Physical exercise is one of the most effective methods for managing obesity, and exercise exerts positive effects on various brain functions. Excessive weight gain is known to be related to the impairment of cognitive function. High-fat diet-induced obesity impairs hippocampal neuroplasticity, which impedes cognitive function, such as learning ability and memory function. In this study, we investigated the effect of treadmill exercise on impairment of cognitive function in relation with hippocampal neuroplasticity using high-fat diet-induced obese mice. After obesity was induced by a 20-week high-fat (60%) diet, treadmill exercise was performed for 12 weeks. In the present results, cognitive function was impaired in the high-fat diet-induced obese mice. Brain-derived neurotrophic factor (BDNF) and tyrosin kinase B (TrkB) expression and cell proliferation were decreased in the high-fat diet-induced obese mice. Treadmill exercise improved cognitive function through enhancing neuroplasticity, including increased expression of BDNF and TrkB and enhanced cell proliferation. The present results suggest that treadmill exercise enhances hippocampal neuroplasticity, and then potentially plays a protective role against obesity-induced cognitive impairment.
Increased body mass index (BMI) raises the risk for Alzheimer’s disease, and this increment of BMI is associated with brain atrophy (
High-fat diet impairs hippocampal structure and functions (
The developmental stages of neurogenesis are characterized by stage-specific markers, such as doublecortin (DCX) (
Neurotrophic factors are critical in regulating the generation, differentiation, and proliferation of neurons, and in maintaining neuronal plasticity. Of these, brain-derived neurotrophic factor (BDNF) is a small dimeric protein and works through high affinity binding with its receptor, tyrosin kinase B (TrkB). BDNF is also implicated in the pathophysiology of several brain injury patients (
Exercise has a positive effect on various brain disorders, including depression, Alzheimer disease, and Parkinson disease (
All animal experimental procedures conformed to the regulations stipulated by the National Institutes of Health and the guidelines of the Korean Academy of Medical Science. This study was approved by the Kyung Hee University Institutional Animal Care and Use Committee (KHUASP [SE]-14-018) (Seoul, Korea). The mice were housed under controlled temperature (20°C±2°C) and lighting (07:00 a.m. to 19:00 p.m.) conditions with food and water available
Exercise protocol was listed in the
In order to evaluate the short-term memory and spatial memory ability, Y-maze test and radial-8-arm maze test were performed, according to the previously describe method (
The animals were sacrificed immediately after determination of radial-8-arm maze test. To prepare the brain slices, the animals were fully anesthetized with diethyl ether after which the mice were transcardially perfused with 50-mM phosphate-buffered saline (PBS) and then fixed with freshly prepared solution of 4% paraformaldehyde in 100-mM phosphate buffer (pH, 7.4). The brains were then removed, postfixed in the same fixative overnight, and transferred into a 30% sucrose solution for cryoprotection. Coronal sections with thicknesses of 40 μm were made using a freezing microtome (Leica, Nussloch, Germany).
BDNF and TrkB expression in the hippocampus was determined by western blot analysis, according to the previously described method (
To detect newly generated cells in the dentate gyrus, BrdU-specific immunohistochemistry was performed, according to the previously described method (
After BrdU staining, the differentiation of BrdU-positive cells was determined on the same section using a mouse antineuronal nucleic antibody (1:1,000; Chemicon International, Temecula, CA, USA). The sections were washed 3 times with PBS, incubated for l hr with a biotinylated antimouse secondary antibody. For staining, the sections were incubated in a reaction mixture consisting of 0.03% DAB and 0.03% hydrogen peroxide for 5 min. The sections were mounted onto gelatin-coated slides, air-dried overnight at room temperature, and coverslips were mounted using Permount (Fisher Scientific, New Jersey, NJ, USA).
