Decreased c-fos responses to dopamine D(1) receptor agonist stimulation in mice deficient for D(3) receptors

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

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

Dopamine D3 Receptor Modulates l-DOPA-Induced Dyskinesia by Targeting D1 Receptor-Mediated Striatal Signaling

The dopamine D3 receptor (D3R) belongs to the dopamine D2-like receptor family and is principally located in the ventral striatum. However, previous studies reported D3R overexpression in the dorsal striatum following l-DOPA treatment in parkinsonian animals. This fact has drawn attention in the importance of D3R in l-DOPA-induced dyskinesia (LID). Here, we used D3R knockout mice to assess the role of D3R in LID and rotational sensitization in the hemiparkinsonian model. Mice lacking D3R presented a reduction in dyskinesia without interfering with the antiparkinsonian l-DOPA effect and were accompanied by a reduction in the l-DOPA-induced rotations. Interestingly, deleting D3R attenuated important molecular markers in the D1R-neurons such as FosB, extracellular signal-regulated kinase, and histone-3 (H3)-activation. Colocalization studies in D1R-tomato and D2R-green fluorescent protein BAC-transgenic mice indicated that l-DOPA-induced D3R overexpression principally occurs in D1R-containing neurons although it is also present in the D2R-neurons. Moreover, D3R pharmacological blockade with PG01037 reduced dyskinesia and the molecular markers expressed in D1R-neurons. In addition, this antagonist further reduced dyskinetic symptoms in D1R heterozygous mice, indicating a direct interaction between D1R and D3R. Together, our results demonstrate that D3R modulates the development of dyskinesia by targeting D1R-mediated intracellular signaling and suggest that decreasing D3R activity may help to ameliorate LID.

Disease-specific heteromerization of G-protein-coupled receptors that target drugs of abuse

Drugs of abuse such as morphine or marijuana exert their effects through the activation of G-protein-coupled receptors (GPCRs), the opioid and cannabinoid receptors, respectively. Moreover, interactions between either of these receptors have been shown to be involved in the rewarding effects of drugs of abuse. Recent advances in the field, using a variety of approaches, have demonstrated that many GPCRs, including opioid, cannabinoid, and dopamine receptors, can form associations between different receptor subtypes or with other GPCRs to form heteromeric complexes. The formation of these complexes, in turn, leads to the modulation of the properties of individual protomers. The development of tools that can selectively disrupt GPCR heteromers as well as monoclonal antibodies that can selectively block signaling by specific heteromer pairs has indicated that heteromers involving opioid, cannabinoid, or dopamine receptors may play a role in various disease states. In this review, we describe evidence for opioid, cannabinoid, and dopamine receptor heteromerization and the potential role of GPCR heteromers in pathophysiological conditions.

Mouse models of genetic effects on cognition: relevance to schizophrenia

Cognitive dysfunction is a core feature of schizophrenia. Growing evidence indicates that a wide variety of genetic mutations and polymorphisms impact cognition and may thus be implicated in various aspects of this mental disorder. Despite differences between human and rodent brain structure and function, genetic mouse models have contributed critical information about brain mechanisms involved in cognitive processes. Here, we summarize discoveries of genetic modifications in mice that impact cognition. Based on functional hypotheses, gene modifications within five model systems are described: 1) dopamine (D1, D2, D3, D4, D5, DAT, COMT, MAO); 2) glutamate (GluR-A, NR1, NR2A, NR2B, GRM2, GRM3, GLAST); 3) GABA (α(5), γ(2), α(4), δGABA(A), GABA(B(1)), GAT1); 4) acetylcholine (nAChRβ2, α7, CHRM1); and 5) calcium (CaMKII-α, neurogranin, CaMKKβ, CaMKIV). We also consider other risk-associated genes for schizophrenia such as dysbindin (DTNBP1), neuregulin (NRG1), disrupted-in-schizophrenia1 (DISC1), reelin and proline dehydrogenase (PRODH). Because of the presumed importance of environmental factors, we further consider genetic modifications within the stress-sensitive systems of corticotropin-releasing factor (CRF), brain-derived neurotrophic factor (BDNF) and the endocannabinoid systems. We highlight the missing information and limitations of cognitive assays in genetically modified mice models relevant to schizophrenia pathology.

