Phrenic afferent projections to the cervical spinal cord in spinal intact and spinal injured rats

Abstract

The phrenic nerve contains both myelinated and unmyelinated afferent projections. Relatively little is known, however, regarding the spinal distribution of these sensory projections. Accordingly, our first goal was to create a map of phrenic afferent projections throughout the cervical spinal cord. Afferents were traced via prolonged phrenic nerve soak with Cascade Blue (CB) in adult Sprague‐Dawley rats. Spinal tissues were harvested, sectioned (40μm) from C2–C7, and processed with routine immunochemical methods. Extensive labeling of afferent projections was observed throughout the ipsilateral dorsal horn (laminae II and III) and intermediate grey region (laminae IV and V) in segments C2–C6. Phrenic afferent projections were crossed the spinal midline in 13% of the tissue sections, and were occasionally observed to reach the contralateral dorsal horn (laminae III). Robust phrenic motoneuron (PhrMN) labeling was observed from C3–C5, and PhrMN dendrites projected across the spinal midline in 30% of tissue sections from C3–C5. Our second objective was to determine if high cervical spinal cord injury (SCI) induced anatomical plasticity in the phrenic sensorimotor system. The phrenic nerve was traced using the same CB method in Sprague‐Dawley rats at 12 weeks following lateral hemisection of the C2 spinal cord. Preliminary analysis indicates that following SCI, phrenic afferents and PhrMN dendrites project across the spinal midline at approximately the same frequency as in spinal‐intact rats (p>0.36). However, we noted that the likelihood of observing phrenic afferent projections extending to the region of the ipsilateral phrenic motor nucleus (laminae VIII‐IX) was increased after SCI (p<0.05). By combining CB labeling with Choline acetyltransferase (ChAT) immunostaining, we verified the presence of laminae VIII‐IX phrenic afferents by observing CB‐positive and ChAT negative fibers. We conclude that phrenic sensory afferent projections 1) are prominent in the ipsilateral C3–C5 dorsal horn and intermediate grey region, 2) can project to the contralateral C3–C5 dorsal horn, and 3) undergo remodeling after high cervical SCI.Support or Funding InformationSPARC OT2 OD023854 (DDF), 1 R01 NS080180‐01A1 (DDF)This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.