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Sadee W. Ligand-Free Signaling of G-Protein-Coupled Receptors: Physiology, Pharmacology, and Genetics. Molecules 2023; 28:6375. [PMID: 37687205 PMCID: PMC10489045 DOI: 10.3390/molecules28176375] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/29/2023] [Revised: 08/28/2023] [Accepted: 08/30/2023] [Indexed: 09/10/2023] Open
Abstract
G-protein-coupled receptors (GPCRs) are ubiquitous sensors and regulators of cellular functions. Each GPCR exists in complex aggregates with multiple resting and active conformations. Designed to detect weak stimuli, GPCRs can also activate spontaneously, resulting in basal ligand-free signaling. Agonists trigger a cascade of events leading to an activated agonist-receptor G-protein complex with high agonist affinity. However, the ensuing signaling process can further remodel the receptor complex to reduce agonist affinity, causing rapid ligand dissociation. The acutely activated ligand-free receptor can continue signaling, as proposed for rhodopsin and μ opioid receptors, resulting in robust receptor activation at low agonist occupancy with enhanced agonist potency. Continued receptor stimulation can further modify the receptor complex, regulating sustained ligand-free signaling-proposed to play a role in opioid dependence. Basal, acutely agonist-triggered, and sustained elevated ligand-free signaling could each have distinct functions, reflecting multi-state conformations of GPCRs. This review addresses basal and stimulus-activated ligand-free signaling, its regulation, genetic factors, and pharmacological implications, focusing on opioid and serotonin receptors, and the growth hormone secretagogue receptor (GHSR). The hypothesis is proposed that ligand-free signaling of 5-HT2A receptors mediate therapeutic effects of psychedelic drugs. Research avenues are suggested to close the gaps in our knowledge of ligand-free GPCR signaling.
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Affiliation(s)
- Wolfgang Sadee
- Cancer Biology and Genetics, College of Medicine, The Ohio State University, Columbus, OH 43210, USA;
- Bioengineering and Therapeutic Sciences, University of California, San Francisco, CA 94158, USA
- Aether Therapeutics Inc., Austin, TX 78756, USA
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Jepkes S, Josee-Lemoy M, Knych H, de Lucena T, Ardeshir A, Stockinger DE. The Pharmacokinetics of Subcutaneous Methylnaltrexone Bromide in Rhesus Macaques ( Macaca mulatta). JOURNAL OF THE AMERICAN ASSOCIATION FOR LABORATORY ANIMAL SCIENCE : JAALAS 2023; 62:260-266. [PMID: 37080736 PMCID: PMC10230534 DOI: 10.30802/aalas-jaalas-22-000111] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/03/2022] [Revised: 01/13/2023] [Accepted: 01/23/2023] [Indexed: 04/22/2023]
Abstract
Opioids are an integral component of pain management for nonhuman primates. These potent analgesics also adverse gastrointestinal (GI) effects that include constipation, bloating, and delayed gastric emptying. Methylnaltrexone bromide (MNTX) is a selective, peripherally acting μ- and κ-opioid receptor antagonist that can be used to mitigate the GI effects associated with opioid administration. Unlike naltrexone, a similar drug in this class, MNTX possesses an N-methyl-quaternary amine group that prevents it from crossing the blood brain barrier. This blockage allows inhibition of peripheral GI opioid receptors without affecting opioid-mediated analgesia in the central nervous system. We conducted a pharmacokinetic analysis of MNTX in serum and CSF of 6 healthy juvenile male rhesus macaques after subcutaneous administration of a 0.15-mg/kg dose. We hypothesized that the macaques would demonstrate a Tmax of 0.5 h, similar to that of humans, and that no MNTX would be detected in the CSF. This treatment resulted in a peak serum concentration of 114 ± 44 ng/mL at 0.25 ± 0.00 h; peak CSF at concentrations were 0.34 ± 0.07 ng/mL at the Tmax. These data show that subcutaneous administration of MNTX to rhesus macaques may block peripheral adverse effects of opioids without interfering with their central analgesic effects.
