D1 and D2 Dopamine Receptors Form Heterooligomers and Cointernalize after Selective Activation of Either Receptor

Rationale and Development of Tavapadon, a D1/D5-Selective Partial Dopamine Agonist for the Treatment of Parkinson's Disease

Currently, available therapeutics for the treatment of Parkinson's disease (PD) fail to provide sustained and predictable relief from motor symptoms without significant risk of adverse events (AEs). While dopaminergic agents, particularly levodopa, may initially provide strong motor control, this efficacy can vary with disease progression. Patients may suffer from motor fluctuations, including sudden and unpredictable drop-offs in efficacy. Dopamine agonists (DAs) are often prescribed during early-stage PD with the expectation they will delay the development of levodopa-associated complications, but currently available DAs are less effective than levodopa for the treatment of motor symptoms. Furthermore, both levodopa and DAs are associated with a significant risk of AEs, many of which can be linked to strong, repeated stimulation of D2/D3 dopamine receptors. Targeting D1/D5 dopamine receptors has been hypothesized to produce strong motor benefits with a reduced risk of D2/D3-related AEs, but the development of D1-selective agonists has been previously hindered by intolerable cardiovascular AEs and poor pharmacokinetic properties. There is therefore an unmet need in PD treatment for therapeutics that provide sustained and predictable efficacy, with strong relief from motor symptoms and reduced risk of AEs. Partial agonism at D1/D5 has shown promise for providing relief from motor symptoms, potentially without the AEs associated with D2/D3-selective DAs and full D1/D5-selective DAs. Tavapadon is a novel oral partial agonist that is highly selective at D1/D5 receptors and could meet these criteria. This review summarizes currently available evidence of tavapadon's therapeutic potential for the treatment of early through advanced PD.

Dopamine Dynamics and Neurobiology of Non-Response to Antipsychotics, Relevance for Treatment Resistant Schizophrenia: A Systematic Review and Critical Appraisal

Treatment resistant schizophrenia (TRS) is characterized by a lack of, or suboptimal response to, antipsychotic agents. The biological underpinnings of this clinical condition are still scarcely understood. Since all antipsychotics block dopamine D2 receptors (D2R), dopamine-related mechanisms should be considered the main candidates in the neurobiology of antipsychotic non-response, although other neurotransmitter systems play a role. The aims of this review are: (i) to recapitulate and critically appraise the relevant literature on dopamine-related mechanisms of TRS; (ii) to discuss the methodological limitations of the studies so far conducted and delineate a theoretical framework on dopamine mechanisms of TRS; and (iii) to highlight future perspectives of research and unmet needs. Dopamine-related neurobiological mechanisms of TRS may be multiple and putatively subdivided into three biological points: (1) D2R-related, including increased D2R levels; increased density of D2Rs in the high-affinity state; aberrant D2R dimer or heteromer formation; imbalance between D2R short and long variants; extrastriatal D2Rs; (2) presynaptic dopamine, including low or normal dopamine synthesis and/or release compared to responder patients; and (3) exaggerated postsynaptic D2R-mediated neurotransmission. Future points to be addressed are: (i) a more neurobiologically-oriented phenotypic categorization of TRS; (ii) implementation of neurobiological studies by directly comparing treatment resistant vs. treatment responder patients; (iii) development of a reliable animal model of non-response to antipsychotics.

Class A and C GPCR Dimers in Neurodegenerative Diseases

Neurodegenerative diseases affect over 30 million people worldwide with an ascending trend. Most individuals suffering from these irreversible brain damages belong to the elderly population, with onset between 50 and 60 years. Although the pathophysiology of such diseases is partially known, it remains unclear upon which point a disease turns degenerative. Moreover, current therapeutics can treat some of the symptoms but often have severe side effects and become less effective in long-term treatment. For many neurodegenerative diseases, the involvement of G proteincoupled receptors (GPCRs), which are key players of neuronal transmission and plasticity, has become clearer and holds great promise in elucidating their biological mechanism. With this review, we introduce and summarize class A and class C GPCRs, known to form heterodimers or oligomers to increase their signalling repertoire. Additionally, the examples discussed here were shown to display relevant alterations in brain signalling and had already been associated with the pathophysiology of certain neurodegenerative diseases. Lastly, we classified the heterodimers into two categories of crosstalk, positive or negative, for which there is known evidence.

Dopamine Receptors: Is It Possible to Become a Therapeutic Target for Depression?

Dopamine and its receptors are currently recognized targets for the treatment of several neuropsychiatric disorders, including Parkinson’s disease, schizophrenia, some drug use addictions, as well as depression. Dopamine receptors are widely distributed in various regions of the brain, but their role and exact contribution to neuropsychiatric diseases has not yet been thoroughly studied. Based on the types of dopamine receptors and their distribution in different brain regions, this paper reviews the current research status of the molecular, cellular and circuit mechanisms of dopamine and its receptors involved in depression. Multiple lines of investigation of these mechanisms provide a new future direction for understanding the etiology and treatment of depression and potential new targets for antidepressant treatments.

Luminescence- and Fluorescence-Based Complementation Assays to Screen for GPCR Oligomerization: Current State of the Art

G protein-coupled receptors (GPCRs) have the propensity to form homo- and heterodimers. Dysfunction of these dimers has been associated with multiple diseases, e.g., pre-eclampsia, schizophrenia, and depression, among others. Over the past two decades, considerable efforts have been made towards the development of screening assays for studying these GPCR dimer complexes in living cells. As a first step, a robust in vitro assay in an overexpression system is essential to identify and characterize specific GPCR–GPCR interactions, followed by methodologies to demonstrate association at endogenous levels and eventually in vivo. This review focuses on protein complementation assays (PCAs) which have been utilized to study GPCR oligomerization. These approaches are typically fluorescence- and luminescence-based, making identification and localization of protein–protein interactions feasible. The GPCRs of interest are fused to complementary fluorescent or luminescent fragments that, upon GPCR di- or oligomerization, may reconstitute to a functional reporter, of which the activity can be measured. Various protein complementation assays have the disadvantage that the interaction between the reconstituted split fragments is irreversible, which can lead to false positive read-outs. Reversible systems offer several advantages, as they do not only allow to follow the kinetics of GPCR–GPCR interactions, but also allow evaluation of receptor complex modulation by ligands (either agonists or antagonists). Protein complementation assays may be used for high throughput screenings as well, which is highly relevant given the growing interest and effort to identify small molecule drugs that could potentially target disease-relevant dimers. In addition to providing an overview on how PCAs have allowed to gain better insights into GPCR–GPCR interactions, this review also aims at providing practical guidance on how to perform PCA-based assays.

