Grand opening of structure-guided design for novel opioids

Discovery of an Ortho-Substituted N-Cyclopropylmethyl-7α-phenyl-6,14-endoethano-tetrahydronorthebaine Derivative as a Selective and Potent Kappa Opioid Receptor Agonist with Subsided Sedative Effect

Research into kappa opioid receptor (KOR) agonists with attenuated central-nervous-system side effects is a critical focus for developing productive and safe analgesics. Herein, a series of ortho-substituted N-cyclopropylmethyl-7α-phenyl-6,14-endoethano-tetrahydronorthebaines were designed, synthesized, and subjected to bioassays. Compound 7a exhibited high subtype selectivity and potent agonistic activity toward KOR (KOR, Ki = 3.9 nM, MOR/KOR = 270, DOR/KOR = 1075; [35S]GTPγS binding, EC50 = 3.4 nM). Additionally, this compound exhibited robust and persistent antinociceptive effects in rodent models with different animal strains (hot plate test, ED50 = 0.20-0.30 mg/kg, i.p.; abdominal constriction test, ED50 = 0.20-0.60 mg/kg, i.p.), with its KOR-mediated mechanism for antinociception firmly established. Notably, compound 7a, unlike conventional KOR agonists, displayed minimal sedation and aversion at the antinociceptive ED50 dose. This feature addresses a crucial limitation in existing KOR agonists, positioning compound 7a as a promising novel therapeutic agent.

The opioid receptor: emergence through millennia of pharmaceutical sciences

Throughout history humanity has searched for an optimal approach to the use of opioids that maximizes analgesia while minimizing side effects. This review reflects upon the conceptualization of the opioid receptor and the critical role that the pharmaceutical sciences played in its revelation. Opium-containing formulations have been delivered by various routes of administration for analgesia and other therapeutic indications for millennia. The concept of a distinct site of opium action evolved as practitioners developed innovative delivery methods, such as intravenous administration, to improve therapeutic outcomes. The introduction of morphine and synthetic opioids engendered the prevalent assumption of a common opioid receptor. Through consideration of structure-activity relationships, spatial geometry, and pharmacological differences of known ligands, the idea of multiple opioid receptors emerged. By accessing the high-affinity property of naloxone, the opioid receptor was identified in central and peripheral nervous system tissue. The endogenous opioid neuropeptides were subsequently discovered. Application of mu-, delta-, and kappa- opioid receptor-selective ligands facilitated the pharmacological characterization and distinctions between the three receptors, which were later cloned and sequenced. Opioid receptor signal transduction pathways were described and attributed to specific physiological outcomes. The crystal structures of mu, delta, kappa, and nociceptin/orphanin FQ receptors bound to receptor-selective ligands have been elucidated. Comparison of these structures reveal locations of ligand binding and engagement of signal transduction pathways. Expanding knowledge regarding the structure and actions of the opioid receptor fuels contemporary strategies for driving the activity of opioid receptors toward maximizing therapeutic and minimizing adverse outcomes.

Post-translational Modifications of Opioid Receptors

Post-translational modifications (PTMs) are key events in signal transduction since they affect protein function by regulating their abundance and/or activity. PTMs involve the covalent attachment of functional groups to specific amino acids. Since they tend to be generally reversible, PTMs serve as regulators of signal transduction pathways. G-protein-coupled receptors (GPCRs) are major signaling proteins that undergo multiple types of PTMs. In this Review, we focus on the opioid receptors, members of GPCR family A, and highlight recent advances in the field that have underscored the importance of PTMs in the functional regulation of these receptors. Since opioid receptor activity plays a central role in the development of tolerance and addiction to morphine and other drugs of abuse, understanding the molecular mechanisms regulating receptor activity is of fundamental importance.

Toward a Universal μ-Agonist Template for Template-Based Alignment Modeling of Opioid Ligands

Opioid ligands are a large group of G-protein-coupled receptor ligands possessing high structural diversity, along with complicated structure-activity relationships (SARs). To better understand their structural correlations as well as the related SARs, we developed the innovative template-based alignment modeling in our recent studies on a variety of opioid ligands. As previously reported, this approach showed promise but also with limitations, which was mainly attributed to the small size of morphine as a template. With this study, we set out to construct an artificial μ-agonist template to overcome this limitation. The newly constructed template contained a largely extended scaffold, along with a few special μ-features relevant to the μ-selectivity of opioid ligands. As demonstrated in this paper, the new template showed significantly improved efficacy in facilitating the alignment modeling of a wide variety of opioid ligands. This report comprises of two main parts. Part 1 discusses the general construction process and the structural features as well as a few typical examples of the template applications and Part 2 focuses on the template refinement and validation.

Unidirectional opioid-cannabinoid cross-tolerance in the modulation of social play behavior in rats

RATIONALE

The endocannabinoid and the endogenous opioid systems interact in the modulation of social play behavior, a highly rewarding social activity abundantly expressed in young mammals. Prolonged exposure to opioid or cannabinoid receptor agonists induces cross-tolerance or cross-sensitization to their acute behavioral effects.

OBJECTIVES AND METHODS

Behavioral and biochemical experiments were performed to investigate whether cross-tolerance or cross-sensitization occurs to the play-enhancing effects of cannabinoid and opioid drugs on social play behavior, and the possible brain substrate involved.

