Evidence against dopamine D1/D2 receptor heteromers

Emerging modes of regulation of neuromodulatory G protein-coupled receptors

In the nervous system, G protein-coupled receptors (GPCRs) control neuronal excitability, synaptic transmission, synaptic plasticity, and, ultimately, behavior through spatiotemporally precise initiation of a variety of signaling pathways. However, despite their critical importance, there is incomplete understanding of how these receptors are regulated to tune their signaling to specific neurophysiological contexts. A deeper mechanistic picture of neuromodulatory GPCR function is needed to fully decipher their biological roles and effectively harness them for the treatment of neurological and psychiatric disorders. In this review, we highlight recent progress in identifying novel modes of regulation of neuromodulatory GPCRs, including G protein- and receptor-targeting mechanisms, receptor-receptor crosstalk, and unique features that emerge in the context of chemical synapses. These emerging principles of neuromodulatory GPCR tuning raise critical questions to be tackled at the molecular, cellular, synaptic, and neural circuit levels in the future.

Age-related changes of dopamine D1 and D2 receptors expression in parvalbumin-positive cells of the orbitofrontal and prelimbic cortices of mice

Dopamine (DA) plays a pivotal role in reward processing, cognitive functions, and emotional regulation. The prefrontal cortex (PFC) is a critical brain region for these processes. Parvalbumin-positive (PV+) neurons are one of the major classes of inhibitory GABAergic neurons in the cortex, they modulate the activity of neighboring neurons, influencing various brain functions. While DA receptor expression exhibits age-related changes, the age-related changes of these receptors in PV+ neurons, especially in the PFC, remain unclear. To address this, we investigated the expression of DA D1 (D1R) and D2 (D2R) receptors in PV+ neurons within the orbitofrontal (OFC) and prelimbic (PrL) cortices at different postnatal ages (P28, P42, P56, and P365). We found that the expression of D1R and D2R in PV+ neurons showed both age- and region-related changes. PV+ neurons in the OFC expressed a higher abundance of D1 than those in the PrL, and those neurons in the OFC also showed higher co-expression of D1R and D2R than those in the PrL. In the OFC and PrL, D1R in PV+ neurons increased from P28 and reached a plateau at P42, then receded to express at P365. Meanwhile, D2R did not show significant age-related changes between the two regions except at P56. These results showed dopamine receptors in the prefrontal cortex exhibit age- and region-specific changes, which may contribute to the difference of these brain regions in reward-related brain functions.

Adenosine A2A and dopamine D2 receptor interaction controls fatigue resistance

Introduction: Caffeine and the selective A2A receptor antagonist SCH58261 both have ergogenic properties, effectively reducing fatigue and enhancing exercise capacity. This study investigates in male Swiss mice the interaction between adenosine A2A receptors and dopamine D2 receptors controlling central fatigue, with a focus on the striatum where these receptors are most abundant. Methods: We employed DPCPX and SCH58261 to antagonize A1 and A2A receptors, caffeine as a non-competitive antagonist for both receptors, and haloperidol as a D2 receptor antagonist; all compounds were tested upon systemic application and caffeine and SCH58261 were also directly applied in the striatum. Behavioral assessments using the open field, grip strength, and treadmill tests allowed estimating the effect of treatments on fatigue. Results and discussion: The results suggested a complex interplay between the dopamine and adenosine systems. While systemic DPCPX had little effect on motor performance or fatigue, the application of either caffeine or SCH58261 was ergogenic, and these effects were attenuated by haloperidol. The intra-striatal administration of caffeine or SCH58261 was also ergogenic, but these effects were unaffected by haloperidol. These findings confirm a role of striatal A2A receptors in the control of central fatigue but suggest that the D2 receptor-mediated control of the ergogenic effects of caffeine and of A2A receptor antagonists might occur outside the striatum. This prompts the need of additional efforts to unveil the role of different brain regions in the control of fatigue.

Engineering, applications, and future perspectives of GPCR-based genetically encoded fluorescent indicators for neuromodulators

This review explores the evolving landscape of G-protein-coupled receptor (GPCR)-based genetically encoded fluorescent indicators (GEFIs), with a focus on their development, structural components, engineering strategies, and applications. We highlight the unique features of this indicator class, emphasizing the importance of both the sensing domain (GPCR structure and activation mechanism) and the reporting domain (circularly permuted fluorescent protein (cpFP) structure and fluorescence modulation). Further, we discuss indicator engineering approaches, including the selection of suitable cpFPs and expression systems. Additionally, we showcase the diversity and flexibility of their application by presenting a summary of studies where such indicators were used. Along with all the advantages, we also focus on the current limitations as well as common misconceptions that arise when using these indicators. Finally, we discuss future directions in indicator engineering, including strategies for screening with increased throughput, optimization of the ligand-binding properties, structural insights, and spectral diversity.

Direct interrogation of context-dependent GPCR activity with a universal biosensor platform

G protein-coupled receptors (GPCRs) are the largest family of druggable proteins in the human genome, but progress in understanding and targeting them is hindered by the lack of tools to reliably measure their nuanced behavior in physiologically-relevant contexts. Here, we developed a collection of compact ONE vector G-protein Optical (ONE-GO) biosensor constructs as a scalable platform that can be conveniently deployed to measure G-protein activation by virtually any GPCR with high fidelity even when expressed endogenously in primary cells. By characterizing dozens of GPCRs across many cell types like primary cardiovascular cells or neurons, we revealed new insights into the molecular basis for G-protein coupling selectivity of GPCRs, pharmacogenomic profiles of anti-psychotics on naturally-occurring GPCR variants, and G-protein subtype signaling bias by endogenous GPCRs depending on cell type or upon inducing disease-like states. In summary, this open-source platform makes the direct interrogation of context-dependent GPCR activity broadly accessible.

Non-canonical interplay between glutamatergic NMDA and dopamine receptors shapes synaptogenesis

Direct interactions between receptors at the neuronal surface have long been proposed to tune signaling cascades and neuronal communication in health and disease. Yet, the lack of direct investigation methods to measure, in live neurons, the interaction between different membrane receptors at the single molecule level has raised unanswered questions on the biophysical properties and biological roles of such receptor interactome. Using a multidimensional spectral single molecule-localization microscopy (MS-SMLM) approach, we monitored the interaction between two membrane receptors, i.e. glutamatergic NMDA (NMDAR) and G protein-coupled dopamine D1 (D1R) receptors. The transient interaction was randomly observed along the dendritic tree of hippocampal neurons. It was higher early in development, promoting the formation of NMDAR-D1R complexes in an mGluR5- and CK1-dependent manner, favoring NMDAR clusters and synaptogenesis in a dopamine receptor signaling-independent manner. Preventing the interaction in the neonate, and not adult, brain alters in vivo spontaneous neuronal network activity pattern in male mice. Thus, a weak and transient interaction between NMDAR and D1R plays a structural and functional role in the developing brain.