The neurobiology of dopamine receptors: evolution from the dual concept to heterodimer complexes

CONTEXT

G protein-coupled receptors (GPCRs) have been classically thought to work as monomeric entities. The current view of their organization, however, assumes that they are part of highly organized molecular complexes, where different receptors and interacting proteins are clustered. These heteromers have peculiar pharmacological, signaling, and trafficking properties. GPCR heteromerization, raising different combinatorial possibilities, thus underlies an unexpected level of diversity within this receptor family.

METHODS

In this paper, we summarize recent data, reported by different research groups, suggesting that the dopamine (DA) D1 receptor forms heteromers with receptors of the same family and with structurally and functionally divergent receptors.

RESULTS AND DISCUSSION

DA D1 and D3 receptors and glutamate NMDA receptors regulate rewarding mechanisms and motivated behavior, modulate emotional and cognitive processes and regulate locomotor activity by extensive cross-talk mechanisms. Co-localization of D1 and D3 receptors and D1 and NMDA receptors in specific neuronal populations in the striatum and nucleus accumbens, moreover, suggested that their cross-talk may involve direct interactions. By using different experimental approaches various groups have, in fact, demonstrated the existence of D1-NMDA and D1-D3 heteromers, in both transfected cell systems and in the straitum, with peculiar pharmacological, signaling, and functional properties. The putative role of the D1-D3 and D1-NMDA heteromers in the physiological regulation of striatal function and in the development of motor dysfunctions will be discussed.

Dysregulation of dopamine-dependent mechanisms as a determinant of hypertension: studies in dopamine receptor knockout mice

Dopamine plays an important role in the pathogenesis of hypertension by regulating epithelial sodium transport and by interacting with vasoactive hormones/humoral factors, such as aldosterone, angiotensin, catecholamines, endothelin, oxytocin, prolactin pro-opiomelancortin, reactive oxygen species, renin, and vasopressin. Dopamine receptors are classified into D(1)-like (D(1) and D(5)) and D(2)-like (D(2), D(3), and D(4)) subtypes based on their structure and pharmacology. In recent years, mice deficient in one or more of the five dopamine receptor subtypes have been generated, leading to a better understanding of the physiological role of each of the dopamine receptor subtypes. This review summarizes the results from studies of various dopamine receptor mutant mice on the role of individual dopamine receptor subtypes and their interactions with other G protein-coupled receptors in the regulation of blood pressure.

Functional interaction between dopamine receptor subtypes for the regulation of c-fos expression

Dopaminergic drugs increase the expression of the proto-oncogene, c-fos, in the brain, which is involved in the coordination of neurobiological changes caused by repeated cocaine or amphetamine use. This study examined the roles of five dopamine receptor subtypes on the c-fos promoter activity. D(1)R or D(5)R significantly increased the expression of c-fos promoter by activating protein kinase A. However, D(2)R, D(3)R, or D(4)R did not show any noticeable effects. The co-expression of D(1)R/D(3)R or D(1)R/D(2)R synergistically activated the basal and agonist-induced expression of the c-fos promoter, respectively. The Ral guanine-nucleotide-dissociation-stimulator-like, which was found to interact with the 3rd cytoplasmic loop of D(3)R, mediated the inhibitory activity of D(3)R in c-fos expression. In summary, the expression of the c-fos promoter was increased by the D1-like receptors and enhanced synergistically by the D2-like receptors via the modulation of cellular cAMP. D(3)R inhibited the expression of the c-fos promoter through an interaction with RGL.

