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γ-Aminobutyric Acid (GABA): Biosynthesis, Role, Commercial Production, and Applications. STUDIES IN NATURAL PRODUCTS CHEMISTRY 2018. [DOI: 10.1016/b978-0-444-64057-4.00013-2] [Citation(s) in RCA: 42] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/24/2022]
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Borghese CM, Ruiz CI, Lee US, Cullins MA, Bertaccini EJ, Trudell JR, Harris RA. Identification of an Inhibitory Alcohol Binding Site in GABAA ρ1 Receptors. ACS Chem Neurosci 2016; 7:100-8. [PMID: 26571107 DOI: 10.1021/acschemneuro.5b00246] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
Alcohols inhibit γ-aminobutyric acid type A ρ1 receptor function. After introducing mutations in several positions of the second transmembrane helix in ρ1, we studied the effects of ethanol and hexanol on GABA responses using two-electrode voltage clamp electrophysiology in Xenopus laevis oocytes. The 6' mutations produced the following effects on ethanol and hexanol responses: small increase or no change (T6'M), increased inhibition (T6'V), and small potentiation (T6'Y and T6'F). The 5' mutations produced mainly increases in hexanol inhibition. Other mutations produced small (3' and 9') or no changes (2' and L277 in the first transmembrane domain) in alcohol effects. These results suggest an inhibitory alcohol binding site near the 6' position. Homology models of ρ1 receptors based on the X-ray structure of GluCl showed that the 2', 5', 6', and 9' residues were easily accessible from the ion pore, with 5' and 6' residues from neighboring subunits facing each other; L3' and L277 also faced the neighboring subunit. We tested ethanol through octanol on single and double mutated ρ1 receptors [ρ1(I15'S), ρ1(T6'Y), and ρ1(T6'Y,I15'S)] to further characterize the inhibitory alcohol pocket in the wild-type ρ1 receptor. The pocket can only bind relatively short-chain alcohols and is eliminated by introducing Y in the 6' position. Replacing the bulky 15' residue with a smaller side chain introduced a potentiating binding site, more sensitive to long-chain than to short-chain alcohols. In conclusion, the net alcohol effect on the ρ1 receptor is determined by the sum of its actions on inhibitory and potentiating sites.
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Affiliation(s)
- Cecilia M. Borghese
- Waggoner
Center for Alcohol and Addiction Research, The University of Texas at Austin, Austin, Texas 78712, United States
| | - Carlos I. Ruiz
- Waggoner
Center for Alcohol and Addiction Research, The University of Texas at Austin, Austin, Texas 78712, United States
| | - Ui S. Lee
- Waggoner
Center for Alcohol and Addiction Research, The University of Texas at Austin, Austin, Texas 78712, United States
| | - Madeline A. Cullins
- Waggoner
Center for Alcohol and Addiction Research, The University of Texas at Austin, Austin, Texas 78712, United States
| | - Edward J. Bertaccini
- Department of Anesthesia & Beckman Program for Molecular and Genetic Medicine, Stanford University, Palo Alto, California 94305, United States
| | - James R. Trudell
- Department of Anesthesia & Beckman Program for Molecular and Genetic Medicine, Stanford University, Palo Alto, California 94305, United States
| | - R. Adron Harris
- Waggoner
Center for Alcohol and Addiction Research, The University of Texas at Austin, Austin, Texas 78712, United States
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Huang SH, Lewis TM, Lummis SC, Thompson AJ, Chebib M, Johnston GA, Duke RK. Mixed antagonistic effects of the ginkgolides at recombinant human ρ1 GABAC receptors. Neuropharmacology 2012; 63:1127-39. [PMID: 22828636 PMCID: PMC3465557 DOI: 10.1016/j.neuropharm.2012.06.067] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/11/2011] [Revised: 06/18/2012] [Accepted: 06/24/2012] [Indexed: 11/26/2022]
Abstract
The diterpene lactones of Ginkgo biloba, ginkgolides A, B and C are antagonists at a range of Cys-loop receptors. This study examined the effects of the ginkgolides at recombinant human ρ(1) GABA(C) receptors expressed in Xenopus oocytes using two-electrode voltage clamp. The ginkgolides were moderately potent antagonists with IC(50)s in the μM range. At 10 μM, 30 μM and 100 μM, the ginkgolides caused rightward shifts of GABA dose-response curves and reduced maximal GABA responses, characteristic of noncompetitive antagonists, while the potencies showed a clear dependence on GABA concentration, indicating apparent competitive antagonism. This suggests that the ginkgolides exert a mixed-type antagonism at the ρ(1) GABA(C) receptors. The ginkgolides did not exhibit any obvious use-dependent inhibition. Fitting of the data to a number of kinetic schemes suggests an allosteric inhibition as a possible mechanism of action of the ginkgolides which accounts for their inhibition of the responses without channel block or use-dependent inhibition. Kinetic modelling predicts that the ginkgolides exhibit saturation of antagonism at high concentrations of GABA, but this was only partially observed for ginkgolide B. It also suggests that there may be different binding sites in the closed and open states of the receptor, with a higher affinity for the receptor in the closed state.
