Selective antagonism at dopamine D3 receptors enhances monoaminergic and cholinergic neurotransmission in the rat anterior cingulate cortex

Adjunctive cariprazine for major depressive disorder: a systematic review and meta-analysis

Converging evidence has suggested that treatment augmentation with a second-generation atypical antipsychotic (SGA) may improve treatment outcomes in major depressive disorder (MDD) patients after an incomplete response to a first-line antidepressant. Cariprazine is a recently approved SGA for MDD augmentation. Herein, we evaluate both continuous (ie, change in depressive symptom severity scores over time) and categorical (ie, remission and response rates) outcomes. Following a full-text review, four randomized controlled trials (RCTs) were included in our meta-analysis, while five studies were included for a qualitative review. Risk ratios (RRs) were calculated for all included randomized controlled studies to determine the relative response and remission rates of cariprazine compared to placebo augmentation. The RR for all-cause dropout was also determined as a proxy for overall acceptability. Two studies found a statistically significant treatment response using cariprazine augmentation. One study observed depressive symptom remission for cariprazine compared to placebo. Our random-effects model revealed moderate antidepressant effects of cariprazine, with a standardized mean difference (SMD) in Montgomery-Åsberg Depression Rating Scale (MADRS) scores of -1.79 (95% CI): -2.89, -0.69). Our pooled response RR and remission RR were calculated as 1.21 (95% CI: 1.05, 1.39, P=0.008) and 0.99 (95% CI: 0.84, 1.17, P=0.91), respectively. The RR for response was statistically significant (P<0.05). However, the RR for remission was not statistically significant. The findings from our meta-analysis include a variable magnitude of effects. Evidence suggests cariprazine may be an effective treatment for MDD; however, further results are needed to clarify this relation.

Unravelling the brain metabolome: A review of liquid chromatography - mass spectrometry strategies for extracellular brain metabolomics

The analysis of the brain extracellular metabolome is of interest for numerous subdomains within neuroscience. Not only does it provide information about normal physiological functions, it is even more of interest for biomarker discovery and target discovery in disease. The extracellular analysis of the brain is particularly interesting as it provides information about the release of mediators in the brain extracellular fluid to look at cellular signaling and metabolic pathways through the release, diffusion and re-uptake of neurochemicals. In vivo samples are obtained through microdialysis, cerebral open-flow microperfusion or solid-phase microextraction. The analytes of potential interest are typically low in concentration and can have a wide range of physicochemical properties. Liquid chromatography coupled to mass spectrometry has proven its usefulness in brain metabolomics. It allows sensitive and specific analysis of low sample volumes, obtained through different approaches. Several strategies for the analysis of the extracellular fluid have been proposed. The most widely used approaches apply sample derivatization, specific stationary phases and/or hydrophilic interaction liquid chromatography. Miniaturization of these methods allows an even higher sensitivity. The development of chiral metabolomics is indispensable, as it allows to compare the enantiomeric ratio of compounds and provides even more challenges. Some limitations continue to exist for the previously developed methods and the development of new, more sensitive methods remains needed. This review provides an overview of the methods developed for sampling and liquid chromatography-mass spectrometry analysis of the extracellular metabolome.

Dysregulated neuromodulation in the anterior cingulate cortex in chronic pain

Chronic pain is a significant global socioeconomic burden with limited long-term treatment options. The intractable nature of chronic pain stems from two primary factors: the multifaceted nature of pain itself and an insufficient understanding of the diverse physiological mechanisms that underlie its initiation and maintenance, in both the peripheral and central nervous systems. The development of novel non-opioidergic analgesic approaches is contingent on our ability to normalize the dysregulated nociceptive pathways involved in pathological pain processing. The anterior cingulate cortex (ACC) stands out due to its involvement in top-down modulation of pain perception, its abnormal activity in chronic pain conditions, and its contribution to cognitive functions frequently impaired in chronic pain states. Here, we review the roles of the monoamines dopamine (DA), norepinephrine (NE), serotonin (5-HT), and other neuromodulators in controlling the activity of the ACC and how chronic pain alters their signaling in ACC circuits to promote pathological hyperexcitability. Additionally, we discuss the potential of targeting these monoaminergic pathways as a therapeutic strategy for treating the cognitive and affective symptoms associated with chronic pain.

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.

Characterization of D3 Autoreceptor Function in Whole Zebrafish Brain with Fast-Scan Cyclic Voltammetry

Zebrafish (Danio rerio) are ideal model organisms for investigating nervous system function, both in health and disease. Nevertheless, functional characteristics of dopamine (DA) release and uptake regulation are still not well-understood in zebrafish. In this study, we assessed D3 autoreceptor function in the telencephalon of whole zebrafish brains ex vivo by measuring the electrically stimulated DA release ([DA]max) and uptake at carbon fiber microelectrodes with fast-scan cyclic voltammetry. Treatment with pramipexole and 7-OH-DPAT, selective D3 autoreceptor agonists, sharply decreased [DA]max. Conversely, SB277011A, a selective D3 antagonist, nearly doubled [DA]max and decreased k, the first-order rate constant for the DA uptake, to about 20% of its original value. Treatment with desipramine, a selective norepinephrine transporter blocker, failed to increase current, suggesting that our electrochemical signal arises solely from the release of DA. Furthermore, blockage of DA uptake with nomifensine-reversed 7-OH-DPAT induced decreases in [DA]max. Collectively, our data show that, as in mammals, D3 autoreceptors regulate DA release, likely by inhibiting uptake. The results of this study are useful in the further development of zebrafish as a model organism for DA-related neurological disorders such as Parkinson's disease, schizophrenia, and drug addiction.

Cariprazine as a treatment for negative psychotic symptoms in first-episode psychosis: case series

Negative psychotic symptoms are among the most disabling features of schizophrenia, and are strongly associated with relatively poor clinical and functional outcomes. However, there are no effective treatments for negative symptoms, and this represents a major unmet clinical need. Recent research has shown that negative symptoms are already present in many patients at illness onset. There is evidence that cariprazine may improve negative symptoms in patients with chronic schizophrenia. However, its utility in treating negative symptoms in the early stage of the disorder is unclear. Here, we report six cases of patients with first-episode psychosis who were treated with cariprazine.

