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Braunscheidel K, Okas M, Woodward JJ. Toluene alters the intrinsic excitability and excitatory synaptic transmission of basolateral amygdala neurons. Front Neurosci 2024; 18:1366216. [PMID: 38595974 PMCID: PMC11002899 DOI: 10.3389/fnins.2024.1366216] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/05/2024] [Accepted: 01/31/2024] [Indexed: 04/11/2024] Open
Abstract
Introduction Inhalant abuse is an important health issue especially among children and adolescents who often encounter these agents in the home. Research into the neurobiological targets of inhalants has lagged behind that of other drugs such as alcohol and psychostimulants. However, studies from our lab and others have begun to reveal how inhalants such as the organic solvent toluene affect neurons in key addiction related areas of the brain including the ventral tegmental area, nucleus accumbens and medial prefrontal cortex. In the present study, we extend these findings and examine the effect of toluene on electrophysiological responses of pyramidal neurons in the basolateral amygdala BLA, a region important for generating emotional and reward based information needed to guide future behavior. Methods Whole-cell patch-clamp electrophysiology recordings of BLA pyramidal neurons in rat brain slices were used to assess toluene effects on intrinsic excitability and excitatory glutamatergic synaptic transmission. Results Acute application of 3 mM but not 0.3 mM toluene produced a small but significant (~20%) increase in current-evoked action potential (AP) firing that reversed following washout of the toluene containing solution. The change in firing during exposure to 3 mM toluene was accompanied by selective changes in AP parameters including reduced latency to first spike, increased AP rise time and decay and a reduction in the fast after-hyperpolization. To examine whether toluene also affects excitatory synaptic signaling, we expressed channelrhodopsin-2 in medial prefrontal cortex neurons and elicited synaptic currents in BLA neurons via light pulses. Toluene (3 mM) reduced light-evoked AMPA-mediated synaptic currents while a lower concentration (0.3 mM) had no effect. The toluene-induced reduction in AMPA-mediated BLA synaptic currents was prevented by the cannabinoid receptor-1 antagonist AM281. Discussion These findings are the first to demonstrate effects of acute toluene on BLA pyramidal neurons and add to existing findings showing that abused inhalants such as toluene have significant effects on neurons in brain regions involved in natural and drug induced reward.
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Affiliation(s)
| | | | - John J. Woodward
- Department of Neuroscience, Medical University of South Carolina, Charleston, SC, United States
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Martynyuk AE, Ju LS, Morey TE, Zhang JQ. Neuroendocrine, epigenetic, and intergenerational effects of general anesthetics. World J Psychiatry 2020; 10:81-94. [PMID: 32477904 PMCID: PMC7243620 DOI: 10.5498/wjp.v10.i5.81] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/24/2019] [Revised: 03/18/2020] [Accepted: 03/26/2020] [Indexed: 02/05/2023] Open
Abstract
The progress of modern medicine would be impossible without the use of general anesthetics (GAs). Despite advancements in refining anesthesia approaches, the effects of GAs are not fully reversible upon GA withdrawal. Neurocognitive deficiencies attributed to GA exposure may persist in neonates or endure for weeks to years in the elderly. Human studies on the mechanisms of the long-term adverse effects of GAs are needed to improve the safety of general anesthesia but they are hampered not only by ethical limitations specific to human research, but also by a lack of specific biological markers that can be used in human studies to safely and objectively study such effects. The latter can primarily be attributed to an insufficient understanding of the full range of the biological effects induced by GAs and the molecular mechanisms mediating such effects even in rodents, which are far more extensively studied than any other species. Our most recent experimental findings in rodents suggest that GAs may adversely affect many more people than is currently anticipated. Specifically, we have shown that anesthesia with the commonly used GA sevoflurane induces in exposed animals not only neuroendocrine abnormalities (somatic effects), but also epigenetic reprogramming of germ cells (germ cell effects). The latter may pass the neurobehavioral effects of parental sevoflurane exposure to the offspring, who may be affected even at levels of anesthesia that are not harmful to the exposed parents. The large number of patients who require general anesthesia, the even larger number of their future unexposed offspring whose health may be affected, and a growing number of neurodevelopmental disorders of unknown etiology underscore the translational importance of investigating the intergenerational effects of GAs. In this mini review, we discuss emerging experimental findings on neuroendocrine, epigenetic, and intergenerational effects of GAs.
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Affiliation(s)
- Anatoly E Martynyuk
- Department of Anesthesiology and the McKnight Brain Institute, University of Florida College of Medicine, Gainesville, FL 32610, United States
| | - Ling-Sha Ju
- Department of Anesthesiology, University of Florida College of Medicine, Gainesville, FL 32610, United States
| | - Timothy E Morey
- Department of Anesthesiology, University of Florida College of Medicine, Gainesville, FL 32610, United States
| | - Jia-Qiang Zhang
- Department of Anesthesiology and Perioperative Medicine, Henan Provincial People’s Hospital, People's Hospital of Zhengzhou University, Zhengzhou 450003, Henan Province, China
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Structural basis for the ethanol action on G-protein-activated inwardly rectifying potassium channel 1 revealed by NMR spectroscopy. Proc Natl Acad Sci U S A 2018; 115:3858-3863. [PMID: 29581303 DOI: 10.1073/pnas.1722257115] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
Ethanol consumption leads to a wide range of pharmacological effects by acting on the signaling proteins in the human nervous system, such as ion channels. Despite its familiarity and biological importance, very little is known about the molecular mechanisms underlying the ethanol action, due to extremely weak binding affinity and the dynamic nature of the ethanol interaction. In this research, we focused on the primary in vivo target of ethanol, G-protein-activated inwardly rectifying potassium channel (GIRK), which is responsible for the ethanol-induced analgesia. By utilizing solution NMR spectroscopy, we characterized the changes in the structure and dynamics of GIRK induced by ethanol binding. We demonstrated here that ethanol binds to GIRK with an apparent dissociation constant of 1.0 M and that the actual physiological binding site of ethanol is located on the cavity formed between the neighboring cytoplasmic regions of the GIRK tetramer. From the methyl-based NMR relaxation analyses, we revealed that ethanol activates GIRK by shifting the conformational equilibrium processes, which are responsible for the gating of GIRK, to stabilize an open conformation of the cytoplasmic ion gate. We suggest that the dynamic molecular mechanism of the ethanol-induced activation of GIRK represents a general model of the ethanol action on signaling proteins in the human nervous system.
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Affiliation(s)
- J. J. Pandit
- Nuffield Department of Anaesthetics; Oxford University Hospitals; Oxford UK
- St John's College; Oxford UK
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McCracken ML, Gorini G, McCracken LM, Mayfield RD, Harris RA, Trudell JR. Inter- and Intra-Subunit Butanol/Isoflurane Sites of Action in the Human Glycine Receptor. Front Mol Neurosci 2016; 9:45. [PMID: 27378846 PMCID: PMC4906044 DOI: 10.3389/fnmol.2016.00045] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/10/2016] [Accepted: 05/26/2016] [Indexed: 11/24/2022] Open
Abstract
Glycine receptors (GlyRs) mediate inhibitory neurotransmission and are targets for alcohols and anesthetics in brain. GlyR transmembrane (TM) domains contain critical residues for alcohol/anesthetic action: amino acid A288 in TM3 forms crosslinks with TM1 (I229) in the adjacent subunit as well as TM2 (S267) and TM4 (Y406, W407, I409, Y410) in the same subunit. We hypothesized that these residues may participate in intra-subunit and inter-subunit sites of alcohol/anesthetic action. The following double and triple mutants of GLRA1 cDNA (encoding human glycine receptor alpha 1 subunit) were injected into Xenopus laevis oocytes: I229C/A288C, I229C/A288C/C290S, A288C/Y406C, A288C/W407C, A288C/I409C, and A288C/Y410C along with the corresponding single mutants and wild-type GLRA1. Butanol (22 mM) or isoflurane (0.6 mM) potentiation of GlyR-mediated currents before and after application of the cysteine crosslinking agent HgCl2 (10 μM) was measured using two-electrode voltage clamp electrophysiology. Crosslinking nearly abolished butanol and isoflurane potentiation in the I229C/A288C and I229C/A288C/C290S mutants but had no effect in single mutants or wild-type. Crosslinking also inhibited butanol and isoflurane potentiation in the TM3-4 mutants (A288C/Y406C, A288C/W407C, A288C/I409C, A288C/Y410C) with no effect in single mutants or wild-type. We extracted proteins from oocytes expressing I229C/288C, A288C/Y410C, or wild-type GlyRs, used mass spectrometry to verify their expression and possible inter-subunit dimerization, plus immunoblotting to investigate the biochemical features of proposed crosslinks. Wild-type GlyR subunits measured about 50 kDa; after crosslinking, the dimeric/monomeric 100:50 kDa band ratio was significantly increased in I229C/288C but not A288C/Y410C mutants or wild-type, providing support for TM1-3 inter-subunit and TM3-4 intra-subunit crosslinking. A GlyR homology model based on the GluCl template provides further evidence for a multi-site model for alcohol/anesthetic interaction with human GLRA1.