A novel perspective on the role of nucleus accumbens neurons in encoding associative learning

The nucleus accumbens (NAc) has been considered a key brain region for encoding reward/aversion and cue-outcome associations. These processes are encoded by medium spiny neurons that express either dopamine receptor D1 (D1-MSNs) or D2 (D2-MSNs). Despite the well-established role of NAc neurons in encoding reward/aversion, the underlying processing by D1-/D2-MSNs remains largely unknown. Recent electrophysiological, optogenetic and calcium imaging studies provided insight on the complex role of D1- and D2-MSNs in these behaviours and helped to clarify their involvement in associative learning. Here, we critically discuss findings supporting an intricate and complementary role of NAc D1- and D2-MSNs in associative learning, emphasizing the need for additional studies in order to fully understand the role of these neurons in behaviour.

Integrative multi-dimensional characterization of striatal projection neuron heterogeneity in adult brain

Striatal projection neurons (SPNs) are traditionally segregated into two subpopulations expressing dopamine (DA) D1-like or D2-like receptors. However, this dichotomy is challenged by recent evidence. Functional and expression studies raise important questions: do SPNs co-express different DA receptors, and do these differences reflect unique striatal spatial distributions and expression profiles? Using RNAscope in mouse striatum, we report heterogenous SPN subpopulations distributed across dorsal-ventral and rostral-caudal axes. SPN subpopulations co-express multiple DA receptors, including D1 and D2 (D1/2R) and D1 and D3. Our integrative approach using single-nuclei multi-omics analyses provides a simple consensus to describe SPNs across diverse datasets, connecting it to complementary spatial mapping. Combining RNAscope and multi-omics shows D1/2R SPNs further separate into distinct subtypes according to spatial organization and conserved marker genes. Each SPN cell type contributes uniquely to genetic risk for neuropsychiatric diseases. Our results bridge anatomy and transcriptomics to offer new understandings of striatal neuron heterogeneity.

Thrombospondin1 mimics rapidly relieve depression via Shank3 dependent uncoupling between dopamine D1 and D2 receptors

Deficits in astrocyte function contribute to major depressive disorder (MDD) and suicide, but the therapeutic effect of directly reactivating astrocytes for depression remains unclear. Here, specific gains and losses of astrocytic cell functions in the medial prefrontal cortex (mPFC) bidirectionally regulate depression-like symptoms. Remarkably, recombinant human Thrombospondin-1 (rhTSP1), an astrocyte-secreted protein, exerted rapidly antidepressant-like actions through tyrosine hydroxylase (Th)/dopamine (DA)/dopamine D2 receptors (D2Rs) pathways, but not dopamine D1 receptors (D1Rs), which was dependent on SH3 and multiple ankyrin repeat domains 3 (Shank3) in the mPFC. TSP1 in the mPFC might have potential as a target for treating clinical depression.

The histamine H3 receptor modulates dopamine D2 receptor-dependent signaling pathways and mouse behaviors

The histamine H3 receptor (H3R) is highly enriched in the spiny projection neurons (SPNs) of the striatum, in both the D1 receptor (D1R)-expressing and D2 receptor (D2R)-expressing populations. A crossantagonistic interaction between H3R and D1R has been demonstrated in mice, both at the behavioral level and at the biochemical level. Although interactive behavioral effects have been described upon coactivation of H3R and D2R, the molecular mechanisms underlying this interaction are poorly understood. Here, we show that activation of H3R with the selective agonist R-(-)-α-methylhistamine dihydrobromide mitigates D2R agonist-induced locomotor activity and stereotypic behavior. Using biochemical approaches and the proximity ligation assay, we demonstrated the existence of an H3R-D2R complex in the mouse striatum. In addition, we examined consequences of simultaneous H3R-D2R agonism on the phosphorylation levels of several signaling molecules using immunohistochemistry. H3R agonist treatment modulated Akt (serine/threonine PKB)-glycogen synthase kinase 3 beta signaling in response to D2R activation via a β-arrestin 2-dependent mechanism in D2R-SPNs but not in D1R-SPNs. Phosphorylation of mitogen- and stress-activated protein kinase 1 and rpS6 (ribosomal protein S6) was largely unchanged under these conditions. As Akt-glycogen synthase kinase 3 beta signaling has been implicated in several neuropsychiatric disorders, this work may help clarify the role of H3R in modulating D2R function, leading to a better understanding of pathophysiology involving the interaction between histamine and dopamine systems.

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.

Roles of Pancreatic Islet Catecholamine Neurotransmitters in Glycemic Control and in Antipsychotic Drug-Induced Dysglycemia

Catecholamine neurotransmitters dopamine (DA) and norepinephrine (NE) are essential for a myriad of functions throughout the central nervous system, including metabolic regulation. These molecules are also present in the pancreas, and their study may shed light on the effects of peripheral neurotransmission on glycemic control. Though sympathetic innervation to islets provides NE that signals at local α-cell and β-cell adrenergic receptors to modify hormone secretion, α-cells and β-cells also synthesize catecholamines locally. We propose a model where α-cells and β-cells take up catecholamine precursors in response to postprandial availability, preferentially synthesizing DA. The newly synthesized DA signals in an autocrine/paracrine manner to regulate insulin and glucagon secretion and maintain glycemic control. This enables islets to couple local catecholamine signaling to changes in nutritional state. We also contend that the DA receptors expressed by α-cells and β-cells are targeted by antipsychotic drugs (APDs)-some of the most widely prescribed medications today. Blockade of local DA signaling contributes significantly to APD-induced dysglycemia, a major contributor to treatment discontinuation and development of diabetes. Thus, elucidating the peripheral actions of catecholamines will provide new insights into the regulation of metabolic pathways and may lead to novel, more effective strategies to tune metabolism and treat diabetes.

Dopamine, Immunity, and Disease

The neurotransmitter dopamine is a key factor in central nervous system (CNS) function, regulating many processes including reward, movement, and cognition. Dopamine also regulates critical functions in peripheral organs, such as blood pressure, renal activity, and intestinal motility. Beyond these functions, a growing body of evidence indicates that dopamine is an important immunoregulatory factor. Most types of immune cells express dopamine receptors and other dopaminergic proteins, and many immune cells take up, produce, store, and/or release dopamine, suggesting that dopaminergic immunomodulation is important for immune function. Targeting these pathways could be a promising avenue for the treatment of inflammation and disease, but despite increasing research in this area, data on the specific effects of dopamine on many immune cells and disease processes remain inconsistent and poorly understood. Therefore, this review integrates the current knowledge of the role of dopamine in immune cell function and inflammatory signaling across systems. We also discuss the current understanding of dopaminergic regulation of immune signaling in the CNS and peripheral tissues, highlighting the role of dopaminergic immunomodulation in diseases such as Parkinson's disease, several neuropsychiatric conditions, neurologic human immunodeficiency virus, inflammatory bowel disease, rheumatoid arthritis, and others. Careful consideration is given to the influence of experimental design on results, and we note a number of areas in need of further research. Overall, this review integrates our knowledge of dopaminergic immunology at the cellular, tissue, and disease level and prompts the development of therapeutics and strategies targeted toward ameliorating disease through dopaminergic regulation of immunity. SIGNIFICANCE STATEMENT: Canonically, dopamine is recognized as a neurotransmitter involved in the regulation of movement, cognition, and reward. However, dopamine also acts as an immune modulator in the central nervous system and periphery. This review comprehensively assesses the current knowledge of dopaminergic immunomodulation and the role of dopamine in disease pathogenesis at the cellular and tissue level. This will provide broad access to this information across fields, identify areas in need of further investigation, and drive the development of dopaminergic therapeutic strategies.

