Mechanism of Toll-like Receptor 4 on Chronic Post-thoracotomy Pain Via Caspase-1 Activation

Objective: Post-operative chronic post-thoracotomy pain (CPTP) has been linked to restrictions in mobility and daily activities. However, its potential causes and optimal therapy have not been well characterized. Here, the purpose of this study was to investigate the role of Toll-like receptor 4 (TLR4) in CPTP rats and its underlying mechanism. Methods: Initially, rat models of CPTP were established. Then, the mechanical withdrawal threshold (MWT) was measured after intrathecal injection of TLR4 antagonist (LPS-RS), TLR4 agonist (LPS-PG), or caspase-1 inhibitor (Ac-YVAD-CMK) in CPTP rats. Levels of TNF-α, IL-6 and IL-1β in the spinal dorsal horn (SDH) were measured by ELISA. TLR4 and caspase-1 were located by immunofluorescence double staining. TLR4 and caspase-1 levels were assessed by qRT-PCR and Western blot. Results: TLR4 and caspase-1 were up-regulated in SDH of CPTP rats. Compared with Sham and non-CPTP groups, MWT was effectively decreased while TNF-α, IL-6 and IL-1β in SDH were increased in CPTP group. Moreover, intrathecal injection of TLR4 antagonist or caspase-1 inhibitor significantly elevated MWT expression and reduced levels of TNF-α, IL-6 and IL-1β in SDH. Additionally, high expression of TLR4 promoted mechanical hyperalgesia and inflammatory response, while intrathecal injection of a mixture of caspase-1 inhibitor and TLR4 agonist reversed the alleviation of caspase-1 inhibitor on the mechanical hyperalgesia and inflammatory response. TLR4 and caspase-1 were co-located in neurons. Conclusion: TLR4 aggravated CPTP in rats by mediating activation of caspase-1 in SDH.