Physiological and molecular characteristics of rat hypothalamic ventromedial nucleus glucosensing neurons

Inhibition by glucose or leptin of hypothalamic neurons expressing neuropeptide Y requires changes in AMP-activated protein kinase activity

AIMS/HYPOTHESIS

Changes in the activity of glucose-excited and glucose-inhibited neurons within the basomedial hypothalamus are key to the central regulation of satiety. However, the molecular mechanisms through which these cells respond to extracellular stimuli remain poorly understood. Here, we investigate the role of 5'-AMP-activated protein kinase (AMPK), a trimeric complex encoded by seven distinct genes of the PRKA family, in the responses to glucose and leptin of each cell type.

METHODS

The activity of isolated rat basomedial hypothalamic neurons was assessed by: (1) recording cellular voltage responses under current clamp; (2) measuring intracellular free Ca(2+) with fluo-3 or fura-2; and (3) developing a neuropeptide Y (NPY) promoter-driven adenovirally produced ratiometric 'pericam' (a green fluorescent protein-based Ca(2+) sensor) to monitor [Ca(2+)] changes selectively in NPY-positive neurons.

RESULTS

The stimulatory effects of decreased (0 or 1.0 vs 15 mmol/l) glucose on glucose-inhibited neurons were mimicked by the AMPK activator, 5-amino-imidazole-4-carboxamide riboside (AICAR) and blocked by the inhibitor Compound C. Similarly, AICAR reversed the inhibitory effects of leptin in the majority of glucose-inhibited neurons. The responses to glucose of Npy-expressing cells, which represented approximately 40 % of all glucose-inhibited neurons, were also sensitive to Compound C or AICAR. Forced changes in AMPK activity had no effect on glucose-excited and non-glucose-responsive neurons.

CONCLUSIONS/INTERPRETATION

Changes in AMPK activity are involved in the responses of glucose-inhibited neurons to large fluctuations in glucose concentration, and possibly also to leptin. This mechanism may contribute to the acute reduction of electrical activity and Ca(2+) oscillation frequency in these, but not other neurons, in the basomedial hypothalamus.

Glucose, insulin, and leptin signaling pathways modulate nitric oxide synthesis in glucose-inhibited neurons in the ventromedial hypothalamus

Glucose-sensing neurons in the ventromedial hypothalamus (VMH) are involved in the regulation of glucose homeostasis. Glucose-sensing neurons alter their action potential frequency in response to physiological changes in extracellular glucose, insulin, and leptin. Glucose-excited neurons decrease, whereas glucose-inhibited (GI) neurons increase, their action potential frequency when extracellular glucose is reduced. Central nitric oxide (NO) synthesis is regulated by changes in local fuel availability, as well as insulin and leptin. NO is involved in the regulation of food intake and is altered in obesity and diabetes. Thus this study tests the hypothesis that NO synthesis is a site of convergence for glucose, leptin, and insulin signaling in VMH glucose-sensing neurons. With the use of the NO-sensitive dye 4-amino-5-methylamino-2',7'-difluorofluorescein in conjunction with the membrane potential-sensitive dye fluorometric imaging plate reader, we found that glucose and leptin suppress, whereas insulin stimulates neuronal nitric oxide synthase (nNOS)-dependent NO production in cultured VMH GI neurons. The effects of glucose and leptin were mediated by suppression of AMP-activated protein kinase (AMPK). The AMPK activator 5-aminoimidazole-4-carboxamide-1-beta-4-ribofuranoside (AICAR) increased both NO production and neuronal activity in GI neurons. In contrast, the effects of insulin on NO production were blocked by the phosphoinositide 3-kinase inhibitors wortmannin and LY-294002. Furthermore, decreased glucose, insulin, and AICAR increase the phosphorylation of VMH nNOS, whereas leptin decreases it. Finally, VMH neurons express soluble guanylyl cyclase, a downstream mediator of NO signaling. Thus NO may mediate, in part, glucose, leptin, and insulin signaling in VMH glucose-sensing neurons.

Glucose sensing by hypothalamic neurones and pancreatic islet cells: AMPle evidence for common mechanisms?

A fuller understanding of the central mechanisms involved in controlling food intake and metabolism is likely to be crucial for developing treatments to combat the growing problem of obesity in Westernised societies. Within the hypothalamus, specialized neurones respond to both appetite-regulating hormones and circulating metabolites to regulate feeding behaviour accordingly. Thus, the activity of hypothalamic glucose-excited and glucose-inhibited neurones is increased or decreased, respectively, by an increase in local glucose concentration. These 'glucose-sensing' neurones may therefore play a key role in the central regulation of food intake and potentially in the regulation of blood glucose concentrations. Whilst the intracellular signalling mechanisms through which glucose-sensing neurones detect changes in the concentration of the sugar have been investigated quite extensively, many elements remain poorly understood. Furthermore, the similarities, or otherwise, with other nutrient-sensing cells, including pancreatic islet cells, are not completely resolved. In this review, we discuss recent advances in this field and explore the potential involvement of AMP-activated protein kinase and other nutrient-regulated protein kinases.

