Dopamine D3 receptor mutant and wild-type mice exhibit identical responses to putative D3 receptor-selective agonists and antagonists

Altered dopaminergic firing pattern and novelty response underlie ADHD-like behavior of SorCS2-deficient mice

Attention deficit hyperactivity disorder (ADHD) is the most frequently diagnosed neurodevelopmental disorder worldwide. Affected individuals present with hyperactivity, inattention, and cognitive deficits and display a characteristic paradoxical response to drugs affecting the dopaminergic system. However, the underlying pathophysiology of ADHD and how this relates to dopaminergic transmission remains to be fully understood. Sorcs2^-/- mice uniquely recapitulate symptoms reminiscent of ADHD in humans. Here, we show that lack of SorCS2 in mice results in lower sucrose intake, indicating general reward deficits. Using in-vivo recordings, we further find that dopaminergic transmission in the ventral tegmental area (VTA) is shifted towards a more regular firing pattern with marked reductions in the relative occurrence of irregular firing in Sorcs2^-/- mice. This was paralleled by abnormal acute behavioral responses to dopamine receptor agonists, suggesting fundamental differences in dopaminergic circuits and indicating a perturbation in the balance between the activities of the postsynaptic dopamine receptor DRD1 and the presynaptic inhibitory autoreceptor DRD2. Interestingly, the hyperactivity and drug response of Sorcs2^-/- mice were markedly affected by novelty. Taken together, our findings show how loss of a candidate ADHD-risk gene has marked effects on dopaminergic circuit function and the behavioral response to the environment.

Neuronal Dopamine D3 Receptors: Translational Implications for Preclinical Research and CNS Disorders

Dopamine (DA), as one of the major neurotransmitters in the central nervous system (CNS) and periphery, exerts its actions through five types of receptors which belong to two major subfamilies such as D1-like (i.e., D1 and D5 receptors) and D2-like (i.e., D2, D3 and D4) receptors. Dopamine D3 receptor (D3R) was cloned 30 years ago, and its distribution in the CNS and in the periphery, molecular structure, cellular signaling mechanisms have been largely explored. Involvement of D3Rs has been recognized in several CNS functions such as movement control, cognition, learning, reward, emotional regulation and social behavior. D3Rs have become a promising target of drug research and great efforts have been made to obtain high affinity ligands (selective agonists, partial agonists and antagonists) in order to elucidate D3R functions. There has been a strong drive behind the efforts to find drug-like compounds with high affinity and selectivity and various functionality for D3Rs in the hope that they would have potential treatment options in CNS diseases such as schizophrenia, drug abuse, Parkinson’s disease, depression, and restless leg syndrome. In this review, we provide an overview and update of the major aspects of research related to D3Rs: distribution in the CNS and periphery, signaling and molecular properties, the status of ligands available for D3R research (agonists, antagonists and partial agonists), behavioral functions of D3Rs, the role in neural networks, and we provide a summary on how the D3R-related drug research has been translated to human therapy.

Selective D2 and D3 receptor antagonists oppositely modulate cocaine responses in mice via distinct postsynaptic mechanisms in nucleus accumbens

The dopamine D3 receptor (D3R) has emerged as a promising pharmacotherapeutic target for the treatment of several diseases including schizophrenia, Parkinson's disease, and substance use disorders. However, studies investigating the D3R's precise role in dopamine neurotransmission or how it may be exploited to modulate responses to drugs of abuse have produced contrasting results, in part because most D3R-targeted compounds often also interact with D2 receptors (D2R). To resolve this issue, we set out to systematically characterize and compare the consequences of selective D2R or D3R antagonists on the behavioral-stimulant properties of cocaine in mice, and to identify putative neurobiological mechanisms underlying their behavior-modifying effects. Pretreatment with the selective D2R antagonist L-741,626 attenuated, while pretreatment with the selective D3R antagonist PG01037 enhanced, the locomotor-activating effects of both acute cocaine administration as well as sensitization following repeated cocaine dosing. While both antagonists potentiated cocaine-induced increases in presynaptic dopamine release, we report for the first time that D3R blockade uniquely facilitated dopamine-mediated excitation of D1-expressing medium spiny neurons in the nucleus accumbens. Collectively, our results demonstrate that selective D3R antagonism potentiates the behavioral-stimulant effects of cocaine in mice, an effect that is in direct opposition to that produced by selective D2R antagonism or nonselective D2-like receptor antagonists, and is likely mediated by facilitating D1-mediated excitation in the nucleus accumbens. These findings provide novel insights into the neuropharmacological actions of D3R antagonists on mesolimbic dopamine neurotransmission and their potential utility as pharmacotherapeutics.

Inducible ablation of dopamine D2 receptors in adult mice impairs locomotion, motor skill learning and leads to severe parkinsonism

Motor execution and planning are tightly regulated by dopamine D1 and D2 receptors present in basal ganglia circuits. Although stimulation of D1 receptors is known to enhance motor function, the global effect of D2 receptor (D2R) stimulation or blockade remains highly controversial, with studies showing increasing, decreasing or no changes in motor activity. Moreover, pharmacological and genetic attempts to block or eliminate D2R have led to controversial results that questioned the importance of D2R in motor function. In this study, we generated an inducible Drd2 null-allele mouse strain that circumvented developmental compensations found in constitutive Drd2^-/- mice and allowed us to directly evaluate the participation of D2R in spontaneous locomotor activity and motor learning. We have found that loss of D2R during adulthood causes severe motor impairments, including hypolocomotion, deficits in motor coordination, impaired learning of new motor routines and spontaneous catatonia. Moreover, severe motor impairment, resting tremor and abnormal gait and posture, phenotypes reminiscent of Parkinson's disease, were evident when the mutation was induced in aged mice. Altogether, the conditional Drd2 knockout model studied here revealed the overall fundamental contribution of D2R in motor functions and explains some of the side effects elicited by D2R blockers when used in neurological and psychiatric conditions, including schizophrenia, bipolar disorder, Tourette's syndrome, dementia, alcohol-induced delusions and obsessive-compulsive disorder.

Implication of dorsostriatal D3 receptors in motivational processes: a potential target for neuropsychiatric symptoms in Parkinson's disease

Beyond classical motor symptoms, motivational and affective deficits are frequently observed in Parkinson’s disease (PD), dramatically impairing the quality of life of patients. Using bilateral 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra pars compacta (SNc) in rats, we have been able to reproduce these neuropsychiatric/non-motor impairments. The present study describes how bilateral 6-OHDA SNc lesions affect the function of the main striatal dopaminergic (DA) receptor subtypes. Autoradiography was used to measure the levels of striatal DA receptors, and operant sucrose self-administration and neuropharmacological approaches were combined to investigate the causal implication of specific DA receptors subtypes in the motivational deficits induced by a dorsostriatal DA denervation. We found that D3 receptors (D₃R) exclusively are down-regulated within the dorsal striatum of lesioned rats. We next showed that infusion of a D₃R antagonist (SB-277011A) in non-lesioned animals specifically disrupts preparatory, but not consummatory behaviors. Our findings reveal an unexpected involvement of dorsostriatal D₃R in motivational processes. They strongly suggest an implication of dorsostriatal D₃R in the neuropsychiatric symptoms observed in PD, highlighting this receptor as a potential target for pharmacological treatment.

Dopamine Receptors: Novel Insights from Biochemical and Genetic Studies

Dopamine receptors are targets for drugs with antipsychotic potency, and they are also the primary target in the treatment of Parkinson’s disease. Molecular cloning has identified five genes that code for dopamine receptors. These receptors belong in two functionally distinct classes of G-protein-coupled receptors, known as the D1 class of receptors (D1 and D5) and the D2 class of receptors (D2, D3, and D4). The diversity of dopamine receptor subtypes that belong to the same functional class, their high degree of structural similarity, and the lack of antagonists with selectivity for each of the individual subtypes have challenged studies on the function of the individual receptor subtypes. This review focuses on the recent progress made with studies on the expression and function of D1, D2, and D3 receptors. It summarizes results of studies that suggest that these receptor proteins are expressed in monomeric and oligomeric forms and reviews results of a growing number of gene-targeting studies that begin to illustrate major differences in the phenotypes of D1-, D2-, and D3-mutant mice.

Pramipexole at a Low Dose Induces Beneficial Effect in the Harmaline-induced Model of Essential Tremor in Rats

AIMS

The aim of the study was to examine the effects of preferential agonists of dopamine D3 receptors: pramipexole and 7-OH-DPAT on the harmaline-induced tremor in rats (a model of essential tremor, ET). To study receptor mechanisms of these drugs, rats were pretreated with dopamine D3 receptor antagonists--SB-277011-A and SR-21502, an antagonist of presynaptic D2/D3 receptors--amisulpride, or a nonselective antagonist of D2-like receptors, haloperidol, at a postsynaptic dose.

