In the study, mice were repeatedly given extremely mild concussive impacts while anesthetized. The brain's response to a single concussion was compared with an injury received daily for 30 days and one received weekly over 30 weeks.
Mice with a single insult temporarily lost 10-15% of their neuronal connections (dendritic spines), but there was no inflammation or cell death. With three days rest, all neuronal connections were restored.
However, those given daily concussions did not show a loss in dendritic spines, and it's thought that the brain habituates to the repeated shocks. It's further suggested that the loss of synapses is actually a protective effect, allowing the brain to regain normal calcium flow. The long-term effect of this not happening is unknown.
Those given a week of rest between each insult did show the normal dendritic spine loss, however.
Additionally, when a mild concussion occurred each day for a month, there was inflammation and damage to the white matter, and this damage continued for months after the last impact.
These findings are consistent with what has been seen in humans, where white matter inflammation has been found to be a long-lasting consequence of TBI.
On a more positive note, in this model of very mild concussion, there was no increase in tau tangles, suggesting this might be limited to more serious injuries.
(2016). Dendritic Spine Loss and Chronic White Matter Inflammation in a Mouse Model of Highly Repetitive Head Trauma.
The American Journal of Pathology.