Receptors for dopamine and somatostatin: formation of hetero-oligomers with enhanced functional activity

The biology and physiology of somatostatin receptors

The biology and therapeutic application of somatostatin and its receptors are reviewed. The focus is on recent literature and patents, especially with regard to the specific function of each somatostatin receptor subtype. Detailed mechanisms mediating the effects of somatostatin and its analogues remain to be elucidated. Nevertheless, progress is being made towards a clear picture of the cellular signalling and physiological changes regulated by somatostatin and its receptors.

Heterodimerization of G-protein-coupled receptors: pharmacology, signaling and trafficking

Although classical models predict that G-protein-coupled receptors (GPCRs) function as monomers, several recent studies acknowledge that GPCRs exist as dimeric or oligomeric complexes. In addition to homodimers, heterodimers between members of the GPCR family (both closely and distantly related) have been reported. In some cases heterodimerization is required for efficient agonist binding and signaling, and in others heterodimerization appears to lead to the generation of novel binding sites. In this article, the techniques used to study GPCR heterodimers, and the 'novel pharmacology' and functional implications resulting from heterodimerization will be discussed.

Lipid-facing correlated mutations and dimerization in G-protein coupled receptors

A correlated mutation analysis has been performed on the aligned protein sequences of a number of class A G-protein coupled receptor families, including the chemokine, neurokinin, opioid, somatostatin, thyrotrophin and the whole biogenic amine family. Many of the correlated mutations are observed flanking or neighbouring conserved residues. The correlated residues have been plotted onto the transmembrane portion of the rhodopsin crystal structure. The structure shows that a significant proportion of the correlated mutations are located on the external (lipid-facing) region of the helices. The occurrence of these highly correlated patterns of change amongst the external residues suggest that they are sites for protein-protein interactions. In particular, it is suggested that the correlated residues may be involved in either large conformational changes, the formation of heterodimers or homodimers (which may be domain swapped) or oligomers required for activation or internalization. The results are discussed in the light of the subtype-specific heterodimerization observed for the chemokine, opioid and somatostatin receptors.

Chronic and acute regulation of Na+/Cl- -dependent neurotransmitter transporters: drugs, substrates, presynaptic receptors, and signaling systems

Na+/Cl- -dependent neurotransmitter transporters, which constitute a gene superfamily, are crucial for limiting neurotransmitter activity. Thus, it is critical to understand their regulation. This review focuses primarily on the norepinephrine transporter, the dopamine transporter, the serotonin transporter, and the gamma-aminobutyric acid transporter GAT1. Chronic administration of drugs that alter neurotransmitter release or inhibit transporter activity can produce persistent compensatory changes in brain transporter number and activity. However, regulation has not been universally observed. Transient alterations in norepinephrine transporter, dopamine transporter, serotonin transporter, and GAT1 function and/or number occur in response to more acute manipulations, including membrane potential changes, substrate exposure, ethanol exposure, and presynaptic receptor activation/inhibition. In many cases, acute regulation has been shown to result from a rapid redistribution of the transporter between the cell surface and intracellular sites. Second messenger systems involved in this rapid regulation include protein kinases and phosphatases, of which protein kinase C has been the best characterized. These signaling systems share the common characteristic of altering maximal transport velocity and/or cell surface expression, consistent with regulation of transporter trafficking. Although less well characterized, arachidonic acid, reactive oxygen species, and nitric oxide also alter transporter function. In addition to post-translational modifications, cytoskeleton interactions and transporter oligomerization regulate transporter activity and trafficking. Furthermore, promoter regions involved in transporter transcriptional regulation have begun to be identified. Together, these findings suggest that Na+/Cl- -dependent neurotransmitter transporters are regulated both long-term and in a more dynamic manner, thereby providing several distinct mechanisms for altering synaptic neurotransmitter concentrations and neurotransmission.

Insect G protein-coupled receptors and signal transduction

G protein-coupled receptors (GPCRs) are seven-transmembrane proteins (7-TM) that transduce extracellular signals into cellular physiological responses through the activation of heterotrimeric guanine nucleotide binding proteins (alpha beta gamma subunits). Their general properties are remarkably well conserved during evolution. Despite this general resemblance, a large variety of different signals are mediated via this category of receptors. Several GPCR-(sub)families have an ancient origin that is situated before the divergence of Protostomian and Deuterostomian animals. Nevertheless, an enormous diversification has occurred since then. The availability of novel sequence information is growing very rapidly as a result of molecular cloning experiments and of metazoan genome (Caenorhabditis elegans, Drosophila melanogaster, Homo sapiens) and EST (expressed sequence tags) sequencing projects. The Drosophila Genome Sequencing Project will certainly have an important impact on insect signal transduction and receptor research. In parallel, convenient expression systems and functional assay procedures will be needed to investigate insect receptor properties and to monitor the effects of natural and artificial ligands. The study of the evolutionary aspects of G protein-coupled receptors and of their signaling pathways will probably reveal insect-specific features. More insight into these features may result in novel methods and practical applications. Arch.

Lack of evidence for cross-competition between vasoactive intestinal peptide and somatostatin at their respective receptors

A possible cross-competition between vasoactive intestinal peptide (VIP) and somatostatin (somatotropin release inhibiting factor; SRIF) and their respective receptors, was investigated at native or recombinant SRIF and VIP/pituitary adenylate cyclase-activating polypeptide (PACAP) receptors. The activity of VIP was examined in radioligand binding assays at mouse sst(1-5), rat sst(1-2) and human sst(1-5) receptors; or at human tumours preferentially expressing each of the five SRIF receptors. Moreover, SRIF was investigated at human tumoral tissues known to exclusively express specific VIP/PACAP receptor(s). VIP had no significant effect on any of the radioligand binding sites of the SRIF receptor family of rat, mouse or human origin tested. Conversely, SRIF did not interfere with the human VIP/PACAP binding sites tested. Taken together, the results cast reservation on the claimed cross-competition between VIP and SRIF at, specifically human sst(3) receptors, or any of the cloned SRIF or VIP/PACAP receptors recognised to date.

