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Activation of the Rat α1β2ε GABA A Receptor by Orthosteric and Allosteric Agonists. Biomolecules 2022; 12:biom12070868. [PMID: 35883422 PMCID: PMC9312946 DOI: 10.3390/biom12070868] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/11/2022] [Revised: 06/10/2022] [Accepted: 06/17/2022] [Indexed: 02/04/2023] Open
Abstract
GABAA receptors are a major contributor to fast inhibitory neurotransmission in the brain. The receptors are activated upon binding the transmitter GABA or allosteric agonists including a number of GABAergic anesthetics and neurosteroids. Functional receptors can be formed by various combinations of the nineteen GABAA subunits cloned to date. GABAA receptors containing the ε subunit exhibit a significant degree of constitutive activity and have been suggested to be unresponsive to allosteric agents. In this study, we have characterized the functional properties of the rat α1β2ε GABAA receptor. We confirm that the α1β2ε receptor exhibits a higher level of constitutive activity than typical of GABAA receptors and show that it is inefficaciously activated by the transmitter and the allosteric agonists propofol, pentobarbital, and allopregnanolone. Manipulations intended to alter ε subunit expression and receptor stoichiometry were largely without effect on receptor properties including sensitivity to GABA and allosteric agonists. Surprisingly, amino acid substitutions at the conserved 9' and 6' positions in the second transmembrane (TM2) domain in the ε subunit did not elicit the expected functional effects of increased constitutive activity and resistance to the channel blocker picrotoxin, respectively. We tested the accessibility of TM2 residues mutated to cysteine using the cysteine-modifying reagent 4-(hydroxymercuri)benzoic acid and found a unique pattern of water-accessible residues in the ε subunit.
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Belelli D, Hales TG, Lambert JJ, Luscher B, Olsen R, Peters JA, Rudolph U, Sieghart W. GABA A receptors in GtoPdb v.2021.3. IUPHAR/BPS GUIDE TO PHARMACOLOGY CITE 2021; 2021. [PMID: 35005623 DOI: 10.2218/gtopdb/f72/2021.3] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
The GABAA receptor is a ligand-gated ion channel of the Cys-loop family that includes the nicotinic acetylcholine, 5-HT3 and strychnine-sensitive glycine receptors. GABAA receptor-mediated inhibition within the CNS occurs by fast synaptic transmission, sustained tonic inhibition and temporally intermediate events that have been termed 'GABAA, slow' [45]. GABAA receptors exist as pentamers of 4TM subunits that form an intrinsic anion selective channel. Sequences of six α, three β, three γ, one δ, three ρ, one ε, one π and one θ GABAA receptor subunits have been reported in mammals [278, 235, 236, 283]. The π-subunit is restricted to reproductive tissue. Alternatively spliced versions of many subunits exist (e.g. α4- and α6- (both not functional) α5-, β2-, β3- and γ2), along with RNA editing of the α3 subunit [71]. The three ρ-subunits, (ρ1-3) function as either homo- or hetero-oligomeric assemblies [359, 50]. Receptors formed from ρ-subunits, because of their distinctive pharmacology that includes insensitivity to bicuculline, benzodiazepines and barbiturates, have sometimes been termed GABAC receptors [359], but they are classified as GABA A receptors by NC-IUPHAR on the basis of structural and functional criteria [16, 235, 236]. Many GABAA receptor subtypes contain α-, β- and γ-subunits with the likely stoichiometry 2α.2β.1γ [168, 235]. It is thought that the majority of GABAA receptors harbour a single type of α- and β - subunit variant. The α1β2γ2 hetero-oligomer constitutes the largest population of GABAA receptors in the CNS, followed by the α2β3γ2 and α3β3γ2 isoforms. Receptors that incorporate the α4- α5-or α 6-subunit, or the β1-, γ1-, γ3-, δ-, ε- and θ-subunits, are less numerous, but they may nonetheless serve important functions. For example, extrasynaptically located receptors that contain α6- and δ-subunits in cerebellar granule cells, or an α4- and δ-subunit in dentate gyrus granule cells and thalamic neurones, mediate a tonic current that is important for neuronal excitability in response to ambient concentrations of GABA [209, 272, 83, 19, 288]. GABA binding occurs at the β+/α- subunit interface and the homologous γ+/α- subunits interface creates the benzodiazepine site. A second site for benzodiazepine binding has recently been postulated to occur at the α+/β- interface ([254]; reviewed by [282]). The particular α-and γ-subunit isoforms exhibit marked effects on recognition and/or efficacy at the benzodiazepine site. Thus, receptors incorporating either α4- or α6-subunits are not recognised by 'classical' benzodiazepines, such as flunitrazepam (but see [356]). The trafficking, cell surface expression, internalisation and function of GABAA receptors and their subunits are discussed in detail in several recent reviews [52, 140, 188, 316] but one point worthy of note is that receptors incorporating the γ2 subunit (except when associated with α5) cluster at the postsynaptic membrane (but may distribute dynamically between synaptic and extrasynaptic locations), whereas as those incorporating the δ subunit appear to be exclusively extrasynaptic. NC-IUPHAR [16, 235, 3, 2] class the GABAA receptors according to their subunit structure, pharmacology and receptor function. Currently, eleven native GABAA receptors are classed as conclusively identified (i.e., α1β2γ2, α1βγ2, α3βγ2, α4βγ2, α4β2δ, α4β3δ, α5βγ2, α6βγ2, α6β2δ, α6β3δ and ρ) with further receptor isoforms occurring with high probability, or only tentatively [235, 236]. It is beyond the scope of this Guide to discuss the pharmacology of individual GABAA receptor isoforms in detail; such information can be gleaned in the reviews [16, 95, 168, 173, 143, 278, 216, 235, 236] and [9, 10]. Agents that discriminate between α-subunit isoforms are noted in the table and additional agents that demonstrate selectivity between receptor isoforms, for example via β-subunit selectivity, are indicated in the text below. The distinctive agonist and antagonist pharmacology of ρ receptors is summarised in the table and additional aspects are reviewed in [359, 50, 145, 223]. Several high-resolution cryo-electron microscopy structures have been described in which the full-length human α1β3γ2L GABAA receptor in lipid nanodiscs is bound to the channel-blocker picrotoxin, the competitive antagonist bicuculline, the agonist GABA (γ-aminobutyric acid), and the classical benzodiazepines alprazolam and diazepam [198].
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Markus F, Angelini C, Trimouille A, Rudolf G, Lesca G, Goizet C, Lasseaux E, Arveiler B, van Slegtenhorst M, Brooks AS, Abou Jamra R, Korenke G, Neidhardt J, Owczarek‐Lipska M. Rare variants in the GABA A receptor subunit ε identified in patients with a wide spectrum of epileptic phenotypes. Mol Genet Genomic Med 2020; 8:e1388. [PMID: 32588540 PMCID: PMC7507344 DOI: 10.1002/mgg3.1388] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/21/2020] [Revised: 05/29/2020] [Accepted: 05/31/2020] [Indexed: 01/08/2023] Open
Abstract
BACKGROUND Epilepsy belongs to a group of chronic and highly heterogeneous brain disorders. Many types of epilepsy and epileptic syndromes are caused by genetic factors. The neural amino acid y-aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the mammalian central nervous system. It regulates activity of channel pores by binding to transmembrane GABA-receptors (GABRs). The GABRs are heteropentamers assembled from different receptor subunits (α1-6, β1-3, γ1-3, δ, ε, θ, π, and ρ1-3). Several epileptic disorders are caused by mutations in genes encoding single GABRs. METHODS We applied trio- and single-whole exome sequencing to search for genetic sequence variants associated with a wide range of epileptic phenotypes accompanied by intellectual disability and/or global developmental delay in the investigated patients. RESULTS We identified four hemizygous sequence variants in the GABAA receptor subunit ε gene (GABRE), including one nonsense (NM_004961.3: c.399C>A, p.Tyr133*), two missense variants (NM_004961.3: c.664G>A, p.Glu222Lys; NM_004961.3: c.1045G>A, p.Val349Ile), and one variant affecting the translation initiation codon (NM_004961.3: c.1A>G, p.Met1?) in four unrelated families. CONCLUSION Our clinical and molecular genetic findings suggest that GABRE is a likely candidate gene for epilepsy. Nevertheless, functional studies are necessary to better understand pathogenicity of the GABRE-mutations and their associations with epileptic phenotypes.