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Affiliation(s)
- Sarah Jepkes
- Primate Medicine Services, California National Primate Research Center, University of California, Davis, California
| | - Marie Josee-Lemoy
- Primate Medicine Services, California National Primate Research Center, University of California, Davis, California
| | - Heather Knych
- California Animal Health and Food Safety Lab Molecular Biosciences, University of California, Davis, California
| | - Thiago de Lucena
- Division of Economics, San Diego State University, San Diego, California; and
| | - Amir Ardeshir
- Infectious Disease Unit, California National Primate Research Center, University of California, Davis, California
| | - Diane E Stockinger
- Primate Medicine Services, California National Primate Research Center, University of California, Davis, California
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Ligand-Free Signaling of G-Protein-Coupled Receptors: Relevance to μ Opioid Receptors in Analgesia and Addiction. Molecules 2022; 27:molecules27185826. [PMID: 36144565 PMCID: PMC9503102 DOI: 10.3390/molecules27185826] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/26/2022] [Revised: 09/04/2022] [Accepted: 09/06/2022] [Indexed: 11/17/2022] Open
Abstract
Numerous G-protein-coupled receptors (GPCRs) display ligand-free basal signaling with potential physiological functions, a target in drug development. As an example, the μ opioid receptor (MOR) signals in ligand-free form (MOR-μ*), influencing opioid responses. In addition, agonists bind to MOR but can dissociate upon MOR activation, with ligand-free MOR-μ* carrying out signaling. Opioid pain therapy is effective but incurs adverse effects (ADRs) and risk of opioid use disorder (OUD). Sustained opioid agonist exposure increases persistent basal MOR-μ* activity, which could be a driving force for OUD and ADRs. Antagonists competitively prevent resting MOR (MOR-μ) activation to MOR-μ*, while common antagonists, such as naloxone and naltrexone, also bind to and block ligand-free MOR-μ*, acting as potent inverse agonists. A neutral antagonist, 6β-naltrexol (6BN), binds to but does not block MOR-μ*, preventing MOR-μ activation only competitively with reduced potency. We hypothesize that 6BN gradually accelerates MOR-μ* reversal to resting-state MOR-μ. Thus, 6BN potently prevents opioid dependence in rodents, at doses well below those blocking antinociception or causing withdrawal. Acting as a ‘retrograde addiction modulator’, 6BN could represent a novel class of therapeutics for OUD. Further studies need to address regulation of MOR-μ* and, more broadly, the physiological and pharmacological significance of ligand-free signaling in GPCRs.
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Safa A, Lau AR, Aten S, Schilling K, Bales KL, Miller VA, Fitzgerald J, Chen M, Hill K, Dzwigalski K, Obrietan K, Phelps MA, Sadee W, Oberdick J. Pharmacological Prevention of Neonatal Opioid Withdrawal in a Pregnant Guinea Pig Model. Front Pharmacol 2021; 11:613328. [PMID: 33716726 PMCID: PMC7953910 DOI: 10.3389/fphar.2020.613328] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/2020] [Accepted: 12/15/2020] [Indexed: 11/19/2022] Open
Abstract
Newborns exposed to prenatal opioids often experience intense postnatal withdrawal after cessation of the opioid, called neonatal opioid withdrawal syndrome (NOWS), with limited pre- and postnatal therapeutic options available. In a prior study in pregnant mice we demonstrated that the peripherally selective opioid antagonist, 6β-naltrexol (6BN), is a promising drug candidate for preventive prenatal treatment of NOWS, and a therapeutic mechanism was proposed based on preferential delivery of 6BN to fetal brain with relative exclusion from maternal brain. Here, we have developed methadone (MTD) treated pregnant guinea pigs as a physiologically more suitable model, enabling detection of robust spontaneous neonatal withdrawal. Prenatal MTD significantly aggravates two classic maternal separation stress behaviors in newborn guinea pigs: calling (vocalizing) and searching (locomotion) - natural attachment behaviors thought to be controlled by the endogenous opioid system. In addition, prenatal MTD significantly increases the levels of plasma cortisol in newborns, showing that cessation of MTD at birth engages the hypothalamic-pituitary-adrenal (HPA) axis. We find that co-administration of 6BN with MTD prevents these withdrawal symptoms in newborn pups with extreme potency (ID50 ∼0.02 mg/kg), at doses unlikely to induce maternal or fetal withdrawal or to interfere with opioid antinociception based on many prior studies in rodents and non-human primates. Furthermore, we demonstrate a similarly high potency of 6BN in preventing opioid withdrawal in adult guinea pigs (ID50 = 0.01 mg/kg). This high potency appears to run counter to our pharmacokinetic studies showing slow 6BN transit of both the placenta and maternal blood brain barrier in guinea pigs, and calls into question the preferential delivery mechanism. Rather, it suggests a novel receptor mechanism to account for the selectively high potency of 6BN to suppress opioid dependence at all developmental stages, even in adults, as compared to its well-established low potency as a classical opioid antagonist. In conclusion, 6BN is an attractive compound for development of a preventive therapy for NOWS.
