Adenylyl cyclase interaction with the D2 dopamine receptor family; differential coupling to Gi, Gz, and Gs

Investigating the ligand agonism and antagonism at the D2long receptor by dynamic mass redistribution

The signalling of the D₂ receptor (D₂R), a G protein-coupled receptor (GPCR), is a complex process consisting of various components. For the screening of D₂R ligands, methods quantifying distinct second messengers such as cAMP or the interaction of the receptor with β-arrestin, are commonly employed. In contrast, a label-free biosensor technology like dynamic mass redistribution (DMR), where it is mostly unknown how the individual signalling pathways contribute to the DMR signal, provides a holistic readout of the complex cellular response. In this study, we report the successful application of the DMR technology to CHO-K1 cells stably expressing the human dopamine D_2long receptor. In real-time kinetic experiments, studies of D₂R reference compounds yielded results for agonists and antagonists that were consistent with those obtained by conventional methods and also allowed a discrimination between partial and full agonists. Furthermore, investigations on the signalling pathway in CHO-K1 hD_2longR cells identified the Gα_i/o protein as the main proximal trigger of the observed DMR response. The present study has shown that the DMR technology is a valuable method for the characterisation of putative new ligands and, due to its label-free nature, suggests its use for deorphanisation studies of GPCRs.

Lipid Rafts and Dopamine Receptor Signaling

The renal dopaminergic system has been identified as a modulator of sodium balance and blood pressure. According to the Centers for Disease Control and Prevention, in 2018 in the United States, almost half a million deaths included hypertension as a primary or contributing cause. Renal dopamine receptors, members of the G protein-coupled receptor family, are divided in two groups: D1-like receptors that act to keep the blood pressure in the normal range, and D2-like receptors with a variable effect on blood pressure, depending on volume status. The renal dopamine receptor function is regulated, in part, by its expression in microdomains in the plasma membrane. Lipid rafts form platforms within the plasma membrane for the organization and dynamic contact of molecules involved in numerous cellular processes such as ligand binding, membrane sorting, effector specificity, and signal transduction. Understanding all the components of lipid rafts, their interaction with renal dopamine receptors, and their signaling process offers an opportunity to unravel potential treatment targets that could halt the progression of hypertension, chronic kidney disease (CKD), and their complications.

Understanding the Binding Specificity of G-Protein Coupled Receptors toward G-Proteins and Arrestins: Application to the Dopamine Receptor Family

G-Protein coupled receptors (GPCRs) are involved in a myriad of pathways key for human physiology through the formation of complexes with intracellular partners such as G-proteins and arrestins (Arrs). However, the structural and dynamical determinants of these complexes are still largely unknown. Herein, we developed a computational big-data pipeline that enables the structural characterization of GPCR complexes with no available structure. This pipeline was used to study a well-known group of catecholamine receptors, the human dopamine receptor (DXR) family and its complexes, producing novel insights into the physiological properties of these important drug targets. A detailed description of the protein interfaces of all members of the DXR family (D1R, D2R, D3R, D4R, and D5R) and the corresponding protein interfaces of their binding partners (Arrs: Arr2 and Arr3; G-proteins: Gi1, Gi2, Gi3, Go, Gob, Gq, Gslo, Gssh, Gt2, and Gz) was generated. To produce reliable structures of the DXR family in complex with either G-proteins or Arrs, we performed homology modeling using as templates the structures of the β2-adrenergic receptor (β2AR) bound to Gs, the rhodopsin bound to Gi, and the recently acquired neurotensin receptor-1 (NTSR1) and muscarinic 2 receptor (M2R) bound to arrestin (Arr). Among others, the work demonstrated that the three partner groups, Arrs and Gs- and Gi-proteins, are all structurally and dynamically distinct. Additionally, it was revealed the involvement of different structural motifs in G-protein selective coupling between D1- and D2-like receptors. Having constructed and analyzed 50 models involving DXR, this work represents an unprecedented large-scale analysis of GPCR-intracellular partner interface determinants. All data is available at www.moreiralab.com/resources/dxr.

Lipid rafts in psychiatry

Lipid microenvironments in the plasma membrane are known to influence many signal transduction pathways. Several of those pathways are critical for both the etiology and treatment of depression. Further, several signaling proteins are modified, covalently, by lipids, a process that alters their interface with the microenvironments mentioned above. This review presents a brief discussion of the interface of the above elements as well as a discussion about the participation of lipids and lipid moieties in the action of antidepressants.

Biased signaling of G protein coupled receptors (GPCRs): Molecular determinants of GPCR/transducer selectivity and therapeutic potential

G protein coupled receptors (GPCRs) convey signals across membranes via interaction with G proteins. Originally, an individual GPCR was thought to signal through one G protein family, comprising cognate G proteins that mediate canonical receptor signaling. However, several deviations from canonical signaling pathways for GPCRs have been described. It is now clear that GPCRs can engage with multiple G proteins and the line between cognate and non-cognate signaling is increasingly blurred. Furthermore, GPCRs couple to non-G protein transducers, including β-arrestins or other scaffold proteins, to initiate additional signaling cascades. Receptor/transducer selectivity is dictated by agonist-induced receptor conformations as well as by collateral factors. In particular, ligands stabilize distinct receptor conformations to preferentially activate certain pathways, designated 'biased signaling'. In this regard, receptor sequence alignment and mutagenesis have helped to identify key receptor domains for receptor/transducer specificity. Furthermore, molecular structures of GPCRs bound to different ligands or transducers have provided detailed insights into mechanisms of coupling selectivity. However, receptor dimerization, compartmentalization, and trafficking, receptor-transducer-effector stoichiometry, and ligand residence and exposure times can each affect GPCR coupling. Extrinsic factors including cell type or assay conditions can also influence receptor signaling. Understanding these factors may lead to the development of improved biased ligands with the potential to enhance therapeutic benefit, while minimizing adverse effects. In this review, evidence for ligand-specific GPCR signaling toward different transducers or pathways is elaborated. Furthermore, molecular determinants of biased signaling toward these pathways and relevant examples of the potential clinical benefits and pitfalls of biased ligands are discussed.

Risky decision-making is associated with impulsive action and sensitivity to first-time nicotine exposure

Excessive risk-taking is common in multiple psychiatric conditions, including substance use disorders. The risky decision-making task (RDT) models addiction-relevant risk-taking in rats by measuring preference for a small food reward vs. a large food reward associated with systematically increasing risk of shock. Here, we examined the relationship between risk-taking in the RDT and multiple addiction-relevant phenotypes. Risk-taking was associated with elevated impulsive action, but not impulsive choice or habit formation. Furthermore, risk-taking predicted locomotor sensitivity to first-time nicotine exposure and resilience to nicotine-evoked anxiety. These data demonstrate that risk preference in the RDT predicts other traits associated with substance use disorder, and may have utility for identification of neurobiological and genetic biomarkers that engender addiction vulnerability.

Regulation of monoamine transporters and receptors by lipid microdomains: implications for depression

Lipid microdomains ("rafts") are dynamic, nanoscale regions of the plasma membrane enriched in cholesterol and glycosphingolipids, that possess distinctive physicochemical properties including higher order than the surrounding membrane. Lipid microdomain integrity is thought to affect neurotransmitter signaling by regulating membrane-bound protein signaling. Among the proteins potentially affected are monoaminergic receptors and transporters. As dysfunction of monoaminergic neurotransmission is implicated in major depressive disorder and other neuropsychiatric conditions, interactions with lipid microdomains may be of clinical importance. This systematic review evaluates what is known about the molecular relationships of monoamine transporter and receptor regulation to lipid microdomains. The PubMed/MeSH database was searched for original studies published in English through August 2017 concerning relationships between lipid microdomains and serotonin, dopamine and norepinephrine transporters and receptors. Fifty-seven publications were identified and assessed. Strong evidence implicates lipid microdomains in the regulation of serotonin and norepinephrine transporters; serotonin 1A, 2A, 3A, and 7A receptors; and dopamine D1 and β2 adrenergic receptors. Results were conflicting or more complex regarding lipid microdomain associations with the dopamine transporter, D2, D3, and D5 receptors; and negative with respect to β1 adrenergic receptors. Indirect evidence suggests that antidepressants, lipid-lowering drugs, and polyunsaturated fatty acids may exert effects on depression and suicide by altering the lipid milieu, thereby affecting monoaminergic transporter and receptor signaling. The lipid composition of membrane subdomains is involved in localization and trafficking of specific monoaminergic receptors and transporters. Elucidating precise mechanisms whereby lipid microdomains modulate monoamine neurotransmission in clinical contexts can have critical implications for pharmacotherapeutic targeting.

