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Extrasynaptic GABA Type a Receptors in the Mechanism of Action of Ethanol

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Extrasynaptic GABA Type a Receptors in the Mechanism of Action of Ethanol Extrasynaptic GABA Type A Receptors in the Mechanism of Action of Ethanol by Dev Chandra B.S., University of Buffalo, 1997 D.M.D., University of Pittsburgh, 2001 Submitted to the Graduate Faculty of the School of Medicine in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Pittsburgh 2008 UNIVERSITY OF PITTSBURGH SCHOOL OF MEDICINE This thesis was presented by Dev Chandra It was defended on January 29, 2008 and approved by Joan M. Lakoski Ph.D., Professor, Department of Pharmacology William C. de Groat Ph.D., Professor, Department of Pharmacology Michael J. Palladino Ph.D., Assistant Professor, Department of Pharmacology A. Paula Monaghan-Nichols Ph.D., Assistant Professor, Department of Neurobiology Gregg E. Homanics Ph.D., Associate Professor, Department of Anesthesiology ii Extrasynaptic GABA Type A Receptors in the Mechanism of Action of Ethanol Dev Chandra, Ph.D. University of Pittsburgh, 2008 The γ-aminobutyric acid (GABA) Type A receptor (GABAA-R) mediates the majority of rapid inhibition in the central nervous system and is the site of action for many clinically used drugs. GABAA-R mediated inhibition can occur via the conventional mechanism - the transient activation of synaptic receptors i.e. phasic inhibition, or via continuous activation of extrasynaptic, high affinity receptors by low concentrations of ambient GABA, leading to “tonic” inhibition. The GABAA-R α4 subunit is expressed at high levels in the dentate gyrus and thalamus and when partnered with the δ subunit, it is suspected to contribute to tonic inhibition. In vitro studies have found that GABAA-Rs containing α4 and δ are highly sensitive to ethanol and to competitive GABAA-R agonists such as gaboxadol and muscimol. In light of these findings, the central hypothesis tested in this thesis was that extrasynaptic GABAA-Rs mediate the depressant effects of these drugs. To provide a model for understanding the precise role of α4 containing GABAA-Rs in drug action, mice were engineered to lack the α4 subunit by targeted disruption of the Gabra4 gene. α4 Subunit knockout mice were viable and superficially indistinguishable from wild-type mice. In electrophysiological recordings, α4 knockout mice showed a lack of tonic inhibition in dentate granule cells and thalamic relay neurons. α4 knockout mice were also less sensitive to the behavioral effects of gaboxadol and muscimol. However, α4 knockout mice did not differ in ethanol-induced changes in anxiety, locomotion, iii ataxia, coordination, analgesia, or thermoregulation. These data demonstrate that tonic inhibition in dentate granule cells and thalamic relay neurons is mediated by extrasynaptic GABAA-Rs containing the α4 subunit and that gaboxadol and muscimol likely achieve their effects via the activation of this GABAA-R subtype. These data also suggest that GABAA-Rs containing the α4 subunit are not necessary for many acute behavioral responses to ethanol. iv TABLE OF CONTENTS ACKNOWLEDGEMENTS ......................................................................................................xiii LIST OF ABBREVIATIONS .................................................................................................... xv 1.0 INTRODUCTION........................................................................................................ 1 1.1 MOLECULAR MECHANISMS OF ETHANOL ............................................ 1 1.2 THE GABA SYSTEM......................................................................................... 3 1.2.1 Synthesis......................................................................................................... 3 1.2.2 GABA release, reuptake and degradation.................................................. 4 1.2.3 GABAergic neurons...................................................................................... 5 1.3 GABA RECEPTORS .......................................................................................... 5 1.3.1 Role of GABA receptors in disease and treatment .................................... 5 1.3.2 GABA Receptor Types ................................................................................. 6 1.4 GABAA RECEPTORS ........................................................................................ 8 1.4.1 Receptor subunits.......................................................................................... 8 1.4.2 Spatial and temporal distribution of subunits ........................................... 9 1.4.3 Receptor subtypes in the brain .................................................................. 10 1.5 GABAA RECEPTOR FUNCTION .................................................................. 