Role of corticotrophin-releasing hormone in the impairment of counterregulatory responses to hypoglycemia

Hypoglycaemia: Still the main drawback of insulin 100 years on: "From man to mouse"

One hundred years on from the initial discovery of insulin, we take this opportunity to reflect on the scientific discoveries that have improved so many lives. From its original crude form, insulin therapy has improved significantly over the past century. Despite this, hypoglycaemia remains an ever-present fear for people with Type 1 diabetes. As such, it is essential that research now looks to minimise the frequency and severity of insulin-induced hypoglycaemia and its complications, some of which can be life-threatening. Over the last century, one thing that has become apparent is the success and need for translational diabetes research. From its origin in dogs, insulin treatment has revolutionised the lives of those with Type 1 diabetes through the coordinated effort of scientists and clinicians. In this review, we recount the more recent research that uses a mouse-to-man approach, specifically in hypoglycaemia research.

Unveiling the sensory and interneuronal pathways of the neuroendocrine connectome in Drosophila

Neuroendocrine systems in animals maintain organismal homeostasis and regulate stress response. Although a great deal of work has been done on the neuropeptides and hormones that are released and act on target organs in the periphery, the synaptic inputs onto these neuroendocrine outputs in the brain are less well understood. Here, we use the transmission electron microscopy reconstruction of a whole central nervous system in the Drosophila larva to elucidate the sensory pathways and the interneurons that provide synaptic input to the neurosecretory cells projecting to the endocrine organs. Predicted by network modeling, we also identify a new carbon dioxide-responsive network that acts on a specific set of neurosecretory cells and that includes those expressing corazonin (Crz) and diuretic hormone 44 (Dh44) neuropeptides. Our analysis reveals a neuronal network architecture for combinatorial action based on sensory and interneuronal pathways that converge onto distinct combinations of neuroendocrine outputs.

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.

Role of hindbrain adenosine 5'-monophosphate-activated protein kinase (AMPK) in hypothalamic AMPK and metabolic neuropeptide adaptation to recurring insulin-induced hypoglycemia in the male rat

Glucose counter-regulatory dysfunction correlates with impaired activation of the hypothalamic metabolic sensor adenosine 5'-monophosphate-activated protein kinase (AMPK). Hypothalamic AMPK is controlled by hindbrain energy status; we examined here whether hindbrain AMPK regulates hypothalamic AMPK and metabolic neurotransmitter maladaptation to recurring insulin-induced hypoglycemia (RIIH). Brain tissue was harvested after single versus serial insulin (I) dosing for Western blot analysis of AMPK, phospho-AMPK (pAMPK), and relevant biosynthetic enzyme/neuropeptide expression in micro-punch dissected arcuate (ARH), ventromedial (VMH), dorsomedial (DMH) nuclei and lateral hypothalamic area (LHA) tissue. The AMPK inhibitor compound c (Cc) or vehicle was administered to the caudal fourth ventricle ahead of antecedent I injections. RIIH caused site-specific elevation (ARH, VMH, LHA) or reduction (DMH) of total AMPK protein versus acute hypoglycemia; Cc respectively exacerbated or attenuated this response in the ARH and VMH. Hindbrain AMPK correspondingly inhibited or stimulated LHA and DMH pAMPK expression during RIIH. RIIH elicited Cc-reversible augmentation of VMH glutamate decarboxylase profiles, but stimulated (ARH pro-opiomelanocortin; LHA orexin-A) or decreased (VMH nitric oxide synthase) other metabolic neurotransmitters without hindbrain sensor involvement. Results demonstrate acclimated up-regulation of total AMPK protein expression in multiple hypothalamic loci during RIIH, and document hindbrain sensor contribution to amplification of this protein profile in the VMH. Concurrent lack of net change in ARH and VMH tissue pAMPK implies adaptive reductions in local sensor activity, which may/may not reflect positive gain in energy state. It remains unclear if 'glucose-excited' VMH GABAergic and/or ARH pro-opiomelanocortin neurons exhibit AMPK habituation to RIIH, and whether diminished sensor activation in these and other mediobasal hypothalamic neurotransmitter populations may contribute to HAAF.

Intracerebroventricular Catalase Reduces Hepatic Insulin Sensitivity and Increases Responses to Hypoglycemia in Rats

Specialized metabolic sensors in the hypothalamus regulate blood glucose levels by influencing hepatic glucose output and hypoglycemic counterregulatory responses. Hypothalamic reactive oxygen species (ROS) may act as a metabolic signal-mediating responses to changes in glucose, other substrates and hormones. The role of ROS in the brain's control of glucose homeostasis remains unclear. We hypothesized that hydrogen peroxide (H₂O₂), a relatively stable form of ROS, acts as a sensor of neuronal glucose consumption and availability and that lowering brain H₂O₂ with the enzyme catalase would lead to systemic responses increasing blood glucose. During hyperinsulinemic euglycemic clamps in rats, intracerebroventricular catalase infusion resulted in increased hepatic glucose output, which was associated with reduced neuronal activity in the arcuate nucleus of the hypothalamus. Electrophysiological recordings revealed a subset of arcuate nucleus neurons expressing proopiomelanocortin that were inhibited by catalase and excited by H₂O₂. During hypoglycemic clamps, intracerebroventricular catalase increased glucagon and epinephrine responses to hypoglycemia, consistent with perceived lower glucose levels. Our data suggest that H₂O₂ represents an important metabolic cue, which, through tuning the electrical activity of key neuronal populations such as proopiomelanocortin neurons, may have a role in the brain's influence of glucose homeostasis and energy balance.

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.

Orexin signaling is necessary for hypoglycemia-induced prevention of conditioned place preference

While the neural control of glucoregulatory responses to insulin-induced hypoglycemia is beginning to be elucidated, brain sites responsible for behavioral responses to hypoglycemia are relatively poorly understood. To help elucidate central control mechanisms associated with hypoglycemia unawareness, we first evaluated the effect of recurrent hypoglycemia on a simple behavioral measure, the robust feeding response to hypoglycemia, in rats. First, food intake was significantly, and similarly, increased above baseline saline-induced intake (1.1 ± 0.2 g; n = 8) in rats experiencing a first (4.4 ± 0.3; n = 8) or third daily episode of recurrent insulin-induced hypoglycemia (IIH, 3.7 ± 0.3 g; n = 9; P < 0.05). Because food intake was not impaired as a result of prior IIH, we next developed an alternative animal model of hypoglycemia-induced behavioral arousal using a conditioned place preference (CPP) model. We found that hypoglycemia severely blunted previously acquired CPP in rats and that recurrent hypoglycemia prevented this blunting. Pretreatment with a brain penetrant, selective orexin receptor-1 antagonist, SB-334867A, blocked hypoglycemia-induced blunting of CPP. Recurrently hypoglycemic rats also showed decreased preproorexin expression in the perifornical hypothalamus (50%) but not in the adjacent lateral hypothalamus. Pretreatment with sertraline, previously shown to prevent hypoglycemia-associated glucoregulatory failure, did not prevent blunting of hypoglycemia-induced CPP prevention by recurrent hypoglycemia. This work describes the first behavioral model of hypoglycemia unawareness and suggests a role for orexin neurons in mediating behavioral responses to hypoglycemia.

Hypothalamic-Pituitary-Adrenal Axis Programming after Recurrent Hypoglycemia during Development

Permanent brain injury is a complication of recurrent hypoglycemia during development. Recurrent hypoglycemia also has adverse consequences on the neuroendocrine system. Hypoglycemia-associated autonomic failure, characterized by ineffective glucose counterregulation during hypoglycemia, is well described in children and adults on insulin therapy for diabetes mellitus. Whether recurrent hypoglycemia also has a programming effect on the hypothalamus-pituitary-adrenal cortex (HPA) axis has not been well studied. Hypoglycemia is a potent stress that leads to increased glucocorticoid secretion in all age groups, including the perinatal period. Other conditions associated with exposure to excess glucocorticoid in the perinatal period have a programming effect on the HPA axis activity. Limited animal data suggest the possibility of similar programming effect after recurrent hypoglycemia in the postnatal period. The age at exposure to hypoglycemia likely determines the HPA axis response in adulthood. Recurrent hypoglycemia in the early postnatal period likely leads to a hyperresponsive HPA axis, whereas recurrent hypoglycemia in the late postnatal period lead to a hyporesponsive HPA axis in adulthood. The age-specific programming effects may determine the neuroendocrine response during hypoglycemia and other stressful events in individuals with history of recurrent hypoglycemia during development.