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Affiliation(s)
- Mandy L McCracken
- Waggoner Center for Alcohol and Addiction Research, The University of Texas at AustinAustin, TX, USA; Integrative Neuroscience Research Branch, Neurobiology of Addiction Section, National Institute on Drug Abuse, National Institutes of HealthBaltimore, MD, USA
| | - Giorgio Gorini
- Waggoner Center for Alcohol and Addiction Research, The University of Texas at Austin Austin, TX, USA
| | - Lindsay M McCracken
- Waggoner Center for Alcohol and Addiction Research, The University of Texas at Austin Austin, TX, USA
| | - R Dayne Mayfield
- Waggoner Center for Alcohol and Addiction Research, The University of Texas at Austin Austin, TX, USA
| | - R Adron Harris
- Waggoner Center for Alcohol and Addiction Research, The University of Texas at Austin Austin, TX, USA
| | - James R Trudell
- Department of Anesthesia and Beckman Program for Molecular and Genetic Medicine, Stanford School of Medicine Stanford, CA, USA
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Kodavanti PRS, Royland JE, Moore-Smith DA, Besas J, Richards JE, Beasley TE, Evansky P, Bushnell PJ. Acute and subchronic toxicity of inhaled toluene in male Long-Evans rats: Oxidative stress markers in brain. Neurotoxicology 2015; 51:10-9. [PMID: 26343380 DOI: 10.1016/j.neuro.2015.09.001] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/18/2015] [Revised: 08/31/2015] [Accepted: 09/01/2015] [Indexed: 10/23/2022]
Abstract
The effects of exposure to volatile organic compounds (VOCs), which are of concern to the EPA, are poorly understood, in part because of insufficient characterization of how human exposure duration impacts VOC effects. Two inhalation studies with multiple endpoints, one acute and one subchronic, were conducted to seek effects of the VOC, toluene, in rats and to compare the effects between acute and subchronic exposures. Adult male Long-Evans rats were exposed to toluene vapor (n=6 per group) at a concentration of 0 or 1019 ± 14 ppm for 6h in the acute study and at 0 ± 0, 10 ± 1.4, 97 ± 7, or 995 ± 43 ppm for 6h/d, 5d/week for 13 weeks in the subchronic study. For the acute study, brains were dissected on ice within 30 min of the end of exposure, while for the subchronic study, brains were dissected 18 h after the last exposure. Frontal cortex, hippocampus, cerebellum, and striatum were assayed for a variety of oxidative stress (OS) parameters including total aconitase (TA), protein carbonyls, glutathione peroxidase (GPX), glutathione reductase (GRD), glutathione transferase (GST), γ-glutamylcysteine synthetase (GCS), superoxide dismutase (SOD), total antioxidants (TAS), NADPH quinone oxidoreductase-1 (NQO1), and NADH ubiquinone reductase (UBIQ-RD) activities using commercially available kits. Following acute exposure, UBIQ-RD, GCS and GRD were increased significantly only in the cerebellum, while TAS was increased in frontal cortex. On the other hand, subchronic exposure affected several OS markers including increases in NQO1 and UBIQ-RD. The effect of subchronic toluene exposure on SOD and TAS was greater in the striatum than in the other brain regions. TA activity (involved in maintaining iron homeostasis and an indicator of DNA damage) was inhibited in striatum and cerebellum, increased in hippocampus, and unchanged in frontal cortex. Protein carbonyls increased significantly in both the frontal cortex and cerebellum. In general, the results showed that acute exposure to toluene affected OS parameters to a lesser extent than did subchronic exposure. These results suggest that toluene exposure induces OS in the brain and this may be a component of an adverse outcome pathway for some of the neurotoxic effects reported following toluene exposure.
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Affiliation(s)
- Prasada Rao S Kodavanti
- Neurotoxicology Branch, NHEERL, ORD, U.S. Environmental Protection Agency, Research Triangle Park, NC, USA.
| | - Joyce E Royland
- Genetic and Cellular Toxicology Branch, NHEERL, ORD, U.S. Environmental Protection Agency, Research Triangle Park, NC, USA
| | - Debra A Moore-Smith
- Neurotoxicology Branch, NHEERL, ORD, U.S. Environmental Protection Agency, Research Triangle Park, NC, USA
| | - Jonathan Besas
- Neurotoxicology Branch, NHEERL, ORD, U.S. Environmental Protection Agency, Research Triangle Park, NC, USA
| | - Judy E Richards
- Cardiopulmonary and Immunotoxicology Branch, NHEERL, ORD, U.S. Environmental Protection Agency, Research Triangle Park, NC, USA
| | - Tracey E Beasley
- Neurotoxicology Branch, NHEERL, ORD, U.S. Environmental Protection Agency, Research Triangle Park, NC, USA
| | - Paul Evansky
- Inhalation Toxicology Facility, NHEERL, ORD, U.S. Environmental Protection Agency, Research Triangle Park, NC, USA
| | - Philip J Bushnell
- Neurotoxicology Branch, NHEERL, ORD, U.S. Environmental Protection Agency, Research Triangle Park, NC, USA
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Shelton KL, Nicholson KL. Benzodiazepine-like discriminative stimulus effects of toluene vapor. Eur J Pharmacol 2014; 720:131-7. [PMID: 24436974 DOI: 10.1016/j.ejphar.2013.10.036] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
In vitro studies show that the abused inhalant toluene affects a number of ligand-gated ion channels.The two most consistently implicated of these are γ-aminobutyric acid type A(GABAA) receptors which are positively modulated by toluene and N-methyl-D-aspartate(NMDA) receptors which are negatively modulated by toluene. Behavioral studies also suggest an interaction of toluene with GABAA and/or NMDA receptors but it is unclear if these receptors underlie the abuse-related intoxicating effects of toluene. Seventeen B6SJLF1/J mice were trained using a two-choice operant drug discrimination procedure to discriminate 10 min of exposure to 2000 ppm toluene vapor from 10 min of exposure to air. The discrimination was acquired in a mean of 65 training sessions. The stimulus effects of 2000 ppm toluene vapor were exposure concentration-dependent but rapidly diminished following the cessation of vapor exposure. The stimulus effects of toluene generalized to the chlorinated hydrocarbon vapor perchloroethylene but not 1,1,2-trichloroethane nor the volatile anesthetic isoflurane. The competitive NMDA antagonist CGS-19755, the uncompetitive antagonist dizocilpine and the glycine-site antagonist L701,324 all failed to substitute for toluene. The classical nonselective benzodiazepines midazolam and chlordiazepoxide produced toluene-like stimulus effects but the alpha 1 subunit preferring positive GABAA modulator zaleplon failed to substitute for toluene. The barbiturates pentobarbital and methohexital and the GABAA positive modulator neurosteroid allopregnanolone did not substitute for toluene. These data suggest that the stimulus effects of toluene may be at least partially mediated by benzodiazepine-like positive allosteric modulation of GABAA receptors containing alpha 2, 3 or 5 subunits.
