Functional selectivity of dopamine receptor agonists. I. Selective activation of postsynaptic dopamine D2 receptors linked to adenylate cyclase

Dopamine D1 receptor-mediated β-arrestin signaling: Insight from pharmacology, biology, behavior, and neurophysiology

The awareness of the potential importance of functional selectivity/biased signaling has led to the discovery of biased compounds as both research tools and novel drugs. A major pan-receptor focus has been to identify GPCR-selective ligands that have bias in G protein-dependent vs. β-arrestin related signaling. Although this field has exploded during the past two decades, it is only recently that highly β-arrestin biased ligands for the dopamine D1 receptor were reported. We now summarize important pharmacological, molecular, and cellular studies relevant to D1-mediated β-arrestin-related signaling. It is intriguing that many results emerged from behavioral and physiological studies implying that bias toward or against D1-mediated β-arrestin either can improve or impair functional outcomes. We discuss the importance of understanding the translatability of cell and animal models to have more precise functional targeting to harness the value of this signaling pathway.

Dopamine D2L receptor density influences the recruitment of β-arrestin2 and Gi1 induced by antiparkinsonian drugs

INTRODUCTION

Brain imaging studies have highlighted that the density of dopamine D₂ receptors markedly fluctuates across the stages of Parkinson's disease and in response to pharmacological treatment. Moreover, receptor density constitutes a molecular determinant for the signaling profile of D₂ receptor ligands. We therefore hypothesized that variations in receptor expression could influence D₂ receptor response to antiparkinsonian drugs, most notably with respect to the recruitment bias between G_i1 and β-arrestin2.

METHODS

The recruitment bias of dopamine, pramipexole, ropinirole, and rotigotine was examined using a nanoluciferase-based biosensor for probing the interactions of the D_2L receptor with either G_i1 or β-arrestin2. The characterization of the functional selectivity of these D₂ receptor agonists was performed at two distinct D_2L receptor densities by taking advantage of a cell model carrying an inducible system that enables the overexpression of the D_2L receptor when exposed to doxycycline.

RESULTS

A high receptor density oriented the balanced signaling profile of dopamine towards a preferential recruitment of G_i1. It also moderated the marked G_i1 and β-arrestin2 biases of pramipexole and rotigotine, respectively. At variance, the G_i1 bias of ropinirole appeared as not being influenced by D_2L receptor density.

CONCLUSIONS

Taken together, these observations highlight receptor density as a key driver of the signaling transducer recruitment triggered by antiparkinsonian agents. Moreover, given the putative beneficial properties of β-arrestin2 in promoting locomotion, this study provides molecular insights that position the arrestin-biased ligand rotigotine as a putatively more beneficial D₂ receptor agonist for the treatment of early and late Parkinson's disease.

Functional Selectivity of Dopamine D1 Receptor Signaling: Retrospect and Prospect

Research progress on dopamine D₁ receptors indicates that signaling no longer is limited to G protein-dependent cyclic adenosine monophosphate phosphorylation but also includes G protein-independent β-arrestin-related mitogen-activated protein kinase activation, regulation of ion channels, phospholipase C activation, and possibly more. This review summarizes recent studies revealing the complexity of D₁ signaling and its clinical implications, and suggests functional selectivity as a promising strategy for drug discovery to magnify the merit of D₁ signaling. Functional selectivity/biased receptor signaling has become a major research front because of its potential to improve therapeutics through precise targeting. Retrospective pharmacological review indicated that many D₁ ligands have some degree of mild functional selectivity, and novel compounds with extreme bias at D₁ signaling were reported recently. Behavioral and neurophysiological studies inspired new methods to investigate functional selectivity and gave insight into the biased signaling of several drugs. Results from recent clinical trials also supported D₁ functional selectivity signaling as a promising strategy for discovery and development of better therapeutics.

Balance between Transmitter Availability and Dopamine D2 Receptors in Prefrontal Cortex Influences Memory Functioning

Insufficient or excessive dopaminergic tone impairs cognitive performance. We examine whether the balance between transmitter availability and dopamine (DA) D2 receptors (D2DRs) is important for successful memory performance in a large sample of adults (n = 175, 64-68 years). The Catechol-O-Methyltransferase polymorphism served as genetic proxy for endogenous prefrontal DA availability, and D2DRs in dorsolateral prefrontal cortex (dlPFC) were measured with [11C]raclopride-PET. Individuals for whom D2DR status matched DA availability showed higher levels of episodic and working-memory performance than individuals with insufficient or excessive DA availability relative to the number of receptors. A similar pattern restricted to episodic memory was observed for D2DRs in caudate. Functional magnetic resonance imaging data acquired during working-memory performance confirmed the importance of a balanced DA system for load-dependent brain activity in dlPFC. Our data suggest that the inverted-U-shaped function relating DA signaling to cognition is modulated by a dynamic association between DA availability and receptor status.

Pharmacological selection of cannabinoid receptor effectors: Signalling, allosteric modulation and bias

The type-1 cannabinoid receptor (CB₁) is a promising drug target for a wide range of diseases. However, many existing and novel candidate ligands for CB₁ have shown only limited therapeutic potential. Indeed, no ligands are currently approved for the clinic except formulations of the phytocannabinoids Δ⁹-THC and CBD and a small number of analogues. A key limitation of many promising CB₁ ligands are their on-target adverse effects, notably including psychoactivity (agonists) and depression/suicidal ideation (inverse agonists). Recent drug development attempts have therefore focussed on altering CB₁ signalling profiles in two ways. Firstly, with compounds that enhance or reduce the signalling of endogenous (endo-) cannabinoids, namely allosteric modulators. Secondly, with compounds that probe the capability of selectively targeting specific cellular signalling pathways that may mediate therapeutic effects using biased ligands. This review will summarise the current paradigm of CB₁ signalling in terms of the intracellular transduction pathways acted on by the receptor. The development of compounds that selectively activate CB₁ signalling pathways, whether allosterically or via orthosteric agonist bias, will also be addressed.

Biased agonism in drug discovery: Is there a future for biased 5-HT1A receptor agonists in the treatment of neuropsychiatric diseases?

Serotonin (5-HT) is one of the fundamental neurotransmitters that contribute to the information essential for an organism's normal, physiological function. Serotonin acts centrally and systemically. The 5-HT_1A receptor is the most widespread serotonin receptor, and participates in many brain-related disorders, including anxiety, depression, and cognitive impairments. The 5-HT_1A receptor can activate several different biochemical pathways and signals through both G protein-dependent and G protein-independent pathways. Preclinical experiments indicate that distinct signaling pathways in specific brain regions may be crucial for antidepressant-like, anxiolytic-like, and procognitive responses. Therefore, the development of new ligands that selectively target a particular signaling pathway(s) could open new possibilities for more effective and safer pharmacotherapy. This review discusses the current state of preclinical studies focusing on the concept of functional selectivity (biased agonism) regarding the 5-HT_1A receptor and its role in antidepressant-like, anxiolytic-like, and procognitive regulation. Such work highlights not only the differential effects of targeted autoreceptors, vs. heteroreceptors, but also the importance of targeting specific downstream intracellular signaling processes, thereby enhancing favorable over unfavorable signaling activation.