Posttranscriptional regulation of adrenal TH gene expression contributes to the maladaptive responses triggered by insulin-induced recurrent hypoglycemia

Acute metabolic stress such as insulin-induced hypoglycemia triggers a counterregulatory response during which the release of catecholamines (epinephrine), the activation of tyrosine hydroxylase (TH) enzyme and subsequent compensatory catecholamine biosynthesis occur in the adrenal medulla. However, recurrent exposure to hypoglycemia (RH), a consequence of tight glycemic control in individuals with type 1 and type 2 diabetes compromises this physiological response. The molecular mechanisms underlying the maladaptive response to repeated glucose deprivation are incompletely understood. We hypothesize that impaired epinephrine release following RH reflects altered regulation of adrenal catecholamine biosynthesis. To test this hypothesis, we compared the effect of single daily (RH) and twice-daily episodes of insulin-induced hypoglycemia (2RH) on adrenal epinephrine release and production in normal rats. Control animals received saline injections under similar conditions (RS and 2RS, respectively). Following 3 days of treatment, we assessed the counterregulatory hormonal responses during a hypoglycemic clamp. Changes in adrenal TH gene expression were also analyzed. The counterregulatory responses, relative TH transcription and TH mRNA levels and Ser40-TH phosphorylation (marker for enzyme activation) were induced to a similar extent in RS, 2RS, and RH groups. In contrast, epinephrine and glucagon responses were attenuated in the 2RH group and this was associated with a limited elevation of adrenal TH mRNA, rapid inactivation of TH enzyme and no significant changes in TH protein. Our results suggest that novel posttranscriptional mechanisms controlling TH mRNA and activated TH enzyme turnover contribute to the impaired epinephrine responses and may provide new therapeutic targets to prevent HAAF.

Direct effects of recurrent hypoglycaemia on adrenal catecholamine release

In Type 1 and advanced Type 2 diabetes mellitus, elevation of plasma epinephrine plays a key role in normalizing plasma glucose during hypoglycaemia. However, recurrent hypoglycaemia blunts this elevation of plasma epinephrine. To determine whether recurrent hypoglycaemia affects peripheral components of the sympatho-adrenal system responsible for epinephrine release, male rats were administered subcutaneous insulin daily for 3 days. These recurrent hypoglycaemic animals showed a smaller elevation of plasma epinephrine than saline-injected controls when subjected to insulin-induced hypoglycaemia. Electrical stimulation of an adrenal branch of the splanchnic nerve in recurrent hypoglycaemic animals elicited less release of epinephrine and norepinephrine than in controls, without a change in adrenal catecholamine content. Responsiveness of isolated, perfused adrenal glands to acetylcholine and other acetylcholine receptor agonists was also unchanged. These results indicate that recurrent hypoglycaemia compromised the efficacy with which peripheral neuronal activity stimulates adrenal catecholamine release and demonstrate that peripheral components of the sympatho-adrenal system were directly affected by recurrent hypoglycaemia.

Hypoglycemia in patients with type 1 diabetes: epidemiology, pathogenesis, and prevention

Hypoglycemia is uncommon in the general, nondiabetic population but occurs frequently in persons with diabetes treated with insulin or insulin secretagogues. Thus, iatrogenic hypoglycemia explains the majority of cases among persons with type 1 diabetes (T1DM). Since T1DM is characterized by absolute insulin dependence, the current imperfections in insulin replacement therapies often lead to a mismatch between caloric supply and circulating insulin levels, thus increasing the risk for glycemic fluctuations. Hypoglycemia is the limiting factor to excellent glycemic control in insulin-treated subjects. Intensification of glycemic control was associated with a 300 % increase in the rate of hypoglycemia in the Diabetes Control and Complications Trial. Recent measurements using continuous glucose monitoring reveal an alarming rate of daytime and nocturnal episodes of hypoglycemia in patients with T1DM. Etiological factors underlying hypoglycemia in T1DM include predictable triggers (skipped meals, exercise, insulin over dosage) as well as defective counterregulation, a component of hypoglycemia-associated autonomic failure.

Long-term, intermittent, insulin-induced hypoglycemia produces marked obesity without hyperphagia or insulin resistance: a model for weight gain with intensive insulin therapy

A major side effect of insulin treatment of diabetes is weight gain, which limits patient compliance and may pose additional health risks. Although the mechanisms responsible for this weight gain are poorly understood, it has been suggested that there may be a link to the incidence of recurrent episodes of hypoglycemia. Here we present a rodent model of marked weight gain associated with weekly insulin-induced hypoglycemic episodes in the absence of diabetes. Insulin treatment caused a significant increase in both body weight and fat mass, accompanied by reduced motor activity, lowered thermogenesis in response to a cold challenge, and reduced brown fat uncoupling protein mRNA. However, there was no effect of insulin treatment on total food intake nor on hypothalamic neuropeptide Y or proopiomelanocortin mRNA expression, and insulin-treated animals did not become insulin-resistant. Our results suggest that repeated iatrogenic hypoglycemia leads to weight gain, and that such weight gain is associated with a multifaceted deficit in metabolic regulation rather than to a chronic increase in caloric intake.

Hypoglycemia-induced increases in thalamic cerebral blood flow are blunted in subjects with type 1 diabetes and hypoglycemia unawareness

The thalamus has been found to be activated during the early phase of moderate hypoglycemia. Here, we tested the hypothesis that this region is less activated during hypoglycemia in subjects with type 1 diabetes (T1DM) and hypoglycemia unawareness relative to controls. Twelve controls (5 F/7 M, age 40 ± 14 years, body mass index 24.2 ± 2.7 kg/m(2)) and eleven patients (7 F/4 M, age 39 ± 13 years, body mass index 26.5 ± 4.4 kg/m(2)) with well-controlled T1DM (A1c 6.8 ± 0.4%) underwent a two-step hyperinsulinemic (2.0 mU/kg per minute) clamp. Cerebral blood flow (CBF) weighted images were acquired using arterial spin labeling to monitor cerebral activation in the midbrain regions. Blood glucose was first held at 95 mg/dL and then allowed to decrease to 50 mg/dL. The CBF image acquisition during euglycemia and hypoglycemia began within a few minutes of when the target blood glucose values were reached. Hypoglycemia unaware T1DM subjects displayed blunting of the physiologic CBF increase that occurs in the thalamus of healthy individuals during the early phase of moderate hypoglycemia. A positive correlation was observed between thalamic response and epinephrine response to hypoglycemia, suggesting that this region may be involved in the coordination of the counter regulatory response to hypoglycemia.

Experimental hypoglycemia is a human model of stress-induced hyperalgesia

Hypoglycemia is a physiological stress that leads to the release of stress hormones, such as catecholamines and glucocorticoids, and proinflammatory cytokines. These factors, in euglycemic animal models, are associated with stress-induced hyperalgesia. The primary aim of this study was to determine whether experimental hypoglycemia in humans would lead to a hyperalgesic state. In 2 separate 3-day admissions separated by 1 to 3 months, healthy study participants were exposed to two 2-hour euglycemic hyperinsulinemic clamps or two 2-hour hypoglycemic hyperinsulinemic clamps. Thermal quantitative sensory testing and thermal pain assessments were measured the day before and the day after euglycemia or hypoglycemia. In contrast to prior euglycemia exposure, prior hypoglycemia exposure resulted in enhanced pain sensitivity to hot and cold stimuli as well as enhanced temporal summation to repeated heat-pain stimuli. These findings suggest that prior exposure to hypoglycemia causes a state of enhanced pain sensitivity that is consistent with stress-induced hyperalgesia. This human model may provide a framework for hypothesis testing and targeted, mechanism-based pharmacological interventions to delineate the molecular basis of hyperalgesia and pain susceptibility.

