Recurrent hypoglycemia reduces the glucose sensitivity of glucose-inhibited neurons in the ventromedial hypothalamus nucleus

A rare case report of reversible glucose counterregulation in an insulinoma patient with type 2 diabetes

CONTEXT

Insulinoma is a neuroendocrine tumor derived from pancreatic β -cells whose clinical manifestation is recurrent hypoglycemia. Insulinoma in a patient with preexisting diabetes is extraordinarily rare, and the unmasking of type 2 diabetes (T2DM) after insulinoma surgery is even rarer.

CASE REPORT

This article reports a 49-year-old male patient with insulinoma that masked the diagnosis of T2DM. The patient was admitted to the hospital with symptoms of hypoglycemia, such as repeated sweating, palpitations, and asthenia for over 4 years. The patient was diagnosed with insulinoma after completing relevant examinations. The emergence of hyperglycemia after the removal of insulinoma is attributable to the coexistence of T2DM. Surprisingly, a reversible decrease in cortisol levels was observed during the diagnostic process. We searched the previously published reports of this type of case from PubMed to determine why type 2 diabetes was covered by insulinoma and why glucocorticoids decreased.

CONCLUSIONS

The diagnosis of T2DM in the patient after surgery may be related to increased food intake and insulin resistance induced by hyperinsulinemia caused by long-term hypoglycemia. The reversible decrease in cortisol levels, not adrenocortical insufficiency during the diagnostic process, may be caused by a transient abnormality in glucose counterregulation.

Serotonergic neurons are involved in the counter-regulatory response to hypoglycemia

OBJECTIVES

Intensive insulin therapy provides optimal glycemic control in patients with diabetes. However, intensive insulin therapy causes so-called iatrogenic hypoglycemia as a major adverse effect. The ventromedial hypothalamus (VMH) has been described as the primary brain area initiating the counter-regulatory response (CRR). Nevertheless, the VMH receives projections from other brain areas which could participate in the regulation of the CRR. In particular, studies suggest a potential role of the serotonin (5-HT) network. Thus, the objective of this study was to determine the contribution of 5-HT neurons in CRR control.

METHODS

Complementary approaches have been used to test this hypothesis in quantifying the level of 5-HT in several brain areas by HPLC in response to insulin-induced hypoglycemia, measuring the electrical activity of dorsal raphe (DR) 5-HT neurons in response to insulin or decreased glucose level by patch-clamp electrophysiology; and measuring the CRR hormone glucagon as an index of the CRR to the modulation of the activity of 5-HT neurons using pharmacological or pharmacogenetic approaches.

RESULTS

HPLC measurements show that the 5HIAA/5HT ratio is increased in several brain regions including the VMH in response to insulin-induced hypoglycemia. Patch-clamp electrophysiological recordings show that insulin, but not decreased glucose level, increases the firing frequency of DR 5-HT neurons in the DR. In vivo, both the pharmacological inhibition of 5-HT neurons by intraperitoneal injection of the 5-HT1A receptor agonist 8-OH-DPAT or the chemogenetic inhibition of these neurons reduce glucagon secretion, suggesting an impaired CRR.

CONCLUSION

Taken together, these data highlight a new neuronal network involved in the regulation of the CRR. In particular, this study shows that DR 5-HT neurons detect iatrogenic hypoglycemia in response to the increased insulin level and may play an important role in the regulation of CRR.

In Vivo Photometry Reveals Insulin and 2-Deoxyglucose Maintain Prolonged Inhibition of VMH Vglut2 Neurons in Male Mice

The ventromedial hypothalamic (VMH) nucleus is a well-established hub for energy and glucose homeostasis. In particular, VMH neurons are thought to be important for initiating the counterregulatory response to hypoglycemia, and ex vivo electrophysiology and immunohistochemistry data indicate a clear role for VMH neurons in sensing glucose concentration. However, the temporal response of VMH neurons to physiologically relevant changes in glucose availability in vivo has been hampered by a lack of available tools for measuring neuronal activity over time. Since the majority of neurons within the VMH are glutamatergic and can be targeted using the vesicular glutamate transporter Vglut2, we expressed cre-dependent GCaMP7s in Vglut2 cre mice and examined the response profile of VMH to intraperitoneal injections of glucose, insulin, and 2-deoxyglucose (2DG). We show that reduced available glucose via insulin-induced hypoglycemia and 2DG-induced glucoprivation, but not hyperglycemia induced by glucose injection, inhibits VMH Vglut2 neuronal population activity in vivo. Surprisingly, this inhibition was maintained for at least 45 minutes despite prolonged hypoglycemia and initiation of a counterregulatory response. Thus, although VMH stimulation, via pharmacological, electrical, or optogenetic approaches, is sufficient to drive a counterregulatory response, our data suggest VMH Vglut2 neurons are not the main drivers required to do so, since VMH Vglut2 neuronal population activity remains suppressed during hypoglycemia and glucoprivation.

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.

Insulin actions on hypothalamic glucose-sensing neurones

Subsequent to the discovery of insulin 100 years ago, great strides have been made in understanding its function, especially in the brain. It is now clear that insulin is a critical regulator of the neuronal circuitry controlling energy balance and glucose homeostasis. This review focuses on the effects of insulin and diabetes on the activity and glucose sensitivity of hypothalamic glucose-sensing neurones. We highlight the role of electrophysiological data in understanding how insulin regulates glucose-sensing neurones. A brief introduction describing the benefits and limitations of the major electrophysiological techniques used to investigate glucose-sensing neurones is provided. The mechanisms by which hypothalamic neurones sense glucose are discussed with an emphasis on those glucose-sensing neurones already shown to be modulated by insulin. Next, the literature pertaining to how insulin alters the activity and glucose sensitivity of these hypothalamic glucose-sensing neurones is described. In addition, the effects of impaired insulin signalling during diabetes and the ramifications of insulin-induced hypoglycaemia on hypothalamic glucose-sensing neurones are covered. To the extent that it is known, we present hypotheses concerning the mechanisms underlying the effects of these insulin-related pathologies. To conclude, electrophysiological data from the hippocampus are evaluated aiming to provide clues regarding how insulin might influence neuronal plasticity in glucose-sensing neurones. Although much has been accomplished subsequent to the discovery of insulin, the work described in our review suggests that the regulation of central glucose sensing by this hormone is both important and understudied.

Repeated hypoglycemia remodels neural inputs and disrupts mitochondrial function to blunt glucose-inhibited GHRH neuron responsiveness

Hypoglycemia is a frequent complication of diabetes, limiting therapy and increasing morbidity and mortality. With recurrent hypoglycemia, the counterregulatory response (CRR) to decreased blood glucose is blunted, resulting in hypoglycemia-associated autonomic failure (HAAF). The mechanisms leading to these blunted effects are only poorly understood. Here, we report, with ISH, IHC, and the tissue-clearing capability of iDISCO+, that growth hormone releasing hormone (GHRH) neurons represent a unique population of arcuate nucleus neurons activated by glucose deprivation in vivo. Repeated glucose deprivation reduces GHRH neuron activation and remodels excitatory and inhibitory inputs to GHRH neurons. We show that low glucose sensing is coupled to GHRH neuron depolarization, decreased ATP production, and mitochondrial fusion. Repeated hypoglycemia attenuates these responses during low glucose. By maintaining mitochondrial length with the small molecule mitochondrial division inhibitor-1, we preserved hypoglycemia sensitivity in vitro and in vivo. Our findings present possible mechanisms for the blunting of the CRR, significantly broaden our understanding of the structure of GHRH neurons, and reveal that mitochondrial dynamics play an important role in HAAF. We conclude that interventions targeting mitochondrial fission in GHRH neurons may offer a new pathway to prevent HAAF in patients with diabetes.