Dopamine: Functions, Signaling, and Association with Neurological Diseases

The dopaminergic system plays important roles in neuromodulation, such as motor control, motivation, reward, cognitive function, maternal, and reproductive behaviors. Dopamine is a neurotransmitter, synthesized in both central nervous system and the periphery, that exerts its actions upon binding to G protein-coupled receptors. Dopamine receptors are widely expressed in the body and function in both the peripheral and the central nervous systems. Dopaminergic signaling pathways are crucial to the maintenance of physiological processes and an unbalanced activity may lead to dysfunctions that are related to neurodegenerative diseases. Unveiling the neurobiology and the molecular mechanisms that underlie these illnesses may contribute to the development of new therapies that could promote a better quality of life for patients worldwide. In this review, we summarize the aspects of dopamine as a catecholaminergic neurotransmitter and discuss dopamine signaling pathways elicited through dopamine receptor activation in normal brain function. Furthermore, we describe the potential involvement of these signaling pathways in evoking the onset and progression of some diseases in the nervous system, such as Parkinson's, Schizophrenia, Huntington's, Attention Deficit and Hyperactivity Disorder, and Addiction. A brief description of new dopaminergic drugs recently approved and under development treatments for these ailments is also provided.

GPCRs and Signal Transducers: Interaction Stoichiometry

Until the late 1990s, class A G protein-coupled receptors (GPCRs) were believed to function as monomers. Indirect evidence that they might internalize or even signal as dimers has emerged, along with proof that class C GPCRs are obligatory dimers. Crystal structures of GPCRs and their much larger binding partners were consistent with the idea that two receptors might engage a single G protein, GRK, or arrestin. However, recent biophysical, biochemical, and structural evidence invariably suggests that a single GPCR binds G proteins, GRKs, and arrestins. Here we review existing evidence of the stoichiometry of GPCR interactions with signal transducers and discuss potential biological roles of class A GPCR oligomers, including proposed homo- and heterodimers.

An update on the physiological and therapeutic relevance of GPCR oligomers

The traditional view on GPCRs held that they function as single monomeric units composed of identical subunits. This notion was overturned by the discovery that GPCRs can form homo- and hetero-oligomers, some of which are obligatory, and can further assemble into receptor mosaics consisting of three or more protomers. Oligomerisation exerts significant impacts on receptor function and physiology, offering a platform for the diversification of receptor signalling, pharmacology, regulation, crosstalk, internalization and trafficking. Given their involvement in the modulation of crucial physiological processes, heteromers could constitute important therapeutic targets for a wide range of diseases, including schizophrenia, Parkinson's disease, substance abuse or obesity. This review aims at depicting the current developments in GPCR oligomerisation research, documenting various class A, B and C GPCR heteromers detected in vitro and in vivo using biochemical and biophysical approaches, as well as recently identified higher-order oligomeric complexes. It explores the current understanding of dimerization dynamics and the possible interaction interfaces that drive oligomerisation. Most importantly, it provides an inventory of the wide range of physiological processes and pathophysiological conditions to which GPCR oligomers contribute, surveying some of the oligomers that constitute potential drug targets. Finally, it delineates the efforts to develop novel classes of ligands that specifically target and tether to receptor oligomers instead of a single monomeric entity, thus ameliorating their ability to modulate GPCR function.

Acute cocaine exposure elicits rises in calcium in arousal-related laterodorsal tegmental neurons

Cocaine has strong reinforcing properties, which underlie its high addiction potential. Reinforcement of use of addictive drugs is associated with rises in dopamine (DA) in mesoaccumbal circuitry. Excitatory afferent input to mesoaccumbal circuitry sources from the laterodorsal tegmental nucleus (LDT). Chronic, systemic cocaine exposure has been shown to have cellular effects on LDT cells, but acute actions of local application have never been demonstrated. Using calcium imaging, we show that acute application of cocaine to mouse brain slices induces calcium spiking in cells of the LDT. Spiking was attenuated by tetrodotoxin (TTX) and low calcium solutions, and abolished by prior exhaustion of intracellular calcium stores. Further, DA receptor antagonists reduced these transients, whereas DA induced rises with similar spiking kinetics. Amphetamine, which also results in elevated levels of synaptic DA, but via a different pharmacological action than cocaine, induced calcium spiking with similar profiles. Although large differences in spiking were not noted in an animal model associated with a heightened proclivity of acquiring addiction‐related behavior, the prenatal nicotine exposed mouse (PNE), subtle differences in cocaine's effect on calcium spiking were noted, indicative of a reduction in action of cocaine in the LDT associated with exposure to nicotine during gestation. When taken together, our data indicate that acute actions of cocaine do include effects on LDT cells. Considering the role of intracellular calcium in cellular excitability, and of the LDT in addiction circuitry, our data suggest that cocaine effects in this nucleus may contribute to the high addiction potential of this drug.

Candidate gene study reveals DRD1 and DRD2 as putative interacting risk factors for youth depression

Alterations in the monoaminergic neurotransmission systems are suspected to be involved in the etiology of neuropsychiatric disorders, including depression. The role of these pathways in the risk of developing depressive symptoms during childhood or adolescence is still not completely clear. This study sought to identify putative genetic factors in genes of serotonergic and dopaminergic systems modulating the level of manifestation of depressive symptoms in children and adolescents. We analyzed 170 single nucleotide polymorphisms (SNPs) in 21 candidate dopaminergic and serotonergic genes in a non-clinical sample of 410 Costa Rican participants of ages between 7 and 18 years, assessing the severity of depressive symptoms through the Child Depression Inventory (CDI). Genotypic and haplotypic associations, as well as epistatic effects, were examined. A significant interaction effect was detected between rs1039089 in conjunction with rs877138 located upstream of the dopamine D1 receptor (DRD1) and the dopamine D2 receptor (DRD2) genes respectively, although no evidence was found for any single variant or haplotype related to a differential liability. This newly described genetic interaction among putative regulatory regions of dopamine receptors could affect the level of manifestation of depressive symptoms through an imbalance of D1-D2 heteromers and modulation of cognitive processes.

Genetic variants of dopamine D2 receptor impact heterodimerization with dopamine D1 receptor

BACKGROUND

The human dopamine D2 receptor gene has three polymorphic variants that alter its amino acid sequence: alanine substitution by valine in position 96 (V96A), proline substitution by serine in position 310 (P310S) and serine substitution by cysteine in position 311 (S311C). Their functional role has never been the object of extensive studies, even though there is some evidence that their occurrence correlates with schizophrenia.

METHODS

The HEK293 cell line was transfected with dopamine D1 and D2 receptors (or genetic variants of the D2 receptor), coupled to fluorescent proteins which allowed us to measure the extent of dimerization of these receptors, using a highly advanced biophysical approach (FLIM-FRET). Additionally, Fluoro-4 AM was used to examine changes in the level of calcium release after ligand stimulation of cells expressing different combinations of dopamine receptors.

RESULTS

Using FLIM-FRET experiments we have shown that in HEK 293 expressing dopamine receptors, polymorphic mutations in the D2 receptor play a role in dimmer formation with the dopamine D1 receptor. The association level of dopamine receptors is affected by ligand administration, with variable effects depending on polymorphic variant of the D2 dopamine receptor. We have found that the level of heteromer formation is reflected by calcium ion release after ligand stimulation and have observed variations of this effect dependent on the polymorphic variant and the ligand.

CONCLUSION

The data presented in this paper support the hypothesis on the role of calcium signaling regulated by the D1-D2 heteromer which may be of relevance for schizophrenia etiology.

