1
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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: 1.5] [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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2
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Gussin H, Tomlinson ID, Cao D, Qian H, Rosenthal S, Pepperberg DR. Quantum dot conjugates of GABA and muscimol: binding to α1β2γ2 and ρ1 GABA(A) receptors. ACS Chem Neurosci 2013; 4:435-43. [PMID: 23509979 PMCID: PMC3605815 DOI: 10.1021/cn300144v] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2012] [Accepted: 12/05/2012] [Indexed: 12/18/2022] Open
Abstract
GABAA receptors are ligand-gated ion channels that mediate inhibitory synaptic signaling in the CNS. Fluorescent probes with the ability to target these receptors can provide insights into receptor location, distribution and dynamics in live cells, while revealing abnormalities in their distribution and dynamics that could occur in a variety of diseases. We have developed fluorescent probes of GABAA receptors that are composed of a CdSe/ZnS core-shell nanocrystal (quantum dot; qdot) conjugated to pegylated derivatives of the GABA receptor agonists GABA and muscimol (GABA-qdots and muscimol-qdots, respectively). Quantitative fluorescence imaging was used to analyze the binding activity of these conjugates to α1β2γ2 GABAA and ρ1 GABAA receptors expressed in Xenopus oocytes. The selectivity of these conjugates for α1β2γ2 GABAA and ρ1 GABAA receptors was determined by their ability to compete with the antagonists bicuculline and methyl-(1,2,3,6-tetrahydropyridin-4-yl)phosphinic acid (TPMPA). Both GABA- and muscimol-qdots exhibited robust binding to both α1β2γ2 and ρ1 GABAA receptors. At α1β2γ2 receptors, pretreatment with bicuculline reduced conjugate binding by ≥8-fold on average, an extent far exceeding the reduction produced by TPMPA (~30%). Conversely, at ρ1 receptors, pretreatment with TPMPA inhibited binding by ~10-fold, an extent greatly exceeding the change produced by bicuculline (~50% or less). These results indicate specific binding of muscimol-qdots and GABA-qdots to α1β2γ2 GABAA and ρ1 GABAA receptors in a manner that preserves the respective pharmacological sensitivities of these receptors to TPMPA and bicuculline, and encourage the use of qdot-conjugated neurotransmitter analogs as labeling agents at GABAA receptors.
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Affiliation(s)
- Hélène
A. Gussin
- Lions of Illinois Eye Research Institute,
Department of Ophthalmology and Visual Sciences, University
of Illinois at Chicago, Chicago, Illinois 60612, United
States
| | - Ian D. Tomlinson
- Department of Chemistry and Departments of Physics, Chemical & Biomolecular
Engineering, and Pharmacology, Vanderbilt
University, Nashville, Tennessee 37235, United States
| | - Dingcai Cao
- Lions of Illinois Eye Research Institute,
Department of Ophthalmology and Visual Sciences, University
of Illinois at Chicago, Chicago, Illinois 60612, United
States
| | - Haohua Qian
- Lions of Illinois Eye Research Institute,
Department of Ophthalmology and Visual Sciences, University
of Illinois at Chicago, Chicago, Illinois 60612, United
States
- National Eye Institute, National Institutes of Health, Bethesda, Maryland 20892,
United States
| | - Sandra
J. Rosenthal
- Department of Chemistry and Departments of Physics, Chemical & Biomolecular
Engineering, and Pharmacology, Vanderbilt
University, Nashville, Tennessee 37235, United States
| | - David R. Pepperberg
- Lions of Illinois Eye Research Institute,
Department of Ophthalmology and Visual Sciences, University
of Illinois at Chicago, Chicago, Illinois 60612, United
States
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3
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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: 0.9] [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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4
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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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5
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Qian H, Ripps H. Focus on molecules: the GABAC receptor. Exp Eye Res 2008; 88:1002-3. [PMID: 18983841 DOI: 10.1016/j.exer.2008.10.005] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/17/2008] [Accepted: 10/01/2008] [Indexed: 10/21/2022]
Affiliation(s)
- Haohua Qian
- Department of Ophthalmology and Visual Sciences, University of Illinois College of Medicine, 1855 West Taylor Street, Chicago, IL 60612, USA.