D1, not D2, dopamine receptor activation dramatically improves MPTP-induced parkinsonism unresponsive to levodopa

Levodopa is the standard-of-care for Parkinson's disease, but continued loss of dopamine neurons with disease progression decreases its bioconversion to dopamine, leading to increased side effects and decreased efficacy. In theory, dopamine agonists could equal levodopa, but no approved oral "dopamine agonist" matches the efficacy of levodopa. There are consistent data in both primate models and in Parkinson's disease showing that selective high intrinsic activity D₁ agonists can equal levodopa in efficacy. There are, however, no data on whether such compounds would be effective in severe disease when levodopa efficacy is low or absent. We compared two approved antiparkinson drugs (levodopa and the D_2/3 agonist bromocriptine) with the experimental selective D₁ full agonist dihydrexidine in two severely parkinsonian MPTP-treated non-human primates. Bromocriptine caused no discernible improvement in parkinsonian signs, whereas levodopa caused a small transient improvement in one of the two subjects. Conversely, the full D₁ agonist dihydrexidine caused a dramatic improvement in both subjects, decreasing parkinsonian signs by ca. 75%. No attenuation of dihydrexidine effects was observed when the two subjects were pretreated with the D₂ antagonist remoxipride. These data provide evidence that selective D₁ agonists may provide profound antiparkinson symptomatic relief even when the degree of nigrostriatal degeneration is so severe that current drugs are ineffective. Until effective disease-modifying therapies are discovered, high intrinsic activity D₁ agonists may offer a major therapeutic advance in improving the quality of life, and potentially the longevity, of late stage Parkinson's patients.

Dopamine receptors in emesis: Molecular mechanisms and potential therapeutic function

Dopamine is a member of the catecholamine family and is associated with multiple physiological functions. Together with its five receptor subtypes, dopamine is closely linked to neurological disorders such as schizophrenia, Parkinson's disease, depression, attention deficit-hyperactivity, and restless leg syndrome. Unfortunately, several dopamine receptor-based agonists used to treat some of these diseases cause nausea and vomiting as impending side-effects. The high degree of cross interactions of dopamine receptor ligands with many other targets including G-protein coupled receptors, transporters, enzymes, and ion-channels, add to the complexity of discovering new targets for the treatment of nausea and vomiting. Using activation status of signaling cascades as mechanism-based biomarkers to foresee drug sensitivity combined with the development of dopamine receptor-based biased agonists may hold great promise and seems as the next step in drug development for the treatment of such multifactorial diseases. In this review, we update the present knowledge on dopamine and dopamine receptors and their potential roles in nausea and vomiting. The pre- and clinical evidence provided in this review supports the implication of both dopamine and dopamine receptor agonists in the incidence of emesis. Besides the conventional dopaminergic antiemetic drugs, potential novel antiemetic targeting emetic protein signaling cascades may offer superior selectivity profile and potency.

Impact of dopamine-related genetic variants on physical activity in old age - a cohort study

OBJECTIVES

The beneficial effects of a physically active lifestyle in aging are well documented. Understanding the factors of importance for physical activity in older adults are therefore essential. Informed by animal and human data linking the dopamine system to motivation and reward processes, we investigated the associations between variations in dopamine genes and objectively measured physical activity and sedentary behaviour. Further, we aimed to verify whether higher age may exacerbate the impact of dopamine genes on physical activity.

METHODS

We analyzed data from 504 older adults, 66-87 years, from the population-based Swedish National study on Aging and Care in Kungsholmen (SNAC-K). Physical activity was measured with activPAL accelerometers and DNA was extracted from blood samples for genotyping. We assessed the effects of three dopamine relevant genetic variations (DRD1, DRD2, and DRD3) on daily time in sedentary behavior, light-intensity physical activity and moderate-to-vigorous physical activity using analyses of covariance, adjusting for sex, age and physical function.

RESULTS

Higher dopamine receptor efficacy was related to moderate-to-vigorous physical activity, but not to light-intensity physical activity or sedentary time. DRD1 explained 2.7% of variance in moderate-to-vigorous physical activity, with more pronounced effect in people aged ≥80 years, about 10% of explained variance.

CONCLUSION

Stronger genetic effects in older adults are in line with the well-established nonlinear effects of dopamine signaling on performance, expected to be exacerbated with aging. Individuals over 80 years, genetically predisposed to lower dopamine receptor efficacy, engaged on average 100 min/week in moderate-to-high physical activity, below the recommended levels beneficial for healthy aging. Our findings highlight that some individuals might need extra support to maintain a physically active lifestyle.

D2 Dopamine Receptor G Protein-Biased Partial Agonists Based on Cariprazine

Functionally selective G protein-coupled receptor ligands are valuable tools for deciphering the roles of downstream signaling pathways that potentially contribute to therapeutic effects versus side effects. Recently, we discovered both Gi/o-biased and β-arrestin2-biased D2 receptor agonists based on the Food and Drug Administration (FDA)-approved drug aripiprazole. In this work, based on another FDA-approved drug, cariprazine, we conducted a structure-functional selectivity relationship study and discovered compound 38 (MS1768) as a potent partial agonist that selectively activates the Gi/o pathway over β-arrestin2. Unlike the dual D2R/D3R partial agonist cariprazine, compound 38 showed selective agonist activity for D2R over D3R. In fact, compound 38 exhibited potent antagonism of dopamine-stimulated β-arrestin2 recruitment. In our docking studies, compound 38 directly interacts with S1935.42 on TM5 but has no interactions with extracellular loop 2, which appears to be in contrast to the binding poses of D2R β-arrestin2-biased ligands. In in vivo studies, compound 38 showed high D2R receptor occupancy in mice and effectively inhibited phencyclidine-induced hyperlocomotion.

[Aripiprazole as dopamine partial agonist model: Basic concepts and clinical impact]

Aripiprazole may be viewed as the prototype of third-generation antipsychotics. This concept is based on the notion of D2 partial agonism, whereas all molecules of first-and second generation were D2 antagonists. After reviewing the basic pharmacological notions linked to such concepts, the mechanisms of action of these molecules are addressed, with a particular focus on functional selectivity and biased ligand. One of the essential pharmacological properties of D2 agonists, and particularity aripiprazole, is their ability to not induce D2 supersensitivity as well as to reverse this supersensitivity when it has been induced by D2 antagonists. In clinical practice, this impacts the choice of treatment in first episode psychosis as well as in refractory schizophrenia. Animal research shows that D2 supersensitivity could contribute to worsen addictive trends. The pharmacokinetic incidence of D2 supersensitivity tends to favour the long-acting forms of partial agonists. The notion of partial agonism could finally lead to design fourth-generation antipsychotics, on the basis on research focusing on functional selectivity.

Characterization of the Neurochemical and Behavioral Effects of Solriamfetol (JZP-110), a Selective Dopamine and Norepinephrine Reuptake Inhibitor

Excessive sleepiness (ES) is associated with several sleep disorders, including narcolepsy and obstructive sleep apnea (OSA). A role for monoaminergic systems in treating these conditions is highlighted by the clinical use of US Food and Drug Administration-approved drugs that act on these systems, such as dextroamphetamine, methylphenidate, modafinil, and armodafinil. Solriamfetol (JZP-110) is a wake-promoting agent that is currently being evaluated to treat ES in patients with narcolepsy or OSA. Clinical and preclinical data suggest that the wake-promoting effects of solriamfetol differ from medications such as modafinil and amphetamine. The goal of the current studies was to characterize the mechanism of action of solriamfetol at monoamine transporters using in vitro and in vivo assays. Results indicate that solriamfetol has dual reuptake inhibition activity at dopamine (DA; IC₅₀ = 2.9 μM) and norepinephrine (NE; IC₅₀ = 4.4 μM) transporters, and this activity is associated in vivo with increased extracellular concentration of DA and NE as measured by microdialysis. Solriamfetol has negligible functional activity at the serotonin transporter (IC₅₀ > 100 μM). Moreover, the wake-promoting effects of solriamfetol are probably owing to activity at DA and NE transporters rather than other neurotransmitter systems, such as histamine or orexin. The dual activity of solriamfetol at DA and NE transporters and the lack of significant monoamine-releasing properties of solriamfetol might explain the differences in the in vivo effects of solriamfetol compared with modafinil or amphetamine. Taken together, these data suggest that solriamfetol may offer an important advancement in the treatment of ES in patients with narcolepsy or OSA.

