Neural tracers have not typically been employed to determine the pathways followed by axons between their perikarya and target tissues. We have adapted the tetramethylbenzidine method for horseradish peroxidase (HRP) to stain fibers en bloc in organs and thus to delineate axonal trajectories. We have also applied this protocol to characterize the pathways that vagal afferents follow to the intestines. The protocol confirms that the proximal segment of the duodenum receives afferents carried in the vagal hepatic branch and demonstrates that vagal afferents innervating the remainder of the small and large intestines course through multiple fascicles derived from the celiac branches of the abdominal vagus. These fascicles divide, intermingle, and reorganize along the abdominal aorta and superior mesenteric artery (SMA), but not along the inferior mesenteric artery, and then project to the intestines with secondary arteries that branch from the SMA. The inferior pancreaticoduodenal, jejunal, middle colic, right colic, and ileocecocolic arteries all carry vagal afferents to segments of the intestines. As the arteries derived from the SMA divide repeatedly into successively finer branches and course to the intestines, the vagal afferent fascicles (typically a pair) running with each arterial branch also divide. These divisions generate sets/pairs of finer fascicles coursing with even the highest order arterial radicles. The vagal fascicles enter the intestinal wall with the vessels and appear to innervate the organ near the point of entry. The results verify the practicality and sensitivity of the en bloc HRP technique and suggest that the protocol could delineate other peripheral pathways.
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We thank Dr. Robert Phillips for his expert help in the production of the figures.
This work was supported by grants from the Fulbright-Hays (Fulbright scholarship) and NSC 42114F Exchange Programs and by NSC86-2418-H-194-003-T to F.B.W., and by the National Institute of Diabetes and Digestive and Kidney Diseases, NIH (DK27627 and DK61317) to T.L.P.
The present observations were reported in part at the 2005 annual meeting of the Society for Neuroscience.
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Wang, F., Powley, T.L. Vagal innervation of intestines: afferent pathways mapped with new en bloc horseradish peroxidase adaptation. Cell Tissue Res 329, 221–230 (2007). https://doi.org/10.1007/s00441-007-0413-7
- Neural tracers
- Visceral afferents
- Rat (Sprague Dawley, male)