Central NMDA enhances hepatic glucose output and non-insulin-mediated glucose uptake by a nonadrenergic mechanism

Does lithium attenuate the liver damage due to oxidative stress and liver glycogen depletion in experimental common bile duct obstruction?

In extrahepatic cholestasis, the molecular mechanisms of liver damage due to bile acid accumulation remain elusive. In this study, the activation of glutamatergic receptors was hypothesized to be responsible for bile acid-induced oxidative stress and liver damage. Recent evidence showed that lithium, as an N-methyl-d-aspartate receptor (NMDAR) GluN2B subunit inhibitor, may act on the glutamate/NMDAR signaling axis. Guinea pigs were assigned to four groups, as sham laparotomy (SL), bile duct ligated (BDL), lithium-treated SL (SL + Li) and lithium-treated BDL (BDL + Li) groups. Cholestasis-induced liver injury was evaluated by aspartate aminotransferase (AST), alanine transaminase (ALT), interleukin-6 (IL-6), tissue malondialdehyde (MDA), copper‑zinc superoxide dismutase and reduced glutathione levels. The liability of glutamate/NMDAR signaling axis was clarified by glutamate levels in both plasma and liver samples, with the production of nitric oxide (NO), as well as with the serum calcium concentrations. Blood glucose, glucagon, insulin levels and glucose consumption rates, in addition to tissue glycogen were measured to evaluate the liver glucose-glycogen metabolism. A high liver damage index (AST/ALT) was calculated in BDL animals in comparison to SL group. In the BDL animals, lithium reduced plasma NO and glutamate in addition to tissue glutamate concentrations, while serum calcium increased. The antioxidant capacities and liver glycogen contents significantly increased, whereas blood glucose levels unchanged and tissue MDA levels decreased 3-fold in lithium-treated cholestatic animals. It was concluded that lithium largely protects the cholestatic hepatocyte from bile acid-mediated damage by blocking the NMDAR-GluN2B subunit.

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.

The hypothalamic clock and its control of glucose homeostasis

The everyday life of mammals, including humans, exhibits many behavioral, physiological and endocrine oscillations. The major timekeeping mechanism for these rhythms is contained in the central nervous system (CNS). The output of the CNS clock not only controls daily rhythms in sleep/wake (or feeding/fasting) behavior but also exerts a direct control over glucose metabolism. Here, we show how the biological clock plays an important role in determining early morning (fasting) plasma glucose concentrations by affecting hepatic glucose production and glucose uptake, as well as glucose tolerance, by determining feeding-induced insulin responses. Recently, large-scale genetic studies in humans provided the first evidence for the involvement of disrupted (clock gene) rhythms in the pathogenesis of type 2 diabetes.

[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.

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.

Route-dependent effects of the non-NMDA receptor antagonist CNQX on plasma corticosterone levels in mice

A non-N-methyl-D-aspartate (non-NMDA) receptor antagonist, 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX), administered intracerebroventricularly (0.1-0.5 microg), significantly inhibited the immobilization stress-induced plasma corticosterone levels in a dose-dependent manner. However, CNQX administered intraperitoneally (1-10 mg/kg) showed a dose-dependent increase of basal plasma corticosterone levels in nonstressed mice and an additive increase of plasma corticosterone levels at the dose of 10 mg/kg in 1 h immobilization-stressed mice. The results suggest that the central and peripheral non-NMDA receptors may be differently involved in the regulation of plasma corticosterone levels in non- and immobilization-stressed mice.