Gs- versus Golf-dependent functional selectivity mediated by the dopamine D1 receptor

The two highly homologous subtypes of stimulatory G proteins Gαs (Gs) and Gαolf (Golf) display contrasting expression patterns in the brain. Golf is predominant in the striatum, while Gs is predominant in the cortex. Yet, little is known about their functional distinctions. The dopamine D₁ receptor (D1R) couples to Gs/olf and is highly expressed in cortical and striatal areas, making it an important therapeutic target for neuropsychiatric disorders. Using novel drug screening methods that allow analysis of specific G-protein subtype coupling, we found that, relative to dopamine, dihydrexidine and N-propyl-apomorphine behave as full D1R agonists when coupled to Gs, but as partial D1R agonists when coupled to Golf. The Gs/Golf-dependent biased agonism by dihydrexidine was consistently observed at the levels of cellular signaling, neuronal function, and behavior. Our findings of Gs/Golf-dependent functional selectivity in D1R ligands open a new avenue for the treatment of cortex-specific or striatum-specific neuropsychiatric dysfunction.

D1-like dopamine receptors are coupled to Golf proteins in the dorsal striatum but Gs in cortical and other areas. Here, the authors demonstrate selective agonism of Gs-coupled versus Golf-coupled D1 receptors.

Mu-Opioid receptor biased ligands: A safer and painless discovery of analgesics?

Biased activation of G-protein-coupled receptors (GPCRs) is shifting drug discovery efforts and appears promising for the development of safer drugs. The most effective analgesics to treat acute pain are agonists of the μ opioid receptor (μ-OR), a member of the GPCR superfamily. However, the analgesic use of opioid drugs, such as morphine, is hindered by adverse effects. Only a few μ-OR agonists have been reported to selectively activate the G_i over β-arrestin signaling pathway, resulting in lower gastrointestinal dysfunction and respiratory suppression. Here, we discuss the strategies that led to the development of biased μ-OR agonists, and potential areas for improvement, with an emphasis on structural aspects of the ligand-receptor recognition process.

Functional selectivity at G-protein coupled receptors: Advancing cannabinoid receptors as drug targets

The phenomenon of functional selectivity, whereby a ligand preferentially directs the information output of a G-protein coupled receptor (GPCR) along (a) particular effector pathway(s) and away from others, has redefined traditional GPCR signaling paradigms to provide a new approach to structure-based drug design. The two principal cannabinoid receptors (CBRs) 1 and 2 belong to the class-A GPCR subfamily and are considered tenable therapeutic targets for several indications. Yet conventional orthosteric ligands (agonists, antagonists/inverse agonists) for these receptors have had very limited clinical utility due to their propensity to incite on-target adverse events. Chemically distinct classes of cannabinergic ligands exhibit signaling bias at CBRs towards individual subsets of signal transduction pathways. In this review, we discuss the known signaling pathways regulated by CBRs and examine the current evidence for functional selectivity at CBRs in response to endogenous and exogenous cannabinergic ligands as biased agonists. We further discuss the receptor and ligand structural features allowing for selective activation of CBR-dependent functional responses. The design and development of biased ligands may offer a pathway to therapeutic success for novel CBR-targeted drugs.

Effects of dopamine D1 and D2 receptor agonists on environmental enrichment attenuated sucrose cue reactivity in rats

RATIONALE

Acute or chronic environmental enrichment (EE) reduces sucrose cue reactivity in rats. This effect may be mediated by dopamine receptors.

OBJECTIVES

We examined whether dopamine D1 or D2 receptor agonism could reverse the EE effect. We also examined whether any reversal effects would vary with the incubation of sucrose craving.

METHODS

Following 10 days (2 h/day) of sucrose self-administration, rats experienced either 1 or 30 days of forced abstinence and either overnight (acute) or 29 day (chronic) EE. D1 (SKF 81297; 0, 0.3, or 1 mg/kg) or D2 (quinpirole; 0, 0.1, or 0.3 mg/kg) agonist was administered systemically immediately prior to a subsequent 2-h cue reactivity test the next day (n = 9-12 per group).

RESULTS

Dose-dependent effects were limited to the day 1 test. High doses of the agonists increased day 1 acute EE cue reactivity to levels comparable to control animals. On the day 30 test, SKF 81297 increased cue reactivity in acute EE, chronic EE, and control rats. In contrast, quinpirole resulted in similar cue reactivity for control and enriched rats, more from a reduction in responding by controls vs. a recovery of responding by EE-experienced rats.

CONCLUSIONS

Both D1 and D2 receptors may be involved in the acute EE-mediated decrease in cue reactivity observed following 1 day of forced abstinence. In contrast, at 30 days of forced abstinence, D1 receptors may be critical in cue reactivity as SKF 81297 was effective at both restoring responding of enriched animals and potentiating responding of controls.

New Concepts in Dopamine D2 Receptor Biased Signaling and Implications for Schizophrenia Therapy

The dopamine D₂ receptor (D₂R) is a G protein-coupled receptor that is a common target for antipsychotic drugs. Antagonism of D₂R signaling in the striatum is thought to be the primary mode of action of antipsychotic drugs in alleviating psychotic symptoms. However, antipsychotic drugs are not clinically effective at reversing cortical-related symptoms, such as cognitive deficits in schizophrenia. While the exact mechanistic underpinnings of these cognitive deficits are largely unknown, deficits in cortical dopamine function likely play a contributing role. It is now recognized that similar to most G protein-coupled receptors, D₂Rs signal not only through canonical G protein pathways but also through noncanonical beta-arrestin2-dependent pathways. We review the current mechanistic bases for this dual signaling mode of D₂Rs and how these new concepts might be leveraged for therapeutic gain to target both cortical and striatal dysfunction in dopamine neurotransmission and hence have the potential to correct both positive and cognitive symptoms of schizophrenia.

Novel Dopamine Therapeutics for Cognitive Deficits in Schizophrenia

Schizophrenia is characterized by profound cognitive deficits that are not alleviated by currently available medications. Many of these cognitive deficits involve dysfunction of the newly evolved, dorsolateral prefrontal cortex (dlPFC). The brains of patients with schizophrenia show evidence of dlPFC pyramidal cell dendritic atrophy, likely reductions in cortical dopamine, and possible changes in dopamine D1 receptors (D1R). It has been appreciated for decades that optimal levels of dopamine are essential for dlPFC working memory function, with many beneficial actions arising from D1R stimulation. D1R are concentrated on dendritic spines in the primate dlPFC, where their stimulation produces an inverted-U dose response on dlPFC neuronal firing and cognitive performance during working memory tasks. Research in both academia and the pharmaceutical industry has led to the development of selective D1 agonists, e.g., the first full D1 agonist, dihydrexidine, which at low doses improved working memory in monkeys. Dihydrexidine has begun to be tested in patients with schizophrenia or schizotypal disorder. Initial results are encouraging, but studies are limited by the pharmacokinetics of the drug. These data, however, have spurred efforts toward the discovery and development of improved or novel new compounds, including D1 agonists with better pharmacokinetics, functionally selective D1 ligands, and D1R positive allosteric modulators. One or several of these approaches should allow optimization of the beneficial effects of D1R stimulation in the dlPFC that can be translated into clinical practice.

