Evidence for antagonist activity of the dopamine D3 receptor partial agonist, BP 897, at human dopamine D3 receptor

Insights into the mechanisms underlying opioid use disorder and potential treatment strategies

Opioid use disorder is a worldwide societal problem and public health burden. Strategies for treating opioid use disorder can be divided into those that target the opioid receptor system and those that target non-opioid receptor systems, including the dopamine and glutamate receptor systems. Currently, the clinical drugs used to treat opioid use disorder include the opioid receptor agonists methadone and buprenorphine, which are limited by their abuse liability, and the opioid receptor antagonist naltrexone, which is limited by poor compliance. Therefore, the development of effective medications with lower abuse liability and better potential for compliance is urgently needed. Based on recent advances in the understanding of the neurobiological mechanisms underlying opioid use disorder, potential treatment strategies and targets have emerged. This review focuses on the progress made in identifying potential targets and developing medications to treat opioid use disorder, including progress made by our laboratory, and provides insights for future medication development. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Low-dose polypharmacology targeting dopamine D1 and D3 receptors reduces cue-induced relapse to heroin seeking in rats

Chemical compounds that target dopamine (DA) D1 or D3 receptors have shown promise as potential interventions in animal models of cue-induced relapse. However, undesirable side effects or pharmacodynamic profiles have limited the advancement of new compounds in preclinical studies when administered as independent treatments. In this series of experiments, we explored the effects of coadministration of a D1-receptor partial agonist (SKF 77434) and a D3-receptor antagonist (NGB 2904) in heroin-seeking rats within a "conflict" model of abstinence and cue-induced relapse. Rats were first trained to press a lever to self-administer heroin, and drug delivery was paired contingently with cues (e.g., light and pump noise). Self-initiated abstinence was facilitated by applying electrical current to the flooring in front of the levers. Lastly, a relapse response was provoked by noncontingent presentation of conditioned cues. Prior to provocation, rats received a systemic injection of SKF 77434, NGB 2904, or a combination of both compounds to assess treatment effects on lever pressing. Results indicated that the coadministration of low (i.e., independently ineffective) doses of both compounds was more effective in reducing cue-induced relapse to heroin seeking than either compound alone, with some evidence of drug synergism. Follow-up studies indicated that this reduction was not due to motoric impairment nor enhanced sensitivity to the electrified flooring and that this treatment did not significantly affect motivation for food. Implications for the treatment of opiate use disorder and recommendations for further research are discussed.

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

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Opioid abuse and overdose have become a national crisis in the USA.

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Although several opioid-based pharmacotherapies are available, they are ineffective in long-term relapse prevention.

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National Institute on Drug Abuse has listed dopamine D3 receptor antagonists as high priority for anti-opioid medication development.

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The novel D3 receptor antagonists (VK4-116, VK4-40) are effective in reducing opioid reward and relapse as well as potentiate opioid analgesia.

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D3 receptor antagonists deserve further studies as new pharmacotherapies for pain and of opioid use disorder.

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.

Computational Study and Modified Design of Selective Dopamine D3 Receptor Agonists

Dopamine D3 receptor (D3 R) is considered as a potential target for the treatment of nervous system disorders, such as Parkinson's disease. Current research interests primarily focus on the discovery and design of potent D3 agonists. In this work, we selected 40 D3 R agonists as the research system. Comparative molecular field analysis (CoMFA) of three-dimensional quantitative structure-activity relationship (3D-QSAR), structure-selectivity relationship (3D-QSSR), and molecular docking was performed on D3 receptor agonists to obtain the details at atomic level. The results indicated that both the CoMFA model (r(2) = 0.982, q(2) = 0.503, rpred2 = 0.893, SEE  = 0.057, F = 166.308) for structure-activity and (r(2) = 0.876, q(2) = 0.436, rpred2 = 0.828, F = 52.645) for structure-selectivity have good predictive capabilities. Furthermore, docking studies on three compounds binding to D3 receptor were performed to analyze the binding modes and interactions. The results elucidate that agonists formed hydrogen bond and hydrophobic interactions with key residues. Finally, we designed six molecules under the guidance of 3D-QSAR/QSSR models. The activity and selectivity of designed molecules have been improved, and ADMET properties demonstrate they have low probability of hepatotoxicity (<0.5). These results from 3D-QSAR/QSSR and docking studies have great significance for designing novel dopamine D3 selective agonists in the future.

The dopamine D3 receptor antagonist YQA14 that inhibits the expression and drug-primed reactivation of morphine-induced conditioned place preference in rats

Increasing evidence suggests that the mesolimbic dopamine system plays a critical role in opioid addiction. However, there is currently no standard drug treatment for opioid addiction. Growing preclinical evidence indicates that the dopamine D3 receptor antagonists are the potential anti-addiction pharmacotherapeutic agents based on in animal models of multiple drug addiction. In this study, we investigated the inhibitory effects of YQA14, a novel dopamine D3 receptor antagonist with a high affinity and selectivity for dopamine D3 receptor, using morphine-induced conditioned place preference (CPP) in rats. The results suggested that YQA14 (6.25-25mg/kg; intraperitoneal, i.p.) decreased the expression of morphine (10mg/kg, s.c.)-induced CPP in a dose-related manner but did not influence the acquisition of morphine-induced CPP. At a 25mg/kg dose of YQA14, it also notably inhibited the reactivation of morphine-priming CPP. These findings suggest that YQA14 is a potential agent for anti-opioid addiction which warrants further study and development.

High-affinity and selective dopamine D₃ receptor full agonists

We have designed, synthesized and evaluated a series of new compounds with the goal to identify potent and selective D(3) ligands. The two most potent and selective new D(3) ligands are compounds 38 and 52, which bind to the D(3) receptors with a K(i) value of <nM and display a selectivity of 450-494 times over the D(2) receptors and >10,000 times over the D(1) receptors. Both 38 and 52 are full agonists with high potency at the D(3) receptor in a D(3) functional assay.

Studies of new fused benzazepine as selective dopamine D3 receptor antagonists using 3D-QSAR, molecular docking and molecular dynamics

In recent years, great interest has been paid to the development of compounds with high selectivity for central dopamine (DA) D3 receptors, an interesting therapeutic target in the treatment of different neurological disorders. In the present work, based on a dataset of 110 collected benzazepine (BAZ) DA D3 antagonists with diverse kinds of structures, a variety of in silico modeling approaches, including comparative molecular field analysis (CoMFA), comparative similarity indices analysis (CoMSIA), homology modeling, molecular docking and molecular dynamics (MD) were carried out to reveal the requisite 3D structural features for activity. Our results show that both the receptor-based (Q(2) = 0.603, R(2) (ncv) = 0.829, R(2) (pre) = 0.690, SEE = 0.316, SEP = 0.406) and ligand-based 3D-QSAR models (Q(2) = 0.506, R(2) (ncv) =0.838, R(2) (pre) = 0.794, SEE = 0.316, SEP = 0.296) are reliable with proper predictive capacity. In addition, a combined analysis between the CoMFA, CoMSIA contour maps and MD results with a homology DA receptor model shows that: (1) ring-A, position-2 and R(3) substituent in ring-D are crucial in the design of antagonists with higher activity; (2) more bulky R(1) substituents (at position-2 of ring-A) of antagonists may well fit in the binding pocket; (3) hydrophobicity represented by MlogP is important for building satisfactory QSAR models; (4) key amino acids of the binding pocket are CYS101, ILE105, LEU106, VAL151, PHE175, PHE184, PRO254 and ALA251. To our best knowledge, this work is the first report on 3D-QSAR modeling of the new fused BAZs as DA D3 antagonists. These results might provide information for a better understanding of the mechanism of antagonism and thus be helpful in designing new potent DA D3 antagonists.

N-(4-{4-[2-(Trifluoromethoxy)phenyl]piperazin-1-yl}butyl)thiophene-2-carboxamide dihydrate

In the title compound, C₂₀H₂₄F₃N₃O₂S·2H₂O, a dopamine D3 ligand, the piperazine ring adopts a chair conformation while the piperazine and benzene rings form a dihedral angle of 47.71 (6)°. In the crystal, mol­ecules are linked by inter­molecular N—H⋯O and O—H⋯O hydrogen bonds. In the mol­ecular structure, the F atoms of the trifluoro­methyl group are disordered over two sites with occupancies of 0.69 (11) and 0.31 (11).

