Date of Award

2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Bioengineering

First Advisor

Christopher Fang-Yen

Abstract

Understanding how the internal state of an organism affects its response to stimuli is an important question of biology and key to understanding human neurobehavioral problems. With its tractable 302 neuron nervous system and complex behavioral repertoire, the roundworm Caenorhabditis elegans is well suited for neurobehavioral studies. In this work, I investigate the modulation of C. elegans touch response. The traditional gentle and harsh (nociceptive) touch assays involve manually delivering a stimulus to 1 mm long animals using an eyebrow or a metal wire, respectively. Using these simple assays, researchers have identified the genes and mechanoreceptor neurons mediating gentle and harsh touch. However, these are two separate manual assays limited in throughput and repeatability of the stimulus. First, I created a multiplexed microfluidic assay that allows gentle and harsh touch response behavior to be compared quantitatively in the same assay. I found that the threshold of harsh touch is about five times the threshold of gentle touch and that, while both responses habituate to repeated stimuli, the gentle touch response depends on the location of the previous touch, while the harsh touch response does not. I also found that gentle touch response is not affected by pre-stimulus velocity. Neuromodulatory states like sleep and stress can also affect touch response. Next, I examined how the response to cellular stressors, which causes an EGF-mediated recovery quiescence called stress-induced sleep (SIS), affects the mechanosensory response. Normally touch causes increased locomotor activity followed by return to baseline behavior. During this study, I observed a period of about 45 seconds of increased quiescence following the initial locomotor response. This behavior had not been documented in the literature, so I named it post-response quiescence (PRQ). I found that PRQ is upregulated following EGF overexpression, mediated, like sleep, by neuropeptide signaling, and requires the sleep active neurons ALA and RIS. While these observations suggested a form of sleep homeostasis, I found that PRQ does not meet two of the four behavioral criteria of sleep: it is not accompanied by a decrease in arousability, and it is not itself under homeostatic regulation. C. elegans touch response is known to be an escape response, and the presence of predator kairomone is known to affect C. elegans behavior. Because of the prevalence of quiescent behavior (freezing and tonic immobility) in vertebrate threat responses, and its upregulation during stress, I argue that PRQ may be a stress-modulated defensive freezing behavior in C. elegans.

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Additional Files

Vid2S3_uFluidic_stimulus.avi (4647 kB)
Vid2S4_uFluidic_omegaTurn.avi (263 kB)
Vid3S1_UVSIS_PRQ.avi (4631 kB)
Vid3S2_control_nonPRQ.avi (5272 kB)
Vid3S3_control_PRQ.avi (5051 kB)
Vid3S4_unc31response.avi (14378 kB)

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