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Shu HJ, Lu X, Bracamontes J, Steinbach JH, Zorumski CF, Mennerick S. Pharmacological and Biophysical Characteristics of Picrotoxin-Resistant, δSubunit-Containing GABA A Receptors. Front Synaptic Neurosci 2021; 13:763411. [PMID: 34867260 PMCID: PMC8636460 DOI: 10.3389/fnsyn.2021.763411] [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: 08/23/2021] [Accepted: 10/26/2021] [Indexed: 11/21/2022] Open
Abstract
GABAA receptors (GABAARs) play a crucial role in inhibition in the central nervous system. GABAARs containing the δ subunit mediate tonic inhibition, have distinctive pharmacological properties and are associated with disorders of the nervous system. To explore this receptor sub-class, we recently developed mice with δ-containing receptors rendered resistant to the common non-competitive antagonist picrotoxin (PTX). Resistance was achieved with a knock-in point mutation (T269Y; T6’Y) in the mouse genome. Here we characterize pharmacological and biophysical features of GABAARs containing the mutated subunit to contextualize results from the KI mice. Recombinant receptors containing δ T6’Y plus WT α4 and WT β2 subunits exhibited 3-fold lower EC50 values for GABA but not THIP. GABA EC50 values in native receptors containing the mutated subunit were in the low micromolar range, in contrast with some published results that have suggested nM sensitivity of recombinant receptors. Rectification properties of δ-containing GABAARs were similar to γ2-containing receptors. Receptors containing δ T6’Y had marginally weaker sensitivity to positive allosteric modulators, likely a secondary consequence of differing GABA sensitivity. Overexpression of δT6’Y in neurons resulted in robust PTX-insensitive IPSCs, suggesting that δ-containing receptors are readily recruited by synaptically released GABA. Overall, our results give context to the use of δ receptors with the T6’Y mutation to explore the roles of δ-containing receptors in inhibition.
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Affiliation(s)
- Hong-Jin Shu
- Department of Psychiatry, Washington University in St. Louis, School of Medicine, St. Louis, MO, United States
| | - Xinguo Lu
- Department of Psychiatry, Washington University in St. Louis, School of Medicine, St. Louis, MO, United States
| | - John Bracamontes
- Department of Anesthesiology, Washington University in St. Louis, School of Medicine, St. Louis, MO, United States
| | - Joe Henry Steinbach
- Department of Anesthesiology, Washington University in St. Louis, School of Medicine, St. Louis, MO, United States.,Taylor Family Institute for Innovative Psychiatric Research, Washington University in St. Louis, School of Medicine, St. Louis, MO, United States
| | - Charles F Zorumski
- Department of Psychiatry, Washington University in St. Louis, School of Medicine, St. Louis, MO, United States.,Taylor Family Institute for Innovative Psychiatric Research, Washington University in St. Louis, School of Medicine, St. Louis, MO, United States.,Department of Neuroscience, Washington University in St. Louis, School of Medicine, St. Louis, MO, United States
| | - Steven Mennerick
- Department of Psychiatry, Washington University in St. Louis, School of Medicine, St. Louis, MO, United States.,Taylor Family Institute for Innovative Psychiatric Research, Washington University in St. Louis, School of Medicine, St. Louis, MO, United States.,Department of Neuroscience, Washington University in St. Louis, School of Medicine, St. Louis, MO, United States
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Sun MY, Ziolkowski L, Mennerick S. δ subunit-containing GABA A IPSCs are driven by both synaptic and diffusional GABA in mouse dentate granule neurons. J Physiol 2020; 598:1205-1221. [PMID: 31951019 DOI: 10.1113/jp279317] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/26/2019] [Accepted: 01/15/2020] [Indexed: 01/10/2023] Open
Abstract
KEY POINTS Current views suggest γ2 subunit-containing GABAA receptors mediate phasic IPSCs while extrasynaptic δ subunits mediate diffusional IPSCs and tonic current. We have re-examined the roles of the two receptor populations using mice with picrotoxin resistance engineered into receptors containing the δ subunit. Using pharmacological separation, we find that in general δ and γ IPSCs are modulated in parallel by manipulations of transmitter output and diffusion, with evidence favouring modestly more diffusional contribution to δ IPSCs. Our findings also reveal that spontaneous δ IPSCs are mainly driven by channel deactivation, rather than by diffusion of GABA. Understanding the functional contributions of the two receptor classes may help us understand the actions of drug therapies with selective effects on one population over the other. ABSTRACT GABAA receptors mediate transmission throughout the central nervous system and typically contain a δ subunit (δ receptors) or a γ2 subunit (γ2 receptors). δ IPSCs decay slower than γ2 IPSCs, but the reasons are unclear. Transmitter diffusion, rebinding, or slow deactivation kinetics of channels are candidates. We used gene editing to confer picrotoxin resistance on δ receptors in mice, then pharmacologically isolated δ receptors in mouse dentate granule cells to explore IPSCs. γ2 and δ components of IPSCs were modulated similarly by presynaptic manipulations and manipulations of transmitter lifetime, suggesting that GABA release recruits δ receptors proportionally to γ2 receptors. δ IPSCs showed more sensitivity to altered transmitter release and to a rapidly dissociating antagonist, suggesting an additional spillover contribution. Reducing GABA diffusion with 5% dextran increased the peak amplitude and decreased the decay of evoked δ IPSCs but had no effect on δ or dual-component (mainly γ2-driven) spontaneous IPSCs, suggesting that GABA actions can be local for both receptor types. Rapid application of varied [GABA] onto nucleated patches from dentate granule cells demonstrated a deactivation rate of δ receptors similar to that of δ spontaneous IPSCs, consistent with the idea that deactivation and local GABA actions drive δ spontaneous IPSCs. Overall, our results indicate that δ IPSCs are activated by both synaptic and diffusional GABA. Our results are consistent with a functional relationship between δ and γ2 GABAA receptors akin to that of slow NMDA and fast AMPA EPSCs at glutamate synapses.