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Affiliation(s)
- Shelley H. Huang
- Discipline of Pharmacology, School of Medical Sciences, Faculty of Medicine, University of Sydney, Australia
| | - Trevor M. Lewis
- School of Medical Sciences, University of New South Wales, Australia
| | - Sarah C.R. Lummis
- Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom
| | - Andrew J. Thompson
- Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom
| | - Mary Chebib
- Faculty of Pharmacy, University of Sydney, Australia
| | - Graham A.R. Johnston
- Discipline of Pharmacology, School of Medical Sciences, Faculty of Medicine, University of Sydney, Australia
| | - Rujee K. Duke
- Discipline of Pharmacology, School of Medical Sciences, Faculty of Medicine, University of Sydney, Australia
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Jones SM, Palmer MJ. Pharmacological analysis of the activation and receptor properties of the tonic GABA(C)R current in retinal bipolar cell terminals. PLoS One 2011; 6:e24892. [PMID: 21949779 PMCID: PMC3174224 DOI: 10.1371/journal.pone.0024892] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/08/2011] [Accepted: 08/22/2011] [Indexed: 11/29/2022] Open
Abstract
GABAergic inhibition in the central nervous system (CNS) can occur via rapid, transient postsynaptic currents and via a tonic increase in membrane conductance, mediated by synaptic and extrasynaptic GABAA receptors (GABAARs) respectively. Retinal bipolar cells (BCs) exhibit a tonic current mediated by GABACRs in their axon terminal, in addition to synaptic GABAAR and GABACR currents, which strongly regulate BC output. The tonic GABACR current in BC terminals (BCTs) is not dependent on vesicular GABA release, but properties such as the alternative source of GABA and the identity of the GABACRs remain unknown. Following a recent report that tonic GABA release from cerebellar glial cells is mediated by Bestrophin 1 anion channels, we have investigated their role in non-vesicular GABA release in the retina. Using patch-clamp recordings from BCTs in goldfish retinal slices, we find that the tonic GABACR current is not reduced by the anion channel inhibitors NPPB or flufenamic acid but is reduced by DIDS, which decreases the tonic current without directly affecting GABACRs. All three drugs also exhibit non-specific effects including inhibition of GABA transporters. GABACR ρ subunits can form homomeric and heteromeric receptors that differ in their properties, but BC GABACRs are thought to be ρ1-ρ2 heteromers. To investigate whether GABACRs mediating tonic and synaptic currents may differ in their subunit composition, as is the case for GABAARs, we have examined the effects of two antagonists that show partial ρ subunit selectivity: picrotoxin and cyclothiazide. Tonic and synaptic GABACR currents were differentially affected by both drugs, suggesting that a population of homomeric ρ1 receptors contributes to the tonic current. These results extend our understanding of the multiple forms of GABAergic inhibition that exist in the CNS and contribute to visual signal processing in the retina.
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Affiliation(s)
- Stefanie M. Jones
- Neuroscience Group, Institute for Science and Technology in Medicine, Keele University, Keele, United Kingdom
| | - Mary J. Palmer
- Neuroscience Group, Institute for Science and Technology in Medicine, Keele University, Keele, United Kingdom
- * E-mail:
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Martínez-Delgado G, Estrada-Mondragón A, Miledi R, Martínez-Torres A. An Update on GABAρ Receptors. Curr Neuropharmacol 2011; 8:422-33. [PMID: 21629448 PMCID: PMC3080597 DOI: 10.2174/157015910793358141] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/12/2009] [Revised: 04/08/2010] [Accepted: 06/21/2010] [Indexed: 01/29/2023] Open
Abstract
The present review discusses the functional and molecular diversity of GABAρ receptors. These receptors were originally described in the mammalian retina, and their functional role in the visual pathway has been recently elucidated; however new studies on their distribution in the brain and spinal cord have revealed that they are more spread than originally thought, and thus it will be important to determine their physiological contribution to the GABAergic transmission in other areas of the central nervous system. In addition, molecular modeling has revealed peculiar traits of these receptors that have impacted on the interpretations of the latest pharmacolgical and biophysical findings. Finally, sequencing of several vertebrate genomes has permitted a comparative analysis of the organization of the GABAρ genes.