Sodium-coupled glucose cotransporters contribute to hypothalamic glucose sensing

Specialized neurons within the hypothalamus have the ability to sense and respond to changes in ambient glucose concentrations. We investigated the mechanisms underlying glucose-triggered activity in glucose-excited neurons, using primary cultures of rat hypothalamic neurons monitored by fluorescence calcium imaging. We found that 35% (738 of 2,139) of the neurons were excited by increasing glucose from 3 to 15 mmol/l, but only 9% (6 of 64) of these glucose-excited neurons were activated by tolbutamide, suggesting the involvement of a ATP-sensitive K(+) channel-independent mechanism. alpha-Methylglucopyranoside (alphaMDG; 12 mmol/l), a nonmetabolizable substrate of sodium glucose cotransporters (SGLTs), mimicked the effect of high glucose in 67% of glucose-excited neurons, and both glucose- and alphaMDG-triggered excitation were blocked by Na(+) removal or by the SGLT inhibitor phloridzin (100 nmol/l). In the presence of 0.5 mmol/l glucose and tolbutamide, responses could also be triggered by 3.5 mmol/l alphaMDG, supporting a role for an SGLT-associated mechanism at low as well as high substrate concentrations. Using RT-PCR, we detected SGLT1, SGLT3a, and SGLT3b in both cultured neurons and adult rat hypothalamus. Our findings suggest a novel role for SGLTs in glucose sensing by hypothalamic glucose-excited neurons.

Metabolic sensing neurons and the control of energy homeostasis

The brain and periphery carry on a constant conversation; the periphery informs the brain about its metabolic needs and the brain provides for these needs through its control of somatomotor, autonomic and neurohumoral pathways involved in energy intake, expenditure and storage. Metabolic sensing neurons are the integrators of a variety of metabolic, humoral and neural inputs from the periphery. Such neurons, originally called "glucosensing", also respond to fatty acids, hormones and metabolites from the periphery. They are integrated within neural pathways involved in the regulation of energy homeostasis. Unlike most neurons, they utilize glucose and other metabolites as signaling molecules to regulate their membrane potential and firing rate. For glucosensing neurons, glucokinase acts as the rate-limiting step in glucosensing while the pathways that mediate responses to metabolites like lactate, ketone bodies and fatty acids are less well characterized. Many metabolic sensing neurons also respond to insulin and leptin and other peripheral hormones and receive neural inputs from peripheral organs. Each set of afferent signals arrives with different temporal profiles and by different routes and these inputs are summated at the level of the membrane potential to produce a given neural firing pattern. In some obese individuals, the relative sensitivity of metabolic sensing neurons to various peripheral inputs is genetically reduced. This may provide one mechanism underlying their propensity to become obese when exposed to diets high in fat and caloric density. Thus, metabolic sensing neurons may provide a potential therapeutic target for the treatment of obesity.

Central insulin action in energy and glucose homeostasis

Insulin has pleiotropic biological effects in virtually all tissues. However, the relevance of insulin signaling in peripheral tissues has been studied far more extensively than its role in the brain. An evolving body of evidence indicates that in the brain, insulin is involved in multiple regulatory mechanisms including neuronal survival, learning, and memory, as well as in regulation of energy homeostasis and reproductive endocrinology. Here we review insulin's role as a central homeostatic signal with regard to energy and glucose homeostasis and discuss the mechanisms by which insulin communicates information about the body's energy status to the brain. Particular emphasis is placed on the controversial current debate about the similarities and differences between hypothalamic insulin and leptin signaling at the molecular level.

Changes in extracellular hypothalamic glucose in relation to feeding

The aim of the present in vivo microdialysis study was to investigate the relation between feeding and changes in glucose concentrations in the rat ventromedial hypothalamus (VMH). Absolute ambient glucose concentrations in VMH were 1.43 mm in non-deprived rats as compared to 0.94 mm after 24-h food deprivation. To examine whether feeding influences hypothalamic glucose, changes of glucose concentration over time were determined relative to a baseline. Experiments were conducted in relation to both, nutritional state (food-deprived rats vs. non-deprived rats) and feeding conditions throughout the experiment (freely feeding rats vs. rats without access to food). The results of this microdialysis study show clearly that glucose concentration in the VMH of rats increases significantly in relation to food intake. The data demonstrate that a 24-h food deprivation before the experiment further augments this increase (up to 350% from baseline) as compared to non-deprived conditions (up to 60% from baseline). However, the magnitude of food related increase in VMH glucose does not correlate with the individual amount of food eaten. In conclusion, the present study shows for the first time that VMH glucose concentrations increase with food intake in the early dark phase, indicating that such changes do not only occur after pharmacological treatment, but also under physiological feeding conditions. The results further indicate that the feeding related increase in VMH glucose depends on the nutritional state of the organism.

Hindbrain catecholamine neurons control multiple glucoregulatory responses

Reduced brain glucose availability evokes an integrated constellation of responses that protect and restore the brain's glucose supply. These include increased food intake, adrenal medullary secretion, corticosterone secretion and suppression of estrous cycles. Our research has focused on mechanisms and neural circuitry underlying these systemic glucoregulatory responses. Using microinjection techniques, we found that localized glucoprivation of hindbrain but not hypothalamic sites, elicited key glucoregulatory responses, indicating that glucoreceptor cells controlling these responses are located in the hindbrain. Selective destruction of hindbrain catecholamine neurons using the retrogradely transported immunotoxin, anti-dopamine beta-hydroxylase conjugated to saporin (DSAP), revealed that spinally-projecting epinephrine (E) or norepinephrine (NE) neurons are required for the adrenal medullary response to glucoprivation, while E/NE neurons with hypothalamic projections are required for feeding, corticosterone and reproductive responses. We also found that E/NE neurons are required for both consummatory and appetitive phases of glucoprivic feeding, suggesting that multilevel collateral projections of these neurons coordinate various components of the behavioral response. Epinephrine or NE neurons co-expressing neuropeptide Y (NPY) may be the neuronal phenotype required for glucoprivic feeding: they increase NPY mRNA expression in response to glucoprivation and are nearly eliminated by DSAP injections that abolish glucoprivic feeding. In contrast, lesion of arcuate nucleus NPY neurons, using the toxin, NPY-saporin, does not impair glucoprivic feeding or hyperglycemic responses. Thus, hindbrain E/NE neurons orchestrate multiple concurrent glucoregulatory responses. Specific catecholamine phenotypes may mediate the individual components of the overall response. Glucoreceptive control of these neurons resides within the hindbrain.

