Dissociation of hypothalamic noradrenergic activity and sympathoadrenal responses to recurrent hypoglycemia

Leptin treatment prevents impaired hypoglycemic counterregulation induced by exposure to severe caloric restriction or exposure to recurrent hypoglycemia

Hypoglycemia-associated autonomic failure (HAAF) is a maladaptive failure in glucose counterregulation in persons with diabetes (PWD) that is caused by recurrent exposure to hypoglycemia. The adipokine leptin is known to regulate glucose homeostasis, and leptin levels fall following exposure to recurrent hypoglycemia. Yet, little is known regarding how reduced leptin levels influence glucose counterregulation, or if low leptin levels are involved in the development of HAAF. The purpose of this study was to determine the effect of hypoleptinemia on the neuroendocrine responses to hypoglycemia. We utilized two separate experimental paradigms known to induce a hypoleptinemic state: 60% caloric restriction (CR) in mice and three days of recurrent hypoglycemia (3dRH) in rats. A sub-set of animals were also treated with leptin (0.5-1.0 μg/g) during the CR or 3dRH periods. Neuroendocrine responses to hypoglycemia were assessed 60 min following an IP insulin injection on the terminal day of the paradigms. CR mice displayed defects in hypoglycemic counterregulation, indicated by significantly lower glucagon levels relative to controls, 13.5 pmol/L (SD 10.7) versus 64.7 pmol/L (SD 45) (p = 0.002). 3dRH rats displayed reduced epinephrine levels relative to controls, 1900 pg/mL (SD 1052) versus 3670 pg/mL (SD 780) (p = 0.030). Remarkably, leptin treatment during either paradigm completely reversed this effect by normalizing glucagon levels in CR mice, 78.0 pmol/L (SD 47.3) (p = 0.764), and epinephrine levels in 3dRH rats, 2910 pg/mL (SD 1680) (p = 0.522). These findings suggest that hypoleptinemia may be a key signaling event driving the development of HAAF and that leptin treatment may prevent the development of HAAF in PWD.

Carvedilol prevents impairment of the counterregulatory response in recurrently hypoglycaemic diabetic rats

AIM

It has been suggested that repeated activation of the adrenergic system during antecedent episodes of hypoglycaemia contributes to the development of counterregulatory failure. We previously reported that treatment with carvedilol, a non-specific β-blocker, prevented the development of counterregulatory failure and improved hypoglycaemia awareness in recurrently hypoglycaemic non-diabetic rats. The current study investigated whether carvedilol has similar benefits in diabetic rats.

METHODS

Recurrently hypoglycaemic streptozotocin-diabetic rats (STZ+RH) were treated with carvedilol for one week prior to undergoing a hypoglycaemic clamp. Hypoglycaemia awareness was evaluated in streptozotocin-diabetic rats made hypoglycaemia unaware using repeated injections of 2-deoxyglucose.

RESULTS

Compared to hypoglycaemia-naïve STZ-diabetic controls, exogenous glucose requirements were more than doubled in the STZ+RH animals and this was associated with a 49% reduction in the epinephrine response to hypoglycaemia. Treating STZ+RH animals with carvedilol improved the epinephrine response to hypoglycaemia. Of note, neither recurrent hypoglycaemia nor carvedilol treatment affected the glucagon response in diabetic animals. Additionally, carvedilol treatment improved the feeding response to insulin-induced hypoglycaemia in diabetic animals made 'hypoglycaemia unaware' using repeated injections of 2-deoxyglucose, suggesting the treatment improved awareness of hypoglycaemia as well.

CONCLUSION

Our data suggest that carvedilol may be useful in preventing impairments of the sympathoadrenal response and the development of hypoglycaemia unawareness during recurring episodes of hypoglycaemia in diabetic animals.

