Human D3 dopamine receptor in the medulloblastoma TE671 cell line: cross-talk between D1 and D3 receptors

Dopamine D1 and D3 receptor interactions in cocaine reward and seeking in rats

RATIONALE

Animal research has demonstrated a role of dopamine D1 and D3 receptors in cocaine reward and seeking.

PURPOSE AND METHODS

Here, we investigated the potential interaction of these two dopamine receptors in cue-induced reinstatement of cocaine seeking, cocaine conditioned place preference (CPP), and cocaine self-administration in rats.

RESULTS

The co-administration of a D3 receptor antagonist, NGB 2904 and a D1 partial agonist, SKF 77434, of doses which when administered individually produced no significant effects, prior to reinstatement or CPP tests significantly reduced lever pressing and time spent in the cocaine-paired environment, suggesting synergistic effects of the combined compounds on cocaine seeking. When given to rats self-administering cocaine under a progressive ratio schedule of reinforcement doses of NGB 2904 which were ineffective alone significantly enhanced the break point-reducing effects of SKF 77434.

CONCLUSIONS

Our results indicate that the combined treatment with a D1 receptor partial agonist and D3 receptor antagonist produces robust decreases in cocaine seeking and reward. This suggests an interaction between dopamine D1 and D3 receptors in cocaine-related behaviors.

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.

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.

Cytochrome P450 aromatase in grey mullet: cDNA and promoter isolation; brain, pituitary and ovarian expression during puberty

In a study towards elucidating the role of aromatases during puberty in female grey mullet, the cDNAs of the brain (muCyp19b) and ovarian (muCyp19a) aromatase were isolated by RT-PCR and their relative expression levels were determined by quantitative real-time RT-PCR. The muCyp19a ORF of 1515bp encoded 505 predicted amino acid residues, while that of muCyp19b was 1485 bp and encoded 495 predicted amino acid residues. The expression level of muCyp19b significantly increased in the brain as puberty advanced; however, its expression level in the pituitary increased only slightly with pubertal development. In the ovary, the muCyp19a expression level markedly increased as puberty progressed. The promoter regions of the two genes were also isolated and their functionality evaluated in vitro using luciferase as the reporter gene. The muCyp19a promoter sequence (650 bp) contained a consensus TATA box and putative transcription factor binding sites, including two half EREs, an SF-1, an AhR/Arnt, a PR and two GATA-3 s. The muCyp19b promoter sequence (2500 bp) showed consensus TATA and CCAAT boxes and putative transcription binding sites, namely: a PR, an ERE, a half ERE, a SP-1, two GATA-binding factor, one half GATA-1, two C/EBPs, a GRE, a NFkappaB, three STATs, a PPAR/RXR, an Ahr/Arnt and a CRE. Basal activity of serially deleted promoter constructs transiently transfected into COS-7, alphaT3 and TE671 cells demonstrated the enhancing and silencing roles of the putative transcription factor binding sites. Quinpirole, a dopamine agonist, significantly reduced the promoter activity of muCyp19b in TE671. The results suggest tissue-specific regulation of the muCyp19 genes and a putative alternative promoter for muCyp19b.

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.

Deletion of dopamine D1 and D3 receptors differentially affects spontaneous behaviour and cocaine-induced locomotor activity, reward and CREB phosphorylation

Co-localization of dopamine D1 and D3 receptors in striatal neurons suggests that these two receptors interact at a cellular level in mediating dopaminergic function including psychostimulant-induced behaviour. To study D1 and D3 receptor interactions in cocaine-mediated effects, cocaine-induced locomotion and reward in mice lacking either D1, D3 or both receptors were analysed. Spontaneous locomotor activity was increased in D1-/- and D1-/-D3-/- mice and D1-/-D3-/- mice did not exhibit habituation of spontaneous rearing activity. Cocaine (20 mg/kg) increased locomotor activity in wild-type and D3-/- mice, failed to stimulate activity in D1-/- mice and reduced activity in D1-/-D3-/- mice. In the conditioned place preference, all groups exhibited reward at 5, 10 and 20 mg/kg of cocaine. D1-/-D3-/- mice did not demonstrate preference at 2.5 mg/kg of cocaine although preference was observed in wild-type, D1-/- and D3-/- mice. The transcription factor cAMP-responsive element binding protein (CREB) is activated by phosphorylation in striatal regions following dopamine receptor activation. Striatal pCREB levels following acute cocaine were increased in wild-type and D3-/- mice and decreased in D1-/- and D1-/-D3-/- mice. After repeated administration of 2.5 mg/kg of cocaine, D1-/- mice had lower pCREB levels in caudate-putamen and nucleus accumbens. Our findings suggest that, although spontaneous and cocaine-induced horizontal activity depended mainly on the presence of the D1 receptor, there may be crosstalk between D1 and D3 receptors in rearing habituation and the perception of cocaine reward at low doses of the drug. Furthermore, alterations in pCREB levels were associated with changes in cocaine-induced locomotor activity but not reward.