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6
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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: 0.6] [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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7
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Study on olfactory function in GABAC receptor/channel ρ1 subunit knockout mice. Neurosci Lett 2007; 427:10-5. [DOI: 10.1016/j.neulet.2007.06.059] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/13/2007] [Revised: 06/25/2007] [Accepted: 06/28/2007] [Indexed: 11/19/2022]
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8
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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: 1.5] [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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9
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Zhu Y, Ripps H, Qian H. A single amino acid in the second transmembrane domain of GABA rho receptors regulates channel conductance. Neurosci Lett 2007; 418:205-9. [PMID: 17398006 PMCID: PMC1942122 DOI: 10.1016/j.neulet.2007.03.020] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/24/2007] [Revised: 03/08/2007] [Accepted: 03/11/2007] [Indexed: 10/23/2022]
Abstract
GABAC receptors, expressed predominately in vertebrate retina, are thought to be formed mainly by GABA rho subunits, each of which exhibits distinct physiological and pharmacological properties. In this study, the receptors formed by perch GABA rho subunits were expressed in HEK cells, and their single channel conductances were determined using noise analysis techniques. The receptors formed by the perch rho1A subunit gate a channel with a conductance of 0.2 pS, whereas the receptors formed by GABA rho2 subunits exhibit much higher channel conductances, i.e., 3.2 and 3.5 pS for perch rho2A and rho2B receptors, respectively. A comparison of the amino acid sequences of the channel-forming TMII regions of the various subunits suggested that a single amino acid at position 2' was a potential site for the large differential in conductance. We found that switching the serine residue at that site in the GABA rho2 subunit to the proline residue present in the rho1 subunit reduced the channel conductance to a level similar to that of the wild type rho1 receptor. Conversely, mutating proline to serine in the amino acid sequence of the rho1 receptor significantly increased its unitary conductance. These results indicate that a single amino acid in the TMII region plays an important role in determining the single channel conductance of the GABAC receptors.
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Affiliation(s)
- Yujie Zhu
- Department of Ophthalmology and Visual Sciences, University of Illinois College of Medicine, 1855 West Taylor Street, Chicago, IL 60612, USA
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10
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Qian H, Pan Y, Choi B, Ripps H. High pH accelerates GABA deactivation on perch-rho1B receptors. Neuroscience 2006; 142:1221-30. [PMID: 16920274 DOI: 10.1016/j.neuroscience.2006.07.003] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/11/2006] [Revised: 07/01/2006] [Accepted: 07/11/2006] [Indexed: 10/24/2022]
Abstract
The ionotropic GABA(C) receptor, formed by GABA rho subunits, is known to be modulated by a variety of endogenous compounds, as well as by changes in pH. In this study, we explore the proton sensitivity of the GABA rho subunits cloned from the perch retina, and report a novel action of high pH on the homomeric receptor formed by one of the GABA rho subunits, the perch-rho(1B) subunit. Raising extracellular pH to 9.5 significantly accelerated GABA deactivation responses elicited from oocytes expressing the perch-rho(1B) subunit, and reduced its sensitivity to GABA. The change in the kinetics of the GABA-offset response occurred without altering the maximum response amplitude, and the reduced GABA sensitivity was independent of membrane potential. Although acidification of the extracellular solution also accelerated GABA deactivation for all other GABA rho receptors examined in this study, the effects of high pH were unique to the homomeric receptor formed by the perch-rho(1B) subunit. In addition, we found that, unlike the effects on the response to the naturally occurring full agonist GABA, the responses elicited by partial agonists (imidazole-4-acetic acid (I4AA) and beta-alanine) in the presence of the high pH solution showed a significant reduction in the maximum response amplitude. When considered in terms of a model describing the activation of GABA(C) receptors, in which pH can potentially affect either the binding affinity or the rate of channel closure, the results were consistent with the view that external alkalization reduces the gating efficiency of the receptor. To identify the proton sensitive domain(s) of the perch-rho(1B) receptor, chimeras were constructed by domain swapping with other perch-rho subunits. Analysis of the pH sensitivities of the various chimeric receptors revealed that the alkaline-sensitive residues are located in the N-terminal region of the perch-rho(1B) subunit.