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

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

Hypothesis-driven medication discovery for the treatment of psychostimulant addiction

Psychostimulant abuse is a serious social and health problem, for which no effective treatments currently exist. A number of review articles have described predominantly 'clinic'-based pharmacotherapies for the treatment of psychostimulant addiction, but none have yet been shown to be definitively effective for use in humans. In the present article, we review various 'hypothesis'- or 'mechanism'-based pharmacological agents that have been studied at the preclinical level and evaluate their potential use in the treatment of psychostimulant addiction in humans. These compounds target brain neurotransmitter or neuromodulator systems, including dopamine (DA), gamma-aminobutyric acid (GABA), endocannabinoid, glutamate, opioid and serotonin, which have been shown to be critically involved in drug reward and addiction. For drugs in each category, we first briefly review the role of each neurotransmitter system in psychostimulant actions, and then discuss the mechanistic rationale for each drug's potential anti-addiction efficacy, major findings with each drug in animal models of psychostimulant addiction, abuse liability and potential problems, and future research directions. We conclude that hypothesis-based medication development strategies could significantly promote medication discovery for the effective treatment of psychostimulant addiction.

Mechanism-based medication development for the treatment of nicotine dependence

Tobacco use is a global problem with serious health consequences. Though some treatment options exist, there remains a great need for new effective pharmacotherapies to aid smokers in maintaining long-term abstinence. In the present article, we first discuss the neural mechanisms underlying nicotine reward, and then review various mechanism-based pharmacological agents for the treatment of nicotine dependence. An oversimplified hypothesis of addiction to tobacco is that nicotine is the major addictive component of tobacco. Nicotine binds to alpha4beta2 and alpha7 nicotinic acetylcholine receptors (nAChRs) located on dopaminergic, glutamatergic and GABAergic neurons in the mesolimbic dopamine (DA) system, which causes an increase in extracellular DA in the nucleus accumbens (NAc). That increase in DA reinforces tobacco use, particularly during the acquisition phase. Enhanced glutamate transmission to DA neurons in the ventral tegmental area appears to play an important role in this process. In addition, chronic nicotine treatment increases endocannabinoid levels in the mesolimbic DA system, which indirectly modulates NAc DA release and nicotine reward. Accordingly, pharmacological agents that target brain acetylcholine, DA, glutamate, GABA, or endocannabonoid signaling systems have been proposed to interrupt nicotine action. Furthermore, pharmacokinetic strategies that alter plasma nicotine availability, metabolism and clearance also significantly alter nicotine's action in the brain. Progress using these pharmacodynamic and pharmacokinetic agents is reviewed. For drugs in each category, we discuss the mechanistic rationale for their potential anti-nicotine efficacy, major findings in preclinical and clinical studies, and future research directions.

N-(4-(4-(2,3-dichloro- or 2-methoxyphenyl)piperazin-1-yl)butyl)heterobiarylcarboxamides with functionalized linking chains as high affinity and enantioselective D3 receptor antagonists

In the present report, the D3 receptor pharmacophore is modified in the 2,3-diCl- and 2-OCH(3)-phenylpiperazine class of compounds with the goal to improve D3 receptor affinity and selectivity. This extension of structure-activity relationships (SAR) has resulted in the identification of the first enantioselective D3 antagonists (R- and S-22) to be reported, wherein enantioselectivity is more pronounced at D3 than at D2, and that a binding region on the second extracellular loop (E2) may play a role in both enantioselectivity and D3 receptor selectivity. Moreover, we have discovered some of the most D3-selective compounds reported to date that show high affinity (K(i) = 1 nM) for D3 and approximately 400-fold selectivity over the D2 receptor subtype. Several of these analogues showed exquisite selectivity for D3 receptors over >60 other receptors, further underscoring their value as in vivo research tools. These lead compounds also have appropriate physical characteristics for in vivo exploration and therefore will be useful in determining how intrinsic activity at D3 receptors tested in vitro is related to behaviors in animal models of addiction and other neuropsychiatric disorders.

The selective dopamine D3 receptor antagonists SB-277011A and NGB 2904 and the putative partial D3 receptor agonist BP-897 attenuate methamphetamine-enhanced brain stimulation reward in rats

RATIONALE

We have previously reported that selective antagonism of brain D3 receptors by SB-277011A or NGB 2904 significantly attenuates cocaine- or nicotine-enhanced brain stimulation reward (BSR).

OBJECTIVE

In the present study, we investigated whether the selective D3 receptor antagonists SB-277011A and NGB 2904 and the putative partial D3 agonist BP-897 similarly reduce methamphetamine (METH)-enhanced BSR.

MATERIALS AND METHODS

Rats were trained to respond for rewarding electrical self-stimulation of the medial forebrain bundle. To assess the degree of drug-induced changes in BSR, a rate-frequency curve shift paradigm was used to measure brain-reward threshold (theta 0).

RESULTS

METH (0.1-0.65 mg/kg, i.p.) dose-dependently lowered ( approximately 10-50%) BSR thresholds, producing an enhancement of BSR. Pretreatment with SB-277011A (12 mg/kg, but not 24 mg/kg, i.p.) significantly attenuated METH-enhanced BSR. NGB 2904 (0.1-1.0 mg/kg, but not 10 mg/kg) also attenuated METH-enhanced BSR. SB-277011A or NGB 2904 alone, at the doses tested, had no effect on BSR. Pretreatment with BP-897 (0.1-5 mg/kg) dose-dependently attenuated METH-enhanced BSR. However, when the dose was increased to 10 mg/kg, BP-897 shifted the stimulation-response curve to the right (inhibited BSR itself) in the presence or absence of METH.

CONCLUSIONS

Selective antagonism of D3 receptors by SB-277011A or NGB 2904 attenuates METH-enhanced BSR in rats, while the METH-enhanced BSR attenuation produced by BP-897 may involve both D3 and non-D3 receptors. These findings support a potential use of selective D3 receptor antagonists for the treatment of METH addiction.

Hybrid approach for the design of highly affine and selective dopamine D3 receptor ligands using privileged scaffolds of biogenic amine GPCR ligands

A series of compounds containing privileged scaffolds of the known histamine H(1) receptor antagonists cetirizine, mianserin, ketotifen, loratadine, and bamipine were synthesized for further optimization as ligands for the related biogenic amine binding dopamine D(3) receptor. A pharmacological screening was carried out at dopamine D(2) and D(3) receptors. In the preliminary testing various ligands have shown moderate to high affinities for dopamine D(3)receptors, for example, N-(4-{4-[benzyl(phenyl)amino]piperidin-1-yl}butylnaphthalen-2-carboxamide (19a) (hD(3)K(i)=0.3 nM; hD(2)K(i)=703 nM), leading to a selectivity ratio of 2343.

Pharmacological actions of NGB 2904, a selective dopamine D3 receptor antagonist, in animal models of drug addiction

As a continuation of our work with SB-277011A, we have examined the effects of another highly elective dopamine (DA) D3 receptor antagonist, N-(4-[4-{2,3-dichlorophenyl}-1-piperazinyl]butyl)-2-fluorenylcarboxamide (NGB 2904), in animal models of addiction. Our results indicate that by systemic administration, NGB 2904 inhibits intravenous cocaine self-administration maintained under a progressive-ratio (PR) reinforcement schedule, cocaine- or cocaine cue-induced reinstatement of cocaine-seeking behavior, and cocaine- or other addictive drug-enhanced brain stimulation reward (BSR). The action of NGB 2904 on PR cocaine self-administration was long-lasting (1-2 days) after a single injection, supporting its potential use in treatment of cocaine addiction. The effects of NGB 2904 in the BSR paradigm were dose-dependent for both NGB 2904 and cocaine; that is, only lower doses of NGB 2904 were effective, and their putative antiaddiction effect could be overcome by increasing the doses of cocaine or other addictive drugs. A dopamine-dependent mechanism is proposed to explain the effects of NGB 2904 on cocaine's actions in these animal models of drug addiction. The data reviewed in this paper suggest that NGB 2904 or other D3-selective antagonists may have potential in controlling motivation for drug-taking behavior or relapse to drug-seeking behavior, but may have a limited role in antagonizing the acute rewarding effects produced by cocaine or other addictive drugs. In addition, NGB 2904 may also act as a useful tool to study the role of D3 receptors in drug addiction.

