The role of tumor necrosis factor-alpha in diffuse axonal injury following fluid-percussive brain injury in rats.

Abstract

The immunolocalization of tumor necrosis factor-alpha (TNF alpha) after diffuse axonal injury (DAI) is demonstrated using a midline fluid percussion rat model (moderate brain injury of 1000 mm Hg was generated) and the effects of TNF alpha on the axolemmal permeability using horseradish peroxidase as a tracer. In addition, the accumulation of beta-amyloid precursor protein (beta-APP) was investigated, which has recently been shown to be a reliable marker for the diagnosis of DAI in cases with fatal head injury. TNF alpha levels in brain tissues from the impact site and the cortex including the corpus callosum, gradually increased during the first 1 h, rose to a maximal elevation at 3 h, gradually decreased at 6 h and decreased further at 24 h. Horseradish peroxidase (HRP) tracer experiments revealed that primary axonal damage appeared as early as 15 min after impact but rapidly recovered and that 1 h after impact, secondary axonal damage occurred in the corpus callosum and the brain stem. By immunoelectron microscopy it was seen that beta-APP accumulated in the axon from 1 h after impact demonstrating that there was functional axonal damage. TNF alpha reactions were detected in the lysosomes of microglia 30 min after impact and 1 h after impact these reactions were mainly detected in the glial cells (such as microglia, astrocytes and oligodendrocytes) in the corpus callosum and the brain stem. It is generally accepted that TNF alpha directly induces primary demyelination and oligodendrocyte apoptosis. Therefore, TNF alpha conveyed from the microglial cells is one cofactor contributing to the formation of the delayed axonal damage observed at these sites. The present study suggests that TNF alpha conveyed from the glial cells may contribute to the pathogenic mechanism of DAI formation following fluid percussive brain injury.