Date of Award

Spring 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Molecular Biophysics and Biochemistry

First Advisor

Koelle, Michael

Abstract

A challenge in neuroscience is understanding how neural circuits generate and sustain behavior over time. Neural circuits consist of interconnected groups of neurons that process and integrate sensory information to perform specific tasks and behaviors. These circuits organize behaviors by maintaining internal states—stable neural activity patterns influenced by sensory and physiological contexts, as discussed in Chapter One. To understand how neural circuits generate internal states, we examined how the well-characterized C. elegans egg-laying circuit transitions between inactive and active states to produce egg-laying behavior. The active internal state lasts approximately 2 to 3 minutes, during which one or more eggs are laid, followed by an inactive state of about 20 minutes when egg laying ceases. The circuit-level mechanisms that control the switch between these two internal states in the C. elegans egg-laying circuit remain poorly understood. Chapter Two describes our research on a pair of exclusively peptidergic PVW neurons, which help initiate the active internal state of the C. elegans egg-laying circuit. PVWs produce previously undescribed branches that terminate in varicosities near the egg-laying muscles and are among the five C. elegans neurons that produce only neuropeptides and no classical neurotransmitters or biogenic amines. We found that stimulating the egg-laying muscles to contract induces calcium activity in PVW varicosities and that stimulating PVW activity enhances egg-laying muscle calcium activity during locomotor body bends. These findings suggest that PVWs and egg-laying muscles form a positive feedback loop: PVW activation stimulates the egg-laying muscles, which in turn activate PVWs. We also demonstrated that while stimulating PVW activity alone is insufficient to trigger egg-laying behavior, silencing PVW activity results in a modest decrease in the number of eggs laid and prolongs the duration of inactive state intervals, suggesting that PVW neurons play a role in generating the active state of the egg-laying circuit. Furthermore, PVWs promote egg-laying behavior under specific environmental conditions that appear to maintain the circuit in its active internal state and favor egg laying. Our model suggests that three positive feedback loops, triggered by mechanical feedback from egg-laying muscle contractions, must converge to transition the circuit from an inactive to an active state. We propose that the HSN, VC, and PVW neurons mediate these three positive feedback loops, allowing the circuit to decisively turn on the active internal state and maintain egg-laying behavior for 2-3 minutes. Chapter Three describes our progress in analyzing the anatomical and ultrastructural features of cells in the C. elegans egg-laying apparatus. We have detailed our efforts to reconstruct a 3D model of the egg-laying apparatus to understand how the morphology of cells enables them to execute specific functions within the egg-laying circuit. These cells include vulval and uterine epithelial cells, vulval and uterine muscles, neuroendocrine cells, and neurons with previously undescribed features. Given that the egg-laying circuit has been studied for decades, we aimed to explain how the spatial organization and connections among these cells allow the egg-laying muscles to coordinate their contractions via gap junctions. The strong preservation of the extracellular space in the egg-laying circuit reveals how these muscle cells might receive neuropeptide signals from neurons and neuroendocrine cells through volume transmission. Chapter Four discusses the broader implications of our work on the C. elegans egg-laying circuit, including future questions that could further reveal the fundamental principles of how neural circuits generate persistent internal states. Since both invertebrates and vertebrates exhibit persistent behavioral states, the mechanisms identified in this model system may offer insights into how internal states are generated and maintained in more complex neural circuits across species.

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