Cloning, characterization and expression of the D2 dopamine receptor from the tilapia pituitary

A full-length cDNA encoding a dopamine receptor (DA-R) was obtained from the pituitary of tilapia (ta). This cDNA encodes a protein of 469 amino acids that exhibits the typical arrangement of GPCR. The taDA-R shows high similarity to the DA-Rs of mullet and fugu, and over 70% similarity to Xenopus, mouse and turkey D2 DA-Rs. Northern blot analysis revealed transcript for a single transcript in the pituitary, of approximately 3 kb. In a Southern analysis, the tilapia probe recognized specific bands in the genomic DNA of both mullet and catfish, suggesting high similarity between the corresponding genes. Phylogenetic analysis clearly aligned the taDA-D2-R with all vertebrate D2-like receptor sequences cloned to date, and it was therefore designated taDA-D2-R. taDA-D2-R was transiently expressed in COS-7 cells together with the reporter construct CRE-luciferase. Addition of the specific D2 dopamine agonists quinpirole or bromocriptine, in the presence of forskolin, led to a dose-dependent decrease in forskolin-induced cAMP levels. Both agonists yielded the same maximal inhibition (around 40%). However, the potency of taDA-D2-R for bromocriptine was higher than for quinpirole. As established for mammalian D2-like receptors, stimulation of the taDA-D2-R with quinpirole triggers pertussis-toxin-sensitive Gi/o-mediated, but not Gs-mediated signaling. In contrast to mammals, PCR analysis gave no evidence of alternative splicing in taDA-D2-R. Pharmacological and genetic manipulation of the taDA-D2-R should enable us to better define its physiological role and to further explore the usefulness of fish as a model system for understanding dopaminergic function in higher organisms.

Aberrant D1 and D3 dopamine receptor transregulation in hypertension

Dopamine plays a role in the regulation of blood pressure by inhibition of sodium transport in renal proximal tubules (RPTs) and relaxation of vascular smooth muscles. Because dopamine receptors can regulate and interact with each other, we studied the interaction of D(1) and D(3) receptors in immortalized RPT cells and mesenteric arteries from Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHRs), and in human coronary artery smooth muscle cells (CASMCs). In WKY rats, the D(1)-like agonist, fenoldopam, increased D(3) receptor protein in a time-dependent and concentration-dependent manner (EC(50)=4.5x10(-9) M, t(1/2)=15.8 hours). In SHRs, fenoldopam (10(-5) M) actually decreased the expression of D(3) receptors. D(1) and D(3) receptor co-immunoprecipitation was increased by fenoldopam (10(-7) M/24 h) in WKY rats but not in SHRs. The effects of fenoldopam in CASMCs were similar as those in WKY RPT cells (ie, fenoldopam increased D(1) and D(3) receptor proteins). Both D(3) (PD128907, Emax=80%+/-6%, pED(50)=5+/-0.1) and D(1)-like receptor (fenoldopam, Emax=81%+/-8%, pED(50)=5+/-0.2, n=12) agonists relaxed mesenteric arterial rings. Co-stimulation of D(1) and D(3) receptors led to additive vasorelaxation in WKY rats, but not in SHRs. D(1) and D(3) receptors interact differently in WKY and SHRs. Altered interactions between D(1) and D(3) receptors may play a role in the pathogenesis of genetic hypertension, including human hypertension, because these receptors also interact in human vascular smooth muscle cells.

Neurochemical changes in dopamine D1, D3 and D1/D3 receptor knockout mice

Neurochemical changes were examined in dopamine D1 receptor knockout (D1(-/-)), dopamine D3 receptor knockout (D3(-/-)) and dopamine D1/D3 receptor double knockout (D1(-/-)D3(-/-)) mice. The level of dopamine D1- and D2-like receptors and gamma-aminobutyric acid (GABA(A)) receptor was assessed by ligand autoradiography and dopamine D1- and D2 receptor, enkephalin, dynorphin and substance P transcripts measured by in situ hybridization. D1(-/-) mice had normal GABA(A) receptor levels, reduced dynorphin and substance P, and increased enkephalin mRNA and dopamine D2-like binding. D1(-/-)D3(-/-) mice evidenced decreased dynorphin and substance P but normal enkephalin expression, whereas dopamine D2-like and GABA(A) receptor binding were increased. Major changes occur in substance P and dynorphin expression in D1(-/-) mice and these changes are unaffected by loss of dopamine D3 receptors. Upregulated dopamine D2-like binding and enkephalin in D1(-/-) mice may be due to decreased dopamine turnover. Upregulated enkephalin in D1(-/-) mice is dependent on functional dopamine D3 receptors.