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Affiliation(s)
- Fenja Markus
- Junior Research GroupGenetics of Childhood Brain MalformationsFaculty VI‐School of Medicine and Health SciencesUniversity of OldenburgOldenburgGermany
- Human GeneticsFaculty VI‐School of Medicine and Health SciencesUniversity of OldenburgOldenburgGermany
| | - Chloé Angelini
- Service de Génétique médicaleCHU de BordeauxBordeauxFrance
| | | | - Gabrielle Rudolf
- CNRS U7104INSERM U1258Institut de Génétique et de Biologie Moléculaire et CellulaireIllkirchFrance
- Service de NeurologieCentre de Références des Maladies Neurogénétique RaresHôpitaux Universitaires de StrasbourgStrasbourgFrance
| | - Gaetan Lesca
- Genetics departmentLyon University Hospital and University of LyonLyonFrance
| | - Cyril Goizet
- Service de Génétique médicaleCHU de BordeauxBordeauxFrance
- CNRS U7104INSERM U1258Institut de Génétique et de Biologie Moléculaire et CellulaireIllkirchFrance
| | | | | | | | - Alice S. Brooks
- Department of Clinical GeneticsErasmus MC University Medical CenterRotterdamThe Netherlands
| | - Rami Abou Jamra
- Institute of Human GeneticsUniversity Medical Center LeipzigLeipzigGermany
| | | | - John Neidhardt
- Human GeneticsFaculty VI‐School of Medicine and Health SciencesUniversity of OldenburgOldenburgGermany
- Research Center Neurosensory ScienceUniversity of OldenburgOldenburgGermany
| | - Marta Owczarek‐Lipska
- Junior Research GroupGenetics of Childhood Brain MalformationsFaculty VI‐School of Medicine and Health SciencesUniversity of OldenburgOldenburgGermany
- Human GeneticsFaculty VI‐School of Medicine and Health SciencesUniversity of OldenburgOldenburgGermany
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O'Neill N, Sylantyev S. The Functional Role of Spontaneously Opening GABA A Receptors in Neural Transmission. Front Mol Neurosci 2019; 12:72. [PMID: 30983968 PMCID: PMC6447609 DOI: 10.3389/fnmol.2019.00072] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/22/2019] [Accepted: 03/08/2019] [Indexed: 12/22/2022] Open
Abstract
Ionotropic type of γ-aminobutyric acid receptors (GABAARs) produce two forms of inhibitory signaling: phasic inhibition generated by rapid efflux of neurotransmitter GABA into the synaptic cleft with subsequent binding to GABAARs, and tonic inhibition generated by persistent activation of extrasynaptic and/or perisynaptic GABAARs by GABA continuously present in the extracellular space. It is widely accepted that phasic and tonic GABAergic inhibition is mediated by receptor groups of distinct subunit composition and modulated by different cytoplasmic mechanisms. Recently, however, it has been demonstrated that spontaneously opening GABAARs (s-GABAARs), which do not need GABA binding to enter an active state, make a significant input into tonic inhibitory signaling. Due to GABA-independent action mode, s-GABAARs promise new safer options for therapy of neural disorders (such as epilepsy) devoid of side effects connected to abnormal fluctuations of GABA concentration in the brain. However, despite the potentially important role of s-GABAARs in neural signaling, they still remain out of focus of neuroscience studies, to a large extent due to technical difficulties in their experimental research. Here, we summarize present data on s-GABAARs functional properties and experimental approaches that allow isolation of s-GABAARs effects from those of conventional (GABA-dependent) GABAARs.
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Affiliation(s)
- Nathanael O'Neill
- Center for Clinical Brain Sciences, University of Edinburgh, Edinburgh, United Kingdom
| | - Sergiy Sylantyev
- Center for Clinical Brain Sciences, University of Edinburgh, Edinburgh, United Kingdom
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Nayak TK, Vij R, Bruhova I, Shandilya J, Auerbach A. Efficiency measures the conversion of agonist binding energy into receptor conformational change. J Gen Physiol 2019; 151:465-477. [PMID: 30635369 PMCID: PMC6445574 DOI: 10.1085/jgp.201812215] [Citation(s) in RCA: 16] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/17/2018] [Accepted: 12/11/2018] [Indexed: 12/19/2022] Open
Abstract
Receptors alternate between resting↔active conformations that bind agonists with low↔high affinity. Here, we define a new agonist attribute, energy efficiency (η), as the fraction of ligand-binding energy converted into the mechanical work of the activation conformational change. η depends only on the resting/active agonist-binding energy ratio. In a plot of activation energy versus binding energy (an "efficiency" plot), the slope gives η and the y intercept gives the receptor's intrinsic activation energy (without agonists; ΔG0). We used single-channel electrophysiology to estimate η for eight different agonists and ΔG0 in human endplate acetylcholine receptors (AChRs). From published equilibrium constants, we also estimated η for agonists of KCa1.1 (BK channels) and muscarinic, γ-aminobutyric acid, glutamate, glycine, and aryl-hydrocarbon receptors, and ΔG0 for all of these except KCa1.1. Regarding AChRs, η is 48-56% for agonists related structurally to acetylcholine but is only ∼39% for agonists related to epibatidine; ΔG0 is 8.4 kcal/mol in adult and 9.6 kcal/mol in fetal receptors. Efficiency plots for all of the above receptors are approximately linear, with η values between 12% and 57% and ΔG0 values between 2 and 12 kcal/mol. Efficiency appears to be a general attribute of agonist action at receptor binding sites that is useful for understanding binding mechanisms, categorizing agonists, and estimating concentration-response relationships.
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Affiliation(s)
- Tapan K Nayak
- Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, NY
| | - Ridhima Vij
- Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, NY
| | - Iva Bruhova
- Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, NY
| | - Jayasha Shandilya
- Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, NY
| | - Anthony Auerbach
- Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, NY
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6
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Sieghart W, Savić MM. International Union of Basic and Clinical Pharmacology. CVI: GABAA Receptor Subtype- and Function-selective Ligands: Key Issues in Translation to Humans. Pharmacol Rev 2018; 70:836-878. [DOI: 10.1124/pr.117.014449] [Citation(s) in RCA: 105] [Impact Index Per Article: 17.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
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O'Neill N, Sylantyev S. Feature Article: Selective modulation of tonically active GABA A receptor functional subgroups by G-proteins and protein kinase C. Exp Biol Med (Maywood) 2018; 243:1046-1055. [PMID: 30205722 DOI: 10.1177/1535370218800980] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
Abstract
IMPACT STATEMENT Here we study intracellular mechanisms which regulate inhibitory signaling delivered through continuously (tonically) open ionotropic receptors of γ-aminobutyric acid (GABA) of dentate gyrus granule cells (DGCs). We found that, apart of classical GABA-A receptors (GABAARs) which can be activated by GABA binding, a significant part of tonic inhibitory current is delivered by newly discovered spontaneously opening GABAARs (s-GABAARs), which enter active state without binding of GABA. We have also found that conventional GABAARs and s-GABAARs are regulated by different intracellular mechanisms, which may overlap and thus induce various signaling repercussions. Our results demonstrate that s-GABAARs play a key role in the mechanism that implements DGCs functional role in the brain. On top of that, since regulatory mechanisms under study are affected in a number of pathological states, our results may have broad implications for treatment of neurological disorders.