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MESH Headings
- Animals
- Binding, Competitive/drug effects
- Binding, Competitive/physiology
- Cell Membrane/drug effects
- Cell Membrane/metabolism
- Female
- GABA Agonists/pharmacology
- Hydrogen-Ion Concentration
- Ion Channel Gating/drug effects
- Ion Channel Gating/physiology
- Ligands
- Neural Inhibition/physiology
- Neurons/metabolism
- Oocytes
- Perches
- Protein Structure, Tertiary/drug effects
- Protein Structure, Tertiary/physiology
- Protons
- Receptors, GABA/chemistry
- Receptors, GABA/drug effects
- Receptors, GABA/metabolism
- Receptors, GABA-B/chemistry
- Receptors, GABA-B/drug effects
- Receptors, GABA-B/metabolism
- Synaptic Transmission/physiology
- Time Factors
- Xenopus
- gamma-Aminobutyric Acid/metabolism
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Affiliation(s)
- H Qian
- Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago, 1855 West Taylor Street, Chicago, IL 60612, USA.
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11
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Liu J, Li GL, Yang XL. An ionotropic GABA receptor with novel pharmacology at bullfrog cone photoreceptor terminals. Neurosignals 2006; 15:13-25. [PMID: 16825800 DOI: 10.1159/000094384] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/12/2005] [Indexed: 11/19/2022] Open
Abstract
Characteristics of ionotropic gamma-aminobutyric acid (GABA) receptors at bullfrog cone terminals were studied by patch clamp techniques in isolated cell and retinal slice preparations. GABA-induced inward currents from isolated cones reversed in polarity at a potential, very close to the chloride equilibrium potential, and they were completely suppressed by picrotoxin. Unexpectedly, the GABA current was dose-dependently potentiated by the well-known GABA(A) receptor antagonist bicuculline (BIC), but was suppressed by gabazine, another GABA(A) antagonist, and imidazole-4-acetic acid (I4AA), a GABA(C) receptor antagonist. Similarly, currents induced by both GABA(A) agonist muscimol and GABA(C) agonist cis-4-aminocrotonic acid (CACA) were also potentiated by BIC. Furthermore, currents induced from cones by GABA and kainate-caused depolarization of horizontal cells in retinal slice preparations were both potentiated by BIC. All these results suggest that the ionotropic GABA receptor at the bullfrog cone terminal exhibits novel pharmacology, distinct from both traditional GABA(A) and GABA(C) receptors.
