Role of capsaicin-sensitive afferent nerves in mucosal injury and injury-induced hyperemia in rat colon.

Exposure of the colonic mucosa to 10% acetic acid results in ischemia and damage of the exposed mucosa and hyperemia in the adjacent unexposed and undamaged mucosa. We hypothesize that this hyperemia is mediated in part by capsaicin-sensitive afferent nerves, and absence of the hyperemia is associated with aggravation of damage in the exposed mucosa. In urethan-anesthetized rats, with the reflectance spectrophotometry probe in contact with the colonic mucosa, index of mucosal oxygen saturation (ISO2) was monitored before and for 5 min after 10% acetic acid was applied to the mucosa surrounding the measuring probe. Histological examination confirmed that the mucosa in contact with the measuring probe was undamaged, but the mucosa exposed to the 10% acetic acid was damaged. In the mucosa in contact with the reflectance spectrophotometry probe, a significant increase in ISO2 was observed in vehicle-pretreated rats, whereas a partial but significant reduction of such hyperemia was observed in capsaicin-pretreated rats. In the mucosa exposed to the 10% acetic acid, the depth of the mucosal lesion, but not the drop in ISO2, was significantly greater in the capsaicin-pretreated rats. The findings are consistent with the hypothesis that the colonic mucosal hyperemia in the undamaged mucosa adjacent to the mucosa damaged by the 10% acetic acid is mediated partially by capsaicin-sensitive afferent nerves. Attenuation of such hyperemia by capsaicin-pretreatment is associated with deeper mucosal damage, suggesting that the capsaicin-sensitive afferent nerves also contribute toward colonic mucosal protection against 10% acetic acid-induced injury in the exposed mucosa.