Date of Award
Doctor of Philosophy (PhD)
The ability to sense changes in temperature is crucial to surviving harsh environments. Over the last decade several ion channels that have been proposed to be cold sensitive have been identified, most notably TRPM8 and TRPA1. Although these molecules have been extensively studied in vitro, their exact roles in cold sensation in vivo are still debated. This uncertainty is in large part due to problems with the standard methods of testing cold sensitivity in vivo, which often rely on subjective measures of cold responsiveness. Experiments using these subjective measures have been repeated by different groups and have yielded conflicting results, leading to this confusion. To address this issue, I developed a novel method, the cold plantar assay (CPA) to objectively assess the cold sensitivity of mice. Once I characterized the assay, I then used it to test the roles of TRPM8 and TRPA1 using genetically modified knockout mice.
Another aspect of temperature sensation is the ability to adjust to changes in ambient temperature. For organisms that live in areas that have seasonal temperature shifts, it is essential to maintain sensitivity to small temperature changes even when the environment as a whole has been heated or cooled by as much as 40C. To detect a 2C change whether the temperature starts at 45C or 15C requires an enormous dynamic range, and sophisticated molecular machinery behind it. The molecular mechanisms that may underlie adaptation have been studied in vitro, but has not been quantified or tested in vivo. In order to study how live mice adapt their thermal response thresholds to changes in the ambient temperature, I modified the CPA and the classical Hargreaves assay to test the temperature sensitivity of mice at different ambient temperatures. Using these assays, I demonstrated that the most important factor for withdrawal from thermal stimuli is the temperature change from baseline, and that this baseline can be rapidly adjusted. Furthermore, I utilized our transgenic knockout mice and pharmacological agents to demonstrate that phospholipid modulation of TRPM8 is essential for the rapid adaptation of cold sensitivity to changing ambient temperatures.
Finally, the last part of this thesis focuses on the different neuronal populations that express those molecules. While many different populations of nociceptive neurons have been identified using immunohistochemical labels, it has been difficult to directly correlate these populations with specific functions. I used a recently developed tool, optogenetics, to activate or silence selected neuronal populations while applying nocifensive stimuli. To deliver the light necessary for optogenetic modulation, I chronically implanted wireless LED devices to deliver the light directly to the axons of the nociceptive neurons. By silencing specific nocifensive populations while delivering to cold stimuli, I assessed which subsets of nociceptive neurons are necessary for full responses to cold stimuli.
Chair and Committee
Robert W. Gereau IV
Timothy Holy, Michael Bruchas, Yu-Qing Cao, Simon Haroutounian,
Brenner, Daniel, "The Molecular and Cellular Basis for Cold Sensation" (2016). Arts & Sciences Electronic Theses and Dissertations. 765.