Pharmacological Interventions to Reduce Electrophysiological Deficits Following Blast Traumatic Brain Injury

Blast-induced traumatic brain injury (bTBI) has been a health concern in both military and civilian populations due to recent military and geopolitical conflicts. Military service members are frequently exposed to single and repeated blasts throughout their training and deployment. As a result of blast exposures, military personnel report symptoms of various neurological and neurosensory deficits. Our group has previously reported decreased long term potentiation (LTP) following either single or repeated bTBI in a rat organotypic hippocampal slice culture (OHSC) model. LTP is a neuronal correlate for learning and memory and is a neurological metric that can be used to evaluate blast injury severity and the efficacy of therapeutic interventions. In the first aim of this thesis, we characterized LTP deficits following repeated bTBI to develop tolerance criteria for blast exposures. We did so by varying the blast injury severity, the inter-blast interval between blasts, and the recovery period following blast exposure. We determined that LTP deficits were compounded as a result of repeated mild bTBI. LTP deficits were attenuated with increasing inter-blast intervals and with increasing recovery periods after injury. Even after three repeated mild bTBIs, LTP spontaneously recovered after 6 days. In the second aim, we investigated the pathological changes in OHSCs following repeated blast exposures. Following injury, we observed robust microglial activation, evidenced by increased expression of the pro-inflammatory marker, CD-68, and decreased expression of the anti-inflammatory marker, CD-206. We also observed increased expression of MIP-1α, IL-1β, MCP-1, IP-10, and RANTES and decreased expression of IL-10 in the acute period after both single and repeated bTBI. Following partial depletion of microglia prior to injury, injury induced LTP deficits were significantly reduced. Lastly, treatment with a novel drug, MW-189, immediately after a repeated bTBI prevented LTP deficits. In the third aim, we investigated changes in inflammatory markers like cyclooxygenase (COX) and tested the efficacy of COX or prostaglandin receptor (EP3R) inhibitors in attenuating LTP deficits. We observed that expression of COX-2 increased 48 hours following repeated blast injury; however, COX-1 expression was unchanged. Following repeated bTBI, EP3R expression was upregulated and cyclic adenosine monophosphate (cAMP) concentration was decreased. Treatment of blast injured OHSCs with a COX-1 specific inhibitor, SC-560, a COX-2 specific inhibitor, rofecoxib, a pan-COX inhibitor, ibuprofen, or an EP3R inhibitor, L-798,106 improved LTP deficits. Delayed treatment with L-798,106 and ibuprofen also improved LTP deficits. Our data suggests that bTBI induced neuroinflammation may be partially responsible for the functional deficits that we have observed in blast-injured OHSCs. Additionally, we also conclude that COX and EP3R inhibition may be viable therapeutic strategies to reduce bTBI induced neurophysiological deficits. In the final aim, we investigated bTBI induced changes to the electrophysiological network of OHSCs. Following blast exposure, sham and injured OHSCs were administered increasing concentrations of bicuculline, a GABAA receptor antagonist. Doing so revealed an increase in connectivity and clustering coefficients in sham slices compared to injured slices. This suggested that the underlying neuronal network of injured slices was dysfunctional. Biologically, this dysfunction could be explained by the decreased expression of GABAA receptor α1 and α5 subunits. A loss of GABAA receptor expression or function may explain the electrophysiological network disruptions that we observed. More work will be required to determine how blast exposure decreases the expression of GABAA receptors and how these receptors may contribute to network deficits. This thesis has expanded upon the tolerance criteria for repeated blast exposures. These studies have also further characterized the pathological changes in microglial activation and explored promising therapeutic pathways that could be used to attenuate functional deficits. Lastly, this thesis has also provided novel ways to interrogate neuronal networks following blast injury, revealing subtle deficits that will need to be explored in more detail.