Discovery of G Protein-Biased D2 Dopamine Receptor Partial Agonists

Biased ligands (also known as functionally selective ligands) of G protein-coupled receptors are valuable tools for dissecting the roles of G protein-dependent and independent signaling pathways in health and disease. Biased ligands have also been increasingly pursued by the biomedical community as promising therapeutics with improved efficacy and reduced side effects compared with unbiased ligands. We previously discovered first-in-class β-arrestin-biased agonists of dopamine D2 receptor (D2R) by extensively exploring multiple regions of aripiprazole, a balanced D2R agonist. In our continuing efforts to identify biased agonists of D2R, we unexpectedly discovered a G protein-biased agonist of D2R, compound 1, which is the first G protein-biased D2R agonist from the aripiprazole scaffold. We designed and synthesized novel analogues to explore two regions of 1 and conducted structure-functional selectivity relationship (SFSR) studies. Here we report the discovery of 1, findings from our SFSR studies, and characterization of novel G protein-biased D2R agonists.

Human Neuropeptide S Receptor Is Activated via a Gαq Protein-biased Signaling Cascade by a Human Neuropeptide S Analog Lacking the C-terminal 10 Residues

Human neuropeptide S (NPS) and its cognate receptor regulate important biological functions in the brain and have emerged as a future therapeutic target for treatment of a variety of neurological and psychiatric diseases. The human NPS (hNPS) receptor has been shown to dually couple to Gαs- and Gαq-dependent signaling pathways. The human NPS analog hNPS-(1-10), lacking 10 residues from the C terminus, has been shown to stimulate Ca(2+)mobilization in a manner comparable with full-length hNPSin vitrobut seems to fail to induce biological activityin vivo Here, results derived from a number of cell-based functional assays, including intracellular cAMP-response element (CRE)-driven luciferase activity, Ca(2+)mobilization, and ERK1/2 phosphorylation, show that hNPS-(1-10) preferentially activates Gαq-dependent Ca(2+)mobilization while exhibiting less activity in triggering Gαs-dependent CRE-driven luciferase activity. We further demonstrate that both Gαq- and Gαs-coupled signaling pathways contribute to full-length hNPS-mediated activation of ERK1/2, whereas hNPS-(1-10)-promoted ERK1/2 activation is completely inhibited by the Gαqinhibitor UBO-QIC but not by the PKA inhibitor H89. Moreover, the results of Ala-scanning mutagenesis of hNPS-(1-13) indicated that residues Lys(11)and Lys(12)are structurally crucial for the hNPS receptor to couple to Gαs-dependent signaling. In conclusion, our findings demonstrate that hNPS-(1-10) is a biased agonist favoring Gαq-dependent signaling. It may represent a valuable chemical probe for further investigation of the therapeutic potential of human NPS receptor-directed signalingin vivo.

Biased signalling: the instinctive skill of the cell in the selection of appropriate signalling pathways

GPCRs (G-protein-coupled receptors) are members of a family of proteins which are generally regarded as the largest group of therapeutic drug targets. Ligands of GPCRs do not usually activate all cellular signalling pathways linked to a particular seven-transmembrane receptor in a uniform manner. The fundamental idea behind this concept is that each ligand has its own ability, while interacting with the receptor, to activate different signalling pathways (or a particular set of signalling pathways) and it is this concept which is known as biased signalling. The importance of biased signalling is that it may selectively activate biological responses to favour therapeutically beneficial signalling pathways and to avoid adverse effects. There are two levels of biased signalling. First, bias can arise from the ability of GPCRs to couple to a subset of the available G-protein subtypes: Gαs, Gαq/11, Gαi/o or Gα12/13. These subtypes produce the diverse effects of GPCRs by targeting different effectors. Secondly, biased GPCRs may differentially activate G-proteins or β-arrestins. β-Arrestins are ubiquitously expressed and function to terminate or inhibit classic G-protein signalling and initiate distinct β-arrestin-mediated signalling processes. The interplay of G-protein and β-arrestin signalling largely determines the cellular consequences of the administration of GPCR-targeted drugs. In the present review, we highlight the particular functionalities of biased signalling and discuss its biological effects subsequent to GPCR activation. We consider that biased signalling is potentially allowing a choice between signalling through ‘beneficial’ pathways and the avoidance of ‘harmful’ ones.

Discovery of D1 Dopamine Receptor Positive Allosteric Modulators: Characterization of Pharmacology and Identification of Residues that Regulate Species Selectivity

The present studies represent the first published report of a dopamine D1 positive allosteric modulator (PAM). D1 receptors have been proposed as a therapeutic target for the treatment of cognitive deficits associated with schizophrenia. However, the clinical utility of orthosteric agonist compounds is limited by cardiovascular side effects, poor pharmacokinetics, lack of D1 selectivity, and an inverted dose response. A number of these challenges may be overcome by utilization of a selective D1 PAM. The current studies describe two chemically distinct D1 PAMs: Compound A [1-((rel-1S,3R,6R)-6-(benzo[d][1,3]dioxol-5-yl)bicyclo[4.1.0]heptan-3-yl)-4-(2-bromo-5-chlorobenzyl)piperazine] and Compound B [rel-(9R,10R,12S)-N-(2,6-dichloro-3-methylphenyl)-12-methyl-9,10-dihydro-9,10-ethanoanthracene-12-carboxamide]. Compound A shows pure PAM activity, with an EC50 of 230 nM and agonist activity at the D2 receptor in D2-expressing human embryonic kidney cells. Compound B shows superior potency (EC50 of 43 nM) and selectivity for D1 versus D2 dopamine receptors. Unlike Compound A, Compound B is selective for human and nonhuman primate D1 receptors, but lacks activity at the rodent (rat and mouse) D1 receptors. Using molecular biology techniques, a single amino acid was identified at position 130, which mediates the species selectivity of Compound B. These data represent the first described D1-selective PAMs and define critical amino acids that regulate species selectivity.

The effect of nicotine induced behavioral sensitization on dopamine D1 receptor pharmacology: An in vivo and ex vivo study in the rat

Behavioral sensitization is a phenomenon which can develop following repeated intermittent administration of a range of psychostimulants, and other compounds, and may model neuroplastic changes seen in addictive processes and neuropsychiatric disease. The aim of the present study was to investigate the effect of dopamine D1 receptor (D1R) ligands on nicotine-induced behavioral sensitization and their molecular consequences in the striatum. Wistar rats were chronically treated (5 days) with vehicle or nicotine (0.4 mg/kg; s.c.) and locomotor activity was measured. Following a 5 day withdrawal period, rats were pretreated with vehicle or the D1R antagonist SCH-23390 (0.03 mg/kg; i.p.) and challenged with nicotine. Either 45 min or 24h post-challenge, the striatum was isolated and ex vivo receptor binding and cAMP accumulation (using LC-MS/MS) were assessed. It was shown that chronic nicotine administration induced the development and expression of locomotor sensitization, of which the latter was blocked by SCH-23390. Nicotine-induced sensitization had no effect on forskolin stimulated cAMP accumulation but increased the efficacy of dopamine for the D1R and decreased the potency of D1R agonists. These effects were antagonized by in vivo pre-challenge with SCH-23390. No effect on D1 receptor binding was observed. Moreover, time dependent effects were observed between tissue taken 45 min and 24h post-challenge. The present findings provide a connection between behavioral sensitization and intracellular cAMP accumulation through the D1R. Together these data suggest that changes in D1R signaling in the dorsal striatum may play an important role in the underlying mechanisms of nicotine-induced behavioral sensitization.