Repeated Activation of Noradrenergic Receptors in the Ventromedial Hypothalamus Suppresses the Response to Hypoglycemia

Activation of the adrenergic system in response to hypoglycemia is important for proper recovery from low glucose levels. However, it has been suggested that repeated adrenergic stimulation may also contribute to counterregulatory failure, but the underlying mechanisms are not known. The aim of this study was to establish whether repeated activation of noradrenergic receptors in the ventromedial hypothalamus (VMH) contributes to blunting of the counterregulatory response by enhancing local lactate production. The VMH of nondiabetic rats were infused with either artificial extracellular fluid, norepinephrine (NE), or salbutamol for 3 hours/day for 3 consecutive days before they underwent a hypoglycemic clamp with microdialysis to monitor changes in VMH lactate levels. Repeated exposure to NE or salbutamol suppressed both the glucagon and epinephrine responses to hypoglycemia compared to controls. Furthermore, antecedent NE and salbutamol treatments raised extracellular lactate levels in the VMH. To determine whether the elevated lactate levels were responsible for impairing the hormone response, we pharmacologically inhibited neuronal lactate transport in a subgroup of NE-treated rats during the clamp. Blocking neuronal lactate utilization improved the counterregulatory hormone responses in NE-treated animals, suggesting that repeated activation of VMH β2-adrenergic receptors increases local lactate levels which in turn, suppresses the counterregulatory hormone response to hypoglycemia.

Sex differences in ventromedial hypothalamic nucleus glucoregulatory transmitter biomarker protein during recurring insulin-induced hypoglycemia

Recurring insulin-induced hypoglycemia (RIIH) in males correlates with maladaptive glucose counter-regulatory collapse and acclimated expression of ventromedial hypothalamic nucleus (VMN) nitric oxide (NO) and γ-aminobutyric acid (GABA) metabolic transmitter biomarkers, e.g., neuronal nitric oxide synthase (nNOS) and glutamate decarboxylase_65/67 (GAD). Hindbrain noradrenergic neurons innervate the VMN, where norepinephrine regulates nNOS and GAD expression. Current research investigated the hypothesis that antecedent hypoglycemia (AH) exposure causes sex-dimorphic habituation of VMN glucoregulatory biomarker proteins between and/or during serial hypoglycemic bouts, and that hindbrain catecholaminergic (CA) signaling may control sex-specific adaptation of one or more of these proteins. Data show that upon recovery from AH, females exhibit CA-mediated reductions in baseline VMN nNOS, GAD, steroidogenic factor-1 (SF-1), and brain-derived neurotrophic factor (BNDF) expression compared to euglycemic profiles. In males, however, AH caused 6-OHDA-insensitive suppression of only basal SF-1 levels in the VMN. VMN transmitter protein acclimation to RIIH was sex-contingent, as differential nNOS, GAD, SF-1, and BDNF responses to a single vs final bout of hypoglycemia occur in males, whereas females show acclimated reactivity of GAD and SF-1 only to renewed hypoglycemia. CA-mediated and -independent habituation of distinctive VMN protein profiles occurred in each sex. Further research is necessary to evaluate, in each sex, effects of altered baseline VMN metabolic neurotransmitter signals on glucose homeostasis as well as non-metabolic functions under the control of those neurochemicals. It would also be insightful to learn if and how sex-contingent habituation of VMN transmitter responses to hypoglycemia contribute to sex-dimorphic patterns of glucose counter-regulation during RIIH.

Experimental Models of Impaired Hypoglycaemia-Associated Counter-Regulation

Impaired awareness of hypoglycaemia (IAH) affects around a quarter of patients with diabetes who receive insulin treatment. This condition is characterised by a progressive reduction in symptomatic and behavioural responses to hypoglycaemia, increasing risk of deeper drops in blood glucose, unconsciousness, and collapse. Thus, patients with IAH experience severe hypoglycaemic episodes more frequently, resulting in significant morbidity and mortality. IAH is thought to develop as a consequence of whole-body adaptations to repeated insulin-induced hypoglycaemia (RH), with widespread deficits in the hypoglycaemia counter-regulatory response (CRR). Despite this important insight, the precise pathophysiology by which RH leads to an attenuated CRR is unknown. Studies into the underlying mechanisms of IAH have employed a variety of protocols in humans and experimental species. The use of animal models has many investigational benefits, including the unprecedented increase in the availability of transgenic strains. However, modelling impaired hypoglycaemia-associated counter-regulation remains challenging and appropriate interpretation of findings across species and protocols even more so. Here, we review the experimental modelling of IAH and impaired hypoglycaemia-associated counter-regulation, with a focus on understanding species-specific variation in glucose homeostasis. This review will aid investigators in interpreting outputs from different studies in IAH and aid progress in the field.