Endogenous activation of group-II metabotropic glutamate receptors inhibits the hypothalamic-pituitary-adrenocortical axis

Systemic injection of the mGlu2/3 receptor antagonist, LY341495 (1 mg/kg, i.p.), increased plasma corticosterone in mice to an extent similar to that induced by the despair test. Treatment with the mGlu2/3 receptor agonist, LY379268 (1 mg/kg, i.p.), or the non-competitive mGlu5 receptor antagonist, MPEP (5 mg/kg, i.p.), failed to induce significant changes in corticosterone levels. Searching for a site of action of LY341495, we examined the expression of mGlu receptor subtypes in the various anatomical regions of the mouse hypothalamic-pituitary-adrenal (HPA) axis. Only mGlu5 and -7 receptor mRNAs were detected in the adrenal gland by RT-PCR, whereas mGlu -1, -3, -4, -5, -7 and -8 receptor mRNAs were detected in the anterior pituitary. All transcripts (with the exception of mGlu5 and mGlu6 receptor mRNAs) were detected in the hypothalamus. However, Western blot analysis showed the presence of mGlu2/3 receptor proteins only in the hypothalamus and not in the anterior pituitary. This was consistent with functional data showing that LY341495 (0.1 and 1 microM) failed to affect ACTH secretion from isolated mouse anterior pituitaries. Moving from these observations, we examined whether LY341495 could activate the HPA axis by inhibiting mGlu2/3 receptors at hypothalamic level. We measured the release of corticotropin releasing hormone (CRH) in isolated mouse hypothalami incubated in the presence of subtype-selective mGlu receptor agonists or antagonists. Among all the drugs we have tested, only LY341495 was able to increase CRH secretion. With high concentrations of LY341495 (1 microM) this increase was similar to that induced by 50 mM K(+). The action of LY341495 was prevented by the combined application of the mGlu2/3 receptor agonist, LY379268. We conclude that group-II mGlu receptors tonically regulate the HPA axis by controlling CRH secretion at hypothalamic level.

Changes in rat serum corticosterone after treatment with metabotropic glutamate receptor agonists or antagonists

From previous work, it appears that glutamate can activate the hypothalamic-pituitary-adrenocortical (HPA) axis by an interaction at either ionotopic or metabotropic (G-protein coupled) receptors. For example, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD), a metabotropic glutamate (mGlu) receptor agonist, has been shown to increase the levels of serum corticosterone in rats. The present study was undertaken to further characterize which of the mGlu receptors are substantially involved in control of the HPA axis. The group I mGlu receptor agonists, 3,5-dihydroxyphenylglycine (DHPG), 1S,3R-ACPD, and 2-chloro-5-hydroxyphenylglycine (CHPG) but not the inactive isomer 1R,3S-ACPD were found to dose-dependently increase serum corticosterone 1 h after intracerebroventricular (i.c.v.) injection in male rats. The relative potency, DHPG (EC50 = 520 nmol) > 1S,3R-ACPD (1.4 micromol) = CHPG (2.7 micromol) >> 1R,3S-ACPD (>> 3 micromol) is consistent with activation of group I (mGlu1/5) receptors. The effects of DHPG were long lasting with substantial elevations in corticosterone remaining for at least 3 h. In a similar manner, the group III mGlu receptor agonists, L-AP4 (4-phosphono-2-aminobutyric acid) and L-SOP (serine-O-phosphate), were found to increase serum corticosterone levels at 1 h. In contrast, the mGlu group II selective agonists LY354740 (10 mg/kg, i.p.) and subtype-selective doses of the group II antagonist LY341495 (1 mg/kg, i.p.) did not significantly elevate serum corticosterone. Given the group I agonists results, it was surprising to find that group I selective and mGlu1 selective antagonists given alone also increased serum corticosterone. As with the agonists, the rise in serum corticosterone with LY393675 (an mGlu1/5 antagonist, EC50 = 20 nmol, i.c.v.) and LY367385 (an mGlu1 antagonist, 325 nmol, i.c.v.) were dose-dependent and consistent with their relative affinity for the group I mGlu receptors. The selective mGlu5 antagonist MPEP [2-methyl-6-(phenylethylnyl)pyridine] increased serum corticosterone but only at high doses (> 30 mg/kg, i.p.). A model involving the high glutamatergic tone on GABAergic interneurons in the paraventricular nucleus of the hypothalamus is discussed as a possible explanation for these results.