Maternal deprivation and handling modify the effect of the dopamine D3 receptor agonist, BP 897 on morphine-conditioned place preference in rats

RATIONALE

Maternal deprivation and handling can lead to a vulnerability to opiate dependence. However, the involvement of the dopamine D3 receptors has not been investigated.

OBJECTIVES

This study analysed the effects of a selective partial D3 receptor agonist, BP 897, on morphine-conditioned place preference (CPP) in deprived and handled rats.

MATERIALS AND METHODS

The effects of BP 897 were studied on the expression and the extinction of morphine CPP. Quantitative autoradiography of D2, D3 receptors and immunoautoradiography of dopamine transporter were performed in some saline- and morphine-treated rats 24 h after the place preference test.

RESULTS

Morphine (5 mg/kg) induced a more prolonged morphine CPP in deprived and handled rats than in control animals. BP 897 (0.5 or 2 mg/kg) enhanced the expression of morphine conditioning in control rats. Same doses did not change morphine conditioning in deprived rats. BP 897 (2 mg/kg) suppressed morphine CPP in handled rats. An increase in basal D2 receptor density in the mesencephalon of handled rats, which was suppressed after morphine CPP, was observed. A decrease in D2 receptor levels in morphine-treated deprived rats occurred in the nucleus accumbens.

CONCLUSIONS

This study shows that maternal deprivation and handling induced a prolonged morphine CPP, and different changes of D2/D3 receptor functioning revealed after morphine CPP. Early manipulations of infant-mother relationships may have different consequences on the balance of opioidergic and dopaminergic neurotransmission and may be of interest to reveal pharmacological properties of dopamine receptor partial agonists or antagonists potentially useful for therapeutic applications.

Action of novel antipsychotics at human dopamine D3 receptors coupled to G protein and ERK1/2 activation

The effects of new generation antipsychotic drugs (APDs) targeting dopamine D(2) and serotonin 5-HT(1A) receptors were compared with typical and atypical APDs on phosphorylation of extracellular signal-regulated kinase 1/2 (ERK 1/2) and measures of G protein activation in CHO cell lines stably expressing the human dopamine D(3) receptor. The preferential dopamine D(3) agonists (+)-7-OH-DPAT and PD128907, like dopamine and quinelorane, efficaciously stimulated ERK 1/2 phosphorylation at dopamine D(3) receptors. In contrast, in [(35)S]GTPgammaS binding experiments, (+)-7-OH-DPAT exhibited partial agonist properties, while PD128907 and quinelorane maintained full agonist properties. The preferential dopamine D(3) ligand BP 897 and the antidyskinetic sarizotan partially activated ERK 1/2 phosphorylation while exerting no agonist activity on GTPgammaS binding, suggesting signal amplification at the MAP kinase level. Antipsychotics differed in their ability to inhibit both agonist-stimulated GTPgammaS binding and ERK 1/2 phosphorylation, but all typical and atypical compounds tested acted as dopamine D(3) receptor antagonists with the exception of n-desmethylclozapine, the active metabolite of clozapine, which partially activated dopamine D(3) receptor-mediated ERK 1/2 phosphorylation. Among the new generation dopamine D(2)/serotonin 5-HT(1A) antipsychotics, only F 15063 and SLV313 acted as pure dopamine D(3) receptor antagonists, bifeprunox was highly efficacious whereas SSR181507 and aripiprazole showed marked partial agonist properties for ERK 1/2 phosphorylation. In contrast, in the GTPgammaS binding study, aripiprazole was devoid of agonist properties and bifeprunox, and to an even lesser extent SSR181507, only weakly stimulated GTPgammaS binding. In summary, these findings underline the differences of dopamine D(3) properties of new generation antipsychotics which may need to be considered in understanding their diverse therapeutic actions.

Discriminative stimulus properties of S32504, a novel D3/D2 receptor agonist and antiparkinson agent, in rats: attenuation by the antipsychotics, aripiprazole, bifeprunox, N-desmethylclozapine, and by selective antagonists at dopamine D2 but not D3 receptors

RATIONALE

Drug-discrimination studies have proven instructive in the characterization of psychotropic agents, a procedure applied herein to the novel antiparkinson agent, S32504. This highly selective agonist at dopamine D(3) and (less potently) D(2) receptors displays potent antiparkinson, neuroprotective and antidepressant properties (Millan et al., J Pharmacol Exp Ther 309:936-950, 2004a; Millan et al., J Pharmacol Exp Ther 309:903-920, 2004b; Millan et al., J Pharmacol Exp Ther 309:921-935, 2004c).

OBJECTIVES

To generate a discriminative stimulus (DS) with S32504 and undertake substitution/antagonism studies with diverse antiparkinson and antipsychotic agents.

MATERIALS AND METHODS

Using a two-lever, fixed-ratio 10 schedule, rats were trained to recognize S32504 (0.04 mg/kg, s.c.) from saline.

RESULTS

S32504 displayed dose-dependent and stereospecific substitution in comparison to its less active racemic form, (+/-) S31411, and to its inactive (-) distomer, S32601. Apomorphine, and the selective D(3)/D(2) receptor agonists, ropinirole, PD128,907, 7-OH-DPAT and CGS15855A, fully (=80%) substituted for S32504, whereas D(4) and D(1)/D(5) receptor agonists were ineffective. The selective D(3) vs D(2) receptor partial agonist, BP897, did not substitute for S32504 and the selective D(3) receptor antagonists, S33084, SB277,011, GR218,231, PNU99194A and S14297, did not block its DS properties. By contrast, S32504 lever selection was blocked by the preferential D(2) vs D(3) receptor antagonists, L741,626 and S23199, and by the D(2)/D(3) antagonists, raclopride and haloperidol. The D(2)/D(3) receptor partial agonists and antipsychotics, aripiprazole, bifeprunox, N-desmethylclozapine and preclamol did not substitute for S32504: indeed, they dose-dependently attenuated its DS properties.

CONCLUSION

The antiparkinson agent, S32504, displays DS properties principally mediated by high-efficacy activation of D(2) receptors Antipsychotics known to act as partial agonists at D(2)/D(3) receptors attenuate DS properties of S32504, actions reflecting their low efficacy at these sites.

Dopamine D3 receptor ligands—Recent advances in the control of subtype selectivity and intrinsic activity

Various pharmacological studies have implicated the dopamine D(3) receptor as an interesting therapeutic target in the treatment of different neurological disorders. Because of these putative therapeutic applications, D(3) receptor ligands with diverse intrinsic activities have been an active field of research in recent years. Separation of purely D(3)-mediated drug effects from effects produced by interactions with similar biogenic amine receptors allows to verify the therapeutic impact of D(3) receptors and to reduce possible side-effects caused by "promiscuous" receptor interactions. The requirement to gain control of receptor selectivity and in particular subtype selectivity has been a challenging task in rational drug discovery for quite a few years. In this review, recently developed structural classes of D(3) ligands are discussed, which cover a broad spectrum of intrinsic activities and show interesting selectivities.

