The brain response to 2-deoxy glucose is blocked by a glial drug

Chemogenetic activation of ventral medullary astrocytes enhances feeding and corticosterone release in response to mild glucoprivation

To investigate the role of glial cells in the regulation of glucoprivic responses in rats, a chemogenetic approach was used to activate astrocytes neighboring catecholamine (CA) neurons in the ventromedial medulla (VLM) where A1 and C1 CA cell groups overlap (A1/C1). Previous results indicate that activation of CA neurons in this region is necessary and sufficient for feeding and corticosterone release in response to glucoprivation. However, it is not known whether astrocyte neighbors of CA neurons contribute to glucoregulatory responses. Hence, we made nanoinjections of AAV5-GFAP-hM3D(Gq)-mCherry to selectively transfect astrocytes in the A1/C1 region with the excitatory designer receptor exclusively activated by designer drugs (DREADDs), hM3D(Gq). After allowing time for DREADD expression, we evaluated the rats for increased food intake and corticosterone release in response to low systemic doses of the antiglycolytic agent, 2-deoxy-d-glucose (2DG), alone and in combination with the hM3D(Gq) activator clozapine-n-oxide (CNO). We found that DREADD-transfected rats ate significantly more food when 2DG and CNO were coadministered than when either 2DG or CNO was injected alone. We also found that CNO significantly enhanced 2DG-induced FOS expression in the A1/C1 CA neurons, and that corticosterone release also was enhanced when CNO and 2DG were administered together. Importantly, CNO-induced activation of astrocytes in the absence of 2DG did not trigger food intake or corticosterone release. Our results indicate that during glucoprivation, activation of VLM astrocytes cells markedly increases the sensitivity or responsiveness of neighboring A1/C1 CA neurons to glucose deficit, suggesting a potentially important role for VLM astrocytes in glucoregulation.

Glial Modulation of Energy Balance: The Dorsal Vagal Complex Is No Exception

The avoidance of being overweight or obese is a daily challenge for a growing number of people. The growing proportion of people suffering from a nutritional imbalance in many parts of the world exemplifies this challenge and emphasizes the need for a better understanding of the mechanisms that regulate nutritional balance. Until recently, research on the central regulation of food intake primarily focused on neuronal signaling, with little attention paid to the role of glial cells. Over the last few decades, our understanding of glial cells has changed dramatically. These cells are increasingly regarded as important neuronal partners, contributing not just to cerebral homeostasis, but also to cerebral signaling. Our understanding of the central regulation of energy balance is part of this (r)evolution. Evidence is accumulating that glial cells play a dynamic role in the modulation of energy balance. In the present review, we summarize recent data indicating that the multifaceted glial compartment of the brainstem dorsal vagal complex (DVC) should be considered in research aimed at identifying feeding-related processes operating at this level.

Astrocyte Clocks and Glucose Homeostasis

The endogenous timekeeping system evolved to anticipate the time of the day through the 24 hours cycle of the Earth's rotation. In mammals, the circadian clock governs rhythmic physiological and behavioral processes, including the daily oscillation in glucose metabolism, food intake, energy expenditure, and whole-body insulin sensitivity. The results from a series of studies have demonstrated that environmental or genetic alterations of the circadian cycle in humans and rodents are strongly associated with metabolic diseases such as obesity and type 2 diabetes. Emerging evidence suggests that astrocyte clocks have a crucial role in regulating molecular, physiological, and behavioral circadian rhythms such as glucose metabolism and insulin sensitivity. Given the concurrent high prevalence of type 2 diabetes and circadian disruption, understanding the mechanisms underlying glucose homeostasis regulation by the circadian clock and its dysregulation may improve glycemic control. In this review, we summarize the current knowledge on the tight interconnection between the timekeeping system, glucose homeostasis, and insulin sensitivity. We focus specifically on the involvement of astrocyte clocks, at the organism, cellular, and molecular levels, in the regulation of glucose metabolism.

