Effects of Isolated and Combined Exposure of the Brain and Lungs to a Laser-Induced Shock Wave(s) on Physiological and Neurological Responses in Rats

Abstract

Blast-induced traumatic brain injury (bTBI) has been suggested to be caused by direct head exposure and by torso exposure to a shock wave (thoracic hypotheses). It is unclear, however, how torso exposure affects the brain in real time. This study applied a mild-impulse laser-induced shock wave(s) (LISW[s]) only to the brain (Group 1), lungs (Group 2), or to the brain and lungs (Group 3) in rats. Because LISWs are unaccompanied by a dynamic pressure in principle, the effects of acceleration can be excluded, allowing analysis of the pure primary mechanism (the effects of a shock wave). For all rat groups, real-time monitoring of the brain and systemic responses were conducted for up to 1 h post-exposure and motor function assessments for up to seven days post-exposure. As reported previously, brain exposure alone caused cortical spreading depolarization (CSD), followed by long-lasting hypoxemia and oligemia in the cortices (Group 1). It was found that even LISW application only to the lungs caused prolonged hypoxemia and mitochondrial dysfunction in the cortices (Group 2). Importantly, features of CSD and mitochondrial dysfunction were significantly exacerbated by combined exposure (Group 3) compared with those caused by brain exposure alone (Group 1). Motor dysfunction was observed in all exposure groups, but their time courses differed depending on the groups. Rats with brain exposure alone exhibited the most evident motor dysfunction at one day post-exposure, and after that, it did not change much for up to seven days post-exposure. Alternatively, two groups of rats with lung exposure (Group 2 and Group 3) exhibited continuously aggravated motor functions for up to seven days post-exposure, suggesting different mechanisms for motor dysfunction caused by brain exposure and that caused by lung exposure. As for the reported thoracic hypotheses, our observations seem to support the volumetric blood surge and vagovagal reflex. Overall, the results of this study indicate the importance of the torso guard to protect the brain and its function.