Pain is multifaceted. TNF has been implicated in the pathogenesis of arthritic pain but the relationship between TNF and other cell types during inflammatory pain is unknown. In the present study, we sought to determine the requirement for local colony-stimulating factor (CSF)-1 signalling, and local and/or systemic neutrophil involvement in TNF-driven pain development.

Inflammatory pain was elicited by intraplantar injection of TNF-α into the left hindpaw and assessed for weight distribution change by incapacitance meter over a 24hr as an indication of pain. Local neutrophil depletion was performed by giving an intraplantar injection of anti-Ly6G mAb at the same time as TNF, while systemic neutrophil depletion was performed by giving an intraperitoneal injection of anti-Ly6G mAb two days prior to local injection of TNF. CSF-1 signalling was also blocked locally by giving an intraplantar injection of anti-CSF-1R mAb at the same time as TNF. For RT-PCR analysis, mice were sacrificed at 4hr and plantar skin tissue was collected for RNA extraction: MPO, TNF, IL-1β and GM-CSF mRNA levels were measured.

TNF-driven pain was transient, with maximal pain observed at 4hrs. Anti-Ly6G mAb had no effect on TNF-evoked pain when it was co-injected with TNF, but systemic depletion of neutrophils using anti-Ly6G mAb abolished pain development, indicating the involvement of systemic neutrophil infiltration during TNF-induced pain. RT-PCR analysis revealed TNF-induced MPO, GM-CSF and IL-1β mRNA gene expression were significantly reduced in mice pretreated with anti-Ly6G mAb. Intraplantar co-administration of the CSF-1R mAb with TNF also prevented pain development.

Collectively this data shows that CSF-1 signalling in resident macrophages is required for TNF-induced pain development and systemic neutrophil infiltration is also required for TNF-induced pain development.