Identification of G protein-biased agonists that fail to recruit β-arrestin or promote internalization of the D1 dopamine receptor

The D1 dopamine receptor (D1R) has been implicated in numerous neuropsychiatric disorders, and D1R-selective ligands have potential as therapeutic agents. Previous studies have identified substituted benzazepines as D1R-selective agonists, but the in vivo effects of these compounds have not correlated well with their in vitro pharmacological activities. A series of substituted benzazepines, and structurally dissimilar D1R-selective agonists, were tested for their functional effects on D1R-mediated cAMP accumulation, D1R-promoted β-arrestin recruitment, and D1R internalization using live cell functional assays. All compounds tested elicited an increase in the level of cAMP accumulation, albeit with a range of efficacies. However, when the compounds were evaluated for β-arrestin recruitment, a subset of substituted benzazepines, SKF83959, SKF38393, SKF82957, SKF77434, and SKF75670, failed to activate this pathway, whereas the others showed similar activation efficacies as seen with cAMP accumulation. When tested as antagonists, the five biased compounds all inhibited dopamine-stimulated β-arrestin recruitment. Further, D1R internalization assays revealed a corroborating pattern of activity in that the G protein-biased compounds failed to promote D1R internalization. Interestingly, the biased signaling was unique for the D1R, as the same compounds were agonists of the related D5 dopamine receptor (D5R), but revealed no signaling bias. We have identified a group of substituted benzazepine ligands that are agonists at D1R-mediated G protein signaling, but antagonists of D1R recruitment of β-arrestin, and also devoid of agonist-induced receptor endocytosis. These data may be useful for interpreting the contrasting effects of these compounds in vitro versus in vivo, and also for the understanding of pathway-selective signaling of the D1R.

A novel method for analyzing extremely biased agonism at G protein-coupled receptors

Seven transmembrane receptors were originally named and characterized based on their ability to couple to heterotrimeric G proteins. The assortment of coupling partners for G protein-coupled receptors has subsequently expanded to include other effectors (most notably the βarrestins). This diversity of partners available to the receptor has prompted the pursuit of ligands that selectively activate only a subset of the available partners. A biased or functionally selective ligand may be able to distinguish between different active states of the receptor, and this would result in the preferential activation of one signaling cascade more than another. Although application of the "standard" operational model for analyzing ligand bias is useful and suitable in most cases, there are limitations that arise when the biased agonist fails to induce a significant response in one of the assays being compared. In this article, we describe a quantitative method for measuring ligand bias that is particularly useful for such cases of extreme bias. Using simulations and experimental evidence from several κ opioid receptor agonists, we illustrate a "competitive" model for quantitating the degree and direction of bias. By comparing the results obtained from the competitive model with the standard model, we demonstrate that the competitive model expands the potential for evaluating the bias of very partial agonists. We conclude the competitive model provides a useful mechanism for analyzing the bias of partial agonists that exhibit extreme bias.

Dopamine invigorates reward seeking by promoting cue-evoked excitation in the nucleus accumbens

Approach to reward is a fundamental adaptive behavior, disruption of which is a core symptom of addiction and depression. Nucleus accumbens (NAc) dopamine is required for reward-predictive cues to activate vigorous reward seeking, but the underlying neural mechanism is unknown. Reward-predictive cues elicit both dopamine release in the NAc and excitations and inhibitions in NAc neurons. However, a direct link has not been established between dopamine receptor activation, NAc cue-evoked neuronal activity, and reward-seeking behavior. Here, we use a novel microelectrode array that enables simultaneous recording of neuronal firing and local dopamine receptor antagonist injection. We demonstrate that, in the NAc of rats performing a discriminative stimulus task for sucrose reward, blockade of either D1 or D2 receptors selectively attenuates excitation, but not inhibition, evoked by reward-predictive cues. Furthermore, we establish that this dopamine-dependent signal is necessary for reward-seeking behavior. These results demonstrate a neural mechanism by which NAc dopamine invigorates environmentally cued reward-seeking behavior.

Biased signaling at chemokine receptors

The ability of G protein-coupled receptors (GPCRs) to activate selective signaling pathways according to the conformation stabilized by bound ligands (signaling bias) is a challenging concept in the GPCR field. Signaling bias has been documented for several GPCRs, including chemokine receptors. However, most of these studies examined the global signaling bias between G protein- and arrestin-dependent pathways, leaving unaddressed the potential bias between particular G protein subtypes. Here, we investigated the coupling selectivity of chemokine receptors CCR2, CCR5, and CCR7 in response to various ligands with G protein subtypes by using bioluminescence resonance energy transfer biosensors monitoring directly the activation of G proteins. We also compared data obtained with the G protein biosensors with those obtained with other functional readouts, such as β-arrestin-2 recruitment, cAMP accumulation, and calcium mobilization assays. We showed that the binding of chemokines to CCR2, CCR5, and CCR7 activated the three Gαi subtypes (Gαi1, Gαi2, and Gαi3) and the two Gαo isoforms (Gαoa and Gαob) with potencies that generally correlate to their binding affinities. In addition, we showed that the binding of chemokines to CCR5 and CCR2 also activated Gα12, but not Gα13. For each receptor, we showed that the relative potency of various agonist chemokines was not identical in all assays, supporting the notion that signaling bias exists at chemokine receptors.

Agonist and antagonist effects of aripiprazole on D₂-like receptors controlling rat brain dopamine synthesis depend on the dopaminergic tone

BACKGROUND

The atypical antipsychotic drug aripiprazole binds with high affinity to a number of G protein coupled receptors, including dopamine D₂ receptors, where its degree of efficacy as a partial agonist remains controversial.

METHODS

We examined the properties of aripiprazole at D₂-like autoreceptors by monitoring the changes of dopamine synthesis in adult rat brain striatal minces incubated ex vivo. The effects of the dopaminergic tone on the properties of aripiprazole were assayed by comparing a basal condition (2 mM K(+), low dopaminergic tone) and a stimulated condition (15 mM K(+), where dopamine release mimics a relatively higher dopaminergic tone). We also used 2 reference compounds: quinpirole showed a clear agonistic activity and preclamol (S-(-)-PPP) showed partial agonism under both basal and stimulated conditions.

RESULTS

Aripiprazole under the basal condition acted as an agonist at D₂-like autoreceptors and fully activated them at about 10 nM, inhibiting dopamine synthesis similarly to quinpirole. Higher concentrations of aripiprazole had effects not restricted to D₂-like autoreceptor activation. Under the stimulated (15 mM K(+)) condition, nanomolar concentrations of aripiprazole failed to decrease dopamine synthesis but could totally block the effect of quinpirole.

CONCLUSIONS

Under high dopaminergic tone, aripiprazole acts as a D₂-like autoreceptor antagonist rather than as an agonist. These data show that, ex vivo, alteration of dopaminergic tone by depolarization affects the actions of aripiprazole on D₂-like autoreceptors. Such unusual effects were not seen with the typical partial agonist preclamol and are consistent with the hypothesis that aripiprazole is a functionally selective D₂R ligand.

Effects of the D1 dopamine receptor agonist dihydrexidine (DAR-0100A) on working memory in schizotypal personality disorder

Pharmacological enhancement of prefrontal D1 dopamine receptor function remains a promising therapeutic approach to ameliorate schizophrenia-spectrum working memory deficits, but has yet to be rigorously evaluated clinically. This proof-of-principle study sought to determine whether the active enantiomer of the selective and full D1 receptor agonist dihydrexidine (DAR-0100A) could attenuate working memory impairments in unmedicated patients with schizotypal personality disorder (SPD). We performed a randomized, double-blind, placebo-controlled trial of DAR-0100A (15 mg/150 ml of normal saline administered intravenously over 30 min) in medication-free patients with SPD (n=16) who met the criteria for cognitive impairment (ie, scoring below the 25th percentile on tests of working memory). We employed two measures of verbal working memory that are salient to schizophrenia-spectrum cognitive deficits, and that clinical data implicate as being associated with prefrontal D1 availability: (1) the Paced Auditory Serial Addition Test (PASAT); and (2) the N-back test (ratio of 2-back:0-back scores). Study procedures occurred over four consecutive days, with working memory testing on Days 1 and 4, and DAR-0100A/placebo administration on Days 2-4. Treatment with DAR-0100A was associated with significantly improved PASAT performance relative to placebo, with a very large effect size (Cohen's d=1.14). Performance on the N-back ratio was also significantly improved; however, this effect rested on both a non-significant enhancement and diminution of 2-back and 0-back performance, respectively; therefore interpretation of this finding is more complicated. DAR-0100A was generally well tolerated, with no serious medical or psychiatric adverse events; common side effects were mild to moderate and transient, consisting mainly of sedation, lightheadedness, tachycardia, and hypotension; however, we were able to minimize these effects, without altering the dose, with supportive measures, eg, co-administered normal saline. Although preliminary, these findings lend further clinical support to the potential of D1 receptor agonists to treat schizophrenia-spectrum working memory impairments. These data suggest a need for further studies with larger group sizes, serum DAR-0100A levels, and a more comprehensive neuropsychological battery.