Chemokine signaling and functional responses: the role of receptor dimerization and TK pathway activation

A broad array of biological responses, including cell polarization, movement, immune and inflammatory responses, and prevention of HIV-1 infection, are triggered by the chemokines, a family of structurally related chemoattractant proteins that bind to specific seven-transmembrane receptors linked to G proteins. Here we discuss one of the early signaling pathways activated by chemokines, the JAK/STAT pathway. Through this pathway, and possibly in conjunction with other signaling pathways, the chemokines promote changes in cellular morphology, collectively known as polarization, required for chemotactic responses. The polarized cell expresses the chemokine receptors at the leading cell edge, to which they are conveyed by rafts, a cholesterol-enriched membrane fraction fundamental to the lateral organization of the plasma membrane. Finally, the mechanisms through which the chemokines promote their effect are discussed in the context of the prevention of HIV-1 infection.

Expression cloning of two genes that together mediate organic solute and steroid transport in the liver of a marine vertebrate

Uptake of organic solutes and xenobiotics by mammalian cells is mediated by ATP-independent transporters, and four families of transporters have now been identified. To search for novel organic solute transporters, a liver cDNA library from an evolutionarily primitive marine vertebrate, the little skate Raja erinacea, was screened for taurocholate transport activity by using Xenopus laevis oocytes. In contrast to the organic anion transporters identified to date, a transport activity was identified in this library that required the coexpression of two distinct gene products, termed organic solute transporter alpha and beta (Ostalpha, Ostbeta). Ostalpha cDNA encodes for a protein of 352 aa and seven putative transmembrane (TM) domains. Ostbeta contains 182 aa and has at least one and perhaps two TM domains. There is no significant sequence identity between Ostalpha and Ostbeta, and only low identity with sequences in the databases; however, Ostalpha bears a resemblance to some G protein-coupled receptors, and Ostbeta exhibits 22% amino acid identity with the C-terminal TM and intracellular domains of protocadherin-gamma, a cell surface glycoprotein. Xenopus oocytes injected with the cRNA for both Ostalpha and Ostbeta, but not each separately, were able to take up taurocholate, estrone sulfate, digoxin, and prostaglandin E(2), but not p-aminohippurate or S-dinitrophenyl glutathione. Transport was sodium-independent, saturable, and inhibited by organic anions and steroids, including the major skate bile salt, scymnol sulfate. These results identify an organic anion transporter composed of a putative seven-helix TM protein and an ancillary membrane polypeptide.

Genetic variations in human G protein-coupled receptors: implications for drug therapy

Numerous genes encode G protein-coupled receptors (GPCRs)-a main molecular target for drug therapy. Estimates indicate that the human genome contains approximately 600 GPCR genes. This article addresses therapeutic implications of sequence variations in GPCR genes. A number of inactivating and activating receptor mutations have been shown to cause a variety of (mostly rare) genetic disorders. However, pharmacogenetic and pharmacogenomic studies on GPCRs are scarce, and therapeutic relevance of variant receptor alleles often remains unclear. Confounding factors in assessing the therapeutic relevance of variant GPCR alleles include 1) interaction of a single drug with multiple closely related receptors, 2) poorly defined binding pockets that can accommodate drug ligands in different orientations or at alternative receptor domains, 3) possibility of multiple receptor conformations with distinct functions, and 4) multiple signaling pathways engaged by a single receptor. For example, antischizophrenic drugs bind to numerous receptors, several of which might be relevant to therapeutic outcome. Without knowing accurately what role a given receptor subtype plays in clinical outcome and how a sequence variation affects drug-induced signal transduction, we cannot predict the therapeutic relevance of a receptor variant. Genome-wide association studies with single nucleotide polymorphisms could identify critical target receptors for disease susceptibility and drug efficacy or toxicity.

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.

Genetic variations and polymorphisms of G protein-coupled receptors: functional and therapeutic implications

G protein-coupled receptors (GPCRs) represent a major class of proteins in the genome of many species, including humans. In addition to the mapping of a number of human disorders to regions of the genome containing GPCRs, a growing body of literature has documented frequently occurring variations (i.e. polymorphisms) in GPCR loci. In this article, we use a domain-based approach to systematically examine examples of genetic variation in the coding and noncoding regions of GPCR loci. Data to date indicate that residues in GPCRs are involved in ligand binding and coupling to G proteins and that regulation can be altered by polymorphisms. Studies of GPCR polymorphisms have also uncovered the functional importance of residues not previously implicated from other approaches that are involved in the function of GPCRs. We predict that studies of GPCR polymorphisms will have a significant impact on medicine and pharmacology, in particular, by providing new means to subclassify patients in terms of both diagnosis and treatment.

Dopamine D2 receptor dimer formation: evidence from ligand binding

We have examined the binding of two radioligands ([(3)H]spiperone and [(3)H]raclopride) to D(2) dopamine receptors expressed in Chinese hamster ovary cells. In saturation binding experiments in the presence of sodium ions, both radioligands labeled a similar number of sites, whereas in the absence of sodium ions [(3)H]raclopride labeled about half the number of sites labeled by [(3)H]spiperone. In competition experiments in the absence of sodium ions, however, raclopride was able to inhibit [(3)H]spiperone binding fully. In saturation analyses with [(3)H]spiperone in the absence of sodium ions raclopride exerted noncompetitive effects, decreasing the number of sites labeled by the radioligand. These data are interpreted in terms of a model where the receptor exists as a dimer, and in the absence of sodium ions, raclopride exerts negative cooperativity across the dimer both for its own binding and the binding of spiperone. A model of the receptor has been produced that provides a good description of the experimental phenomena described here.

Sparks and puffs in oligodendrocyte progenitors: cross talk between ryanodine receptors and inositol trisphosphate receptors

Investigating how calcium release from the endoplasmic reticulum (ER) is triggered and coordinated is crucial to our understanding of how oligodendrocyte progenitor cells (OPs) develop into myelinating cells. Sparks and puffs represent highly localized Ca(2+) release from the ER through ryanodine receptors (RyRs) and inositol trisphosphate receptors (IP(3)Rs), respectively. To study whether sparks or puffs trigger Ca(2+) waves in OPs, we performed rapid high-resolution line scan recordings in fluo-4-loaded OP processes. We found spontaneous and evoked sparks and puffs, and we have identified functional cross talk between IP(3)Rs and RyRs. Local events evoked using the IP(3)-linked agonist methacholine (MeCh) showed significantly different morphology compared with events evoked using the caffeine analog 3,7-dimethyl-1-propargylxanthine (DMPX). Pretreatment with MeCh potentiated DMPX-evoked events, whereas inhibition of RyRs potentiated events evoked by low concentrations of MeCh. Furthermore, activation of IP(3)Rs but not RyRs was critical for Ca(2+) wave initiation. Using immunocytochemistry, we show OPs express the specific Ca(2+) release channel subtypes RyR3 and IP(3)R2 in patches along OP processes. RyRs are coexpressed with IP(3)Rs in some patches, but IP(3)Rs are also found alone. This differential distribution pattern may underlie the differences in local and global Ca(2+) signals mediated by these two receptors. Thus, in OPs, interactions between IP(3)Rs and RyRs determine the spatial and temporal characteristics of calcium signaling, from microdomains to intracellular waves.