Recurrent hypoglycemia is associated with loss of activation in rat brain cingulate cortex

A subset of people with diabetes fail to mount defensive counterregulatory responses (CRR) to hypoglycemia. Although the mechanisms by which this occurs remain unclear, recurrent exposure to hypoglycemia may be an important etiological factor. We hypothesized that loss of CRR to recurrent exposure to hypoglycemia represents a type of stress desensitization, in which limbic brain circuitry involved in modulating stress responses might be implicated. Here, we compared activation of limbic brain regions associated with stress desensitization during acute hypoglycemia (AH) and recurrent hypoglycemia (RH). Healthy Sprague Dawley rats were exposed to either acute or recurrent 3-d hypoglycemia. We also examined whether changes in neuronal activation were caused directly by the CRR itself by infusing epinephrine, glucagon, and corticosterone without hypoglycemia. AH increased neuronal activity as quantified by c-fos immunoreactivity (FOS-IR) in the cingulate cortex and associated ectorhinal and perirhinal cortices but not in an adjacent control area (primary somatosensory cortex). FOS-IR was not observed after hormone infusion, suggesting that AH-associated activation was caused by hypoglycemia rather than by CRR. Importantly, AH FOS-IR activation was significantly blunted in rats exposed to RH. In conclusion, analogous with other models of stress habituation, activation in the cingulate cortex and associated brain areas is lost with exposure to RH. Our data support the hypothesis that limbic brain areas may be associated with the loss of CRR to RH in diabetes.

Brain glucose sensors play a significant role in the regulation of pancreatic glucose-stimulated insulin secretion

As patients decline from health to type 2 diabetes, glucose-stimulated insulin secretion (GSIS) typically becomes impaired. Although GSIS is driven predominantly by direct sensing of a rise in blood glucose by pancreatic β-cells, there is growing evidence that hypothalamic neurons control other aspects of peripheral glucose metabolism. Here we investigated the role of the brain in the modulation of GSIS. To examine the effects of increasing or decreasing hypothalamic glucose sensing on glucose tolerance and insulin secretion, glucose or inhibitors of glucokinase, respectively, were infused into the third ventricle during intravenous glucose tolerance tests (IVGTTs). Glucose-infused rats displayed improved glucose handling, particularly within the first few minutes of the IVGTT, with a significantly lower area under the excursion curve within the first 10 min (AUC0-10). This was explained by increased insulin secretion. In contrast, infusion of the glucokinase inhibitors glucosamine or mannoheptulose worsened glucose tolerance and decreased GSIS in the first few minutes of IVGTT. Our data suggest a role for brain glucose sensors in the regulation of GSIS, particularly during the early phase. We propose that pharmacological agents targeting hypothalamic glucose-sensing pathways may represent novel therapeutic strategies for enhancing early phase insulin secretion in type 2 diabetes.

Radix astragali (huangqi) as a treatment for defective hypoglycemia counterregulation in diabetes

Radix astragali is a herbal remedy used in China to treat patients with diabetes exposed to repeated episodes of hypoglycemia. The physiological basis or validity of this approach is not clear. In the present study, we examine the effect of pre-treatment with Radix astragali on hormonal counterregulatory responses to hypoglycemia in normal male Sprague-Dawley rats. Four groups of rodents were studied. In two of these groups, rodents were pre-treated for 3 days with either intravenous Radix astragali or control solution and, subsequently, while awake and unrestrained, underwent an in vivo hyperinsulinemic hypoglycemic (50 mg/dl) clamp study. The rodents in other two groups were pre-treated for 7 days with either intravenous Radix astragali or control solution. In addition, for the last 3-days of their treatment, the rats were subjected to a once-daily episode of insulin-induced hypoglycemia. Upon completion of this protocol, each rat underwent a controlled in vivo hyperinsulinemic hypoglycemic (50 mg/dl) clamp study. Radix astragali was shown to amplify the glucose counterregulatory response to hypoglycemia in both untreated and recurrently hypoglycemic rats. Immunocytochemistry studies suggested this might reflect increased neural activation in two key central glucose-sensing regions, the paraventricular hypothalamus and the nucleus tractus solitarius. Based on these rodent studies, we conclude that Radix astragali pre-treatment can amplify the counterregulatory response to hypoglycemia through a mechanism that may involve the central glucose-sensing regions. Future studies to examine the potential therapeutic benefit of Radix astragali in rodent models of type 1 diabetes are warranted.

Recurrent hypoglycemia increases hypothalamic glucose phosphorylation activity in rats

The mechanisms underpinning impaired defensive counterregulatory responses to hypoglycemia that develop in some people with diabetes who suffer recurrent episodes of hypoglycemia are unknown. Previous work examining whether this is a consequence of increased glucose delivery to the hypothalamus, postulated to be the major hypoglycemia-sensing region, has been inconclusive. Here, we hypothesized instead that increased hypothalamic glucose phosphorylation, the first committed intracellular step in glucose metabolism, might develop following exposure to hypoglycemia. We anticipated that this adaptation might tend to preserve glucose flux during hypoglycemia, thus reducing detection of a falling glucose. We first validated a model of recurrent hypoglycemia in chronically catheterized (right jugular vein) rats receiving daily injections of insulin. We confirmed that this model of recurrent insulin-induced hypoglycemia results in impaired counterregulation, with responses of the key counterregulatory hormone, epinephrine, being suppressed significantly and progressively from the first day to the fourth day of insulin-induced hypoglycemia. In another cohort, we investigated the changes in brain glucose phosphorylation activity over 4 days of recurrent insulin-induced hypoglycemia. In keeping with our hypothesis, we found that recurrent hypoglycemia markedly and significantly increased hypothalamic glucose phosphorylation activity in a day-dependent fashion, with day 4 values 2.8 ± 0.6-fold higher than day 1 (P < .05), whereas there was no change in glucose phosphorylation activity in brain stem and frontal cortex. These findings suggest that the hypothalamus may adapt to recurrent hypoglycemia by increasing glucose phosphorylation; and we speculate that this metabolic adaptation may contribute, at least partly, to hypoglycemia-induced counterregulatory failure.

Neuroendocrine responses to hypoglycemia

The counterregulatory response to hypoglycemia is a complex and well-coordinated process. As blood glucose concentration declines, peripheral and central glucose sensors relay this information to central integrative centers to coordinate neuroendocrine, autonomic, and behavioral responses and avert the progression of hypoglycemia. Diabetes, both type 1 and type 2, can perturb these counterregulatory responses. Moreover, defective counterregulation in the setting of diabetes can progress to hypoglycemia unawareness. While the mechanisms that underlie the development of hypoglycemia unawareness are not completely known, possible causes include altered sensing of hypoglycemia by the brain and/or impaired coordination of responses to hypoglycemia. Further study is needed to better understand the intricacies of the counterregulatory response and the mechanisms contributing to the development of hypoglycemia unawareness.

The role of valproate in metabolic disturbances in bipolar disorder patients

BACKGROUND

Our previous report showed that patients with bipolar disorder (BD) have higher prevalence of hypertriglyceridemia, low high-density lipoprotein cholesterol (HDL) and obesity in Taiwan. To confirm whether the metabolic disturbances is associated with the disease itself or the medications used for treating BD, we further compared the metabolic status among the valproate (VPA) treated BD patients, drug-free BD patients and healthy controls in Taiwan.

METHOD

This cross-sectional study included 119 healthy controls and 77 BD patients diagnosed according to the DSMIV-TR criteria in a university hospital. Among the diseased group, 25 remitted BD patients were drug-free (BD-F), and 52 of them were treated with VPA (BD-VPA). Their body mass index (BMI), plasma glucose levels and plasma lipid profiles were measured.

RESULTS

Plasma fasting glucose, insulin, total cholesterol, triglyceride, high-density lipoprotein cholesterol, and low-density lipoprotein cholesterol (LDL) levels were significantly different among BD-VPA, the BD-F, and the healthy control groups. Valproate treatment was associated with significant higher plasma insulin, triglyceride, and BMI levels as well as lower fasting glucose and HDL levels. However, these biochemical indexes did not differ significantly between the BD-F and the healthy control groups.

CONCLUSION

These results provide further evidence that VPA treatment for BD may increase the risk of metabolic disturbances. The risk may be reduced after discontinuing VPA medication.