Effects of desensitized nicotinic receptors on rotational behavior in a 6-hydroxydopamine model of Parkinson's disease

The purpose of this study was to investigate the effects of desensitized nicotinic acetylcholine receptors (nAChRs) on rotational behavior in the unilateral 6-hydroxydopamine (6-OHDA) model of Parkinson's disease (PD). When rats were treated with different doses of nicotine, nAChRs were observed in activated, subacute desensitized, acute desensitized, and chronic desensitized states. The rotational behavior of the hemiparkinsonian rats was determined when nAChRs were in the activated or different desensitized states. The results showed that hemiparkinsonian rats exhibited no significant changes in apomorphine-induced rotation when brain nAChRs were in an activated state. However, hemiparkinsonian rats displayed a significant reduction in apomorphine-induced rotational behavior when brain nAChRs were in subacute, acute, or chronic desensitized states induced by repeated administration of nicotine. When nAChRs were blocked by the nAChR antagonist mecamylamine, the behavior of the hemiparkinsonian rats worsened. These results suggest that desensitized nAChRs can lead to behavioral improvement in the 6-OHDA rat model of PD.

Behavioral sensitization, alternative splicing, and d3 dopamine receptor-mediated inhibitory function

Behavioral sensitization, the progressive and enduring augmentation of certain behaviors following repetitive drug use, alters rodent locomotion in a long-standing manner. The same dopamine pathways playing an important role in drug dependence and psychosis also play a critical role in sensitization. Individual dopamine receptor subtypes have markedly different functional responses to stimulation, with D3 dopamine receptor stimulation inhibiting rodent locomotion. The D3 receptor has highest affinity of the dopamine receptor subtypes for dopamine, and is occupied to a greater degree following stimulant drug administration. D3 receptor activity may be regulated through the expression of an alternatively spliced, truncated receptor isoform (termed 'D3nf') altering receptor localization and function via dimerization with the full-length subunit. The expected physiological response to repetitive drug administration is tolerance. Tolerance of D3 receptor inhibition of locomotion would contribute to sensitization to stimulant drugs. We hypothesize that repetitive D3 receptor stimulation contributes to the development of behavioral sensitization through decreased responsivity of D3-receptor-mediated locomotor inhibition. Increased D3nf expression may direct altered receptor localization and subsequent release of D3-receptor-mediated inhibition, contributing to the expression of sensitization. These hypotheses follow directly from the affinities of the receptor subtypes for dopamine; dopamine concentrations following stimulant administration; the effects of individual dopamine receptor subtype stimulation on locomotion; and the expected homeostatic response of the system to perturbation by drug. Clarifying these mechanisms underlying sensitization may suggest new interventions for neuropsychiatric conditions in which dopamine plays an important role, including psychosis, drug dependence, and Parkinson's disease. This information may also elucidate a previously unrecognized mechanism regulating receptor trafficking and desensitization.

Characterization of the desensitization properties of five dopamine receptor subtypes and alternatively spliced variants of dopamine D2 and D4 receptors

Proper regulation of brain dopaminergic activity is essential for maintaining normal mental functions. In this study, the regulatory properties of five different dopamine receptor subtypes and alternative splicing variants of dopamine D2 and D4 were examined. The stimulation of D1R, D2R, D5R but not D3R, D4R caused the robust translocation of beta-arrestin to the plasma membrane. When D1R or D3R were co-expressed with D2R, D1R significantly inhibited the sequestration of D2R, suggesting that the inhibitory effects of D1R on the D2R sequestration could explain the synergistic activity between two receptors. The sequestration of alternatively spliced isoforms of D2R was differently regulated by GRKs and beta-arrestins. Three alternative splicing variants of D4R produced a similar level of beta-arrestin translocation, and the studies with the deletion mutants of D4R within the third cytoplasmic loop revealed that the regions containing the SH3-binding domains are responsible for the beta-arrestin translocation.

Conversion of the modulatory actions of dopamine on spinal reflexes from depression to facilitation in D3 receptor knock-out mice