Update on the Mechanism of Action of Aripiprazole: Translational Insights into Antipsychotic Strategies Beyond Dopamine Receptor Antagonism

Dopamine partial agonism and functional selectivity have been innovative strategies in the pharmacological treatment of schizophrenia and mood disorders and have shifted the concept of dopamine modulation beyond the established approach of dopamine D2 receptor (D2R) antagonism. Despite the fact that aripiprazole was introduced in therapy more than 12 years ago, many questions are still unresolved regarding the complexity of the effects of this agent on signal transduction and intracellular pathways, in part linked to its pleiotropic receptor profile. The complexity of the mechanism of action has progressively shifted the conceptualization of this agent from partial agonism to functional selectivity. From the induction of early genes to modulation of scaffolding proteins and activation of transcription factors, aripiprazole has been shown to affect multiple cellular pathways and several cortical and subcortical neurotransmitter circuitries. Growing evidence shows that, beyond the consequences of D2R occupancy, aripiprazole has a unique neurobiology among available antipsychotics. The effect of chronic administration of aripiprazole on D2R affinity state and number has been especially highlighted, with relevant translational implications for long-term treatment of psychosis. The hypothesized effects of aripiprazole on cell-protective mechanisms and neurite growth, as well as the differential effects on intracellular pathways [i.e. extracellular signal-regulated kinase (ERK)] compared with full D2R antagonists, suggest further exploration of these targets by novel and future biased ligand compounds. This review aims to recapitulate the main neurobiological effects of aripiprazole and discuss the potential implications for upcoming improvements in schizophrenia therapy based on dopamine modulation beyond D2R antagonism.

Dopamine D1 receptor signaling: does GαQ-phospholipase C actually play a role?

Despite numerous studies showing therapeutic potential, no central dopamine D1 receptor ligand has ever been approved, because of potential limitations, such as hypotension, seizures, and tolerance. Functional selectivity has been widely recognized as providing a potential mechanism to develop novel therapeutics from existing targets, and a highly biased, functionally selective D1 ligand might overcome some of the past limitations. SKF-83959 [6-chloro-3-methyl-1-(m-tolyl)-2,3,4,5-tetrahydro-1H-benzo[d]azepine-7,8-diol] is reported to be a highly biased D1 ligand, having full agonism at D1-mediated activation of phospholipase C (PLC) signaling (via GαQ) and antagonism at D1-mediated adenylate cyclase signaling (via GαOLF/S). For this reason, numerous studies have used this compound to elucidate the physiologic role of D1-PLC signaling, including a novel molecular mechanism (GαQ-PLC activation via D1-D2 heterodimers). There is, however, contradictory literature that suggests that SKF-83959 is actually a partial agonist at both D1-mediated adenylate cyclase and β-arrestin recruitment. Moreover, the D1-mediated PLC stimulation has also been questioned. This Minireview examines 30 years of relevant literature and proposes that the data strongly favor alternate hypotheses: first, that SKF-83959 is a typical D1 partial agonist; and second, that the reported activation of PLC by SKF-83959 and related benzazepines likely is due to off-target effects, not actions at D1 receptors. If these hypotheses are supported by future studies, it would suggest that caution should be used regarding the role of PLC and downstream pathways in D1 signaling.

Structure-activity relationships of privileged structures lead to the discovery of novel biased ligands at the dopamine D₂ receptor

Biased agonism at GPCRs highlights the potential for the discovery and design of pathway-selective ligands and may confer therapeutic advantages to ligands targeting the dopamine D2 receptor (D2R). We investigated the determinants of efficacy, affinity, and bias for three privileged structures for the D2R, exploring changes to linker length and incorporation of a heterocyclic unit. Profiling the compounds in two signaling assays (cAMP and pERK1/2) allowed us to identify and quantify determinants of biased agonism at the D2R. Substitution on the phenylpiperazine privileged structures (2-methoxy vs 2,3-dichloro) influenced bias when the thienopyridine heterocycle was absent. Upon inclusion of the thienopyridine unit, the substitution pattern (4,6-dimethyl vs 5-chloro-6-methoxy-4-methyl) had a significant effect on bias that overruled the effect of the phenylpiperazine substitution pattern. This latter observation could be reconciled with an extended binding mode for these compounds, whereby the interaction of the heterocycle with a secondary binding pocket may engender bias.

Biased ligands at G-protein-coupled receptors: promise and progress

Drug discovery targeting G protein-coupled receptors (GPCRs) is no longer limited to seeking agonists or antagonists to stimulate or block cellular responses associated with a particular receptor. GPCRs are now known to support a diversity of pharmacological profiles, a concept broadly referred to as functional selectivity. In particular, the concept of ligand bias, whereby a ligand stabilizes subsets of receptor conformations to engender novel pharmacological profiles, has recently gained increasing prominence. This review discusses how biased ligands may deliver safer, better tolerated, and more efficacious drugs, and highlights several biased ligands that are in clinical development. Biased ligands targeting the angiotensin II type 1 receptor and the μ opioid receptor illustrate the translation of the biased ligand concept from basic biology to clinical drug development.

Discovery and characterization of a G protein-biased agonist that inhibits β-arrestin recruitment to the D2 dopamine receptor

A high-throughput screening campaign was conducted to interrogate a 380,000+ small-molecule library for novel D2 dopamine receptor modulators using a calcium mobilization assay. Active agonist compounds from the primary screen were examined for orthogonal D2 dopamine receptor signaling activities including cAMP modulation and β-arrestin recruitment. Although the majority of the subsequently confirmed hits activated all signaling pathways tested, several compounds showed a diminished ability to stimulate β-arrestin recruitment. One such compound (MLS1547; 5-chloro-7-[(4-pyridin-2-ylpiperazin-1-yl)methyl]quinolin-8-ol) is a highly efficacious agonist at D2 receptor-mediated G protein-linked signaling, but does not recruit β-arrestin as demonstrated using two different assays. This compound does, however, antagonize dopamine-stimulated β-arrestin recruitment to the D2 receptor. In an effort to investigate the chemical scaffold of MLS1547 further, we characterized a set of 24 analogs of MLS1547 with respect to their ability to inhibit cAMP accumulation or stimulate β-arrestin recruitment. A number of the analogs were similar to MLS1547 in that they displayed agonist activity for inhibiting cAMP accumulation, but did not stimulate β-arrestin recruitment (i.e., they were highly biased). In contrast, other analogs displayed various degrees of G protein signaling bias. These results provided the basis to use pharmacophore modeling and molecular docking analyses to build a preliminary structure-activity relationship of the functionally selective properties of this series of compounds. In summary, we have identified and characterized a novel G protein-biased agonist of the D2 dopamine receptor and identified structural features that may contribute to its biased signaling properties.

