The potential role of dopamine D₃ receptor neurotransmission in cognition

Abstract

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.

1. Introduction

Cognition involves a number of mental processes that include attention, memory, language comprehension and expression, problem solving, and decision making. Cognition is indispensable for understanding information, applying knowledge, and changing preferences. Cognitive dysfunction is common in individuals with neuropsychiatric disorders, such as schizophrenia, mood disorders, Parkinson’s disease (PD), autism and Alzheimer’s disease (AD), even though the characteristic manifestations and pathophysiology of these disorders are different (Millan et al., 2012). Importantly, cognitive dysfunction has a negative impact on social functioning, independent community living, employment and quality of life (QOL) (Demirtas-Tatlidede et al., 2013; Green, 2007; Green et al., 2000, 2004; Millan et al., 2012). As such, the discovery of effective treatments to improve cognition is essential for improving QOL in individuals with neuropsychiatric disorders.

Although pro-cognitive drugs, such as donepezil (a cholinesterase inhibitor) or memantine (a weak N-methyl-D-aspartate receptor antagonist), are clinically available for AD, these agents have only short-term effects on behavioral and cognitive test scores compared with placebo (Kaduszkiewicz et al., 2005; McShane et al., 2006; Saddichha and Pandey, 2008). No disease-modifying effects have been shown in AD and no preventative treatments exist (Ballard et al., 2011). Many candidate pro-cognitive drugs have been investigated in clinical trials for schizophrenia, including AZD3480 (an α4β2 central neuronal nicotinic receptor agonist) (Velligan et al., 2012), dimebon (a serotonin 5HT-6 receptor antagonist) (Morozova et al., 2012), EVP-6124, TC-5619 (α7 nicotinic receptor partial agonists) (Lieberman et al., 2013; Prickaerts et al., 2012; Tregellas et al., 2011), rimonabant (a cannabinoid-1 receptor antagonist) (Boggs et al., 2012), and rosiglitazone (a peroxisome proliferator-activated receptor-γagonist) (Yi et al., 2012). Unfortunately, these drugs lack enough efficacy for therapeutic utilization. Thus, there is an urgent need to identify new therapeutic strategies for cognitive dysfunction in neuropsychiatric disorders.

The dopaminergic (DAergic) system has been implicated in cognitive function through animal and human research, including studies of molecular genetics and neuroimaging (Backman et al., 2006; Cole et al., 2012). Interestingly, animal and human studies have demonstrated that there is an inverted-U curve between DAergic signaling and cognition, where too little or too much DAergic signaling impairs cognitive performance. Mid-level DAergic signaling appears necessary for optimal cognitive performance (Baunez and Robbins, 1999; Boussaoud and Kermadi, 1997; Cools and D’Esposito, 2012; Glickstein et al., 2005).

The present article reviews the data on dopamine (DA) D₃ receptors as a target for novel pro-cognitive treatments. Initially, the role of the DAergic system in cognition is presented. Then, the D₃ receptor predominant brain regions related to cognitive function are discussed, in particular, the limbic regions that modulate memory, emotions and motivation (Gurevich and Joyce, 1999; Murray et al., 1992, 1994). Next, the characteristics and in vivo quantification of D₃ receptors are described and the relationship between D₃ receptors and cognition from previously reported animal and human studies are reviewed. Finally, D₃ receptor antagonists are proposed as a pro-cognitive therapy for cognitive dysfunction in individuals with neuropsychiatric disorders.

2. Search strategies and selection criteria

PubMed, Medline, EMBASE and references from relevant studies, review articles and books were searched using the terms ‘dopamine,’ ‘D3 receptor’ and ‘cognition.’ Only publications written in English pertaining to the relationship between D₃ receptors and cognition were selected. The search yielded 164 articles, which formed the empirical basis of this review. The last search was conducted on April 14, 2013. Cross-referencing of the identified publications was also performed. The literature search was conducted independently by two of the authors (S.N. and A.G.).

3. Characteristics and in vivo quantification of dopamine D₃ receptors

Dopamine receptors are divided into two subclasses, D₁- and D₂-like receptor families. The D₁-like receptor family contains the D₁ and D₅ receptors, and the D₂-like receptor family contains the D₂, D₃, and D₄ receptors (Ilani et al., 2001; Le Foll et al., 2009). As a member of the D₂-like receptor family, D₃ is a G-protein coupled receptor (GPCR). In the simplest conceptualization, GPCRs work as a switch. When a G-protein is attached to the cellular side of a GPCR, the GPCR exists in high-affinity for its ligand, but when the G-protein detaches from the GPCR, the GPCR has very low affinity for DA. D₃ receptors are unique among the D₂-like receptors, exhibiting sustained high affinity for DA (>20-fold higher than D₂ receptors), suggesting that D₃ receptors in vivo may be occupied by endogenous DA for extended periods of time, leading to high spontaneous activation of D₃ receptors (Richtand et al., 2001; Vanhauwe et al., 2000). Unlike D₂ receptors, D₃ receptors can be stimulated by tonic DA levels in the brain due to their high affinity for DA (Sokoloff et al., 1990), and may attenuate any effects of DA fluctuation related to phasic DA release. Accordingly, small changes in the number or function of D₃ receptors may lead to dramatic effects on synaptic transmission, suggesting that D₃ receptors could be critical modulators of normal DAergic function, and consequently, cognition (as described below). Other DA receptors such as D₁, D₂, D₄ and D₅ are also involved in cognition. As the focus of this review is D₃ receptors and cognition, please see El-Ghundi et al. (2007), Floresco and Magyar (2006), and Takahashi et al. (2012) for a review on other DA receptors and cognition.