Heteromeric association creates a P2Y-like adenosine receptor

Adenosine and its endogenous precursor ATP are main components of the purinergic system that modulates cellular and tissue functions via specific adenosine and ATP receptors (P1 and P2 receptors), respectively. Although adenosine inhibits excitability and ATP functions as an excitatory transmitter in the central nervous system, little is known about the ability of P1 and P2 receptors to form new functional structures such as a heteromer to control the complex purinergic cascade. Here we have shown that G(i/o) protein-coupled A1 adenosine receptor (A1R) and Gq protein-coupled P2Y1 receptor (P2Y1R) coimmunoprecipitate in cotransfected HEK293T cells, suggesting the oligomeric association between distinct G protein-coupled P1 and P2 receptors. A1R and P2Y2 receptor, but not A1R and dopamine D2 receptor, also were found to coimmunoprecipitate in cotransfected cells. A1R agonist and antagonist binding to cell membranes were reduced by coexpression of A1R and P2Y1R, whereas a potent P2Y1R agonist adenosine 5'-O-(2-thiotriphosphate) (ADPbetaS) revealed a significant potency to A1R binding only in the cotransfected cell membranes. Moreover, the A1R/P2Y1R coexpressed cells showed an ADPbetaS-dependent reduction of forskolin-evoked cAMP accumulation that was sensitive to pertussis toxin and A1R antagonist, indicating that ADPbetaS binds A1R and inhibits adenylyl cyclase activity via G(i/o) proteins. Also, a high degree of A1R and P2Y1R colocalization was demonstrated in cotransfected cells by double immunofluorescence experiments with confocal laser microscopy. These results suggest that oligomeric association of A1R with P2Y1R generates A1R with P2Y1R-like agonistic pharmacology and provides a molecular mechanism for an increased diversity of purine signaling.

The C-terminal domains of the GABA(b) receptor subunits mediate intracellular trafficking but are not required for receptor signaling

GABA(B) receptors are G-protein-coupled receptors that mediate slow synaptic inhibition in the brain and spinal cord. These receptors are heterodimers assembled from GABA(B1) and GABA(B2) subunits, neither of which is capable of producing functional GABA(B) receptors on homomeric expression. GABA(B1,) although able to bind GABA, is retained within the endoplasmic reticulum (ER) when expressed alone. In contrast, GABA(B2) is able to access the cell surface when expressed alone but does not couple efficiently to the appropriate effector systems or produce any detectable GABA-binding sites. In the present study, we have constructed chimeric and truncated GABA(B1) and GABA(B2) subunits to explore further GABA(B) receptor signaling and assembly. Removal of the entire C-terminal intracellular domain of GABA(B1) results in plasma membrane expression without the production of a functional GABA(B) receptor. However, coexpression of this truncated GABA(B1) subunit with either GABA(B2) or a truncated GABA(B2) subunit in which the C terminal has also been removed is capable of functional signaling via G-proteins. In contrast, transferring the entire C-terminal tail of GABA(B1) to GABA(B2) leads to the ER retention of the GABA(B2) subunit when expressed alone. These results indicate that the C terminal of GABA(B1) mediates the ER retention of this protein and that neither of the C-terminal tails of GABA(B1) or GABA(B2) is an absolute requirement for functional coupling of heteromeric receptors. Furthermore although GABA(B1) is capable of producing GABA-binding sites, GABA(B2) is of central importance in the functional coupling of heteromeric GABA(B) receptors to G-proteins and the subsequent activation of effector systems.

Selective inhibition of beta(2)-adrenergic receptor-mediated cAMP generation by activation of the P2Y(2) receptor in mouse pineal gland tumor cells

Rhythmic noradrenergic signaling from the hypothalamic clock in the suprachiasmatic nucleus to the pineal gland causes an increase in intracellular cAMP which regulates the circadian fluctuation of melatonin synthesis. The activation of phospholipase C (PLC)-coupled P2Y(2) receptors upon treatment with ATP and UTP exclusively inhibited the isoproterenol-stimulated cAMP production in mouse pineal gland tumor cells. However, the activation of other PLC-coupled receptors including P2Y(1) and bombesin receptors had little or no effect on the isoproterenol-stimulated cAMP production. Also, ATP did not inhibit cAMP production caused by forskolin, prostaglandin E(2), or the adenosine analog NECA. These results suggest a selective coupling between signalings of P2Y(2) and beta(2)-adrenergic receptors. The binding of [(3)H]CGP12177 to beta(2)-adrenergic receptors was not effected by the presence of ATP or UTP. Ionomycin decreased the isoproterenol-stimulated cAMP production, whereas phorbol 12-myristate 13-acetate slightly potentiated the isoproterenol response. Chelation of intracellular Ca(2+), however, had little effect on the ATP-induced inhibition of cAMP production, while it completely reversed the ionomycin-induced inhibition. Treatment of cells with pertussis toxin almost completely blocked the inhibitory effect of nucleotides. Pertussis toxin also inhibited the nucleotide-induced increase in intracellular Ca(2+) and inositol 1,4,5-trisphosphate production by 30-40%, suggesting that the ATP-mediated inhibition of the cAMP generation and the partial activation of PLC are mediated by pertussis toxin-sensitive G(i)-protein. We conclude that one of the functions of P2Y(2) receptors on the pineal gland is the selective inhibition of beta-adrenergic receptor-mediated signaling pathways via the inhibitory G-proteins.

Peptides as receptor ligand drugs and their relationship to G-coupled signal transduction

Peptides act as effector agents that regulate and/or mediate physiological processes, serving as hormones, neurotransmitters and signal transducing factors. The low molecular weight peptides affect receptor-mediated events, which influence cardiovascular, gastrointestinal and neurocranial systems. While some peptides have been marketed as drugs, many have served as leads or templates for the development of non-peptide drugs that mimic peptide actions. This review presents the advantages and disadvantages of using peptides as drugs that bind as ligands to cell-surface receptors and considers their applications in such events. The value of both the peptides and their mimics is based on their participation in the biomodulation of physiological processes, which frequently employ scaffolding proteins acting in a cascading sequence of protein-to-protein interactions. The peptides bind to G-coupled surface receptors to initiate a signal that is transduced to the interior of the cell through multiple layers of phosphorylating enzymes and binding proteins. Peptides have been further employed to identify the molecular targets of signal transduction, the uncoupling of which might provide a means for various disease therapies. The exploitation of such peptide-mediated signal pathways, which are of primary importance to tumour cells, may provide an attractive strategy for anticancer therapy in the future.

