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Iqbal F, Thompson AJ, Riaz S, Pehar M, Rice T, Syed NI. Anesthetics: from modes of action to unconsciousness and neurotoxicity. J Neurophysiol 2019; 122:760-787. [PMID: 31242059 DOI: 10.1152/jn.00210.2019] [Citation(s) in RCA: 23] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
Modern anesthetic compounds and advanced monitoring tools have revolutionized the field of medicine, allowing for complex surgical procedures to occur safely and effectively. Faster induction times and quicker recovery periods of current anesthetic agents have also helped reduce health care costs significantly. Moreover, extensive research has allowed for a better understanding of anesthetic modes of action, thus facilitating the development of more effective and safer compounds. Notwithstanding the realization that anesthetics are a prerequisite to all surgical procedures, evidence is emerging to support the notion that exposure of the developing brain to certain anesthetics may impact future brain development and function. Whereas the data in support of this postulate from human studies is equivocal, the vast majority of animal research strongly suggests that anesthetics are indeed cytotoxic at multiple brain structure and function levels. In this review, we first highlight various modes of anesthetic action and then debate the evidence of harm from both basic science and clinical studies perspectives. We present evidence from animal and human studies vis-à-vis the possible detrimental effects of anesthetic agents on both the young developing and the elderly aging brain while discussing potential ways to mitigate these effects. We hope that this review will, on the one hand, invoke debate vis-à-vis the evidence of anesthetic harm in young children and the elderly, and on the other hand, incentivize the search for better and less toxic anesthetic compounds.
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Affiliation(s)
- Fahad Iqbal
- Vi Riddell Pain Program, Alberta Children's Hospital Research Institute, Hotchkiss Brain Institute, Cumming School of Medicine, University of Calgary, Calgary, Alberta, Canada
| | - Andrew J Thompson
- Vi Riddell Pain Program, Alberta Children's Hospital Research Institute, Hotchkiss Brain Institute, Cumming School of Medicine, University of Calgary, Calgary, Alberta, Canada.,Department of Neuroscience, Faculty of Science, University of Calgary, Calgary, Alberta, Canada
| | - Saba Riaz
- Vi Riddell Pain Program, Alberta Children's Hospital Research Institute, Hotchkiss Brain Institute, Cumming School of Medicine, University of Calgary, Calgary, Alberta, Canada
| | - Marcus Pehar
- Vi Riddell Pain Program, Alberta Children's Hospital Research Institute, Hotchkiss Brain Institute, Cumming School of Medicine, University of Calgary, Calgary, Alberta, Canada
| | - Tiffany Rice
- Department of Anesthesiology, Perioperative and Pain Medicine, Alberta Children's Hospital, University of Calgary, Calgary, Alberta, Canada
| | - Naweed I Syed
- Vi Riddell Pain Program, Alberta Children's Hospital Research Institute, Hotchkiss Brain Institute, Cumming School of Medicine, University of Calgary, Calgary, Alberta, Canada
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2
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Inhibitory modulation of the cough reflex by acetylcholine in the caudal nucleus tractus solitarii of the rabbit. Respir Physiol Neurobiol 2018; 257:93-99. [PMID: 29369803 DOI: 10.1016/j.resp.2018.01.011] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/23/2017] [Revised: 01/08/2018] [Accepted: 01/15/2018] [Indexed: 12/16/2022]
Abstract
A cholinergic system has been described in the nucleus tractus solitarii (NTS). However, no information is available on the role played by acetylcholine (ACh) in the modulation of the cough reflex within the caudal NTS that has an important function in cough regulation. We addressed this issue making use of bilateral microinjections (30-50 nl) of 10 mM ACh combined with 5 mM physostigmine as well as of 10 mM mecamylamine or 10 mM scopolamine into the caudal NTS of pentobarbital sodium-anesthetized, spontaneously breathing rabbits. Microinjections of ACh/physostigmine caused depressant effects on the cough reflex induced by mechanical and chemical stimulation of the tracheobronchial tree. They also elicited transient increases in respiratory frequency and decreases in abdominal activity. These effects were prevented by scopolamine, but not by mecamylamine. The results show for the first time that ACh exerts an inhibitory modulation of the cough reflex through muscarinic receptors within the caudal NTS. They also may provide hints for novel antitussive approaches.