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Jatczak-Śliwa M, Terejko K, Brodzki M, Michałowski MA, Czyzewska MM, Nowicka JM, Andrzejczak A, Srinivasan R, Mozrzymas JW. Distinct Modulation of Spontaneous and GABA-Evoked Gating by Flurazepam Shapes Cross-Talk Between Agonist-Free and Liganded GABA A Receptor Activity. Front Cell Neurosci 2018; 12:237. [PMID: 30210295 PMCID: PMC6121034 DOI: 10.3389/fncel.2018.00237] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/25/2018] [Accepted: 07/17/2018] [Indexed: 11/13/2022] Open
Abstract
GABAA receptors (GABAARs) play a crucial inhibitory role in the CNS. Benzodiazepines (BDZs) are positive modulators of specific subtypes of GABAARs, but the underlying mechanism remains obscure. Early studies demonstrated the major impact of BDZs on binding and more recent investigations indicated gating, but it is unclear which transitions are affected. Moreover, the upregulation of GABAAR spontaneous activity by BDZs indicates their impact on receptor gating but the underlying mechanisms remain unknown. Herein, we investigated the effect of a BDZ (flurazepam) on the spontaneous and GABA-induced activity for wild-type (WT, α1β2γ2) and mutated (at the orthosteric binding site α1F64) GABAARs. Surprisingly, in spite of the localization at the binding site, these mutations increased the spontaneous activity. Flurazepam (FLU) upregulated this activity for mutants and WT receptors to a similar extent by affecting opening/closing transitions. Spontaneous activity affected GABA-evoked currents and is manifested as an overshoot after agonist removal that depended on the modulation by BDZs. We explain the mechanism of this phenomenon as a cross-desensitization of ligand-activated and spontaneously active receptors. Moreover, due to spontaneous activity, FLU-pretreatment and co-application (agonist + FLU) protocols yielded distinct results. We provide also the first evidence that GABAAR may enter the desensitized state in the absence of GABA in a FLU-dependent manner. Based on our data and model simulations, we propose that FLU affects agonist-induced gating by modifying primarily preactivation and desensitization. We conclude that the mechanisms of modulation of spontaneous and ligand-activated GABAAR activity concerns gating but distinct transitions are affected in spontaneous and agonist-evoked activity.
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Affiliation(s)
- Magdalena Jatczak-Śliwa
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, Wrocław, Poland.,Department of Molecular Physiology and Neurobiology, University of Wrocław, Wrocław, Poland
| | - Katarzyna Terejko
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, Wrocław, Poland
| | - Marek Brodzki
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, Wrocław, Poland.,Department of Molecular Physiology and Neurobiology, University of Wrocław, Wrocław, Poland
| | - Michał A Michałowski
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, Wrocław, Poland.,Department of Molecular Physiology and Neurobiology, University of Wrocław, Wrocław, Poland
| | - Marta M Czyzewska
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, Wrocław, Poland
| | - Joanna M Nowicka
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, Wrocław, Poland
| | - Anna Andrzejczak
- Department of Molecular Physiology and Neurobiology, University of Wrocław, Wrocław, Poland
| | | | - Jerzy W Mozrzymas
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, Wrocław, Poland
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MacKenzie G, Maguire J. Neurosteroids and GABAergic signaling in health and disease. Biomol Concepts 2015; 4:29-42. [PMID: 25436563 DOI: 10.1515/bmc-2012-0033] [Citation(s) in RCA: 39] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/30/2012] [Accepted: 10/12/2012] [Indexed: 11/15/2022] Open
Abstract
Endogenous neurosteroids such as allopregnanolone, allotetrahydrodeoxycorticosterone, and androstanediol are synthesized either de novo in the brain from cholesterol or are generated from the local metabolism of peripherally derived progesterone or corticosterone. Fluctuations in neurosteroid concentrations are important in the regulation of a number of physiological responses including anxiety and stress, reproductive, and sexual behaviors. These effects are mediated in part by the direct binding of neurosteroids to γ-aminobutyric acid type-A receptors (GABAARs), resulting in the potentiation of GABAAR-mediated currents. Extrasynaptic GABAARs containing the δ subunit, which contribute to the tonic conductance, are particularly sensitive to low nanomolar concentrations of neurosteroids and are likely their preferential target. Considering the large charge transfer generated by these persistently open channels, even subtle changes in neurosteroid concentrations can have a major impact on neuronal excitability. Consequently, aberrant levels of neurosteroids have been implicated in numerous disorders, including, but not limited to, anxiety, neurodegenerative diseases, alcohol abuse, epilepsy, and depression. Here we review the modulation of GABAAR by neurosteroids and the consequences for health and disease.
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10
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Hengen KB, Nelson NR, Stang KM, Johnson SM, Smith SM, Watters JJ, Mitchell GS, Behan M. Daily isoflurane exposure increases barbiturate insensitivity in medullary respiratory and cortical neurons via expression of ε-subunit containing GABA ARs. PLoS One 2015; 10:e0119351. [PMID: 25748028 PMCID: PMC4352015 DOI: 10.1371/journal.pone.0119351] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/19/2014] [Accepted: 01/12/2015] [Indexed: 11/23/2022] Open
Abstract
The parameters governing GABAA receptor subtype expression patterns are not well understood, although significant shifts in subunit expression may support key physiological events. For example, the respiratory control network in pregnant rats becomes relatively insensitive to barbiturates due to increased expression of ε-subunit-containing GABAARs in the ventral respiratory column. We hypothesized that this plasticity may be a compensatory response to a chronic increase in inhibitory tone caused by increased central neurosteroid levels. Thus, we tested whether increased inhibitory tone was sufficient to induce ε-subunit upregulation on respiratory and cortical neurons in adult rats. Chronic intermittent increases in inhibitory tone in male and female rats was induced via daily 5-min exposures to 3% isoflurane. After 7d of treatment, phrenic burst frequency was less sensitive to barbiturate in isoflurane-treated male and female rats in vivo. Neurons in the ventral respiratory group and cortex were less sensitive to pentobarbital in vitro following 7d and 30d of intermittent isoflurane-exposure in both male and female rats. The pentobarbital insensitivity in 7d isoflurane-treated rats was reversible after another 7d. We hypothesize that increased inhibitory tone in the respiratory control network and cortex causes a compensatory increase in ε-subunit-containing GABAARs.
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Affiliation(s)
- Keith B. Hengen
- Neuroscience Training Program, University of Wisconsin, Madison, Madison, Wisconsin, United States of America
- * E-mail:
| | - Nathan R. Nelson
- Department of Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin, Madison, Madison, Wisconsin, United States of America
| | - Kyle M. Stang
- Department of Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin, Madison, Madison, Wisconsin, United States of America
| | - Stephen M. Johnson
- Department of Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin, Madison, Madison, Wisconsin, United States of America
| | - Stephanie M. Smith
- Department of Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin, Madison, Madison, Wisconsin, United States of America
| | - Jyoti J. Watters
- Department of Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin, Madison, Madison, Wisconsin, United States of America
| | - Gordon S. Mitchell
- Department of Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin, Madison, Madison, Wisconsin, United States of America
| | - Mary Behan
- Department of Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin, Madison, Madison, Wisconsin, United States of America
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Abrupt changes in pentobarbital sensitivity in preBötzinger complex region, hypoglossal motor nucleus, nucleus tractus solitarius, and cortex during rat transitional period (P10-P15). Respir Physiol Neurobiol 2014; 207:61-71. [PMID: 25550216 DOI: 10.1016/j.resp.2014.12.016] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/23/2014] [Revised: 12/19/2014] [Accepted: 12/22/2014] [Indexed: 01/01/2023]
Abstract
On postnatal days P10-P15 in rat medulla, neurotransmitter receptor subunit composition shifts toward a more mature phenotype. Since medullary GABAARs regulate cardiorespiratory function, abrupt alterations in GABAergic synaptic inhibition could disrupt homeostasis. We hypothesized that GABAARs on medullary neurons become more resistant to positive allosteric modulation during P10-P15. Medullary and cortical slices from P10 to P20 rats were used to record spontaneous action potentials in pre-Botzinger Complex (preBötC-region), hypoglossal (XII) motor nucleus, nucleus tractus solitarius (NTS), and cortex during exposure to pentobarbital (positive allosteric modulator of GABAARs). On P14, pentobarbital resistance abruptly increased in preBötC-region and decreased in NTS, but these changes in pentobarbital resistance were not present on P15. Pentobarbital resistance decreased in XII motor nucleus during P11-P15 with a nadir at P14. Abrupt changes in pentobarbital resistance indicate changes in GABAergic receptor composition and function that may compensate for potential increased GABAergic inhibition and respiratory depression that occurs during this key developmental transitional period.