D₃ receptors are expressed both as autoreceptors on DA neurons and as post-synaptic receptors (Diaz et al., 1995, 2000; Levesque et al., 1992). They are present in the ventral striatum (nucleus accumbens (NAc)) and other limbic areas (Bouthenet et al., 1991; Sokoloff et al., 1990). There are low D₃ receptor levels in the dorsal striatum and a variety of cortical regions, including the frontal cortex in humans (Hall et al., 1996; Suzuki et al., 1998) and nonhuman primates (Morissette et al., 1998); whereas the dorsal striatum is almost entirely devoid of D₃ receptor expression in rodents. D₃ receptors are also found in the islands of Calleja and cerebellum (Diaz et al., 2000; Levesque et al., 1992). It is important to note that there are significant inter-species differences in the distribution of D₃ receptors in the central nervous system, which may limit generalization of D₃ receptor-mediated behaviors from one species to another (Levant, 1998).

Some of the brain structures expressing D₃ receptors encompass a proposed feedback loop for modulating attention, memory, emotions, motivation and reward. The output neurons of the NAc have a high density of D₃ receptors and receive DAergic innervation from the ventral tegmental area (VTA). The outputs of the NAc reach the entorhinal and prefrontal cortices (PFC) after relaying in the ventral pallidum and mediodorsal thalamus. In turn, the shell of the NAc receives projections from the cerebral cortex, hippocampus and amygdala and projects to the VTA (Sokoloff et al., 2006). The D₃ receptors in the VTA and substantia nigra may function as autoreceptors and affect DAergic feed-forward loops; potentially influencing theta oscillations that seem essential for coordinating neuronal activity among these loops (Fujisawa and Buzsaki, 2011). These feed-forward loops have a unique configuration, and the preferential distribution of D₃ receptors in the regions that modulate attention, memory and emotions (Gurevich et al., 1997) suggests that D₃ receptors may play a role in regulating cognitive function.

Our group has developed [¹¹C]-(+)-4-propyl-9-hydroxy-naphthoxazine ([¹¹C]-(+)–PHNO) (Wilson et al., 2005), the only available radiotracer for imaging D₃ receptors with a 53-fold D₃/D₂ receptor selectivity in vivo in humans (Gross and Drescher, 2012). [¹¹C]-(+)–PHNO is a D_2/3 PET agonist with high in vitro affinity for both D₂ and D₃ receptors, but shows preferential in vivo affinity and selectivity for D₃ receptors in D₃-receptor brain regions. PET studies in mice and baboons using [¹¹C]-(+)-PHNO in the presence and absence of a D₃ receptor antagonist, SB-277011, confirm that D₃ receptors are highly expressed and quantifiable in vivo in the ventral pallidum, substantia nigra, thalamus, and habenula; to a lesser extent in the ventral striatum; and negligibly in the dorsal caudate and putamen (Rabiner et al., 2009). The development of [¹¹C]-(+)-PHNO as a D₃ receptor PET ligand provides the possibility of in vivo exploration of the role of D₃ receptors in normal and abnormal cognition. The use of [¹¹C]-(+)-PHNO, however, is limited by its non-selective binding to both D₃ and D₂ receptors, chiefly because D₂ receptors tend to prevail in brain regions where there is an abundance of D₃ receptors (Gurevich and Joyce, 1999; Suzuki et al., 1998). The binding of the [¹¹C]-(+)-PHNO may also be influenced by endogenous DA levels due to its higher affinity for D₃ receptors, especially in the presence of compounds that block auto-receptors, which enhance endogenous DA release (Gross and Drescher, 2012). Thus, a highly selective D₃ receptor antagonist PET tracer is still needed as [¹¹C]-(+)-PHNO may not be a suitable radiotracer to detect D₃ receptors in areas with low D₃ receptor concentrations or in regions where D₂ receptors prevail over D₃ receptors.

