Treadmill exercise improves cognitive function and facilitates nerve growth factor signaling by activating mitogen-activated protein kinase/extracellular signal-regulated kinase1/2 in the streptozotocin-induced diabetic rat hippocampus.

This study aimed to investigate the effects of regular treadmill exercise on nerve growth factor (NGF) expression, the improvement of cognitive function in the hippocampus of diabetic rats, and to understand the molecular mechanisms through which the relevant signaling factors act. We investigated the effects of regular treadmill exercise for 6 weeks on NGF, tyrosine kinase receptor A (TrkA), p75 receptor, phosphatidylinositol 3-kinase (PI3-K), mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase 1/2 (Erk1/2), cyclic AMP response element-binding protein (CREB), and caspase-3 protein levels; we also assessed cell survival and cognitive function. Forty male Sprague-Dawley rats were divided into four groups: (1) normal control group (NCG: n=10); (2) normal exercise group (NEG: n=10); (3) diabetes control group (DCG: n=10), and (4) diabetes exercise group (DEG: n=10). Diabetes was induced by injecting streptozotocin (STZ; 55 mg/kg dissolved in 0.05 M citrate buffer, pH 4.5, i.p.) into rats. Rats were subjected to treadmill exercise for 5 days a week over 6 weeks, and the speed of the treadmill was gradually increased. In a passive avoidance test, the retention latency in the DCG was significantly shorter than that in the DEG (P<0.05). Increased 5-bromo-2'-deoxyuridine-5'-mono-phosphate (BrdU)-labeled cells (P<0.001) and significant increases in NGF and TrkA protein levels were observed in the hippocampal dentate gyrus in the NEG and DEG (P<0.001 and P<0.01, respectively). The p75 receptor protein level significantly increased in the NEG but decreased in the DCG (P<0.001). The p-PI3-K and t-CREB protein levels significantly increased in the NEG (P<0.001 and P<0.05, respectively), whereas t-Erk1/2 significantly decreased in the DCG (P<0.01, P<0.01, respectively). p-Erk1/2 and p-CREB protein levels significantly increased in the NEG and DEG (P<0.001, P<0.001, and P<0.01, respectively). Caspase-3 protein levels significantly increased in the DCG (P<0.001). These results show that treadmill exercise improves cognitive function, increases the number of BrdU-labeled cells, and increases NGF levels, by the activation of the MAPK/Erk1/2 signaling pathway in the hippocampus of diabetic rats.