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Affiliation(s)
- Min-Yu Sun
- Department of Psychiatry, Washington University School of Medicine, 660 S. Euclid Ave, Box 8134, St Louis, MO, 63110, USA
| | - Luke Ziolkowski
- Department of Psychiatry, Washington University School of Medicine, 660 S. Euclid Ave, Box 8134, St Louis, MO, 63110, USA
| | - Steven Mennerick
- Department of Psychiatry, Washington University School of Medicine, 660 S. Euclid Ave, Box 8134, St Louis, MO, 63110, USA.,Department of Neuroscience, Washington University School of Medicine, 660 S. Euclid Ave, Box 8134, St Louis, MO, 63110, USA.,Taylor Family Institute for Innovative Psychiatric Research, Washington University School of Medicine, 660 S. Euclid Ave, Box 8134, St Louis, MO, 63110, USA
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3
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Chemogenetic Isolation Reveals Synaptic Contribution of δ GABA A Receptors in Mouse Dentate Granule Neurons. J Neurosci 2018; 38:8128-8145. [PMID: 30076210 DOI: 10.1523/jneurosci.0799-18.2018] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/26/2018] [Revised: 07/18/2018] [Accepted: 07/24/2018] [Indexed: 11/21/2022] Open
Abstract
Two major GABAA receptor classes mediate ionotropic GABA signaling, those containing a δ subunit and those with a γ2 subunit. The classical viewpoint equates γ2-containing receptors with IPSCs and δ-containing receptors with tonic inhibition because of differences in receptor localization, but significant questions remain because the populations cannot be pharmacologically separated. We removed this barrier using gene editing to confer a point mutation on the δ subunit in mice, rendering receptors containing the subunit picrotoxin resistant. By pharmacologically isolating δ-containing receptors, our results demonstrate their contribution to IPSCs in dentate granule neurons and weaker contributions to thalamocortical IPSCs. Despite documented extrasynaptic localization, we found that receptor localization does not preclude participation in isolated IPSCs, including mIPSCs. Further, phasic inhibition from δ subunit-containing receptors strongly inhibited summation of EPSPs, whereas tonic activity had little impact. In addition to any role that δ-containing receptors may play in canonical tonic inhibition, our results highlight a previously underestimated contribution of δ-containing receptors to phasic inhibition.SIGNIFICANCE STATEMENT GABAA receptors play key roles in transient and tonic inhibition. The prevailing view suggests that synaptic γ2-containing GABAA receptors drive phasic inhibition, whereas extrasynaptic δ-containing receptors mediate tonic inhibition. To re-evaluate the impact of δ receptors, we took a chemogenetic approach that offers a sensitive method to probe the synaptic contribution of δ-containing receptors. Our results reveal that localization does not strongly limit the contribution of δ receptors to IPSCs and that δ receptors make an unanticipated robust contribution to phasic inhibition.
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Feng HJ, Forman SA. Comparison of αβδ and αβγ GABA A receptors: Allosteric modulation and identification of subunit arrangement by site-selective general anesthetics. Pharmacol Res 2017; 133:289-300. [PMID: 29294355 DOI: 10.1016/j.phrs.2017.12.031] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/18/2017] [Revised: 12/28/2017] [Accepted: 12/29/2017] [Indexed: 12/27/2022]
Abstract
GABAA receptors play a dominant role in mediating inhibition in the mature mammalian brain, and defects of GABAergic neurotransmission contribute to the pathogenesis of a variety of neurological and psychiatric disorders. Two types of GABAergic inhibition have been described: αβγ receptors mediate phasic inhibition in response to transient high-concentrations of synaptic GABA release, and αβδ receptors produce tonic inhibitory currents activated by low-concentration extrasynaptic GABA. Both αβδ and αβγ receptors are important targets for general anesthetics, which induce apparently different changes both in GABA-dependent receptor activation and in desensitization in currents mediated by αβγ vs. αβδ receptors. Many of these differences are explained by correcting for the high agonist efficacy of GABA at most αβγ receptors vs. much lower efficacy at αβδ receptors. The stoichiometry and subunit arrangement of recombinant αβγ receptors are well established as β-α-γ-β-α, while those of αβδ receptors remain controversial. Importantly, some potent general anesthetics selectively bind in transmembrane inter-subunit pockets of αβγ receptors: etomidate acts at β+/α- interfaces, and the barbiturate R-5-allyl-1-methyl-5-(m-trifluoromethyl-diazirynylphenyl) barbituric acid (R-mTFD-MPAB) acts at α+/β- and γ+/β- interfaces. Thus, these drugs are useful as structural probes in αβδ receptors formed from free subunits or concatenated subunit assemblies designed to constrain subunit arrangement. Although a definite conclusion cannot be drawn, studies using etomidate and R-mTFD-MPAB support the idea that recombinant α1β3δ receptors may share stoichiometry and subunit arrangement with α1β3γ2 receptors.
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Affiliation(s)
- Hua-Jun Feng
- Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, and Department of Anesthesia, Harvard Medical School, Boston, MA 02114, USA.
| | - Stuart A Forman
- Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, and Department of Anesthesia, Harvard Medical School, Boston, MA 02114, USA.
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Falk-Petersen CB, Søgaard R, Madsen KL, Klein AB, Frølund B, Wellendorph P. Development of a Robust Mammalian Cell-based Assay for Studying Recombinant α 4 β 1/3 δ GABA A Receptor Subtypes. Basic Clin Pharmacol Toxicol 2017; 121:119-129. [PMID: 28299900 DOI: 10.1111/bcpt.12778] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/06/2017] [Accepted: 03/10/2017] [Indexed: 11/29/2022]
Abstract
δ-Containing GABAA receptors are located extrasynaptically and mediate tonic inhibition. Their involvement in brain physiology positions them as interesting drug targets. There is thus a continued interest in establishing reliable recombinant expression systems for δ-containing GABAA receptors. Inconveniently, the recombinant expression of especially α4 β1/3 δ receptors has been found to be notoriously difficult, resulting in mixed receptor populations and/or stoichiometries and differential pharmacology depending on the expression system used. With the aim of developing a facile and robust 96-well format cell-based assay for extrasynaptic α4 β1/3 δ receptors, we have engineered and validated a HEK293 Flp-In™ cell line stably expressing the human GABAA δ-subunit. Upon co-transfection of α4 and β1/3 subunits, at optimized ratios, we have established a well-defined system for expressing α4 β1/3 δ receptors and used the fluorescence-based FLIPR Membrane Potential (FMP) assay to evaluate their pharmacology. Using the known reference compounds GABA and THIP, ternary α4 β1/3 δ and binary α4 β1/3 receptors could be distinguished based on potency and kinetic profiles but not efficacy. As expected, DS2 was able to potentiate only δ-containing receptors, whereas Zn2+ had an inhibitory effect only at binary receptors. By contrast, the hitherto reported δ-selective compounds, AA29504 and 3-OH-2'MeO6MF, were non-selective. The expression system was further validated using patch clamp electrophysiology, in which the superagonism of THIP was confirmed. The established FMP assay set-up, based on transient expression of human α4 and β1/3 subunits into a δ-subunit stable HEK293 Flp-In™ cell line, portrays a simple 96-well format assay as a useful supplement to electrophysiological recordings on δ-containing GABAA receptors.