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Affiliation(s)
- Jian Liu
- Institute of Neurobiology, Institute of Brain Science, Fudan University, Shanghai, PR China
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12
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Pan Y, Ripps H, Qian H. Random assembly of GABA rho1 and rho2 subunits in the formation of heteromeric GABA(C) receptors. Cell Mol Neurobiol 2006; 26:289-305. [PMID: 16767514 DOI: 10.1007/s10571-006-9001-8] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/17/2005] [Accepted: 02/08/2006] [Indexed: 10/24/2022]
Abstract
1. Various combinations of the rho subunits (rho(1A), rho(1B), rho(2A), rho(2B)) of GABA(C) receptors cloned from white perch retina were expressed in Xenopus oocytes, and electrophysiological and pharmacological methods were used to test their ability to co-assemble into heteromeric receptors. Simultaneous injection of the two subunits, irrespective of their relative proportions, led invariably to the formation of a preponderance of heteromeric receptors. 2. The GABA deactivation responses elicited from these cells could be described by a single exponential decay, and their pharmacological responses deviated significantly from those expected of a simple mixture of two homomeric rho(1) and rho(2) receptors. In contrast, a double exponential function comprising fast and slow components was required to fit the GABA deactivation responses elicited from oocytes sequentially expressing rho(1) and rho(2) subunits, a condition that favors the formation of a mixture of homomeric rho(1) and rho(2) receptors. 3. Both the GABA-response kinetics and the sensitivity to picrotoxin of the heteromeric perch rho(1B)rho(2A) receptor varied with the proportion of the subunit RNA injected, indicating there is no fixed stoichiometry for their co-assembly into heteromeric rho(1)rho(2) receptors. 4. If native GABA(C) receptors in retinal neurons behave in a similar manner as in the oocyte expression system, these finding suggest that the properties of their GABA(C) receptors are likely to be influenced by the transcription/translation efficiency of GABA rho subunit genes.
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Affiliation(s)
- Yi Pan
- Department of Ophthalmology and Visual Sciences, University of Illinois, Chicago, USA
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13
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Abstract
1. The present review gives an overview of studies conducted on GABAC receptors over the past 10 years since the author started at the University of Sydney. It concentrates on the structure-activity relationship profiles of the receptor and how these studies were used to: (i) develop selective GABAC receptor ligands; and (ii) understand the impact of amino acid changes on GABAC receptor pharmacology and function. 2. Structure-activity relationship studies involving variations of both ligands and their receptor targets are vital to the discovery of drugs that interact selectively with particular native and mutant receptor subtypes. Such agents may be useful for treating anxiety, depression, epilepsy and memory related disorders, such as Alzheimer's disease.
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Affiliation(s)
- M Chebib
- Faculty of Pharmacy, University of Sydney, Sydney, New South Wales, Australia
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14
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Pan Y, Khalili P, Ripps H, Qian H. Pharmacology of GABAC receptors: responses to agonists and antagonists distinguish A- and B-subtypes of homomeric rho receptors expressed in Xenopus oocytes. Neurosci Lett 2004; 376:60-5. [PMID: 15694275 DOI: 10.1016/j.neulet.2004.11.024] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/05/2004] [Revised: 11/01/2004] [Accepted: 11/11/2004] [Indexed: 10/26/2022]
Abstract
GABA(C) receptors, expressed predominantly in vertebrate retina, are thought to be formed mainly by GABA rho subunits. Five GABA rho subunits have been cloned from white perch retina, four of which form functional homooligomeric receptors when expressed in Xenopus oocytes. These rho subtypes, classified as rho1A, rho1B, rho2A and rho2B receptors based on amino acid sequence alignment, exhibit distinct temporal and pharmacological properties. To examine further the pharmacological properties associated with the various rho receptor subtypes, we investigated the effects of a selective GABA(C) receptor antagonist, TPMPA, on the GABA-mediated activity of receptors formed in Xenopus oocytes by the four GABA rho subunits. In addition, we recorded the activation profiles of beta-alanine, taurine, and glycine, three amino acids that modulate neuronal activity in various parts of the CNS and are purported to be rho receptor agonists. TPMPA effectively inhibited GABA-elicited responses on A-type receptors, whereas B-type receptors exhibited a relatively low sensitivity to the drug. A-type and B-type receptors also displayed distinctly different reactions to agonists. Both taurine and glycine-activated the B-type receptors, whereas these agents had no detectable effect on A-type receptors. Similarly, beta-alanine evoked large responses from B-type receptors, but was far less effective on A-type receptors. These results indicate that, in addition to the characteristic response properties identified previously, there is a pattern of pharmacological reactions that further distinguishes the A- and B-subtypes of GABA rho receptor.