Thromboxane A2 in the paraventricular hypothalamic nucleus mediates glucoprivation-induced adrenomedullary outflow

Glucoprivation stimulates a rapid sympathetic response to release and/or secrete catecholamines into the bloodstream. However, the central regulatory mechanisms involving adrenoceptors and prostanoids production in the paraventricular hypothalamic nucleus (PVN) that are responsible for the glucoprivation-induced elevation of plasma catecholamines are still unresolved. In this study, we aimed to clarify whether glucoprivation-induced activation of noradrenergic neurons projecting to the PVN can induce α- and/or β-adrenergic receptor activation and prostanoids production in the PVN to elevate plasma catecholamine levels. We examined the effects of α- and β-adrenergic receptor antagonists, a cyclooxygenase inhibitor, a thromboxane A synthase inhibitor, and a PGE₂ subtype EP₃ receptor antagonist on intravenously administered 2-deoxy-D-glucose (2-DG)-induced elevation of noradrenaline in the PVN and plasma levels of catecholamine in freely moving rats. In addition, we examined whether intravenously administered 2-DG can increase prostanoids levels in the PVN microdialysates. Intracerebroventricular (i.c.v.) pretreatment with phentolamine (a non-selective α-adrenergic receptor antagonist) suppressed the 2-DG-induced increase in the plasma level of adrenaline, whereas i.c.v. pretreatment with propranolol (a non-selective β-adrenergic receptor antagonist) suppressed the 2-DG-induced elevation of the plasma level of noradrenaline. I.c.v. pretreatment with indomethacin (a cyclooxygenase inhibitor) and furegrelate (a thromboxane synthase inhibitor) attenuated the 2-DG-induced elevations of both noradrenaline and adrenaline levels. Furthermore, 2-DG administration elevated the thromboxane B₂ level, a metabolite of thromboxane A₂ in PVN microdialysates. Our results suggest that glucoprivation-induced activation of α- and β-adrenergic receptor in the brain including the PVN and then thromboxane A₂ production in the PVN, which are essential for the 2-DG-induced elevations of both plasma adrenaline and noradrenaline levels.

Carvedilol prevents counterregulatory failure and impaired hypoglycaemia awareness in non-diabetic recurrently hypoglycaemic rats

AIMS/HYPOTHESIS

This study evaluates whether the non-selective β-blocker, carvedilol, can be used to prevent counterregulatory failure and the development of impaired awareness of hypoglycaemia (IAH) in recurrently hypoglycaemic rats.

METHODS

Sprague Dawley rats were implanted with vascular catheters and intracranial guide cannulas targeting the ventromedial hypothalamus (VMH). These animals underwent either three bouts of insulin-induced hypoglycaemia or received three saline injections (control group) over 3 days. A subgroup of recurrently hypoglycaemic animals was treated with carvedilol. The next day, the animals underwent a hypoglycaemic clamp with microdialysis without carvedilol treatment to evaluate changes in central lactate and hormone levels. To assess whether carvedilol prevented IAH, we treated rats that had received repeated 2-deoxyglucose (2DG) injections to impair their awareness of hypoglycaemia with carvedilol and measured food intake in response to insulin-induced hypoglycaemia as a surrogate marker for hypoglycaemia awareness.

RESULTS

Compared with the control group, recurrently hypoglycaemic rats had a ~1.7-fold increase in VMH lactate and this was associated with a 75% reduction in the sympathoadrenal response to hypoglycaemia. Treatment with carvedilol restored VMH lactate levels and improved the adrenaline (epinephrine) responses. In 2DG-treated rats compared with control animals receiving saline, food intake was reduced in response to hypoglycaemia and increased with carvedilol treatment.

CONCLUSIONS/INTERPRETATION

We conclude that carvedilol may be a useful therapy to prevent counterregulatory failure and improve IAH.

