Brain glycogen supercompensation in the mouse after recovery from insulin-induced hypoglycemia

Astrocyte Glycogen Is a Major Source of Hypothalamic Lactate in Rats With Recurrent Hypoglycemia

Lactate is an important metabolic substrate for sustaining brain energy requirements when glucose supplies are limited. Recurring exposure to hypoglycemia (RH) raises lactate levels in the ventromedial hypothalamus (VMH), which contributes to counterregulatory failure. However, the source of this lactate remains unclear. The current study investigates whether astrocytic glycogen serves as the major source of lactate in the VMH of RH rats. By decreasing the expression of a key lactate transporter in VMH astrocytes of RH rats, we reduced extracellular lactate concentrations, suggesting excess lactate was locally produced from astrocytes. To determine whether astrocytic glycogen serves as the major source of lactate, we chronically delivered either artificial extracellular fluid or 1,4-dideoxy-1,4-imino-d-arabinitol to inhibit glycogen turnover in the VMH of RH animals. Inhibiting glycogen turnover in RH animals prevented the rise in VMH lactate and the development of counterregulatory failure. Lastly, we noted that RH led to an increase in glycogen shunt activity in response to hypoglycemia and elevated glycogen phosphorylase activity in the hours following a bout of hypoglycemia. Our data suggest that dysregulation of astrocytic glycogen metabolism following RH may be responsible, at least in part, for the rise in VMH lactate levels.

ARTICLE HIGHLIGHTS

Astrocytic glycogen serves as the major source of elevated lactate levels in the ventromedial hypothalamus (VMH) of animals exposed to recurring episodes of hypoglycemia. Antecedent hypoglycemia alters VMH glycogen turnover. Antecedent exposure to hypoglycemia enhances glycogen shunt activity in the VMH during subsequent bouts of hypoglycemia. In the immediate hours following a bout of hypoglycemia, sustained elevations in glycogen phosphorylase activity in the VMH of recurrently hypoglycemic animals contribute to sustained elevations in local lactate levels.

Insulin enhances contextual fear memory independently of its effect in increasing plasma adrenaline

AIMS

Adrenaline enhances contextual fear memory consolidation possibly by activating liver β₂-adrenoceptors causing transient hyperglycaemia. Contrastingly, insulin-induced hypoglycaemia may culminate in blood adrenaline increment, hidering the separation of each hormone's action in contextual fear memory. Therefore, an Ad-deficient mouse model was used aiming to investigate if contextual fear memory consolidation following insulin administration requires or not subsequent increases in plasma adrenaline, which occurs in response to insulin-induced hypoglycemia.

MAIN METHODS

Fear conditioning was performed in wild-type (WT) and adrenaline-deficient (Pnmt-KO) male mice (129 × 1/SvJ) treated with insulin (2 U/kg, intraperitoneal (i.p.)) or vehicle (0.9 % NaCl (i.p.)). Blood glucose was quantified. Catecholamines were quantified using HPLC with electrochemical detection. Quantitative real-time polymerase chain reaction was used to assess mRNA expression of hippocampal Nr4a1, Nr4a2, Nr4a3, and Bdnf genes.

KEY FINDINGS

Insulin-treated WT mice showed increased freezing behaviour when compared to vehicle-treated WT mice. Also, plasma dopamine, noradrenaline, and adrenaline increased in this group. Insulin-treated Pnmt-KO animals showed increased freezing behaviour when compared with respective vehicle. However, no changes in plasma or tissue catecholamines were identified in insulin-treated Pnmt-KO mice when compared with respective vehicle. Furthermore, insulin-treated Pnmt-KO mice presented increased Bdnf mRNA expression when compared to vehicle-treated Pnmt-KO mice.

SIGNIFICANCE

Concluding, enhanced freezing behaviour after insulin treatment, even in adrenaline absence, may indicate a key role of insulin in contextual fear memory. Insulin may cause central molecular changes promoting contextual fear memory formation and/or retrieval. This work may indicate a further role of insulin in the process of contextual fear memory modulation.

Ghrelin does not impact the blunted counterregulatory response to recurrent hypoglycemia in mice

Introduction

Recurrent episodes of insulin-induced hypoglycemia in patients with diabetes mellitus can result in hypoglycemia-associated autonomic failure (HAAF), which is characterized by a compromised response to hypoglycemia by counterregulatory hormones (counterregulatory response; CRR) and hypoglycemia unawareness. HAAF is a leading cause of morbidity in diabetes and often hinders optimal regulation of blood glucose levels. Yet, the molecular pathways underlying HAAF remain incompletely described. We previously reported that in mice, ghrelin is permissive for the usual CRR to insulin-induced hypoglycemia. Here, we tested the hypothesis that attenuated release of ghrelin both results from HAAF and contributes to HAAF.

Methods

C57BL/6N mice, ghrelin-knockout (KO) + control mice, and GhIRKO (ghrelin cell-selective insulin receptor knockout) + control mice were randomized to one of three treatment groups: a "Euglycemia" group was injected with saline and remained euglycemic; a 1X hypoglycemia ("1X Hypo") group underwent a single episode of insulin-induced hypoglycemia; a recurrent hypoglycemia ("Recurrent Hypo") group underwent repeated episodes of insulin-induced hypoglycemia over five successive days.

