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Behavioural and genetic dissociation of faciliatatory processes in Caenorhabditis elegans Yu, Alex Jian Tong

Abstract

The simplest forms of learning are non-associative learning. In non-associative learning, both sensitization and dishabituation cause an increase in the behavioural response. A strong and/or surprising stimulus can enhance a naïve response above the baseline in sensitization, or facilitate a previously decremented response in dishabituation. While a lot has been done to unravel the cellular and molecular mechanisms for sensitization, the mechanism(s) for dishabituation remains elusive. Sensitization and dishabituation were considered as the same facilitatory process for decades, but recent evidence suggests that they may be two separate processes. The focus of this thesis was to investigate whether sensitization and dishabituation are the same or two different processes. In this research, sensitization and dishabituation of the response to optogenetic stimulation of ASH nociceptor neurons by a mechanosensory tap stimulus were characterized and compared in a series of behavioural paradigms in C. elegans. It was found that sensitization and dishabituation were produced in four ASH response components with different time-dependent dynamics. Using a candidate gene approach, the effects of several genes on sensitization and dishabituation of the ASH response were examined. It was found that genes played differential roles in mediating the two facilitatory processes; genes that were critical for sensitization did not affect dishabituation. Taken together, these findings strongly suggest that sensitization and dishabituation are two facilitatory processes mediated by distinct genetic and molecular pathways. This research deepens our understanding of the complex mechanisms of “simple” forms of learning.

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