An opioid peptide analog relieves inflammatory allodynia by the direct inhibition of microglial activation: a μ opioid receptor independent effect

Background and Purpose: Opioids are widely used in the treatment of moderate and severe pain. Nociceptive stimulation and classical opioids have been reported to potentially promote microglial activation and neuroinflammation, which also reduces the analgesic effect of opioid drugs and causes chronic pain sensitization. The aim of this study was to demonstrate whether the novel opioid agonist MEL-0614 could inhibit activated microglia and neuroinflammation while facilitating recovery from persistent pain. Experimental approach: Mice were administered lipopolysaccharide and formalin to induce allodynia. Von Frey test was used to detect the anti-allodynia effect of MEL-0614 before and after LPS and formalin injection. In the spinal cord, the levels of proinflammatory cytokines and microglial activation were determined after MEL-0614 administration. BV2 and primary microglia were cultured to further explore the effect of MEL-0614 on LPS-induced microglial activation and key signalling pathways involved. Key results: MEL-0614 prevented and reversed allodynia induced by LPS and formalin in vivo, which was not inhibited by the μ opioid receptor antagonist CTAP. MEL-0614 also downregulated the activation of microglia and related proinflammatory cytokines in the spinal cord. Additionally, in BV2 and primary microglia, MEL-0614 inhibited the LPS-induced upregulation of proinflammatory factors, which was unaffected by CTAP. The NOD-like receptor protein 3-related signalling pathway may be involved in the interaction between MEL-0614 and microglia. Conclusion and Implications: The opioid agonist MEL-0614 inhibited the activation of microglia and the subsequent upregulation of proinflammatory factors both in vivo and in vitro. Notably, this effect is not mediated by the opioid receptors.