Date of Award
Winter 11-23-2023
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Locusts are a historically important model organism, and have been at the core of many discoveries related to olfaction. In recent years they have become less popular compared to organisms with genetic toolkits such as drosophila, but they have many advantages for studying principles of olfactory chemosensation and related behaviors. In this thesis there are two primary focuses; the development of a surface-bound odorant treadmill (odor-trail treadmill) for studying trail-following behaviors, and the second is the development of tools and procedures to enable simultaneous neural and behavioral recording in locusts. Addressing the first, odor-trail following has been observed in many organisms, but the classical example is the pheromone trail-following behaviors in ants. However, when studying larger organisms with sporadic movement bouts such as locusts, previously used approaches represented severe constraints. To address these issues I developed and constructed a treadmill where the locust is able to explore an odor trail printed on a belt, and the belt can be advanced to provide practically-unlimited exploration in one direction. Novel features include the dynamic printing of the odorant onto the belt before it enters the behavioral arena, thereby allowing arbitrary odorant patterns, and closed-loop control of speed based on the location of the locust within the chamber. I then used the treadmill to examine locust following behavior. While I found that locusts engaged in trail-following behaviors under both visible and dark (IR) conditions, as well as on both straight and more complex zig-zag pattern trails, for simplicity data on trail following in an odorant panel was collected using straight trails under visible conditions. I collected 15 minutes of behavior for both scented and control trials from over 600 locusts, and analyzed trail proximity and their movement behaviors. Trail-following behavior was quantified using two different metrics, the first using the proximity to the trail during the trial (Trail Proximity Index, or TPI), and the second using the ratio of the proximity during the odor trial to that during the control trial (Trail Following Index, or TFI). Largely similar odorants comprised the most attractive and least attractive subsets of each metric, although more repellent odorants reached significance under TFI than TPI, and only a single attractive odorant (Geraniol 10%) showed significant attraction. Using coarse measurements of velocity, and finer-grained measurements by classifying movement periods by tortuosity, I found that the most significant difference was the strategies used by male and female locusts. Male locusts increased average velocity much more slowly than female locusts, and engaged in more stationary and tortuous bouts than females. In contrast, females tended to move towards the edge of the arena, and advance along the treadmill in a straight line. Moreover, attractive odorants elicited more tortuous movement, although there was no significant correlation of expression levels of any of the four levels with TFI. The second half of the thesis focuses on tool and method development for performing simultaneous neural and behavioral recordings. The majority of previous work in locusts was performed separately, presenting several constraints; separate recordings constrain comparisons to averages of each response. This means that variance in neural response or behavior cannot be compared to each other, and that potential state and other modulations of response cannot be evaluated. To address these challenges we developed an improved minimally invasive surgical preparation, and a 2-chamber and trackball behavioral assay that utilized it. Using the new surgical methods and tools we were able to perform a proof of concept by identifying explosive odorants accurately and rapidly. We then developed a 2-chamber assay and performed simultaneous neural and behavioral recordings from ambulatory locust preparations. In these preparations we were able to find significant differences in spiking activity and odor preference, however the lack of control over the stimulus meant that there were few instances where changes in concentration could be analyzed, and the long electrode tether introduced noise from movement limiting neural resolution. We then developed a trackball preparation and corresponding surgical tools and approaches to address these issues; by holding the locust and electrode steady movement noise was minimized. Furthermore, the consistent position of the locust meant previously established methods for odor delivery could be used. Seven locusts were prepared and simultaneous neural and behavioral responses recorded for ten trials of four odorants. To evaluate neural recording quality, the same method of identifying odorants using a discriminant was used, and neural responses were sufficiently clear for accurate identification. Furthermore, significant movement changes were found in response to Hexanol 1% stimuli, and a significant correlation was found with increases in lateral movement towards the odor stimulus and magnitude of neural response increase.
Language
English (en)
Chair
Barani Raman