Assessment of the effects of acute and repeated exposure to blast overpressure in rodents: toward a greater understanding of blast and the potential ramifications for injury in humans exposed to blast

Abstract

Mild traumatic brain injury (mTBI) resulting from exposure to improvised explosive devices (IEDs) has fueled a requirement to develop animals models that mirror this condition using exposure to blast overpressure (BOP). En route to developing a model of repeated exposure to BOP we sought to initially characterize the effects of acute BOP exposure in rodents, focusing specifically on the levels of BOP exposure that produced clinical mTBI symptoms. We first measured BOP effects on gross motor function on a balance beam. Separate groups of unanesthetized rats were exposed (in different orientations) to 36.6, 74.5, and 116.7 kPa BOP exposure inside a pneumatically driven shock tube. Results demonstrated that rats exposed to 116.7 kPa demonstrated transient alterations or loss of consciousness indicated by a transient loss of righting and by increased latencies on the balance beam. The 116.7 kPa exposure was the threshold for overt pathology for acute BOP exposure with approximately 30% of rats presenting with evidence of subdural hemorrhage and cortical contusions. All animals exposed to 116.7 kPa BOP manifested evidence of significant pulmonary hemorrhage. Anterograde memory deficits were observed in rats exposed to 74.5 kPa facing the BOP wave and rats exposed to 116.7 kPa in the lateral (side) orientation. We next assessed repeated exposure to either lateral or frontal 36.6 kPa BOP in anesthetized rats, once per day for 12 days. Results showed that repeated exposure in the frontal, but not side, orientation to the BOP wave produced a transitory learning deficit on a Morris water maze task as shown by significantly longer latencies to reach the submerged platform in the second and third blocks of a four block session. Implications of these data are discussed in relation to the manifestation of mTBI in military personnel exposed to IEDs. Finally, we suggest that there are multiple types of long-term brain injury from blast exposure.

Keywords: blast injury; concussion; improvised explosive devices; loss or alteration of consciousness; memory; orientation; pathology; traumatic brain injury.

Figure 1

(A–C) Balance beam walking after exposure to 36.6, 74.5, or 116.7 kPa: mean and SEM latency to transit 200 cm on a 2.5-cm thick balance beam. Once asymptotic performance baseline (B) performance was achieved rats were exposed to 36.6, 74.5, or 116.7 kPa BOP in three different orientation conditions (facing, sideways or facing away from the blast wave) and were subsequently assessed on the balance beam at short and long intervals after exposure.

Figure 2

(A) Retrograde effects of BOP: median (and standard interquartile range), total time on the white (TTW) side of the black–white shuttle box during a 10 min test session 24 h after passive avoidance training followed immediately by exposure to varying intensities of BOP. BOP exposed rats were either facing or sideways orientation to the blast wave. (B) Anterograde Effects of BOP: median (and standard interquartile range), TTW side of the black–white shuttle box during a 10 min test session 24 h after passive avoidance training was administered after exposure to varying intensities of BOP. BOP exposed rats were either facing or sideways orientation to the blast wave. Significance is with a Mann–Whitney U pairwise test for non-parametric data with significance criteria of p < 0.05.

Figure 3

Effects of repeated BOP exposure on the Morris water maze: mean latency time to find the submerged platform in four blocked trials presented in a single day. BOP groups were exposed to 36.6 kPa BOP in a shock tube while under anesthesia once per day for 12 days. Training on the Morris water maze task occurred 24 h after the 12th BOP exposure. *Indicates significance at p < 0.05.

Figure 4

APP immunohistochemistry – frontal overpressure exposure: APP immunohistochemistry is seen with animals exposed to 12 sessions of 36.6 kPa blast overpressure exposure (A,B,E,F,I,J,M,N) versus SHAM injured controls (C,D,G,H,K,L,O,P). Photomontages of multiple photomicrographs of frontal (A,C), mid (E,G), and posterior (I,K) portions of the cerebrum are shown in coronal section. Sagittal photomicrograph of the brainstem is seen in plates (M,O). Magnifications of corresponding boxes within photomontages are seen in plates (B,D,F,H,J,L,N,P). Within all of the plates, no evidence of traumatic injury or difference between experimentally injured and SHAM injured controls is seen.