Structural Plasticity of GABAergic and Glutamatergic Terminals in the Ventral Motor and Caudal Intralaminar Thalamic Nuclei in MPTP-treated Parkinsonian Monkeys Open Access

Abstract

In primates, the parvocellular part of the ventral anterior nucleus (VApc) and the centromedian nucleus (CM) are the thalamic targets of inputs from the internal globus pallidus (GPi). Neurons in VApc and CM also receive glutamatergic projections from the cerebral cortex, and GABAergic inputs from the reticular nucleus (RTN) and interneurons. In MPTP-treated monkeys, the firing rate and pattern of VApc neurons is altered. To examine whether structural changes in synaptic connectivity are associated with these dysfunctions, we used light- and electron microscopy (LM and EM) immunohistochemical and tracing methods to assess changes in GABAergic and glutamatergic synaptic networks of the VApc and CM in parkinsonian monkeys.

At the LM level, the intensity of immunostaining for GABAergic markers in VApc and CM did not significantly differ between normal and parkinsonian monkeys, suggesting that there are no gross changes in the GABAergic innervation to these nuclei in the parkinsonian state. At the EM level, we identified 3 types of terminals in VApc and CM: Terminals forming asymmetric synapses (As-type), which originate mostly from the cerebral cortex, terminals forming single symmetric synapses (S1-type), representing GABAergic inputs from the RTN and interneurons, and terminals forming multiple symmetric synapses (S2-type) which originate from the GPi. The average density of As-type terminals outnumbered that of S1 and S2-type terminals in VApc and CM of normal and parkinsonian monkeys. Compared to untreated animals, the density of As-type terminals was lower in the VApc of parkinsonian monkeys, while that of S1 and S2-type terminals was the same. The pattern of synaptic connectivity of the three terminal subtypes did not differ between both groups. However, the proportion of S2-type terminals in contact with vesicle-filled dendrites of GABAergic interneurons was higher in VApc and CM of parkinsonian monkeys. Preliminary evidence from 20 reconstructed pallidothalamic terminals suggest an increase in the flat area and surface area of GABAergic synapses in parkinsonian animals. Together, these findings suggest that the parkinsonian state is associated with plasticity in the prevalence and microcircuitry of glutamatergic corticothalamic afferents and GABAergic pallidothalamic inputs, which may contribute to the pathophysiology of the basal ganglia-thalamocortical system in parkinsonism.

Table of Contents

ABSTRACT II

ACKNOWLEDGEMENTS IV

TABLE OF CONTENTS VI

LIST OF FIGURES IX

LIST OF TABLES X

CHAPTER 1: INTRODUCTION & BACKGROUND 1

1.1 INTRODUCTION 2

1.2 FUNCTIONAL CIRCUITRY OF THE BASAL GANGLIA 3

1.2.1 General Organization of the Basal Ganglia 3

1.2.2 Basal Ganglia-Thalamocortical network 4

1.2.2.1 Direct and Indirect Pathways 4

1.3 THALAMUS: FUNCTIONAL COMPARTMENTALIZATION, CELLULAR COMPOSITION, AND CONNECTIONS 8

1.3.1 Thalamic compartmentalization and nomenclature 8

1.3.1.1 Anatomical Organization of Thalamus 8

1.3.1.2 Motor Thalamus 12

1.3.1.3 Caudal Intralaminar Thalamus 13

1.3.2 Motor Thalamic neurons 14

1.3.2.1 Relay Cells: Glutamatergic thalamocortical neurons 15

1.3.2.2 GABAergic Interneurons 16

1.3.2.3 Reticular Thalamic Nucleus Neurons 18

1.3.3 Ventral Motor Thalamus: afferent connections 19

1.3.3.1 Corticothalamic system 19

1.3.3.2 Pallidothalamic system 21

1.3.4 CM/Pf: afferent connections 23

1.3.5 RTN: afferent connections 24

1.3.6 Ventral Motor Thalamus: efferent connections 24

1.3.6.1 Thalamocortical system 24

1.3.7 CM/Pf: efferent connections 27

1.3.7.1 Thalamocortical system 27

1.3.7.2 Thalamostriatal system 28

1.4  PARKINSON’S DISEASE 29

1.4.1 MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) model of Parkinson’s disease 30

1.4.1.1 The MPTP-treated monkey model of Parkinson’s disease 30

1.4.2 Pathophysiology of Parkinson’s disease and the involvement of the thalamus 32

1.4.2.1 BG-thalamocortical network in Parkinson’s disease 32

1.4.2.2 Firing abnormalities in Parkinson’s disease 34

1.5 GOALS OF THESIS 35

1.5.1 Specific Aim 1 36

1.5.2 Specific Aim 2 37

CHAPTER 2 : STRUCTURAL PLASTICITY OF GABAERGIC AND GLUTAMATERGIC NETWORKS IN THE MOTOR THALAMUS OF MPTP-TREATED PARKINSONIAN MONKEYS 38