Neurochemical evidence supporting dopamine D1-D2 receptor heteromers in the striatum of the long-tailed macaque: changes following dopaminergic manipulation

Although it has long been widely accepted that dopamine receptor types D1 and D2 form GPCR heteromers in the striatum, the presence of D1–D2 receptor heteromers has been recently challenged. In an attempt to properly characterize D1–D2 receptor heteromers, here we have used the in situ proximity ligation assay (PLA) in striatal sections comprising the caudate nucleus, the putamen and the core and shell territories of the nucleus accumbens. Experiments were carried out in control macaques as well as in MPTP-treated animals (with and without dyskinesia). Obtained data support the presence of D1–D2 receptor heteromers within all the striatal subdivisions, with the highest abundance in the accumbens shell. Dopamine depletion by MPTP resulted in an increase of D1–D2 density in caudate and putamen which was normalized by levodopa treatment. Two different sizes of heteromers were consistently found, thus suggesting that besides individual heteromers, D1–D2 receptor heteromers are sometimes organized in macromolecular complexes made of a number of D1–D2 heteromers. Furthermore, the PLA technique was combined with different neuronal markers to properly characterize the identities of striatal neurons expressing D1–D2 heteromers. We have found that striatal projection neurons giving rise to either the direct or the indirect basal ganglia pathways expressed D1–D2 heteromers. Interestingly, macromolecular complexes of D1–D2 heteromers were only found within cholinergic interneurons. In summary, here we provide overwhelming proof that D1 and D2 receptors form heteromeric complexes in the macaque striatum, thus representing a very appealing target for a number of brain diseases involving dopamine dysfunction.

Polypharmacology of dopamine receptor ligands

Most neurological diseases have a multifactorial nature and the number of molecular mechanisms discovered as underpinning these diseases is continuously evolving. The old concept of developing selective agents for a single target does not fit with the medical need of most neurological diseases. The development of designed multiple ligands holds great promises and appears as the next step in drug development for the treatment of these multifactorial diseases. Dopamine and its five receptor subtypes are intimately involved in numerous neurological disorders. Dopamine receptor ligands display a high degree of cross interactions with many other targets including G-protein coupled receptors, transporters, enzymes and ion channels. For brain disorders like Parkinsońs disease, schizophrenia and depression the dopaminergic system, being intertwined with many other signaling systems, plays a key role in pathogenesis and therapy. The concept of designed multiple ligands and polypharmacology, which perfectly meets the therapeutic needs for these brain disorders, is herein discussed as a general ligand-based concept while focusing on dopaminergic agents and receptor subtypes in particular.

Age-Dependent D1-D2 Receptor Coactivation in the Lateral Orbitofrontal Cortex Potentiates NMDA Receptors and Facilitates Cognitive Flexibility

The orbitofrontal cortex (OFC) integrates information about the environment to guide decision-making. Glutamatergic synaptic transmission mediated through N-methyl-d-aspartate receptors is required for optimal functioning of the OFC. Additionally, abnormal dopamine signaling in this region has been implicated in impulsive behavior and poor cognitive flexibility. Yet, despite the high prevalence of psychostimulants prescribed for attention deficit/hyperactivity disorder, there is little information on how dopamine modulates synaptic transmission in the juvenile or the adult OFC. Using whole-cell patch-clamp recordings in OFC pyramidal neurons, we demonstrated that while dopamine or selective D2-like receptor (D2R) agonists suppress excitatory synaptic transmission of juvenile or adult lateral OFC neurons; in juvenile lateral OFC neurons, higher concentrations of dopamine can target dopamine receptors that couple to a phospholipase C (PLC) signaling pathway to enhance excitatory synaptic transmission. Interfering with the formation of a putative D1R-D2R interaction blocked the potentiation of excitatory synaptic transmission. Furthermore, targeting the putative D1R-D2R complex with a biased agonist, SKF83959, not only enhanced excitatory synaptic transmission in a PLC-dependent manner, but also improved the performance of juvenile rats on a reversal-learning task. Our results demonstrate that dopamine signaling in the lateral OFC differs between juveniles and adults, through potential crosstalk between dopamine receptor subtypes.

G Protein-Coupled Receptor Multimers: A Question Still Open Despite the Use of Novel Approaches

Heteromerization of G protein-coupled receptors (GPCRs) can significantly change the functional properties of involved receptors. Various biochemical and biophysical methodologies have been developed in the last two decades to identify and functionally evaluate GPCR heteromers in heterologous cells, with recent approaches focusing on GPCR complex stoichiometry and stability. Yet validation of these observations in native tissues is still lagging behind for the majority of GPCR heteromers. Remarkably, recent studies, particularly some involving advanced fluorescence microscopy techniques, are contributing to our current knowledge of aspects that were not well known until now, such as GPCR complex stoichiometry and stability. In parallel, a growing effort is being applied to move the field forward into native systems. This short review will highlight recent developments to study the stoichiometry and stability of GPCR complexes and methodologies to detect native GPCR dimers.

Dopamine D1 and corticotrophin-releasing hormone type-2α receptors assemble into functionally interacting complexes in living cells

BACKGROUND AND PURPOSE

Dopamine and corticotrophin-releasing hormone (CRH; also known as corticotrophin-releasing factor) are key neurotransmitters in the interaction between stress and addiction. Repeated treatment with cocaine potentiates glutamatergic transmission in the rat basolateral amygdala/cortex pathway through a synergistic action of D1 -like dopamine receptors and CRH type-2α receptors (CRF2 α receptors). We hypothesized that this observed synergism could be instrumented by heteromers containing the dopamine D1 receptor and CRF2 α receptor.

EXPERIMENTAL APPROACH

D1 /CRF2 α receptor heteromerization was demonstrated in HEK293T cells using co-immunoprecipitation, BRET and FRET assays, and by using the heteromer mobilization strategy. The ability of D1 receptors to signal through calcium, when singly expressed or co-expressed with CRF2 α receptors, was evaluated by the calcium mobilization assay.

KEY RESULTS

D1 /CRF2 α receptor heteromers were observed in HEK293T cells. When singly expressed, D1 receptors were mostly located at the cell surface whereas CRF2 α receptors accumulated intracellularly. Interestingly, co-expression of both receptors promoted D1 receptor intracellular and CRF2 α receptor cell surface targeting. The heteromerization of D1 /CRF2 α receptors maintained the signalling through cAMP of both receptors but switched D1 receptor signalling properties, as the heteromeric D1 receptor was able to mobilize intracellular calcium upon stimulation with a D1 receptor agonist.

CONCLUSIONS AND IMPLICATIONS

D1 and CRF2 α receptors are capable of heterodimerization in living cells. D1 /CRF2 α receptor heteromerization might account, at least in part, for the complex physiological interactions established between dopamine and CRH in normal and pathological conditions such as addiction, representing a new potential pharmacological target.