RESULTS

The play-enhancing effects induced by systemic administration of JZL184, which inhibits the hydrolysis of the endocannabinoid 2-AG, were suppressed in animals repeatedly pretreated with the opioid receptor agonist morphine. Conversely, acute morphine administration increased social play in rats pretreated with vehicle or with either JZL184 or the cannabinoid agonist WIN55,212-2. Acute administration of JZL184 increased the activation of both CB1 receptors and their effector Akt in the nucleus accumbens and prefrontal cortex, brain regions important for the expression of social play. These effects were absent in animals pretreated with morphine. Furthermore, only animals repeatedly treated with morphine and acutely administered with JZL184 showed reduced activation of CB1 receptors and Akt in the amygdala.

CONCLUSIONS

The present study demonstrates a dynamic opioid-cannabinoid interaction in the modulation of social play behavior, occurring in limbic brain areas strongly implicated in social play behavior. A better understanding of opioid-cannabinoid interactions in social play contributes to clarify neurobiological aspects of social behavior at young age, which may provide new therapeutic targets for social dysfunctions.

VEGAS as a Platform for Facile Directed Evolution in Mammalian Cells

Directed evolution, artificial selection toward designed objectives, is routinely used to develop new molecular tools and therapeutics. Successful directed molecular evolution campaigns repeatedly test diverse sequences with a designed selective pressure. Unicellular organisms and their viral pathogens are exceptional for this purpose and have been used for decades. However, many desirable targets of directed evolution perform poorly or unnaturally in unicellular backgrounds. Here, we present a system for facile directed evolution in mammalian cells. Using the RNA alphavirus Sindbis as a vector for heredity and diversity, we achieved 24-h selection cycles surpassing 10^-3 mutations per base. Selection is achieved through genetically actuated sequences internal to the host cell, thus the system's name: viral evolution of genetically actuating sequences, or "VEGAS." Using VEGAS, we evolve transcription factors, GPCRs, and allosteric nanobodies toward functional signaling endpoints each in less than 1 weeks' time.

Pharmacological Assays for Investigating the NOP Receptor

The nociceptin/orphanin FQ (N/OFQ) peptide receptor (NOP) is a G protein-coupled receptor involved in the regulation of several physiological functions and pathological conditions. Thus, researchers from academia and industry are pursuing NOP to discover and study novel pharmacological entities. In a multidisciplinary effort of pharmacologists, medicinal chemists, and molecular and structural biologists the mechanisms of NOP activation and inhibition have been, at least partially, disentangled. Here, we review the in vitro methodologies employed, which have contributed to our understanding of this target. We hope this chapter guides the reader through the mostly established assay platforms to investigate NOP pharmacology, and gives some hints taking advantage from what has already illuminated the function of other GPCRs. We analyzed the pharmacological results obtained with a large panel of NOP ligands investigated in several assays including receptor binding, stimulation of GTPγS binding, decrease of cAMP levels, calcium flux stimulation via chimeric G proteins, NOP/G protein and NOP/β-arrestin interaction, label-free assays such as dynamic mass redistribution, and bioassays such as the electrically stimulated mouse vas deferens.

Targeting opioid receptor signaling in depression: do we need selective κ opioid receptor antagonists?

The opioid receptors are a family of G-protein coupled receptors (GPCRs) with close structural homology. The opioid receptors are activated by a variety of endogenous opioid neuropeptides, principally β-endorphin, dynorphins, leu- and met-enkephalins. The clinical potential of targeting opioid receptors has largely focused on the development of analgesics. However, more recent attention has turned to the role of central opioid receptors in the regulation of stress responses, anhedonia and mood. Activation of the κ opioid receptor (KOP) subtype has been shown in both human and rodent studies to produce dysphoric and pro-depressive like effects. This has led to the idea that selective KOP antagonists might have therapeutic potential as antidepressants. Here we review data showing that mixed μ opioid (MOP) and KOP antagonists have antidepressant-like effects in rodent behavioural paradigms and highlight comparable studies in treatment-resistant depressed patients. We propose that developing multifunctional ligands which target multiple opioid receptors open up the potential for fine-tuning hedonic responses mediated by opioids. This alternative approach towards targeting multiple opioid receptors may lead to more effective treatments for depression.

Computational insights into the subtype selectivity and "message-address-efficacy" mechanisms of opioid receptors through JDTic binding and unbinding

In drug design and discovery, binding affinity and selectivity are two basic properties of a drug candidate. Opioid receptors (ORs) are the main targets of strong analgesics. Like some other class A members of G-protein-coupled receptors (GPCRs), ORs exhibit complex selectivity on their ligands. The diversity of binding activity and selectivity among opioids has deeply attracted researchers for a long time. To investigate the subtype selectivity of μ, δ and κ ORs in detail, using the κ-selective antagonist JDTic as a probe, we performed a series of computational simulations, including molecular dynamics and metadynamics, on JDTic-μ/δ/κ-OR complexes. From the simulations, we found that the decisive factor of JDTic selectivity on the μ-subtype was the 2.63 position, which affected the efficacy of JDTic through changing the dynamics of the Q2.60 residue. In addition to the 2.63-position residue, the 7.35 position was the other crucial aspect of JDTic selectivity for the δ-subtype. Based on the results, we suggest a new concept, the "message-address-efficacy" hypothesis, to explain the relationships among the affinity, selectivity and function between ORs and opioids. Thus, all the detailed dynamics of JDTic-bound ORs might be helpful to deeply understand the subtype selectivity and binding mechanisms of other GPCRs.