GHRH neurons in the arcuate nucleus are activated by glucose deprivation; however, repeated hypoglycemia blunts activation, remodels inputs, and disrupts mitochondrial fusion.

Changes in neuronal activity across the mouse ventromedial nucleus of the hypothalamus in response to low glucose: Evaluation using an extracellular multi-electrode array approach

The hypothalamic ventromedial nucleus (VMN) is involved in maintaining systemic glucose homeostasis. Neurophysiological studies in rodent brain slices have identified populations of VMN glucose-sensing neurones: glucose-excited (GE) neurones, cells which increased their firing rate in response to increases in glucose concentration, and glucose-inhibited (GI) neurones, which show a reduced firing frequency in response to increasing glucose concentrations. To date, most slice electrophysiological studies characterising VMN glucose-sensing neurones in rodents have utilised the patch clamp technique. Multi-electrode arrays (MEAs) are a state-of-the-art electrophysiological tool enabling the electrical activity of many cells to be recorded across multiple electrode sites (channels) simultaneously. We used a perforated MEA (pMEA) system to evaluate electrical activity changes across the dorsal-ventral extent of the mouse VMN region in response to alterations in glucose concentration. Because intrinsic (ie, direct postsynaptic sensing) and extrinsic (ie, presynaptically modulated) glucosensation were not discriminated, we use the terminology 'GE/presynaptically excited by an increase (PER)' and 'GI/presynaptically excited by a decrease (PED)' in the present study to describe responsiveness to changes in extracellular glucose across the mouse VMN. We observed that 15%-60% of channels were GE/PER, whereas 2%-7% were GI/PED channels. Within the dorsomedial portion of the VMN (DM-VMN), significantly more channels were GE/PER compared to the ventrolateral portion of the VMN (VL-VMN). However, GE/PER channels within the VL-VMN showed a significantly higher basal firing rate in 2.5 mmol l^-1 glucose than DM-VMN GE/PER channels. No significant difference in the distribution of GI/PED channels was observed between the VMN subregions. The results of the present study demonstrate the utility of the pMEA approach for evaluating glucose responsivity across the mouse VMN. pMEA studies could be used to refine our understanding of other neuroendocrine systems by examining population level changes in electrical activity across brain nuclei, thus providing key functional neuroanatomical information to complement and inform the design of single-cell neurophysiological studies.

Ventromedial hypothalamus glucose-inhibited neurones: A role in glucose and energy homeostasis?

The ventromedial hypothalamus (VMH) plays a complex role in glucose and energy homeostasis. The VMH is necessary for the counter-regulatory response to hypoglycaemia (CRR) that increases hepatic gluconeogenesis to restore euglycaemia. On the other hand, the VMH also restrains hepatic glucose production during euglycaemia and stimulates peripheral glucose uptake. The VMH is also important for the ability of oestrogen to increase energy expenditure. This latter function is mediated by VMH modulation of the lateral/perifornical hypothalamic area (lateral/perifornical hypothalamus) orexin neurones. Activation of VMH AMP-activated protein kinase (AMPK) is necessary for the CRR. By contrast, VMH AMPK inhibition favours decreased basal glucose levels and is required for oestrogen to increase energy expenditure. Specialised VMH glucose-sensing neurones confer the ability to sense and respond to changes in blood glucose levels. Glucose-excited (GE) neurones increase and glucose-inhibited (GI) neurones decrease their activity as glucose levels rise. VMH GI neurones, in particular, appear to be important in the CRR, although a role for GE neurones cannot be discounted. AMPK mediates glucose sensing in VMH GI neurones suggesting that, although activation of these neurones is important for the CRR, it is necessary to silence them to lower basal glucose levels and enable oestrogen to increase energy expenditure. In support of this, we found that oestrogen reduces activation of VMH GI neurones in low glucose by inhibiting AMPK. In this review, we present the evidence underlying the role of the VMH in glucose and energy homeostasis. We then discuss the role of VMH glucose-sensing neurones in mediating these effects, with a strong emphasis on oestrogenic regulation of glucose sensing and how this may affect glucose and energy homeostasis.

Repetitive hypoglycemia reduces activation of glucose-responsive neurons in C1 and C3 medullary brain regions to subsequent hypoglycemia

The impaired ability of the autonomic nervous system to respond to hypoglycemia is termed "hypoglycemia-associated autonomic failure" (HAAF). This life-threatening phenomenon results from at least two recent episodes of hypoglycemia, but the pathology underpinning HAAF remains largely unknown. Although naloxone appears to improve hypoglycemia counterregulation under controlled conditions, hypoglycemia prevention remains the current mainstay therapy for HAAF. Epinephrine-synthesizing neurons in the rostroventrolateral (C1) and dorsomedial (C3) medulla project to the subset of sympathetic preganglionic neurons that regulate peripheral epinephrine release. Here we determined whether or not C1 and C3 neuronal activation is impaired in HAAF and whether or not 1 wk of hypoglycemia prevention or treatment with naloxone could restore C1 and C3 neuronal activation and improve HAAF. Twenty male Sprague-Dawley rats (250-300 g) were used. Plasma epinephrine levels were significantly increased after a single episode of hypoglycemia (n = 4; 5,438 ± 783 pg/ml vs. control 193 ± 27 pg/ml, P < 0.05). Repeated hypoglycemia significantly reduced the plasma epinephrine response to subsequent hypoglycemia (n = 4; 2,179 ± 220 pg/ml vs. 5,438 ± 783 pg/ml, P < 0.05). Activation of medullary C1 (n = 4; 50 ± 5% vs. control 3 ± 1%, P < 0.05) and C3 (n = 4; 45 ± 5% vs. control 4 ± 1%, P < 0.05) neurons was significantly increased after a single episode of hypoglycemia. Activation of C1 (n = 4; 12 ± 3%, P < 0.05) and C3 (n = 4; 19 ± 5%, P < 0.05) neurons was significantly reduced in the HAAF groups. Hypoglycemia prevention or treatment with naloxone did not restore the plasma epinephrine response or C1 and C3 neuronal activation. Thus repeated hypoglycemia reduced the activation of C1 and C3 neurons mediating adrenal medullary responses to subsequent bouts of hypoglycemia.

A voltage-dependent depolarization induced by low external glucose in neurons of the nucleus of the tractus solitarius: interaction with KATP channels

KEY POINTS

Neurons from the brainstem nucleus of the tractus solitarius (NTS) participate in the counter-regulatory mechanisms in response to hypoglycaemia. ATP-sensitive potassium (K_ATP ) channels are expressed in NTS neurons, and are partially open at rest in normoglycaemic 5 mM glucose. In normoglycaemic conditions, most NTS neurons depolarize in response to low external glucose (0.5 mM), via a voltage-dependent mechanism. Conversely, most NTS neurons incubated in hyperglycaemic 10 mM glucose do not respond to low glucose due to a more positive resting membrane potential caused by the closure of K_ATP channels following increased intracellular metabolic ATP. Our findings show that in hyperglycaemic conditions, NTS neurons failed to sense rapid changes in external glucose, which could be related to hypoglycaemia-associated autonomic failure.