Research Resource: Real-Time Analysis of Somatostatin and Dopamine Receptor Signaling in Pituitary Cells Using a Fluorescence-Based Membrane Potential Assay

Stable somatostatin analogues and dopamine receptor agonists are the mainstay for the pharmacological treatment of functional pituitary adenomas; however, only a few cellular assays have been developed to detect receptor activation of novel compounds without disrupting cells to obtain the second messenger content. Here, we adapted a novel fluorescence-based membrane potential assay to characterize receptor signaling in a time-dependent manner. This minimally invasive technique provides a robust and reliable read-out for ligand-induced receptor activation in permanent and primary pituitary cells. The mouse corticotropic cell line AtT-20 endogenously expresses both the somatostatin receptors 2 (sst2) and 5 (sst5). Exposure of wild-type AtT-20 cells to the sst2- and sst5-selective agonists BIM-23120 and BIM-23268, respectively, promoted a pertussis toxin- and tertiapin-Q-sensitive reduction in fluorescent signal intensity, which is indicative of activation of G protein-coupled inwardly rectifying potassium (GIRK) channels. After heterologous expression, sst1, sst3, and sst4 receptors also coupled to GIRK channels in AtT-20 cells. Similar activation of GIRK channels by dopamine required overexpression of dopamine D2 receptors (D2Rs). Interestingly, the presence of D2Rs in AtT-20 cells strongly facilitated GIRK channel activation elicited by the sst2-D2 chimeric ligand BIM-23A760, suggesting a synergistic action of sst2 and D2Rs. Furthermore, stable somatostatin analogues produced strong responses in primary pituitary cultures from wild-type mice; however, in cultures from sst2 receptor-deficient mice, only pasireotide and somatoprim, but not octreotide, induced a reduction in fluorescent signal intensity, suggesting that octreotide mediates its pharmacological action primarily via the sst2 receptor.

Efficacy of Hybrid Tetrahydrobenzo[d]thiazole Based Aryl Piperazines D-264 and D-301 at D₂ and D₃ Receptors

In elucidating the role of pharmacodynamic efficacy at D3 receptors in therapeutic effectiveness of dopamine receptor agonists, the influence of study system must be understood. Here two compounds with D3 over D2 selectivity developed in our earlier work, D-264 and D-301, are compared in dopamine receptor-mediated G-protein activation in striatal regions of wild-type and D2 receptor knockout mice and in CHO cells expressing D2 or D3 receptors. In caudate-putamen of D2 knockout mice, D-301 was ~3-fold more efficacious than D-264 in activating G-proteins as assessed by [(35)S]GTPγS binding; in nucleus accumbens, D-301 stimulated G-protein activation whereas D-264 did not. In contrast, the two ligands exerted similar efficacy in both regions of wild-type mice, suggesting both ligands activate D2 receptors with similar efficacy. In D2 and D3 receptor-expressing CHO cells, D-264 and D-301 appeared to act in the [(35)S]GTPγS assay as full agonists because they produced maximal stimulation equal to dopamine. Competition for [(3)H]spiperone binding was then performed to determine Ki/EC50 ratios as an index of receptor reserve for each ligand. Action of D-301, but not D-264, showed receptor reserve in D3 but not in D2 receptor-expressing cells, whereas dopamine showed receptor reserve in both cell lines. Gαo1 is highly expressed in brain and is important in D2-like receptor-G protein coupling. Transfection of Gαo1 in D3- but not D2-expressing CHO cells led to receptor reserve for D-264 without altering receptor expression levels. D-301 and dopamine exhibited receptor reserve in D3-expressing cells both with and without transfection of Gαo1. Altogether, these results indicate that D-301 has greater intrinsic efficacy to activate D3 receptors than D-264, whereas the two compounds act on D2 receptors with similar intrinsic efficacy. These findings also suggest caution in interpreting Emax values from functional assays in receptor-transfected cell models without accounting for receptor reserve.

Microbes & neurodevelopment--Absence of microbiota during early life increases activity-related transcriptional pathways in the amygdala

The mammalian amygdala is a key emotional brain region for eliciting social behaviour, critically involved in anxiety and fear-related behaviours, and hence a focus of research on neurodevelopmental and stress-related disorders such as autism and anxiety. Recently, increasing evidence implicates host-microbe interactions in the aetiology of these conditions. Germ-free (GF) mice, devoid of any microbiota throughout organismal maturation, are a well-established tool to study the effects of absence of the microbiota on host physiology. A growing body of independently replicated findings confirm that GF animals demonstrate altered anxiety-related behaviour and impaired social behaviour. However, the underlying mechanisms of this interaction and the nature of the pathways involved are only insufficiently understood. To further elucidate the molecular underpinnings of microbe-brain interaction, we therefore exploited unbiased genome-wide transcriptional profiling to determine gene expression in the amygdala of GF and GF mice that have been colonised after weaning. Using RNA-sequencing and a comprehensive downstream analysis pipeline we studied the amygdala transcriptome and found significant differences at the levels of differential gene expression, exon usage and RNA-editing. Most surprisingly, we noticed upregulation of several immediate early response genes such as Fos, Fosb, Egr2 or Nr4a1 in association with increased CREB signalling in GF mice. In addition, we found differential expression and recoding of several genes implicated in brain physiology processes such as neurotransmission, neuronal plasticity, metabolism and morphology. In conclusion, our data suggest altered baseline neuronal activity in the amygdala of germ-free animals, which is established during early life and may have implications for understanding development and treatment of neurodevelopmental disorders.

Activation of the D4 dopamine receptor attenuates proliferation and migration of vascular smooth muscle cells through downregulation of AT1a receptor expression

Angiotensin (Ang) II has an important role in the vascular smooth muscle cell (VSMC) proliferation and migration and subsequently in the development of vascular diseases, whereas dopamine has the opposite effect. Previous studies have shown an interaction between dopamine and AT(1) receptors in the kidney. The dopamine D(4) receptor is expressed in arteries and has an inhibitory effect on VSMC proliferation. We hypothesized that the D(4) receptor, through its interaction with the AT(1a) receptor, may have an inhibitory effect on Ang II-mediated VSMC proliferation and migration, which could have a pivotal role in hypertension-induced vascular remodeling. In the current study, we found that Ang II markedly induced the proliferation and migration of A10 cells, which was inhibited by the D(4) receptor agonist PD168077. The activation of the D(4) receptor by PD168077 inhibited AT(1a) receptor expression in a concentration- and time-dependent manner. These effects were attenuated by silencing the D(4) receptor with a D(4) receptor-targeting small interfering RNA. The D(4) receptor-mediated inhibition of AT(1) receptor function involved protein kinase A (PKA). The activation of the D(4) receptor by PD168077 increased PKA activity in A10 cells, and the presence of a PKA inhibitor (PKA inhibitor 14-22, 10(-7) mol l(-1) per 24 h) blocked the inhibitory effect of the D(4) receptor on AT(1) receptor expression and function. The inhibitory effect of the D(4) receptor on AT(1) receptor expression and function was preserved in VSMCs (primary culture) from spontaneously hypertensive rats relative to VSMCs from Wistar-Kyoto rats. In conclusion, our data provide insight into the regulatory role of the D(4) receptor on AT(1a) receptor expression and function in VSMCs and suggest that targeting the action of the D(4) receptor may represent an effective therapeutic approach for the treatment of cardiovascular diseases.

