Establishment of a new anal sphincter injury model in rats based on cardiotoxin

Background/purpose Mainstream models for anal sphincter injury use large animals. We developed a simple and stable anal sphincter injury model in a small animal (i.e., rats) to obtain manometry measurements by using a miniaturized probe and applying cardiotoxin. Methods The histological structure of the anal canal was evaluated by using manometry in normal rats (n = 40). We damaged the internal and external anal sphincters by locally administering snake poison (cardiotoxin; 20 uM, 100 μL 8 points). We evaluated the anal canal function through manometry measurements (n = 5) and examined the histology using hematoxylin-eosin staining (at each time point, n = 3; total n = 15). Results The manometry parameters and structure of the anal canal of normal rats were similar to those of humans, because rats have resting pressure, rectoanal reflex in the manometry, and an external and internal anal sphincter. After inducing injury, the following findings were observed: rhythmic wave loss and a remarkable reduction in the anal sphincter resting pressure; and local bleeding and advanced infiltration of the inflammatory cells (day 1) and the loss of muscle fibers (day 3). Conclusion This new rat model will contribute to increasing the knowledge on the anal canal.