Background: Chronic exposure to high altitude (HA) can lead to maladaptive changes in immune function, vascularization, and neurotransmission, resulting in an increased risk for neurological impairment. Whether the occurrence of mild traumatic brain injury (mTBI) at HA exacerbates neurobehavioral deficits remains uncertain. Therefore, an experimental design was employed to determine the impact of mTBI in the context of HA (hypobaric hypoxia), using established murine models of repetitive close head injury and simulated HA at 5000 m.
Methods: Mice were exposed to HA or sea level (SL) for a 12-week period, after which time they were randomized to sham or mTBI groups. For mTBI, three closed head injuries were administered 24 h apart. Longitudinal assessment of neuroimaging biomarkers using a battery of positron emission tomography and magnetic resonance imaging modalities were employed to identify the effects of chronic HA exposure (12 weeks) and mTBI in male C57BL/6J mice, with assessments conducted at baseline, following 12 weeks of HA and two weeks post TBI. Assessment of HA and injury associated cognitive deficits (novel object recognition, and fear memory recall) were assessed from day 5-9 following TBI and histopathology (biomarkers of tau, gliosis, and Iba1 staining) was assessed in the hippocampus, cortex, and thalamus from brains of these animals taking after the final imaging session. Further protein quantification was conducted in tissues from the ipsilateral left hippocampus for phosphorylated tau, glial fibril acid protein (GFAP; astrogliosis), ionized calcium binding adaptor molecule 1 (Iba1; microglia activation), cleaved caspase 3 (apoptosis), the excitation/inhibition ratios were evaluated using the surrogate markers of plasticity, glutamatergic 2RB subunit containing NMDA receptors and GluR2 containing AMPA receptors.
Results: Histopathology revealed astrogliosis and increased microglial activation at the site of injury, with vascular/white matter pathology associated with HA (see Cramer et al. 2019). Within the hippocampus GFAP was elevated in both mTBI groups relative to sham controls, but HA did not further augment this. Iba1 was reduced following HA exposure relative to the SL group, but this decrease was not detectable in those animals in the mTBI group. Decreased contextual fear memory and reduced discrimination of novel objects were apparent in animals maintained at HA for 12 weeks and also in those animals in the mTBI groups. There were no synergistic effects of mTBI on HA associated cognitive dysfunction. Cerebral blood flow was markedly increased following 12 weeks of HA, as was global brain volume, magnetization transfer ratio and trace within the cortex, hippocampus, and thalamus, suggestive of widespread edema. Fractional anisotropy and standardized uptake values were decreased overall with HA exposure, indicative of white matter pathology. The increase in cerebral blood flow and global volume observed following HA, was dramatically reduced in HA-mTBI animals two weeks post injury relative to sham controls.
Discussion/
Conclusion: The data show that both long term HA exposure and mTBI at both SL and HA can negatively impact cognitive and induce changes at a cellular level within the ipsilateral hippocampus, which is a critical brain region regulating cognitive performance. Although no synergistic effects of HA and TBI were noted on behavior or biochemistry/pathology, discrete compounding alterations detectable by imaging modalities were apparent. Such longitudinal evaluation with these imaging modalities will serve as increasingly important screening tools capable of identifying functional deficits that occur overtime. Overall, these data support further investigation of the long term impact of concurrent HA exposure and mTBI and the use of multiple tools to evaluate increased risk for neurological and behavioral impairments later in life.
Keywords: High altitude, repetitive closed head injury, mild traumatic brain injury, cognition, longitudinal imaging, PET, MRI, astrogliosis, microglia, hippocampus
