Dopamine D3 Receptor, Cognition and Cognitive Dysfunctions in Neuropsychiatric Disorders: From the Bench to the Bedside

The dopamine D3 receptor (D3R) plays a prominent role in the modulation of cognition in healthy individuals, as well as in the pathophysiological mechanism underlying the cognitive deficits affecting patients suffering from neuropsychiatric disorders. At a therapeutic level, a growing body of evidence suggests that the D3R blockade enhances cognitive and thus it may be an optimal therapeutic strategy against cognitive dysfunctions. However, this is not always the case because other ligands targeting the D3R, and behaving as partial agonists or biased agonists, may exert their pro-cognitive effect by maintaining adequate level of dopamine in key brain areas tuning cognitive performances. In this chapter, we review and discuss preclinical and clinical findings with the aim to remark the crucial role of the D3R in cognition and to strengthen the message that drugs targeting D3R may be excellent cognitive enhancers for the treatment of several neuropsychiatric and neurological disorders.

The Role of Dopamine D3 Receptors in Tobacco Use Disorder: A Synthesis of the Preclinical and Clinical Literature

Tobacco smoking is a significant cause of preventable morbidity and mortality globally. Current pharmacological approaches to treat tobacco use disorder (TUD) are only partly effective and novel approaches are needed. Dopamine has a well-established role in substance use disorders, including TUD, and there has been a long-standing interest in developing agents that target the dopaminergic system to treat substance use disorders. Dopamine has 5 receptor subtypes (DRD1 to DRD5). Given the localization and safety profile of the dopamine receptor D3 (DRD3), it is of therapeutic potential for TUD. In this chapter, the preclinical and clinical literature investigating the role of DRD3 in processes relevant to TUD will be reviewed, including in nicotine reinforcement, drug reinstatement, conditioned stimuli and cue-reactivity, executive function, and withdrawal. Similarities and differences in findings from the animal and human work will be synthesized and findings will be discussed in relation to the therapeutic potential of targeting DRD3 in TUD.

Dual-acting agents for improving cognition and real-world function in Alzheimer's disease: Focus on 5-HT6 and D3 receptors as hubs

To date, there are no interventions that impede the inexorable progression of Alzheimer's disease (AD), and currently-available drugs cholinesterase (AChE) inhibitors and the N-Methyl-d-Aspartate receptor antagonist, memantine, offer only modest symptomatic benefit. Moreover, a range of mechanistically-diverse agents (glutamatergic, histaminergic, monoaminergic, cholinergic) have disappointed in clinical trials, alone and/or in association with AChE inhibitors. This includes serotonin (5-HT) receptor-6 antagonists, despite compelling preclinical observations in rodents and primates suggesting a positive influence on cognition. The emphasis has so far been on high selectivity. However, for a multi-factorial disorder like idiopathic AD, 5-HT6 antagonists possessing additional pharmacological actions might be more effective, by analogy to "multi-target" antipsychotics. Based on this notion, drug discovery programmes have coupled 5-HT6 blockade to 5-HT4 agonism and inhibition of AchE. Further, combined 5-HT6/dopamine D3 receptor (D3) antagonists are of especial interest since D3 blockade mirrors 5-HT6 antagonism in exerting broad-based pro-cognitive properties in animals. Moreover, 5-HT6 and dopamine D3 antagonists promote neurocognition and social cognition via both distinctive and convergent actions expressed mainly in frontal cortex, including suppression of mTOR over-activation and reinforcement of cholinergic and glutamatergic transmission. In addition, 5-HT6 blockade affords potential anti-anxiety, anti-depressive and anti-epileptic properties, and antagonising 5-HT6 receptors may be associated with neuroprotective ("disease-modifying") properties. Finally D3 antagonism may counter psychotic episodes and D3 receptors themselves offer a promising hub for multi-target agents. The present article reviews the status of "R and D" into multi-target 5-HT6 and D3 ligands for improved treatment of AD and other neurodegenerative disorders of aging. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Enhancing cognition through pharmacological and environmental interventions: Examples from preclinical models of neurodevelopmental disorders