METHODS

For tremor measurement, fully automated force plate actimeters were used and data were analyzed using fast Fourier transform.

RESULTS

Harmaline (15 mg/kg ip)-triggered tremor was manifested by an increase in the power within 9-15 Hz band (AP2). Pramipexole administered at a low (0.1 mg/kg sc), but not higher doses (0.3 and 1 mg/kg sc), and 7-OH-DPAT (0.1, 0.3, and 1 mg/kg sc) reversed the harmaline-increased AP2. None of the examined dopamine antagonists: SB-277011-A (10 mg/kg ip), SR-21502 (15 mg/kg ip), haloperidol (0.5 mg/kg ip), or amisulpride (1 mg/kg ip) influenced the above effect of dopamine agonists.

CONCLUSION

The present study indicates that pramipexole reduces the harmaline-induced tremor, which may suggest its beneficial effects in ET patients. However, mechanisms underlying its action are still unclear and need further examination.

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.

The role of dopamine D(3) receptors in antipsychotic activity and cognitive functions

Dopamine D(3) receptors have a pre- and postsynaptic localization in brain stem nuclei, limbic parts of the striatum, and cortex. Their widespread influence on dopamine release, on dopaminergic function, and on several other neurotransmitters makes them attractive targets for therapeutic intervention. The signaling pathways of D(3) receptors are distinct from those of other members of the D(2)-like receptor family. There is increasing evidence that D(3) receptors can form heteromers with dopamine D(1), D(2), and probably other G-protein-coupled receptors. The functional consequences remain to be characterized in more detail but might open new interesting pharmacological insight and opportunities. In terms of behavioral function, D(3) receptors are involved in cognitive, social, and motor functions, as well as in filtering and sensitization processes. Although the role of D(3) receptor blockade for alleviating positive symptoms is still unsettled, selective D(3) receptor antagonism has therapeutic features for schizophrenia and beyond as demonstrated by several animal models: improved cognitive function, emotional processing, executive function, flexibility, and social behavior. D(3) receptor antagonism seems to contribute to atypicality of clinically used antipsychotics by reducing extrapyramidal motor symptoms; has no direct influence on prolactin release; and does not cause anhedonia, weight gain, or metabolic dysfunctions. Unfortunately, clinical data with new, selective D(3) antagonists are still incomplete; their cognitive effects have only been communicated in part. In vitro, virtually all clinically used antipsychotics are not D(2)-selective but also have affinity for D(3) receptors. The exact D(3) receptor occupancies achieved in patients, particularly in cortical areas, are largely unknown, mainly because only nonselective or agonist PET tracers are currently available. It is unlikely that a degree of D(3) receptor antagonism optimal for antipsychotic and cognitive function can be achieved with existing antipsychotics. Therefore, selective D(3) antagonism represents a promising mechanism still to be fully exploited for the treatment of schizophrenia, cognitive deficits in schizophrenia, and comorbid conditions such as substance abuse.

Yawning and locomotor behavior induced by dopamine receptor agonists in mice and rats

Dopaminergic (DA) agonist-induced yawning in rats seems to be mediated by DA D3 receptors, and low doses of several DA agonists decrease locomotor activity, an effect attributed to presynaptic D2 receptors. Effects of several DA agonists on yawning and locomotor activity were examined in rats and mice. Yawning was reliably produced in rats, and by the cholinergic agonist, physostigmine, in both the species. However, DA agonists were ineffective in producing yawning in Swiss-Webster or DA D2R and DA D3R knockout or wild-type mice. The drugs significantly decreased locomotor activity in rats at one or two low doses, with activity returning to control levels at higher doses. In mice, the drugs decreased locomotion across a 1000-10 000-fold range of doses, with activity at control levels (U-91356A) or above control levels [(+/-)-7-hydroxy-2-dipropylaminotetralin HBr, quinpirole] at the highest doses. Low doses of agonists decreased locomotion in all mice except the DA D2R knockout mice, but were not antagonized by DA D2R or D3R antagonists (L-741 626, BP 897, or PG01037). Yawning does not provide a selective in-vivo indicator of DA D3R agonist activity in mice. Decreases in mouse locomotor activity by the DA agonists seem to be mediated by D2 DA receptors.

The dopamine D3/D2 agonist (+)-PD-128,907 [(R-(+)-trans-3,4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano[4,3-b]-1,4-oxazin-9-ol)] protects against acute and cocaine-kindled seizures in mice: further evidence for the involvement of D3 receptors

Previous findings have demonstrated a protective role for dopamine D(3)/D(2) receptor agonists in the convulsant and lethal effects of acutely administered cocaine. Data are provided here to establish that the protection occurs through a D(3)-linked mechanism and that protection is extended to seizure kindling. The D(3) antagonist SB-277011-A [4-quinolinecarboxamide,N-[trans-4-[2-(6-cyano-3,4-dihydro-2(1H)-isoquinolinyl)ethyl]-cyclohexyl]-(9CI)] prevented the anticonvulsant effect of the D(3)/D(2) receptor agonist (+)-PD-128,907 [(R-(+)-trans-3,4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano[4,3-b]-1,4-oxazin-9-ol)] on cocaine-induced seizures. The protection afforded by the D(3)/D(2) agonist, (+)-PD-128,907, was eliminated in D(3) receptor-deficient mice. In D(2) receptor knockout mice, the anticonvulsant effects of (+)-PD-128,907 were preserved. (+)-PD-128,907 also prevented the acquisition and expression of cocaine-kindled seizures engendered by repeated daily dosing with 60 mg/kg cocaine. (+)-PD-128,907 also blocked the seizures induced in mice fully seizure kindled to cocaine. Although repeated dosing with cocaine increased the potency of cocaine to produce seizures and lethality (decreased ED(50) values), daily coadministration of (+)-PD-128,907 significantly prevented this potency shift. In mice treated daily with cocaine and (+)-PD-128,907, the density, but not the affinity, of D(3) receptors was increased. The specificity with which (+)-PD-128,907 acts upon this cocaine-driven process was demonstrated by the lack of a significant effect of (+)-PD-128,907 on seizure kindling to a GABA(A) receptor antagonist, pentylenetetrazol. Taken together and with literature findings, the data indicate that dopamine D(3) receptors function in the initiation of a dampening mechanism against the toxic effects of cocaine, a finding that might have relevance to psychiatric disorders of drug dependence, schizophrenia, and bipolar depression.

Divergent effect of the selective D3 receptor agonist pd-128,907 on locomotor activity in Roman high- and low-avoidance rats: relationship to NGFI-A gene expression in the Calleja islands

RATIONALE

The inbred Roman high- (RHA-I) and low-avoidance (RLA-I) rats, differing in dopaminergic activity and novelty/substance-seeking profiles, may be a suitable model to study the implication of the dopaminergic system in vulnerability to drug abuse. Differences in D3 receptor binding recently described between the two strains (Guitart-Masip M, Johansson B, Fernández-Teruel A, Cañete T, Tobeña A, Terenius L, Giménez-Llort L, Neuroscience 142:1231-1243, 2006b) may be important in shaping the aforementioned differences in novelty seeking.

OBJECTIVE

The aim of the present work was to study the effect of D3 receptor activation on novelty-induced locomotor activity in these two strains of rats.

MATERIALS AND METHODS

We administered saline and PD-128,907 (0.01 and 0.1 mg/kg), a putative D3 receptor agonist, to the Roman rats and studied the locomotor activity when animals were placed in a novel environment. Thereafter, by means of in situ hybridization, nerve growth factor inducible clone A (NGFI-A) mRNA was measured in the striatum and the Calleja islands of these animals.

RESULTS

We found that RLA-I rats showed stronger locomotor inhibition than RHA-I rats after PD-128,907 administration. Moreover, RLA-I rats showed stronger reduction of NGFI-A mRNA in the Calleja islands than RHA-I rats.

CONCLUSIONS

These results, together with previous findings, suggest that differences in D3 receptor expression in the Calleja islands may contribute to the divergent behavioral effect of PD-128,907 administration in the two strains of Roman rats.

