Exposure of the thorax to a sublethal blast wave causes a hydrodynamic pulse that leads to perivenular inflammation in the brain

Abstract

Traumatic brain injury (TBI) caused by an explosive blast (blast-TBI) is postulated to result, in part, from transvascular transmission to the brain of a hydrodynamic pulse (a.k.a., volumetric blood surge, ballistic pressure wave, hydrostatic shock, or hydraulic shock) induced in major intrathoracic blood vessels. This mechanism of blast-TBI has not been demonstrated directly. We tested the hypothesis that a blast wave impacting the thorax would induce a hydrodynamic pulse that would cause pathological changes in the brain. We constructed a Thorax-Only Blast Injury Apparatus (TOBIA) and a Jugular-Only Blast Injury Apparatus (JOBIA). TOBIA delivered a collimated blast wave to the right lateral thorax of a rat, precluding direct impact on the cranium. JOBIA delivered a blast wave to the fluid-filled port of an extracorporeal intravenous infusion device whose catheter was inserted retrograde into the jugular vein, precluding lung injury. Long Evans rats were subjected to sublethal injury by TOBIA or JOBIA. Blast injury induced by TOBIA was characterized by apnea and diffuse bilateral hemorrhagic injury to the lungs associated with a transient reduction in pulse oximetry signals. Immunolabeling 24 h after injury by TOBIA showed up-regulation of tumor necrosis factor alpha, ED-1, sulfonylurea receptor 1 (Sur1), and glial fibrillary acidic protein in veins or perivenular tissues and microvessels throughout the brain. The perivenular inflammatory effects induced by TOBIA were prevented by ligating the jugular vein and were reproduced using JOBIA. We conclude that blast injury to the thorax leads to perivenular inflammation, Sur1 up-regulation, and reactive astrocytosis resulting from the induction of a hydrodynamic pulse in the vasculature.

Keywords: ED-1; GFAP; Sur1, TNF-α; blast-TBI; perivenular inflammation.

FIG. 1.

Thorax-Only Blast Injury Apparatus (TOBIA) and the blast wave produced by TOBIA. (A and B) Overview (A) and close-up view (B) of TOBIA with the rat positioned for blast injury. BDC, blast dissipation chamber; BDCTI, blast dissipation chamber thorax interface. (C) Blast wave produced by TOBIA shown at low and high temporal resolution; note that the specific characteristics of the blast wave, including the fast initial peak overpressure (1), the underpressure (2), and the secondary slower overpressure (3), resemble closely the characteristic features of a free-field explosive blast (see Fig. 2 in Ling and colleagues). Color image is available online at www.liebertpub.com/neu

FIG. 2.

Blast injury induced by Thorax-Only Blast Injury Apparatus (TOBIA) is associated with pulmonary, but not brain, hemorrhagic injury. (A and B) Box plot of duration of apnea (A) and relationship between apnea duration and O₂ saturation, measured by pulse oxymetry, in sham-injured (0 sec apnea) and TOBIA-injured rats 1 min after blast (B); data from 5 and 13 rats in the sham and TOBIA groups, respectively. Box-plot symbols: box, 25th and 75th percentiles;×, 1st and 99th percentiles; line, median; small square, mean; ρ, Pearson's correlation coefficient. (C) Time course of O₂ saturation (mean±standard error), measured by pulse oxymetry, in sham-injured (empty squares) and TOBIA-injured rats (empty circles); data from the same rats as in (A). *p<0.05; **p<0.01. (D–F) Images of whole lungs after perfusion (D) and in hematoxylin and eosin sections after blast exposure from TOBIA (E) or after sham injury (F). (G and H) Dorsal (G) and mid-sagittal (H) views of perfused brain after blast exposure from TOBIA. In (D–H), the images shown are representative of findings in the same rats as in (A). Color image is available online at www.liebertpub.com/neu

FIG. 3.

Perivenular neuroinflammation induced by Thorax-Only Blast Injury Apparatus (TOBIA). (A–C) Coimmunolabeling for TNF-α (green) and laminin (red) showing up-regulation of TNF-α in perivenular tissues after blast injury induced by TOBIA in hippocampus (HC) (A) and in hypothalamus (HT) (B), but not in a sham-injured rat (C). (D) Bar graph showing the abundance of perivenular TNF-α in cortex (CTX), hippocampus, and hypothalamus, compared to sham (sham data from three regions combined). All bars, 50 μm; % ROI, percent region of interest. **p<0.01; five veins per region from 5 rats in the sham and TOBIA groups. (E–G) Coimmunolabeling for TNF-α (green) and ED-1 (red) showing up-regulation of ED-1 in the wall of vessels that also display abundant TNF-α up-regulation after blast injury induced by TOBIA. Arrows point to ED-1-positive cells outside of the endothelial layer, including one with a thin process typical of microglia (G). Data shown are representative of findings in the same rats as in (A–D). TNF-α, tumor necrosis factor alpha. Color image is available online at www.liebertpub.com/neu

FIG. 4.

