinical manifestations: age of onset, PD stage, dopamine responsiveness, and survival time after onset; showed a better correlation with striatal D1R + D3R densities combined compared to D1R or D3R alone.

Interpretation

There is a possibility that we may infer the results in diagnosis, treatment, and prognosis of PD by detecting D1R + D3R as opposed to using dopamine D1 or D3 receptors alone. This is especially true for elderly patients with low D2R expression as is common in this disease.

Radiolabeled 6-(2, 3-Dichlorophenyl)-N4-methylpyrimidine-2, 4-diamine (TH287): A Potential Radiotracer for Measuring and Imaging MTH1

MTH1 (MutT homolog 1) or NUDT1 (Nudix Hydrolase 1), also known as oxidized purine nucleoside triphosphatase, has potential as a biomarker for monitoring cancer progression and quantifying target engagement for relevant therapies. In this study, we validate one MTH1 inhibitor TH287 as a PET MTH1 radiotracer. TH287 was radiolabeled with tritium and the binding of [³H]TH287 to MTH1 was evaluated in live glioblastoma cells (U251MG) through saturation and competitive binding assays, together with in vitro enzymatic assays. Furthermore, TH287 was radiolabeled with carbon-11 for in vivo microPET studies. Saturation binding assays show that [³H]TH287 has a dissociation constant (K_d) of 1.97 ± 0.18 nM, B_max of 2676 ± 122 fmol/mg protein for U251MG cells, and n_H of 0.98 ± 0.02. Competitive binding assays show that TH287 (K_i: 3.04 ± 0.14 nM) has a higher affinity for MTH1 in U251MG cells compared to another well studied MTH1 inhibitor: (S)-crizotinib (K_i: 153.90 ± 20.48 nM). In vitro enzymatic assays show that TH287 has an IC₅₀ of 2.2 nM in inhibiting MTH1 hydrolase activity and a K_i of 1.3 nM from kinetics assays, these results are consistent with our radioligand binding assays. Furthermore, MicroPET imaging shows that [¹¹C]TH287 gets into the brain with rapid clearance from the brain, kidney, and heart. The results presented here indicate that radiolabeled TH287 has favorable properties to be a useful tool for measuring MTH1 in vitro and for further evaluation for in vivo PET imaging MTH1 of brain tumors and other central nervous system disorders.

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

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

The interactions of dopamine and oxidative damage in the striatum of patients with neurodegenerative diseases

The striatum with a number of dopamine containing neurons, receiving projections from the substantia nigra and ventral tegmental area; plays a critical role in neurodegenerative diseases of motor and memory function. Additionally, oxidative damage to nucleic acid may be vital in the development of age-associated neurodegeneration. The metabolism of dopamine is recognized as one of the sources of reactive oxygen species through the Fenton mechanism. The proposed interactions of oxidative insults and dopamine in the striatum during the progression of diseases are the hypotheses of most interest to our study. This study investigated the possibility of significant interactions between these molecules that are involved in the late-stage of Alzheimer's disease (AD), Parkinson disease (PD), Parkinson disease dementia, dementia with Lewy bodies, and controls using ELISA assays, autoradiography, and mRNA in situ hybridization assay. Interestingly, lower DNA/RNA oxidative adducts levels in the caudate and putamen of diseased brains were observed with the exception of an increased DNA oxidative product in the caudate of AD brains. Similar changes were found for dopamine concentration and vesicular monoamine transporter 2 densities. We also found that downstream pre-synaptic dopamine D1 Receptor binding correlated with dopamine loss in Lewy body disease groups, and RNA damage and β-site APP cleaving enzyme 1 in the caudate of AD. This is the first demonstration of region-specific alterations of DNA/RNA oxidative damage which cannot be viewed in isolation, but rather in connection with the interrelationship between different neuronal events; chiefly DNA oxidative adducts and density of vesicular monoamine transporter 2 densities in AD and PD patients.

Translocator protein in late stage Alzheimer's disease and Dementia with Lewy bodies brains

OBJECTIVE

Increased translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor (PBR), in glial cells of the brain has been used as a neuroinflammation marker in the early and middle stages of neurodegenerative diseases, such as Alzheimer's disease (AD) and Dementia with Lewy Bodies (DLB). In this study, we investigated the changes in TSPO density with respect to late stage AD and DLB.

METHODS

TSPO density was measured in multiple regions of postmortem human brains in 20 different cases: seven late stage AD cases (Braak amyloid average: C; Braak tangle average: VI; Aged 74-88, mean: 83 ± 5 years), five DLB cases (Braak amyloid average: C; Braak tangle average: V; Aged 79-91, mean: 84 ± 4 years), and eight age-matched normal control cases (3 males, 5 females: aged 77-92 years; mean: 87 ± 6 years). Measurements were taken by quantitative autoradiography using [3 H]PK11195 and [3 H]PBR28.

RESULTS

No significant changes were found in TSPO density of the frontal cortex, striatum, thalamus, or red nucleus of the AD and DLB brains. A significant reduction in TSPO density was found in the substantia nigra (SN) of the AD and DLB brains compared to that of age-matched healthy controls.

INTERPRETATION

This distinct pattern of TSPO density change in late stage AD and DLB cases may imply the occurrence of microglia dystrophy in late stage neurodegeneration. Furthermore, TSPO may not only be a microglia activation marker in early stage AD and DLB, but TSPO may also be used to monitor microglia dysfunction in the late stage of these diseases.