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Affiliation(s)
- Alireza Safa
- Department of Neuroscience, The Ohio State University Wexner Medical Center, Columbus, OH, United States
| | - Allison R. Lau
- Department of Psychology, California National Primate Research Center, Animal Behavior Graduate Group, University of California, Davis, CA, United States
| | - Sydney Aten
- Department of Neuroscience, The Ohio State University Wexner Medical Center, Columbus, OH, United States
| | - Karl Schilling
- Anatomisches Institute, Anatomie und Zellbiologie, Rheinische Friedrich-Wilhelms-Universität, Bonn, Germany
| | - Karen L. Bales
- Department of Psychology, California National Primate Research Center, Animal Behavior Graduate Group, University of California, Davis, CA, United States
| | - Victoria A. Miller
- Department of Neuroscience, The Ohio State University Wexner Medical Center, Columbus, OH, United States
| | - Julie Fitzgerald
- Department of Neuroscience, The Ohio State University Wexner Medical Center, Columbus, OH, United States
| | - Min Chen
- Division of Pharmaceutics and Pharmaceutical Chemistry, College of Pharmacy, The Ohio State University, Columbus, OH, United States
| | - Kasey Hill
- Division of Pharmaceutics and Pharmaceutical Chemistry, College of Pharmacy, The Ohio State University, Columbus, OH, United States
| | - Kyle Dzwigalski
- Division of Pharmaceutics and Pharmaceutical Chemistry, College of Pharmacy, The Ohio State University, Columbus, OH, United States
| | - Karl Obrietan
- Department of Neuroscience, The Ohio State University Wexner Medical Center, Columbus, OH, United States
| | - Mitch A. Phelps
- Division of Pharmaceutics and Pharmaceutical Chemistry, College of Pharmacy, The Ohio State University, Columbus, OH, United States
| | - Wolfgang Sadee
- Department of Cancer Biology and Genetics, Ohio State University Wexner Medical Center, Columbus, OH, United States
- Aether Therapeutics Inc., Austin, TX, United States
| | - John Oberdick
- Department of Neuroscience, The Ohio State University Wexner Medical Center, Columbus, OH, United States
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Sadee W, Oberdick J, Wang Z. Biased Opioid Antagonists as Modulators of Opioid Dependence: Opportunities to Improve Pain Therapy and Opioid Use Management. Molecules 2020; 25:E4163. [PMID: 32932935 PMCID: PMC7571197 DOI: 10.3390/molecules25184163] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2020] [Revised: 09/01/2020] [Accepted: 09/04/2020] [Indexed: 12/20/2022] Open
Abstract
Opioid analgesics are effective pain therapeutics but they cause various adverse effects and addiction. For safer pain therapy, biased opioid agonists selectively target distinct μ opioid receptor (MOR) conformations, while the potential of biased opioid antagonists has been neglected. Agonists convert a dormant receptor form (MOR-μ) to a ligand-free active form (MOR-μ*), which mediates MOR signaling. Moreover, MOR-μ converts spontaneously to MOR-μ* (basal signaling). Persistent upregulation of MOR-μ* has been invoked as a hallmark of opioid dependence. Contrasting interactions with both MOR-μ and MOR-μ* can account for distinct pharmacological characteristics of inverse agonists (naltrexone), neutral antagonists (6β-naltrexol), and mixed opioid agonist-antagonists (buprenorphine). Upon binding to MOR-μ*, naltrexone but not 6β-naltrexol suppresses MOR-μ*signaling. Naltrexone blocks opioid analgesia non-competitively at MOR-μ*with high potency, whereas 6β-naltrexol must compete with agonists at MOR-μ, accounting for ~100-fold lower in vivo potency. Buprenorphine's bell-shaped dose-response curve may also result from opposing effects on MOR-μ and MOR-μ*. In contrast, we find that 6β-naltrexol potently prevents dependence, below doses affecting analgesia or causing withdrawal, possibly binding to MOR conformations relevant to opioid dependence. We propose that 6β-naltrexol is a biased opioid antagonist modulating opioid dependence at low doses, opening novel avenues for opioid pain therapy and use management.
