Collateral Projections of Corticostriatal Neurons in Primates: A Main Source of Inputs to the Pontine Nuclei and High-Order Thalamic Nuclei Público

Abstract

Despite significant advances in our understanding of the motor connectome in rodent, our knowledge of the neuropathological changes that affect the morphology and connections of cortical projection neurons in non-human primates remains rudimentary. Extratelencephalic tract (ET) neurons, a type of cortical neuron from layer 5b of the motor cortex, send collateral projections to a number of motor areas in the brain. The lack of efficient tools to characterize the projection targets of specific populations of ET neurons in primates has significantly hampered progress in this area. Current research is limited by single-cell filling and lack of retrograde labeling of axon collaterals which has led to conflicting results on the projection of ET neurons to different brain regions. The recent development of a designer vector variant of recombinant adeno-associated virus serotype 2 (AAV2-retro), with highly potent and selective retrograde transduction properties, addresses some of these limitations. Our findings demonstrate that AAV2-retro is a highly reliable tool to retrogradely label corticostriatal neurons in rhesus monkeys. A major feature that distinguishes this technique, from other retrograde tracing methods, is the prominent labeling of axon collaterals that originate from labeled neurons allowing for the analysis of the connectome of these cells.

Our preliminary data suggests that corticostriatal neurons that project to the putamen give rise to axon collaterals that terminate in the pontine nuclei in rhesus monkeys. After AAV2-retro injections in the putamen, profuse retrograde labeling was found in motor cortices and clusters of labeled axon terminals were seen in the pontine and pulvinar nuclei. Preliminary electron microscopy data from two monkeys showed that these axon terminals form asymmetric (putatively, excitatory glutamatergic) synapses with dendritic or vesicle-filled profiles, suggesting their cortical origin. Double immunolabeling of one of the monkeys indicates labeled terminals are positive for vGluT-1 which further confirms the cortical origin. These results suggest cortical neurons send efference copies of motor commands to the striatum, pons, and thalamus. Further work will determine how this connection is altered in a nonhuman primate model of Parkinson’s disease.

Table of Contents

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

Axon Collateralization…...………………………………………………..………………1

The Motor Connectome and Species Differences…………………….…………………..1

Corticostriatal Neurons……………………………………………………………………4

Study Goals………………………………………………………………….………….....6

Methods……………………………………………………………………………………...…….8

Viral Vector-Mediated Labeling of Cortical Neurons……………………………………..8

Animals………………………………………………………………………………..…..8

Immunohistochemistry……………………………………………………………………9

Light Microscopy…………………………………………...………………….….9

Electron Microscopy……………………………………………..………………10

Immunogold for Electron Microscopy…………………………………………...11

Image Analysis……………………………………………………………………...……12

Light Microscopy…………………………………………………...……………12

Electron Microscopy…………………………………………………..…………12

Statistical Analysis……………………………………………………………………….13

Results……………………………………………………………………………………………14

Localization of Viral Vector Labeling by Light Microscopy……………...……………..14

Ultrastructure Characterization of Corticopontine and Corticothalamic Projections–Electron Microscopy……………………………………...…………...……17

Ultrastructure Characterization of Corticopontine Terminals labeled by M1 Anterograde AAV Injection vs Putamen Retrograde AAV Injection………………….………………22

Discussion……………………………………………………………………………………..…24

Collaterals of Corticostriatal Axonal Projections Throughout the Brain………...………24

Immunoreactivity for vGluT1 in Corticostriatal Projection Terminals in Pontine Nuclei and Verification of Morphology in Comparison to Corticopontine and Corticothalamic Neurons…………………………………………………………………………..………25

Differences in Morphology of Postsynaptic Targets Between Collaterals of Corticostriatal Projections in Pontine and Pulvinar Nuclei……………………………...………………26

Comparison of Morphology Between Corticostriatal, Corticopontine, and Corticothalamic Terminals……………………………………………………….………27

Limitations for Current Study and Future Directions……………………………………28

References………………………………………………………………………………………..30

About this Honors Thesis

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School	Emory College
Department	Neuroscience and Behavioral Biology
Degree	B.S.
Submission	Honors Thesis
Language	English
Research Field	Biology, Neuroscience
Palavra-chave	Pontine nuclei Corticostriatal neurons Pulvinar nuclei
Committee Chair / Thesis Advisor	Dr. Yoland Smith, Emory University
Committee Members	Dr. Adriana Galvan, Emory University Dr. Dieter Jaeger, Emory University

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