In this review we discuss the role of environmental and pharmacological treatments to enhance cognition with special regards to neurodevelopmental related disorders and aging. How the environment influences brain structure and function, and the interactions between rearing conditions and gene expression, are fundamental questions that are still poorly understood. We propose a model that can explain some of the discrepancies in findings for effects of environmental enrichment on outcome measures. Evidence of a direct causal correlation of nootropics and treatments that enhanced cognition also will be presented, and possible molecular mechanisms that include neurotrophin signaling and downstream pathways underlying these processes are discussed. Finally we review recent findings achieved with a wide set of behavioral and cognitive tasks that have translational validity to humans, and should be useful for future work on devising appropriate therapies. As will be discussed, the collective findings suggest that a combinational therapeutic approach of environmental enrichment and nootropics could be particularly successful for improving learning and memory in both developmental disorders and normal aging.

The frontal cortex as a network hub controlling mood and cognition: Probing its neurochemical substrates for improved therapy of psychiatric and neurological disorders

The highly-interconnected and neurochemically-rich frontal cortex plays a crucial role in the regulation of mood and cognition, domains disrupted in depression and other central nervous system disorders, and it is an important site of action for their therapeutic control. For improving our understanding of the function and dysfunction of the frontal cortex, and for identifying improved treatments, quantification of extracellular pools of neuromodulators by microdialysis in freely-moving rodents has proven indispensable. This approach has revealed a complex mesh of autoreceptor and heteroceptor interactions amongst monoaminergic pathways, and led from selective 5-HT reuptake inhibitors to novel classes of multi-target drugs for treating depression like the mixed α₂-adrenoceptor/5-HT reuptake inhibitor, S35966, and the clinically-launched vortioxetine and vilazodone. Moreover, integration of non-monoaminergic actions resulted in the discovery and development of the innovative melatonin receptor agonist/5-HT_2C receptor antagonist, Agomelatine. Melatonin levels, like those of corticosterone and the "social hormone", oxytocin, can now be quantified by microdialysis over the full 24 h daily cycle. Further, the introduction of procedures for measuring extracellular histamine and acetylcholine has provided insights into strategies for improving cognition by, for example, blockade of 5-HT₆ and/or dopamine D₃ receptors. The challenge of concurrently determining extracellular levels of GABA, glutamate, d-serine, glycine, kynurenate and other amino acids, and of clarifying their interactions with monoamines, has also been resolved. This has proven important for characterizing the actions of glycine reuptake inhibitors that indirectly augment transmission at N-methyl-d-aspartate receptors, and of "glutamatergic antidepressants" like ketamine, mGluR5 antagonists and positive modulators of AMPA receptors (including S47445). Most recently, quantification of the neurotoxic proteins Aβ42 and Tau has extended microdialysis studies to the pathogenesis of neurodegenerative disorders, and another frontier currently being broached is microRNAs. The present article discusses the above themes, focusses on recent advances, highlights opportunities for clinical "translation", and suggests avenues for further progress.

Dopamine D3 Receptor Antagonists as Potential Therapeutics for the Treatment of Neurological Diseases

D3 receptors represent a major focus of current drug design and development of therapeutics for dopamine-related pathological states. Their close homology with the D2 receptor subtype makes the development of D3 selective antagonists a challenging task. In this review, we explore the relevance and therapeutic utility of D3 antagonists or partial agonists endowed with multireceptor affinity profile in the field of central nervous system disorders such as schizophrenia and drug abuse. In fact, the peculiar distribution and low brain abundance of D3 receptors make them a valuable target for the development of drugs devoid of motor side effects classically elicited by D2 antagonists. Recent research efforts were devoted to the conception of chemical templates possibly endowed with a multi-target profile, especially with regards to other G-protein-coupled receptors (GPCRs). A comprehensive overview of the recent literature in the field is herein provided. In particular, the evolution of the chemical templates has been tracked, according to the growing advancements in both the structural information and the refinement of the key pharmacophoric elements. The receptor/multireceptor affinity and functional profiles for the examined compounds have been covered, together with their most significant pharmacological applications.

Targeting the dopamine D3 receptor: an overview of drug design strategies

INTRODUCTION

Dopamine is a neurotransmitter widely distributed in both the periphery and the central nervous system (CNS). Its physiological effects are mediated by five closely related G protein-coupled receptors (GPCRs) that are divided into two major subclasses: the D1-like (D1, D5) and the D2-like (D2, D3, D4) receptors. D3 receptors (D3Rs) have the highest density in the limbic areas of the brain, which are associated with cognitive and emotional functions. These receptors are therefore attractive targets for therapeutic management.