Brain glucosamine boosts protective glucoprivic feeding

The risk of iatrogenic hypoglycemia is increased in diabetic patients who lose defensive glucoregulatory responses, including the important warning symptom of hunger. Protective hunger symptoms during hypoglycemia may be triggered by hypothalamic glucose-sensing neurons by monitoring changes downstream of glucose phosphorylation by the specialized glucose-sensing hexokinase, glucokinase (GK), during metabolism. Here we investigated the effects of intracerebroventricular (ICV) infusion of glucosamine (GSN), a GK inhibitor, on food intake at normoglycemia and protective feeding responses during glucoprivation and hypoglycemia in chronically catheterized rats. ICV infusion of either GSN or mannoheptulose, a structurally different GK inhibitor, dose-dependently stimulated feeding at normoglycemia. Consistent with an effect of GSN to inhibit competitively glucose metabolism, ICV coinfusion of d-glucose but not l-glucose abrogated the orexigenic effect of ICV GSN at normoglycemia. Importantly, ICV infusion of a low GSN dose (15 nmol/min) that was nonorexigenic at normoglycemia boosted feeding responses to glucoprivation in rats with impaired glucose counterregulation. ICV infusion of 15 nmol/min GSN also boosted feeding responses to threatened hypoglycemia in rats with defective glucose counterregulation. Altogether our findings suggest that GSN may be a potential therapeutic candidate for enhancing defensive hunger symptoms during hypoglycemia.

Amplified hormonal counterregulatory responses to hypoglycemia in rats after systemic delivery of a SUR-1-selective K(+) channel opener?

OBJECTIVE

In glucose-sensing neurons, ATP-sensitive K(+) channels (K(ATP) channels) are thought to translate metabolic signals into an alteration in neuronal firing rates. Because these neurons express the Kir6.2/SUR-1 isoform of the K(ATP) channel, we sought to examine the therapeutic potential of the SUR-1-selective potassium channel opener (KCO), NN414, to amplify counterregulatory response to hypoglycemia.

RESEARCH DESIGN AND METHODS

In vivo dose-response studies with NN414 delivered intravenously to normal Sprague-Dawley rats before the induction of controlled hypoglycemia were performed. Based on these studies, the potential for NN414 to restore counterregulatory responses in chronically cannulated nondiabetic and diabetic BB rats was explored using the in vivo hyperinsulinemic-hypoglycemic clamp technique.

RESULTS

NN414 delivered systemically amplified epinephrine responses during acute hypoglycemia and showed a persisting effect to amplify the epinephrine response when given 24 h before the hypoglycemic study. Local delivery of a potassium-channel blocker to the ventromedial hypothalamus reversed the effects of systemic NN414. In addition, NN414 amplified the epinephrine response to hypoglycemia in both nondiabetic and diabetic BB rats with defective hormonal counterregulation.

CONCLUSIONS

These studies demonstrate in a variety of rodent models that systemic delivery of Kir6.2/SUR-1-selective KCOs enhance the glucose counterregulatory response to insulin-induced hypoglycemia. Future studies in human subjects are now required to determine their potential as a therapy for hypoglycemia-associated autonomic failure in type 1 diabetes.

The mechanisms that underlie glucose sensing during hypoglycaemia in diabetes

Hypoglycaemia is a frequent and greatly feared side-effect of insulin therapy, and a major obstacle to achieving near-normal glucose control. This review will focus on the more recent developments in our understanding of the mechanisms that underlie the sensing of hypoglycaemia in both non-diabetic and diabetic individuals, and how this mechanism becomes impaired over time. The research focus of my own laboratory and many others is directed by three principal questions. Where does the body sense a falling glucose? How does the body detect a falling glucose? And why does this mechanism fail in Type 1 diabetes? Hypoglycaemia is sensed by specialized neurons found in the brain and periphery, and of these the ventromedial hypothalamus appears to play a major role. Neurons that react to fluctuations in glucose use mechanisms very similar to those that operate in pancreatic B- and A-cells, in particular in their use of glucokinase and the K(ATP) channel as key steps through which the metabolic signal is translated into altered neuronal firing rates. During hypoglycaemia, glucose-inhibited (GI) neurons may be regulated by the activity of AMP-activated protein kinase. This sensing mechanism is disturbed by recurrent hypoglycaemia, such that counter-regulatory defence responses are triggered at a lower glucose level. Why this should occur is not yet known, but it may involve increased metabolism or fuel delivery to glucose-sensing neurons or alterations in the mechanisms that regulate the stress response.

Effect of acute and recurrent hypoglycemia on changes in brain glycogen concentration

Our objective was to evaluate whether excessive brain glycogen deposition might follow episodes of acute hypoglycemia (AH) and thus play a role in the hypoglycemia-associated autonomic failure seen in diabetic patients receiving intensive insulin treatment. We determined brain glucose and glycogen recovery kinetics after AH and recurrent hypoglycemia (RH), an established animal model of counterregulatory failure. A single bout of insulin-induced AH or RH for 3 consecutive days was used to deplete brain glucose and glycogen stores in rats. After microwave fixation and glycogen extraction, regional recovery kinetics in the brain was determined using a biochemical assay. Both AH and RH treatments reduced glycogen levels in the cerebellum, cortex, and hypothalamus from control levels of 7.78 +/- 0.55, 5.4 +/- 0.38, and 4.45 +/- 0.37 micromol/g, respectively, to approximately 50% corresponding to a net glycogen utilization rate between 0.6 and 1.2 micromol/g.h. After hypoglycemia, glycogen levels returned to baseline within 6 h in both the AH and the RH group. However, recovery of brain glycogen tended to be faster in rats exposed to RH. This effect followed more rapid recovery of brain glucose levels in the RH group, despite similar blood glucose levels in both groups. There was no statistically significant increase above baseline glycogen levels in either group. In particular, brain glycogen was not increased 24 h after the last of recurrent episodes of hypoglycemia, when a significant counterregulatory defect could be documented during a hyperinsulinemic hypoglycemic clamp study. We conclude that glycogen supercompensation is not a major contributory factor to the pathogenesis of hypoglycemia-associated autonomic failure.

Stimulation of both type I and type II corticosteroid receptors blunts counterregulatory responses to subsequent hypoglycemia in healthy man

Antecedent increases of corticosteroids can blunt counterregulatory responses to subsequent stress. Our aim was to determine whether prior activation of type I corticosteroid (mineralocorticoid) or type II corticosteroid (glucocorticoid) receptors blunts counterregulatory responses to subsequent hypoglycemia. Healthy volunteers participated in five randomized 2-day protocols. Day 1 involved morning and afternoon 2-h hyperinsulinemic (9 pmol.kg(-1).min(-1)) euglycemic clamps (PE; n = 14), hypoglycemic clamps (PH; n = 14), or euglycemic clamps with oral fludrocortisone (PE + F; type I agonist, 0.2 mg, n = 14), oral dexamethasone (PE + D; type II agonist, 0.75 mg, n = 13), or both (PE + F + D; n = 14). Day 2 was identical in all protocols and consisted of a 2-h hyperinsulinemic hypoglycemic clamp. Day 2 insulin (625 +/- 40 pmol/l) and glucose (2.9 +/- 0.1 mmol/l) levels were similar among groups. Levels of epinephrine, norepinephrine, glucagon, growth hormone, and MSNA were significantly blunted by prior activation of both type I and type II corticosteroid receptors to PE. Prior activation of both corticosteroid receptors also significantly blunted NEFA during subsequent hypoglycemia. Thus, levels of a wide spectrum of key counterregulatory mechanisms (neuroendocrine, ANS, and metabolic) were blunted by antecedent pharmacological stimulation of either type I or type II corticosteroid receptors in healthy man. These data suggest that activation of type I corticosteroid receptors in man can have acute and profound regulating effects on physiological stress in man. Both type I and type II corticosteroid receptors may be involved in the multiple mechanisms controlling counterregulatory responses to hypoglycemia in healthy man.

