Relationship (?) Between Histological and Functional Measures in Moderate Contusion SCI in Adult Rat

Abstract

To create a spinal cord injury (SCI), most laboratories implement a standardized method; however, considerable variability in injury size and associated functional deficits can occur across subjects. The present study was initiated to explore relationships between the histological features of spinal cord (SC) lesion and the associated functional deficits in locomotor, respiratory, and lower urinary tract (LUT) function in a widely used rat contusion SCI model that is generated using an Infinite Horizon impactor. To this end, adult female rats (n=8) received a moderate mid‐thoracic contusion SCI (200 kdyn, T8), and the impact displacement and force were recorded. Indices of functional recovery included: (1) BBB open field locomotor scoring at 1‐week post SCI, (2) total volume of expressed urine during the first 2‐weeks post SCI, and (3) diaphragm EMG activity and reflex micturition behavior and residual urine volume recorded during an acute cystometry experiment at 4‐weeks post SCI, after which the bladder was removed and weighed. Following fixation (4% PFA) and cryoprotection of the SC, 20 μm thick horizonal sections were obtained and counterstained with cresyl violet and Cyanine R / FeCl3 to visualize cell bodies and myelin, respectively, and the section at the level of the central canal was used as the site for preliminary analysis of the extent of lesion damage and degree of white matter sparing. The lesion area ranged from 27–49% and was measured by outlining the perimeter of the lesion site using a threshold tool and then normalized (%) to the area of a standardized 7 mm rostral‐caudal length region of SC that included the injury site. Myelin thickness was measured at 3 sites at the level of the injury and at the rostral and caudal ends of the tissue (uninjured) on each side of the SC, and the injury site measurements were normalized to the average myelin thickness of the corresponding uninjured side. A spared myelin index (%) using the larger minimum myelin thickness from the right or left side of the SC was then determined and ranged from 19–91%. For generation of the SCI, impactor displacement varied from 1287 to 1728 μm, with larger displacements being associated with larger injury sites and less myelin sparing. The lesion area, however, was not a strong predictor of myelin sparing, particularly in cases with asymmetrical lesions (n=2). Both lesion area and spared myelin index were reasonable predictors of post SCI recovery of locomotor (BBB scores) and LUT (expressed urine volume) function but not in predicting the degree of dysfunction in reflex micturition (increased residual volume) and bladder hypertrophy or inspiratory burst measures and their variability. These preliminary results suggest that factors in addition to the histological characteristics of the lesion site contribute to the degree of functional deficits/recovery following SCI.Support or Funding InformationDOD CDMRP W81XWH‐17‐1‐0260; NIH NS096514; NYS DOH SCIRB C32088GG