MOR Ligands Research Articles

Understanding and Predicting Ligand Efficacy in the μ-Opioid Receptor through Quantitative Dynamical Analysis of Complex Structures.

The μ-opioid receptor (MOR) is a G-protein coupled receptor involved in nociception and the primary target of opioid drugs. Understanding the relationships among the ligand structure, receptor dynamics, and efficacy in activating MOR is crucial for drug discovery and development. Here, we use coarse-grained normal-mode analysis to predict ligand-induced changes in receptor dynamics with the Quantitative Dynamics Activity Relationship (QDAR) DynaSig-ML methodology, training a LASSO regression model on the entropic signatures (ESs) computed from ligand-receptor complexes. We train and validate the methodology using a data set of 179 MOR ligands with experimentally measured efficacies split into strictly chemically different cross-validation sets. By analyzing the coefficients of the ES LASSO model, we identified key residues involved in MOR activation, several of which have mutational data supporting their role in MOR activation. Additionally, we explored a contact-only LASSO model based on ligand-protein interactions. While the model showed predictive power, it failed at predicting efficacy for ligands with low structural similarity to the training set, emphasizing the importance of receptor dynamics for predicting ligand-induced receptor activation. Moreover, the low computational cost of our approach, at 3 CPU s per ligand-receptor complex, opens the door to its application in large-scale virtual screening contexts. Our work contributes to a better understanding of dynamics-function relationships in the μ-opioid receptor and provides a framework for predicting ligand efficacy based on ligand-induced changes in receptor dynamics.

Benzylpiperidine derivatives as new dual μ-opioid and σ1 receptor ligands with potent antinociceptive effects

Dual-acting μ-opioid receptor (MOR)/sigma-1 receptor (σ1R) ligands have displayed promise in exerting robust antinociceptive effects while reducing opioid-related side effects. To discover safer and more effective analgesics, we designed, prepared, and evaluated 30 benzylpiperidine derivatives as dual MOR and σ1R ligands. The obtained benzylpiperidine analogs were tested for MOR and σ1R binding affinity in vitro. The best compound 52 showed high affinity for both MOR [Ki (MOR) = 56.4 nM] and σ1R [Ki (σ1R) = 11.0 nM] and produced potent antinociceptive effects in the abdominal contraction test (ED50 = 4.04 mg/kg in mice), carrageenan-induced inflammatory pain model (ED50 = 6.88 mg/kg in mice), formalin test (ED50 = 13.98 mg/kg in rats) and complete Freund’s adjuvant (CFA)-induced chronic pain model (ED50 = 7.62 mg/kg in mice). Moreover, 52 had less MOR-related adverse effects than oxycodone, including constipation, acute hyperlocomotion and physical dependence. The above results suggested that 52 may be a promising candidate for the development of safer analgesics.

Thiophenpiperazine amide derivatives as new dual MOR and σ1R ligands for the treatment of pain

A new series of thiophenpiperazine amide derivatives as potent dual ligands for the μ-opioid (MOR) and sigma-1 (σ1R) receptors are reported. Compound 23 exhibited good affinity to σ1R (Ki = 44.7 ± 7.05 nM) and high selectivity to σ2R. Furthermore, Compound 23 exerted MOR agonism and σ1R antagonism and potent analgesic activity in animal moldes (the abdominal constriction test (ED50 = 3.83 mg/kg) and carrageenan-induced inflammatory hyperalgesia model (ED50 = 5.23 mg/kg)). We obtained new dual ligands that might serve as starting points for preparing targeted tools. Furthermore, 23 may be a useful chemical probe for understanding more fully analgesic effects associated with MOR agonism and σ1R antagonism.

Towards Multitargeted Ligands as Pain Therapeutics: Dual Ligands of the Cavα2δ-1 Subunit of Voltage-Gated Calcium Channel and the μ-Opioid Receptor.

