Repeat traumatic brain injury in the juvenile rat is associated with increased axonal injury and cognitive impairments

Abstract

Among the enormous population of head-injured children and young adults are a growing subpopulation who experience repeat traumatic brain injury (RTBI). The most common cause of RTBI in this age group is sports-related concussions, and athletes who have experienced a head injury are at greater risk for subsequent TBI, with consequent long-term cognitive dysfunction. While several animal models have been proposed to study RTBI, they have been shown to either produce injuries too severe, were conducted in adults, involved craniotomy, or failed to show behavioral deficits. A closed head injury model for postnatal day 35 rats was established, and single and repeat TBI (1-day interval) were examined histologically for axonal injury and behaviorally by the novel object recognition (NOR) task. The results from the current study demonstrate that an experimental closed head injury in the rodent with low mortality rates and absence of gross pathology can produce measurable cognitive deficits in a juvenile age group. The introduction of a second injury 24 h after the first impact resulted in increased axonal injury, astrocytic reactivity and increased memory impairment in the NOR task. The histological evidence demonstrates the potential usefulness of this RTBI model for studying the impact and time course of RTBI as it relates to the pediatric and young adult population. This study marks the first critical step in experimentally addressing the consequences of concussions and the cumulative effects of RTBI in the developing brain.

Fig. 1

RTBI model. Anesthetized animal is placed against a wooden L-shaped frame secured within a student stereotaxic frame.

Fig. 2

β-APP immunohistochemistry of the grey-white matter junction under the impact site in single TBI (a, c) and RTBI (b, d) animals at 1 day after the last injury. Inset: region from which the photos were taken.

Fig. 3

Average number of β-APP-positive blebs from 3 regions of the grey-white matter junction in 3 coronal sections of single TBI and RTBI animals. RTBI animals showed a significantly greater number of β-APP-positive immunolabeling than single-impact animals. ∗∗ p < 0.01.

Fig. 4

GFAP-positive labeling in the grey-white matter junction at the site of impact in ipsilateral sham (a), single impact (b), ipsilateral RTBI (c) and contralateral RTBI (d).

Fig. 5

Average number of entries (a), time spent (c) and distance traveled per zone (d) during the open field test 1 day after the last injury. Columns: means. Whiskers: SEM. b Diagram of the zones.

Fig. 6

a Diagram of the novel object recognition task. b, c Percent time spent with novel object with intertrial period of 1 h (b) and 24 h (c) at day 2 after injury. ∗∗ p < 0.05 relative to sham animals.