Portal vein hypoglycemia is essential for full induction of hypoglycemia-associated autonomic failure with slow-onset hypoglycemia

Antecedent hypoglycemia leads to impaired counterregulation and hypoglycemic unawareness. To ascertain whether antecedent portal vein hypoglycemia impairs portal vein glucose sensing, thereby inducing counterregulatory failure, we compared the effects of antecedent hypoglycemia, with and without normalization of portal vein glycemia, upon the counterregulatory response to subsequent hypoglycemia. Male Wistar rats were chronically cannulated in the carotid artery (sampling), jugular vein (glucose and insulin infusion), and mesenteric vein (glucose infusion). On day 1, the following three distinct antecedent protocols were employed: 1) HYPO-HYPO: systemic hypoglycemia (2.52 +/- 0.11 mM); 2) HYPO-EUG: systemic hypoglycemia (2.70 +/- 0.03 mM) with normalization of portal vein glycemia (portal vein glucose = 5.86 +/- 0.10 mM); and 3) EUG-EUG: systemic euglycemia (6.33 +/- 0.31 mM). On day 2, all groups underwent a hyperinsulinemic-hypoglycemic clamp in which the fall in glycemia was controlled so as to reach the nadir (2.34 +/- 0.04 mM) by minute 75. Counterregulatory hormone responses were measured at basal (-30 and 0) and during hypoglycemia (60-105 min). Compared with EUG-EUG, antecedent hypoglycemia (HYPO-HYPO) significantly blunted the peak epinephrine (10.44 +/- 1.35 vs. 15.75 +/- 1.33 nM: P = 0.01) and glucagon (341 +/- 16 vs. 597 +/- 82 pg/ml: P = 0.03) responses to next-day hypoglycemia. Normalization of portal glycemia during systemic hypoglycemia on day 1 (HYPO-EUG) prevented blunting of the peak epinephrine (15.59 +/- 1.43 vs. 15.75 +/- 1.33 nM: P = 0.94) and glucagon (523 +/- 169 vs. 597 +/- 82 pg/ml: P = 0.66) responses to day 2 hypoglycemia. Consistent with hormonal responses, the glucose infusion rate during day 2 hypoglycemia was substantially elevated in HYPO-HYPO (74 +/- 12 vs. 49 +/- 4 micromol x kg(-1) x min(-1); P = 0.03) but not HYPO-EUG (39 +/- 7 vs. 49 +/- 4 micromol x kg(-1) x min(-1): P = 0.36). Antecedent hypoglycemia local to the portal vein is required for the full induction of hypoglycemia-associated counterregulatory failure with slow-onset hypoglycemia.

Antecedent hindbrain glucoprivation does not impair the counterregulatory response to hypoglycemia

Recurrent hypoglycemia impairs hormonal counterregulatory responses (CRRs) to further bouts of hypoglycemia. The hypothalamus and hindbrain are both critical for sensing hypoglycemia and triggering CRRs. Hypothalamic glucose sensing sites are implicated in the pathogenesis of defective CRRs; however, the contribution of hindbrain glucose sensing has not been elucidated. Using a rat model, we compared the effect of antecedent glucoprivation targeting hindbrain or hypothalamic glucose sensing sites with the effect of antecedent recurrent hypoglycemia on CRR to hypoglycemia induced 24 h later. Recurrent hypoglycemia decreased sympathoadrenal (1,470 +/- 325 vs. 3,811 +/- 540 pg/ml in controls [t = 60 min], P = 0.001) and glucagon secretion (222 +/- 43 vs. 494 +/- 56 pg/ml in controls [t = 60]), P = 0.003) in response to hypoglycemia. Antecedent 5-thio-glucose (5TG) injected into the hindbrain did not impair sympathoadrenal (3,806 +/- 344 pg/ml [t = 60]) or glucagon (513 +/- 56 pg/ml [t = 60]) responses to subsequent hypoglycemia. However, antecedent 5TG delivered into the third ventricle was sufficient to blunt CRRs to hypoglycemia. These results show that hindbrain glucose sensing is not involved in the development of defective CRRs. However, neural substrates surrounding the third ventricle are particularly sensitive to glucoprivic stimulation and may contribute importantly to the development of defective CRRs.

Metabolic consequences of exercise-associated autonomic failure

Type I diabetes mellitus patients who tightly control blood glucose levels toward normal have increased frequency of hypoglycemia, a major barrier to physical activity. This article presents the hypothesis that dysfunctional autonomic regulation of metabolism after recent bouts of exercise or hypoglycemia contributes to exercise-induced hypoglycemia in these patients.

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.

Corticotrophin-releasing factor receptors within the ventromedial hypothalamus regulate hypoglycemia-induced hormonal counterregulation

Recurrent episodes of hypoglycemia impair sympathoadrenal counterregulatory responses (CRRs) to a subsequent episode of hypoglycemia. For individuals with type 1 diabetes, this markedly increases (by 25-fold) the risk of severe hypoglycemia and is a major limitation to optimal insulin therapy. The mechanisms through which this maladaptive response occurs remain unknown. The corticotrophin-releasing factor (CRF) family of neuropeptides and their receptors (CRFR1 and CRFR2) play a critical role in regulating the neuroendocrine stress response. Here we show in the Sprague-Dawley rat that direct in vivo application to the ventromedial hypothalamus (VMH), a key glucose-sensing region, of urocortin I (UCN I), an endogenous CRFR2 agonist, suppressed (approximately 55-60%), whereas CRF, a predominantly CRFR1 agonist, amplified (approximately 50-70%) CRR to hypoglycemia. UCN I was shown to directly alter the glucose sensitivity of VMH glucose-sensing neurons in whole-cell current clamp recordings in brain slices. Interestingly, the suppressive effect of UCN I-mediated CRFR2 activation persisted for at least 24 hours after in vivo VMH microinjection. Our data suggest that regulation of the CRR is largely determined by the interaction between CRFR2-mediated suppression and CRFR1-mediated activation in the VMH.

Glucocorticoid-deficient corticotropin-releasing hormone knockout mice maintain glucose requirements but not autonomic responses during repeated hypoglycemia

Glucocorticoids have been implicated in hypoglycemia-induced autonomic failure but also contribute to normal counterregulation. To determine the influence of normal and hypoglycemia-induced levels of glucocorticoids on counterregulatory responses to acute and repeated hypoglycemia, we compared plasma catecholamines, corticosterone, glucagon, and glucose requirements in male wild-type (WT) and glucocorticoid-deficient, corticotropin-releasing hormone knockout (CRH KO) mice. Conscious, chronically cannulated, unrestrained WT and CRH KO mice underwent a euglycemic (Prior Eu) or hypoglycemic clamp (Prior Hypo) on day 1 followed by a hypoglycemic clamp on day 2 (blood glucose both days, 65 +/- 1 mg/dl). Baseline epinephrine and glucagon were similar, and norepinephrine was elevated, in CRH KO vs. WT mice. CRH KO corticosterone was almost undetectable (<1.5 microg/dl) and unresponsive to hypoglycemia. CRH KO glucose requirements were significantly higher during day 1 hypoglycemia despite epinephrine and glucagon responses that were comparable to or greater than those in WT. Hyperinsulinemic euglycemia did not increase hormones or glucose requirements above baseline. On day 2, Prior Hypo WT had significantly higher glucose requirements and significantly lower corticosterone and glucagon responses. Prior Hypo and Prior Eu CRH KO mice had similar day 2 glucose requirements. However, Prior Hypo CRH KO mice had significantly lower day 2 epinephrine and norepinephrine vs. Prior Eu CRH KO and tended to have lower glucagon than on day 1. We conclude that glucocorticoid insufficiency in CRH KO mice correlates with 1) impaired counterregulation during acute hypoglycemia and 2) complex effects after repeated hypoglycemia, neither preventing decreased hormone responses nor worsening glucose requirements.

Activation of AMP-activated protein kinase within the ventromedial hypothalamus amplifies counterregulatory hormone responses in rats with defective counterregulation

Defective counterregulatory responses (CRRs) to hypoglycemia are associated with a marked increase in the risk of severe hypoglycemia. The mechanisms leading to the development of defective CRRs remain largely unknown, although they are associated with antecedent hypoglycemia. Activation of AMP-activated protein kinase (AMPK) in the ventromedial hypothalamus (VMH) amplifies the counterregulatory increase in glucose production during acute hypoglycemia. To examine whether activation of AMPK in the VMH restores defective CRR, controlled hypoglycemia ( approximately 2.8 mmol/l) was induced in a group of 24 Sprague-Dawley rats, all of which had undergone a 3-day model of recurrent hypoglycemia before the clamp study. Before the acute study, rats were microinjected to the VMH with either 5-aminoimidazole-4-carboxamide (AICAR; n=12), to activate AMPK, or saline (n=12). In a subset of rats, an infusion of H(3)-glucose was additionally started to calculate glucose turnover. Stimulation of AMPK within the VMH was found to amplify hormonal CRR and increase endogenous glucose production. In addition, analysis of tissue from both whole hypothalamus and VMH showed that recurrent hypoglycemia induces an increase in the gene expression of AMPK alpha(1) and alpha(2). These findings suggest that the development of novel drugs designed to selectively activate AMPK in the VMH offer a future therapeutic potential for individuals with type 1 diabetes who have defective CRRs to hypoglycemia.