The synthesis and pharmacological activity of a new series of dual ligands combining activities towards the α2δ-1 subunit of voltage-gated calcium channels (Cavα2δ-1) and the μ-opioid receptor (MOR) as novel pain therapeutics are reported. A careful exploration of the pharmacophores related to both targets, which in principle had few common characteristics, led to the design of novel compounds exhibiting both activities. The construction of the dual ligands started from published Cavα2δ-1 ligands, onto which MOR ligand pharmacophoric elements were added. This exercise led to new amino-acidic substances with good affinities on both targets as well as good metabolic and physicochemical profiles and low potential for drug-drug interactions. A representative compound, (2S,4S)-4-(4-chloro-3-(((cis)-4-(dimethylamino)-4-phenylcyclohexyl)methyl)-5-fluorophenoxy)pyrrolidine-2-carboxylic acid, displayed promising analgesic activities in several in vivo pain models as well as a reduced side-effect profile in relation to morphine.

Novel Receptor-Binding-Based Assay for the Detection of Opioids in Human Urine Samples.

Consumption of opioids is a growing global health problem. The gold standard for drugs of abuse screening is immunochemical assays. However, this method comes with some disadvantages when screening for a wide variety of opioids. Detection of the binding of a compound at the human μ-opioid receptor (MOR) offers a promising alternative target. Here, we set up a urine assay to allow for detection of compounds that bind at the MOR, thus allowing the assay to be utilized as a screening tool for opioid intake. The assay is based on the incubation of MOR-containing cell membranes with the selective MOR-ligand DAMGO and urine. After filtration, the amount of DAMGO in the eluate is analyzed by liquid chromatography tandem mass spectroscopy (LC-MS/MS). The absence of DAMGO in the eluate corresponds to a competing MOR ligand in the urine sample, thus indicating opiate/opioid intake by the suspect. Sensitivity and specificity were determined by the analysis of 200 consecutive forensic routine casework urine samples. A pronounced displacement of DAMGO was observed in 29 of the 35 opiate/opioid-positive samples. Detection of fentanyl intake proved to be the most challenging aspect. Applying a cut-off value of, e.g., 10% DAMGO binding would lead to a sensitivity of 83% and a specificity of 95%. Consequently, the novel assay proved to be a promising screening tool for opiate/opioid presence in urine samples. The nontargeted approach and possible automation of the assay make it a promising alternative to conventional methods.

Activation of Brainstem mu‐Opioid Receptors (mORs) Increases Respiratory‐Related Motor Burst Amplitude in Neonatal Rat Brainstem‐Spinal Cord Preparations

Mu‐opioid receptors (mORs) located on brainstem respiratory neurons are part of an intricate peptidergic neuromodulatory system that regulates breathing. The endogenous role of mORs in respiratory motor control, however, is poorly understood. To explore this question, very low (nanomolar) concentrations of mOR agonist drugs were bath‐applied to neonatal rat (P1‐P3) brainstem‐spinal cord preparations while measuring respiratory‐related motor output from spinal C4‐C5 rootlets containing phrenic motor neuron axons. When DAMGO (synthetic mOR agonist) or endomorphin‐2 (endogenous mOR ligand) were bath‐applied for 15 min at 20‐100 nM, respiratory motor burst amplitude increased dramatically with a modest decrease in burst frequency. Burst amplitude facilitated by 45 ± 38% and 32 ± 24% (mean ± stdev) above baseline for 50 nM DAMGO (n=8) and 100 nM endomorphin‐2 (n=10), respectively (p<0.001). To test whether respiratory motor output on thoracic ventral roots was altered by mOR activation, brainstem‐spinal cord preparations were isolated to include spinal segments C1‐T8. Bath‐applied DAMGO (50 nM) induced burst amplitude facilitation on thoracic roots T4‐T6 and cervical roots C4‐C5 by 62 ± 19% and 38 ± 36%, respectively (p<0.012). To test whether mOR‐induced burst facilitation is due to brainstem or spinal mechanisms, a split‐bath chamber with a barrier at C1‐C2 was used to separately bathe the brainstem and spinal cord compartments. DAMGO (50 nM) application to the spinal cord (C2‐C8) had little effect on respiratory motor output measured from C4‐C5 rootlets, whereas DAMGO application to the brainstem increased C4‐C5 motor output by 34 ± 19% (p<0.004). These data suggest that mOR activation in the brainstem can transform the respiratory network to produce a motor pattern with decreased frequency balanced by increased amplitude, with relatively little change in overall neural output. Given that mOR‐induced facilitation was observed in both cervical and thoracic motor neuron pools, we hypothesize that burst amplitude facilitation is due to mOR activation on rhythm‐generating neurons or premotor neurons in the brainstem. This study reveals that endogenous mOR activation modulates the neural control of breathing in a manner that is potentially different from respiratory depression, which is more commonly associated with opioids.