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Affiliation(s)
- Yi Pan
- Department of Ophthalmology and Visual Sciences, University of Illinois College of Medicine, 1855 West Taylor Street, Chicago, IL 60612, USA
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15
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Carland JE, Moore AM, Hanrahan JR, Mewett KN, Duke RK, Johnston GAR, Chebib M. Mutations of the 2' proline in the M2 domain of the human GABAC rho1 subunit alter agonist responses. Neuropharmacology 2004; 46:770-81. [PMID: 15033337 DOI: 10.1016/j.neuropharm.2003.11.027] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/23/2003] [Revised: 11/11/2003] [Accepted: 11/25/2003] [Indexed: 10/26/2022]
Abstract
Mutations of the proline residue at the 2' position (P2') within the second transmembrane (M2) domain of the gamma-aminobutyric acid(C) (GABA(C)) rho1 subunit are known to produce receptors with altered pharmacology. In the present study, P2' was mutated to alanine (rho1P2'A), phenylalanine (rho1P2'F), glycine (rho1P2'G) and serine (rho1P2'S). Mutant receptors were characterized using a range of agonists, partial agonists and antagonists. rho1P2'A, rho1P2'G and rho1P2'S receptors were less susceptible than wild-type receptors to agonist activation. Most notably, the partial agonists, (+/-)-trans-2-(aminomethyl)cyclopropanoic acid ((+/-)-TAMP) and imidazole-4-acetic acid (I4AA) were converted to antagonists at rho1P2'G and rho1P2'S receptors and the partial agonist CACA acted as an antagonist at rho1P2'S receptors. In contrast, rho1P2'F receptors were more prone to activation by agonists. A correlation was observed between the pharmacological properties of the mutant receptors and the hydrophobicity of each residue. Unlike the agonists or partial agonists, the affinity of competitive antagonists, (1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid (TPMPA) and 4,5,6,7-tetrahydroisoxazole[4,5-c]pyridine-3-ol (THIP), did not change significantly between wild-type and mutant receptors. Thus, the results suggest that the agonist/competitive antagonist binding site(s) were not significantly affected by the mutations, but that receptor activation properties altered such that the more hydrophobic the residue at the 2' position, the more prone the receptor is to agonist activation.
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Affiliation(s)
- Jane E Carland
- Faculty of Pharmacy, A15, University of Sydney, Sydney, NSW 2006, Australia
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16
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Vigh J, Solessio E, Morgans CW, Lasater EM. Ionic mechanisms mediating oscillatory membrane potentials in wide-field retinal amacrine cells. J Neurophysiol 2003; 90:431-43. [PMID: 12649310 DOI: 10.1152/jn.00092.2003] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Particular types of amacrine cells of the vertebrate retina show oscillatory membrane potentials (OMPs) in response to light stimulation. Historically it has been thought the oscillations arose as a result of circuit properties. In a previous study we found that in some amacrine cells, the ability to oscillate was an intrinsic property of the cell. Here we characterized the ionic mechanisms responsible for the oscillations in wide-field amacrine cells (WFACs) in an effort to better understand the functional properties of the cell. The OMPs were found to be calcium (Ca2+) dependent; blocking voltage-gated Ca2+ channels eliminated the oscillations, whereas elevating extracellular Ca2+ enhanced them. Strong intracellular Ca2+ buffering (10 mM EGTA or bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid) eliminated any attenuation in the OMPs as well as a Ca2+-dependent inactivation of the voltage-gated Ca2+ channels. Pharmacological and immunohistochemical characterization revealed that WFACs express L- and N-type voltage-sensitive Ca2+ channels. Block of the L-type channels eliminated the OMPs, but omega-conotoxin GVIA did not, suggesting a different function for the N-type channels. The L-type channels in WFACs are functionally coupled to a set of calcium-dependent potassium (K(Ca)) channels to mediate OMPs. The initiation of OMPs depended on penitrem-A-sensitive (BK) K(Ca) channels, whereas their duration is under apamin-sensitive (SK) K(Ca) channel control. The Ca2+ current is essential to evoke the OMPs and triggering the K(Ca) currents, which here act as resonant currents, enhances the resonance as an amplifying current, influences the filtering characteristics of the cell membrane, and attenuates the OMPs via CDI of the L-type Ca2+ channel.
