The GABAA receptor is a critical part of the sensingmachinery that triggers homeostatic plasticity of synaptic strengthand intrinsic excitability. Public

Wilhelm, Jennifer Caldwell (2008)

Permanent URL: https://etd.library.emory.edu/concern/etds/nv935379c?locale=fr
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Abstract

Generating and maintaining stability is a problem that neural networks must manage during development. Whereas Hebbian forms of plasticity allow for fast experience-dependent modification of synapses, these changes in synaptic strength can lead to the destabilization of neuronal firing rates if long-term potentiation and long-term depression are unconstrained. A new set of mechanisms, termed homeostatic plasticity, aids in the stabilization of network spiking activity by restraining the average amount of spiking activity within a cell or a network of cells within a set range. Experimentally homeostatic plasticity has been examined by pushing spiking activity to extremes; either all activity is blocked or activity is highly enhanced. Studies in multiple experimental systems have found that chronically reducing spiking activity in synaptically connected neurons triggers coordinated increases in excitatory synaptic strength as well as increases in intrinsic cellular excitability. These changes are thought to be homeostatic as they act to increase activity to recover normal levels. The mechanisms by which cells monitor spiking activity and trigger compensatory changes in synaptic strength are not well understood. Cells could monitor the average level of membrane depolarization or the activation of neurotransmitter receptors as a proxy for activity. Few studies have been able to separate the roles of membrane depolarization and neurotransmission. However, using the developing chick embryonic spinal cord, we are able to block either excitatory GABAAergic or glutamatergic transmission without significantly altering membrane depolarization. We found that blocking GABAA receptors, but not glutamate receptors, triggered compensatory increases in synaptic strength and cellular excitability similar to those demonstrated after activity block. This suggests a special role for the GABAA receptor as a critical part of the machinery that senses changes in activity and triggers compensatory mechanisms.

Table of Contents

1 CHAPTER 1: GENERAL INTRODUCTION

3 Homeostatic regulation of neuronal firing rates

5 Homeostatic regulation of activity: scaling of quantal amplitudes

8 Homeostatic synaptic scaling: possible mechanisms

11 Homeostatic synaptic scaling: potential signaling molecules

14 Homeostatic regulation of activity: changes in mPSC frequency

16 Possible sensors for activity

18 Differences between homeostatic and Hebbian synaptic plasticity

20 Homeostatic plasticity: homeostatic changes in cellular excitability

22 Spontaneous network activity (SNA) in developing systems

25 SNA after neurotransmitter block in the in vitro chick spinal cord

27 SNA and homeostatic synaptic plasticity

28 Dissertation objectives

36 CHAPTER 2: GABAA TRANSMISSION IS A CRITICAL STEP IN THE PROCESS OF TRIGGERING HOMEOSTATIC INCREASES IN QUANTAL AMPLITUDE

37 ABSTRACT

38 INTRODUCTION

41 RESULTS

41 Activity block in ovo

41 Activity block increased SNA frequency in vitro

42 Activity block increased mPSC amplitude

46 Neurotransmitter block in ovo

47 Neurotransmitter block does not alter SNA frequency in vitro

48 No change in mPSCs following AMPA/NMDA receptor block

49 GABAA receptor block increased mPSC amplitude

51 12-hour GABAA block did not induce changes in mPSC amplitude

53 Activity block, but not GABAA receptor block, accelerates the recovery from post-episode depression.

55 DISCUSSION

56 Transiently reducing SNA does not increase mPSC amplitudes

58 Changes to mPSC amplitude following neurotransmitter block

60 Activity and neurotransmission regulate different aspects of homeostasis

61 Recovery of SNA in ovo is not due to changes in quantal amplitude

63 Concluding remarks

63 EXPERIMENTAL PROCEDURES

63 Dissection

64 Lidocaine infusion

64 Pharmacological blockade of synaptic transmission

65 Whole cell electrophysiology: voltage clamp

65 Extracellular and intracellular solutions

65 Identification and characterization of spontaneous postsynaptic currents

67 Statistics

68 REMAINING QUESTIONS AND FUTURE DIRECTIONS

91 CHAPTER 3: BLOCK OF GABAA RECEPTORS TRIGGERS HOMEOSTATIC RECOVERY OF SPONTANEOUS NETWORK ACTIVITY BY CHANGES IN CELLULAR EXCITABILITY