To visualize DCX expression, immunohistochemistry for DCX in the hippocampus was performed, according to the previously described method (
For confirming the expression of BDNF and TrkB, the detected bands were calculated densitometrically using Molecular Analyst, ver. 1.4.1 (Bio-Rad). The number of BrdU-positive and DCX- positive cells in the dentate gyrus was counted hemilaterally under a light microscope (Olympus, Tokyo, Japan), and they were expressed as the numbers of cells per square millimeter in the dentate gyrus. The area of the dentate gyrus was measured by Image-Pro Plus image analysis system (Media Cyberbetics Inc., Silver Spring, MD, USA). The data were analyzed with one-way analysis of variance and then Duncan
The percentage of the spontaneous alteration in the Y-maze test was lower in the high-fat diet-induced obese mice than control mice (
The expression of BDNF and TrkB in the hippocampus was analyzed (
Cell differentiation in hippocampal dentate gyrus was analyzed by DCX-positive cells (
Cell proliferation in hippocampal dentate gyrus was analyzed by BrdU-positive cells (
In the present study, short-term and spatial memory was decreased by high-fat diet for 32 weeks. The previous study suggested that high-fat diet deteriorated hippocampus-dependent spatial learning ability before the occurring the metabolic disorders, such as hypercholesterolemia and hyperinsulinemia (
In the present study, reduced hippocampal neuroplasticity caused impaired memory function in the high-fat diet-induced obese mice. BDNF and TrkB expression and cell proliferation were decreased in the high-fat diet-induced obese mice. Exercise is one of the most effective methods managing obesity and exercise prevents dementia or impairment of cognitive function. In the present study, treadmill exercise for 12 weeks starting 20 weeks after high-fat diet improved cognitive function, and this improvement of cognitive function was accompanied by increased expression of BDNF and TrkB with enhanced cell proliferation. In animal models of obesity and diabetes, exercise increased hippocampal BDNF expression and enhanced cell proliferation and differentiation (
In the high-fat diet-induced obese mice, impaired cognitive function might be ascribed to the decrement in hippocampal neuroplasticity. Treadmill exercise activated hippocampal neuroplasticity, resulting in improvement in cognitive function in the high-fat diet-induced obese mice. The present results suggest that treadmill exercise enhances hippocampal neuroplasticity, and then potentially plays a protective role against obesity-induced cognitive impairment.
This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2013 S1A5B5A01031478).
CONFLICT OF INEREST
No potential conflict of interest relevant to this article was reported.
Effect of treadmill exercise on short-term and spatial memory. (A) Short-term memory in Y-maze task, (B) correct number in the radial-8-arm maze test, and (C) error number in the radial-8-arm maze test. CON, control group; CON+EX, control and exercise group; HFD, high-fat diet group; HFD+EX, high-fat diet and exercise group. Data are expressed as the mean±standard error of the mean. *
Effect of treadmill exercise on brain-derived neurotrophic factor (BDNF) and tyrosin kinase B (TrkB) in hippocampus. CON, control group; CON+EX, control and exercise group; HFD, high-fat diet group; HFD+EX, high-fat diet and exercise group. Data are expressed as the mean±standard error of the mean. *
Effect of treadmill exercise on cell differentiation in the hippocampal dentate gyrus. Upper panel: Photomicrographs of doublecortin (DCX)-positive cells (immunohistochemistry). The scale bar represents 50 μm. Lower panel: The number of DCX-positive cells in each group. CON, control group; CON+EX, control and exercise group; HFD, high-fat diet group; HFD+EX, high-fat diet and exercise group. Data are expressed as the mean±standard error of the mean. *
Effect of treadmill exercise on cell proliferation in the hippocampal dentate gyrus. Upper panel: Photomicrographs of 5-bromo-2′-deoxyridine (BrdU)-positive cells (immunohistochemistry). The scale bar represents 50 μm. Lower panel: The number of BrdU-positive cells in each group. CON, control group; CON+EX, control and exercise group; HFD, high-fat diet group; HFD+EX, high-fat diet and exercise group. Data are expressed as the mean±standard error of the mean. *
Treadmill exercise started 20 weeks after the intake of high fat diet
Exercise period (wk) | Warm-up (m/min) | Exercise (m/min) | Cool-down (m/min) | Time (min) |
---|---|---|---|---|
0–2 | 10 | 30 | ||
3–4 | 10 | 40 | ||
5–6 | 5 | 13 | 5 | 30 |
7–8 | 13 | 40 | ||
9–10 | 16 | 40 | ||
11–12 | 16 | 50 |