Analogues of Arylamide Phenylpiperazine Ligands To Investigate the Factors Influencing D3 Dopamine Receptor Bitropic Binding and Receptor Subtype Selectivity

We have previously reported on the ability of arylamide phenylpiperazines to bind selectively to the D3 versus the D2 dopamine receptor subtype. For these studies, we used LS-3-134 as the prototypic arylamide phenylpiperazine ligand because it binds with high affinity at D3 dopamine receptor (0.17 nM) and exhibits >150-fold D3 vs D2 receptor binding selectivity. Our goal was to investigate how the composition and size of the nonaromatic ring structure at the piperazine position of substituted phenylpiperazine analogues might influence binding affinity at the human D2 and D3 dopamine receptors. Two factors were identified as being important for determining the binding affinity of bitropic arylamide phenylpiperazines at the dopamine D3 receptor subtype. One factor was the strength of the salt bridge between the highly conserved residue Asp^3.32 with the protonated nitrogen of the nonaromatic ring at the piperazine position. The second factor was the configuration of the unbound ligand in an aqueous solution. These two factors were found to be related to the logarithm of the affinities using a simple correlation model, which could be useful when designing high affinity subtype selective bitropic ligands. While this model is based upon the interaction of arylamide phenylpiperazines with the D2 and D3 D2-like dopamine receptor subtypes, it provides insights into the complexity of the factors that define a bitropic mode of the binding at GPCRs.

[O-methyl-11C]N-(4-(4-(3-Chloro-2-methoxyphenyl)-piperazin-1-yl)butyl)-1H-indole-2-carboxamide ([11C]BAK4-51) Is an Efflux Transporter Substrate and Ineffective for PET Imaging of Brain D₃ Receptors in Rodents and Monkey

Selective high-affinity antagonists for the dopamine D₃ receptor (D₃R) are sought for treating substance use disorders. Positron emission tomography (PET) with an effective D₃R radioligand could be a useful tool for the development of such therapeutics by elucidating pharmacological specificity and target engagement in vivo. Currently, a D₃R-selective radioligand does not exist. The D₃R ligand, N-(4-(4-(3-chloro-2-methoxyphenyl)piperazin-1-yl)butyl)-1H-indole-2-carboxamide (BAK4-51, 1), has attractive properties for PET radioligand development, including full antagonist activity, very high D₃R affinity, D₃R selectivity, and moderate lipophilicity. We labeled 1 with the positron-emitter carbon-11 (t_1/2 = 20.4 min) in the methoxy group for evaluation as a radioligand in animals with PET. However, [¹¹C]1 was found to be an avid substrate for brain efflux transporters and lacked D₃R-specific signal in rodent and monkey brain in vivo.

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

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

The role of beta-arrestin2 in shaping fMRI BOLD responses to dopaminergic stimulation

RATIONALE

The dopamine D₂ receptor (D₂R) couples to inhibitory G_i/o proteins and is targeted by antipsychotic and antiparkinsonian drugs. Beta-arrestin2 binds to the intracellular regions of the agonist-occupied D₂R to terminate G protein activation and promote internalization, but also to initiate downstream signaling cascades which have been implicated in psychosis. Functional magnetic resonance imaging (fMRI) has proven valuable for measuring dopamine receptor-mediated changes in neuronal activity, and might enable beta-arrestin2 function to be studied in vivo.

OBJECTIVES

The present study examined fMRI blood oxygenation level dependent (BOLD) signal changes elicited by a dopamine agonist in wild-type (WT) and beta-arrestin2 knockout (KO) mice, to investigate whether genetic deletion of beta-arrestin2 prolongs or otherwise modifies D₂R-dependent responses.

METHODS

fMRI BOLD data were acquired on a 9.4 T system. During scans, animals received 0.2 mg/kg apomorphine, i.v. In a subset of experiments, animals were pretreated with 2 mg/kg of the D₂R antagonist, eticlopride.

RESULTS

Following apomorphine administration, BOLD signal decreases were observed in caudate/putamen of WT and KO animals. The time course of response decay in caudate/putamen was significantly slower in KO vs. WT animals. In cingulate cortex, an initial BOLD signal decrease was followed by a positive response component in WT but not in KO animals. Eticlopride pretreatment significantly reduced apomorphine-induced BOLD signal changes.

CONCLUSIONS

The prolonged striatal response decay rates in KO animals might reflect impaired D₂R desensitization, consistent with the known function of beta-arrestin2. Furthermore, the apomorphine-induced positive response component in cingulate cortex may depend on beta-arrestin2 signaling downstream of D₂R.

