Potassium channel dysfunction in hypothalamic glucose-receptive neurones of obese Zucker rats

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.

Effects of Leptin on Hypothalamic Arcuate Neurons in Wistar and Zucker Rats: An In Vitro Study

Leptin, a product of the ob gene, decreases food intake and body weight in both Wistar and Zucker obese rats when administered centrally or peripherally. To examine whether these leptin effects might be mediated through a neuropeptide Y (NPY) signaling pathway in the medial part of the arcuate nucleus of the hypothalamus (vmARC), the effects of leptin on vmARC neurons in Wistar and Zucker obese rats were examined electrophysiologically using brain slice preparations. Bath application of leptin inhibited about 60% of the vmARC neurons recorded in slices from Wistar rats. Similar inhibitory effects of leptin on vmARC neurons were also observed under low-Ca2+, high-Mg2+ Ringer's solution. However, inhibitory effects were almost absent under Ringer's solution containing a protein kinase C inhibitor, chelerythrine chloride. In slices from Zucker obese rats, leptin inhibited only about 25% of the vmARC neurons recorded, and the proportion of neurons inhibited was significantly smaller for these rats than for Wistar rats. These results suggest that reductions in food intake and body weight induced by leptin in both Wistar and Zucker obese rats are partly mediated via inhibition of an NPY signaling pathway in the vmARC.

Ion Channels in Obesity: Pathophysiology and Potential Therapeutic Targets

Obesity is a multifactorial disease related to metabolic disorders and associated with genetic determinants. Currently, ion channels activity has been linked to many of these disorders, in addition to the central regulation of food intake, energetic balance, hormone release and response, as well as the adipocyte cell proliferation. Therefore, the objective of this work is to review the current knowledge about the influence of ion channels in obesity development. This review used different sources of literature (Google Scholar, PubMed, Scopus, and Web of Science) to assess the role of ion channels in the pathophysiology of obesity. Ion channels present diverse key functions, such as the maintenance of physiological homeostasis and cell proliferation. Cell biology and pharmacological experimental evidences demonstrate that proliferating cells exhibit ion channel expression, conductance, and electrical properties different from the resting cells. Thereby, a large variety of ion channels has been identified in the pathogenesis of obesity such as potassium, sodium, calcium and chloride channels, nicotinic acetylcholine receptor and transient receptor potential channels. The fundamental involvement of these channels on the generation of obesity leads to the progress in the knowledge about the mechanisms responsible for the obesity pathophysiology, consequently emerging as new targets for pharmacological modulation.

Modulation of olfactory sensitivity and glucose-sensing by the feeding state in obese Zucker rats

The Zucker fa/fa rat has been widely used as an animal model to study obesity, since it recapitulates most of its behavioral and metabolic dysfunctions, such as hyperphagia, hyperglycemia and insulin resistance. Although it is well established that olfaction is under nutritional and hormonal influences, little is known about the impact of metabolic dysfunctions on olfactory performances and glucose-sensing in the olfactory system of the obese Zucker rat. In the present study, using a behavioral paradigm based on a conditioned olfactory aversion, we have shown that both obese and lean Zucker rats have a better olfactory sensitivity when they are fasted than when they are satiated. Interestingly, the obese Zucker rats displayed a higher olfactory sensitivity than their lean controls. By investigating the molecular mechanisms involved in glucose-sensing in the olfactory system, we demonstrated that sodium-coupled glucose transporters 1 (SGLT1) and insulin dependent glucose transporters 4 (GLUT4) are both expressed in the olfactory bulb (OB). By comparing the expression of GLUT4 and SGLT1 in OB of obese and lean Zucker rats, we found that only SGLT1 is regulated in genotype-dependent manner. Next, we used glucose oxidase biosensors to simultaneously measure in vivo the extracellular fluid glucose concentrations ([Gluc]ECF) in the OB and the cortex. Under metabolic steady state, we have determined that the OB contained twice the amount of glucose found in the cortex. In both regions, the [Gluc]ECF was 2 fold higher in obese rats compared to their lean controls. Under induced dynamic glycemia conditions, insulin injection produced a greater decrease of [Gluc]ECF in the OB than in the cortex. Glucose injection did not affect OB [Gluc]ECF in Zucker fa/fa rats. In conclusion, these results emphasize the importance of glucose for the OB network function and provide strong arguments towards establishing the OB glucose-sensing as a key factor for sensory olfactory processing.