Norepinephrine regulation of ventromedial hypothalamic nucleus metabolic transmitter biomarker and astrocyte enzyme and receptor expression: Impact of 5' AMP-activated protein kinase

The ventromedial hypothalamic energy sensor AMP-activated protein kinase (AMPK) maintains glucostasis via neurotransmitter signals that diminish [γ-aminobutyric acid] or enhance [nitric oxide] counter-regulation. Ventromedial hypothalamic nucleus (VMN) 'fuel-inhibited' neurons are sensitive to astrocyte-generated metabolic substrate stream. Norepinephrine (NE) regulates astrocyte glycogen metabolism in vitro, and hypoglycemia intensifies VMN NE activity in vivo. Current research investigated the premise that NE elicits AMPK-dependent adjustments in VMN astrocyte glycogen metabolic enzyme [glycogen synthase (GS); glycogen phosphorylase (GP)] and gluco-regulatory neuron biomarker [glutamate decarboxylase_65/67 (GAD); neuronal nitric oxide synthase (nNOS); SF-1] protein expression in male rats. We also examined whether VMN astrocytes are directly receptive to NE and if noradrenergic input regulates cellular sensitivity to the neuro-protective steroid estradiol. Intra-VMN NE correspondingly augmented or reduced VMN tissue GAD and nNOS protein despite no change in circulating glucose, data that imply that short-term exposure to NE promotes persistent improvement in VMN nerve cell energy stability. The AMPK inhibitor Compound C (Cc) normalized VMN nNOS, GS, and GP expression in NE-treated animals. NE caused AMPK-independent down-regulation of alpha₂-, alongside Cc-reversible augmentation of beta₁-adrenergic receptor protein profiles in laser-microdissected astrocytes. NE elicited divergent adjustments in astrocyte estrogen receptor-beta (AMPK-unrelated reduction) and GPR-30 (Cc-revocable increase) proteins. Outcomes implicate AMPK in noradrenergic diminution of VMN nitrergic metabolic-deficit signaling and astrocyte glycogen shunt activity. Differentiating NE effects on VMN astrocyte adrenergic and estrogen receptor variant expression suggest that noradrenergic regulation of glycogen metabolism may be mediated, in part, by one or more receptors characterized here by sensitivity to this catecholamine.

The Effects of Insulin-Induced Hypoglycaemia on Tyrosine Hydroxylase Phosphorylation in Rat Brain and Adrenal Gland

In this study we investigated the effects of insulin-induced hypoglycaemia on tyrosine hydroxylase (TH) protein and TH phosphorylation in the adrenal gland, C1 cell group, locus coeruleus (LC) and midbrain dopaminergic cell groups that are thought to play a role in response to hypoglycaemia and compared the effects of different concentrations of insulin in rats. Insulin (1 and 10 U/kg) treatment caused similar reductions in blood glucose concentration (from 7.5-9 to 2-3 mmol/L); however, plasma adrenaline concentration was increased 20-30 fold in response to 10 U/kg insulin and only 14 fold following 1 U/kg. Time course studies (at 10 U/kg insulin) revealed that in the adrenal gland, Ser31 phosphorylation was increased between 30 and 90 min (4-5 fold), implying that TH was activated to increase catecholamine synthesis in adrenal medulla to replenish the stores. In the brain, Ser19 phosphorylation was limited to certain dopaminergic groups in the midbrain, while Ser31 phosphorylation was increased in most catecholaminergic regions at 60 min (1.3-2 fold), suggesting that Ser31 phosphorylation may be an important mechanism to maintain catecholamine synthesis in the brain. Comparing the effects of 1 and 10 U/kg insulin revealed that Ser31 phosphorylation was increased to similar extent in the adrenal gland and C1 cell group in response to both doses whereas Ser31 and Ser19 phosphorylation were only increased in response to 1 U/kg insulin in LC and in response to 10 U/kg insulin in most midbrain regions. Thus, the adrenal gland and some catecholaminergic brain regions become activated in response to insulin administration and brain catecholamines may be important for initiation of physiological defences against insulin-induced hypoglycaemia.