Effects of RGH-237 [N-{4-[4-(3-Aminocarbonyl-phenyl)-piperazin-1-yl]-butyl}-4-bromo-benzamide], an Orally Active, Selective Dopamine D3 Receptor Partial Agonist in Animal Models of Cocaine Abuse

Dopamine D(3) receptor partial agonism has been suggested as a potential therapeutic intervention in cocaine addiction. RGH-237 [N-{4-[4-(3-aminocarbonyl-phenyl)-piperazin-1-yl]-butyl}-4-bromo-benzamide] was identified as a novel selective dopamine D(3) receptor partial agonist and used for testing this hypothesis in animal models. The compound showed nanomolar affinity to human (K(i) = 6.7 nM) and rat (K(i) = 1.6 nM) D(3) receptors with an intrinsic activity of approximately 50%. It possessed several hundredfold selectivity over the D(2) receptor. The molecule bound with moderate (100-250 nM) affinity to 5-hydroxytryptamine 1A (5-HT(1A)) and nonselectively labeled opiate receptors. RGH-237 proved to be practically inactive on more than 40 other targets, including monoaminergic, cholinergic, GABAergic, and glutamatergic receptors. In rats orally administered RGH-237 was well and rapidly absorbed yielding 41% oral bioavailability. At its pharmacologically active dose (10 mg/kg p.o.), the brain concentration of RGH-237 reached 110 ng/g. Its blood and brain levels were sustained for 3 h. RGH-237 at the oral dose of 10 mg/kg moderately but significantly inhibited the acquisition of cocaine-induced place preference, although by itself, it had no place-conditioning effect. The compound did not affect fixed ratio 1 cocaine self-administration. In a reinstatement paradigm of cocaine self-administration, the compound potently and dose-dependently blocked the cue-induced cocaine-seeking behavior of rats at 10 and 30 mg/kg oral doses. RGH-237 did not affect seeking activity for natural rewards, such as sucrose and water. It did not exert notable effect on spontaneous motor activity of rats. Our results demonstrate that selective D(3) partial agonists may be an effective therapeutic means in the treatment of cocaine abuse.

The selective dopamine D3 receptor antagonist SB-277011A reduces nicotine-enhanced brain reward and nicotine-paired environmental cue functions

Increasing evidence suggests that enhanced dopamine (DA) neurotransmission in the nucleus accumbens (NAc) may play a role in mediating the reward and reinforcement produced by addictive drugs and in the attentional processing of drug-associated environmental cues. The meso-accumbens DA system is selectively enriched with DA D3 receptors, a DA receptor subtype increasingly implicated in reward-related brain and behavioural processes. From a variety of evidence, it has been suggested that selective DA D3 receptor antagonism may be a useful pharmacotherapeutic approach for treating addiction. The present experiments tested the efficacy of SB-277011A, a selective DA D3 receptor antagonist, in rat models of nicotine-enhanced electrical brain-stimulation reward (BSR), nicotine-induced conditioned locomotor activity (LMA), and nicotine-induced conditioned place preference (CPP). Nicotine was given subcutaneously within the dose range of 0.25-0.6 mg/kg (nicotine-free base). SB-277011A, given intraperitoneally within the dose range of 1-12 mg/kg, dose-dependently reduced nicotine-enhanced BSR, nicotine-induced conditioned LMA, and nicotine-induced CPP. The results suggest that selective D3 receptor antagonism constitutes a new and promising pharmacotherapeutic approach to the treatment of nicotine dependence.

The structural evolution of dopamine D3 receptor ligands: structure-activity relationships and selected neuropharmacological aspects

"Evolution consists largely of molecular tinkering."-Following the famous concept of the molecular geneticist and medicine Nobel laureate François Jacob, in this review we describe the structural evolution of dopamine D3 receptor ligands from the natural agonist dopamine (DA) to highly potent and subtype selective new agents by bioisosteric tinkering with well-established and privileged or novel and fancy chemical functionalities and scaffolds. Some of the more than 200 ligands presented herein have already achieved therapeutic or scientific value up to now, some will most likely achieve it in the future. Hence, great importance is not only attached to the relationship between structure and activity of the ligands, but also to their utility as pharmacological tools in animal models or as therapeutics in patients with neurological diseases or other disorders.

Effect of dopamine D3 antagonists on PPI in DBA/2J mice or PPI deficit induced by neonatal ventral hippocampal lesions in rats

Schizophrenic patients typically exhibit impairment of sensorimotor gating, which can be modeled in animal models such as the test of prepulse inhibition of startle response (PPI) in rodents. It has been found that antipsychotics enhanced PPI in DBA mice and reversed the PPI deficit induced by neonatal ventral hippocampal (NVH) lesions in rats. However, the relative involvement of D(3) and D(2) receptors in these effects is unknown since all antipsychotics are D(2)/D(3) antagonists with limited binding preference at D(2) receptors. Therefore, in the current study, we investigated the influence of several dopamine antagonists with higher selectivity at D(3) vs D(2) receptors on PPI in DBA/2J mice and in NVH-lesioned rats. The PPI in DBA/2J mice was enhanced by the nonselective D(2)/D(3) antagonists, haloperidol at 0.3-3 mg/kg, or risperidone at 0.3-1 mg/kg, while PPI-enhancing effects were observed after the administration of higher doses of the preferential D(3)/D(2) antagonist, BP 897 at 8 mg/kg, and the selective D(3) antagonists, SB 277011 at 30 mg/kg and A-437203 at 30 mg/kg. No effect was observed following the treatment with the selective D(3) antagonist, AVE 5997 up to 30 mg/kg. The PPI deficits induced by NVH lesions were reversed by haloperidol but not by the more selective D(3) antagonists, A-437203 and AVE 5997. BP 897 enhanced PPI nonselectivity, that is, in both lesioned and nonlesioned rats. In summary, the present study indicates that PPI-enhancing effects induced by antipsychotics in DBA/2J mice and in NVH-lesioned rats are unlikely to be mediated by D(3) receptors.

The novel dopamine D3 receptor antagonist NGB 2904 inhibits cocaine's rewarding effects and cocaine-induced reinstatement of drug-seeking behavior in rats

Accumulating evidence indicates that dopamine (DA) D(3) receptor 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 novel D(3)-selective antagonist NGB 2904 (N-(4-[4-{2,3-dichlorophenyl}-1-piperazinyl]butyl)-3-fluorenylcarboxamide) on cocaine self-administration, cocaine-enhanced brain stimulation reward (BSR), and cocaine-triggered reinstatement of drug-seeking behavior in male Long-Evans rats. We found that: (1) acute intraperitoneal (i.p.) administration of NGB 2904 (0.1-10 mg/kg) failed to alter cocaine self-administration (0.5 mg/kg/infusion) under fixed-ratio 2 (FR2) reinforcement, but 1 or 5 mg/kg NGB 2904 significantly lowered the break-point for cocaine self-administration under progressive-ratio (PR) reinforcement; (2) cocaine (1, 2, and 10 mg/kg) significantly enhanced electrical BSR (decreased brain reward thresholds), while NGB 2904 significantly inhibited the enhancement of BSR elicited by 2 mg/kg, but not 10 mg/kg of cocaine; (3) NGB 2904 alone neither maintained self-administration behavior nor altered brain reward thresholds; and (4) NGB 2904 significantly inhibited cocaine-triggered reinstatement of extinguished drug-seeking behavior, but not sucrose-plus-sucrose-cue-triggered reinstatement of sucrose-seeking behavior. Overall, these data show that the novel D(3)-selective antagonist NGB 2904 attenuates cocaine's rewarding effects as assessed by PR self-administration, BSR, and cocaine-triggered reinstatement of cocaine-seeking behavior. Owing to these properties and to its lack of rewarding effects (as assessed by BSR and by substitution during drug self-administration), NGB 2904 merits further investigation as a potential agent for treatment of cocaine addiction.