Consequences of Acute or Chronic Methylphenidate Exposure Using Ex Vivo Neurochemistry and In Vivo Electrophysiology in the Prefrontal Cortex and Striatum of Rats

Methylphenidate (MPH) is among the main drugs prescribed to treat patients with attention-deficit and hyperactivity disease (ADHD). MPH blocks both the norepinephrine and dopamine reuptake transporters (NET and DAT, respectively). Our study was aimed at further understanding the mechanisms by which MPH could modulate neurotransmitter efflux, using ex vivo radiolabelled neurotransmitter assays isolated from rats. Here, we observed significant dopamine and norepinephrine efflux from the prefrontal cortex (PFC) after MPH (100 µM) exposure. Efflux was mediated by both dopamine and norepinephrine terminals. In the striatum, MPH (100 µM) triggered dopamine efflux through both sodium- and vesicular-dependent mechanisms. Chronic MPH exposure (4 mg/kg/day/animal, voluntary oral intake) for 15 days, followed by a 28-day washout period, increased the firing rate of PFC pyramidal neurons, assessed by in vivo extracellular single-cell electrophysiological recordings, without altering the responses to locally applied NMDA, via micro-iontophoresis. Furthermore, chronic MPH treatment resulted in decreased efficiency of extracellular dopamine to modulate NMDA-induced firing activities of medium spiny neurons in the striatum, together with lower MPH-induced (100 µM) dopamine outflow, suggesting desensitization to both dopamine and MPH in striatal regions. These results indicate that MPH can modulate neurotransmitter efflux in brain regions enriched with dopamine and/or norepinephrine terminals. Further, long-lasting alterations of striatal and prefrontal neurotransmission were observed, even after extensive washout periods. Further studies will be needed to understand the clinical implications of these findings.

The Signaling and Pharmacology of the Dopamine D1 Receptor

The dopamine D1 receptor (D1R) is a Gα_s/olf-coupled GPCR that is expressed in the midbrain and forebrain, regulating motor behavior, reward, motivational states, and cognitive processes. Although the D1R was initially identified as a promising drug target almost 40 years ago, the development of clinically useful ligands has until recently been hampered by a lack of suitable candidate molecules. The emergence of new non-catechol D1R agonists, biased agonists, and allosteric modulators has renewed clinical interest in drugs targeting this receptor, specifically for the treatment of motor impairment in Parkinson's Disease, and cognitive impairment in neuropsychiatric disorders. To develop better therapeutics, advances in ligand chemistry must be matched by an expanded understanding of D1R signaling across cell populations in the brain, and in disease states. Depending on the brain region, the D1R couples primarily to either Gα_s or Gα_olf through which it activates a cAMP/PKA-dependent signaling cascade that can regulate neuronal excitability, stimulate gene expression, and facilitate synaptic plasticity. However, like many GPCRs, the D1R can signal through multiple downstream pathways, and specific signaling signatures may differ between cell types or be altered in disease. To guide development of improved D1R ligands, it is important to understand how signaling unfolds in specific target cells, and how this signaling affects circuit function and behavior. In this review, we provide a summary of D1R-directed signaling in various neuronal populations and describe how specific pathways have been linked to physiological and behavioral outcomes. In addition, we address the current state of D1R drug development, including the pharmacology of newly developed non-catecholamine ligands, and discuss the potential utility of D1R-agonists in Parkinson's Disease and cognitive impairment.

Behavioral and Neuroanatomical Consequences of Cell-Type Specific Loss of Dopamine D2 Receptors in the Mouse Cerebral Cortex

Developmental dysregulation of dopamine D2 receptors (D2Rs) alters neuronal migration, differentiation, and behavior and contributes to the psychopathology of neurological and psychiatric disorders. The current study is aimed at identifying how cell-specific loss of D2Rs in the cerebral cortex may impact neurobehavioral and cellular development, in order to better understand the roles of this receptor in cortical circuit formation and brain disorders. We deleted D2R from developing cortical GABAergic interneurons (Nkx2.1-Cre) or from developing telencephalic glutamatergic neurons (Emx1-Cre). Conditional knockouts (cKO) from both lines, Drd2^fl/fl, Nkx2.1-Cre⁺ (referred to as GABA-D2R-cKO mice) or Drd2^fl/fl, Emx1-Cre⁺ (referred to as Glu-D2R-cKO mice), exhibited no differences in simple tests of anxiety-related or depression-related behaviors, or spatial or nonspatial working memory. Both GABA-D2R-cKO and Glu-D2R-cKO mice also had normal basal locomotor activity, but GABA-D2R-cKO mice expressed blunted locomotor responses to the psychotomimetic drug MK-801. GABA-D2R-cKO mice exhibited improved motor coordination on a rotarod whereas Glu-D2R-cKO mice were normal. GABA-D2R-cKO mice also exhibited spatial learning deficits without changes in reversal learning on a Barnes maze. At the cellular level, we observed an increase in PV+ cells in the frontal cortex of GABA-D2R-cKO mice and no noticeable changes in Glu-D2R-cKO mice. These data point toward unique and distinct roles for D2Rs within excitatory and inhibitory neurons in the regulation of behavior and interneuron development, and suggest that location-biased D2R pharmacology may be clinically advantageous to achieve higher efficacy and help avoid unwanted effects.

Disrupting D1-NMDA or D2-NMDA receptor heteromerization prevents cocaine's rewarding effects but preserves natural reward processing

Addictive drugs increase dopamine in the nucleus accumbens (NAc), where it persistently shapes excitatory glutamate transmission and hijacks natural reward processing. Here, we provide evidence, from mice to humans, that an underlying mechanism relies on drug-evoked heteromerization of glutamate N-methyl-d-aspartate receptors (NMDAR) with dopamine receptor 1 (D1R) or 2 (D2R). Using temporally controlled inhibition of D1R-NMDAR heteromerization, we unraveled their selective implication in early phases of cocaine-mediated synaptic, morphological, and behavioral responses. In contrast, preventing D2R-NMDAR heteromerization blocked the persistence of these adaptations. Interfering with these heteromers spared natural reward processing. Notably, we established that D2R-NMDAR complexes exist in human samples and showed that, despite a decreased D2R protein expression in the NAc, individuals with psychostimulant use disorder display a higher proportion of D2R forming heteromers with NMDAR. These findings contribute to a better understanding of molecular mechanisms underlying addiction and uncover D2R-NMDAR heteromers as targets with potential therapeutic value.

