Contribution of excitatory amino acids to hypoglycemic counter-regulation

Pathophysiology and aetiology of hypoglycaemic crises

Hypoglycaemia is a common, life-threatening complication that occurs as a component of a wide variety of disease processes. Despite its frequent occurrence, information concerning the aetiology, characteristics and outcomes of hypoglycaemic crises in veterinary medicine is limited. This review summarises the current understanding of the pathophysiology of hypoglycaemia, the body's counter-regulatory response, underlying aetiologies, diagnosis and treatment. Disease mechanisms are discussed and published evidence in veterinary literature regarding prognostic indicators, prevalence, diagnosis and treatment is examined for hypoglycaemia-related disease processes including insulinoma, glucose-lowering toxins and medications.

Potential approaches to prevent hypoglycemia-associated autonomic failure

Clear health benefits are associated with intensive glucose control in type 1 diabetes mellitus (T1DM). However, maintaining near-normal glycemia remains an elusive goal for many patients, in large part owing to the risk of severe hypoglycemia. In fact, recurrent episodes of hypoglycemia lead to 'hypoglycemia-associated autonomic failure' (HAAF), characterized by defective counter-regulatory responses to hypoglycemia. Extensive studies to understand the mechanisms underlying HAAF have revealed multiple potential etiologies, suggesting various approaches to prevent the development of HAAF. In this review, we present an overview of the literature focused on pharmacological approaches that may prevent the development of HAAF. The purported underlying mechanisms of HAAF include: 1) central mechanisms (opioid receptors, ATP-sensitive K+(KATP) channels, adrenergic receptors, serotonin selective receptor inhibitors, γ-aminobuyric acid receptors, N-methyl D-aspartate receptors); 2) hormones (cortisol, estrogen, dehydroepiandrosterone (DHEA) or DHEA sulfate, glucagon-like peptide-1) and 3) nutrients (fructose, free fatty acids, ketones), all of which have been studied vis-à-vis their ability to impact the development of HAAF. A careful review of the current literature reveals many promising therapeutic approaches to treat or reduce this important limitation to optimal glycemic control.

Sex-Specific Life Course Changes in the Neuro-Metabolic Phenotype of Glut3 Null Heterozygous Mice: Ketogenic Diet Ameliorates Electroencephalographic Seizures and Improves Sociability

We tested the hypothesis that exposure of glut3+/- mice to a ketogenic diet ameliorates autism-like features, which include aberrant behavior and electrographic seizures. We first investigated the life course sex-specific changes in basal plasma-cerebrospinal fluid (CSF)-brain metabolic profile, brain glucose transport/uptake, glucose and monocarboxylate transporter proteins, and adenosine triphosphate (ATP) in the presence or absence of systemic insulin administration. Glut3+/- male but not female mice (5 months of age) displayed reduced CSF glucose/lactate concentrations with no change in brain Glut1, Mct2, glucose uptake or ATP. Exogenous insulin-induced hypoglycemia increased brain glucose uptake in glut3+/- males alone. Higher plasma-CSF ketones (β-hydroxybutyrate) and lower brain Glut3 in females vs males proved protective in the former while enhancing vulnerability in the latter. As a consequence, increased synaptic proteins (neuroligin4 and SAPAP1) with spontaneous excitatory postsynaptic activity subsequently reduced hippocampal glucose content and increased brain amyloid β1-40 deposition in an age-dependent manner in glut3+/- males but not females (4 to 24 months of age). We then explored the protective effect of a ketogenic diet on ultrasonic vocalization, sociability, spatial learning and memory, and electroencephalogram seizures in male mice (7 days to 6 to 8 months of age) alone. A ketogenic diet partially restored sociability without affecting perturbed vocalization, spatial learning and memory, and reduced seizure events. We conclude that (1) sex-specific and age-dependent perturbations underlie the phenotype of glut3+/- mice, and (2) a ketogenic diet ameliorates seizures caused by increased cortical excitation and improves sociability, but fails to rescue vocalization and cognitive deficits in glut3+/- male mice.

