IRF8-Cathepsin S/PAR2 axis-mediated spinal microglia-neuron crosstalk is responsible for the exacerbation of postsurgical pain induced by preoperative stress

Background: Preoperative stress has been reported to be a risk factor for chronic postsurgical pain (CPSP), and spinal microglia might play a critical role in stress-induced chronification of postsurgical pain. However, the underlying molecular mechanism is still poorly understood.

Methods: A mice model of single prolonged stress (SPS) combined with a plantar incision was used in the present study. The Von Frey test, the elevated plus maze test, and the open field test were applied to assess the pain- and anxiety-like behaviors. The effect of SPS and plantar incision, as well as the underlying mechanisms of CPSP, were determined by utilizing immunohistochemistry, immunofluorescence staining, and Western blotting.

Results: In the present study, we demonstrated that previous SPS exposure resulted in anxiety-like behaviors and exacerbated postsurgical pain. Concomitantly, preoperative SPS promoted microglial activation, increased Cathepsin S (CatS)-PAR2 signaling, and enhanced neuronal activation and the expression of synapse-associated proteins in the spinal dorsal horn of incisional mice. The immunofluorescence staining showed that the CatS was mainly co-expressed in microglia and PAR2 existed in neurons. Our behavioral pharmacology experiments confirmed that spinal microglia and CatS-PAR2 signaling played a critical role in the development of SPS-exacerbated postsurgical pain. By activating PAR2, intrathecal CatS administration was able to induce prolongation of postsurgical pain and enhanced spinal neuronal activation as well as the expression of synapse-associated proteins. We further found that the expression of IRF8 in the spinal cord of incisional mice was also enhanced by SPS. Genetic inhibition of IRF8 by intrathecal IRF8 siRNA injection significantly prevented SPS-exacerbated postsurgical pain. Furthermore, the increased microglial activation, upregulated CatS-PAR2 signaling, and enhanced neuronal activation along with increased synapse-associated proteins expression following SPS and incision were also dramatically suppressed by IRF8 siRNA.

Conclusions: Our data suggested that IRF8-CatS-PAR2 axis-mediated spinal microglia-neuron crosstalk is responsible for the exacerbation of postsurgical pain induced by preoperative stress.

Cathepsin S; Interferon regulatory factor 8; Microglia; Postsurgical pain; Protease-activated receptor-2; Single prolonged stress.