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Abstract
Rapid activation of postsynaptic GABAA receptors (GABAARs) is crucial in many neuronal functions, including the synchronization of neuronal ensembles and controlling the precise timing of action potentials. Although the γ2 subunit is believed to be essential for the postsynaptic clustering of GABAARs, synaptic currents have been detected in neurons obtained from γ2(-/-) mice. To determine the role of the γ2 subunit in synaptic GABAAR enrichment, we performed a spatially and temporally controlled γ2 subunit deletion by injecting Cre-expressing viral vectors into the neocortex of GABAARγ2(77I)lox mice. Whole-cell recordings revealed the presence of miniature IPSCs in Cre(+) layer 2/3 pyramidal cells (PCs) with unchanged amplitudes and rise times, but significantly prolonged decays. Such slowly decaying currents could be evoked in PCs by action potentials in presynaptic fast-spiking interneurons. Freeze-fracture replica immunogold labeling revealed the presence of the α1 and β3 subunits in perisomatic synapses of cells that lack the γ2 subunit. Miniature IPSCs in Cre(+) PCs were insensitive to low concentrations of flurazepam, providing a pharmacological confirmation of the lack of the γ2 subunit. Receptors assembled from only αβ subunits were unlikely because Zn(2+) did not block the synaptic currents. Pharmacological experiments indicated that the αβγ3 receptor, rather than the αβδ, αβε, or αβγ1 receptors, was responsible for the slowly decaying IPSCs. Our data demonstrate the presence of IPSCs and the synaptic enrichment of the α1 and β3 subunits and suggest that the γ3 subunit is the most likely candidate for clustering GABAARs at synapses in the absence of the γ2 subunit.
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Hengen KB, Nelson NR, Stang KM, Johnson SM, Crader SM, Watters JJ, Mitchell GS, Behan M. Increased GABA(A) receptor ε-subunit expression on ventral respiratory column neurons protects breathing during pregnancy. PLoS One 2012; 7:e30608. [PMID: 22303446 PMCID: PMC3269439 DOI: 10.1371/journal.pone.0030608] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2011] [Accepted: 12/24/2011] [Indexed: 12/14/2022] Open
Abstract
GABAergic signaling is essential for proper respiratory function. Potentiation of this signaling with allosteric modulators such as anesthetics, barbiturates, and neurosteroids can lead to respiratory arrest. Paradoxically, pregnant animals continue to breathe normally despite nearly 100-fold increases in circulating neurosteroids. ε subunit-containing GABAARs are insensitive to positive allosteric modulation, thus we hypothesized that pregnant rats increase ε subunit-containing GABAAR expression on brainstem neurons of the ventral respiratory column (VRC). In vivo, pregnancy rendered respiratory motor output insensitive to otherwise lethal doses of pentobarbital, a barbiturate previously used to categorize the ε subunit. Using electrode array recordings in vitro, we demonstrated that putative respiratory neurons of the preBötzinger Complex (preBötC) were also rendered insensitive to the effects of pentobarbital during pregnancy, but unit activity in the VRC was rapidly inhibited by the GABAAR agonist, muscimol. VRC unit activity from virgin and post-partum females was potently inhibited by both pentobarbital and muscimol. Brainstem ε subunit mRNA and protein levels were increased in pregnant rats, and GABAAR ε subunit expression co-localized with a marker of rhythm generating neurons (neurokinin 1 receptors) in the preBötC. These data support the hypothesis that pregnancy renders respiratory motor output and respiratory neuron activity insensitive to barbiturates, most likely via increased ε subunit-containing GABAAR expression on respiratory rhythm-generating neurons. Increased ε subunit expression may be critical to preserve respiratory function (and life) despite increased neurosteroid levels during pregnancy.
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Affiliation(s)
- Keith B Hengen
- Neuroscience Training Program, University of Wisconsin, Madison, Wisconsin, United States of America.
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14
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Herbison AE, Moenter SM. Depolarising and hyperpolarising actions of GABA(A) receptor activation on gonadotrophin-releasing hormone neurones: towards an emerging consensus. J Neuroendocrinol 2011; 23:557-69. [PMID: 21518033 PMCID: PMC3518440 DOI: 10.1111/j.1365-2826.2011.02145.x] [Citation(s) in RCA: 180] [Impact Index Per Article: 13.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
The gonadotrophin-releasing hormone (GnRH) neurones represent the final output neurones of a complex neuronal network that controls fertility. It is now appreciated that GABAergic neurones within this network provide an important regulatory influence on GnRH neurones. However, the consequences of direct GABA(A) receptor activation on adult GnRH neurones have been controversial for nearly a decade now, with both hyperpolarising and depolarising effects being reported. This review provides: (i) an overview of GABA(A) receptor function and its investigation using electrophysiological approaches and (ii) re-examines the past and present results relating to GABAergic regulation of the GnRH neurone, with a focus on mouse brain slice data. Although it remains difficult to reconcile the results of the early studies, there is a growing consensus that GABA can act through the GABA(A) receptor to exert both depolarising and hyperpolarising effects on GnRH neurones. The most recent studies examining the effects of endogenous GABA release on GnRH neurones indicate that the predominant action is that of excitation. However, we are still far from a complete understanding of the effects of GABA(A) receptor activation upon GnRH neurones. We argue that this will require not only a better understanding of chloride ion homeostasis in individual GnRH neurones, and within subcellular compartments of the GnRH neurone, but also a more integrative view of how multiple neurotransmitters, neuromodulators and intrinsic conductances act together to regulate the activity of these important cells.
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Affiliation(s)
- A E Herbison
- Centre for Neuroendocrinology and Department of Physiology, University of Otago School of Medical Sciences, Dunedin, New Zealand.
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15
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Liao IH, Corbett BA, Gilbert DL, Bunge SA, Sharp FR. Blood gene expression correlated with tic severity in medicated and unmedicated patients with Tourette Syndrome. Pharmacogenomics 2011; 11:1733-41. [PMID: 21142917 DOI: 10.2217/pgs.10.160] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022] Open
Abstract
BACKGROUND Tourette Syndrome (TS) has been linked to both genetic and environmental factors. Gene-expression studies provide valuable insight into the causes of TS; however, many studies of gene expression in TS do not account for the effects of medication. MATERIALS & METHODS To investigate the effects of medication on gene expression in TS patients, RNA was isolated from the peripheral blood of 20 medicated TS subjects (MED) and 23 unmedicated TS subjects (UNMED), and quantified using whole-genome Affymetrix microarrays. RESULTS D2 dopamine receptor expression correlated positively with tic severity in MED but not UNMED. GABA(A) receptor ε subunit expression negatively correlated with tic severity in UNMED but not MED. Phenylethanolamine N-methyltransferase expression positively correlated with tic severity in UNMED but not MED. CONCLUSION Modulation of tics by TS medication is associated with changes in dopamine, norepinephrine and GABA pathways.
