Ketotifen ameliorates capsaicin-augmented acetic acid-induced colitis

Capsaicin-sensitive afferent neurons are involved in maintaining the integrity of the gastro-intestinal mucosa. These neurons are closely apposed to mast cells and could, therefore, lead to their activation. In the present study, the role of capsaicin-sensitive neurons in the pathogenesis of experimental colitis and the possible involvement of mast cells and nitric oxide were evaluated. Rats were treated with capsaicin subcutaneously, 20, 30, and 50 mg/kg, on three consecutive days, a regimen shown to ablate primary afferent neurons. Colitis was induced two weeks later by flushing 2 ml 5% acetic acid into the proximal colon. Control rats received saline, acetic acid, or capsaicin alone. Another group of rats received ketotifen (100 μg/100 g body wt × 2/day) intragastrically 48 hr prior to damage induction and thereafter. Rats were sacrificed 24 hr after damage induction, the colon isolated, damage assessed, explants were organ-cultured for 24 hr for determination of nitric oxide generation, and mucosa extracted for determination of leukotriene B₄ generation and nitric oxide synthase activity. Significant increases in colonic weight, nitric oxide synthase activity, and nitric oxide and leukotriene B₄ generation accompanied the near tripling of acetic acid-induced damage in capsaicin-pretreated rats. Ketotifen pretreatment significantly decreased the macroscopic damage and the increase in colonic weight. The protection provided by ketotifen was accompanied by a significant decrease in leukotriene B₄ generation and nitric oxide synthase activity (80%). The correlation between the extent of colonic damage, nitric oxide synthase activity, and nitric oxide generation in acetic acid-induced colitis in capsaicin-pretreated rats suggests possible involvement of nitric oxide in its pathogenesis. The protective effect of ketotifen in capsaicin-pretreated rats indicates that mast cell activation is not prevented by ablation of afferent neurons.