Metabolic imprinting: critical impact of the perinatal environment on the regulation of energy homeostasis

Epidemiological studies in humans suggest that maternal undernutrition, obesity and diabetes during gestation and lactation can all produce obesity in offspring. Animal models have allowed us to investigate the independent consequences of altering the pre- versus post-natal environments on a variety of metabolic, physiological and neuroendocrine functions as they effect the development in the offspring of obesity, diabetes, hypertension and hyperlipidemia (the 'metabolic syndrome'). During gestation, maternal malnutrition, obesity, type 1 and type 2 diabetes and psychological, immunological and pharmacological stressors can all promote offspring obesity. Normal post-natal nutrition can reduce the adverse impact of some of these pre-natal factors but maternal high-fat diets, diabetes and increased neonatal access to food all enhance the development of obesity and the metabolic syndrome in offspring. The outcome of these perturbations of the perinatal environmental is also highly dependent upon the genetic background of the individual. Those with an obesity-prone genotype are more likely to be affected by factors such as maternal obesity and high-fat diets than are obesity-resistant individuals. Many perinatal manipulations appear to promote offspring obesity by permanently altering the development of central neural pathways, which regulate food intake, energy expenditure and storage. Given their strong neurotrophic properties, either excess or an absence of insulin and leptin during the perinatal period are likely to be effectors of these developmental changes. Because obesity is associated with an increased morbidity and mortality and because of its resistance to treatment, prevention is likely to be the best strategy for stemming the tide of the obesity epidemic. Such prevention should begin in the perinatal period with the identification and avoidance of factors which produce permanent, adverse alterations in neural pathways which control energy homeostasis.

Brain glucose sensing: a subtle mechanism

PURPOSE OF REVIEW

Brain nutrient sensing allows a fine regulation of different physiological functions, such as food intake and blood glucose, related to energy homeostasis. Glucose sensing is the most studied function and a parallel has been made between the cellular mechanisms involved in pancreatic beta cells and neurons.

RECENT FINDINGS

Two types of glucosensing neurons have been characterized--those for which the activity is proportional to changes in glucose concentration and those for which the activity is inversely proportional to these changes. A new level of complexity has recently been demonstrated, as the response and the mechanism appear to vary in function according to the level of the glucose change. For some of the responses, the detection is probably not at the level of the neuron itself, but astrocytes also appear to be involved, indicating a coupling between the two types of cells. Finally, numerous data have demonstrated the modulation of glucose sensing by other nutrients, in particular fatty acids, hormones (insulin, leptin and ghrelin) and peptides (neuropeptide Y). This implies a common pathway in which AMPkinase may play a crucial role.

SUMMARY

Recent observations in brain nutrient sensing indicate subtle mechanisms, with different cellular and molecular mechanisms involved. This fact would explain the discrepancies reported in the expression of different proteins (glucose transporters, hexokinases, channels). Astrocytes may be involved in one type of response, thus adding a new level of complexity.

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

Recurrent hypoglycemia blunts the brain's ability to sense and respond to subsequent hypoglycemic episodes. Glucose-sensing neurons in the ventromedial hypothalamus nucleus (VMN) are well situated to play a role in hypoglycemia detection. VMN glucose-inhibited (GI) neurons, which decrease their firing rate as extracellular glucose increases, are extremely sensitive to decreased extracellular glucose. We hypothesize that recurrent hypoglycemia decreases the glucose sensitivity of VMN GI neurons. To test our hypothesis, 14- to 21-day-old Sprague-Dawley rats were subcutaneously injected with regular human insulin (4 U/kg) or saline (control) for three consecutive days. Blood glucose levels 1 h after insulin injection on day 3 were significantly lower than on day 1, reflecting an impaired ability to counteract hypoglycemia. On day 4, the glucose sensitivity of VMN GI neurons was measured using conventional whole cell current-clamp recording. After recurrent insulin-induced hypoglycemia, VMN GI neurons only responded to a glucose decrease from 2.5 to 0.1, but not 0.5, mM. Additionally, lactate supplementation also decreased glucose sensitivity of VMN GI neurons. Thus our findings suggest that decreases in glucose sensitivity of VMN GI neurons may contribute to the impairments in central glucose-sensing mechanisms after recurrent hypoglycemia.