Descending monoaminergic systems modulate spinal cord function, yet spinal dopaminergic actions are poorly understood. Using the in vitro lumbar cord, we studied the effects of dopamine and D2-like receptor ligands on spinal reflexes in wild-type (WT) and D3-receptor knock-out mice (D3KO). Low dopamine levels (1 microM) decreased the monosynaptic "stretch" reflex (MSR) amplitude in WT animals and increased it in D3KO animals. Higher dopamine concentrations (10-100 microM) decreased MSR amplitudes in both groups, but always more strongly in WT. Like low dopamine, the D3 receptor agonists pergolide and PD 128907 reduced MSR amplitude in WT but not D3KO mice. Conversely, D3 receptor antagonists (GR 103691 and nafadotride) increased the MSR in WT but not in D3KO mice. In comparison, D2-preferring agonists bromocriptine and quinpirole depressed the MSR in both groups. Low dopamine (1-5 microM) also depressed longer-latency (presumably polysynaptic) reflexes in WT but facilitated responses in D3KO mice. Additionally, in some experiments (e.g., during 10 microM dopamine or pergolide in WT), polysynaptic reflexes were facilitated in parallel to MSR depression, demonstrating differential modulatory control of these reflex circuits. Thus, low dopamine activates D3 receptors to limit reflex excitability. Moreover, in D3 ligand-insensitive mice, excitatory actions are unmasked, functionally converting the modulatory action of dopamine from depression to facilitation. Restless legs syndrome (RLS) is a CNS disorder involving abnormal limb sensations. Because RLS symptoms peak at night when dopamine levels are lowest, are relieved by D3 agonists, and likely involve increased reflex excitability, the D3KO mouse putatively explains how impaired D3 activity could contribute to this sleep disorder.

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.

In vivo CREB phosphorylation mediated by dopamine and NMDA receptor activation in mouse hippocampus and caudate nucleus

The pattern of CREB phosphorylation was investigated in the caudate nucleus and hippocampus 10 min or 3 h after i.p. injection of dopamine or NMDA receptor agonists alone, or in combination with antagonists. Ten minutes after C57BL/6 J mice were injected with either the dopamine D1 receptor agonist SKF-38393 hydrobromide or NMDA, immunoreactivity of phosphorylated CREB (pCREB) was significantly increased in all parts of the caudate nucleus but not in hippocampal regions. However, 3 h after the injection of SKF-38393, pCREB levels in the caudate nucleus did not differ significantly from the pCREB levels in control animals, whereas pCREB levels were still elevated 3 h after NMDA injection. Except for the D1 receptor antagonist SCH-23390, which induced CREB phosphorylation in the caudate nucleus, dopamine and NMDA receptor antagonists had little effect on pCREB levels by themselves. However, the NMDA receptor antagonist CGS-19755 injected i.p. blocked both the NMDA- and SKF-38393-induced rise of pCREB levels in the caudate nucleus. Similarly, the D1 receptor antagonist SCH-23390 inhibited the effects produced by SKF-38393 or NMDA. Interestingly, the D2 receptor antagonist sulpiride also blocked the SKF-38393-triggered rise of pCREB. The results demonstrated that NMDA and dopamine receptors modulate pCREB levels in the caudate nucleus and suggest mutual permissive roles for both receptors.

Effect of Methamphetamine on Cognition and Repetitive Motor Behavior of Mice Deficient for Dopamine D2and D3Receptors

Mice deficient for dopamine D2 and D3 receptors exhibit blunted D(1)-receptor responses to agonist stimulation. This blunted D1-receptor activity is prominent in the medial prefrontal cortex (mPFC) and results in a significantly impaired performance of the mutants in a test for spatial working memory. A single dose of methamphetamine (METH; 5 mg/kg i.p.), however, elicits a long-lasting increase in agonist-stimulated D1 receptor activity in the mPFC. In D2 mutants, this increase reaches wild-type levels, and the working memory of METH-treated mutants is completely rescued. In D3 mutants, however, the METH-induced increase in D1-receptor activity remains below wild-type levels and does not result in improved working memory performance. D2 and D3 mutants also differ in their locomotor responses to METH. Repeated administration of this drug (5 mg/kg administered three times at 2-h intervals) leads to a transition from horizontal hyperlocomotion to excessive orofacial stereotypy (taffy pulling) only in wild type and D3 mutants. In both genotypes, this transition is accompanied by a change in the relative ratios of striatal neuronal activation in two neurochemically distinct compartments, with striosomal neuronal activation exceeding that of the striatal matrix during stereotypy. Both the stereotypic response to METH and the associated predominant activation of neurons located in striosomes require D2-receptor expression. These studies indicate a differential requirement for D1- and D2-like receptor activation in mediating the effects of METH on cognitive and motor function.

