Role of the dopamine D3 receptor in reactivity to cocaine-associated cues in mice

Enhancement of memory consolidation by an avoidance conditioned stimulus: Modulation by the D3 receptor

Conditioned stimuli (CS) paired with foot-shock can enhance memory consolidation. Because the dopamine D3R has been implicated in mediating various responses to CSs, the current study explored its potential role in modulation of memory consolidation by an avoidance CS. Male Sprague-Dawley rats trained to avoid foot-shocks in a two-way signalled active avoidance task (8 sessions, 30 trials per session, 0.8 mA foot-shock) were pre-treated with the D3R antagonist NGB-2904 (Vehicle, 0.1 or 5 mg/kg) and exposed to the CS immediately after the sample phase of an object recognition memory task. Discrimination ratios were assessed 72 h later. Immediate, but not delayed (6 h), post-sample exposure to the CS enhanced object recognition memory and this effect was dose-dependently blocked by NGB-2904. Control experiments with the beta-noradrenergic receptor antagonist propranolol (10 or 20 mg/kg) and D2R antagonist pimozide (0.2 or 0.6 mg/kg) indicated that NGB-2904 targeted post-training memory consolidation. Exploring the pharmacological selectivity of the D3R effect, it was found that: 1) 5 mg/kg NGB-2904 blocked conditioned memory modulation produced by post-sample exposure to a "weak" CS (one day of avoidance training) and concurrent stimulation of catecholamine activity by 10 mg/kg bupropion; 2) post-sample exposure to a "weak" CS and concurrent administration of the D3R agonist 7-OH-DPAT (1 mg/kg) enhanced consolidation of object memory. Finally, because 5 mg/kg NGB-2904 had no effect on modulation by avoidance training in the presence of foot-shocks, the findings herein support the hypothesis that the D3R plays an important role in modulation of memory consolidation by CSs.

Selective D3 receptor antagonism modulates neural response during negative emotional processing in substance dependence

Introduction

Negative affective states contribute to the chronic-relapsing nature of addiction. Mesolimbic dopamine D3 receptors are well placed to modulate emotion and are dysregulated in substance dependence. Selective antagonists might restore dopaminergic hypofunction, thus representing a potential treatment target. We investigated the effects of selective D3 antagonist, GSK598809, on the neural response to negative emotional processing in substance dependent individuals and healthy controls.

Methodology

Functional MRI BOLD response was assessed during an evocative image task, 2 h following acute administration of GSK598809 (60 mg) or placebo in a multi-site, double-blind, pseudo-randomised, cross-over design. Abstinent drug dependent individuals (DD, n = 36) comprising alcohol-only (AO, n = 19) and cocaine-alcohol polydrug (PD, n = 17) groups, and matched controls (n = 32) were presented with aversive and neutral images in a block design (contrast of interest: aversive > neutral). Whole-brain mixed-effects and a priori ROI analyses tested for group and drug effects, with identical models exploring subgroup effects.

Results

No group differences in task-related BOLD signal were identified between DD and controls. However, subgroup analysis revealed greater amygdala/insular BOLD signal in PD compared with AO groups. Following drug administration, GSK598809 increased BOLD response across HC and DD groups in thalamus, caudate, putamen, and pallidum, and reduced BOLD response in insular and opercular cortices relative to placebo. Multivariate analyses in a priori ROIs revealed differential effects of D3 antagonism according to subgroup in substantia nigra; GSK598809 increased BOLD response in AO and decreased response in PD groups.

Conclusion

Acute GSK598809 modulates the BOLD response to aversive image processing, providing evidence that D3 antagonism may impact emotional regulation. Enhanced BOLD response within D3-rich mesolimbic regions is consistent with its pharmacology and with attenuation of substance-related hypodopaminergic function. However, the lack of group differences in task-related BOLD response and the non-specific effect of GSK598809 between groups makes it difficult to ascertain whether D3 antagonism is likely to be normalising or restorative in our abstinent populations. The suggestion of differential D3 modulation between AO and PD subgroups is intriguing, raising the possibility of divergent treatment responses. Further study is needed to determine whether D3 antagonism should be recommended as a treatment target in substance dependence.

A new class of Benzothiophene morpholine analogues with high selectivity and affinity were designed and evaluated for anti-drug addiction

To probe the mechanism of dopamine receptors in drug addiction and look for potential new methods for treating this disease, we have designed and synthesized benzothiophene morpholine analogues that were considered as dopamine D3 receptor selective ligands. Radioligand binding assay was used to determine the binding affinity of target compounds. Members of this class have great selectivity and binding affinity in D3 receptor. In addition, the ability of these compounds to mitigate the symptoms of addiction from opioids was investigated in animal behavior patterns, and we have found that two compounds (18a and 18d) have good affinity in the D3R and exhibit the efficacy of anti-drug addiction in morphine dependent mice induced by naloxone.

Identification of the Risk Genes Associated With Vulnerability to Addiction: Major Findings From Transgenic Animals

Understanding risk factors for substance use disorders (SUD) can facilitate medication development for SUD treatment. While a rich literature exists discussing environmental factors that influence SUD, fewer articles have focused on genetic factors that convey vulnerability to drug use. Methods to identify SUD risk genes include Genome-Wide Association Studies (GWAS) and transgenic approaches. GWAS have identified hundreds of gene variants or single nucleotide polymorphisms (SNPs). However, few genes identified by GWAS have been verified by clinical or preclinical studies. In contrast, significant progress has been made in transgenic approaches to identify risk genes for SUD. In this article, we review recent progress in identifying candidate genes contributing to drug use and addiction using transgenic approaches. A central hypothesis is if a particular gene variant (e.g., resulting in reduction or deletion of a protein) is associated with increases in drug self-administration or relapse to drug seeking, this gene variant may be considered a risk factor for drug use and addiction. Accordingly, we identified several candidate genes such as those that encode dopamine D₂ and D₃ receptors, mGluR₂, M₄ muscarinic acetylcholine receptors, and α₅ nicotinic acetylcholine receptors, which appear to meet the risk-gene criteria when their expression is decreased. Here, we describe the role of these receptors in drug reward and addiction, and then summarize major findings from the gene-knockout mice or rats in animal models of addiction. Lastly, we briefly discuss future research directions in identifying addiction-related risk genes and in risk gene-based medication development for the treatment of addiction.

A Historical Perspective on the Dopamine D3 Receptor

Before 1990, the multiplicity of dopamine receptors beyond D1 and D2 had remained a controversial concept, despite its substantial clinical implications, at a time when it was widely accepted that dopamine interacted with only two receptor subtypes, termed D1 and D2, differing one from the other by their pharmacological specificity and opposite effects on adenylyl cyclase. It was also generally admitted that the therapeutic efficacy of antipsychotics resulted from blockade of D2 receptors. Thanks to molecular biology techniques, the D3 receptor could be characterized as a distinct molecular entity having a restricted anatomical gene expression and different signaling, which could imply peculiar functions in controlling cognitive and emotional behaviors. Due to the structural similarities of D2 and D3 receptors, the search for D3-selective compounds proved to be difficult, but nevertheless led to the identification of fairly potent and in vitro and in vivo selective compounds. The latter permitted to confirm a role of D3 receptors in motor functions, addiction, cognition, and schizophrenia, which paved the way for the development of new drugs for the treatment of psychiatric disorders.

