Mild Traumatic Brain Injury Impairs Episodic Memory In Rats

Mild traumatic brain injury (mTBI) is the most common type of traumatic brain injury. Symptoms following mTBI fall into physical, emotional, sleep, and cognitive categories, with memory deficits being a commonly documented sequelae. Whereas many animal models of mTBI exist, relatively few studies have examined the cognitive deficits of mTBI with human-like cognitive tasks. The Wayne State University Closed Head Weight Drop Model recapitulates critical physical elements of sport-related concussions and trauma-based mTBI. However, until now, this model has not previously been evaluated using a human-like memory task. Rats were trained in an odor-based item-in-context task that dissociates episodic and non-episodic memory (Panoz-Brown et al., Current Biology, 2016). The animals then underwent either a weight drop or a sham procedure. After the manipulation, animals were assessed in the item-in-context task. Episodic memory was significantly impaired in the injured rats by over 10% but not in the sham rats. Non-episodic memory was not impaired in either group. Additionally, a time-course immunohistochemical analysis of the hippocampus was performed to examine possible time-dependent changes in ionized calcium-binding adaptor molecule 1 (iba1), a marker of activated microglia/macrophages and glial fibrillary acidic protein (GFAP), a marker of astrocytes. Concussion injury was associated with time-dependent morphological changes in astrocytes and microglia in injured rats compared to sham rats. This study is the first to document episodic memory impairment in an animal model of mTBI.