Impulse noise and brain injury. An experimental study in rats

Abstract

Abstract. Pressure waves, experienced as impulse noise, affect many body organs, although most attention has been paid to the inner ear. Impulse noise is produced by e.g. explosions, heavy weapons, and airbag deployment. There are reports on neurological handicaps, but little is known about the effects on the central nervous system (CNS) and threshold levels for brain damage have not been assessed. The focus of this thesis was to investigate whether exposure to impulse noise affected the CNS. The impulse noise was generated by detonation of explosives. The effect on the brain was demonstrated with immunohistochemical markers for the neurofilament heavy subunit, _ -amyloid precursor protein (_-APP), immediate early genes (IEGs), c-Jun, c-Fos, and c-Myc. Reactive astrocytes and microglial cells were also visualized. Apoptosis was demonstrated with the TUNEL technique. Possible induction of leakage of marker proteins from neurons and glial cells was as well investigated.Neurons were damaged or killed in brain regions important for higher functions such as memory and learning. Immediate early gene products (IEG) appeared in increased amounts in theseneurons, which also contained _-APP, in accordance with events in posttraumatic and degenerating brain tissue. Furthermore, the neuronal cytoskeleton was deranged as reflected by the altered phosphorylated heavy subunit of neurofilament (p-NFH) pattern and thus the capability for the neurons to perform normal functions and to communicate. Some of these changes might be reversible, but there was a certain loss of neurons by apoptosis in several brain regions. Microglial cells andastrocytes were affected, either primarily by the impulse noise or secondarily due to the nerve cell damage. Brain tissue seemed to protect against impulse noise, as the most extensive effects weredemonstrable in brain regions facing the exposure source. Dose response conditions were revealed for the impulse noise effects on the brain. The time course of changes in the parameters was in agreement with that reported for posttraumatic brain reactions after other types of brain injury. Additionally, both neurons and astrocytes became transiently leaky to cytosolic proteins.The studies show that impulse noise above certain levels affected the brain, inducing reactive and degenerative changes.