Granulocyte-macrophage colony-stimulating factor receptor expression in clinical pain disorder tissues and role in neuronal sensitization

INTRODUCTION: Granulocyte-macrophage colony-stimulating factor receptor (GM-CSFR) is highly expressed in peripheral macrophages and microglia, and is involved in arthritis and cancer pain in animal models. However, there is limited information on GM-CSFR expression in human central nervous system (CNS), peripheral nerves, or dorsal root ganglia (DRG), particularly in chronic pain conditions. OBJECTIVES: Immunohistochemistry was used to quantify GM-CSFR expression levels in human tissues, and functional sensory effects of GM-CSF were studied in cultured DRG neurons. RESULTS: Granulocyte-macrophage colony-stimulating factor receptor was markedly increased in microglia at lesional sites of multiple sclerosis spinal cords (P = 0.01), which co-localised with macrophage marker CD68 (P = 0.009). In human DRG, GM-CSFR was expressed in a subset of small/medium diameter cells (30%) and few large cells (10%), with no significant change in avulsion-injured DRG. In peripheral nerves, there was a marked decrease in axonal GM-CSFR after chronic painful nerve injury (P = 0.004) and in painful neuromas (P = 0.0043); CD-68–positive macrophages were increased (P = 0.017) but did not appear to express GM-CSFR. Although control synovium showed absent GM-CSFR immunostaining, this was markedly increased in macrophages of painful osteoarthritis knee synovium. Granulocyte-macrophage colony-stimulating factor receptor was expressed in 17 ± 1.7% of small-/medium-sized cultured adult rat DRG neurons, and in 27 ± 3.3% of TRPV1-positive neurons. Granulocyte-macrophage colony-stimulating factor treatment sensitized capsaicin responses in vitro, which were diminished by p38 MAPK or TrkA inhibitors. CONCLUSION: Our findings support GM-CSFR as a therapeutic target for pain and hypersensitivity in clinical CNS and peripheral inflammatory conditions. Although GM-CSFR was decreased in chronic painful injured peripheral nerves, it could mediate CNS neuroinflammatory effects, which deserves study.