ABSTRACT

The nucleus of the tractus solitarius (NTS) is an integrative centre for autonomic counter-regulatory responses to hypoglycaemia. K_ATP channels link the metabolic status of the neuron to its excitability. Here we investigated the influence of K_ATP channels on the membrane potential of NTS neurons in normo- and hyperglycaemic external glucose concentrations, and after switching to a hypoglycaemic concentration, using in vitro electrophysiological recordings in brainstem slices. We found that in normoglycaemic (5 mM) glucose, tolbutamide, a K_ATP channel antagonist, depolarized the membrane of most neurons, and this effect was observed in more hyperpolarized neurons. All neurons hyperpolarized after pharmacological activation of K_ATP channels. Most NTS neurons depolarized in the presence of low glucose (0.5 mM), and this effect was only seen in hyperpolarized neurons. The effect of glucose was caused by a cationic current with a reversal potential around -50 mV. In the presence of hyperglycaemic glucose (10 mM), neurons were more depolarized, and fewer neurons responded to K_ATP blockage. Application of 0.5 mM glucose solution to these neurons depolarized the membrane only in more hyperpolarized neurons. We conclude that NTS neurons present with K_ATP channels open at rest in normoglycaemic conditions, and their membrane potential is affected by extracellular glucose. Moreover, NTS neurons depolarize the membrane in response to the application of a low glucose solution, but this effect is occluded by membrane depolarization triggered by K_ATP blockage. Our data suggest a homeostatic regulation of the membrane potential by external glucose, and a possible mechanism related to the hypoglycaemia-associated autonomic failure.

A proteomics-metabolomics approach indicates changes in hypothalamic glutamate-GABA metabolism of adult female rats submitted to intrauterine growth restriction

PURPOSE

Intrauterine growth restriction (IUGR) has been shown to induce the programming of metabolic disturbances and obesity, associated with hypothalamic derangements. The present study aimed at investigating the effects of IUGR on the protein and metabolite profiles of the hypothalamus of adult female rats.

METHODS

Wistar rats were mated and either had ad libitum access to food (control group) or received only 50% of the control intake (restricted group) during the whole pregnancy. Both groups ate ad libitum throughout lactation. At 4 months of age, the control and restricted female offspring was euthanized for blood and tissues collection. The hypothalami were processed for data independent acquisition mass spectrometry-based proteomics or targeted mass spectrometry-based metabolomics.

RESULTS

The adult females submitted to IUGR showed increased glycemia and body adiposity, with normal body weight and food intake. IUGR modulated significantly 28 hypothalamic proteins and 7 hypothalamic metabolites. The effects of IUGR on hypothalamic proteins and metabolites included downregulation of glutamine synthetase, glutamate decarboxylase, glutamate dehydrogenase, isocitrate dehydrogenase, α-ketoglutarate, and up-regulation of NADH dehydrogenase and phosphoenolpyruvate. Integrated pathway analysis indicated that IUGR affected GABAergic synapse, glutamate metabolism, and TCA cycle, highly interconnected pathways whose derangement has potentially multiple consequences.

CONCLUSION

The present findings suggested that the effects of IUGR on GABA/glutamate-glutamine cycle may be involved in the programming of obesity and hyperglycemia in female rats.

Activation of µ-opioid receptors in the rostral ventrolateral medulla blocks the sympathetic counterregulatory response to glucoprivation

Activation of neurons in the rostral ventrolateral medulla (RVLM) following glucoprivation initiates sympathoadrenal activation, adrenaline release, and increased glucose production. Here, we aimed to determine the role of RVLM µ-opioid receptors in the counterregulatory response to systemic glucoprivation. Experiments were performed in pentobarbital sodium anesthetized male Sprague-Dawley rats ( n = 30). Bilateral activation of RVLM µ-opioid receptors with [d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO) (8 mM, 50 nl) depressed adrenal sympathetic nerve activity for ~60 min ( n = 6; Δ49.9 ± 5.8%, P < 0.05). The counterregulatory response to glucoprivation (measured by adrenal sympathetic efferent nerve activity) induced by 2-deoxyglucose (2-DG) ( n = 6; Δ63.6 ± 16.5%, P < 0.05) was completely blocked 60 min after DAMGO microinjections ( n = 6; Δ10.2 ± 3.5%, P < 0.05). Furthermore, DAMGO pretreatment attenuated the increase in blood glucose levels after 2-DG infusion ( n = 6; 6.1 ± 0.7mmol/l vs. baseline 5.2 ± 0.3mmol/l, P > 0.05) compared with 2-DG alone ( n = 6; 7.6 ± 0.4mmol/l vs. baseline 6.0 ± 0.4mmol/l, P < 0.05). Thus, activation of RVLM µ-opioid receptors attenuated the neural efferent response to glucoprivation and reduced glucose production.

Recurrent hypoglycemia inhibits the counterregulatory response by suppressing adrenal activity

Hypoglycemia activates the counterregulatory response (CRR), a neural-endocrine reflex that restores euglycemia. Although effective if occasionally activated, repeated induction of the CRR leads to a decline in responsiveness and prolonged exposure to hypoglycemia. The mechanism underlying this impairment is not known. We found that the reduction in epinephrine release that characterizes a suppressed CRR involves a long-lasting form of sympatho-adrenal synaptic plasticity. Using optogenetically evoked catecholamine release, we show that recurrent hypoglycemia reduced the secretory capacity of mouse adrenal chromaffin cells. Single activation of the CRR increased the adrenal levels of tyrosine hydroxylase (TH), the rate-limiting enzyme for catecholamine synthesis, but this was prevented by repeated activation. In contrast, the level of neuropeptide Y (NPY), an adrenal cotransmitter, remained elevated after recurrent hypoglycemia. Inhibition of NPY or Y1 signaling, either transgenically or pharmacologically, prevented the attenuation of both TH expression and epinephrine release. These results indicate that impairment of the CRR involves suppressed activity at the adrenal level. Interfering with the peripheral NPY-dependent negative feedback loop may provide a way to avoid the pathophysiological consequences of recurrent hypoglycemia which are common in the diabetic state.

Hypoglycemia: Role of Hypothalamic Glucose-Inhibited (GI) Neurons in Detection and Correction

Hypoglycemia is a profound threat to the brain since glucose is its primary fuel. As a result, glucose sensors are widely located in the central nervous system and periphery. In this perspective we will focus on the role of hypothalamic glucose-inhibited (GI) neurons in sensing and correcting hypoglycemia. In particular, we will discuss GI neurons in the ventromedial hypothalamus (VMH) which express neuronal nitric oxide synthase (nNOS) and in the perifornical hypothalamus (PFH) which express orexin. The ability of VMH nNOS-GI neurons to depolarize in low glucose closely parallels the hormonal response to hypoglycemia which stimulates gluconeogenesis. We have found that nitric oxide (NO) production in low glucose is dependent on oxidative status. In this perspective we will discuss the potential relevance of our work showing that enhancing the glutathione antioxidant system prevents hypoglycemia associated autonomic failure (HAAF) in non-diabetic rats whereas VMH overexpression of the thioredoxin antioxidant system restores hypoglycemia counterregulation in rats with type 1 diabetes.We will also address the potential role of the orexin-GI neurons in the arousal response needed for hypoglycemia awareness which leads to behavioral correction (e.g., food intake, glucose administration). The potential relationship between the hypothalamic sensors and the neurocircuitry in the hindbrain and portal mesenteric vein which is critical for hypoglycemia correction will then be discussed.