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Affiliation(s)
- Christina B Falk-Petersen
- Department of Drug Design and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Rikke Søgaard
- Department of Drug Design and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Kenneth L Madsen
- Molecular Neuropharmacology and Genetics Laboratory, Lundbeck Foundation Center for Biomembranes in Nanomedicine, Department of Neuroscience and Pharmacology, Faculty of Health and Medical Sciences, The Panum Institute, University of Copenhagen, Copenhagen, Denmark
| | - Anders B Klein
- Department of Drug Design and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Bente Frølund
- Department of Drug Design and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
| | - Petrine Wellendorph
- Department of Drug Design and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
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Wongsamitkul N, Baur R, Sigel E. Toward Understanding Functional Properties and Subunit Arrangement of α4β2δ γ-Aminobutyric Acid, Type A (GABAA) Receptors. J Biol Chem 2016; 291:18474-83. [PMID: 27382064 DOI: 10.1074/jbc.m116.738906] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/18/2016] [Indexed: 11/06/2022] Open
Abstract
GABAA receptors are pentameric ligand-gated channels mediating inhibitory neurotransmission in the CNS. α4βxδ GABAA receptors are extrasynaptic receptors important for tonic inhibition. The functional properties and subunit arrangement of these receptors are controversial. We predefined subunit arrangement by using subunit concatenation. α4, β2, and δ subunits were concatenated to dimeric, trimeric, and, in some cases, pentameric subunits. We constructed in total nine different receptor pentamers in at least two different ways and expressed them in Xenopus oocytes. The δ subunit was substituted in any of the five positions in the α1β2 receptor. In addition, we investigated all receptors with the 2:2:1 subunit stoichiometry for α4, β2, and δ. Several functional receptors were obtained. Interestingly, all of these receptors had very similar EC50 values for GABA in the presence of the neurosteroid 3α, 21-dihydroxy-5α-pregnan-20-one (THDOC). All functional receptors containing δ subunits were sensitive to 4-chloro-N-[2-(2-thienyl)imidazo[1,2-a]pyridin-3-yl]benzamide (DS2). Moreover, none of the receptors was affected by ethanol up to 30 mm These properties recapitulate those of non-concatenated receptors expressed from a cRNA ratio of 1:1:5 coding for α4, β2, and δ subunits. We conclude that the subunit arrangement of α4β2δ GABAA receptors is not strongly predefined but is mostly satisfying the 2:2:1 subunit stoichiometry for α4, β2, and δ subunits and that several subunit arrangements result in receptors with similar functional properties tuned to physiological conditions.
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Affiliation(s)
- Nisa Wongsamitkul
- From the Institute of Biochemistry and Molecular Medicine, University of Bern, 3012 Bern, Switzerland
| | - Roland Baur
- From the Institute of Biochemistry and Molecular Medicine, University of Bern, 3012 Bern, Switzerland
| | - Erwin Sigel
- From the Institute of Biochemistry and Molecular Medicine, University of Bern, 3012 Bern, Switzerland
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7
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Jaenisch N, Liebmann L, Guenther M, Hübner CA, Frahm C, Witte OW. Reduced tonic inhibition after stroke promotes motor performance and epileptic seizures. Sci Rep 2016; 6:26173. [PMID: 27188341 PMCID: PMC4870642 DOI: 10.1038/srep26173] [Citation(s) in RCA: 39] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/25/2016] [Accepted: 04/28/2016] [Indexed: 01/19/2023] Open
Abstract
Stroke survivors often recover from motor deficits, either spontaneously or with the support of rehabilitative training. Since tonic GABAergic inhibition controls network excitability, it may be involved in recovery. Middle cerebral artery occlusion in rodents reduces tonic GABAergic inhibition in the structurally intact motor cortex (M1). Transcript and protein abundance of the extrasynaptic GABAA-receptor complex α4β3δ are concurrently reduced (δ-GABAARs). In vivo and in vitro analyses show that stroke-induced glutamate release activates NMDA receptors, thereby reducing KCC2 transporters and down-regulates δ-GABAARs. Functionally, this is associated with improved motor performance on the RotaRod, a test in which mice are forced to move in a similar manner to rehabilitative training sessions. As an adverse side effect, decreased tonic inhibition facilitates post-stroke epileptic seizures. Our data imply that early and sometimes surprisingly fast recovery following stroke is supported by homeostatic, endogenous plasticity of extrasynaptic GABAA receptors.
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Affiliation(s)
- Nadine Jaenisch
- Hans-Berger Department of Neurology, Jena University Hospital, D-07747 Jena, Germany
| | - Lutz Liebmann
- Institute of Human Genetics, Jena University Hospital, D-07743 Jena, Germany
| | - Madlen Guenther
- Hans-Berger Department of Neurology, Jena University Hospital, D-07747 Jena, Germany
| | - Christian A. Hübner
- Institute of Human Genetics, Jena University Hospital, D-07743 Jena, Germany
| | - Christiane Frahm
- Hans-Berger Department of Neurology, Jena University Hospital, D-07747 Jena, Germany
| | - Otto W. Witte
- Hans-Berger Department of Neurology, Jena University Hospital, D-07747 Jena, Germany
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8
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High and low GABA sensitivity α4β2δ GABAA receptors are expressed in Xenopus laevis oocytes with divergent stoichiometries. Biochem Pharmacol 2016; 103:98-108. [DOI: 10.1016/j.bcp.2015.12.021] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/23/2015] [Accepted: 12/29/2015] [Indexed: 12/11/2022]
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9
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Salari AA, Bakhtiari A, Homberg JR. Activation of GABA-A receptors during postnatal brain development increases anxiety- and depression-related behaviors in a time- and dose-dependent manner in adult mice. Eur Neuropsychopharmacol 2015; 25:1260-74. [PMID: 25983020 DOI: 10.1016/j.euroneuro.2015.04.022] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 09/11/2014] [Revised: 04/16/2015] [Accepted: 04/24/2015] [Indexed: 10/23/2022]
Abstract
Disturbances of the gamma-amino butyric acid-ergic (GABAergic) system during postnatal development can have long-lasting consequences for later life behavior, like the individual's response to stress. However, it is unclear which postnatal windows of sensitivity to GABA-ergic modulations are associated with what later-life behavioral outcomes. Therefore, we sought to determine whether neonatal activation of the GABA-A receptor during two postnatal periods, an early window (postnatal day 3-5) and a late window (postnatal day 14-16), can affect anxiety- and depression-related behaviors in male mice in later life. To this end, mice were treated with either saline or muscimol (50, 100, 200, 300 and 500μg/kg) during the early and late postnatal periods. An additional group of mice was treated with the GABA-A receptor antagonist bicuculline+muscimol. When grown to adulthood male mice were exposed to behavioral tests to measure anxiety- and depression-related behaviors. Baseline and stress-induced corticosterone (CORT) levels were also measured. The results indicate that early postnatal and to a lesser extent later postnatal exposure to the GABA-A receptor agonist muscimol increased anxiety-like behavior and stress-induced CORT levels in adults. Moreover, the early postnatal treatment with muscimol increased depression-like behavior with increasing baseline CORT levels. The anxiogenic and depression-like later-life consequences could be antagonized by bicuculline. Our findings suggest that GABA-A receptor signaling during early-life can influence anxiety- and depression-related behaviors in a time- and dose-dependent manner in later life. Our findings help to increase insight in the developmental mechanisms contributing to stress-related disorders.
