PAG neuronal NMDARs activation mediated morphine-induced hyperalgesia by HMGB1-TLR4 dependent microglial inflammation

Area of Science:

• Neuroscience
• Immunology
• Pharmacology

Background:

• Opioid-induced hyperalgesia (OIH) is a significant clinical challenge limiting morphine's analgesic efficacy.
• The precise mechanisms underlying OIH, particularly the roles of Toll-like receptor 4 (TLR4) and glutamate receptors in the periaqueductal gray (PAG), remain incompletely understood.

Purpose of the Study:

• To elucidate the association between TLR4 and N-methyl-D-aspartate Receptors (NMDARs) activation in the PAG during OIH.
• To investigate the role of microglia and specific molecular pathways in morphine-induced hyperalgesia.

Main Methods:

• Utilized a mouse model of OIH, observing microglia activation, TLR4/p65 NF-κB signaling, and NMDAR subunit phosphorylation in the PAG.
• Employed cell cultures (BV2, N2A) to examine TLR4 activation by High-mobility group box 1 (HMGB1) and its downstream effects on NMDARs.
• Intervened using TLR4 inhibition, knockdown, and knockout (Lentivirus-GFP-TLR4 shRNA) to assess their impact on OIH.

Main Results:

• Morphine induced microglia activation, increased TLR4/p65 NF-κB, and pro-inflammatory cytokines in the PAG and BV2 cells.
• Morphine activated TLR4 via HMGB1, not MD2, leading to microglia-derived inflammation that phosphorylated NMDAR subunits (NR1, NR2B) in adjacent neurons.
• TLR4 knockout in the PAG reversed these molecular changes and significantly alleviated OIH symptoms.

Conclusions:

• Morphine-induced OIH involves TLR4 activation in PAG microglia, mediated by HMGB1.
• Activated microglia release inflammatory factors that phosphorylate NMDARs in neurons, increasing excitability and contributing to OIH.
• Targeting the TLR4/NMDAR pathway in the PAG presents a promising therapeutic strategy for managing OIH.