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Affiliation(s)
- Jozsef Vigh
- Department of Ophthalmology and Visual Sciences, John Moran Eye Center, University of Utah, Health Sciences Center, Salt Lake City, Utah 84132, USA
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17
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Gamel-Didelon K, Kunz L, Fohr KJ, Gratzl M, Mayerhofer A. Molecular and physiological evidence for functional gamma-aminobutyric acid (GABA)-C receptors in growth hormone-secreting cells. J Biol Chem 2003; 278:20192-5. [PMID: 12660236 DOI: 10.1074/jbc.m301729200] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The neurotransmitter gamma-aminobutyric acid (GABA), released by hypothalamic neurons as well as by growth hormone- (GH) and adrenocorticotropin-producing cells, is a regulator of pituitary endocrine functions. Different classes of GABA receptors may be involved. In this study, we report that GH cells, isolated by laser microdissection from rat pituitary slices, possess the GABA-C receptor subunit rho2. We also demonstrate that in the GH adenoma cell line, GH3, GABA-C receptor subunits are not only expressed but also form functional channels. GABA-induced Cl- currents were recorded using the whole cell patch clamp technique; these currents were insensitive to bicuculline (a GABA-A antagonist) but could be induced by the GABA-C agonist cis-4-aminocrotonic acid. In contrast to typical GABA-C mediated currents in neurons, they quickly desensitized. Ca2+i recordings were also performed on GH3 cells. The application of either GABA or cis-4-aminocrotonic acid led to Ca2+ transients of similar amplitude, indicating that the activation of GABA-C receptors in GH3 cells may cause membrane depolarization, opening of voltage-gated Ca2+ channels, and a subsequent Ca2+ influx. Our results point at a role for GABA in pituitary GH cells and disclose an additional pathway to the one known via GABA-B receptors.
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18
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Jentsch TJ, Stein V, Weinreich F, Zdebik AA. Molecular structure and physiological function of chloride channels. Physiol Rev 2002; 82:503-68. [PMID: 11917096 DOI: 10.1152/physrev.00029.2001] [Citation(s) in RCA: 941] [Impact Index Per Article: 42.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Cl- channels reside both in the plasma membrane and in intracellular organelles. Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of electrical excitability. Their physiological roles are impressively illustrated by various inherited diseases and knock-out mouse models. Thus the loss of distinct Cl- channels leads to an impairment of transepithelial transport in cystic fibrosis and Bartter's syndrome, to increased muscle excitability in myotonia congenita, to reduced endosomal acidification and impaired endocytosis in Dent's disease, and to impaired extracellular acidification by osteoclasts and osteopetrosis. The disruption of several Cl- channels in mice results in blindness. Several classes of Cl- channels have not yet been identified at the molecular level. Three molecularly distinct Cl- channel families (CLC, CFTR, and ligand-gated GABA and glycine receptors) are well established. Mutagenesis and functional studies have yielded considerable insights into their structure and function. Recently, the detailed structure of bacterial CLC proteins was determined by X-ray analysis of three-dimensional crystals. Nonetheless, they are less well understood than cation channels and show remarkably different biophysical and structural properties. Other gene families (CLIC or CLCA) were also reported to encode Cl- channels but are less well characterized. This review focuses on molecularly identified Cl- channels and their physiological roles.
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Affiliation(s)
- Thomas J Jentsch
- Zentrum für Molekulare Neurobiologie Hamburg, Universität Hamburg, Hamburg, Germany.
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19
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Affiliation(s)
- H Qian
- Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago, 1855 West Taylor Srteet, Chicago, IL 60612, USA.