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16
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Hengen KB, Gomez TM, Stang KM, Johnson SM, Behan M. Changes in ventral respiratory column GABAaR ε- and δ-subunits during hibernation mediate resistance to depression by EtOH and pentobarbital. Am J Physiol Regul Integr Comp Physiol 2010; 300:R272-83. [PMID: 21084677 DOI: 10.1152/ajpregu.00607.2010] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
During hibernation in the 13-lined ground squirrel, Ictidomys tridecemlineatus, the cerebral cortex is electrically silent, yet the brainstem continues to regulate cardiorespiratory function. Previous work showed that neurons in slices through the medullary ventral respiratory column (VRC) but not the cortex are insensitive to high doses of pentobarbital during hibernation, leading to the hypothesis that GABA(A) receptors (GABA(A)R) in the VRC undergo a seasonal modification in subunit composition. To test whether alteration of GABA(A)R subunits are responsible for hibernation-associated pentobarbital insensitivity, we examined an array of subunits using RT-PCR and Western blots and identified changes in ε- and δ-subunits in the medulla but not the cortex. Using immunohistochemistry, we confirmed that during hibernation, the expression of ε-subunit-containing GABA(A)Rs nearly doubles in the VRC. We also identified a population of δ-subunit-containing GABA(A)Rs adjacent to the VRC that were differentially expressed during hibernation. As δ-subunit-containing GABA(A)Rs are particularly sensitive to ethanol (EtOH), multichannel electrodes were inserted in slices of medulla and cortex from hibernating squirrels and EtOH was applied. EtOH, which normally inhibits neuronal activity, excited VRC but not cortical neurons during hibernation. This excitation was prevented by bicuculline pretreatment, indicating the involvement of GABA(A)Rs. We propose that neuronal activity in the VRC during hibernation is unaffected by pentobarbital due to upregulation of ε-subunit-containing GABA(A)Rs on VRC neurons. Synaptic input from adjacent inhibitory interneurons that express δ-subunit-containing GABA(A)Rs is responsible for the excitatory effects of EtOH on VRC neurons during hibernation.
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Affiliation(s)
- K B Hengen
- Neuroscience Training Program, University of Wisconsin, Madison, Wisconsin, USA
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17
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Tang X, Hernandez CC, Macdonald RL. Modulation of spontaneous and GABA-evoked tonic alpha4beta3delta and alpha4beta3gamma2L GABAA receptor currents by protein kinase A. J Neurophysiol 2009; 103:1007-19. [PMID: 19939957 DOI: 10.1152/jn.00801.2009] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Protein kinase A (PKA) has been reported to regulate synaptic alphabetagamma gamma-aminobutyric acid type A (GABA(A)) receptor currents, but whether PKA regulates GABA(A) receptor peri- and extrasynaptic tonic currents is unknown. GABA(A) receptors containing alpha4 subunits are important in mediating tonic inhibition and exist as both alpha4betadelta and alpha4betagamma receptors in the brain. To mimic GABA-independent and GABA-dependent tonic currents, we transfected HEK 293T cells with alpha4beta3delta or alpha4beta3gamma2L subunits and recorded spontaneous currents in the absence of applied GABA and steady-state currents in the presence of 1 muM GABA. Both alpha4beta3delta and alpha4beta3gamma2L receptors displayed spontaneous currents, but PKA activation increased spontaneous alpha4beta3delta currents substantially more than spontaneous alpha4beta3gamma2L currents. The increase in spontaneous alpha4beta3delta currents was due to an increase in single-channel open frequency. In contrast, PKA activation did not alter steady-state tonic currents recorded in the presence of 1 muM GABA. We concluded that PKA had a GABA concentration-dependent effect on alpha4beta3delta and alpha4beta3gamma2L currents. In the absence of GABA, spontaneous alpha4beta3delta and, to a lesser extent, alpha4beta3gamma2L currents could provide a basal, tonic current that could be regulated by PKA. With increasing concentrations of extracellular GABA, however, tonic alpha4beta3delta and alpha4beta3gamma2L currents would become more GABA dependent and less PKA sensitive. Thus in brain regions with fluctuating extracellular GABA levels, the dynamic range of GABA-activated tonic currents would be set by PKA and the increase in tonic current produced by increasing GABA would be reduced by PKA-mediated phosphorylation. When ambient GABA reaches micromolar concentrations, PKA would have no effect on steady-state tonic currents.
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Affiliation(s)
- Xin Tang
- Neuroscience Program, Vanderbilt University Medical Center, Nashville, TN 37232, USA
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18
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Sasaki K, Matsuo M, Maeda T, Zaitsu M, Hamasaki Y. Febrile seizures: characterization of double-stranded RNA-induced gene expression. Pediatr Neurol 2009; 41:114-8. [PMID: 19589459 DOI: 10.1016/j.pediatrneurol.2009.03.003] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/01/2008] [Revised: 02/23/2009] [Accepted: 03/04/2009] [Indexed: 11/29/2022]
Abstract
An association has long been suspected between febrile seizures and interleukin-1beta, the most potent endogenous pyrogen. Interleukin-1beta production increases after double-stranded RNA stimulation in leukocytes of febrile seizure patients. To elucidate the genetics of the immune response, the gene expression pattern after double-stranded RNA stimulation was investigated using DNA microarray. Compared with the control group, expression of the genes ACCN4 (sodium channel), KCNC3 (potassium channel), GABRE (gamma-aminobutyric acid receptor epsilon subunit), RIPK2 (receptor interacting protein kinase-2), TLR4 (toll-like receptor-4), IL26 (interleukin-26), and TNF (tumor necrosis factor), and CASP1 (caspase-1) was increased in the febrile seizure group (P < 0.01). Because RIPK2 and CASP1 are associated with interleukin-1beta production, increased expression might cause increased interleukin-1beta production in the febrile seizure patients. The induced expression of several ion channel genes by double-stranded RNA may affect neuronal excitability which leads to seizure susceptibility during infection.
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Affiliation(s)
- Kazuya Sasaki
- Department of Pediatrics, Faculty of Medicine, Saga University, Saga, Japan
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19
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Belujon P, Baufreton J, Grandoso L, Boué-Grabot E, Batten TFC, Ugedo L, Garret M, Taupignon AI. Inhibitory transmission in locus coeruleus neurons expressing GABAA receptor epsilon subunit has a number of unique properties. J Neurophysiol 2009; 102:2312-25. [PMID: 19625540 DOI: 10.1152/jn.00227.2009] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
Fast inhibitory synaptic transmission in the brain relies on ionotropic GABA(A) receptors (GABA(A)R). Eighteen genes code for GABA(A)R subunits, but little is known about the epsilon subunit. Our aim was to identify the synaptic transmission properties displayed by native receptors incorporating epsilon. Immunogold localization detected epsilon at synaptic sites on locus coeruleus (LC) neurons. In situ hybridization revealed prominent signals from epsilon, and mRNAs, some low beta1 and beta3 signals, and no gamma signal. Using in vivo extracellular and in vitro patch-clamp recordings in LC, we established that neuron firing rates, GABA-activated currents, and mIPSC charge were insensitive to the benzodiazepine flunitrazepam (FLU), in agreement with the characteristics of recombinant receptors including an epsilon subunit. Surprisingly, LC provided binding sites for benzodiazepines, and GABA-induced currents were potentiated by diazepam (DZP) in the micromolar range. A number of GABA(A)R ligands significantly potentiated GABA-induced currents, and zinc ions were only active at concentrations above 1 muM, further indicating that receptors were not composed of only alpha and beta subunits, but included an epsilon subunit. In contrast to recombinant receptors including an epsilon subunit, GABA(A)R in LC showed no agonist-independent opening. Finally, we determined that mIPSCs, as well as ensemble currents induced by ultra-fast GABA application, exhibited surprisingly slow rise times. Our work thus defines the signature of native GABA(A)R with a subunit composition including epsilon: differential sensitivity to FLU and DZP and slow rise time of currents. We further propose that alpha(3,) beta(1/3,) and epsilon subunits compose GABA(A)R in LC.
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Affiliation(s)
- P Belujon
- University Bordeaux, Centre National de la Recherche Scientifique Unité Mixte de Recherche, Bordeaux, France
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20
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Abstract
Most sedative-hypnotics used in insomnia treatment target the gamma-aminobutyric acid (GABA)(A) receptors. A vast repertoire of GABA(A) receptor subtypes has been identified and displays specific electrophysiological and functional properties. GABA(A)-mediated inhibition traditionally refers to 'phasic' inhibition, arising from synaptic GABA(A) receptors which transiently inhibit neurons. However, there is growing evidence that peri- or extra-synaptic GABA(A) receptors are continuously activated by low GABA concentrations and mediate a 'tonic' conductance. This slower type of signaling appears to play a key role in controlling cell excitability. This review aims at summarizing recent knowledge on GABA transmission, including the emergence of tonic conductance, and highlighting the importance of GABA(A) receptor heterogeneity. The mechanism of action of sedative-hypnotic drugs and their effects on sleep and the electroencephalogram will be reported. Furthermore, studies using genetically engineered mice will be emphasized, providing insights into the role of GABA(A) receptors in mechanisms underlying physiological and pharmacological sleep. Finally, we will address the potential of GABA(A) receptor pharmacology for the treatment of insomnia.