Mutation of three residues in the third intracellular loop of the dopamine D2 receptor creates an internalization-defective receptor

Arrestins mediate desensitization and internalization of G protein-coupled receptors and also direct receptor signaling toward heterotrimeric G protein-independent signaling pathways. We previously identified a four-residue segment (residues 212-215) of the dopamine D2 receptor that is necessary for arrestin binding in an in vitro heterologous expression system but that also impairs receptor expression. We now describe the characterization of additional mutations at that arrestin binding site in the third intracellular loop. Mutating two (residues 214 and 215) or three (residues 213-215) of the four residues to alanine partially decreased agonist-induced recruitment of arrestin3 without altering activation of a G protein. Arrestin-dependent receptor internalization, which requires arrestin binding to β2-adaptin (the β2 subunit of the clathrin-associated adaptor protein AP2) and clathrin, was disproportionately affected by the three-residue mutation, with no agonist-induced internalization observed even in the presence of overexpressed arrestin or G protein-coupled receptor kinase 2. The disjunction between arrestin recruitment and internalization could not be explained by alterations in the time course of the receptor-arrestin interaction, the recruitment of G protein-coupled receptor kinase 2, or the receptor-induced interaction between arrestin and β2-adaptin, suggesting that the mutation impairs a property of the internalization complex that has not yet been identified.

D1-like dopamine receptors downregulate Na+-K+-ATPase activity and increase cAMP production in the posterior gills of the blue crab Callinectes sapidus

Dopamine-mediated regulation of Na(+)-K(+)-ATPase activity in the posterior gills of some crustaceans has been reported to be involved in osmoregulation. The dopamine receptors of invertebrates are classified into three groups based on their structure and pharmacology: D1- and D2-like receptors and a distinct invertebrate receptor subtype (INDR). We tested the hypothesis that a D1-like receptor is expressed in the blue crab Callinectes sapidus and regulates Na(+)-K(+)-ATPase activity. RT-PCR, using degenerate primers, showed the presence of D1βR mRNA in the posterior gill. The blue crab posterior gills showed positive immunostaining for a dopamine D5 receptor (D5R or D1βR) antibody in the basolateral membrane and cytoplasm. Confocal microscopy showed colocalization of Na(+)-K(+)-ATPase and D1βR in the basolateral membrane. To determine the effect of D1-like receptor stimulation on Na(+)-K(+)-ATPase activity, intact crabs acclimated to low salinity for 6 days were given an intracardiac infusion of the D1-like receptor agonist fenoldopam, with or without the D1-like receptor antagonist SCH23390. Fenoldopam increased cAMP production twofold and decreased Na(+)-K(+)-ATPase activity by 50% in the posterior gills. This effect was blocked by coinfusion with SCH23390, which had no effect on Na(+)-K(+)-ATPase activity by itself. Fenoldopam minimally decreased D1βR protein expression (10%) but did not affect Na(+)-K(+)-ATPase α-subunit protein expression. This study shows the presence of functional D1βR in the posterior gills of euryhaline crabs chronically exposed to low salinity and highlights the evolutionarily conserved function of the dopamine receptors on sodium homeostasis.

Characterisation of AmphiAmR11, an amphioxus (Branchiostoma floridae) D2-dopamine-like G protein-coupled receptor

The evolution of the biogenic amine signalling system in vertebrates is unclear. However, insights can be obtained from studying the structures and signalling properties of biogenic amine receptors from the protochordate, amphioxus, which is an invertebrate species that exists at the base of the chordate lineage. Here we describe the signalling properties of AmphiAmR11, an amphioxus (Branchiostoma floridae) G protein-coupled receptor which has structural similarities to vertebrate α₂-adrenergic receptors but which functionally acts as a D₂ dopamine-like receptor when expressed in Chinese hamster ovary -K1 cells. AmphiAmR11 inhibits forskolin-stimulated cyclic AMP levels with tyramine, phenylethylamine and dopamine being the most potent agonists. AmphiAmR11 also increases mitogen-activated protein kinase activity and calcium mobilisation, and in both pathways, dopamine was found to be more potent than tyramine. Thus, differences in the relative effectiveness of various agonists in the different second messenger assay systems suggest that the receptor displays agonist-specific coupling (biased agonism) whereby different agonists stabilize different conformations of the receptor which lead to the enhancement of one signalling pathway over another. The present study provides insights into the evolution of α₂-adrenergic receptor signalling and support the hypothesis that α₂-adrenergic receptors evolved from D₂-dopamine receptors. The AmphiAmR11 receptor may represent a transition state between D₂-dopamine receptors and α₂-adrenergic receptors.

Capture of D2 dopamine receptor signaling complexes in striatal cells for mass spectrometry proteomic analysis

In recent years advancements in proteomic techniques have contributed to the understanding of protein interaction networks (Interactomes) in various cell types. Today, high throughput proteomics promises to define virtually all of the components of a signaling and a regulatory network within cells for various molecules including membrane-spanning receptors. The D2 dopamine receptor (D2R) is a primary mediator of dopamine transmission in the brain. Signaling through D2Rs has been linked to dopamine-mediated effects on motivation, reward, locomotion and addiction to drugs of abuse. In the striatum, the D2R is a key mediatory of dopamine transmission. Actions on this receptor are an important pharmacological property of various drugs including typical antipsychotics and drugs of abuse. Here we provide an approach for the identification protein interaction networks of the D2R within striatal cells. We discuss key assays and techniques, such as cellular membrane protein fractionation, western blot analysis, magnetic bead coimmunoprecipitation, and liquid chromatography electrospray ionization (LC-ESI) mass spectrometry, that can be used for the isolation and characterization of D2R protein interaction networks. This approach presents a reliable method for the identification and characterization of D2R signaling within cells.

Acute administration of dopaminergic drugs has differential effects on locomotion in larval zebrafish

Altered dopaminergic signaling causes behavioral changes in mammals. In general, dopaminergic receptor agonists increase locomotor activity, while antagonists decrease locomotor activity. In order to determine if zebrafish (a model organism becoming popular in pharmacology and toxicology) respond similarly, the acute effects of drugs known to target dopaminergic receptors in mammals were assessed in zebrafish larvae. Larvae were maintained in 96-well microtiter plates (1 larva/well). Non-lethal concentrations (0.2-50 μM) of dopaminergic agonists (apomorphine, SKF-38393, and quinpirole) and antagonists (butaclamol, SCH-23390, and haloperidol) were administered at 6 days post-fertilization (dpf). An initial experiment identified the time of peak effect of each drug (20-260 min post-dosing, depending on the drug). Locomotor activity was then assessed for 70 min in alternating light and dark at the time of peak effect for each drug to delineate dose-dependent effects. All drugs altered larval locomotion in a dose-dependent manner. Both the D1- and D2-like selective agonists (SKF-38393 and quinpirole, respectively) increased activity, while the selective antagonists (SCH-23390 and haloperidol, respectively) decreased activity. Both selective antagonists also blunted the response of the larvae to changes in lighting conditions at higher doses. The nonselective drugs had biphasic effects on locomotor activity: apomorphine increased activity at the low dose and at high doses, while butaclamol increased activity at low to intermediate doses, and decreased activity at high doses. This study demonstrates that (1) larval zebrafish locomotion can be altered by dopamine receptor agonists and antagonists, (2) receptor agonists and antagonists generally have opposite effects, and (3) drugs that target dopaminergic receptors in mammals appear, in general, to elicit similar locomotor responses in zebrafish larvae.