AREAS COVERED

This review summarizes the functional and pharmacological characteristics of D3Rs, including the design and clinical relevance of full agonists, partial agonists and antagonists, as well as the capacity of these receptors to form active homodimers, heterodimers or higher order receptor complexes as pharmacological targets in several neurological and neurodegenerative disorders.

EXPERT OPINION

The high sequence homology between D3R and the D2-type challenges the development of D3R-selective compounds. The design of new D3R-preferential ligands with improved physicochemical properties should provide a better pharmacokinetic/bioavailability profile and lesser toxicity than is found with existing D3R ligands. It is also essential to optimize D3R affinity and, especially, D3R vs. D2-type binding and functional selectivity ratios. Developing allosteric and bitopic ligands should help to improve the D3R selectivity of these drugs. As most evidence points to the ability of GPCRs to form homomers and heteromers, the most promising therapeutic strategy in the future is likely to involve the application of heteromer-selective drugs. These selective ligands would display different affinities for a given receptor depending on the receptor partners within the heteromer. Therefore, designing novel compounds that specifically target and modulate D1R-D3R heteromers would be an interesting approach for the treatment of levodopa (L-DOPA)-induced dyskinesias.

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.

The dopamine D₃-preferring D₂/D₃ dopamine receptor partial agonist, cariprazine, reverses behavioural changes in a rat neurodevelopmental model for schizophrenia

Current antipsychotic medication is largely ineffective against the negative and cognitive symptoms of schizophrenia. One promising therapeutic development is to design new molecules that balance actions on dopamine D2 and D3 receptors to maximise benefits and limit adverse effects. This study used two rodent paradigms to investigate the action of the dopamine D3-preferring D3/D2 receptor partial agonist cariprazine. In adult male rats, cariprazine (0.03-0.3 mg/kg i.p.), and the atypical antipsychotic aripiprazole (1-3 mg/kg i.p.) caused dose-dependent reversal of a delay-induced impairment in novel object recognition (NOR). Treating neonatal rat pups with phencyclidine (PCP) and subsequent social isolation produced a syndrome of behavioural alterations in adulthood including hyperactivity in a novel arena, deficits in NOR and fear motivated learning and memory, and a reduction and change in pattern of social interaction accompanied by increased ultrasonic vocalisations (USVs). Acute administration of cariprazine (0.1 and 0.3 mg/kg) and aripiprazole (3 mg/kg) to resultant adult rats reduced neonatal PCP-social isolation induced locomotor hyperactivity and reversed NOR deficits. Cariprazine (0.3 mg/kg) caused a limited reversal of the social interaction deficit but neither drug affected the change in USVs or the deficit in fear motivated learning and memory. Results suggest that in the behavioural tests investigated cariprazine is at least as effective as aripiprazole and in some paradigms it showed additional beneficial features further supporting the advantage of combined dopamine D3/D2 receptor targeting. These findings support recent clinical studies demonstrating the efficacy of cariprazine in treatment of negative symptoms and functional impairment in schizophrenia patients.

Novel dimensions of D3 receptor function: Focus on heterodimerisation, transactivation and allosteric modulation

The brain׳s complexity derives not only from the way the intricate network of neurons is wired, but also by protein complexes that recognize and decode chemical information. G protein-coupled receptors (GPCRs) represent the most abundant family of proteins mediating neurotransmission in the brain, and their ability to form homo- and heteromers which amplifies the scope for synaptic communication and fine-tuning. Dopamine receptors are important drug targets and members of both the D1/D5 and D2/D3/D4 receptor families form homo- and heteromers. The present article focuses on D3 receptor homo- and heteromers, in particular, those formed in association with their D2 counterparts. We highlight the binding profiles and mechanisms of interaction with D3-D3 homomers and D3-D2 heteromers of: first, the PET ligand and potent agonist [(11)C]-(+)-PHNO; second, the novel, bitopic/allosteric dopamine D3 receptor antagonist, SB269,652; and third, diverse partial agonists like antipsychotic and aripiprazole. Molecular mechanisms of interplay between the two protomers of heteromeric D3-D2 complexes are likewise discussed: for example, "transactivation", whereby recruitment of one member of a heteromer harnesses signalling pathways is normally coupled to the other protomer. Finally, D1 receptor heteromers are also taken into consideration in deciphering the nature of interfaces required to stabilize dimeric assemblies and permit their interaction with G proteins. Improved understanding of D3 as well as D2 and D1 receptor complexes should yield important insights into their physiological roles and pathological significance, and permit the development of novel drug classes with potentially distinctive functional profiles and improved therapeutic windows.