Potential Mechanisms for Why Not All Antipsychotics Are Able to Occupy Dopamine D3 Receptors in the Brain in vivo

Dysfunctions of the dopaminergic system are believed to play a major role in the core symptoms of schizophrenia such as positive, negative, and cognitive symptoms. The first line of treatment of schizophrenia are antipsychotics, a class of medications that targets several neurotransmitter receptors in the brain, including dopaminergic, serotonergic, adrenergic and/or muscarinic receptors, depending on the given agent. Although the currently used antipsychotics display in vitro activity at several receptors, majority of them share the common property of having high/moderate in vitro affinity for dopamine D₂ receptors (D₂Rs) and D₃ receptors (D₃Rs). In terms of mode of action, these antipsychotics are either antagonist or partial agonist at the above-mentioned receptors. Although D₂Rs and D₃Rs possess high degree of homology in their molecular structure, have common signaling pathways and similar in vitro pharmacology, they have different in vivo pharmacology and therefore behavioral roles. The aim of this review, with summarizing preclinical and clinical evidence is to demonstrate that while currently used antipsychotics display substantial in vitro affinity for both D₃Rs and D₂Rs, only very few can significantly occupy D₃Rs in vivo. The relative importance of the level of endogenous extracellular dopamine in the brain and the degree of in vitro D₃Rs receptor affinity and selectivity as determinant factors for in vivo D₃Rs occupancy by antipsychotics, are also discussed.

Developments in Biological Mechanisms and Treatments for Negative Symptoms and Cognitive Dysfunction of Schizophrenia

The causal mechanisms and treatment for the negative symptoms and cognitive dysfunction in schizophrenia are the main issues attracting the attention of psychiatrists over the last decade. The first part of this review summarizes the pathogenesis of schizophrenia, especially the negative symptoms and cognitive dysfunction from the perspectives of genetics and epigenetics. The second part describes the novel medications and several advanced physical therapies (e.g., transcranial magnetic stimulation and transcranial direct current stimulation) for the negative symptoms and cognitive dysfunction that will optimize the therapeutic strategy for patients with schizophrenia in future.

Pharmacotherapy of schizophrenia: Mechanisms of antipsychotic accumulation, therapeutic action and failure

Schizophrenia is a multi-dimensional disorder with a complex and mostly unknown etiology, leading to a severe decline in life quality. Antipsychotic drugs (APDs) remain beneficial interventions in the treatment of the disorder, but vary significantly in binding profile, clinical effects and adverse reactions. The present review summarizes the main principles of APD mechanisms of action with a particular focus on recent findings in APD accumulation and its role in the therapeutic efficacy and treatment failure. High and low doses of APDs were shown to be effective in different dimensions of antipsychotic-like behaviour in rodent models. Efficacy of the APDs correlates with high dopamine D2 receptor occupancy, which occurs quickly after drug administration. However, onset and peak of action are delayed up to several days or weeks. APD accumulation via acidic trapping in synaptic vesicles is considered to underlie the time course of APD action. Use-dependent exocytosis, co-release with dopamine and serotonin and inhibition of ion channels impact on the neuronal transmission and determine effects of APDs. Disruption in accumulating properties leads to diminished APD effects. In addition, long-term APD administration at therapeutic doses leads to treatment failure both in animal models and in humans. APD failure was associated with treatment induced neuroadaptations, including a decline in extracellular dopamine levels, dopamine transporter upregulation, and altered neuronal firing. However, enhanced synaptic vesicle release has also been reported. APD loss of efficacy may be reversed through inhibition of the dopamine transporter or switching the administration regimen from continuous to intermittent. Thus, manipulating the accumulation properties of APDs, changes in the administration regimen and doses, or co-administration with dopamine transporter inhibitors may be considered to yield benefits in the development of new effective strategies in the treatment of schizophrenia.

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.

Preferential Effects of Cariprazine on Counteracting the Disruption of Social Interaction and Decrease in Extracellular Dopamine Levels Induced by the Dopamine D3 Receptor Agonist, PD-128907 in Rats: Implications for the Treatment of Negative and Depressive Symptoms of Psychiatric Disorders

The negative and cognitive symptoms of schizophrenia and related disorders may be due to reduced dopaminergic tone in cortical brain areas. Alteration in the function of dopamine (DA) D₃ receptors may play a role in this cortical hypofunctionality and underlie the deficits in social behaviors and cognitive functions in schizophrenia. Cariprazine is a potent DA D₃-preferring D₃/D₂ receptor partial agonist that is approved for the treatment of schizophrenia and bipolar disorder. The objective of the study was to compare the abilities of cariprazine, aripiprazole (another DA receptor partial agonist with more D₂ receptor preference), and ABT-925 (a selective DA D₃ antagonist) to counteract the social deficit and neurochemical alterations induced by the D₃ receptor-preferring agonist (+)-PD 128907 (PD) in rats. Administration of PD (0.16 mg/kg; s.c.) induced a marked (-72%) but short-lasting disruption of the defensive social aggregation behavior (huddling) in the first 10-min period. Cariprazine at all doses (0.1, 0.3, 1 mg/kg; p.o.) almost completely abolished the PD-induced disruption of huddling. Likewise, ABT-925 (3 mg/kg; p.o.) and to a lesser extent aripiprazole (20 mg/kg; p.o.) were effective in blocking the PD-induced disruption of huddling. As measured by microdialysis, the highest dose of cariprazine prevented a PD-induced decrease in DA levels (40-80 min post PD dose) in the medial prefrontal cortex (mPFC), whereas aripiprazole did not have a significant effect. ABT-925 significantly counteracted the effect of PD at 80 min post-dose. In the nucleus accumbens (nAcc) shell, the highest dose of cariprazine, as well as ABT-925 and aripiprazole, significantly reversed the PD-induced decrease in DA levels. Taken together, these data provide behavioral and in vivo neurochemical evidence for the preferential DA D₃ receptor action of cariprazine in the rat. This property of cariprazine may offer therapeutic benefits against the cognitive deficits and negative/depressive symptoms of schizophrenia and related disorders.

Negative Symptoms of Schizophrenia: New Prospects of Cariprazine Treatment

Background

Cariprazine is a new piperazine derivative atypical antipsychotic, like aripiprazole and brexpiprazole. It has been approved for treating schizophrenia in many countries and has recently been included on the List of Essential Medicines in Russia. Unlike most other atypical antipsychotics, it shows high in vivo occupancy of dopamine D2 and D3 receptors at clinically relevant doses. In animal models, cariprazine has demonstrated dopamine D3 receptor-dependent pro-cognitive and anti-anhedonic effects, suggesting its potential for treating negative symptoms. This review summarizes the efficacy of cariprazine in the treatment of negative symptoms of schizophrenia.