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Shelton KL, Nicholson KL. Pharmacological classification of the abuse-related discriminative stimulus effects of trichloroethylene vapor. ACTA ACUST UNITED AC 2014; 3:235839. [PMID: 25202471 DOI: 10.4303/jdar/235839] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Inhalants are distinguished as a class primarily based upon a shared route of administration. Grouping inhalants according to their abuse-related in vivo pharmacological effects using the drug discrimination procedure has the potential to provide a more relevant classification scheme to the research and treatment community. Mice were trained to differentiate the introceptive effects of the trichloroethylene vapor from air using an operant procedure. Trichloroethylene is a chlorinated hydrocarbon solvent once used as an anesthetic as well as in glues and other consumer products. It is now primarily employed as a metal degreaser. We found that the stimulus effects of trichloroethylene were similar to those of other chlorinated hydrocarbon vapors, the aromatic hydrocarbon toluene and the vapor anesthetics methoxyflurane and isoflurane. The stimulus effects of trichloroethylene overlapped with those of the barbiturate methohexital, to a lesser extent the benzodiazepine midazolam and to ethanol. NMDA antagonists, the kappa opioid agonist U50,488 and the mixed 5-HT agonist mCPP largely failed to substitute for trichloroethylene. These data suggest that stimulus effects of chlorinated hydrocarbon vapors are mediated at least partially by GABAA receptor positive modulatory effects.
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Affiliation(s)
- Keith L Shelton
- Department of Pharmacology and Toxicology, Virginia Commonwealth University School of Medicine, P.O. Box 980613, Richmond, Virginia 23298-0613
| | - Katherine L Nicholson
- Department of Pharmacology and Toxicology, Virginia Commonwealth University School of Medicine, P.O. Box 980613, Richmond, Virginia 23298-0613
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Beckley JT, Woodward JJ. Volatile solvents as drugs of abuse: focus on the cortico-mesolimbic circuitry. Neuropsychopharmacology 2013; 38:2555-67. [PMID: 23954847 PMCID: PMC3828545 DOI: 10.1038/npp.2013.206] [Citation(s) in RCA: 36] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/12/2013] [Revised: 08/12/2013] [Accepted: 08/13/2013] [Indexed: 12/15/2022]
Abstract
Volatile solvents such as those found in fuels, paints, and thinners are found throughout the world and are used in a variety of industrial applications. However, these compounds are also often intentionally inhaled at high concentrations to produce intoxication. While solvent use has been recognized as a potential drug problem for many years, research on the sites and mechanisms of action of these compounds lags behind that of other drugs of abuse. In this review, we first discuss the epidemiology of voluntary solvent use throughout the world and then consider what is known about their basic pharmacology and how this may explain their use as drugs of abuse. We next present data from preclinical and clinical studies indicating that these substances induce common addiction sequelae such as dependence, withdrawal, and cognitive impairments. We describe how toluene, the most commonly studied psychoactive volatile solvent, alters synaptic transmission in key brain circuits such as the mesolimbic dopamine system and medial prefrontal cortex (mPFC) that are thought to underlie addiction pathology. Finally, we make the case that activity in mPFC circuits is a critical regulator of the mesolimbic dopamine system's ability to respond to volatile solvents like toluene. Overall, this review provides evidence that volatile solvents have high abuse liability because of their selective effects on critical nodes of the addiction neurocircuitry, and underscores the need for more research into how these compounds induce adaptations in neural circuits that underlie addiction pathology.
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Affiliation(s)
- Jacob T Beckley
- Department of Neurosciences, Medical University of South Carolina, Charleston, SC, USA,Center for Drug and Alcohol Programs, Department of Psychiatry/Neurosciences, Medical University of South Carolina, Charleston, SC, USA
| | - John J Woodward
- Department of Neurosciences, Medical University of South Carolina, Charleston, SC, USA,Center for Drug and Alcohol Programs, Department of Psychiatry/Neurosciences, Medical University of South Carolina, Charleston, SC, USA,Department of Neurosciences, Medical University of South Carolina, IOP 4 North, 67 President Street, MSC 861, Charleston, SC 29425, USA, Tel: +(843) 792 5225, Fax: +(843) 792 7353, E-mail:
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Spurny R, Billen B, Howard RJ, Brams M, Debaveye S, Price KL, Weston DA, Strelkov SV, Tytgat J, Bertrand S, Bertrand D, Lummis SCR, Ulens C. Multisite binding of a general anesthetic to the prokaryotic pentameric Erwinia chrysanthemi ligand-gated ion channel (ELIC). J Biol Chem 2013; 288:8355-8364. [PMID: 23364792 PMCID: PMC3605653 DOI: 10.1074/jbc.m112.424507] [Citation(s) in RCA: 86] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
Pentameric ligand-gated ion channels (pLGICs), such as nicotinic acetylcholine, glycine, γ-aminobutyric acid GABAA/C receptors, and the Gloeobacter violaceus ligand-gated ion channel (GLIC), are receptors that contain multiple allosteric binding sites for a variety of therapeutics, including general anesthetics. Here, we report the x-ray crystal structure of the Erwinia chrysanthemi ligand-gated ion channel (ELIC) in complex with a derivative of chloroform, which reveals important features of anesthetic recognition, involving multiple binding at three different sites. One site is located in the channel pore and equates with a noncompetitive inhibitor site found in many pLGICs. A second transmembrane site is novel and is located in the lower part of the transmembrane domain, at an interface formed between adjacent subunits. A third site is also novel and is located in the extracellular domain in a hydrophobic pocket between the β7–β10 strands. Together, these results extend our understanding of pLGIC modulation and reveal several specific binding interactions that may contribute to modulator recognition, further substantiating a multisite model of allosteric modulation in this family of ion channels.
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Affiliation(s)
- Radovan Spurny
- Laboratory of Structural Neurobiology, Department of Cellular and Molecular Medicine, KU Leuven, Herestraat 49, PB 601, B-3000 Leuven, Belgium
| | - Bert Billen
- Laboratory of Structural Neurobiology, Department of Cellular and Molecular Medicine, KU Leuven, Herestraat 49, PB 601, B-3000 Leuven, Belgium
| | - Rebecca J Howard
- Waggoner Center for Alcohol and Addiction Research, The University of Texas, Austin, Texas 78712
| | - Marijke Brams
- Laboratory of Structural Neurobiology, Department of Cellular and Molecular Medicine, KU Leuven, Herestraat 49, PB 601, B-3000 Leuven, Belgium
| | - Sarah Debaveye
- Laboratory of Structural Neurobiology, Department of Cellular and Molecular Medicine, KU Leuven, Herestraat 49, PB 601, B-3000 Leuven, Belgium
| | - Kerry L Price
- Department of Biochemistry, University of Cambridge, Tennis Court Road, Cambridge CB 1QW, United Kingdom
| | - David A Weston
- Department of Biochemistry, University of Cambridge, Tennis Court Road, Cambridge CB 1QW, United Kingdom
| | - Sergei V Strelkov
- Laboratory of Biocrystallography, KU Leuven, Herestraat 49, PB 822, B-3000 Leuven, Belgium
| | - Jan Tytgat
- Laboratory of Toxicology, KU Leuven, Herestraat 49, PB 922, B-3000 Leuven, Belgium
| | - Sonia Bertrand
- HiQScreen Sàrl, 15 rue de l'Athénée, Case Postale 209, CH-1211 Geneva 12, Switzerland
| | - Daniel Bertrand
- HiQScreen Sàrl, 15 rue de l'Athénée, Case Postale 209, CH-1211 Geneva 12, Switzerland
| | - Sarah C R Lummis
- Department of Biochemistry, University of Cambridge, Tennis Court Road, Cambridge CB 1QW, United Kingdom
| | - Chris Ulens
- Laboratory of Structural Neurobiology, Department of Cellular and Molecular Medicine, KU Leuven, Herestraat 49, PB 601, B-3000 Leuven, Belgium.