Evaluation of the role of N-methyl-D-aspartate (NMDA) receptors in insulin secreting beta-cells

The possibility that antagonism of N-methyl-D-aspartate (NMDA) receptors represent a novel drug target for diabetes prompted the current studies probing NMDA receptor function in the detrimental actions of homocysteine on pancreatic beta-cell function. Cellular insulin content and release, changes in membrane potential and intracellular Ca(2+) and gene expression were assessed following acute (20min) and long-term (18h) exposure of pancreatic clonal BRIN-BD11 beta-cells to known NMDA receptor modulators in the absence and presence of cytotoxic concentrations of homocysteine. As expected, acute or long-term exposure to homocysteine significantly suppressed basal and secretagogue-induced insulin release. In addition, NMDA reduced glucose-stimulated insulin secretion (GSIS). Interestingly, the selective NMDA receptor antagonist, MK-801, had no negative effects on GSIS. The effects of the NMDA receptor modulators were largely independent of effects on membrane depolarisation and increases of intracellular Ca(2+). However, combined culture of the NMDA antagonist, MK-801, with homocysteine did enhance intracellular Ca(2+) levels. Actions of NMDA agonists/antagonists and homocysteine on signal transduction pathways were independent of changes in cellular insulin content, cell viability, DNA damage or expression of key beta-cell genes. Taken together, the data support a role for NMDA receptors in controlling pancreatic beta-cell function. However, modulation of NMDA receptor function was unable to prevent the detrimental beta-cell effects of homocysteine.

The Relationship of Glucokinase Activator–induced Hypoglycemia with Arteriopathy, Neuronal Necrosis, and Peripheral Neuropathy in Nonclinical Studies

Glucokinase activators (GKAs) are being developed for the treatment of type 2 diabetes. The toxicity of 4 GKAs (PF-04279405, PF-04651887, piragliatin, and PF-04937319) was assessed in mice, rats, dogs, and/or monkeys. GKAs were administered for 2 to 8 weeks. Standard endpoints, glucose, and insulin were assessed. All compounds produced varying degrees of hypoglycemia in all species. Brain neuronal necrosis and/or peripheral neuropathy were observed with most compounds. These findings are consistent with literature reports linking hypoglycemia with nervous system effects. Arteriopathy, mainly of cardiac vessels, was observed at a low frequency in monkey and/or dog. Arteriopathy occurred only at doses that produced severe and prolonged periods of repeated hypoglycemia. Since this lesion occurred in multiple studies with structurally distinct GKAs, these results suggested arteriopathy was related to GKA pharmacology. The morphological characteristics of the arteriopathy were consistent with that produced by experimental catecholamine administration. We hypothesize that the prolonged periods of hypoglycemia resulted in increased local and/or systemic concentrations of catecholamines via a counterregulatory and/or stress-related mechanism. Alternatively, prolonged hypoglycemia may have resulted in endothelial dysfunction leading to arteriopathy. This risk can be managed in human patients in clinical studies by careful glucose monitoring and intervention to avoid prolonged episodes of hypoglycemia.

Interactive effects of neonatal exposure to monosodium glutamate and aspartame on glucose homeostasis

BACKGROUND

Recent evidence suggests that the effects of certain food additives may be synergistic or additive. Aspartame (ASP) and Monosodium Glutamate (MSG) are ubiquitous food additives with a common moiety: both contain acidic amino acids which can act as neurotransmitters, interacting with NMDA receptors concentrated in areas of the Central Nervous System regulating energy expenditure and conservation. MSG has been shown to promote a neuroendocrine dysfunction when large quantities are administered to mammals during the neonatal period. ASP is a low-calorie dipeptide sweetener found in a wide variety of diet beverages and foods. However, recent reports suggest that ASP may promote weight gain and hyperglycemia in a zebrafish nutritional model.

METHODS

We investigated the effects of ASP, MSG or a combination of both on glucose and insulin homeostasis, weight change and adiposity, in C57BL/6 J mice chronically exposed to these food additives commencing in-utero, compared to an additive-free diet. Pearson correlation analysis was used to investigate the associations between body characteristics and variables in glucose and insulin homeostasis.