Glucose-sensing neurons of the hypothalamus

Specialized subgroups of hypothalamic neurons exhibit specific excitatory or inhibitory electrical responses to changes in extracellular levels of glucose. Glucose-excited neurons were traditionally assumed to employ a 'beta-cell' glucose-sensing strategy, where glucose elevates cytosolic ATP, which closes KATP channels containing Kir6.2 subunits, causing depolarization and increased excitability. Recent findings indicate that although elements of this canonical model are functional in some hypothalamic cells, this pathway is not universally essential for excitation of glucose-sensing neurons by glucose. Thus glucose-induced excitation of arcuate nucleus neurons was recently reported in mice lacking Kir6.2, and no significant increases in cytosolic ATP levels could be detected in hypothalamic neurons after changes in extracellular glucose. Possible alternative glucose-sensing strategies include electrogenic glucose entry, glucose-induced release of glial lactate, and extracellular glucose receptors. Glucose-induced electrical inhibition is much less understood than excitation, and has been proposed to involve reduction in the depolarizing activity of the Na+/K+ pump, or activation of a hyperpolarizing Cl- current. Investigations of neurotransmitter identities of glucose-sensing neurons are beginning to provide detailed information about their physiological roles. In the mouse lateral hypothalamus, orexin/hypocretin neurons (which promote wakefulness, locomotor activity and foraging) are glucose-inhibited, whereas melanin-concentrating hormone neurons (which promote sleep and energy conservation) are glucose-excited. In the hypothalamic arcuate nucleus, excitatory actions of glucose on anorexigenic POMC neurons in mice have been reported, while the appetite-promoting NPY neurons may be directly inhibited by glucose. These results stress the fundamental importance of hypothalamic glucose-sensing neurons in orchestrating sleep-wake cycles, energy expenditure and feeding behaviour.

Insulin and leptin induce Glut4 plasma membrane translocation and glucose uptake in a human neuronal cell line by a phosphatidylinositol 3-kinase- dependent mechanism

The insulin-sensitive glucose transporter Glut4 is expressed in brain areas that regulate energy homeostasis and body adiposity. In contrast with peripheral tissues, however, the impact of insulin on Glut4 plasma membrane (PM) translocation in neurons is not known. In this study, we examined the role of two anorexic hormones (leptin and insulin) on Glut4 translocation in a human neuronal cell line that express endogenous insulin and leptin receptors. We show that insulin and leptin both induce Glut4 translocation to the PM of neuronal cells and activate glucose uptake. Wortmannin, a specific inhibitor of phosphatidylinositol 3-kinase, totally abolished insulin- and leptin-dependent Glut4 translocation and stimulation of glucose uptake. Thus, Glut4 translocation is a phosphatidylinositol 3-kinase-dependent mechanism in neuronal cells. Next, we investigated the impact of chronic insulin and leptin treatments on Glut4 expression and translocation. Chronic exposure of neuronal cells to insulin or leptin down-regulates Glut4 proteins and mRNA levels and abolishes the acute stimulation of glucose uptake in response to acute insulin or leptin. In addition, chronic treatment with either insulin or leptin impaired Glut4 translocation. A cross-desensitization between insulin and leptin was apparent, where exposure to insulin affects leptin-dependent Glut4 translocation and vice versa. This cross-desensitization could be attributed to the increase in suppressor of cytokine signaling-3 expression, which was demonstrated in response to each hormone. These results provide evidence to suggest that Glut4 translocation to neuronal PM is regulated by both insulin and leptin signaling pathways. These pathways might contribute to an in vivo glucoregulatory reflex involving a neuronal network and to the anorectic effect of insulin and leptin.

Postnatal environment overrides genetic and prenatal factors influencing offspring obesity and insulin resistance

There is growing evidence that the postnatal environment can have a major impact on the development of obesity and insulin resistance in offspring. We postulated that cross-fostering obesity-prone offspring to lean, obesity-resistant dams would ameliorate their development of obesity and insulin resistance, while fostering lean offspring to genetically obese dams would lead them to develop obesity and insulin resistance as adults. We found that obesity-prone pups cross-fostered to obesity-resistant dams remained obese but did improve their insulin sensitivity as adults. In contrast, obesity-resistant pups cross-fostered to genetically obese dams showed a diet-induced increase in adiposity, reduced insulin sensitivity, and associated changes in hypothalamic neuropeptide, insulin, and leptin receptors, which might have contributed to their metabolic defects. There was a selective increase in insulin levels and differences in fatty acid composition of obese dam milk which might have contributed to the increased adiposity, insulin resistance, and hypothalamic changes in obesity-resistant cross-fostered offspring. These results demonstrate that postnatal factors can overcome both genetic predisposition and prenatal factors in determining the development of adiposity, insulin sensitivity, and the brain pathways that mediate these functions.

Evidence from glut2-null mice that glucose is a critical physiological regulator of feeding

A role for glucose in the control of feeding has been proposed, but its precise physiological importance is unknown. Here, we evaluated feeding behavior in glut2-null mice, which express a transgenic glucose transporter in their beta-cells to rescue insulin secretion (ripglut1;glut2-/- mice). We showed that in the absence of GLUT2, daily food intake was increased and feeding initiation and termination following a fasting period were abnormal. This was accompanied by suppressed regulation of hypothalamic orexigenic and anorexigenic neuropeptides expression during the fast-to-refed transition. In these conditions, however, there was normal regulation of the circulating levels of insulin, leptin, or glucose but a loss of regulation of plasma ghrelin concentrations. To evaluate whether the abnormal feeding behavior was due to suppressed glucose sensing, we evaluated feeding in response to intraperitoneal or intracerebroventricular glucose or 2-deoxy-D-glucose injections. We showed that in GLUT2-null mice, feeding was no longer inhibited by glucose or activated by 2-deoxy-D-glucose injections and the regulation of hypothalamic neuropeptide expression by intracerebroventricular glucose administration was lost. Together, these data demonstrate that absence of GLUT2 suppressed the function of central glucose sensors, which control feeding probably by regulating the hypothalamic melanocortin pathway. Furthermore, inactivation of these glucose sensors causes overeating.