Evidence that hindbrain astrocytes in the rat detect low glucose with a glucose transporter 2-phospholipase C-calcium release mechanism

Astrocytes generate robust cytoplasmic calcium signals in response to reductions in extracellular glucose. This calcium signal, in turn, drives purinergic gliotransmission, which controls the activity of catecholaminergic (CA) neurons in the hindbrain. These CA neurons are critical to triggering glucose counter-regulatory responses (CRRs) that, ultimately, restore glucose homeostasis via endocrine and behavioral means. Although the astrocyte low-glucose sensor involvement in CRR has been accepted, it is not clear how astrocytes produce an increase in intracellular calcium in response to a decrease in glucose. Our ex vivo calcium imaging studies of hindbrain astrocytes show that the glucose type 2 transporter (GLUT2) is an essential feature of the astrocyte glucosensor mechanism. Coimmunoprecipitation assays reveal that the recombinant GLUT2 binds directly with the recombinant Gq protein subunit that activates phospholipase C (PLC). Additional calcium imaging studies suggest that GLUT2 may be connected to a PLC-endoplasmic reticular-calcium release mechanism, which is amplified by calcium-induced calcium release (CICR). Collectively, these data help outline a potential mechanism used by astrocytes to convert information regarding low-glucose levels into intracellular changes that ultimately regulate the CRR.

Hindbrain astrocytes and glucose counter-regulation

Hindbrain astrocytes are emerging as critical components in the regulation of homeostatic functions by either modulating synaptic activity or serving as primary detectors of physiological parameters. Recent studies have suggested that the glucose counter-regulation response (CRR), a critical defense against hypoglycemic emergencies, is dependent on glucoprivation-sensitive astrocytes in the hindbrain. This subpopulation of astrocytes produces a robust calcium signal in response to glucopenic stimuli. Both ex vivo and in vivo evidence suggest that low-glucose sensitive astrocytes utilize purinergic gliotransmission to activate catecholamine neurons in the hindbrain that are critical to the generation of the integrated CRR. Lastly, reports in the clinical literature suggest that an uncontrolled activation of CRR may as part of the pathology of severe traumatic injury. Work in our laboratory also suggests that this pathological hyperglycemia resulting from traumatic injury may be caused by the action of thrombin (generated by tissue trauma or bleeding) on hindbrain astrocytes. Similar to their glucopenia-sensitive neighbors, these hindbrain astrocytes may trigger hyperglycemic responses by their interactions with catecholaminergic neurons.

Hypothalamic Control of Systemic Glucose Homeostasis: The Pancreas Connection

Maintenance of glucose homeostasis is mandatory for organismal survival. It is accomplished by complex and coordinated interplay between glucose detection mechanisms and multiple effector systems. The brain, in particular homeostatic regions such as the hypothalamus, plays a crucial role in orchestrating such a highly integral response. We review here current understanding of how the hypothalamus senses glucose availability and participates in systemic glucose homeostasis. We provide an update of the relevant signaling pathways and neuronal subsets involved, as well as of the mechanisms modulating metabolic processes in peripheral tissues such as liver, skeletal muscle, fat, and especially the pancreas. We also discuss the relevance of these networks in human biology and prevalent metabolic conditions such as diabetes and obesity.

Response of catecholaminergic neurons in the mouse hindbrain to glucoprivic stimuli is astrocyte dependent

Hindbrain catecholaminergic (CA) neurons are required for critical autonomic, endocrine, and behavioral counterregulatory responses (CRRs) to hypoglycemia. Recent studies suggest that CRR initiation depends on hindbrain astrocyte glucose sensors (McDougal DH, Hermann GE, Rogers RC. Front Neurosci 7: 249, 2013; Rogers RC, Ritter S, Hermann GE. Am J Physiol Regul Integr Comp Physiol 310: R1102-R1108, 2016). To test the proposition that hindbrain CA responses to glucoprivation are astrocyte dependent, we utilized transgenic mice in which the calcium reporter construct (GCaMP5) was expressed selectively in tyrosine hydroxylase neurons (TH-GCaMP5). We conducted live cell calcium-imaging studies on tissue slices containing the nucleus of the solitary tract (NST) or the ventrolateral medulla, critical CRR initiation sites. Results show that TH-GCaMP5 neurons are robustly activated by a glucoprivic challenge and that this response is dependent on functional astrocytes. Pretreatment of hindbrain slices with fluorocitrate (an astrocytic metabolic suppressor) abolished TH-GCaMP5 neuronal responses to glucoprivation, but not to glutamate. Pharmacologic results suggest that the astrocytic connection with hindbrain CA neurons is purinergic via P2 receptors. Parallel imaging studies on hindbrain slices of NST from wild-type C57BL/6J mice, in which astrocytes and neurons were prelabeled with a calcium reporter dye and an astrocytic vital dye, show that both cell types are activated by glucoprivation but astrocytes responded significantly sooner than neurons. Pretreatment of these hindbrain slices with P2 antagonists abolished neuronal responses to glucoprivation without interruption of astrocyte responses; pretreatment with fluorocitrate eliminated both astrocytic and neuronal responses. These results support earlier work suggesting that the primary detection of glucoprivic signals by the hindbrain is mediated by astrocytes.