Feeding condition and the relative contribution of different dopamine receptor subtypes to the discriminative stimulus effects of cocaine in rats

RATIONALE

The contribution of dopamine receptor subtypes in mediating the discriminative stimulus effects of cocaine is not fully established. Many drug discrimination studies use food to maintain responding, necessitating food restriction, which can alter drug effects.

OBJECTIVE

This study established stimulus control with cocaine (10 mg/kg) in free-feeding and food-restricted rats responding under a schedule of stimulus shock termination (SST) and in food-restricted rats responding under a schedule of food presentation to examine whether feeding condition or the reinforcer used to maintain responding impacts the effects of cocaine.

METHOD

Dopamine receptor agonists and antagonists were examined for their ability to mimic or attenuate, respectively, the effects of cocaine.

RESULTS

Apomorphine, quinpirole, and lisuride occasioned >90 % responding on the cocaine-associated lever in free-feeding rats responding under a schedule of SST; apomorphine, but not quinpirole or lisuride, occasioned >90 % responding on the cocaine lever in food-restricted rats responding under a schedule of SST. In food-restricted rats responding for food these drugs occasioned little cocaine lever responding and were comparatively more potent in decreasing responding. In free-feeding rats, the effects of cocaine were attenuated by the D2/D3 receptor antagonist raclopride and the D3 receptor-selective antagonist PG01037. In food-restricted rats, raclopride and the D2 receptor-selective antagonist L-741,626 attenuated the effects of cocaine. Raclopride antagonized quinpirole in all groups while PG01037 antagonized quinpirole only in free-feeding rats.

CONCLUSION

These results demonstrate significant differences in the discriminative stimulus of cocaine that are due to feeding conditions and not to the use of different reinforcers across procedures.

Increasing dopamine D2 receptor expression in the adult nucleus accumbens enhances motivation

A decrease in dopamine D2 receptor (D2R) binding in the striatum is one of the most common findings in disorders that involve a dysregulation of motivation, including obesity, addiction and attention deficit hyperactivity disorder. As disruption of D2R signaling in the ventral striatum--including the nucleus accumbens (NAc)--impairs motivation, we sought to determine whether potentiating postsynaptic D2R-dependent signaling in the NAc would improve motivation. In this study, we used a viral vector strategy to overexpress postsynaptic D2Rs in either the NAc or the dorsal striatum. We investigated the effects of D2R overexpression on instrumental learning, willingness to work, use of reward value representations and modulation of motivation by reward associated cues. Overexpression of postsynaptic D2R in the NAc selectively increased motivation without altering consummatory behavior, the representation of the value of the reinforcer, or the capacity to use reward associated cues in flexible ways. In contrast, D2R overexpression in the dorsal striatum did not alter performance on any of the tasks. Thus, consistent with numerous studies showing that reduced D2R signaling impairs motivated behavior, our data show that postsynaptic D2R overexpression in the NAc specifically increases an animal's willingness to expend effort to obtain a goal. Taken together, these results provide insight into the potential impact of future therapeutic strategies that enhance D2R signaling in the NAc.

Imaging addiction: D2 receptors and dopamine signaling in the striatum as biomarkers for impulsivity

Dependence to drugs of abuse is closely associated with impulsivity, or the propensity to choose a lower, but immediate, reward over a delayed, but more valuable outcome. Here, we review clinical and preclinical studies showing that striatal dopamine signaling and D2 receptor levels - which have been shown to be decreased in addiction - directly impact impulsivity, which is itself predictive of drug self-administration. Based on these studies, we propose that the alterations in D2 receptor binding and dopamine release seen in imaging studies of addiction constitute neurobiological markers of impulsivity. Recent studies in animals also show that higher striatal dopamine signaling at the D2 receptor is associated with a greater willingness to expend effort to reach goals, and we propose that this same relationship applies to humans, particularly with respect to recovery from addiction. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

New concepts in pharmacological efficacy at 7TM receptors: IUPHAR review 2

The present-day concept of drug efficacy has changed completely from its original description as the property of agonists that causes tissue activation. The ability to visualize the multiple behaviours of seven transmembrane receptors has shown that drugs can have many efficacies and also that the transduction of drug stimulus to various cellular stimulus-response cascades can be biased towards some but not all pathways. This latter effect leads to agonist 'functional selectivity', which can be favourable for the improvement of agonist therapeutics. However, in addition, biased agonist potency becomes cell type dependent with the loss of the monotonic behaviour of stimulus-response mechanisms, leading to potential problems in agonist quantification. This has an extremely important effect on the discovery process for new agonists since it now cannot be assumed that a given screening or lead optimization assay will correctly predict therapeutic behaviour. This review discusses these ideas and how new approaches to quantifying agonist effect may be used to circumvent the cell type dependence of agonism. This article, written by a corresponding member of the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC-IUPHAR), reviews our current understanding of the interaction of ligands with seven transmembrane receptors. Further information on these pharmacological concepts is being incorporated into the IUPHAR/BPS database GuideToPharmacology.org.

Phenylpiperazine derivatives with selectivity for dopamine D3 receptors modulate cocaine self-administration in rats

This study examined cocaine self-administration after pretreatments with three structurally related compounds that bind selectively to dopamine D3 receptors (D3Rs) relative to the D2 receptor subtype (D2Rs) and exhibit varying intrinsic activities in the forskolin-stimulated adenylyl cyclase assay. The compounds are: a) WC10, a D3R weak partial agonist/antagonist with 42-fold D3R:D2R selectivity, b) WC26, a 51-fold selective D3R partial agonist, c) WC44, a 23-fold selective D3R agonist. Rats were stabilized on a multiple variable-interval 60-s (VI60) schedule with alternating components of sucrose (45 mg pellets) or cocaine reinforcement (0.375 mg/kg, IV) and then tested for effects of the WC compounds (0.0, 1.0, 3.0, 5.6, or 10.0 mg/kg, IP). Another cohort was trained to self-administer cocaine (0.75 mg/kg, IV) on a VI60 schedule then tested with various doses of cocaine available (0.0-1.5 mg/kg, IV) following pretreatment with WC10 (5.6 or 10.0 mg/kg) or WC44 (10.0 mg/kg). WC10 and WC26 decreased both cocaine and sucrose reinforcement rates at the 10.0 mg/kg dose, whereas WC44 decreased only cocaine reinforcement rate at this dose. Furthermore, WC26 and WC44 increased response latency for cocaine but not sucrose. In the cocaine dose-response experiment, WC10 and WC44 flattened the dose-effect function of cocaine reinforcement rate. All compounds decreased spontaneous locomotion. WC10 and WC26 also reduced cocaine-induced locomotion. These results support the targeting of D3Rs for treatments for cocaine dependence. WC26 and WC44, in particular, show promise as they increased the latency to respond for cocaine but not sucrose, suggesting selective reduction of the motivation for cocaine.

Structure-functional selectivity relationship studies of β-arrestin-biased dopamine D₂ receptor agonists

Functionally selective G protein-coupled receptor (GPCR) ligands, which differentially modulate canonical and noncanonical signaling, are extremely useful for elucidating key signal transduction pathways essential for both the therapeutic actions and side effects of drugs. However, few such ligands have been created, and very little purposeful attention has been devoted to studying what we term: "structure-functional selectivity relationships" (SFSR). We recently disclosed the first β-arrestin-biased dopamine D(2) receptor (D(2)R) agonists UNC9975 (44) and UNC9994 (36), which have robust in vivo antipsychotic drug-like activities. Here we report the first comprehensive SFSR studies focused on exploring four regions of the aripiprazole scaffold, which resulted in the discovery of these β-arrestin-biased D(2)R agonists. These studies provide a successful proof-of-concept for how functionally selective ligands can be discovered.