RESULTS

ASP alone (50 mg/Kgbw/day) caused an increase in fasting blood glucose of 1.6-fold, together with reduced insulin sensitivity during an Insulin Tolerance Test (ITT) P < 0.05. Conversely MSG alone decreased blood triglyceride and total cholesterol (T-CHOL) levels. The combination of MSG (120 mg/Kgbw/day) and ASP elevated body weight, and caused a further increase in fasting blood glucose of 2.3-fold compared to Controls (prediabetic levels); together with evidence of insulin resistance during the ITT (P < 0.05). T-CHOL levels were reduced in both ASP-containing diets in both genders. Further analysis showed a strong correlation between body weight at 6 weeks, and body weight and fasting blood glucose levels at 17 weeks, suggesting that early body weight may be a predictor of glucose homeostasis in later life.

CONCLUSIONS

Aspartame exposure may promote hyperglycemia and insulin intolerance. MSG may interact with aspartame to further impair glucose homeostasis. This is the first study to ascertain the hyperglycemic effects of chronic exposure to a combination of these commonly consumed food additives; however these observations are limited to a C57BL/6 J mouse model. Caution should be applied in extrapolating these findings to other species.

BAD modulates counterregulatory responses to hypoglycemia and protective glucoprivic feeding

Hypoglycemia or glucoprivation triggers protective hormonal counterregulatory and feeding responses to aid the restoration of normoglycemia. Increasing evidence suggests pertinent roles for the brain in sensing glucoprivation and mediating counterregulation, however, the precise nature of the metabolic signals and molecular mediators linking central glucose sensing to effector functions are not fully understood. Here, we demonstrate that protective hormonal and feeding responses to hypoglycemia are regulated by BAD, a BCL-2 family protein with dual functions in apoptosis and metabolism. BAD-deficient mice display impaired glycemic and hormonal counterregulatory responses to systemic glucoprivation induced by 2-deoxy-D-glucose. BAD is also required for proper counterregulatory responses to insulin-induced hypoglycemia as evident from significantly higher glucose infusion rates and lower plasma epinephrine levels during hyperinsulinemic hypoglycemic clamps. Importantly, RNA interference-mediated acute knockdown of Bad in the brain provided independent genetic evidence for its relevance in central glucose sensing and proper neurohumoral responses to glucoprivation. Moreover, BAD deficiency is associated with impaired glucoprivic feeding, suggesting that its role in adaptive responses to hypoglycemia extends beyond hormonal responses to regulation of feeding behavior. Together, these data indicate a previously unappreciated role for BAD in the control of central glucose sensing.

Memantine prevents hypoglycemia-induced decrements of the cerebral energy status in healthy subjects

CONTEXT

The risk to develop dementia is significantly increased in diabetes mellitus. Memantine, an N-methyl-D-aspartate receptor antagonist, which is clinically applied in dementia, has been shown to exert neuroprotective effects under hypoglycemic conditions in rats.

OBJECTIVE

We hypothesized that memantine may prevent hypoglycemia-induced decrements in the cerebral high-energy phosphate, i.e. ATP, metabolism to exert its neuroprotective action under these conditions.

DESIGN AND PARTICIPANTS

In a randomized, double-blind crossover design, we applied memantine vs. placebo in 16 healthy male subjects and examined the cerebral high-energy phosphate metabolism by (31)phosphor magnetic resonance spectroscopy, hormonal counterregulation, and neurocognitive performance during hypoglycemic glucose clamp conditions.

RESULTS

We found increments in hormonal counterregulation and reduced neurocognitive performance during hypoglycemia (P < 0.05). Cerebral ATP levels increased upon hypoglycemia in the memantine condition as compared with placebo (P = 0.006) and remained higher after renormalizing blood glucose concentrations (P = 0.018), which was confirmed by ATP to inorganic phosphate ratio (P = 0.046). Phosphocreatine levels and phosphocreatine to inorganic phosphate ratio remained stable throughout the experiments and did not differ between conditions (P > 0.1 for both).