Computational insights into the G-protein-biased activation and inactivation mechanisms of the μ opioid receptor

The μ opioid receptor (OR), a member of the class A subfamily of G-protein coupled receptors (GPCRs), is a major target for the treatment of pain. G-protein biased μ-OR agonists promise to be developed as analgesics. Thus, TRV130, the first representative μ-OR ligand with G-protein bias, has entered into phase III clinical trials. To identify the detailed G-protein-biased activation and inactivation mechanisms of the μ-OR, we constructed five μ-OR systems that were in complexes with the G-protein-biased agonists TRV130 and BU72, the antagonists β-FNA and naltrexone, as well as the free receptor. We performed a series of conventional molecular dynamics simulations and analyses of G-protein-biased activation and inactivation mechanisms of μ-OR. Our results, together with previously reported mutation results, revealed the operating mode of the activation switch composed of residues W^6.48 and Y^7.43 (Ballesteros/Weinstein numbering), the activity of which was responsible for down- and up-regulation, respectively, of the β-arrestin signaling, which in turn affected G-protein-biased activation of μ-OR. TRV130 was found to stabilize W^6.48 by interacting with Y^7.43. In addition, we obtained useful information regarding μ-OR-biased activation, such as strong stabilization of W^7.35 through a hydrophobic ring interaction in the TRV130 system. These findings may facilitate understanding of μ-OR biased activation and the design of new biased ligands for GPCRs.

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.

Synthesis and opioid receptor binding of indium (III) and [111In]-labeled macrocyclic conjugates of diprenorphine: novel ligands designed for imaging studies of peripheral opioid receptors

Radiolabeled diprenorphine (DPN) and analogs are widely used ligands for non-invasive brain imaging of opioid receptors. To develop complementary radioligands optimized for studies of the peripheral opioid receptors, we prepared a pair of hydrophilic DPN derivatives, conjugated to the macrocyclic chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), for complexation with trivalent metals. The non-radioactive indium (III) complexes, tethered to the C6-oxygen position of the DPN scaffold by 6- to 9-atom spacers, displayed high affinities for binding to μ, δ and κ opioid receptors in vitro. Use of the 9-atom linker conferred picomolar affinities equipotent to those of the parent ligand DPN. The [¹¹¹In]-labeled complexes were prepared in good yield (>70%), with high radiochemical purity (~99%) and high specific radioactivity (>4000 mCi/μmol). Their log D_7.4 values were -2.21 to -1.66. In comparison, DPN is lipophilic, with a log D_7.4 of +2.25. Further study in vivo is warranted to assess the suitability of these [¹¹¹In]-labeled DPN-DOTA conjugates for imaging trials.

Proposed Mode of Binding and Action of Positive Allosteric Modulators at Opioid Receptors

Available crystal structures of opioid receptors provide a high-resolution picture of ligand binding at the primary (“orthosteric”) site, that is, the site targeted by endogenous ligands. Recently, positive allosteric modulators of opioid receptors have also been discovered, but their modes of binding and action remain unknown. Here, we use a metadynamics-based strategy to efficiently sample the binding process of a recently discovered positive allosteric modulator of the δ-opioid receptor, BMS-986187, in the presence of the orthosteric agonist SNC-80, and with the receptor embedded in an explicit lipid–water environment. The dynamics of BMS-986187 were enhanced by biasing the potential acting on the ligand–receptor distance and ligand–receptor interaction contacts. Representative lowest-energy structures from the reconstructed free-energy landscape revealed two alternative ligand binding poses at an allosteric site delineated by transmembrane (TM) helices TM1, TM2, and TM7, with some participation of TM6. Mutations of amino acid residues at these proposed allosteric sites were found to either affect the binding of BMS-986187 or its ability to modulate the affinity and/or efficacy of SNC-80. Taken together, these combined experimental and computational studies provide the first atomic-level insight into the modulation of opioid receptor binding and signaling by allosteric modulators.

Single Amino Acid Variation Underlies Species-Specific Sensitivity to Amphibian Skin-Derived Opioid-like Peptides

It has been suggested that the evolution of vertebrate opioid receptors (ORs) follow a vector of increased functionality. Here, we test this idea by comparing human and frog ORs. Interestingly, some of the most potent opioid peptides known have been isolated from amphibian skin secretions. Here we show that such peptides (dermorphin and deltorphin) are highly potent in the human receptors and inactive in frog ORs. The molecular basis for the insensitivity of the frog ORs to these peptides was studied using chimeras and molecular modeling. The insensitivity of the delta OR (DOR) to deltorphin was due to variation of a single amino acid, Trp7.35, which is a leucine in mammalian DORs. Notably, Trp7.35 is completely conserved in all known DOR sequences from lamprey, fish, and amphibians. The deltorphin-insensitive phenotype was verified in fish. Our results provide a molecular explanation for the species selectivity of skin-derived opioid peptides.