Regulation of dopamine transporter function and cell surface expression by D3 dopamine receptors

D(3) dopamine receptors are expressed by dopamine neurons and are implicated in the modulation of presynaptic dopamine neurotransmission. The mechanisms underlying this modulation remain ill defined. The dopamine transporter, which terminates dopamine transmission via reuptake of released neurotransmitter, is regulated by receptor- and second messenger-linked signaling pathways. Whether D3 receptors regulate dopamine transporter function is unknown. We addressed this issue using a fluorescent imaging technique that permits real time quantification of dopamine transporter function in living single cells. Accumulation of the fluorescent dopamine transporter substrate trans-4-[4-(dimethylamino)styryl]-1-methylpyridinium (ASP(+)) in human embryonic kidney cells expressing human dopamine transporter was saturable and temperature-dependent. In cells co-expressing dopamine transporter and D3 receptors, the D2/D3 agonist quinpirole produced a rapid, concentration-dependent, and pertussis toxin-sensitive increase of ASP(+) uptake. Similar agonist effects were observed in Neuro2A cells and replicated in human embryonic kidney cells using a radioligand uptake assay in which binding to and activation of D3 receptors by [(3)H]dopamine was prevented. D3 receptor stimulation activated phosphoinositide 3-kinase and MAPK. Inhibition of either kinase prevented the quinpirole-induced increase in uptake. D3 receptor activation differentially affected dopamine transporter function and subcellular distribution depending on the duration of agonist exposure. Biotinylation experiments revealed that the rapid increase of uptake was associated with increased cell surface and decreased intracellular expression and increased dopamine transporter exocytosis. In contrast, prolonged agonist exposure reduced uptake and transporter cell surface expression. These results demonstrate that D3 receptors regulate dopamine transporter function and identify a novel mechanism by which D3 receptors regulate extracellular dopamine concentrations.

The dopamine D3 receptor antagonist NGB 2904 increases spontaneous and amphetamine-stimulated locomotion

The dopamine D3 receptor is believed to play an important role in regulation of rodent locomotor behavior, and has been proposed as a therapeutic target for substance abuse, psychotic disorders, and Parkinson's disease. One model of dopamine D3 receptor function, based on studies utilizing D3 receptor knockout mice and D3 receptor-preferring agonists, proposes that D3 receptor stimulation is inhibitory to psychostimulant-induced locomotion, in opposition to the effects of concurrent dopamine D1 and D2 receptor stimulation. Recent progress in medicinal chemistry has led to the development of highly-selective dopamine D3 receptor antagonists. In order to extend our understanding of D3 dopamine receptor's behavioral functions, we determined the effects of the highly-selective dopamine D3 receptor antagonist NGB 2904 on amphetamine-stimulated and spontaneous locomotion in wild-type and dopamine D3 receptor knockout mice. NGB 2904 (26.0 microg/kg s.c.) enhanced amphetamine-stimulated locomotion in wild-type mice, but had no measurable effect in dopamine D3 receptor knockout mice. Of a range of doses (0.026 microg-1.0 mg/kg) given acutely or once daily for seven days, the highest dose of NGB 2904 (1.0 mg/kg) stimulated spontaneous locomotion in wild-type mice, but was without measurable effect in dopamine D3 receptor knockout mice. These behavioral effects of NGB 2904 contrast with those described for other highly D3 receptor-selective antagonists, which have not previously demonstrated an effect on spontaneous locomotor activity. In combination, these data add to the behavioral profile of this novel D3 receptor ligand and provide further support for a role for dopamine D3 receptor inhibitory function in the modulation of rodent locomotion.

Evidence for the role of dopamine D3 receptors in oral operant alcohol self-administration and reinstatement of alcohol-seeking behavior in mice

The present study examined the effects of the acute intraperitoneal (i.p.) administration of the selective dopamine (DA) D(3) receptor antagonist SB-277011A (10, 20 or 30 mg/kg i.p.) on the oral operant self-administration of alcohol in male C57BL/6N mice. These effects were compared with those of naltrexone (0.5, 1 and 2 mg/kg i.p.) and acamprosate (100, 200 and 400 mg/kg i.p.). Compared with vehicle, the acute administration of SB-277011A (10 or 20 mg/kg) did not significantly alter the operant self-administration of alcohol, whereas the 30 mg/kg dose significantly reduced alcohol intake (g/kg), the number of reinforcers, and the number of active lever presses. The oral self-administration of alcohol was not significantly altered by the acute administration of either naltrexone or acamprosate, compared with vehicle-treated mice. SB-277011A, naltrexone and acamprosate were also tested in a model of drug/cue-triggered reinstatement of alcohol-seeking behavior. In this model, neither naltrexone (2 mg/kg) nor acamprosate (400 mg/kg) prevented relapse to alcohol-seeking behavior. In contrast, SB-277011A significantly reduced reinstatement of alcohol seeking in a dose-dependent manner. Provided these results can be extrapolated to humans, they suggest that selective DA D(3) receptor antagonists may be useful in the pharmacotherapeutic management of alcohol intake and prevention of relapse to alcohol-seeking behavior.

Tonic dopaminergic stimulation impairs associative learning in healthy subjects

Endogenous dopamine plays a central role in salience coding during associative learning. Administration of the dopamine precursor levodopa enhances learning in healthy subjects and stroke patients. Because levodopa increases both phasic and tonic dopaminergic neurotransmission, the critical mechanism mediating the enhancement of learning is unresolved. We here probed how selective tonic dopaminergic stimulation affects associative learning. Forty healthy subjects were trained in a novel vocabulary of 45 concrete nouns over the course of 5 consecutive training days in a prospective, randomized, double-blind, placebo-controlled design. Subjects received the tonically stimulating dopamine-receptor agonist pergolide (0.1 mg) vs placebo 120 min before training on each training day. The dopamine agonist significantly impaired novel word learning compared to placebo. This learning decrement persisted up to the last follow-up 4 weeks post-training. Subjects treated with pergolide also showed restricted emotional responses compared to the PLACEBO group. The extent of 'flattened' affect with pergolide was related to the degree of learning inhibition. These findings suggest that tonic occupation of dopamine receptors impairs learning by competition with phasic dopamine signals. Thus, phasic signaling seems to be the critical mechanism by which dopamine enhances associative learning in healthy subjects and stroke patients.

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.

Regulation of CART mRNA in the rat nucleus accumbens via D3 dopamine receptors

A variety of studies indicate that CART in the nucleus accumbens (NAcc) is involved in the action of psychostimulants. In order to understand in more detail if and how dopamine is involved in the regulation of CART mRNA in the NAcc, the present studies of individual receptors were performed. The D1 agonist, dihydrexidine, and the D1 antagonist, SCH23,390, were administered separately and in combination to adult male rats; however, no changes were found in CART mRNA as measured by in situ hybridization. The D2/3 agonist, quinpirole, was administered either separately or in combination with the D2 selective antagonist, L741,626, or the D3 selective antagonist, GR103,691. Quinpirole produced a decrease in CART mRNA of up to 43%. This effect was blocked by pretreatment with the D3 antagonist GR103, 691, but not by the D2 antagonist, L741,626. CART peptide levels showed a similar decrement after acute quinpirole. CART mRNA levels in the NAcc of D3 mutant mice were found to be higher than that in wild-type animals, but treating the mutants with quinpirole failed to produce a decrease in CART expression like that observed in wild-type rodents. These findings demonstrate that CART is regulated by dopamine in the NAcc, at least partly by D3 dopamine receptors.

Dopamine D3 receptor modulation of dopamine efflux in the rat nucleus accumbens

The effect of antipsychotics on electrically evoked dopamine efflux in the rat nucleus accumbens core and shell was investigated, using in vitro fast cyclic voltammetry. In the nucleus accumbens core, the dopamine D2/D3 receptor agonist, (+/-)7-OH-DPAT ((+/-)-2-dipropylamino-7-hydroxy-1,2,3,4-tetrahydronaphthalene), inhibited dopamine efflux with a pEC50 of 8.1. Clozapine, haloperidol, sulpiride and the selective dopamine D3 receptor antagonist, SB-277011-A, had no effect on dopamine efflux per se but all attenuated the (+/-)7-OH-DPAT-induced-inhibition of dopamine efflux, with pA2 values of 6.6, 7.9, 7.0 and 7.6, respectively. In the nucleus accumbens shell, (+/-)7-OH-DPAT inhibited dopamine efflux with a pEC50 of 8.3. Clozapine and SB-277011-A had no effect on dopamine efflux. In contrast, haloperidol and sulpiride significantly increased dopamine efflux through a D2 receptor-mediated mechanism. Clozapine, haloperidol, sulpiride and SB-277011-A attenuated the (+/-)7-OH-DPAT-induced inhibition with pA2 values of 7.3, 8.6, 7.6 and 8.2, respectively. These data demonstrate that dopamine efflux is modulated by both dopamine D2 and D3 receptors in the rat nucleus accumbens.