Blockade of GABA(A) receptors in the ventromedial hypothalamus further stimulates glucagon and sympathoadrenal but not the hypothalamo-pituitary-adrenal response to hypoglycemia

Hypoglycemia provokes a multifaceted counterregulatory response involving the sympathoadrenal system, stimulation of glucagon secretion, and the hypothalamo-pituitary-adrenal axis that is commonly impaired in diabetes. We examined whether modulation of inhibitory input from gamma-aminobutyric acid (GABA) in the ventromedial hypothalamus (VMH), a major glucose-sensing region within the brain, plays a role in affecting counterregulatory responses to hypoglycemia. Normal Sprague-Dawley rats had carotid artery and jugular vein catheters chronically implanted, as well as bilateral steel microinjection guide cannulas inserted down to the level of the VMH. Seven to 10 days following surgery, the rats were microinjected with artificial extracellular fluid, the GABA(A) receptor agonist muscimol (1 nmol/side), or the GABA(A) receptor antagonist bicuculline methiodide (12.5 pmol/side) before being subjected to a hyperinsulinemic-hypoglycemic (2.5 mmol/l) glucose clamp for 90 min. Following VMH administration of bicuculline methiodide, glucose infusion rates were significantly suppressed, whereas muscimol raised glucose infusion rates significantly compared with controls. Glucagon and epinephrine responses were elevated with the antagonist and suppressed with the agonist compared with controls. Corticosterone responses, however, were unaffected by either administration of the agonist or antagonist into the VMH. These data demonstrate that modulation of the GABAergic system in the VMH alters both glucagon and sympathoadrenal, but not corticosterone, responses to hypoglycemia. Our findings are consistent with the hypothesis that GABAergic inhibitory tone within the VMH can modulate glucose counterregulatory responses.

Regulation of glucagon secretion by glucose transporter type 2 (glut2) and astrocyte-dependent glucose sensors

Ripglut1;glut2-/- mice have no endogenous glucose transporter type 2 (glut2) gene expression but rescue glucose-regulated insulin secretion. Control of glucagon plasma levels is, however, abnormal, with fed hyperglucagonemia and insensitivity to physiological hypo- or hyperglycemia, indicating that GLUT2-dependent sensors control glucagon secretion. Here, we evaluated whether these sensors were located centrally and whether GLUT2 was expressed in glial cells or in neurons. We showed that ripglut1;glut2-/- mice failed to increase plasma glucagon levels following glucoprivation induced either by i.p. or intracerebroventricular 2-deoxy-D-glucose injections. This was accompanied by failure of 2-deoxy-D-glucose injections to activate c-Fos-like immunoreactivity in the nucleus of the tractus solitarius and the dorsal motor nucleus of the vagus. When glut2 was expressed by transgenesis in glial cells but not in neurons of ripglut1;glut2-/- mice, stimulated glucagon secretion was restored as was c-Fos-like immunoreactive labeling in the brainstem. When ripglut1;glut2-/- mice were backcrossed into the C57BL/6 genetic background, fed plasma glucagon levels were also elevated due to abnormal autonomic input to the alpha cells; glucagon secretion was, however, stimulated by hypoglycemic stimuli to levels similar to those in control mice. These studies identify the existence of central glucose sensors requiring glut2 expression in glial cells and therefore functional coupling between glial cells and neurons. These sensors may be activated at different glycemic levels depending on the genetic background.

The rise, fall, and resurrection of the ventromedial hypothalamus in the regulation of feeding behavior and body weight

Early researchers found that lesions of the ventromedial hypothalamus (VMH) resulted in hyperphagia and obesity in a variety of species including humans, which led them to designate the VMH as the brain's "satiety center." Many researchers later dismissed a role for the VMH in feeding behavior when Gold claimed that lesions restricted to the VMH did not result in overeating and that obesity was observed only with lesions or knife cuts that extended beyond the borders of the VMH and damaged or severed the ventral noradrenergic bundle (VNAB) or paraventricular nucleus (PVN). However, anatomical studies done both before and after Gold's study did not replicate his results with lesions, and in nearly every published direct comparison of VMH lesions vs. PVN or VNAB lesions, the group with VMH lesions ate substantially more food and gained twice as much weight. Several other important differences have also been found between VMH and both PVN and VNAB lesion-induced obesity. Concerns regarding (a) motivation to work for food and (b) the effects of nonirritative lesions have also been addressed and answered in many studies. Lesion studies with weanling rats and adult pair-tube-fed rats, as well as recent studies of knockout mice deficient in the orphan nuclear receptor steroidogenic factor 1, indicate that VMH lesion-induced obesity is in large part a metabolic obesity (due to autonomic nervous system disorders) independent of hyperphagia. However, there is ample evidence that the VMH also plays a primary role in feeding behavior. Neuroimaging studies in humans have shown a marked increase in activity in the area of the VMH during feeding. The VMH has a large population of glucoresponsive neurons that dynamically respond to blood glucose levels and numerous histamine, dopamine, serotonin, and GABA neurons that respond to feeding-related stimuli. Recent studies have implicated melanocortins in the VMH regulation of feeding behavior: food intake decreases when arcuate nucleus pro-opiomelanocortin (POMC) neurons activate VMH brain-derived neurotrophic factor (BDNF) neurons. Moderate hyperphagia and obesity have also been observed in female rats with damage to the efferent projections from the posterodorsal amygdala to the VMH. Hypothalamic obesity can result from damage to either the POMC or BDNF neurons. The concept of hypothalamic feeding and satiety centers is outdated and unnecessary, and progress in understanding hypothalamic mechanisms of feeding behavior will be achieved only by appreciating the different types of neural and blood-borne information received by the various nuclei, and then attempting to determine how this information is integrated to obtain a balance between energy intake and energy output.