On 'polypharmacy' and multi-target agents, complementary strategies for improving the treatment of depression: a comparative appraisal

Major depression is a heterogeneous disorder, both in terms of symptoms, ranging from anhedonia to cognitive impairment, and in terms of pathogenesis, with many interacting genetic, epigenetic, developmental and environmental causes. Accordingly, it seems unlikely that depressive states could be fully controlled by a drug possessing one discrete mechanism of action and, in the wake of disappointing results with several classes of highly selective agent, multi-modal treatment concepts are attracting attention. As concerns pharmacotherapy, there are essentially two core strategies. First, multi-target antidepressants that act via two or more complementary mechanisms and, second, polypharmacy, which refers to co-administration of two distinct drugs, usually in separate pills. Both multi-target agents and polypharmacy ideally couple a therapeutically unexploited action to a clinically established mechanism in order to enhance efficacy, moderate side-effects, accelerate onset of action and treat a broader range of symptoms. The melatonin MT1/MT2 agonist and 5-HT(2C) antagonist, agomelatine, which is effective in the short- and long-term treatment of depression, exemplifies the former approach, while evidence-based polypharmacy is illustrated by the adjunctive use of second-generation antipsychotics with serotonin reuptake inhibitors for treatment of resistant depression. Histone acetylation and methylation, ghrelin signalling, inflammatory modulators, metabotropic glutamate-7 receptors and trace amine-associated-1 receptors comprise attractive substrates for new multi-target and polypharmaceutical strategies. The present article outlines the rationale underpinning multi-modal approaches for treating depression, and critically compares and contrasts the pros and cons of established and potentially novel multi-target vs. polypharmaceutical treatments. On balance, the former appear the most promising for the elaboration, development and clinical implementation of innovative concepts for the more effective management of depression.

Negative symptoms of schizophrenia: clinical characteristics, pathophysiological substrates, experimental models and prospects for improved treatment

Schizophrenia is a complex and multifactorial disorder generally diagnosed in young adults at the time of the first psychotic episode of delusions and hallucinations. These positive symptoms can be controlled in most patients by currently-available antipsychotics. Conversely, they are poorly effective against concomitant neurocognitive dysfunction, deficits in social cognition and negative symptoms (NS), which strongly contribute to poor functional outcome. The precise notion of NS has evolved over the past century, with recent studies - underpinned by novel rating methods - suggesting two major sub-domains: "decreased emotional expression", incorporating blunted affect and poverty of speech, and "avolition", which embraces amotivation, asociality and "anhedonia" (inability to anticipate pleasure). Recent studies implicate a dysfunction of frontocortico-temporal networks in the aetiology of NS, together with a disruption of cortico-striatal circuits, though other structures are also involved, like the insular and parietal cortices, amygdala and thalamus. At the cellular level, a disruption of GABAergic-glutamatergic balance, dopaminergic signalling and, possibly, oxytocinergic and cannibinoidergic transmission may be involved. Several agents are currently under clinical investigation for the potentially improved control of NS, including oxytocin itself, N-Methyl-d-Aspartate receptor modulators and minocycline. Further, magnetic-electrical "stimulation" strategies to recruit cortical circuits and "cognitive-behavioural-psychosocial" therapies likewise hold promise. To acquire novel insights into the causes and treatment of NS, experimental study is crucial, and opportunities are emerging for improved genetic, pharmacological and developmental modelling, together with more refined readouts related to deficits in reward, sociality and "expression". The present article comprises an integrative overview of the above issues as a platform for this Special Issue of European Neuropsychopharmacology in which five clinical and five preclinical articles treat individual themes in greater detail. This Volume provides, then, a framework for progress in the understanding - and ultimately control - of the debilitating NS of schizophrenia.