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Affiliation(s)
- Ali-Akbar Salari
- Drug Applied Research Center, Tabriz University of Medical Sciences, Tabriz, Iran; Laboratory of Neuropsychopharmacology and Psychoneuroimmunology, Hayyan Research Institute, University of Tabriz, Tabriz, Iran.
| | - Amir Bakhtiari
- Department of Microbiology, Faculty of Sciences, Karaj Branch, Islamic Azad University, Alborz, Iran
| | - Judith R Homberg
- Department of Cognitive Neuroscience, Centre for Neuroscience, Donders Institute for Brain, Cognition, and Behaviour, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands
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Liu K, Jounaidi Y, Forman SA, Feng HJ. Etomidate uniquely modulates the desensitization of recombinant α1β3δ GABA(A) receptors. Neuroscience 2015; 300:307-13. [PMID: 26028470 DOI: 10.1016/j.neuroscience.2015.05.051] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/01/2015] [Revised: 05/18/2015] [Accepted: 05/20/2015] [Indexed: 11/24/2022]
Abstract
Central GABA(A) receptors mediate GABAergic phasic and tonic inhibition. While synaptic αβγ GABA(A) receptors primarily mediate phasic inhibition, extrasynaptic αβδ receptors play an important role in mediating tonic inhibition. Etomidate is a general anesthetic that produces its effects by enhancing GABA(A) receptor activity. We previously showed that etomidate modulates the gating of oocyte-expressed αβγ and αβδ receptors with similar overall allosteric impact, but different pharmacological patterns. In αβγ receptors, etomidate enhances apparent GABA sensitivity (reduces GABA EC50), modestly increases maximal GABA efficacy, and slows current deactivation without affecting desensitization (Zhong et al., 2008). In αβδ receptors characterized by low GABA efficacy, etomidate dramatically increases responses to both low and maximal GABA. The effects of etomidate on desensitization and deactivation of αβδ receptors are unknown. To investigate the kinetic effects of etomidate on α1β3δ receptors of defined subunit arrangement, we expressed concatenated trimer (β3-α1-δ) and dimer (β3-α1) GABA(A) receptor subunit assemblies in human embryonic kidney (HEK)293T cells and recorded whole-cell voltage-clamp currents during rapid external solution exchanges. As expected, etomidate substantially increased maximal GABA-induced currents and prolonged deactivation. Moreover, desensitization was significantly decreased by etomidate. During prolonged GABA applications, etomidate enhanced steady-state currents more than peak currents. Thus, etomidate enhances tonic GABAergic inhibition through extrasynaptic αβδ receptors by both augmenting gating and reducing desensitization.
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Affiliation(s)
- K Liu
- Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital and Harvard Medical School, Boston, MA, USA; Department of Anesthesia, China-Japan Friendship Hospital, Beijing, China
| | - Y Jounaidi
- Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital and Harvard Medical School, Boston, MA, USA
| | - S A Forman
- Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital and Harvard Medical School, Boston, MA, USA.
| | - H-J Feng
- Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital and Harvard Medical School, Boston, MA, USA.
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11
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Hammer H, Ebert B, Jensen HS, Jensen AA. Functional characterization of the 1,5-benzodiazepine clobazam and its major active metabolite N-desmethylclobazam at human GABA(A) receptors expressed in Xenopus laevis oocytes. PLoS One 2015; 10:e0120239. [PMID: 25798598 PMCID: PMC4370687 DOI: 10.1371/journal.pone.0120239] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/16/2014] [Accepted: 02/04/2015] [Indexed: 12/13/2022] Open
Abstract
The 1,5-benzodiazepine clobazam is indicated for the adjunctive treatment of seizures associated with Lennox-Gastaut syndrome in patients 2 years of age or older in the United States, and for treatment of anxiety and various forms of epilepsy elsewhere. Clobazam has been reported to exhibit different in vivo adverse effects and addiction liability profile than the classic 1,4-benzodiazepines. In this study, it was investigated whether the in vitro pharmacological properties of clobazam and its major active metabolite N-desmethylclobazam could explain some of these clinical differences. The functional properties of the two 1,5-benzodiazepines were characterized at the human γ-aminobutyric acid type A receptor (GABAAR) subtypes α1β2γ2S, α2β2γ2S, α3β2γ2S, α5β2γ2S and α6β2δ expressed in Xenopus laevis oocytes by use of two-electrode voltage-clamp electrophysiology and compared to those exhibited by the 1,4-benzodiazepine clonazepam. All three compounds potentiated GABA EC20-evoked responses through the α1,2,3,5β2γ2S GABAARs in a reversible and concentration-dependent manner, with each displaying similar EC50 values at the four subtypes. Furthermore, the degrees of potentiation of the GABA EC20 currents through the four receptors mediated by saturating modulator concentrations did not differ substantially for any of the three benzodiazepines. The three compounds were substantially less potent (200-3900 fold) as positive allosteric modulators at the α6β2δ GABAAR than at the α1,2,3,5β2γ2S receptors. Interestingly, however, clobazam and especially N-desmethylclobazam were highly efficacious potentiators of α6β2δ receptor signaling. Although this activity component is unlikely to contribute to the in vivo effects of clobazam/N-desmethylclobazam, the 1,5-benzodiazepine could constitute an interesting lead for novel modulators targeting this low-affinity binding site in GABAARs. In conclusion, the non-selective modulation exerted by clobazam, N-desmethylclobazam and clonazepam at the α1β2γ2S, α2β2γ2S, α3β2γ2S and α5β2γ2S GABAARs indicate that the observed clinical differences between clobazam and 1,4-benzodiazepines are likely to arise from factors other than their respective pharmacological properties at the GABAARs as investigated here.
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Affiliation(s)
- Harriet Hammer
- Department of Drug Design and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
| | | | | | - Anders A. Jensen
- Department of Drug Design and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark
- * E-mail:
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12
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Villumsen IS, Wellendorph P, Smart TG. Pharmacological characterisation of murine α4β1δ GABAA receptors expressed in Xenopus oocytes. BMC Neurosci 2015; 16:8. [PMID: 25887256 PMCID: PMC4359537 DOI: 10.1186/s12868-015-0148-4] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/18/2014] [Accepted: 02/20/2015] [Indexed: 11/15/2022] Open
Abstract
Background GABAA receptor subunit composition has a profound effect on the receptor’s physiological and pharmacological properties. The receptor β subunit is widely recognised for its importance in receptor assembly, trafficking and post-translational modifications, but its influence on extrasynaptic GABAA receptor function is less well understood. Here, we examine the pharmacological properties of a potentially native extrasynaptic GABAA receptor that incorporates the β1 subunit, specifically composed of α4β1δ and α4β1 subunits. Results GABA activated concentration-dependent responses at α4β1δ and α4β1 receptors with EC50 values in the nanomolar to micromolar range, respectively. The divalent cations Zn2+ and Cu2+, and the β1-selective inhibitor salicylidine salicylhydrazide (SCS), inhibited GABA-activated currents at α4β1δ receptors. Surprisingly the α4β1 receptor demonstrated biphasic sensitivity to Zn2+ inhibition that may reflect variable subunit stoichiometries with differing sensitivity to Zn2+. The neurosteroid tetrahydro-deoxycorticosterone (THDOC) significantly increased GABA-initiated responses in concentrations above 30 nM for α4β1δ receptors. Conclusions With this study we report the first pharmacological characterisation of various GABAA receptor ligands acting at murine α4β1δ GABAA receptors, thereby improving our understanding of the molecular pharmacology of this receptor isoform. This study highlights some notable differences in the pharmacology of murine and human α4β1δ receptors. We consider the likelihood that the α4β1δ receptor may play a role as an extrasynaptic GABAA receptor in the nervous system.