92 ABSTRACT

93 INTRODUCTION

95 RESULTS

95 GABAA receptor block increases intrinsic excitability

97 GABAA receptor block increases INa

99 GABAA receptor block does not alter ICa

99 GABAA receptor block decreases IA and IK(Ca)

101 GABAA receptor block differentially regulates ionic current densities

102 K+ current densities do not appear to co-regulate

103 DISCUSSION

104 Reduced GABAergic transmission and network excitability both trigger increased cellular excitability

106 Coordinated regulation of ion channels

107 GABAA receptor as a sensor for cellular excitability and synaptic strength

108 Homeostatic changes in cellular excitability, not synaptic strength, lead to recovery of activity

109 Recovery of SNA after glutamatergic block

110 Concluding remarks

111 EXPERIMENTAL PROCEDURES

111 Pharmacological blockade of activity and synaptic transmission

112 Dissection

112 Whole cell electrophysiology: current and voltage clamp

113 Extracellular and intracellular solutions

114 Chemicals and drugs

114 Statistics

116 REMAINING QUESTIONS AND FUTURE DIRECTIONS

132 CHAPTER 4: GENERAL DISCUSSION

134 Homeostatic regulation of cellular excitability and quantal amplitude

135 The GABAA receptor as a critical signal during development

136 Non-specific drug effects are unlikely to trigger homeostatic processes

137 Is the GABAA receptor a bidirectional sensor for changes in activity?

139 Potential signaling pathways initiated by GABAA receptor block

143 Other sensors are necessary for the homeostatic regulation of activity

145 Is neurotransmission part of the sensing machinery in mature neurons?

146 Concluding remarks

149 REFERENCES

FIGURES AND TABLES

1 CHAPTER 1: GENERAL INTRODUCTION

29 Figure 1.1 Isolated STG neurons can produce rhythmic bursting activity

30 Figure 1.2 Alterations to network activity produces bidirectional compensatory changes in quantal amplitude

32 Figure 1.3 Homeostatic synaptic plasticity: a closed feedback loop?

33 Figure 1.4 Activity dependent regulation of multiple factors that contribute to the scaling of quantal amplitudes.

34 Figure 1.5 GABAergic signaling is depolarizing in early development

35 Figure 1.6 Episodes of SNA in the in vitro chick spinal cord

36 CHAPTER 2: GABAA TRANSMISSION IS A CRITICAL STEP IN THE PROCESS OF TRIGGERING HOMEOSTATIC INCREASES IN QUANTAL AMPLITUDE

73 Figure 2.1 Activity block in ovo

74 Figure 2.2 Activity block increases SNA frequency in vitro

75 Figure 2.3 Activity block increases GABA- and AMPA-mPSCs

79 Figure 2.4 Neurotransmitter block in ovo

80 Figure 2.5 Neurotransmitter block does not alter SNA frequency in vitro

81 Figure 2.6 Glutamate receptor block does not alter mPSCs

82 Figure 2.7 GABAA receptor block increases mPSCs

84 Figure 2.8 12 hours of GABAA receptor block does not alter mPSCs

85 Figure 2.9 Activity block, but not GABAA receptor block, accelerates recovery from post-episode depression

87 Figure 2.10 Schematic of experimental procedure

89 Figure 2.11 Identification of populations of mPSCs in motoneurons

90 Figure 2.12 mPSCs can be identified by decay kinetics

77 Table 2.1 mPSC properties: amplitude, frequency, and kinetics

78 Table 2.2 Passive membrane properties

88 Table 2.3 Acute TTX does not alter PSC properties

91 CHAPTER 3: BLOCK OF GABAA RECEPTORS TRIGGERS HOMEOSTATIC RECOVERY OF SPONTANEOUS NETWORK ACTIVTY THROUGH COMPENSATORY CHANGES IN CELLULAR EXCITABILITY

118 Figure 3.1 GABAA receptor block decreased spike threshold

121 Figure 3.2 INa is increased by GABAA receptor block

122 Figure 3.3 ICa is not altered by GABAA receptor block

123 Figure 3.4 IA is decreased by GABAA receptor block

125 Figure 3.5 IK(Ca) is decreased by GABAA receptor block

127 Figure 3.6 Current densities are differentially regulated

128 Figure 3.7 IA and IK(Ca) do not co-regulate

129 Figure 3.8. Activity recovery occur via increased cellular excitability

131 Figure 3.9 GABAA receptor and activity block trigger similar changes

120 Table 3.1 Membrane properties of motoneurons

132 CHAPTER 4: GENERAL DISCUSSION

149 REFERENCES

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