title: 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.
authors.affiliation: National Trauma Research Institute, The Alfred Hospital, Melbourne, Victoria, Australia.
authors.forename: Sarah C
authors.initials: SC
authors.lastname: Hellewell
authors.forename: Edwin B
authors.initials: EB
authors.lastname: Yan
authors.forename: Doreen A
authors.initials: DA
authors.lastname: Agyapomaa
authors.forename: Nicole
authors.initials: N
authors.lastname: Bye
authors.forename: M Cristina
authors.initials: MC
authors.lastname: Morganti-Kossmann
mesh_terms: D016564:Amyloid beta-Protein Precursor / Q000378:metabolism; D000818:Animals; D015703:Antigens, CD / Q000378:metabolism; D015214:Antigens, Differentiation, Myelomonocytic / Q000378:metabolism; D001784:Blood Gas Analysis; D001794:Blood Pressure / Q000502:physiology; D001921:Brain / Q000473:pathology; D001930:Brain Injuries / Q000150:complications / Q000473:pathology*; D003337:Corpus Callosum / Q000378:metabolism; D020833:Diffuse Axonal Injury / Q000473:pathology*; D019084:Fluorescent Antibody Technique, Indirect; D005911:Gliosis / Q000473:pathology; D002534:Hypoxia, Brain / Q000209:etiology / Q000473:pathology*; D007150:Immunohistochemistry; D019344:Lactic Acid / Q000097:blood; D008262:Macrophage Activation / Q000502:physiology; D008297:Male; D017628:Microglia / Q000473:pathology*; D016900:Neurofilament Proteins / Q000378:metabolism; D011712:Pyramidal Tracts / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D000097382:CD68 Molecule
chemical_list: D016564:Amyloid beta-Protein Precursor; D015703:Antigens, CD; D015214:Antigens, Differentiation, Myelomonocytic; D019344:Lactic Acid; D016900:Neurofilament Proteins; C067980:CD68 antigen, human; D000097382:CD68 Molecule