Neuronal and astrocytic disruption during traumatic brain injury described using longitudinal imagin

Abstract

In Traumatic Brain Injury (TBI) mechanical forces applied to the cranium and brain cause irreversible primary neuronal and astroglial damage associated with terminal dendritic beading and spine loss. Edema, which quickly develops after TBI, causes an increase in intracranial pressure, which can cause secondary injury in the peri-contusional area. Spreading depolarizations have also been shown to occur after TBI in humans as well as in animal models, contributing to secondary injury. However, the impact of spreading depolarizations on real-time injury to dendrites and astrocytes in the pericontusional area during edema development is unknown. We used in vivo twophoton laser scanning microscopy (2PLSM) in mouse somatosensory cortex via an optical window to monitor yellow fluorescent protein expressing neurons and enhanced green fluorescent protein expressing astrocytes in the peri-contusional area after mild TBI. Loss of capillary blood flow preceded dendritic beading, which developed slowly over the ·next several hours. This indicated that acute injury to dendrites was gated by the degree of ischemia as a result of developing edema. Astrocytes were swollen (cytotoxic edema) due to ion and 'rater uptake and remained swollen up to 24 hours. When spreading depolarizations were repeatedly induced by pressure-injecting 1 M KCI with a Picospritzer outside the imaging area, dendrites underwent rapid beading and recovery coinciding with passage of spreading depolarizations, as was confirmed with electrophysiological recordings in the vicinity of imaged dendrites. However, each s:ubsequent I spreading depolarization resulted in a smaller percentage of dendrites that were I able to fully recover until ultimately all were terminally injured. Thus our data suggests that accumulated stress resulting from spreading depolarizations expedites acute dendritic injury in the peri-contusional area, worsening secondary damage following TBI. We also propose that persistent astroglial swelling in the peri-contusional area may be harmful and decreases astroglial maintenance of normal homeostatic function and possibly contributes greatly to the negative outcome of TBI as astrocytes fail to provide neuronal support.