Differential distribution of D3 dopamine receptors in the brains of several mammalian species

Effects of selective dopamine D3 receptor partial agonist/antagonists on oxycodone self-administration and antinociception in monkeys

Recent studies suggest that dopamine D3 receptors (D3R) may be a therapeutic target for opioid use disorders (OUD). This study examined the effects of the D3R partial agonist (±)VK4-40 and the D3R-selective antagonist (±)VK4-116, compared to the mu-opioid receptor antagonist naltrexone (NTX), in nonhuman primate models of OUD and antinociception. Adult male and female (N = 4/sex) cynomolgus monkeys were trained to self-administer oxycodone (0.003-0.1 mg/kg/injection) first under a fixed-ratio (FR) and then a progressive-ratio (PR) schedule of reinforcement during daily 1- and 4-hr sessions, respectively. Under the FR schedule, intravenous NTX (0.01-0.1 mg/kg), (±)VK4-116 (1.0-10 mg/kg), and (±)VK4-40 (1.0-10 mg/kg) were studied in combination with the peak oxycodone dose and a dose on the descending limb of the dose-effect curve; NTX and (±)VK4-40 were also studied at the peak of the PR dose-response curve (N = 4). Following saline extinction, each compound was examined on oxycodone-induced reinstatement. Finally, these compounds were assessed in adult male rhesus monkeys (N = 3) in a warm-water (38 °C, 50 °C, 54 °C) tail withdrawal assay. NTX decreased responding on the peak of the FR oxycodone dose-response curve, but increased responding on the descending limb. (±)VK4-40, but not (±)VK4-116, significantly decreased peak oxycodone self-administration; (±)VK4-40 did not increase responding on the descending limb. NTX and (±)VK4-40, but not (±)VK4-116, attenuated oxycodone-induced reinstatement. Under PR responding, NTX and (±)VK4-40 decreased breakpoints. Oxycodone-induced antinociception was attenuated by NTX, but not by (±)VK4-40 or (±)VK4-116. Together, these results suggest that further research evaluating the effects of (±)VK4-40 as a novel pharmacotherapy for OUD is warranted.

Ventral striatal islands of Calleja neurons bidirectionally mediate depression-like behaviors in mice

The ventral striatum is a reward center implicated in the pathophysiology of depression. It contains islands of Calleja, clusters of dopamine D3 receptor-expressing granule cells, predominantly in the olfactory tubercle (OT). These OT D3 neurons regulate self-grooming, a repetitive behavior manifested in affective disorders. Here we show that chronic restraint stress (CRS) induces robust depression-like behaviors in mice and decreases excitability of OT D3 neurons. Ablation or inhibition of these neurons leads to depression-like behaviors, whereas their activation ameliorates CRS-induced depression-like behaviors. Moreover, activation of OT D3 neurons has a rewarding effect, which diminishes when grooming is blocked. Finally, we propose a model that explains how OT D3 neurons may influence dopamine release via synaptic connections with OT spiny projection neurons (SPNs) that project to midbrain dopamine neurons. Our study reveals a crucial role of OT D3 neurons in bidirectionally mediating depression-like behaviors, suggesting a potential therapeutic target.

Current Perspectives on Selective Dopamine D3 Receptor Antagonists/Partial Agonists as Pharmacotherapeutics for Opioid and Psychostimulant Use Disorders

Over three decades of evidence indicate that dopamine (DA) D3 receptors (D3R) are involved in the control of drug-seeking behavior and may play an important role in the pathophysiology of substance use disorders (SUD). The expectation that a selective D3R antagonist/partial agonist would be efficacious for the treatment of SUD is based on the following key observations. First, D3R are distributed in strategic areas belonging to the mesolimbic DA system such as the ventral striatum, midbrain, and ventral pallidum, which have been associated with behaviors controlled by the presentation of drug-associated cues. Second, repeated exposure to drugs of abuse produces neuroadaptations in the D3R system. Third, the synthesis and characterization of highly potent and selective D3R antagonists/partial agonists have further strengthened the role of the D3R in SUD. Based on extensive preclinical and preliminary clinical evidence, the D3R shows promise as a target for the development of pharmacotherapies for SUD as reflected by their potential to (1) regulate the motivation to self-administer drugs and (2) disrupt the responsiveness to drug-associated stimuli that play a key role in reinstatement of drug-seeking behavior triggered by re-exposure to the drug itself, drug-associated environmental cues, or stress. The availability of PET ligands to assess clinically relevant receptor occupancy by selective D3R antagonists/partial agonists, the definition of reliable dosing, and the prospect of using human laboratory models may further guide the design of clinical proof of concept studies. Pivotal clinical trials for more rapid progression of this target toward regulatory approval are urgently required. Finally, the discovery that highly selective D3R antagonists, such as R-VK4-116 and R-VK4-40, do not adversely affect peripheral biometrics or cardiovascular effects alone or in the presence of oxycodone or cocaine suggests that this class of drugs has great potential in safely treating psychostimulant and/or opioid use disorders.

A connectomics-based taxonomy of mammals

Mammalian taxonomies are conventionally defined by morphological traits and genetics. How species differ in terms of neural circuits and whether inter-species differences in neural circuit organization conform to these taxonomies is unknown. The main obstacle to the comparison of neural architectures has been differences in network reconstruction techniques, yielding species-specific connectomes that are not directly comparable to one another. Here, we comprehensively chart connectome organization across the mammalian phylogenetic spectrum using a common reconstruction protocol. We analyse the mammalian MRI (MaMI) data set, a database that encompasses high-resolution ex vivo structural and diffusion MRI scans of 124 species across 12 taxonomic orders and 5 superorders, collected using a unified MRI protocol. We assess similarity between species connectomes using two methods: similarity of Laplacian eigenspectra and similarity of multiscale topological features. We find greater inter-species similarities among species within the same taxonomic order, suggesting that connectome organization reflects established taxonomic relationships defined by morphology and genetics. While all connectomes retain hallmark global features and relative proportions of connection classes, inter-species variation is driven by local regional connectivity profiles. By encoding connectomes into a common frame of reference, these findings establish a foundation for investigating how neural circuits change over phylogeny, forging a link from genes to circuits to behaviour.

D3 Receptors and PET Imaging

This chapter encapsulates a short introduction to positron emission tomography (PET) imaging and the information gained by using this technology to detect changes of the dopamine 3 receptor (D3R) at the molecular level in vivo. We will discuss available D3R radiotracers, emphasizing [11C]PHNO. The focus, however, will be on PET findings in conditions including substance abuse, obesity, traumatic brain injury, schizophrenia, Parkinson's disease, and aging. Finally, there is a discussion about progress in producing next-generation selective D3R radiotracers.

Ventral striatal islands of Calleja neurons control grooming in mice

The striatum comprises multiple subdivisions and neural circuits that differentially control motor output. The islands of Calleja (IC) contain clusters of densely packed granule cells situated in the ventral striatum, predominantly in the olfactory tubercle (OT). Characterized by expression of the D3 dopamine receptor, the IC are evolutionally conserved, but have undefined functions. Here, we show that optogenetic activation of OT D3 neurons robustly initiates self-grooming in mice while suppressing other ongoing behaviors. Conversely, optogenetic inhibition of these neurons halts ongoing grooming, and genetic ablation reduces spontaneous grooming. Furthermore, OT D3 neurons show increased activity before and during grooming and influence local striatal output via synaptic connections with neighboring OT neurons (primarily spiny projection neurons), whose firing rates display grooming-related modulation. Our study uncovers a new role of the ventral striatum's IC in regulating motor output and has important implications for the neural control of grooming.

Bivalent ligands promote endosomal trafficking of the dopamine D3 receptor-neurotensin receptor 1 heterodimer

Bivalent ligands are composed of two pharmacophores connected by a spacer of variable size. These ligands are able to simultaneously recognize two binding sites, for example in a G protein-coupled receptor heterodimer, resulting in enhanced binding affinity. Taking advantage of previously described heterobivalent dopamine-neurotensin receptor ligands, we demonstrate specific interactions between dopamine D3 (D₃R) and neurotensin receptor 1 (NTSR1), two receptors with expression in overlapping brain areas that are associated with neuropsychiatric diseases and addiction. Bivalent ligand binding to D₃R-NTSR1 dimers results in picomolar binding affinity and high selectivity compared to the binding to monomeric receptors. Specificity of the ligands for the D₃R-NTSR1 receptor pair over D₂R-NTSR1 dimers can be achieved by a careful choice of the linker length. Bivalent ligands enhance and stabilize the receptor-receptor interaction leading to NTSR1-controlled internalization of D₃R into endosomes via recruitment of β-arrestin, highlighting a potential mechanism for dimer-specific receptor trafficking and signalling.

