Post-Traumatic Hypoxia Exacerbates Brain Tissue Damage: Analysis of Axonal Injury and Glial Responses

Abstract

Traumatic brain injury (TBI) resulting in poor neurological outcome is predominantly associated with diffuse brain damage and secondary hypoxic insults. Post-traumatic hypoxia is known to exacerbate primary brain injury; however, the underlying pathological mechanisms require further elucidation. Using a rat model of diffuse traumatic axonal injury (TAI) followed by a post-traumatic hypoxic insult, we characterized axonal pathology, macrophage/microglia accumulation, and astrocyte responses over 14 days. Rats underwent TAI alone, TAI followed by 30 min of hypoxia (TAI + Hx), hypoxia alone, or sham-operation (n = 6/group). Systemic hypoxia was induced by ventilating rats with 12% oxygen in nitrogen, resulting in a ∼ 50% reduction in arterial blood oxygen saturation. Brains were assessed for axonal damage, macrophage/microglia accumulation, and astrocyte activation at 1, 7, and 14 days post-treatment. Immunohistochemistry with axonal damage markers (β-amyloid precursor protein [β-APP] and neurofilament) showed strong positive staining in TAI + Hx rats, which was most prominent in the corpus callosum (retraction bulbs 69.8 ± 18.67; swollen axons 14.2 ± 5.25), and brainstem (retraction bulbs 294 ± 118.3; swollen axons 50.3 ± 20.45) at 1 day post-injury. Extensive microglia/macrophage accumulation detected with the CD68 antibody was maximal at 14 days post-injury in the corpus callosum (macrophages 157.5 ± 55.48; microglia 72.71 ± 20.75), and coincided with regions of axonal damage. Astrocytosis assessed with glial fibrillary acidic protein (GFAP) antibody was also abundant in the corpus callosum and maximal at 14 days, with a trend toward an increase in TAI + Hx animals (18.99 ± 2.45 versus 13.56 ± 0.81; p = 0.0617). This study demonstrates for the first time that a hypoxic insult following TAI perpetuates axonal pathology and cellular inflammation, which may account for the poor neurological outcomes seen in TBI patients who experience post-traumatic hypoxia.

Repetitive Mild Traumatic Brain Injury in a Mouse Model Produces Learning and Memory Deficits Accompanied by Histological Changes

“…These results are consistent with other studies that showed astrocytosis peaking between 10 and 14 days following concussion in rodents. 42,49,50 This increase in gliosis is also believed to coincide with macrophage accumulation, which has also been shown to peak at 10 days post-injury. Our data demonstrate that repetitive, noninvasive mTBI in the mouse resulted in a graded injury response, with the extent of reactive astrocytosis increased in animals exposed to repetitive impacts.…”

Structural brain injury in patients with disorders of consciousness: A voxel-based morphometry study

“…These groups were matched for age. The time a patient spent in the DOC at the day of scanning was used as a covariate (the natural logarithm of the time spent in DOC in days was used; the natural logarithm was chosen, as, given known biological secondary responses to primary brain injury [33,[60][61][62], the effect of time on structural brain injury was expected to be strongest in the initial weeks). The structural brain injury correlating to this covariate was examined.…”

Moderately Elevated Intracranial Pressure after Diffuse Traumatic Brain Injury is Associated with Exacerbated Neuronal Pathology and Behavioral Morbidity in the Rat

“…As the amount of axotomy is commonly used to evaluate the magnitude of diffuse TBI, 31,32 we first analyzed the number of inured axons, visualized via APP accumulation within axonal swellings, 6 hours after TBI (sham n ¼ 2 animals, TBI n ¼ 4, TBI þ ICP elevation n ¼ 5). In accordance with the previous reports, no APP þ axonal swellings were seen in sham-injured controls ( Figure 3A), whereas TBI alone and TBI þ ICP-elevation showed numerous APP þ swellings within layers V and VI of the somatosensory cortex (Figure 3) Figures 3B and 3C).…”