Date of Award
Doctor of Philosophy (PhD)
During chemotaxis, animals compute spatial information about odor gradients to make navigational choices for finding or avoiding an odor source. The challenge to the neural circuitry is to interpret and respond to odor concentrations that change over time as animals traverse a gradient. In this thesis, I ask how a nervous system regulates spatial navigation by studying the chemotaxis response of Caenorhabditis elegans to diacetyl. A behavioral analysis demonstrated that AWA sensory neurons drive chemotaxis over several orders of magnitude in odor concentration, providing an entry point for dissecting the mechanistic basis of chemotaxis at the level of neural activity. Precise microfluidic stimulation enabled me to dissociate space from time in the olfactory input to characterize how odor sensing relates to behavior. I systematically measured neuronal responses to odor in the diacetyl chemotaxis circuit, aided by a newly developed imaging system with flexible stimulus delivery and elevated throughput. I found reliable sensory responses to the behaviorally relevant range of odor concentrations. I then followed odor-evoked activity to downstream interneurons that integrate sensory input. Adaptation of neuronal responses to odor yielded a highly sensitive response to small increases in odor concentration at the interneuron level, providing a mechanism for efficient gradient sensing during klinokinesis. Adaptation dynamics at the sensory level were stimulus-dependent and cell-autonomously altered in several classes of mutant animals. Behavioral responses to different concentrations of diacetyl resulted from overlapping contributions from multiple sensory neurons. AWA was specifically required for orientation behavior in response to small increases in odor concentration that are encountered in shallow gradients, demonstrating functional specialization amongst sensory neurons for stimulus characteristics. This work sheds light on an algorithm underlying acute behavioral computation and its biological implementation. The experimental results are presented in two parts: Chapter 2 describes the development of a microscope for high-throughput imaging of neuronal activity in Caenorhabditis elegans. I present a characterization of chemosensory responses to odor and its correlation with behavior. This work has been published (Larsch et al., 2013). Chapter 3 describes the functional architecture of the AWA chemosensory circuit and the role of adaptation in maintaining sensitivity over a wide range of stimulus intensities. This work is currently being prepared for publication.
Larsch, Johannes, "A Mechanism for Spatial Orientation Based on Sensory Adaptation in Caenorhabditis Elegans" (2015). Student Theses and Dissertations. 270.