The Cerebellar Dopaminergic System

In the central nervous system (CNS), dopamine (DA) is involved in motor and cognitive functions. Although the cerebellum is not been considered an elective dopaminergic region, studies attributed to it a critical role in dopamine deficit-related neurological and psychiatric disorders [e.g., Parkinson's disease (PD) and schizophrenia (SCZ)]. Data on the cerebellar dopaminergic neuronal system are still lacking. Nevertheless, biochemical studies detected in the mammalians cerebellum high dopamine levels, while chemical neuroanatomy studies revealed the presence of midbrain dopaminergic afferents to the cerebellum as well as wide distribution of the dopaminergic receptor subtypes (DRD1-DRD5). The present review summarizes the data on the cerebellar dopaminergic system including its involvement in associative and projective circuits. Furthermore, this study also briefly discusses the role of the cerebellar dopaminergic system in some neurologic and psychiatric disorders and suggests its potential involvement as a target in pharmacologic and non-pharmacologic treatments.

Neuronal Dopamine D3 Receptors: Translational Implications for Preclinical Research and CNS Disorders

Dopamine (DA), as one of the major neurotransmitters in the central nervous system (CNS) and periphery, exerts its actions through five types of receptors which belong to two major subfamilies such as D1-like (i.e., D1 and D5 receptors) and D2-like (i.e., D2, D3 and D4) receptors. Dopamine D3 receptor (D3R) was cloned 30 years ago, and its distribution in the CNS and in the periphery, molecular structure, cellular signaling mechanisms have been largely explored. Involvement of D3Rs has been recognized in several CNS functions such as movement control, cognition, learning, reward, emotional regulation and social behavior. D3Rs have become a promising target of drug research and great efforts have been made to obtain high affinity ligands (selective agonists, partial agonists and antagonists) in order to elucidate D3R functions. There has been a strong drive behind the efforts to find drug-like compounds with high affinity and selectivity and various functionality for D3Rs in the hope that they would have potential treatment options in CNS diseases such as schizophrenia, drug abuse, Parkinson’s disease, depression, and restless leg syndrome. In this review, we provide an overview and update of the major aspects of research related to D3Rs: distribution in the CNS and periphery, signaling and molecular properties, the status of ligands available for D3R research (agonists, antagonists and partial agonists), behavioral functions of D3Rs, the role in neural networks, and we provide a summary on how the D3R-related drug research has been translated to human therapy.

Association of dopamine D2/3 receptor binding potential measured using PET and [11C]-(+)-PHNO with post-mortem DRD2/3 gene expression in the human brain

Open access post-mortem transcriptome atlases such as the Allen Human Brain Atlas (AHBA) can inform us about mRNA expression of numerous proteins of interest across the whole brain, while in vivo protein binding in the human brain can be quantified by means of neuroreceptor positron emission tomography (PET). By combining both modalities, the association between regional gene expression and receptor distribution in the living brain can be approximated. Here, we compare the characteristics of D2 and D3 dopamine receptor distribution by applying the dopamine D2/3 receptor agonist radioligand [11C]-(+)-PHNO and human gene expression data. Since [11C]-(+)-PHNO has a higher affinity for D3 compared to D2 receptors, we hypothesized that there is a stronger relationship between D2/3 non-displaceable binding potentials (BPND) and D3 mRNA expression. To investigate the relationship between D2/3 BPND and mRNA expression of DRD2 and DRD3 we performed [11C]-(+)-PHNO PET scans in 27 healthy subjects (12 females) and extracted gene expression data from the AHBA. We also calculated D2/D3 mRNA expression ratios to imitate the mixed D2/3 signal of [11C]-(+)-PHNO. In accordance with our a priori hypothesis, a strong correlation between [11C]-(+)-PHNO and DRD3 expression was found. However, there was no significant correlation with DRD2 expression. Calculated D2/D3 mRNA expression ratios also showed a positive correlation with [11C]-(+)-PHNO binding, reflecting the mixed D2/3 signal of the radioligand. Our study supports the usefulness of combining gene expression data from open access brain atlases with in vivo imaging data in order to gain more detailed knowledge on neurotransmitter signaling.

Coexistence of D3 R typical and atypical signaling in striatonigral neurons during dopaminergic denervation. Correlation with D3 nf expression changes

Dopamine D₃ R are widely expressed in basal ganglia where interact with D₁ R. D₃ R potentiate cAMP accumulation and GABA release stimulated by D₁ R in striatonigral neurons through "atypical" signaling. During dopaminergic denervation, D₃ R signaling changes to a "typical" in which antagonizes the effects of D₁ R, the mechanisms of this switching are unknown. D₃ nf splice variant regulates membrane anchorage and function of D₃ R and decreases in denervation; thus, it is possible that D₃ R signaling switching correlates with changes in D₃ nf expression and increases of membranal D₃ R that mask D₃ R atypical effects. We performed experiments in unilaterally 6-hydroxydopamine lesioned rats and found a decrease in mRNA and protein of D₃ nf, but not of D₃ R in the denervated striatum. Proximity ligation assay showed that D₃ R-D₃ nf interaction decreased after denervation, whereas binding revealed an increased B_max in D₃ R. The new D₃ R antagonized cAMP accumulation and GABA release stimulated by D₁ R; however, in the presence of N-Ethylmaleimide (NEM), to block G_i protein signaling, activation of D₃ R produced its atypical signaling stimulating D₁ R effects. Finally, we investigated if the typical and atypical effects of D₃ R modulating GABA release are capable of influencing motor behavior. Injections of D₃ R agonist into denervated nigra decreased D₁ R agonist-induced turning behavior but potentiated it in the presence of NEM. Our data indicate the coexistence of D₃ R typical and atypical signaling in striatonigral neurons during denervation that correlated with changes in the ratio of expression of D₃ nf and D₃ R isoforms. The coexistence of both atypical and typical signaling during denervation influences motor behavior.

On the relationship of first-episode psychosis to the amphetamine-sensitized state: a dopamine D2/3 receptor agonist radioligand study

Schizophrenia is characterized by increased behavioral and neurochemical responses to dopamine-releasing drugs. This prompted the hypothesis of psychosis as a state of "endogenous" sensitization of the dopamine system although the exact basis of dopaminergic disturbances and the possible role of prefrontal cortical regulation have remained uncertain. To show that patients with first-episode psychosis release more dopamine upon amphetamine-stimulation than healthy volunteers, and to reveal for the first time that prospective sensitization induced by repeated amphetamine exposure increases dopamine-release in stimulant-naïve healthy volunteers to levels observed in patients, we collected data on amphetamine-induced dopamine release using the dopamine D2/3 receptor agonist radioligand [11C]-(+)-PHNO and positron emission tomography. Healthy volunteers (n = 28, 14 female) underwent a baseline and then a post-amphetamine scan before and after a mildly sensitizing regimen of repeated oral amphetamine. Unmedicated patients with first-episode psychosis (n = 21; 6 female) underwent a single pair of baseline and then post-amphetamine scans. Furthermore, T1 weighted magnetic resonance imaging of the prefrontal cortex was performed. Patients with first-episode psychosis showed larger release of dopamine compared to healthy volunteers. After sensitization of healthy volunteers their dopamine release was significantly amplified and no longer different from that seen in patients. Healthy volunteers showed a negative correlation between prefrontal cortical volume and dopamine release. There was no such relationship after sensitization or in patients. Our data in patients with untreated first-episode psychosis confirm the "endogenous sensitization" hypothesis and support the notion of impaired prefrontal control of the dopamine system in schizophrenia.