Mesocorticolimbic Dopamine Pathways Across Adolescence: Diversity in Development

Mesocorticolimbic dopamine circuity undergoes a protracted maturation during adolescent life. Stable adult levels of behavioral functioning in reward, motivational, and cognitive domains are established as these pathways are refined, however, their extended developmental window also leaves them vulnerable to perturbation by environmental factors. In this review, we highlight recent advances in understanding the mechanisms underlying dopamine pathway development in the adolescent brain, and how the environment influences these processes to establish or disrupt neurocircuit diversity. We further integrate these recent studies into the larger historical framework of anatomical and neurochemical changes occurring during adolescence in the mesocorticolimbic dopamine system. While dopamine neuron heterogeneity is increasingly appreciated at molecular, physiological, and anatomical levels, we suggest that a developmental facet may play a key role in establishing vulnerability or resilience to environmental stimuli and experience in distinct dopamine circuits, shifting the balance between healthy brain development and susceptibility to psychiatric disease.

Sex Differences in Dopamine Receptors and Relevance to Neuropsychiatric Disorders

Dopamine is an important neurotransmitter that plays a key role in neuropsychiatric illness. Sex differences in dopaminergic signaling have been acknowledged for decades and have been linked to sex-specific heterogeneity in both dopamine-related behaviours as well as in various neuropsychiatric disorders. However, the overall number of studies that have evaluated sex differences in dopamine signaling, both in health and in these disorders, is low. This review will bring together what is known regarding sex differences in innate dopamine receptor expression and function, as well as highlight the known sex-specific roles of dopamine in addiction, depression, anxiety, schizophrenia, and attention deficit hyperactivity disorder. Due to differences in prognosis, diagnosis, and symptomatology between male and female subjects in disorders that involve dopamine signaling, or in responses that utilize pharmacological interventions that target dopamine receptors, understanding the fundamental sex differences in dopamine receptors is of vital importance for the personalization of therapeutic treatment strategies.

Long-term functional alterations following prenatal GLP-1R activation

Evidence supporting the use of glucagon-like peptide-1 (GLP-1) analogues to pharmacologically treat disorders beyond type 2 diabetes and obesity is increasing. However, little is known about how activation of the GLP-1 receptor (GLP-1R) during pregnancy affects maternal and offspring outcomes. We treated female C57Bl/6 J mice prior to conception and throughout gestation with a long-lasting GLP-1R agonist, Exendin-4. While GLP-1R activation has significant effects on food and drug reward, depression, locomotor activity, and cognition in adults, we found few changes in these domains in exendin-4-exposed offspring. Repeated injections of Exendin-4 had minimal effects on the dams and may have enhanced maternal care. Offspring exposed to the drug weighed significantly more than their control counterparts during the preweaning period and demonstrated alterations in anxiety-like outcomes, which indicate a developmental role for GLP-1R modulation in the stress response that may be sex-specific.

Dopamine D2 receptor overexpression in the nucleus accumbens core induces robust weight loss during scheduled fasting selectively in female mice

Anorexia nervosa (AN) is an eating disorder observed predominantly in women and girls that is characterized by a low body-mass index, hypophagia, and hyperactivity. Activity-based anorexia (ABA), which refers to the weight loss, hypophagia, and hyperactivity exhibited by rodents exposed to both running wheels and scheduled fasting, provides a model for aspects of AN. Increased dopamine D2/D3 receptor binding in the anteroventral striatum has been reported in AN patients. We virally overexpressed D2Rs on nucleus accumbens core (D2R-OE_NAc) neurons that endogenously express D2Rs, and tested mice of both sexes in the open field test, ABA paradigm, and intraperitoneal glucose tolerance test (IGTT). D2R-OE_NAc did not alter baseline body weight, but increased locomotor activity in the open field across both sexes. During constant access to food and running wheels, D2R-OE_NAc mice of both sexes increased food intake and ran more than controls. However, when food was available only 7 h a day, only female D2R-OE_NAc mice rapidly lost 25% of their initial body weight, reduced food intake, and substantially increased wheel running. Surprisingly, female D2R-OE_NAc mice also rapidly lost 25% of their initial body weight during scheduled fasting without wheel access and showed no changes in food intake. In contrast, male D2R-OE_NAc mice maintained body weight during scheduled fasting. D2R-OE_NAc mice of both sexes also showed glucose intolerance in the IGTT. In conclusion, D2R-OE_NAc alters glucose metabolism in both sexes but drives robust weight loss only in females during scheduled fasting, implicating metabolic mechanisms in this sexually dimorphic effect.

Characterization of dopamine D2 receptor coupling to G proteins in postmortem brain of subjects with schizophrenia

Background

Alterations of dopamine D₁ (D1R) and D₂ receptor (D2R) are proposed in schizophrenia but brain neuroimaging and postmortem studies have shown controversial results in relation to D1R and D2R density. Besides, scarce information on the functionality of brain D1R and D2R is available. The present study characterized G-protein activation by D1R and D2R agonists in postmortem human brain. Furthermore, D2R functional status was compared between schizophrenia and control subjects.

Methods

G-protein receptor coupling was assessed in control caudate nucleus and frontal cortex by [³⁵S]GTPγS-binding stimulation induced by increasing concentrations (10^–10–10^–3 M) of dopamine, and the selective dopaminergic agonists SKF38393 (D1R) and NPA (D2R). Concentration–response curves to NPA stimulation of [³⁵S]GTPγS binding were analyzed in antipsychotic-free (n = 10) and antipsychotic-treated (n = 7) schizophrenia subjects and matched controls (n = 17).

Results

In caudate, [³⁵S]GTPγS-binding responses to agonists were compatible with the existence of functional D2R. In contrast, stimulations in cortex showed responses that did not correspond to D1R or D2R. [³⁵S]GTPγS-binding activation by NPA in caudate displayed biphasic curves with similar profile in schizophrenia (EC_50H = 7.94 nM; EC_50L = 7.08 μM) and control (EC_50H = 7.24 nM; EC_50L = 15.14 μM) subjects. The presence or absence of antipsychotic medication did not influence the pharmacological parameters.

Conclusions

Feasibility of functional evaluation of dopamine receptors in postmortem human brain by conventional [³⁵S]GTPγS-binding assays appears to be restricted to signalling through inhibitory G_i/o proteins. These findings provide functional information about brain D2R status in subjects with schizophrenia and do not support the existence of D2R supersensitive in this mental disorder.