Brain metabolic sensing and metabolic signaling at the level of an astrocyte

Astrocytes support neuronal function by providing essential structural and nutritional support, neurotransmitter trafficking and recycling and may also contribute to brain information processing. In this article we review published results and report new data suggesting that astrocytes function as versatile metabolic sensors of central nervous system (CNS) milieu and play an important role in the maintenance of brain metabolic homeostasis. We discuss anatomical and functional features of astrocytes that allow them to detect and respond to changes in the brain parenchymal levels of metabolic substrates (oxygen and glucose), and metabolic waste products (carbon dioxide). We report data suggesting that astrocytes are also sensitive to circulating endocrine signals-hormones like ghrelin, glucagon-like peptide-1 and leptin, that have a major impact on the CNS mechanisms controlling food intake and energy balance. We discuss signaling mechanisms that mediate communication between astrocytes and neurons and consider how these mechanisms are recruited by astrocytes activated in response to various metabolic challenges. We review experimental data suggesting that astrocytes modulate the activities of the respiratory and autonomic neuronal networks that ensure adaptive changes in breathing and sympathetic drive in order to support the physiological and behavioral demands of the organism in ever-changing environmental conditions. Finally, we discuss evidence suggesting that altered astroglial function may contribute to the pathogenesis of disparate neurological, respiratory and cardiovascular disorders such as Rett syndrome and systemic arterial hypertension.

Hindbrain cytoglucopenia-induced increases in systemic blood glucose levels by 2-deoxyglucose depend on intact astrocytes and adenosine release

The hindbrain contains critical neurocircuitry responsible for generating defensive physiological responses to hypoglycemia. This counter-regulatory response (CRR) is evoked by local hindbrain cytoglucopenia that causes an autonomically mediated increase in blood glucose, feeding behavior, and accelerated digestion; that is, actions that restore glucose homeostasis. Recent reports suggest that CRR may be initially triggered by astrocytes in the hindbrain. The present studies in thiobutabarbital-anesthetized rats show that exposure of the fourth ventricle (4V) to 2-deoxyglucose (2DG; 15 μmol) produced a 35% increase in circulating glucose relative to baseline levels. While the 4V application of the astrocytic signal blocker, fluorocitrate (FC; 5 nmol), alone, had no effect on blood glucose levels, 2DG-induced increases in glucose were blocked by 4V FC. The 4V effect of 2DG to increase glycemia was also blocked by the pretreatment with caffeine (nonselective adenosine antagonist) or a potent adenosine A1 antagonist (8-cyclopentyl-1,3-dipropylxanthine; DPCPX) but not the NMDA antagonist (MK-801). These results suggest that CNS detection of glucopenia is mediated by astrocytes and that astrocytic release of adenosine that occurs after hypoglycemia may cause the activation of downstream neural circuits that drive CRR.

The role of astrocytes in the hypothalamic response and adaptation to metabolic signals

The hypothalamus is crucial in the regulation of homeostatic functions in mammals, with the disruption of hypothalamic circuits contributing to chronic conditions such as obesity, diabetes mellitus, hypertension, and infertility. Metabolic signals and hormonal inputs drive functional and morphological changes in the hypothalamus in attempt to maintain metabolic homeostasis. However, the dramatic increase in the incidence of obesity and its secondary complications, such as type 2 diabetes, have evidenced the need to better understand how this system functions and how it can go awry. Growing evidence points to a critical role of astrocytes in orchestrating the hypothalamic response to metabolic cues by participating in processes of synaptic transmission, synaptic plasticity and nutrient sensing. These glial cells express receptors for important metabolic signals, such as the anorexigenic hormone leptin, and determine the type and quantity of nutrients reaching their neighboring neurons. Understanding the mechanisms by which astrocytes participate in hypothalamic adaptations to changes in dietary and metabolic signals is fundamental for understanding the neuroendocrine control of metabolism and key in the search for adequate treatments of metabolic diseases.