Synthesis, biological, and structural explorations of a series of μ-opioid receptor (MOR) agonists with high G protein signaling bias

Biased agonism refers to the ability of compounds to drive preferred signaling pathways and avoid adverse signaling pathways in a ligand-dependent manner for some G-protein–coupled receptors. It is thought that the separation of therapeutic efficacy (e.g., analgesia) from adverse effects (e.g., respiration depression) can be achieved through the design of biased MOR agonists and one example is the recently approved MOR biased agonist oliceridine (TRV130). However, oliceridine only demonstrates modest beneficial effects as compared to other opioids in terms of therapeutic/adverse effect balance. One possibility attributable to the modest success of oliceridine is its limited bias, and as such developing MOR ligands with a more biased agonism profile could in theory further improve the beneficial effects of the ligands. Here, we rationally designed and synthesized a series of derivatives as potent highly biased MOR agonists (19a-v) through the modification and structure-activity relationship study of TRV130. This novel synthetic molecule, LPM3480392 (19m), demonstrated improved in vitro biased agonism (EC50 = 0.35 nM, Emax = 91.4%) with no measured β-arrestin recruitment (EC50 > 30000 nM, Emax = 1.6%), good brain penetration (B/P ratio = 4.61, 0.25 h post-IV dosing 2.0 mg/kg), a favorable pharmacokinetic profile (distribution volume = 10766 mL/kg, t1/2 = 1.9 h) and produced potent antinociceptive effect with reduced respiratory suppression (sO2(%) = 92.17, 0.32 mg/kg, SC) as compared to TRV130. LPM3480392 has completed preclinical studies and is currently under clinical development (CTR20210370) as an analgesic for the treatment of moderate to severe pain.

Encoding mu-opioid receptor biased agonism with interaction fingerprints.

Opioids are potent painkillers,however, their therapeutic use requires close medical monitoring to diminish the risk of severe adverse effects. The G-protein biased agonists of the μ-opioid receptor (MOR) have shown safer therapeutic profiles than non-biased ligands. In this work, we performed extensive all-atom molecular dynamics simulations of two markedly biased ligands and a balanced reference molecule. From those simulations, we identified a protein-ligand interaction fingerprint that characterizes biased ligands. Then, we built and virtually screened a database containing 68,740 ligands with proven or potential GPCR agonistic activity. Exemplary molecules that fulfill the interacting pattern for biased agonism are showcased, illustrating the usefulness of this work for the search of biased MOR ligands and how this contributes to the understanding of MOR biased signaling.

Label-free cell phenotypic study of opioid receptors and discovery of novel mu opioid ligands from natural products

