Abstract

Many organisms rely on olfaction to navigate their environment in response to surrounding chemical cues. Within the olfactory system, first, external signals are transduced into neural activity patterns that drive a diverse array of behavioral responses. However, sensory processing alone does not fully account for this variability. Fluctuations in neuromodulator levels are equally critical in shaping how olfactory information is encoded and translated into behavior. The goal of this dissertation is to investigate how changes in the neurochemistry within the olfactory circuit influence neural signaling and, ultimately, behavioral responses. To achieve this, I investigated how three neuromodulators: serotonin, dopamine, and octopamine modulate each component of the circuit and lead to observed behavioral changes. This comprehensive study involved a detailed electrophysiological analysis of two major components of the antennal lobe: projection neurons (PNs) and local neurons (LNs), behavior, and neural circuit modeling. First, I examined how all three neuromodulators affect the intrinsic properties of these neurons, as well as their responses to a variety of odorants. Notably, the results indicate that serotonin uniquely alters PN activity by transforming baseline firing into bursting patterns and enhancing odor-evoked responses in a non-odor-specific manner. In contrast, both dopamine and octopamine decreased baseline firing rate in PNs, but differentially affected odor-evoked responses: dopamine increased, whereas octopamine decreased them. While the findings from PNs were particularly compelling, I extended the investigation to explore the role of inhibitory GABAergic LNs in mediating these effects. First, I found that LNs exhibit two distinct types of odor-evoked responses – depolarization or hyperpolarization. Moreover, I found that only dopamine significantly influenced the LN odor-evoked response by reducing the depolarization and enhancing hyperpolarization. In contrast, serotonin and octopamine appear to act either directly on PNs or indirectly via presynaptic terminals of olfactory receptor neurons (ORNs), since no effect on LNs was observed. To connect these physiological findings to behavior, I examined how these neuromodulators affect one of the simplest olfactory-driven behaviors in locusts, the palp opening response (POR), known to be an appetitive reaction to odorants. Unlike the physiological results, serotonin exhibited odor-specific effects on POR, enhancing responses to appetitive odorants such as hexanol (a food-related odor) while reducing responses to aversive odorants such as linalool (common insecticide). In contrast, dopamine and octopamine injections produced behavioral changes that closely mirrored their effects on PN odor-evoked responses. Finally, I integrated these findings into a conceptual neural model that hypothesizes the existence of two subgroups of PNs and LNs. In this model, increased activity of one PN subgroup drives POR initiation, whereas increased activity of the other suppresses it. The hypothesis of two LN subtypes was supported by distinct electrophysiological profiles observed during experiments and through subsequent analyses. In sum, this work provides new insight into the organization and connectivity of the olfactory circuit in locusts by probing it with serotonin, dopamine, and octopamine, which dynamically shape olfactory-driven behavior.

Committee Chair

Baranidharan Raman

Committee Members

Dennis Barbour; Gaia Tavoni; Ismael Seanez; Shrikanth Singamaneni

Degree

Doctor of Philosophy (PhD)

Author's Department

Biomedical Engineering

Author's School

McKelvey School of Engineering

Document Type

Dissertation

Date of Award

2-3-2026

Language

English (en)

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