Prenatal Exposure to Methamphetamine: Up-Regulation of Brain Receptor Genes

Methamphetamine (METH) is a widespread illicit drug. If it is taken by pregnant women, it passes through the placenta and just as it affects the mother, it can impair the development of the offspring. The aim of our study was to identify candidates to investigate for changes in the gene expression in the specific regions of the brain associated with addiction to METH in rats. We examined the various areas of the central nervous system (striatum, hippocampus, prefrontal cortex) for signs of impairment in postnatal day 80 in experimental rats, whose mothers had been administered METH (5 mg/kg/day) during the entire gestation period. Changes in the gene expression at the mRNA level were determined by two techniques, microarray and real-time PCR. Results of two microarray trials were evaluated by LIMMA analysis. The first microarray trial detected either up-regulated or down-regulated expression of 2189 genes in the striatum; the second microarray trial detected either up-regulated or down-regulated expression of 1344 genes in the hippocampus of prenatally METH-exposed rats. We examined the expression of 10 genes using the real-time PCR technique. Differences in the gene expression were counted by the Mann–Whitney U-test. Significant changes were observed in the cocaine- and amphetamine-regulated transcript prepropeptide, tachykinin receptor 3, dopamine receptor D3 gene expression in the striatum regions, in the glucocorticoid nuclear receptor Nr3c1 gene expression in the prefrontal cortex and in the carboxylesterase 2 gene expression in the hippocampus of prenatally METH-exposed rats. The microarray technique also detected up-regulated expression of trace amine-associated receptor 7 h gene in the hippocampus of prenatally METH-exposed rats. We have identified susceptible genes; candidates for the study of an impairment related to methamphetamine addiction in the specific regions of the brain.

Differential Dopamine D1 and D3 Receptor Modulation and Expression in the Spinal Cord of Two Mouse Models of Restless Legs Syndrome

Restless Legs Syndrome (RLS) is often and successfully treated with dopamine receptor agonists that target the inhibitory D3 receptor subtype, however there is no clinical evidence of a D3 receptor dysfunction in RLS patients. In contrast, genome-wide association studies in RLS patients have established that a mutation of the MEIS1 gene is associated with an increased risk in developing RLS, but the effect of MEIS1 dysfunction on sensorimotor function remain unknown. Mouse models for a dysfunctional D3 receptor (D3KO) and Meis1 (Meis1KO) were developed independently, and each animal expresses some features associated with RLS in the clinic, but they have not been compared in their responsiveness to treatment options used in the clinic. We here confirm that D3KO and Meis1KO animals show increased locomotor activities, but that only D3KO show an increased sensory excitability to thermal stimuli. Next we compared the effects of dopaminergics and opioids in both animal models, and we assessed D1 and D3 dopamine receptor expression in the spinal cord, the gateway for sensorimotor processing. We found that Meis1KO share most of the tested behavioral properties with their wild type (WT) controls, including the modulation of the thermal pain withdrawal reflex by morphine, L-DOPA and D3 receptor (D3R) agonists and antagonists. However, Meis1KO and D3KO were behaviorally more similar to each other than to WT when tested with D1 receptor (D1R) agonists and antagonists. Subsequent Western blot analyses of D1R and D3R protein expression in the spinal cord revealed a significant increase in D1R but not D3R expression in Meis1KO and D3KO over WT controls. As the D3R is mostly present in the dorsal spinal cord where it has been shown to modulate sensory pathways, while activation of the D1Rs can activate motoneurons in the ventral spinal cord, we speculate that D3KO and Meis1KO represent two complementary animal models for RLS, in which the mechanisms of sensory (D3R-mediated) and motor (D1R-mediated) dysfunctions can be differentially explored.

Development of a population pharmacokinetic model to predict brain distribution and dopamine D2 receptor occupancy of raclopride in non-anesthetized rat

BACKGROUND

Raclopride is a selective antagonist of the dopamine D2 receptor. It is one of the most frequently used in vivo D2 tracers (at low doses) for assessing drug-induced receptor occupancy (RO) in animals and humans. It is also commonly used as a pharmacological blocker (at high doses) to occupy the available D2 receptors and antagonize the action of dopamine or drugs on D2 in preclinical studies. The aims of this study were to comprehensively evaluate its pharmacokinetic (PK) profiles in different brain compartments and to establish a PK-RO model that could predict the brain distribution and RO of raclopride in the freely moving rat using a LC-MS based approach.

METHODS

Rats (n=24) received a 10-min IV infusion of non-radiolabeled raclopride (1.61μmol/kg, i.e. 0.56mg/kg). Plasma and the brain tissues of striatum (with high density of D2 receptors) and cerebellum (with negligible amount of D2 receptors) were collected. Additional microdialysis experiments were performed in some rats (n=7) to measure the free drug concentration in the extracellular fluid of the striatum and cerebellum. Raclopride concentrations in all samples were analyzed by LC-MS. A population PK-RO model was constructed in NONMEM to describe the concentration-time profiles in the unbound plasma, brain extracellular fluid and brain tissue compartments and to estimate the RO based on raclopride-D2 receptor binding kinetics.

RESULTS

In plasma raclopride showed a rapid distribution phase followed by a slower elimination phase. The striatum tissue concentrations were consistently higher than that of cerebellum tissue throughout the whole experimental period (10-h) due to higher non-specific tissue binding and D2 receptor binding in the striatum. Model-based simulations accurately predicted the literature data on rat plasma PK, brain tissue PK and D2 RO at different time points after intravenous or subcutaneous administration of raclopride at tracer dose (RO <10%), sub-pharmacological dose (RO 10%-30%) and pharmacological dose (RO >30%).

CONCLUSION

For the first time a predictive model that could describe the quantitative in vivo relationship between dose, PK and D2 RO of raclopride in non-anesthetized rat was established. The PK-RO model could facilitate the selection of optimal dose and dosing time when raclopride is used as tracer or as pharmacological blocker in various rat studies. The LC-MS based approach, which doses and quantifies a non-radiolabeled tracer, could be useful in evaluating the systemic disposition and brain kinetics of tracers.

Genetic blockade of the dopamine D3 receptor enhances hippocampal expression of PACAP and receptors and alters their cortical distribution

Dopamine D3 receptors (D3Rs) are implicated in several aspects of cognition, but their role in aversive conditioning has only been marginally uncovered. Investigations have reported that blockade of D3Rs enhances the acquisition of fear memories, a phenomenon tightly linked to the neuropeptide pituitary adenylate cyclase-activating peptide (PACAP). However, the impact of D3R ablation on the PACAPergic system in regions critical for the formation of new memories remains unexplored. To address this issue, levels of PACAP and its receptors were compared in the hippocampus and cerebral cortex (CX) of mice devoid of functional D3Rs (D3R(-/-)) and wild-types (WTs) using a series of comparative immunohistochemical and biochemical analyses. Morphometric and stereological data revealed increased hippocampal area and volume in D3R(-/-) mice, and augmented neuronal density in CA1 and CA2/3 subfields. PACAP levels were increased in the hippocampus of D3R(-/-) mice. Expression of PACAP receptors was also heightened in mutant mice. In the CX, PACAP immunoreactivity (IR), was restricted to cortical layer V in WTs, but was distributed throughout layers IV-VI in D3R(-/-) mice, along with increased mRNAs, protein concentration and staining scores. Consistently, PAC1, VPAC1 and VPAC2 IRs were variably redistributed in CX, with a general upregulation in cortical layers II-IV in knockout animals. Our interpretation of these findings is that disturbed dopamine neurotransmission due to genetic D3R blockade may enhance the PACAP/PAC1-VPAC axis, a key endogenous system for the processing of fear memories. This could explain, at least in part, the facilitated acquisition and consolidation of aversive memories in D3R(-/-) mice.

Dopamine D3 receptor binding of (18)F-fallypride: Evaluation using in vitro and in vivo PET imaging studies

Identification of dopamine D3 receptors (D3R) in vivo is important to understand several brain functions related to addiction. The goal of this work was to identify D3R binding of the dopamine D2 receptor (D2R)/D3R imaging agent, (18)F-fallypride. Brain slices from male Sprague-Dawley rats (n = 6) and New Zealand White rabbits (n = 6) were incubated with (18)F-fallypride and D3R selective agonist (R)-7-OH-DPAT (98-fold D3R selective). Rat slices were also treated with BP 897 (68-fold D3R selective partial agonist) and NGB 2904 (56-fold D3R selective antagonist). In vivo rat studies (n = 6) were done on Inveon PET using 18-37 MBq (18)F-fallypride and drug-induced displacement by (R)-7-OH-DPAT, BP 897 and NGB 2904. PET/CT imaging of wild type (WT, n = 2) and D2R knock-out (KO, n = 2) mice were carried out with (18)F-fallypride. (R)-7-OH-DPAT displaced binding of (18)F-fallypride, both in vitro and in vivo. In vitro, at 10 nM (R)-7-OH-DPAT, (18)F-fallypride binding in the rat ventral striatum (VST) and dorsal striatum (DST) and rabbit nucleus accumbens were reduced by ∼10-15%. At 10 μM (R)-7-OH-DPAT all regions in rat and rabbit were reduced by ≥85%. In vivo reductions for DST and VST before and after (R)-7-OH-DPAT were: low-dose (0.015 mg kg(-1)) DST -22%, VST -29%; high-dose (1.88 mg kg(-1)) DST -58%, VST -77%, suggesting D3R/D2R displacement. BP 897 and NGB 2904 competed with (18)F-fallypride in vitro, but unlike BP 897, NGB 2904 did not displace (18)F-fallypride in vivo. The D2R KO mice lacked (18)F-fallypride binding in the DST. In summary, our findings suggest that up to 20% of (18)F-fallypride may be bound to D3R sites in vivo.