KKHA-761, a potent D3 receptor antagonist with high 5-HT1A receptor affinity, exhibits antipsychotic properties in animal models of schizophrenia

KKHA-761, 1-{4-[3-(3,4-dimethoxy-phenyl)-isoxazol-5-yl]-butyl}-4-(2-methoxy-phenyl)-piperazine, has a high affinity (Ki=3.85 nM) for human dopamine D3 receptor with about 70-fold selectivity over the human dopamine D(2L) receptor (Ki=270 nM). KKHA-761 also showed high affinity for cloned human 5-HT1A receptor (Ki=6.4 nM). KKHA-761 exhibited D3 and 5-HT1A receptor antagonist activities in vitro, reversing dopamine- or 5-HT-mediated stimulation of [35S]GTPrS binding. The in vivo pharmacological profile of KKHA-761 was compared with both typical and atypical antipsychotics including clozapine and haloperidol. Apomorphine-induced dopaminergic behavior, cage climbing, in mice was potently blocked by a single administration (i.p.) of KKHA-761 (ID50=4.06 mg/kg) or clozapine (ID50=4.0 mg/kg). Cocaine- or MK-801-induced hyperactivity in animals was markedly inhibited by KKHA-761 or clozapine. In addition, KKHA-761 significantly reversed the disruption of prepulse inhibition (PPI) produced by apomorphine in mice, indicating the antidopaminergic or antipsychotic activity of KKHA-761 in mice. However, KKHA-761 was inactive in the forced swimming behavioral despair model in mice, suggesting lack of antidepressant properties. KKHA-761 attenuated the hypothermia induced by a selective dopamine D3 agonist, 7-OH-DPAT, in mice, whereas clozapine enhanced it. Moderate doses of both KKHA-761 and clozapine did not increase serum prolactin levels in rats. Lower doses of, however, haloperidol significantly increased prolactin secretion. KKHA-761 did not induce cataleptic response up to 20 mg/kg, but significant catalepsy was shown at lower doses of clozapine and haloperidol. Furthermore, KKHA-761 showed a low incidence of rotarod ataxia (TD50=34.4 mg/kg, i.p.) in mice. The present results, therefore, suggest that KKHA-761 is a potent antipsychotic agent with combined dopamine D3 and serotonin 5-HT1A receptors modulation activity, which may further enhance its therapeutic potential for anxiety, psychotic depression, and other related disorders.

Brain stimulation and morphine reward deficits in dopamine D2 receptor-deficient mice

RATIONALE

The rewarding effects of lateral hypothalamic brain stimulation, various natural rewards, and several drugs of abuse are attenuated by D1 or D2 dopamine receptor (D1R or D2R) antagonists. Much of the evidence for dopaminergic involvement in rewards is based on pharmacological agents with limited or "relative" selectivity for dopamine receptor subtypes. Genetically engineered animal models provide a complementary approach to pharmacological investigations.

OBJECTIVES

In the present study, we explored the contribution of dopamine D2Rs to (1) brain stimulation reward (BSR) and (2) the potentiation of this behavior by morphine and amphetamine using D2R-deficient mice.

METHODS

Wild-type (D2Rwt), heterozygous (D2Rhet), and D2R knockout (D2Rko) mice were trained to turn a wheel for rewarding brain stimulation. Once equivalent rate-frequency curves were established, morphine-induced (0, 1.0, 3.0, and 5.6 mg/kg s.c.) and amphetamine-induced (0, 1.0, 2.0, and 4.0 mg/kg i.p.) potentiations of BSR were determined.

RESULTS

The D2Rko mice required approximately 50% more stimulation than the D2Rwt mice did. With the equi-rewarding levels of stimulation current, amphetamine potentiated BSR equally across the three genotypes. In contrast, morphine potentiated rewarding stimulation in the D2Rwt, had no effect in the D2Rhet, and antagonized rewarding stimulation in the D2Rko mice.

CONCLUSIONS

D2R elimination decreases, but does not eliminate, the rewarding effects of lateral hypothalamic stimulation. After compensation for this deficit, amphetamine continues to potentiate BSR, while morphine does not.

Phenotypic studies on dopamine receptor subtype and associated signal transduction mutants: insights and challenges from 10 years at the psychopharmacology-molecular biology interface

BACKGROUND

Mutants with targeted gene deletion ('knockout') or insertion (transgenic) of D1, D2, D3, D4 and D5 dopamine (DA) receptor subtypes are complemented by an increasing variety of double knockout and transgenic-'knockout' models, together with knockout of critical components of DA receptor signalling cascades such as G alpha(olf)[G gamma7], adenylyl cyclase type 5, PKA [RIIbeta] and DARPP-32. However, it is increasingly recognised that these molecular techniques have a number of inherent limitations. Furthermore, there are poorly understood methodological factors that contribute to inconsistent phenotypic findings between laboratories.

OBJECTIVE

This review seeks to document the impact of DA receptor subtype and related transduction mutants on our understanding of the behavioural roles of these entities, primarily at the level of unconditioned psychomotor behaviour.

METHODS

It includes ethologically based and orofacial movement studies in our own laboratories, since these are the only studies to systematically compare each of the D1, D2, D3, D4 and D5 receptor and DARPP-32 signal transduction 'knockouts'.

DISCUSSION

There is a particular emphasis on identifying methodological factors that might influence phenotypic effects and account for inconsistencies. The findings are offered empirically to (1) specify the extent of phenotypic diversity among individual DA receptor subtypes and transduction components and (2) indicate relationships between D1, D2, D3, D4 and D5 receptor subtype proteins, associated G alpha(i)/G alpha(s)/G alpha(olf)[G gamma7]-adenylyl cyclase type 5-PKA [RIIbeta]-DARPP-32 signalling cascades and behaviour. The findings are also offered heuristically as a base for such phenotypic comparisons at additional levels of behaviour so that a yet more complete phenotypic profile might emerge.

Dopamine D3 receptor antagonists as therapeutic agents

The behavioral and pathophysiological role of the dopamine D(3) receptor, which was deduced from anatomical, lesion and drug treatment studies in the ten years following cloning of the receptor, indicated that its functions differed from those of the D(2) receptor. There is increasingly strong evidence that D(3) receptor antagonists will be effective antipsychotic agents. In this regard, an amelioration of the negative and cognitive symptoms of schizophrenia holds the most promise for D(3) receptor antagonists, a concept currently under clinical evaluation. In addition, D(3) receptors could be involved in behavioral sensitization and the potential application of D(3) receptor antagonists in the treatment of drug abuse is undergoing intensive experimental investigation.

Dopamine agonist-induced yawning in rats: a dopamine D3 receptor-mediated behavior

