Regulation of astrocyte metabolism by mitochondrial translocator protein 18 kDa.

The mitochondrial translocator protein 18 kDa (TSPO) has been linked to functions from steroidogenesis to regulation of cellular metabolism and is an attractive therapeutic target for chronic CNS inflammation. Studies in Leydig cells and microglia indicate that TSPO function may vary between cells depending on their specialized roles. Astrocytes are critical for providing trophic and metabolic support in the brain. Recent work has highlighted that TSPO expression increases in astrocytes under inflamed conditions and may drive astrocyte reactivity. Relatively little is known about the role TSPO plays in regulating astrocyte metabolism and whether this protein is involved in immunometabolic processes in these cells. Using TSPO-deficient (TSPO-/- ) mouse primary astrocytes in vitro (MPAs) and a human astrocytoma cell line (U373 cells), we performed extracellular metabolic flux analyses. We found that TSPO deficiency reduced basal cellular respiration and attenuated the bioenergetic response to glucopenia. Fatty acid oxidation was increased, and lactate production was reduced in TSPO-/- MPAs and U373 cells. Co-immunoprecipitation studies revealed that TSPO forms a complex with carnitine palmitoyltransferase 1a in U373 and MPAs, presenting a mechanism wherein TSPO may regulate FAO in these cells. Compared to TSPO+/+ cells, in TSPO-/- MPAs we observed attenuated tumor necrosis factor release following 3 h lipopolysaccharide (LPS) stimulation, which was enhanced at 24 h post-LPS stimulation. Together these data suggest that while TSPO acts as a regulator of metabolic flexibility, TSPO deficiency does not appear to modulate the metabolic response of MPAs to inflammation, at least in response to the model used in this study.

Impact of chemogenetic activation of dorsal vagal complex astrocytes in mice on adaptive glucoregulatory responses.

The dorsal vagal complex (DVC) regulates diverse aspects of physiology including food intake and blood glucose homeostasis. Astrocytes play an active role in regulating DVC function and, by extension, physiological parameters. DVC astrocytes in ex vivo slices respond to low tissue glucose. The response of neurons to low glucose is conditional on intact astrocyte signalling in slice preparations, suggesting astrocytes are primary sensors of glucose deprivation (glucoprivation). Based on these published findings we hypothesised that in vivo DVC astrocyte manipulation with chemogenetics would be sufficient to alter physiological responses that control blood glucose. We found that 2-h after systemic 2-DG-induced glucoprivation there were no observable changes in morphology of glial fibrillary acidic protein (GFAP)-immunoreactive DVC cells, specifically those in the nucleus of the solitary tract (NTS). Chemogenetic activation of DVC astrocytes was sufficient to suppress nocturnal food intake by reducing both meal size and meal number and this manipulation also suppressed 2-DG-induced glucoprivic food intake. Chemogenetic activation of DVC astrocytes did not increase basal blood glucose nor protect against insulin-induced hypoglycaemia. In male mice, chemogenetic DVC astrocyte activation did not alter glucose tolerance. In female mice, the initial glucose excursion was reduced in a glucose tolerance test, suggesting enhanced glucose absorption. Based on our data and published work, we propose that DVC astrocytes may play an indispensable homeostatic role, that is, are necessary to maintain the function of glucoregulatory neuronal circuitry, but alone their bulk activation is not sufficient to result in adaptive glucoregulatory responses. It is possible that there are state-dependent effects and/or DVC astrocyte subsets that have this specialised role, but this was unresolvable using the experimental approaches employed here.

Glutamate Prevents Altered Mitochondrial Function Following Recurrent Low Glucose in Hypothalamic but Not Cortical Primary Rat Astrocytes.

