Physical Exercise Improves Cognitive Outcomes in 2 Models of Transient Cerebral Ischemia
Background and Purpose—Preclinical studies suggest that exercise can enhance cognition after cerebral ischemia but often use long training regiments and test cognition during or acutely after training. The cognitive changes may result from enhanced physical fitness and may only provide acute benefit. We sought to determine whether a short period of exercise after cerebral ischemia could improve cognitive outcomes when measured days after completion of exercise training in 2 cerebral ischemia models.
Methods—Focal or global cerebral ischemia was induced in Sprague-Dawley rats. Rats recovered (3–4 days) then were subject to no exercise (0 m/min), mild (6 m/min), moderate (10 m/min), or heavy (15–18 m/min) treadmill exercise (5–6 days). Cognition was tested 8 to 10 days after the last exercise session with hippocampal-dependent contextual fear conditioning.
Results—A short training period of moderate exercise enhanced cognitive function for a week after exercise completion in both models of cerebral ischemia.
Conclusions—Utilization of this exercise paradigm can further the elucidation of exercise-mediated factors involved in cognitive recovery independent of changes in physical fitness.
Survivors of stroke will be at an enhanced risk for accelerated cognitive decline, yet there remains no well-accepted therapy for cognitive recovery.1 Physical exercise (PE) after stroke improves motor functions in patients2 and animal models.3 A recent meta-analysis suggested that PE improves cognitive performance in models of acquired brain injuries.4 However, studies that examine the ability of PE to promote cognitive recovery after cerebral ischemia use weeks of PE training and test cognition during or immediately after the PE period.4 These study designs cannot distinguish whether the effects of PE are because of enhancements in overall animal fitness or an acute effect of exercise intervention. It remains unclear whether a short bout of PE after cerebral ischemia has prolonged positive effects.
In this proof-of-concept study, we aimed to determine whether a short bout of PE training initiated after cerebral ischemia can improve hippocampal-dependent contextual fear memory tested for a week after the last exercise session.
Materials and Methods
Animal Care and Ischemia Surgeries
All experiments were performed in accordance with the Guide for Care and Use of Laboratory Animals and approved by the University of Miami’s Institutional Animal Care and Use Committee. Experimental timeline is indicated in Figure 1. Detailed experimental methods, including randomization, blinding, and allocation concealment, are in the online-only Data Supplement (http://stroke.ahajournals.org). Focal cerebral ischemia was achieved with intraluminal suture blockage of the middle cerebral artery for 90 minutes.5,6 Global cerebral ischemia was induced with 8-minute asphyxia cardiac arrest.5,7 Physiological parameters were maintained within normal ranges during surgeries (online-only Data Supplement).
Subacute Treadmill Exercise
After 3 to 4 days of recovery, adult (3-month-old), male Sprague-Dawley rats (Charles River Laboratories, Wilmington, MA) were exercised on a treadmill (0.59 mA aversion grid; Columbus Instruments, Columbus, OH) for 5 to 6 days (Figure 1). A blinded investigator randomized rats to speeds selected based on our experience and other reports8,9: no exercise (0 m/min), mild (6 m/min), moderate (10 m/min), or heavy exercise (15–18 m/min) with sample sizes for each speed: n=11, n=11, n=10, and n=9, respectively, for focal ischemia and n=10, n=5, n=7, and n=5, respectively, for global ischemia. After focal ischemia, heavy exercise was set at 18 m/min. However, rats were unable to run at 18 m/min after global ischemia; so 15 m/min was used for heavy exercise. Daily PE consisted of a 2-minute warm-up at 5 m/min, then 30 minutes of exercise at designated speed. Rats randomized to the 0 m/min group remained on a stationary treadmill for 30 minutes. Animals that refused to run were excluded from analysis (n=2; global ischemia with heavy exercise).
Contextual Fear Conditioning
Eighteen days after surgery (8–10 days after exercise), hippocampal-dependent contextual fear conditioning was performed.7 Rats were placed into the operant conditioning chamber (Coulbourn Instruments, Whitehall, PA) for 370 seconds with a 2 second 1.5 mA shock administered at second 340. The following day, rats remained in the chamber for 480 seconds with no shock administered. Amount of time freezing was quantified (FreezeFrame; Coulbourn Instruments) before the shock on the first day (baseline) and for the entire duration on the second day (contextual). Animals that froze >50% on the first day were excluded from further analysis (n=1; global cerebral ischemia with moderate exercise).