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Kirson D, Todorovic J, Mihic SJ. Positive allosteric modulators differentially affect full versus partial agonist activation of the glycine receptor. J Pharmacol Exp Ther 2012; 342:61-70. [PMID: 22473615 DOI: 10.1124/jpet.112.191486] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Taurine acts as a partial agonist at the glycine receptor (GlyR) in some brain regions such as the hippocampus, striatum, and nucleus accumbens. Ethanol, volatile anesthetics, and inhaled drugs of abuse are all known positive allosteric modulators of GlyRs, but their effects on taurine-activated GlyRs remain poorly understood, especially their effects on the high concentrations of taurine likely to be found after synaptic release. Two-electrode voltage-clamp electrophysiology in Xenopus laevis oocytes was used to compare the enhancing effects of ethanol, anesthetics, and inhalants on human homomeric α1-GlyR activated by saturating concentrations of glycine versus taurine. Allosteric modulators had negligible effects on glycine-activated GlyR while potentiating taurine-activated currents. In addition, inhaled anesthetics markedly enhanced desensitization rates of taurine- but not glycine-activated receptors. Our findings suggest that ethanol, volatile anesthetics, and inhalants differentially affect the time courses of synaptic events at GlyR, depending on whether the receptor is activated by a full or partial agonist.
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Affiliation(s)
- Dean Kirson
- Waggoner Center for Alcohol and Addiction Research, and Institute for Neuroscience, University of Texas at Austin, Austin, Texas, USA
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GABAA-positive modulator selective discriminative stimulus effects of 1,1,1-trichloroethane vapor. Drug Alcohol Depend 2012; 121:103-9. [PMID: 21924562 PMCID: PMC3257377 DOI: 10.1016/j.drugalcdep.2011.08.016] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/08/2011] [Revised: 08/17/2011] [Accepted: 08/18/2011] [Indexed: 11/22/2022]
Abstract
BACKGROUND The abuse-related behavioral effects of inhalant vapors are poorly understood but probably involve multiple neurotransmitter receptor mechanisms. The present study examined the receptor systems responsible for transducing the discriminative stimulus of the abused chlorinated hydrocarbon 1,1,1-trichloroethane (TCE) in mice. METHODS Thirty mice were trained to discriminate 10 min of 12,000 ppm TCE vapor exposure from air using an operant procedure. Substitution tests were then conduced with positive GABA(A) receptor modulators and/or NMDA receptor antagonists. RESULTS The nonselective benzodiazepines midazolam and diazepam produced 62% and 61% and the barbiturate pentobarbital produced 68% TCE-lever selection. Zaleplon, an alpha1 subunit-preferring positive GABA(A) receptor benzodiazepine-site positive modulator resulted in 29% TCE-lever selection. The direct extrasynaptic GABA(A) agonist gaboxodol (THIP) and the GABA reuptake inhibitor tiagabine failed to substitute for TCE. No substitution was elicited by a competitive (CGS-19755), noncompetitive (dizocilpine) or glycine-site (L701,324) NMDA antagonist. The mixed benzodiazepine/noncompetitive NMDA antagonist anesthetic Telazol and the anticonvulsant valproic acid exhibited low levels of partial substitution for TCE (38% and 39%, respectively). Ethanol and nitrous oxide failed to substitute for TCE. CONCLUSIONS The results suggest that the discriminative stimulus effects of TCE are fairly selectively mediated by positive modulation of GABA(A) receptors. The failure of gaboxadol to substitute and the poor substitution by zaleplon suggests that extrasynaptic GABA(A) receptors as well as GABA(A) receptors containing alpha1 subunits and are not involved in transducing the discriminative stimulus of TCE. Studies with additional GABA(A) benzodiazepine-site positive modulators will be necessary to confirm and extend these findings.
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van Thriel C, Westerink RHS, Beste C, Bale AS, Lein PJ, Leist M. Translating neurobehavioural endpoints of developmental neurotoxicity tests into in vitro assays and readouts. Neurotoxicology 2011; 33:911-24. [PMID: 22008243 DOI: 10.1016/j.neuro.2011.10.002] [Citation(s) in RCA: 68] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/29/2011] [Accepted: 10/04/2011] [Indexed: 12/11/2022]
Abstract
The developing nervous system is particularly vulnerable to chemical insults. Exposure to chemicals can result in neurobehavioural alterations, and these have been used as sensitive readouts to assess neurotoxicity in animals and man. Deconstructing neurobehaviour into relevant cellular and molecular components may allow for detection of specific neurotoxic effects in cell-based systems, which in turn may allow an easier examination of neurotoxic pathways and modes of actions and eventually inform the regulatory assessment of chemicals with potential developmental neurotoxicity. Here, current developments towards these goals are reviewed. Imaging genetics (CB) provides new insights into the neurobiological correlates of cognitive function that are being used to delineate neurotoxic mechanisms. The gaps between in vivo neurobehaviour and real-time in vitro measurements of neuronal function are being bridged by ex vivo measurements of synaptic plasticity (RW). An example of solvent neurotoxicity demonstrates how an in vivo neurological defect can be linked via the N-methyl-d-aspartate (NMDA)-glutamate receptor as a common target to in vitro readouts (AB). Axonal and dendritic morphology in vitro proved to be good correlates of neuronal connectivity and neurobehaviour in animals exposed to polychlorinated biphenyls and organophosphorus pesticides (PJL). Similarly, chemically induced changes in neuronal morphology affected the formation of neuronal networks on structured surfaces. Such network formation may become an important readout for developmental neurotoxicity in vitro (CvT), especially when combined with human neurons derived from embryonic stem cells (ML). We envision that future in vitro test systems for developmental neurotoxicity will combine the above approaches with exposure information, and we suggest a strategy for test system development and cell-based risk assessment.
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Affiliation(s)
- Christoph van Thriel
- Neurobehavioural Toxicology and Chemosensation, IfADo - Leibniz Research Centre for Working Environment and Human Factors, Dortmund, Germany.
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Hester SD, Johnstone AF, Boyes WK, Bushnell PJ, Shafer TJ. Acute toluene exposure alters expression of genes in the central nervous system associated with synaptic structure and function. Neurotoxicol Teratol 2011; 33:521-9. [DOI: 10.1016/j.ntt.2011.07.008] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/19/2011] [Revised: 07/07/2011] [Accepted: 07/20/2011] [Indexed: 10/17/2022]
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Functional prokaryotic-eukaryotic chimera from the pentameric ligand-gated ion channel family. Proc Natl Acad Sci U S A 2011; 108:12143-8. [PMID: 21730130 DOI: 10.1073/pnas.1104494108] [Citation(s) in RCA: 75] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Pentameric ligand-gated ion channels (pLGICs), which mediate chemo-electric signal transduction in animals, have been recently found in bacteria. Despite clear sequence and 3D structure homology, the phylogenetic distance between prokaryotic and eukaryotic homologs suggests significant structural divergences, especially at the interface between the extracellular (ECD) and the transmembrane (TMD) domains. To challenge this possibility, we constructed a chimera in which the ECD of the bacterial protein GLIC is fused to the TMD of the human α1 glycine receptor (α1GlyR). Electrophysiology in Xenopus oocytes shows that it functions as a proton-gated ion channel, thereby locating the proton activation site(s) of GLIC in its ECD. Patch-clamp experiments in BHK cells show that the ion channel displays an anionic selectivity with a unitary conductance identical to that of the α1GlyR. In addition, pharmacological investigations result in transmembrane allosteric modulation similar to the one observed on α1GlyR. Indeed, the clinically active drugs propofol, four volatile general anesthetics, alcohols, and ivermectin all potentiate the chimera while they inhibit GLIC. Collectively, this work shows the compatibility between GLIC and α1GlyR domains and points to conservation of the ion channel and transmembrane allosteric regulatory sites in the chimera. This provides evidence that GLIC and α1GlyR share a highly homologous 3D structure. GLIC is thus a relevant model of eukaryotic pLGICs, at least from the anionic type. In addition, the chimera is a good candidate for mass production in Escherichia coli, opening the way for investigations of "druggable" eukaryotic allosteric sites by X-ray crystallography.
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Chau PL. New insights into the molecular mechanisms of general anaesthetics. Br J Pharmacol 2010; 161:288-307. [PMID: 20735416 PMCID: PMC2989583 DOI: 10.1111/j.1476-5381.2010.00891.x] [Citation(s) in RCA: 75] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2010] [Revised: 04/27/2010] [Accepted: 04/30/2010] [Indexed: 01/22/2023] Open
Abstract
This paper provides new insights of how general anaesthetic research should be carried out in the future by an analysis of what we know, what we do not know and what we would like to know. I describe previous hypotheses on the mechanism of action of general anaesthetics (GAs) involving membranes and protein receptors. I provide the reasons why the GABA type A receptor, the NMDA receptor and the glycine receptor are strong candidates for the sites of action of GAs. I follow with a review on attempts to provide a mechanism of action, and how future research should be conducted with the help of physical and chemical methods.