I.c.v. administration of the nonsteroidal glucocorticoid receptor antagonist, CP-472555, prevents exacerbated hypoglycemia during repeated insulin administration

Hypoglycemia elicits an integrated array of CNS-mediated counterregulatory responses, including activation of the hypothalamic-pituitary-adrenal axis. The role of antecedent adrenocortical hypersecretion in impaired glucose counterregulation remains controversial. The present studies utilized the selective, nonsteroidal glucocorticoid receptor antagonist, CP-472555, as a pharmacological tool to investigate the hypothesis that hypoglycemic hypercorticosteronemia modulates CNS efferent autonomic and neuroendocrine motor responses to recurring insulin-induced hypoglycemia via glucocorticoid receptor-dependent mechanisms. Groups of adult male rats were injected s.c. with either one or four doses of the intermediate-acting insulin, Humulin neutral protamine Hagedorn (NPH), on as many days, while controls were injected with diluent alone. Animals injected with four doses of insulin were pretreated by i.c.v. administration of graded doses of the glucocorticoid receptor antagonist or vehicle alone prior to the first three doses of insulin. Repeated daily injection of NPH exacerbated hypoglycemia, attenuated patterns of glucagon and epinephrine secretion, and diminished neuronal transcriptional activation in discrete CNS metabolic loci, including the lateral hypothalamic area, dorsomedial hypothalamic nucleus, paraventricular hypothalamic nucleus, and nucleus of the solitary tract. While i.c.v. delivery of 25 or 100 ng doses of CP-472555 did not alter any of these parameters, animals treated with 500 ng exhibited circulating glucose, glucagon, and epinephrine levels that were similar to those in rats injected with one dose of insulin, as well as a reversal of recurring insulin-induced hypoglycemia-associated reductions in Fos immunolabeling in the lateral hypothalamic area, dorsomedial hypothalamic nucleus, and paraventricular hypothalamic nucleus. These results provide unique pharmacological evidence that antecedent activation of central glucocorticoid receptor is required for exacerbation of hypoglycemia during recurring insulin-induced hypoglycemia, and that these receptors mediate modulatory effects of hypoglycemic hypercorticosteronemia on autonomic efferent responses to recurring insulin-induced hypoglycemia. The data also suggest that neurons in central loci characterized here by antagonist-mediated overturn of recurring insulin-induced hypoglycemia-induced decreases in neuronal transcriptional activation may be direct or indirect substrates for this hormonal modulation action.

Antecedent hypercortisolemia is not primarily responsible for generating hypoglycemia-associated autonomic failure

Hypoglycemia-associated autonomic failure (HAAF) occurs commonly in patients with longstanding diabetes, placing affected patients at increased risk for severe hypoglycemia. Previous studies have suggested that hypoglycemia-induced hypercortisolemia may be responsible for blunting subsequent sympathoadrenal responses to hypoglycemia; however, this view remains highly controversial. In this work, we sought to better define the role of antecedent hypercortisolemia in generating HAAF, using two complimentary experimental models in nondiabetic human subjects: 1) antecedent hydrocortisone infusions (simulating physiologic cortisol responses to hypoglycemia) and 2) antecedent hypoglycemia, with and without concurrent blockade of endogenous cortisol production using oral metyrapone. Our results showed no effect of antecedent hypercortisolemia on epinephrine responses to subsequent hypoglycemia (area under the curve/time 280 +/- 53 vs. 337 +/- 57 pg/ml, P = 0.16). Of particular importance, selective blockade of endogenous cortisol production during antecedent hypoglycemia had no effect on subsequent counterregulatory responses to hypoglycemia. Compared with epinephrine responses following antecedent euglycemia (area under the curve/time 312 +/- 38 pg/ml), epinephrine responses were comparably blunted following antecedent hypoglycemia, regardless of whether concurrent metyrapone blockade was employed (198 +/- 28 vs. 192 +/- 28 pg/ml, P = NS). Similar results were obtained for glucagon and ACTH levels. Considered together, these observations provide strong evidence that hypoglycemia-induced hypercortisolemia is not primarily responsible for the generation of HAAF.

Cognitive and neural hippocampal effects of long-term moderate recurrent hypoglycemia

Recurrent hypoglycemia is the most feared complication of intensive insulin therapy for type 1 diabetes. Study of the cognitive impact of recurrent hypoglycemia in humans has been hampered by difficulty in controlling for prior glycemic history and diabetes status; there have been no prospective studies. We used a rat model of recurrent hypoglycemia with hypoglycemia for 3 h, once weekly, from 1 month of age. At 4, 8, and 12 months of age, cohorts were tested on a hippocampally dependent spatial memory task, during which hippocampal extracellular fluid (ECF) glucose and lactate were measured using microdialysis. At 4 months, recurrent hypoglycemia improved euglycemic task performance (76 +/- 4 vs. 64 +/- 3% for controls) and reversed the task-associated dip in ECF glucose seen in controls. However, recurrent hypoglycemia impaired performance in animals tested when hypoglycemic (45 +/- 4 vs. 55 +/- 2%). Recurrent hypoglycemia preserved euglycemic task performance across age: at 12 months, both task performance (62%) and ECF glucose changes in euglycemic recurrently hypoglycemic animals resembled those of 4-month-old control animals, whereas control animals' performance deteriorated to chance (44%) by 8 months. At 12 months, hippocampal slice physiology was assessed, with results paralleling the cognitive findings: slices from recurrently hypoglycemic rats showed improved gamma-aminobutyric acid (GABA)ergic inhibition at euglycemia but much greater loss of this tone at low bath glucose. Our data show that moderate weekly hypoglycemia prevented age-related decline in hippocampally cognitive function and cognitive metabolism, at least when euglycemic. The impact of recurrent hypoglycemia on cognition is multifaceted and includes both metabolic and electrophysiological components.

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.

Feeding and neuroendocrine responses after recurrent insulin-induced hypoglycemia

Prior exposure to hypoglycemia impairs neuroendocrine counterregulatory responses (CRR) during subsequent hypoglycemia. Defective CRR to hypoglycemia is a component of the clinical syndrome hypoglycemia-associated autonomic failure (HAAF). Hypoglycemia also potently stimulates food intake, an important behavioral CRR. Because the increased feeding response to hypoglycemia is behavioral and not hormonal, we hypothesized that it may be regulated differently with recurrent bouts of hypoglycemia. To test this hypothesis, we simultaneously evaluated neuroendocrine CRR and food intake in rats experiencing one or three episodes of insulin-induced hypoglycemia. As expected, recurrent hypoglycemia significantly reduced neuroendocrine hypoglycemic CRR. Epinephrine (E), norepinephrine (NE) and glucagon responses 120 min after insulin injection were significantly reduced in recurrent hypoglycemic rats, relative to rats experiencing hypoglycemia for the first time. Despite these neuroendocrine impairments, food intake was significantly elevated above baseline saline intake whether rats were experiencing a first (hypoglycemia: 3.4+/-0.4 g vs. saline: 0.94+/-0.3 g, P<0.05) or third hypoglycemic episode (hypoglycemia: 3.8+/-0.3 g vs. saline: 1.2+/-0.3 g, P<0.05). These findings demonstrate that food intake elicited in response to hypoglycemia is not impaired as a result of recurrent hypoglycemia. Thus, neuroendocrine and behavioral (stimulation of food intake) CRR are differentially regulated by recurrent hypoglycemia experience.