All data are represented as mean±SEM. Sample size calculations are detailed in the online-only Data Supplement. Data were analyzed with 1-way ANOVA and comparisons between individual exercise groups were quantified with Bonferroni post hoc tests. Significance was assessed as P<0.05.
At baseline, there were no differences in the amount of time freezing between groups subject to focal (P=0.6597; Figure 2A) or global (P=0.2635; Figure 2C) cerebral ischemia. Contextual freezing was significantly different among groups (P=0.0078 for focal; P=0.0037 for global; Figure 2B and 2D) with animals randomized to no exercise (0 m/min) freezing the least suggesting worse memory. After focal ischemia, rats subjected to moderate exercise (10 m/min) had a significant enhancement in duration of freezing compared with nonexercised rats (P<0.01; Figure 2B). Moderate exercise had a similar effect after global cerebral ischemia enhancing contextual fear memory as compared with the nonexercised controls (P<0.05; Figure 2D). Heavy exercise (15 m/min) after global cerebral ischemia also enhanced percent time freezing (P<0.01).
Although there is no treatment for cognitive dysfunction after stroke, prolonged PE can improve cognitive recovery after cerebral ischemia in animal models.4 Using 2 models of cerebral ischemia and 3 speeds of treadmill exercise, we show that a brief bout of moderate exercise initiated after cerebral ischemia can enhance memory tested during a week after the last exercise session. This suggests that postischemia, exercise-mediated cognitive benefits are unlikely a result of changes in physical fitness or acute exercise effects. Although future studies are needed to determine the duration of enhanced fear memory, the fact that this enhancement was evident for a week after the last exercise session suggests it is long lasting. Additionally, our finding that the fastest treadmill speed (18 m/min) was unable to improve cognition is similar to that of others where exercise at 8 m/min, but not 20 m/min, resulted in an acute improvement in spatial memory.8,9 Future studies that aim to understand exercise-mediated cognitive improvements after cerebral ischemia should use speeds between 8 and 15 m/min.
After stroke, there is a period of enhanced plasticity with extensive remodeling occurring in the peri-infarct cortex.10,11 Targeted, task-specific motor training can improve motor function and mold this redeveloping motor network.12 The ability to translate this approach to improving cognitive functioning after stroke has been limited. Several brain regions are involved in cognition necessitating a multitargeted rehabilitation intervention. Additionally, ischemia-induced enhancement of plasticity may be limited further from the infarct.13 Here, we tested cognition with hippocampal-dependent contextual fear conditioning.7 Our previous studies find no gross neuronal loss in the hippocampus after focal ischemia5 and extensive hippocampal neuronal loss after global ischemia.14 Despite these injury differences, PE improved hippocampal-dependent cognitive functioning suggesting that PE may be a multitargeted therapy that can improve cognitive centers regardless of ischemic insult.
In this proof-of-concept study, we find that an early, short bout of moderate exercise in 2 models of cerebral ischemia promotes an enhancement of cognitive function for a week after the last exercise session. Without the confounding effects of prolonged exercise training on overall fitness or acute effects of recent exercise, our experimental paradigm can be utilized for the identification of biomarkers or soluble mediators of lasting exercise benefits.
Sources of Funding
This work was supported by the American Heart Association Fellowship 15PRE2236000 (H.M. Stradecki-Cohan); American Heart Association/American Stroke Association Bugher Foundation 14BFSC17690007 (Drs Sacco, Wright, and Perez-Pinzon), National Institutes of Health/National Institute of Neurological Disorders and Stroke R01 NS34773-15, and NS45676-08 (Dr Perez-Pinzon); Lois Pope Leaders in Furthering Education Fellows Program (H.M. Stradecki-Cohan); and the Evelyn F. McKnight Brain Research Foundation.
Guest Editor for this article was Christoph Kleinschnitz, MD.
The online-only Data Supplement is available with this article at http://stroke.ahajournals.org/lookup/suppl/doi:10.1161/STROKEAHA.117.017296/-/DC1.
- Received March 10, 2017.
- Revision received April 27, 2017.
- Accepted May 23, 2017.
- © 2017 American Heart Association, Inc.
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