Background: μ opioid receptors (μORs) are expressed by neurons and inflammatory cells, and mediate immune response. We tested whether activation of peripheral μORs ameliorates the acute and delayed phase of colitis. Methods: C57BL/6J mice were treated with 3% dextran sodium sulfate (DSS) in water, 5 days with or without the peripherally acting μOR agonist, [D-Ala2, N-Me-Phe4, Gly5-ol]-Enkephalin (DAMGO) or with DAMGO+μOR antagonist at day 2-5, then euthanized. Other mice received DSS followed by water for 4 weeks, or DSS with DAMGO starting at day 2 of DSS for 2 or 3 weeks followed by water, then euthanized at 4 weeks. Disease activity index (DAI), histological damage, and myeloperoxidase assay (MPO), as index of neutrophil infiltration, were evaluated. Cytokines and μOR mRNAs were measured with RT-PCR, and nuclear factor-kB (NF-kB), the antiapoptotic factor Bcl-xL, and caspase 3 and 7 with Western blot. Key Results: DSS induced acute colitis with elevated DAI, tissue damage, apoptosis and increased MPO, cytokines, μOR mRNA, and NF-kB. DAMGO significantly reduced DAI, inflammatory indexes, cytokines, caspases, and NF-kB, and upregulated Bcl-xL, effects prevented by μOR antagonist. In DSS mice plus 4 weeks of water, DAI, NF-kB, and μOR were normal, whereas MPO, histological damage, and cytokines were still elevated; DAMGO did not reduce inflammation, and did not upregulate Bcl-xL. Conclusions & Inferences: μOR activation ameliorated the acute but not the delayed phase of DSS colitis by reducing cytokines, likely through activation of the antiapoptotic factor, Bcl-xL, and suppression of NF-kB, a potentiator of inflammation. μ opioid receptors (μORs) mediate a variety of biological processes, including gastrointestinal functions and immune response. This study showed that activation of peripheral μOR with DAMGO, a selective μOR agonist, reduces the inflammatory response and disease activity index in acute experimental colitis through a mechanism involving the activation of the antiapoptotic factor, Bcl-xL, a member of the Bcl-2 family of apoptosis regulator proteins, and the suppression of the transcription nuclear factor-kB (shown here), which in turns reduces the release of pro-inflammatory cytokines and immune cells infiltration. By contrast, the delayed phase of colitis, which is characterized by lower level of inflammation, does not appear to benefit for additional treatment with peripherally acting μOR agonists as indicated by the lack of effect of DAMGO on the expression of Bcl-xL and NF-kB.
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