Towards improved animal models for evaluating social cognition and its disruption in schizophrenia: the CNTRICS initiative

Social cognition refers to processes used to monitor and interpret social signals from others, to decipher their state of mind, emotional status and intentions, and select appropriate social behaviour. Social cognition is sophisticated in humans, being embedded with verbal language and enacted in a complex cultural environment. Its disruption characterises the entire course of schizophrenia and is correlated with poor functional outcome. Further, deficits in social cognition are related to impairment in other cognitive domains, positive symptoms (paranoia and delusions) and negative symptoms (social withdrawal and reduced motivation). In light of the significance and inadequate management of social cognition deficits, there is a need for translatable experimental procedures for their study, and identification of effective pharmacotherapy. No single paradigm captures the multi-dimensional nature of social cognition, and procedures for assessing ability to infer mental states are not well-developed for experimental therapeutic settings. Accordingly, a recent CNTRICS meeting prioritised procedures for measuring a specific construct: "acquisition and recognition of affective (emotional) states", coupled to individual recognition. Two complementary paradigms for refinement were identified: social recognition/preference in rodents, and visual tracking of social scenes in non-human primates (NHPs). Social recognition is disrupted in genetic, developmental or pharmacological disease models for schizophrenia, and performance in both procedures is improved by the neuropeptide oxytocin. The present article surveys a broad range of procedures for studying social cognition in rodents and NHPs, discusses advantages and drawbacks, and focuses on development of social recognition/preference and gaze-following paradigms for improved study of social cognition deficits in schizophrenia and their potential treatment.

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.

Pharmacological treatment of schizophrenia: a critical review of the pharmacology and clinical effects of current and future therapeutic agents

Since the introduction of chlorpromazine and throughout the development of the new-generation antipsychotic drugs (APDs) beginning with clozapine, the D(2) receptor has been the target for the development of APDs. Pharmacologic actions to reduce neurotransmission through the D(2) receptor have been the only proven therapeutic mechanism for psychoses. A number of novel non-D(2) mechanisms of action of APDs have been explored over the past 40 years but none has definitively been proven effective. At the same time, the effectiveness of treatments and range of outcomes for patients are far from satisfactory. The relative success of antipsychotics in treating positive symptoms is limited by the fact that a substantial number of patients are refractory to current medications and by their lack of efficacy for negative and cognitive symptoms, which often determine the level of functional impairment. In addition, while the newer antipsychotics produce fewer motor side effects, safety and tolerability concerns about weight gain and endocrinopathies have emerged. Consequently, there is an urgent need for more effective and better-tolerated antipsychotic agents, and to identify new molecular targets and develop mechanistically novel compounds that can address the various symptom dimensions of schizophrenia. In recent years, a variety of new experimental pharmacological approaches have emerged, including compounds acting on targets other than the dopamine D(2) receptor. However, there is still an ongoing debate as to whether drugs selective for singe molecular targets (that is, 'magic bullets') or drugs selectively non-selective for several molecular targets (that is, 'magic shotguns', 'multifunctional drugs' or 'intramolecular polypharmacy') will lead to more effective new medications for schizophrenia. In this context, current and future drug development strategies can be seen to fall into three categories: (1) refinement of precedented mechanisms of action to provide drugs of comparable or superior efficacy and side-effect profiles to existing APDs; (2) development of novel (and presumably non-D(2)) mechanism APDs; (3) development of compounds to be used as adjuncts to APDs to augment efficacy by targeting specific symptom dimensions of schizophrenia and particularly those not responsive to traditional APD treatment. In addition, efforts are being made to determine if the products of susceptibility genes in schizophrenia, identified by genetic linkage and association studies, may be viable targets for drug development. Finally, a focus on early detection and early intervention aimed at halting or reversing progressive pathophysiological processes in schizophrenia has gained great influence. This has encouraged future drug development and therapeutic strategies that are neuroprotective. This article provides an update and critical review of the pharmacology and clinical profiles of current APDs and drugs acting on novel targets with potential to be therapeutic agents in the future.

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.

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.

