Suppression of TLR4-MyD88 signaling pathway attenuated chronic mechanical pain in a rat model of endometriosis

Background As a classic innate immunity pathway, Toll-like receptor 4 (TLR4) signaling has been intensively investigated for its function of pathogen recognition. The receptor is located not only on immune cells but also on sensory neurons and spinal glia. Recent studies revealed the involvement of neuronal TLR4 in different types of pain. However, the specific role of TLR4 signaling in the pain symptom of endometriosis (EM) remains obscure. Methods The rat endometriosis model was established by transplanting uterine horn tissue into gastrocnemius. Western blotting and/or immunofluorescent staining were applied to detect high mobility group box 1 (HMGB1), TLR4, myeloid differentiation factor-88 adaptor protein (MyD88), and nuclear factor kappa-B-p65 (NF-kappa B-p65) expression, as well as the activation of astrocyte and microglia. The antagonist of TLR4 (LPS-RS-Ultra, LRU) and MyD88 homodimerization inhibitory peptide (MIP) were intrathecally administrated to assess the behavioral effects of blocking TLR4 signaling on endometriosis-related pain. Results Mechanical hyperalgesia was observed at the graft site, while HMGB1 was upregulated in the implanted uterine tissue, dorsal root ganglion (DRG), and spinal dorsal horn (SDH). Compared with sham group, upregulated TLR4, MyD88, and phosphorylated NF-kappa B-p65 were detected in the DRG and SDH in EM rats. The activation of astrocytes and microglia in the SDH was also confirmed in EM rats. Intrathecal application of LRU and MIP alleviated mechanical pain on the graft site of EM rats, with decreased phosphorylation of NF-kappa B-p65 in the DRG and reduced activation of glia in the SDH. Conclusions HMGB1-TLR4-MyD88 signaling pathway in the DRG and SDH may involve in endometriosis-related hyperpathia. Blockade of TLR4 and MyD88 might serve as a potential treatment for pain in endometriosis.