Document Type

Dissertation

Date of Award

5-31-2020

Degree Name

Doctor of Philosophy in Biology - (Ph.D.)

Department

Federated Department of Biological Sciences

First Advisor

Gal Haspel

Second Advisor

Farzan Nadim

Third Advisor

Daphne F. Soares

Fourth Advisor

Eric Scott Fortune

Fifth Advisor

Andrew M. Leifer

Abstract

Inhibition plays important roles in modulating neural activities at different levels from small synapses to brain regions, and different systems from sensory to motor. To achieve translocation, locomotor systems produce alternation of antagonist muscles, including axial posture and limb movement and alternation. In the nematode C. elegans, a cross-inhibition circuit, involving excitatory cholinergic and inhibitory GABAergic motoneurons, is believed to generate the dorsoventral alternation of body-wall muscles that supports backward undulatory locomotion. This dissertation challenges this prevalent hypothesis, delves into studying different roles of inhibition, and depicts the expression pattern and functional role of ionotropic GABAA receptor, UNC-49, in motoneurons and body-wall muscles in the locomotion circuit.

This dissertation demonstrates that the shrinking phenotype, formerly demonstrated only by harsh touch to the head of GABA transmission mutants, is exhibited by wild type as well as mutant animals in response to harsh touches to the head or tail and that only GABA transmission mutants show slow locomotion post-stimulation. Impairment of GABA transmission, either genetically or optogenetically, induces lower undulation frequency and lower translocation speed during crawling and swimming in both directions. GABAergic motoneurons’ activity pattern is different during low and high undulation frequency. During low undulation frequency, GABAergic VD and DD motoneurons show a similar activity pattern; while during high undulation frequency their activity alternate. The experimental results suggest at least three possible roles for inhibition that we test with a computational model - cross-inhibition or disinhibition of body-wall muscles, and inhibitory reset or disinhibition of cholinergic VA and VB motoneurons - that could underlie fast undulatory locomotion in C. elegans.

This dissertation also shows the distribution of ionotropic GABAA receptor, UNC-49, in motoneurons and body-wall muscle cells, and demonstrates the contribution of UNC-49 to fast undulatory locomotion at different elements of the locomotion circuit. Besides formerly reported expression in body-wall muscles, UNC-49 also expresses in GABAergic motoneurons and the promoter unc-49p induces gene transcription in cholinergic VA and VB motoneurons. Tissue-specific rescue demonstrates the UNC-49 in body-wall muscles plays the most important role in sustaining fast undulatory locomotion.

The experimental optimization in the dissertation projects is elaborated, including synchronized two-channel calcium imaging, big plasmid construction using Gibson Assembly, and NeuroPAL imaging using confocal microscopy.

The results of this dissertation demonstrate that GABAergic motoneurons are not necessary for dorsoventrally undulatory locomotion in C. elegans, while they are critical for sustaining fast locomotion. Ionotropic GABAA receptor, UNC-49, expresses in body-wall muscles as well as GABAergic motoneurons and cholinergic VA and VB motoneurons but contributes to sustaining fast undulatory locomotion mostly via synapses from GABAergic motoneurons to body-wall muscle cells.

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