Astrocytes contribute to glutamatergic signalling, which is required for hypoglycaemia counterregulation and is impaired by recurrent insulin-induced hypoglycaemia. This study examined the glutamate response of astrocytes when challenged with acute and recurrent low glucose (RLG) exposure. The metabolic responses of cortical (CRTAS) and hypothalamic (HTAS) primary rat astrocytes were measured in acute and recurrent low glucose using extracellular flux analyses. RLG caused mitochondrial adaptations in both HTAS and CRTAS, many of which were attenuated by glutamate exposure during low glucose (LG) treatments. We observed an increase in capacity of HTAS to metabolise glutamine after RLG exposure. Demonstrating astrocytic heterogeneity in the response to LG, CRTAS increased cellular acidification, a marker of glycolysis in LG, whereas this decreased in HTAS. The directional change in intracellular Ca2+ levels of each cell type, correlated with the change in extracellular acidification rate (ECAR) during LG. Further examination of glutamate-induced Ca2+ responses in low glucose treated CRTAS and HTAS identified sub-populations of glucose-excited- and glucose-inhibited-like cells with differing responses to glutamate. Lastly, release of the gliotransmitter ATP by HTAS was elevated by RLG, both with and without concurrent glutamate exposure. Therefore, hypothalamic astrocytes adapt to RLG by increasing glutamate uptake and oxidation in a manner that prevents RLG-induced mitochondrial adaptations.

Time-optimized feeding is beneficial without enforced fasting.

Time-restricted feeding (TRF) studies underscore that when food is consumed during the daily cycle is important for weight gain/loss because the circadian clock rhythmically modulates metabolism. However, the interpretation of previous TRF studies has been confounded by study designs that introduced an extended period of enforced fasting. We introduce a novel time-optimized feeding (TOF) regimen that disentangles the effects of phase-dependent feeding from the effects of enforced fasting in mice, as well as providing a laboratory feeding protocol that more closely reflects the eating patterns of humans who usually have 24 hour access to food. Moreover, we test whether a sudden switch from ad libitum food access to TRF evokes a corticosterone (stress) response. Our data indicate that the timing of high-fat feeding under TOF allows most of the benefit of TRF without obligatory fasting or evoking a stress response. This benefit occurs through stable temporal coupling of carbohydrate/lipid oxidation with feeding. These results highlight that timing the ingestion of calorically dense foods to optimized daily phases will enhance lipid oxidation and thereby limit fat accumulation.

Absence of the mitochondrial translocator protein 18 kDa in mice does not affect body weight or food intake responses to altered energy availability.

Changes in mitochondrial function in a variety of cells/tissues are critical for orchestrating systemic energy homeostasis and are linked to the development of obesity and many of its comorbidities. The mitochondrial translocator protein of 18 kDa (TSPO) is expressed in organs throughout the body, including the brain, liver, adipose tissue, gonads and adrenal glands, where it is implicated in regulating steroidogenesis and cellular metabolism. Prior work from our group and others has shown that, in rodents, TSPO levels are altered in adipose tissue by obesity and that modulation of TSPO activity may impact systemic glucose homeostasis. Furthermore, in vitro studies in a variety of cell types have implicated TSPO in mediating cellular energetics and substrate utilisation. Although mice with germline global TSPO deficiency (TSPO-/- ) have no reported changes in body weight under standard husbandry conditions, we hypothesised that, given the roles of TSPO in regulating mitochondrial function and cellular metabolic flexibility, these animals may have alterations in their systemic response to altered energy availability, either nutritional excess or insufficiency. In agreement with published work, compared to wild-type (TSPO+/+ ) littermates, TSPO-/- mice of both sexes did not exhibit differences in body weight on standard chow. Furthermore, following a 12-hour overnight fast, there was no difference in weight loss or compensatory food intake during re-feeding. Five weeks of feeding a high-fat diet (HFD) did not reveal any impact of the absence of TSPO on body weight gain in either male or female mice. Basal blood glucose levels and glucose clearance in a glucose tolerance test were influenced by feeding a HFD diet but not by genotype. In conclusion, in the paradigms examined, germline global deletion of TSPO did not change the physiological response to deviations in systemic energy availability at the whole organism level.

Brain-Body Control of Glucose Homeostasis-Insights From Model Organisms.