The potential for selective pharmacological therapies through biased receptor signaling

The discovery that not all agonists uniformly activate cellular signaling pathways (biased signaling) has greatly changed the drug discovery process for agonists and the strategy for treatment of disease with agonists. Technological advances have enabled complex receptor behaviors to be viewed independently and through these assays, the bias for an agonist can be quantified. It is predicted that therapeutic phenotypes will be linked, through translational studies, to quantified scales of bias to guide medicinal chemists in the drug discovery process.

Selective mode of action of guanidine-containing non-peptides at human NPFF receptors

The binding pocket of both NPFF receptors was investigated, focusing on subtype-selective behavior. By use of four nonpeptidic compounds and the peptide mimetics RF9 and BIBP3226, agonistic and antagonistic properties were characterized. A set of Ala receptor mutants was generated. The binding pocket was narrowed down to the upper part of transmembrane helices V, VI, VII and the extracellular loop 2. Positions 5.27 and 6.59 have been shown to have a strong impact on receptor activation and were suggested to form an acidic, negatively charged binding pocket in both NPFF receptor subtypes. Additionally, position 7.35 was identified to play an important role in functional selectivity. According to docking experiments, the aryl group of AC-216 interacts with position 7.35 in the NPFF(1) but not in the NPFF(2) receptor. These results provide distinct insights into the receptor specific binding pockets, which is necessary for the development of drugs to address the NPFF system.

Identification and characterization of a novel class of atypical dopamine receptor agonists

PURPOSE

The D3 dopamine receptor exhibits tolerance and slow response termination (SRT) properties that are not exhibited by the closely-related D2 dopamine receptor. We previously demonstrated that the induction of tolerance elicits a unique conformational change in the D3 receptor. Here we tested the hypothesis that the tolerance and SRT properties of the D3 receptor are ligand-dependent.

METHODS

We used pharmacophore modeling and in silico screening approaches coupled with electrophysiological and biochemical methods to identify and functionally characterize the novel dopamine receptor agonists.

RESULTS

We identified cis-8-OH-PBZI (PBZI), FAUC73 and an additional novel compound, ES609, which although they are full D3 receptor agonists, do not induce the tolerance and SRT properties of the D3 receptor. In addition, PBZI has full intrinsic activity at D2L, is a partial agonist at D2S and exhibits functional selectivity at D4.2 dopamine receptors. ES609 is a partial agonist at D2S, D2L and D4.2 receptors, and exhibits functional selectivity at D2L and D4.2 dopamine receptors.

CONCLUSION

We have discovered a novel class of atypical dopamine receptor agonists that include three structurally dissimilar compounds. These new agonists will help determine the physiological and pathophysiological relevance of D3 receptor tolerance and SRT properties.

Opposing effects on the phrenic motor pathway attributed to dopamine-D1 and -D3/D2 receptor activation

Previous in vivo studies revealed that dopamine-D1-agonists elevate excitability of ventral respiratory column (VRC) neurons and increase discharge activity in the phrenic motor output through actions in the brainstem. In this in vivo study performed on pentobarbital-anesthetized cats, we show that D1-agonists (SKF-38393, dihydrexidine) given intravenously enhanced discharge activity in VRC inspiratory neurons and the phrenic nerve in two stages; discharge intensity first increased to a peak and then discharge duration increased. Cross-correlation analysis of VRC inspiratory neuron and phrenic nerve discharges showed that both stages increased strength of coupling between medullary inspiratory neurons and the phrenic motoneuron output. Intracellular recording and microiontophoresis experiments indicated that D1-agonists produced their stimulatory effects indirectly through actions on synaptic inputs to VRC inspiratory neurons. Because other laboratories have provided evidence that dopamine acting on other types of receptors depresses respiratory neuron excitability we tested the effects of piribedil, an agonist that activates receptors of the generally depressant D3/D2-dopamine receptor family, on phrenic nerve activity. Piribedil depressed phrenic nerve inspiratory discharge intensity, prolonged discharge duration, slowed burst frequency and slowed rate of action potential augmentation. The effects of piribedil were partially counteracted by intravenous injection of dihydrexidine. We propose that under normal, steady state conditions, D1-receptor-mediated excitatory modulation of phrenic motor output overrides D3/D2-receptor mediated inhibition.

Receptor conformations involved in dopamine D(2L) receptor functional selectivity induced by selected transmembrane-5 serine mutations

Although functional selectivity is now widely accepted, the molecular basis is poorly understood. We have studied how aspects of transmembrane region 5 (TM5) of the dopamine D(2L) receptor interacts with three rationally selected rigid ligands (dihydrexidine, dinapsoline, and dinoxyline) and the reference compounds dopamine and quinpirole. As was expected from homology modeling, mutation of three TM5 serine residues to alanine (S5.42A, S5.43A, S5.46A) had little effect on antagonist affinity. All three mutations decreased the affinity of the agonist ligands to different degrees, S5.46A being somewhat less affected. Four functions [adenylate cyclase (AC), extracellular signal-regulated kinase 1/2 phosphorylation (MAPK), arachidonic acid release (AA), and guanosine 5'-O-(3-thio)triphosphate binding (GTPγS)] were assessed. The intrinsic activity (IA) of quinpirole was unaffected by any of the mutations, whereas S5.42A and S5.46A mutations abolished the activity of dopamine and the three rigid ligands, although dihydrexidine retained IA at MAPK function only with S5.42A. Remarkably, S5.43A did not markedly affect IA for AC and MAPK for any of the ligands and eliminated AA activity for dinapsoline and dihydrexidine but not dinoxyline. These data suggest that this mutation did not disrupt the overall conformation or signaling ability of the mutant receptors but differentially affected ligand activation. Computational studies indicate that these D(2) agonists stabilize multiple receptor conformations. This has led to models showing the stabilized conformations and interhelical and receptor-ligand contacts corresponding to the different activation pathways stabilized by various agonists. These data provide a basis for understanding D(2L) functional selectivity and rationally discovering functionally selective D(2) drugs.

Increasing the density of the D2L receptor and manipulating the receptor environment are required to evidence the partial agonist properties of aripiprazole

The clinical efficacy of aripiprazole in the treatment of psychosis relies on a partial agonism at D2 receptors. As the expression of this receptor differs physiologically between pre- and post-synaptic sites and is affected by pathological conditions or pharmacological treatments, it appears difficult to predict the clinical response to partial agonists. In addition, the response to this novel antipsychotic was shown to depend on the cell-line and the pathway analyzed, suggesting a functional selective profile at the D2 receptor. This study aims at examining the influence of receptor density and ionic environment on the pharmacological properties of aripiprazole. A cell line was developed in which the expression of the recombinant D2 receptor can be tightly manipulated using doxycycline and sodium butyrate. The potency and efficacy of aripiprazole and other reference D2 receptor ligands were examined in [35S]GTPγS binding assays, in buffers containing either NaCl or N-methyl-D-glucamine (NMDG) which is proposed to enhance G protein coupling. Increasing the density of D2 receptors considerably enhanced the [35S]GTPγS binding induced by dopamine and the full agonist NPA. In maximally induced cells, the agonist properties of the partial agonist (-)-3-PPP was revealed in a buffer containing NaCl, whereas the response to aripiprazole was not evidenced. Substituting NMDG for NaCl promoted the response to dopamine and (-)3-PPP and was proven efficient to reveal the partial agonist profile of aripiprazole. While NMDG substitution for NaCl strongly enhanced receptor-G protein coupling, these ionic manipulations are likely to influence receptor conformations, thereby modulating the activation of signaling pathways. Our data obtained with partial agonists acting at the D2 receptor suggest that these changes in the experimental conditions could contribute to reveal the functional selective profile of GPCR ligands. They also emphasize that the properties of functional selective ligands do not only depend on receptor density but also on the surrounding environment which likely differs between brain structures.

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.