Abstracts

Rationale:
Neuronal hyperactivation after traumatic brain injury (TBI) due to excessive glutamate release from dead and dying neurons coupled with glutamate transporter downregulation1 contributes to excitotoxic damage well beyond the initial insult and often leads to post-traumatic epilepsy (PTE). We have shown that TBI in rats results in progressive loss of cortical inhibition due to increased oxidative stress, and preferential damage to highly metabolically active parvalbumin-positive inhibitory interneurons (PVI) and their supportive extracellular matrix, the peri-neuronal nets (PNN).2, 3 Given the prominent role of oxidative stress in post-TBI pathophysiology, we now test whether N-acetyl cysteine (NAC) mitigates the extent of injury and markers of epileptic activity on EEG in the rat fluid percussion injury (FPI) model.  References: 1. Goodrich, G.S., et al., J Neurotrauma, 2013. 30(16): p. 1434-41. 2. Hsieh, T.H., et al., Cereb Cortex, 2017. 27(12): p. 5509-5524. 3. Hameed, M.Q., et al., Cereb Cortex, 2019. 29(11): p. 4506-4518.
Method:
Adult male rats received moderate FPI (n=52), and were divided into 2 groups: FPI-NAC, to receive NAC (100mg/kg/day) in drinking water, or FPI-water, plain water only, for 6 weeks after injury. To ensure accurate dosing immediately post-injury, NAC (FPI-NAC) or saline (FPI-water) was administered via intraperitoneal injection for the first 2 days after injury. A sham group (n=24) received all surgical procedures except FPI and received plain water. Reduced and oxidized cortical glutathione disulfide (GSH, GSSG) levels were measured 3 and 6 weeks after injury using HPLC (n=5-6 per group). A third cohort (n=7-8 per group) was systemically perfused with paraformaldehyde at 6 weeks for immunostaining of PVIs and PNNs. A fourth cohort, (n=7 per group) was implanted with wireless EEG transmitters and recorded for weekly recording.
Results:
GSH:GSSG indicates the redox state, with higher ratios revealing a more reducing environment and lower ratios being more oxidizing. We observed a decrease in the ratio 3 weeks (p< 0.05) and 6 weeks post-injury (p< 0.05) in FPI-water rats compared to sham. NAC treatment normalized these changes in the ratio both 3 (n.s) and 6 weeks (n.s) after FPI compared to sham rats. PVI count in perilesional cortex was significantly reduced 6 weeks after injury in FPI-water rats compared to both sham (p< 0.05) and FPI-NAC (p< 0.05), and NAC treatment normalized PVI counts relative to sham rats (n.s). There was no impact of FPI and NAC rescue on PVI count in contralesional cortex. PNN+ cell count was significantly reduced in both perilesional and contralesional cortex after FPI in untreated rats compared to sham controls, with a more pronounced perilesional effect (peri: p< 0.01, contra: p< 0.05). The perilesional reduction of PNN+ cell count was also blocked by NAC treatment. Injury-associated gain of power in the EEG delta band (0.5-4 Hz) 1, 3 and 6 weeks after TBI in FPI-water rats compared to sham controls (p< 0.0001) was also absent with NAC treatment.
Conclusion:
While a direct effect on seizures was not observed in our rats, the preserved PVI and PNN counts, and mitigation of gain of EEG delta power indicate a capacity for subacute NAC treatment to preserve cortical inhibitory tone and interrupt post-traumatic epileptogenesis.
Funding:
:This work was funded by the National Institute of Neurological Disorders and Stroke (NINDS) of the National Institutes of Health (NIH) 5R01NS088583 (AR) and the Boston Children’s Hospital Translational Research Program (AR)