Signaling Mechanisms of the D3Dopamine Receptor

A substantial body of evidence shows the capacity of the dopamine D3 receptor to couple functionally to G proteins when expressed in an appropriate milieu in heterologous expression systems. In these systems, activation of D3 receptors inhibits adenylate cyclase, modulates ion flow through potassium and calcium channels, and activates kinases, most notably mitogen-activated protein kinase. Coupling to Gi/Go is implicated in many of these effects, but other G proteins may contribute. Studies with chimeric receptors implicate the third intracellular loop in the mediation of agonist-induced signal transduction. Finally, D3-preferring drugs modulate expression of c-fos in neuronal cultures and brain. Signaling mechanisms of the D3 receptor in brain, however, remain to be definitively determined.

Identifying vulnerability markers in prodromal patients: a step in the right direction for schizophrenia prevention

Research has shown that many of the long-term deficits that are observable in schizophrenia populations are present prior to the emergence of psychotic symptoms. Recent research suggests schizophrenia has a "prodromal" period, whereby significant changes from premorbid functioning can be observed. Accurate classification of this period could have far-reaching implications for schizophrenia prevention. This article aims to provide an indepth evaluation of the perceived benefits of vulnerability marker research in this unique phase. It is hoped that identification of such markers may improve the predictive potency of prodromal criteria, and perhaps pave the way for future screening and primary prevention strategies.

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.

Focused motor stereotypies do not require enhanced activation of neurons in striosomes

Stereotypic motor behavior is a widespread phenomenon of many neurologic and psychiatric disorders. Studies on the mechanisms controlling motor stereotypies have focused on the role of dopamine in modulating the activity of basal ganglia neuronal circuits, and recent results demonstrated that stereotypic motor responses characteristic of psychomotor stimulant sensitization correlate with an enhanced activation of neurons located in striatal striosomes that substantially exceeds that of the surrounding matrix. The present study tested whether predominant striosomal activation is a general predictor for stereotypy. Wild-type and dopamine D(2) and D(3) receptor knockout mice were treated either three times with methamphetamine (METH; 3 x 5 mg/kg every 2 hours) or once with a full D(1) agonist. Depending on the genotype, both treatments elicit the same focused stereotypy (taffy pulling). Repeated METH-treatment elicits intense stereotypy in wild-type and D(3) mutants but not in D(2) single and D(2)/D(3) double mutants. The stereotypic response of wild-type and D(3) mutants correlates with a predominant activation of neurons located in striosomes. No striosomal predominance is detected in METH-treated D(2) single and D(2)/D(3) double mutants. In contrast, D(2) single and D(2)/D(3) double mutants exhibited the most severe stereotypic response to D(1)-agonist treatment. However, this treatment did not result in enhanced striosomal activation. Thus, whereas the expression of stereotypy in response to repeated METH treatment requires D(2) receptor expression, D(2) receptor expression diminishes stereotypic responses to an acute dose of a D(1) agonist. Enhanced striosomal activation, however, is a reliable indicator of D(1)- and D(2)-receptor coactivation but not a predictor for repetitive motor behavior in general.

Comparative phenotypic resolution of spontaneous, D2-like and D1-like agonist-induced orofacial movement topographies in congenic mutants with dopamine D2 vs. D3 receptor ?knockout?

Using a novel system, the role of D2-like dopamine receptors in distinct topographies of orofacial movement was assessed in mutant mice with congenic D2 vs. D3 receptor knockout, and compared with findings in D1A mutants. Under spontaneous conditions, D2 mutants evidenced increased vertical jaw movements and unaltered horizontal jaw movements, with reductions in tongue protrusions and incisor chattering; in D3 mutants, only incisor chattering was reduced. Given previous evidence that D1A mutants show reduced horizontal but not vertical jaw movements, this indicates that apparent oppositional D1-like:D2-like interactions in the regulation of composited jaw movements may in fact reflect the independent actions of D2 receptors to inhibit vertical jaw movements and of D1A receptors to facilitate horizontal jaw movements. Effects of the D2-like agonist RU 24213 to exert greater reduction in horizontal than in vertical jaw movements were not altered prominently in either D2 or D3 mutants. The D1-like agonists A 68930 and SK&F 83959 induced vertical jaw movements, tongue protrusions, and incisor chattering; induction of tongue protrusions by A 68930 was reduced in D2 mutants. D2 receptors exert topographically specific regulation of orofacial movements in a manner distinct from their D1A counterparts, while D3 receptors exert only minor regulation of such movements.