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MESH Headings
- Anesthetics, General/adverse effects
- Anesthetics, General/chemistry
- Anesthetics, General/pharmacology
- Animals
- Biomedical Research/methods
- Biomedical Research/trends
- Brain/drug effects
- Brain/metabolism
- Humans
- Models, Molecular
- Molecular Structure
- Point Mutation
- Protein Binding
- Receptors, GABA-A/chemistry
- Receptors, GABA-A/genetics
- Receptors, GABA-A/metabolism
- Receptors, N-Methyl-D-Aspartate/chemistry
- Receptors, N-Methyl-D-Aspartate/genetics
- Receptors, N-Methyl-D-Aspartate/metabolism
- Stereoisomerism
- Structure-Activity Relationship
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Affiliation(s)
- P-L Chau
- Bioinformatique Structurale, CNRS URA 2185, Institut Pasteur, Paris, France.
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Shelton KL. Discriminative stimulus effects of inhaled 1,1,1-trichloroethane in mice: comparison to other hydrocarbon vapors and volatile anesthetics. Psychopharmacology (Berl) 2009; 203:431-40. [PMID: 18972104 PMCID: PMC2790320 DOI: 10.1007/s00213-008-1380-8] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/31/2008] [Accepted: 10/13/2008] [Indexed: 01/17/2023]
Abstract
RATIONALE Because the toxicity of many inhalants precludes evaluation in humans, drug discrimination, an animal model of subjective effects, can be used to gain insights on their poorly understood abuse-related effects. OBJECTIVES The purpose of the present study was to train a prototypic inhalant that has known abuse liability, 1,1,1-trichloroethane (TCE), as a discriminative stimulus in mice, and compare it to other classes of inhalants. MATERIALS AND METHODS Eight B6SJLF1/J mice were trained to discriminate 10 min of exposure to 12,000 ppm inhaled TCE vapor from air and seven mice were trained to discriminate 4,000 ppm TCE from air. Tests were then conducted to characterize the discriminative stimulus of TCE and to compare it to representative aromatic and chlorinated hydrocarbon vapors, volatile halogenated anesthetics as well as an odorant compound. RESULTS Only the 12,000 ppm TCE versus the air discrimination group exhibited sufficient discrimination accuracy for substitution testing. TCE vapor concentration- and exposure time-dependently substituted for the 12,000 ppm TCE vapor training stimulus. Full substitution was produced by trichloroethylene, toluene, enflurane, and sevoflurane. Varying degrees of partial substitution were produced by the other volatile test compounds. The odorant, 2-butanol, did not produce any substitution for TCE. CONCLUSIONS The discriminative stimulus effects of TCE are shared fully or partially by chlorinated and aromatic hydrocarbons as well as by halogenated volatile anesthetics. However, these compounds can be differentiated from TCE both quantitatively and qualitatively. It appears that the degree of similarity is not solely a function of chemical classification but may also be dependent upon the neurochemical effects of the individual compounds.
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Affiliation(s)
- Keith L. Shelton
- Department of Pharmacology and Toxicology, Virginia Commonwealth University School of Medicine, Richmond VA 23298-0613
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Himmel HM. Mechanisms Involved in Cardiac Sensitization by Volatile Anesthetics: General Applicability to Halogenated Hydrocarbons? Crit Rev Toxicol 2008; 38:773-803. [DOI: 10.1080/10408440802237664] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
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Zacny JP, Walker DJ, Drum M, Sells M. Choice of sevoflurane and its subjective and psychomotor effects in light and moderate drinkers. Drug Alcohol Depend 2008; 94:101-8. [PMID: 18077105 PMCID: PMC2288701 DOI: 10.1016/j.drugalcdep.2007.10.010] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/30/2007] [Revised: 10/18/2007] [Accepted: 10/18/2007] [Indexed: 11/30/2022]
Abstract
BACKGROUND Sevoflurane, an inhalant of the volatile anesthetic class, has neurobiological and behavioral effects in common with abused inhalants and ethanol. We sought to determine if choice for subanesthetic doses of sevoflurane, and its subjective and psychomotor effects, would differ as a function of alcohol-drinking status in healthy volunteers. METHODS The effects of four concentrations of sevoflurane (0, 0.2, 0.4, 0.8% sevoflurane in oxygen) were studied in 16 light drinkers and 16 moderate drinkers. During each of four sessions, subjects sampled a concentration of sevoflurane and 100% O(2) (placebo) for 10 min each. Subjective and psychomotor testing commenced 5 min into each sampling trial. Later, within the session, subjects chose nine times, once every 5 min, among sevoflurane (e.g., "Agent A"), placebo (e.g., "Agent B," 100% O(2)), or neither (and were administered 100% O(2), identified as "drug-free air"). RESULTS Choice for sevoflurane at the 0.4% concentration was significantly higher in the moderate drinkers than in the light drinkers. A number of subjective effects reported during inhalation of sevoflurane were markedly lower in the moderate-drinking group than in the light-drinking group. However, psychomotor impairment induced by sevoflurane was similar in magnitude in both groups. CONCLUSIONS Alcohol-drinking status affected sevoflurane choice. The results are consistent with several studies comparing light and heavier drinkers, using other drugs. Although both drinking groups were similarly impaired by sevoflurane, the moderate drinkers reported less of a subjective response than light drinkers, suggestive of cross-tolerance.
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Affiliation(s)
- James P. Zacny
- Department of Anesthesia and Critical Care The University of Chicago Chicago. Illinois USA
| | - Diana J. Walker
- Department of Anesthesia and Critical Care The University of Chicago Chicago. Illinois USA
| | - Melinda Drum
- Department of Health Studies The University of Chicago Chicago. Illinois USA
| | - Molly Sells
- Department of Anesthesia and Critical Care The University of Chicago Chicago. Illinois USA
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Milutinovic PS, Yang L, Cantor RS, Eger EI, Sonner JM. Anesthetic-like modulation of a gamma-aminobutyric acid type A, strychnine-sensitive glycine, and N-methyl-d-aspartate receptors by coreleased neurotransmitters. Anesth Analg 2007; 105:386-92. [PMID: 17646495 DOI: 10.1213/01.ane.0000267258.17197.7d] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023]
Abstract
INTRODUCTION A mechanism of anesthesia has recently been proposed which predicts that coreleased neurotransmitters may modulate neurotransmitter receptors for which they are not the native agonist in a manner similar to anesthetics. METHODS We tested this prediction by applying acetylcholine to a NR1/NR2A N-methyl-d-aspartate receptor, glycine to a wild-type alpha(1)beta(2) and anesthetic-resistant alpha(1)(S270I)beta(2) gamma-amino-butyric acid (GABA) type A receptor, and GABA to a homomeric alpha(1) wild type and anesthetic-resistant alpha(1) S267I glycine receptor. Receptors were expressed in Xenopus laevis oocytes and studied using two-electrode voltage clamping. RESULTS We found inhibition of N-methyl-d-aspartate receptor function by acetylcholine, enhancement of glycine receptor function by GABA, and enhancement of GABA type A receptor function by glycine. As expected of compounds with anesthetic activity, GABA showed far less potentiation (enhancement) of the function of the anesthetic-resistant S267I glycine receptor than that of the wild-type receptor. Glycine potentiated the function of wild-type GABA type A receptors but inhibited the function of the anesthetic-resistant S270I GABA type A receptor. CONCLUSIONS These results show that neurotransmitters that are coreleased onto anesthetic-sensitive receptors may modulate the function of receptors for which they are not the native agonist via an anesthetic-like mechanism. These findings lend support to a recent theory of anesthetic action.
