Abstract
Brain glycogen localized in astrocytes produces lactate via cAMP signaling, which regulates memory functions and endurance capacity. Exhaustive endurance exercise with hypoglycemia decreases brain glycogen, although the mechanism underlying this phenomenon remains unclear. Since insulin-induced hypoglycemia decreases brain glycogen, this study tested the hypothesis that hypoglycemia mediates exercise-induced brain glycogen decrease. To test the hypothesis, the effects of insulin- and exhaustive exercise-induced hypoglycemia on brain glycogen levels were compared using the microwave irradiation method in adult Wistar rats. The insulin challenge and exhaustive exercise induced similar levels of severe hypoglycemia. Glycogen in the hypothalamus and cerebellum decreased similarly with the insulin challenge and exhaustive exercise; however, glycogen in the cortex, hippocampus, and brainstem of the exercise group were lower compared with the insulin group. Brain lactate and cAMP levels in the hypothalamus and cerebellum increased similarly with the insulin challenge and exhaustive exercise, but those in the cortex, hippocampus, and brainstem of the exercise group were higher compared with the insulin group. Blood glucose correlated positively with brain glycogen, but the slope of regression lines was greater in the exercise group compared with the insulin group in the cortex, hippocampus, and brainstem, but not the hypothalamus and cerebellum. These findings support the hypothesis that hypoglycemia mediates the exercise-induced reduction in brain glycogen, at least in the hypothalamus and cerebellum. However, glycogen reduction during exhaustive endurance exercise in the cortex, hippocampus, and brainstem is not due to hypoglycemia alone, implicating the role of exercise-specific neuronal activity in brain glycogen decrease.