Results

Recurrent hypoglycemia exaggerated the reduction in blood glucose (by ~30%) and attenuated the elevations in plasma levels of the CRR hormones glucagon (by 64.5%) and epinephrine (by 52.9%) in C57BL/6N mice compared to a single hypoglycemic episode. Yet, plasma ghrelin was equivalently reduced in "1X Hypo" and "Recurrent Hypo" C57BL/6N mice. Ghrelin-KO mice exhibited neither exaggerated hypoglycemia in response to recurrent hypoglycemia, nor any additional attenuation in CRR hormone levels compared to wild-type littermates. Also, in response to recurrent hypoglycemia, GhIRKO mice exhibited nearly identical blood glucose and plasma CRR hormone levels as littermates with intact insulin receptor expression (floxed-IR mice), despite higher plasma ghrelin in GhIRKO mice.

Conclusions

These data suggest that the usual reduction of plasma ghrelin due to insulin-induced hypoglycemia is unaltered by recurrent hypoglycemia and that ghrelin does not impact blood glucose or the blunted CRR hormone responses during recurrent hypoglycemia.

Time-dependent changes in hippocampal and striatal glycogen long after maze training in male rats

Long-lasting biological changes reflecting past experience have been studied in and typically attributed to neurons in the brain. Astrocytes, which are also present in large number in the brain, have recently been found to contribute critically to learning and memory processing. In the brain, glycogen is primarily found in astrocytes and is metabolized to lactate, which can be released from astrocytes. Here we report that astrocytes themselves have intrinsic neurochemical plasticity that alters the availability and provision of metabolic substrates long after an experience. Rats were trained to find food on one of two versions of a 4-arm maze: a hippocampus-sensitive place task and a striatum-sensitive response task. Remarkably, hippocampal glycogen content increased while striatal levels decreased during the 30 days after rats were trained to find food in the place version, but not the response version, of the maze tasks. A long-term consequence of the durable changes in glycogen stores was seen in task-by-site differences in extracellular lactate responses activated by testing on a working memory task administered 30 days after initial training, the time when differences in glycogen content were most robust. These results suggest that astrocytic plasticity initiated by a single experience may augment future availability of energy reserves, perhaps priming brain areas to process learning of subsequent experiences more effectively.

Mental Fatigue Impairs Endurance Performance: A Physiological Explanation

Mental fatigue reflects a change in psychobiological state, caused by prolonged periods of demanding cognitive activity. It has been well documented that mental fatigue impairs cognitive performance; however, more recently, it has been demonstrated that endurance performance is also impaired by mental fatigue. The mechanism behind the detrimental effect of mental fatigue on endurance performance is poorly understood. Variables traditionally believed to limit endurance performance, such as heart rate, lactate accumulation and neuromuscular function, are unaffected by mental fatigue. Rather, it has been suggested that the negative impact of mental fatigue on endurance performance is primarily mediated by the greater perception of effort experienced by mentally fatigued participants. Pageaux et al. (Eur J Appl Physiol 114(5):1095-1105, 2014) first proposed that prolonged performance of a demanding cognitive task increases cerebral adenosine accumulation and that this accumulation may lead to the higher perception of effort experienced during subsequent endurance performance. This theoretical review looks at evidence to support and extend this hypothesis.

Central Mechanisms of Glucose Sensing and Counterregulation in Defense of Hypoglycemia

Glucose homeostasis requires an organism to rapidly respond to changes in plasma glucose concentrations. Iatrogenic hypoglycemia as a result of treatment with insulin or sulfonylureas is the most common cause of hypoglycemia in humans and is generally only seen in patients with diabetes who take these medications. The first response to a fall in glucose is the detection of impending hypoglycemia by hypoglycemia-detecting sensors, including glucose-sensing neurons in the hypothalamus and other regions. This detection is then linked to a series of neural and hormonal responses that serve to prevent the fall in blood glucose and restore euglycemia. In this review, we discuss the current state of knowledge about central glucose sensing and how detection of a fall in glucose leads to the stimulation of counterregulatory hormone and behavior responses. We also review how diabetes and recurrent hypoglycemia impact glucose sensing and counterregulation, leading to development of impaired awareness of hypoglycemia in diabetes.

Recurrent moderate hypoglycemia exacerbates oxidative damage and neuronal death leading to cognitive dysfunction after the hypoglycemic coma

Moderate recurrent hypoglycemia (RH) is frequent in Type 1 diabetes mellitus (TIDM) patients who are under intensive insulin therapy increasing the risk for severe hypoglycemia (SH). The consequences of RH are not well understood and its repercussions on neuronal damage and cognitive function after a subsequent episode of SH have been poorly investigated. In the current study, we have addressed this question and observed that previous RH during seven consecutive days exacerbated oxidative damage and neuronal death induced by a subsequent episode of SH accompanied by a short period of coma, in the parietal cortex, the striatum and mainly in the hippocampus. These changes correlated with a severe decrease in reduced glutathione content (GSH), and a significant spatial and contextual memory deficit. Administration of the antioxidant, N-acetyl-L-cysteine, (NAC) reduced neuronal death and prevented cognitive impairment. These results demonstrate that previous RH enhances brain vulnerability to acute hypoglycemia and suggests that this effect is mediated by the decline in the antioxidant defense and oxidative damage. The present results highlight the importance of an adequate control of moderate hypoglycemic episodes in TIDM.

Structure and Regulation of Glycogen Synthase in the Brain

Brain glycogen synthesis is a regulated, multi-step process that begins with glucose transport across the blood brain barrier and culminates with the actions of glycogen synthase and the glycogen branching enzyme to elongate glucose chains and introduce branch points in a growing glycogen molecule. This review focuses on the synthesis of glycogen in the brain, with an emphasis on glycogen synthase, but draws on salient studies in mammalian muscle and liver as well as baker's yeast, with the goal of providing a more comprehensive view of glycogen synthesis and highlighting potential areas for further study in the brain. In addition, deficiencies in the glycogen biosynthetic enzymes which lead to glycogen storage diseases in humans are discussed, highlighting effects on the brain and discussing findings in genetically modified animal models that recapitulate these diseases. Finally, implications of glycogen synthesis in neurodegenerative and other diseases that impact the brain are presented.

