Cognitive Function Following Traumatic Brain Injury: Effects of Injury Severity and Recovery Period in a Parasagittal Fluid-Percussive Injury Model

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Abstract
Previous work in this laboratory has demonstrated that rats show substantial deficits on the cued and hidden versions of the Morris water maze, as well as an apparent time-dependent recovery over a period of months, following moderate parasagittal fluid-percussion (FP) injury. However, the longitudinal nature of those studies precluded definitive statements regarding recovery because of the possible confound of practice-dependent improvements in performance. The present experiments were undertaken to address this issue and to investigate more closely the relationship between impact severity and posttraumatic learning/memory deficits, which have not been examined thoroughly in this model. Separate groups of rats were subjected to mild (1.1 to 1.4 atm), moderate (1.8 to 2.1 atm), or severe (2.2 to 2.7 atm) FP injury (or sham surgery) and tested on several water maze tasks at either 5 days or 8 weeks after injury. Moderately and severely injured animals showed impairment in acquisition of the hidden platform task at both time points. Cued platform task performance was impaired significantly in severely injured animals 8 weeks after insult. Mildly injured animals exhibited no significant deficits on either task at either time point. The results indicate that deficits on the hidden platform task are more robust than those on the cued platform task, and that performance on both tasks is dependent on injury severity. They also indicate that the learning/memory deficits in this model are relatively enduring, suggesting that the model is a reasonable one for assessing potential treatment regimens.