Methods

A literature search of databases covering international and Russian journals, for articles published between 1st January 2010 and 1st June 2020.

Results

Cariprazine demonstrated at least comparable efficacy in the treatment of schizophrenia symptoms to active comparators including risperidone, olanzapine or aripiprazole. The drug has a good safety profile. It appeared to be associated with a lower risk of metabolic syndromes and most extrapyramidal symptoms. The positive effect of cariprazine on the negative symptoms of schizophrenia may be associated with the elimination of secondary negative symptoms. However, of all the atypical antipsychotics to date, only cariprazine has a convincingly, methodologically robust proven advantage over risperidone in eliminating the predominant negative symptoms of schizophrenia. Yet only four studies have investigated the effect of cariprazine on the negative symptoms of schizophrenia. There is a lack of research into its direct impact on emotional-volitional disorders, anhedonia, cognitive symptoms and personality changes. However, there is evidence to suggest cariprazine is effective in treatment-resistant cases, but this requires further confirmation.

Conclusion

Cariprazine is an effective and well-tolerated agent for the treatment of schizophrenia and may be effective in cases where other antipsychotics have failed. Cariprazine has been shown to have a positive effect on negative symptoms. Further studies are needed to collect more data on long-term treatment of schizophrenia and especially negative symptoms.

Negative Symptoms in Schizophrenia: A Review and Clinical Guide for Recognition, Assessment, and Treatment

Schizophrenia is frequently a chronic and disabling disorder, characterized by heterogeneous positive and negative symptom constellations. The objective of this review was to provide information that may be useful for clinicians treating patients with negative symptoms of schizophrenia. Negative symptoms are a core component of schizophrenia that account for a large part of the long-term disability and poor functional outcomes in patients with the disorder. The term negative symptoms describes a lessening or absence of normal behaviors and functions related to motivation and interest, or verbal/emotional expression. The negative symptom domain consists of five key constructs: blunted affect, alogia (reduction in quantity of words spoken), avolition (reduced goal-directed activity due to decreased motivation), asociality, and anhedonia (reduced experience of pleasure). Negative symptoms are common in schizophrenia; up to 60% of patients may have prominent clinically relevant negative symptoms that require treatment. Negative symptoms can occur at any point in the course of illness, although they are reported as the most common first symptom of schizophrenia. Negative symptoms can be primary symptoms, which are intrinsic to the underlying pathophysiology of schizophrenia, or secondary symptoms that are related to psychiatric or medical comorbidities, adverse effects of treatment, or environmental factors. While secondary negative symptoms can improve as a consequence of treatment to improve symptoms in other domains (ie, positive symptoms, depressive symptoms or extrapyramidal symptoms), primary negative symptoms generally do not respond well to currently available antipsychotic treatment with dopamine D₂ antagonists or partial D₂ agonists. Since some patients may lack insight about the presence of negative symptoms, these are generally not the reason that patients seek clinical care, and clinicians should be especially vigilant for their presence. Negative symptoms clearly constitute an unmet medical need in schizophrenia, and new and effective treatments are urgently needed.

Cariprazine for the treatment of bipolar depression: a review

INTRODUCTION

Depressive symptoms and episodes dominate the course of bipolar disorder. The morbidity of bipolar disorder is disproportionately mediated by depressive symptoms; economic costs of bipolar disorder are also disproportionately due to unremitting depressive symptoms. Relatively few treatment options have established unequivocal efficacy in the treatment of bipolar depression. Herein we review evidence regarding the efficacy of the D₃ preferring D₂/D₃ partial agonist cariprazine in the treatment of adults with bipolar depression. Areas covered: Randomized controlled trials that sought to determine the efficacy, tolerability, and safety of cariprazine in adults with bipolar I depression. Expert opinion: The available evidence from clinical trials indicates that cariprazine is effective at treating bipolar depression wherein treatment for bipolar depression remains an unmet need in bipolar disorder. Cariprazine has demonstrated good tolerability and safety profiles in bipolar disorder. Furthermore, cariprazine may be effective in improving both anhedonia and cognitive dysfunction. Long term prevention studies in bipolar depression, as well as separate studies evaluating efficacy in adults with bipolar II depression, are needed.

Efficacy of cariprazine on negative symptoms in patients with acute schizophrenia: A post hoc analysis of pooled data

Although currently approved antipsychotics exert efficacy on positive symptoms of schizophrenia, treatments for negative symptoms remain a major unmet need. Post hoc analyses were used to investigate the possible efficacy of cariprazine in patients with moderate/severe negative symptoms of schizophrenia and no predominance of positive symptoms. Data were pooled from 2 randomized, double-blind, placebo- and active-controlled cariprazine studies in patients with acute schizophrenia (NCT00694707, NCT01104766). Analyses included data from a subset of patients with a Positive and Negative Syndrome Scale factor score for negative symptoms (PANSS-FSNS) ≥24, PANSS factor score for positive symptoms (PANSS-FSPS) ≤19, and scores of ≥4 on ≥2 of 3 PANSS items (blunted affect [N1], passive/apathetic social withdrawal [N4], lack of spontaneity/flow of conversation [N6]). Changes from baseline to week 6 in PANSS-FSNS were evaluated in the following treatment groups: placebo (n = 79), cariprazine 1.5-3 (n = 94) and 4.5-6 mg/d (n = 66), risperidone 4 mg/d (n = 34), or aripiprazole 10 mg/d (n = 44). Significant differences were observed versus placebo for cariprazine (1.5-3 mg/d, P = .0179; 4.5-6 mg/d, P = .0002) and risperidone (P = .0149), but not aripiprazole (P = .3265), and versus aripiprazole for cariprazine 4.5-6 mg/d (P = .0197). After adjusting for positive symptom changes, differences versus placebo remained statistically significant for cariprazine (1.5-3 mg/d, P = .0322; 4.5-6 mg/d, P = .0038) but not for risperidone (P = .2204). PANSS-FSNS response (≥20% reduction from baseline) rates were significantly higher with cariprazine (1.5-3 mg/d = 54.3%, P = .0194; 4.5-6 mg/d = 69.7%, P = .0001) than placebo (35.4%). In patients with acute schizophrenia and moderate/severe negative symptoms, cariprazine was associated with significantly greater improvement in negative symptoms compared with placebo and aripiprazole, warranting further exploration of the efficacy of cariprazine on negative symptoms.

