The role of enhanced cutaneous IL-1β signaling in a rat tibia fracture model of complex regional pain syndrome

Tibia fracture in rats initiates a syndrome resembling the complex regional pain syndrome type I. Accumulating evidence indicates that IL-1 beta is involved ill the modulation of nociceptive information and it acts as an intermediate inflammatory mediator via up-regulation of NGF. We hypothesized that I signaling might mediate the development of the CRPS-like changes after tibial fracture, either directly or by stimulating NGF expression. Rats underwent distal tibia fracture and casting for 4 weeks and were chronically treated with an IL-1 receptor antagonist (IL-1ra). Nociceptive testing and assessment of edema and hindpaw warmth were performed at baseline and after cast removal. Bone microarchitecture was evaluated by micro-computed tomography. Confocal immunofluorescence and in situ hybridization techniques were used to evaluate changes in the cutaneous expression of IL-1 beta at 4 weeks post-fracture. The nociceptive and vascular effects of intraplantar IL-1 beta injections were evaluated in intact rats at different time points after injection. We found that: ( I) IL-1ra reduced fracture-induced nociceptive sensitization, but did not decrease hindpaw edema or warmth, (2) fracture chronically up-regulated IL-1 beta mRNA and protein expression in hindpaw skin keratinocytes, (3) IL-1 beta intraplantar injection induced mechanical allodynia in a dose-dependent manner and stimulated keratinocyte NGF expression in the hindpaw skin, and (4) intraplantar injection of NGF-induced nociceptive sensitization. Collectively, these results indicate that Cutaneous IL-1 beta signaling can contribute to chronic regional nociceptive sensitization after fracture, possibly by stimulating NGF over-expression in keratinocytes. Our data also highlight the importance of the keratinocyte as the primary Source of post-traumatic IL-1 beta over-expression. Published by Elsevier B.V. on behalf of International Association for the Study of Pain.