State-Dependent Changes in Brain Glycogen Metabolism

A fundamental understanding of glycogen structure, concentration, polydispersity and turnover is critical to qualify the role of glycogen in the brain. These molecular and metabolic features are under the control of neuronal activity through the interdependent action of neuromodulatory tone, ionic homeostasis and availability of metabolic substrates, all variables that concur to define the state of the system. In this chapter, we briefly describe how glycogen responds to selected behavioral, nutritional, environmental, hormonal, developmental and pathological conditions. We argue that interpreting glycogen metabolism through the lens of brain state is an effective approach to establish the relevance of energetics in connecting molecular and cellular neurophysiology to behavior.

Basal fatty acid oxidation increases after recurrent low glucose in human primary astrocytes

AIMS/HYPOTHESIS

Hypoglycaemia is a major barrier to good glucose control in type 1 diabetes. Frequent hypoglycaemic episodes impair awareness of subsequent hypoglycaemic bouts. Neural changes underpinning awareness of hypoglycaemia are poorly defined and molecular mechanisms by which glial cells contribute to hypoglycaemia sensing and glucose counterregulation require further investigation. The aim of the current study was to examine whether, and by what mechanism, human primary astrocyte (HPA) function was altered by acute and recurrent low glucose (RLG).

METHODS

To test whether glia, specifically astrocytes, could detect changes in glucose, we utilised HPA and U373 astrocytoma cells and exposed them to RLG in vitro. This allowed measurement, with high specificity and sensitivity, of RLG-associated changes in cellular metabolism. We examined changes in protein phosphorylation/expression using western blotting. Metabolic function was assessed using a Seahorse extracellular flux analyser. Immunofluorescent imaging was used to examine cell morphology and enzymatic assays were used to measure lactate release, glycogen content, intracellular ATP and nucleotide ratios.

RESULTS

AMP-activated protein kinase (AMPK) was activated over a pathophysiologically relevant glucose concentration range. RLG produced an increased dependency on fatty acid oxidation for basal mitochondrial metabolism and exhibited hallmarks of mitochondrial stress, including increased proton leak and reduced coupling efficiency. Relative to glucose availability, lactate release increased during low glucose but this was not modified by RLG. Basal glucose uptake was not modified by RLG and glycogen levels were similar in control and RLG-treated cells. Mitochondrial adaptations to RLG were partially recovered by maintaining euglycaemic levels of glucose following RLG exposure.

CONCLUSIONS/INTERPRETATION

Taken together, these data indicate that HPA mitochondria are altered following RLG, with a metabolic switch towards increased fatty acid oxidation, suggesting glial adaptations to RLG involve altered mitochondrial metabolism that could contribute to defective glucose counterregulation to hypoglycaemia in diabetes.

Inhibition of glycogen phosphorylase stimulates ventromedial hypothalamic nucleus AMP-activated protein kinase: Activity and neuronal nitric oxide synthase protein expression in male rats

The glucose polymer glycogen is a vital fuel reserve in the brain. The mediobasal hypothalamic energy sensor AMP‐activated protein kinase (AMPK) maintains glucostasis via neurotransmitter mechanisms that suppress [γ‐aminobutyric acid; GABA] or stimulate [nitric oxide; steroidogenic factor‐1 (SF1)] counter‐regulatory outflow. This study investigated whether glycogen‐derived fuel supply is a critical screened variable in ventromedial hypothalamic nucleus (VMN) monitoring of neuro‐metabolic stability during glucostasis and/or insulin (I)‐induced hypoglycemia. Adult male rats were pretreated by intra‐VMN infusion of the glycogen phosphorylase inhibitor 1,4‐dideoxy‐1,4‐imino‐D‐arabinitol (DAB) before sc vehicle or I injection. Western blot analyses of micropunch‐dissected VMN tissue from euglycemic animals showed DAB augmentation of phosphoAMPK (pAMPK), neuronal nitric oxide synthase (nNOS), and SF‐1, but not glutamate decarboxylase_65/67 (GAD) protein. Combinatory DAB/I treatment did not further enhance AMPK activity but significantly amplified nNOS expression relative to DAB alone. Hypoglycemic stimulation of corticosterone, but not glucagon release was prevented by DAB. Results imply that glycogen‐derived substrate fuel provision represses VMN AMPK activity and neurotransmitter signals of metabolic deficiency. Progressive augmentation of nNOS protein by DAB/I versus DAB/V intimates that “fuel‐inhibited” nitrergic neurons may exhibit increasing sensitivity to disrupted glycogen breakdown during glucoprivation versus glucostasis. nNOS and GAD reactivity to DAB/I, but not I implies that acute glycogen utilization during hypoglycemia may be sufficiently robust to avert effects on local metabolic sensory signaling. DAB/I upregulation of GAD alongside prevention of hypercorticosteronemia suggests that indicators of metabolic sufficiency may occur secondary to local compensatory adaptations to severe restriction of glucose‐derived energy.

Aquaporin-4 deletion ameliorates hypoglycemia-induced BBB permeability by inhibiting inflammatory responses

Background

Severe hypoglycemia induces brain edema by upregulating aquaporin-4 (AQP4) expression and by degrading tight junctions. Acute severe hypoglycemia induces a proinflammatory environment that may contribute to a disruption in the epithelial barrier by decreasing tight junction protein expression. Interestingly, the altered AQP4 expression has been considered to play a critical role in neuroinflammation during acute brain injury. It has been shown that AQP4 deletion reduces brain inflammation in AQP4-null mice after intracerebral LPS injection. However, the effect of AQP4 deletion regarding protection against hypoglycemia-induced blood-brain barrier (BBB) breakdown is unknown.