The atypical antipsychotic blonanserin reverses (+)-PD-128907- and ketamine-induced deficit in executive function in common marmosets

Antagonism of the dopamine D3 receptor is considered a promising strategy for the treatment of cognitive impairment associated with schizophrenia. We have previously reported that the atypical antipsychotic blonanserin, a dopamine D2/D3 and serotonin 5-HT2A receptor antagonist, highly occupies dopamine D3 receptors at its antipsychotic dose range in rats. In the present study, we evaluated the effects of blonanserin on executive function in common marmosets using the object retrieval with detour (ORD) task. The dopamine D3 receptor-preferring agonist (+)-PD-128907 at 1mg/kg decreased success rate in the difficult trial, but not in the easy trial. Since the difference between the two trials is only cognitive demand, our findings indicate that excess activation of dopamine D3 receptors impairs executive function in common marmosets. Blonanserin at 0.1mg/kg reversed the decrease in success rate induced by (+)-PD-128907 in the difficult trial. This finding indicates that blonanserin has beneficial effect on executive function deficit induced by activation of the dopamine D3 receptor in common marmosets. Next, and based on the glutamatergic hypothesis of schizophrenia, the common marmosets were treated with the N-methyl-d-aspartate (NMDA) receptor antagonist ketamine. Ketamine at sub-anesthetic doses decreased success rate in the difficult trial, but not in the easy trial. Blonanserin at 0.1mg/kg reversed the decrease in success rate induced by ketamine in the difficult trial. The findings of this study suggest that blonanserin might have beneficial effect on executive dysfunction in patients with schizophrenia.

Dopamine's Actions in Primate Prefrontal Cortex: Challenges for Treating Cognitive Disorders

The prefrontal cortex (PFC) elaborates and differentiates in primates, and there is a corresponding elaboration in cortical dopamine (DA). DA cells that fire to both aversive and rewarding stimuli likely project to the dorsolateral PFC (dlPFC), signaling a salient event. Since 1979, we have known that DA has an essential influence on dlPFC working memory functions. DA has differing effects via D1 (D1R) versus D2 receptor (D2R) families. D1R are concentrated on dendritic spines, and D1/5R stimulation produces an inverted U-shaped dose response on visuospatial working memory performance and Delay cell firing, the neurons that generate representations of visual space. Optimal levels of D1R stimulation gate out "noise," whereas higher levels, e.g., during stress, suppress Delay cell firing. These effects likely involve hyperpolarization-activated cyclic nucleotide-gated channel opening, activation of GABA interneurons, and reduced glutamate release. Dysregulation of D1R has been related to cognitive deficits in schizophrenia, and there is a need for new, lower-affinity D1R agonists that may better mimic endogenous DA to enhance mental representations and improve cognition. In contrast to D1R, D2R are primarily localized on layer V pyramidal cell dendrites, and D2/3R stimulation speeds and magnifies the firing of Response cells, including Response Feedback cells. Altered firing of Feedback neurons may relate to positive symptoms in schizophrenia. Emerging research suggests that DA may have similar effects in the ventrolateral PFC and frontal eye fields. Research on the orbital PFC in monkeys is just beginning and could be a key area for future discoveries.

Dysbindin-1 modifies signaling and cellular localization of recombinant, human D₃ and D₂ receptors

Dystrobrevin binding protein-1 (dysbindin-1), a candidate gene for schizophrenia, modulates cognition, synaptic plasticity and frontocortical circuitry and interacts with glutamatergic and dopaminergic transmission. Loss of dysbindin-1 modifies cellular trafficking of dopamine (DA) D2 receptors to increase cell surface expression, but its influence upon signaling has never been characterized. Further, the effects of dysbindin-1 upon closely related D3 receptors remain unexplored. Hence, we examined the impact of dysbindin-1 (isoform A) co-expression on the localization and coupling of human D2L and D3 receptors stably expressed in Chinese hamster ovary or SH-SY5Y cells lacking endogenous dysbindin-1. Dysbindin-1 co-transfection decreased cell surface expression of both D3 and D2L receptors. Further, while their affinity for DA was unchanged, dysbindin-1 reduced the magnitude and potency of DA-induced adenylate cylase recruitment/cAMP production. Dysbindin-1 also blunted the amplitude of DA-induced phosphorylation of ERK1/2 and Akt at both D2L and D3 receptors without, in contrast to cAMP, affecting the potency of DA. Interference with calveolin/clathrin-mediated processes of internalization prevented the modification by dysbindin-1 of ERK1/2 and adenylyl cyclase stimulation at D2L and D3 receptors. Finally, underpinning the specificity of the influence of dysbindin-1 on D2L and D3 receptors, dysbindin-1 did not modify recruitment of adenylyl cyclase by D1 receptors. These observations demonstrate that dysbindin-1 influences cell surface expression of D3 in addition to D2L receptors, and that it modulates activation of their signaling pathways. Accordingly, both a deficiency and an excess of dysbindin-1 may be disruptive for dopaminergic transmission, supporting its link to schizophrenia and other CNS disorders. Dysbindin-1, a candidate gene for schizophrenia, alters D2 receptors cell surface expression. We demonstrate that dysbindin-1 expression also influences cell surface levels of D3 receptors. Further, Dysbindin-1 reduces DA-induced adenylate cylase recruitment/cAMP production and modifies major signaling pathways (Akt and extracellular signal-regulated kinases1/2 (ERK1/2)) of both D2 and D3 receptors. Dysbindin-1 modulates thus D2 and D3 receptor signaling, supporting a link to schizophrenia.

Clozapine improves the orienting of attention in schizophrenia

Attentional deficits are prominent in the cognitive profile of patients with schizophrenia. However, it remains unclear whether treatment with clozapine, an atypical antipsychotic and first-line intervention used to reduce positive and negative symptoms of psychosis, improves the attentional functions. We used the revised attention network test to measure alerting, orienting, and executive control of attention both pre- and post-treatment with clozapine in patients with schizophrenia (n=32) and compared performance to healthy controls (n=32). Results revealed that there were deficits in all three attentional functions pre-treatment, and while clozapine improved the orienting function in patients with schizophrenia, there was no evidence for improvement in the alerting and executive control of attention. The enhancement of the orienting function by clozapine may increase the ability of patients with schizophrenia to orient towards objects and thoughts of interest.

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.