Thioredoxin-1 Overexpression in the Ventromedial Nucleus of the Hypothalamus Preserves the Counterregulatory Response to Hypoglycemia During Type 1 Diabetes in Male Rats

We previously showed that the glutathione precursor, N-acetylcysteine (NAC), prevented hypoglycemia-associated autonomic failure (HAAF) and impaired activation of ventromedial hypothalamus (VMH) glucose-inhibited (GI) neurons by low glucose after recurrent hypoglycemia (RH) in nondiabetic rats. However, NAC does not normalize glucose sensing by VMH GI neurons when RH occurs during diabetes. We hypothesized that recruiting the thioredoxin (Trx) antioxidant defense system would prevent HAAF and normalize glucose sensing after RH in diabetes. To test this hypothesis, we overexpressed Trx-1 (cytosolic form of Trx) in the VMH of rats with streptozotocin (STZ)-induced type 1 diabetes. The counterregulatory response (CRR) to hypoglycemia in vivo and the activation of VMH GI neurons in low glucose using membrane potential sensitive dye in vitro was measured before and after RH. VMH Trx-1 overexpression normalized both the CRR and glucose sensing by VMH GI neurons in STZ rats. VMH Trx-1 overexpression also lowered the insulin requirement to prevent severe hyperglycemia in STZ rats. However, like NAC, VMH Trx-1 overexpression did not prevent HAAF or normalize activation of VMH GI neurons by low glucose in STZ rats after RH. We conclude that preventing HAAF in type 1 diabetes may require the recruitment of both antioxidant systems.

Glucose-Sensing in the Reward System

Glucose-sensing neurons are neurons that alter their activity in response to changes in extracellular glucose. These neurons, which are an important mechanism the brain uses to monitor changes in glycaemia, are present in the hypothalamus, where they have been thoroughly investigated. Recently, glucose-sensing neurons have also been identified in brain nuclei which are part of the reward system. However, little is known about the molecular mechanisms by which they function, and their role in the reward system. We therefore aim to provide an overview of molecular mechanisms that have been studied in the hypothalamic glucose-sensing neurons, and investigate which of these transporters, enzymes and channels are present in the reward system. Furthermore, we speculate about the role of glucose-sensing neurons in the reward system.

Brain GLUT4 Knockout Mice Have Impaired Glucose Tolerance, Decreased Insulin Sensitivity, and Impaired Hypoglycemic Counterregulation

GLUT4 in muscle and adipose tissue is important in maintaining glucose homeostasis. However, the role of insulin-responsive GLUT4 in the central nervous system has not been well characterized. To assess its importance, a selective knockout of brain GLUT4 (BG4KO) was generated by crossing Nestin-Cre mice with GLUT4-floxed mice. BG4KO mice had a 99% reduction in GLUT4 protein expression throughout the brain. Despite normal feeding and fasting glycemia, BG4KO mice were glucose intolerant, demonstrated hepatic insulin resistance, and had reduced glucose uptake in the brain. In response to hypoglycemia, BG4KO mice had impaired glucose sensing, noted by impaired epinephrine and glucagon responses and impaired c-fos activation in the hypothalamic paraventricular nucleus. Moreover, in vitro glucose sensing of glucose-inhibitory neurons from the ventromedial hypothalamus was impaired in BG4KO mice. In summary, BG4KO mice are glucose intolerant, insulin resistant, and have impaired glucose sensing, indicating a critical role for brain GLUT4 in sensing and responding to changes in blood glucose.

RD Lawrence Lecture 2015 Old habits are hard to break: lessons from the study of hypoglycaemia

Despite the introduction of newer technologies and improved insulin formulations, recurrent hypoglycaemia continues to affect the lives of many people with Type 1 and Type 2 diabetes. Developing strategies or therapies designed to prevent or minimize hypoglycaemia risk is of utmost importance to help individuals safely achieve glycaemic targets. Novel, educational or behavioural approaches need to be based on a clear understanding of the mechanisms underpinning both the detection of hypoglycaemia and why repeated exposure to hypoglycaemia leads to the development of a clinical syndrome referred to as impaired awareness of hypoglycaemia. In the present review, I propose that impaired awareness of hypoglycaemia may represent a form of learning called habituation, a response that, at a cellular level, represents a biological adaptation designed to protect the organism from future exposure to that stressor. In diabetes, this survival response to low glucose is, however, overwhelmed by high systemic insulin levels resulting from exogenous insulin therapy, leading to progressively more severe hypoglycaemia. A recognition of the underlying mechanism means that the development of impaired awareness of hypoglycaemia can perhaps be better understood and explained to individuals with diabetes, and novel therapeutic approaches such as dishabituation or cognitive behavioural therapies can be considered.

Ventromedial hypothalamic glucose sensing and glucose homeostasis vary throughout the estrous cycle

OBJECTIVE

17β-Estradiol (17βE) regulates glucose homeostasis in part by centrally mediated mechanisms. In female rodents, the influence of the ovarian cycle on hypoglycemia counterregulation and glucose tolerance is unclear. We found previously that in prepubertal females, 17βE modulates glucose sensing in nonadapting glucose-inhibited (GI) and adapting GI (AdGI) neurons within the ventrolateral portion of the ventromedial nucleus (VL-VMN). Nonadapting GI neurons persistently decrease their activity as glucose increases while AdGI neurons transiently respond to a glucose increase. To begin to understand if endogenous fluctuations in estrogen levels across the estrous cycle impact hypothalamic glucose sensing and glucose homeostasis, we assessed whether hypoglycemia counterregulation and glucose tolerance differed across the phases of the estrous cycle. We hypothesized that the response to insulin-induced hypoglycemia (IIH) and/or glucose tolerance would vary throughout the estrous cycle according to changes in 17βE availability. Moreover, that these changes would correlate with estrous-dependent changes in the glucose sensitivity of VL-VMN glucose-sensing neurons (GSNs).

METHODS

These hypotheses were tested in female mice by measuring the response to IIH, glucose tolerance and the glucose sensitivity of VL-VMN GSNs during each phase of the estrous cycle. Furthermore, a physiological brain concentration of 17βE seen during proestrus was acutely applied to brain slices isolated on the day of diestrous and the response to low glucose in VL-VMN GSNs was assayed.

RESULTS

The response to IIH was strongest during diestrous. The response of nonadapting GI and AdGI neurons to a glucose decrease from 2.5 to 0.5mM also peaked during diestrous; an effect which was blunted by the addition of 17βE. In contrast, the glucose sensitivity of the subpopulation of GSNs which are excited by glucose (GE) was not affected by estrous phase or exogenous 17βE application.

CONCLUSION

These data suggest that physiological fluctuations in circulating 17βE levels across the estrous cycle lead to changes in hypothalamic glucose sensing and the response to IIH.

