Exploring Toll-like receptor 4 as a target for opioids in microglia

Opioids are the most widely used class of analgesics for acute and chronic pain; however, their chronic use is accompanied by side effects and complications. Although the mechanisms underlying these phenomena are not fully understood, neuroinflammation is likely to contribute to analgesic tolerance and opioid-induced hyperalgesia. Indeed, opioids have been shown to induce neuroinflammation and microglia activation, leading to the production and release of numerous pain and inflammatory agents. Furthermore, given the central role of Toll-like receptor 4 (TLR4) in the activation of innate immune cells, including microglia, this receptor has been proposed as an off-target site for opioid action.

With the aim to validate TLR4 as an opioid target, we studied the effect of fentanyl on LPS-induced TLR4 activation in primary microglia cells. The production and release of different proinflammatory mediators were investigated in microglia by real-time PCR and ELISA. Fentanyl increased mRNA levels of IL-1β, IL-6, TNF-α, iNOS, and MCP1/CCL2, as well as the LPS-induced secretion of IL-1β and TNF-α. Next, we analyzed the role of TLR4 in mediating the fentanyl effect using ciprofloxacin and CLI-095, two inhibitors of TLR4 signaling, previously identified by our group and others. Ciprofloxacin and CLI-095 completely reduced the release of proinflammatory cytokines by microglia stimulated with LPS and fentanyl, suggesting that TLR4 could be the target of the proinflammatory activity of fentanyl.

These results provide new insight into the mechanism of the proinflammatory activity of fentanyl, which involves the activation of TLR4 signaling. Our findings might facilitate the development of new molecules active in modulating opioid-mediated TLR4 signaling, for future development of effective and safe pharmacological agents for pain management.