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Fourati Z, Ruza RR, Laverty D, Drège E, Delarue-Cochin S, Joseph D, Koehl P, Smart T, Delarue M. Barbiturates Bind in the GLIC Ion Channel Pore and Cause Inhibition by Stabilizing a Closed State. J Biol Chem 2016; 292:1550-1558. [PMID: 27986812 DOI: 10.1074/jbc.m116.766964] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/08/2016] [Revised: 12/06/2016] [Indexed: 12/12/2022] Open
Abstract
Barbiturates induce anesthesia by modulating the activity of anionic and cationic pentameric ligand-gated ion channels (pLGICs). Despite more than a century of use in clinical practice, the prototypic binding site for this class of drugs within pLGICs is yet to be described. In this study, we present the first X-ray structures of barbiturates bound to GLIC, a cationic prokaryotic pLGIC with excellent structural homology to other relevant channels sensitive to general anesthetics and, as shown here, to barbiturates, at clinically relevant concentrations. Several derivatives of barbiturates containing anomalous scatterers were synthesized, and these derivatives helped us unambiguously identify a unique barbiturate binding site within the central ion channel pore in a closed conformation. In addition, docking calculations around the observed binding site for all three states of the receptor, including a model of the desensitized state, showed that barbiturates preferentially stabilize the closed state. The identification of this pore binding site sheds light on the mechanism of barbiturate inhibition of cationic pLGICs and allows the rationalization of several structural and functional features previously observed for barbiturates.
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Affiliation(s)
- Zaineb Fourati
- From the Unité de Dynamique Structurale des Macromolécules, UMR 3528 du CNRS, Institut Pasteur, 75015 Paris, France
| | - Reinis Reinholds Ruza
- From the Unité de Dynamique Structurale des Macromolécules, UMR 3528 du CNRS, Institut Pasteur, 75015 Paris, France
| | - Duncan Laverty
- the Department of Neuroscience, Physiology and Pharmacology, University College London, London WC1E 6BT, United Kingdom
| | - Emmanuelle Drège
- the UMR 8076 du CNRS, BioCIS, Faculté de Pharmacie, Université Paris Sud, 92296 Chatenay-Malabry, France
| | - Sandrine Delarue-Cochin
- the UMR 8076 du CNRS, BioCIS, Faculté de Pharmacie, Université Paris Sud, 92296 Chatenay-Malabry, France
| | - Delphine Joseph
- the UMR 8076 du CNRS, BioCIS, Faculté de Pharmacie, Université Paris Sud, 92296 Chatenay-Malabry, France
| | - Patrice Koehl
- the Department of Computer Science, University of California, Davis, California 95616
| | - Trevor Smart
- the Department of Neuroscience, Physiology and Pharmacology, University College London, London WC1E 6BT, United Kingdom.
| | - Marc Delarue
- From the Unité de Dynamique Structurale des Macromolécules, UMR 3528 du CNRS, Institut Pasteur, 75015 Paris, France.