CONCLUSION

Our data demonstrate that memantine preserves the cerebral energy status during experimentally induced hypoglycemia in healthy subjects. An improved neuronal energy status may thus be involved in the neuroprotective effect under these conditions and may qualify memantine as potential future option to combat cognitive impairments and dementia in diabetes.

Blocking AMPA receptor signalling by caroverine infusion does not affect counter-regulation of hypoglycaemia in healthy men

AIMS/HYPOTHESIS

Glutamatergic pathways are assumed to play a critical role in the hormonal stress response to hypoglycaemia. In rats, glutamate signalling at the amino-3-hydroxy-5-methyl-4-isoxazol propionate (AMPA) receptor contributes to hormone release induced by behavioural stressors. We hypothesised that blocking the AMPA receptor by caroverine in healthy men would impair their perception of neuroglycopenia and thereby diminish hormonal counter-regulation as well as symptoms of hypoglycaemia, as a model of stress.

METHODS

In a balanced double-blind study, two hypoglycaemic clamp sessions (mean blood glucose 2.4 mmol/l for 50 min) were performed in ten healthy men during intravenous administration of 80 mg caroverine or placebo. We assessed concentrations of counter-regulatory hormones as well as subjective symptoms related to hypoglycaemia.

RESULTS

AMPA receptor antagonisation by caroverine did not influence the perception of neuroglycopenic and autonomic hypoglycaemia-associated symptoms (p > 0.39 for all). Notwithstanding, caroverine did increase basal and counter-regulatory glucagon secretion (p < 0.002) and slightly enhanced counter-regulatory growth hormone concentrations (p = 0.07). Counter-regulatory release of ACTH, cortisol, adrenaline (epinephrine) and noradrenaline (norepinephrine) did not differ between conditions (p > 0.11 for all).

CONCLUSIONS/INTERPRETATION

Antagonising AMPA receptor signalling by caroverine infusion failed to diminish and even slightly amplified counter-regulatory hormone release during hypoglycaemia in healthy men. The discrepancy with previous findings in rats may be due to different dosages or administration routes and calls for further investigations on the role of AMPA receptor signalling in hypoglycaemia counter-regulation in humans.

Blocking NMDA receptor signaling does not decrease hormonal counterregulation to hypoglycemia in humans

In animals, blocking of glutamate signaling at the N-methyl-D-aspartate (NMDA) receptor reduces the neuroendocrine counterregulation to hypoglycemia. Hence, it has been proposed that increased excitatory glutamatergic input to the hypothalamus signals enforced central nervous energy demand under conditions of reduced supply. We examined the effect of the NMDA receptor antagonist memantine on hypoglycemia counterregulation in healthy humans. Hypoglycemic clamp experiments were performed in 10 healthy men after oral administration of 20 mg memantine and placebo. Counterregulatory hormones were measured during baseline and a clamp period of 120 min with hypoglycemia of 2.4 mmol/l lasting for 50 min. In addition, symptoms related to glycemic changes were assessed. Unexpectedly, the counterregulatory responses to hypoglycemia of adrenocorticotropin, cortisol and epinephrine were not decreased but tended to be increased by memantine, while norepinephrine and growth hormone were not affected. Glucagon levels were increased by memantine treatment during baseline and throughout the hypoglycemic period. After memantine administration, subjects also experienced more neuroglycopenic symptoms during hypoglycemia, whereas differences in autonomic symptoms did not reach significance. Contrasting with findings in animals, blocking the NMDA receptor does not decrease the counterregulatory responses to hypoglycemia in humans. Our data do not support the view that in humans, enhanced glutamate signaling during hypoglycemia supports the satisfaction of increased central nervous energy demands by enforcing hormonal counterregulation.

[Metabolic syndrome. Origin within the central nervous system?]

