Exploring homeostatic regulation in neuronal systems: Insights from cortical cultures, embryonic chick spinal cord, and the Fmr1 KO mouse model Public

Abstract

Homeostatic plasticity encompasses a set of mechanisms that are crucial for stabilizing various

characteristics of neural activity despite any perturbations the nervous system might encounter. This

dissertation explores homeostatic regulation and plasticity in three different systems: neuronal

cortical cultures, the embryonic chick spinal cord, and the Fragile X Syndrome (FXS) mouse model.

In the first study, we investigated the effects of GABAergic blockade on neuronal firing in mouse

cortical cultures and motoneurons in the embryonic chick spinal cord. After conducting a

comprehensive analysis of various spiking activity characteristics, we found that the response to

GABAergic blockade was variable across many spiking features, including burst frequency and

overall spike frequency. However, the spike rate within a burst consistently increased and then

returned to baseline control levels within hours in both systems, suggesting that this feature is

robustly homeostatically maintained. In the second study, we used a mouse model of FXS, the Fmr1

KO mouse, to examine if there are impairments in homeostatic plasticity following unilateral

whisker deprivation in layer 5/6 of the barrel cortex. Our results demonstrate significant deficits in

the recruitment of excitatory and inhibitory neurons, both at baseline and following whisker

deprivation. In addition, we observed a change in the sensitivity of excitatory neurons at a later

developmental time point. Together, these two studies provide insights into how networks maintain

stable activity levels through homeostatic plasticity mechanisms, and how perturbations affect

normal spiking activity, in both in vitro and in vivo experimental models.

Table of Contents

1 Introduction ……………………………………………………………….. 1

1.1 Homeostatic plasticity ……………………………………………………………. 1

1.1.1 Homeostatic synaptic plasticity …………………………………………… 2

1.1.2 Homeostatic intrinsic plasticity……………………………………………. 3

1.1.3 Importance of homeostatic plasticity in neuron function and development . 4

1.1.4 Homeostatic plasticity in the embryonic chick spinal cord ………………... 5

1.1.5 Homeostatic regulation of spiking characteristics ………………………… 8

1.1.6 Homeostatic plasticity in neurodevelopmental disorders ……………….... 10

1.2 Fragile X Syndrome (FXS) ………………………………………………………. 12

1.2.1 Genetic basis and prevalence ……………………………………………. 12

1.2.2 Anatomical, cognitive, and behavioral clinical manifestations …………… 13

1.2.3 Current therapeutic approaches …………………………………………. 15

1.2.4 Functions of FMRP in the nervous system ……………………………… 16

1.2.5 Experimental models of FXS ……………………………………………. 18

1.2.6 mGluR theory of FXS ………………………………………………….... 19

1.2.7 Impaired baseline and homeostatic plasticity in FXS …………………….. 21

1.3 Barrel Cortex ……………………………………………………………………. 22

1.3.1 Overview of the whisker-responsive circuit …………………………….... 23

1.3.2 Deficits in the barrel cortex in FXS models …………………………….... 27

1.3.4 Importance of using sensory systems in studying plasticity ……………… 29

1.4 Dissertation aims and hypotheses ……………………………………………….. 31

2 Homeostatic Regulation of Spike Rate within Bursts in Two Distinct Preparations …… 33

2.1 Abstract …………………………………………………………………………. 33

2.2 Introduction …………………………………………………………………….. 34

2.3 Results …………………………………………………………………………... 35

2.3.1 SRWB is homeostatically restored following GABAergic blockade in cortical cultures ….. 35

2.3.2 SRWB is homeostatically restored following GABAergic blockade in the isolated chick embryo spinal cord ….. 47

2.4 Discussion ………………………………………………………………………. 56

2.4.1 Variability in firing rate properties following GABAergic blockade …….... 56

2.4.2 Homeostasis of SRWB in both preparations …………………………….. 58

2.5 Experimental Procedures ………………………………………………………... 61

2.5.1 Cell culture ……………………………………………………………… 61

2.5.2 Spinal cord dissection …………………………………………………… 62

2.5.3 Electrophysiology recordings ……………………………………………. 62

2.5.4 Calcium imaging ………………………………………………………… 63

2.5.5 Data analysis …………………………………………………………….. 64

2.5.6 Statistical analysis ………………………………………………………... 67

3 Failure to homeostatically recruit layer 5/6 neurons during whisker stimulation in Fmr1 KO mice… 68

3.1 Abstract …………………………………………………………………………. 68

3.2 Introduction …………………………………………………………………….. 68

3.3 Results …………………………………………………………………………... 70

3.3.1 P16 WT RS neurons respond to whisker stimulation more than KO RS neurons …..…. 71

3.3.2 P16 2-day whisker-deprived WT and KO neurons increased responsiveness, but only WT neurons increased neuronal recruitment …... 74

3.3.3 P21 KO RS neurons exhibit an increased threshold to whisker stimulation compared to WT neurons …………….... 77

3.3.4 P21 7-day whisker-deprived WT and KO neurons increased responsiveness, but only WT neurons increased neuronal recruitment …... 79

3.3.5 WT and KO FS neurons respond similarly to RS neurons at P16 ……….. 81

3.4 Discussion ………………………………………………………………………. 82

3.4.1 Baseline differences in WT and KO neurons ………………………………. 83

3.4.2 Plasticity in WT neurons ………………………………………………….... 85

3.4.3 Plasticity in KO neurons …………………………………………………... 86

3.5 Experimental Procedures ………………………………………………………... 88

3.5.1 Mice ……………………………………………………………………... 88

3.5.2 Whisker deprivation ……………………………………………………... 89

3.5.3 Electrophysiology recordings ……………………………………………. 90

3.5.4 Whisker stimulation ……………………………………………………... 91

3.5.5 Histology ………………………………………………………………... 91

3.5.6 Analysis …………………………………………………………………. 91

3.5.7 Code and data availability ………………………………………………... 92

4 Discussions and Future Directions …………………………………….. 94

4.1 General discussion and future directions for Chapter 2 ……………….…………. 94

4.1.1 Variability and degeneracy of firing properties …………………………... 94

4.1.2 Significance of SRWB homeostasis ……………………………………… 95

4.1.3 Future directions ………………………………………………………... 97

4.2 General discussion and future directions for Chapter 3 ……………………….... 101

4.2.1 Developmental changes in sensory processing in the Fmr1 KO model … 101

4.2.2 Impaired recruitment of neurons at baseline and following perturbations in the Fmr1 KO ……….... 102

4.2.3 Future directions ……………………………………………………….. 105

4.3 What aspect of neuronal activity is homeostatically maintained ……………….... 108

5 References ………………………………………………………………. 110

About this Dissertation

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School	Laney Graduate School
Department	Biological and Biomedical Sciences
Subfield / Discipline	Neuroscience
Degree	Ph.D.
Submission	Dissertation
Language	English
Research Field	Biology, Neuroscience
Mot-clé	embyronic spinal cord Fragile X Syndrome homeostatic plasticity neuroscience barrel cortex mouse cortical cultures
Committee Chair / Thesis Advisor	Peter Wenner, Emory University
Committee Members	Garrett Stanley, Georgia Tech Astrid Prinz, Emory University Nicholas Au Yong, Emory University Malavika Murugan, Emory University

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