Dopamine Receptors and Neurodegeneration

Dopamine (DA) is one of the major neurotransmitters and participates in a number of functions such as motor coordination, emotions, memory, reward mechanism, neuroendocrine regulation etc. DA exerts its effects through five DA receptors that are subdivided in 2 families: D1-like DA receptors (D1 and D5) and the D2-like (D2, D3 and D4). All DA receptors are widely expressed in the central nervous system (CNS) and play an important role in not only in physiological conditions but also pathological scenarios. Abnormalities in the DAergic system and its receptors in the basal ganglia structures are the basis Parkinson's disease (PD), however DA also participates in other neurodegenerative disorders such as Huntington disease (HD) and multiple sclerosis (MS). Under pathological conditions reorganization of DAergic system has been observed and most of the times, those changes occur as a mechanism of compensation, but in some cases contributes to worsening the alterations. Here we review the changes that occur on DA transmission and DA receptors (DARs) at both levels expression and signals transduction pathways as a result of neurotoxicity, inflammation and in neurodegenerative processes. The better understanding of the role of DA receptors in neuropathological conditions is crucial for development of novel therapeutic approaches to treat alterations related to neurodegenerative diseases.

Identifying Medication Targets for Psychostimulant Addiction: Unraveling the Dopamine D3 Receptor Hypothesis

The dopamine D3 receptor (D3R) is a target for developing medications to treat substance use disorders. D3R-selective compounds with high affinity and varying efficacies have been discovered, providing critical research tools for cell-based studies that have been translated to in vivo models of drug abuse. D3R antagonists and partial agonists have shown especially promising results in rodent models of relapse-like behavior, including stress-, drug-, and cue-induced reinstatement of drug seeking. However, to date, translation to human studies has been limited. Herein, we present an overview and illustrate some of the pitfalls and challenges of developing novel D3R-selective compounds toward clinical utility, especially for treatment of cocaine abuse. Future research and development of D3R-selective antagonists and partial agonists for substance abuse remains critically important but will also require further evaluation and development of translational animal models to determine the best time in the addiction cycle to target D3Rs for optimal therapeutic efficacy.

Dopamine agonists early monotherapy for the delay of development of levodopa-induced dyskinesias

Dyskinesias are common, often disabling motor complications emerging in Parkinson's disease following chronic levodopa treatment. Common views associate the development of dyskinesias both with progressive loss of striatal dopamine nerve terminals and with intermittent delivery of the short half-life levodopa. Thus, according to continuous dopaminergic stimulation theory, dopamine agonists having half-lifes longer than levodopa would minimize the risk of the development of dyskinesias. The article highlights some interesting aspects of the clinical trials testing dopamine agonists monotherapy as a strategy that can reduce the risk of motor complications, and raises some concerns in terms of their early use in Parkinson's disease treatment to prevent or delay dyskinesia. Finally, we emphasize the need for reconsideration of arguments against use of levodopa as a starting therapy for Parkinson's disease.

Dopamine D3 receptors in the basolateral amygdala and the lateral habenula modulate cue-induced reinstatement of nicotine seeking

Dopamine D3 receptors are implicated in cue-induced relapse to drug seeking. We have previously shown that systemic administration of a selective D3 antagonist reduces cue-induced reinstatement of nicotine seeking in rats. The current study sought to investigate potential neural substrates mediating this effect. The D3 antagonist SB-277011-A (0.01-1 μg/0.5 μl/side) infused into the basolateral amygdala or the lateral habenula, but not the nucleus accumbens, significantly attenuated cue-induced reinstatement of nicotine seeking. Moreover, infusion of SB-277011-A (1 μg/0.5 μl/side) into the basolateral amygdala or lateral habenula had no effect on food self-administration. Together with the finding that systemic SB-277011-A had no effect on extinction responding, this suggests that the effects observed here were on reinstatement and cue seeking, and not due to nonspecific motor activation or contextual-modified residual responding. The further finding of binding of [(125)I]7-OH-PIPAT to D3 receptors in the lateral habenula and in the basolateral amygdala is consistent with an important role of D3 receptors in these areas in nicotine seeking. It was also found that systemic administration of the selective D2 antagonist L741626 decreased cue-induced reinstatement, consistent with a role of D2 and D3 receptors in modulating this behavior. The current study supports an important role for D3 receptors in the basolateral amygdala and lateral habenula in cue-induced reinstatement.

Further characterization of quinpirole-elicited yawning as a model of dopamine D3 receptor activation in male and female monkeys

The dopamine (DA) D3 receptor (D3R) has been associated with impulsivity, pathologic gambling, and drug addiction, making it a potential target for pharmacotherapy development. Positron emission tomography studies using the D3R-preferring radioligand [(11)C]PHNO ([(11)C](+)-propyl-hexahydro-naphtho-oxazin) have shown higher binding potentials in drug abusers compared with control subjects. Preclinical studies have examined D3R receptor activation using the DA agonist quinpirole and the unconditioned behavior of yawning. However, the relationship between quinpirole-elicited yawning and D3R receptor availability has not been determined. In Experiment 1, eight drug-naive male rhesus monkeys were scanned with [(11)C]PHNO, and the ability of quinpirole (0.01-0.3 mg/kg i.m.) to elicit yawning was examined. Significant positive (globus pallidus) and negative (caudate nucleus, putamen, ventral pallidum, and hippocampus) relationships between D3R receptor availability and quinpirole-induced yawns were noted. Experiment 2 replicated earlier findings that a history of cocaine self-administration (n = 11) did not affect quinpirole-induced yawning and extended this to examine monkeys (n = 3) with a history of methamphetamine (MA) self-administration and found that monkeys with experience self-administering MA showed greater potency and significantly higher quinpirole-elicited yawning compared with controls. Finally, quinpirole-elicited yawning was studied in drug-naive female monkeys (n = 6) and compared with drug-naive male monkeys (n = 8). Sex differences were noted, with quinpirole being more potent and eliciting significantly more yawns in males compared with females. Taken together these findings support the use of quinpirole-elicited yawning as a behavioral tool for examining D3R activation in monkeys and that both drug history and sex may influence individual sensitivity to the behavioral effects of D3R compounds.

The potential role of dopamine D₃ receptor neurotransmission in cognition

Currently available treatments have limited pro-cognitive effects for neuropsychiatric disorders, such as schizophrenia, Parkinson's disease and Alzheimer's disease. The primary objective of this work is to review the literature on the role of dopamine D₃ receptors in cognition, and propose dopamine D₃ receptor antagonists as possible cognitive enhancers for neuropsychiatric disorders. A literature search was performed to identify animal and human studies on D₃ receptors and cognition using PubMed, MEDLINE and EMBASE. The search terms included "dopamine D₃ receptor" and "cognition". The literature search identified 164 articles. The results revealed: (1) D₃ receptors are associated with cognitive functioning in both healthy individuals and those with neuropsychiatric disorders; (2) D₃ receptor blockade appears to enhance while D₃ receptor agonism seems to impair cognitive function, including memory, attention, learning, processing speed, social recognition and executive function independent of age; and (3) D₃ receptor antagonists may exert their pro-cognitive effect by enhancing the release of acetylcholine in the prefrontal cortex, disinhibiting the activity of dopamine neurons projecting to the nucleus accumbens or prefrontal cortex, or activating CREB signaling in the hippocampus. These findings suggest that D₃ receptor blockade may enhance cognitive performance in healthy individuals and treat cognitive dysfunction in individuals with a neuropsychiatric disorder. Clinical trials are needed to confirm these effects.