Design, synthesis and preliminary bioactivity evaluation of bitopic benzopyranomorpholine analogues as selective dopamine D3 receptor ligands as anti-drug addiction therapeutic agents

Three series of bitopic benzopyranomorpholine analogues were designed, synthesized, and evaluated as a novel class of selective ligands for the dopamine D3 receptor. Binding affinities of target compounds were determined using the method of radioligand binding assay. Most compounds demonstrated considerable binding affinities and selectivity for D3 receptor. Besides, the compounds were screened for their ability to alleviate withdrawal symptoms of opioid addiction in animal behavioral models. The results showed that compound 20h displayed nanomolar affinity for the D3R, and exhibited anti-drug addiction efficacy in the animal model of of naloxone-induced withdrawal symptoms in morphine-dependent mice.

Distinct Regulation of Dopamine D3 Receptor in the Basolateral Amygdala and Dentate Gyrus during the Reinstatement of Cocaine CPP Induced by Drug Priming and Social Stress

Relapse in the seeking and intake of cocaine is one of the main challenges when treating its addiction. Among the triggering factors for the recurrence of cocaine use are the re-exposure to the drug and stressful events. Cocaine relapse engages the activity of memory-related nuclei, such as the basolateral amygdala (BLA) and the hippocampal dentate gyrus (DG), which are responsible for emotional and episodic memories. Moreover, D3 receptor (D3R) antagonists have recently arisen as a potential treatment for preventing drug relapse. Thus, we have assessed the impact of D3R blockade in the expression of some dopaminergic markers and the activity of the mTOR pathway, which is modulated by D3R, in the BLA and DG during the reinstatement of cocaine-induced conditioned place preference (CPP) evoked by drug priming and social stress. Reinstatement of cocaine CPP paralleled an increasing trend in D3R and dopamine transporter (DAT) levels in the BLA. Social stress, but not drug-induced reactivation of cocaine memories, was prevented by systemic administration of SB-277011-A (a selective D3R antagonist), which was able, however, to impede D3R and DAT up-regulation in the BLA during CPP reinstatement evoked by both stress and cocaine. Concomitant with cocaine CPP reactivation, a diminution in mTOR phosphorylation (activation) in the BLA and DG occurred, which was inhibited by D3R blockade in both nuclei before the social stress episode and only in the BLA when CPP reinstatement was provoked by a cocaine prime. Our data, while supporting a main role for D3R signalling in the BLA in the reactivation of cocaine memories evoked by social stress, indicate that different neural circuits and signalling mechanisms might mediate in the reinstatement of cocaine-seeking behaviours depending upon the triggering stimuli.

New Drugs, Old Targets: Tweaking the Dopamine System to Treat Psychostimulant Use Disorders

The abuse of illicit psychostimulants such as cocaine and methamphetamine continues to pose significant health and societal challenges. Despite considerable efforts to develop medications to treat psychostimulant use disorders, none have proven effective, leaving an underserved patient population and unanswered questions about what mechanism(s) of action should be targeted for developing pharmacotherapies. As both cocaine and methamphetamine rapidly increase dopamine (DA) levels in mesolimbic brain regions, leading to euphoria that in some can lead to addiction, targets in which this increased dopaminergic tone may be mitigated have been explored. Further, understanding and targeting mechanisms underlying relapse are fundamental to the success of discovering medications that reduce the reinforcing effects of the drug of abuse, decrease the negative reinforcement or withdrawal/negative affect that occurs during abstinence, or both. Atypical inhibitors of the DA transporter and partial agonists/antagonists at DA D3 receptors are described as two promising targets for future drug development.

(±)VK4-40, a novel dopamine D3 receptor partial agonist, attenuates cocaine reward and relapse in rodents

BACKGROUND AND PURPOSE

Despite widespread abuse of cocaine, there are no approved treatments for cocaine use disorder. Chronic cocaine use is associated with up-regulated dopamine D₃ receptor expression in the brain. Therefore, most D₃ -based medication development has focused on D₃ antagonists. However, D₃ antagonists do not attenuate cocaine intake under "easy" self-administration conditions, when response requirements are low. We evaluated a novel, highly selective and metabolically stable D₃ partial agonist, (±)VK4-40, for its efficacy in reducing cocaine intake and relapse to drug seeking.

EXPERIMENTAL APPROACH

The impact of (±)VK4-40 on cocaine intake and relapse was evaluated using intravenous self-administration procedures under a fixed-ratio 2 reinforcement schedule and cocaine-primed reinstatement conditions in rats. Optogenetic brain-stimulation reward procedures were used to evaluate the interaction of (±)VK4-40 and cocaine in the mesolimbic dopamine system in mice. Sucrose self-administration in rats and a conditioned place preference paradigm in mice were used to evaluate the abuse potential of (±)VK4-40 alone and other unwanted effects.

KEY RESULTS

(±)VK4-40 dose-dependently reduced cocaine self-administration and cocaine-primed reinstatement of drug-seeking behaviour. (±)VK4-40 also inhibited cocaine-enhanced brain-stimulation reward caused by optogenetic stimulation of dopamine neurons in the ventral tegmental area. (±)VK4-40 alone decreased brain-stimulation reward but produced neither conditioned place preference nor place aversion. This new D₃ partial agonist also failed to alter oral sucrose self-administration.

CONCLUSION AND IMPLICATIONS

The novel D₃ partial agonist, (±)VK4-40 attenuates cocaine reward and relapse in rodents, without significant unwanted effects. These findings support further investigation of D₃ partial agonists as putative treatments for cocaine use disorder.

Dopamine D3 receptor-based medication development for the treatment of opioid use disorder: Rationale, progress, and challenges

Opioid abuse and related overdose deaths continue to rise in the United States, contributing to the current national opioid crisis. Although several opioid-based pharmacotherapies are available (e.g., methadone, buprenorphine, naloxone), they show limited effectiveness in long-term relapse prevention. In response to the opioid crisis, the National Institute on Drug Abuse proposed a list of pharmacological targets of highest priority for medication development for the treatment of opioid use disorders (OUD). Among these are antagonists of dopamine D3 receptors (D3R). In this review, we first review recent progress in research of the dopamine hypothesis of opioid reward and abuse and then describe the rationale and recent development of D3R ligands for the treatment of OUD. Herein, an emphasis is placed on the effectiveness of newly developed D3R antagonists in the animal models of OUD. These new drug candidates may also potentiate the analgesic effects of clinically used opioids, making them attractive as adjunctive medications for pain management and treatment of OUD.

Prenatal nicotine sex-dependently alters adolescent dopamine system development

Despite persistent public health initiatives, many women continue to smoke during pregnancy. Since maternal smoking has been linked to persisting sex-dependent neurobehavioral deficits in offspring, some consider nicotine to be a safer alternative to tobacco during pregnancy, and the use of electronic nicotine delivery systems is on the rise. We presently show, however, that sustained exposure to low doses of nicotine during fetal development, approximating plasma levels seen clinically with the nicotine patch, produces substantial changes in developing corticostriatal dopamine systems in adolescence. Briefly, pregnant dams were implanted on gestational day 4 with an osmotic minipump that delivered either saline (GS) or nicotine (3 mg/kg/day) (GN) for two weeks. At birth, pups were cross-fostered with treatment naïve dams and were handled daily. Biochemical analyses, signaling assays, and behavioral responses to cocaine were assessed on postnatal day 32, representative of adolescence in the rodent. GN treatment had both sex-dependent and sex-independent effects on prefrontal dopamine systems, altering Catechol-O-methyl transferase (COMT)-dependent dopamine turnover in males and norepinephrine transporter (NET) binding expression in both sexes. GN enhanced cocaine-induced locomotor activity in females, concomitant with GN-induced reductions in striatal dopamine transporter (DAT) binding. GN enhanced ventral striatal D2-like receptor expression and G-protein coupling, while altering the roles of D2 and D3 receptors in cocaine-induced behaviors. These data show that low-dose prenatal nicotine treatment sex-dependently alters corticostriatal dopamine system development, which may underlie clinical deficits seen in adolescents exposed to tobacco or nicotine in utero.

