Graded unilateral cervical spinal cord injury and respiratory motor recovery

Abstract

We examined the potential contribution of ventromedial (VM) tissue sparing to respiratory recovery following chronic (1 mo) unilateral C2 spinal cord injury (SCI) in rats. Preserved white matter ipsilateral to the injury was quantitatively expressed relative to contralateral white matter. The ipsilateral-to-contralateral white matter ratio was 0 after complete C2 hemisection (C2HS) and 0.23 ±0.04 with minimal VM sparing. Inspiratory (breath•min −1 ) and phrenic frequency (burst•min −1 ), measured by plethysmography (conscious rats) and phrenic neurograms (anesthetized rats) respectively, were both lower with minimal VM sparing (p<0.05 vs. C2HS). Tidal volume also was greater in minimal VM sparing rats during a hypercapnic challenge (p<0.05 vs. C2HS). In other C2 hemilesioned rats with more extensive VM matter sparing (ipsilateral-to-contralateral white matter ratio = 0.55±0.05), respiratory deficits were indicated at 1 mo post-injury by reduced ventilation during hypercapnic challenge (p<0.05 vs. uninjured). Anterograde (ventral respiratory column-tospinal cord) neuroanatomical tracing studies showed that descending respiratory projections from the brainstem are present in VM tissue. We conclude that even relatively minimal sparing of VM tissue after C2 hemilesion can alter respiratory outcomes. In addition, respiratory deficits can emerge in the adult rat after high cervical SCI even when relatively extensive VM sparing occurs.

A Murine Model of Cervical Spinal Cord Injury to Study Post-lesional Respiratory Neuroplasticity

“…Especially, when respiratory neuroplasticity is studied, Fuller et al 21 showed that the amount of respiratory recovery following a C2 injury was correlated with the remaining spared ventral white matter.…”

“…The subsequent amount of respiratory recovery (phrenic nerve activity and diaphragm activity) following a C2 injury is correlated with the remaining ventrolateral spared white matter 21 . Since the injury is "handmade" and requires some practice from the surgeon, the extent of each injury has to be checked by histological techniques (fixation of the tissue with paraformaldehyde 4%, frozen tissue slicing, cresyl violet staining) to determine the exact size of damaged tissue ( Figure 2B).…”

Contribution of the spontaneous crossed-phrenic phenomenon to inspiratory tidal volume in spontaneously breathing rats

“…Our anesthesia and injury methods have been previously described (14,17). Rats were anesthetized by injection of xylazine (10 mg/kg sq) and ketamine (140 mg/kg ip; Fort Dodge Animal Health).…”

“…These procedures were adapted from our prior publications (9,14,17,29,41). Isoflurane anesthesia (3-4% in O 2) was induced in a closed chamber followed by intraperitoneal injection of urethane (1.6 g/kg; Sigma, St. Louis, MO).…”

“…All C2HS lesions were confirmed to extend to the spinal midline as previously described (14,17,40). At the conclusion of the phrenicotomy experiment, rats were euthanized by systemic perfusion with saline followed by 4% paraformaldehyde (Sigma).…”