Functional characterization of G-protein-coupled receptors: a bioinformatics approach

Complex molecular and cellular mechanisms regulate G protein-coupled receptors (GPCRs). It is suggested that proteins intrinsically disordered regions (IDRs) are to play a role in GPCR's intra and extracellular regions plasticity, due to their potential for post-translational modification and interaction with other proteins. These regions are defined as lacking a stable three-dimensional (3D) structure. They are rich in hydrophilic and charged, amino acids and are capable to assume different conformations which allow them to interact with multiple partners. In this study we analyzed 75 GPCR involved in synaptic transmission using computational tools for sequence-based prediction of IDRs within a protein. We also evaluated putative ligand-binding motifs using receptor sequences. The disorder analysis indicated that neurotransmitter GPCRs have a significant amount of disorder in their N-terminus, third intracellular loop (3IL) and C-terminus. About 31%, 39% and 53% of human GPCR involved in synaptic transmission are disordered in these regions. Thirty-three percent of receptors show at least one predicted PEST motif, this being statistically greater than the estimate for the rest of human GPCRs. About 90% of the receptors had at least one putative site for dimerization in their 3IL or C-terminus. ELM instances sampled in these domains were 14-3-3, SH3, SH2 and PDZ motifs. In conclusion, the increased flexibility observed in GPCRs, added to the enrichment of linear motifs, PEST and heteromerization sites, may be critical for the nervous system's functional plasticity.

Heteromeric dopamine receptor signaling complexes: emerging neurobiology and disease relevance

The pharmacological modification of dopamine transmission has long been employed as a therapeutic tool in the treatment of many mental health disorders. However, as many of the pharmacotherapies today are not without significant side effects, or they alleviate only a particular subset of symptoms, the identification of novel therapeutic targets is imperative. In light of these challenges, the recognition that dopamine receptors can form heteromers has significantly expanded the range of physiologically relevant signaling complexes as well as potential drug targets. Furthermore, as the physiology and disease relevance of these receptor heteromers is further understood, their ability to exhibit pharmacological and functional properties distinct from their constituent receptors, or modulate the function of endogenous homomeric receptor complexes, may allow for the development of alternate therapeutic strategies and provide new avenues for drug design. In this review, we describe the emerging neurobiology of the known dopamine receptor heteromers, their physiological relevance in brain, and discuss the potential role of these receptor complexes in neuropsychiatric disease. We highlight their value as targets for future drug development and discuss innovative research strategies designed to selectively target these dopamine receptor heteromers in the search for novel and clinically efficacious pharmacotherapies.

Prostaglandin E receptor EP1 forms a complex with dopamine D1 receptor and directs D1-induced cAMP production to adenylyl cyclase 7 through mobilizing G(βγ) subunits in human embryonic kidney 293T cells

The mechanism underlying the crosstalk between multiple G protein-coupled receptors remains poorly understood. We previously reported that prostaglandin E receptor EP1 facilitates dopamine D1 receptor signaling in striatal slices and promotes behavioral responses induced by D1 receptor agonists. Here, using human embryonic kidney (HEK)-293T cells expressing D1 and EP1, we have analyzed the mechanism underlying EP1-mediated facilitation of D1 receptor signaling. Fluorescent immunostaining showed that EP1 and D1 receptors are partly colocalized in the cells, and coprecipitation experiments revealed a molecular complex of EP1 and D1 receptors. Treatment of the cells with 17S,17,20-dimethyl-2,5-ethano-6-oxo-PGE₁ (ONO-DI-004), an EP1-selective agonist, enhanced cAMP production induced by D1 agonists (±)-6-chloro-2,3,4,5-tetrahydro-1-phenyl-1H-3-benzazepine hydrobromide (SKF-81297) and 6-chloro-2,3,4,5-tetrahydro-1-(3-methylphenyl)-3-(2-propenyl)-1H-3-benzazepine-7,8-diol hydrobromide (SKF-83822). Although this facilitative effect of EP1 stimulation was not affected by pharmacologic blockade of EP1-induced Ca²⁺ increase, it was blocked by overexpression of G(tα) as a G(βγ) scavenger. Consistently, depletion of adenylyl cyclase (AC) 7, a G(βγ)-sensitive AC isoform, abolished the facilitative action of EP1 on D1-induced cAMP production. Notably, neither G(tα) overexpression nor AC7 depletion affected cAMP production induced by D1 stimulation alone. In contrast, depletion of AC6, another AC isoform, reduced cAMP production induced by D1 stimulation alone, but spared its facilitation by EP1 stimulation. Collectively, these data suggest that, through complex formation with D1, EP1 signaling directs the D1 receptor through G(βγ) to be coupled to AC7, an AC isoform distinct from those used by the D1 receptor alone, in HEK-293T cells.

D1-D2 dopamine receptor synergy promotes calcium signaling via multiple mechanisms

The D(1) dopamine receptor (D(1)R) has been proposed to form a hetero-oligomer with the D(2) dopamine receptor (D(2)R), which in turn results in a complex that couples to phospholipase C-mediated intracellular calcium release. We have sought to elucidate the pharmacology and mechanism of action of this putative signaling pathway. Dopamine dose-response curves assaying intracellular calcium mobilization in cells heterologously expressing the D(1) and D(2) subtypes, either alone or in combination, and using subtype selective ligands revealed that concurrent stimulation is required for coupling. Surprisingly, characterization of a putative D(1)-D(2) heteromer-selective ligand, 6-chloro-2,3,4,5-tetrahydro-3-methyl-1-(3-methylphenyl)-1H-3-benzazepine-7,8-diol (SKF83959), found no stimulation of calcium release, but it did find a broad range of cross-reactivity with other G protein-coupled receptors. In contrast, SKF83959 appeared to be an antagonist of calcium mobilization. Overexpression of G(qα) with the D(1) and D(2) dopamine receptors enhanced the dopamine-stimulated calcium response. However, this was also observed in cells expressing G(qα) with only the D1R. Inactivation of Gi or Gs with pertussis or cholera toxin, respectively, largely, but not entirely, reduced the calcium response in D(1)R and D(2)R cotransfected cells. Moreover, sequestration of G(βγ) subunits through overexpression of G protein receptor kinase 2 mutants either completely or largely eliminated dopamine-stimulated calcium mobilization. Our data suggest that the mechanism of D(1)R/D(2)R-mediated calcium signaling involves more than receptor-mediated G(q) protein activation, may largely involve downstream signaling pathways, and may not be completely heteromer-specific. In addition, SKF83959 may not exhibit selective activation of D(1)-D(2) heteromers, and its significant cross-reactivity to other receptors warrants careful interpretation of its use in vivo.