Glucokinase is a critical regulator of ventromedial hypothalamic neuronal glucosensing

To test the hypothesis that glucokinase is a critical regulator of neuronal glucosensing, glucokinase activity was increased, using a glucokinase activator drug, or decreased, using RNA interference combined with calcium imaging in freshly dissociated ventromedial hypothalamic nucleus (VMN) neurons or primary ventromedial hypothalamus (VMH; VMN plus arcuate nucleus) cultures. To assess the validity of our approach, we first showed that glucose-induced (0.5-2.5 mmol/l) changes in intracellular Ca(2+) concentration ([Ca(2+)](i)) oscillations, using fura-2 and changes in membrane potential (using a membrane potential-sensitive dye), were highly correlated in both glucose-excited and -inhibited neurons. Also, glucose-excited neurons increased (half-maximal effective concentration [EC(50)] = 0.54 mmol/l) and glucose-inhibited neurons decreased (half-maximal inhibitory concentration [IC(50)] = 1.12 mmol/l) [Ca(2+)](i) oscillations to incremental changes in glucose from 0.3 to 5 mmol/l. In untreated primary VMH neuronal cultures, the expression of glucokinase mRNA and the number of demonstrable glucosensing neurons fell spontaneously by half over 12-96 h without loss of viable neurons. Transfection of neurons with small interfering glucokinase RNA did not affect survival but did reduce glucokinase mRNA by 90% in association with loss of all demonstrable glucose-excited neurons and a 99% reduction in glucose-inhibited neurons. A pharmacological glucokinase activator produced a dose-related increase in [Ca(2+)](i) oscillations in glucose-excited neurons (EC(50) = 0.98 mmol/l) and a decrease in glucose-inhibited neurons (IC(50) = 0.025 micromol/l) held at 0.5 mmol/l glucose. Together, these data support a critical role for glucokinase in neuronal glucosensing.

Neuronal responses to transient hypoglycaemia in the dorsal vagal complex of the rat brainstem

Several regions of the mammalian brain contain glucosensing neurones. In vivo studies have suggested that those located in the hypothalamus and lower brainstem are involved in glucoprivic feeding and homeostatic control of blood glucose. We have identified and characterized hypoglycaemia-sensitive neurones in the dorsal vagal complex of the brainstem using in situ hybridization, single-cell RT-PCR and whole-cell patch-clamp recordings from rat brainstem slices. Approximately 80% of neurones did not respond to hypoglycaemia (changing artificial cerebrospinal fluid (ACSF) glucose from 10 mM to 0 mM) within 5 min (non-responsive: NR). Another 10% depolarized within 155+/-31 s (mean+/-s.e.m.) of glucose removal (glucose-inhibited: GI), and the remaining neurones hyperpolarized within 53+/-7 s (glucose-excited: GE). The hyperpolarization was reversed by the KATP channel blocker tolbutamide. Single-cell RT-PCR revealed that GI and GE, but not NR, cells expressed glucokinase (GLK). In contrast, SUR1, a KATP channel subunit, was expressed in GE and some NR cells. In situ hybridization with biotin-labelled riboprobes in the dorsal vagal complex revealed ubiquitous expression of SUR1, and widespread, but sparse, expression of GLK. Identification of astrocytes using a GFAP (glial fibrillary acidic protein) antibody showed that GLK and GFAP were not colocalized. In summary, we have demonstrated that GI and GE neurones exist in the brainstem and that GLK is essential for their function. It seems likely that GE neurones work in a way analogous to pancreatic beta-cells in that they require both GLK and KATP channels.

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.

Neuronal insulin signal transduction mechanisms in diabetes phenotypes

The hippocampus is an important integration center for learning and memory in the mammalian central nervous system (CNS) and is particularly sensitive and responsive to changes in insulin and glucose concentrations. Insulin administration improves cognitive performance in a variety of physiological and pathophysiological settings, including diabetes phenotypes. Our previous studies demonstrated that hyperglycemia produces behavioral, neuroanatomical and neurochemical changes in the adult rat hippocampus that are indicative of accelerated brain aging. In addition, the trafficking of insulin-sensitive glucose transporters (GLUTs) is impaired in experimental models of diabetes. Such results suggest that insulin receptor (IR) signaling may be disrupted in diabetes phenotypes, although the signaling mechanisms utilized by neurons are not clearly defined. To this end, we have employed in vivo and in vitro approaches to determine the insulin signaling pathways utilized by neurons. These methodologies provide insight into the signaling mechanisms utilized by neuronal IRs and ultimately will allow for determination of the IR signaling deficits that may contribute to accelerated brain aging in the hippocampus of diabetic subjects.

Monocarboxylate transporters in the central nervous system: distribution, regulation and function

Monocarboxylate transporters (MCTs) are proton-linked membrane carriers involved in the transport of monocarboxylates such as lactate, pyruvate, as well as ketone bodies. They belong to a larger family of transporters composed of 14 members in mammals based on sequence homologies. MCTs are found in various tissues including the brain where three isoforms, MCT1, MCT2 and MCT4, have been described. Each of these isoforms exhibits a distinct regional and cellular distribution in rodent brain. At the cellular level, MCT1 is expressed by endothelial cells of microvessels, by ependymocytes as well as by astrocytes. MCT4 expression appears to be specific for astrocytes. By contrast, the predominant neuronal monocarboxylate transporter is MCT2. Interestingly, part of MCT2 immunoreactivity is located at postsynaptic sites, suggesting a particular role of monocarboxylates and their transporters in synaptic transmission. In addition to variation in expression during development and upon nutritional modifications, new data indicate that MCT expression is regulated at the translational level by neurotransmitters. Understanding how transport of monocarboxylates is regulated could be of particular importance not only for neuroenergetics but also for areas such as functional brain imaging, regulation of food intake and glucose homeostasis, or for central nervous system disorders such as ischaemia and neurodegenerative diseases.