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Affiliation(s)
- Wolfgang Sadee
- Department of Cancer Biology and Genetics, College of Medicine, The Ohio State University, Columbus, OH 43210, USA
- Aether Therapeutics Inc., 4200 Marathon Blvd. Austin, TX 78756, USA
- Pain and Addiction Research Center, University of California San Francisco, San Francisco, CA 94158, USA
| | - John Oberdick
- Department of Neuroscience, College of Medicine, The Ohio State University Wexner Medical Center, Columbus, OH 43210, USA;
| | - Zaijie Wang
- Departments of Pharmaceutical Sciences and Neurology, University of Illinois at Chicago. Chicago, IL 60612, USA;
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Saunders NR, Dziegielewska KM, Møllgård K, Habgood MD. Physiology and molecular biology of barrier mechanisms in the fetal and neonatal brain. J Physiol 2018; 596:5723-5756. [PMID: 29774535 PMCID: PMC6265560 DOI: 10.1113/jp275376] [Citation(s) in RCA: 74] [Impact Index Per Article: 12.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/13/2018] [Accepted: 03/12/2018] [Indexed: 12/11/2022] Open
Abstract
Properties of the local internal environment of the adult brain are tightly controlled providing a stable milieu essential for its normal function. The mechanisms involved in this complex control are structural, molecular and physiological (influx and efflux transporters) frequently referred to as the 'blood-brain barrier'. These mechanisms include regulation of ion levels in brain interstitial fluid essential for normal neuronal function, supply of nutrients, removal of metabolic products, and prevention of entry or elimination of toxic agents. A key feature is cerebrospinal fluid secretion and turnover. This is much less during development, allowing greater accumulation of permeating molecules. The overall effect of these mechanisms is to tightly control the exchange of molecules into and out of the brain. This review presents experimental evidence currently available on the status of these mechanisms in developing brain. It has been frequently stated for over nearly a century that the blood-brain barrier is not present or at least is functionally deficient in the embryo, fetus and newborn. We suggest the alternative hypothesis that the barrier mechanisms in developing brain are likely to be appropriately matched to each stage of its development. The contributions of different barrier mechanisms, such as changes in constituents of cerebrospinal fluid in relation to specific features of brain development, for example neurogenesis, are only beginning to be studied. The evidence on this previously neglected aspect of brain barrier function is outlined. We also suggest future directions this field could follow with special emphasis on potential applications in a clinical setting.
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Affiliation(s)
- Norman R. Saunders
- Department of Pharmacology and TherapeuticsUniversity of MelbourneParkvilleVictoriaAustralia
- Department of Cellular and Molecular Medicine, Faculty of Health and Medical SciencesUniversity of CopenhagenBlegdamsvej 3CopenhagenDenmark
| | - Katarzyna M. Dziegielewska
- Department of Pharmacology and TherapeuticsUniversity of MelbourneParkvilleVictoriaAustralia
- Department of Cellular and Molecular Medicine, Faculty of Health and Medical SciencesUniversity of CopenhagenBlegdamsvej 3CopenhagenDenmark
| | - Kjeld Møllgård
- Department of Pharmacology and TherapeuticsUniversity of MelbourneParkvilleVictoriaAustralia
- Department of Cellular and Molecular Medicine, Faculty of Health and Medical SciencesUniversity of CopenhagenBlegdamsvej 3CopenhagenDenmark
| | - Mark D. Habgood
- Department of Pharmacology and TherapeuticsUniversity of MelbourneParkvilleVictoriaAustralia
- Department of Cellular and Molecular Medicine, Faculty of Health and Medical SciencesUniversity of CopenhagenBlegdamsvej 3CopenhagenDenmark
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Review: The blood-brain barrier; protecting the developing fetal brain. Placenta 2017; 54:111-116. [DOI: 10.1016/j.placenta.2016.12.005] [Citation(s) in RCA: 75] [Impact Index Per Article: 10.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/30/2016] [Revised: 12/01/2016] [Accepted: 12/05/2016] [Indexed: 11/18/2022]
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