Design, synthesis, and structure-activity relationship studies of a series of [4-(4-carboxamidobutyl)]-1-arylpiperazines: insights into structural features contributing to dopamine D3 versus D2 receptor subtype selectivity

Antagonist and partial agonist modulators of the dopamine D3 receptor (D3R) have emerged as promising therapeutics for the treatment of substance abuse and neuropsychiatric disorders. However, development of druglike lead compounds with selectivity for the D3 receptor has been challenging because of the high sequence homology between the D3R and the dopamine D2 receptor (D2R). In this effort, we synthesized a series of acylaminobutylpiperazines incorporating aza-aromatic units and evaluated their binding and functional activities at the D3 and D2 receptors. Docking studies and results from evaluations against a set of chimeric and mutant receptors suggest that interactions at the extracellular end of TM7 contribute to the D3R versus D2R selectivity of these ligands. Molecular insights from this study could potentially enable rational design of potent and selective D3R ligands.

What is the role of the D3 receptor in addiction? A mini review of PET studies with [(11)C]-(+)-PHNO

The chronic use of drugs, including psychostimulants such as cocaine and amphetamine, has been associated with low D2/3 dopamine receptor availability, which in turn has been linked to poor clinical outcome. In contrast, recent studies focused on the D3 receptor (a member of the D2-like receptor family) suggest that chronic exposure to stimulant drugs can up-regulate this receptor subtype, which, in preclinical models, is linked to dopamine system sensitization - a process hypothesized to contribute to relapse in addiction. In this mini review we present recent human data suggesting that the D3 receptor may contribute to core features of addiction, and discuss the usefulness of the PET imaging probe [(11)C]-(+)-PHNO in investigating this question.

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.

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.

Occupancy of brain dopamine D3 receptors and drug craving: a translational approach

Selective dopamine D(3) receptor (D(3)R) antagonists prevent reinstatement of drug-seeking behavior and decrease the rewarding effects of contextual cues associated with drug intake preclinically, suggesting that they may reduce drug craving in humans. GSK598809 is a selective D(3)R antagonist recently progressed in Phase I trials. The aim of this study was to establish a model, based on the determination of the occupancy of brain D(3)Rs (O(D(3))(R)) across species, to predict the ability of GSK598809 to reduce nicotine-seeking behavior in humans, here assessed as cigarette craving in smokers. Using ex vivo [(125)I](R)-trans-7-hydroxy-2-[N-propyl-N-(3'-iodo-2'-propenyl)amino] tetralin ([(125)I]7OH-PIPAT) autoradiography and [(11)C]PHNO positron emission tomography, we demonstrated a dose-dependent occupancy of the D(3)Rs by GSK598809 in rat, baboon, and human brains. We also showed a direct relationship between O(D(3))(R) and pharmacokinetic exposure, and potencies in line with the in vitro binding affinity. Likewise, GSK598809 dose dependently reduced the expression of nicotine-induced conditioned place preference (CPP) in rats, with an effect proportional to the exposure and O(D(3))(R) at every time point, and 100% effect at O(D(3))(R) values 72%. In humans, a single dose of GSK598809, giving submaximal levels (72-89%) of O(D(3))(R), transiently alleviated craving in smokers after overnight abstinence. These data suggest that either higher O(D(3))(R) is required for a full effect in humans or that nicotine-seeking behavior in CPP rats only partially translates into craving for cigarettes in short-term abstinent smokers. In addition, they provide the first clinical evidence of potential efficacy of a selective D(3)R antagonist for the treatment of substance-use disorders.

Noradrenergic versus dopaminergic modulation of impulsivity, attention and monitoring behaviour in rats performing the stop-signal task: possible relevance to ADHD

RATIONALE

Deficient response inhibition is a prominent feature of many pathological conditions characterised by impulsive and compulsive behaviour. Clinically effective doses of catecholamine reuptake inhibitors are able to improve such inhibitory deficits as measured by the stop-signal task (SST) in humans and other animals. However, the precise therapeutic mode of action of these compounds in terms of their relative effects on dopamine (DA) and noradrenaline (NA) systems in prefrontal cortical and striatal regions mediating attention and cognitive control remains unclear.

OBJECTIVES

We sought to fractionate the effects of global catecholaminergic manipulations on SST performance by using receptor-specific compounds for NA or DA. The results are described in terms of the effects of modulating specific receptor subtypes on various behavioural measures such as response inhibition, perseveration, sustained attention, error monitoring and motivation.

RESULTS

Blockade of α2-adrenoceptors improved sustained attention and response inhibition, whereas α1 and β1/2 adrenergic receptor antagonists disrupted go performance and sustained attention, respectively. No relevant effects were obtained after targeting DA D1, D2 or D4 receptors, while both a D3 receptor agonist and antagonist improved post-error slowing and compulsive nose-poke behaviour, though generally impairing other task measures.

CONCLUSIONS

Our results suggest that the use of specific pharmacological agents targeting α2 and β noradrenergic receptors may improve existing treatments for attentional deficits and impulsivity, whereas DA D3 receptors may modulate error monitoring and perseverative behaviour.

In vivo occupancy of dopamine D3 receptors by antagonists produces neurochemical and behavioral effects of potential relevance to attention-deficit-hyperactivity disorder

Dopamine D(3) receptors have eluded definitive linkage to neurologic and psychiatric disorders since their cloning over 20 years ago. We report a new method that does not employ a radiolabel for simultaneously defining in vivo receptor occupancy of D(3) and D(2) receptors in rat brain after systemic dosing using the tracer epidepride (N-[[(2S)-1-ethylpyrrolidin-2-yl]methyl]-5-iodo-2,3-dimethoxybenzamide). Decreases in epidepride binding in lobule 9 of cerebellum (rich in D(3) receptors) were compared with nonspecific binding in the lateral cerebellum. The in vivo occupancy of the dopamine D(3) receptors was dose dependently increased by SB-277011A (trans-N-[4-[2-(6-cyano-1,2,3,4-tetrahydroisoquinolin-2-yl)ethyl]cyclohexyl]-4-quinolinecarboxamide) and U99194 (2,3-dihydro-5,6-dimethoxy- N,N-dipropyl-1H-inden-2-amine). Both antagonists increased extracellular levels of acetylcholine (ACh) in the medial prefrontal cortex of rats and modified brain-tissue levels of ACh and choline. Consistent with these findings, the D(3) receptor antagonists enhanced the acquisition of learning of rats either alone or in the presence of the norepinephrine uptake blocker reboxetine as with the attention-deficit-hyperactivity disorder (ADHD) drug methylphenidate. Like reboxetine, the D(3) receptor antagonists also prevented deficits induced by scopolamine in object recognition memory of rats. Mice in which the dopamine transporter (DAT) has been deleted exhibit hyperactivity that is normalized by compounds that are effective in the treatment of ADHD. Both D(3) receptor antagonists decreased the hyperactivity of DAT(-/-) mice without affecting the activity of wild type controls. The present findings indicate that dopamine D(3) receptor antagonists engender cognition-enhancing and hyperactivity-dampening effects. Thus, D(3) receptor blockade could be considered as a novel treatment approach for cognitive deficits and hyperactivity syndromes, including those observed in ADHD.