A specific role for the dopamine D3 receptor in behavior has yet to be elucidated. We now report that dopamine D2/D3 agonists elicit dose-dependent yawning behavior in rats, resulting in an inverted U-shaped dose-response curve. A series of experiments was directed toward the hypothesis that the induction of yawning is a D3 receptor-mediated effect, whereas the inhibition of the yawning observed at higher doses is due to competing D2 receptor activity. We compared several dopaminergic agonists with a range of in vitro D3 selectivity, including PD-128,907 [(S)-(+)-(4aR, 10bR)-3,4,4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano-[4,3-b]-1,4-oxazin-9-ol HCl], PD-128,908 [(R)-(-)-(4aS,10bS)-3,4,4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano-[4,3-b]-1,4-oxazin-9-ol HCl], quinelorane [(5aR-trans)-5,5a,6,7,8, 9,9a,10-octahydro-6-propylpyrido[2,3-g]quinazolin-2-amine dihydrochloride], pramipexole (N'-propyl-4,5,6,7-tetrahydrobenzothiazole-2,6-diamine), 7-OH-DPAT [(+/-)-7-hydroxy-2-dipropylaminotetralin HBr], quinpirole [trans-(-)-(4aR)-4,4a,5,6,7,8, 8a,9-octahydro-5-propyl-1H-pyrazolo[3,4-g]quinoline HCl], bromocriptine [(+)-2-bromo-12'-hydroxy-2'-(1-methylethyl)-5'-(2-methylpropyl) ergotaman-3',6'-18-trione methanesulfonate], and apomorphine [(R)-(-)-5,6,6a,7-tetrahydro-6-methyl-4H-dibenzo-[de,g]quinoline-10,11-diol HCl] with respect to their ability to induce yawning in rats. A series of D2/D3 antagonists differing in selectivity for D3 over D2 receptors were evaluated for their ability to alter the effects of the dopamine agonists. The antagonists L-741,626 (3-[4-(4-chlorophenyl)-4-hydroxypiperidin-l-yl]methyl-1H-indole), haloperidol (4-[4-(4-chlorophenyl)-4-hydroxy-1-piperidinyl]-1-(4-fluorophenyl)-1-butanone HCl), nafadotride (N-[(1-butyl-2-pyrrolidinyl)methyl]-4-cyano-1-methoxy-2-naphtha-lenecarboxamide), U99194 (2,3-dihydro-5,6-dimethoxy-N,N-dipropyl-1H-inden-2-amine maleate), SB-277011A (trans-N-[4-[2-(6-cyano-1,2,3,4-tetrahydroisoquinolin-2-yl)ethyl]cyclohexyl]-4-quinolinecarboxamide), and PG01037 (N-{4-[4-(2,3-dichlorophenyl)-piperazin-1-yl]-trans-but-2-enyl}-4-pyridine-2-yl-benzamide HCl) were used to determine effects on dose-response curves for D2/D3 agonist-induced yawning. In addition, the potential contribution of cholinergic and/or serotonergic mechanisms to the yawning response was investigated using a series of pharmacological tools including scopolamine [(a,S)-a-(hydroxymethyl)benzeneacetic acid (1a,2b,4b,5a,7b)-9-methyl-3-oxa-9-azatricyclo[3.3.1.02,4]-non7-yl ester hydrobromide], mianserin (1,2,3,4,10,14b-hexahydro-2-methyldibenzo[c,f]pyrazino[1,2-a]azepine HCl), and the D3-preferring antagonists nafadotride, U99194, SB-277011A, and PG01037 to differentially modulate yawning induced by PD-128,907, physostigmine [(3aS)-cis-1,2,3,3a,8,8a-hexahydro-1,3a,8-trimethylpyrrolo[2,3-b]indol-5-ol methylcarbamate hemisulfate], and N-[3-(trifluoromethyl)phenyl]piperazine HCl. The results of these experiments provide convergent evidence that dopamine D2/D3 agonist-induced yawning is a D3 agonist-mediated behavior, with subsequent inhibition of yawning being driven by competing D2 agonist activity. Thus, dopamine agonist-induced yawning may represent an in vivo method for selectively identifying D3 and D2 receptor-mediated activities.

[Central dopamine receptors: general considerations (Part 1)]

Specific protein G-coupled receptors mediate the effects of dopamine in its projection areas. Five dopaminergic receptors have been cloned, characterized and classified in two families, the D1-like family (D1- and D5-receptor subtypes) and the D2-like family (D2-, D3- and D4-receptor subtypes). These five dopamine receptor subtypes are characterized by their diversity in terms of transduction, distribution, pharmacology, functions, and regulation, leading to pleiotropic pathophysiological and pharmacological involvements in neuropsychiatric disorders underlain by a deregulation of the dopaminergic system.

Phenotypic analysis of dopamine receptor knockout mice; recent insights into the functional specificity of dopamine receptor subtypes

The functional specificity of dopamine receptor subtypes remains incompletely understood, in part due to the absence of highly selective agonists and antagonists. Phenotypic analysis of dopamine receptor knockout mice has been instrumental in identifying the role of dopamine receptor subtypes in mediating dopamine's effects on motor function, cognition, reward, and emotional behaviors. In this article, we provide an update of recent studies in dopamine receptor knockout mice and discuss the limitations and future promise of this approach.

Mapping the effects of the selective dopamine D2/D3 receptor agonist quinelorane using pharmacological magnetic resonance imaging

Dopamine agonists with a high affinity for D2 and D3 receptors have a biphasic effect on rodent locomotion, inducing hypolocomotion at low doses and hyperlocomotion at higher doses. Controversy surrounds the role of the D3 receptor in mediating the hypolocomotor response to low agonist doses. This study examines patterns of neuronal activation induced by varying doses of the D2/D3 receptor agonist quinelorane using blood oxygen level dependent (BOLD) pharmacological magnetic resonance imaging (phMRI), and compares them with corresponding behavioural responses. Quinelorane (3 microg/kg) induced hypolocomotion in rats naive to the testing environment, and in phMRI experiments increased neuronal activity within the anterior olfactory nuclei, nucleus accumbens and islets of Calleja, regions containing a high density of D3 receptors. A 30 microg/kg dose of quinelorane resulted in biphasic locomotor effects, with initial hypolocomotion followed by sustained hyperlocomotion. phMRI indicated that this higher dose increased cerebral activity within limbic and olfactory regions, as did the lower drug dose, but induced additional activation in the caudate-putamen and globus pallidus, areas dense in D2 receptors but containing few D3 receptors. The more restricted pattern of activation at low agonist doses and close temporal relationship between behavioural and BOLD signal responses to quinelorane suggest that those nuclei most dense in D3 receptors play a key role in mediating the hypolocomotor effects of quinelorane. However, the presence of D3 receptors in activated brain regions may be coincidental, and further studies are required to show definitively which class of receptors mediates agonist-induced hypolocomotion. In contrast, the activation of D2 receptors within the striatum appears necessary for quinelorane-induced hyperlocomotion.

Effects of dopamine D3 receptor antagonists on spontaneous and agonist-reduced motor activity in NMRI mice and Wistar rats: comparative study with nafadotride, U 99194A and SB 277011

Studies investigating the role of the dopamine D3 receptor in the regulation of motor activity of rodents have used several ligands; however, there have been few comparative studies using agonist-antagonist interactions. In the present study, we compared the effects of dopamine D3 antagonists with different levels of selectivity over D2 receptors (nafadotride, U 99194A and SB 277011) on motor activity as well as on agonist-induced hypoactivity, in mice and rats. Horizontal and vertical movements were measured in photocell activity cages. 7-Hydroxy-2-(di-n-propylamino)tetralin (7-OH-DPAT) and PD 128907 were used as dopaminergic agonists. Both dose-dependently inhibited motor activity in mice and vertical activity in rats, while decreasing horizontal activity of rats at doses of 0.01 and 0.1 mg/kg s.c., with no effect (7-OH-DPAT) or stimulation (PD 128907) at the 1 mg/kg dose. In mice habituated to the activity cage, nafadotride (0.1-3 mg/kg i.p.) caused a dose-dependent decrease in motor activity but did not affect the hypomotility evoked by either 7-OH-DPAT (0.1 mg/kg) or PD 128907 (0.1 mg/kg). In habituated rats it had no significant effect on motor activity and was not able to antagonize the hypoactivity caused by PD 128907 (0.1 mg/kg s.c.). U 99194A (5, 10 and 20 mg/kg s.c.) dose-dependently and significantly increased motor activity in mice and inhibited the effects of both agonists. In rats, nafadotride produced considerable motor stimulation and significantly inhibited the PD 128907-induced decrease in horizontal, but not in vertical, activity. SB 277011 (15-45 mg/kg p.o.) significantly increased motor activity in mice and partially blocked the action of 7-OH-DPAT on vertical, but not on horizontal, activity while against PD 128907, its significant inhibitory effect was restricted to a single dose (20 mg/kg). In habituated rats, SB 277011 (13.5, 20 and 30 mg/kg p.o.) exerted no significant effects on motor activity and did not antagonize the hypoactivity caused by PD 128907. Considerable species differences and movement-type differences (horizontal versus vertical) were observed between the effects of the tested dopamine D2/D3 ligands on motor activity in rodents. The antagonists also differed markedly in the robustness of their action. The poorly D3 selective antagonist, nafadotride, had little effect on motor behaviour. The moderately selective U 99194A exerted marked stimulatory effects on motility, and potently inhibited the actions of agonists. SB 277011, a highly selective dopamine D3 receptor antagonist, showed limited ability to influence the motor activity of rodents.

Dopamine and alcoholism: neurobiological basis of ethanol abuse

The role of the dopamine (DA) system in brain reward mechanisms and the development of substance abuse has been well established. We review earlier animal and human studies on DA and alcoholism with some relevant issues relating to those studies. The present animal and human data suggest several alterations in the DA system in the context of alcoholism. Receptor studies imply that DA D(2) receptor density and function are lower at least among type 1 alcoholics, which suggests that they could benefit from drugs that enhance DAergic activity, such as partial DA agonists. These drugs could help to restore suboptimal levels of DAergic activity by reducing both the craving for alcohol in abstinence and the euphoria subsequent to alcohol's release of DA in the nucleus accumbens (NAC), thus providing negative reinforcement for relapse.