New Technologies for Elucidating Opioid Receptor Function

Recent advances in technology, including high resolution crystal structures of opioid receptors, novel chemical tools, and new genetic approaches have provided an unparalleled palette of tools for deconstructing opioid receptor actions in vitro and in vivo. Here we provide a brief description of our understanding of opioid receptor function from both molecular and atomic perspectives, as well as their role in neural circuits in vivo. We then show how insights into the molecular details of opioid actions can facilitate the creation of functionally selective (biased) and photoswitchable opioid ligands. Finally, we describe how newly engineered opioid receptor-based chemogenetic and optogenetic tools, and new mouse lines, are expanding and transforming our understanding of opioid function and, perhaps, paving the way for new therapeutics.

Molecular switches of the κ opioid receptor triggered by 6'-GNTI and 5'-GNTI

The κ opioid receptor (κOR) is a member of G-protein-coupled receptors, and is considered as a promising drug target for treating neurological diseases. κOR selective 6'-GNTI was proved to be a G-protein biased agonist, whereas 5'-GNTI acts as an antagonist. To investigate the molecular mechanism of how these two ligands induce different behaviors of the receptor, we built two systems containing the 5'-GNTI-κOR complex and the 6'-GNTI-κOR complex, respectively, and performed molecular dynamics simulations of the two systems. We observe that transmembrane (TM) helix 6 of the κOR rotates about 4.6(o) on average in the κOR-6'-GNTI complex. Detailed analyses of the simulation results indicate that E297(6.58) and I294(6.55) play crucial roles in the rotation of TM6. In the simulation of the κOR-5'-GNTI system, it is revealed that 5'-GNTI can stabilize TM6 in the inactive state form. In addition, the kink of TM7 is stabilized by a hydrogen bond between S324(7.47) and the residue V69(1.42) on TM1.

Modulation of the Interaction between a Peptide Ligand and a G Protein-Coupled Receptor by Halogen Atoms

Systematic halogenation of two native opioid peptides has shown that halogen atoms can modulate peptide-receptor interactions in different manners. First, halogens may produce a steric hindrance that reduces the binding of the peptide to the receptor. Second, chlorine, bromine, or iodine may improve peptide binding if their positive σ-hole forms a halogen bond interaction with negatively charged atoms of the protein. Lastly, the negative electrostatic potential of fluorine can interact with positively charged atoms of the protein to improve peptide binding.

Pharmacological Profile of Nociceptin/Orphanin FQ Receptors Interacting with G-Proteins and β-Arrestins 2

Nociceptin/orphanin FQ (N/OFQ) controls several biological functions by selectively activating an opioid like receptor named N/OFQ peptide receptor (NOP). Biased agonism is emerging as an important and therapeutically relevant pharmacological concept in the field of G protein coupled receptors including opioids. To evaluate the relevance of this phenomenon in the NOP receptor, we used a bioluminescence resonance energy transfer technology to measure the interactions of the NOP receptor with either G proteins or β-arrestin 2 in the absence and in presence of increasing concentration of ligands. A large panel of receptor ligands was investigated by comparing their ability to promote or block NOP/G protein and NOP/arrestin interactions. In this study we report a systematic analysis of the functional selectivity of NOP receptor ligands. NOP/G protein interactions (investigated in cell membranes) allowed a precise estimation of both ligand potency and efficacy yielding data highly consistent with the known pharmacological profile of this receptor. The same panel of ligands displayed marked differences in the ability to promote NOP/β-arrestin 2 interactions (evaluated in whole cells). In particular, full agonists displayed a general lower potency and for some ligands an inverted rank order of potency was noted. Most partial agonists behaved as pure competitive antagonists of receptor/arrestin interaction. Antagonists displayed similar values of potency for NOP/Gβ₁ or NOP/β-arrestin 2 interaction. Using N/OFQ as reference ligand we computed the bias factors of NOP ligands and a number of agonists with greater efficacy at G protein coupling were identified.

What can crystal structures of aminergic receptors tell us about designing subtype-selective ligands?

G protein-coupled receptors (GPCRs) are integral membrane proteins that represent an important class of drug targets. In particular, aminergic GPCRs interact with a significant portion of drugs currently on the market. However, most drugs that target these receptors are associated with undesirable side effects, which are due in part to promiscuous interactions with close homologs of the intended target receptors. Here, based on a systematic analysis of all 37 of the currently available high-resolution crystal structures of aminergic GPCRs, we review structural elements that contribute to and can be exploited for designing subtype-selective compounds. We describe the roles of secondary binding pockets (SBPs), as well as differences in ligand entry pathways to the orthosteric binding site, in determining selectivity. In addition, using the available crystal structures, we have identified conformational changes in the SBPs that are associated with receptor activation and explore the implications of these changes for the rational development of selective ligands with tailored efficacy.

Opioid receptors: Structural and mechanistic insights into pharmacology and signaling

Opioid receptors are important drug targets for pain management, addiction, and mood disorders. Although substantial research on these important subtypes of G protein-coupled receptors has been conducted over the past two decades to discover ligands with higher specificity and diminished side effects, currently used opioid therapeutics remain suboptimal. Luckily, recent advances in structural biology of opioid receptors provide unprecedented insights into opioid receptor pharmacology and signaling. We review here a few recent studies that have used the crystal structures of opioid receptors as a basis for revealing mechanistic details of signal transduction mediated by these receptors, and for the purpose of drug discovery.