Challenges in Modelling Hypoglycaemia-Associated Autonomic Failure: A Review of Human and Animal Studies

Recurrent insulin-induced hypoglycaemia is a major limitation to insulin treatment in diabetes patients leading to a condition called hypoglycaemia-associated autonomic failure (HAAF). HAAF is characterised by reduced sympathoadrenal response to subsequent hypoglycaemia thereby predisposing the patients to severe hypoglycaemia that can lead to coma or even death. Despite several attempts being made, the mechanism of HAAF is yet to be clearly established. In order for the mechanism of HAAF to be elucidated, establishing a human/animal model of the phenomenon is the foremost requirement. Several research groups have attempted to reproduce the phenomenon in diabetic and nondiabetic humans and rodents and reported variable results. The success of the phenomenon is marked by a significant reduction in plasma adrenaline response to subsequent hypoglycaemic episode relative to that of the antecedent hypoglycaemic episode. A number of factors such as the insulin dosage, route of administration, fasting conditions, blood sampling methods and analyses, depth, duration, and number of antecedent hypoglycaemic episodes can impact the successful reproduction of the phenomenon and thus have to be carefully considered while developing the protocol. In this review, we have outlined the protocols followed by different research groups to reproduce the phenomenon in diabetic and nondiabetic humans and rodents including our own observations in rats and discussed the factors that have to be given careful consideration in reproducing the phenomenon successfully.

Acute hypoglycemia causes depressive-like behaviors in mice

Reports in humans advocate a link between hypoglycemia and altered mood. Such observations, however, have not been mechanistically explored. Here we examined depressive-like behaviors in mice resulting from acute hypoglycemia. Mice were fasted for 12 hours and then administered intraperitoneal insulin to induce a blood glucose nadir of 50 mg/dL at 0.75 hour after injection that by 2 hours postinjection had returned to normal. The behaviors of locomotion, forced swim, saccharin preference, and novel object recognition were subsequently examined. Mice made hypoglycemic showed depressive-like behaviors 24 hours after resolution of hypoglycemia as evidenced by increased immobility in the forced swim test (FST) and reduced saccharin preference. Movement and memory were not impacted by hypoglycemia 24 hours after its resolution. By 48 hours posthypoglycemia, depressive-like behaviors resolved. In contrast, neither peripheral insulin administration without resultant hypoglycemia nor intracerebroventricular insulin administration altered performance in the FST. The antidepressants fluoxetine and desipramine prevented hypoglycemia-induced immobility in the FST, as did the antiadrenergic agents phentolamine, metoprolol, and butoxamine. Epinephrine and norepinephrine administration caused increased immobility in the FST at 24 hours postadministration that subsequently resolved by 48 hours. These data indicate that, in mice, acute hypoglycemia through adrenergic pathways caused depressive-like behaviors that exist well beyond the resolution of hypoglycemia.

Rate of fall in blood glucose and recurrent hypoglycemia affect glucose dynamics and noradrenergic activation in the ventromedial hypothalamus

Noradrenergic activity in the ventromedial hypothalamus (VMH) is increased and activates a sympathoadrenal response during hypoglycemia. How the rate at which hypoglycemia develops affects local glucose concentrations and norepinephrine (NE) release was evaluated by placing microdialysis probes into the VMH of male Sprague-Dawley rats receiving insulin (20 mU·kg(-1)·min(-1)) and variable glucose infusions. During a first episode of hypoglycemia, interstitial glucose concentrations in the VMH generally declined at the same rate as plasma glucose; however, the faster hypoglycemia developed, the greater the magnitude of the initial NE release in the VMH (r(2) = 0.72, P < 0.001). Following recurrent episodes of hypoglycemia, VMH glucose decreased at a slower rate than plasma glucose, and the initial NE release was attenuated at the same rates of blood glucose decline. The plasma glucose threshold for the initial NE release in VMH was similar for all groups (∼3.23 mM); however, the VMH glucose threshold was stimulated and was lower when blood glucose declined more slowly (0.86 ± 0.06 vs. 1.06 ± 0.04 mmol/l, P < 0.01). The timing of the initial increase in NE release in VMH corresponded with an increase in plasma epinephrine during the first episode of hypoglycemia but not following recurrent hypoglycemia. Although a decrease in VMH glucose concentration is required for noradrenergic activation in VMH, there does not appear to be a set glucose threshold within the VMH for activation of this response.