Methods

An acute severe hypoglycemic stress model was established via injection of 4 unit/kg body weight of insulin. Evans blue (EB) staining and water measurement were used to assess BBB permeability. Western blot, reverse transcription polymerase chain reaction, and immunofluorescence were used to detect the expression of related proteins. The production of cytokines was assessed via enzyme-linked immunosorbent assay.

Results

Hypoglycemia-induced brain edema and BBB leakage were reduced in AQP4^−/− mice. AQP4 deletion upregulated PPAR-γ and inhibited proinflammatory responses. Moreover, knockdown of aquaporin-4 by small interfering RNA in astrocytes co-cultured with endothelial cells effectively reduced transendothelial permeability and degradation of tight junctions. Treatment with PPAR-γ inhibitors showed that upregulation of PPAR-γ was responsible for the protective effect of AQP4 deletion under hypoglycemic conditions.

Conclusions

Our data suggest that AQP4 deletion protects BBB integrity by reducing inflammatory responses due to the upregulation of PPAR-γ expression and attenuation of proinflammatory cytokine release. Reduction in AQP4 may be protective in acute severe hypoglycemia.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1203-8) contains supplementary material, which is available to authorized users.

Accuracy of 5 Point-of-Care Glucometers in C57BL/6J Mice

Despite few published studies that assess the accuracy of glucometers in laboratory animals, glucometers are commonly used in animal research. We set out to determine the accuracy of 5 point-of-care glucometers (POCG) when used to evaluate murine whole blood, plasma, and serum samples. The POCG tested included one veterinary device (POCG A) and 4 humanuse instruments (POCG B through E). Whole blood, plasma, and serum samples from 50 female C57BL/6J mice were analyzed on all POCG, and serum was analyzed on a reference biochemical analyzer. The mean blood glucose concentration (BGC) measured in whole blood by using POCG A was greater than that on the biochemical analyzer, whereas the mean BGC in whole blood according to POCG B through E did not differ significantly from that on the biochemical analyzer. Mean BGC in plasma and serum did not differ between POCG B and E and the biochemical analyzer, whereas the plasma and serum BGC values from POCG C and D were greater than the mean BGC from the biochemical analyzer. The accuracy of each POCG for each sample type was evaluated by analyzing mean differences, correlations, and Bland-Altman graphs. We found that the 4 human-use POCG are appropriate for use with whole blood from female C57BL/6J mice, whereas only 2 of the evaluated POCG were sufficiently accurate for use with plasma or serum.

Melatonin ameliorates hypoglycemic stress-induced brain endothelial tight junction injury by inhibiting protein nitration of TP53-induced glycolysis and apoptosis regulator

Severe hypoglycemia has a detrimental impact on the cerebrovasculature, but the molecular events that lead to the disruption of the integrity of the tight junctions remain unclear. Here, we report that the microvessel integrity was dramatically compromised (59.41% of wild-type mice) in TP53-induced glycolysis and apoptosis regulator (TIGAR) transgenic mice stressed by hypoglycemia. Melatonin, a potent antioxidant, protects against hypoglycemic stress-induced brain endothelial tight junction injury in the dosage of 400 nmol/L in vitro. FRET (fluorescence resonance energy transfer) imaging data of endothelial cells stressed by low glucose revealed that TIGAR couples with calmodulin to promote TIGAR tyrosine nitration. A tyrosine 92 mutation interferes with the TIGAR-dependent NADPH generation (55.60% decreased) and abolishes its protective effect on tight junctions in human brain microvascular endothelial cells. We further demonstrate that the low-glucose-induced disruption of occludin and Caludin5 as well as activation of autophagy was abrogated by melatonin-mediated blockade of nitrosative stress in vitro. Collectively, we provide information on the detailed molecular mechanisms for the protective actions of melatonin on brain endothelial tight junctions and suggest that this indole has translational potential for severe hypoglycemia-induced neurovascular damage.

Neuroprotective potential of astroglia

Astroglia are the homoeostatic cells of the central nervous system, which participate in all essential functions of the brain. Astrocytes support neuronal networks by handling water and ion fluxes, transmitter clearance, provision of antioxidants, and metabolic precursors and growth factors. The critical dependence of neurons on constant support from the astrocytes confers astrocytes with intrinsic neuroprotective properties. On the other hand, loss of astrocytic support or their pathological transformation compromises neuronal functionality and viability. Manipulating neuroprotective functions of astrocytes is thus an important strategy to enhance neuronal survival and improve outcomes in disease states. © 2017 The Authors Journal of Neuroscience Research Published by Wiley Periodicals, Inc.

Cerebral glycogen in humans following acute and recurrent hypoglycemia: Implications on a role in hypoglycemia unawareness

Supercompensated brain glycogen levels may contribute to the development of hypoglycemia-associated autonomic failure (HAAF) following recurrent hypoglycemia (RH) by providing energy for the brain during subsequent periods of hypoglycemia. To assess the role of glycogen supercompensation in the generation of HAAF, we estimated the level of brain glycogen following RH and acute hypoglycemia (AH). After undergoing 3 hyperinsulinemic, euglycemic and 3 hyperinsulinemic, hypoglycemic clamps (RH) on separate occasions at least 1 month apart, five healthy volunteers received [1-¹³C]glucose intravenously over 80+ h while maintaining euglycemia. ¹³C-glycogen levels in the occipital lobe were measured by ¹³C magnetic resonance spectroscopy at ∼8, 20, 32, 44, 56, 68 and 80 h at 4 T and glycogen levels estimated by fitting the data with a biophysical model that takes into account the tiered glycogen structure. Similarly, prior ¹³C-glycogen data obtained following a single hypoglycemic episode (AH) were fitted with the same model. Glycogen levels did not significantly increase after RH relative to after euglycemia, while they increased by ∼16% after AH relative to after euglycemia. These data suggest that glycogen supercompensation may be blunted with repeated hypoglycemic episodes. A causal relationship between glycogen supercompensation and generation of HAAF remains to be established.