Ethnopharmacological relevanceMu opioid receptor (MOR) is mainly a drug target for analgesia. Opioid-like agonists such as morphine have been clinically used for analgesia but have potential adverse effects. MOR antagonists have been demonstrated to alleviate these side effects. Plants (Carthamus tinctorius L, Cynanchum otophyllum C. K. Schneid., Coffea arabica L., Prinsepia utilis Royle and Lepidium meyenii Walp.) and Ganoderma fungi (Ganoderma hainanense J. D. Zhao, Ganoderma capense (Lloyd) Teng, Ganoderma cochlear (Blume et Nees) Bres., Ganoderma resinaceum Boud and Ganoderma applanatum (Pers.) Pat.) are traditional medicines with beneficial effects on immunoregulation, analgesia and the nervous system, but whether MORs are engaged in their effects remains unknown. Aim of the studyThis work aimed to identify MOR ligands among compounds isolated from the above-mentioned 10 species, and to investigate selectivity against four opioid receptor subtypes. By analyzing the structure-activity relationship and off-target effects, we could provide a new direction for the future development of MOR drugs. Materials and methodsFour opioid receptor subtype models, including MOR, delta (DOR), kappa (KOR) and nop (NOR), were established with a label-free phenotypic dynamic mass redistribution assay to systematically profile the pharmacological properties of known ligands. Then, 82 natural compounds derived from the 10 species were screened against MOR to identify new ligands. The selectivity of the new ligands was characterized against the four subtypes, and off-target effects were also investigated on eight G protein-coupled receptors (GPCRs). ResultsThe pharmacological properties of known ligands on transfected HEK293T-MOR, HEK293-DOR, HEK293-KOR and HEK293-NOR cell lines were characterized. Seven compounds purified from Ganoderma cochlear (Blume et Nees) Bres. and Carthamus tinctorius L were MOR antagonists with micromolar potency. Among them, compound 35 showed the strongest antagonistic activity on MOR with an IC50 value of 10.0 ± 3.0 μM. To a certain extent, these seven new antagonists, exhibited antagonistic activity on the other opioid receptor subtypes, and they had almost no effect on other GPCRs, including CB1, CB2, M2 and beta2AR. Additionally, a compound from Lepidium meyenii Walp. displayed MOR agonistic activity. ConclusionsThe established screening models opened new avenues for the discovery and evaluation of opioid receptor ligand selectivity. Together, the novel MOR antagonists and agonists will enrich the inventory of MOR ligands and benefit related therapies.

Preclinical Studies on Nalfurafine (TRK-820), a Clinically Used KOR Agonist.

Nalfurafine has been used clinically in Japan for treatment of itch in kidney dialysis patients and in patients with chronic liver diseases. A one-year post-marketing study showed nalfurafine to be safe and efficacious without producing side effects of typical KOR agonists such as anhedonia and psychotomimesis. In this chapter, we summarize in vitro characterization and in vivo preclinical studies on nalfurafine. In vitro, nalfurafine is a highly potent and moderately selective KOR full agonist; however, whether it is a biased KOR agonist is a matter of debate. In animals, nalfurafine produced anti-pruritic effects in a dose range lower than that caused side effects, including conditioned place aversion (CPA), hypolocomotion, motor incoordination, consistent with the human data. In addition, nalfurafine showed antinociceptive effects in several pain models at doses that did not cause the side effects mentioned above. It appears to be effective against inflammatory pain and mechanical pain, but less so against thermal pain, particularly high-intensity thermal pain. U50,488H and nalfurafine differentially modulated several signaling pathways in a brain region-specific manners. Notably, U50,488H, but not nalfurafine, activated the mTOR pathway, which contributed to U50,488H-induced CPA. Because of its lack of side effects associated with typical KOR agonists, nalfurafine has been investigated as a combination therapy with an MOR ligand for pain treatment and for its effects on opioid use disorder and alcohol use disorder, and results indicate potential usefulness for these indications. Thus, although in vitro data regarding uniqueness of nalfurafine in terms of signaling at the KOR are somewhat equivocal, in vivo results support the assertion that nalfurafine is an atypical KOR agonist with a significantly improved side-effect profile relative to typical KOR agonists.

Intrinsic Efficacy of Opioid Ligands and Its Importance for Apparent Bias, Operational Analysis, and Therapeutic Window.

Evidence from several novel opioid agonists and knockout animals suggests that improved opioid therapeutic window, notably for analgesia versus respiratory depression, is a result of ligand bias downstream of activation of the µ-opioid receptor (MOR) toward G protein signaling and away from other pathways, such as arrestin recruitment. Here, we argue that published claims of opioid bias based on application of the operational model of agonism are frequently confounded by failure to consider the assumptions of the model. These include failure to account for intrinsic efficacy and ceiling effects in different pathways, distortions introduced by analysis of amplified (G protein) versus linear (arrestin) signaling mechanisms, and nonequilibrium effects in a dynamic signaling cascade. We show on both theoretical and experimental grounds that reduced intrinsic efficacy that is unbiased across different downstream pathways, when analyzed without due considerations, does produce apparent but erroneous MOR ligand bias toward G protein signaling, and the weaker the G protein partial agonism is the greater the apparent bias. Experimentally, such apparently G protein-biased opioids have been shown to exhibit low intrinsic efficacy for G protein signaling when ceiling effects are properly accounted for. Nevertheless, such agonists do display an improved therapeutic window for analgesia versus respiratory depression. Reduced intrinsic efficacy for G proteins rather than any supposed G protein bias provides a more plausible, sufficient explanation for the improved safety. Moreover, genetic models of G protein-biased opioid receptors and replication of previous knockout experiments suggest that reduced or abolished arrestin recruitment does not improve therapeutic window for MOR-induced analgesia versus respiratory depression. SIGNIFICANCE STATEMENT: Efforts to improve safety of µ-opioid analgesics have focused on agonists that show signaling bias for the G protein pathway versus other signaling pathways. This review provides theoretical and experimental evidence showing that failure to consider the assumptions of the operational model can lead to large distortions and overestimation of actual bias. We show that low intrinsic efficacy is a major determinant of these distortions, and pursuit of appropriately reduced intrinsic efficacy should guide development of safer opioids.

