Direct Application of the TNF-Alpha Inhibitor, Etanercept, Does Not Affect CGRP Expression and Phenotypic Change of DRG Neurons Following Application of Nucleus Pulposus Onto Injured Sciatic Nerves in Rats

Study Design. Immunohistological and behavioral analysis of the effect of a tumor necrosis factor alpha (TNF-&agr;) inhibitor in an injured-nerve model. Objective. To examine the effect of direct application of a TNF-&agr; inhibitor (etanercept) on injured-nerve pain caused by nucleus pulposus. Summary and Background Data. TNF-&agr; is thought to play a crucial role in radicular pain. Calcitonin gene-related peptide (CGRP) is an inflammatory neuropeptide found in small sensory neurons. We have reported that CGRP appears in medium and large dorsal root ganglion (DRG) neurons that transmit proprioception in physiologic conditions. The purpose of the current study was to examine the change in behavior and phenotypic change of CGRP-immunoreactive DRG neurons by the TNF-&agr; inhibitor, etanercept, in a disc herniation model. Methods. For the injured-nerve model, nucleus pulposus was applied to the sciatic nerve and the sciatic nerve pinched. Saline (10 &mgr;L; n = 10), as a control, or etanercept (150 &mgr;g: n = 10) were applied to sciatic nerves simultaneously. Mechanical allodynia was examined. Immunohistochemistry was used to examine CGRP expression in L5 DRGs. Results. Significant mechanical allodynia for 10 days was seen in the injured-nerve group compared with sham-operated animals. Etanercept ameliorated the mechanical allodynia slightly on day 2; however, there was no effect on other days. CGRP immunoreactivity was upregulated in the L5 DRG neurons of injured-nerve groups compared with the sham-operated group (P < 0.01). However, etanercept did not affect CGRP expression after nerve injury (P > 0.05). Proportions of CGRP- immunoreactive medium and large neurons were not significantly different in the nerve injury + saline group compared with the injury + etanercept group (P > 0.05). Conclusion. Our results indicate that direct application of a TNF-&agr; inhibitor had a small effect on acute pain behavior and may not be effective for suppression of inflammatory peptides in the current disc-herniation model.

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