Glycogen Supercompensation in the Rat Brain After Acute Hypoglycemia is Independent of Glucose Levels During Recovery

Patients with diabetes display a progressive decay in the physiological counter-regulatory response to hypoglycemia, resulting in hypoglycemia unawareness. The mechanism through which the brain adapts to hypoglycemia may involve brain glycogen. We tested the hypothesis that brain glycogen supercompensation following hypoglycemia depends on blood glucose levels during recovery. Conscious rats were submitted to hypoglycemia of 2 mmol/L for 90 min and allowed to recover at different glycemia, controlled by means of i.v. glucose infusion. Brain glycogen concentration was elevated above control levels after 24 h of recovery in the cortex, hippocampus and striatum. This glycogen supercompensation was independent of blood glucose levels in the post-hypoglycemia period. In the absence of a preceding hypoglycemia insult, brain glycogen concentrations were unaltered after 24 h under hyperglycemia. In the hypothalamus, which controls peripheral glucose homeostasis, glycogen levels were unaltered. Overall, we conclude that post-hypoglycemia glycogen supercompensation occurs in several brain areas and its magnitude is independent of plasma glucose levels. By supporting brain metabolism during recurrent hypoglycemia periods, glycogen may have a role in the development of hypoglycemia unawareness.

Insulin neuroprotection and the mechanisms

Objective:

To analyze the mechanism of neuroprotection of insulin and which blood glucose range was benefit for insulin exerting neuroprotective action.

Data Sources:

The study is based on the data from PubMed.

Study Selection:

Articles were selected with the search terms “insulin”, “blood glucose”, “neuroprotection”, “brain”, “glycogen”, “cerebral ischemia”, “neuronal necrosis”, “glutamate”, “γ-aminobutyric acid”.

Results:

Insulin has neuroprotection. The mechanisms include the regulation of neurotransmitter, promoting glycogen synthesis, and inhibition of neuronal necrosis and apoptosis. Insulin could play its role in neuroprotection by avoiding hypoglycemia and hyperglycemia.

Conclusions:

Intermittent and long-term infusion insulin may be a benefit for patients with ischemic brain damage at blood glucose 6–9 mmol/L.

Metabolic Alterations Associated to Brain Dysfunction in Diabetes

From epidemiological studies it is known that diabetes patients display increased risk of developing dementia. Moreover, cognitive impairment and Alzheimer's disease (AD) are also accompanied by impaired glucose homeostasis and insulin signalling. Although there is plenty of evidence for a connection between insulin-resistant diabetes and AD, definitive linking mechanisms remain elusive. Cerebrovascular complications of diabetes, alterations in glucose homeostasis and insulin signalling, as well as recurrent hypoglycaemia are the factors that most likely affect brain function and structure. While difficult to study in patients, the mechanisms by which diabetes leads to brain dysfunction have been investigated in experimental models that display phenotypes of the disease. The present article reviews the impact of diabetes and AD on brain structure and function, and discusses recent findings from translational studies in animal models that link insulin resistance to metabolic alterations that underlie brain dysfunction. Such modifications of brain metabolism are likely to occur at early stages of neurodegeneration and impact regional neurochemical profiles and constitute non-invasive biomarkers detectable by magnetic resonance spectroscopy (MRS).

Brain Glycogen Decreases During Intense Exercise Without Hypoglycemia: The Possible Involvement of Serotonin

Brain glycogen stored in astrocytes, a source of lactate as a neuronal energy source, decreases during prolonged exercise with hypoglycemia. However, brain glycogen dynamics during exercise without hypoglycemia remain unknown. Since intense exercise increases brain noradrenaline and serotonin as known inducers for brain glycogenolysis, we hypothesized that brain glycogen decreases with intense exercise not accompanied by hypoglycemia. To test this hypothesis, we employed a well-established acute intense exercise model of swimming in rats. Rats swam for fourteen 20 s bouts with a weight equal to 8 % of their body mass and were sacrificed using high-power (10 kW) microwave irradiation to inactivate brain enzymes for accurate detection of brain glycogen and monoamines. Intense exercise did not alter blood glucose, but did increase blood lactate levels. Immediately after exercise, brain glycogen decreased and brain lactate increased in the hippocampus, cerebellum, cortex, and brainstem. Simultaneously, serotonin turnover in the hippocampus and brainstem mutually increased and were associated with decreased brain glycogen. Intense swimming exercise that does not induce hypoglycemia decreases brain glycogen associated with increased brain lactate, implying an importance of glycogen in brain energetics during intense exercise even without hypoglycemia. Activated serotonergic regulation is a possible underlying mechanism for intense exercise-induced glycogenolysis at least in the hippocampus and brainstem.

