Critical Periods in the Development of Amygdala Inhibition: Effects of Prenatal Stress Open Access
Ehrlich, David Edward (2014)
Abstract
The amygdala plays a key role in emotional processing, and dysfunction of the amygdala is implicated in a variety of psychiatric disorders. Growing evidence suggests many disorders, including anxiety, depression, autism, and schizophrenia, are neurodevelopmental in origin. During development, brain circuits undergo "sensitive periods" when environmental factors have potent effects that include long-term consequences for emotional processing. We hypothesized that a risk factor for psychiatric disorders, prenatal stress, alters the trajectory of amygdala maturation and thereby influences emotional outcomes. In order to test this hypothesis, however, it was necessary to first understand how the amygdala typically develops. While a number of gross structural and functional changes have been identified in the normally developing amygdala, no studies have characterized amygdala development in terms of the function of neurons. Here, we report the first findings of changes to electrophysiology of the developing amygdala. We used patch clamp in ex vivo rat brain slices to describe the electrophysiology of amygdala neurons, discovering a number of changes. Amygdala neurons exhibited profound maturation of their intrinsic properties from birth through infancy, including reduced excitability, faster action potentials produced at higher rates, and altered expression of membrane currents and the ion channels that mediate them. Developing amygdala neurons also showed corresponding morphological changes, including expansion of dendritic arbors and the emergence of dendritic spines. In addition, synapses for the neurotransmitter gamma-aminobutyric acid (GABA) exhibited changes to kinetics, excitability, and synaptic plasticity. GABA regulates a variety of neurodevelopmental processes, including cell proliferation, migration, and differentiation, as well as synapse maturation, suggesting perturbation of the GABA system could alter amygdala development. We found that GABAergic function in the amygdala was altered by exposure to prenatal stress, with deficits emerging during infancy that preceded long-term changes to neuron excitability. Furthermore, these changes to amygdala maturation correspond with deficits in amygdala-dependent, emotional behavior. In sum, we have provided the first account of physiological development of amygdala neurons, and identified ways in which this process is perturbed by a risk factor for psychiatric illness.
Table of Contents
Abbreviations...1
Chapter 1: The Basolateral Amygdala and Early-Life Stress in Anxiety Disorder Etiology...2
1.1 Neurodevelopmental Basis of Anxiety Disorders...3
1.1.1 Emergence of Anxiety Disorders in Children and Adolescents...3
1.1.2 Early Life Sensitivity to Stress...4
1.1.3 'Critical Periods' in Development as Windows of Vulnerability...10
1.2 Juvenile and Adult Anxiety Disorders Involve Dysfunction of the Amygdala...11
1.2.1 Implicating Amygdala Dysfunction in Anxiety Disorders...12
1.2.2 The BLA in the Responses to Acute and Chronic Stress...14
1.2.3 GABA, a Key Regulator of BLA Function...16
1.3 Maturation of Amygdala Physiology and Function...20
1.3.1 A Critical Period for Amygdala Influence on Emotional Development...21
1.3.2 Development of the Human Amygdala...22
1.3.3 Development of the Rodent Amygdala...23
1.4 Impact of Early Life Experience on Amygdala Development...33
1.4.1 Early Life Stress and the Trajectory of Amygdala Development...34
1.4.2 Experiential Factors Promoting Resiliency...41
1.5 Conceptual Summary...43
Chapter 2: Postnatal development of electrophysiological properties of principal neurons in the rat basolateral amygdala...47
2.1 Abstract...48
2.2 Introduction...49
2.3 Methods...50
2.3.1 Ethical approval...50
2.3.2 Animals...51
2.3.3 Slice preparation...51
2.3.4 Patch clamp recording...51
2.3.5 Data Analysis...52
2.3.6 Membrane Properties and Intrinsic Currents...53
2.3.7 Action Potentials and Spike Trains...54
2.3.8 Resonance and Oscillations...55
2.3.9 Statistics...55
2.4 Results...57
2.4.1 Postnatal Maturation of Passive Membrane Properties...57
2.4.2 Postnatal Maturation of Intrinsic Currents...58
2.4.3 Postnatal Maturation of Spiking...63
2.4.4 No Effect of Sex on Postnatal Changes in Physiological Properties...66
2.5 Discussion...66
2.5.1 Maturation of passive membrane properties...67
2.5.2 Maturation of membrane potential oscillations and resonance...68
2.5.3 Maturation of Ih and its contribution to resonance...70
2.5.4 Maturation of trains of action potentials...72
2.5.5 Maturation of action potentials and AHPs...73