Essential conservation of D1 mutant phenotype at the level of individual topographies of behaviour in mice lacking both D1 and D3 dopamine receptors

RATIONALE

In the absence of agonists and antagonists evidencing appropriate selectivities, individual and interactive properties of D(1) and D(3) dopamine receptors would be illuminated most powerfully by their co-deletion.

OBJECTIVES

To define and contrast the behavioural phenotype of D(1)/D(3) double knockout mice in comparison with wild types, and with individual D(1) and D(3 )mutants.

METHODS

Behavioural phenotype was characterised using an ethologically based topographical technique.

RESULTS

On comparison with wild types, D(1)/D(3) double mutants were characterised topographically as follows: increases in sniffing and locomotion, which evidenced delayed habituation; reductions in rearing free, rearing seated, grooming, chewing and stillness. Though the D(1)/D(3) double mutant ethogram comprised elements of both single mutant D(1) and D(3) lines, this phenotype was largely reflective of the D(1) mutant component.

CONCLUSIONS

Distinct patterns of initial exploratory behaviour and of temporal change over subsequent habituation were evident across the three genotypes, with particular conservation of the D(1) phenotype in D(1)/D(3 )double mutants. Under the present conditions, there was little systematic evidence for D(1):D(3) interactions in the regulation of these aspects of behaviour.

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.

Pharmacological and molecular characterisation of SK3 channels in the TE671 human medulloblastoma cell line

The expression of the small conductance calcium-activated potassium channels SK1, SK2 and SK3 was investigated in the TE671 human medulloblastoma cell line using RT-PCR and transcripts were detected only for SK3. Immunodetection experiments confirmed this result, demonstrating the presence of the SK3 protein. This potassium channel was characterised in TE671 cells using whole-cell patch-clamp recordings. Voltage steps to -100 mV from a holding potential of 0 mV in equimolar 140 mM intra- and extracellular K(+) (K(+)(in/out)) elicited an inward current. The reversal potential of this current shifted 56.6 mV per 10-fold increase in K(+)(out) thus suggesting K(+) selectivity. This current was dependent on the concentration of Ca(2+)(in) with an EC(50) of 104.2 nM. A pharmacological characterisation of this current revealed that it was not blocked by 1 microM charybdotoxin (ChTX), 0.3 microM iberiotoxin (IbTX) or 10 microM clotrimazole (CLT) and only modestly inhibited (<50%) by 30 nM scyllatoxin (ScTX), 200 microM dequalinium chloride (Deq) or 300 microM d-tubocurarine (d-TC). The non-selective SK blocker d-TC blocked the current with an IC(50) of 43.2 microM while apamin blocked the current to a much greater extent (87.8% at 1 microM) with an IC(50) of 4.3 nM. Furthermore, the current was significantly increased (132.6+/-5.2%, n=7) by 500 microM 1-ethyl-2-benzimidazolinone (EBIO). Collectively, these data demonstrate the presence of an endogenous SK3 channel in human TE671 cells.

Dopamine receptor-coupling defect in hypertension

Dopamine synthesized in non-neural tissues, eg, renal proximal tubule, functions in an autocrine or paracrine manner. The effects of dopamine are transduced by two classes of receptors (D1- and D2-like) that belong to the superfamily of G protein-coupled receptors. In genetic hypertension, the D1 receptor, a member of the D1-like receptor family, is uncoupled from its G protein complex, resulting in a decreased ability to regulate renal sodium transport. The impaired D1 receptor/G protein coupling in renal proximal tubules in genetic hypertension is secondary to abnormal phosphorylation and desensitization of the D1 receptor caused by activating single nucleotide polymorphisms of a G protein-coupled receptor kinase, GRK type 4.

Role of dopamine receptors in the kidney in the regulation of blood pressure

Regulation by dopamine of cardiovascular function, renal function and systemic blood pressure regulation is multifaceted. Each of the five dopamine receptor subtypes participates in the regulation of blood pressure by mechanisms specific for the subtype. Some receptors regulate blood pressure by influencing the central or peripheral nervous system; others influence epithelial transport and regulate the secretion and receptors of several humoral agents. The D1, D3, and D4 receptors interact with the renin-angiotensin system, while the D2 and D5 receptors interact with the sympathetic nervous system to regulate blood pressure.

