Vanessa H. Routh

Insulin activates ATP-sensitive K + channels in hypothalamicneurons of lean, but not obese rats

Insulin and leptin receptors are present in hypothalamic regions that control energy homeostasis, and these hormones reduce food intake and body weight in lean, but not obese, Zucker rats. Here we demonstrate that insulin, like leptin, hyperpolarizes lean rat hypothalamic glucose-responsive (GR) neurons by opening KATP channels. These findings suggest hypothalamic KATP channel function is crucial to physiological regulation of food intake and body weight.

Brain glucose sensing and body energy homeostasis: role in obesity and diabetes

The brain has evolved mechanisms for sensing and regulating glucose metabolism. It receives neural inputs from glucosensors in the periphery but also contains neurons that directly sense changes in glucose levels by using glucose as a signal to alter their firing rate. Glucose-responsive (GR) neurons increase and glucose-sensitive (GS) decrease their firing rate when brain glucose levels rise. GR neurons use an ATP-sensitive K+ channel to regulate their firing. The mechanism regulating GS firing is less certain. Both GR and GS neurons respond to, and participate in, the changes in food intake, sympathoadrenal activity, and energy expenditure produced by extremes of hyper- and hypoglycemia. It is less certain that they respond to the small swings in plasma glucose required for the more physiological regulation of energy homeostasis. Both obesity and diabetes are associated with several alterations in brain glucose sensing. In rats with diet-induced obesity and hyperinsulinemia, GR neurons are hyporesponsive to glucose. Insulin-dependent diabetic rats also have abnormalities of GR neurons and neurotransmitter systems potentially involved in glucose sensing. Thus the challenge for the future is to define the role of brain glucose sensing in the physiological regulation of energy balance and in the pathophysiology of obesity and diabetes.

Glucose-sensing neurons: are they physiologically relevant?

Glucose homeostasis is of paramount concern to the brain since glucose is its primary fuel. Thus, the brain has evolved mechanisms to sense and respond to changes in glucose levels. The efferent aspects of the central nervous system response to hypoglycemia are relatively well understood. In addition, it is accepted that the brain regulates food intake and energy balance. Obesity and diabetes both result from and cause alterations in the central nervous system function. Thus, it is reasonable to hypothesize that the brain also regulates daily glucose homeostasis and energy balance. However, little is known about how the brain actually senses and responds to changes in extracellular glucose. While there are neurons in the brain that change their action potential frequency in response to changes in extracellular glucose, most studies of these neurons have been performed using glucose levels that are outside the physiologic range of extracellular brain glucose. Thus, the physiologic relevance of these glucose-sensing neurons is uncertain. However, recent studies show that glucose-sensing neurons do respond to physiologic changes in extracellular glucose. This review will first investigate the data regarding physiologic glucose levels in the brain. The various subtypes of physiologically relevant glucose-sensing neurons will then be discussed. Based on the relative glucose sensitivity of these subtypes of glucose-sensing neurons, possible roles in the regulation of glucose homeostasis are hypothesized. Finally, the question of whether these neurons are only glucose sensors or whether they play a more integrated role in the regulation of energy balance will be considered.

Characterization of Glucosensing Neuron Subpopulations in the Arcuate Nucleus: Integration in Neuropeptide Y and Pro-Opio Melanocortin Networks?

Four types of responses to glucose changes have been described in the arcuate nucleus (ARC): excitation or inhibition by low glucose concentrations <5 mmol/l (glucose-excited and -inhibited neurons) and by high glucose concentrations >5 mmol/l (high glucose–excited and –inhibited neurons). However, the ability of the same ARC neuron to detect low and high glucose concentrations has never been investigated. Moreover, the mechanism involved in mediating glucose sensitivity in glucose-inhibited neurons and the neurotransmitter identity (neuropeptide Y [NPY] or pro-opio melanocortin [POMC]) of glucosensing neurons has remained controversial. Using patch-clamp recordings on acute mouse brain slices, successive extracellular glucose changes greater than and less than 5 mmol/l show that glucose-excited, high glucose–excited, glucose-inhibited, and high glucose–inhibited neurons are different glucosensing cell subpopulations. Glucose-inhibited neurons directly detect decreased glucose via closure of a chloride channel. Using transgenic NPY–green fluorescent protein (GFP) and POMC-GFP mice, we show that 40% of NPY neurons are glucose-inhibited neurons. In contrast, <5% of POMC neurons responded to changes in extracellular glucose >5 mmol/l. In vivo results confirm the lack of glucose sensitivity of POMC neurons. Taken together, hypo- and hyperglycemia are detected by distinct populations of glucosensing neurons, and POMC and NPY neurons are not solely responsible for ARC glucosensing.

