Cervical dorsal rhizotomy (CDR) enlarges phrenic motoneurons in rats

We hypothesized that CDR (C3-C5) would alter the morphology of phrenic motoneurons, reflecting spinal plasticity. Adult male rats were studied 28 days after CDR (N=6) or sham surgery (N=6). Phrenic motoneurons were labeled by intramuscular injection of cholera toxin B-fragment (CTB) three days before the rats were sacrificed. CTB labeled phrenic motoneurons were visualized using an immunohistochemical reaction with a Cy-3 conjugated secondary antibody. Longitudinal sections were imaged using a Bio Rad MRC500 confocal microscope. A series of optical slices (0.6 mm) through the phrenic motoneuron pool were digitized and transferred to a Sun workstation where optical slices were spatially reconstructed using ANALYZE software. Cell volumes and surface areas for ̃80 motoneurons were measured and corrected for the estimated Z-axis distortion, based on 3-D reconstruction of fluorescent microspheres of known diameter. CDR increased cell volumes and surface areas of the motoneurons in comparison to sham-operated rats. These structural adaptations suggest that the intrinsic excitability of phrenic motoneurons may decrease following CDR; however, this hypothesis remains to be tested directly.