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Poliacek I, Rose MJ, Pitts TE, Mortensen A, Corrie LW, Davenport PW, Bolser DC. Central administration of nicotine suppresses tracheobronchial cough in anesthetized cats. J Appl Physiol (1985) 2014; 118:265-72. [PMID: 25477349 DOI: 10.1152/japplphysiol.00075.2014] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023] Open
Abstract
We tested the hypothesis that nicotine, which acts peripherally to promote coughing, might inhibit reflex cough at a central site. Nicotine was administered via the vertebral artery [intra-arterial (ia)] to the brain stem circulation and by microinjections into a restricted area of the caudal ventral respiratory column in 33 pentobarbital anesthetized, spontaneously breathing cats. The number of coughs induced by mechanical stimulation of the tracheobronchial airways; amplitudes of the diaphragm, abdominal muscle, and laryngeal muscles EMGs; and several temporal characteristics of cough were analyzed after administration of nicotine and compared with those during control and recovery period. (-)Nicotine (ia) reduced cough number, cough expiratory efforts, blood pressure, and heart rate in a dose-dependent manner. (-)Nicotine did not alter temporal characteristics of the cough motor pattern. Pretreatment with mecamylamine prevented the effect of (-)nicotine on blood pressure and heart rate, but did not block the antitussive action of this drug. (+)Nicotine was less potent than (-)nicotine for inhibition of cough. Microinjections of (-)nicotine into the caudal ventral respiratory column produced similar inhibitory effects on cough as administration of this isomer by the ia route. Mecamylamine microinjected in the region just before nicotine did not significantly reduce the cough suppressant effect of nicotine. Nicotinic acetylcholine receptors significantly modulate functions of brain stem and in particular caudal ventral respiratory column neurons involved in expression of the tracheobronchial cough reflex by a mecamylamine-insensitive mechanism.
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Affiliation(s)
- I Poliacek
- Department of Physiological Sciences, College of Veterinary Medicine, University of Florida, Gainesville, Florida; and Comenius University in Bratislava, Jessenius Faculty of Medicine in Martin, Institute of Medical Biophysics, Martin, Slovak Republic
| | - M J Rose
- Department of Physiological Sciences, College of Veterinary Medicine, University of Florida, Gainesville, Florida; and
| | - T E Pitts
- Department of Physiological Sciences, College of Veterinary Medicine, University of Florida, Gainesville, Florida; and
| | - A Mortensen
- Department of Physiological Sciences, College of Veterinary Medicine, University of Florida, Gainesville, Florida; and
| | - L W Corrie
- Department of Physiological Sciences, College of Veterinary Medicine, University of Florida, Gainesville, Florida; and
| | - P W Davenport
- Department of Physiological Sciences, College of Veterinary Medicine, University of Florida, Gainesville, Florida; and
| | - D C Bolser
- Department of Physiological Sciences, College of Veterinary Medicine, University of Florida, Gainesville, Florida; and
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Hamouda AK, Stewart DS, Chiara DC, Savechenkov PY, Bruzik KS, Cohen JB. Identifying barbiturate binding sites in a nicotinic acetylcholine receptor with [3H]allyl m-trifluoromethyldiazirine mephobarbital, a photoreactive barbiturate. Mol Pharmacol 2014; 85:735-46. [PMID: 24563544 DOI: 10.1124/mol.113.090985] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
At concentrations that produce anesthesia, many barbituric acid derivatives act as positive allosteric modulators of inhibitory GABAA receptors (GABAARs) and inhibitors of excitatory nicotinic acetylcholine receptors (nAChRs). Recent research on [(3)H]R-mTFD-MPAB ([(3)H]R-5-allyl-1-methyl-5-(m-trifluoromethyldiazirinylphenyl)barbituric acid), a photoreactive barbiturate that is a potent and stereoselective anesthetic and GABAAR potentiator, has identified a second class of intersubunit binding sites for general anesthetics in the α1β3γ2 GABAAR transmembrane domain. We now characterize mTFD-MPAB interactions with the Torpedo (muscle-type) nAChR. For nAChRs expressed in Xenopus oocytes, S- and R-mTFD-MPAB inhibited ACh-induced currents with IC50 values of 5 and 10 µM, respectively. Racemic mTFD-MPAB enhanced the equilibrium binding of [(3)H]ACh to nAChR-rich membranes (EC50 = 9 µM) and inhibited binding of the ion channel blocker [(3)H]tenocyclidine in the nAChR desensitized and resting states with IC50 values of 2 and 170 µM, respectively. Photoaffinity labeling identified two binding sites for [(3)H]R-mTFD-MPAB in the nAChR transmembrane domain: 1) a site within the ion channel, identified by photolabeling in the nAChR desensitized state of amino acids within the M2 helices of each nAChR subunit; and 2) a site at the γ-α subunit interface, identified by photolabeling of γMet299 within the γM3 helix at similar efficiency in the resting and desensitized states. These results establish that mTFD-MPAB is a potent nAChR inhibitor that binds in the ion channel preferentially in the desensitized state and binds with lower affinity to a site at the γ-α subunit interface where etomidate analogs bind that act as positive and negative nAChR modulators.
