Abstracts

Rationale: To test whether epileptogenic loss of cortical inhibition follows repetitive mild  closed head injury (rmCHI). Methods: Young adult (postnatal day 60) male mice were anesthetized, sustained horizontally and exposed to rmCHI using a solid weight (54 g dropped from 106 cm above the head) for a single impact daily for five consecutive days. In one cohort (n=4 injured, 4 sham), mice were implanted with microdialysis cannulae positioned in the somatosensory cortex after injury, and the concentration of extracellular gamma aminobutyric acid (GABA) and glutamate (Glu) were measured weekly in the dialysate. A second cohort (n=6 injured, 6 sham) was systemically perfused with 4% paraformaldehyde, and brain tissues processed for immunostaining of GABAergic, parvalbumin-expressing interneurons (PVIs) at one and six weeks after injury. Results: One week after rmCHI, extracellular Glu increased to 92% of sham control, which then returned to baseline by the second week after injury. Glu levels remained at the baseline in the subsequent four-week latent period after which extracellular GABA reduced to 50% of sham control. Parallel immunostaining revealed a 29% increase in PVI density (p<0.03) in the somatosensory cortex one week after injury. However, PVI density declined to 60% (p<0.009) of sham control six weeks after injury. Conclusions: We reported progressive biochemical and anatomical changes in the cortex after rmCHI in mice. The initial increase in extracellular Glu and PVI density may set up an excitation/inhibition (E/I) imbalance in local circuits at the early phase of disease progression after rmCHI. Chronically, this E/I imbalance leads to a loss of parvalbumin cells and failure in cortical inhibition mediated by GABA. Together, these changes result in increased vulnerability to unprovoked seizures, consistent with increased epilepsy risk in mild traumatic brain injury in human patients (Huguenard et al. 2016, Keret et al. 2017). Our results provide insights into the potential cellular and molecular mechanisms of epileptogenesis after rmCHI, as well as identify novel therapeutic targets for post-traumatic epilepsy. Funding: This work was part of the Football Players Health Study at Harvard, funded by a grant from the National Football League Players Association (NFLPA). The content is solely the responsibility of the authors. It does not necessarily represent the official views of Harvard Medical School, Harvard University, its affiliates, Boston Children’s Hospital, or NFLPA.