Chronic exposure to KATP channel openers results in attenuated glucose sensing in hypothalamic GT1-7 neurons

Individuals with Type 1 diabetes (T1D) are often exposed to recurrent episodes of hypoglycaemia. This reduces hormonal and behavioural responses that normally counteract low glucose in order to maintain glucose homeostasis, with altered responsiveness of glucose sensing hypothalamic neurons implicated. Although the molecular mechanisms are unknown, pharmacological studies implicate hypothalamic ATP-sensitive potassium channel (K_ATP) activity, with K_ATP openers (KCOs) amplifying, through cell hyperpolarization, the response to hypoglycaemia. Although initial findings, using acute hypothalamic KCO delivery, in rats were promising, chronic exposure to the KCO NN414 worsened the responses to subsequent hypoglycaemic challenge. To investigate this further we used GT1-7 cells to explore how NN414 affected glucose-sensing behaviour, the metabolic response of cells to hypoglycaemia and K_ATP activity. GT1-7 cells exposed to 3 or 24 h NN414 exhibited an attenuated hyperpolarization to subsequent hypoglycaemic challenge or NN414, which correlated with diminished K_ATP activity. The reduced sensitivity to hypoglycaemia was apparent 24 h after NN414 removal, even though intrinsic K_ATP activity recovered. The NN414-modified glucose responsiveness was not associated with adaptations in glucose uptake, metabolism or oxidation. K_ATP inactivation by NN414 was prevented by the concurrent presence of tolbutamide, which maintains K_ATP closure. Single channel recordings indicate that NN414 alters K_ATP intrinsic gating inducing a stable closed or inactivated state. These data indicate that exposure of hypothalamic glucose sensing cells to chronic NN414 drives a sustained conformational change to K_ATP, probably by binding to SUR1, that results in loss of channel sensitivity to intrinsic metabolic factors such as MgADP and small molecule agonists.

Estrogens modulate ventrolateral ventromedial hypothalamic glucose-inhibited neurons

Objective

Brain regulation of glucose homeostasis is sexually dimorphic; however, the impact sex hormones have on specific neuronal populations within the ventromedial hypothalamic nucleus (VMN), a metabolically sensitive brain region, has yet to be fully characterized. Glucose-excited (GE) and -inhibited (GI) neurons are located throughout the VMN and may play a critical role in glucose and energy homeostasis. Within the ventrolateral portion of the VMN (VL-VMN), glucose sensing neurons and estrogen receptor (ER) distributions overlap. We therefore tested the hypothesis that VL-VMN glucose sensing neurons were sexually dimorphic and regulated by 17β-estradiol (17βE).

Methods

Electrophysiological recordings of VL-VMN glucose sensing neurons in brain slices isolated from age- and weight-matched female and male mice were performed in the presence and absence of 17βE.

Results

We found a new class of VL-VMN GI neurons whose response to low glucose was transient despite continued exposure to low glucose. Heretofore, we refer to these newly identified VL-VMN GI neurons as ‘adapting’ or AdGI neurons. We found a sexual dimorphic response to low glucose, with male nonadapting GI neurons, but not AdGI neurons, responding more robustly to low glucose than those from females. 17βE blunted the response of both nonadapting GI and AdGI neurons to low glucose in both males and females, which was mediated by activation of estrogen receptor β and inhibition of AMP-activated kinase. In contrast, 17βE had no impact on GE or non-glucose sensing neurons in either sex.

Conclusion

These data suggest sex differences and estrogenic regulation of VMN hypothalamic glucose sensing may contribute to the sexual dimorphism in glucose homeostasis.

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Hypothalamic glucose sensitivity is sexually dimorphic.

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17βE blunts activation of glucose inhibited neurons in low glucose.

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Estrogen regulation of glucose sensing may mediate sexual dimorphisms in glucose homeostasis.

Leptin acts in the brain to influence hypoglycemic counterregulation: disparate effects of acute and recurrent hypoglycemia on glucagon release

Leptin has been shown to diminish hyperglycemia via reduced glucagon secretion, although it can also enhance sympathoadrenal responses. However, whether leptin can also inhibit glucagon secretion during insulin-induced hypoglycemia or increase epinephrine during acute or recurrent hypoglycemia has not been examined. To test whether leptin acts in the brain to influence counterregulation, hyperinsulinemic hypoglycemic (∼45 mg/dl) clamps were performed on rats exposed to or not exposed to recurrent hypoglycemia (3 days, ∼40 mg/dl). Intracerebroventricular artificial cerebral spinal fluid or leptin was infused during the clamp. During acute hypoglycemia, leptin decreased glucagon responses by 51% but increased epinephrine and norepinephrine by 24 and 48%, respectively. After recurrent hypoglycemia, basal plasma leptin levels were undetectable. Subsequent brain leptin infusion during hypoglycemia paradoxically increased glucagon by 45% as well as epinephrine by 19%. In conclusion, leptin acts within the brain to diminish glucagon secretion during acute hypoglycemia but increases epinephrine, potentially limiting its detrimental effects during hypoglycemia. Exposure to recurrent hypoglycemia markedly suppresses plasma leptin, whereas exogenous brain leptin delivery enhances both glucagon and epinephrine release to subsequent hypoglycemia. These data suggest that recurrent hypoglycemia may diminish counterregulatory responses in part by reducing brain leptin action.

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.

Peripheral and central glucose sensing in hypoglycemic detection

Hypoglycemia poses a serious threat to the integrity of the brain, owing to its reliance on blood glucose as a fuel. Protecting against hypoglycemia is an extended network of glucose sensors located within the brain and in the periphery that serve to mediate responses restoring euglycemia, i.e., counterregulatory responses. This review examines the various glucose sensory loci involved in hypoglycemic detection, with a particular emphasis on peripheral glucose sensory loci and their contribution to hypoglycemic counterregulation.

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.

Hypothalamic glucose sensing: making ends meet

The neuroendocrine system governs essential survival and homeostatic functions. For example, growth is needed for development, thermoregulation maintains optimal core temperature in a changing environment, and reproduction ensures species survival. Stress and immune responses enable an organism to overcome external and internal threats while the circadian system regulates arousal and sleep such that vegetative and active functions do not overlap. All of these functions require a significant portion of the body's energy. As the integrator of the neuroendocrine system, the hypothalamus carefully assesses the energy status of the body in order to appropriately partition resources to provide for each system without compromising the others. While doing so the hypothalamus must ensure that adequate glucose levels are preserved for brain function since glucose is the primary fuel of the brain. To this end, the hypothalamus contains specialized glucose sensing neurons which are scattered throughout the nuclei controlling distinct neuroendocrine functions. We hypothesize that these neurons play a key role in enabling the hypothalamus to partition energy to meet these peripheral survival needs without endangering the brain's glucose supply. This review will first describe the varied mechanisms underlying glucose sensing in neurons within discrete hypothalamic nuclei. We will then evaluate the way in which peripheral energy status regulates glucose sensitivity. For example, during energy deficit such as fasting specific hypothalamic glucose sensing neurons become sensitized to decreased glucose. This increases the gain of the information relay when glucose availability is a greater concern for the brain. Finally, changes in glucose sensitivity under pathological conditions (e.g., recurrent insulin-hypoglycemia, diabetes) will be addressed. The overall goal of this review is to place glucose sensing neurons within the context of hypothalamic control of neuroendocrine function.