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Affiliation(s)
- Ayman K Hamouda
- Department of Neurobiology, Harvard Medical School, Boston, Massachusetts (A.K.H., D.C.C., J.B.C.); Department of Anesthesia, Critical Care, and Pain Medicine, Massachusetts General Hospital, Boston, Massachusetts (D.S.S.); and Department of Medicinal Chemistry and Pharmacognosy, University of Illinois at Chicago, Chicago, Illinois (P.Y.S., K.S.B.)
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Wang H, Zhang Y, Li ST. The effect of local anesthetics on the inhibition of adult muscle-type nicotinic acetylcholine receptors by nondepolarizing muscle relaxants. Eur J Pharmacol 2010; 630:29-33. [DOI: 10.1016/j.ejphar.2009.12.028] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/28/2009] [Revised: 12/03/2009] [Accepted: 12/15/2009] [Indexed: 10/20/2022]
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Arias HR, Bhumireddy P, Bouzat C. Molecular mechanisms and binding site locations for noncompetitive antagonists of nicotinic acetylcholine receptors. Int J Biochem Cell Biol 2006; 38:1254-76. [PMID: 16520081 DOI: 10.1016/j.biocel.2006.01.006] [Citation(s) in RCA: 101] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/27/2005] [Revised: 01/03/2006] [Accepted: 01/12/2006] [Indexed: 01/16/2023]
Abstract
Nicotinic acetylcholine receptors are pentameric proteins that belong to the Cys-loop receptor superfamily. Their essential mechanism of functioning is to couple neurotransmitter binding, which occurs at the extracellular domain, to the opening of the membrane-spanning cation channel. The function of these receptors can be modulated by structurally different compounds called noncompetitive antagonists. Noncompetitive antagonists may act at least by two different mechanisms: a steric and/or an allosteric mechanism. The simplest idea representing a steric mechanism is that the antagonist molecule physically blocks the ion channel. On the other hand, there exist distinct allosteric mechanisms. For example, noncompetitive antagonists may bind to the receptor and stabilize a nonconducting conformational state (e.g., resting or desensitized state), and/or increase the receptor desensitization rate. Barbiturates, dissociative anesthetics, antidepressants, and neurosteroids have been shown to inhibit nicotinic receptors by allosteric mechanisms and/or by open- and closed-channel blockade. Receptor modulation has proved to be highly complex for most noncompetitive antagonists. Noncompetitive antagonists may act by more than one mechanism and at distinct sites in the same receptor subtype. The binding site location for one particular molecule depends on the conformational state of the receptor. The mechanisms of action and binding affinities of noncompetitive antagonists differ among nicotinic receptor subtypes. Knowledge of the structure of the nicotinic acetylcholine receptor, the location of its noncompetitive antagonist binding sites, and the mechanisms of inhibition will aid the design of new and more efficacious drugs for treatment of neurological diseases.
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Affiliation(s)
- Hugo R Arias
- Department of Pharmaceutical Sciences, College of Pharmacy, Western University of Health Sciences, 309 E. Second Street, Pomona, CA 91766-1854, USA.