Tight regulation of blood glucose is essential for long term health. Blood glucose levels are defended by the correct function of, and communication between, internal organs including the gastrointestinal tract, pancreas, liver, and brain. Critically, the brain is sensitive to acute changes in blood glucose level and can modulate peripheral processes to defend against these deviations. In this mini-review we highlight select key findings showcasing the utility, strengths, and limitations of model organisms to study brain-body interactions that sense and control blood glucose levels. First, we discuss the large platform of genetic tools available to investigators studying mice and how this field may yet reveal new modes of communication between peripheral organs and the brain. Second, we discuss how rats, by virtue of their size, have unique advantages for the study of CNS control of glucose homeostasis and note that they may more closely model some aspects of human (patho)physiology. Third, we discuss the nascent field of studying the CNS control of blood glucose in the zebrafish which permits ease of genetic modification, large-scale measurements of neural activity and live imaging in addition to high-throughput screening. Finally, we briefly discuss glucose homeostasis in drosophila, which have a distinct physiology and glucoregulatory systems to vertebrates.

Brain Permeable AMP-Activated Protein Kinase Activator R481 Raises Glycaemia by Autonomic Nervous System Activation and Amplifies the Counterregulatory Response to Hypoglycaemia in Rats.

Aim: We evaluated the efficacy of a novel brain permeable "metformin-like" AMP-activated protein kinase activator, R481, in regulating glucose homeostasis. Materials and Methods: We used glucose sensing hypothalamic GT1-7 neuronal cells and pancreatic αTC1.9 α-cells to examine the effect of R481 on AMPK pathway activation and cellular metabolism. Glucose tolerance tests and hyperinsulinemic-euglycemic and hypoglycemic clamps were used in Sprague-Dawley rats to assess insulin sensitivity and hypoglycemia counterregulation, respectively. Results: In vitro, we demonstrate that R481 increased AMPK phosphorylation in GT1-7 and αTC1.9 cells. In Sprague-Dawley rats, R481 increased peak glucose levels during a glucose tolerance test, without altering insulin levels or glucose clearance. The effect of R481 to raise peak glucose levels was attenuated by allosteric brain permeable AMPK inhibitor SBI-0206965. This effect was also completely abolished by blockade of the autonomic nervous system using hexamethonium. During hypoglycemic clamp studies, R481 treated animals had a significantly lower glucose infusion rate compared to vehicle treated controls. Peak plasma glucagon levels were significantly higher in R481 treated rats with no change to plasma adrenaline levels. In vitro, R481 did not alter glucagon release from αTC1.9 cells, but increased glycolysis. Non brain permeable AMPK activator R419 enhanced AMPK activity in vitro in neuronal cells but did not alter glucose excursion in vivo. Conclusions: These data demonstrate that peripheral administration of the brain permeable "metformin-like" AMPK activator R481 increases blood glucose by activation of the autonomic nervous system and amplifies the glucagon response to hypoglycemia in rats. Taken together, our data suggest that R481 amplifies the counterregulatory response to hypoglycemia by a central rather than a direct effect on the pancreatic α-cell. These data provide proof-of-concept that central AMPK could be a target for future drug development for prevention of hypoglycemia in diabetes.

Astrocytes in the nucleus of the solitary tract: Contributions to neural circuits controlling physiology.

The nucleus of the solitary tract (NTS) is the primary brainstem centre for the integration of physiological information from the periphery transmitted via the vagus nerve. In turn, the NTS feeds into downstream circuits regulating physiological parameters. Astrocytes are glial cells which have key roles in maintaining CNS tissue homeostasis and regulating neuronal communication. Recently an increasing number of studies have implicated astrocytes in the regulation of synaptic transmission and physiology. This review aims to highlight evidence for a role for astrocytes in the functions of the NTS. Astrocytes maintain and modulate NTS synaptic transmission contributing to the control of diverse physiological systems namely cardiovascular, respiratory, glucoregulatory, and gastrointestinal. In addition, it appears these cells may have a role in central control of feeding behaviour. As such these cells are a key component of signal processing and physiological control by the NTS.

The metabolic response to inflammation in astrocytes is regulated by nuclear factor-kappa B signaling.

