Beyond Reflex: Nociception, Neural Circuits, and the Transformation of Threat into Memory in Drosophila melanogaster

Abstract

A fly flinches, jumps, runs—not randomly, but in a sequence carved by neurons that have been waiting for danger. For any animal, survival depends on knowing when the world has become dangerous—and reacting fast enough to avoid harm. This ability, called nociception, begins with specialized sensory neurons that detect mechanical, thermal, or chemical threats and ends with circuits that orchestrate escape and shape future behavior. In Drosophila melanogaster, nociception has been dissected in detail in larvae, but the adult system remains largely uncharted: how are its nociceptors wired, how do they drive different forms of escape, and how are their sensory properties tuned to the life of the adult fly? This thesis traces the anatomy and logic of escape in Drosophila melanogaster, revealing how a specific class of abdominal nociceptors initiates a spectrum of behaviors that begin with milliseconds of alarm and end with seconds of altered state.In this thesis, I combine connectomics, optogenetics, calcium imaging, single-cell RNA sequencing, and computational modeling to build a multi-level understanding of adult nociceptor function. I begin with a behavioral screen for neurons capable of driving aversion, focusing on a genetically defined subset of abdominal class IV multidendritic neurons (md). Connectomic reconstruction shows that md neurons project to two distinct downstream pathways: circuits for rapid, reflexive escape behaviors (running, jumping) and ascending circuits for arousal and behavioral modulation. Within the ascending pathway, a pair of inhibitory/peptidergic neurons emerges as a candidate mechanism for regulating both the magnitude and persistence of nociceptive responses, balancing sensitivity with stability. I next examine the molecular identity of md neurons. Single-cell RNA sequencing reveals a shift from the larval polymodal profile toward a mechanosensory-biased expression program, with strong enrichment of Piezo, ppk, and ppk26, and downregulation of thermosensory channels such as TrpA1 and Painless. Yet functional imaging reveals robust heat responses but no detectable mechanosensory activity, pointing to post-transcriptional regulation or context-dependent tuning. Together, these findings define the first steps of a complete circuit map for adult Drosophila nociceptors and show how parallel pathways coordinate rapid escape with longer-term state modulation. They also reveal that molecular identity does not always predict sensory function—reminding us that in neural systems, what a cell expresses and what it does are related, but not the same. This work offers both a detailed model for nociceptive control in a compact brain and a broader framework for thinking about how evolution balances urgency, persistence, and sensitivity in the face of danger. We also begin to discuss the ethical implications this type of study might present to us.