Brain glycogen supercompensation after different conditions of induced hypoglycemia and sustained swimming in rainbow trout (Oncorhynchus mykiss)

Brain glycogen is depleted when used as an emergency energy substrate. In mammals, brain glycogen levels rebound to higher than normal levels after a hypoglycemic episode and a few hours after refeeding or administration of glucose. This phenomenon is called glycogen supercompensation. However, this mechanism has not been investigated in lower vertebrates. The aim of this study was therefore to determine whether brain glycogen supercompensation occurs in the rainbow trout brain. For this purpose, short-term brain glucose and glycogen contents were determined in rainbow trout after being subjected to the following experimental conditions: i) a 5-day or 10-day fasting period and refeeding; ii) a single injection of insulin (4 mg kg(-1)) and refeeding; and iii) sustained swimming and injection of glucose (500 mg kg(-1)). Food deprivation during the fasting periods and insulin administration both induced a decrease in glucose and glycogen levels in the brain. However, only refeeding after 10 days of fasting significantly increased the brain glycogen content above control levels, in a clear short-term supercompensation response. Unlike in mammals, prolonged exercise did not alter brain glucose or glycogen levels. Furthermore, brain glycogen supercompensation was not observed after glucose administration in fish undergoing sustained swimming. To our knowledge, this is the first study providing direct experimental evidence for the existence of a short-term glycogen supercompensation response in a teleost brain, although the response was only detectable after prolonged fasting.

Hypoglycemia-associated changes in the electroencephalogram in patients with type 1 diabetes and normal hypoglycemia awareness or unawareness

Hypoglycemia is associated with increased activity in the low-frequency bands in the electroencephalogram (EEG). We investigated whether hypoglycemia awareness and unawareness are associated with different hypoglycemia-associated EEG changes in patients with type 1 diabetes. Twenty-four patients participated in the study: 10 with normal hypoglycemia awareness and 14 with hypoglycemia unawareness. The patients were studied at normoglycemia (5-6 mmol/L) and hypoglycemia (2.0-2.5 mmol/L), and during recovery (5-6 mmol/L) by hyperinsulinemic glucose clamp. During each 1-h period, EEG, cognitive function, and hypoglycemia symptom scores were recorded, and the counterregulatory hormonal response was measured. Quantitative EEG analysis showed that the absolute amplitude of the θ band and α-θ band up to doubled during hypoglycemia with no difference between the two groups. In the recovery period, the θ amplitude remained increased. Cognitive function declined equally during hypoglycemia in both groups and during recovery reaction time was still prolonged in a subset of tests. The aware group reported higher hypoglycemia symptom scores and had higher epinephrine and cortisol responses compared with the unaware group. In patients with type 1 diabetes, EEG changes and cognitive performance during hypoglycemia are not affected by awareness status during a single insulin-induced episode with hypoglycemia.

In vivo Magnetic Resonance Spectroscopy of cerebral glycogen metabolism in animals and humans

Glycogen serves as an important energy reservoir in the human body. Despite the abundance of glycogen in the liver and skeletal muscles, its concentration in the brain is relatively low, hence its significance has been questioned. A major challenge in studying brain glycogen metabolism has been the lack of availability of non-invasive techniques for quantification of brain glycogen in vivo. Invasive methods for brain glycogen quantification such as post mortem extraction following high energy microwave irradiation are not applicable in the human brain. With the advent of (13)C Magnetic Resonance Spectroscopy (MRS), it has been possible to measure brain glycogen concentrations and turnover in physiological conditions, as well as under the influence of stressors such as hypoglycemia and visual stimulation. This review presents an overview of the principles of the (13)C MRS methodology and its applications in both animals and humans to further our understanding of glycogen metabolism under normal physiological and pathophysiological conditions such as hypoglycemia unawareness.

Effect of insulin-induced hypoglycaemia on the peripheral nervous system: focus on adaptive mechanisms, pathogenesis and histopathological changes

Insulin-induced hypoglycaemia (IIH) is a common acute side effect in type 1 and type 2 diabetic patients, especially during intensive insulin therapy. The peripheral nervous system (PNS) depends on glucose as its primary energy source during normoglycaemia and, consequently, it may be particularly susceptible to IIH damage. Possible mechanisms for adaption of the PNS to IIH include increased glucose uptake, utilisation of alternative energy substrates and the use of Schwann cell glycogen as a local glucose reserve. However, these potential adaptive mechanisms become insufficient when the hypoglycaemic state exceeds a certain level of severity and duration, resulting in a sensory-motor neuropathy with associated skeletal muscle atrophy. Large myelinated motor fibres appear to be particularly vulnerable. Thus, although the PNS is not an obligate glucose consumer, as is the brain, it appears to be more prone to IIH than the central nervous system when hypoglycaemia is not severe (blood glucose level ≤ 2 mm), possibly reflecting a preferential protection of the brain during periods of inadequate glucose availability. With a primary focus on evidence from experimental animal studies investigating nondiabetic IIH, the present review discusses the effect of IIH on the PNS with a focus on adaptive mechanisms, pathogenesis and histological changes.

Neurons have an active glycogen metabolism that contributes to tolerance to hypoxia

Glycogen is present in the brain, where it has been found mainly in glial cells but not in neurons. Therefore, all physiologic roles of brain glycogen have been attributed exclusively to astrocytic glycogen. Working with primary cultured neurons, as well as with genetically modified mice and flies, here we report that-against general belief-neurons contain a low but measurable amount of glycogen. Moreover, we also show that these cells express the brain isoform of glycogen phosphorylase, allowing glycogen to be fully metabolized. Most importantly, we show an active neuronal glycogen metabolism that protects cultured neurons from hypoxia-induced death and flies from hypoxia-induced stupor. Our findings change the current view of the role of glycogen in the brain and reveal that endogenous neuronal glycogen metabolism participates in the neuronal tolerance to hypoxic stress.