Salviae miltiorrhizae radix increases dopamine release of rat and pheochromocytoma PC12 cells

The Radix of Salvia miltiorrhiza Bunge (Labiatae) (SMR), an eminent herb, is often included as an ingredient in various herbal remedies recommended for vascular circulation therapies. The present study investigated the effect of SMR on dopaminergic neurotransmission. Various extracts prepared from the stems of SMR were tested for cytotoxic activity on pheochromocytoma PC12 cells using the XTT assay method. The ethanol extract (IC50 > 100 microg/mL), water extract (IC50 > 100 microg/mL) and chloroform (IC50 = 90 microg/mL) fraction exhibited weak cytotoxic activity. However, the butanol (IC50 = 80 microg/mL) and ethyl acetate (EtOAc; IC50 = 70 microg/mL) fractions exhibited strong cytotoxic activity. Also, the extracts and fractions were investigated for dopamine release effects. The EtOAc fraction showed a stronger stimulatory effect on dopamine release activity than the other fractions. The effect of the crude EtOAc fraction (50 microg/mL) of SMR on K+ (20 mm)-stimulated dopamine (DA) release from rat striatal slices was compared with amphetamine (10(-4) m) using high-performance liquid chromatography with electrochemical detection to measure endogenous DA. The EtOAc fraction significantly increased K+ -stimulated DA release (p < 0.001) from rat striatal slices when compared with K+ -stimulated alone. The EtOAc fraction potentiated the effect of amphetamine on K+ -stimulated DA release (p < 0.001) when compared with amphetamine alone. To examine whether in vitro the EtOAc fraction treatment induces DA release in PC12 cells, the role of protein kinases was investigated in the induction of the EtOAc fraction-mediated events by using inhibitors of protein kinase C (PKC), mitogen activated protein kinase (MAP kinase) or protein kinase A (PKA). The PKC inhibitors chelerythrine (50 nm and 100 nm) and Ro31-8220 (100 nm) and the MAP kinase kinase inhibitor, PD98059 (20 microm), inhibited the ability of the EtOAc fraction of SMR to elicit the EtOAc fraction-stimulated DA release. The PKC activator, 12-O-tetradecanoyl phorbol 13-acetate (TPA, 100 nm) mimicked the ability of the EtOAc fraction of SMR to elicit DA release. In contrast, a selective PKA inhibitor, 50 microm Rp-8-Br-cAMP, blocked the development of EtOAc fraction-stimulated DA release. It was demonstrated that the EtOAc fraction of SMR stimulated DA release. Therefore the mechanism by which the EtOAc fraction of SMR induced the enhancement in EtOAc fraction-stimulated DA release is apparent.

Salviae Miltiorrhizae BGE Radix Increases Rat Striatal K+-Stimulated Dopamine Release and Activates the Dopamine Release with Protection Against Hydrogen Peroxide-Induced Injury in Rat Pheochromocytoma PC12 Cells

The present study investigated the effect of the medicinal plant Salviae miltiorrhizae radix (SMR) on dopaminergic neurotransmission in comparison with amphetamine. The effect of SM (0.1 g/ml) on K(+) (20 mM)-stimulated dopamine (DA) release from rat striatal slices was compared with amphetamine (10(-4) M). Amphetamine and SMR significantly increased K(+)-stimulated DA release (P<0.001) from rat striatal slices when compared with K(+)-stimulated alone. On the other hand, to examine whether in vitro SMR treatment induces DA release in PC12 cells, the role of protein kinases has been investigated in the induction of the SMR-mediated events by using inhibitors of protein kinase C (PKC), mitogen activated protein kinase (MAP kinase) or protein kinase A (PKA). PKC inhibitors chelerythrine (50 and 100 nM), Ro31-8220 (100 nM) and the MAP kinase inhibitor, PD98059 (20 microM) inhibited the ability of SMR to elicit the SMR-stimulated DA release. The direct-acting PKC activator, 12-O-tetradecanoyl phorbol 13-acetate (TPA, 100 nM) mimicked the ability of SMR to elicit DA release. On the contrary, a selective PKA inhibitor, 50 microM Rp-8-Br-cAMP, blocked the development of SMR-stimulated DA release. The results demonstrated that SMR may stimulate DA release and that SMR-induced increases in MAP kinase and PKC are important for induction of the enhancement in transporter-mediated DA release and PKA was also required for the enhancement in SMR-stimulated DA release. SMR treatment (0.1-10 microg/ml) to the hydrogen peroxide (H(2)O(2))-treated PC12 cells activated the enzyme activities such as catalase, superoxide dismutase and glutathione peroxidase, and decreased the malondialdehyde level, indicating that SMR has also protective effects against free radical-induced cell toxicity. Therefore, the mechanism by which SMR induces the enhancement in SMR-stimulated DA release is apparent. It remains to be determined whether the effect of SMR on DA function is important in its therapeutic use in the treatment of drug addiction.

Selective antagonist at D3 receptors, but not non-selective partial agonists, influences the expression of cocaine-induced conditioned place preference in free-feeding rats

The non-selective dopamine (DA) D(3) partial agonist BP 897 influenced rats' seeking behavior induced by cocaine-associated cues but there are contradictions about its ability to modulate cocaine-induced conditioned place preference (CPP), and mechanisms involved. We therefore re-evaluated its activity on both acquisition and expression of these behaviors, taking into consideration the actual brain concentrations of unchanged drug and its potential active metabolite 1-(2-methoxyphenyl)-piperazine (oOCH(3)PP), as well as its negative motivational properties. BP 897 induced conditioned place aversion (CPA) at 3 mg/kg, but not at 0.3 and 1 mg/kg. However, in this range of amply spaced doses BP 897 did not affect the acquisition and expression of cocaine (10 mg/kg i.p.) CPP in rats, although its brain concentrations were well above those affecting in vitro D(3) receptors. Concentrations of oOCH(3)PP were below the limits of quantification of the analytical procedure. As concerns the expression behavior, its structurally and pharmacologically related derivative N-[4-[4-(2-methoxyphenyl)piperazin-1-yl]butyl]benzo[b]furan-2-carboxamide (1 and 3 mg/kg, i.p.) also had no such effect. By contrast, the selective D(3) receptor antagonist SB-277011-A (3 mg/kg, i.p.) antagonized the expression of cocaine-induced CPP, supporting the suggestion that "full" antagonist activity at D(3) receptors is necessary to prevent 10 mg/kg cocaine-induced place conditioning in free-feeding rats.

The role of central dopamine D3 receptors in drug addiction: a review of pharmacological evidence

The cDNA for the dopamine D3 receptor was isolated and characterized in 1990. Subsequent studies have indicated that D3 receptors, as well as D3 receptor mRNA, are primarily localized in limbic regions in mammals. This finding led to the postulate that D3 receptors may be involved in drug dependence and addiction. However, this hypothesis has been difficult to test due to the lack of compounds with high selectivity for central D3 receptors. The interpretation of results from studies using mixed D2/D3 agonists and/or antagonists is problematic because these agents have low selectivity for D3 over D2 receptors and it is likely that their actions are primarily related to D2 receptor antagonism and possibly interaction with other neurotransmitter receptors. Currently, with the synthesis and characterization of new highly selective D3 receptor antagonists such as SB-277011-A this difficulty has been surmounted. The purpose of the present article is to review, for the first time, the effects of various putative D3 receptor selective compounds in animal models of drug dependence and addiction. The results obtained with highly selective D3 receptor antagonists such as SB-277011-A, SB-414796, and NGB-2904 indicate that central D3 receptors may play an important role in drug-induced reward, drug-taking, and cue-, drug-, and stress-induced reinstatement of drug-seeking behavior. Provided these results can be extrapolated to human drug addicts, they suggest that selective DA D3 receptor antagonists may prove effective as potential pharmacotherapeutic agents to manage drug dependence and addiction.

Acute administration of SB-277011A, NGB 2904, or BP 897 inhibits cocaine cue-induced reinstatement of drug-seeking behavior in rats: role of dopamine D3 receptors

Recent studies have shown that the novel dopamine (DA) D3 receptor antagonists SB-277011A and NGB 2904 inhibit cocaine- and/or stress-induced reinstatement of drug-seeking behavior. The present study sought to determine if SB-277011A, NGB 2904, or BP-897 (a mixed D3 agonist/antagonist) similarly inhibit cocaine-associated cue-induced reinstatement of drug-seeking behavior. Long-Evans rats were allowed to self-administer cocaine. Each cocaine infusion was paired with discrete conditioned cue-light and tone. Subsequently, drug-seeking (i.e., lever-pressing) behavior was extinguished in the absence of cocaine and cocaine-associated cues. Rats were then tested for cue-induced reinstatement of drug-seeking. We found that cocaine-associated cues evoked robust reinstatement of lever-pressing. Acute intraperitoneal (i.p.) administration of SB-277011A (6, 12, or 24 mg/kg) produced a dose-dependent inhibition of cue-induced reinstatement of drug-seeking behavior by 35, 65, and 85%, respectively, compared to vehicle-treated animals. Acute i.p. administration of NGB 2904 (0.1, 1.0, or 5.0 mg/kg) produced a 45, 30, and 70% inhibition of cue-induced reinstatement, respectively, compared to vehicle-treated animals. Acute i.p. administration of either 0.1 or 1 mg/kg of BP 897 did not produce a significant effect on cue-induced reinstatement, whereas a dose of 3 mg/kg produced a 70% inhibition of cue-induced reinstatement. These findings, combined with previous data, suggest that DA D3 receptor antagonism may underlie the inhibitory effects of SB-277011A and NGB 2904 on cocaine cue-induced reinstatement, while the effects of BP 897 may involve D3 and non-D3 receptor mechanisms.