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Affiliation(s)
- Inge S Villumsen
- Department of Neuroscience, Physiology & Pharmacology, University College London, Gower Street, London, WC1E 6BT, UK. .,Department of Drug Design and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen, Fruebjergvej 3, 2100, Copenhagen, Denmark.
| | - Petrine Wellendorph
- Department of Drug Design and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen, Fruebjergvej 3, 2100, Copenhagen, Denmark.
| | - Trevor G Smart
- Department of Neuroscience, Physiology & Pharmacology, University College London, Gower Street, London, WC1E 6BT, UK.
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Eaton MM, Bracamontes J, Shu HJ, Li P, Mennerick S, Steinbach JH, Akk G. γ-aminobutyric acid type A α4, β2, and δ subunits assemble to produce more than one functionally distinct receptor type. Mol Pharmacol 2014; 86:647-56. [PMID: 25238745 DOI: 10.1124/mol.114.094813] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Native γ-aminobutyric acid (GABA)A receptors consisting of α4, β1-3, and δ subunits mediate responses to the low, tonic concentration of GABA present in the extracellular milieu. Previous studies on heterologously expressed α4βδ receptors have shown a large degree of variability in functional properties, including sensitivity to the transmitter. We studied properties of α4β2δ receptors employing free subunits and concatemeric constructs, expressed in Xenopus oocytes, HEK 293 cells, and cultured hippocampal neurons. The expression system had a strong effect on the properties of receptors containing free subunits. The midpoint of GABA activation curve was 10 nM for receptors in oocytes versus 2300 nM in HEK cells. Receptors activated by the steroid alfaxalone had an estimated maximal open probability of 0.6 in oocytes and 0.01 in HEK cells. Irrespective of the expression system, receptors resulting from combining the tandem construct β2-δ and a free α4 subunit exhibited large steroid responses. We propose that free α4, β2, and δ subunits assemble in different configurations with distinct properties in oocytes and HEK cells, and that subunit linkage can overcome the expression system-dependent preferential assembly of free subunits. Hippocampal neurons transfected with α4 and the picrotoxin-resistant δ(T269Y) subunit showed large responses to alfaxalone in the presence of picrotoxin, suggesting that α4βδ receptors may assemble in a similar configuration in neurons and oocytes.
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Affiliation(s)
- Megan M Eaton
- Department of Anesthesiology (M.M.E., J.B., P.L., J.H.S., G.A.), Department of Psychiatry (H.-J.S., S.M.), and Taylor Family Institute for Innovative Psychiatric Research (S.M., J.H.S., G.A.), Washington University School of Medicine, St. Louis, Missouri
| | - John Bracamontes
- Department of Anesthesiology (M.M.E., J.B., P.L., J.H.S., G.A.), Department of Psychiatry (H.-J.S., S.M.), and Taylor Family Institute for Innovative Psychiatric Research (S.M., J.H.S., G.A.), Washington University School of Medicine, St. Louis, Missouri
| | - Hong-Jin Shu
- Department of Anesthesiology (M.M.E., J.B., P.L., J.H.S., G.A.), Department of Psychiatry (H.-J.S., S.M.), and Taylor Family Institute for Innovative Psychiatric Research (S.M., J.H.S., G.A.), Washington University School of Medicine, St. Louis, Missouri
| | - Ping Li
- Department of Anesthesiology (M.M.E., J.B., P.L., J.H.S., G.A.), Department of Psychiatry (H.-J.S., S.M.), and Taylor Family Institute for Innovative Psychiatric Research (S.M., J.H.S., G.A.), Washington University School of Medicine, St. Louis, Missouri
| | - Steven Mennerick
- Department of Anesthesiology (M.M.E., J.B., P.L., J.H.S., G.A.), Department of Psychiatry (H.-J.S., S.M.), and Taylor Family Institute for Innovative Psychiatric Research (S.M., J.H.S., G.A.), Washington University School of Medicine, St. Louis, Missouri
| | - Joe Henry Steinbach
- Department of Anesthesiology (M.M.E., J.B., P.L., J.H.S., G.A.), Department of Psychiatry (H.-J.S., S.M.), and Taylor Family Institute for Innovative Psychiatric Research (S.M., J.H.S., G.A.), Washington University School of Medicine, St. Louis, Missouri
| | - Gustav Akk
- Department of Anesthesiology (M.M.E., J.B., P.L., J.H.S., G.A.), Department of Psychiatry (H.-J.S., S.M.), and Taylor Family Institute for Innovative Psychiatric Research (S.M., J.H.S., G.A.), Washington University School of Medicine, St. Louis, Missouri
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14
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Patel B, Mortensen M, Smart TG. Stoichiometry of δ subunit containing GABA(A) receptors. Br J Pharmacol 2014; 171:985-94. [PMID: 24206220 PMCID: PMC3925037 DOI: 10.1111/bph.12514] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/01/2013] [Revised: 10/02/2013] [Accepted: 10/24/2013] [Indexed: 12/19/2022] Open
Abstract
BACKGROUND AND PURPOSE Although the stoichiometry of the major synaptic αβγ subunit-containing GABAA receptors has consensus support for 2α:2β:1γ, a clear view of the stoichiometry of extrasynaptic receptors containing δ subunits has remained elusive. Here we examine the subunit stoichiometry of recombinant α4β3δ receptors using a reporter mutation and a functional electrophysiological approach. EXPERIMENTAL APPROACH Using site-directed mutagenesis, we inserted a highly characterized 9' serine to leucine mutation into the second transmembrane (M2) region of α4, β3 and δ subunits that increases receptor sensitivity to GABA. Whole-cell, GABA-activated currents were recorded from HEK-293 cells co-expressing different combinations of wild-type (WT) and/or mutant α4(L297S), β3(L284S) and δ(L288S) subunits. KEY RESULTS Recombinant receptors containing one or more mutant subunits showed increased GABA sensitivity relative to WT receptors by approximately fourfold, independent of the subunit class (α, β or δ) carrying the mutation. GABA dose-response curves of cells co-expressing WT subunits with their respective L9'S mutants exhibited multiple components, with the number of discernible components enabling a subunit stoichiometry of 2α, 2β and 1δ to be deduced for α4β3δ receptors. Varying the cDNA transfection ratio by 10-fold had no significant effect on the number of incorporated δ subunits. CONCLUSIONS AND IMPLICATIONS Subunit stoichiometry is an important determinant of GABAA receptor function and pharmacology, and δ subunit-containing receptors are important mediators of tonic inhibition in several brain regions. Here we demonstrate a preferred subunit stoichiometry for α4β3δ receptors of 2α, 2β and 1δ.