Effect of insulin-induced hypoglycaemia on the central nervous system: evidence from experimental studies

Insulin-induced hypoglycaemia (IIH) is a major acute complication in type 1 as well as in type 2 diabetes, particularly during intensive insulin therapy. The brain plays a central role in the counter-regulatory response by eliciting parasympathetic and sympathetic hormone responses to restore normoglycaemia. Brain glucose concentrations, being approximately 15-20% of the blood glucose concentration in humans, are rigorously maintained during hypoglycaemia through adaptions such as increased cerebral glucose transport, decreased cerebral glucose utilisation and, possibly, by using central nervous system glycogen as a glucose reserve. However, during sustained hypoglycaemia, the brain cannot maintain a sufficient glucose influx and, as the cerebral hypoglycaemia becomes severe, electroencephalogram changes, oxidative stress and regional neuronal death ensues. With particular focus on evidence from experimental studies on nondiabetic IIH, this review outlines the central mechanisms behind the counter-regulatory response to IIH, as well as cerebral adaption to avoid sequelae of cerebral neuroglycopaenia, including seizures and coma.

Hypoglycemia-associated autonomic failure in healthy humans: comparison of two vs three periods of hypoglycemia on hypoglycemia-induced counterregulatory and symptom response 5 days later

CONTEXT

Hypoglycemia-associated autonomic failure (HAAF) limits the ability of patients with diabetes to achieve target glycemia. Animal models have provided insights into the pathogenesis of HAAF, but a robust human model of HAAF in which recurrent hypoglycemia impacts the counterregulatory responses to hypoglycemia days later is lacking.

OBJECTIVE

The aim of this study was to determine the impact of two or three episodes of moderate hypoglycemia on counterregulatory responses to subsequent hypoglycemia induced 5 days later.

DESIGN AND SUBJECTS

Six healthy subjects participated in each of the two study protocols. In both protocol 1 and 2, subjects underwent two 2-hour hypoglycemic clamp studies during the morning and afternoon of day 1. In protocol 2, subjects underwent an additional third hypoglycemic clamp during the morning of day 2. All subjects in both protocols underwent a final hypoglycemic clamp on the morning of day 5.

RESULTS

In protocol 1, there were no significant differences in the hypoglycemia-induced hormone response or in symptoms scores between the mornings of days 1 and 5. In protocol 2, hypoglycemia-induced epinephrine (P = .02) and cortisol (P = .04) secretions were significantly lower on day 5 compared with day 1, whereas glucagon (P = .08) and norepinephrine (P = .59) were not different. Also in protocol 2, neurogenic (P = .02) and neuroglycopenic (P = .04) symptoms during hypoglycemia were decreased on day 5 compared with day 1.

CONCLUSION

These results demonstrate that exposure of healthy humans to three 2-hour hypoglycemic episodes over 30 hours leads to significant blunting in counterregulatory and symptom response to subsequent hypoglycemia on day 5.

Metabolic Flux and Compartmentation Analysis in the Brain In vivo

Through significant developments and progresses in the last two decades, in vivo localized nuclear magnetic resonance spectroscopy (MRS) became a method of choice to probe brain metabolic pathways in a non-invasive way. Beside the measurement of the total concentration of more than 20 metabolites, (1)H MRS can be used to quantify the dynamics of substrate transport across the blood-brain barrier by varying the plasma substrate level. On the other hand, (13)C MRS with the infusion of (13)C-enriched substrates enables the characterization of brain oxidative metabolism and neurotransmission by incorporation of (13)C in the different carbon positions of amino acid neurotransmitters. The quantitative determination of the biochemical reactions involved in these processes requires the use of appropriate metabolic models, whose level of details is strongly related to the amount of data accessible with in vivo MRS. In the present work, we present the different steps involved in the elaboration of a mathematical model of a given brain metabolic process and its application to the experimental data in order to extract quantitative brain metabolic rates. We review the recent advances in the localized measurement of brain glucose transport and compartmentalized brain energy metabolism, and how these reveal mechanistic details on glial support to glutamatergic and GABAergic neurons.

Effects of hypoglycaemia on neuronal metabolism in the adult brain: role of alternative substrates to glucose

Hypoglycaemia is characterized by decreased blood glucose levels and is associated with different pathologies (e.g. diabetes, inborn errors of metabolism). Depending on its severity, it might affect cognitive functions, including impaired judgment and decreased memory capacity, which have been linked to alterations of brain energy metabolism. Glucose is the major cerebral energy substrate in the adult brain and supports the complex metabolic interactions between neurons and astrocytes, which are essential for synaptic activity. Therefore, hypoglycaemia disturbs cerebral metabolism and, consequently, neuronal function. Despite the high vulnerability of neurons to hypoglycaemia, important neurochemical changes enabling these cells to prolong their resistance to hypoglycaemia have been described. This review aims at providing an overview over the main metabolic effects of hypoglycaemia on neurons, covering in vitro and in vivo findings. Recent studies provided evidence that non-glucose substrates including pyruvate, glycogen, ketone bodies, glutamate, glutamine, and aspartate, are metabolized by neurons in the absence of glucose and contribute to prolong neuronal function and delay ATP depletion during hypoglycaemia. One of the pathways likely implicated in the process is the pyruvate recycling pathway, which allows for the full oxidation of glutamate and glutamine. The operation of this pathway in neurons, particularly after hypoglycaemia, has been re-confirmed recently using metabolic modelling tools (i.e. Metabolic Flux Analysis), which allow for a detailed investigation of cellular metabolism in cultured cells. Overall, the knowledge summarized herein might be used for the development of potential therapies targeting neuronal protection in patients vulnerable to hypoglycaemic episodes.