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Affiliation(s)
- Gustavo Martínez-Delgado
- Instituto de Neurbiología, Departamento de Neurobiología Celular y Molecular, Laboratorio D15, Campus UNAM Juriquilla. Querétaro 76230, México
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Jones SM, Palmer MJ. Activation of the tonic GABAC receptor current in retinal bipolar cell terminals by nonvesicular GABA release. J Neurophysiol 2009; 102:691-9. [PMID: 19494193 DOI: 10.1152/jn.00285.2009] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Within the second synaptic layer of the retina, bipolar cell (BC) output to ganglion cells is regulated by inhibitory input to BC axon terminals. GABA(A) receptors (GABA(A)Rs) mediate rapid synaptic currents in BC terminals, whereas GABA(C) receptors (GABA(C)Rs) mediate slow evoked currents and a tonic current, which is strongly regulated by GAT-1 GABA transporters. We have used voltage-clamp recordings from BC terminals in goldfish retinal slices to determine the source of GABA for activation of these currents. Inhibition of vesicular release with concanamycin A or tetanus toxin significantly inhibited GABA(A)R inhibitory postsynaptic currents and glutamate-evoked GABA(A)R and GABA(C)R currents but did not reduce the tonic GABA(C)R current, which was also not dependent on extracellular Ca(2+). The tonic current was strongly potentiated by inhibition of GABA transaminase, under both normal and Ca(2+)-free conditions, and was activated by exogenous taurine; however inhibition of taurine transport had little effect. The tonic current was unaffected by GAT-2/3 inhibition and was potentiated by GAT-1 inhibition even in the absence of vesicular release, indicating that it is unlikely to be evoked by reversal of GABA transporters or by ambient GABA. In addition, GABA release does not appear to occur via hemichannels or P2X(7) receptors. BC terminals therefore exhibit two forms of GABA(C)R-mediated inhibition, activated by vesicular and by nonvesicular GABA release, which are likely to have distinct functions in visual signal processing. The tonic GABA(C)R current in BC terminals exhibits similar properties to tonic GABA(A)R and glutamate receptor currents in the brain.
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Affiliation(s)
- S M Jones
- Neuroscience Group, Institute for Science and Technology in Medicine, Keele University, Keele, Staffordshire ST5 5BG, United Kingdom
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Xie A, Song X, Ripps H, Qian H. Cyclothiazide: a subunit-specific inhibitor of GABAC receptors. J Physiol 2008; 586:2743-52. [PMID: 18420703 DOI: 10.1113/jphysiol.2008.153346] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
We tested the effects of cyclothiazide (CTZ), an agent used to block desensitization of AMPA-type glutamate receptors, on heterologously expressed GABA(C) receptors formed by homomeric rho subunits. CTZ inhibition of GABA(C) receptors was subunit specific; it produced a dose-dependent reduction of the GABA-elicited current on homomeric rho2 receptors with an IC(50) of about 12 microm, but had no significant effect on homomeric rho1 receptors. This differential sensitivity was attributable to a single amino acid located on the second transmembrane domain of the rho subunits. Mutating the residue at this position from serine to proline on the rho2 subunit eliminated CTZ sensitivity, whereas switching proline to serine on the rho1 subunit made the receptor CTZ sensitive. The inhibitory properties of CTZ were consistent with its action as a channel blocker on the receptors formed by rho2 subunits. The effect showed a small degree of voltage dependence, and was due mainly to a non-competitive mechanism that reduced the maximum response elicited by GABA. In addition, the prominent membrane current rebound when co-application of GABA and CTZ was terminated suggests that the binding site for CTZ on the GABA(C) receptor is distinct from that for GABA, and that CTZ acts as a non-competitive antagonist on the GABA(C) receptor. CTZ inhibited the open channel of the GABA(C) receptor with a time constant of about 0.4 s, but the kinetics were approximately 10-fold slower when GABA is absent. The ability of CTZ to interact with various types of neurotransmitter receptors indicates that the drug has multiple actions in the CNS.
