Effects of melatonin on the expression of iNOS and COX-2 in rat models of colitis

Abstract

Aim: To investigate the effects of melatonin (MT) on the expression of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) in rat models of colitis.

Methods: Healthy adult Sprague-Dawlay (SD) rats of both sexes, weighing 280+/-30 g, were employed in the present study. The rat models of colitis were induced by either acetic acid or 2,4,6-trinitrobenzene sulfonic acid (TNBS) enemas. The experimental animals were randomly divided into melatonin treatment and model control group that were intracolicly treated daily with melatonin at doses of 2.5, 5.0, 10.0 mg/kg(-1) and equal amount of saline respectively from 24 h following induction of colitis in rats inflicted with acetic acid enema and the seventh day in rats with TNBS to the end of study. A normal control group of rats treated with neither acetic acid nor TNBS but saline enema was also included in the study. On the 28(th) day of the experiment, the rat colon mucosal damage index (CDMI) was calculated, and the colonic prostaglandin E(2) (PGE(2)), nitric oxide (NO), as well as the iNOS and COX-2 expression were also determined biochemically or immunohistochemically.

Results: CDMI increased to 2.87+/-0.64 and 3.12+/-1.12 respectively in rats treated with acetic acid and TNBS enema, which was in accordance with the significantly elevated colonic NO and PGE(2) contents, as well as the up-regulated colonic iNOS and COX-2 expression in both of the two rat models of colitis. With treatment by melatonin at the doses of 5.0 and 10.0 mg/kg(-1), CDMI in both models of rat colitis was significantly decreased (P<0.05-0.01), which accorded synchronously and unanimously with the reduced colonic NO and PGE(2) content, as well as the down-regulated expression of colonic iNOS and COX-2.

Conclusion: Melatonin has a protective effect on colonic injury induced by both acetic acid and TNBS enemas, which is probably via a mechanism of local inhibition of iNOS and COX-2 expression in colonic mucosa.

Figure 1

Abnormal expression of iNOS in colonic tissue of rats with cloitis induced by both acetic acid and TNBS enemas and its improvement by melatonin. A. Immunohistochemical localization of iNOS in normal control, which was manifested as fine brown granules distributed mainly in the cytoplasm of histocytes, neutrophils and smooth muscle cells. B. Positively stained granules for iNOS were significantly increased in both number and intensity in colonic tissue of model control rats. (a) Acetic acid treated rats; (b) TNBS treated rats. C. The colonic iNOS expression was significantly reduced in both acetic acid (d) and TNBS (e) treated rats after intervened with 10.0 mg•kg^-1 of melatonin.

Figure 2

Abnormal expression of COX-2 in colonic tissue of rats with colitis induced by acetic acid or TNBS enema and its improvement by melatonin, which was in accordance with the observation in clonic iNOS expression. A. Positively stained COX-2 granules in colonic tissue of normal control rats. B. iNOS expression was significantly increased as manifested by the augmented and intensified positively stained granules in colonic tissue of model control rats. (a) Acetic acid treated rats; (b) TNBS treated rats. C. The colonic COX-2 expression was significantly reduced in both acetic acid (d) and TNBS (e) treated rats after intervened with 10.0 mg•kg^-1 of melatonin.