Effects of selective dopamine receptor subtype agonists on cardiac contractility and regional haemodynamics in rats

1. Activation of dopamine (DA) receptors produces cardiovascular responses such as vasodilation and hypotension. However, knowledge of the role of specific dopamine receptor subtypes (especially D3 and D4) in the cardiovascular system is limited. The objective of the present study was to characterize the haemodynamic and cardiac responses to agonists with selectivity for D1, D2, D3 and D4 receptor subtypes. 2. Inactin-anaesthetized rats were instrumented to measure regional haemodynamic and cardiac contractility responses with slow intravenous infusion of agonists. 3. Fenoldopam (a D1 receptor agonist) decreased (P < 0.05) renal vascular resistance beginning at a dose of 3 micromol/kg. Infusion of PNU-95666E (a D2 receptor agonist) produced dose-dependent decreases (P < 0.05) in mean arterial pressure (MAP), heart rate (HR) and hindquarter vascular resistance (HQVR). Administration of BP897 (a partial D3 receptor agonist) decreased (P < 0.05) MAP and HQVR at 3 micromol/kg. PD168077 (a D4 receptor agonist) caused significant increases in HQVR at 1 micromol/kg. None of the compounds tested elicited significant changes in cardiac contractility. 4. Using selective agonists of dopamine receptor subtypes, the present studies characterize distinct cardiovascular effects in anaesthetized rats. Consistent with its well-defined effects as a D1 receptor agonist, fenoldopam administration resulted in renal vasodilation. Similar to earlier studies using the non-selective D2-like receptor agonist quinpirole, selective agonism at the D2 receptor using PNU-95666E resulted in bradycardia, hindquarter vasodilation and decreases in arterial pressure. Partial agonism at the D3 receptor with BP897 had no effect on heart rate, but did produce depressor responses driven by decreases in HQVR. Conversely, agonism of the D4 receptor using PD168077 resulted in modest hindquarter vasoconstriction that was not dose dependent. Hence, by comparison, agonism of the D4 receptor has little effect in the cardiovascular system of the rat relative to the other dopamine receptor subtype agonists tested.

Intra-BLA or Intra-NAc Infusions of the Dopamine D₃ Receptor Partial Agonist, BP 897, Block Intra-NAc Amphetamine Conditioned Activity

Recent studies have shown that both systemic and intra-nucleus accumbens (NAc) or intra-amygdala administration of dopamine D3 receptor ligands modulate reward-related learning. A previous study (H. Aujla, H. Sokoloff, & R. J. Beninger. 2002) showed that systemic administration of the partial dopamine D3 receptor agonist BP 897 selectively blocked the expression, but not the acquisition, of amphetamine-conditioned activity. This suggested the hypothesis that intra-NAc or intra-basolateral amygdala (BLA) BP 897 would attenuate the expression, but not the acquisition, of amphetamine-conditioned activity. Rats were habituated to activity-monitoring chambers for 5 days, for 1 hr each day. Conditioning occurred on the next 3 days, followed by a single 1-hr test session. Intra-NAc or intra-BLA infusions of BP 897 during test, but not during conditioning, attenuated intra-NAc amphetamine conditioned activity. Results indicate that the ability of BP 897 to attenuate the expression of conditioned activity is mediated in part by the NAc and BLA.

Blockade of mesolimbic dopamine D3 receptors inhibits stress-induced reinstatement of cocaine-seeking in rats

RATIONALE

The dopamine (DA) D3 receptor is preferentially expressed in the mesolimbic system. We have previously shown that selective D3 receptor blockade by the novel D3 antagonist SB-277011A inhibits cocaine's reinforcing action and cocaine-induced reinstatement of cocaine-seeking behavior.

OBJECTIVE

In the present study, we investigated whether SB-277011A similarly inhibits stress-induced reinstatement of cocaine-seeking behavior.

METHODS

Rats were allowed to self-administer cocaine (0.5 mg/kg per infusion, 3 h per session) for 10-14 days, followed by a once-daily extinction session for 7-14 days during which saline was substituted for cocaine. Extinction criteria were fewer than ten lever-presses per 3-h session for at least 3 consecutive days. After cocaine-seeking behavior was extinguished, each animal was tested twice for footshock-stress-induced reinstatement, once with vehicle (25% hydroxypropyl-beta-cyclodextrin) and once with one of three doses of SB-277011A in counterbalanced fashion.

RESULTS

During the last 3 days of cocaine self-administration (SA), active lever-presses were approximately 100 per session under fixed-ratio 2 reinforcement (approximately 25 mg/kg cocaine per session). After extinction, intermittent footshock (10 min, 0.5 mA, 0.5 s on with a mean inter-shock interval of 40 s) robustly reinstated the cocaine-seeking behavior (8.4+/-3.6 active lever-presses in last extinction session to 35.3+/-5.2 in animals after footshock stress). Intraperitoneal (i.p.) injections of SB-277011A (3, 6, and 12 mg/kg) dose-dependently blocked stress-induced reinstatement of cocaine-seeking. Reinstatement was also blocked by microinjections of SB-277011A (1.5 microg/0.5 microl per side) bilaterally into the nucleus accumbens, but not into the dorsal striatum.

CONCLUSIONS

The mesolimic DA D3 receptor plays an important role in mediating stress-induced reinstatement.

Modulation of the locomotor activating effects of the noncompetitive NMDA receptor antagonist MK801 by dopamine D2/3 receptor agonists in mice

The noncompetitive NMDA receptor antagonist MK801 (dizocilpine) produces behavioral stimulation mediated, in part, through indirect activation of the dopamine (DA) system. Previous reports indicate that D2/3 agonists inhibit MK801-induced stereotypies; however, it is unclear if these agonists also attenuate MK801-induced locomotion. As such, the ability of the D2/3 agonists, quinelorane and quinpirole, and the partial D3 agonist, BP897, to attenuate the locomotor activating effects of MK801 was examined in mice. MK801 (0.1-1.0 mg/kg) produced a biphasic effect on total distance traveled with the intermediate dose of 0.3 mg/kg producing the greatest stimulation. The increase in MK801-induced total distance traveled was attenuated by the coadministration of quinelorane and quinpirole at doses that alone had no effect on activity. Similarly, the partial D3 agonist, BP897, blocked the effects of MK801. The D3-preferring antagonist, nafadotride, reversed the attenuation of quinelorane and partially reversed the attenuation of quinpirole. The D2-preferring antagonist, eticlopride, reversed the attenuating effects of quinelorane, but was not effective against quinpirole. Nafadotride and eticlopride were ineffective against the attenuating effects of BP897 on MK801-induced locomotion. Because BP897 is a partial agonist it was tested against quinelorane/MK801 and quinpirole/MK801 combinations. BP897 reversed the attenuating effects of quinelorane, but not those of quinpirole on MK801's effects. These results demonstrate that the DA system, through D2/3 receptor activation, modulates the locomotor activating effects produced by noncompetitive NMDA receptor blockade.