Molecular Dynamics Simulations to Investigate How PZM21 Affects the Conformational State of the μ-Opioid Receptor Upon Activation.

Opioid analgesics such as morphine have indispensable roles in analgesia. However, morphine use can elicit side effects such as respiratory depression and constipation. It has been reported that G protein-biased agonists as substitutes for classic opioid agonists can alleviate (or even eliminate) these side effects. The compounds PZM21 and TRV130 could be such alternatives. Nevertheless, there are controversies regarding the efficacy and G protein-biased ability of PZM21. To demonstrate a rationale for the reduced biasing agonism of PZM21 compared with that of TRV130 at the molecular level, we undertook a long-term molecular dynamics simulation of the μ-opioid receptor (MOR) upon the binding of three ligands: morphine, TRV130, and PZM21. We found that the delayed movement of the W2936.48 (Ballesteros–Weinstein numbering) side chain was a factor determining the dose-dependent agonism of PZM21. Differences in conformational changes of W3187.35, Y3267.43, and Y3367.53 in PZM21 and TRV130 explained the observed differences in bias between these ligands. The extent of water movements across the receptor channel was correlated with analgesic effects. Taken together, these data suggest that the observed differences in conformational changes of the studied MOR–ligand complexes point to the low-potency and lower bias effects of PZM21 compared with the other two ligands, and they lay the foundation for the development of G protein-biased agonists.

Endogenous opioid peptides and brain development: Endomorphin-1 and Nociceptin play a sex-specific role in the control of oligodendrocyte maturation and brain myelination.

The generation of fully functional oligodendrocytes, the myelinating cells of the central nervous system, is preceded by a complex maturational process. We previously showed that the timing of oligodendrocyte differentiation and rat brain myelination were altered by perinatal exposure to buprenorphine and methadone, opioid analogs used for the management of pregnant addicts. Those observations suggested the involvement of the μ-opioid receptor (MOR) and the nociceptin/orphanin FQ receptor (NOR). However, it remained to be determined if these receptors and their endogenous ligands could indeed control the timing of myelination under normal physiological conditions of brain development. We now found that the endogenous MOR ligand endomorphin-1 (EM-1) exerts a striking stimulatory action on cellular and morphological maturation of rat pre-oligodendrocytes, but unexpectedly, these effects appear to be restricted to the cells from the female pups. Critically, this stimulation is abolished by coincubation with the endogenous NOR ligand nociceptin. Furthermore, NOR antagonist treatment of 9-day-old female pups results in accelerated brain myelination. Interestingly, the lack of sex-dependent differences in developmental brain levels of EM-1 and nociceptin, or oligodendroglial expression of MOR and NOR, suggests that the observed sex-specific responses may be highly dependent on important intrinsic differences between the male and female oligodendrocytes. The discovery of a significant effect of EM-1 and nociceptin in the developing female oligodendrocytes and brain myelination, underscores the need for further studies investigating brain sex-related differences and their implications in opioid use and abuse, pain control, and susceptibility and remyelinating capacity in demyelinating disease as multiple sclerosis.