Morphine-induced alterations of immune status are blocked by the dopamine D2-like receptor agonist 7-OH-DPAT

Morphine administration produces profound effects on the immune system, including reductions in natural killer cell activity, mitogen-induced lymphocyte proliferation, and cytokine production. Although it has been established that the activation of central nervous system (CNS) micro-opioid receptors by morphine induces immunomodulation, little is known about the neural mechanisms underlying such processes. Interestingly, it has been shown that the dopamine (DA) D2-like receptor agonist 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) blocks the effect of morphine on a number of behaviors that are mediated by central dopamine pathways. The present study examined whether dopamine is involved in the immunomodulatory effects of morphine. In separate experiments, 7-OH-DPAT was administered either systemically (subcutaneous, s.c.) or centrally (intracerebroventricularly, i.c.v.) prior to morphine treatment in male Lewis rats. The results demonstrate that both systemic and central administration of 7-OH-DPAT attenuate the suppressive effect of morphine on several measures of immune status. Overall, these findings provide the first evidence that CNS dopaminergic mechanisms are directly involved in morphine-induced immunomodulation.

Behavioral effects of dopamine agonists and antagonists in MPTP-lesioned D3 receptor knockout mice

To test the modulatory role of D(3) receptors in normal and dopamine-depleted mice, D(3) receptor KO mice and wild-type (WT) littermates were administered saline, L-dopa/carbidopa (20/2 mg/kg ip), a preferential D(3)>D(2) agonist S32504, a D1+D(2)/D(3) agonist apomorphine, a selective D(3) antagonist S33084, or apomorphine with S33084 prior to and after administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). We monitored lines crossed in a 55-min session, average number of rears, and average number of grooming bouts. MPTP treatment produced equivalent 70% losses of dopamine fibers in the caudate putamen (CPu) and nucleus accumbens (NAC) of WT and D(3) KO mice as compared to their control (vehicle injected) counterparts. D(3) receptors were absent in KO mice, and the number of D(3) receptors was unaffected by MPTP-induced loss of DA terminals in WT mice. The results support a lack of involvement of the D(3) receptor for D1:D2 receptor-mediated behavioral activity (synergy). First, S32504 inhibited all behaviors and to a similar degree in D(3) KO and WT mice. Second, S33084 at the higher concentration increased number of lines crossed in response to high dose apomorphine in both D(3) KO and WT mice. Third, in nonlesioned mice, apomorphine-induced gnawing stereotypies were inhibited by S33084 in both D(3) KO and WT mice. Interestingly, the inhibition of apomorphine-induced gnawing was not apparent in MPTP-lesioned mice, and this stereotypy was elevated in D(3) KO-MPTP-lesioned mice. Thus, the suppressive effects of S32504 could be via D2 autoreceptor inhibition of DA release, and D2 receptor blockade by S33084 leads to release of that inhibition. This may be more apparent in MPTP-lesioned partially DA denervated mice.

Ethanol consumption in mice with a targeted disruption of the dopamine-3 receptor gene

Considerable evidence suggests that the mesolimbic dopaminergic system is an important substrate for the rewarding effects of ethanol consumption. Previous data have demonstrated that pharmacological agents that alter dopamine signaling also influence the self-administration of ethanol. The present experiments were designed to assess the role of the dopamine-3 receptor (D3-R) on voluntary ethanol consumption in C57BL/6 mice. Mice with targeted disruption of the D3-R gene (D3-R - /-) were compared to wild-type controls in an ethanol intake paradigm. In Experiment 1, mice had 24-hour access to ethanol each day in a two-bottle choice paradigm for a period of 7 days per concentration. The concentrations tested were 3, 6, 10 and 15%. In Experiment 2, mice had I hour of access to ethanol each day in a two-bottle paradigm for a period of 7 days per concentration. The same concentrations in Experiment I were compared in Experiment 2. In Experiment 3 we sought to test the development of a conditioned taste aversion (CTA) after receiving an intraperitoneal (ip.) injection of 2.0 g/kg ethanol. In Experiment 4, blood ethanol levels where assessed following a 2.0 g/kg ip.injection of ethanol. Experiment 5 assessed taste preference for saccharine and quinine in wild-type and D3-R -/- mice. Contrary to our predictions, both D3-R -/- and wild-types on a CS57BL/6 background had similar intakes of ethanol, at all concentrations tested, in the 24-hour and 1-hour intake paradigms. Wild-type and D3-R -/- mice respond to injected ethanol similarly by developing a conditioned taste aversion. Metabolic analysis revealed mutant mice are slower in metabolizing a bolus injection of ethanol. Lastly, wild-type and D3-R -/- mice showed similar consumption to increasing concentration of both sweet and bitter tastes. These data suggest that deletion of the D3-R gene does not increase ethanol consumption above that found on the C57BL/6 genetic background. Furthermore, the D3-R -/- mice adequately learn a CTA to ethanol and do not ham differing taste reactivity to saccharin or quinine. However, D3-R -/- mice do appear to have a slower rate of ethanol metabolism.

The dopamine D3 receptor mediates locomotor hyperactivity induced by NMDA receptor blockade

N-methyl-D-aspartate (NMDA)/glutamate receptor antagonists, like phencyclidine, generate schizophrenic-like symptoms in humans and behavioural abnormalities in animals, such as hyperactivity. We investigated the role of the dopamine D(3) receptor in locomotor hyperactivity produced in mice by dizocilpine (MK-801), another NMDA receptor antagonist, at a low dose (0.12 mg/kg). BP 897, a highly D(3) receptor-selective partial agonist, or nafadotride, a preferential D(3) receptor antagonist, both at low doses (1 mg/kg and lower), had no effects on spontaneous activity and completely inhibited MK-801-induced hyperactivity. Clozapine, an atypical antipsychotic, produced the same effect as BP 897 and nafadotride. Haloperidol, a typical antipsychotic, reduced both spontaneous activity and MK-801-induced hyperactivity. In D(3) receptor knockout mice, MK-801-induced hyperactivity was weaker than that observed in wild-type mice while BP 897 and nafadotride were inactive. On the contrary, the effects of clozapine and haloperidol, which target multiple receptors in addition to the D(3) receptor, were almost completely preserved in D(3) receptor knockout mice. Our results show that hyperactivity produced by a low dose of MK-801 is dependent upon D(3) receptor stimulation and constitutes the first simple response to assess the in vivo activity of D(3) receptor-selective drugs. In addition, since D(3) receptor antagonists and antipsychotics produced very similar effects, our results add to the growing evidence suggesting that D(3) receptor blockade might produce antipsychotic effects.

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.

Evidence for regulation of body temperature in rats by dopamine D2 receptor and possible influence of D1 but not D3 and D4 receptors

The dopamine D(3) receptor agonist PD 128907 decreased body temperature in the rat. The selective dopamine D(3) and D(4) receptor antagonists, A-437203 and L-745,870, respectively, did not prevent this effect. In contrast, PD 128907-induced hypothermia was antagonized by SCH 23390, a selective D(1) receptor antagonist, and by L-741,626, a selective D(2) receptor antagonist. Moreover, the selective D(2) receptor agonist trihydroxy-N-n-propylnoraporphine (TNPA) elicited a robust hypothermia which was prevented by pretreatment with L-741,626 but not by A-437203. In agreement with previous data obtained in D(3) knock-out mice, present results suggest that D(2) rather than D(3) receptors mediate dopamine receptor agonist-induced hypothermia in rats. In addition, it appears that both D(1) and D(2) receptors may be involved in a cooperative manner.

D(3) dopamine receptors are down-regulated in amphetamine sensitized rats and their putative antagonists modulate the locomotor sensitization to amphetamine

D(3) dopamine receptor agonists inhibit locomotor activity in rodents and modulate the reinforcing effect of psychostimulants; however, their functional role during behavioral sensitization remains unclear. In the present study, we intend to investigate if D(3) dopamine receptors alter during the amphetamine sensitization and test if manipulation of D(3) receptors would affect the development of locomotor sensitization to amphetamine. We have found that D(3) dopamine receptors are down-regulated in the limbic forebrain in chronic amphetamine-treated (5 mg/kg x 7 days) animals. The levels of both D(3) receptor protein (B(max) value) and mRNA decreased significantly in the behaviorally sensitized rats compared to the saline-treated controls. When animals were co-administered a putative D(3) receptor antagonist (U99194A or GR103691; 20 microg x 7 days; intracerebroventricle) and amphetamine (5 mg/kg x 7 days, i.p.), the locomotor sensitization to amphetamine was significantly inhibited. However, when the putative D(3) receptor antagonist U99194A was administered during the amphetamine withdrawal period at day 10, it did not affect the development of locomotor sensitization. Furthermore, pretreatment with the preferential D(3) agonist 7-hydroxydipropylaminotetralin partially blocked the inhibitory effect of U99194A on locomotor sensitization. These data prove the participation of D(3) dopamine receptors in the development of amphetamine sensitization and, in addition, suggest a potential application for D(3) antagonists in the prevention of amphetamine addiction.