Dopamine receptor interacting proteins: targeting neuronal calcium sensor-1/D2 dopamine receptor interaction for antipsychotic drug development

D2 dopamine receptors (D2Rs) represent an important class of receptors in the pharmacological development of novel therapeutic drugs for the treatment of schizophrenia. Recent research into D2R signaling suggests that receptor properties are dependent on interaction with a cohort of dopamine receptor interacting proteins (DRIPs) within a macromolecular structure termed the signalplex. One component of this signalplex is neuronal calcium sensor 1 (NCS-1) a protein found to regulate the phosphorylation, trafficking, and signaling profile of the D2R in neurons. It has also been found that NCS-1 can contribute to the pathology of schizophrenia and may play a role in the efficacy of antipsychotic drug medication in the brain. In this review we discuss how the selective targeting of a DRIP, such as NCS-1, can be utilized as a novel strategy of drug design for the creation of new therapeutics for a disease such as schizophrenia. Using a fluorescence polarization assay we explore how the ability to detect changes in D2R/NCS-1 interaction can be exploited as an effective screening tool in the isolation and development of lead compounds for antipsychotic drug development. This line of work explores a novel direction in targeting D2Rs via their signalplex components and supports the notion that receptor interacting proteins represent an emerging new class of molecular targets for pharmacological drug development.

Molecular determinants of A2AR-D2R allosterism: role of the intracellular loop 3 of the D2R

In the CNS, an antagonistic interaction has been shown between adenosine A(2A) and dopamine D(2) receptors (A(2A)Rs and D(2)Rs) that may be relevant both in normal and pathological conditions (i.e., Parkinson's disease). Thus, the molecular determinants mediating this receptor-receptor interaction have recently been explored, as the fine tuning of this target (namely the A(2A)R/D(2)R oligomer) could possibly improve the treatment of certain CNS diseases. Here, we used a fluorescence resonance energy transfer-based approach to examine the allosteric modulation of the D(2)R within the A(2A)R/D(2)R oligomer and the dependence of this receptor-receptor interaction on two regions rich in positive charges on intracellular loop 3 of the D(2)R. Interestingly, we observed a negative allosteric effect of the D(2)R agonist quinpirole on A(2A)R ligand binding and activation. However, these allosteric effects were abolished upon mutation of specific arginine residues (217-222 and 267-269) on intracellular loop 3 of the D(2)R, thus demonstrating a major role of these positively charged residues in mediating the observed receptor-receptor interaction. Overall, these results provide structural insights to better understand the functioning of the A(2A)R/D(2)R oligomer in living cells.

D1 dopamine receptor signaling is modulated by the R7 RGS protein EAT-16 and the R7 binding protein RSBP-1 in Caenoerhabditis elegans motor neurons

Dopamine signaling modulates voluntary movement and reward-driven behaviors by acting through G protein-coupled receptors in striatal neurons, and defects in dopamine signaling underlie Parkinson's disease and drug addiction. Despite the importance of understanding how dopamine modifies the activity of striatal neurons to control basal ganglia output, the molecular mechanisms that control dopamine signaling remain largely unclear. Dopamine signaling also controls locomotion behavior in Caenorhabditis elegans. To better understand how dopamine acts in the brain we performed a large-scale dsRNA interference screen in C. elegans for genes required for endogenous dopamine signaling and identified six genes (eat-16, rsbp-1, unc-43, flp-1, grk-1, and cat-1) required for dopamine-mediated behavior. We then used a combination of mutant analysis and cell-specific transgenic rescue experiments to investigate the functional interaction between the proteins encoded by two of these genes, eat-16 and rsbp-1, within single cell types and to examine their role in the modulation of dopamine receptor signaling. We found that EAT-16 and RSBP-1 act together to modulate dopamine signaling and that while they are coexpressed with both D1-like and D2-like dopamine receptors, they do not modulate D2 receptor signaling. Instead, EAT-16 and RSBP-1 act together to selectively inhibit D1 dopamine receptor signaling in cholinergic motor neurons to modulate locomotion behavior.

Role of dopamine receptors in ADHD: a systematic meta-analysis

The dopaminergic system plays a pivotal role in the central nervous system via its five diverse receptors (D1-D5). Dysfunction of dopaminergic system is implicated in many neuropsychological diseases, including attention deficit hyperactivity disorder (ADHD), a common mental disorder that prevalent in childhood. Understanding the relationship of five different dopamine (DA) receptors with ADHD will help us to elucidate different roles of these receptors and to develop therapeutic approaches of ADHD. This review summarized the ongoing research of DA receptor genes in ADHD pathogenesis and gathered the past published data with meta-analysis and revealed the high risk of DRD5, DRD2, and DRD4 polymorphisms in ADHD.

Mapping structural determinants within third intracellular loop that direct signaling specificity of type 1 corticotropin-releasing hormone receptor

The type 1 corticotropin-releasing hormone receptor (CRH-R1) influences biological responses important for adaptation to stressful stimuli, through activation of multiple downstream effectors. The structural motifs within CRH-R1 that mediate G protein activation and signaling selectivity are unknown. The aim of this study was to gain insights about important structural determinants within the third intracellular loop (IC3) of the human CRH-R1α important for cAMP and ERK1/2 pathways activation and selectivity. We investigated the role of the juxtamembrane regions of IC3 by mutating amino acid cassettes or specific residues to alanine. Although simultaneous tandem alanine mutations of both juxtamembrane regions Arg²⁹²-Met²⁹⁵ and Lys³¹¹-Lys³¹⁴ reduced ligand binding and impaired signaling, all other mutant receptors retained high affinity binding, indistinguishable from wild-type receptor. Agonist-activated receptors with tandem mutations at the proximal or distal terminal segments enhanced activation of adenylyl cyclase by 50–75% and diminished activation of inositol trisphosphate and ERK1/2 by 60–80%. Single Ala mutations identified Arg²⁹², Lys²⁹⁷, Arg³¹⁰, Lys³¹¹, and Lys³¹⁴ as important residues for the enhanced activation of adenylyl cyclase, partly due to reduced inhibition of adenylyl cyclase activity by pertussis toxin-sensitive G proteins. In contrast, mutation of Arg²⁹⁹ reduced receptor signaling activity and cAMP response. Basic as well as aliphatic amino acids within both juxtamembrane regions were identified as important for ERK1/2 phosphorylation through activation of pertussis toxin-sensitive G proteins as well as G_q proteins. These data uncovered unexpected roles for key amino acids within the highly conserved hydrophobic N- and C-terminal microdomains of IC3 in the coordination of CRH-R1 signaling activity.

G(i/o) protein-dependent and -independent actions of Pertussis Toxin (PTX)

Pertussis toxin (PTX) is a typical A-B toxin. The A-protomer (S1 subunit) exhibits ADP-ribosyltransferase activity. The B-oligomer consists of four subunits (S2 to S5) and binds extracellular molecules that allow the toxin to enter the cells. The A-protomer ADP-ribosylates the α subunits of heterotrimeric G_i/o proteins, resulting in the receptors being uncoupled from the G_i/o proteins. The B-oligomer binds proteins expressed on the cell surface, such as Toll-like receptor 4, and activates an intracellular signal transduction cascade. Thus, PTX modifies cellular responses by at least two different signaling pathways; ADP-ribosylation of the Gα_i/o proteins by the A-protomer (G_i/o protein-dependent action) and the interaction of the B-oligomer with cell surface proteins (G_i/o protein-independent action).

