Exhaustive endurance exercise activates brain glycogen breakdown and lactate production more than insulin-induced hypoglycemia

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 to the insulin group. Blood glucose correlated positively with brain glycogen, but the slope of regression lines was greater in the exercise group compared to the insulin group in the cortex, hippocampus, and brainstem, but not the hypothalamus and cerebellum. 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 to the insulin group. 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.

Glycogen Phosphorylase Isoform Regulation of Ventromedial Hypothalamic Nucleus Gluco-Regulatory Neuron 5′-AMP-Activated Protein Kinase and Transmitter Marker Protein Expression

Brain glycogen is remodeled during metabolic homeostasis and provides oxidizable L-lactate equivalents. Brain glycogen phosphorylase (GP)-brain (GPbb; AMP-sensitive) and -muscle (GPmm; norepinephrine-sensitive) type isoforms facilitate stimulus-specific control of glycogen disassembly. Here, a whole animal model involving stereotactic-targeted delivery of GPmm or GPbb siRNA to the ventromedial hypothalamic nucleus (VMN) was used to investigate the premise that these variants impose differential control of gluco-regulatory transmission. Intra-VMN GPmm or GPbb siRNA administration inhibited glutamate decarboxylate65/67 (GAD), a protein marker for the gluco-inhibitory transmitter γ--aminobutyric acid (GABA), in the caudal VMN. GPbb knockdown, respectively overturned or exacerbated hypoglycemia-associated GAD suppression in rostral and caudal VMN. GPmm siRNA caused a segment-specific reversal of hypoglycemic augmentation of the gluco-stimulatory transmitter indicator, neuronal nitric oxide synthase (nNOS). In both cell types, GP siRNA down-regulated 5′-AMP-activated protein kinase (AMPK) during euglycemia, but hypoglycemic suppression of AMPK was reversed by GPmm targeting. GP knockdown elevated baseline GABA neuron phosphoAMPK (pAMKP) content, and amplified hypoglycemic augmentation of pAMPK expression in each neuron type. GPbb knockdown increased corticosterone secretion in eu- and hypoglycemic rats. Outcomes validate efficacy of GP siRNA delivery for manipulation of glycogen breakdown in discrete brain structures in vivo, and document VMN GPbb control of local GPmm expression. Results document GPmm and/or -bb regulation of GABAergic and nitrergic transmission in discrete rostro-caudal VMN segments. Contrary effects of glycogenolysis on metabolic-sensory AMPK protein during eu- versus hypoglycemia may reflect energy state-specific astrocyte signaling. Amplifying effects of GPbb knockdown on hypoglycemic stimulation of pAMPK infer that glycogen mobilization by GPbb limits neuronal energy instability during hypoglycemia.

Dynamic Variations in Brain Glycogen are Involved in Modulating Isoflurane Anesthesia in Mice

General anesthesia severely affects the metabolites in the brain. Glycogen, principally stored in astrocytes and providing the short-term delivery of substrates to neurons, has been implicated as an affected molecule. However, whether glycogen plays a pivotal role in modulating anesthesia–arousal remains unclear. Here, we demonstrated that isoflurane-anesthetized mice exhibited dynamic changes in the glycogen levels in various brain regions. Glycogen synthase (GS) and glycogen phosphorylase (GP), key enzymes of glycogen metabolism, showed increased activity after isoflurane exposure. Upon blocking glycogenolysis with 1,4-dideoxy-1,4-imino-D-arabinitol (DAB), a GP antagonist, we found a prolonged time of emergence from anesthesia and an enhanced δ frequency in the EEG (electroencephalogram). In addition, augmented expression of glycogenolysis genes in glycogen phosphorylase, brain (Pygb) knock-in (PygbH11/H11) mice resulted in delayed induction of anesthesia, a shortened emergence time, and a lower ratio of EEG-δ. Our findings revealed a role of brain glycogen in regulating anesthesia–arousal, providing a potential target for modulating anesthesia.