The contribution of hypothalamic macroglia to the regulation of energy homeostasis

The hypothalamus is critical for the regulation of energy homeostasis. Genetic and pharmacologic studies have identified a number of key hypothalamic neuronal circuits that integrate signals controlling food intake and energy expenditure. Recently, studies have begun to emerge demonstrating a role for non-neuronal cell types in the regulation of energy homeostasis. In particular the potential importance of different glial cell types is increasingly being recognized. A number of studies have described changes in the activity of hypothalamic macroglia (principally astrocytes and tanycytes) in response to states of positive and negative energy balance, such as obesity and fasting. This article will review these studies and discuss how these findings are changing our understanding of the cellular mechanisms by which energy homeostasis is regulated.

Hindbrain glucoprivation effects on gastric vagal reflex circuits and gastric motility in the rat are suppressed by the astrocyte inhibitor fluorocitrate

Fasting and hypoglycemia elicit powerful gastrointestinal contractions. Whereas the relationship between utilizable nutrient and gastric motility is well recognized, the explanation of this phenomenon has remained incomplete. A relatively recent controversial report suggested that astrocytes in the dorsal hindbrain may be the principal detectors of glucoprivic stimuli. Our own studies also show that a subset of astrocytes in the solitary nucleus (NST) is activated by low glucose. It is very likely that information about glucopenia may directly impact gastric control because the hindbrain is also the location of the vago-vagal reflex circuitry regulating gastric motility. Our in vivo single unit neurophysiological recordings in intact rats show fourth ventricular application of 2-deoxyglucose (2-DG) inhibits NST neurons and activates dorsal motor nucleus (DMN) neurons involved in the gastric accommodation reflex. Additionally, as shown in earlier studies, either systemic insulin or central 2-DG causes an increase in gastric motility. These effects on motility were blocked by fourth ventricle pretreatment with the astrocyte inactivator fluorocitrate. Fluorocitrate administered alone has no effect on gastric-NST or -DMN neuron responsiveness, or on gastric motility. These results suggest that glucoprivation-induced increases in gastric motility are dependent on intact hindbrain astrocytes.

Effect of insulin-induced hypoglycaemia on the central nervous system: evidence from experimental studies

Insulin-induced hypoglycaemia (IIH) is a major acute complication in type 1 as well as in type 2 diabetes, particularly during intensive insulin therapy. The brain plays a central role in the counter-regulatory response by eliciting parasympathetic and sympathetic hormone responses to restore normoglycaemia. Brain glucose concentrations, being approximately 15-20% of the blood glucose concentration in humans, are rigorously maintained during hypoglycaemia through adaptions such as increased cerebral glucose transport, decreased cerebral glucose utilisation and, possibly, by using central nervous system glycogen as a glucose reserve. However, during sustained hypoglycaemia, the brain cannot maintain a sufficient glucose influx and, as the cerebral hypoglycaemia becomes severe, electroencephalogram changes, oxidative stress and regional neuronal death ensues. With particular focus on evidence from experimental studies on nondiabetic IIH, this review outlines the central mechanisms behind the counter-regulatory response to IIH, as well as cerebral adaption to avoid sequelae of cerebral neuroglycopaenia, including seizures and coma.

Astrocytes in the nucleus of the solitary tract are activated by low glucose or glucoprivation: evidence for glial involvement in glucose homeostasis

Glucose homeostasis is maintained through interplay between central and peripheral control mechanisms which are aimed at storing excess glucose following meals and mobilizing these same stores during periods of fasting. The nucleus of the solitary tract (NST) in the dorsal medulla has long been associated with the central detection of glucose availability and the control of glucose homeostasis. Recent evidence has emerged which supports the involvement of astrocytes in glucose homeostasis. The aim of the present study was to investigate whether NST-astrocytes respond to physiologically relevant decreases in glucose availability, in vitro, as well as to the presence of the glucoprivic compound 2-deoxy-D-Glucose. This report demonstrates that some NST-astrocytes are capable of responding to low glucose or glucoprivation by increasing cytoplasmic calcium; a change that reverses with restoration of normal glucose availability. While some NST-neurons also demonstrate an increase in calcium signaling during low glucose availability, this effect is smaller and somewhat delayed compared to those observed in adjacent astrocytes. TTX did not abolish these hypoglycemia mediated responses of astrocytes, suggesting that NST-astrocytes may be directly sensing low glucose levels as opposed to responding to neuronal detection of hypoglycemia. Thus, chemodetection of low glucose by NST-astrocytes may play an important role in the autonomic regulation of glucose homeostasis.