Defective counterregulation and hypoglycemia unawareness in diabetes: mechanisms and emerging treatments

For people with diabetes, hypoglycemia remains the limiting factor in achieving glycemic control. This article reviews recent advances in how the brain senses and responds to hypoglycemia. Novel mechanisms by which individuals with insulin-treated diabetes develop hypoglycemia unawareness and impaired counterregulatory responses are outlined. Prevention strategies for reducing the incidence of hypoglycemia are discussed.

Brain glycogen content and metabolism in subjects with type 1 diabetes and hypoglycemia unawareness

Supercompensated brain glycogen may contribute to the development of hypoglycemia unawareness in patients with type 1 diabetes by providing energy for the brain during periods of hypoglycemia. Our goal was to determine if brain glycogen content is elevated in patients with type 1 diabetes and hypoglycemia unawareness. We used in vivo (13)C nuclear magnetic resonance spectroscopy in conjunction with [1-(13)C]glucose administration in five patients with type 1 diabetes and hypoglycemia unawareness and five age-, gender-, and body mass index-matched healthy volunteers to measure brain glycogen content and metabolism. Glucose and insulin were administered intravenously over ∼51 hours at a rate titrated to maintain a blood glucose concentration of 7 mmol/L. (13)C-glycogen levels in the occipital lobe were measured at ∼5, 8, 13, 23, 32, 37, and 50 hours, during label wash-in and wash-out. Newly synthesized glycogen levels were higher in controls than in patients (P<0.0001) for matched average blood glucose and insulin levels, which may be due to higher brain glycogen content or faster turnover in controls. Metabolic modeling indicated lower brain glycogen content in patients than in controls (P=0.07), implying that glycogen supercompensation does not contribute to the development of hypoglycemia unawareness in humans with type 1 diabetes.

The physiology and pathophysiology of the neural control of the counterregulatory response

Despite significant technological and pharmacological advancements, insulin replacement therapy fails to adequately replicate β-cell function, and so glucose control in type 1 diabetes mellitus (T1D) is frequently erratic, leading to periods of hypoglycemia. Moreover, the counterregulatory response (CRR) to falling blood glucose is impaired in diabetes, leading to an increased risk of severe hypoglycemia. It is now clear that the brain plays a significant role in the development of defective glucose counterregulation and impaired hypoglycemia awareness in diabetes. In this review, the basic intracellular glucose-sensing mechanisms are discussed, as well as the neural networks that respond to and coordinate the body's response to a hypoglycemic challenge. Subsequently, we discuss how the body responds to repeated hypoglycemia and how these adaptations may explain, at least in part, the development of impaired glucose counterregulation in diabetes.

Brain glycogen supercompensation following exhaustive exercise

Brain glycogen localized in astrocytes, a critical energy source for neurons, decreases during prolonged exhaustive exercise with hypoglycaemia. However, it is uncertain whether exhaustive exercise induces glycogen supercompensation in the brain as in skeletal muscle. To explore this question, we exercised adult male rats to exhaustion at moderate intensity (20 m min(-1)) by treadmill, and quantified glycogen levels in several brain loci and skeletal muscles using a high-power (10 kW) microwave irradiation method as a gold standard. Skeletal muscle glycogen was depleted by 82-90% with exhaustive exercise, and supercompensated by 43-46% at 24 h after exercise. Brain glycogen levels decreased by 50-64% with exhaustive exercise, and supercompensated by 29-63% (whole brain 46%, cortex 60%, hippocampus 33%, hypothalamus 29%, cerebellum 63% and brainstem 49%) at 6 h after exercise. The brain glycogen supercompensation rates after exercise positively correlated with their decrease rates during exercise. We also observed that cortical and hippocampal glycogen supercompensation were sustained until 24 h after exercise (long-lasting supercompensation), and their basal glycogen levels increased with 4 weeks of exercise training (60 min day(-1) at 20 m min(-1)). These results support the hypothesis that, like the effect in skeletal muscles, glycogen supercompensation also occurs in the brain following exhaustive exercise, and the extent of supercompensation is dependent on that of glycogen decrease during exercise across brain regions. However, supercompensation in the brain preceded that of skeletal muscles. Further, the long-lasting supercompensation of the cortex and hippocampus is probably a prerequisite for their training adaptation (increased basal levels), probably to meet the increased energy demands of the brain in exercising animals.

Noninvasive measurement of brain glycogen by nuclear magnetic resonance spectroscopy and its application to the study of brain metabolism

Glycogen is the reservoir for glucose in the brain. Beyond the general agreement that glycogen serves as an energy source in the central nervous system, its exact role in brain energy metabolism has yet to be elucidated. Experiments performed in cell and tissue culture and animals have shown that glycogen content is affected by several factors, including glucose, insulin, neurotransmitters, and neuronal activation. The study of in vivo glycogen metabolism has been hindered by the inability to measure glycogen noninvasively, but, in the past several years, the development of a noninvasive localized (13) C nuclear magnetic resonance (NMR) spectroscopy method has allowed the study of glycogen metabolism in the conscious human. With this technique, (13) C-glucose is administered intravenously, and its incorporation into and washout from brain glycogen is tracked. One application of this method has been to the study of brain glycogen metabolism in humans during hypoglycemia: data have shown that mobilization of brain glycogen is augmented during hypoglycemia, and, after a single episode of hypoglycemia, glycogen synthesis rate is increased, suggesting that glycogen stores rebound to levels greater than baseline. Such studies suggest that glycogen may serve as a potential energy reservoir in hypoglycemia and may participate in the brain's adaptation to recurrent hypoglycemia and eventual development of hypoglycemia unawareness. Beyond this focused area of study, (13) C NMR spectroscopy has a broad potential for application in the study of brain glycogen metabolism and carries the promise of a better understanding of the role of brain glycogen in diabetes and other conditions.