The relationship between cognition and D₃ receptors in healthy individuals is inconsistently reported (Ball et al., 1998; Czermak et al., 2009; Keri et al., 2005; Lane et al., 2008). While the D₃ receptor gene, DRD3, was not associated with enhanced general cognitive ability or reasoning skill (Ball et al., 1998; Gong et al., 2011), the DRD3 Ser/Ser genotype was linked to fewer perseverative errors during the Wisconsin Card Sorting Test (WCST) (Lane et al., 2008). In contrast, elderly individuals carrying the DRD3 Ser/Gly genotype had more benefit from multimodal cognitive training than the carriers of the Ser/Ser genotype (Pieramico et al., 2012). Only one study has directly investigated whether differences in midbrain D₃ receptor availability are associated with functional interactions between large-scale networks and brain regions involved in cognition in healthy individuals (Cole et al., 2012). Combining [¹¹C]-(+)-PHNO PET and resting-state functional magnetic resonance imaging (fMRI), this study of healthy individuals demonstrated that high midbrain D₃ receptor availability is associated with reduced functional connectivity between the orbitofrontal cortex (OFC) and frontopar-ietal networks, which are implicated in executive control and salience processing (Seeley et al., 2007). This result suggests that D₃ receptor availability can modulate the pathway underlying cognitive control in healthy individuals.

4. Potential mechanisms between dopamine D₃ receptors and cognition

The preclinical evidence suggests that D₃ receptors influence cognition by modulating PFC function despite the relatively few D₃ receptors in this region (Loiseau and Millan, 2009; Watson et al., 2012a). PET studies of baboons have shown that D₃ receptor-mediated regional cerebral blood flow responses are restricted to the prefrontal and limbic cortices (Black et al., 2002). Executive function as measured with a PFC-dependent task is enhanced in D₃ receptor knockout (KO) mice and is accompanied by increased c-fos expression in the PFC (Glickstein et al., 2005). Bilateral microinjection of D₃ receptor antagonists into the PFC of rats improves social recognition, social discrimination and object recognition, while microinjection into the NAc or striatum has no effect (Loiseau and Millan, 2009; Watson et al., 2012a). Moreover, the blockade of D₃ receptor enhances the release of acetylcholine (ACh) in the PFC of rats (Gobert et al., 1995; Lacroix et al., 2003, 2006; Millan and Brocco, 2008; Millan et al., 1995, 2007), whereas D₃ receptor agonists do not increase ACh levels (Gobert et al., 2003). The mechanism by which D3 antagonism results in elevated cortical ACh levels is unclear. Given a high expression of D₃ receptors in the thalamus and a lack of D3 receptor expression in the PFC, modulation of thalamocortical projections via D3 antagonism may indirectly enhance the release of ACh in the PFC. This, in turn, may facilitate the PFC’s top-down control of subcortical brain regions that process cognitive cues (Loiseau and Millan, 2009; Millan et al., 2007; Perio et al., 1989; Soffie and Bronchart, 1988; Watson et al., 2012a; Winslow and Camacho, 1995).

DAergic system enhancement by D₃ receptor antagonists may contribute to the cognitive control exerted by the frontal cortex through other mechanisms (Sesack and Grace, 2010). D₃ receptor KO mice have extracellular levels of DA in the NAc that are twice as high as those of wild type (WT) mice (Joseph et al., 2002; Koeltzow et al., 1998; Le Foll et al., 2005a). Low, D₃-selective doses of preferential D₃ receptor agonists decrease DA synthesis measured by microdialysis in the mesolimbic area in rats (Pugsley et al., 1995). The antisense mutation of D₃ receptors increases DA turnover in the limbic forebrain and NAc in rats (Nissbrandt et al., 1995). In contrast, selective D₃ receptor antagonists block the inhibitory effect of D₃ receptor agonism on DA release and synthesis in the frontal cortex (Banasikowski et al., 2010; Gobert et al., 1995, 1996; Millan et al., 2008). Lastly, a combined PET-fMRI study showed that high mid-brain D₃ receptor availability was associated with reduced functional connectivity between the OFC and frontoparietal networks implicated in executive control in healthy individuals (Cole et al., 2012). These findings suggest that D₃ receptors can modulate cortical control of cognitive functions via their inhibitory effect on mesocortical DAergic activity (Gross and Drescher, 2012). Further work is needed to elucidate the mechanism of the relationship between D₃ receptors, the frontal cortex and cognition, as the PFC has relatively few D₃ receptors. A recent study reported that D₃ receptors may control N-methyl-D-aspartate (NMDA) receptor signaling by acting on pyramidal cells either directly at post-synaptic levels in the NAc or indirectly at presynaptic levels in the PFC. The D₃ receptor selective antagonist, F17141, reversed hyperactivity and social interaction deficits induced by NMDA receptor blockade by MK-801 in mice (Sokoloff et al., 2013). Thus, glutamatergic–D₃ receptor interactions may shed light on these relationships.