Perivenular neuroinflammation induced by Thorax-Only Blast Injury Apparatus (TOBIA) is associated with up-regulation of Sur1. (A–C) Coimmunolabeling for laminin (green) and Sur1 (red) along with superimposed images (right), showing up-regulation of Sur1 in veins and perivenular tissues in cortex (CTX) (A), hippocampus (HC) (B), and hypothalamus (HT) (C) after blast injury induced by TOBIA. (D–F) Superimposed images of sections coimmunolabeled for laminin (green) and Sur1 (red) showing absence of Sur1 in perivenular tissues in cortex (D), hippocampus (E), and hypothalamus (F) after sham injury. (G) Bar graph showing the abundance of Sur1 in perivenular tissues of the regions indicated after blast injury induced by TOBIA versus sham injury (sham data from three regions combined); 5 veins per region from 5 and 8 rats in the sham and TOBIA groups, respectively. % ROI, percent region of interest. **p<0.01. (H) Section of cortex immunolabeled for Sur1 showing Sur1 up-regulation in elongated structures consistent with microvessels. Data shown in (H) are representative of findings in 3 rats. Sur1, sulfonylurea receptor 1. Color image is available online at www.liebertpub.com/neu

FIG. 5.

Perivenular neuroinflammation induced by Thorax-Only Blast Injury Apparatus (TOBIA) requires patency of the internal jugular vein (IJ). (A) Hydrodynamic pulse recorded in the jugular vein produced by TOBIA shown at low and high temporal resolution; note the fast pressure transient with general features similar to those of the blast wave recorded in air (see Fig. 1C). (B and C) Coimmunolabeling for GFAP (green) and Sur1 (red), showing prominent up-regulation of Sur1 in veins, and up-regulation of GFAP in perivenular tissues of the hippocampus on the side with a patent IJ (B), compared to weak expression on the side with a ligated IJ, after blast induced by TOBIA; asterisks denote veins. (D) Bar graph showing a quantitative analysis of Sur1 and GFAP in or around hippocampal veins from the side of the patent (P-IJ) versus the ligated (L-IJ) internal jugular. **p<0.01; ***p<0.001; 21 and 30 veins from patent-IJ versus ligated-IJ sides, respectively, in 3 rats. GFAP, glial fibrillary acidic protein; Sur1, sulfonylurea receptor 1. Color image is available online at www.liebertpub.com/neu

FIG. 6.

Jugular-Only Blast Injury Apparatus (JOBIA). (A and B) Views of the fluid-filled vascular access port used for retrograde catheterization of the internal jugular vein. When the port—instead of the thorax—is exposed to a blast from Thorax-Only Blast Injury Apparatus (TOBIA), the catheter delivers a hydrodynamic pulse to the internal jugular vein without injuring the lungs. (C) Overview of JOBIA with the rat positioned behind a barrier to protect it from direct blast exposure. BDC, blast dissipation chamber; VP, vascular port. (D) Hydrodynamic pulse recorded from the end of the catheter of the vascular access port when the port is exposed to a blast, shown at low and high temporal resolution; note the fast pressure transient with general features similar to those of the hydrodynamic pulse recorded in the internal jugular vein with blast exposure of the thorax (see Fig. 5A). Color image is available online at www.liebertpub.com/neu

FIG. 7.

Perivenular neuroinflammation induced by Thorax-Only Blast Injury Apparatus (TOBIA) is reproduced by Jugular-Only Blast Injury Apparatus (JOBIA). (A and B) Hippocampal sections immunolabeled for Sur1 showing up-regulation of Sur1 in veins of the perforant pathway by the hydrodynamic pulse induced by JOBIA (A), but not on the contralateral side (B). Arrows point to veins in the perforant pathway. DG, dentate gyrus; CA3, cornu ammonis region 3. (C–E) Superimposed images of sections coimmunolabeled for laminin or Sur1 (green), and TNF-α or GFAP (red), as indicated, of hippocampal tissues ipsilateral (C and D) and contralateral (E) to the hydrodynamic pulse induced by JOBIA, showing prominent neuroinflammation induced by the hydrodynamic pulse. (F) Bar graph showing the abundance of GFAP and TNF-α in perivenular tissues of the perforant pathway associated with the hydrodynamic pulse induced by JOBIA. % ROI, percent region of interest; 15 contralateral and nine ipsilateral veins from 3 rats. **p<0.01; ***p<0.001. Sur1, sulfonylurea receptor 1; GFAP, glial fibrillary acidic protein; TNF-α, tumor necrosis factor alpha. Color image is available online at www.liebertpub.com/neu

FIG. 8.

Summary hypothesis of events induced by Thorax-Only Blast Injury Apparatus (TOBIA) and Jugular-Only Blast Injury Apparatus (JOBIA) that result in cerebral perivenular inflammation. The blast waves produced by cartridge detonation with both TOBIA and JOBIA are identical (black arrows). The blast wave in air impacts a fluid-filled structure (gray shading) that is oriented orthogonally—the superior (sup.) vena cava in TOBIA and the access port in JOBIA—thereby generating a hydrodynamic pulse (green arrows) that propagates rostrally and can be monitored in the jugular (jug.) vein by a high-frequency pressure transducer. The waveforms actually recorded with TOBIA and JOBIA are shown at identical scales (the first 1000 msec and peak overpressures of 50–60 mm Hg). TOBIA and JOBIA produce similar fast initial peak overpressures, but only TOBIA generates a strong negative underpressure, possibly as a result of the downward thrust of the diaphragm (small black arrows). The peak overpressures, which are similar with TOBIA and JOBIA, are believed to be responsible for depositing kinetic energy (small red arrows) in cerebral veins that results in the similar biological responses of perivenular inflammation with both devices. Color image is available online at www.liebertpub.com/neu