D2/D3 dopamine receptor heterodimers exhibit unique functional properties

Evidence for heterodimerization has recently been provided for dopamine D(1) and adenosine A(1) receptors as well as for dopamine D(2) and somatostatin SSTR(5) receptors. In this paper, we have studied the possibility that D(2) and D(3) receptors interact functionally by forming receptor heterodimers. Initially, we split the two receptors at the level of the third cytoplasmic loop into two fragments. The first, containing transmembrane domains (TM) I to V and the N-terminal part of the third cytoplasmic loop, was named D(2trunk) or D(3trunk), and the second, containing the C-terminal part of the third cytoplasmic loop, TMVI and TMVII, and the C-terminal tail, was named D(2tail) or D(3tail). Then we defined the pharmacological profiles of the homologous (D(2trunk)/D(2tail) and D(3trunk)/D(3tail)) as well as of the heterologous (D(2trunk)/D(3tail) and D(3trunk)/D(2tail)) cotransfected receptor fragments. The pharmacological profile of the cross-cotransfected fragments was different from that of the native D(2) or D(3) receptors. In most cases, the D(3trunk)/D(2tail) was the one with the highest affinity for most agonists and antagonists. Moreover, we observed that all of these receptor fragments reduced the expression of the wild type dopamine D(2) and D(3) receptors, suggesting that D(2) and D(3) receptors can form complexes with these fragments and that these complexes bind [(3)H]nemonapride less efficiently or are not correctly targeted to the membrane. In a second set of experiments, we tested the ability of the split and the wild type receptors to inhibit adenylyl cyclase (AC) types V and VI. All of the native and split receptors inhibited AC-V and AC-VI, with the exception of D(3), which was unable to inhibit AC-VI. We therefore studied the ability of D(2) and D(3) to interact functionally with one another to inhibit AC-VI. We found that with D(2) alone, R-(+)-7-hydroxydypropylaminotetralin hydrobromide inhibited AC-VI with an IC(50) of 2.05 +/- 0.15 nm, while in the presence of D(2) and D(3) it inhibited AC-VI with an IC(50) of 0.083 +/- 0.011 nm. Similar results were obtained with a chimeric cyclase made from AC-V and AC-VI. Coimmunoprecipitation experiments indicate that D(2) and D(3) receptors are capable of physical interaction.

Mutant G protein alpha subunit activated by Gbeta gamma: a model for receptor activation?

How receptors catalyze exchange of GTP for GDP bound to the Galpha subunit of trimeric G proteins is not known. One proposal is that the receptor uses the G protein's betagamma heterodimer as a lever, tilting it to pull open the guanine nucleotide binding pocket of Galpha. To test this possibility, we designed a mutant Galpha that would bind to betagamma in the tilted conformation. To do so, we excised a helical turn (four residues) from the N-terminal region of alpha(s), the alpha subunit of G(S), the stimulatory regulator of adenylyl cyclase. In the presence, but not in the absence, of transiently expressed beta(1) and gamma(2), this mutant (alpha(s)Delta), markedly stimulated cAMP accumulation. This effect depended on the ability of the coexpressed beta protein to interact normally with the lip of the nucleotide binding pocket of alpha(s)Delta. We substituted alanine for an aspartate in beta(1) that binds to a lysine (K206) in the lip of the alpha subunit's nucleotide binding pocket. Coexpressed with alpha(s)Delta and gamma(2), this mutant, beta(1)-D228A, elevated cAMP much less than did beta(1)-wild type; it did bind to alpha(s)Delta normally, however, as indicated by its unimpaired ability to target alpha(s)Delta to the plasma membrane. We conclude that betagamma can activate alpha(s) and that this effect probably involves both a tilt of betagamma relative to alpha(s) and interaction of beta with the lip of the nucleotide binding pocket. We speculate that receptors use a similar mechanism to activate trimeric G proteins.

Advances in the molecular understanding of GABA(B) receptors

The molecular nature of the metabotropic GABA(B) receptor was for some time a mystery, however it was recently discovered that two related G-protein-coupled receptors have to heterodimerize to form the functional GABA(B) receptor at the cell surface. This review discusses the most recent findings in the rapidly expanding field of GABA(B) receptor research, and includes a summary of all splice variants of both receptor subunits identified to date. It also evaluates emerging evidence that certain splice variants might play a role in determining pharmacologically distinguishable receptors, and reviews receptor localization at the sub-cellular level and involvement in neuronal development.

Dopamine D2 and D3 receptors are linked to the actin cytoskeleton via interaction with filamin A

We have used a yeast two-hybrid approach to uncover protein interactions involving the D2-like subfamily of dopamine receptors. Using the third intracellular loop of the D2S and D3 dopamine receptors as bait to screen a human brain cDNA library, we identified filamin A (FLN-A) as a protein that interacts with both the D2 and D3 subtypes. The interaction with FLN-A was specific for the D2 and D3 receptors and was independently confirmed in pull-down and coimmunoprecipitation experiments. Deletion mapping localized the dopamine receptor-FLN-A interaction to the N-terminal segment of the D2 and D3 dopamine receptors and to repeat 19 of FLN-A. In cultures of dissociated rat striatum, FLN-A and D2 receptors colocalized throughout neuronal somata and processes as well as in astrocytes. Expression of D2 dopamine receptors in FLN-A-deficient M2 melanoma cells resulted in predominant intracellular localization of the D2 receptors, whereas in FLN-A-reconstituted cells, the D2 receptor was predominantly localized at the plasma membrane. These results suggest that FLN-A may be required for proper cell surface expression of the D2 dopamine receptors. Association of D2 and D3 dopamine receptors with FLN-A provides a mechanism whereby specific dopamine receptor subtypes may be functionally linked to downstream signaling components via the actin cytoskeleton.