Another potential mechanism by which D₃ receptor antagonists may improve cognition is cAMP/PKA/CREB signaling in the hippocampus, which has a modest density of D₃ receptors (Basile et al., 2006; Bouthenet et al., 1991; Khan et al., 1998; Richtand et al., 1995; Stanwood et al., 2000). D₃ receptor antagonists do not appear to influence ACh levels in the hippocampus as they do in the PFC (Bouthenet et al., 1991; Joyce, 2001; Joyce and Millan, 2005; Stanwood et al., 2000). Aged D₃ receptor KO mice showed better spatial memory performance than age-matched WT mice along with a higher degree of hippocampal CREB phosphorylation, which may have neuroprotective effects on memory consolidation (Lee et al., 2005). Whereas no difference was found in the level of CREB phosphorylation in the PFC between the aged D₃ receptor KO mice and age-matched WT mice (Swant et al., 2008; Taubenfeld et al., 1999; Walton et al., 1999; Xing et al., 2010b). Thus, the cognitive effects of D₃ receptors may be attributable to the activation of cAMP/PKA/CREB signaling in the hippocampus.

Alternatively, given that D₃ receptor stimulation potentiates D₁ receptor-mediated behavioral effects (Le Foll et al., 2009; Marcellino et al., 2008), the interaction between D₃ and co-expressed D₁ receptors may cause dysregulation of CREB signaling in the aged hippocampus (Xing et al., 2010b). In support of this, D₁ receptor KO mice do not acquire spatial memory or show activation of the underlying CREB signaling pathways compared to the WT mice; however, D₃ receptor KO mice exhibited normal learning abilities and normal activation of these signaling pathways. The results suggest that D₁ receptors, but not D₃ receptors, may be critical for hippocampus-dependent spatial learning.

In summary, the above findings suggest that D₃ receptors may influence cognition by modulating PFC function (Loiseau and Millan, 2009; Watson et al., 2012a) and by regulating CREB signaling in the hippocampus (Xing et al., 2010b); however, further research into this relationship is required.

5. Dopamine D₃ receptors and cognition in neuropsychiatric disorders with cognitive dysfunction

Dopamine D₃ receptors are implicated in the pathophysiology of neuropsychiatric disorders that are commonly accompanied by cognitive dysfunction, including schizophrenia (Dubertret et al., 1998; Gross and Drescher, 2012; Keefe and Harvey, 2012), drug addiction (Khaled et al., 2010; Le Foll et al., 2002, 2003, 2005b), PD (Bezard et al., 2003; Boileau et al., 2009), dementia (Sokoloff et al., 2006), mood disorders (Sokoloff et al., 2006), and autism (de Krom et al., 2009), although not consistently (Chiaroni et al., 2000; Cosentino et al., 2009; Heidbreder and Newman, 2010; Kim et al., 2008; Kumar and Patel, 2007; Serretti et al., 2000). However, only a few studies have directly explored the relationship between D₃ receptors and cognition in these conditions.

5.1. Dopamine D₃ receptors in Alzheimer’s disease

Dementia is associated with D₃ receptor anomalies. Piggott et al. (1999) demonstrated that D₃ receptor binding is increased in the striatum of AD when compared to healthy controls in a [3H]-7-OH-DPAT autoradiography study of postmortem brains. Preclinical studies have found that the brains of D₃ receptor KO mice have reduced levels of neurofibromin (NF1). The heterozygous loss of NF1 causes neurofibromatosis type I disease and increases amyloid precursor protein (APP) levels, which are involved in the pathogenesis of AD. These findings suggest that a link between NF1, APP and D₃ receptors may contribute to cognitive dysfunction in spatial learning and memory in neurofibromatosis type 1 disease (Castorina et al., 2011; Donarum et al., 2006).

5.2. Dopamine D₃ receptors in schizophrenia

Some genetic and postmortem studies have shown elevated D₃ receptor expression in the central nervous system and blood lymphocytes in individuals with schizophrenia (Gurevich et al., 1997; Ilani et al., 2001). Our group found no difference in D₃ receptor availability as measured with [¹¹C]-(+)-PHNO PET between unmedicated individuals with schizophrenia and healthy controls. The limitations of [¹¹C]-(+)-PHNO are described in the section “Characteristics and in vivo quantification of dopamine D3 receptors” (Graff-Guerrero et al., 2009).

A few studies have explored the relationship between cognitive performance and DRD3 Ser9Gly polymorphisms in individuals with schizophrenia. Szekeres et al. (2004) found that Ser/Ser carriers perform better on the WCST than Gly carriers, which is indicative of better working memory performance. Keri et al. (2005) found that Gly carriers have more efficient striatal habit learning both in healthy controls and in individuals with schizophrenia. In contrast, Bombin et al. (2008) showed that Ser/Ser carriers perform better on an executive functioning task than Gly/Gly carriers. Conversely, Rybakowski et al. (2005) did not find any association between working memory and the various DRD3 polymorphisms. Given the fact that the Ser allele is associated with a lower affinity for DA than the Gly allele (Jeanneteau et al., 2006; Lundstrom and Turpin, 1996; Lundstrom et al., 1998), these results indicate that the lower DRD3 Ser/Ser affinity for DA and higher DA-mediated response may be associated with better frontal/executive functioning, such as set-shifting.