Glucosensing neurons in the ventromedial hypothalamic nucleus (VMN) and hypoglycemia-associated autonomic failure (HAAF)

Hypoglycemia is a profound threat to the brain since glucose is its preferred fuel. Thus, decreases in plasma glucose must be sensed and appropriate hormonal and neuroendocrine responses generated to restore glucose to safe levels (i.e. counterregulatory responses (CRR) to hypoglycemia). Recurrent hypoglycemia impairs these protective mechanisms, resulting in a potentially life‐threatening condition known as hypoglycemia‐associated autonomic failure (HAAF). During HAAF, the glycemic threshold is reset so that glucose levels must fall further before the CRR is initiated. The brain plays a critical role in sensing hypoglycemia and initiating the CRR. Additionally, many neurons may sense changes in plasma and extracellular glucose. However, the way in which central glucose sensing is integrated to lead to effective initiation of the CRR is unknown. Furthermore, the mechanisms by which this system becomes impaired during HAAF are also unknown. Glucosensing neurons in the ventromedial hypothalamic nucleus (VMN) are poised to serve an integrative function in glucose homeostasis. First, they sense glucose. Second, the VMN receives input from other glucose‐sensing areas. Finally, the VMN projects to areas linked to the regulation of the sympathoadrenal system that mediates the CRR. This review discusses VMN glucosensing neurons relative to their capacity to play a role in the regulation of the CRR and the generation of HAAF. Glucosensing neurons in the hindbrain as well as peripheral glucosensors are also considered. Copyright

Brain glucosensing and the K ATP channel

An ATP-sensitive K+ channel in glucose-responsive neurons is shown to be required for the emergency response to severe glucose deprivation, but not necessarily for normal feeding.

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 14C-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.

Low-affinity sulfonylurea binding sites reside on neuronal cell bodies in the brain.

The antidiabetic sulfonylurea drugs bind to sites associated with an ATP-sensitive potassium (Katp) channel on cell bodies and terminals of neurons which increase their firing rates or transmitter release when glucose concentrations rise or sulfonylureas are present. High-affinity sulfonylurea binding sites are concentrated in areas such as the substantia nigra (SN) where glucose and sulfonylureas increase transmitter release from GABA neurons. But there is a paucity of high-affinity sites in areas such as the hypothalamic ventromedial nucleus (VMN) where many neurons increase their activity when glucose rises. Here we assessed both high- and low--affinity sulfonylurea binding autoradiographically with 20 nM [3H]glyburide in the presence of absence of Gpp(NH)p. Neurotoxin lesions with 6-hydroxydopamine (6-OHDA), 5,7-dihydroxytryptamine (5,7-DHT) and ibotenic acid were used to elucidate the cellular location of the two sites in the VMN, SN and locus coeruleus (LC). In the VMN, 25% of the sites were of low affinity. Neither 6-OHDA nor 5,7-DHT affected [3H]glyburide binding, while ibotenic acid reduced the number of VMN neurons and abolished low-affinity without changing high-affinity binding. In cell-attached patches of isolated VMN neurons, both 10 mM glucose and 100 microM glyburide decreased the open probability of the Katp channel suggesting that the low-affinity binding site resides on these neurons. In the SN pars reticulata, ibotenic acid reduced the number of neurons and high-affinity [3H]glyburide binding was decreased by 20%, while 6-OHDA had no effect. In the SN pars compacta, both 6-OHDA and ibotenic acid destroyed endogenous dopamine neurons and selectivity ablated low-affinity binding. In the LC, 6-OHDA destroyed norepinephrine neurons and abolished low-affinity binding. These data suggest that low-affinity sulfonylurea binding sites reside on cell bodies on VMN, SN dopamine and LC norepinephrine neuron cell bodies and that high-affinity sites may be on axon terminals of GABA neurons in the SN.

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.

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 bodys 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 brains 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.