Experience during adolescence shapes brain development: From synapses and networks to normal and pathological behavior

Adolescence is a period of dramatic neural reorganization creating a period of vulnerability and the possibility for the development of psychopathology. The maturation of various neural circuits during adolescence depends, to a large degree, on one's experiences both physical and psychosocial. This occurs through a process of plasticity which is the structural and functional adaptation of the nervous system in response to environmental demands, physiological changes and experiences. During adolescence, this adaptation proceeds upon a backdrop of structural and functional alterations imparted by genetic and epigenetic factors and experiences both prior to birth and during the postnatal period. Plasticity entails an altering of connections between neurons through long-term potentiation (LTP) (which alters synaptic efficiency), synaptogenesis, axonal sprouting, dendritic remodeling, neurogenesis and recruitment (Skaper et al., 2017). Although most empirical evidence for plasticity derives from studies of the sensory systems, recent studies have suggested that during adolescence, social, emotional, and cognitive experiences alter the structure and function of the networks subserving these domains of behavior. Each of these neural networks exhibits heightened vulnerability to experience-dependent plasticity during the sensitive periods which occur in different circuits and different brain regions at specific periods of development. This report will summarize some examples of adaptation which occur during adolescence and some evidence that the adolescent brain responds differently to stimuli compared to adults and children. This symposium, "Experience during adolescence shapes brain development: from synapses and networks to normal and pathological behavior" occurred during the Developmental Neurotoxicology Society/Teratology Society Annual Meeting in Clearwater Florida, June 2018. The sections will describe the maturation of the brain during adolescence as studied using imaging technologies, illustrate how plasticity shapes the structure of the brain using examples of pathological conditions such as Tourette's' syndrome and attention deficit hyperactivity disorder, and a review of the key molecular systems involved in this plasticity and how some commonly abused substances alter brain development. The role of stimulants used in the treatment of attention deficit hyperactivity disorder (ADHD) in the plasticity of the reward circuit is then described. Lastly, clinical data promoting an understanding of peer-influences on risky behavior in adolescents provides evidence for the complexity of the roles that peers play in decision making, a phenomenon different from that in the adult. Imaging studies have revealed that activation of the social network by the presence of peers at times of decision making is unique in the adolescent. Since normal brain development relies on experiences which alter the functional and structural connections between cells within circuits and networks to ultimately alter behavior, readers can be made aware of the myriad of ways normal developmental processes can be hijacked. The vulnerability of developing adolescent brain places the adolescent at risk for the development of a life time of abnormal behaviors and mental disorders.

Modulation of stress- and cocaine prime-induced reinstatement of conditioned place preference after memory extinction through dopamine D3 receptor

Accumulating evidence indicates that dopamine (DA) D3 receptor (DAD3R) antagonists appear highly promising in attenuating cocaine reward and relapse in preclinical models of addiction. In the present study, we investigated the effects of the selective DAD3R antagonist SB-277011-A on the reinstatement of cocaine-induced conditioned place preference (CPP) produced by a priming dose of cocaine, by social defeat stress and by two kinds of physiological stressors (restraint and tail pinch) in male adult mice. We also explored reinstatement-related plasma corticosterone levels (as marker of stress response) and the effects of blocking DAD3R. Administration of SB-277011-A (24 or 48 mg/kg i.p.) did not modify conditioned reinstatement of cocaine seeking triggered by cocaine prime. By contrast, we found that the vulnerability to reinstatement of the CPP of defeated animals that have undergone CPP extinction was abolished by the DAD3R antagonist (24 mg/kg) given 30 min before the test session. Reactivation of the CPP response produced by physiological stress stimuli was also attenuated by SB-277011-A (48 mg/kg i.p.). On the other hand, the blockade of DAD3R significantly prevented the increased corticosterone release during reinstatement of cocaine-induced CPP that was seen in social defeated animals, in mice suffering physiological stress and after cocaine prime. Present results demonstrate a modulation by DAD3R of the reactivation of the incentive value of cocaine-associated cues induced by social and physiological stress stimuli, which was associated to a glucocorticoid-dependent mechanism. Our results also point to a possible potential therapeutic use of selective DAD3R antagonists for the prevention of stress-induced cocaine-seeking and relapse.

Selective dopamine D3 receptor antagonist YQA14 inhibits morphine-induced behavioral sensitization in wild type, but not in dopamine D3 receptor knockout mice

Increasing preclinical evidence demonstrates that dopamine D3 receptor (D3R) antagonists are a potential option for the treatment of drug addiction. The reinstatement of the addiction can be triggered by environmental stimuli that acquire motivational salience through repeated associations with the drug's effects. YQA14 is a novel D3R antagonist that has exhibited pharmacotherapeutic efficacy in reducing cocaine and amphetamine reward and relapse to drug seeking in mice. In this study we investigated the effects of YQA14 on morphine-induced context-specific locomotor sensitization in mice. We showed that repeated injection of YQA14 (6.25-25 mg/kg every day ip) prior to morphine (10 mg/kg every day sc) not only inhibited the acquisition, but also significantly attenuated the expression of morphine-induced locomotor sensitization. Furthermore, in the expression phase, one single injection of YQA14 (6.25-25 mg/kg, ip) dose-dependently inhibited the expression of morphine-induced behavioral sensitization. Moreover, YQA14 inhibited the expression of morphine-induced behavioral sensitization in wild mice (WT), but not in D3R knockout (D3R^-/-) mice in the expression phase. In addition, D3R^-/- mice also displayed the reduction in the expression phase compared with WT mice. In summary, this study demonstrates that blockade or knockout of the D3R inhibits morphine-induced behavior sensitization, suggesting that D3R plays an important role in the pathogenesis and etiology of morphine addiction, and it might be a potential target for clinical management of opioid addiction.

Dopamine D1 and D3 receptor polypharmacology as a potential treatment approach for substance use disorder

In the search for efficacious pharmacotherapies to treat cocaine addiction much attention has been given to agents targeting dopamine D1 or D3 receptors because of the involvement of these receptors in drug-related behaviors. D1-like and D3 receptor partial agonists and antagonists have been shown to reduce drug reward, reinstatement of drug seeking and conditioned place preference in rodents and non-human primates. However, translation of these encouraging results to clinical settings has been limited due to a number of factors including toxicity, poor pharmacokinetic properties and extrapyramidal and sedative side effects. This review highlights the role of D1 and D3 receptors in drug reward and seeking, the discovery of D1-D3 heteromers and their potential as targets in the treatment of addiction.

The effects of buspirone on occupancy of dopamine receptors and the rat gambling task

BACKGROUND

The dopamine D3 receptor (DRD₃) has been proposed as a target for drug development for the treatment of addictive disorders. Recently, the anxiolytic buspirone has been shown to have affinity for DRD₃ and DRD₄, and interest in repurposing it for addictive disorders has grown.

METHODS

Binding of [³H]-(+)-PHNO in the rat cerebellum and striatum was used to measure occupancy by buspirone of DRD₃ or DRD₂, respectively. Effects of buspirone in the rat gambling task (rGT) and the five-choice serial reaction time task (5-CSRTT) were examined.

RESULTS

Buspirone occupied both the DRD₂ and DRD₃ at high doses and the DRD₃, but not the DRD₂, in the narrow dose range of 3 mg/kg. At 10 mg/kg, a disruption of performance on rGT was observed. All measures of performance on the rGT, except for perseverations, were affected at 3 mg/kg. On the 5-CSRTT, omissions were increased. Impairments in the rGT were not mimicked by the effects induced by satiation. Further, buspirone did not impair food-maintained responding under a progressive ratio schedule of reinforcement at any dose, suggesting that the effects of buspirone on the rGT cannot be explained by non-selective actions.