5.3. Dopamine D₃ receptors in addictions

Dopamine D₃ receptors may play an important role in the pathophysiology of substance use disorders. D₃ receptors are distributed in the mesolimbic DA system where they may mediate the influence of drug-associated cues on drug-seeking behaviors (for reviews, please see Heidbreder (2013); Heidbreder et al. (2005)). A PET study found that methamphetamine polydrug abusers had higher [¹¹C]-(+)-PHNO binding in the D₃ receptor-rich regions, indicating that the D₃ receptor might be upregulated in this population (Boileau et al., 2012).

Few studies have explored the relationship between cognitive function and DRD3 Ser9Gly polymorphisms in individuals with substance use disorders. An association was found between a single nucleotide polymorphism of the DRD3 gene (rs6280TC) and cognitive dysfunction in human immunodeficiency virus (HIV)-positive individuals who were dependent on methamphetamine (Gupta et al., 2011). The study found that DRD3 Gly allele carriers have more cognitive dysfunction than the Ser/Ser carriers in the presence of recent methamphetamine use. Although no association was found between the DRD3 gene and HIV viral load in this study, simian models of the neurological complications of acquired immune deficiency syndrome showed that methamphetamine administration may increase the viral load in the frontal lobe, caudate, and hippocampus (Marcondes et al., 2010). It was proposed that DA enhances replication of the HIV virus in macrophages within the central nervous system. Due to the Gly allele’s higher affinity for DA than the Ser allele, the presence of Gly may facilitate increased replication of the virus by methamphetamine, which, in turn, may induce greater cognitive dysfunction. Further research is required to determine the utility of DRD3 genotype as a biological marker in clinical settings.

The relationship between DRD3 and cognition has also been explored in alcohol-dependent subjects. Homozygosity for the DRD3 Ser9Gly polymorphism is significantly increased in alcohol-dependent individuals with low cognitive impulsiveness. Impulsivity secondary to alcohol intoxication may, in part, be related to increased DAergic activity, which was proposed to foster dependence in individuals with low cognitive impulsiveness (Limosin et al., 2005).

In summary, the findings from the few post-mortem, neuroimaging, and genetic studies point to a relationship between D₃ receptors and cognitive dysfunction in individuals with neuropsychiatric disorders.

6. Pharmacological dopamine D₃ receptor intervention and cognition in humans

Few data have been published on the effects of selective DA D₃ receptor antagonists on cognitive dysfunction in humans due to the lack of selective D₃ receptor antagonists available on the market. Although many antipsychotics have high affinity for D₃ as well as D₂ receptors, the high affinity of endogenous DA for D₃ receptors has been postulated to result in only minimal or no D₃ receptor occupancy by antipsychotics in DA rich areas (Graff-Guerrero et al., 2009; Gross and Drescher, 2012; Mizrahi et al., 2011; Schotte et al., 1996). Mugnaini et al. (2013) demonstrated that abstinent smokers took significantly longer to color-name words related to smoking than to color-name neutral control words in the Stroop test (Mugnaini et al., 2013). This attentional bias of the abstinent smokers was partially reversed by a selective D₃ receptor antagonist, GSK598809. ABT-925 is the only selective D₃ receptor antagonist that has survived for testing in a clinical phase II trial in individuals with schizophrenia. Although it did not show significant clinical improvement, positive effects were found in individuals with the G allele of the DRD3 Ser9Gly polymorphism (Bhathena et al., 2011). Further, ABT-925 showed significant effect on executive function and emotion recognition in the same sample (Gross et al., 2013). Most of the evidence is derived from the effects of preferential D₃ receptor-agonists, pramipexole (5- to 7-fold D₃/D₂ and D₄ receptor selectivity) (Brusa et al., 2003; Hubble, 2000; Rektorova et al., 2005; Samuels et al., 2006a, b, 2007) and rotigotine (20-fold D₃/D₂ and 100-fold D₃/D₁ receptor selectivity) (Bunten and Happe, 2006; Sanford and Scott, 2011; Scheller et al., 2009) on cognition in healthy individuals and those with PD or bipolar disorder. Thus far, the cognitive effects of preferential D₃ receptor agonists in humans are mixed (Brusa et al., 2005). Rotigotine, for example, did not significantly influence cognition in individuals with PD (Trenkwalder et al., 2011); while pramipexole improved working memory in individuals with cognitively-impaired PD (Costa et al., 2009) and euthymic bipolar disorder (Burdick et al., 2012). In other studies, pramipexole had deleterious or no effects on working memory in healthy individuals (Ersche et al., 2011; Hamidovic et al., 2008) and impaired reversal learning in individuals with PD (Cools et al., 2006). Moreover, ropinirole (10-fold D₃/D₂ and D₄ receptor selectivity) induced cognitive adverse events less frequently than pramipexole in individuals with early PD (Zagmutt and Tarrants, 2012). These inconsistent findings may derive from the fact that preferential D₃ receptor agonists have other mechanisms, such as 5-HT_1A agonism (e.g., rotigotine). Further, preferential D₃ receptor agonists can contribute to excess DAergic activity based on the baseline DAergic activity, which may have negative effects on cognitive function. For example, in individuals with PD there was an inverted U-shaped curve relationship between DAergic signaling and cognition (Svenningsson et al., 2012).