Dopamine D3 receptor deletion increases tissue plasminogen activator (tPA) activity in prefrontal cortex and hippocampus

Considerable evidence indicates that dopamine (DA) influences tissue plasminogen activator (tPA)-mediated proteolytic processing of the precursor of brain-derived neurotrophic factor (proBDNF) into mature BDNF (mBDNF). However, specific roles in this process for the dopamine D3 receptor (D3R) and the underlying molecular mechanisms are yet to be fully characterized. In the present study, we hypothesized that D3R deletion could influence tPA activity in the prefrontal cortex and hippocampus. Using D3R knockout (D3(-/-)) mice, we show that receptor inactivation is associated with increased tPA expression/activity both in the prefrontal cortex and, to a greater extent, in the hippocampus. Augmented tPA expression in D3(-/-) mice correlated with increased BDNF mRNA levels, plasmin/plasminogen protein ratio and the conversion of proBDNF into mBDNF, as well as enhanced tPA and mBDNF immunoreactivity, as determined by quantitative real time polymerase chain reaction (qRT-PCR), immunoblot and immunohistochemistry. In addition, when compared to wild-type controls, D3(-/-) mice exhibited increased basal activation of the canonical cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA)-driven Akt/cAMP-response element-binding protein (CREB) signaling cascade, as determined by the increased Akt phosphorylation both at Thr304 and Ser473 residues, of DA and cAMP-regulated protein of 32kDa (DARPP-32) at Thr34 and a phosphorylation state-dependent inhibition of glycogen synthetase kinase-3β (GSK-3β) at Ser9, a substrate of Akt whose constitutive function impairs normal CREB transcriptional activity through phosphorylation at its Ser129 residue. Accordingly, CREB phosphorylation at Ser133 was significantly increased in D3(-/-) mice, whereas the GSK-3β-dependent phosphorylation at Ser129 was diminished. Altogether, our finding reveals that mice lacking D3Rs show enhanced tPA proteolytic activity on BDNF which may involve, at least in part, a potentiated Akt/CREB signaling, possibly due to hindered GSK-3β activity.

Effects of the 5HT2C antagonist SB242084 on the pramipexole-induced potentiation of water contrafreeloading, a putative animal model of compulsive behavior

RATIONALE

In rats, quinpirole, a dopaminergic D2/D3 receptor agonist, elicits both hyperdipsia and water "contrafreeloading" (CFL), a putative model of compulsivity. The role of D3 receptors in this effect remains unclear. Clomipramine (CIM) was found to contrast both hyperdipsia and CFL, but the role of serotonin in this effect requires further investigation.

OBJECTIVES

We studied the effects of the preferential D3 agonist pramipexole (PPX) in both models. Furthermore, we tested the sensitivity of PPX-induced CFL to CIM and to the 5HT2c antagonist SB242084.

METHODS

In experiment 1, drinking was measured at 2 and 5 h after eight daily injections of PPX (0 to 1.0 mg/kg intraperitoneally). In the CFL study, every other third lever press, the rat was reinforced by the delivery of water. On days 1-6, water was only available upon lever pressing. On days 7-15, choice between response-contingent and free access was provided. PPX doses as in the experiment 1 were given. In two further experiments, PPX (0.5 mg/kg) was administered alone or in combination with CIM (5 or 10 mg/kg) or SB242084 (0.3 or 1.0 mg/kg).

RESULTS

PPX did not produce hyperdipsia but enhanced spontaneous CFL. SB242084 attenuated PPX-induced CFL more effectively than CIM, restoring the preference for free access to water.

CONCLUSIONS

CFL, but not polydipsia, was induced by preferential D3 activation, an effect prevented by 5HT2c receptor blockade. Since PPX interferes with decision making and 5HT2c receptor supersensitivity is involved in the expression of compulsive behaviors, this study supports the compulsive nature of dopaminergic-induced CFL.

Receptor-specific modulation of risk-based decision making by nucleus accumbens dopamine

The nucleus accumbens (NAc) serves as an integral node within cortico-limbic circuitry that regulates various forms of cost-benefit decision making. The dopamine (DA) system has also been implicated in enabling organisms to overcome a variety of costs to obtain more valuable rewards. However, it remains unclear how DA activity within the NAc may regulate decision making involving reward uncertainty. This study investigated the contribution of different DA receptor subtypes in the NAc to risk-based decision making, assessed with a probabilistic discounting task. In well-trained rats, D1 receptor blockade with SCH 23,390 decreased preference for larger, uncertain rewards, which was associated with enhanced negative-feedback sensitivity (ie, an increased tendency to select a smaller/certain option after an unrewarded risky choice). Treatment with a D1 agonist (SKF 81,297) optimized decision making, increasing choice of the risky option when reward probability was high, and decreasing preference under low probability conditions. In stark contrast, neither blockade of NAc D2 receptors with eticlopride, nor stimulation of these receptors with quinpirole or bromocriptine influenced risky choice. In comparison, infusion of the D3-preferring agonist PD 128,907 decreased reward sensitivity and risky choice. Collectively, these results show that mesoaccumbens DA refines risk-reward decision biases via dissociable mechanisms recruiting D1 and D3, but not D2 receptors. D1 receptor activity mitigates the effect of reward omissions on subsequent choices to promote selection of reward options that may have greater long-term utility, whereas excessive D3 receptor activity blunts the impact that larger/uncertain rewards have in promoting riskier choices.

The role of dopamine D(3) receptors in antipsychotic activity and cognitive functions

Dopamine D(3) receptors have a pre- and postsynaptic localization in brain stem nuclei, limbic parts of the striatum, and cortex. Their widespread influence on dopamine release, on dopaminergic function, and on several other neurotransmitters makes them attractive targets for therapeutic intervention. The signaling pathways of D(3) receptors are distinct from those of other members of the D(2)-like receptor family. There is increasing evidence that D(3) receptors can form heteromers with dopamine D(1), D(2), and probably other G-protein-coupled receptors. The functional consequences remain to be characterized in more detail but might open new interesting pharmacological insight and opportunities. In terms of behavioral function, D(3) receptors are involved in cognitive, social, and motor functions, as well as in filtering and sensitization processes. Although the role of D(3) receptor blockade for alleviating positive symptoms is still unsettled, selective D(3) receptor antagonism has therapeutic features for schizophrenia and beyond as demonstrated by several animal models: improved cognitive function, emotional processing, executive function, flexibility, and social behavior. D(3) receptor antagonism seems to contribute to atypicality of clinically used antipsychotics by reducing extrapyramidal motor symptoms; has no direct influence on prolactin release; and does not cause anhedonia, weight gain, or metabolic dysfunctions. Unfortunately, clinical data with new, selective D(3) antagonists are still incomplete; their cognitive effects have only been communicated in part. In vitro, virtually all clinically used antipsychotics are not D(2)-selective but also have affinity for D(3) receptors. The exact D(3) receptor occupancies achieved in patients, particularly in cortical areas, are largely unknown, mainly because only nonselective or agonist PET tracers are currently available. It is unlikely that a degree of D(3) receptor antagonism optimal for antipsychotic and cognitive function can be achieved with existing antipsychotics. Therefore, selective D(3) antagonism represents a promising mechanism still to be fully exploited for the treatment of schizophrenia, cognitive deficits in schizophrenia, and comorbid conditions such as substance abuse.

Dopamine D3 receptor antagonists: a patent review (2007 - 2012)

INTRODUCTION

The synthesis and characterization of new highly potent and selective dopamine (DA) D3 receptor antagonists has permitted to characterize the role of the DA D3 receptor in the control of drug-seeking behavior and in the pathophysiology of impulse control disorders and schizophrenia.

AREAS COVERED

In the present review, the authors will first describe most recent classes of DA D3 receptor antagonists by reviewing about 43 patent applications during the 2007 - 2012 period; they will then outline the biological rationale in support of the use of selective DA D3 receptor antagonists in the treatment of drug addiction, impulse control disorders and schizophrenia.

EXPERT OPINION

The strongest clinical application and potential for selective DA D3 receptor antagonists lies in the reduction of drug-induced incentive motivation, the attenuation of drug's rewarding efficacy and the reduction in reinstatement of drug-seeking behavior triggered either by re-exposure to the drug itself, re-exposure to environmental cues that had been previously associated with drug-taking behavior or stress. The selectivity of these antagonists together with reduced lipophilicity (minimizing unspecific binding), increased brain penetration and improved physico-chemical profile are all key factors for clinical efficacy and safety.