Supplementary Information

The online version contains supplementary material available at 10.1007/s43440-021-00305-4.

Heteromerization of dopaminergic receptors in the brain: Pharmacological implications

Dopamine exerts its physiological effects through two subtypes of receptors, i.e. the receptors of the D₁ family (D_1R and D_5R) and the D₂ family (D_2R, D_3R, and D_4R), which differ in their pattern of distribution, affinity, and signaling. The D₁-like subfamily (D_1R and D_5R) are coupled to Gα_s/olf proteins to activate adenylyl cyclase whereas the D₂-like receptors are coupled to Gα_i/o subunits and suppress the activity of adenylyl cyclase. Dopamine receptors are capable of forming homodimers, heterodimers, and higher-order oligomeric complexes, resulting in a change in the individual protomers' recognition, signaling, and pharmacology. Heteromerization has the potential to modify the canonical pharmacological features of individual monomeric units such as ligand affinity, activation, signaling, and cellular trafficking through allosteric interactions, reviving the field and introducing a new pharmacological target. Since heteromers are expressed and formed in a tissue-specific manner, they could provide the framework to design selective and effective drug candidates, such as brain-penetrant heterobivalent drugs and interfering peptides, with limited side effects. Therefore, heteromerization could be a promising area of pharmacology research, as it could contribute to the development of novel pharmacological interventions for dopamine dysregulated brain disorders such as addiction, schizophrenia, cognition, Parkinson's disease, and other motor-related disorders. This review is articulated based on the three criteria established by the International Union of Basic and Clinical Pharmacology for GPCR heterodimers (IUPHAR): evidence of co-localization and physical interactions in native or primary tissue, presence of a new physiological and functional property than the individual protomers, and loss of interaction and functional fingerprints upon heterodimer disruption.

Dysregulation of brain dopamine systems in major depressive disorder

Major depressive disorder (MDD or depression) is a debilitating neuropsychiatric syndrome with genetic, epigenetic, and environmental contributions. Depression is one of the largest contributors to chronic disease burden; it affects more than one in six individuals in the United States. A wide array of cellular and molecular modifications distributed across a variety of neuronal processes and circuits underlie the pathophysiology of depression-no established mechanism can explain all aspects of the disease. MDD suffers from a vast treatment gap worldwide, and large numbers of individuals who require treatment do not receive adequate care. This mini-review focuses on dysregulation of brain dopamine (DA) systems in the pathophysiology of MDD and describing new cellular targets for potential medication development focused on DA-modulated micro-circuits. We also explore how neurodevelopmental factors may modify risk for later emergence of MDD, possibly through dopaminergic substrates in the brain.

The Tail of the Striatum: From Anatomy to Connectivity and Function

The dorsal striatum, the largest subcortical structure of the basal ganglia, is critical in controlling motor, procedural, and reinforcement-based behaviors. Although in mammals the striatum extends widely along the rostro-caudal axis, current knowledge and derived theories about its anatomo-functional organization largely rely on results obtained from studies of its rostral sectors, leading to potentially oversimplified working models of the striatum as a whole. Recent findings indicate that the extreme caudal part of the striatum, also referred to as the tail of striatum (TS), represents an additional functional domain. Here, we provide an overview of past and recent studies revealing that the TS displays a heterogeneous cell-type-specific organization, and a unique input-output connectivity, which poises the TS as an integrator of sensory processing.

Dopamine D2 autoreceptor interactome: Targeting the receptor complex as a strategy for treatment of substance use disorder

Dopamine D2 autoreceptors (D2ARs), located in somatodendritic and axon terminal compartments of dopamine (DA) neurons, function to provide a negative feedback regulatory control on DA neuron firing, DA synthesis, reuptake and release. Dysregulation of D2AR-mediated DA signaling is implicated in vulnerability to substance use disorder (SUD). Due to the extreme low abundance of D2ARs compared to postsynaptic D2 receptors (D2PRs) and the lack of experimental tools to differentiate the signaling of D2ARs from D2PRs, the regulation of D2ARs by drugs of abuse is poorly understood. The recent availability of conditional D2AR knockout mice and newly developed virus-mediated gene delivery approaches have provided means to specifically study the function of D2ARs at the molecular, cellular and behavioral levels. There is a growing revelation of novel mechanisms and new proteins that mediate D2AR activity, suggesting that D2ARs act cooperatively with an array of membrane and intracellular proteins to tightly control DA transmission. This review highlights D2AR-interacting partners including transporters, G-protein-coupled receptors, ion channels, intracellular signaling modulators, and protein kinases. The complexity of the D2AR interaction network illustrates the functional divergence of D2ARs. Pharmacological targeting of multiple D2AR-interacting partners may be more effective to restore disrupted DA homeostasis by drugs of abuse.

Aberrant Dopamine Receptor Signaling Plays Critical Role in the Impairment of Striatal Neurons in Experimental Cerebral Malaria

One-fourth survivors of cerebral malaria (CM) retain long-term cognitive and behavioral deficits. Structural abnormalities in striatum are reported in 80% of children with CM. Dopamine receptors (D1 and D2) are widely expressed in striatal medium spiny neurons (MSNs) that regulate critical physiological functions related to behavior and cognition. Dysregulation of dopamine receptors alters the expression of downstream proteins such as dopamine- and cAMP-regulated phosphoprotein (DARPP), Ca²⁺/calmodulin-dependent protein kinase II alpha (CaMKIIα), and p25/cyclin-dependent kinase 5 (cdk5). However, the role of dopamine receptor signaling dysfunction on the outcome of striatal neuron degeneration is unknown underlying the pathophysiology of CM. Using experimental CM (ECM), the present study attempted to understand the role of aberrant dopamine receptor signaling and its possible relation in causing MSNs morphological impairment. The effect of antimalarial drug artemether (ARM) rescue therapy was also assessed after ECM on the outcome of dopamine receptors downstream signaling. ECM was induced in C57BL/6 mice (male and female) infecting with Plasmodium berghei ANKA (PbA) parasite that reiterates the clinical setting of CM. We demonstrated that ECM caused a significant increase in the expression of D1, D2 receptors, phosphorylated DARPP, p25, cdk5, CaMKIIα, and D1-D2 heteromers. A substantial increase in neuronal damage observed in the dorsolateral striatum region of ECM brains (particularly in MSNs) as revealed by increased Fluoro-Jade C staining, reduced dendritic spine density, and impaired dendritic arborization with varicosities. While the ARM rescue therapy significantly altered the effects of ECM induced dopamine receptor signaling dysfunction and neurodegeneration. Overall, our data suggest that dysregulation of dopamine receptor signaling plays an important role in the degeneration of MSNs, and the ARM rescue therapy might provide better insights to develop effective therapeutic strategies for CM.

