Electrophysiological and Pathological Characterization of the Period of Heightened Vulnerability to Repetitive Injury in an in Vitro Stretch Model

Clinical studies suggest that repeat exposures to mild traumatic brain injury (mTBI) or concussion, such as sports-related mTBI, result in verbal, memory, and motor deficits that can progressively worsen and take longer for recovery with each additional concussion. Pre-clinical studies suggest that mild mechanical injury of the brain can initiate a period of heightened vulnerability during which the brain is more susceptible to a subsequent mild injury. It is unknown how long this period of heightened vulnerability lasts and, as a result, appropriate return-to-play guidelines for athletes who have sustained sports-related mTBI could be better clarified. To better understand this pathology and define the duration of heightened vulnerability to subsequent exposure, we employed a well-defined stretch injury model to mechanically stimulate organotypic hippocampal slice cultures (OHSCs) and evaluated both electrophysiological and pathological markers of injury. We found that an initial mild stretch initiated a period of heightened vulnerability to a subsequent stretch that lasted at least 24 h. Two mild stretch injuries delivered 24 h apart significantly increased tissue injury, including cell death, damage to dendrites, increased nitrite production, astrogliosis, and loss of long-term potentiation (LTP). Cell loss, dendrite damage, and nitrite production were not significantly increased when the inter-injury interval was increased to 72 h; however, LTP deficits and astrogliosis persisted. An interval of 144 h was sufficient to prevent the detrimental effects of repetitive stretch. Improved understanding of the brain's response to repetitive mTBI in vitro may aid in translational studies, informing rest periods for the injured athlete.