Oligomerization of G-protein-coupled transmitter receptors

Examples of G-protein-coupled receptors that can be biochemically detected in homo- or heteromeric complexes are emerging at an accelerated rate. Biophysical approaches have confirmed the existence of several such complexes in living cells and there is strong evidence to support the idea that dimerization is important in different aspects of receptor biogenesis and function. While the existence of G-protein-coupled-receptor homodimers raises fundamental questions about the molecular mechanisms involved in transmitter recognition and signal transduction, the formation of heterodimers raises fascinating combinatorial possibilities that could underlie an unexpected level of pharmacological diversity, and contribute to cross-talk regulation between transmission systems. Because G-protein-coupled receptors are major pharmacological targets, the existence of dimers could have important implications for the development and screening of new drugs. Here, we review the evidence supporting the existence of G-protein-coupled-receptor dimerization and discuss its functional importance.

Oligomerisation of G-protein-coupled receptors

A range of approaches have recently provided evidence that G-protein-coupled receptors can exist as oligomeric complexes. Both homo-oligomers, comprising multiple copies of the same gene product, and hetero-oligomers containing more than one receptor have been detected. In several, but not all, examples, the extent of oligomerisation is regulated by the presence of agonist ligands, and emerging evidence indicates that receptor hetero-oligomers can display distinct pharmacological characteristics. A chaperonin-like role for receptor oligomerisation in effective delivery of newly synthesised receptors to the cell surface is a developing concept, and recent studies have employed a series of energy-transfer techniques to explore the presence and regulation of receptor oligomerisation in living cells. However, the majority of studies have relied largely on co-immunoprecipitation techniques, and there is still little direct information on the fraction of receptors existing as oligomers in intact cells.

A novel approach for the identification of protein-protein interaction with integral membrane proteins

Protein-protein interaction plays a major role in all biological processes. The currently available genetic methods such as the two-hybrid system and the protein recruitment system are relatively limited in their ability to identify interactions with integral membrane proteins. Here we describe the development of a reverse Ras recruitment system (reverse RRS), in which the bait used encodes a membrane protein. The bait is expressed in its natural environment, the membrane, whereas the protein partner (the prey) is fused to a cytoplasmic Ras mutant. Protein-protein interaction between the proteins encoded by the prey and the bait results in Ras membrane translocation and activation of a viability pathway in yeast. We devised the expression of the bait and prey proteins under the control of dual distinct inducible promoters, thus enabling a rapid selection of transformants in which growth is attributed solely to specific protein-protein interaction. The reverse RRS approach greatly extends the usefulness of the protein recruitment systems and the use of integral membrane proteins as baits. The system serves as an attractive approach to explore novel protein-protein interactions with high specificity and selectivity, where other methods fail.

The role of the D(2) dopamine receptor (D(2)R) in A(2A) adenosine receptor (A(2A)R)-mediated behavioral and cellular responses as revealed by A(2A) and D(2) receptor knockout mice

The A(2A)R is largely coexpressed with D(2)Rs and enkephalin mRNA in the striatum where it modulates dopaminergic activity. Activation of the A(2A)R antagonizes D(2)R-mediated behavioral and neurochemical effects in the basal ganglia through a mechanism that may involve direct A(2A)R-D(2)R interaction. However, whether the D(2)R is required for the A(2A)R to exert its neural function is an open question. In this study, we examined the role of D(2)Rs in A(2A)R-induced behavioral and cellular responses, by using genetic knockout (KO) models (mice deficient in A(2A)Rs or D(2)Rs or both). Behavioral analysis shows that the A(2A)R agonist 2-4-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine reduced spontaneous as well as amphetamine-induced locomotion in both D(2) KO and wild-type mice. Conversely, the nonselective adenosine antagonist caffeine and the A(2A)R antagonist 8-(3-chlorostyryl)caffeine produced motor stimulation in mice lacking the D(2)R, although the stimulation was significantly attenuated. At the cellular level, A(2A)R inactivation counteracted the increase in enkephalin expression in striatopallidal neurons caused by D(2)R deficiency. Consistent with the D(2) KO phenotype, A(2A)R inactivation partially reversed both acute D(2)R antagonist (haloperidol)-induced catalepsy and chronic haloperidol-induced enkephalin mRNA expression. Together, these results demonstrate that A(2A)Rs elicit behavioral and cellular responses despite either the genetic deficiency or pharmacological blockade of D(2)Rs. Thus, A(2A)R-mediated neural functions are partially independent of D(2)Rs. Moreover, endogenous adenosine acting at striatal A(2A)Rs may be most accurately viewed as a facilitative modulator of striatal neuronal activity rather than simply as an inhibitory modulator of D(2)R neurotransmission.

Oligomerization of the human serotonin transporter and of the rat GABA transporter 1 visualized by fluorescence resonance energy transfer microscopy in living cells

Recent biochemical studies indicate that the serotonin transporter can form oligomers. We investigated whether the human serotonin transporter (hSERT) can be visualized as an oligomer in the plasma membrane of intact cells. For this purpose, we generated fusion proteins of hSERT and spectral variants of the green fluorescent protein (cyan and yellow fluorescent proteins, CFP and YFP, respectively). When expressed in human embryonic kidney 293 cells, the resulting fusion proteins (CFP-hSERT and YFP-hSERT) were efficiently inserted into the plasma membrane and were functionally indistinguishable from wild-type hSERT. Oligomers were visualized by fluorescence resonance energy transfer microscopy in living cells using two complementary methods, i.e. ratio imaging and donor photobleaching. Interestingly, oligomerization was not confined to hSERT; fluorescence resonance energy transfer was also observed between CFP- and YFP-labeled rat gamma-aminobutyric acid transporter. The bulk of serotonin transporters was recovered as high molecular weight complexes upon gel filtration in detergent solution. In contrast, the monomers of CFP-hSERT and YFP-hSERT were essentially undetectable. This indicates that the homo-oligomeric form is the favored state of hSERT in living cells, which is not significantly affected by coincubation with transporter substrates or blockers. Based on our observations, we conclude that constitutive oligomer formation might be a general property of Na(+)/Cl(-)-dependent neurotransmitter transporters.

Heterodimerization of G-protein-coupled receptors in the CNS

Over the last year the combinations of G-protein-coupled receptors that are known to form heterodimeric complexes has rapidly increased. For example, dopamine receptors can dimerize with both somatostatin and adenosine receptors. These studies have been aided by improved technologies to monitor protein/protein interactions in living cells. Crosstalk at the level of the receptors might explain some of the known physiological interactions of these neurotransmitter systems and also provide new approaches for therapeutic intervention.