Dopamine and renal function and blood pressure regulation

Dopamine is an important regulator of systemic blood pressure via multiple mechanisms. It affects fluid and electrolyte balance by its actions on renal hemodynamics and epithelial ion and water transport and by regulation of hormones and humoral agents. The kidney synthesizes dopamine from circulating or filtered L-DOPA independently from innervation. The major determinants of the renal tubular synthesis/release of dopamine are probably sodium intake and intracellular sodium. Dopamine exerts its actions via two families of cell surface receptors, D1-like receptors comprising D1R and D5R, and D2-like receptors comprising D2R, D3R, and D4R, and by interactions with other G protein-coupled receptors. D1-like receptors are linked to vasodilation, while the effect of D2-like receptors on the vasculature is variable and probably dependent upon the state of nerve activity. Dopamine secreted into the tubular lumen acts mainly via D1-like receptors in an autocrine/paracrine manner to regulate ion transport in the proximal and distal nephron. These effects are mediated mainly by tubular mechanisms and augmented by hemodynamic mechanisms. The natriuretic effect of D1-like receptors is caused by inhibition of ion transport in the apical and basolateral membranes. D2-like receptors participate in the inhibition of ion transport during conditions of euvolemia and moderate volume expansion. Dopamine also controls ion transport and blood pressure by regulating the production of reactive oxygen species and the inflammatory response. Essential hypertension is associated with abnormalities in dopamine production, receptor number, and/or posttranslational modification.

Inhibitory effect of the D(3) dopamine receptor on insulin receptor expression and function in vascular smooth muscle cells

BACKGROUND

Vascular smooth muscle cell (VSMC) proliferation is regulated by numerous hormones and humoral factors. Our previous study found that stimulation of D(1)-like dopamine receptors inhibited insulin receptor expression and function in VSMCs. We hypothesize that there is also an interaction between D(3) dopamine and insulin receptors, i.e., stimulation of the D(3) receptor inhibits insulin receptor expression and function.

METHODS

Receptor expression was determined by immunoblotting, immunohistochemisty, and reverse transcriptase-PCR; VSMC proliferation was determined by 3-(4,5-dimethylthiazol-2-yl)-diphenyl-tetrazolium bromide (MTT) assay and cell number.

RESULTS

Insulin receptor protein is increased in the aorta of D(3) receptor deficient mice. Stimulation of the D(3) receptor inhibited insulin receptor mRNA and protein expression and insulin-mediated VSMC proliferation, and increased protein kinase A (PKA) activity, insulin receptor phosphorylation, and degradation in immortalized aortic VSMCs (A10 cells). These effects were blocked by a PKA inhibitor, indicating that the D(3) receptor-mediated decrease in insulin receptor expression was related to a decrease in transcription/post-transcription and increased degradation, involving PKA signaling.

CONCLUSIONS

D(3) receptor stimulation may be a target to reduce the adverse effect of insulin in hypertension by inhibition of insulin receptor expression and function in arterial VSMCs.

Development of a cyclic adenosine monophosphate assay for Gi-coupled G protein-coupled receptors by utilizing the endogenous calcitonin activity in Chinese hamster ovary cells

Activation of G(i)-coupled G protein-coupled receptor (GPCRs) by their ligands leads to inhibition of adenylyl cyclase (AC) and reduction of cyclic adenosine monophosphate (cAMP) levels in cells. The traditional cAMP assay for G(i)-coupled GPCRs commonly uses forskolin, a nonspecific AC activator, to increase the basal cAMP level in cells to create an assay window for ligand detection. However, there is still a need to develop a nonforskolin-based cAMP assay because of the challenges inherent in titrating the concentration of forskolin to achieve a reliable assay window, along with issues related to the cAMP-independent effects of forskolin. Herein, we describe such an assay by utilizing the endogenous activity of the calcitonin receptor in Chinese hamster ovary (CHO) cells. The calcitonin receptor is a G(s)-coupled GPCR that, when activated by calcitonin, leads to the stimulation of AC and increases cAMP in cells. Thus, we use calcitonin, instead of forskolin, to increase the basal cAMP level in CHO cells to achieve an assay window. We demonstrated that calcitonin peptides robustly increased cAMP accumulation in several CHO cell lines stably expressing well-known G(i)-coupled GPCRs, such as the Dopamine D2 receptor, the Opioid μ receptor, or the Cannabinoid receptor-1. Agonists of these G(i)-coupled GPCRs attenuated calcitonin-induced cAMP production in their receptor stable cell lines. On the other hand, antagonists and/or inverse agonists blocked the effects of their agonists on calcitonin-induced cAMP production. This calcitonin-based cAMP assay has been demonstrated to be sensitive and robust and exhibited acceptable assay windows (signal/noise ratio) and, thus, can be applied to screen for agonists and antagonists/inverse agonists of G(i)-coupled GPCRs in high-throughput screening formats.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

G protein-coupled receptor kinase 4 (GRK4) regulates the phosphorylation and function of the dopamine D3 receptor

During conditions of moderate sodium excess, the dopaminergic system regulates blood pressure and water and electrolyte balance by engendering natriuresis. Dopamine exerts its effects on dopamine receptors, including the dopamine D(3) receptor. G protein-coupled receptor kinase 4 (GRK4), whose gene locus (4p16.3) is linked to essential hypertension, desensitizes the D(1) receptor, another dopamine receptor. This study evaluated the role of GRK4 on D(3) receptor function in human proximal tubule cells. D(3) receptor co-segregated in lipid rafts and co-immunoprecipitated and co-localized in human proximal tubule cells and in proximal and distal tubules and glomeruli of kidneys of Wistar Kyoto rats. Bimolecular fluorescence complementation and confocal microscopy revealed that agonist activation of the receptor initiated the interaction between D(3) receptor and GRK4 at the cell membrane and promoted it intracellularly, presumably en route to endosomal trafficking. Of the four GRK4 splice variants, GRK4-gamma and GRK4-alpha mediated a 3- and 2-fold increase in the phosphorylation of agonist-activated D(3) receptor, respectively. Inhibition of GRK activity with heparin or knockdown of GRK4 expression via RNA interference completely abolished p44/42 phosphorylation and mitogenesis induced by D(3) receptor stimulation. These data demonstrate that GRK4, specifically the GRK4-gamma and GRK4-alpha isoforms, phosphorylates the D(3) receptor and is crucial for its signaling in human proximal tubule cells.

Multiple roles of Gi/o protein-coupled receptors in control of action potential secretion coupling in pituitary lactotrophs

G(i/o) protein-coupled receptors, signaling through G protein-dependent and protein-independent pathways, have prominent effects on secretion by modulating calcium signaling and regulating the size of the releasable secretory pool, the rates of exocytosis and endocytosis, and de novo synthesis. Pituitary cells fire action potentials spontaneously, and the associated calcium influx is sufficient to maintain prolactin (PRL) release but not gonadotropin release at high and steady levels for many hours. Such secretion, termed intrinsic, spontaneous, or basal, reflects fusion of secretory vesicles triggered by the cell type-specific pattern of action potentials. In lactotrophs, activation of endothelin ET(A) and dopamine D(2) receptors causes inhibition of spontaneous electrical activity and basal adenylyl cyclase activity accompanied with inhibition of basal PRL release. Agonist-induced inhibition of cAMP production and firing of action potentials is abolished in cells with blocked pertussis toxin (PTX)-sensitive G(i/o) signaling pathway. However, agonist-induced inhibition of PRL release is only partially relieved in such treated cells, indicating that both receptors also inhibit exocytosis downstream of cAMP/calcium signaling. The PTX-insensitive step in agonist-induced inhibition of PRL release is not affected by inhibition of phosphoinositide 3-kinase and glycogen synthase kinase-3 but is partially rescued by downregulation of the G(z)alpha expression. Thus, ET(A) and D(2) receptors inhibit basal PRL release not only by blocking electrical activity but also by desensitizing calcium-secretion coupling.