2.1 INTRODUCTION 39

2.2 MATERIALS AND METHODS 40

2.2.1 Animals 40

2.2.2 MPTP treatment and assessment of Parkinsonism 41

2.2.3 Anterograde labeling of pallidothalamic terminals 43

2.2.4 Tissue collection 44

2.2.5 Immunohistochemistry 44

2.2.5.1 Tissue processing for microscopy 44

2.2.5.2 Striatal tyrosine hydroxylase (TH) immunostaining 46

2.2.5.3 Post-embedding GABA immunostaining 46

2.2.5.4 vGAT and GAD67 immunoperoxidase labeling 47

2.2.5.5 GFP immunoperoxidase labeling 47

2.2.5.6 vGluT1 immunostaining 47

2.2.6 Analysis of material 48

2.2.6.1 Striatal TH immunostaining intensity measurements 48

2.2.6.2 Density of vGAT and GAD67-staining 49

2.2.6.3 Density of GABAergic and glutamatergic terminals 50

2.2.6.4 Size of glutamatergic terminals 50

2.2.6.5 Pattern of synaptic connectivity of GABAergic and glutamatergic terminals 51

2.2.6.6 Relative prevalence of dendritic profiles 51

2.2.6.7 Proportion of vGluT1-labeled terminals and of anterogradely labeled GPi terminals in contact with GABAergic interneurons 52

2.3 RESULTS 54

2.3.1 Nigrostriatal dopamine denervation in MPTP-treated monkeys 54

2.3.2 GAD67 and vGAT Immunolabeling in VApc and CM 54

2.3.3 Types of GABAergic and glutamatergic terminals in VApc and CM 54

2.3.4 Relative prevalence and preferred post-synaptic targets of GABAergic and glutamatergic terminals in the VApc and CM 59

2.3.5 Size of glutamatergic and GABAergic terminals in the VApc and CM of MPTP-treated monkeys 63

2.3.6 Corticothalamic and pallidothalamic synapses onto putative interneuron dendrites 64

2.4 DISCUSSION 69

2.4.1 Synaptic Organization of VApc and CM in Control vs. Parkinsonian Monkeys 69

2.4.2 GABAergic innervation of CM in the parkinsonian state 71

2.4.3 Plastic remodeling of the corticothalamic projection to VApc and CM in the parkinsonian state 72

2.4.4 Plasticity of Cortical and Pallidal Inputs to Putative GABAergic Interneurons in MPTP-treated Parkinsonian Monkeys 74

CHAPTER 3 : ULTRASTRUCTURAL FEATURES OF SINGLE PALLIDOTHALAMIC TERMINALS IN CONTROL AND PARKINSONIAN MONKEYS VISUALIZED USING 3D ELECTRON MICROSCOPIC RECONSTRUCTION APPROACHES 76

3.1 INTRODUCTION 77

3.2 MATERIALS AND METHODS 79

3.2.1 Animals 79

3.2.2 MPTP treatment and assessment 79

3.2.3 Anterograde labeling of pallidothalamic terminals 80

3.2.4 Immunohistochemistry 80

3.2.4.1 GFP immunoperoxidase labeling 80

3.2.5 Ultrathin serial sectioning electron microscopy analysis and 3D reconstruction 80

3.2.6 Analysis of material 82

3.3 RESULTS 83

3.3.1 General ultrastructural characteristics of pallidothalamic terminals in a control and a MPTP-treated monkey 83

3.3.2 Number of Synapses per GPi Terminals in Control and Parkinsonian Monkeys 87

3.4 DISCUSSION 90

3.4.1 3D reconstruction utilizing SBF/SEM 90

3.4.2 Anatomical and functional characteristics of pallidothalamic system 91

3.4.3 Changes in number and size of GABAergic pallidothalamic synapses in parkinsonian monkeys 93

CHAPTER 4 : CONCLUSIONS & IMPLICATIONS 94

4.1 SUMMARY OF MAIN FINDINGS 95

4.2 IMPLICATIONS 96

4.2.1 Influence of synaptic plasticity in the corticothalamic system 96

4.2.2 Reorganization of GABAergic interneurons and projection neurons in the basal ganglia receiving thalamus 97

4.2.3 Structural changes in GABAergic system in Parkinson’s disease in the VApc 98

4.3 CONCLUDING REMARKS 99

4.4 FUTURE DIRECTIONS 100

CHAPTER 5 : REFERENCES 102

 

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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
Keyword	MPTP Basal Ganglia Synaptic Plasticity Parkinson disease Connectivity Motor Thalamus
Committee Chair / Thesis Advisor	Smith, Yoland, Emory University Wichmann, Thomas, Emory University
Committee Members	Bolam, Paul, University of Oxford Tansey, Malu, Emory University Jaeger, Dieter, Emory University

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