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Affiliation(s)
- Raphaëlle Winsky-Sommerer
- Section for Chronobiology and Sleep Research, Institute of Pharmacology and Toxicology, University of Zurich, Zurich, Switzerland.
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21
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Price TJ, Cervero F, Gold MS, Hammond DL, Prescott SA. Chloride regulation in the pain pathway. BRAIN RESEARCH REVIEWS 2009; 60:149-70. [PMID: 19167425 PMCID: PMC2903433 DOI: 10.1016/j.brainresrev.2008.12.015] [Citation(s) in RCA: 191] [Impact Index Per Article: 12.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Accepted: 12/29/2008] [Indexed: 12/18/2022]
Abstract
Melzack and Wall's Gate Control Theory of Pain laid the theoretical groundwork for a role of spinal inhibition in endogenous pain control. While the Gate Control Theory was based on the notion that spinal inhibition is dynamically regulated, mechanisms underlying the regulation of inhibition have turned out to be far more complex than Melzack and Wall could have ever imagined. Recent evidence indicates that an exquisitely sensitive form of regulation involves changes in anion equilibrium potential (E(anion)), which subsequently impacts fast synaptic inhibition mediated by GABA(A), and to a lesser extent, glycine receptor activation, the prototypic ligand gated anion channels. The cation-chloride co-transporters (in particular NKCC1 and KCC2) have emerged as proteins that play a critical role in the dynamic regulation of E(anion) which in turn appears to play a critical role in hyperalgesia and allodynia following peripheral inflammation or nerve injury. This review summarizes the current state of knowledge in this area with particular attention to how such findings relate to endogenous mechanisms of hyperalgesia and allodynia and potential applications for therapeutics based on modulation of intracellular Cl(-) gradients or pharmacological interventions targeting GABA(A) receptors.
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Affiliation(s)
| | - Fernando Cervero
- McGill University, Department of Anesthesia, McGill Centre for Research on Pain,
| | | | - Donna L Hammond
- University of Iowa, Department of Anesthesia, Department of Pharmacology,
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22
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Pape JR, Bertrand SS, Lafon P, Odessa MF, Chaigniau M, Stiles JK, Garret M. Expression of GABA(A) receptor alpha3-, theta-, and epsilon-subunit mRNAs during rat CNS development and immunolocalization of the epsilon subunit in developing postnatal spinal cord. Neuroscience 2009; 160:85-96. [PMID: 19249336 DOI: 10.1016/j.neuroscience.2009.02.043] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/19/2008] [Revised: 02/03/2009] [Accepted: 02/19/2009] [Indexed: 12/13/2022]
Abstract
Ionotropic GABA(A) receptors are heteromeric structures composed of a combination of five from at least 16 different subunits. Subunit genes are expressed in distinct cell types at specific times during development. The most abundant native GABA(A) receptors consist of alpha1-, beta2-, and gamma2-subunits that are co-expressed in numerous brain areas. alpha3-, theta-, And epsilon-subunits are clustered on the X chromosome and show striking overlapping expression patterns throughout the adult rat brain. To establish whether these subunits are temporally and spatially co-expressed, we used in situ hybridization to analyze their expression throughout rat development from embryonic stage E14 to postnatal stage P12. Each transcript exhibited a unique or a shared regional and temporal developmental expression profile. The thalamic expression pattern evolved from a restricted expression of epsilon and theta transcripts before birth, to a theta and alpha3 expression at birth, and finally to a grouped epsilon, theta and alpha3 expression postpartum. However, strong similarities occurred, such as a grouped expression of the three subunits within the hypothalamus, tegmentum and pontine nuclei throughout the developmental process. At early stages of development (E17), epsilon and theta appeared to have a greater spatial distribution before the dominance of the alpha3 subunit transcript around birth. We also revealed expression of alpha3, theta, and epsilon in the developing spinal cord and identified neurons that express epsilon in the postnatal dorsal horn, intermediolateral column and motoneurons. Our findings suggest that various combinations of alpha3-, theta- and epsilon-subunits may be assembled at a regional and developmental level in the brain.
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Affiliation(s)
- J-R Pape
- Université de Bordeaux, CNRS, UMR 5228, France
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23
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Olsen RW, Sieghart W. International Union of Pharmacology. LXX. Subtypes of gamma-aminobutyric acid(A) receptors: classification on the basis of subunit composition, pharmacology, and function. Update. Pharmacol Rev 2008; 60:243-60. [PMID: 18790874 DOI: 10.1124/pr.108.00505] [Citation(s) in RCA: 788] [Impact Index Per Article: 49.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
In this review we attempt to summarize experimental evidence on the existence of defined native GABA(A) receptor subtypes and to produce a list of receptors that actually seem to exist according to current knowledge. This will serve to update the most recent classification of GABA(A) receptors (Pharmacol Rev 50:291-313, 1998) approved by the Nomenclature Committee of the International Union of Pharmacology. GABA(A) receptors are chloride channels that mediate the major form of fast inhibitory neurotransmission in the central nervous system. They are members of the Cys-loop pentameric ligand-gated ion channel (LGIC) superfamily and share structural and functional homology with other members of that family. GABA(A) receptors are assembled from a family of 19 homologous subunit gene products and form numerous, mostly hetero-oligomeric, pentamers. Such receptor subtypes with properties that depend on subunit composition vary in topography and ontogeny, in cellular and subcellular localization, in their role in brain circuits and behaviors, in their mechanisms of regulation, and in their pharmacology. We propose several criteria, which can be applied to all the members of the LGIC superfamily, for including a receptor subtype on a list of native hetero-oligomeric subtypes. With these criteria, we develop a working GABA(A) receptor list, which currently includes 26 members, but will undoubtedly be modified and grow as information expands. The list is divided into three categories of native receptor subtypes: "identified," "existence with high probability," and "tentative."
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Affiliation(s)
- Richard W Olsen
- Department of Molecular and Medical Pharmacology, Geffen School of Medicine at UCLA, Room CHS 23-120, 650 Young Drive South, Los Angeles, CA 90095-1735, USA.
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24
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The promiscuous role of the epsilon subunit in GABAA receptor biogenesis. Mol Cell Neurosci 2008; 37:610-21. [DOI: 10.1016/j.mcn.2007.12.011] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/01/2007] [Revised: 11/21/2007] [Accepted: 12/06/2007] [Indexed: 11/18/2022] Open
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25
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Walker MC, Semyanov A. Regulation of excitability by extrasynaptic GABA(A) receptors. Results Probl Cell Differ 2008; 44:29-48. [PMID: 17671772 DOI: 10.1007/400_2007_030] [Citation(s) in RCA: 74] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/16/2023]
Abstract
Not only are GABA(A) receptors activated transiently by GABA released at synapses, but high affinity, extrasynaptic GABA(A) receptors are also activated by ambient, extracellular GABA as a more persistent form of signalling (often termed tonic inhibition). Over the last decade tonic GABA(A) receptor-mediated inhibition and the properties of GABA(A) receptors mediating this signalling have received increasing attention. Tonic inhibition is present throughout the central nervous system, but is expressed in a cell-type specific manner (e.g. in interneurons more so than in pyramidal cells in the hippocampus, and in thalamocortical neurons more so than in reticular thalamic neurons in the thalamus). As a consequence, tonic inhibition can have a complex effect on network activity. Tonic inhibition is not fixed but can be modulated by endogenous and exogenous modulators, such as neurosteroids, and by developmental, physiological and pathological regulation of GABA uptake and GABA(A) receptor expression. There is also growing evidence that tonic currents play an important role in epilepsy, sleep (also actions of anaesthetics and sedatives), memory and cognition. Therefore, drugs specifically aimed at targeting the extrasynaptic receptors involved in tonic inhibition could be a novel approach to regulating both physiological and pathological processes.
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Affiliation(s)
- Matthew C Walker
- Institute of Neurology, University College London, Queen Square, London, UK.