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Affiliation(s)
- B Patel
- Department of Neuroscience, Physiology and Pharmacology, University College LondonLondon, UK
| | - M Mortensen
- Department of Neuroscience, Physiology and Pharmacology, University College LondonLondon, UK
| | - T G Smart
- Department of Neuroscience, Physiology and Pharmacology, University College LondonLondon, UK
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Carver CM, Reddy DS. Neurosteroid interactions with synaptic and extrasynaptic GABA(A) receptors: regulation of subunit plasticity, phasic and tonic inhibition, and neuronal network excitability. Psychopharmacology (Berl) 2013; 230:151-88. [PMID: 24071826 PMCID: PMC3832254 DOI: 10.1007/s00213-013-3276-5] [Citation(s) in RCA: 171] [Impact Index Per Article: 15.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/29/2013] [Accepted: 08/29/2013] [Indexed: 12/25/2022]
Abstract
RATIONALE Neurosteroids are steroids synthesized within the brain with rapid effects on neuronal excitability. Allopregnanolone, allotetrahydrodeoxycorticosterone, and androstanediol are three widely explored prototype endogenous neurosteroids. They have very different targets and functions compared to conventional steroid hormones. Neuronal γ-aminobutyric acid (GABA) type A (GABA(A)) receptors are one of the prime molecular targets of neurosteroids. OBJECTIVE This review provides a critical appraisal of recent advances in the pharmacology of endogenous neurosteroids that interact with GABA(A) receptors in the brain. Neurosteroids possess distinct, characteristic effects on the membrane potential and current conductance of the neuron, mainly via potentiation of GABA(A) receptors at low concentrations and direct activation of receptor chloride channel at higher concentrations. The GABA(A) receptor mediates two types of inhibition, now characterized as synaptic (phasic) and extrasynaptic (tonic) inhibition. Synaptic release of GABA results in the activation of low-affinity γ2-containing synaptic receptors, while high-affinity δ-containing extrasynaptic receptors are persistently activated by the ambient GABA present in the extracellular fluid. Neurosteroids are potent positive allosteric modulators of synaptic and extrasynaptic GABA(A) receptors and therefore enhance both phasic and tonic inhibition. Tonic inhibition is specifically more sensitive to neurosteroids. The resulting tonic conductance generates a form of shunting inhibition that controls neuronal network excitability, seizure susceptibility, and behavior. CONCLUSION The growing understanding of the mechanisms of neurosteroid regulation of the structure and function of the synaptic and extrasynaptic GABA(A) receptors provides many opportunities to create improved therapies for sleep, anxiety, stress, epilepsy, and other neuropsychiatric conditions.
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Affiliation(s)
- Chase Matthew Carver
- Department of Neuroscience and Experimental Therapeutics, College of Medicine, Texas A&M University Health Science Center, 2008 Medical Research and Education Building, 8447 State Highway 47, Bryan, TX, 77807-3260, USA
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16
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Karim N, Wellendorph P, Absalom N, Bang LH, Jensen ML, Hansen MM, Lee HJ, Johnston GA, Hanrahan JR, Chebib M. Low nanomolar GABA effects at extrasynaptic α4β1/β3δ GABAA receptor subtypes indicate a different binding mode for GABA at these receptors. Biochem Pharmacol 2012; 84:549-57. [DOI: 10.1016/j.bcp.2012.05.017] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/04/2012] [Revised: 05/18/2012] [Accepted: 05/21/2012] [Indexed: 10/28/2022]
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17
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You H, Kozuska JL, Paulsen IM, Dunn SM. Benzodiazepine modulation of the rat GABAA receptor α4β3γ2L subtype expressed in Xenopus oocytes. Neuropharmacology 2010; 59:527-33. [DOI: 10.1016/j.neuropharm.2010.07.011] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/03/2010] [Revised: 06/23/2010] [Accepted: 07/08/2010] [Indexed: 10/19/2022]
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18
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Adusei DC, Pacey LKK, Chen D, Hampson DR. Early developmental alterations in GABAergic protein expression in fragile X knockout mice. Neuropharmacology 2010; 59:167-71. [PMID: 20470805 DOI: 10.1016/j.neuropharm.2010.05.002] [Citation(s) in RCA: 107] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/08/2010] [Revised: 03/26/2010] [Accepted: 05/05/2010] [Indexed: 11/19/2022]
Abstract
Fragile X syndrome is the most common heritable form of mental retardation. It is caused by silencing of the Fmr1 gene and the absence of the encoded protein. The purpose of this study was to examine global protein expression levels of GABA(A) and GABA(B) receptors, and GABAergic enzymes and trafficking proteins in fragile X knockout mice during brain maturation. Quantitative western blotting of homogenates of forebrain revealed that the levels of GABA(A) beta1 and beta3, GABA(B)-R1, NKCC1, KCC2, gephyrin and ubiquilin were not significantly different from wild-type mice at any of the postnatal time points examined. In contrast, the GABA(A) receptor alpha1, beta2, and delta subunits, and the GABA enzymes GABA transaminase and succinic semialdehyde dehydrogenase were down-regulated during postnatal development, while GAD65 was up-regulated in the adult knockout mouse brain. The GABA(A) receptor alpha1 and beta2 subunits displayed a divergent pattern of developmental expression whereby alpha1 was reduced in the immature brain but regained a level of expression similar to wild-type mice by adulthood, while the expression of beta2 was similar to wild-types at postnatal day 5 but reduced at day 12 and in the adult brain. The GABA(A) receptor delta subunit and the GABA catabolic enzymes GABA transaminase and succinic semialdehyde dehydrogenase were simultaneously but transiently decreased only at postnatal day 12. Our results demonstrate that GABA(A) receptor subunits and GABA enzymes display complex patterns of changes during brain development suggesting that dynamic interactions may occur between GABA transmitter levels and GABA receptors in fragile X syndrome.
