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Published on: January 7, 2020
Mu opioid receptor activation modulates Toll like receptor 4 in murine macrophages
Silvia Franchi1, Sarah Moretti, Mara Castelli
1Dipartimento di Farmacologia Chemioterapia e Tossicologia medica, Università degli Studi di Milano, Via Vanvitelli 32, 20129 Milano, Italy.
Abstract:
Opioids have been shown to affect both innate and adaptive immunity. We previously showed that morphine affects the macrophage production of pro-inflammatory cytokines after LPS in a NFkB dependent manner. Toll like receptors (TLRs) play a crucial role in the signaling pathways which lead to NFkB activation. TLR4 is considered the Lipopolysaccaride (LPS) receptor. The data here presented show that, in murine macrophages, morphine impacts on the immune function acting on the early step of pathogen recognition. Morphine, when added to RAW 264.7 cells and when injected into mice (s.c. 20mg/kg) is in fact able to decrease TLR4 both at mRNA and protein level in RAW cells and peritoneal macrophages. In the same cells, the mu opioid receptor (MOR) antagonist Naltrexone increases TLR4 levels, thus suggesting a role of the endogenous opioid system in TLR4 regulation. The effect of the two drugs is moreover lost in case of co-administration. Experiments with MOR KO mice and with DAMGO (MOR specific agonist) confirm that the effect of morphine on TLR4 mRNA in peritoneal macrophages is due to the MOR activation. Moreover the effect on TLR4 is blocked by PTX thus indicating the involvement of a G(i) protein after MOR binding. This work unveils a clear link between MOR activation and TLR4, suggesting a new possible mechanism at the basis of the peripheral immunosuppressive effect of opioids.
Insights
Morphine suppresses immune responses by decreasing Toll-like receptor 4 (TLR4) in macrophages. This occurs via mu-opioid receptor (MOR) activation, revealing a new pathway for opioid-induced immunosuppression.
Area of Science:
- Immunology
- Pharmacology
- Neuroscience
Background:
- Opioids modulate innate and adaptive immunity.
- Morphine affects macrophage pro-inflammatory cytokine production via NF-kB.
- Toll-like receptors (TLRs), particularly TLR4, are critical for pathogen recognition and NF-kB activation.
Purpose of the Study:
- To investigate the impact of morphine on Toll-like receptor 4 (TLR4) expression and function in murine macrophages.
- To elucidate the role of the mu-opioid receptor (MOR) in morphine's effect on TLR4.
- To identify the signaling pathway involved in MOR-mediated regulation of TLR4.
Main Methods:
- Morphine treatment of RAW 264.7 cells and murine models.
- Assessment of TLR4 mRNA and protein levels.
- Administration of MOR antagonist Naltrexone and MOR-specific agonist DAMGO.
- Experiments using MOR knockout (KO) mice.
- Pertussis toxin (PTX) treatment to assess G(i) protein involvement.
Main Results:
- Morphine decreased both mRNA and protein levels of TLR4 in macrophages.
- Naltrexone increased TLR4 levels, suggesting endogenous opioid system regulation.
- Co-administration of morphine and Naltrexone abolished the effects.
- MOR activation was confirmed as the mechanism for morphine's effect on TLR4.
- The effect was blocked by PTX, indicating G(i) protein signaling.
Conclusions:
- Morphine directly impacts pathogen recognition by reducing TLR4 expression in macrophages.
- Mu-opioid receptor activation is central to this immunosuppressive effect.
- This study reveals a novel mechanism linking MOR activation to TLR4 regulation, contributing to opioid-induced peripheral immunosuppression.
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