Pharmacological or genetic blockade of the dopamine D3 receptor increases cell proliferation in the hippocampus of adult mice

Dopamine plays an important role in cellular processes controlling the functional and structural plasticity of neurons, as well as their generation and proliferation, both in the developing and the adult brain. The precise roles of individual dopamine receptors subtypes in adult neurogenesis remain poorly defined, although D3 receptors are known to be involved in neurogenesis in the subventricular zone. By contrast, very few studies have addressed the influence of dopamine and D3 receptors upon neurogenesis in the subgranular zone of the hippocampus, an issue addressed herein employing constitutive D3 receptor knockout mice, or chronic exposure to the preferential D3 receptor antagonist, S33138. D3 receptor knockout mice revealed increased baseline levels of cell proliferation and ongoing neurogenesis, as measured both using Ki-67 and doublecortin, whereas there was no difference in cell survival as measured by BrdU (5-bromo-2'-deoxyuridine). Chronic administration of S33138 was shown to be functionally active in enhancing levels of the plasticity-related molecule, delta-FosB, in the D3 receptor-rich nucleus accumbens. In accordance with the stimulated neurogenesis seen in D3 receptor knockout mice, S33138 increased proliferation in wild-type mice. These observations suggest that D3 receptors exert a tonic, constitutive inhibitory influence upon adult hippocampal neurogenesis.

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.

Cognitive dysfunction in psychiatric disorders: characteristics, causes and the quest for improved therapy

Studies of psychiatric disorders have traditionally focused on emotional symptoms such as depression, anxiety and hallucinations. However, poorly controlled cognitive deficits are equally prominent and severely compromise quality of life, including social and professional integration. Consequently, intensive efforts are being made to characterize the cellular and cerebral circuits underpinning cognitive function, define the nature and causes of cognitive impairment in psychiatric disorders and identify more effective treatments. Successful development will depend on rigorous validation in animal models as well as in patients, including measures of real-world cognitive functioning. This article critically discusses these issues, highlighting the challenges and opportunities for improving cognition in individuals suffering from psychiatric disorders.

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.

Animal models of schizophrenia

Developing reliable, predictive animal models for complex psychiatric disorders, such as schizophrenia, is essential to increase our understanding of the neurobiological basis of the disorder and for the development of novel drugs with improved therapeutic efficacy. All available animal models of schizophrenia fit into four different induction categories: developmental, drug-induced, lesion or genetic manipulation, and the best characterized examples of each type are reviewed herein. Most rodent models have behavioural phenotype changes that resemble 'positive-like' symptoms of schizophrenia, probably reflecting altered mesolimbic dopamine function, but fewer models also show altered social interaction, and learning and memory impairment, analogous to negative and cognitive symptoms of schizophrenia respectively. The negative and cognitive impairments in schizophrenia are resistant to treatment with current antipsychotics, even after remission of the psychosis, which limits their therapeutic efficacy. The MATRICS initiative developed a consensus on the core cognitive deficits of schizophrenic patients, and recommended a standardized test battery to evaluate them. More recently, work has begun to identify specific rodent behavioural tasks with translational relevance to specific cognitive domains affected in schizophrenia, and where available this review focuses on reporting the effect of current and potential antipsychotics on these tasks. The review also highlights the need to develop more comprehensive animal models that more adequately replicate deficits in negative and cognitive symptoms. Increasing information on the neurochemical and structural CNS changes accompanying each model will also help assess treatments that prevent the development of schizophrenia rather than treating the symptoms, another pivotal change required to enable new more effective therapeutic strategies to be developed.

Are impairments of time perception in schizophrenia a neglected phenomenon?

Based on clinical, phenomenological and neurobiological observations, psychiatrists often report a deficit in time estimation in patients with schizophrenia. Cognitive models of time estimation in healthy subjects have been proposed and developed for approximately 30 years. The current theory in the field of time perception, which is supported by a connectionist model, postulates that temporal judgement is based upon a pacemaker-counter device that depends mostly upon memory and attentional resources. The pacemaker emits pulses that are accumulated in a counter, and the number of pulses determines the perceived length of an interval. Patients with schizophrenia are known to display attentional and memory dysfunctions. Moreover, dopamine regulation mechanisms are involved in both the temporal perception processes and schizophrenia. Thus, it is still unclear if temporal impairments in schizophrenia are related to a specific disturbance in central temporal processes or are due to certain cognitive problems, such as attentional and memory dysfunctions, or biological abnormalities. The authors present a critical literature review on time perception in schizophrenia that covers topics from psychopathology to neuroscience. Temporal perception appears to play a key role in schizophrenia and to be partially neglected in the current literature. Future research is required to better ascertain the underlying mechanisms of time perception impairments in schizophrenia.