Insulin signaling in the central nervous system: a critical role in metabolic homeostasis and disease from C. elegans to humans

Insulin and its signaling systems are implicated in both central and peripheral mechanisms governing the ingestion, distribution, metabolism, and storage of nutrients in organisms ranging from worms to humans. Input from the environment regarding the availability and type of nutrients is sensed and integrated with humoral information (provided in part by insulin) regarding the sufficiency of body fat stores. In response to these afferent inputs, neuronal pathways are activated that influence energy flux and nutrient metabolism in the body and ensure reproductive competency. Growing evidence supports the hypothesis that reduced central nervous system insulin signaling from either defective secretion or action contributes to the pathogenesis of common metabolic disorders, including diabetes and obesity, and may therefore help to explain the close association between these two disorders. These considerations implicate insulin action in the brain, an organ previously considered to be insulin independent, as a key determinant of both glucose and energy homeostasis.

Role of neuronal energy status in the regulation of adenosine 5'-monophosphate-activated protein kinase, orexigenic neuropeptides expression, and feeding behavior

Nutrient sensing in the hypothalamus is tightly related to food intake regulation. However, the mechanisms by which the nutrient-sensing cells of the brain translate this signal of energy need into feeding behavior via regulation of neuropeptide expression are not known. To address this issue, we investigated two neuronal cell lines expressing agouti-related protein (AgRP), ex vivo hypothalamic tissues, and in vivo whole animals. Maintaining cells in a low cellular ATP concentration generated by low glucose, 2-deoxyglucose (2-DG), ATP synthesis inhibitor, and 5-aminoimidazole-4-carboxamide 1-beta-d-ribofuranoside increased phosphorylation of AMP-activated protein kinase (AMPK) and increased AgRP expression, whereas maintaining cells in high ATP status by high glucose and pyruvate supplementation in 2-DG-treated cells decreased phosphorylation of AMPK and decreased AgRP expression. Overexpression of a dominant-inhibitory mutant of AMPK significantly decreased low-glucose- or 2-DG-induced AgRP expression. Furthermore, ex vivo hypothalamus culture in high glucose concentrations decreased both expression and phosphorylation of AMPK and expression of both AgRP and neuropeptide Y, whereas pyruvate supplementation suppressed a 2-DG-induced AgRP expression. Finally, our in vivo studies clearly show that central administration of pyruvate dramatically delayed 2-DG-induced food intake. These data indicate that modulation of ATP levels in neuronal cells triggers a cascade of events via AMPK that modulate feeding behavior to restore energy status of cells.

A new ATP-sensitive K+ channel-independent mechanism is involved in glucose-excited neurons of mouse arcuate nucleus

Glucose is known to modify electrical activity of neurons in different hypothalamic areas such as the arcuate nucleus (ARC) or the ventromedian nucleus. In these structures, it has been demonstrated that glucose-induced excitation of neurons involves ATP-sensitive K(+) (K(ATP)) channel closure. The aim of the present study was to determine whether ARC neurons were able to detect high extracellular glucose concentrations and which mechanisms were involved in this detection by using whole-cell and cell-attached patch-clamp techniques in acute mouse brain slices. An increase from 5 to 20 mmol/l glucose stimulated 19% and inhibited 9% of ARC neurons. Because of the high-glucose concentrations used, we called these neurons high-glucose-excited (HGE) and high-glucose-inhibited (HGI) neurons, respectively. Glucose-induced depolarization of HGE neurons was not abolished by tetrodotoxin treatment and was correlated with an increase of membrane conductance that reversed at approximately 20 mV. Experiments with diazoxide, pinacidil, or tolbutamide showed that K(ATP) channels were present and functional in most of the ARC neurons but were mostly closed at 5 mmol/l glucose. Moreover, HGE neurons were also present in ARC of Kir6.2 null mice. These results suggested that ARC neurons have the ability to sense higher glucose concentrations than 5 mmol/l through a new K(ATP) channel-independent mechanism.

Obesity-prone rats have preexisting defects in their counterregulatory response to insulin-induced hypoglycemia

Rats that develop diet-induced obesity (DIO) on a 31% fat [high-energy (HE)] diet have defective sensing and responding to altered glucose levels compared with diet-resistant (DR) rats. Thus we postulated that they would also have defective counterregulatory responses (CRR) to insulin-induced hypoglycemia (IIH). Chow-fed selectively bred DIO and DR rats underwent three sequential 60-min bouts of IIH separated by 48 h. Glucose levels fell comparably, but DIO rats had 22–29% lower plasma epinephrine (Epi) levels during the first two bouts than DR rats. By the third trial, despite comparable Epi levels, DIO rats had lower 30-min glucose levels and rebounded less than DR rats 85 min after intravenous glucose. Although DIO rats gained more carcass and fat weight after 4 wk on an HE diet than DR rats, they were unaffected by prior IIH. Compared with controls, DR rats with prior IIH and HE diet had higher arcuate nucleus neuropeptide Y (50%) and proopiomelanocortin (POMC; 37%) mRNA and an inverse correlation ( r = 0.85; P = 0.004) between POMC expression and body weight gain on the HE diet. These data suggest that DIO rats have a preexisting defect in their CRR to IIH but that IIH does not affect the expression of their hypothalamic neuropeptides or weight gain as it does in DR rats.