All efforts based on current concepts of obesity have failed to stop the epidemic. Hitherto, the question of body mass regulation focused on regulatory principles centered on the hypothalamus. We present the novel view that the brain (cerebral hemispheres, hypothalamus) requests energy in an active manner from the body (allocation) or the environment (food intake). Disruption of one of the cerebral energy request pathways is highly relevant to the development of obesity, metabolic syndrome and diabetes type 2. We have reviewed the literature from this new perspective, putting the brain as the focal midpoint of all metabolic activity.

Propoxyphene-induced hypoglycemia in renal failure

OBJECTIVE

To present a case of symptomatic hypoglycemia induced by propoxyphene.

METHODS

The historical features, results of laboratory evaluations, and clinical course of a man with end-stage renal disease in whom hypoglycemia developed during treatment with propoxyphene are described.

RESULTS

A 54-year-old man with chronic renal failure had recurrent episodes of hypoglycemia (plasma glucose level, 40 mg/dL). While he continued treatment with propoxyphene, 58 hours into a 72-hour fast the plasma glucose concentration was 38 mg/dL, in conjunction with a beta-hydroxybutyric acid level of 0.9 mmol/L and inappropriately elevated plasma insulin, serum C-peptide, and proinsulin levels. A plasma drug screen was negative for sulfonylureas. A selective arterial calcium stimulation test yielded negative results. A 72-hour fast after discontinuation of propoxyphene therapy resulted in no hypoglycemia, and he experienced no hypoglycemic episodes for at least 2 years after withdrawal of the propoxyphene treatment.

CONCLUSION

The importance of obtaining a thorough medication history in a patient with renal failure and hypoglycemic episodes is highlighted by this case of propoxyphene-induced hypoglycemia. The mechanism of this effect is not known, although non-micro receptor agonism or noncompetitive N-methyl-D-aspartate receptor antagonism may have a role in causing hypoglycemia.

The selfish brain: competition for energy resources

Although the brain constitutes only 2% of the body mass, its metabolism accounts for 50% of total body glucose utilization. This delicate situation is aggravated by the fact that the brain depends on glucose as energy substrate. Thus, the contour of a major problem becomes evident: how can the brain maintain constant fluxes of large amounts of glucose to itself in the presence of powerful competitors as fat and muscle tissue. Activity of cortical neurons generates an "energy on demand" signal which eventually mediates the uptake of glucose from brain capillaries. Because energy stores in the circulation (equivalent to ca. 5 g glucose) are also limited, a second signal is required termed "energy on request"; this signal is responsible for the activation of allocation processes. The term "allocation" refers to the activation of the "behavior control column" by an input from the hippocampus-amygdala system. As far as eating behavior is concerned the behavior control column consists of the ventral medial hypothalamus (VMH) and periventricular nucleus (PVN). The PVN represents the central nucleus of the brain's stress systems, the hypothalamus-pituitary-adrenal (HPA) axis and the sympathetic nervous system (SNS). Activation of the sympatico-adrenal system inhibits glucose uptake by peripheral tissues by inhibiting insulin release and inducing insulin resistance and increases hepatic glucose production. With an inadequate "energy on request" signal neuroglucopenia would be the consequence. A decrease in brain glucose can activate glucose-sensitive neurons in the lateral hypothalamus (LH) with the release of orexigenic peptides which stimulate food intake. If the energy supply of the brain depends on activation of the LH rather than on increased allocation to the brain, an increase in body weight is evitable. An increase in fat mass will generate feedback signals as leptin and insulin, which activate the arcuate nucleus. Activation of arcuate nucleus in turn will stimulate the activity of the PVN in a way similar to the activation by the hippocampus-amydala system. The activity of PVN is influenced by the hippocampal outflow which in turn is the consequence of a balance of low-affinity and high-affinity glucocorticoid receptors. This set-point can permanently be displaced by extreme stress situations, by starvation, exercise, hormones, drugs or by endocrine-disrupting chemicals. Disorders in the "energy on request" process will influence the allocation of energy and in so doing alter the body mass of the organism. In this "selfish brain theory" the neocortex and the limbic system play a central role in the pathogenesis of diseases, such as anorexia nervosa, obesity and diabetes mellitus type II. From these considerations it appears that the primary disturbance in obesity is a displacement of the hippocampal set-point of the system. The resulting permanent activation of the feedback system must result in a likewise permanent activation of the sympatico-adrenal system, which induces insulin resistance, hypertension and the other components of the metabolic syndrome. Available therapies for treatment of the metabolic syndrome (blockade of alpha- and beta-adrenergic receptors, insulin and insulin secretagogues) interfere with mechanisms, which must be considered compensatory. This explains why these therapies are disappointing in the long run. New therapeutic strategies based on the "selfish brain theory" will be discussed.