Mechanisms of hypoglycemia-associated autonomic failure and its component syndromes in diabetes

Iatrogenic hypoglycemia is a problem for people with diabetes. It causes recurrent morbidity, and sometimes death, as well as a vicious cycle of recurrent hypoglycemia, precluding maintenance of euglycemia over a lifetime of diabetes. Improved therapeutic approaches that will minimize both hypo- and hyperglycemia will be based on insight into the pathophysiology of glucoregulation, specifically glucose counterregulation, in insulin-deficient (type 1 and advanced type 2) diabetes. In such patients, hypoglycemia is the result of the interplay of relative or absolute therapeutic insulin excess and compromised physiological (the syndrome of defective glucose counterregulation) and behavioral (the syndrome of hypoglycemia unawareness) defenses against falling plasma glucose concentrations. The concept of hypoglycemia-associated autonomic failure (HAAF) in diabetes posits that recent antecedent iatrogenic hypoglycemia causes both defective glucose counterregulation (by reducing epinephrine responses to a given level of subsequent hypoglycemia in the setting of absent decrements in insulin and absent increments in glucagon) and hypoglycemia unawareness (by reducing sympathoadrenal and the resulting neurogenic symptom responses to a given level of subsequent hypoglycemia) and thus a vicious cycle of recurrent hypoglycemia. The clinical impact of HAAF is well established in type 1 diabetes; it also affects those with advanced type 2 diabetes. It is now known to be largely reversible, by as little as 2-3 weeks of scrupulous avoidance of hypoglycemia, in most affected patients. However, the mechanisms of HAAF and its component syndromes are largely unknown. Loss of the glucagon secretory response, a key feature of defective glucose counterregulation, is plausibly explained by insulin deficiency, specifically loss of the decrement in intraislet insulin that normally signals glucagon secretion as glucose levels fall. Reduced neurogenic symptoms, a key feature of hypoglycemia unawareness, are largely the result of reduced sympathetic neural responses to falling glucose levels. The mechanism by which hypoglycemia shifts the glycemic thresholds for sympathoadrenal activation to lower plasma glucose concentrations, the key feature of both components of HAAF, is not known. It does not appear to be the result of the release of a systemic mediator (e.g., cortisol, epinephrine) during antecedent hypoglycemia or of increased blood-to-brain glucose transport (although increased transport of alternative fuels is conceivable). It is likely the result of alterations of brain metabolism. Although there is an array of clues, the specific alteration remains to be identified. While the research focus has been largely on the hypothalamus, hypoglycemia is now known to activate widespread brain regions, including the medial prefrontal cortex. The possibility that HAAF could be the result of posthypoglycemic brain glycogen supercompensation has also been raised. Finally, there appear to be diverse causes of HAAF. In addition to recent antecedent hypoglycemia, these include exercise- and sleep-related HAAF. Clearly, a unifying mechanism of HAAF would need to incorporate these causes as well. Pending the prevention and cure of diabetes, critical fundamental, translational, and outcomes research is needed if we are to eliminate hypoglycemia from the lives of people affected by diabetes.

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.

Activation of ATP-sensitive K+ channels in the ventromedial hypothalamus amplifies counterregulatory hormone responses to hypoglycemia in normal and recurrently hypoglycemic rats

The mechanism(s) by which glucosensing neurons detect fluctuations in glucose remains largely unknown. In the pancreatic beta-cell, ATP-sensitive K+ channels (K ATP channels) play a key role in glucosensing by providing a link between neuronal metabolism and membrane potential. The present study was designed to determine in vivo whether the pharmacological opening of ventromedial hypothalamic K ATP channels during systemic hypoglycemia would amplify hormonal counterregulatory responses in normal rats and those with defective counterregulation arising from prior recurrent hypoglycemia. Controlled hypoglycemia (approximately 2.8 mmol/l) was induced in vivo using a hyperinsulinemic (20 mU x kg(-1) x min(-1)) glucose clamp technique in unrestrained, overnight-fasted, chronically catheterized Sprague-Dawley rats. Immediately before the induction of hypoglycemia, the rats received bilateral ventromedial hypothalamic microinjections of either the potassium channel openers (KCOs) diazoxide and NN414 or their respective controls. In normal rats, both KCOs amplified epinephrine and glucagon counterregulatory responses to hypoglycemia. Moreover, diazoxide also amplified the counterregulatory responses in a rat model of defective hormonal counterregulation. Taken together, our data suggest that the K ATP channel plays a key role in vivo within glucosensing neurons in the ventromedial hypothalamus in the detection of incipient hypoglycemia and the initiation of protective counterregulatory responses. We also conclude that KCOs may offer a future potential therapeutic option for individuals with insulin-treated diabetes who develop defective counterregulation.

Combined corticotropin-releasing hormone and glucocorticoid deficiency does not enhance counterregulatory responses after recurrent hypoglycemia in mice

Glucocorticoids and corticotropin-releasing hormone (CRH) have been proposed to inhibit counterregulatory responses to recurrent hypoglycemia. We used the CRH knockout (CRH KO) mouse to test the hypothesis that combined CRH and glucocorticoid deficiency would prevent development of counterregulatory deficits after repeated hypoglycemia. To develop a mouse model of recurrent hypoglycemia, we first tested the effects of daily lente insulin injection on counterregulatory responses to acute hypoglycemia in male C57BL/6 mice. Treatment with up to 250 U/kg per day lente insulin resulted in significantly greater decreases in plasma glucose, suggestive of impaired counterregulation, after hypoglycemia induced by acute insulin injection. Plasma catecholamine responses to hypoglycemia in repeatedly hypoglycemic C57BL/6 mice were unexpectedly higher than in naive mice, which we interpreted as a compensatory response to the greater decreases in plasma glucose. Lente insulin doses had to be reduced (50-75 U/kg per day) for CRH KO mice to survive repeated hypoglycemia. Wild-type (WT) mice treated with 50 to 75 U/kg per day lente insulin exhibited enhanced sympathetic activity after hypoglycemia, resembling the compensatory responses associated with impaired glucose homeostasis in C57BL/6 mice treated with 250 U/kg per day lente insulin. During acute hypoglycemia, CRH KO mice maintained higher plasma glucose levels that correlated with higher plasma norepinephrine and greater glycogen mobilization. Recurrent hypoglycemia did not enhance liver glycogen depletion or the markedly impaired glucocorticoid and epinephrine responses to hypoglycemia in CRH KO mice. Previously hypoglycemic CRH KO mice also did not display the further increases in sympathetic activity that in WT mice were suggestive of compensation for impaired counterregulation. Despite the apparent resistance of CRH KO mice to the counterregulatory effects of repeated hypoglycemia, their greater mortality after hypoglycemia tolerated by WT mice indicates that combined CRH and glucocorticoid deficiency does not significantly improve counterregulation after repeated hypoglycemia.

Recurrent insulin-induced hypoglycemia causes site-specific patterns of habituation or amplification of CNS neuronal genomic activation

Antecedent hypoglycemia is a primary factor in hypoglycemia-associated autonomic failure, a pathophysiological condition characterized by impaired glucose counterregulatory function. Conventional therapeutic strategies involving administration of intermediate dosage-release formulations of insulin in the management of insulin-dependent diabetes mellitus result in frequent iatrogenic hypoglycemia. This study investigated the neuroanatomical location, direction, and magnitude of CNS neuronal genomic activation by singular versus repeated induction of hypoglycemic bouts of greater than 6 h duration achieved by administration of the intermediate-acting insulin, humulin neutral protamine Hagedorn (NPH). Adult male rats injected subcutaneously with Humulin NPH exhibited robust immunolabeling for the nuclear transcription factor, Fos, in discrete telencephalic, diencephalic, midbrain, and caudal hindbrain loci in a pattern that was not identical to that described for regular insulin. Administration of four doses of insulin on as many days significantly diminished or extinguished Fos immunostaining within the parvocellular hypothalamic paraventricular nucleus, lateral hypothalamic area, dorsomedial hypothalamic nucleus, thalamic paraventricular nucleus, nucleus tractus solitarius, and area postrema, but did not modify labeling of other metabolic loci. However, numbers of Fos-immunoreactivity-positive magnocellular neurons in the hypothalamic paraventricular and supraoptic nuclei were significantly increased after the second and fourth insulin doses, relative to the single-dose group. Concurrent observations of exacerbated hypoglycemia and modified patterns of glucoregulatory hormone secretion after serial injections of intermediate-acting insulin suggest that central mechanisms governing compensatory endocrine responses, specifically glucagon, become habituated to repetitive hypoglycemia of extended duration. Resultant alterations in CNS-islet and -adrenomedullary output and hypothalamic-pituitary-adrenal activity may reflect diminished neuronal activation within one or more of the brain loci characterized here by nonuniform transcriptional activation. The current studies provide a neuroanatomical foundation for further investigation of the neurochemical phenotypes and interconnectivity of functionally adaptive neurons, underlying cellular and molecular mechanisms of diminished or enhanced activation, as well as the impact of these modified cellular responses on glucose counterregulation during administration of intermediate-acting insulin.