CONCLUSIONS

Although buspirone had effects on the rGT at the dose that selectively occupied the DRD₃, the effects found do not parallel those found in previous studies of the effects of selective DRD₃ antagonists on the rGT. Thus, buspirone may impair performance on the rGT through actions at multiple receptor sites.

Cocaine-induced synaptic structural modification is differentially regulated by dopamine D1 and D3 receptors-mediated signaling pathways

Synaptic plasticity plays a critical role in cocaine addiction. The dopamine D1 and D3 receptors differentially regulate the cocaine-induced gene expression, structural remodeling and behavioral response. However, how these two receptors coordinately mediate the ultra-structural changes of synapses after cocaine exposure and whether these changes are behaviorally relevant are still not clear. Here, using quantitative electron microscopy, we show that D1 and D3 receptors have distinct roles in regulating cocaine-induced ultra-structural changes of synapses in the nucleus accumbens and caudoputamen. Pre-treatment of cocaine-treated mice with D3 receptor antagonist NGB2904 resulted in an increase in the ratio of total and asymmetric synapse to neuron and in the length of postsynaptic densities, compared with cocaine treatment alone. In contrast, pre-treatment of cocaine-treated mice with D1 receptor antagonist SCH23390 caused a reduction in synapse-to-neuron ratio and in postsynaptic densities length. Similarly, NGB2904 and SCH23390 showed opposite/differential effects on cocaine-induced structural plasticity, conditioned place preference and locomotor activity and signaling activation, including the activation of ERK, CREB and NR1 and the expression of c-fos and Cdk5. Therefore, we provide direct electron microscopy evidence that dopamine D1 and D3 receptors reciprocally regulate the ultra-structural changes of synapses following chronic exposure to cocaine. In addition, our data suggest that D1 and D3 receptors may regulate cocaine-induced ultra-structural changes and behavior responses by impact on structural plasticity and signaling transduction.

Opiate exposure state controls dopamine D3 receptor and cdk5/calcineurin signaling in the basolateral amygdala during reward and withdrawal aversion memory formation

The dopamine (DA) D3 receptor (D3R) is highly expressed in the basolateral nucleus of the amygdala (BLA), a neural region critical for processing opiate-related reward and withdrawal aversion-related memories. Functionally, D3R transmission is linked to downstream Cdk5 and calcineurin signaling, both of which regulate D3R activity states and play critical roles in memory-related synaptic plasticity. Previous evidence links D3R transmission to opiate-related memory processing, however little is known regarding how chronic opiate exposure may alter D3R-dependent memory mechanisms. Using conditioned place preference (CPP) and withdrawal aversion (conditioned place aversion; CPA) procedures in rats, combined with molecular analyses of BLA protein expression, we examined the effects of chronic opiate exposure on the functional role of intra-BLA D3R transmission during the acquisition of opiate reward or withdrawal aversion memories. Remarkably, we report that the state of opiate exposure during behavioural conditioning (opiate-naïve/non-dependent vs. chronically exposed and in withdrawal) controlled the functional role of intra-BLA D3R transmission during the acquisition of both opiate reward memories and withdrawal-aversion associative memories. Thus, whereas intra-BLA D3R blockade had no effect on opiate reward memory formation in the non-dependent state, blockade of intra-BLA D3R transmission prevented the formation of opiate reward and withdrawal aversion memory in the chronically exposed state. This switch in the functional role of D3R transmission corresponded to significant increases in Cdk5 phosphorylation and total expression levels of calcineurin, and a corresponding decrease in intra-BLA D3R expression. Inhibition of either intra-BLA Cdk5 or calcineurin reversed these effects, switching intra-BLA associative memory formation back to a D3R-independent mechanism.

Effects of Acute and Chronic Treatments with Dopamine D2 and D3 Receptor Ligands on Cocaine versus Food Choice in Rats

Dopamine D₃ receptor ligands are potential medications for psychostimulant addiction. Medication assessment may benefit from preclinical studies that evaluate chronic medication effects on choice between an abused drug and an alternative, nondrug reinforcer. This study compared acute and chronic effects of dopamine D₂- and D₃-preferring ligands on choice between intravenous cocaine and palatable food in rats. Under baseline conditions, cocaine maintained dose-dependent increases in cocaine choice and reciprocal decreases in food choice. Acutely, the D₂ agonist R-(-)-norpropylapomorphine (NPA) and antagonist L-741,626 [3-[[4-(4-chlorophenyl)-4-hydroxypiperidin-l-yl]methyl-1H-indole] produced leftward and rightward shifts in cocaine dose-effect curves, respectively, whereas the partial agonist terguride had no effect. All three drugs dose-dependently decreased food-maintained responding. Chronically, the effects of R-(-)-norpropylapomorphine and L-741,626 on cocaine self-administration showed marked tolerance, whereas suppression of food-reinforced behavior persisted. Acute effects of the D₃ ligands were less systematic and most consistent with nonselective decreases in cocaine- and food-maintained responding. Chronically, the D₃ agonist PF-592,379 [5-[(2R,5S)-5-methyl-4-propylmorpholin-2-yl]pyridin-2-amine] increased cocaine choice, whereas an intermediate dose of the D₃ antagonist PG01037 [N-[(E)-4-[4-(2,3-dichlorophenyl)piperazin-1-yl]but-2-enyl]-4-pyridin-2-ylbenzamide] produced a therapeutically desirable decrease in cocaine choice early in treatment; however, tolerance to this effect developed, and lower and higher doses were ineffective. D₃ ligands failed to significantly modify total cocaine intake but caused persistent decreases in food intake. Thus, D₂-and D₃-preferring ligands showed distinct profiles, consistent with different pharmacological actions. In addition, these results highlight the role of acute versus chronic treatment as a determinant of test drug effects. With the possible exception of the D₃ antagonist PG01037, no ligand was promising in terms of cocaine addiction treatment.

Examining the role of dopamine D2 and D3 receptors in Pavlovian conditioned approach behaviors

Elucidating the neurobiological mechanisms underlying individual differences in the extent to which reward cues acquire the ability to act as incentive stimuli may contribute to the development of successful treatments for addiction and related disorders. We used the sign-tracker/goal-tracker animal model to examine the role of dopamine D2 and D3 receptors in the propensity to attribute incentive salience to reward cues. Following Pavlovian training, wherein a discrete lever-cue was paired with food reward, rats were classified as sign- or goal-trackers based on the resultant conditioned response. We examined the effects of D2/D3 agonists, 7-OH-DPAT (0.01-0.32mg/kg) or pramipexole (0.032-0.32mg/kg), the D2/D3 antagonist raclopride (0.1mg/kg), and the selective D3 antagonist, SB-277011A (6 or 24mg/kg), on the expression of sign- and goal-tracking conditioned responses. The lever-cue acquired predictive value and elicited a conditioned response for sign- and goal-trackers, but only for sign-trackers did it also acquire incentive value. Following administration of either 7-OH-DPAT, pramipexole, or raclopride, the performance of the previously acquired conditioned response was attenuated for both sign- and goal-trackers. For sign-trackers, the D2/D3 agonist, 7-OH-DPAT, also attenuated the conditioned reinforcing properties of the lever-cue. The selective D3 antagonist did not affect either conditioned response. Alterations in D2/D3 receptor signaling, but not D3 signaling alone, transiently attenuate a previously acquired Pavlovian conditioned response, regardless of whether the response is a result of incentive motivational processes. These findings suggest activity at the dopamine D2 receptor is critical for a reward cue to maintain either its incentive or predictive qualities.