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MESH Headings
- 4-Aminobutyrate Transaminase/genetics
- 4-Aminobutyrate Transaminase/metabolism
- Adaptor Proteins, Signal Transducing
- Adaptor Proteins, Vesicular Transport/genetics
- Adaptor Proteins, Vesicular Transport/metabolism
- Age Factors
- Animals
- Animals, Newborn
- Autophagy-Related Proteins
- Carrier Proteins/genetics
- Carrier Proteins/metabolism
- Fragile X Mental Retardation Protein/genetics
- Gene Expression Regulation, Developmental/genetics
- Membrane Proteins/genetics
- Membrane Proteins/metabolism
- Mice
- Mice, Inbred C57BL
- Mice, Knockout
- Receptors, GABA-A/genetics
- Receptors, GABA-A/metabolism
- Receptors, GABA-B/genetics
- Receptors, GABA-B/metabolism
- Sodium-Potassium-Chloride Symporters/genetics
- Sodium-Potassium-Chloride Symporters/metabolism
- Solute Carrier Family 12, Member 2
- Symporters/genetics
- Symporters/metabolism
- gamma-Aminobutyric Acid/genetics
- gamma-Aminobutyric Acid/metabolism
- K Cl- Cotransporters
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Affiliation(s)
- Daniel C Adusei
- Department of Pharmaceutical Sciences, Leslie Dan Faculty of Pharmacy, University of Toronto, 144 College St., Toronto, Ontario, Canada M5S 3M2
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19
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Wagoner KR, Czajkowski C. Stoichiometry of expressed alpha(4)beta(2)delta gamma-aminobutyric acid type A receptors depends on the ratio of subunit cDNA transfected. J Biol Chem 2010; 285:14187-94. [PMID: 20233712 DOI: 10.1074/jbc.m110.104257] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The gamma-aminobutyric acid type A receptor (GABA(A)R) is the target of many depressants, including benzodiazepines, anesthetics, and alcohol. Although the highly prevalent alphabetagamma GABA(A)R subtype mediates the majority of fast synaptic inhibition in the brain, receptors containing delta subunits also play a key role, mediating tonic inhibition and the actions of endogenous neurosteroids and alcohol. However, the fundamental properties of delta-containing GABA(A)Rs, such as subunit stoichiometry, are not well established. To determine subunit stoichiometry of expressed delta-containing GABA(A)Rs, we inserted the alpha-bungarotoxin binding site tag in the alpha(4), beta(2), and delta subunit N termini. An enhanced green fluorescent protein tag was also inserted into the beta(2) subunit to shift its molecular weight, allowing us to separate subunits using SDS-PAGE. Tagged alpha(4)beta(2)delta GABA(A)Rs were expressed in HEK293T cells using various ratios of subunit cDNA, and receptor subunit stoichiometry was determined by quantitating fluorescent alpha-bungarotoxin bound to each subunit on Western blots of surface immunopurified tagged GABA(A)Rs. The results demonstrate that the subunit stoichiometry of alpha(4)beta(2)delta GABA(A)Rs is regulated by the ratio of subunit cDNAs transfected. Increasing the ratio of delta subunit cDNA transfected increased delta subunit incorporation into surface receptors with a concomitant decrease in beta(2) subunit incorporation. Because receptor subunit stoichiometry can directly influence GABA(A)R pharmacological and functional properties, considering how the transfection protocols used affect subunit stoichiometry is essential when studying heterologously expressed alpha(4)beta(2)delta GABA(A)Rs. Successful bungarotoxin binding site tagging of GABA(A)R subunits is a novel tool with which to accurately quantitate subunit stoichiometry and will be useful for monitoring GABA(A)R trafficking in live cells.
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Affiliation(s)
- Kelly R Wagoner
- Molecular and Cellular Pharmacology Graduate Program, University of Wisconsin at Madison, Madison, Wisconsin 53711, USA
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20
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Prototypic GABA(A) receptor agonist muscimol acts preferentially through forebrain high-affinity binding sites. Neuropsychopharmacology 2010; 35:999-1007. [PMID: 20032968 PMCID: PMC2823376 DOI: 10.1038/npp.2009.203] [Citation(s) in RCA: 53] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
Muscimol has been regarded as a universal agonist for all gamma-aminobutyric acid type A receptor (GABA(A)-R) subtypes. However, brain regional distribution of muscimol's high-affinity binding sites greatly differs from those of other binding sites of the GABA(A)-R. To test whether behavioral effects of muscimol correlated with the density of high-affinity [(3)H]muscimol binding, we examined several GABA(A)-R subunit gene-modified mouse lines: alpha1, alpha4, or delta-knockouts (KO), alpha4+delta-double KO, and Thy1.2 promoter-driven alpha6 transgenic mice (Thy1alpha6). We determined the high-affinity [(3)H]muscimol binding in brain sections by quantitative autoradiography and sedative/ataxic effects induced in vivo by muscimol using a constant speed rotarod. alpha4-KO mice had reduced [(3)H]muscimol binding in the caudate-putamen, thalamus, and hippocampus, and were less sensitive to the behavioral impairment by muscimol. Similarly, delta-KO mice also had reduced binding to forebrain regions and a lower behavioral sensitivity to muscimol than their wild-type controls. In contrast, alpha1-KO mice had unaltered behavioral sensitivity to muscimol and unaltered [(3)H]muscimol binding, even though previous studies have demonstrated dramatically reduced binding to various other GABA(A)-R sites in these mice. Finally, Thy1alpha6 mice exhibited increased behavioral sensitivity to muscimol, and to another direct GABA-site agonist gaboxadol, and increased [(3)H]muscimol binding in the cerebral cortex and hippocampus. Thus, the differences in sedative and motor-impairing actions of muscimol in various mouse models correlated with the level of forebrain high-affinity [(3)H]muscimol binding. These data suggest that a small special population of GABA(A)-Rs, most likely extrasynaptic non-alpha1-containing receptors, strongly contributes to the in vivo pharmacological effects of muscimol.
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Mortensen M, Ebert B, Wafford K, Smart TG. Distinct activities of GABA agonists at synaptic- and extrasynaptic-type GABAA receptors. J Physiol 2010; 588:1251-68. [PMID: 20176630 DOI: 10.1113/jphysiol.2009.182444] [Citation(s) in RCA: 126] [Impact Index Per Article: 9.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022] Open
Abstract
The activation characteristics of synaptic and extrasynaptic GABA(A) receptors are important for shaping the profile of phasic and tonic inhibition in the central nervous system, which will critically impact on the activity of neuronal networks. Here, we study in isolation the activity of three agonists, GABA, muscimol and 4,5,6,7-tetrahydoisoxazolo[5,4-c]pyridin-3(2H)-one (THIP), to further understand the activation profiles of alpha 1 beta 3 gamma 2, alpha 4 beta 3 gamma 2 and alpha 4 beta 3 delta receptors that typify synaptic- and extrasynaptic-type receptors expressed in the hippocampus and thalamus. The agonists display an order of potency that is invariant between the three receptors, which is reliant mostly on the agonist dissociation constant. At delta subunit-containing extrasynaptic-type GABA(A) receptors, both THIP and muscimol additionally exhibited, to different degrees, superagonist behaviour. By comparing whole-cell and single channel currents induced by the agonists, we provide a molecular explanation for their different activation profiles. For THIP at high concentrations, the unusual superagonist behaviour on alpha 4 beta 3 delta receptors is a consequence of its ability to increase the duration of longer channel openings and their frequency, resulting in longer burst durations. By contrast, for muscimol, moderate superagonist behaviour was caused by reduced desensitisation of the extrasynaptic-type receptors. The ability to specifically increase the efficacy of receptor activation, by selected exogenous agonists over that obtained with the natural transmitter, may prove to be of therapeutic benefit under circumstances when synaptic inhibition is compromised or dysfunctional.
