Mild traumatic brain injury (MTBI) leads to spatial learning deficits

Abstract

Primary objective: The aim of this study was to investigate the effect of mild and severe TBI on young male Wistar rats' spatial learning.

Research design: Randomized repeated measure experimental design was used to examine spatial learning in three independent animal groups.

Methods and procedures: Twenty-four (severe n = 9, mild n = 8, sham n = 7) male rats were included in the study. Animals received controlled mild (1.5 mm), severe (2.5 mm) cortical impact injury or sham surgery. Spatial learning was assessed daily using a modified Morris water maze test, 20 days post-trauma, for 5 consecutive days. Percentage time travelled within each quadrant and escape latency were calculated. All animals' hippocampal brain regions were examined post-injury using neuron (MAP2) and pre-synaptic protein (Synaptophysin) biomarkers.

Main outcomes and results: It took the animals with mild injury until day 3 to reach the platform; and animals with mild and severe injury spent significantly less time in the target quadrant than the sham. The hippocampal neuron numbers differed proportionately between animals with severe and mild injury, but the percentage of synaptophysin density was significantly less in the dentate gyrus of both animals with mild and severe injury than sham group.

Conclusion: Persistent spatial learning deficits exist after mild TBI; these deficits appear equivalent to deficits exhibited after a more severe injury.

Figure 1

Different platform locations within the North East quadrant (target quadrant). The platform or target location was moved every trial within the North East quadrant in the fourlocations shown in the figure. Location 1, 2, 3 and 4 were fixed for all animals.

Figure 2

Mean percentage lesion volume by group. The mean percentage lesion volume of animals with severe injury was 15.95% ±1.69 (n = 8). The mean percentage lesion volume of animals with mild injury was 4.85% ±0.58 (n = 7). The difference between the two groups is statistically significant (p<0.05).

Figure 3

Representative brain sections of each animal group. Images show significantly smaller cortical and adjacent structures damage between the groups. (a) Sham. (b) Mild injury. (c) Severe injury.

Figure 4

Mean swim speed to target location per group, mild, severe and sham, in a water maze task over 5 testing days. The swim speed of the three animal groups did not differ significantly from day 1–5. The animals with severe injury maintained the same speed from day 1–5; however, the animals with mild injury and sham reached the platform faster on day 5. Nonetheless, the difference between the three groups was not statistically significant.

Figure 5

Mean escape latency (seconds) and percentage time spent in target quadrant per group; mild, severe and sham, in a water maze task over 5 testing days. The difference in escape latency between animals with mild injury and sham animals on days 4 and 5 was statistically significant (p < 0.05). The difference in escape latency between animals with severe injury and sham groups on days 4 and 5 is statistically significant (p < 0.05). The difference in escape latency between animals with severe and animals with mild injury is statistically significant on day 5 (p < 0.05). The difference in percentage time spent in the target quadrant between animals with severe injury and sham animals on days 2, 3, 4 and 5 was statistically significant (p < 0.05). The difference in percentage time spent in the target quadrant between animals with mild injury and sham animals on days 2, 3 and 5 was statistically significant (p <0.05). The difference in percentage time spent in the target quadrant between animals with severe injury and animals with mild injury was statistically significant on days 1 and 3 (p < 0.05).

Figure 6

Sham, mild and severe groups; mean percentage time (fourr trials/day) spent in the target quadrant and percentage time spent in target quadrant on trial one each day over 5 testing days. The difference in percentage time spent in target quadrant on Trial 1 and the mean of the four trials per day was statistically significant for the sham group on days 1, 2 and 3 (p < 0.05), indicating that by day 4 and 5 the learning and retention across trials was achieved and equivalent. Whereas, the difference in percentage time in target quadrant on trial 1 and the mean of the four trials per day was statistically significant (p < 0.05) for the animals with mild injury on days 3, 4 and 5, indicating that memory overnight of the spatial location was impaired; however, they retained some shorter-term spatial learning capacity across trials. On the other hand the difference between trial one and the mean time spent in the target quadrant of the animals with severe injury was statistically significant (p < 0.05), only on day 2 indicating a more severe spatial learning and memory impairment.

Figure 7

Escape latency and percentage time spent in target quadrant performance normalized to trial 1 of day 1 per group; sham, mild and severe. The sham animals escape latency decreased from 1 to 0.2 by day 5, the animals with mild injury' escape latency decreased from 1 to 0.5; however, the animals with severe injury' escape latency decreased only from 1 to 0.8. The percentage time the sham animals spent in the target quadrant increased from 1 to 2.5 on day 5. The percentage time the animals with mild injury spent in the target quadrant increased from 1 to 0.5; however, the animals with severe injury spent approximately the same percentage time in the target quadrant from day 1 to 5.