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Arias HR, Kem WR, Trudell JR, Blanton MP. Unique general anesthetic binding sites within distinct conformational states of the nicotinic acetylcholine receptor. INTERNATIONAL REVIEW OF NEUROBIOLOGY 2003; 54:1-50. [PMID: 12785284 DOI: 10.1016/s0074-7742(03)54002-8] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
General anesthesia is a complex behavioral state provoked by the pharmacological action of a broad range of structurally different hydrophobic molecules called general anesthetics (GAs) on receptor members of the genetically linked ligand-gated ion channel (LGIC) superfamily. This superfamily includes nicotinic acetylcholine (AChRs), type A and C gamma-aminobutyric acid (GABAAR and GABACR), glycine (GlyR), and type 3 5-hydroxytryptamine (5-HT3R) receptors. This review focuses on recent advances in the localization of GA binding sites on conformationally and compositionally distinct AChRs. The experimental evidence outlined in this review suggests that: 1. Several neuronal-type AChRs might be targets for the pharmacological action of distinct GAs. 2. The molecular components of a specific GA binding site on a certain receptor subtype are different from the structural determinants of the locus for the same GA on a different receptor subtype. 3. There are unique binding sites for distinct GAs in the same receptor protein. 4. A GA can activate, potentiate, or inhibit an ion channel, indicating the existence of more than one binding site for the same GA. 5. The affinity of a specific GA depends on the conformational state of the receptor. 6. GAs inhibition channels by at least two mechanisms, an open-channel-blocking and/or an allosteric mechanism. 7. Certain GAs may inhibit AChR function by competing for the agonist binding sites or by augmenting the desensitization rate.
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Affiliation(s)
- Hugo R Arias
- Department of Pharmaceutical Sciences, College of Pharmacy, Western University of Health Sciences, Pomona, California 91766, USA
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9
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Arneric S. Cholinergic Approaches to Pain Therapy. Pain 2003. [DOI: 10.1201/9780203911259.ch61] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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10
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Papke RL. Enhanced inhibition of a mutant neuronal nicotinic acetylcholine receptor by agonists: protection of function by (E)-N-methyl-4-(3-pyridinyl)-3-butene-1-amine (TC-2403). J Pharmacol Exp Ther 2002; 301:765-73. [PMID: 11961083 DOI: 10.1124/jpet.301.2.765] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Inhibition of neuronal nicotinic receptors can be regulated by sequence in the beta subunit second transmembrane domain (TM2). The incorporation of a beta4(6'F10'T) subunit, which contains sequence from the muscle beta subunit at the TM2 6' and 10' positions of the neuronal beta4 subunit, increases the loss of receptor responsiveness after the application of acetylcholine (ACh), nicotine, or 3-(2,4-dimethoxybenzylidene)-anabaseine (DMXB), an alpha7-selective partial agonist. Inhibition of receptor responsiveness following agonist exposure may occur through either an enhancement of desensitization, increased channel block by an agonist, or alternatively via allosteric modulation. Although DMXB produces very little activation of either alpha3beta4 or alpha3beta4(6'F10'T) receptors, DMXB shows an enhanced use-and voltage-dependent inhibition of alpha3beta4(6'F10'T) receptors compared with wild-type. In contrast, the alpha4beta2-selective agonist (E)-N-methyl-4-(3-pyridinyl)-3-butene-1-amine (TC-2403, previously identified as RJR-2403) shows increased activation of alpha3beta4(6'F10'T) receptors compared with alpha3beta4 receptors (as related to ACh activation) but with no significant increase in antagonist activity. The interaction between the binding of local anesthetics and the functional inhibition produced by these agonists was evaluated. The binding of the local anesthetics to their inhibitory sites does not affect inhibitory effects of DMXB and nicotine. However, TC-2403 can protect receptor function from the inhibitory effects of other agonists, suggesting that TC-2403, as well as agonists that cause inhibition, may be binding to an allosteric site, either promoting or inhibiting channel opening. The ability of TC-2403 to protect receptor function from agonist-induced inhibition may point toward valuable new combination drug therapies.
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Affiliation(s)
- Roger L Papke
- Department of Pharmacology and Therapeutics, University of Florida, Gainesville, Florida 32610-0267, USA.