Rationale in support of the use of selective dopamine D₃ receptor antagonists for the pharmacotherapeutic management of substance use disorders

Growing evidence indicates that dopamine (DA) D(3) receptors are involved in the control of drug-seeking behavior and may play an important role in the pathophysiology of substance use disorders. First, DA D(3) receptors are distributed in strategic areas belonging to the mesolimbic DA system such as the ventral striatum, midbrain, and pallidum, which have been associated with behaviors controlled by the presentation of drug-associated cues. Second, repeated exposure to drugs of abuse has been shown to produce neuroadaptations in the DA D(3) system. Third, the synthesis and characterization of highly potent and selective DA D(3) receptor antagonists has permitted to further define the role of the DA D(3) receptor in drug addiction. Provided that the available preclinical and preliminary clinical evidence can be translated into clinical proof of concept in human, selective DA D(3) receptor antagonists show promise for the treatment of substance use disorders as reflected by their potential to (1) regulate the motivation to self-administered drugs under schedules of reinforcement that require an increase in work demand and (2) disrupt the responsiveness to drug-associated stimuli that play a key role in the reinstatement of drug-seeking behavior triggered by re-exposure to the drug itself, re-exposure to environmental cues that had been previously associated with drug-taking behavior, or stress.

Direct and indirect interactions of the dopamine D₃ receptor with glutamate pathways: implications for the treatment of schizophrenia

This article, based on original data as well as on previously reported preclinical and clinical data that are reviewed, describes direct and indirect interactions of the D(3) receptor with N-methyl-D-aspartate receptor (NMDA) signaling and their functional consequences and therapeutic implications for schizophrenia. D(3) receptor immunoreactivity at ultrastructural level with electron microscopy was identified at presumably glutamatergic, asymmetric synapses of the medium-sized spiny neurons of the nucleus accumbens. This finding supports the existence of a direct interaction of the D(3) receptor with glutamate, in line with previously described interactions with NMDA signaling involving Ca(2+)/calmodulin-dependent protein kinase II at post-synaptic densities (Liu et al. 2009). Indirect interactions of the D(3) receptor with glutamate could involve a negative control exerted by the D(3) receptor on mesocortical dopamine neurons and the complex regulation of the glutamatergic pyramidal cells by dopamine in the prefrontal cortex. This could be exemplified here by the regulation of pyramidal cell activity in conditions of chronic NMDA receptor blockade with dizocilpine (MK-801). BP897, a D(3) receptor-selective partial agonist, reversed the dysregulation of cortical c-fos mRNA expression and pyramidal cell hyperexcitability, as measured by paired-pulse electrophysiology. At the behavioral level, blockade of the D(3) receptor, by known D(3) receptor antagonists or the novel D(3) receptor-selective antagonist F17141, produces antipsychotic-like effects in reversing hyperactivity and social interaction deficits induced by NMDA receptor blockade by MK-801 in mice. The glutamate-D(3) receptor interactions described here offer a conceptual framework for developing new D(3) receptor-selective drugs, which may appear as an original, efficacious, and safe way to potentially indirectly target glutamate in schizophrenia.

Blockade of dopamine D₃ but not D₂ receptors reverses the novel object discrimination impairment produced by post-weaning social isolation: implications for schizophrenia and its treatment

Dopamine D₃ receptors are densely expressed in mesolimbic projection areas, and selective antagonists enhance cognition, consistent with their potential therapeutic use in the treatment of schizophrenia. This study examines the effect of dopamine D₃ vs. D₂ receptor antagonists on the cognitive impairment and hyperactivity produced by social isolation of rat pups, in a neurodevelopmental model of certain deficits of schizophrenia. Three separate groups of male Lister hooded rats were group-housed or isolation-reared from weaning. Six weeks later rats received either vehicle or the dopamine D₃ selective antagonist, S33084 (0.04 and 0.16 mg/kg), the preferential D₃ antagonist, S33138 (0.16 and 0.63 mg/kg) or the preferential D₂ antagonist, L-741,626 (0.63 mg/kg) s.c. 30 min prior to recording; horizontal locomotor activity in a novel arena for 60 min and, the following day, novel object discrimination using a 2-h inter-trial interval. Isolation rearing induced locomotor hyperactivity in a novel arena and impaired novel object discrimination compared to that in group-housed littermates. Both S33084 and S33138 restored novel object discrimination deficits in isolation-reared rats without affecting discrimination in group-housed controls. By contrast, L-741,626 impaired novel object discrimination in group-housed rats, without affecting impairment in isolates. S33084 (0.16 mg/kg), S33138 and, less markedly, L741,626 reduced the locomotor hyperactivity in isolates without attenuating activity in group-housed controls. Selective blockade of dopamine D₃ receptors reverses the visual recognition memory deficit and hyperactivity produced by isolation rearing. These data support further investigation of the potential use of dopamine D₃ receptor antagonists to treat schizophrenia.

Receptor targets for antidepressant therapy in bipolar disorder: an overview

The treatment of bipolar depression is one of the most challenging issues in contemporary psychiatry. Currently only quetiapine and the olanzapine-fluoxetine combination are officially approved by the FDA against this condition. The neurobiology of bipolar depression and the possible targets of bipolar antidepressant therapy remain relatively elusive. We performed a complete and systematic review to identify agents with definite positive or negative results concerning efficacy followed by a second systematic review to identify the pharmacodynamic properties of these agents. The comparison of properties suggests that the stronger predictors for antidepressant efficacy in bipolar depression were norepinephrine alpha-1, dopamine D1 and histamine antagonism, followed by 5-HT2A, muscarinic and dopamine D2 and D3 antagonism and eventually by norepinephrine reuptake inhibition and 5HT-1A agonism. Serotonin reuptake which constitutes the cornerstone in unipolar depression treatment does not seem to play a significant role for bipolar depression. Our exhaustive review is compatible with a complex model with multiple levels of interaction between the major neurotransmitter systems without a single target being either necessary or sufficient to elicit the antidepressant effect in bipolar depression.