Influence of VMH fuel sensing on hypoglycemic responses

Hypoglycemia produces complex neural and hormonal responses that restore glucose levels to normal. Glucose, metabolic substrates and their transporters, neuropeptides and neurotransmitters alter the firing rate of glucose-sensing neurons in the ventromedial hypothalamus (VMH); these monitor energy status and regulate the release of neurotransmitters that instigate a suitable counter-regulatory response. Under normal physiological conditions, these mechanisms maintain blood glucose concentrations within narrow margins. However, antecedent hypoglycemia and diabetes can lead to adaptations within the brain that impair counter-regulatory responses. Clearly, the mechanisms employed to detect and regulate the response to hypoglycemia, and the pathophysiology of defective counter-regulation in diabetes, are complex and need to be elucidated to permit the development of therapies that prevent or reduce the risk of hypoglycemia.

Continuous hypothalamic K(ATP) activation blunts glucose counter-regulation in vivo in rats and suppresses K(ATP) conductance in vitro

AIMS/HYPOTHESIS

Acute systemic delivery of the sulfonylurea receptor (SUR)-1-specific ATP-sensitive K(+) channel (K(ATP)) opener, NN414, has been reported to amplify glucose counter-regulatory responses (CRRs) in rats exposed to hypoglycaemia. Thus, we determined whether continuous NN414 could prevent hypoglycaemia-induced defective counter-regulation.

METHODS

Chronically catheterised male Sprague-Dawley rats received a continuous infusion of NN414 into the third ventricle for 8 days after implantation of osmotic minipumps. Counter-regulation was examined by hyperinsulinaemic-hypoglycaemic clamp on day 8 after three episodes of insulin-induced hypoglycaemia (recurrent hypoglycaemia [RH]) on days 5, 6 and 7. In a subset of rats exposed to RH, NN414 infusion was terminated on day 7 to wash out NN414 before examination of counter-regulation on day 8. To determine whether continuous NN414 exposure altered K(ATP) function, we used the hypothalamic glucose-sensing GT1-7 cell line, which expresses the SUR-1-containing K(ATP) channel.

RESULTS

Continuous exposure to NN414 in the setting of RH increased, rather than decreased, the glucose infusion rate (GIR), as exemplified by attenuated adrenaline (epinephrine) secretion. Termination of NN414 on day 7 with subsequent washout for 24 h partially diminished the GIR. The same duration of exposure of GT1-7 cells to NN414 substantially reduced K(ATP) conductance, which was also reversed on washout of the agonist. The suppression of K(ATP) current was not associated with reduced channel subunit mRNA or protein levels.

CONCLUSIONS/INTERPRETATION

These data indicate that continuous K(ATP) activation results in suppressed CRRs to hypoglycaemia in vivo, which in vitro is associated with the reversible conversion of KATP into a stable inactive state.

Hypothalamic S-nitrosylation contributes to the counter-regulatory response impairment following recurrent hypoglycemia

AIMS

Hypoglycemia is a severe side effect of intensive insulin therapy. Recurrent hypoglycemia (RH) impairs the counter-regulatory response (CRR) which restores euglycemia. During hypoglycemia, ventromedial hypothalamus (VMH) production of nitric oxide (NO) and activation of its receptor soluble guanylyl cyclase (sGC) are critical for the CRR. Hypoglycemia also increases brain reactive oxygen species (ROS) production. NO production in the presence of ROS causes protein S-nitrosylation. S-nitrosylation of sGC impairs its function and induces desensitization to NO. We hypothesized that during hypoglycemia, the interaction between NO and ROS increases VMH sGC S-nitrosylation levels and impairs the CRR to subsequent episodes of hypoglycemia. VMH ROS production and S-nitrosylation were quantified following three consecutive daily episodes of insulin-hypoglycemia (RH model). The CRR was evaluated in rats in response to acute insulin-induced hypoglycemia or via hypoglycemic-hyperinsulinemic clamps. Pretreatment with the anti-oxidant N-acetyl-cysteine (NAC) was used to prevent increased VMH S-nitrosylation.

RESULTS

Acute insulin-hypoglycemia increased VMH ROS levels by 49±6.3%. RH increased VMH sGC S-nitrosylation. Increasing VMH S-nitrosylation with intracerebroventricular injection of the nitrosylating agent S-nitroso-L-cysteine (CSNO) was associated with decreased glucagon secretion during hypoglycemic clamp. Finally, in RH rats pre-treated with NAC (0.5% in drinking water for 9 days) hypoglycemia-induced VMH ROS production was prevented and glucagon and epinephrine production was not blunted in response to subsequent insulin-hypoglycemia.

CONCLUSION

These data suggest that NAC may be clinically useful in preventing impaired CRR in patients undergoing intensive-insulin therapy.

Revisiting the Ventral Medial Nucleus of the Hypothalamus: The Roles of SF-1 Neurons in Energy Homeostasis

Obesity, diabetes, and other metabolic complications are growing concerns for public health and could lead to detrimental life-threatening conditions. Neurons whose activities are required for energy and glucose homeostasis are found in a number of hypothalamic nuclei. In the early twentieth century, the ventral medial nucleus of the hypothalamus (VMH) was the first site reported to play a prominent role in the regulation of energy homeostasis through control of food intake and energy expenditure. Recent studies using sophisticated genetic tools have further highlighted the importance of the VMH and have extended our understanding of the physiological role of the nucleus in regulation of energy homeostasis. These genetic studies were preceded by the identification of steroidogenic factor-1 (SF-1) as a marker of the VMH. This review focuses on the emerging homeostatic roles of the SF-1 neurons in the VMH discovered through the use of genetic models, particularly highlighting the control of energy, and glucose homeostasis.

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

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

Glutamate dehydrogenase 1 and SIRT4 regulate glial development

Congenital hyperinsulinism/hyperammonemia (HI/HA) syndrome is caused by an activation mutation of glutamate dehydrogenase 1 (GDH1), a mitochondrial enzyme responsible for the reversible interconversion between glutamate and α-ketoglutarate. The syndrome presents clinically with hyperammonemia, significant episodic hypoglycemia, seizures, and frequent incidences of developmental and learning defects. Clinical research has implicated that although some of the developmental and neurological defects may be attributed to hypoglycemia, some characteristics cannot be ascribed to low glucose and as hyperammonemia is generally mild and asymptomatic, there exists the possibility that altered GDH1 activity within the brain leads to some clinical changes. GDH1 is allosterically regulated by many factors, and has been shown to be inhibited by the ADP-ribosyltransferase sirtuin 4 (SIRT4), a mitochondrially localized sirtuin. Here we show that SIRT4 is localized to mitochondria within the brain. SIRT4 is highly expressed in glial cells, specifically astrocytes, in the postnatal brain and in radial glia during embryogenesis. Furthermore, SIRT4 protein decreases in expression during development. We show that factors known to allosterically regulate GDH1 alter gliogenesis in CTX8 cells, a novel radial glial cell line. We find that SIRT4 and GDH1 overexpression play antagonistic roles in regulating gliogenesis and that a mutant variant of GDH1 found in HI/HA patients accelerates the development of glia from cultured radial glia cells.