7. Pharmacological dopamine D₃ receptor intervention and cognition in animals

Numerous animal studies have explored the cognitive effects of pharmacological interventions targeting D₃ receptors. Table 1 summarizes the cognitive domains that have been investigated. Interventions have included both D₃ receptor agonists and antagonists (Gross and Drescher, 2012). Cognitive dysfunction was produced through several distinct perturbations, including the use of drugs such as scopolamine, inter-trial intervals, isolation rearing, and aging. D₃ receptor antagonists either enhanced or had no impact on cognitive performance, including the agents S33084 (Loiseau and Millan, 2009), S33138 (Millan and Brocco, 2008), SB277011 (Loiseau and Millan, 2009), (+)S14297 (Millan et al., 2007), nefadotride (Sigala et al., 1997), RGH-1756 (Laszy et al., 2005), U-99194A (Laszy et al., 2005), and RG-15 (Gyertyan et al., 2008). In contrast, the D₃ receptor agonists impaired cognitive performance in WT mice, including PD128907 (Watson et al., 2012a) and 7-OH-DPAT (Bernaerts and Tirelli, 2003). Moreover, the D₃ receptor antagonist, S33138, reversed age-related cognitive decline in a delayed matching-to-sample task of working memory in old rhesus monkeys, suggesting that D₃ receptor blockade is pro-cognitive independent of age (Millan et al., 2010). The specificity of D₃ receptors for their pro-cognitive effects is supported by the failure of L741,626, a preferential D₂ antagonist, to improve cognition after it was infused in the frontal cortex (Loiseau and Millan, 2009).

Table 1.

Pharmacological dopamine D3 receptor intervention and cognition in animals.

Procedure	Perturbation	Drugs	Effect on D3R	Species	Effects	Principle cognitive domain	References
Social recognition	Delay (2 h)	S33084, S33138, SB-277011, (+)S14297	Antagonist	Rat	Improvement of deficit	Mainly olfactory, memory (delay) and attention (scopolamine)	Millan et al. (2007, 2008), Millan and Brocco (2008), Loiseau and Millan (2009)
	Scopolamine	S33084, S33138, SB-277011, (+)S14297	Antagonist	Rat	Improvement of deficit		Millan et al. (2007, 2008), Millan and Brocco (2008)
	None	S33084, SB-277011	Antagonist	Rat	No effect		Millan et al. (2007), Watson et al. (2012a)
	None	PD128907, 7-OH-DPAT	Agonist	Rat	Impairment		Millan et al. (2007), Watson et al. (2012a)
Social discrimination	Delay (30 min)	S33084, S33138	Antagonist	Rat	Improvement of deficit	Selective attention (mainly olfactory)	Millan et al. (2010), Watson et al. (2012a
5-Choice serial reaction time	None	S33138	Antagonist	Rat	No effect	Attention (accuracy), speed of processing (latency)	Millan et al. (2008), Millan and Brocco (2008)
Passive avoidance	Scopolamine	S33138, Nefadotride	Antagonist	Mouse, rat	Improvement of deficit	Aversive/associative learning	Sigala et al. (1997), Millan et al. (2008), Millan and Brocco (2008)
	Dizocilipine	S33138	Antagonist	Mouse	Improvement of deficit		Millan et al. (2008), Millan and Brocco (2008)
	Delay (3 h)	7-OH-DPAT	Agonist	Chick	Impairment		Hale and Crowe (2002)
	Delay (3 h)	U-99194+7-OH-DPAT	Antagonist+agonist	Chick	No effect		Hale and Crowe (2002)
	None	S33138	Antagonist	Rat	No effect		Millan et al. (2008), Millan and Brocco (2008)
	None	U-99194A	Antagonist	Mouse	Enhancement		Bernaerts and Tirelli (2003)
	None	7-OH-DPAT	Agonist	Mouse	Impairment		Ukai (1997), Bernaerts and Tirelli (2003)
Object recognition	Isolation rearing	S33084, S33138	Antagonist	Rat	Improvement of deficit	Visual, declarative memory, attention	Watson et al. (2012b)
	Delay (4 h)	S33084, S33138	Antagonist	Rat	Improvement of deficit		Millan et al. (2010), Watson et al. (2012a)
	Delay (2 min)	PD128907	Agonist	Rat	Impairment		Watson et al. (2012a)
	Delay (2 min)	S33084+PD128907	Antagonist+agonist	Rat	No effect		Watson et al. (2012a)
Variable delayed response	Chronic, low-dose MPTP	S33138	Antagonist	Rhesus monkey	Improvement of deficit with short delays	Attention (short delays), spatial working memory (long delays)	Millan et al. (2010)
Attentional set shifting	Chronic, low-dose MPTP	S33138	Antagonist	Rhesus monkey	Improvement of deficit	Executive function (cognitive flexibility)	Millan et al. (2010)
Spatial discrimination and serial reversal learning	None	Nefadotride	Antagonist	Rat	No effect	Spatial reversal learning	Boulougouris et al. (2009)
Delayed matching to sample	Aging (27 years)	S33138	Antagonist	Rhesus monkey	Improvement of deficit with long delays	Visual working memory, attention	Millan et al. (2010)
Reversal visual object discrimination	None	7-OH-DPAT	Agonist	Marmoset	Impairment	Reversal object learning	Smith et al. (1999)
Water labyrinth	FG-7142	BP-897	Partial agonist	Rat	Improvement of deficit	Spatial learning	Laszy et al. (2005)
	Scopolamine	BP-897	Partial agonist	Rat	No effect		Laszy et al. (2005)
	FG-7142	SB277011, RGH-1756, U-99194A	Antagonist	Rat	Improvement of deficit		Laszy et al. (2005)
	Scopolamine/diazepam	SB277011, RGH-1756, U-99194A, RG-15	Antagonist	Rat	Improvement of deficit		Laszy et al. (2005), Gyertyan et al. (2008)