Comparative pharmacology of human dopamine D(2)-like receptor stable cell lines coupled to calcium flux through Galpha(qo5)

The goal of this study was to develop a new approach to study the pharmacology of the dopamine D(4) receptor that could be used in comparative studies with dopamine D(2) and D(3) receptors. Stable HEK-293 cell lines co-expressing recombinant human D(2L), D(3) or D(4) receptors along with Galpha(qo5) cDNA were prepared. Dopamine induced a robust, transient calcium signal in these cell lines with EC(50)s for D(2L), D(3) and D(4) of 18.0, 11.9 and 2.2 nM, respectively. Reported D(4)-selective agonists CP226269 and PD168077 were potent, partial D(4) agonists exhibiting 31-1700-fold selectivity for D(4) over D(3) or D(2). Non-selective D(2)-like agonists apomorphine and quinpirole showed full efficacy but did not discriminate across the three receptors. D(3)-selective agonists 7-hydroxy-DPAT and PD128907 were potent but non-selective D(2)-like agonists. The reported D(3) partial agonist BP-897 exhibited minimal agonist activity at D(3) but was a potent D(3) antagonist and a partial D(4) agonist. Other D(2)-like antagonists, haloperidol, clozapine, and domperidone showed concentration-dependent inhibition of dopamine responses at all three receptors with K(i) ranging from 0.05 to 48.3 nM. The D(3) selective antagonist S33084 and D(4)-selective antagonist L-745870 were highly selective for D(3) and D(4) receptors with K(b) of 0.7 and 0.1 nM, respectively. Stable co-expression of D(2)-like receptors with chimeric Galpha(qo5) proteins in HEK-293 cells is an efficient method to study receptor activation in a common cellular background and an efficient method for direct comparison of ligand affinity and efficacy across human D(2L), D(3) and D(4) receptors.

Modulation of the discriminative stimulus effects of mu opioid agonists in rats: I. Effects of dopamine D2/3 antagonists

Mu opioid receptor agonists such as morphine stimulate the release of dopamine (DA) in various brain regions. These increases in DA are thought to be involved in some of the behavioral effects of mu agonists. The present study was designed to examine the modulatory actions of two D2/3 antagonists (nafadotride and eticlopride), the D2/3 partial agonist BP897, the D1/2 antagonist flupenthixol, and the D1 antagonist SCH23390 on the discriminative stimulus effects of the mu partial agonist nalbuphine and the higher-efficacy mu agonists heroin, methadone and morphine, in rats trained to discriminate heroin from water. Both nafadotride and eticlopride attenuated the effects of the mu agonists, whereas BP897 was effective against nalbuphine and partially effective against morphine. Flupenthixol attenuated the heroin-like discriminative stimulus effects of heroin and morphine, although not as completely as nafadotride or eticlopride. SCH23390 was least effective and produced little attenuation. These results demonstrate that the discriminative stimulus effects of mu agonists in rats are more readily attenuated by drugs that block D2-like, rather than D1-like, receptors.

Effects of a dopamine D3 receptor ligand, BP 897, on acquisition and expression of food-, morphine-, and cocaine-induced conditioned place preference, and food-seeking behavior in rats

The present study addressed the role of dopaminergic D(3) receptors (D(3)R) in motivational processes in rats. The effects of the selective D(3)R partial agonist, BP 897 (0.25-1 mg/kg, i.p.), on the establishment and the expression of conditioned place preference (CPP) supported by food, morphine (4 mg/kg, s.c.), or cocaine (2 mg/kg, s.c.) were investigated using an unbiased, one-compartment, place-conditioning procedure. When administered alone, BP 897 (0.05-2 mg/kg, i.p.) did not support CPP; on the contrary, conditioned place avoidance (CPA) was observed at 1 mg/kg, suggesting that this dose of BP 897 could be perceived as aversive. When given before each cocaine injection during the conditioning phase, BP 897 (1 mg/kg) prevented the establishment of CPP, and a single administration of BP 897 (0.5 and 1 mg/kg) before the test session impaired the expression of cocaine CPP. In contrast, neither the establishment nor the expression of food- and morphine-CPP were significantly altered by BP 897 (up to 1 mg/kg), whereas the full but less selective D(3)/D(2)R agonists, 7-OH-DPAT (0.5-2 mug/kg, s.c.) and quinelorane (1 mug/kg, s.c.), prevented the acquisition of food CPP. In a within-session extinction schedule of lever pressing for food, BP 897 (0.06-2 mg/kg) was ineffective in potentiating response reinstatement induced by the noncontingent delivery of two food pellets, in contrast with quinelorane and 7-OH-DPAT where previous studies showed to be efficient in this respect (Duarte et al, 2003). These results indicate that BP 897 has no positive appetitive value on its own, and that a moderate degree of stimulation of D(3)R is not sufficient to modulate food-primed food-seeking behavior or alter incentive motivation for food, morphine, and/or their associated cues. However, D(3)R are likely involved in the perception of the rewarding value of cocaine and cocaine-paired cues. This suggests that the appetitive effects of cocaine are subserved by mechanisms different, at least in part, from those of morphine and food, and that D(3)R play a role only in the former.

N-(4-[4-(2,3-dichlorophenyl)piperazin-1-yl]butyl, butenyl and butynyl)arylcarboxamides as novel dopamine D(3) receptor antagonists

The dopamine D(3) receptor subtype has been targeted as a potential neurochemical modulator of the behavioral actions of psychomotor stimulants, such as cocaine. Previous synthetic studies provided structural requirements for high affinity binding to D(3) receptors which included a 2,3-dichloro-phenylpiperazine linked to an arylamido function via a butyl chain. To reduce lipophilicity of these agents and further investigate optimal conformation, a second series of 15 novel ligands was designed that included heteroaromatic substitution and unsaturated alkyl linkers. These compounds were synthesized and evaluated for binding at rat D(3) and D(2) receptors stably expressed in Sf9 cells. D(3) binding affinities ranged from K(i)=0.6-1080 nM, with a broad range of D(3)/D(2) selectivities (2-97). The discovery of potent, selective and bioavailable D(3) receptor ligands will provide essential molecular probes to elucidate the role D(3) receptors play in the psychomotor stimulant and reinforcing effects of cocaine.

Selective antagonism at dopamine D3 receptors prevents nicotine-triggered relapse to nicotine-seeking behavior

Drugs of abuse, including, nicotine have been shown to enhance brain reward functions in the mesocortico-limbic dopamine (DA) system in general, and the nucleus accumbens in particular. The latter occupies a prominent position in the ventral striatum and expresses a high density of DA D(3) receptors. As such, the present study aimed at investigating the effect of the selective D(3) receptor antagonist SB-277011-A on both the stable maintenance of intravenous nicotine self-administration and nicotine-triggered relapse to nicotine-seeking behavior in the rat. SB-277011-A (3-10 mg/kg i.p.) significantly reduced reinstatement of nicotine-seeking behavior without affecting nicotine self-administration per se. These results suggest that DA D(3) receptors are involved in the reinstatement of nicotine-seeking behavior independently of any interaction with the primary reinforcing effects of nicotine itself. These findings point toward the potential use of selective DA D(3) receptor antagonists for the pharmacotherapeutic management of relapse to drug-seeking behaviors.

Cocaine-seeking behavior in response to drug-associated stimuli in rats: involvement of D3 and D2 dopamine receptors

Previous studies employed a second-order schedule paradigm maintained by cocaine reinforcement to show that BP897, a dopamine D(3) partial agonist, selectively modulated drug-seeking behavior. We investigated its effect on drug-seeking behavior induced by presentation of stimuli associated with and predictive of cocaine availability after a period of extinction and in the absence of any further cocaine. Male rats were trained to associate discriminative stimuli (S(D)) with the availability of intravenous (i.v.) 0.25 mg/0.1 ml/infusion cocaine (S(D+)) or no-reward (S(D-)) saline solution. Each infusion of cocaine or saline was followed by a response-cue signaling 20-s time-out (TO). After meeting the self-administration training criterion rats were placed on extinction conditions during which i.v. solutions and S(D)s were withheld. Every other 3 days on which rats met the extinction criterion, reinstatement tests were conducted, presenting the S(D+) or S(D-) noncontingently together with a contingent presentation of cocaine- or saline-cues signaling 20-s TO. Regardless of the order of presentation or the nature of the stimuli (auditory or visual), cocaine-associated but not saline-associated stimuli reinstated responding on the previously active lever. Presentation of cocaine-associated stimuli induced lasting drug-seeking behavior for at least eight test sessions. BP897 (1.0 mg/kg i.p.) significantly attenuated this behavior. Since it has been reported that BP897 can interact with a panel of different receptors with high affinity, we evaluated the effects of 7-OH-DPAT, an agonist to D(3) receptors, raclopride, a preferential antagonist to D(2) receptors, and WAY 100,635, an antagonist at 5-HT(1A) receptors, on drug-seeking behavior. 7-OH-DPAT (0.1-3.0 mg/kg i.p.) had biphasic effects on reinstatement induced by the cocaine-associated cues, low dosages reducing and high dosages increasing the impact of cocaine-associated stimuli on rats' behavior. Raclopride (0.1, 0.3 mg/kg s.c.) completely prevented drug-seeking behavior induced by the reintroduction of cocaine-associated stimuli. WAY 100,635 (0.1-1.0 mg/kg s.c.) had no effect on this behavior. These results, while confirming that the partial agonist at the D(3) receptors, BP897, might be a useful medication, also suggest a role of D(2) receptors in cue-induced cocaine-seeking behavior.