Addiction-related genes in gambling disorders: new insights from parallel human and pre-clinical models

Neurobiological research supports the characterization of disordered gambling (DG) as a behavioral addiction. Recently, an animal model of gambling behavior was developed (rat gambling task, rGT), expanding the available tools to investigate DG neurobiology. We investigated whether rGT performance and associated risk gene expression in the rat's brain could provide cross-translational understanding of the neuromolecular mechanisms of addiction in DG. We genotyped tagSNPs (single-nucleotide polymorphisms) in 38 addiction-related genes in 400 DG and 345 non-DG subjects. Genes with P<0.1 in the human association analyses were selected to be investigated in the animal arm to determine whether their mRNA expression in rats was associated with the rat's performance on the rGT. In humans, DG was significantly associated with tagSNPs in DRD3 (rs167771) and CAMK2D (rs3815072). Our results suggest that age and gender might moderate the association between CAMK2D and DG. Moderation effects could not be investigated due to sample power. In the animal arm, only the association between rGT performance and Drd3 expression remained significant after Bonferroni correction for 59 brain regions. As male rats were used, gender effects could not be investigated. Our results corroborate previous findings reporting the involvement of DRD3 receptor in addictions. To our knowledge, the use of human genetics, pre-clinical models and gene expression as a cross-translation paradigm has not previously been attempted in the field of addictions. The cross-validation of human findings in animal models is crucial for improving the translation of basic research into clinical treatments, which could accelerate neurobiological and pharmacological investigations in addictions.

Preclinical evaluation of a promising C-11 labeled PET tracer for imaging phosphodiesterase 10A in the brain of living subject

Phosphodiesterase 10A (PDE10A) plays a key role in the regulation of brain striatal signaling. A PET tracer for PDE10A may serve as a tool to evaluate PDE10A expression in vivo in central nervous system disorders with striatal pathology. Here, we further characterized the binding properties of a previously reported radioligand we developed for PDE10A, [(11)C]TZ1964B, in rodents and nonhuman primates (NHPs). The tritiated counterpart [(3)H]TZ1964B was used for in vitro binding characterizations in rat striatum homogenates and in vitro autoradiographic studies in rat brain slices. The carbon-11 labeled [(11)C]TZ1964B was utilized in the ex vivo autoradiography studies for the brain of rats and microPET imaging studies for the brain of NHPs. MicroPET scans of [(11)C]TZ1964B in NHPs were conducted at baseline, as well as with using a selective PDE10A inhibitor MP-10 for either pretreatment or displacement. The in vivo regional target occupancy (Occ) was obtained by pretreating with different doses of MP-10 (0.05-2.00 mg/kg). Both in vitro binding assays and in vitro autoradiographic studies revealed a nanomolar binding affinity of [(3)H]TZ1964B to the rat striatum. The striatal binding of [(3)H]TZ1964B and [(11)C]TZ1964B was either displaced or blocked by MP-10 in rats and NHPs. Autoradiography and microPET imaging confirmed that the specific binding of the radioligand was found in the striatum but not in the cerebellum. Blocking studies also confirmed the suitability of the cerebellum as an appropriate reference region. The binding potentials (BPND) of [(11)C]TZ1964B in the NHP striatum that were calculated using either the Logan reference model (LoganREF, 3.96 ± 0.17) or the simplified reference tissue model (SRTM, 4.64 ± 0.47), with the cerebellum as the reference region, was high and had good reproducibility. The occupancy studies indicated a MP-10 dose of 0.31 ± 0.09 mg/kg (LoganREF)/0.45 ± 0.17mg/kg (SRTM) occupies 50% striatal PDE10A binding sites. Studies in rats and NHPs demonstrated radiolabeled TZ1964B has a high binding affinity and good specificity for PDE10A, as well as favorable in vivo pharmacokinetic properties and binding profiles. Our data suggests that [(11)C]TZ1964B is a promising radioligand for in vivo imaging PDE10A in the brain of living subject.

Analysis of Human Dopamine D3 Receptor Quaternary Structure

The dopamine D3 receptor is a class A, rhodopsin-like G protein-coupled receptor that can form dimers and/or higher order oligomers. However, the molecular basis for production of these complexes is not well defined. Using combinations of molecular modeling, site-directed mutagenesis, and homogenous time-resolved FRET, the interfaces that allow dopamine D3 receptor monomers to interact were defined and used to describe likely quaternary arrangements of the receptor. These were then compared with published crystal structures of dimeric β1-adrenoreceptor, μ-opioid, and CXCR4 receptors. The data indicate important contributions of residues from within each of transmembrane domains I, II, IV, V, VI, and VII as well as the intracellular helix VIII in the formation of D3-D3 receptor interfaces within homo-oligomers and are consistent with the D3 receptor adopting a β1-adrenoreceptor-like quaternary arrangement. Specifically, results suggest that D3 protomers can interact with each other via at least two distinct interfaces: the first one comprising residues from transmembrane domains I and II along with those from helix VIII and a second one involving transmembrane domains IV and V. Moreover, rather than existing only as distinct dimeric species, the results are consistent with the D3 receptor also assuming a quaternary structure in which two transmembrane domain I-II-helix VIII dimers interact to form a "rhombic" tetramer via an interface involving residues from transmembrane domains VI and VII. In addition, the results also provide insights into the potential contribution of molecules of cholesterol to the overall organization and potential stability of the D3 receptor and possibly other GPCR quaternary structures.