Comparative study of perturbations of peripheral markers in different stressors in rats

Stress has been implicated in the etiopathogenesis of several diseases. In the present study, the effects of acute (AS), chronic (CS), and chronic unpredictable stress (CUS) were studied on the ulcer index, adrenal gland mass, and biochemical and hormonal changes in rats. The stress was provided in the form of immobilization-immobilization for 150 min, once only, and for 10 consecutive days in CS and CUS. In CUS, animals received variable unpredictable stressors. Immediately after stress, animals were decapitated, blood was collected, and plasma was separated for the estimation of plasma glucose, triglyceride, cholesterol, creatine kinase (CK), corticosterone, and insulin. The adrenal gland and stomach were also dissected for mass and ulcer scoring, respectively. AS significantly increased the ulcer index, plasma glucose, CK, corticosterone, and insulin. CS and CUS significantly increased the ulcer index, adrenal gland mass, and corticosterone. In CS, a significant decrease in plasma triglyceride and cholesterol levels was found, but in CUS only cholesterol was decreased significantly. High CK activity and hyperglycemia maintain the energy demands of metabolism, and elevated corticosterone desensitizes the insulin receptor in AS. In CS and CUS, prolonged elevation of corticosterone shifts metabolism to utilization of lipids as a secondary substrate by gluconeogenesis. From our experiment, it is clear that AS causes maximum activation of energy metabolism, which becomes specific after habituation in prolonged CS. These biochemical manipulations in the body by using different types of stressors are good markers that can be of great use to understand, target, and manage stress-induced etiologies.

The selfish brain: competition for energy resources

The brain occupies a special hierarchical position in the organism. It is separated from the general circulation by the blood-brain barrier, has high energy consumption and a low energy storage capacity, uses only specific substrates, and it can record information from the peripheral organs and control them. Here we present a new paradigm for the regulation of energy supply within the organism. The brain gives priority to regulating its own adenosine triphosphate (ATP) concentration. In that postulate, the peripheral energy supply is only of secondary importance. The brain has two possibilities to ensure its energy supply: allocation or intake of nutrients. The term 'allocation' refers to the allocation of energy resources between the brain and the periphery. Neocortex and the limbic-hypothalamus-pituitary-adrenal (LHPA) system control the allocation and intake. In order to keep the energy concentrations constant, the following mechanisms are available to the brain: (1) high and low-affinity ATP-sensitive potassium channels measure the ATP concentration in neurons of the neocortex and generate a 'glutamate command' signal. This signal affects the brain ATP concentration by locally (via astrocytes) stimulating glucose uptake across the blood-brain barrier and by systemically (via the LHPA system) inhibiting glucose uptake into the muscular and adipose tissue. (2) High-affinity mineralocorticoid and low-affinity glucocorticoid receptors determine the state of balance, i.e. the setpoint, of the LHPA system. This setpoint can permanently and pathologically be displaced by extreme stress situations (chronic metabolic and psychological stress, traumatization, etc.), by starvation, exercise, infectious diseases, hormones, drugs, substances of abuse, or chemicals disrupting the endocrine system. Disorders in the 'energy on demand' process or the LHPA-system can influence the allocation of energy and in so doing alter the body mass of the organism. In summary, the presented model includes a newly discovered 'principle of balance' of how pairs of high and low-affinity receptors can originate setpoints in biological systems. In this 'Selfish Brain Theory', the neocortex and limbic system play a central role in the pathogenesis of diseases such as anorexia nervosa and obesity.