Functional and molecular heterogeneity of D2R neurons along dorsal ventral axis in the striatum

Action control is a key brain function determining the survival of animals in their environment. In mammals, neurons expressing dopamine D2 receptors (D2R) in the dorsal striatum (DS) and the nucleus accumbens (Acb) jointly but differentially contribute to the fine regulation of movement. However, their region-specific molecular features are presently unknown. By combining RNAseq of striatal D2R neurons and histological analyses, we identified hundreds of novel region-specific molecular markers, which may serve as tools to target selective subpopulations. As a proof of concept, we characterized the molecular identity of a subcircuit defined by WFS1 neurons and evaluated multiple behavioral tasks after its temporally-controlled deletion of D2R. Consequently, conditional D2R knockout mice displayed a significant reduction in digging behavior and an exacerbated hyperlocomotor response to amphetamine. Thus, targeted molecular analyses reveal an unforeseen heterogeneity in D2R-expressing striatal neuronal populations, underlying specific D2R's functional features in the control of specific motor behaviors.

Dopamine modulates individual differences in avoidance behavior: A pharmacological, immunohistochemical, neurochemical and volumetric investigation

Avoidance behavior is a hallmark in pathological anxiety disorders and results in impairment of daily activities. Individual differences in avoidance responses are critical in determining vulnerability or resistance to anxiety disorders. Dopaminergic activation is implicated in the processing of avoidance responses; however, the mechanisms underlying these responses are unknown. In this sense, we used a preclinical model of avoidance behavior to investigate the possibility of an intrinsic differential dopaminergic pattern between good and poor performers. The specific goal was to assess the participation of dopamine (DA) through pharmacological manipulation, and we further evaluated the effects of systemic injections of the dopaminergic receptor type 1 (D1 antagonist - SCH23390) and dopaminergic receptor type 2 (D2 antagonist - sulpiride) antagonists in the good performers. Additionally, we evaluated the effects of intra-amygdala microinjection of a D1 antagonist (SCH23390) and a D2 antagonist (sulpiride) in good performers as well as intra-amygdala microinjection of a D1 agonist (SKF38393) and D2 agonist (quinpirole) in poor performers. Furthermore, we quantified the contents of dopamine and metabolites (3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA)) in the amygdala, evaluated the basal levels of tyrosine hydroxylase expression (catecholamine synthesis enzyme) and measured the volume of the substantia nigra, ventral tegmental area and locus coeruleus. Our results showed that it could be possible to convert animals from good to poor performers, and vice versa, by intra-amygdala (basolateral and central nucleus) injections of D1 receptor antagonists in good performers or D2 receptor agonists in poor performers. Additionally, the good performers had lower levels of DOPAC and HVA in the amygdala, an increase in the total volume of the amygdala (AMG), substantia nigra (SN), ventral tegmental area (VTA) and locus coeruleus (LC), and an increase in the number of tyrosine hydroxylase-positive cells in SN, VTA and LC, which positively correlates with the avoidance behavior. Taken together, our data show evidence for a dopaminergic signature of avoidance performers, emphasizing the role of distinct dopaminergic receptors in individual differences in avoidance behavior based on pharmacological, immunohistochemical, neurochemical and volumetric analyses. Our findings provide a better understanding of the role of the dopaminergic system in the execution of avoidance behavior.

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The role of dopamine in individual differences in avoidance behavior.

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Dopamine modulates avoidance behavior.

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Dopaminergic evidence of individual difference in avoidance behavior.

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Good and poor avoiders distinction based on dopaminergic signature.

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Dopaminergic signature of avoidance performers: poor versus good avoiders.

Where Is Dopamine and how do Immune Cells See it?: Dopamine-Mediated Immune Cell Function in Health and Disease

Dopamine is well recognized as a neurotransmitter in the brain, and regulates critical functions in a variety of peripheral systems. Growing research has also shown that dopamine acts as an important regulator of immune function. Many immune cells express dopamine receptors and other dopamine related proteins, enabling them to actively respond to dopamine and suggesting that dopaminergic immunoregulation is an important part of proper immune function. A detailed understanding of the physiological concentrations of dopamine in specific regions of the human body, particularly in peripheral systems, is critical to the development of hypotheses and experiments examining the effects of physiologically relevant dopamine concentrations on immune cells. Unfortunately, the dopamine concentrations to which these immune cells would be exposed in different anatomical regions are not clear. To address this issue, this comprehensive review details the current information regarding concentrations of dopamine found in both the central nervous system and in many regions of the periphery. In addition, we discuss the immune cells present in each region, and how these could interact with dopamine in each compartment described. Finally, the review briefly addresses how changes in these dopamine concentrations could influence immune cell dysfunction in several disease states including Parkinson's disease, multiple sclerosis, rheumatoid arthritis, inflammatory bowel disease, as well as the collection of pathologies, cognitive and motor symptoms associated with HIV infection in the central nervous system, known as NeuroHIV. These data will improve our understanding of the interactions between the dopaminergic and immune systems during both homeostatic function and in disease, clarify the effects of existing dopaminergic drugs and promote the creation of new therapeutic strategies based on manipulating immune function through dopaminergic signaling. Graphical Abstract.

Identification of Synthetic Activators of Cancer Cell Migration by Hybrid Deep Learning

Deep convolutional neural networks (CNNs) are a method of choice for image recognition. Herein a hybrid CNN approach is presented for molecular pattern recognition in drug discovery. Using self-organizing map images of molecular pharmacophores as input, CNN models were trained to identify chemokine receptor CXCR4 modulators with high accuracy. This machine learning classifier identified first-in-class synthetic CXCR4 full agonists. The receptor-activating effects were confirmed by intracellular cAMP response and in a phenotypic spheroid invasion assay of medulloblastoma cell invasion. Additional macromolecular targets of the small molecules were predicted in silico and tested in vitro, revealing modulatory effects on dopamine receptors and CCR1. These results positively advocate the applicability of molecular image recognition by CNNs to ligand-based virtual compound screening, and demonstrate the complementarity of machine intelligence and human expert knowledge.

Strategies for the Treatment of Parkinson's Disease: Beyond Dopamine

Parkinson’s disease (PD) is the second-leading cause of dementia and is characterized by a progressive loss of dopaminergic neurons in the substantia nigra alongside the presence of intraneuronal α-synuclein-positive inclusions. Therapies to date have been directed to the restoration of the dopaminergic system, and the prevention of dopaminergic neuronal cell death in the midbrain. This review discusses the physiological mechanisms involved in PD as well as new and prospective therapies for the disease. The current data suggest that prevention or early treatment of PD may be the most effective therapeutic strategy. New advances in the understanding of the underlying mechanisms of PD predict the development of more personalized and integral therapies in the years to come. Thus, the development of more reliable biomarkers at asymptomatic stages of the disease, and the use of genetic profiling of patients will surely permit a more effective treatment of PD.