12 1.5.1 Phasic inhibition.......................................................................................... 14 1.5.2 GABAA-R subtypes in phasic inhibition................................................... 14 v 1.5.3 Tonic inhibition........................................................................................... 15 1.5.4 Receptor subunits involved in tonic inhibition......................................... 16 1.6 GABAA RECEPTOR PHARMACOLOGY.................................................... 18 1.6.1 General considerations ............................................................................... 18 1.6.2 Benzodiazepines and GABAA-R subtype specific interactions............... 19 1.6.3 Pharmacology – Direct GABAA receptor agonists................................... 21 1.6.3.1 Extrasynaptic receptors and direct GABAA-R agonists ................. 22 1.6.4 Extrasynaptic GABAA receptors in the pharmacology of ethanol......... 23 1.7 ANIMAL MODELS TO STUDY GABAA-R PHARMACOLOGY ............. 26 1.8 THE α4 SUBUNIT OF THE GABAA-R.......................................................... 27 1.9 GAPS IN OUR KNOWLEDGE ABOUT α4 .................................................. 28 1.10 CREATING GENE KNOCKOUT MICE....................................................... 29 2.0 SPECIFIC AIMS........................................................................................................ 33 3.0 CREATION OF GABAA-R α4 SUBUNIT MUTANT MICE............................... 35 3.1 INTRODUCTION ............................................................................................. 35 3.2 METHODS......................................................................................................... 36 3.2.1 Overview of creating Gabra4 mutant mice............................................... 36 3.2.2 Design of α4 subunit gene targeting vector .............................................. 41 3.2.3 Targeting of α4 in Embryonic Stem Cells ................................................ 41 3.2.4 Production of Mice from Targeted ES Cells ............................................ 43 3.2.5 Reverse Transcriptase- Polymerase Chain Reaction (RT-PCR)............ 43 3.2.6 Western Blot Analysis................................................................................. 44 3.2.7 Elevated Plus-Maze Test ............................................................................ 44 vi 3.2.8 Open Field Activity..................................................................................... 45 3.3 RESULTS ........................................................................................................... 45 3.3.1 Creation of Vector and ES Cell Targeting: .............................................. 45 3.3.2 ES Cell Targeting........................................................................................ 46 3.3.3 Production of α4 mutant mice from targeted ES Cells ........................... 46 3.3.4 Gross Characterization of f/f (KO) mice .................................................. 47 3.3.5 RNA Analysis .............................................................................................. 48 3.3.6 Protein Analysis .......................................................................................... 49 3.3.7 Behavioral Characterization...................................................................... 50 3.4 SUMMARY, DISCUSSION AND FUTURE DIRECTIONS ........................ 52 3.4.1 Summary...................................................................................................... 52 3.4.2 Creation and Characterization of α4 KO mice........................................ 52 3.4.3 Future uses of α4 subunit KO mice........................................................... 54 3.4.4 Conditional α4 Knockout mice .................................................................. 56 4.0 THE EXTRASYNAPTIC GABAA-R IN THE MOLECULAR MECHANISMS OF GABOXADOL AND MUSCIMOL .................................................................................... 58 4.1 INTRODUCTION ............................................................................................. 58 4.1.1 Tonic inhibition and α4 containing GABAA-Rs ....................................... 58 4.1.2 Gaboxadol and α4/δ GABAA-R pharmacology........................................ 59 4.1.3 Aims of this study........................................................................................ 61 4.2 MATERIALS AND METHODS...................................................................... 61 4.2.1 Mice .............................................................................................................. 61 4.2.2 Fixed Speed Rotarod................................................................................... 62 vii 4.2.3 Radiant Tail Flick Assay
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