The dopamine D3 receptor antagonist, SR 21502, facilitates extinction of cocaine conditioned place preference

BACKGROUND

Pharmacotherapeutic agents that could facilitate extinction of cocaine cues would be useful in the treatment of cocaine addiction. We tested whether SR 21502, a selective dopamine (DA) D3 receptor antagonist, can facilitate extinction of cocaine conditioned place preference (CPP) in rats.

METHODS

In experiment 1, cocaine (10mg/kg) CPP was first established and then extinguished. During the extinction phase the rats were injected with SR 21502 and placed in the previously cocaine-paired compartment for four sessions and vehicle in the other compartment on four alternating sessions. The rats were then tested again for cocaine CPP. In experiment 2, different groups of rats were trained to associate SR 21502 with one compartment and saline with the other.

RESULTS

In experiment 1, the animals spent significantly more time in the cocaine-paired compartment after cocaine conditioning than they did before conditioning. Subsequently, the animals treated with SR 21502 during the extinction phase spent significantly less time in the cocaine-paired compartment than the vehicle group. In experiment 2, animals conditioned with SR 21502 preferred neither side of the CPP apparatus, indicating that SR 21502 produced no effects of its own.

CONCLUSIONS

These findings suggest that treatment with SR 21502, a DA D3 receptor antagonist, in the presence of cocaine cues can facilitate extinction of cocaine CPP and further suggest that this compound might be an effective cocaine addiction treatment.

The selective dopamine D3 receptor antagonist, SR 21502, reduces cue-induced reinstatement of heroin seeking and heroin conditioned place preference in rats

BACKGROUND

Because the role of dopamine (DA) D3 receptors has been investigated primarily in relation to cocaine-related behaviors little is known of the role of these receptors in heroin seeking.

PURPOSES

To investigate the effect of the selective DA D3 receptor antagonist, SR 21502, on cue-induced reinstatement of heroin seeking and heroin conditioned place preference (CPP).

METHODS

In experiment 1, rats were trained to self-administer intravenous heroin for 15 days followed by extinction. Following extinction animals were treated with one of several SR 21502 doses (0, 7.5, 10 or 15mg/kg) and a cue-induced reinstatement test was conducted. In experiment 2, animals were conditioned to experience heroin in one compartment of a CPP apparatus and saline in the other. On the test day animals were treated with 0, 3.75, 7.5, 10 or 15mg/kg of SR 21502 and tested for their CPP.

RESULTS

The results from experiment 1 showed a significant dose-related reduction in cue-induced reinstatement of active lever pressing in the 7.5 and 10mg groups and an absence of the reinstatement effect in the 15mg group. In experiment 2, animals treated with vehicle or 3.75mg of SR 21502 showed significant heroin place preferences but those treated with the higher doses showed no CPP.

CONCLUSIONS

Our findings suggest that DA D3 receptors play a significant role in heroin approach behaviors driven by conditioned stimuli. As such, we propose that SR 21502 holds potential as an effective pharmacotherapeutic agent for relapse prevention and should be studied further.

Neuronal circuitry underlying the impact of D3 receptor ligands in drug addiction

Since the cloning of the D3 receptor in the early 1990s, there has been a great deal of interest in this receptor as a possible therapeutic target for drug addiction. The development of a D3 ligand suitable for use in humans has remained elusive, so the study of the function of the D3 receptor and its possible therapeutic efficacy has largely been restricted to animals. Pre-clinical studies have established that systemic administration of D3 ligands, particularly antagonists and partial agonists, can alter drug-seeking in animals. Despite over a decade of research, few studies have investigated the effects of intra-cerebral infusion of D3 ligands on drug-seeking. In the present review, these studies are summarized, which have largely focused on stimulus-controlled behaviors. Converging evidence from studies of D3 receptor expression, Fos and pharmacological Magnetic Resonance Imaging (phMRI) is also provided to delineate some of the D3 brain systems involved in drug-seeking and taking. The data so far indicate that different brain systems may be involved in different types of stimulus control as well as drug taking.

Lentiviral vector-mediated dopamine d3 receptor modulation in the rat brain impairs alcohol intake and ethanol-induced conditioned place preference

BACKGROUND

It has been reported that dopamine D3 receptor (D3R) knockout mice display similar ethanol (EtOH) consumption compared to wild types. In addition, studies with D3R pharmacological targeting were inconclusive.

METHODS

In the current study, we used both gain- and loss-of-function approaches to test the effects of central D3R manipulation on voluntary alcohol intake and EtOH-induced conditioned place preference (CPP) in rats. To this aim, we developed a lentiviral-mediated gene transfer approach to examine whether D3R knockdown (LV-siD3R) or overexpression (LV-D3R) in the nucleus accumbens (NAcc) is sufficient to modulate voluntary alcohol consumption and EtOH-CPP.

RESULTS

Using the standard 2-bottle choice drinking paradigm and an unbiased CPP procedure, our results indicated that, like the D3R selective antagonist SB-277011-A, LV-siD3R attenuated voluntary alcohol consumption. In contrast, LV-D3R increased EtOH intake with no effect on total fluid intake. Similarly, the D3R agonist 7-OH-DPAT also exacerbated EtOH intake. Interestingly, neither pharmacological nor genetic manipulation of D3R activity affected saccharin and quinine consumption and preference. More importantly, we report that LV-siD3R blocked, whereas LV-D3R exacerbated, EtOH-CPP.

CONCLUSIONS

These results support the notion that the D3R plays an important role in alcohol reward in rats and suggest that a key threshold range of D3R levels is associated with impaired alcohol consumption. Taken together, these findings demonstrate that the D3R is an essential component of the molecular pathways underlying the reinforcing properties of alcohol. Thus, medications targeting the D3Rs may be beneficial to tackle EtOH abuse and alcoholism in humans.

Smoking and BDNF Val66Met polymorphism in male schizophrenia: a case-control study

Some recent studies show an association between a functional polymorphism of BDNF gene (Val66Met) and the susceptibility to nicotine dependence and we hypothesized that this polymorphism was associated with smoking in both schizophrenia patients and healthy controls. The BDNF Val66Met gene polymorphism was genotyped in 690 chronic male schizophrenia patients (smoker/nonsmoker = 522/169) and 628 male controls (smoker/nonsmoker = 322/306) using a case-control design. Nicotine dependence (ND) was assessed by the cigarettes smoked per day (CPD), the Heaviness of Smoking Index (HSI), and the Fagerstrom Test for ND (FTND). Patients also were rated on the Positive and Negative Syndrome Scale (PANSS). The results showed no significant differences in BDNF Val66Met genotype and allele distributions between the patients and healthy controls or between smokers and nonsmokers in either patients or healthy controls alone. In patient groups, however, the smokers with the Met allele had significantly higher HSI scores (Met/Met: 2.8 ± 1.7 vs. Met/Val: 2.2 ± 1.7 vs. Val/Val: 2.0 ± 1.6, p < 0.01) and a trend toward a significantly higher FTND score (p = 0.09) than those with the Val/Val genotype. In addition, the smokers showed significantly lower PANSS negative symptom and total scores, longer duration of illness and more hospitalizations (all p < 0.05). In the control group, the smokers with the Met allele started smoking significantly earlier than those with the Val/Val genotype (both p < 0.05). These results suggest that the BDNF Val66Met polymorphism may affect a smoker's response to nicotine in both schizophrenia and healthy controls from a Chinese Han population, but with differential effects in different aspects of smoking behaviors.