In vitro and in vivo pharmacological characterization of nociceptin/orphanin FQ tetrabranched derivatives

BACKGROUND AND PURPOSE

An innovative chemical approach, named peptide welding technology (PWT), allows the synthesis of multibranched peptides with extraordinary high yield, purity and reproducibility. With this approach, three different tetrabranched derivatives of nociceptin/orphanin FQ (N/OFQ) have been synthesized and named PWT1-N/OFQ, PWT2-N/OFQ and PWT3-N/OFQ. In the present study we investigated the in vitro and in vivo pharmacological profile of PWT N/OFQ derivatives and compared their actions with those of the naturally occurring peptide.

EXPERIMENTAL APPROACH

The following in vitro assays were used: receptor and [(35)S]-GTPγS binding, calcium mobilization in cells expressing the human N/OFQ peptide (NOP) receptor, or classical opioid receptors and chimeric G proteins, electrically stimulated mouse vas deferens bioassay. In vivo experiments were performed; locomotor activity was measured in normal mice and in animals with the NOP receptor gene knocked out [NOP(-/-)].

KEY RESULTS

In vitro PWT derivatives of N/OFQ behaved as high affinity potent and rather selective full agonists at human recombinant and animal native NOP receptors. In vivo PWT derivatives mimicked the inhibitory effects exerted by the natural peptide on locomotor activity showing 40-fold higher potency and extremely longer lasting action. The effects of PWT2-N/OFQ were no longer evident in NOP(-/-) mice.

CONCLUSIONS AND IMPLICATIONS

The results showed that the PWT can be successfully applied to the peptide sequence of N/OFQ to generate tetrabranched derivatives characterized by a pharmacological profile similar to the native peptide and associated with a higher potency and marked prolongation of action in vivo.

Role of opioid receptor heterodimerization in pain modulation and tolerance development

Protein to protein interactions leading to homo/heteromerization of receptor is well documented in literature. These interactions leading to dimeric/oligomers formation of receptors are known to modulate their function, particularly in case of G-protein coupled receptors. The opioid receptor heteromers having changed pharmacological properties than the constituent protomers provides preferences for novel drug targets that could lead to potential analgesic activity devoid of tolerance and physical dependence. Heterodimerization of opioid receptors appears to generate novel binding properties with improved specificity and lack of side effects. Further the molecules which can interact simultaneously to both the protomers of the heteromer, or to both the binding sites (orthosteric and allosteric) of a receptor protein could be potential therapeutic molecules. This review highlights the recent advancements in exploring the plausible role of heteromerization of opioid receptors in induction of tolerance free antinociception.

Histidine(7.36(305)) in the conserved peptide receptor activation domain of the gonadotropin releasing hormone receptor couples peptide binding and receptor activation

Transmembrane helix seven residues of G protein-coupled receptors (GPCRs) couple agonist binding to a conserved receptor activation mechanism. Amino-terminal residues of the GnRH peptide determine agonist activity. We investigated GnRH interactions with the His(7.36(305)) residue of the GnRH receptor, using functional and computational analysis of modified GnRH receptors and peptides. Non-polar His(7.36(305)) substitutions decreased receptor affinity for GnRH four- to forty-fold, whereas GnRH signaling potency was more decreased (~150-fold). Uncharged polar His(7.36(305)) substitutions decreased GnRH potency, but not affinity. [2-Nal(3)]-GnRH retained high affinity at receptors with non-polar His(7.36(305)) substitutions, supporting a role for His(7.36(305)) in recognizing Trp(3) of GnRH. Compared with GnRH, [2-Nal(3)]-GnRH potency was lower at the wild type GnRH receptor, but unchanged or higher at mutant receptors. Results suggest that His(7.36(305)) of the GnRH receptor forms two distinct interactions that determine binding to Trp(3) and couple agonist binding to the conserved transmembrane domain network that activates GPCRs.

Modulation of opioid-induced feeding behavior by endogenous nitric oxide in neonatal layer-type chicks

The current study was designed to evaluate the effects of central administration of L-arginine (The precursor of nitric oxide), N(G)-nitro-L-arginine methyl ester (L-NAME), a nitric oxide (NO) synthase inhibitor, selective opioid receptor agonists and involvement of central nitrergic/opioidergic systems on feeding behavior in neonatal layer-type chicks. The results of this study showed that the intracerebroventricular (ICV) injection of L-arginine (400 and 800 nmol) significantly decreased food intake (P < 0.001) but the injection of 200 nmol L-arginine had no effect on cumulative food intake in FD3 chickens (P > 0.05). The ICV injection of L-NAME (200 and 400 nmol) increased food intake (P < 0.001) but 100 nmol of L-NAME had no significant effect (P > 0.05). On the other hand, the co-injection of 100 nmol L-NAME significantly attenuated the anorexigenic effect of 800 nmol L-arginine (P < 0.001). Moreover, the food intake of chicks was significantly decreased by ICV injection of DAMGO (μ-opioid receptor agonist, 125 pmol) (P < 0.001) while both DPDPE (δ-opioid receptor agonist, 40 pmol) and U-50488H (κ-opioid receptor agonist, 30 nmol) significantly stimulated food intake (P < 0.001). In addition, the hypophagic effect of DAMGO was significantly amplified by administration of L-arginine (P < 0.001) while the administration of L-NAME attenuated the hypophagic effect of DAMGO (P < 0.001). In contrast, co-injection of L-arginine or L-NAME with DPDPE had no effect on the hyperphagia induced by DPDPE as well as the hyperphagic effect of U-50488H on food intake was not affected by concurrent injection of L-arginine or L-NAME (P > 0.05). These results suggest that nitrergic and opioidergic systems have an important role on feeding behavior in the CNS of neonatal layer-type chicks and it seems that interaction between them is mediated by μ-opioid receptor.