Self-regulation therapy to reproduce drug effects: a suggestion technique to change personality and the DRD3 gene expression

This study proposes a strategy, based on self-regulation therapy, to change personality and its biological substrate, the DRD3 gene expression. It has been demonstrated that acute doses of stimulating drugs, like methylphenidate, are able to change personality and the expression of certain genes in the short term. On the other hand, self-regulation therapy has been proven to reproduce the effects of drugs. Thus, it is feasible to hope that self-regulation therapy is equally effective as methylphenidate in changing personality and the gene expression. This is a preliminary study with a single-case experimental design with replication in which 2 subjects participated. The results and potential implications for research and psychotherapy are discussed.

Mathematical modelling of [¹¹C]-(+)-PHNO human competition studies

The D(2)/D(3) agonist radioligand [(11)C]-(+)-PHNO is currently the most suitable D(3) imaging agent available, despite its limited selectivity for the D(3) over the D(2). Given the collocation of D(2) and D(3) receptors, and generally higher densities of D(2), the separation of D(2) and D(3) information from [(11)C]-(+)-PHNO PET data are somewhat complex. This complexity is compounded by recent data suggesting that [(11)C]-(+)-PHNO PET scans might be routinely performed in non-tracer conditions (with respect to D(3) receptors), and that the cerebellum (used as a reference region) might manifest some displaceable binding signal. Here we present the modelling and analysis of data from two human studies which employed an adequate dose range of selective D(3) antagonists (GSK598809 and GSK618334) to interrogate the [(11)C]-(+)-PHNO PET signal. Models describing the changes observed in the PET volume of distribution (V(T)) and binding potential (BP(ND)) were used to identify and quantify a [(11)C]-(+)-PHNO mass dose effect at the D(3), and displaceable signal in the cerebellum, as well as providing refined estimates of regional D(3) fractions of [(11)C]-(+)-PHNO BP(ND). The dose of (+)-PHNO required to occupy half of the available D(3) receptors (ED(50)(PHNO,D3)) was estimated as 40ng/kg, and the cerebellum BP(ND) was estimated as 0.40. These findings confirm that [(11)C]-(+)-PHNO human PET studies are in fact routinely performed under non-tracer conditions. This suggests that (+)-PHNO injection masses should be minimised and tightly controlled in order to mitigate the mass dose effect. The specific binding detected in the cerebellum was modest but could have a significant effect, for example on estimates of D(3) potency in drug occupancy studies. A range of methods for the analysis of future [(11)C]-(+)-PHNO data, incorporating models for the effects quantified here, were developed and evaluated. The comparisons and conclusions drawn from these can inform the design and analysis of future PET studies with [(11)C]-(+)-PHNO.

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.

Cariprazine (RGH-188), a potent D3/D2 dopamine receptor partial agonist, binds to dopamine D3 receptors in vivo and shows antipsychotic-like and procognitive effects in rodents

We investigated the in vivo effects of orally administered cariprazine (RGH-188; trans-N-{4-[2-[4-(2,3-dichlorophenyl)-piperazin-1-yl]-ethyl]-cyclohexyl}-N',N'-dimethyl-urea), a D(3)/D(2) dopamine receptor partial agonist with ∼10-fold preference for the D(3) receptor. Oral bioavailability of cariprazine at a dose of 1mg/kg in rats was 52% with peak plasma concentrations of 91ng/mL. Cariprazine 10mg/kg had good blood-brain barrier penetration, with a brain/plasma AUC ratio of 7.6:1. In rats, cariprazine showed dose-dependent in vivo displacement of [(3)H](+)-PHNO, a dopamine D(3) receptor-preferring radiotracer, in the D(3) receptor-rich region of cerebellar lobules 9 and 10. Its potent inhibition of apomorphine-induced climbing in mice (ED(50)=0.27mg/kg) was sustained for 8h. Cariprazine blocked amphetamine-induced hyperactivity (ED(50)=0.12mg/kg) and conditioned avoidance response (CAR) (ED(50)=0.84mg/kg) in rats, and inhibited the locomotor-stimulating effects of the noncompetitive NMDA antagonists MK-801 (ED(50)=0.049mg/kg) and phencyclidine (ED(50)=0.09mg/kg) in mice and rats, respectively. It reduced novelty-induced motor activity of mice (ED(50)=0.11mg/kg) and rats (ED(50)=0.18mg/kg) with a maximal effect of 70% in both species. Cariprazine produced no catalepsy in rats at up to 100-fold dose of its CAR inhibitory ED(50) value. Cariprazine 0.02-0.08mg/kg significantly improved the learning performance of scopolamine-treated rats in a water-labyrinth learning paradigm. Though risperidone, olanzapine, and aripiprazole showed antipsychotic-like activity in many of these assays, they were less active against phencyclidine and more cataleptogenic than cariprazine, and had no significant effect in the learning task. The distinct in vivo profile of cariprazine may be due to its higher affinity and in vivo binding to D(3) receptors versus currently marketed typical and atypical antipsychotics.

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.

PG01037, a novel dopamine D3 receptor antagonist, inhibits the effects of methamphetamine in rats

Our previous studies have shown that the selective dopamine D(3) receptor antagonists SB-277011A or NGB 2904 significantly attenuate cocaine self-administration under a progressive-ratio reinforcement schedule and cocaine-, methamphetamine- or nicotine-enhanced brain stimulation reward. However, the poor bioavailability of SB-277011A has limited its potential use in humans. In the present study, we investigated the effects of the novel D(3) receptor antagonist PG01037 on methamphetamine self-administration, methamphetamine-associated cue-induced reinstatement of drug seeking and methamphetamine-enhanced brain stimulation reward. Rats were allowed to intravenously self-administer methamphetamine under fixed-ratio 2 and progressive-ratio reinforcement conditions, and then the effects of PG01037 on methamphetamine self-administration and cue-induced reinstatement were assessed. Additional groups of rats were trained for intracranial electrical brain stimulation reward and the effects of PG01037 and methamphetamine on brain stimulation reward were assessed. Acute intraperitoneal administration of PG01037 (3, 10, 30 mg/kg) failed to alter methamphetamine or sucrose self-administration under fixed-ratio 2 reinforcement, but significantly lowered the break-point levels for methamphetamine or sucrose self-administration under progressive-ratio reinforcement. In addition, PG01037 significantly inhibited methamphetamine-associated cue-triggered reinstatement of drug-seeking behavior and methamphetamine-enhanced brain stimulation reward. These data suggest that the novel D(3) antagonist PG01037 significantly attenuates the rewarding effects as assessed by progressive-ratio self-administration and brain stimulation reward, and inhibits methamphetamine-associated cue-induced reinstatement of drug-seeking behavior These findings support the potential use of PG01037 or other selective D(3) antagonists in the treatment of methamphetamine addiction.

Imaging brain regional and cortical laminar effects of selective D3 agonists and antagonists

RATIONALE

Dopamine D3 receptors (D3R) may be important therapeutic targets for both drug abuse and dyskinesias in Parkinson's disease; however, little is known about their functional circuitry.

OBJECTIVES

We wished to determine if D3R antagonists SB-277011 and PG-01037 and D3R-preferring agonist 7-OH-DPAT are D3R selective in vivo. We further wished to characterize the response to D3R drugs using whole brain imaging to identify novel D3R circuitry.

METHODS

We investigated D3R circuitry in rats using pharmacologic MRI and challenge with selective D3R antagonists and agonist at various doses to examine regional changes in cerebral blood volume (CBV). We compared regional activation patterns with D2R/D3R agonists, as well as with prior studies of mRNA expression and autoradiography.

RESULTS

D3R antagonists induced positive CBV changes and D3R agonist negative CBV changes in brain regions including nucleus accumbens, infralimbic cortex, thalamus, interpeduncular region, hypothalamus, and hippocampus (strongest in subiculum). All D3R-preferring drugs showed markedly greater responses in nucleus accumbens than in caudate/putamen consistent with D3R selectivity and contrary to what was observed with D2R agonists. At high doses of D3R agonist, functional changes were differentiated across cortical laminae, with layer V-VI yielding positive CBV changes and layer IV yielding negative CBV changes. These results are not inconsistent with differential D1R and D3R innervation in these layers respectively showed previously using post-mortem techniques.

CONCLUSIONS

MRI provides a new tool for testing the in vivo selectivity of novel D3R dopaminergic ligands where radiolabels may not be available. Further, the functional D3R circuitry strongly involves hypothalamus and subiculum as well as the limbic striatum.

