Background and purpose
Enriched environment (EE) has been shown to increase neurogenesis in normal adult rodents. Likewise, numerous studies have also shown that cerebral ischemic insult induces neurogenesis. In light of our recent finding suggesting that neurogenesis might contribute to functional recovery after cerebral ischemia, we intend to maximize neurogenesis by increasing the survival of newborn cells in the hippocampus. The aim of this study was to determine the effect of EE and learning on neurogenic activity and the survival of newborn neurons in the dentate gyrus (DG) following ischemic stroke. The effect of running on the treadmill, a widely used rehabilitation therapy, was also explored in rats with focal stroke.
Methods
Rats were subjected to 90-min right distal middle cerebral artery occlusion (MCAO) and sham surgery, and labeled with BrdU on days 4–7 after MCAO. NMDA was microinjected into layer V cortex to study the effect of excitatoxicity on denate gyrus progenitor cell proliferation. One to two weeks following MCAO, rats were then placed in standard environment (SE), EE, or subjected to 30-min daily treadmill running for various periods of time from 2 weeks to 2 months. In all groups, the number of BrdU, Ki67 and double cortin immunoreactive cells were quantified in the subgranular zone (SGZ) and granule cell layer (GCL). The number of new neurons was also determined by quantifying the cells double positive for BrdU and NeuN immunostaining. Neuronal density was determined in the ischemic penumbra medial to cortical lesion by unbiased stereology.
Results
Cerebral focal ischemia induces a transient increase of cell proliferation and neuroblasts in both ipsilateral and contralateral DG SGZ. NMDA microinjected into layer V of cerebral cortex also triggers an increase of cell proliferation in the DG SGZ to a lesser extent compared to MCAO. Majority of the newborn cells induced by cerebral ischemia in the SGZ and GCL died within 2 weeks following cell division with approximately two thirds of the surviving new cells expressing neuronal marker NeuN. An increased survival of newborn cells after MCAO is observed in both EE and running groups, compared to SE. However, EE but not running, restores the total number of neuroblasts after MCAO compared to sham operation. EE also results in the highest newborn neurons compared to running or SE. Furthermore, EE but not running, significantly increases the neuronal density in the ischemic penumbra. DG progenitor cell proliferation induced by running is fast and transient compared to the effect of EE,. At 2 months following treatment, running no longer increases progenitor cell proliferation, whereas EE still significantly increases the number of Ki67 immunoreactive cells in the SGZ of both MCAO and sham rats.
Conclusions
EE enhances neurogenesis, restores neuroblast production, and promote neuronal survival in the ischemic penumbra after MCAO, while running only increases newborn neuron survival. Further study is needed to delineate the mechanisms mediating EE and running enhanced neurogenesis.