Sex differences in the effectiveness of buprenorphine to decrease rates of responding in rhesus monkeys

Sex differences in μ-opioid receptor (MOR) agonist-induced antinociception have been reported in nonhuman primates. The degree to which μ-opioid receptor agonist sex differences in nonhuman primates extend to other behavioral endpoints remains unknown. The present study compared the behavioral effects of three MOR ligands (fentanyl, buprenorphine, and naltrexone) that varied in efficacy to stimulate [S]-GTPγS binding (from highest to lowest: fentanyl, buprenorphine, and naltrexone) in male and female rhesus monkeys. Male (n=3) and female (n=3) monkeys were trained to respond under a fixed-ratio 10 schedule of food presentation during daily sessions consisting of multiple components. Once rates of responding were stable, cumulative dose-effect functions were determined for intramuscular fentanyl (0.00032-0.032 mg/kg), buprenorphine (0.001-1 mg/kg), and naltrexone (0.01-0.1 mg/kg). Fentanyl dose-dependently decreased rates of responding in both sexes and the corresponding ED50 values were not significantly different. Buprenorphine dose-dependently decreased rates of responding in females, but not males. Naltrexone did not significantly alter behavior in either females or males. Overall, these results suggest that the expression of sex differences in MOR pharmacology depends upon both the efficacy of the MOR ligand and the behavioral endpoint.

Two delta opioid receptor subtypes are functional in single ventral tegmental area neurons, and can interact with the mu opioid receptor

The mu and delta opioid receptors (MOR and DOR) are highly homologous members of the opioid family of GPCRs. There is evidence that MOR and DOR interact, however the extent to which these interactions occur in vivo and affect synaptic function is unknown. There are two stable DOR subtypes: DPDPE sensitive (DOR1) and deltorphin II sensitive (DOR2); both agonists are blocked by DOR selective antagonists. Robust motivational effects are produced by local actions of both MOR and DOR ligands in the ventral tegmental area (VTA). Here we demonstrate that a majority of both dopaminergic and non-dopaminergic VTA neurons express combinations of functional DOR1, DOR2, and/or MOR, and that within a single VTA neuron, DOR1, DOR2, and MOR agonists can differentially couple to downstream signaling pathways. As reported for the MOR agonist DAMGO, DPDPE and deltorphin II produced either a predominant K+ dependent hyperpolarization or a Cav2.1 mediated depolarization in different neurons. In some neurons DPDPE and deltorphin II produced opposite responses. Excitation, inhibition, or no effect by DAMGO did not predict the response to DPDPE or deltorphin II, arguing against a MOR-DOR interaction generating DOR subtypes. However, in a subset of VTA neurons the DOR antagonist TIPP-Ψ augmented DAMGO responses; we also observed DPDPE or deltorphin II responses augmented by the MOR selective antagonist CTAP. These findings directly support the existence of two independent, stable forms of the DOR, and show that MOR and DOR can interact in some neurons to alter downstream signaling.

Binding mode analyses of NAP derivatives as mu opioid receptor selective ligands through docking studies and molecular dynamics simulation

Mu opioid receptor selective antagonists are highly desirable because of their utility as pharmacological probes for receptor characterization and functional studies. Furthermore, the mu opioid receptors act as an important target in drug abuse and addiction treatment. Previously, we reported NAP as a novel lead compound with high selectivity and affinity towards the mu opioid receptor. Based on NAP, we have synthesized its derivatives and further characterized their binding affinities and selectivity towards the receptor. NMP and NGP were identified as the two most selective MOR ligands among NAP derivatives. In the present study, molecular modeling methods were applied to assess the dual binding modes of NAP derivatives, particularly on NMP and NGP, in three opioid receptors, in order to analyze the effects of structural modifications on the pyridyl ring of NAP on the binding affinity and selectivity. The results indicated that the steric hindrance, electrostatic, and hydrophobic effects caused by the substituents on the pyridyl ring of NAP contributed complimentarily on the binding affinity and selectivity of NAP derivatives to three opioid receptors. Analyses of these contributions provided insights on future design of more potent and selective mu opioid receptor ligands.