Contribution of excitatory amino acids to hypoglycemic counter-regulation

We determined the contribution of central N-methyl-D-aspartate (NMDA) receptor activation to the neuro-endocrine counter-regulatory response to insulin-induced hypoglycemia. Glucose kinetics, gluconeogenic substrate balance and counter-regulatory hormonal responses were determined in two groups of conscious dogs fitted with chronic vascular catheters and intracerebroventricular (i.c.v.) cannula. Peripheral insulin infusion (5 mU/kg per min for 3 h) decreased plasma glucose levels 40% and increased the rate of glucose appearance (R(a)) 2-fold. This was associated with significant increases in net hepatic uptake of glycerol and lactate, without any change in the net hepatic uptake of alanine. i.c.v. pretreatment with MK-801, an NMDA receptor antagonist, blunted (50%) the rise in glucose R(a) as well as the increase in the net hepatic uptake of glycerol and lactate. Hypoglycemia increased plasma cortisol (3-fold to 14.3+/-1 mg/dl) and epinephrine levels (14-fold to 3811+/-172 pg/ml), and this stress response was attenuated (30% and 60%, respectively) by MK-801 pretreatment. In controls, MK-801 did not alter the increase in norepinephrine or glucagon elicited by hypoglycemia. These results indicate that during hypoglycemia, central excitatory amino acids contribute to the modulation of the glucoregulatory response through activation of NMDA receptors, resulting in stimulation of the sympathoadrenal and hypothalamic-pituitary adrenal axis. This mechanism appears to play an important role in the sustained elevation in hepatic glucose production during hypoglycemia.

Sustained effects of repeated restraint stress on muscle and adipocyte metabolism in high-fat-fed rats

Repeated restraint stress 3 h/day for 3 days in rats causes a temporary hypophagia but a sustained weight loss. We investigated whether poststress changes in peripheral tissue metabolism contributed to these responses. One day after the last restraint, insulin sensitivity, measured by oral glucose tolerance test, was improved in restrained rats. Restraint and pair-fed rats weighed less than controls, but body fat content was the same in all groups. Muscle glucose uptake, measured in vitro, was not changed by treatment, whereas in vitro adipocyte glucose uptake was substantially inhibited only in restrained rats. Adipocytes from restrained rats had elevated rates of fatty acid oxidation but not fatty acid esterification, indicating a shift in energy supply from glucose to fatty acids. Five days after the last restraint, the reduced weight of restrained and pair-fed rats resulted from loss of both lean and fat tissue. These results demonstrate that restraint caused sustained, tissue-specific changes in metabolism that may contribute to changes in body composition and body weight of the rats.

Contribution of excitatory amino acids to morphine-induced metabolic alterations

Previous studies have indicated that excitatory amino acids are involved in the analgesic and addictive properties of morphine. However, their role in the morphine-induced alterations in glucose metabolism is not known. This study assessed the contribution of NMDA receptor activation to the morphine-induced hormonal and metabolic alterations in conscious unrestrained chronically catheterized rats. Whole body glucose flux was assessed with a primed constant intravenous infusion of [3-3H]glucose in rats pretreated with the NMDA-receptor antagonist MK-801 (0.25 mg/kg, intraarterial) or an equal volume (1.5 ml) of sterile saline (0.9%) administered 15 min prior to i.c.v. injection of H2O (Con; 5 microliters) or morphine sulfate (80 micrograms). No significant alterations were noted in metabolic and hormonal parameters of H2O injected rats. i.c.v. morphine increased the plasma glucose concentration (60%), hepatic glucose production (Ra; 60%) and whole body glucose utilization (Rd; 53%), but did not alter the glucose metabolic clearance rate (MCR). MK-801 alone resulted in transient hyperglycemia (25%), stimulation of glucose Ra (60%) and glucose Rd (53%), and a significant (30%) increase in MCR. MK-801 pretreatment blunted the morphine-induced hyperglycemia and the increased glucose Ra and Rd. Morphine increased the plasma concentration of epinephrine (4-fold), norepinephrine (2-fold) and corticosterone (67%); however, no alterations in plasma insulin and glucagon were detected. MK-801 pretreatment, blunted the morphine-induced increase in corticosterone and norepinephrine, and elicited a significant rise in insulin concentrations. These results indicate that activation of the NMDA receptors contributes to the morphine-induced hyperglycemia and hormonal alterations. Furthermore, this response appears partially mediated by activation of sympathetic outflow and suppression of insulin release, which is blunted by inhibition of NMDA receptors.