L-tetrahydropalmatine inhibits methamphetamine-induced locomotor activity via regulation of 5-HT neuronal activity and dopamine D3 receptor expression

Methamphetamine (METH) is a psychomotor stimulant that produces hyperlocomotion in rodents. l-tetrahydropalmatine (l-THP) is an active ingredient found in Corydalis ternata which has been used as a traditional herbal preparation in Asian countries for centuries, however, the effect of l-THP on METH-induced phenotypes largely unknown. In this study, to evaluate the effect of l-THP on METH-induced psychotropic effects, rats were pretreated with l-THP (10 and 15 mg/kg) before acute METH injection, following which the total distance the rats moved in an hour was measured. To clarify a possible mechanism underlying the effect of l-THP on METH-induced behavioral changes, dopamine receptor mRNA expression levels in the striatum of the rats was measured following the locomotor activity study. In addition, the effect of l-THP (10 and 15 mg/kg) on serotonergic (5-HTergic) neuronal pathway activation was studied by measurement of 5-HT (80 μg/10μl/mouse)-induced head twitch response (HTR) in mice. l-THP administration significantly inhibited both hyperlocomotion in rats and HTR in mice. l-THP inhibited climbing behavior-induced by dopaminergic (DAergic) neuronal activation in mice. Furthermore, l-THP attenuated the decrease in dopamine D3 receptor mRNA expression levels in the striatum of the rats induced by METH. These results suggest that l-THP can ameliorate behavioral phenotype induced by METH through regulation of 5-HT neuronal activity and dopamine D3 receptor expression.

Stimulant-induced dopamine increases are markedly blunted in active cocaine abusers

Dopamine signaling in nucleus accumbens is essential for cocaine reward. Interestingly, imaging studies have reported blunted dopamine increases in striatum (assessed as reduced binding of [(11)C]raclopride to D2/D3 receptors) in detoxified cocaine abusers. Here, we evaluate whether the blunted dopamine response reflected the effects of detoxification and the lack of cocaine-cues during stimulant exposure. For this purpose we studied 62 participants (43 non-detoxified cocaine abusers and 19 controls) using positron emission tomography and [(11)C]raclopride (radioligand sensitive to endogenous dopamine) to measure dopamine increases induced by intravenous methylphenidate and in 24 of the cocaine abusers, we also compared dopamine increases when methylphenidate was administered concomitantly with a cocaine cue-video versus a neutral-video. In controls, methylphenidate increased dopamine in dorsal (effect size 1.4; P<0.001) and ventral striatum (location of accumbens) (effect size 0.89; P<0.001), but in cocaine abusers methylphenidate's effects did not differ from placebo and were similar whether cocaine-cues were present or not. In cocaine abusers despite the markedly attenuated dopaminergic effects, the methylphenidate-induced changes in ventral striatum were associated with intense drug craving. Our findings are consistent with markedly reduced signaling through D2 receptors during intoxication in active cocaine abusers regardless of cues exposure, which might contribute to compulsive drug use.

Recent methods for measuring dopamine D3 receptor occupancy in vivo: importance for drug development

There is considerable interest in developing highly selective dopamine (DA) D3 receptor ligands for a variety of mental health disorders. DA D3 receptors have been implicated in Parkinson's disease, schizophrenia, anxiety, depression, and substance use disorders. The most concrete evidence suggests a role for the D3 receptor in drug-seeking behaviors. D3 receptors are a subtype of D2 receptors, and traditionally the functional role of these two receptors has been difficult to differentiate. Over the past 10-15 years a number of compounds selective for D3 over D2 receptors have been developed. However, translating these findings into clinical research has been difficult as many of these compounds cannot be used in humans. Therefore, the functional data involving the D3 receptor in drug addiction mostly comes from pre-clinical studies. Recently, with the advent of [(11)C]-(+)-PHNO, it has become possible to image D3 receptors in the human brain with increased selectivity and sensitivity. This is a significant innovation over traditional methods such as [(11)C]-raclopride that cannot differentiate between D2 and D3 receptors. The use of [(11)C]-(+)-PHNO will allow for further delineation of the role of D3 receptors. Here, we review recent evidence that the role of the D3 receptor has functional importance and is distinct from the role of the D2 receptor. We then introduce the utility of analyzing [(11)C]-(+)-PHNO binding by region of interest. This novel methodology can be used in pre-clinical and clinical approaches for the measurement of occupancy of both D3 and D2 receptors. Evidence that [(11)C]-(+)-PHNO can provide insights into the function of D3 receptors in addiction is also presented.

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

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

Dopamine D3 receptor is necessary for ethanol consumption: an approach with buspirone

Mesolimbic dopamine (DA) controls drug- and alcohol-seeking behavior, but the role of specific DA receptor subtypes is unclear. We tested the hypothesis that D3R gene deletion or the D3R pharmacological blockade inhibits ethanol preference in mice. D3R-deficient mice (D3R(-/-)) and their wild-type (WT) littermates, treated or not with the D3R antagonists SB277011A and U99194A, were tested in a long-term free choice ethanol-drinking (two-bottle choice) and in a binge-like ethanol-drinking paradigm (drinking in the dark, DID). The selectivity of the D3R antagonists was further assessed by molecular modeling. Ethanol intake was negligible in D3R(-/-) and robust in WT both in the two-bottle choice and DID paradigms. Treatment with D3R antagonists inhibited ethanol intake in WT but was ineffective in D3R(-/-) mice. Ethanol intake increased the expression of RACK1 and brain-derived neurotrophic factor (BDNF) in both WT and D3R(-/-); in WT there was also a robust overexpression of D3R. Thus, increased expression of D3R associated with activation of RACK1/BDNF seems to operate as a reinforcing mechanism in voluntary ethanol intake. Indeed, blockade of the BDNF pathway by the TrkB selective antagonist ANA-12 reversed chronic stable ethanol intake and strongly decreased the striatal expression of D3R. Finally, we evaluated buspirone, an approved drug for anxiety disorders endowed with D3R antagonist activity (confirmed by molecular modeling analysis), that resulted effective in inhibiting ethanol intake. Thus, DA signaling via D3R is essential for ethanol-related reward and consumption and may represent a novel therapeutic target for weaning.

Neural and psychological mechanisms underlying compulsive drug seeking habits and drug memories--indications for novel treatments of addiction

This review discusses the evidence for the hypothesis that the development of drug addiction can be understood in terms of interactions between Pavlovian and instrumental learning and memory mechanisms in the brain that underlie the seeking and taking of drugs. It is argued that these behaviours initially are goal-directed, but increasingly become elicited as stimulus-response habits by drug-associated conditioned stimuli that are established by Pavlovian conditioning. It is further argued that compulsive drug use emerges as the result of a loss of prefrontal cortical inhibitory control over drug seeking habits. Data are reviewed that indicate these transitions from use to abuse to addiction depend upon shifts from ventral to dorsal striatal control over behaviour, mediated in part by serial connectivity between the striatum and midbrain dopamine systems. Only some individuals lose control over their drug use, and the importance of behavioural impulsivity as a vulnerability trait predicting stimulant abuse and addiction in animals and humans, together with consideration of an emerging neuroendophenotype for addiction are discussed. Finally, the potential for developing treatments for addiction is considered in light of the neuropsychological advances that are reviewed, including the possibility of targeting drug memory reconsolidation and extinction to reduce Pavlovian influences on drug seeking as a means of promoting abstinence and preventing relapse.