Glucose sensing neurons in the ventromedial hypothalamus

Neurons whose activity is regulated by glucose are found in a number of brain regions. Glucose-excited (GE) neurons increase while glucose-inhibited (GI) neurons decrease their action potential frequency as interstitial brain glucose levels increase. We hypothesize that these neurons evolved to sense and respond to severe energy deficit (e.g., fasting) that threatens the brains glucose supply. During modern times, they are also important for the restoration of blood glucose levels following insulin-induced hypoglycemia. Our data suggest that impaired glucose sensing by hypothalamic glucose sensing neurons may contribute to the syndrome known as hypoglycemia-associated autonomic failure in which the mechanisms which restore euglycemia following hypoglycemia become impaired. On the other hand, increased responses of glucose sensing neurons to glucose deficit may play a role in the development of Type 2 Diabetes Mellitus and obesity. This review will discuss the mechanisms by which glucose sensing neurons sense changes in interstitial glucose and explore the roles of these specialized glucose sensors in glucose and energy homeostasis.

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.

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.

Repeated glucoprivation delayed hyperphagic responses while activating neuropeptide Y neurons in rats

It is well known that glucoprivation induces the release of counterregulatory hormones such as glucagon, and that the response is attenuated when the stimuli are repeated. Glucoprivation also activates orexigenic neurons and induces hyperphagic responses, although it remains unclear whether these responses are attenuated in repeated glucoprivation. In this study, we examined time course changes in feeding as well as activities of orexigenic neuropeptide Y (NPY) neurons in repeated glucoprivation in rats. Either 2-deoxy-d-glucose (2DG), which blocks glucose utilization, or isotonic saline (control) was injected subcutaneously to rats for 14 days, and food consumption for 1 and 2h after injection was monitored throughout the experiment. While 2DG injection induced robust feeding responses during the first 1h after injection, the response was gradually attenuated and the food consumption was significantly less on days 12-14 compared to that on day 1. On the other hand, food consumption during 2h after 2DG injection was not changed significantly for 14 days. The transcriptional activities of NPY neurons in the arcuate nucleus and C1/A1 region of the hindbrain, measured by intronic in situ hybridization, were significantly enhanced after repeated 2DG injection for 14 days, while the feeding responses to intracerebroventricular injection of NPY were significantly less in the 2DG-repeated group compared to the saline-repeated group. It is thus demonstrated that repeated glucoprivation delayed hyperphagic responses while activating NPY neurons in rats. Our data also suggest that decreased feeding responses to NPY might be at least partially responsible for the delayed response.

Hypothalamic nitric oxide in hypoglycemia detection and counterregulation: a two-edged sword

Hypoglycemia is the main complication for patients with type 1 diabetes mellitus receiving intensive insulin therapy. In addition to the obvious deleterious effects of acute hypoglycemia on brain function, recurrent episodes of hypoglycemia (RH) have an even more insidious effect. RH impairs the ability of the brain to detect and initiate an appropriate counterregulatory response (CRR) to restore euglycemia in response to subsequent hypoglycemia. Knowledge of mechanisms involved in hypoglycemia detection and counterregulation has significantly improved over the past 20 years. Glucose sensitive neurons (GSNs) in the ventromedial hypothalamus (VMH) may play a key role in the CRR. VMH nitric oxide (NO) production has recently been shown to be critical for both the CRR and glucose sensing by glucose-inhibited neurons. Interestingly, downstream effects of NO may also contribute to the impaired CRR after RH. In this review, we will discuss current literature regarding the molecular mechanisms by which VMH GSNs sense glucose. Putative roles of GSNs in the detection and initiation of the CRR will then be described. Finally, hypothetical mechanisms by which VMH NO production may both facilitate and subsequently impair the CRR will be discussed.

Multiple hypothalamic circuits sense and regulate glucose levels

The hypothalamus monitors body energy status in part through specialized glucose sensing neurons that comprise both glucose-excited and glucose-inhibited cells. Here we discuss recent work on the elucidation of neurochemical identities and physiological significance of these hypothalamic cells, including caveats resulting from the currently imprecise functional and molecular definitions of glucose sensing and differences in glucose-sensing responses obtained with different experimental techniques. We discuss the recently observed adaptive glucose-sensing responses of orexin/hypocretin-containing neurons, which allow these cells to sense changes in glucose levels rather than its absolute concentration, as well as the glucose-sensing abilities of melanin-concentrating hormone, neuropeptide Y, and proopiomelanocortin-containing neurons and the recent data on the role of ventromedial hypothalamic steroidogenic factor-1 (SF-1)/glutamate-containing cells in glucose homeostasis. We propose a model where orexin/hypocretin and SF-1/glutamate neurons cooperate in stimulating the sympathetic outflow to the liver and pancreas to increase blood glucose, which in turn provides negative feedback inhibition to these cells. Orexin/hypocretin neurons also stimulate feeding and reward seeking and are activated by hunger and stress, thereby providing a potential link between glucose sensing and goal-oriented behavior. The cell-type-specific neuromodulatory actions of glucose in several neurochemically distinct hypothalamic circuits are thus likely to be involved in coordinating higher brain function and behavior with autonomic adjustments in blood glucose levels.

Brain insulin action regulates hypothalamic glucose sensing and the counterregulatory response to hypoglycemia

OBJECTIVE

An impaired ability to sense and appropriately respond to insulin-induced hypoglycemia is a common and serious complication faced by insulin-treated diabetic patients. This study tests the hypothesis that insulin acts directly in the brain to regulate critical glucose-sensing neurons in the hypothalamus to mediate the counterregulatory response to hypoglycemia.

RESEARCH DESIGN AND METHODS

To delineate insulin actions in the brain, neuron-specific insulin receptor knockout (NIRKO) mice and littermate controls were subjected to graded hypoglycemic (100, 70, 50, and 30 mg/dl) hyperinsulinemic (20 mU/kg/min) clamps and nonhypoglycemic stressors (e.g., restraint, heat). Subsequently, counterregulatory responses, hypothalamic neuronal activation (with transcriptional marker c-fos), and regional brain glucose uptake (via (14)C-2deoxyglucose autoradiography) were measured. Additionally, electrophysiological activity of individual glucose-inhibited neurons and hypothalamic glucose sensing protein expression (GLUTs, glucokinase) were measured.

RESULTS

NIRKO mice revealed a glycemia-dependent impairment in the sympathoadrenal response to hypoglycemia and demonstrated markedly reduced (3-fold) hypothalamic c-fos activation in response to hypoglycemia but not other stressors. Glucose-inhibited neurons in the ventromedial hypothalamus of NIRKO mice displayed significantly blunted glucose responsiveness (membrane potential and input resistance responses were blunted 66 and 80%, respectively). Further, hypothalamic expression of the insulin-responsive GLUT 4, but not glucokinase, was reduced by 30% in NIRKO mice while regional brain glucose uptake remained unaltered.

CONCLUSIONS

Chronically, insulin acts in the brain to regulate the counterregulatory response to hypoglycemia by directly altering glucose sensing in hypothalamic neurons and shifting the glycemic levels necessary to elicit a normal sympathoadrenal response.