Rethinking schizophrenia in the context of normal neurodevelopment

The schizophrenia brain is differentiated from the normal brain by subtle changes, with significant overlap in measures between normal and disease states. For the past 25 years, schizophrenia has increasingly been considered a neurodevelopmental disorder. This frame of reference challenges biological researchers to consider how pathological changes identified in adult brain tissue can be accounted for by aberrant developmental processes occurring during fetal, childhood, or adolescent periods. To place schizophrenia neuropathology in a neurodevelopmental context requires solid, scrutinized evidence of changes occurring during normal development of the human brain, particularly in the cortex; however, too often data on normative developmental change are selectively referenced. This paper focuses on the development of the prefrontal cortex and charts major molecular, cellular, and behavioral events on a similar time line. We first consider the time at which human cognitive abilities such as selective attention, working memory, and inhibitory control mature, emphasizing that attainment of full adult potential is a process requiring decades. We review the timing of neurogenesis, neuronal migration, white matter changes (myelination), and synapse development. We consider how molecular changes in neurotransmitter signaling pathways are altered throughout life and how they may be concomitant with cellular and cognitive changes. We end with a consideration of how the response to drugs of abuse changes with age. We conclude that the concepts around the timing of cortical neuronal migration, interneuron maturation, and synaptic regression in humans may need revision and include greater emphasis on the protracted and dynamic changes occurring in adolescence. Updating our current understanding of post-natal neurodevelopment should aid researchers in interpreting gray matter changes and derailed neurodevelopmental processes that could underlie emergence of psychosis.

Reversal of quinpirole inhibition of ventral tegmental area neurons is linked to the phosphatidylinositol system and is induced by agonists linked to G(q)

Putative dopaminergic (pDAergic) ventral tegmental area neurons play an important role in brain pathways related to addiction. Extended exposure of pDAergic neurons to moderate concentrations of dopamine (DA) results in a time-dependent decrease in sensitivity of pDAergic neurons to DA inhibition, a process called dopamine inhibition reversal (DIR). We have shown that DIR is mediated by phospholipase C and conventional protein kinase C through concurrent stimulation of D2 and D1-like receptors. In the present study, we further characterized this phenomenon by using extracellular recordings in brain slices to examine whether DIR is linked to phosphatidylinositol (PI) or adenylate cyclase (AC) second-messenger pathways. A D1-like dopaminergic agonist associated with PI turnover (SKF83959), but not one linked to AC (SKF83822), promoted reversal of inhibition produced by quinpirole, a dopamine D2-selective agonist. Other neurotransmitter receptors linked to PI turnover include serotonin 5-HT(2), α(1)-adrenergic, neurotensin, and group I metabotropic glutamate (mGlu) receptors. Both serotonin and neurotensin produced significant reversal of quinpirole inhibition, but agonists of α(1)-adrenergic and group I mGlu receptors failed to significantly reverse quinpirole inhibition. These results indicate that some agonists that stimulate PI turnover can facilitate desensitization of D2 receptors but that there may be other factors in addition to PI that control that interaction.

Reversal of dopamine inhibition of dopaminergic neurons of the ventral tegmental area is mediated by protein kinase C

Adaptation of putative dopaminergic (pDA) neurons in the ventral tegmental area (VTA) to drugs of abuse may alter information processing related to reward and reinforcement and is an important factor in the development of addiction. We have demonstrated that prolonged increases in the concentration of dopamine (DA) result in a time-dependent decrease in sensitivity of pDA neurons to DA, which we termed DA inhibition reversal (DIR). In this study, we used extracellular recordings to examine factors mediating DIR. A 40 min administration of DA (2.5-10 μM), but not the DA D2 receptor agonist quinpirole (50-200 nM), resulted in inhibition of neuronal firing followed by DIR. In the presence of 100 nM cocaine, inhibition followed by DIR was seen with much lower DA concentrations. Reversal of quinpirole inhibition could be induced by an activator of protein kinase C, but not of protein kinase A. Inhibitors of protein kinase C or phospholipase C blocked the development of DIR. Disruption of intracellular calcium release also prevented DIR. Reduction of extracellular calcium or inhibition of store-operated calcium entry blocked DIR, but the L-type calcium channel blocker nifedipine did not. DIR was age-dependent and not seen in pDA VTA neurons from rat pups younger than 15 days postnatally. Our data indicate that DIR is mediated by protein kinase C, and implicate a conventional protein kinase C. This characterization of DIR gives insight into the regulation of autoinhibition of pDA VTA neurons, and the resulting long-term alteration in information processing related to reward and reinforcement.

G protein-coupled receptors: walking hand-in-hand, talking hand-in-hand?

Most cells express a panel of different G protein-coupled receptors (GPCRs) allowing them to respond to at least a corresponding variety of extracellular ligands. In order to come to an integrative well-balanced functional response these ligand-receptor pairs can often cross-regulate each other. Although most GPCRs are fully capable to induce intracellular signalling upon agonist binding on their own, many GPCRs, if not all, appear to exist and function in homomeric and/or heteromeric assemblies for at least some time. Such heteromeric organization offers unique allosteric control of receptor pharmacology and function between the protomers and might even unmask 'new' features. However, it is important to realize that some functional consequences that are proposed to originate from heteromeric receptor interactions may also be observed due to intracellular crosstalk between signalling pathways of non-associated GPCRs.

CODA-RET reveals functional selectivity as a result of GPCR heteromerization

Here we present a novel method that combines protein complementation with resonance energy transfer to study conformational changes in response to activation of a defined G protein-coupled receptor heteromer, and we apply the approach to the putative dopamine D1-D2 receptor heteromer. Remarkably, the potency of the D2 receptor (D2R) agonist R(–)-Propylnorapomorphine (NPA) to change the Gαi conformation via the D2R protomer in the D1-D2 heteromer was enhanced 10-fold relative to that observed in the D2R homomer. In contrast, the potencies of the D2R agonists dopamine and quinpirole were the same in the homomer and heteromer. Thus, we have uncovered a molecular mechanism for functional selectivity, in which a drug acts differently at a GPCR protomer depending on the identity of the second protomer that participates in forming the signaling unit, opening the door to enhanced pharmacological specificity through targeting differences between homomeric and heteromeric signaling.

Dopamine D1-D2 receptor heteromer signaling pathway in the brain: emerging physiological relevance

Dopamine is an important catecholamine neurotransmitter modulating many physiological functions, and is linked to psychopathology of many diseases such as schizophrenia and drug addiction. Dopamine D1 and D2 receptors are the most abundant dopaminergic receptors in the striatum, and although a clear segregation between the pathways expressing these two receptors has been reported in certain subregions, the presence of D1-D2 receptor heteromers within a unique subset of neurons, forming a novel signaling transducing functional entity has been shown. Recently, significant progress has been made in elucidating the signaling pathways activated by the D1-D2 receptor heteromer and their potential physiological relevance.

Gene-gene interaction analyses between NMDA receptor subunit and dopamine receptor gene variants and clozapine response

AIMS

To investigate the possible association and gene-gene interaction effects of polymorphisms in NMDA receptor subunit (GRIN1, GRIN2A and GRIN2B) and dopamine receptor (DRD1, DRD2 and DRD3) genes with clozapine response.

MATERIALS & METHODS

GRIN1 rs11146020 (G1001C), GRIN2A GT-repeat and GRIN2B rs10193895 (G-200T) polymorphisms were tested for association in a Caucasian (n = 183) and an African-American (n = 49) sample using χ(2) and ANOVA tests. Logistic regression and two-way ANOVA were used to explore gene-gene interaction effects with dopamine receptor gene variants.

RESULTS & CONCLUSION

This study does not support the involvement of the NMDA receptor subunit gene polymorphisms in clozapine response. All tests for an association were negative. Gene-gene interaction analyses however yielded promising leads, including an observed effect between DRD1 rs686 and DRD3 Ser9Gly polymorphisms on clozapine response (p = 0.002).