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20
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Lasater EM, Liu Y. Properties of turtle retinal ganglion cell GABA receptors. PROGRESS IN BRAIN RESEARCH 2001; 131:319-31. [PMID: 11420952 DOI: 10.1016/s0079-6123(01)31026-9] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/20/2023]
Affiliation(s)
- E M Lasater
- Moran Eye Center, University of Utah Health Sciences Center, University of Utah, 50 North Medical Drive, Salt Lake City, UT 84132, USA.
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21
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Vitanova L, Kupenova P, Haverkamp S, Popova E, Mitova L, Wässle H. Immunocytochemical and electrophysiological characterization of GABA receptors in the frog and turtle retina. Vision Res 2001; 41:691-704. [PMID: 11248259 DOI: 10.1016/s0042-6989(00)00294-7] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
The expression of GABA receptors (GABARs) was studied in frog and turtle retinae. Using immunocytochemical methods, GABA(A)Rs and GABA(C)Rs were preferentially localized to the inner plexiform layer (IPL). Label in the IPL was punctate indicating a synaptic clustering of GABARs. Distinct, but weaker label was also present in the outer plexiform layer. GABA(A)R and GABA(C)R mediated effects were studied by recording electroretinograms (ERGs) and by the application of specific antagonists. Bicuculline, the GABA(A)R antagonist, produced a significant increase of the ERG. Picrotoxin, when co-applied with saturating doses of bicuculline, caused a further increase of the ERG due to blocking of GABA(C)Rs. The putative GABA(C)R antagonist Imidazole-4-acidic acid (I4AA) failed to antagonize GABA(C)R mediated inhibition and, in contrast, appeared rather as an agonist of GABARs.
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Affiliation(s)
- L Vitanova
- Department of Physiology, Medical University, Sofia, Bulgaria
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22
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Zhang D, Pan ZH, Awobuluyi M, Lipton SA. Structure and function of GABA(C) receptors: a comparison of native versus recombinant receptors. Trends Pharmacol Sci 2001; 22:121-32. [PMID: 11239575 DOI: 10.1016/s0165-6147(00)01625-4] [Citation(s) in RCA: 101] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
In less than a decade our knowledge of the GABA(C) receptor, a new type of Cl(-)-permeable ionotropic GABA receptor, has greatly increased based on studies of both native and recombinant receptors. Careful comparison of properties of native and recombinant receptors has provided compelling evidence that GABA receptor rho-subunits are the major molecular components of GABA(C) receptors. Three distinct rho-subunits from various species have been cloned and the pattern of their expression in the retina, as well as in various brain regions, has been established. The pharmacological profile of GABA(C) receptors has been refined and more specific drugs have been developed. Molecular determinants that underlie functional properties of the receptors have been assigned to specific amino acid residues in rho-subunits. This information has helped determine the subunit composition of native receptors, as well as the molecular basis underlying subtle variations among GABA(C) receptors in different species. Finally, GABA(C) receptors play a unique functional role in retinal signal processing via three mechanisms: (1) slow activation; (2) segregation from other inhibitory receptors; and (3) contribution to multi-neuronal pathways.
