Connectomic mapping of pharyngeal and gut sensory circuits in adult

Feeding is regulated by both external sensory signals, such as taste, and internal sensory signals originating from the pharynx and gut. The recent completion of the Full Adult Fly Brain (FAFB) connectome offers an opportunity to map these sensory inputs and their downstream circuits. While the external gustatory receptor neurons have been relatively well characterized, the internal pharyngeal and gut sensory neurons remain less understood. Here, we systemically identify their axonal projections in the FAFB connectome and examine their downstream circuits. We find that the stomodeal nerve, which carries afferent signals from the gastrointestinal tract to the brain, contains multiple types of sensory axons with distinct morphology and downstream output connections. In addition, we identify sensory axons derived from different pharyngeal sense organs and find that chemosensory and mechanosensory axons arborize in distinct regions of the subesophageal zone. Characterization of the second- and third-order neurons reveals the major brain regions that receive input from pharyngeal and gut sensory neurons. Interestingly, a subset of these internal sensory neurons forms monosynaptic connections with various motor neurons and endocrine cells, suggesting that internal signals from the pharynx and gut may directly influence feeding-related motor programs and endocrine output. Together, our study delineates the pharyngeal and gut sensory circuits, laying a foundation for future studies on how internal sensory signals regulate feeding behavior and endocrine functions.