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Urban BW, Bleckwenn M, Barann M. Interactions of anesthetics with their targets: non-specific, specific or both? Pharmacol Ther 2006; 111:729-70. [PMID: 16483665 DOI: 10.1016/j.pharmthera.2005.12.005] [Citation(s) in RCA: 63] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/23/2005] [Accepted: 12/23/2005] [Indexed: 01/11/2023]
Abstract
What makes a general anesthetic a general anesthetic? We shall review first what general anesthesia is all about and which drugs are being used as anesthetics. There is neither a unique definition of general anesthesia nor any consensus on how to measure it. Diverse drugs and combinations of drugs generate general anesthetic states of sometimes very different clinical quality. Yet the principal drugs are still considered to belong to the same class of 'general anesthetics'. Effective concentrations of inhalation anesthetics are in the high micromolar range and above, and even for intravenous anesthetics they do not go below the micromolar range. At these concentrations, many molecular and higher level targets are affected by inhalation anesthetics, fewer probably by intravenous anesthetics. The only physicochemical characteristic shared by anesthetics is the correlation of their anesthetic potencies with hydrophobicity. These correlations depend on the group of general anesthetics considered. In this review, anesthetic potencies for many different targets are plotted against octanol/water partition coefficients as measure of hydrophobicity. Qualitatively, similar correlations result, suggesting several but weak interactions with proteins as being characteristic of anesthetic actions. The polar interactions involved are weak, being roughly equal in magnitude to hydrophobic interactions. Generally, intravenous anesthetics are noticeably more potent than inhalation anesthetics. They differ considerably more between each other in their interactions with various targets than inhalation anesthetics do, making it difficult to come to a decision which of these should be used in future studies as representative 'prototypical general anesthetics'.
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Affiliation(s)
- Bernd W Urban
- Klinik für Anästhesiologie und Operative Intensivmedizin, Universitätsklinikum Bonn, Sigmund-Freud-Strasse 25, D-53127 Bonn, Germany.
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Roberts MT, Phelan R, Erlichman BS, Pillai RN, Ma L, Lopreato GF, Mihic SJ. Occupancy of a single anesthetic binding pocket is sufficient to enhance glycine receptor function. J Biol Chem 2005; 281:3305-11. [PMID: 16361257 DOI: 10.1074/jbc.m502000200] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
Alcohols and volatile anesthetics enhance the function of inhibitory glycine receptors (GlyRs). This is hypothesized to occur by their binding to a pocket formed between the transmembrane domains of individual alpha1 GlyR subunits. Because GlyRs are pentameric, it follows that each GlyR contains up to five alcohol/anesthetic binding sites, with one in each subunit. We asked how many subunits per pentamer need be bound by drug in order to enhance receptor-mediated currents. A cysteine mutation was introduced at amino acid serine 267 (S267C) in the transmembrane 2 domain as a tool to block GlyR potentiation by some anesthetic drugs and to provide a means for covalent binding by the small, anesthetic-like thiol reagent propyl methanethiosulfonate. Xenopus laevis oocytes were co-injected with various ratios of wild-type (wt) to S267C alpha1 GlyR cDNAs in order to express heteromeric receptors with a range of wt:mutant subunit stoichiometries. The enhancement of GlyR currents by 200 mm ethanol and 1.5 mm chloroform was positively correlated with the number of wt subunits found in heteromeric receptors. Furthermore, currents from oocytes injected with high ratios of wt to S267C cDNAs (up to 200:1) were significantly and irreversibly enhanced following propyl methanethiosulfonate labeling and washout, demonstrating that drug binding to a single subunit in the receptor pentamer is sufficient to induce enhancement of GlyR currents.
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Affiliation(s)
- Michael T Roberts
- Waggoner Center for Alcohol and Addiction Research, Section of Neurobiology, Institutes for Neuroscience and Cell and Molecular Biology, The University of Texas at Austin, Austin, Texas 78712
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Lobo IA, Harris RA. Sites of alcohol and volatile anesthetic action on glycine receptors. INTERNATIONAL REVIEW OF NEUROBIOLOGY 2005; 65:53-87. [PMID: 16140053 DOI: 10.1016/s0074-7742(04)65003-3] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/11/2023]
Affiliation(s)
- Ingrid A Lobo
- Institute for Cellular and Molecular Biology, Waggoner Center for Alcohol and Addiction Research, Section of Neurobiology, University of Texas at Austin, Austin, Texas 78712, USA
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Shafer TJ, Bushnell PJ, Benignus VA, Woodward JJ. Perturbation of Voltage-Sensitive Ca2+ Channel Function by Volatile Organic Solvents. J Pharmacol Exp Ther 2005; 315:1109-18. [PMID: 16109744 DOI: 10.1124/jpet.105.090027] [Citation(s) in RCA: 52] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The mechanisms underlying the acute neurophysiological and behavioral effects of volatile organic compounds (VOCs) remain to be elucidated. However, the function of neuronal ion channels is perturbed by VOCs. The present study examined effects of toluene (TOL), trichloroethylene (TCE), and perchloroethylene (PERC) on whole-cell calcium current (ICa) in nerve growth factor-differentiated pheochromocytoma (PC12) cells. All three VOCs affected ICa in a reversible, concentration-dependent manner. At +10-mV test potentials, VOCs inhibited ICa, whereas at test potentials of -20 and -10 mV, they potentiated it. The order of potency for inhibition (IC50) was PERC (270 microM) > TOL (720 microM) > TCE (1525 microM). VOCs also changed ICa inactivation kinetics from a single- to double-exponential function. Voltage-ramp experiments suggested that VOCs shifted ICa activation in a hyperpolarizing direction; this was confirmed by calculating the half-maximal voltage of activation (V1/2, act) in the absence and presence of VOCs using the Boltzman equation. V(1/2, act) was shifted from approximately -2 mV in control to -11, -12, and -16 mV by TOL, TCE, and PERC, respectively. Similarly, VOCs shifted the half-maximal voltage of steady-state inactivation (V1/2, inact) from approximately -16 mV in control to -32, -35, and -20 mV in the presence of TOL, TCE, and PERC, respectively. Inhibition of ICa by TOL was confirmed in primary cultures of cortical neurons, where 827 microM TOL inhibited current by 61%. These data demonstrate that VOCs perturb voltage-sensitive Ca2+ channel function in neurons, an effect that could contribute to the acute neurotoxicity of these compounds.
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Affiliation(s)
- Timothy J Shafer
- Neurotoxicology Division, MD-B105-05, NHEERL, ORD, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711, USA.
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Bushnell PJ, Shafer TJ, Bale AS, Boyes WK, Simmons JE, Eklund C, Jackson TL. Developing an exposure-dose-response model for the acute neurotoxicity of organic solvents: overview and progress on in vitro models and dosimetry. ENVIRONMENTAL TOXICOLOGY AND PHARMACOLOGY 2005; 19:607-614. [PMID: 21783533 DOI: 10.1016/j.etap.2004.12.026] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
We are developing an exposure-dose-response (EDR) model for volatile organic compounds (VOCs) to predict acute effects of VOCs on nervous system function from exposure data (concentration and duration of inhalation). This model contains both toxicokinetic and toxicodynamic components. One advantage of the EDR model will be its ability to relate in vitro effects of solvents on cellular ion channels (putative targets) to in vivo effects, using a combination of physiologically-based toxicokinetic (PBTK) modeling (to estimate VOC concentrations in the blood and brain) and in vitro studies to clarify the mode of action of the VOCs. Recent work in vitro has focused on quantifying the inhibitory effects of toluene, trichloroethylene (TCE) and perchloroethylene (PERC) on ion channel currents. All three VOCs inhibit current through voltage-sensitive calcium channels (VSCCs) in pheochromocytoma cells; PERC blocked calcium currents and altered the current-voltage relationship at lower concentrations than did toluene or TCE. Recombinant nicotinic acetylcholine receptors (nAChRs), expressed in Xenopus oocytes, were also inhibited by PERC and toluene in a concentration-dependent manner. PERC inhibited α7 receptors more than α4β2 receptors in recombinant human and rat nAChRs. However, human and rat α7 receptors were equally sensitive to PERC and TOL. These in vitro studies will be used to identify an appropriate neuronal receptor system to serve as an index of acute effects of VOCs in vivo. The PBTK model incorporates physiological input parameters derived from radiotelemetered heart rate data from rats performing operant tests of cognitive and motor functions. These studies should improve predictions of target organ concentrations of inhaled VOCs in subjects actively performing behavioral tests over a range of physical activity levels.