[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

4-(Dimethylamino)-N-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butyl)benzamide (WC-10), a N-phenyl piperazine analog, displays high affinity and moderate selectivity for dopamine D(3) receptors versus dopamine D(2) receptors (Chu et al. [2005] Bioorg Med Chem 13:77-87). In this study, WC-10 was radiolabeled with tritium (specific activity = 80 Ci/mmol), and quantitative autoradiography studies were conducted using rhesus monkey and Sprague-Dawley rat brain sections. K(d) values for the binding of [3H]WC-10 to D(3) receptors obtained from quantitative autoradiography with rhesus monkey and rat brain sections are in agreement with K(d) values obtained from cloned human and rat receptors (Xu et al. [2009] Synapse 63:717-728). The D(2) selective antagonist [3H]raclopride binds with 11-fold higher affinity to human HEK D(2L) (K(d) = 1.6 nM) than HEK D(3) (K(d) = 18 nM) receptors; [3H]raclopride binds to rat Sf9 rD(2L) receptors with a K(d) of 6.79 nM, a value that is 4-fold lower than binding to human HEK D(2L) receptors and 2.5-fold higher than binding to rat Sf9 rD(3) receptors. In vitro quantitative autoradiography studies with [3H]WC-10 and [3H]raclopride were conducted on adult rat and rhesus monkey brain sections. A mathematical model for calculating the absolute densities of dopamine D(2) and D(3) receptors based on the in vitro receptor binding data of [3H]WC-10 and [3H]raclopride was developed.

The antipsychotics olanzapine, risperidone, clozapine, and haloperidol are D2-selective ex vivo but not in vitro

In a recent human [(11)C]-(+)-PHNO positron emission tomography study, olanzapine, clozapine, and risperidone occupied D2 receptors in striatum (STR), but, despite their similar in vitro D2 and D3 affinities, failed to occupy D3 receptors in globus pallidus. This study had two aims: (1) to characterize the regional D2/D3 pharmacology of in vitro and ex vivo [(3)H]-(+)-PHNO binding sites in rat brain and (2) to compare, using [(3)H]-(+)-PHNO autoradiography, the ex vivo and in vitro pharmacology of olanzapine, clozapine, risperidone, and haloperidol. Using the D3-selective drug SB277011, we found that ex vivo and in vitro [(3)H]-(+)-PHNO binding in STR is exclusively due to D2, whereas that in cerebellar lobes 9 and 10 is exclusively due to D3. Surprisingly, the D3 contribution to [(3)H]-(+)-PHNO binding in the islands of Calleja, ventral pallidum, substantia nigra, and nucleus accumbens was greater ex vivo than in vitro. Ex vivo, systemically administered olanzapine, risperidone, and haloperidol, at doses occupying approximately 80% D2, did not occupy D3 receptors. Clozapine, which also occupied approximately 80% of D2 receptors ex vivo, occupied a smaller percentage of D3 receptors than predicted by its in vitro pharmacology. Across brain regions, ex vivo occupancy by antipsychotics was inversely related to the D3 contribution to [(3)H]-(+)-PHNO binding. In contrast, in vitro occupancy was similar across brain regions, independent of the regional D3 contribution. These data indicate that at clinically relevant doses, olanzapine, clozapine, risperidone, and haloperidol are D2-selective ex vivo. This unforeseen finding suggests that their clinical effects cannot be attributed to D3 receptor blockade.

A functional fast scan cyclic voltammetry assay to characterize dopamine D2 and D3 autoreceptors in the mouse striatum

Dopamine D2 and D3 autoreceptors are located on pre-synaptic terminals and are known to control the release and synthesis of dopamine. Dopamine D3 receptors have a fairly restricted pattern of expression in the mammalian brain. Their localization in the nucleus accumbens core and shell is of particular interest because of their association with the rewarding properties of drugs of abuse. Using background subtracted fast scan cyclic voltammetry, we investigated the effects of dopamine D2 and D3 agonists on electrically stimulated dopamine release and uptake rates in the mouse caudate-putamen and nucleus accumbens core and shell. The dopamine D2 agonists (-)-quinpirole hydrochloride and 5,6,7,8-Tetrahydro-6-(2-propen-1-yl)-4H-thiazolo[4,5-d]azepin-2-amine dihydrochloride (B-HT 920) had the same dopamine release inhibition effects on caudate-putamen and nucleus accumbens (core and shell) based on their EC(50) and efficacies. This suggests that the dopamine D2 autoreceptor functionality is comparable in all three striatal regions investigated. The dopamine D3 agonists (4aR,10bR)-3,4a,4,10b-Tetrahydro-4-propyl-2H,5H-[1]benzopyrano-[4,3-b]-1,4-oxazin-9-ol hydrochloride ((+)-PD 128907) and (+/-)-7-Hydroxy-2-dipropylaminotetralin hydrobromide (7-OH-DPAT) had a significantly greater effect on dopamine release inhibition in the nucleus accumbens shell than in caudate-putamen. This study confirms that, the dopamine D3 autoreceptor functionality is greater in the nucleus accumbens shell followed by the nucleus accumbens core, with the caudate-putamen having the least. Neither dopamine D2 nor D3 agonists affected the uptake rates in nucleus accumbens but concentrations greater than 0.3 muM lowered the uptake rate in caudate-putamen. To validate our method of evaluating dopamine D2 and D3 autoreceptors, sulpiride (D2 antagonist) and nafadotride (D3 antagonist) were used to reverse the effects of the dopamine agonists to approximately 100% of the pre-agonist dopamine release concentration. Finally, these results demonstrate a functional voltammetric assay that characterizes dopamine D2-like agonist as either D2- or D3-preferring agonists by taking advantage of the unique receptor density within the striatum.

Astrocyte expression of D2-like dopamine receptors in the prefrontal cortex

The dopaminergic system is of crucial importance for understanding human behavior and the pathogenesis of many psychiatric and neurological conditions. The majority of studies addressing the localization of dopamine receptors (DR) examined the expression of DR in neurons, while its expression, precise anatomical localization and possible function in glial cells have been largely neglected. Here we examined the expression of D2-like family of DR in neuronal and glial cells in the normal human brain using immunocytochemistry and immunofluorescence. Tissue samples from the right orbitomedial (Brodmann’s areas 11/12), dorsolateral (areas 9/46) and dorsal medial (area 9) prefrontal cortex were taken during autopsy from six subjects with no history of neurological or psychiatric disorders, formalin-fixed, and embedded in paraffin. The sections were stained using novel anti-DRD2, anti-DRD3, and anti-DRD4 monoclonal antibodies. Adjacent sections were labeled with an anti-GFAP (astroglial marker) and an anti-CD68 antibody (macrophage/microglial marker). The pyramidal and non-pyramidal cells of all three regions analyzed had strong expression of DRD2 and DRD4, whereas DRD3 were very weakly expressed. DRD2 were more strongly expressed in layer III compared to layer V pyramidal neurons. In contrast, DRD4 receptors had a stronger expression in layer V neurons. The most conspicuous finding was the strong expression of DRD2, but not DRD3 or DRD4, receptors in the white matter fibrous astrocytes and in layer I protoplasmic astrocytes. Weak DRD2-immunoreactivity was also observed in protoplasmic astrocytes in layers III and V. These results suggest that DR-expressing astrocytes directly participate in dopaminergic transmission of the human prefrontal cortex.

The ventral basal ganglia, a selection mechanism at the crossroads of space, strategy, and reward

The basal ganglia are often conceptualised as three parallel domains that include all the constituent nuclei. The 'ventral domain' appears to be critical for learning flexible behaviours for exploration and foraging, as it is the recipient of converging inputs from amygdala, hippocampal formation and prefrontal cortex, putatively centres for stimulus evaluation, spatial navigation, and planning/contingency, respectively. However, compared to work on the dorsal domains, the rich potential for quantitative theories and models of the ventral domain remains largely untapped, and the purpose of this review is to provide the stimulus for this work. We systematically review the ventral domain's structures and internal organisation, and propose a functional architecture as the basis for computational models. Using a full schematic of the structure of inputs to the ventral striatum (nucleus accumbens core and shell), we argue for the existence of many identifiable processing channels on the basis of unique combinations of afferent inputs. We then identify the potential information represented in these channels by reconciling a broad range of studies from the hippocampal, amygdala and prefrontal cortex literatures with known properties of the ventral striatum from lesion, pharmacological, and electrophysiological studies. Dopamine's key role in learning is reviewed within the three current major computational frameworks; we also show that the shell-based basal ganglia sub-circuits are well placed to generate the phasic burst and dip responses of dopaminergic neurons. We detail dopamine's modulation of ventral basal ganglia's inputs by its actions on pre-synaptic terminals and post-synaptic membranes in the striatum, arguing that the complexity of these effects hint at computational roles for dopamine beyond current ideas. The ventral basal ganglia are revealed as a constellation of multiple functional systems for the learning and selection of flexible behaviours and of behavioural strategies, sharing the common operations of selection-by-disinhibition and of dopaminergic modulation.