Sigma-2 receptor binding is decreased in female, but not male, APP/PS1 mice

The sigma-2 receptor is a steroid-binding membrane-associated receptor which has been implicated in cell survival. Sigma-2 has recently been shown to bind amyloid-β (Aβ) oligomers in Alzheimer's disease (AD) brain. Furthermore, blocking this interaction was shown to prevent or reverse the effects of Aβ to cause cognitive impairment in mouse models and synaptic loss in neuronal cultures. In the present work, the density of sigma-2 receptors was measured in a double transgenic mouse model of amyloid-β deposition (APP/PS1). Comparisons were made between males and females and between transgenic and wt animals. Sigma-2 receptor density was assessed by quantitative autoradiography performed on coronal brain slices using [(3)H]N-[4-(3,4-dihydro-6,7-dimethoxyisoquinolin-2(1H)-yl)butyl]-2-methoxy-5-methyl-benzamide ([(3)H]RHM-1), which has a 300-fold selectivity for the sigma-2 receptor over the sigma-1 receptor. The translocator protein of 18 kDa (TSPO) is expressed on activated microglia and is a marker for neuroinflammation. TSPO has been found to be upregulated in neurodegenerative disorders, including AD. Therefore, in parallel with the sigma-2 autoradiography experiments, we measured TSPO expression using the selective radioligand, [(3)H]PBR28. We also quantified Aβ plaque burden in the same animals using a monoclonal antibody raised against aggregated Aβ. Sigma-2 receptor density was significantly decreased in piriform and motor cortices as well as striata of 16-month old female, but not male, APP/PS1 mice as compared to their wt counterparts. [(3)H]PBR28 binding and immunostaining for Aβ plaques were significantly increased in piriform and motor cortices of both male and female transgenic mice. In striatum however, significant increases were observed only in females.

The role of dopamine and dopaminergic pathways in dystonia: insights from neuroimaging

Background

Dystonia constitutes a heterogeneous group of movement abnormalities, characterized by sustained or intermittent muscle contractions causing abnormal postures. Overwhelming data suggest involvement of basal ganglia and dopaminergic pathways in dystonia. In this review, we critically evaluate recent neuroimaging studies that investigate dopamine receptors, endogenous dopamine release, morphology of striatum, and structural or functional connectivity in cortico-basal ganglia-thalamo-cortical and related cerebellar circuits in dystonia.

Method

A PubMed search was conducted in August 2014.

Results

Positron emission tomography (PET) imaging offers strong evidence for altered D2/D3 receptor binding and dopaminergic release in many forms of idiopathic dystonia. Functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI) data reveal likely involvement of related cerebello-thalamo-cortical and sensory-motor networks in addition to basal ganglia.

Discussion

PET imaging of dopamine receptors or transmitter release remains an effective means to investigate dopaminergic pathways, yet may miss factors affecting dopamine homeostasis and related subcellular signaling cascades that could alter the function of these pathways. fMRI and DTI methods may reveal functional or anatomical changes associated with dysfunction of dopamine-mediated pathways. Each of these methods can be used to monitor target engagement for potential new treatments. PET imaging of striatal phosphodiesterase and development of new selective PET radiotracers for dopamine D3-specific receptors and Mechanistic target of rampamycin (mTOR) are crucial to further investigate dopaminergic pathways. A multimodal approach may have the greatest potential, using PET to identify the sites of molecular pathology and magnetic resonance methods to determine their downstream effects.

Striatal dopamine D1-like receptor binding is unchanged in primary focal dystonia

BACKGROUND

Multiple studies have demonstrated decreases in striatal D2-like (D2, D3) radioligand binding in primary focal dystonias. Although most investigations have focused on D2-specific receptors (D2R), a recent study suggests that the decreased D2-like binding may be due to a D3-specific (D3R) abnormality. However, only limited data exist on the role of D1-specific receptors (D1R) and the D1R-mediated pathways within basal ganglia in dystonia. Metabolic positron emission tomography (PET) data in primary generalized dystonia suggest resting state over activity in the D1R-mediated direct pathway, leading to excessive disinhibition of motor cortical areas. This work investigated whether striatal D1-like receptors are affected in primary focal dystonias.

METHODS

Striatal-specific (caudate and putamen) binding of the D1-like radioligand [(11)C]NNC 112 was measured using PET in 19 patients with primary focal dystonia (cranial, cervical, or arm) and 18 controls.

RESULTS

No statistically significant difference was detected in striatal D1-like binding between the two groups. The study had 91% power to detect a 20% difference, indicating that false-negative results were unlikely.

CONCLUSIONS

Because [(11)C]NNC 112 has high affinity for D1-like receptors, very low affinity for D2-like receptors, and minimal sensitivity to endogenous dopamine levels, we conclude that D1-like receptor binding is not impaired in these primary focal dystonias.

Regulation of dopamine presynaptic markers and receptors in the striatum of DJ-1 and Pink1 knockout rats

Pathogenic autosomal recessive mutations in the DJ-1 (Park7) or the PTEN-induced putative kinase 1 (Pink1 or PARK6) genes are associated with familial Parkinson's disease (PD). It is not well known regarding the pathological mechanisms involving the DJ-1 and Pink1 mutations. Here we characterized DJ-1 and Pink1 knockout rats both through expression profiling and using quantitative autoradiography to measure the densities of the dopamine D1, D2, D3 receptors, vesicular monoamine transporter type-2 (VMAT2) and dopamine transporter (DAT) in the striatum of transgenic rats and wild type controls. Expression profiling with a commercially available array of 84 genes known to be involved in PD indicated that only the target gene was significantly downregulated in each transgenic rat model. D1 receptor, VMAT2, and DAT were measured using [(3)H]SCH23390, [(3)H]dihydrotetrabenazine, and [(3)H]WIN35428, respectively. No significant changes were observed in the density of DAT in either model. Although the densities of VMAT2 and D1 receptor were unchanged in Pink1 knockout, but both were increased in DJ-1 knockout rats. The densities of D2 and D3 receptors, determined by mathematical analysis of binding of radioligands [(3)H]WC-10 and [(3)H]raclopride, were significantly increased in both knockout models. These distinctive changes in the expression of dopamine presynaptic markers and receptors in the striatum may reflect different compensatory regulation of dopamine system in DJ-1 versus Pink1 knockout rat models of familial PD.

