Tian-Zhi Guo, Tzuping Wei, Wen-Wu Li, Xiang-Qi Li, J. David Clark, Wade S. Kingery
A tibia fracture cast immobilized for 4 weeks can induce exaggerated substance P (SP) and CGRP signaling and neuropeptide-dependent nociceptive and inflammatory changes in the hindlimbs of rats similar to those seen in complex regional pain syndrome (CRPS). Four weeks of hindlimb cast immobilization can also induce nociceptive and vascular changes resembling CRPS. To test our hypothesis that immobilization alone could cause exaggerated neuropeptide signaling and inflammatory changes we tested 5 cohorts of rats; 1) controls, 2) tibia fracture and hindlimb casted, 3) hindlimb casted, no fracture, 4) tibia fracture with intrameduallary pinning, no cast, and 5) tibia fracture with intrameduallary pinning and hindlimb casting. After 4 weeks the casts were removed and hindlimb allodynia, unweighting, warmth, edema, sciatic nerve neuropeptide content, cutaneous and spinal cord inflammatory mediator levels, and spinal c-Fos activation were measured. After fracture with casting there was allodynia, unweighting, warmth, edema, increased sciatic nerve SP and CGRP, increased skin NK1 receptors and keratinocyte proliferation, increased in inflammatory mediator expression in the hindpaw skin (TNF-α, IL-1β, IL-6, NGF) and cord (IL-1β, NGF), and increased spinal c-Fos activation. These same changes were observed after cast immobilization alone, except spinal IL-1β levels were not increased. Treating cast only rats with an NK1 receptor antagonist inhibited development of nociceptive and inflammatory changes. Four weeks after fracture with pinning all nociceptive and vascular changes had resolved and there were no increases in neuropeptide signaling or inflammatory mediator expression.