Abbreviations: D3R=dopamine D3 receptor.

D₃ receptors may also participate in the DA-related pro-cognitive effects of angiotensin AT4 receptor agonists through unclear mechanisms (Braszko, 2010). A partial D_3/2 receptor agonist that preferentially binds to presynaptic receptors abolished the memory-enhancing effects of angiotensin AT4 receptor agonists (Stragier et al., 2007).

In summary, animal studies suggest that D₃ receptor blockade enhances cognitive function, while D₃ receptor activation impairs cognition independent of age, which would justify clinical trials to examine the effect of D₃ receptor antagonists on neuropsychiatric disorders (Laszy et al., 2005).

8. Genetic dopamine D₃ receptor intervention (D₃ receptor knockout) and cognition in animals

The performance of D₃ receptor KO mice on various cognitive tasks, such as the two-choice perceptual discrimination test, SND, the step-through passive-avoidance test, T-maze and the delayed alternation test, were also examined to further characterize the roles of the D₃ receptor (Table 2) (Chourbaji et al., 2008; Glickstein et al., 2002, 2005; Micale et al., 2010; Watson et al., 2012a; Xing et al., 2010b).

Table 2.

Genetic dopamine D3 receptor intervention (D3R knockout) and cognition in animals.

Procedure	Perturbation	Mice	Drug	Effects	Principle cognitive domain	References
Social discrimination	Delay (1/2 h)	D3R KO	None	Enhancement	Selective attention (mainly olfactory)	Watson et al. (2012a)
	Delay (1/2 h)	D3R KO	S33084 (D3R antagonist)	No effect		Watson et al. (2012a)
	Delay (1/2 h)	WT	S33084 (D3R antagonist)	Improvement		Watson et al. (2012a)
Passive avoidance	None	D3R KO	None	Improvement	Aversive/associative learning	Micale et al. (2010)
Attentional set shifting	None	D3R KO	None	Enhancement	Executive function (cognitive flexibility)	Glickstein et al. 2005
T-maze	None	D3R KO	None	No effect	Spatial working memory	Chourbaji et al. (2008)
Delayed alternation	None	D3R KO	None	Impairment		Glickstein et al. 2002
Morris water maze	Aging	D3R KO	None	Enhancement		Xing et al. (2010a, 2010b)

Abbreviations: D3R=dopamine D3 receptor, KO=knockout, WT=wild type.

D₃ receptor KO mice showed better set-shifting and social discrimination than WT mice (Glickstein et al., 2005; Watson et al., 2012a). While the D₃ receptor antagonist, S33084, improved social discrimination in WT mice, it had no effect on the performance of the D₃ receptor KO mice (Watson et al., 2012a). This suggests that S33084-mediated improvement can be explained by D₃ receptor antagonism, and not by other non-specific effects of S33084. In addition, a stereological assessment of the set-shifting test-induced neuronal expression of c-fos in the anterior cingulate and prelimbic/infralimbic cortices revealed higher activation in the D₃ receptor KO mice than in the WT mice (Glickstein et al., 2005). These findings indicate that the increased set-shifting performance in the D₃ receptor KO mice correlates with the magnitude of the activation of the PFC.

Aged D₃ receptor KO mice displayed ameliorated age-related spatial memory dysfunction and smaller decreases in hippocampal CREB activation in comparison with aged WT mice (Xing et al., 2010b). Given that CREB may have neuroprotective effects and that the persistent elevation of hippocampal CREB may be associated with memory consolidation, these findings suggest that D₃ receptor-regulated CREB signaling in the hippocampus may be involved in these age-associated cognitive alterations (Xing et al., 2010b). In contrast, the D₃ receptor KO mice were impaired or showed no difference compared to the WT mice in spatial working memory (Chourbaji et al., 2008; Glickstein et al., 2002). Of note, spatial working memory was partially rescued by a D₁ receptor agonist, which caused a parallel increase in neuronal DRD3 Ser9Gly l c-fos expression in the PFC (Glickstein et al., 2002). This indicates that this cognitive task is partially dependent on optimal functioning of the D1R (Millan et al., 2010; Watson et al., 2012a).