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Affiliation(s)
- Philip J Bushnell
- Neurotoxicology Division, National Health and Environmental Effects Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711, USA
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Walker DJ, Beckman NJ, Zacny JP. Reinforcing and subjective effects of the volatile anesthetic, sevoflurane. Drug Alcohol Depend 2004; 76:191-201. [PMID: 15488343 DOI: 10.1016/j.drugalcdep.2004.05.002] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/28/2004] [Revised: 05/03/2004] [Accepted: 05/04/2004] [Indexed: 11/28/2022]
Abstract
Sevoflurane is a volatile anesthetic that is chemically similar to volatile substances of abuse and can be safely administered to humans in laboratory research. In this study, the reinforcing and subjective effects of five concentrations of sevoflurane (0, 0.2, 0.4, 0.6, 0.8% sevoflurane in O2) were studied in 20 non-drug-abusers. During each of five sessions, subjects sampled a concentration of sevoflurane and 100% O2 (placebo) for 10 min each. Later, within the session, they chose nine times, once every 5 min, among sevoflurane (e.g. "Agent A"), placebo (e.g. "Agent B"), or neither (and were administered 100% O2, identified as "drug-free air"). Although "neither" was selected most frequently, mean preference ratios (sevoflurane choices/[sevoflurane choices+placebo choices]) and total sevoflurane choice peaked at the 0.4% concentration. Choice patterns varied across subjects, with some subjects never choosing sevoflurane and other subjects showing monotonic increasing or bitonic concentration-choice functions. Concentration-related increases in subjective effects were observed, including effects that are putatively associated with abuse liability. Ratings of drug liking and of wanting to inhale the drug again were positively correlated with sevoflurane choice. This study shows that sevoflurane can function as a reinforcer and produce abuse liability-related subjective effects in some healthy volunteers.
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Affiliation(s)
- Diana J Walker
- Department of Anesthesia and Critical Care, University of Chicago, 5841 S. Maryland Ave., MC 4028, Chicago, IL 60637, USA.
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Bale AS, Meacham CA, Benignus VA, Bushnell PJ, Shafer TJ. Volatile organic compounds inhibit human and rat neuronal nicotinic acetylcholine receptors expressed in Xenopus oocytes. Toxicol Appl Pharmacol 2004; 205:77-88. [PMID: 15885267 DOI: 10.1016/j.taap.2004.09.011] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/11/2004] [Revised: 09/17/2004] [Accepted: 09/21/2004] [Indexed: 01/16/2023]
Abstract
The relative sensitivity of rats and humans to volatile organic compounds (VOCs) such as toluene (TOL) and perchloroethylene (PERC) is unknown and adds to uncertainty in assessing risks for human exposures to VOCs. Recent studies have suggested that ion channels, including nicotinic acetylcholine receptors (nAChRs), are targets of TOL effects. However, studies comparing TOL effects on human and rat ligand-gated ion channels have not been conducted. To examine potential toxicodynamic differences between these species, the sensitivity of human and rat nAChRs to TOL was assessed. Since PERC has similar effects, in vivo, to TOL, effects of PERC on nAChR function were also examined. Two-electrode voltage-clamp techniques were utilized to measure acetylcholine-induced currents in neuronal nAChRs (alpha4beta2, alpha3beta2, and alpha7) expressed in Xenopus oocytes. PERC (0.065 mM) inhibited alpha7 nAChR currents by 60.1 +/- 4.0% (human, n = 7) and 40 +/- 3.5% (rat, n = 5), and inhibited alpha4beta2 nAChR currents by 42.0 +/- 5.2% (human, n = 6) and 52.2 +/- 5.5% (rat, n = 8). Likewise, alpha3beta2 nAChRs were significantly inhibited by 62.2 +/- 3.8% (human, n = 7) and 62.4 +/- 4.3% (rat, n = 8) in the presence of 0.065 mM PERC. TOL also inhibited both rat and human alpha7, alpha4beta2, and alpha3beta2 nAChRs. Statistical analysis indicated that although there was not a species (human vs. rat) difference with PERC (0.0015-0.065 mM) or TOL (0.03-0.9 mM) inhibition of alpha7, alpha4beta2, or alpha3beta2 nAChRs, all receptor types were more sensitive to PERC than TOL. These results demonstrate that human and rat nACh receptors represent a sensitive target for VOCs. This toxicodynamic information will help decrease the uncertainty associated with animal to human extrapolations in the risk assessment of VOCs.
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Affiliation(s)
- Ambuja S Bale
- Neurotoxicology Division, National Health and Environmental Effects Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711, USA
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Abstract
The glycine receptor chloride channel (GlyR) is a member of the nicotinic acetylcholine receptor family of ligand-gated ion channels. Functional receptors of this family comprise five subunits and are important targets for neuroactive drugs. The GlyR is best known for mediating inhibitory neurotransmission in the spinal cord and brain stem, although recent evidence suggests it may also have other physiological roles, including excitatory neurotransmission in embryonic neurons. To date, four alpha-subunits (alpha1 to alpha4) and one beta-subunit have been identified. The differential expression of subunits underlies a diversity in GlyR pharmacology. A developmental switch from alpha2 to alpha1beta is completed by around postnatal day 20 in the rat. The beta-subunit is responsible for anchoring GlyRs to the subsynaptic cytoskeleton via the cytoplasmic protein gephyrin. The last few years have seen a surge in interest in these receptors. Consequently, a wealth of information has recently emerged concerning GlyR molecular structure and function. Most of the information has been obtained from homomeric alpha1 GlyRs, with the roles of the other subunits receiving relatively little attention. Heritable mutations to human GlyR genes give rise to a rare neurological disorder, hyperekplexia (or startle disease). Similar syndromes also occur in other species. A rapidly growing list of compounds has been shown to exert potent modulatory effects on this receptor. Since GlyRs are involved in motor reflex circuits of the spinal cord and provide inhibitory synapses onto pain sensory neurons, these agents may provide lead compounds for the development of muscle relaxant and peripheral analgesic drugs.
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Affiliation(s)
- Joseph W Lynch
- School of Biomedical Sciences, Univ. of Queensland, Brisbane QLD 4072, Australia.
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Lu H, Xu TL. The general anesthetic pentobarbital slows desensitization and deactivation of the glycine receptor in the rat spinal dorsal horn neurons. J Biol Chem 2002; 277:41369-78. [PMID: 12196533 DOI: 10.1074/jbc.m206768200] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Although many general anesthetics have been found to produce anesthetic and analgesic effects by augmenting GABA(A) receptor (GABA(A)R) function, the role of the glycine receptor (GlyR) in this process is not fully understood at the neuronal level in the spinal cord. We investigated the effects of a barbiturate general anesthetic, pentobarbital (PB), on the glycinergic miniature inhibitory postsynaptic currents (mIPSCs) and the responses to exogenously applied glycine, or taurine, a low affinity GlyR agonist, by using the whole-cell patch-clamp technique in the rat spinal dorsal horn neurons isolated using a novel mechanical method. Bath application of 30 microm PB significantly prolonged the decay time constant of the spontaneous glycinergic mIPSC without changing its amplitude and frequency. Co-application of 0.3 mm PB reduced the peak amplitude, affected the macroscopic desensitization and deactivation of the response to externally applied Gly in a concentration-dependent manner. In addition, the recovery of Gly response from desensitization was also prolonged by PB. However, PB did not change the desensitization and deactivation kinetics of the taurine-induced response. The GABA(A)R antagonist bicuculline (10 microm) did not affect the effect of PB on the Gly response. Thus, PB prolonged the spinal glycinergic mIPSCs by slowing desensitization and deactivation of GlyR. Two other structurally different intravenous anesthetics, i.e. propofol (10 microm) and etomidate (3 microm), prolonged the duration of the glycinergic mIPSC in the rat spinal dorsal horn neurons. In conclusion, on GlyR-Cl(-) channel complexes there may exist action site(s) of intravenous general anesthetics. GlyR and glycinergic neurotransmission may play an important role in the modulation of general anesthesia in the mammalian spinal cord.
