Although morphine is a highly effective and widely used opioid analgesic, morphine also causes neuorinflammation, which can reduce its analgesic effect. Wang et al. report that this inflammatory response is mediated by morphine binding to MD-2, an extracellular component of the Toll-like receptor 4 (TLR4) complex that also binds lipopolysaccharide (LPS). Morphine bound to MD-2 thus triggered TLR4 inflammatory signaling. In vitro binding assays showed that morphine bound to purified human MD-2 and competed with LPS and curcumin, a fluorescent probe that binds MD-2. Molecular dynamics simulation of morphine and MD-2 suggested that morphine binding caused conformational changes similar to those induced by LPS. In cells overexpressing tagged forms of TLR4 and MD-2, morphine induced the oligomerization of the proteins, which is necessary for signal initiation. In primary cultures of central nervous system endothelial cells, which endogenously express TLR4 and MD-2, morphine triggered TLR4 signaling (as indicated by phosphorylation of the mitogen-activated protein kinases p38 and ERK) and induction of mRNAs encoding inflammatory mediators, such as interleukin-1β. Morphine also induced proinflammatory signaling in BV-2 microglia, and these responses were blunted by knockdown of TLR4 or MD-2. The analgesic effects of morphine were potentiated in mice genetically deficient in TLR4 or its adaptor protein MyD88 compared with that in wild-type controls. Analysis of spinal cord samples from the knockout mice confirmed enhanced p38 phosphorylation in response to morphine. The proinflammatory signaling induced by morphine in cultured BV-2 cells was abolished by two small-molecule inhibitors, one of which targeted TLR4 and prevented interaction with MD-2 and the other of which targeted MD-2 and prevented binding of morphine. Furthermore, intrathecal injection of either of the small-molecule inhibitors with morphine into rats potentiated the analgesic effect of morphine. Similar in vivo studies with TLR4-knockout mice showed that the potentiation of morphine’s analgesic effects by the TLR4 and MD-2 inhibitors was lost. Thus, MD-2 is a morphine receptor, and binding to this receptor stimulates TLR4 signaling to produce the inflammatory effects of morphine.
X. Wang, L. C. Loram, K. Ramos, A. J. de Jesus, J. Thomas, K. Cheng, A. Reddy, A. A. Somogyi, M. R. Hutchinson, L. R. Watkins, H. Yin, Morphine activates neuroinflammation in a manner parallel to endotoxin. Proc. Natl. Acad. Sci. U.S.A. 109, 6325–6330 (2012). [Abstract] [Full Text]