Influence of social isolation in the rat on serotonergic function and memory--relevance to models of schizophrenia and the role of 5-HT₆ receptors

There is increasing awareness of the importance that early environmental factors have on brain development and their role in the neurobiology of neurodevelopmental disorders including schizophrenia. The isolation reared rat attempts to model adverse effects that human social isolation (absence of social contact) can have on normal brain development. The isolation reared rat also models aspects of schizophrenia including the development of persistent learning and memory deficits. This short review concentrates on the effects of isolation rearing on cognition, including deficits in novel object discrimination, and the neural mechanisms that may underlie this impairment. There is evidence that a key effect of social isolation may be loss of neuronal plasticity combined with change in the functional state of various cortical and hippocampal neurotransmitters, including glutamate and serotonin. Reduced glutamate function may underlie the deficits in novel object discrimination, which can be reversed by administration of a 5-HT(6) receptor antagonist. This suggests that the 5-HT(6) antagonists may act by reducing 5-HT(6) receptor mediated activation of GABA, resulting in glutamate disinhibition. Thus drugs acting at 5-HT(6) receptors may offer a novel approach to treat neurodevelopmental cognitive symptoms, including those seen in schizophrenia.

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.

Imaging dopamine D3 receptors in the human brain with positron emission tomography, [11C]PHNO, and a selective D3 receptor antagonist

BACKGROUND

Dopamine D(3) receptors are involved in the pathophysiology of several neuropsychiatric conditions. [(11)C]-(+)-PHNO is a radiolabeled D(2) and D(3) agonist, suitable for imaging the agonist binding sites (denoted D(2HIGH) and D(3)) of these receptors with positron emission tomography (PET). PET studies in nonhuman primates documented that, in vivo, [(11)C]-(+)-PHNO displays a relative selectivity for D(3) compared with D(2HIGH) receptor sites and that the [(11)C]-(+)-PHNO signal is enriched in D(3) contribution compared with conventional ligands such as [(11)C] raclopride.

METHODS

To define the D(3) contribution (f(PHNO)(D3)) to [(11)C]-(+)-PHNO binding potential (BP(ND)) in healthy humans, 52 PET scans were obtained in 19 healthy volunteers at baseline and following oral administration of various doses of the selective D(3) receptor antagonist, GSK598809.

RESULTS

The impact of GSK598809 on [(11)C]-(+)-PHNO was regionally selective. In dorsal regions of the striatum, GSK598809 did not significantly affect [(11)C]-(+)-PHNO BP(ND) (f(PHNO)(D3) approximately 0%). Conversely, in the substantia nigra, GSK598809 dose-dependently reduced [(11)C]-(+)-PHNO binding to nonspecific level (f(PHNO)(D3) approximately 100%). In ventral striatum (VST), globus pallidus and thalamus (THA), [(11)C]-(+)-PHNO BP(ND) was attributable to a combination of D(2HIGH) and D(3) receptor sites, with f(PHNO)(D3) of 26%, 67% and 46%, respectively. D(3) receptor binding potential (BP(ND)(D3)) was highest in globus pallidus (1.90) and substantial nigra (1.39), with lower levels in VST (.77) and THA (.18) and negligible levels in dorsal striatum.

CONCLUSIONS

This study elucidated the pharmacologic nature of the [(11)C]-(+)-PHNO signal in healthy subjects and provided the first quantification of D(3) receptor availability with PET in the living human brain.

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.

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.

The preferential dopamine D3 receptor antagonist S33138 inhibits cocaine reward and cocaine-triggered relapse to drug-seeking behavior in rats