Inhibitory effect of D1-like and D3 dopamine receptors on norepinephrine-induced proliferation in vascular smooth muscle cells

The sympathetic nervous system plays an important role in the regulation of blood pressure. There is increasing evidence for positive and negative interactions between dopamine and adrenergic receptors; the activation of the alpha-adrenergic receptor induces vasoconstriction, whereas the activation of dopamine receptor induces vasorelaxation. We hypothesize that the D1-like receptor and/or D3 receptor also inhibit alpha1-adrenergic receptor-mediated proliferation in vascular smooth muscle cells (VSMCs). In this study, VSMC proliferation was determined by measuring [3H]thymidine incorporation, cell number, and uptake of 3-(4,5-dimethylthiazol-2-yl)-diphenyltetrazolium bromide (MTT). Norepinephrine increased VSMC number and MTT uptake, as well as [3H]thymidine incorporation via the alpha1-adrenergic receptor in aortic VSMCs from Sprague-Dawley rats. The proliferative effects of norepinephrine were attenuated by the activation of D1-like receptors or D3 receptors, although a D1-like receptor agonist, fenoldopam, and a D3 receptor agonist, PD-128907, by themselves, at low concentrations, had no effect on VSMC proliferation. Simultaneous stimulation of both D1-like and D3 receptors had an additive inhibitory effect. The inhibitory effect of D3 receptor was via protein kinase A, whereas the D1-like receptor effect was via protein kinase C-zeta. The interaction between alpha1-adrenergic and dopamine receptors, especially D1-like and D3 receptors in VSMCs, could be involved in the pathogenesis of hypertension.

Dopamine inhibits basal prolactin release in pituitary lactotrophs through pertussis toxin-sensitive and -insensitive signaling pathways

Dopamine D2 receptors signal through the pertussis toxin (PTX)-sensitive G(i/o) and PTX-insensitive G(z) proteins, as well as through a G protein-independent, beta-arrestin/glycogen synthase kinase-3-dependent pathway. Activation of these receptors in pituitary lactotrophs leads to inhibition of prolactin (PRL) release. It has been suggested that this inhibition occurs through the G(i/o)-alpha protein-mediated inhibition of cAMP production and/or G(i/o)-betagamma dimer-mediated activation of inward rectifier K(+) channels and inhibition of voltage-gated Ca(2+) channels. Here we show that the dopamine agonist-induced inhibition of spontaneous Ca(2+) influx and release of prestored PRL was preserved when cAMP levels were elevated by forskolin treatment. We further observed that dopamine agonists inhibited both spontaneous and depolarization-induced Ca(2+) influx in untreated but not in PTX-treated cells. This inhibition was also observed in cells with blocked inward rectifier K(+) channels, suggesting that the dopamine effect on voltage-gated Ca(2+) channel gating is sufficient to inhibit spontaneous Ca(2+) influx. However, agonist-induced inhibition of PRL release was only partially relieved in PTX-treated cells, indicating that dopamine receptors also inhibit exocytosis downstream of voltage-gated Ca(2+) influx. The PTX-insensitive step in agonist-induced inhibition of PRL release was not affected by the addition of wortmannin, an inhibitor of phosphatidylinositol 3-kinase, and lithium, an inhibitor of glycogen synthase kinase-3, but was attenuated in the presence of phorbol 12-myristate 13-acetate, which inhibits G(z) signaling pathway in a protein kinase C-dependent manner. Thus, dopamine inhibits basal PRL release by blocking voltage-gated Ca(2+) influx through the PTX-sensitive signaling pathway and by desensitizing Ca(2+) secretion coupling through the PTX-insensitive and protein kinase C-sensitive signaling pathway.

Presynaptic receptors for dopamine, histamine, and serotonin

Presynaptic receptors for dopamine, histamine and serotonin that are located on dopaminergic, histaminergic and sertonergic axon terminals, respectively, function as autoreceptors. Presynaptic receptors also occur as heteroreceptors on other axon terminals. Auto- and heteroreceptors mainly affect Ca(2+) -dependent exocytosis from the receptor-bearing nerve ending. Some additionally subserve other presynaptic functions.Presynaptic dopamine, histamine and serotonin receptors are involved in various (patho)physiological conditions. Examples are the following:Dopamine autoreceptors play a role in Parkinson's disease, schizophrenia and drug addiction. Dopamine heteroreceptors affecting the release of acetylcholine and of amino acid neurotransmitters in the basal ganglia are also relevant for Parkinson's disease. Peripheral dopamine heteroreceptors on postganglionic sympathetic terminals influence heart rate and vascular resistance through modulation of noradrenaline release. Blockade of histamine autoreceptors increases histamine synthesis and release and may support higher CNS functions such as arousal, cognition and learning. Peripheral histamine heteroreceptors on C fiber and on postganglionic sympathetic fiber terminals diminish neuropeptide and noradrenaline release, respectively. Both inhibititory effects are beneficial in myocardial ischemia. The inhibition of neuropeptide release also explains the antimigraine effects of some agonists of presynaptic histamine receptors. Upregulation of presynaptic serotonin autoreceptors is probably involved in the pathogenesis of major depression. Correspondingly, antidepressant treatments can be linked with a reduced density of 5-HT autoreceptors. 5-HT Heteroreceptor activation diminishes acetylcholine and GABA release and may therefore increase anxiety. In the periphery, presynaptic 5-HT heteroreceptor agonists shorten migraine attacks by inhibition of the release of neuropeptides from trigeminal afferents, apart from their constrictive action on meningeal vessels.

A proteomic approach to receptor signaling: Molecular mechanisms and therapeutic implications derived from discovery of the dopamine D2 receptor signalplex

Recent research in cell signaling has shown that the assembly of G protein coupled receptors into signaling complexes or signalplexes represents the primary mechanism by which receptor-mediated signaling is established and maintained. In this review, we summarize the current state of knowledge regarding protein interactions that comprise the dopamine D2 receptor signalplex within the brain. Studies based on conventional and advanced two-hybrid methodologies, as well as bioinformatic and computational analysis of sequence information from completed genomes have demonstrated interactions between dopamine D2 receptors and a cohort of dopamine receptor interacting proteins (DRIPs). DRIP interactions appear to regulate key aspects of receptor function including the signaling and membrane trafficking of dopamine D2 receptors. Disruptions or modifications of the signalplex, using membrane permeant competing peptide or dominant negative approaches, may represent promising new strategies for the selective targeting of the dopamine D2 receptor in cells and in native tissue. DRIP interactions provide a novel platform for understanding the mechanisms of dopamine receptor signaling, and for the potential development of novel treatments for brain disease.

Multiple genes and factors associated with bipolar disorder converge on growth factor and stress activated kinase pathways controlling translation initiation: implications for oligodendrocyte viability

Famine and viral infection, as well as interferon therapy have been reported to increase the risk of developing bipolar disorder. In addition, almost 100 polymorphic genes have been associated with this disease. Several form most of the components of a phosphatidyl-inositol signalling/AKT1 survival pathway (PIK3C3, PIP5K2A, PLCG1, SYNJ1, IMPA2, AKT1, GSK3B, TCF4) which is activated by growth factors (BDNF, NRG1) and also by NMDA receptors (GRIN1, GRIN2A, GRIN2B). Various other protein products of genes associated with bipolar disorder either bind to or are affected by phosphatidyl-inositol phosphate products of this pathway (ADBRK2, HIP1R, KCNQ2, RGS4, WFS1), are associated with its constituent elements (BCR, DUSP6, FAT, GNAZ) or are downstream targets of this signalling cascade (DPYSL2, DRD3, GAD1, G6PD, GCH1, KCNQ2, NOS3, SLC6A3, SLC6A4, SST, TH, TIMELESS). A further pathway relates to endoplasmic reticulum-stress (HSPA5, XBP1), caused by problems in protein glycosylation (ALG9), growth factor receptor sorting (PIK3C3, HIP1R, SYBL1), or aberrant calcium homoeostasis (WFS1). Key processes relating to these pathways appear to be under circadian control (ARNTL, CLOCK, PER3, TIMELESS). DISC1 can also be linked to many of these pathways. The growth factor pathway promotes protein synthesis, while the endoplasmic reticulum stress pathway, and other stress pathways activated by viruses and cytokines (IL1B, TNF, Interferons), oxidative stress or starvation, all factors associated with bipolar disorder risk, shuts down protein synthesis via control of the EIF2 alpha and beta translation initiation complex. For unknown reasons, oligodendrocytes appear to be particularly prone to defects in the translation initiation complex (EIF2B) and the convergence of these environmental and genomic signalling pathways on this area might well explain their vulnerability in bipolar disorder.

