Changes in calcitonin gene-related peptide expression following joint immobilization in rats

Long-term immobilization by casting can occasionally cause pathologic pain states in the immobilized side. The underlying neurophysiological mechanisms of immobilization-related pain are not well understood. For this reason, we specifically examined changes of calcitonin gene-related peptide (CGRP) expression in the dorsal root ganglion (DRG), spinal dorsal horn and posterior nuclei (cuneate nuclei) in a long-term immobilization model following casting for 5 weeks. A plastic cast was wrapped around the right limb from the forearm to the forepaw to keep wrist joint at 90 degrees of flexion. In this model, CGRP in immobilized (ipsilateral) side was distributed in larger DRG neurons compared with contralateral side, even though the number of CGRP-immunoreactive (CGRP-IR) neurons did not differ. Spinal laminae III-V, not laminae I-II in ipsilateral side showed significantly high CGRP expression relative to contralateral side. CGRP expression in cuneate nuclei was not significantly different between ipsilateral and contralateral sides. Long-term immobilization by casting may induce phenotypic changes in CGRP expression both in DRG and spinal deep layers, and these changes are partly responsible for pathological pain states in immobilized side.