Interactions of the opioid and cannabinoid systems in reward: Insights from knockout studies

The opioid system consists of three receptors, mu, delta, and kappa, which are activated by endogenous opioid peptides (enkephalins, endorphins, and dynorphins). The endogenous cannabinoid system comprises lipid neuromodulators (endocannabinoids), enzymes for their synthesis and their degradation and two well-characterized receptors, cannabinoid receptors CB1 and CB2. These systems play a major role in the control of pain as well as in mood regulation, reward processing and the development of addiction. Both opioid and cannabinoid receptors are coupled to G proteins and are expressed throughout the brain reinforcement circuitry. Extending classical pharmacology, research using genetically modified mice has provided important progress in the identification of the specific contribution of each component of these endogenous systems in vivo on reward process. This review will summarize available genetic tools and our present knowledge on the consequences of gene knockout on reinforced behaviors in both systems, with a focus on their potential interactions. A better understanding of opioid-cannabinoid interactions may provide novel strategies for therapies in addicted individuals.

Opioid receptor desensitization: mechanisms and its link to tolerance

Opioid receptors (OR) are part of the class A of G-protein coupled receptors and the target of the opiates, the most powerful analgesic molecules used in clinic. During a protracted use, a tolerance to analgesic effect develops resulting in a reduction of the effectiveness. So understanding mechanisms of tolerance is a great challenge and may help to find new strategies to tackle this side effect. This review will summarize receptor-related mechanisms that could underlie tolerance especially receptor desensitization. We will focus on the latest data obtained on molecular mechanisms involved in opioid receptor desensitization: phosphorylation, receptor uncoupling, internalization, and post-endocytic fate of the receptor.

In vivo neuronal co-expression of mu and delta opioid receptors uncovers new therapeutic perspectives

Opioid receptors belong to the G protein coupled receptor family. They modulate brain function at all levels of neural integration and therefore impact on autonomous, sensory, emotional and cognitive processing. In vivo functional interaction between mu and delta opioid receptors are known to take place though it is still debated whether interactions occur at circuitry, cellular or molecular level. Also, the notion of receptor crosstalk via mu-delta heteromers is well documented in vitro but in vivo evidence remains scarce. To identify neurons in which receptor interactions could take place, we designed a unique double mutant knock-in mouse line that expresses functional red-fluorescent mu receptors and green-fluorescent delta receptors. We mapped mu and delta receptor distribution and co-localization throughout the nervous system and created the first interactive brain atlas with concomitant mu-delta visualization at subcellular resolution (http://mordor.ics-mci.fr/). Mu and delta receptors co-localize in neurons from subcortical networks but are mainly detected in separate neurons in the forebrain. Also, co-immunoprecipitation experiments indicated physical proximity in the hippocampus, a prerequisite to mu-delta heteromerization. Altogether, data suggest that mu-delta functional interactions take place at systems level for high-order emotional and cognitive processing whereas mu-delta may interact at cellular level in brain networks essential for survival, which has potential implications for innovative drug design in pain control, drug addiction and eating disorders.

Heteromers of μ-δ opioid receptors: new pharmacology and novel therapeutic possibilities

Several studies suggest that heteromerization between μ (MOP) and δ (DOP) opioid receptors modulates the signalling properties of the individual receptors. For example, whereas activation of MOP receptors by an agonist induces G protein-mediated signalling, the same agonist induces β-arrestin-mediated signalling in the context of the MOP-DOP receptor heteromer. Moreover, heteromer-mediated signalling is allosterically modulated by a combination of MOP and DOP receptor ligands. This has implications in analgesia given that morphine-induced antinociception can be potentiated by DOP receptor ligands. Recently reagents selectively targeting the MOP-DOP receptor heteromer such as bivalent ligands, antibodies or membrane permeable peptides have been generated; these reagents are enabling studies to elucidate the contribution of endogenously expressed heteromers to analgesia as well as to the development of side-effects associated with chronic opioid use. Recent advances in drug screening technology have led to the identification of a MOP-DOP receptor heteromer-biased agonist that activates both G protein-mediated and β-arrestin-mediated signalling. Moreover, this heteromer-biased agonist exhibits potent antinociceptive activity but with reduced side-effects, suggesting that ligands targeting the MOP-DOP receptor heteromer form a basis for the development of novel therapeutics for the treatment of pain. In this review, we summarize findings regarding the biological and functional characteristics of the MOP-DOP receptor heteromer and the in vitro and in vivo properties of heteromer-selective ligands.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

In vivo opioid receptor heteromerization: where do we stand?

