Topographical Organization and Morphology of Calcitonin Gene‐Related Peptide (CGRP) Axons in the Flat‐Mounts of Whole Mouse Stomach

Abstract

Nociceptive afferents innervate the stomach and send nociceptive signals centrally to the brain and locally to the enteric nervous system. Nociceptive afferents can be detected with a variety of different markers (e.g., CGRP, SP, TRPV1). However, the distribution and morphological structure of nociceptive axons and terminals have not yet been well determined in the flat‐mounts of the whole stomach. In this study, we used calcitonin gene‐related peptide (CGRP) as a marker to label nociceptive afferent axons and terminals in the in the flat mounts of the whole ventral and dorsal stomachs of mouse (n = 6/each side, 3‐5 months). We applied a combination of state‐of‐the‐art techniques, including confocal microscopy, Zeiss Imager microscopy, flat‐mount tissue processing of the whole organ and immunohistochemistry, Neurolucida 360 tracing and integration of the tracing data into a 3D stomach scaffold to determine the distribution and morphology of CGRP‐IR axons and terminals in the whole stomach. We found that 1) CGRP‐IR axons formed extensive terminal networks in both ventral and dorsal stomachs. 2) CGRP‐IR axons dramatically innervated the blood vessels. 3) In longitudinal and circular muscles, CGRP‐IR varicose axons ran in parallel with the direction of the muscles. 4) CGRP‐IR axons formed a complex network between the longitudinal and circular muscle layers. 5) In the myenteric ganglia, CGRP‐IR axons formed varicose terminal contacts with individual myenteric neurons. 6) We did not observe any significant CGRP‐IR myenteric neurons. 7) CGRP‐IR axon innervation of the blood vessels were traced and digitized and integrated into a 3D scaffold. This is the first time that we provided a topographical map of CGRP‐IR innervation of the whole stomach at single cell/axons resolution. The work provides an anatomical foundation for functional studies of CGRP‐IR axons in various regions of the stomach and their remodeling in diseases. The first two authors contributed equally to this work.