Cellular biology of somatostatin receptors

Somatostatin, and the recently discovered neuropeptide cortistatin, exert their physiological actions via a family of six G protein-coupled receptors (sst1, sst2A, sst2B, sst3, sst4, sst5). Following the cloning of somatostatin receptors significant advances have been made in our understanding of their molecular, pharmacological and signaling properties although much progress remains to be done to define their physiological role in vivo. In this review, the present knowledge regarding neuroanatomical localization, signal transduction pathways, desensitization and internalization properties of somatostatin receptors is summarized. Evidence that somatostatin receptors can form homo- and heterodimers and can physically interact with members of the SSTRIP/Shank/ProSAP1/CortBP1 family is also discussed.

Gonadotropin-releasing hormone receptor microaggregation. Rate monitored by fluorescence resonance energy transfer

Gonadotropin-releasing hormone (GnRH) regulates pituitary gonadotropin release and is a therapeutic target for human and animal reproductive diseases. In the present study we have utilized the technique of fluorescence resonance energy transfer to monitor the rate of GnRH receptor-receptor interactions. This technique relies on the observation that the degree of physical intimacy of molecules can be assessed by the tendency of proximal fluorophores to exchange energy. Our data indicate that GnRH agonist, but not antagonist, occupancy of the GnRH receptor promotes physical intimacy (microaggregation) between receptors. The time course indicates that this occurs promptly (<1 min) after occupancy and persists for at least 80 min and within the physiologically relevant range of the releasing hormone. The process measured is not inhibited by 0.1 mm vinblastin, 2 microm cytochalasin D, or 3 mm EGTA, an observation that distinguishes it from macroaggregation (patching, capping, and internalization). These observations, along with reports from other laboratories, are consonant with a growing body of evidence that indicates that microaggregation is an early event following agonist occupancy of the receptor and part of the mechanism by which effector regulation occurs.

Oligomerization of opioid receptors with beta 2-adrenergic receptors: a role in trafficking and mitogen-activated protein kinase activation

G-protein-coupled receptors (GPCRs) have recently joined the list of cell surface receptors that dimerize. Dimerization has been shown to alter the ligand-binding, signaling, and trafficking properties of these receptors. Recent studies have shown that GPCRs heterodimerize with closely related members, resulting in the modulation of their function. In this study, we have attempted to determine whether members of GPCR superfamilies that couple to different families of G-proteins can associate and form oligomers. We chose the beta2 adrenergic receptor that couples to stimulatory G-proteins and delta & kappa opioid receptors that couple to inhibitory G-proteins. beta2 and delta receptors undergo robust agonist-mediated endocytosis, whereas kappa receptors do not. We find that when coexpressed, beta2 receptors can form heteromeric complexes with both delta and kappa receptors. This heterooligomerization does not significantly alter the ligand binding or coupling properties of the receptors. However, it affects the trafficking properties of the receptors. For example, we find that delta receptors, when coexpressed with beta2 receptors, undergo isoproterenol-mediated endocytosis. Conversely, beta2 receptors in these cells undergo etorphine-mediated endocytosis. However, beta2 receptors, when coexpressed with kappa receptors, undergo neither opioid- nor isoproterenol-mediated endocytosis. Moreover, these cells exhibit a substantial decrease in the isoproterenol-induced phosphorylation of mitogen-activated protein kinases. Taken together, these results provide direct evidence of heteromerization of GPCRs that couple to different types of G-proteins, which results in the modulation of receptor trafficking and signal transduction.

Oligomerization of opioid receptors with beta 2-adrenergic receptors: a role in trafficking and mitogen-activated protein kinase activation

G-protein-coupled receptors (GPCRs) have recently joined the list of cell surface receptors that dimerize. Dimerization has been shown to alter the ligand-binding, signaling, and trafficking properties of these receptors. Recent studies have shown that GPCRs heterodimerize with closely related members, resulting in the modulation of their function. In this study, we have attempted to determine whether members of GPCR superfamilies that couple to different families of G-proteins can associate and form oligomers. We chose the beta2 adrenergic receptor that couples to stimulatory G-proteins and delta & kappa opioid receptors that couple to inhibitory G-proteins. beta2 and delta receptors undergo robust agonist-mediated endocytosis, whereas kappa receptors do not. We find that when coexpressed, beta2 receptors can form heteromeric complexes with both delta and kappa receptors. This heterooligomerization does not significantly alter the ligand binding or coupling properties of the receptors. However, it affects the trafficking properties of the receptors. For example, we find that delta receptors, when coexpressed with beta2 receptors, undergo isoproterenol-mediated endocytosis. Conversely, beta2 receptors in these cells undergo etorphine-mediated endocytosis. However, beta2 receptors, when coexpressed with kappa receptors, undergo neither opioid- nor isoproterenol-mediated endocytosis. Moreover, these cells exhibit a substantial decrease in the isoproterenol-induced phosphorylation of mitogen-activated protein kinases. Taken together, these results provide direct evidence of heteromerization of GPCRs that couple to different types of G-proteins, which results in the modulation of receptor trafficking and signal transduction.

Clinicopathological value of somatostatin type 2A and estrogen receptor immunoreactivity in human breast carcinoma

Somatostatin type 2A (sstr2A) and estrogen receptor (ER) are interrelated regulatory receptors present in normal breast epithelium and in a population of breast carcinomas. ER mediates growth stimulatory effects of estrogens whereas sstr2A mediates growth inhibitory actions of somatostatin. However, much work has been devoted to elucidate the biological role of ER, little is known about sstr2A in breast cancer. In the present study we examined immunoreactivity of sstr2A and ER in 64 breast carcinomas in correlation with tumor size and histological grade (HG), presence of lymph node metastasis (LNM), Nottingham prognostic index (NPI), and the patients' age. ER and sstr2A immunoreactivity were present in 78% and 63% of the breast carcinomas, respectively. Ninety percent of tumors immunoreactive for sstr2A were simultaneously immunoreactive for ER. ER immunoreactivity correlated significantly with lower HG (p = 0.03) and better NPI (p = 0.02). sstr2A immunoreactivity correlated significantly with lower HG (p = 0.012) but not with NPI (p = 0.26). There was no correlation of sstr2A immunoreactivity and tumor size, patients' chronological age or LNM. The results confirm prognostic value of ER immunohistochemistry in breast carcinoma. However sstr2A cannot substitute ER for prognostic evaluation, sstr2A immunoreactivity being significantly associated with lower HG seems to represent an independent prognostic factor in breast carcinoma.