Opioid receptors are highly homologous GPCRs that modulate brain function at all levels of neural integration, including autonomous, sensory, emotional and cognitive processing. Opioid receptors functionally interact in vivo, but the underlying mechanisms involving direct receptor-receptor interactions, affecting signalling pathways or engaging different neuronal circuits, remain unsolved. Heteromer formation through direct physical interaction between two opioid receptors or between an opioid receptor and a non-opioid one has been postulated and can be characterized by specific ligand binding, receptor signalling and trafficking properties. However, despite numerous studies in heterologous systems, evidence for physical proximity in vivo is only available for a limited number of opioid heteromers, and their physiopathological implication remains largely unknown mostly due to the lack of appropriate tools. Nonetheless, data collected so far using endogenous receptors point to a crucial role for opioid heteromers as a molecular entity that could underlie human pathologies such as alcoholism, acute or chronic pain as well as psychiatric disorders. Opioid heteromers therefore stand as new therapeutic targets for the drug discovery field.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

A mu-delta opioid receptor brain atlas reveals neuronal co-occurrence in subcortical networks

Opioid receptors are G protein-coupled receptors (GPCRs) that modulate brain function at all levels of neural integration, including autonomic, sensory, emotional and cognitive processing. Mu (MOR) and delta (DOR) opioid receptors functionally interact in vivo, but whether interactions occur at circuitry, cellular or molecular levels remains unsolved. To challenge the hypothesis of MOR/DOR heteromerization in the brain, we generated redMOR/greenDOR double knock-in mice and report dual receptor mapping throughout the nervous system. Data are organized as an interactive database offering an opioid receptor atlas with concomitant MOR/DOR visualization at subcellular resolution, accessible online. We also provide co-immunoprecipitation-based evidence for receptor heteromerization in these mice. In the forebrain, MOR and DOR are mainly detected in separate neurons, suggesting system-level interactions in high-order processing. In contrast, neuronal co-localization is detected in subcortical networks essential for survival involved in eating and sexual behaviors or perception and response to aversive stimuli. In addition, potential MOR/DOR intracellular interactions within the nociceptive pathway offer novel therapeutic perspectives.

Molecular Perspectives for mu/delta Opioid Receptor Heteromers as Distinct, Functional Receptors

Opioid receptors are the sites of action for morphine and the other opioid drugs. Abundant evidence now demonstrates that different opioid receptor types can physically associate to form heteromers. Understandings of the nature, behavior, and role of these opioid receptor heteromers are developing. Owing to their constituent monomers’ involvement in analgesia, mu/delta opioid receptor (M/DOR) heteromers have been a particular focus of attention. There is now considerable evidence demonstrating M/DOR to be an extant and physiologically relevant receptor species. Participating in the cellular environment as a distinct receptor type, M/DOR availability is complexly regulated and M/DOR exhibits unique pharmacology from that of other opioid receptors (ORs), including its constituents. M/DOR appears to have a range of actions that vary in a ligand- (or ligands-) dependent manner. These actions can meaningfully affect the clinical effects of opioid drugs: strategies targeting M/DOR may be therapeutically useful. This review presents and discusses developments in these understandings with a focus on the molecular nature and activity of M/DOR in the context of therapeutic potentials.

Investigational peptide and peptidomimetic μ and δ opioid receptor agonists in the relief of pain

INTRODUCTION

Current methods for treating prolonged and neuropathic pain are inadequate and lead to toxicities that greatly diminish quality of life. Therefore, new approaches to the treatment of pain states are needed to address these problems.

AREAS COVERED

The review primarily reviews approaches that have been taken in the peer-reviewed literature of multivalent ligands that interact with both μ and δ opioid receptors as agonists, and in some cases, also with pharmacophores for antagonist ligands that interact with other receptors as antagonists to block pain.

EXPERT OPINION

Although there are a number of drugs currently on the market for the treatment of pain; none of them are 100% successful. In the authors' opinion, it is clear that new directions and modalities are needed to better address the treatment of prolonged and neuropathic pain; one drug or class clearly is not the answer for all pain therapy. Undoubtedly, there are many different phenotypes of prolonged and neuropathic pain and this should be one avenue to further develop appropriate therapies.

Molecular control of δ-opioid receptor signalling

Opioids represent widely prescribed and abused medications, although their signal transduction mechanisms are not well understood. Here we present the 1.8 Å high-resolution crystal structure of the human δ-opioid receptor (δ-OR), revealing the presence and fundamental role of a sodium ion in mediating allosteric control of receptor functional selectivity and constitutive activity. The distinctive δ-OR sodium ion site architecture is centrally located in a polar interaction network in the seven-transmembrane bundle core, with the sodium ion stabilizing a reduced agonist affinity state, and thereby modulating signal transduction. Site-directed mutagenesis and functional studies reveal that changing the allosteric sodium site residue Asn 131 to an alanine or a valine augments constitutive β-arrestin-mediated signalling. Asp95Ala, Asn310Ala and Asn314Ala mutations transform classical δ-opioid antagonists such as naltrindole into potent β-arrestin-biased agonists. The data establish the molecular basis for allosteric sodium ion control in opioid signalling, revealing that sodium-coordinating residues act as 'efficacy switches' at a prototypic G-protein-coupled receptor.

