Blast-induced neurotrauma leads to neurochemical changes and neuronal degeneration in the rat hippocampus

Methodology and evaluation of intracranial pressure response in rats exposed to complex shock waves

Studies on blast neurotrauma have focused on investigating the effects of exposure to free-field blast representing the simplest form of blast threat scenario without considering any reflecting surfaces. However, in reality personnel are often located within enclosures or nearby reflecting walls causing a complex blast environment, that is, involving shock reflections and/or compound waves from different directions. The purpose of this study was to design a complex wave testing system and perform a preliminary investigation of the intracranial pressure (ICP) response of rats exposed to a complex blast wave environment (CBWE). The effects of head orientation in the same environment were also explored. Furthermore, since it is hypothesized that exposure to a CBWE would be more injurious as compared to a free-field blast wave environment (FFBWE), a histological comparison of hippocampal injury (cleaved caspase-3 and glial fibrillary acidic protein (GFAP)) was conducted in both environments. Results demonstrated that, regardless of orientation, peak ICP values were significantly elevated over the peak static air overpressure. Qualitative differences could be noticed compared to the ICP response in rats exposed to simulated FFBWE. In the CBWE scenario, after the initial loading the skull/brain system was not allowed to return to rest and was loaded again reaching high ICP values. Furthermore, results indicated consistent and distinct ICP-time profiles according to orientation, as well as distinctive values of impulse associated with each orientation. Histologically, cleaved caspase-3 positive cells were significantly increased in the CBWE as compared to the FFBWE. Overall, these findings suggest that the geometry of the skull and the way sutures are distributed in the rats are responsible for the difference in the stresses observed. Moreover, this increase stress contributes to correlation of increased injury in the CBWE.

Screening of biochemical and molecular mechanisms of secondary injury and repair in the brain after experimental blast-induced traumatic brain injury in rats

Abstract Explosive blast-induced traumatic brain injury (TBI) is the signature insult in modern combat casualty care and has been linked to post-traumatic stress disorder, memory loss, and chronic traumatic encephalopathy. In this article we report on blast-induced mild TBI (mTBI) characterized by fiber-tract degeneration and axonal injury revealed by cupric silver staining in adult male rats after head-only exposure to 35 psi in a helium-driven shock tube with head restraint. We now explore pathways of secondary injury and repair using biochemical/molecular strategies. Injury produced ∼25% mortality from apnea. Shams received identical anesthesia exposure. Rats were sacrificed at 2 or 24 h, and brain was sampled in the hippocampus and prefrontal cortex. Hippocampal samples were used to assess gene array (RatRef-12 Expression BeadChip; Illumina, Inc., San Diego, CA) and oxidative stress (OS; ascorbate, glutathione, low-molecular-weight thiols [LMWT], protein thiols, and 4-hydroxynonenal [HNE]). Cortical samples were used to assess neuroinflammation (cytokines, chemokines, and growth factors; Luminex Corporation, Austin, TX) and purines (adenosine triphosphate [ATP], adenosine diphosphate, adenosine, inosine, 2'-AMP [adenosine monophosphate], and 5'-AMP). Gene array revealed marked increases in astrocyte and neuroinflammatory markers at 24 h (glial fibrillary acidic protein, vimentin, and complement component 1) with expression patterns bioinformatically consistent with those noted in Alzheimer's disease and long-term potentiation. Ascorbate, LMWT, and protein thiols were reduced at 2 and 24 h; by 24 h, HNE was increased. At 2 h, multiple cytokines and chemokines (interleukin [IL]-1α, IL-6, IL-10, and macrophage inflammatory protein 1 alpha [MIP-1α]) were increased; by 24 h, only MIP-1α remained elevated. ATP was not depleted, and adenosine correlated with 2'-cyclic AMP (cAMP), and not 5'-cAMP. Our data reveal (1) gene-array alterations similar to disorders of memory processing and a marked astrocyte response, (2) OS, (3) neuroinflammation with a sustained chemokine response, and (4) adenosine production despite lack of energy failure-possibly resulting from metabolism of 2'-3'-cAMP. A robust biochemical/molecular response occurs after blast-induced mTBI, with the body protected from blast and the head constrained to limit motion.

Changes in mouse cognition and hippocampal gene expression observed in a mild physical- and blast-traumatic brain injury

Warfare has long been associated with traumatic brain injury (TBI) in militarized zones. Common forms of TBI can be caused by a physical insult to the head-brain or by the effects of a high velocity blast shock wave generated by the detonation of an explosive device. While both forms of trauma are distinctly different regarding the mechanism of trauma induction, there are striking similarities in the cognitive and emotional status of survivors. Presently, proven effective therapeutics for the treatment of either form of TBI are unavailable. To be able to develop efficacious therapies, studies involving animal models of physical- and blast-TBI are required to identify possible novel or existing medicines that may be of value in the management of clinical events. We examined indices of cognition and anxiety-like behavior and the hippocampal gene transcriptome of mice subjected to both forms of TBI. We identified common behavioral deficits and gene expression regulations, in addition to unique injury-specific forms of gene regulation. Molecular pathways presented a pattern similar to that seen in gene expression. Interestingly, pathways connected to Alzheimer's disease displayed a markedly different form of regulation depending on the type of TBI. While these data highlight similarities in behavioral outcomes after trauma, the divergence in hippocampal transcriptome observed between models suggests that, at the molecular level, the TBIs are quite different. These models may provide tools to help define therapeutic approaches for the treatment of physical- and blast-TBIs. Based upon observations of increasing numbers of personnel displaying TBI related emotional and behavioral changes in militarized zones, the development of efficacious therapies will become a national if not a global priority.

Effects of blast-induced neurotrauma on the nucleus accumbens

Blast-induced neurotrauma (BINT) leads to deterioration at the cellular level, with adverse cognitive and behavioral outcomes. The nucleus accumbens (NAC) plays an important role in reward, addiction, aggression, and fear pathways. To identify the molecular changes and pathways affected at an acute stage in the NAC, this study focused on a time course analysis to determine the effects of blast on neurochemical and apoptotic pathways. By using a rodent model of BINT, acute damage to the NAC was assessed by proton magnetic resonance spectroscopy (¹H-MRS), high-performance liquid chromatography, immunohistochemistry, and Western blotting. The results demonstrated ongoing neuroprotective effects from elevated levels of Bcl-2, an antiapoptotic marker, at 24 hr and N-acetyl aspartate glutamate at 48 hr following blast exposure. Selective loss of serotonin levels at 24 hr, increased levels of inflammation (elevated glycerophosphocholine at 48 and 72 hr), and increased levels of glial fibrillary acidic protein were also observed at 24 and 48 hr, leading to disruptive energy status. Furthermore, active cell death was indicated by the increased levels of the apoptotic marker Bax, decreased actin levels, and signs excitotoxicity (glutamate/creatine). In addition, increased levels of caspase-3, an apoptotic marker, confirm active cell death in NAC. It is hypothesized that blast overpressure causes inflammation and neurochemical changes that trigger apoptosis in NAC. This cascade of events may lead to stress-related behavioral outcomes and psychiatric sequelae.