(L-449) Acute to Chronic Pain Following Blast Neurotrauma

Introduction: Traumatic brain injuries (TBI) have become more prevalent in the military population with over 80% of the reported TBIs being classified as mild [Siedhoff, et al. 2022]. However, even in mild cases the long-term effects can still appear such as chronic headaches/pain, behavioral deficits, and drug abuse. Veterans have reported chronic headaches/migraines that are persisting months and even years following repeated blast-tbis. According to Veteran Affairs, over 20% of post-9/11 servicemembers suffer from chronic headaches and migraines.  There is a need to examine the underlying mechanisms that contribute to the chronification of headaches following bTBI.  The aim of this study was to determine the influence that repeated blast exposures have on chronic pain outcomes and anxiety-like behaviors.

Materials and Methods: Male Sprague Dawley rats (n=10/group; 9 weeks of age at time of injury) were exposed to repeated blasts using an Advanced Blast Simulator at Virginia Tech’s Center for Injury Biomechanics. Anesthetized animals were subjected to three blast events with an inter-blast interval of one hour. Sham controls underwent all of the same procedures with the exception of the blast insults. Following bTBI, the animals were monitored for 4 and 12 weeks with regular testing to assess for chronic pain and anxiety-like behavior.  Assessments to examine facial pain sensitivity (hyperalgesia) were performed at 48-hours and weekly following blast injury. Facial hyperalgesia testing was performed using Von Frey filaments (bending force ranging from 10 to 300 g), the tip of the monofilament was applied with increasing force to the periorbital region of the face for 1 second, 3 times per filament until a response was seen. Anxiety-like behaviors (open field test) were tested monthly, animals were recorded in the open field arena for 5-minutes each. Testing for social deficits (triple-chamber) was performed once at the end of the respective timepoints. For the triple chamber test, animals had a 10-minute habituation period before being introduced to a stranger rat for 10 minutes and then being introduced to a second stranger rat for another 10 minutes. Brains were collected and stained through immunohistochemistry with GFAP, Substance P, CGRP, and TMEM-119. All statistical analyses were performed using GraphPad prism version 9 software, with statistical differences between groups and multiple time points being assessed. 

Results, Conclusions, and Discussions: The blast insult introduced a positive average overpressure of 19 ± 1.87 psi with a positive phase duration of 2.30 ± 0.06 ms. Chronic pain and anxiety-like behaviors were observed in animals following repeated blast exposure. Starting 48-hours following blasts, there was a significant decrease in maximum force response to the Von Frey filaments of bTBI animals compared to sham animals indicating facial sensitivity during hyperalgesia testing (p < 0.05). At 4-weeks, the blast group showed significant levels of hyperawareness based on in-zone frequency (p < 0.05). At 12-weeks, blast animals showed significant sociability deficits based on time spent in the chamber with the stranger and around the sniff zone of the stranger rat (p < 0.05). At 12-weeks, there were increased levels of Substance P and TMEM-119 in the cortex in blast compared to sham based on signficant integrative density and area fraction (p < 0.05). There were also increased concentration of Substance P and CGRP in the pons region in blast compared to sham based on significant integrative density and area fraction (p < 0.05).
These findings suggest that acute pain was detected in blast animals at 48-hours post blast then developed into chronic pain by the week 2 time-point. Anxiety-like behaviors were also demonstrated in a similar timeframe. There was no significant astrogliosis at 12-weeks, however substance P has shown to increase the concentration of reactive microglia at this same timepoint. Future studies will be expanding on the behavior assessments to further delineate the development of chronic pain such as hot/cold plate. There is also limited research in females and thus they will be an included population in assessing how blast neurotrauma affects the body both behaviorally and pathologically.
Acknowledgements (Optional): The authors would like to thank the Center for Injury Biomechanics at Virginia Tech for the support.

References (Optional): H. Siedhoff et al., “Perspectives on Primary Blast Injury of the Brain: Translational Insights Into Non-inertial Low-Intensity Blast Injury,” Frontiers in Neurology, vol. 12, Jan. 2021, doi: 10.3389/fneur.2021.818169.