Structure and dynamics of G-protein coupled receptors

G-protein coupled receptors (GPCRs) are seven helical transmembrane proteins that mediate cell-to-cell communication. They also form the largest superfamily of drug targets. Hence detailed studies of the three dimensional structure and dynamics are critical to understanding the functional role of GPCRs in signal transduction pathways, and for drug design. In this chapter we compare the features of the crystal structures of various biogenic amine receptors, such as β1 and β2 adrenergic receptors, dopamine D3 receptor, M2 and M3 muscarinic acetylcholine receptors. This analysis revealed that conserved residues are located facing the inside of the transmembrane domain in these GPCRs improving the efficiency of packing of these structures. The NMR structure of the chemokine receptor CXCR1 without any ligand bound, shows significant dynamics of the transmembrane domain, especially the helical kink angle on the transmembrane helix6. The activation mechanism of the β2-adrenergic receptor has been studied using multiscale computational methods. The results of these studies showed that the receptor without any ligand bound, samples conformations that resemble some of the structural characteristics of the active state of the receptor. Ligand binding stabilizes some of the conformations already sampled by the apo receptor. This was later observed in the NMR study of the dynamics of human β2-adrenergic receptor. The dynamic nature of GPCRs leads to a challenge in obtaining purified receptors for biophysical studies. Deriving thermostable mutants of GPCRs has been a successful strategy to reduce the conformational heterogeneity and stabilize the receptors. This has lead to several crystal structures of GPCRs. However, the cause of how these mutations lead to thermostability is not clear. Computational studies are beginning to shed some insight into the possible structural basis for the thermostability. Molecular Dynamics simulations studying the conformational ensemble of thermostable mutants have shown that the stability could arise from both enthalpic and entropic factors. There are regions of high stress in the wild type GPCR that gets relieved upon mutation conferring thermostability.

Targeting opioid receptors with pharmacological chaperones

G protein-coupled receptors (GPCRs) are polytopic membrane proteins that have a pivotal role in cellular signaling. Like other membrane proteins, they fold in the endoplasmic reticulum (ER) before they are transported to the plasma membrane. The ER quality control monitors the folding process and misfolded proteins and slowly folding intermediates are targeted to degradation in the cytosol via the ubiquitin-proteasome pathway. The high efficiency of the quality control machinery may lead to the disposal of potentially functional receptors. This is the major underlying course for loss-of-function conformational diseases, such as retinitis pigmentosa, nephrogenic diabetes insipidus and early onset obesity, which involve mutant GPCRs. During the past decade, it has become increasingly evident that small-molecular lipophilic and pharmacologically selective receptor ligands, called pharmacological chaperones (PCs), can rescue these mutant receptors from degradation by stabilizing newly synthesized receptors in the ER and enhancing their transport to the cell surface. This has raised the interesting prospect that PCs might have therapeutic value for the treatment of conformational diseases. At the same time, accumulating evidence has indicated that wild-type receptors might also be targeted by PCs, widening their therapeutic potential. This review focuses on one GPCR subfamily, opioid receptors that have been useful models to unravel the mechanism of action of PCs. In contrast to most other GPCRs, compounds that act as PCs for opioid receptors, including widely used opioid drugs, target wild-type receptors and their common natural variants.

Molecular details of the activation of the μ opioid receptor

Molecular details of μ opioid receptor activations were obtained using molecular dynamics simulations of the receptor in the presence of three agonists, three antagonists, and a partial agonist and on the constitutively active T279K mutant. Agonists have a higher probability of direct interactions of their basic nitrogen (N) with Asp147 as compared with antagonists, indicating that direct ligand-Asp147 interactions modulate activation. Medium-size substituents on the basic N of antagonists lead to steric interactions that perturb N-Asp147 interactions, while additional favorable interactions occur with larger basic N substituents, such as in N-phenethylnormorphine, restoring N-Asp147 interactions, leading to agonism. With the orvinols, the increased size of the C19 substituent in buprenorphine over diprenorphine leads to increased interactions with residues adjacent to Asp147, partially overcoming the presence of the cyclopropyl N substituent, such that buprenorphine is a partial agonist. Results also indicate different conformational properties of the intracellular regions of the transmembrane helices in agonists versus antagonists.

Discovery of a novel selective kappa-opioid receptor agonist using crystal structure-based virtual screening

Kappa-opioid (KOP) receptor agonists exhibit analgesic effects without activating reward pathways. In the search for nonaddictive opioid therapeutics and novel chemical tools to study physiological functions regulated by the KOP receptor, we screened in silico its recently released inactive crystal structure. A selective novel KOP receptor agonist emerged as a notable result and is proposed as a new chemotype for the study of the KOP receptor in the etiology of drug addiction, depression, and/or pain.

Catecholamine receptors: prototypes for GPCR-based drug discovery

Drugs acting at G protein-coupled receptors (GPCRs) constitute ~40% of those in current clinical use. GPCR-based drug discovery remains at the forefront of drug development, especially for new treatments for psychiatric illness and neurological disease. Here, the basic framework of GPCR signaling learned through the elucidation of catecholamine receptor signaling through G proteins and β-arrestins, and X-ray crystallographic structure determination is reviewed. In silico docking studies developed in tandem with confirmatory empirical data gathering from binding and signaling experiments have allowed this basic framework to be expanded to drug hunting through predictive in silico searching as well as high-throughput and high-content screening approaches. For efforts moving forward for the deployment of new GPCR-acting drugs, collaborative efforts between industry and government/academic research in target validation at the molecular and cellular levels have become progressively more common. Polypharmacological approaches have become increasingly available for learning more about the mechanisms of GPCR-targeted drugs, based on interaction not with a single, but with a wide range of GPCR targets. These approaches are likely to aid in drug repurposing efforts, yield valuable insight on the side effects of currently employed drugs, and allow for a clearer picture of the actual targets of "atypical" drugs used in a variety of therapeutic contexts.