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.

Mice lacking dopamine D2 and D3 receptors have spatial working memory deficits

Mice deficient for dopamine D(2) and D(3) receptors exhibit blunted c-fos responses to D(1) agonist stimulation. Stereologic cell counting revealed decreased numbers of medial prefrontal cortex neurons that express Fos immunoreactivity in all layers, particularly in the prelimbic and anterior cingulate subregions. Pretreatment of these mutants with a single, low dose of methamphetamine (METH) led to a sustained increase in the number of neurons that express Fos immunoreactivity in response to a D(1) agonist challenge, which was most significant in prelimbic and anterior cingulate subregions. The increased c-fos responses reached wild-type-like levels in METH-pretreated D(2) mutants but remained submaximal in METH-pretreated D(3) mutants. Additional studies tested the performance of wild type and mutants in a delayed alternation test, a cognitive task critically dependent on optimal activation of prefrontal cortical D(1) receptors by synaptically released dopamine. Both D(2) and D(3) mutants exhibited deficits in their spatial working memory, with increasing impairments at increasing delays. Whereas METH pretreatment rescued the spatial working memory of D(2) mutants, it had no effect on D(3) mutants. These data suggest that the sustained improvement of spatial working memory in METH-pretreated D(2) mutants is attributable to D(1) receptor-mediated mechanisms.

A single dose of methamphetamine rescues the blunted dopamine D(1)-receptor activity in the neocortex of D(2)- and D(3)-receptor knockout mice

Knockout mice deficient for dopamine D(2) and D(3) receptors exhibit blunted c-fos responses to D(1)-agonist stimulation. A single dose of methamphetamine (METH), however, leads to a long-term reversal of these blunted c-fos responses in both mutants, and the same effect is obtained with a single administration of a full D(1)-agonist. Consistent with the predominant c-fos expression in the neocortex induced by METH itself, METH pretreatment leads to the largest D(1)-agonist-stimulated c-fos responses in the neocortex of these mutants. For example, a pronounced blunting of neocortical c-fos responses is detected in the prefrontal cortex, a region in which D(1) receptors play a critical role in working memory. METH pretreated mutants, however, exhibit robust c-fos responses in this region that are indistinguishable from wild type. Recent studies indicate that different mechanisms operate in brains of D(2) and D(3) mutants to lead to decreased D(1)-receptor activity. For example, drug-naive D(2), but not D(3), mutants show significantly decreased G protein activation in response to D(1)-agonist stimulation, and METH pretreatment also rescues this abnormal molecular phenotype. Moreover, although the protein phosphatases (PP) 1/2A and 2B play a critical role in modulating G protein activation in wild type, their effect is either diminished (PP1/2A) or abolished (2B) in D(2) mutants. Interestingly however, METH pretreatment does not rescue the activities of these phosphatases in the mutants, suggesting that the long-term effects of a single dose of METH are mediated via effector systems that act downstream of G protein activation.

Family and twin strategies as a head start in defining prodromes and endophenotypes for hypothetical early-interventions in schizophrenia

In an effort to share the experiences of 'genotype-hunters'-who have approached the difficult task of forecasting future schizophrenia in the young offspring or other relatives of index cases, in new samples guided by the prior probabilities of 15% in offspring or 50% in identical co-twins-with 'early-interventionists'-who focus on purported prodromal symptoms in children who would be treated pharmacologically to prevent the development of schizophrenia-we provide a focused review that emphasizes the hazards of false positives in both approaches. Despite the advantages prospective high-risk strategies have had from clinical and laboratory findings that implicate some prodromal signs and endophenotypes, e.g. attention, memory, and information processing evaluations, the yields are not sufficient for practical applications involving antipsychotic drugs for undiagnosed children. Even more caution than usual is required, given the suggestions that the developing neocortex is vulnerable to dopaminergic exposure.

