Abstracts

Rationale: Objective: To evaluate magnetic resonance spectroscopy (MRS) as an in vivo tool to detect early biomarkers of post-traumatic epileptogenesis (PTEgen) in a rat traumatic brain injury (TBI) model. Background: In the rodent model, the earliest PTEgen stages are characterized by perilesional glutamate excitotoxicity and oxidative cell injury, which lead to continuing neuronal injury and death. Rising extracellular glutamate levels inhibit the transport of cystine into astrocytes via the glutamate/cystine antitransporter (SLC7A11 or xCT). This inhibition decreases the amount of cystine available for conversion to cysteine, the rate-limiting precursor for glutathione (GSH) synthesis. A high reduced:oxidized GSH ratio (GSH:GSSG) is essential for maintaining the neuronal redox state and mitigating oxidative stress after TBI. Notably, in the first days after injury, parvalbumin-positive inhibitory interneurons are injured preferentially in the post-traumatic cortex, as they are highly metabolically active and thus particularly sensitive to oxidative stress. An in vivo tool to serially measure regional metabolites, specifically glutamate and GSH remains an unmet need.Methods: Adult male rats underwent an epileptogenic lateral fluid percussion injury (FPI) and were studied by MRS on 7T Bruker rodent MRI at 1 hour with 2 x 2 x 2 mm voxels using PointRESolved Spectroscopy (PRESS) localization (echo time/repetition time: 20ms, 2.5 sec), water suppression, Fast Automatic Shimming Technique by Mapping Along Projections (FASTMAP) with 256 repetitions with regions of interest over lesional, perilesional and contralesional cortex. MRS data was analyzed using linear combinations model (LCmodel, Provencher), fitted to a basis set simulated at the same echo time, and normalized to a water reference. Metabolite measures with Cramer-Rao lower bound of less than 20% standard deviation were used for analysis. Rats were either sacrificed within 6 hours of TBI for tissue processing or received a follow-up MRS on Day 7 and then sacrificed for tissue studies. Tissue from lesional cortex was processed for high performance liquid chromatography (HPLC) to measure GSH and its metabolites.Results: Following acute injury (N=4), glutamate levels were elevated 20% above baseline as measured by MRS at day 0 and rose to 35% by day 7 in perilesional tissue. GSH, as measured by MRS was elevated 40 % above baseline acutely on Day 0 at the site of the lesion and subsequently decreased 12 % below baseline by day 7 (N = 4). This corresponded to a robust decrease in the ratio of reduced GSH to its oxidized form in lesional (10/0.05 nMol/mg) compared with contralateral (7.5/0.075 nMol/mg) cortex at Day 7 (p < 0.05), as measured by HPLC.Conclusions: Serial MRS measures may detect pathologic alterations in glutamate and GSH homeostasis from time of TBI through at least the first week of PTEgen. These findings highlight opportunities to intervene in the early post-TBI period with neuroprotective antioxidant agents that may maintain endogenous antioxidant reserve.