Evolution of neuronal and astroglial disruption in the peri-contusional cortex of mice revealed by in vivo two-photon imaging

Abstract

In traumatic brain injury mechanical forces applied to the cranium and brain cause irreversible primary neuronal and astroglial damage associated with terminal dendritic beading and spine loss representing acute damage to synaptic circuitry. Oedema develops quickly after trauma, raising intracranial pressure that results in a decrease of blood flow and consequently in cerebral ischaemia, which can cause secondary injury in the peri-contusional cortex. Spreading depolarizations have also been shown to occur after traumatic brain injury in humans and in animal models and are thought to accelerate and exacerbate secondary tissue injury in at-risk cortical territory. Yet, the mechanisms of acute secondary injury to fine synaptic circuitry within the peri-contusional cortex after mild traumatic brain injury remain unknown. A mild focal cortical contusion model in adult mouse sensory-motor cortex was implemented by the controlled cortical impact injury device. In vivo two-photon microscopy in the peri-contusional cortex was used to monitor via optical window yellow fluorescent protein expressing neurons, enhanced green fluorescent protein expressing astrocytes and capillary blood flow. Dendritic beading in the peri-contusional cortex developed slowly and the loss of capillary blood flow preceded terminal dendritic injury. Astrocytes were swollen indicating oedema and remained swollen during the next 24 h throughout the imaging session. There were no recurrent spontaneous spreading depolarizations in this mild traumatic brain injury model; however, when spreading depolarizations were repeatedly induced outside the peri-contusional cortex by pressure-injecting KCl, dendrites undergo rapid beading and recovery coinciding with passage of spreading depolarizations, as was confirmed with electrophysiological recordings in the vicinity of imaged dendrites. Yet, accumulating metabolic stress resulting from as few as four rounds of spreading depolarization significantly added to the fraction of beaded dendrites that were incapable to recover during repolarization, thus facilitating terminal injury. In contrast, similarly induced four rounds of spreading depolarization in another set of control healthy mice caused no accumulating dendritic injury as dendrites fully recovered from beading during repolarization. Taken together, our data suggest that in the mild traumatic brain injury the acute dendritic injury in the peri-contusional cortex is gated by the decline in the local blood flow, most probably as a result of developing oedema. Furthermore, spreading depolarization is a specific mechanism that could accelerate injury to synaptic circuitry in the metabolically compromised peri-contusional cortex, worsening secondary damage following traumatic brain injury.

The pathophysiology of repetitive concussive traumatic brain injury in experimental models; new developments and open questions

“…However, diolistic labeling of dendritic spines has not been used to assess the effects of TBI to our knowledge. Multiphoton microscopy in mice carrying fluorescent reporter genes in sparse neuronal subsets are beginning to be used to assess dendritic spine dynamics after TBI in vivo (Sword et al, 2013). Importantly, none of these assessments of dendritic spine integrity have been applied in animal models of repetitive concussive TBI to our knowledge.…”

Suppression of spreading depolarization and stabilization of dendritic spines by GLYX-13, an NMDA receptor glycine-site functional partial agonist

“…The long-term functional significance of this surprising lability of dendritic spine morphology to cognitive function, and future susceptibility to a range of pathologies from Alzheimer's to epilepsy to migraine, is unknown. However, it seems a reasonable hypothesis that the complete recovery of spine size at their former locations following SD that is promoted by GLYX-13 is likely to be therapeutically beneficial in preventing long-term negative sequelae of migraine, and other generators of SDs such as traumatic brain injury (Hartings et al, 2009(Hartings et al, , 2014Sword et al, 2013), ischemia (Risher et al, 2010) and seizures (Takano et al, 2007).…”

Acute Brain Trauma in Mice Followed By Longitudinal Two-photon Imaging

“…Therefore a limitation of the present model is that it does not provide information about mechanisms of closed head injury. Recently Sword and coauthors reported multiphoton imaging results on cell behavior in pericontusional cortex 6 . Taking into account that closed head injury is a more frequent medical case, the methodological approach reported by the authors is highly promising to complement traditional research methods in the field of impact brain trauma 11 .…”

“…Using this powerful tool, we can visualize dynamic morphological and functional changes in living brain under posttraumatic conditions. The advantages of in vivo two-photon microscopy in studying brain injury were recently demonstrated by Kirov and colleagues 6 . Using a mild focal cortical contusion model, these authors showed that acute dendritic injury in the pericontusional cortex is gated by the decline in the local blood flow.…”