Exercise reverses learning deficits induced by hippocampal injury by promoting neurogenesis

Cognitive impairment is a prevalent and debilitating consequence of various neurological conditions, including stroke. Hippocampal atrophy, characterized by a reduction in the volume of the hippocampus—a brain region critical for learning and memory—is frequently associated with cognitive decline after stroke. The hippocampus is unique in its capacity for adult neurogenesis, the generation of new neurons throughout adulthood. Prior research has established a strong link between adult neurogenesis and spatial learning, suggesting that boosting this process could potentially enhance cognitive recovery. Exercise has emerged as a powerful stimulus for adult neurogenesis. Previous studies have demonstrated that exercise improves cognitive performance in humans post-stroke and enhances neurogenesis in animal models. However, the commonly used middle cerebral artery occlusion (MCAO) model in rodent stroke research presents challenges due to short survival periods and motor deficits that complicate the assessment of long-term recovery. To overcome these limitations, this study utilized a novel mouse model of unilateral hippocampal injury induced by ET-1 injection, resulting in a measurable learning deficit without significant motor impairment. This model allowed for the long-term evaluation of whether voluntary exercise could promote neurogenesis and improve cognitive function following hippocampal injury.

The existing literature highlights the importance of the hippocampus in learning and memory, particularly spatial learning. Studies have shown a correlation between hippocampal neurogenesis and cognitive function, with reduced neurogenesis associated with cognitive decline. Conversely, strategies to enhance neurogenesis, such as exercise, have been shown to improve learning and memory. While several studies have investigated the effects of exercise on cognitive function following stroke, the results have been mixed, partly due to the limitations of existing stroke models. The MCAO model, although widely used, often presents confounding motor deficits that hinder the accurate assessment of cognitive recovery. The need for a more suitable model that allows for prolonged survival and the evaluation of long-term cognitive recovery motivated the present research. This study aims to build upon the existing findings by utilizing a new model to specifically investigate the role of exercise-induced neurogenesis in reversing learning deficits following unilateral hippocampal damage.

Female C57BL/6 mice were used. Unilateral hippocampal injury was induced by injecting endothelin-1 (ET-1) into the right hippocampus, while the left hippocampus received a vehicle injection (PBS). The severity of the hippocampal lesion was assessed by measuring hippocampal volume and the area of the dentate gyrus granule cell layer. Spatial learning was evaluated using the active place avoidance (APA) task, a test where mice learn to avoid a specific location in a cylinder to prevent receiving a foot shock. Mice were tested before (APA1) and after (APA2) the ET-1 injection to establish baseline and post-injury performance. A subset of mice with similar APA2 performance was then randomly assigned to either a voluntary running group (Run) or a control group (No Run) for 21 days. BrdU was administered to label newly proliferated cells during the final four days of the running period. After 21 days, mice were again tested in the APA task (APA3) with altered visual cues. To determine the role of neurogenesis, some mice were treated with diphtheria toxin (DT) either systemically or unilaterally in the hippocampus to ablate newly formed neurons, after which mice were retested (APA3 and APA4). Hippocampal neurogenesis was assessed using immunohistochemistry to quantify BrdU-positive cells, and cells co-expressing BrdU with the neuronal markers DCX (immature neurons) and NeuN (mature neurons). Motor function was assessed using the Rota-rod test, activity monitoring, and grip strength measurements before and after surgery. Statistical analyses included repeated measures two-way ANOVA, paired t-tests, and unpaired t-tests.

Intrahippocampal ET-1 injection resulted in a significant reduction in hippocampal volume and the area of the dentate gyrus granule cell layer. Mice with ET-1-induced lesions showed severe deficits in spatial learning in the APA task (APA2) compared to control mice. However, voluntary wheel running for 21 days significantly improved spatial learning performance in the ET-1-injected mice (APA3). This improvement was associated with a significant increase in neurogenesis, indicated by higher numbers of BrdU-positive cells, BrdU+/NeuN+ cells, and DCX-positive cells in both the injured and contralateral hippocampi. Importantly, systemic ablation of newborn neurons using DT in DCXDTR mice completely abolished the exercise-induced improvement in learning, confirming the critical role of new neurons in cognitive recovery. Furthermore, unilateral ablation of newborn neurons in either the injured or uninjured hippocampus only temporarily delayed the recovery of spatial learning, suggesting that increased neurogenesis in either hemisphere is sufficient for functional recovery. The ET-1-induced hippocampal injury did not significantly impair motor function. There was also a significant correlation between the number of BrdU-positive cells and the number of shocks received in the APA task, further supporting the link between neurogenesis and learning.

This study demonstrates that voluntary exercise can effectively reverse learning deficits caused by unilateral hippocampal injury, and this effect is entirely dependent on exercise-induced neurogenesis. The finding that unilateral ablation of newborn neurons only temporarily delayed recovery, while bilateral ablation completely blocked recovery, underscores the remarkable plasticity of the brain. These findings suggest that the increased neurogenesis is not just about repairing the damaged area, but could also involve compensatory mechanisms in the intact hemisphere. The use of a novel model of hippocampal injury that avoided motor deficits provided a more accurate assessment of the cognitive effects of exercise and neurogenesis. This research supports the therapeutic potential of exercise in enhancing cognitive recovery after hippocampal damage. The underlying mechanisms of how exercise promotes neurogenesis and whether this involves factors like BDNF, growth hormone or serotonin warrant further investigation.

This research demonstrates the restorative potential of voluntary exercise in ameliorating learning deficits following unilateral hippocampal injury. The study confirms that the beneficial effects of exercise are entirely dependent on increased neurogenesis. The finding that neurogenesis in either the injured or uninjured hippocampus is sufficient for recovery highlights the brain’s capacity for functional compensation. This study’s findings provide compelling evidence supporting the use of exercise as a therapeutic strategy to enhance cognitive function after hippocampal damage. Future studies should investigate the underlying molecular mechanisms and explore the therapeutic potential of targeting these pathways to further optimize cognitive recovery.

This study used a model of unilateral hippocampal injury in young, healthy female mice. The results may not generalize to older animals with co-morbidities or to humans. The study primarily focused on spatial learning. Future research should investigate the effects of exercise on other cognitive domains. The use of DT for neuronal ablation may have had non-specific effects. Further research should validate the findings using alternative methods.