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Affiliation(s)
- D Zhang
- Center for Neuroscience and Aging, The Burnham Institute, 10901 North Torrey Pines Road, La Jolla, CA 92037, USA
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23
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Martínez-Torres A, Demuro A, Miledi R. GABAρ1/GABA
A
α1 receptor chimeras to study receptor desensitization. Proc Natl Acad Sci U S A 2000; 97:3562-6. [PMID: 10725369 PMCID: PMC16279 DOI: 10.1073/pnas.97.7.3562] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
γ-Aminobutyrate type C (GABA
C
) receptors are ligand-gated ion channels that are expressed preponderantly in the vertebrate retina and are characterized, among other things, by a very low rate of desensitization and resistance to the specific GABA
A
antagonist bicuculline. To examine which structural elements determine the nondesensitizing character of the human homomeric ρ1 receptor, we used a combination of gene chimeras and electrophysiology of receptors expressed in
Xenopus
oocytes. Two chimeric genes were constructed, made up of portions of the ρ1-subunit and of the α1-subunit of the GABA
A
receptor. When expressed in
Xenopus
oocytes, one chimeric gene (ρ1/α1) formed functional homooligomeric receptors that were fully resistant to bicuculline and were blocked by the specific GABA
C
antagonist (1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid and by zinc. Moreover, these chimeric receptors had a fast-desensitizing component, even faster than that of heterooligomeric GABA
A
receptors, in striking contrast to the almost nil desensitization of wild-type ρ1 (wt ρ1) receptors. To see whether the fast-desensitizing characteristic of the chimera was determined by the amino acids forming the ion channels, we replaced the second transmembrane segment (TM2) of ρ1 by that of the α1-subunit of GABA
A
. Although the α1-subunit forms fast-desensitizing receptors when coexpressed with other GABA
A
subunits, the sole transfer of the α1TM2 segment to ρ1 was not sufficient to form desensitizing receptors. All this suggests that the slow-desensitizing trait of ρ1 receptors is determined by a combination of several interacting domains along the molecule.
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Affiliation(s)
- A Martínez-Torres
- Laboratory of Cellular and Molecular Neurobiology, Department of Neurobiology and Behavior, University of California, Irvine, CA 92697, USA
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24
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Abstract
gamma-Aminobutyrate type C (GABA(C)) receptors are ligand-gated ion channels that are expressed preponderantly in the vertebrate retina and are characterized, among other things, by a very low rate of desensitization and resistance to the specific GABA(A) antagonist bicuculline. To examine which structural elements determine the nondesensitizing character of the human homomeric rho1 receptor, we used a combination of gene chimeras and electrophysiology of receptors expressed in Xenopus oocytes. Two chimeric genes were constructed, made up of portions of the rho1-subunit and of the alpha1-subunit of the GABA(A) receptor. When expressed in Xenopus oocytes, one chimeric gene (rho1/alpha1) formed functional homooligomeric receptors that were fully resistant to bicuculline and were blocked by the specific GABA(C) antagonist (1,2,5, 6-tetrahydropyridine-4-yl)methylphosphinic acid and by zinc. Moreover, these chimeric receptors had a fast-desensitizing component, even faster than that of heterooligomeric GABA(A) receptors, in striking contrast to the almost nil desensitization of wild-type rho1 (wt rho1) receptors. To see whether the fast-desensitizing characteristic of the chimera was determined by the amino acids forming the ion channels, we replaced the second transmembrane segment (TM2) of rho1 by that of the alpha1-subunit of GABA(A). Although the alpha1-subunit forms fast-desensitizing receptors when coexpressed with other GABA(A) subunits, the sole transfer of the alpha1TM2 segment to rho1 was not sufficient to form desensitizing receptors. All this suggests that the slow-desensitizing trait of rho1 receptors is determined by a combination of several interacting domains along the molecule.
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25
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Abstract
In the conventional view, GABA acts at either ionotropic GABAA or metabotropic GABAB receptors. Recently, novel ionotropic GABA receptors that are composed of rho-subunits have been identified in the vertebrate retina. These bicuculline- and baclofen-insensitive GABA receptors are frequently called GABAC, following an early suggestion by Graham Johnston and colleagues. An IUPHAR committee has recommended that the term GABAC be avoided and subclassifies the retinal receptors as GABAA0r. However, new evidence regarding the pharmacology, structure, function, genetics and cellular localization of ionotropic GABA receptors strengthens the case for the existence of two major classes of these receptors, GABAA and GABAC.
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Affiliation(s)
- J Bormann
- Department of Cell Physiology, Ruhr-University Bochum, D-44780 Bochum, Germany.
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