Brain region-dependent effects of dexamethasone on counterregulatory responses to hypoglycemia in conscious rats

The aim of this study was to determine whether activation of central type II glucocorticoid receptors can blunt autonomic nervous system counterregulatory responses to subsequent hypoglycemia. Sixty conscious unrestrained Sprague-Dawley rats were studied during 2-day experiments. Day 1 consisted of either two episodes of clamped 2-h hyperinsulinemic (30 pmol x kg(-1) x min(-1)) hypoglycemia (2.8 +/- 0.1 mM; n = 12), hyperinsulinemic euglycemia (6.2 +/- 0.1 mM; n = 12), hyperinsulinemic euglycemia plus simultaneous lateral cerebroventricular infusion of saline (24 microl/h; n = 8), or hyperinsulinemic euglycemia plus either lateral cerebral ventricular infusion (n = 8; LV-DEX group), fourth cerebral ventricular (n = 10; 4V-DEX group), or peripheral (n = 10; P-DEX group) infusion of dexamethasone (5 microg/h), a specific type II glucocorticoid receptor analog. For all groups, day 2 consisted of a 2-h hyperinsulinemic (30 pmol x kg(-1) x min(-1)) or hypoglycemic (2.9 +/- 0.2 mM) clamp. The hypoglycemic group had blunted epinephrine, glucagon, and endogenous glucose production in response to subsequent hypoglycemia. Consequently, the glucose infusion rate to maintain the glucose levels was significantly greater in this group vs. all other groups. The LV-DEX group did not have blunted counterregulatory responses to subsequent hypoglycemia, but the P-DEX and 4V-DEX groups had significantly lower epinephrine and norepinephrine responses to hypoglycemia compared with all other groups. In summary, peripheral and fourth cerebral ventricular but not lateral cerebral ventricular infusion of dexamethasone led to significant blunting of autonomic counterregulatory responses to subsequent hypoglycemia. These data suggest that prior activation of type II glucocorticoid receptors within the hindbrain plays a major role in blunting autonomic nervous system counterregulatory responses to subsequent hypoglycemia in the conscious rat.

Effects of low and moderate antecedent exercise on counterregulatory responses to subsequent hypoglycemia in type 1 diabetes

Antecedent moderate-intensity exercise has been shown to blunt autonomic, neuroendocrine, and metabolic counterregulatory responses to subsequent hypoglycemia in nondiabetic individuals. The aims of the current study were to determine 1) whether this occurs in type 1 diabetic patients and 2) whether the degree of blunting is dependent on exercise intensity. Twenty-seven type 1 diabetic patients (13 women and 14 men) were studied during a single-step, 2-h hyperinsulinemic (9 pmol x kg(-1) x min(-1))-hypoglycemic (approximately 2.8 mmol/l) clamp 1 day after two 90-min exercise bouts at 30% (n = 11) or at 50% (n = 11) Vo(2max) or after no prior stress (control subjects, n = 25). After prior exercise at both 30 and 50% Vo(2max), epinephrine (1,959 +/- 553 and 1,528 +/- 424 vs. 3,420 +/- 424 pmol/l, respectively; P < 0.05) and pancreatic polypeptide (97 +/- 32 and 98 +/- 8 vs. 223 +/- 32 pmol/l, respectively; P < 0.05) responses to subsequent hypoglycemia were significantly lower compared with those of control subjects. Endogenous glucose production was significantly lower, while glucose utilization and, consequently, the exogenous glucose infusion rate needed to maintain hypoglycemia were significantly greater after both exercise intensities compared with that of control subjects. Muscle sympathetic nerve activity was significantly reduced by prior exercise of both intensities at baseline (16 +/- 4 and 22 +/- 4 vs. 31 +/- 3 bursts/min) and during hypoglycemia (22 +/- 4 and 27 +/- 5 vs. 41 +/- 3 bursts/min) compared with that of control subjects (P < 0.05). Total hypoglycemic symptoms were also significantly lower (P < 0.05) in both exercise groups compared with the control group. In summary, repeated episodes of prolonged exercise of both low and moderate intensities blunted key autonomic (epinephrine and pancreatic polypeptide) and metabolic (endogenous glucose production and peripheral glucose uptake) counterregulatory responses to next-day hypoglycemia in type 1 diabetes.

The effects of dehydroepiandrosterone sulfate on counterregulatory responses during repeated hypoglycemia in conscious normal rats

We previously determined that both antecedent hypoglycemia and elevated cortisol levels blunt neuroendocrine and metabolic responses to subsequent hypoglycemia in conscious, unrestrained rats. The adrenal steroid dehydroepiandrosterone sulfate (DHEA-S) has been shown in several studies to oppose corticosteroid action. The purpose of this study was to determine if DHEA-S could preserve counterregulatory responses during repeated hypoglycemia. We studied 40 male Sprague-Dawley rats during a series of 2-day protocols. Day 1 consisted of two 2-h episodes of 1) hyperinsulinemic (30 pmol. kg(-1). min(-1)) euglycemia (6.2 +/- 0.2 mmol/l; n = 12; ANTE EUG), 2) hyperinsulinemic euglycemia (6.0 +/- 0.1 mmol/l; n = 8) plus simultaneous intravenous infusion of DHEA-S (30 mg/kg; ANTE EUG + DHEA-S), 3) hyperinsulinemic hypoglycemia (2.8 +/- 0.1 mmol/l; n = 12; ANTE HYPO), or 4) hyperinsulinemic hypoglycemia (2.8 +/- 0.1 mmol/l; n = 8) with simultaneous intravenous infusion of DHEA-S (30 mg/kg; ANTE HYPO + DHEA-S). Day 2 consisted of a single 2-h hyperinsulinemic hypoglycemic (2.8 +/- 0.1 mmol/l) clamp. During the final 30 min of day 2, hypoglycemia norepinephrine levels were significantly lower in the ANTE HYPO group versus the ANTE HYPO + DHEA-S group (2.0 +/- 0.2 vs. 3.3 +/- 0.6 nmol/l; P < 0.05). In addition, epinephrine (8 +/- 1 vs. 17 +/- 2, 14 +/- 3, and 15 +/- 3 nmol/l), glucagon (91 +/- 8 vs. 273 +/- 36, 231 +/- 42, and 297 +/- 48 ng/l), and corticosterone (1,255 +/- 193 vs. 1,915 +/- 212, 1,557 +/- 112, and 1,668 +/- 119 pmol/l) were significantly lower in the ANTE HYPO group versus the ANTE EUG, ANTE EUG + DHEA-S, and ANTE HYPO + DHEA-S groups (P < 0.05). Endogenous glucose production was also significantly less in the ANTE HYPO group versus the ANTE EUG, ANTE EUG + DHEA-S, and ANTE HYPO + DHEA-S groups (13 +/- 5 vs. 32 +/- 3, 38 +/- 7, and 29 +/- 8 micro mol/l. kg(-1). min(-1); P < 0.05). Consequently, the amount of exogenous glucose needed to maintain the glycemic level during the clamp studies was significantly higher in the ANTE HYPO versus the ANTE EUG, ANTE EUG + DHEA-S, and ANTE HYPO + DHEA-S groups (57 +/- 8 vs. 22 +/- 5, 18 +/- 6, and 18 +/- 3 micro mol/l. kg(-1). min(-1); P < 0.05). In summary, day-1 antecedent hypoglycemia blunted neuroendocrine and metabolic responses to next-day hypoglycemia. However, simultaneous DHEA-S infusion during antecedent hypoglycemia preserved neuroendocrine and metabolic counterregulatory responses during subsequent hypoglycemia in conscious rats.