Detecting G protein-coupled receptor complexes in postmortem human brain with proximity ligation assay and a Bayesian classifier

Despite the controversy regarding the existence and physiological relevance of class A G protein-coupled receptor dimerization, there is substantial evidence for functional interactions between the dopamine D2 receptor (D2R) and the adenosine A2A receptor (A2AR). A2AR-D2R complexes have been detected in rodent brains by proximity ligation assay; however, their existence in the human brain has not been demonstrated. In this study, we used Brightfield proximity ligation assay, combined with a systematic sampling and a parameter-free naive Bayesian classifier, and demonstrated proximity between the D2R and the A2AR in the adult human ventral striatum, consistent with their colocalization within complexes and the possible existence of D2R-A2AR heteromers. These methods are applicable to the relative quantification of proximity of two proteins, as well as the expression levels of individual proteins.

Brightfield proximity ligation assay was used to assess the expression of G protein-coupled receptors and their proximity in postmortem adult human brains. A novel automated machine learning method (Bayesian optimized PLA signal sorting) was developed to automatically quantify Brightfield proximity ligation assay data.

Luciferase complementation based-detection of G-protein-coupled receptor activity

Protein complementation assays (PCA) are used as pharmacological tools, enabling a wide array of applications, ranging from studies of protein-protein interactions to second messenger effects. Methods to detect activities of G protein-coupled receptors (GPCRs) have particular relevance for drug screening. Recent development of an engineered luciferase NanoLuc created the possibility of generating a novel PCA, which in turn could open a new avenue for developing drug screening assays. Here we identified a novel split position for NanoLuc and demonstrated its use in a series of fusion constructs to detect the activity of GPCRs. The split construct can be applied to a variety of pharmacological screening systems.

Dopamine-induced functional activation of Gαq mediated by dopamine D1-like receptor in rat cerebral cortical membranes

Although multiple roles of dopamine through D₁-like (D₁ and D₅) and D₂-like (D₂, D₃, and D₄) receptors are initiated primarily through stimulation or inhibition of adenylyl cyclase via G_s/olf or G_i/o, respectively, there have been many reports indicating diverse signaling mechanisms that involve alternative G protein coupling. In this study, dopamine-induced Gα_q activation in rat brain membranes was investigated. Agonist-induced Gα_q activation was assessed by increase in guanosine-5'-O-(3-[³⁵S]thio)triphosphate ([³⁵S]GTPγS) binding to Gα_q determined by [³⁵S]GTPγS binding/immunoprecipitation assay in rat brain membranes. Dopamine-stimulated Gα_q functionality was highest in cortex as compared to hippocampus or striatum. In cerebral cortical membranes, this effect was mimicked by benzazepine derivatives with agonist properties at dopamine D₁-like receptors, that is, SKF83959, SKF83822, R(+)-SKF81297, R(+)-SKF38393, and SKF82958, but not by the compounds with dopamine D₂-like receptor agonist properties except for aripiprazole. Against expectation, stimulatory effects were also induced by SKF83566, R(+)-SCH23390, and pergolide. The pharmacological profiling by using a series of antagonists indicated that dopamine-induced response was mediated through dopamine D₁-like receptor, which was distinct from the receptor involved in 5-HT-induced response (5-HT_2A receptor). Conversely, the responses induced by SKF83566, R(+)-SCH23390, and pergolide were most likely mediated by 5-HT_2A receptor, but not by dopamine D₁-like receptor. Caution should be paid when interpreting the experimental data, especially in behavioral pharmacological research, in which SKF83566 or R(+)-SCH23390 is used as a standard selective dopamine D₁-like receptor antagonist. Also, possible clinical implications of the agonistic effects of pergolide on 5-HT_2A receptor has been mentioned.

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.

Disentangling the diverse roles of dopamine D2 receptors in striatal function and behavior

Dopamine D2 receptors (D2Rs) mediate many of the actions of dopamine in the striatum, ranging from movement to the effortful pursuit of reward. Yet despite significant advances in linking D2Rs to striatal functions with pharmacological and genetic strategies in animals, how dopamine orchestrates its myriad actions on different cell populations -each expressing D2Rs- remains unclear. Furthermore, brain imaging and genetic studies in humans have consistently associated striatal D2R alterations with various neurological and neuropsychiatric disorders, but how and which D2Rs are involved in each case is poorly understood. Therefore, a critical first step is to engage in a refined and systematic investigation of the impact of D2R function on specific striatal cells, circuits, and behaviors. Here, I will review recent efforts, primarily in animal models, aimed at unlocking the complex and heterogeneous roles of D2Rs in striatum.

Intracerebral injection of R-(-)-Apomorphine into the nucleus accumbens decreased carbachol-induced 22-kHz ultrasonic vocalizations in rats

Rats can produce ultrasonic vocalizations (USVs) in a variety of different contexts that signal their emotional state to conspecifics. Under distress, rats can emit 22-kHz USVs, while during positive pro-social interactions rats can emit frequency-modulated (FM) 50-kHz USVs. It has been previously reported that rats with increasing emission of FM 50-kHz USVs in anticipation of rewarding electrical stimulation or positive pro-social interaction decrease the number of emitted 22-kHz USVs. The purpose of the present investigation was to determine, in a pharmacological-behavioural experiment, if the positive emotional arousal of the rat indexed by the number of emitted FM 50-kHz USVs can decrease the magnitude of a subsequent negative emotional state indexed by the emission of 22-kHz USVs. To induce a positive emotional state, an intracerebral injection of a known D₁/D₂ agonist R-(-)-apomorphine (3.0 μg/0.3 μl) into the medial nucleus accumbens shell was used, while a negative emotional state was induced by intracerebral injection of carbachol (1.0 μg/0.3 μl), a known broad-spectrum muscarinic agonist, into the anterior hypothalamic-medial preoptic area. Our results demonstrated that initiation of a positive emotional state was able to significantly decrease the magnitude of subsequently expressed negative emotional state measured by the number of emitted 22-kHz USVs. The results suggest the neurobiological substrates that initiate positive emotional state indirectly antagonize the brain regions that initiate negative emotional states.

Constraints on GPCR Heterodimerization Revealed by the Type-4 Induced-Association BRET Assay

G-protein-coupled receptors (GPCRs) comprise the largest and most pharmacologically important family of cell-surface receptors encoded by the human genome. In many instances, the distinct signaling behavior of certain GPCRs has been explained in terms of the formation of heteromers with, for example, distinct signaling properties and allosteric cross-regulation. Confirmation of this has, however, been limited by the paucity of reliable methods for probing heteromeric GPCR interactions in situ. The most widely used assays for GPCR stoichiometry, based on resonance energy transfer, are unsuited to reporting heteromeric interactions. Here, we describe a targeted bioluminescence resonance energy transfer (BRET) assay, called type-4 BRET, which detects both homo- and heteromeric interactions using induced multimerization of protomers within such complexes, at constant expression. Using type-4 BRET assays, we investigate heterodimerization among known GPCR homodimers: the CXC chemokine receptor 4 and sphingosine-1-phosphate receptors. We observe that CXC chemokine receptor 4 and sphingosine-1-phosphate receptors can form heterodimers with GPCRs from their immediate subfamilies but not with more distantly related receptors. We also show that heterodimerization appears to disrupt homodimeric interactions, suggesting the sharing of interfaces. Broadly, these observations indicate that heterodimerization results from the divergence of homodimeric receptors and will therefore likely be restricted to closely related homodimeric GPCRs.