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.

In vivo quantification of regional dopamine-D3 receptor binding potential of (+)-PHNO: Studies in non-human primates and transgenic mice

Examination of dopamine-D3 (D3) receptors with positron emission tomography (PET) have been hampered in the past by the lack of a PET ligand with sufficient selectivity for D3 over dopamine-D2 (D2) receptors. The two types co-localize in the brain, with D2 density significantly higher than D3, hence nonselective PET ligands inform on D2, rather than D3 status. [(11)C]-(+)-PHNO is a novel PET ligand with a preferential affinity for D3 over D2. We used the selective D3 antagonist, SB-277011 to dissect regional fractions of the [(11)C]-(+)-PHNO signal attributable to D3 and D2 in primate brain. The results were compared with quantitative autoradiography with (3)H-(+)-PHNO in wild-type, D2-knock-out, and D3-knock-out mice examined at baseline and following administration of SB-277011. Both sets of results converged to indicate a predominant D3-related component to (+)-PHNO binding in extra-striatal regions, with binding in the midbrain being entirely attributable to D3. The midbrain is thus an excellent target region to examine D3 receptor occupancy with [(11)C]-(+)-PHNO PET in vivo.

[(3)H]4-(Dimethylamino)-N-[4-(4-(2-methoxyphenyl)piperazin- 1-yl)butyl]benzamide, a selective radioligand for dopamine D(3) receptors. I. In vitro characterization

4-(Dimethylamino)-N-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butyl)benzamide (WC-10), a N-phenyl piperazine analog, has been shown to have high affinity and selectivity for dopamine D(3) receptors versus dopamine D(2) receptors (Chu et al. [2005] Bioorg Med Chem 13:77-87). In this study, WC-10 was radiolabeled with tritium (specific activity = 80 Ci/mmol) and [(3)H]WC-10 binding to genetically cloned dopamine D(2L) and D(3) receptors was evaluated in vitro. [(3)H]WC-10 binds with a 66-fold higher affinity to human HEK D(3) than HEK D(2L) receptors, with a dissociation constant (K(d)) of 1.2 nM at HEK D(3) receptors. However, [(3)H]WC-10 binds to rat Sf9 rD(3) receptors with a K(d) of 3.9 nM, a value that is 3-fold lower than binding to human HEK D(3) receptors and 40-fold value higher than binding to rat Sf9 rD(2L) receptors. The K(d) values obtained from saturation binding experiments were consistent with the results determined from kinetic (k(on) and k(off)) studies. The pharmacologic profiles of a series of dopaminergic drugs for inhibiting the binding of [(3)H]WC-10 to D(3) receptors was in agreement with previously reported data. In vitro autoradiography studies of rat and monkey brains show that [(3)H]WC-10 labeled D(3) sites in the striatal region.

First human evidence of d-amphetamine induced displacement of a D2/3 agonist radioligand: A [11C]-(+)-PHNO positron emission tomography study

Imaging the competition between D(2/3) radioligands and endogenous dopamine is so far the only way to measure dopamine release in the living human brain. The dopamine D(2) receptor exists in a high (D(2)(high)) and a low-affinity state for dopamine. Under physiological conditions, dopamine is expected to bind to D(2)(high) only. [(11)C]-(+)-4-propyl-9-hydroxynaphthoxazine ((+)-PHNO) is the first D(2/3) agonist radioligand for positron emission tomography (PET) imaging in humans. Since [(11)C]-(+)-PHNO is expected to bind preferentially to D(2)(high), it should be particularly vulnerable to competition with endogenous dopamine. Nine healthy subjects participated in two PET scans, one after administration of d-amphetamine and one after placebo. [(11)C]-(+)-PHNO PET test re-test variability was determined in 11 healthy subjects. Binding potentials (BPs) were calculated for caudate, putamen, ventral striatum, and globus pallidus. d-Amphetamine led to a significant decrease of [(11)C]-(+)-PHNO BPs in caudate (-13.2%), putamen (-20.8%), and ventral striatum (-24.9%), but not in globus pallidus (-6.5%). d-Amphetamine-induced displacement correlated with serum d-amphetamine levels in all regions but caudate. This is the first report on competition between endogenous dopamine and a D(2/3) agonist radioligand in humans. [(11)C]-(+)-PHNO PET might be a superior measure for release of endogenous dopamine than PET employing conventional D(2/3) antagonist radioligands.

Positron emission tomography quantification of [11C]-(+)-PHNO binding in the human brain

The kinetic modeling of [11C]-(+)-PHNO binding to the dopamine D2/3 receptors in six human volunteers using positron emission tomography (PET) is described. [11C]-(+)-PHNO is the first agonist radioligand for the D2/3 in humans and as expected showed high uptake in caudate, putamen, globus pallidus (GP) and ventral striatum, and low uptake in cerebellum. A two-tissue compartment model (2CM) with four parameters was necessary to adequately fit time-activity data in all regions. Although a 2CM provided an excellent estimation of total distribution volumes, which were highly correlated with those obtained with the invasive Logan approach, it provided a poor identification of the k3/k4 ratios. Coupling K1/k2 between brain regions (Method C) or fixing K1/k2 to the value obtained in cerebellum (Method D) enabled more stable estimates of k3/k4 as compared with an unconstrained 2CM. The k3/k4 obtained with Method D ranged from 0.12+/-0.03 in cerebellum to 3.93+/-0.77 in GP and were similar to those obtained when coupling K1/k2. Binding potentials (BPs) obtained using the simplified reference tissue model (BP(SRTM)) ranged from 2.08+/-0.34 in caudate to 3.55+/-0.78 in GP and were highly correlated with k3/k4 estimates obtained with Method D (r=0.98). However, BP(SRTM) were 11%+/-5% lower than values obtained with Method D. BPs derived using the noninvasive Logan approach were slightly lower but not significantly different than BP(SRTM). This study demonstrates that [11C]-(+)-PHNO can be used for the quantitative measurement of D2/3 densities and should enable further studies of potential D2/3 dysregulation in several important psychiatric and neurologic illnesses.

The dopamine D3 receptor antagonist NGB 2904 increases spontaneous and amphetamine-stimulated locomotion

The dopamine D3 receptor is believed to play an important role in regulation of rodent locomotor behavior, and has been proposed as a therapeutic target for substance abuse, psychotic disorders, and Parkinson's disease. One model of dopamine D3 receptor function, based on studies utilizing D3 receptor knockout mice and D3 receptor-preferring agonists, proposes that D3 receptor stimulation is inhibitory to psychostimulant-induced locomotion, in opposition to the effects of concurrent dopamine D1 and D2 receptor stimulation. Recent progress in medicinal chemistry has led to the development of highly-selective dopamine D3 receptor antagonists. In order to extend our understanding of D3 dopamine receptor's behavioral functions, we determined the effects of the highly-selective dopamine D3 receptor antagonist NGB 2904 on amphetamine-stimulated and spontaneous locomotion in wild-type and dopamine D3 receptor knockout mice. NGB 2904 (26.0 microg/kg s.c.) enhanced amphetamine-stimulated locomotion in wild-type mice, but had no measurable effect in dopamine D3 receptor knockout mice. Of a range of doses (0.026 microg-1.0 mg/kg) given acutely or once daily for seven days, the highest dose of NGB 2904 (1.0 mg/kg) stimulated spontaneous locomotion in wild-type mice, but was without measurable effect in dopamine D3 receptor knockout mice. These behavioral effects of NGB 2904 contrast with those described for other highly D3 receptor-selective antagonists, which have not previously demonstrated an effect on spontaneous locomotor activity. In combination, these data add to the behavioral profile of this novel D3 receptor ligand and provide further support for a role for dopamine D3 receptor inhibitory function in the modulation of rodent locomotion.