Sweet talk in the brain: glucosensing, neural networks, and hypoglycemic counterregulation

Glucose is the primary fuel for the vast majority of cells, and animals have evolved essential cellular, autonomic, endocrine, and behavioral measures to counteract both hypo- and hyperglycemia. A central component of these counterregulatory mechanisms is the ability of specific sensory elements to detect changes in blood glucose and then use that information to produce appropriate counterregulatory responses. Here we focus on the organization of the neural systems that are engaged by glucosensing mechanisms when blood glucose concentrations fall to levels that pose a physiological threat. We employ a classic sensory-motor integrative schema to describe the peripheral, hindbrain, and hypothalamic components that make up counterregulatory mechanisms in the brain. We propose that models previously developed to describe how the forebrain modulates autonomic reflex loops in the hindbrain offer a reasoned framework for explaining how counterregulatory neural mechanisms in the hypothalamus and hindbrain are structured.

Antecedent short-term central nervous system administration of estrogen and progesterone alters counterregulatory responses to hypoglycemia in conscious male rats

The aim of this study was to test the hypothesis that antecedent short-term administration of estradiol or progesterone into the central nervous system (CNS) reduces levels of neuroendocrine counterregulatory hormones during subsequent hypoglycemia. Conscious unrestrained male Sprague-Dawley rats were studied during randomized 2-day experiments. Day 1 consisted of an 8-h lateral ventricle infusion of estradiol (1 mug/mul; n = 9), progesterone (1 mug/mul; n = 9), or saline (0.2 mul/min; n = 10). On day 2, a 2-h hyperinsulinemic (30 pmol.kg(-1).min(-1)) hypoglycemic (2.9 +/- 0.2 mM) clamp was performed on all rats. Central administration of estradiol on day 1 resulted in significantly lower plasma epinephrine levels during hypoglycemia compared with saline, whereas central administration of progesterone resulted in increased levels of plasma norepinephrine and decreased levels of corticosterone both at baseline and during hypoglycemia. Glucagon responses during hypoglycemia were unaffected by prior administration of estradiol or progesterone. Endogenous glucose production following day 1 estradiol was significantly lower during day 2 hypoglycemia, and consequently, the glucose infusion rate to maintain the glycemia was significantly greater after estradiol administration compared with saline. These data suggest that 1) CNS administration of both female reproductive hormones can have rapid effects in modulating levels of counterregulatory hormones during subsequent hypoglycemia in conscious male rats, 2) forebrain administration of reproductive hormones can significantly reduce pituitary adrenal and sympathetic nervous system drive during hypoglycemia, 3) reproductive steroid hormones produce differential effects on sympathetic nervous system activity during hypoglycemia, and 4) reduction of epinephrine resulted in significantly blunted metabolic counterregulatory responses during hypoglycemia.

From bedside to bench and back again: research issues in animal models of human disease

To improve outcomes for patients with many serious clinical problems, multifactorial research approaches by nurse scientists, including the use of animal models, are necessary. Animal models serve as analogies for clinical problems seen in humans and must meet certain criteria, including validity and reliability, to be useful in moving research efforts forward. This article describes research considerations in the development of rodent models. As the standard of diabetes care evolves to emphasize intensive insulin therapy, rates of severe hypoglycemia are increasing among patients with type 1 and type 2 diabetes mellitus. A consequence of this change in clinical practice is an increase in rates of two hypoglycemia-related diabetes complications: hypoglycemia-associated autonomic failure (HAAF) and resulting hypoglycemia unawareness. Work on an animal model of HAAF is in an early developmental stage, with several labs reporting different approaches to model this complication of type 1 diabetes mellitus. This emerging model serves as an example illustrating how evaluation of validity and reliability is critically important at each stage of developing and testing animal models to support inquiry into human disease.