Dopamine receptor functions: lessons from knockout mice [corrected]

In the past few years, a number of laboratories have used gene targeting via homologous recombination to generate mice deficient for key molecules involved in dopaminergic (DAergic) transmission. This tremendous effort has resulted in the successful generation and characterization of mice deficient for the neurotransmitter DA, the main terminator of DAergic neurotransmission (the DA transporter), and all five subtypes of DA receptors. This review summarizes the results from studies of the various DA receptor knockout mice and of mice deficient in proteins that mediate DA receptor signaling. It focuses on a comparison of the locomotor phenotypes and responses to drugs of abuse (psychostimulants), and reviews the results of anatomic studies examining the morphological and neurochemical differentiation of the striatum in these mutants. Moreover, an overview of recently published results highlighting the physiological relevance of the interaction between different DA receptors and between DA receptors and other neurotransmitter receptors in the modulation of behavioral and molecular responses to DAergic stimulation is presented. Finally, in view of the recently discovered heteroligomeric assemblies of neurotransmitter receptors that involve DA receptor subtypes, the potential value of knockout mice as a tool for testing the in vivo significance of these heteroligomeric receptors is discussed.

Dopamine D3 receptor as a therapeutic target for antipsychotic and antiparkinsonian drugs

The cloning of the gene for the D3 receptor and subsequent identification of its distribution in brain and pharmacology allowed for serious consideration of the possibility that it might be a target for drugs used to treat schizophrenia and Parkinson's disease (PD). That is because it is highly expressed in limbic regions of the brain, exhibits low expression in motor divisions, and has pharmacologic similarity to the D2 receptor. Thus, antipsychotics that were presumed to block D2 receptors also had high affinity for the D3 receptor. Dopamine agonists used to treat the clinical symptoms of PD also have high affinity for the D3 receptor, and two D3 receptor-preferring agonists were found to be effective for treatment of PD. Many compounds achieving high potency and selectivity are now available, but few have reached clinical testing. Recent findings with respect to the anatomy of this receptor in human brain, altered expression in schizophrenia and PD, and biological models to study its function support the proposal that it is a target for development of drugs to alleviate symptoms in neuropsychiatric and neurologic disorders. Because of distinct aspects of regulation of the D3 receptor, it represents a unique target for therapeutic intervention in schizophrenia without high potential for unintended side effects such as tardive dyskinesia. It may also be that D3 receptor agonists can provide neuroprotective effects in PD and can modify clinical symptoms that D2 receptor-preferring agonists cannot provide.

The psychopharmacology-molecular biology interface: exploring the behavioural roles of dopamine receptor subtypes using targeted gene deletion ('knockout')

In the absence of selective agonists and antagonists able to discriminate between individual members of the D1-like and D2-like families of dopamine receptor subtypes, functional parcellation has remained problematic. 'Knockout' of these subtypes by targeted gene deletion offers a new approach to evaluating their roles in the regulation of behaviour. Like any new technique, 'knockout' has associated with it a number of methodological limitations that are now being addressed in a systematic manner. Studies on the phenotype of D1(A/1), D(1B/5), D2, D3 and D4 'knockouts' at the level of spontaneous and agonist/antagonist-induced behaviour are reviewed, in terms of methodological issues, neuronal implications and potential clinical relevance. Dopamine receptor subtype 'knockout' is a nascent technology that is now beginning to fulfil its potential. It is being complemented by more systematic phenotypic characterisation at the level of behaviour and additional, molecular biologically-based approaches.