The dopamine d1-d2 receptor heteromer in striatal medium spiny neurons: evidence for a third distinct neuronal pathway in Basal Ganglia

Dopaminergic signaling within the basal ganglia has classically been thought to occur within two distinct neuronal pathways; the direct striatonigral pathway which contains the dopamine D1 receptor and the neuropeptides dynorphin (DYN) and substance P, and the indirect striatopallidal pathway which expresses the dopamine D2 receptor and enkephalin (ENK). A number of studies have also shown, however, that D1 and D2 receptors can co-exist within the same medium spiny neuron and emerging evidence indicates that these D1/D2-coexpressing neurons, which also express DYN and ENK, may comprise a third neuronal pathway, with representation in both the striatonigral and striatopallidal projections of the basal ganglia. Furthermore, within these coexpressing neurons it has been shown that the dopamine D1 and D2 receptor can form a novel and pharmacologically distinct receptor complex, the dopamine D1–D2 receptor heteromer, with unique signaling properties. This is indicative of a functionally unique role for these neurons in brain. The aim of this review is to discuss the evidence in support of a novel third pathway coexpressing the D1 and D2 receptor, to discuss the potential relevance of this pathway to basal ganglia signaling, and to address its potential value, and that of the dopamine D1–D2 receptor heteromer, in the search for new therapeutic strategies for disorders involving dopamine neurotransmission.

Dopamine D4 receptor oligomerization--contribution to receptor biogenesis

Dopamine D(4) receptors (D(4) Rs) are G protein-coupled receptors that play a role in attention and cognition. In the present study, we investigated the dimerization properties of this receptor. Western blot analysis of the human D(4.2)R, D(4.4)R and D(4.7)R revealed the presence of higher molecular weight immunoreactive bands, which might indicate the formation of receptor dimers and multimers. Homo- and heterodimerization of the receptors was confirmed by co-immunoprecipitation and bioluminescence resonance energy transfer studies. Although dimerization of a large number of G protein-coupled receptors has been described, the functional importance often remains to be elucidated. Folding efficiency is rate-limiting for D(4)R biogenesis and quality control in the endoplasmic reticulum plays an important role for D(4)R maturation. Co-immunoprecipitation and immunofluorescence microscopy studies using wild-type and a nonfunctional D(4.4)R folding mutant show that oligomerization occurs in the endoplasmic reticulum and that this plays a role in the biogenesis and cell surface targeting of the D(4)R. The different polymorphic repeat variants of the D(4)R display differential sensitivity to the chaperone effect. In the present study, we show that this is also reflected by bioluminescence resonance energy transfer saturation assays, suggesting that the polymorphic repeat variants have different relative affinities to form homo- and heterodimers. In summary, we conclude that D(4)Rs form oligomers with different affinities and that dimerization plays a role in receptor biogenesis.

Separation and reformation of cell surface dopamine receptor oligomers visualized in cells

We previously showed that dopamine receptors existed as homo- and heterooligomers, in cells and in brain tissue. We developed a method designed to study the formation and regulation of G protein coupled receptor (GPCR) oligomers in cells, using a GPCR into which a nuclear localization sequence (NLS) had been inserted. Unlike wildtype GPCRs, in the presence of agonist/antagonist ligands the GPCR-NLS is retained at the cell surface, and following ligand removal, the GPCR-NLS translocated from the cell surface. The D(1) dopamine receptor expressed with either D(2)-NLS or D(1-)NLS receptors translocated to the nucleus, indicating hetero- or homo-oligomerization with the NLS-containing receptor. Using these tools, we now demonstrate that D(1)-D(2) dopamine heterooligomers can be disrupted and the component receptors separated by dopamine and selective agonists that occupied one or both binding pockets. Subsequent agonist removal allowed the reformation of the heterooligomer. D(1) receptor homooligomers could also be disrupted by agonist, but at higher concentrations than that required for the disruption of the D(1)-D(2) heteromer. Dopamine D(1) or D(2) receptor antagonists had no effect on the integrity of the homo- or heterooligomer. We have also determined that the D(1)-D(2) heterooligomer contains D(1) homooligomers. These studies indicate that the populations of dopamine receptor oligomers at the cell surface are subject to conformational changes following agonist occupancy and are likely dynamically regulated following agonist activation.

Heteromerization of G protein-coupled receptors: relevance to neurological disorders and neurotherapeutics

Because G protein-coupled receptors (GPCRs) are numerous, widely expressed and involved in major physiological responses, they represent a relevant therapeutic target for drug discovery, particularly regarding pharmacological treatments of neurological disorders. Among the biological phenomena regulating receptor function, GPCR heteromerization is an important emerging area of interest and investigation. There is increasing evidence showing that heteromerization contributes to the pharmacological heterogeneity of GPCRs by modulating receptor ontogeny, activation and recycling. Although in many cases the physiological relevance of receptor heteromerization has not been fully established, the unique pharmacological and functional properties of heteromers are likely to lead to new strategies in clinical medicine. This review describes the main GPCR heteromers and their implications for major neurological disorders such as Parkinson's disease, schizophrenia and addiction. A better understanding of molecular mechanisms underlying drug interactions related to the targeting of receptor heteromers could provide more specific and efficient therapeutic agents for the treatment of brain diseases.

Dopamine receptor homooligomers and heterooligomers in schizophrenia

Over the past two decades the dopamine D2 receptor has been undoubtedly the most widely studied dopamine receptor for the therapeutic treatment of schizophrenia, as the majority of antipsychotics exhibit antagonism at this receptor. However, the cognitive symptoms of the disorder are mostly resistant to the majority of available antipsychotic treatments and, as a result, there is a critical need to develop novel therapies that ameliorate all symptoms. The recognition that dopamine receptors, such as all G protein-coupled receptors (GPCRs), exist as oligomeric complexes has provided new avenues for drug design in the search for novel therapies. Furthermore, that it is now known that dopamine receptors can form heteromers, such as the dopamine D1-D2 receptor heteromer, with pharmacology and function distinct from its constituent receptors, has significantly expanded the range of potential drug targets. The aim of this review is to discuss the therapeutic relevance of these dopamine receptor oligomers to schizophrenia and to address the potential value of dopamine receptor heteromers in the search for new therapeutic strategies.