Inhibition of central GABAA receptors enhances hepatic glucose production and peripheral glucose uptake

Previous studies have demonstrated that intracerebroventricular (ICV) injection of bicuculline methiodide (BMI), a gamma-aminobutyric acid receptor antagonist, increases plasma glucose concentrations. The purpose of the present study was to determine whether the hyperglycemic response was due to an increased rate of hepatic glucose production (HGP) or a change in the rate of glucose utilization. In vivo glucose flux was assessed in catheterized, conscious overnight fasted rats using [3-3H]glucose. ICV injection of BMI (10 nmol) increased glucose levels 60% after 30 min. This hyperglycemia resulted from a rapid increase in HGP that exceeded an increased rate of glucose utilization. No alteration in the glucose metabolic clearance rate, an index of the avidity of the body's tissues for glucose, was detected in BMI-injected rats. BMI enhanced both hepatic gluconeogenesis and glycogenolysis, since the reduction in liver glycogen (19 mumol/g liver) could not totally account for all of the increased HGP. These metabolic alterations were associated with sustained increases in circulating concentrations of corticosterone, glucagon and catecholamines. Prior adrenalectomy completely abolished the BMI-induced increase in glucose flux and the reduction in tissue glycogen, despite the persistent hyperglucagonemia. These data indicate that, in the fasted condition, the hyperglycemia produced by central administration of BMI results from an increased rate of HGP (both gluconeogenesis and glycogenolysis) and not a reduction in the ability of tissues to use glucose. The concomitant elevation in glucose disposal was the result of an increased mass action effect. The enhanced glucose metabolic response to BMI appears mediated exclusively by an increased secretion of epinephrine from the adrenal medulla.

Contribution of central and peripheral adrenergic stimulation to IL-1 alpha-mediated glucoregulation

The present study determined the contribution of central adrenoceptors and the peripheral sympathetic nervous system in regulating the hormonal and glucose metabolic response to intracerebroventricular injection of interleukin (IL)-1 alpha. After an overnight fast, hepatic glucose production (HGP) and peripheral glucose uptake (GU) were assessed in catheterized conscious unrestrained rats using [3-3H]glucose. Intracerebroventricular injection of IL-1 alpha (100 ng) produced a hyperglycemia that resulted from an early increase in HGP (108%) that exceeded a smaller elevation (82%) in GU. Intracerebroventricular injection of the alpha- and beta-adrenergic antagonists phentolamine and propranolol before IL-1 alpha blunted the glucose metabolic response 30-50%. This attenuated response was associated with normalization of the IL-1 alpha-induced hyperglucagonemia and hyperinsulinemia and a 50-60% reduction in the incremental increase in plasma catecholamines. In contrast to central administration, systemic infusion of adrenergic blockers completely prevented the IL-1 alpha-induced increases in plasma glucose, as well as HGP and GU. In these rats, the elevated plasma levels of insulin, glucagon, and corticosterone produced by intracerebroventricular injection of IL-1 alpha were still present. The results indicate that 1) the enhanced whole body glucose metabolism seen after central administration of IL-1 alpha is mediated by increased sympathoadrenal activity and 2) the IL-1 alpha-induced increase in pancreatic insulin and glucagon secretion as well as part of the peripheral catecholamine release is mediated by central adrenoreceptors.