An isoform-specific interaction of the membrane anchors affects mammalian adenylyl cyclase type V activity

The nine membrane-bound mammalian adenylyl cyclases (ACs) contain two highly diverged membrane anchors, M1 and M2, with six transmembrane spans each and two conserved cytosolic domains which coalesce into a pseudoheterodimeric catalytic unit. Previously, the catalytic segments, bacterially expressed as soluble proteins, were characterized extensively whereas the function of the membrane domains remained unexplored. Using the catalytic C1 and C2 domains of AC type V we employed the membrane anchors from type V and VII ACs for construction of enzymes with duplicated, inverted, fully swapped and chimeric membrane anchors. Further, in the M1 membrane domain individual transmembrane spans were removed or exchanged between type V and VII ACs. The constructs were expressed in HEK293 cells, the expression levels and membrane localization was assessed by Western blotting. Cell-free basal, forskolin-, GTP gamma S-and G(s alpha)/GTP gamma S-stimulated AC activities were determined. The results demonstrate that enzymatic activities were only maintained when the M1 and M2 membrane domains were derived from either AC V or VII. Constructs with chimeric membrane domains, i.e. M1 from type V and M2 from type VII AC or vice versa, were essentially inactive although the expression levels and membrane localization appeared to be normal. The data indicate a functionally important interaction of the membrane domains of ACs in that they seem to interact in a pair-like, isoform delimited manner. This interaction directly impinges on the formation of the catalytic interface. We propose that protein-protein interactions of the AC membrane domains may constitute another, yet unexplored level of AC regulation.

Differential targeting of beta -adrenergic receptor subtypes and adenylyl cyclase to cardiomyocyte caveolae. A mechanism to functionally regulate the cAMP signaling pathway

Differential modes for beta(1)- and beta(2)-adrenergic receptor (AR) regulation of adenylyl cyclase in cardiomyocytes is most consistent with spatial regulation in microdomains of the plasma membrane. This study examines whether caveolae represent specialized subdomains that concentrate and organize these moieties in cardiomyocytes. Caveolae from quiescent rat ventricular cardiomyocytes are highly enriched in beta(2)-ARs, Galpha(i), protein kinase A RIIalpha subunits, caveolin-3, and flotillins (caveolin functional homologues); beta(1)-ARs, m(2)-muscarinic cholinergic receptors, Galpha(s), and cardiac types V/VI adenylyl cyclase distribute between caveolae and other cell fractions, whereas protein kinase A RIalpha subunits, G protein-coupled receptor kinase-2, and clathrin are largely excluded from caveolae. Cell surface beta(2)-ARs localize to caveolae in cardiomyocytes and cardiac fibroblasts (with markedly different beta(2)-AR expression levels), indicating that the fidelity of beta(2)-AR targeting to caveolae is maintained over a physiologic range of beta(2)-AR expression. In cardiomyocytes, agonist stimulation leads to a marked decline in the abundance of beta(2)-ARs (but not beta(1)-ARs) in caveolae. Other studies show co-immunoprecipitation of cardiomyocytes adenylyl cyclase V/VI and caveolin-3, suggesting their in vivo association. However, caveolin is not required for adenylyl cyclase targeting to low density membranes, since adenylyl cyclase targets to low buoyant density membrane fractions of HEK cells that lack prototypical caveolins. Nevertheless, cholesterol depletion with cyclodextrin augments agonist-stimulated cAMP accumulation, indicating that caveolae function as negative regulators of cAMP accumulation. The inhibitory interaction between caveolae and the cAMP signaling pathway as well as domain-specific differences in the stoichiometry of individual elements in the beta-AR signaling cascade represent important modifiers of cAMP-dependent signaling in the heart.

Ectodomain shedding, translocation and synthesis of SorLA are stimulated by its ligand head activator

The single transmembrane receptor SorLA is the mammalian orthologue of the head activator-binding protein, HAB, from hydra. The human neuronal precursor cell line NT2 and the neuroendocrine cell line BON produce head activator (HA) and respond to HA by entry into mitosis and cell proliferation. They express SorLA, and bind HA with nanomolar affinity. HA coupled to Sepharose is able to precipitate SorLA specifically proving that SorLA binds HA. Using antisera directed against extra- and intracellular epitopes we find SorLA as membrane receptor and as soluble protein released from cells into the culture medium. Cell lines differ strongly in processing of SorLA, with NT2 cells expressing SorLA mainly as membrane receptor, whereas release predominates in BON cells. Soluble SorLA lacks the intracellular domain and is shed from the transmembrane protein by a metalloprotease. Release from cells and brain slices is stimulated by HA and by phorbol ester, and it is blocked by a metalloprotease inhibitor and by lowering the temperature to 20 degrees C. Blockade of SorLA shedding and treatment of cells with SorLA antisense oligonucleotides lead to a decrease in the rate of cell proliferation. From this we conclude that SorLA is necessary to mediate the mitogenic effect of endogenous HA. HA enhances the translocation of SorLA from internal membranes to the cell surface and its internalization. In addition, HA stimulates SorLA synthesis hinting at an autocatalytic feedback loop in which the ligand activates production, processing, and translocation of its receptor.

A novel family of candidate pheromone receptors in mammals

Pheromonal activation of the vomeronasal organ (VNO) elicits genetically preprogrammed behaviors and physiological changes in mammals. We have identified a novel gene family encoding over one hundred VNO specific receptors, the V3Rs. V3R sequences are highly similar to each other and appear distantly related to the putative pheromone receptors, V1Rs, and the taste receptors, T2Rs. Within the VNO, V3R-positive neurons are distinct from neurons expressing the pheromone receptor families V1R and V2R. The V3Rs are likely to represent a new large family of pheromone receptors in mammals. Multiple V3R-related human sequences have been identified, including one clone retaining the capacity to create a complete and functional transcript. Our data uncover a striking complexity in the molecular and cellular organization of the VNO and provide an essential framework for the study of pheromone signaling in mammals.

Receptor cross talk: ligand-gated ion channels start to communicate

Evidence is mounting for a direct mode of interaction between neurotransmitter receptors that modulates the activity of the interacting proteins. Brandon and Moss discuss two examples in which direct interactions between neurotransmitter receptors appear to regulate the receptors independently of the signal transduction cascades they activate. This type of interaction has important implications for rapid modulation of receptor function and the integration of multiple signals at a synapse.

Signalling mechanisms: a decade of signalling

Knowledge of signaling mechanisms has increased dramatically during the past decade, particularly in the areas of development, biochemical signaling cascades, synaptic transmission and ion channel biophysics.