Hetero-dimerization of serotonin 5-HT(2A) and dopamine D(2) receptors

In the present study, detailed information is presented on the hetero-dimerization of the serotonin 5-HT(2A) receptor and the dopamine D(2) receptor. Biophysical approaches (fluorescence spectroscopy as well as fluorescence lifetime microscopy) were used to determine the degree of fluorescence resonance energy transfer (FRET) between cyan and yellow fluorescent protein labeled receptor variants co-expressed in human embryonic kidney 293 cells (HEK293). Recorded data demonstrate the existence of energy transfer between the wild-type forms of 5-HT(2A)R and D(2)R, pointing toward the formation of hetero-5-HT(2A)R/D(2)R dimers and homo-5-HT(2A)R/5-HT(2A)R dimers. Moreover, the present study investigates the role of specific motifs (one containing adjacent arginine residues (217RRRRKR222) in the third intracellular loop (ic3) of D(2)R, and the other consisting of acidic glutamate residues (454EE455) in the C-tail of (5-HT(2A)R) in the formation of noncovalent complexes between these receptors. Our results suggest that these regions of 5-HT(2A)R and D(2)R may be involved in the interaction between these two proteins. On the other hand, the above-mentioned motifs do not play an important role in the homo-dimerization of these receptors. Furthermore, we estimated the influence of specific receptor ligands on the dimerization processes. Agonists (DOI and quinpirole) and antagonists (ketanserin and butaclamol) cause different effects on FRET efficiency depending on whether homo- or hetero-complexes are present. These data may have therapeutic implications, since (using the immunofluorescence double labeling protocols) the co-localization of these two receptors was demonstrated in the medial prefrontal cortex and pars reticulate of the substantia nigra of the rat brain.

Dopamine D1-D2 receptor Heteromer-mediated calcium release is desensitized by D1 receptor occupancy with or without signal activation: dual functional regulation by G protein-coupled receptor kinase 2

We identified that activation of the G(q)-linked dopamine D1-D2 receptor hetero-oligomer generates a PLC-dependent intracellular calcium signal. Confocal FRET between endogenous dopamine D1 and D2 receptors in striatal neurons confirmed a physical interaction between them. Pretreatment with SKF 83959, which selectively activates the D1-D2 receptor heteromer, or SKF 83822, which only activates the D1 receptor homo-oligomer, led to rapid desensitization of the D1-D2 receptor heteromer-mediated calcium signal in both heterologous cells and striatal neurons. This desensitization response was mediated through selective occupancy of the D1 receptor binding pocket. Although SKF 83822 was unable to activate the D1-D2 receptor heteromer, it still permitted desensitization of the calcium signal. This suggested that occupancy of the D1 receptor binding pocket by SKF 83822 resulted in conformational changes sufficient for desensitization without heteromer activation. Bioluminescence resonance energy transfer and co-immunoprecipitation studies indicated an agonist-induced physical association between the D1-D2 receptor heteromeric complex and GRK2. Increased expression of GRK2 led to a decrease in the calcium signal with or without prior exposure to either SKF 83959 or SKF 83822. GRK2 knockdown by siRNA led to an increase in the signal after pretreatment with either agonist. Expression of the catalytically inactive and RGS (regulator of G protein signaling)-mutated GRK2 constructs each led to a partial recovery of the GRK2-attenuated calcium signal. These results indicated that desensitization of the dopamine D1-D2 receptor heteromer-mediated signal can occur by agonist occupancy even without activation and is dually regulated by both the catalytic and RGS domains of GRK2.

Pharmacology of signaling induced by dopamine D(1)-like receptor activation

Dopamine D(1)-like receptors consisting of D(1) and D(5) subtypes are intimately implicated in dopaminergic regulation of fundamental neurophysiologic processes such as mood, motivation, cognitive function, and motor activity. Upon stimulation, D(1)-like receptors initiate signal transduction cascades that are mediated through adenylyl cyclase or phosphoinositide metabolism, with subsequent enhancement of multiple downstream kinase cascades. The latter actions propagate and further amplify the receptor signals, thus predisposing D(1)-like receptors to multifaceted interactions with various other mediators and receptor systems. The adenylyl cyclase response to dopamine or selective D(1)-like receptor agonists is reliably associated with the D(1) subtype, while emerging evidence indicates that the phosphoinositide responses in native brain tissues may be preferentially mediated through stimulation of the D(5) receptor. Besides classic coupling of each receptor subtype to specific G proteins, additional biophysical models are advanced in attempts to account for differential subcellular distribution, heteromolecular oligomerization, and activity-dependent selectivity of the receptors. It is expected that significant advances in understanding of dopamine neurobiology will emerge from current and anticipated studies directed at uncovering the molecular mechanisms of D(5) coupling to phosphoinositide signaling, the structural features that might enhance pharmacological selectivity for D(5) versus D(1) subtypes, the mechanism by which dopamine may modulate phosphoinositide synthesis, the contributions of the various responsive signal mediators to D(1) or D(5) interactions with D(2)-like receptors, and the spectrum of dopaminergic functions that may be attributed to each receptor subtype and signaling pathway.

Restoration of amphetamine-induced locomotor sensitization in dopamine D1 receptor-deficient mice

RATIONALE AND OBJECTIVES

Amphetamine-induced sensitization is thought to involve dopamine D(1) receptors. Using mice lacking dopamine D(1) receptors (D (1) (-/-) ), we found that they exhibited blunted sensitization to low doses of amphetamine, while others using different treatment and testing regimens reported inconsistent results. We investigated whether experimental variables, alteration in gene expression or cholinergic input played a role in amphetamine-induced responses.

METHODS

D (1) (-/-) and wild-type (D (1) (+/+) ) mice pretreated with amphetamine (1 mg/kg, 3-7 days) or various doses of nicotine (chronically but intermittently) were challenged with amphetamine (0.7 and/or 1 mg/kg) after short and long abstinence periods. Expression of brain-derived neurotrophic factor (BDNF) and phosphorylated c-AMP response element binding protein (p-CREB) genes were measured under basal conditions and after acute or repeated amphetamine treatments.

RESULTS

D (1) (-/-) mice failed to exhibit amphetamine-induced sensitization following short-term treatments and long abstinence periods, but expressed sensitization following prolonged amphetamine treatment or a shorter abstinence period. Basal expression of p-CREB (but not BDNF) was higher in D (1) (-/-) than D (1) (+/+) mice and was reduced after amphetamine treatment. Prolonged nicotine pretreatment augmented locomotor responses to amphetamine in both genotypes and restored sensitization in D (1) (-/-) mice.

CONCLUSIONS

D(1) receptors were necessary for induction, but may not be necessary for expression of amphetamine-induced sensitization at low doses. The manifestation of amphetamine sensitization depended on the duration of treatment and length of the withdrawal period. Cholinergic-nicotinic stimulation restored amphetamine-induced sensitization in D (1) (-/-) mice. Enhanced basal expression of p-CREB in D (1) (-/-) mice may represent an adaptive mechanism related to lack of D(1) receptors.

Calcium signaling cascade links dopamine D1-D2 receptor heteromer to striatal BDNF production and neuronal growth

Although the perturbation of either the dopaminergic system or brain-derived neurotrophic factor (BDNF) levels has been linked to important neurological and neuropsychiatric disorders, there is no known signaling pathway linking these two major players. We found that the exclusive stimulation of the dopamine D1-D2 receptor heteromer, which we identified in striatal neurons and adult rat brain by using confocal FRET, led to the activation of a signaling cascade that links dopamine signaling to BDNF production and neuronal growth through a cascade of four steps: (i) mobilization of intracellular calcium through Gq, phospholipase C, and inositol trisphosphate, (ii) rapid activation of cytosolic and nuclear calcium/calmodulin-dependent kinase IIalpha, (iii) increased BDNF expression, and (iv) accelerated morphological maturation and differentiation of striatal neurons, marked by increased microtubule-associated protein 2 production. These effects, although robust in striatal neurons from D5(-/-) mice, were absent in neurons from D1(-/-) mice. We also demonstrated that this signaling cascade was activated in adult rat brain, although with regional specificity, being largely limited to the nucleus accumbens. This dopaminergic pathway regulating neuronal growth and maturation through BDNF may have considerable significance in disorders such as drug addiction, schizophrenia, and depression.