Counterregulatory deficits occur within 24 h of a single hypoglycemic episode in conscious, unrestrained, chronically cannulated mice

Hypoglycemia-induced counterregulatory failure is a dangerous complication of insulin use in diabetes mellitus. Controlled hypoglycemia studies in gene knockout models, which require the use of mice, would aid in identifying causes of defective counterregulation. Because stress can influence counterregulatory hormones and glucose homeostasis, we developed glucose clamps with remote blood sampling in conscious, unrestrained mice. Male C57BL/6 mice implanted with indwelling carotid artery and jugular vein catheters were subjected to 2 h of hyperinsulinemic glucose clamps 24 h apart, with a 6-h fast before each clamp. On day 1, blood glucose was maintained (euglycemia, 178 +/- 4 mg/dl) or decreased to 62 +/- 1 mg/dl (hypoglycemia) by insulin (20 mU x kg(-1) x min(-1)) and variable glucose infusion. Donor blood was continuously infused to replace blood sample volume. Baseline plasma epinephrine (32 +/- 8 pg/ml), corticosterone (16.1 +/- 1.8 microg/dl), and glucagon (35 +/- 3 pg/ml) were unchanged during euglycemia but increased significantly during hypoglycemia, with a glycemic threshold of approximately 80 mg/dl. On day 2, all mice underwent a hypoglycemic clamp (blood glucose, 64 +/- 1 mg/dl). Compared with mice that were euglycemic on day 1, previously hypoglycemic mice had significantly higher glucose requirements and significantly lower plasma glucagon and corticosterone (n = 6/group) on day 2. Epinephrine tended to decrease, although not significantly, in repeatedly hypoglycemic mice. Pre- and post-clamp insulin levels were similar between groups. We conclude that counterregulatory responses to acute and repeated hypoglycemia in unrestrained, chronically cannulated mice reproduce aspects of counterregulation in humans, and that repeated hypoglycemia in mice is a useful model of counterregulatory failure.

Cortical Fluoro-Jade staining and blunted adrenomedullary response to hypoglycemia after noncoma hypoglycemia in rats

Intensive insulin therapy in patients with type 1 diabetes mellitus reduces long-term complications; however, intensive therapy is also associated with a three-fold increase in hypoglycemic episodes. The present study in conscious rats characterizes the physiologic and neuropathologic consequences of a single episode of moderate hypoglycemia. In this model, intravenous insulin is used to reduce plasma glucose to 30 to 35 mg/dL for 75 mins. This single hypoglycemic insult acutely induces hypoglycemia-associated autonomic failure (HAAF), with epinephrine responses to hypoglycemia reduced more than 36% from control. Neuropathology after this insult includes the appearance of dying cells, assessed with the marker Fluoro-jade B (FJ). After hypoglycemic insult, FJ+ cells were consistently seen in subdivisions of the medial prefrontal cortex, the orbital cortex, and the piriform cortex. There was a significant correlation between depth of hypoglycemia and number of FJ+ cells, suggesting that there is a critical threshold below which vulnerable cells begin to die. These data suggest that there is a population of cells that are vulnerable to moderate levels of hypoglycemia commonly experienced by patients with insulin-treated diabetes. These cells, which may be neurons, are primarily found in cortical regions implicated in visceral perception and autonomic control, raising the possibility that their loss contributes to clinically reported deficits in autonomic and perceptual responses to hypoglycemia.

Hypoglycemia-induced noradrenergic activation in the VMH is a result of decreased ambient glucose

During insulin-induced hypoglycemia, there is an increase in extracellular norepinephrine (NE) in the ventromedial hypothalamus (VMH). This brain area is known to play an important role in integrated hormonal and behavioral responses to systemic hypoglycemia. Selective glucoprivation restricted to the VMH is both necessary and sufficient to initiate secretion of counterregulatory hormones. The present study was designed to investigate whether increased release of NE in the VMH depends on detection of glucoprivation localized in this area. In awake, chronically catheterized male Sprague-Dawley rats, extracellular NE in the VMH was monitored using 1-mm microdialysis probes perfused with Krebs Ringer buffer (KRB) or KRB + 100 mM d-glucose (d-Glc). During insulin-induced hypoglycemia (glycemic nadir approximately 2.4 mM) extracellular NE was increased to >160% of baseline (P < 0.01) only in the KRB + insulin group. There was no increase in NE from baseline when glucose was added to the perfusate to maintain euglycemia at the periprobe environment. The sympathoadrenal response to hypoglycemia, present in the KRB + insulin group, was attenuated in the d-Glc + insulin group. The present results confirm that noradrenergic activation in the VMH during systemic hypoglycemia depends on detection of glucoprivation locally in this area. These data provide additional support for the importance of increased noradrenergic activity in the VMH in the counterregulatory hormonal responses to hypoglycemia.

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.