In summary, D₃ receptor KO mice showed better performance than WT mice in measures of selective attention (mainly olfactory), aversive/associative learning, spatial memory and executive function (cognitive flexibility) independent of age. In addition, D₃ receptor antagonists enhanced the cognitive performance of the WT mice and did not affect the cognitive performance of the D₃ receptor KO mice, supporting the finding from pharmacological interventions that D₃ receptor blockade enhances cognitive function.

9. Implications for dopamine D₃ receptor antagonists as pro-cognitive agents

Deficits across multiple cognitive domains are common in neuropsychiatric disorders (Millan et al., 2012), contributing to worse clinical outcomes, social dysfunction, poorer QOL and greater caregivers’ burden (Demirtas-Tatlidede et al., 2013). Despite this impact, the cognitive enhancing effects of currently available treatments are limited. Various pro-cognitive drug candidates, including AZD3480 (Velligan et al., 2012), dimebon (Morozova et al., 2012), EVP-6124 (Prickaerts et al., 2012), rimonabant (Boggs et al., 2012), rosiglitazone (Yi et al., 2012) and TC-5619 (Lieberman et al., 2013), have been investigated in clinical trials for neuropsychiatric disorders; however, most of them have failed to show clinically relevant effects. Thus, there is an urgent need to identify new therapeutic strategies that target cognitive dysfunction. There is great heterogeneity in the pathological mechanisms underlying symptom expression in neuropsychiatric disorders. Even within the same condition, subtypes present symptomatically different with regard to the presence and severity of psychosis, mood, anxiety, and cognitive impairment (e.g. schizophrenia spectrum disorders), depending on individual host factors and age of illness onset (Millan et al., 2012). Likewise, cognitive dysfunction is common to different neuropsychiatric disorders, suggesting that treatment interventions aimed at specific targets may be effective for cognitive dysfunction common across the different disorders (Millan et al., 2012).

As described in the present review, D₃ receptor antagonists hold promise as pro-cognitive drugs across a variety of cognitive domains, and are particularly attractive for age-related cognitive dysfunction, due to the improved side effect profile over traditional DA antagonists, which are associated with extrapyramidal symptoms, tardive dyskinesia, and metabolic effects (Millan and Brocco, 2008). Selective D₃ receptor antagonists were not associated with catalepsy (Millan et al., 1995, 2000; Reavill et al., 2000) and were shown to counteract this effect of haloperidol in rodents (Gyertyan and Saghy, 2007). Thus, unlike most dopamine blocking agents, selective D₃ receptor antagonists do not appear to be associated with negative/cognitive symptoms and may also treat these adverse effects attributable to antipsychotics (Gross and Drescher, 2012).

This review has to be considered in light of the limitations within the literature. First, this review only focuses on the relationship between D₃ receptors and cognition. Many other mechanisms, from receptors to complex networks, should be taken into consideration when exploring cognitive treatments for neuropsychiatric disorders, assuming the possibility that some cognitive domains may be related to D₃ receptor signaling, while others may not, such as exploratory abilities and hippocampus-dependent spatial learning (Xing et al., 2010a; Zhu et al., 2010). Thus, D₃ receptor antagonists should also be studied as adjuncts to other cognitive treatments. Second, few studies have specifically addressed the relationship between D₃ receptors and cognition in humans. Most of the studies referred to in this review were animal studies that used cognitive modeling procedures, which may not reflect human cognition. Further, few animal models of cognitive dysfunction in the context of neuropsychopathology exist compared with those available for normal cognition. This hampers direct translation from animal models to real world clinical settings. Third, the results obtained from D₃ receptor KO mice may be due to the effects of a lack of D₃ receptors on adaptive developmental processes (Glickstein et al., 2005). Fourth, although D₃ receptor antagonists are promising as pro-cognitive drugs, few data have been published on the effects of selective D₃ receptor antagonists on cognitive dysfunction in humans. This is likely due to the lack of selective D₃ receptor antagonists available on the market.

In conclusion, human studies indicate that a relationship exists between D₃ receptors and cognitive dysfunction in individuals with neuropsychiatric disorders. Preclinical studies suggest that D₃ receptor antagonists may improve cognitive performance by enhancing the release of ACh in the PFC, disinhibiting the activity of DA neurons projecting to the NAc or PFC, or activating CREB signaling in the hippocampus. Given that currently available treatments have limited value for the management of cognitive dysfunction, D₃ receptor antagonists are worthy candidates for the enhancement of cognitive function in individuals with neuropsychiatric disorders. Clinical trials are needed to confirm their effects.