Regulation of dopamine D₃ receptor in the striatal regions and substantia nigra in diffuse Lewy body disease

The regulation of D₃ receptor has not been well documented in diffuse Lewy body disease (DLBD). In this study, a novel D₃-preferring radioligand [(3)H]WC-10 and a D₂-preferring radioligand [(3)H]raclopride were used and the absolute densities of the dopamine D₃ and D₂ receptors were determined in the striatal regions and substantia nigra (SN) from postmortem brains from five cases of DLBD, which included dementia with Lewy bodies (DLB, n=4) and Parkinson disease dementia (PDD, n=1). The densities of the dopamine D₁ receptor, vesicular monoamine transporter 2 (VMAT2), and dopamine transporter (DAT) were also measured by quantitative autoradiography using [(3)H]SCH23390, [(3)H]dihydrotetrabenazine, and [(3)H]WIN35428, respectively. The densities of these dopaminergic markers were also measured in the same brain regions in 10 age-matched control cases. Dopamine D₃ receptor density was significantly increased in the striatal regions including caudate, putamen and nucleus accumbens (NAc). There were no significant changes in the dopamine D₁ and D₂ receptor densities in any brain regions measured. VMAT2 and DAT densities were reduced in all the brain regions measured in DLB/PDD, however, the significant reduction was found in the putamen for DAT and in the NAc and SN for VMAT2. The decrease of dopamine pre-synaptic markers implies neuronal loss in the substantia nigra pars compacta (SNpc) in these DLB/PDD cases, while the increase of D₃ receptors in striatal regions could be attributed to dopaminergic medication history and psychiatric states such as hallucinations. Whether it also reflects compensatory regulation upon dopaminergic denervation warrants further confirmations on larger populations.

Fos expression in response to dopamine D3-preferring phenylpiperazine drugs given with and without cocaine

WC 44 and WC 10 are phenylpiperazines with low (23 fold) to moderate (42 fold) selectivity for dopamine D3 receptors (D3Rs) over D2Rs, respectively. WC 44 is a full D3R agonist in the forskolin-stimulated adenylyl cyclase (AC) assay, whereas WC 10 has little efficacy. In contrast to their opposite effects in the AC assay, these drugs often produce similar behavioral effects, suggesting that the AC assay does not predict the efficacy of these drugs in vivo. Here, we examined whether Fos protein expression induced by these drugs would be more consistent with their behavioral effects in vivo. Rats received either vehicle, WC 10 (5.6 mg/kg, i.p.), WC 44 (10.0 mg/kg, i.p), cocaine (10.0 mg/kg, i.p.), or cocaine with WC 10 (5.6 mg/kg, i.p.) or with WC 44 (10.0 mg/kg, i.p). Locomotion was monitored for 90 min and the brains were harvested for immunohistochemistry. Both WC 10 and WC 44 decreased spontaneous and cocaine-induced locomotion. Both compounds also increased Fos expression relative to saline in the dorsal striatum and nucleus accumbens core and shell, and relative to cocaine alone in the nucleus accumbens shell. The findings suggest that even though these compounds have different efficacy in the AC bioassy, they produce similar brain activation and attenuation of cocaine hyperlocomotion. Together with our previous research demonstrating that these compounds down-shift the cocaine self-administration dose-effect function, the findings support the idea that D3R-selective compounds may be useful for cocaine dependence medications development.

Positron emission tomography imaging of dopamine D2 receptors using a highly selective radiolabeled D2 receptor partial agonist

A series of microPET imaging studies were conducted in anesthetized rhesus monkeys using the dopamine D2-selective partial agonist, [(11)C]SV-III-130. There was a high uptake in regions of brain known to express a high density of D2 receptors under baseline conditions. Rapid displacement in the caudate and putamen, but not in the cerebellum, was observed after injection of the dopamine D2/3 receptor nonselective ligand S(-)-eticlopride at a low dosage (0.025mg/kg/i.v.); no obvious displacement in the caudate, putamen and cerebellum was observed after the treatment with a dopamine D3 receptor selective ligand WC-34 (0.1mg/kg/i.v.). Pretreatment with lorazepam (1mg/kg, i.v. 30min) to reduce endogenous dopamine prior to tracer injection resulted in unchanged binding potential (BP) values, a measure of D2 receptor binding in vivo, in the caudate and putamen. d-Amphetamine challenge studies indicate that there is a significant displacement of [(11)C]SV-III-130 by d-Amphetamine-induced increases in synaptic dopamine levels.