Inflammation and metabolism are intrinsically linked with inflammatory stimuli inducing metabolic changes in cells and, in turn, metabolic capacity determining cellular inflammatory responses. Although well characterized in peripheral immune cells there is comparatively less known about these "immunometabolic" responses in astrocytes. In this study, we tested the hypothesis that the astrocytic inflammatory response driven by nuclear factor-kappa B (NF-κB) signaling is dependent on glycolytic metabolism. Using mouse primary cortical astrocyte cultures, we assessed changes in cellular metabolism after exposure to lipopolysaccharide (LPS), with cytokine ELISAs and immunoblotting being used to measure inflammatory responses. Results indicate temporally distinct metabolic adaptations to pro-inflammatory stimulation in astrocytes: 3 hr LPS treatment increased glycolysis but did not alter mitochondrial metabolism, while following 24 hr of LPS treatment we observed increased oxidative phosphorylation, and decreased glycolytic capacity and glucose uptake, partly due to reduced glucose transporter 1 expression. Inhibition of NF-κB signaling with the IKK-beta inhibitor TPCA-1 prevented the LPS induced changes to glycolysis and oxidative phosphorylation. Furthermore, TPCA-1 treatment altered both glycolysis and oxidative phosphorylation independently from inflammatory stimulation, indicating a role for NF-κB signaling in regulation of basal metabolism in astrocytes. Inhibition of glycolysis with 2-deoxyglucose significantly attenuated LPS-induced cytokine release and NF-κB phosphorylation, indicating that intact glycolysis is required for the full inflammatory response to LPS. Together our data indicate that astrocytes display immunometabolic responses to acute LPS stimulation which may represent a potential therapeutic target for neuroinflammatory disorders.

Immunometabolic Changes in Glia - A Potential Role in the Pathophysiology of Obesity and Diabetes.

Chronic low-grade inflammation is a feature of the pathophysiology of obesity and diabetes in the CNS as well as peripheral tissues. Glial cells are critical mediators of the response to inflammation in the brain. Key features of glia include their metabolic flexibility, sensitivity to changes in the CNS microenvironment, and ability to rapidly adapt their function accordingly. They are specialised cells which cooperate to promote and preserve neuronal health, playing important roles in regulating the activity of neuronal networks across the brain during different life stages. Increasing evidence points to a role of glia, most notably astrocytes and microglia, in the systemic regulation of energy and glucose homeostasis in the course of normal physiological control and during disease. Inflammation is an energetically expensive process that requires adaptive changes in cellular metabolism and, in turn, metabolic intermediates can also have immunomodulatory actions. Such "immunometabolic" changes in peripheral immune cells have been implicated in contributing to disease pathology in obesity and diabetes. This review will discuss the evidence for a role of immunometabolic changes in glial cells in the systemic regulation of energy and glucose homeostasis, and how this changes in the context of obesity and diabetes.

Regulation of food intake by astrocytes in the brainstem dorsal vagal complex.

A role for glial cells in brain circuits controlling feeding has begun to be identified with hypothalamic astrocyte signaling implicated in regulating energy homeostasis. The nucleus of the solitary tract (NTS), within the brainstem dorsal vagal complex (DVC), integrates vagal afferent information from the viscera and plays a role in regulating food intake. We hypothesized that astrocytes in this nucleus respond to, and influence, food intake. Mice fed high-fat chow for 12 hr during the dark phase showed NTS astrocyte activation, reflected in an increase in the number (65%) and morphological complexity of glial-fibrillary acidic protein (GFAP)-immunoreactive cells adjacent to the area postrema (AP), compared to control chow fed mice. To measure the impact of astrocyte activation on food intake, we delivered designer receptors exclusively activated by designer drugs (DREADDs) to DVC astrocytes (encompassing NTS, AP, and dorsal motor nucleus of the vagus) using an adeno-associated viral (AAV) vector (AAV-GFAP-hM3Dq_mCherry). Chemogenetic activation with clozapine-N-oxide (0.3 mg/kg) produced in greater morphological complexity in astrocytes and reduced dark-phase feeding by 84% at 4 hr postinjection compared with vehicle treatment. hM3Dq-activation of DVC astrocytes also reduced refeeding after an overnight fast (71% lower, 4 hr postinjection) when compared to AAV-GFAP-mCherry expressing control mice. DREADD-mediated astrocyte activation did not impact locomotion. hM3Dq activation of DVC astrocytes induced c-FOS in neighboring neuronal feeding circuits (including in the parabrachial nucleus). This indicates that NTS astrocytes respond to acute nutritional excess, are involved in the integration of peripheral satiety signals, and can reduce food intake when activated.