Astrocytes in the hindbrain detect glucoprivation and regulate gastric motility

Glucoprivation is a strong signal for the initiation of gastrointestinal contractions. While this relationship between utilizable nutrient levels and gastric motility has been recognized for more than 100 years, the explanation of this phenomenon has remained incomplete. Using widely differing approaches, recent work has suggested that the hindbrain is responsible for this chemoreflex effect. Surprisingly, astrocytes may be the main glucodetector elements under hypoglycemic conditions. Our own work using in vitro live cell calcium imaging shows that astrocytes in the NST increase cytoplasmic calcium in a concentration dependent manner in reaction to reductions in glucose. This effect is reversed on restoration of normal glucose concentrations. In vivo single unit neurophysiological recordings show that brainstem neurons driving gastric motility are activated by glucoprivic stimuli. Studies in intact animals verify that both dorsal medullary and systemic glucoprivation significantly increases gastric motility. Astrocyte inactivation with fluorocitrate blocks the pro-motility effects of glucoprivation. Thus, it appears that intact astrocyte signaling may be essential to glucoregulatory control over gastric motility.

Sensing of glucose in the brain

The brain, and in particular the hypothalamus and brainstem, have been recognized for decades as important centers for the homeostatic control of feeding, energy expenditure, and glucose homeostasis. These structures contain neurons and neuronal circuits that may be directly or indirectly activated or inhibited by glucose, lipids, or amino acids. The detection by neurons of these nutrient cues may become deregulated, and possibly cause metabolic diseases such as obesity and diabetes. Thus, there is a major interest in identifying these neurons, how they respond to nutrients, the neuronal circuits they form, and the physiological function they control. Here I will review some aspects of glucose sensing by the brain. The brain is responsive to both hyperglycemia and hypoglycemia, and the glucose sensing cells involved are distributed in several anatomical sites that are connected to each other. These eventually control the activity of the sympathetic or parasympathetic nervous system, which regulates the function of peripheral organs such as liver, white and brown fat, muscle, and pancreatic islets alpha and beta cells. There is now evidence for an extreme diversity in the sensing mechanisms used, and these will be reviewed.

Impaired branched chain amino acid metabolism alters feeding behavior and increases orexigenic neuropeptide expression in the hypothalamus

Elevation of dietary or brain leucine appears to suppress food intake via a mechanism involving mechanistic target of rapamycin, AMPK, and/or branched chain amino acid (BCAA) metabolism. Mice bearing a deletion of mitochondrial branched chain aminotransferase (BCATm), which is expressed in peripheral tissues (muscle) and brain glia, exhibit marked increases in circulating BCAAs. Here, we test whether this increase alters feeding behavior and brain neuropeptide expression. Circulating and brain levels of BCAAs were increased two- to four-fold in BCATm-deficient mice (KO). KO mice weighed less than controls (25·9 vs 20·4 g, P<0·01), but absolute food intake was relatively unchanged. In contrast to wild-type mice, KO mice preferred a low-BCAA diet to a control diet (P<0·05) but exhibited no change in preference for low- vs high-protein (HP) diets. KO mice also exhibited low leptin levels and increased hypothalamic Npy and Agrp mRNA. Normalization of circulating leptin levels had no effect on either food preference or the increased Npy and Agrp mRNA expression. If BCAAs act as signals of protein status, one would expect reduced food intake, avoidance of dietary protein, and reduction in neuropeptide expression in BCATm-KO mice. Instead, these mice exhibit an increased expression of orexigenic neuropeptides and an avoidance of BCAAs but not HP. These data thus suggest that either BCAAs do not act as physiological signals of protein status or the loss of BCAA metabolism within brain glia impairs the detection of protein balance.

Astrocytes as brain interoceptors

Astrocytes form a vascular-neuronal interface and provide CNS neural networks with essential structural and metabolic support. They embrace all penetrating arterioles and capillaries, enwrap multiple neuronal somata, thousands of individual synapses, and upon activation release gliotransmitters (ATP, glutamate and D-serine) capable of modulating neuronal activity. The aim of this brief report is to review recent data implicating astrocytes in the brain mechanisms responsible for the detection of different sensory modalities and transmitting sensory information to the relevant neural networks controlling vital behaviours. The concept of astrocytes as brain interoceptors is strongly supported by our recent data obtained from studies of the central nervous mechanisms underlying the chemosensory control of breathing. At the level of the medulla oblongata, astrocytes indeed act as functional central respiratory chemoreceptors, sensing changes in the arterial blood and brain levels of /pH and then imparting these changes on the activity of the respiratory network to induce adaptive changes in lung ventilation.