Dopamine D1, D2, D3 receptors, vesicular monoamine transporter type-2 (VMAT2) and dopamine transporter (DAT) densities in aged human brain

The dopamine D(1), D(2), D(3) receptors, vesicular monoamine transporter type-2 (VMAT2), and dopamine transporter (DAT) densities were measured in 11 aged human brains (aged 77-107.8, mean: 91 years) by quantitative autoradiography. The density of D(1) receptors, VMAT2, and DAT was measured using [(3)H]SCH23390, [(3)H]dihydrotetrabenazine, and [(3)H]WIN35428, respectively. The density of D(2) and D(3) receptors was calculated using the D(3)-preferring radioligand, [(3)H]WC-10 and the D(2)-preferring radioligand [(3)H]raclopride using a mathematical model developed previously by our group. Dopamine D(1), D(2), and D(3) receptors are extensively distributed throughout striatum; the highest density of D(3) receptors occurred in the nucleus accumbens (NAc). The density of the DAT is 10-20-fold lower than that of VMAT2 in striatal regions. Dopamine D(3) receptor density exceeded D(2) receptor densities in extrastriatal regions, and thalamus contained a high level of D(3) receptors with negligible D(2) receptors. The density of dopamine D(1) linearly correlated with D(3) receptor density in the thalamus. The density of the DAT was negligible in the extrastriatal regions whereas the VMAT2 was expressed in moderate density. D(3) receptor and VMAT2 densities were in similar level between the aged human and aged rhesus brain samples, whereas aged human brain samples had lower range of densities of D(1) and D(2) receptors and DAT compared with the aged rhesus monkey brain. The differential density of D(3) and D(2) receptors in human brain will be useful in the interpretation of PET imaging studies in human subjects with existing radiotracers, and assist in the validation of newer PET radiotracers having a higher selectivity for dopamine D(2) or D(3) receptors.

Characterization of extrastriatal D2 in vivo specific binding of [¹⁸F](N-methyl)benperidol using PET

PET imaging studies of the role of the dopamine D2 receptor family in movement and neuropsychiatric disorders are limited by the use of radioligands that have near-equal affinities for D2 and D3 receptor subtypes and are susceptible to competition with endogenous dopamine. By contrast, the radioligand [¹⁸F]N-methylbenperidol ([¹⁸F]NMB) has high selectivity and affinity for the D2 receptor subtype (D2R) and is not sensitive to endogenous dopamine. Although [¹⁸F]NMB has high binding levels in striatum, its utility for measuring D2R in extrastriatal regions is unknown. A composite MR-PET image was constructed across 14 healthy adult participants representing average NMB uptake 60 to 120 min after [¹⁸F]NMB injection. Regional peak radioactivity was identified using a peak-finding algorithm. FreeSurfer and manual tracing identified a priori regions of interest (ROI) on each individual's MR image and tissue activity curves were extracted from coregistered PET images. [¹⁸F]NMB binding potentials (BP(ND) s) were calculated using the Logan graphical method with cerebellum as reference region. In eight unique participants, extrastriatal BP(ND) estimates were compared between Logan graphical methods and a three-compartment kinetic tracer model. Radioactivity and BP(ND) levels were highest in striatum, lower in extrastriatal subcortical regions, and lowest in cortical regions relative to cerebellum. Age negatively correlated with striatal BP(ND) s. BP(ND) estimates for extrastriatal ROIs were highly correlated across kinetic and graphical methods. Our findings indicate that PET with [¹⁸F]NMB measures specific binding in extrastriatal regions, making it a viable radioligand to study extrastriatal D2R levels in healthy and diseased states.

No differential regulation of dopamine transporter (DAT) and vesicular monoamine transporter 2 (VMAT2) binding in a primate model of Parkinson disease

Radioligands for DAT and VMAT2 are widely used presynaptic markers for assessing dopamine (DA) nerve terminals in Parkinson disease (PD). Previous in vivo imaging and postmortem studies suggest that these transporter sites may be regulated as the numbers of nigrostriatal neurons change in pathologic conditions. To investigate this issue, we used in vitro quantitative autoradioradiography to measure striatal DAT and VMAT2 specific binding in postmortem brain from 14 monkeys after unilateral internal carotid artery infusion of 1-Methyl-4-Phenyl-1,2,3,6-tetrahydropyridine (MPTP) with doses varying from 0 to 0.31 mg/kg. Quantitative estimates of the number of tyrosine hydroxylase (TH)-immunoreactive (ir) neurons in substantia nigra (SN) were determined with unbiased stereology, and quantitative autoradiography was used to measure DAT and VMAT2 striatal specific binding. Striatal VMAT2 and DAT binding correlated with striatal DA (r(s) = 0.83, r(s) = 0.80, respectively, both with n = 14, p<0.001) but only with nigra TH-ir cells when nigral cell loss was 50% or less (r = 0.93, n = 8, p = 0.001 and r = 0.91, n = 8, p = 0.002 respectively). Reduction of VMAT2 and DAT striatal specific binding sites strongly correlated with each other (r = 0.93, n = 14, p<0.0005). These similar changes in DAT and VMAT2 binding sites in the striatal terminal fields of the surviving nigrostriatal neurons demonstrate that there is no differential regulation of these two sites at 2 months after MPTP infusion.

