Extensive respiratory plasticity after cervical spinal cord injury in rats: Axonal sprouting and rerouting of ventrolateral bulbospinal pathways

Spinal cord injury (SCI) causes an interruption of descending motor and autonomic nervous tracts. However, a partial injury, and particularly a unilateral section, is generally followed by spontaneous locomotor and respiratory recovery. Although locomotor functional recovery has been correlated to spontaneous anatomical plasticity of the corticospinal tract, the remodeling of the bulbospinal tract that sustains respiratory improvement is unknown and has therefore been investigated here after chronic lateral cervical injury in rats (90 days post-lesion by comparison to 7 days post-lesion). We show that chronic lateral C2 SCI leads both to a decreased thickness of the ipsilateral ventrolateral funiculus at sus and sub-lesional levels and to an opposite effect on the contralateral side. At C1 level, the number of ventrolateral bulbospinal fibers, stained with anterograde tracer was reduced within the ipsilateral ventrolateral funiculi while collateral arborization toward the gray matter and growth associated protein-43 levels was increased. At C2 lesional level, fibers rerouting toward the gray matter were also identified for 5% of the axotomized axon terminals. Despite these chronic sprouting processes respiratory bulbospinal projections to ipsilateral phrenic nucleus remained poor (less than 10% compared to non-injured conditions). Retrograde labeling of projections onto the phrenic nucleus revealed, after chronic injury, an increased recruitment of C1 propriospinal interneurons which moreover received more contacts from bulbospinal collaterals. This chronic remodeling was correlated with chronic diaphragm recovery under conditions of respiratory stress. Thus, despite extensive axonal loss and absence of direct phrenic reinnervation by bulbospinal respiratory neurons, sprouting processes toward cervical propriospinal neurons may contribute to the observed partial respiratory recovery.