Dopamine receptors and transporters in the brain reward circuits of type 1 and 2 alcoholics measured with human whole hemisphere autoradiography

The role of the dopamine system in brain reward mechanisms and development of substance abuse is well-established with nucleus accumbens as a key structure in mediating these effects. Several studies on alcoholism have indicated defects in dopaminergic neurotransmission and alterations in dopamine receptor densities. However, it has remained unclear if the substance abuse-related dopaminergic defect is specifically associated with a certain receptor subtype. The aim of this study was to compare putative alterations of dopamine D(1,) D(2), and D(3) receptors in nucleus accumbens, amygdala, and substantia nigra among alcoholics and controls. We studied the densities of dopamine D(1) and D(3) receptors in brains of 9 type 1 alcoholics, 8 type 2 alcoholics, and 10 healthy controls by using postmortem human whole hemisphere autoradiography. The mean densities of dopamine D(1) and D(3) receptors were at the same level in all study groups. Combining these with our previous results, our data suggest that among type 1 alcoholics dopamine transporters are lower in nucleus accumbens and dopamine D(2), but not D(1) or D(3) receptors in nucleus accumbens and amygdala. Further, the densities of all these dopamine-binding sites among type 2 alcoholics are at the level of healthy controls. The results suggest that lower dopamine receptor density is specific for D(2) receptor and for type 1 alcoholism, which supports Cloninger's neurogenetic model of two alcoholic subtypes, and indicates the importance of classifying these subgroups separately when issues related to dopaminergic activity are studied.

Elevated D3 dopamine receptor mRNA in dopaminergic and dopaminoceptive regions of the rat brain in response to morphine

As opiates increase dopamine transmission, we measured the effects of morphine on dopamine-related genes using a real-time optic PCR assay that reliably detects small differences in mRNA in discrete brain regions. Tissue from dopaminoceptive and dopaminergic brain regions was collected from rats injected twice daily for 7 days with saline or increasing doses of morphine. Tissues were assayed for D1, D2 and D3 dopamine receptor mRNAs (D1R, D2R and D3R), as well as for mRNAs for tyrosine hydroxylase (TH) and the dopamine transporter (DAT). The neuron-associated mRNAs for SNAP-25 and synaptophysin, as well as the glial-associated mRNA for S100-beta and three 'housekeeping' mRNAs, were also measured. As reported previously by others, there was no alteration in D1R mRNA and a 25% decrease in D2R mRNA in the caudate-putamen, 2 h after the final morphine injection. Importantly, in the same RNA extracts, D3R mRNA showed significant increases of 85% in the caudate-putamen and 165% in the ventral midbrain, including the substantia nigra and ventral tegmental area. There were no other significant morphine effects. Mapping of brain regions in saline control rats agreed with previous studies, including showing the presence of low abundance TH mRNA and the absence of DAT mRNA in the caudate-putamen. The finding that chronic, intermittent injections of morphine caused an increase in D3R mRNA extends our understanding of the ability of D3R agonists to reduce the effects of morphine.

7-OH-DPAT and PD 128907 selectively activate the D3 dopamine receptor in a novel environment

The D3 dopamine receptor is expressed primarily in limbic brain areas, and appears to play an inhibitory role in rodent locomotor behavior. Evidence suggests a potential role for the D3 receptor in the pathology of neuropsychiatric disease. Progress in elucidating D3 receptor function has been hampered, however, by a lack of well-characterized, selective ligands and by conflicting information regarding the behavioral phenotype of D3 receptor knockout mice. Here, we describe studies evaluating the behavioral effects of (+/-)-7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) and PD 128907, two D3 receptor agonists whose in vivo selectivity has been a topic of considerable controversy. We demonstrate that both compounds inhibit locomotion under novel environmental conditions in wild-type (WT) mice, but are without measurable behavioral effect under identical conditions in D3 receptor knockout mice. Additionally, we demonstrate that at low, D3 selective doses, these compounds are without behavioral effect in both WT and D3 receptor knockout mice that have acclimated to the testing environment. These findings suggest that D3 receptor stimulation inhibits novelty-stimulated locomotion, and establish conditions for the use of 7-OH-DPAT and PD 128907 as D3 receptor agonists in vivo. Potential implications of these observations are discussed.

Nafadotride administration increases D1 and D1/D2 dopamine receptor mediated behaviors

The administration of nafadotride, given at doses known to block the D3 dopamine receptors (0.75, 1.5, 3 mg/kg i.p.) increased locomotor activity both in naive and habituated rats and counteracted the hypothermia but not the hypomotility induced by a low dose of the putative D3 dopamine agonist (+/-)-7-hydroxy-2-(di-N-propylamino)-tetralin (7-OH-DPAT; 0.04 mg/kg). Nafadotride did not antagonize either the motor effects induced by different doses of the D2 agonist quinpirole (0.05 and 0.3 mg/kg) or the hypermotility induced by 7-OH-DPAT given at a dose (0.32 mg/kg) stimulating D2 dopamine receptors. The same nafadotride doses potentiated the grooming behavior induced by the D1 dopamine agonist SKF 38393 (10 mg/kg i.p.) as well as the stereotyped response to the D1/D2 agonist apomorphine (0.5 mg/kg s.c.). Stereotyped behavior was also observed in rats concomitantly treated with nafadotride and the D2 agonist quinpirole. As the activation of D1 dopamine receptors plays an important role in the occurrence of stereotypies, the results suggest that the blockade of D3 receptors by nafadotride could have favored D1/D2 dopamine receptor-mediated behaviors by potentiating D1 receptor function.

Phenotypic, ethologically based resolution of spontaneous and D(2)-like vs D(1)-like agonist-induced behavioural topography in mice with congenic D(3) dopamine receptor "knockout"

Uncertainty as to the functional role of the D(3) dopamine receptor, due primarily to a paucity of selective agonists or antagonists, is being addressed in mice with targeted gene deletion ("knockout") thereof. This study describes, for the first time, the phenotype of congenic D(3)-null mice. Initially, 129/Sv x C57BL/6 D(3)-null mice were backcrossed 14 times onto C57BL/6; they were then assessed using an ethologically based approach which resolves all topographies of behaviour within the mouse repertoire. The ethogram of D(3)-null mice, on comparison with wildtypes, was characterised by no alteration in any topography of behaviour over an initial period of exploration; subsequent assessment over several hours revealed only increased rearing among females due to delayed habituation. Low doses of the selective D(2)-like agonist RU 24213 (0.016-0.25 mg/kg) inhibited topographies of exploratory behaviour; this effect was diminished in D(3)-null mice only when investigated following prolonged habituation, and then only for certain topographies of behaviour, primarily sniffing and rearing. High doses of RU 24213 (0.1-12.5 mg/kg) induced stereotyped sniffing and "ponderous" locomotion, while the selective D(1)-like agonist SK&F 83959 (0.016-2.0 mg/kg) promoted characteristic grooming syntax; these effects did not differ materially between the genotypes. When examined topographically on an essentially congenic C57BL/6 background (<0.005% 129/Sv), the resultant phenotype indicated essential conservation of the mouse ethogram, high-dose D(2)-like stimulatory effects, and D(1)-like stimulatory effects in the absence of D(3) receptors. A role for D(3) receptors in inhibitory processes appeared topographically circumscribed and only when baseline levels of behaviour were low.