BP 897, a selective dopamine D3 receptor ligand with therapeutic potential for the treatment of cocaine-addiction

BP 897 is a potent (K(i) = 0.92 nM) dopamine D(3) receptor compound developed for the treatment of cocaine abuse and craving. BP 897 has a high selectivity for the dopamine D(3) versus D(2) receptors (70-fold) and a moderate affinity for 5-HT(1A) receptors, (K(i) = 84 nM), adrenergic-alpha(1) (K(i) = 60 nM) and -alpha(2) adrenoceptors (K(i) = 83 nM). BP 897 displays significant intrinsic activity at the human dopamine D(3) receptor by decreasing forskolin-stimulated cAMP levels and by stimulating mitogenesis of dopamine D(3)-expressing NG108-15 cells. Although these findings suggest that BP 897 is a partial agonist, recent studies in Chinese Hamster Ovary (CHO) cells with expressed dopamine D(3) receptors demonstrated that BP 897 is devoid of any intrinsic activity but potently inhibits dopamine agonist effects (pIC(50) = 9.43 and 9.51) in agonist-induced acidification rate or increase of GTPgammaS binding, respectively. In addition, BP 897 inhibits in vivo (EC(50) = 1.1 mg/kg, i.v.) agonist-induced decrease of firing rate of dopaminergic neurons in the substantia nigra. It has been clearly shown that BP 897, 1 mg/kg, i.p., reduces cocaine-seeking behavior in rats, without producing reinforcement on its own. In rhesus monkeys, BP 897 is not self-administered (up to 30 microg/kg, i.v.) but reduces cocaine self-administration. The potential usefulness of BP 897 in the treatment of drug-seeking behavior is further supported by its effects in drug conditioning models. Although BP 897 reduces L-DOPA-induced dyskinesia in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys, it provokes a return of parkinsonian symptoms. At high doses BP 897 has been reported to produce catalepsy in rats. Pharmacokinetic and toxicological data have not yet been published. These interesting preclinical findings with BP 897 provide additional validation for dopamine D(3) receptor as a therapeutic target for the treatment of cocaine abuse and its associated central nervous system (CNS) disorders. BP 897 recently entered phase II clinical studies.

Attenuation of cue-controlled cocaine-seeking by a selective D3 dopamine receptor antagonist SB-277011-A

Conditioned stimuli (CS) previously paired with drugs of abuse can elicit cravings in humans, relapse to drug use, and can also reinforce drug-seeking behavior in both humans and animals, events that are believed to be subserved in part by activation of the mesolimbic dopamine system. Converging anatomical, pharmacological, and behavioral evidence implicates dopamine D(3) receptors in the mechanisms underlying cue-controlled behaviors. The purpose of the present study was therefore to investigate the effects on cocaine-seeking behavior of a novel D(3) receptor antagonist, SB-277011-A, which is 100-fold more selective for D(3) over D(2) dopamine receptors. We have established previously that second-order schedules of reinforcement provide an animal model of cue-controlled drug-seeking both prior to and after cocaine has been self-administered. SB-277011-A dose-dependently decreased cocaine-seeking maintained by a cocaine-associated conditioned reinforcer in both the first, drug-free interval and also following self-administration of cocaine. At higher doses, SB-277011-A also increased the latency to receive the first CS presentation and cocaine infusion, thereby decreasing the number of cocaine infusions self-administered under the second-order schedule of reinforcement. SB-277011-A had no effect on cocaine intake under an FR-1 schedule of reinforcement, or on responding for sucrose under a second-order schedule of reinforcement, at any dose tested. These results therefore suggest that D(3) dopamine receptors may be critically involved in cue-controlled drug-seeking behavior independently of any interaction with the reinforcing effects of cocaine itself, and may therefore provide a therapeutic target in the treatment of relapse to cocaine use induced by CSs.

Dopamine D3 receptor antagonism inhibits cocaine-seeking and cocaine-enhanced brain reward in rats

dopamine D3 receptor is preferentially localized to the mesocorticolimbic dopaminergic system and has been hypothesized to play a role in cocaine addiction. To study the involvement of the D3 receptor in brain mechanisms and behaviors commonly assumed to be involved in the addicting properties of cocaine, the potent and selective D3 receptor antagonist trans-N-[4-[2-(6-cyano-1,2,3,4-tetrahydroisoquinolin-2-yl)ethyl] cyclohexyl]-4-quinolininecarboxamide (SB-277011-A) was administered to laboratory rats, and the following measures were assessed: (1) cocaine-enhanced electrical brain-stimulation reward, (2) cocaine-induced conditioned place preference, and (3) cocaine-triggered reinstatement of cocaine seeking behavior. Systemic injections of SB-277011-A were found to (1) block enhancement of electrical brain stimulation reward by cocaine, (2) dose-dependently attenuate cocaine-induced conditioned place preference, and (3) dose-dependently attenuate cocaine-triggered reinstatement of cocaine seeking behavior. Thus, D3 receptor blockade attenuates both the rewarding effects of cocaine and cocaine-induced drug-seeking behavior. These data suggest an important role for D3 receptors in mediating the addictive properties of cocaine and suggest that blockade of dopamine D3 receptors may constitute a new and useful target for prospective pharmacotherapies for cocaine addiction.

A herbal medicine used in the treatment of addiction mimics the action of amphetamine on in vitro rat striatal dopamine release

Thunbergia laurifolia Linn. (TH) and Simplocos racemosa Roxb. (SY) are herbal medicines used in the treatment of drug addiction without scientific support for their mechanism of action. The present study investigated the effects of these medicinal plants on dopaminergic neurotransmission in comparison with amphetamine. The effect of crude water extracts (0.1 g/ml) of TH and SY on K(+) (20 mM)-stimulated dopamine release from rat striatal slices were compared with amphetamine (10(-4) M) using high-performance liquid chromatography with electrochemical detection to measure endogenous dopamine. Amphetamine and TH, but not SY, significantly increased K(+)-stimulated dopamine release (P < 0.001) from rat striatal slices when compared with K(+)-stimulated alone. TH potentiated the effect of amphetamine on K(+)-stimulated dopamine release (P < 0.001) when compared with amphetamine alone. The results indicate that TH may stimulate dopamine release in the same manner as amphetamine. It remains to be determined whether the effect of these extracts on dopamine function is important in their therapeutic use in the treatment of drug addiction.

A dopamine D3 receptor partial agonist blocks the expression of conditioned activity

The partial dopamine D3 receptor agonist BP 897 attenuates cocaine seeking suggesting that BP 897 will attenuate conditioned activity to environmental stimuli paired with amphetamine. During conditioning, amphetamine (2.0 mg/kg) stimulated activity and co-treatment with BP 897 (1.0 mg/kg) had no effect. In the saline test, groups conditioned with amphetamine or amphetamine plus BP 897 showed conditioned activity. Treatment with BP 897 in the test following conditioning with saline produced no significant effect but following conditioning with amphetamine BP 897 blocked conditioned activity. Results extend previous findings that BP 897 attenuates responding for cocaine-paired stimuli to amphetamine-paired stimuli in a different paradigm and support the potential of BP 897 as a therapeutic agent for the prevention of drug seeking.