The benefits of voluntary physical exercise after traumatic brain injury on rat's object recognition memory: A comparison of different temporal schedules

Physical exercise can reduce the cognitive decline associated with traumatic brain injury, yet little is known about the optimal administration schedules. Here, different protocols of voluntary wheel running were evaluated for their effects on object recognition memory (ORM), neuroprotection (NeuN⁺ cells), microglial reactivity (Iba1 staining) and neurogenesis (DCX⁺ cells) after controlled cortical impact injury (CCI). CCI-lesioned rats were divided into a sedentary group and three exercise groups: early discontinued exercise (3 weeks of exercise initiated 4 days post-injury, followed by 4 weeks in a sedentary state); delayed exercise (3 weeks of exercise initiated 4 weeks post-injury), and early continuous exercise (7 weeks of exercise starting 4 days post-injury). The deficits induced by CCI in a 24 h ORM test were reversed in the delayed exercise group and reduced in the early discontinued and early continuous groups. The early discontinued protocol also reduced the loss of NeuN⁺ cells in the hilus, while attenuated microglial reactivity was found in the dorsal hippocampus of both the early exercising groups. Running at the end of the experiment increased the number of DCX⁺ cells in the early continuous and delayed groups, and an inverted U-shaped relationship was found between the mean daily exercise time and the amount of neurogenesis. Thus, exercise had benefits on memory both when it was commenced soon and later after injury, although the neural mechanisms implicated differed. Accordingly, the effects of exercise on memory and neurogenesis appear to not only depend on the specific temporal schedule but also, they may be influenced by the amount of daily exercise.