Heteromerization of dopamine D2 receptors with dopamine D1 or D5 receptors generates intracellular calcium signaling by different mechanisms

The repertoire of signal transduction pathways activated by dopamine in brain includes the increase of intracellular calcium. However the mechanism(s) by which dopamine activated this important second messenger system was/were unknown. Although we showed that activation of the D5 dopamine receptor increased calcium concentrations, the restricted anatomic distribution of this receptor made this unlikely to be the major mechanism in brain. We have identified novel heteromeric dopamine receptor complexes that are linked to calcium signaling. The calcium pathway activated through the D1-D2 receptor heteromer involved coupling to Gq, through phospholipase C and IP(3) receptors to result in a rise in intracellular calcium. The calcium rise activated through the D2-D5 receptor heteromer involved a small rise in intracellular calcium through the Gq pathway that triggered a store-operated channel mediated influx of extracellular calcium. These novel receptor heteromeric complexes, for the first time, establish the link between dopamine action and rapid calcium signaling.

Differential distributions and trafficking properties of dopamine D1 and D5 receptors in nerve cells

OBJECTIVE

To explore the possible differential trafficking properties of the dopamine D1-like receptor subtypes, D1 receptor and D5 receptor.

METHODS

To visualize distributions of dopamine D1-like receptor subtypes at subcellular level, the yellow and cyan variants of green fluorescent protein (GFP) were used to tag D1 and D5 receptors. After transfection with the tagged dopamine receptors, the neuroblastoma cells NG108-15 were treated with D1 agonist SKF38393 or acetylcholine (ACh). Then we observed the subcellular distributions of the tagged receptors under the confocal microscopy and tried to determine trafficking properties by comparing their distribution patterns before and after the drug treatment.

RESULTS

In resting conditions, D1 receptors located in the plasma membrane of NG108-15 cells, while D5 receptors located in both plasma membrane and cytosol. With the pre-treatment of SKF38393, the subcellular distribution of D1 receptors was changed. The yellow particle-like fluorescence of tagged D1 receptors appeared in the cytosol, indicating that D1 receptors were internalized into cytosol from the cell surface. Same situation also occurred in ACh pre-treatment. In contrast, the subcellular distribution of D5 receptors was not changed after SKF38393 or ACh treatment, indicating that D5R was not translocated to cell surface. Interestingly, when D1 and D5 receptors were co-expressed in the same cell, both kept their distinct subcellular distribution patterns and the trafficking properties.

CONCLUSION

Our present study reveals that in NG108-15 nerve cells, dopamine D1 and D5 receptors exhibit differential subcellular distribution patterns, and only D1 receptor has a marked trafficking response to the drug stimulation. We further discuss the potential role of the differential trafficking properties of D1-like receptors in complex modulation of DA signaling.

G protein-coupled receptor hetero-dimerization: contribution to pharmacology and function

The concept that G protein-coupled receptors (GPCRs) can form hetero-dimers or hetero-oligomers continues to gain experimental support. However, with the exception of the GABA(B) receptor and the sweet and umami taste receptors few reported examples meet all of the criteria suggested in a recent International Union of Basic and Clinical Pharmacology sponsored review (Pin et al., 2007) that should be required to define distinct and physiologically relevant receptor species. Despite this, there are many examples in which pairs of co-expressed GPCRs reciprocally modulate their function, trafficking and/or ligand pharmacology. Such data are at least consistent with physical interactions between the receptor pairs. In recent times, it has been suggested that specific GPCR hetero-dimer or hetero-oligomer pairs may represent key molecular targets of certain clinically effective, small molecule drugs and there is growing interest in efforts to identify ligands that may modulate hetero-dimer function selectively. The current review summarizes key recent developments in these topics.

Studies on the role of the receptor protein motifs possibly involved in electrostatic interactions on the dopamine D1 and D2 receptor oligomerization

We investigated the influence of an epitope from the third intracellular loop (ic3) of the dopamine D(2) receptor, which contains adjacent arginine residues (217RRRRKR222), and an acidic epitope from the C-terminus of the dopamine D(1) receptor (404EE405) on the receptors' localization and their interaction. We studied receptor dimer formation using fluorescence resonance energy transfer. Receptor proteins were tagged with fluorescence proteins and expressed in HEK293 cells. The degree of D(1)-D(2) receptor heterodimerization strongly depended on the number of Arg residues replaced by Ala in the ic3 of D(2)R, which may suggest that the indicated region of ic3 in D(2)R might be involved in interactions between two dopamine receptors. In addition, the subcellular localization of these receptors in cells expressing both receptors D(1)-cyan fluorescent protein, D(2)-yellow fluorescent protein, and various mutants was examined by confocal microscopy. Genetic manipulations of the Arg-rich epitope induced alterations in the localization of the resulting receptor proteins, leading to the conclusion that this epitope is responsible for the cellular localization of the receptor. The lack of energy transfer between the genetic variants of yellow fluorescent protein-tagged D(2)R and cyan fluorescent protein-tagged D(1)R may result from differing localization of these proteins in the cell rather than from the possible role of the D(2)R basic domain in the mechanism of D(1)-D(2) receptor heterodimerization. However, we find that the acidic epitope from the C-terminus of the dopamine D(1) receptor is engaged in the heterodimerization process.

Dopamine enhances fast excitatory synaptic transmission in the extended amygdala by a CRF-R1-dependent process

A common feature of drugs of abuse is their ability to increase extracellular dopamine levels in key brain circuits. The actions of dopamine within these circuits are thought to be important in reward and addiction-related behaviors. Current theories of addiction also posit a central role for corticotrophin-releasing factor (CRF) and an interaction between CRF and monoaminergic signaling. One region where drugs of abuse promote robust rises in extracellular dopamine levels is the bed nucleus of the stria terminalis (BNST), a CRF-rich component of the extended amygdala. We find that dopamine rapidly enhances glutamatergic transmission in the BNST through activation of a combination of D(1)- and D(2)-like receptors. This enhancement is activity-dependent and requires the downstream action of CRF receptor 1 (CRF-R1), suggesting that dopamine induces CRF release through a local network mechanism. Furthermore, we found that both in vivo and ex vivo cocaine induced a dopamine receptor and CRF-R1-dependent enhancement of a form of NMDA receptor-dependent short-term potentiation in the BNST. These data highlight a direct and rapid interaction between dopamine and CRF systems that regulates excitatory transmission and plasticity in a brain region key to reinforcement and reinstatement. Because a rise in extracellular dopamine levels in the BNST is a shared consequence of multiple classes of drugs of abuse, this suggests that the CRF-R1-dependent enhancement of glutamatergic transmission in this region may be a common key feature of substances of abuse.