Conditioning effects of repetitive mild neurotrauma on motor function in an animal model of focal brain injury

A weight drop model of brain injury was used to determine the effects of repetitive mild brain injury on motor function, heat shock protein and glial fibrillary acidic protein expression in the anesthetized, adult male, Sprague-Dawley rat. Repetitive mild brain injury was produced when animals received a series of three mild injuries spaced three days apart. A separate group of repetitive mild injured animals also received a subsequent severe brain injury between three and five days after the last mild injury. All animals were trained on a beam-walking test prior to surgery. The mild, repetitive mild and repetitive mild plus severe brain injury groups showed no motor deficits in the beam-walking test, whereas the animals with only severe brain injury showed significant motor deficits (increase in number of footslips) in the beam-walking test that recovered within eight days after injury. Both repetitive mild plus severe injury and severe injury only animals had cortical necrotic cavities of similar size in the region of the hindlimb motor cortex. Both the repetitive mild and severe brain-injured animals had marked heat shock protein 27kDa and glial fibrillary acidic protein staining in the cerebral cortex. Fluoro-Jade, heat shock protein 27kDa and 72kDa labeling indicated that there were widespread effects on cortical, subcortical and spinal neurons and glial cells after repetitive mild brain injury. These results suggest that repetitive mild brain injury conditions the brain so that subsequent brain injury at the same site has no effect on motor function. Furthermore, repetitive mild injury-induced activation of processes distant to the primary injury site may have a role in activation of secondary sites involved in recovery of motor function.