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.

Arthropod D2 receptors positively couple with cAMP through the Gi/o protein family

The pyloric network is an important model system for understanding neuromodulation of rhythmic motor behaviors like breathing or walking. Dopamine (DA) differentially modulates neurons within the pyloric network. However, while the electrophysiological actions of DA have been well characterized, nothing is known about the signaling events that mediate its effects. We have begun a molecular characterization of DA receptors (DARs) in this invertebrate system. Here, we describe the cloning and characterization of the lobster D(2) receptor, D(2 alpha Pan). We found that when expressed in HEK cells, the D(2 alpha Pan) receptor is activated by DA, but not other monoamines endogenous to the lobster nervous system. This receptor positively couples with cAMP through multiple Gi/o proteins via two discrete pathways: 1) a G alpha mediated inhibition of adenylyl cyclase (AC), leading to a decrease in cAMP and 2) a G beta gamma-mediated activation of phospholipase C beta (PLC beta), leading to an increase in cAMP. Alternate splicing alters the potency and efficacy of the receptor, but does not affect monoamine specificity. Finally, we show that arthropod D(2) receptor coupling with cAMP varies with the cellular milieu.

Gz proteins are functionally coupled to dopamine D2-like receptors in vivo

The receptors that couple to the G protein Gz in vivo are still relatively unknown. In this study, we investigated the effects of various dopamine receptor agonists in a mouse deficient in the alpha subunit of Gz. The dopamine D1-like receptor agonist SKF38393 stimulated comparable locomotor activity in both wildtype mice and mice lacking Galphaz. In contrast, the dopamine D2-like receptor agonist quinpirole suppressed locomotor activity in both groups of mice, but this suppression was significantly smaller in Galphaz knockout mice. Consistent with these behavioural observations, quinpirole inhibition of dopamine release in the forebrain nucleus accumbens evoked by electrical stimulation of dopamine axons was significantly attenuated in mice lacking Galphaz. In addition, hypothermia and adrenocorticotropic hormone release resulting from activation of dopamine D2-like receptors were also significantly reduced in Galphaz knockout mice. However, adrenocorticotropic hormone secretion induced by corticotrophin releasing hormone and the serotonin 1A receptor agonist 8-hydroxy-dipropylamino-tetralin were similar between wildtype and Galphaz knockout mice. Western blot analysis showed that the expression levels of Galphai, Galphao, Galphas, Galphaq and Gbeta were the same in the brains of mice of both genotypes. Overall, our data suggest that Gz proteins are functionally coupled to dopamine D2-like receptors in vivo.

D2-class dopamine receptor inhibition of NMDA currents in prefrontal cortical neurons is platelet-derived growth factor receptor-dependent

NMDA receptor function is modulated by both G-protein-coupled receptors and receptor tyrosine kinases. In acutely isolated rat hippocampal neurons, direct activation of the platelet-derived growth factor (PDGF) receptor or transactivation of the PDGF receptor by D4 dopamine receptors inhibits NMDA-evoked currents in a phospholipase C (PLC)-dependent manner. We have investigated further the ability of D2-class dopamine receptors to modulate NMDA-evoked currents in isolated rat prefrontal cortex (PFC). We have demonstrated that, similar to isolated hippocampal neurons, the application of PDGF-BB or quinpirole to isolated PFC neurons induces a slow-onset and long-lasting inhibition of NMDA-evoked currents. However, in contrast to hippocampal neurons, the inhibition of NMDA-evoked currents by quinpirole in PFC neurons is dependent upon D2/3, rather than D4, dopamine receptors. In PFC slices, application of both PDGF-BB and quinpirole induced a phosphorylation of the PDGF receptor at the PLCgamma binding and activation site, Tyr1021. The PDGF receptor kinase inhibitor, tyrphostin A9, and the D2/3 dopamine receptor antagonist, raclopride, inhibited quinpirole-induced Tyr1021 phosphorylation. These finding suggest that quinpirole treatment inhibits NMDAR signaling via PDGF receptor transactivation in both the hippocampus and the PFC, and that the effects of quinpirole in these regions are mediated by D4 and D2/3 dopamine receptors, respectively.

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

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

Review of evidence that posttransplantation psychiatric treatment commonly affects prolactin levels and thereby influences graft fate

Delirium, depression and other psychiatric difficulties are commonly encountered by posttransplantation patients, and antipsychotic medicines are frequently used to treat these difficulties. This article reviews previous research data concerning the immunological effects of these medicines, with particular focus on the consequences of prolactin elevation. Unproven but of concern is that these effects may influence graft fate. Older antipsychotic medicines such as haloperidol and chlorpromazine have a high likelihood of elevating prolactin. Prolactin is an immunologically active molecule generally promoting bone marrow function. This may be of benefit post-stem-cell transplant, helping engraftment, but could further rejection of solid-organ transplants. Elevated prolactin is implicated in the facilitation of graft-versus-host disease. Aripiprazole is the antipsychotic medicine least likely to increase prolactin (and may actually decrease prolactin); risperidone, the most likely to increase prolactin. Olanzapine, quetiapine and ziprazadone are antipsychotic medicines with a lower likelihood of elevating prolactin. Older ("neuroleptic") antipsychotics, such as chlorpromazine, droperidol and haloperidol, perphenazine and many others, are likely to elevate serum prolactin. Among antidepressants, most serotonin reuptake inhibitors, with the exception of sertraline, can slightly elevate prolactin. The atypical (i.e., alone in their class) antidepressants bupropion and mirtazapine are prolactin neutral. The immunological consequences of psychiatric medicines should be considered when treating transplant patients for delirium, depression and thought disorders; in addition, if elevation of prolactin is thought to be of immunological importance during psychiatric treatment, then it should be monitored and treated. The dopamine agonists used to treat Parkinson's disease--bromocriptine, pergolide, pramipexole, ropinerole--usually reverse antipsychotic-induced prolactin increases without compromising psychiatric effectiveness.

Molecular cloning and characterization of crustacean type-one dopamine receptors: D1alphaPan and D1betaPan

Dopamine (DA) differentially modulates identified neurons in the crustacean stomatogastric nervous system (STNS). While the electrophysiological actions of DA have been well characterized, little is known about the dopaminergic transduction cascades operating in this system. As a first step toward illuminating the molecular underpinnings of dopaminergic signal transduction in the crustacean STNS, we have cloned and characterized two type-one DA receptors (DARs) from the spiny lobster (Panulirus interruptus): D(1alphaPan) and D(1betaPan). We found that the structure and function of these arthropod DARs are well conserved across species. Using a heterologous expression system, we determined that DA, but not serotonin, octopamine, tyramine or histamine activates these receptors. When stably expressed in HEK cells, the D(1alphaPan) receptor couples with Gs, and DA elicits an increase in [cAMP]. The D(1betaPan) receptor responds to DA with a net increase in [cAMP] that is mediated by Gs and Gz.

Enhanced effect of dopaminergic stimulation on prepulse inhibition in mice deficient in the alpha subunit of G(z)

RATIONALE

G(z) is a member of the G(i) G protein family associated with dopamine D2-like receptors; however, its functions remain relatively unknown. The aim of the present study was to investigate prepulse inhibition (PPI) of acoustic startle, locomotor hyperactivity and dopamine D2 receptor binding in mice deficient in the alpha subunit of G(z).

