Role of substance P signaling in enhanced nociceptive sensitization and local cytokine production after incision

Substance P (SP) signaling facilitates nociceptive sensitization in various inflammatory and chronic pain models and we postulated that SP signaling might also contribute to the development of post-incisional hyperalgesia. These studies used mice with a deletion of the pre-protachykinin A gene (ppt-A−/−) which codes for SP to determine the role of SP signaling in post-incisional pain and in the increased cytokine and nerve growth factor (NGF) expression observed in the incised skin. SP deficient ppt-A−/− mice displayed reduced mechanical allodynia and heat hyperalgesia compared to the wild-type (wt) mice at all post-incision time points, despite similar baseline values (p < 0.001). Furthermore, the NK-1 receptor antagonist LY303870 attenuated mechanical allodynia produced by incision in the wt mice (p < 0.001). Incision also up-regulated IL-6, TNF-α and KC levels but not IL-1β after 2 h in the wt mice skin. However, ppt-A−/− mice had more skin NGF levels 2 h post-incision. Subcutaneous hind paw SP injection produced acute and transient elevations of IL-1β, IL-6, and KC but modest elevations in TNF-α levels in the wt mice. Systemic LY303870 reversed the SP-induced elevations of these cytokines. Hind paw injection of IL-6 and NGF dose dependently produced less mechanical allodynia in the ppt-A−/− compared to wt mice. Additionally, SP produced mechanical allodynia in a dose-dependent fashion in wt mice. Therefore, SP supports nociceptive sensitization after hind paw incision and potentially participates directly in modulating the intensity of inflammatory response in peri-incisional tissue.