Abstract
BACKGROUND: Methylmercury (MeHg) causes damage specifically in cerebrocortical neurons, but not in hippocampal neurons. In our previous studies using cultured neurons, we found that brain-derived neurotrophic factor (BDNF), which is prominently present in hippocampal neurons, plays a key role in resistance to MeHg neurotoxicity. Our findings, combined with recent findings that moderate exercise increases BDNF in the brain, led us to hypothesize that moderate exercise protects against MeHg-induced neurotoxicity by inducing BDNF expression. METHODS: C57 black 6NJcl (C57BL/6NJcl) male mice were used to evaluate the effects of treadmill exercise (a moderate exercise) on the neurotoxicity of MeHg exposure at 1.5 mg/kg/day. The effects of treadmill exercise on MeHg neurotoxicity were evaluated through neurobehavioral, neuropathological, and biochemical analyses using brain tissue, blood, and muscle tissue. RESULTS: Treadmill exercise had a significant inhibitory effect on the neurological symptoms associated with apoptotic neuronal death and subsequent cerebrocortical neuron loss induced by MeHg exposure. In the cerebral cortex, treadmill exercise significantly increased BDNF levels and activated the neuroprotective-related BDNF-tropomyosin receptor kinase (Trk) B and p44/42 mitogen-activated protein kinase (MAPK) pathways along with significantly suppressing the neuronal cell death-associated p38 MAPK pathway. Furthermore, treadmill exercise significantly increased fibronectin type III domain containing 5 (FNDC5) expression in the muscle tissue and elevated ed the concentration of its metabolite, irisin, in the blood. CONCLUSIONS: These results suggest that treadmill exercise increases BDNF in the brain and suppresses neurotoxic pathways, ultimately protecting against MeHg neurotoxicity. Moreover, the increase of BDNF in the brain may be attributed to the exercise-induced increased expression of FNDC5 in muscle tissue from where it is released into the blood as irisin and finally transferred into the brain and promoted BDNF production.