The response of glucose-excited neurones in the ventromedial hypothalamus to decreased glucose is enhanced in a murine model of type 2 diabetes mellitus

Obesity and type 2 diabetes mellitus (T2DM) are associated with dysfunctional insulin signalling and impaired central glucose sensing. Glucose sensing neurones reside in key areas of the brain involved in glucose and energy homeostasis (e.g. ventromedial hypothalamus; VMH). We have recently shown that insulin attenuates the ability of glucose-excited (GE) neurones to sense decreased glucose. We hypothesise that this effect of insulin on VMH GE neurones is impaired during T2DM when insulin signalling is dysfunctional. To test our hypotheses, we used whole cell patch clamp recording techniques to evaluate the effects of insulin on VMH GE neurones in brain slices from wild-type and diabetic (db/db) mice. The effects of decreasing glucose from 2.5 to 0.1 mM on VMH GE neurones were similar in wild-type and db/db mice. However, decreasing glucose from 2.5 to 0.5 mM decreased the action potential frequency, membrane potential and input resistance of VMH GE neurones to a significantly greater extent in db/db versus wild-type mice. Furthermore, insulin (5 nM) blunted the effects of decreased glucose in wild-type, but not db/db mice. These differences in both glucose and insulin sensitivity between wild-type and db/db mice were completely ameliorated by the insulin sensitiser, Compound 2 (300 nM). These data are consistent with our hypothesis that impaired insulin signalling in T2DM sensitises VMH GE neurones to decreased glucose.

Ventromedial hypothalamic nitric oxide production is necessary for hypoglycemia detection and counterregulation

OBJECTIVE

The response of ventromedial hypothalamic (VMH) glucose-inhibited neurons to decreased glucose is impaired under conditions where the counterregulatory response (CRR) to hypoglycemia is impaired (e.g., recurrent hypoglycemia). This suggests a role for glucose-inhibited neurons in the CRR. We recently showed that decreased glucose increases nitric oxide (NO) production in cultured VMH glucose-inhibited neurons. These in vitro data led us to hypothesize that NO release from VMH glucose-inhibited neurons is critical for the CRR.

RESEARCH DESIGN AND METHODS

The CRR was evaluated in rats and mice in response to acute insulin-induced hypoglycemia and hypoglycemic clamps after modulation of brain NO signaling. The glucose sensitivity of ventromedial nucleus glucose-inhibited neurons was also assessed.

RESULTS

Hypoglycemia increased hypothalamic constitutive NO synthase (NOS) activity and neuronal NOS (nNOS) but not endothelial NOS (eNOS) phosphorylation in rats. Intracerebroventricular and VMH injection of the nonselective NOS inhibitor N(G)-monomethyl-l-arginine (l-NMMA) slowed the recovery to euglycemia after hypoglycemia. VMH l-NMMA injection also increased the glucose infusion rate (GIR) and decreased epinephrine secretion during hyperinsulinemic/hypoglycemic clamp in rats. The GIR required to maintain the hypoglycemic plateau was higher in nNOS knockout than wild-type or eNOS knockout mice. Finally, VMH glucose-inhibited neurons were virtually absent in nNOS knockout mice.

CONCLUSIONS

We conclude that VMH NO production is necessary for glucose sensing in glucose-inhibited neurons and full generation of the CRR to hypoglycemia. These data suggest that potentiating NO signaling may improve the defective CRR resulting from recurrent hypoglycemia in patients using intensive insulin therapy.

AMP-activated protein kinase and nitric oxide regulate the glucose sensitivity of ventromedial hypothalamic glucose-inhibited neurons

The mechanisms by which glucose regulates the activity of glucose-inhibited (GI) neurons in the ventromedial hypothalamus (VMH) are largely unknown. We have previously shown that AMP-activated protein kinase (AMPK) increases nitric oxide (NO) production in VMH GI neurons. We hypothesized that AMPK-mediated NO signaling is required for depolarization of VMH GI neurons in response to decreased glucose. In support of our hypothesis, inhibition of neuronal nitric oxide synthase (nNOS) or the NO receptor soluble guanylyl cyclase (sGC) blocked depolarization of GI neurons to decreased glucose from 2.5 to 0.7 mM or to AMPK activation. Conversely, activation of sGC or the cell-permeable analog of cGMP, 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP), enhanced the response of GI neurons to decreased glucose, suggesting that stimulation of NO-sGC-cGMP signaling by AMPK is required for glucose sensing in GI neurons. Interestingly, the AMPK inhibitor compound C completely blocked the effect of sGC activation or 8-Br-cGMP, and 8-Br-cGMP increased VMH AMPKalpha2 phosphorylation. These data suggest that NO, in turn, amplifies AMPK activation in GI neurons. Finally, inhibition of the cystic fibrosis transmembrane regulator (CFTR) Cl(-) conductance blocked depolarization of GI neurons to decreased glucose or AMPK activation, whereas decreased glucose, AMPK activation, and 8-Br-cGMP increased VMH CFTR phosphorylation. We conclude that decreased glucose triggers the following sequence of events leading to depolarization in VMH GI neurons: AMPK activation, nNOS phosphorylation, NO production, and stimulation of sGC-cGMP signaling, which amplifies AMPK activation and leads to closure of the CFTR.

The rate-limiting step for glucose transport into the hypothalamus is across the blood-hypothalamus interface

Specialized glucosensing neurons are present in the hypothalamus, some of which neighbor the median eminence, where the blood-brain barrier has been reported leaky. A leaky blood-brain barrier implies high tissue glucose levels and obviates a role for endothelial glucose transporters in the control of hypothalamic glucose concentration, important in understanding the mechanisms of glucose sensing We therefore addressed the question of blood-brain barrier integrity at the hypothalamus for glucose transport by examining the brain tissue-to-plasma glucose ratio in the hypothalamus relative to other brain regions. We also examined glycogenolysis in hypothalamus because its occurrence is unlikely in the potential absence of a hypothalamus-blood interface. Across all regions the concentration of glucose was comparable at a given plasma glucose concentration and was a near linear function of plasma glucose. At steady-state, hypothalamic glucose concentration was similar to the extracellular hypothalamic glucose concentration reported by others. Hypothalamic glycogen fell at a rate of approximately 1.5 micromol/g/h and remained present in substantial amounts. We conclude for the hypothalamus, a putative primary site of brain glucose sensing that: the rate-limiting step for glucose transport into brain cells is at the blood-hypothalamus interface, and that glycogenolysis is consistent with a substantial blood -to- intracellular glucose concentration gradient.

Mechanisms of the blunting of the sympatho-adrenal response: a theory

Development of therapeutic measures to reduce the risk of potentially fatal episodes of hypoglycaemia and thus to achieve the full benefits of intensive insulin therapy in diabetic patients requires a complete understanding of the multi-factorial mechanisms for repeated hypoglycaemia-induced blunting of the sympatho-adrenal response (BSAR). After critical analysis of the hypotheses, this review paper suggests a heuristic theory. This theory suggests two mechanisms for the BSAR, each involving a critical role for the central brain noradrenergic system. Furthermore, this theory also suggests that the lateral hypothalamus (LH) plays an important role in this phenomenon. Within the framework of this theory, explanations for 1) sexual dimorphism in the adrenomedullary response (AR), 2) dissociation in the blunting of the AR and the sympathetic response (SR) and 3) antecedent exercise-induced blunting of the AR are provided. In addition, habituation of orexin-A neurons is suggested to cause defective awakening. Moreover, potential therapeutics measures have been also suggested that will reduce or prevent severe episodes of hypoglycaemia.