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Nishizawa N, Shirasaki T, Nakao S, Matsuda H, Shingu K. The Inhibition of the N-Methyl-d-Aspartate Receptor Channel by Local Anesthetics in Mouse CA1 Pyramidal Neurons. Anesth Analg 2002. [DOI: 10.1213/00000539-200202000-00017] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
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12
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Nishizawa N, Shirasaki T, Nakao S, Matsuda H, Shingu K. The inhibition of the N-methyl-D-aspartate receptor channel by local anesthetics in mouse CA1 pyramidal neurons. Anesth Analg 2002; 94:325-30, table of contents. [PMID: 11812692 DOI: 10.1097/00000539-200202000-00017] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Abstract
UNLABELLED Although the effects of local anesthetics on sodium channels and various other channels and receptors have been intensively investigated, there is little information available about their effects on N-methyl-D-aspartate (NMDA) receptors. We examined the effects of four local anesthetics (procaine, tetracaine, bupivacaine, and lidocaine) on NMDA-induced currents by using a whole-cell patch-clamp technique in dissociated mouse hippocampal pyramidal neurons. Procaine and tetracaine produced a reversible and concentration-dependent inhibition of NMDA-induced currents, but lidocaine showed little inhibition at 1 mM or less. The half-maximal inhibition values (mM; mean +/- SEM) for procaine, tetracaine, bupivacaine, and lidocaine at -60 mV were 0.296 +/- 0.031, 0.637 +/- 0.044, 2.781 +/- 0.940 (extrapolated data), and 7.766 +/- 14.093 (extrapolated data), respectively. Procaine 0.2 mM reduced the maximal NMDA-induced currents without affecting the 50% effective concentration values for NMDA. The inhibition by procaine exhibited voltage dependence and was more effective at negative potentials. These results indicate a noncompetitive antagonism of procaine on NMDA receptors and suggest that the inhibition is the result of a channel-blocking mechanism. IMPLICATIONS We examined the effects of four local anesthetics (procaine, tetracaine, bupivacaine, and lidocaine) on NMDA-induced currents by using a whole-cell patch-clamp technique in dissociated mouse hippocampal pyramidal neurons. Both procaine and tetracaine produced a reversible and concentration-dependent inhibition of the NMDA-induced currents.
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Affiliation(s)
- Nobuyasu Nishizawa
- Department of Anesthesiology, Kansai Medical University, Moriguchi-City, Osaka, Japan
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14
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Dilger JP, Boguslavsky R, Barann M, Katz T, Vidal AM. Mechanisms of barbiturate inhibition of acetylcholine receptor channels. J Gen Physiol 1997; 109:401-14. [PMID: 9089445 PMCID: PMC2217072 DOI: 10.1085/jgp.109.3.401] [Citation(s) in RCA: 55] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
We used patch clamp techniques to study the inhibitory effects of pentobarbital and barbital on nicotinic acetylcholine receptor channels from BC3H-1 cells. Single channel recording from outside-out patches reveals that both drugs cause acetylcholine-activated channel events to occur in bursts. The mean duration of gaps within bursts in 2 ms for 0.1 mM pentobarbital and 0.05 ms for 1 mM barbital. In addition, 1 mM barbital reduces the apparent single channel current by 15%. Both barbiturates decrease the duration of openings within a burst but have only a small effect on the burst duration. Macroscopic currents were activated by rapid perfusion of 300 microM acetylcholine to outside-out patches. The concentration dependence of peak current inhibition was fit with a Hill function; for pentobarbital, Ki = 32 microM, n = 1.09; for barbital, Ki = 1900 microM, n = 1.24. Inhibition is voltage independent. The kinetics of inhibition by pentobarbital are at least 30 times faster than inhibition by barbital (3 ms vs. < 0.1 ms at the Ki). Pentobarbital binds > or = 10-fold more tightly to open channels than to closed channels; we could not determine whether the binding of barbital is state dependent. Experiments performed with both barbiturates reveal that they do not compete for a single binding site on the acetylcholine receptor channel protein, but the binding of one barbiturate destabilizes the binding of the other. These results support a kinetic model in which barbiturates bind to both open and closed states of the AChR and block the flow of ions through the channel. An additional, lower-affinity binding site for pentobarbital may explain the effects seen at > 100 microM pentobarbital.