Glial cells of the nucleus tractus solitarius as partners of the dorsal hindbrain regulation of energy balance: a proposal for a working hypothesis

While the evidences emphasizing the role of astroglial cells in numerous aspects of information processing within the brain merges, the literature dealing with the involvement of this cell population in the signalization involved in feeding behavior and energetic homeostasis remains scarce. Nevertheless, some clues are now available indicating that glia could play a dynamic role in the regulation of energy balance, and that strengthening research effort in this field may further our understanding of the mechanisms controlling feeding behaviour. In the present review, we have summarized recent data indicating that the multifaceted glial compartment of the brainstem should be considered in future research aimed at identifying feeding-related processes operating at this level.

Brain glucose sensing, counterregulation, and energy homeostasis

Neuronal circuits in the central nervous system play a critical role in orchestrating the control of glucose and energy homeostasis. Glucose, beside being a nutrient, is also a signal detected by several glucose-sensing units that are located at different anatomical sites and converge to the hypothalamus to cooperate with leptin and insulin in controlling the melanocortin pathway.

Desperately seeking sugar: glial cells as hypoglycemia sensors

A life-saving response to hypoglycemia requires rapid sensing of decreases in glycemia and consequent brisk glucagon secretion. Preceding studies have shown that mice lacking glucose transporter type 2 (GLUT2) lose this response. In this issue of the JCI, Marty et al. report that glucose sensing and consequent pancreatic glucagon secretion are restored by re-expression of GLUT2 in glial but not neuronal cells. A new, glucose-sensing role is ascribed to GLUT2-expressing glial cells.

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.

An astrocyte toxin influences the pattern of breathing and the ventilatory response to hypercapnia in neonatal rats

Recent in vitro data suggest that astrocytes may modulate respiration. To examine this question in vivo, we treated 5-day-old rat pups with methionine sulfoximine (MS), a compound that alters carbohydrate and glutamate metabolism in astrocytes, but not neurons. MS-treated pups displayed a reduced breathing frequency (f) in baseline conditions relative to saline-treated pups. Hypercapnia (5% CO(2)) increased f in both groups, but f still remained significantly lower in the MS-treated group. No differences between treatment groups in the responses to hypoxia (8% O(2)) were observed. Also, MS-treated rats showed an enhanced accumulation of glycogen in neurons of the facial nucleus, the nucleus ambiguus, and the hypoglossal nucleus, structures that regulate respiratory activity and airway patency. An altered transfer of nutrient molecules from astrocytes to neurons may underlie these effects of MS, although direct effects of MS upon neurons or upon peripheral structures that regulate respiration cannot be completely ruled out as an explanation.

GLUT2 immunoreactivity in Gomori-positive astrocytes of the hypothalamus

A specialized subtype of astrocyte, the Gomori-positive (GP) astrocyte, is unusually abundant and prominent in the arcuate nucleus of the hypothalamus. GP astrocytes possess cytoplasmic granules derived from degenerating mitochondria. GP granules are highly stained by Gomori's chrome alum hematoxylin stain, by the Perl's reaction for iron, or by toluidine blue. The source of the oxidative stress causing mitochondrial damage in GP astrocytes is uncertain, but such damage could arise from the oxidative metabolism of glucose transported into astrocytes by high-capacity GLUT2 glucose transporters. In accord with this hypothesis, the reported anatomical distribution of astrocytes staining positively for GLUT2 glucose transporters closely matches that of GP astrocytes. To examine whether or not these two staining procedures detect the same population of astrocytes, immunocytochemistry was performed on semithin sections to detect GLUT2 protein and sections were then stained with toluidine blue to detect GP granules. It was determined that GP astrocytes are frequently immunoreactive for the GLUT2 transporter protein. These data support the possibility that GP astrocytes may have an important influence upon the reactivity of the hypothalamus to glucose and that a specialized glucose metabolism may in part underlie the development of mitochondrial abnormalities in hypothalamic GP astrocytes.