Selective blockade of dopamine D3 receptors enhances while D2 receptor antagonism impairs social novelty discrimination and novel object recognition in rats: a key role for the prefrontal cortex

Dopamine D(3) receptor antagonists exert pro-cognitive effects in both rodents and primates. Accordingly, this study compared the roles of dopamine D(3) vs D(2) receptors in social novelty discrimination (SND), which relies on olfactory cues, and novel object recognition (NOR), a visual-recognition task. The dopamine D(3) receptor antagonist, S33084 (0.04-0.63 mg/kg), caused a dose-related reversal of delay-dependent impairment in both SND and NOR procedures in adult rats. Furthermore, mice genetically deficient in dopamine D(3) receptors displayed enhanced discrimination in the SND task compared with wild-type controls. In contrast, acute treatment with the preferential dopamine D(2) receptor antagonist, L741,626 (0.16-5.0 mg/kg), or with the dopamine D(3) agonist, PD128,907 (0.63-40 μg/kg), caused a dose-related impairment in performance in rats in both tasks after a short inter-trial delay. Bilateral microinjection of S33084 (2.5 μg/side) into the prefrontal cortex (PFC) of rats increased SND and caused a dose-related (0.63-2.5 μg/side) improvement in NOR, while intra-striatal injection (2.5 μg/side) had no effect on either. In contrast, bilateral microinjection of L741,626 into the PFC (but not striatum) caused a dose-related (0.63-2.5 μg/side) impairment of NOR. These observations suggest that blockade of dopamine D(3) receptors enhances both SND and NOR, whereas D(3) receptor activation or antagonism of dopamine D(2) receptor impairs cognition in these paradigms. Furthermore, these actions are mediated, at least partly, by the PFC. These data have important implications for exploitation of dopaminergic mechanisms in the treatment of schizophrenia and other CNS disorders, and support the potential therapeutic utility of dopamine D(3) receptor antagonism.

The acute administration of the selective dopamine D(3) receptor antagonist SB-277011A reverses conditioned place aversion produced by naloxone precipitated withdrawal from acute morphine administration in rats

We examined the effect of SB-277011A, a selective D(3) receptor antagonist, on the conditioned place aversion (CPA) response associated with naloxone-induced withdrawal from acute morphine administration in male Sprague-Dawley rats. Morphine (5.6 mg/kg i.p.) was given, followed 4 hrs later by naloxone (0.3 mg/kg i.p.) and prior to placing the animals in one specific chamber of the test apparatus. All animals were subjected to 2 of these trials. A significant CPA occurred in animals that received an i.p. injection of vehicle 30 minutes prior to the measurement of chamber preference. The pretreatment of animals (30 minutes prior to testing) with 3 mg/kg i.p. of SB-277011A did not significantly alter the CPA compared to animals treated with vehicle (1 ml/kg i.p. of deionized distilled water). In contrast, the acute pretreatment of animals with 6, 12 or 24 mg/kg i.p. of SB-277011A significantly decreased the CPA compared to vehicle-treated animals. In fact, the 12 and 24 mg/kg doses of SB-277011A significantly increased the time spent in the chamber where animals were paired with morphine and naloxone. These results suggest that the selective antagonism of D(3) receptors attenuates the CPA produced by a model of naloxone-induced withdrawal from acute morphine dependence.

Recent advances in the search for D3- and D4-selective drugs: probes, models and candidates

Dopamine D(2)-like receptors (including D(2), D(3) and D(4)) belong to the 'rhodopsin-like' family of G protein-coupled receptors (GPCRs), which represent the largest group of targets for bioactive molecules. Due to their high sequence similarity, the design of subtype-selective ligands requires rational and effective strategies. The general formula of 1,4-disubstituted aromatic piperidines and piperazines (1,4-DAPs) was extracted from classical dopaminergic drugs. The biological properties of this compound family are encoded by an aromatic head group that controls intrinsic activity, an amine moiety and a lipophilic appendage. D(3)- and D(4)-selective molecular probes and drug candidates were generated from the general formula of 1,4-DAP. Formal structural rearrangement led to investigational drugs beyond the 1,4-DAP structure. The very recent publication of the X-ray crystal structure of D(3) should facilitate efficient discovery of unprecedented chemotypes. However, the development of D(3)-selective agonists, functionally selective ligands and the exploitation of homo- and heteromers remain challenging.

The dopamine D3 receptor antagonist, S33138, counters cognitive impairment in a range of rodent and primate procedures

Although dopamine D(3) receptor antagonists have been shown to enhance frontocortical cholinergic transmission and improve cognitive performance in rodents, data are limited and their effects have never been examined in primates. Accordingly, we characterized the actions of the D(3) receptor antagonist, S33138, in rats and rhesus monkeys using a suite of procedures in which cognitive performance was disrupted by several contrasting manipulations. S33138 dose-dependently (0.01-0.63 mg/kg s.c.) blocked a delay-induced impairment of novel object recognition in rats, a model of visual learning and memory. Further, S33138 (0.16-2.5 mg/kg s.c.) similarly reduced a delay-induced deficit in social novelty discrimination in rats, a procedure principally based on olfactory cues. Adult rhesus monkeys were trained to perform cognitive procedures, then chronically exposed to low doses of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine which produced cognitive impairment without motor disruption. In an attentional set-shifting task of cognitive flexibility involving an extra-dimensional shift, deficits were reversed by S33138 (0.04 and 0.16 mg/kg p.o.). S33138 also significantly improved accuracy (0.04 and 0.16 mg/kg p.o.) at short (but not long) delays in a variable delayed-response task of attention and working memory. Finally, in a separate set of experiments performed in monkeys displaying age-related deficits, S33138 significantly (0.16 and 0.63 mg/kg p.o.) improved task accuracies for long delay intervals in a delayed matching-to-sample task of working memory. In conclusion, S33138 improved performance in several rat and primate procedures of cognitive impairment. These data underpin interest in D(3) receptor blockade as a strategy for improving cognitive performance in CNS disorders like schizophrenia and Parkinson's disease.