Neuronal glucosensing: what do we know after 50 years?

Glucosensing neurons are specialized cells that use glucose as a signaling molecule to alter their action potential frequency in response to variations in ambient glucose levels. Glucokinase (GK) appears to be the primary regulator of most neuronal glucosensing, but other regulators almost certainly exist. Glucose-excited neurons increase their activity when glucose levels rise, and most use GK and an ATP-sensitive K(+) channel as the ultimate effector of glucose-induced signaling. Glucose-inhibited (GI) neurons increase their activity at low glucose levels. Although many use GK, it is unclear what the final pathway of GI neuronal glucosensing is. Glucosensing neurons are located in brain sites and respond to and integrate a variety of hormonal, metabolic, transmitter, and peptide signals involved in the regulation of energy homeostasis and other biological functions. Although it is still uncertain whether daily fluctuations in blood glucose play a specific regulatory role in these physiological functions, it is clear that large decreases in glucose availability stimulate food intake and counterregulatory responses that restore glucose levels to sustain cerebral function. Finally, glucosensing is altered in obesity and after recurrent bouts of hypoglycemia, and this altered sensing may contribute to the adverse outcomes of these conditions. Thus, although much is known, much remains to be learned about the physiological function of brain glucosensing neurons.

Nicotine infusion into rat ventromedial nuclei and effects on monoaminergic system

Nicotine increases satiety and reduces food intake (FI). We hypothesize that nicotine influences FI via alteration of serotonin (5HT) and dopamine (DA) concentration in ventromedial nucleus (VMN) and lateral hypothalamic area (LHA). Microdialysis cannulas were implanted into ipsilateral VMN and contralateral LHA. Nicotine or vehicle was infused for 60 min into VMN of overnight food-deprived rats, followed by ad lib food for 40 min. Hypothalamic changes in 5HT and DA concentrations were measured every 20 min. Intra-VMN nicotine induced a long-lasting increase in 5HT concentration and an increase in DA for a short duration in the VMN, associated with an increase in 5HT in the LHA. Our data suggest that the nicotine-induced hypophagia correlates with VMN and LHA monoaminergic changes.

The regulation of glucose-excited neurons in the hypothalamic arcuate nucleus by glucose and feeding-relevant peptides

Glucosensing neurons in the hypothalamic arcuate nucleus (ARC) were studied using electrophysiological and immunocytochemical techniques in neonatal male Sprague-Dawley rats. We identified glucose-excited and -inhibited neurons, which increase and decrease, respectively, their action potential frequency (APF) as extracellular glucose levels increase throughout the physiological range. Glucose-inhibited neurons were found predominantly in the medial ARC, whereas glucose-excited neurons were found in the lateral ARC. ARC glucose-excited neurons in brain slices dose-dependently increased their APF and decreased their ATP-sensitive K+ channel (KATP channel) currents as extracellular glucose levels increased from 0.1 to 10 mmol/l. However, glucose sensitivity was greatest as extracellular glucose decreased to <2.5 mmol/l. The glucokinase inhibitor alloxan increases KATP single-channel currents in glucose-excited neurons in a manner similar to low glucose. Leptin did not alter the activity of ARC glucose-excited neurons. Although insulin did not affect ARC glucose-excited neurons in the presence of 2.5 mmol/l (steady-state) glucose, they were stimulated by insulin in the presence of 0.1 mmol/l glucose. Neuropeptide Y (NPY) inhibited and alpha-melanocyte-stimulating hormone stimulated ARC glucose-excited neurons. ARC glucose-excited neurons did not show pro-opiomelanocortin immunoreactivity. These data suggest that ARC glucose-excited neurons may serve an integrative role in the regulation of energy balance.

Third ventricular alloxan reversibly impairs glucose counterregulatory responses

Glucokinase (GK) is hypothesized to be the critical glucosensor of pancreatic beta-cells and hypothalamic glucosensing neurons. To understand the role of GK in glucoprivic counterregulatory responses, we injected alloxan, a GK inhibitor and toxin, into the third ventricle (3v) to target nearby GK-expressing neurons. Four and 6 days after 3v, but not 4v, alloxan injection, alloxan-treated rats ate only 30% and their blood glucose area under the curve was only 28% of saline controls' after systemic 2-deoxy-D-glucose. In addition, their hyperglycemic response to hindbrain glucoprivation induced with 5-thio-glucose was impaired, whereas fasting blood glucose levels and food intake after an overnight fast were elevated. These impaired responses were associated with the destruction of 3v tanycytes, reduced glial fibrillary acidic protein-immunoreactivity surrounding the 3v, neuronal swelling, and decreased arcuate nucleus neuropeptide Y (NPY) mRNA. Nevertheless, hypothalamic GK mRNA was significantly elevated. Two weeks after alloxan injection, 3v tanycyte destruction was reversed along with restoration of feeding and hyperglycemic responses to both systemic and hindbrain glucoprivation. At this time there were significant decreases in GK, NPY, and proopiomelanocortin mRNA. Thus, neural substrates near and around the 3v affected by alloxan may be critically involved in the expression of these glucoprivic responses.