Effect of central administration of carnosine and its constituents on behaviors in chicks

Even though their contents in the brain are high, the function of brain carnosine and its constituents has not been clarified. Both carnosine and anserine inhibited food intake in a dose dependent fashion when injected intracerebroventricularly. The constituents of carnosine, beta-alanine (beta-Ala) and l-histidine (His), also inhibited food intake, but their effects were weaker than carnosine itself. Co-administration with beta-Ala and His inhibited food intake similar to carnosine, but also altered other behaviors. Injection of carnosine induced hyperactivity and increased plasma corticosterone level, whereas beta-Ala plus His induced hypoactivity manifested as sleep-like behavior. This later effect seemed to be derived from beta-Ala, not His. These results suggest that central carnosine may act in the brain of chicks to regulate brain function and/or behavior in a manner different from its constituents.

D-chiro-Inositol enhances effects of hypothalamic toxin gold-thioglucose

D-chiro-Inositol (DCI) enhances reproductive function in insulin-resistant women with polycystic ovarian disease and enhances the effects of insulin in the periphery, suggesting that this compound may act in part by sensitizing the hypothalamus to effects of insulin. Effects of gold-thioglucose (GTG) to produce hypothalamic lesions and subsequent obesity are insulin-dependent, suggesting that responses to GTG may be a marker of hypothalamic sensitivity to insulin. To assess these hypotheses, the present study assessed if DCI would enhance the ability of a subthreshhold dose of GTG to produce hypothalamic lesions and subsequent obesity. At the subthreshhold dose used (0.4 mg/kg i.p.), injection of GTG produced no subsequent effect on body weight compared to saline; similarly, at the dose of DCI used (10 mg/kg/day in drinking water), DCI produced no effect on body weight. In contrast, when given to mice exposed to DCI, this dose of GTG produced significant increase in body weight and evidence of an enhanced medial arcuate hypothalamic lesion.

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.

Anatomical relationship between specialized astrocytes and leptin-sensitive neurones

We have previously reported that a specialized subpopulation of astrocytes in the arcuate nucleus of the hypothalamus show an unusually intense immunoreactivity for brain fatty acid binding protein (bFABP). Since bFABP has been shown to regulate the activity of an enzyme, fatty acid synthase, that has a potent influence upon the regulation of feeding by the hypothalamus, it was of interest to determine if bFABP + astrocytes are positioned to potentially influence the activity of feeding-regulating neurones. In this study, we examined the anatomical relationship between specialized arcuate astrocytes immunoreactive for bFABP and feeding-regulating neurones that are responsive to leptin and which are immunoreactive for the transcription factor STAT3. The results show that both cell types are abundant in the arcuate nucleus of the hypothalamus and are frequently closely adjacent to each other. This study provides an anatomical basis for the possibility that specialized arcuate astrocytes regulate the function of leptin-sensitive, feeding-regulating neurones of the arcuate nucleus.

In vivo and in vitro glycogenic effects of methionine sulfoximine are different in two inbred strains of mice

We investigated the relationship between brain glycogen anabolism and methionine sulfoximine (MSO)-induced seizures in two inbred mouse strains that presented differential susceptibility to the convulsant. CBA/J was considered a MSO-high-reactive strain and C57BL/6J a MSO-low-reactive strain. Accordingly, the dose of MSO needed to induce seizures in CBA/J mice is lower than that in C57BL/6J mice, and CBA/J mice which had seizures, died during the first convulsion. In addition, the time--course of the MSO effect is faster in CBA/J mice than that in C57BL/6J mice. Analyses were performed in C57BL/6J and CBA/J mice after administration of 75 (subconvulsive dose) and 40 mg/kg of MSO (subconvulsive dose, not lethal dose), respectively. In the preconvulsive period, MSO induced an increase in the brain glycogen content of C57BL/6J mice only. Twenty-four hours after MSO administration, the brain glycogen content increased in both strains. The activity and expression of fructose-1,6-bisphosphatase, the last key enzyme of the gluconeogenic pathway, were increased in MSO-treated C57BL/6J mice as compared to control mice, at all experimental time points, whereas they were increased in CBA/J mice only 24 h after MSO administration. These latter results correspond to CBA/J mice that did not have seizures. Interestingly, the differences observed in vivo were consistent with results in primary cultured astrocytes from the two strains. This data suggests that the metabolism impairment, which was not a consequence of seizures, could be related to the difference in seizure susceptibility between the two strains, depending on their genetic background.