METHODS

We used automated startle boxes to assess startle and PPI after treatment with saline, amphetamine, apomorphine or MK-801. We used photocell cages to quantitate locomotor activity after amphetamine treatment. Dopamine D2 receptor density was determined by autoradiography.

RESULTS

Startle responses and baseline PPI were not different between the Galpha(z) knockout mice and wild-type controls (average PPI 46+/-4 vs 49+/-3%, respectively). Amphetamine treatment caused a marked disruption of PPI in Galpha(z) knockouts (average PPI 22+/-2%), but less so in controls (average PPI 42+/-3%). Similar genotype-dependent responses were seen after apomorphine treatment (average PPI 23+/-3% vs 40+/-3%), but not after MK-801 treatment (average PPI 29+/-5 vs 33+/-2%). Amphetamine-induced locomotor hyperactivity was greater in Galpha(z) knockouts than in controls. There was no difference in the density of dopamine D2 receptors in nucleus accumbens.

CONCLUSIONS

Mice deficient in the alpha subunit of G(z) show enhanced sensitivity to the disruption of PPI and locomotor hyperactivity caused by dopaminergic stimulation. These results suggest a possible role for G(z) in neuropsychiatric illnesses with presumed dopaminergic hyperactivity, such as schizophrenia.

Paralemmin interacts with D3 dopamine receptors: implications for membrane localization and cAMP signaling

Paralemmin is a novel lipid-anchored protein, which is highly expressed in neuronal plasma membranes. In this study, we demonstrate that paralemmin specifically interacts with the third intracellular loop of the D3 dopamine receptor. Utilizing co-immunoprecipitation and glutathione-S-transferase (GST) pulldown strategies, we demonstrate that paralemmin interacts exclusively with D3, but not D2 or D4 dopamine receptors or beta-adrenergic receptors. Immunocytochemistry demonstrated co-localization of paralemmin and D3 receptor in vivo in hippocampus and cerebellum and in vitro in glial and neuronal cultures. Deletion mutational analysis indicates that amino acids 154-230 of paralemmin strongly interacted with amino acids 211-227 and 281-330 of the third intracellular loop of D3 receptor. The consequences of these interactions were investigated by co-expression in HEK293 cells. Cell surface biotinylation experiments demonstrate that paralemmin decreased D3 receptor concentration at the plasma membrane. Consistent with this observation, paralemmin expression decreased dopamine-stimulated adenylate cyclase activity. However, paralemmin also decreased basal, isoproterenol and forskolin-stimulated adenylate cyclase activity, suggesting a more general cellular function for paralemmin. Taken together, paralemmin has been implicated as a potent modulator of cellular cAMP signaling within the brain.

A role for the distal carboxyl tails in generating the novel pharmacology and G protein activation profile of mu and delta opioid receptor hetero-oligomers

Opioid receptor pharmacology in vivo has predicted a greater number of receptor subtypes than explained by the profiles of the three cloned opioid receptors, and the functional dependence of the receptors on each other shown in gene-deleted animal models remains unexplained. One mechanism for such findings is the generation of novel signaling complexes by receptor hetero-oligomerization, which we previously showed results in significantly different pharmacology for mu and delta receptor hetero-oligomers compared with the individual receptors. In the present study, we show that deltorphin-II is a fully functional agonist of the mu-delta heteromer, which induced desensitization and inhibited adenylyl cyclase through a pertussis toxin-insensitive G protein. Activation of the mu-delta receptor heteromer resulted in preferential activation of Galpha(z), illustrated by incorporation of GTPgamma(35)S, whereas activation of the individually expressed mu and delta receptors preferentially activated Galpha(i). The unique pharmacology of the mu-delta heteromer was dependent on the reciprocal involvement of the distal carboxyl tails of both receptors, so that truncation of the distal mu receptor carboxyl tail modified the delta-selective ligand-binding pocket, and truncation of the delta receptor distal carboxyl tail modified the mu-selective binding pocket. The distal carboxyl tails of both receptors also had a significant role in receptor interaction, as evidenced by the reduced ability to co-immunoprecipitate when the carboxyl tails were truncated. The interaction between mu and delta receptors occurred constitutively when the receptors were co-expressed, but did not occur when receptor expression was temporally separated, indicating that the hetero-oligomers were generated by a co-translational mechanism.

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.

The dopamine D4 receptor gene 48-base-pair-repeat polymorphism and mood disorders: a meta-analysis

BACKGROUND

We conducted a meta-analysis to re-evaluate the role of the dopamine D4 receptor gene 48-base-pair- repeat (DRD4) polymorphism in mood disorders.

METHODS

DRD4 allele frequencies were compared between 917 patients with unipolar (UP) or bipolar affective disorder (BP) and 1164 control subjects from 12 samples, using the Cochrane Review Manager.

RESULTS

An association was found between all mood disorder groups and DRD4.2. After correcting for multiple testing, the association between DRD4.2 and BP dropped to insignificance; however, the evidence of an association between the DRD4.2 allele and UP (p < .001) and the combined group (p < .001) remained. There was no evidence for heterogeneity or publication bias.

CONCLUSIONS

These findings suggest that the DRD4.2 allele is a risk allele for depression symptomatology. Meta-analysis may be a valuable objective tool for a quantitative summary of evidence for association studies in psychiatric genetics.

Phosphodiesterase inhibitors: a novel mechanism for receptor-independent antipsychotic medications

OVERVIEW

All current antipsychotic medications work by binding to Gi-coupled dopamine (DA) D2 receptors. Such medications are thought to affect cellular function primarily by decreasing DA-mediated regulation of intracellular cyclic adenosine monophosphate (cAMP).However, several studies indicate that cAMP signal transduction abnormalities in schizophrenia may not be limited to D2-containing cells. The current study examines the potential of using non-receptor-based agents that modify intracellular signal transduction as potential antipsychotic medications.

METHODS

The indirect DA agonist amphetamine has been used to model the auditory sensory processing deficits in schizophrenia. Such pharmacologically induced abnormalities are reversed by current antipsychotic treatments. This study examines the ability of the phosphodiesterase-4 inhibitor, rolipram, to reverse amphetamine-induced abnormalities in auditory-evoked potentials that are characteristic of schizophrenia.

RESULTS

Rolipram reverses amphetamine-induced reductions in auditory-evoked potentials.

CONCLUSION

This finding could lead to novel approaches to receptor-independent treatments for schizophrenia.

Dopamine Receptor Signaling

The D1-like (D1, D5) and D2-like (D2, D3, D4) classes of dopamine receptors each has shared signaling properties that contribute to the definition of the receptor class, although some differences among subtypes within a class have been identified. D1-like receptor signaling is mediated chiefly by the heterotrimeric G proteins Galphas and Galphaolf, which cause sequential activation of adenylate cyclase, cylic AMP-dependent protein kinase, and the protein phosphatase-1 inhibitor DARPP-32. The increased phosphorylation that results from the combined effects of activating cyclic AMP-dependent protein kinase and inhibiting protein phosphatase 1 regulates the activity of many receptors, enzymes, ion channels, and transcription factors. D1 or a novel D1-like receptor also signals via phospholipase C-dependent and cyclic AMP-independent mobilization of intracellular calcium. D2-like receptor signaling is mediated by the heterotrimeric G proteins Galphai and Galphao. These pertussis toxin-sensitive G proteins regulate some effectors, such as adenylate cyclase, via their Galpha subunits, but regulate many more effectors such as ion channels, phospholipases, protein kinases, and receptor tyrosine kinases as a result of the receptor-induced liberation of Gbetagamma subunits. In addition to interactions between dopamine receptors and G proteins, other protein:protein interactions such as receptor oligomerization or receptor interactions with scaffolding and signal-switching proteins are critical for regulation of dopamine receptor signaling.