Neurotensin gene expression and behavioral responses following administration of psychostimulants and antipsychotic drugs in dopamine D(3) receptor deficient mice

Exposure to psychostimulants and antipsychotics increases neurotensin (NT) gene expression in the striatum and nucleus accumbens. To investigate the contribution of D(3) receptors to these effects we used mice with targeted disruption of the D(3) receptor gene. Basal NT mRNA expression was similar in D(3) receptor mutant mice and wild-type animals. Acute administration of haloperidol increased NT gene expression in the striatum in D(3)+/+, D(3)+/- and D(3)-/- mice. Similarly, acute cocaine and amphetamine induced NT mRNA expression in the nucleus accumbens shell and olfactory tubercle to a comparable extent in D(3) mutants and wild-type mice. Daily injection of cocaine for seven days increased NT mRNA in a restricted population of neurons in the dorsomedial caudal striatum of D(3)+/+ mice, but not in D(3)-/- and D(3)+/- animals. No differences were observed between D(3) receptor mutant mice and wild-type littermates in the locomotor activity and stereotyped behaviors induced by repeated cocaine administration. These findings demonstrate that dopamine D(3) receptors are not necessary for the acute NT mRNA response to drugs of abuse and antipsychotics but appear to play a role in the regulation of NT gene induction in striatal neurons after repeated cocaine. In addition, our results indicate that the acute locomotor response to cocaine and development of psychostimulant-induced behavioral sensitization do not require functional D(3) receptors.

A single dose of methamphetamine leads to a long term reversal of the blunted dopamine D1 receptor-mediated neocortical c-fos responses in mice deficient for D2 and D3 receptors

Dopamine D(1) receptors play an essential role in the induction of expression of the immediate-early gene c-fos in response to pharmacological stimuli. In the forebrain of wild-type mice, administration of a D(1) receptor agonist leads to c-fos mRNA expression levels that are substantially higher than corresponding levels expressed after indirect stimulation of dopamine receptors with methamphetamine. In mice deficient for D(2) and D(3) receptors, c-fos mRNA levels expressed in response to D(1) agonist administration are significantly blunted. However, a single dose of methamphetamine (5 mg/kg) leads to a long lasting reversal of the blunted c-fos responses in these mutants. In the forebrain, this reversal is restricted to the neocortex. Moreover, methamphetamine also enhances c-fos expression levels in preadolescent wild-type mice that normally express low c-fos mRNA in response to D(1) agonist stimulation. Thus, a single dose of methamphetamine leads to a long term increase in D(1) receptor-dependent c-fos responses in brains with either low (preadolescent mice) or blunted (adult D(2) and D(3) mutant mice) c-fos expression levels. A similar long term reversal of the blunted c-fos responses is achieved with a single dose of a full D(1) agonist. These results indicate that the constitutive inactivation of D(2) and D(3) receptors leads to a decrease in agonist-promoted D(1) receptor activity that can be reversed by intermittent agonist stimulation.

Modification of dopamine D(1) receptor knockout phenotype in mice lacking both dopamine D(1) and D(3) receptors

Experimental evidence suggests that dopamine D(1) and D(3) receptors may interact in an opposing or synergistic fashion. To investigate interactions between both receptors in behaviour, we have used dopamine D(1) and D(3) receptor knockout mice to generate mice lacking both receptors. D(1)(-/-)D(3)(-/-) mice were viable, fertile and showed no gross morphological abnormalities. In an open field, they exhibited lower activity than wild-type, D(1)(-/-) and D(3)(-/-) mice. D(1)(-/-)D(3)(-/-) mice performed equally poorly in the rotarod and Morris water maze tasks as their D(1)(-/-) littermates. Basal locomotor activity and anxiety-like behaviour were normal in D(1)(-/-)D(3)(-/-) mice. Combined deletion of both receptors abolished the exploratory hyperactivity and anxiolytic-like behaviour of dopamine D(3) receptor mutant phenotype and further attenuated the low exploratory phenotype of D(1)(-/-) mice. These results imply an interaction of both receptors in the expression of exploratory behaviour in a novel environment, and the need for the presence of intact dopamine D(1) receptor for the expression of certain behaviours manifested in dopamine D(3) receptor mutant phenotype. In addition, dopamine D(1) receptor, but not dopamine D(3) receptor, is involved in the ability to perform on the rotarod and spatial learning.