We have previously reported that selective dopamine (DA) D3 receptor antagonists are effective in a number of animal models of drug addiction, but not in intravenous drug self-administration, suggesting a limited ability to modify drug reward. In the present study, we evaluated the actions ofS33138, a novel partially selective D3 receptor antagonist, in animal models relevant to drug addiction. S33138, at doses of 0.156 or 0.625 mg/kg (i.p.), attenuated cocaine-enhanced brain-stimulation reward (BSR), and the highest dose tested (2.5 mg/kg) produced a significant aversive-like rightward shift in BSR rate-frequency reward functions. Further, S33138 produced biphasic effects on cocaine self-administration, i.e., a moderate dose (2.5 mg/kg, p.o.) increased, while a higher dose (5 mg/kg, p.o.) inhibited, cocaine self-administration. The increase in cocaine self-administration likely reflects a compensatory response to a partial reduction in drug reward after S33138. In addition, S33138 (0.156-2.5 mg/kg, p.o.) also dose-dependently inhibited cocaine-induced reinstatement of drug-seeking behavior. The reduction in cocaine-enhanced BSR and cocaine-triggered reinstatement produced by lower effective doses (e.g., 0.156 or 0.625 mg/kg) of 533138 is unlikely due to impaired locomotion, as lower effective doses of S33138 decreased neither Ymax levels in the BSR paradigm, rotarod performance, nor locomotion. However, the higher doses (2.5 or 5 mg/kg) of S33138 also significantly inhibited sucrose self-administration and rotarod performance, suggesting non-D3 receptor-mediated effects on non-drug reward and locomotion. These data suggest that lower doses of S33138 interacting essentially with D3 receptors have pharmacotherapeutic potential in treatment of cocaine addiction, while higher doses occupying D2 receptors may influence locomotion and non-drug reward.

Blockade of dopamine D(3) receptors in frontal cortex, but not in sub-cortical structures, enhances social recognition in rats: similar actions of D(1) receptor agonists, but not of D(2) antagonists

Though D(3) receptor antagonists can enhance cognitive function, their sites of action remain unexplored. This issue was addressed employing a model of social recognition in rats, and the actions of D(3) antagonists were compared to D(1) agonists that likewise possess pro-cognitive properties. Infusion of the highly selective D(3) antagonists, S33084 and SB277,011 (0.04-2.5 microg/side), into the frontal cortex (FCX) dose-dependently reversed the deficit in recognition induced by a delay. By contrast, the preferential D(2) antagonist, L741,626 (0.63-5.0) had no effect. The action of S33084 was regionally specific inasmuch as its injection into the nucleus accumbens or striatum was ineffective. A similar increase of recognition was obtained upon injection of the D(1) agonist, SKF81297 (0.04-0.63), into the FCX though it was also active (0.63) in the nucleus accumbens. These data suggest that D(3) receptors modulating social recognition are localized in FCX, and underpin their pertinence as targets for antipsychotic agents.

Dual- and triple-acting agents for treating core and co-morbid symptoms of major depression: novel concepts, new drugs

The past decade of efforts to find improved treatment for major depression has been dominated by genome-driven programs of rational drug discovery directed toward highly selective ligands for nonmonoaminergic agents. Selective drugs may prove beneficial for specific symptoms, for certain patient subpopulations, or both. However, network analyses of the brain and its dysfunction suggest that agents with multiple and complementary modes of action are more likely to show broad-based efficacy against core and comorbid symptoms of depression. Strategies for improved multitarget exploitation of monoaminergic mechanisms include triple inhibitors of dopamine, serotonin (5-HT) and noradrenaline reuptake, and drugs interfering with feedback actions of monoamines at inhibitory 5-HT(1A), 5-HT(1B) and possibly 5-HT(5A) and 5-HT(7) receptors. Specific subsets of postsynaptic 5-HT receptors mediating antidepressant actions are under study (e.g., 5-HT(4) and 5-HT(6)). Association of a clinically characterized antidepressant mechanism with a nonmonoaminergic component of activity is an attractive strategy. For example, agomelatine (a melatonin agonist/5-HT(2C) antagonist) has clinically proven activity in major depression. Dual neurokinin(1) antagonists/5-HT reuptake inhibitors (SRIs) and melanocortin(4) antagonists/SRIs should display advantages over their selective counterparts, and histamine H(3) antagonists/SRIs, GABA(B) antagonists/SRIs, glutamatergic/SRIs, and cholinergic agents/SRIs may counter the compromised cognitive function of depression. Finally, drugs that suppress 5-HT reuptake and blunt hypothalamo-pituitary-adrenocorticotrophic axis overdrive, or that act at intracellular proteins such as GSK-3beta, may abrogate the negative effects of chronic stress on mood and neuronal integrity. This review discusses the discovery and development of dual- and triple-acting antidepressants, focusing on novel concepts and new drugs disclosed over the last 2 to 3 years.