Gait Abnormalities and Inflammatory Cytokines in an Autologous Nucleus Pulposus Model of Radiculopathy

Study Design. The authors investigated gait abnormalities and mechanical hypersensitivity associated with invertebral disc herniation in a rat model of radiculopathy. Further evaluation involved assessing how nucleus pulposus (NP) injury affected systemic cytokine expression and molecular changes at the dorsal root ganglion (DRG). Objective. The objective of this work was to describe the gait and behavioral changes in an animal model of discherniation induced radiculopathy. A second objective included examining how these functional changes correlated with neuroinflammation and autoreactive lymphocyte immune activation. Summary of Background Data. Animal models of radiculopathy describe demyelination, slowed nerve conduction, and heightened pain sensitivity after application of autologous NP to the DRG. The quantitative impact of disc herniation on animal locomotion has not been investigated. Further, while local inflammation occurs at the injury site, the role of autoimmune cytokines reactive against previously immune-sequestered NP requires investigation. Methods. NP-treated animals (n = 16) received autologous tail NP placed onto the L5 DRG exposed by unilateral facetectomy, and control animals (n = 16) underwent exposure only. At weekly time points, animals were evaluated for mechanical allodynia, thermal hyperalgesia, and gait characteristics through digitized video analysis. Serum cytokine content was measured after animal sacrifice, and immunohistochemistry tested DRG tissue for mediators of inflammation and immune activation. Results. Sensory testing revealed mechanical allodynia in the affected limb of NP-treated rats compared with sham animals (P < 0.01) at all time points. Gait analysis reflected functional locomotive consequences of marked asymmetry (P = 0.048) and preference to bear weight on the contralateral limb (duty factor imbalance, P < 0.01) at early time points. Equivalent serum cytokine expression occurred in both groups, confirming the local inflammatory nature of this disease model. Immunohistochemistry of the sectioned DRGs revealed equivalent postsurgical inflammatory activation (interleukin 23, P = 0.47) but substantial early immune activation in the NP-treated group (interleukin 17, P = 0.01). Conclusion. This model of radiculopathy provides evidence of altered gait in a model of noncompressive disc herniation. Systemic inflammation was absent, but mechanical allodynia, local inflammation, and autoreactive immune activation were observed. Future work will involve therapeutic interventions to rescue animals from the phenotype of inflammatory radiculopathy.