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

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

Dopamine D3 receptor ligands for drug addiction treatment: update on recent findings

The dopamine D3 receptor is located in the limbic area and apparently mediates selective effects on motivation to take drugs and drug-seeking behaviors, so that there has been considerable interest on the possible use of D3 receptor ligands to treat drug addiction. However, only recently selective tools allowing studying this receptor have been developed. This chapter presents an overview of findings that were presented at a symposium on the conference Dopamine 2013 in Sardinia in May 2013. Novel neurobiological findings indicate that drugs of abuse can lead to significant structural plasticity in rodent brain and that this is dependent on the availability of functional dopamine D3 autoreceptor, whose activation increased phosphorylation in the ERK pathway and in the Akt/mTORC1 pathway indicating the parallel engagement of a series of intracellular signaling pathways all involved in cell growth and survival. Preclinical findings using animal models of drug-seeking behaviors confirm that D3 antagonists have a promising profile to treat drug addiction across drugs of abuse type. Imaging the D3 is now feasible in human subjects. Notably, the development of (+)-4-propyl-9-hydroxynaphthoxazine ligand used in positron emission tomography (PET) studies in humans allows to measure D3 and D2 receptors based on the area of the brain under study. This PET ligand has been used to confirm up-regulation of D3 sites in psychostimulant users and to reveal that tobacco smoking produces elevation of dopamine at the level of D3 sites. There are now novel antagonists being developed, but also old drugs such as buspirone, that are available to test the D3 hypothesis in humans. The first results of clinical investigations are now being provided. Overall, those recent findings support further exploration of D3 ligands to treat drug addiction.

Dopamine D3 and 5-HT1B receptor dysregulation as a result of psychostimulant intake and forced abstinence: Implications for medications development

Addiction to psychostimulants, including cocaine and amphetamine, is associated with dysregulation of dopamine and serotonin (5-HT) neurotransmitter systems. Neuroadaptations in these systems vary depending on the stage of the drug taking-abstinence-relapse cycle. Consequently, the effects of potential treatments that target these systems may vary depending on whether they are given during abstinence or relapse. In this review, we discuss evidence that dopamine D3 receptors (D3Rs) and 5-HT1B receptors (5-HT1BRs) are dysregulated in response to both chronic psychostimulant use and subsequent abstinence. We then review findings from preclinical self-administration models which support targeting D3Rs and 5-HT1BRs as potential medications for psychostimulant dependence. Potential side effects of the treatments are discussed and attention is given to studies reporting positive treatment outcomes that depend on: 1) whether testing occurs during self-administration versus abstinence, 2) whether escalation of drug self-administration has occurred, 3) whether the treatments are given repeatedly, and 4) whether social factors influence treatment outcomes. We conclude that D3/D2 agonists may decrease psychostimulant intake; however, side effects of D3/D2R full agonists may limit their therapeutic potential, whereas D3/D2R partial agonists have fewer undesirable side effects. D3-selective antagonists may not reduce psychostimulant intake during relapse, but nonetheless, may decrease motivation for seeking psychostimulants with relatively few side-effects. 5-HT1BR agonists provide a striking example of treatment outcomes that are dependent on the stage of the addiction cycle. Specifically, these agonists initially increase cocaine's reinforcing effects during maintenance of self-administration, but after a period of abstinence they reduce psychostimulant seeking and the resumption of self-administration. In conclusion, we suggest that factors contributing to dysregulation of monoamine systems, including drug history, abstinence, and social context, should be considered when evaluating potential treatments to better model treatment effects in humans. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

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.

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

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

Blockade of dopamine D3 receptors in the nucleus accumbens and central amygdala inhibits incubation of cocaine craving in rats

Cue-induced drug seeking progressively increases over time of withdrawal from drug self-administration in rats, a phenomenon called 'incubation of craving'. The underlying mechanisms have been linked to increased expression of brain-derived neurotrophic factor and GluR2-lacking AMPA receptors in the mesolimbic dopamine (DA) system and also to increased extracellular signal-regulated kinase activation in the central amygdala (CeA). However, it remains unclear whether any DA mechanism is also involved in incubation of craving. Recent research demonstrates that cue-induced cocaine seeking appears to parallel increased DA D3 , but not D1 or D2 , receptor expression in the nucleus accumbens (NAc) of rats over time of withdrawal, suggesting possible involvement of D3 receptors (D3 Rs) in incubation of cocaine craving. Here, we report that systemic or local administration of SB-277011A, a highly selective D3 R antagonist, into the NAc (core and shell) or the CeA, but not the dorsal striatum or basolateral amygdala, significantly inhibits expression of incubation of cocaine craving in rats after 2-30 days of withdrawal from previous cocaine self-administration but had no effect on sucrose-seeking behavior in rats after 10-30 days of withdrawal. These data suggest that DA D3 Rs in both the NAc and the CeA play an important role in incubation of cocaine craving in rats and support the potential utility of D3 R antagonists in the treatment of cocaine addiction.

The neurobiology of imagination: possible role of interaction-dominant dynamics and default mode network

This work aims at presenting some hypotheses about the potential neurobiological substrate of imagery and imagination. For the present purposes, we will define imagery as the production of mental images associated with previous percepts, and imagination as the faculty of forming mental images of a novel character relating to something that has never been actually experienced by the subject but at a great extent emerges from his inner world. The two processes appear intimately related and imagery can arguably be considered as one of the main components of imagination. In this proposal, we argue that exaptation and redeployment, two basic concepts capturing important aspects of the evolution of biological structures and functions (Anderson, 2007), could also be useful in explaining imagery and imagination. As far as imagery is concerned it is proposed that neural structures originally implicated in performing certain functions, e.g., motor actions, can be reused for the imagery of the virtual execution of that function. As far as imagination is concerned we speculate that it can be the result of a “tinkering” that combines and modifies stored perceptual information and concepts leading to the creation of novel “mental objects” that are shaped by the subject peculiar inner world. Hence it is related to his self-awareness. The neurobiological substrate of the tinkering process could be found in a hierarchical model of the brain characterized by a multiplicity of functional modules (FMs) that can be assembled according to different spatial and temporal scales. Thus, it is surmised that a possible mechanism for the emergence of imagination could be represented by modulatory mechanisms controlling the perviousness of “modifiers” along the communication channels within and between FMs leading to their dynamically reassembling into novel configurations.

Dopamine D(3) receptor deletion or blockade attenuates cocaine-induced conditioned place preference in mice

The dopamine (DA) D3 receptor (D3R) has received much attention in medication development for treatment of addiction. However, the functional role of the D3R in drug reward and addiction has been a matter of debate. We recently reported that D3 receptor-knockout (D3(-/-)) mice display increased vulnerability to cocaine self-administration, which we interpret as a compensatory response to attenuated cocaine reward after D3R deletion. Here we report that D3(-/-) mice displayed attenuated cocaine-induced conditioned place response (CPP) compared to wild-type mice. Similarly, blockade of brain D3Rs by YQA-14, a novel DA D3 receptor antagonist, significantly and dose-dependently inhibits acquisition and expression of cocaine-induced CPP in WT mice, but not in D3(-/-) mice. These findings suggest that: 1) D3Rs play an important role in mediating cocaine's rewarding effects; and 2) YQA-14 is a highly potent and selective D3R antagonist in vivo, which deserves further study as a candidate for treatment of cocaine addiction.

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

INTRODUCTION

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

AREAS COVERED

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

EXPERT OPINION

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

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

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

Increased vulnerability to cocaine in mice lacking dopamine D3 receptors

Neuroimaging studies using positron emission tomography suggest that reduced dopamine D(2) receptor availability in the neostriatum is associated with increased vulnerability to drug addiction in humans and experimental animals. The role of D(3) receptors (D(3)Rs) in the neurobiology of addiction remains unclear, however. Here we report that D(3)R KO (D(3)(-/-)) mice display enhanced cocaine self-administration and enhanced motivation for cocaine-taking and cocaine-seeking behavior. This increased vulnerability to cocaine is accompanied by decreased dopamine response to cocaine secondary to increased basal levels of extracellular dopamine in the nucleus accumbens, suggesting a compensatory response to decreased cocaine reward in D(3)(-/-) mice. In addition, D(3)(-/-) mice also display up-regulation of dopamine transporters in the striatum, suggesting a neuroadaptative attempt to normalize elevated basal extracellular dopamine. These findings suggest that D(3)R deletion increases vulnerability to cocaine, and that reduced D(3)R availability in the brain may constitute a risk factor for the development of cocaine addiction.

