Abstract
BackgroundPain is one critical hallmark of inflammatory responses. A large number of studies have demonstrated that stromal cell-derived factor 1 (SDF1, also named as CXCL12) and its cognate receptor C-X-C chemokine receptor type 4 (CXCR4) play an important role in immune reaction and inflammatory processes. However, whether and how SDF1–CXCR4 signaling is involved in inflammatory pain remains unclear.MethodsUnder the intraplantar (i.pl.) bee venom (BV) injection-induced persistent inflammatory pain state, the changes of SDF1 and CXCR4 expression and cellular localization in the rat dorsal root ganglion (DRG) were detected by immunofluorescent staining. The role of SDF1 and CXCR4 in the hyperexcitability of primary nociceptor neurons was assessed by electrophysiological recording. Western blot analysis was used to quantify the DRG Nav1.8 and phosphorylation of ERK (pERK) expression. Behavioral tests were conducted to evaluate the roles of CXCR4 as well as extracellular signal-regulated kinase (ERK) and Nav1.8 in the BV-induced persistent pain and hypersensitivity.ResultsWe showed that both SDF1 and CXCR4 were dramatically up-regulated in the DRG in i.pl. BV-induced inflammatory pain model. Double immunofluorescent staining showed that CXCR4 was localized in all sizes (large, medium, and small) of DRG neuronal soma, while SDF1 was exclusively expressed in satellite glial cells (SGCs). Electrophysiological recording showed that bath application with AMD3100, a potent and selective CXCR4 inhibitor, could reverse the hyperexcitability of medium- and small-sized DRG neurons harvested from rats following i.pl. BV injection. Furthermore, we demonstrated that the BV-induced ERK activation and Nav1.8 up-regulation in the DRG could be blocked by pre-antagonism against CXCR4 in the periphery with AMD3100 as well as by blockade of ERK activation by intrathecal (i.t.) or intraplantar (i.pl.) U0126. At behavioral level, the BV-induced persistent spontaneous pain as well as primary mechanical and thermal hypersensitivity could also be significantly suppressed by blocking CXCR4 and Nav1.8 in the periphery as well as by inhibition of ERK activation at the DRG level.ConclusionsThe present results suggest that peripheral inflammatory pain state can trigger over release of SDF1 from the activated SGCs in the DRG by which SGC-neuronal cross-talk is mediated by SDF1–CXCR4 coupling that result in subsequent ERK-dependent Nav1.8 up-regulation, leading to hyperexcitability of tonic type of the primary nociceptor cells and development and maintenance of persistent spontaneous pain and hypersensitivity.
Highlights
Pain is one critical hallmark of inflammatory responses
The present results suggest that peripheral inflammatory pain state can trigger over release of SDF1 from the activated satellite glial cells (SGCs) in the dorsal root ganglion (DRG) by which SGC-neuronal cross-talk is mediated by SDF1–CXCR4 coupling that result in subsequent extracellular signal-regulated kinase (ERK)-dependent Nav1.8 up-regulation, leading to hyperexcitability of tonic type of the primary nociceptor cells and development and maintenance of persistent spontaneous pain and hypersensitivity
Because the bee venom (BV)-induced increase in SDF1-like immunoreactivity was not seen in the DRG neuronal profiles, but was seen in intercellular space, we performed a double immunofluorescent labeling for SDF1 and glial fibrillary acidic protein (GFAP), a marker for satellite glial cells of the DRG
Summary
A large number of studies have demonstrated that stromal cell-derived factor 1 (SDF1, named as CXCL12) and its cognate receptor C-X-C chemokine receptor type 4 (CXCR4) play an important role in immune reaction and inflammatory processes. Immunohistochemical studies found that the expression level of SDF1 and CXCR4 was changed in the dorsal root ganglion (DRG) cells in the unilateral sciatic nerve injury (CCI)-induced pain model and the up-regulation of CXCR4 lasted at least for 2 weeks [15]. In the spinal cord injury-induced central neuropathic pain model, KnerlichLukoschus and colleagues demonstrated that SDF1 and CXCR4 expression was continuously increased from 2 to 42 days at the spinal cord level [16]. To date, the underlying mechanisms of SDF1/CXCR4 involved in the chronic and persistent pain remain unclear
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