Topographical Organization and Morphology of Substance‐P Axons in the Flat‐Mounts of the Mouse Whole Stomach

Abstract

More than 50 million Americans suffer from chronic pain. However, the anatomical and physiological mechanisms of peripheral nociceptive processes have not been well elucidated which has seriously impeded the progress of designing novel bioengineering manipulations/treatments for chronic pain. In this study, we performed a comprehensive topographical mapping of pain‐related neural circuitry in the flat‐mount of whole mouse stomach (male, n=8, 3‐5 months). We used Substance P (SP) as a marker for nociceptive axons and applied a combination of state‐of‐the‐art techniques, including flat‐mount tissue processing and immunohistochemistry of the whole stomach, confocal microscopy, Zeiss Imager microscopy to determine the distribution and morphology of SP‐IR axons and terminals in the whole stomach. We found that 1) SP‐IR axons formed extensive terminal networks in both the ventral and dorsal stomachs. 2) SP‐IR axons were much denser in the antrum and corpus regions than in the fundus and cardia. 3) SP‐IR varicose axons ran in parallel with the circular and longitudinal muscle layers. 4) SP‐IR axons innervated blood vessels. 5) SP‐IR varicose axons formed terminals wrapping around individual myenteric neurons. 6) There were no confirmed SP‐IR myenteric neurons. 7) SP‐IR afferent innervation of the stomach showed a similar pattern of SP‐IR axons and terminals between the ventral and dorsal stomachs. In control mice (n=8), we injected tracer DiI into the ventral and dorsal stomach muscular layers or Fluorogold injection (i.p) and found that the main extrinsic source of SP‐IR afferent axons in the stomach was from the T7‐T11 DRG and to a lesser extent the VNG, but not from the celiac ganglia and dorsal motor nucleus of vagus. Our data provide for the first time a comprehensive topographical map for SP‐IR axons and terminals in the whole stomach with single cell/axon/synapse resolution. This work will contribute to a 3D digital representation of a brain‐stomach nociceptive atlas in a stomach scaffold, thus providing a novel anatomical foundation for functional mapping of nociceptive afferent axons and their pathological remodeling in the stomach. The first two authors contributed equally to this work.