DARPP-32 in the orchestration of responses to positive natural stimuli

Dopamine- and cAMP-regulated phosphoprotein (M_r 32 kDa, DARPP-32) is an integrator of multiple neuronal signals and plays a crucial role particularly in mediating the dopaminergic component of the systems involved in the evaluation of stimuli and the ensuing elaboration of complex behavioral responses (e.g., responses to reinforcers and stressors). Dopamine neurons can fire tonically or phasically in distinct timescales and in specific brain regions to code different behaviorally relevant information. Dopamine signaling is mediated mainly through the regulation of adenylyl cyclase activity, stimulated by D1-like or inhibited by D2-like receptors, respectively, that modulates cAMP-dependent protein kinase (PKA) function. The activity of DARPP-32 is finely regulated by its phosphorylation at multiple sites. Phosphorylation at the threonine (Thr) 34 residue by PKA converts DARPP-32 into an inhibitor of protein phosphatase 1, while the phosphorylation at the Thr75 residue turns it into an inhibitor of PKA. Thus, DARPP-32 is critically implicated in regulating striatal output in response to the convergent pathways that influence signaling of the cAMP/PKA pathway. This review summarizes some of the landmark and recent studies of DARPP-32-mediated signaling in the attempt to clarify the role played by DARPP-32 in the response to rewarding natural stimuli. Particularly, the review deals with data derived from rodents studies and discusses the involvement of the cAMP/PKA/DARPP-32 pathway in: 1) appetitive food-sustained motivated behaviors, 2) motivated behaviors sustained by social reward, 3) sexual behavior, and 4) responses to environmental enrichment.

Understanding GPCR dimerization

Initially G protein-coupled receptors, GPCRs, were thought to act as monomers, but recently strong evidence has been gathered indicating that they are capable of forming homo- and heterodimers or higher order oligomeric complexes, and that the dimerization phenomenon can modulate the pharmacological response and function of these receptors. In this chapter we point to the great potential of alternative therapeutic approach targeted at GPCR dimers, which is especially important in the field of neuropsychopharmacology. We also included a brief description of methods used for studying the phenomenon of GPCR oligomerization, with particular attention paid to the proximity ligation assay, PLA, the procedure which allows the study of interactions between receptors not only in vitro but also in vivo, with good anatomical resolution, what is especially important in the studies of various GPCRs involved in central neurotransmission.

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.

Essential Control of the Function of the Striatopallidal Neuron by Pre-coupled Complexes of Adenosine A2A-Dopamine D2 Receptor Heterotetramers and Adenylyl Cyclase

The central adenosine system and adenosine receptors play a fundamental role in the modulation of dopaminergic neurotransmission. This is mostly achieved by the strategic co-localization of different adenosine and dopamine receptor subtypes in the two populations of striatal efferent neurons, striatonigral and striatopallidal, that give rise to the direct and indirect striatal efferent pathways, respectively. With optogenetic techniques it has been possible to dissect a differential role of the direct and indirect pathways in mediating "Go" responses upon exposure to reward-related stimuli and "NoGo" responses upon exposure to non-rewarded or aversive-related stimuli, respectively, which depends on their different connecting output structures and their differential expression of dopamine and adenosine receptor subtypes. The striatopallidal neuron selectively expresses dopamine D₂ receptors (D2R) and adenosine A_2A receptors (A2AR), and numerous experiments using multiple genetic and pharmacological in vitro, in situ and in vivo approaches, demonstrate they can form A2AR-D2R heteromers. It was initially assumed that different pharmacological interactions between dopamine and adenosine receptor ligands indicated the existence of different subpopulations of A2AR and D2R in the striatopallidal neuron. However, as elaborated in the present essay, most evidence now indicates that all interactions can be explained with a predominant population of striatal A2AR-D2R heteromers forming complexes with adenylyl cyclase subtype 5 (AC5). The A2AR-D2R heteromer has a tetrameric structure, with two homodimers, which allows not only multiple allosteric interactions between different orthosteric ligands, agonists, and antagonists, but also the canonical Gs-Gi antagonistic interaction at the level of AC5. We present a model of the function of the A2AR-D2R heterotetramer-AC5 complex, which acts as an integrative device of adenosine and dopamine signals that determine the excitability and gene expression of the striatopallidal neurons. The model can explain most behavioral effects of A2AR and D2R ligands, including the psychostimulant effects of caffeine. The model is also discussed in the context of different functional striatal compartments, mainly the dorsal and the ventral striatum. The current accumulated knowledge of the biochemical properties of the A2AR-D2R heterotetramer-AC5 complex offers new therapeutic possibilities for Parkinson's disease, schizophrenia, SUD and other neuropsychiatric disorders with dysfunction of dorsal or ventral striatopallidal neurons.

Activation of Dopamine D1-D2 Receptor Complex Attenuates Cocaine Reward and Reinstatement of Cocaine-Seeking through Inhibition of DARPP-32, ERK, and ΔFosB

A significant subpopulation of neurons in rat nucleus accumbens (NAc) coexpress dopamine D1 and D2 receptors, which can form a D1-D2 receptor complex, but their relevance in addiction is not known. The existence of the D1-D2 heteromer in the striatum of rat and monkey was established using in situ PLA, in situ FRET and co-immunoprecipitation. In rat, D1-D2 receptor heteromer activation led to place aversion and abolished cocaine CPP and locomotor sensitization, cocaine intravenous self-administration and reinstatement of cocaine seeking, as well as inhibited sucrose preference and abolished the motivation to seek palatable food. Selective disruption of this heteromer by a specific interfering peptide induced reward-like effects and enhanced the above cocaine-induced effects, including at a subthreshold dose of cocaine. The D1-D2 heteromer activated Cdk5/Thr75-DARPP-32 and attenuated cocaine-induced pERK and ΔFosB accumulation, together with inhibition of cocaine-enhanced local field potentials in NAc, blocking thus the signaling pathway activated by cocaine: D1R/cAMP/PKA/Thr34-DARPP-32/pERK with ΔFosB accumulation. In conclusion, our results show that the D1-D2 heteromer exerted tonic inhibitory control of basal natural and cocaine reward, and therefore initiates a fundamental physiologic function that limits the liability to develop cocaine addiction.