Functional homomers and heteromers of dopamine D2L and D3 receptors co-exist at the cell surface

Human dopamine D(2long) and D(3) receptors were modified by N-terminal addition of SNAP or CLIP forms of O(6)-alkylguanine-DNA-alkyltransferase plus a peptide epitope tag. Cells able to express each of these four constructs only upon addition of an antibiotic were established and used to confirm regulated and inducible control of expression, the specificity of SNAP and CLIP tag covalent labeling reagents, and based on homogenous time-resolved fluorescence resonance energy transfer, the presence of cell surface D(2long) and D(3) receptor homomers. Following constitutive expression of reciprocal constructs, potentially capable of forming and reporting the presence of cell surface D(2long)-D(3) heteromers, individual clones were assessed for levels of expression of the constitutively expressed protomer. This was unaffected by induction of the partner protomer and the level of expression of the partner required to generate detectable cell surface D(2long)-D(3) heteromers was defined. Such homomers and heteromers were found to co-exist and using a reconstitution of function approach both homomers and heteromers of D(2long) and D(3) receptors were shown to be functional, potentially via trans-activation of associated G protein. These studies demonstrate the ability of dopamine D(2long) and D(3) receptors to form both homomers and heteromers, and show that in cells expressing each subtype a complex mixture of homomers and heteromers co-exists at steady state. These data are of potential importance both to disorders in which D(2long) and D(3) receptors are implicated, like schizophrenia and Parkinson disease, and also to drugs exerting their actions via these sites.

Sleep deprivation differentially affects dopamine receptor subtypes in mouse striatum

The effects of sleep deprivation on dopaminergic systems remain elusive, in part due to the lack of selective ligands for dopamine receptor subtypes. We examined D1, D2, and D3 receptor density in the mouse brain after sleep deprivation by receptor autoradiography using [H]SCH 23390 for D1R, [H]raclopride for D2R, and [H]WC-10 for D3R (a novel D3R-selective compound developed in our laboratory, not previously reported in mouse). Sleep-deprived mice showed a significant decrease in D1R, no change in D2R, and a significant increase in D3R binding in striatum. This pattern of dopamine receptor changes was not seen in mice subjected to restraint stress, suggesting specificity to sleep. These data provide evidence that brain dopaminergic circuits are remodeled after sleep deprivation.

PET imaging for attention deficit preclinical drug testing in neurofibromatosis-1 mice

Attention system abnormalities represent a significant barrier to scholastic achievement in children with neurofibromatosis-1 (NF1). Using a novel mouse model of NF1-associated attention deficit (ADD), we demonstrate a presynaptic defect in striatal dopaminergic homeostasis and leverage this finding to apply [(11)C]-raclopride positron-emission tomography (PET) in the intact animal. While methylphenidate and l-Deprenyl correct both striatal dopamine levels on PET imaging and defective attention system function in Nf1 mutant mice, pharmacologic agents that target de-regulated cyclic AMP and RAS signaling in these mice do not. These studies establish a robust preclinical model to evaluate promising agents for NF1-associated ADD.

Synthesis and characterization of selective dopamine D₂ receptor ligands using aripiprazole as the lead compound

A series of compounds structurally related to aripiprazole (1), an atypical antipsychotic and antidepressant used clinically for the treatment of schizophrenia, bipolar disorder, and depression, have been prepared and evaluated for affinity at D(₂-like) dopamine receptors. These compounds also share structural elements with the classical D(₂-like) dopamine receptor antagonists, haloperidol, N-methylspiperone, domperidone and benperidol. Two new compounds, 7-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butoxy)-3,4-dihydroquinolin-2(1H)-one oxalate (6) and 7-(4-(4-(2-(2-fluoroethoxy)phenyl)piperazin-1-yl)butoxy)-3,4-dihydroquinolin-2(1H)-one oxalate (7) were found to (a) bind to the D₂ receptor subtype with high affinity (K(i) values < 0.3 nM), (b) exhibit >50-fold D₂ versus D₃ receptor binding selectivity and (c) be partial agonists at both the D₂ and D₃ receptor subtype.

Endogenous dopamine (DA) competes with the binding of a radiolabeled D₃ receptor partial agonist in vivo: a positron emission tomography study

A series of microPET imaging studies were conducted in anesthetized rhesus monkeys using the dopamine D₃-selective partial agonist, [¹⁸F]5. There was variable uptake in regions of brain known to express a high density of D₃ receptors under baseline conditions. Pretreatment with lorazepam (1 mg/kg, i.v. 30 min) to reduce endogenous dopamine activity before tracer injection resulted in a dramatic increase in uptake in the caudate, putamen, and thalamus, and an increase in the binding potential (BP) values, a measure of D₃ receptor binding in vivo. These data indicate that there is a high level of competition between [¹⁸F]5 and endogenous dopamine for D₃ receptors in vivo.

Effect of cyclosporin A on the uptake of D3-selective PET radiotracers in rat brain

INTRODUCTION

Four benzamide analogs having a high affinity and selectivity for D(3) versus D(2) receptors were radiolabeled with (11)C or (18)F for in vivo evaluation.

METHODS

Precursors were synthesized, and the four D(3) selective benzamide analogs were radiolabeled. The tissue distribution and brain uptake of the four compounds were evaluated in control rats and rats pretreated with cyclosporin A, a modulator of P-glycoprotein and an inhibitor of other ABC efflux transporters that contribute to the blood brain barrier. Micro-positron emission tomographic (PET) imaging was carried out for [(11)C]6 in a control and a cyclosporin A pretreated rat.