Dopamine receptors: from structure to behavior

The responses obtained with drugs that act at dopamine receptors depend on the spectrum of receptors stimulated, the pattern of stimulation and the neuronal signal-transduction pathways that are activated. In the absence of drugs that reliably discriminate between the various cloned receptors, elucidating the role of these receptors has largely relied on molecular genetic approaches that include expression of genes for receptors in cell lines and manipulation of this expression in animal models. Connecting molecular events that occur consequent to receptor stimulation with the resulting physiological effects entails bridging a complex network of interactions. This article reviews the current understanding of the molecular, cellular and systemic consequences of activation of the different dopamine receptors.

Modulation of pituitary somatostatin receptor subtype (sst1-5) messenger ribonucleic acid levels by changes in the growth hormone axis

The role of individual components of the hypothalamic-pituitary-GH axis in the modulation of pituitary somatostatin (SRIF) receptor subtype (sst1-5) synthesis was assessed using multiplex RT-PCR to measure receptor messenger RNA (mRNA) levels in normal rats and spontaneous dwarf rats (SDRs). In SDRs, a strain with no immunodetectable GH, pituitary sst1 and sst2 mRNA levels were elevated, sst5 mRNA levels were reduced, and sst3 and sst4 mRNA levels did not significantly differ from those in normal controls. Treatment of SDRs with GH (72 h), but not insulin-like growth factor I, significantly decreased sst2 mRNA levels and increased sst4 and sst5 mRNA levels above vehicle-treated control levels. To test whether more rapid changes in circulating GH levels could alter SRIF receptor subtype expression, normal rats were infused (iv) with GH-releasing hormone (GHRH) for 4 h in the presence or absence of SRIF antiserum. GHRH infusion increased pituitary sst1 and sst2 and decreased sst5, but had no effect on sst3 and sst4 mRNA levels. Immunoneutralization of SRIF, which produced a rise in circulating GH levels, did not alter basal or GHRH-mediated SRIF receptor subtype expression. These observations indicate that acute suppression of SRIF tone does not regulate pituitary SRIF receptor subtype mRNA levels in vivo. The possibility that elevated circulating GH concentrations induced by GHRH infusion were responsible for the observed changes in SRIF receptor subtype mRNA levels was examined by infusing SDRs with GHRH for 4 h. GHRH did not increase sst1 mRNA levels in SDRs above their already elevated value. However, GHRH infusion produced an increase in sst2 and a decrease in sst5 mRNA levels similar to those observed in normal rats, indicating that the acute effects of GHRH on SRIF receptor subtype expression are independent of circulating GH levels. Primary rat pituitary cell cultures were incubated with GHRH (10 nM) or forskolin (10 microM) for 4 h to determine whether GHRH could directly mediate SRIF receptor subtype mRNA. GHRH treatment increased sst1 and sst2 mRNA levels and decreased sst5 mRNA levels, but had no effect on sst3 and sst4, similar to the results in vivo. The effect of forskolin mimicked that of GHRH on sst1, sst2, and sst5 mRNA, suggesting that GHRH acts through cAMP to directly mediate gene transcription or mRNA stability of these SRIF receptor subtypes. In addition, forskolin reduced sst3 and sst4 expression. These results strongly suggest that rat pituitary sst1, sst2, and sst5 mRNA levels are regulated both in vivo and in vitro by GHRH. The stimulatory action of GHRH on sst1 and sst2 and the inhibitory action on sst5 indicate that these receptor subtypes have independent and unique roles in the modulation of pituitary GH release.

Control of dendritic field formation in Drosophila: the roles of flamingo and competition between homologous neurons

Neurons elaborate dendrites with stereotypic branching patterns, thereby defining their receptive fields. These branching patterns may arise from properties intrinsic to the neurons or competition between neighboring neurons. Genetic and laser ablation studies reported here reveal that different multiple dendritic neurons in the same dorsal cluster in the Drosophila embryonic PNS do not compete with one another for dendritic fields. In contrast, when dendrites from homologous neurons in the two hemisegments meet at the dorsal midline in larval stages, they appear to repel each other. The formation of normal dendritic fields and the competition between dendrites of homologous neurons require the proper expression level of Flamingo, a G protein-coupled receptor-like protein, in embryonic neurons. Whereas Flamingo functions downstream of Frizzled in specifying planar polarity, Flamingo-dependent dendritic outgrowth is independent of Frizzled.

Dopamine D1 and adenosine A1 receptors form functionally interacting heteromeric complexes

The possible molecular basis for the previously described antagonistic interactions between adenosine A(1) receptors (A(1)R) and dopamine D(1) receptors (D(1)R) in the brain have been studied in mouse fibroblast Ltk(-) cells cotransfected with human A(1)R and D(1)R cDNAs or with human A(1)R and dopamine D(2) receptor (long-form) (D(2)R) cDNAs and in cortical neurons in culture. A(1)R and D(1)R, but not A(1)R and D(2)R, were found to coimmunoprecipitate in cotransfected fibroblasts. This selective A(1)R/D(1)R heteromerization disappeared after pretreatment with the D(1)R agonist, but not after combined pretreatment with D(1)R and A(1)R agonists. A high degree of A(1)R and D(1)R colocalization, demonstrated in double immunofluorescence experiments with confocal laser microscopy, was found in both cotransfected fibroblast cells and cortical neurons in culture. On the other hand, a low degree of A(1)R and D(2)R colocalization was observed in cotransfected fibroblasts. Pretreatment with the A(1)R agonist caused coclustering (coaggregation) of A(1)R and D(1)R, which was blocked by combined pretreatment with the D(1)R and A(1)R agonists in both fibroblast cells and in cortical neurons in culture. Combined pretreatment with D(1)R and A(1)R agonists, but not with either one alone, substantially reduced the D(1)R agonist-induced accumulation of cAMP. The A(1)R/D(1)R heteromerization may be one molecular basis for the demonstrated antagonistic modulation of A(1)R of D(1)R receptor signaling in the brain. The persistence of A(1)R/D(1)R heteromerization seems to be essential for the blockade of A(1)R agonist-induced A(1)R/D(1)R coclustering and for the desensitization of the D(1)R agonist-induced cAMP accumulation seen on combined pretreatment with D(1)R and A(1)R agonists, which indicates a potential role of A(1)R/D(1)R heteromers also in desensitization mechanisms and receptor trafficking.

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.