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Affiliation(s)
- An Xie
- Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago, 1855 W. Taylor Street, Chicago, IL 60612, USA
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Wang J, Lester HA, Dougherty DA. Establishing an ion pair interaction in the homomeric rho1 gamma-aminobutyric acid type A receptor that contributes to the gating pathway. J Biol Chem 2007; 282:26210-6. [PMID: 17606618 DOI: 10.1074/jbc.m702314200] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
gamma-Aminobutyric acid type A (GABA(A)) receptors are members of the Cys-loop superfamily of ligand-gated ion channels. Upon agonist binding, the receptor undergoes a structural transition from the closed to the open state, but the mechanism of gating is not well understood. Here we utilized a combination of conventional mutagenesis and the high precision methodology of unnatural amino acid incorporation to study the gating interface of the human homopentameric rho1 GABA(A) receptor. We have identified an ion pair interaction between two conserved charged residues, Glu(92) in loop 2 of the extracellular domain and Arg(258) in the pre-M1 region. We hypothesize that the salt bridge exists in the closed state by kinetic measurements and free energy analysis. Several other charged residues at the gating interface are not critical to receptor function, supporting previous conclusions that it is the global charge pattern of the gating interface that controls receptor function in the Cys-loop superfamily.
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Affiliation(s)
- Jinti Wang
- Division of Chemistry and Chemical Engineering and Division of Biology, California Institute of Technology, Pasadena, California 91125, USA
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Ramsey DJ, Ripps H, Qian H. Streptozotocin-induced diabetes modulates GABA receptor activity of rat retinal neurons. Exp Eye Res 2007; 85:413-422. [PMID: 17662714 PMCID: PMC2001264 DOI: 10.1016/j.exer.2007.06.005] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/04/2007] [Revised: 05/29/2007] [Accepted: 06/01/2007] [Indexed: 11/22/2022]
Abstract
Neural deficits suggestive of involvement of the GABA signaling pathway can often be detected early in the course of diabetic retinopathy, a leading cause of blindness in the United States. To examine in greater detail the nature of the neuronal changes associated with hyperglycemia, we investigated GABA receptor activity on retinal bipolar cells in streptozotocin-induced diabetic rats; cells from age-matched normal rats served as controls. Patch-clamp recordings from isolated rod-bipolar cells revealed that diabetes enhanced the whole cell currents elicited by GABA. Responses of the GABA(C) receptor, the predominant GABA receptor on rat rod bipolar cells, exhibited a greater sensitivity to GABA, larger maximum current responses, slower response kinetics, and a smaller single channel conductance among diabetic cells relative to those recorded from normal controls. Compared with the properties of homomeric rho1 and heteromeric rho1rho2 receptors formed in a heterologous expression system, these results suggested that there was a greater contribution from the rho1 subunit in the GABA(C) receptor-mediated response of diabetic cells. The levels of mRNA, measured with real-time RT-PCR, were consistent with this finding. There was a significant enhancement in the ratio of rho1/rho2 subunit expression in the retina of diabetic animals, although the levels of GABA rho1 subunit expression were comparable in diabetic and normal retinas. Taken together, the results suggest that diabetes modifies the subunit composition of the GABA(C) receptor on retinal neurons, most likely through its effect on the efficacy of gene transcription.
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Affiliation(s)
- David J. Ramsey
- Department of Ophthalmology and Visual Sciences, University of Illinois College of Medicine, 1855 West Taylor Street, Chicago, Illinois 60612, USA
- Department of Physiology and Biophysics, University of Illinois College of Medicine, 1855 West Taylor Street, Chicago, Illinois 60612, USA
- Department of Health Policy and Administration, University of Illinois College of Medicine, 1855 West Taylor Street, Chicago, Illinois 60612, USA
| | - Harris Ripps
- Department of Ophthalmology and Visual Sciences, University of Illinois College of Medicine, 1855 West Taylor Street, Chicago, Illinois 60612, USA
- Department of Physiology and Biophysics, University of Illinois College of Medicine, 1855 West Taylor Street, Chicago, Illinois 60612, USA
- Department of Anatomy and Cell Biology, University of Illinois College of Medicine, 1855 West Taylor Street, Chicago, Illinois 60612, USA
| | - Haohua Qian
- Department of Ophthalmology and Visual Sciences, University of Illinois College of Medicine, 1855 West Taylor Street, Chicago, Illinois 60612, USA
- Department of Physiology and Biophysics, University of Illinois College of Medicine, 1855 West Taylor Street, Chicago, Illinois 60612, USA
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