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

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

Astrocytes in neuroendocrine systems: An overview.

A class of glial cell, astrocytes, is highly abundant in the central nervous system (CNS). In addition to maintaining tissue homeostasis, astrocytes regulate neuronal communication and synaptic plasticity. There is an ever-increasing appreciation that astrocytes are involved in the regulation of physiology and behaviour in normal and pathological states, including within neuroendocrine systems. Indeed, astrocytes are direct targets of hormone action in the CNS, via receptors expressed on their surface, and are also a source of regulatory neuropeptides, neurotransmitters and gliotransmitters. Furthermore, as part of the neurovascular unit, astrocytes can regulate hormone entry into the CNS. This review is intended to provide an overview of how astrocytes are impacted by and contribute to the regulation of a diverse range of neuroendocrine systems: energy homeostasis and metabolism, reproduction, fluid homeostasis, the stress response and circadian rhythms.

Basal fatty acid oxidation increases after recurrent low glucose in human primary astrocytes.

AIMS/HYPOTHESIS: Hypoglycaemia is a major barrier to good glucose control in type 1 diabetes. Frequent hypoglycaemic episodes impair awareness of subsequent hypoglycaemic bouts. Neural changes underpinning awareness of hypoglycaemia are poorly defined and molecular mechanisms by which glial cells contribute to hypoglycaemia sensing and glucose counterregulation require further investigation. The aim of the current study was to examine whether, and by what mechanism, human primary astrocyte (HPA) function was altered by acute and recurrent low glucose (RLG). METHODS: To test whether glia, specifically astrocytes, could detect changes in glucose, we utilised HPA and U373 astrocytoma cells and exposed them to RLG in vitro. This allowed measurement, with high specificity and sensitivity, of RLG-associated changes in cellular metabolism. We examined changes in protein phosphorylation/expression using western blotting. Metabolic function was assessed using a Seahorse extracellular flux analyser. Immunofluorescent imaging was used to examine cell morphology and enzymatic assays were used to measure lactate release, glycogen content, intracellular ATP and nucleotide ratios. RESULTS: AMP-activated protein kinase (AMPK) was activated over a pathophysiologically relevant glucose concentration range. RLG produced an increased dependency on fatty acid oxidation for basal mitochondrial metabolism and exhibited hallmarks of mitochondrial stress, including increased proton leak and reduced coupling efficiency. Relative to glucose availability, lactate release increased during low glucose but this was not modified by RLG. Basal glucose uptake was not modified by RLG and glycogen levels were similar in control and RLG-treated cells. Mitochondrial adaptations to RLG were partially recovered by maintaining euglycaemic levels of glucose following RLG exposure. CONCLUSIONS/INTERPRETATION: Taken together, these data indicate that HPA mitochondria are altered following RLG, with a metabolic switch towards increased fatty acid oxidation, suggesting glial adaptations to RLG involve altered mitochondrial metabolism that could contribute to defective glucose counterregulation to hypoglycaemia in diabetes.

Regulation of energy rheostasis by the melanocortin-3 receptor.

Like most homeostatic systems, adiposity in mammals is defended between upper and lower boundary conditions. While leptin and melanocortin-4 receptor (MC4R) signaling are required for defending energy set point, mechanisms controlling upper and lower homeostatic boundaries are less well understood. In contrast to the MC4R, deletion of the MC3R does not produce measurable hyperphagia or hypometabolism under normal conditions. However, we demonstrate that MC3R is required bidirectionally for controlling responses to external homeostatic challenges, such as caloric restriction or calorie-rich diet. MC3R is also required for regulated excursion from set point, or rheostasis, during pregnancy. Further, we demonstrate a molecular mechanism: MC3R provides regulatory inputs to melanocortin signaling, acting presynaptically on agouti-related protein neurons to regulate γ-aminobutyric acid release onto anorexigenic MC4R neurons, exerting boundary control on the activity of MC4R neurons. Thus, the MC3R is a critical regulator of boundary controls on melanocortin signaling, providing rheostatic control on energy storage.