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Affiliation(s)
- Martin Mortensen
- Department of Neuroscience, Physiology and Pharmacology, University College London, Gower Street, London WC1E 6BT, UK
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22
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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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23
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Joshi S, Kapur J. Slow intracellular accumulation of GABA(A) receptor delta subunit is modulated by brain-derived neurotrophic factor. Neuroscience 2009; 164:507-19. [PMID: 19665523 DOI: 10.1016/j.neuroscience.2009.08.008] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/17/2009] [Revised: 08/03/2009] [Accepted: 08/04/2009] [Indexed: 11/26/2022]
Abstract
GABA(A) receptors composed of the gamma2 and delta subunits have distinct properties, functions and subcellular localization, and pathological conditions differentially modulate their surface expression. Recent studies demonstrate that acute seizure activity accelerated trafficking of the gamma2 and beta2/3 subunits but not that of the delta subunit. The trafficking of the gamma2 and beta2/3 subunits is relatively well understood but that of the delta subunit has not been studied. We compared intracellular accumulation of the delta and gamma2 subunits in cultured hippocampal neurons using an antibody feeding technique. Intracellular accumulation of the delta subunit peaked between 3 and 6 h, whereas, maximum internalization of the gamma2 subunit took 30 min. In the organotypic hippocampal slice cultures internalization of the delta subunit studied using a biotinylation assay revealed highest accumulation between 3 and 5 h and that of the gamma2 subunit between 15 and 45 min. The surface half-life of the delta subunit was 171 min in cultured hippocampal neurons and 102 min in the organotypic hippocampal slice cultures. In the subsequent studies, internalization of the delta subunit was found to be dependent on network activity but independent of ligand-binding. Brain-derived neurotrophic factor (BDNF) reduced buildup of the delta subunit in the cytoplasmic compartments and increased its surface expression, and this BDNF effect was independent of network activity. BDNF effect was mediated by activation of TrkB receptors, PLCgamma and PKC. Increase in the basal PKC activity augmented cell surface stability of the delta subunit. These results suggest that rate of intracellular accumulation of the delta subunit was distinct and modulated by BDNF.
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Affiliation(s)
- S Joshi
- Department of Neurology, Box 800394, University of Virginia, Health Sciences Center, Charlottesville, VA 22908, USA
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24
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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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25
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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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26
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Anseloni VCZ, Gold MS. Inflammation-induced shift in the valence of spinal GABA-A receptor-mediated modulation of nociception in the adult rat. THE JOURNAL OF PAIN 2008; 9:732-8. [PMID: 18467182 DOI: 10.1016/j.jpain.2008.03.004] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/29/2008] [Revised: 03/11/2008] [Accepted: 03/12/2008] [Indexed: 11/18/2022]
Abstract
UNLABELLED The objective of this study was to assess the impact of persistent inflammation on spinal gamma-aminobutyric acid-A (GABA-A) receptor-mediated modulation of evoked nociceptive behavior in the adult rat. Nocifensive threshold was assessed with von Frey filaments applied to the dorsal surface of the hind paw. The GABA-A receptor agonist muscimol, the antagonist gabazine, the benzodiazepine receptor agonist midazolam, and antagonists PK11195 and flumazenil were administered spinally in the presence and absence of complete Freund's adjuvant (CFA)-induced inflammation. In naive rats, muscimol increased and gabazine decreased nociceptive threshold. After CFA, the effects of these compounds were reversed: Low doses of muscimol exacerbated the inflammation-induced decrease in nociceptive threshold and gabazine increased nociceptive threshold. Midazolam increased nociceptive threshold both in the presence and absence of inflammation. Flumazenil but not PK11195 blocked the analgesic effects of midazolam. These findings indicate that inflammation-induced changes in GABA-A signaling are complex and are likely to involve several distinct mechanisms. Rectifying the changes in GABA-A signaling may provide effective relief from hypersensitivity observed in the presence of inflammation. PERSPECTIVE An inflammation-induced shift in spinal GABA-A receptor signaling from inhibition to excitation appears to underlie inflammatory pain and hypersensitivity. Use of GABA-A receptor selective general anesthetics in association with therapeutic interventions may be contraindicated. More importantly, rectifying the changes in GABA-A signaling may provide effective relief from inflammatory hypersensitivity.
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Affiliation(s)
- Vanessa C Z Anseloni
- Department of Biomedical Sciences, School of Dentistry, Baltimore, Maryland, USA
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Barrera NP, Betts J, You H, Henderson RM, Martin IL, Dunn SMJ, Edwardson JM. Atomic force microscopy reveals the stoichiometry and subunit arrangement of the alpha4beta3delta GABA(A) receptor. Mol Pharmacol 2007; 73:960-7. [PMID: 18079275 DOI: 10.1124/mol.107.042481] [Citation(s) in RCA: 71] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023] Open
Abstract
The GABA(A) receptor is a chloride-selective ligand-gated ion channel of the Cys-loop superfamily. The receptor consists of five subunits arranged pseudosymmetrically around a central pore. The predominant form of the receptor in the brain contains alpha(1)-, beta(2)-, and gamma(2)-subunits in the arrangement alphabetaalphagammabeta, counter-clockwise around the pore. GABA(A) receptors containing delta-instead of gamma-subunits, although a minor component of the total receptor population, have interesting properties, such as an extrasynaptic location, high sensitivity to GABA, and potential association with conditions such as epilepsy. They are therefore attractive targets for drug development. Here we addressed the subunit arrangement within the alpha(4)beta(3)delta form of the receptor. Different epitope tags were engineered onto the three subunits, and complexes between receptors and anti-epitope antibodies were imaged by atomic force microscopy. Determination of the numbers of receptors doubly decorated by each of the three antibodies revealed a subunit stoichiometry of 2alpha:2beta:1delta. The distributions of angles between pairs of antibodies against the alpha- and beta-subunits both had peaks at around 144 degrees , indicating that these pairs of subunits were nonadjacent. Decoration of the receptor with ligands that bind to the extracellular domain (i.e., the lectin concanavalin A and an antibody that recognizes the beta-subunit N-terminal sequence) showed that the receptor preferentially binds to the mica extracellular face down. Given this orientation, the geometry of complexes of receptors with both an antibody against the delta-subunit and Fab fragments against the alpha-subunits indicates a predominant subunit arrangement of alphabetaalphadeltabeta, counter-clockwise around the pore when viewed from the extracellular space.
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Affiliation(s)
- Nelson P Barrera
- Department of Pharmacology, University of Cambridge, Tennis Court Road, Cambridge CB2 1PD, United Kingdom
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