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Affiliation(s)
- J P Dilger
- Department of Anesthesiology, University at Stony Brook, New York 11794-8480, USA.
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Yost CS, Winegar BD. Potency of agonists and competitive antagonists on adult- and fetal-type nicotinic acetylcholine receptors. Cell Mol Neurobiol 1997; 17:35-50. [PMID: 9118208 DOI: 10.1023/a:1026325020191] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
1. The potency of agonists and competitive antagonists on the two expressed forms of the nicotinic acetylcholine receptor (adult or junctional subtype, epsilon-AChR; fetal or extrajunctional subtype, gamma-AChR) have not previously been compared systematically in homogeneous receptor preparations. 2. Each subtype of the receptor was expressed separately in Xenopus oocytes by cytoplasmic injection of combinations of RNA transcribed in vitro. The presence of each type of receptor was confirmed by single-channel recordings. Expressing oocytes were assayed using discontinuous, single-electrode voltage clamp by measuring peak currents in response to test compounds. 3. The extrajunctional subtype was more potently activated by the nicotinic agonist dimethylphenyl piperazinium iodide (DMPP) than was the junctional form. There was no statistically significant difference in potency between the two subtypes for other nicotinic agonists (nicotine, cytisine and succinylcholine). The rank order of potency for epsilon-AChR was succinylcholine > cytisine > DMPP > nicotine, and that for gamma-AChR was DMPP > cytisine > succinylcholine > nicotine. 4. Two agonists (cytisine and succinylcholine) displayed six- to eight-fold greater intrinsic activity in activating epsilon-AChR over gamma-AChR. There was no difference between the two forms of receptor in efficacy for nicotine. 5. The extrajunctional form was much more potently inhibited by the steroidal competitive antagonist pancuronium than was the junctional receptor. However, there was no significant difference in potency of inhibition by the curariform drug atracurium. 6. Contrary to previous reports, there is no consistent relation between the effect of agonists and antagonists and the subtype of receptor. These data suggest that the resistance or sensitivity to these agents seen in various clinical settings are related to other cellular factors.
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Affiliation(s)
- C S Yost
- Department of Anesthesia, University of California, San Francisco 94143-0648, USA
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Brioni JD, Decker MW, Sullivan JP, Arneric SP. The pharmacology of (-)-nicotine and novel cholinergic channel modulators. ADVANCES IN PHARMACOLOGY (SAN DIEGO, CALIF.) 1996; 37:153-214. [PMID: 8891102 DOI: 10.1016/s1054-3589(08)60950-3] [Citation(s) in RCA: 100] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
Advances in the understanding of the molecular biology and pharmacology of nAChRs may provide targets for the development of novel and selective modulators of nAChRs in the brain. This contention is supported by the dissimilar behavioral effects observed following systemic administration of currently available nicotinic ligands. The concept of multiple subtypes of nAChRs is not unique, as evidenced by the pharmacology of other ligand-gated ion channels, such as GABA-A receptor, which also exist in multiple subtypes. At present, with respect to the nAChRs, relatively few of the subtypes identified have been cloned from human tissue and pharmacologically evaluated, but several groups are focusing their research efforts in this direction. With a thorough understanding of the pharmacological and functional characteristics of more of the putative human nAChR subtypes, this knowledge will facilitate the discovery of more efficacious and less toxic ChCMs that may provide potential novel therapeutic agents for a variety of CNS conditions.
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Affiliation(s)
- J D Brioni
- Abbott Laboratories, Abbott Park, Illinois 60064, USA
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