Glucose sensing mechanisms in hypothalamic cell models: glucose inhibition of AgRP synthesis and secretion

Glucose-sensing neurons play a role in energy homeostasis, yet how orexigenic neurons sense glucose remains unclear. As models of glucose-inhibited (GI) neurons, mHypoE-29/1 and mHypoA-NPY/GFP cells express the essential orexigenic neuropeptide AgRP and glucose sensing machinery. Exposure to increasing concentrations of glucose or the glucose analog 2-deoxyglucose (2-DG) results in a decrease in AgRP mRNA levels. Taste receptor, Tas1R2 mRNA expression was reduced by glucose, whereas 2-DG reduced Tas1R3 mRNA levels. Increasing glucose concentrations elicited a rise in Akt and neuronal nitric oxide synthase (nNOS) phosphorylation, CaMKKβ levels, and a reduction of AMP-kinase alpha phosphorylation. Inhibitors of NOS and the cystic fibrosis transmembrane conductance regulator (CFTR) prevented a decrease in AgRP secretion with glucose, suggesting a pivotal role for nNOS and the CFTR in glucose-sensing. These models possess the hallmark characteristics of GI neurons, and can be used to disentangle the mechanisms by which orexigenic neurons sense glucose.

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.

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.

Cerebral insulin increases brain response to glucose

The hypothalamus participates in the regulation of carbohydrate metabolism involving a feedback loop between the brain and the periphery in which glucose-sensitive hypothalamic areas appear to be involved. We have previously shown that a glucose injection (9 mg/kg) in the carotid artery toward the brain, in an amount that did not modify glycaemia, caused a rapid and transient increase in plasma insulin concentrations. To determine whether central insulin could influence this response, we investigated the change in central glucose-induced insulin secretion in intracerebroventricular (i.c.v) insulin-injected rats and in hyperinsulinaemic obese Zucker rats. Central glucose-induced insulin secretion was increased by 50% in i.c.v. insulin-injected rats compared to control rats. When a similar test was performed at a lower dose of glucose (3 mg/kg), a significant insulin secretion was observed only in rats submitted to a prior central insulin injection. These data indicate an increase in the brain response to glucose after insulin treatment. Using an identical lower glucose dose, we also demonstrated an enhanced brain glucose sensitivity in hyperinsulinaemic and insulin-resistant obese Zucker rats. Together, these results indicate that acute i.c.v. insulin or pathological hyperinsulinaemic state (i.e. obese Zucker rat) modulates the nervous control of insulin secretion by increasing the brain response to glucose.

Brain glucose sensing mechanism and glucose homeostasis

PURPOSE OF REVIEW

Glucose homeostasis must be finely regulated. Changes in glucose levels elicit a complex neuroendocrine response that prevents or rapidly corrects hyper- or hypoglycemia. It is well established that different parts of the brain, particularly the hypothalamus and the brain stem, are important centres involved in the monitoring of glucose status and the regulation of feeding. The pioneering work of Mayer, including his proposal of the glucostatic theory, has recently received experimental support from the molecular, electro-physiological and physiological fields.

RECENT FINDINGS

Making the analogy with the beta cell of the islet of Langerhans, it has been proposed that glucose sensing could be assured in some cells of the brain by proteins such as glucose transporter 2, glucokinase and the ATP-dependent potassium channel. Furthermore, some pathological conditions such as diabetes and obesity have been shown to alter this glucose sensing system.

SUMMARY

These findings could lead to a better understanding of metabolic disorders, with hypoglycemia possibly being the most deleterious.

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.

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.