The influence of seasonal light on the circadian system
Tackenberg, Michael Christian
The biological response to changes in seasonal light has been the subject of scientific interest for nearly 100 years. Over time, the significant role of the circadian pacemaker in mediating the mammalian response to seasonal light changes has become clearer. Despite knowledge of a role in transducing this light signal into a physiological response, the exact mechanism of that transduction and the specific components of the light signal that induce those changes have yet to be fully defined. In this dissertation, the distinct components of seasonal light (duration, timing, onset-offset interval) are examined for their ability to induce persistent changes in two circadian behavioral properties implicated in seasonal responses: locomotor activity duration and free-running period. We find that the onset-offset interval is critical for determining activity duration, while net phase shift direction and light interval are critical for determining period. Further, we find that a change to phase distribution underlies changes to activity duration but not to free-running period. To interrogate the cellular mechanism of these changes, the role of a particular set of circadian clock neurons, the VIPergic neurons of the SCN, in inducing proximal changes in activity duration is examined. We find that VIPergic neuron activation by optogenetics induces proximal changes in activity duration. Lastly, a set of techniques for the assessment of relevant circadian properties both in vivo and ex vivo are presented. These include a more precise and accurate period-independent method to measure phase shifts, a higher-throughput computational approach to calculating activity duration, and a strategy for quantifying phase dispersal across bioluminescent SCN cultures. The results and techniques presented in this dissertation represent an advance in our understanding of the underlying induction of seasonal responses (both in cues and in cellular mechanism) as well as a step forward in the accurate quantification of relevant properties.