Dopamine autoreceptor regulation of release and uptake in mouse brain slices in the absence of D(3) receptors

The effects of the dopamine D(3) receptor, a putative autoreceptor, have been investigated by comparing behavioral and neurochemical properties of wild-type mice and mice with a genetic deletion of the D(3) receptor. The D(3) knock-out mice were modestly hyper-responsive to a novel environment relative to wild-type mice, and, consistent with this, quantitative in vivo microdialysis revealed elevated striatal dopamine extracellular levels. The dynamic actions of autoreceptors on electrically evoked dopamine release were examined in striatal brain slices from these animals and monitored with fast scan cyclic voltammetry at carbon-fiber microelectrodes. Quinpirole, a dopamine receptor agonist with potency at both D(2) and D(3) receptors, inhibited evoked dopamine in a dose-dependent manner with a slightly higher dose required in the knock-out animals (EC(50) of 60+/-10 nM in wild-type animals and 130+/-40 in D(3) knock-out animals; both curves had a Hill slope near 2). Dopamine synthesis inhibition with alpha-methyl-p-tyrosine caused released dopamine levels to decrease in each genotype. However, regulation of secretion by autoreceptors was still operant. Dose-response curves to quinpirole were unchanged in D(3) knock-out tissue, but secretion-regulated release exhibited a Hill slope decreased to 1 in the wild-type animals. In both genotypes, similar quinpirole-evoked increases in uptake rate were evident following synthesis inhibition. These data are consistent with the D(3) receptor having a small but significant role as a dopamine autoreceptor that partially regulates secretion, but not synthesis, in the caudate-putamen.

D(3) receptor ligands modulate extracellular dopamine clearance in the nucleus accumbens

An involvement of the D(3) dopamine receptor in the regulation of extracellular dopamine has been suggested. However, the mechanisms mediating this effect are unclear. We have used the technique of no net flux microdialysis under transient conditions to examine the influence of the D(3) -preferring agonist (+)-PD128907 upon extracellular dopamine levels in the nucleus accumbens of the mouse. (+)-PD 128907 (0.1 mg/kg intraperitoneally) significantly decreased extracellular dopamine. This decrease was associated with a marked increase in the extraction fraction, which suggests an increase in dopamine clearance. The ability of D(3) -preferring compounds to modulate dopamine uptake was investigated in vitro using rotating disk electrode voltammetry. (+)-PD 128907 (10 nm) significantly increased the initial clearance rate of 3 microm dopamine in rat nucleus accumbens tissue suspensions. Kinetic analysis revealed no change in the apparent K (m) of uptake but it showed a 33% increase in V (max). In contrast, the D(3) antagonist GR 103691 (10 nm) significantly decreased dopamine uptake. Consistent with the low levels of D(3) receptors in the dorsal striatum, neither compound affected uptake in tissue suspensions from this brain region. These data indicate that D(3) receptor activation increases dopamine uptake in the nucleus accumbens and suggest that this receptor subtype can regulate extracellular dopamine by modulating the DA transporter activity.

Role of dopamine D2-like receptors in cocaine self-administration: studies with D2 receptor mutant mice and novel D2 receptor antagonists

Dopamine receptor subtypes have been classified generally as D1-like (e.g., D1, D5) or D2-like (D2, D3, D4), and converging evidence suggests that D2-like receptors may be especially important in mediating the abuse-related effects of cocaine. However, it has been difficult to differentiate the roles of the D2-like receptor subtypes in the behavioral effects of cocaine because of the relatively low selectivity of drugs for D2, D3, and D4 receptors in vivo. The goal of the present series of studies was to investigate the contributions of D2-like receptor subtypes in the reinforcing effects of cocaine using new genetic and pharmacological tools. First, we evaluated cocaine self-administration behavior, and related effects of nonselective D2-like drugs, in mutant mice that lack the D2 receptor but express D3 and D4 receptors. When high doses of cocaine on the descending limb of the cocaine dose-effect function were available, D2 mutant mice self-administered at higher rates than their heterozygous or wild-type littermates, but the ascending limb of the cocaine dose-effect function did not differ between genotypes. Elevated rates of drug intake were not attributable to nonspecific increases in response rate, because response rates maintained by presentation of a range of food concentrations were significantly lower in D2 mutant mice than in wild-type mice. In wild-type mice, pretreatment with the D2-like antagonist eticlopride increased rates of self-administration of high doses of cocaine, and the D2-like agonist quinelorane served as a positive reinforcer when substituted for cocaine. However, these effects of eticlopride and quinelorane were not observed in mice that lacked the D2 receptor. Next, we compared the effects of novel antagonists selective for different D2 receptor subtypes on cocaine self-administration behavior in outbred rats. In rats, a D2 selective antagonist increased rates of self-administration of high doses of cocaine and also combinations of cocaine and the D2-like agonist quinelorane, whereas D3/D4 antagonists were ineffective. Collectively, these findings suggest that the D2 receptor is not necessary for cocaine self-administration, but this receptor subtype is involved in mechanisms that limit rates of high-dose cocaine self-administration. Our results also suggest that D3 and D4 receptors do not play major roles in the modulation of cocaine self-administration by D2-like drugs.

Toward a molecular understanding of psychostimulant actions using genetically engineered dopamine receptor knockout mice as model systems

A major focus in studying the progression and prevention of addictive diseases has been to understand the molecular and cellular mechanisms underlying drug addiction. The brain dopaminergic system plays a central role in reward and motivation and is thought to be the main neural substrate for the actions of abusive drugs. We have used the gene targeting technology to generate dopamine D1 and D3 receptor knockout mice and used these mice as model systems to gain a molecular understanding of acute effects of psychostimulants cocaine and amphetamine. The use of a combined approach involving behavioral, electrophysiological as well as molecular studies has allowed us to define initially the roles of dopamine D1 and D3 receptors in the acute effects of psychostimulants and will enable us to understand mechanisms underlying their chronic actions in the future.

IL-1 receptor type I gene expression in the amygdala of inflammatory susceptible Lewis and inflammatory resistant Fischer rats

Lewis (LEW/N) and Fischer (F344/N) rats have different responses to inflammatory and behavioral stressors due to differences in hypothalamus-pituitary-adrenal (HPA) axis function. For example, LEW/N rats are more sensitive to restraint, inflammation and experimentally induced autoimmunity due to decreased HPA activity. The HPA axis response to peripheral inflammation is mediated, at least in part, by IL-1beta and its receptor, IL-1 type I (IL-1RI). Here, we studied the distribution of IL-1RI mRNA in the brains of LEW/N and F344/N rats, and demonstrated that IL-1RI mRNA expression has significantly increased in the basolateral nucleus (BLA) of the amygdala of LEW/N rats. These findings suggest that strain-specific HPA axis responses may be mediated by extrahypothalamic pathways.

Common discriminative stimulus properties in rats of muscarinic antagonists, clozapine and the D3 preferring antagonist PNU-99194a: an analysis of possible mechanisms

Dopamine D3 receptors have been implicated in the aetiology of schizophrenia and the actions of antipsychotic drugs. The initial studies reported here assessed the involvement of such receptors in the in vivo actions of the atypical antipsychotic clozapine and the putative D3-preferring antagonist PNU-99194A in drug discrimination assays. Rats trained to discriminate clozapine consistently generalized to PNU-99194A in two separate studies. However, four other putative D3-preferring antagonists (PD 152255, (+)-S14297, nafadotride and (+)-AJ 76) did not induce generalization to clozapine. In rats trained to discriminate PNU-99194A, which has been suggested to induce a stimulus mediated specifically by D3 antagonism, the D3-preferring antagonist (+)-UH 232 and clozapine both induced full generalization. However, the PNU-99194A-trained animals also generalized fully to the muscarinic antagonists scopolamine and trihexyphenidyl. A possible explanation for the symmetrical generalization observed between clozapine and PNU-99194A is that these drugs have common muscarinic antagonist actions, since muscarinic antagonists have been reported to substitute for clozapine in numerous prior studies. However, in vitro receptor binding studies with M1-M5 receptors indicated that (with the possible exception of the M4 receptor), no muscarinic receptor subtype had high affinity for both clozapine, PNU-99194A and scopolamine. In addition, other binding studies indicated that whereas clozapine and PNU-99194A had high affinity for the D3 receptor, scopolamine did not. It is therefore concluded that: (1) The generalization seen between clozapine, PNU-99194A and muscarinic antagonists may be mediated by common effects 'downstream' from either muscarinic or D3 receptors; (2) D3 antagonism does not play a critical role in the clozapine stimulus (since D3-preferring antagonists did not consistently induce generalization to clozapine); (3) although D3 antagonism plays a role in the PNU-91994A stimulus (since the D3-preferring antagonist (+)-UH 232 induced full generalization, in accord with results from prior studies with other D3-preferring antagonists, the PNU-99194A stimulus also has commonalities with that induced by muscarinic antagonists and clozapine. The in vivo differences observed between PNU-99194A and other D3-preferring antagonists should be borne in mind when this agent is used as a tool to study D3 receptor functioning in vivo. The similarities between the PNU-99194A and clozapine stimuli suggest tentatively that compounds with a profile like PNU-99194A may have antipsychotic actions similar to clozapine. Some preclinical data are suggestive of such effects of PNU-99194A.