2.5.6 Maturation of amygdala connectivity and neuronal morphology...75
Chapter 3: Morphology and Ion Channel Expression of Developing Principal Neurons in the Rat Basolateral Amygdala...95
3.1 Abstract...96
3.2 Introduction...97
3.3 Methods...98
3.3.1 Ethical approval...98
3.3.2 Animals...98
3.3.3 Slice Physiology...99
3.3.4 Patch clamp recording...99
3.3.5 Histochemical Processing...100
3.3.6 Neuronal reconstruction and Data Analysis...101
3.3.7 Single Cell RT-PCR...102
3.3.8 Statistics...102
3.4 Results...103
3.4.1 Soma size...103
3.4.2 Growth and Retraction of Dendritic Arbor...104
3.4.3 Maturation of Dendritic Branching...106
3.4.4 Developmental Emergence of Dendritic Spines...107
3.4.5 Expression of Ion Channel Transcripts...108
3.5 Discussion...111
3.5.1 Somatic Development...112
3.5.2 Dendritic Morphology...112
3.5.3 Dendritic Spine Emergence...115
3.5.4 Voltage-gated Ion Channel Expression...117
Chapter 4: Postnatal maturation of GABAergic transmission in the rat basolateral amygdala...136
4.1 Abstract...137
4.2 Introduction...138
4.3 Methods...141
4.3.1 Ethical approval...141
4.3.2 Animals...141
4.3.3 Slice preparation...141
4.3.4 Whole-cell patch clamp...142
4.3.5 Spontaneous inhibitory postsynaptic currents...143
4.3.6 Stimulation-evoked postsynaptic potentials and currents...144
4.3.7 Picospritzer response...145
4.3.8 Single-cell and whole tissue RT-PCR...146
4.3.9 Statistics...146
4.4 Results...147
4.4.1 Compound synaptic response to local electrical stimulation...147
4.4.2 Kinetics of fast synaptic inhibition of BLA principal neurons...148
4.4.3 Depolarized reversal potential of GABAA receptors in immature BLA principal neurons...151
4.4.4 Short-term plasticity of GABAA IPSCs...152
4.4.5 Spontaneous GABA activity is rhythmically organized throughout the first postnatal month...154
4.5 Discussion...154
4.5.1 Shift from depolarizing to hyperpolarizing GABAA transmission...155
4.5.2 Development of a GABAergic shunt of the network response...157
4.5.3 Faster IPSCs with age...159
4.5.4 Short-term synaptic depression of GABAA IPSCs in immature BLA...161
Chapter 5: The Developmental Trajectory of Amygdala Neuron Excitability and GABAergic Transmission are Altered by Prenatal Stress...183
5.1 Abstract...184
5.2 Introduction...186
5.3 Methods...189
5.3.1 Ethical Approval...189
5.3.2 Animals...189
5.3.3 Prenatal Stress...189
5.3.4 Electrophysiology...190
5.3.5 Quantitative RT-PCR...194
5.3.6 Behavioral Testing...195
5.3.7 Statistics...196
5.4 Results...198
5.4.1 Reduced emotionality in adult and juvenile PS rats...198
5.4.2 PS altered intrinsic properties of BLA principal neurons during development...200
5.4.3 PS reduced excitability of BLA principal neurons in adulthood...201
5.4.4 PS altered the developmental trajectory of GABAergic transmission in the BLA...202
5.4.5 PS reduced BLA expression of the GABAA receptor α1 subunit during a development critical period...204
5.5 Discussion...205
5.5.1 PS Reduces Anxiety-Like Behavior and May Reduce Sociability...205
5.5.2 PS Alters Electrophysiological Properties of Developing BLA Neurons...207
5.5.3 PS Alters GABAergic Transmission and Receptor Expression in the BLA...209
Chapter 6: Spike-Timing Precision and Neuronal Synchrony are Enhanced by an Interaction between Synaptic Inhibition and Membrane Oscillations in the Amygdala...239
6.1 Abstract...240
6.2 Introduction...241
6.3 Methods...243
6.3.1 Animals and housing conditions...243
6.3.2 Electrophysiological procedures...244
6.3.3 Data and statistical analysis...247
6.4 Results...248
6.4.1 Primate BLA principal neurons receive spontaneous, synchronized, rhythmic IPSPs that coordinate action potential timing...248
6.4.2 Compound IPSPs enhance spike-timing precision in rat BLA principal neurons...251
6.4.3 Compound IPSPs synchronize the firing activity of multiple BLA principal neurons...252
6.4.4 Compound IPSPs facilitate an intrinsic membrane potential oscillation in BLA principal neurons...253
6.4.5 The membrane potential oscillation is sensitive to modulation of its component currents...254
6.5 Discussion...258
6.5.1 Synchronized inhibition drives coordinated activity of BLA principal neurons...258
6.5.2 Resonance frequency and intrinsic membrane oscillations in BLA principal neurons...261
6.5.3 Implications for learning and memory...264
Chapter 7: Discussion...286
7.1 Summary of Results...287
7.2 Integration of Findings...289
7.2.1 Importance of Studying Developmental Trajectories...289
7.2.2 Potential Impact of Prenatal Stress on Emotion Via Amygdala Network Oscillations...290
7.2.3 Applying Critical Period Concepts to BLA Development...292
References...299
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