RESULTS

All four compounds showed low brain uptake in control rats at 5 and 30 min post-injection; despite recently reported rat behavioral studies conducted on analogs 6 (WC-10) and 7 (WC-44). Following administration of cyclosporin A, increased brain uptake was observed with all four PET radiotracers at both 5 and 30 min post-intravenous injection. An increase in brain uptake following modulation/inhibition of the ABC transporters was also observed in the microPET study.

CONCLUSIONS

These data suggest that D3 selective conformationally-flexible benzamide analogs which contain a N-2-methoxyphenylpiperazine moiety are substrates for P-glycoprotein or other adenosine triphosphate (ATP)-binding cassette transporters expressed at the blood-brain barrier, and that PET radiotracers containing this pharmacophore may display low brain uptake in rodents due to the action of these efflux transporters.

Evaluation of the D3 dopamine receptor selective agonist/partial agonist PG01042 on L-dopa dependent animal involuntary movements in rats

The substituted 4-phenylpiperazine D3 dopamine receptor selective antagonist PG01037 ((E)-N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)but-2-enyl)-4-(pyridin-2-yl)benzamide) was reported to attenuate L-dopa-associated abnormal involuntary movements (AIMs) in unilaterally lesioned rats, a model of L-dopa-dependent dyskinesia in patients with Parkinson's Disease (Kumar et al., 2009a). We now report that PG01042 (N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)butyl)-4-(pyridin-3-yl)benzamide), which is a D3 dopamine receptor selective agonist for adenylyl cyclase inhibition and a partial agonist for mitogenesis, is also capable of attenuating AIMs scores. The intrinsic activity of PG01037 and PG01042 were determined using a) a forskolin-dependent adenylyl cyclase inhibition assay and b) an assay for agonist-associated mitogenesis. It was observed that the in vivo efficacy of PG01042 increased when administered by intraperitoneal (i.p.) injection simultaneously with L-dopa/benserazide (8 mg/kg each), as compared to a 60 min or 30 min pretreatment. PG01042 was found to attenuate AIM scores in these animals in a dose dependent manner. While PG01042 did not effectively inhibit SKF 81297-dependent AIMs, it inhibited apomorphine-dependent AIM scores. Rotarod studies indicate that PG01042 at a dose of 10 mg/kg did not adversely affect motor coordination of the unilaterally lesioned rats. Evaluation of lesioned rats using a cylinder test behavioral paradigm indicated that PG01042 did not dramatically attenuate the beneficial effects of L-dopa. These studies and previously published studies suggest that both D3 dopamine receptor selective antagonists, partial agonists and agonists, as defined by an adenylyl cyclase inhibition assay and a mitogenic assay, are pharmacotherapeutic candidates for the treatment of L-dopa-associated dyskinesia in patients with Parkinson's Disease.

Synthesis, biological evaluation and radiolabelling by 18F-fluoroarylation of a dopamine D3-selective ligand as prospective imaging probe for PET

Radical (18)F-fluoroarylation with fluorine-18-labelled arenediazonium chlorides has been successfully applied to the radiochemical synthesis of the dopamine D(3)-selective ligand SH 317 ([(18)F]8). SH 317 has been evaluated as a new PET ligand candidate by in vivo experiments.

Decreased striatal dopamine receptor binding in primary focal dystonia: a D2 or D3 defect?

Dystonia is an involuntary movement disorder characterized by repetitive patterned or sustained muscle contractions causing twisting or abnormal postures. Several lines of evidence suggest that abnormalities of dopaminergic pathways contribute to the pathophysiology of dystonia. In particular, dysfunction of D2-like receptors that mediate function of the indirect pathway in the basal ganglia may play a key role. We have demonstrated with positron emission tomography that patients with primary focal cranial or hand dystonia have reduced putamenal specific binding of [(18)F]spiperone, a nonselective D2-like radioligand with nearly equal affinity for serotonergic 5-HT(2A) sites. We then repeated the study with [(18)F]N-methyl-benperidol (NMB), a more selective D2-like receptor radioligand with minimal affinity for 5-HT(2A). Surprisingly, there was no decrease in NMB binding in the putamen of subjects with dystonia. Our findings excluded reductions of putamenal uptake greater than 20% with 95% confidence intervals. The analysis of the in vitro selectivity of NMB and spiperone demonstrated that NMB was highly selective for D2 receptors relative to D3 receptors (200-fold difference in affinity), whereas spiperone has similar affinity for all three of the D2-like receptor subtypes. These findings when coupled with other literature suggest that a defect in D3, rather than D2, receptor expression may be associated with primary focal dystonia.

Synthesis and characterization of selective dopamine D2 receptor antagonists. 2. Azaindole, benzofuran, and benzothiophene analogs of L-741,626

A series of indole, 7-azaindole, benzofuran, and benzothiophene compounds have been prepared and evaluated for affinity at D2-like dopamine receptors. These compounds share structural elements with the classical D2-like dopamine receptor antagonists haloperidol, N-methylspiperone and benperidol. Two new compounds, 4-(4-iodophenyl)-1-((4-methoxy-1H-indol-3-yl)methyl)piperidin-4-ol (6) and 4-(4-iodophenyl)-1-((5-methoxy-1H-indol-3-yl)methyl)piperidin-4-ol (7), were found to have high affinity to and selectivity for D2 versus D3 receptors. Changing the aromatic ring system from an indole to other heteroaromatic ring systems reduced the D2 binding affinity and the D2 versus D3 selectivity.