Elevation of the Chemokine Pair CXCL10/CXCR3 Initiates Sequential Glial Activation and Crosstalk During the Development of Bimodal Inflammatory Pain after Spinal Cord Ischemia Reperfusion

Abstract

Background/Aims: Spinal microglia and astrocytes are the main responders to the inflammatory cascade and process pain through various neural interactions. CXCL10 is a late-phase protein that accelerates arteriogenesis during reperfusion through CXCR3. However, the early-phase expression (within 72 h postoperatively) of CXCL10 and CXCR3 during the development of ischemia-reperfusion (IR)-induced inflammatory pain remains unclear. We investigated whether this chemokine pair participates in glial interactions during early-phase IR injury. Methods: A rat model was induced by an 8-min occlusion of the aortic arch. Temporal assessments of mechanical and thermal allodynia and the protein levels of CXCL10 and CXCR3 were determined through measurements of paw withdrawal thresholds (PWTs) and paw withdrawal latencies (PWLs) and Western blotting assays. The co-localization of various cells with glial cells was detected by double immunofluorescence. The effects of CXCL10/CXCR3 on glial interactions were explored by intrathecal treatment with specific inhibitors (AMD487, minocycline and fluorocitrate) and recombinant CXCL10, and subsequent release of cytokines was assessed by ELISAs. Results: The IR injury initiated bimodal allodynia within 72 h of reperfusion, as illustrated by two W-shape trends in the PWTs and PWLs with two minima at 12 and 48 h post-IR. Allodynia was highly correlated with overexpression of CXCL10 and CXCR3, which were expressed in microglia at the early stage and in both microglia and astrocytes at the late stage, as shown by increased CXCL10 and CXCR3 immunoreactivities and double-labeled cells. AMD487 and minocycline injections exerted comparable inhibitory effects on CXCR3 and Iba-1 and on GFAP immunoreactivity at 12 and 48 h post-IR, and these inhibitory effects were only observed at 48 h following fluorocitrate injection. The levels of TNF-α and IL-6 showed variations in concert with the changes in Iba-1 and GFAP immunoreactivities. Recombinant CXCL10 injection reversed the abovementioned effects. Conclusion: The results showed that CXCL10/CXCR3 are involved in bimodal inflammatory pain during early-phase IR injury. The sequential activation of and crosstalk between microglia and astrocytes mediated through CXCR3 upregulation suggested that treatments targeting specific cell types are important in post-IR allodynia.