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26
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Abstract
Incorporation of the epsilon subunit into the GABAA receptor has been suggested to confer unusual, but variable, biophysical and pharmacological characteristics to both recombinant and native receptors. Due to their structural similarity with the gamma subunits, epsilon subunits have been assumed to substitute at the single position of the gamma subunit in assembled receptors. However, prior work suggests that functional variability in epsilon-containing receptors may reflect alternative sites of incorporation and of not just one, but possibly multiple epsilon subunits in the pentameric receptor complex. Here we present data indicating that increased expression of epsilon, in conjunction with alpha2 and beta3 subunits, results in expression of GABAA receptors with correspondingly altered rectification, deactivation and levels of spontaneous openings, but not increased total current density. We also provide data that the epsilon subunit, like the beta3 subunit, can self-export and data from chimeric receptors suggesting that similarities between the assembly domains of the beta3 and the epsilon subunits may allow the epsilon subunit to replace the beta, as well as the gamma, subunit. The substitution of an epsilon for a beta, as well as the gamma subunit and formation of receptors with alternative patterns of assembly with respect to epsilon incorporation may underlie the observed variability in both biophysical and pharmacological properties noted not only in recombinant, but also in native receptors.
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Affiliation(s)
- Brian L Jones
- Department of Physiology, Dartmouth Medical School, Hanover, New Hampshire 03755, USA
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27
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Pytel M, Wójtowicz T, Mercik K, Sarto-Jackson I, Sieghart W, Ikonomidou C, Mozrzymas JW. 17 β-estradiol modulates GABAergic synaptic transmission and tonic currents during development in vitro. Neuropharmacology 2007; 52:1342-53. [PMID: 17418284 DOI: 10.1016/j.neuropharm.2007.01.014] [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: 11/14/2006] [Revised: 01/17/2007] [Accepted: 01/18/2007] [Indexed: 10/23/2022]
Abstract
Estrogens exert a variety of modulatory effects on the structure and function of the nervous system. In particular, 17 beta-estradiol was found to affect GABAergic inhibition in adult animals but its action on GABAergic currents during development has not been elucidated. In the present study, we investigated the effect of 17 beta-estradiol on hippocampal neurons developing in vitro. In this model, mIPSC kinetics showed acceleration with age along with increased alpha1 subunit expression, similarly as in vivo. Long-term treatment with 17 beta-estradiol increased mIPSC amplitudes in neurons cultured for 6-8 and 9-11DIV and prolonged the mIPSC decaying phase only in the 9-11DIV group. The time needed for the onset of 17 beta-estradiol effect on mIPSC amplitude was approximately 48 h. In the period of 9-11DIV, treatment with 17 beta-estradiol strongly reduced the tonic conductance activated by low GABA concentrations. The effects of 17 beta-estradiol on mIPSCs and tonic conductance were not correlated with any change in expression of considered GABAAR subunits (alpha1-3, alpha5-6, gamma2) while alpha4 and delta subunits were at the detection limit. In conclusion, we provide evidence that 17 beta-estradiol differentially affects the phasic and tonic components of GABAergic currents in neurons developing in vitro.
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Affiliation(s)
- Maria Pytel
- Laboratory of Neuroscience, Department of Biophysics, Wroclaw Medical University, Chalubinskiego 3, 50-368 Wroclaw, Poland
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28
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Farrant M, Kaila K. The cellular, molecular and ionic basis of GABA(A) receptor signalling. PROGRESS IN BRAIN RESEARCH 2007; 160:59-87. [PMID: 17499109 DOI: 10.1016/s0079-6123(06)60005-8] [Citation(s) in RCA: 266] [Impact Index Per Article: 15.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/10/2023]
Abstract
GABA(A) receptors mediate fast synaptic inhibition in the CNS. Whilst this is undoubtedly true, it is a gross oversimplification of their actions. The receptors themselves are diverse, being formed from a variety of subunits, each with a different temporal and spatial pattern of expression. This diversity is reflected in differences in subcellular targetting and in the subtleties of their response to GABA. While activation of the receptors leads to an inevitable increase in membrane conductance, the voltage response is dictated by the distribution of the permeant Cl(-) and HCO(3)(-) ions, which is established by anion transporters. Similar to GABA(A) receptors, the expression of these transporters is not only developmentally regulated but shows cell-specific and subcellular variation. Untangling all these complexities allows us to appreciate the variety of GABA-mediated signalling, a diverse set of phenomena encompassing both synaptic and non-synaptic functions that can be overtly excitatory as well as inhibitory.
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Affiliation(s)
- Mark Farrant
- Department of Pharmacology, UCL (University College London), Gower Street, London WC1E 6BT, UK.
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29
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McCartney MR, Deeb TZ, Henderson TN, Hales TG. Tonically Active GABAA Receptors in Hippocampal Pyramidal Neurons Exhibit Constitutive GABA-Independent Gating. Mol Pharmacol 2006; 71:539-48. [PMID: 17090706 DOI: 10.1124/mol.106.028597] [Citation(s) in RCA: 72] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Phasic and tonic inhibitory currents of hippocampal pyramidal neurons exhibit distinct pharmacological properties. Picrotoxin and bicuculline methiodide inhibited both components, consistent with a role for GABAA receptors; however, gabazine, at a concentration that abolished miniature GABAergic inhibitory postsynaptic currents and responses to exogenous GABA, had no effect on tonic currents. Because all GABA-activated GABAA receptors in pyramidal neurons are gabazine-sensitive, it follows that tonic currents are not GABA-activated. Furthermore, picrotoxin-sensitive spontaneous single-channel events recorded from outside-out patches had the same chord conductance as GABA-activated channels and were gabazine-resistant. Therefore, we hypothesize that GABAA receptors, constitutively active in the absence of GABA, mediate tonic current; the failure of gabazine to block tonic current reflects a lack of negative intrinsic efficacy of the antagonist. We compared the negative efficacies of bicuculline and gabazine using the general anesthetic propofol to directly activate GABAA receptors native to pyramidal neurons or alpha1beta3gamma2 receptors recombinantly expressed in human embryonic kidney 293 cells. Propofol activated gabazine-resistant, bicuculline-sensitive currents when applied to either preparation. Although gabazine had negligible efficacy as an inhibitor of propofol-activated currents, it prevented inhibition by bicuculline, which acts as an inverse agonist inhibiting GABA-independent gating. Recombinant alpha1beta1/3gamma2 receptors also mediated agonist-independent tonic currents that were resistant to gabazine and inhibited by bicuculline. Thus, gabazine is a competitive antagonist with negligible negative efficacy and is therefore unable to inhibit GABAA receptors that are active in the absence of GABA because of either anesthetic or spontaneous gating. Moreover, spontaneously active GABAA receptors mediate gabazine-resistant tonic currents in pyramidal neurons.
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Affiliation(s)
- Melissa R McCartney
- Department of Pharmacology, The George Washington University, 2300 Eye Street NW, Washington, DC 20037, USA.
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Mizielinska S, Greenwood S, Connolly CN. The role of GABAA receptor biogenesis, structure and function in epilepsy. Biochem Soc Trans 2006; 34:863-7. [PMID: 17052216 DOI: 10.1042/bst0340863] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Maintaining the correct balance in neuronal activation is of paramount importance to normal brain function. Imbalances due to changes in excitation or inhibition can lead to a variety of disorders ranging from the clinically extreme (e.g. epilepsy) to the more subtle (e.g. anxiety). In the brain, the most common inhibitory synapses are regulated by GABAA (γ-aminobutyric acid type A) receptors, a role commensurate with their importance as therapeutic targets. Remarkably, we still know relatively little about GABAA receptor biogenesis. Receptors are constructed as pentameric ion channels, with α and β subunits being the minimal requirement, and the incorporation of a γ subunit being necessary for benzodiazepine modulation and synaptic targeting. Insights have been provided by the discovery of several specific assembly signals within different GABAA receptor subunits. Moreover, a number of recent studies on GABAA receptor mutations associated with epilepsy have further enhanced our understanding of GABAA receptor biogenesis, structure and function.