Cannabinoids for the treatment of schizophrenia? A balanced neurochemical framework for both adverse and therapeutic effects of cannabis use

Recent studies have found that cannabinoids may improve neuropsychological performance, ameliorate negative symptoms, and have antipsychotic properties for a subgroup of the schizophrenia population. These findings are in contrast to the longstanding history of adverse consequences of cannabis use, predominantly on the positive symptoms, and a balanced neurochemical basis for these opposing views is lacking. This paper details a review of the neurobiological substrates of schizophrenia and the neurochemical effects of cannabis use in the normal population, in both cortical (in particular prefrontal) and subcortical brain regions. The aim of this paper is to provide a holistic neurochemical framework in which to understand how cannabinoids may impair, or indeed, serve to ameliorate the positive and negative symptoms as well as cognitive impairment. Directions in which future research can proceed to resolve the discrepancies are briefly discussed.

From the cell to the clinic: a comparative review of the partial D₂/D₃receptor agonist and α2-adrenoceptor antagonist, piribedil, in the treatment of Parkinson's disease

Though L-3,4-dihydroxyphenylalanine (L-DOPA) is universally employed for alleviation of motor dysfunction in Parkinson's disease (PD), it is poorly-effective against co-morbid symptoms like cognitive impairment and depression. Further, it elicits dyskinesia, its pharmacokinetics are highly variable, and efficacy wanes upon long-term administration. Accordingly, "dopaminergic agonists" are increasingly employed both as adjuncts to L-DOPA and as monotherapy. While all recognize dopamine D(2) receptors, they display contrasting patterns of interaction with other classes of monoaminergic receptor. For example, pramipexole and ropinirole are high efficacy agonists at D(2) and D(3) receptors, while pergolide recognizes D(1), D(2) and D(3) receptors and a broad suite of serotonergic receptors. Interestingly, several antiparkinson drugs display modest efficacy at D(2) receptors. Of these, piribedil displays the unique cellular signature of: 1), signal-specific partial agonist actions at dopamine D(2)and D(3) receptors; 2), antagonist properties at α(2)-adrenoceptors and 3), minimal interaction with serotonergic receptors. Dopamine-deprived striatal D(2) receptors are supersensitive in PD, so partial agonism is sufficient for relief of motor dysfunction while limiting undesirable effects due to "over-dosage" of "normosensitive" D(2) receptors elsewhere. Further, α(2)-adrenoceptor antagonism reinforces adrenergic, dopaminergic and cholinergic transmission to favourably influence motor function, cognition, mood and the integrity of dopaminergic neurones. In reviewing the above issues, the present paper focuses on the distinctive cellular, preclinical and therapeutic profile of piribedil, comparisons to pramipexole, ropinirole and pergolide, and the core triad of symptoms that characterises PD-motor dysfunction, depressed mood and cognitive impairment. The article concludes by highlighting perspectives for clarifying the mechanisms of action of piribedil and other antiparkinson agents, and for optimizing their clinical exploitation.

Quantification of acetylcholine, an essential neurotransmitter, in brain microdialysis samples by liquid chromatography mass spectrometry

Chemical neurotransmission has been the subject of intensive investigations in recent years. Acetylcholine is an essential neurotransmitter in the central nervous system as it has an effect on alertness, memory and learning. Enzymatic hydrolysis of acetylcholine in the synaptic cleft is fast and quickly metabolizes to choline and acetate by acetylcholinesterase. Hence the concentration in the extracellular fluid of the brain is low (0.1-6 nm). Techniques such as microdialysis are routinely employed to measure acetylcholine levels in living brain systems and the microdialysis sample volumes are usually less than 50 microL. In order to develop medicine for the diseases associated with cognitive dysfunction like mild cognitive impairment, Alzheimer's disease, schizophrenia and Parkinson's disease, or to study the mechanism of the illness, it is important to measure the concentration of acetylcholine in the extracellular fluid of the brain. Recently considerable attention has been focused on the development of chromatographic-mass spectrometric techniques to provide more sensitive and accurate quantification of acetylcholine collected from in-vivo brain microdialysis experiments. This review will provide a brief overview of acetylcholine biosynthesis, microdialysis technique and liquid chromatography mass spectrometry, which is being used to quantitate extracellular levels of acetylcholine.

Current perspectives on selective dopamine D(3) receptor antagonists as pharmacotherapeutics for addictions and related disorders

Repeated exposure to drugs of abuse produces long-term molecular and neurochemical changes that may explain the core features of addiction, such as the compulsive seeking and taking of the drug, as well as the risk of relapse. A growing number of new molecular and cellular targets of addictive drugs have been identified, and rapid advances are being made in relating those targets to specific behavioral phenotypes in animal models of addiction. In this context, the pattern of expression of the dopamine (DA) D(3) receptor in the rodent and human brain and changes in this pattern in response to drugs of abuse have contributed primarily to direct research efforts toward the development of selective DA D(3) receptor antagonists. Growing preclinical evidence indicates that these compounds may actually regulate the motivation to self-administer drugs and disrupt drug-associated cue-induced craving. This report will be divided into three parts. First, preclinical evidence in support of the efficacy of selective DA D(3) receptor antagonists in animal models of drug addiction will be reviewed. The effects of mixed DA D(2)/D(3) receptor antagonists will not be discussed here because most of these compounds have low selectivity at the D(3) versus D(2) receptor, and their efficacy profile is related primarily to functional antagonism at D(2) receptors and possibly interactions with other neurotransmitter systems. Second, major advances in medicinal chemistry for the identification and optimization of selective DA D(3) receptor antagonists and partial agonists will be analyzed. Third, translational research from preclinical efficacy studies to so-called proof-of-concept studies for drug addiction indications will be discussed.

Antipsychotic drug development

Schizophrenia typically manifests itself with a wide array of symptoms--positive, negative, cognitive, and affective--and may also involve neurodevelopmental and neurodegenerative aspects. Each of these symptom dimensions may be derived from pathology at one or more receptor types, localized in different regions of the brain. The absence of a single therapeutic target for schizophrenia has therefore prompted the de-emphasis of selective "magic bullets" and a critical re-examination of the intramolecular polypharmacy afforded by antipsychotics. In this chapter, we present a review of some of the receptor targets that are currently thought to mediate symptoms of schizophrenia, and discuss their possible implications for future antipsychotic drug development. Therapeutic strategies for schizophrenia that successfully exploit the multifunctionality of antipsychotics will take into account the entire receptor activity "portfolio" of the agent and provide a total therapeutic response that, like the elephant of the Buddhist parable, is greater than the sum of its parts.