CNS Targets of Adipokines.

Our understanding of adipose tissue as an endocrine organ has been transformed over the last 20 years. During this time, a number of adipocyte-derived factors or adipokines have been identified. This article will review evidence for how adipokines acting via the central nervous system (CNS) regulate normal physiology and disease pathology. The reported CNS-mediated effects of adipokines are varied and include the regulation of energy homeostasis, autonomic nervous system activity, the reproductive axis, neurodevelopment, cardiovascular function, and cognition. Due to the wealth of information available and the diversity of their known functions, the archetypal adipokines leptin and adiponectin will be focused on extensively. Other adipokines with established CNS actions will also be discussed. Due to the difficulties associated with studying CNS function on a molecular level in humans, the majority of our knowledge, and as such the studies described in this paper, comes from work in experimental animal models; however, where possible the relevant data from human studies are also highlighted. © 2017 American Physiological Society. Compr Physiol 7:1359-1406, 2017.

AMP-activated protein kinase (AMPK) activator A-769662 increases intracellular calcium and ATP release from astrocytes in an AMPK-independent manner.

AIM: To test the hypothesis that, given the role of AMP-activated protein kinase (AMPK) in regulating intracellular ATP levels, AMPK may alter ATP release from astrocytes, the main sources of extracellular ATP (eATP) within the brain. MATERIALS AND METHODS: Measurements of ATP release were made from human U373 astrocytoma cells, primary mouse hypothalamic (HTAS) and cortical astrocytes (CRTAS) and wild-type and AMPK α1/α2 null mouse embryonic fibroblasts (MEFs). Cells were treated with drugs known to modulate AMPK activity: A-769662, AICAR and metformin, for up to 3 hours. Intracellular calcium was measured using Fluo4 and Fura-2 calcium-sensitive fluorescent dyes. RESULTS: In U373 cells, A-769662 (100 µM) increased AMPK phosphorylation, whereas AICAR and metformin (1 mM) induced a modest increase or had no effect, respectively. Only A-769662 increased eATP levels, and this was partially blocked by AMPK inhibitor Compound C. A-769662-induced increases in eATP were preserved in AMPK α1/α2 null MEF cells. A-769662 increased intracellular calcium in U373, HTAS and CRTAS cells and chelation of intracellular calcium using BAPTA-AM reduced A-769662-induced eATP levels. A-769662 also increased ATP release from a number of other central and peripheral endocrine cell types. CONCLUSIONS: AMPK is required to maintain basal eATP levels but is not required for A-769662-induced increases in eATP. A-769662 (>50 µM) enhanced intracellular calcium levels leading to ATP release in an AMPK and purinergic receptor independent pathway.

Evidence for a novel functional role of astrocytes in the acute homeostatic response to high-fat diet intake in mice.

OBJECTIVE: Introduction of a high-fat diet to mice results in a period of voracious feeding, known as hyperphagia, before homeostatic mechanisms prevail to restore energy intake to an isocaloric level. Acute high-fat diet hyperphagia induces astrocyte activation in the rodent hypothalamus, suggesting a potential role of these cells in the homeostatic response to the diet. The objective of this study was to determine physiologic role of astrocytes in the acute homeostatic response to high-fat feeding. METHODS: We bred a transgenic mouse model with doxycycline-inducible inhibition of NFkappaB (NFκB) signaling in astrocytes to determine the effect of loss of NFκB-mediated astrocyte activation on acute high-fat hyperphagia. ELISA was used to measure the levels of markers of astrocyte activation, glial-fibrillary acidic protein (GFAP) and S100B, in the medial basal hypothalamus. RESULTS: Inhibition of NFκB signaling in astrocytes prevented acute high-fat diet-induced astrocyte activation and resulted in a 15% increase in caloric intake (P < 0.01) in the first 24 h after introduction of the diet. CONCLUSIONS: These data reveal a novel homeostatic role for astrocytes in the acute physiologic regulation of food intake in response to high-fat feeding.

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.