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Affiliation(s)
- S Mizielinska
- Neuroscience Institute, Ninewells Medical School, University of Dundee, Dundee DD1 9SY, UK
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Bouairi E, Kamendi H, Wang X, Gorini C, Mendelowitz D. Multiple types of GABAA receptors mediate inhibition in brain stem parasympathetic cardiac neurons in the nucleus ambiguus. J Neurophysiol 2006; 96:3266-72. [PMID: 16914614 DOI: 10.1152/jn.00590.2006] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Recent work suggests neurons can have different types of gamma-aminobutyric acid type A (GABA(A)) receptors that mediate phasic inhibitory postsynaptic currents (IPSCs) and tonic currents. This study examines the diversity of GABAergic synaptic currents in parasympathetic cardioinhibitory neurons that receive rhythmic bursts of GABAergic neurotransmission. Focal application of gabazine (25 microM) to cardiac vagal neurons in vitro did not change the frequency of firing in spontaneously active neurons or the resting membrane potential; however, picrotoxin (100 microM) significantly depolarized cardiac vagal neurons and increased their firing. Similarly, gabazine (25 microM) selectively blocked GABAergic IPSCs but did not change holding current in cardiac vagal neurons, whereas picrotoxin (100 microM) not only blocked GABAergic IPSCs but also rapidly decreased the tonic current. Because the tonic current could be attributable to activation of GABA receptors by ambient GABA or, alternatively, spontaneous opening of constitutively active GABA channels, an antagonist for the GAT-1 GABA transporter NO-711 (10 microM) was applied to distinguish between these possibilities. NO-711 did not significantly alter the holding current in these neurons. The benzodiazepine flunitrazepam (1 microM) significantly increased the tonic current and GABAergic IPSC decay time; surprisingly, however, in the presence of gabazine flunitrazepam failed to elicit any change. These results suggest cardiac vagal neurons possess gabazine-sensitive GABA(A) receptors that mediate phasic synaptic currents, a gabazine-insensitive but picrotoxin-sensitive extrasynaptic tonic current that when blocked depolarizes and increases the firing rate of cardiac vagal neurons, and benzodiazepines recruit a third type of GABA(A) receptor that is sensitive to gabazine and augments the extrasynaptic tonic current.
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Affiliation(s)
- Euguenia Bouairi
- Department of Pharmacology and Physiology, The George Washington University, 2300 Eye Street NW, Washington, DC 20037, USA
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Jones BL, Whiting PJ, Henderson LP. Mechanisms of anabolic androgenic steroid inhibition of mammalian epsilon-subunit-containing GABAA receptors. J Physiol 2006; 573:571-93. [PMID: 16543268 PMCID: PMC1779744 DOI: 10.1113/jphysiol.2006.106534] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022] Open
Abstract
GABAergic transmission regulates the activity of gonadotrophin-releasing hormone (GnRH) neurons in the preoptic area/hypothalamus that control the onset of puberty and the expression of reproductive behaviours. One of the hallmarks of illicit use of anabolic androgenic steroids (AAS) is disruption of behaviours under neuroendocrine control. GnRH neurons are among a limited population of cells that express high levels of the epsilon-subunit of the GABAA receptor. To better understand the actions of AAS on neuroendocrine mechanisms, we have characterized modulation of GABAA receptor-mediated currents in mouse native GnRH neurons and in heterologous cells expressing recombinant alpha2beta3epsilon-receptors. GnRH neurons exhibited robust currents in response to millimolar concentrations of GABA and a picrotoxin (PTX)-sensitive, bicuculline-insensitive current that probably arises from spontaneous openings of GABAA receptors. The AAS 17alpha-methyltestosterone (17alpha-MeT) inhibited spontaneous and GABA-evoked currents in GnRH neurons. For recombinant alpha2beta3epsilon-receptors, 17alpha-MeT inhibited phasic and tonic GABA-elicited responses, accelerated desensitization and slowed paired pulse response recovery. Single channel analysis indicated that GABA-evoked events could be described by three open dwell components and that 17alpha-MeT enhanced residence in the intermediate dwell state. This AAS also inhibited a PTX-sensitive, spontaneous current (open probability, approximately 0.15-0.2) in a concentration-dependent fashion (IC50 approximately 9 microm). Kinetic modelling indicated that the inhibition induced by 17alpha-MeT occurs by an allosteric block in which the AAS interacts preferentially with a closed state and promotes accumulation in that state. Finally, studies with a G302S mutant epsilon-subunit suggest that this residue within the transmembrane domain TM2 plays a role in mediating AAS binding and modulation. In sum, our results indicate that inclusion of the epsilon-subunit significantly alters the profile of AAS modulation and that this allosteric inhibition of native GnRH neurons should be considered with regard to AAS disruption of neuroendocrine control.
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Affiliation(s)
- Brian L Jones
- Department of Physiology, Dartmouth Medical School, Hanover, New Hampshire 03755, USA
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Ranna M, Sinkkonen ST, Möykkynen T, Uusi-Oukari M, Korpi ER. Impact of epsilon and theta subunits on pharmacological properties of alpha3beta1 GABAA receptors expressed in Xenopus oocytes. BMC Pharmacol 2006; 6:1. [PMID: 16412217 PMCID: PMC1363348 DOI: 10.1186/1471-2210-6-1] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2005] [Accepted: 01/13/2006] [Indexed: 11/29/2022] Open
Abstract
Background γ-Aminobutyric acid type A (GABAA) receptors provide the main inhibitory control in the brain. Their heterogeneity may make it possible to precisely target drug effects to selected neuronal populations. In situ hybridization using rat brain sections has revealed a unique expression of GABAA receptor ε and θ subunit transcripts in the locus coeruleus, where they are accompanied at least by α3, α2, β1 and β3 subunits. Here, we studied the pharmacology of the human α3β1, α3β1ε, α3β1θ and α3β1εθ receptor subtypes expressed in Xenopus oocytes and compared them with the γ2 subunit-containing receptors. Results The GABA sensitivites and effects of several positive modulators of GABAA receptors were studied in the absence and the presence of EC25 GABA using the two-electrode voltage-clamp method. We found 100-fold differences in GABA sensitivity between the receptors, α3β1ε subtype being the most sensitive and α3β1γ2 the least sensitive. Also gaboxadol dose-response curves followed the same sensitivity rank order, with EC50 values being 72 and 411 μM for α3β1ε and α3β1γ2 subtypes, respectively. In the presence of EC25 GABA, introduction of the ε subunit to the receptor complex resulted in diminished modulatory effects by etomidate, propofol, pregnanolone and flurazepam, but not by pentobarbital. Furthermore, the α3β1ε subtype displayed picrotoxin-sensitive spontaneous activity. The θ subunit-containing receptors were efficiently potentiated by the anesthetic etomidate, suggesting that θ subunit could bring the properties of β2 or β3 subunits to the receptor complex. Conclusion The ε and θ subunits bring additional features to α3β1 GABAA receptors. These receptor subtypes may constitute as novel drug targets in selected brain regions, e.g., in the brainstem locus coeruleus nuclei.
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Affiliation(s)
- Martin Ranna
- Institute of Biomedicine, Pharmacology, Biomedicum Helsinki, POB 63 (Haartmaninkatu 8), FI-00014 University of Helsinki, Finland
| | - Saku T Sinkkonen
- Institute of Biomedicine, Pharmacology, Biomedicum Helsinki, POB 63 (Haartmaninkatu 8), FI-00014 University of Helsinki, Finland
| | - Tommi Möykkynen
- Institute of Biomedicine, Pharmacology, Biomedicum Helsinki, POB 63 (Haartmaninkatu 8), FI-00014 University of Helsinki, Finland
| | - Mikko Uusi-Oukari
- Department of Pharmacology and Clinical Pharmacology, University of Turku, Itäinen Pitkäkatu 4, FI-20520 Turku, Finland
| | - Esa R Korpi
- Institute of Biomedicine, Pharmacology, Biomedicum Helsinki, POB 63 (Haartmaninkatu 8), FI-00014 University of Helsinki, Finland
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