Morphine creates a neuroinflammatory response and enhances release of the proinflammatory cytokines like interleukin-1β (IL-1β), which compromises morphine analgesia as well as induces morphine tolerance. In this study, we attempted to investigate the mechanisms of morphine induced IL-1β synthesis and release. Microglial cells were treated with morphine (100 μM) once daily for 3 days. Control groups underwent the same procedure but received sterile saline injection instead of morphine. Toll-like receptor 4 (TLR4) and P2X4 receptor (P2X4R) signaling were analyzed using Western blot; immunofluorescence was used to detect the signaling of CD68; real-time RT-PCR and ELISA kit was used to measure the messenger RNA and protein synthesis and release level of IL-1β. Morphine enhanced IL-1β synthesis and P2X4R protein expression. TLR4 were responsible for morphine-induced IL-1β synthesis, while morphine-induced IL-1β release was via P2X4R. Morphine-induced IL-1β release is mediated by endocytosis of TLR4. These results indicated that TLR4 and P2X4R pathways mediated IL-1β synthesis and release in microglia followed chronic morphine. TLR4 internalization is the main mechanism of morphine-induced microglia activation and IL-1β release.
Keywords: Interleukin-1β; Microglia; Morphine; P2X4 receptors; Toll-like receptor 4.