Impaired renal D(1)-like and D(2)-like dopamine receptor interaction in the spontaneously hypertensive rat

D(1)-like (D(1), D(5)) and D(2)-like (D(2), D(3), D(4)) dopamine receptors interact in the kidney to produce a natriuresis and a diuresis. Disruption of D(1) or D(3) receptors in mice results in hypertension that is caused, in part, by a decreased ability to excrete an acute saline load. We studied D(1)-like and D(2)-like receptor interaction in anesthetized spontaneously hypertensive rats (SHR) by the intrarenal infusion of Z-1046 (a novel dopamine receptor agonist with rank order potency of D(3)> or =D(4)>D(2)>D(5)>D(1)). Z-1046 increased glomerular filtration rate (GFR), urine flow, and sodium excretion in normotensive Wistar-Kyoto rats but not in SHRs. The lack of responsiveness to Z-1046 in SHRs was not an epiphenomenon, because intrarenal cholecystokinin infusion increased GFR, urine flow, and sodium excretion to a similar extent in the two rat strains. We conclude that renal D(1)-like and D(2)-like receptor interaction is impaired in SHRs. The impaired D(1)-like and D(2)-like receptor interaction in SHRs is not caused by alterations in the coding sequence of the D(3) receptor, the D(2)-like receptor expressed in rat renal tubules that has been shown to be involved in sodium transport. Because the diuretic and natriuretic effects of D(1)-like receptors are, in part, caused by an interaction with D(2)-like receptors, it is possible that the decreased Z-1046 action in SHRs is secondary to the renal D(1)-like receptor dysfunction in this rat strain.

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.

Characterization of the 5' flanking region of the rat D(3) dopamine receptor gene

The D(3) dopamine receptor has a restricted regional distribution in brain and is regulated by dopaminergic agents. Additionally, the D(3) gene is implicated in the pathogenesis of several neuropsychiatric disorders or in their response to pharmacological agents. Elucidating its transcription control mechanisms is therefore of interest in order to explain these biological features of the D(3) gene. In this study, the 5' flanking region of the rat D(3) gene was characterized by isolating the 5' end of its cDNA as well as 4.6 kb of genomic sequence. Analysis of this region revealed the presence of two new exons 196-bp and 120-bp long, separated by an 855-bp intron, located several kilobases upstream of the previously published coding exons. Thus, current evidence indicates that the rat D(3) gene is organized into eight exons. Transcription initiation site was determined by primer extension analysis and repeated rounds of 5' RACE and was found to localize at a pyrimidine-rich consensus 'initiator' sequence, similar to the rat D(2) gene. The D(3) promoter lacks TATA or CAAT boxes but unlike that of other dopamine receptor genes has only 52% GC content. Functional analysis of D(3) promoter deletion mutants fused to a reporter gene in TE671 cells, which endogenously express this gene, revealed strong transcriptional activity localized within 36 nucleotides upstream of transcription start site, and a potent silencer between bases --37 and --537. The D(3) promoter is inactive in C6 and COS7 cells. We conclude that the D(3) gene, similar to the closely related D(2) gene, is transcribed from a tissue specific promoter which is under intense negative control.

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.

Dopamine D(3) receptor is selectively and transiently expressed in the developing whisker barrel cortex of the rat

The rodent primary somatosensory cortex (SI) contains a map of the body surface, the most conspicuous part of which are "barrels," neuronal aggregates in layer IV that receive somatotopic projections from whiskers on the rodent's snout. We report that the D(3) dopamine receptor (D(3)R) is selectively and transiently expressed in SI during the first 2 weeks of postnatal development. D(3)R binding sites and mRNA overlap completely and are limited to layer IV of SI. D(3)R/mRNA are organized in a pattern corresponding to somatotopic representations of the body (e.g., whiskers, jaws, paws, etc.) with the highest expression in the barrel field. D(3) mRNA is first detected at postnatal day (P)4, increases rapidly until P7-10, and sharply decreases after P14. D(3)R binding sites are detectable at P6, peak at P14, and decline afterwards. D(1), D(2), D(4), or D(5) mRNAs display dissimilar expression pattern. D(1) mRNA is mostly confined to infragranular layers throughout the cortex. D(4) mRNA expression in layer IV rises by 4 weeks postnatal, when D(3)R expression is virtually undetectable. Quantitative analysis of D(3) mRNA expression demonstrates that the proportion of D(3) mRNA-positive cells decreases between P7 and P14, whereas mRNA concentration per cell remains stable. Moreover, D(3)R number continues to rise, whereas mRNA levels begin to decline. Thus, a process limiting D(3)R expression to fewer cells may occur that also induces changes in post-transcriptional regulation of D(3)R expression in remaining cells. These findings indicate that dopamine acting via D(3)R may play an important role in the development or function of the SI.