Chronic Δ⁹-tetrahydrocannabinol exposure induces a sensitization of dopamine D₂/₃ receptors in the mesoaccumbens and nigrostriatal systems

Δ⁹-tetrahydrocannabinol (THC), through its action on cannabinoid type-1 receptor (CB₁R), is known to activate dopamine (DA) neurotransmission. Functional evidence of a direct antagonistic interaction between CB₁R and DA D₂-receptors (D₂R) suggests that D₂R may be an important target for the modulation of DA neurotransmission by THC. The current study evaluated, in rodents, the effects of chronic exposure to THC (1 mg/kg/day; 21 days) on D₂R and D₃R availabilities using the D₂R-prefering antagonist and the D₃R-preferring agonist radiotracers [¹⁸F]fallypride and [³H]-(+)-PHNO, respectively. At 24 h after the last THC dose, D₂R and D₃R densities were significantly increased in midbrain. In caudate/putamen (CPu), THC exposure was associated with increased densities of D₂R with no change in D₂R mRNA expression, whereas in nucleus accumbens (NAcc) both D₃R binding and mRNA levels were upregulated. These receptor changes, which were completely reversed in CPu but only partially reversed in NAcc and midbrain at 1 week after THC cessation, correlated with an increased functionality of D₂/₃R in vivo, based on findings of increased locomotor suppressive effect of a presynaptic dose and enhanced locomotor activation produced by a postsynaptic dose of quinpirole. Concomitantly, the observations of a decreased gene expression of tyrosine hydroxylase in midbrain together with a blunted psychomotor response to amphetamine concurred to indicate a diminished presynaptic DA function following THC. These findings indicate that the early period following THC treatment cessation is associated with altered presynaptic D₂/₃R controlling DA synthesis and release in midbrain, with the concurrent development of postsynaptic D₂/₃R supersensitivity in NAcc and CPu. Such D₂/₃R neuroadaptations may contribute to the reinforcing and habit-forming properties of THC.

Reduced expression of haloperidol conditioned catalepsy in rats by the dopamine D3 receptor antagonists nafadotride and NGB 2904

Haloperidol, a dopamine (DA) D2 receptor-preferring antagonist, produces catalepsy whereby animals maintain awkward posture for a period of time. Sub-threshold doses of haloperidol fail to produce catalepsy initially, however, when the drug is given repeatedly in the same test environment, gradual day-to-day increases in catalepsy are observed. More importantly, if sensitized rats are injected with saline instead of haloperidol they continue to be cataleptic in the test environment suggesting that environment-drug associations may play a role. DA D3 receptors have been implicated in a number of conditioned behaviors. We were interested if DA D3 receptors contribute to catalepsy sensitization and conditioning in rats. We tested this hypothesis using the DA D3 receptor-selective antagonist NGB 2904 (0.5, 1.8 mg/kg) and the DA D3 receptor-preferring antagonist nafadotride (0.1, 0.5 mg/kg). For 10 consecutive conditioning days rats were treated with one of the D3 receptor antagonists alone or in combination with haloperidol (0.25 mg/kg) and tested for catalepsy, quantified by the time a rat remained with its forepaws on a horizontal bar. On test day (day 11), rats were injected with saline or the D3 receptor antagonist and tested for conditioned catalepsy in the previously drug-paired environment. Rats treated with NGB 2904 or nafadotride alone did not develop catalepsy. Rats treated with haloperidol or haloperidol plus NGB 2904 or nafadotride developed catalepsy sensitization with repeated conditioning. When injected with saline they continued to exhibit catalepsy in the test environment--now conditioned. On the other hand, NGB 2904 (1.8 mg/kg) or nafadotride (0.5 mg/kg) given on the test day (after sensitization to haloperidol) significantly attenuated the expression of conditioned catalepsy. Our data suggest that the D3 receptor antagonist NGB 2904 (1.8 mg/kg) and nafadotride (0.5 mg/kg) significantly attenuate conditioned catalepsy in rats when given in test but not when given during sensitization. Results implicate DA D3 receptors in regulating the expression of conditioned catalepsy.

Phenylpiperazine derivatives with selectivity for dopamine D3 receptors modulate cocaine self-administration in rats

This study examined cocaine self-administration after pretreatments with three structurally related compounds that bind selectively to dopamine D3 receptors (D3Rs) relative to the D2 receptor subtype (D2Rs) and exhibit varying intrinsic activities in the forskolin-stimulated adenylyl cyclase assay. The compounds are: a) WC10, a D3R weak partial agonist/antagonist with 42-fold D3R:D2R selectivity, b) WC26, a 51-fold selective D3R partial agonist, c) WC44, a 23-fold selective D3R agonist. Rats were stabilized on a multiple variable-interval 60-s (VI60) schedule with alternating components of sucrose (45 mg pellets) or cocaine reinforcement (0.375 mg/kg, IV) and then tested for effects of the WC compounds (0.0, 1.0, 3.0, 5.6, or 10.0 mg/kg, IP). Another cohort was trained to self-administer cocaine (0.75 mg/kg, IV) on a VI60 schedule then tested with various doses of cocaine available (0.0-1.5 mg/kg, IV) following pretreatment with WC10 (5.6 or 10.0 mg/kg) or WC44 (10.0 mg/kg). WC10 and WC26 decreased both cocaine and sucrose reinforcement rates at the 10.0 mg/kg dose, whereas WC44 decreased only cocaine reinforcement rate at this dose. Furthermore, WC26 and WC44 increased response latency for cocaine but not sucrose. In the cocaine dose-response experiment, WC10 and WC44 flattened the dose-effect function of cocaine reinforcement rate. All compounds decreased spontaneous locomotion. WC10 and WC26 also reduced cocaine-induced locomotion. These results support the targeting of D3Rs for treatments for cocaine dependence. WC26 and WC44, in particular, show promise as they increased the latency to respond for cocaine but not sucrose, suggesting selective reduction of the motivation for cocaine.

Increased brain-derived neurotrophic factor (BDNF) expression in the ventral tegmental area during cocaine abstinence is associated with increased histone acetylation at BDNF exon I-containing promoters

Recent evidence suggests that the persistence of cocaine seeking during periods of protracted drug abstinence following chronic cocaine exposure is mediated, in part, by neuroadaptations in the mesolimbic dopamine system. Specifically, incubation of cocaine-seeking behavior coincides with increased brain-derived neurotrophic factor (BDNF) protein expression in the ventral tegmental area (VTA). However, the molecular mechanisms that regulate time-dependent changes in VTA BDNF protein expression during cocaine abstinence are unclear. The goal of these experiments was to determine whether VTA BDNF transcript levels are altered following cocaine abstinence and identify the molecular mechanisms regulating cocaine-induced changes in VTA BDNF transcription. Rats were allowed to self-administer cocaine (0.25 mg/infusion, i.v.) for 14 days on a fixed-ratio schedule of reinforcement followed by 7 days of forced drug abstinence. BDNF protein and exon I-containing transcripts were significantly increased in the VTA of cocaine-experienced rats following 7 days of forced drug abstinence compared to yoked saline controls. Cocaine-induced changes in BDNF mRNA were associated with increased acetylation of histone 3 and binding of CREB-binding protein to exon I-containing promoters in the VTA. Taken together, these results suggest that drug abstinence following cocaine self-administration remodels chromatin in the VTA resulting in increased expression of BDNF, which may contribute to neuroadaptations underlying cocaine craving and relapse.