Hypertonic saline protects the axo-myelinic interface acutely after contusive SCI and improves postural stability and...

The development of periaxonal swellings and separation of the axo-myelinic interface are prominent features acutely post-contusive spinal cord injury (SCI) and are associated with ongoing secondary degeneration of myelinated fibers. However, the molecular and cellular mechanisms remain poorly understood. Given the temporal overlap between vascular edema formation after SCI and periaxonal swelling, we hypothesized that hypertonic saline (HTS) treatment would reduce periaxonal swelling and protect myelinated fibers after SCI. To test this hypothesis, we used longitudinal intravital two-photon excitation microscopy to simultaneously image dorsal column axons and their myelin sheath using Thy1^YFP+ mice and the lipophilic fluorescent dye Nile red respectively. We found that low-dose HTS (3%) given at 1, 3, or 6 h following a contusive SCI (T13, 30 kilodyne, IH Impactor) significantly reduced periaxonal swellings and increased axonal survival at 24 h compared to normal saline (NS, 0.9% NaCl) treated controls. In distinction, delayed treatment of 5% HTS beginning at 6 h after SCI was less effective. To determine whether 3% HTS treatment initiated at 6 h following a T9, 50 kilodyne contusive SCI improves neurological recovery we used a standard open-field behavioral test (Basso Mouse Scale, BMS) and horizontal ladder. We also applied Motion Sequencing (MoSeq), an unsupervised machine learning method, to further assess behavioral changes due to SCI and the effects of HTS treatment. BMS scoring, and proportional analysis of BMS subscores revealed subtle improvements in functional recovery between HTS and NS treatment. In addition, MoSeq analysis unveiled novel differences in behavior between normal and SCI mice and identified enhanced recovery trajectories in locomotor behaviors following 3% HTS treatment. Collectively, these findings indicate that low-dose HTS confers acute protection to myelinated fibers following SCI through attenuation of edema-associated alterations within the periaxonal space and preservation of axonal integrity. However, the extent to which HTS treatment alone translates into robust functional recovery remains to be determined.