Functional μ-Opioid-Galanin Receptor Heteromers in the Ventral Tegmental Area

The μ-opioid receptor (MOR) localized in the ventral tegmental area (VTA) plays a key role in the reinforcing and addictive properties of opioids. With parallel in vitro experiments in mammalian transfected cells and in situ and in vivo experiments in rat VTA, we demonstrate that a significant population of these MORs form functional heteromers with the galanin receptor subtype Gal1 (Gal1R), which modulate the activity of the VTA dopaminergic neurons. The MOR-Gal1R heteromer can explain previous results showing antagonistic galanin-opioid interactions and offers a new therapeutic target for the treatment of opioid use disorder.

An LP1 analogue, selective MOR agonist with a peculiar pharmacological profile, used to scrutiny the ligand binding domain

The hypothesis that central analgesia with reduced side effects is obtainable by occupying an ‘allosteric’ site in the MOR ligand binding domain requires the development of new ligands with peculiar pharmacological profile to be used as tools. New benzomorphan derivatives, analogues of LP1, a multitarget MOR agonist/DOR antagonist, were designed to examine in depth MOR ligand binding domain. Compound 5, bearing a diphenylic N-substituent on the benzomorphan nucleus, showed an affinity (Kiμ=0.5±0.2nM) comparable to that of LP1 and a better selectivity versus DOR and KOR. It elicits antinociceptive effects in ex vivo (GPI) and in vivo. This new compound engages receptor amino acidic residues not reached by LP1 and by other established MOR ligands. Molecular modeling studies, conducted on 5 and on several reference compounds, allowed us to propose possible residues in the MOR ligand binding domain essential for their interactions with ‘orthosteric’ and ‘allosteric’ binding sites.

The µ Opioid Receptor and Ligands Acting at the µ Opioid Receptor, as Therapeutics and Potential Therapeutics

Although the µ opioid receptor (MOR) was pharmacologically and biochemically identified in binding studies forty years ago, its structure, function, and true complexity only have emerged after its cloning in 1993. Continuous efforts from many laboratories have greatly advanced our understanding of MORs, ranging from their anatomic distribution to cellular and molecular mechanisms, and from cell lines to in vivo systems. The MOR is recognized as the main target for effective pain relief, but its involvement in many other physiological functions has also been recognized. This review provides a synopsis on the history of research on MORs and ligands acting at the MOR with the focus on their clinical and potential use as therapeutic drugs, or as valuable research tools. Since the elucidation of the chemical structure of morphine and the characterization of endogenous opioid peptides, research has stimulated the development of new generations of MOR ligands with distinct pharmacological profiles (agonist, antagonist, mixed agonist/antagonist and partial agonist) or site of action (central/peripheral). Discovery of therapeutically useful morphine-like drugs and innovative drugs with new scaffolds, with several outstanding representatives, is discussed. Extensive efforts on modifications of endogenous peptides to attain stable and MOR selective analogs are overviewed with stimulating results for the development of peptide-based pharmaceuticals. With pharmacophore modeling as an important tool in drug discovery, application of modern computational methodologies for the development of morphinans as new MOR ligands is described. Moreover, the crystal structure of the MOR available today will enable the application of structure-based approaches to design better drugs for the management of pain, addiction and other human diseases, where MORs play a key role.

Neuroanatomical distribution of μ-opioid receptor mRNA and binding in monogamous prairie voles (Microtus ochrogaster) and non-monogamous meadow voles (Microtus pennsylvanicus)

The opiate system has long been implicated in the rewarding properties of social interactions. In particular, the μ-opioid receptor (MOR) mediates multiple forms of social attachment, including the attachment of offspring to the mother and social bonding between mates. We have previously shown that MOR in the caudate-putamen is involved in partner preference formation in monogamous prairie voles. Here, using in situ hybridization and receptor autoradiography, we mapped in detail the distribution of MOR mRNA and ligand binding in monogamous prairie vole brains and compared MOR binding density with that of promiscuous meadow vole brains. Comparison of MOR binding in these closely related species with distinctly different social behavior revealed that while the distribution of MOR is similar, prairie voles have significantly higher densities of MOR than meadow voles in a majority of regions in the forebrain, including the caudate-putamen, nucleus accumbens shell, lateral septum and several thalamic nuclei, including the anteroventral and anteromedial thalamic nuclei. These differences in MOR expression between prairie and meadow voles could potentially contribute to species differences in behavior, including social attachment.