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Affiliation(s)
- Hui Lu
- Laboratory of Receptor Pharmacology, Department of Neurobiology and Biophysics, University of Science and Technology of China, Hefei 230027, People's Republic of China
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Wilkemeyer MF, Menkari CE, Charness ME. Novel antagonists of alcohol inhibition of l1-mediated cell adhesion: multiple mechanisms of action. Mol Pharmacol 2002; 62:1053-60. [PMID: 12391267 DOI: 10.1124/mol.62.5.1053] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
1-Octanol antagonizes ethanol inhibition of L1-mediated cell adhesion and prevents ethanol teratogenesis in mouse whole embryo culture. Herein, we identify a new series of alcohol antagonists and study their mechanism of action. Cell aggregation assays were carried out in ethanol-sensitive, human L1-transfected NIH/3T3 cells in the absence and presence of 100 mM ethanol or 2 mM 1-butanol and candidate antagonists. Antagonist potency for 1-alcohols increased progressively over 5 log orders from 1-pentanol (C5) to 1-dodecanol (C12). Antagonist potency declined from 1-dodecanol (C12) to 1-tridecanol (C13), and 1-tetradecanol (C14) and 1-pentadecanol (C15) were inactive. The presence and position of a double bond in the 1-butanol molecule determined whether a compound was a full agonist (1-butanol), a mixed agonist-antagonist (2-buten-1-ol), or an antagonist (3-buten-1-ol). Increasing the concentration of agonist (1-butanol or ethanol) overcame the antagonism of 3-buten-1-ol, benzyl alcohol, cyclopentanol, and 3-pentanol, but not that of 4-methyl-1-pentanol, 2-methyl-2-pentanol, 1-pentanol, 2-pentanol, 1-octanol, and 2,6-di-isopropylphenol (propofol), suggesting that the mechanisms of antagonism may differ between these groups of compounds. These findings suggest that selective straight, branched, and cyclic alcohols may act at multiple, discrete sites to antagonize the actions of ethanol and 1-butanol on L1-mediated cell-cell adhesion.
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Laube B, Maksay G, Schemm R, Betz H. Modulation of glycine receptor function: a novel approach for therapeutic intervention at inhibitory synapses? Trends Pharmacol Sci 2002; 23:519-27. [PMID: 12413807 DOI: 10.1016/s0165-6147(02)02138-7] [Citation(s) in RCA: 177] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
Transmitter-gated ion channels mediate rapid synaptic transmission in the CNS and constitute important targets for many neuroactive drugs. Inhibitory glycine receptors (GlyRs) are members of the nicotinic acetylcholine receptor superfamily and inhibit neuronal firing by opening Cl(-) channels following agonist binding. In this article, we discuss recent developments in GlyR pharmacology, delineate the receptor domains that are involved in binding of agonists and allosteric modulators, and present a molecular model of the extracellular architecture of the receptor. The recent discovery of compounds that act preferentially on specific GlyR isoforms and the differential expression of these isoforms in distinct regions of the developing and adult CNS show considerable promise towards the development of drugs that act in defined glycine-mediated pathways. In particular, compounds that can potentiate GlyR function should provide leads for novel muscle relaxants in addition to sedative and analgesic agents.
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Affiliation(s)
- Bodo Laube
- Dept. of Neurochemistry, Max-Planck-Institute for Brain Research, Deutschordenstrasse 46, 60528, Frankfurt, Germany
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Beckstead MJ, Phelan R, Trudell JR, Bianchini MJ, Mihic SJ. Anesthetic and ethanol effects on spontaneously opening glycine receptor channels. J Neurochem 2002; 82:1343-51. [PMID: 12354281 DOI: 10.1046/j.1471-4159.2002.01086.x] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
Strychnine-sensitive glycine receptors mediate inhibitory neurotransmission occurring in the brain stem and spinal cord. Alcohols, volatile anesthetics and inhaled drugs of abuse are positive allosteric modulators of glycine receptor function, normally enhancing function only in the presence of glycine. A complication in studying allosteric actions on ligand-gated ion channels is in the dissection of their effects on neurotransmitter binding from their effects on channel opening. Mutation of an aspartate residue at position 97 to arginine in the glycine receptor alpha1 subunit simulated the effects of glycine binding, producing receptors that exhibited tonic channel opening in the absence of neurotransmitter; i.e. these receptors demonstrated a dissociation of channel opening from neurotransmitter binding. In these receptors, ethanol, enflurane, chloroform, halothane, 1,1,1-trichloroethane and toluene elicited inward currents in the absence of glycine. We previously identified mutations on ligand-gated ion channels that eliminate ethanol, anesthetic and inhalant actions (such as S267I on alpha1 glycine receptors). The double mutant (D97R and S267I) receptors were both constitutively active and resistant to the enhancing effects of ethanol and enflurane. These data demonstrate that ethanol and volatile anesthetics can affect glycine receptor channel opening independently of their effects on enhancing neurotransmitter binding.
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Affiliation(s)
- Michael J Beckstead
- Department of Physiology and Pharmacology, Wake Forest University School of Medicine, Winston-Salem, North Carolina, USA
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Krasowski MD, Hong X, Hopfinger AJ, Harrison NL. 4D-QSAR analysis of a set of propofol analogues: mapping binding sites for an anesthetic phenol on the GABA(A) receptor. J Med Chem 2002; 45:3210-21. [PMID: 12109905 PMCID: PMC2864546 DOI: 10.1021/jm010461a] [Citation(s) in RCA: 81] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
A training set of 27 propofol (2,6-diisopropylphenol) analogues was used to construct four-dimensional (4D) quantitative structure-activity relationship (QSAR) models for three screens of biological activity: loss of righting reflex (LORR) in tadpoles, enhancement of agonist activity at the gamma-aminobutyric acid type A (GABA(A)) receptor, and direct (agonist-independent) activation of the receptor. The three resulting 4D-QSAR models are almost identical in form, and all suggest three key ligand-receptor interaction sites. The formation of an intermolecular hydrogen bond involving the proton of the ligand -OH group is the most important binding interaction. A hydrophobic pocket binding interaction involving the six-substituent is the second most significant binding site, and a similar hydrophobic pocket binding interaction near the two-substituent is the third postulated binding site from the 4D-QSAR models. A test set of eight compounds was used to evaluate the tadpole LORR 4D-QSAR model. Those compounds highly congeneric to the training set compounds were accurately predicted. However, compounds exploring substituent sites and/or electronic structures different from the training set were less well-predicted. Overall, the results show a striking similarity between the models of the sites responsible for anesthesia and those mediating effects of the training set of propofol analogues on the GABA(A) receptor; it follows that the GABA(A) receptor is therefore the likely site of propofol's anesthetic action.
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Affiliation(s)
- Matthew D Krasowski
- Department of Anesthesia and Critical Care, University of Chicago Medical Center, 5841 South Maryland Avenue, Chicago, IL 60637, USA
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Trudell JR, Bertaccini E. Molecular modelling of specific and non-specific anaesthetic interactions. Br J Anaesth 2002; 89:32-40. [PMID: 12173239 DOI: 10.1093/bja/aef157] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
There has been rapid progress in molecular modelling in recent years. The convergence of improved software for molecular mechanics and dynamics, techniques for chimeric substitution and site-directed mutations, and the first x-ray structures of transmembrane ion channels have made it possible to build and test models of anaesthetic binding sites. These models have served as guides for site-directed mutagenesis and as starting points for understanding the molecular dynamics of anaesthetic-site interactions. Ligand-gated ion channels are targets for inhaled anaesthetics and alcohols in the central nervous system. The inhibitory strychnine-sensitive glycine and gamma-aminobutyric acid type A receptors are positively modulated by anaesthetics and alcohols; site-directed mutagenesis techniques have identified amino acid residues important for the action of volatile anaesthetics and alcohols in these receptors. Key questions are whether these amino acid mutations form part of alcohol- or anaesthetic-binding sites or if they alter protein stability in a way that allows anaesthetic molecules to act remotely by non-specific mechanisms. It is likely that molecular modelling will play a major role in answering these questions.
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Affiliation(s)
- J R Trudell
- Department of Anaesthesia, Beckman Program for Molecular and Genetic Medicine, Stanford University, Stanford, CA 94305-5117, USA
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35
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Metzler DE, Metzler CM, Sauke DJ. Chemical Communication Between Cells. Biochemistry 2001. [DOI: 10.1016/b978-012492543-4/50033-7] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
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