Role of the δ-Opioid Receptor in 2 Murine Models of Colitis

Crohn disease and ulcerative colitis, collectively referred to as inflammatory bowel disease (IBD), are chronic inflammatory disorders of the gastrointestinal tract. Currently, the etiology of IBD is unknown, and immunosuppressive therapies have become the standard of care to reduce the inflammation; however, these agents only induce remission 50% of the time in patients and can have serious side effects. Recently, endogenous opioids and opioid receptors have been shown to play a role in the mediation of inflammation. In addition, opioid receptor blockade with a nonselective antagonist, naltrexone, has been shown to reduce colitis in both murine models and human subjects. The goal of the current study was to determine if the antiinflammatory effects of naltrexone are mediated through the delta (δ) opioid receptor. Male C57BL/6NCrl (6 to 8 wk.; n = 110) and female BALB/cAnNCrl (6-8 wk.; n = 91) mice were studied using 2 animal models of chemically induced colitis: dextran sodium sulfate (DSS) and 2, 4, 6-trinitrobenzenesulfonic acid (TNBS). The selective δ-receptor antagonists naltrindole and 7-benzylidenenaltrexone were administered to examine the role of the δ-opioid receptor in colonic inflammation. The quantitative measurement of colitis activity, colon weight and length, Hct, WBC count, and gross and microscopic aberrations were analyzed. Administration of naltrexone in the DSS colitis model significantly improved overall disease activity indices on day 5 of therapy. The use of δ-antagonists and naltrexone had limited to no effect on TNBS colitis. Similar findings were obtained by using the DSS colitis model. Based on the current findings, the authors conclude that naltrexone therapy has limited effect on the improvement of colitis in 2 murine models; however, the δ-opioid receptor was not responsible for mediating the effects.